Electron precipitation associated with geomagnetic activity: Balloon observation of X ray flux in South Atlantic anomaly

Abstract

A stratospheric balloon experiment was conducted on December 20, 1994, with an X ray 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 X ray fluxes which axe associated with simultaneous decreases in geomagnetic H field component. The most prominent event with an X ray emission equivalent to ≈1/4 of the diffuse cosmic X ray flux permitted the determination of the spectrum in the energy range of 18.6 to 120 keV. The inferred electron spectrum showed a steep low‐energy component below 150 keV (E0=16 keV). The electron flux estimates are lower compared to earlier balloon results obtained at the time of a severe magnetic storm. The inferred fluxes from our balloon experiment are consistent with the electron fluxes observed by the S3–2 satellite and are lower than the OGO 5 satellite measurements, both obtained in the SAA at L= 1.3. Further, the estimates are compatible 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 experiment, 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 X ray flux event is associated with two succesive southward turnings of Bz component of the interplanetary magnetic field (IMF). Precipitation of essentially low‐energy electrons below 150 keV occurred at the first northward reversal of IMF Bz and after the latter northward reversal higher energy electrons (> 150 keV) also precipitated. We suggest the diffusion of particles from higher L regions into flux tubes connected to the SAA region, due to electric field fluctuations associated with succession of substorms, in a process similar to the “enhanced radial diffusion” proposed by Lyons and Schulz [1989]. To compare or to relate the inferred fluxes from the balloon experiment with the satellite measurements and for a better understanding of the precipitation of electrons during magnetic disturbances, it is necessary to have additional experimental investigations in the SAA.