Thomson scattering in the lower corona in the presence of sunspots |
|
Pascal Saint-Hilaire Submitted: 2021-10-12 20:54
Polarized scattered light from low (few tens of Mm altitudes) coronal transients has been recently reported in SDO/HMI observations. In a classic paper, Minnaert (1930) provided an analytic theory of polarization via electron scattering in the corona. This work assumed axisymmetric input from the photosphere with a single-parameter limb-darkening function. This diagnostic has recently been used to estimate free electron number and mass of HMI transients
near the solar limb, but it applies equally well to any coronal material, at any height. Here we extend this work numerically to incorporate sunspots, which can strongly effect the polarization properties of the scattered light in the low corona.
Sunspot effects are explored first for axisymmetric model cases, and then applied to the full description of two sunspot groups as observed by HMI. We find that, 1) As previously reported by Minnaert, limb darkening has a strong influence, usually increasing the level of linear polarization tangential to the limb; 2) Unsurprisingly, the effects of the sunspot generally increase at the lower
scatterer altitudes, the larger the sunspot is, and the closer to their center the scatterer subpoint is; 3) Assuming the Stokes Q>0 basis to be tangential to the limb, sunspots typically decrease the Stokes Q/I polarization and the perceived electron densities below the spotless case, sometimes dramatically; 4) Typically, a sizeable non-zero Stokes U/I polarization component will appear when a sunspot’s influence becomes non-negligible. However, that is not true in rare cases of extreme symmetry (e.g. scattering mass at the center of an axisymetric sunspot). The tools
developed here are generally applicable to an arbitrary image input.
Authors: Pascal Saint-Hilaire, Juan-Carlos Martinez Oliveros, Hugh S. Hudson
Projects: None
|
Publication Status: Accepted in ApJ
Last Modified: 2021-10-13 13:06
|
 
 
|
|
Precise Formation-Flying Telescope in Target-Centric Orbit: the Solar Case |
|
Pascal Saint-Hilaire Submitted: 2020-09-24 09:50
We present the general concept of a telescope with optics and detectors mounted on two separate spacecrafts, in orbit around the telescope’s target (scopocentric or target-centric orbit), and using propulsion to maintain the Target-Optics-Detector alignment and Optics-Detector distance. Specifically, we study the case of such a telescope with the Sun as the target, orbiting at ∼1 AU. We present a simple differential acceleration budget for maintaining Target-Optics-Detector alignment and Optics-Detector distance, backed by simulations of the orbital dynamics, including solar radiation pressure and influence of the planets. Of prime interest are heliocentric orbits (such as Earth-trailing/leading orbits or Distant Retrograde Orbits), where thrust requirement to maintain formation is primarily in a single direction (either sun-ward or anti-sunward), can be quite minuscule (a few m/s/year), and preferably met by constant-thrust engines such as solar electric propulsion or even by solar sailing via simple extendable and/or orientable flaps or rudders.
Authors: Saint-Hilaire, Pascal; Marchese, Jeffrey E.
Projects: None
|
Publication Status: Journal of the Astronautical Sciences
Last Modified: 2020-09-28 12:25
|
 
 
|
|
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
|
 
 
|
|
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
|
 
 
|
|
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
|
 
 
|
|
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 profile 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
|
 
 
|
|
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
|
 
 
|
|
|
Key
|
 | Go to main E-Print page. |
 | Download Preprint. |
 | Submitters Homepage. |
 | Edit Entry. |
 | Delete abstract. |
|
|
|