Observation and modeling of energetic particles at synchronous orbit on July 29, 1977

Abstract

In the 12 hours following a worldwide storm sudden commencement at 0027 UT on July 29 there was a series of at least four magnetospheric substorms, the last and largest of which exhibited an expansion phase onset at ∼1200 UT. Data from six spacecraft in three general local time groupings (0300, 0700, and 1300 LT) were examined, and vector magnetic field data and energetic electron and ion data from ∼15 keV to >2 MeV were employed. Four primary types of studies were carried out: (1) timing and morphology of energetic particle injections; (2) variation of particle phase space densities f(µ = p²/2mB), using local magnetic field and particle flux data; (3) measurement of boundary motions, using high‐energy ion gradient anisotropies; and (4) adiabatic modeling, which included injection, large‐scale convection, corotation, and gradient drifts. For the 1200 UT substorm it is concluded that there was a substantial flux dropout in a broad sector near local midnight because of a large‐scale boundary motion, followed by a recovery to a predropout configuration. There were then several subsequent injection events with distinct onsets (extending as far eastward as 0300 LT), for which ion anisotropy information suggests an inward motion of particles from outside of geostationary orbit. Particle drift information reveals that these particles drifted azimuthally completely around the earth. It is also concluded from the phase space density studies that ‘fresh’ particles with magnetic moments of up to at least several hundred MeV/gauss were injected near geostationary orbit. The present adiabatic convection model can explain the observed injection of large magnetic moment particles from the plasma sheet into synchronous orbit, although physical elements of the normal model must be altered somewhat.

Ion flux dropout observed near dipolarization front

“…The dropout does not follow the same mechanism as that observed at the geosynchronous orbit or in the radiation belt for various reasons such as large-scale boundary motions [29], loss to the atmosphere, or loss by magnetopause shadowing [30], and it is not attributable to tail lobe encounter [31]. We believe the formation of the ion flux dropout has a close relationship with the formation of DF.…”

Prompt energization of relativistic and highly relativistic electrons during a substorm interval: Van Allen Probes observations

“…In particular, Baker et al [1982] report modeling results showing that to get deep injections of energetic particles into the inner magnetosphere, one needs to have a localized "wedge" region of strong inductive electric fields (they presumed the substorm current wedge region). Such fields allow very energetic particles to penetrate inward rapidly enough to avoid the effects of gradient-curvature drift forces.…”

On the origin of flux dropouts near geosynchronous orbit during the growth phase of substorms: 1. Betatron effects

“…Then, in order to assign a weight to each test particle, the ions were there traced forward in time until they cross the midnight meridian in the initial steady state configuration representing the pregrowth phase magnetosphere. This latter step is necessary to determine the initial distribution function using model fluxes at the midnight meridian [Buck et al, 1973;Baker et al, 1982]. The left panel of Figure 6 shows the spectra measured by the ATS6 satellite at 6.6 RE [Baker et al, 1982].…”