252 research outputs found
Modelling and Interpreting The Effects of Spatial Resolution on Solar Magnetic Field Maps
Different methods for simulating the effects of spatial resolution on
magnetic field maps are compared, including those commonly used for
inter-instrument comparisons. The investigation first uses synthetic data, and
the results are confirmed with {\it Hinode}/SpectroPolarimeter data. Four
methods are examined, one which manipulates the Stokes spectra to simulate
spatial-resolution degradation, and three "post-facto" methods where the
magnetic field maps are manipulated directly. Throughout, statistical
comparisons of the degraded maps with the originals serve to quantify the
outcomes. Overall, we find that areas with inferred magnetic fill fractions
close to unity may be insensitive to optical spatial resolution; areas of
sub-unity fill fractions are very sensitive. Trends with worsening spatial
resolution can include increased average field strength, lower total flux, and
a field vector oriented closer to the line of sight. Further-derived quantities
such as vertical current density show variations even in areas of high average
magnetic fill-fraction. In short, unresolved maps fail to represent the
distribution of the underlying unresolved fields, and the "post-facto" methods
generally do not reproduce the effects of a smaller telescope aperture. It is
argued that selecting a method in order to reconcile disparate spatial
resolution effects should depend on the goal, as one method may better preserve
the field distribution, while another can reproduce spatial resolution
degradation. The results presented should help direct future inter-instrument
comparisons.Comment: Accepted for publication in Solar Physics. The final publication
(including full-resolution figures) will be available at
http://www.springerlink.co
The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Optimization of the Spectral Line Inversion Code
The Very Fast Inversion of the Stokes Vector (VFISV) is a Milne-Eddington
spectral line inversion code used to determine the magnetic and thermodynamic
parameters of the solar photosphere from observations of the Stokes vector in
the 6173 A Fe I line by the Helioseismic and Magnetic Imager (HMI) onboard the
Solar Dynamics Observatory (SDO). We report on the modifications made to the
original VFISV inversion code in order to optimize its operation within the HMI
data pipeline and provide the smoothest solution in active regions. The changes
either sped up the computation or reduced the frequency with which the
algorithm failed to converge to a satisfactory solution. Additionally, coding
bugs which were detected and fixed in the original VFISV release, are reported
here.Comment: Accepted for publication in Solar Physic
The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: SHARPs -- Space-weather HMI Active Region Patches
A new data product from the Helioseismic and Magnetic Imager (HMI) onboard
the Solar Dynamics Observatory (SDO) called Space-weather HMI Active Region
Patches (SHARPs) is now available. SDO/HMI is the first space-based instrument
to map the full-disk photospheric vector magnetic field with high cadence and
continuity. The SHARP data series provide maps in patches that encompass
automatically tracked magnetic concentrations for their entire lifetime; map
quantities include the photospheric vector magnetic field and its uncertainty,
along with Doppler velocity, continuum intensity, and line-of-sight magnetic
field. Furthermore, keywords in the SHARP data series provide several
parameters that concisely characterize the magnetic-field distribution and its
deviation from a potential-field configuration. These indices may be useful for
active-region event forecasting and for identifying regions of interest. The
indices are calculated per patch and are available on a twelve-minute cadence.
Quick-look data are available within approximately three hours of observation;
definitive science products are produced approximately five weeks later. SHARP
data are available at http://jsoc.stanford.edu and maps are available in either
of two different coordinate systems. This article describes the SHARP data
products and presents examples of SHARP data and parameters.Comment: 27 pages, 7 figures. Accepted to Solar Physic
Response to "Comment on `Resolving the 180deg Ambiguity in Solar Vector Magnetic Field Data: Evaluating the Effects of Noise, Spatial Resolution, and Method Assumptions'"
We address points recently discussed in Georgoulis (2011) in reference to
Leka et al. (2009b). Most importantly, we find that the results of Georgoulis
(2011) support a conclusion of Leka et al. (2009b): that limited spatial
resolution and the presence of unresolved magnetic structures can challenge
ambiguity- resolution algorithms. Moreover, the findings of both Metcalf et al.
(2006) and Leka et al. (2009b) are confirmed in Georgoulis (2011): a method's
performance can be diminished when the observed field fails to conform to that
method's assumptions. The implication of boundaries in models of solar magnetic
structures is discussed; we confirm that the distribution of the field
components in the model used in Leka et al. (2009b) is closer to what is
observed on the Sun than what is proposed in Georgoulis (2011). It is also
shown that method does matter with regards to simulating limited spatial
resolution and avoiding an inadvertent introduction of bias. Finally, the
assignment of categories to data- analysis algorithms is revisited; we argue
that assignments are only useful and elucidating when used appropriately.Comment: Accepted for publication in Solar Physic
Multiresolution analysis of active region magnetic structure and its correlation with the Mt. Wilson classification and flaring activity
Two different multi-resolution analyses are used to decompose the structure
of active region magnetic flux into concentrations of different size scales.
Lines separating these opposite polarity regions of flux at each size scale are
found. These lines are used as a mask on a map of the magnetic field gradient
to sample the local gradient between opposite polarity regions of given scale
sizes. It is shown that the maximum, average and standard deviation of the
magnetic flux gradient for alpha, beta, beta-gamma and beta-gamma-delta active
regions increase in the order listed, and that the order is maintained over all
length-scales. This study demonstrates that, on average, the Mt. Wilson
classification encodes the notion of activity over all length-scales in the
active region, and not just those length-scales at which the strongest flux
gradients are found. Further, it is also shown that the average gradients in
the field, and the average length-scale at which they occur, also increase in
the same order. Finally, there are significant differences in the gradient
distribution, between flaring and non-flaring active regions, which are
maintained over all length-scales. It is also shown that the average gradient
content of active regions that have large flares (GOES class 'M' and above) is
larger than that for active regions containing flares of all flare sizes; this
difference is also maintained at all length-scales.Comment: Accepted for publication in Solar Physic
Comment on "Resolving the 180-deg Ambiguity in Solar Vector Magnetic Field Data: Evaluating the Effects of Noise, Spatial Resolution, and Method Assumptions"
In a recent paper, Leka at al. (Solar Phys. 260, 83, 2009)constructed a
synthetic vector magnetogram representing a three-dimensional magnetic
structure defined only within a fraction of an arcsec in height. They rebinned
the magnetogram to simulate conditions of limited spatial resolution and then
compared the results of various azimuth disambiguation methods on the resampled
data. Methods relying on the physical calculation of potential and/or
non-potential magnetic fields failed in nearly the same, extended parts of the
field of view and Leka et al. (2009) attributed these failures to the limited
spatial resolution. This study shows that the failure of these methods is not
due to the limited spatial resolution but due to the narrowly defined test
data. Such narrow magnetic structures are not realistic in the real Sun.
Physics-based disambiguation methods, adapted for solar magnetic fields
extending to infinity, are not designed to handle such data; hence, they could
only fail this test. I demonstrate how an appropriate limited-resolution
disambiguation test can be performed by constructing a synthetic vector
magnetogram very similar to that of Leka et al. (2009) but representing a
structure defined in the semi-infinite space above the solar photosphere. For
this magnetogram I find that even a simple potential-field disambiguation
method manages to resolve the ambiguity very successfully, regardless of
limited spatial resolution. Therefore, despite the conclusions of Leka et al.
(2009), a proper limited-spatial-resolution test of azimuth disambiguation
methods is yet to be performed in order to identify the best ideas and
algorithms.Comment: Solar Physics, in press (19 pp., 5 figures, 2 tables
What is the relationship between photospheric flow fields and solar flares?
We estimated photospheric velocities by separately applying the Fourier Local
Correlation Tracking (FLCT) and Differential Affine Velocity Estimator (DAVE)
methods to 2708 co-registered pairs of SOHO/MDI magnetograms, with nominal
96-minute cadence and ~2" pixels, from 46 active regions (ARs) from 1996-1998
over the time interval t45 when each AR was within 45^o of disk center. For
each magnetogram pair, we computed the average estimated radial magnetic field,
B; and each tracking method produced an independently estimated flow field, u.
We then quantitatively characterized these magnetic and flow fields by
computing several extensive and intensive properties of each; extensive
properties scale with AR size, while intensive properties do not depend
directly on AR size. Intensive flow properties included moments of speeds,
horizontal divergences, and radial curls; extensive flow properties included
sums of these properties over each AR, and a crude proxy for the ideal Poynting
flux, the total |u| B^2. Several magnetic quantities were also computed,
including: total unsigned flux; a measure of the amount of unsigned flux near
strong-field polarity inversion lines, R; and the total B^2. Next, using
correlation and discriminant analysis, we investigated the associations between
these properties and flares from the GOES flare catalog, when averaged over
both t45 and shorter time windows, of 6 and 24 hours. We found R and total |u|
B^2 to be most strongly associated with flares; no intensive flow properties
were strongly associated with flares.Comment: 57 pages, 13 figures; revised content; added URL to manuscript with
higher-quality image
The Influence of Spatial Resolution on Nonlinear Force-Free Modeling
The nonlinear force-free field (NLFFF) model is often used to describe the
solar coronal magnetic field, however a series of earlier studies revealed
difficulties in the numerical solution of the model in application to
photospheric boundary data. We investigate the sensitivity of the modeling to
the spatial resolution of the boundary data, by applying multiple codes that
numerically solve the NLFFF model to a sequence of vector magnetogram data at
different resolutions, prepared from a single Hinode/SOT-SP scan of NOAA Active
Region 10978 on 2007 December 13. We analyze the resulting energies and
relative magnetic helicities, employ a Helmholtz decomposition to characterize
divergence errors, and quantify changes made by the codes to the vector
magnetogram boundary data in order to be compatible with the force-free model.
This study shows that NLFFF modeling results depend quantitatively on the
spatial resolution of the input boundary data, and that using more highly
resolved boundary data yields more self-consistent results. The free energies
of the resulting solutions generally trend higher with increasing resolution,
while relative magnetic helicity values vary significantly between resolutions
for all methods. All methods require changing the horizontal components, and
for some methods also the vertical components, of the vector magnetogram
boundary field in excess of nominal uncertainties in the data. The solutions
produced by the various methods are significantly different at each resolution
level. We continue to recommend verifying agreement between the modeled field
lines and corresponding coronal loop images before any NLFFF model is used in a
scientific setting.Comment: Accepted to ApJ; comments/corrections to this article are welcome via
e-mail, even after publicatio
The X10 Flare on 2003 October 29: Triggered by Magnetic Reconnection between Counter-Helical Fluxes?
Vector magnetograms taken at Huairou Solar Observing Station (HSOS) and Mees
Solar Observatory (MSO) reveal that the super active region (AR) NOAA 10486 was
a complex region containing current helicity flux of opposite signs. The main
positive sunspots were dominated by negative helicity fields, while positive
helicity patches persisted both inside and around the main positive sunspots.
Based on a comparison of two days of deduced current helicity density,
pronounced changes were noticed which were associated with the occurrence of an
X10 flare that peaked at 20:49 UT, 2003 October 29. The average current
helicity density (negative) of the main sunspots decreased significantly by
about 50. Accordingly, the helicity densities of counter-helical patches
(positive) were also found to decay by the same proportion or more. In
addition, two hard X-ray (HXR) `footpoints' were observed by the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI} during the flare in the 50-100
keV energy range. The cores of these two HXR footpoints were adjacent to the
positions of two patches with positive current helicity which disappeared after
the flare. This strongly suggested that the X10 flare on 2003 Oct. 29 resulted
from reconnection between magnetic flux tubes having opposite current helicity.
Finally, the global decrease of current helicity in AR 10486 by ~50% can be
understood as the helicity launched away by the halo coronal mass ejection
(CME) associated with the X10 flare.Comment: Solar Physics, 2007, in pres
Two-Dimensional Spectroscopy of Photospheric Shear Flows in a Small delta Spot
In recent high-resolution observations of complex active regions,
long-lasting and well-defined regions of strong flows were identified in major
flares and associated with bright kernels of visible, near-infrared, and X-ray
radiation. These flows, which occurred in the proximity of the magnetic neutral
line, significantly contributed to the generation of magnetic shear. Signatures
of these shear flows are strongly curved penumbral filaments, which are almost
tangential to sunspot umbrae rather than exhibiting the typical radial
filamentary structure. Solar active region NOAA 10756 was a moderately complex,
beta-delta sunspot group, which provided an opportunity to extend previous
studies of such shear flows to quieter settings. We conclude that shear flows
are a common phenomenon in complex active regions and delta spots. However,
they are not necessarily a prerequisite condition for flaring. Indeed, in the
present observations, the photospheric shear flows along the magnetic neutral
line are not related to any change of the local magnetic shear. We present
high-resolution observations of NOAA 10756 obtained with the 65-cm vacuum
reflector at Big Bear Solar Observatory (BBSO). Time series of
speckle-reconstructed white-light images and two-dimensional spectroscopic data
were combined to study the temporal evolution of the three-dimensional vector
flow field in the beta-delta sunspot group. An hour-long data set of consistent
high quality was obtained, which had a cadence of better than 30 seconds and
sub-arcsecond spatial resolution.Comment: 23 pages, 6 gray-scale figures, 4 color figures, 2 tables, submitted
to Solar Physic
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