Thermal and kinetic coronal rain diagnostics with MgII h & k lines

We performed the temperature diagnostics using inversions of data from nine spectroscopic observations obtained with the IRIS spectrograph in the MgII h & k lines. The sensitivity to the temperature of the emission peaks of these lines was exploited to determine the temperature of the coronal rain plasma using inversions of the spectroscopic profiles. Additional relationships between different spectral features of these lines, derived from the use of 3D radiative transfer line synthesis applied to simulations, were employed in order to derive the line-of-sight (LoS) velocities in different parts of the coronal rain plasma. For the first time, spectroscopic inversions of coronal rain were successfully performed. Temperatures derived from the inversions yield coronal rain clump temperatures at the formation height of the emission peaks of the MgII h & k lines in the range between 5000 and 7000 K. This narrow range of values remains consistent among all the different observations used in this work. We obtained LoS velocities of up to 40 km/s, which are consistent with the motion of the plasma being mostly constrained to the plane of the sky, as the coronal rain was mostly detected shortly after its formation and the observations took place in the disc. Furthermore, velocity diagnostics led to the detection of larger velocities at higher layers of the coronal rain plasma in some cases. This increased velocity seems to indicate that at some point (at least) during the fall of coronal rain clumps towards the chromosphere, the material in the upper part of the coronal rain plasma is falling with greater velocity than the material below it. The conditions of the temperature and density of the coronal rain plasma where the Mg II h line forms appear to be slightly different that those of the Mg II k line, with the former found at slightly colder and denser parts of the plasma

Magnetic field inference in active region coronal loops using coronal rain clumps

Aims. We aim to infer information about the magnetic field in the low solar corona from coronal rain clumps using high-resolution spectropolarimetric observations in the Ca II 8542 Å line obtained with the Swedish 1 m Solar Telescope. Methods. The weak-field approximation (WFA) provides a simple tool to obtain the line-of-sight component of the magnetic field from spectropolarimetric observations. We adapted a method developed in a previous paper in order to assess the different conditions that must be satisfied in order to properly use the WFA for the data at hand. We also made use of velocity measurements in order to estimate the plane-of-the-sky magnetic field component, so that the magnetic field vector could be inferred. Results. We have inferred the magnetic field vector from a data set totalling 100 spectral scans in the Ca II 8542 Å line, containing an off-limb view of the lower portion of catastrophically cooled coronal loops in an active region. Our results, albeit limited by the cadence and signal-to-noise ratio of the data, suggest that magnetic field strengths of hundreds of Gauss, even reaching up to 1000 G, are omnipresent at coronal heights below 9 Mm from the visible limb. Our results are also compatible with the presence of larger magnetic field values such as those reported by previous works. However, for large magnetic fields, the Doppler width from coronal rain is not that much larger than the Zeeman width, thwarting the application of the WFA. Furthermore, we have determined the temperature, T, and microturbulent velocity, ξ, of coronal rain clumps and off-limb spicules present in the same data set, and we have found that the former ones have narrower T and ξ distributions, their average temperature is similar, and coronal rain has microturbulent velocities smaller than those of spicules

Temperature diagnostics of chromospheric fibrils

Context. Chromospheric fibrils are thin and elongated structures that connect nearby photospheric magnetic field concentrations of opposite polarities. Aims. We assess the possibilities and drawbacks related to the use of current instrumentation and inversion techniques to infer the thermodynamic structure of chromospheric fibrils. Methods. We employed spectroscopic observations obtained in the Ca ii 854.2 nm line with the CRISP instrument at the Swedish 1-m Solar Telescope and in coordination with observations in the ultraviolet Mg ii h & k lines taken with the IRIS satellite. We studied the temperature sensitivity of these chromospheric lines to properly invert their spectral profiles with the Stockholm inversion Code and determine the temperature, line-of-sight velocity, and microturbulent velocity of manually traced chromospheric fibrils present in the field of view. Results. Fibril-like structures show a very particular dependence of their temperature as a function of the position along their length. Their temperatures at the detected footpoints are, on average, 300 K higher than the temperature at the midpoint. The temperature variation appears to be almost symmetrical in shape, with partially traced fibrils showing a similar trend for the temperature variation. Additionally, the response of the Ca ii 854.2 nm line core to variations of the temperature for the inverted models of the atmosphere in fibril areas seems to be insufficient to properly resolve the aforementioned temperature structure. Only the addition of more temperature sensitive lines such as the Mg ii h & k lines would make it possible to properly infer the thermodynamic properties of chromospheric fibrils. Comparisons between the results obtained here and in previous studies focused on bright Ca ii K fibrils yield great similarities between these structures in terms of their temperature

Ubiquitous hundred-Gauss magnetic fields in solar spicules

Aims. We use high-resolution spectropolarimetric observations in the Ca ii 8542 A line obtained with the SST to study the magnetic field in solar spicules. Methods. The equations that result from the application of the Weak Field Approximation (WFA) to the radiative transfer equations are used to infer the LOS component of the magnetic field (BLOS). Two restrictive conditions are imposed on the Stokes I and V profiles at each pixel before they can be used in a Bayesian inversion to compute its BLOS. Results. The LOS magnetic field component has been inferred in six data sets totalling 448 spectral scans in the Ca ii 8542 A line and containing both active region and quiet Sun areas, with values of hundreds of G being abundantly inferred. There seems to be no difference, from the statistical point of view, between the magnetic field strength of spicules in the quiet Sun or near an active region. On the other hand, the BLOS distributions present smaller values on the disk than off-limb, a fact that can be explained by the effect of superposition on the chromosphere of on-disk structures. We find that spicules in the vicinity of a sunspot have a magnetic field polarity (i.e. north or south) equal to that of the sunspot. This paper also contains an analysis of the effect of off-limb overlapping structures on the observed Stokes I and V parameters and the BLOS obtained from the WFA. It is found that this value is equal to or smaller than the largest LOS magnetic field components of the two structures. In addition, using random BLOS, Doppler velocities and line intensities of these two structures leads in ~ 50% of the cases to Stokes I and V parameters unsuitable to be used with the WFA. Conclusions. Our results present a scarcity of LOS magnetic field components smaller than some 50 G, which must not be taken as evidence against the existence of these magnetic field strengths in spicules