Stereoscopic measurements of flares from PHOBOS and GOES

A unique approach to observing the Sun stereoscopically in soft X-rays was afforded by the PHOBOS mission to Mars during 1989. Concurrent measurements of two flares from two widely separated spacecraft allowed us to obtain estimates of each flare's height above the solar surface. The requirement was that the flare had to be over the limb as observed by one spacecraft and on the visible disk as viewed by the other. The first flare occurred on March 4, when the active region was beyond the east limb as observed by GOES (at Earth), but on the disk as viewed by PHOBOS (at Mars). The second flare, on March 15, was on the disk for GOES, but beyond the west limb for PHOBOS. We believe that the same extraordinary active region, 5395, was responsible for both events. Soft X-ray photometers on each spacecraft contained two broad-band channels. The two-channel data were used to computeflare (assumed isothermal) plasma temperatures. The sharply peaked flare on March 4 indicated essentially identical maximum electron temperatures (∼ 13 Mk) at both spacecraft, confirming that the hottest plasma was indeed concentrated at the highest (unocculted) part of the loop. However, in the case of the long-duration March 15 flare, whose loop was in apparent upwards motion, the partially occulted flare indicated substantially cooler temperatures. This finding suggests that the hot core of this flare may have been below the limb, or that the partially occulted flux originated not from post-flare loops but from an independent, higher X-ray arch. The PHOBOS and GOES X-ray photometers were intercompared in July 1988, soon after the PHOBOS launch, to establish relative calibration parameters.