European Solar Physics Online Seminar Archive

Sunspots are the longest known manifestation of solar activity and their magnetic nature has been known for more than a century. Despite this, the boundary between umbrae and penumbrae, the two fundamental sunspot regions, has hitherto been solely defined by an intensity threshold. We now unveil the empirical law of the magnetic nature of the umbra-penumbra boundary in stable sunspots: an invariant vertical component of the magnetic field. We study the magnetic nature of umbra-penumbra boundaries in sunspots of different sizes, morphology, evolutionary stage, and phase of the solar cycle. We use a sample of 88 scans of Hinode/SOT spectropolarimeter to infer the magnetic field properties at the umbral boundaries. We define these boundaries by an intensity threshold and perform a statistically analysis of the magnetic field properties at these boundaries. We statistically prove that the umbra-penumbra boundary in stable sunspots is characterised by an invariant value of the vertical component of the magnetic field: The vertical component of the magnetic field strength does not depend on the umbra size, its morphology, and phase of the solar cycle. With statistical Bayesian inference, we find that the vertical component of the magnetic field strength is, with 99\% likelihood, in the range of 1849-1885 G with the most probable value of 1867 G. In contrast, the magnetic field strength and inclination averaged along individual boundaries are found to be dependent on the umbral size: The larger the umbra, the stronger and more horizontal the magnetic field at its boundary is. [more]

Propagating slow magneto-acoustic waves are regularly observed in the solar corona, particularly in sunspot related loop structures. These waves exhibit rapid damping as they propagate along the loops. Several physical and geometrical effects were found to produce the observed decay in the wave amplitude. It has also been shown that the damping is frequency dependent. A majority of the observed characteristics have been attributed to damping by thermal conduction in the solar corona. Although it is believed that these waves originate in the photosphere, their damping behaviour in the sub-coronal layers is relatively less studied. Using high spatial and temporal resolution images of a sunspot, we investigated propagation and damping characteristics of slow magnetoacoustic waves up to transition region heights. The major conclusions from this study will be discussed in the talk which include: 1) The energy flux in slow waves estimated from the relative amplitudes decays gradually right from the photosphere even when the oscillation amplitude is increasing. 2) The damping displayed by slow waves is frequency dependent well below coronal heights. 3) A spatial comparison of power spectra across the umbra highlights enhancement of high-frequency waves near the umbral center. [more]

Ellerman bombs and UV bursts are transient brightenings that are ubiquitously observed in the lower atmospheres of active and emerging flux regions. While some Ellerman bombs display clear UV burst signatures, not all have correlated UV signal or vice versa, suggesting the underlying atmospheric and magnetic properties may differ between events. As both are believed to pinpoint sites of magnetic reconnection in reconfiguring fields, understanding their occurrence and detailed evolution may provide helpful insights in the overall evolution of active regions. Here we present results from observations and inversions of SST/CRISP and CHROMIS, as well as IRIS data of these transient events. At unprecedented spatial resolution the CHROMIS Ca II H & K observations reveal dynamic fine structure suggesting a plasmoid-mediated reconnection process. We investigate several cases, combining information from the Mg II h & k and Ca II 8542Å and H & K lines in order to infer the temperature stratification and magnetic field configuration within which these events occur. I’ll address the difficulties of successfully inverting their Si IV profiles and will discuss our results in light of the current debate on the connection between UV bursts and Ellerman bombs, their occurrence heights and in particular the temperatures that they may (or may not) reach. [more]

We present preliminary results derived from the analysis of spectropolarimetric measurements of active region AR12546, which represents one of the largest sunspots to have emerged onto the solar surface over the last 20 years. The region was observed with full-Stokes scans of the Fe I 617.3 nm and Ca II 854.2 nm lines with the Interferometric BIdimensional Spectrometer (IBIS) instrument at the Dunn Solar Telescope over an uncommon, extremely long time interval exceeding three hours. We show preliminary results from the phase lag analysis of different quantities and discuss the results in terms of the literature on the subject and MHD wave propagation theory. [more]