Author name code: acton
ADS astronomy entries on 2022-09-14
author:"Acton, Loren W."
------------------------------------------------------------------------
Title: Solar Cycle Variation of Coronal Temperature, Emission Measure,
and Soft X-Ray Irradiance Observed with Yohkoh Soft X-Ray Telescope
Authors: Takeda, Aki; Acton, Loren; Albanese, Nicole
Bibcode: 2019ApJ...887..225T
Altcode:
This paper presents the solar soft X-ray irradiance (0.3-3.0 nm)
obtained from the mission-long full-Sun X-ray images observed with
the Soft X-ray Telescope (SXT) of the Yohkoh mission (1991-2001). The
irradiance was calculated using filter-ratio temperatures and emission
measures (EMs) from the full-Sun integrated X-ray signals employing
a coronal spectrum synthesized with CHIANTI atomic database version
8.0. Dependence of the results on the assumption of elemental abundances
is investigated. The SXT irradiance for a shorter wavelength range was
also calculated and compared with the low energy flux (0.1-0.8 nm)
of the X-Ray Sensor on board the GOES satellite. To incorporate the
spatial information of the observed corona, we tried the alternative
method to derive irradiance by applying the filter-ratio method to the
images of northern hemispheric corona instead of spatially integrated
signals from there. The EM weighted average of the spatially resolved
temperatures turned out to be generally 20%-30% lower than those derived
from the integrated signals, while the total of spatially resolved EMs
are accordingly 1.5-2 times higher. This trend is enhanced when solar
activity is the lowest in early 1996. The irradiance obtained from the
alternative method does not significantly vary from the result from
the integrated signals. This provides some validation for the simpler
full-Sun integrated method. The temporal variation of the EMs of cool
(<1.5 MK), medium, and hot (>2.5 MK) components indicates that
the ratio of the hot component relative to the medium component is
higher in the ascending phase (mid 1998-2001) than in the descending
phase (1992-1995) of the solar activity cycle.
Title: A Modified Kirkpatrick-Baez Design for a Practical Astronomical
X-ray Telescope
Authors: Longcope, Dana; Acton, Loren W.; Kankelborg, Charles
Bibcode: 2019AAS...23430101L
Altcode:
Kirkpatrick-Baez (K-B) optics offer a means of imaging soft x-rays
with modest resolution and a multi-arc-minute field of view at a cost
far below the conventional Wolter design. Such a low-cost system could
be useful for dedicated, long time-line observation of astronomical
x-ray sources from orbit. A K-B telescope consists of crossed arrays
of parabolic mirrors at grazing incidence. The classic K-B design is
subject to significant aberration, arising from interplay between
the focusing of the fore and aft mirror arrays. We demonstrate
here a modified K-B design with aberrations reduced by an order of
magnitude. We show, furthermore, that it is possible to construct
such a system by constraining flat "slats" of commercially-available
glass in precision machined grooves. The slats deform into shapes which
adequately approximate the optimal figures, thereby yielding focusing
better than the best version of the classic K-B design. The result is a
new approach that greatly simplifies the task of achieving both useful
resolution and high effective area for x-ray astronomy applications.
Title: Recalibration of the Soft X-Ray Telescope Onboard Yohkoh
Authors: Acton, Loren W.
Bibcode: 2018SoPh..293..137A
Altcode:
We present a new derivation of the X-ray spectral sensitivity of the
Soft X-ray Telescope (SXT) experiment onboard Yohkoh. The recalibration
is based upon the hypothesis that, during the first 15 months of the
mission, an absorbing material gradually built up on the entrance
filters of the telescope. We have also re-evaluated the times and
sizes of ruptures of the SXT entrance filters. The impact of this
recalibration on derived filter-ratio temperature, emission measure,
and calculated spectral irradiance is substantial, especially for SXT
data prior to November 1992.
Title: On-Orbit Performance and Calibration of the Soft X-Ray
Telescope on Yohkoh
Authors: Acton, Loren W.
Bibcode: 2016SoPh..291..643A
Altcode: 2016SoPh..tmp...22A
This paper documents details of the on-orbit performance, data problem
solving, and calibration of the Soft X-ray Telescope (SXT) experiment
on Yohkoh. This information is important to a full understanding of
the strengths and weaknesses of the SXT data set. The paper begins
with summaries of SXT calibration issues and how they have been
addressed, operational anomalies experienced during the mission, and
a brief discussion of the SXT optical train. The following section
on the accuracy of Yohkoh pointing determination provides information
important for alignment of SXT images with each other and with other
solar data. The remainder of the paper gives details of work by the
experiment team to understand and ameliorate the many instrument
anomalies and changes which impacted the scientific data.
Title: X-Ray Searches for Solar Axions
Authors: Hudson, H. S.; Acton, L. W.; DeLuca, E. E.; Hannah, I. G.;
Reardon, K.; Van Bibber, K.
Bibcode: 2012ASPC..455...25H
Altcode: 2012arXiv1201.4607H
Axions generated thermally in the solar core can convert nearly directly
to X-rays as they pass through the solar atmosphere via interaction with
the magnetic field. The result of this conversion process would be a
diffuse centrally-concentrated source of few-keV X-rays at disk center;
it would have a known dimension, of order 10% of the solar diameter, and
a spectral distribution resembling the blackbody spectrum of the solar
core. Its spatial structure in detail would depend on the distribution
of mass and field in the solar atmosphere. The brightness of the source
depends upon these factors as well as the unknown coupling constant
and the unknown mass of the axion; this particle is hypothetical and
no firm evidence for its existence has been found yet. We describe the
solar magnetic environment as an axion/photon converter and discuss
the upper limits obtained by existing and dedicated observations from
three solar X-ray observatories: Yohkoh, RHESSI, and Hinode.
Title: An Invitation to the Improved Yohkoh Legacy data Archive
Authors: Takeda, A.; Acton, L. W.; McKenzie, D.; Yoshimura, K.;
Freeland, S. L.
Bibcode: 2010AGUFMSH11B1645T
Altcode:
The Yohkoh Legacy data Archive (YLA) is a unique source of solar X-ray
images and spectra obtained with the Yohkoh satellite(1991-2001). The
archive consists of four levels of data products from raw to highly
value added, with ample amount of documentation and user-friendly web
interface (http://solar.physics.montana.edu/ylegacy). The YLA is also
unique in that quality and variety of the products and services have
been constantly improved through these nine years after the completion
of the satellite mission. Our recent improvements are: (1) Completion of
the stray light correction, applied to the data after the SXT entrance
filter failure in 1992. (2) Completion of the satellite attitude data
correction. (3) E-mail based individual user support (E-consultant
service). (4) New FITS header definition with World Coordinate System
compliance. (5) Update of SXT response function based on the latest
atomic data and models (Chianti 6.0.1). (6) Introduction of a new
data category, Level 3, for co-aligned composite images suitable for
browsing in movie mode. The main body of our archive (data from the
Soft X-ray Telescope) are provided in both Yohkoh-specific (XDA) and
FITS formats. The XDA format is convenient for analysis with existing
SSW/Yohkoh software that runs under IDL. The more general FITS format
enables use of advanced SSW applications developed in the various
missions after Yohkoh. FITS products do not specifically require IDL
to read and view images, which allows our products to be accessed from
a wide range of communities. Our products can be accessed through our
quick-look and data-search web services, and also through the Virtual
Solar Observatory data search. Since 2009, YLA is funded as one of
NASA's Resident Archives in Virtual Observatories for Heliophysics Data
program. Through the funds, we take responsibility for maintaining
the best corrected data sets, and providing the easy access and user
support. The top page of the YLA web interface
Title: The Yohkoh Legacy Archive
Authors: Acton, L. W.; Takeda, A.; McKenzie, D. E.
Bibcode: 2008AGUFMSA53A1563A
Altcode:
Yohkoh was a Japan/US/UK mission for the study of high energy
processes on the sun. Scientific operation extended from September
1991 until 14 December 2001, nearly an entire solar activity
cycle. Observations included full-disk soft and hard x-ray imaging,
hard x-ray spectroscopy, and high resolution flare spectroscopy in
S XV, Ca XIX, Fe XXV and Fe XXVI from the Bent Crystal Spectrometer
(BCS). The Yohkoh Legacy Archive (YLA) brings together all Yohkoh
observational data along with extensive documentation required for
a full understanding of instrumentation, mission operations, and
data reduction and correction. Extensive meta-data aid the user in
efficiently accessing the data base. Creation of the YLA has been the
work of 8 years; the top objective has been to present the extensive
Yohkoh database in a form fully usable for scientists or students who
are unfamiliar with Yohkoh instrumentation. The YLA may be accessed
at http://solar.physics.montana.edu/ylegacy or through the Virtual
Solar Observatory (VSO), although the VSO capability is still under
development. Data from the Yohkoh hard x-ray instruments and BCS are
presented in flare list formats. The Soft X-ray Telescope (SXT) images
are available in quantitative and movie formats. This long, uniform,
archive of SXT images is especially useful for solar cycle studies as
well as high resolution soft x-ray flare studies. Examples of YLA data
products and research enabled by the archive will be presented.
Title: Searching the X-ray Sun For Solar Axions
Authors: Hudson, Hugh S.; Acton, L. W.; DeLuca, E. E.; Hannah, I. G.;
Hurford, G. J.; Lin, R. P.; Reardon, K. P.; van Bibber, K.
Bibcode: 2008AAS...212.0402H
Altcode: 2008BAAS...40..193H
The axion is a hypothetical weakly-interacting elementary particle. The
solar core may produce a copious axion flux via the Primakoff
effect. This same process can also convert a tiny fraction of the
axions back into photons via interaction with the magnetic field
threading the solar atmosphere. The spectral signature of the emitted
X-rays is determined mainly by the temperature of the solar core, and
the spatial distribution also depends strongly on the solar magnetic
field in the back-conversion process. The X-ray intensity thus varies as
Gaγγ(∫BperpdL)2,
where Bperp is the perpendicular component of the
chromospheric and coronal magnetic field in the appropriate zone
for interaction and photon escape, and Gaγγ
is the (unknown) coupling constant, dependent on the (unknown) mass
of the axion. We describe observational tests suitable for solar
X-ray imagers and discuss projects now under way with Yohkoh/SXT,
RHESSI, and Hinode/XRT. The successful detection of axions would have
implications for basic physics and for cosmological dark matter. It
would also help us to characterize the ill-understood extension of the
solar magnetic field into the chromosphere and corona. We specifically
focus on applying the existing understanding of solar (and stellar)
magnetism to this problem.
Title: Yohkoh SXT Full-Resolution Observations of Sigmoids: Structure,
Formation, and Eruption
Authors: Canfield, Richard C.; Kazachenko, Maria D.; Acton, Loren W.;
Mackay, D. H.; Son, Ji; Freeman, Tanya L.
Bibcode: 2007ApJ...671L..81C
Altcode:
We study the structure of 107 bright sigmoids using full-resolution
(2.5" pixels) images from the Yohkoh Soft X-Ray Telescope (SXT)
obtained between 1991 December and 2001 December. We find that none of
these sigmoids are made of single loops of S or inverse-S shape; all
comprise a pattern of multiple loops. We also find that all S-shaped
sigmoids are made of right-bearing loops and all inverse-S-shaped
sigmoids of left-bearing loops, without exception. We co-align the SXT
images with Kitt Peak magnetograms to determine the magnetic field
directions in each sigmoid. We use a potential-field source surface
model to determine the direction of the overlying magnetic field. We
find that sigmoids for which the relative orientation of these two
fields has a parallel component outnumber antiparallel ones by more than
an order of magnitude. We find that the number of sigmoids per active
region varies with the solar cycle in a manner that is consistent with
this finding. Finally, those few sigmoids that are antiparallel erupt
roughly twice as often as those that are parallel. We briefly discuss
the implications of these results in terms of formation and eruption
mechanisms of flux tubes and sigmoids.
Title: Low Cost X-ray Optics for Studying StellarDynamo Cycles
Authors: Rust, Thomas; Acton, L.; Kankelborg, C.; Martens, P.
Bibcode: 2007AAS...210.2302R
Altcode: 2007BAAS...39..128R
Comparison of measured coronal X-ray variability over stellar magnetic
dynamo cycles with theoretical models will yield new understanding
of the solar magnetic dynamo cycle. We present the results of a study
comparing surface roughnesses of three candidate materials for use as
glancing angle X-ray reflectors. This work is part of a continuing
effort by MSU's Solar Physics Group and Space Science Engineering
Laboratory (SSEL) to design and build large aperture, low cost X-ray
optics for space experiments. The MSU proposed SADE (Starspot and
Dynamo Explorer) instrument would use arrays of nested Kirkpatrick-Baez
reflectors, called STAX (Sade Telescope Array for X-rays), for long
term measurements of soft X-ray fluxes from about a hundred nearby
solar-type stars. The advantage of the STAX design is that it uses
"off the shelf" materials bent to shape, which is far cheaper and
easier to manufacture than the polished cylindrical optics typically
used in X-ray telescopes. In order to determine whether off the shelf
materials satisfy the stringent surface smoothness requirements for
glancing angle reflectors, we have undertaken an atomic force microscope
investigation of three candidate materials. In addition, we compare
the measured and theoretical diffraction pattern of our existing STAX
test article. This comparison will provide insight into the suitability
of the candidate material, as well as the feasibility of maintaining
proper shape over the surface of the reflector by constraining the
edges in precision machined grooves. This work is supported by a grant
from MSU/NASA EPSCOR.
Title: Understanding coronal heating and solar wind acceleration:
Case for in situ near-Sun measurements
Authors: McComas, D. J.; Velli, M.; Lewis, W. S.; Acton, L. W.;
Balat-Pichelin, M.; Bothmer, V.; Dirling, R. B.; Feldman, W. C.;
Gloeckler, G.; Habbal, S. R.; Hassler, D. M.; Mann, I.; Matthaeus,
W. H.; McNutt, R. L.; Mewaldt, R. A.; Murphy, N.; Ofman, L.; Sittler,
E. C.; Smith, C. W.; Zurbuchen, T. H.
Bibcode: 2007RvGeo..45.1004M
Altcode:
The solar wind has been measured directly from 0.3 AU outward,
and the Sun's atmosphere has been imaged from the photosphere out
through the corona. These observations have significantly advanced our
understanding of the influence of the Sun's varying magnetic field on
the structure and dynamics of the corona and the solar wind. However,
how the corona is heated and accelerated to produce the solar wind
remains a mystery. Answering these fundamental questions requires
in situ observations near the Sun, from a few solar radii (R S
) out to ~20 R S , where the internal, magnetic, and
turbulent energy in the coronal plasma is channeled into the bulk energy
of the supersonic solar wind. A mission to make such observations has
long been a top priority of the solar and space physics community. The
recent Solar Probe study has proven that such a mission is technically
feasible and can be accomplished within reasonable resources.
Title: The EUV Unresolved Corona
Authors: Cirtain, Jonathan; Martens, P. C. H.; Acton, L. W.; Weber,
Mark
Bibcode: 2006SoPh..235..295C
Altcode:
The unresolved corona for three active regions (ARs) above the solar
limb is investigated. Intensities measured for ions formed above 1 MK
are presented as a function of height above the solar surface. The
observed decrease in intensity with altitude is found to be best
fit by an exponential. Furthermore, this exponential decrease is
approximately the decrease in emission expected for a hydrostatic
planar geometry atmosphere, where the scale height temperature is
dependent on the dynamics of the AR. For two of the ARs analyzed, we
have found that the best-fit temperature derived from the exponential
fits is consistent with an isothermal hydrostatic unresolved corona.
Title: The Amplitude of Stellar X-ray Cycles
Authors: Martens, P. C. H.; Chattterjee, P.; Saar, S. H.; Acton,
L. W. A.
Bibcode: 2005AAS...20710405M
Altcode: 2005BAAS...37R1334M
A decade worth of data from the Soft X-ray Telescope onboard the
Japan/US/UK mission Yohkoh shows that the Sun has a variation of
non-flaring disk-integrated soft X-ray emission (0.4 - 2.4 keV) of
about a factor 30 over its sunspot cycle. To date no cyclic variation
of that magnitude has been observed in other late-type stars. We
show that this negative result is partly explained by the inclusion of
EUV emission in stellar observations done with ROSAT: we calculated
what the solar signal would be if observed in the ROSAT passband and
found a cycle amplitude of a factor two to three. That leaves open
the question of the cycle amplitude in soft X-rays for solar-type
stars. To adress this we analysed ROSAT data for the energy band
above the C-K edge, but found no sufficiently frequent observations of
individual stars. The next best approach is to compare the measured soft
X-ray flux for singly observed stars with the average flux predicted
from the star's Rossby number: if cycles with amplitudes as large as
that of the Sun exist, the residual fluxes should be statistically
significant. From a sample of about 15 single and cyclic late-type
stars (as evidenced from Ca-K data) we find that with 99.6% certainty
the residual fluxes are due to X-ray cycles that are similar to or
greater than that of the Sun.
Title: Solar Probe: Humanity's First Visit to a Star (Invited)
Authors: McComas, D. J.; Velli, M.; Lewis, W. S.; Acton, L. W.;
Balat-Pichelin, M.; Bothmer, V.; Dirling, R. B.; Eng, D. A.; Feldman,
W. C.; Gloeckler, G.; Guhathakurtha, M.; Habbal, S. R.; Hassler, D. M.;
Mann, I.; Maldonado, H. M.; Matthaeus, W. H.; McNutt, R. L.; Mewaldt,
R. A.; Murphy, N.; Ofman, L.; Potocki, K. A.; Sittler, E. C.; Smith,
C. W.; Zurbuchen, T. H.
Bibcode: 2005ESASP.592..279M
Altcode: 2005ESASP.592E..42M; 2005soho...16E..42M
No abstract at ADS
Title: Scattering of Solar X-Rays by Jupiter and Saturn
Authors: Cravens, T. E.; Clark, J. D.; Bhardwaj, A.; Elsner, R.;
Waite, J. H.; Acton, L.; Maurellis, A. N.; Gladstone, R.
Bibcode: 2005AGUSM.P44A..05C
Altcode:
Soft X-ray emission has been observed from the disks of both Jupiter
and Saturn, as well as from the auroral regions of these planets. The
low-latitude disk emission as observed by ROSAT, the Chandra X-Ray
Observatory, and XMM-Newton appears to be uniformly distributed across
the disk and to be correlated with solar activity. These characteristics
suggest that the source of the disk x-rays are: (1) elastic scattering
of solar X-rays by atmospheric neutrals (2) absorption of solar X-rays
in the carbon K-shell followed by fluorescent emission. The carbon atoms
are found in methane molecules located below the homopause. In this
paper we present the results of calculations of the scattering albedo
and of the emitted x-ray intensity for a range of atmospheric abundances
and for a number of solar irradiance spectra. The model calculations
are compared with recent x-ray observations of Jupiter and Saturn.
Title: Magnetic Field Topology of Sigmoids
Authors: Son, J. H.; Canfield, R. C.; Acton, L. W.
Bibcode: 2004AAS...205.1004S
Altcode: 2004BAAS...36R1349S
Sigmoids are studied due to their eruptive nature, which affects the
Earth and the space atmosphere. The shape of the sigmoid (S-shaped or
inverse S-shaped) is an indicator of eruption. The origin of this shape
has been the topic of many research papers. One such paper by Fan and
Gibson, The Emergence of a Twisted Magnetic Flux Tube Into a Preexisting
Coronal Arcade, appeared in 2003. Fan and Gibson argue that a sigmoid
with left-handed twist has left-handed writhe, which gives the sigmoid
its S-shape and right-handed twist the inverse S-shape. Our study
determined that there is no correlation between a sigmoid's handedness
and shape as claimed in the paper by Fan and Gibson. Doing a statistical
study observing the topology of the sigmoid using the data from Yohkoh
Soft X-ray Telescope, we classified each sigmoid by its shape, twist,
and magnetic field lines. We found that 23% of our data was right-handed
and S-shaped, 33% was left-handed and S-shaped, 22% was right-handed and
inverse S-shaped, and 22% was left-handed and inverse-S shaped. Thus,
we found no systematic relationship between the handedness and shape
of the sigmoid -- in disagreement with Fan and Gibson.
Title: The Diffuse EUV Corona
Authors: Cirtain, J. W.; Martens, P. C. H.; Acton, L. W.; Scott, J. T.
Bibcode: 2004AAS...204.7304C
Altcode: 2004BAAS...36..800C
Recent investigations have shown that the EUV corona could have two
primary components; one at relatively static high temperature ( 2
MK) and another dynamic at a low temperature (below 1 MK). These two
coronal constituents are within the temperature ranges observed by
the Transition Region and Coronal Explorer (TRACE), and are easily
resolved using the spectral capabilities of the Coronal Diagnostic
Spectrometer (CDS) on SoHO. We analyze an Active Region above the limb
and determine emission measure scaling laws for the high temperature
components. We also provide an estimate of the physical properties
of the cool loops observed during the study. Through the use of the
scaling laws we have developed a method to subtract the static coronal
component from TRACE images. We then use the resultant TRACE 173 Å
images to determine the changes in temperature and density in these
loops as a function of time. This allows us to calculate the heating
rate for the dynamic cool structures. This work is supported in
part via NASA GSRP fellowship (425175)
Title: Solar Extreme Ultraviolet and X-ray Irradiance Variations
Authors: Woods, Tom; Acton, Loren W.; Bailey, Scott; Eparvier, Frank;
Garcia, Howard; Judge, Darrell; Lean, Judith; Mariska, John T.;
McMullin, Don; Schmidtke, Gerhard; Solomon, Stanley C.; Tobiska,
W. Kent; Warren, Harry P.; Viereck, Rodney
Bibcode: 2004GMS...141..127W
Altcode:
The solar extreme ultraviolet (EUV) radiation at wavelengths shortward
of 120 nm is a primary energy source for planetary atmospheres and
is also a tool for remote sensing of the planets. For such aeronomic
studies, accurate values of the solar EUV irradiance are needed over
time periods of minutes to decades. There has been a variety of solar
EUV irradiance measurements since the 1960s, but most of the recent
observations have been broadband measurements in the X-ray ultraviolet
(XUV) at wavelengths shortward of 35 nm. A summary of the solar EUV
irradiance measurements and their variability during the last decade is
presented. One of the most significant new solar irradiance results is
the possibility that the irradiance below 20 nm is as much as a factor
of 4 higher than the reference Atmospheric Explorer E (AE-E) spectra
established in the 1970s and 1980s. The primary short-term irradiance
variability is caused by the solar rotation, which has a mean period
of 27 days. The primary long-term variability is related to the solar
dynamo and is known best by the 11-year sunspot cycle. The solar cycle
variability as a function of wavelength can be characterized as 20% to
70% between 120 and 65 nm and as a factor of 1.5 to 10 between 65 and 1
nm. The variability of the total solar EUV irradiance, integrated from
0 to 120 nm, is estimated to be 30-40% for a large 27-day rotational
period and a factor of about 2 for the 11-year solar cycle during the
recent, rather active, solar cycles.
Title: The Relationship Between X-Ray Radiance and Magnetic Flux
Authors: Pevtsov, Alexei A.; Fisher, George H.; Acton, Loren W.;
Longcope, Dana W.; Johns-Krull, Christopher M.; Kankelborg, Charles
C.; Metcalf, Thomas R.
Bibcode: 2003ApJ...598.1387P
Altcode:
We use soft X-ray and magnetic field observations of the Sun (quiet
Sun, X-ray bright points, active regions, and integrated solar disk)
and active stars (dwarf and pre-main-sequence) to study the relationship
between total unsigned magnetic flux, Φ, and X-ray spectral radiance,
LX. We find that Φ and LX exhibit a very nearly
linear relationship over 12 orders of magnitude, albeit with significant
levels of scatter. This suggests a universal relationship between
magnetic flux and the power dissipated through coronal heating. If the
relationship can be assumed linear, it is consistent with an average
volumetric heating rate Q~B/L, where B is the average field strength
along a closed field line and L is its length between footpoints. The
Φ-LX relationship also indicates that X-rays provide a
useful proxy for the magnetic flux on stars when magnetic measurements
are unavailable.
Title: Do Stellar X-ray Observations Provide Evidence For Solar-like
Cycles?
Authors: Stern, R. A.; Alexander, D.; Acton, L. W.
Bibcode: 2003csss...12..906S
Altcode:
Utilizing 10 years of Yohkoh data, spanning nearly a complete solar
cycle, we investigate the statistical variations of solar X-ray fluxes
in the stellar context. The Yohkoh soft X-ray data can be described
by the combination of a smoothly varying function representing the
solar cycle plus a lognormal distribution representing the day-to-day
variability in the lowest energy bands. Using data from the SXT filter
which most closely resembles the ROSAT PSPC or Einstein IPC bandpasses,
we examine the distribution of two ``snapshot'' samples of the Sun's
X-ray emission taken at varying points in the cycle. Comparison
with the ROSAT and Einstein ``snapshots'' of Hyades G stars strongly
suggests that these more active ``suns'' have very long cycles, weak
or no cycles, or cycles which are integral or sub-multiples of the
solar cycle.
Title: SADE: The starspot and dynamo explorer
Authors: Martens, P. C. H.; Acton, L. W. A.; Klumpar, D.; Kankelborg,
C.; Stern, R. A.; Peres, G.; Culhane, J. L.
Bibcode: 2003AdSpR..32.1123M
Altcode:
We propose a mission called SADE, the Starspot And Dynamo Explorer, to
study dynamo activity in nearby late-type stars. The onboard instruments
will be a Ca-K telescope for magnetically dominated chromospheric
emission, and an X-ray grazing incidence telescope to study coronal
emission. We design the mission for a life-time of 15 years or longer
to capture a full activity cycle for most solar-type stars. We aim to
firmly establish the spectrum of the relation between chromospheric
and corona' emission in late-type stars, and capture one or more stars
going into or coming out of a Maunder type minimum. Operation costs will
be kept to a minimum by automating mission operations to a maximum,
and have the science operations be carried out by students at Montana
State University.
Title: Book Review: JOURNEY FROM THE CENTER OF THE SUN / Princeton
University Press, 2002
Authors: Acton, Loren W.
Bibcode: 2003PhT....56b..62A
Altcode:
No abstract at ADS
Title: Journey from the Center of the Sun
Authors: Zirker, Jack B.; Acton, Loren W.
Bibcode: 2003PhT....56b..62Z
Altcode:
No abstract at ADS
Title: The Point Spread Function of the Yohkoh Soft X-ray Telescope
Authors: McKenzie, D. E.; Gburek, S.; Acton, L. W.; Martens, P. C.
Bibcode: 2002AAS...200.5502M
Altcode: 2002BAAS...34Q.732M
The point spread function (PSF) of the Yohkoh solar observatory's Soft
X-ray Telescope has two primary components, a sharply defined core
and a diffuse wing due to photon scattering. Because the extent of the
PSF is significantly wider than a single pixel, its characterization is
useful for improvement of the quality of the SXT images. We will present
results from analyses of the two PSF components, and demonstrate our
best model of the core and scattering wing of the SXT point spread
function. An example of PSF deconvolution to remove the effects of
photon scattering will be given.
Title: The Yohkoh Galileo Project
Authors: Davey, A. R.; Acton, L. W.
Bibcode: 2002AAS...200.6812D
Altcode: 2002BAAS...34..757D
The Japan/US/UK Yohkoh mission was launched on 29 August 1991 and
ceased acquiring solar observations on 14 December 2001. Over the decade
the mission returned a record of energetic solar coronal and activity
phenomena of high quality and enduring value. In order to assure the
usability of Yohkoh data for generations of future scientists we plan to
create a durable and readily accessible archive of Yohkoh data products
and descriptive and explanatory documentation. We call the effort to
preserve and document the Yohkoh data archive the YOHKOH GALILEO PROJECT
in honor of Galileo Galilei, whose 17th-century sunspot observations
are still scientifically useful today. The ten years of observations
by Yohkoh provide a unique, high-quality, record of the evolution
high-energy solar phenomena over an entire sunspot cycle. These data
will be mined for decades, if not centuries, for studies of solar
activity, its control of space weather and the sun-earth connection,
and properties of magnetically active astrophysical objects. The Galileo
Project is being undertaken by the same team of U.S., Japanese, and
U.K. scientists who cooperatively conducted the observational phase of
the mission and the GSFC Solar Data Analysis Center where the primary
public archive of Yohkoh data are located.
Title: Persistent Coronal Streamers and the Identification of
Sunspot Clusters
Authors: Li, Jing; LaBonte, Barry; Acton, Loren; Slater, Greg
Bibcode: 2002ApJ...565.1289L
Altcode:
We use limb synoptic plots to study long-lived features of the lower
solar corona. The most persistent features are the polar sinusoids,
which are generated by streamers associated with active regions. We
find that the lifetimes of these structures (up to about 10 solar
rotations) are much longer than the lifetimes of individual sunspots
(typically less than one solar rotation). The long lifetimes of
the polar sinusoids are due to clusters of spatially related but
noncontemporaneous spots. The continuous emergence of sunspots and
magnetic flux from spot clusters in the photosphere provides the
long life spans of the coronal streamers. Two-thirds of the ~180
sunspots recorded in the southern hemisphere in a 1-year period
near the 1996-1997 solar minimum were members of noncontemporaneous
clusters. The clusters suggest large-scale, long-lived structures in
the subphotospheric magnetic field from which sunspots emerge.
Title: SADE, the Student Astrophysical Dynamo Explorer
Authors: Martens, P.; Acton, L.; Klumpar, D.; Stern, R.; Peres, G.;
Culhane, L.
Bibcode: 2002cosp...34E1298M
Altcode: 2002cosp.meetE1298M
In soft x-rays the solar coronal radiance varies by a factor of 10-30
over the solar activity cycle. A similar variation in most stars in
the existing x-ray database has not been found (Stern 2001); even
stars which exhibit chromospheric activity cycles show only marginal
evidence for X-ray cycles. This is rather puzzling as the time span
and multiple coverage of the x-ray sky should reveal at least a hint of
such a pronounced cyclical variation. We propose a mission called the
Student Astrophysical Dynamo Explorer to measure the x-ray brightness
of about 75 stars once every 5 days for up to 15 years. Selection of
prime stars takes into account location (avoid eclipse), rotation rate,
Ca-K observations, and magnetic field strength, to focus on the best
candidates for dynamo studies. We baseline a nested 4-5 mirror system
with 200 cm^2 geometric area, with a 1.5 to 2 meter focal length, 15
arcsec on-axis resolution, and Au or Ni coatings. The strawman detector
is a back-illuminated CCD of 512x512 pixels, with pixels that can be
large as a 15 arcseconds. Available exposure time per star per visit
is about an hour and a half. We are exploring the option of adding
a visible light detector for astroseismology. To minimize operations
cost for this long duration mission we envisage tracking and commanding
from a simple ground station at Montana State University, operated by
students under the auspices of MSU's Space Science and Engineering Lab
(SSEL).
Title: Large-Scale and Long-Lived Coronal Structures Detected in
Limb Synoptic Maps
Authors: Li, J.; Labonte, B.; Acton, L.; Slater, G.
Bibcode: 2002mwoc.conf..333L
Altcode:
The presentation will summarize our recent work on the large-scale,
long-lived coronal streamers based on limb synoptic maps made from
Yohkoh/SXT and SOHO/EIT images. The number of properties of such
coronal streamers have been revealed on the limb synoptic maps: 1. They
originate from active regions and have large extent in both latitude
and altitude. 2. Because they are large in 3-dimension, they largely
control the visibility of polar hole regions. When the active region
is in close side of the sun, then may cover the polar holes. When
the active region is at far side of the sun, they can be seen above
the solar limb by projections; 3. The life time of such large-scaled
coronal streamers can be up to 10 solar rotations; 4. These structures
are associated with underlying, non-contemporaneous spot clusters;
5. Individual spots in the clusters are short-lived, but contribute
magnetic flux collectively to form the long-lived coronal features.
Title: The active Sun
Authors: Alexander, D.; Acton, L. W.
Bibcode: 2002css1.book.1089A
Altcode:
No abstract at ADS
Title: Ten Years of Yohkoh and its Current Status: A Brief Summary
Authors: Kosugi, T.; Acton, L. W.
Bibcode: 2002mwoc.confD...7K
Altcode:
No abstract at ADS
Title: The Yohkoh Public Outreach Project
Authors: Larson, M. B.; Slater, T.; McKenzie, D.; Acton, L.; Alexander,
D.; Lemen, J.; Freeland, S.; Metcalf, T.
Bibcode: 2002mwoc.conf..117L
Altcode:
The NASA funded Yohkoh Public Outreach Project (YPOP) began in 1995
with the goal of providing public access to high quality Yohkoh
SXT data via the World Wide Web. The project utilizes the intrinsic
excitement of the SXT data, and in particular the SXT movies, to develop
science learning tools and classroom activities. The WWW site at URL:
http://www.lmsal.com/YPOP/ uses a movie theater theme to highlight
available Yohkoh movies in a format that is entertaining and inviting
to non-scientists and well received by scientists. We will discuss
the wide range of people YPOP has reached over the past six years,
as well as lessons learned during the development of the project.
Title: The Yohkoh Public Outreach Project: A Space Science Resource
for Formal and Informal Education
Authors: Lemen, J. R.; Alexander, D.; Metcalf, T. R.; Freeland, S. L.;
Acton, L. W.; Larson, M.; McKenzie, D.; Slater, T.
Bibcode: 2001AGUFMED12A0160L
Altcode:
The Yohkoh Public Outreach Project (YPOP) is a NASA-funded web site
maintained by scientists and educators at Lockheed Martin Solar and
Astrophysics Lab. and Montana State University. YPOP includes a range
of activities for youngsters, parents, teachers and anyone interested
in learning more about the Sun. YPOP utilizes a number of approaches
to the dissemination of solar data which incorporates elements of both
formaleducation, via a number of lesson plans and classroom activities,
and informal education, via access to the latest solar images, a
solar tour, and updated movies. This combination has proved extremely
effective in providing quality access to scientific data for a broad
audience with a wide range of interests. The Yohkoh Public Outreach
Project can be found at http://www.LMSAL.com/YPOP.
Title: Soft X-Ray Luminosity and Photospheric Magnetic Field in
Quiet Sun
Authors: Pevtsov, Alexei A.; Acton, Loren W.
Bibcode: 2001ApJ...554..416P
Altcode:
We use full-disk soft X-ray data from Yohkoh and Kitt Peak daily
magnetograms to study the coronal irradiance and photospheric magnetic
field remote from active regions between 1991 November and 1998
December. For every image of our data set we extract three areas
4°×4° in size centered at N00° W00°, N50° W00°, and S50°
W00° and compute X-ray irradiance and unsigned magnetic flux for
each of these areas. Between 1991 (active Sun) and 1996 (quiet Sun)
the X-ray irradiance at the heliographic center decreased by more than
a factor of 7, while the magnetic flux decreased by only a factor of
2. A similar tendency is observed for our high-latitude samples. Apart
from the cycle-related variations, all three areas of quiet Sun
exhibit significant nonperiodic changes in X-ray irradiance. These
variations occur on 9-12 month intervals and clearly correlate with
changes in sunspot activity. Similar variations are present in the
total X-ray irradiance averaged over the solar disk. By contrast, the
magnetic fluxes from the photosphere beneath these same areas show no
corresponding variations on this time scale. In our opinion, coronal
heating models based on the reconnection of quiet-Sun magnetic elements
(variously called chromospheric network, ``magnetic carpet,'' or ``salt
and pepper'' field) can at best account for a minimal contribution to
heating the million-degree corona observed by the Yohkoh soft X-ray
telescope. We conclude that the X-ray irradiance in the quiet Sun
(at least in the Yohkoh temperature range, >2 MK) is primarily
associated with the strong magnetic fields of active regions, not with
weak photospheric fields. The association, however, is not direct. We
interpret the enhanced X-ray irradiance above the quiet Sun not as a
result of the coronal ``canopy'' formed by the active-region magnetic
field above the quiet-Sun areas, but as the large-scale relaxation
process in the corona triggered by the evolution of active regions. To
further support this conclusion, we show examples of active and quiet
hemispheres in 1996 with similar weak-field properties but greatly
different global X-ray emission and a pronounced change in X-ray
irradiance over the entire visible hemisphere that was associated with
the emergence of a single small active region.
Title: The global solar corona defined by newly emerged flux at
low latitudes
Authors: Li, J.; LaBonte, B.; Acton, L. W.; Slater, G. L.
Bibcode: 2001AGUSM..SH41B05L
Altcode:
The global structure of the solar corona changes greatly from solar
minimum to maximum. To understand the details of this restructuring,
we analyzed the full disk Yohkoh/SXT images taken during the years
1996 to 1998. We found that the global coronal structures are largely
defined by a handful of active regions which emerged at low latitudes
in the first year of the new solar cycle following solar minimum. The
rotation rates of the structures agree with photospheric differential
rotation rate of their associated active region. Following the first
year of the solar cycle, newly emerged flux tends to appear near the
old flux, which may give the impression of the existence of an active
longitude; however, the coronal structure is clearly associated in
space and time with the new flux, not some average location.
Title: Temperature Tomography of the Soft X-Ray Corona: Measurements
of Electron Densities, Tempuratures, and Differential Emission
Measure Distributions above the Limb
Authors: Aschwanden, Markus J.; Acton, Loren W.
Bibcode: 2001ApJ...550..475A
Altcode:
We analyze long-exposure and off-pointing Yohkoh/SXT data of
the solar corona observed on 1992 August 26. We develop a new
(temperature) tomography method that is based on a forward-fitting
method of a four-parameter model to the observed soft X-ray fluxes
F1(h) and F2(h) of two SXT wavelength filters
as a function of height h. The model is defined in terms of a
differential emission measure (DEM) distribution dEM(h, T)/dT,
which includes also a temperature dependence of density scale
heights λn(T)=qλλT and allows
us to quantify deviations (qλ≠1) from hydrostatic
equilibrium (qλ=1). This parametrization facilitates a
proper line-of-sight integration and relates the widely used filter
ratio temperature TFR to the peak of the DEM distribution. A
direct consequence of the multi-scale height atmosphere is that the
filter ratio temperature TFR(h) is predicted to increase
with height, even if all magnetic field lines are isothermal. Our
model fitting reveals that coronal holes and quiet-Sun regions are
in perfect hydrostatic equilibrium but that coronal streamers have
a scale height that exceeds the hydrostatic scale height by a factor
of up to qλ<~2.3, which underscores the dynamic nature
of coronal streamers. Our density measurements in coronal holes are
slightly lower than most of the white-light polarized brightness
inversions and seem to come closer to the requirements of solar wind
models. Our DEM model provides also a physical framework for the
semiempirical Baumbach-Allen formula and quantifies the temperature
ranges and degree of hydrostaticity of the K, L, and F coronae.
Title: Solar Polar Rays Are Not Polar
Authors: Li, J.; Jewitt, D.; Labonte, B.; Acton, L.
Bibcode: 2001IAUS..203..404L
Altcode:
In this poster we discuss the nature of polar rays, based on limb
synoptic images constructed from Yohkoh/SXT and SOHO/EIT data. In
the literature, polar rays and polar plumes are often mentioned
interchangably. We find that polar rays are projection effects caused
by hot plasma from equatorial active areas and are not physically
associated with the coronal polar holes. Instead, the rise in number
and strength of polar rays toward solar activity maximum is responsible
for hiding the polar holes and polar plumes. We will present the limb
synoptic maps and simple physical models to lead to this result.
Title: A Method to Determine the Heating Mechanisms of the Solar
Corona
Authors: Priest, E. R.; Foley, C. R.; Heyvaerts, J.; Arber, T. D.;
Mackay, D.; Culhane, J. L.; Acton, L. W.
Bibcode: 2000ApJ...539.1002P
Altcode:
One of the paradigms about coronal heating has been the belief that the
mean or summit temperature of a coronal loop is completely insensitive
to the nature of the heating mechanisms. However, we point out that
the temperature profile along a coronal loop is highly sensitive to
the form of the heating. For example, when a steady state heating
is balanced by thermal conduction, a uniform heating function makes
the heat flux a linear function of distance along the loop, while
T7/2 increases quadratically from the coronal footpoints;
when the heating is concentrated near the coronal base, the heat flux
is small and the T7/2 profile is flat above the base;
when the heat is focused near the summit of a loop, the heat flux
is constant and T7/2 is a linear function of distance
below the summit. It is therefore important to determine how the
heat deposition from particular heating mechanisms varies spatially
within coronal structures such as loops or arcades and to compare it
to high-quality measurements of the temperature profiles. We propose
a new two-part approach to try and solve the coronal heating problem,
namely, first of all to use observed temperature profiles to deduce the
form of the heating, and second to use that heating form to deduce the
likely heating mechanism. In particular, we apply this philosophy to
a preliminary analysis of Yohkoh observations of the large-scale solar
corona. This gives strong evidence against heating concentrated near the
loop base for such loops and suggests that heating uniformly distributed
along the loop is slightly more likely than heating concentrated at
the summit. The implication is that large-scale loops are heated in
situ throughout their length, rather than being a steady response
to low-lying heating near their feet or at their summits. Unless
waves can be shown to produce a heating close enough to uniform, the
evidence is therefore at present for these large loops more in favor
of turbulent reconnection at many small randomly distributed current
sheets, which is likely to be able to do so. In addition, we suggest
that the decline in coronal intensity by a factor of 100 from solar
maximum to solar minimum is a natural consequence of the observed
ratio of magnetic field strength in active regions and the quiet Sun;
the altitude of the maximum temperature in coronal holes may represent
the dissipation height of Alfvén waves by turbulent phase mixing;
and the difference in maximum temperature in closed and open regimes
may be understood in terms of the roles of the conductive flux there.
Title: Simultaneous SOHO andYohkoh Observations of a Small Solar
Active Region
Authors: Griffiths, N. W.; Fisher, G. H.; Woods, D. T.; Acton, L. W.;
Siegmund, O. H. W.
Bibcode: 2000ApJ...537..481G
Altcode:
We present new results from observations of a small active region
taken with the SUMER spectrograph and Extreme Ultraviolet Imaging
Telescope (EIT) on SOHO and with the Soft X-Ray Telescope (SXT) on
Yohkoh. The SUMER study features line and continuum emission covering
a wide range of temperatures and includes the density-sensitive O
IV] λ1400 multiplet. A more extensive analysis of quiet Sun data
presented in a previous paper is also included. The presence of
a power-law relationship between emission-line power and electron
density is confirmed, although the exponents in the active region are
slightly higher than those found in the quiet Sun. These power-law
relationships suggest that the volume filling factor decreases with
increasing density and indicate possible differences between emitting
material in active regions and the quiet Sun. We study active-region
emission measures from both the SUMER and SXT data sets. For the active
region as a whole, SXT temperatures and emission measures appear to
fit smoothly onto the emission-measure distribution determined from
cooler transition region and coronal lines in the SUMER spectra. We
find no significant variation in the shape of the lower transition
region emission-measure distribution for different subregions of the
data set. This reinforces the conclusion from the original quiet Sun
paper that ensembles of ``cool loops'' are unlikely to be the source
of this emission unless the structures are considerably smaller than
the 1" spatial resolution of SUMER.
Title: Global Solar Corona Revealed by Time Series Observations
Authors: Li, Jing; Kuhn, J.; LaBonte, B.; Raymond, J. C.; Acton, L. W.
Bibcode: 2000ApJ...538..415L
Altcode:
Time series observations at UV (Extreme Ultraviolet Imaging
Telescope/Solar and Heliospheric Observatory) and X-ray (Soft
X-Ray Telescope/Yohkoh) wavelengths reveal properties of the global
solar corona that are not easily identified in a single image. A
median-filtering technique that rejects features varying with time
is used to isolate background corona. The coronal hole boundaries,
polar plumes, and polar rays in the inner corona are clearly seen in
Extreme Ultraviolet Imaging Telescope data over 5 months during the
last solar minimum (1996 January through May). For the first time,
we provide physical evidence for coronal hole boundaries in the
inner corona. The observations show clearly that the polar coronal
holes expand divergently with height. A simple latitudinal and radial
electron density distribution for the inner corona is found.
Title: Soft X-ray Luminosity and Photospheric Magnetic Field in
Quiet Sun.
Authors: Pevtsov, A. A.; Acton, L. W.
Bibcode: 2000SPD....31.1306P
Altcode: 2000BAAS...32..845P
We are using full disk soft X-ray data from Yohkoh and Kitt Peak
daily magnetograms to study the coronal luminosity and photospheric
magnetic field in the quiet Sun between 1991 November and 1998
December. For every image of our data set we extract three areas
4 by 4 solar degrees in size centered at 00N00W, 50N00W and 50S00W
and compute X-ray luminosity and unsigned magnetic flux for each of
these areas. Between 1991 (active Sun) and 1996 (quiet Sun) the X-ray
luminosity at the heliographic center decreases by more than a factor
of 7...while the magnetic flux decreases by only a factor of 2. A
similar tendency is observed for our high latitude samples. Apart
from the cycle-related variations, all three areas of quiet Sun
exhibit significant non-periodic changes in X-ray luminosity. These
variations occur on 9-12 month intervals and clearly correlate
with increase/decrease in sunspot activity. Similar variations are
present in the total X-ray irradiance averaged over the solar disk. On
the contrary, the magnetic fluxes from the same areas of quiet Sun
show no corresponding variations on this time scale. In our opinion,
coronal heating models based on the reconnection of quiet sun magnetic
elements (variously called chromospheric network, "magnetic carpet"
or "salt and pepper" field) can not explain the million degree corona
observed by the Yohkoh soft X-ray telescope. We conclude that the X-ray
luminosity in the quiet Sun (at least in the Yohkoh temperature range,
>2 MK) is primarily associated with the strong magnetic fields of
active regions, not with weak photospheric fields. To further support
this conclusion, we show one example of a dramatic change in X-ray
luminosity over the entire visible corona that was associated with
the emergence of a single small active region.
Title: Physical properties of solar inner corona revealed by time
series observations made by EIT and SXT.
Authors: Li, J.; Labonte, B. J.; Acton, L. W.
Bibcode: 2000BAAS...32R.815L
Altcode:
No abstract at ADS
Title: Shapes, Colors and Motions on the Sun
Authors: Acton, L. W.
Bibcode: 2000AAS...196.5801A
Altcode: 2000BAAS...32..765A
My scientific research has been propelled by overriding curiosities
about: What does it look like? What is its spectrum? How does it move
and change? Fortunately, for the solar astronomer, it is increasingly
possible to satisfy such curiosities in exquisite detail. In this
lecture I will trace the evolution of our knowledge of solar coronal
morphology and spectroscopy since the dawn of the space age. Most of
what we see is shaped and driven by solar magnetism. The magnetic
topology must needs be represented by cartoons which grasp the
salient points of the structures before the astrophysics, of such a
complex region as a magnetized stellar atmosphere, can be modelled
and understood. I shall endeavor to illustrate how the observations,
and the cartoons, are improving. I am grateful for NASA funding,
the primary support of my research over 36 years.
Title: Evolution of the Solar Corona From Cycle 22 to Cycle 23 As
Revealed by X-ray Limb Synoptic Maps
Authors: Slater, G. L.; Freeland, S. L.; LaBonte, B. J.; Li, J.;
Acton, L. W.
Bibcode: 2000SPD....31.0228S
Altcode: 2000BAAS...32..815S
How does the solar corona vary within a solar cycle? We present time
series observations assembled from the Yohkoh Soft X-ray Telescope
(SXT) full mission image database which reveal coronal structures in the
inner corona that are not easily identifiable in individual images. We
have generated limb synoptic maps prepared from SXT data taken over
8 years (1992 through 1999). This period covers the decay phase of
solar cycle 22 and the rise phase of solar cycle 23. The SXT images
have recently been re-calibrated using more precise techniques. We
will address such topics as the variation of the polar coronal holes,
the lifetimes of active regions and associated streamers, and the
nature of polar plumes in the two phases of the solar cycle.
Title: Physical Properties of Solar Inner Corona Revealed by Time
Series Observations made by EIT and SXT
Authors: Li, J.; LaBonte, B. J.; Acton, L. W.
Bibcode: 2000SPD....31.0227L
Altcode: 2000BAAS...32..815L
We present a useful way to study how the solar corona varies with
time. Observations by EIT and SXT are used to construct limb synoptic
maps. The maps show limb emissions as functions of polar angle and
time. We identify and interpret various persistent structures such as
coronal hole boundaries, polar rays, and polar plumes seen on these
maps. The physical properties of these structures are discussed.An
empirical 3-dimensional electron density model for the background
corona will be introduced. This model was obtained through the use of
a median-filtering technique on the EIT coronal images obtained during
solar minimum (January through May 1996). This work was supported by
NASA grant NAG5-4941 and by a subcontract with LMSAL in support of
NASA contract NAS8-40801 for YOHKOH SXT.
Title: Jovian X-ray emission from solar X-ray scattering
Authors: Maurellis, Ahilleas N.; Cravens, Thomas E.; Gladstone,
G. Randall; Waite, J. Hunter; Acton, Loren W.
Bibcode: 2000GeoRL..27.1339M
Altcode:
Soft x-ray emissions with brightnesses of about 0.01-0.2 Rayleighs
have been observed from both the equatorial and auroral regions of
Jupiter. It has been proposed that the equatorial emission, like the
auroral emission, may be largely due to precipitation of energetic
heavy ions into the atmosphere [Waite et al., 1997]. In this paper
we model two alternative mechanisms for low-latitude x-ray emission:
(1) elastic scattering of solar x-rays by atmospheric neutrals,
(2) fluorescent scattering of carbon K-shell x-rays from methane
molecules located below the jovian homopause. Our modeled brightnesses
agree, up to a factor of two, with the bulk of low-latitude ROSAT
measurements. This suggests that solar photon scattering (approximately
90% elastic scattering) may act in conjunction with energetic heavy
ion precipitation to generate jovian equatorial x-ray emission.
Title: A Method for Characterizing Rotation Rates in the Soft
X-Ray Corona
Authors: Weber, M. A.; Acton, L. W.; Alexander, D.; Kubo, S.; Hara, H.
Bibcode: 1999SoPh..189..271W
Altcode:
Differential rotation rates of soft X-ray features in the solar
corona are quantified by a method of harmonic filtering using
the Lomb-Scargle periodogram. This approach leads reasonably to a
quantitative discrimination between uncertainty estimates and spectral
leakage of the fundamental rotation frequency due to the presence
of multiple rotating tracers. Mean rotation rates as a function of
latitude and year are calculated for the years 1992-1997 (roughly
the declining phase of the last solar activity cycle). The corona is
found to have a small but measurable latitudinal gradient in rotation
rate. The presence of multiple features places a lower bound of 1-2%
on the relative uncertainties with which a `mean' rotation rate can
be measured. The results are compared with autocorrelation estimates
and found to agree within 1σ.
Title: Deriving solar X ray irradiance from Yohkoh observations
Authors: Acton, Loren W.; Weston, David C.; Bruner, Marilyn E.
Bibcode: 1999JGR...10414827A
Altcode:
The soft X ray telescope (SXT) on Yohkoh produces calibrated images
which are used to derive X ray irradiance and coronal temperature. In
this paper we compute the mean temperature and spectral irradiance for
25 coronal differential emission measure (DEM) models ranging from very
quiet solar conditions to the main phase of large flares. These results
are compared to what would have been derived from an SXT observation
of the solar conditions represented by the DEM models. After applying
an empirical correction algorithm, derived as part of this work, 24
of 25 SXT filter-ratio temperatures fall within 0.1 in log10(T) of
the mean DEM temperatures. The irradiance error depends both on the
chosen spectral band and the spectrum, here represented by a single
plasma temperature. As expected, the irradiance is more accurate and
less sensitive to spectrum for bands that match the band pass of the
sensor. Even a crude color temperature approximation to the coronal X
ray spectrum greatly improves the accuracy of derived radiance over
values derived for an assumed temperature of 3×106K:
the mean temperature of all DEM. An RMS improvement of a factor of
12-17 was found for the cases studied. For two bands, which match
the sensor band pass, the RMS accuracy is better than 10%. Yohkoh X
ray irradiance data, converted with SXT color temperatures, will be
placed in the National Geophysical Data Center for use by the solar
and atmospheric physics communities.
Title: The transition region and coronal explorer
Authors: Handy, B. N.; Acton, L. W.; Kankelborg, C. C.; Wolfson, C. J.;
Akin, D. J.; Bruner, M. E.; Caravalho, R.; Catura, R. C.; Chevalier,
R.; Duncan, D. W.; Edwards, C. G.; Feinstein, C. N.; Freeland, S. L.;
Friedlaender, F. M.; Hoffmann, C. H.; Hurlburt, N. E.; Jurcevich,
B. K.; Katz, N. L.; Kelly, G. A.; Lemen, J. R.; Levay, M.; Lindgren,
R. W.; Mathur, D. P.; Meyer, S. B.; Morrison, S. J.; Morrison, M. D.;
Nightingale, R. W.; Pope, T. P.; Rehse, R. A.; Schrijver, C. J.;
Shine, R. A.; Shing, L.; Strong, K. T.; Tarbell, T. D.; Title, A. M.;
Torgerson, D. D.; Golub, L.; Bookbinder, J. A.; Caldwell, D.; Cheimets,
P. N.; Davis, W. N.; Deluca, E. E.; McMullen, R. A.; Warren, H. P.;
Amato, D.; Fisher, R.; Maldonado, H.; Parkinson, C.
Bibcode: 1999SoPh..187..229H
Altcode:
The Transition Region and Coronal Explorer (TRACE) satellite, launched
2 April 1998, is a NASA Small Explorer (SMEX) that images the solar
photosphere, transition region and corona with unprecedented spatial
resolution and temporal continuity. To provide continuous coverage
of solar phenomena, TRACE is located in a sun-synchronous polar
orbit. The ∼700 Mbytes of data which are collected daily are made
available for unrestricted use within a few days of observation. The
instrument features a 30-cm Cassegrain telescope with a field of view
of 8.5×.5 arc min and a spatial resolution of 1 arc sec (0.5 arc sec
pixels). TRACE contains multilayer optics and a lumogen-coated CCD
detector to record three EUV wavelengths and several UV wavelengths. It
observes plasmas at selected temperatures from 6000 K to 10 MK with
a typical temporal resolution of less than 1 min.
Title: Making YOHKOH SXT Images Available to the Public: The YOHKOH
Public Outreach Project
Authors: Larson, M. B.; McKenzie, D.; Slater, T.; Acton, L.; Alexander,
D.; Freeland, S.; Lemen, J.; Metcalf, T.
Bibcode: 1999AAS...194.7024L
Altcode: 1999BAAS...31..941L
The NASA funded Yohkoh Public Outreach Project (YPOP) provides public
access to high quality Yohkoh SXT data via the World Wide Web. The
products of this effort are available to the scientific research
community, K-12 schools, and informal education centers including
planetaria, museums, and libraries. The project utilizes the intrinsic
excitement of the SXT data, and in particular the SXT movies, to develop
science learning tools and classroom activities. The WWW site at URL:
http://solar.physics.montana.edu/YPOP/ uses a movie theater theme to
highlight available Yohkoh movies in a format that is entertaining and
inviting to non-scientists. The site features informational tours of
the Sun as a star, the solar magnetic field, the internal structure
and the Sun's general features. The on-line Solar Classroom has proven
very popular, showcasing hand-on activities about image filtering,
the solar cycle, satellite orbits, image processing, construction of a
model Yohkoh satellite, solar rotation, measuring sunspots and building
a portable sundial. The YPOP Guestbook has been helpful in evaluating
the usefulness of the site with over 300 detailed comments to date.
Title: Potential Field Source Surface Simulations of Soft X-ray
Corona Variability During the Solar Cycle
Authors: Lean, J. L.; Wang, Y. -M.; Mariska, J. T.; Acton, L. W.
Bibcode: 1999AAS...194.9208L
Altcode: 1999BAAS...31..987L
Magnetic fields that emerge in the solar photosphere and extend upwards
into the corona are associated with coronal heating. Some studies have
determined empirically that coronal brightness depends directly on
photospheric field strength, whereas others relate the brightness to the
length of the loops or to the sheering of opposite polarity fields. We
use the potential field source surface (PFSS) model of Wang and Sheeley
(ApJ, 392, 310, 1992) to investigate the applicability of a range of
quantitative associations between photospheric magnetic fields and the
global brightness of the non-flaring soft X-ray corona, recorded in full
disk X-rays images made by the SXT on Yohkoh. The model extrapolates all
photospheric magnetic field lines, in both active regions and smaller
scale features, into the corona. For an assigned coronal temperature of
1.5E6 K, the model determines coronal density by assuming hydrostatic
equilibrium along each closed field line and using adopted scaling
laws to relate the footpoint density to the magnetic field and/or
loop length. Integrating the brightness along the line of sight then
permits direct simulation of the independently measured SXT full disk
coronal images. With the NSO Carrington magnetic field maps as input,
the PFSS simulations can account for 85 global X-ray corona during
the six years from 1992 to 1997. This agreement is achieved using
a constant coronal temperature and a function that depends on both
the absolute strength of the photospheric magnetic field footprints,
and on the inverse loop length. Despite the overall good agreement
of the simulations and observations, significant differences occur
during some Carrington rotations. Simulations that utilize inputs
from three independent ground-based observatories (NSO, WSO and MWO)
can also at times differ significantly from each other. NASA Office
of Space Science has funded this work.
Title: SOHO and YOHKOH Observations of a Small Active Region
Authors: Fisher, G. H.; Griffiths, N. W.; Woods, D. T.; Acton, L. W.;
Siegmund, O. H. W.
Bibcode: 1999AAS...194.1603F
Altcode: 1999BAAS...31..850F
We describe the observation of a small active region on 19 April
1997 using the SXT telescpe on Yohkoh, the EIT telescope on SOHO,
and the SUMER spectrograph on SOHO. The SUMER observation resulted in
a rastered image of approximately 13 by 70 arcseconds, with a spatial
resolution of approximately 1 square arcsecond. The SUMER data included
transition region lines of N and O, as well as chromospheric continuum
emission and Fe XII emission from the corona. Density sensitive O IV
lines, in conjunction with the other observed emission allows us to
simultaneously determine density and emission measure distributions. We
study the variation of density, emission measure, and emitting volume
within the observed region and compare our results with a similar
study of the Quiet Sun.
Title: A Stable Filament Cavity with a Hot Core
Authors: Hudson, H. S.; Acton, L. W.; Harvey, K. L.; McKenzie, D. E.
Bibcode: 1999ApJ...513L..83H
Altcode:
We present observations of a long-lived solar filament cavity with
soft X-ray sources along its axis. This structure appeared above the
southern polar crown polarity-inversion line for approximately three
rotations during 1997 June-August, centered at a west-limb passage on
approximately July 3. At the limb, the Yohkoh soft X-ray data showed
a bright region situated above and around the projected filament
location but near the axis of the cavity. We describe measurements
of the geometry of the cavity, which we interpret as a flux rope that
is partially embedded in the photosphere, and use the Yohkoh data to
describe the physical parameters of the structure. We find that the
core consists of an unresolved mass of filamentary substructures, with a
volume filling factor significantly less than unity for the soft X-ray
telescope (SXT) resolution. The core has a higher temperature than the
cavity surrounding it, ruling out explanations in terms of a transition
region supported by thermal conduction. Transient activity occurred in
the polar crown region, but no detectable destabilization or eruption
of the cavity structure resulted from it. We suggest that the bright
structure at the core of the cavity corresponds to higher altitude
coronal segments of the field lines that support the filament material.
Title: Super-X: a soft x-ray telescope for Solar-B
Authors: Acton, Loren W.; Bookbinder, Jay A.; Bruner, Marilyn E.;
Golub, Leon; Lemen, James R.
Bibcode: 1998SPIE.3442..140A
Altcode:
This paper describes the conceptual design of a soft x-ray
telescope, super-x, which we will propose for the Japan/US/UK Solar-B
mission. Super-X will break new ground in both angular resolution
and solar coronal temperature discrimination. The telescope design
is based upon the successful transition region and coronal explorer
instrument. It features four XUV spectral channels spanning the 0.3 to
20 MK temperature range with an angular resolution of approximately 0.27
seconds of arc. We will describe considerations affecting spectral line
selection and some details of the characteristics of the instrument.
Title: Physical Structure of a Coronal Streamer in the Closed-Field
Region as Observed from UVCS/SOHO and SXT/Yohkoh
Authors: Li, J.; Raymond, J. C.; Acton, L. W.; Kohl, J. L.; Romoli,
M.; Noci, G.; Naletto, G.
Bibcode: 1998ApJ...506..431L
Altcode:
We analyze a coronal helmet streamer observed on 1996 July 25 using
instruments aboard two solar spacecraft, the Ultraviolet Coronagraph
Spectrometer (UVCS) on board Solar and Heliospheric Observatory
(SOHO) and the Soft X-Ray Telescope (SXT) on board Yohkoh. We derive
temperatures and electron densities at 1.15 R⊙ from
SXT/Yohkoh observations. At this height, the streamer temperature is
about log T (K) = 6.28 +/- 0.05, and the electron density is about
log ne(cm-3) = 8.09 +/- 0.26, while at 1.5
R⊙ a temperature of log T (K) = 6.2 and a density of log
ne(cm-3) = 7.1 are obtained by UVCS/SOHO. Within
the measurement uncertainty this suggests a constant temperature from
the base of the streamer to 1.5 R⊙. Electron density
measurements suggest that the gas in the streamer core is close to
hydrostatic equilibrium. Comparison with potential field models for the
magnetic field suggests a plasma β larger than 1 in the closed-field
region in the streamer. In deriving electron densities and temperatures
from the SXT/Yohkoh data, we include the effects of abundance anomalies
on the SXT filter response. We use the elemental abundances derived from
the UVCS/SOHO observations to estimate the first ionization potential
and gravitational settling effects. We then give the set of abundances
for the solar corona, which agrees with our observations. In addition,
we analyzed the SXT data from 6 consecutive days. We found that from
1996 July 22 to July 27, the physical properties of the streamer are
nearly constant. We conclude that we may be observing the same loop
system over 6 days.
Title: Nitric oxide abundance in the mesosphere/lower thermosphere
region: Roles of solar soft X rays, suprathermal N(4S)
atoms, and vertical transport
Authors: Swaminathan, P. K.; Strobel, D. F.; Kupperman, D. G.;
Kumar, C. Krishna; Acton, L.; DeMajistre, R.; Yee, J. -H.; Paxton,
L.; Anderson, D. E.; Strickland, D. J.; Duff, J. W.
Bibcode: 1998JGR...10311579S
Altcode:
This paper carefully examines the inability of photochemical
models to account for the large nitric oxide densities of
~108cm-3 at ~105 km obtained from IR, UV, and
microwave measurements. A detailed and up-to-date photochemical model
is constructed that incorporates measured YOHKOH soft X ray fluxes, hot
N atom chemistry with an energy dependent thermalization cross section
and seven reaction sources, and laboratory-constrained N(2D)
yields. The resulting model which has well-constrained chemistry
compared to past models fails to generate high enough NO densities
in comparison with the most reliable measurements of absolute NO
concentrations in the lower thermosphere. The sensitivity of the model
results and the known uncertainties in the inputs are used to identify
where future efforts should be focused. A deficit remains despite an
increase in the vertical mixing rates in the lower thermosphere from
the very low Kzz profile used in our calculations and/or an
increase in the N(2D) yield from electron impact dissociation
of N2 from its nominal value of 0.54 to 0.62. The sensitivity
of NO profiles to the nascent energy distributions of the atmospheric
sources of suprathermal N atoms is illustrated by including the
thermalization of suprathermal N atoms with an updated thermalization
cross section. The diurnally averaged NO concentration at 105 km is
enhanced by factors of 1.2 and 2.6 when the energy distributions of the
N atoms from electron impact dissociation of N2 are chosen
with peaks near 0.6 eV or 3-4 eV, but deficits of factors of ~7 and ~3,
respectively, remain. There is higher sensitivity to vertical transport
than to variations of chemistry within known uncertainties.
Title: Nature of the heating mechanism for the diffuse solar corona
Authors: Priest, E. R.; Foley, C. R.; Heyvaerts, J.; Arber, T. D.;
Culhane, J. L.; Acton, L. W.
Bibcode: 1998Natur.393..545P
Altcode:
The temperature of the Sun's outer atmosphere (the corona) exceeds that
of the solar surface by about two orders of magnitude, but the nature
of the coronal heating mechanisms has long been a mystery. The corona
is a magnetically dominated environment, consisting of a variety of
plasma structures including X-ray bright points, coronal holes and
coronal loops. The latter are closed magnetic structures that occur
over a range of scales and are anchored at each end in the solar
surface. Large-scale regions of diffuse emission are made up of many
long coronal loops. Here we present X-ray observations of the diffuse
corona from which we deduce its likely heating mechanism. We find that
the observed variation in temperature along a loop is highly sensitive
to the spatial distribution of the heating. From a comparison of
the observations and models we conclude that uniform heating gives
the best fit to the loop temperature distribution, enabling us to
eliminate previously suggested mechanisms of low-lying heating near
the footpoints of a loop. Our findings favour turbulent breaking and
reconnection of magnetic field lines as the heating mechanism of the
diffuse solar corona.
Title: First Results from the TRACE Mission
Authors: Title, A.; Tarbell, T.; Schrijver, C.; Wolfson, J.; Shine,
R.; Hurlburt, N.; Golub, L.; Deluca, E.; Bookbinder, J.; Handy, B.;
Acton, L.; Harrison, R.; Delaboudinere, J. -P.
Bibcode: 1998AAS...192.1507T
Altcode: 1998BAAS...30..841T
The TRACE spacecraft was launched on 1 April and all systems are
functioning as designed. The initial outgassing period will conclude
on 20 April and the science program will then begin. TRACE is a UV-EUV
imager with one arc second spatial resolution and is capable of taking
images with a cadence as high as two seconds. We will present images
and image sequences. We hope to present initial comparisons of magnetic
evolution and transition region and coronal brightenings.
Title: Studies of Coronal Temperature
Authors: Acton, L. W.; Lemen, J. R.
Bibcode: 1998ASSL..229...15A
Altcode: 1998opaf.conf...15A
No abstract at ADS
Title: Yohkoh Soft X-Ray Determination of Plasma Parameters in a
Polar Coronal Hole
Authors: Foley, Carl R.; Culhane, J. Leonard; Acton, Loren W.
Bibcode: 1997ApJ...491..933F
Altcode:
The Yohkoh Soft X-Ray Telescope (SXT) has been used to study the
emission from a coronal hole surrounding the north pole of the
Sun. Stronger emission from closed coronal structures in the line of
sight can interfere with attempts to measure properties of coronal hole
plasma. SXT observations indicate that the north polar region was free
of such contamination on 1992 October 3. Measured X-ray intensities,
corrected for background and scattered X-rays, are compared with a
theoretical coronal hole model. They are found to be broadly consistent
with the model predictions for variation of intensity with height and
for limb brightening, although the electron density is lower than
would be appropriate for model predictions based on solar maximum
densities. Electron temperatures estimated by the filter ratio method
are also consistent with the model and with an in situ estimate of the
maximum electron temperature in the solar wind by the Ulysses ionic
charge composition experiment.
Title: X-Ray Network Flares of the Quiet Sun
Authors: Krucker, Säm; Benz, Arnold O.; Bastian, T. S.; Acton,
Loren W.
Bibcode: 1997ApJ...488..499K
Altcode:
Temporal variations in the soft X-ray (SXR) emission and the radio
emission above the solar magnetic network of the quiet corona are
investigated using Yohkoh SXR images with deep exposure and VLA
observations in the centimeter radio range. The SXR data show several
brightenings, with an extrapolated occurrence probability of one
brightening per 3 seconds on the total solar surface. During the roughly
10 minutes of enhanced flux, total radiative losses of the observed
plasma are around 1025 ergs per event. These events are more
than an order of magnitude smaller than previously reported X-ray bright
points or active region transient brightenings. For all of the four
SXR events with simultaneous radio observations, a corresponding radio
source correlating in space and time can be found. There are several
similarities between solar flares and the SXR/radio events presented in
this paper. (1) Variations in temperature and emission measure during
the SXR enhancements are consistent with evaporation of cooler material
from the transition region and the chromosphere. (2) The ratio of the
total energies radiated in SXR and radio frequencies is similar to
that observed in flares. (3) At least one radio event shows a degree
of polarization as high as 35%. (4) In three out of four substructures
the centimeter radio emission peaks several tens of seconds earlier
than in the SXR emission. (5) The associated radio emission tends to
be more structured and to have faster rise times. These events thus
appear to be flare-like and are called network flares.
Title: Coronal Heating and the Vertical Temperature Structure of
the Quiet Corona
Authors: Wheatland, M. S.; Sturrock, P. A.; Acton, L. W.
Bibcode: 1997ApJ...482..510W
Altcode:
The radial variation of temperature in the inner corona is examined
using long-exposure Yohkoh images of two regions of diffuse (quiet)
corona. The results indicate a steady radial increase of temperature for
both regions, out to 0.7 and 0.95 solar radii above the limb. We find
that the filter-ratio data for the two regions is well fitted in each
case by integration over the line of sight of a spherically symmetric
model atmosphere in hydrostatic equilibrium and with a temperature
profile due to a conserved inward heat flux. An Abel inversion process
is also applied to the data, and this gives results consistent with
the spherically symmetric, conserved-heat flux model. These results
imply that the nonthermal energy responsible for heating these regions
of the quiet corona is being deposited beyond the observed range of
heights. However, the diffuse regions we examined are believed to be
partly closed-field regions, and so, the radial models require careful
interpretation. We discuss the implications for coronal heating in
these regions.
Title: The Solar-B Mission
Authors: Antiochos, Spiro; Acton, Loren; Canfield, Richard; Davila,
Joseph; Davis, John; Dere, Kenneth; Doschek, George; Golub, Leon;
Harvey, John; Hathaway, David; Hudson, Hugh; Moore, Ronald; Lites,
Bruce; Rust, David; Strong, Keith; Title, Alan
Bibcode: 1997STIN...9721329A
Altcode:
Solar-B, the next ISAS mission (with major NASA participation), is
designed to address the fundamental question of how magnetic fields
interact with plasma to produce solar variability. The mission has
a number of unique capabilities that will enable it to answer the
outstanding questions of solar magnetism. First, by escaping atmospheric
seeing, it will deliver continuous observations of the solar surface
with unprecedented spatial resolution. Second, Solar-B will deliver the
first accurate measurements of all three components of the photospheric
magnetic field. Solar-B will measure both the magnetic energy driving
the photosphere and simultaneously its effects in the corona. Solar-B
offers unique programmatic opportunities to NASA. It will continue an
effective collaboration with our most reliable international partner. It
will deliver images and data that will have strong public outreach
potential. Finally, the science of Solar-B is clearly related to the
themes of origins and plasma astrophysics, and contributes directly
to the national space weather and global change programs.
Title: Solar origin of the 26-day periodicity observed by Ulysses
Authors: Bai, T.; Hoeksema, J. T.; Weber, M.; Acton, L. W.
Bibcode: 1997JGR...102.9793B
Altcode:
The Ulysses spacecraft discovered that the interplanetary magnetic
sector structure went through a major restructuring in mid-1992. The
observed recurrence period changed from about 25.4 days to about 26.2
days. Another interesting discovery is that the solar wind speed,
energetic particle fluxes, and interplanetary magnetic field all varied
quasiperiodically with a similar 26.2-day period during Ulysses'
midlatitude passage south of the ecliptic in 1992-1993. In order to
find the solar origins of these interplanetary phenomena, we compared
Ulysses observations with relevant solar data. According to our study
the global pattern of the open magnetic field lines originating in the
photosphere changed drastically in June 1992, and this resulted in a
major restructuring of the interplanetary sector structure. After that
time the magnetic field pattern in the midlatitude and high-latitude
zones of the southern hemisphere was dominated by two large unipolar
regions (covering the entire longitude interval) that rotated with
a synodic period of about 28.5 days until mid-1993. Because the
heliographic longitude of the spacecraft remained the same while it
approached the Sun, the 26.2-day period seen by Ulysses is equivalent
to the terrestrial synodic period of 28.5 days. By analyzing soft X
ray data observed by the Yohkoh satellite we confirm the existence of
a stable lobe protruding from the polar coronal hole. This protrusion
persisted from 1992 until the end of the study in mid-1995.
Title: Co-Temporal Evolution of Magnetic Sources of Coronal and
Chromospheric/Photospheric Irradiance Variability
Authors: Lean, J. L.; Mariska, J. T.; Acton, L. W.
Bibcode: 1997SPD....28.1401L
Altcode: 1997BAAS...29R.917L
Magnetic features emergent from the Sun's convection zone modulate the
global radiative output throughout the solar atmosphere. Space-based
radiometric monitoring by instruments onboard Yohkoh and UARS have
tracked the decline in coronal soft X-rays concurrent with chromospheric
and photospheric UV global fluxes from high activity in 1991 to the
present solar minimum. Although these different global emissions can
at times vary in quite different ways they each exhibit components of
variability traceable to common magnetic variability sources, primarily
sunspots, bright plages and active network. Comparisons of the SXT
soft X-ray images of the corona with groundbased Ca K images of the
chromosphere/ photosphere permit the establishment of relationships
between these common magnetic sources in solar atmosphere regimes
with quite different temperatures and densities. While the impact
of large active regions seen in both the coronal and chromospheric
images provides the dominant modulation of the emissions (although with
different spectral signatures traceable to different spatial scales),
more diffuse, extended magnetic sources also contribute to solar cycle
changes in the overall background global emission of the corona as
well as the chromosphere/photosphere.
Title: Using the WWW to Make YOHKOH SXT Images Available to the
Public: The YOHKOH Public Outreach Project
Authors: Larson, M.; McKenzie, D.; Slater, T.; Acton, L.; Alexander,
D.; Freeland, S.; Lemen, J.; Metcalf, T.
Bibcode: 1997SPD....28.0231L
Altcode: 1997BAAS...29..898L
The Yohkoh Public Outreach Project (YPOP) is funded by NASA as one of
the Information Infrastructure Technology and Applications Cooperative
Agreement Teams to create public access to high quality Yohkoh SXT data
via the World Wide Web. These products are being made available to the
scientific research community, K-12 schools, and informal education
centers including planetaria, museums, and libraries. The project aims
to utilize the intrinsic excitement of the SXT data, and in particular
the SXT movies, to develop science learning tools and classroom
activities. The WWW site at URL: http://www.space.lockheed.com/YPOP/
uses a movie theater theme to highlight available Yohkoh movies in a
non-intimidating and entertaining format for non-scientists. The site
features lesson plans, 'solar' activities, slide shows and, of course,
a variety of movies about the Sun. Classroom activities are currently
undergoing development with a team of scientists and K-12 teachers
for distribution in late 1997. We will display the products currently
online, which include a solar classroom with activities for teachers,
background resources, and a virtual tour of our Sun.
Title: Conversion of YOHKOH X-ray Observations to Spectral Radiance
Authors: Acton, L.; Weston, D.; Bruner, M.
Bibcode: 1997SPD....28.0274A
Altcode: 1997BAAS...29..906A
The soft x-ray telescope on YOHKOH has returned some 50-100 full-disk
images of the sun per day since October 1991. This data set is useful
as a source of information on soft x-ray radiance from the sun and
irradiance of the atmosphere of the earth. The purpose of this paper is
to examine the errors inherent in deriving absolute spectral radiance
from the YOHKOH observations. Precise conversion of broad band x-ray
measurements to spectral radiance requires a knowledge of the spectrum
of the radiation. For solar measurements the spectrum is usually
known only in approximation and theoretical spectral models are used
to compute spectral radiance. A further problem is our poor knowledge
of the ever-changing distribution of temperature and emission measure
of the solar corona. Only if the distribution of emission measure with
temperature (the Differential Emission measure or DEM) is known can
spectral models be computed which reflect the true solar x-ray spectrum
with good accuracy. We have attempted to estimate the uncertainty
in x-ray spectral radiance derived from YOHKOH data by comparing the
``true'' and measured radiance values for solar DEM distributions from
the literature. Apart from possible systematic error the YOHKOH-derived
radiances are quite good (15% accuracy) for the primary spectral
passband of the telescope. Extrapolating to spectral bands at longer
wavelengths of interest to aeronomy gives increasing error.
Title: The TRACE Mission
Authors: Wolfson, J.; Bruner, M.; Jurcevich, B.; Lemen, J.; Schrijver,
K.; Shine, R.; Strong, K.; Tarbell, T.; Title, A.; Golub, L.;
Bookbinder, J.; Deluca, E.; Acton, L.; Handy, B.; Kankelborg, C.;
Fisher, R.
Bibcode: 1997SPD....28.0143W
Altcode: 1997BAAS...29..887W
The TRACE (Transition Region and Coronal Explorer) mission will explore
the connections between fine-scale magnetic fields and plasma structures
in the coronal, transition zone and temperature minimum regions of the
sun. TRACE will collect images of solar plasmas at temperatures from
10(4) to 10(7) K, with one arc second spatial resolution and excellent
temporal resolution and continuity. With a scheduled launch date of 15
December 1997, the mission will emphasize collaborative observations
with SoHO, enabling simultaneous observations of high-resolution images,
spectra, and magnetograms. The 30 cm aperture TRACE telescope uses four
normal-incidence coatings for the EUV and UV on quadrants of the primary
and secondary mirrors. Interference filters further isolate 5 different
UV bands. The images are co-aligned and internally stabilized against
spacecraft jitter. A 1024 x 1024 lumigen-coated CCD detector collects
images over an 8.5 x 8.5 arc minute field-of-view. LMATC, SAO, and GSFC
built the TRACE instrument, which was integrated with the GSFC-produced
SMEX spacecraft on 28 February (just over two years from the start
of its development). It will be put into a Sun-synchronous orbit and
operated in coordination with the SoHO Experiment Operations Facility at
GSFC. We are committed to maintaining a publicly accessible data base
for TRACE data. Browsing and data set requesting capabilities will be
provided at Web site www.space.lockheed.com/TRACE/TRACElinks.html. This
site already contains a large volume of information on the mission
including preliminary scientific observing programs and directions
as to how to participate in the mission now and in the future. This
project is supported by NASA contract NAS5-38099.
Title: Fine structure of the X-ray and radio emissions of the quiet
solar corona.
Authors: Benz, A. O.; Krucker, S.; Acton, L. W.; Bastian, T. S.
Bibcode: 1997A&A...320..993B
Altcode:
Two deep soft X-ray exposures of a quiet region on the Sun were made
with the SXT telescope on board the Yohkoh satellite on 20 Feb 1995. We
report on the spatial X-ray fine structure. Regions of enhanced X-ray
emission, more than two orders of magnitude fainter than previously
reported X-ray bright points, are loosely associated with bipolar
regions in the magnetic network. The power spectrum of quiet X-ray
images at small spatial scales is similar to that of active regions,
but exhibits a kink at a scale of =~25,000km, possibly connected to
the supergranular structure. The spatial X-ray structures in the
time averaged image amount to an rms amplitude which is 6% of the
mean value. The X-ray structures correlate with contemporaneous radio
maps obtained by the VLA at wavelengths of 1.3, 2.0, and 3.6cm. The
amplitude of the brightness variations in the images increases with
radio wavelength, i.e., with increasing height. The cross-correlation
coefficient with the absolute magnetic field strength, however,
generally decreases with height, consistent with the idea of bipolar
regions in the network and of the magnetic field deviating from
vertical in the upper chromosphere. The X-ray observations require an
enhanced pressure in the corona above the magnetic network, but suggest
similar temperatures. Model calculations show that, under a constant
temperature, an rms density increase (relative to that in the cell
interior) ranging from about 20% in the chromosphere to 60% in the low
corona is sufficient to explain the observed standard deviations due
to the spatial structures in radio waves and soft X-rays, respectively.
Title: Using the World Wide Web to Make YOHKOH SXT Images Available
to the Public: The YOHKOH Public Outreach Project
Authors: McKenzie, D.; Larson, M. B.; Slater, T.; Acton, L.; Alexander,
D.; Freeland, S.; Lemen, J.; Metcalf, T.
Bibcode: 1997ESASP.404..561M
Altcode: 1997cswn.conf..561M
No abstract at ADS
Title: Limb Events Observed by YOHKOH and Coronal Mass Ejections:
A Filamentary Soft X-ray Structure on 5 October 1996
Authors: Watari, S.; Watanabe, Takashi; Acton, L. W.; Hudson, H. S.
Bibcode: 1997ESASP.404..725W
Altcode: 1997cswn.conf..725W
No abstract at ADS
Title: X-Ray/Radio Network Flares of the Quiet Sun
Authors: Benz, Arnold O.; Krucker, Sam; Acton, Loren W.; Bastian, T. S.
Bibcode: 1997IAUJD..19E...1B
Altcode:
The temporal variations in the soft X-ray (SXR) emission and the
radio emission above the solar magnetic network of the quiet corona
have been investigated using Yohkoh SXR images with deep exposure and
VLA observations in the centimetric radio range. The SXR data show
several brightenings with an extrapolated occurrence probability of
one brightening per 3 seconds on the total solar surface. During the
roughly 10 minutes of enhanced flux, the total radiative losses of
the observed plasma are betwee () n 0.6 and 2.4 cdot 1026
erg per event. These events are more than an order of magnitude smaller
than previously reported X-ray bright points or active region transient
brightenings. For all of the four SXR events with simultaneous radio
observations, a corresponding radio source correlating in space
and time can be found. There are several similarities between these
SXR/radio events and regular solar flares. These events thus appear
to be flare-like and are called network flares. We will report also
on very recent work using SOHO's EIT and CDS experiments combined with
VLA and Kitt Peak observations.
Title: Differential rotation rates in the soft X-ray solar corona
Authors: Weber, M.; Alexander, D.; Acton, L. W.
Bibcode: 1997MmSAI..68..495W
Altcode:
No abstract at ADS
Title: A Long-Duration Solar Flare with Mass Ejection and Global
Consequences
Authors: Hudson, H. S.; Acton, L. W.; Freeland, S. L.
Bibcode: 1996ApJ...470..629H
Altcode:
We report observations of a long-duration flare with mass ejection
from the corona, using the Yohkoh soft X-ray telescope (SXT). This
flare occurred 1994 November 13 near disk center during quiet solar
conditions, with excellent temporal coverage of both the core activity
in the active region itself and of the global corona. The initial
X-ray images reveal two arcades of cusped magnetic loops, connected
via a series of thin loops. These loops rise rapidly during the
increasing phase of soft X-ray flare brightness. In its final state,
the flare has the configuration of postflare loops with a cusp. Large
regions of the X-ray corona appear to empty during the evolution of the
event. We suggest that this corresponds a coronal mass ejection (CME)
seen in soft X-rays. Its detection in the SXT images is consistent with
the finding that material participating in a CME exists at elevated
coronal temperatures (2.8 x 106 K in this case) before
the ejection. We estimate a mass >4 x 1014 g for the
ejected material. The X-ray morphology of the event has strong points
of similarity with the classical reconnection picture of long-duration
event (LDE) formation, but there are significant discrepancies: there
is no observed inward flow during the rise phase, the expansions are
multiple and appear to be nonradial, and none of the observed motions
suggest a reconnection jet. We note the subsequent occurrence of very
large scale coronal disturbances, including regions near the boundaries
of coronal holes at both poles. We suggest that this global disturbance
implies a perturbation reaching as far outward as the heliospheric
neutral sheet. The exciter would require a horizontal velocity of
approximately 200 km s-1 in such a case, consistent with
the projected velocity of the plasma cloud that we identify with a
CME in the process of launching.
Title: Spectral Averaging of the Point Spread Function for Yohkoh's
SXT
Authors: Weston, David C.; Acton, Loren W.
Bibcode: 1996SoPh..168..215W
Altcode:
The point spread function of the soft X-ray telescope (SXT)
aboard the Yohkoh spacecraft is a Moffat function with elliptical
characteristics. This function has two parameters, a and b, that are
wavelength dependent in the X-ray region of interest. Since most SXT
data analysis is performed with respect to plasma temperature rather
than wavelength, it is useful to spectrally average these two parameters
over wavelength with temperature as the free variable. The results of
this spectral averaging are given here for users of SXT data.
Title: Solar identification of solar-wind disturbances observed
at Ulysses
Authors: Lemen, J. R.; Acton, L. W.; Alexander, D.; Galvin, A. B.;
Harvey, K. L.; Hoeksema, J. T.; Zhao, X.; Hudson, H. S.
Bibcode: 1996AIPC..382...92L
Altcode:
The Ulysses polar passages are producing a unique set of observations of
solar-wind disturbances at high heliographic latitudes. In this paper
we use the Yohkoh soft X-ray telescope (SXT) to locate some of these
events, as defined by the Ulysses/SWICS data, in the solar corona. Of 8
events, we identify two with flares, three with front-side large arcade
events, two with far-side events, and one was not seen in the Ulysses
data. The arcade events generally resemble long-duration flares seen
in active regions, but are larger, slower, and cooler. We present
Yohkoh images of each of these events. In the large arcade events
(see Alexander et al., 1996, for a detailed look at one of them) the
magnetic morphology at the location of the Yohkoh arcade is generally
consistent with the development of a large system of loops. Some of
the identifications are ambiguous, and we summarize the reasons for
this. From the SWICS data we have obtained ionization temperatures for
several events, and find that they have no obvious pattern in relation
to the X-ray temperatures; this may be expected on the basis that the
interplanetary plasma cloud is physically distinct from the plasma
trapped in the corona. Soft X-ray observations of the solar corona
show occasional occurrences of large-scale brightenings in the form
of arcades of loops. Such structures have been known since Skylab
(e.g., Sturrock, 1980), and have a clear relationship with coronal
mass ejections (e.g., Kahler, 1977). We now may study this phenomenon
statistically with the much more comprehensive Yohkoh observations;
with Yohkoh movies we can also begin to extend our knowledge to the
three-dimensional development of the structures. At the same time
Ulysses has sampled the latitude dependence of the interplanetary
effects. With this paper we introduce this subject and provide a
preliminary listing of events from the passage of Ulysses through
high heliographic latitudes. The starting point of the present
survey is a list of interplanetary plasma clouds (IPC's) derived
from Ulysses/SWICS data. These are essentially the same as the events
termed CMEs by Gosling et al. (1994a, 1994b). For this identification
the presence of bidirectional streaming in the suprathermal electron
distribution is one of the main criteria. We note that there are no
direct coronagraph observations, however. The Yohkoh observations
were examined at the apparent time of origin of each Ulysses event,
resulting in some clear and some less-certain identifications. We
also studied the ionization temperatures of the IPC material as a
beginning step to give the identifications a physical basis. There
has been little study thus far of the Yohkoh soft X-ray observations
in relationship to CMEs, which we believe to be closely related to
the interplanetary disturbances. Hiei et al. (1993) reported the only
Yohkoh event yet studied in conjunction with white-light coronagraph
observations. However Klimchuk et al. (1994) showed that X-ray eruptive
phenomena with parameters similar to those of CMEs occur frequently at
the limb, and there have been several studies of individual eruptive
events (e.g., Watanabe et al., 1992). Presently there is no systematic
knowledge of the X-ray coronal counterparts of CMEs, and the survey
represented here is part of the effort to rectify this situation.
Title: Yohkoh/SXT soft x-ray observations of sudden mass loss from
the solar corona
Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Freeland, S. L.;
Lemen, J. R.; Harvey, K. L.
Bibcode: 1996AIPC..382...88H
Altcode:
With soft X-ray imaging we can study the entire coronal volume, except
for cold inclusions such as prominences, as a function of time. This
should allow us to observe the origins of coronal mass ejections. We
report here an initial survey of the Yohkoh/SXT observations at the
times of reported or apparent mass ejections: three LDE flare events
and two large-scale arcade formations. For each of the events we
can easily detect sudden coronal dimming, which we interpret as the
launch interval of a CME. In one of the flare events we have found
a well-defined plasma cloud, apparently formed from a set of loop
structures, which rises and disappears during the growth phase of the
flare emission. Its mass amounted to some 4×1014 g with
a density of 3×108 cm-3 and a temperature of
2.8 MK before its disappearance.
Title: The solar origins of two high-latitude interplanetary
disturbances
Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Harvey, K. L.;
Kahler, S. W.; Kurokawa, H.; Lemen, J. R.
Bibcode: 1996AIPC..382...84H
Altcode:
Two extremely similar interplanetary forward/reverse shock events,
with bidirectional electron streaming, were detected by Ulysses in
1994 [Gosling et al., 1994]. Both events resulted in geomagnetic
storms and presumably were associated with coronal mass ejections. In
this paper we use the Yohkoh soft X-ray observations to characterize
the conditions in the lower corona at the times appropriate for the
launching of these two events. We find two strikingly different solar
events to be the likeliest candidates: an LDE flare on 20 Feb. 1994,
and a extremely large-scale arcade event on 14 April 1994.
Title: An Analytical Model for Fluted Sunspots and a New
Interpretation of Evershed Flow and X-Ray Anemones
Authors: Martens, Petrus C. H.; Hurlburt, Neal E.; Title, Alan M.;
Acton, Loren W.
Bibcode: 1996ApJ...463..372M
Altcode:
We present a force-free constant-α model for the magnetic field in and
above so-called "fluted" sunspots. This model is motivated by recent
high-resolution observations of Title et al. at the Swedish Solar
Observatory in La Palma. They observed that the inclination angle of
the magnetic field in the penumbra of sunspots oscillates rapidly
with azimuth, with a period of about 60 and an amplitude of about
18°. They further find that there is little variation in the radial
direction and in absolute field strength. The resulting phenomenon
of interlocking high- and low-inclination field lines was called
"flutedness. In our model, the parameters are chosen to reproduce
the La Palma magnetograms, and an analytical expression is obtained for
the three-dimensional magnetic field emanating from the sunspot's umbra
and penumbra. The model correctly reproduces the azimuthal variation in
inclination angle, as well as the mean constancy of the magnetic field
strength, and the appearance of a highly corrugated neutral line on
the limb side of off-center sunspots. We find that the "flutedness"
results in a highly complex topology in a boundary layer extending
from the photo sphere into the chromosphere, while the coronal field is
uniform. Title et al. demonstrated that the Evershed flow occurs
in regions of nearly horizontal magnetic field, and tacitly assumed,
as is done in most of the literature, that the dark filaments in which
the flow is observed form individual magnetic flux tubes. Our magnetic
field solution suggests that the regions of nearly horizontal field
at the photo spheric boundary may not form individual magnetic flux
tubes, but rather a series of short horizontal loops bridging a neutral
line that is stretched in the radial direction along the penumbra,
up to the outer penumbral boundary. Hence, the Evershed flow could not
be a simple siphon flow in the radial direction, but would consist of
phase-coordinated flows along the many short loops bridging the neutral
line. However, the assumption of a force-free field breaks down in
this region of the atmosphere, and the topology suggested by it may
not materialize in reality. We further demonstrate that there
are large variations in the photospheric cross sections of coronal
loops, due to the complexity of the field near their photospheric
footpoints. Under the assumption of constant energy input per unit
surface area into these loops, the variation in cross section is
qualitatively consistent with the variation in X-ray brightness of
loops in penumbral "anemones" observed by Yohkoh.
Title: TRACE: the Transition Region and Coronal Explorer
Authors: Schrijver, C.; Title, A.; Acton, L.; Bruner, M.; Fischer,
R.; Golub, L.; Harrison, R.; Lemen, J.; Rosner, R.; Scharmer, G.;
Scherrer, P.; Strong, K.; Tarbell, T.; Wolfson, J.
Bibcode: 1996AAS...188.6704S
Altcode: 1996BAAS...28..934S
The TRACE mission is designed to obtain images of the solar
transition region and corona of unprecedented quality. With these
images we will be able to explore quantitatively the connections
between the photospheric magnetic field and the associated hot and
tenuous structures in the outer atmosphere. The TRACE telescope has
an aperture of 30 cm, and will observe an 8.5 x 8.5 arcminute field of
view with a resolution of one arcsecond. Finely tuned coatings on four
quadrants on the primary and secondary normal--incidence mirrors will
allow observations in narrow EUV and UV spectral bands. The passbands
are set to Fe IX, XII, and XV lines in the EUV band, while filters
allow observations in C IV, Ly alpha , and the UV continuum using
the UV mirror quadrant. The data thus cover temperatures from 10(4)
K up to 10(7) K. The Sun--synchronous orbit allows long intervals of
uninterrupted viewing. Observations at different wavelengths can be
made in rapid succession with an alignment of 0.1 arcsec. Coordinated
observing with TRACE, SoHO and YOHKOH will give us the first opportunity
to observe all temperature regimes in the solar atmosphere, including
magnetograms, simultaneously from space. TRACE is currently scheduled
to be launched in October 1997. More information can be found on the
web at ``http://pore1.space.lockheed.com/TRACE/welcome.html''.
Title: Yokhoh Soft X-Ray Telescope Images of the Diffuse Solar Corona
Authors: Sturrock, P. A.; Wheatland, M. S.; Acton, L. W.
Bibcode: 1996ApJ...461L.115S
Altcode:
During the interval 1992 May 3--15, an extended region (out to 1.5
solar radii) of diffuse, stable corona crossed the northeast limb
of the Sun. This region underlaid a coronal streamer as revealed by
the Mauna Loa Coronagraph of the High Altitude Observatory. During
this passage, the soft X-ray telescope on Yohkoh obtained a number of
high-quality pairs of images, closely spaced in time, through the two
thinnest analysis filters. Analysis of these data indicates that (1)
the temperature increases steadily with height and (2) the variation
of temperature with radius is consistent with a conserved inward
heat flux. These results imply that the magnetic field configuration
was substantially open out to 1.5 solar radii and that there was no
significant coronal heating below that height in that region. It appears
that this region was being heated by nonthermal energy deposited beyond
1.5 solar radii.
Title: A high-temperature component in coronal holes observed with
YOHKOH SXT
Authors: Hara, H.; Tsuneta, S.; Acton, L. W.; Bruner, M. E.; Lemen,
J. R.; Ogawara, Y.
Bibcode: 1996AdSpR..17d.231H
Altcode: 1996AdSpR..17..231H
Temperatures of coronal holes are estimated from several sets of soft
X-ray images taken through various broad-band filters with the Soft
X-ray Telescope (SXT) aboard Yohkoh. The effect of scattered X-rays from
bright regions surrounding a temperature determination area, especially
those from nearby active regions, is carefully removed with the point
spread function derived from the post-launch data. An isothermal
approximation is applied to thus corrected data. The temperatures of
coronal holes near the disk center are found to be 1.8 - 2.4 x 10^6
K, which is almost the same as those derived for quiet regions. The
emission measures in coronal holes are estimated to be 10^25.5-26.2
cm^-5, about ten times smaller than in quiet regions. We conclude that
temperatures in coronal holes do not differ from those in quiet regions,
and that the depression in soft X-ray intensity of coronal hole regions
results from a lower density by a factor of 3 than quiet regions. We
propose that the coronal hole component observed with the SXT is not
the same one which is observed with the Skylab EUV instrument. An
X-ray intensity from a coronal hole is independently confirmed by the
eclipse observation on 1993 November 13, and consistent with intensities
derived from the scattering correction.
Title: Reconnection and Field Line Shrinkage in Solar Flares
Authors: Forbes, T. G.; Acton, L. W.
Bibcode: 1996ApJ...459..330F
Altcode:
We use images of flare loops taken by the Soft X-ray Telescope (SXT)
on Yohkoh to estimate the decrease in height that open field lines
undergo after they have reconnected to form closed loops. Following
previous practice, we refer to this decrease as field line shrinkage
and assume that intensity structures trace out the field lines. For
this study, we examine two long-duration events near the limb which
have flare loops that continually grow with time. The shrinkage is
determined by comparing the height of a field line when it lies at the
outermost edge of the flare loop system with the height it has later on
when it lies at the innermost edge. We find that the field lines shrink
by about 20% of their initial height in one flare and by about 32%
in the other. These values are within 5% of the shrinkage predicted by
a simple model of the reconnecting field which assumes that the field
is potential everywhere except for a current sheet extending upward
from the top of the loops. Numerical integration of the model density
along the line of sight implies that most of the discrepancy between
the observations and the theory is due to projection effects which
occur when an arcade of loops is viewed at an arbitrary angle. Both
flares have bright regions at the top of the loops, but in one flare
the lower part of the region is cooler and denser than the rest of
the loop, while in the other flare it is not. Consideration of the
mapping of the bright regions to the footpoint of the loops implies
that the cool region is formed by a thermal instability downstream of
a reconnection outflow in the uppermost part of the loop. The absence
of a cool, dense region in the other flare may be caused by the fact
that it is a very weak event with temperatures and densities too low
to trigger a thermal instability.
Title: Statistical Study of Solar X-Ray Jets Observed with the YOHKOH
Soft X-Ray Telescope
Authors: Shimojo, Masumi; Hashimoto, Shizuyo; Shibata, Kazunari;
Hirayama, Tadashi; Hudson, Hugh S.; Acton, Loren W.
Bibcode: 1996PASJ...48..123S
Altcode:
We have found 100 X-ray jets in the database of full Sun images taken
with the Soft X-ray Telescope (SXT) aboard Yohkoh during the period
from 1991 November through 1992 April. A statistical study for these
jets results in the following characteristics: 1)\ Most are associated
with small flares (microflares--subflares) at their footpoints. 2)\ The
lengths lie in the range of a few times 10(4) --4 times 10(5) km. 3)\
The widths are 5 times 10(3) --10(5) km. 4)\ The apparent velocities are
10--1000 km s(-1) with an average velocity of about 200 km s(-1) . 5)\
The lifetime of the jet extends to ~ 10 hours and the distribution
of the observed lifetime is a power law with an index of ~ 1.2. 6)\
76% of the jets show constant or converging shapes; the width of the
jet is constant or decreases with distance from the footpoint. The
converging type tends to be generated with an energetic footpoint
event and the constant type by a wide energy range of the footpoint
event. 7)\ Many jets ( ~ 68%) appear in or near to active regions
(AR). Among the jets ejected from bright-point like features in ARs,
most ( ~ 86%) are observed to the west of the active region. 8)\ 27%
of the jets show a gap ( > 10(4) km) between the exact footpoint of
the jet and the brightest part of the associated flare. 9)\ The X-ray
intensity distribution along an X-ray jet often shows an exponential
decrease with distance from the footpoint. This exponential intensity
distribution holds from the early phase to the decay phase.
Title: Comparison of YOHKOH X-ray and other solar activity parameters
for November 1991 to November 1995
Authors: Acton, L.
Bibcode: 1996ASPC..109...45A
Altcode: 1996csss....9...45A
No abstract at ADS
Title: Interpretation of SXT Data Concerning the Diffuse Corona
Authors: Sturrock, P. A.; Wheatland, M. S.; Acton, L. W.
Bibcode: 1996mpsa.conf..417S
Altcode: 1996IAUCo.153..417S
No abstract at ADS
Title: Coronal Structures; Local and Global
Authors: Acton, L. W.
Bibcode: 1996mpsa.conf....3A
Altcode: 1996IAUCo.153....3A
No abstract at ADS
Title: Temperature Structure of the Diffuse Corona
Authors: Foley, C. R.; Culhane, J. L.; Acton, L. W.; Lemen, J. R.
Bibcode: 1996mpsa.conf..419F
Altcode: 1996IAUCo.153..419F
No abstract at ADS
Title: The band of solar wind variability at low heliographic
latitudes near solar activity minimum: Plasma results from the
Ulysses rapid latitude scan
Authors: Gosling, J. T.; Bame, S. J.; Feldman, W. C.; McComas,
D. J.; Phillips, J. L.; Goldstein, B.; Neugebauer, M.; Burkepile,
J.; Hundhausen, A. J.; Acton, L.
Bibcode: 1995GeoRL..22.3329G
Altcode:
Near solar activity minimum large variations in the quiescent solar
wind flow are confined to a narrow latitude band centered near the
heliographic equator. During Ulysses' recent rapid latitude scan this
band was ∼43° wide. Flow parameters poleward of the band in the
opposite solar hemispheres were nearly the same. Main entry into the
band of variable solar wind was via a shock disturbance most likely
associated with over-expansion of a coronal mass ejection event. Wind
variability within the band was associated primarily with longitudinal
structure in the solar corona and solar rotation; high-speed streams
observed there were associated with locations where the polar coronal
holes extended equatorward toward the Ulysses orbit. Observations
indicate that the polar coronal holes at this time occupied only
∼13% of the low corona, yet a nearly uniform high-speed wind (average
speed ∼750 km s-1) filled ∼63% of the heliosphere. This
indicates the holes expanded by a factor of ∼4.8 from the low corona
to interplanetary space. Much of this lateral expansion occurred beyond
1.74 solar radii from Sun center.
Title: Comparison of YOHKOH x-ray coronal events with ULYSSES
interplanetary events
Authors: Lemen, J. R.; Acton, L. W.; Alexander, D.; Galvin, A. B.;
Harvey, K. L.; Hoecksema, J. T.; Zhao, X.; Hudson, H.
Bibcode: 1995sowi.conf...58L
Altcode:
The Yohkoh soft X-ray telescope (SXT) has observed several
largescale eruptive events per year for the first three years
of observations (Aug. 1991 - Nov. 1994) Such events are most
prominent at high latitudes, but resemble long-duration flare
events seen in active regions. Some of the high-latitude events
have now been identified in the Ulysses/SWICS data base during the
Ulysses south polar passage. There are puzzling examples of solar
events with no interplanetary counterparts. A comparison of coronal
and interplanetary events can lead to better models for mapping
interplanetary disturbances back to their source location, especially
by combining Yohkoh morphology with three-dimensional representations
of the coronal magnetic field. In this paper we describe the parameters
of the hot plasma seen by SXT. There is clear evidence for non radial
motion in specific events. We present comparisons between the ionization
temperature of the interplanetary plasma with that observed at the
Sun in cases where this is possible.
Title: Yohkoh/SXT soft x-ray observations of sudden mass loss from
the solar corona
Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Freeland, S. L.;
Lemen, J. R.; Harvey, K. L.
Bibcode: 1995sowi.confR..58H
Altcode:
Direct X-ray observations allow us to estimate the hot coronal mass
before and after a flare or other disturbance of the type leading to
a coronal mass ejection. The sudden disappearance of a large coronal
structure (scale greater than 105 km) gives evidence that an ejection
has occurred, if the time scales are much shorter than the conductive
or radiative cooling times for such structures. A flare also typically
adds large amounts of new material to the corona via evaporation
resulting from the coronal energy release. This provides a competing
mechanism that makes the estimation of the total mass loss somewhat
difficult. We note that the X-ray observations have the advantage of
covering the entire corona rather than the limb regions unlike the
coronagraph observations. We have identified two examples of coronal
mass disappearances. before and during long duration flare events on
21 Feb. 1992 (on the E limb) and 13 Nov. 1994 (near disk center). In
latter case the total mass amounted to some 4 x 1014 g with a
density of 3 x 108cm-3 and a temperature of 2.8 MK
before its disappearance. This corresponds to a radiative cooling time
of some 104 S. much longer than the observed time of disappearance. We
therefore suggest that these sudden mass disappearances correspond with
coronal mass ejections (CMEs), and suggest that further data analysis
will be able to confirm this by comparison with optical observations
of specific CMEs.
Title: The solar origins of two high-latitude interplanetary
disturbances
Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Harvey, K. L.;
Kurokawa, H.; Kahler, S.; Lemen, J. R.
Bibcode: 1995sowi.confS..58H
Altcode:
Two extremely similar interplanetary forward/reverse shock events,
with bidirectional electron streaming were detected by Ulysses in
1994. Ground-based and Yohkoh/SXT observations show two strikingly
different solar events that could be associated with them: an LDE flare
on 20 Feb. 1994, and a extremely large-scale eruptive event on 14 April
1994. Both events resulted in geomagnetic storms and presumably were
associated with coronal mass ejections. The sharply contrasting nature
of these solar events argues against an energetic causal relationship
between them and the bidirectional streaming events observed by Ulysses
during its S polar passage. We suggest instead that for each pair of
events. a common solar trigger may have caused independent instabilities
leading to the solar and interplanetary phenomena.
Title: Observations of Coronal Temperature Structure by Yohkoh
Authors: Acton, L. W.; Culhane, J. L.; Lemen, J. R.; Sturrock, P. A.
Bibcode: 1995SPD....26..615A
Altcode: 1995BAAS...27..964A
No abstract at ADS
Title: Temperature Structure of the Diffuse Corona
Authors: Foley, C. A.; Acton, L. W.; Culhane, J. L.; Lemen, J. R.
Bibcode: 1995SPD....26..716F
Altcode: 1995BAAS...27R.969F
No abstract at ADS
Title: The point spread function of the soft X-ray telescope aboard
Yohkoh
Authors: Martens, Petrus C.; Acton, Loren W.; Lemen, James R.
Bibcode: 1995SoPh..157..141M
Altcode:
The point spread function of the SXT telescope aboardYohkoh has been
measured in flight configuration in three different X-ray lines
at White Sands Missile Range. We have fitted these data with an
elliptical generalization of the Moffat function. Our fitting method
consists of χ2 minimizationin Fourier space, especially
designed for matching of sharply peaked functions. We find excellent
fits with a reduced χ2 of order unity or less for single
exposure point spread functions over most of the CCD. Near the edges
of the CCD the fits are less accurate due to vignetting. From fitting
results with summation of multiple exposures we find a systematic
error in the fitting function of the order of 3% near the peak of the
point spread function, which is close to the photon noise for typical
SXT images in orbit. We find that the full width to half maximum and
fitting parameters vary significantly with CCD location. However, we
also find that point spread functions measured at the same location
are consistent to one another within the limit determined by photon
noise. A `best' analytical fit to the PSF as function of position
on the CCD is derived for use in SXT image enhancement routines. As
an aside result we have found that SXT can determine the location of
point sources to about a quarter of a 2.54 arc sec pixel.
Title: Differential Rotation in the Solar Corona
Authors: Weber, M.; Alexander, D.; Acton, L. W.
Bibcode: 1995SPD....26..714W
Altcode: 1995BAAS...27..969W
No abstract at ADS
Title: Correlated brightness variations in solar radiative output
from the photosphere to the corona
Authors: Lean, J. L.; Mariska, J. T.; Strong, K. T.; Hudson, H. S.;
Acton, L. W.; Rottman, G. J.; Woods, T. N.; Willson, R. C.
Bibcode: 1995GeoRL..22..655L
Altcode:
Correlated brightness variations are shown to occur in time series of
coronal soft X-rays exclusive of prominent active regions, chromospheric
ultraviolet radiation, and the photospheric total solar irradiance
corrected for sunspot effects. These temporal correlations suggest that
upwardly extending magnetic fields may have a large scale impact on the
solar atmosphere in addition to their demonstrable role of generating
localized active regions. The correlations have implications for
improving and extending solar spectrum variability models.
Title: Interpretation of SXT Data Concerning the Diffuse Corona
Authors: Sturrock, P. A.; Acton, L. W.
Bibcode: 1995SPD....26..616S
Altcode: 1995BAAS...27..964S
No abstract at ADS
Title: Coordinated X-ray and Hα Observations of Eruptive Flares
Authors: Wülser, J. -P.; Canfield, R. C.; Sakao, T.; Acton, L. W.
Bibcode: 1995SPD....26.1315W
Altcode: 1995BAAS...27..990W
No abstract at ADS
Title: The 1991 October 24 Flare: A Challenge for Standard Models
Authors: de La Beaujardiere, J. -F.; Canfield, R. C.; Hudson, H. S.;
Wulser, J. -P.; Acton, L.; Kosugi, T.; Masuda, S.
Bibcode: 1995ApJ...440..386D
Altcode:
The M9.8 solar flare of 1991 October 24 22:30 UT presents several
interesting characteristics: (1) energy release starts high in
the corona; (2) the primary chromospheric ribbons are initially
well separated and do not move apart at an observable rate; (3) no
evidence is found for an erupting filament or other driver. To explain
this flare, we consider several canonical flare models, including a
filament eruption, a confined filament eruption, current interruption,
and interacting loops. We conclude that none of these scenarios
unequivocally explains this flare. Two possibilities which cannot be
ruled out are (1) the eruption of a filament unobservable in H-alpha
which starts high in the corona and produces no ribbon motions smaller
than our detection threshold and no perceptible expansion of the coronal
X-ray source, and (2) energy release due to spontaneous, propagating
reconnection which allows the system to essentially brighten in place.
Title: Differential rotation in the solar corona
Authors: Weber, M.; Acton, Loren W.; Alexander, David
Bibcode: 1994ESASP.373..405W
Altcode: 1994soho....3..405W
No abstract at ADS
Title: Temperatures of Coronal Holes Observed with the YOHKOH SXT
Authors: Hara, Hirohisa; Tsuneta, Saku; Acton, Loren W.; Bruner,
Marilyn E.; Lemen, James R.; Ogawara, Yoshiaki
Bibcode: 1994PASJ...46..493H
Altcode:
Temperatures of coronal holes have been estimated from several sets
of soft X-ray images taken through various broad-band filters with the
Soft X-ray Telescope (SXT) aboard Yohkoh. Since coronal holes are dark
areas, a detailed examination of the telescope point spread function,
which is slightly dependent on the X-ray wavelength, is of crucial
importance. The calibration is made using post-launch data, and the
effect of scattered X-rays from bright regions surrounding coronal
holes, especially those from nearby active regions, is carefully
removed. An isothermal approximation is applied to the thus-corrected
data. The temperatures of coronal holes near the disk center are found
to be 1.8--2.4 times 10(6) K, which is almost the same as those derived
for quiet regions not including active regions. The emission measures
in coronal holes are estimated to be 10(25.5--26.2) cm(-5) , about ten
times smaller than those of quiet regions. We conclude that temperatures
in coronal holes do not differ from those in quiet regions, and that
the depression in the soft X-ray intensity of coronal hole regions
results from a lower density by a factor of 3 than quiet regions.
Title: Long Duration Events in Magnetic Arcades and Large Loops
Authors: Fludra, A.; Jakimiec, J.; Tomczak, M.; Culhane, J. L.; Acton,
L. W.
Bibcode: 1994kofu.symp..393F
Altcode:
A number of long duration flares, with decay time between 1 and
17 hours, have been analysed using the Yohkoh Soft X-ray Telescope
images and spectra from the Bragg Crystal Spectrometer. X-ray images
suggest that these events typically occur in the following magnetic
field configurations: magnetic arcade, expanding arch, or large loops
triggered and heated up by a low-lying, compact, impulsive flare
located below these high loops. A continued energy release is observed
during decay of these events. Single loop flare models should not be
indiscriminately applied to analysis of thermodynamics of these flares
due to their more complex structure and restructuring of the magnetic
field, and often increasing height.
Title: Temperature Analysis of the Post-Flare Loops of June 25-26,
1992
Authors: Anwer, B.; Hiei, E.; Hudson, H. S.; Acton, L. W.; Lemen,
J.; Metcalf, T. R.
Bibcode: 1994kofu.symp..137A
Altcode:
We have performed an analysis of temperatures and emission measures
of thermal plasma on a post--flare loop system following an X3.9
flare of June 25, 1992, at 20:14 UT in NOAA active region 7205 near
the west limb (N09, W67). The filter ratio method was applied to the
data sets taken using the Al 0.1 micron (thin Al) and Al 12 micron
(thick Al) filters of the Yohkoh Soft X-ray Telescope (SXT). We found
that the plasma temperature of the top of loops was in the range 5 -
8 x 10^6 K and log emission measure between 44.6 and 46.7 cm^(-3)
for data sets taken from 22:56:57 UT of June 25 to 09:00 UT of June
26. Furthermore, the occurrence of a C1-class flare at the top of the
flare loops increased the plasma temperature from 5.5 x 10^6 K to 6.6
x 10^6 K at 06:57:11 UT. The loops top was much brighter than the legs
and footpoints, with delta_T was about 0.1 x 10^6 K.
Title: Observations of Enhanced Coronal Heating in Sheared MAgnetic
Fields
Authors: Moore, R. T.; Porter, J.; Roumeliotis, G.; Tsuneta, S.;
Shimizu, T.; Sturrock, P. A.; Acton, L. W.
Bibcode: 1994kofu.symp...89M
Altcode:
From superposition of Yohkoh SXT images on MSFC vector magnetograms of
two active regions, we find: (1) coronal heating is enhanced at sites of
strong magnetic shear, and (2) this heating is produced by microflares.
Title: Morphology of the 10 Million Degree Plasma in Solar Flares
and the Failure of the Chromospheric Evaporation Model
Authors: Seely, J. F.; Feldman, U.; Doschek, G. A.; Strong, K. T.;
Acton, L. W.; Uchida, Y.; Tsuneta, S.
Bibcode: 1994kofu.symp..177S
Altcode:
The SXT images of over fifty C, M, and X type flares which occurred
between October 1991 and February 1993 were analyzed. For each flare,
the 10 million degree emitting region was typically found to be located
at the loop top in the first well-exposed flare image recorded during
the rise phase (within 1 to 2 minutes after flare onset), in images
recorded near the intensity peak, and in images recorded during most of
the decay phase. For the November 2 1992 limb flare, the loop top was
bright for 24 hours. For a few flares, the brightness of the footpoints
in the onset images was comparable to the brightness of the loop top,
but the loop top brightness rapidly increased relative to the footpoints
and remained intense for the duration of the flare. The brightest region
at the loop top was very small throughout the flare, often as small as
a single pixel (1800x1800 km). The conclusions are that the energy is
deposited in a small volume at the top of the flaring loop structure,
the heating mechanism acts over a period of up to tens of hours, and
the hot plasma is confined at the top of the loop structure. These
results are not explained by the traditional chromospheric evaporation
model of solar flares.
Title: Microflaring at the Feet of Large Active Region Loops
Authors: Porter, J.; Moore, R. T.; Roumeliotis, G.; Shimizu, T.;
Tsuneta, S.; Sturrock, P. A.; Acton, L. W.
Bibcode: 1994kofu.symp...65P
Altcode:
By superposing Yohkoh SXT images on an MSFC magnetogram of an active
region, we find that the brightest loops in the bipolar magnetic
envelope spanning the active region are rooted near a compact site
of mixed polarity and microflaring. Apparently, the enhanced coronal
heating in these high loops is a consequence of the microflaring and/or
related magnetic activity at this end site.
Title: Multispectral Observations of Chromospheric Evaporation in
the 1991 November 15 X-Class Solar Flare
Authors: Wuelser, Jean-Pierre; Canfield, Richard C.; Acton, Loren W.;
Culhane, J. Leonard; Phillips, Andrew; Fludra, Andrzej; Sakao, Taro;
Masuda, Satoshi; Kosugi, Takeo; Tsuneta, Saku
Bibcode: 1994ApJ...424..459W
Altcode:
We analyze simultaneous H(alpha) images and spectra (from Mees
Solar Observatory), and soft and hard X-ray images and spectra (from
YOHKOH) during the early phase of an X1.5/3B flare. We investigate
the morphological relationship between chromospheric downflows,
coronal upflows, and particle precipitation sites, and the energetic
relationship between conductive heating, nonthermal particle heating,
and the chromospheric response. We find that the observations
consistently fit the chromospheric evaporation model. In particular,
we demonstrate that the observed upflowing coronal and downflowing
chromospheric plasma components originate in the same locations,
and we show that our unique set of optical and X-ray observations
can clearly distinguish between conductively driven and electron beam
driven evaporation.
Title: The Morphology of the 10 7 K Plasma in Solar
Flares. I. Nonimpulsive Flares
Authors: Feldman, U.; Seely, J. F.; Doschek, G. A.; Strong, K. T.;
Acton, L. W.; Uchida, Y.; Tsuneta, S.
Bibcode: 1994ApJ...424..444F
Altcode:
In this paper we have analyzed images of 48 C-, M-, and X-type
flares which occurred between 1991 October and 1993 February. The
images were recorded by the soft X-ray telescope (SXT) flown on the
Japanese Yohkoh spacecraft. The spatial resolution of the recorded
images is about 2.5 sec. In each of the recorded flares the brightest
regions emitting the 107 K radiation were analyzed and
evaluate. The 107 K emitting region was found to be located
at loop tops in the first well-exposed flare image recorded during the
rise phase (within 1-2 minutes after flare onset), in images recorded
near intensity peak, and in those recorded during most of the decay
phase. Occasionally, in the begining of the event when the total
flare intensity is low, the brightness of the footpoints may rival the
brightness of the loop top. However, in these cases it is expected that
the temperature of the loop top is considerably higher. The emitting
region, even during flare peak, is very small (often smaller than a
single SXT pixel 1800 x 1800 km).
Title: Morphology of Active Region Transient Brightenings with the
YOHKOH Soft X-Ray Telescope
Authors: Shimizu, Toshifumi; Tsuneta, Saku; Acton, Loren W.; Lemen,
James R.; Ogawara, Yoshiaki; Uchida, Yutaka
Bibcode: 1994ApJ...422..906S
Altcode:
Frequent transient X-ray brightenings occur in solar active regions. The
Yohkoh Soft X-ray Telescope observed 142 transient brightenings
during an interval of time in late 1991 October. We classify them
in terms of morphology and time evolution: (1) simultaneous multiple
loop brightenings are more often seen than brightenings of single and
pointlike structures; (2) for multiple-loop brightenings, the loops
tend to brighten from their footpoints and/or the apparent contact
point in the initial phase of transient brightenings, followed by
the brightening of the entire loops; (3) more than one-half of the
multiple-loop brightenings have Y-type configurations in which the
apparent contact points are located close to their footpoints. Though
transient brightenings show great variety in morphology, these
results suggest that most of them are due to the magnetic interaction
of multiple loops. X-ray emission from the footpoints in the early
phase suggests that the hot plasma in the brightening loops comes from
chromospheric matter or low-temperature coronal matter present around
the bases of the coronal loops prior to the brightening. Enhanced X-ray
emission at the contact points implies local plasma heating by magnetic
interaction. The predominance of the Y-type configuration suggests that
the interaction of coronal loops tends to occur near the footpoints.
Title: An Analytical Model for Fluted Sunspots and its Relation with
Evershed Flow and X-Ray Anemone
Authors: Hurlburt, Neal E.; Martens, Petrus C.; Title, Alan M.;
Acton, Loren
Bibcode: 1994ASPC...68..300H
Altcode: 1994sare.conf..300H
No abstract at ADS
Title: General Structure of the X-ray Corona (invited)
Authors: Acton, L. W.
Bibcode: 1994scs..conf...69A
Altcode: 1994IAUCo.144...69A
This paper will comment on the structure, changes and heating of the
X-ray corona as revealed by the Yohkoh observations.
Title: Filament Tether Cutting Before a Major Eruptive Flare
Authors: Canfield, R. C.; Blais, K. A.; Reardon, K. P.; Acton, Loren;
Kurokawa, H.
Bibcode: 1994ASPC...68..411C
Altcode: 1994sare.conf..411C
No abstract at ADS
Title: Coronal Eruptions Observed by YOHKOH
Authors: Klimchuk, J. A.; Acton, L. W.; Harvey, K. L.; Hudson, H. S.;
Kluge, K. L.; Sime, D. G.; Strong, K. T.; Watanabe, Ta.
Bibcode: 1994xspy.conf..181K
Altcode:
No abstract at ADS
Title: Optical and SXT Observations of the x9 Flare of Nov. 1992
Authors: Ichimoto, K.; Sakurai, T.; Nishino, Y.; Noguchi, M.; Shinoda,
K.; Yamaguchi, A.; Kumagai, K.; Hirayama, T.; Tsuneta, S.; Acton, L.
Bibcode: 1994xspy.conf..259I
Altcode:
No abstract at ADS
Title: Eruptive-Prominence Related Coronal Disturbances Observed
with YOHKOH SXT
Authors: Watanabe, T.; Kozuka, Y.; Ohyama, M.; Kojima, M.; Yamaguchi,
K.; Watari, S.; Tsuneta, S.; Joselyn, J. A.; Harvey, K. L.; Acton,
L. W.; Klimchuk, J. A.
Bibcode: 1994step.conf...85W
Altcode:
No abstract at ADS
Title: The X Flare of 15 November, 1991: Preflare Flux Emergence,
Heating and Filament Eruption
Authors: Canfield, R. C.; Blais, K. A.; McClymont, A. N.; Metcalf,
T. R.; Reardon, K. P.; Wülser, J. -P.; Acton, L. W.; Kurokawa, H.;
Hirayama, T.
Bibcode: 1994xspy.conf..153C
Altcode:
No abstract at ADS
Title: Temperature of Coronal Holes Measured by YOHKOH SXT
Authors: Hara, H.; Tsuneta, S.; Acton, L. W.; Lemen, J. R.; Ogawara, Y.
Bibcode: 1994xspy.conf..217H
Altcode:
No abstract at ADS
Title: Coronal/Interplanetary Disturbances Associated with a Solar
Filament Disappearance on September 28, 1991
Authors: Watanabe, T.; Kozuka, Y.; Ohyama, M.; Kojima, M.; Yamaguchi,
K.; Watari, S.; Tsuneta, S.; Joselyn, J. A.; Harvey, K. L.; Acton,
L. W.; Klimchuk, J. A.
Bibcode: 1994step.conf...89W
Altcode:
No abstract at ADS
Title: Temperature and Density Structure of a Solar Flare Observed
by the YOHKOH SXT and HXT
Authors: McTiernan, J.; Kane, S.; Loran, J.; Lemen, J.; Acton, L.;
Hara, H.; Tsuneta, S.; Kosugi, T.
Bibcode: 1994xspy.conf..255M
Altcode:
No abstract at ADS
Title: Hα and X-Ray Signatures of Chromospheric Evaporation Observed
during the Early Phase of the 15 November 1991 Flare
Authors: Wülser, J. -P.; Canfield, R. C.; Acton, L. W.; Culhane,
J. L.; Phillips, A.; Fludra, A.; Sakao, T.; Masuda, S.; Kosugi, T.
Bibcode: 1994xspy.conf...75W
Altcode:
No abstract at ADS
Title: Non-Thermal Effects in Slow Solar Flares
Authors: Hudson, H. S.; Acton, L. W.; Sterling, A. C.; Tsuneta, S.;
Fishman, J.; Meegan, C.; Paciesas, W.; Wilson, R.
Bibcode: 1994xspy.conf..143H
Altcode:
No abstract at ADS
Title: A Loop-Loop Interaction Observed with YOHKOH SXT
Authors: Akioka, M.; Acton, L. W.; Hudson, H. S.
Bibcode: 1994xspy.conf..241A
Altcode:
No abstract at ADS
Title: Morphological Evolution of the Post-Flare Loops of June
25-26, 1992
Authors: Anwar, B.; Hiei, E.; Hudson, H. S.; Acton, L. W.; Metacalf,
T.; Lemen, J.; Martens, P.
Bibcode: 1994xspy.conf..121A
Altcode:
No abstract at ADS
Title: Energy Release Topology in Solar Active Regions: Soft X-Ray
Observation from YOHKOH
Authors: Cheng, C. -C.; Acton, L.
Bibcode: 1994xspy.conf...83C
Altcode:
No abstract at ADS
Title: Rapid Sunspot Motion during a Major Solar Flare
Authors: Anwar, B.; Acton, L. W.; Hudson, H. S.; Makita, M.; McClymont,
A. N.; Tsuneta, S.
Bibcode: 1993SoPh..147..287A
Altcode:
A major solar flare on 15 November, 1991 produced a striking
perturbation in the position and shape of the sunspot related most
closely to the flare. We have studied these perturbations by use of the
aspect-sensor images from the Soft X-ray Telescope on board YOHKOH,
and with ground-based data from the Mees Solar Observatory. The
perturbation occurred during the impulsive phase of the flare, with
a total displacement on the order of 1 arc sec. The apparent velocity
of approximately 2 km s−1 exceeds that typically reported
for sunspot proper motions even in flare events. We estimate that the
magnetic energy involved in displacing the sunspot amounted to less
than 4 × 1030 ergs, comparable to the radiant energy from
the perturbed region. Examination of the Mees Observatory data shows
that the spot continued moving at lower speed for a half-hour after
the impulsive phase. The spot perturbation appears to have been a
result of the coronal restructuring and flare energy release, rather
than its cause.
Title: The 1992 January 5 Flare at 13.3 UT: Observations from YOHKOH
Authors: Doschek, G. A.; Strong, K. T.; Bentley, R. D.; Brown, C. M.;
Culhane, J. L.; Fludra, A.; Hiei, E.; Lang, J.; Mariska, J. T.;
Phillips, K. J. H.; Pike, C. D.; Sterling, A. C.; Watanabe, T.; Acton,
L. W.; Bruner, M. E.; Hirayama, T.; Tsuneta, S.; Rolli, E.; Kosugi,
T.; Yoshimori, M.; Hudson, H. S.; Metcalf, T. R.; Wuelser, J. -P.;
Uchida, Y.; Ogawara, Y.
Bibcode: 1993ApJ...416..845D
Altcode:
We discuss X-ray spectra and soft X-ray images of an M1.9 flare that
occurred on 1992 January 5 near 13.3 UT. These data were obtained
with instrumentation on the Japanese Yohkoh spacecraft. They cover
the entire rise phase of the flare. To supplement these data we have
ground-based magnetograms and Hα spectroheliograms. We calculate
the electron temperature and emission measure of the flare as a
function of time during the early rise phase using X-ray spectral
line intensities and line ratios. Using spectral line widths, line
profile asymmetries, and wavelength shifts due to the Doppler effect,
we calculate the dynamical properties of the flare. The time development
of the morphology of the flare, as revealed by the soft X-ray images
and the Hα spectroheliograms, and the physical quantities inferred
from the X-ray spectra, are compared with chromospheric evaporation
models. There is an enhancement of blueshifted emission that is closely
correlated with the hard X-ray bursts. Heating of one loop in the flare
is consistent with a conduction-evaporation model, but heating is found
in several structures that do not appear to be physically associated
with each other. No standard evaporation model can adequately explain
all of the observations.
Title: Temperature and Density Structure of the 1991 November 2 Flare
Observed by the YOHKOH Soft X-Ray Telescope and Hard X-Ray Telescope
Authors: McTiernan, James M.; Kane, Sharad R.; Loran, Jon M.; Lemen,
James R.; Acton, Loren W.; Hara, Hirohisa; Tsuneta, Saku; Kosugi, Takeo
Bibcode: 1993ApJ...416L..91M
Altcode:
No abstract at ADS
Title: YOHKOH and the Mysterious Solar Flares
Authors: Petersen, Carolyn C.; Bruner, Marilyn; Acton, Loren; Ogawara,
Yoshiaki
Bibcode: 1993S&T....86...20P
Altcode:
Japan's Yohkoh soft/hard X-ray and gamma-ray telescope-equipped
spacecraft has thus far yielded unprecedentedly detailed views of the
solar corona; attention is here given to a 10-minute time-lapse film
sequence of solar coronal activity which dramatizes the expansion
of observation capabilities Yohkoh offers. It is in this way seen
that powerful eruptions at the limb of the sun often evolve into
helmet-shaped streamers which are clearly visible in X-rays for
more than 1 million km above the solar surface. Also, activity in one
location can quickly affect activity in another. Yohkoh-based flare-loop
studies give attention to the association between soft X-ray loops and
'kernels' observed in red hydrogen light.
Title: The Confined Two-Ribbon Flare of 1991 October 24
Authors: de La Beaujardiere, J. F.; Canfield, R. C.; Hudson, H. S.;
Wuelser, J. -P.; Kosugi, T.; Masuda, S.; Acton, L. W.
Bibcode: 1993BAAS...25.1178D
Altcode:
No abstract at ADS
Title: Filament Tether Cutting Before a Major Eruptive Flare
Authors: Canfield, R. C.; Blais, K. A.; McClymont, A. N.; Metcalf,
T. R.; Reardon, K. P.; Wuelser, J. -P.; Acton, L. W.; Kurokawa, H.
Bibcode: 1993BAAS...25.1188C
Altcode:
No abstract at ADS
Title: Loop Interactions in Solar Active Regions Observed from
SXT/Yohkoh
Authors: Cheng, C. -C.; Acton, L. W.; SXT/YOHKOH Team
Bibcode: 1993BAAS...25.1178C
Altcode:
No abstract at ADS
Title: Multitemperature Observations of an Emerging Flux Region
Authors: Bruner, M. E.; Acton, L. W.; Brown, W. A.; Lemen, J. R.;
Shine, R.; Strong, K. T.; Tarbell, T.; Dulk, G.; Tsuneta, S.; Bastian,
T.; Dame, L.
Bibcode: 1993BAAS...25.1179B
Altcode:
No abstract at ADS
Title: The Yohkoh Software and Database System
Authors: Morrison, M. D.; Freeland, S. L.; Lemen, J. R.; Acton, L. W.;
Bentley, R. D.
Bibcode: 1993BAAS...25R1188M
Altcode:
No abstract at ADS
Title: Observations of an Emerging Flux Region
Authors: Brown, W. A.; Acton, L. W.; Bruner, M. E.; Lemen, J. R.;
Shine, R.; Strong, K. T.; Tarbell, T.; Dulk, G.; Tsuneta, S.; Bastian,
T.; Dame, L.
Bibcode: 1993BAAS...25Q1214B
Altcode:
No abstract at ADS
Title: The Dynamics of Coronal Holes as Determined From X-ray Synoptic
Maps Derived From SXT Imagery
Authors: Slater, G. L.; Linford, G. A.; Strong, K. T.; Acton, L. W.;
Tsuneta, S.; Hara, H.; Takahashi, T.; Hiei, H.; Kubo, M.; Harvey,
K.; Bornmann, P.; McIntosh, P. S.; Sime, D.; Watari, S.
Bibcode: 1993BAAS...25.1179S
Altcode:
No abstract at ADS
Title: The Extended and Diffuse X-Ray Corona Observed by Yohkoh-SXT
Authors: Lemen, J. R.; Slater, G. L.; Hudson, H. S.; Acton, L. W.
Bibcode: 1993BAAS...25.1179L
Altcode:
No abstract at ADS
Title: Solar Coronal Magnetic Field Topology Inferred from High
Resolution Optical and X-ray Movies
Authors: Tarbell, T.; Frank, Z.; Hurlburt, N.; Morrison, M.; Shine,
R.; Title, A.; Acton, L.
Bibcode: 1993BAAS...25.1208T
Altcode:
No abstract at ADS
Title: Yohkoh-SXT Observations from the Spartan and Nixt Max91
Campaign
Authors: Morrison, M.; Bruner, M.; Freeland, S.; Lemen, J.; Linford,
G.; Nitta, N.; Slater, G.; Strong, K.; Hara, H.; Kano, R.; Shimizu,
T.; Tsuneta, S.; Hudson, H.; Ogawara, Y.; Kosugi, T.; Sakao, T.;
Watanabe, T.; Takeda, A.; Acton, L.
Bibcode: 1993BAAS...25.1213M
Altcode:
No abstract at ADS
Title: X ray jets in the solar corona: Observations with YOHKOH Soft
X Ray Telescope
Authors: Shibata, Kazunari; Ishido, Y.; Acton, L.; Strong, K.;
Hirayama, T.; Uchida, Yutaka; McAllister, A.; Matsumoto, R.; Tsuneta,
Saku; Shimizu, T.
Bibcode: 1993ppcn.conf..207S
Altcode:
The discovery of x-ray jet like features using the soft x-ray telescope
onboard Yohkoh satellite is reported. The origin of newly discovered
x-ray jets and the relation to known solar jets at other wavelengths
are discussed. The jets are associated with flare like bright points,
flaring emerging flux regions, or flaring active regions. They appear
to recur at the same place. The following characteristics are found:
in some cases, a dark void appears after ejection at the footpoint of
the jet; some jets show structure which suggests a helical magnetic
field configuration along the jet; one of the jets associated with a
flaring bright point was found to be identified as an H alpha surge. In
this case, the x-ray bright point is situated just on H alpha bright
point at the footpoint of the surge. The top of the surge is not bright
in x-rays.
Title: Solar Coronal Magnetic Field Topology Inferred from High
Resolution Optical and X-ray Movies
Authors: Tarbell, T.; Frank, Z.; Hurlburt, N.; Morrison, M.; Shine,
R.; Title, A.; Acton, L.
Bibcode: 1993AAS...182.4805T
Altcode: 1993BAAS...25R.880T
We are using high resolution digital movies of solar active regions
in optical and X-ray wavelengths to study solar flares and other
transients. The optical movies were collected at the Swedish Solar
Observatory on La Palma using the Lockheed tunable filtergraph
system, in May - July, 1992. They include longitudinal and transverse
magnetograms, H-alpha Doppler and intensity images at many wavelengths,
Ca K, Na D, and white light images. Simultaneous X-ray images from
Yohkoh are available much of the time. We are learning several ways to
establish the connectivity of some coronal magnetic field lines. Some
of the clues available are: magnetic footpoint polarities and transverse
field directions; H-alpha fibrils and loops seen in several wavelengths;
proper motion and Doppler shifts of blobs moving along field lines;
footpoint brightening in micro-flares; spreading of flare ribbons
during gradual phases of flares; X-ray morphology and correlations with
H-alpha; and draining of flare loops. Examples of each of these will
be shown on video. This work is supported by NASA Contracts NASW-4612
and NAS8-37334 and by Lockheed Independent Research Funds.
Title: Observations of X-ray Jets Using YOHKOH Soft X-Ray Telescope
Authors: Shibata, K.; Ishido, Y.; Acton, L.; Strong, K.; Hirayama,
T.; Uchida, Y.; McAllister, A.; Matsumoto, R.; Tsuneta, S.; Shimizu,
T.; Hara, H.; Sakurai, T.; Ichimoto, K.; Nishino, Y.; Ogawara, Y.
Bibcode: 1993ASPC...46..343S
Altcode: 1993IAUCo.141..343S; 1993mvfs.conf..343S
No abstract at ADS
Title: White-light movies of the solar photosphere from the SOUP
instrument on Spacelab 2 (Advances in Space Research 1986)
Authors: Title, A. M.; Tarbell, T. D.; Simon, G. W.; Acton, L.;
Duncan, D.; Ferguson, S.; Finch, M.; Frank, Z.; Kelly, G.; Lindgren,
R.; Morrill, M.; Pope, T.; Reeves, R.; Rehse, R.; Shine, R.; Topka,
K.; Harvey, J.; Leibacher, J.; Livingston, W.; November, L.
Bibcode: 1993inas.book..100T
Altcode:
No abstract at ADS
Title: Microflaring at the feet of large active region loops
Authors: Porter, Jason; Moore, Ron; Roumeliotis, George; Shimizu,
Toshifumi; Tsuneta, Saku; Sturrock, Peter; Acton, Loren
Bibcode: 1993STIN...9670891P
Altcode:
By superposing Yohkoh SXT images on an MSFC magnetogram of an active
region, we find that the brightest loops in the bipolar magnetic
envelope spanning the active region are rooted near a compact site
of mixed polarity and microflaring. Apparently, the enhanced coronal
heating in these high loops is a consequence of the microflaring and/or
related magnetic activity at this end site.
Title: Erratum - the Morphology of 20X10/6K Plasma in Large
Non-Impulsive Solar Flares
Authors: Acton, L. W.
Bibcode: 1992PASJ...44..691A
Altcode:
No abstract at ADS
Title: Comparison between YOHKOH Soft X-ray Images and 3D MHD
Simulations of Solar Emerging Flux Regions
Authors: Matsumoto, R.; Tajima, T.; Kaisig, M.; Shibata, K.; Ishido,
Y.; Tsuneta, S.; Shimizu, T.; Kawai, G.; Kurokawa, H.; Akioka, M.;
Acton, L.; Strong, K.; Nitta, N.
Bibcode: 1992AAS...181.8109M
Altcode: 1992BAAS...24.1253M
The soft X-ray telescope on the Yohkoh mission enabled us to observe
the evolution of emerging flux regions (EFR) in coronal X-rays with
high spatial and temporal resolution. Furthermore, we now have enough
computing capability to perform three-dimensional MHD simulation
of EFRs with sufficient spacial resolution to study details of the
flux emergence process. These new tools provide the opportunity to
investigate the physics involved in the formation of coronal loops
in much more detail. We carried out 3D MHD simulations of emerging
magnetic flux regions under various intial conditions; (1) a horizontal
magnetic flux sheet, (2) a bundle of horizontal flux tubes, and (3)
a flux sheet with sheared magnetic fields. Numerical results show that
coronal magnetic loops are formed due to the enhanced buoyancy resulting
from gas precipitating along magnetic field lines. The interchange modes
help to produce a fine fibrous structure perpendicular to the magnetic
field direction in the linear stage, while the undular modes determine
the overall loop structure. We observe in 3D simulations that during the
ascendance of loops the bundle of flux tubes, or even the flux sheet,
developes into dense filaments pinched between magnetic loops. We
also find that magnetic field lines are twisted by the vortex motion
produced by the horizontal expansion of magnetic loops. Our numerical
results may explain the observed signatures such as (1) the spacial
relation between soft X-ray loops and Hα arch filaments obtained by
coordinated observation between Yohkoh and ground-based observatories
(Kawai et al. 1992), (2) the rate of increase in size of soft X-ray
loops in EFRs (Ishido et al. 1992), (3) emergence of twisted magnetic
loops, and (4) the threshold flux for formation of chromospheric arch
filament systems (AFS).
Title: Flare-Related Relaxation of Magnetic Shear as Observed with
the Soft X-Ray Telescope of YOHKOH and with Vector Magnetographs
Authors: Sakurai, Takashi; Shibata, Kazunari; Ichimoto, Kiyoshi;
Tsuneta, Saku; Acton, Loren W.
Bibcode: 1992PASJ...44L.123S
Altcode:
The soft X-ray Telescope of Yohkoh observed an M-class flare on 1992
February 6. As the flare progressed, an initially sheared coronal loop
structure was seen to evolve toward a relaxed magnetic configuration. In
association with this evolution, the vector magnetograph observations
detected a decrease in the electric currents.
Title: White-Light Flares Observed by YOHKOH
Authors: Hudson, Hugh S.; Acton, Loren W.; Hirayama, Tadashi; Uchida,
Yutaka
Bibcode: 1992PASJ...44L..77H
Altcode:
The Yohkoh observatory is producing a first sample of white-light
flares observed from space. We present observations of four of them,
all X-class events. The Yohkoh (SXT) white-light data typically
have a 12-s cadence for images with 2.''46 pixels over a field of
view of 2.'62 in one of two broad-band optical filters, and the 1991
November 15 flare produced a brightness increase of about 38% over the
photospheric brightness in the 30 Angstroms \ passband filter centered
at 4308 Angstroms. The white-light flare morphology in the best-observed
flares displays a double ``footpoint'' character, establishing a close
relationship with the compact magnetic flux tubes involved with both
hard and soft X-ray emissions. The ``footpoint'' brightnesses may vary
independently with time. We describe the data in the context of the
soft and hard X-ray observations simultaneously carried out on board
the Yohkoh satellite, emphasizing energetics and timing.
Title: Effective Geometrical Thickness and Electron Density of a
Flare of 1991 December 2 Observed with the Soft X-Ray Telescope of
YOHKOH and Coronagraph
Authors: Ichimoto, Kiyoshi; Hirayama, Tadashi; Yamaguchi, Asami;
Kumagai, Kazuyoshi; Tsuneta, Saku; Hara, Hirohisa; Acton, Loren W.;
Bruner, Marilyn E.
Bibcode: 1992PASJ...44L.117I
Altcode:
A very small geometrical thickness of 1000 km was found for an M3.7
flare of 1991 December 2, which occurred beyond the limb. While
the Yohkoh soft X-ray telescope gives the emission measure, a new
coronagraph which can observe continuum images at 6630- Angstroms gives
the electron column density originating from electron scattering of a
10(7) K flare plasma. The reasoning for the latter is because [Fe XIV]
5303 Angstroms images show a much different shape compared with X-ray
and the 6630- Angstroms continuum, and there was no Hα emission. From
these we obtained an electron density of 4times 10(10) cm(-3) and the
above-mentioned small length in the line of sight. Since the apparent
width of a flaring plasma both in soft X-ray and continuum images
measured parallel to the limb is ~ 4 times 10(4) km at a height of 7
times 10(4) km, the smallness of the effective length of 1000 km is
striking, and may have significant bearing on the energy conversion
of the flare. A brief discussion concerning the origin of the mass
and magnetic morphology is given.
Title: Observations of X-Ray Jets with the YOHKOH Soft X-Ray Telescope
Authors: Shibata, Kazunari; Ishido, Yoshinori; Acton, Loren W.; Strong,
Keith T.; Hirayama, Tadashi; Uchida, Yutaka; McAllister, Alan H.;
Matsumoto, Ryoji; Tsuneta, Saku; Shimizu, Toshifumi; Hara, Hirohisa;
Sakurai, Takashi; Ichimoto, Kiyoshi; Nishino, Yohei; Ogawara, Yoshiaki
Bibcode: 1992PASJ...44L.173S
Altcode:
Time series of Soft X-ray Telescope images have revealed many X-ray
jets in the solar corona. The typical size of a jet is 5 times 10(3)
--4 times 10(5) km, the translational velocity is 30--300 km s(-1) ,
and the corresponding kinetic energy is estimated to be 10(25) --10(28)
erg. Many of the jets are associated with flares in X-ray bright points,
emerging flux regions, or active regions. They sometimes occur several
times from the same X-ray feature. In some cases, a dark void appears
after ejection at the footpoint of the jet. The void seems to be
the result of a change in the topology of the X-ray emitting plasma,
perhaps due to magnetic reconnection. Some jets show a structure which
suggests a helical magnetic field configuration along the jet. One
of the jets associated with a flaring bright point was identified as
being an Hα surge. In this case, the X-ray bright point is situated
just on the Hα bright point at the footpoint of the surge. The top
of the surge is not bright in X-rays. We briefly discuss the origin
of these newly discovered X-ray jets.
Title: The Morphology of 20times 10(6) K Plasma in Large Non-Impulsive
Solar Flares
Authors: Acton, Loren W.; Feldman, Uri; Bruner, Marilyn E.; Doschek,
George A.; Hirayama, Tadashi; Hudson, Hugh S.; Lemen, James R.;
Ogawara, Yoshiaki; Strong, Keith T.; Tsuneta, Saku
Bibcode: 1992PASJ...44L..71A
Altcode:
We have examined images of 10 flares observed by the Soft X-ray
Telescope on-board the Yohkoh spacecraft. These images show that the
hottest portion of the soft X-ray flare is located in compact regions
that appear to be situated at the tops of loops. These compact regions
form at, or shortly after, flare onset, and persist well into the decay
phase of the flares. In some cases, the compact regions are only a
few thousand kilometers in size and are small compared to the lengths
of flaring loops. This is inconsistent with the smoother intensity
distribution along the loops expected from models of chromospheric
evaporation.
Title: The Structure of the Coronal Soft X-Ray Source Associated
with the Dark Filament Disappearance of 1991 September 28 Using the
YOHKOH Soft X-Ray Telescope
Authors: McAllister, Alan; Uchida, Yutaka; Tsuneta, Saku; Strong,
Keith T.; Acton, Loren W.; Hiei, Eijiro; Bruner, Marilyn E.; Watanabe,
Takashi; Shibata, Kazunari
Bibcode: 1992PASJ...44L.205M
Altcode:
The internal structure of an X-ray emitting elongated object
appearing in association with Hα -dark filament disappearance of 1991
September 28 was analyzed with the help of a fine-structure enhancing
technique. We present a description of the soft X-ray structures and
their evolution, while focusing on the central, brightest part of the
structure, which is also the most difficult to resolve. We start with
the idea of applying the standard ``eruption-reconnection'' models
of Hα double-ribbon flares with filament disappearences in order to
explain this event as the appearence of an arcade of loops across the
initial dark filament position, with a row of hot spots at reconnection
sites along the loop tops. Our study of the Yohkoh Soft X-ray images,
including their fine-structure enhancement, the making of an accurately
aligned movie, and a preliminary comparison with Hα data, however,
has led us to question the applicability of these models to this type
of event. It seems, rather, that much of the bright structure comprises
heated pre-existing loops, which interact where they cross each other
in a complex ``knot'' at the northern end of the disappearing dark
filament. The bright part turns out to comprise highly sheared loops,
perhaps being pushed up by the slowly rising axial field of the dark
filament, which is, however, contained under the arcade, rather than
having broken through it. We conclude that there is a strong possibility
that much of the dark filament mass remains in the heated unwinding
axial field and briefly discuss the theoretical implications.
Title: The X Flare of 1991 November 15: Coordinated Mees/Yohkoh
Observations
Authors: Canfield, Richard C.; Hudson, Hugh S.; Leka, K. D.; Mickey,
Donald L.; Metcalf, Thomas R.; Wuelser, Jean-Pierre; Acton, Loren W.;
Strong, Keith T.; Kosugi, Takeo; Sakao, Taro; Tsuneta, Saku; Culhane,
J. Leonard; Phillips, Andrew; Fludra, Andrzej
Bibcode: 1992PASJ...44L.111C
Altcode:
This is a preliminary report on two unique new results from coordinated
observations at Mees Solar Observatory and Yohkoh of the X1.5 flare
of 1991 November 15, using vector magnetograms, Hα imaging spectra,
X-ray images, and X-ray spectra. First, we find a close spatial
relationship between Hα redshifts and X-rays from a flare loop and
its footpoints at a time of large X-ray blueshifts. Second, we find
that impulsive-phase hard X-rays originate in regions that are near,
but not coincident with, the peaks of the vertical electrical current
density distribution in AR 6919.
Title: The Status of YOHKOH in Orbit: an Introduction to the Initial
Scientific Results
Authors: Ogawara, Yoshiaki; Acton, Loren W.; Bentley, Robert D.;
Bruner, Marilyn E.; Culhane, J. Leonard; Hiei, Eijiro; Hirayama,
Tadashi; Hudson, Hugh S.; Kosugi, Takeo; Lemen, James R.; Strong, Keith
T.; Tsuneta, Saku; Uchida, Yutaka; Watanabe, Tetsuya; Yoshimori, Masato
Bibcode: 1992PASJ...44L..41O
Altcode:
In this introductory article accompanying the initial scientific
papers from the Yohkoh mission, we briefly summarize the design
and in-orbit function of the spacecraft and its four scientific
instruments. Although these initial results include mainly studies
based upon individual Yohkoh experiments at this early stage, there
are also analyses of combined data sets provided by several on-board
and ground-based instruments in progress. The results presented here,
and anticipated future results, suggest that the Yohkoh observations
with their comprehensive coverage of solar high-energy phenomena will
come to represent a significant milestone in the progress of solar
physics. This will be true not only regarding flares, but also for
fainter coronal structures and even coronal holes.
Title: Coronal/Interplanetary Disturbances Associated with
Disappearing Solar Filaments
Authors: Watanabe, Takashi; Kozuka, Yukio; Ohyama, Masamitsu; Kojima,
Masayoshi; Yamaguchi, Kisuke; Watari, Shin-Ichi; Tsuneta, Saku;
Joselyn, Jo A.; Harvey, Karen L.; Acton, Loren W.; Klimchuk, James. A.
Bibcode: 1992PASJ...44L.199W
Altcode:
We discuss two examples of coronal/interplanetary disturbances
associated with the disappearance of a 35(deg) long quiescent filament
occurring near the solar disk center on 1991 September 28 (McAllister
et al. 1992, Publ. Astron. Soc. Japan, 44, L205) and with a 25(deg)
long eruptive prominence at the eastern solar limb taking place on
1991 November 7. Bright soft X-ray arcades were observed for both
cases with the Yohkoh SXT, about 2--3 hr after the onset of each
Hα event. For the erupting prominence on November 7, the arcade
did not appear before the prominence reached a height of about 0.3
solar radii above the limb. This suggests that magnetic reconnection
occurred below the relevant Hα structures. A transient coronal hole
was formed in the immediate vicinity of the disappearing filament on
September 28. Formation of the new coronal hole is suggested to be a
cause of the filament disappearance. An interplanetary disturbance was
detected by radio scintillation (IPS) observations immediately after
the filament disappeared.
Title: The YOHKOH mission for high-energy solar physics
Authors: Acton, L.; Tsuneta, S.; Ogawara, Y.; Bentley, R.; Bruner, M.;
Canfield, R.; Culhane, L.; Doschek, G.; Hiei, E.; Hirayama, T. Hudson,
H.; Kosugi, T.; Lang, J.; Lemen, J.; Nishimura, J.; Makishima, K.;
Uchida, Y.; Watanabe, T.
Bibcode: 1992Sci...258..618A
Altcode: 1992Sci...258..591A
Data on solar flare mechanisms and the sun's corona will be generated
by Japan's Yohkoh satellite's X-ray imaging sensors and X-ray and
gamma-ray spectrometers. It is noted that the X-ray corona above active
regions expands, in some cases almost continually, in contradiction of
the widely accepted model of magnetohydrostatic equilibrium in such
regions. Flaring X-ray bright points have been discovered to often
involve ejecta into an adjacent, much larger and fainter magnetic loop,
which brightens along its length at speeds up to 1000 km/sec.
Title: Detailed Comparison between Hα and YOHKOH Soft X-Ray Images
of a Confined Two-Ribbon Flare
Authors: Kurokawa, Hiroki; Kawai, Goro; Kitai, Reizaburo; Funakoshi,
Yasuhiro; Nakai, Yoshihiro; Tsuneta, Saku; Kosugi, Takeo; Enome,
Shinzo; Acton, Loren W.; Ogawara, Yoshiaki
Bibcode: 1992PASJ...44L.129K
Altcode:
The spatial and temporal relationships between soft X-ray loops
and Hα flare Kernels were studied for the 1B (M2.6) flare of 1991
December 5. Most of flare energy was released inside three soft X-ray
loops whose footpoints were bright as Hα Kernels. We suggest that the
interaction among pre-existing or emerging magnetic loops is essential
for the successive energy release in a confined two-ribbon flare.
Title: High-Temperature Plasmas in Active Regions Observed with the
Soft X-Ray Telescope aboard YOHKOH
Authors: Hara, Hirohisa; Tsuneta, Saku; Lemen, James R.; Acton,
Loren W.; McTiernan, James M.
Bibcode: 1992PASJ...44L.135H
Altcode:
High-temperature plasmas reaching 5--6times 10(6) K in solar
active regions have been found with the soft X-ray telescope aboard
Yohkoh. NOAA region 6919 was investigated in detail using five different
X-ray filters: The temperature of a bright loop in the active region
is 5.7times 10(6) K, with an emission measure of 5.0times 10(28)
cm(-5) ; in a fainter part of the region plasma, we find 5.0times
10(6) \ K and 4.0times 10(27) cm(-5) . This indicates that such
high-temperature plasmas exist in the active region, irrespective of
the brightness. Another observation of the quiet corona was conducted in
order to investigate the reliability of a temperature analysis with the
same filter pairs which show such high temperatures in active regions:
The inferred temperature was 2.7times 10(6) K, and the emission measure
1.3times 10(26) cm(-5) , which is consistent with the typical results
of Skylab. Therefore, the high-temperature plasmas in solar active
regions are considered to be real.
Title: Global Restructuring of the Coronal Magnetic Fields Observed
with the YOHKOH Soft X-Ray Telescope
Authors: Tsuneta, Saku; Takahashi, Tetsuo; Acton, Loren W.; Bruner,
Marilyn E.; Harvey, Karen L.; Ogawara, Yoshiaki
Bibcode: 1992PASJ...44L.211T
Altcode:
We present an example of the large-scale ``restructuring" of a polar
coronal magnetic structure taking place over a time period of 20
hr. A large-scale closed-loop arcade appears to be created from an
open field structure formed in association with the disappearance of a
polar crown filament. The loops increase their height with time. The
loop formation propagates westward, and a cusp structure, inside
of which is bright in X-rays, is seen on the west limb. This global
structural change of the coronal magnetic field appears to take place
only through a non-explosive, quasi-steady magnetic reconnection. The
Yohkoh soft X-ray images show that the solar corona is full of such
global restructuring, suggesting that magnetic reconnection is a
primary device for the general coronal magnetic evolution.
Title: Transient Brightenings in Active Regions Observed by the Soft
X-Ray Telescope on YOHKOH
Authors: Shimizu, Toshifumi; Tsuneta, Saku; Acton, Loren W.; Lemen,
James R.; Uchida, Yutaka
Bibcode: 1992PASJ...44L.147S
Altcode:
The Soft X-ray Telescope (SXT) aboard the Yohkoh satellite has revealed
that active regions show many compact loop brightenings which we call
``active-region transient brightenings.'' The released energy by an
``active-region transient brightening'' is considerably less than
10(29) erg, which is the low end of the subflare energy range. Small
soft X-ray enhancements observed by the GOES satellites are identified
to relatively intense ``active-region transient brightenings.'' The
transient brightening occurs on the average of one every ~ 3 min in
``active'' active regions and down to one every ~ 1 hr in ``quieter''
active regions. This suggests that the transient brightening is a
very common phenomenon in active regions and that the magnetic loops
in active regions are far from static.
Title: Hard X-Ray Imaging Observations by YOHKOH of the 1991 November
15 Solar Flare
Authors: Sakao, Taro; Kosugi, Takeo; Masuda, Satoshi; Inda, Mika;
Makishima, Kazuo; Canfield, Richard C.; Hudson, Hugh S.; Metcalf,
Thomas R.; Wuelser, Jean-P.; Acton, Loren W.; Ogawara, Yoshiaki
Bibcode: 1992PASJ...44L..83S
Altcode:
We report on hard X-ray imaging observations of the 1991 November 15
flare with the HXT instrument aboard {Yohkoh}. Distributions of the hard
X-ray sources at various stages of the flare, together with an overlay
of the white-light flare, are presented. Attention is concentrated on
the behavior of hard X-ray sources during the impulsive phase. The
hard X-ray source appeared initially as a single source near the
magnetic neutral line, then evolved into a double-source shape with the
separation increasing with time. We believe that this is evidence for a
multiple loop system flaring successively with a rising energy-release
site. At the minima between the individual spikes of the time profile,
the hard X-rays at 20--30 keV were concentrated near the apex of the
flaring loop, whereas the hard X-rays above 30 keV originated from the
footpoints. These observations are compared with the existing models.
Title: Observation of a Solar Flare at the Limb with the YOHKOH Soft
X-Ray Telescope
Authors: Tsuneta, Saku; Hara, Hirohisa; Shimizu, Toshifumi; Acton,
Loren W.; Strong, Keith T.; Hudson, Hugh S.; Ogawara, Yoshiaki
Bibcode: 1992PASJ...44L..63T
Altcode:
A long-enduring soft X-ray flare at the solar limb was well observed
by the Soft X-ray Telescope aboard the Yohkoh spacecraft from its
pre-flare stage through the post-flare phase. A ``helmet streamer"
arch appears several hours prior to the flare, in association with a
continuous expansion and restructuring of the active-region magnetic
structure. This arch then starts to flare, and increases its height and
footpoint separation at v = 10--30 km s(-1) . The arch has a complex
temperature structure in the rising phase, whereas the outer arches
have systematically higher temperatures in the decay phase. Magnetic
reconnection in a neutral sheet at the loop top, created by pre-flare
magnetic restructuring, would explain this type of flare.
Title: Comparison between Hα and YOHKOH Soft X-Ray Images of Emerging
Flux Regions
Authors: Kawai, Goro; Kurokawa, Hiroki; Tsuneta, Saku; Shimizu,
Toshifumi; Shibata, Kazunari; Acton, Loren W.; Strong, Keith T.;
Nitta, Nariaki
Bibcode: 1992PASJ...44L.193K
Altcode:
We carried out a detailed comparison between Hα and Yohkoh Soft X-ray
(SXR) images of three emerging flux regions. The main results are:
(1) In general, SXR bright features coincide well in space with Hα
arch filament systems in the emerging flux regions (EFR). (2) Some
young and active parts of EFRs are especially bright in SXR. (3)
The SXR structures related to EFR show fairly rapid changes in both
brightness and shape. These results are consistent with the model that
the emerging cool loops of EFRs evolve into hot coronal loops through
some heating processes.
Title: X-Ray Observations from Yohkoh of the Energy Release Topologies
in Solar Active Regions
Authors: Cheng, C. -C.; Acton, L.; YOHKOH Team
Bibcode: 1992AAS...181.5504C
Altcode: 1992BAAS...24.1211C
No abstract at ADS
Title: White--Light Flares Observed by YOHKOH
Authors: Hudson, H. S.; Wulser, J. -P.; Acton, L.; Uchida, Y.
Bibcode: 1992AAS...180.2309H
Altcode: 1992BAAS...24..761H
The YOHKOH observatory is producing a first sample of white-light
flares observed from space. We present observations of three of
these flares, 1991 Oct. 27, 1991 Nov. 15, and 1991 Dec. 3. Of these,
the 1991 Nov. 15 was also well-observed with H-alpha spectroscopic
imaging observations at Mees Observatory, Haleakala. The YOHKOH (SXT)
white-light data typically have a 12-second cadence for images with
2.46 arc-sec pixels over a field of view of 2.62 arc min in one of two
broad-band optical filters, and the Nov. 15 flare produced a brightness
excess of about 25\ We describe the YOHKOH white-light observations in
terms of morphology and flare energetics in the context of the X-ray
observations. For the Nov. 15 flare, we find that the H-alpha emission
wing spectroheliograms match closely with the continuum images. Such
observations permit tests of models (``electron precipitation'')
in which the non--thermal electrons responsible for the hard X-ray
bremsstrahlung also excite the upper photosphere by direct heating and
ionization. We find by comparison with the hard X-ray data that this
mechanism is viable, but confirm the need for at least one additional
mechanism for the excitation of the continuum.
Title: November 15, 1991 X Flare -- The Movie: Hα , Soft X-rays,
and Hard X-rays and Magnetic Fields
Authors: Wulser, J. -P.; Acton, L.; Sakao, T.; Canfield, R.; Kosugi,
T.; Slater, G.; Strong, K.; Tsuneta, S.
Bibcode: 1992AAS...180.3003W
Altcode: 1992BAAS...24..775W
The X1.5/3B flare on 1991 November 15, 22:33 UT was well observed
by the Hα Imaging Spectrograph and the Vector Magnetograph
(Stokes Polarimeter) at Mees Solar Observatory, and by the Soft-
and Hard X-ray Telescopes (SXT and HXT) aboard YOHKOH. We have
combined this multispectral dataset into a series of temporally and
spatially co-aligned video movies and analyzed the morphological
and temporal relationships of the various flare emissions. The
earliest manifestations of this flare include unresolved preflare SXR
brightenings very close to the magnetic neutral line and preflare
motions of filaments seen in Hα . In the flare core, SXR and Hα
emission show moving and rotating coronal structures which we interpret
as a successive brightening of adjacent loops during the main phase
of the flare. The HXR source shows much more dramatic variability
than the SXR source, and they are clearly not cospatial. On the other
hand, there is a close spatial relationship between the HXR and Hα
blue wing emission sites. The Hα , HXR, and SXR images all point to
acceleration and heating in a region that starts close to the neutral
line and moves outward during each HXR burst and during the gradual
phase. Spectacular mass ejections are seen in both SXR and Hα , with
clear unwinding of tightly coiled structures, acceleration of X-ray
and Hα material to velocities of order 1000 km/s, and a striking
thermal bifurcation between hot and cold plasma.
Title: Early Results from the YOHKOH Soft X-ray Telescope
Authors: Bruner, M. E.; Acton, L. W.; Lemen, J.; Hirayama, T.;
Tsuneta, S.
Bibcode: 1992AAS...180.2301B
Altcode: 1992BAAS...24..759B
The The Soft X-ray Telescope on the Yohkoh satellite, launched by
Japan on August 30, 1992, has proved to be a resounding success. It
is providing a wealth of new information and many surprises, both on
flares and on the behavior of the solar corona. Performance of the
telescope has met or exceed our most optimistic expectations and
it appears to be in perfect focus. Unlike the Skylab instruments,
the Yohkoh telescope is not limited by a finite supply of film,
permitting long sequences of images to be made with relatively high
time resolution. Repetition rates for a given exposure / filter
combination are typically a few seconds per frame to a few minutes
per frame, depending on the selected field size. Movies assembled from
long exposure sequences have shown the corona to be even more dynamic
than expected. Major re-structuring, involving large fractions of the
visible corona, can take place in an hour or two. Smaller regions are
even more dynamic, changing almost continuously. Movies, created from
long exposure sequences, have demonstrated the fundamental importance of
large-scale coronal loops in connecting widely separated regions such
that activity in one region quickly affects the physical conditions
at remote sites. The images also show that the majority of the loops
have nearly constant cross sections along their lengths, rather than
one that increases with height. Several X-class flares have been
observed; the surprising result is that they do not appear to be very
dynamic in soft X-rays. The flare kernels seem to consist of compact
loop structures that brighten and then fade without changing size or
shape. Bright points are not as prominent as in the Skylab images;
a result of using a CCD (a linear detector) rather than film which has
a logarithmic response. The other instruments on Yohkoh are producing
equally exciting results; it seems clear that the Yohkoh mission will
produce many major advances in our knowledge of the flare mechanism.
Title: Soft X-ray Blueshifts and Hα Redshifts in the November 15,
1991 X-Class Flare
Authors: Wulser, J. -P.; Acton, L.; Canfield, R.; Culhane, L.; Fludra,
A.; Masuda, S.; Phillips, A.; Sakao, T.
Bibcode: 1992AAS...180.1805W
Altcode: 1992BAAS...24..755W
We analyzed simultaneous Hα line profile observations (from Mees Solar
Observatory), CaXIX line profile observations (from the Bragg Crystal
Spectrometer aboard YOHKOH), and soft- and hard X-ray images (from SXT
and HXT on YOHKOH) of the November 15, 1991 X-class flare. The observed
CaXIX emission shows strong blueshifts very early in the flare. The soft
X-ray images suggest that this blueshifted emission originates from
one or more of three localized soft X-ray bright points. At the same
time, the Hα line profile shows redshifted and blueshifted emission
in several locations. Two Hα redshift kernels are associated with the
two brightest soft X-ray sources. On the basis of their relationship
to the magnetic neutral line and their subsequent development, we
conclude that these Hα redshift kernels are the footpoints of a
single loop, which initially exhibits two soft X-ray bright points
in the loop legs. The results suggest that chromospheric evaporation
is the responsible mechanism for the observed Hα redshifts and CaXIX
blueshifts in the early stage of the flare. The independent hard X-ray
(HXT) and Hα observations both indicate that this chromospheric
evaporation is not associated with strong non-thermal electron
precipitation. The third soft X-ray bright point, the faintest of the
three, is not associated with redshifted Hα emission. This bright spot
develops into a connection between the second Hα redshift kernel,
and another Hα kernel with strong blueshifts. The Hα blueshift is
associated with a mass ejection phenomenon.
Title: Temperature Structure of Solar Flares Observed by the
YOHKOH SXT
Authors: McTiernan, J. M.; Kane, S. R.; Loran, J. M.; Lemen, J. R.;
Acton, L. W.; Hara, H.; Tsuneta, S.
Bibcode: 1992AAS...180.3002M
Altcode: 1992BAAS...24Q.775M
Hot plasmas from several solar flares have been observed by the Soft
X-ray Telescope (SXT) on board the Yohkoh satellite. For a sample of
flares observed by the SXT with a variety of X-ray filters, we have
calculated temperature and emission measures as functions of space and
time. Initial results from this analysis show the following: (1) The
flare plasmas range in temperarure from several million degrees K up to
greater than 20 million degrees K, depending on the individual event;
(2) The region with the higest temperature does not coincide with
the brightest region. For example, for the flare of 15 November 1991
(2238 UT) the temperature was typically 8-9 million degrees K on the
bright kernels, with temperatures of 15-25 million degrees K on the
edges of the bright regions. The average temperature for the flare was
approximately 10 million degrees K. A preliminary interpretation of
these observational results in terms of the temperature and density
structure inside a magnetic loop will be presented.
Title: Electric Currents and Hard X-ray Images in the X Class Flare
of November 15, 1991
Authors: Metcalf, T. R.; Sakao, T.; Acton, L. W.; Canfield, R. C.;
Hudson, H. S.; Inda, M.; Kosugi, T.; Wulser, J. P.
Bibcode: 1992AAS...180.3005M
Altcode: 1992BAAS...24..776M
We present co-aligned observations of hard x-rays observed with the
Hard X-ray Telescope (HXT) on board the YOHKOH spacecraft and vertical
electric currents derived from a vector magnetogram obtained at the
Mees Solar Observatory, Haleakala, Hawaii. Previous work comparing
the wings of the Hα line to vertical electric currents has suggested
that electron precipitation in flares occurs at the edges of these
currents. The Stark wings of Hα were interpreted as a signature of
non-thermal electrons penetrating the relatively dense chromosphere and
used as a proxy for direct observation of the non-thermal electrons. The
hard X-rays used in this study provide a direct determination of
the locations of the electron energy losses. In the X class flare
of November 15, 1991, we find the same relation between hard X-ray
emission and vertical electric currents as was found between Hα Stark
wing emission and vertical currents: the hard x-ray emission occurs
predominantly at the edges of the vertical current sites, and not
spatially on top of these currents. Canfield, R. C., de La Beaujardiere,
J., and Leka, K. D., in ``The Physics of Solar Flares", ed. Culhane and
Jordan, The Royal Society, London, 1991 Canfield, R. C., Leka, K. D.,
and Wulser,J. P., in ``Flare Physics in Solar Activity Maximum 22",
ed. Uchida, Canfield, Watanabe, and Hiei, Springer-Verlag, Berlin, 1991
Title: Design Rationale of the Solar Ultraviolet Network / Sun
Authors: Dame, L.; Acton, L.; Bruner, M. E.; Connes, P.; Cornwell,
T. J.; Curdt, W.; Foing, B. H.; Hammer, R.; Harrison, R.; Heyvaerts,
J.; Karabin, M.; Marsch, E.; Martic, M.; Mattic, W.; Muller, R.;
Patchett, B.; Roca-Cortes, T.; Rutten, R. J.; Schmidt, W.; Title,
A. M.; Tondello, G.; Vial, J. C.; Visser, H.
Bibcode: 1992ESOC...39..995D
Altcode: 1992hrii.conf..995D
No abstract at ADS
Title: The Soft X-ray Telescope for the SOLAR-A mission
Authors: Tsuneta, S.; Acton, L.; Bruner, M.; Lemen, J.; Brown, W.;
Caravalho, R.; Catura, R.; Freeland, S.; Jurcevich, B.; Morrison,
M.; Ogawara, Y.; Hirayama, T.; Owens, J.
Bibcode: 1991SoPh..136...37T
Altcode:
The Soft X-ray Telescope (SXT) of the SOLAR-A mission is designed
to produce X-ray movies of flares with excellent angular and time
resolution as well as full-disk X-ray images for general studies. A
selection of thin metal filters provide a measure of temperature
discrimination and aid in obtaining the wide dynamic range required for
solar observing. The co-aligned SXT aspect telescope will yield optical
images for aspect reference, white-light flare and sunspot studies,
and, possibly, helioseismology. This paper describes the capabilities
and characteristics of the SXT for scientific observing.
Title: SOLAR-A Reformatted Data Files and Observing Log
Authors: Morrison, M. D.; Lemen, J. R.; Acton, L. W.; Bentley, R. D.;
Kosugi, T.; Tsuneta, S.; Ogawara, Y.; Watanabe, T.
Bibcode: 1991SoPh..136..105M
Altcode:
All of the SOLAR-A telemetry data will be reformatted before
distribution to the analysis computers and the various users. This
paper gives an overview of the files which will be created and the
format and organization which the files will use. The organization
has been chosen to be efficient in space, to ease access to the data,
and to allow for the data to be transportable to different machines. An
observing log file will be created automatically using the reformatted
data files as the input. It will be possible to perform searches with
the observing log to list cases where instruments are in certain modes
and/or seeing certain signal levels.
Title: The O VII Soft X-Ray Spectrum and Its Application to Hot
Plasmas in Astrophysics
Authors: Gabriel, A. H.; Bely-Dubau, F.; Faucher, P.; Acton, L. W.
Bibcode: 1991ApJ...378..438G
Altcode:
The paper presents a revised theory and atomic model for the line
intensities emitted by O VII, taking into account all of the processes
responsible for the emission. This is used to provide a revision of the
density measurements made for solar active regions and during flares, as
well as an attempt to understand the spectrum of the Puppis A supernova
remnant. In order to explain the strange intensity ratios observed
from Puppis A, previous authors have proposed an interpretation based
upon a high-temperature thermal plasma in a nonequilibrium ionization
state. An alternative model is presented here, based upon the assumed
presence of a proportion of fast, nonthermal electrons imbedded in an
otherwise thermal plasma at a temperature below 10 to the 6th K. This
can adequately explain the observations without the necessity of
invoking departures from the ionization balance.
Title: Radiation concerns for the Solar-A soft x-ray telescope
Authors: Acton, Loren W.; Morrison, Mons D.; Janesick, James R.;
Elliott, Tom S.
Bibcode: 1991SPIE.1447..123A
Altcode:
The charge-coupled device (CCD) camera of the Soft X-ray Telescope
(SXT) for the Japanese Solar-A Mission utilizes a 1024 X 1024 virtual
phase CCD manufactured by Texas Instruments in Japan. This sensor
will be subject to radiation in the form of trapped protons from
the earth's radiation belts and soft x-rays (0.2-4 keV) in the solar
image. Proton damage produces 'dark spikes' or pixels of enhanced dark
current. This can be characterized in terms of the average increase
in dark current as a function of proton fluence and predicted through
proton transfer calculations. During the preparation of this camera it
has been discovered that exposure to soft x-rays creates 'permanent'
ionization damage in the gate insulator, resulting in flat-band
shift, dark current increase, loss of charge transfer efficiency,
and, ultimately, total unpinning of the sensor. It has been found
that ultra-violet, and to a lesser degree, visible-light flooding
photo-emits free electrons into the gate oxide which 'anneals' the
damage, restoring proper operation of the CCD.
Title: A solar interferometric mission for ultrahigh resolution
imaging and spectroscopy: SIMURIS
Authors: Damé, L.; Acton, L.; Bruner, M.; Connes, P.; Cornwell, T.;
Foing, B. H.; Heyvaerts, J.; Lemaire, P.; Martić, M.; Muller, R.;
Porteneuve, J.; Roca Cortés, T.; Riehl, J.; Rutten, R.; Séchaud,
M.; Smith, P.; Thorne, A. P.; Title, A. M.; Vial, J. -C.; Visser,
H.; Weigelt, G.
Bibcode: 1991AdSpR..11a.383D
Altcode: 1991AdSpR..11..383D
SIMURIS is an interferometric investigation of the very fine structure
of the solar atmosphere from the photosphere to the corona. It was
proposed to ESA /1/, November 30 1989, for the Next Medium Size
Mission - M2, and accepted in February 1990 for an Assessment Study
in the context of the Space Station. The main scientific objectives
will be outlined, and the ambitious model payload featuring the Solar
Ultraviolet Network (SUN), a 2 m long monolithic array of 4 telescopes
of Ø20 cm, and the Imaging Fourier Transform Spectrometer (IFTS),
an UV and Visible Imaging Fourier Transform Spectrometer coupled to
a Ø40 cm Gregory, described.
Title: Solar physics at ultrahigh resolution from the space station
with the Solar Ultraviolet Network (SUN)
Authors: Damé, L.; Acton, L.; Bruner, M.; Connes, P.; Cornwell,
T.; Foing, B.; Heyvaerts, J.; Lemaire, P.; Martić, M.; Muller, R.;
Roca Cortés, T.; Riehl, J.; Rutten, R.; Title, A. M.; Vial, J. -C.;
Visser, H.; Weigelt, G.
Bibcode: 1991AdSpR..11e.267D
Altcode: 1991AdSpR..11..267D
The SUN experiment is a UV and visible Space Interferometer aimed at
ultra-high resolution in the solar atmosphere. It has been proposed
to ESA as part of the SIMURIS Mission Proposal which has recently
been accepted for an Assessment Study in the framework of the
Space Station. The 4 × 20 cm telescopes of the SUN linear array are
non-redundantly placed to cover a 2 m baseline, and the instrument makes
full use of stabilized interferometry potential, the 4 telescopes being
co-aligned and co-phased on a reference field on the sun. After a brief
outline of the scientific objectives, the concept of the instrument
is described, and its image reconstruction potential is illustrated.
Title: Soft X-ray Telescope (SXT) (Extended Abstract)
Authors: Tsuneta, Saku; Acton, Loren
Bibcode: 1991LNP...387...18T
Altcode: 1991fpsa.conf...18T
No abstract at ADS
Title: Imaging capabilities of SXT for Solar-A
Authors: Lemen, J. R.; Acton, L. W.; Brown, W. A.; Bruner, M. E.;
Catura, R. C.; Strong, K. T.; Watanabe, T.
Bibcode: 1991AdSpR..11e..69L
Altcode: 1991AdSpR..11...69L
The Soft X-ray Telescope (SXT) is a grazing-incidence solar X-ray
telescope which will be flown on the Japanese Solar-A satellite. The
Solar-A satellite, which is scheduled for launch in 1991, will carry
an ensemble of instruments designed to study the Sun during the next
solar sunspot maximum. The SXT will be the first high-resolution
X-ray telescope since Skylab to be flown on an extended-duration
mission. Measurements have been conducted to determine the focal length,
point spread function, and effective area of the SXT mirror. The results
indicate a half-power diameter of 4.9 arcsec and an effective area
of 1.33 cm2 at 13.3 Å. The mirror achieves high-contrast
imaging with very little X-ray scattering. The telescope fwhm is 3.2
arcsec at 8.32 Å. A coaligned aspect telescope operating at visible
wavelengths will permit imaging of magnetic plage or solar continuum
radiation with the same plate scale as the X-ray images.
Title: Plasma diagnostic capabilities of the Soft X-Ray Telescope
on Solar-A
Authors: Strong, K. T.; Acton, L. W.; Brown, W. A.; Claflin, E. S.;
Lemen, J. R.; Tsuneta, S.
Bibcode: 1991AdSpR..11e..73S
Altcode: 1991AdSpR..11...73S
We present the predicted response of the Solar-A Soft X-Ray Telescope
(SXT) to various solar targets. From prelaunch calibrations of the
SXT flight mirror, X-ray filters, and CCD detector, we are able to
predict exposure times and image cadence for a representative range of
temperatures and emission measures of the coronal plasma. We find that
the SXT is very sensitive; it should be able to observe active regions
and flares at its nominal cadence (2 s) with exposure times ranging
from 0.0001 to 1 s. The SXT temperature diagnostic capabilities are
presented for various combinations of the X-ray filters. SXT data can
be used to reproduce accurately the temperature and emission measure
of an isothermal plasma. However, if there is a wide distribution
of temperatures in a pixel, reconstructing the original differential
emission measure distribution becomes more uncertain.
Title: Capabilities and Limitations of SOLAR-A
Authors: Acton, L. W.; Hudson, H. S.
Bibcode: 1991LNP...387...28A
Altcode: 1991fpsa.conf...28A
The purpose of this paper is to present an overview useful to
scientists, not familiar with Solar-A, who are interested in the
capabilities of the mission for solar research. In order to keep
the paper to a manageable size it will be assumed that the reader is
generally familiar both with the experimental techniques of Solar-A
and current research in solar high-energy physics. We do not provide
detailed technical descriptions of the Solar-A instruments. We do,
however, briefly discuss the capabilities of Solar-A in the context
of solar activity research in the 1990's.
Title: High Resolution Solar Physics from the Space Station with
Interferometric Techniques: The Solar Ultraviolet Network (SUN) -
Instrument &Objectives
Authors: Damé, L.; Acton, L.; Bruner, M.; Connes, P.; Cornwell, T.;
Foing, B.; Heyvaerts, J.; Jalin, R.; Lemaire, Ph.; Martic, M.; Moreau,
B.; Muller, R.; Roca Cortés, T.; Riehl, J.; Rutten, R.; Title, A. M.;
Vial, J. -C.; Visser, H.; Weigelt, G.
Bibcode: 1990PDHO....7..262D
Altcode: 1990dysu.conf..262D; 1990ESPM....6..262D
No abstract at ADS
Title: X-Ray calibration of a virtual-phase 1024 X 1024 CCD
Authors: Catura, Richard C.; Lemen, James R.; Morrison, Mons D.;
Windt, David L.; Jordan, W. C.; Acton, Loren W.
Bibcode: 1989SPIE.1159..578C
Altcode:
Results are presented on a calibration with X-rays of a
front-illuminated virtual phase CCD with a 1024 x 1024 pixel array,
performed as a part of the Solar-A preparation, which is a joint
Japanese-U.S.-UK space project scheduled for a launch in August
1991. In the experiment, absolute quantum efficiency (QE) of a virtual
CCD was measured at 14 wavelengths between 5.4 and 67.7 A, and its
flat field responses to the illumination by C-K and Al-K X-rays
were investigated together with its imaging properties in visible
light. Higher than expected QE measurements were obtained at soft X-ray
and EUV wavelengths; these are considered to be caused by fluorescence
occurring in the absorbing layers on the CCD-entrance aperture.
Title: Solar Ultraviolet Network: an interferometric investigation
of the fundamental solar astrophysical scales
Authors: Dame, Luc; Moreau, Bernard G.; Cornwell, Timothy J.;
Visser, H.; Title, Alan M.; Acton, Loren W.; Aime, Claude; Braam,
Bart M.; Bruner, Marilyn E.; Connes, Pierre; Faucherre, Michel; Foing,
B. H.; Haisch, Bernhard M.; Hoekstra, Roel; Heyvaerts, Jean; Jalin,
Rene; Lemaire, Philippe; Martic, Milena; Muller, R.; Noens, J. C.;
Porteneuve, Jacques; Schulz-Luepertz, E.; von der Luehe, Oskar
Bibcode: 1989SPIE.1130..126D
Altcode:
The Solar UV Network (SUN) presently proposed is an interferometric
system, based on the principles of stabilized interferometry, which
will be capable of solar observations with spatial resolutions better
than 0.013 arcsec. SUN will consist of four 20-cm diameter telescopes
aligned nonredundantly on a 2-m baseline. SUN is judged to be ideally
deployable by the NASA Space Station, if implemented on a pointing
platform whose performance is of the order of the Instrument Pointing
System flown on Spacelab 2. The compact, nonredundant configuration of
SUN's telescopes will allow high-resolution imaging of a 2 x 2 arcsec
field on the solar disk.
Title: Report from solar physics
Authors: Walker, A. B. C.; Acton, L.; Brueckner, G.; Chupp, E. L.;
Hudson, H. S.; Roberts, W.
Bibcode: 1989stss.work...31W
Altcode:
A discussion of the nature of solar physics is followed by a brief
review of recent advances in the field. These advances include: the
first direct experimental confirmation of the central role played
by thermonuclear processes in stars; the discovery that the 5-minute
oscillations of the Sun are a global seismic phenomenon that can be
used as a probe of the structure and dynamical behavior of the solar
interior; the discovery that the solar magnetic field is subdivided into
individual flux tubes with field strength exceeding 1000 gauss. Also
covered was a science strategy for pure solar physics. Brief discussions
are given of solar-terrestrial physics, solar/stellar relationships,
and suggested space missions.
Title: MAX 1991. The active sun: A plan for pursuing the study of
the active sun at the time of the next maximum in solar activity,
January 1985
Authors: Acton, L.
Bibcode: 1989STIN...9020030A
Altcode:
The results of the discusions of a working group for the definition of
a program for the forthcoming crest of solar activity, 1990 to 1993
are presented. The MAX '91 program described are intended to achieve
important scientific goals within the context of the natural solar
variability. The heart of the MAX '91 program is a series of campaigns
oriented towards specific scientific problems, and taking place in the
solar maximum period 1990 to 1993. These campaigns will take advantage
of the load-carrying capability of the Space Shuttle to fly instruments
with observational capabilities very different from those of the Solar
Maximum Mission. Various combinations of instruments appropriate to
the specific scientific problem of a given campaign would be flown on
a Shuttle sortie mission.
Title: The Soft X-ray Telescope for SOLAR-A
Authors: Brown, W. A.; Acton, L. W.; Bruner, M. E.; Lemen, J. R.;
Strong, K. T.
Bibcode: 1989BAAS...21..862B
Altcode:
No abstract at ADS
Title: The soft X-ray telescope for the solar A mission.
Authors: Bruner, M. E.; Acton, L. W.; Brown, W. A.; Stern, R. A.;
Hirayama, T.; Tsuneta, S.; Watanabe, T.; Ogawara, Y.
Bibcode: 1989GMS....54..187B
Altcode: 1989sspp.conf..187B; 1989opss.conf..187B
The Solar A mission, being conducted by the Japanese Institute
for Astronautical and Space Sciences, is a project to study solar
flares using a cluster of instruments on an orbiting satellite. It
is scheduled to be launched in September or October of 1991. The
emphasis of the mission is on imaging and spectroscopy of hard and
soft X-rays. The Soft X-Ray Telescope (SXT), one of two major imaging
instruments on the satellite, is a joint U.S.-Japan project. It is
being prepared at Lockheed under NASA sponsorship. The electronic
control system for the SXT is based on microprocessors and is a joint
effort between Lockheed and the National Astronomical Observatory of
Japan (NAOJ). The SXT uses a glancing incidence telescope of 1.55 m
effective focal length to form images in the 0.25 to 3.0 keV range on
a 1024×1024 virtual phase CCD detector. A selection of thin metallic
filters located near the focal plane provides the capability for
electron temperature diagnostics. Knowledge of the alignment of soft
X-ray images with respect to features observable in visible light
is provided by a coaxially mounted aspect telescope which forms its
image on the CCD sensor when the thin metallic filter is replaced by
an appropriate glass filter. A novel mechanical design has permitted
a very lightweight structure that remains stiff enough to survive the
severe launch environment. Other Solar A instruments include a hard
X-ray telescope, a Bragg crystal spectrometer, a wide band spectrometer,
and a radiation belt monitor.
Title: The soft X ray telescope for Solar-A
Authors: Brown, W. A.; Acton, L. W.; Bruner, M. E.; Lemen, J. R.;
Strong, K. T.
Bibcode: 1989dots.work..158B
Altcode:
The Solar-A satellite being prepared by the Institute for Sapce and
Astronautical Sciences (ISAS) in Japan is dedicated to high energy
observations of solar flares. The Soft X Ray Telescope (SXT) is being
prepared to provide filtered images in the 2 to 60 A interval. The
flight model is now undergoing tests in the 1000 foot tunnel at
MSFC. Launch will be in September 1991. Earlier resolution and
efficiency tests on the grazing incidence mirror have established its
performance in soft x rays. The one-piece, two mirror grazing incidence
telescope is supported in a strain free mount separated from the focal
plane assembly by a carbon-epoxy metering tube whose windings and
filler are chosen to minimize thermal and hygroscopic effects. The
CCD detector images both the x ray and the concentric visible light
aspect telescope. Optical filters provide images at 4308 and 4700
A. The SXT will be capable of producing over 8000 of the smallest
partial frame images per day, or fewer but larger images, up to 1024 x
1024 pixel images. Image sequence with two or more of the five x ray
analysis filters, with automatic exposure compensation to optimize
the charge collection by the CCD detector, will be used to provide
plasma diagnostics. Calculations using a differential emission measure
code were used to optimize filter selection over the range of emission
measure variations and to avoid redundancy, but the filters were chosen
primarily to give ratios that are monotonic in plasma temperature.
Title: SMM X-ray polychromator
Authors: Strong, Keith T.; Haisch, Bernhard M.; Lemen, James R.;
Acton, L. W.; Bawa, H. S.; Claflin, E. S.; Freeland, S. L.; Slater,
G. L.; Kemp, D. L.; Linford, G. A.
Bibcode: 1988lock.reptR....S
Altcode:
The range of observing and analysis programs accomplished with the
X-Ray Polychromator (XRP) instruments during the decline of solar
cycle 21 and the rise of the solar cycle 22 is summarized. Section
2 describes XRP operations and current status. This is meant as
a guide on how the instrument is used to obtain data and what its
capabilities are for potential users. The science section contains
a series of representative abstracts from recently published papers
on major XRP science topics. It is not meant to be a complete list
but illustrates the type of science that can come from the analysis
of the XRP data. There then follows a series of appendixes that
summarize the major data bases that are available. Appendix A is a
complete bibliography of papers and presentations produced using XRP
data. Appendix B lists all the spectroscopic data accumulated by the
Flat Crystal Spectrometer (FCS). Appendix C is a compilation of the
XRP flare catalogue for events equivalent to a GOES C-level flare or
greater. It lists the start, peak and end times as well as the peak
Ca XIX flux.
Title: Soft X-ray images of the solar corona using normal incidence
optics.
Authors: Bruner, M. E.; Haisch, B. M.; Brown, W. A.; Acton, L. W.;
Underwood, J. H.
Bibcode: 1988JPhys..49..115B
Altcode: 1988IAUCo.102..115B
A solar coronal loop system has been photographed in soft X-rays using
a normal incidence telescope based on multilayer mirror technology. The
image was recorded during a rocket flight on 1985 October 25, and was
dominated by emission lines arising from the Si XII spectrum. The
rocket also carried a high resolution soft X-ray spectrograph that
confirmed the presence of Si XII line radiation in the source.
Title: The diagnostics of astrophysical plasmas, using the oxygen
VII soft X-ray lines.
Authors: Gabriel, A. H.; Bely-Dubau, F.; Faucher, P.; Acton, L. W.
Bibcode: 1988JPhys..49..235G
Altcode: 1988IAUCo.102..235G
The authors present a revised theory and atomic model for the line
intensities emitted by O VII, taking into account all of the processes
responsible for the emission. This is used to provide a revision of
the density measurements made during solar flares, as well as in an
attempt to understand the spectrum of the Puppis A supernova remnant.
Title: Analytic Capabilities of the Soft X-Ray Telescope for Solar-A
Authors: Brown, W. A.; Bruner, M. E.; Acton, L. W.; Lemen, J. R.
Bibcode: 1988BAAS...20..710B
Altcode:
No abstract at ADS
Title: Observed and theoretical spectra in the 10 - 100 Å interval.
Authors: Brown, W. A.; Bruner, M. E.; Acton, L. W.
Bibcode: 1988JPhys..49..259B
Altcode: 1988IAUCo.102..259B
The soft X-ray spectra of solar flares recorded in two sounding rocket
flights in 1982 and 1985 are compared with predicted spectra.
Title: Variability of solar mesogranulation
Authors: Simon, G. W.; November, L. J.; Acton, L. W.; Ferguson, S. H.;
Shine, R. A.; Tarbell, T. D.; Title, A. M.; Topka, K. P.; Zirin, H.
Bibcode: 1988AdSpR...8g.169S
Altcode: 1988AdSpR...8..169S
From white-light photographs of solar granulation obtained with the
SOUP instrument on Space Shuttle Flight STS-19 we have measured the
motions of granules using local correlation tracking techniques. The
granules are organized into larger-scale structures (mesogranular and
supergranular) which exhibit outflow from upwellings, convergence into
sinks, as well as significant vorticity. Magnetic fields follow these
same flow patterns. We describe these velocity structures, and suggest
that their effect on magnetic field structures may be important to
the solar flare buildup process.
Title: Correlation Lifetimes of Quiet and Magnetic Granulation from
the SOUP Instrument on Spacelab 2
Authors: Title, A.; Tarbell, T.; Topka, K.; Acton, L.; Duncan, D.;
Ferguson, S.; Finch, M.; Frank, Z.; Kelly, G.; Lindgren, R.; Morrill,
M.; Pope, T.; Reeves, R.; Rehse, R.; Shine, R.; Simon, G.; Harvey,
J.; Leibacher, J.; Livingston, W.; November, L.; Zirker, J.
Bibcode: 1988ApL&C..27..141T
Altcode:
The time sequences of diffraction limited granulation images obtained by
the Solar Optical Universal Polarimeter on Spacelab 2 are presented. The
uncorrection autocorrelation limetime in magnetic regions is dominated
by the 5-min oscillation. The removal of this oscillation causes the
autocorrelation lifetime to increase by more than a factor of 2. The
results suggest that a significant fraction of granule lifetimes are
terminated by nearby explosions. Horizontal displacements and transverse
velocities in the intensity field are measured. Lower limits to the
lifetime in the quiet and magnetic sun are set at 440 s and 950 s,
respectively.
Title: The SOLAR-A soft X-ray telescope experiment
Authors: Acton, L.; Bruner, M.; Brown, W.; Lemen, J.; Hirayama, T.;
Tsuneta, S.; Watanabe, T.; Ogawara, Y.
Bibcode: 1988AdSpR...8k..93A
Altcode: 1988AdSpR...8...93A
The Japanese SOLAR-A mission for the study of high energy solar physics
is timed to observe the sun during the next activity maximum. This small
spacecraft includes a carefully coordinated complement of instruments
for flare studies. In particular, the soft X-ray telescope (SXT)
will provide X-ray images of flares with higher sensitivity and time
resolution than have been available before. This paper describes the
scientific capabilities of the SXT and illustrates it application to
the study of an impulsive compact flare.
Title: Extreme ultra-violet filtergrams and X-ray spectroscopy of
active regions and flares from TRC/XSST rocket campaigns
Authors: Foing, B. H.; Martic, M.; Bonnet, R. M.; Bruner, M. E.;
Acton, L. W.; Brown, W. A.
Bibcode: 1988AdSpR...8k.153F
Altcode: 1988AdSpR...8..153F
Ultraviolet filtergrams and X ray spectra were taken by the joint
LPSP/Lockheed rocket experiment TRC/XSST during four international
campaigns involving also ground observations. XSST spectra in the
range 8 to 97A were obtained on the 13 July 1982 flare. From the EUV
filtergrams obtained by the TRC (Transition Region Camera), we could
derive calibrated fluxes in the 220nm and 160 nm continua, in the Ly
alpha and C IV lines, spanning a range of temperatures from medium
photosphere (5 000K), temperature minimum (4 200K), and transition
region (20 000 and 100 000 K). For active regions and a flare, the TRC
fluxes are compared to other data and to corresponding calculations
from semiempirical models. Finally, the scientific prospects of an
advanced TRC/XSST payload onboard future missions are discussed.
Title: The relation between convection flows and magnetic structure
at the solar surface
Authors: Simon, G. W.; November, L. J.; Acton, L. W.; Title, A. M.;
Tarbell, T. D.; Topka, K. P.; Shine, R. A.; Ferguson, S. H.; Weiss,
N. O.; Zirin, H.
Bibcode: 1988AdSpR...8k.133S
Altcode: 1988AdSpR...8..133S
We describe recent results from the comparison of data from the Solar
Optical Universal Polarimeter instrument on Spacelab 2 and magnetograms
from Big Bear Solar Observatory. We show that the Sun's surface velocity
field governs the structure of the observed magnetic field over the
entire solar surface outside sunspots and pores. We attempt to describe
the observed flows by a simple axisymmetric plume model. Finally,
we suggest that these observations may have important implications
for the prediction of solar flares, mass ejections, and coronal heating.
Title: X-Ray Photographs of a Solar Active Region with a Multilayer
Telescope at Normal Incidence
Authors: Underwood, J. H.; Bruner, M. E.; Haisch, B. M.; Brown, W. A.;
Acton, L. W.
Bibcode: 1987Sci...238...61U
Altcode:
An astronomical photograph was obtained with a multilayer x-ray
telescope. A 4-centimeter tungsten-carbon multilayer mirror was flown
as part of an experimental solar rocket payload, and successful
images were taken of the sun at normal incidence at a wavelength
of 44 angstroms. Coronal Si-XII emission from an active region was
recorded on film; as expected, the structure is very similar to that
observed at O-VIII wavelengths by the Solar Maximum Mission flat crystal
spectrometer at the same time. The small, simple optical system used
in this experiment appears to have achieved a resolution of 5 to 10
arc seconds.
Title: Sunspot observations from the SOUP instrument on Spacelab 2.
Authors: Shine, R. A.; Title, A. M.; Tarbell, T. D.; Acton, L.; Duncan,
D.; Ferguson, S. H.; Finch, M.; Frank, Z.; Kelly, G.; Lindgren, R.
Bibcode: 1987NASCP2483..133S
Altcode: 1987tphr.conf..133S
A series of white light images obtained by the SOUP instrument on
Spacelab 2 of active region 4682 on August 5, 1985 were analyzed
in the area containing sunspots. Although the umbra of the spot is
underexposed, the film is well exposed in the penumbral regions. These
data were digitally processed to remove noise and to separate p-mode
oscillations from low velocity material motions. The results of this
preliminary investigation include: (1) proper motion measurements of
a radial outflow in the photospheric granulation pattern just outside
the penumbra; (2) discovery of occasional bright structures (streakers)
that appear to be ejected outward from the penumbra; (3) broad dark
clouds moving outward in the penumbra in addition to the well known
bright penumbral grains moving inward; (4) apparent extensions and
contractions of penumbral filaments over the photosphere; and (5)
observation of a faint bubble or loop-like structure which seems to
expand from two bright penumbral filaments into the photosphere.
Title: First results on quiet and magnetic granulation from SOUP.
Authors: Title, A. M.; Tarbell, T. D.; Acton, L.; Duncan, D.; Ferguson,
S. H.; Finch, M.; Frank, Z.; Kelly, G.; Lindgren, R.; Morrill, M.
Bibcode: 1987NASCP2483...55T
Altcode: 1987tphr.conf...55T
The flight of Solar Optical Universal Polarimeter (SOUP) on Spacelab
2 allowed the collection of time sequences of diffraction limited (0.5
arc sec) granulation images with excellent pointing (0.003 arc sec) and
completely free of the distortion that plagues groundbased images. The
p-mode oscillations are clearly seen in the data. Using Fourier
transforms in the temporal and spatial domain, it was shown that the
p-modes dominate the autocorrelation lifetime in magnetic regions. When
these oscillations are removed the autocorrelation lifetime is found
to be 500 sec in quiet and 950 sec in magnetic regions. In quiet
areas exploding granules are seen to be common. It is speculated that
a significant fraction of granule lifetimes are terminated by nearby
explosions. Using local correlation tracking techniques it was able to
measure horizontal displacements, and thus transverse velocities, in
the magnetic field. In quiet sun it is possible to detect both super
and mesogranulation. Horizontal velocities are as great as 1000 m/s
and the average velocity is 400 m/s. In magnetic regions horizontal
velocities are much less, about 100 m/s.
Title: Real Time Science From Spacelab 2
Authors: Bartoe, J. -D. F.; Acton, L. W.
Bibcode: 1986BAAS...18..661B
Altcode:
No abstract at ADS
Title: Electron Density Diagnostics in the 10--100 Angstrom Interval
for a Solar Flare
Authors: Brown, W. A.; Bruner, M. E.; Acton, L. W.; Mason, H. E.
Bibcode: 1986ApJ...301..981B
Altcode:
Electron density measurements from spectral-line diagnostics are
reported for a solar flare on July 13, 1982, 1627 UT. The spectrogram,
covering the 10-95 A interval, contained usable lines of helium-like
ions C V, N VI, O VII, and Ne IX which are formed over the temperature
interval 0.7-3.5 x 10 to the 6th K. In addition, spectral-line ratios of
Si IX, Fe XIV, and Ca XV were compared with new theoretical estimates
of their electron density sensitivity to obtain additional electron
density diagnostics. An electron density of 3 x 10 to the 10th/cu
cm was obtained. The comparison of these results from helium-like
and other ions gives confidence in the utility of these tools for
solar coronal analysis and will lead to a fuller understanding of the
phenomena observed in this flare.
Title: XSST/TRC rocket observations of 13 July 1982 flare.
Authors: Foing, B. H.; Bonnet, R. M.; Dame, L.; Bruner, M.; Acton,
L. W.; Brown, W. A.
Bibcode: 1986lasf.conf..319F
Altcode: 1986lasf.symp..319F
The authors analyse the UV filtergrams of the 13 July
1982 solar flare, taken by the Transition Region Camera,
during the third flight of the joint Lockheed/LPSP rocket
experiment XSST/TRC. From the calibrated intensities of the flare
components, they estimate directly the Lyα line flux (from 230 to
650×103erg cm-2s-1sr-1),
differentially the C IV line flux (from 30 to 130×103erg
cm-2s-1sr-1), and the excess of 160 nm
continuum temperature brightness (from 100 to 250K) over the underlying
plage. No detectable variation is observed in the 220-nm channel
formed in the medium photosphere. These values are small compared to
other observed or calculated equivalent quantities from Machado model
of flare F1. The authors estimate the corresponding power required to
heat the temperature minimum accordingly over the 1200 Mm2
area, to be 3.6×1025erg s-1 for this small
X-ray C6 flare, 7 minutes after the ground based observed flare maximum.
Title: White-light movies of the solar photosphere from the soup
instrument on spacelab 2
Authors: Title, A. M.; Tarbell, T. D.; Simon, G. W.; Acton, L.;
Duncan, D.; Ferguson, S.; Finch, M.; Frank, Z.; Kelly, G.; Lindgren,
R.; Morrill, M.; Pope, T.; Reeves, R.; Rehse, R.; Shine, R.; Topka,
K.; Harvey, J.; Leibacher, J.; Livingston, W.; November, L.
Bibcode: 1986AdSpR...6h.253T
Altcode: 1986AdSpR...6..253T
We present initial results on solar granulation, pores and sunspots
from the white-light films obtained by the Solar Optical Universal
Polarimeter (SOUP) instrument on Spacelab 2. SOUP contains a
30-cm Cassegrain telescope, an active secondary mirror for image
stabilization, and a white-light optical system with 35-mm film
and video cameras. Outputs from the fine guidance servo provided
engineering data on the performance of the ESA Instrument Pointing
System (IPS). Several hours of movies were taken at various
disk and limb positions in quiet and active regions. The images
are diffraction-limited at 0.5 arc second resolution and are, of
course, free of atmospheric seeing and distortion. Properties of the
granulation in magnetic and non-magnetic regions are compared and are
found to differ significantly in size, rate of intensity variation,
and lifetime. In quiet sun on the order of fifty percent of the area
has at least one ``exploding granule'' occurring in it during a 25
minute period. Local correlation tracking has detected several types
of transverse flows, including systematic outflow from the penumbral
boundary of a spot, motion of penumbral filaments, and cellular flow
patterns of supergranular and mesogranular size. Feature tracking has
shown that in quiet sun the average granule fragment has a velocity
of about one kilometer per second.
Title: Energy release topology in a multiple-loop solar flare
Authors: Cheng, C. -C.; Pallavicini, R.; Acton, L. W.;
Tandberg-Hanssen, E.
Bibcode: 1985ApJ...298..887C
Altcode:
The temporal and spatial structures of the UV and X-ray emissions
and the magnetic field configuration in the November 12, 1980 flare
observed from SMM have been studied. The UV observations were done in
the O V and Fe XXI lines with a spatial resolution of 10 arcsec. The
observations show that the impulsive UV bursts, and also the hard
X-ray bursts by their temporal correlation with the impulsive O V
emission, occurred in small localized kernels. By comparing the O V,
Fe XXI, and X-ray raster images of the flare with the magnetogram,
these emission kernels were identified as footpoints of interacting
magnetic flux loops. The temporal evolution of the O V/Fe XXI emission
shows that there was considerable preheating in the flare plasma some
8-9 minutes prior to the onset of the main hard X-ray bursts. The
results are interpreted as indicating that the primary flare energy
release occurred in a highly sheared multiloop structure, which lies
along a magnetic neutral line. By either beam particle propagation or
convective motion, flare energy is transported via a common footpoint to
another loop which brightened later. The preheating of the flare plasma
is shown to create a more favorable environment for energetic particle
acceleration which resulted in the main impulsive hard X-ray bursts.
Title: Interpretation of the oxygen VII soft X-ray spectrum from
the Puppis-A supernova remnant.
Authors: Gabriel, A. H.; Acton, L. W.; Bely-Dubau, F.; Faucher, P.
Bibcode: 1985ESASP.239..137G
Altcode: 1985cxrs.work..137G
The soft X-ray line spectrum from Puppis-A recorded with the
Einstein observatory shows line ratios different from those
anticipated. Previous interpretations were based upon high temperatures
>5×106K and departures from ionization equilibrium. It
is shown that an alternative model can fit the observations, in which
1% of fast electrons are present in an otherwise thermal plasma at 1
to 2×106K. This plasma can be in steady-state ionization
balance.
Title: Rocket spectrogram of a solar flare in the 10-100 A region.
Authors: Acton, L. W.; Bruner, M. E.; Brown, W. A.; Fawcett, B. C.;
Schweizer, W.; Speer, R. J.
Bibcode: 1985ApJ...291..865A
Altcode:
The soft (10-100 A) X-ray spectrum of an M-class solar flare was
observed with a high-resolution (0.02 A) rocket-borne spectrograph on
1982 July 13. The spectrum samples an area of 600/sq arcsec on the sun,
centered on or near the brightest X-ray feature of the flare. Several
hundred emission lines characteristic of temperatures from about
0.5 to 7 x 10 to the 6th K have been photographically recorded. All
but three of the stronger lines have been identified. It is argued
that previous identification of the line at 17.62 A as iron Ly-alpha
is incorrect. Spectral lines from nickel, iron, chromium, calcium,
sulphur, silicon, aluminium, magnesium, neon, oxygen, nitrogen, and
carbon are tabulated and discussed with extensive reference to earlier
work. Absolute line intensities are given and the calibration of the
telescope-spectrograph is discussed.
Title: Flare Electron Densities Using X-ray Line Ratios
Authors: Brown, W. A.; Bruner, M. E.; Acton, L. W.; Mason, H. E.
Bibcode: 1985BAAS...17R.629B
Altcode:
No abstract at ADS
Title: Soft X-ray polychromator for the Solar Maximum Mission
Authors: Haisch, B. M.; Levay, M.; Stern, R. A.; Strong, K. T.;
Wolfson, C. J.; Acton, L. W.
Bibcode: 1984lock.rept.....H
Altcode:
The XRP was designed to measure the following temporal and spatial
properties of the active and flaring Sun: electron temperature,
departures from steady state, ion kinetic temperatures, and electron
density. The Bent Crystal Spectrometer (BCS) is capable of measuring
the broadening and blue shifts often observed in the impulsive phase of
flares. The six simultaneous line fluxes indicative of six different
temperatures of formation observable by the Flat Crystal Spectrometer
(FCS) allows the derivation of the differential emission measure of
the plasma at each raster point. During the operational periods of the
XRP hundreds of flares of C-level (GOES classification) were observed
and brighter in both the FCS and BCS, including 5 X-flares. Associated
theoretical work in atomic physics, stimulated in part by the promise
of XRP measurements, has benefitted from the experimental data on
solar plasmas which the XRP has provided in abundance.
Title: A consistent picture of coronal and chromospheric processes
in a well-observed solar flare
Authors: Gunkler, T. A.; Canfield, R. C.; Acton, L. W.; Kiplinger,
A. L.
Bibcode: 1984ApJ...285..835G
Altcode:
The solar flare of 15:22 UT on June 24, 1980 is analyzed using
simultaneous observations in hard X-rays, soft X-rays, and H-alpha line
profiles obtained from instruments aboard the Solar Maximum Mission
and ground-based instruments. The theoretical H-alpha profiles of
Canfield, Gunkler, and Ricchiazzi (1984) are used to analyze the
H-alpha data, and the work of Hummer and Rybicki (1968) is used to
provide qualitative velocity information. The soft X-ray data are
employed to obtain coronal measurements of parameters of interest,
while the flux and spectrum of the hard X-rays are used to calculate
the peak power of nonthermal electrons. Various flare phenomena are
studied, including heating of the chromosphere by nonthermal electrons,
enhanced coronal pressure, enhanced thermal conduction, chromospheric
evaporation and mass motion. It is shown that the observations strongly
suggest a scenario in which two large magnetic loop systems interact
to provide the flare energy.
Title: X-ray, Ultraviolet, Optical and Magnetic Structure in and
near an Active Region
Authors: Haisch, B. M.; Tarbell, T. D.; Bruner, M. E.; Acton, L. W.;
Bonnet, R. M.; Hagyard, M. J.
Bibcode: 1984BAAS...16Q1002H
Altcode:
No abstract at ADS
Title: SMM observations of K-alpha radiation from fluorescence of
photospheric iron by solar flare X-rays
Authors: Parmar, A. N.; Culhane, J. L.; Rapley, C. G.; Wolfson, C. J.;
Acton, L. W.; Phillips, K. J. H.; Dennis, B. R.
Bibcode: 1984ApJ...279..866P
Altcode:
High-resolution Fe K-alpha spectra near 1.94 A observed during solar
flares with the Bent Crystal Spectrometer on the Solar Maximum Mission
are presented. The evidence for two possible excitation mechanisms,
electron impact and fluorescence, is examined. It is found that the
fluorescence mechanism satisfactorily describes the results, while the
observations do not support electron collisional excitation of the Fe
K-alpha transitions in low ionization stages (II-XII) of iron. Using
Bai's model of the fluorescent excitation process, the photospheric iron
abundance relative to that of hydrogen is estimated to be 5-6 x 10 to
the -5th. The mean height of the soft X-ray source producing the K-alpha
fluorescence is calculated on the basis of this model for about 40 large
flares. The solar K-alpha lines are found to be about 25 percent wider
than those measured in the laboratory. Weak line features observed at
wavelengths shorter than that of the K-alpha lines are discussed.
Title: The Transition Region Camera Experiment: High Resolution
Ultra-Violet Filtergrams of the Sun
Authors: Foing, B. H.; Bonnet, R. M.; Acton, L. W.; Brown, W. A.;
Bruner, M.; Decandim, M.
Bibcode: 1984ssdp.conf...99F
Altcode:
Three series of high resolution UV pictures of the Sun have been
obtained during the three flights of the Transition Region Camera
which took place on July 3, 1979, September 23, 1980 and July 13,
1982. These pictures reveal many new structures which were never
seen before on Ly alpha spectro-heliograms and on UV pictures of the
temperature minimum region. The scientific objectives, instrumentation,
flight conditions and preliminary results are described.
Title: Results from the X-ray polychromator on SMM
Authors: Culhane, J. L.; Acton, L. W.; Gabriel, A. H.
Bibcode: 1984MmSAI..55..673C
Altcode:
Observations of the soft X-ray emitting plasma by means of the
X-Ray Polychromator (XRP) on the Solar Maximum Mission satellite are
described. The scientific advances achieved by use of the XRP are
in the areas of: (1) flare morphology, (2) spectroscopy and plasma
diagnostics, (3) chromospheric evaporation and the physics of flare
loops, (4) studies of the microwave emission mechanisms of active
regions, (5) the fluorescent excitation of Fe II K-alpha radiation,
(6) measurement of variations of calcium abundance for X-ray plasmas,
and (7) soft X-ray observations of spray transients. The findings in
each of these areas are discussed.
Title: Performance of a solar soft X-ray spectrograph-telescope.
Authors: Brown, W. A.; Bruner, M. E.; Acton, L. W.
Bibcode: 1984AnIPS...6...60B
Altcode:
No abstract at ADS
Title: Spatial and Temporal Development of the Impulsive and Gradual
Phases in a Solar Flare Observed with SMM
Authors: Cheng, C. -C.; Acton, L. W.; Tandberg-Hanssen, E.
Bibcode: 1983BAAS...15R.918C
Altcode:
No abstract at ADS
Title: A Consistent Picture of Coronal and Chromospheric Processes
in a Well-Observed Flare
Authors: Gunkler, T. A.; Canfield, R. C.; Acton, L. W.; Kiplinger,
A. L.
Bibcode: 1983BAAS...15Q.919G
Altcode:
No abstract at ADS
Title: Transport and Containment of Plasmas Particles and Energy
Within Flares
Authors: Acton, L. W.; Brown, W. A.; Bruner, M. E. C.; Haisch, B. M.;
Strong, K. T.
Bibcode: 1983SoPh...86...79A
Altcode:
Results from the analysis of flares observed by the Solar Maximum
Mission (SMM) and a recent rocket experiment are discussed. We
find evidence for primary energy release in the corona through the
interaction of magnetic structures, particle and plasma transport
into more than a single magnetic structure at the time of a flare
and a complex and changing magnetic topology during the course of a
flare. The rocket data are examined for constraints on flare cooling,
within the context of simple loop models. These results form a basis
for comments on the limitations of simple loop models for flares.
Title: Closed coronal structures. V - Gasdynamic models of flaring
loops and comparison with SMM observations
Authors: Pallavicini, R.; Peres, G.; Serio, S.; Vaiana, G.; Acton,
L.; Leibacher, J.; Rosner, R.
Bibcode: 1983ApJ...270..270P
Altcode:
The hydrodynamic response of confined magnetic structures to strong
heating perturbations is investigated by means of a time-dependent
one-dimensional colde which incorporates the energy, momentum, and
mass conservation equations. The entire atmospheric structure from the
chromosphere to the corona is taken into account. Models with different
energy input, heating time dependence, preflare conditions and heating
location have been examined in the numerical simulations. The
result of model calculations are compared with observations of flares
obtained with the X-ray Polychromator experiment on the Solar Maximum
Mission. These include light curves of spectral lines formed over a
wide range of coronal flare temperatures, as well as determinations
of Doppler shifts for the high temperature plasma. Several examples
are used to illustrate the range of the observational variation. It is shown that the predictions of the numerical simulations
are in good overall agreement with the observed evolution of the
flare coronal plasma. The model reproduces correctly the temporal
profile of X-ray spectral lines and -- to first order at least --
their relative intensities. The upflow velocities predicted by
model calculations are in agreement with the observed blueshifts,
supporting the interpretation of the blueshifts as due to evaporation
of chromospheric material. The possibility of using the comparison
of model predictions with observations to derive information on the
processes of energy release and transfer in solar flares is discussed.
Title: An X-Ray Empirical Model of a Solar Flare Loop
Authors: Bruner, M.; Brown, W. A.; Acton, L. W.; Strong, K. T.
Bibcode: 1983BAAS...15..708B
Altcode:
No abstract at ADS
Title: Magnetic flux tubes and local heating in the solar temperature
minimum region.
Authors: Foing, B. H.; Bonnet, R. M.; Bruner, M. E.; Acton, L. W.
Bibcode: 1983BAAS...15..620F
Altcode:
No abstract at ADS
Title: High resolution large area modular array of reflectors (LAMAR)
WolterType I X-ray telescope for Spacelab.
Authors: Catura, R. C.; Acton, L. W.; Brown, W. A.; Gilbreth, C. W.;
Springer, L. A.; Vieira, J. R.; Culhane, J. L.; Mason, I. W.; Siegmund,
O.; Patrick, T. J.; Sheather, P. H.; Pounds, K. A.; Cooke, B. A.;
Evans, K.; Pye, J.; Smith, G.; Wells, A.; Spragg, J. E.; Whitford,
C. H.; Garmire, G.; Margon, B.; Fabian, A.
Bibcode: 1983OptEn..22..132C
Altcode:
A Wolter Type I x-ray telescope, intended both for astronomical
observations and to serve as a prototype module for the large area
modular array of reflectors (LAMAR) mission, is now in definition study
under NASA's Spacelab program. The five mirror telescope presently
being designed is to have a blur circle radius of 20 arc sec and an
effective area of about 400 cm2 at 1/4 keV, 200 cm2 in the 0.5 to
2 keV range, and 50 cm2 between 2 and 5 keV. Future expansion to a
full ten mirror telescope will approximately double these effective
areas. A rotary interchange mechanism will allow either of two imaging
proportional counters (IPCs) to be placed at the telescope focus; one
operating between 0.15 and 2 keV and the other optimized for the 0.6
to 6 keV energy range. During flight, the telescope will utilize an
instrument pointing system for a series of observations lasting from
six minutes to several hours. This investigation has dual objectives:
The primary objective is scientific and involves observational study
of galactic and extragalactic x-ray sources, extending the work of the
Einstein Observatory to much fainter sources and to higher energies. The
second objective is to provide an assessment of the cost and improved
performance of utilizing Wolter Type l x-ray optics for the LAMAR
mission and to extend the technology for producing these optics to
still higher angular resolution and toward lower cost.
Title: Chromospheric evaporation in a well-observed compact flare
Authors: Acton, L. W.; Leibacher, J. W.; Canfield, R. C.; Gunkler,
T. A.; Hudson, H. S.; Kiplinger, A. L.
Bibcode: 1982ApJ...263..409A
Altcode:
Hudson and Ohki (1972) pointed out that the increase of the soft
X-ray emission measure during flares might be accounted for in two
different ways, either by 'coronal condensation', or by what they termed
'chromospheric rarefaction', now more commonly called 'chromospheric
evaporation'. They ruled out coronal condensation on the basis of
cornal mass content arguments. Moore et al. (1980) found it highly
probable that the bulk of the mass of the soft X-ray emitting plasma is
supplied during the rise phase by chromospheric evaporation from the
feet of the soft X-ray loops. On the other hand, Cheng et al. (1981)
argued that chromospheric evaporation is not important as a source
of soft X-ray plasma. The present investigation is concerned with
an event in which direct chromospheric observations contradict the
conclusions reached by Cheng et al. Up to now chromospheric evaporation
has always been an inference, without compelling positive evidence. In
the current investigation, observations are considered which constitute
such evidence.
Title: Observation of Flare Density Using Helium-Like Ion Lines
Authors: Brown, W. A.; Bruner, M. E.; Acton, L. W.
Bibcode: 1982BAAS...14..898B
Altcode:
No abstract at ADS
Title: 12-95 Angstrom Spectrum of a Solar Flare
Authors: Bruner, M. E.; Acton, L. W.; Brown, W. A.
Bibcode: 1982BAAS...14..924B
Altcode:
No abstract at ADS
Title: Flare Classification - Fact or Fancy - a Review of the Days
Discussion
Authors: Acton, L. W.
Bibcode: 1982Obs...102..123A
Altcode:
No abstract at ADS
Title: Rocket photographs of fine structure and wave patterns in
the solar temperature minimum
Authors: Bonnet, R. M.; Decaudin, M.; Foing, B.; Bruner, M.; Acton,
L. W.; Brown, W. A.
Bibcode: 1982A&A...111..125B
Altcode:
A new series of high resolution pictures of the sun has been obtained
during the second flight of the Transition Region Camera which occurred
on September 23, 1980. The qualitative analysis of the results indicates
that a substantial portion of the solar surface at the temperature
minimum radiates in non-magnetic regions and from features below 1
arcsec in size. Wave patterns are observed on the 160 nm temperature
minimum pictures. They are absent on the Lyman alpha pictures. Their
physical characteristics are compatible with those of gravitational
and acoustic waves generated by exploding granules.
Title: Impulsive Phase of Flares in Soft X-Ray Emission
Authors: Antonucci, E.; Gabriel, A. H.; Acton, L. W.; Culhane, J. L.;
Doyle, J. G.; Leibacher, J. W.; Machado, M. E.; Orwig, L. E.; Rapley,
C. G.
Bibcode: 1982SoPh...78..107A
Altcode:
Observations using the Bent Crystal Spectrometer instrument on the
Solar Maximum Mission show that turbulence and blue-shifted motions
are characteristic of the soft X-ray plasma during the impulsive phase
of flares, and are coincident with the hard X-ray bursts observed
by the Hard X-ray Burst Spectrometer. A method for analysing the
Ca XIX and Fe XXV spectra characteristic of the impulsive phase
is presented. Non-thermal widths and blue-shifted components in the
spectral lines of Ca XIX and Fe XXV indicate the presence of turbulent
velocities exceeding 100 km s-1 and upward motions of
300-400 km s-1.
Title: Solar flare X-ray spectra from the Solar Maximum Mission Flat
Crystal Spectrometer
Authors: Phillips, K. J. H.; Fawcett, B. C.; Kent, B. J.; Gabriel,
A. H.; Leibacher, J. W.; Wolfson, C. J.; Acton, L. W.; Parkinson,
J. H.; Culhane, J. L.; Mason, H. E.
Bibcode: 1982ApJ...256..774P
Altcode:
High-resolution solar X-ray spectra obtained with the Flat Crystal
Spectrometer aboard the Solar Maximum Mission from two solar flares and
a nonflaring active region are analyzed. The 1-22 A region was observed
during the flare on 1980 August 25, while smaller spectral regions were
repeatedly covered during the 1980 November 5 flare. Voigt profiles
were fitted to spectral lines to derive accurate wavelengths and to
resolve blends. During the August 25 flare, 205 lines were found in
the range 5.68-18.97 A, identifications being provided for all but 40
(mostly weak) lines. Upper limits to flare densities are derived from
various line ratios, the hotter (about 10 to the 7th K) ions giving an
electron density of less than 10 to the 12th per cu cm for the August
25 flare. Other observed line ratios (e.g., in Fe XVII and Mg XII)
indicate a need for revisions in theoretical calculations.
Title: Towards understanding solar flares
Authors: Acton, L. W.
Bibcode: 1982LockH.......38A
Altcode:
Instrumentation and spacecraft payloads developed at Lockheed for
solar flare studies are reviewed, noting the significance of the
observations for adding to a data base for eventual prediction of
the occurrence of flares and subsequent radiation hazards to people
in space. Developmental work on the two solar telescopes on board
the Skylab pallet was performed at a Lockheed facility, as was the
fabrication of very-large-area proportional counter for flights on
the Aerobee rocket in 1967. The rocket work led to the fabrication of
the Mapping X Ray Heliometer on the Orbiting Solar Observatory and the
X Ray Polychromator for the Solar Maximum Mission. The Polychromator
consists of a bent crystal spectrometer for high time resolution flare
studies over a wide field of view, and a flat crystal spectrometer
for simultaneous polychromatic imaging at 7 different X ray wavelengths.
Title: Observations of a POST Flare Radio Burst in X-Rays
Authors: Svestka, Z.; Hoyng, P.; van Tend, W.; Boelee, A.; de Jager,
C.; Stewart, R. T.; Acton, L. W.; Bruner, E. C.; Gabriel, A. H.;
Rapley, C. G.; de Jager, C.; LaFleur, H.; Nelson, G.; Simnett, G. M.;
van Beek, H. F.; Wagner, W. J.
Bibcode: 1982SoPh...75..305S
Altcode:
More than six hours after the two-ribbon flare of 21 May 1980, the
hard X-ray spectrometer aboard the SMM imaged an extensive arch above
the flare region which proved to be the lowest part of a stationary
post-flare noise storm recorded at the same time at Culgoora. The X-ray
arch extended over 3 or more arc minutes to a projected distance of
95 000 km, and its real altitude was most probably between 110 000
and 180 000 km. The mean electron density in the cloud was close to
109 cm−3 and its temperature stayed for many
hours at a fairly constant value of about 6.5 × 106 K. The
bent crystal spectrometer aboard the SMM confirms that the arch emission
was basically thermal. Variations in brightness and energy spectrum at
one of the supposed footpoints of the arch seem to correlate in time
with radio brightness suggesting that suprathermal particles from
the radio noise regions dumped in variable quantities into the low
corona and transition layer; these particles may have contributed to
the population of the arch, after being trapped and thermalized. The
arch extended along the H∥ = 0 line thus apparently
hindering any upward movement of the upper loops reconnected in the
flare process. There is evidence from Culgoora that this obstacle may
have been present above the flare since 15-30 min after its onset.
Title: Direct evidence for chromospheric evaporation in a
well-observed compact flare
Authors: Canfield, R. C.; Gunkler, T. A.; Hudson, H. S.; Acton, L. W.;
Leibacher, J. W.; Kiplinger, A. L.
Bibcode: 1982AdSpR...2k.145C
Altcode: 1982AdSpR...2..145C
Observations of the solar flare of May 7, 1980 using several Solar
Maximum Mission instruments are presented as an investigation of the
phenomenon of chromospheric evaporation. The total amount of plasma
at temperatures greater than 2 x 10 to the 6th K were determined from
the X-ray data, and the amount of plasma that was evaporated from
the chromosphere was determined from the H-alpha data. The H-alpha
profiles indicate that for the flare as a whole, at the time of peak
soft X-ray emission measure, the number of atoms evaporated from the
chromosphere was 7 x 10 to the 37th. The soft X-ray emission measure
of 1 x 10 to the 49th/cu cm, coupled with the flare volume estimate
of 10 to the 26th cu cm, indicates that there were 3 x 10 to the 37th
electrons in the soft X-ray plasma with temperatures greater than 2 x
10 to the 6th K. These results indicate that enough material had been
evaporated from the chromosphere to account for the X-ray plasma. Taken
together, the H-alpha, soft X-ray, and hard X-ray images indicate that
chromospheric evaporation is driven both by flare-accelerated electrons
during the impulsive phase and by conduction during the thermal phase.
Title: 12-95 Å Spectrum of a Solar Flare Solar Soft X-Ray Flare
Spectra.
Authors: Bruner, M. E.; Brown, W. A.; Acton, L. W.
Bibcode: 1982uxsa.coll...13B
Altcode: 1982IAUCo..73...13B
No abstract at ADS
Title: Observational Studies of Chromospheric Evaporation
Authors: Acton, L. W.
Bibcode: 1982sofl.symp...27A
Altcode:
No abstract at ADS
Title: High resolution large area modular array of reflectors /LAMAR/
Wolter type I X-ray telescope for Spacelab
Authors: Catura, R. C.; Acton, L. W.; Brown, W. A.; Gilbreth, C. W.;
Springer, L. A.; Vieira, J. R.; Culhane, J. L.; Mason, I. W.; Siegmund,
O.; Patrick, T. J.
Bibcode: 1982SPIE..284..169C
Altcode:
The Spacelab Wolter type I X-ray telescope, which is intended for
both astronomical observations and the functional verification of
the future Large Area Modular Array of Reflectors (LAMAR) concept,
comprises five mirrors and is designed to have a blur circle radius
of 20 arcsec, with effective areas of (1) 400 sq cm at 0.25 keV,
(2) 200 sq cm in the 0.5-2.0 keV range, and (3) 50 sq cm between
2 and 5 keV. A rotary interchange mechanism allows either of two
imaging proportional counters to be placed at the telescope focus. The
telescope's primary objective is the observational study of galactic
and extragalactic X-ray sources, extending the work of the Einstein
Observatory to fainter sources and higher energies. Secondarily, the
costs and performance to be expected from the use of this telescope
type in the LAMAR mission will be assessed.
Title: X-Ray Flare Spectroscopy: SMM Observations and Loop Modeling.
Authors: Acton, L.; Pallavicini, R.; Peres, G.; Vaiana, G.
Bibcode: 1982uxsa.coll....1A
Altcode: 1982IAUCo..73....1A
No abstract at ADS
Title: Fluorescent excitation of photospheric Fe K-α emission during
solar flares
Authors: Parmar, A. N.; Culhane, J. L.; Rapley, C. G.; Phillips,
K. J. H.; Wolfson, C. J.; Acton, L. W.; Dennis, B. R.
Bibcode: 1982AdSpR...2k.151P
Altcode: 1982AdSpR...2..151P
The Bent Crystal Spectrometer on the NASA Solar Maximum Mission
satellite provides high spectral and temporal resolution observations of
the Fe Kα lines. We have analysed spectra from almost 50 solar flares
that occurred during 1980. These data strongly support fluorescent
excitation of photospheric iron by photons of E > 7.11 keV emitted
by the hot coronal plasma produced during the flare. After comparison
of the data with a model, we discuss the observed Kα line widths,
estimates of the size of the emitting region, the height of the coronal
source and the photospheric iron abundance.
Title: Direct Evidence for Chromospheric Evaporation in a
Well-Observed Compact Flare
Authors: Canfield, R. C.; Acton, L. W.; Gunkler, T. A.; Hudson, H. S.;
Kiplinger, A. L.; Leibacher, J. W.
Bibcode: 1981BAAS...13R.819C
Altcode:
No abstract at ADS
Title: X-ray line widths and coronal heating
Authors: Acton, L. W.; Wolfson, C. J.; Joki, E. G.; Culhane, J. L.;
Rapley, C. G.; Bentley, R. D.; Gabriel, A. H.; Phillips, K. J. H.;
Hayes, R. W.; Antonucci, E.
Bibcode: 1981ApJ...244L.137A
Altcode:
Preliminary results of spectroscopy and imaging of a solar active region
and flare plasma in soft X-ray emission lines are presented. Observed
X-ray line widths in a nonflaring active region are broader than the
Doppler width corresponding to the local electron temperature. An
analysis of 41 soft X-ray flares within a single active region reveals
a preference for flares to occur at locations that already show
enhanced X-ray emission and to favor magnetic complexity over high
gradient. However, flares do not appear to be directly responsible
for the heating and X-ray production of the active regions.
Title: Observations of the limb solar flare on 1980 April 30 with
the SMM X-ray polychromator
Authors: Gabriel, A. H.; Phillips, K. J. H.; Acton, L. W.; Wolfson,
C. J.; Culhane, J. L.; Rapley, C. G.; Bentley, R. D.; Kayat, M. A.;
Jordan, C.; Antonucci, E.
Bibcode: 1981ApJ...244L.147G
Altcode:
No abstract at ADS
Title: X-ray spectra of solar flares obtained with a high-resolution
bent crystal spectrometer
Authors: Culhane, J. L.; Rapley, C. G.; Bentley, R. D.; Gabriel,
A. H.; Phillips, K. J.; Acton, L. W.; Wolfson, C. J.; Catura, R. C.;
Jordan, C.; Antonucci, E.
Bibcode: 1981ApJ...244L.141C
Altcode:
Preliminary results obtained for three solar flares with the bent
crystal spectrometer on the SMM are presented. Resonance and satellite
lines of Ca XIX and XVIII and Fe XXV and XXIV are observed together
with the Fe XXVI Lyman-alpha line. Plasma properties are deduced from
line ratios and evidence is presented for changes of line widths
coincident with the occurrence of a hard X-ray impulsive burst. Fe
K-alpha spectra from a disk center and a limb flare agree with the
predictions of a fluorescence excitation model. However, a transient
Fe K-alpha burst observed in a third flare may be explained by the
collisional ionization of cool iron by energetic electrons.
Title: Solar maximum mission experiment: Early results from the soft
X-ray polychromator experiment
Authors: Gabriel, A. H.; Antonucci, E.; Phillips, K. J. H.; Culhane,
J. L.; Bentley, R. D.; Parmar, A. N.; Rapley, C. G.; Acton, L. W.;
Leibacher, J. W.; Wolfson, C. J.; Strong, K. T.; Jordan, C.
Bibcode: 1981AdSpR...1m.267G
Altcode: 1981AdSpR...1Q.267G
The X-Ray Polychromator experiment has been in operation on the
SMM satellite for over three months. It is observing flares and
active regions in the wavelength range 1Å to 23Å using a number
of different modes. These include polychromatic imaging, high
resolution line profiles, high dispersion spectra, and light curves
with high time-resolution. Data are described together with some of
the preliminary analysis and interpretation.
Title: Solar Maximum Mission experiment: early results from the soft
X-ray polychromator experiment.
Authors: Gabriel, A. H.; Culhane, J. L.; Acton, L. W.; Antonucci,
E.; Bentley, R. D.; Jordan, C.; Leibacher, J. W.; Parmar, A. N.;
Phillips, K. J. H.; Rapley, C. G.; Wolfson, C. J.; Strong, K. T.
Bibcode: 1981hea..conf..267G
Altcode:
No abstract at ADS
Title: High resolution large area modular array of reflectors (LAMAR)
Wolter Type I x-ray telescope for Spacelab
Authors: Catura, R. C.; Acton, L. W.; Brown, W. A.; Gilbreth, C. W.;
Springer, L. A.; Vieira, J. R.; Culhane, J. L.; Mason, I. W.; Siegmund,
O.; Patrick, T. J.; Sheather, P. H.; Pounds, K. A.; Cooke, B. A.;
Evans, K.; Pye, J.; Smith, G.; Wells, A.; Spragg, J. E.; Whitford,
C. H.; Franks, A.; Gale, B.; Lindsey, K.; Stedman, M.; Garmire, G.;
Margon, B.; Fabian, A.
Bibcode: 1981SPIE..284..169C
Altcode:
No abstract at ADS
Title: Large-aperture high-resolution X-ray collimator for the Solar
Maximum Mission
Authors: Nobles, R. A.; Acton, L. W.; Joki, E. G.; Leibacher, J. W.;
Peterson, R. C.
Bibcode: 1980ApOpt..19.2957N
Altcode:
A description is presented of a flight-qualified large-aperture 12
x 12-arcsec angular resolution multigrid X-ray collimator developed
for the Solar Maximum Mission (SMM) flat crystal spectrometer. This
collimator, designed for the 1.4-22.4-A wavelength range, utilizes an
optical bench/metering structure to align and support prealigned grid
subassemblies. The optical bench is a lightweight, rigid, and stable
aluminum honeycomb structure. The grids are of a compound and bimetallic
design, having 63.5-micron square holes on an 88.9-micron spacing in
8-micron thick gold, which is in turn supported by a 76-micron thick
Invar grid having 600-micron square holes on a 739-micron spacing. The
small apertures in the gold provide the 12-arcsec collimation with
the Invar grids providing wide angle off-axis blocking out to an
approximately 35 arcmin view angle. The collimator has seven individual
channels, four of a 5.1- x 10-cm area and three of a 1.3- x 10-cm
area. Laboratory measurements gave an average angular resolution of
12.5 arcsec FWHM with 0.259 transmission for the large-area channels
and 12.0 arcsec and 0.200 transmission for the small-area channels. A
thermal filter composed of two layers of approximately 1000-A thick
aluminum prevents solar heating of the front collimator grids by
absorbing longer wavelength radiation while passing most of the X
radiation in the band of interest.
Title: A Small Impulsive Flare
Authors: Acton, L. W.; Leibacher, J. W.
Bibcode: 1980BAAS...12..904A
Altcode:
No abstract at ADS
Title: The Lockheed OSO-8 program. Analysis of data from the mapping
X-ray heliometer experiment
Authors: Acton, L. W.; Wolfson, C. J.; Datlowe, D. W.; Mosher, J. M.;
Roethig, D. T.; Smith, K. L.
Bibcode: 1980lock.reptR....A
Altcode:
The final report describes the extent of the analysis effort, and
other activities associated with the preservation and documentation
of the data set are described. The main scientific results, which are
related to the behavior of individual solar activity regions in the
energy band 1.5 - 15 keV, are summarized, and a complete bibliography
of publications and presentations is given. Copies of key articles
are also provided.
Title: X-rays, filament activity and flare prediction
Authors: Mosher, J. M.; Acton, L. W.
Bibcode: 1980SoPh...66..105M
Altcode:
127 hr of high-resolution Hα movies of young active regions have
been compared with simultaneous 1.5-15 keV X-ray measurements from the
Mapping X-Ray Heliometer experiment on OSO-8, with particular attention
to preflare periods and to the possibility of X-ray emission associated
with filament activity during that time. The period studied included
8 confirmed flares or subflares, 16 unreported events of comparable
magnitude, and numerous examples of filament activity. We found no
evidence for X-ray emission from areas of enhanced filament activity
unless simultaneous brightenings were present in Hα. In addition,
we detected no peculiar behavior of either filaments or X-rays during
the period of approximately 20 min preceding these small flares which,
even in retrospect, would have allowed them to be `predicted'.
Title: High-resolution Lyman-alpha filtergrams of the sun
Authors: Bonnet, R. M.; Decaudin, M.; Bruner, E. C., Jr.; Acton,
L. W.; Brown, W. A.
Bibcode: 1980ApJ...237L..47B
Altcode:
The results of an experiment, conducted jointly by the Lockheed Palo
Alto Research Laboratory and the Laboratoire de Physique Stellaire et
Planetaire du CNRS, which investigated the transition-region plasma
and the geometry of coronal active regions, in relation to models of
the high-temperature layers, are presented. A Black Brant rocket was
used to obtain 1-arc sec resolution L-alpha pictures of the sun, which
revealed small scale features not seen previously at this wavelength,
that delineate the geometry of the magnetic field in the chromosphere
and in the corona. It is concluded that these observations might
provide a new way of observing the upper chromosphere and corona,
and that they provide direct evidence of the inhomogeneous character
of the chromosphere and of the dominant role of the magnetic field
Title: The soft X-ray polychromator for the Solar Maximum Mission.
Authors: Acton, L. W.; Culhane, J. L.; Gabriel, A. H.; Bentley,
R. D.; Bowles, J. A.; Firth, J. G.; Finch, M. L.; Gilbreth, C. W.;
Guttridge, P.; Hayes, R. W.; Joki, E. G.; Jones, B. B.; Kent, B. J.;
Leibacher, J. W.; Nobles, R. A.; Patrick, T. J.; Phillips, K. J. H.;
Rapley, C. G.; Sheather, P. H.; Sherman, J. C.; Stark, J. P.; Springer,
L. A.; Turner, R. F.; Wolfson, C. J.
Bibcode: 1980SoPh...65...53A
Altcode:
The 1.4-22.4 Å range of the soft X-ray spectrum includes a multitude
of emission lines which are important for the diagnosis of plasmas
in the 1.5-50 million degree temperature range. In particular, the
hydrogen and helium-like ions of all abundant solar elements with Z
> 7 have their primary transitions in this region and these are
especially useful for solar flare and active region studies. The soft
X-ray polychromator (XRP) is a high resolution experiment working
in this spectral region. The XRP consists of two instruments with
a common control, data handling and power system. The bent crystal
spectrometer is designed for high time resolution studies in lines of Fe
I-Fe XXVI and Ca XIX. The flat crystal scanning spectrometer provides
for 7 channel polychromatic mapping of flares and active regions in
the resonance lines of O VIII, Ne IX, Mg XI, Si XIII, S XV, Ca XIX,
and Fe XXV with 14″ spatial resolution. In its spectral scanning
mode it covers essentially the entire 1.4-22.5 Å region.
Title: X-ray spectrometer spectrograph telescope system.
Authors: Bruner, E. C., Jr.; Acton, L. W.; Brown, W. A.; Salat, S. W.;
Franks, A.; Schmidtke, G.; Schweizer, W.; Speer, R. J.
Bibcode: 1980OptEn..19..433B
Altcode:
We now describe a new sounding rocket payload that has been developed
for x-ray spectroscopic studies of the solar corona. The instrument
incorporates a grazing incidence Rowland mounted grating spectrograph
and an extreme off-axis parabolic sector telescope to isolate regions
of the sun of order 1 X10 arc seconds in size. The focal surface of
the spectrograph is shared by photographic and photoelectric detection
systems, with the latter serving as a part of the rocket pointing system
control loop. Fabrication and alignment of the optical system are based
on high precision machining and mechanical metrology techniques. The
spectrograph covers the 10 to 50 angstrom interval and has a resolution
of 16 milliangstroms in the current version. Modifications planned for
future flights will improve the resolution to around 5 milliangstroms,
permitting line widths to be measured.
Title: Ganging up on the Sun
Authors: Frost, K. J.; Acton, L. W.
Bibcode: 1980SciN..117..404F
Altcode:
No abstract at ADS
Title: State-of-the-art Space Telescope Digicon performance data
Authors: Ginaven, R. O.; Choisser, J. P.; Acton, L.; Wysoczanski,
W.; Alting-Mees, H. R.; Smith, R. D., II; Beaver, E. A.; Eck, H. J.;
Delamere, A.; Shannon, J. L.
Bibcode: 1980SPIE..217...55G
Altcode:
The Digicon has been chosen as the detector for the High Resolution
Spectrograph and the Faint Object Spectrograph of the Space
Telescope. Both tubes are 512 channel, parallel-output devices and
feature CsTe photocathodes on MgF2 faceplates. Using a computer-assisted
test facility, the tubes have been characterized with respect to diode
array performance, photocathode response (1100-9000 A), and imaging
capability. Data are presented on diode dark current and capacitance
distributions, pulse height resolution, photocathode quantum efficiency,
uniformity and blemishes, dark count rate, distortion, resolution,
and crosstalk.
Title: Comparison of Wolter I and Kirkpatrick-Baez X-ray optics for
a Spacelab LAMAR facility.
Authors: Catura, R. C.; Brown, W. A.; Acton, L. W.
Bibcode: 1980OptEn..19..602C
Altcode:
An x-ray astronomical observatory called the LAMAR, utilizing multiple
grazing incidence x-ray telescopes for high sensitivity observations,
is being considered by NASA for a Spacelab facility. A LAMAR utilizing
Wolter Type I x-ray optics figured by diamond turning is described
and its performance compared with a similar facility involving x-ray
optics of Kirkpatrick-Baez design. Effective areas, imaging properties,
and relative sensitivities of these two LAMAR facilities have been
calculated with the aid of computer ray tracing codes. We conclude that
the two optical designs provide comparable effective areas. Therefore,
the ability to achieve the highest possible angular resolution within
cost constraints will be decisive in the choice of x-ray optics for
the LAMAR.
Title: High Resolution Lyman Alpha Filtergrams of the Sun
Authors: Bonnet, R. M.; Decaudin, M.; Bruner, E. C.; Acton, L. W.;
Brown, W. A.
Bibcode: 1979BAAS...11..640B
Altcode:
No abstract at ADS
Title: Reduced Solar X-ray Data from OSO-8
Authors: Mosher, J. M.; Acton, L. W.; Wolfson, C. J.; Smith, K. L.
Bibcode: 1979BAAS...11..710M
Altcode:
No abstract at ADS
Title: Analysis of X-ray and EUV spectra of solar active regions
Authors: Strong, K. T.; Acton, L. W.
Bibcode: 1979lock.reptQ....S
Altcode:
Data acquired by two flights of an array of six Bragg crystal
spectrometers on an Aerobee rocket to obtain high spatial and spectral
resolution observations of various coronal features at soft X-ray
wavelengths (9-23A) were analyzed. The various aspects of the analysis
of the X-ray data are described. These observations were coordinated
with observations from the experiments on the Apollo Telescope Mount
and the various data sets were related to one another. The Appendices
contain the published results, abstracts of papers, computer code
descriptions and preprints of papers, all produced as a result of this
research project.
Title: Rocket studies of solar corona and transition region
Authors: Acton, L. W.; Bruner, E. C., Jr.; Brown, W. A.; Nobles, R. A.
Bibcode: 1979lock.reptQ....A
Altcode:
The XSST (X-Ray Spectrometer/Spectrograph Telescope) rocket payload
launched by a Nike Boosted Black Brant was designed to provide
high spectral resolution coronal soft X-ray line information on
a spectrographic plate, as well as time resolved photo-electric
records of pre-selected lines and spectral regions. This spectral
data is obtained from a 1 x 10 arc second solar region defined by
the paraboloidal telescope of the XSST. The transition region camera
provided full disc images in selected spectral intervals originating
in lower temperature zones than the emitting regions accessible to the
XSST. A H-alpha camera system allowed referencing the measurements
to the chromospheric temperatures and altitudes. Payload flight and
recovery information is provided along with X-ray photoelectric and UV
flight data, transition camera results and a summary of the anomalies
encountered. Instrument mechanical stability and spectrometer pointing
direction are also examined.
Title: An investigation of the 1.9 Å feature in solar-flare X-ray
spectra.
Authors: Parkinson, J. H.; Veck, N. J.; Ashfield, M. E. C.; Culhane,
J. L.; Ku, W. H. -M.; Lemen, J. R.; Novic, R.; Acton, L. W.; Wolfson,
C. J.
Bibcode: 1979ApJ...231..551P
Altcode:
The 1.9 A feature, observed in the X-ray spectra of three solar flares
with the Columbia University and Lockheed Palo Alto Research Laboratory
spectrometers on the OSO 8 satellite, is shown to be due to a blend of
1s-2p transitions in a range of Fe ions. In the temperature range 9-16
x -10 to the 6th K, the feature has a mean wavelength of 1.900 + or -
0.009 A and is 0.04 A wider than a single line, indicating that the
main contributors are Fe XIX-Fe XXII. Most of the emission originates
from the dielectronic recombination process, and when inner-shell
excitation is included together with normal collisional excitation, the
observed intensity of the feature can be accounted for adequately. For
these events, if the electron density is below approximately 10 to the
12th/cu cm, deviations from ionization equilibrium will be significant
for ions more highly ionized than Fe XXI.
Title: Comparison of Wolter-I and Kirkpatrick-Baez X-ray optics for
a Spacelab LAMAR facility
Authors: Catura, R. C.; Brown, W. A.; Acton, L. W.
Bibcode: 1979SPIE..184...73C
Altcode:
An X-ray astronomical observatory called the LAMAR, utilizing multiple
grazing incidence X-ray telescopes for high sensitivity observations,
is being considered by NASA for a Spacelab facility. A LAMAR utilizing
Wolter Type I X-ray optics figured by diamond turning is described
and its performance compared with a similar facility involving X-ray
optics of Kirkpatrick-Baez design. Effective areas, imaging properties
and relative sensitivities of these two LAMAR facilities have been
calculated with the aid of computer ray tracing codes. We conclude that
the two optical designs provide comparable effective areas. Therefore,
the ability to achieve the highest possible angular resolution within
cost constraints will be decisive in the choice of X-ray optics for
the LAMAR.
Title: Discussion
Authors: Acton, L. W.; Engvold, O.; Heasley, J. N.; Heyvaerts, J.;
Hirayama, T.; Kundu, M. R.; Leroy, J. L.; Malville, J.; Rust, D. M.;
Zirin, H.
Bibcode: 1979phsp.coll...31A
Altcode: 1979IAUCo..44...31A
No abstract at ADS
Title: Discussion
Authors: Acton, L. W.; Engvold, O.; Milkey, R. W.; Orrall, F. Q.;
Zirin, H.
Bibcode: 1979phsp.coll..354A
Altcode: 1979IAUCo..44..354A
No abstract at ADS
Title: Cosmic X-ray telescope for ARIES rocket observations
Authors: Catura, R. C.; Acton, L. W.; Berthelsdorf, R.; Culhane,
J. L.; Sanford, P. W.; Franks, A.
Bibcode: 1979SPIE..184...23C
Altcode:
A rocket-borne Wolter Type I X-ray telescope having a focal length
of 2.3m, an entrance aperture of 66cm and a geometrical area of
380cm2 is nearing completion. The telescope mirrors are formed by
diamond turning their figures into forged aluminum substrates of
5083 alloy. These diamond-turned substrates are subsequently plated
with a thin coating of electroless nickel and polished to obtain the
final X-ray reflecting surfaces. Details of the rocket payload, the
X-ray telescope, its calculated response and the experience gained
in selecting the mirror substrate alloy are discussed and the current
status of the telescope is reviewed.
Title: State-of-the-art Space Telescope digicon performance data.
Authors: Ginaven, R. O.; Choisser, J. P.; Acton, L. W.; Wysoczanski,
W.; Alting-Mees, H. R.; Smith, R. D., II; Beaver, E. A.; Eck, H. J.;
Delamere, W. A.; Shannon, J. L.
Bibcode: 1979SPIE..197...55G
Altcode:
No abstract at ADS
Title: Discussion
Authors: Acton, L. W.; Gaizauskas, V.
Bibcode: 1979phsp.coll..274A
Altcode: 1979IAUCo..44..274A
No abstract at ADS
Title: Discussion
Authors: Acton, L. W.; Brueckner, J.; Heyvaerts, J.; Maltby, P.;
Spicer, D. S.
Bibcode: 1979phsp.coll..314A
Altcode: 1979IAUCo..44..314A
No abstract at ADS
Title: Paraboloidal X-ray telescope mirror for solar coronal
spectroscopy
Authors: Brown, W. A.; Bruner, E. C., Jr.; Acton, L. W.; Franks, A.;
Stedman, M.; Speer, R. J.
Bibcode: 1979SPIE..184..278B
Altcode:
The telescope mirror for the X-ray Spectrograph Spectrometer Telescope
System is a sixty degree sector of an extreme off-axis paraboloid
of revolution. It was designed to focus a coronal region 1 by 10
arc seconds in size on the entrance slit of the spectrometer after
reflection from the gold surface. This paper discusses the design,
manufacture, and metrology of the mirror, the methods of precision
mechanical metrology used to focus the system, and the mounting system
which locates the mirror and has proven itself through vibration
tests. In addition, the results of reflection efficiency measurements,
alignment tolerances, and ray trace analysis of the effects of
misalignment are considered.
Title: Testing of Space Telescope 512-channel Digicon
Authors: Ginaven, R. O.; Choisser, J. P.; Acton, L.; Wysoczanski,
W.; Hartung, W.; Beaver, E. A.
Bibcode: 1979SPIE..203...36G
Altcode:
A test facility has been designed and built for testing Space Telescope
Digicons consisting of 512 parallel photon-counting channels. The
facility is designed to measure the critical parameters of Digicons
based on the expected performance of the detector: capacitance ranging
from one to ten picofarads, single photoelectron pulses consisting of
7,000 electrons each, image distortions in micrometers, leakage currents
in picoamperes, and background count rates of one photoelectron per
hundred seconds per diode channel. The test flow chart is presented
and results from several tests are briefly discussed.
Title: X-Ray Emission Associated with Filament Activity.
Authors: Acton, L. W.; Mosher, J. M.
Bibcode: 1979phsp.coll..269A
Altcode: 1979phsp.conf..269A; 1979IAUCo..44..269A
The temporal and spatial relationships of activated filaments, soft
X-ray production, and H-alpha flares are investigated. The events
chosen for study are from a sample of 114 hours of the very best,
high-resolution H-alpha records of active regions gleaned from about 150
days of observations of active regions by Big Bear Solar Observatory,
all with simultaneous mapping X-ray heliometer coverage. In the final
data set, 20 flares are chosen for detailed study (10 significant
flares and 10 smaller flarelike events). The data are examined for
preflare filament activation and preflare X-ray enhancements with a
view to discover a link between the two phenomena. To the limits set
by the data, no convincing evidence for consistent preflare effects
in H-alpha or X-rays is found. Aside from the 'disparition brusque',
it seems clear that filament activity is not directly responsible for
any dramatic effects in X-rays, and even there, the enhancement comes
after the filament has disappeared.
Title: Discussion
Authors: Acton, L. W.; Engvold, O.; Landman, D. A.
Bibcode: 1979phsp.coll...47A
Altcode: 1979IAUCo..44...47A
No abstract at ADS
Title: X-ray spectrometer spectrograph telescope system
Authors: Bruner, E. C., Jr.; Acton, L. W.; Brown, W. A.; Salat, S. W.;
Franks, A.; Schmidtke, G.; Schweizer, W.; Speer, R. J.
Bibcode: 1979SPIE..184..270B
Altcode:
A new sounding rocket payload that has been developed for X-ray
spectroscopic studies of the solar corona is described. The instrument
incorporates a grazing incidence Rowland mounted grating spectrograph
and an extreme off-axis paraboloic sector feed system to isolate
regions of the sun of order 1 x 10 arc seconds in size. The focal
surface of the spectrograph is shared by photographic and photoelectric
detection systems, with the latter serving as a part of the rocket
pointing system control loop. Fabrication and alignment of the optical
system is based on high precision machining and mechanical metrology
techniques. The spectrograph has a resolution of 16 milliangstroms and
modifications planned for future flights will improve the resolution
to 5 milliangstroms, permitting line widths to be measured.
Title: Discussion
Authors: Acton, L. W.; Heasley, J. N.; Sahal-Bréchot, S.; Stenflo, J.
Bibcode: 1979phsp.coll...91A
Altcode: 1979IAUCo..44...91A
No abstract at ADS
Title: Discussion
Authors: Acton, L. W.; Anzer, U.; Engvold, O.; Martin, S. F.; Pneuman,
G. W.; Rust, D. M.; Tandberg-Hanssen, E.; Zirin, H.
Bibcode: 1979phsp.coll..164A
Altcode: 1979IAUCo..44..164A
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.
Bibcode: 1979phsp.coll..225A
Altcode: 1979IAUCo..44..225A
No abstract at ADS
Title: Temperature and ionization balance dependence of O VII
line ratios.
Authors: Acton, L. W.; Brown, W. A.
Bibcode: 1978ApJ...225.1065A
Altcode:
The forbidden-plus-intersystem to resonance line ratio (G)
for the heliumlike ion O VII is calculated, taking into account
cascades, blended satellite lines, and radiative plus dielectric
recombination. It is noted that G is of particular use for investigating
radiative-transfer effects and nonequilibrium ionization in the solar
corona and that the calculations are applicable to a low-density
optically thin Maxwellian plasma. The temperature dependence of G
is considered for the case of a steady-state equilibrium plasma,
and the effect of departures from ionization equilibrium on G is
examined. It is found that G is quite insensitive to temperature
over the range from 600,000 to 6 million K for a steady-state plasma,
but that recombinations may be suppressed or dominant, depending on
the relative abundance of O VIII, for a plasma in which the state of
ionization is not in equilibrium with the electron temperature. This
latter effect is shown to be capable of causing large variations in
G that are dependent on electron temperature.
Title: Radiative transfer of X-rays in the solar corona.
Authors: Acton, L. W.
Bibcode: 1978ApJ...225.1069A
Altcode:
The problem of resonance scattering of X-ray emission lines in the solar
corona is investigated. For the resonance lines of some helium-like
ions, significant optical depths are reached over distances small
compared with the size of typical coronal features. A general integral
equation for the transfer of resonance-line radiation under solar
coronal conditions is derived. This expression is in a form useful
for modeling the complex three-dimensional temperature and density
structure of coronal active regions. The transfer equation is then cast
in a form illustrating the terms which give rise to the attenuation
or enhancement of the resonance-line intensity. The source function
for helium-like oxygen (O VII) under coronal conditions is computed
and discussed in terms of the relative importance of scattering.
Title: Does the emission measure decrease during the start of a soft
X-ray flare?
Authors: Wolfson, C. J.; Acton, L. W.; Datlowe, D. W.
Bibcode: 1978SoPh...59..373W
Altcode:
Soft X-ray flare observations, interpreted as the emission from a
single temperature plasma, frequently indicate a significant decrease
in the inferred emission measure. It is shown that this effect results
naturally from the isothermal assumption, and is eliminated when the
preflare contribution to the total emission is removed.
Title: Soft X-Ray Observations during the Preflare Phase of the
Solar Flare Phenomenon.
Authors: Wolfson, C. J.; Acton, L. W.; Leibacher, J. W.
Bibcode: 1978BAAS...10..456W
Altcode:
No abstract at ADS
Title: Solar X-ray spectroscopy with bent crystal spectrometer (BCS).
Authors: Rapley, C. G.; Culhane, J. L.; Acton, L. W.; Catura, R. C.;
Joki, E. C.; Bakke, J. C.
Bibcode: 1978nisa.symp..121R
Altcode: 1978nisa.conf..121R
No abstract at ADS
Title: Iron-line X-ray emission from solar plasma: comments on
ionization equilibrium and line excitation.
Authors: Acton, L. W.; Catura, R. C.; Roethig, D. T.
Bibcode: 1977ApJ...218..881A
Altcode:
X-ray spectra of two small solar flares in the l 5 keV region
have been obtained with a high- sensitivity proportional counter
spectrometer. These data have been analyzed to obtain the temperature,
emission measure, and 1.9 A (6.6 keV) iron-line flux for 20 time
intervals during the declining phase of the flares. We observe
the iron-line flux to be systematically stronger than predicted by
the theory most commonly applied in such analyses. The discrepancy
increases toward lower temperatures to a factor of 100 at 9 x 106
K. When the recent ionization equilibrium calculations of Jacobs et
al. for iron are used in the analysis, the discrepancy is reduced by
a factor of 10. We suggest that inclusion of inner-shell excitation
of iron in stages of ionization below Fe xxiv may account for the
remaining excess of low-temperature iron-line flux. If this hypothesis
is correct, however, then the ability to discriminate between ionization
equilibrium calculations with these low-spectral-resolution data is
nullified. Subject headings: plasmas - Sun: flares - X-rays: spectra
Title: Early evolution of an X-ray emitting solar active region.
Authors: Wolfson, C. J.; Acton, L. W.; Leibacher, J. W.; Roethig, D. T.
Bibcode: 1977SoPh...55..181W
Altcode:
The birth and early evolution of a solar active region has been
investigated using X-ray observations from the Lockheed Mapping X-Ray
Heliometer on board the OSO-8 spacecraft. X-ray emission is observed
within three hours of the first detection of Hα plage. At that time, a
plasma temperature of 4 × 106 K in a region having a density
of the order of 1010 cm−3 is inferred. During
the fifty hours following birth almost continuous flares or flare-like
X-ray bursts are superimposed on a monotonically increasing base level
of X-ray emission produced by plasma with a temperature of the order 3
× 106 K. If we assume that the X-rays result from heating
due to dissipation of current systems or magnetic field reconnection,
we conclude that flare-like X-ray emission soon after active region
birth implies that the magnetic field probably emerges in a stressed
or complex configuration.
Title: Bent crystal spectrometer for solar x-ray spectroscopy
Authors: Rapley, C. G.; Culhane, J. L.; Acton, L. W.; Catura, R. C.;
Joki, E. G.; Bakke, J. C.
Bibcode: 1977RScI...48.1123R
Altcode:
A bent crystal Bragg spectrometer is described which provides continuous
and simultaneous coverage for all x-ray wavelengths within its spectral
range. This instrument provides high spectral resolution but involves no
precision crystal rocking or position encoding mechanisms and therefore
may be highly ruggedized. Principles of operation are discussed and
its application to solar x-ray spectroscopy is described.
Title: Evolution of the X-ray emitting corona preceding and after
major solar events
Authors: Wolfson, C. J.; Acton, L. W.; Roethig, D. T.; Walt, M.
Bibcode: 1977cosp.meetR....W
Altcode:
Soft X-ray emission from the sun during STIP Interval II, observed with
the Lockheed Mapping X-ray Heliometer on the NASA OSO-8 satellite,
is presented. In examining the emission versus time for extended
intervals around the times of the Class 1B flare on March 28, 1976,
and the Class 1B flare on April 30, 1976, we find significantly more
low level flare activity prior to the major flares than after. Twelve
modest X-ray bursts are investigated and no compelling case of a
preflare brightening phase is observed. Preliminary correlations with
the time history of emitted solar particles are discussed.
Title: Solar X-ray spectroscopy with a Bent Crystal Spectrometer /BCS/
Authors: Rapley, C. G.; Culhane, J. L.; Acton, L. W.; Catura, R. C.;
Joki, E. C.; Bakke, J. C.
Bibcode: 1977cosp.meetR....R
Altcode:
A high-resolution Bent Crystal Spectrometer (BCS) which provides
simultaneous and continuous time coverage of a range of wavelengths is
described; its application to spectral studies of solar X-ray emission
is discussed. Design of the BCS and its accompanying position-sensitive
detector is considered. Calibrations of the crystal curvature, the
angle between a crystal reference plane and the collimator axis, and
the position response of the position-sensitive proportional counter
in determining absolute wavelengths are also mentioned. An active
solar region spectrum obtained during an airborne BCS test is compared
to wavelength data derived from theory or previous experiments, and
excellent agreement is found. Specifications for a BCS system to be
used in the Solar Maximum Mission satellite of 1979 are given.
Title: X-Ray Observations of the Virgo X-Ray Source.
Authors: Catura, R. C.; Acton, L. W.
Bibcode: 1976BAAS....8..554C
Altcode:
No abstract at ADS
Title: Resonance Scattering of X-rays in the Solar Corona.
Authors: Acton, L. W.; Brown, W. A.; Catura, R. C.; Strong, K. T.
Bibcode: 1976BAAS....8..556A
Altcode:
No abstract at ADS
Title: Instrumentation for solar and cosmic X-ray spectroscopy.
Authors: Acton, L. W.; Catura, R. C.
Bibcode: 1976SSI.....2..445A
Altcode:
Five flight instruments for high resolution X-ray spectroscopy are
described. They include one- and two-dimensionally collimated solar
rocket X-ray spectrometers, a 1-23 A X-ray polychromator for the Solar
Maximum Mission, a collimated crystal spectrometer for cosmic X-ray
studies, and a grating spectrometer/spectrograph with paraboloidal
telescope for 10 to 50 A solar X-ray research. The discussion includes
factors motivating design and use, examples of results for those
already flown, and potential for future studies.
Title: Design and Fabrication of Multigrid X-Ray Collimators
Authors: Acton, L. W.; Joki, E. G.; Salmon, R. J.
Bibcode: 1976SSI.....2..159A
Altcode:
The paper considers the design and fabrication of multigrid collimators
for use in X-ray astronomy. The most important collimator performance
criteria are minimum off-band transmission or leakage, and maximum
on-band transmission. Lockheed experience with multigrid collimator
is summarized: (1) an Oda type with one-dimensional collimation of
1.7 arc minute FWHM and using only the central transmission band,
(2) an Oda type with 2 arc minute one-dimensional collimation using
up to 27 transmission bands, each separated by 42 arc min. and (3)
a modified Parkinson type with two-dimensional collimation of 40 arc
sec. Attention is given to grid materials, precision, plating, hole
quality, and results of acceptance testing.
Title: Proceedings of the symposium on the techniques of solar and
cosmic X-ray spectroscopy. Held at Holmbury, England, 22 and 23
May 1975.
Authors: Acton, L. W.; Culhane, J. L.
Bibcode: 1976SSI.....2....1A
Altcode: 1976STIA...7647776A
Papers are presented on the spectroscopic analysis of solar and cosmic
X-ray spectra, the absolute calibration of the reflection integral
of Bragg X-ray analyzer crystals, the design and construction of
modulation collimators, and Si(Li) X-ray astronomy aboard the HEAO-B
satellite. Attention is also given to the OSO 8 instrument for stellar
and solar X-ray spectroscopy and polarimetry, a soft X-ray spectrometer
for diffuse sources, instrumentation for solar spectrophotometry at
extreme ultraviolet and soft X-ray wavelengths, the calibration of
the Ariel 5 Bragg spectrometer and a European X-ray spectroscopy and
polarimetry payload for Spacelab. Individual items are announced in
this issue.
Title: X-ray structure of the Puppis supernova remnant.
Authors: Catura, R. C.; Acton, L. W.
Bibcode: 1976ApJ...207L.163C
Altcode:
The angular distribution of X-ray emission from the Puppis supernova
remnant was investigated with a focusing X-ray collector during a
rocket observation. Extent of the X-ray emitting region along the
direction of scan is 42 (+ or - 4) arcmin and is well correlated with
the size of the Puppis A radioshell. The existence of a feature within
the remnant is confirmed whose X-ray emission is strongly localized,
and its spectrum is observed to be softer than the rest of the remnant.
Title: Spectroscopic Studies of the Solar Corona at X-Ray Wavelengths:
Discussion
Authors: Vaiana, G.; Acton, L. W.
Bibcode: 1976RSPTA.281..390V
Altcode:
No abstract at ADS
Title: Spectroscopic Studies of the Solar Corona at X-Ray Wavelengths
Authors: Acton, L. W.; Catura, R. C.
Bibcode: 1976RSPTA.281..383A
Altcode: 1976RSLPT.281..383A
The spatial distribution of the emission in several X-ray lines is
discussed with emphasis on temperature dependence and association with
active regions. New results are presented for the trio of helium-like
O VII lines which demonstrate (1) a spatial variation in the density
dependent forbidden to intersystem line ratio, and (2) a strong spatial
variation in the intensity of the O VII resonance line relative to
the optically forbidden transitions. The second effect appears to be
caused by resonance scattering by material in the line of sight.
Title: Intense 0.19 nm Line Emission From a Small Solar Flare
Authors: Acton, L. W.; Catura, R. C.
Bibcode: 1976BAAS....8..375A
Altcode:
No abstract at ADS
Title: The Early Evolution of X-Ray Producing Active Regions
Authors: Wolfson, C. J.; Acton, L. W.; Newkirk, L. L.; Roethig, D. T.;
Smith, K. L.
Bibcode: 1976BAAS....8..317W
Altcode:
No abstract at ADS
Title: Intense soft X-ray flux from Her X-1.
Authors: Catura, R. C.; Acton, L. W.
Bibcode: 1976NASSP.389..119C
Altcode: 1976NASSP.589..119C; 1976xrbi.nasa..119C
An intense flux of soft X-rays extending up to at least 1 keV has
been observed from Her X-1. If the soft X-ray intensity is corrected
for interstellar absorption the luminosity between 0.16 and 1 keV is
comparable to that from 2-10 keV. The soft X-rays are modulated with
the 1.24 sec period observed at higher energies but are approximately
180 deg out of phase with the high energy flux. These results extend
the detection of this flux to higher energy, a different binary phase,
and to a time 19 periods (of the 35 day cycle) later. These observations
suggest that this soft emission is a stable feature in the spectrum
of this source during its X-ray on state and that this emission is
local to Her X-1.
Title: Techniques of Solar and Cosmic X-ray Spectroscopy
Authors: Acton, L. W.; Culhane, J. L.
Bibcode: 1976SSI.....2.....A
Altcode:
No abstract at ADS
Title: Intense soft X-ray flux from Hercules X-1.
Authors: Catura, R. C.; Acton, L. W.
Bibcode: 1975ApJ...202L...5C
Altcode:
An intense flux of soft X-rays extending up to at least 1 keV has been
observed from Her X-1. If the soft X-ray intensity is corrected for
interstellar absorption, the luminosity in the 0.16-0.28 keV band is
comparable to that from 2 to 10 keV. This confirms the conclusion of
Shulman et al. (1975) but extends the detection of this flux to higher
energy, a different binary phase, and to a time 19 periods later in
the 35-day cycle. These observations suggest that this soft emission
may be a stable feature in the spectrum of Her X-1 during its X-ray
on state. It appears difficult to account for the high soft X-ray
luminosity by current simple models of the X-ray source.
Title: Analysis of Aerobee 17.012 data
Authors: Catura, R. C.; Acton, L. W.
Bibcode: 1975lock.reptT....C
Altcode:
The results are summarized of reduction and analysis of data acquired
during the flight of NASA-Aerobee 17.012CG. This rocket was launched
from White Sands Missile Range on 6 April 1974 at 0410 U.T. The primary
instrument in the experiment payload was a singly focussing x-ray
reflector system sensitive in the range from 0.18 to 3 keV. X-ray
detectors for this system consisted of two gas flow proportional
counters with fields of view defined by apertures of 0.1 deg and 0.3
deg at the reflector focus. A Bragg spectrometer utilizing KAP crystals
was included in the payload with the objective of detecting the Lyman
alpha line of 0 VIII at 18.97A in the Puppis supernova remanant. A
35 mm camera photographed the star field every 1.6 sec throughout the
flight to provide aspect information.
Title: INVITED - Solar Flares.
Authors: Acton, L. W.
Bibcode: 1975BAAS....7..410A
Altcode:
No abstract at ADS
Title: Intense Soft X-Ray Flux from Her X-1
Authors: Catura, R. C.; Acton, L. W.
Bibcode: 1975BAAS....7..430C
Altcode:
No abstract at ADS
Title: Oxygen to Neon Abundance Ratio in the Solar Corona
Authors: Acton, L. W.; Catura, R. C.; Joki, E. G.
Bibcode: 1975BAAS....7..356A
Altcode:
No abstract at ADS
Title: Evidence for X-ray emission from Capella.
Authors: Catura, R. C.; Acton, L. W.; Johnson, H. M.
Bibcode: 1975ApJ...196L..47C
Altcode:
X-ray emission in the range from 0.2 to 1.6 keV has been detected from
an area of the sky which contains the binary star system Capella. The
X-ray source is at most a few arc minutes in extent and shows no
spectral turnover at low energy, consistent with a nearby source. We
suggest Capella as the source of this emission and that this object
belongs to a new class of galactic X-ray sources with a luminosity of
10 to the 31st to 10 to the 34th ergs per sec. Emission from this class
of objects is variable, predominantly below 2 keV, and originates from
nearby stellar objects.
Title: Observation of soft X-rays from cosmic sources
Authors: Catura, R. C.; Acton, L. W.
Bibcode: 1975lock.reptQ....C
Altcode:
A binary X-ray source, an extended extragalactic X-ray source and
several nearby stars were surveyed for X-ray emission. The energy
spectrum and time structure of X-ray flux from the binary source, Her
X-l, was investigated in the range from 0.15 to 6 KeV. This source was
observed at a binary phase of 0.18 with the system near elongation
normal to the line of sight. Intense pulsations were observed in
optical emission lines near this binary phase. The spectrum and angular
distribution of X-ray emission from the X-ray source in the Virgo
Cluster of Galaxies, near M 87, was also observed. In addition, the
stars Alpha Leo, Zeta Her, and Epsilon Vir were investigated. Epsilon
Aur and Alpha Aur were also scanned. These stars were studied since
there is increasing evidence that such objects may be transient sources
of soft X-rays.
Title: Spectrum and Structure of X-ray Emission from Puppis A.
Authors: Catura, R. C.; Acton, L. W.
Bibcode: 1975BAAS....7R.246C
Altcode:
No abstract at ADS
Title: Spatially Resolved X-Ray Spectra of Coronal Active Regions
Authors: Catura, R. C.; Acton, L. W.; Joki, E. G.; Rapley, C. G.;
Culhane, J. L.
Bibcode: 1975IAUS...68...67C
Altcode:
No abstract at ADS
Title: Rocket observations of the Perseus X-ray source
Authors: Catura, R. C.; Acton, L. W.
Bibcode: 1975xris.conf..970C
Altcode:
The spectrum and angular distribution of soft X-ray emission from
the source in the Perseus cluster has been observed. A prominent
feature of this source is a point-like component at the position of
NGC 1275. Preliminary analysis indicates that the spectrum of this
source does not rise as steeply at energies below 1 keV as has been
reported previously.
Title: Solar X-ray spectroscopy with spatial resolution
Authors: Acton, L. W.; Catura, R. C.; Joki, E. G.
Bibcode: 1975xris.conf.1008A
Altcode:
Preliminary results from a solar X-ray spectroscopic experiment are
presented. The data emphasize the nonhomogeneous and multithermal nature
of the corona. The oxygen-to-neon abundance ratio in the corona is found
to be about 4.7, as derived from the O VIII-to-Ne IX resonance-line
photon flux ratio.
Title: Oxygen-to-neon abundance ratio in the solar corona.
Authors: Acton, L. W.; Catura, R. C.; Joki, E. G.
Bibcode: 1975ApJ...195L..93A
Altcode:
The oxygen-to-neon abundance ratio in the solar corona is determined
to be 4.7 + or - 1.5 on the basis of a relatively model-independent
analysis utilizing 25 separate measurements of the O VIII to Ne
IX resonance line ratio. The stated uncertainty includes random and
possible systematic experimental error in the data but does not include
possible error in the calculated flux ratios which have been fitted to
the observations. This abundance ratio is smaller than most published
results based upon EUV or X-ray observations but is in good agreement
with direct measurements of flare cosmic rays.
Title: Evidence for X-Ray Emission from Capella.
Authors: Catura, R. C.; Acton, L. W.; Johnson, H. M.
Bibcode: 1974BAAS....6..445C
Altcode:
No abstract at ADS
Title: The Extended X-Ray Source in Virgo and its Relation to M87
Authors: Catura, R. C.; Acton, L. W.; Johnson, H. M.; Zaumen, W. T.
Bibcode: 1974ApJ...190..521C
Altcode:
Spectral data in the 0.2-1.6 keV range are presented for the extended
X-ray source in the Virgo cluster. When combined with Uharu data,
the simplest function which fits the composite spectrum from 0.2 to 10
keV is one describing bremsstrahlung from an isothermal plasma at 36
>c 106 K. Production of the X-rays by inverse Compton scattering
is also considered and models relating the X-ray source to M87 are
discussed. Subject headings: galaxies, individual - X-ray sources
Title: Cooling of Solar Flare Plasmas
Authors: Zaumen, W. T.; Acton, L. W.
Bibcode: 1974SoPh...36..139Z
Altcode:
A simple model for the cooling of solar flare plasmas is
considered. This model predicts that an increase in emission measure
with decreasing temperature is a general feature of a cooling flare. The
results are compared to solar flare data.
Title: Preliminary Results of X-Ray Observations from an ATM Support
Rocket
Authors: Catura, R. C.; Acton, L. W.; Zaumen, W. T.
Bibcode: 1974BAAS....6S.285C
Altcode:
No abstract at ADS
Title: The Lockheed OSO-I Experiment: Instrument Capabilities and
Observing Plans
Authors: Wolfson, C. J.; Acton, L. W.; Catura, R. C.
Bibcode: 1974BAAS....6..298W
Altcode:
No abstract at ADS
Title: The solar X-ray spectrum.
Authors: Culhane, J. L.; Acton, L. W.
Bibcode: 1974ARA&A..12..359C
Altcode:
The characteristics of X-ray emission from the normal corona are
considered, giving attention to questions of spatial and spectral
distribution. Aspects of X-ray emission from solar flares are
also investigated. Details of X-ray spectroscopy for coronal plasma
diagnosis are discussed, taking into account techniques, the diagnosis
of equilibrium plasmas, and the diagnosis of nonthermal events. The
study of continuum emission and line emission is reported along with
investigations of departures from equilibrium.
Title: A Conductive Cooling Model for a Confined Solar Flare Plasma
(presented by L.W. Acton)
Authors: Acton, L. W.; Zaumen, W. T.
Bibcode: 1974IAUS...57..479A
Altcode:
No abstract at ADS
Title: X-Ray Spectra of Multi-Temperature Plasmas
Authors: Zaumen, W. T.; Acton, L. W.; Catura, R. C.
Bibcode: 1973BAAS....5Q.282Z
Altcode:
No abstract at ADS
Title: Spectrum of the X-Ray Source at M 87.
Authors: Catura, R. C.; Acton, L. W.; Johnson, H. M.; Zaumen, W. T.;
Fisher, P. C.
Bibcode: 1973BAAS....5...33C
Altcode:
No abstract at ADS
Title: X-ray study of solar plage regions and a small flare.
Authors: Acton, L. W.; Catura, R. C.; Culhane, J. L.
Bibcode: 1973spre.conf..781A
Altcode: 1973spre....2..781A
No abstract at ADS
Title: Coronal survey in X-rays of O vii and Ne ix
Authors: Acton, L. W.; Catura, R. C.; Meyerott, A. J.; Wolfson, C. J.;
Culhane, J. L.
Bibcode: 1972SoPh...26..183A
Altcode:
We report some results of a rocket experiment flown on 29 April,
1971. A survey of the solar corona was carried out with a pair of
collimated Bragg spectrometers to study the resonance, intersystem and
forbidden line emission from the helium-like ions O VII (22 Å) and
Ne IX (13 Å). In the direction of dispersion the collimator provided
a field of view of 1.7'. Also, the continuum radiation near 3 Å was
monitored by a collimated proportional counter within a view angle
of 4.2'. The observed X-ray emission came from the general corona,
seven plage regions, and one dynamic feature- the late stage of a small
flare. From the intensity of the O VII and Ne IX resonance lines the
electron temperature and emission measure of the individual emitting
regions are derived on the basis of two models, one (a) in which the
region is assumed to be isothermal and another (b) in which the emission
measure decreases exponentially with increasing temperature. The
latter model, which is the most adequate of the two, yields for the
electron temperature of the time-varying feature 2-3 × 106
K, for the other active regions 1.5-2.5 × 106 K, and for
the general corona 1.3-1.7 × 106 K. The Ne IX emitting
regions are about 1.5 times as hot as the O VII regions. The emission
measure ranges from 0.4-2.3 × 1048 cm−3 for
all active regions and is about 2 × 1049 cm−3
for one hemisphere of the general corona above 106 K. From
an analysis of the ratio, R, of the forbidden and intersystem lines
of O VII we conclude that none of the regions producing these lines
at the time of the rocket flight had electron densities exceeding
about 3 × 109 cm−3. Our data demonstrate
a dependence of R upon temperature in agreement with the theory of
Blumenthal et al. (1971). The wavelengths for the intersystem, the
1s22s2Se−1s2p2s2P0
satellite, and the forbidden transition show in the case of Ne IX
improved agreement with predictions. The observed strength of the
satellite lines for both O VII and Ne IX agrees with the predictions
of Gabriel's (1972) theory, which attributes their formation to
dielectronic recombination.
Title: Mapping the Solar Corona in X-Ray Lines of O VII and NE IX
Authors: Catura, R. C.; Acton, L. W.; Meyerott, A. J.; Culhane, J. L.
Bibcode: 1972SSRv...13..742C
Altcode: 1972IAUCo..14..742C
No abstract at ADS
Title: Analysis of X-Ray Line Emission from Individual Solar Active
Regions
Authors: Catura, R. C.; Acton, L. W.; Wolfson, C. J.; Culhane, J. L.
Bibcode: 1972BAAS....4R.379C
Altcode:
No abstract at ADS
Title: Helium-like Line Emission from Coronal Features
Authors: Acton, L. W.; Catura, R. C.; Meyerott, A. J.; Culhane, J. L.
Bibcode: 1971NPhS..233...75A
Altcode: 1971Natur.233...75A
SINCE Gabriel and Jordan1 proposed the theory
relating the relative intensity of the inter-system
(3P-1S0) and forbidden
(3S1 - 1S0) lines of
helium-like ions to the electron density of the emitting volume there
has been growing interest in using this type of information to determine
electron densities in the solar corona2-5. We present here
the preliminary results of a rocket experiment designed to study the
OVII and NeIX line emission from discrete coronal features. These
results indicate that electron densities in normal, non-flare, coronal
features are below the low density limits (6 × 109 and 1
× 1011 cm-3 respectively) given by Freeman et
al.5 for these particular ions.
Title: Rocket Prototype of an X-Ray Optical System for Surveying
and Locating Cosmic X-Ray Sources
Authors: Fisher, P. C.; Acton, L. W.; Catura, R. C.; Kirkpatrick,
P.; Meyerott, A. J.; Roethig, D. T.
Bibcode: 1971IAUS...41..182F
Altcode:
No abstract at ADS
Title: Simultaneous Rocket and Satellite Observations of the Solar
X-Ray Spectrum in the 3-15-keV Range
Authors: Culhane, J. L.; Acton, L. W.; Catura, R. C.
Bibcode: 1970BAAS....2Q.307C
Altcode:
No abstract at ADS
Title: X-Ray Line Emission from Scorpius X-1
Authors: Acton, L. W.; Catura, R. C.; Culhane, J. L.; Fisher, P. C.
Bibcode: 1970ApJ...161L.175A
Altcode:
Observations of the Sco X-1 source with a large-area proportional
counter suggest the presence of iron-line emission in the spectrum of
the source at an energy of 6.6 + 0.2 keV. The counts due to the line
are in excess of the continuum and background counts by 6 standard
deviations. A comparison of the observed line-to-continuum ratio with
calculations yields a value consistent with a normal cosmic abundance
of iron in the source.
Title: Localization of Solar X-ray Emission at Energies above 3 keV
Authors: Catura, R. C.; Acton, L. W.; Fisher, P. C.
Bibcode: 1970Natur.227...55C
Altcode:
SOLAR, X-ray emission becomes increasingly localized to active regions
as the photon energy increases. Photographs of the Sun at X-ray
energies less than 0.5 keV show emission from the entire corona, while
those sensitive in the 1-3 keV interval show the X-ray emission to be
concentrated in condensed regions1,2 which are correlated
with features apparent at both optical and radio wavelengths. If this
trend were to continue as expected, X-rays at energies above 3 keV
would be emitted exclusively from active regions with little or no
contribution from the general corona.
Title: X-Ray Photometry of M1
Authors: Acton, L. W.; Catura, R. C.; Fisher, P. C.; Roethig, D. T.
Bibcode: 1970PASP...82..653A
Altcode:
No abstract at ADS
Title: A simplified thermal continuum functionfor the X-ray emission
from coronal plasmas
Authors: Culhane, J. L.; Acton, L. W.
Bibcode: 1970MNRAS.151..141C
Altcode:
At temperatures below x io6 0K, the free-bound process makes an
increasingly significant contribution to the total continuum flux. While
the free-free spectrum may be represented by a relatively simple
expression, a detailed calculation of the spectrum requires a lengthy
summation process over all the ionization stages and levels of the
abundant coronal ions. A simple empirical expression is presented which
gives the magnitude and spectral slope of the free-bound contribution
over the energy interval - for the temperature range 4 2ooxIo60K. This
expression gives results that agree to better than 15 per cent with
more detailed calculations of the thermal continuous spectrum.
Title: Lockheed Solar Observatory, Saugus, California. Report
1968-1969.
Authors: Acton, L. W.
Bibcode: 1970BAAS....2...72A
Altcode:
No abstract at ADS
Title: Spectrum of the Crab x-Ray Source from 4 to 40 ke V
Authors: Acton, Loren W.; Catura, Richard C.; Fisher, Philip C.
Bibcode: 1969BAAS....1R.231A
Altcode:
No abstract at ADS
Title: Lockheed Solar Observatory
Authors: Acton, Loren W.
Bibcode: 1969SoPh....6..485A
Altcode:
No abstract at ADS
Title: Lockheed Solar Observatory, Saugus, California. Report
1967-1968.
Authors: Acton, L. W.
Bibcode: 1969BAAS....1...56A
Altcode:
No abstract at ADS
Title: X-Ray and Microwave Emission of the Sun with Special Reference
to the Events of July, 1961
Authors: Acton, Loren W.
Bibcode: 1968ApJ...152..305A
Altcode:
X-ray (X < 14 A), microwave (X = 10 7 cm), optical, and ionospheric
observations are utilized to- gether with theoretical X-ray spectra to
derive physical models of some X-ray emitting volumes on the Sun. The
following kinds of emitting regions are studied: (1) a large "permanent"
coronal condensation; (2) a flare in its flash phase; (3) a yellow-line
condensation associated with a post-flare loop-prominence system. Most
of the observational data were gathered during the month of July,
1961. This period presented the opportunity to study the X-ray emission
of the Sun over a wide range of solar activity and at the same time
unambiguously to identify the sources of the emission. Both thermal
and non-thermal emission processes seem to be required to account for
the X-ray emis- sion from flares. This study indicates that it is not
necessary to invoke temperatures higher than 4-5 X 106 O K to account
for the thermal radiation of flares. The prolonged enhancements of soft
X-rays which follow some large flares appear to be thermal emission from
the same "sporadic" coronal condensations which emit the yellow-line
of Ca xv. In the case of the yellow-line condensation studied here,
the observa- tions are best fitted if 1 per cent of the volume contains
90 per cent of the material
Title: Observations of Energetic X-Rays from Quiescent Solar Active
Regions
Authors: Acton, L. W.; Fisher, Philip C.
Bibcode: 1968IAUS...35..432A
Altcode:
No abstract at ADS
Title: Observations of Galactic X-Ray Sources
Authors: Fisher, Philip C.; Jordan, Willard C.; Meyerott, Arthur J.;
Acton, Loren W.; Roethig, Douglas T.
Bibcode: 1968ApJ...151....1F
Altcode:
The final results of a 1964-1965 survey for galactic X-ray sources
in the interval 330° <1" < 16O~ are summarized here. Attention
was restricted to relatively low galactic latitudes. Fifteen discrete
sources have been located to an accuracy of about ± 1~, and another
eight sources have been tentatively identified and located. Seven
of the twenty-three sources had a 4-8-keY X-ray luminosity which
varied significantly over the 1-year span of the measurements. The
intensities of at least eight of the brighter sources were not
found to have varied appreciably with time Seven of the brighter and
relatively time-invariant sources at the lowest galactic latitude
were found to lie within the interval 358° <i~1 < 17°. The
observations have been used to prepare a rudimentary color-magnitude
diagram; color being defined as the response of a specified detector
to 5-10-keY photons divided by that detector's response to 2 8-5.0-keY
photons. Evaluation of the color-magnitude diagram's information and the
two galactic position coordinates of the X-ray sources indicates that
seven relatively time-invariant sources having 358° <111 < 17°
may all have had an effective temperature of `~-5 X ~° K. This is the
reported effective temperature of Sco X-1 and Cyg XR-2, which have been
found by other workers to have a nova-like character. It appears that a
single physical phenomenon is occurring in these particular sources, and
that this phe- nomenon has a nova-like character at optical wavelengths
and can on occasion he found in Population I objects. I. INTRODuCTIO
Title: Spectrographic Observations of the Flare of July 20, 1961
Authors: Zirin, Harold; Acton, Loren W.
Bibcode: 1967ApJ...148..501Z
Altcode:
Spectroscopic observations of the late stages of a limb flare which
occurred on July 20,1961, are presented. Because these observations
were made with an achromatic coronagraph, accurate line intensity
ratios are possible. The He I and He ii line ratios indicate a
recombination-cascade spectrum. Measurements of cbntinuum densities
give values around 1010 in the post-flare loop region and 1011 or more
in the flare. Ca xv line widths correspond to 4.3 X 100 K. A magnetic
loop structure which guides successive surges is studied.
Title: X-Ray Spectra of Several Cosmic Sources
Authors: Fisher, Philip C.; Jordan, Willard C.; Meyerott, Arthur J.;
Acton, Loren W.; Roethig, Douglas T.
Bibcode: 1967ApJ...147.1209F
Altcode:
No abstract at ADS
Title: The Interesting Loop Prominence of July 20, 1961
Authors: Acton, L. W.
Bibcode: 1966PASP...78R.438A
Altcode:
No abstract at ADS
Title: Resolution of X-Ray Sources at Low Galactic Longitude
Authors: Fisher, Philip C.; Jordan, Willard C.; Meyerott, Arthur J.;
Acton, Loren W.; Roethig, Douglas T.
Bibcode: 1966Natur.211..920F
Altcode:
No abstract at ADS
Title: Observations of Cosmic X-Rays
Authors: Fisher, Philip C.; Johnson, Hugh M.; Jordan, Willard C.;
Meyerott, Arthur J.; Acton, Loren W.
Bibcode: 1966ApJ...143..203F
Altcode:
Results are presented for eight X-ray sources observed on an October
1,1964, rocket flight. The most detailed information is for the
brightest source in Scorpius which has been located at a = 16"l 4"'
+ 1"' a = - 15"36' l 15'. Although seven of the eight sources lie
inside the longitude interval 344" in ½ 16", no measurable quantity
of 4-keV X-rays is associated with the positions of Kepler's supernova
or the galactic center. The spectrum of the brightest Scorpius source
is peaked in the 4- range. For all sources, more flux is observed in
the 4-8-keV interval than in the 8-12-keV interval. The results are
compared with measurements of others.
Title: Contribution of Characteristic X-rays to the Radiation of
Solar Flares
Authors: Acton, Loren W.
Bibcode: 1965Natur.207..737A
Altcode:
ATOMS bombarded by energetic electrons may experience ionization through
the loss of a K-shell electron. Part of the subsequent rearrangement
of the ion to its ground-state involves the movement of an outer
(usually an L- or M-shell) electron to fill the K-shell vacancy. The
energy released in this transition appears in one of two forms. Either
a photon is emitted (characteristic radiation) or one or more outer
electrons are expelled from the ion (the Auger process). The purpose
of this communication is to point out thatcharacteristic X-rays
emitted as a result of such inner-electron transitions may make an
important contribution to the X-ray emission of solar flares. It is
not yet known with certainty whether the radiation from flares in
the λλ 1-3 Å region is predominantly of thermal1,2 or
non-thermal3 origin. The detection of the characteristic
radiation of iron atoms in the X-radiation from flares would indicate
that non-thermal processes are important in the production of X-rays
in this wavelength range.
Title: Solar X-Ray Observations by Injun 1
Authors: van Allen, J. A.; Frank, L. A.; Maehlum, B.; Acton, L. W.
Bibcode: 1965JGR....70.1639V
Altcode:
On the basis of 48 random observations of the sun from June through
December 1961 with a thin mica window Geiger tube on satellite Injun 1,
it is found that: (a) The `quiet day' solar X-ray flux at 1 astronomical
unit in the wavelength range λ < 14 A had a minimum value of
0.5×10-3 erg (cm2 sec)-1, a value
which is approximately independent of the assumed blackbody temperature
of the emitting areas of the sun over the temperature range 0.8 to
8×106 °K. (b) The solar X-ray flux underwent substantial
time variation, from the lowest value quoted in (a) to the highest value
on July 20 during a Hα flare of importance 3. The X-ray flux λ <
14 A on July 20 was 0.07 erg (cm2 sec)-1. (c)
Three other high intensity values also appeared to be associated with
visual flares, but no convincing relationship between intensity and
flare activity was found among the other 44 cases. (d) The median flux
observed in the 48 random observations was 1.7×10-3 erg
(cm2 sec)-1 for λ < 14 A.
Title: X-Radiation of the Sun.
Authors: Acton, Loren Wilber
Bibcode: 1965PhDT.........5A
Altcode:
No abstract at ADS
Title: Observations of Solar X-Ray Emission in the 8 to 20 A Band
Authors: Acton, L. W.; Chubb, T. A.; Kreplin, R. W.; Meekins, J. F.
Bibcode: 1963JGR....68.3335A
Altcode:
This paper reports 101 observations of the solar X-ray flux in the 8-20
A band. These observations were made by the Solar Radiation 3 satellite
from June 29 to November 26, 1961. The fluxes range from a high value
of greater than 0.4 erg cm-2 sec-1, during a
3+ limb flare, to below the limit of detectability, 0.002
erg cm-2 sec-1. The intensity of this radiation
is correlated with general solar activity and is greatly enhanced
during some flares. The intensity is observed to change by a factor of
2 without any apparent change in solar activity. X-ray enhancements
may persist for hours after the end of large flares. Comparison of
sudden ionospheric disturbances and the X-ray enhancements indicates
that the spectrum of the X radiation differs from one flare to the
next and with time during a given flare event.
Title: Effect of Intermittent Operation of Flare Patrols on Observed
Flare Frequency.
Authors: Acton, Loren
Bibcode: 1962AJ.....67Q.109A
Altcode:
It is not required that an observing station observe a solar flare
throughout its entire lifetime in order to report the flare. For this
reason it is necessary to consider the breaks in observing time as well
as the actual observing time when calculating flare frequency. This
consideration has been neglected in most earlier studies involving the
occurrence frequency of solar flares. We used the following method,
similar to that of Svestka (Bull. Astron. Inst. Czech. 7, 9-18,
1954), in calculating the "effective observing time" (E.O.T.) for
flares of each importance for each observing station. E O T =A O
T $Zt tB + t0N, where A.O.T. actual observing time; Bj number of
breaks in observing of duration tj t0 N total number of breaks with
duration t#t0 t0 mean lifetime of flares of a given importance. After
analyzing some 300 flares, each reported by two or more observing
stations, we adopted the mean flare lifetimes given in Table I. The
percentage changes in observing times range from a low of 4% for
1-flares observed at Capetown to a high of 243 % for 3+ flares
observed at the Dunsink Observatory. The mean value for importance
2 flares is about 60%. The application of these corrections yields
significant changes in calculations involving the fre- TABLE I. Mean
flare lifetime as a function of importance. Imp 1- 1 1+ 2- 2 2+ 3- 3
3+ to (min) to 20 35 50 60 65 70 75 80 quency of occurrence of solar
flares. Two examples of such changes are: (1) The shape of the curve
of flare frequency vs flare importance is changed, the percentage
decrease in flares per hour being larger for the larger flares. (2)
The disparity of reported flares per hour between different observing
stations (Dodson and Hedeman, J. Ceophys. Research 65, 123, 1960)
is considerably lessened when one considers the number of flares per
"effective" observing hour rather than the number of flares per actual
observing hour. % for 1-flares observed at Capetown to a high of 243 %
for 3+ flares observed at the Dunsink Observatory. The mean value for
importance 2 flares is about 60%. The application of these corrections
yields significant changes in calculations involving the fre- TABLE
I. Mean flare lifetime as a function of importance. Imp 1- 1 1+ 2-
2 2+ 3- 3 3+ to (min) to 20 35 50 60 65 70 75 80 quency of occurrence
of solar flares. Two examples of such changes are: (1) The shape of the
curve of flare frequency vs flare importance is changed, the percentage
decrease in flares per hour being larger for the larger flares. (2)
The disparity of reported flares per hour between different observing
stations (Dodson and Hedeman, J. Ceophys. Research 65, 123, 1960)
is considerably lessened when one considers the number of flares per
"effective" observing hour rather than the number of flares per actual
observing hour. % for 1-flares observed at Capetown to a high of 243 %
for 3+ flares observed at the Dunsink Observatory. The mean value for
importance 2 flares is about 60%. The application of these corrections
yields significant changes in calculations involving the fre- TABLE
I. Mean flare lifetime as a function of importance. Imp 1- 1 1+ 2-
2 2+ 3- 3 3+ to (min) to 20 35 50 60 65 70 75 80 quency of occurrence
of solar flares. Two examples of such changes are: (1) The shape of the
curve of flare frequency vs flare importance is changed, the percentage
decrease in flares per hour being larger for the larger flares. (2)
The disparity of reported flares per hour between different observing
stations (Dodson and Hedeman, J. Ceophys. Research 65, 123, 1960)
is considerably lessened when one considers the number of flares per
"effective" observing hour rather than the number of flares per actual
observing hour.
Title: Some Relationships between Short-Wave Fadeouts, Magnetic
Crochets and Solar Flares
Authors: Acton, Loren W.
Bibcode: 1961JGR....66.3060A
Altcode:
The purpose of this letter is to report the results of a study of
correlations between the three classes of short-wave fadeouts (SWF)
and the disk distribution and importance of the associated solar
flares. We will also consider relationships between the type of SWF
and the occurrence of simultaneous magnetic crochets (sometimes called
`solar flare effects' or SFE's). Short-wave fadeouts are reported
by the Central Radio Propagation Laboratory of the National Bureau
of Standards, Boulder Laboratories, in its monthly tabulations of
solar-geophysical data (F series, part B). They are arbitrarily divided
into three classes according to the rapidity and general character of
the recorded field strength decrease. The three types of fadeouts are
illustrated in Figure 1 and described below.