Spacecraft platforms and systems interact sometimes strongly with the space plasma environment. Due to their mobility, charged particles(electrons, ions) hit external surfaces and , in synergy with the ionization effect by solar UV or high energy particles, lead to the accumulation of a net electrical of charge build up an electrostatic potential difference between the satellite and the plasma, able to reach many kV negative or a few tens volts positive. We investigated characteristics of ionosphere plasmas and spacecraft surface voltage.
The ionosphere has a nearly bell-shaped electron density profile with a gradient length between 10 and 40km. In the polar region the density profile is nearly aligned along the magnetic field. Its maximum density usually occurs at a height of 200-300km, where the ionization is typically 1-10%. Since the neutral density decrease exponentially with a scale height of 7km, the electron-neutral and ion-neutral collisions decrease with heights above 75km and electron cyclotron motion and collisionless behavior become important while the ions become collisionless at approximately 125km. In the low ionosphere, an object in space plasma tends to a few V negative on the surface. Due to contact and electrostatic interaction, a spacecraft in the ionosphere plasma environment will collect a large number of charged particles (electrons, ions). Adding to the ionization effects, these various currents will define an electrical balance for the spacecraft-plasma system. The charged spacecraft will repel particles of the same sign (electrostatic barrier) and attract particles of opposite sign. The result will be the creation of a non neutral plasma region around the vehicle called the sheath. The sheath has been investigated by many lab plasma researchers and fusion research laboratory. We employ a one-dimensional electrostatic particle-in-cell code, with collisions incorporated using the Monte Carlo method. While it has been reported previously that the presheath potential drop increases with the ion collision frequency, the effects of electron collisions were not explored. In the collisional case, we investigated floating potential variation with ion-neutral collision frequency. The effects of electron collisions in the plasma sheath are also investigated. We compared plasma sheath formation with only ion-neutral collision effect with one considering ion-neutral collision and electron-neutral collision effect. We note that the reduced electron flux, caused by electron collisions, make the presheath potential shallower as the required ion drift motions to balance the electron current are smaller.
Secondly, We researched into plasma sheath formation with the dependence of the plasma parameters on the external magnetic field intensity and magnetic field angle grazing on plasma system. We also studied plasma-sheath relation with collision effect and magnetic field effect.
Thirdly, our simulation system treated with collisional plasma sheath structure considering lab plasma system parameters.
Finally, We studied plasma floating potential and plasma drift velocity and temperature between 80km and 400km ionosphere region. We compared observed satellite surface voltage with PIC simulation result of space plasma environment from 140km to 400km. The numerical approximation of floating potentials with altitude has been calculated. The data of ionosphere plasma environment of 80km and 100km altitude shows nearly collisional case of plasma. From this region, presheath potential drop increase with collisional frequency due to plasma neutral interactions. Collisionless plasma environment should be formed above 125km region. In this area, floating potential drop has dependence on ion to electron temperature ratio. Electron temperature has a good influence on floating potential drop with the altitude between 140km and 400km. The atmospheric electrical conductivity is nearly isotropic below 70km, but above that altitude the electron-neutral collision frequency becomes smaller than the electron gyro frequency and above 140km , ion-neutral collision frequency becomes smaller than the ion gyro frequency , so that the electrical conductivity becomes anisotropic. Above 140km, ion gyro frequency becomes larger than ion neutral collision frequency and magnetic field effects are dominant over above this altitude. We included magnetic field effect on plasma sheath formation from 140km to 400km ionosphere region. The inclusion of magnetic field shows sheath thickness and sheath potential variation are significantly affected by magnetic field intensity , especially in the case when the magnetic field is parallel to the surface or intersects the surface at small angles. When magnetic angle 8 is large ,magnetic field becomes almost perpendicular to the acoustic motion. Therefore, the magnetic force on charged particles will be more , which in turn makes (due to gyration) the particles deviate from the propagation direction, so a less number of charged particles can move to the sheath. Since the density of charged particles will be required to shield the electric field of the electrode. Hence , the sheath thickness will be more in this case. On the other hand, as 8 decreases, the effect of the magnetic field on plasma motion becomes weak. Hence , the sheath thickness will decrease.
The formation of the sheath and the characteristic variation of the sheath potential and thickness can be obtained from the result of PIC simulation on ionospheric plasma environment using non magnetized plasma and magnetized plasma system.
