Interplanetary magnetic field structure and the variations of the ELFand VLF emissions in the topside ionosphere.

The Intercosmos-13 data obtained when measuring ELF and VLF emission amplitudes during the vernal equinox of 1975 at auroral latitudes and over the polar caps are compared with certain IMF parameters, namely the polarity of the sector structure and the signs and magnitudes of the By and Bz components of the IMF as defined in the Solar Ecliptic coordinate systems (Nishida, 1978). The comparison shows that: (1) the positive polarity of the IMF sector structure (when the IMF vector is directed toward the earth) involves an enhanced probability of the detection of larger emission field intensities (greater than 25-30 dB); (2) the emission median intensity is approximately 20 dB higher at By greater than 0 than at By less than 0; (3) the 0.72 kHz emission median intensity in the polar caps at night-side auroral latitudes is lower when Bz is greater than 0 than when Bz is less than 0; and (4) at vernal equinox there is no north-south asymmetry in the dependence of ELF and VLF emission intensity on the IMF parameters.