A Search for Low Energy Protons in Flares =========================================

fl117.metcalf03
Posted:  23-Apr-93
Updated: 08-Apr-96
Events specified: flares in October 1992 (see text)

Metcalf, Mickey, Canfield, Wuelser, Hudson, Acton, Hirayama, Kosugi, Yoshimori, Tsuneta, Simnett

Project Description:

This project uses H-alpha data from the University of Hawaii's Imaging Vector Magnetograph to search for low energy protons in solar flares. It is expected that 10-100 keV proton beams in flares will produce linear impact polarization of the H-alpha line. Such polarization has been observed previously by Henoux et al (Ap.J.Supp., 73, 303, 1990) and by Metcalf et al. (The Compton Observatory Science Workshop, NASA Conference Publication 3137, 536, 1992).

The interpretation of H-alpha flare polarization as impact polarization from a low energy proton beam has yet to be conclusively proved, however. The goal of this experiment is to understand the environment in which the flare occurs to aid in identifying of the cause of the observed polarization.

There are two characteristics of impact polarization which allow us to uniquely identify it. First, the polarization should be directed towards the center of the solar disk for a vertical proton beam and second, the polarization should depend on the heliocentric angle of the observation as (sin(theta))^2. We will look for these effects.

Goals

The goals of the study are:

Search for linear polarization in the observed flares. (Mees)

     Demonstrate whether the polarization is or is not impact polarization.

Is the polarization directed towards the center of the solar disk (Mees)? Does the degree of polarization increase for flares closer to the limb (Mees)? Is the linear polarization consistent with the gamma rays (WBS)? What energy protons are required in the corona to to expect 10-100 keV protons in the chromosphere (Ne, loop geometry) (SXT)? 3) If the observed polarization is impact polarization, determine the

   characteristics of the low energy proton population and how it relates
   to other flare emissions. Is the polarization coincident with electron
   precipitation sites (HXT, Mees)?  What is the temporal and spatial 
   relationship between the H-alpha polarization and the SXR's and HXR's
   (Mees,HXT,SXT,WBS,BCS)?

Data

Required Observations/Analysis technique:

   Determine pre-flare conditions: T, Ne (SXT,WBS)       
      
 Observe linear H-alpha polarization in flares (Mees)       
      
 Observe electron precipitation sites (Mees, HXT)       
      
 Determine location of flare energy release as a function of time      
     (SXT,HXT,WBS).

The data analysis and interpretation that will be done are:

         Pre
flare conditions: WBS team member, U.Hawaii, SXT team member     
            
 H
alpha polarization: U. Hawaii     
            
 Electron precipitation sites: U. Hawaii, HXT team member     
            
 Site of energy release: U. Hawaii, HXT team member     
            
 Interpretation: All

We observed a large number of flares during October, 1992 with the IVM and will search for evidence of impact polarization in these flares. The most notable are:

Beg Max End OPT --Peak Radio Flux--- Assoc. YYMMDD DOY HHMM HHMM HHMM XRAY INP LAT LON NOAA 2.7 8.8 15.4 Report

921019 293  1751  1759  1811 M1.1  1F N06 E23 7315                    III      
921023 297  0054  0103  0115 M4.1  2B S00 W20 7315   100   340   250  II,V     
921026 300  1754  1816  1935 M4.5  2B S24 W11 7321    93   900   630  III      
921027 301  0143  0150  0159 M1.1                           59   100  III      
921030 304  1659  1816  1916 X1.7  2B S22 W61 7321  7700  9000  4000  III,IV,L,

We have analyzed the October 23 flare in some detail and we do find linear polarization in the H-alpha line during the flare.

Update 08-Apr-96

The project, A Search for Low Energy Protons in Flares (fl117.metcalf03) is now complete. A paper on this subject will be submitted to ApJ in the next few days once the figures are finalized.

The abstract and the full text of the paper can be accessed from http://www.solar.ifa.hawaii.edu/~metcalf/Papers/preprints.html

Abstract

We present observations of linear polarization in the H-alpha spectral line during five solar flares observed in 1992 October using the University of Hawaii Imaging Vector Magnetograph. In five of five flares, ranging in soft x-ray class from C2.0 to M4.1, linear polarization was observed in the impulsive phase and is consistent with the predictions of impact polarization by a nearly vertical beam of deka-keV protons. In particular, linear polarization was observed along the direction from the flare site to the center of the solar disk. We also observed a polarization component perpendicular to this direction which is consistent with impact polarization from horizontally circling photo-electrons. To further nterpret the observations, we co-aligned images from the Yohkoh Hard X-ray Telescope (HXT) and from the Soft X-ray Telescope (SXT) with the polarization maps. With this dataset we make a strong, though not yet certain, case for the presence of proton beams in solar flares.