Vol.2. ~ II, pp. 185-158, 1983 0273-1177/83/1 10155-04$3.00/() ein~od in (;r~1 Brit;i .Ailr iglits reserved. Copyright © COSPAR IMPACT LINEAR POLARIZATION OBSERVED IN A UV CHROMOSPHERIC LINE DURING A SOLAR FLARE J. C. Henoux~’, G. Chambe*, M. Semel*, B. Woodgate**, R. Shine** and J. Beckers*** *Ohsert,a(oire de Paris, Meudon, France **Goddard Space Flight Center, Greenbelt, MD, U.S.A. ** *U,ziversily of Arizona, Tucson, AZ, U.S.A. ABSTRACT Linear polarization was observed in the SI 1437A line in bright flaring points during the soft X—ray emission. The degree of polarization is about 25% and is detected at a signal to noise ratio of 2.9. The polarized electric vector is directed towards disk center to within 30• This polarization could be due to collisional excitation of SI by energetic electrons beamed in the vertical direction. We suggest that the heat flux in the region inter- connecting the transition zone to the high chromosphere during the gradual phase of a flare could lead to an anisotropic excitation. Then the observed polarization would be due to vertical motions of the transition zone sweeping the preexisting chromosphere. INTRODUCTION The assumption is usually made that energy is supplied to the chromosphere from the flaring corona via heat conduction, electron bombardment or X—ray irradiation. The importance of these modes of energy transfer is widely accepted through theoretical con- siderations. However there is no direct observational evidence of any of these mechanisms in flaring chromospheres. The detection of energetic electrons with an anisotropic velo- city distribution in the chromosphere could permit a discrimination between them. An observation of impact line polarization can give the main direction of anisotropy for the energetic electron distribution function. Atoms collisionally excited by elec- trons emit line radiation which may be linearly polarized. By selecting a line linearly polarized, at threshold, in the beam direction, vertical heat conduction and electron beams would produce linear polarization with the electric vector directed towards disk center. On the other hand X—ray irradiation from above with axial symmetry would pro- duce linear polarization with a direction of polarization perpendicular to disk center direction. In this paper we report the first observation of linear polarization in a chromospheric liv line during a solar flare. OBSERVATIONAL TECHNIQUE Selected line — The SI line 1436.9 A ( 3P 0 — 3D 1) was used for the following reasons: — The line is a chromospheric line. Its excitation potential is high (8.63 cv). Hence the line intensity is very sensitive to the population of energetic electrons. — The polarization at threshold is high. From Beckers’ evaluation (Henze [1]) this threshold polarization (ego) is 100%. — The line profile does not show any self absorption in flares. Instrumentation — We used the Ultraviolet Spectrometer and Polarimeter (UVSP) on the Solar Maximum Mission. This instrument is described by Woodgate et al. [21. A more de- tailed description of the Polarimeter can be found in Calvert et al. (3]. The telescope is an aplanatic Gregorian which focuses the sun on the entrance slit of an Ebert—Fastie spectrometer. Magnesium fluoride waveplates can be inserted behind the entrance slit and rotated in steps of 22.5°. A linear polarizer following the rotating waveplate is required to serve as an analyzer. The linear polarizer for the waveplate used is the spectrometer grating. 155