Charging of Large Structures in Space with Application to the Solar Sail Spacecraft

Some Important Charging Effects at Low Altitudes Introduction T HE ionosphere as a rule is a rather benign environment as far as charging of spacecraft is concerned. The ionospheric plasma is relatively cool and dense in comparison with other plasmas in space. The plasma density ranges from values on the order of 10 cm" to values as low as or even below 10 cm~ , with temperatures well below 1 eV (11,600 K). However, there can be large fluxes of energetic electrons in the polar ionosphere. The possibility that these auroral fluxes could charge a spacecraft to large potentials makes this subject a matter of concern. However, even moderate charging of spacecraft can complicate the interpretation of data from scientific experiments on satellites in the ionosphere. Figures 1 and 2 show how two important length scales vary in the ionosphere. Figure 1 shows that the Debye length ranges from about 1 mm to a few centimeters. The electrostatic sheath thickness about a charged object is on the order of a few Debye lengths. Thus, in the ionosphere, the Debye length is very small compared to a large space structure. Figure 2 shows typical electron and proton gyroradii in the ionosphere. Again, the electron gyroradius is small, on the order of a few centimeters to 1 m. The proton gyroradius is larger, up to tens of meters in size, and thus can be comparable to the dimensions of large space structures. The charging of objects in the ionosphere is characterized by extreme anisotropy in the charging currents. The ion current is dominated by the motion of the spacecraft in its motion through the plasma and hence the ion current occurs largely on the leading surfaces of such an object. The electron current does not vary as strongly as the ion current but is affected by the VxB and wake effects which cause it to also vary with position on the spacecraft surface. The photoemission current varies with surface orientation with respect to sunlight. Possibility of Large Potentials in the Polar Ionosphere Katz and Park studied the possible charging effects of large electron fluxes in the auroral ionosphere on a spacecraft. They took an energetic electron flux o f ix lO" 4 A/m at an energy of 3 keV and examined its effect on an orbiting sphere in the ionospheric plasma with a density of 10 cm~ at a temperature of about 0.1 eV. In their model, the large negative current due to the energetic electrons was balanced primarily by the positive current due to ion collection. The significant result obtained by Katz and Parks was that the sphere would charge to large negative potentials approaching the energy of the auroral electrons. This occurred because the increase in ion current necessary to balance the electron current was limited by the sheath thickness. The sheath thickness grows slowly with the potential drop across the sheath when the Debye length is small and, hence, a large potential drop is required to obtain the necessary ion current. This result is illustrated in Fig. 3 which shows how the equilibrium potential of the sphere increases with its radius. In regions where the plasma concentration is small, the potentials are correspondingly greater. A more precise calculation must include secondary electron currents and photoemission but, in principle, it is possible to have large potentials in regions where large energetic electron fluxes occur. It is important to know the frequency of occurrence of large fluxes of energetic electrons in the polar ionosphere.