Evaluating Mean Magnetic Field in Flare Loops

Abstract

We analyze multiple-wavelength observations of a two-ribbon flare exhibiting ap- parent expansion motion of the flare ribbons in the lower atmosphere and rising motion of X-ray emission at the top of newly-formed flare loops. We evaluate magnetic reconnection rate in terms of VrBr by measuring the ribbon-expansion velocity (Vr) and the chromospheric magnetic field (Br) swept by the ribbons. We also measure the velocity (Vt) of the apparent rising motion of the loop-top X-ray source, and estimate the mean magnetic field (Bt )a t the top of newly-formed flare loops using the relationVtBt �≈� VrBr� , namely, conservation of reconnection flux along flare loops. For this flare, Bt is found to be 120 and 60 G, respec- tively, during two emission peaks five minutes apart in the impulsive phase. An estimate of the magnetic field in flare loops is also achieved by analyzing the microwave and hard X-ray spectral observations, yielding B = 250 and 120 G at the two emission peaks, respectively. The measured B from the microwave spectrum is an appropriately-weighted value of mag- netic field from the loop top to the loop leg. Therefore, the two methods to evaluate coronal magnetic field in flaring loops produce fully-consistent results in this event.