European Solar Physics Online Seminar

Following an initiative by the University of Oslo the MPS will participate in the "European Solar Physics Online Seminar" series (ESPOS). Details can be found here: https://folk.uio.no/tiago/espos/
The aim of this video conference series is to promote ideas more widely with a specialized audience, and give some exposure to cutting-edge research for students and other young researchers that do not regularly travel to conferences. The ESPOS series is planned to take place every second Thursday at 11am.

Room: Auditorium
In this talk we will focus on diagnostic potential of the spectral region around D lines of Sodium. We will first outline our approach to non-lte inversions, and present a method for computation of response functions in non-local thermodynamic equilibrium. We will then discuss the sensitivity of Sodium D lines to the atmospheric parameters and present some example inversions of that spectral region. [more]
At the interface between the Sun's surface and million-degree outer atmosphere or corona lies the chromosphere. At 10,000K it is much cooler than the corona, but also many orders of magnitude denser. The chromosphere processes all magneto-convective energy that drives the heating of the million-degree outer atmosphere or corona, and requires a heating rate that is at least as large as that required for the corona. Yet many questions remain about what drives the chromospheric dynamics and energetics and how these are connected to the transition region and corona. The Interface Region Imaging Spectrograph (IRIS) is a NASA small explorer satellite that was launched in 2013 to study how the Sun's magneto-convection powers the low solar atmosphere. I will review recent results from IRIS in which observations and models are compared to study the role of small-scale magnetic fields in the generation of violent jets and how these jets feed plasma into the transition region and hot corona. [more]
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]

ESP Online Seminar: What can numerical simulations tell us about the mechanism of solar and stellar activity? (J. Warnecke)

ESPOS
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]
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]
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]

ESP Online Seminar: Searching for the Origin of Flares in M dwarfs (L. Doyle)

ESPOS
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]
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]

ESP Online Seminar: Asymmetric Solar Waveguides: Theory and Observations (M. Allcock )

ESPOS
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]
Solar observations offer both a rich interdisciplinary laboratory on fundamental astrophysics and precious tools for Space Weather applications. The involved plasma processes determine a complex radio emission picture that could be efficiently explored through single-dish imaging at high frequencies. In particular, mapping the brightness temperature of the free-free radio emission in the centimetre and millimetre range is an effective tool to characterise the vertical structure of the solar atmosphere.In this presentation I disclose the continuum imaging of chromosphere and corona in K-band (18-26.5 GHz) performed with the 32-m diameter Medicina Radio Telescope and the 64-m diameter Sardinia Radio Telescope (SRT), as a first scientific demonstration test for the potentialities of Italian single-dish antennas in this field. These observations proved that the antennas and K-band receivers are stable during solar pointing and could provide full mapping of the solar disk in about 1 hour exposure using state-of-the-art imaging techniques. This study will be useful for the assessment of observation parameters aiming at studying in detail the chromospheric brightness temperature of the quiet Sun, the solar flares and the sunspots; in perspective, a contribution will be provided to Space Weather monitoring networks and forecast, filling different gaps that presently exist in the worldwide observing scenario. [more]
We present high spatial resolution narrow-band images in three different chromospheric spectral lines, including Ca II K with the new CHROMospheric Imaging Spectrometer installed at the Swedish 1-m Solar Telescope. These observations feature a unipolar region enclosed in a supergranular cell, and located 68º off the disk-centre. The observed cell exhibits a radial arrangement of the fibrils which recalls of a chromospheric rosette. However, in this case, the convergence point of the fibrils is located at the very centre of the supergranular cell. Our study aims to show how the chromosphere appears in this peculiar region and retrieve its magnetic field and velocity distribution. In the centre of the cell, we measured a significant blue-shift in the Ca II K nominal line core associated to an intensity enhancement. We interpreted it as the product of a strong velocity gradient along the line of sight. In this talk, we will discuss the techniques employed to obtain magnetic field maps so close to the limb and suggest a possible configuration that takes into account also the measured velocity within the unipolar region. [more]
To understand the links between the distribution of the prominence plasma, the configuration of its magnetic field and the observations of prominence/filament fine structures obtained in UV/EUV, optical and radio domains from various vantage points, we need complex 3D prominence models. We have developed two such models which combine 3D magnetic field configurations of an entire prominence with a detailed description of the prominence plasma distributed along hundreds of fine structures. The first 3D Whole-Prominence Fine Structure (WPFS) model, developed by Gunár & Mackay (2015), uses a magnetic field configuration obtained from non-linear force-free field simulations of Mackay & van Ballegooijen (2009). The second WPFS model was developed by Gunár, Dudík, Aulanier, Schmieder & Heinzel (2018). The model employs a magnetic field configuration of a polar crown prominence based on the linear force-free field modelling approach designed by Aulanier & Démoulin (1998) which allows us to calculate linear magneto-hydrostatic extrapolations from photospheric flux distributions. The prominence plasma in both models is located in magnetic dips that occur naturally in the predominantly horizontal prominence magnetic field. This plasma has a realistic distribution of the density and temperature, including the prominence-corona transition region. The models thus provide comprehensive information about the 3D distribution of the prominence plasma and magnetic field which can be consistently studied both as a prominence on the limb and as a filament on the disk. These models can be visualized for example in the H-alpha spectral line. Together with the models, we will present some of their capabilities which allow us to study the evolution of prominences/filaments or to analyze the true and apparent shapes and motions of the prominence fine structures. [more]
The Astronomical Observatory of the Coimbra University has a collection of solar observations on a daily basis, since 1926. We obtain regular observations of the full solar disk using a classical spectroheliograph, in the spectral lines of Ca II and Halpha (this one only after 1989). Until 2007 the acquisition was based on photographic plates and films. This data is digitized and public. Since then, a 12-bit CCD camera is operational. Nowadays, the local weather conditions allows observations in more than 300 days/year. This data is, particularly, suitable for solar cycle studies. In this talk, we briefly present a history of these observations; we discuss recent results using the data; and we present some perspectives for the near future. The ubiquitous presence of small magnetic elements in the Quiet Sun represents a prominent coupling between the photosphere and the upper layers of the Sun’s atmosphere. Small magnetic element tracking has been widely used to study the transport and diffusion of the magnetic field on the solar photosphere. From the analysis of the displacement spectrum of these tracers, it has been recently agreed that a regime of super-diffusivity dominates the solar surface. In this talk we will focus on the analysis of the bipolar magnetic pairs in the solar photosphere and their diffusion properties, using a 25-h dataset from the HINODE satellite. Interestingly, the displacement spectrum for bipolar couples behaves similarly to the case where all magnetic pairs are considered. We also measure, from the same dataset, the magnetic emergence rate of the bipolar magnetic pairs and we interpret them as the magnetic footpoints of emerging magnetic loops. The measured magnetic emergence rate is used to constrain a simplified model that mimics the advection on the solar surface and evolves the position of a great number of loops, taking into account emergence, reconnection and cancellation events. In particular we compute the energy released by the reconnection between different magnetic loops in the nano-flares energy range. Our model gives a quantitative estimate of the energy released by the reconfiguration of the magnetic loops in a quiet Sun area as a function of height in the solar atmosphere, from hundreds of Km above the photosphere up to the corona, suggesting that an efficiency of ~10% in the energy deposition might sustain the million degree corona. [more]

ESP Online Seminar: 2D and 3D Kinematic Analysis of an Ideal-MHD Prominence Eruption (Thomas Rees-Crockford)

ESPOS
We carry out multi-dimensional kinematic analysis of a prominence eruption in order to characterise the role of eruptive ideal-MHD instabilities. Using SDO/AIA and STEREO/EUVI-A we reconstruct the leading edge of the prominence in 3D, as observed between 26-Feb-2013 20:30:00 UT and 27-Feb-2013 05:45:00 UT. We use a novel semi-automated, dual, edge detection method to precisely detect the leading edge and create height-time profiles from SDO/AIA image sequences in He II 30.4 nm, to analyse the kinematics of erupting plasma along radial slits intersecting the leading edge coordinates. Constraining the power index parameter of fitted functions characterizing the linear and non-linear phases of the eruption, we investigate a set of fits of the eruption profile across all slits and identify the best fit in order to compare different eruption mechanisms. We also parameterise the onset time of the acceleration phase in order to confine the start time of the torus instability. For the first time, 3D kinematic analysis has identified a significant delay in the onset time of the acceleration phase together with a corresponding critical height at which acceleration starts to occur, as a function of position along the leading edge, which is in remarkable agreement with the determination of the critical height according to the decay index governing the torus instability. [more]
Since the work of Carlin et al. (2012), more studies have investigated, with the help of MHD models of the solar chromosphere, the behavior of scattering linear polarization (LP) in presence of macroscopic motions and weak magnetic fields. The results of the spatio-temporal simulations revealed a variable spectral morphology in the polarization of chromospheric lines, as well as modulations of LP amplitude that affect Hanle diagnosis, and also several situations with clear potential for diagnosing solar and stellar atmospheres. While much of these results resorts in the dynamic variations of radiation field anisotropy along the (also dynamic) formation region, they also pose new questions that seem to transcend the role of the anisotropy. On the other hand, two problems slow down further research in this area. One is the SNR reached with current telescopes, which is insufficient to compare observations with time-resolved simulations of scattering polarization. And the other is the extension and complexity of the formation region of the polarization signals, which often demands analysis of multidimensional simulations to extract conclusions. In this regard, I seek to develop a standard model for polarization that is precise enough to explain the signals but simple enough to provide analyses without relying in MHD models. We will start this seminar by summarizing the physical situation concerning the formation of dynamical LP in the external solar layers and by supporting the need of investigating the circular polarization. We will then focus on the novel exploration of NLTE circular polarization considering atomic orientation and velocity gradients. This approach leads to a better understanding of the formation of polarization by avoiding the limb-darkening modulation (introduced by the anisotropy in LP) and by allowing effective comparisons with observations. To explain this I will advance some observational results and I will present the first version of a simple and insightful model for explaining NLTE polarization signals. A particularity of the approach here proposed is the association of the zeroes of the emergent polarization spectrum with its morphology and with the properties of the scattering medium. [more]

ESP Online Seminar: Solar irradiance variability and surface magnetism (Kok Leng Yeo)

ESPOS
The variation in solar irradiance is commonly assumed to be driven by its surface magnetism. Until recently, this assumption could not be verified conclusively as models of solar irradiance variability based on solar surface magnetism have to be calibrated to solar irradiance measurements. Making use of realistic three-dimensional magnetohydrodynamic simulations of the solar atmosphere and state-of-the-art full-disk magnetograms from SDO, we developed a model of total solar irradiance (TSI) that does not require any such calibration. The modelled TSI variability is therefore, unlike preceding models, independent of TSI measurements. The model replicates over 95% of the observed variability over the lifetime of SDO, confirming the relationship to solar surface magnetism and leaving limited scope for alternative drivers of solar irradiance variability (at least over the time scales examined, that is, days to years). [more]
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