Issue |
A&A
Volume 374, Number 3, August II 2001
|
|
---|---|---|
Page(s) | 1127 - 1134 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361:20010784 | |
Published online | 15 August 2001 |
Polarization of the hydrogen H
line in solar flares
Contribution of the local polarized radiation field and effect of the spectral index of the proton energy distribution
1
Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy e-mail: vogt@na.astro.it
2
Observatoire de Paris-Meudon, DAMAp, CNRS UMR 8588 (Département Atomes et Molécules en Astrophysique), 5 Place Jules Janssen, 92195 Meudon Cedex, France e-mail: Sylvie.Sahal-Brechot@obspm.fr, V.Bommier@obspm.fr
Corresponding author: E. Vogt, etienne@astro.gla.ac.uk
Received:
3
April
2001
Accepted:
1
June
2001
Linear polarization of the hydrogen Hα line was observed during solar
flares. The polarization vector is directed towards disk center and its degree
is of the order of 5% . The best explanation for this polarization is anisotropic collisional
excitation of the level of hydrogen by vertical beams of protons with an
energy greater than a few keV. However, previous calculations gave an expected
polarization degree of 2.5% or less, a factor of two below the observations.
In this paper, the theoretical model for the formation of the line polarization
has been refined, including the effect of polarization in the local radiation
field that is created by hydrogen proton anisotropic excitation.
We have also increased the spectral index of the proton energy distribution from
4 to 5, giving more weight to the low energy protons which are the most efficient
for impact polarization, without ionizing the atmosphere too much.
It is found that the inclusion of the polarization of the local radiation field
does not increase the Hα polarization very significantly; however, going
from a spectral index of 4 to 5 results in an expected polarization degree of
4.5% , compatible with the observations.
Key words: Sun: flares / polarization / atomic processes / line: formation
© ESO, 2001
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