X-Ray Substorm

Abstract

Energetic electrons which impinge into the polar upper atmosphere generate brems-strahlung X-rays when they collide with upper atmospheric particles. Balloon-borne detectors have been used to observe X-rays at about the 30 km level. However, because of the layer of atmosphere between the height at which the X-rays are generated and the height of the balloon, the X-rays suffer both photoelectric absorption and Compton scattering; further, these effects are energy-dependent. For these reasons, the procedure of obtaining the original energy spectrum of incoming electrons from the observed X-ray spectrum is not necessarily straightforward. Since our interest in this monograph is a synoptic aspect of the X-ray bursts, we shall not be concerned with details of the data reduction procedure (for details, see cf. Brown, 1966; Barcus and Rosenberg, 1966). Nevertheless, it is worthwhile to have some idea as to the effects of the atmosphere. For this purpose, we show in Figure 72 a model calculation by Barcus and Rosenberg (1966); they assumed the differential intensity of an incident exponential electron spectrum dI e/dE = (κ/E 0) exp (- E/E 0) with E 0 = 30 keV and computed the thick-target bremsstrahlung spectrum dI x/dE at the production height. Then, by taking into account photoelectric absorption and Compton scattering, they computed the X-ray spectrum dI x/dE at a typical balloon depth 10 g cm-2; a uniform isotropic precipitation over an area 104 km2 above the balloon is assumed. It can be seen that there occurs a sharp cut-off of the X-rays for energies below 40 keV and thus that balloon data must be carefully analyzed for energies less than 50 keV.