A sunspot penumbra is observationally examined to reveal properties of small-scale flow structures and how they are related to the filamentary magnetic structures and the Evershed flow. We also study how the photospheric dynamics is related to chromospheric activities. The study is based on data analysis of spectro-polarimetric observations of photospheric Fe I lines with the Solar Optical Telescope aboard Hinode in a sunspot penumbra at different heliocentric angles. Vector magnetic fields and velocities are derived using the spectro-polarimetric data and a Stokes inversion technique. An observation with a Ca II H filtergram co-spatial and co-temporal with the spectro-polarimetric one is also used to study possible chromospheric responses. We find small patches with downflows at photospheric layers. The downflow patches have a size of 0.5" or smaller and have a geometrical configuration different from that of the Evershed flow. The downflow velocity is about 1 km/s at lower photspheric layers, and is almost zero in the upper layers. Some of the downflow patches are associated with brightenings seen in Ca II H images. The downflows are possible observational signatures of downward flows driven by magnetic reconnection in the interlaced magnetic field configuration, where upward flows make brightenings in the chromosphere. Another possibility is that they are concentrated downward flows of overturning magnetoconvection.
Sunspot penumbrae show high-velocity patches along the periphery. The high-velocity downflow patches are believed to be the return channels of the Evershed flow. We aim to investigate their structure in detail using Hinode SOT/SP observations. We employ Fourier interpolation in combination with spatially coupled height dependent LTE inversions of Stokes profiles to produce high-resolution, height-dependent maps of atmospheric parameters of these downflows and investigate their properties. High-speed downflows are observed over a wide range of viewing angles. They have supersonic line-of-sight …
We study the presence of reversed polarity magnetic flux in sunspot penumbra. We applied a new regularized method to deconvolve spectropolarimetric data observed with the spectropolarimeter SP onboard Hinode. The new regularization is based on a principal component decomposition of the Stokes profiles. The resulting Stokes profiles were inverted to infer the magnetic field vector using SIR. We find, for the first time, reversed polarity fields at the border of many bright penumbral filaments in the whole penumbra.
Spatial distributions of the dominant oscillation frequency obtained for four sunspots show a feature shared by all the analysed levels of the solar atmosphere in these sunspots. This feature located in the inner penumbrae indicates that this region has favourable conditions for 2.5-4 mHz oscillation propagation. This agrees with the fact that the spectral composition of the oscillations at three atmospheric heights (FeI 6173Å, 1700Å, and He II 304Å) in this region are similar. There have been previous evidence of particular similarities along height of photospheric magnetic field strength, li…
To investigate the penumbral plasma flow on a small scale, spectropolari- metric data of sunspots recorded by HINODE was used. Maps of Doppler velocities were created by evaluating the bisector in the line-wing, thereby visualizing the flow pattern in the low photosphere where the Evershed effect is most pronounced. In penumbrae close to the disk center, the vertical component of the Evershed flow dominates. The latter consists of a series of elongated up-flow patterns extending radially through the entire center-side penumbra at a constant azimuth. Along this structure, strong up-flows appear…
Aims: We search for penumbral magnetic fields of opposite polarity and for their correspondence with downflows. Methods: We used spectropolarimetric HINODE data of a spot very close to disk center to suppress the horizontal velocity components as much as possible. We focus our study on 3-lobe Stokes V profiles. Results: From forward modeling and inversions, we show that 3-lobe profiles testify to the presence of opposite magnetic fields. They occur predominately in the mid and outer penumbra and are associated with downflows in the deep layers of the photosphere. Conclusions. Standard magnetog…
The satellite Hinode has recently revealed penumbral structures with a magnetic polarity opposite to the main sunspot polarity. They may be a direct confirmation of magnetic field lines and mass flows returning to the solar interior throughout the penumbra, a configuration previously inferred from interpretation of observed Stokes profile asymmetries. The paper points out the relationship between the reverse polarity features found by Hinode, and the model Micro-Structured Magnetic Atmospheres (MISMAs) proposed for sunspots. We show how the existing model MISMAs produce strongly redshifted rev…
Recent spectro-polarimetric observations of a sunspot showed the formation of bipolar magnetic patches in the mid penumbra and their propagation toward the outer penumbral boundary. The observations were interpreted as being caused by sea-serpent magnetic fields near the solar surface (Sainz Dalda & Bellot Rubio 2008). In this Letter, we develop a 3D radiative MHD numerical model to explain the sea-serpent structure and the wave-like behavior of the penumbral magnetic field lines. The simulations reproduce the observed behavior, suggesting that the sea-serpent phenomenon is a consequence of ma…
We have employed a penumbral model, that includes the Evershed flow and convective motions inside penumbral filaments, to reproduce the azimuthal variation of the net circular polarization (NCP) in sunspot penumbrae at different heliocentric angles for two different spectral lines. The theoretical net circular polarization fits the observations as satisfactorily as penumbral models based on flux-tubes. The reason for this is that the effect of convective motions on the NCP is very small compared to the effect of the Evershed flow. In addition, the NCP generated by convective upflows cancels ou…
Downflows at supersonic speeds have been observed in the transition region (TR) above sunspots for more than three decades. These downflows are often seen in different TR spectral lines above sunspots. We have performed a statistical investigation of these downflows using a large sample which was missing earlier. The Interface Region Imaging Spectrograph (IRIS) has provided a wealth of observational data of sunspots at high spatial and spectral resolution in the past few years. We have identified sixty datasets obtained with IRIS raster scans. Using an automated code, we identified the locatio…
High resolution spectropolarimetric observations of 3 sunspots taken with Hinode demonstrate the existence of supersonic downflows at or close to the umbra-penumbra boundary which have not been reported before. These downflows are confined to large patches, usually encompassing bright penumbral filaments, and have lifetimes of more than 14 hr. The presence of strong downflows in the center-side penumbra near the umbra rules out an association with the Evershed flow. Chromospheric filtergrams acquired close to the time of the spectropolarimetric measurements show large, strong, and long-lived b…
We report three-minute oscillations in the solar chromosphere driven by a strong downflow event in a sunspot. We used the Fast Imaging Solar Spectrograph of the 1.6 m New Solar Telescope and the Interface Region Imaging Spectrograph (IRIS). The strong downflow event is identified in the chromospheric and transition region lines above the sunspot umbra. After the event, oscillations occur at the same region. The amplitude of the Doppler velocity oscillations is 2 km/s, and gradually decreases with time. In addition, the period of the oscillations gradually increases from 2.7 minutes to 3.3 minu…
We report the discovery of supersonic downflows in a sunspot light bridge using measurements taken with the spectropolarimeter on board the Hinode satellite. The downflows occur in small patches close to regions where the vector magnetic field changes orientation rapidly, and are associated with anomalous circular polarization profiles. An inversion of the observed Stokes spectra reveals velocities of up to 10 km/s, making them the strongest photospheric flows ever measured in light bridges. Some (but not all) of the downflowing patches are cospatial and cotemporal with brightness enhancements…
Recent high resolution spectropolarimetric observations from Hinode detected the presence of supersonic downflows in a sunspot light bridge (Louis et al. 2009). These downflows occurred in localized patches, close to regions where the field azimuth changed by a large value. This apparent discontinuity in the field azimuth was seen along a thin ridge running along the western edge of the light bridge. Some, but not all, of these downflowing patches were co-spatial with chromospheric brightness enhancements seen in Ca II H filtergrams. The presence of magnetic inhomogeneities at scales of 0.3 ar…
High resolution spectropolarimetric observations by Hinode reveal the existence of supersonic downflows at the umbra-penumbra boundary of 3 sunspots that was recently reported by Louis et al. (2010). These downflows are observed to be co-spatial with bright penumbral filaments and occupy an area greater than 1.6 arcsec$^2$. They are located at the center-side penumbra and have the same polarity as the sunspot which suggests that they are not associated with the Evershed Flow. In this paper we describe the supersonic velocities observed in NOAA AR 10923 and discuss the photospheric as well as c…
Recently, there have been some reports of unusually strong photospheric magnetic fields (which can reach values of over 7 kG) inferred from Hinode SOT/SP sunspot observations within penumbral regions. These superstrong penumbral fields are even larger than the strongest umbral fields on record and appear to be associated with supersonic downflows. The finding of such fields has been controversial since they seem to show up only when spatially coupled inversions are performed. Here, we investigate and discuss the reliability of those findings by studying in detail observed spectra associated wi…
Daily records of sunspot group areas compiled by the Royal Observatory, Greenwich, from May of 1874 through 1976 indicate a curious history for the penumbral areas of the smaller sunspot groups. On average, the ratio of penumbral area to umbral area in a sunspot group increases from 5 to 6 as the total sunspot group area increases from 100 to 2000 microHem (a microHem is a millionth the area of a solar hemisphere). This relationship does not vary substantially with sunspot group latitude or with the phase of the sunspot cycle. However, for the sunspot groups with total areas <100 microHem, thi…
Super-sonic downflows have been observed in transition region spectra above numerous sunspots; however, little research has been conducted to date into how persistent these signatures are within sunspots on time-scales longer than a few hours. Here, we analyse the lead sunspot of AR 12526 to infer the properties and evolution of super-sonic downflows occurring within it. Sixteen large, dense raster scans sampled by IRIS are analysed. These rasters tracked the lead sunspot of AR 12526 at discrete times between the 27th March 2016 and the 2nd April 2016. One sit-and-stare observation acquired on…
We report spectropolarimetric observations of a supersonic downflow impacting the lower atmosphere within a large sunspot umbra. This work is an extension of Schad et al. 2016 using observations acquired in the He I 10830 Angstrom triplet by the Facility Infrared Spectropolarimeter. Downflowing material accelerating along a cooled coronal loop reaches peak speeds near 200 km s$^{-1}$ and exhibits both high speed emission and absorption within the umbra, which we determine to be a consequence of the strong height dependence of the radiatively-controlled source function above the sunspot umbra. …
Recent numerical simulations and observations of sunspots show a significant amount of opposite polarity magnetic field within the sunspot penumbra. Most of the opposite polarity field is associated with convective downflows. We present an analysis of 3D MHD simulations through forward modeling of synthetic Stokes profiles of the Fe\sci 6301.5 Å~ and Fe\sci 6302.5 Å~ lines). The synthetic Stokes profiles are spatially and spectrally degraded considering typical instrument properties. Line bisector shifts of the Fe\sci 6301.5 Å~ line are used to determine line-of-sight velocities. Far wing magn…
The motion of dark striations across bright filaments in a sunspot penumbra has become an important new diagnostic of convective gas flows in penumbral filaments. The nature of these striations has, however, remained unclear. Here we present an analysis of small scale motions in penumbral filaments in both simulations and observations. The simulations, when viewed from above, show fine structure with dark lanes running outwards from the dark core of the penumbral filaments. The dark lanes either occur preferentially on one side or alternate between both sides of the filament. We identify this …
We study the velocity structure of penumbral filaments in the deep photosphere to obtain direct evidence for the convective nature of sunspot penumbrae. A sunspot was observed at high spatial resolution with the 1-m Swedish Solar Telescope in the deep photospheric C I 5380 Å absorption line. The Multi-Object Multi-Frame Blind Deconvolution (MOMFBD) method is used for image restoration and straylight is filtered out. We report here the discovery of clear redshifts in the C I 5380 Å line at multiple locations in sunspot penumbral filaments. For example, bright head of filaments show larger conce…
Phase perturbations due to inclined surface magnetic field of active region strength are calculated numerically in quiet Sun and simple sunspot models in order to estimate and compare the direct and indirect (thermal) effects of the fields on helioseismic waves. It is found that the largest direct effects occur in highly inclined field characteristic of penumbrae, and scale roughly linearly with magnetic field strength. The combined effects of sunspot magnetic and thermal anomalies typically yield negative travel-time perturbations in penumbrae. Travel-time shifts in umbrae depend on details o…
Downflows with high velocities occur in the transition region above many sunspots; however, how these signatures evolve over short time-scales in both spatial and spectral terms is still unknown. In this article, we investigate the evolution of downflows detected within the transition region on time-scales of the order minutes and search for clues as to their formation mechanisms in co-temporal imaging data. The high-resolution spectral and imaging data used to identify downflows here were sampled by IRIS on the 20th and 21st May 2015. Imaging data from the Hinode and SDO satellites provided c…
We study the temporal evolution of downflows observed at the lateral edges of penumbral filaments in a sunspot located very close to the disk center. Our analysis is based on a sequence of nearly diffraction-limited scans of the Fe I 617.3 nm line taken with the CRisp Imaging Spectro-Polarimeter at the Swedish 1 m Solar Telescope. We compute Dopplergrams from the observed intensity profiles using line bisectors and filter the resulting velocity maps for subsonic oscillations. Lateral downflows appear everywhere in the center-side penumbra as small, weak patches of redshifts next to or along th…
We investigate a small-scale ($\approx$ 1.5 Mm along the slit), supersonic downflow of about 90 km s$^{-1}$ in the transition region above the light-bridged sunspot umbra in AR 11836. The observations were obtained with the Interface Region Spectrograph (IRIS) on 2013 September 2, from 16:40 to 17:59 UT. The downflow shows up as red-shifted "satellite" lines of the Si IV and O IV transition region lines and is remarkably steady over the observing period of nearly 80 min. The downflow is not visible in the chromospheric lines, which only show an intensity enhancement at the location of the down…
We present an analysis of twisting motions in penumbral filaments in sunspots located at heliocentric angles from $30^\circ$ to $48^\circ$ using three time series of blue continuum images obtained by the Broadband Filter Imager (BFI) onboard {\it Hinode}. The relations of the twisting motions to the filament brightness and the position within the filament and within the penumbra, respectively, are investigated. Only certain portions of the filaments show twisting motions. In a statistical sense, the part of the twisting portion of a filament located closest to the umbra is brightest and posses…
In this work we selected one particular fibril from a high resolution solar chromosphere observation from the Dutch Open Telescope, and tried to obtain a broad picture of the intricate mechanism that might be incorporated in the multiple layer of the Solar atmosphere in high cadence multi-wavelength observation. We analyzed the changingvfibril patter using multi-wavelength tomography, which consists of both H$α$ line center \& the blue wing, Doppler-signal, Ca II H, and the G-band. We have found that the intermittent ejected material through fibril from Doppler images has clearly shown oscilla…
We have observed formation of penumbrae on a pore in the active region NOAA10838 using Dunn Solar Telescope at NSO,Sunpot,USA. Simultaneous observations using different instruments (DLSP,UBF,Gband and CaK) provide us with vector magnetic field at photosphere, intensity images and Doppler velocity at different heights from photosphere to chromosphere. Results from our analysis of this particular data-set suggests that penumbrae are formed as a result of relaxation of magnetic field due to a flare happening at the same time. Images in \Halpha\ show the flare (C 2.9 as per GOES) and vector magnet…
We study the velocity field of umbral dots at a resolution of 0.14". Our analysis is based on full Stokes spectropolarimetric measurements of a pore taken with the CRISP instrument at the Swedish 1-m Solar Telescope. We determine the flow velocity at different heights in the photosphere from a bisector analysis of the Fe I 630 nm lines. In addtion, we use the observed Stokes Q, U, and V profiles to characterize the magnetic properties of these structures. We find that most umbral dots are associated with strong upflows in deep photospheric layers. Some of them also show concentrated patches of…
Ellerman bombs are regions with enhanced Balmer line wing emission and mark magnetic reconnection in the deep solar atmosphere in active regions and quiet Sun. They are often found in regions where opposite magnetic polarities are in close proximity. Recent high resolution observations suggest that Ellerman bombs are more prevalent than thought before. We aim to determine the occurrence of Ellerman bombs in the penumbra of sunspots. We analyze high spatial resolution observations of sunspots in the Balmer H-alpha and H-beta lines as well as auxiliary continuum channels obtained with the Swedis…
We investigate subsurface moat flow system around symmetrical sunspots of McIntosh type H and compare it to the flow system within supergranular cells. Representatives of both types of flows are constructed by means of statistical averaging of flow maps obtained by time--distance helioseismic inversions. We find that moat flows around H-type sunspots replace the supergranular flows but there are two principal differences between the two phenomena: The moat flow is asymmetrical, probably due to proper motion of sunspots with respect to the local frame of rest, while the flow in the supergranula…
The ratio of penumbral to umbral area of sunspots is an important topic for solar and geophysical studies. Hathaway (Solar Physics, 286, 347, 2013) found a curious behaviour in this parameter for small sunspot groups (areas smaller than 100 millionths of solar hemisphere, msh) using records from Royal Greenwich Observatory (RGO). Hathaway showed that penumbra-umbra ratio decreased smoothly from more than 7 in 1905 to lower than 3 by 1930 and then increased to almost 8 in 1961. Thus, Hathaway proposed the existence of a secular variation in the penumbra-umbra area ratio. In order to confirm tha…
In the past two decades, the complex nature of sunspots has been disclosed with high-resolution observations. One of the most important findings is the "uncombed" penumbral structure, where a more horizontal magnetic component carrying most of Evershed Flows is embedded in a more vertical magnetic background (Solanki & Montavon 1993). The penumbral bright grains are locations of hot upflows and dark fibrils are locations of horizontal flows that are guided by nearly horizontal magnetic field. On the other hand, it was found that flares may change the topology of sunspots in $δ$ configuration: …
We perform NLTE inversions in a large set of umbral flashes, including the dark fibrils visible within them, and in the quiescent umbra by using the inversion code NICOLE on a set of full Stokes high-resolution Ca II 8542 A observations of a sunspot at disk center. We find that the dark structures have Stokes profiles that are distinct from those of the quiescent and flashed regions. They are best reproduced by atmospheres that are more similar to the flashed atmosphere in terms of velocities, even if with reduced amplitudes. We also find two sets of solutions that finely fit the flashed profi…
We present a series of high-resolution sunspot simulations that cover a time span of up to 100 hours. The simulation domain extends about 18 Mm in depth beneath the photosphere and 98 Mm horizontally. We use open boundary conditions that do not maintain the initial field structure against decay driven by convective motions. We consider two setups: A sunspot simulation with penumbra, and a "naked-spot" simulation in which we removed the penumbra after 20 hours through a change in the magnetic top boundary condition. While the sunspot has an Evershed outflow of 3-4 km/s, the naked spot is surrou…
Solar magnetic fields are generated in the solar interior and pop up at the solar surface to form active regions. As the magnetic field appears on the surface, it forms various structures like small magnetic elements, pores, and sunspots. The nature of these formation processes is largely unknown. In this contribution we elaborate on the formation of sunspots and particularly on the formation of penumbrae. We report on observations that we obtained at the German VTT on July 4, 2009 on the formation of the spot in AR 11024. This data set is accomplished with data from SOHO/MDI which offers an e…
We investigate the vertical gradient of the magnetic field of sunspots in the photospheric layer. Independent observations were obtained with the SOT/SP onboard the Hinode spacecraft and with the TIP-2 mounted at the VTT. We apply state-of-the-art inversion techniques to both data sets to retrieve the magnetic field and the corresponding vertical gradient. In the sunspot penumbrae we detected patches of negative vertical gradients of the magnetic field strength, i.e.,the magnetic field strength decreases with optical depth in the photosphere. The negative gradient patches are located in the in…
We investigated the vertical penumbral plasma flow on small spatial scales using data recorded by the spectropolarimeter of the solar optical telescope onboard Hinode. To this end we computed maps of apparent Doppler velocities by comparing the spectral position of the Fe I 630.15 nm & Fe I 630.25 nm lines with the averaged line profiles of the quiet Sun. To visualize the flow pattern in the low photosphere, we used a bisector of the wing of the absorption lines. Due to the small heliocentric angle (3 < Theta < 9) of our data sets, the horizontal component of the Evershed flow (EF) does not co…
Plasma loops or plumes rooted in sunspot umbrae often harbor downflows with speeds of 100 km/s. These downflows are supersonic at transition region temperatures of 0.1 MK. The source of these flows is not well understood. We aim to investigate the source of sunspot supersonic downflows (SSDs) in AR 12740 using simultaneous spectroscopic and imaging observations. We identified SSD events from multiple raster scans of a sunspot by the Interface Region Imaging Spectrograph, and calculated the electron densities, mass fluxes and velocities of these SSDs. The EUV images provided by the AIA onboard …
IRIS data allow us to study the solar transition region (TR) with an unprecedented spatial resolution of 0.33 arcsec. On 2013 August 30, we observed bursts of high Doppler shifts suggesting strong supersonic downflows of up to 200 km/s and weaker, slightly slower upflows in the spectral lines Mg II h and k, C II 1336 Å, Si IV 1394 Å, and 1403 Å, that are correlated with brightenings in the slitjaw images (SJIs). The bursty behavior lasts throughout the 2 hr observation, with average burst durations of about 20 s. The locations of these short-lived events appear to be the umbral and penumbral f…
Context. A significant part of the penumbral magnetic field returns below the surface in the very deep photosphere. For lines in the visible, a large portion of this return field can only be detected indirectly by studying its imprints on strongly asymmetric and three-lobed Stokes V profiles. Infrared lines probe a narrow layer in the very deep photosphere, providing the possibility of directly measuring the orientation of magnetic fields close to the solar surface. Aims. We study the topology of the penumbral magnetic field in the lower photosphere, focusing on regions where it returns below …
Inversions of spectropolarimetric observations of penumbral filaments deliver the stratification of different physical quantities in an optical depth scale. However, without establishing a geometrical height scale their three-dimensional geometrical structure can not be derived. This is crucial in understanding the correct spatial variation of physical properties in the penumbral atmosphere and to provide insights into the mechanism capable of explaining the observed penumbral brightness. The aim of this work is to determine a global geometrical height scale in the penumbra by minimizing the d…
Observations with the Interface Region Imaging Spectrograph (IRIS) have revealed numerous sub-arcsecond bright dots in the transition region above sunspots. These bright dots are seen in the 1400Å and 1330Å slit-jaw images. They are clearly present in all sunspots we investigated, mostly in the penumbrae, but also occasionally in some umbrae and light bridges. The bright dots in the penumbrae typically appear slightly elongated, with the two dimensions being 300--600 km and 250--450 km, respectively. The long sides of these dots are often nearly parallel to the bright filamentary structures in…
Light bridges are the most prominent manifestation of convection in sunspots. The brightest representatives are granular light bridges composed of features that appear to be similar to granules. An in-depth study of the convective motions, temperature stratification, and magnetic field vector in and around light bridge granules is presented with the aim of identifying similarities and differences to typical quiet-Sun granules. Spectropolarimetric data from the Hinode Solar Optical Telescope were analyzed using a spatially coupled inversion technique to retrieve the stratified atmospheric param…
We discuss NICOLE inversions of Fe I 630.15 nm and 630.25 nm Stokes spectra from a sunspot penumbra recorded with the CRISP imaging spectropolarimeter on the Swedish 1-m Solar Telescope at a spatial resolution close to 0.15". We report on narrow radially extended lanes of opposite polarity field, located at the boundaries between areas of relatively horizontal magnetic field (the intra-spines) and much more vertical field (the spines). These lanes harbor convective downflows of about 1 km/s. The locations of these downflows close to the spines agree with predictions from the convective gap mod…
We investigate the properties of orphan penumbrae, which are photospheric filamentary structures observed in active regions near polarity inversion lines that resemble the penumbra of regular sunspots but are not connected to any umbra. We use Hinode data from the Solar Optical Telescope to determine the properties of orphan penumbrae. Spectropolarimetric data are employed to obtain the vector magnetic field and line-of-sight velocities in the photosphere. Magnetograms are used to study the overall evolution of these structures, and G-band and Ca II H filtergrams are to investigate their brigh…
We present observations of a precursory signature that would be helpful for understanding the formation process of sunspot penumbrae. The Hinode Solar Optical Telescope successfully captured the entire evolution of a sunspot from the pore to a large well-developed sunspot with penumbra in an emerging flux region appeared in NOAA Active Region 11039. We found an annular zone (width 3"-5") surrounding the umbra (pore) in Ca II H images before the penumbra is formed around the umbra. The penumbra was developed as if to fill the annular zone. The annular zone shows weak magnetogram signals, meanin…
Penumbrae are known to be area of mainly horizontal magnetic field surrounding umbrae of relatively large and mature sunspots. In this paper, we observationally studied the formation of penumbrae in NOAA10978, where several penumbral formations were observed in G-band images of SOT/Hinode. Thanks to the continuous observation by Hinode, we could morphologically follow the evolution of sunspots and found that there are several paths to the penumbral formation: (1) Active accumulation of magnetic flux, (2) Rapid emergence of magnetic field, and (3) Appearance of twisted or rotating magnetic tube…
Context. Recent discoveries of intensity correlated downflows in the interior of a sunspot penumbra provide direct evidence for overturning convection, adding to earlier strong indications of convection from filament dynamics observed far from solar disk center, and supporting recent simulations of sunspots. Aims. Using spectropolarimetric observations obtained at a spatial resolution approaching 0'.'1 with the Swedish 1-m Solar Telescope (SST) and its spectropolarimeter CRISP, we investigate whether the convective downflows recently discovered in the C i line at 538.03 nm can also be detected…
Sunspots are concentrations of magnetic field on the visible solar surface that strongly affect the convective energy transport in their interior and surroundings. The filamentary outer regions (penumbrae) of sunspots show systematic radial outward flows along channels of nearly horizontal magnetic field. These flows were discovered 100 years ago and are present in all fully developed sunspots. Using a comprehensive numerical simulation of a sunspot pair, we show that penumbral structures with such outflows form when the average magnetic field inclination to the vertical exceeds about 45 degre…
High resolution spectropolarimetric data obtained by HINODE is used to characterize the small-scale penumbral magnetic field as well as the vertical and horizontal component of the Evershed effect (EE). The results demonstrate that the penumbral velocity field differs significantly from that of the quiet Sun. Morphological studies yield elongated upflow channels in the inner penumbra and round downflows in the outer penumbra. These flows are identified as the sources and the sinks of the EE. Remarkable is the high plasma velocity in these sinks, which is much larger when compared to the quiet …
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 and survey their characteristics. We analyze spectro-polarimetric scans of parts of 13 ARs obtained in the infrared He I 10830 Å triplet formed in the upper chromosphere recorded with the GREGOR Infrared Spectrograph (GRIS) mounted at the GREGOR solar telescope. We retrieve the line-of-sight velo…
The flare-prolific active region NOAA 10930 offered both a developing delta-spot and a decaying satellite sunspot of opposite polarity. The objective of this study is to characterize the photometric decay of the satellite sunspot and the evolution of photospheric and chromospheric horizontal proper motions in its surroundings. We apply the local correlation tracking technique to a 16-hour time-series of Hinode G-band and CaIIH images and study the horizontal proper motions in the vicinity of the satellite sunspot on 2006 December 7. Decorrelation times were computed to measure the lifetime of …
Penumbral microjets (PJs) are transient narrow bright features in the chromosphere of sunspot penumbrae, first characterized by Katsukawa et al (2007) using the \CaII\ H-line filter on {\it Hinode}'s Solar Optical Telescope (SOT). It was proposed that the PJs form as a result of reconnection between two magnetic components of penumbra (spines and interspines), and that they could contribute to the transition region (TR) and coronal heating above sunspot penumbrae. We propose a modified picture of formation of PJs based on recent results on internal structure of sunspot penumbral filaments. Usi…
We applied automatic identification of sunspot umbrae and penumbrae to daily observations from the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) to study their magnetic flux (B) and area (A). The results confirm a previously known logarithmic relationship between the area of sunspots and their maximum flux density. In addition, we find that the relation between average magnetic flux (Bavg) and sunspot area shows a bimodal distribution: for small sunspots and pores (A < 20 millionth of solar hemisphere, MSH), Bavg = 800 G (gauss), and for large sunspots (A > 1…
Context. The Evershed effect, a nearly horizontal outflow of material seen in the penumbrae of sunspots in the photospheric layers, is a common characteristic of well-developed penumbrae, but is still not well understood. Even less is known about photospheric horizontal inflows in the penumbra, also known as counter Evershed flows. Aims. Here we present a rare feature observed in the penumbra of the main sunspot of AR NOAA 10930. This spot displays the normal Evershed outflow in most of the penumbra, but harbors a fast photospheric inflow of material over a large sector of the disk-center penu…
We report on the discovery of mostly horizontal field channels just outside sunspot penumbrae (in the so-called `moat' region) that are seen to sustain supersonic flows (line-of-sight component of 6 km s{-1}). The spectral signature of these supersonic flows corresponds to circular polarization profiles with an additional, satellite, third lobe of the same sign as the parent sunspot' Stokes V blue lobe, for both downflows and upflows. This is consistent with an outward directed flow that we interpret as the continuation of the magnetized Evershed flow outside sunspots at supersonic speeds. In …
