On the estimate of magnetic non-potentiality of sunspots derived using Hinode SOT/SP observations: Effect of polarimetric noise

The accuracy of Milne-Eddington (ME) inversions, used to retrieve the magnetic field vector, depends upon the signal-to-noise ratio (SNR) of the spectro-polarimetric observations. The SNR in real observations varies from pixel to pixel, therefore the accuracy of the field vector also varies over the map. The aim of this work is to study the effect of polarimetric noise on the inference of magnetic field vector and the magnetic non-potentiality of a real sunspot. To this end, we use Hinode SOT/SP vector magnetogram of a real sunspot NOAA 10933 as an input to generate synthetic Stokes profiles under ME model assumptions. We then add normally-distributed polarimetric noise of the level 0.5\% of continuum intensity to these synthetic profiles and invert them again using ME code. This process is repeated 100 times with different realizations of noise. It is found that within most of the sunspot area (> 90% area) the spread in the (i) field strength is less than 8 Gauss, (ii) field inclination is less than 1 degree, and (iii) field azimuth is less than 5 degrees. Further, we determine the uncertainty in the magnetic non-potentiality of a sunspot as determined by the force-free parameter alpha_g and Spatially Averaged Signed Shear Angle (SASSA). It is found that for the sunspot studied here these parameters are alpha_g = -3.5 +/- 0.37 (x 10^{-9} m^{-1}) and SASSA = -1.68 +/- 0.014 degrees. This suggests that the SASSA is a less dispersion non-potentiality parameter as compared to alpha_g. Further, we examine the effect of increasing noise levels viz. 0.01, 0.1, 0.5 and 1% of continuum intensity and find that SASSA is less vulnerable to noise as compared to alpha_g parameter.Comment: Astrophysical Journal (In Press) 29 pages, 5 figures (scatterplots do not appear in soft-copy but appear on laser-printer

Evolution of helicity in NOAA 10923 over three consecutive solar rotations

We have studied the evolution of magnetic helicity and chirality in an active region over three consecutive solar rotations. The region when it first appeared was named NOAA10923 and in subsequent rotations it was numbered NOAA 10930, 10935 and 10941. We compare the chirality of these regions at photospheric, chromospheric and coronal heights. The observations used for photospheric and chromospheric heights are taken from Solar Vector Magnetograph (SVM) and H_alpha imaging telescope of Udaipur Solar Observatory (USO), respectively. We discuss the chirality of the sunspots and associated H_alpha filaments in these regions. We find that the twistedness of superpenumbral filaments is maintained in the photospheric transverse field vectors also. We also compare the chirality at photospheric and chromospheric heights with the chirality of the associated coronal loops, as observed from the HINODE X-Ray Telescope.Comment: 8 pages, 4 figure

On the flare induced seismicity in the active region NOAA 10930 and related enhancement of global waves in the sun

A major flare (of class X3.4) occurred on 13 December 2006 in the active region NOAA 10930. The energy released during flares is also known to induce acoustic oscillations in the Sun. Here, we analyze the line-of-sight velocity patterns in this active region during the X3.4 flare using the Dopplergrams obtained by GONG instrument. We have also analyzed the disk-integrated velocity observations of the Sun obtained by GOLF instrument onboard SOHO spacecraft as well as full-disk collapsed velocity signals from GONG observations during this flare to study any possible connection between the flare related changes seen in the local and global velocity oscillations in the Sun. We apply wavelet transform to the time series of the localized velocity oscillations as well as the global velocity oscillations in the Sun spanning the flare event. The line-of-sight velocity shows significant enhancement in some localized regions of the penumbra of this active region during the flare. The affected region is seen to be away from the locations of the flare ribbons and the hard X-ray footpoints. The sudden enhancement in this velocity seems to be caused by the Lorentz force driven by the "magnetic jerk" in the localized penumbral region. Application of wavelet analysis to these flare induced localized seismic signals show significant enhancement in the high-frequency domain (5-8 mHz) and a feeble enhancement in the p-mode oscillations (2-5 mHz) during the flare. On the other hand, the wavelet analysis of GOLF velocity data and the full-disk collapsed GONG velocity data spanning the flare event indicate significant post-flare enhancements in the high-frequency global velocity oscillations in the Sun. We find indications of a connection between flare induced localized seismic signals and the excitation of global high-frequency oscillations in the Sun.Comment: 29 pages, 7 figures, To appear in The Astrophysical Journa

First Use of Synoptic Vector Magnetograms for Global Nonlinear, Force-Free Coronal Magnetic Field Models

Context. The magnetic field permeating the solar atmosphere is generally thought to provide the energy for much of the activity seen in the solar corona, such as flares, coronal mass ejections (CMEs), etc. To overcome the unavailability of coronal magnetic field measurements, photospheric magnetic field vector data can be used to reconstruct the coronal field. Currently, there are several modelling techniques being used to calculate three-dimensional field lines into the solar atmosphere. Aims. For the first time, synoptic maps of a photospheric-vector magnetic field synthesized from the vector spectromagnetograph (VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) are used to model the coronal magnetic field and estimate free magnetic energy in the global scale. The free energy (i.e., the energy in excess of the potential field energy) is one of the main indicators used in space weather forecasts to predict the eruptivity of active regions. Methods. We solve the nonlinear force-free field equations using an optimization principle in spherical geometry. The resulting threedimensional magnetic fields are used to estimate the magnetic free energy content E(sub free) = E(sub nlfff) E(sub pot), which is the difference of the magnetic energies between the nonpotential field and the potential field in the global solar corona. For comparison, we overlay the extrapolated magnetic field lines with the extreme ultraviolet (EUV) observations by the atmospheric imaging assembly (AIA) on board the Solar Dynamics Observatory (SDO). Results. For a single Carrington rotation 2121, we find that the global nonlinear force-free field (NLFFF) magnetic energy density is 10.3% higher than the potential one. Most of this free energy is located in active regions

Magnetic Non-Potentiality of Solar Active Regions and Peak X-Ray Flux of the Associated Flares

Predicting the severity of the solar eruptive phenomena like flares and Coronal Mass Ejections (CMEs) remains a great challenge despite concerted efforts for several decades. The advent of high quality vector magnetograms obtained from Hinode (SOT/SP) has increased the possibility of meeting this challenge. In particular, the Spatially Averaged Signed Shear Angle (SASSA) seems to be an unique parameter to quantify the non-potentiality of the active regions. We demonstrate the usefulness of SASSA for predicting the flare severity. For this purpose we present case studies of the evolution of magnetic non-potentiality using 115 vector magnetograms of four active regions namely ARs NOAA 10930, 10960, 10961 and 10963 during December 08-15, 2006, June 03-10, 2007, June 28-July 5, 2007 and July 10-17, 2007 respectively. The NOAA ARs 10930 and 10960 were very active and produced X and M class flares respectively, along with many smaller X-ray flares. On the other hand, the NOAA ARs 10961 and 10963 were relatively less active and produced only very small (mostly A and B-class) flares. For this study we have used a large number of high resolution vector magnetograms obtained from Hinode (SOT/SP). The analysis shows that the peak X-ray flux of the most intense solar flare emanating from the active regions depends on the magnitude of the SASSA at the time of the flare. This finding of the existence of a lower limit of SASSA for a given class of X-ray flare will be very useful for space weather forecasting. We have also studied another non-potentiality parameter called mean weighted shear angle (MWSA) of the vector magnetograms along with SASSA. We find that the MWSA does not show such distinction as the SASSA for upper limits of GOES X-Ray flux of solar flares, however both the quantities show similar trends during the evolution of all active regions studied.Comment: 25 pages, 5 figures, accepted for publication in the Astrophysical Journa

Evolution of Currents of Opposite Signs in the Flare Productive Solar Active Region NOAA 10930

Analysis of a time series of high spatial resolution vector magnetograms of the active region NOAA 10930 available from SOT/SP on-board Hinode revealed that there is a mixture of upward and downward currents in the two foot-points of an emerging flux-rope. The flux emergence rate is almost the same in both the polarities. We observe that along with an increase in magnetic flux, the net current in each polarity increases initially for about three days after which it decreases. This net current is characterized by having exactly opposite signs in each polarities while its magnitude remains almost the same most of the time. The decrease of net current in both the polarities is due to the increase of current having a sign opposite to that of the net current. The dominant current, with same sign as the net current, is seen to increase first and then decreases during the major X-class flares. Evolution of non-dominant current appears to be a necessary condition for a flare initiation. The above observations can have a plausible explanation in terms of the superposition of two different force-free states resulting in non-zero Lorentz force in the corona. This Lorentz force then push the coronal plasma and might facilitate the magnetic reconnection required for flares. Also, the evolution of the net current is found to follow the evolution of magnetic shear at the polarity inversion line.Comment: 24 pages, 6 figures, Submitted to Astrophysical Journa