ELECTRON-PRECIPITATION ASSOCIATED WITH GEOMAGNETIC-ACTIVITY - BALLOONOBSERVATION OF X-RAY FLUX IN SOUTH ATLANTIC ANOMALY

A stratospheric balloon experiment was conducted on December 20, 1994, with an Xray detector to measure fluxes of precipitation electrons in the South Atlantic anomaly (SAA) region of Brazil. For the first time in the SAA, this detector system of high sensitivity monitored three events of increases in Xray fluxes which are associated with simultane ous decreases in geomagnetic H field component. The most prominent eve nt with an Xray emission equivalent to approximate to 1/4 of the diffu se cosmic Xray flux permitted the determination of the spectrum in the energy range of 18.6 to 120 keV. The inferred electron spectrum showe d a steep low-energy component below 150 keV (E-o=16 keV). The electro n flux estimates are lower compared to earlier balloon results obtaine d at the time of a severe magnetic storm. The inferred fluxes from our balloon experiment are consistent with the electron fluxes observed b y the S3-2 satellite and are lower than the O GO 5 satellite measureme nts, both obtained in the SAA at L=1.3. Further, the estimates are com patible with the P78-1 satellite fluxes present in the drift loss cone at L=1.25. The presence of sufficient electron fluxes at L=1.3 region in the SAA, and the similarity between the electron spectrum observed by the satellite experiments and that inferred in our balloon experim ent, prompted us to investigate the possibility of the access of these electrons present at satellite altitudes into L=1.13 regions during a mild storm. Interestingly, this prominent Xray flux event is associat ed with two succesive southward turnings of B-z component of the inter planetary magnetic field (IMF). Precipitation of essentially low-energ y electrons below 150 keV occurred at the first northward reversal of IMF B-z and after the latter northward reversal higher energy electron s (> 150 keV) also precipitated. We suggest the diffusion of particles from higher L regions into flux tubes connected to the SAA region, du e to electric field fluctuations associated with succession of substor ms, in a process similar to the ''enhanced radial diffusion'' proposed by Lyons and Schulz [1989]. To compare or to relate the inferred flux es from the balloon experiment with the satellite measurements and for a better understanding of the precipitation of electrons during magne tic disturbances, it is necessary to have additional experimental inve stigations in the SAA.