Sunspot Penumbra Research Articles

Mapping the Longitudinal Magnetic Field in the Atmosphere of an Active Region Plage from the Inversion of the Near-ultraviolet CLASP2.1 Spectropolarimetric Data

We apply the HanleRT Tenerife Inversion Code to the spectropolarimetric observations obtained by the Chromospheric Layer Spectropolarimeter. This suborbital space experiment measured the variation with wavelength of the four Stokes parameters in the near-ultraviolet spectral region of the Mg ii h and k lines over a solar disk area containing part of an active region plage and the edge of a sunspot penumbra. We infer the stratification of the temperature, the electron density, the line-of-sight velocity, the microturbulent velocity, and the longitudinal component of the magnetic field from the observed intensity and circular polarization profiles. The inferred model atmosphere shows larger temperature and electron density in the plage and the superpenumbra regions than in the quiet regions. The shape of the plage region in terms of its brightness is similar to the pattern of the inferred longitudinal component of the magnetic field in the chromosphere, as well as to that of the overlying moss observed by the Atmospheric Imaging Assembly in the 171 Å band, which suggests a similar magnetic origin for the heating in both the plage and the moss region. Moreover, this heating is particularly significant in the regions with larger inferred magnetic flux. In contrast, in the superpenumbra, the regions with larger electron density and temperature are usually found in between these regions with larger magnetic flux, suggesting that the details of the heating mechanism in the chromosphere of the superpenumbra may be different from those in the plage, but with the magnetic field still playing a key role.

Characterizing Solar Center-to-limb Radial-velocity Variability with SDO

Stellar photospheric inhomogeneities are a significant source of noise, which currently precludes the discovery of Earth-mass planets orbiting Sun-like stars with the radial-velocity (RV) method. To complement several previous studies that have used ground- and spaced-based facilities to characterize the RV of the Sun, here we characterize the center-to-limb variability (CLV) of solar RVs arising from various solar-surface inhomogeneities observed by the Solar Dynamics Observatory (SDO)/Helioseismic Magnetic Imager and SDO/Atmospheric Imaging Assembly. By using various SDO observables to classify pixels and calculate line-of-sight velocities as a function of pixel classification and limb angle, we show that each identified feature type, including the umbrae and penumbrae of sunspots, quiet-Sun magnetoconvective cells, magnetic network, and plage, exhibit distinct and complex CLV signatures, including a notable limb-angle dependence in the observed suppression of convective blueshift for magnetically active regions. We discuss the observed distributions of velocities by identified region type and limb angle, offer interpretations of the physical phenomena that shape these distributions, and emphasize the need to understand the RV signatures of these regions as astrophysical signals, rather than simple (un)correlated noise processes.

Very Long-periodic Pulsations Detected Simultaneously in a White-light Flare and Sunspot Penumbra

We investigate the origin of very long-periodic pulsations in the white-light emission of an X6.4 flare on 2024 February 22 (SOL2024-02-22T22:08), which occurred at the edge of a sunspot group. The flare white-light fluxes reveal four successive and repetitive pulsations, which are simultaneously measured by the Helioseismic and Magnetic Imager and the White-light Solar Telescope. A quasi-period of 8.6−1.9+1.5 minutes, determined by the Morlet wavelet transform, is detected in the visible continuum channel. The modulation depth, which is defined as the ratio between the oscillatory amplitude and its long-term trend, is smaller than 0.1%, implying that the quasi-periodic pulsation (QPP) feature is a weak wave process. Imaging observations show that the X6.4 flare occurs near a sunspot group. Moreover, the white-light brightening is located in sunspot penumbra, and a similar quasi-period of about 8.5 −1.8+1.6 minutes is identified in one penumbral location of the nearest sunspot. The map of Fourier power distribution suggests that a similar periodicity is universally existing in most parts of the penumbra that is close to the penumbral–photospheric boundary. Our observations support the scenario that the white-light QPP is probably modulated by the slow-mode magnetoacoustic gravity wave leaking from the sunspot penumbra.

Efficiency of solar microflares in accelerating electrons when rooted in a sunspot

Context. The spectral shape of the X-ray emission in solar flares varies with the event size, with small flares generally exhibiting softer spectra than large events, indicative of a relatively lower number of accelerated electrons at higher energies. Aims. We investigate two microflares of GOES classes A9 and C1 (after background subtraction) observed by STIX onboard Solar Orbiter with exceptionally strong nonthermal emission. We complement the hard X-ray imaging and spectral analysis by STIX with co-temporal observations in the (E)UV and visual range by AIA and HMI to investigate what makes these microflares so efficient in high-energy particle acceleration. Methods. We made a preselection of events in the STIX flare catalog based on the ratio of the thermal to nonthermal quicklook X-ray emission. The STIX spectrogram science data were used to perform spectral fitting to identify the non-thermal and thermal components. The STIX X-ray images were reconstructed to analyze the spatial distribution of the precipitating electrons and the hard X-ray emission they produce. The EUV images from SDO/AIA and SDO/HMI LOS magnetograms were analyzed to better understand the magnetic environment and the chromospheric and coronal response. For the A9 event, EOVSA microwave observations were available, allowing for image reconstruction in the radio domain. Results. We performed case studies of two microflares observed by STIX on October 11, 2021 and November 10, 2022, which showed unusually hard microflare X-ray spectra with power-law indices of the electron flux distributions of δ = (2.98 ± 0.25) and δ = (4.08 ± 0.23), during their non-thermal peaks and photon energies up to 76 keV and 50 keV, respectively. For both events under study, we found that one footpoint is located within a sunspot covering areas with mean magnetic flux densities in excess of 1500 G, suggesting that the hard electron spectra are caused by the strong magnetic fields the flare loops are rooted in. Additionally, we revisited a previously published unusually hard RHESSI microflare and found that in this event, there was also one flare kernel located within a sunspot, which corroborates the result from the two hard STIX microflares under study in this work. Conclusions. The characteristics of the strong photospheric magnetic fields inside the sunspot umbrae and penumbrae where flare loops are rooted play an important role in the generation of exceptionally hard X-ray spectra in these microflares.

Temporal and Periodic Analysis of Penumbra–Umbra Ratio for the Last Four Solar Cycles

We investigate the long-term dynamic behavior of the sunspot penumbra to umbra area ratio by analyzing the Debrecen Photoheliographic Data (DPD) of sunspot groups during the period 1976–2017 (Solar Cycles 21–24). We consider all types of spots and find that the average penumbra–umbra ratio does not exhibit any significant variation with spot latitudes, solar-cycle phases as well as sunspot-cycle strengths. However, the behavior of this ratio is different when we consider the latitudinal distribution of the northern and southern hemispheres separately. Our analysis indicates that for daily total sunspot area the average spot ratio varies from 5.5 to 6.5 and for very large sunspots (> 5000 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$\\mu$\\end{document}Hem; one μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$\\mu$\\end{document}Hem is 10−6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$10^{-6}$\\end{document} the area of visual solar hemisphere) its value rises to about 8.3. In the case of the group-sunspot area, the average spot ratio is ∼6.76. Furthermore, we found that this ratio exhibits a trend for both smaller (area <100 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$\\mu$\\end{document}Hem) and large (area > 100 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$\\mu$\\end{document}Hem) sunspots. Finally, we report the periodic and quasiperiodic variations present in this ratio time series after applying the multitaper method (MTM) and Morlet-wavelet technique. We found that along with the ∼11-year solar-cycle period, the penumbra to umbra area ratio also shows several midterm variations, specifically, Rieger-type and quasibiennial periodicities. We also found that Rieger-type periods occur in all cycles, but the temporal evolution and the modulation of these types of periodicities are different in different solar cycles.

The relation between magnetic field inclination and the apparent motion of penumbral grains

Context. The bright heads of penumbral filaments, penumbral grains (PGs), show apparent horizontal motions inward, toward the umbra, or outward, away from the umbra. Aims. We aim to ascertain statistically whether the direction of PGs’ apparent motion is related to the inclination of the surrounding magnetic field. Methods. We used spectropolarimetric observations of five sunspot penumbrae to compare magnetic inclinations inside PGs with those in their surroundings. The data were taken by three observatories: the Hinode satellite, the Swedish Solar Telescope, and the GREGOR solar telescope. The direction of PGs’ motion was determined by feature tracking. The atmospheric conditions in PGs and their surroundings, including magnetic field information, were retrieved by means of height-stratified spectropolarimetric inversions. Results. Out of a sample of 444 inward-moving PGs and 269 outward-moving ones, we show that 43% of the inward-moving PGs have a magnetic inclination larger by 8° ±4° than the inclination in their surroundings and 51% of the outward-moving PGs have an inclination smaller by 13° ±7° than the surrounding one. The opposite relation of inclinations is observed in only one fifth of the inward- and outward-moving PGs. Conclusions. Rising hot plasma in PGs surrounded by a less inclined magnetic field may adapt its trajectory to be more vertical, causing an inward apparent motion of PGs. Conversely, it may be dragged by a more horizontal surrounding magnetic field such that an outward apparent motion is observed.

Observations of the Chromospheric Evershed Flow of Sunspot Penumbra with the Application of the Self-organizing Map Technique

The sunspot penumbra is usually observed in the photosphere and it is of particular interest for its magnetoconvection, which seems to transport the heat from the top of the convection zone into the solar atmosphere. It is well known that the penumbra magnetic field extends into the upper layers of the solar atmosphere forming the so-called superpenumbra. Thanks to the application of the Self-organizing Map technique to a spectral data set containing monochromatic images acquired along the Ca ii 854.2 nm and Hα 656.28 nm lines, we were able to segment the penumbra and to measure the plasma velocity along the chromospheric portions of penumbral filaments. We found that the head, body, and tail of penumbral filaments show vertical flows compatible with the persistence of the Evershed flow. Instead, the inverse Evershed flow has been observed only in the outer portion of the superpenumbra. We found that two opposite Evershed regimes work next to each other, without overlapping, and both contribute to the downflow around sunspots. These results confirm the uncombed model of the sunspot penumbra and provide some hints that the downflow around sunspots may be ascribed to the magnetic field dragging the plasma down.

Expulsion of Counter Evershed Flows from Sunspot Penumbrae

In addition to the Evershed flow directed from the umbra toward the outer boundary of a sunspot, under special circumstances a counter Evershed flow (CEF) in the opposite direction also occurs. We aim to characterize the proper motions and evolution of three CEFs observed by the Solar Optical Telescope on board the Japanese Hinode spacecraft and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We use state-of-the-art inversions of the radiative-transfer equation of polarized light applied to spectropolarimetric observations of the Fe i line pair around 630 nm. The three CEFs appeared within the penumbra. Two of the CEF structures, as part of their decay process, were found to move radially outwards through the penumbra parallel to the penumbral filaments with speeds, deduced from their proper motions, ranging between 65 and 117 m s−1. In these two cases, a new spot appeared in the moat of the main sunspot after the CEFs reached the outer part of the penumbra. Meanwhile, the CEFs moved away from the umbra, and their magnetic field strengths decreased. The expulsion of these two CEFs seems to be related to the normal Evershed flow. The third CEF appeared to be dragged by the rotation of a satellite spot. Chromospheric brightenings were found to be associated with the CEFs, and those CEFs that reached the umbra–penumbra boundary showed enhanced chromospheric activity. The two CEFs, for which line-of-sight velocity maps were available during their formation phase, appear as intrusions into the penumbra. They may be associated with magnetic flux emergence.

The role of the chromospheric magnetic canopy in the formation of a sunspot penumbra

Context. While it is conjectured that a chromospheric canopy plays a role in penumbra formation, it has been difficult to find observational evidence of the connectivity between the photosphere and the chromosphere. Aims. We investigate the existence of a chromospheric canopy as a necessary condition for the formation of a penumbra. Another aim is to find the origin of the inclined magnetic fields. Methods. Spectropolarimetric observations of NOAA AR 12776 from the GRIS spectrograph at the GREGOR telescope were analyzed. Atmospheric parameters were obtained from the deep photospheric Ca I 10 839 Å line (VFISV inversion code), the mostly photospheric Si I 10 827 Å line (SIR inversion code), and the chromospheric He I 10 830 Å triplet (HAZEL inversion code). We compared the photospheric and chromospheric magnetic topology of a sunspot sector with a fully-fledged penumbra to a sector where no penumbra formed. Additionally, imaging data from the BBI instrument, attached to the GREGOR telescope, in TiO-band and G-band were analyzed. Results. In the deepest atmospheric layers, we find that the magnetic properties (inclination and field strength distribution) measured on the sunspot sector with fully-fledged penumbra are similar to those measured on the sector without penumbra. However, in higher layers the magnetic properties are different. In the region showing no penumbra, almost vertical chromospheric magnetic fields are observed. Additionally, thin filamentary structures with a maximum width of 0.1″ are seen in photospheric high-resolution TiO-band images in this region. Conclusions. The existence of a penumbra is found to be determined by the conditions in the chromosphere. This indicates that a chromospheric canopy is a necessary condition for the formation of a penumbra. However, our results demonstrate that inclined fields in the chromospheric canopy are not needed for the development of inclined fields in the photosphere. We question the “fallen magnetic flux tubes” penumbra formation scenario and favor a scenario in which inclined fields emerge from below the surface and are blocked by the overlying chromospheric canopy.

Automatic segmentation of the fine structures of sunspots in high-resolution solar images

Context. With the development of large-aperture ground-based solar telescopes and the adaptive optics system, the resolution of the obtained solar images has become increasingly higher. In the high-resolution photospheric images, the fine structures (umbra, penumbra, and light bridge) of sunspots can be observed clearly. The research of the fine structures of sunspots can help us to understand the evolution of solar magnetic fields and to predict eruption phenomena that have significant impacts on the Earth, such as solar flares. Therefore, algorithms for automatically segmenting the fine structures of sunspots in high-resolution solar image will greatly facilitate the study of solar physics. Aims. This study is aimed at proposing an automatic fine-structure segmentation method for sunspots that is accurate and requires little time. Methods. We used the superpixel segmentation to preprocess a solar image. Next, the intensity information, texture information, and spatial location information were used as features. Based on these features, the Gaussian mixture model was used to cluster different superpixels. According to different intensity levels of the umbra, penumbra, and quiet photosphere, the clusters were classified into umbra, penumbra, and quiet-photosphere areas. Finally, the morphological method was used to extract the light-bridge area. Results. The experimental results show that the method we propose can segment the fine structures of sunspots quickly and accurately. In addition, the method can process high-resolution solar images from different solar telescopes and generates a satisfactory segmentation performance.

First looks at solar active regions with ALMA

During the first few years of observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA), the scientific community has acquired a number of observational datasets targeting various structures in active regions, including sunspot umbra and penumbra, active region pores, and plages. In this paper we review the results obtained from the extensive analysis of these interferometric millimeter data, together with the coordinated observations from IRIS, SDO, IBIS, and Hinode, that reveal information on the chromospheric thermal structure above active regions and properties of small-scale heating events near magnetic field concentrations. We discuss the properties of waves (especially the three-minute oscillations) in sunspots, plage, and network. We speculate how high-resolution millimeter data can supplement spectral line observations in the visible and UV and can improve chromospheric spectroscopic inversions. We identify challenges in the interpretation of the millimeter continuum emission due to the complex, non-local and time-dependent processes that determine the electron density through the chromosphere. Finally we overview the prospects for future active regions observations with ALMA during the ascending phase of the solar cycle.

Clarifying Physical Properties of Magnetic Fields in Sunspots

We demonstrate that the radial magnetic-field component at the outer boundary of the sunspot penumbra is about 550 Mx cm$^{-2}$ independent of the sunspot area and the maximum magnetic field in the umbra. The mean magnetic-field intensity in sunspots grows slightly as the sunspot area increases up to 500 -- 1000 millionth of visual hemisphere (m.v.h.) and may reach about 900 -- 2000 Mx cm$^{-2}$. The total magnetic flux weakly depends on the maximum field strength in a sunspot and is determined by the spottedness, i.e. the sunspot number and the total sunspot area; however, the relation between the total flux and the sunspot area is substantially nonlinear. We suggest an explicit parametrization for this relation. The contribution of the magnetic flux associated with sunspots to the total magnetic flux is small, not achieving more than 20% even at the maximum of the solar activity.

Chinese Sunspot Drawings and Their Digitization—(VII) Sunspot Penumbra to Umbra Area Ratio Using the Hand-Drawing Records from Yunnan Observatories

The ratio of penumbral to umbra area of sunspots plays a crucial role in the solar physics fields, especially for understanding the origin and evolution of the solar activity cycle. By analyzing the recently digitized sunspot drawings observed from Yunnan Observatories (1957–2021), we investigate the long-term variation of the penumbral to umbra area ratio of sunspots. An automatic extraction method, based on the maximum between-class variance and the morphological discrimination, is used to accurately extract penumbra and umbra and to calculate the ratio over six solar cycles (cycle 19–24). The expected value of the ratio of penumbra to umbra area is found to be 6.63 ± 0.98, and it does not exhibit any systematic variation with sunspot latitudes and phases. The average ratio fluctuates from 5 to 7.5 per year and the overall trend has decreased after 1999 compared to the previous one. The ratio of sunspot penumbra to umbra area satisfies the log-normal distribution, implying that its variation is related to the evolution of the photospheric magnetic field. Our results are consistent with previous works.

Horizontal motions in sunspot penumbrae

Context. A model of penumbral filaments represented by magnetoconvective cells was derived recently from spectropolarimetric observations. This model resolves many of the inconsistencies found in the relations between intensity, magnetic, and velocity patterns in sunspot penumbrae. Aims. High-resolution observations of horizontal motions in the penumbra are needed to complement the concept of penumbrae obtained from spectropolarimetry. Time series of intensity images of a large sunspot in AR 10634 acquired with the Swedish Solar Telescope in the G band and red continuum are analysed. The two simultaneous time series last six hours and five minutes. Methods. Horizontal motions of penumbral grains (PGs), structures in dark bodies of filaments, the outer penumbral border, and G-band bright points are measured in time slices that cover the whole width of the penumbra and the neighbouring granulation. The spatial and temporal resolutions are 90 km and 20.1 s, respectively. Results. In the inner penumbra, PGs move towards the umbra (inwards) with a mean speed of −0.7 km s−1. The direction of motion changes from inwards to outwards at approximately 60% of the penumbral width, and the mean speed increases gradually in the outer penumbra, approaching 0.5 km s−1. This speed is also typical of an expansion of the penumbra–granulation border during periods that typically last one hour and are followed by a fast contraction. The majority of the G-band bright points moves away from the sunspot, with a typical speed of 0.6 km s−1. High outward speeds, 3.6 km s−1 on average, are observed in dark bodies of penumbral filaments. Conclusions. According to the model of penumbral filaments, it is suggested that the speeds detected in the dark bodies of filaments are associated with the Evershed flow and that the opposite directions of PG motions in the inner and outer penumbrae may be explained by the interaction of rising plasma in filament heads with a surrounding, differently inclined magnetic field.

Magnetized supersonic downflows in the chromosphere

The chromosphere above active regions (ARs) on the Sun hosts magnetized supersonic downflows. Studies of these supersonic downflows help to decipher the magnetic fine structure and dynamics of the chromosphere. We perform a statistical analysis of the magnetized supersonic downflows in a number of ARs at different evolutionary stages and survey their characteristics. We analyze spectro-polarimetric scans of parts of 13 ARs obtained in the infrared He I 10 830 Å triplet formed in the upper chromosphere recorded with the GREGOR Infrared Spectrograph mounted at the GREGOR solar telescope. We retrieve the line-of-sight velocities and the magnetic field vector using the HELIX+ inversion code that assumes Milne-Eddington atmospheres. We find magnetized supersonic downflows in all the ARs, with larger area coverage by such flows in ARs observed during their emerging phase. The fact that supersonic downflows were detected in all scans, albeit only covering a small fraction, 0.2–6.4%, of the observed field-of-view, suggests that they are a comparatively common phenomenon in the upper chromospheres of ARs. The supersonic downflows are found to be associated with many AR features, such as pores, sunspot umbrae, sunspot penumbrae, light bridges, plages, He I loops as part of arch filament systems characteristic of emerging fields, and filaments. Although several mechanisms are identified to be causing the supersonic downflows, by far the most common one appears to be the draining of plasma along the legs of rising magnetic loops. The loops mainly drain into forming pores. The line-of-sight velocities of the supersonic downflows reach values of up to 49 km s−1, and the velocity distribution shows multiple populations. Almost 92% of these supersonic downflows coexist with a subsonic flow component. The weaker, more horizontal fields associated with the supersonic component suggests that it is formed above the subsonic component.

Similarities of magnetoconvection in the umbra and in the penumbra of sunspots

Context. It is unclear why there is a rather sharp boundary in sunspots between the umbra and the penumbra. Both regions exhibit magnetoconvection, which manifests in penumbral filaments in the penumbra and in umbral dots in the umbra. Aims. Here we compare the physical properties of umbral dots and penumbral filaments. Our goal is to understand how the properties of these convective features change across the boundary between the umbra and the penumbra and how this is related to the rapid increase in brightness at the umbra-penumbra boundary. Methods. We derived ensemble averages of the physical properties of different types of convective features based on observations of two sunspots with Hinode. Results. There are strong similarities between the convective features in the outer parts of the umbra and the ones in the penumbra, with most physical parameters being smooth and continuous functions of the length of the features. Conclusions. Our results indicate that the transition in brightness from the umbra to the penumbra is solely caused by an increased effectiveness of magnetoconvection within individual convective cells. There is no significant difference in the number density of convective elements between the outer umbra and the inner penumbra. Penumbral filaments exhibit a larger area and a higher brightness compared to umbral dots. It is still unclear how exactly the underlying magnetic field causes the increase in the size and brightness of convective features in the penumbra.

Penumbral decay observed in active region NOAA 12585

Context. The physical conditions leading the sunspot penumbra decay are poorly understood so far. Aims. We investigate the photospheric magnetic and velocity properties of a sunspot penumbra during the decay phase to advance the current knowledge of the conditions leading to this process. Methods. A penumbral decay was observed with the CRISP instrument at the Swedish 1 m Solar Telescope on 2016 September 4 and 5 in the active region NOAA 12585. During these days, full-Stokes spectropolarimetric scans along the Fe I 630 nm line pair were acquired over more than one hour. We inverted these observations with the VFISV code to obtain the evolution of the magnetic and velocity properties. We complement the study with data from instruments on board the Solar Dynamics Observatory and Hinode space missions. Results. The studied penumbra disappears progressively in time and space. The magnetic flux evolution seems to be linked to the presence of moving magnetic features (MMFs). Decreasing Stokes V signals are observed. Evershed flows and horizontal fields were detected even after the disappearance of the penumbral sector. Conclusions. The analyzed penumbral decay seems to result from the interaction between opposite polarity fields in type III MMFs and penumbra, while the presence of overlying canopies regulates the evolution in the different penumbral sectors.

Evolution of solar surface inflows around emerging active regions

Context.Solar active regions are associated with Evershed outflows in sunspot penumbrae, moat outflows surrounding sunspots, and extended inflows surrounding active regions. Extended inflows have been identified around established active regions with various methods. The evolution of these inflows and their dependence on active region properties as well as their effect on the global magnetic field are not yet understood.Aims.We aim to understand the evolution of the average inflows around emerging active regions and to derive an empirical model for these inflows. We expect that this can be used to better understand how the inflows act on the diffusion of the magnetic field in active regions.Methods.We analyzed horizontal flows at the surface of the Sun using local correlation tracking of solar granules observed in continuum images of the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We measured average flows of a sample of 182 isolated active regions up to seven days before and after their emergence onto the solar surface with a cadence of 12 h. About half of the active regions in the sample developed sunspots with moat flows in addition to the surrounding inflows. We investigated the average inflow properties with respect to active region characteristics of total flux and latitude. We fit a model to these observed inflows for a quantitative analysis.Results.We find that converging flows of about 20–30 m s−1are first visible one day prior to emergence, in agreement with recent results. These converging flows are present regardless of the active region properties of latitude or flux. We confirm a recently found prograde flow of about 40 m s−1at the leading polarity during emergence. We find that the time after emergence when the latitudinal inflows increase in amplitude depends on the flux of the active region, ranging from one to four days after emergence and increasing with flux. The largest extent of the inflows is up to about 7 ± 1° away from the center of the active region within the first six days after emergence. The inflow velocities have amplitudes of about 50 m s−1.

A deep learning method to estimate magnetic fields in solar active regions from photospheric continuum images

Context. The magnetic field is the underlying cause of solar activities. Spectropolarimetric Stokes inversions have been routinely used to extract the vector magnetic field from observations for about 40 years. In contrast, the photospheric continuum images have an observational history of more than 100 years. Aims. We suggest a new method to quickly estimate the unsigned radial component of the magnetic field, |Br|, and the transverse field, Bt, just from photospheric continuum images (I) using deep convolutional neural networks (CNN). Methods. Two independent models, that is, I versus |Br| and I versus Bt, are trained by the CNN with a residual architecture. A total of 7800 sets of data (I, Br and Bt) covering 17 active region patches from 2011 to 2015 from the Helioseismic and Magnetic Imager are used to train and validate the models. Results. The CNN models can successfully estimate |Br| as well as Bt maps in sunspot umbra, penumbra, pore, and strong network regions based on the evaluation of four active regions (test datasets). From a series of continuum images, we can also detect the emergence of a transverse magnetic field quantitatively with the trained CNN model. The three-day evolution of the averaged value of the estimated |Br| and Bt from continuum images follows that from Stokes inversions well. Furthermore, our models can reproduce the nonlinear relationships between I and |Br| as well as Bt, explaining why we can estimate these relationships just from continuum images. Conclusions. Our method provides an effective way to quickly estimate |Br| and Bt maps from photospheric continuum images. The method can be applied to the reconstruction of the historical magnetic fields and to future observations for providing the quick look data of the magnetic fields.

How rare are counter Evershed flows?

One of the main characteristics of sunspot penumbrae is the radially outward-directed Evershed flow. Only recently have penumbral regions been reported with similar characteristics to normal penumbral filaments but with an opposite direction of the flow. Such flows directed toward the umbra are known as counter Evershed flows (CEFs). We aim to determine the occurrence frequency of CEFs in active regions (ARs) and to characterize their lifetime and the prevailing conditions in the ARs. We analyzed the continuum images, Dopplergrams, and magnetograms recorded by SDO/HMI of 97 ARs that appeared from 2011 to 2017. We followed the ARs for 9.6 ± 1.4 days on average. We found 384 CEFs in total, with a median value of six CEFs per AR. Counter Evershed flows are a rather common feature, occurring in 83.5% of all ARs regardless of the magnetic complexity of the AR. However, CEFs were only observed, on average, during 5.9% of the mean total duration of all the observations analyzed here. The lifetime of CEFs follows a log-normal distribution with a median value of 10.6−6.0+12.4 h. In addition, we report two populations of CEFs, those that are associated with light bridges and those that are not. We explain that the rarity of reports of CEFs in the literature is a result of highly incomplete coverage of ARs with spectropolarimetric data. By using the continuous observations now routinely available from space, we are able to overcome this limitation.