Geostationary orbit and s/c

spaceweb@oulu.fi - last update: 7 May 1998, 1620 UT (RR)

The geostationary or geosynchronous orbit is located at approximately 6.6 Re geocentric distance in the geographical equatorial plane. In it, centrifugal force just balances gravity for a spacecraft that is in a circular orbit with a period of one orbit per day (i.e., at fixed geographic longitude). The first operational geosynchronous satellite (Syncom 2) was launched in 1963, and nowadays there are typically several spacecraft in this orbit continuosly.

The geosynchronous orbit tends to skim the inner boundary of the plasma sheet. This means that the surrounding plasma/magnetic field region varies a lot with the magnetic activity. During quiet times, the satellites are typically at dipolar like field lines earthward of the plasma sheet, while during major magnetic storms they are, when close to midnight, clearly within its stretched field lines. Because the geographic equatorial plane is tilted relative to the magnetic equatorial plane, geostationary orbit lies between -11° and 11° magnetic latitude. This means, with the stretched field line geometry, that the geostationary satellites may be at field lines threading distances considerably father away than 6.6 Re in the tail

• hard
  • 300-2000 keV, T=200 keV, n=1x10(-4) cm(-3)
  • n decreases, often drastically, during magnetic storms, while T changes less
  • at geosynchronous orbit, there is a weak diurnal variation on n, showing larger density at noon (lower L-shells) than at midnight (higher L-shells); this indicates existense of a large radial density gradient
  • density gradients, in turn, indicate that outward diffusion should be operating as a regulator for the hard population
  • temperature gradient is weaker than the density gradient, so the diurnal variation is not clear in T
  • also a quasiperiodicity at 27-day solar rotation period is observed in n
• soft
  • 30-300 keV, T=25 keV, n=5x10(-3) cm(-3)
  • n increases drastically (hundredfold or more) during substorm particle injections, also T varies

Particle injections occur during substorm expansion phase. The energy dependent drift of these particles around the Earth can be monitored by satellites at different magnetic local times. From the dispersion one can calculate estimates for the time t(0) for the injection and the longitudinal width of the injection region, using either several satellites or the drift echoes (Belian et al., 1978). Drift echoes are formed when particles complete more than one circle around the Earth. (Note, however, that injections are not confined to the geostationary orbit only, and that they can be seen as close to Earth as 4.3 Re or so.)

References

• Belian, R. D., D. N. Baker, P. R. Higbie, and E. W. Hones, High-resolution energetic particle measurements at 6.6 Re, 2. High-energy proton drift echoes, J. Geophys. Res., 83, 4857-4862, 1978.
• Cayton, T. E., R. D. Belian, S. P. Gary, T. A. Fritz, and D. N. Baker, Energetic electron components at geosynchronous orbit, Geophys. Res. Lett., 16, 147-150, 1989.

Data sources:

• LANL data
• GOES data

See also:

• Space Physics Text Book of Oulu