Actually, the surface charging will stop before then, since these are secondary currents which act to reduce the net (negative minus positive) charging current. These secondary current components (Figure 6.2-2) include secondary electrons (due to both primary electron and due to primary protons) and photoelectrons (due to ultraviolet light from the sun). Since all of these secondary electrons are ejected from the surface of the spacecraft, they reduce the net flow of negative charge to that surface. Some numbers will illustrate this effect. If the primary electron current density reaching an uncharged surface is lnamp/cm^ (10“9 amp/cm2, a typical value) the primary proton current density will be ^0.014 n amp/cm2. The secondary electron current densities due to the primary proton and electrons will be on the order of ^0.024 n amp/cm2 and "v0.5 n amp/cm2 respectively. The photoelectron current density will be ^0.5 n amp/cm, independent of the other current densities. The net current density will be ^0.45 n amp/cm2 for aluminum surfaces in the dark; M3 n amp/cm for such surfaces in the light. Other materials may charge more or less, depending on their secondary electron emission characteristics. Because of the small surface capacitances of most spacecraft (typically ^10~H farad/cm2), a net charging current of <10“9 amp/cm^ will produce an initial charging rate of £100 volts/sec. If a spacecraft surface can tolerate 10^ volts before breaking down, it will take £100 seconds to reach the breakdown voltage. Subsequent breakdowns (discharges) will occur at time intervals <100 seconds since a discharge will not generally deplete the surface charge completely. What usually happens is that small areas of an insulated surface (£100 cm^) will discharge as a unit, and the spacecraft insulated surface area (>1 m2) is sufficiently large to produce such £1 joule discharge approximately every second. Figure 6.2-2. Secondary Electron Effects
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