plasma generated during polar magnetic substorms (disturbances in the earth's magnetic field observed and near the magnetic poles at the surface of the earth). The usual consequence of the charging and subsequent electrical breakdown of the surfaces of the spacecraft is uncontrolled switching of one or more circuits. These phenomena occurred primarily (^60 percent) in the midnight to dawn gradient of the orbits - just the gradient into which electrons from the hot plesme will go when the geomegnetic field teers the plesme epert. The meesurements of e plesme detector on board the ATS-S spacecraft not only provided data on the characteristics of the hot plasma but also explained many of these anomalous switchings on board the satellite near synchronous altitudes. Hot plasma has the following characteristics: • Density of electrons ~ density of protons • Temperature of electrons « temperature of protons; therefore, thermal velocity of electrons ~ 40 x thermal velocity of protons. • Directed velocity of electrons^ directed velocity of protons; therefore, directed velocity of electrons « 1/6 x thermal velocity of electrons and directed velocity of protons ~ 6 x thermal velocity of protons. • Spacecraft charges negatively until current density of electrons ®current density of protons Note: All of these velocities are >> spacecraft velocity. A plasma is a collection of equal numbers of particles with plus and minus electrical charges which may be described by two types of motion: thermal and directed. The thermal motion results in no net displacement of the particles (on the average) and the energy distribution of the thermal motions of the particles is approximately Maxwellian. The directed motions results in a net displacement of the plasma as a whole, and all particles have the same directed velocity. Since electrons and protons in the hot plasma responsible for spacecraft charging have comparable thermal energies, the protons (because their rest mass is ^1836 times that of the electrons) have thermal velocities of ^1//1836 those of the protons. It is these higher thermal velocities of the electrons which accounts for the fact that they___ impact the surface of any uncharged spacecraft immersed in the plasma ^/1836 = 42.8 times as often as the protons do. However, assuming comparable fractions of the electrons and the protons which hit the surface of the spacecraft and stick on it, the surface will acquire a negative charge. This negative charge makes it harder for additional electrons to reach the spacecraft surface and easier for additional protons to do so. Thus the primary surface current density due to the electrons decreases while the primary surface current density due to the protons increases as the spacecraft charges up. When the primary electron current density (initially ^42.8 times the primary proton current density) has been reduced to that of the increased primary proton current density the surface charging must stop.
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