European Solar Physics Online Seminar Archive

Surges are ubiquitous cool ejections in the solar atmosphere that often appear associated with other interesting phenomena such as UV bursts or coronal jets. Recent observations from the Interface Region Imaging Spectrograph show that surges, although traditionally related to chromospheric lines such as H I 6563 Å or Ca II 8542 Å, can exhibit enhanced emission in Si IV and, as a consequence, lead to spectral profiles that are brighter than for the average transition region. However, a theoretical explanation to understand that behaviour was missing. In this talk, we analyse the response of the transition region to surge phenomena. To that end, we carried out two 2.5D radiative-MHD numerical experiments using the Bifrost code and including the non-equilibrium ionisation of silicon and oxygen. In the experiments, a cool and dense surge is obtained as a consequence of magnetic flux emergence. We find that non-equilibrium is key to understand why surges show enhanced emissivity in transition region lines. Studying the properties of emitting surge plasma, we point out the important role of the optically thin radiative cooling and heat conduction for the non-equilibrium ionisation. Furthermore, through the calculation of synthetic spectra of O IV, we provide predictions for future observations. [more]

Sunspots are the longest known manifestation of solar activity and their magnetic nature has been known for more than a century. Despite this, the boundary between umbrae and penumbrae, the two fundamental sunspot regions, has hitherto been solely defined by an intensity threshold. We now unveil the empirical law of the magnetic nature of the umbra-penumbra boundary in stable sunspots: an invariant vertical component of the magnetic field. We study the magnetic nature of umbra-penumbra boundaries in sunspots of different sizes, morphology, evolutionary stage, and phase of the solar cycle. We use a sample of 88 scans of Hinode/SOT spectropolarimeter to infer the magnetic field properties at the umbral boundaries. We define these boundaries by an intensity threshold and perform a statistically analysis of the magnetic field properties at these boundaries. We statistically prove that the umbra-penumbra boundary in stable sunspots is characterised by an invariant value of the vertical component of the magnetic field: The vertical component of the magnetic field strength does not depend on the umbra size, its morphology, and phase of the solar cycle. With statistical Bayesian inference, we find that the vertical component of the magnetic field strength is, with 99\% likelihood, in the range of 1849-1885 G with the most probable value of 1867 G. In contrast, the magnetic field strength and inclination averaged along individual boundaries are found to be dependent on the umbral size: The larger the umbra, the stronger and more horizontal the magnetic field at its boundary is. [more]

Ellerman bombs and UV bursts are transient brightenings that are ubiquitously observed in the lower atmospheres of active and emerging flux regions. While some Ellerman bombs display clear UV burst signatures, not all have correlated UV signal or vice versa, suggesting the underlying atmospheric and magnetic properties may differ between events. As both are believed to pinpoint sites of magnetic reconnection in reconfiguring fields, understanding their occurrence and detailed evolution may provide helpful insights in the overall evolution of active regions. Here we present results from observations and inversions of SST/CRISP and CHROMIS, as well as IRIS data of these transient events. At unprecedented spatial resolution the CHROMIS Ca II H & K observations reveal dynamic fine structure suggesting a plasmoid-mediated reconnection process. We investigate several cases, combining information from the Mg II h & k and Ca II 8542Å and H & K lines in order to infer the temperature stratification and magnetic field configuration within which these events occur. I’ll address the difficulties of successfully inverting their Si IV profiles and will discuss our results in light of the current debate on the connection between UV bursts and Ellerman bombs, their occurrence heights and in particular the temperatures that they may (or may not) reach. [more]

The magnetic field in the Sun undergoes a cyclic modulation with a reversal typically every 11 years due to a dynamo operating under the surface. Also, other solar-like stars exhibit magnetic activity, most of them with much higher levels compared to the Sun. Some of these stars show cyclic modulation of their activity similar to the Sun. The rotational dependence of activity and cycle length suggests a common underlying dynamo mechanism.Here we present results of 3D MHD convective dynamo simulations of slowly and rapidly rotating solar-type stars, where the interplay between convection and rotation self-consistently drives a large-scale magnetic field. With the help of the test-field method, we are able to measure the turbulent transport coefficients in these simulations and therefore get insights about the dynamo mechanism operating in these simulations. It allows us to derive a scaling of the cycle period with the relevant effects of the dynamo.We discuss how magnetic helicity is a key quantity connecting the stellar convection zone with the stellar surface and stellar coronae. Magnetic helicity is produced in the convection zone of stars via a dynamo in the presence of convection and rotation. At the surface, it plays an important role in the formation process of active regions. In the corona, it is believed to be essential for the release of energy leading to the eruption of plasma via coronal mass ejections and is thought to play an important role in the heating process of the coronal plasma. Numerical simulations of stellar convection zones and the solar corona allow us to investigate this process. [more]

Propagating slow magneto-acoustic waves are regularly observed in the solar corona, particularly in sunspot related loop structures. These waves exhibit rapid damping as they propagate along the loops. Several physical and geometrical effects were found to produce the observed decay in the wave amplitude. It has also been shown that the damping is frequency dependent. A majority of the observed characteristics have been attributed to damping by thermal conduction in the solar corona. Although it is believed that these waves originate in the photosphere, their damping behaviour in the sub-coronal layers is relatively less studied. Using high spatial and temporal resolution images of a sunspot, we investigated propagation and damping characteristics of slow magnetoacoustic waves up to transition region heights. The major conclusions from this study will be discussed in the talk which include: 1) The energy flux in slow waves estimated from the relative amplitudes decays gradually right from the photosphere even when the oscillation amplitude is increasing. 2) The damping displayed by slow waves is frequency dependent well below coronal heights. 3) A spatial comparison of power spectra across the umbra highlights enhancement of high-frequency waves near the umbral center. [more]

Ellerman bombs and UV bursts are transient brightenings that are ubiquitously observed in the lower atmospheres of active and emerging flux regions. While some Ellerman bombs display clear UV burst signatures, not all have correlated UV signal or vice versa, suggesting the underlying atmospheric and magnetic properties may differ between events. As both are believed to pinpoint sites of magnetic reconnection in reconfiguring fields, understanding their occurrence and detailed evolution may provide helpful insights in the overall evolution of active regions. Here we present results from observations and inversions of SST/CRISP and CHROMIS, as well as IRIS data of these transient events. At unprecedented spatial resolution the CHROMIS Ca II H & K observations reveal dynamic fine structure suggesting a plasmoid-mediated reconnection process. We investigate several cases, combining information from the Mg II h & k and Ca II 8542Å and H & K lines in order to infer the temperature stratification and magnetic field configuration within which these events occur. I’ll address the difficulties of successfully inverting their Si IV profiles and will discuss our results in light of the current debate on the connection between UV bursts and Ellerman bombs, their occurrence heights and in particular the temperatures that they may (or may not) reach. [more]

Long-period intensity pulsations have been recently detected in coronal loops with EUV images of both SoHO/EIT (Auchère et al., 2014) and SDO/AIA (Froment et al., 2015). These pulsations have been interpreted as resulting from thermal non-equilibrium (TNE), thus providing a signature of a highly-stratified and quasi-constant heating at the loops footpoints (Froment et al., 2017; Auchère et al., 2016). Depending on the adequacy between the geometry of the loop and the characteristics of the heating, this can result in either complete (down to chromospheric temperatures) or incomplete (> 1 MK) condensation and evaporation cycles, that are responsible for the observed intensity pulsations. Using 1D hydrodynamic simulations, Froment et al. (2017, 2018) were able to reproduce the observed pulsations, with incomplete condensation for the active region studied in their previous paper. The simulations also predict periodic plasma flows along the loops footpoints, with velocities up to 40 km/s. We try to detect these flows by using time series of spatially resolved spectra from the EUV spectrometer Hinode/EIS. We systematically search for EIS datasets that correspond to the observation of pulsation events among the 3000+ that were detected in AIA data, between 2010 and 2016. For the 11 datasets that are found, we derive series of Doppler velocity maps, which allows us to track the evolution of the plasma velocity in the loop over several pulsation periods. We then compare these data to the results of previous simulations and observations. We detect the signature of flows along some loops that have velocity patterns consistent with the predictions from the simulations. However, the expected pulsations in velocity cannot be identified in any of the datasets that we analysed, either due to insufficient temporal resolution, or to line of sight ambiguities combined with low signal to noise. [more]

A giant solar filament was visible on the solar surface between 8-23 November 2011. The filament stretched over more than half a solar diameter. Multi-wavelength data from the SDO instrument AIA (171, 193, 304, and 211 Å) were used to examine counter-streaming flows within the spine of the filament. H-alpha images from the Kanzelhöhe Solar Observatory provided context information. We applied local correlation tracking (LCT) to a two-hour AIA time series from 16 November 2011 to derive horizontal flow velocities of the filament. To enhance the contrast of the AIA images, we employed noise adaptive fuzzy equalization (NAFE), allowing us to identify and quantify counter-streaming flows in the filament. We detected counter-streaming flows in the filament, visible in the time-lapse movies of all the examined AIA wavelength bands. Using time-lapse movies we found that these persistent flows lasted for at least two hours. Furthermore, by applying LCT to the images we clearly determined counter-streaming flows in time series of 171 Å and 193 Å images. In the 304 Å wavelength band we found only minor indications for counter-streaming flows with LCT, while in the 211 Å wavelength band the counter-streaming flows are not detectable. The average horizontal flows reach mean flow speeds of 0.5 km/s. The highest horizontal flow speeds are identified in the 171 Å band with flow speeds of up to 2.5 km/s. The results are averaged over a time series of 90 min. Because the LCT sampling window has a finite width, spatial degradation cannot be avoided, leading to lower estimates of the flow velocities as compared to feature tracking or Doppler measurements. The counter-streaming flows cover about 15-20% of the whole area of the EUV filament channel and are located in the central part of the spine. In conclusion, we confirm counter-streaming flows are omnipresent also in giant quiet-Sun filaments. [more]

We report multi-wavelength ultraviolet observations taken with the IRIS satellite, concerning the emergence phase in the upper chromosphere and transition region of an emerging flux region (EFR) embedded in the unipolar plage of active region NOAA 12529. IRIS data are complemented by full-disk, simultaneous observations of the Solar Dynamics Observatory satellite, relevant to the photosphere and the corona. The photospheric configuration of the EFR is also analysed by measurements taken with the spectropolarimeter onboard the Hinode satellite, when the EFR was fully developed. Recurrent intense brightenings that resemble UV bursts, with counterparts in all coronal passbands, are identified at the edges of the EFR. Jet activity is also found at chromospheric and coronal levels, near the observed brightness enhancement sites. Analysis of the IRIS line profiles reveals heating of dense plasma in the low solar atmosphere and the driving of bi-directional, high-velocity flows with speeds up to 100 km/s at the same locations. Comparing these signatures with previous observations and numerical models, we suggest evidence of several long-lasting, small-scale magnetic reconnection episodes between the emerging bipole and the ambient field. This process leads to the cancellation of a pre-existing photospheric flux concentration of the plage with the opposite polarity flux patch of the EFR. Moreover, the reconnection appears to occur higher in the atmosphere than usually found in UV bursts, explaining the observed coronal counterparts. [more]

We present an overview of K2 short cadence observations for 32 M dwarfswhich have spectral types between M0-L1. All of the stars in our sampleshowed flares with the most energetic reaching 3x10^34 ergs. As previousstudies have found, we find rapidly rotating stars tend to show moreflares, with evidence for a decline in activity in stars with rotationperiods longer than approximately 10 days. We determined the rotationalphase of each flare and performed a simple statistical test on oursample to determine whether the phase distribution of the flares israndom or if there is a preference for phase. We find none show apreference for the rotational phase of the flares. If the analogybetween the physics of solar and stellar flares holds and these eventsoccur from active regions which typically host spots, then you wouldexpect to see more flares during the rotation minimum where the starspotis most visible. However, this is not the case with our sample and infact all of our stars show flares at all rotational phases, suggestingthese flares are not all originating from one dominant starspot on thesurface of the stars. We outline three scenarios which could explain thelack of a correlation between the number of flares and the stellarrotation phase. [more]

The strong enhancement of the ultraviolet emission during solar flares is usually taken as an indication of plasma heating in the low solar atmosphere caused by the deposition of the energy released during these events. Images taken with broadband ultraviolet filters by the Transition Region and Coronal Explorer (TRACE) and Atmospheric Imaging Assembly (AIA 1600 and 1700 Å) have revealed the morphology and evolution of flare ribbons in great detail. However, the spectral content of these images is still largely unknown. Without the knowledge of the spectral contribution to these UV filters, the use of these rich imaging datasets is severely limited. Aiming to solve this issue, we estimate the spectral contributions of the AIA UV flare and plage images using high-resolution spectra in the range 1300 to 1900 Å from the Skylab NRL SO82B spectrograph. We find that the flare excess emission in AIA 1600 Å is composed of the C IV 1550 Å doublet (26%), Si I continua (20%), with smaller contributions from many other chromospheric lines such as C I 1561 and 1656 Å multiplets, He II 1640 Å, Si II 1526 and 1533 Å. For the AIA 1700 Å band, C I 1656 Å multiplet is the main contributor (38%), followed by He II 1640 (17%), and accompanied by a multitude of other chromospheric lines, with minimal contribution from the continuum. Our results can be generalised to state that the AIA UV flare excess emission is of chromospheric origin, while plage emission is dominated by photospheric continuum emission in both channels. [more]

We present preliminary results derived from the analysis of spectropolarimetric measurements of active region AR12546, which represents one of the largest sunspots to have emerged onto the solar surface over the last 20 years. The region was observed with full-Stokes scans of the Fe I 617.3 nm and Ca II 854.2 nm lines with the Interferometric BIdimensional Spectrometer (IBIS) instrument at the Dunn Solar Telescope over an uncommon, extremely long time interval exceeding three hours. We show preliminary results from the phase lag analysis of different quantities and discuss the results in terms of the literature on the subject and MHD wave propagation theory. [more]

While surface waves propagating at tangential discontinuities have been studied in great detail, few studies have been dedicated to the investigation of the nature of waves at contact discontinuities, i.e. plasma discontinuity, where the background magnetic field crosses the interface between two media. In this talk, I will show that by introducing magnetic field inclination, the frequency of waves is rendered complex, where the imaginary part describes wave attenuation, due to lateral energy leakage. We investigate the eigenvalue and initial value problem and determine the conditions of transition from contact with the tangential discontinuity. Finally, I will present an investigation into the effect of magnetic field inclination on magnetic Rayleigh-Taylor instability. [more]

Are solar MHD waveguides symmetric? It is convenient to assume that they are. The solar physics community is familiar with the traditional notion of sausage and kink waves, which propagate along waveguides in the solar atmosphere that we assume are symmetric. In this talk, we drop this assumption and motivate the study of MHD wave propagation in asymmetric waveguides from theoretical and observational viewpoints. We discuss the implications that asymmetric waveguides have for mode identification, highlighting the observational ambiguity between waves in symmetric and asymmetric waveguides, which becomes a crucial consideration when implementing magneto-seismology diagnostics. We present a novel technique for solar magneto-seismology that utilises the observed asymmetry of MHD waves to diagnose background parameters of the solar atmosphere that are difficult to measure using traditional methods. We present a preliminary application of this technique to chromospheric fibrils as a proof-of-concept and discuss the potential further application to prominences, elongated magnetic bright points, and sunspot light walls. [more]

The emergence of magnetic flux through the photosphere and into the outer solar atmosphere produces, amongst many other phenomena, the appearance of Ellerman bombs (EBs) in the photosphere. EBs are observed in the wings of Hα and are highly likely to be due to reconnection in the photosphere, below the chromospheric canopy. But signs of the reconnection process are also observed in several other spectral lines, typical of the chromosphere or transition region. An example are the UV bursts observed in the transition region lines of Si IV. In this work we analyse high-cadence coordinated observations between the Swedish 1-m Solar Telescope (SST) and the IRIS spacecraft in order to study the possible relationship between reconnection events at different layers in the atmosphere, and in particular, the timing history between them. High-cadence, high-resolution Hα images from the SST provide us with the positions, timings and trajectories of Ellerman bombs in an emerging flux region. Simultaneous co-aligned IRIS slit-jaw images at 2796Å, 1400Å and 1330Å and detailed Mg II and Si IV spectra from the fast spectrograph raster allow us to study the possible chromospheric/transition region counterparts of those photospheric Ellerman bombs. Our main goal is to study whether there is a temporal and spatial relationship between the appearance of an EB and the appearance of a UV burst. [more]

Are some parts of the Interplanetary Magnetic Field’s (IMF) neutral line more flare energetic than others? What are Hale Sector Boundaries (HSBs) and are they connected with flares? Do they have anything to do with Active Longitudes? In this work, I will discuss how RHESSI flares are associated with structures in the solar magnetic field termed as HSBs. If you think of the large-scale domains of different polarity that the IMF is formed of, they the parts of the boundary between them, that have the same polarity change as the sunspots back at the Sun. As the polarity of sunspots follows Hale’s law, the HSB of a particular polarity change will only occur in one hemisphere per cycle, and then alternate in the next cycle. It has previously been shown that HSBs coincide with stronger magnetic fields and more frequent flare occurrence (Dittmer 1975, Svalgaard & Wilcox 1976, Svalgaard et al. 2011). I will explain how we extended this work through solar cycles 23 and 24 using RHESSI flare locations from2002 to 2016. We compared these flares to the HSBs determined using two different methods. One uses the polarity change at the Earth to estimate when the HSB was at solar central meridian and the other uses Potential Field Source Surface (PFSS) extrapolations to identify the HSB for all times. We found that for both Cycle 23 and 24 more than 40% of non-limb flares were located near a HSB, a correlation that varies with cycle phase and hemisphere. I will describe how this evolves with time and the potential of these approaches for assisting flare forecasting. We then used the locations of HSBs calculated with the first method,using Earth-based data, to a Carrington rotation system and comparedthem with the migration paths of Active Longitudes as show in Gyenge et al. (2016). We found that there are times where they overlap, but that is not happening in a consistent manner. They often move at different rates relative to each other (and the Carrington solar rotation rate) and these vary over each Cycle. [more]

The Wilson depression is the difference in geometric height of the layer of unit continuum optical depth between the sunspot umbra and the quiet Sun. Measuring the Wilson depression is important for understanding the geometry of sunspots. Current methods suffer from systematic effects or need to make assumptions on the geometry of the magnetic field. This leads to large systematic uncertainties of the derived Wilson depressions. Here we present a method for deriving the Wilson depression that only requires the information about the magnetic field that are accessible by spectropolarimetry and that does not rely on assumptions on the geometry of sunspots or on its magnetic field. Our method is based on minimizing the divergence of the magnetic field vector derived from spectropolarimetric observations. We focus on large spatial scales only in order to reduce the number of free parameters. We test the performance of our method using synthetic Hinode data derived from two sunspot simulations. We find that the maximum and the umbral averaged Wilson depression for both spots determined with our method typically lies within 100 km of the true value obtained from the simulations. In addition, we apply the method to spots from the Hinode sunspot database at MPS. The derived Wilson depressions (500-700 km) are consistent with results typically obtained from the Wilson effect. In our sample, larger spots with a stronger magnetic field exhibit a higher Wilson depression than smaller spots. [more]