ar096.pevtsov02 Posted: 1-Sep-96 Updated: Events specified: AR 7926
Alexei Pevtsov, Richard Canfield
We propose to use Advanced Stokes Polarimeter vector magnetograms and Yohkoh SXT images to study the variation of magnetic energy and helicity of decaying active region NOAA 7926 as well as the luminosity and temperature of the corona integrated over this active region.
In general, several different mechanisms may heat the solar corona, i.e., magnetic field dissipation, Alfven waves, electric current dissipation, and photospheric random motions of the footpoints of the magnetic flux tubes.
Conservation of the magnetic helicity also may be important. For linear force-free magnetic field Hm=Int( alpha^{-1} B^2) dV = alpha^{-1} Em, where B - magnetic induction, Hm - magnetic helicity, Em - magnetic energy, alpha = curl B / B, and Int stands for volume integral over dV.
Dissipating force-free fields are expected to evolve gradually to potential fields. Together with magnetic helicity conservation that means dissipation of magnetic energy, which may heat the corona above active region (Berger, 1987).
The basic questions we address here are: Is magnetic helicity conserved during the dissipation of the magnetic field of this active region? What is the role of helicity conservation for coronal heating?
We observed decaying active region NOAA 7926 during 4 days using Advanced Stokes Polarimeter at NSO taking one magnetogram every 30 minutes. The active region (second rotation of the NOAA 7918) showed very low flare activity. The main sunspot gradually disappeared during period of observations. From tentative examination of the magnetograms we got an impression that there are no significant proper motions of the pores in the active region. Thus, we expect that this active region is a good candidate to study magnetic field dissipation.
First, we plan to study how the force-free field alpha-coefficient evolves and how magnetic helicity is conserved. Using the virial theorem we will compute the energy of the magnetic field and compare its variation with coronal luminosity and temperature averaged over the whole active region. The averaged luminosity and temperature will be computed for several fixed (luminosity- temperature) intervals to separate quiet corona and coronal loops. We also will study proper motions of the sunspot and pores of this active region to determine relative role of such motions in coronal heating.
We plan to use the routine SXT full disk images for this study. will study proper motions of the sunspot and pores of this active region to determine relative role of such motions in coronal heating.
We plan to use the routine SXT full disk images for this study.