Analysis of sudden variations in photospheric magnetic fields during a   large flare and their influences in the solar atmosphere
  The solar active region NOAA 11719 produced a large two-ribbon flare on 11 April 2013. We have investigated the sudden variations in the photospheric magnetic fields in this active region during the flare employing the magnetograms obtained in the spectral line Fe I 6173 Angstrom by the Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO). The analysis of the line-of-sight magnetograms from HMI show sudden and persistent magnetic field changes at different locations of the active region before the onset of the flare and during the flare. The vector magnetic field observations available from HMI also show coincident variations in the total magnetic field strength and its inclination angle at these locations. Using the simultaneous Dopplergrams obtained from HMI, we observe perturbations in the photospheric Doppler signals following the sudden changes in the magnetic fields in the aforementioned locations. The power spectrum analysis of these velocity signals show enhanced acoustic power in these affected locations during the flare as compared to the pre-flare condition. Accompanying these observations, we have also used the near-simultaneous chromospheric observations obtained in the spectral line H-alpha 6562.8 Angstrom by the Global Oscillation Network Group (GONG) to study the evolution of flare ribbons and intensity oscillations in the active region. The H-alpha intensity oscillations also show enhanced oscillatory power during the flare in the aforementioned locations. These results indicate that the transient Lorentz force associated with the sudden changes in the magnetic fields could drive the localized photospheric and chromospheric oscillations, like the flare-induced oscillations in the solar atmosphere. 
Brajesh Kumar, A. Raja Bayanna, P. Venkatakrishnan, and Shibu K.   Mathew
astro-ph.SR