VLF WAVE MODES DETERMINED BY THE HEIGHT GAINS MEASURED DURING THE EXTENDED-LIFE-BALLOON-BORNE-OBSERVATORY TEST FLIGHT

The changes with altitude of the vertical electric field from seven VL F ground-based transmitters were measured during the ascent of the Ext ended Life Balloon-Borne Observatory (ELBBO) balloon test flight from Ft. Sumner, New Mexico. The altitude profiles of the VLF vertical elec tric field allowed us to determine the propagation modes based on the relative amplitude changes of the VLF signals with height. Three of th e VLF transmitters had signals which crossed through the dawn terminat or on their paths to the balloon's receiver. The observed height gains from these three stations show that at least two models of sunrise mo dal interference, those proposed by Crombie [1964] and by Kaiser [1968 ], are occurring. For these three cases a time and altitude-varying he ight gain signal (the ratio E(z)/E(0) for altitude z above the surface z = 0) was measured during the balloon's 70-min ascent to an altitude of 28.5 km. This method of balloon-measured VLF height gains allows a high level of discrimination between modes, which is shown to be a us eful method for studying the phenomena of sunrise and sunset terminato r effects on VLF modal interference.