European Solar Physics Online Seminar Archive

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]

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]

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]

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]

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