Search for evidence of alpha-particle beams during a solar flare observed by the coronal diagnostic spectrometer aboard the Solar and Heliospheric Observatory

We observed NOAA Active Region 9090 (N13 degrees, W39 degrees) with the Cor onal Diagnostic Spectrometer (CDS) and the Extreme-Ultraviolet Imaging Tele scope (EIT) aboard the Solar and Heliospheric Observatory spacecraft betwee n 18:17 and 21:09 UT on 2000 July 24 to search for evidence of alpha-partic le beams during solar flares. Theoretically, an alpha-particle beam will ma nifest itself during the impulsive phase of a flare through an enhancement in the red wing of the He II Ly alpha (lambda 303.782) emission line withou t a corresponding blue wing enhancement. This enhancement is due to downstr eaming nonthermal alpha particles undergoing charge-exchange with chromosph eric neutral hydrogen atoms to form downstreaming nonthermal He II ions. Ly alpha radiation emitted from these downstreaming ions is Doppler-shifted i nto the red wing of the Ly alpha line. Our CDS observing program acquired h igh time resolution (9.7 s) 4 " x 4' slit spectra between 590 and 630 Angst rom where we observed He II Ly alpha in second , order (lambda 607.564). Th e CDS and EIT observations reveal that AR 9090 underwent significant intens ity fluctuations prior to a sudden drastic increase (impulsive phase) aroun d 20:00 UT. The GOES satellite reports a C3.8 event in this region from 19: 57 to 20:05 UT. We fitted the spectral background and emission-line profile s for each CDS spectrum in our observed sequence. Density- and temperature- insensitive intensity ratios of O IV and Mg X lines generally agree with th eir theoretical values before and after the sudden intensity increase, whic h supports a reliable relative radiometric calibration for CDS, but differ significantly from their theoretical values during the flare impulsive phas e. This may indicate line blending with unknown components, line blending w ith second-order C IV and Fe XV lines, or loss of ionization equilibrium. M ost important, however, we find that although the red and blue wing backgro unds for He II Ly alpha remain relatively constant during most of our obser vation, the blue wing undergoes a more significant enhancement during the i mpulsive phase than does the red wing. This effect is opposite to that expe cted in the presence of an alpha-particle beam. Furthermore, blended spectr al line features that mimic the expected nonthermal redshifted He II Ly alp ha beam signal are understood in terms of well-known emission-line componen ts. Thus, we find no evidence for the presence of alpha-particle beams in o ur observations. We estimate an upper limit of similar to 250 ergs cm(-2) s (-1) sr(-1) Angstrom (-1) for the non-thermal redshifted peak spectral inte nsity due to an alpha-particle beam prior to the impulsive phase.