A comparison of the Mercury and Earth magnetospheres: Electron measurements and substorm time scales
Abstract
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 - E/E 0) near E0 and has a power law ( j∼ E- κ) high-energy tail. While E0 at Earth is ∼7 E0 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 B tail (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.
- Publication:
-
Icarus
- Pub Date:
- September 1987
- DOI:
- 10.1016/0019-1035(87)90040-6
- Bibcode:
- 1987Icar...71..448C
- Keywords:
-
- Earth Magnetosphere;
- Electron Distribution;
- Electron Energy;
- Magnetic Storms;
- Mercury (Planet);
- Planetary Magnetic Fields;
- Planetary Magnetospheres;
- Energetic Particles;
- Flux (Rate);
- Mariner 10 Space Probe;
- Plasma Spectra;
- Relativistic Electron Beams;
- Solar System;
- MERCURY (PLANET);
- COMPARISONS;
- MAGNETOSPHERE;
- TIME SCALE;
- ELECTRONS;
- PLASMA;
- DISTRIBUTION;
- DYNAMICS;
- ENERGY;
- SPECTRUM;
- CONVECTION;
- INJECTION;
- MODELS;
- FLUX;
- DIAGRAMS;
- CALCULATIONS;
- MAGNETIC FIELDS;
- SPACECRAFT OBSERVATIONS;
- MARINER 10;
- EQUIPMENT;
- SIMULATIONS