Observations of nonthermal and thermal hard X-ray spikes in an M-class flare

We report a study of an M2.3 flare, which occurred on 2002 September 9 in NOAA Active Region 0105. The observation was made at Big Bear Solar Observatory (BBSO) at the wavelength of Hα -1.3 Å, with a cadence of 40 ms. The flare was also observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Three kernels appear in Hα - 1.3 Å images, two of which are conjugate kernels. The light curves of the two conjugate kernels show a correlation with that of hard X-rays (HXRs) in the 25-50 keV band; however, the third one does not. We use HXR images and spectra and the time delay between the Hα blue-wing emissions and HXRs to distinguish between the thermal and nonthermal character of spikes in the light curve. Among the four spikes we studied, the second spike is thermal, while the other three are nonthermal. What characterizes the thermal spike is the long time delay (∼10 s), the lack of HXR "footpoint" emission, and the absence of a power-law component in HXR imaging spectra. In addition, the spectrum of this spike is only well fitted by a double-temperature model with a hot (∼25.5 MK) component and a superhot (∼47.1 MK) component with different emission measures. The nonthermal spikes are characterized by short time delays ≲4 s, the RHESSI time resolution used in this paper), HXR footpoint emission, and power-law spectra. The relative Hα blue-wing intensities during the periods of the three nonthermal spikes are evaluated in terms of beam parameters: power-law index and X-ray flux at 10 keV. Comparison with the observations shows that the first spike of the early impulsive phase did not produce the predicted Hα intensity.