The 22 May 2007 B-class flare: new insights from Hinode observations

¹ DAMTP, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, UK
e-mail: g.del-zanna@damtp.cam.ac.uk
² NASA Goddard Space Flight Center, Solar Physics Lab., Code 671, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
³ Dept. of Computational and Data Sciences, George Mason University, 4400 University Drive, MSN 6A2, Fairfax, VA 22030, USA
⁴ Department of Physics and Astronomy, Rice University, Houston, TX 77005, USA
⁵ Nobeyama Solar Radio Observatory, National Astronomical Observatory of Japan, Minamisaku, Nagano 384-1305, Japan
⁶ Unit of Synergetic Studies for Space, Kyoto University, Yamashina, Kyoto 607-8471, Japan

Received: 30 April 2010
Accepted: 9 October 2010

Abstract

We present multi-wavelength observations of a small B-class flare which occurred on the Sun on 2007 May 22. The observations include data from Hinode, GOES, TRACE and the Nobeyama Radioheliograph. We obtained spatially and spectrally-resolved information from the Hinode EUV Imaging Spectrometer (EIS) during this event. The temporal and temperature coverage of the EIS observations provides new insights into our understanding of chromospheric evaporation and cooling. The flare showed many “typical” features, such as brightenings in the ribbons, hot (10 MK) loop emission and subsequent cooling. We also observed a new feature, strong (up to 170 km s^-1) blue-shifted emission in lines formed around 2–3 MK, located at the footpoints of the 10 MK coronal emission and within the ribbons. Electron densities at 2 MK in the kernels are high, of the order of 10¹¹ cm^-3, suggesting a very narrow layer where the chromospheric evaporation occurs. We have run a non-equilibrium hydrodynamic numerical simulation using the HYDRAD code to study the cooling of the 10 MK plasma, finding good agreement between the predicted and observed temperatures, densities and ion populations. Line blending for some potentially useful diagnostic lines for flares, which are observed with Hinode/EIS, is also discussed.