Publications (2)0 Total impact
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ABSTRACT: We describe observations of a white-light flare (SOL2011-02-24T07:35:00,
M3.5) close to the limb of the Sun, from which we obtain estimates of the
heights of the optical continuum sources and those of the associated hard X-ray
sources.For this purpose we use hard X-ray images from the Reuven Ramaty High
Energy Spectroscopic Imager (RHESSI), and optical images at 6173 \AA from the
Solar Dynamics Observatory (SDO). We find that the centroids of the
impulsive-phase emissions in white light and hard X-rays (30-80 keV) match
closely in central distance (angular displacement from Sun center), within
uncertainties of order 0.2". This directly implies a common source height for
these radiations, strengthening the connection between visible flare continuum
formation and the accelerated electrons. We also estimate the absolute heights
of these emissions, as vertical distances from Sun center. Such a direct
estimation has not been done previously, to our knowledge. Using a simultaneous
195 \AA image from the Solar-Terrestrial RElations Observatory (STEREO-B)
spacecraft to identify the heliographic coordinates of the flare footpoints, we
determine mean heights above the photosphere (as normally defined; \tau = 1 at
5000 \AA) of 305 \pm 170 km and 195 \pm 70 km, respectively, for the centroids
of the hard X-ray (HXR) and white light (WL) footpoint sources of the flare.
These heights are unexpectedly low in the atmosphere, and are consistent with
the expected locations of \tau = 1 for the 6173 \AA and the ~40 keV photons
observed, respectively.
06/2012;
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ABSTRACT: Low-frequency solar and interplanetary radio bursts are generated at
frequencies below the ionospheric plasma cutoff and must therefore be measured
in space, with deployable antenna systems. The problem of measuring both the
general direction and polarization of an electromagnetic source is commonly
solved by iterative fitting methods such as linear regression that deal
simultaneously with both directional and polarization parameters. We have
developed a scheme that separates the problem of deriving the source direction
from that of determining the polarization, avoiding iteration in a
multi-dimensional manifold. The crux of the method is to first determine the
source direction independently of concerns as to its polarization. Once the
source direction is known, its direct characterization in terms of Stokes
vectors in a single iteration if desired, is relatively simple. This study
applies the source-direction determination to radio signatures of flares
received by STEREO. We studied two previously analyzed radio type III bursts
and found that the results of the eigenvalue decomposition technique are
consistent with those obtained previously by Reiner et al. (Solar Phys. 259,
255, 2009). For the type III burst observed on 7 December 2007, the difference
in travel times from the derived source location to STEREO A and B is the same
as the difference in the onset times of the burst profiles measured by the two
spacecraft. This is consistent with emission originating from a single,
relatively compact source. For the second event of 29 January 2008, the
relative timing does not agree, suggesting emission from two sources separated
by 0.1 AU, or perhaps from an elongated region encompassing the apparent source
locations.
05/2012;
