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The Transition Region Above Sunspots Hui Tian* , Tanmoy Samanta and Jingwen Zhang

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The Transition Region Above Sunspots Hui Tian* , Tanmoy Samanta and Jingwen Zhang Tian et al. Geosci. Lett. (2018) 5:4 https://doi.org/10.1186/s40562-018-0103-1 REVIEW Open Access The transition region above sunspots Hui Tian* , Tanmoy Samanta and Jingwen Zhang Abstract Over decades, sunspots and their fne structures have been studied in detail at the photospheric level with diferent ground-based telescopes, as the surface of the Sun primarily emits light in the visible wavelengths. For a very long period, the upper atmosphere above the sunspot regions, especially the transition region (TR) above sunspots where the plasma emits light in the far ultraviolet (FUV) and extreme ultraviolet (EUV), has been poorly understood. In the past decades after the development of space instrumentations, FUV and EUV observations have uncovered many secrets of the TR above sunspots. In this paper, we present a brief review of research results about the TR structures and dynamics obtained through imaging and spectroscopic observations of sunspots in the past ~ 20 years. Though these observations have gathered remarkable and detailed information and greatly improved our understanding of the TR above the sunspots, paradoxically, they leave us with many new questions which should be answered in the future. Keywords: Sunspots, Solar transition region, Chromosphere Introduction penumbra surrounding the umbra. Numerous umbral Sunspots are the darkest regions with the strongest mag- dots are often found inside the dark core, while the most netic felds and lowest temperatures on the surface of prominent structures in the penumbra are the so-called the Sun. Tough observed by humanity for thousands of penumbral flaments. Systematic outward fows, termed years, detailed investigation of the long-term evolution of Evershed fows (Evershed 1909), are usually present in sunspots started only 400 years ago, when Galileo Gali- the penumbra. Some sunspots also have light bridges, lei invented the astronomical telescope. Te number of which are bright lane-like structures dividing the umbra sunspots strongly varies on a timescale of about 11 years, into two or more parts. It is believed that umbral dots, which is known as the sunspot cycle or solar cycle. Cor- penumbral flaments, and light bridges are formed as respondingly, the occurring frequency of large-scale solar a result of vigorous convective motions. For a detailed activities such as solar fares and coronal mass ejections description of these substructures and dynamics, we refer also changes with such an 11-year cycle. As a result, the to the comprehensive review by Solanki (2003). Some of Earth’s magnetosphere and ionosphere are disturbed by these substructures and dynamics have been well repro- the same cycle. Te total electromagnetic radiation of the duced through radiative magnetohydrodynamic (MHD) Sun is also closely related to the 11-year sunspot cycle. simulations (see a review by Rempel and Schlichenmaier For instance, the extreme ultraviolet (EUV) and far ultra- 2011). violet (FUV) emission of the Sun varies signifcantly over Most studies of sunspots are focused on the photo- a solar cycle. spheric structures and dynamics. For a very long period, Modern high-resolution observations by large-aper- the upper atmosphere in sunspot regions, especially ture telescopes have revealed detailed substructures the transition region (TR) above the sunspots, has been and short-term dynamics of sunspots. A sunspot often poorly understood. Tis is mainly due to the fact that consists of a dark core termed umbra and a less dark there were only very limited numbers of TR observa- tions in the sunspot regions. By defnition, the TR is the interface region between the chromosphere and corona, *Correspondence: [email protected] where the temperature increases from roughly 2 104 to School of Earth and Space Sciences, Peking University, Beijing 100871, 5 × China 8 × 10 K (e.g., Tian 2017). TR probing relies mainly on © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Tian et al. Geosci. Lett. (2018) 5:4 Page 2 of 8 imaging and spectroscopic observations of the EUV and FUV emission, primarily in the spectral range of 400– 1600 Å. In the past ~ 20 years, observations from mainly three EUV/FUV spectrographs have greatly improved our understanding of the TR. Te frst two instruments are the Solar Ultraviolet Measurements of Emitted Radia- tion (SUMER, Wilhelm et al. 1995) and Coronal Diagnos- tic Spectrometer (CDS, Harrison et al. 1995) onboard the Solar and Heliospheric Observatory (SOHO) launched in late 1995. Te SUMER instrument has a very broad wavelength coverage, from roughly 660 to 1610 Å. Hun- dreds of strong TR lines have been observed by SUMER. Te spatial resolution of SUMER observations is about 2′′–3′′. Unfortunately, most SUMER observations were performed in the frst few years after the launch of SOHO, when the solar activity was relatively low. As a result, there are limited numbers of sunspot observa- tions by SUMER. For more than one solar cycle, spec- tra of some strong TR lines have been acquired in many CDS observations. However, the CDS instrument was mainly designed for coronal studies and the spatial reso- lution of the data is of much poorer quality than that of SUMER. Despite that, SUMER and CDS observations have revealed signifcant new insight into the structures and dynamics of the TR above sunspots. Our knowledge of the TR above sunspots has been greatly enhanced after Fig. 1 Images of the photosphere (upper left, IRIS SJI 2832 Å), chro- the launch of the Interface Region Imaging Spectrograph mosphere (upper right, IRIS SJI 2796 Å), TR (lower left, IRIS SJI 1400 Å) and lower corona (lower right, SDO/AIA 171 Å) in a sunspot region. (IRIS; De Pontieu et al. 2014) mission in June 2013. As These images were taken on July 20, 2013. The feld of view of each a UV spectrograph with slit-jaw imaging (SJI) capabil- image has a size of 50″ 42″ ity, IRIS has been performing both imaging and spectro- × scopic observations of the TR with unprecedented high ′′ ′′ resolution (0.3 –0.4 ) since July 2013. Te imaging capa- the umbra. Such enhanced TR emission features are bility of IRIS allows us to capture highly dynamic events very common in sunspot umbrae. Te 171 Å flter of the occurring in the TR and chromosphere above the sun- Atmospheric Imaging Assembly (AIA, Lemen et al. 2012) spots. Te high spectral resolution permits the accurate onboard the Solar Dynamics Observatory (SDO) sam- detection of plasma fows with diferent speeds in the ples emission from mainly several Fe X/Fe IX lines, which sunspots’ atmosphere. 5 are formed around 9 × 10 K (lower corona). Te 171 Å Here, we review our current knowledge of the TR image presented in Fig. 1 reveals some far reaching loops structures and dynamics above sunspots, mainly based that appear to be rooted in the sunspot umbra. on observations from the SUMER, CDS and IRIS instru- Sunspot plumes are the most prominent features in ments. We will also discuss what new questions should the TR above sunspots. Tey often exhibit as compact be answered in the future to better our understanding of and very bright features in the TR spectral lines within the TR above sunspots. sunspots (Foukal et al. 1974; Foukal 1976; Solanki 2003). SUMER observations have revealed the presence of Average properties of the TR above sunspots these plumes in a wide range of temperatures. Normally, Figure 1 shows images of diferent layers of a sunspot’s these sunspot plumes are characterized by signifcantly atmosphere. Emission structures of the photosphere and enhanced intensities of spectral lines with formation chromosphere appear to be similar, both revealing a dark temperatures between 105 and 106 K (in the upper TR umbra and a less dark penumbra. Te IRIS 1400 Å flter and lower corona). Dammasch et al. (2008) suggested samples mainly the Si IV 1394 and 1403 Å lines that are 4 that sunspot plumes are just the common footpoints of formed at a temperature of 8 × 10 K under the condi- many far reaching loops. Tese plumes are often found tion of ionization equilibrium. From the 1400 Å image, to be associated with signifcant redshifts of spectral lines we can see some patches of enhanced emission within formed in the upper TR such as the Ne VIII 770 Å line Tian et al. Geosci. Lett. (2018) 5:4 Page 3 of 8 (e.g., Brynildsen et al. 2001a; Marsch et al. 2004; Brosius Fig. 2 shows a patch of blueshift inside the umbra in both and Landi 2005; Dammasch et al. 2008). Te line profles the C IV 1548 Å and Ne VIII 770 Å lines that are formed 5 5 are often entirely redshifted by ~ 30 km/s at locations of at a temperature of about 10 and 6 × 10 K, respectively. sunspot plumes (see Fig. 2). Marsch et al. (2008) found Te magnitude of the blueshift appears to increase with that these redshifts are associated with the lower part the formation temperature. Magnetic feld extrapolation of the legs of large-scale magnetic loops that are recon- also indicates that the blueshifted region is associated structed based on magnetic feld measurement in the with open feld lines, suggesting that these blueshifts are photosphere. Tere are two proposed mechanisms for possible signatures of the initial acceleration of the solar the generation of these downfows: by siphon fows along wind (Marsch et al. 2004). far reaching loops (Brynildsen et al.
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