Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes

Context. The solar chromosphere and the lower transition region are believed to play a crucial role in the heating of the solar corona. Models that describe the chromosphere (and the lower transition region), accounting for its highly dynamic and structured character are, so far, found to be lacking. This is partly due to the breakdown of complete frequency redistribution (CRD) in the chromospheric layers and also because of the difficulty in obtaining complete sets of observations that adequately constrain the solar atmosphere at all relevant heights. Aims. We aim to obtain semi-empirical model atmospheres that reproduce the features of the Mg II h&k line profiles that sample the middle chromosphere with focus on a sunspot. Methods. We used spectropolarimetric observations of the Ca II 8542 Å spectra obtained with the Swedish 1 m Solar Telescope and used NICOLE inversions to obtain semi-empirical model atmospheres for different features in and around a sunspot. These were used to synthesize Mg II h&k spectra using the RH1.5D code, which we compared with observations taken with the Interface Region Imaging Spectrograph (IRIS). Results. Comparison of the synthetic profiles with IRIS observations reveals that there are several areas, especially in the penumbra of the sunspot, where most of the observed Mg II h&k profiles are very well reproduced. In addition, we find that supersonic hot down-flows, present in our collection of models in the umbra, lead to synthetic profiles that agree well with the IRIS Mg II h&k profiles, with the exception of the line core. Conclusions. We put forward and make available four semi-empirical model atmospheres. Two for the penumbra, reflecting the range of temperatures obtained for the chromosphere, one for umbral flashes, and a model representative of the quiet surroundings of a sunspot.

NVST observations of collision-induced apparent fan-shaped jets

ABSTRACT Using high-quality H α observations from the New Vacuum Solar Telescope, we first report apparent fan-shaped jets (AFJs) generated during the interaction between primary fan-shaped jets (FJs) and nearby facula magnetic structure. The primary FJs were intermittently launched from a sunspot penumbra with negative-polarity magnetic fields in active region 12740 on 2019 May 6, accompanied by impulsive brightenings at the base. While the propagating FJ encountered and collided with the negative-polarity magnetic structure of the west facula, the density of jet material was enhanced to the east of the facula. Meanwhile, the jet structures exhibited a deflection towards the north-west at the jet–facula collision location. Then the primary FJ evolved into two parts, with one part being reflected away from the facula and the other part forming an AFJ. Easily distinguished from the primary FJ, the ejecting AFJ was more ordered and had an apparent end at the facula. The AFJ was impulsively accelerated to speeds of 100 km s−1, and reached lengths of up to 40 Mm. The observed AFJ had a similar morphology to the fan-shaped quasi-separatrix layer (QSL) between the penumbra and facula magnetic systems, implying that the material of the AFJ was mainly guided by the fan plane of the QSL. We suggest that the collision does not cause a change in the field-line connectivity and only leads to the redistribution of jet material.

Long-term variation of sunspot penumbra to umbra area ratio

A typical sunspot, as seen in white-light intensity images, has a two part structure: a dark umbra and a lighter penumbra. Such distinction primarily arises due to the different orientations of magnetic fields in these two regions. In this study, we use the Kodaikanal white-light digitized data archive to analyze the long-term evolution of umbral and penumbal area. We used an ‘automated algorithm’ to uniquely identify the sunspot umbra (including the calculation of penumbra to umbra ratio) from these digitized intensity images. Our analysis reveals that the ratio increases slightly with the increase of sunspot area upto 100 μHem but eventually settles down to a constant value after that. This study, not only allows us to better understand the evolution of an individual spot and its corresponding magnetic field but this is also beneficial for solar dynamo studies which aim to reproduce such structures using a MHD theory.