[1] The vertical ion drift is important for understanding and modeling the electrodynamics at low latitudes. Measurements from the ion drift meter on the Defense Meteorological Satellite Program (DMSP) F15 are used to examine longitudinal variations in the vertical ion drift at the dip equator in the topside ionosphere. Local time was restricted to 0930 and the data were organized by month for 2001 and 2002. Two features were found contributing to the longitudinal variations in the electrodynamics. Meridional winds contribute to the equatorial vertical ion drift at the 830 km magnetic apex due to their dynamo action at the foot points of the magnetic field lines in the F region at magnetic latitudes near 15°. Electric fields producing a downward perturbation drift at 0930 are produced by hemispheric differences in the dynamo current due primarily to the different orientation of the magnetic meridian relative to the terminator. This produces the seasonally dependent variation as a function of longitude that is observed. A wavenumber-4 longitudinal variation also appears to be present throughout the year but is more influential during equinox conditions. This variation also shows a seasonal cycle, shifting east during northern summer and west during northern winter. Further analysis would be required to isolate the characteristics of a wavenumber-4 driver from the wavenumber-4 component of the Fourier series used here to fit the data.
During an Extravehicular Activity (EVA), if the Extravehicular Mobility Unit (EMU) makes galvanic contact with the International Space Station (ISS), a negative Floating Potential (FP) can lead to an arcing hazard when it exceeds -45.5 V, and a positive FP can produce a DC current high enough to stimulate the astronaut's muscles (5 mA), and also cause a hazard. The Boeing Space Environments team developed and utilizes a Plasma Interaction Model (PIM) in order to calculate the ISS FP based on the plasma environment, ISS velocity, geomagnetic field, solar array and ISS orientation, and solar array regulation to support EVA planning operations. Presently, the model excludes the sheath capacitance, resulting in the total potential drop being across the dielectric surface of the vehicle. Data from the Floating Potential Measurement Unit (FPMU) show this assumption to be generally true. However, Rapid Charging Events (RCE) are often observed in the FPMU data at eclipse exit when the electron number density, Ne, is low (<5e10 m -3). During these events, the FP can rise more than 40 V in one to five seconds. There is then a relaxation phase where the FP drops back to the normal FP values. The PIM model is not capable of producing these RCEs. It was thought that the inclusion of the sheath in PIM could improve the charging predictions, particularly as related to RCEs. A parametric study was performed to determine what portion of the measured FP is across the sheath for a range of Ne experienced by the ISS, and if the inclusion of the sheath in PIM is necessary. Results show that the potential drop across the sheath is negligible at times when the Ne > 1e11 m -3 . However, there appears to be a transitional region between 1e10 m -3 and 1e11 m -3 where the sheath capacitance becomes more significant. During those conditions the potential drop across the sheath can be larger than the potential drop across the dielectric for short periods (1-5 seconds). These results agree remarkably well with measurements made by the FPMU. The inclusion of the sheath explains why high charging measurements occur when the Ne is low at eclipse exit and even times when the solar arrays are not a significant driver (i.e., potentials often rise as the ISS flies through spread-F). Results also show that the RCEs are not a safety concern because the potential drop across the dielectric surface does not exceed -45.5 V. In that case, the EMU would not arc. This gives high confidence in the low probability of an arcing hazard occurring.Background:
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