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Author name code: sheeley
ADS astronomy entries on 2022-09-14
author:"Sheeley, Neil R."
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Title: The Sun's Mean Line-of-Sight Field
Authors: Sheeley, Neil R., Jr
2022arXiv220803216S Altcode:
We regard the Sun-as-a-star magnetic field (i.e. the mean field) as
a filter for the spherical harmonic components of the photospheric
field, and calculate the transmission coefficients of this filter. The
coefficients for each harmonic, $Y_{l}^{m}$, are listed in three tables
according to their dependence on $B_{0}$, the observer's latitude in
the star's polar coordinate system. These coefficients are used to
interpret the 46-yr sequence of daily mean-field measurements at the
Wilcox Solar Observatory. We find that the non-axisymmetric part of the
field originates in the $Y_{1}^{1}$, $Y_{2}^{2}$, and a combination of
the $Y_{3}^{3}$ and $Y_{3}^{1}$ harmonic components. The axisymmetric
part of the field originates in $Y_{2}^{0}$ plus a $B_{0}$-dependent
combination of the $Y_{1}^{0}$ and $Y_{3}^{0}$ components. The
power spectrum of the field has peaks at frequencies corresponding
to the ~27-day synodic equatorial rotation period and its second
and third harmonics. Each of these peaks has fine structure on its
low-frequency side, indicating magnetic patterns that rotate slowly
under the influence of differential rotation and meridional flow. The
sidebands of the fundamental mode resolve into peaks corresponding
to periods of ~28.5 and ~30 days, which tend to occur at the start
of sunspot maximum, whereas the ~27-day period tends to occur toward
the end of sunspot maximum. We expect similar rotational sidebands
to occur in magnetic observations of other Sun-like stars and to be
a useful complement to asteroseismology studies of convection and
magnetic fields in those stars.
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Title: Acceleration of Coronal Mass Ejection Plasma in the Low Corona
as Measured by the Citizen CATE Experiment
Authors: Penn, Matthew J.; Baer, Robert; Walter, Donald; Pierce,
Michael; Gelderman, Richard; Ursache, Andrei; Elmore, David; Mitchell,
Adrianna; Kovac, Sarah; Hare, Honor; McKay, Myles; Jensen, Logan;
Watson, Zachary; Conley, Mike; Powers, Lynn; Lazarova, Marianna;
Wright, Joseph; Young, David; Isberner, Fred; Hart, C. Alexandra;
Sheeley, N. R., Jr.; Penn, Debbie; Allen-Penn, Kate; Alder, Bruce;
Alder, Ryan; Hall-Conley, Geri; Gerdes, David; Weber, Katherine;
Johnson, Jeffrey; Matzek, Gerald; Somes, Steven; Sobnosky, Rob;
McGowen, Robert; Meo, Michael; Proctor, Damani; Wessinger, Charlie;
Schilling, Jeannine; Kerr, Jay; Beltzer-Sweeney, Alexander; Falatoun,
Alex; Higgins, David; Boyce, Grady; Hettick, Jared; Blanco, Philip;
Dixon, Scott; Ardebilianfard, Sepehr; Boyce, Pat; Lighthill, Richard;
Lighthill, Denese; Anderson, David; Anderson, Mine; Schad, Thomas;
Smith, Sonna; Jensen, Declan; Allen, Anthony; Smith, Donavan; Brandon,
Gage; Earp, Joe; Earp, Jane; Blair, Bob; Claver, Chuck F.; Claver,
Jennifer A.; Claver, Ryan H.; Hoops, Danielle; Rivera, Esteban;
Gibson, Llanee; Hiner, Martin; Lann, Rein; Miller, Shaedyn; Briggs,
Burton; Davis, Karan; Jackson, Brian; Kautzsch, Kaleb; Sandidge,
Wesley; Lucas, Russell; Gregg, Duane; Kamenetzky, Julia; Rivera,
Tiffany; Shaw, Joe; Scherrer, Bryan; Sandbak, Dylan; McFate, Richard;
Harris, Wilson; Brasier, Zachery; McNeil, Stephen; Jensen, Jack;
Jensen, Makai; Moore, Mason; Temple, Alexandria; Vanderhorst, Thomas;
Kautz, Richard; Bellorado, Orion; Jenkins, LaVor R.; Pantuso, Corey;
Carey, Marley; Byrnes, Josh; Scholtens, Kyle; Web, Julian; Baker,
Brain; Barngrover, Katie; Hathaway, Drew; Smith, Kallen; Chandler,
Kellyn; Hinkle, Lydia; Chandler, Ione; Gisler, Galen; Benner, Jack;
Mas, Madison; Rogers, Maya; Moore, Prescott; Pelofske, Elijah;
Gulley, Stephen; Short, Beth; Crooker, Isabel; Hammock, Jennifer;
Cardenas, Katsina; Cardenas, Kateri; Wellman, Jennifer; Roy, Mark;
Meyer, Joe; Brough, Jalynne; Brough, Kameron; Nelson, Tim; Nelson,
Zack; Russell, Caleb; Bautz, Theresa; Weitzel, Eric; Team; Wistisen,
Michele; Aagard, Shae; Whipps, Zachary; Neuroth, Logan; Poste, Dawson;
Worthen, Connor; Gosain, Sanjay; Steward, Mark; Gosain, Vanshita;
Gosain, Ruchi; Jorgensen, Janet; Doucette, Eleanor; Doucette, Reba;
Iwen, Elliott; Cochran, Alexus; Stith, James; Scribner, Doug; Kenney,
Austen; Pisciotti, Kolby; Pease, Irene; Cynamon, Samuel; Cynamon,
Charles; Cynamon, Dawn; Tolbert, Bart; Dupree, Jean A.; Weremeichik,
Jeremy; Pindell, Nathan; Stives, Kristen; Simacek, Thomas K.; Simacek,
Yolanta G.; Simacek, Anne L.; Boeck, Wayne; Boeck, Andreea; Ryan,
Austin; Wierzorec, Gabriel; Klebe, Dimitri; Costanza, Bryan; Cerny,
Arnie; Schmale, Trevor; Hoffman, Tessa; Streeter, Sam; Erickson,
Jack; McClellan, Michele; Erickson, Ella; Brettell, Brynn; Shoffner,
Savannah; McClellan, Emilie; VanVoorhis, Julie; Bramhall, Cole; Stelly,
Daniel; Bee, Bentley; Acevedo, Bruno; Kroeger, Madison; Trumpenski,
Ben; Sump, Nolan; Brook, Liam; Ernzen, Jagert; Lewis, Jessica;
Maderak, Ryan; Kennedy, Charles; Dembinski, David; Wright, Rita;
Foster, Michael; Ahmadbasir, Mohammad; Laycox, Monty; Foster, James;
Orr, Ethan; Staab, Ashley; Speck, Angela; Baldridge, Sean; Kegley,
Lucy; Bavlnka, Jordan; Ballew, Thomas; Callen, Bruce; Ojakangas,
Gregory; Bremer, Mark; Angliongto, Maryanne; Redecker, Mark; Bremer,
Chris; Hill, Peggy; Rodgers, Michael; Duncan, Jordan; Fincher, Sam;
Nielsen, Ben; Hasler, Samantha; Shivelbine, Taylor; Howard, Tyler;
Midden, Chris; Patrick, Sean; Glenn, Kerry; Mandrell, Chris; Dawson,
Kyle; Cortez, Margaret; Levsky, Alyssa; Gallaba, Dinuka; Perrone,
Mason; Taylor, Jasmyn; Yanamandra-Fisher, Padma A.; Harper, Howard;
Adams, Lindsay; Springer, Michaela; Menard, BillyJoe; Boggs, Dylan;
Lynch, Caitlin; Watson, Jacob; York, Andi; Matthews, David; Brown,
Kiley; Garrison, Dylan; Mangin, Jonathan; Mangin, Isaac; Birriel,
Jennifer; Birriel, Ignacio; Yess, Capp; Anderson, Jesse; Caudill,
Ethan; Smith, Allyn; Buckner, Spencer; Longhurst, Russ; Fagan, Ben;
Nations, Christian; DiMatties, Jeffrey; Thompson, Patricia; Garrison,
David; Garrison, Thomas; Garrison, William; Kidd, Mary; Baker, Maria;
Ledford, Mary-Beth; Winebarger, Amy; Freed, Michael; Church, Morgyn;
Dickens, Jim; Anderson, Bob; Smith, Ned; Dorsey, Lynne; Justice, Doug;
Zavala, Daniel; Stockbridge, Zach; Brittain, Sean; Jensen, Stanley;
Leiendecker, Harrison; Thompson, Erin; Deady, Michelle; Quinn-Hughes,
Kelly; Slimmer, David; Granger, Valerie; LaRoche, Michael; Hill
LaRoche, Serena; Manspeaker, Rachel; Nguyen, Peter; Smith, Daniel;
Payne, Jim; Zissett, Jerry; Roberts, Arianna M.; Roberts, Gabrielle
W.; Roberts, Harrison; Riddle, Amy; Ursache, Corina; Ursache, Elena
2020PASP..132a4201P Altcode:
The citizen Continental-America Telescopic Eclipse (CATE) Experiment
was a new type of citizen science experiment designed to capture a time
sequence of white-light coronal observations during totality from 17:16
to 18:48 UT on 2017 August 21. Using identical instruments the CATE
group imaged the inner corona from 1 to 2.1 RSun with 1.″43 pixels
at a cadence of 2.1 s. A slow coronal mass ejection (CME) started on
the SW limb of the Sun before the total eclipse began. An analysis
of CATE data from 17:22 to 17:39 UT maps the spatial distribution of
coronal flow velocities from about 1.2 to 2.1 RSun, and shows the CME
material accelerates from about 0 to 200 km s<SUP>-1</SUP> across this
part of the corona. This CME is observed by LASCO C2 at 3.1-13 RSun
with a constant speed of 254 km s<SUP>-1</SUP>. The CATE and LASCO
observations are not fit by either constant acceleration nor spatially
uniform velocity change, and so the CME acceleration mechanism must
produce variable acceleration in this region of the corona.
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Title: Memorable Events During a Research Career
Authors: Sheeley, Neil R.
2019JGRA..124.4949S Altcode:
In this paper, I sketch a path through my research career in solar and
heliospheric physics, recalling some memorable events and discoveries
that occurred along the way. This chain of events begins with an
influential Time magazine article in 1955 and progresses through a
summer at Bell Labs, 9 years at Caltech, 7 years at the Kitt Peak
National Observatory, 43 years at the Naval Research Laboratory, and
ends with a digitized map of the Sun's Ca II K-Line emission in 1919
when the AGU was born. Accidents and puzzling results are often the
keys to progress and should be examined carefully.
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Title: Observational Evidence for the Associated Formation of Blobs
and Raining Inflows in the Solar Corona
Authors: Sanchez-Diaz, Eduardo; Rouillard, Alexis P.; Davies, Jackie
A.; Lavraud, Benoit; Sheeley, Neil R.; Pinto, Rui F.; Kilpua, Emilia;
Plotnikov, Illya; Genot, Vincent
2017EGUGA..19.4855S Altcode:
The origin of the Slow Solar Wind (SSW) is still a topic of much
debate. The continual emergence of small transient structures from
helmet streamers is thought to constitute an important sources of the
SSW. Determining the height at which these transients are released
is an important factor in determining the conditions under which the
SSW forms. To this end, we have carried out a multipoint analysis of
small transient structures released from a north-south tilted helmet
streamer into the slow solar wind over a broad range of position
angles during Carrington Rotation 2137. During this period of time,
the catalogue in Sheeley & Wang (2014) reported a big number of
transient structures collapsing back toward the Sun, referred to as
"raining inflows". Combining the remote-sensing observations taken
by the Solar-TErrestrial RElations Observatory (STEREO) mission with
coronagraphic observations from the SOlar and Heliospheric Observatory
(SOHO) spacecraft, we show that the release of such small transient
structures (often called blobs), which subsequently move away from
the Sun, is associated with the concomitant formation of raining
inflows. This is the first direct association between outflowing blobs
and raining inflows, which locates the formation of blobs above the
helmet streamers and gives strong support that the blobs are released
by magnetic reconnection. This work was made with the funding from
the HELCATS project under the FP7 EU contract number 606692
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Title: Tracking the Magnetic Flux in and around Sunspots
Authors: Sheeley, N. R., Jr.; Stauffer, J. R.; Thomassie, J. C.;
Warren, H. P.
2017ApJ...836..144S Altcode:
We have developed a procedure for tracking sunspots observed by the
Helioseismic and Magnetic Imager on the Solar Dynamics Observatory
and for making curvature-corrected space/time maps of the associated
line-of-sight magnetic field and continuum intensity. We apply
this procedure to 36 sunspots, each observed continuously for nine
days around its central meridian passage time, and find that the
proper motions separate into two distinct components depending on
their speeds. Fast (∼3-5 km s<SUP>-1</SUP>) motions, comparable
to Evershed flows, are produced by weak vertical fluctuations of the
horizontal canopy field and recur on a timescale of 12-20 min. Slow
(∼0.3-0.5 km s<SUP>-1</SUP>) motions diverge from a sunspot-centered
ring whose location depends on the size of the sunspot, occurring in the
mid-penumbra for large sunspots and at the outer edge of the penumbra
for small sunspots. The slow ingoing features are contracting spokes of
a quasi-vertical field of umbral polarity. These inflows disappear when
the sunspot loses its penumbra, and may be related to inward-moving
penumbral grain. The slow outgoing features may have either polarity
depending on whether they originate from quasi-vertical fields of umbral
polarity or from the outer edge of the canopy. When a sunspot decays,
the penumbra and canopy disappear, and the moat becomes filled with
slow outflows of umbral polarity. We apply our procedure to decaying
sunspots, to long-lived sunspots, and to numerical simulations of a
long-lived sunspot by Rempel.
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Title: Observational Evidence for the Associated Formation of Blobs
and Raining Inflows in the Solar Corona
Authors: Sanchez-Diaz, E.; Rouillard, A. P.; Davies, J. A.; Lavraud,
B.; Sheeley, N. R.; Pinto, R. F.; Kilpua, E.; Plotnikov, I.; Genot, V.
2017ApJ...835L...7S Altcode: 2016arXiv161205487S
The origin of the slow solar wind is still a topic of much debate. The
continual emergence of small transient structures from helmet
streamers is thought to constitute one of the main sources of the slow
wind. Determining the height at which these transients are released
is an important factor in determining the conditions under which the
slow solar wind forms. To this end, we have carried out a multipoint
analysis of small transient structures released from a north-south
tilted helmet streamer into the slow solar wind over a broad range
of position angles during Carrington Rotation 2137. Combining the
remote-sensing observations taken by the Solar-TErrestrial RElations
Observatory (STEREO) mission with coronagraphic observations from the
SOlar and Heliospheric Observatory (SOHO) spacecraft, we show that
the release of such small transient structures (often called blobs),
which subsequently move away from the Sun, is associated with the
concomitant formation of transient structures collapsing back toward
the Sun; the latter have been referred to by previous authors as
“raining inflows.” This is the first direct association between
outflowing blobs and raining inflows, which locates the formation of
blobs above the helmet streamers and gives strong support that the
blobs are released by magnetic reconnection.
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Title: The Stereo Electron Spikes and the Interplanetary Magnetic
Field
Authors: Jokipii, J. R.; Sheeley, N. R., Jr.; Wang, Y. M.; Giacalone,
J.
2016AGUFMSH51G..06J Altcode:
A recent paper (Klassen etal, 2015) discussed observations of a spike
event of 55-65 keV electrons which occurred very nearly simultaneously
at STEREO A and STEREO B, which at the time were separated in longitude
by 38 degrees. The authors associated the spikes with a flare at the
Sun near the footpoint of the nominal Archimedean spiral magnetic
field line passing through STEREO A. The spike at STEREO A was delayed
by 2.2 minutes from that at STEREOB. We discuss the observations in
terms of a model in which the electrons, accelerated at the flare,
propagate without significant scattering along magnetic field lines
which separate or diverge as a function of radial distance from the
Sun. The near simultaneity of the spikes at the two spacecraft is a
natural consequence of this model. We interpret the divergence of the
magnetic field lines as a consequence of field-line random walk and
flux-tube expansion. We show that the field-line random walk in the
absence of flux-tube expansion produces an rms spread of field lines
significantly less than that which is required to produce to observed
divergence. We find that observations of the solar wind and its source
region at the time of the event can account for the observations in
terms of propagation along interplanetary magnetic field-lines. Klassen,
A., Dresing, N., Gomez-Herrero, R, and Heber, B., A&A 580, A115
(2015) Financial support for NS and YMW was provided by NASA and CNR.
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Title: Can collisions regulate the He++ abundance of the slow
solar wind?
Authors: Sanchez Diaz, Eduardo; Blelly, A. P. Rouillard. L.; Pinto,
R.; Lavraud, B.; Segura, K.; Tao, C.; Sheeley, N. R.; Plotnikov, I.
2016shin.confE..80S Altcode:
At solar maximum, the slow solar wind presents an enhanced alpha
abundance compared to solar minimum (Aellig et al., 2001; Kasper et al.,
2007, 2012). Sanchez-Diaz et al. (2016) found that the yearly averages
of alpha abundance in the slow solar wind is very well correlated
with the yearly averages of the proton mass flux right above the
transition region. This correlation is especially remarkable for the
very slow solar wind (V<300 km/s), where this proton flux is very
high and variable (5 times bigger than at solar minimum and one order
of magnitude bigger than in the fast solar wind). We hypothesized that
the alpha abundance might be enhanced due to a non-negligible amount of
Coulomb collisions between protons and alphas due to this high proton
flux. To explore the role of H+ ? He++ collisions in the acceleration of
He++ ions, we input the proton temperature and expansion factor profiles
resulting from the combination of a Potential Field Source Model (PFSS)
and a 1D hydrodynamic solar wind model described in Pinto et al. (2009)
into a collisional two fluid hydrostatic model. The model assumes that
there is no heating for the He++ ions in the very slow solar wind. We
evaluate the possible role of Coulomb collisions on the escape of He++
for different geometries and boundary conditions. We find that there
is a region right above the transition region where the collisions
have the effect of transferring momentum from the protons to the
alphas. This is the region where the hydrogen may be accelerated by
collisions with hydrogen. In the upper Corona it is the alphas that
would transfer momentum to the protons and the collisions have the
effect of slowing the alphas down. <P />This study was carried out as
part of the HELCATS FP7 project
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Title: Multipoint observation of coronal inflows
Authors: Sanchez Diaz, Eduardo; Rouillard, A.; Davies, J.; Lavraud,
B.; Plotnikov, I.; Sheeley, N. R.; Pinto, R.
2016shin.confE.112S Altcode:
Corotating Interaction Regions (CIRs) are filled with numerous
small-scale transients. Outward moving blobs have been commonly
observed in LASCO and SECCHI (Plotnikov et al. 2016; Rouillard et
al., 2008, 2009, 2010; Sheeley et al., 1997). Inward moving blobs,
or inflows, have been observed in LASCO C2 over the last two solar
cycles (Wang et al., 1999a,b; Sheeley et al., 2001,2002,2007,2014). On
rare occasions, inflows and outflows are observed together and in
apparent association. Outward moving blobs and inflows are thought
to be the products of the same process: magnetic reconnection at
around 4-5 solar radii producing an inflow-outflow pair. However,
only one isolated signature (either inflow or outflow) is reported
in most observations. The second signature is thought to usually lie
outside the field-of-view (FOV) of the instruments, or below their
sensitivity limit. <P />Here we show observations of the same CIR by
combining the SECCHI and LASCO instrument onboard STEREO and SOHO. The
FOV of SECCHI-COR2 allows us to see the outflow part associated to
the inflows. This outflow can be followed all the way out in the
Heliosphere with SECHHI-HI1 and HI2. During May and June 2013, the
magnetic field topology of the corona, with a nearly vertical neutral
line, and the lack of CMEs was optimal to observe blobs and inflows at
all latitudes. We present a 3D reconstruction of all the small-scale
transients, both inflows and outflows, entrained by this CIR. This
includes the first observation of inflows with COR2 and the first
multipoint observation of the inflows entrained by the same CIR from
two satellites. We evaluate the presumed connection between coronal
inflows, in-out pairs and blobs and discuss the origin of blobs and
their likely association to a continuous magnetic reconnection process
in the Solar Corona. <P />This study was carried out as part of the
HELCATS FP7 project
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Title: The very slow solar wind: Properties, origin and variability
Authors: Sanchez-Diaz, Eduardo; Rouillard, Alexis P.; Lavraud,
Benoit; Segura, Kevin; Tao, Chihiro; Pinto, Rui; Sheeley, N. R.;
Plotnikov, Illya
2016JGRA..121.2830S Altcode:
Solar wind slower than 300 km/s, hereafter termed very slow solar
wind (VSSW), is seldom observed at 1 AU. It was, however, commonly
measured inside 0.7 AU by the two Helios spacecraft, particularly
during solar maximum. Magnetohydrodynamic (MHD) modeling reveals that
the disappearance of VSSW at 1 AU is the result of its interaction
with faster solar wind. The acceleration and compression of the VSSW
contributes to the observed highly variable structure of the slow
solar wind at 1 AU. The VSSW usually contains the heliospheric plasma
sheet and current sheet. It has higher density and lower temperature
than the regular slow solar wind, extending the known scaling laws
below 300 km/s. Its helium abundance increases with solar activity
even more significantly than the slow solar wind. Contrary to faster
solar winds, the helium ions in the VSSW are slower than the dominant
protons. Combining a Potential Field Source Surface (PFSS) model with
ballistic back tracing, we study the source region of the VSSW. We
show that the proton density flux for the VSSW is much higher than
for the faster winds, particularly at solar maximum.
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Title: Geographically Distributed Citizen Scientist Training for
the 2017 Citizen CATE Experiment
Authors: Gelderman, Richard; Penn, Matt; Baer, Robert; Isberner,
Fred; Pierce, Michael; Walter, Donald K.; Yanamandra-Fisher, Padma;
Sheeley, Neil R.
2016AAS...22734902G Altcode:
The solar eclipse of 21 August 2017 will be visible to over a half
billion people across the entire North American continent. The roughly
100-mile wide path of totality, stretching from Oregon to South
Carolina, will be the destination for tens of millions of people. In
the decades since 1979, when the last total solar eclipse was visible
from the continental USA, the phenomenon of Internet enabled citizen
science has grown to be an accepted mode for science. The Citizen
Continental-America Telescopic Eclipse (Citizen CATE) experiment
has been funded as one of the three 2017 eclipse related NASA STEM
agreements to engage citizen scientists in a unique, cutting-edge
solar physics experiment. Teams across the USA will be trained to use
standardized refracting telescope and digital imager set-ups to observe
the solar corona during the eclipse, acquiring multiple exposures to
create one high dynamic range image. After observing during the eclipse,
the CATE volunteers will upload the combined image to a cloud-storage
site and the CATE team will then work to properly orient and align all
the images collected from across the continent to produce a continuous
90-minutes movie. A time-compressed first cut of the entire sequence
will be made available to media outlets on the same afternoon of
the eclipse, with hope that high quality images will encourage the
most accurate coverage of this Great American Eclipse. We discuss
overall the project, as well as details of the initial tests of the
prototype set-up (including in the Faroe Islands during the March 2015
total solar eclipse) and plans for the future night-time and day-time
observing campaigns, and for a handful of observing teams positioned
for overlapping observations of the March 2016 total solar eclipse in
the South Pacific.
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Title: Coronal Mass Ejections and the Solar Cycle Variation of the
Sun's Open Flux
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
2015ApJ...809L..24W Altcode: 2021arXiv210407238W
The strength of the radial component of the interplanetary magnetic
field (IMF), which is a measure of the Sun’s total open flux, is
observed to vary by roughly a factor of two over the 11 year solar
cycle. Several recent studies have proposed that the Sun’s open
flux consists of a constant or “floor” component that dominates
at sunspot minimum, and a time-varying component due to coronal mass
ejections (CMEs). Here, we point out that CMEs cannot account for
the large peaks in the IMF strength which occurred in 2003 and late
2014, and which coincided with peaks in the Sun’s equatorial dipole
moment. We also show that near-Earth interplanetary CMEs, as identified
in the catalog of Richardson and Cane, contribute at most ∼30% of the
average radial IMF strength even during sunspot maximum. We conclude
that the long-term variation of the radial IMF strength is determined
mainly by the Sun’s total dipole moment, with the quadrupole moment
and CMEs providing an additional boost near sunspot maximum. Most of
the open flux is rooted in coronal holes, whose solar cycle evolution
in turn reflects that of the Sun’s lowest-order multipoles.
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Title: The Recent Rejuvenation of the Sun's Large-scale Magnetic
Field: A Clue for Understanding Past and Future Sunspot Cycles
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
2015ApJ...809..113S Altcode:
The quiet nature of sunspot cycle 24 was disrupted during the second
half of 2014 when the Sun’s large-scale field underwent a sudden
rejuvenation: the solar mean field reached its highest value since
1991, the interplanetary field strength doubled, and galactic cosmic
rays showed their strongest 27-day modulation since neutron-monitor
observations began in 1957; in the outer corona, the large increase of
field strength was reflected by unprecedentedly large numbers of coronal
loops collapsing inward along the heliospheric current sheet. Here, we
show that this rejuvenation was not caused by a significant increase in
the level of solar activity as measured by the smoothed sunspot number
and CME rate, but instead was caused by the systematic emergence of flux
in active regions whose longitudinal distribution greatly increased the
Sun’s dipole moment. A similar post-maximum increase in the dipole
moment occurred during each of the previous three sunspot cycles,
and marked the start of the declining phase of each cycle. We note
that the north-south component of this peak dipole moment provides
an early indicator of the amplitude of the next cycle, and conclude
that the amplitude of cycle 25 may be comparable to that of cycle 24,
and well above the amplitudes obtained during the Maunder Minimum.
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Title: Using Running Difference Images to Track Proper Motions of
XUV Coronal Intensity on the Sun
Authors: Sheeley, N. R., Jr.; Warren, H. P.; Lee, J.; Chung, S.;
Katz, J.; Namkung, M.
2014ApJ...797..131S Altcode:
We have developed a procedure for observing and tracking proper
motions of faint XUV coronal intensity on the Sun and have applied
this procedure to study the collective motions of cellular plumes and
the shorter-period waves in sunspots. Our space/time maps of cellular
plumes show a series of tracks with the same 5-8 minute repetition
times and ~100 km s<SUP>-1</SUP> sky-plane speeds found previously
in active-region fans and in coronal hole plumes. By synchronizing
movies and space/time maps, we find that the tracks are produced by
elongated ejections from the unipolar flux concentrations at the bases
of the cellular plumes and that the phases of these ejections are
uncorrelated from cell to cell. Thus, the large-scale motion is not a
continuous flow, but is more like a system of independent conveyor belts
all moving in the same direction along the magnetic field. In contrast,
the proper motions in sunspots are clearly waves resulting from periodic
disturbances in the sunspot umbras. The periods are ~2.6 minutes, but
the sky-plane speeds and wavelengths depend on the heights of the waves
above the sunspot. In the chromosphere, the waves decelerate from 35-45
km s<SUP>-1</SUP> in the umbra to 7-8 km s<SUP>-1</SUP> toward the outer
edge of the penumbra, but in the corona, the waves accelerate to ~60-100
km s<SUP>-1</SUP>. Because chromospheric and coronal tracks originate
from the same space/time locations, the coronal waves must emerge from
the same umbral flashes that produce the chromospheric waves.
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Title: Coronal Inflows during the Interval 1996-2014
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
2014ApJ...797...10S Altcode:
We extend our previous counts of coronal inflows from the 5 yr interval
1996-2001 to the 18 yr interval 1996-2014. By comparing stackplots
of these counts with similar stackplots of the source-surface
magnetic field and its longitudinal gradient, we find that the
inflows occur in long-lived streams with counting rates in excess of
18 inflows per day at sector boundaries where the gradient exceeds
0.22 G rad<SUP>-1</SUP>. These streams are responsible for the high
(86%) correlation between the inflow rate and the longitudinal
field gradient. The overall inflow rate was several times larger in
sunspot cycle 23 than it has been so far in cycle 24, reflecting the
relatively weak source-surface fields during this cycle. By comparison,
in cycles 21-22, the source-surface field and its gradient had bursts
of great strength, as if large numbers of inflows occurred during
those cycles. We find no obvious relation between inflows and coronal
mass ejections (CMEs) on timescales of days to weeks, regardless of the
speeds of the CMEs, and only a 60% correlation on timescales of months,
provided the CMEs are fast (V > 600 km s<SUP>-1</SUP>). We conclude
that most of the flux carried out by CMEs is returned to the Sun via
field line reconnection well below the 2.0 R <SUB>⊙</SUB> inner limit
of the LASCO field of view, and that the remainder accumulates in the
outer corona for an eventual return at sector boundaries.
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Title: Origin of the Wang-Sheeley-Arge Solar Wind Model
Authors: Sheeley, Neil R.
2014AAS...22420302S Altcode:
A correlation between solar wind speed at Earth and the amount of field
line expansion in the corona was verified in 1989 using 22 years of
solar and interplanetary observations. This talk will trace the history
of this discovery from its birth 15 years earlier in the Skylab era
to its current use as a space weather forecasting technique. This
research was supported by NASA and ONR.
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Title: Memories
Authors: Sheeley, N. R., Jr.
2013ASPC..478...85S Altcode:
This paper highlights some events in solar research, beginning at
Caltech in 1959 when Leighton described a technique for obtaining
high-resolution observations of solar magnetic fields, and extending to
1971 when his ideas were being pursued with the new spectroheliograph
and 82 cm solar image at the Kitt Peak National Observatory.
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Title: Using Coronal Cells to Infer the Magnetic Field Structure
and Chirality of Filament Channels
Authors: Sheeley, N. R., Jr.; Martin, S. F.; Panasenco, O.; Warren,
H. P.
2013ApJ...772...88S Altcode: 2013arXiv1306.2273S
Coronal cells are visible at temperatures of ~1.2 MK in Fe XII
coronal images obtained from the Solar Dynamics Observatory and Solar
Terrestrial Relations Observatory spacecraft. We show that near a
filament channel, the plumelike tails of these cells bend horizontally
in opposite directions on the two sides of the channel like fibrils
in the chromosphere. Because the cells are rooted in magnetic flux
concentrations of majority polarity, these observations can be used
with photospheric magnetograms to infer the direction of the horizontal
field in filament channels and the chirality of the associated magnetic
field. This method is similar to the procedure for inferring the
direction of the magnetic field and the chirality of the fibril pattern
in filament channels from Hα observations. However, the coronal cell
observations are easier to use and provide clear inferences of the
horizontal field direction for heights up to ~50 Mm into the corona.
---------------------------------------------------------
Title: Fe XII Stalks and the Origin of the Axial Field in Filament
Channels
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Stenborg, G.
2013ApJ...770...72W Altcode:
Employing Fe XII images and line-of-sight magnetograms, we deduce
the direction of the axial field in high-latitude filament channels
from the orientation of the adjacent stalklike structures. Throughout
the rising phase of the current solar cycle 24, filament channels
poleward of latitude 30° overwhelmingly obeyed the hemispheric
chirality rule, being dextral (sinistral) in the northern (southern)
hemisphere, corresponding to negative (positive) helicity. During
the deep minimum of 2007-2009, the orientation of the Fe XII stalks
was often difficult to determine, but no obvious violations of the
rule were found. Although the hemispheric trend was still present
during the maximum and early declining phase of cycle 23 (2000-2003),
several high-latitude exceptions were identified at that time. From
the observation that dextral (sinistral) filament channels form
through the decay of active regions whose Fe XII features show
a counterclockwise (clockwise) whorl, we conclude that the axial
field direction is determined by the intrinsic helicity of the active
regions. In contrast, generation of the axial field component by the
photospheric differential rotation is difficult to reconcile with the
observed chirality of polar crown and circular filament channels, and
with the presence of filament channels along the equator. The main role
of differential rotation in filament channel formation is to expedite
the cancellation of flux and thus the removal of the transverse field
component. We propose further that, rather than being ejected into
the heliosphere, the axial field is eventually resubmerged by flux
cancellation as the adjacent unipolar regions become increasingly mixed.
---------------------------------------------------------
Title: The Solar Wind and Interplanetary Field during Very Low
Amplitude Sunspot Cycles
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
2013ApJ...764...90W Altcode:
Cosmogenic isotope records indicate that a solar-cycle modulation
persists through extended periods of very low sunspot activity. One
immediate implication is that the photospheric field during such grand
minima did not consist entirely of ephemeral regions, which produce
a negligible amount of open magnetic flux, but continued to have a
large-scale component originating from active regions. Present-day
solar and heliospheric observations show that the solar wind mass
flux and proton density at the coronal base scale almost linearly
with the footpoint field strength, whereas the wind speed at Earth is
uncorrelated with the latter. Thus a factor of ~4-7 reduction in the
total open flux, as deduced from reconstructions of the interplanetary
magnetic field (IMF) during the Maunder Minimum, would lead to a
similar decrease in the solar wind densities, while leaving the
wind speeds largely unchanged. We also demonstrate that a decrease
in the strengths of the largest active regions during grand minima
will reduce the amplitude of the Sun's equatorial dipole relative to
the axial component, causing the IMF strength to peak near sunspot
minimum rather than near sunspot maximum, a result that is consistent
with the phase shift observed in the <SUP>10</SUP>Be record during the
Maunder Minimum. Finally, we discuss the origin of the 5 yr periodicity
sometimes present in the cosmogenic isotope data during low and medium
amplitude cycles.
---------------------------------------------------------
Title: Calibrating 100 Years of Polar Faculae Measurements:
Implications for the Evolution of the Heliospheric Magnetic Field
Authors: Muñoz-Jaramillo, Andrés; Sheeley, Neil R.; Zhang, Jie;
DeLuca, Edward E.
2012ApJ...753..146M Altcode: 2013arXiv1303.0345M
Although the Sun's polar magnetic fields are thought to provide
important clues for understanding the 11 year sunspot cycle, including
the observed variations of its amplitude and period, the current
database of high-quality polar field measurements spans relatively
few sunspot cycles. In this paper, we address this deficiency by
consolidating Mount Wilson Observatory polar faculae data from four
data reduction campaigns, validating it through a comparison with
facular data counted automatically from Michelson Doppler Imager (MDI)
intensitygrams, and calibrating it against polar field measurements
taken by the Wilcox Solar Observatory and average polar field and
total polar flux calculated using MDI line-of-sight magnetograms. Our
results show that the consolidated polar facular measurements are in
excellent agreement with both polar field and polar flux estimates,
making them an ideal proxy to study the evolution of the polar
magnetic field. Additionally, we combine this database with sunspot
area measurements to study the role of the polar magnetic flux in the
evolution of the heliospheric magnetic field (HMF). We find that there
is a strong correlation between HMF and polar flux at solar minimum
and that, taken together, polar flux and sunspot area are better
at explaining the evolution of the HMF during the last century than
sunspot area alone.
---------------------------------------------------------
Title: The Longitudinal Properties of a Solar Energetic Particle
Event Investigated Using Modern Solar Imaging
Authors: Rouillard, A. P.; Sheeley, N. R.; Tylka, A.; Vourlidas,
A.; Ng, C. K.; Rakowski, C.; Cohen, C. M. S.; Mewaldt, R. A.; Mason,
G. M.; Reames, D.; Savani, N. P.; StCyr, O. C.; Szabo, A.
2012ApJ...752...44R Altcode:
We use combined high-cadence, high-resolution, and multi-point imaging
by the Solar-Terrestrial Relations Observatory (STEREO) and the Solar
and Heliospheric Observatory to investigate the hour-long eruption of a
fast and wide coronal mass ejection (CME) on 2011 March 21 when the twin
STEREO spacecraft were located beyond the solar limbs. We analyze the
relation between the eruption of the CME, the evolution of an Extreme
Ultraviolet (EUV) wave, and the onset of a solar energetic particle
(SEP) event measured in situ by the STEREO and near-Earth orbiting
spacecraft. Combined ultraviolet and white-light images of the lower
corona reveal that in an initial CME lateral "expansion phase," the
EUV disturbance tracks the laterally expanding flanks of the CME,
both moving parallel to the solar surface with speeds of ~450 km
s<SUP>-1</SUP>. When the lateral expansion of the ejecta ceases, the
EUV disturbance carries on propagating parallel to the solar surface
but devolves rapidly into a less coherent structure. Multi-point
tracking of the CME leading edge and the effects of the launched
compression waves (e.g., pushed streamers) give anti-sunward speeds
that initially exceed 900 km s<SUP>-1</SUP> at all measured position
angles. We combine our analysis of ultraviolet and white-light images
with a comprehensive study of the velocity dispersion of energetic
particles measured in situ by particle detectors located at STEREO-A
(STA) and first Lagrange point (L1), to demonstrate that the delayed
solar particle release times at STA and L1 are consistent with the
time required (30-40 minutes) for the CME to perturb the corona over a
wide range of longitudes. This study finds an association between the
longitudinal extent of the perturbed corona (in EUV and white light)
and the longitudinal extent of the SEP event in the heliosphere.
---------------------------------------------------------
Title: Calibration Of a Century of Polar Field Measurements and
what this Tells us About the Long-term Variability of the Solar and
Heliospheric Magnetic Field
Authors: Munoz-Jaramillo, Andres; Sheeley, N. R.; Zhang, J.; DeLuca,
E. E.
2012AAS...22012304M Altcode:
In addition to the well known 11-year periodicity, the solar cycle also
presents long-term modulations of its amplitude and period which play
a determinant role in the evolution of space weather and climate. To
this date, the efforts at understanding long-term solar variability
have focused on the active parts of the cycle using sunspot properties
as their main source of data. However, the recent extend minimum of
sunspot cycle 23 has shown us that the quiet parts of the cycle are
equally important and thus long-term databases complementary to sunspot
properties are necessary. <P />Here we show how to consolidate Mount
Wilson Observatory polar faculae data from four observational campaigns
(1906-1964, Sheeley 1966; 1960-1975, Sheeley 1976; 1975-1990, Sheeley
1991; 1985-2007, Sheeley 2008), validate it through a comparison
with facular data counted automatically from MDI intensitygrams,
and calibrate it against polar field measurements taken by the Wilcox
Solar Observatory (1977-2011) and average polar field and total polar
flux calculated using MDI line-of-sight magnetograms (1996-2011). <P
/>We also show that the consolidated polar facular measurements are
in excellent agreement with both polar field and polar flux estimates,
making them an ideal proxy to study the evolution of the polar magnetic
field since 1906 and use this proxy to study the role of polar flux in
the evolution of the solar cycle and the Heliospheric Magnetic Field
(HMF).
---------------------------------------------------------
Title: Coronal Cells
Authors: Sheeley, N. R., Jr.; Warren, H. P.
2012ApJ...749...40S Altcode:
We have recently noticed cellular features in Fe XII 193 Å images of
the 1.2 MK corona. They occur in regions bounded by a coronal hole
and a filament channel, and are centered on flux elements of the
photospheric magnetic network. Like their neighboring coronal holes,
these regions have minority-polarity flux that is ~0.1-0.3 times
their flux of majority polarity. Consequently, the minority-polarity
flux is "grabbed" by the majority-polarity flux to form low-lying
loops, and the remainder of the network flux escapes to connect with
its opposite-polarity counterpart in distant active regions of the
Sun. As these regions are carried toward the limb by solar rotation,
the cells disappear and are replaced by linear plumes projecting toward
the limb. In simultaneous views from the Solar Terrestrial Relations
Observatory and Solar Dynamics Observatory spacecraft, these plumes
project in opposite directions, extending away from the coronal hole in
one view and toward the hole in the other view, suggesting that they are
sky-plane projections of the same radial structures. We conclude that
these regions are composed of closely spaced radial plumes, extending
upward like candles on a birthday cake and visible as cells when seen
from above. We suppose that a coronal hole has this same discrete,
cellular magnetic structure, but that it is not seen until the
encroachment of opposite-polarity flux closes part or all of the hole.
---------------------------------------------------------
Title: On the Nature of the Solar Wind from Coronal Pseudostreamers
Authors: Wang, Y. -M.; Grappin, R.; Robbrecht, E.; Sheeley, N. R., Jr.
2012ApJ...749..182W Altcode:
Coronal pseudostreamers, which separate like-polarity coronal holes, do
not have current sheet extensions, unlike the familiar helmet streamers
that separate opposite-polarity holes. Both types of streamers taper
into narrow plasma sheets that are maintained by continual interchange
reconnection with the adjacent open magnetic field lines. White-light
observations show that pseudostreamers do not emit plasma blobs; this
important difference from helmet streamers is due to the convergence
of like-polarity field lines above the X-point, which prevents the
underlying loops from expanding outward and pinching off. The main
component of the pseudostreamer wind has the form of steady outflow
along the open field lines rooted just inside the boundaries of the
adjacent coronal holes. These flux tubes are characterized by very
rapid expansion below the X-point, followed by reconvergence at greater
heights. Analysis of an idealized pseudostreamer configuration shows
that, as the separation between the underlying holes increases, the
X-point rises and the expansion factor f <SUB>ss</SUB> at the source
surface increases. In situ observations of pseudostreamer crossings
indicate wind speeds v ranging from ~350 to ~550 km s<SUP>-1</SUP>,
with O<SUP>7 +</SUP>/O<SUP>6 +</SUP> ratios that are enhanced compared
with those in high-speed streams but substantially lower than in the
slow solar wind. Hydrodynamic energy-balance models show that the
empirical v-f <SUB>ss</SUB> relation overestimates the wind speeds
from nonmonotonically expanding flux tubes, particularly when the
X-point is located at low heights and f <SUB>ss</SUB> is small. We
conclude that pseudostreamers produce a "hybrid" type of outflow that
is intermediate between classical slow and fast solar wind.
---------------------------------------------------------
Title: Observations of Reconnecting Flare Loops with the Atmospheric
Imaging Assembly
Authors: Warren, Harry P.; O'Brien, Casey M.; Sheeley, Neil R., Jr.
2011ApJ...742...92W Altcode: 2011arXiv1109.2474W
Perhaps the most compelling evidence for the role of magnetic
reconnection in solar flares comes from the supra-arcade downflows that
have been observed above many post-flare loop arcades. These downflows
are thought to be related to highly non-potential field lines that have
reconnected and are propagating away from the current sheet. We present
new observations of supra-arcade downflows taken with the Atmospheric
Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO). The
morphology and dynamics of the downflows observed with AIA provide new
evidence for the role of magnetic reconnection in solar flares. With
these new observations we are able to measure downflows originating
at larger heights than in previous studies. We find, however,
that the initial velocities measured here (~144 km s<SUP>-1</SUP>)
are well below the Alfvén speed expected in the lower corona, and
consistent with previous results. We also find no evidence that the
downflows brighten with time, as would be expected from chromospheric
evaporation. These observations suggest that simple two-dimensional
models cannot explain the detailed observations of solar flares.
---------------------------------------------------------
Title: The longitudinal properties of solar energetic particle events
investigated using modern solar imaging.
Authors: Rouillard, A. P.; Sheeley, N. R.; Tylka, A. J.; Vourlidas,
A.; Ng, C. K.; Mason, G. M.; Cohen, C. M.
2011AGUFMSH33D..03R Altcode:
On 2011 March 21, the Solar-Terrestrial Relations Observatory
(STEREO) imaged the launch (at ~02:10UT) and outward propagation of
a fast (>1300 km s-1) and wide (latitudinal extent greater than
90 degrees) coronal mass ejection (CME) which emerged W135 degrees
(behind the west-limb as viewed from near-Earth spacecraft). Protons
with energies exceeding 90 MeV were first detected by the STEREO-Ahead
(STA) spacecraft and a few minutes later by near-Earth orbiting
spacecraft. Velocity dispersion analyses of energetic electrons, protons
and heavier ions, put the solar particle release (SPR) times along
magnetic field lines connected to STA at ~02:30UT and along magnetic
field lines connected to near-Earth spacecraft at ~ 03:00UT. The
spatial evolution of a pressure wave and its associated shock forming
around the CME could be tracked using (critical) high-cadence and
high-resolution STA (extreme ultraviolet and white-light) images. We
demonstrate that the delay between the SEP onset at STA and Earth is
consistent with the time required for the pressure wave to propagate
from the launch-site of the CME to the base of coronal streamers that
are magnetically connected to near-Earth spacecraft. By considering
measured shock speeds and inferred shock geometries along different
longitudes and by deriving spectra of energetic protons, this study
also presents some interpretation of the longitudinal variability of
the SEP event in terms of the evolution of the compression wave. Time
permitting, we will also summarise the results of similar analyses
carried out for the other energetic CME events in 2010 and 2011.
---------------------------------------------------------
Title: Observations of the White Light Corona from Solar Orbiter
and Solar Probe Plus
Authors: Howard, R. A.; Thernisien, A. F.; Vourlidas, A.; Plunkett,
S. P.; Korendyke, C. M.; Sheeley, N. R.; Morrill, J. S.; Socker,
D. G.; Linton, M. G.; Liewer, P. C.; De Jong, E. M.; Velli, M. M.;
Mikic, Z.; Bothmer, V.; Lamy, P. L.
2011AGUFMSH43F..06H Altcode:
The SoloHI instrument on Solar Orbiter and the WISPR instrument on Solar
Probe+ will make white light coronagraphic images of the corona as the
two spacecraft orbit the Sun. The minimum perihelia for Solar Orbiter
is about 60 Rsun and for SP+ is 9.5 Rsun. The wide field of view of the
WISPR instrument (about 105 degrees radially) corresponds to viewing
the corona from 2.2 Rsun to 20 Rsun. Thus the entire Thomson hemisphere
is contained within the telescope's field and we need to think of
the instrument as being a traditional remote sensing instrument and
then transitioning to a local in-situ instrument. The local behavior
derives from the fact that the maximum Thomson scattering will favor
the electron plasma close to the spacecraft - exactly what the in-situ
instruments will be sampling. SoloHI and WISPR will also observe
scattered light from dust in the inner heliosphere, which will be an
entirely new spatial regime for dust observations from a coronagraph,
which we assume to arise from dust in the general neighborhood of about
half way between the observer and the Sun. As the dust grains approach
the Sun, they evaporate and do not contribute to the scattering. A
dust free zone has been postulated to exist somewhere inside of 5 Rsun
where all dust is evaporated, but this has never been observed. The
radial position where the evaporation occurs will depend on the
precise molecular composition of the individual grains. The orbital
plane of Solar Orbiter will gradually increase up to about 35 degrees,
enabling a very different view through the zodiacal dust cloud to test
the models generated from in-ecliptic observations. In this paper we
will explore some of the issues associated with the observation of
the dust and will present a simple model to explore the sensitivity
of the instrument to observe such evaporations.
---------------------------------------------------------
Title: Interpreting the Properties of Solar Energetic Particle Events
by Using Combined Imaging and Modeling of Interplanetary Shocks
Authors: Rouillard, A. P.; Odstřcil, D.; Sheeley, N. R.; Tylka, A.;
Vourlidas, A.; Mason, G.; Wu, C. -C.; Savani, N. P.; Wood, B. E.;
Ng, C. K.; Stenborg, G.; Szabo, A.; St. Cyr, O. C.
2011ApJ...735....7R Altcode:
Images of the solar corona obtained by the Solar-Terrestrial Relations
Observatory (STEREO) provide high-cadence, high-resolution observations
of a compression wave forming ahead of a fast (940 km s<SUP>-1</SUP>)
coronal mass ejection (CME) that erupted at ~9:00 UT on 2010 April
03. The passage of this wave at 1 AU is detected in situ by the Advanced
Composition Explorer and Wind spacecraft at 08:00 UT on April 05 as a
shock followed by a turbulent and heated sheath. These unprecedented and
complementary observations of a shock-sheath region from the Sun to 1 AU
are used to investigate the onset of a Solar Energetic Particle (SEP)
event measured at the first Lagrange point (L1) and at STEREO-Behind
(STB). The spatial extent, radial coordinates, and speed of the ejection
are measured from STEREO observations and used as inputs to a numerical
simulation of the CME propagation in the background solar wind. The
simulated magnetic and plasma properties of the shock and sheath region
at L1 agree very well with the in situ measurements. These simulation
results reveal that L1 and STB are magnetically connected to the western
and eastern edges of the driven shock, respectively. They also show
that the 12 hr delay between the eruption time of the ejection and the
SEP onset at L1 corresponds to the time required for the bow shock to
reach the magnetic field lines connected with L1. The simulated shock
compression ratio increases along these magnetic field lines until
the maximum flux of high-energy particles is observed.
---------------------------------------------------------
Title: The Solar Origin of Small Interplanetary Transients
Authors: Rouillard, A. P.; Sheeley, N. R., Jr.; Cooper, T. J.; Davies,
J. A.; Lavraud, B.; Kilpua, E. K. J.; Skoug, R. M.; Steinberg, J. T.;
Szabo, A.; Opitz, A.; Sauvaud, J. -A.
2011ApJ...734....7R Altcode:
In this paper, we present evidence for magnetic transients with
small radial extents ranging from 0.025 to 0.118 AU measured in
situ by the Solar-Terrestrial Relations Observatory (STEREO) and the
near-Earth Advanced Composition Explorer (ACE) and Wind spacecraft. The
transients considered in this study are much smaller (<0.12 AU)
than the typical sizes of magnetic clouds measured near 1 AU (~0.23
AU). They are marked by low plasma beta values, generally lower magnetic
field variance, short timescale magnetic field rotations, and are all
entrained by high-speed streams by the time they reach 1 AU. We use
this entrainment to trace the origin of these small interplanetary
transients in coronagraph images. We demonstrate that these magnetic
field structures originate as either small or large mass ejecta. The
small mass ejecta often appear from the tip of helmet streamers as
arch-like structures and other poorly defined white-light features (the
so-called blobs). However, we have found a case of a small magnetic
transient tracing back to a small and narrow mass ejection erupting
from below helmet streamers. Surprisingly, one of the small magnetic
structures traces back to a large mass ejection; in this case, we show
that the central axis of the coronal mass ejection is along a different
latitude and longitude to that of the in situ spacecraft. The small
size of the transient is related to the in situ measurements being
taken on the edges or periphery of a larger magnetic structure. In
the last part of the paper, an ejection with an arch-like aspect is
tracked continuously to 1 AU in the STEREO images. The associated in
situ signature is not that of a magnetic field rotation but rather
of a temporary reversal of the magnetic field direction. Due to its
"open-field topology," we speculate that this structure is partly formed
near helmet streamers due to reconnection between closed and open
magnetic field lines. The implications of these observations for our
understanding of the variability of the slow solar wind are discussed.
---------------------------------------------------------
Title: Carrington Maps of Ca II K-line Emission for the Years
1915-1985
Authors: Sheeley, N. R., Jr.; Cooper, T. J.; Anderson, J. R. L.
2011ApJ...730...51S Altcode:
We have used Mount Wilson Observatory calcium K-line images, digitized
and flat fielded by the solar group at UCLA, to construct Carrington
maps of Ca II 3934 Å intensity for the years 1915-1985. These maps have
a spatial resolution comparable to the resolution of Carrington maps
of the magnetic field observed at Kitt Peak and a spectral resolution
comparable to the width of the K<SUB>232</SUB> emission. Consequently,
they provide a way of tracking the spatial distribution of magnetic
flux from the present back to the year 1915. In this paper, we compare
some of the recent K-line maps with corresponding maps of magnetic
field, and show examples of K-line maps during earlier sunspot
cycles when high-quality magnetograms were not available. Then, we
use butterfly diagrams and super-synoptic displays to summarize the
long-term evolution of the patterns of calcium intensity. Although each
cycle has characteristics that are similar to the others, cycle 19 is
remarkable for its broad latitudinal distribution of active regions,
for its giant poleward surges of flux, and for the emergence of a
north-south asymmetry that lasted 10 years.
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Title: The Wide Field Imager for Solar PRobe (WISPR)
Authors: Plunkett, S. P.; Howard, R. A.; Vourlidas, A.; Korendyke,
C. M.; Socker, D. G.; Morrill, J. S.; Sheeley, N. R.; Linton, M.;
Liewer, P. C.; de Jong, E. M.; Mikic, Z.
2010AGUFMSH11B1622P Altcode:
The Wide Field Imager for Solar PRobe (WISPR) will image the
Thomson-scattered light from the coronal plasma in the inner corona,
with unprecedented spatial resolution, cadence, and sensitivity. WISPR
follows on the SECCHI Heliospheric Imager (HI) aboard the STEREO
mission, and addresses all four key objectives in the Solar Probe Plus:
Report of the STDT (2008): (1) Determine the structure and dynamics of
the magnetic fields at the sources of the fast and slow solar wind, (2)
Trace the flow of energy that heats the solar corona and accelerates
the SW, (3) explore the mechanisms that accelerate and transport
energetic particles, (4) explore dusty plasma phenomena and their
influence on the solar wind and energetic particle formation. Situated
in the ram direction of the Solar Probe Plus (SPP) spacecraft (S/C),
WISPR will have the unique ability to image the coronal structures when
they are close to the Sun, as they approach, and as they pass over the
spacecraft. As a remote sensor, WISPR will connect the structures close
to the Sun to the spacecraft and provide important spatial and temporal
information; measuring, for example, the properties of the structures
generating the shocks and SEPs that will be measured in a few minutes at
the S/C. Since the S/C is embedded in the corona, WISPR and the in situ
instruments will measure for the first time the same plasma. Also as
the SPP transits through the corona, the rapidly-varying viewpoint and
high spatial resolution of WISPR will enable tomographic imaging of the
corona, and lead to higher fidelity and finer scale 3D reconstructions
than are possible with the STEREO mission or single-view rotational
tomography. The wide field of view will include at times other inner
heliospheric probes (e.g. Solar Orbiter) and will image the outflowing
wind that is impinging on that probe. In addition to this standard
imaging mode, WISPR opens a new capability for imaging instruments,
the measurement of pressure turbulence by employing a high cadence mode
(~1 sec) to image a small region in the corona. For the first time,
the slopes of the power spectral density from images can be compared
directly to the density and magnetic field fluctuations seen in situ as
a function of coronal spatial structure and heliocentric distance. In
addition, the 1 sec cadence can be generated anywhere within the WISPR
field, enabling the study of the transition of the solar wind injected
at the tops of the helmet streamers to inertial dissipation scales.
---------------------------------------------------------
Title: White Light and In Situ Comparison of a Forming Merged
Interaction Region
Authors: Rouillard, A. P.; Lavraud, B.; Sheeley, N. R.; Davies, J. A.;
Burlaga, L. F.; Savani, N. P.; Jacquey, C.; Forsyth, R. J.
2010ApJ...719.1385R Altcode:
The images taken by the Heliospheric Imager (HI) instruments, part of
the SECCHI imaging package on board the pair of STEREO spacecraft,
provide information on the radial and latitudinal evolution of the
plasma transported by coronal mass ejections (CMEs). In this case
study, a CME, appearing near 15 UT on 2007 November 15 in SECCHI
coronagraph images, leads to the formation of two out-flowing density
structures (DSs) in the heliosphere. The analysis of time-elongation
maps constructed from images obtained by the HI instruments shows that
these DSs were propagating along the Sun-Earth line. A direct comparison
of HI images and in situ measurements taken near Earth could therefore
be performed. These two DSs are separated by a cavity associated with
little brightness variation or equivalently little electron density
variation. In situ measurements made in the solar wind near Earth on
2007 November 20 show that this cavity corresponds to a magnetic cloud
(MC). While the leading DS is related to the sheath in front of the MC,
the second DS is located on the sunward side of the MC where high-speed
solar wind from a coronal hole catches up and interacts with the MC. We
conclude that HI observes the sub-structures of a merged interaction
region (MIR), a region of the interplanetary medium where the total
solar wind pressure is greatly enhanced by the interaction of an MC
with the ambient solar wind. This MIR caused the largest geomagnetic
storm in 2007.
---------------------------------------------------------
Title: Observations of the magnetic field and plasma in the
heliosheath by Voyager 2 from 2007.7 to 2009.4
Authors: Burlaga, L. F.; Ness, N. F.; Wang, Y. -M.; Sheeley, N. R.;
Richardson, J. D.
2010JGRA..115.8107B Altcode: 2010JGRA..11508107B
The density and temperature profiles of the plasma measured by Voyager
2 (V2) behind the termination shock changed abruptly near 2008.6
from relatively large average values and large fluctuations during
2007.7 to 2008.6 (interval A) to relatively low average values and
very small-amplitude fluctuations during 2008.6 to 2009.4 (interval
B). This paper shows that the change in the magnetic field strength B(t)
was less abrupt than the plasma changes, and the fluctuations of the
magnetic field strength in interval B were of moderate amplitude, with
indications of a quasiperiodic structure in part of the interval. The
magnetic field was directed away from the sun (positive polarity)
∼ 78% ± 5% of the time in both interval A and interval B, changing
in an irregular way from positive to negative polarities throughout
the interval. The polarity distribution indicates that the minimum
latitudinal extent of the heliospheric current sheet (HCS) was
near V2 throughout the interval, consistent with the extrapolated
minimum latitudes of the HCS computed from solar magnetic field
observations. Thus, V2 was observing magnetic fields from the southern
polar coronal hole most of the time. The distribution of B was lognormal
in interval A and Gaussian interval B.
---------------------------------------------------------
Title: On the "Extended" Solar Cycle in Coronal Emission
Authors: Robbrecht, E.; Wang, Y. -M.; Sheeley, N. R., Jr.; Rich, N. B.
2010ApJ...716..693R Altcode:
Butterfly diagrams (latitude-time plots) of coronal emission show a
zone of enhanced brightness that appears near the poles just after
solar maximum and migrates toward lower latitudes; a bifurcation seems
to occur at sunspot minimum, with one branch continuing to migrate
equatorward with the sunspots of the new cycle and the other branch
heading back to the poles. The resulting patterns have been likened to
those seen in torsional oscillations and have been taken as evidence
for an extended solar cycle lasting over ~17 yr. In order to clarify
the nature of the overlapping bands of coronal emission, we construct
butterfly diagrams from green-line simulations covering the period
1967-2009 and from 19.5 nm and 30.4 nm observations taken with the
Extreme-Ultraviolet Imaging Telescope during 1996-2009. As anticipated
from earlier studies, we find that the high-latitude enhancements mark
the footpoint areas of closed loops with one end rooted outside the
evolving boundaries of the polar coronal holes. The strong underlying
fields were built up over the declining phase of the cycle through
the poleward transport of active-region flux by the surface meridional
flow. Rather than being a precursor of the new-cycle sunspot activity
zone, the high-latitude emission forms a physically distinct, U-shaped
band that curves upward again as active-region fields emerge at
midlatitudes and reconnect with the receding polar-hole boundaries. We
conclude that the so-called extended cycle in coronal emission is a
manifestation not of early new-cycle activity, but of the poleward
concentration of old-cycle trailing-polarity flux by meridional flow.
---------------------------------------------------------
Title: What's So Peculiar about the Cycle 23/24 Solar Minimum?
Authors: Sheeley, N. R., Jr.
2010ASPC..428....3S Altcode: 2010arXiv1005.3834S
Traditionally, solar physicists become anxious around solar minimum,
as they await the high-latitude sunspot groups of the new cycle. Now,
we are in an extended sunspot minimum with conditions not seen in recent
memory, and interest in the sunspot cycle has increased again. In this
paper, I will describe some of the characteristics of the current solar
minimum, including its great depth, its extended duration, its weak
polar magnetic fields, and its small amount of open flux. Flux transport
simulations suggest that these characteristics are a consequence of
temporal variations of the Sun's large-scale meridional circulation.
---------------------------------------------------------
Title: Tracking Streamer Blobs into the Heliosphere
Authors: Sheeley, N. R., Jr.; Rouillard, A. P.
2010ApJ...715..300S Altcode: 2010arXiv1006.5379S
In this paper, we use coronal and heliospheric images from the Solar
Terrestrial Relations Observatory (STEREO) spacecraft to track streamer
blobs into the heliosphere and to observe them being swept up and
compressed by the fast wind from low-latitude coronal holes. From an
analysis of their elongation/time tracks, we discover a "locus of
enhanced visibility" where neighboring blobs pass each other along
the line of sight and their corotating spiral is seen edge-on. The
detailed shape of this locus accounts for a variety of east-west
asymmetries and allows us to recognize the spiral of blobs by its
signatures in the STEREO images: in the eastern view from STEREO-A,
the leading edge of the spiral is visible as a moving wavefront where
foreground ejections overtake background ejections against the sky
and then fade. In the western view from STEREO-B, the leading edge is
only visible close to the Sun-spacecraft line where the radial path
of ejections nearly coincides with the line of sight. In this case,
we can track large-scale waves continuously back to the lower corona
and see that they originate as face-on blobs.
---------------------------------------------------------
Title: Formation and Evolution of Coronal Holes Following the
Emergence of Active Regions
Authors: Wang, Y. -M.; Robbrecht, E.; Rouillard, A. P.; Sheeley,
N. R., Jr.; Thernisien, A. F. R.
2010ApJ...715...39W Altcode:
The low level of solar activity over the past four years has provided
unusually favorable conditions for tracking the formation and evolution
of individual coronal holes and their wind streams. Employing
extreme-ultraviolet images recorded with the Solar Terrestrial
Relations Observatory during 2007-2009, we analyze three cases
in which small coronal holes first appear at the edges of newly
emerged active regions and then expand via flux transport processes,
eventually becoming attached to the polar holes. The holes form
gradually over timescales comparable to or greater than that for
the active regions to emerge, without any obvious association with
coronal mass ejections. The evolving hole areas coincide approximately
with the footpoints of open field lines derived from potential-field
source-surface extrapolations of the photospheric field. One of these
coronal-hole systems, centered at the equator and maintained by a
succession of old-cycle active regions emerging in the same longitude
range, persists in one form or another for up to two years. The other
two holes, located at midlatitudes and originating from new-cycle
active regions, become strongly sheared and decay away after a few
rotations. The hole boundaries and the small active-region holes, both
of which are sources of slow wind, are observed to undergo continual
short-term (lsim1 day) fluctuations on spatial scales comparable to
that of the supergranulation. From in situ measurements, we identify
a number of plasma sheets associated with pseudostreamers separating
holes of the same polarity.
---------------------------------------------------------
Title: Intermittent release of transients in the slow solar wind:
1. Remote sensing observations
Authors: Rouillard, A. P.; Davies, J. A.; Lavraud, B.; Forsyth, R. J.;
Savani, N. P.; Bewsher, D.; Brown, D. S.; Sheeley, N. R.; Davis,
C. J.; Harrison, R. A.; Howard, R. A.; Vourlidas, A.; Lockwood, M.;
Crothers, S. R.; Eyles, C. J.
2010JGRA..115.4103R Altcode: 2010JGRA..11504103R
The Heliospheric Imager (HI) instruments on board the STEREO spacecraft
are used to analyze the solar wind during August and September 2007. We
show how HI can be used to image the streamer belt and, in particular,
the variability of the slow solar wind which originates inside and
in the vicinity of the streamer belt. Intermittent mass flows are
observed in HI difference images, streaming out along the extension of
helmet streamers. These flows can appear very differently in images:
plasma distributed on twisted flux ropes, V-shaped structures, or
“blobs.” The variety of these transient features may highlight the
richness of phenomena that could occur near helmet streamers: emergence
of flux ropes, reconnection of magnetic field lines at the tip of
helmet streamers, or disconnection of open magnetic field lines. The
plasma released with these transient events forms part of the solar
wind in the higher corona; HI observations show that these transients
are frequently entrained by corotating interaction regions (CIRs),
leading to the formation of larger, brighter plasma structures in HI
images. This entrainment is used to estimate the trajectory of these
plasma ejecta. In doing so, we demonstrate that successive transients
can be entrained by the same CIR in the high corona if they emanate
from the same corotating source. Some parts of the streamers are more
effective sources of transients than others. Surprisingly, evidence
is given for the outflow of a recurring twisted magnetic structure,
suggesting that the emergence of flux ropes can be recurrent.
---------------------------------------------------------
Title: On the Weakening of the Polar Magnetic Fields during Solar
Cycle 23
Authors: Wang, Y. -M.; Robbrecht, E.; Sheeley, N. R., Jr.
2009ApJ...707.1372W Altcode:
The Sun's polar fields are currently ~40% weaker than they were during
the previous three sunspot minima. This weakening has been accompanied
by a corresponding decrease in the interplanetary magnetic field (IMF)
strength, by a ~20% shrinkage in the polar coronal-hole areas, and by
a reduction in the solar-wind mass flux over the poles. It has also
been reflected in coronal streamer structure and the heliospheric
current sheet, which only showed the expected flattening into the
equatorial plane after sunspot numbers fell to unusually low values
in mid-2008. From latitude-time plots of the photospheric field,
it has long been apparent that the polar fields are formed through
the transport of trailing-polarity flux from the sunspot latitudes
to the poles. To address the question of why the polar fields are
now so weak, we simulate the evolution of the photospheric field and
radial IMF strength from 1965 to the present, employing a surface
transport model that includes the effects of active region emergence,
differential rotation, supergranular convection, and a poleward bulk
flow. We find that the observed evolution can be reproduced if the
amplitude of the surface meridional flow is varied by as little as 15%
(between 14.5 and 17 m s<SUP>-1</SUP>), with the higher average speeds
being required during the long cycles 20 and 23.
---------------------------------------------------------
Title: A solar storm observed from the Sun to Venus using the STEREO,
Venus Express, and MESSENGER spacecraft
Authors: Rouillard, A. P.; Davies, J. A.; Forsyth, R. J.; Savani,
N. P.; Sheeley, N. R.; Thernisien, A.; Zhang, T. -L.; Howard, R. A.;
Anderson, B.; Carr, C. M.; Tsang, S.; Lockwood, M.; Davis, C. J.;
Harrison, R. A.; Bewsher, D.; Fränz, M.; Crothers, S. R.; Eyles,
C. J.; Brown, D. S.; Whittaker, I.; Hapgood, M.; Coates, A. J.; Jones,
G. H.; Grande, M.; Frahm, R. A.; Winningham, J. D.
2009JGRA..114.7106R Altcode: 2009JGRA..11407106R
The suite of SECCHI optical imaging instruments on the STEREO-A
spacecraft is used to track a solar storm, consisting of several coronal
mass ejections (CMEs) and other coronal loops, as it propagates from
the Sun into the heliosphere during May 2007. The 3-D propagation
path of the largest interplanetary CME (ICME) is determined from the
observations made by the SECCHI Heliospheric Imager (HI) on STEREO-A
(HI-1/2A). Two parts of the CME are tracked through the SECCHI images, a
bright loop and a V-shaped feature located at the rear of the event. We
show that these two structures could be the result of line-of-sight
integration of the light scattered by electrons located on a single
flux rope. In addition to being imaged by HI, the CME is observed
simultaneously by the plasma and magnetic field experiments on the Venus
Express and MESSENGER spacecraft. The imaged loop and V-shaped structure
bound, as expected, the flux rope observed in situ. The SECCHI images
reveal that the leading loop-like structure propagated faster than
the V-shaped structure, and a decrease in in situ CME speed occurred
during the passage of the flux rope. We interpret this as the result
of the continuous radial expansion of the flux rope as it progressed
outward through the interplanetary medium. An expansion speed in the
radial direction of ∼30 km s<SUP>-1</SUP> is obtained directly from
the SECCHI-HI images and is in agreement with the difference in speed
of the two structures observed in situ. This paper shows that the flux
rope location can be determined from white light images, which could
have important space weather applications.
---------------------------------------------------------
Title: Radial and solar cycle variations of the magnetic fields in
the heliosheath: Voyager 1 observations from 2005 to 2008
Authors: Burlaga, L. F.; Ness, N. F.; Acuña, M. H.; Wang, Y. -M.;
Sheeley, N. R.
2009JGRA..114.6106B Altcode: 2009JGRA..11406106B
We discuss the magnetic field strength B(t) and polarity observed by
Voyager 1 (V1) in the heliosheath at the heliographic latitude ≈34°
as it moved away from the Sun from 2005 through 2008.82 (where 2008.0
is the beginning of 1 January 2008). The pattern of the polarity of the
magnetic field changed from alternating positive and negative polarities
to predominantly negative polarities (magnetic fields pointing along
the Archimedean spiral field angle toward the Sun) at ≈2006.23). This
transition indicates that the latitudinal extent of the heliospheric
current sheet (HCS) was decreasing in the supersonic solar wind, as
expected for the declining phase of the solar cycle, and as predicted
by extrapolation of the magnetic neutral line near the photosphere to
the position of V1. However, the polarity was not uniformly negative
in during 2008, in contrast to the predicted polarity. This difference
suggests that the maximum latitudinal extent of the HCS was tending
to increase in the northern hemisphere in the heliosheath, while it
was decreasing in the supersonic solar wind. The large-scale magnetic
field strength B(t) was observed by V1 from 2005 through 2008.82. During
this interval of decreasing solar activity toward solar minimum, B(t)
at 1 AU was decreasing, and the solar wind speed V at the latitude of
V1 was increasing. Adjusting the temporal profile of B(t) observed by V1
for the solar cycle variations of B and V in the supersonic solar wind,
we find that the radial gradient of B(R) in heliosheath from the radial
distance R = 94.2 AU to 107.9 AU between 2005.0 and 2008.82 was 0.0017
nT/AU ≤ grad B ≤ 0.0055 nT/AU, or grad B = (0.0036 ± 0.0019) nT/AU.
---------------------------------------------------------
Title: Remote-sensing Observations of the Corona and Solar Wind
Authors: Sheeley, Neil R., Jr.
2009AAS...21410701S Altcode:
On June 25, 1908, George Ellery Hale used the 60-foot Tower Telescope
on Mount Wilson to make the first measurements of magnetic fields
in sunspots. This began a series of studies that led to Hale's Law
of sunspot polarities and established the Mount Wilson Observatory
as a leading center of solar magnetic field research. The magnetic
aura was still present in 1962 when I began solar research there as a
Caltech graduate student. Mount Wilson astronomer Horace Babcock and
his father had invented the solar magnetograph, discovered the polar
fields of the Sun, and observed their reversal near the 1958 sunspot
maximum. Caltech physicist Robert Leighton had added new instrumentation
to the Mount Wilson spectroheliograph and obtained high-resolution
maps of the magnetic field. Babcock had just published his classic
paper on the topology of the field and its 22-year cycle. The paper
contained a sketch, illustrating the coronal field-line reconnection,
which he thought must occur in response to changes of the photospheric
field. Some loops flew away in the yet-to-be-discovered solar wind
and other loops collapsed back to the Sun. <P />In this talk, I will
present new observations from the SOHO and STEREO spacecraft, which
show such coronal changes. Loops stretch out in the expanding corona
and tear away from the Sun like drops from a leaky faucet. Simultaneous
observations with different perspectives show that the detached loops
are really helices in 3-D. Off-pointed heliospheric imagers allow us
to track these ejections outward past planets (including Earth) and
comets, and to observe their compression into a heliospheric spiral,
as a consequence of longitudinal speed gradients on the rotating
Sun. And XUV observations of the solar disk show brightness changes
associated with reconnections high in the corona, like auroral displays
in the magnetosphere.
---------------------------------------------------------
Title: A Multispacecraft Analysis of a Small-Scale Transient Entrained
by Solar Wind Streams
Authors: Rouillard, A. P.; Savani, N. P.; Davies, J. A.; Lavraud, B.;
Forsyth, R. J.; Morley, S. K.; Opitz, A.; Sheeley, N. R.; Burlaga,
L. F.; Sauvaud, J. -A.; Simunac, K. D. C.; Luhmann, J. G.; Galvin,
A. B.; Crothers, S. R.; Davis, C. J.; Harrison, R. A.; Lockwood, M.;
Eyles, C. J.; Bewsher, D.; Brown, D. S.
2009SoPh..256..307R Altcode:
The images taken by the Heliospheric Imagers (HIs), part of the
SECCHI imaging package onboard the pair of STEREO spacecraft,
provide information on the radial and latitudinal evolution of the
plasma compressed inside corotating interaction regions (CIRs). A
plasma density wave imaged by the HI instrument onboard STEREO-B was
found to propagate towards STEREO-A, enabling a comparison between
simultaneous remote-sensing and in situ observations of its structure to
be performed. In situ measurements made by STEREO-A show that the plasma
density wave is associated with the passage of a CIR. The magnetic
field compressed after the CIR stream interface (SI) is found to have
a planar distribution. Minimum variance analysis of the magnetic field
vectors shows that the SI is inclined at 54° to the orbital plane of
the STEREO-A spacecraft. This inclination of the CIR SI is comparable
to the inclination of the associated plasma density wave observed by
HI. A small-scale magnetic cloud with a flux rope topology and radial
extent of 0.08 AU is also embedded prior to the SI. The pitch-angle
distribution of suprathermal electrons measured by the STEREO-A SWEA
instrument shows that an open magnetic field topology in the cloud
replaced the heliospheric current sheet locally. These observations
confirm that HI observes CIRs in difference images when a small-scale
transient is caught up in the compression region.
---------------------------------------------------------
Title: Two Years of the STEREO Heliospheric Imagers. Invited Review
Authors: Harrison, Richard A.; Davies, Jackie A.; Rouillard, Alexis
P.; Davis, Christopher J.; Eyles, Christopher J.; Bewsher, Danielle;
Crothers, Steve R.; Howard, Russell A.; Sheeley, Neil R.; Vourlidas,
Angelos; Webb, David F.; Brown, Daniel S.; Dorrian, Gareth D.
2009SoPh..256..219H Altcode:
Imaging of the heliosphere is a burgeoning area of research. As
a result, it is awash with new results, using novel applications,
and is demonstrating great potential for future research in a wide
range of topical areas. The STEREO (Solar TErrestrial RElations
Observatory) Heliospheric Imager (HI) instruments are at the heart
of this new development, building on the pioneering observations of
the SMEI (Solar Mass Ejection Imager) instrument aboard the Coriolis
spacecraft. Other earlier heliospheric imaging systems have included
ground-based interplanetary scintillation (IPS) facilities and the
photometers on the Helios spacecraft. With the HI instruments, we now
have routine wide-angle imaging of the inner heliosphere, from vantage
points outside the Sun-Earth line. HI has been used to investigate the
development of coronal mass ejections (CMEs) as they pass through the
heliosphere to 1 AU and beyond. Synoptic mapping has also allowed us to
see graphic illustrations of the nature of mass outflow as a function
of distance from the Sun - in particular, stressing the complexity of
the near-Sun solar wind. The instruments have also been used to image
co-rotating interaction regions (CIRs), to study the interaction of
comets with the solar wind and CMEs, and to witness the impact of CMEs
and CIRs on planets. The very nature of this area of research - which
brings together aspects of solar physics, space-environment physics,
and solar-terrestrial physics - means that the research papers are
spread among a wide range of journals from different disciplines. Thus,
in this special issue, it is timely and appropriate to provide a review
of the results of the first two years of the HI investigations.
---------------------------------------------------------
Title: Remote-sensing Observations of the Corona and Solar Wind
Authors: Sheeley, Neil R., Jr.
2009SPD....40.2801S Altcode:
On June 25, 1908, George Ellery Hale used the 60-foot Tower Telescope
on Mount Wilson to make the first measurements of magnetic fields
in sunspots. This began a series of studies that led to Hale's Law
of sunspot polarities and established the Mount Wilson Observatory
as a leading center of solar magnetic field research. The magnetic
aura was still present in 1962 when I began solar research there as a
Caltech graduate student. Mount Wilson astronomer Horace Babcock and
his father had invented the solar magnetograph, discovered the polar
fields of the Sun, and observed their reversal near the 1958 sunspot
maximum. Caltech physicist Robert Leighton had added new instrumentation
to the Mount Wilson spectroheliograph and obtained high-resolution
maps of the magnetic field. Babcock had just published his classic
paper on the topology of the field and its 22-year cycle. The paper
contained a sketch, illustrating the coronal field-line reconnection,
which he thought must occur in response to changes of the photospheric
field. Some loops flew away in the yet-to-be-discovered solar wind
and other loops collapsed back to the Sun. <P />In this talk, I will
present new observations from the SOHO and STEREO spacecraft, which
show such coronal changes. Loops stretch out in the expanding corona
and tear away from the Sun like drops from a leaky faucet. Simultaneous
observations with different perspectives show that the detached loops
are really helices in 3-D. Off-pointed heliospheric imagers allow us
to track these ejections outward past planets (including Earth) and
comets, and to observe their compression into a heliospheric spiral,
as a consequence of longitudinal speed gradients on the rotating
Sun. And XUV observations of the solar disk show brightness changes
associated with reconnections high in the corona, like auroral displays
in the magnetosphere.
---------------------------------------------------------
Title: White-light and in-situ observations of an Earth-impacting
CME using the STEREO spacecraft
Authors: Rouillard, A. P.; Savani, N. P.; Sheeley, N. R.; Burlaga,
L. F.; Lavraud, B.; Davies, J. A.; Forsyth, R. J.; Davis, C.;
Lockwood, M.
2009EGUGA..11.8052R Altcode:
We use the heliospheric imagers (HIs) onboard the STEREO A and B
spacecraft to analyse the propagation of an Earth-impacting Coronal
Mass Ejection (CME) during November 2007. The transient is observed
continuously in white-light images from the Sun to 1 AU. The 3-D
propagation path is determined and a method is outlined to obtain the
acceleration profile of the transient. The CME comprises two large
density increases in white-light images. Comparison of white-light
and in-situ observations shows that the flux-rope is embedded inside
these two density increases. The evolution of the CME brightness in
white-light images is discussed in terms of the kinematic evolution
of the transient and its interaction with the ambient solar wind.
---------------------------------------------------------
Title: The Structure of Streamer Blobs
Authors: Sheeley, N. R., Jr.; Lee, D. D. -H.; Casto, K. P.; Wang,
Y. -M.; Rich, N. B.
2009ApJ...694.1471S Altcode:
We have used Sun-Earth Connection Coronal and Heliospheric Investigation
observations obtained from the STEREO A and B spacecraft to study
complementary face-on and edge-on views of coronal streamers. The
face-on views are analogous to what one might see looking down on a flat
equatorial streamer belt at sunspot minimum, and show streamer blobs
as diffuse arches gradually expanding outward from the Sun. With the
passage of time, the legs of the arches fade, and the ejections appear
as a series of azimuthal structures like ripples on a pond. The arched
topology is similar to that obtained in face-on views of streamer
disconnection events (including in/out pairs and streamer blowout
mass ejections), and suggests that streamer blobs have the helical
structure of magnetic flux ropes.
---------------------------------------------------------
Title: Understanding the Geomagnetic Precursor of the Solar Cycle
Authors: Wang, Y. -M.; Sheeley, N. R.
2009ApJ...694L..11W Altcode:
Geomagnetic activity late in the sunspot cycle has been used
successfully to forecast the amplitude of the following cycle. This
success is somewhat surprising, however, because the recurrent
high-speed wind streams that trigger the activity are not proxies of
the Sun's polar fields, whose strength is a critical factor in many
solar dynamo models. Instead, recurrent geomagnetic activity signals
increases in the Sun's equatorial dipole moment, which decays on the
~1-2 yr timescale of the surface meridional flow and does not survive
into the next cycle. In accordance with the original empirical method
of Ohl, we therefore argue that solar cycle predictions should be based
on the minimum level of geomagnetic activity, which is determined by
the Sun's axial dipole strength, not on the peak activity during the
declining phase of the cycle. On physical grounds, we suggest that
an even better indicator would be the total open flux (or strength of
the radial interplanetary field component) at sunspot minimum, which
in turn can be derived from the historical aa index by removing the
contribution of the solar wind speed. This predictor yields a peak
yearly sunspot number R <SUB>max</SUB> = 97 ± 25 for solar cycle 24.
---------------------------------------------------------
Title: A synoptic view of solar transient evolution in the inner
heliosphere using the Heliospheric Imagers on STEREO
Authors: Davies, J. A.; Harrison, R. A.; Rouillard, A. P.; Sheeley,
N. R.; Perry, C. H.; Bewsher, D.; Davis, C. J.; Eyles, C. J.; Crothers,
S. R.; Brown, D. S.
2009GeoRL..36.2102D Altcode:
By exploiting data from the STEREO/heliospheric imagers (HI) we extend
a well-established technique developed for coronal analysis by producing
time-elongation plots that reveal the nature of solar transient activity
over a far more extensive region of the heliosphere than previously
possible from coronagraph images. Despite the simplicity of these plots,
their power in demonstrating how the plethora of ascending coronal
features observed near the Sun evolve as they move antisunward is
obvious. The time-elongation profile of a transient tracked by HI can,
moreover, be used to establish its angle out of the plane-of-the-sky an
illustration of such analysis reveals coronal mass ejection material
that can be clearly observed propagating out to distances beyond
1AU. This work confirms the value of the time-elongation format in
identifying/characterising transient activity in the inner heliosphere,
whilst also validating the ability of HI to continuously monitor solar
ejecta out to and beyond 1AU.
---------------------------------------------------------
Title: Remote Sensing of the Slow Solar Wind
Authors: Sheeley, N. R.
2008AGUFMSH43B..05S Altcode:
We are using SECCHI (Sun Earth Connection Coronal and Heliospheric
Imager) observations from the STEREO (Solar Terrestrial Relations
Observatory) spacecraft to construct elongation/time maps of material
moving outward from about 2 solar radii to distances beyond the orbit
of Earth. These maps span all position angles within the low-latitude
fields of the Heliospheric Imagers and provide synoptic coverage of the
motions observed since April 2007. These motions include the gradual
acceleration of streamer blobs to solar wind speeds in the range
300-400 km/s. They also include streamer detachments and eruptions,
in which the inflating streamers stretch until they separate into
collapsing loops and outgoing arches that reach terminal speeds in
the range 300-500 km/s. During 2008, the two STEREO spacecraft have
separated by 68 degrees, and are now providing complementary views of
these ejecta. Recent observations show that some of the streamer blobs
and ejections have the helical topology expected for magnetic flux
ropes produced by field line reconnection in the corona and solar wind.
---------------------------------------------------------
Title: On the Solar Origins of Open Magnetic Fields in the Heliosphere
Authors: Rust, David M.; Haggerty, Dennis K.; Georgoulis, Manolis K.;
Sheeley, Neil R.; Wang, Yi-Ming; DeRosa, Marc L.; Schrijver, Carolus J.
2008ApJ...687..635R Altcode:
A combination of heliospheric and solar data was used to identify open
magnetic fields stretching from the lower corona to Earth orbit. 35
near-relativistic electron beams detected at the ACE spacecraft
"labeled" the heliospheric segments of the open fields. An X-ray
flare occurred <20 minutes before injection of the electrons
in 25 events. These flares labeled the solar segment of the open
fields. The flares occurred in western-hemisphere active regions (ARs)
with coronal holes whose polarity agreed with the polarity of the
beam-carrying interplanetary fields in 23 of the 25 events. We conclude
that electron beams reach 1 AU from open AR fields adjacent to flare
sites. The Wang & Sheeley implementation of the potential-field
source-surface model successfully identified the open fields in
36% of cases. Success meant that the open fields reached the source
surface within 3 heliographic deg of the interplanetary magnetic field
connected to ACE at 1 AU. Inclusion of five near misses improves
the success rate to 56%. The success rate for the Schrijver &
DeRosa PFSS implementation was 50%. Our results suggest that, even
if the input magnetic data are updated frequently, the PFSS models
succeed in only ~50% of cases to identify the coronal segment of open
fields. Development of other techniques is in its infancy.
---------------------------------------------------------
Title: A Century of Polar Faculae Variations
Authors: Sheeley, N. R., Jr.
2008ApJ...680.1553S Altcode:
The numbers of faculae at the poles of the Sun have been estimated from
white-light images obtained at the Mount Wilson Observatory during
1985-2006 and combined with prior estimates extending back to 1906,
when the observations began. The combined data show an 11 yr cyclic
variation with faculae maxima occurring during sunspot minima in each
of the past 10 sunspot cycles. Also, these numbers of polar faculae are
well correlated with the line-of-sight component of the polar magnetic
field measured at the Wilcox Solar Observatory since 1976. The numbers
of polar faculae show a secular decrease since 1986, suggesting that
the polar fields are now weaker than they have been during any cycle
in the past century.
---------------------------------------------------------
Title: A Century of Polar Faculae Variations
Authors: Sheeley, N. R.
2008AGUSMSP23A..04S Altcode:
The numbers of faculae at the poles of the Sun have been estimated
from white-light images obtained at the Mount Wilson Observatory
during 1985-2006 and combined with prior estimates extending back
to 1906 when the observations began. The combined data show an 11-yr
cyclic variation with faculae maxima occurring during sunspot minima
in each of the past 10 sunspot cycles. Also, these numbers of polar
faculae are well correlated with the line-of-sight component of the
polar magnetic field measured at the Wilcox Solar Observatory since
1976. The numbers of polar faculae show a secular decrease since 1986,
suggesting that the polar fields are now weaker than they have been
during any cycle in the past century.
---------------------------------------------------------
Title: Heliospheric Images of the Solar Wind at Earth
Authors: Sheeley, N. R., Jr.; Herbst, A. D.; Palatchi, C. A.; Wang,
Y. -M.; Howard, R. A.; Moses, J. D.; Vourlidas, A.; Newmark, J. S.;
Socker, D. G.; Plunkett, S. P.; Korendyke, C. M.; Burlaga, L. F.;
Davila, J. M.; Thompson, W. T.; St. Cyr, O. C.; Harrison, R. A.;
Davis, C. J.; Eyles, C. J.; Halain, J. P.; Wang, D.; Rich, N. B.;
Battams, K.; Esfandiari, E.; Stenborg, G.
2008ApJ...675..853S Altcode:
During relatively quiet solar conditions throughout the spring and
summer of 2007, the SECCHI HI2 white-light telescope on the STEREO
B solar-orbiting spacecraft observed a succession of wave fronts
sweeping past Earth. We have compared these heliospheric images with
in situ plasma and magnetic field measurements obtained by near-Earth
spacecraft, and we have found a near perfect association between the
occurrence of these waves and the arrival of density enhancements
at the leading edges of high-speed solar wind streams. Virtually
all of the strong corotating interaction regions are accompanied by
large-scale waves, and the low-density regions between them lack such
waves. Because the Sun was dominated by long-lived coronal holes and
recurrent solar wind streams during this interval, there is little
doubt that we have been observing the compression regions that are
formed at low latitude as solar rotation causes the high-speed wind
from coronal holes to run into lower speed wind ahead of it.
---------------------------------------------------------
Title: SECCHI Observations of the Sun's Garden-Hose Density Spiral
Authors: Sheeley, N. R., Jr.; Herbst, A. D.; Palatchi, C. A.; Wang,
Y. -M.; Howard, R. A.; Moses, J. D.; Vourlidas, A.; Newmark, J. S.;
Socker, D. G.; Plunkett, S. P.; Korendyke, C. M.; Burlaga, L. F.;
Davila, J. M.; Thompson, W. T.; St. Cyr, O. C.; Harrison, R. A.;
Davis, C. J.; Eyles, C. J.; Halain, J. P.; Wang, D.; Rich, N. B.;
Battams, K.; Esfandiari, E.; Stenborg, G.
2008ApJ...674L.109S Altcode:
The SECCHI HI2 white-light imagers on the STEREO A and B spacecraft
show systematically different proper motions of material moving outward
from the Sun in front of high-speed solar wind streams from coronal
holes. As a group of ejections enters the eastern (A) field of view,
the elements at the rear of the group appear to overrun the elements
at the front. (This is a projection effect and does not mean that the
different elements actually merge.) The opposite is true in the western
(B) field; the elements at the front of the group appear to run away
from the elements at the rear. Elongation/time maps show this effect
as a characteristic grouping of the tracks of motion into convergent
patterns in the east and divergent patterns in the west, consistent
with ejections from a single longitude on the rotating Sun. Evidently,
we are observing segments of the "garden-hose" spiral made visible
when fast wind from a low-latitude coronal hole compresses blobs of
streamer material being shed at the leading edge of the hole.
---------------------------------------------------------
Title: Global structure and dynamics of large-scale fluctuations in
the solar wind: Voyager 2 observations during 2005 and 2006
Authors: Burlaga, L. F.; Ness, N. F.; Acũna, M. H.; Wang, Y. -M.;
Sheeley, N. R.; Wang, C.; Richardson, J. D.
2008JGRA..113.2104B Altcode:
The Voyager 2 (V2) observations of daily averages of the solar
wind during 2005 and 2006 from 75.3 AU to 81.6 AU between ~25.7°S
and 27.1°S show both a step-like trend in the speed V(t) and
“large-scale fluctuations” of the magnetic field strength B, speed
V, density N and temperature T. The distribution functions of B,
N, and NV<SUP>2</SUP> observed by V2 are lognormal and that of V
is approximately Gaussian. We introduce a method for specifying
the boundary conditions at all latitudes (except near the poles)
on a Sun-centered surface of radius of 1 AU, based on solar magnetic
field observations. This paper uses only the boundary conditions at
the latitude of V2 and a 1-D time-dependent MHD model to calculate
the radial evolution of the large-scale fluctuations of B(t), V(t)
and N(t) at distances between 1 and 90 AU. This model explains the
V2 observations of a lognormal distribution of B and the Gaussian
distribution of V, but not the observed lognormal distributions of
N and NV<SUP>2</SUP>. The lognormal distribution of B observed by V2
was produced primarily by dynamical processes beyond 1 AU.
---------------------------------------------------------
Title: Limits to the Radiative Asymmetry of the Quiet Solar Disk
Authors: Livingston, W.; Sheeley, N. R., Jr.
2008ApJ...672.1228L Altcode:
Precise data on the uniformity of photospheric radiation over the solar
disk seems not to exist. Such information is necessary to separate
the radiative behavior of the quiet basal atmosphere from the active
(magnetic) atmosphere. Is the latter the sole source of known irradiance
variation? How uniform can a solar-like stellar disk be? To obtain this
information we have made monochromatic scans along the central meridian
of the quiet Sun using single element detectors which do not require
flat fielding. The scans were in continua and in selected Fraunhofer
lines ranging from 3134 to 46880 Å the observational epoch was near
solar minimum: 2006 October to 2007 February. The meridian was chosen
to avoid rotational Doppler shifts. We extract the asymmetry between
the north and south hemispheres and present it as our main product. In
the near-infrared and visible continuum, averaging over granulation and
avoiding sunspots, we found that such asymmetry was as low as 0.05%
(at 34168 Å on 2007 February 8). In the violet and ultraviolet this
asymmetry typically increases to 1%. Asymmetry is larger in the cores
of the medium strong photospheric and chromospheric lines, which refer
to higher levels in the atmosphere, and may reach 15%. The contrast of
faculae increases in the blue (and with improved spatial resolution or
seeing), and is the probable source for the measured asymmetries. We
also find that line core scans are in general flatter than continuum
scans.
---------------------------------------------------------
Title: A Streamer Ejection with Reconnection Close to the Sun
Authors: Sheeley, N. R., Jr.; Warren, H. P.; Wang, Y. -M.
2007ApJ...671..926S Altcode:
We previously described coronal events that expand gradually outward
over an interval of 1-2 days and then suddenly tear apart in the
coronagraph's 2-6 R<SUB>solar</SUB> field of view to form an outgoing
flux rope and an inward system of collapsing loops. Now, we combine
LASCO white-light images of the outer corona with spectrally resolved
EIT images of the inner corona to describe a similar event for which the
separation occurs closer to the Sun. The evolution of this 2006 July
1-2 event had four phases: (1) an expansion phase in which magnetic
loops rise slowly upward and increase the amount of open flux in the
adjacent polar coronal hole and in the low-latitude hole of opposite
polarity; (2) a stretching phase in which the legs of the rising
loops pinch together to form a current sheet; (3) a transition phase
in which field line reconnection produces an outgoing flux rope and a
hot cusp of new loops; and (4) an end phase in which the reconnected
loops become visible at lower temperatures, and the outgoing flux rope
plows through the slow material ahead of it to form a traveling bow
wave. During this time, the photospheric field was relatively weak and
unchanging, as if the eruption had a nonmagnetic origin. We suppose
that coronal heating gradually overpowers magnetic tension and causes
the streamer to separate into a system of collapsing loops and a flux
rope that is carried outward in the solar wind.
---------------------------------------------------------
Title: Secchi Observations of Mass Flows in the Inner Heliosphere
Authors: Sheeley, N. R.; Herbst, A. D.; Palatchi, C. A.; Wang, Y.
2007AGUFMSH42A..01S Altcode:
We use SECCHI (Sun Earth Connection Coronal and Heliospheric
Investigation) observations to construct "height/time" maps of material
flows in the heliosphere from the Sun to elongation angles beyond
90 degrees. Close to the Sun, we see accelerating tracks of ejecta
(like streamer blobs) close to the sky plane. Farther from the Sun,
we see groups of parallel tracks, some near the sky plane and some
out of the plane. At the greatest elongations, we see a collection of
non-parallel tracks, some merging and some crossing, and mainly from
directions well out of the sky plane. These maps and their associated
time- lapse movies are providing a new view of mass flows and their
interactions in the inner heliosphere.
---------------------------------------------------------
Title: The Solar Eclipse of 2006 and the Origin of Raylike Features
in the White-Light Corona
Authors: Wang, Y. -M.; Biersteker, J. B.; Sheeley, N. R., Jr.;
Koutchmy, S.; Mouette, J.; Druckmüller, M.
2007ApJ...660..882W Altcode:
Solar eclipse observations have long suggested that the white-light
corona is permeated by long fine rays. By comparing photographs of
the 2006 March 29 total eclipse with current-free extrapolations of
photospheric field measurements and with images from the Solar and
Heliospheric Observatory (SOHO), we deduce that the bulk of these
linear features fall into three categories: (1) polar and low-latitude
plumes that overlie small magnetic bipoles inside coronal holes,
(2) helmet streamer rays that overlie large loop arcades and separate
coronal holes of opposite polarity, and (3) “pseudostreamer” rays
that overlie twin loop arcades and separate coronal holes of the
same polarity. The helmet streamer rays extend outward to form the
plasma sheet component of the slow solar wind, while the plumes and
pseudostreamers contribute to the fast solar wind. In all three cases,
the rays are formed by magnetic reconnection between closed coronal
loops and adjacent open field lines. Although seemingly ubiquitous
when seen projected against the sky plane, the rays are in fact rooted
inside or along the boundaries of coronal holes.
---------------------------------------------------------
Title: Limits To The Radiative Asymmetry Of The Quiet Solar Disk
Authors: Livingston, W. C.; Sheeley, N. R.
2007AAS...210.2503L Altcode: 2007BAAS...39..131L
Precise data on the uniformity of photospheric radiation over the solar
disk seems not to exist. Such may be needed for the future detection
of planets crossing solar-like stellar disks, for example. To obtain
this information we have made monochromatic scans along the central
meridian of the quiet Sun using single element detectors which do not
require ’flat fielding’. The scans were in continua and selected
Fraunhofer lines ranging from 3129 to 46880 A; the observational epoch
was near solar minimum: Oct 2006 to Feb 2007. The meridian was chosen
to avoid rotational doppler shifts. We extract the asymmetry between
the N and S hemispheres and this is our main product. In the near IR
and visible continuum, averaging over granulation and discounting
sunspots, such asymmetry is as low as 0.01%; 0.005% at 34168 A on
8 Feb 2007. In the violet and UV this increases to 1%. In the cores
of medium strength photospheric lines and in chromospheric lines the
asymmetry is up to 15%. Faculae are the probable source of our measured
quiet disk asymmetries, and the continuum at 34168 A is favorable for
this reason. Line core scans are in general flatter than continuum
scans because they sample thinner, higher layers of the atmosphere,
where the temperature gradient is less.
---------------------------------------------------------
Title: Coronal Pseudostreamers
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Rich, N. B.
2007ApJ...658.1340W Altcode:
In a recent study of the 2006 solar eclipse, we noted that there are
two kinds of coronal streamers: “helmet streamers,” which separate
coronal holes of opposite magnetic polarity, and “pseudostreamers,”
which overlie twin loop arcades and separate holes of the same
polarity. It is well known that the heliospheric plasma and current
sheets represent the outward extension of helmet streamers. Using
white-light data from the Large Angle and Spectrometric Coronagraph
(LASCO), we here show that pseudostreamers likewise have plasma sheet
extensions, across which the polarity does not reverse; these multiple
sheets contribute significantly to the brightness of the K corona,
although their internal densities tend to be lower than those in
the heliospheric plasma sheet. We use current-free extrapolations of
photospheric field measurements to simulate the observed brightness
patterns in the outer corona, including the contributions of both
helmet streamer and pseudostreamer plasma sheets. Running-difference
images show that pseudostreamers are relatively quiescent, resembling
large-scale plumes; preliminary analysis suggests flow speeds as
high as 200 km s<SUP>-1</SUP> at heliocentric distances of only ~3
R<SUB>solar</SUB>, supporting the prediction (based on their low
flux tube divergence rates) that pseudostreamers are sources of fast
solar wind.
---------------------------------------------------------
Title: In/Out Pairs and the Detachment of Coronal Streamers
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
2007ApJ...655.1142S Altcode:
We previously described coronal events that originate in the 2-6
R<SUB>solar</SUB> field of view of the LASCO white-light coronagraph
and involve the simultaneous ejection of material inward toward the
Sun and outward away from it. Now, in a study of more than 160 in/out
pairs, we have found that these features are density enhancements at
the leading and trailing edges of depletions that occur when slowly
rising coronal structures separate from the Sun. The outward component
is shaped like a large arch with both ends attached to the Sun, and the
inward component is often resolved into loops. We also found about 60
additional events in which the outward components began near the edge of
the occulting disk and inward components were not visible, as if these
events were in/out pairs that originated below the 2 R<SUB>solar</SUB>
radius of the occulting disk. We conclude that in/out pairs belong to a
broad class of streamer detachments, which include “streamer blowout”
coronal mass ejections, and we suppose that all of these events occur
when rising magnetic loops reconnect to produce an outgoing helical
flux rope and an ingoing arcade of collapsing loops.
---------------------------------------------------------
Title: Sources of the Solar Wind at Ulysses during 1990-2006
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
2006ApJ...653..708W Altcode:
The Ulysses spacecraft is now well into its third polar orbit around
the Sun. Using stackplot displays, we summarize the wind speeds and
interplanetary sector polarities recorded by Ulysses since its launch
in 1990 and relate the observed patterns to the global evolution of
open magnetic regions (coronal holes) over the solar cycle. We verify
that the wind speeds are inversely correlated with the rate of flux-tube
divergence in the corona, as derived from a current-free extrapolation
of the measured photospheric field. We identify the source of each of
the long-lived, high-speed streams encountered by Ulysses and discuss
their formation, evolution, and rotational properties.
---------------------------------------------------------
Title: Observations of Flux Rope Formation in the Outer Corona
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
2006ApJ...650.1172W Altcode:
In previous studies employing the Large Angle and Spectrometric
Coronagraph (LASCO), we identified a class of white-light ejections
that separate into incoming and outgoing components at distances of
~3-5 R<SUB>solar</SUB> from Sun center. These events, of which up to
several per month are observed during high solar activity, are generally
preceded by a gradual outward expansion of faint loops over a period
of a day or more. The expansion terminates when the streamer material,
in the form of an elongated stalk or a sheetlike structure, suddenly
tears apart. The collapsing material is sometimes recognizable as a
collection of loops, while the ejected component is usually poorly
resolved. Here we describe a streamer detachment observed on 2005
December 11, in which the outgoing component can be clearly identified
as a cylindrical flux rope with its ends anchored in the Sun. Based
on simple three-dimensional white-light reconstructions, we conclude
that in/out pairs in general represent the pinching off of streamer
loop arcades to form flux ropes, as seen from different viewing angles.
---------------------------------------------------------
Title: Role of the Sun's Nonaxisymmetric Open Flux in Cosmic-Ray
Modulation
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Rouillard, A. P.
2006ApJ...644..638W Altcode:
We reexamine the empirical relationship between the Sun's open magnetic
flux and the cosmic-ray (CR) intensity over the solar cycle. The
single parameter that correlates best with the inverted CR rate is
found to be the nonaxisymmetric or longitudinally varying component
of the total open flux, rather than the sunspot number or the rate
of coronal mass ejections (CMEs). The nonaxisymmetric open flux in
turn tracks the evolution of the Sun's equatorial dipole component,
which is a function of both the strength and the longitudinal
distribution of sunspot activity. Year-long peaks in the equatorial
dipole strength coincide with steplike decreases in the CR intensity
and with the formation of global merged interaction regions (GMIRs)
in the outer heliosphere. During these periods, nonaxisymmetric open
flux (in the form of low-latitude coronal holes) is created through the
organized emergence of large active regions, resulting in the global
injection of magnetic energy into the heliosphere. At the same time,
strengthenings of the equatorial dipole are generally accompanied
by large increases in the number of fast CMEs. Rotationally induced,
compressional interactions between the nonaxisymmetric open flux, fast
CMEs, and high-speed streams then give rise to outward-propagating
diffusive barriers that extend over all longitudes and to a latitude
(>~45°) again determined by the equatorial dipole strength.
---------------------------------------------------------
Title: Solar physics: Back to the next solar cycle
Authors: Wang, Y. -M.; Sheeley, N. R.
2006NatPh...2..367W Altcode:
Many solar physicists expect the peak sunspot activity during the
next solar cycle to be at its weakest in almost a century. A recent
prediction to the contrary could turn this prevailing wisdom on
its head.
---------------------------------------------------------
Title: Probing Solar Open Magnetic Fields With Near-relativistic
Electron Beams.
Authors: Haggerty, Dennis K.; Rust, D.; Sheeley, N. R.; Wang, Y.
2006SPD....37.1102H Altcode: 2006BAAS...38R.238H
To achieve better understanding of our Sun-Earth environment,
NASA's Living with a Star (LWS) program addresses question that
cut across discipline boundaries. We present preliminary results on
probing solar open magnetic fields with near-relativistic electron
beams. This effort is directed at the major question: What determines
the topology and evolution of the magnetic fields that stretch from the
surface of the Sun to the outer boundary of the heliosphere? During
nine years of operation, nearly an entire solar cycle, the ACE/EPAM
instrument has measured well over 600 near-relativistic electron
events. Approximately 30% of these electron events are impulsive
with beam-like anisotropies and are predominantly from flares on the
western hemisphere. Near-relativistic electrons are accelerated in the
low corona and are released onto open coronal field lines, where they
propagate nearly scatter-free out to 1 AU. Near-relativistic electrons
are ideal probes of coronal open field lines because the transit time
to 1 AU is 10 minutes as compared to lower energy ions that spend more
time in the interplanetary medium and are therefore much more subject
to transport processes. In this work we use observations of electrons
from 1 AU, observations of various electromagnetic emissions associated
with electron acceleration at the Sun, and observations and models
of open magnetic field lines at the Sun to pinpoint the location of
electron acceleration.
---------------------------------------------------------
Title: Carrington Maps of the Upper Photosphere
Authors: Sheeley, N. R., Jr.; Warren, H. P.
2006ApJ...641..611S Altcode:
We have used images of the Sun's disk, obtained in the 6767 Å
continuum with the Michelson Doppler Interferometer (MDI) on the
Solar and Heliospheric Observatory (SOHO), to make Carrington
maps of the upper photosphere during the years 1996-2005. Each map
is constructed from observations near the limb where the continuum
radiation originates relatively high in the photosphere and faculae have
their greatest visibility. Consequently, the Carrington maps resemble
spectroheliograms in temperature-sensitive photospheric lines and show
the global distribution of faculae and all but the smallest sunspots
(which are obscured by overlying faculae). A time-lapse sequence of the
combined east-limb and west-limb maps shows the emergence of active
regions and the evolution of large-scale patterns of faculae with an
average temporal resolution of 14 days during the sunspot cycle. Also,
a longitudinally averaged butterfly diagram of these maps shows that in
each hemisphere there is a facula-free zone separating the old-cycle
polar field from trailing-polarity flux that is migrating poleward
from the sunspot belts. These facula-free zones coincide with the
neutral zones of the axisymmetric component of photospheric magnetic
field and their arrival at the poles in 2001 marks the reversal of the
polar fields. We think that this mapmaking technique can be applied
to white-light images obtained daily at the Mount Wilson Observatory
since 1905 and that the resulting Carrington maps will provide details
about the polar-field reversal process during past sunspot cycles when
high-quality magnetograms were unavailable.
---------------------------------------------------------
Title: Multi-Wavelength Observations of CMEs and Associated Phenomena.
Report of Working Group F
Authors: Pick, M.; Forbes, T. G.; Mann, G.; Cane, H. V.; Chen, J.;
Ciaravella, A.; Cremades, H.; Howard, R. A.; Hudson, H. S.; Klassen,
A.; Klein, K. L.; Lee, M. A.; Linker, J. A.; Maia, D.; Mikic,
Z.; Raymond, J. C.; Reiner, M. J.; Simnett, G. M.; Srivastava, N.;
Tripathi, D.; Vainio, R.; Vourlidas, A.; Zhang, J.; Zurbuchen, T. H.;
Sheeley, N. R.; Marqué, C.
2006SSRv..123..341P Altcode: 2006SSRv..tmp...60P
This chapter reviews how our knowledge of CMEs and CME-associated
phenomena has been improved, since the launch of the SOHO mission,
thanks to multi-wavelength analysis. The combination of data obtained
from space-based experiments and ground based instruments allows us
to follow the space-time development of an event from the bottom of
the corona to large distances in the interplanetary medium. Since CMEs
originate in the low solar corona, understanding the physical processes
that generate them is strongly dependant on coordinated multi-wavelength
observations. CMEs display a large diversity in morphology and kinematic
properties, but there is presently no statistical evidence that those
properties may serve to group them into different classes. When a CME
takes place, the coronal magnetic field undergoes restructuring. Much
of the current research is focused on understanding how the corona
sustains the stresses that allow the magnetic energy to build up and
how, later on, this magnetic energy is released during eruptive flares
and CMEs. Multi-wavelength observations have confirmed that reconnection
plays a key role during the development of CMEs. Frequently, CMEs
display a rather simple shape, exhibiting a well known three-part
structure (bright leading edge, dark cavity and bright knot). These
types of events have led to the proposal of the ‘`standard model’'
of the development of a CME, a model which predicts the formation
of current sheets. A few recent coronal observations provide some
evidence for such sheets. Other more complex events correspond to
multiple eruptions taking place on a time scale much shorter than the
cadence of coronagraph instruments. They are often associated with
large-scale dimming and coronal waves. The exact nature of these waves
and the physical link between these different manifestations are not
yet elucidated. We also discuss what kind of shocks are produced during
a flare or a CME. Several questions remain unanswered. What is the
nature of the shocks in the corona (blast-wave or piston-driven?) How
they are related to Moreton waves seen in Hα? How they are related
to interplanetary shocks? The last section discusses the origin of
energetic electrons detected in the corona and in the interplanetary
medium. “Complex type III-like events,”which are detected at
hectometric wavelengths, high in the corona, and are associated with
CMEs, appear to originate from electrons that have been accelerated
lower in the corona and not at the bow shock of CMEs. Similarly,
impulsive energetic electrons observed in the interplanetary medium
are not the exclusive result of electron acceleration at the bow shocks
of CMEs; rather they have a coronal origin.
---------------------------------------------------------
Title: Multi-Wavelength Observations of CMEs and Associated Phenomena
Authors: Pick, M.; Forbes, T. G.; Mann, G.; Cane, H. V.; Chen, J.;
Ciaravella, A.; Cremades, H.; Howard, R. A.; Hudson, H. S.; Klassen,
A.; Klein, K. L.; Lee, M. A.; Linker, J. A.; Maia, D.; Mikic,
Z.; Raymond, J. C.; Reiner, M. J.; Simnett, G. M.; Srivastava, N.;
Tripathi, D.; Vainio, R.; Vourlidas, A.; Zhang, J.; Zurbuchen, T. H.;
Sheeley, N. R.; Marqué, C.
2006cme..book..341P Altcode:
This chapter reviews how our knowledge of CMEs and CME-associated
phenomena has been improved, since the launch of the SOHO mission,
thanks to multi-wavelength analysis. The combination of data obtained
from space-based experiments and ground based instruments allows us
to follow the space-time development of an event from the bottom of
the corona to large distances in the interplanetary medium. Since
CMEs originate in the low solar corona, understanding the physical
processes that generate them is strongly dependant on coordinated
multi-wavelength observations. CMEs display a large diversity in
morphology and kinematic properties, but there is presently no
statistical evidence that those properties may serve to group them
into different classes. When a CME takes place, the coronal magnetic
field undergoes restructuring. Much of the current research is focused
on understanding how the corona sustains the stresses that allow the
magnetic energy to build up and how, later on, this magnetic energy is
released during eruptive flares and CMEs. Multiwavelength observations
have confirmed that reconnection plays a key role during the development
of CMEs. Frequently, CMEs display a rather simple shape, exhibiting a
well known three-part structure (bright leading edge, dark cavity and
bright knot). These types of events have led to the proposal of the
"standard model" of the development of a CME, a model which predicts
the formation current sheets. A few recent coronal observations provide
some evidence for such sheets. Other more complex events correspond
to multiple eruptions taking place on a time scale much shorter than
the cadence of coronagraph instruments. They are often associated with
large-scale dimming and coronal waves. The exact nature of these waves
and the physical link between these different manifestations are not
yet elucidated. We also discuss what kind of shocks are produced during
a flare or a CME. Several questions remain unanswered. What is the
nature of the shocks in the corona (blast-wave or piston-driven?) How
they are related to Moreton waves seen in Hα? How they are related
to interplanetary shocks? The last section discusses the origin of
energetic electrons detected in the corona and in the interplanetary
medium. "Complex type III-like events," which are detected at
hectometric wavelengths, high in the corona, and are associated with
CMEs, appear to originate from electrons that have been accelerated
lower in the corona and not at the bow shock of CMEs. Similarly,
impulsive energetic electrons observed in the interplanetary medium
are not the exclusive result of electron acceleration at the bow shocks
of CMEs; rather they have a coronal origin.
---------------------------------------------------------
Title: Surface Evolution of the Sun's Magnetic Field: A Historical
Review of the Flux-Transport Mechanism
Authors: Sheeley, Neil R., Jr.
2005LRSP....2....5S Altcode:
This paper reviews our attempts to understand the transport of
magnetic flux on the Sun from the Babcock and Leighton models to
the recent revisions that are being used to simulate the field over
many sunspot cycles. In these models, the flux originates in sunspot
groups and spreads outward on the surface via supergranular diffusion;
the expanding patterns become sheared by differential rotation, and
the remnants are carried poleward by meridional flow. The net result
of all of the flux eruptions during a sunspot cycle is to replace the
initial polar fields with new fields of opposite polarity. A central
issue in this process is the role of meridional flow, whose relatively
low speed is near the limit of detection with Doppler techniques. A
compelling feature of Leighton's original model was that it reversed the
polar fields without the need for meridional flow. Now, we think that
meridional flow is central to the reversal and to the dynamo itself.
---------------------------------------------------------
Title: The Transport of Photospheric Magnetic Flux (Invited)
Authors: Sheeley, N. R., Jr.
2005ESASP.592..233S Altcode: 2005soho...16E..35S; 2005ESASP.592E..35S
No abstract at ADS
---------------------------------------------------------
Title: Modeling the Sun's Magnetic Field and Irradiance since 1713
Authors: Wang, Y. -M.; Lean, J. L.; Sheeley, N. R., Jr.
2005ApJ...625..522W Altcode:
We use a flux transport model to simulate the evolution of the
Sun's total and open magnetic flux over the last 26 solar cycles
(1713-1996). Polar field reversals are maintained by varying the
meridional flow speed between 11 and 20 m s<SUP>-1</SUP>, with the
poleward-directed surface flow being slower during low-amplitude
cycles. If the strengths of the active regions are fixed but their
numbers are taken to be proportional to the cycle amplitude, the
open flux is found to scale approximately as the square root of the
cycle amplitude. However, the scaling becomes linear if the number of
active regions per cycle is fixed but their average strength is taken
to be proportional to the cycle amplitude. Even with the inclusion
of a secularly varying ephemeral region background, the increase in
the total photospheric flux between the Maunder minimum and the end of
solar cycle 21 is at most ~one-third of its minimum-to-maximum variation
during the latter cycle. The simulations are compared with geomagnetic
activity and cosmogenic isotope records and are used to derive a
new reconstruction of total solar irradiance (TSI). The increase in
cycle-averaged TSI since the Maunder minimum is estimated to be ~1 W
m<SUP>-2</SUP>. Because the diffusive decay rate accelerates as the
average spacing between active regions decreases, the photospheric
magnetic flux and facular brightness grow more slowly than the sunspot
number and TSI saturates during the highest amplitude cycles.
---------------------------------------------------------
Title: Solar Subsurface Flows of Active Region AR~0696
Authors: Komm, R.; Howe, R.; Donaldson Hanna, K.; Hill, F.; Sheeley, N.
2005AGUSMSP24A..02K Altcode:
We use a ring-diagram analysis to determine the subsurface flows in
the upper 16~Mm of the convection zone from high-resolution Global
Oscillation Network Group (GONG) data obtained during the first two
weeks of November 2004. The active region AR~0696 emerges near the
eastern limb and moves across the disk during this time period. The
region produced several terrestrially effective flares and halo
CMEs during its transit across the disk. During its disk passage,
AR~0696 is the only large active region in the northern hemisphere
and almost the only flare producing region on the sun. This makes it a
good candidate for investigating the relation between active regions,
their flare activity, and associated subsurface flows. We will present
the latest results. This work was supported by NASA grant NAG 5-11703.
---------------------------------------------------------
Title: Global Structure of the Out-of-ecliptic Solar Wind
Authors: Whang, Y.; Wang, Y.; Sheeley, N.; Burlaga, L.
2005AGUSMSH13A..03W Altcode:
We use the observed photospheric field maps and the wind speed
observed from Ulysses to study the out-of-ecliptic solar wind. The
model calculates the wind speed from the rate of magnetic flux-tube
expansion factor using a conversion function that is determined by
least-squares fit of all currently available data from Ulysses. Using
the best-fit conversion function we investigate the global solar
wind at all latitudes, from 90º south to 90º north, covering a
36-year period from 1968 through 2003. The results complement and expand
upon earlier studies conducted with IPS and other in situ spacecraft
observations. The rotationally averaged wind speed is a function of
two parameters: the heliolatitude and the phase of the solar cycle. The
out-of-ecliptic solar wind has a recurrent stable structure, the average
wind speed varies like a sine square of latitude profile spanning more
than 5 years during the declining phase and solar minimum in each solar
cycle. Near solar maximum the structure of the out-of-ecliptic solar
wind is in a transient state lasting 2 to 3 years when the stable
structure breaks down during the disappearance and reappearance of
the polar coronal holes. We also report the Ulysses observations of
the stable and transient structure.
---------------------------------------------------------
Title: Global structure of the out-of-ecliptic solar wind
Authors: Whang, Y. C.; Wang, Y. -M.; Sheeley, N. R.; Burlaga, L. F.
2005JGRA..110.3103W Altcode: 2005JGRA..11003103W
We use the observed photospheric field maps and the wind speed observed
from Ulysses to study the out-of-ecliptic solar wind. The model
calculates the wind speed from the rate of magnetic flux tube expansion
factors using a conversion function that is determined by least squares
fit of all currently available data from Ulysses. Using the best fit
conversion function, we investigate the global solar wind covering
a 36-year period from 1968 through 2003. The results complement and
expand upon earlier studies conducted with interplanetary scintillation
and other in situ spacecraft observations. The rotationally averaged
wind speed is a function of two parameters: the heliolatitude and the
phase of the solar cycle. The out-of-ecliptic solar wind has a recurrent
stable structure, and the average wind speed varies like a sine square
of latitude profile spanning more than 5 years during the declining
phase and solar minimum in each solar cycle. Ulysses has observed
this stable structure in its first polar orbit in 1992-1997. Near
solar maximum the structure of the out-of-ecliptic solar wind is in a
transient state lasting 2-3 years when the stable structure breaks down
during the disappearance and reappearance of the polar coronal holes.
---------------------------------------------------------
Title: The Origin of Postflare Loops
Authors: Sheeley, N. R., Jr.; Warren, H. P.; Wang, Y. -M.
2004ApJ...616.1224S Altcode:
We apply a tracking technique, previously developed to study motions
in the outer corona by Sheeley, Walters, Wang, and Howard, to 195 Å
filtergrams obtained with the Transition Region and Coronal Explorer
(TRACE) satellite and obtain height-time maps of the motions in the
hot (10-20 MK) plasma clouds above postflare loop systems. These
maps indicate the following two main characteristics. (1) Within the
plasma cloud, the motions are downward at speeds of approximately 4
km s<SUP>-1</SUP>. The cloud itself grows with time, its upper layers
being replenished by the arrival and deceleration of fast inflows and
its lower layers disappearing when they cool to form the tops of new
postflare loops. (2) Early in these events, the inward motions are
turbulent, showing a variety of dark elongated features resembling
“tadpoles” and some bright features. Later, the inflows are visible
as dark collapsing loops, changing from initially cusp-shaped features
to rounder loops as they move inward. Their speeds initially lie in the
range 100-600 km s<SUP>-1</SUP> but decrease to 4 km s<SUP>-1</SUP>
in about 3 minutes, corresponding to an average deceleration ~1500 m
s<SUP>-2</SUP>. Combining these observations with similar observations
obtained at reconnection sites in the outer corona by the Large Angle
Spectrometric Coronagraph (LASCO), we conclude that postflare loops are
the end result of the formation, filling, deceleration, and cooling
of magnetic loops produced by the reconnection of field lines blown
open in the flare. The formation of collapsing loops occurs in the
dark tadpoles; the filling of these initially dark loops occurs via
chromospheric evaporation, which also contributes to the deceleration
of the loops; and the radiative cooling ultimately resolves the loops
into sharply defined structures.
---------------------------------------------------------
Title: Footpoint Switching and the Evolution of Coronal Holes
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
2004ApJ...612.1196W Altcode:
We discuss the role of footpoint exchanges between open and closed
magnetic field lines (also known as “interchange reconnection”)
in the formation and rotational evolution of coronal holes. Such
exchanges cause open flux to jump from one location to another when
active regions emerge; they also act to untie the rotation of coronal
holes from that of the underlying plasma. We introduce a quantitative
measure of the footpoint exchange rate and apply it to a variety of
idealized configurations. During the formation of coronal holes,
footpoint switching dominates over the creation of new open flux
if the background (or polar) field is strong compared to that of
the emerging active region, so the latter acts to change mainly the
direction rather than the magnitude of the Sun's dipole vector. The
principal role of footpoint exchanges is to counteract the subsequent
rotational shearing of the holes; this result is accomplished by means
of continual sideways displacements of open and closed field lines along
the hole boundaries. Because the timescale for rotational shearing
(~3 months) is less than that for the decay of the Sun's large-scale
nonaxisymmetric field (~1 yr), interchange reconnection is expected
on average to dominate over the closing down of flux throughout the
solar cycle.
---------------------------------------------------------
Title: The termination shock near 35° latitude
Authors: Whang, Y. C.; Burlaga, L. F.; Wang, Y. -M.; Sheeley, N. R.
2004GeoRL..31.3805W Altcode: 2004GeoRL..3103805W
The termination shock moves outwards and inwards over timescales of a
solar cycle in response to the variations in the average solar wind
speed. The amplitude is greater than 50 AU near 35° latitude; the
maximum (minimum) distance occurs during the rising (declining) phase
of the solar cycle. Shock parameters are distinctly different when
the shock moves outwards or inwards. During the period of high-speed
(low-speed) solar wind, the shock moves outward (inward) and the shock
is weaker (stronger). This study assumes that the first crossing of
Voyager 1 with the termination shock occurred at 85.5 AU on 2002.6. If
Voyager 1 did cross the shock in 2002.6, the spacecraft would likely
cross the shock at least two more times before 2010, but no second
crossing would occur close to 2003.1. If Voyager 1 did not cross the
shock in mid-2002, it might still do so before 2005.
---------------------------------------------------------
Title: The termination shock near 35 degrees latitude
Authors: Whang, Y. C.; Burlaga, L. F.; Wang, Y.; Sheeley, N. R.
2003AGUFMSH11C1115W Altcode:
The paper calculates the solar cycle variation of the termination
shock near 35 degrees latitude. The solution assumes that Voyager 1
crossed the termination shock at 85.5 AU on 2002.6. At mid latitudes,
the termination shock is an oblique shock; the shock location is
unambiguously solar cycle dependent with amplitude greater than 50
AU. The maximum (minimum) distance occurs during the rising (declining)
phase of the solar cycle. During the period of high-speed (low-speed)
solar wind, the termination shock moves outward (inward) and the
shock is weaker (stronger). The calculation provides the shock speed,
the shock strength, the preshock and postshock solar wind speed, and
the flow speed in the shock frame of reference. The shock parameters
are distinctly different depending on whether the shock moves outward
or inward. If Voyager 1 did cross the termination shock in 2002.6, it
would likely cross the shock coming back to the solar wind near 98 AU
in 2006 during the next outward motion of the termination shock, while
in 2010 the spacecraft would cross the termination shock back into the
heliosheath near 113 AU during the inward motion of the termination
shock near the maximum of Cycle 24. If Voyager 1 did not cross the
termination shock in mid-2002, it might still do so before 2005.
---------------------------------------------------------
Title: Relating Interplanetary Helium Variation to Coronal Magnetic
Fields and Solar Wind Formation
Authors: Kasper, J. C.; Lazarus, A. J.; Steinberg, J. T.; Riley, P.;
Sheeley, N.; Wang, Y.
2003AGUFMSH11D1136K Altcode:
We have recently identified two new features in the variation of the
relative abundance of helium in the solar wind relative to hydrogen
as observed by the Wind spacecraft. First, the He/H ratio is a
linear function of speed for solar wind speeds ranging from 250-550
km/s. This signal is most evident during the previous solar minimum
from 1995 through 1997. Secondly, during that same quiet interval we
have identified a six-month periodic modulation of the He/H ratio which
can be seen at all solar wind speeds. This modulation is in phase with
the orbit of the Earth about the Sun, and maxima in He/H occur at the
times when Earth is most distant from the heliographic equator. As
we shall demonstrate, these effects are partly in agreement with
theoretical models of the magnetic topology of the corona and of the
formation of the solar wind. However, these features present challenges
to the currently accepted paradigm of solar wind, with helium-poor wind
emerging from coronal streamers and fast, helium-rich wind emerging from
coronal holes. The abundance of helium is a sensitive test of models
of solar wind formation and of the connection between photospheric,
coronal, and interplanetary magnetic fields. The observed variations of
He/H will be discussed in terms of existing models of coronal magnetic
field topology and solar wind acceleration. This research is supported
by the NSF/SHINE grant ATM-0327723.
---------------------------------------------------------
Title: On the Topological Evolution of the Coronal Magnetic Field
During the Solar Cycle
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
2003ApJ...599.1404W Altcode:
Using an axisymmetric model that includes the effects of flux
emergence and surface transport processes, we calculate the evolution
of the photospheric magnetic field over the solar cycle and derive
a corresponding sequence of coronal configurations by means of a
potential-field source-surface extrapolation. By identifying magnetic
neutral points and tracking changes in the total flux within each
topological domain, we construct an overall picture of how open
and closed flux is transported as the coronal field reverses its
polarity:1. During the rising phase of the cycle, an X-point forms
above the emerging flux (represented by a bipole structure) in each
hemisphere, and the overlying, opposite-polarity field lines are
“stripped away” (reconnected to each side); at the same time, as
the Sun's axial dipole strength decreases, open field lines from the
polar coronal holes begin to merge at the equator and close down.2. As
the rate of flux emergence peaks, the X-point rises toward the source
surface and the bipole opens up, forming a trailing-polarity hole
on its poleward side, which evolves into the new-cycle polar hole;
the leading-polarity open flux on the equatorward side of the bipole
progressively closes down by merging with its opposite-hemisphere
counterpart.3. Later in the declining phase of the cycle, the
opposite-hemisphere bipoles begin to reconnect with each other at an
equatorial X-point, producing long trailing-polarity loops that rise
toward the source surface and continue to feed flux into the new-cycle
polar holes, and short leading-polarity loops that collapse toward the
photosphere and eventually submerge.We compare the case in which the
transport of the photospheric field is by supergranular diffusion alone
with that in which both diffusion and a 20 m s<SUP>-1</SUP> poleward
flow are present; the latter model is shown to reproduce more closely
the coronal topologies inferred from the observed photospheric field.
---------------------------------------------------------
Title: Voyager 1 Studies of the HMF to 81 AU During the Ascending
Phase of Solar Cycle 23
Authors: Burlaga, L. F.; Ness, N. F.; Wang, Y. -M.; Sheeley, N. R.
2003AIPC..679...39B Altcode:
The paper analyzes the magnetic field strength B and polarity
observed in the distant heliosphere from 1996 to early 2001 and will be
discussed in relation to the variation of B from 1978 through 1996. The
observations extend the results of Burlaga et al. [1]. The polarity of
the heliospheric magnetic field (HMF) from 1997 to early 2001 is studied
in relation to the extrapolated position of the heliospheric current
sheet (HCS). These observations of polarity extend the earlier results
of Burlaga et al. [2] and Burlaga and Ness [3]. The V1 observations
of the heliospheric magnetic field strength B agree with Parker's
model of the global heliospheric magnetic field from 1 to 81 AU and
from 1978 to 2001, when one considers the solar cycle variations in
the source magnetic field strength and the latitude/time variation in
the solar wind speed. Parker's model, without adjustable parameters,
describe the general tendency for B to decrease with increasing distance
R from the Sun, and the solar cycle time variations causing the three
broad increases of B around 1980, 1990, and 2000, and the minima of B
in 1987 and 1997. The variation of magnetic polarity observed by V1
and V2 was caused by the increasing latitudinal width of the sector
zone with increasing solar activity, which in turn was related to
the increasing maximum latitudinal extent and the decreasing minimum
latitudinal extent of the footpoints of the HCS.
---------------------------------------------------------
Title: Linear Polarization Measurements of Chromospheric Emission
Lines
Authors: Sheeley, N. R., Jr.; Keller, C. U.
2003ApJ...594.1085S Altcode:
We have used the Zurich Imaging Stokes Polarimeter (ZIMPOL I)
with the McMath-Pierce 1.5 m main telescope on Kitt Peak to obtain
linear polarization measurements of the off-limb chromosphere with a
sensitivity better than 1×10<SUP>-5</SUP>. We found that the off-disk
observations require a combination of good seeing (to show the emission
lines) and a clean heliostat (to avoid contamination by scattered light
from the Sun's disk). When these conditions were met, we obtained the
following principal results:1. Sometimes self-reversed emission lines
of neutral and singly ionized metals showed linear polarization caused
by the transverse Zeeman effect or by instrumental cross talk from the
longitudinal Zeeman effect in chromospheric magnetic fields. Otherwise,
these lines tended to depolarize the scattered continuum radiation
by amounts that ranged up to 0.2%.2. Lines previously known to show
scattering polarization just inside the limb (such as the Na I λ5889 D2
and the He I λ5876 D3 lines) showed even more polarization above the
Sun's limb, with values approaching 0.7%.3. The O I triplet at λ7772,
λ7774, and λ7775 showed a range of polarizations. The λ7775 line,
whose maximum intrinsic polarizability, P<SUB>max</SUB>, is less than
1%, revealed mainly Zeeman contributions from chromospheric magnetic
fields. However, the more sensitive λ7772 (P<SUB>max</SUB>=19%) and
λ7774 (P<SUB>max</SUB>=29%) lines had relatively strong scattering
polarizations of approximately 0.3% in addition to their Zeeman
polarizations. At times of good seeing, the polarization spectra
resolve into fine structures that seem to be chromospheric spicules.
---------------------------------------------------------
Title: Modeling the Sun's Large-Scale Magnetic Field during the
Maunder Minimum
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
2003ApJ...591.1248W Altcode:
We use a flux transport model to simulate the evolution of the Sun's
magnetic dipole moment, polar fields, and open flux under Maunder
minimum conditions. Even when the rate of active region emergence
is taken to be a factor of ~30 smaller than in recent solar cycles,
regular polarity oscillations of the axial dipole and polar fields
can be maintained if the speed of the poleward surface flow is reduced
from ~20 to ~10 m s<SUP>-1</SUP> and the source flux emerges at very
low latitudes (~10°). The axial dipole is then found to have an
amplitude of the order of 0.5 G, as compared with ~4 G during solar
cycle 21. The strength of the radial interplanetary field component
at Earth is estimated to be in the range ~0.3-0.7 nT, about a factor
of 7 lower than contemporary values. We discuss the implications of
these weak fields for our understanding of geomagnetic activity and
cosmic-ray modulation during the Maunder minimum.
---------------------------------------------------------
Title: On the Fluctuating Component of the Sun's Large-Scale
Magnetic Field
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
2003ApJ...590.1111W Altcode:
The Sun's large-scale magnetic field and its proxies are known to
undergo substantial variations on timescales much less than a solar
cycle but longer than a rotation period. Examples of such variations
include the double activity maximum inferred by Gnevyshev, the large
peaks in the interplanetary field strength observed in 1982 and 1991,
and the 1.3-1.4 yr periodicities detected over limited time intervals
in solar wind speed and geomagnetic activity. We consider the question
of the extent to which these variations are stochastic in nature. For
this purpose, we simulate the evolution of the Sun's equatorial dipole
strength and total open flux under the assumption that the active
region sources (BMRs) are distributed randomly in longitude. The
results are then interpreted with the help of a simple random walk
model including dissipation. We find that the equatorial dipole and open
flux generally exhibit multiple peaks during each 11 yr cycle, with the
highest peak as likely to occur during the declining phase as at sunspot
maximum. The widths of the peaks are determined by the timescale τ~1
yr for the equatorial dipole to decay through the combined action of
meridional flow, differential rotation, and supergranular diffusion. The
amplitudes of the fluctuations depend on the strengths and longitudinal
phase relations of the BMRs, as well as on the relative rates of flux
emergence and decay. We conclude that stochastic processes provide a
viable explanation for the “Gnevyshev gaps” and for the existence
of quasi periodicities in the range ~1-3 yr.
---------------------------------------------------------
Title: Solar Wind Speed and Temperature Outside 10 AU and the
Termination Shock
Authors: Whang, Y. C.; Burlaga, L. F.; Wang, Y. -M.; Sheeley,
N. R., Jr.
2003ApJ...589..635W Altcode:
In this paper we first present a series of pickup proton solar wind
solutions following the fluid motion in the upwind direction to show
that the wind speed V and temperature T, at a given r outside 10 AU,
are primarily functions of the 1 AU wind speed V<SUB>0</SUB>. This
relationship is attributed to the accumulated effects of the
pickup proton process on the heating and deceleration of the solar
wind. Because pickup protons are expected to have similar effects on
the solar wind at all latitudes in the upwind side of the heliosphere,
in the second part of the paper, the two formulae V(r, V<SUB>0</SUB>)
and T(r, V<SUB>0</SUB>) are extended to study the termination shock at
35° latitude. Wang and Sheeley have an empirical model for calculating
the 1 AU wind speed V<SUB>0</SUB> from the observed photospheric
field. We use the simulated wind speed V<SUB>0</SUB> to calculate V
and T outside 60 AU following the fluid motion; then we can study the
solar cycle variation of the termination shock. The shock location near
35° is unambiguously dependent on the solar cycle, with a period of
approximately 1 solar cycle; the amplitude for variation of the shock
location is greater than 50 AU. The new result supports the idea that
the first encounter of Voyager 1 with the termination shock may occur
during the declining phase of cycle 23. After the first encounter, the
spacecraft will cross the shock two more times over a period of 8 years.
---------------------------------------------------------
Title: The Solar Wind and Its Magnetic Sources at Sunspot Maximum
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
2003ApJ...587..818W Altcode:
We use in situ measurements from the Advanced Composition
Explorer and magnetograph data from the National Solar
Observatory to relate the properties of the solar wind during
1998-2002 to its source region magnetic fields. The great bulk
of the solar maximum wind is characterized by low proton speeds
(v<SUB>p</SUB>~420 km s<SUP>-1</SUP>) and high oxygen charge state
ratios (n<SUB>O<SUP>7+</SUP></SUB>/n<SUB>O<SUP>6+</SUP></SUB>~0.3). This
slow wind originates from small, sheared open-field regions located
near active regions and characterized by very large flux tube expansion
factors (f<SUB>exp</SUB>>>10) and high footpoint field strengths
(B<SUB>0</SUB>~30 G). In contrast, the occasional high-speed streams
emanate from weak-field regions (B<SUB>0</SUB>~5 G) with small expansion
factors (f<SUB>exp</SUB>~4) and show relatively low charge state
ratios (n<SUB>O<SUP>7+</SUP></SUB>/n<SUB>O<SUP>6+</SUP></SUB>~0.1)
their proton velocities (v<SUB>p</SUB>~550 km s<SUP>-1</SUP>) are
substantially reduced by interactions with the surrounding sea of
low-speed wind. We attribute the high freeze-in temperatures of the
slow wind to enhanced heating taking place in the low corona in the
presence of the very strong, rapidly diverging source fields, which
are found to be correlated with high mass and energy flux densities
at the coronal base.
---------------------------------------------------------
Title: Modeling the Sun's polar fields and open flux during the
Maunder Minimum
Authors: Wang, Y.; Lean, J.; Sheeley, N.
2003EAEJA.....2424W Altcode:
We use a flux transport model to simulate the evolution of the
Sun's polar fields and open magnetic flux under Maunder Minimum
conditions. Even when the rate of active region emergence is taken
to be a factor of 50--100 smaller than in recent cycles, we find that
11 yr polarity oscillations of the polar fields can be maintained if
the speed of the poleward surface flow is reduced from 20 m/s to 10
m/s and the magnetic flux is assumed to emerge at low latitudes. The
strength of the radial IMF component is inferred to be in the range
∼0.2--0.6 nT, about a factor of 10 weaker than at present but with a
stronger solar-cycle modulation. We discuss the relationship between
our simulation results and what is known observationally about the
Maunder Minimum.
---------------------------------------------------------
Title: Meridional Flow and the Solar Cycle Variation of the Sun's
Open Magnetic Flux
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Lean, J.
2002ApJ...580.1188W Altcode:
We simulate the evolution of the Sun's large-scale magnetic field
during solar cycle 21, including the effect of surface transport
processes and active region emergence. As an important new constraint
on the model, we have scaled our source fluxes upward to be consistent
with the average measured strength of the interplanetary magnetic
field (IMF). By adopting a poleward bulk flow of amplitude ~20-25 m
s<SUP>-1</SUP> together with a supergranular diffusion rate of ~500
km<SUP>2</SUP> s<SUP>-1</SUP>, we are then able to match the observed
variation of the Sun's polar fields and open magnetic flux. The high
meridional flow speeds, peaking at low latitudes, prevent the buildup
of an overly strong axisymmetric dipole component at sunspot minimum,
while accounting for the giant poleward surges of flux and accompanying
polar field fluctuations observed near sunspot maximum. The present
simulations also reproduce the large peak in the equatorial dipole
and IMF strength recorded in 1982.
---------------------------------------------------------
Title: Polarity reversal of the solar magnetic field during cycle 23
Authors: Wang, Y. -M.; Sheeley, N. R.; Andrews, M. D.
2002JGRA..107.1465W Altcode:
Using magnetograph data, coronagraph observations, and source surface
extrapolations, we examine the evolution of the photospheric magnetic
field from 1996 through the 2000-2001 polarity reversal and show how
this evolution is reflected in coronal holes, coronal streamers, the
heliospheric current sheet (HCS), and the solar wind. The photospheric
polarity reversal is completed in the more active Northern Hemisphere
in late 2000 and then in the Southern Hemisphere in 2001. The polar
coronal holes disappear in 2000 and start to re-form in 2001; during
this interval, most of the open magnetic flux resides in the active
region latitudes, where small coronal holes with strong footpoint
fields generate predominantly slow solar wind. The nondipolar nature
of the large-scale coronal field at sunspot maximum gives rise to
complex streamer/HCS topologies, in which a four-sector structure and
even a secondary, detached current sheet with cylindrical geometry are
sometimes present. Comparison of the potential field extrapolations with
coronagraph and Ulysses observations suggests that the magnetograph
measurements may have underestimated the strength of the south polar
field during late 2000.
---------------------------------------------------------
Title: The effect of increasing solar activity on the Sun's total
and open magnetic flux during multiple cycles: Implications for
solar forcing of climate
Authors: Lean, J. L.; Wang, Y. -M.; Sheeley, N. R.
2002GeoRL..29.2224L Altcode: 2002GeoRL..29x..77L
We investigate the relationship between solar irradiance and cosmogenic
isotope variations by simulating with a flux transport model the effect
of solar activity on the Sun's total and open magnetic flux. As the
total amount of magnetic flux deposited in successive cycles increases,
the polar fields build up, producing a secular increase in the open
flux that controls the interplanetary magnetic field which modulates
the cosmic ray flux that produces cosmogenic isotopes. Non-axisymmetric
fields at lower latitudes decay on time scales of less than a year; as
a result the total magnetic flux at the solar surface, which controls
the Sun's irradiance, lacks an upward trend during cycle minima. This
suggests that secular increases in cosmogenic and geomagnetic proxies
of solar activity may not necessarily imply equivalent secular trends
in solar irradiance. Questions therefore arise about the interpretation
of Sun-climate relationships, which typically assume that the proxies
imply radiative forcing.
---------------------------------------------------------
Title: Solar Wind Speed and Temperature Outside 10 AU and the
Termination Shock
Authors: Whang, Y. C.; Burlaga, L. F.; Wang, Y.; Sheeley, N. R.
2002AGUFMSH11A0376W Altcode:
We study the speed V and temperature T of the solar wind in the
distant heliosphere obtained from pickup proton magnetohydrodynamic
equations that treat interstellar pickup protons and solar wind protons
as two distinct proton species. The study uses the 27-day average Omni
plasma and magnetic field data as input. We obtain 336 sets of numerical
solutions covering the period from the solar minimum of Cycle 20 in 1976
to the maximum of Cycle 23; each represents the extrapolation for the
real solar wind conditions at 1 AU to the distant heliosphere following
the fluid motion. The result shows that outside 10 AU near the ecliptic
V and T at a given r are primarily functions of the 1 AU wind speed. The
maximum of the 1 AU wind speed is 615 km/s occurred on March 1989, the
minimum at 327 km/s occurred on February 1999. A straight line can be
used to represent the relation between V and the 1 AU speed for each
given r; and a parabola can represent the relation for T at each given
r. These relations are attributed to the accumulated effects of pickup
proton process in the distant heliosphere. Because pickup protons are
expected to have similar effects on the solar wind at all latitudes
on the upwind side of the heliosphere, the relationships for V and T
are extended to study the solar cycle and latitudinal variation for
the heliocentric distance of the termination shock. Wang and Sheeley
have an empirical model to calculate the 1 AU wind speed as a function
of latitude and longitude from the observed photospheric field. The
calculated latitudinal variation is consistent with the observational
results from Ulysses. We use the simulated 1 AU speed to calculate V
and T at varying r following the fluid motion. Then we can calculate
the solar cycle variation of the shock distance at all latitudes over
a 26-year's period. The averaged distance increases with the latitude;
from ecliptic to the pole the distance increases by a factor of 2.
---------------------------------------------------------
Title: Characteristics of Coronal Inflows
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
2002ApJ...579..874S Altcode:
This paper describes coronal inflows observed with the Large Angle
Spectrometric Coronagraph (LASCO). The inflows are not seen above
5.5 R<SUB>solar</SUB>, which appears to be a “point of no return”
for the Sun's plasmas and fields. Below this height, most inflows
seem to indicate magnetic flux that is returning to the Sun after its
reconnection at sector boundaries. Some inflows have characteristics
(like fast, oppositely directed ejections of material) that are
easily interpreted in terms of conventional models of field line
reconnection. However, the overwhelming majority of coronal inflows
have a more complex behavior that typically includes the following
characteristics:1. The birth of a very weak, localized density
enhancement about 4-5 R<SUB>solar</SUB> from Sun center and its
initially slow downward motion along a coronal ray;2. Acceleration
to a maximum speed of 50-100 km s<SUP>-1</SUP>, and the formation
of a sinking column;3. Deceleration and the appearance of a dark
depletion tail, visible against the bright background structures in
the lower corona;4. The formation of a stretched loop below about
2.5 R<SUB>solar</SUB>.We suppose that the initial downward motion is
a preparatory stage for reconnection, which occurs in the depleted
region in the wake of the sinking column and is later revealed by the
formation of a stretched loop in the lower corona.
---------------------------------------------------------
Title: Heliospheric magnetic field strength and polarity from 1 to
81 AU during the ascending phase of solar cycle 23
Authors: Burlaga, L. F.; Ness, N. F.; Wang, Y. -M.; Sheeley, N. R.
2002JGRA..107.1410B Altcode:
The Voyager 1 (V1) observations of the heliospheric magnetic field
strength B agree with Parker's model of the global heliospheric magnetic
field from 1 to 81.0 AU and from 1978 to 2001.34 when one considers
the solar cycle variations in the source magnetic field strength and
the latitude/time variation in the solar wind speed. In particular,
Parker's model, without adjustable parameters, describes the general
tendency for B to decrease with increasing distance R from the Sun, the
three broad increases of B around 1980, 1990, and 2000, and the minima
of B in 1987 and 1997. During 1987 and 1997, B appears to be lower than
Parker's model predicts, but that can be attributed to the presence
of a heliospheric vortex street at these times and/or uncertainty
in the observations. There is no evidence for a significant flux
deficit increasing monotonically from 1 to 81.0 AU. By extrapolating
these results and considering the limitations of the observations,
V1 should continue to make useful measurements during the next few
years at least. The magnetic field polarity in the distant heliosphere
at V1 and Voyager 2 (V2) changed during the ascending phase of solar
cycle 23. In the Northern Hemisphere, V1 observed a decrease in the
percentage of positive polarities from ≈100% during 1997 to ≈50%
during 2000. In the Southern Hemisphere, V2 observed the opposite
behavior, an increase in the percentage of positive polarities from
≈0% during 1997 to ≈50% during 2000. The variation of magnetic
polarity observed by V1 and V2 was caused by the increasing latitudinal
width of the sector zone with increasing solar activity, which in
turn was related to the increasing maximum latitudinal extent and
the decreasing minimum latitudinal extent of the footprints of the
heliospheric current sheet (HCS). There was a tendency for the speed
and proton temperature to decrease and the density to increase at V2
from 1997 (when it observed flows from polar coronal holes) to 2001
(when it observed more complex and dynamic flows).
---------------------------------------------------------
Title: Sunspot activity and the long-term variation of the Sun's
open magnetic flux
Authors: Wang, Y. -M.; Sheeley, N. R.
2002JGRA..107.1302W Altcode:
The interplanetary magnetic field (IMF) originates in open magnetic
regions of the Sun (coronal holes), which in turn form mainly through
the emergence and dispersal of active region fields. The radial IMF
strength is proportional to the total open flux Φ<SUB>open</SUB>, which
can be estimated from source surface extrapolations of the measured
photospheric field, after correction for magnetograph saturation
effects. We derive the long-term variation of Φ<SUB>open</SUB> during
1971-2000 and discuss its relation to sunspot activity. The average
value of Φ<SUB>open</SUB> was ∼20-30% higher during 1976-1996 than
during 1971-1976 and 1996-2000, with major peaks occurring in 1982
and 1991. Near sunspot minimum, most of the open flux resides in the
large polar coronal holes, whereas at sunspot maximum it is rooted
in relatively small, low-latitude holes located near active regions
and characterized by strong footpoint fields; since the decrease in
the total area occupied by holes is offset by the increase in their
average field strengths, Φ<SUB>open</SUB> remains roughly constant
between activity minimum and maximum, unlike the total photospheric
flux Φ<SUB>tot</SUB>. The long-term variation of Φ<SUB>open</SUB>
approximately follows that of the Sun's total dipole strength, with a
contribution from the magnetic quadrupole around sunspot maximum. Global
fluctuations in sunspot activity lead to increases in the equatorial
dipole strength and hence to enhancements in Φ<SUB>open</SUB> and
the IMF strength lasting typically ∼1 year. We employ simulations to
clarify the role of active region emergence and photospheric transport
processes in the evolution of the open flux. Representing the initial
field configuration by one or more bipolar magnetic regions (BMRs), we
calculate its subsequent evolution under the influence of differential
rotation, supergranular convection, and a poleward bulk flow. The
initial value of Φ<SUB>open</SUB> is determined largely by the
equatorial dipole strength, which in turn depends on the longitudinal
phase relations between the BMRs. As the surface flow carries the BMR
flux to higher latitudes, the equatorial dipole is annihilated on a
timescale of ∼1 year by the combined effect of rotational shearing and
turbulent diffusion. The remaining flux becomes concentrated around the
poles, and Φ<SUB>open</SUB> approaches a limiting value that depends
on the axisymmetric dipole strengths of the original BMRs. The polar
coronal holes thus represent the long-lived, axisymmetric remnant of
the active regions that emerged earlier in the cycle.
---------------------------------------------------------
Title: A comparison of mean density and microscale density
fluctuations in a CME at 10 R<SUB>solar</SUB>
Authors: Lynch, B. J.; Coles, W. A.; Sheeley, N. R.
2002GeoRL..29.1913L Altcode: 2002GeoRL..29s..19L
We have observed intensity scintillation (IPS) of the radio source 0854
+ 201 at 8 GHz on August 2, 2000 during the passage of a coronal mass
ejection (CME) across the line of sight. The source was at a distance
of 10 R<SUB>solar</SUB> over the north solar pole. Simultaneous
observations with the LASCO C3 instrument allow us to model the
mean density N<SUB>e</SUB> and the microscale density fluctuations
δN<SUB>e</SUB> within the CME. We find that N<SUB>e</SUB> increased
by a factor of 2.18 but δN<SUB>e</SUB> increased by only 1.76, so
the ratio δN<SUB>e</SUB>/N<SUB>e</SUB> is 19% smaller than in the
pre-CME slow wind. During the passage of the CME a short burst of
enhanced turbulence doubled the IPS variance but was not visible in
the C3 images. This was likely caused by a thin flux tube crossing
the line of sight. Detailed modeling indicates that the diameter of
the tube was 41,000 km and its density was 14.5 times the CME density.
---------------------------------------------------------
Title: Role of a Variable Meridional Flow in the Secular Evolution
of the Sun's Polar Fields and Open Flux
Authors: Wang, Y. -M.; Lean, J.; Sheeley, N. R., Jr.
2002ApJ...577L..53W Altcode:
We use a magnetic flux transport model to simulate the evolution of
the Sun's polar fields and open flux during solar cycles 13 through 22
(1888-1997). The flux emergence rates are assumed to scale according to
the observed sunspot-number amplitudes. We find that stable polarity
oscillations can be maintained if the meridional flow rate is allowed
to vary from cycle to cycle, with higher poleward speeds occurring
during the more active cycles. Our model is able to account for a
doubling of the interplanetary field strength since 1900, as deduced
by Lockwood, Stamper, & Wild from the geomagnetic aa index. We
confirm our earlier conclusion that such a doubling of the open flux
does not imply that the base level of the total photospheric flux has
increased significantly over the last century.
---------------------------------------------------------
Title: Coronal White-Light Jets near Sunspot Maximum
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
2002ApJ...575..542W Altcode:
During the 1996-1997 activity minimum, the Large Angle and Spectrometric
Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO)
recorded numerous jetlike ejections above the Sun's polar regions. In
a previous study, we showed that these white-light ejections were
the outward extensions of extreme-ultraviolet (EUV) jets, which in
turn originated from flaring bright points inside the polar coronal
holes. Here we investigate a number of jetlike events observed with
LASCO during the current sunspot maximum. To identify the solar surface
counterparts of these events, we again use Fe XII λ195 images obtained
by the EUV Imaging Telescope on SOHO. The white-light jets in our sample
have angular widths of ~3°-7° and velocities typically of order 600 km
s<SUP>-1</SUP> they tend to be brighter and wider than the polar jets
observed near sunspot minimum and are distributed over a much greater
range of latitudes. Many of the ejections are recurrent in nature and
originate from active regions located inside or near the boundaries
of nonpolar coronal holes. We deduce that the jet-producing regions
consist of systems of closed magnetic loops partially surrounded by
open fields; perturbations in the closed fields caused them to reconnect
with the overlying open flux, releasing the trapped energy in the form
of jetlike ejections. In some events, the core of the active region
erupts, producing fast, collimated ejections with widths of up to ~15°.
---------------------------------------------------------
Title: Solar Energetic Particle Production by Shocks in Fast and
Slow Solar Wind Structures
Authors: Kahler, S. W.; Reames, D. V.; Sheeley, N. R., Jr.
2002AAS...200.3702K Altcode: 2002BAAS...34..694K
Gradual solar energetic particle (SEP) events at 1 AU are produced
by coronal and interplanetary shocks driven by coronal mass ejections
(CMEs). Shocks from fast (V > 900 km/s) CMEs should be produced more
easily in slow solar wind regions where the flow and fast-mode MHD wave
speeds are low and less easily in fast solar wind regions where those
speeds are high. We might therefore expect to observe more intense SEP
events at 1 AU when the Earth lies in a slow wind region than when it
lies in a fast wind region. While stream-stream interactions wash out
the slow-fast stream boundaries in the solar wind speed profiles at
1 AU, the O+7/O+6 signatures of the streams are unchanged at 1 AU. We
use the 20 MeV proton intensities from the EPACT instrument on Wind,
the associated CMEs observed with the Lasco coronagraph on SOHO,
and the ACE SWICS/SWIMS solar wind values of O+7/O+6 to look for
variations of peak SEP intensities as a function of O+7/O+6. We find
no significant dependence of the SEP intensities on O+7/O+6 for either
poorly connected or well connected CME source regions or for different
CME speed ranges. While a broad range of angular widths are associated
with fast (V > 900 km/s) CMEs, we find that no fast CMEs with widths
< 60 degrees are associated with SEP events. On the other hand,
nearly all fast halo CMEs are associated with SEP events. Thus the CME
widths are more important in SEP production than previously thought,
but the solar wind source regions in which SEPs are produced are not
a significant factor.
---------------------------------------------------------
Title: Observations of Core Fallback during Coronal Mass Ejections
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
2002ApJ...567.1211W Altcode:
White-light observations made with the Large Angle and Spectrometric
Coronagraph (LASCO) during the present solar maximum have revealed a
multitude of faint, inward-moving features at heliocentric distances of
r~2-6 R<SUB>solar</SUB>. Most of these structures appear to originate
above r~3 R<SUB>solar</SUB> and may be signatures of the closing-down
of magnetic flux at the boundaries of coronal holes or in the aftermath
of coronal mass ejections (CMEs). Here, we present observations of a
different type of inflow, in which material within the bright core of
a CME collapses back toward the Sun after rising to heights of r~2.5-6
R<SUB>solar</SUB>. We have identified roughly 20 such fallback events
during 1998-2001. The core structures, which have the form of loops or
concave-outward flux ropes, ascend into the coronagraph field of view
beyond 2 R<SUB>solar</SUB> with speeds of ~100-400 km s<SUP>-1</SUP>
but return with speeds of only ~50-200 km s<SUP>-1</SUP>. The initial
deceleration rates of ~20-100 m s<SUP>-2</SUP> are comparable to the
local gravitational deceleration GM<SUB>solar</SUB>/r<SUP>2</SUP>
but continually decrease with time. The associated CMEs tend to be
impulsive but relatively slow, with the leading front moving outward
at ~250-450 km s<SUP>-1</SUP> and often showing some deceleration. It
is thus not surprising that some fraction of the core material fails
to reach escape speeds, remaining bound to the Sun by gravitational
and magnetic tension forces. We suggest that the dynamical behavior
of the core may be determined in part by momentum exchanges with the
background medium, which consists of ongoing outflows of CME material,
ambient solar wind, and inflow streams. In particular, we attribute the
asymmetry of the up-down trajectories to the action of such drag forces,
whose direction changes from inward to outward as the core decelerates.
---------------------------------------------------------
Title: Coronal Mass Ejections Associated with Impulsive Solar
Energetic Particle Events
Authors: Kahler, S. W.; Reames, D. V.; Sheeley, N. R., Jr.
2001ApJ...562..558K Altcode:
An impulsive solar energetic particle (SEP) event observed on the
Wind spacecraft on 2000 May 1 was associated with an impulsive solar
active region M1 X-ray flare. The timing and position of a fast (v=960
km s<SUP>-1</SUP>), narrow CME observed in the LASCO coronagraph on
SOHO make clear the connection between the CME and the flare and SEP
event. Impulsive SEP events have long been associated with impulsive
flares, but only gradual SEP events have thus far been found to be
associated with CMEs. A comparison of impulsive SEP events with CME
observations from the Solwind and LASCO coronagraphs revealed further
good cases of narrow (10°-40°) CMEs associated with impulsive SEP
events. A recent model of impulsive flares includes jets or plasmoids
that are ejected upward from magnetic reconnection sites over active
regions and might therefore be expected to appear in exceptional
cases as faint and narrow CMEs in coronagraphs. We suggest that this
model allows us to understand better SEP production and propagation
in impulsive flares.
---------------------------------------------------------
Title: Coronal Inflows and Sector Magnetism
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
2001ApJ...562L.107S Altcode:
We have remeasured the rate of coronal inflows during 1996-2001,
taking into account multiple occurrences per day, and have displayed the
results in Carrington stack plots to study their long-term behavior. The
stack plots show long-lived recurrence patterns related to the sectorial
component of the Sun's large-scale field. In particular, most inflows
are parts of streams that last for several months and occur where the
sectorial field has strong gradients. The occurrence rate occasionally
approaches ~1 hr<SUP>-1</SUP> near sunspot maximum when the sectorial
field is strong and the streamer belt is greatly distorted from its
flattened equatorial configuration. The link between coronal inflows
and sector magnetism suggests that the inflows are by-products of
a global recycling process in which nonaxisymmetric open flux is
provided by active regions and dissipated by differential rotation,
supergranular diffusion, and meridional flow.
---------------------------------------------------------
Title: Associations of Accelerating CMEs with Solar Energetic
Particle Events
Authors: Kahler, S.; Sheeley, N.; Reames, D.
2001AGUSM..SH22B03K Altcode:
Gradual solar energetic particle (SEP) events are well associated with
fast coronal mass ejections (CMEs). The times of significant E >
10 MeV SEP events observed with the Goddard Space Flight Center EPACT
detector on the Wind spacecraft have been compared with CME observations
from the Lasco coronagraph on the SOHO spacecraft. As earlier studies
have shown, a correlation exists between peak SEP intensities and
the measured speeds of associated CMEs. Of the CMEs associated with
SEP events in the period 1996 to 2000, we find 9 CMEs for which the
height-time plots of the leading edges show accelerations of at least
13 m/s/s. The heights at which those CMEs attained speeds of 600 km/s
ranged from 7 to 20 Ro. The peak 20 MeV intensities of the 9 SEP events
are relatively low compared with all gradual SEP events of the same
period. We compare the energy spectra and solar event associations of
these 9 SEP events with those of the SEP events associated with CMEs
of uniform speeds.
---------------------------------------------------------
Title: Coronal Inflows and the Sun's Nonaxisymmetric Open Flux
Authors: Sheeley, N. R., Jr.; Knudson, T. N.; Wang, Y. -M.
2001ApJ...546L.131S Altcode:
Wang et al. recently described white-light coronagraph observations
of faint coronal features moving inward toward the Sun at heliocentric
distances of 2-6 R<SUB>solar</SUB>. In a study of these inflows during
1996-2000, we have found that they occur along bends of the coronal
streamer belt and are especially common when the magnetic field has
a four-sector structure. The measured inflow rate is dominated by
episodic bursts that are correlated with the occurrence of nonpolar
coronal holes and other indicators of the Sun's nonaxisymmetric open
flux. However, the inflow rate has only a broad long-term correlation
with conventional indicators of solar activity like the sunspot
number and coronal mass ejection rate. We conclude that most inflows
indicate collapsing field lines that occur as nonpolar coronal holes
are subjected to photospheric motions and the eruptions of new flux.
---------------------------------------------------------
Title: The dynamical nature of coronal streamers
Authors: Wang, Y. -M.; Sheeley, N. R.; Socker, D. G.; Howard, R. A.;
Rich, N. B.
2000JGR...10525133W Altcode:
Recent high-sensitivity imaging of the Sun's white-light corona from
space has revealed a variety of unexpected small-scale phenomena,
including plasma blobs that are ejected continually from the
cusplike bases of streamers, fine raylike structures pervading the
outer streamer belt, and inflows that occur mainly during times of
high solar activity. These phenomena can be interpreted as different
manifestations of magnetic field line reconnection, in which plasma
and magnetic flux are exchanged between closed and open field regions
of the corona. The observations provide new insights into a number
of long-standing questions, including the origin of the streamer
material in the outer corona, the sources of the slow solar wind, and
the mechanisms that regulate the interplanetary magnetic field strength.
---------------------------------------------------------
Title: Detection of coronal mass ejection associated shock waves in
the outer corona
Authors: Sheeley, N. R.; Hakala, W. N.; Wang, Y. -M.
2000JGR...105.5081S Altcode:
White light coronal images from the Large-Angle Spectrometric
Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO)
spacecraft show disturbances propagating away from high-speed coronal
mass ejections (CMEs). The disturbances are faintly visible ahead
of the ejected material at the noses of the CMEs but are strongly
visible along the flanks and rear ends, where they produce kinks in
the streamers and other raylike features that extend in all directions
from the Sun at this phase of the sunspot cycle. The kinks decelerate
as they move radially outward along the rays, apparently indicating the
slowing of the entire wave front as it passes by. For a fast CME seen
head on (or tail on) the deceleration occurs at virtually all position
angles around the occulting disk. However, for a CME seen obliquely
the speed varies strongly with position angle, being fast and uniform
near the nose but slower and decelerating at the sides and rear where
the deflected rays are more inclined from the sky plane and farther
from the Sun. The initial speeds (~800-1400 km/s) are faster than the
nominal MHD speed (~600 km/s) at these heights, implying that these
disturbances are shock waves, made visible like “amber waves of grain”
[Bates, 1895] in the field of coronal rays around the Sun.
---------------------------------------------------------
Title: Understanding the evolution of the Sun's open magnetic flux
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Lean, J.
2000GeoRL..27..621W Altcode:
The large-scale magnetic field of the Sun, including the open flux that
extends into the interplanetary medium, originates in active regions
but is redistributed over the photosphere by differential rotation,
supergranular convection, and poleward meridional flow. We use
simulations to clarify the role of the surface transport processes
in the evolution of the total open flux, Φ<SUB>open</SUB>,
which determines the strength of the radial interplanetary field
component. Representing the initial photospheric field configuration
by one or more bipolar magnetic regions (BMRs), we show that
Φ<SUB>open</SUB> varies approximately as the net dipole strength,
determined by vectorially summing the dipole moments of the individual
BMRs. As meridional flow carries the BMR flux to higher latitudes,
the equatorial dipole component is annihilated on a timescale ∼1
yr by the combined effect of rotational shearing and supergranular
diffusion. The remaining flux becomes concentrated around the poles,
and Φ<SUB>open</SUB> approaches a limiting value that depends on
the axisymmetric dipole strength of the original active regions. We
discuss the implications of these results for the solar cycle evolution
of Φ<SUB>open</SUB>.
---------------------------------------------------------
Title: The long-term variation of the Sun's open magnetic flux
Authors: Wang, Y. -M.; Lean, J.; Sheeley, N. R., Jr.
2000GeoRL..27..505W Altcode:
The interplanetary magnetic field (IMF) has its origin in open magnetic
regions of the Sun (coronal holes). The location of these regions
and their total open flux Φ<SUB>open</SUB> can be inferred from
current-free extrapolations of the observed photospheric field. We
derive the long-term variation of Φ<SUB>open</SUB> during 1971-1998
and discuss its causes. Near sunspot minimum, the open flux originates
mainly from the large polar coronal holes, whereas at sunspot maximum
it is rooted in small, lower-latitude holes characterized by very high
field strengths; the total amount of open flux thus remains roughly
constant between sunspot minimum and maximum. Through most of the
cycle, the variation of Φ<SUB>open</SUB> closely follows that of the
Sun's total dipole strength, showing much less dependence on the total
photospheric flux or the sunspot number. However, episodic increases in
large-scale sunspot activity lead to strengthenings of the equatorial
dipole component, and hence to enhancements in Φ<SUB>open</SUB>
and the IMF strength lasting typically ∼1 yr.
---------------------------------------------------------
Title: Evolution of coronal streamer structure during the rising
phase of solar cycle 23
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Rich, N. B.
2000GeoRL..27..149W Altcode:
White-light images of the outer corona recorded with the Large Angle
Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric
Observatory (SOHO) show a rapid widening of the streamer belt during
1998 and early 1999. The observed streamer structure and its evolution
from rotation to rotation are reproduced with a model in which the
Thomson-scattering electrons are concentrated within a narrow layer
centered around the heliospheric current sheet. The latitudinal
spreading of the streamer belt is shown to be a consequence of the
increased rate of magnetic flux emergence in the sunspot latitudes,
which led to a weakening of the Sun’s axisymmetric dipole moment, to
a rapid growth in the nonaxisymmetric components of the coronal field,
and hence to a strong tilting and warping of the plasma/current sheet.
---------------------------------------------------------
Title: Continuous tracking of coronal outflows: Two kinds of coronal
mass ejections
Authors: Sheeley, N. R.; Walters, J. H.; Wang, Y. -M.; Howard, R. A.
1999JGR...10424739S Altcode:
We have developed a new technique for tracking white-light coronal
intensity features and have used this technique to construct continuous
height/time maps of coronal ejecta as they move outward through the
2-30R<SUB>s</SUB> field of view of the Large-Angle Spectrometric
Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO)
spacecraft. Displayed as gray-scale images, these height/time maps
provide continuous histories of the motions along selected radial paths
in the corona and reveal a variety of accelerating and decelerating
features, including two principal types of coronal mass ejections
(CMEs): (1) Gradual CMEs, apparently formed when prominences and their
cavities rise up from below coronal streamers: When seen broadside,
these events acquire balloon-like shapes containing central cores,
and their leading edges accelerate gradually to speeds in the range
400-600 km/s before leaving the 2-30R<SUB>s</SUB> field of view. The
cores fall behind with speeds in the range 300-400 km/s. Seen along
the line of sight, these events appear as smooth halos around the
occulting disk, consistent with head-on views of optically thin bubbles
stretched out from the Sun. At the relatively larger radial distances
seen from this “head-on” perspective, gradually accelerating CMEs
fade out sooner and seem to reach a constant speed more quickly than
when seen broadside. Some suitably directed gradual CMEs are associated
with interplanetary shocks and geomagnetic storms. (2) Impulsive CMEs,
often associated with flares and Moreton waves on the visible disk: When
seen broadside, these CMEs move uniformly across the 2-30R<SUB>s</SUB>
field of view with speeds typically in excess of 750 km/s. At the
relatively larger radial distances seen from a head-on perspective,
impulsive events tend to have a more ragged structure than the gradual
CMEs and show clear evidence of deceleration, sometimes reducing their
speeds from 1000 to 500 km/s in 1 hour. Such decelerations are too large
to represent ballistic motions in the Sun's gravitational field but
might be caused by shock waves, sweeping up material far from the Sun.
---------------------------------------------------------
Title: Using LASCO observations to infer solar wind speed near the sun
Authors: Sheeley, N. R.
1999AIPC..471...41S Altcode: 1999sowi.conf...41S
No abstract at ADS
---------------------------------------------------------
Title: Coronagraph observations of inflows during high solar activity
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Howard, R. A.; Cyr,
O. C. St.; Simnett, G. M.
1999GeoRL..26.1203W Altcode:
Since the start of the SOHO mission three years ago, the Large Angle
Spectrometric Coronagraph (LASCO) has recorded numerous examples of
small, faint features moving inward through the corona. The inflows
are observed at heliocentric distances of 2-4 R<SUB>s</SUB> and became
increasingly common during 1998, as solar and coronal mass ejection
(CME) activity increased. The inward-moving structures, which are most
easily detected in running difference movies, often have a cusplike
appearance and tend to leave a density depletion in their wake; the
downward velocities range from less than 20 km s<SUP>-1</SUP> to over
100 km s<SUP>-1</SUP>. The downflows are observed typically ∼1 day
after the passage of a CME, and coexist side by side with continuing
outflows of streamer material. We interpret these small-scale events
as observational signatures of the gradual closing-down of magnetic
flux dragged outward by CMEs or other transient outflows.
---------------------------------------------------------
Title: Streamer disconnection events observed with the LASCO
coronagraph
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Howard, R. A.; Rich,
N. B.; Lamy, P. L.
1999GeoRL..26.1349W Altcode:
We present Large Angle Spectrometric Coronagraph (LASCO) observations
of two events that suggest magnetic disconnection in coronal
streamers. During the 1-2 days preceding each event, successions of
narrow looptops are seen rising slowly through the 2-6 R<SUB>S</SUB>
field of view, forming a bright streamer stalk which continues to
elongate with time. As the streamer becomes ever more constricted, it
eventually severs at a heliocentric distance of ∼4 R<SUB>S</SUB>. The
lower part of the stalk collapses back to form a cusplike structure
extending to ∼3 R<SUB>S</SUB>, while the disconnected segment is
observed as a kink or density enhancement that propagates outward with
a speed of order 200 km s<SUP>-1</SUP>. We interpret these non-CME
events as transient openings and closings of magnetic flux rooted at
the boundaries of coronal holes.
---------------------------------------------------------
Title: The Increase in Mass of CMEs due to Propagation
Authors: Howard, R. A.; Dere, K. P.; Sheeley, N. R., Jr.; Subramanian,
P.; Vourlidas, A.; Wang, D.
1999AAS...19410102H Altcode: 1999BAAS...31..998H
The question we would like to address is whether coronal mass ejections
snowplow ambient material. Some of the CMEs observed by the LASCO
coronagraph on SOHO have a clearly defined loop-like front, meaning
that the trailing edge of the front can be clearly defined. We measure
the mass in the front of the CME in this subclass. We find that for
some of the events, the mass in the leading edge increases, implying
that the CME is indeed "snowplowing" ambient material. If there is a
significant increase in mass, then the CME frontal speed might decrease
to conserve momentum. We estimate the amount of ambient material,
using a model of coronal densities, and find that it is consistent
with the mass increase. We can also estimate the height in the corona,
below the occulting disk, from where the original material in the CME
is released. These concepts as well as conditions for when the mass
increases will be discussed.
---------------------------------------------------------
Title: Filament Eruptions near Emerging Bipoles
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1999ApJ...510L.157W Altcode:
It has been suggested in previous studies that quiescent prominences
and filaments erupt preferentially in the vicinity of emerging
magnetic flux. We describe three such filament eruptions observed
during 1998 with the Extreme-ultraviolet Imaging Telescope on the
Solar and Heliospheric Observatory. Comparison with magnetograms and
with potential field extrapolations suggests that the new bipole
enables the eruption by diverting the flux overlying the filament
sideways or to greater heights, allowing the filament to rise out of
its channel. Because eruptions sometimes occur in the absence of any
observable flux emergence, however, we conclude that new flux may act
as a strong catalyst but is not a necessary condition for filament
destabilization.
---------------------------------------------------------
Title: Scattering polarization in the chromosphere
Authors: Keller, C. U.; Sheeley, N. R., Jr.
1999ASSL..243...17K Altcode: 1999sopo.conf...17K
No abstract at ADS
---------------------------------------------------------
Title: Observations of Correlated White-Light and Extreme-Ultraviolet
Jets from Polar Coronal Holes
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Socker, D. G.; Howard,
R. A.; Brueckner, G. E.; Michels, D. J.; Moses, D.; St. Cyr, O. C.;
Llebaria, A.; Delaboudinière, J. -P.
1998ApJ...508..899W Altcode:
Time-lapse sequences of white-light images recorded with the Large
Angle Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric
Observatory (SOHO) frequently show long, narrow structures moving
outward over the Sun's polar regions at high apparent speeds. By
comparing the LASCO observations with Fe XII λ195 spectroheliograms
made with the Extreme-ultraviolet Imaging Telescope (EIT) on SOHO
between 1997 April and 1998 February, we have identified 27 correlated
white-light and extreme-ultraviolet (EUV) jet events. In each case,
the EUV jet was observed near the limb of the polar coronal hole 20-60
minutes before the corresponding white-light jet was registered in the
coronagraph's 2-6 R<SUB>⊙</SUB> field of view. The jets originate
near flaring EUV bright points and are presumably triggered by field
line reconnection between magnetic bipoles and neighboring unipolar
flux. The leading edges of the white-light jets propagate outward at
speeds of 400-1100 km s<SUP>-1</SUP>, whereas the bulk of their material
travels at much lower velocities averaging around 250 km s<SUP>-1</SUP>
at heliocentric distances of 2.9-3.7 R<SUB>⊙</SUB>. These lower
velocities may reflect the actual outflow speeds of the background
polar wind.
---------------------------------------------------------
Title: Heliospheric magnetic field strength out to 66 AU: Voyager
1, 1978-1996
Authors: Burlaga, L. F.; Ness, N. F.; Wang, Y. -M.; Sheeley, N. R.
1998JGR...10323727B Altcode:
We discuss Voyager 1 (V1) observations of the heliospheric magnetic
field strength from 1978 through 1996. During this period the distance
of V1 from the Sun increased from ~3 AU to 66 AU and its heliographic
latitude increased from ~5°S to 33°N. The magnetic field strength
profile observed by V1 is consistent with Parker's spiral field model
when one considers (1) the solar cycle variation of the observed
magnetic field strength at 1 AU, B<SUB>1</SUB>(t) (which is a measure
of the source field strength) and (2) the latitudinal and solar cycle
variations of the solar wind speed, V(t,θ). Both B<SUB>1</SUB>(t) and
V(t,θ) make significant contributions to the variation of the magnetic
field strength variations observed by V1. There is no evidence for a
“magnetic flux deficit” increasing with distance from the Sun. There
is a solar cycle variation of the magnetic field strength in the outer
heliosphere, which will affect the modulation of cosmic rays.
---------------------------------------------------------
Title: Large-scale coronal heating by the small-scale magnetic field
of the Sun
Authors: Schrijver, C. J.; Title, A. M.; Harvey, K. L.; Sheeley,
N. R.; Wang, Y. -M.; van den Oord, G. H. J.; Shine, R. A.; Tarbell,
T. D.; Hurlburt, N. E.
1998Natur.394..152S Altcode:
Magnetic fields play a crucial role in heating the outer atmospheres
of the Sun and Sun-like stars, but the mechanisms by which magnetic
energy in the photosphere is converted to thermal energy in the corona
remain unclear. Observations show that magnetic fields emerge onto
the solar surface as bipolar regions with a broad range of length
scales. On large scales, the bipolar regions survive for months before
dispersing diffusively. On the smaller scales, individual bipolar
regions disappear within days but are continuously replenished by new
small flux concentrations, resulting in a sustained state of mixed
polarity. Here we determine the rate of emergence of these small
bipolar regions and we argue that the frequent magnetic reconnections
associated with these regions (an unavoidable consequence of continued
flux replacement) will heat the solar atmosphere. The model that
describes the details of these mixed-polarity regions is complementary
to the traditional diffusion model for large-scale flux dispersal and
a combination of the two should lead to a more complete understanding
of the role of magnetic fields in stellar atmospheres.
---------------------------------------------------------
Title: Origin of Streamer Material in the Outer Corona
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Walters, J. H.; Brueckner,
G. E.; Howard, R. A.; Michels, D. J.; Lamy, P. L.; Schwenn, R.;
Simnett, G. M.
1998ApJ...498L.165W Altcode:
We investigate the nature and origin of the outward-moving density
inhomogeneities (“blobs”) detected previously with the Large Angle and
Spectrometric Coronagraph on the Solar and Heliospheric Observatory. The
blobs are concentrated around the thin plasma layer that surrounds
the heliospheric current sheet and that constitutes the outer streamer
belt; they represent only a small, fluctuating component of the total
density within the plasma sheet. As noted before in Sheeley et al.,
blobs are characterized by low speeds and are continually emitted
from the elongated tips of helmet streamers at 3-4 R<SUB>solar</SUB>
from Sun center. We suggest that both the blobs and the plasma sheet
itself represent closed-field material injected into the solar wind as
a result of footpoint exchanges between the stretched helmet-streamer
loops and neighboring open field lines. The plasma sheet is thus
threaded by newly reconnected, open magnetic field lines, which lend
the white-light streamer belt its filamentary appearance. Since in
situ observations at 1 AU show that the slow wind (with speeds below
500 km s<SUP>-1</SUP>) spreads over an angular extent much greater
than the <~3° width of the plasma sheet, we deduce that a major
component of this wind must originate outside the helmet streamers
(i.e., from just inside coronal holes).
---------------------------------------------------------
Title: White-Light Coronal Mass Ejections: A New Perspective from
LASCO
Authors: St. Cyr, O. C.; Howard, R. A.; Simnett, G. M.; Gurman, J. B.;
Plunkett, S. P.; Sheeley, N. R.; Schwenn, R.; Koomen, M. J.; Brueckner,
G. E.; Michels, D. J.; Andrews, M.; Biesecker, D. A.; Cook, J.; Dere,
K. P.; Duffin, R.; Einfalt, E.; Korendyke, C. M.; Lamy, P. L.; Lewis,
D.; Llebaria, A.; Lyons, M.; Moses, J. D.; Moulton, N. E.; Newmark,
J.; Paswaters, S. E.; Podlipnik, B.; Rich, N.; Schenk, K. M.; Socker,
D. G.; Stezelberger, S. T.; Tappin, S. J.; Thompson, B.; Wang, D.
1997ESASP.415..103S Altcode: 1997cpsh.conf..103S
No abstract at ADS
---------------------------------------------------------
Title: The high-latitude solar wind near sunspot Maximum
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1997GeoRL..24.3141W Altcode:
We use an empirical relation between solar wind speed and coronal
flux-tube expansion to predict what Ulysses might have seen had it
flown over the solar poles during 1989-1991 instead of 1994-1996. The
wind speed patterns, derived from solar magnetograph data, show the
following characteristics: (1) high-speed streams having recurrence
rates of 28-29 days and originating from midlatitude extensions of the
polar coronal holes dominate the rising phase of the sunspot cycle
(1987-1989) (2) the persistent high-speed polar wind disappears and
low-speed wind is found at all latitudes during 1989-1990 (3) very fast,
episodic “polar jets” are generated as active region fields surge
to the poles at the time of polar field reversal (1990-1991). The wind
speed patterns that Ulysses encounters during its second polar orbit
are expected to show the same general characteristics.
---------------------------------------------------------
Title: Solar Wind Stream Interactions and the Wind Speed-Expansion
Factor Relationship.
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Phillips, J. L.; Goldstein,
B. E.
1997ApJ...488L..51W Altcode:
Previous studies have shown that the solar wind speed observed near
Earth is inversely correlated with the divergence rate of magnetic flux
tubes near the Sun. We test the global validity of this relationship
by employing Ulysses wind speed measurements during 1990-1997 as well
as inecliptic data for 1976-1997. When the correspondence between wind
speeds and expansion factors is adjusted to yield optimal agreement with
the high-latitude Ulysses measurements, the model matches the overall
patterns of fast and slow wind near the ecliptic but predicts too much
very fast wind there. We show how this discrepancy can be resolved
by taking account of wind stream interactions, where we apply a crude
algorithm based on the propagation times of neighboring wind parcels;
the interactions reduce the amount of very fast wind at latitudes
where slow wind is present. We also test and reject an alternative
model in which the wind speed is assumed to be a function only of
angular distance from the heliospheric neutral sheet.
---------------------------------------------------------
Title: Origin and Evolution of Coronal Streamer Structure During
the 1996 Minimum Activity Phase
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Howard, R. A.; Kraemer,
J. R.; Rich, N. B.; Andrews, M. D.; Brueckner, G. E.; Dere, K. P.;
Koomen, M. J.; Korendyke, C. M.; Michels, D. J.; Moses, J. D.;
Paswaters, S. E.; Socker, D. G.; Wang, D.; Lamy, P. L.; Llebaria,
A.; Vibert, D.; Schwenn, R.; Simnett, G. M.
1997ApJ...485..875W Altcode:
We employ coronal extrapolations of solar magnetograph data to interpret
observations of the white-light streamer structure made with the LASCO
coronagraph in 1996. The topological appearance of the streamer belt
during the present minimum activity phase is well described by a model
in which the Thomson-scattering electrons are concentrated around a
single, warped current sheet encircling the Sun. Projection effects
give rise to bright, jet-like structures or spikes whenever the current
sheet is viewed edge-on multiple spikes are seen if the current sheet is
sufficiently wavy. The extreme narrowness of these features in polarized
images indicates that the scattering layer is at most a few degrees
wide. We model the evolution of the streamer belt from 1996 April to
1996 September and show that the effect of photospheric activity on
the streamer belt topology depends not just on the strength of the
erupted magnetic flux, but also on its longitudinal phase relative
to the background field. Using flux transport simulations, we also
demonstrate how the streamer belt would evolve during a prolonged
absence of activity.
---------------------------------------------------------
Title: The Green Line Corona and Its Relation to the Photospheric
Magnetic Field
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Hawley, S. H.; Kraemer,
J. R.; Brueckner, G. E.; Howard, R. A.; Korendyke, C. M.; Michels,
D. J.; Moulton, N. E.; Socker, D. G.; Schwenn, R.
1997ApJ...485..419W Altcode:
Images of the green line corona made with the LASCO C1 coronagraph
on SOHO are analyzed by applying current-free extrapolations to the
observed photospheric field. The Fe XIV λ5303 emission is shown to
be closely related to the underlying photospheric field strength. By
modeling the observed intensity patterns as a function of latitude and
height above the solar limb, we derive an approximate scaling law of the
form n<SUB>foot</SUB> ~ <B<SUB>foot</SUB>><SUP>0.9</SUP>, where
n<SUB>foot</SUB> is the density of the green line-emitting plasma and
<B<SUB>foot</SUB>> is the average field strength at the footprints
of the coronal loop. The observed high-latitude enhancements in the
green line corona are attributed to the poleward concentration of the
large-scale photospheric field. The strongest such enhancements occur
where the high-latitude unipolar fields become reconnected to active
region flux at lower latitudes; the global emission pattern rotates
quasi-rigidly at the rate of the dominant active region complex. The
validity of the current-free approximation is assessed by comparing
the topology of the observed and simulated green line structures.
---------------------------------------------------------
Title: Measurements of Flow Speeds in the Corona Between 2 and 30
R<SUB>⊙</SUB>
Authors: Sheeley, N. R.; Wang, Y. -M.; Hawley, S. H.; Brueckner,
G. E.; Dere, K. P.; Howard, R. A.; Koomen, M. J.; Korendyke, C. M.;
Michels, D. J.; Paswaters, S. E.; Socker, D. G.; St. Cyr, O. C.;
Wang, D.; Lamy, P. L.; Llebaria, A.; Schwenn, R.; Simnett, G. M.;
Plunkett, S.; Biesecker, D. A.
1997ApJ...484..472S Altcode:
Time-lapse sequences of white-light images, obtained during sunspot
minimum conditions in 1996 by the Large Angle Spectrometric Coronagraph
on the Solar and Heliospheric Observatory, give the impression of
a continuous outflow of material in the streamer belt, as if we
were observing Thomson scattering from inhomogeneities in the solar
wind. Pursuing this idea, we have tracked the birth and outflow of
50-100 of the most prominent moving coronal features and find that:
<P />1. They originate about 3-4 R<SUB>⊙</SUB> from Sun center as
radially elongated structures above the cusps of helmet streamers. Their
initial sizes are about 1 R<SUB>⊙</SUB> in the radial direction and
0.1 R<SUB>⊙</SUB> in the transverse direction. <P />2. They move
radially outward, maintaining constant angular spans and increasing
their lengths in rough accord with their speeds, which typically
double from 150 km s<SUP>-1</SUP> near 5 R<SUB>⊙</SUB> to 300 km
s<SUP>-1</SUP> near 25 R<SUB>⊙</SUB>. <P />3. Their individual speed
profiles v(r) cluster around a nearly parabolic path characterized
by a constant acceleration of about 4 m s<SUP>-2</SUP> through most
of the 30 R<SUB>⊙</SUB> field of view. This profile is consistent
with an isothermal solar wind expansion at a temperature of about
1.1 MK and a sonic point near 5 R<SUB>⊙</SUB>. <P />Based on their
relatively small initial sizes, low intensities, radial motions, slow
but increasing speeds, and location in the streamer belt, we conclude
that these moving features are passively tracing the outflow of the
slow solar wind.
---------------------------------------------------------
Title: Association of Extreme-Ultraviolet Imaging Telescope (EIT)
Polar Plumes with Mixed-Polarity Magnetic Network
Authors: Wang, Y. -M.; Sheeley, N. R.; Dere, K. P.; Duffin, R. T.;
Howard, R. A.; Michels, D. J.; Moses, J. D.; Harvey, J. W.; Branston,
D. D.; Delaboudinière, J. -P.; Artzner, G. E.; Hochedez, J. F.;
Defise, J. M.; Catura, R. C.; Lemen, J. R.; Gurman, J. B.; Neupert,
W. M.; Newmark, J.; Thompson, B.; Maucherat, A.; Clette, F.
1997ApJ...484L..75W Altcode:
SOHO EIT spectroheliograms showing the polar coronal holes during the
present sunspot minimum are compared with National Solar Observatory
(Kitt Peak) magnetograms taken in Fe I λ8688 and Ca II λ8542. The
chromospheric λ8542 magnetograms, obtained on a routine, near-daily
basis since 1996 June, reveal the Sun's strong polar fields with
remarkable clarity. We find that the Fe IX λ171 polar plumes occur
where minority-polarity flux is in contact with flux of the dominant
polarity inside each polar hole. Moreover, the locations of “plume
haze” coincide approximately with the patterns of brightened He
II λ304 network within the coronal hole. The observations appear
to be consistent with mechanisms of plume formation involving
magnetic reconnection between unipolar flux concentrations and nearby
bipoles. The fact that minority-polarity fields constitute only a small
fraction of the total magnetic flux within the polar holes suggests
that plumes are not the main source of the high-speed polar wind.
---------------------------------------------------------
Title: Using LASCO Observations to Infer Solar Wind Flow Near the Sun
Authors: Sheeley, N. R., Jr.; Brueckner, G. E.; Dere, K. P.; Howard,
R. A.; Korendyke, C. M.; Michels, D. J.; Socker, D. G.; Koomen, M. J.;
Paswaters, S. E.; Wang, D.; Lamy, P. L.; Llebaria, A.; Schwenn, R.;
St Cyr, O. C.; Simnett, G. M.; Plunkett, S.; Biesecker, D. A.
1997SPD....28.0301S Altcode: 1997BAAS...29..907S
We have continued to track individual coronal features as they become
detached from helmet streamers and move outward from the Sun. The
composite speed profile for 50-100 features has a parabolic shape
with a constant acceleration of about 4 m/s(2) over the 2-30 R field
of view. This well-determined speed profile contrasts strongly with
the nearly uniform scatterplot obtained for about 50 nominal coronal
mass ejections (CMEs), and suggests that these detached bits of coronal
“debris” are passively tracing the speed of the slow solar wind. We
have also begun the more difficult task of tracking outflow along
polar plumes and will summarize these results as of June 1997.
---------------------------------------------------------
Title: Richard Tousey 1908-1997
Authors: Koomen, M. J.; Hunter, W. R.; Sheeley, N. R.
1997CIBu..140....4K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Near-Sun Magnetic Fields and the Solar Wind
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Phillips, J. L.
1997cwh..conf..459S Altcode: 2006mslp.conf..459S
No abstract at ADS
---------------------------------------------------------
Title: Obituary: Robert B. Leighton, 1919-1997
Authors: Neugebauer, Gerry; Peck, Charles W.; Sheeley, Neil; Trimble,
Virginia
1997BAAS...29.1477N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Obituary: Richard Tousey, 1908-1997
Authors: Koomen, M. J.; Hunter, W. R.; Sheeley, N. R., Jr.
1997BAAS...29.1494K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: ULYSSES plasma parameters: latitudinal, radial, and temporal
variations.
Authors: Goldstein, B. E.; Neugebauer, M.; Phillips, J. L.; Bame, S.;
Gosling, J. T.; McComas, D.; Wang, Y. -M.; Sheeley, N. R.; Suess, S. T.
1996A&A...316..296G Altcode:
Observations by the Ulysses SWOOPS plasma experiment are used
to investigate spatial and temporal gradients during the mission,
with emphasis on more recent high latitude observations including the
recent South Pole to North Pole passage during solar minimum. Compared
to lower latitudes, the high latitude solar wind had higher average
speed, proton temperature, and momentum flux, and lower number flux
density. As the average momentum flux observed in the high speed wind
was 21% greater than at the equator, during solar minimum the distance
to the heliopause will be comparatively less in the solar equatorial
plane than over the poles. The long term temporal gradients of momentum
flux over the life of the mission are considerably larger than the
latitudinal gradient observed by Ulysses during solar minimum. A
modest North-South high latitude asymmetry is observed in the plasma
parameters; the velocity is on the average 13km/s to 24km/s greater at
Northern latitudes than at Southern, and temperature is also higher. The
North-South temperature asymmetry is greater than can be explained by
the North-South velocity difference and the dependence of solar wind
temperature upon speed. The power law dependence of temperature on
heliocentric distance, r, at high latitudes is in range r^-0.81^ to
r^-1.03^, where r^-0.81^ is the Southern latitude result and r^-1.03^
the Northern. The parameter T/n^1/2^, where T is temperature and n
is proton number density, can be better predicted from speed than can
temperature alone. Comparison with calculations based on source models
and magnetograph data indicate that the expansion of open coronal
field lines close to the Sun was greater in the Southern hemisphere
than in the Northern; this anticorrelation with the expansion factor
is consistent with previous observational and theoretical work.
---------------------------------------------------------
Title: Elemental Abundance Variations in the Solar Atmosphere
Authors: Sheeley, N. R., Jr.
1996ApJ...469..423S Altcode:
Skylab solar images in the transition region lines of neon, magnesium,
and calcium have been used to trace elemental abundance variations
in sunspots and the quiet Sun. Sunspots are invariably accompanied
by spikelike features, enriched in elements of low first ionization
potential (FIP) such as magnesium and calcium, and extending
outward from the penumbras. Material with the normal, unenriched,
photospheric-like composition is sometimes seen over the umbra, but
it is only seen in the presence of very bright chromospheric emission
associated with flares or emerging flux. The salt-and-pepper fields
of the quiet Sun give rise to small-scale structures, enhanced in
the lines of both helium and neon, and having the "photospheric"
composition. However, enrichments of low-Fl P elements are sometimes
found at unipolar flux concentrations in coronal holes, and occasionally
they have very large enrichment factors. These observations suggest
that the composition depends on whether the plasma is coronal or not
and that the fractionation process is somehow related to the production
of coronal material.
---------------------------------------------------------
Title: The Magnetic Nature of Coronal Holes
Authors: Wang, Yi-Ming; Hawley, Scott H.; Sheeley, Neil R., Jr.
1996Sci...271..464W Altcode:
Solar wind streams originate from low-density, magnetically open
regions of the sun's corona, known as coronal holes. The locations,
areal sizes, rotation, and solar-cycle evolution of these regions
can be reproduced and understood by applying simple extrapolation
models to measurements of the photospheric magnetic field. The
surprisingly rigid rotation displayed by many coronal holes suggests
that field-line reconnection occurs continually in the corona, despite
the high electrical conductivity of the coronal plasma. The magnetic
field strengths and field-line divergence rates in coronal holes
can be related empirically to the bulk speed and the mass and energy
flux densities of the solar wind plasma. Such relations may help to
illuminate the physical processes responsible for heating the corona
and driving the solar wind.
---------------------------------------------------------
Title: Coronal Plumes and Their Relationship to Network Activity
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1995ApJ...452..457W Altcode:
Using Skylab extreme-ultraviolet spectroheliograms, we address the
question of what lies under a coronal plume. Plumes and their base
areas, both inside polar coronal holes and within lower latitude holes
near central meridian, are identified in the Mg IX λ368 emission
line. While some (usually spike-shaped) plumes show a strongly enhanced
Mg IX core, others (sheetlike in appearance) are characterized by a
much more diffuse base which may extend over several supergranules. The
base areas are found to contain collections of compact (although not
always intense) Ne VII λ465 features, whose locations in turn generally
coincide with enhancements in the He II λ304 network inside the coronal
hole. Bright plumes always show intense network features within their
base areas, but the converse does not hold: not every Ne VII or He II
bright point has an associated Mg IX plume. By comparing the locations
of plumelike Mg IX "haze" in a lower latitude hole with a simultaneous
high-resolution magnetogram, we infer that coronal plumes occur near
regions of mixed magnetic polarity. We suggest a mechanism for plume
formation, whereby small bipoles within a coronal hole reconnect with
unipolar flux concentrations located at network junctions.
---------------------------------------------------------
Title: Coronal flux-tube expansion and the polar wind
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Phillips, J. L.
1995AdSpR..16i.365W Altcode: 1995AdSpR..16..365W
Empirical and theoretical studies indicate an inverse correlation
between the areal expansion rate of magnetic flux tubes near the
Sun and the solar wind speed far from the sun. This relationship is
combined with solar magnetograph measurements to predict the wind
speed structures at high latitudes, and the results are compared with
observations now being carried out by Ulysses. Based on the evolution
of the polar fields during previous sunspot cycles, we also discuss
how the high-latitude wind is likely to evolve between 1994 and the
next solar maximum. Our main predictions are as follows: (1) As the
cycle declines, the fastest wind streams are expected to be centered
at mid-latitudes (above the polar-hole extensions), not at the poles
themselves. (2) The fastest wind at the poles is predicted to occur
not at sunspot minimum, when the polar fields are strongest and large
axisymmetric polar coronal holes are present, but just after sunspot
maximum, when the polar fields undergo their polarity reversal.
---------------------------------------------------------
Title: Solar Implications of ULYSSES Interplanetary Field Measurements
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1995ApJ...447L.143W Altcode:
Recent observations by the Ulysses magnetometer team have shown that the
strength of the radial interplanetary field component, |B<SUB>r</SUB>| ,
is essentially independent of latitude, a result which implies that the
heliospheric currents are confined entirely to thin sheets. Using such
a current sheet model, we extrapolate the observed photospheric field to
1 AU and compare the predicted magnitude and sign of Br with spacecraft
measurements during 1970--1993. Approximate agreement can be obtained
if the solar magnetograph measurements in the Fe I lambda 5250 line are
scaled upward by a latitude-dependent factor, similar to that derived
by Ulrich from a study of magnetic saturation effects. The correction
factor implies sharply peaked polar fields near sunspot minimum,
with each polar coronal hole having a mean field strength of 10 G.
---------------------------------------------------------
Title: Comparing ULYSSES wind speed with coronal flux-tube expansion
factor
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Phillips, J. L.; Bame,
S. J.; Goldstein, B. E.
1995sowi.confR..63S Altcode:
We have been comparing measurements of solar wind speed at the
Ulysses spacecraft with coronal flux-tube expansion rates, derived from
photospheric field measurements using a current-free coronal model. The
large-scale patterns of derived speed have continued to reproduce the
observed patterns from launch through south polar passage to the present
40S latitude of the spacecraft. The fastest non-transient wind speeds
of approx. 860 km/s were encountered at midlatitudes en route to the
south pole, rather than during polar passage when the peak speeds were
approx. 820 km/s. Although this result is in qualitative agreement with
the idea that the wind speed is controlled by the coronal flux-tube
expansion rate, the 40 km/s difference is significantly smaller than
the 100-150 km/s difference based on our in-ecliptic calibration. This
paper will summarize our attempts to resolve this discrepancy and will
show the observational status of our coronal/interplanetary comparison
at the time of the meeting.
---------------------------------------------------------
Title: Identification of Low-Latitude Coronal Plumes in
Extreme-Ultraviolet Spectroheliograms
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1995ApJ...446L..51W Altcode:
Using Skylab extreme ultraviolet images of the solar disk, we have
identified plumelike features inside low-latitude coronal holes
undergoing limb passage. Like their polar counterparts, these diffuse
Mg IX structures are located above enhancements in the weak neon and
helium background emission within the coronal hole. We conclude that
coronal plumes are not unique to the polar regions but may occur in
open magnetic regions at any latitude.
---------------------------------------------------------
Title: Source regions of the solar wind
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1995sowi.conf...31W Altcode:
Using Skylab XUV data, we examine some properties of the source regions
of the solar wind. In particular, we discuss the physical nature of
polar plumes and their relationship to the polar wind, the nature of
the source regions of the slow solar wind, and the relationship between
abundance anomalies (the FIP effect) determined from the Skylab data
and the sources of fast and slow wind.
---------------------------------------------------------
Title: Observations of Elemental Abundance Variations in the Solar
Atmosphere
Authors: Sheeley, N. R.
1995SPD....26..610S Altcode: 1995BAAS...27Q.963S
No abstract at ADS
---------------------------------------------------------
Title: A Volcanic Origin for High-FIP Material in the Solar Atmosphere
Authors: Sheeley, N. R., Jr.
1995ApJ...440..884S Altcode:
Photospheric magnetograms and Skylab images in the lines of Ne VI and
Mg VI near 400 Å have been used to study the relation between magnetic
field topology and solar composition. Although plasma with the neon-rich
photospheric composition is rare in the corona, it always occurs where
new flux is emerging. This suggests that the neon richness of the
plasma reflects its recent photospheric origin, not its closed-loop
magnetic topology as has been assumed in the past. Such a "volcanic"
process would be consistent with our observations of sunspots, which
have the Ne-rich composition when they lie next to a neutral line
where flux is emerging, but have the Mg-rich composition when they
are surrounded by a "buffer zone" of like-polarity flux.
---------------------------------------------------------
Title: The Rotation of Photospheric Magnetic Fields: A Random Walk
Transport Model
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1994ApJ...430..399W Altcode:
In an earlier study of solar differential rotation, we showed that
the transport of magnetic flux across latitudes acts to establish
quasi-stationary patterns, therby accounting for the observed rigid
rotation of the large-scale photospheric field. In that paper, the
effect of supergranular convection was represented by a continuum
diffusion, limiting the applicability of the calculations to large
spatial scales. Here we extend the model to scales comparable to that
of the supergranulation itself by replacing the diffusive transport
with a discrete random walk process. Rotation curves are derived
by cross-correlating the simulated photospheric field maps for a
variety of time lags and spatial resolutions. When the lag between
maps is relatively short less than or approximately = 15 days), the
midlatitude correlation functions show two distinct components: a broad
feature associated with the large-scale unipolar patterns and a narrow
feature originating from small magnetic structures encompossing from
one to several supergranular cells. By fitting the broad component
we obtain the rigid rotation profile of the patterns, whereas by
fitting the narrow component, we recover the differential rate of
the photospheric plasma itself. For time lags of 1 month or greater,
only the broad feature associated with the long-lived patterns remains
clearly identifiable in the simulations.
---------------------------------------------------------
Title: Ulysses at 50° south: constant immersion in the high-speed
solar wind
Authors: Phillips, J. L.; Balogh, A.; Bame, S. J.; Goldstein, B. E.;
Gosling, J. T.; Hoeksema, J. T.; McComas, D. J.; Neugebauer, M.;
Sheeley, N. R., Jr.; Wang, Y. -M.
1994GeoRL..21.1105P Altcode:
We present speed observations from the Ulysses solar wind plasma
experiment through 50° south latitude. The pronounced speed modulation
arising from solar rotation and the tilt of the heliomagnetic current
sheet has nearly disappeared. Ulysses is now observing wind speeds
in the 700 to 800 km s<SUP>-1</SUP> range, with a magnetic polarity
indicating an origin in the large south polar coronal hole. The
strong compressions, rarefactions, and shock waves previously seen have
weakened or disappeared. Occasional coronal mass ejections characterized
by low plasma density caused by radial expansion have been observed. The
coronal configuration was simple and stable in 1993, indicating that
the observed solar wind changes were caused by increasing spacecraft
latitude. Trends in prevailing speed with increasing latitude support
previous findings. A decrease in peak speed southward of 40° latitude
may indicate that the fastest solar wind comes from the equatorial
extensions of the polar coronal holes.
---------------------------------------------------------
Title: Global evolution of interplanetary sector structure, coronal
holes, and solar wind streams during 1976-1993: Stackplot displays
based on solar magnetic observations
Authors: Wang, Y. M.; Sheeley, N. R., Jr.
1994JGR....99.6597W Altcode:
We use potential field calculations and solar magnetic observations
during 1976-1993 to infer the evolution of interplanetary sector
structure, coronal holes, and solar wind streams at heliographic
latitudes ranging from 80°S to 80°N. The results are presented in
the form of stackplots, which show long-lived patterns that rotate
quasi-rigidly at rates determined by the photospheric distribution
of nonaxisymmetric magnetic flux. The fastest wind streams and their
coronal hole sources form slowly rotating patterns near the poles just
after sunspot maximum but migrate to lower latitudes and tend to rotate
at near-equatorial rates as sunspot activity declines.
---------------------------------------------------------
Title: Returning to the random walk
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
1994ASIC..433..379S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Understanding the Rotation of Coronal Holes
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1993ApJ...414..916W Altcode:
In an earlier study we found that the rotation of coronal holes
could be understood on the basis of a nearly current-free coronal
field, with the holes representing open magnetic regions. In this
paper we illustrate the model by focusing on the case of CH1, the
rigidly rotating boot-shaped hole observed by Skylab. We show that the
interaction between the polar fields and the flux associated with active
regions produces distortions in the coronal field configuration and
thus in the polar-hole boundaries; these distortions corotate with the
perturbing nonaxisymmetric flux. In the case of CH1, positive-polarity
field lines in the northern hemisphere 'collided' with like-polarity
field lines fanning out from a decaying active region complex located
just below the equator, producing a midlatitude corridor of open field
lines rotating at the rate of the active region complex. Sheared coronal
holes result when nonaxisymmetric flux is present at high latitudes,
or equivalently, when the photospheric neutral line extends to high
latitudes. We demonstrate how a small active region, rotating at the
local photospheric rate, can drift through a rigidly rotating hole
like CH1. Finally, we discuss the role of field-line reconnection in
maintaining a quasi-potential coronal configuration.
---------------------------------------------------------
Title: Coronal Flux-Tube Expansion and the Solar Wind Speed at Ulysses
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Bame, S. J.; Phillips,
J. L.; Goldstein, B. E.
1993BAAS...25.1203S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flux Emergence and the Evolution of Large-Scale Photosphenc
Field Patterns (Abstract)
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1993ASPC...46..487W Altcode: 1993mvfs.conf..487W; 1993IAUCo.141..487W
No abstract at ADS
---------------------------------------------------------
Title: A New Determination of the Solar Rotation Rate
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Nash, A. G.
1992ApJ...401..378S Altcode:
We use 'stackplot' displays to compare observations of the photospheric
magnetic field during sunspot cycle 21 with simulations based on the
flux-transport model. Adopting nominal rates of diffusion, differential
rotation, and meridional flow, we obtain slanted patterns similar
to those of the observed field, even when the sources of flux are
assigned random longitudes in the model. At low latitudes, the slopes
of the nearly vertical patterns of simulated field are sensitive to
the rotation rate used in the calculation, and insensitive to the
rates of diffusion and flow during much of the sunspot cycle. Good
agreement between the observed and simulated patterns requires a
synodic equatorial rotation period of 26.75 +/- 0.05 days.
---------------------------------------------------------
Title: Obituary: Ana Gomes Nash, 1957-1992
Authors: Sheeley, Neil R.; Wang, Yi-Ming
1992BAAS...24.1329S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On Potential Field Models of the Solar Corona
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1992ApJ...392..310W Altcode:
It is shown that the line-of-sight matching procedure involved in
potential field models of the solar corona do not make good use of the
available data because there is strong evidence that the magnetic field
is nearly radial, and therefore nonpotential, at the photosphere. It is
argued that the observed photospheric field should first be corrected
for line-of-sight projection and then matched to the radial component
of the potential field. It is shown that this procedure yields much
stronger polar fields than the standard method and produces better
agreement with high-latitude coronal holes and with white-light
structures in the outer corona. The relationship of both methods to
the observed inclination angles of polar plumes is also discussed.
---------------------------------------------------------
Title: The relationship between solar wind speed and the areal
expansion factor
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1992sws..coll..125W Altcode:
Empirical studies indicate that the solar wind speed at Earth is
inversely correlated with the divergence rate of the coronal magnetic
field. This result suggests that the mechanical energy flux at the
coronal base (in the form of Alfven waves, for example) is roughly
constant within open field regions.
---------------------------------------------------------
Title: The Flux-Transport Model and Its Implications
Authors: Sheeley, N. R., Jr.
1992ASPC...27....1S Altcode: 1992socy.work....1S
No abstract at ADS
---------------------------------------------------------
Title: Coronal holes and solar wind streams during the sunspot cycle
Authors: Sheeley, N. R., Jr.
1992sws..coll..263S Altcode:
Complementary synoptic observations of the Sun and interplanetary
space have been obtained nearly continuously for more than two
sunspot cycles and have led to new ideas about the origin of the
solar wind. These observations show an inverse correlation between
wind speed at Earth and magnetic flux tube expansion in the corona,
with fast wind originating from slowly diverging tubes and vice
versa. Although this result is consistent with the Skylab-era concept
that fast wind originates from the center of a large isolated coronal
hole, it implies that the wind may be even faster at the facing edges
of like-polarity holes where the flux-tubes converge as they begin
their outward extension. Thus, very fast wind ought to originate from
the high-latitude edges of the circumpolar holes soon after sunspot
maximum and from the mid-latitude necks of the polar-hole lobes during
the declining phase of the cycle. The observed inverse correlation may
be understood physically in terms of a model in which Alfven waves
boost the wind to high speed provided that the wave energy flux is
distributed approximately uniformly at the coronal base.
---------------------------------------------------------
Title: A New Solar Cycle Model Including Meridional Circulation
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Nash, A. G.
1991ApJ...383..431W Altcode:
A kinematic model is presented for the solar cycle which includes
not only the transport of magnetic flux by supergranular diffusion
and a poleward bulk flow at the sun's surface, but also the effects
of turbulent diffusion and an equatorward 'return flow' beneath
the surface. As in the earlier models of Babcock and Leighton, the
rotational shearing of a subsurface poloidal field generates toroidal
flux that erupts at the surface in the form of bipolar magnetic
regions. However, such eruptions do not result in any net loss of
toroidal flux from the sun (as assumed by Babcock and Leighton);
instead, the large-scale toroidal field is destroyed both by 'unwinding'
as the local poloidal field reverses its polarity, and by diffusion as
the toroidal flux is transported equatorward by the subsurface flow
and merged with its opposite hemisphere counterpart. The inclusion
of meridional circulation allows stable oscillations of the magnetic
field, accompanied by the equatorward progression of flux eruptions,
to be achieved even in the absence of a radial gradient in the angular
velocity. An illustrative case in which a subsurface flow speed of
order 1 m/s and subsurface diffusion rate of order 10 sq km/s yield
22-yr oscillations in qualitative agreement with observations.
---------------------------------------------------------
Title: Out-of-ecliptic tests of the inverse correlation between
solar wind speed and coronal expansion factor
Authors: Sheeley, N. R., Jr.; Swanson, E. T.; Wang, Y. -M.
1991JGR....9613861S Altcode:
In this paper we address the question of whether out-of-ecliptic
measurements satisfy the inverse correlation between wind speed at
1 AU and flux tube divergence in the corona, already found from
measurements in the ecliptic. Using the in-ecliptic calibration,
we derive out-of-ecliptic speeds from coronal expansion factors
determined from global observations of photospheric field and their
current-free coronal extension. These derived speeds are compared with
speeds inferred from interplanetary scintillation measurements during
1972-1988 and with in situ speeds measured by the Pioneer 11 spacecraft
at 16°N latitude during 1984-1988. These three sets of wind speed
show the same overall variation with latitude and time during the
sunspot cycle, with higher latitudes having more years of fast wind
than lower latitudes and all latitudes having slow wind at sunspot
maximum. Although some detailed discrepancies are also present, the
overall agreement is comparable to that achieved in the ecliptic plane.
---------------------------------------------------------
Title: Magnetic Flux Transport and the Sun's Dipole Moment: New
Twists to the Babcock-Leighton Model
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1991ApJ...375..761W Altcode:
The mechanisms that give rise to the sun's large-scale poloidal
magnetic field are explored in the framework of the Babcock-Leighton
(BL) model. It is shown that there are in general two quite distinct
contributions to the generation of the 'alpha effect': the first is
associated with the axial tilts of the bipolar magnetic regions as they
erupt at the surface, while the second arises through the interaction
between diffusion and flow as the magnetic flux is dispersed over the
surface. The general relationship between flux transport and the BL
dynamo is discussed.
---------------------------------------------------------
Title: Polar Faculae: 1906--1990
Authors: Sheeley, N. R., Jr.
1991ApJ...374..386S Altcode:
The numbers of faculae at the sun's poles have been estimated from
white-light images obtained at the Mount Wilson Observatory during
1970-1990 and have been combined with previous measurements extending
back to 1906 when the observations began. The combined measurements
now span four complete 22 year cycles and show the following: (1) the
numbers of north and south polar faculae were about 50 percent larger
around sunspot minimum in 1986 than in 1976, but were still smaller
than some of the very large numbers that occurred near sunspot minima in
earlier cycles; (2) in 1974, the number of south polar faculae exhibited
a short-lived increase which coincided with the arrival of a surge of
trailing-polarity flux at the pole, suggesting that similar poleward
surges may have been responsible for previously unexplained bursts of
faculae such as the one that occurred at the south pole in 1959; and
(3) the numbers of polar faculae have been highly correlated with the
Wilcox Solar Observatory polar field strengths since these magnetic
measurements began in 1976.
---------------------------------------------------------
Title: Why Fast Solar Wind Originates from Slowly Expanding Coronal
Flux Tubes
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1991ApJ...372L..45W Altcode:
Empirical studies indicate that the solar wind speed at earth is
inversely correlated with the divergence rate of the coronal magnetic
field. It is shown that this result is consistent with simple wind
acceleration models involving Alfven waves, provided that the wave
energy flux at the coronal base is taken to be roughly constant within
open field regions.
---------------------------------------------------------
Title: Deriving Solar Wind Speed from Solar Magnetic Field
Measurements
Authors: Nash, A. G.; Wang, Y. -M.; Sheeley, N. R., Jr.
1991BAAS...23..821N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Field Configurations Associated with Fast Solar Wind
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
1991SoPh..131..165S Altcode:
In this paper, we consider the implications of the observed inverse
correlation between solar wind speed at Earth and the expansion rate
of the Sun-Earth flux tube as it passes through the corona. We find
that the coronal expansion rate depends critically on the large-scale
photospheric field distribution around the footpoint of the flux tube,
with the smallest expansions occurring in tubes that are rooted near
a local minimum in the field. This suggests that the fastest wind
streams originate from regions where large coronal holes are about to
break apart and from the facing edges of adjacent like-polarity holes,
whose field lines converge as they transit the corona. These ideas
lead to the following predictions: Weak holes and fragmentary holes
can be sources of very fast wind.
---------------------------------------------------------
Title: Photospheric and coronal magnetic fields
Authors: Sheeley, N. R., Jr.
1991RvGeo..29S.925S Altcode: 1991RvGeS..29..925S; 1991RvGeo..29..925S
Research on small-scale and large-scale photospheric and coronal
magnetic fields during 1987-1990 is reviewed, focusing on observational
studies. Particular attention is given to the new techniques, which
include the correlation tracking of granules, the use of highly
Zeeman-sensitive infrared spectral lines and multiple lines to deduce
small-scale field strength, the application of long integration
times coupled with good seeing conditions to study weak fields,
and the use of high-resolution CCD detectors together with computer
image-processing techniques to obtain images with unsurpassed spatial
resolution. Synoptic observations of large-scale fields during the
sunspot cycle are also discussed.
---------------------------------------------------------
Title: The solar activity cycle.
Authors: Rabin, Douglas M.; DeVore, C. R.; Sheeley, Neil R., Jr.;
Harvey, Karen L.; Hoeksema, J. T.
1991sia..book..781R Altcode:
Study of the solar cycle is entering a new era dominated by objective,
precise measurements of magnetic, velocity and radiation fields over the
surface of the Sun. This review emphasizes observations of photospheric
magnetic flux during cycle 21 (1976 - 1986) and how these measurements
have been used to model the cyclic variability of the heliospheric
magnetic field. Indices of solar activity are discussed in terms of
their potential to figure in theoretical or empirical models. Other
recent data, such as measurements of large-scale surface flows and
information on the Sun's internal rotation from helioseismology, as
well as the magnetic flux observations, are considered in the context
of Babcock's phenomenological model of the solar cycle: can this model
still serve? Is there anything better to replace it?
---------------------------------------------------------
Title: Magnetic Flux Transport and the Sunspot-Cycle Evolution of
Coronal Holes and Their Wind Streams
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1990ApJ...365..372W Altcode:
The relationships between magnetic flux transport from active regions
and the formation and evolution of coronal holes are examined through
numerical simulations. The model utilized is based on the assumption
that coronal holes represent open field regions, and that the solar-wind
speed at 1 AU is universely correlated with the divergence rate of the
coronal field. The evolution of coronal holes and wind streams during
1980 - 1990 is discussed, along with flux transport and the evolution
of open field regions, and focus is placed on declining, rising, and
maximum phases. It is concluded that supergranular diffusion spreads
active region flux over the solar surface and wipes out pockets of
mixed polarity, thus creating unipolar areas containing open field
lines; differential rotation spreads flux in longitude and it combines
with diffusion to create axisymmetric polar holes from the original
active-region fields; and meridional flow accelerates the decay of
low-latitude holes by carrying flux to midlatitudes.
---------------------------------------------------------
Title: Latitudinal distribution of solar-wind speed from magnetic
observations of the Sun
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Nash, A. G.
1990Natur.347..439W Altcode:
Empirical studies suggest a close relationship between the solar-wind
speed near the Earth and the magnetic structure of the solar corona. The
correlation can be used to infer the latitudinal distribution of wind
speed at different phases of the sunspot cycle, and to identify the
sources of fast, high-latitude wind streams such as those that might
be encountered by the Ulysses spacecraft on its journey toward the
solar poles during 1992-1995.
---------------------------------------------------------
Title: Solar Wind Speed and Coronal Flux-Tube Expansion
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1990ApJ...355..726W Altcode:
The hypothesis that the solar wind speed at 1 AU and the rate of
magnetic flux-tube expansion in the corona are inversely correlated
is shown to be consistent with observations extending over the last
22 years. This empirical relationship allows the daily wind speeds
at earth to be predicted from a current-free extrapolation of the
observed photospheric field into the corona. The narrow boundaries
of high-speed wind streams are attributed to steep gradients in
the flux-tube expansion rates at the edges of coronal holes. When a
heliospheric current sheet is included in the model, it is found that
the flux tubes near the hole axis, although diverging more slowly
than those near the hole boundary in the corona, have undergone the
greatest net expansion at 1 AU, an effect consistent with the low
densities within high-speed streams.
---------------------------------------------------------
Title: Coronal Mass Ejections and the Injection Profiles of Solar
Energetic Particle Events
Authors: Kahler, S. W.; Reames, D. V.; Sheeley, N. R., Jr.
1990ICRC....5..183K Altcode: 1989ICRC....5..183K; 1990ICRC...21e.183K
Previous studies using Skylab and Solwind coronagraph observations have
shown that almost all E greater than 10 MeV solar energetic proton (SEP)
events are associated with the occurrence of a coronal mass ejection
(CME). These earlier studies did not address the relationship between
the position of the associated CME and the timing of the injection
of particles into the interplanetary medium. Ten cases are selected
in which a SEP event observed with the GSFC detectors on the IMP 8 or
ISEE 3 spacecraft was correlated to a CME well observed by the Solwind
coronagraph. The height of the leading edge of the CME is compared with
the particle injection profiles for several energy ranges using the
solar release times for the particles. The derived injection profiles
are found to be increasing and sometimes reaching maximum while the
associated CMEs are at heights of 2-10 Ro.
---------------------------------------------------------
Title: Evolution of the Sun's Polar Fields during Sunspot Cycle 21:
Poleward Surges and Long-Term Behavior
Authors: Wang, Y. -M.; Nash, A. G.; Sheeley, N. R., Jr.
1989ApJ...347..529W Altcode:
Longitudinally averaged observations of the photospheric field during
1976-1986 are analyzed using a flux transport model. The way in
which source eruptions, supergranular diffusion, and meridional flow
collaborate to produce strong, highly concentrated polar fields near
sunspot minimum is clarified as follows: (1) widespread eruptions
of individual bipolar magnetic regions, with their leading polarity
flux equatorward of their trailing polarity flux, collectively
establish a large-scale separation of polarities in latitude; (2) the
low-latitude, leading polarity flux diffuses across the equator and
merges with its opposite hemisphere counterpart; and (3) meridional
flow carries the resulting surplus of trailing polarity flux to
the poles, and concentrates it there against the spreading effect
of diffusion. Episodic 'surges' of flux to the poles are induced
by fluctuations in the source eruption rate. Simulations indicate
that relatively weak, trailing polarity surges may occur even in a
steady flow field. However, in order to account for the giant surges
of alternating polarity and the resulting oscillations in the polar
field strength observed during 1980-1982, both accelerated flow and
enhanced eruption rates are required.
---------------------------------------------------------
Title: Two Eruptive Prominences and a CME on 1982FEB9
Authors: Wang, J. L.; Nelson, G. J.; Sheeley, N. R., Jr.; Howard,
R. A.; Koomen, M. J.; Michels, D. J.; Kawabata, K.; Ogawa, H.
1989AcApS...9..260W Altcode: 1989AcApS...9..265W
No abstract at ADS
---------------------------------------------------------
Title: The Effect of Newly Erupting Flux on the Polar Coronal Holes
Authors: Sheeley, N. R.; Wang, Y. -M.; Harvey, J. W.
1989SoPh..119..323S Altcode:
He I 10830 Å images show that early in sunspot cycles 21 and 22,
large bipolar magnetic regions strongly affected the boundaries of the
nearby polar coronal holes. East of each eruption, the hole boundary
immediately contracted poleward, leaving a band of enhanced helium
network. West of the eruption, the boundary remained diffuse and
gradually expanded equatorward into the leading, like-polarity part of
the bipolar magnetic region. Comparisons between these observations
and simulations based on a current-free coronal model suggest that:
The Sun's polar magnetic fields are confined to relatively small caps of
high average field strength, apparently by a poleward meridional flow.
---------------------------------------------------------
Title: Coronal Mass Ejections and Associated X-Ray Flare Durations
Authors: Kahler, S. W.; Sheeley, N. R., Jr.; Liggett, M.
1989ApJ...344.1026K Altcode:
It is found that 22 percent of a sample of M1 or greater impulsive
soft X-ray flares were associated with coronal mass ejections (CMEs)
observed in the Solwind coronagraph. These flares were more energetic
than similar impulsive flares without CMEs, and the associated CMEs were
narrow (5-40 deg) in angular width. A survey of all CMEs associated
with M1 or greater X-ray flares reveals a good correlation between
flare duration CME angular width. The H-alpha characteristics of
impulsive, CME-associated flares suggest that they are not the dynamic
or eruptive flares presumed to be associated with CMEs, but rather,
are confined flares. The H-alpha flare locations are neither centered
under the CME legs. The disparity in size scales between the CMEs and
their associated flares leaves the basis of the correlation between
CME width and X-ray flare duration unresolved.
---------------------------------------------------------
Title: Magnetic Flux Transport on the Sun
Authors: Wang, Y. -M.; Nash, A. G.; Sheeley, N. R., Jr.
1989Sci...245..712W Altcode:
Although most of the magnetic flux observed on the sun originates in
the low-latitude sunspot belts, this flux is gradually dispersed over
a much wider range of latitudes by supergranular convective motions
and meridional circulation. Numerical simulations show how these
transport processes interact over the 11-year sunspot cycle to produce
a strong “topknot” polar field, whose existence near sunspot minimum
is suggested by the observed strength of the interplanetary magnetic
field and by the observed areal extent of polar coronal holes. The
required rates of diffusion and flow are consistent with the decay
rates of active regions and with the rotational properties of the
large-scale solar magnetic field.
---------------------------------------------------------
Title: Average Properties of Bipolar Magnetic Regions during Sunspot
CYCLE-21
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1989SoPh..124...81W Altcode:
We examine the statistical properties of some 2700 bipolar magnetic
regions (BMRs) with magnetic fluxes ≥3 × 10<SUP>20</SUP> Mx which
erupted during 1976-1986. Empirical rules were used to estimate
the fluxes visually from daily magnetograms obtained at the National
Solar Observatory/Kitt Peak. Our analysis shows the following: (i) the
average flux per BMR declined between 1977 and 1985; (ii) the average
tilts of BMRs relative to the east-west line increase toward higher
latitudes; (iii) weaker BMRs had larger root-mean-square tilt angles
than stronger BMRs at all latitudes; (iv) over the interval 1976-1986,
BMRs with their leading poles equatorward of their trailing poles
contributed a total of 4 times as much flux as BMRs with `inverted'
tilts, but the relative amount of flux contributed by BMRs with
inverted or zero tilts increased as the sunspot cycle progressed;
(v) only 4% of BMRs had `reversed' east-west polarity orientations;
(vi) although the northern hemisphere produced far more flux during
the rising phase of the sunspot cycle, the southern hemisphere largely
compensated for this imbalance during the declining phase; (vii)
southern-hemisphere BMRs erupted at systematically higher latitudes
than northern-hemisphere ones through most of sunspot cycle 21.
---------------------------------------------------------
Title: Implications of a Strongly Peaked Polar Magnetic Field
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; DeVore, C. R.
1989SoPh..124....1S Altcode:
Using the flux-transport equation in the absence of sources, we study
the relation between a highly peaked polar magnetic field and the
poleward meridional flow that concentrates it. If the maximum flow
speed ν<SUB>m</SUB> greatly exceeds the effective diffusion speed
κ/R, then the field has a quasi-equilibrium configuration in which the
poleward convection of flux via meridional flow approximately balances
the equatorward spreading via supergranular diffusion. In this case,
the flow speed ν(θ) and the magnetic field B(θ) are related by
the steady-state approximation ν(θ) ≃ (κ/R)B'(θ)/B(θ) over
a wide range of colatitudes θ from the poles to midlatitudes. In
particular, a general flow profile of the form sin<SUP>p</SUP>θ
cos<SUP>q</SUP>θ which peaks near the equator (q ≪ p) will correspond
to a cos<SUP>n</SUP>θ magnetic field at high latitudes only if p =
1 and ν<SUB>m</SUB> = n κ/R. Recent measurements of n ∼ 8 and κ
∼ 600 km<SUP>2</SUP> s<SUP>−1</SUP> would then give ν<SUB>m</SUB>
∼ 7 m s<SUP>−1</SUP>.
---------------------------------------------------------
Title: The Evolution of the Sun's Polar Magnetic Field
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Nash, A. G.
1989BAAS...21..827S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Average Magnetic Properties of Active Regions during Sunspot
Cycle 21
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1989BAAS...21Q.827W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Giant solar arches and coronal mass ejections in November 1980
Authors: Svestka, Zdenek F.; Jackson, Bernard V.; Howard, Russell A.;
Sheeley, Neil R., Jr.
1989SoPh..122..131S Altcode:
Using data from the SOLWIND coronagraph and photometers aboard
HELIOS-A we examine coronal mass ejections from an active region
which produced a series of giant post-flare coronal arches. HXIS
X-ray observations reveal that in several cases underlying flares
did not disrupt these arch structures, but simply revived them,
enhancing their temperature, density and brightness. Thus we are
curious to know how these quasi-stationary X-ray structures could
survive in the corona in spite of recurrent appearances of powerful
dynamic flares below them. We have found reliable evidence that two
dynamic flares which clearly revived the preexisting giant arch were
not associated with any mass ejection. After two other flares, which
were associated with mass ejections, the arch might have been newly
formed when the ejection was over. In one of these cases, however,
the arch had typical characteristics of a revived structure so that
it is likely that it survived a powerful mass ejection nearby. In a
magnetic configuration of the arch which results from potential-field
modelling (Figure 1(b)) such a survival seems possible.
---------------------------------------------------------
Title: Coronal mass ejections and coronal structures.
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Stewart,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1989epos.conf..493H Altcode:
The work of this team was concerned with modelling of post-flare arches,
the reconnection theory of flares, the slow variation of coronal
structure, and the coronal and interplanetary detection, evolution,
and consequences of mass ejections.
---------------------------------------------------------
Title: The solar origin of long-term variations of the interplanetary
magnetic field strength
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1988JGR....9311227W Altcode:
Spacecraft measurements over the past two sunspot cycles have shown
that the average strength of the interplanetary magnetic field (IMF)
undergoes surprisingly modest long-term variation, unlike the total
magnetic flux observed on the Sun. Our attempt to model the IMF during
sunspot cycle 21, based on a current-free extrapolation of the observed
photospheric field out to a fixed source surface where the field lines
become radial, yields calculated IMF intensities which vary by an order
of magnitude and which are far too low near sunspot minimum. We obtain
much better agreement with a model containing both heliospheric sheet
currents, which deflect polar flux toward the ecliptic, and volume
currents, which maintain a residual latitudinal gradient in the IMF
intensity. In order to match the observed IMF intensity levels, however,
the measured photospheric fields had to be scaled up by approximately a
factor of 2. Our composite model has the following main consequences:
(1) The source of the radial component of the IMF may be represented
to a first approximation by the dipole component of the photospheric
field. (2) The radial IMF intensity is strongest in the direction of
the dipole axis, which is aligned with the Sun's rotation axis near
sunspot minimum but tilts toward the ecliptic near sunspot maximum. (3)
The average strength of the photospheric field above latitude 55° is
of order 10 G around sunspot minimum.
---------------------------------------------------------
Title: Mechanisms for the Rigid Rotation of Coronal Holes
Authors: Nash, A. G.; Sheeley, N. R., Jr.; Wang, Y. -M.
1988SoPh..117..359N Altcode:
We show that the rotation of coronal holes can be understood in
terms of a current-free model of the coronal magnetic field, in which
holes are the footpoint locations of open field lines. The coronal
field is determined as a function of time by matching its radial
component to the photospheric flux distribution, whose evolution is
simulated including differential rotation, supergranular diffusion,
and meridional flow. We find that ongoing field-line reconnection allows
the holes to rotate quasi-rigidly with their outer-coronal extensions,
until their boundaries become constrained by the neutral line of
the photospheric field as it winds up to form stripes of alternating
magnetic polarity. This wind-up may be significantly retarded by a
strong axisymmetric field component which forces the neutral line to
low latitudes; it is also gradually halted by the cross-latitudinal
transport of flux via supergranular diffusion and a poleward bulk
flow. We conclude that a strong axisymmetric field component is
responsible for the prolonged rigid rotation of large meridional holes
during the declining phase of the sunspot cycle, but that diffusion and
flow determine the less rigid rotation observed near sunspot maximum,
when the holes corotate with their confining polarity stripes.
---------------------------------------------------------
Title: The Quasi-rigid Rotation of Coronal Magnetic Fields
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Nash, A. G.; Shampine,
L. R.
1988ApJ...327..427W Altcode:
Assuming that the coronal magnetic field can be approximated by
a current-free extension of the photospheric field, the authors
use spherical harmonic analysis and numerical simulations to
study its rotational properties. In the outer corona, they find
that the rotation rate is determined by three principal factors:
1. "Coronal filtering". 2. Global averages of the photospheric
rotation rate. 3. Ongoing source eruptions. These principles allow
to understand the observationally inferred rotational properties of
the outer coronal field. The overall rigidity of the rotation profile
reflects the tendency for the photosphere's non-axisymmetric flux to
be concentrated toward lower latitudes, where the rotational shear
is small; increased curvature and asymmetry occur during the rising
phase of the sunspot cycle because of the presence of higher latitude
flux. The coronal rotation rate shows a progressive acceleration due
to the equatorward migration of sunspots.
---------------------------------------------------------
Title: A Model for Long-Term Variation of Interplanetary Magnetic
Field-Strenght
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
1988BAAS...20..705W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Two eruptive prominences and a CME on February 9, 1982.
Authors: Wang, J. L.; Nelson, G. J.; Sheeley, N. R., Jr.; Howard,
R. A.; Koomen, M. J.; Michels, D. J.; Kawabata, K.; Ogawa, H.
1988sscd.conf..547W Altcode:
Two H-alpha eruptive prominences located at N<SUB>10</SUB>W<SUB>90</SUB>
and S<SUB>14</SUB>W<SUB>90</SUB>, respectively, were observed
simultaneously on 9 February 1982. A CME corresponding to the northern
one was observed by the satellite P<SUB>78-1</SUB>. No CME was seen
for the much larger southern prominence which was associated with an
intense radio burst. A comparison of the two eruptive prominences shows
that the shape of a eruptive prominence and its change with time could
be an important factor in its association with a CME.
---------------------------------------------------------
Title: Energetic interpanetary shocks, radio emission, and coronal
mass ejections
Authors: Cane, H. V.; Sheeley, N. R., Jr.; Howard, R. A.
1987JGR....92.9869C Altcode:
The interplanetary shocks which generate detectable low-frequency
(<1 MHz) radio emission, represent as a group, the most energetic
shocks produced by the sun. For all interplanetary (IP) shocks which
generated so-called IP type II events, we find when observations were
available, that the associated solar events involved fast (>500 km/s)
coronal mass ejections (CMEs). In comparison with the set of all CMEs
detected by the Solwind coronagraph the CMEs associated with IP type
II events are the most massive and energetic. The majority (>50%)
belong to the structural classes described by the Solwind researchers
as “curved front” or “halo.” Evidence presented suggest that these
are the same class viewed from a different persective. Our results are
consistent with there being a close relationship between interplanetary
shocks and fast CMEs.
---------------------------------------------------------
Title: Sunspot - Cycle Variations of the Interplanetary Field
Strength: Implications for Coronal Models
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; DeVore, C. R.
1987BAAS...19.1133W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Mechanism for the Rigid Rotation of Coronal Holes
Authors: Nash, A. G.; Sheeley, N. R., Jr.; Wang, Y. -M.
1987BAAS...19.1133N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar activity and heliosphere-wide cosmic ray modulation
in mid-1982
Authors: Cliver, E. W.; Mihalov, J. D.; Sheeley, N. R., Jr.; Howard,
R. A.; Koomen, M. J.; Schwenn, R.
1987JGR....92.8487C Altcode:
A major episode of flare activity in June and July 1982 was accompanied
by a pair of heliosphere-wide cosmic ray modulation events. In each
case, a large Forbush decrease (FD) at earth was followed in turn
by apparently related decreases at Pioneer 11 (P11) and Pioneer 10
(P10). The Pioneer spacecraft were separated by ~155° in ecliptic
longitude. We reviewed white light coronagraph and near-sun (<=1 AU)
satellite data to identify plausible solar origins of these modulation
events. The first widespread intensity decrease (FD 1) can be attributed
to the combined effects of a backside flare on June 3 from solar active
region 18382/18383, located 23° in ecliptic longitude from Pioneer 10,
and a visible disk flare from 18405 on June 6, when this region was
9° from Pioneer 11. The second widespread modulation event during this
period (FD 2) may be linked to flares from active region 18474 on July
12 and 22. The July 12 flare was located 34° in azimuth from Pioneer
11, and the July 22 flare was 24° from Pioneer 10. Since even fast
shocks would take ~1 month to propagate to Pioneer 11 (12 AU) and ~2
months to reach Pioneer 10 (28 AU) in mid-1982, these “one-to-one”
associations must be regarded with caution. The processes of entrainment
and coalescence can cause a given traveling interplanetary disturbance
to lose its identify enroute to the outer heliosphere. The fact that
we were able to identify plausible solar flare candidates for each of
the four Forbushlike decreases observed at the Pioneer satellites (two
each at P10 and P11), however, removes the need to invoke a chock from
a single flare as the sole cause of either FD 1 (at both P10 and P11)
or FD 2. Such single-flare scenarios have recently been suggested by
several investigators to account for the widespread intensity decreases
in mid-1982. Instead, the heliosphere-wide modulation during this
period appears to result primarily from a sustained episode of powerful
flares from a relatively narrow range of active solar longitude. <P />A
significant fraction (1/2 to 3/4) of the major coronal mass ejections
(CMEs) and near-sun shocks observed during June and July 1982 originated
in flares occurring in a 45° band of Carrington longitude. Because
of solar rotation these flares occur over the full range of ecliptic
longitude and can generate an outward propagating shell of CMEs and
shocks that encompasses the sun to produce the observed azimuthal
symmetry in the cosmic ray modulation. The prolonged high-speed wind
stream at P10 in the second half of 1982 may have resulted, at least
in part, from the coalescence of a series of fast transient streams
directed toward that distant spacecraft into an extended compound
stream.
---------------------------------------------------------
Title: The Origin of Rigidly Rotating Magnetic Field Patterns on
the Sun
Authors: Sheeley, N. R., Jr.; Nash, A. G.; Wang, Y. -M.
1987ApJ...319..481S Altcode:
Using analytical calculations and numerical simulations, it is shown
that a meridional component of magnetic-flux transport will offset
the shearing effect of differential rotation and give rise to rigidly
rotating patterns of large-scale magnetic field. The nonaxisymmetric
field attains a striped polarity pattern which rotates rigidly like a
barber pole while its individual small-scale flux elements rotate at
the differential rate of the latitudes they are crossing. On the sun,
the meridional transport is provided by supergranular diffusion possibly
assisted by a small poleward flow. New sources of flux retard this
process and exclude the rigid rotation from the sunspot belts until
well into the declining phase of the sunspot cycle. This mechanism
accounts for a number of heretofore unexplained phenomena including
the tendency for coronal holes to rotate rigidly during the declining
phase of the sunspot cycle.
---------------------------------------------------------
Title: The Origin of Rigidly Rotating Solar Magnetic Field Patterns
Authors: Nash, A. G.; Sheeley, N. R., Jr.; Wang, Y. -M.
1987BAAS...19..938N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Rotation of Coronal Holes during Sunspot Cycle 21
Authors: Harvey, J.; Sheeley, N. R., Jr.
1987BAAS...19Q.935H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Quasi-Rigid Rotation of Coronal Magnetic Fields
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Nash, A. G.; Shampine,
L. R.
1987BAAS...19..939W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simulations of the Sun's Polar Magnetic Fields during Sunspot
CYCLE-21
Authors: DeVore, C. Richard; Sheeley, Neil R., Jr.
1987SoPh..108...47D Altcode:
Regarding new bipolar magnetic regions as sources of flux,
we have simulated the evolution of the radial component of the
solar photospheric magnetic field during 1976-1984 and derived the
corresponding evolution of the line-of-sight polar fields as seen from
Earth. The observed timing and strength of the polar-field reversal
during cycle 21 can be accounted for by supergranular diffusion alone,
for a diffusion coefficient of 800 km<SUP>2</SUP> s<SUP>-1</SUP>. For
an assumed 300 km<SUP>2</SUP> s<SUP>-1</SUP> rate of diffusion, on the
other hand, a poleward meridional flow with a moderately broad profile
and a peak speed of 10 m s<SUP>-1</SUP> reached at about 5° latitude
is required to obtain agreement between the simulated and observed
fields. Such a flow accelerates the transport of following-polarity flux
to the polar caps, but also inhibits the diffusion of leading-polarity
flux across the equator. For flows faster than about 10 m s<SUP>-1</SUP>
the latter effect dominates, and the simulated polar fields reverse
increasingly later and more weakly than the observed fields.
---------------------------------------------------------
Title: Interpreting Coronal Evolution in Terms of the Eruption and
Transport of Photospheric Magnetic Fields
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Nash, A. G.; Shampine,
L. R.
1987sowi.conf..322S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Correlation of CME Angular Sizes and Soft X-Ray Time
Scales of Solar Eruptive Events
Authors: Kahler, S. W.; Sheeley, N. R., Jr.; Howard, R. A.
1987sowi.conf..232K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Energetic Proton Events and Coronal Mass Ejections Near
Solar Minimum
Authors: Kahler, S. W.; Cliver, E. W.; Cane, H. V.; McGuire, R. E.;
Reames, D. V.; Sheeley, N. R., Jr.; Howard, R. A.
1987ICRC....3..121K Altcode: 1987ICRC...20c.121K; 1987ICRC....3..121C
No abstract at ADS
---------------------------------------------------------
Title: Interplanetary Effects of Coronal Mass Ejections
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Steward,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1986epos.conf.6.52H Altcode: 1986epos.confF..52H
No abstract at ADS
---------------------------------------------------------
Title: Interplanetary shocks preceded by solar filament eruptions
Authors: Cane, H. V.; Kahler, S. W.; Sheeley, N. R., Jr.
1986JGR....9113321C Altcode:
We discuss the solar and interplanetary characteristics of six
interplanetary shock and energetic particle events associated with
the eruptions of solar filaments lying outside active regions. The
events are characterized by the familiar double-ribbon Hα brightenings
observed with large flares, but only very weak soft X ray and microwave
bursts. Both impulsive phases and metric type II bursts are absent in
all six events. The energetic particles observed near the earth appear
to be accelerated predominantly in the interplanetary shocks. The
interplanetary shock speeds are lower and the longitudinal extents
considerably less than those of flare-associated shocks. Three of the
events were associated with unusual enhancements of singly ionized
helium in the solar wind following the shocks. These enhancements
appear to be direct detections of the cool filament material expelled
from the corona. We suggest that these events are part of a spectrum
of solar eruptive events which include both weaker events and large
flares. Despite their unimpressive and unreported solar signatures,
the quiescent filament eruptions can result in substantial space and
geophysical disturbances.
---------------------------------------------------------
Title: Initiations of Coronal Mass Ejections
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Steward,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1986epos.conf.6.27H Altcode: 1986epos.confF..27H
No abstract at ADS
---------------------------------------------------------
Title: The Slowly Varying Corona Near Solar Activity Maximum
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Steward,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1986epos.conf.6.57H Altcode: 1986epos.confF..57H
No abstract at ADS
---------------------------------------------------------
Title: Modelling of Coronal Mass Ejections and POST Flare Arches
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Steward,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1986epos.conf6.366H Altcode: 1986epos.confF.366H
No abstract at ADS
---------------------------------------------------------
Title: Simulations of the Gross Solar Magnetic Field during
SUNSPOT-CYCLE-21
Authors: Sheeley, Neil R., Jr.; DeVore, C. Richard; Shampine, Lauree R.
1986SoPh..106..251S Altcode:
Regarding new bipolar magnetic regions as sources of flux, we
have simulated the evolution of the radial component of the solar
photospheric magnetic field during 1976-1984 with a spatial resolution
of about 34 000 km, and have derived the corresponding evolution of
its absolute value averaged over the visible disk. For nominal values
of the transport parameters, this simulated gross field is in close,
though imperfect, agreement with the observed gross field and its
associated indices of solar activity. By analyzing the response of the
simulated gross field to variations in the transport parameters and the
source properties, we find that the simulated field originates in newly
erupted bipolar regions. The lifetimes of these regions are almost
always less than 3 mo. Consequently, the strength of the simulated
gross field is a measure of the current level of solar activity, and
any recurrent patterns with lifetimes in excess of 6 mo must reflect
the continuing eruption of new flux at `active longitudes' rather than
the persistence of old flux in long-lived magnetic structures.
---------------------------------------------------------
Title: Solar Gradual Hard X-Ray Bursts and Associated Phenomena
Authors: Cliver, E. W.; Dennis, B. R.; Kiplinger, A. L.; Kane, S. R.;
Neidig, D. F.; Sheeley, N. R., Jr.; Koomen, M. J.
1986ApJ...305..920C Altcode:
White-light coronagraph, H-alpha and radio data are presented as well
as hard X-ray data for a sample of 10 gradual hard X-ray bursts (GHBs)
in an attempt to better understand the nature of these events. It is
found that: (1) the hard X-ray photon energy spectrum began to harden
near the onset of the GHBs and continued in this fashion during the
decay phase; (2) a coronal mass ejection (CME) occurred in association
with at least nine of the GHBs; (3) the GHBs occurred in the late phase
of major flares; (4) the centimeter wavelength bursts associated with
the GHBs had relatively low frequency spectral maxima, and in relation
to the observed hard X-ray emission, they were microwave-rich; (5) the
associated decimetric bursts showed significant intensity variations
on time scales ranging from 0.1 to approximately greater than 1 minute;
and (6) the GHBs were most strongly associated with type IV events. It
is concluded that the acceleration and trapping of radiating electrons
occurs in the postflare loop systems following CMEs.
---------------------------------------------------------
Title: Comet 1983 XX (Solwind 6)
Authors: Howard, R.; Koomen, M.; Michels, D.; Sheeley, N.; Marsden,
B. G.
1986IAUC.4229....1H Altcode:
R. Howard, M. Koomen, D. Michels and N. Sheeley, Naval Research
Laboratory, report identification of another comet in the SOLWIND
coronagraph data. The apparent heliocentric separation F (solar radii)
and position angle O (counted from the solar north pole) have been
converted to R.A. and Decl. by the undersigned. On the sixth data
frame the coma was behind the occulting disk. 1983 UT F O R.A. (1950.0)
Decl. Sept. 24.863 8.1 230.5 11 54.4 - 0 50 24.870 7.8 230.4 11 54.8
- 0 49 24.878 7.6 231.2 11 55.0 - 0 47 24.885 7.3 230.6 11 55.3 - 0
47 24.892 7.2 230.5 11 55.5 - 0 47 25.062 < 2.5 231.8 (12 00.9 -
0 32) Preliminary examination of the raw data suggests that SOLWIND
6 was brighter than SOLWIND 2-5 (but certainly not SOLWIND 1). The
tail was still quite bright on Sept. 25.08 and present but fainter on
Sept. 25.14. Careful scrutiny of subsequent data reveals no trace of
the comet reappearing from behind the occulted area. The line of sight
was clearly very nearly in the comet's orbital plane. Computations
by the undersigned suggest that the comet was very probably a member
of the Kreutz group, a representative set of orbital elements being
as follows: T = 1983 Sept. 25.19 ET, Peri. = 78.39, Node = 357.94,
Incl. = 143.95 (equinox 1950.0), q = 0.0076 AU.
---------------------------------------------------------
Title: Properties of Metre-Wavelength Solar Bursts Associated with
Coronal Mass Ejections
Authors: Robinson, R. D.; Stewart, R. T.; Sheeley, N. R., Jr.; Howard,
R. A.; Koomen, J.; Michels, D. J.
1986SoPh..105..149R Altcode:
An investigation is made to determine the relationship between a
coronal mass ejection (CME) and the characteristics of associated
metre-wave activity. It is found that (1) the CME width and leading
edge velocity can be highly influential in determining the intensity,
spectral complexity and frequency coverage of both type II and continuum
bursts; (2) the presence of a CME is possibly a necessary condition for
the production of a metric continuum event and (3) metric continuum
bursts as well as intense, complex type II events are preferentially
associated with strong, long lasting soft X-ray events.
---------------------------------------------------------
Title: The Origin of the 28-DAY to 29-DAY Recurrent Patterns of the
Solar Magnetic Field
Authors: Sheeley, N. R., Jr.; DeVore, C. R.
1986SoPh..104..425S Altcode:
Numerical simulations of the Sun's mean line-of-sight magnetic field
suggest an origin for the 28-to 29-day recurrent patterns of the
field and its associated interplanetary phenomena. The patterns are
caused by longitudinal fluctuations in the eruption of new magnetic
flux, the transport of this flux to mid latitudes by supergranular
diffusion and meridional flow, and the slow rotation of the resulting
flux distributions at the 28- to 29-day periods characteristic of
those latitudes.
---------------------------------------------------------
Title: SOLWIND observations of coronal mass ejections during 1979-1985
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.
1986sfcp.nasa..241S Altcode:
Coronal observations have been processed for parts of each year during
the interval 1979-1985. Around sunspot maximum, coronal mass ejections
(CMEs) occurred at the rate of approximately 2 per day, and had a wide
range of physical and morphological properties. During the recent years
of relatively low sunspot number, CMEs occurred at the rate of only
0.2 per day, and were dominated by the class of so-called streamer
blowout. These special CMEs maintained a nearly constant occurrence
rate of roughly 0.1 per day during the entire interval.
---------------------------------------------------------
Title: Complimentary Aspects of the Mean and Gross Solar Magnetic
Fields
Authors: DeVore, C. R.; Sheeley, N. R., Jr.
1986BAAS...18..710D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Filament Eruptions and Energetic Particle Events
Authors: Kahler, S. W.; Cliver, E. W.; Cane, H. V.; McGuire, R. E.;
Stone, R. G.; Sheeley, N. R., Jr.
1986ApJ...302..504K Altcode:
The 1981 December 5 solar filament eruption that is associated with an
energetic (E greater than 50 MeV) particle event observed at 1 AU. The
eruption was photographed in H-alpha and was observed by the Solwind
whitelight coronagraph on P78-1. It occurred well away from any solar
active region and was not associated with an impulsive microwave burst,
indicating that magnetic complexity and a detectable impulsive phase
are not required for the production of a solar energetic particle (SEP)
event. No metric type II or IV emission was observed, but an associated
interplanetary type II burst was detected by the low-frequency radio
experiment on ISEE 3. The December 5 and two other SEP events lacking
evidence for low coronal shocks had unusually steep energy spectra
(gamma greater than 3.5). In terms of shock acceleration, this suggests
that shocks formed relatively high in the corona may produce steeper
energy spectra than those formed at lower altitudes. It is noted that
the filament itself maybe one source of the ions accelerated to high
energies, since it is the only plausible coronal source of the He(+)
ions observed in SEP events.
---------------------------------------------------------
Title: The Decay of the Mean Solar Magnetic Field
Authors: Sheeley, N. R., Jr.; DeVore, C. R.
1986SoPh..103..203S Altcode:
We have analyzed the effects that differential rotation and a
hypothetical meridional flow would have on the evolution of the Sun's
mean line-of-sight magnetic field as seen from Earth. By winding the
large-scale field into strips of alternating positive and negative
polarity, differential rotation causes the mean-field amplitude to decay
and the mean-field rotation period to acquire the value corresponding to
the latitude of the surviving unwound magnetic flux. For a latitudinally
broad two-sector initial field such as a horizontal dipole, the decay
is rapid for about 5 rotations and slow with a t<SUP>−1/2</SUP>
dependence thereafter. If a poleward meridional flow is present,
it will accelerate the decay by carrying the residual flux to high
latitudes where the line-of-sight components are small. The resulting
decay is exponential with an e-folding time of 0.75 yr (10 rotations)
for an assumed 15 m s<SUP>−1</SUP> peak meridional flow speed.
---------------------------------------------------------
Title: Coronal mass ejection associated with the stationary post-flare
arch of 21 22 May 1980
Authors: McCabe, Marie K.; Švestka, Zdeněk F.; Howard, Russell A.;
Jackson, Bernard V.; Sheeley, Neil R.
1986SoPh..103..399M Altcode:
By using a combination of X-ray (HXIS), Hα (Haleakala), white-light
corona (Solwind), and zodiacal light (Helios) images on 21-22 May, 1980
we demonstrate, and try to explain, the co-existence of a coronal mass
ejection with a stationary post-flare coronal arch. The mass ejection
was seen, both by Solwind and Helios, in prolongation of the path of
a powerful spray, whereas the active region filament did not erupt. A
tentative comparison is made with other occurrences of stationary,
or quasi-stationary post-flare coronal arches.
---------------------------------------------------------
Title: Erratum - Numerical Simulations of Largescale Solar Magnetic
Fields
Authors: DeVore, C. R.; Sheeley, N. R., Jr.; Boris, J. P.; Young,
T. R., Jr.; Harvey, K. L.
1986AuJPh..39..115D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar gradual hard X-ray bursts: Observations and an
interpretation
Authors: Cliver, E. W.; Dennis, B. R.; Kiplinger, A.; Kane, S.;
Neidig, D. F.; Sheeley, N.; Koomen, M.
1986AdSpR...6f.249C Altcode: 1986AdSpR...6..249C
A recent study of solar gradual hard X-ray bursts is summarized. The
data are interpreted in terms of a model involving the acceleration
and trapping of electrons in post flare loop systems following coronal
mass ejections. A controversy about the classification of the metric
continuum that typically accompanies gradual hard X-ray events is
addressed.
---------------------------------------------------------
Title: Coronal mass ejections and coronal structures.
Authors: Hildner, E.; Bassi, J.; Bougeret, J. L.; Duncan, R. A.;
Gary, D. E.; Gergely, T. E.; Harrison, R. A.; Howard, R. A.; Illing,
R. M. E.; Jackson, B. V.; Kahler, S. W.; Kopp, K.; Low, B. C.; Lantos,
P.; Phillips, K. J. H.; Poletto, G.; Sheeley, N. R., Jr.; Stewart,
R. T.; Svestka, Z.; Waggett, P. W.; Wu, S. T.
1986NASCP2439....6H Altcode:
Contents: 1. Introduction. 2. Observations. 3. Initiation of
coronal mass ejections - observations. 4. Modelling of coronal mass
ejections and post-flare arches. 5. Interplanetary effects of coronal
mass ejections. 6. The slowly varying corona near solar activity
maximum. 7. Summary.
---------------------------------------------------------
Title: The Solar Cycle Dependence of Coronal Mass Ejections
Authors: Howard, R. A.; Sheeley, N. R., Jr.; Michels, D. J.; Koomen,
M. J.
1986ASSL..123..107H Altcode: 1986shtd.symp..107H
The Solwind white light coronagraph on P78-1 has been making routine
observations of the solar corona since March 28, 1979. Data from the
1984/1985 time period has just been analyzed. During this interval,
a period of low-solar-activity coronal mass ejections (CMEs) occurred
at the rate of 0.2-0.4/day, in contrast to the rate of 1.8/day during
the period around solar maximum, 1979-1981. The rate of equatorial
CMEs also dropped by the same amount during this period. A class of
CMEs, 'streamer blowouts', occurred at the same rate during the two
epochs. Many of the parameters associated with CMEs, their type, their
angular span, central latitude, mass, speed, and energy, have changed
from their distributions at solar maximum. During the 1984/1985 period,
CMEs are confined to low latitudes, rarely reaching the high latitudes
seen during the maximum years. They are smaller, slower, less massive,
and less energetic.
---------------------------------------------------------
Title: Solwind observations of coronal mass ejections during 1979
- 1985.
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.
1986NASCP2421..241S Altcode:
Around sunspot maximum, coronal mass ejections (CMEs) occurred at the
rate of approximately 2 per day, and had a wide range of physical and
morphological properties. During the recent years of relatively low
sunspot number, CMEs occurred at the rate of only 0.2 per day, and
were dominated by the class of so-called "streamer blowouts." These
special CMEs maintained a nearly constant occurrence rate of roughly
0.1 per day during the entire interval.
---------------------------------------------------------
Title: Simulations of Magnetic-Flux Transport in Solar Active Regions
Authors: DeVore, C. R.; Sheeley, N. R., Jr.; Boris, J. P.; Young,
T. R., Jr.; Harvey, K. L.
1985SoPh..102...41D Altcode:
We simulate the evolution of several observed solar active regions by
solving a transport equation for magnetic flux at the photosphere. The
rates of rotation, meridional flow, and diffusion of the flux are
determined self-consistently in the calculations. Our findings
are in good quantitative agreement with previous measures of the
rotation rate and diffusion constant associated with photospheric
magnetic fields. Although our meridional velocities are consistent
in direction and magnitude with recently reported poleward flows,
relatively large uncertainties in our velocity determinations make
this result inconclusive.
---------------------------------------------------------
Title: Probable Sungrazing Comets
Authors: Sheeley, N. R., Jr.; Howard, R.; Koomen, M.; Michels, D.;
Marsden, B. G.
1985IAUC.4129....1S Altcode:
N. R. Sheeley, Jr., Naval Research Laboratory, reports observations
of two more probable sungrazing comets (cf. IAUC 3640, 3718, 3719)
in the coronagraphic data from the P78-1 SOLWIND satellite. R. Howard,
M. Koomen and D. Michels were also involved with these observations of
what are tentatively called SOLWIND 4 and 5, and the Central Bureau
in fact received the provisional data below some months before the
deliberate destruction of the satellite on Sept. 13. Improved positions
are anticipated for all five objects. The apparent heliocentric
separation F (solar radii) and position angle O have been converted
to R.A. and Decl. by the undersigned. SOLWIND 4 UT F O R.A. (1950.0)
Decl. 1981 Nov. 3.999 10.5 205.0 14 29.5 -17 42 4.038 9.7 205.0 14
30.0 -17 31 4.105 8.4 203.0 14 31.2 -17 15 4.171 7.2 200.6 14 32.3
-17 01 4.238 5.8 197.1 14 33.5 -16 43 4.304 4.6 195.5 14 34.3 -16
26 4.371 3.3? 188.5? 14 35.4 -16 08 SOLWIND 5 UT F O R.A. (1950.0)
Decl. 1984 July 28.302 6.7 243.4 8 22.4 +18 15 28.309 6.6 243.8 8 22.5
+18 16 28.316 6.4 244.2 8 22.7 +18 18 28.324 6.4 244.9 8 22.7 +18 19
28.331 5.9 246.0 8 23.2 +18 24 28.368 4.8 247.6 8 24.4 +18 32 28.375
4.6 248.3 8 24.6 +18 34 28.383 4.4 249.2 8 24.8 +18 36 28.390 4.2 250.2
8 25.0 +18 38 28.397 3.9 250.8 8 25.3 +18 41 28.435 2.7 258.5 8 26.6
+18 52 28.443 2.7 259.0 8 26.6 +18 52 Computations by the undersigned
suggest that SOLWIND 5 is a member of the Kreutz group with T = 1984
July 28.48 ET, Peri. = 62.28, Node = 337.30, Incl. = 139.14 (equinox
1950.0), q = 0.0044 AU. Of all the comets SOLWIND 4 is least likely
to belong to the Kreutz group, and the following very hypothetical
elements have been derived: T = 1981 Nov. 4.6 ET, Peri. = 97.2, Node =
25.6, Incl. = 113.7, q = 0.008 AU.
---------------------------------------------------------
Title: Coronal mass ejections: 1979-1981
Authors: Howard, R. A.; Sheeley, N. R., Jr.; Michels, D. J.; Koomen,
M. J.
1985JGR....90.8173H Altcode:
In an examination of the Solwind coronagraph images obtained during
the interval March 28, 1979, to December 31, 1981, we have identified
998 coronal mass ejections and recorded their structural classes,
central latitudes, latitudinal spans, speeds, excess brightnesses, and
relative importances. A statistical analysis revealed the following
general results. (1) The properties of coronal mass ejections (CMEs)
depended strongly on their structure. Curved front, halo, and complex
CMEs were the most energetic, and single spike, streamer blowout, and
diffuse fan CMEs were the least energetic. CMEs occurred over a wide
range of position angles, broadly centered on the equator, and had an
average angular span of 45°. The leading edge moved at an average
of approximately 470 km/s, and the average ejected mass and kinetic
energy were 4.1×10<SUP>15</SUP> g and 3.5×10<SUP>30</SUP> erg,
respectively. The average CME proton flux at the equator at 1 AU was
2.2×10<SUP>7</SUP> cm<SUP>-2</SUP> s<SUP>-1</SUP> or approximately 5%
of the measured in situ flux during 1971-1976. (2) During 1979-1981,
the average occurrence rate was 1.8/day for all CMEs, 0.9/day for
“major” CMEs, and 0.15/day for all CMEs that crossed the equator and
had an angular span of at least 45°. (3) The temporal variations in
the CME occurrence rate did not show an obvious persistent relation to
the variations in the sunspot number on time scales ranging from 7 to
180 days. During 1979-1981 the maximum in the 180-day average CME rate
peaked in the second half of 1980, whereas the 180-day average sunspot
number peaked during the firt half of 1980. <P />The 180-day average
rate of fast CMEs (speeds of at least 800 km/s) had a monotonic increase
that seemed to be more closely associated with the occurrence rate of
large solar flares than with the variation of the sunspot number.
---------------------------------------------------------
Title: Simulations of the Mean Solar Magnetic Field during Sunspot
CYCLE-21
Authors: Sheeley, N. R., Jr.; DeVore, C. R.; Boris, J. P.
1985SoPh...98..219S Altcode:
Regarding new bipolar magnetic regions as sources of flux, we have
computed the evolution of the photospheric magnetic field during
1976-1984 and derived the corresponding evolution of the mean
line-of-sight field as seen from Earth. We obtained a good, but
imperfect, agreement between the observed mean field and the field
computed for a nominal choice of flux transport parameters. Also,
we determined the response of the computed mean field to variations
in the transport parameters and the source properties. The results
lead us to regard the mean-field evolution as a random-walk process
with dissipation. New eruptions of flux produce the random walk,
and together differential rotation, meridional flow (if present), and
diffusion provide the dissipation. The net effect of each new source
depends on its strength and orientation (relative to the strength and
orientation of the mean field) and on the time elapsed before the next
eruption (relative to the decay time of the field). Thus the mean field
evolves principally due to the contributions of the larger sources,
which produce a strong, gradually evolving field near sunspot maximum
but a weak, sporadically evolving field near sunspot minimum.
---------------------------------------------------------
Title: White Light and Radio Sounding Observations of Coronal
Transients
Authors: Bird, M. K.; Volland, H.; Howard, R. A.; Koomen, M. J.;
Michels, D. J.; Sheeley, N. R., Jr.; Amstrong, J. W.; Seidel, B. L.;
Stelzried, C. T.; Woo, R.
1985SoPh...98..341B Altcode:
A concerted search for coronal transients was conducted with the
`Solwind' coronagraph during the solar occultations of the two
Helios spacecraft in October/November 1979. The polarization angle
and bandwidth of the linearly polarized S-band downlink signal were
monitored at the three 64-m tracking stations of the NASA Deep
Space Network to determine coronal Faraday rotation and spectral
broadening. A one-to-one correspondence could be established between
abrupt disturbances in the two signal parameters and the passage of
a white-light transient through the signal ray path from spacecraft
to Earth. The white-light morphology and the additional information
provided by the radio sounding coverage are presented for each of the
five distinct events recorded. Although no specific example could
be observed in sufficient detail in both white light and Faraday
rotation to derive the small-scale magnetic structure, some qualitative
descriptions of the orientation and rough estimates of the magnitude
of the transient magnetic field could be made.
---------------------------------------------------------
Title: Energetic Protons from a Disappearing Solar Filament
Authors: Kahler, S. W.; Cliver, E. W.; Cane, H. V.; McGuire, R. E.;
Stone, R. G.; Sheeley, N. R., Jr.
1985ICRC....4...94K Altcode: 1985ICRC...19d..94K
A solar energetic (E 50 MeV) particle (SEP) event observed at 1 AU began
about 15000 UT on 1981 December 5. This event was associated with a fast
coronal mass ejection observed with the Solwind coronagraph on the P78-1
satellite. No metric type 2 or type 4 burst was observed, but a weak
interplanetary type 2 burst was observed with the low frequency radio
experiment on the International Sun-Earth Explorer-3 satellite. The mass
ejection was associated with the eruption of a large solar quiescent
filament which lay well away from any active regions. The eruption
resulted in an H alpha double ribbon structure which straddled the
magnetic inversion line. No impulsive phase was obvious in either
the H alpha or the microwave observations. This event indicates that
neither a detectable impulsive phase nor a strong or complex magnetic
field is necessary for the production of energetic ions.
---------------------------------------------------------
Title: Helios spacecraft and earth perspective observations of three
looplike solar mass ejection transients
Authors: Jackson, B. V.; Howard, R. A.; Sheeley, N. R., Jr.; Michels,
D. J.; Koomen, M. J.; Illing, R. M. E.
1985JGR....90.5075J Altcode:
Three looplike mass ejection transients observed from earth with the
SOLWIND coronagraph and the solar maximum mission coronagraph are
imaged by Helios spacecraft zodiacal light photometers. Because the
Helios spacecraft are not earth orbiting, views of these ejections
from the two perspectives allow conclusions to be drawn about their
three-dimensional shapes. The mass ejection of May 24, 1979, in
Helios data is concentrated in an outer structure followed by bright
features separated by a region of depleted material. The ejections of
June 18 and 29, 1980, appear restricted in position angle in Helios
observations to less and the same, respectively, as in coronagraph
observations. The observations imply that the ejections essentially
retain their basic structure and speed out to heights (0.2-0.4 AU)
observed by the Helios spacecraft.
---------------------------------------------------------
Title: The Frequency of Long-Duration Solar X-Ray Events
Authors: Koomen, M. J.; Sheeley, N. R., Jr.; Howard, R. A.; Michels,
D. J.
1985SoPh...97..375K Altcode:
It is shown that the long-duration X-ray events tend to ignore the
sunspot cycle. This is particularly true for events with durations of
6 hr or more.
---------------------------------------------------------
Title: High-resolution X-ray spectra of solar flares. VII - A
long-duration X-ray flare associated with a coronal mass ejection
Authors: Kreplin, R. W.; Doschek, G. A.; Feldman, U.; Sheeley, N. R.,
Jr.; Seely, J. F.
1985ApJ...292..309K Altcode:
It has been recognized that very long duration X-ray events
(lasting several hours) are frequently associated with coronal mass
ejection. Thus, Sheeley et al. (1983) found that the probability of the
occurrence of a coronal mass ejection (CME) increases monotonically
with the X-ray event duration time. It is pointed out that the
association of long-duration, or long-decay, X-ray events (LDEs) with
CMEs was first recognized from analysis of solar images obtained by
the X-ray telescopes on Skylab and the Naval Research Laboratory (NRL)
slitless spectroheliograph. Recently high-resolution Bragg crystal
X-ray spectrometers have been flown on three spacecraft, including the
Department of Defense P78-1 spacecraft, the NASA Solar Maximum Mission
(SMM), and the Japanese Hinotori spacecraft. In the present paper,
P78-1 X-ray spectra of an LDE which had its origin behind the solar
west limb on November 14, 1980 is presented. The obtained data make
it possible to estimate temperatures of the hottest portion of the
magnetic loops in which the emission arises.
---------------------------------------------------------
Title: A comparison of solar helium-3-rich events with type II bursts
and coronal mass ejections
Authors: Kahler, S.; Reames, D. V.; Sheeley, N. R., Jr.; Howard,
R. A.; Michels, D. J.; Koomen, M. J.
1985ApJ...290..742K Altcode:
The authors ask whether the energetic particles of <SUP>3</SUP>He-rich
events are accelerated in the same process as that resulting in
particles of normal-abundance events. They first present a list
of 66 <SUP>3</SUP>He-rich events observed with the Goddard Space
Flight Center particle detector on ISEE 3. It is then shown that
these events are not statistically associated with either of the two
common signatures of normal-abundance events, metric type II bursts
and coronal mass ejections. This indicates that enhanced abundance
events may be produced only in the impulsive phases of flares, while
normal abundance events are produced in subsequent flare shock waves.
---------------------------------------------------------
Title: The Mass Distribution of Coronal Mass Ejections
Authors: Jackson, B. V.; Howard, R. A.; Koomen, M. J.; Michels, D. J.;
Sheeley, N. R., Jr.
1985BAAS...17..636J Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Stochastic Model for the Mean Solar Magnetic Field During
the Sunspot Cycle
Authors: DeVore, C. R.; Sheeley, N. R., Jr.
1985BAAS...17..642D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Numerical Simulations of the Mean Solar Magnetic Field During
the Sunspot Cycle
Authors: Sheeley, N. R., Jr.; DeVore, C. R.
1985BAAS...17R.642S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Doppler scintillation observations of interplanetary shocks
within 0.3 AU
Authors: Woo, Richard; Armstrong, J. W.; Sheeley, N. R.; Howard,
R. A.; Koomen, M. J.; Michels, D. J.; Schwenn, R.
1985JGR....90..154W Altcode:
Near-sun spacecraft Doppler scintillation observations have been
combined with Solwind coronagraph and Helios 1 plasma measurements to
provide more definitive measurements of the evolution and propagation
of interplanetary shock waves between the sun and earth orbit than
have been available from previous observations. This study shows that
substantial deceleration of fast shocks (shock speeds exceeding 1000 km
s<SUP>-</SUP><SUP>1</SUP>) takes place near the sun and that the amount
of deceleration increases with shock speed. This is consistent with
the significantly lower and rather narrow range of shock velocities
observed by direct spacecraft near earth orbit. When coronal mass
ejection (CME) speeds are available for the fast shocks, they are
considerably lower than the speeds measured farther out but near the
sun. This implies that either the fast shocks first accelerate before
decelerating on their way out from the sun (assuming the CME front
is identified with the shock) or the CME speeds do not represent and
substantially underestimate the shock speeds in the outer corona. If
the CME speeds underestimate the shock speeds of the fast shocks, they
do not appear to do so for the slow shocks. If the shocks are being
driven over distances indicated by the acceleration region or to the
point where deceleration begins, then their velocity profiles imply
that the slower shocks are being driven farther out than the faster
shocks. The analysis of one piston-driven shock shows the velocity of
the contact surface is about 0.58 that of the shock front velocity.
---------------------------------------------------------
Title: Numerical simulations of large-scale solar magnetic fields
Authors: DeVore, C. R.; Boris, J. P.; Young, T. R., Jr.; Sheeley,
N. R.; Harvey, K. L.
1985AuJPh..38..999D Altcode:
The authors have solved numerically a transport equation which describes
the evolution of the large-scale magnetic field of the Sun. Data
derived from solar magnetic observations are used to initialize the
computations and to account for the emergence of new magnetic flux
during the sunspot cycle. The authors' objective is to assess the
ability of the model to reproduce the observed evolution of the field
patterns. They discuss recent results from simulations of individual
active regions over a few solar rotations and of the magnetic field
of the Sun over sunspot cycle 21.
---------------------------------------------------------
Title: Coronal mass ejections and interplanetary shocks
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Michels, D. J.; Koomen,
M. J.; Schwenn, R.; Muehlhaeuser, K. H.; Rosenbauer, H.
1985JGR....90..163S Altcode:
A comparison between Solwind observations of coronal mass ejections
(CME's) and Helios 1 observations of interplanetary shocks during
1979-1982 indicates that 72% of the shocks were associated with large,
low-latitude mass ejections on the nearby limb. Most of the associated
CME's has speeds in excess of 500 km/s, but some of them had speeds in
the range 200-400 km/s. An additional 26% of the shocks may have been
associated with CME's, but we were less confident of these associations
because the sizes and locations of the CME's did not seem appreciably
different from those of the numerous CME's without Helios shocks. Only
2% of the shocks clearly lacked CME's. As the average level of sunspot
activity declined during 1982, the shock frequency also declined, but
the observed shocks and some of their associated CME's had unusually
high speeds well in excess of 1000 km/s.
---------------------------------------------------------
Title: Characteristics of coronal mass ejections associated with
solar frontside and backside metric type II bursts
Authors: Kahler, S. W.; Cliver, E. W.; Sheeley, N. R.; Howard, R. A.;
Koomen, M. J.; Michels, D. J.
1985JGR....90..177K Altcode:
We compare fast (v>=500 km s<SUP>-</SUP><SUP>1</SUP>) coronal
mass ejections (CME's) with reported metric type II bursts to study
the properties of CME's associated with coronal shocks. We confirm
an earlier report of fast frontside CME's with no associated metric
type II bursts and calculate that 33+/-15% of all fast frontside CME's
are not associated with such bursts. Faster CME's are more likely to
be associated with type II bursts, as expected from the hypothesis
of piston-driven shocks. However, CME brightness and associated
peak 3-cm burst intensity are also important factors, as might be
inferred from the Wagner and MacQueen (1983) view of type II shocks
decoupled from associated CME's. We use the equal visibility of solar
frontside and backside CME's to deduce the observability of backside
type II bursts. We calculate that 23+/-7% of all backside type II bursts
associated with fast CME's can be observed at the earth and that 13+/-4%
of all type II bursts originate in backside flares. CME speed again
is the most important factor in the observability of backside type
II bursts.
---------------------------------------------------------
Title: Associations beteen coronal mass ejections and solar energetic
proton events
Authors: Kahler, S. W.; Sheeley, N. R., Jr.; Howard, R. A.; Michels,
D. J.; Koomen, M. J.; McGuire, R. E.; von Rosenvinge, T. T.; Reames,
D. V.
1984JGR....89.9683K Altcode:
We have used data from the Naval Research Laboratory (NRL) white
light coronograph on the P78-1 spacecraft and energetic (E>4 MeV)
proton data from the Goddard Space Flight Center (GSFC) detectors
on the IMP 8 and ISEE 3 spacecraft to investigate the association
between proton events originating in flares and coronal mass ejections
(CME's). The primary data were 50 prompt proton events observed between
April 1979 and February 1982 for which reduced coronograph data were
available. H alpha flares could be confidently associated with 27 of
these events, and in 26 of these 27 cases an associated CME was found,
indicating a high but not perfect association of prompt proton events
with CME's. Peak proton fluxes correlate with both the speeds and the
angular sizes of the associated CME's. We show that the CME speeds do
not significantly correlate with CME angular sizes, so that the peak
proton fluxes are correlated with two independent CME parameters. With
larger angular sizes, CME's are more likely to be loops and fans
rather than jets and spikes and are more likely to intersect the
ecliptic. Which of these factors is important to the peak proton flux
correlation cannot be determined from the data. We find weak evidence
that steeper proton spectra are associated with faster and wider
CME's. Two of the 50 proton events of the study and two additional
events, all with no associated CME's share common characteristics:
relatively short duration (~1) day proton events with low fluxes,
parent flares with short (~10 min) soft x ray duration, close magnetic
connection to the earth, and gamma ray and metric type II emission.
---------------------------------------------------------
Title: Coronal Mass Ejections and Sudden Filament Disappearances
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Harvey, K. L.
1984BAAS...16..930S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Characteristics of Flares Producing Metric Type-II Bursts
and Coronal Mass Ejections
Authors: Kahler, S.; Sheeley, N. R., Jr.; Howard, R. A.; Koomen,
M. J.; Michels, D. J.
1984SoPh...93..133K Altcode:
We attempt to study the origin of coronal shocks by comparing several
flare characteristics for two groups of flares: those with associated
metric type II bursts and coronal mass ejections (CMEs) and those
with associated metric type II bursts but no CMEs. CMEs accompany
about 60% of all flares with type II bursts for solar longitudes
greater than 30°, where CMEs are well observed with the NRL Solwind
coronagraph. Hα flare areas, 1-8 Å X-ray fluxes, and impulsive 3 cm
fluxes are all statistically smaller for events with no CMEs than for
events with CMEs. It appears that both compact and large mass ejection
flares are associated with type II bursts. The events with no CMEs
imply that at least many type II shocks are not piston-driven, but
the large number of events of both groups with small 3 cm bursts does
not support the usual assumption that type II shocks are produced by
large energy releases in flare impulsive phases. The poor correlation
between 3 cm burst fluxes and the occurrence of type II bursts may be
due to large variations in the coronal Alfvén velocity.
---------------------------------------------------------
Title: The concentration of the large-scale solar magnetic field by
a meridional surface flow
Authors: DeVore, C. R.; Boris, J. P.; Sheeley, N. R., Jr.
1984SoPh...92....1D Altcode:
We calculate analytical and numerical solutions to the magnetic flux
transport equation in the absence of new bipolar sources of flux, for
several meridional flow profiles and a range of peak flow speeds. We
find that a poleward flow with a broad profile and a nominal 10 m
s<SUP>−1</SUP> maximum speed concentrates the large-scale field
into very small caps of less than 15° half-angle, with average field
strengths of several tens of gauss, contrary to observations. A flow
which reaches its peak speed at a relatively low latitude and then
decreases rapidly to zero at higher latitudes leads to a large-scale
field pattern which is consistent with observations. For such a flow,
only lower latitude sunspot groups can contribute to interhemispheric
flux annihilation and the resulting decay and reversal of the polar
magnetic fields.
---------------------------------------------------------
Title: Associations between coronal mass ejections and metric type
II bursts
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Michels, D. J.; Robinson,
R. D.; Koomen, M. J.; Stewart, R. T.
1984ApJ...279..839S Altcode:
A statistical comparison of metric type II bursts and coronal
mass ejections (CMEs) during 1979-1982 was carried out. Type II
bursts without CMEs were associated with short-lived (0.5 hr) soft
X-ray events, but not with interplanetary shocks at the Helios 1
spacecraft. Type II bursts with CMEs were associated with longer-lived
X-ray events (3 hr on the average) and interplanetary shocks, and
the CMEs had speeds greater than 400 km/s. CMEs without metric type
II bursts were divided equally into groups faster and slower than 455
km/s. The faster CMEs were associated with interplanetary shocks, some
of which originated on the visible disk where metric type II bursts
should have been observed if they had occurred. These results suggest
that (1) shocks without CMEs have a relatively impulsive origin and may
die out sooner than many shocks with CMEs which are piston driven, and
(2) either some fast CMEs do not reach shock-producing super-Alfvenic
speeds until they leave the lower corona where the metric emission
originates, or these CMEs form shocks that are unable to excite type
II emission in the lower corona.
---------------------------------------------------------
Title: The Propagation of Transient Solar Events
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Michels, D. J.; Koomen,
M. J.
1984BAAS...16Q.454S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Temporal Variation of Coronal Mass Ejections During
1979-1981
Authors: Howard, R. A.; Sheeley, N. R., Jr.; Michels, D. J.; Koomen,
M. J.
1984BAAS...16..454H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Energetic Proton Events Unassociated with Coronal
Mass Ejections
Authors: Kahler, S.; Evenson, P.; McGuire, R. E.; Reames, D. V.;
von Rosenvinge, T. T.; Sheeley, N. R., Jr.; Howard, R. A.; Koomen,
M. J.; Michels, D. J.
1984BAAS...16..453K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Synoptic observations of coronal transients and their
interplanetary consequences
Authors: Michels, D. J.; Sheeley, N. R., Jr.; Howard, R. A.; Koomen,
M. J.; Schwenn, R.; Mulhauser, K. H.; Rosenbauer, H.
1984AdSpR...4g.311M Altcode: 1984AdSpR...4..311M
A small coronagraph has been placed in orbit to monitor the sun's
outer corona from 2.5 to 10.0 solar radii, and five years of nearly
continuous synoptic observations have now been completed. Rapid and
sensitive image processing techniques have been developed to screen
the data for transient phenomena, particularly coronal mass ejections
(CMEs). About 50,000 coronal images have been examined, out of a
five-year total of 68,000, and a standardized listing of more than 1,200
coronal transients for the period 1979-1982 has been prepared. These
data have been analysed in the light of other available information,
particularly on conditions in the interplanetary plasma. The dynamical
characteristics of the active corona, as they are beginning to emerge
from the data, are presented. We find that coronal mass ejections
exercise significant influence on the interplanetary solar wind. They
are the source of disturbances that are frequent and energetic,
that tend to be somewhat focussed, that often reach shock intensity,
and that propagate to large heliocentric distances, sometimes causing
major geomagnetic storms.
---------------------------------------------------------
Title: Evolution of solar magnetic flux.
Authors: Boris, J. P.; DeVore, C. R.; Golub, L.; Howard, R. F.; Low,
B. C.; Sheeley, N. R., Jr.; Simon, G. W.; Tsinganos, K. C.
1984NASRP1120....3B Altcode:
Contents: Introduction. Appearance of magnetic flux: models for flux
emergence, unexplained observations. Dynamics of surface magnetic
flux: magnetic flux transport, magnetic flux structure. Disappearance
of magnetic flux: theoretical considerations, observations of flux
disappearance. Summary.
---------------------------------------------------------
Title: Coronal transients and their interplanetary effects.
Authors: Hundhausen, A. J.; Burlaga, L. F.; Feldman, W. C.; Gosling,
J. T.; Hildner, E.; House, L. L.; Howard, R. A.; Krieger, A. S.;
Kundu, M. R.; Low, B. C.; Sheeley, N. R., Jr.; Steinolfson, R. S.;
Stewart, R. T.; Stone, R. G.; Wu, S. T.
1984NASRP1120....6H Altcode:
Contents: 1. Introduction. 2. Background material: Ancient history -
solar flares and geomagnetic storms. Modern history - interplanetary
shock waves. Coronal transients or mass ejections. 3. The present:
Theoretical models. New observations of coronal mass ejections. 4. The
future: Solar origins. Interplanetary effects.
---------------------------------------------------------
Title: Evidence for Directivity of Coronal Transients
Authors: Michels, D. J.; Howard, R. A.; Sheeley, N. R., Jr.; Koomen,
M. J.
1984sii..conf..319M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Long-term synoptic observations of Solar coronal mass ejections
Authors: Michels, D. J.; Howard, R. A.; Koomen, M. J.; Sheeley, N. R.
1984stp..conf..367M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Gle-Associated Flare of 21 August 1979
Authors: Cliver, E. W.; Kahler, S. W.; Cane, H. V.; Koomen, M. J.;
Michels, D. J.; Howard, R. A.; Sheeley, N. R., Jr.
1984sii..conf..205C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The statistical properties of coronal mass ejections during
1979-1981
Authors: Howard, R. A.; Sheeley, N. R.; Koomen, M. J.; Michels, D. J.
1984AdSpR...4g.307H Altcode: 1984AdSpR...4..307H
The Solwind coronagraph on the P78-1 earth-orbiting satellite has been
monitoring the Sun routinely at 10-minute intervals during the 5-year
interval from April, 1979 to the present. In a statistical analysis of
about 1000 mass ejections observed through the end of 1981, we find an
average occurrence rate of 1.8 mass ejections per day. Histograms of
speed, central latitude, angular span, brightness, and other parameters
have been constructed, and properties such as shape classification have
been tabulated. These characteristics are summarized for these years
near sunspot maximum. The average speed and mass estimate are found to
be similar to those found at the declining phase of the previous sunspot
cycle. The angular span and central latitude distributions are quite
different than seen during the declining phase, and are very dependent
upon structural class. The fluctuations in the occurrence rate of CMEs
does not seem to match the fluctuations in the sunspot number. There
is a tendency for high speed CMEs to occur more frequently in 1981
than in 1980, and more frequently in 1980 than in 1979.
---------------------------------------------------------
Title: Radio and Visible Light Observations of a Coronal Arcade
Transient
Authors: Gergely, T. E.; Kundu, M. R.; Erksine, F. T., III; Sawyer, C.;
Wagner, W. J.; Illing, R.; House, L. L.; McCabe, M. K.; Stewart, R. T.;
Nelson, G. J.; Koomen, M. J.; Michels, D.; Howard, R.; Sheeley, N.
1984SoPh...90..161G Altcode:
We discuss simultaneous visible-light and radio observations of
a coronal transient that occurred on 9 April, 1980. Visible-light
observations of the transient and the associated erupting prominence
were available from the Coronagraph/Polarimeter carried aboard SMM,
the P78-1 coronagraph, and from the Haleakala Observatory. Radio
observations of the related type III-II-IV bursts were available from
the Clark Lake and Culgoora Observatories. The transient was extremely
complex; we suggest that an entire coronal arcade rather than just a
single loop participated in the event. Type III burst sources observed
at the beginning of the event were located along a nearby streamer,
which was not disrupted, but was displaced by the outmoving loops. The
type II burst showed large tangential motion, but unlike such sources
usually do, it had no related herringbone structure. A moving type
IV burst source can be associated with the most dense feature of the
white-light transient.
---------------------------------------------------------
Title: Coronal Transients Observed during Solar Occultation of the
HELIOS Spacecraft in STIP Interval VIII
Authors: Bird, M. K.; Volland, H.; Howard, R. A.; Koomen, M. J.;
Michels, D. J.; Sheeley, N. R., Jr.; Armstrong, J. W.; Seidel, B. L.;
Stelzried, C. T.; Woo, R.
1984sii..conf..101B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The GLE-associated flare of 21 August, 1979
Authors: Cliver, E. W.; Kahler, S. W.; Cane, H. V.; Koomen, M. J.;
Michels, D. J.; Howard, R. A.; Sheeley, N. R., Jr.
1983SoPh...89..181C Altcode:
We use a variety of ground-based and satellite measurements to identify
the source of the ground level event (GLE) beginning near 06∶30 UT on
21 August, 1979 as the 2B flare with maximum at ∼06∶15 UT in McMath
region 16218. This flare differed from previous GLE-associated flares
in that it lacked a prominent impulsive phase, having a peak ∼9 GHz
burst flux density of only 27 sfu and a ≳20 keV peak hard X-ray flux
of ≲3 × 10<SUP>-6</SUP> ergs cm<SUP>-2</SUP>s<SUP>-1</SUP>. Also,
McMath 16218 was magnetically less complex than the active regions in
which previous cosmic-ray flares have occurred, containing essentially
only a single sunspot with a rudimentary penumbra. The flare was
associated with a high speed (≳700 km s<SUP>-1</SUP>) mass ejection
observed by the NRL white light coronagraph aboard P78-1 and a shock
accelerated (SA) event observed by the low frequency radio astronomy
experiment on ISEE-3.
---------------------------------------------------------
Title: Associations between coronal mass ejections and interplanetary
shocks.
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.; Schwenn, R.; Muhlhauser, K. H.; Rosenbauer, H.
1983NASCP.2280.693S Altcode: 1983sowi.conf..693S
The authors are in the process of comparing nearly continuous
complementary coronal observations and interplanetary plasma
measurements for the years 1979 - 1982. Their preliminary results
show that almost all low-latitude high-speed coronal mass ejections
(CME's) were associated with shocks at HELIOS 1. Some suitably directed
low-speed CME's were clearly associated with shocks while others may
have been associated with disturbed plasma (such as NCDE's) without
shocks. A few opposite-hemisphere CME's associated with great flares
also seemed to have been associated with shocks at HELIOS.
---------------------------------------------------------
Title: Associations between coronal mass ejections and soft X-ray
events
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.
1983ApJ...272..349S Altcode:
The association between white light coronal mass ejections (CMEs)
and full-disk X ray events have been examined as a function of
X ray duration during the recent years of high sunspot activity
(1979-1981). On a time scale of hours, no duration interval has been
found that separates X ray events into two distinct classes depending
on whether or not they have associated CMEs. Rather, the tendency for
long-duration X ray events to have associated CMEs reflects the fact
that, as X ray duration increases, the differential distribution of
events without CMEs falls off faster than the distriution of X ray
events with CMEs.
---------------------------------------------------------
Title: The Correlation of Coronal Mass Ejections with Energetic
Flare Proton Events
Authors: Kahler, S. W.; McGuire, R. E.; Reames, D. V.; von Rosenvinge,
T. T.; Sheeley, N. R., Jr.; Howard, R. A.; Michels, D. J.; Koomen,
M. J.
1983ICRC....4....6K Altcode: 1983ICRC...18d...6K
Proton events of energies of at least 4 MeV presumed due to solar
flares are compared with coronal mass ejections (CMEs) observed with
an orbiting coronagraph. H alpha flares are associated with 27 of
the 50 flare proton events of the study. Each of these 27 flares
is then associated temporally and spatially with a CME, confirming
the earlier conclusion, based on Skylab data, that a CME may be a
necessary condition for a flare proton event. Peak 4-22 MeV proton
fluxes correlate with both the speeds and the angular sizes of the
associated CMEs. CMEs of larger angular sizes are more likely to
be loops or fans rather than jets or spikes and are more likely to
intersect the ecliptic.
---------------------------------------------------------
Title: Helios Images of a Coronal Mass Ejection Transient
Authors: Jackson, B. V.; Howard, R. A.; Koomen, M. J.; Michels, D. J.;
Sheeley, N. R.
1983BAAS...15..705J Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Major Mass Ejection Rate From Three Space Coronagraphs
Authors: Sawyer, C.; Howard, R.; Sheeley, N.; Koomen, M.; Michels, D.
1983BAAS...15..706S Altcode: 1983BAAS...15..683M
No abstract at ADS
---------------------------------------------------------
Title: On the Nature of Coronal Mass Ejections During the Period 28
March 1979 through 30 June 1981
Authors: Howard, R. A.; Sheeley, N. R., Jr.; Koomen, M. J.; Michels,
D. J.
1983BAAS...15R.703H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The solar corona on 31 July, 1981
Authors: Fisher, R. R.; Lacey, L. R.; Rock, K. A.; Yasukawa, E. A.;
Sheeley, N. R., Jr.; Michels, D. J.; Howard, R. A.; Koomen, M. J.;
Bagrov, A.
1983SoPh...83..233F Altcode:
Various instruments were used to observe the solar corona near or at
the time of total eclipse, 31 July, 1981. The High Altitude Observatory
(HAO) coronal eclipse camera and the MK-III K-coronameter recorded the
lower portions of the corona; the distribution of white light material
above 3 R<SUB>⊙</SUB> was observed with the Naval Research Laboratory
(NRL) satellite coronagraph on P78-1. These data sets are used to
describe coronal structure and to identify coronal active regions. The
polar coronal holes, as developed at this time in the solar cycle, were
offset from the poles of rotation; both were seen displaced eastward
on eclipse day. High latitude streamers appear in all three data sets,
extending from the base of the corona outward to at least eight solar
radii from Sun center. At least two transients were observed by the NRL
experiment on the eclipse day, but it is likely that no transient was in
progress during any observation along the eclipse path. A distribution
of the white-light corona, derived from synoptic K-coronameter data,
is given.
---------------------------------------------------------
Title: Meridional Flows and Magnetic Flux Transport on the Sun
Authors: Boris, J. P.; Sheeley, N. R., Jr.; Young, T. R., Jr.; DeVore,
C. R.; Harvey, K. L.
1983BAAS...15R.701B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal Mass Ejections and Interplanetary Disturbances
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.; Schween, R.; Muhlauser, K. H.; Rosenbauer, H.
1983BAAS...15..699S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of Major Flares in He I 10830
Authors: Harvey, K. L.; Recely, F.; Sheeley, N. R., Jr.; Howard,
R. A.; Koomen, M. J.; Michels, D. J.
1983BAAS...15..712H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Associations between Coronal Mass Ejections and Solar Energetic
Proton Events
Authors: Kahler, S. W.; Sheeley, N. R., Jr.; Howard, R. A.; Koomen,
M. J.; Michels, D. J.; McGuire, R. E.; von Rosenvinge, T. T.; Reames,
D. V.
1983BAAS...15..699K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A White-Light / Fex / Hα Coronal Transient Observation to
10-SOLAR-RADII
Authors: Wagner, W. J.; Illing, R. M. E.; Sawyer, C. B.; House, L. L.;
Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels, D. J.;
Smartt, R. N.; Dryer, M.
1983SoPh...83..153W Altcode:
Multi-telescope observations of the coronal transient of 15-16
April, 1980 provide simultaneous data from the Solar Maximum Mission
Coronagraph/Polarimeter, the Solwind Coronagraph, and the new Emission
Line Coronagraph of the Sacramento Peak Observatory. An eruptive
prominence-associated white light transient is for the first time
seen as an unusual wave or brightening in Fe Xgl6374 (but not in
Fe XIVgl5303). Several interpretations of this fleeting enhancement
are offered.
---------------------------------------------------------
Title: A quantitative study of magnetic flux transport on the Sun
Authors: Sheeley, N. R., Jr.; Boris, J. P.; Young, T. R., Jr.; DeVore,
C. R.; Harvey, K. L.
1983IAUS..102....2S Altcode:
A computational model, based on diffusion, differential rotation,
and meridional flow, has been developed to simulate the transport
of magnetic flux on the Sun. Using Kitt Peak magnetograms as input,
as have determined a best-fit diffusion constant by comparing the
computed and observed fields at later times. This paper presents
the initial results of a project to simulate the transport of solar
magnetic flux using diffusion, differential rotation, and meridional
flow. The study concerns the evolution of large-scale fields on a time
scale of weeks of years, and ignores the rapid changes that accompany
the emergence of new magnetic regions and the day-to-day changes of
the supergranular network itself.
---------------------------------------------------------
Title: A Quantitative Study of Magnetic Flux Transport on the Sun
Authors: Sheeley, N. R., Jr.; Boris, J. P.; Young, T. R., Jr.; DeVore,
C. R.; Harvey, K. L.
1983IAUS..102..273S Altcode:
A computational model, based on diffusion, differential rotation, and
meridional circulation, has been developed to simulate the transport
of magnetic flux on the Sun. Using Kitt Peak magnetograms as input,
the authors have determined a best-fit diffusion constant by comparing
the computed and observed fields at later times. The results suggest
that diffusion may be fast enough to account for the observed polar
magnetic field reversal without requiring a significant assist from
meridional currents.
---------------------------------------------------------
Title: The observation of a coronal transient directed at Earth.
Authors: Howard, R. A.; Michels, D. J.; Sheeley, N. R., Jr.; Koomen,
M. J.
1982ApJ...263L.101H Altcode:
The paper reports the observation of a large coronal transient that
can only be interpreted as a three-dimensional structure. Its form
is one which has not been observed before: a gradually expanding,
sun-centered disk of excess brightness, whose projected radius increased
from 4 to 8 solar radii during 0832-0958 UT on November 27, 1979. This
earth-directed transient was the source of an interplanetary shock
wave that reached ISEE 3 at 0649 UT, November 30, and earth at 0738
UT, November 30. Fitting the shock speed at ISEE 3 and the average
transit speed from the sun to ISEE 3 to a power law of the form V =
(V<SUB>0)(r</SUB> exp -n), it is found that V<SUB>0</SUB> = 1980 km/s
and n = 0.294, in good agreement with shock wave models. The shock
speed predicted by the power law at 10 solar radii is 1000 km/s,
in good agreement with the estimated frontal speed of the transient.
---------------------------------------------------------
Title: A magnetic cloud and a coronal mass ejection
Authors: Burlaga, L. F.; Klein, L.; Sheeley, N. R., Jr.; Michels,
D. J.; Howard, R. A.; Koomen, M. J.; Schwenn, R.; Rosenbauer, H.
1982GeoRL...9.1317B Altcode:
An interplanetary magnetic cloud observed by the Helios 1 spacecraft
was found to be associated with a coronal mass ejection observed by the
NRL Solwind coronagraph on the spacecraft P78-1. The magnetic cloud was
observed on June 20, 1980 when Helios 1 was at 0.54 AU and nearly 90°
west of the earth-sun line. This was associated with a large loop-like
coronal mass ejection observed over the west limb on June 18, 1980,
moving toward Helios 1. The speed of the front of the event at Helios
1 was (470 ± 10) km/s, which is close to the mean transit speed (∽
500 km/s). The magnetic cloud was similar to others described in the
literature: The magnetic field strength was higher than average; the
density was relatively low; the magnetic pressure greatly exceeded
the ion thermal pressure; and the magnetic field direction changed
through the cloud by rotating parallel to a plane which was highly
inclined with respect to the ecliptic.
---------------------------------------------------------
Title: Simultaneous radio scattering and white light observations
of a coronal transient
Authors: Woo, R.; Armstrong, J. W.; Sheeley, N. R., Jr.; Howard,
R. A.; Michels, D. J.; Koomen, M. J.
1982Natur.300..157W Altcode:
Reports that a coronal transient observed by the Solwind coronagraph
off the west limb of the sun was also seen in the spectral broadening
observations of the Helois-2 2.3 GHz radio signal. These simultaneous
data are compared and applied to studies of the shock front. UT
difference images show a time difference of up to 2 h between shock and
white light occurrences. Helios and Voyager shock speed measurements
are consistent, implying that the event observed by Helios is a
shock. These coronal transient data show that the shock front is ahead
of the white light front, but additional observations of larger events
will contribute information to the understanding of coronal transient
evolution as they propagate away from the sun.
---------------------------------------------------------
Title: Coronagraphic observations of two new sungrazing comets
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.
1982Natur.300..239S Altcode:
We recently reported the discovery<SUP>1</SUP> of a comet
(Howard-Koomen-Michels: 1979 XI) that apparently collided with the
Sun on 30 August 1979. We now report observations of two additional
sungrazers that encountered the Sun on 27 January 1981 and 20 July 1981,
respectively. Like comet 1979 XI, these two new comets seem to have
been members of the Kreutz group of sungrazers<SUP>2-4</SUP>, and like
1979 XI the new comets did not reappear after their encounters with
the Sun. The discovery of three previously unreported comets during
the initial 2.3 yr of our satellite coronal observations leads us to
suspect that sungrazers are much more common than one might suppose
from the list of only nine known sungrazers observed during the years
1668-1970<SUP>5,6</SUP>.
---------------------------------------------------------
Title: A Model for the Evolution of Large-Scale Magnetic Fields in
the Solar Photosphere
Authors: DeVore, C. R.; Boris, J. P.; Young, T. R., Jr.; Sheeley,
N. R., Jr.; Harvey, K. L.
1982BAAS...14..978D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Probable Sungrazing Comet
Authors: Michels, D. J.; Sheeley, N.; Harlow, F.; Marsden, B. G.
1982IAUC.3718....1M Altcode:
D. J. Michels, Naval Research Laboratory. reports that continued
analysis of the P78-l SOLWIND data has revealed a second sungrazing
comet (cf. IAUC 3640, 3647). The 15 observations below were obtained
as the comet approached the sun on 1981 Jan. 26-27. The discovery
team Includes N. Sheeley, 0. Roberts and F. Harlow. Calculations
at N.R.L. Indicate that the observations are consistent with a
Kreutz-type orbit and that the comet possibly hit the sun. The object
brightened from ~ mag 0.0 at 8 solar radii to mag -2.5 or brighter at
3 solar radii, apparently with a fading to mag +1.0 near 5.5 solar
radii. Contrary to the situation with the earlier object (comet
1979 XI), there was this time no evidence of residue from the comet
after its presumed perihelion passage. 1981 UT rho theta R. A. (1950)
Decl. Jan. 26.860 8.07 118.0 20 43.40 -19 21.3 26.867 7.87 117.8 20
43.22 -19 19.8 26.874 7.83 117.6 20 43.22 -19 19.1 26.881 7.63 117.1
20 43.10 -19 16.9 26.888 7.30 116.8 20 42.74 -19 14.7 26.927 6.12 115.6
20 41.72 -19 06.4 26.934 5.97 115.3 20 41.66 -19 05.1 26.941 5.93 114.8
20 41.66 -19 04.1 26.947 5.67 114.0 20 41.42 -19 01.7 26.954 5.41 113.7
20 41.18 -19 00.0 26.994 4.01 109.7 20 39.92 -18 49.6 27.001 3.71 109.4
20 39.62 -18 48.4 27.008 3.66 109.0 20 39.62 -18 47.7 27.014 3.40 107.9
20 39.38 -18 45.9 27.021 3,05 105.6 20 39.02 -18 43.1 The heliocentric
separations rho are given in units of the instantaneous solar radius,
and the position angle theta is measured counterclockwise from the
sun's north pole. The followIng parabolic orbital elements, derived by
the undersigned on the assumption that the comet's perihelion direction
agrees with the Kreutz group, satisfy the observations within 2'.5: T =
1981 Jan. 27.076 ET Peri. = 71.982 Node = 349.967 1950.0 q = 0.00488 AU
(= 1.05 Rs) Incl. = 142.738
---------------------------------------------------------
Title: Magnetic measurements of coronal holes during 1975 1980
Authors: Harvey, K. L.; Sheeley, N. R., Jr.; Harvey, J. W.
1982SoPh...79..149H Altcode:
Photospheric magnetic fluxes and average field strengths have been
measured beneath 33 coronal holes observed on 63 occasions during
1975-1980. The principal result is that low-latitude holes contained
3 times more flux near sunspot maximum than near minimum despite the
fact that their sizes were essentially the same. Average magnetic
field strengths ranged from 3-36 G near sunspot maximum compared to
1-7 G near minimum. Evidently the low-latitude coronal holes received
a proportion of the extra flux that was available at low latitudes
near sunspot maximum.
---------------------------------------------------------
Title: The coronal field lines of an evolving bipolar magnetic region
Authors: Sheeley, N. R., Jr.
1982ApJ...255..316S Altcode:
A simple potential field model is presented to illustrate that
loops of magnetic flux rise upward through the corona during the
relatively short growth phase of a bipolar magnetic region but contract
back to the sun's surface during the much longer decay phase of the
photospheric region. To reconcile this behavior with the unidirectional,
solar-wind-driven convection of flux outward from the sun, one must
postulate the existence of an X-type neutral line in the middle corona
where open field lines can be converted back to closed ones.
---------------------------------------------------------
Title: Observations of coronal structure during sunspot maximum.
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.; Harvey, K. L.; Harvey, J. W.
1982SSRv...33..219S Altcode:
This paper presents some of the results that have been obtained from
the Kitt Peak observations of coronal holes and the NRL observations
of coronal transients during the recent years near sunspot maximum
(1979 1981). On the average, low-latitude coronal holes of comparable
size contained 3 times more flux near sunspot maximum than near the
previous minimum. In the outer corona, transients occurred at the
observed rate of at least 2 per day, and quiet conditions persisted
during less than 15 % of the observed days. We describe a sample of
the more than 800 events that we have observed so far, including the
observation of a comet apparently colliding with the Sun.
---------------------------------------------------------
Title: The observation of a coronal transient directed at earth.
Authors: Michels, D. J.; Howard, R. A.; Koomen, M. J.; Sheeley,
N. R., Jr.
1982BAAS...14R.572M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of a Comet on Collision Course with the Sun
Authors: Michels, D. J.; Sheeley, N. R.; Howard, R. A.; Koomen, M. J.
1982Sci...215.1097M Altcode:
A brilliant new comet (1979 XI: Howard-Koomen-Michels) was discovered
in data from the Naval Research Laboratory's orbiting SOLWIND
coronagraph. An extensive sequence of pictures, telemetered from
the P78-1 satellite, shows the coma, accompanied by a bright and
well-developed tail, passing through the coronagraph's field of view at
a few million kilometers from the sun. Preliminary orbital calculations
based on the observed motion of the comet's head and morphology of the
tail indicate that this previously unreported object is a sungrazing
comet and may be one of the group of Kreutz sungrazers. It appears from
the data that the perihelion distance was less than 1 solar radius,
so that the cometary nucleus encountered dense regions of the sun's
atmosphere, was completely vaporized, and did not reappear after the
time of closest approach to the sun. After this time, however, cometary
debris, scattered into the ambient solar wind, caused a brightening
of the corona over one solar hemisphere and to heliocentric distances
of 5 to 10 solar radii.
---------------------------------------------------------
Title: Probable Sungrazing Comet
Authors: Howard, R.; Koomen, N.; Michels, D. J.; Sheeley, N.; Marsden,
B. G.
1981IAUC.3640....1H Altcode:
Images of a probable comet have been found at the Naval Research
Laboratory, Washington, on coronagraph exposures obtained from
the satellite P78-1 in 1979. The object was found by R. Howard as
a result of instrumentation developed and operated by N. Koomen and
D. J. Michels. The following heliocentric separations rho (in units of
the instantaneous solar radius) and position angles theta (measured
counterclockwise from the sun's north pole) have been derived by
N. Sheeley and converted to R.A. and Decl. by the undersigned. The head
of the object was somewhat brighter than Venus, and a tail was directed
roughly away from the center of the sun. 1979 UT rho theta R.A. (1950)
Decl. Aug. 30.789 5.96 234.2 10 26.47 + 8 45.1 30.796 5.67 233.8 10
26.80 + 8 45.5 30.802 5.27 234.3 10 27.22 + 8 47.8 30.809 5.16 234.6
10 27.36 + 8 48.6 30.816 5.09 235.4 10 27.43 + 8 49.8 30.856 3.65 235.7
10 29.07 + 8 54.8 30.867 3.11 236.1 10 29.67 + 8 56.9 30.885 2.56 239.2
10 30.28 + 9 00.7 At the last observation the object's head was at the
edge of the coronagraph's occulting disk. On the next exposure, taken at
Aug. 30.989 UT, the tail is still present, and during the next several
hours cometary material evidently diffused around to p.a. ~ 360o. The
comet's orbit cannot be unequivocally determined, but computations by
the undersigned suggest that a retrograde solution is to be preferred,
for this better explains the previous failure to detect the comet in a
twilit sky. Other possible coronagraphic or hitherto unreported visual
detections of the object would of course be very useful. Retrograde
orbit solutions show some resemblance to the orbits of the members
of the Kreutz sungrazing comet group; there would seem to be a good
chance that the comet hit the sun (for the head was not detected after
perihelion). The following possible orbital solution has been selected
solely because of its general resemblance to the Kreutz-type orbits:
T = 1979 Aug. 30.92 ET Peri. = 83.42 Node = 9.81 1950.0 q = 0.001 AU
(assumed) Incl. = 142.42
---------------------------------------------------------
Title: Coronal holes, solar wind streams, and geomagnetic disturbances
during 1978 and 1979
Authors: Sheeley, N. R., Jr.; Harvey, J. W.
1981SoPh...70..237S Altcode:
We have extended our long-term study of coronal holes, solar wind
streams, and geomagnetic disturbances through the rising phase of
sunspot cycle 21 into the era of sunspot maximum. During 1978 and
1979, coronal holes reflected the influence of differential rotation,
and existed within a slowly-evolving large-scale pattern despite the
relatively high level of sunspot activity. The long-lived 28.5-day
pattern is not produced by a rigidly-rotating quasi-stationary
structure on the Sun, but seems to be produced by a non-stationary
migratory process associated with solar differential rotation. The
association between coronal holes and solar wind speed enhancements at
Earth continues to depend on the latitude of the holes (relative to
the heliographic latitude of Earth), but even the best associations
since 1976 have speeds of only 500-600 km s<SUP>-1</SUP> rather than
the values of 600-700 km s<SUP>-1</SUP> that usually occurred during
the declining phase of sunspot cycle 20.
---------------------------------------------------------
Title: A Comet on Collision Course with the Sun: Observations on
August 30-31, 1979
Authors: Howard, R. A.; Koomen, M. J.; Michels, D. J.; Sheeley,
N. R., Jr.
1981BAAS...13..875H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Comet on Collision Course with the Sun: Orbital Data Deduced
from the Observations of Aug 30-31, 1979
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.
1981BAAS...13..875S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronagraph Observations of a Sun-directed Comet, Aug. 30-31,
1979: Images, Analysis and Photometry
Authors: Koomen, M. J.; Michels, D. J.; Sheeley, N. R., Jr.; Howard,
R. A.
1981BAAS...13R.891K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Comet on Collision Course with the Sun: Dynamical
Interpretation of the Observations of Aug 30-31, 1979
Authors: Michels, D. J.; Sheeley, N. R., Jr.; Howard, R. A.; Koomen,
M. J.
1981BAAS...13..875M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A comet on collision course with the sun: orbital data deduced
from the observations of August 30 - 31, 1979.
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.
1981BAAS...13R.875S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A comet on collision course with the sun: observations on
August 30 - 31, 1979.
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.
1981BAAS...13Q.875S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The influence of differential rotation on the equatorial
component of the sun's magnetic dipole field
Authors: Sheeley, N. R., Jr.
1981ApJ...243.1040S Altcode:
This paper examines the effect that solar differential rotation would
have on a hypothetical large-scale equatorial dipole field. The evolving
large-scale field pattern is expressed as a series of non-axisymmetric
moments. As time increases, power is transferred to progressively
higher order moments. In the 27d rotating coordinate system, each
moment undergoes a small retrograde drift which remains nearly uniform
until that mode begins to fade. The synodic rotation periods of the
first few moments are comparable to the observed 28.5d period of the
sun's large-scale field near sunspot maximum. Differential rotation
may be the source of this 28.5d period, but the eruption of new flux
is necessary to keep the pattern going.
---------------------------------------------------------
Title: Coronal Transients Near Sunspot Maximum
Authors: Poland, A. I.; Howard, R. A.; Koomen, M. J.; Michels, D. J.;
Sheeley, N. R., Jr.
1981SoPh...69..169P Altcode:
The Naval Research Laboratory's most recent Earth-orbiting coronagraph,
called Solwind, has been observing the Sun's outer corona (2.6-10.0
R<SUB>⊙</SUB>) at 10-min intervals since March 28, 1979. These
observations provide the first comprehensive view of coronal transients
near the peak of a sunspot cycle. Six, well-defined transients in
our quick-look data have masses ranging from 7 × 10<SUP>14</SUP>
g to 2 × 10<SUP>16</SUP> g and outward speeds ranging from 150 km
s<SUP>−1</SUP> to 900 km s<SUP>−1</SUP>. These values are comparable
to the ones that were obtained with the OSO-7 and Skylab observations
during the declining phase of the last sunspot cycle. Although the
amount of quick-look data is not sufficient to provide meaningful
statistics, the coronal transients near sunspot maximum seem to occur
with a greater frequency and a wider latitude range than the transients
during the declining phase of the cycle. In both eras, there is a good,
but imperfect, association between the occurrence of coronal transients
and surface phenomena such as eruptive prominences and flares.
---------------------------------------------------------
Title: The overall structure and evolution of active regions.
Authors: Sheeley, N. R., Jr.
1981sars.work...17S Altcode:
The evolutionary characteristics and structure of the magnetic fields
and atmospheric phenomena associated with the development of a solar
magnetic region are discussed. Bipolar magnetic regions are introduced
as the source of all solar magnetic fields, formed as bundles of
magnetic flux rise to and break through the solar surface and spread
throughout the photosphere, chromosphere and corona. The photospheric
magnetic region is shown to be characterized by bipolar flux regions
of various sizes emerging rapidly for a few days, then expanding and
decreasing in flux for several months and fragmenting. Sunspots and
faculae are considered as tracers of the magnetic regions in the upper
photosphere or lower chromosphere, while chromospheric tracers include
arch filaments, field transition arches, long chromospheric threads,
disk filaments and dark fibrils in chromospheric lines. The transition
region and lower corona exhibit a multithermal plasma distribution,
with low-temperature plasmas confined to the footprints and legs of
magnetic field lines and high-temperature plasmas originating in loops
or systems of unresolved loops. The bipolar magnetic region is also
shown to interact with its surroundings to produce an interconnected
field line pattern.
---------------------------------------------------------
Title: Coronal Evolution and Transients: 1979-1980
Authors: Howard, R. A.; Koomen, M. J.; Sheeley, N. R.; Michels, D. J.
1980BAAS...12R.898H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Solar Origin of An Interplanetary Shock
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.; Rompolt, B.; Schwenn, R. W.; Mihalov, J. D.
1980BAAS...12..920S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal Holes and the Sun's Mean Magnetic Field
Authors: Harvey, K. L.; Sheeley, N. R., Jr.
1980BAAS...12..918H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Three-Coronagraph Record from 1 to 10 R<SUB>O</SUB> of the
Energetics of a Coronal Transient
Authors: Wagner, W. J.; Sawyer, C.; Illing, R. M. E.; House, L. L.;
Querfeld, C. W.; Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.;
Michels, D. J.; Smartt, R. N.
1980BAAS...12Q.902W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The equatorial latitude of auroral activity during 1972 1977
Authors: Sheeley, N. R., Jr.; Howard, R. A.
1980SoPh...67..189S Altcode:
The equatorial latitude of auroral activity has been derived from both
electron and optical observations with the DMSP satellites. Virtually
all of the observations that were obtained during the 5-year interval
June 1972-September 1977 have been used to construct a nearly continuous
plot of invariant geomagnetic latitude versus time.
---------------------------------------------------------
Title: Recent Satellite Observations of the Corona
Authors: Koomen, M. J.; Sheeley, N. R., Jr.; Michels, D. J.; Howard,
R. A.
1980S&T....60..102K Altcode:
Observations of the solar corona by the DOD satellite P78-1 are
discussed and compared with previous findings by OSO-7 and Skylab. The
satellite carries a coronagraph and several other solar instruments,
including X-ray spectrometers for analyzing the spectra of solar
flares and for monochromatic imaging of X-ray emission from active
regions. Computer analysis of the pictures obtained in a three-day
span in May 1979 shows that many of the day-to-day changes in the
streamers result from the rapid ejection of coronal matter, which
disrupts the streamers and leaves gradually fading structures in their
place. The findings of P78-1 as well as OSO-7 and Skylab seem to agree
on a connection between eruptive prominences and coronal transients,
but the exact nature of the association is not yet clear.
---------------------------------------------------------
Title: The observation of a high-latitude coronal transient
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Koomen, M. J.; Michels,
D. J.; Poland, A. I.
1980ApJ...238L.161S Altcode:
On 1979 September 27 NRL's earth-orbiting coronagraph (Solwind)
observed a coronal mass ejection whose outward direction in the plane
of the sky was within 12 deg of the sun's north polar axis. Such
high-latitude transients were not observed by the OSO 7 and Skylab
coronagraphs during the declining phase of the last sunspot cycle,
but may be relatively common near sunspot maximum.
---------------------------------------------------------
Title: Initial observations with the SOLWIND coronagraph
Authors: Sheeley, N. R., Jr.; Michels, D. J.; Howard, R. A.; Koomen,
M. J.
1980ApJ...237L..99S Altcode:
The NRL Solwind coronagraph has provided the first systematic,
temporally (10 minute intervals) resolved view of the outer solar
corona near the peak of a sunspot cycle. It observes a 2.6-10.0 solar
radii annular field of view with a spatial resolution of 1.25 arc
min, and the Solwind observations allow the study of the evolution
of streamers at high latitude for the first time. It is noted that
most coronal structures changed significantly from day to day, which
changes were associated with the ejection of material outward across
the coronagraph's field of view. Attention is given to a transient
which was observed to have a well-defined curved front which advanced
outward across the field of view at an essentially constant speed of
500 km/s, but unlike most Skylab loop-type mass ejections, left no
remnants 24 hours later.
---------------------------------------------------------
Title: Temporal Variations of Loop Structures in the Solar Atmosphere
Authors: Sheeley, N. R., Jr.
1980SoPh...66...79S Altcode:
Unique timelapse sequences of Skylab/ATM spectroheliograms reveal the
following characteristics of normal (i.e. non-flare) loop structures
in the solar atmosphere: At the 0.5 × 10<SUP>6</SUP> K temperature
of Ne VII, emission is concentrated into individual spiky structures
that project 10<SUP>4</SUP>-10<SUP>5</SUP> km from their magnetic
footpoints and live on the order of 30 min.
---------------------------------------------------------
Title: Simultaneous Measurement of Coronal Faraday Rotation and
Total Electron Content During Solar Occultation of PSRO525+21
Authors: Howard, R. A.; Bird, M. K.; Koomen, M. J.; Michels, D. J.;
Sheeley, N. R., Jr.
1980BAAS...12..545H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The evolution of the polar coronal holes
Authors: Sheeley, N. R., Jr.
1980SoPh...65..229S Altcode:
He I 10830 Å synoptic maps, obtained at the Kitt Peak National
Observatory during 1974-1979, show that the Sun's polar coronal
holes have contracted significantly during 1977-1978. Prior to the
accelerated increase of sunspot activity in mid-1977, the area of
each polar cap was on the order of 8% of the Sun's total surface area
(4πR<SUP>2</SUP>), whereas toward the end of 1978 these areas fell
below 2% of 4πR<SUP>2</SUP>. Synoptic polar plots show that the
vestigual holes had irregular shapes and were often well removed from
the poles themselves. These results are consistent with the changes
that one would expect when the polar magnetic fields are weakening
just prior to sunspot maximum.
---------------------------------------------------------
Title: Coronal Holes, Solar Wind Streams, and Geomagnetic Disturbances
During 1978 and 1979
Authors: Sheeley, N. R., Jr.; Harvey, J. W.
1980BAAS...12R.545S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Lateral Expansion of the August 14, 1979 Coronal Transient
Authors: Koomen, M. J.; Michels, D. J.; Howard, R. A.; Sheeley,
N. R., Jr.
1980BAAS...12..515K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Contraction and Disappearance of the Polar Coronal Holes
Authors: Sheeley, N. R., Jr.
1980NASCP2098..219S Altcode: 1980sscs.nasa..219S
He I 10830 A spectroheliograms obtained during 1974-1979 were used to
study the evolution of the Sun's polar coronal holes. Synoptic polar
plots show that the holes have decreased in size to the point that
only vestigual holes remain and even these remnants fluctuate with
detailed sunspot activity.
---------------------------------------------------------
Title: Satellite observations of the outer corona near sunspot maximum
Authors: Michels, D. J.; Howard, R. A.; Koomen, M. J.; Sheeley,
N. R., Jr.
1980IAUS...86..439M Altcode: 1980IAUS...89..439M
This paper shows the first satellite observations of the Sun's white
light corona (2.6 R_⊙ - 10.0 R_⊙) during the active phase of a
sunspot cycle. Since March 28, 1979, these observations have been
obtained routinely with a spatial resolution of 1.25 arc min and a
repetition rate of 10 minutes during the one-hour sunlit portion of
each 97-minute satellite orbital period. As an illustration of these
new observations, we show the coronal changes associated with the
great mass of May 8, 1979.
---------------------------------------------------------
Title: The solar mass ejection of 8 May 1979
Authors: Michels, D. J.; Howard, R. A.; Koomen, M. J.; Sheeley, N. R.,
Jr.; Rompolt, B.
1980IAUS...91..387M Altcode:
The solar mass ejection of May 8, 1979 is characterized using images
from the NRL SOLWIND coronagraph on board the P78-1 satellite. The
eruption of the polar crown filament began at about 0810 UT and reached
maximum distance of 1.5 solar radii from the center of the sun at 1021
UT. The eruptive prominence subsequently faded, and some of the material
fell back to the solar surface. Vestiges of the H-alpha prominence
disappeared by 1120 UT. The bulk of the prominence material displayed
radial velocities averaging 40 km/sec with 165 km/sec the maximum
velocity observed. From 1028 UT to 1246 UT, the leading edge of the
ejecta moved outward with a constant radial velocity of approximately
500 km/sec. Projected back to the solar limb, this indicated an onset
near 0950 UT.
---------------------------------------------------------
Title: Solar observations with a new earth-orbiting coronagraph
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Michels, D. J.; Koomen,
M. J.
1980IAUS...91...55S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Rapid changes in the fine structure of a coronal "Bright point"
and a small coronal "active region".
Authors: Sheeley, N. R., Jr.; Golub, L.
1979SoPh...63..119S Altcode:
A coronal `bright point' is resolved into a pattern of emission which,
at any given time, consists of 2 or 3 miniature loops (each ∼2500
km in diameter and ∼12 000 km long). During the half-day lifetime
of the `bright point' individual loops evolved on a time scale ∼6
min. A small `ctive region' seemed to evolve in this way, but the
occasional blurring together of several loops made it difficult to
follow individual changes.
---------------------------------------------------------
Title: Coronal Holes and Solar Magnetic Fields (Article published in
the special issues: Proceedings of the Symposium on Solar Terrestrial
Physics held in Innsbruck, May- June 1978. (pp. 137-538))
Authors: Harvey, J. W.; Sheeley, N. R., Jr.
1979SSRv...23..139H Altcode:
Since 1972, nearly continuous observations of coronal holes and their
associated photospheric magnetic fields have been made using a variety
of satellite and ground-based equipment. The results of comparisons
of these observations are reviewed and it is demonstrated that the
structure and evolution of coronal holes is basically governed by the
large-scale distribution of photospheric magnetic flux. Non-polar
holes form in the decaying remnants of bipolar magnetic regions in
areas with a large-scale flux imbalance. There is strong indirect
evidence that the magnetic field in coronal holes is always open to
interplanetary space but not all open-field regions have associated
coronal holes. The well-observed declining phase of the last solar
cycle was characterized by stable magnetic field and coronal hole
patterns which were associated with recurrent, high-speed wind streams
and interplanetary magnetic field patterns at the Earth. The ascending
phase of the current cycle has been characterized by transient magnetic
field and coronal hole patterns which tend to occur at high solar
latitudes. This shift in magnetic field and coronal hole patterns
has resulted in a less obvious and more complicated association with
high-speed wind streams at the Earth.
---------------------------------------------------------
Title: Comparison of Satellite and Rocket Coronal Observations around
April 13, 1979
Authors: Howard, R. A.; Michels, D. J.; Koomen, M. J.; Sheeley, N. R.,
Jr.; Kohl, J. L.; Munro, R. H.; Weiser, H.
1979BAAS...11..408H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Discussion
Authors: Anzer, U.; Raadu, M. A.; Rompolt, B.; Rust, D. M.; Sheeley,
N.; Spicer, D. S.
1979phsp.coll..171A Altcode: 1979IAUCo..44..171A
No abstract at ADS
---------------------------------------------------------
Title: Discussion
Authors: Engvold, O.; Gaizauskas, Gaizauskas; Rust, D. M.; Sheeley,
N.; Tandberg-Hanssen, E.; Ohman, Y.; Zirin, H.
1979phsp.coll..207E Altcode: 1979IAUCo..44..207E
No abstract at ADS
---------------------------------------------------------
Title: Discussion
Authors: Acton, L. W.; Kundu, M. R.; Maltby, P.; Malville, J.; Orrall,
F. Q.; Sheeley, N.; Spicer, D. S.
1979phsp.coll..225A Altcode: 1979IAUCo..44..225A
No abstract at ADS
---------------------------------------------------------
Title: Discussion
Authors: Anzer, U.; Chiuderi-Drago, F.; Kundu, M. R.; Leroy, J. L.;
Malville, J.; Rompolt, B.; Sheeley, N.; Stenflo, J.; Tandberg-Hanssen,
E.; Öhman, Y.
1979phsp.coll...77A Altcode: 1979IAUCo..44...77A
No abstract at ADS
---------------------------------------------------------
Title: Coronal holes, solar wind streams, and geomagnetic activity
during the new sunspot cycle.
Authors: Sheeley, N. R., Jr.; Harvey, J. W.
1978SoPh...59..159S Altcode:
The paper presents results obtained for 1976-1977 using daily He I 10830
A spectroheliograms and photospheric magnetograms. It was found that as
the magnetic field patterns changed, the solar atmosphere evolved from
a structure with a few large long-lived low-latitude coronal holes to
one with numerous small short-lived high-latitude holes. High-latitude
holes recurred with a synodic rotation period of 28-29 days instead of
the 27-day period already known to be characteristic of low-latitude
holes. A Bartels display of the occurrence of holes, wind speed,
and geomagnetic activity is considered.
---------------------------------------------------------
Title: Coronal holes, solar wind streams, and geomagnetic activity
during the new sunspot cycle
Authors: Harvey, J. W.; Sheeley, N. R.
1978SoPh...59..159H Altcode:
We have extended our previous study of coronal holes, solar wind
streams, and geomagnetic disturbances from the declining phase
(1973-1975) of sunspot cycle 20 through sunspot minimum (1976)
into the rising phase (1977) of cycle 21. Using daily He I 10830
Å spectroheliograms and photospheric magnetograms, we found the
following results: As the magnetic field patterns changed, the solar
atmosphere evolved from a structure having a few, large, long-lived,
low-latitude coronal holes to one having numerous small, short-lived,
high-latitude holes (in addition to the polar holes which persisted
throughout this 5-year interval).
---------------------------------------------------------
Title: The equatorward extent of auroral activity during 1973 1974
Authors: Sheeley, N. R., Jr.
1978SoPh...58..405S Altcode:
The equatorward boundary of auroral activity during 1973-1974 has
been derived from DMSP photographs and their associated `auroral
analysis records'. On a time scale of days, the equatorward position
of the northern auroral oval varied in phase with the average level of
geomagnetic activity. In general, this variation was associated with the
occurrence of solar flares and coronal holes. On a time scale of hours,
the equatorward position of the oval correlated with the AE index of
substorm activity and with the strength of the southward component of
the interplanetary magnetic field.
---------------------------------------------------------
Title: Coronal Holes, Solar Wind Streams and Geomagnetic Activity
During the New Solar Cycle.
Authors: Sheeley, N. R.; Harvey, J. W.
1978BAAS...10Q.461S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Extreme ultraviolet observations of coronal
holes. II. Association of holes with solar magnetic fields and a
model for their formation during the solar cycle.
Authors: Bohlin, J. D.; Sheeley, N. R., Jr.
1978SoPh...56..125B Altcode:
Extreme-ultraviolet Skylab and ground-based solar magnetic field
data have been combined to study the origin and evolution of coronal
holes. It is shown that holes exist only within the large-scale
unipolar magnetic cells into which the solar surface is divided at
any given time. A well-defined boundary zone usually exists between
the edge of a hole and the neutral line which marks the edge of its
magnetic cell. This boundary zone is the region across which a cell
is connected by magnetic arcades with adjacent cells of opposite
polarity. Three pieces of observational evidence are offered to
support the hypothesis that the magnetic lines of force from a hole
are open. Kitt Peak magnetograms are used to show that, at least on
a relative scale, the average field strengths within holes are quite
variable, but indistinguishable from the field strengths in other
quiet parts of the Sun's surface.
---------------------------------------------------------
Title: A survey of coronal holes and their solar wind associations
throughout sunspot cycle 20.
Authors: Broussard, R. M.; Sheeley, N. R., Jr.; Tousey, R.; Underwood,
J. H.
1978SoPh...56..161B Altcode:
To gain insight into the relationships between solar activity, the
occurrence and variability of coronal holes, and the association of
such holes with solar wind features such as high-velocity streams,
a study of the period 1963-1974 was made. This period corresponds
approximately with sunspot cycle 20. The primary data used for this
work consisted of X-ray and XUV solar images obtained from rockets. The
investigation revealed that: The polar coronal holes prominent at solar
minimum, decreased in area as solar activity increased and were small
or absent at maximum phase. This evolution exhibited the same phase
difference between the two hemispheres that was observed in other
indicators of activity.
---------------------------------------------------------
Title: A comparison of He ii 304 Å and He i 10 830 Å
spectroheliograms
Authors: Harvey, J. W.; Sheeley, N. R., Jr.
1977SoPh...54..343H Altcode:
Spectroheliograms were obtained simultaneously in the He II 304 Å
emission line and the He I 10 830 Å absorption line with an angular
resolution of approximately 5″. A negative print of the 304 Å
image is matched with a positive print of the 10 830 Å image so that
corresponding features of the chromospheric network (including active
regions) appear identical in the two images. Differences between these
images include the facts that: Disk filaments and limb darkening are
strongly visible in the 10 830 Å positive image, but they are weakly
visible (as lightenings) in the 304 Å negative image.
---------------------------------------------------------
Title: A pictorial comparison of interplanetary magnetic field
polarity, solar wind speed, and geomagnetic disturbance index during
the sunspot cycle.
Authors: Sheeley, N. R., Jr.; Asbridge, J. R.; Bame, S. J.; Harvey,
J. W.
1977SoPh...52..485S Altcode:
Observations of interplanetary magnetic field polarity, solar wind
speed, and geomagnetic disturbance index (C9) during the years
1962-1975 are compared in a 27-day pictorial format that emphasizes
their associated variations during the sunspot cycle. This display
accentuates graphically several recently reported features of solar
wind streams including the fact that the streams were faster, wider,
and longer-lived during 1962-1964 and 1973-1975 in the declining
phase of the sunspot cycle than during intervening years (Bame et al.,
1976; Gosling et al., 1976). The display reveals strikingly that these
high-speed streams were associated with the major, recurrent patterns
of geomagnetic activity that are characteristic of the declining
phase of the sunspot cycle. Finally, the display shows that during
1962-1975 the association between long-lived solar wind streams and
recurrent geomagnetic disturbances was modulated by the annual variation
(Burch, 1973) of the response of the geomagnetic field to solar wind
conditions. The phase of this annual variation depends on the polarity
of the interplanetary magnetic field in the sense that negative sectors
of the interplanetary field have their greatest geomagnetic effect in
northern hemisphere spring, and positive sectors have their greatest
effect in the fall. During 1965-1972 when the solar wind streams
were relatively slow (500 km s<SUP>-1</SUP>), the annual variation
strongly influenced the visibility of the corresponding geomagnetic
disturbance patterns.
---------------------------------------------------------
Title: Analysis of Solar-Flare Plasmas Using EUV and X-Ray Data.
Authors: Tandberghanssen, E.; Sheeley, N.; Smith, J. B.
1977BAAS....9..311T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations During the Impulsive Phase of the August 7,
1973, Solar Flare.
Authors: Sheeley, N. R., Jr.; Kane, S. R.; Vorpahl, J. A.; Chapman,
G. A.
1977BAAS....9..311S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Comparison of He II 304 Å and He I 10830 Å
Spectroheligrams.
Authors: Harvey, J. W.; Sheeley, N. R., Jr.
1977BAAS....9..325H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An improved measurement of a spectrogram of a "gap".
Authors: Chapman, G. A.; Sheeley, N. R., Jr.
1977SoPh...51...61C Altcode:
A spectrogram of the 5250 Å region previously obtained during a period
of excellent seeing has been remeasured and calibrated by reference
to the preliminary KPNO photometric atlas. This analysis, using the
logarithm of opacitance instead of an H-D plot, has reduced some of
the effects of scattered light and shows greater facular line contrast.
---------------------------------------------------------
Title: Observations of Coronal Holes Throughout Sunspot Cycle 20.
Authors: Broussard, R. M.; Underwood, J. H.; Tousey, R.; Sheeley,
N. R., Jr.
1976BAAS....8..557B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal holes, solar wind streams, and recurrent geomagnetic
disturbances: 1973 1976
Authors: Sheeley, N. R., Jr.; Harvey, J. W.; Feldman, W. C.
1976SoPh...49..271S Altcode:
Observations of coronal holes, solar wind streams, and geomagnetic
disturbances during 1973-1976 are compared in a 27-day pictorial format
which shows their long-term evolution. The results leave little doubt
that coronal holes are related to the high-speed streams and their
associated recurrent geomagnetic disturbances. In particular, these
observations strongly support the hypothesis that coronal holes are
the solar origin of the high-speed streams observed in the solar wind
near the ecliptic plane.
---------------------------------------------------------
Title: High-latitude observations of solar wind streams and coronal
holes
Authors: Ricket, B. J.; Sime, D. G.; Crockett, W. R.; Tousey, R.;
Sheeley, N. R., Jr.
1976JGR....81.3845R Altcode:
Interplanetary scintillation observations of the solar wind velocity
during 1973 and the first part of 1974 reveal several corotating
high-speed streams. These streams, of heliographic latitudes from +40°
to -60°, have been mapped back to the vicinity of the sun and have been
compared with coronal holes identified in wide band XUV solar images
taken during the manned portions of the Skylab mission. There is some
evidence that the high-speed streams are preferentially associated with
coronal holes and that they can spread out from the hole boundaries
up to about 20° in latitude. However, this association is not one
to one; streams are observed which do not map back to coronal holes,
and holes are observed which do not lie at the base of streams. To the
extent that a statistical interpretation is possible the association
is not highly significant, but individual consideration of streams and
holes suggests that the statistical result is biased somewhat against
a strong correlation.
---------------------------------------------------------
Title: Polar faculae during the interval 1906-1975
Authors: Sheeley, N. R., Jr.
1976JGR....81.3462S Altcode:
In a previous study (Sheeley, 1964, 1966) the numbers of faculae at
the poles of the sun were counted and used to estimate the strengths
of the polar magnetic fields during the period 1906-1964. The present
paper extends this study to include the recent period 1964-1975. The
new observations show the following: (1) The polar fields are stronger
now than they have been for a decade. At the south pole, most of this
increase in strength occurred in 1973 during the Skylab mission. At
the north pole the change occurred gradually during 1972 and 1973. (2)
Except for a relatively brief interval during 1959 the south polar
field has been weaker during the last magnetic cycle than it has been
during any cycle since this uniform set of observations began at Mount
Wilson in 1906.
---------------------------------------------------------
Title: Energy released by the interaction of coronal magnetic fields.
Authors: Sheeley, N. R., Jr.
1976SoPh...47..173S Altcode:
Comparisons between coronal spectroheliograms and photospheric
magnetograms are presented to support the idea that as coronal magnetic
fields interact, a process of field line reconnection usually takes
place as a natural way of preventing magnetic stresses from building
up in the lower corona. This suggests that the energy which would have
been stored in stressed fields is continuously released as kinetic
energy of material being driven aside to make way for the reconnecting
fields. However, this kinetic energy is negligible compared to the
thermal energy of the coronal plasma. Therefore, it appears that these
slow adjustments of coronal magnetic fields cannot account for even the
normal heating of the corona, much less the energetic events associated
with solar flares.
---------------------------------------------------------
Title: Coronal Changes Associated with a Disappearing Filament
Authors: Sheeley, N. R., Jr.; Bohlin, J. D.; Brueckner, G. E.; Purcell,
J. D.; Scherrer, V. E.; Tousey, R.; Smith, J. B., Jr.; Speich, D. M.;
Tandberg Hanssen, E.; Wilson, R. M.; de Loach, A. C.; Hoover, R. B.;
McGuire, J. P.
1975SoPh...45..377S Altcode:
This paper describes Skylab/ATM observations of the events associated
with a disappearing filament near the center of the solar disk on
January 18, 1974. As the filament disappeared, the nearby coronal
plasma was heated to a temperature in excess of 6 × 10<SUP>6</SUP>K. A
change in the pattern of coronal emission occurred during the 11/3 hr
period that the soft X-ray flux was increasing. This change seemed to
consist of the formation and apparent expansion of a loop-like coronal
structure which remained visible until its passage around the west limb
several days later. The time history of the X-ray and microwave radio
flux displayed the well-known gradual-rise-and-fall (GRF) signature,
suggesting that this January 18 event may have properties characteristic
of a wide class of X-ray and radio events.
---------------------------------------------------------
Title: The Calculation of Force-Free Fields from Discrete Flux
Distributions
Authors: Sheeley, N. R., Jr.; Harvey, J. W.
1975SoPh...45..275S Altcode:
This paper presents particularly simple mathematical formulas for the
calculation of force-free fields of constant α from the distribution
of discrete sources on a flat surface. The advantage of these formulas
lies in their physical simplicity and the fact that they can be easily
used in practice to calculate the fields. The disadvantage is that they
are limited to fields of `sufficiently small α'. These formulas may be
useful in the study of chromospheric magnetic fields by the comparison
of high-resolution Hα photographs and photospheric magnetograms.
---------------------------------------------------------
Title: Skylab/ATM Observations of Transient Events Having the GRF
X-Ray and Microwave Character.
Authors: Sheeley, N. R., Jr.; Bohlin, J. D.; Scherrer, V. E.;
Tousey, R.
1975BAAS....7..429S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Sun's Polar Caps as Coronal Holes: Their Sizes, Evolution,
and Phenomenology During the Skylab Mission.
Authors: Bohlin, J. D.; Rubenstein, D. M.; Sheeley, N. R., Jr.
1975BAAS....7..457B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: X-Ray Event of August 13-15, 1973.
Authors: Scherrer, V.; Sandlin, G.; Sheeley, N.; Tousey, R.
1975BAAS....7..430S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A newly observed solar feature: macrospicules in He II 304 Å.
Authors: Bohlin, J. D.; Vogel, S. N.; Purcell, J. D.; Sheeley, N. R.,
Jr.; Tousey, R.; Vanhoosier, M. E.
1975ApJ...197L.133B Altcode:
He II 304 A spectroheliograms, obtained with the NRL extreme-ultraviolet
slitless spectrograph during the Skylab mission, show spikelike
structures at the sun's polar limb which resemble the familiar
H-alpha spicules. However, the relatively large size and long life
of these He II features has led to distinguishing them by the name
'macrospicules'. The macrospicules appear as protuberances or jets,
ranging from 5 to over 60 sec in length, from 5 to 30 sec in width,
and from 5 to over 40 minutes in lifetime. Perhaps the most radical
departure from H-alpha spicules is that macrospicules occur only within
the chromospheric boundaries of coronal holes. Thus macrospicules
are most easily visible over the solar poles due to the coronal holes
normally present there, and much less frequently at lower latitudes
during limb passage of relatively rare, low-latitude coronal holes.
---------------------------------------------------------
Title: Polar Plumes in XUV Emission-Line Corona
Authors: Bohlin, J. D.; Purcell, J. D.; Sheeley, N. R., Jr.; Tousey, R.
1975BAAS....7..356B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Macro-Spicules in He II 304 Å Over the Sun's Polar Cap
Authors: Bohlin, J. D.; Vogel, S. N.; Purcell, J. D.; Sheeley, N. R.;
Tousey, R.; van Hoosier, M. E.
1975BAAS....7R.354B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The High-Energy Limb Event of January 17, 1974
Authors: Tousey, R.; Bohlin, J. D.; Moe, O. K.; Purcell, J. D.;
Sheeley, N. R.
1975BAAS....7..348T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The reconnection of magnetic field lines in the solar corona.
Authors: Sheeley, N. R., Jr.; Bohlin, J. D.; Brueckner, G. E.; Purcell,
J. D.; Scherrer, V. E.; Tousey, R.
1975ApJ...196L.129S Altcode:
Skylab XUV coronal spectroheliograms and photospheric magnetograms
are compared. This comparison shows that, as new bipolar magnetic
fields emerge through the solar surface into the corona, the new
coronal fields interact with the old ones in a manner that suggests
the reconnection of the field lines.
---------------------------------------------------------
Title: Interpreting XUV Spectroheliograms in Terms of Coronal Magnetic
Field Structures
Authors: Sheeley, N. R., Jr.; Bohlin, J. D.; Brueckner, G. E.; Purcell,
J. D.; Scherrer, V. E.; Tousey, R.
1975BAAS....7Q.346S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: XUV Observations of Coronal Magnetic Fields
Authors: Sheeley, N. R., Jr.; Bohlin, J. D.; Brueckner, G. E.; Purcell,
J. D.; Scherrer, V.; Tousey, R.
1975SoPh...40..103S Altcode:
Spectroheliograms obtained with the Naval Research Laboratory's
Extreme Ultraviolet Spectrograph (S082A) on Skylab are compared with
Kitt Peak National Observatory magnetograms. A principal result is
the characteristic reconnection of flux from an emerging bipolar
magnetic region to previously existing flux in its vicinity. Examples
of the disappearance of magnetic flux from the solar atmosphere are
also shown. The results of a particularly simple, potential field
calculation are shown for comparison with the Skylab observations.
---------------------------------------------------------
Title: Structure of the sun's polar cap at wavelengths 240 - 600 Å.
Authors: Bohlin, J. D.; Sheeley, N. R.; Tousey, R.
1975spre.conf..651B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The 1175 Å to 1900 Å Ultraviolet Spectrum of Solar Flares
Authors: Brueckner, G. E.; Bohlin, J. D.; Moe, O. K.; Nicolas, K. R.;
Purcell, J. D.; Scherrer, V. E.; Sheeley, N. R., Jr.; Tousey, R.
1974BAAS....6..285B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Preliminary Study of Coronal Structures by Means of
Time-Lapse Photography
Authors: Sheeley, N. R.; Bohlin, J. D.; Brueckner, G. E.; Purcell,
J. D.; Scherrer, V.; Tousey, R.
1974BAAS....6Q.294S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Cinematographic Observations for ATM and their Comparison
with some ATM Results
Authors: Zirin, H.; Holt, J.; Brueckner, G. E.; Bohlin, J. D.; Purcell,
J. D.; Scherrer, V. E.; Sheeley, N. R.; Tousey, R.
1974BAAS....6R.298Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Preliminary Results from the Nrl/atm Instruments from
SKYLAB SL/2
Authors: Tousey, R.; Bartoe, J. D. F.; Bohilin, J. D.; Brueckner,
G. E.; Purcell, J. D.; Scherrer, V. E.; Schumacher, R. J.; Sheeley,
N. R.; Vanhoosier, M. E.
1974IAUS...57..491T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The eruptive prominence of August 21, 1973 observed from
Skylab in the white light corona and in the He II 304 Å chromosphere.
Authors: Poland, A. I.; Bohlin, J. D.; Brueckner, G. E.; Purcell,
J. D.; Scherrer, V. E.; Sheeley, N. R.; Tousey, R.
1974BAAS....6..219P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Eruptive Prominence of August 21, 1973 Observed from
Skylab in the White Light Corona and in the He II 304Å Chromosphere.
Authors: Poland, A. I.; Bohlin, J. D.; Brueckner, G. E.; Purcell,
J. D.; Scherrer, V. E.; Sheeley, N. R.; Tousey, R.
1974BAAS....6..220P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Preliminary Study of the Extreme Ultraviolet
Spectroheliograms from Skylab
Authors: Tousey, R.; Bartoe, J. D. F.; Bohlin, J. D.; Brueckner, G. E.;
Purcell, J. D.; Scherrer, V. E.; Sheeley, N. R., Jr.; Schumacher,
R. J.; Vanhoosier, M. E.
1973SoPh...33..265T Altcode:
Some of the first observations obtained with the Naval Research
Laboratory's Extreme Ultraviolet Spectrograph (S082A) during the first
Skylab mission are presented and compared with magnetograms and other
ground-based data. The instrument is a slitless objective-type grating
spectrograph covering 170-630 Å and described in Solar Phys.27, 251
(1972). Chromospheric network, loop prominences, active regions,
a flare, limb brightening, XUV bright points, and `coronal holes'
are among the phenomena shown and discussed.
---------------------------------------------------------
Title: Magnetic Outflow - A Stage in the Development of an Active
Region
Authors: Allen, R.; Edberg, S.; Labonte, B.; Sheeley, N. R.
1973BAAS....5Q.268A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of the Horizontal Velocity Field Surrounding
Sunspots
Authors: Sheeley, N. R., Jr.
1972SoPh...25...98S Altcode:
During the summer and fall of 1971, Doppler spectroheliograms were
obtained for several sunspots located near the solar limb. These
observations confirm a previous result based on the study of only a
few sunspots that in the plage-free photosphere surrounding sunspots
the spatially-averaged, horizontal flow tends to be outward at 0.5-1.0
km s<SUP>−1</SUP> for distances typically 10000-20000 km beyond the
outer boundary of the penumbra. It is suggested that these material
motions are the means by which small-scale fragments of magnetic flux
are carried away from sunspots.
---------------------------------------------------------
Title: The Photospheric Velocity Field in and Around Sunspots
Authors: Sheeley, N. R., Jr.
1972BAAS....4Q.391S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Time behavior of Ca ii K<SUB>2v </SUB> spectral features in
non-magnetic regions of the solar disk
Authors: Liu, S. Y.; Sheeley, N. R.; Smith, Elske V. P., Jr.
1972SoPh...23..289L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: New Observations of Solar Magnetic and Brightness Fields
Authors: Sheeley, N. R., Jr.; Liu, S. Y.
1972lfpm.conf..285S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A comparison of the intensity variations of the CN photospheric
and K line chromospheric network with time
Authors: Liu, S. Y.; Sheeley, N. R., Jr.
1971SoPh...20..282L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Using CN λ 3883 spectroheliograms to map weak photospheric
magnetic fields
Authors: Sheeley, N. R., Jr.
1971SoPh...20...19S Altcode:
By photographically averaging time sequences of high-resolution CN λ
3883 spectroheliograms, the noise level due to the rapidly fluctuating
intensity of the solar background has been reduced significantly. Very
faint faculae that are lost in the noise on a single frame are easily
visible on such an enhanced picture. A comparison between these
enhanced spectroheliograms and a photoelectric magnetogram suggests
that the brightness-magnetic field correlation extends to much weaker
field strengths and fainter faculae than can be detected on a single,
high quality CN λ 3883 spectroheliogram.
---------------------------------------------------------
Title: Two-Dimensional Observations of the Velocity Fields in and
around Sunspots
Authors: Sheeley, N. R., Jr.; Bhatnagar, A.
1971SoPh...19..338S Altcode:
Doppler spectroheliograms of sunspots and their surroundings have
been obtained with a spatial resolution approaching one second
of arc and a time resolution of 20 s per frame. Observations of 5
sunspots, located 18°, 45°, 56°, 60° and 72° from the disk center
respectively, showed considerable long-lived fine structure and, in
particular, indicated the following: The Evershed outflow terminated
in spoke-like structures that constitute the ragged outer boundary of
the penumbra. Some of these spokes extended more than 8000 km beyond
the average outer boundary.
---------------------------------------------------------
Title: Measurements of the Oscillatory and Slowly-Varying Components
of the Solar Velocity Field
Authors: Sheeley, N. R., Jr.; Bhatnagar, A.
1971SoPh...18..379S Altcode:
Spectroheliograms with high spatial resolution are presented to
illustrate the decomposition of the solar velocity field into its
oscillatory and slowly-varying components. An analysis of data obtained
in the lines FEIλ 5434 and FeIIλ 4924 yield essentially the same
principal results: Spectroheliograms of the oscillatory component
have a mottled appearance of rising and falling elements ranging from
2000 km to 3000 km in size. These elements oscillate vertically with
a period in the range 275-300 s and an amplitude of 0.5 km/s. Although
most oscillations last two cycles some have been observed for as many
as four cycles.
---------------------------------------------------------
Title: The Reduction of the Solar Velocity Field into Its Oscillatory
and Slowly-Varying Components
Authors: Sheeley, N. R., Jr.; Bhatnagar, A.
1971SoPh...18..195S Altcode:
Spectroheliogram movies of the solar velocity field have been made in
the λ4924 line of FeII with a time resolution of 20 sec/frame and a
spatial resolution in the range 1-2 sec of arc. A conventional doppler
movie has been used to generate two additional movies which show
the slowly-varying and oscillatory components of the velocity field
separately. A basic result is the simplicity of the field patterns
into which the relatively complex velocity field can be decomposed.
---------------------------------------------------------
Title: The Time Dependence of Magnetic, Velocity and Intensity Fields
in the Solar Atmosphere
Authors: Sheeley, N. R., Jr.
1971IAUS...43..310S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simultaneous Measurements of Magnetic Fields and Brightness
Fields Using a 4-Image Spectroheliograph
Authors: Sheeley, N. R., Jr.; Engvold, O.
1970SoPh...12...69S Altcode:
The use of an auxiliary beamsplitter with the Kitt Peak 15-foot
spectroheliograph permits spectroheliograms to be taken simultaneously
in 4 identical images of the sun. By using two of these images
for a Zeeman spectroheliogram, a third image for a FeI λ4071
spectroheliogram, and the fourth image for a 6107Å continuum
spectroheliogram, simultaneous measurements of magnetic fields and
brightness fields have been obtained. Within the limits of intensity
variations imposed by doppler shifts and brightness fluctuations
of the continuum, a quantitative relation does exist between the
measured values of brightness and magnetic field strength of the
photospheric network. For intensities measured +0.12 Å from the core
of FeI λ4071, this relation is ln(1 +ΔI/I) = α¦B<SUB>∥</SUB>¦,
whereB<SUB>∥</SUB> refers to the component of magnetic field normal to
the solar surface,ΔI/I is the fractional excess of brightness of the
magnetic regions relative to the brightness of non-magnetic regions,
and α = (6±2)%/100 gauss.
---------------------------------------------------------
Title: The Evolution of the Photospheric Network
Authors: Sheeley, N. R., Jr.
1969SoPh....9..347S Altcode:
A time-lapse sequence of spectroheliograms in the bandhead of CN at
λ3883 reveals the following behavior of the photospheric network
with time: There is a steady flow of bright `points' (≃ 1000 km in
diameter) laterally outward from sunspots at speeds on the order of 1
km.sec<SUP>−1</SUP>. After traveling about 10 000 km from a sunspot
they either conglomerate to form fragments of the photospheric network
or disappear.
---------------------------------------------------------
Title: Spectroheliograms in Fe II λ4924
Authors: Sheeley, N. R., Jr.; Engvold, O.
1969BAAS....1T.292S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Photospheric Network
Authors: Chapman, G. A.; Sheeley, N. R., Jr.
1968SoPh....5..442C Altcode:
Spectroheliograms, obtained in certain Fraunhofer lines with the
82-cm solar image at the Kitt Peak National Observatory, show a bright
photospheric network having the following properties: It resembles,
but does not coincide with, the chromospheric network, the structure
of the photospheric network being finer and more delicate than the
relatively coarse structure of the chromospheric network.
---------------------------------------------------------
Title: Correlations Between Brightness Fields and Magnetic Fields
on the Sun
Authors: Chapman, G. A.; Sheeley, N. R., Jr.
1968IAUS...35..161C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of Small-Scale Solar Magnetic Fields
Authors: Sheeley, N. R., Jr.
1967SoPh....1..171S Altcode:
Spectrograms, obtained during moments of good seeing with the high
spatial resolution afforded by the 80-cm solar image at the Kitt Peak
National Observatory, show the following: Magnetic fields of several
hundred gauss occur in tiny areas easily as small as 500 km in extent
in regions of the solar surface sometimes well removed from areas of
sunspot activity.
---------------------------------------------------------
Title: The Average Profile of the Solar K-Line during the Sunspot
Cycle
Authors: Sheeley, N. R., Jr.
1967ApJ...147.1106S Altcode:
Measurements of the fraction of the solar disk covered by bright
K2 emission at various phases of the sunspot cycle are presented
together with measurements of the K2 emission intensity of various
solar features. It is estimated that at the time of the last sunspot
maximum approximately 20 per cent of the visible disk was covered by
K2 emission with intensity at least three times that of the quiet
disk. This corresponds to a 40 per cent increase in the average
intensity of K2 emission at the time of sunspot maximum over the
average intensity at the time of sunspot minimum. A smaller variation
associated with solar rotation is alsd estimated. Average K-spectra
for the separate hemispheres of the Sun, as they currently appear,
are presented. These spectra show considerable differences which are
attributed to the differences in solar activity presently existing in
the two hemispheres. It is concluded that, if one regards the Sun as a
point source of light, as it would appear from the vicinity of a distant
star, there would be a significant variation in the intensity of its
K2 emission with time corresponding to the variation of the sunspot
number with time. This result emphasizes the possibility of finding
"sunspot cycles" in stars by observing the intensity of K2 emission.
---------------------------------------------------------
Title: Measurements of Solar Magnetic Fields
Authors: Sheeley, N. R., Jr.
1966ApJ...144..723S Altcode:
Using the photographic technique of Leighton, high spatial resolution
measurements of photospheric magnetic fields have been obtained
with the following chief results: 1. Regarding sunspot groups as
sources of magnetic flux and defining a "source flux" that may be
determined uniquely for a sunspot group from flux measurements, we
found a variation in source fluxes from group to group ranging from
less than 0.2 SFU (1 SFU [solar flux unit] = 1021 maxwells) for very
"small" spot groups to more than 20 SFU for very "large" groups. This
variation of source flux with sunspot group "size" is described by
the rule-of-thumb = 1.2 Am, where is the source flux in SFU and Am
is the group size in 1018 cm2 defined as the maximum area attained
by the sunspot group during its development. 2. Zeeman photographs
emphasize that as a bipolar magnetic region (BMR) develops in time and
as its magnetic flux spreads over a progressively larger area, the flux
density does not decrease smoothly from the vicinity of the source to
the outer limit of detectable flux but is distributed in successively
smaller bits and fragments. Moreover, measurements obtained from these
Zeeman photographs reveal magnetic field strengths of 200-700 gauss in
these bits and fragments, showing that fields of a few hundred gauss
are not uncommon for small magnetic features even in quiet regions
of the Sun. 3. Numbers of polar faculae have been calibrated to give
magnetic flux on the polar caps of the Sun as a function of time during
the period 1905-1964. The fluxes vary cyclicly with time approximately
90 out of phase with the variation of the sunspot number for the whole
solar disk with time during the same period (provided the sunspot
number is given a polarity corresponding to the magnetic polarity
of the following sunspots of the relevant hemisphere). The maxima of
the polar fluxes vary considerably from cycle to cycle (just as the
maxima of the sunspot number vary considerably from cycle to cycle),
maximum fluxes ranging from 6 to 21 SFU, with 12 SFU being a typical
maximum polar flux during 1905-1964. These results are consistent with
the hypothesis that emerging BMR's are the sources of all the flux on
the solar surface, and that the random walk plus differential rotation
is the dominant mechanism for the distribution of the flux provided
by these sources. More important, these measurements provide a means
of testing this hypothesis in more detail than has been possible
heretofore.
---------------------------------------------------------
Title: Measurements of Solar Magnetic Fields.
Authors: Sheeley, Neil Rolfson, Jr.
1965PhDT.........4S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Polar Faculae during the Sunspot Cycle.
Authors: Sheeley, Neil R., Jr.
1964ApJ...140..731S Altcode:
The numbers of north and south polar faculae have been counted for the
period 1935-1963. The chief result is a variation of the numbers of
polar faculae with time, approximately 180 out of phase with the time
variation of the sunspot number for the whole solar disk. If the number
of polar faculae and sunspot number both are plotted versus time with
"polarities," the important feature appears that the numbers of polar
faculae lag behind the sunspot number by approximately 90 . This is
consistent with the supposition (Leighton 1963, 1964) that the sunspots
provide a source of magnetic flux which is carried to the poles in a
few years' time by means of a random-walk process associated with the
convective supergranulation (Leighton, Noyes, and Simon 1962; Simon
1963) Comparison between the numbers of polar facuiae and measurements
of the polar magnetic field since 1952 (Babcock 1961; Howard 1963)
suggests that the past observations of polar faculae can be used to
extend measurements of polar magnetic field prior to 1952.
