Plasma and energetic electron flux variations in the Mercury 1 C event: Evidence for a magnetospheric boundary layer

Near the outbound magnetopause crossing during the first encounter of Mariner 10 with the planet Mercury on March 29, 1974, large intensity, ∼6 s quasi‐periodic variations in the intensity‐time profile of the charged particle experiment's electron counting rate appeared as a series of peaks and valleys. The peaks have previously been interpreted as quasi‐periodic burst sequences of mildly relativistic (>175 keV) electrons, caused in one case by episodic ∼6‐s magnetotail substorm reconnection events and in another case by multiple encounters with a substorm energized electron population drifting around Mercury with an ∼6 s drift period. In this paper, we offer a new and fundamentally different interpretation of the Mariner 10 energetic electron, plasma electron, and magnetic field data near the outbound magnetopause at Mercury 1. We show that magnetosheath‐like boundary layer plasma was observed up to ∼360 km (∼0.15 R M ) planetward of the dawn magnetopause crossing as sensed by the magnetometer. We show that observations of substorm enhanced >35 keV electron flux (that previously interpreted as >175 keV electrons) associated with the hot tenuous plasma sheet population were interleaved with ∼6 s period observations of a cold dense boundary layer plasma associated with a much lower >35 keV electron flux. We argue that the ∼6 s temporal signature is due to variation of the thickness and/or position of the boundary layer plasma population. This explanation of the ∼6‐s variations, based upon our analysis of the coincident responses of the magnetic field experiment and two independent charged particle instruments (at their highest temporal resolutions), finds a direct analogue in observations of Earth's magnetospheric boundary layer, although the time scales are significantly shorter at Mercury.