Motion of 3-6 keV Nonthermal Sources Along a Flare Loop

Observations of nonthermal X-ray sources are critical to studying electron acceleration and transport in solar flare. Strong thermal emission radiated from the pre-heated plasma before the flare impulsive phase often makes it difficult to detect low-energy nonthermal X-ray sources which are mainly produced by low-energy nonthermal electrons. Knowledge of the distribution of these low-energy nonthermal electrons is particularly important in determining the total nonthermal electron energy in solar flares. We report one `early impulsive flare' in which impulsive hard X-ray emission was seen early in the flare before the soft X-ray emission had risen significantly, indicating very limited plasma pre-heating. Early in the flare, RHESSI < 25 keV images show coronal sources that moved down and then up along a flare loop. In particular, the 3-6 keV source appeared as one coronal source at the start of the flare, and then it split into two coronal sources moving down along the two legs of the flare loop. After reaching the two footpoints at the hard X-ray peak, the two sources moved back up to the looptop again. RHESSI images and spectra all indicate that nonthermal emission dominated at energies as low as 3-6 keV early in the flare. We suggest that variations of low-energy cutoff proportional to the hard X-ray fluxes and soft-hard-soft evolution of nonthermal electron spectra could both result in the accelerated electrons reaching lower altitude along the legs of the flare loops and hence result in the observed downward and upward motions of the nonthermal sources.