Early rocket observations of auroral Bremsstrahlung and its absorption in the mesosphere

Abstract

In the summer and autumn of 1957 the author and his colleagues at the University of Iowa conducted 10 successful balloon‐launched rocket (rockoon) flights of Geiger‐Mueller tubes during two shipboard expeditions, one to the arctic and the other to the antarctic. Summit altitudes ranged from 77 to 130 km. One flight was conducted in the equatorial zone, six in the northern auroral zone, and three in the southern auroral zone. Detailed results of each flight are presented. Auroral bremsstrahlung was detected on eight of the nine high‐latitude flights. By virtue of physical shielding, the detectors were insensitive to photons of energy E < 8 keV. Hence our derived absolute spectra refer to the high‐energy tail of the auroral electron spectrum and the resulting bremsstrahlung. For the energy range E > 8 keV, we find a typical e‐folding energy E* of about 10 keV for a differential photon number spectrum of the form dn/dE = A exp(−E/E*) with A in units of photons (cm² s keV)−1. Essentially, the same value of E* is obtained by two complementary methods: (1) The dependence of counting rate on atmospheric depth Q and (2) the inverse ratio of counting rates of an “unshielded” tube and one with an added shield. A detailed discussion of the bases for interpretation of the flight data is given in an extended appendix. For a representative set of data (flight 64) we find an omnidirectional flux of downward moving photons of 2.4×104 (cm² s)−1 at Q = 0, the integral of the above spectrum from E = 0 to E = ∞ with the explicit understanding that this result does not include the presumably much greater flux of photons having E < 8 keV and a much steeper spectrum. The corresponding electron number flux striking the top of the atmosphere is 9.3×107 electrons (cm² s)−1 with an e‐folding energy T* = 30 keV. The corresponding energy flux is 4.5 erg (cm² s)−1. Both of the two latter fluxes represent the integral from electron energy T = 0 to T = ∞ of the high‐energy tail of the electron spectrum, again with the explicit understanding that the estimates do not include the steeply rising spectrum for T ≲ 15 keV.