2,611 research outputs found
Bootstrapping the Coronal Magnetic Field with STEREO: I. Unipolar Potential Field Modeling
We investigate the recently quantified misalignment of between the 3-D geometry of stereoscopically triangulated
coronal loops observed with STEREO/EUVI (in four active regions) and
theoretical (potential or nonlinear force-free) magnetic field models
extrapolated from photospheric magnetograms. We develop an efficient method of
bootstrapping the coronal magnetic field by forward-fitting a parameterized
potential field model to the STEREO-observed loops. The potential field model
consists of a number of unipolar magnetic charges that are parameterized by
decomposing a photospheric magnetogram from MDI. The forward-fitting method
yields a best-fit magnetic field model with a reduced misalignment of
. We evaluate also stereoscopic
measurement errors and find a contribution of , which constrains the residual misalignment to
, which is likely
due to the nonpotentiality of the active regions. The residual misalignment
angle of the potential field due to nonpotentiality is found to
correlate with the soft X-ray flux of the active region, which implies a
relationship between electric currents and plasma heating.Comment: 12 figures, manuscript submitted to ApJ, 2010 Apr 2
Coronal Loop Oscillations Observed with AIA - Kink-Mode with Cross-Sectional and Density Oscillations
A detailed analysis of a coronal loop oscillation event is presented, using
data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO) for the first time. The loop oscillation event occurred on
2010 Oct 16, 19:05-19:35 UT, was triggered by an M2.9 GOES-class flare, located
inside a highly inclined cone of a narrow-angle CME. This oscillation event had
a number of unusual features: (i) Excitation of kink-mode oscillations in
vertical polarization (in the loop plane); (ii) Coupled cross-sectional and
density oscillations with identical periods; (iii) no detectable kink amplitude
damping over the observed duration of four kink-mode periods ( min);
(iv) multi-loop oscillations with slightly () different periods;
and (v) a relatively cool loop temperature of MK. We employ a
novel method of deriving the electron density ratio external and internal to
the oscillating loop from the ratio of Alfv\'enic speeds deduced from the flare
trigger delay and the kink-mode period, i.e.,
. The coupling of the kink mode and
cross-sectional oscillations can be explained as a consequence of the loop
length variation in the vertical polarization mode. We determine the exact
footpoint locations and loop length with stereoscopic triangulation using
STEREO/EUVI-A data. We model the magnetic field in the oscillating loop using
HMI/SDO magnetogram data and a potential field model and find agreement with
the seismological value of the magnetic field, G, within a
factor of two.Comment: ApJ (in press, accepted May 10, 2011
Observational evidence of resonantly damped propagating kink waves in the solar corona
In this Letter we establish clear evidence for the resonant absorption
damping mechanism by analyzing observational data from the novel Coronal
Multi-Channel Polarimeter (CoMP). This instrument has established that in the
solar corona there are ubiquitous propagating low amplitude (1 km
s) Alfv\'{e}nic waves with a wide range of frequencies. Realistically
interpreting these waves as the kink mode from magnetohydrodynamic (MHD) wave
theory, they should exhibit a frequency dependent damping length due to
resonant absorption, governed by the TGV relation showing that transversal
plasma inhomogeneity in coronal magnetic flux tubes causes them to act as
natural low-pass filters. It is found that observed frequency dependence on
damping length (up to about 8 mHz) can be explained by the kink wave
interpretation and furthermore, the spatially averaged equilibrium parameter
describing the length scale of transverse plasma density inhomogeneity over a
system of coronal loops is consistent with the range of values estimated from
TRACE observations of standing kink modes
Morphological feature extraction for statistical learning with applications to solar image data
Abstract: Many areas of science are generating large volumes of digital image data. In order to take full advantage of the high-resolution and high-cadence images modern technology is producing, methods to automatically process and analyze large batches of such images are needed. This involves reducing complex images to simple representations such as binary sketches or numerical summaries that capture embedded scientific information. Using techniques derived from mathematical morphology, we demonstrate how to reduce solar images into simple ‘sketch ’ representations and numerical summaries that can be used for statistical learning. We demonstrate our general techniques on two specific examples: classifying sunspot groups and recognizing coronal loop structures. Our methodology reproduces manual classifications at an overall rate of 90 % on a set of 119 magnetogram and white light images of sunspot groups. We also show that our methodology is competitive with other automated algorithms at producing coronal loop tracings and demonstrate robustness through noise simulations. 2013 Wile
Solar Stereoscopy with STEREO/EUVI A and B spacecraft from small (6 deg) to large (170 deg) spacecraft separation angles
We performed for the first time stereoscopic triangulation of coronal loops
in active regions over the entire range of spacecraft separation angles
(, and
). The accuracy of stereoscopic correlation depends mostly on the
viewing angle with respect to the solar surface for each spacecraft, which
affects the stereoscopic correspondence identification of loops in image pairs.
From a simple theoretical model we predict an optimum range of , which is also experimentally confirmed. The best
accuracy is generally obtained when an active region passes the central
meridian (viewed from Earth), which yields a symmetric view for both STEREO
spacecraft and causes minimum horizontal foreshortening. For the extended
angular range of we find a mean 3D
misalignment angle of of stereoscopically
triangulated loops with magnetic potential field models, and for a force-free field model, which is partly caused by
stereoscopic uncertainties . We predict optimum
conditions for solar stereoscopy during the time intervals of 2012--2014,
2016--2017, and 2021--2023.Comment: Solar Physics, (in press), 22 pages, 9 figure
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