Absence of linear polarization in H<i>α</i>emission of solar flares

Bianda, M.; Benz, A. O.; Stenflo, J. O.; Küveler, Gerd; Ramelli, R.

doi:10.1051/0004-6361:20041654

Cited by 32 publications

(33 citation statements)

References 21 publications

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“…But no observation to date employed chromospheric polarimetry to unambiguously trace the magnetic field changes. Former chromospheric polarization measurements mostly focused on linear polarization and the search for "impact" polarization (e.g., Henoux et al 1990;Bianda et al 2005;Judge et al 2015). In this paper, we report the first direct observations of chromospheric magnetic field changes that were observed through spectropolarimetry of the Ca II 8542Å line during an X-class flare and we compare them to the photospheric changes of the same flare.…”

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confidence: 99%

First Detection of Chromospheric Magnetic Field Changes During an X1-Flare

Kleint

2016

ApJ

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Stepwise changes of the photospheric magnetic field, which often becomes more horizontal, have been observed during many flares. Previous interpretations include coronal loops that contract and it has been speculated that such jerks could be responsible for sunquakes. Here we report the detection of stepwise chromospheric line-of-sight magnetic field (B LOS ) changes obtained through spectropolarimetry of Ca II 8542Å with DST/IBIS during the X1-flare SOL20140329T17:48. They are stronger (<640 Mx cm −2 ) and appear in larger areas than their photospheric counterparts (<320 Mx cm −2 ). The absolute value of B LOS more often decreases than increases. Photospheric changes are predominantly located near a polarity inversion line, chromospheric changes near footpoints of loops. The locations of changes are near, but not exactly co-spatial to hard X-ray (HXR) emission and neither to enhanced continuum emission, nor a small sunquake. Enhanced chromospheric and coronal emission is observed in nearly all locations that exhibit changes of B LOS , but the emission also occurs in many locations without any B LOS changes. Photospheric and chromospheric changes of B LOS show differences in timing, sign, and size and seem independent of each other. A simple model of contracting loops yields changes of opposite sign to those observed. An explanation for this discrepancy could be increasing loop sizes or loops that untwist in a certain direction during the flare. It is yet unclear which processes are responsible for the observed changes, their timing, size and locations, especially considering the incoherence between photosphere and chromosphere.

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confidence: 99%

First Detection of Chromospheric Magnetic Field Changes During an X1-Flare

Kleint

2016

ApJ

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“…Trujillo Bueno et al (2002) were also able to determine the magnetic field vector of the observed filament by modeling the Stokes Q and U profiles (produced by the forward-scattering Hanle effect) and the Stokes V profiles (caused by the longitudinal Zeeman effect). Observations in filaments carried out by Bianda et al (2006) in the H i 6563 Å and He i 5876 Å lines showed similar polarization signatures.…”

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confidence: 53%

Observations of the forward scattering Hanle effect in the Ca I 4227 Å line

Bianda¹,

Ramelli²,

Anusha³

et al. 2011

A&A

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Chromospheric magnetic fields are notoriously difficult to measure. The chromospheric lines are broad, while the fields are producing a minuscule Zeeman-effect polarization. A promising diagnostic alternative is provided by the forward-scattering Hanle effect, which can be recorded in chromospheric lines such as the He i 10 830 Å and the Ca i 4227 Å lines. We present a set of spectropolarimetric observations of the full Stokes vector obtained near the center of the solar disk in the Ca i 4227 Å line with the ZIMPOL polarimeter at the IRSOL observatory. We detect a number of interesting forward-scattering Hanle effect signatures, which we model successfully using polarized radiative transfer. Here we focus on the observational aspects, while a separate companion paper deals with the theoretical modeling.

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“…Another surprising result was that no coherent radio emission was detected in 17% of hard X-ray flares , since both emissions are produced by electrons of similar energy and occasionally coincide with subsecond accuracy (Arzner & Benz 2005). A puzzle is also the absence of linear polarization in Ha emission, which limits the anisotropy of energetic protons and refutes earlier positive reports (Bianda et al 2005).…”

Section: Rhessi Observationsmentioning

confidence: 78%

Astrophysics in 2005

Trimble

Aschwanden

Hansen

2006

PUBL ASTRON SOC PAC

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ABSTRACT. We bring you, as usual, the Sun and Moon and stars, plus some galaxies and a new section on astrobiology. Some highlights are short (the newly identified class of gamma-ray bursts, and the Deep Impact on Comet 9P/Tempel 1), some long (the age of the universe, which will be found to have the Earth at its center), and a few metonymic, for instance the term "down-sizing" to describe the evolution of star formation rates with redshift.

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Absence of linear polarization in Hαemission of solar flares

Cited by 32 publications

References 21 publications

First Detection of Chromospheric Magnetic Field Changes During an X1-Flare

First Detection of Chromospheric Magnetic Field Changes During an X1-Flare

Observations of the forward scattering Hanle effect in the Ca I 4227 Å line

Astrophysics in 2005

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