David J. Gorney

Precipitating relativistic electrons - Their long-term effect on stratospheric odd nitrogen levels

Using electron count rate data at geostationary orbit, daily energy spectra, extending from 30 keV to 15 MeV, have been developed for trapped relativistic electrons at 6.6 RE These spectra have been used to model the flux of these electrons into the atmosphere at 120 km. Energy deposition calculations permit daily sources of HOx and NOy to be calculated at auroral and subauroral latitudes due to relativistic electron precipitation (REP) for the period June 13, 1979, through June 4, 1988. Both short-term and long-term source variations are quite large over the period considered. The long-term variation of the NOy source is found to reach a maximum in late 1984 and early 1985, with significant declines thereafter. Daily Solar Backscattered Ultraviolet (SBUV) O3 data show a significant response to these precipitation events. Two-dimensional model calculations have been carried out for the period 1979 to 1990 with REP effects included through June 4, 1988. Results suggest that globally integrated NOy has increased by 35–40% from 1979 to early 1985 with declines thereafter. The largest long-term increases are found in the lower stratosphere at the high latitudes. Comparisons of Limb Infrared Monitor of the Stratosphere (LIMS), Solar Mesospheric Explorer (SME), Stratospheric Aerosol and Gas Experiment (SAGE), and SAGE II NO2 data are consistent with these calculations. The results suggest that a significant contribution to the anomalously large and unexplained global O3 declines between 1979 and 1985 has been made by the catalytic destruction of O3 by odd nitrogen in the lower stratosphere at mid to high latitudes. The results also provide evidence for a clear and strong linkage between solar variability, the state of the magnetosphere, and the chemical climatological state of the middle and lower atmosphere.

Stormtime transport of ring current and radiation belt ions

This is an investigation of stormtime particle transport that leads to formation of the ring current. Our method is to trace the guiding-center motion of representative ions (having selected first adiabatic invariants µ) in response to model substorm-associated impulses in the convection electric field. We compare our simulation results qualitatively with existing analytically tractable idealizations of particle transport (direct convective access and radial diffusion) in order to assess the limits of validity of these approximations. For µ ≲ 10 MeV/G (E ≲ 110 keV at L ≈ 3) the ion drift period on the final (ring-current) drift shell of interest (L ≈ 3) exceeds the duration of the main phase of our model storm, and we find that the transport of ions to this drift shell is appropriately idealized as direct convective access, typically from open drift paths. Ion transport to a final closed drift path from an open (plasma-sheet) drift trajectory is possible for those portions of that drift path that lie outside the mean stormtime separatrix between closed and open drift trajectories. For µ ∼ 10-25 MeV/G (110 keV ≲ E ≲ 280 keV at L ≈ 3) the drift period at L ≈ 3 is comparable to the postulated 3-hr duration of the storm, and the mode of transport is transitional between direct convective access and transport that resembles radial diffusion. (This particle population is transitional between the ring current and radiation belt). For µ ≳ 25 MeV/G (radiation-belt ions having E ≳ 280 keV at L ≈ 3) the ion drift period is considerably shorter than the main phase of a typical storm, and ions gain access to the ring-current region essentially via radial diffusion. By computing the mean and mean-square cumulative changes in 1/L among (in this case) 12 representative ions equally spaced in drift time around the steady-state drift shell of interest (L ≈ 3), we have estimated (from both our forward and our time-reversed simulations) the time-integrated radial-diffusion coefficients DLLsim for particles having selected values of µ ≳ 15 MeV/G. The results agree surprisingly well with the predictions (DLLql) of quasilinear radial-diffusion theory, despite the rather brief duration (≈ 3 hr) of our model storm and despite the extreme variability (with frequency) of the spectral-density function that characterizes the applied electric field during our model storm. As expected, the values of DLLsim deduced (respectively) from our forward and time-reversed simulations agree even better with each other and with DLLsim when the impulse amplitudes which characterize the individual substorms of our model storm are systematically reduced.

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.

Cross-polar cap potential difference, auroral electrojet indices, and solar wind parameters

The cross-polar cap potential difference Φ (KRM) is estimated from ground magnetic perturbation data through the magnetometer inversion method of Kamide, Richmond, and Matsushita (KRM), combined with an “empirical” ionospheric conductance distribution estimated from the DMSP X ray image data. A significant correlation is found between Φ (KRM) and the AE(12) index; Φ (KRM, in kilovolts) = 36 + 0.082 AE(12, in nanoteslas) with the correlation coefficient being 0.80. Φ (KRM) is then compared with the potential difference estimated from a more direct method of the satellite electric field measurements [Weimer et al., 1990] and also with Φ (IMF) based on solar wind parameters [Reiff and Luhmann, 1986]. Φ (IMF) is found to be linearly correlated with Φ (KRM), as Φ (IMF) = 29.8 + 0.999 Φ (KRM), with the highest correlation obtained for a 40-min lag in the interplanetary magnetic field (IMF). Note that Φ (IMF) is systematically larger than Φ (KRM) by 30 kV, suggesting the possibility that the theoretical method overestimates the cross-polar cap potential difference. During steady southward IMF periods where steady Φ (IMF) variations are expected, significant fluctuations in calculated Φ (KRM) values are obtained. Since the decrease in Φ (KRM) is closely associated with enhancements in auroral particle precipitation during these periods, a highly correlative relation between Φ (IMF) and Φ (KRM) cannot be deduced unless the phases of substorms are taken into account. The overall high correlation between them, however, supports the view expressed by Wolf et al. [1986] that directly driven processes are more important than unloading processes during disturbed periods.

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.

Ion radial diffusion in an electrostatic impulse model for stormtime ring current formation

Guiding-center simulations of stormtime transport of ring-current and radiation-belt ions having first adiabatic invariants μ ≳ 15 MeV/G (E ≳ 165 keV at L ∼ 3) are surprisingly well described (typically within a factor of ≲ 4) by the quasilinear theory of radial diffusion. This holds even for the case of an individual model storm characterized by substorm-associated impulses in the convection electric field, provided that the actual spectrum of the electric field is incorporated in the quasilinear theory. Correction of the quasilinear diffusion coefficient DLLql for drift-resonance broadening (so as to define DLLrb) reduced the typical discrepancy with the diffusion coefficients DLLsim deduced from guiding-center simulations of representative-particle trajectories to a factor ∼3. The typical discrepancy was reduced to a factor ∼ 1.4 by averaging DLLsim, DLLql, and DLLrb over an ensemble of model storms characterized by different (but statistically equivalent) sets of substorm-onset times.

Imagers for the magnetosphere, aurora, and plasmasphere

We present a small Explorer mission, Imagers for the Magnetosphere, Aurora, and Plasmasphere (IMAP), to provide the first global magnetospheric images that will allow a systematic study of major regions of the magnetosphere, their dynamics, and their interactions. The mission objective is to obtain simultaneous images of the inner magnetosphere (ring current and trapped particles), the plasmasphere, the aurora, and auroral upflowing ions. The instruments are (1) a Low Energy Neutral Particle Imager for imaging H and O atoms, separately, in the energy range of ~1 to 30 keV, in several energy passbands; (2) an Energetic Neutral Particle Imager for imaging H atoms in the energy range ~15 to 200 keV and, separately, O atoms in the energy range ~60 to 200 keV, each in several energy passbands; (3) an Extreme-Ultraviolet Imager to obtain images of the plasmasphere (the distribution of cold He + ) by means of He + (30.4 nm) emissions; and (4) a Far-Ultraviolet Imaging Monochromator to provide images of the aurora and the geocorona. All images will be obtained with time and spatial resolutions appropriate to the global and macroscale structures to be observed. IMAP promises new quantitative analyses that will provide great advances in insight and knowledge of global and macroscale magnetospheric parameters. The results expected from IMAP will provide the first large-scale visualization of the ring current, the trapped ion populations, the plasmasphere, and the upflowing auroral ion population. Such images, coupled with simultaneously obtained auroral images, will also provide the initial opportunity to globally interconnect these major magnetospheric regions. The time sequencing of IMAP images will also provide the initial large-scale visualization of magnetospheric dynamics, both in space and time.

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

An Auroral X Ray Imaging Spectrometer

A scanning X-ray spectrometer was flown aboard the United States Air Force Defense Meteorological Satellite Program (DMSP-F6) satellite to image X-ray production in the earths atmosphere. One of the main objectives of this experiment was to image auroral signatures associated with electron precipitation at energies above a few keV. A brief description of the instrument is given and a sequence of auroral images is shown to demonstrate on-orbit performance and to illustrate the use of such data to monitor ionization density perturbations in the earths atmosphere.

Relativistic magnetospheric electrons: Lower ionospheric conductivity and long-term atmospheric variability

Abstract Long-term (1979–88) observations of relativistic electrons in the earths outer magnetosphere show a strong solar cycle dependence with a prominent intensity maximum during the approach to solar minimum (1983–85). This population therefore corresponds to the presence of high-speed solar wind streams emanating from solar coronal holes. Using a numerical code, we have calculated the precipitating electron energy deposition in the earths upper and middle atmosphere. Observed events (typically persisting several days) would have maximum effect in the 40–60 km altitude range. We suggest that this electron population could play an important long-term role in modulating lower D-region ionization and middle atmospheric ozone chemistry. We describe methods of observing middle atmospheric and lower ionospheric effects of the electrons. A particularly promising approach may involve the monitoring of global Schumann resonance modes which are sensitive to global changes in the properties of the earth-ionosphere cavity. Ongoing work indicates that Schumann resonance properties are moderately correlated with the flux of precipitating relativistic electrons thus offering the possibility of continuously monitoring this aspect of magnetosphere-atmosphere coupling.