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.

Ogo-5 Pitch Angle Measurements

West, 1979

Polar Pitch Angle MeasurementsSelesnick and Blake, 2002

8

4

0

-4

-8

-4-8 840Dipole L shell

Pitch Angle (degrees)

Pitch Angle (degrees)

Ele

ctro

n F

lux

Ele

ctro

n F

lux

B

p

Normal Distribution

Butterfly Distribution2.2 MeV

.

90

avespinj

j

Distinctive pitch angle distributions result from drift shell splitting in Earth’s asymmetric magnetic field combined with the radial gradient of electron flux

GOES 10 and 12 are 3-axis stabilized

GOES 11 is in storage and spinning

Spin period: ~ 8 min

Electron data: ~10.2 s resolutionMagnetic 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

GOES 11 Measures Pitch Angle Distributions

20 minutes of data

Pitch Angle Distributions are strongly peaked at 90ºnear local noon

B

p

20 minutes of data

Pitch Angle Distributions have minima at 90º near local midnight

Characterize Pitch Angle With sinm Function

Fit measurements over ~1 spin (8 min) to a sinm function

GOES 11 September 14, 20031800 – 1808 UT

0500 – 0508 UT

Day Sector:m ~ 0.9

Night Sector:m ~ -0.7

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.

Local Time Dependence is Similar During High and Low Fluxes

GOES 11 September, 2003

Pitch Angle Distribution Versus Magnetic Field

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.

GOES 11 September, 2003Dusk local timesDawn local times

September 15-17, 2003

September 15

September 17

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

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