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J.W. Brosius (Catholic U), K.J.H. Phillips (NRC)
We present light curves and Doppler velocity measurements for a GOES class M2 solar flare observed at high time resolution with SOHO's CDS (9.8 s) and Yohkoh's BCS (9.0 s) in NOAA Region 9433 on 2001 April 24. Coordinated imagery with SOHO's EIT and the TRACE satellite reveal that the CDS slit was centered on the flare commencement site. EUV spectra from CDS include the preflare quiescent phase, two precursors, the flare's impulsive and peak phases, and its slow decline. Soft X-ray spectra from BCS were obtained during the impulsive through decline phases. We find that (1) the average wavelengths of O III, O IV, O V, Ne VI, and He II lines measured during the preflare quiescent phase are equal (within the measurement uncertainties) to those measured during the late decline phase, indicating that they can be used as reference standards against which to measure Doppler velocities during the flare; (2) the EUV lines of O III, O IV, O V, and He II exhibit upflows around 40 km/s during both precursor events, suggesting small-scale chromospheric evaporation; (3) the Fe XIX EUV intensity rises and stays above its preflare noise level during the precursors; (4) the maximum upflow velocities measured in Fe XIX with CDS (64 km/s) and in Ca XIX (65 km/s) and S XV (78 km/s) with BCS occur during the flare impulsive phase, and are simultaneous within the instrumental time resolutions; (5) the EUV lines of O III, O IV, O V, and He II exhibit downflows around 40 km/s during the flare impulsive phase, suggesting momentum balance between the hot upflowing material and the cool downflowing material. Our observations are consistent with energy transport by nonthermal particle beams in chromospheric evaporation theory. JWB acknowledges NASA support through grant NAG5-11757; KJHP acknowledges support through an NRC Research Associateship.
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Bulletin of the American Astronomical Society, 36 #2
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