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

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 D 3 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.

Using the StokesVwidths of Fe I lines for diagnostics of the intrinsic solar photospheric magnetic field

“…The SVW method with the 6301 line appears to be as reliable as MLR with 5247/5250 pair (Figure 4d), the estimations reveal typical point spread of around 200-250 G and the systematic error between 100-200 G. Importantly, these relatively low errors are nearly constant in a wide range of field values, from 200 to 1900 G. The MLR method with the 6301/6302 pair appears to be less reliable ( Figure 4c), showing a slightly bigger spread, 250-350 G, and a noticeable systematic error: it seems to quickly saturate above ∼1500 G. Still, the MLR estimations with this pair are acceptable in the range below 1200-1300 G. This is consistent with the earlier analysis by Gordovskyy et al (2018), although quite surprising, taking into account the difference in line formation depths in this pair (see e.g. Khomenko & Collados, 2007).…”

“…Of course, the reliability of the considered methods depends on the calibration. The calibration used in this and other similar studies (see Khomenko & Collados, 2007;Smitha and Solanki, 2017;Gordovskyy et al, 2018) is based on the vertically-uniform magnetic field. Taking into account that the magnetic field in the photosphere is expected to have substantial vertical gradients, this calibration may result in significant systematic error.…”

“…However, this method is computationally expensive and is usually applied to relatively small photospheric areas, consisting of up to 10 3 -10 4 pixels (e.g. Kalewicz & Bommier, 2019;Kuckein, 2019). Hence, it cannot be realistically used to analyse big photospheric area, such as a large active region (10 4 -10 6 pixels), or the whole Sun.…”

24 synoptic maps of average magnetic field in 296 prominences measured by the Hanle effect during the ascending phase of solar cycle 21

“…The numerical values of each prominence location on the solar surface and the observed magnetic field vector coordinates are provided in a table available at CDS. Recent observations with a better spatial resolution (Schmieder et al 2014a;Levens et al 2016a,b;Kalewicz & Bommier 2019) display a rather homogeneous magnetic field across quiescent prominences, so that the consideration of their average field makes sense.…”

“…The method is outlined in Bommier et al (1981) and the results are given in Bommier (2014). The method was also successfully applied by Kalewicz & Bommier (2019) to a prominence that was observed more recently with spatial resolution with the French THÉMIS telescope settled on the European Izaña site on the island of Tenerife (Canary Islands, Spain).…”