© 2008 The Aerospace Corporation Workshop on Coupling of Thunderstorms and Lightning to Near-Earth...

© 2008 The Aerospace Corporation

Workshop on Coupling of Thunderstorms and Lightning to Near-Earth Space

University of Corsica, 23-27 June 2008

SAMPEX - Searches for Lightning-Induced Relativistic

Electron Precipitation

J. B. Blake1 and U. Inan2

1 The Aerospace Corporation2 Stanford University




Objective of Talk

• SAMPEX has observed trapped and precipitating magnetospheric electrons on a continuous basis since launch in July 1992– Precipitation from interactions with the residual atmosphere

– Precipitation by magnetospheric waves

– Scattering by whistlers generated by lightning

• However SAMPEX searches for energetic electrons (several MeV and above) accelerated by lightning have not resulted in convincing evidence for their existence in the magnetosphere

• What are the strengths/weaknesses of the SAMPEX search




Outline

• SAMPEX as a detector of rapidly varying relativistic electron fluxes in near-Earth space– Sensor characteristics– SAMPEX coverage in B, L space

• Microbursts - relativistic electron precipitation on sub-second timescales– Commonly observed phenomena illustrating SAMPEX

performance

• Gammaburst Observation• Thunderstorm/Lightning associated electron

precipitation




SAMPEX

• Launched July 1992• Duty cycle well over 90%,

still in operation• Altitude 500-600 km• Inclination 82 degrees• Zenith pointing at high

latitudes, field-line pointing at low L

• Four energetic particle detectors




SAMPEX

• For present purposes only two instrument/channels need be considered:– PET/P1

• Electrons > 500 keV, eG ~ 5 cm2 sr, sampled 10 times/sec

– HILT/SSD• Electrons > 1.05 MeV, eG ~ 100 cm2 sr,

sampled 50 times/sec




HILT Sensor




Key Facts About SAMPEX

• The altitude of SAMPEX is such that the spacecraft is in the Drift-Loss-Cone (DLC) ~ 85% of the time– In the DLC only recently scatter or injected

electrons can be seen– The lifetime of these electrons is no more

than tens of minutes and they will be lost after longitudinal drift to the western edge of the South Atlantic Magnetic Anomaly




Drift Loss Cone Issues




Drift Loss Cone Issues




Microburst Examples




Microburst Examples




Microburst Examples




Microburst Examples










Microburst Probability, f(L)

• Peak probability for L values between 5 & 6

• In the slot region (2<L<3) probability is very low

• In the inner zone (L<2) no events seen




Low-Latitude Microbursts

• Found by a statistical search

• Trio of bursts at unusually low L, ~ 2

• Spectral information indicates no electrons at several MeV




Low-Latitude Microbursts

• Typical isolated microburst– Periodicity at bounce

frequency• Observed periodicity

agrees with calculated to better than 1%

– Amplitude decreases with time

– Other smaller features often seen




SGR 1806-20

• An intense gammaburst is easily seen in the SAMPEX high rate channels

• Single events can be easily found by a computer search for statistically significant variations




Storm-Associated Electron Precipitation

• Electrons > 150 keV observed in DLC over three day period associated with storms/lightning at low latitude

• J. B. Blake, U. S. Inan, M. Walt, T. F. Bell, J. Bortnik, D. L. Chenette, H. J. Christian,, Lightning-induced energetic electron flux enhancements in the drift loss cone, J. Geophys. Res 106, 2001.




Storm-Associated Electron Precipitation

• Precipitation took place in multiple L-shells

• Precipitation shells seen at same L shells and with comparable magnitude in both hemispheres




Summary• SAMPEX observes relativistic electron fluxes with a 20 ms

temporal resolution

• SAMPEX database now is 16 years long with almost continuous

coverage

• SAMPEX database has been search by several scientists using a variety of techniques to find short, energetic transients

• There are no cases where convincing observations were made of highly relativistic electrons (several MeV and above)




Summary• SAMPEX is very sensitive to relativistic electrons

– ~ 100 cm2 sr at 1 MeV– Several hundred cm2 sr at several MeV

• However SAMPEX is not very sensitive to gammas, by design.

• SAMPEX observations set stringent conditions upon a hypothesis that lightning accelerates and injects multi-MeV electrons into the magnetosphere

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