Radiation Belt Loss at the Magnetopause T. G. Onsager, J. C. Green, H. J. Singer, G. D. Reeves, S. Bourdarie Suggest a pitch-angle dependence of magnetopause loss and an inward motion of electron drift paths to remain inside of the magnetopause Goal: Understand the effectiveness of transport across the magnetopause as a loss mechanism for outer radiation belt electrons Use measurements from GOES and LANL geosynchronous satellites to observe the change in electron flux due to motion of the magnetopause inside of geosynchronous orbit Observe a local-time dependence of flux decrease High fluxes remain at geosynchronous orbit after the magnetopause moves outward

Solar Wind Conditions Associated with Flux Dropouts: -Increased solar wind pressure -Increased southward IMF

Summers et al., JGR, 1998 Geosynchronous Orbit Electron Trajectories Earthward motion of the magnetopause Outward motion of particles (Dst Effect) Pitch-angle scattering into the atmosphere Causes of Radiation Belt Loss

UT (hours) March 27, 2001 Solar Wind Conditions Associated With Magnetopause Crossing 12 Hours of Data Magnetopause crossing associated with increasing dynamic pressure and southward IMF Note: ACE data not shifted in time to account for the propagation time to Earth Noon

Local-Time Dependence of Magnetopause-Induced Dropout -GOES-8:located post-noon does not cross magnetopause, but observes flux decrease first -GOES-10:located near noon crosses magnetopause, and observes flux decrease after GOES-8 March 27, 2001

Local-Time Dependence of Flux Recovery: -GOES-8:Flux recovers while GOES-10 is still in the magnetosheath -GOES-10:Flux recovers with magnetopause crossing Flux change from before to after magnetopause crossing is about a factor of 10. March 27, 2001

Expected Local-Time Dependence of Flux Loss at Geosynchronous Orbit As magnetopause moves earthward, flux dropouts should first be seen at midnight and last at noon 90º Pitch Angles For small pitch angles, flux should decrease first at noon, later at midnight

Los Alamos Geosynchronous Energetic Particle Data Spin-Averaged Differential Flux (#/cm 2 /s/sr/keV) Noon Midnight March 27, 2001 Hours (UT) Earthward motion of the magnetopause is expected to reduce the flux at geosynchronous orbit near midnight before reducing the flux at noon. However – flux near midnight is not reduced, even when the magnetopause is inside geosynchronous orbit at noon. 1.2 MeV

Preferential Loss is Observed for Large Pitch Angle Electrons GOES-11 measures pitch angle distributions while spinning -Magnetopause was encountered post noon -90º pitch angle electrons decreases by about a factor of 10 -Field-aligned electrons decreased only slightly Local Noon

Summary of Observations: -Electron flux at 16 LT decreases before the magnetopause reaches geosynchronous orbit at noon -No decrease in flux is seen at noon until the magnetopause crosses the satellite -No flux decrease is seen at midnight when the magnetopause is inside geosynchronous orbit at noon. -After the magnetopause retreats outside of geosynchronous orbit, considerable electron flux remains (a factor of ~10 below the initial levels).

Suggested Explanation of Observations: -Electron flux at 16 LT decreases before the magnetopause reaches geosynchronous orbit at noon -Large pitch angle particles are lost at the magnetopause -No decrease in flux is seen at noon until the magnetopause crosses the satellite -Expected with maximum compression at noon -No flux decrease is seen at midnight when the magnetopause is inside geosynchronous orbit at noon. -Small and intermediate pitch angles are not lost -After the magnetopause retreats outside of geosynchronous orbit, considerable electron flux remains (a factor of ~10 below the initial levels). -Electron trajectories move earthward as the magnetosphere compresses – electrons are not lost