Harry C. Koons

A neural network model of the relativistic electron flux at geosynchronous orbit

A neural network has been developed to model the temporal variations of relativistic (>3 MeV) electrons at geosynchronous orbit based on model inputs consisting of 10 consecutive days of the daily sum of the planetary magnetic index ΣKp. The neural network (in essence, a nonlinear prediction filter) consists of three layers of neurons, containing 10 neurons in the input layer, 6 neurons in a hidden layer, and 1 output neuron. The output is a prediction of the daily-averaged electron flux for the tenth day. The neural network was trained using 62 days of data from July 1, 1984, through August 31, 1984, from the SEE spectrometer on the geosynchronous spacecraft 1982-019. The performance of the model was measured by comparing model outputs with measured fluxes over a 6-year period from April 19, 1982, to June 4, 1988. For the entire data set the rms logarithmic error of the neural network is 0.76, and the average logarithmic error is 0.58. The neural network is essentially zero biased, and for accumulation intervals of 3 days or longer the average logarithmic error is less than 0.1. The neural network provides results that are significantly more accurate than those from linear prediction filters. The model has been used to simulate conditions which are rarely observed in nature, such as long periods of quiet (ΣKp = 0) and ideal impulses. It has also been used to make reasonably accurate day-ahead forecasts of the relativistic electron flux at geosynchronous orbit.

Relationship between electrostatic discharges on spacecraft P78-2 and the electron environment

Abstract : The relationship between the energetic electron environment and electrostatic discharges on the P78-2 (SCATHA) spacecraft has been examined. Internal discharges occur near perigee while surface discharges occur about uniformly over the radial range covered by SCATHA, 5.5 to 8 Re. The surface discharges peak at local midnight and decrease toward local morning while the internal discharges have a broad occurrence maximum centered on local noon. Both types are far more likely to occur when the Earths magnetic field is disturbed. Although few surface discharges occur when the Planetary Magnetic Index, Kp, is less than 4, a modest number of internal discharges occur under these quiet to normal conditions. In all cases where internal discharges occurred under quiet to normal conditions, a period of very disturbed conditions had occurred a few days earlier. Surface discharges have a strong tendency to occur when the flux of electrons with energies of 10s of keV is high while internal discharges occur when the flux of electrons with energies of 100s of keV is high, and the flux of 10s of keV electrons is low. A significant correlation has been found between the occurrence of discharges on SCATHA and an estimate of the energetic electron fluence at geosynchronous orbit obtained from a neural network model of the relativistic electron flux at geosynchronous orbit. At predicted daily fluences greater than 1010 elec/sq cm almost all discharges result from internal charging. The incidence of internal discharges at predicted daily fluences above 1010 elec/sq cm is sufficiently high to warrant the use of this predictor to issue warnings for real time satellite operations.

A Severe Spacecraft-Charging Event on SCATHA in September 1982,

Abstract : On September 22, 1982, 29 large amplitude electrostatic discharges were detected by the Pulse Analyzer onboard the SCATHA satellite. Seventeen of these pulses exceeded the maximum voltage discrimination level, which was set to 7.4 volts. This was the worst instance of electrostatic discharges encountered to date by the SCATHA satellite. Three different spacecraft anomalies occurred on SCATHA that day, the most serious being a two-minute loss of data. During this same time period, the Surface Potential Monitor experiment aboard the satellite measured the largest differential surface charging observed in the data since the satellites launch in January 1979. Keywords: Electrostatic discharges; Spacecraft anomalies; Spacecraft charging.

Spatial and temporal correlation of spacecraft surface charging in geosynchronous orbit

Surface-charging sensors are used on spacecraft to measure the potential between a sample material on the surface and the spacecraft frame. The frame is taken to be electrical ground. A study has been performed to determine how far away a surface-charging sensor can be in local time in geosynchronous orbit and still provide reasonable situational awareness about hazardous surface-charging conditions to a nearby spacecraft. The study was conducted using surface-charging sensor data from four INTELSAT and two New Skies Satellites communications satellites and electron temperature data from one electrostatic analyzer provided by Los Alamos National Laboratory for a spacecraft in geosynchronous orbit. The distances between pairs of nearby spacecraft covered a range from 5.5 to 34 deg in longitude or 0.4 to 2.3 h in local time. We have cross-correlated data from pairs of spacecraft for two large surface-charging events in April and July 2000 and also plotted scatter diagrams of the data for pairs of vehicles for the entire year 2000. For the two large events we find a relatively high, normalized, cross-correlation coefficient indicating a significant degree of temporal correlation with time lags varying from 0 to 22 min. When looking at scatter diagrams from many more events over a one-year period, we find no apparent organization in the data. There appears to be no obvious relationship between the potentials measured simultaneously on adjacent vehicles during charging events. We also found no amplitude correlation between the electron temperature measured on one vehicle and the surface potential measured on another located only a few degrees away in longitude. However, individual charging events are usually (but not always) encountered within the same time period on nearby spacecraft. We conclude that surface potential measurements for the purposes of anomaly diagnosis or situational awareness of potentially hazardous environmental conditions cannot be made on nearby vehicles but must be made on each spacecraft in geosynchronous orbit.

Substorms and magnetic storms from the satellite charging perspective

Abstract Substorms and magnetic storms generate significant space weather effects in the inner magnetosphere. They change the dose rates experienced by satellites in many orbits and are directly linked to the occurrence of satellite charging. Substorms inject hot plasma into the nightside magnetosphere. The drifting electron component of this hot plasma can charge the surfaces of the satellites leasing to electrostatic discharges, the associated satellite anomalies and sometimes failures. These occur in regions that are consistent with the expected motions of the substorm injected particles. The high-energy electron enhancements following many magnetic storms can be sufficient to cause charging of shielded satellite elements. Not all magnetic storms result in flux enhancements sufficient to cause such internal charging. Because the induced voltages from the internal charging are usually not directly measured, the anomalies they cause are more difficult to link to the space environment and magnetic storms. However, the anomaly statistics are sufficient to show linkage in a several cases.

Spacecraft environmental anomalies expert system

Abstract : An expert system has been developed by The Aerospace Corporation, Space and Environment Technology Center for use in the diagnosis of satellite anomalies caused by the space environment. The expert system is designed to determine the probable cause of an anomaly from the following candidates: surface charging, bulk charging, single-event effects, total radiation dose, and space-plasma effects. Such anomalies depend on the orbit of the satellite, the local plasma and radiation environment (which is highly variable), the satellite-exposure time, and the hardness of the circuits and components in the satellite. The expert system is a rule-based system that uses the Texas Instruments Personal Consultant Plus expert-system shell. The expert systems knowledgebase includes about 200 rules, as well as a number of databases that contain information on spacecraft and their orbits, previous spacecraft anomalies, and the environment. Space environment, Satellite anomalies, Expert system, Spacecraft charging, Single-event upsets, Radiation dose

Electron plasma waves in the solar wind: AMPTE/IRM and UKS observations

Selected events of plasma wave and electromagnetic emissions in the earths electron fore-shock region have been studied. Strong emissions are observed in the plasma-wave band when the site of the satellite is magnetically connected to the bow shock. These emissions are generally highly fluctuating. Under certain conditions one observes electromagnetic radiation at the second harmonic produced locally. Electromagnetic emission generated at a position far away from the site of the spacecraft is occasionally detected giving rise to remote sensing of the bow shock. These emissions are related to energetic electron fluxes.

Plasma Waves Stimulated by Electron Beams in the Lab and in the Auroral Ionosphere

Energetic electron beams are frequently used as active probes of space plasmas. Often the assumed test particle nature of these electrons is violated when the electron beam stimulates plasma wave emissions. Such complex phenomena have been observed on rockets and satellites and are being modeled in laboratory plasmas. The large vacuum chamber at NASA Johnson Space Center in Houston, Texas has been used for modeling F-region type ionospheric plasmas. A VLF receiver has been flown into an auroral plasma and the spectra from this flight will be compared to VLF spectra obtained in the NASA/JSC laboratory chamber. The electron beam is believed to have produced beam plasma discharge (BPD) on the rocket similar to that seen in the lab. At times during the rocket flight the electron beam was operated at 4 kilovolts and the electron current modulated at 3 kilohertz from 0 to 80 milli-amps. This resulted in the beam pulsing in and out of BPD and a variety of propagating wave modes.

Extremely Low Frequency Wave Analyzer

Abstract : This report describes the Extremely Low Frequency Wave Analyzer flown as part of the LASSII (Low Altitude Satellite Studies of Ionospheric Irregularities) experiment aboard the CRRES (Chemical Release and Radiation Effects Satellite). This instrument measures electrostatic and electromagnetic waves in the ambient Ionosphere and during the CRRES chemical releases.

Electrostatic Instabilities in Multicomponent Plasmas

A commonly employed heuristic method for evaluating phase velocities and growth rates of ion and electron plasma oscillations is generalized here to multicomponent plasmas. The new formulation exhibits the correct Galilean transformation properties.