The interrelationship between the >130 keV electron trapping boundary, the VHF radar backscatter, and the visual aurora

A coordinated program between satellite, VHF radar, and optical auroral observations was carried out in Alaska during the spring of 1972. On 18 days in February, March, and April the positions of the >130-keV isotropic and cutoff trapping boundaries in the evening sector of the oval were determined on satellite 1971-89a. On 9 of these days, simultaneous VHF 50-MHz auroral radar backscatter echoes were observed to lie equatorward of the cutoff trapping boundary regardless of the phase of the auroral substorm. The analysis of these data with respect to substorm phase, Dst, and position of the aurora shows that the motion of the trapping boundary is a dynamic feature of the substorm that can be monitored and approximately located by the VHF radar echoes.