E. R. Sanchez

The low-latitude boundary layer and the boundary plasma sheet at low altitude: Prenoon precipitation regions and convection reversal boundaries

The dayside zone of soft precipitation can be divided into four distinct types of plasma regimes, each corresponding to the respective magnetospheric source region: the cusp, the mantle, the low-latitude boundary layer (LLBL), and the dayside extension of the BPS. Based on a detailed spectral study, including comparisons with nonsimultaneous ISEE 1 satellite LLBL data, we identify regions of LLBL-type plasma in the DMSP data set and compare these plasma boundaries with convection reversal boundaries (CRBs) as determined by either Sondrestrom or the drift meter instrument on board the DMSP F9 spacecraft. The nine cases considered are all in the prenoon local time sector. We find that in eight of the nine cases the CRB occurs within the LLBL as expected, generally near to, but not coincident with, the equatorward edge of the LLBL-type plasma. In our sample set, chosen for cases with latitudinally wide, easily identifiable LLBL signatures, the average latitudinal width was 1.85° magnetic latitude. The CRB, defined as the onset of steady antisunward convection, occurred about 30% of this width beyond the equatorward onset of LLBL-type particles. The most equatorward portion of the region with LLBL-type plasma usually had near-zero or erratic convection and may correspond to the “stagnation region” reported from ISEE observations. The potential drop observed across the low-altitude LLBL is roughly estimated to be typically ∼5 keV. A summary is given on how the various high-altitude sources can be identified when plasma regions are observed at low altitude in the dayside auroral oval.

The role of precipitation losses in producing the rapid early recovery phase of the Great Magnetic Storm of February 1986

The possible role of precipitation losses in eroding stormtime ring current is subject to debate. To explore this controversy, the recovery phase of the February 6–10, 1986, great magnetic storm is examined, when intense ion precipitation was observed at midlatitudes by NOAA-6 and DMSP satellites. This storm period is particularly interesting because the ring current exhibits distinctive two-phase decay as seen in the Dst index, the early rapid timescale decay corresponding to the intense ion precipitation period described above. Hamilton et al. [1988] concluded, from close agreement between the observed timescale for ring current decay and the theoretical timescale for O+ charge exchange loss, that rapid early recovery phase of this storm resulted from the charge exchange loss of high energy O+; the second and longer decay phase was equated with H+ charge exchange loss. A model of the ring current evolution during this great magnetic storm [Fok et al., 1995] failed to reproduce the observed ring current decay rates, a puzzling result because charge exchange losses were well represented in the ring current model and initial and boundary conditions were taken from the same data set used in the Hamilton et al. [1988] study. A simple energy balance calculation for the global ring current is carried out using (1) either an energy input predicted from upstream solar wind parameters or one calculated from the drift-loss model output, (2) collisional loss timescales extracted from the drift-loss model, and (3) precipitation losses estimated from NOAA-6 and DMSP observations. The energy balance model replicates the evolution of the ring current energy content derived from Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) observations when ion precipitation losses are included and model energy input function is reduced to agree with predictions based upon upstream solar wind parameters. The O+ charge exchange losses and observed global precipitation losses were of equal magnitude in early recovery of the ring current during this great magnetic storm. Later longer decay timescales in the model resulted from a combination of O+ and H+ charge exchange losses; O+ charge exchange losses remained important throughout the model time interval. The present model produces agreement with the AMPTE/CCE estimates of ring current kinetic energy content versus time. Disagreement between the Dst* inferred from the AMPTE/CCE particle measurements and the observed Dst* is an interesting issue needing further explanation.

Low‐altitude observations of the evolution of substorm injection boundaries

We have analyzed the properties of the evolution of the ion and electron precipitation and the magnetic field perturbations in the auroral oval during substorms using the low-altitude polar-orbiting satellites DMSP F6 and F7. We show two examples where the magnetosphere-ionosphere coupling, based on the character of the particle precipitation and field-aligned current regions, exhibits different substorm responses. The main difference in the coupling response is reflected in the intensification of the ion keV plasma sheet precipitation flux. In one example (November 29, 1984) the intensification is confined to a narrow latitudinal region at the poleward edge of the region 2 current system. In the other example (November 1, 1984) the intensification encompasses a much wider latitudinal span over which typical estimated ion thermal energies are 15 keV and where there is a proliferation of discrete electron precipitation with peak energies of 500 eV. During the recovery phase, that region develops a very uniform diffuse electron precipitation that can last for long periods (>1 hour) subjected to electrostatic potential drops of ≈450 eV. There are, however, characteristics common to both examples. For instance, the energy distribution in the equatorward edge of the midnight auroral oval ion precipitation is transformed a few minutes into the expansion phase from that expected qualitatively from a steady state earthward convection into an energy distribution where the flux of keV ions is dramatically reduced and only ions and electrons with energies below 1 keV are detected. This precipitation region is colocated with a system of region 2 polarity field-aligned currents and has a well-defined poleward boundary that defines the location where the average energy of the ion and electron precipitation starts to decrease monotonically with decreasing latitude. Also, during the expansion phase the intense diffuse electron precipitation that occurs poleward of the region 2 system coincides with strong region 1 field-aligned currents. Inside this region, ion and electron flux depletion (≈2° wide) develops around the magnetic latitude where the enhancement of the westward electrojet is first detected. During expansion and recovery additional ion precipitation appears in the midnight and dusk passes equatorward of the electron equatorward precipitation boundary in the two cases presented here. The similarity between the ion spectra of this ion precipitation and the spectra of the plasma sheet suggests that portions of plasma sheet plasma become detached from the bulk of the plasma sheet population. The equatorward detachments are separated from the rest of the plasma sheet by a dispersionless boundary. The ion energy cutoff of the dusk detachments exhibits the dispersion in latitude characteristic of steady state earthward transport produced by an enhanced convection electric field. This suggests that the particle transport in the magnetosphere is disrupted at onset in the inner magnetosphere where region 2 currents are generated but continues unimpeded in the earthward region.

Observations of persistent dayside F region electron temperature enhancements associated with soft magnetosheathlike precipitation

A series of experiments with the Sondrestrom incoherent scatter (IS) radar (66.99°N latitude, 50.95°W longitude) were designed to examine F region structure in the dayside auroral oval in order to search for plasma signatures from magnetospheric regions such as the cusp, boundary plasma sheet, low-latitude boundary layer, and mantle. This IS radar mode, optimized to search for ionospheric features which remain fixed in local time, was coordinated with overflights of the DMSP F-10 satellite on 2 days in September 1992. For both study days, IS radar data show persistent (∼ 7 hour), enhanced T e regions at 300 to 500 km. These enhanced T e regions evolve during periods in which relatively unstructured, laminar N e densities are observed and thus are not merely the result of a structured suppression of electron cooling. The cores of these T e enhancements were observed at latitudes and magnetic local times corresponding to DMSP satellite measurement of soft (< 100 eV) cusplike precipitation. These T e hotspots move systematically equatorward with increasing geomagnetic activity and display a sharp field-aligned equatorward edge at the location of satellite cusp detection. Unlike prior IS radar/satellite cusp investigations, no significant N e enhancements were measured coincident with T e hotspots. A simple ionospheric model is invoked to confirm that such soft cusplike precipitation does not significantly alter the magnitude of the ambient plasma density, and we argue that cusp detection based on collocated N e and T e enhancements is seldom possible. The local time persistence of the T e enhancements, beyond the typical cusp widths, suggests an association with additional dayside magnetospheric regimes such as the low-latitude boundary layer. Both latitudinal and vertical T e gradients maximize at the location of satellite cusp detection, suggesting that the heat source is a divergence of magnetospheric heat flux on freshly reconnected geomagnetic field lines.

Solar cycle variation of plasma mass density in the outer magnetosphere: Magnetoseismic analysis of toroidal standing Alfvén waves detected by Geotail

We study the variation of plasma mass density in the outer magnetosphere over a solar cycle using mass density estimated from the frequency of fundamental toroidal standing Alfven waves observed by the Geotail spacecraft. We identify wave events using ion bulk velocity data covering 1995–2006 and use events in the 0400–0800 magnetic local time sector for statistical analysis. We find that the F10.7 index is a dominant controlling factor of the mass density. For the equatorial mass density ρeq* that is normalized to the value at L = 11, we obtain an empirical formula logρeq*=−0.136+1.78×10−3F10.7, where the units of ρeq* and F10.7 are amu cm−3 and solar flux units (sfu; 1 sfu =10−22 W m−2 Hz−1), respectively. This formula indicates that ρeq* changes by a factor of 1.8, if F10.7 changes from 70 sfu (solar minimum) to 210 sfu (solar maximum). A formula derived in a similar manner using GOES magnetometer data indicates that, for the same range of F10.7, the mass density at L ∼ 7 varies by a factor of 4.1 We attribute the smaller factor at L = 11 to the lower O+/H+ number density ratio at higher L, the stronger F10.7 dependence of the O+ outflow rate than the H+ outflow rate, and entry of solar wind H+ ions to the outer magnetosphere.

Energy transfer between the ionosphere and magnetosphere during the January 1997 CME event

This Letter shows that transient changes in convection and precipitation patterns can significantly influence the electromagnetic energy transfer between the magnetosphere and the ionosphere. At the times of the transient events during the 10 January 1997 storm that was produced by a Coronal Mass Ejection (CME)-related magnetic cloud, the Sondrestrom incoherent scatter radar measured the local electrodynamic response of the high-latitude ionosphere. Important discrepancies between the net electrodynamic energy flux and the electromagnetic energy flux estimated for a stationary atmosphere suggest that the assumption of the ionosphere as a purely resistive load is often inadequate during these transient events as neutral winds cannot conform to the new orientation of the electric field and thus impact the load/generator characteristics of the ionosphere.

Toward an observational synthesis of substorm models : Precipitation regions and high-latitude convection reversals observed in the nightside auroral oval by DMSP satellites and HF radars

A combination of simultaneous measurements from high, low and ground altitude instruments has been used to infer the rapid evolution of the coupled nightside magnetosphere-ionosphere during substorms. Reversals from an eastward zonal convection to westward zonal convection become apparent inside the volume of the substorm bulge a few minutes after the intensification of the westward electrojet. These reversals persist for periods of 10–20 min and appear to have a one-to-one correspondence with the occurrence of dipolarizations. The changes in convection are accompanied by changes in precipitation. Flux depletion regions (FDR) are measured by the DMSP satellites inside the surge, near the equatorward portion of the westward electrojet intensification. The poleward boundary of every FDR is collocated with a convection reversal and an arc intensification that marks a poleward transition into a region initially dominated by intense discrete electron precipitation and velocity dispersed ion structures (VDIS). Convection in the FDR constitutes an eastward electrojet channel that may produce a transient recovery signature as observed by ground magnetometers. The observations of FDRs and the fast westward flows that accompany them are consistent with the scenario of a rarefaction and/or a neutral line in the near-earth tail that produces fast earthward flows after the breakup. The arc intensification at the poleward boundary of the depletion region and the collocated transient convection reversal favor an enhancement of the magnetosphere-ionosphere coupling and thus the continuation of the substorm expansion in a multiple cell convection system. Arguments are presented to explain how a series of pseudobreakups modify the near Earth magnetic field into an increasingly dipolar geometry until a breakup is possible.

Incoherent scatter radar-FAST satellite common volume observations of upflow-to-outflow conversion

Incoherent scatter radar measurements from the Sondrestrom Research Facility and the European Incoherent Scatter Svalbard radar have been combined with all-sky images, polar convection measurements, and FAST particle and field measurements to quantify the contribution of different magnetosphere-ionosphere coupling processes to the extraction efficiency of ions from the ionosphere. Upflowing ions are traced from their source vertically and horizontally to determine where and when they are likely to intersect the acceleration region observed by FAST. The duration and location of auroral emissions are used to estimate the size and duration of the acceleration region. The upflow-to-outflow efficiency is estimated for three periods of polar cap boundary intensifications and streamers during substorm recovery and steady magnetospheric convection. The extraction efficiency of conics ranges between 0.1%, for the lowest amplitude of broadband extremely low frequency waves, and 5%, for the highest-amplitude waves sampled. Simultaneous measurements of all-sky images and magnetic field-aligned radar measurements show that the most intense ion upflux occurs adjacent to the boundary of intense electron precipitation characteristic of polar cap boundary intensifications and streamers, suggesting that the most efficient acceleration mechanisms couple ionospheric heating at F region altitude with dispersive Alfven waves that grow from horizontal gradients in electric field and conductivity.

Diagnostics of an artificial relativistic electron beam interacting with the atmosphere

We use a Monte Carlo model to simulate the interaction of a beam of relativistic (0.5-10 MeV) electrons with the upper atmosphere as they are injected downward from a notional high-altitude (thermospheric/ionospheric) injection platform. The beam parameters, defined by realistic parameters of a compact linear accelerator, are used to create a distribution of thousands of electrons. Each electron is injected downward from 300 km altitude toward the dense atmosphere, where it undergoes elastic and inelastic collisions, leading to secondary ionization, optical emissions, and X-rays via bremsstrahlung. In this report we describe the model initialization (i.e., development of the electron distribution), essential features of the Monte Carlo model, and secondary outputs, including optical emissions, X-ray fluxes, secondary ionization, and backscattered energetic electron fluxes. Optical emissions are propagated to the ground through the lower atmosphere, including the effects of atmospheric absorption and scattering, to estimate the brightness of the emission column for a given beam current and energy. Similarly, X-ray fluxes are propagated to hypothetical detectors on balloons and satellites. Secondary ionization is used to estimate the radar signal returns from various ground-based radar facilities. Finally, simulated backscattered electron fluxes are measured at the injection location. The simulation results show that each of these diagnostics should be readily detectable by appropriate instruments.

Modification of the loss cone for energetic particles

The optimal pitch angle which maximizes the penetration distance, along the magnetic field, of relativistic charged particles injected from the midplane of an axisymmetric field is investigated analytically and numerically. Higher-order terms of the magnetic moment invariant are necessary to correctly determine the mirror point of trapped energetic particles, and therefore the loss cone. The modified loss cone resulting from the inclusion of higher-order terms is no longer entirely defined by the pitch angle but also by the phase angle of the particle at the point of injection. The optimal orientation of the injection has a nonzero component perpendicular to the magnetic field line, and is in the plane tangential to the flux surface. Numerical integration of particle orbits were carried out for a relativistic electron in a dipole field, showing agreement with analytic expressions. The results are relevant to experiments, which are concerned with injection of relativistic beams into the atmosphere from aboard a spacecraft in the magnetosphere.