Magnetic field dissipation during the solar flare 23rd July 2002

Valentina Zharkova ,  Bradford University

We analyze the SOHO/MDI high-cadence magnetic field variations occurring prior and during the 2002 July 23 flare $2B/X4.8$
    and their association with the hard X-ray and $H_{\alpha}$ emission observed with RHESSI and BBSO. The line-of-sight (LOS)
    magnetic field variations associated with the flare 23 July 2002 occurred in a bipolar area around the magnetic inversion line,
    or apparent neutral line (ANL), located with the automated morphological edge detection technique.
    There were 4 hard X-ray sources observed by RHESSI payload in different
    energy bands, three of them were associated with
    magnetic field changes. In the hard X-ray footpoints located on the south-east of the ANL with the strongest negative LOS magnetic
    field and north-west from the ANL with positive polarity, the irreversible descrease of magnetic field (or increase in absolute value)
    was measured starting 2 minutes prior the flare, decreasing by 120 G and staying at this level after the flare offset.
    The magnetic field changes in the whole area were irreversible showing an increase of magnetic flux from $6.7\times 10^{21}$ Mx
    up to $7.9\times10^{21}$ Mx lasting for 6 minutes until 00:30:00 UT when the magnetic field returned to noise fluctuations
    about the new magnitude. Before the flare occurrence there were 9 pre-existing sources of a strong
    magnetic field of the positive and negative polarities about 400-600 G and 6 new emerging sources: 2 negative of -580 G and
    -380 G and 4 positive of 100-160 G which were found to be associated with the 3 hard X-ray footpoints. The emerging negative
    sources appeared in a close location and at the time when the ANL was moved towards the positive polarity with
    a speed of 250 km/s. Based on the global magnetic reconnection models, the rates of magnetic field changes
    (a reconnection rate) deduced from the total area around the ANL and
    from the area of the hard X-ray footpoint with irreversible changes were found to be about $3.1\times10^{18}$ Mx/s,
    the average Poynting flux was$5\cdot 10^{11}$ $erg/cm^2/s$. This resulted in the magnetic energy of
    $\sim 1.2\times 10^{31}$ erg released during this flare in one magnetic source associated with the hard X-ray
    footpoint, which can be increased to $\sim 2.3\cdot 10^{31}$ erg, if the other two footpoints are taken into
    account.