Plasma environments at high altitudes and in polar orbits result in electrostatic charging of spacecraft surfaces and at low altitude cause leakage of power from exposed solar arrays and electromagnetic perturbations.
Spacecraft Charging at High Altitude
The electrostatic charging of spacecraft surfaces is the result of the spacecraft attempting to achieve a balance of currents to surfaces corresponding to an equilibrium state:
Here, Ie and Ii are currents of ambient electrons and ions, Ise and Ib are the secondary currents emitted by the surface as a result (secondary emission (SE), backscatter and ion-induced SE), Iph is the photoemission current and IR the bulk currents to the surface. These currents are obviously functions of the environment which is complex but can often be simplified for analysis to a Maxwellian or double-Maxwellian distribution. During geomagnetic substorms, hot plasma (10-30 keV) is introduced at geostationary orbit. The currents also depend on surface potentials and electric fields on and around the spacecraft and on the many material properties which are not always well-behaved. In the simple case where we only consider the ambient terms, Ie + Ii = 0 a surface floats at 2 to 3 times the electron temperature (in Volts). This results from the higher current of more mobile electrons, requiring a high negative surface charge to repel them. However, the modifying factors can be very important in space.
The important material properties are:
- Dielectric thickness;
- Dielectric constant;
- Dielectric resistivity - this is not generally a constant in space but is illumination, temperature, radiation and field-dependent;
- Surface resistivity;
- Secondary electron emission yield as a function of incident electron or ion energy;
- Photoemission current (from solar illumination).
It is clear that testing and characterisation of materials are crucial for analysis.
Plasma Interactions at Low Altitudes
The low-altitude plasma is normally cold and dense. This means that the plasma effectively screens any spacecraft-generated electric field and that surface charging to high potential level is normally not possible. However, the spacecraft velocity is mesosonic - it is faster than the ion thermal speed but lower than the electron thermal speed. Deep wakes form behind spacecraft since ions are not fast enough to refill this region while a ram ion current is collected on the `front'. Surfaces or bodies in the wake can charge if exposed to energetic electron fluxes such as found during auroral events.
Another plasma-induced problem at low altitude is that elevated-voltage systems can drive current through the local plasma. The current can be larger than expected because of plasma sheath geometry effects - this is snapover.
Finally, plasma interacts with electromagnetic waves. The dispersive properties and non-homogeneity of the ionospheric plasma can be a concern for several space-based systems, e.g. telecommunication device, global positioning systems and RADAR.
Last update: 29 March 2007