Two fluid dynamics in solar prominences

Abstract

Solar prominences contain a significant number of neutral species. The dynamics of the ionised and neutral fluids composing the prominence plasma can be slightly different if the collisional coupling is not strong enough. The differential dynamics can be discerned by tracing line-of-sight velocities using observational techniques. Large-scale velocities can be quantified by measuring the global local and instantaneous displacement of spectral lines by the Doppler effect. Small-scale velocities leave their imprint on the width of spectral lines. In addition, these small-scale velocities can have a thermal (pure stochastic motion) nature or a non-thermal (small-scale unresolved instabilities, high-frequency waves, etc.) origin. For this work, we used one spectral line of ionised and two spectral lines of neutral elements to measure the resolved and unresolved velocities in a prominence with the aim to investigate the possible decoupling of the observed charged and neutral species. A faint prominence was observed with the German Vacuum Tower Telescope (VTT) on June 17, 2017. Time series consisting of repeated ten-position scans over the prominence were performed while simultaneously recording the intensity spectra of the 854.2 nm 656.28 nm, and 587.56 nm lines. The line-of-sight velocities and the Doppler width of the three spectral lines were determined at every spatial position and temporal moment. To make sure all spectral lines were sampling the same plasma volume, we applied selection criteria to identify locations with optically thin plasma. In addition, asymmetric or double-peaked profiles were also excluded for the analysis, since (even in an optically thin regime) they are indicative of the presence of strong velocity gradients or multiple components in the line of sight. Thus, only optically thin, symmetric, single-lobed profiles were retained for this study. As an additional reliability test of the selection criteria, we have also compared our results with optical thickness calculations. After the application of all the selection criteria, only a region close to the prominence border met all requirements. The velocities of the three spectral lines turned out to be very similar over this region, with the ionised showing velocity excursions systematically larger compared to those of the neutral lines of and at some moments. The latter was found to be much closer to each other. Most of the velocity differences were below 1 The analysis of the Doppler widths indicated that the line shows an excess of unresolved motions. We cannot establish whether these velocities are related to a different temperature of the ions or to unresolved small-scale motions due to any non-thermal mechanism. The dynamics of the ionised and neutral plasma components in the observed prominence were very close to one another. The differences found may indicate that a localised decoupling between ions and neutrals may appear at particular spatial locations or instants of time. Indications of different unresolved motions between those species have also been obtained.

Future prospects for partially ionized solar plasmas: the prominence case

“…The dynamics associated with multi-fluid plasma physics has been predicted to play a role over very small scales, often below the observational limits of our instrumentation (<1 min, 10 km, e.g. [4,68]). However, as illustrated previously, it may also have an effect on the stability and lifetime cycle of the whole prominence.…”

“…The interpretation of this result depends on the assumed hypotheses. Assuming the non-thermal component to be larger for ions than for neutrals, [4] measured line widths consistent with a unique kinetic temperature. Other authors, instead, found different thermal components for the two species under the assumption of a zero non-thermal motion (e.g.…”

“…Other authors, instead, found different thermal components for the two species under the assumption of a zero non-thermal motion (e.g. [4,[73][74][75]). Knowing that prominences are made up of dynamic fine, probably unresolved, structures, we expect to observe the result of the superposition motions, both within the instrument's resolution and, under an optically thin plasma assumption, along the line of sight.…”

“…[2]. Summarizing, it is of great theoretical and observational importance to fully understand the differences introduced by the use of multi-fluid approaches versus the single-fluid approximation, which could lead to completely new physics, the observational signatures [4] of which could be detected by present and forthcoming observational facilities.…”

Mixing, heating and ion-neutral decoupling induced by Rayleigh–Taylor instability in prominence-corona transition regions

“…However, the spatial resolution, limited either by instrument capabilities or atmospheric seeing conditions, hinders the detection of the finest structures. Numerous authors have endeavoured to identify potential dynamical decoupling between plasma components by measuring sets of ionized and neutral spectral lines targeting optically thin prominence plasma [11][12][13][14][15][16]. Despite inherent uncertainties in these observations, straining the sensitivity limits of the instrumentation used, all studies, except [12], have reported a slight excess in the amplitude of ion variations over neutrals.…”