THEMIS as particle detector: Spectropolarimetry of solar flares

The progressive phases of three solar flares have beenobserv ed with THEMIS in July 2000, using the multiline spectropolarimetric MTR mode. A preliminary analysis of the characteristics of the polarization of the Hα and Hβ lines observed at the beginning of the progressive phase of one of these flares is presented

Imaging Spectroscopy of a White-Light Solar Flare

We report observations of a white-light solar flare (SOL2010-06-12T00:57, M2.0) observed by the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO) and the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). The HMI data give us the first space-based high-resolution imaging spectroscopy of a white-light flare, including continuum, Doppler, and magnetic signatures for the photospheric FeI line at 6173.34{\AA} and its neighboring continuum. In the impulsive phase of the flare, a bright white-light kernel appears in each of the two magnetic footpoints. When the flare occurred, the spectral coverage of the HMI filtergrams (six equidistant samples spanning \pm172m{\AA} around nominal line center) encompassed the line core and the blue continuum sufficiently far from the core to eliminate significant Doppler crosstalk in the latter, which is otherwise a possibility for the extreme conditions in a white-light flare. RHESSI obtained complete hard X-ray and \Upsilon-ray spectra (this was the first \Upsilon-ray flare of Cycle 24). The FeI line appears to be shifted to the blue during the flare but does not go into emission; the contrast is nearly constant across the line profile. We did not detect a seismic wave from this event. The HMI data suggest stepwise changes of the line-of-sight magnetic field in the white-light footpoints.Comment: 14 pages, 7 figures, Accepted by Solar Physic

X-Ray Spectroscopy of Stars

(abridged) Non-degenerate stars of essentially all spectral classes are soft X-ray sources. Low-mass stars on the cooler part of the main sequence and their pre-main sequence predecessors define the dominant stellar population in the galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solar corona. X-ray emission from cool stars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronal plasma. Coronal structure, its thermal stratification and geometric extent can be interpreted based on various spectral diagnostics. New features have been identified in pre-main sequence stars; some of these may be related to accretion shocks on the stellar surface, fluorescence on circumstellar disks due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot stars clearly dominate the interaction with the galactic interstellar medium: they are the main sources of ionizing radiation, mechanical energy and chemical enrichment in galaxies. High-energy emission permits to probe some of the most important processes at work in these stars, and put constraints on their most peculiar feature: the stellar wind. Here, we review recent advances in our understanding of cool and hot stars through the study of X-ray spectra, in particular high-resolution spectra now available from XMM-Newton and Chandra. We address issues related to coronal structure, flares, the composition of coronal plasma, X-ray production in accretion streams and outflows, X-rays from single OB-type stars, massive binaries, magnetic hot objects and evolved WR stars.Comment: accepted for Astron. Astrophys. Rev., 98 journal pages, 30 figures (partly multiple); some corrections made after proof stag

MOLÉCULES DANS LES ATMOSPHÈRES STELLAIRES

Les molécules sont abondantes dans les étoiles froides. Elles influent sur la stratification de l'atmosphère et le mode de transfert de l'énergie et on doit donc en tenir compte lors de la recherche des conditions physiques dans les atmosphères stellaires froides. Par ailleurs la spectroscopie moléculaire permet la mesure des abondances des éléments légers et fournit donc des données aux théories de nucléosynthèse et d'évolution stellaire.Molecules are abundant in cold stellar atmospheres. They have an effect upon the thermal stratification of the atmosphere and the energy transfert ; so one must include them in any research of the physical conditions in cold stellar atmospheres. From the study of molecular spectrum it is possible to know the abundances of light elements which are data for the nucleo-synthesis and stellar evolution theories

First detection of return currents in solar flares by spectropolarimetry with THEMIS

Using THEMIS French-Italien telescope with the MTR mode, the Hydrogen Hα and Hβ lines have been observed to be linearly polarized up to a few percent by impact during the impulsive phase of two solar flares associated with high-frequency radio pulses. Two privileged directions of linear polarization are present, respectively radial (in the disk center to flare direction) and tangential (perpendicular to the radial direction). This 90 degree change in the linear polarization direction is interpreted as due to the chromospheric return current generated by the penetration of a non-thermal electron beam into the chromosphere

Impact H

Electron beams, bombarding the dense chromospheric layers during solar flares, carry electric currents which need to be neutralized by so-called return currents. Return currents are formed by background plasma electrons having an anisotropic velocity distribution. Thus they can generate impact Hα line polarization. First, a numerical method of computation of the impact Hα line polarization for an arbitrary electron distribution function is presented. Then the polarization due to return current electrons associated with beam electrons is computed. For low electron beam fluxes, the return current is low and the polarization is only due to the electron beam, i.e. it is perpendicular to the electron beam direction and it reaches -8.0%. Increasing the return current and the beam flux leads to a change of orientation of the polarization by 90° and the polarization degree can even reach a maximum of +22.4%. But this change and the maximum of the polarization require very high electron beam fluxes of $1.79 \times 10^{12}$ and $4.8 \times 10^{12}$ ergs cm-2 s-1, respectively. Therefore plasma processes, which can reduce the high-energy flux requirement for the polarization change observations, are briefly discussed