Spectropolarimetric fluctuations in a sunspot chromosphere

Abstract

The instrumental advances made in this new era of 4 m class solar telescopes with unmatched spectropolarimetric accuracy and sensitivity will enable the study of chromospheric magnetic fields and their dynamics with unprecedented detail. In this regard, spectropolarimetric diagnostics can provide invaluable insight into magneto-hydrodynamic (MHD) wave processes. MHD waves and, in particular, Alfvénic fluctuations associated with particular wave modes were recently recognized as important mechanisms not only for the heating of the outer layers of the Sun’s atmosphere and the acceleration of the solar wind, but also for the elemental abundance anomaly observed in the corona of the Sun and other Sun-like stars (also known as first ionization potential) effect. Here, we take advantage of state-of-the-art and unique spectropolarimetric Interferometric BIdimensional Spectrometer observations to investigate the relation between intensity and circular polarization (CP) fluctuations in a sunspot chromosphere. Our results show a clear link between the intensity and CP fluctuations in a patch which corresponds to a narrow range of magnetic field inclinations. This suggests the presence of Alfvénic perturbations in the sunspot. This article is part of the Theo Murphy meeting issue ‘High-resolution wave dynamics in the lower solar atmosphere’.

Alfvénic Perturbations in a Sunspot Chromosphere Linked to Fractionated Plasma in the Corona

“…IBIS full Stokes spectropolarimetric scans were used to identify possible signatures of magnetic field oscillations in circular polarization (CP) measurements in the umbra. A full account of the observations and data reduction techniques is provided in Stangalini et al (2018), Murabito et al (2020), Houston et al (2020), andStangalini et al (2021). Here, we precis the aspects that are relevant to this analysis.…”

“…To identify possible intrinsic magnetic field oscillations in the same IBIS data set, Stangalini et al (2021) performed a specific phase lag analysis between the CP and the core intensity of the Ca II. Figure 2(c) shows the Stokes V max /I cont CP map saturated at 0.3.…”

“…4. Alfvénic Perturbations in the sunspot Chromosphere Stangalini et al (2021) found the presence of Alfvénic perturbations in the sunspot chromosphere in the 3 minute band of the IBIS Ca II time series (see Section 2.2). The frequency band corresponding to this period, which is also the dominant period in the solar chromosphere (e.g., Khomenko & Collados 2015, and references therein), is smaller than the ion-cyclotron frequency, and thus in the regime appropriate for the model of Laming (2015).…”

“…The detection of magnetic oscillations associated with MHD modes is a difficult task, as opacity effects or instrumental crosstalk with other physical quantities can easily mimic the effect of these oscillations (e.g., Khomenko & Collados 2015;Joshi & de la Cruz Rodríguez 2018, and references therein). One way to disentangle intrinsic magnetic oscillations in the solar atmosphere is through the investigation of the phase lag between the polarization signals associated with magnetic field disturbances and other physical quantities such as intensity and Doppler velocity (Stangalini et al 2018(Stangalini et al , 2021. Oscillating physical quantities associated with MHD waves may have different phase relations depending on the MHD mode and the propagation state of the wave (Fujimura & Tsuneta 2009;Hinode Review Team et al 2019), thus the analysis of the phase relations between them can be exploited for their identification (Stangalini et al 2018) as well as the identification of the specific mode producing them.…”

“…We find that the elemental abundance variation in the corona above the sunspot is highly structured, with extremes in the level of fractionation among the distinct loop populations. The distribution of the highly fractionated plasma appears to be correlated with the spatial locations at which intrinsic magnetic oscillations are identified in nearly simultaneous high spatial resolution spectropolarimetric observations of the solar chromosphere (Stangalini et al 2021). Magnetic field modeling is used to investigate the connectivities of the loop populations within the sunspot in order to seek an understanding of the distribution of plasma composition observed there.…”