Low Orbit Aircraft Radiation Environment And The Surface Charging Effect Research

Low Earth Orbit(LEO) is one of the most important orbits for spacecraft atpresent. In LEO region, the high energetic particle and plasma are the major spaceenvironment factors which could cause spacecraft system failures and anomalies.Investigating the characteristics of space environment and effects to spacecraft canprovide the technical basis for spacecraft design to defend the space environmentthreat, and is also benefit to space environment research. In this dissertation, thefeature about high energy proton environment of inner radiation belt in the lowaltitude region is investigated at first. Then the research about plasma effects onLEO spacecraft are carried out, which include the analysis of spacecraft surfacecharging and the simulation of the current leakage effect.The NOAA-15 high energy proton observation from 1998 to 2011 is used toanalyze the effect of solar cycle activity on high energy proton flux. The statisticresearch indicates that there is an inverse correlative relationship between theproton flux in inner radiation belt and solar activity. This anti-correlation is relatedto geomagnetic coordinates ?? and ??, and more significant with the increasing of?? and decreasing of ??. There is also a phase lag between the solar activity andthe proton flux. This hysteresis effect is more obvious in the region with smaller?? or larger ??. The lag can reach one year in some region. This hysteresis effectmeans it takes a long time to reach the dynamic balance between the source andthe loss for the proton of inner radiation belt in the low altitude region. Theunbalance between the source and loss is the reason why the intensity of protonflux at the same solar activity is different. The comparison with the result of AP8model indicates the energetic proton flux from AP8 is higher than the satellite’sobservation in the region with large B, which suggests the Ap8 model may overstatethe proton flux enhancement at inner radiation belt in the low altitude region ifonly the long-term variation of magnetic field considered .The technique for evaluating spacecraft surface charging is developed in onedimension condition with the geometric model of spacecraft simplified to a sphere.This method is based on the application of the current balance equation and empirical formula of secondary current. The variation of surface charging lev-els with plasma conditions in LEO and material properties was studied by usingthis method. The result shows that the spacecraft charging level has little cor-relation with precipitation electron total flux while it obviously increases withelectron fluxes above 10KeV. When the electron fluxes above 10KeV below 108cm?2s?1sr?1, the spacecraft can be hardly charged to high level (several tens ofVolts to thousands of Volts). The background thermal plasma restrains the surfacecharging level. The surface charging level decreases with thermal plasma density,and there is a threshold value of plasma density beyond which high level surfacecharging can not occur. The sunlight can hinder surface charging. The mostimportant material property affecting the surface charging is the maximum sec-ondary electron emission yield for electrons. When the emission yield is beyond 3,the spacecraft can not be charged high level. If there are more than two materialsover the surface of spacecraft, the charging status will be influenced by each otherand the charging result is determined by the conductivity among surface materials.When it is insulated among surface materials or the conductivity are not good, itis still possible for the high level charging in some small local area although themost surface of spacecraft adopted the resisting charging materials. When all thematerials over the surface are conductive, the whole spacecraft charging level canstill be affected if the regional surface materials processing is inappropriate.The last part of this dissertation is the development for calculating currentleakage effect of high voltage solar array in LEO. The method is based on currentbalance, a simplified solar array structure model and an empirical current collectionmodel of solar array. Using this method, the relationships between power losscaused by current leakage and plasma environment, solar array voltage, the areaof exposed conductor are analyzed. The research shows the power loss of solararray declines quickly with the increasing of orbital altitude. The current leakagein the region among 300-400 kilometers is most serious because of the high plasmadensity. The power loss increases exponentially with the voltage of solar array.Meanwhile, we also noticed that the power loss is far less than 1% of total poweras the voltage of solar array is below 200V. The power loss of the solar arrayincreases linearly with the area of exposed conductor. Based on this result, the current leakage effect can be reduced by decreasing the conductor’s exposure areaof solar array.