Flare-induced magnetic field changes in the chromosphere
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It is well known that flares cause changes in the azimuthal direction of chromospheric magnetic field lines (e.g. Zirin, 1983). It is less well known that flares also cause changes in the inclination angle of chromospheric magnetic field lines (Bruzek, 1975). Inclination angle changes are notable in that horizontal field lines take the form of fibrils, while vertical field lines take the form of plages (Marsh, 1976). This study examines a complete sample of large flares to determine when the field inclination changes during the flare. The Hard X-ray Burst Spectrometer Event Listing (Dennis et. al, 1985) was searched for events with total counts > 105 and start times > 14 UT but <24 UT. Big Bear Solar Observatory Hα films were examined to identify which X-ray events show large horizontal ribbon motion over regions of fibrils. Of the 7 events found, 6 contain areas of the chromosphere that have the magnetic field direction turned from horizontal to vertical. The change in field direction at a given location occurs after the arrival of the ribbon, often 103 to 104 s after the flare start. No change in the chromosphere is seen before the ribbon arrival.
These observations show that field lines involved in the flare do not spend more than a few minutes in “open” configurations before reconnecting to new partners, independent of the flare duration. The transit time of an Alfvén wave down a magnetic field line is only ≈102 s. Flare models that postulate field line disconnection or opening which lasts longer than the Alfvén time are ruled out, since the chromospheric footpoints are observed not to change before the ribbon arrival (field line energization). The captive filament eruption model (Moore and LaBonte, 1980) also implies field line alterations lasting the duration of the flare, and is thus ruled out. Successful flare models should have the behavior of a brushfire, with the free energy in one bush (field line) released only after the burning (flare energy deposition) in its neighbor raises its own temperature over the ignition point (inductively raises the free energy density over the flare threshold). The flare continues until the inductive energy density increase is small enough to be contained by the normal nonflare processes.
A full description of the observations and their implications will appear in a future paper. This work is supported by NASA Grant NSG 7536.
KeywordsFlare Field Line Free Energy Density Magnetic Field Change Filament Eruption
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