Transition Region Downflows in the Impulsive Phase of Solar Flares

Abstract

We present observations of four flares that occurred during coordinated observations between the Coronal Diagnostic Spectrometer (CDS) on board SOHO and the Domeless Solar Telescope (DST) at Hida Observatory. We studied the evolution of relative Doppler velocities in the flare kernels by using He I (3.5; 10(4) K), O v (2:2; 105 K), and Mg IX (1:0; 106 K) spectra obtained with high time cadence (42 s) SOHO CDS observations and the H alpha monochromatic images obtained with the DST. We found that the transition region plasma of O v showed strong downward velocities up to 87 km s(-1) simultaneously with the downflows in the lower temperature chromospheric emissions in He I and H alpha during the impulsive phase of all four flares. From these results we suggest that the downflows in the transition region and the chromosphere are a common feature in the impulsive phase of flares. For the MgIX line we did not detect any significant change in velocity, which suggests that the 10(6) K plasma was close to the intermediate temperature between the upflowing plasma (10(7) K) and the downflowing plasma (10(4)-10(5) K). These are important for understanding the dynamics of the solar atmosphere in response to the sudden energy deposition of a flare.