An investigation of the linear polarization in emission features of active regions

Abstract

A study has been made of linear polarization in 'moustaches' using the He and Hfl lines. A Wollaston prism and an achromatic 2/2-plate were employed in the study. In the course of the observations, the major axis of the plate was placed at 0 ~ and 22?5 with respect to the horizontal axis of the installation. The Stokes parameters J, Q, and U were recorded, thus enabling the degree of polarization and the field azimuth to be determined. During the first three minutes of the existence of the moustaches, the two lines revealed a linear polarization of about 7~o located at the center of the lines. No polarization was found during the later stage of the moustaches' lifetimes. Whenever the linear polarization is present, there is a tendency for the polarization to decrease rapidly, in accompaniment of a turning of the azimuth. Several mechanisms for interpreting the polarization observed in the moustaches are examined. Some suggestions are made in support of the excitation of hydrogen atoms in the moustaches initiated by a high-energy electron flux or vertical heat conduction.

The viewing angle effect on Hα-line impact polarisation in impulsive solar events

“…1−3 of Zharkova & Syniavskii 2000) the impact polarisation appears only near the line cores whereas the wings are fully depolarised by collisions with thermal electrons that is in agreement with the observations by Firstova (1986). The calculations also reveal that the simulated H α -line profiles are rather asymmetric with the maximuma corresponding the main transitions for which ∆L = ∆J (3d 5/2 − 2p 3/2 , 3d 3/2 − 2p 1/2 and 3p 3/2 − 2s 1/2 ).…”

“…Since moustaches are known to be very sensitive to seeing conditions and the image resolution varies from 0.5 to 1.2 arcsec, relatively short exposure times (0.05 and 0.1 s) are selected in order to minimize image tremor. In addition, normalization of the H α line profiles to the continuum in each strip is carried out that helps to decrease the instrumental polarization (Firstova 1986). The sample Stokes parameters I and Q/I of moustaches are presented in Fig.…”

“…The degree of polarisation measured in the H α line centre in flares ranges over 3−5%, in some rare cases exceeding 10% (Chambe & Henoux 1979;Henoux & Semel 1981;Henoux 1991;Firstova & Bulatov 1996). In moustaches, considerable polarisation was also established the H α line center seen to be about 7% (Firstova 1986) or in the range in 3−10% (Babin & Koval 1985−1988Firstova et al 1999;Kashapova 2002). Other observations of 32 moustaches by a different instrument reported that the polarisation, which does not exceed 8% (Rust & Keil 1992), with the bulk of them (about 87%) not exceeding 2%.…”

“…In the observations by the direction of this plane is considered to coincide with the flare-to-disk centre direction leading to a positive polarisation, whereas some observations by Firstova & Bulatov (1996) show the plane is perpendicular to this direction resulting in negative polarisation. In different moustaches the polarisation planes can very often have opposite orientations or even not have a well defined direction on the solar disk centre (Firstova 1986;Firstova et al 1999). …”

Eclipse on the Coral Sea: Cycle 24 Ascending

“…Linear line-core polarization attributed to the beam impact became the diagnostic for this notion [103] In our opinion these authors failed to recognize that EBs are photospheric and occur without anything happening overhead. We note that in the example of [103] the polarization is not cospatial with the moustache, and that the excess emission profiles of [104] have no dip at line center. We suspect that the measured line core polarizations stem from flaring arch filaments, possibly through the mechanism of [112].…”