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Register a new userReduced Microwave Brightness Temperature in a Sunspot Atmosphere due to Open Magnetic Fields
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Motivated by dark coronal lanes in SOHO / EIT 284 {AA} EUV observations we construct and optimize an atmosphere model of the AR 8535 sunspot by adding a cool and dense component in the volume of plasma along open field lines determined using the Potential Field Source Surface (PFSS) extrapolation. Our model qualitatively reproduces the observed reduced microwave brightness temperature in the northern part of the sunspot in the VLA observations from 13 May 1999 and provides a physical explanation for the coronal dark lanes. We propose application of this method to other sunspots with such observed dark regions in EUV or soft X-rays and with concurrent microwave observations to determine the significance of open field regions. The connection between open fields and the resulting plasma temperature and density change is of relevance for slow solar wind source investigations.
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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
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 Swedish 1-m Solar Telescope and apply the k-means clustering technique to systematically detect and characterize Ellerman Bombs. Features with all the defining characteristics of Ellerman bombs are found in large numbers over the entire penumbra. The true prevalence of these events is only fully appreciated in the H-beta line due to highest spatial resolution and lower chromospheric opacity. We find that the penumbra hosts some of the highest Ellerman bomb densities, only surpassed by the moat in the immediate surroundings of the sunspot. Some penumbral Ellerman bombs show flame morphology and rapid dynamical evolution. Many penumbral Ellerman bombs are fast moving with typical speed of 3.7 km/s and sometimes more than 10 km/s. Many penumbral Ellerman bombs migrate from the inner to the outer penumbra over hundreds of km and some continue moving beyond the outer penumbral boundary into the moat. Many penumbral Ellerman bombs are found in the vicinity of regions with opposite magnetic polarity. We conclude that reconnection is a near continuous process in the low atmosphere of the penumbra of sunspots as manifest in the form of penumbral Ellerman bombs. These are so prevalent that they may be a major sink of sunspot magnetic energy.
We use high spatial and temporal resolution observations, simultaneously obtained with the New Vacuum Solar Telescope and Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, to investigate the high-frequency oscillations above a sunspot umbra. A novel time--frequency analysis method, namely the synchrosqueezing transform (SST), is employed to represent their power spectra and to reconstruct the high-frequency signals at different solar atmospheric layers. A validation study with synthetic signals demonstrates that SST is capable to resolving weak signals even when their strength is comparable with the high-frequency noise. The power spectra, obtained from both SST and the Fourier transform, of the entire umbral region indicate that there are significant enhancements between 10 and 14 mHz (labeled as 12 mHz) at different atmospheric layers. Analyzing the spectrum of a photospheric region far away from the umbra demonstrates that this 12~mHz component exists only inside the umbra. The animation based on the reconstructed 12 mHz component in AIA 171 AA illustrates that an intermittently propagating wave first emerges near the footpoints of coronal fan structures, and then propagates outward along the structures. A time--distance diagram, coupled with a subsonic wave speed ($sim$ 49 km s$^{-1}$), highlights the fact that these coronal perturbations are best described as upwardly propagating magnetoacoustic slow waves. Thus, we first reveal the high-frequency oscillations with a period around one minute in imaging observations at different height above an umbra, and these oscillations seem to be related to the umbral perturbations in the photosphere.
Sunspots on the surface of the Sun are the observational signatures of intense manifestations of tightly packed magnetic field lines, with near-vertical field strengths exceeding 6,000 G in extreme cases. It is well accepted that both the plasma density and the magnitude of the magnetic field strength decrease rapidly away from the solar surface, making high-cadence coronal measurements through traditional Zeeman and Hanle effects difficult since the observational signatures are fraught with low-amplitude signals that can become swamped with instrumental noise. Magneto-hydrodynamic (MHD) techniques have previously been applied to coronal structures, with single and spatially isolated magnetic field strengths estimated as 9-55 G. A drawback with previous MHD approaches is that they rely on particular wave modes alongside the detectability of harmonic overtones. Here we show, for the first time, how omnipresent magneto-acoustic waves, originating from within the underlying sunspot and propagating radially outwards, allow the spatial variation of the local coronal magnetic field to be mapped with high precision. We find coronal magnetic field strengths of 32 +/- 5 G above the sunspot, which decrease rapidly to values of approximately 1 G over a lateral distance of 7000 km, consistent with previous isolated and unresolved estimations. Our results demonstrate a new, powerful technique that harnesses the omnipresent nature of sunspot oscillations to provide magnetic field mapping capabilities close to a magnetic source in the solar corona.
We present a study of wave processes occurring in solar active region NOAA 11131 on 2010 December 10, captured by the Solar Dynamics Observatory in the 1600A, 304A, and 171A channels. For spectral analysis we employed pixelised wavelet filtering together with a developed digital technique based on empirical mode decomposition. We studied the 3-minute wave dynamics to obtain relationships with the magnetic structuring of the underlying sunspot. We found that during development of wave trains the motion path occurred along a preferential direction, and that the broadband wavefronts can be represented as a set of separate narrowband oscillation sources. These sources become visible as the waves pass through the umbral inhomogeneities caused by the differing magnetic field inclination angles. We found the spatial and frequency fragmentation of wavefronts, and deduced that the combination of narrowband spherical and linear parts of the wavefronts provide the observed spirality. Maps of the magnetic field inclination angles confirm this assumption. We detect the activation of umbral structures as the increasing of oscillations in the sources along the front ridge. Their temporal dynamics are associated with the occurrence of umbral flashes. Spatial localisation of the sources is stable over time and depends on the oscillation period. We propose that these sources are the result of wave paths along the loops extending outwards from the magnetic bundles of the umbra.
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