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A comparison of the Mercury and Earth magnetospheres: Electron measurements and substorm time scales

1987; Elsevier BV; Volume: 71; Issue: 3 Linguagem: Inglês

10.1016/0019-1035(87)90040-6

1090-2643

S. P. Christon,

Geomagnetism and Paleomagnetism Studies

Similarities between plasma electron distributions and large-scale dynamics of the hermean and terrestrial magnetospheres are investigated by direct comparison. The plasma electron energy spectrum typical of the hermean premidnight cool plasma region is similar to a cool central plasma sheet electron spectrum measured in an analogous region of the terrestrial magnetotail. Both spectral shapes are similar to a kappa (κ) distribution, in which the flux j appears Maxwellian (j∼Ee−EE0) near E0 and has a power law (j∼E−κ) high-energy tail. While E0 at Earth is ∼7E0 at Mercury, κis∼7 in both cases. A model kappa distribution incorporating convective flow is used to simulate observed plasma electron spectral variations near the Mariner 10-Mercury 1 A event, suggesting that convection is stronger prior to, rather than during, the A event. This is inconsistent with the convective injection model proposed to explain the two hermean relativistic electron flux enhancements, A and Btail (D. N. Baker, Geophys. Res. Lett. 13 (1986), 789–792). Comparison of observed and predicted flux suggests that convective injection alone does not describe the hermean relativistic electron events. The hermean postmidnight energetic electron B and B′ events (each similar to terrestrial substorm dynamic injections at 6.6 Re) appear to be multiple onsets during a hermean substorm. Comparisons suggest that the B event may have been as locally intense as the terrestrial dynamic injections. Four substorm time scales at Mercury, derived primarily from the electron measurements, are on the average ∼33±11 times shorter than those at Earth. A fifth time scale, estimated from the other four, facilitates a self-consistent interpretation of previously unexplained particle and field variations in the hermean magnetosphere in terms of substorm activity.