Nonlinear force-free coronal magnetic stereoscopy

Getting insights into the 3D structure of the solar coronal magnetic field have been done in the past by two completely different approaches: (1.) Nonlinear force-free field (NLFFF) extrapolations, which use photospheric vector magnetograms as boundary condition. (2.) Stereoscopy of coronal magnetic loops observed in EUV coronal images from different vantage points. Both approaches have their strength and weaknesses. Extrapolation methods are sensitive to noise and inconsistencies in the boundary data and the accuracy of stereoscopy is affected by the ability of identifying the same structure in different images and by the separation angle between the view directions. As a consequence, for the same observational data, the computed 3D coronal magnetic field with the two methods do not necessarily coincide. In an earlier work (Paper I) we extended our NLFFF optimization code by the inclusion of stereoscopic constrains. The method was successfully tested with synthetic data and within this work we apply the newly developed code to a combined data-set from SDO/HMI, SDO/AIA and the two STEREO spacecraft. The extended method (called S-NLFFF) contains an additional term that monitors and minimizes the angle between the local magnetic field direction and the orientation of the 3D coronal loops reconstructed by stereoscopy. We find that prescribing the shape of the 3D stereoscopically reconstructed loops the S-NLFFF method leads to a much better agreement between the modeled field and the stereoscopically reconstructed loops. We also find an appreciable decrease by a factor of two in the angle between the current and the magnetic field which indicates the improved quality of the force-free solution obtained by S-NLFFF.Comment: 9 pages, 7 figure

Kelvin-Helmholtz instability of a coronal streamer

The shear-flow-driven instability can play an important role in energy transfer processes in coronal plasma. We present for the first time the observation of a kink-like oscillation of a streamer probably caused by the streaming kinkmode Kelvin-Helmholtz instability. The wave-like behavior of the streamer was observed by Large Angle and Spectrometric Coronagraph Experiment (LASCO) C2 and C3 aboard SOlar and Heliospheric Observatory (SOHO). The observed wave had a period of about 70 to 80 minutes, and its wavelength increased from 2 Rsun to 3 Rsun in about 1.5 hours. The phase speeds of its crests and troughs decreased from 406 \pm 20 to 356 \pm 31kms^{-1} during the event. Within the same heliocentric range, the wave amplitude also appeared to increase with time. We attribute the phenomena to the MHD Kelvin-Helmholtz instability which occur at a neutral sheet in a fluid wake. The free energy driving the instability is supplied by the sheared flow and sheared magnetic field across the streamer plane. The plasma properties of the local environment of the streamer were estimated from the phase speed and instability threshold criteria.Comment: ApJ, accepte

Polar plumes' orientation and the Sun's global magnetic field

We characterize the orientation of polar plumes as a tracer of the large-scale coronal magnetic field configuration. We monitor in particular the north and south magnetic pole locations and the magnetic opening during 2007-2008 and provide some understanding of the variations in these quantities. The polar plume orientation is determined by applying the Hough-wavelet transform to a series of EUV images and extracting the key Hough space parameters of the resulting maps. The same procedure is applied to the polar cap field inclination derived from extrapolating magnetograms generated by a surface flux transport model. We observe that the position where the magnetic field is radial (the Sun's magnetic poles) reflects the global organization of magnetic field on the solar surface, and we suggest that this opens the possibility of both detecting flux emergence anywhere on the solar surface (including the far side) and better constraining the reorganization of the corona after flux emergence

Analysis of UV and EUV emission from impacts on the Sun after 2011 June 7 eruptive flare

On 2011 June 7 debris from a large filament eruption fell back to the Sun causing bright ultraviolet (UV) and extreme ultraviolet (EUV) splashes across the surface. These impacts may give clues on the process of stellar accretion. The aim is to investigate how the impact emission is influenced by structures in the falling ejecta and at the solar surface. We determine the UV and EUV light curves of a sample of impacts. The ballistic impact velocity is estimated from the ejection and landing times and, where possible, compared with the velocity derived by tracking the downflows in SDO/AIA and STEREO/EUVI images. Estimates of the column density before impact are made from the darkness of the falling plasma in the 193 A channel. The impact velocities were between 230 and 450 km/s. All impacts produced bright EUV emission at the impact site but bright UV was only observed when the impacting fragments reached the chromosphere. There was no clear relation between EUV intensity and kinetic energy. Low UV to EUV intensity ratios (I{UV}/I{EUV}) were seen (i) from impacts of low column-density fragments, (ii) when splashes, produced by some impacts, prevented subsequent fragments from reaching the chromosphere, and (iii) from an impact in an active region. The earliest impacts with the lowest velocity (~250 km/s) had the highest I{UV}/I{EUV}. The I{UV}/I{EUV} decreases with impact velocity, magnetic field at the impact site, and EUV ionising flux. Many of the infalling fragments dissipate above the chromosphere either due to ionisation and trapping in magnetic structures, or to them encountering a splash from an earlier impact. If the same happens in accreting stars then the reduced X-ray compared to optical emission that has been observed is more likely due to absorption by the trailing stream than locally at the impact site.Comment: 10 pages, 14 figures To be published in A&

Comparisons of CME morphological characteristics derived from five 3D reconstruction methods

We compare different methods to reconstruct the three-dimensional (3D) CME morphology. The explored methods include geometric localisation, mask fitting, forward modeling, polarisation ratio and local correlation tracking plus triangulation. The five methods are applied to the same CME event, which occurred on August 7 2010. Their corresponding results are presented and compared, especially in their propagation direction and spatial extent in 3D. We find that mask fitting and geometric localisation method produce consistent results. Reconstructions including three-view observations are more precise than reconstructions done with only two views. Compared to the forward modeling method, in which a-priori shape of the CME geometry is assumed, mask fitting has more flexibility. Polarisation ratio method makes use of the Thomson scattering geometry. We find spatially the 3D CME derived from mask fitting lies mostly in the overlap region obtained with the polarisation method from COR2 A and B. In addition, mask fitting can help resolve the front/back ambiguity inherent in the polarisation ratio method. However, local correlation tracking plus triangulation did not show a consistent result with the other four methods. For reconstructions of a diffuse CME, when the separation angle between STEREO A and B is large, finding two corresponding points in a STEREO image pair becomes very difficult. Excluding the local correlation tracking method, the latitude of the CME's centre of gravity derived from the other methods deviates within one degree and longitude differs within 19 degrees.Comment: to appear in Solar Physic

Preprocessing of vector magnetograph data for a non-linear force-free magnetic field reconstruction

Knowledge regarding the coronal magnetic field is important for the understanding of many phenomena, like flares and coronal mass ejections. Because of the low plasma beta in the solar corona the coronal magnetic field is often assumed to be force-free and we use photospheric vector magnetograph data to extrapolate the magnetic field into the corona with the help of a non-linear force-free optimization code. Unfortunately the measurements of the photospheric magnetic field contain inconsistencies and noise. In particular the transversal components (say Bx and By) of current vector magnetographs have their uncertainties. Furthermore the magnetic field in the photosphere is not necessary force-free and often not consistent with the assumption of a force-free field above. We develop a preprocessing procedure to drive the observed non force-free data towards suitable boundary conditions for a force-free extrapolation. As a result we get a data set which is as close as possible to the measured data and consistent with the force-free assumption.Comment: 20 pages, 5 figure

Electron Density Reconstruction of Solar Coronal Mass Ejections Based on a Genetic Algorithm: Method and Application

We present a new method to reconstruct the three-dimensional electron density of coronal mass ejections (CMEs) based on a genetic algorithm, namely the genetic reconstruction method (GRM). GRM is first applied to model CMEs with different orientations and shapes. A set of analytic model CMEs from Gibson and Low is employed to produce synthetic CME images for GRM reconstruction. Model CMEs with longitudes of 0, 45, 90, 135, 180 degrees and latitudes of 0, 15, 30, 45 degrees are used to test the performance of GRM. The model CMEs are obscured with a simulated occulter of a coronagraph to determine the influence of CME brightness incompleteness. We add random noise to some synthetic CME images to test the performance of GRM. The CME reconstructions are carried out using synthetic data from Solar Terrestrial Relations Observatory (STEREO) A and B with a separation angle of 90 degrees and from STEREO A and the Solar and Heliospheric Observatory (SOHO) with a separation angle of 73 degrees. The Pearson correlation coefficient and the mean relative absolute deviation are calculated to analyze the similarities in brightness and electron density between the model and reconstructed CMEs. Comparisons based on the similarity analysis under various conditions stated above give us valuable insights into the advantages and limitations of GRM reconstruction. The method is then applied to real coronagraph data from STEREO A and B, and SOHO on 2013 September 30.Comment: 27 pages, 17 figures, 1 table. Published on Ap

Polar plumes' orientation and the Sun's global magnetic field

We characterize the orientation of polar plumes as a tracer of the large-scale coronal magnetic field configuration. We monitor in particular the north and south magnetic pole locations and the magnetic opening during 2007-2008 and provide some understanding of the variations in these quantities. The polar plume orientation is determined by applying the Hough-wavelet transform to a series of EUV images and extracting the key Hough space parameters of the resulting maps. The same procedure is applied to the polar cap field inclination derived from extrapolating magnetograms generated by a surface flux transport model. We observe that the position where the magnetic field is radial (the Sun's magnetic poles) reflects the global organization of magnetic field on the solar surface, and we suggest that this opens the possibility of both detecting flux emergence anywhere on the solar surface (including the far side) and better constraining the reorganization of the corona after flux emergence.DL