For nearly three decades, physicists have considered the ionospheric response to gravity waves at mid-latitudes a solved problem. Since elaborated by Hooke in 1968, the assumption that the ionosphere acts as a passive tracer of field-aligned neutral atmosphere dynamics has remained the basic foundation of explanations of wave coupling in the mid-latitude thermosphere-ionosphere system. However, a variety of observations made over the past 25 years by satellites, radar, remote sensing, and optical imaging suggest the need to reevaluate this assumption. In this paper, I present a new theory of gravity wave-ionosphere interaction that offers to account for the bulk of these observations. The theory is based on the response of the global electrical circuit to F region conductivity variations and suggests that the nighttime, mid-latitude ionosphere may react electrodynamically to gravity wave forcing in a nearly ubiquitous fashion. If true, the theory could explain the considerable electric field fluctuations observed in the nighttime mid-latitude ionosphere, commonly noted features of traveling ionospheric disturbances, and the occurrence of violent mid-latitude spread F events over the MU radar in Japan. Further progress in this area will come from continued observations of the mid-latitude ionosphere using instrument clusters at incoherent scatter radar facilities and new theoretical developments in modeling the global distributions of instability processes and three-dimensional turbulent plasma instability cascades.

The Earth's Bow Shock in Motion

The Earth's bow shock is its first defense against the onslaught of solar particles (protons and electrons) that hit us every day. It is here that the solar particle flow (or solar wind) is slowed, heated, and partially deflected around the Earth's magnetosphere. The bow shock affects the solar wind and the solar wind, in return, affects the bow shock. The bow shock moves in response to solar wind variations and may experience oscillations as solar wind structures modulate and pass throught the shock. Space craft in the vicinity of the bow shock frequently experience multiple crossings due to this motion. In order to study these phenomena we use the unique orbit of Geotail (near Earth phase) in which Geotail is in the vicinity of the bow shock twice in its ~5 day orbit, and in fact skims along either the dawn side or the dusk side bow shock for approximately a month (twice a year). Either Wind or IMP8 (or both) are upstream to monitor the solar wind during these crossings.

This seminar will include a general review of shock formation, a discussion of kinds of shocks, and the differences between quasi-parallel and quasi-perpendicular shocks. There will also be a more detailed review of the shape and character of the Earth's bow shock and how this changes with changing solar wind parameters, the differences between the dawn and dusk side, and the impact of solar wind structures striking the bow shock. After the review, recent results using Geotail data will be presented.

Magnetic Helicity is Conserved in Ideal Magnetohydrodynamics-But What About the Real World

Magnetic helicity is a curious topological quantity which is conserved in the time evolution of an ideal (i.e. perfectly conducting) magnetohydrodynamic flow. This "topological conservation law" has found much application in the analysis of plasmas and, in particular, solar flux ropes. However, it would be nice to have a way of quantifying the "approximate conservation of magnetic helicity" in a plasma of large but finite conductivity.

This talk has three parts. First, we review some topological aspects of three dimensional vector fields and give a "metric-free" description of the helicity of a solenoidal vector field. Second, we look at three and four dimensional aspects of Maxwell's equations to see how infinite conductivity implies the conservation of magnetic helicity in ideal magnetohydrodynamics. Third and finally, we combine the first two parts to show that a clear articulation of topological aspects of three dimensional vector fields enables us to derive a correction factor which relates the time rate of change of magnetic helicity to a current helicity associated with flux ropes when

Nonlinear Wave-Driven Currents In The E-Region Ionosphere: Simulations, Movie and Theories.

The electrojet is an ionospheric current strong enough to deflect a compass needle. This current flows along the Earth's magnetic equator and in the auroral ionosphere within the E-region (90 - 120 km in altitude). Two plasma instabilities disrupt the flow of the electrojet current: the modified two-stream (Farley-Buneman) and the gradient-drift instabilities. I shall argue that both these instabilities nonlinearly drive D.C. currents in the E-region ionosphere. These currents flow parallel to, and with a comparable magnitude to, the fundamental Pederson current. Hence, wave-driven currents act to discharge the electrojet, effectively reducing the resistivity of the E-region. This talk will review the physics of E-region waves, show a number of results from simulations of the two-stream instability, describe the nonlinear behavior leading to DC currents, and discuss a few implications of this nonlinear current.

Field-aligned Currents and Parallel Electric Fields in the Plasma sheet Boundary Layer

The plasma sheet boundary layer is the transition region from the geomagnetic tail lobes to the central plasma sheet. In the context of a Dungey picture of the magnetosphere, it occupies the Earthward side of magnetic field lines intercepting the nightside reconnection region and comprises plasma recently transported from the lobes onto closed field lines connected to Earth. Observations show that a persistent characteristic feature of the layer is fast Earthward and anti-Earthward flows, a few hundred kilometers per second, with Earthward flows in the outermost part of the layer and anti-Earthward flows occurring toward the central plasma sheet. The layer also contains field-aligned currents with current sheet densities estimated to be as high as a few hundred mA/m mapped to the ionosphere. The current sheets are narrow in latitude and are directed both into and out of the ionosphere but inward currents, embedded in a larger scale Region 1 current system, predominate before and after midnight. Parallel electric fields that accelerate auroral electrons are observed in the evening sector in conjunction with the current sheets.

This talk outlines a mathematical model of the plasma sheet boundary layer. The convection electric field in the plasma sheet boundary layer drives field-aligned currents. The currents are non-MHD Hall currents, of order mc/q in fluid theory. We present a fluid theory of the currents in a two-dimensional magnetic field model containing an X-type reconnection line. There are two contributions to the currents: those arising from the Earthward rate of change of the duskward ion drift velocity and finite Larmor radius contributions. In a uniform dawn-dusk electric field, the current is driven most efficiently where the magnetic field is weak, on plasma sheet boundary layer field lines that are near the reconnection line. In principle, the current is modified and limited by magnetospheric-ionospheric coupling; in reconnection region and comprises plasma recently transported from the lobes onto closed field lines connected to Earth. Observations show that a persistent characteristic feature of the layer is fast Earthward and anti-Earthward flows, a few hundred kilometers per second, with Earthward flows in the outermost part of the layer and anti-Earthward flows occurring toward the central plasma sheet. The layer also contains field-aligned currents with current sheet densities estimated to be as high as a few hundred mA/m mapped to the ionosphere. The current sheets are narrow in latitude and are directed both into and out of the ionosphere but inward currents, embedded in a larger scale Region 1 current system, predominate before and after midnight. Parallel electric fields that accelerate auroral electrons are observed in the evening sector in conjunction with the current sheets.

This talk outlines a mathematical model of the plasma sheet boundary layer. The convection electric field in the plasma sheet boundary layer drives field-aligned currents. The currents are non-MHD Hall currents, of order mc/q in fluid theory. We present a fluid theory of the currents in a two-dimensional magnetic field model containing an X-type reconnection line. There are two contributions to the currents: those arising from the Earthward rate of change of the duskward ion drift velocity and finite Larmor radius contributions. In a uniform dawn-dusk electric field, the current is driven most efficiently where the magnetic field is weak, on plasma sheet boundary layer field lines that are near the reconnection line. In principle, the current is modified and limited by magnetospheric-ionospheric coupling; in practice, the ionospheric Pederson conductivity is so high that the coupling has virtually no effect. The current into the ionosphere can be about as large 1 MA, the detailed value depending on the size of the ion diffusion region. Parallel potential drops are of the order of 1 kV.

CCD Image Measurements of Small and Large-Scale Gravity Waves in the Earth's Upper Atmosphere

Results from two different imaging systems designed to investigate faint optical emissions from the Earth's upper atmospheric nightglow layers (altitude ~ 80-100 km) will be presented. An all-sky, multi-wavelength CCD imager has been used to investigate wave signatures in the near infrared OH, and visible wavelength Na (589.2 nm) and OI (557.7 nm) emissions to determine the spatial characteristics of short period (<1 hour) quasi-monochromatic gravity waves. Examples of the images together with an investigation of 'wavelength-period' trends in the data recorded at several sites at equatorial, low and mid-latitudes will be presented. The second CCD camera was designed to map mesospheric temperature with a precision of better than 2 K and represents a new development. This instrument is currently being operated alongside a Na temperature lidar at Fort Collins, Colorado to investigate long period variations in the OH(6,2) band rotational temperature and intensity. Observations to date have revealed a persistent quasi-eight hour oscillation with a 10 day mean amplitude of ~5 K and maximum nightly amplitude of up to 7.5 K. The mean phase of this oscillation differs by 180 degrees from Spring to Fall, strongly suggesting a tidal origin. The results of these studies will be discussed with reference to other measurements in the literature.

Auroral X-ray Imaging and Spectroscopy: Remote-Sensing of Precipitating Electrons on a Global Scale

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) aboard NASA's Polar satellite has provided the first-ever global-scale images of the precipitation patterns of energetic electrons in the earth's auroral zone. As such it has provided a new view of auroral processes which has proven to be a valuable complement to the visible and ultra-violet auroral imagers. PIXIE accomplishes this using a pinhole camera method and a 3-dimensional position-sensitive proportional counter which provides the position and energy of each detected x-ray. An overview of the PIXIE investigation will be provided, summarizing the performance of this new instrument and describing new results which have been obtained in the ISTP era.

Magnetospheric Electric Field Driven Particle Transport and Acceleration.

Long before the discovery of the magnetosphere, Hannes Alfven's 1950 book "Cosmical Electrodynamics" had already described many of the key roles electric fields play in space plasmas. Unfortunately, it has proven to be very difficult to measure such electric fields. The particle and wave activity often associated with processes driven by electric fields can make these fields even more difficult to measure. Two techniques that are often employed are: the use of devices such as wire booms to measure potential differences; and the use of particle detectors to measure various aspects of ambient particle distribution functions. A third technique is now being employed by the Electron Drift Instrument (EDI) on the Equator-S spacecraft: two electron beams are aimed in the two directions that allow the beam electrons to return to sensors on the spacecraft after gyrating and drifting in the ambient fields. The firing directions of the electron guns and the times of flight are used independently to measure the electron drift during one gyroperiod and thus determine the perpendicular component of the electric field. The early attempts to measure magnetospheric electric fields will be discussed briefly. The use of each of these techniques on the Polar and Equator-S spacecraft will then be described and illustrated by preliminary results produced by EDI.

Galileo Energetic Particle Detector (EPD) Observations at Jupiter

The NASA Galileo mission was launched in October, 1989, and has been in orbit around Jupiter since December, 1995. The EPD instrument on the Galileo spacecraft contains two separate bi-directional telescopes. The Low Energy Magnetospheric Measurement System (LEMMS) measures the energy spectra of ions above 20 keV (and electrons above 15 keV), while the Composition Measurement System (CMS) measures energetic ion spectra and composition above energies ranging from 80 keV for protons to 10 keV/nucleon for sulfur. This time-of-flight based measurement extends direct composition determination about a factor of ten below equivalent Voyager energy thresholds. The EPD science team is studying energetic particle composition, spectra, and dynamics throughout the huge Jovian magnetosphere, from the Io torus to the deep Jovian magnetotail. In the mission to date there have been (unique to the Galileo mission) close fly-bys of each of the Jovian Galilean satellites. As seen in their effect on Jovian energetic particle fluxes, each has a unique character. There are striking bi-directional energetic electron beams at Io, Europa perturbs ambient fluxes significantly, Ganymede has a mini-magnetosphere of its own, coupled to the enveloping Jovian magnetosphere, and Callisto simply acts as an absorber. The Jovian magnetosphere itself is complex and dynamic; in energetic fluxes protons dominate in the inner magnetosphere, but beyond about 20Rj sulfur dominates (and sulfur always dominates energetic ion pressure). In the inner magnetosphere dynamic, substorm-like particle injections are seen, andin the outer magnetosphere corotational anisotropies give way to tailward anisotropies. This talk will discuss EPD observations during the orbital tour and at the Galilean satellite encounters. There is more information on the Galileo mission at http://www.jpl.nasa.gov/galileo/ and on EPD at http://sd-www.jhuapl.edu/Galileo_EPD/

Hawkeye's View of the High-Latitude Magnetosphere

The NASA Langley/University of Iowa Hawkeye spacecraft (Explorer 52) was launched on June 3, 1974. It operated continuously in a polar orbit with an apogee of almost 21 RE over the north pole and re-entered on April 28, 1978 (nearly four years later) after 667 orbits. Hawkeye was one of the first spacecraft that carried a full complement of space physics instruments: a plasma wave receiver (VLF), a fluxgate magnetometer (MAG), and a low energy proton-electron differential energy analyzer (LEPEDEA). The broad science objectives of the Hawkeye spacecraft were to survey the high latitude polar regions (cusp, auroral zone, mantle, bow shock and magnetopause) of the Earth's magnetosphere. Because of Hawkeye's unique orbit, its data sets have now been archived at the NASA NSSDC for public access (http://nssdc.gsfc.nasa.gov/ hawkeye/hawkeye.html). This presentation will discuss some of the recent results from surveying and analyzing the Hawkeye observations on the exterior cusp, high-latitude magnetospheric boundary and high-latitude reconnection processes.

First Results of the Equator-S Ion Composition Instrument

The Equator-S Ion Composition (ESIC) sensor on the Equator-S satellite measures the 3-dimensional distribution functions of the major ion species in the magnetosphere and magnetosheath over the energy range 20--40000 eV. The composition information is important for determining the source of the ions, as well as for differentiating between possible acceleration mechanisms. Using a combination of a top-cap electrostatic analyzer followed by an acceleration of 15--20 kV, and a time-of-flight measurement, the instrument can resolve H+, He++, He+, O++, O+ and molecular ions. An RPA at the entrance to the electrostatic analyser can optionally be used to extend the energy range down to the spacecraft potential. A versatile on-board processing system calculates proton moments with 1.5 s (1-spin) time resolution, and 4-species moments with 6 s resolution. The time resolution of the 3D distribution functions depends on bit-rate and instrument mode, but during high bit rate, they are also transmitted with spin period resolution. The satellite was launched on Dec 2, 1997, into a 500 x 67300 km equatorial orbit, and the ESIC instrument became fully operational on Jan 19, 1998. Initial results include high resolution measurements of the dayside magnetopause, showing ion transport into the boundary layers, and observations of ion injections into the inner magnetosphere during a magnetic storm.

This page is maintained by Alicia Eccles.

Last Update: May 1, 1998.