Linear Prediction Filter Analysis of Relativistic Electron Properties at 6.6 R E

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. A9, PAGES 15,133-15,140, SEPTEMBER I, 1990 Linear Prediction Filter Analysis of Relativistic Electron Properties at 6.6 R E D. N. BAKER NASA Goddard Space Flight Center, Greenbelt, Maryland R.

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. A9, PAGES 15,133-15,140, SEPTEMBER I, 1990 Linear Prediction Filter Analysis of Relativistic Electron Properties at 6.6 R E D. N. BAKER NASA Goddard Space Flight Center, Greenbelt, Maryland R. L. MCPHERRON Institute ofgeophysics and Planetary Physics, University ofcalifornia, Los Angeles T. E. CAYTON AND R. W. KLEBESADEL Los Alamos National Laboratory, Los Alamos. New Mexico Electrons with energies >500 kev in the Earth's outer magnetosphere exhibit very hard energy spectra and highly variable absolute intensities. The fluxes often show a strong 27-day periodicity which is related to recurrent solar wind stream variations at I AU. We have used available solar wind speed data as well as continuous geomagnetic indices such as Kp and AE in order to characterize the relationship of relativistic electrons to these geophysical parameters. The present analysis emphasizes data taken in and employs electron measurements from 3 to 40 MeV at geostationary orbit along with geomagnetic data from the National Geophysical Data Center CD ROM. The method of linear prediction filter (LPF) analysis is used to characterize and predict the general relationship between solar wind or geomagnetic indices as input time series and electron properties as the output time series. Filters are found that generally decrease strongly at zero lag time and then peak strongly at lags of 2-3 days. Cross-covariance analyses show strong correlative peaks between electron fluxes and geomagnetic parameters at multiples of 13 and 27 days. The present analysis allows enhanced understanding of the relativistic electron behavior on both short and long time scales and permits improved prediction of both high-altitude spacecraft operational environments and magnetosphereatmosphere coupling relationships. ; i INTRODUCTION High-energy electrons in Earth's magnetosphere are of both scientific and practical interest. On the one hand, it is important to know where and how energetic electrons are accelerated; the Earth's environs are the best, most accessible region in which to study such general cosmic particle acceleration processes. Once electrons are accelerated to high energies, they can playa significant role in coupling the magnetosphere with the middle atmosphere [Baker et al. 1986]. Such coupling comes about as these electrons precipitate deeply into the mesosphere and stratosphere [Baker et al., 1987a; L. B. Callis et ai., Precipitating relativistic electrons: Their long-term effect on stratospheric odd nitrogen levels, submitted to Journal of Geophysical Research, 1989]. The middle and long time scale variations introduced by virtue of relativistic electron flux changes in the magnetosphere may offer a mechanism for modulating atmospheric chemistry and electrodynamics properties on solar rotation (27-day) and solar cycle oi-year) time scales. On the other hand, the practical significance of multi-mev electrons relates to their effects on spacecraft operations in the outer magnetosphere. A variety of analyses have demonstrated that high-energy electrons can be a principal cause of spacecraft subsystem upsets and/or complete satellite failures [e.g., Reagan et al., 1983; Baker et al., 1987b]. Relativistic electrons can penetrate deeply into spacecraft Copyright 1990 by the American Geophysical Union. Paper number 90JA o /90/90ja-00856$05.00 components and cabling, causing the so-called "deep dielectric charging" phenomenon. The subsequent materials breakdown and discharge processes within spacecraft subsystems have been inferred to account for many operational problems [Reagan et al., 1983]. Prior studies of high-energy electrons at geostationary orbit (6.6 R E ) have shown that the electron flux variations are often very large [Paulikas and Blake, 1979]. Moreover, the flux changes are often relatively periodic and are related to recurrent solar wind streams in the interplanetary medium [Baker et al., 1986J. This quasi-periodic character of the relativistic electrons has suggested that enhancements in the electron fluxes could be predicted with reasonable confidence [see Nagai, 1988]. If so, there would accrue many potential benefits for those studying magnetospheric particle populations and for those operating spacecraft within the, magnetospheric confines. It is well known that solar wind velocity variations are highly correlated with solar wind density and temperature changes, such that an overall, systematic pattern of solar wind structure often can be discerned [Borrini et al., 1981]. The correlated pattern of solar wind properties is often most clearly developed for recurrent solar wind streams which emerge from solar coronal holes [e.g., Feldman et al., 1978]. In many cases, a high-speed stream from a coronal hole may persist for many solar rotations, thus giving rise to solar wind changes near Earth with a period of 27 days. Solar wind velocity, density, and magnetic field changes near 1 AU are the dominant causes of geomagnetic activity variations [e.g., Arnoldy, 1971; Akasofu, 1981, and refer- 15,133

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DIN EN : (E) DIN EN 16603-10-04:2015-05 (E) Space engineering - Space environment; English version EN 16603-10-04:2015 Foreword... 12 Introduction... 13 1 Scope... 14 2 Normative references... 15 3 Terms, definitions