Magnetic field vector ambiguity resolution in a quiescent prominence observed on two consecutive days

T Kalewicz,V Bommier

doi:10.1051/0004-6361/201935488

Abstract

Context. Magnetic field vector measurements are always ambiguous, that is, two or more field vectors are solutions of the observed polarisation. Aims. The aim of the present paper is to solve the ambiguity by comparing the ambiguous field vectors obtained in the same prominence observed on two consecutive days. The effect of the solar rotation is to modify the scattering angle of the prominence radiation, which modifies the symmetry of the ambiguous solutions. This method, which is a kind of tomography, was successfully applied in the past to the average magnetic field vector of 20 prominences observed at the Pic du Midi. The aim of the present paper is to apply this method to a prominence observed with spatial resolution at the THÉMIS telescope (European site at Izaña, Tenerife Island). Methods. The magnetic field vector is measured by interpretation of the Hanle effect observed in the He I D3 5875.6 Å line, within the horizontal field vector hypothesis for simplicity. The ambiguity is first solved by comparing the two pairs of solutions obtained for a “big pixel” determined by averaging the observed Stokes parameters in a large region at the prominence centre. Each pixel is then disambiguated by selecting the closest solution in a propagation from the prominence centre to the prominence boundary. Results. The results previously obtained on averaged prominences are all recovered. The polarity is found to be inverse with a small angle of about −21° between the magnetic field vector and the long axis of the filament. The magnetic field strength of about 6 G is found to slightly increase with height, as previously observed. The new result is the observed decrease with height, of the absolute value of the angle between the magnetic field vector and the long axis of the filament. Conclusions. This result is in excellent agreement with prominence magnetohydrodynamical models.

Highlights

IntroductionLeroy et al 1977) chose the He i D3 5875.6 Å line for this purpose because this line is absent from the incident photospheric spectrum, and Doppler dimming or brightening is avoided
Following the pioneering work by Hyder (1965) showing the possible existence of the Hanle effect in solar prominences, measurements of the effect were undertaken in order to diagnose the magnetic field in these objects. Leroy (1977; see Leroy et al 1977) chose the He i D3 5875.6 Å line for this purpose because this line is absent from the incident photospheric spectrum, and Doppler dimming or brightening is avoided
A table of Hanle sensitivity for a series of spectral lines can be found in Sahal-Brechot (1981), where it can be seen that the He i D3 line sensitivity lies around 6 G

Summary

Introduction

Leroy et al 1977) chose the He i D3 5875.6 Å line for this purpose because this line is absent from the incident photospheric spectrum, and Doppler dimming or brightening is avoided. This line has the well-adapted Hanle sensitivity for determining the prominence magnetic field, which was found to be on the order of 6 G. Leroy measured the Hanle effect in about 400 quiescent prominences at the Pic du Midi coronagraph, during the ascending phase of solar cycle XXI (1976−1982), but without any spatial or spectral resolution in these objects.

Objectives

Methods

Results