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Observations of linear polarization in a solar coronal loop prominence system near 6173 A  

Pascal Saint-Hilaire   Submitted: 2014-03-25 22:18

White-light observations by the Solar Dynamics Observatory's Helioseismic and Magnetic Imager of a loop-prominence system occurring in the aftermath of an X-class flare on 2013 May 13 near the eastern solar limb show a linearly polarized component, reaching up to 20% at an altitude of 33 Mm, about the maximal amount expected if the emission were due solely to Thomson scattering of photospheric light by the coronal material. The mass associated with the polarized component was 8.2x1014 g. At 15 Mm altitude, the brightest part of the loop was 3(±0.5)% linearly polarized, only about 20% of that expected from pure Thomson scattering, indicating the presence of an additional unpolarized component at wavelengths near Fe I (617.33 nm). We estimated the free electron density of the white-light loop system to possibly be as high as 1.8x1012 cm-3.

Authors: Pascal Saint-Hilaire, Jesper Schou, Juan-Carlos Martinez Oliveros, Hugh S. Hudson, Säm Krucker, Hazel Bain, Sebastien Couvidat.
Projects: RHESSI,SDO-HMI

Publication Status: Accepted for publication by ApJL.
Last Modified: 2014-03-26 12:36
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A decade of solar Type III radio bursts observed by the Nançay Radioheliograph 1998-2008  

Pascal Saint-Hilaire   Submitted: 2012-11-15 18:22

We present a statistical survey of almost 10 000 radio Type III bursts observed by the Nançay Radioheliograph from 1998 to 2008, covering nearly a full solar cycle. In particular, sources sizes, positions, and fluxes were examined. We find an east-west asymmetry in source positions which could be attributed to a 6(±)1 degree eastward tilt of the magnetic field, that source FWHM sizes s roughly follow a solar-cycle averaged distribution dN/ds = 14 f-3.3 s-4 arcmin-1 day-1, and that source fluxes closely follow a solar-cycle averaged dN/dS = 0.34 f-2.9 S-1.7 sfu-1 day-1 distribution (when f is in GHz, s in arcmin, and S in sfu). Fitting a barometric density profile yields a temperature of 0.6 MK, while a solar wind-like (propto h-2) density profile yields a density of 1.2x106 cm-3 at an altitude of 1 RS, assuming harmonic emission. Finally, we found that the solar-cycle averaged radiated Type III energy could be similar in magnitude to that radiated by nanoflares via non-thermal bremsstrahlung processes, and we hint at the possibility that escaping electron beams might carry as much energy away from the corona as is introduced into it by accelerated nanoflare electrons.

Authors: Pascal Saint-Hilaire, Nicole Vilmer, Alain Kerdraon
Projects: Nançay Radioheliograph

Publication Status: Accepted for publication in ApJ.
Last Modified: 2012-11-20 20:24
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Allen Telescope Array Multi-Frequency Observations of the Sun  

Pascal Saint-Hilaire   Submitted: 2011-11-29 22:49

We present the first observations of the Sun with the Allen Telescope Array(ATA). We used up to six frequencies, from 1.43 to 6 GHz, and baselines from 6to 300 m. To our knowledge, these are the fi?rst simultaneous multifrequencyfull-Sun maps obtained at microwave frequencies without mosaicing. Theobservations took place when the Sun was relatively quiet, although at leastone active region was present each time. We present multi-frequency fluxbudgets for each sources on the Sun. Outside of active regions, assumingoptically thin bremsstrahlung (free-free) coronal emission on top of anoptically thick ~10 000 K chromosphere, the multi-frequency information can becondensed into a single, frequency-independent, 'coronal bremsstrahlungcontribution function' [EM/sqrt(T)] map. This technique allows the separationof the physics of emission as well as a measurement of the density structure ofthe corona. Deviations from this simple relationship usually indicate thepresence of an additional gyroresonance-emission component, as is typical inactive regions.

Authors: P. Saint-Hilaire, G.J. Hurford, G. Keating, G.C. Bower, C. Gutierrez-Kraybill
Projects: None

Publication Status: Accepted for publication in Solar Physics
Last Modified: 2011-11-30 08:15
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Statistically-derived flaring chromospheric-coronal density structure from non-thermal X-ray observations of the Sun  

Pascal Saint-Hilaire   Submitted: 2010-08-26 18:27

For the first time, we have used RHESSI's spatial and energy resolution to determine the combined chromospheric and coronal density pro file of the flaring solar atmosphere in a statistical manner, using a dataset of 838 flares observable in hard X-rays above 25 keV. Assuming the thick-target beam model, our ''average flaring atmosphere'' was found to have density scale heights of 131±16 km at low altitudes (chromosphere, up to ~1-1.5 Mm above photosphere), and of 5-6 Mm at high altitudes (corona, above ~2-3 Mm). Assuming a unit step change in ionization level, modeling yields a height of 1.3±0.2 Mm for the transition between fully neutral to fully-ionized atmosphere. Furthermore, centroids of emission above 50 keV, produced by electrons of similar or higher energies, are located mostly in a small region ~0.5 Mm in vertical extent, where neutral densities are beyond 3x1013 cm-3.

Authors: P. Saint-Hilaire, Säm Krucker, Robert P. Lin
Projects: RHESSI

Publication Status: ApJ, accepted on 2010/08/18.
Last Modified: 2010-08-27 09:01
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X-RAY EMISSION FROM THE BASE OF A CURRENT SHEET IN THE WAKE OF A CORONAL MASS EJECTION  

Pascal Saint-Hilaire   Submitted: 2009-05-20 11:29

Following a coronal mass ejection (CME) which started on 2002 November 26, RHESSI observed for 12 hr an X-ray source above the solar limb, at altitudes between 0.1 and 0.3 R_S above the photosphere. The Geostationary Operational Environmental Satellite baseline was remarkably high throughout this event. The X-ray source?s temperature peaked around 10?11 MK, and its emission measure increased throughout this time interval. Higher up, at 0.7 R_S, hot (initially >8 MK) plasma has been observed by Ultraviolet Coronograph Spectrometer on Solar and Heliospheric Observatory for 2.3 days. This hot plasma was interpreted as the signature of a current sheet (CS) trailing the CME. The thermal energy content of the X-ray source is more than an order of magnitude larger than in the CS. Hence, it could be the source of the hot plasma in the CS, although CS heating by magnetic reconnection within it cannot be discounted. To better characterize the X-ray spectrum, we have used novel techniques (back-projection-based and visibility-based) for long integration (several hours) imaging spectroscopy. There is no observed non-thermal hard X-ray bremsstrahlung emission, leading to the conclusion that there is either very little particle acceleration occurring in the vicinity of this post-flare X-ray source, or that either the photon spectral index would have had to be uncharacteristically (in flare parlance) high (γ >= 8) and/or the low-energy cutoff very low (Ec <= 6 keV).

Authors: P. Saint-Hilaire, S. Krucker, and R.P. Lin
Projects: RHESSI

Publication Status: Accepted for publication on 2009/04/24.
Last Modified: 2009-05-20 13:26
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Thermal and non-thermal energies in solar flares  

Pascal Saint-Hilaire   Submitted: 2005-03-03 08:33

The energy of the thermal flare plasma and the kinetic energy of the non-thermal electrons in 14 hard X-ray peaks from 9 medium-sized solar flares have been determined from RHESSI observations. The emissions have been carefully separated in the spectrum. The turnover or cutoff in the low-energy distribution of electrons has been studied by simulation and fitting, yielding a reliable lower limit to the non-thermal energy. It remains the largest contribution to the error budget. Other effects, such as albedo, non-uniform target ionization, hot target, and cross-sections on the spectrum have been studied. The errors of the thermal energy are about equally as large. They are due to the estimate of the flare volume, the assumption of the filling factor, and energy losses. Within a flare, the non-thermal/thermal ratio increases with accumulation time, as expected from loss of thermal energy due to radiative cooling or heat conduction. Our analysis suggests that the thermal and non-thermal energies are of the same magnitude. This surprising result may be interpreted by an efficient conversion of non-thermal energy to hot flare plasma.

Authors: Pascal Saint-Hilaire and Arnold O. Benz
Projects: RHESSI

Publication Status: To be published in Astronomy & Astrophysics
Last Modified: 2005-03-03 08:33
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The RHESSI Experimental Data Center  

Pascal Saint-Hilaire   Submitted: 2002-09-19 15:18

The RHESSI Experimental Data Center (HEDC) at ETH Zurich aims to facilitate the use of RHESSI data. It explores new ways to speed up browsing and selecting events such as solar flares. HEDC provides pre-processed data for on-line use and allows basic data processing remotely over the Internet. In this article, we describe the functionality and contents of HEDC, as well as first experiences by users. HEDC can be accessed at http://www.hedc.ethz.ch.

Authors: Pascal Saint-Hilaire, Christoph von Praun, Etzard Stolte,Gustavo Alonso, Arnold O. Benz and Thomas Gross
Projects:

Publication Status: Solar Physics (in press)
Last Modified: 2002-09-23 14:36
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Abstracts by Author
Observations of linear polarization in a solar coronal loop prominence system near 6173 A
A decade of solar Type III radio bursts observed by the Nancay Radioheliograph 1998-2008
Allen Telescope Array Multi-Frequency Observations of the Sun
Statistically-derived flaring chromospheric-coronal density structure from non-thermal X-ray observations of the Sun
X-RAY EMISSION FROM THE BASE OF A CURRENT SHEET IN THE WAKE OF A CORONAL MASS EJECTION
Thermal and non-thermal energies in solar flares
The RHESSI Experimental Data Center

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