Larry R. Lyons

Proton aurora and substorm intensifications

Ground based measurements from the CANOPUS array of meridian scanning photometers and precipitating ion and electron data from the DMSP F9 satellite show that the electron arc which brightens to initiate substorm intensifications is formed within a region of intense proton precipitation that is well equatorward (approximately four to six degrees) of the nightside open-closed field line boundary. The precipitating protons are from a population that is energized via earthward convection from the magnetotail into the dipolar region of the magnetosphere and may play an important role in the formation of the electron arcs leading to substorm intensifications on dipole-like field lines.

Formation of the stable auroral arc that intensifies at substorm onset

In a companion paper, the authors present observational evidence that the stable, growth-phase auroral arc that intensifies at substorm expansion phase onset often forms on magnetic field lines that map to within approximately 1 to 2 R(sub e) of synchronous. The equatorial plasma pressure is 1 to 10 nPa in this region, which can give a cross-tail current greater than 0.1 A/m. In this paper, they propose that the arc is formed by a perpendicular magnetospheric-current divergence that results from a strong dawn-to-dusk directed pressure gradient in the vicinity of magnetic midnight. They estimate that the current divergence is sufficiently strong that a greater than 1 kV field-aligned potential drop is required to maintain ionospheric-current continuity. They suggest that the azimuthal pressure gradient results from proton drifts in the vicinity of synchronous orbit that are directed nearly parallel to the cross-tail electric field.

Ion radial diffusion in an electrostatic impulse model for stormtime ring current formation

Guiding-center simulations of stormtime transport of ring-current and radiation-belt ions having first adiabatic invariants μ ≳ 15 MeV/G (E ≳ 165 keV at L ∼ 3) are surprisingly well described (typically within a factor of ≲ 4) by the quasilinear theory of radial diffusion. This holds even for the case of an individual model storm characterized by substorm-associated impulses in the convection electric field, provided that the actual spectrum of the electric field is incorporated in the quasilinear theory. Correction of the quasilinear diffusion coefficient DLLql for drift-resonance broadening (so as to define DLLrb) reduced the typical discrepancy with the diffusion coefficients DLLsim deduced from guiding-center simulations of representative-particle trajectories to a factor ∼3. The typical discrepancy was reduced to a factor ∼ 1.4 by averaging DLLsim, DLLql, and DLLrb over an ensemble of model storms characterized by different (but statistically equivalent) sets of substorm-onset times.

Observations of plasma sheet expansion at substorm onset, R = 15 to 22 Re

We have used a large number of auroral magnetograms to identify four isolated substorms and estimate their onset times. At the onsets, ISEE-1 was in the vicinity of magnetic midnight at radial distances of 15.6 to 21.8 Re and very near the outer boundary of the plasma sheet. We find that, for each event, the plasma sheet expanded, and the magnetic field dipolarized at the inferred onset time. Our most definitive event occurred while ISEE was at a geocentric radial distance of 21.8 Re. This result conflicts with previous understanding, though further verification of the result is required. Our observations show very similar characteristics to those observed at synchronous orbit, and they are consistent with an extension of a portion of the substorm current wedge to the radial distance of the satellite. If this explanation is correct, ISEE must have been within the longitude range of the substorm current wedge at the onsets.

Energy content of stormtime ring current from phase space mapping simulations

We perform a phase space mapping study to estimate the enhancement in energy content that results from stormtime particle transport in the equatorial magnetosphere. Our pre-storm phase space distribution is based on a steady-state transport model. Using results from guiding-center simulations of ion transport during model storms having main phases of 3 hr, 6 hr, and 12 hr, we map phase space distributions of ring current protons from the pre-storm distribution in accordance with Liouvilles theorem. We find that transport can account for the entire ten to twenty-fold increase in magnetospheric particle energy content typical of a major storm if a realistic stormtime enhancement of the phase space density f is imposed at the nightside tail plasma sheet (represented by an enhancement of f at the neutral line in our model)

An explanation for tailward flows with positive Bz in the distant tail neutral sheet during quiet times

ISEE-3 and GEOTAIL observations have revealed that at x = −210 to −220 RE, the z-component of magnetic field across the tail neutral sheet is positive, and the plasma flow is tailward during geomagnetically quiet times. Observations have also shown that the y- and z-components of the interplanetary magnetic field (IMF) partially penetrate into the magnetotail for both positive and negative IMF Bz. We have evaluated the topology of the magnetospheric magnetic field expected from a partial penetration of the IMF when it has a positive z-component. We find that, in addition to the closed and purely IMF field lines that are generally acknowledged to exist for a southward IMF Bz, there is a region at intermediate radial distances of open field lines that cross the tail neutral sheet before exiting the magnetosphere. This region lies between x ≈ −150 RE and x ≈ −325 RE for the representative IMF orientation and model that we have used. We suggest that the quiet-time observations from ISEE-3 and GEOTAIL of tailward flow and positive Bz across the distant tail neutral sheet were obtained during positive IMF Bz conditions and in the region of open field lines. Such an explanation would also account for the GEOTAIL observation that magnetosheath-like plasma associated with positive Bz is common in the distant tail plasma sheet beyond 150 RE but not closer to the Earth.

DE particle and field observations during the poleward expansion of an auroral surge through the plasma sheet

A substorm surge on September 25, 1981 was observed by the imagers on the high-altitude DE-1 spacecraft and encountered by the coplanar, low-altitude DE-2 spacecraft. This case is unique among those that we examined in that DE-2 traversed the surge during the ∼10–15 min period of its rapid poleward motion through the plasma sheet. Because of this, the DE-2 measurements provide a critical test for the recent proposal that the substorm expansion phase is due to an anti-sunward-propagating reduction in the large-scale magnetospheric electric field imparted to the magnetosphere from the solar wind [Lyons, 1995]. A specific prediction of this theory is that the electric field reduction in the equatorial magnetosphere maps to the ionosphere as a poleward-moving electric field reduction. In the ionosphere, large growth-phase electric fields are expected poleward of the active surge aurora, and significantly weaker electric fields are expected equatorward of the active aurora. This electric field pattern is expected to propagate poleward through the plasma sheet with the surge. The DE-2 measurements show significant southwestward electric fields within the portion of the plasma sheet poleward of the active auroral region, and greatly reduced electric fields within, the heated central plasma sheet that was left behind by the narrow region of poleward-moving, active aurora. We also find weak electric fields equatorward of the region of active aurora in typical DE-2 passes over auroral surges, where the region of active aurora had moved poleward to very near the magnetic separatrix prior to the satellite pass. These observations agree strongly with the Lyons [1995] predictions. Consistent with previous observations, the DE-2 measurements also show strong and variable electric fields within the region of active surge; however these fields are not specifically addressed by the theory.