Author name code: hagenaar
ADS astronomy entries on 2022-09-14
author:"Hagenaar, Hermance J."
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Title: Detection of Flux Emergence, Splitting, Merging, and
Cancellation of Network Fields. II. Apparent Unipolar Flux Change
and Cancellation
Authors: Iida, Y.; Hagenaar, H. J.; Yokoyama, T.
Bibcode: 2015ApJ...814..134I
Altcode: 2015arXiv151004764I
In this second paper in the series, we investigate occurrence
frequencies of apparent unipolar processes, cancellation, and
emergence of patch structures in quiet regions. Apparent unipolar
events are considerably more frequent than cancellation and emergence,
per our definition, which is consistent with Lamb et al. Furthermore,
we investigate the frequency distributions of changes in flux during
apparent unipolar processes and find that they concentrate around the
detection limit of the analysis. Combining these findings with the
results of our previous paper, Iida et al., which found that merging
and splitting are more dominant than emergence and cancellation, these
results support the understanding that apparent unipolar processes
are actually interactions with and among patches below the detection
limit and that there still are numerous flux interactions between the
flux range in this analysis and below the detection limit. We also
investigate occurrence frequency distributions of flux decrease during
cancellation. We found a relatively strong dependence, 2.48 ± 0.26
as a power-law index. This strong dependence on flux is consistent
with the model, which was suggested in the previous paper.
Title: Occurrence rates of merging, splitting, and cancellation in
quiet regions on the solar surface
Authors: Iida, Y.; Hagenaar, H. J.; Yokoyama, T.
Bibcode: 2012AGUFMSH13C2262I
Altcode:
We report the magnetic activities' occurrence rates, namely merging,
splitting, and cancellation, as functions of magnetic flux content per
involved patch in quiet regions on the solar surface. The structure of
magnetic field on the solar surface is important not only because it
has the important roles in the various solar activities, such as the
coronal heating, the X-ray bright points, the solar jets and the solar
dynamo, but also because it is the only actual magneto-convection system
on the stellar surface which we can observe. Recent high-resolution
observations reveal that more amount of magnetic flux is contained in
quiet regions than in active regions, which suggests the importance
of magnetic field in quiet regions for the understanding of the total
flux transport of the Sun. Parnell et al. (2009) investigate frequency
distribution of flux content and find the power-law distribution with
an index of -1.85. Two scenarios of its maintenance are suggested. One
is that it directly represents the frequency distribution of flux
content supplied from below the solar surface. The other is that it
is maintained by the surface magnetic activities, namely emergence,
splitting, merging, and cancellation. To distinct these scenarios,
it is necessary to investigate the occurrences of these activities and
supplied flux amount. We try the quantification of them by using the
auto-detection code developed by the authors. We use two data sets
of line-of-sight magnetograms in quiet regions obtained by Solar
Optical Telescope (SOT)/ Narrowband Filter Imager (NFI) on board
Hinode satellite. One has the high temporal cadence (~1 minutes)
and the other has the long observational period (~140 hours). Nearly
1600 and 20000 patches are tracked for each polarities in the data
sets, respectively. We obtained the power-law distribution of flux
content per patch with indexes of -1.79 and -1.93 respectively, which
are consistent with the result by Parnell et al. (2009). The total
occurrence rate of magnetic activities are investigated. We found
that the occurrence of merging and splitting is larger than those of
emergence and cancellation by one-order of magnitude, which means that
the frequency distribution of flux content is maintained by the surface
activities not by flux supply from below the photosphere. Further we
investigate the flux dependence of their occurrence. The occurrence
probability distributions of merging and splitting do not have the
significant difference from those with constant time scales. As for the
cancellation, we find the steep power-law distribution with an index
of -2.48, which is the same value with that of emergence reported by
Thornton & Parnell (2011). From the above results, we conclude
that the frequency dependence of magnetic flux amount per patch is
maintained by the surface activities, merging and splitting. Further,
We suggest a new picture of the flux maintenance in quiet regions. 1)
Frequency distribution of the flux content is maintained to a power-law
distribution by merging and splitting on the solar surface. 2) The
frequency of cancellation is interpreted as a result of collisions of
patches under the motions in random direction with constant velocities,
which may be driven by convective motions. 3) Most of emergences are
re-emergences of submerged loops recognized as cancellations.
Title: Signatures of Moving Magnetic Features in and above the
Photosphere
Authors: Hagenaar, H.; Shine, R.; Ryutova, M.; Dalda, A. S.
Bibcode: 2012ASPC..454..181H
Altcode:
Hinode/SOT observations of NOAA AR 10933 from 2007 Jan 4 16:14 UT -
Jan 6 22:20 UT are used to study MMFs (moving magnetic features) in the
periphery of the region's large sunspot and the surrounding moat. The
data consist of a nearly continuous set of Fe 6302 Å Stokes V images
with sets of G band and Ca II H filtergrams at various cadences, FOV's,
and resolutions plus some SpectroPolarimeter (SP) scans. We also used
TRACE images in 171 Å to follow any possible signatures at higher
temperatures. We applied automatic object recognition and tracking
to the MMFs as seen in the Fe 6302 Å Stokes V images. An SP scan
was used to determine the line profiles for several paths. Reliable
inversions have not yet been done, but we find a few locations of
possible supersonic downflows from the Stokes IQUV line profiles. The
population of MMFs on the East side of the sunspot is much higher
than on the opposite side, mostly involving a large number of mixed
polarity MMFs. Consequently, the chromosphere shows strongly enhanced
brightenings with a clear pattern: enhanced brightenings in Ca H
outline the locations where opposite polarity MMFs meet. This activity
does not prevent formation of active low lying “closed” loops at
coronal temperatures seen in the TRACE 171 Å line. The other side,
with fewer MMFs, shows a pattern that we found earlier: regions with
an MMF deficiency show long living “open” coronal loops. This work
was supported by NASA contract NNM07AA01C.
Title: Detection of Flux Emergence, Splitting, Merging, and
Cancellation of Network Field. I. Splitting and Merging
Authors: Iida, Y.; Hagenaar, H. J.; Yokoyama, T.
Bibcode: 2012ApJ...752..149I
Altcode: 2012arXiv1204.5261I
Frequencies of magnetic patch processes on the supergranule boundary,
namely, flux emergence, splitting, merging, and cancellation,
are investigated through automatic detection. We use a set of
line-of-sight magnetograms taken by the Solar Optical Telescope
(SOT) on board the Hinode satellite. We found 1636 positive patches
and 1637 negative patches in the data set, whose time duration is
3.5 hr and field of view is 112'' × 112''. The total numbers of
magnetic processes are as follows: 493 positive and 482 negative
splittings, 536 positive and 535 negative mergings, 86 cancellations,
and 3 emergences. The total numbers of emergence and cancellation are
significantly smaller than those of splitting and merging. Further,
the frequency dependence of the merging and splitting processes on the
flux content are investigated. Merging has a weak dependence on the
flux content with a power-law index of only 0.28. The timescale for
splitting is found to be independent of the parent flux content before
splitting, which corresponds to ~33 minutes. It is also found that
patches split into any flux contents with the same probability. This
splitting has a power-law distribution of the flux content with an
index of -2 as a time-independent solution. These results support
that the frequency distribution of the flux content in the analyzed
flux range is rapidly maintained by merging and splitting, namely,
surface processes. We suggest a model for frequency distributions of
cancellation and emergence based on this idea.
Title: Detection of Flux Emergence, Splitting, Merging, and
Cancellation in the Quiet Sun
Authors: Iida, Y.; Hagenaar, H.; Yokoyama, T.
Bibcode: 2012ASPC..455..169I
Altcode: 2011arXiv1102.1238I
We investigate the frequency of magnetic activities, namely flux
emergence, splitting, merging, and cancellation, through an automatic
detection in order to understand the generation of the power-law
distribution of magnetic flux reported by Parnell et al. (2009). Quiet
Sun magnetograms observed in the Na I 5896 Å line by the Hinode Solar
Optical Telescope is used in this study. The longitudinal fluxes of
the investigated patches range from ≍ 1017 Mx to ≍
1019 Mx. Emergence and cancellation are much less frequent
than merging and splitting. The time scale for splitting is found to be
≍ 33 minutes and independent of the flux contained in the splitting
patch. Moreover magnetic patches split into any flux contents with an
equal probability. It is shown that such a fragmentation process leads
to a distribution with a power-law index -2. Merging has a very weak
dependence on flux content, with a power-law index of only -0.33. These
results suggest that (1) magnetic patches are fragmented by splitting,
merging, and tiny cancellation; and (2) flux is removed from the
photosphere through tiny cancellations after these fragmentations.
Title: Flares Producing Well-organized Post-flare Arcades (Slinkies)
Have Early Precursors
Authors: Ryutova, M. P.; Frank, Z.; Hagenaar, H.; Berger, T.
Bibcode: 2011ApJ...733..125R
Altcode:
Exploding loop systems producing X-ray flares often, but not always,
bifurcate into a long-living, well-organized system of multi-threaded
loop arcades resembling solenoidal slinkies. The physical conditions
that cause or prevent this process are not known. To address this
problem, we examined most of the major (X-class) flares that occurred
during the last decade and found that the flares that bifurcate into
long-living slinky arcades have different signatures than those that
do not "produce" such structures. The most striking difference is that,
in all cases of slinky formation, GOES high energy proton flux becomes
significantly enhanced 10-24 hr before the flare occurs. No such effect
was found prior to the "non-slinky" flares. This fact may be associated
with the difference between energy production by a given active region
and the amount of energy required to bring the entire system into
the form of well-organized, self-similar loop arcades. As an example
illustrating the process of post-flare slinky formation, we present
observations taken with the Hinode satellite, in several wavelengths,
showing a time sequence of pre-flare and flare activity, followed by
the formation of dynamically stable, well-organized structures. One
of the important features revealed is that post-flare coronal slinky
formation is preceded by scale invariant structure formation in the
underlying chromosphere/transition region. We suggest that the observed
regularities can be understood within the framework of self-organized
critical dynamics characterized by scale invariant structure formation
with critical parameters largely determined by energy saturation
level. The observed regularities per se may serve as a long-term
precursor of strong flares and may help to study predictability of
system behavior.
Title: Solar Magnetic Tracking. III. Apparent Unipolar Flux Emergence
in High-resolution Observations
Authors: Lamb, D. A.; DeForest, C. E.; Hagenaar, H. J.; Parnell,
C. E.; Welsch, B. T.
Bibcode: 2010ApJ...720.1405L
Altcode:
Understanding the behavior of weak magnetic fields near the detection
limit of current instrumentation is important for determining the
flux budget of the solar photosphere at small spatial scales. Using
0farcs3-resolution magnetograms from the Solar Optical Telescope's
Narrowband Filter Imager (NFI) on the Hinode spacecraft, we confirm
that the previously reported apparent unipolar magnetic flux emergence
seen in intermediate-resolution magnetograms is indeed the coalescence
of previously existing flux. We demonstrate that similar but smaller
events seen in NFI magnetograms are also likely to correspond to
the coalescence of previously existing weak fields. The uncoalesced
flux, detectable only in the ensemble average of hundreds of these
events, accounts for 50% of the total flux within 3 Mm of the detected
features. The spatial scale at which apparent unipolar emergence can
be directly observed as coalescence remains unknown. The polarity of
the coalescing flux is more balanced than would be expected given the
imbalance of the data set, however without further study we cannot
speculate whether this implies that the flux in the apparent unipolar
emergence events is produced by a granulation-scale dynamo or is
recycled from existing field.
Title: Erratum: "The Dependence of Ephemeral Region Emergence on
Local Flux Imbalance" (2008, ApJ,
678, 541)
Authors: Hagenaar, Hermance J.; DeRosa, Marc L.; Schrijver, Carolus J.
Bibcode: 2010ApJ...715..696H
Altcode:
We have discovered an error in the labeling of Figure 5. The importance
of the figure is to indicate the dependence of flux emergence on local
flux (im-) balance. However, the scales of the figures were incorrect,
causing a discrepancy between Table 2 and Figure 5(a). The corrected
Figure 5 appears below. The change does not affect the conclusion.
Title: Moving Magnetic Features and the Flow Pattern around Sunspots
Authors: Hagenaar, H. J.; Shine, R. A.
Bibcode: 2009AGUFMSH51A1255H
Altcode:
Studies of Moving Magnetic Features indicate a Spoke Pattern around
Sunspots. We investigate this flow pattern further on Hinode/ SOT data.
Title: A Power-Law Distribution of Solar Magnetic Fields Over More
Than Five Decades in Flux
Authors: Parnell, C. E.; DeForest, C. E.; Hagenaar, H. J.; Johnston,
B. A.; Lamb, D. A.; Welsch, B. T.
Bibcode: 2009ApJ...698...75P
Altcode:
Solar flares, coronal mass ejections, and indeed phenomena on all
scales observed on the Sun, are inextricably linked with the Sun's
magnetic field. The solar surface is covered with magnetic features
observed on many spatial scales, which evolve on differing timescales:
the largest features, sunspots, follow an 11-year cycle; the smallest
seem to follow no cycle. Here, we analyze magnetograms from Solar and
Heliospheric Observatory (SOHO)/Michelson Doppler Imager (full disk
and high resolution) and Hinode/Solar Optical Telescope to determine
the fluxes of all currently observable surface magnetic features. We
show that by using a "clumping" algorithm, which counts a single
"flux massif" as one feature, all feature fluxes, regardless of flux
strength, follow the same distribution—a power law with slope -1.85
± 0.14—between 2 × 1017 and 1023 Mx. A power
law suggests that the mechanisms creating surface magnetic features
are scale-free. This implies that either all surface magnetic features
are generated by the same mechanism, or that they are dominated by
surface processes (such as fragmentation, coalescence, and cancellation)
in a way which leads to a scale-free distribution.
Title: A Power-law Distribution of Solar Magnetic Fields Over More
Than Five Decades in Flux
Authors: Parnell, Clare; DeForest, C. E.; Hagenaar, H. J.; Johnston,
B. A.; Lamb, D. A.; Welsch, B. T.
Bibcode: 2009SPD....40.0603P
Altcode:
The surface of the Sun is covered with magnetic features observed
on many spatial scales, which evolve on differing time scales: the
largest features, sunspots, follow an 11 year cycle; the smallest
apparently follow no cycle. Magnetograms from SoHO/MDI (full disk and
high-resolution) and Hinode/SOT are analysed to determine the fluxes
of all currently observable surface magnetic features. To identify
features we use a 'clumping' algorithm, which defines a single feature
as a group of contiguous, same-sign pixels, each of which exceeds an
absolute flux cutoff. We show that, using this feature identification
method, all feature fluxes, regardless of flux strength, follow the
same distribution - a power-law with slope -1.85±0.14 - between 2x
1017 and 1023 Mx. This result implies that the
processes that determine the spatial structure of surface magnetic
features are scale-free. Hence, suggesting that either all surface
magnetic features are generated by the same mechanism, or that their
spatial structure is dominated by processes in the interior or at the
surface (e.g., fragmentation, coalescence and cancellation) that produce
a scale-free distribution. We will discuss the likelihood of these two
mechanisms for generating the powerlaw distribution of feature fluxes.
Title: Quiet-Sun: A Comparison of MDI and SOT Fluxes
Authors: Parnell, C. E.; Deforest, C. E.; Hagenaar, H. J.; Lamb,
D. A.; Welsch, B. T.
Bibcode: 2008ASPC..397...31P
Altcode:
The SOT-NFI on Hinode has both higher resolution and better sensitivity
than MDI on SOHO. Line-of-sight magnetograms of the quiet Sun taken
simultaneously by both MDI and SOT are investigated to show how the
observed flux differs between the two instruments. We find that: (i)
the total unsigned flux observed by SOT is approximately 50% greater
than that observed by MDI and (ii) the total signed flux remains
approximately constant. Thus, the extra flux observed by SOT is made
up of equal amounts of positive and negative flux. By comparing the
observed flux distributions from MDI and SOT we find that the extra flux
is contained in features with fluxes less than the smallest observed
by MDI. Indeed, the smallest features in SOT have just ≥ 10^{16} Mx,
a factor of thirty less than the smallest observed by MDI. The
distributions of feature fluxes observed by the two instruments are
also compared. We find that by using a `clumping' algorithm, which
counts a single `flux massif' as one feature, the fluxes in MDI and
SOT follow the same distribution - a power-law - between 2× 10^{17}
and 10^{20} Mx. Thus, the mechanism producing network and intranetwork
features appears to be the same. Furthermore, the power-law index of
this distribution is found to be -1.85. This value is neither the
Kolomogrov -5/3 slope of hydrodynamic turbulence nor the Krichenen
-2 slope of magneto-hydrodynamic turbulence, although both of these
numbers may be within the error bars of our analysis.
Title: Magnetic Flux Emergence on Different Scales
Authors: Hagenaar, H.; Cheung, M.
Bibcode: 2008ESPM...12.2.53H
Altcode:
Magnetic flux emerges on the Sun on many different scales, from
weak intranetwork to network concentrations and (ephemeral) active
regions. Methods previously developed to recognize regions of
magnetic emergence on MDI Full Disk magnetograms fail when applied to
Hinode/SOT Stokes maps: the resolution is so much higher that simple
bipoles on MDI are observed as collections of fragments. We present
a new method for the automatic detection and characterization of
flux emergence on a range of scales. Our findings are compared with
simulations and discuss the implications for our understanding of
emerging flux ropes.
Title: The Dependence of Ephemeral Region Emergence on Local Flux
Imbalance
Authors: Hagenaar, Hermance J.; DeRosa, Marc L.; Schrijver, Carolus J.
Bibcode: 2008ApJ...678..541H
Altcode:
We investigate the distribution and evolution of existing and emerging
magnetic network elements in the quiet-Sun photosphere. The ephemeral
region emergence rate is found to depend primarily on the imbalance of
magnetic flux in the area surrounding its emergence location, such that
the rate of flux emergence is lower within strongly unipolar regions by
at least a factor of 3 relative to flux-balanced quiet Sun. As coronal
holes occur over unipolar regions, this also means that ephemeral
regions occur less frequently there, but we show that this is an
indirect effect—independent of whether the region is located within
an open-field coronal hole or a closed-field quiet region. We discuss
the implications of this finding for near-photospheric dynamo action and
for the coupling between closed coronal and open heliospheric fields.
Title: The Small-Scale Field Measured With Hinode/SOT and Feature
Tracking: Where is the mixed- polarity flux?
Authors: Deforest, C. E.; Lamb, D. A.; Berger, T.; Hagenaar, H.;
Parnell, C.; Welsch, B.
Bibcode: 2008AGUSMSP51D..01D
Altcode:
We report on the results of the first feature tracking study of
the solar magnetic field with Hinode/SOT. We processed a SOT Na-D
line-of-sight magnetogram sequence with five different magnetic
tracking codes. The SOT data allow us to probe the evolving magnetic
field on the granular scale for hours at a time, something that was
not possible with either ground-based observations (which are limited
to short periods of good seeing) or prior space-based observations
(which are limited to arcsecond spatial scales). We find that the field
is much less mixed than previously supposed: while Hinode resolves
small-scale structure within features that, to SOHO/MDI, would
appear as monolithic flux concentrations, this substructure has but
a single sign. Furthermore, the average distance between identifiable
flux concentrations of opposite sign remains nearly unchanged at the
higher resolution, a result that is quite surprising in light of the
common picture of a sea of strong mixed-polarity flux concentrations
dotting the inter-granular lanes. We discuss possible mechanisms for
this surprising result, and implications for the small-scale dynamo.
Title: Solar Magnetic Tracking. II. The Apparent Unipolar Origin of
Quiet-Sun Flux
Authors: Lamb, D. A.; DeForest, C. E.; Hagenaar, H. J.; Parnell,
C. E.; Welsch, B. T.
Bibcode: 2008ApJ...674..520L
Altcode:
We investigate the origin of small-scale flux concentrations in the
quiet Sun. In apparent violation of the physical requirement for
flux balance, 94% of the features containing newly detected flux
are unipolar at a resolution of 1.2''. We analyze 2619 of these
apparent unipolar emergences in an image sequence from the SOHO MDI
magnetograph and compare the ensemble average to a model of asymmetric
bipolar emergence that could in principle hide opposing flux under
the noise floor of MDI. We examine the statistical consequences of
this mechanism and find that it cannot be responsible for more than
a small fraction of the unipolar emergences. We conclude that the
majority of the newly detected flux in the quiet Sun is instead due
to the coalescence of previously existing but unresolved flux into
concentrations that are large and strong enough to be detected. We
estimate the rate of coalescence into arcsecond-scale magnetic
features averaged over the solar surface to be 7 × 1021
Mx hr-1, comparable to the reported flux injection rate
due to ephemeral regions. This implies that most flux in the solar
network has been processed by very small scale shredding, emergence,
cancellation, and/or coalescence that is not resolved at 1.2'', and
it suggests that currently unresolved emergences may be at least as
important as ephemeral region emergences to the overall flux budget.
Title: Ephemeral Bipolar Regions in Coronal Holes
Authors: Hagenaar, H.; Schrijver, C.; De Rosa, M.
Bibcode: 2008ASPC..383..343H
Altcode:
We investigate the distribution and evolution of magnetic network
elements in quiet Sun with or without coronal holes. Ephemeral region
emergence rates are found to depend on the degree of imbalance of
magnetic flux, but independent of whether there is a coronal hole or
not. We discuss the implications of this finding for near-photospheric
dynamo action and for the coupling between closed coronal and open
heliospheric fields.
Title: Fine Scale Magnetic Fields in and around a Decaying Active
Region
Authors: Sankarasubramanian, K.; Hagenaar, H.
Bibcode: 2008arXiv0801.2820S
Altcode:
High spatial resolution spectro-polarimetric observation of a decaying
spot was observed with the Diffraction Limited Spectro-Polarimeter. The
spatial resolution achieved was close to the diffraction limit (0."18)
of the Dunn Solar Telescope. The fine scales present inside the decaying
active region as well as surrounding areas were studied. Two interesting
phenomenon observed are: (i) Canopy like structures are likely to be
present in the umbral dots as well as in the light bridges providing
evidence for field-free intrusion, (ii) There are opposite polarity
loops present outside of the spot and some of them connects to the
main spot and the surrounding magnetic features.
Title: Fine scale magnetic fields of a decaying active region
Authors: Sankarasubramanian, K.; Hagenaar, H.
Bibcode: 2007BASI...35..427S
Altcode:
High spatial resolution spectro-polarimetric observation of a decaying
spot was observed with the Diffraction Limited Spectro-Polarimeter. The
spatial resolution achieved was close to the diffraction limit (0.''18)
of the Dunn Solar Telescope. The fine scales present inside the decaying
active region as well as surrounding areas were studied. Two interesting
phenomenon observed are: (i) Canopy like structures are likely to be
present in the umbral dots as well as in the light bridges providing
evidence for field-free intrusion, (ii) There are opposite polarity
loops present outside of the spot and some of them connects to the
main spot and the surrounding magnetic features.
Title: Feature Tracking of Hinode Magnetograms
Authors: Lamb, D.; Deforest, C. E.; Hagenaar, H. J.; Parnell, C. E.;
Welsch, B. T.
Bibcode: 2007AGUFMSH53A1066L
Altcode:
We present results of applying feature tracking to a sequence of Hinode
magnetograms. The single line wing Na D 5896 magnetograms have a high
signal-to-noise ratio, allowing the detection of flux approximately
30 times weaker than in MDI magnetograms. We find evidence that, even
with Hinode's improved resolution and sensitivity, we do not always
detect the bipolar emergence of new magnetic flux. This suggests that
we have not reached the ultimate resolution to observe the fundamental
flux generation processes in the photosphere.
Title: Fine Structure of the Net Circular Polarization in a Sunspot
Penumbra
Authors: Tritschler, A.; Müller, D. A. N.; Schlichenmaier, R.;
Hagenaar, H. J.
Bibcode: 2007ApJ...671L..85T
Altcode: 2007arXiv0710.4545T
We present novel evidence for fine structure observed in the
net circular polarization (NCP) of a sunspot penumbra based on
spectropolarimetric measurements utilizing the Zeeman-sensitive Fe
I 630.2 nm line. For the first time we detect filamentary organized
fine structure of the NCP on spatial scales that are similar to the
inhomogeneities found in the penumbral flow field. We also observe an
additional property of the visible NCP, a zero-crossing of the NCP
in the outer parts of the center-side penumbra, which has not been
recognized before. In order to interpret the observations we solve the
radiative transfer equations for polarized light in a model penumbra
with embedded magnetic flux tubes. We demonstrate that the observed
zero-crossing of the NCP can be explained by an increased magnetic
field strength inside magnetic flux tubes in the outer penumbra combined
with a decreased magnetic field strength in the background field. Our
results strongly support the concept of the uncombed penumbra.
Title: Magnetic Solitons: Unified Mechanism for Moving Magnetic
Features
Authors: Ryutova, M.; Hagenaar, H.
Bibcode: 2007SoPh..246..281R
Altcode:
In a highly dynamic environment with sources and sinks of energy,
flux tubes do not in general obey local conservation laws, nor do the
ensembles of flux tubes that exhibit collective phenomena. We use the
approach of energetically open dissipative systems to study nonlinear
waves in flux tubes and their role in the dynamics of the overlying
atmosphere. We present results of theoretical and observational studies
of the properties of moving magnetic features (MMFs) around sunspots and
the response of the overlying atmosphere to various types of MMFs. We
show that all types of MMFs, often having conflicting properties,
can be described on a unified basis by employing the model of shocks
and solitons propagating along the penumbral filaments co-aligned with
Evershed flows. The model is also consistent with the response of the
upper atmosphere to individual MMFs, which depends on their type. For
example, soliton-type bipolar MMFs mainly participate in the formation
of a moat and do not carry much energy into the upper atmosphere,
whereas shock-like MMFs, with the appearance of single-polarity
features, are often associated with chromospheric jets and microflares.
Title: Solar Magnetic Tracking. I. Software Comparison and Recommended
Practices
Authors: DeForest, C. E.; Hagenaar, H. J.; Lamb, D. A.; Parnell,
C. E.; Welsch, B. T.
Bibcode: 2007ApJ...666..576D
Altcode: 2007arXiv0704.2921D
Feature tracking and recognition are increasingly common tools for
data analysis, but are typically implemented on an ad hoc basis
by individual research groups, limiting the usefulness of derived
results when selection effects and algorithmic differences are not
controlled. Specific results that are affected include the solar
magnetic turnover time, the distributions of sizes, strengths, and
lifetimes of magnetic features, and the physics of both small scale flux
emergence and the small-scale dynamo. In this paper, we present the
results of a detailed comparison between four tracking codes applied
to a single set of data from SOHO/MDI, describe the interplay between
desired tracking behavior and parameterization tracking algorithms,
and make recommendations for feature selection and tracking practice
in future work.
Title: Magnetic Network Formation Due to Sub-arcsecond Flux Processing
Authors: Lamb, Derek; DeForest, C. E.; Parnell, C. E.; Hagenaar,
H. J.; Welsch, B. T.
Bibcode: 2007AAS...210.9213L
Altcode: 2007BAAS...39Q.210L
Kinematic models of solar magnetic network formation typically
employ the breakup of ephemeral regions by granular and supergranular
flow. We show that the coalescence of sub-arcsecond-scale magnetic
flux concentrations into features detectable with MDI is responsible
for injecting as much flux into the magnetic network as the published
emergence rate of ephemeral regions. We also show that the few fresh
bipoles we do detect have no preferential alignment, and thus violate
Joy's law at the arcsecond scale. These two items suggest that at least
half of the flux that makes its way into the network has been processed
at spatial scales below 1 arcsecond, indicative of a local dynamo.
Title: Anticorrelation between Moving Magnetic Features and Coronal
Loop Formation
Authors: Ryutova, M. P.; Hagenaar, H.; Title, A.
Bibcode: 2007ApJ...656L..45R
Altcode:
We study a possible connection of moving magnetic features (MMFs)
and the overlying atmosphere using several sets of multiwavelength
observations of sunspot areas from the photosphere to the corona. We
find that as a collective phenomenon, very intense MMF formation
anticorrelates with the presence of large-scale ``stable'' coronal
loops: such loops are rooted at the side of sunspots with no or few
MMFs rather than at the side of the penumbra/moat highly populated by
MMFs. Conjectures to help understand the observed correlation between
the preferable site of coronal loops and the deficiency of MMFs are
discussed.
Title: Moving Magnetic Features Observed in Photosphere, Chromosphere,
and Transition Region
Authors: Hagenaar, H. J.; Frank, Z. A.
Bibcode: 2006ESASP.617E..64H
Altcode: 2006soho...17E..64H
No abstract at ADS
Title: Moving Magnetic Features around Sunspots
Authors: Hagenaar, Hermance J.; Shine, Richard A.
Bibcode: 2005ApJ...635..659H
Altcode:
We study statistical properties of small-scale magnetic features
around sunspots using time sequences of high-resolution magnetograms
of eight sunspots made with the Michelson Doppler Imager (MDI)
on board SOHO. Flow maps around the spots are also derived from
cross-correlation analysis of MDI continuum or TRACE white light and
used for comparison of photospheric flow patterns with the tracks of
moving magnetic features. An automated algorithm to find and track
unipolar concentrations of magnetic field was developed. Depending on
the velocity, size, and distance from the spot, a selected subset of all
concentrations can be identified as moving magnetic features (MMFs). Our
method finds 4-24 MMFs per hour around the spots, with higher counts
for larger sunspots. After being first detected, the MMFs have an
average flux content <Φ0>=2.5×1018
Mx. Their average lifetime is about 1 hr, but it takes a
concentration only tmax=25 minutes to reach its maximum
flux content of about <Φmax>=6.1×1018
Mx. MMFs are found to transport a net flux out of a spot at a rate of
(0.4-6.2)×1019 Mx hr-1: if sunspots were to decay
only by outflowing MMFs, it would take a sunspot of 1022
Mx one to several weeks to completely disassemble. The MMFs have
an initial velocity of 0>=1.8 km s-1, faster than the
average moat flow. Before merging into the moat region or surrounding
network, they travel a distance <Δ>=3.5 Mm. The tracks of the
individual MMFs correlate with the direction of local plasma flows and
sometimes display a spokelike pattern around the sunspots. We find an
average initial size 0>=1.7 Mm2, but the distribution
of sizes suggests features with a diameter of only 600-1000 km, which
would not be recognized by our algorithm. Comparison of a cotemporal,
cospatial magnetogram made with the Swedish Vacuum Solar Telescope
(SVST) on La Palma (with 12 times the spatial resolution) indicates
that unipolar magnetic fluxes in the MDI magnetogram may be comprised
of smaller elements with both polarities.
Title: Moving Magnetic Features around Sunspots
Authors: Hagenaar, H. J.; Shine, R. S.
Bibcode: 2005ESASP.600E..60H
Altcode: 2005dysu.confE..60H; 2005ESPM...11...60H
No abstract at ADS
Title: Unified Mechanism for the Formation of Moving Magnetic Features
Authors: Ryutova, M. P.; Hagenaar, H. J.
Bibcode: 2005AGUSMSP31A..04R
Altcode:
In the highly dynamic environment around sunspots there are small
scale magnetic features, MMF's, that show clear regularities and may
be thus categorized according their observed properties. For now there
are at least 4 types of "MMF's" (Moving Magnetic Features). Type I
MMF's are compact pairs of opposite polarity elements that may emerge
anywhere in penumbra or moat region and move radially outward gradually
separating; their velocities exceed the velocities of ambient flows,
and their inner " foot" shares the sunspot's polarity. Type II MMF's
are seen as unipolar features of the same polarity as the sunspot,
moving outward from the sunspot with higher velocities than type
I. Type III MMF's are also seen as unipolar features but have the
polarity opposite to the sunspot's and travel with higher velocities
than the other two types of MMF's. Recently the "type IV" features
were observed in a sunspot formation region, that appear as compact
bipoles flowing into sunspots and with an inner foot of a polarity
opposite to the sunspot's. These were dubbed the MDF's (Moving Dipolar
Features). The observed properties of all types of MMF's clearly
violate the energy and momentum conservation laws, and thus require
the application of physical mechanisms adequate for energetically open
systems. Such mechanisms have been applied to type I and type II MMF's
(Ryutova, Shine, Title, and Sakai, 1998, ApJ, 492, 402) with a good
agreement between the theory and observations. Here we show that the
same approach not only explains the origin, structure and dynamics of
MDF's and type III MMF's, but consolidates all types of MMF's into one
scheme. Theoretical results are compared with the observed properties
of MMF's using time series of several data sets.
Title: Destruction Mechanisms of Quiet-Sun Magnetic Flux
Authors: Lamb, D. A.; Deforest, C. E.; Hagenaar, H. J.; Parnell,
C. E.; Welsch, B. T.
Bibcode: 2005AGUSMSP41B..02L
Altcode:
We use SWAMIS, a freely available magnetic feature tracking suite,
to analyze the destruction of solar small-scale magnetic flux. We
track a sequence of high resolution MDI magnetograms to find the
destruction rates in a patch of quiet sun. We state criteria for
defining the individual magnetochemical destruction mechanisms
of merging, cancellation, and disappearance, and determine the
contribution of each process to the removal of detected flux from the
photosphere. Destruction mechanisms are important to determine because,
together with formation mechanisms, they provide information as to
the nature of the small-scale dynamo. We present preliminary results
and discuss the implications of these rates on models of quiet-sun
magnetic flux generation.
Title: Moving Magnet Features around Sunspots
Authors: Hagenaar, H. J.; Shine, R. A.
Bibcode: 2003AGUFMSH42B0544H
Altcode:
Moving magnetic features (MMF's) associated with small-scale emerging
fluxes near the sunspot penumbra are believed to play an important
role in mass and energy flow near sunspots. Since their discovery 30
year ago, only a few theoretical interpretations have been proposed on
the real identity of MMF's: they may be associated with field lines
detached from a decaying spot, or with closed magnetic loops. MMF's
have remained a difficult subject for observations, requiring high
spatial resolution movies for at least several hours. Coronal emission
does not show immediate response to the birth and disappearance of
individual MMF's; and the role of MMF's in the dynamics of upper layers
of the atmosphere remains unclear. We present the results of recent,
multi-wavelength observations designed to study the dynamics of MMF's
from the time of their emergence to the moment when they merge into
network or moat. Vector magnetograms made with the Dunn Telescope at
Sunspot, NM, are co-aligned with MDI magnetograms, and TRACE 1600 Å
and Fe IX/X 171 Å images, showing field orientation at the site of
emergence, and the response of the chromosphere and corona to dynamic
changes in the MMF's. These data allow meaningful statistics on MMF's
and their relation to sunspot evolution.
Title: The Properties of Small Magnetic Regions on the Solar Surface
and the Implications for the Solar Dynamo(s)
Authors: Hagenaar, Hermance J.; Schrijver, Carolus J.; Title, Alan M.
Bibcode: 2003ApJ...584.1107H
Altcode:
We find that bipolar active regions that emerge onto the Sun's surface
are part of a smoothly decreasing frequency distribution that spans
almost 4 orders of magnitude in flux and 8 orders of magnitude in
frequency. Distributions of emergence latitude and dipole orientation
narrow from nearly uniform for the smallest observed ephemeral regions
(~5×1018 Mx) up to narrowly distributed about the mean for
the largest active regions (close to 1022 Mx), while the
emergence frequency increases smoothly and rapidly with decreasing
flux. At the low end of the flux spectrum, the cycle variation in
emergence frequency is at most a factor of 1.5, in antiphase with
the cycle variation of close to an order of magnitude for the large
active regions. We discuss a scenario in which the ephemeral regions
with fluxes below ~30×1018 Mx have their origin in a
turbulent dynamo, largely independent of the global sunspot cycle. Our
empirical findings are based on a combination of previously published
work on active regions and large ephemeral regions, complemented
here with an analysis of the photospheric magnetic field outside
active regions, as observed in SOHO/MDI full-disk magnetograms taken
from the most recent sunspot minimum in 1996 to about 1 yr after
sunspot maximum in 2001. We find that the spectrum of the emerging
bipoles with fluxes (6-30)×1018 Mx can be approximated
throughout this period by a fixed exponential distribution with
an e-folding scale of (5.3+/-0.1)×1018 Mx. We confirm
that the ephemeral regions are an important source of flux for the
quiet magnetic network, in particular for the smallest scales; the
larger scale patterns are dominated by flux dispersing from decaying
active regions. As the variation of these two sources is nearly in
antiphase, the flux contained in the quiet-Sun network shows little
overall variation: the flux spectrum and the total absolute flux for
network concentrations with fluxes <~20×1018 Mx are
essentially independent of cycle phase. For network concentrations with
fluxes >~30×1018 Mx, mostly found in regions populated
substantially by decayed active regions, the network flux distribution
approaches an exponential for which the e-folding scale increases with
sunspot activity from ~20×1018 Mx to ~33×1018
Mx, as the total flux in this component varies in phase with the sunspot
cycle. A comparison of the flux-emergence rate with the network flux
implies an overall mean replacement time for flux in quiet Sun of
8-19 hr.
Title: Ephemeral Regions on a Sequence of Full-Disk Michelson Doppler
Imager Magnetograms
Authors: Hagenaar, Hermance J.
Bibcode: 2001ApJ...555..448H
Altcode:
Ephemeral regions are small-scale, bipolar regions of magnetic field,
emerging all over the solar surface. As structures ephemeral regions
are short-lived they can be recognized for 4.4 hr on average. This
paper examines a 3.5 day sequence of full-disk Michelson Doppler
Imager (MDI) magnetograms in order to estimate the importance of
ephemeral regions to the total magnetic flux budget on the sun. The
data were taken in 1997 October, which was around the minimum of
solar cycle 22, and early in cycle 23. An algorithm was developed
to automatically recognize ephemeral regions on this sequence of
magnetograms. Assuming uniform emergence over the entire solar surface,
the total amount of flux emerging in ephemeral regions is estimated
to be 5×1023 Mx-1. This rate of emergence is
sufficient to replace the magnetic field in quiet sun in 14 hr. In
total, 38,000 ephemeral regions are found, with an absolute flux
Φ in the range (2.6-407)×1018 Mx. The distribution
function of their fluxes follows an exponential with an average of
11.3×1018 Mx. This relatively low flux content may be
due to the fact that these ephemeral regions are detected before
they have reached a maximum. After their first recognition, they
increase in flux with a rate of typically dΦ/dt=1.6×1015
Mx s-1. Only 60% of the ephemeral regions are found to have
the orientation expected in cycle 22. After emergence, the outer borders
of the ephemeral regions expand from a size of 8.9 Mm, with a velocity
of 2.3 km s-1. No particular pattern can be recognized from
a map of all locations of flux emergence. From a χ2 test it
is found that the emergences occur randomly, on a scale below 20 Mm. On
larger scales some order is found, but its origin remains unknown.
Title: Chromosphere: Network
Authors: Rutten, R.; Hagenaar, H.
Bibcode: 2000eaa..bookE1994R
Altcode:
The chromospheric network on the Sun was discovered by HALE in 1892
with his newly invented spectroheliograph. It is a patchy pattern
(Hale called it a `reticuled structure') covering the solar disk
outside active regions that appears when the Sun is imaged in spectral
diagnostics formed in the CHROMOSPHERE (figure 1)....
Title: Recognition of Ephemeral Regions on a sequence of full-disk
MDI magnetograms
Authors: Hagenaar, H. J.
Bibcode: 2000SPD....31.0123H
Altcode: 2000BAAS...32..805H
Ephemeral regions are bipolar regions of magnetic flux, emerging all
over the solar surface, with a typical lifetime of 4.4 hrs. Ephemeral
regions and active regions are distinguished by a rather arbitrary
size limit of 2.5 square degrees. In this paper, a 3.5 day sequence
of full disk magnetograms is studied, taken by the MDI-instrument
on SOHO. An algorithm is developed to recognize ephemeral regions
on these magnetograms. The chosen data set was taken with a high
frequency of, on average, one magnetogram per minute, during a
total time span of 3.5 days. In each image, all coherent magnetic
concentrations are selected. Subsequently, two opposite-polarity
concentrations of comparable total flux content, situated closer to
one another than 10 Mm, are interpreted as bipoles. These bipoles
can be associated with newly emerged ephemeral regions, but may
also be the result of chance encounters of previously existing
opposite-polarity concentrations. Pairs of images are compared,
interspaced by 95 - 100 minutes, in order to determine which bipoles
do not overlap magnetic concentrations in the previous image, within
a radius of 19 Mm. We find a total number of (1-2)104
ephemeral regions, on 1/4 of the total solar surface. The total
amount of flux emerging in ephemeral regions is estimated to be of
order 1016}-10{17 Mx/sec. This flux, emerging
in ephemeral regions, provides a significant fraction of the total
flux on the solar surface. We discuss the implications for the total
flux balance on the entire sun. This work was supported by the SOI/MDI
project at Stanford and LMSAL (grant NAG5-3077).
Title: Dispersal of Magnetic Flux in the Quiet Solar Photosphere
Authors: Hagenaar, H. J.; Schrijver, C. J.; Title, A. M.; Shine, R. A.
Bibcode: 1999ApJ...511..932H
Altcode:
We study the random walk of magnetic flux concentrations on two
sequences of high-resolution magnetograms, observed with the Michelson
Doppler Imager on board SOHO. The flux contained in the concentrations
ranges from |Φ|=1018 Mx to |Φ|=1019 Mx, with
an average of |Φ|=2.5×1018 Mx. Larger concentrations tend
to move slower and live longer than smaller ones. On short timescales,
the observed mean-square displacements are consistent with a random
walk, characterized by a diffusion coefficient D(t<10 ks)=70-90
km2 s-1. On longer timescales, the diffusion
coefficient increases to D(t>30 ks)=200-250 km2
s-1, approaching the measurements for a five-day set of Big
Bear magnetograms, D~=250 km2 s-1. The transition
between the low and large diffusion coefficients is explained with
a model and simulations of the motions of test particles, subject to
random displacements on both the granular and supergranular scales,
simultaneously. In this model, the supergranular flow acts as a
negligible drift on short timescale, but dominates the granular
diffusion on longer timescales. We also investigate the possibility
that concentrations are temporarily confined, as if they were caught
in supergranular vertices, that form short-lived, relatively stable
environments. The best agreement of model and data is found for step
lengths of 0.5 and 8.5 Mm, associated evolution times of 14 minutes
and 24 hr, and a confinement time of no more than a few hours. On
our longest timescale, DSim(t>105)-->285
km2 s-1, which is the sum of the small- and
large-scale diffusion coefficients. Models of random walk diffusion on
the solar surface require a larger value: DWang=600+/-200
km2 s-1. One possible explanation for the
difference is a bias in our measurements to the longest lived, and
therefore slower concentrations in our data sets. Another possibility
is the presence of an additional, much larger diffusive scale.
Title: Flows and magnetic patterns on the solar surface
Authors: Hagenaar, Hermance Jacqueline Mandy
Bibcode: 1999PhDT.......279H
Altcode:
No abstract at ADS
Title: Modeling the distribution of magnetic fluxes in field
concentrations in a solar active region
Authors: Schrijver, Carolus J.; Title, Alan M.; Hagenaar, Hermance J.;
Shine, Richard A.
Bibcode: 1997SoPh..175..329S
Altcode:
Much of the magnetic field in solar and stellar photospheres is
arranged into clusters of `flux tubes', i.e., clustered into compact
areas in which the intrinsic field strength is approximately a
kilogauss. The flux concentrations are constantly evolving as they
merge with or annihilate against other concentrations, or fragment
into smaller concentrations. These processes result in the formation
of concentrations containing widely different fluxes. Schrijver et
al. (1997, Paper I) developed a statistical model for this distribution
of fluxes, and tested it on data for the quiet Sun. In this paper we
apply that model to a magnetic plage with an average absolute flux
density that is 25 times higher than that of the quiet network studied
in Paper I. The model result matches the observed distribution for the
plage region quite accurately. The model parameter that determines the
functional form of the distribution is the ratio of the fragmentation
and collision parameters. We conclude that this ratio is the same in the
magnetic plage and in quiet network. We discuss the implications of this
for (near-)surface convection, and the applicability of the model to
stars other than the Sun and as input to the study of coronal heating.
Title: Sustaining the Quiet Photospheric Network: The Balance of
Flux Emergence, Fragmentation, Merging, and Cancellation
Authors: Schrijver, Carolus J.; Title, Alan M.; van Ballegooijen,
Adriaan A.; Hagenaar, Hermance J.; Shine, Richard A.
Bibcode: 1997ApJ...487..424S
Altcode:
The magnetic field in the solar photosphere evolves as flux
concentrations fragment in response to sheared flows, merge when they
collide with others of equal polarity, or (partially) cancel against
concentrations of opposite polarity. Newly emerging flux replaces the
canceled flux. We present a quantitative statistical model that is
consistent with the histogram of fluxes contained in concentrations
of magnetic flux in the quiet network for fluxes exceeding ~2 ×
1018 Mx, as well as with estimated collision frequencies
and fragmentation rates. This model holds for any region with weak
gradients in the magnetic flux density at scales of more than a few
supergranules. We discuss the role of this dynamic flux balance (i)
in the dispersal of flux in the photosphere, (ii) in sustaining the
network-like pattern and mixed-polarity character of the network, (iii)
in the formation of unipolar areas covering the polar caps, and (iv) on
the potential formation of large numbers of very small concentrations
by incomplete cancellation. Based on the model, we estimate that as
much flux is cancelled as is present in quiet-network elements with
fluxes exceeding ~2 × 1018 Mx in 1.5 to 3 days, which is
compatible with earlier observational estimates. This timescale is
close to the timescale for flux replacement by emergence in ephemeral
regions, so that this appears to be the most important source of flux
for the quiet-Sun network; based on the model, we cannot put significant
constraints on the amount of flux that is injected on scales that are
substantially smaller than that of the ephemeral regions. We establish
that ephemeral regions originate in the convection zone and are not
merely the result of the reemergence of previously cancelled network
flux. We also point out that the quiet, mixed-polarity network is
generated locally and that only any relatively small polarity excess
is the result of flux dispersal from active regions.
Title: On the Dynamics of Magnetic Flux Concentrations in Quiet
Photospheric Network.
Authors: Sakai, J. I.; Ryutova, M.; Schrijver, K.; Shine, R.; Tarbell,
T.; Berger, T.; Title, A.; Hagenaar, H.
Bibcode: 1997SPD....28.0260S
Altcode: 1997BAAS...29..904S
Magnetic flux concentrations in the quiet photospheric network show
a complex dynamics which includes merging of colliding fluxes, the
"total" or partial cancellation of neighboring fluxes, fragmentation
and others. We propose a mechanism to explain the observed phenomena
based on the idea that magnetic flux concentrations in the photospheric
network are essentially non-collinear. We show that non-collinearity
of colliding fluxes leads to the whole new class of effects which are
observed; for example, the apparent cancellation of opposite polarity
fluxes turns into the formation of horizontal magnetic fluxes (which
later may appear as a new weaker bipoles) and is accompanied by the
shock formation and mini-flares. In the case of shock formation
the reconnection area becomes a source of the acoustic emission;
mini-flares may be seen as bright points. The energetics of these
processes strongly depends on geometry of "collision" and physical
parameters of colliding fluxes. For example, if colliding fluxes have
comparable and "small" cross sections, the reconnection results in
complete reorganization of their magnetic fields; if merging fluxes
are large enough or considerably different, magnetic flux may be
only partially reconnected and partially survived. Reconnection of
non-collinear equal polarity fluxes leads to the "scattering" processes
which include the fragmentation into several smaller fluxes if initially
colliding concentrations carried different amount of magnetic flux. We
give the example of numerical simulation for the case of merging and
fragmentation process occurring during the collision of collinear
"strong" and "weak" magnetic flux concentrations. The calculation
results shown to be consistent with observational data from both
the SOHO/MDI instrument and the Swedish Vacuum Solar Telescope on
La Palma. This research is supported by NASA contract NAG5-3077 at
Stanford University and the MDI contract PR 9162 at Lockheed.
Title: The Distribution of Cell Sizes of the Solar Chromospheric
Network
Authors: Hagenaar, Hermance J.; Schrijver, Carolus J.; Title, Alan M.
Bibcode: 1997ApJ...481..988H
Altcode:
This paper studies the cellular pattern of the supergranular network. We
present an algorithm to draw a surface-filling cell pattern on an
uninterrupted two-day sequence of Ca II K filtergrams with a 1 nm
bandpass. The 60° × 40° field of view contains both quiet and
enhanced network and plages. The algorithm uses a threshold-independent
method of steepest descent on spatially smoothed and time-averaged
images. We determine the distribution function of cell areas and
find a broad, asymmetric spectrum of areas. The distribution is
found to be invariant for different spatial smoothings if the cell
areas are normalized to a unit mean. It is this invariance that
leads us to believe we have determined the intrinsic distribution of
cell areas. Extrapolation of the average cell size to zero spatial
smoothing yields a characteristic cell diameter of L = 13-18 Mm. This
is roughly half the generally quoted supergranular length scale L ~
32 Mm as determined with autocorrelation methods. The difference
in characteristic cell size reflects the application of a different
measurement method: the autocorrelation method as used by Simon &
Leighton and others is preferentially weighted towards relatively
large cells. We find no significant dependence of cell size on local
magnetic flux density.
Title: A search for interaction between magnetic fields and
supergranular flows in the network based on MDI observations
Authors: Schrijver, C. J.; Shine, R. A.; Title, A. M.; Hagenaar,
H. J.; Hurlburt, N. E.; Tarbell, T. D.; Simon, G. W.
Bibcode: 1997SPD....28.0243S
Altcode: 1997BAAS...29..901S
We study the supergranular flow field and its temporal evolution in
the quiet Sun as observed with the Michelson Doppler Imager on board
SOHO. We use the intensity images to derive the flow fields using
local correlation tracking. The data sets span one to two days with a
one--minute cadence. We separate areas with a relatively high filling
factor for magnetic concentrations from areas with a low magnetic
filling factor in order to study to what extent the flows influence
the magnetic network in the quiet Sun and vice versa. This work is
supported by NASA Grant NAG5-3077 at Stanford and Lockheed Martin,
and by AFOSR and the Fellows Program of AF Phillips Lab at NSO/SP
Title: Dispersal of magnetic flux in the quiet network as observed
on a day-long magnetogram sequences observed with MDI on SOHO
Authors: Hagenaar, H. J.; Schrijver, C. J.; Shine, R. A.; Title, A. M.
Bibcode: 1997SPD....28.0244H
Altcode: 1997BAAS...29..901H
We study the dynamic behavior of magnetic flux elements in the quiet
solar network using high--resolution magnetograms observed with the
Michelson Doppler Imager on board SOHO. We track concentrations of
magnetic flux in uninterrupted time sequences spanning 20 to 45 hours
in order to study the dispersal of magnetic elements in the turbulent
photospheric flows. We measure the displacements and derive average
speeds as a function of time. The displacements are compared to a
random walk model. This work is supported by NASA Grant NAG5-3077 at
Stanford and Lockheed.
Title: The dynamic nature of the supergranular network
Authors: Title, A. M.; Schrijver, C. J.; van Ballegooijen, A. A.;
Hagenaar, H. J.; Shine, R. A.
Bibcode: 1997SPD....28.0242T
Altcode: 1997BAAS...29..900T
The magnetic field in the quiet solar photosphere evolves as flux
concentrations fragment in response to sheared flows, merge when they
collide with others of equal polarity, or (partially) cancel against
concentrations of opposite polarity. Newly emerging flux, mostly in
ephemeral regions, replaces the canceled flux in a matter of a few
days. We present a quantitative statistical model to describe the
resulting histogram of fluxes contained in concentrations of magnetic
flux in the quiet network. We discuss this dynamic flux balance with
respect to (i) the potential dispersal of flux in the photosphere
as a function of ephemeral-region properties, (ii) sustaining the
network--like pattern and mixed--polarity character of the network,
and (iii) the formation of unipolar areas covering the polar caps. We
establish that ephemeral regions are not the result of the re-emergence
of previously cancelled network flux. Moreover, their emergence cannot
be correlated to the emergence of active regions but must instead
be relatively homogeneous. We also point out that the bulk of the
quiet, mixed-polarity network is generated locally, and that only any
relatively small polarity excess is the result of flux dispersal from
active regions.
Title: On the Patterns of the Solar Granulation and Supergranulation
Authors: Schrijver, Carolus J.; Hagenaar, Hermance J.; Title, Alan M.
Bibcode: 1997ApJ...475..328S
Altcode:
We study the cellular patterns of the white light granulation and of the
chromospheric Ca II K supergranular network. We apply a gradient-based
tessellation algorithm to define the cell outlines. The geometry of
the patterns formed by the associated granular and supergranular
flows are very similar, in spite of the substantial difference in
length scale. We compare these patterns to generalized Voronoi foams
and conclude that both convective patterns are very nearly compatible
with an essentially random distribution of upflow centers, with the
downflow boundaries determined by the competing strengths of outflows
of neighboring upwellings. There appears to be a slight clustering
in upflow positions for the granulation, consistent with the granular
evolution. This slight preference for large granules to be surrounded
by somewhat smaller ones makes the granular and supergranular patterns
differ enough to allow a correct identification in three out of four
cases by eye. The model analogy suggests that the range in outflow
strengths is remarkably small. The patterns appear to be rather
insensitive to the details of the competing forces that establish the
pattern of the downflow network: similar patterns result under very
different conditions, so that little can be learned about the details
of the forces involved by studying the geometry of these patterns only.
Title: On the dynamics of magnetic flux concentrations in quiet
photospheric network.
Authors: Sakai, J. I.; Ryutova, M.; Schrijver, K.; Shine, R. A.;
Tarbell, T. D.; Berger, T. E.; Title, A. M.; Hagenaar, H. J.
Bibcode: 1997BAAS...29T.904S
Altcode:
No abstract at ADS
Title: Dynamics of the Chromospheric Network: Mobility, Dispersal,
and Diffusion Coefficients
Authors: Schrijver, Carolus J.; Shine, Richard A.; Hagenaar, Hermance
J.; Hurlburt, Neal E.; Title, Alan M.; Strous, Louis H.; Jefferies,
Stuart M.; Jones, Andrew R.; Harvey, John W.; Duvall, Thomas L., Jr.
Bibcode: 1996ApJ...468..921S
Altcode:
Understanding the physics behind the dispersal of photo spheric magnetic
flux is crucial to studies of magnetoconvection, dynamos, and stellar
atmospheric activity. The rate of flux dispersal is often quantified by
a diffusion coefficient, D. Published values of D differ by more than a
factor of 2, which is more than the uncertainties allow. We propose that
the discrepancies between the published values for D are the result of
a correlation between the mobility and flux content of concentrations of
magnetic flux. This conclusion is based on measurements of displacement
velocities of Ca II K mottles using an uninterrupted 2 day sequence
of filtergrams obtained at the South Pole near cycle minimum. We
transform the Ca II K intensity to an equivalent magnetic flux density
through a power-law relationship defined by a comparison with a nearly
simultaneously observed magnetogram. One result is that, wherever the
network is clearly defined in the filtergrams, the displacement vectors
of the mottles are preferentially aligned with the network, suggesting
that network-aligned motions are more important to field dispersal than
deformation of the network pattern by cell evolution. The rms value
of the inferred velocities, R = <|v|2>½,
decreases with increasing flux, Φ, contained in the mottles, from R
≍ 240 m s-1 down to 140 s-1. The value of R(Φ)
appears to be independent of the flux surrounding the concentration,
to the extreme that it does not matter whether the concentration is
in a plage or in the network. The determination of a proper effective
diffusion coefficient requires that the function R(Φ) be weighted
by the number density n(Φ) of mottles that contain a total flux. We
find that n(Φ) decreases exponentially with Φ and propose a model
of continual random splitting and merging of concentrations of flux to
explain this dependence. Traditional methods used to measure D tend to
be biased toward the larger, more sluggish flux concentrations. Such
methods neglect or underestimate the significant effects of the
relatively large number of the more mobile, smaller concentrations. We
argue that the effective diffusion coefficient for the dispersal of
photo spheric magnetic flux is ∼600 km2 s-1.
Title: What is the size scale of the solar supergranular network?
Authors: Hagenaar, H.; Schrijver, C.; Title, A.
Bibcode: 1996AAS...188.0201H
Altcode: 1996BAAS...28..820H
We developed an algorithm to outline the chromospheric network on a
2-day sequence of Ca II K observations made from the South Pole, in
order to study the sizes of supergranulation cells. We find an average
cell diameter that is substantially smaller than the generally quoted
value of 30--35 Mm, as first determined by Simon and Leighton (1964)
from autocorrelation curves of the line--of--sight velocities. We
argue that the autocorrelation method is preferentially weighted
towards large cells, which results in an estimated size that is
approximately 1.5 to 2 times larger than the true average cell
diameter. A comparable difference should occur in studies of the size
scale of the chromospheric network. In addition, we find that secondary
maxima to the autocorrelation peaks of the Doppler signal imply that
the correlation between cell size and flow velocity is weak at best. If
such a correlation should exist, it would be too weak to affect the
spacing of the secondary maxima of the autocorrelation function.