Analysis of the penumbra formation in sunspots

Abstract

The formation of the penumbra in sunspots is a physical process which involve the coupling between the plasma and the magnetic field in different layers of the solar atmosphere. Its study requires long time series of observations carried out with high temporal, spatial, and spectral resolution. Moreover, due to few available datasets of this phase of the sunspot evolution, the physical processes at the base of the penumbra formation are still unclear. For this reason in this thesis I performed some observational analysis of the penumbra formation using high resolution data acquired during an observing campaign at the Richard B. Dunn Solar Telescope (NSO) and data taken by HMI onboard of SDO.

Two main aspects have been investigated: the location of the stable settlement of the first penumbra filaments and the transition from inverse to classical Evershed flow.

Using the high resolution images I observed the first settlement of the penumbra filaments in the two polarities of AR NOOA 11490. Before the penumbra formation the pore of the preceding polarity exhibits an annular zone characterized by a magnetic field greater than 1000 G, having an (upside down) ballerina skirt structure of the magnetic field. In this case, the penumbra starts to form in the side away from the opposite polarity, in agreement with the observations of Schlichenmaier et al. (2010). On the other hand, using the same dataset, I showed that in the following polarity of the AR NOAA 11490 a stable penumbra forms in the area facing the opposite polarity, located below an AFS, i.e. in a flux emergence region.

Moreover, considering a sample of other six ARs observed by HMI, I found that there is no preferred location for the penumbra formation.

I interpreted the formation of the penumbra as due to the field lines of the magnetic canopy, already existing at a higher level of the solar atmosphere and overlying the pore, which sink down into the photosphere and below the solar surface. In fact, in this case there is a non-zero probability of finding near-horizontal field also in the region between the two main sunspots, as shown by the recent simulations of MacTaggart et al. (2016).

Concerning the transition from inverse to classical Evershed flow, in the preceding polarity of AR NOAA 11490 I found changes in the direction of the LOS velocity field during the formation of the first penumbral sector. In about 1-3 hours the LOS velocity became coherent with the Evershed flow pattern while the penumbra was completely formed. I also found observational evidences of this transition in most of the pores of my sample observed by HMI. Therefore, I proposed a new model to explain this transition in the velocity field, based on the presence of small U-loops, which are able to drive a siphon flow toward the pore, i.e., corresponding to the inverse Evershed flow, before the penumbra formation.

The thesis is organized as follows: in Chapter 1 I provide a brief introduction on the characteristics of the solar active regions. Chapter 2 describes the main features observed in solar sunspots with particular attention to the penumbra and its formation, taking into account both the observational and theoretical point of view. The used instruments, the data and their method of analysis have been described in Chapter 3. Chapters 4 and 5 report the analysis of the data concerning the preceding and following polarities of AR NOAA 11490, respectively. The study carried out using a sample of 6 ARs observed by HMI is reported in Chapter 6. A discussion containing a possible scenario for the penumbra formation and the transition from the inverse to the classical Evershed flow is reported in Chapter 7. The conclusions drawn from the results obtained in this thesis are reported in Chapter 8.