Acceleration of charged particles in Mercury's magnetosphere

Abstract

High‐intensity bursts of electrons with energies up to ∼600 keV were discovered during two nightside encounters of the Mariner 10 spacecraft with the planet Mercury. During the first encounter, Mariner 10 passed through the magnetotail plasma sheet region on March 29, 1974, and during the third encounter, on March 16, 1975, the spacecraft passed through the near‐polar region. Initial reports on these results (Simpson et al., 1974), which were based on 6‐ and 1.2‐s averages of the counting rates, strongly indicated that impulsive electron acceleration, probably by magnetic field reconnection, occurred in the magnetotail of Mercury and that this phenomenon was similar to Earth's substorm effect. The present paper is a detailed analysis of the encounter measurements based on the highest time resolution available for the particle counting rates, namely 0.6‐s intervals, and the correlation of the counting rates with the 0.04‐s measurements of the magnetic field. This analysis shows that the time‐intensity characteristics of the charged particle bursts, whether occurring in sequences of bursts or as single bursts, are similar for both encounters; namely, (1) individual burst onsets are ∼0.5 s per decade of rising intensity, (2) exponential decay time constants are ∼1.5 s per decade, (3) the repetition period for the sequences of bursts is ∼5 to 6 s, and (4) the maximum observed intensity of successive bursts in a sequence decreases exponentially. These observations are not artifacts of the encounter measurements but are characteristics of the particles which arrive at the point of detection promptly from the acceleration source region. Magnetosonic waves have been found near the magnetotail plasma sheet which were associated with the onsets of the two most intense burst sequences. Assuming that these waves are similar to the injection fronts of substorms described at Earth and that they indicate the presence of a neutral sheet current, an estimate of the maximum cross‐tail potential yields <2×104 V. Thus this mechanism could not account for the acceleration of the high‐energy electrons. Furthermore, acceleration mechanisms based on stochastic processes also appear to be excluded. On the other hand, the direct evidence at Mercury for the acceleration of electrons to 0.5 MeV within 1‐2 s argues for acceleration in an inductive electric field within the plasma sheet of Mercury's magnetotail produced during explosive reconnection of the magnetic field. It is pointed out that instabilities leading to explosive reconnection, such as via the tearing mode instability, could provide the required free energy for particle acceleration. The problem of how this mechanism could explain the observed quasi‐periodic electron burst sequences at Mercury is discussed. From measurements of the total electron energy in a sequence of bursts it is found that for 1% efficiency in the acceleration mechanism the required total magnetic free energy is of the order of 1012 joules, depending upon the assumed electron energy spectrum and the mode of particle propagation. From comparisons with the impulsive acceleration of charged particles to the order of MeV energies in the earth's magnetotail during magnetic substorms reported in recent years it is concluded that the rates of acceleration at Mercury are a factor of 200 to 300 times more rapid. The study reported here provides strong evidence for acceleration during explosive magnetic field reconnection within Mercury's magnetotail and places stringent, new constraints on the development of theories to explain the physics of the high‐energy charged particle acceleration in the magnetotails of planets.