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Radiation Belt Electron Pitch Angle Measurements from the GOES Satellites

Radiation Belt Electron Pitch Angle Measurements from the GOES Satellites. T. G. Onsager, J. C. Green, and H. J. Singer. NOAA Geostationary Operational Environmental Satellites (GOES): Normally 3-axis stabilized – Spinning while in on-orbit storage.

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Radiation Belt Electron Pitch Angle Measurements from the GOES Satellites

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  1. Radiation Belt Electron Pitch Angle Measurements from the GOES Satellites T. G. Onsager, J. C. Green, and H. J. Singer NOAA Geostationary Operational Environmental Satellites (GOES): Normally 3-axis stabilized – Spinning while in on-orbit storage • Demonstration of pitch angle measurements from GOES • Statistical characterization of pitch angle distributions • Variability of pitch angle distributions during radiation belt enhancement and loss Multiple years of geosynchronous electron pitch angle distributions are becoming available to investigate radiation belt dynamics.

  2. B  p Ogo-5 Pitch Angle Measurements Distinctive pitch angle distributions result from drift shell splitting in Earth’s asymmetric magnetic field combined with the radial gradient of electron flux West, 1979 Normal Distribution Electron Flux Pitch Angle (degrees) Polar Pitch Angle Measurements Selesnick and Blake, 2002 2.2 MeV Butterfly Distribution 8 4 Electron Flux 0 -4 -8 Pitch Angle (degrees) -8 -4 0 4 8 Dipole L shell

  3. GOES 11 Measures Pitch Angle Distributions GOES 10 and 12 are 3-axis stabilized GOES 11 is in storage and spinning Spin period: ~ 8 min Electron data: ~10.2 s resolution Magnetic field: ~ 0.5 s resolution Electron detector has a wide acceptance aperture - ~ 90º Periodic measurements from late 1998 to the present Pitch angle measurements from GOES 11 can be applied to data on GOES 10 and 12

  4. B  p Pitch Angle Distributions are strongly peaked at 90º near local noon 20 minutes of data

  5. Pitch Angle Distributions have minima at 90º near local midnight 20 minutes of data

  6. Characterize Pitch Angle With sinm Function GOES 11 September 14, 2003 1800 – 1808 UT Day Sector: m ~ 0.9 Night Sector: m ~ -0.7 0500 – 0508 UT Fit measurements over ~1 spin (8 min) to a sinm function

  7. Pitch Angle Distribution versus Local Time GOES 11 September, 2003 Pitch angle index is determined every 8 minutes (one spin) and binned in local time. Pitch angle distributions are peaked at 90º near noon and have minima at 90º on the night side.

  8. Local Time Dependence is Similar During High and Low Fluxes GOES 11 September, 2003

  9. Dusk local times Dawn local times Pitch Angle Distribution Versus Magnetic Field GOES 11 September, 2003 Pitch angle index is strongly correlated with local magnetic field strength. Local magnetic field can be used to estimate pitch angle distribution when pitch angle measurements are not available.

  10. September 15-17, 2003 Flux dropout and recovery occurred over ~ 2 day interval Pitch angle distributions remained peaked at 90º near dusk as fluxes decreased – indicating a highly distorted magnetic field Fluxes recover abruptly as an isotropic distribution – perhaps indicating no preferred pitch angle for local acceleration September 15 September 17

  11. Summary • GOES satellites which spin during on-orbit storage are acquiring a large data set of pitch angle distributions of radiation belt electrons • Data are being used to study the statistic properties of the geosynchronous electron distributions and their variability during abrupt enhancements and loss • Characterization of the pitch angle distributions as sinmα shows a strong correlation with the local magnetic field magnitude, yielding a relationship to estimate the pitch angle distribution at geosynchronous orbit • Pitch angle distributions during flux enhancement and loss events will improve our future efforts to understand radiation belt source and loss processes

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