RHESSI and TRACE Observations of the 21 April 2002 X1.5 Flare

Abstract

Observations of the X1.5 flare on 21 April 2002 are reviewed using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Transition Region and Coronal Explorer (TRACE). The major findings are as follows: (1) The 3–25 keV X-rays started < 4 min before the EUV (195 Å) emission suggesting that the initial energy release heated plasma directly to ≳20 MK, well above the 1.6 MK needed to produce the Fexu (195 Å) line. (2) Using coaligned 12–25 keV RHESSI and TRACE images, further evidence is found for the existence of hot (15–20 MK) plasma in the 195 Å passband. This hot, diffuse emission is attributed to the presence of the Fe xxiv (192 Å) line within the TRACE 195 Å passband. (3) The 12–25 keV source centroid moves away from the limb with an apparent velocity of ~ 9.9 km s⁻¹, slowing to ~ 1.7 km s⁻¹ after 3 hours, its final altitude being ~ 140 Mm after ~ 12 hours. This suggests that the energy release site moves to higher altitudes in agreement with classical flare models. (4) The 50–100 keV emission correlates well with EUV flare ribbons, suggesting thick-target interactions at the footpoints of the magnetic arcade. The 50–100 keV time profile matches the time derivative of the GOES light curve (Neupert effect), which suggests that the same electrons that produced the thick-target hard X-ray emission also heat the plasma seen in soft X-rays. (5) X-ray footpoint emission has an E⁻³ spectrum down to ~ 10 keV suggesting a lower electron cutoff energy than previously thought. (6) The hard X-ray (25–200 keV) peaks have FWHM durations of ~ 1 min suggesting a more gradual energy release process than expected. (7) The TRACE images reveal a bright symmetric front propagating away from the main flare site at speeds of ≥ 120 km s⁻¹. This may be associated with the fast CME observed several minutes later by LASCO. (8) Dark sinuous lanes are observed in the TRACE images that extend almost radially from the post-flare loop system. This ‘fan of spines’ becomes visible well into the decay phase of the flare and shows evidence for both lateral and downward motions.