empirical model of the low energy plasma in the inner magnetosphere n.
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Empirical Model of the Low-Energy Plasma in the Inner Magnetosphere

Empirical Model of the Low-Energy Plasma in the Inner Magnetosphere

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Empirical Model of the Low-Energy Plasma in the Inner Magnetosphere

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  1. Empirical Model of the Low-Energy Plasma in the Inner Magnetosphere J. L. Roeder, M. W. Chen, J. F. Fennell Space Sciences Applications Laboratory The Aerospace Corporation Los Angeles, CA USA R. Friedel Los Alamos National Laboratory Los Alamos, NM USA

  2. Effects of Low-Energy Particles on Materials • Low-energy (~0.1-100 keV) particle environment induces severe effects on satellite surface materials • Ions cause cumulative changes • Sputtering (collisional ejection of atoms) • Ion implantation with resulting chemical reactions • Higher mass ions tend to be more effective for damage • Synergetic effects of ions with electrons and solar ultraviolet radiation • Extrapolation of NASA radiation belt models to low energies is too uncertain • Example is Tedlar material with UV coating subjected to a simulated GEO environment

  3. Statistical Model of Particle Flux Environment • Model average omni directional particle flux spectrum as function of spatial position within Earth’s magnetosphere • Fly through spatial model to accumulate an average ion flux spectrum over any orbit (similar to NASA AE8 and AP8) • Data from CAMMICE/MICS and Hydra particle instruments on NASA Polar satellite • Polar launched in 1996 into 9 x 2 RE orbit at 98° inclination • Available data • CAMMICE/MICS data from March 1996 – April 2002 • Hydra data from March 1996 – present • Model constructed in 3-d magnetic coordinates (L, magnetic latitude, magnetic local time) from IGRF model magnetic field • Compare average spectrum of low energy particles with high energy spectra predicted by NASA models

  4. Polar has limited coverage of trapped particle fluxes trapped near equator • Equatorial crrossings start near L ~ 3 and move outward over the mission duration

  5. CAMMICE/MICS Model • MICS measures H-Fe ion fluxes of 1-200 keV/q with mass and charge state separation • Major ion species in inner magnetosphere: H+ and O+ • MICS has a single field-of-view direction providing limited angular coverage so map in pitch angle to magnetic equator • Averaged 5-minute intervals over 3.5 years: February 1996 – October 1999 (174 M data points) • 12 energies • 4 particle species: H+, O+, He+, and He++ • Spatial bins: • 18 equatorial pitch angles • 16 values of L over range 2–10 • Two-hour bins in magnetic local time • Map pitch angle data from equator to latitude of target orbit • Average all angles for omni directional flux spectrum

  6. CAMMICE/MICS Equatorial Pitch Angle Distributions • Total ion flux predominately H+ except at lowest energy • Ion flux at  = 90° trapped at magnetic equator; flux at lower and higher pitch angles are ions that mirror at higher and lower latitudes • Gaps at  = 90° for L>5 from poor Polar equatorial coverage • Fitting procedure fills equatorial data gaps (dotted lines) • Distributions harden for lower L shells due to energization of ions as they drift inward

  7. Statistical Variation of Ion Flux • Huge variation of ion flux with geomagnetic activity over 4 orders of magnitude • Average (mean) flux marked by solid white line • Dotted lines mark average plus and minus standard deviation • Relative standard deviation ~100–200%

  8. Radial and Local Time Variations of H+ Flux • Equatorially trapped H+ flux (90° pitch angles) versus L and magnetic local time • Symmetric local time distributions at highest energy • Dusk – premidnight bulge for lower energies • Dusk sector peak consistent with the known ion transport processes which turn ions drift inward from the nightside and turn eastward • Peak fluxes at L~4 with sharp inside boundary at L~3 • Small amounts of contamination from very high energy protons at inner boundary near L~2–2.5

  9. Radial Variation of H+ Flux Spectra • Equatorially trapped H+ flux spectra versus L at magnetic local time • Model validated against published model from AMPTE/CCE data [Milillo et al., 1999] • Peak magnitudes agree with factor of 2 • Many but not all spectral and spatial features agree • Faint spectral peaks at 35 keV due imperfect calibration match between low and high energy MICS channels. Negligible effects on orbital averaged flux

  10. Comparison of AMPTE/CCE with CAMMICE/MICS Protons

  11. Radial and Local Time Variations of O+ Flux • Equatorially trapped O+ flux (90° pitch angles) versus L and magnetic local time • L and local time distributions of O+ ions very similar to H+ • Symmetric local time distributions at highest energy • Dusk – premidnight bulge for lower energies • O+ spectrum much softer than H+; very few ions above 50 keV • At lowest energy O+ ion flux comparable or larger than H+ flux

  12. Radial Variation of O+ Flux Spectra • Equatorially trapped O+ flux spectra versus L at magnetic local time • No published model available for validation; Milillo et al. [1999] analyzed only H+ • Calibration match between low and high energy O+ channels is better than H+, so no obvious spectral artifacts • Lack of high energy O+ compared with H+ is consistent with the increase in the charge exchange loss process with energy for O+

  13. CAMMICE/MICS Ion Flux Time Series for GPS Orbit • One-year average omni directional total ion flux for GPS orbit • Ephemeris at 1-minute resolution • Satellite magnetic coordinates and total ion flux shown for typical 2-day interval from the year • H+ flux at most energies decreases rapidly with radius. So, as upper limit, flux for L>10 set equal to value at L=10. Possible problem with low energy O+

  14. Hydra Model • Six years of data averaged (March 1996 – early 2002) • Fluxes accumulated in spatial bins • L (0.1 width) • magnetic local time (1 hour) • magnetic latitude (10 deg) • Spectra interpolated to fixed set of energies to match data from various instrument operational modes • Spectra re-interpolated over 50 bins (0.02–14 keV) • Hydra model only for high inclination orbits such as GPS. Gaps in Polar orbit coverage at low latitudes not yet filled by interpolation or modeling.

  15. Hydra Ion Flux Distribution in GPS Orbit • Distribution of ion flux versus energy measured by Hydra averaged over GPS orbit • High time resolution of Hydra instrument results in large number of measurement points for each bin • Ion flux spectrum monotonically decreasing with energy • Statistical variation is over 3 orders of magnitude, similar to MICS variation. Relative standard deviation 100 – 200%

  16. Average Particle Flux Spectrum in GPS Orbit • H+ dominant above a few keV but O+ comparable to H+ at 1–2 keV • CAMMICE/MICS H+ matches AP-8 spectrum ~50–200 keV • Hydra ions smoothly join to CAMMICE/MICS H+ and O+ at 1 keV • Standard deviation of flux is 100–150% of average value • Negligible variation of flux between GPS orbital planes • Ion mass composition for E < 1 keV uncertain

  17. Average Flux Spectrum in GEO • CAMMICE/MICS and NASA models averaged over arbitrary GEO trajectory • Compared with LANL model [Korth et al., 1999] • GEO measurements from all of 1996 • Magnetospheric Plasma Analyzer (MPA) data at 1 eV–40 keV assumed protons • Excellent agreement between models • Much softer high energy H+ spectra, as expected

  18. Average Ion Flux Spectrum in GEO and GPS Orbit • Negligible difference in O+ spectrum for two orbits • Hardening of H+ spectrum consistent with acceleration of ions as they drift inward into high magnetic regions • Lack of energetic O+ ions at GPS due to the known increase in O+ charge exchange losses at high energy

  19. Summary • Hydra, CAMMICE and AP8 can specify average spectrum over energy range from 10 eV to 5 MeV • Excellent agreement between all ion models • O+ ion flux equal to proton flux at 1–2 keV and H+ dominates at higher energies • Almost identical O+ spectrum in GEO and GPS orbits • Use Polar TIMAS data to extend ion composition below 1 keV and CEPPAD IPS to extend protons up to 1.5 MeV • Compare models of Polar data with other missions including CRRES and SCATHA