Antarctic substorm events observed by sounding rockets, ionization of the D- and E-regions by auroral electrons

Abstract

The ionization structure of the auroral arc was measured on a sounding rocket which penetrated into a bright auroral arc. The E-region electron density becomes large (2 ∼ 5 × 10 5 el/cm 3 only in the moving auroral arc, whose N 2 + 4278 Å brightness is 1 ∼ 2·5 kR. The electron density in the D-region beneath the lower boundary of the arc (75 ∼ 98 km in altitude) is also considerably enhanced to 2 ∼ 5 × 10 4 el/cm 3. The observed E-region electron density can be interpreted theoretically as due to the direct ionization by precipitating electrons, whose energy spectrum is approximately represented by an exponential type having the characteristic energy of 2 keV. The correlation between the electron density and the N 2 + 4278 Å brightness can be reasonably explained by considering the simultaneous effects on the ionization and the optical excitation caused by the primary electrons having a flux of 9 × 10 9 el/cm 2/sec per 1 kR of the 4278 Å emission. Further analyses using the electron density data from four other sounding rockets have shown that the D-region ionization has good correlations to the cosmic noise absorption (CNA) and the magnetic substorm activities observed simultaneously at the ground station, whereas it has poor correlation to the same quantity of the E-region measured in the same experiment. It is found that the observed D-region ionization is much larger than that predicted by the theory which takes into account the Bremsstrahlung X-ray ionization along with the direct impact ionization when it is applied to the precipitating electron flux spectrum consistent to the E-region ionization and optical excitation. After all the present experimental results suggest a dual nature of the electron precipitation spectrum in the substorm, i.e. the softer part which is localized in the auroral arc and the harder part which is spatially wide-spread over the substorm area.