Mariarita Murabito¹, Francesca Zuccarello¹, Salvo L. Guglielmino¹, Paolo Romano²
1. Dipartimento di Fisica e Astronomia – Sezione Astrofisica, Università di Catania, Via S. Sofia 78, I-95125 Catania, Italy
2. INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, I-95125 Catania, Italy

The presence of the penumbra distinguishes a mature sunspot from a pore in active regions (ARs). However, owing to the scarcity of high-resolution observations acquired during the transition between these two phases of the sunspot evolution, the physical processes at the base of the penumbra formation are still unclear.

Before the era of high-resolution observations, Zwaan^[1] reported that the penumbra encircles the umbra sector by sector fulfilling the gap formed in the side facing the opposite polarity of the AR when no more magnetic flux emerges. Recently, some studies have contributed to advance our knowledge of the formation of the penumbra. In this respect, high-resolution observations reported by Schlichenmaier et al.^[2] confirmed that the penumbra forms in sectors and, although filaments form in the area towards the opposite polarity of the AR, they are not stable and disappear. This fact led to the conclusion that the flux emergence prevents the settlement of the penumbra. However, in contrast with previous observations, Murabito et al.^[3] observed the formation of stable penumbral filaments in a region of flux emergence between two main polarities of an AR.

It is also worthwhile recalling that a mature sunspot is characterized by a flow pattern known as Evershed flow. Recently, Murabito et al.^[4] highlighted that its onset occurs during the penumbra formation phase.

Figure 1| Cartoon representing the different type of the penumbra formation side. White and black oval represent the following and preceding protospot, respectively. Grey ovals represent the first stable penumbral sector.

Thanks to the possibility of acquiring full-disk observations over a long-time interval, HMI is surely the best instrument, with its moderate spatial resolution (1”), to follow the entire evolution of sunspots during their passage across the solar disk. For this reason, in order to understand where the penumbra starts to form and the connection with the onset of the Evershed flow, we analyzed twelve ARs that appeared on the solar disk in 2011 and 2012, using data acquired by the HMI instrument. We choose ARs with the simplest magnetic field bipolar configuration: β-type. We classified the ARs into two groups, depending on where the first stable penumbral filaments appear (see cartoon in Figure 1). Stability of the penumbral filaments was fixed upon applying a threshold of 10 times the granular life time.

Figure 2| Continuum intensity maps of two examples of twelve selected ARs. Left, preceding spot of AR 11150 classified as B-type; Right, preceding spot of AR 11640 classified as A-type. The black and orange arrows indicate the opposite polarity region and the side where the first penumbral sector appears, respectively.

Concerning the side where the penumbra starts to form, we found that eight spots formed the first stable penumbral sector in the region between the two opposite polarities, nine spots on the opposite side (two examples are shown in Figure 2). Contrary to what was reported by Zwaan^[1], we also found that the last penumbral sector does not always appear on the side facing the opposite polarity of the AR (see the left panel of Figure 2). Furthermore, the analysis showed that the time needed to form the penumbra seems to be related to the side where the penumbra appears. In particular, we found that the penumbra formation in B-type spots seems quicker (about 21 hr) than in the A-type spots (about 39 hr).

Figure 3| Zoom of the continuum and LOS velocity maps of the following sunspot of AR 11610.

With regard to the flow pattern, eleven sunspots showed an inverse Evershed flow (i.e., a plasma motion directed toward the protospot border) before the penumbra formation, which changes within 1-6 hours into the classical Evershed flow as soon as the penumbra forms (an example is shown in Figure 3). This confirms the observational evidence that the appearance of penumbral filaments is correlated with the transition from the inverse Evershed flow. Furthermore, this result supports the model proposed by Murabito et al.^[3] to explain this transition in the velocity field.

Our findings are supported by a recent sunspot simulation by Chen et al.^[5].

For more details of this work, please refer to our recent publication:
Murabito, M., Zuccarello, F., Guglielmino, S. L., Romano, P., 2018, ApJ, 855, 58

References

[1] Zwaan, C. 1992, NATO Advanced Science Institutes (ASI) Series C, 375, 75
[2] Schlichenmaier, R., Rezaei, R., Bello González, N., & Waldmann, T.A. 2010b, A&A, 512, L1
[3] Murabito, M., Romano, P., Guglielmino, S. L., Zuccarello, F. & Solanki, S. K. 2016, ApJ, 285, 75
[4] Murabito, M., Romano, P., Guglielmino, S. L., & Zuccarello, F. 2017, ApJ, 834, 76
[5] Chen, F., Rempel, M., & Fan, Y. 2017, ApJ, 846, 149