Radiation Belt Sources and Losses Driven by Solar Wind Variability

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1 Radiation elt Sources and Losses Driven by Solar Wind Variability Terry Onsager, NOAA Space nvironment Center Janet Green, University o Colorado, LASP Geo Reeves, Los Alamos National Laboratory and Scot lkington, Anthony Chan, Yue Fei, Howard Singer, T. Obara

2 What is the electron lux here, how do the electrons get here, what happens to them along way, and where do they go? Solar wind super halo varies by ~ orders o magnitude Solar wind speed is highly correlated with radiation belt lux levels IMF z controls coupling o solar wind to the magnetosphere

3 Phase Space Density or Magnetic Moment: M MeV/G Solar Wind lectrons j p p c mc M p m p c MeV G 5 nt 9 kev j x MeV p c 9 1 s j 1 cm ssr kev 1 s MeV

4 Solar Wind lectron Flux is Highly Variable at High nergies WIND lectron Flux and Solar Wind Speed [Li et al., 1997] WIND lectron Spectra - Davin Larson cm s sr MeV -1 7 kev, 66. kev, 18 kev Vsw lectron lux shows some correlation with solar wind speed Phase space density at M MeV/G can vary by a actor o 5 5 nt 9 kev j 5x1 5x1 cm ssr kev 4 s MeV

5 Phase Space Density in the Solar Wind Versus in the Plasma Sheet Solar Wind lectrons Plasma Sheet lectrons IS 1 11 R Downtail rom arth D. Larson Williams et al., nt nt 9 kev kev j 5x1 cm ssr kev j cm ssr kev 5x1 4 s MeV 7x1 5 s MeV

6 Phase Space Density in the Plasma Sheet Versus in the Radiation elt M 1 MeV/G 1-8 Geosynchronous Orbit IS 1 11 R Downtail rom arth Plasma Sheet: Hilmer et al., Williams et al., 199 Plasma sheet electron and ion heating was associated with current sheet disruption and ield dipolarization. Phase space density in the near- arth plasma sheet is comparable to phase space density at geosynchronous orbit. M MeV / G m e j p c 1 e s km Geosynchronous Orbit: M 1 MeV / G m s s km 6 km 6

7 1 x x For M MeV/G: Fairield, 1968 Solar Wind Plasma Sheet Radiation elt s MeV The plasma sheet may at times have suicient lux to supply the radiation belt Is the radial gradient o phase space density at geostationary orbit consistent with an electron source inside or outside o this location?

8 stimating the Radial Gradient o Phase Space Density at Geostationary Orbit Geosynchronous Satellites Measure lectrons in Dierent L-shells

9 Convert lux to phase space density: p j mc c p Compare phase space density at ixed 1 st adiabatic invariant: G MeV c p m p M Restrict analysis to uatorially mirroring particles: nd invariant

10 An assumption: o o e / > j d J For a measured integral lux: > p d J ' mc J c >

11 m uator α α sin, Another assumption: m uator α α α sin,, sinα sin α 1 α α m m sin, m mc J c > Two ree parameters: o and m

12 πk aφ L

13 m mc J c > > > m J J

14 9 8 / 9 L 9 L 8 e L o

15

16 Key Factors Associated With Radiation elt Depletion Crossing o the heliospheric current sheet Southward turning and increase in total IMF Onset o geomagnetic activity ollowing prolonged quiet conditions Increase in solar wind dynamic pressure 5 vents Days From Geosynchronous Flux Dropout

17 Solar Wind Conditions Associated with Flux Dropout

18 Geomagnetic Activity Associated with Flux Dropout

19 stimated Magnitude o Dst ect at Geosynchronous Orbit Kim and Chan, 1997 Initial/Final Flux Adiabatic ect o Dst on Geosynchronous lectron Flux lectron nergy MeV Dst Flux Reduction Factor -5 nt ~ -1 nt ~ 15 MeV electron lux is expected to decrease by less than a actor o or Dst < 5 nt

20 Flux Depletion Occurs Throughout the Radiation elts Precipitation into the atmosphere is observed at the dropout

21 Summary nergetic electron lux in the solar wind super halo varies by orders o magnitude, with some correlation with speed Considerable energization occurs in the plasma sheet taking phase space density close to the levels in the radiation belts Radial gradient at geosynchronous orbit is positive increasing outward at times Radiation belt luxes are remarkably stable and insensitive to solar wind speed under quiet geomagnetic conditions Following prolonged quiet conditions the radiation belts are remarkably sensitive to pressure enhancements and moderate geomagnetic activity