S. F. Fung

Plasma density distribution along the magnetospheric field: RPI observations from IMAGE

A new technique is introduced that remotely measures the plasma density profile in the plasmasphere. Radio plasma imager (RPI) echo observations provide echo delay time as function of frequency, from which the plasma density as function of position along the magnetic field line can be calculated. An example from the nightside plasmasphere (L=3) shows the density having its minimum value near the equator and rapidly increasing densities along the field line above 40° magnetic latitude. The density increases at a faster rate toward the ionosphere than the field strength. The index of the power law of the density as a function of field strength increases from a few tenths near the equator to close to unity near 40° and greater than 2 near the ionosphere.

First results from the Radio Plasma Imager on IMAGE

The Radio Plasma Imager (RPI) is a 3 kHz to 3 MHz radio sounder, incorporating modern digital processing techniques and long electronically-tuned antennas, that is flown to large radial distances into the high-latitude magnetosphere on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Clear echoes, similar to those observed by ionospheric topside sounders, are routinely observed from the polar-cap ionosphere by RPI even when IMAGE is located at geocentric distances up to approximately 5 Earth radii. Using an inversion technique, these echoes have been used to determine electron-density distributions from the polar-cap ionosphere to the location of the IMAGE satellite. Typical echoes from the plasmapause boundary, observed from outside the plasmasphere, are of a diffuse nature indicating persistently irregular structure. Echoes attributed to the cusp and the magnetopause have also been identified, those from the cusp have been identified more often and with greater confidence.

Configuration of high‐latitude and high‐altitude boundary layers

We report comprehensive statistical results for 4 years of Hawkeye observations totaling 1757 boundary crossings. Our objective is to use the full set of Hawkeye plasma, magnetic field, and plasma wave data to identify every cusp-associated data interval (entry layer, cusp, plasma mantle), to spatially map these regions, and to isolate the primary variables affecting their occurrence frequency and location. We find that SM coordinates best order the angular position of cusp region data intervals and that GSM coordinates are better for ordering in radial distance, especially for the plasma mantle. Dipole tilt and external pressure are the primary variables affecting high-latitude and high-altitude boundary configuration. Compared to these, the effects of IMF parameters are minor although discernable when adequate corrections are made for dipole tilt and pressure. New results on cusp flaring and indentation of the high-latitude boundary are obtained by limiting this data set in pressure, varying dipole tilt ranges, and examining changes in boundary configuration in both SM and GSM coordinates. We find that as the dipole tilts more toward the oncoming magnetosheath plasma flow, indentation becomes enlarged and the cusp outflow region, the plasma mantle, becomes more flared out relative to the Earth-Sun line. In contrast, as the dipole tilts away from the Sun direction, cusp indentation is reduced but the cusp outflow region remains flared out compared to boundary shapes inferred from low-latitude observations. A semiempirical magnetopause model by Boardsen et al. [this issue] compares very well with the high-latitude boundary layer observations reported here including a test for hemispheric symmetry, which is assumed by the model.

The feasibility of radio sounding in the magnetosphere

A radio sounder outside the plasmasphere could provide nearly continuous remote density measurements of the magnetopause and plasmasphere, as well as other important density features elsewhere in this region. Using digital integration and tuned reception at frequencies from a few kilohertz to a few megahertz with 400-m to 500-m tip-to-tip dipole antennas and 10 W transmitter power, such a sounder would be capable of 10% density resolution and 500 to 1300 km spatial resolution in only a few minutes at distances of up to 4 RE. By providing such detailed observations of its principal density structures, such a sounder would then clearly revolutionize magnetospheric research.

Exterior and interior polar cusps: Observations from Hawkeye

Hawkeye plasma, magnetic field, and plasma wave instruments directly sampled the throat of the northern polar cusp as the orientation of the interplanetary magnetic field (IMF) changed from southward to northward on July 3, 1974. Two distinct regions in the polar cusp were identified based on magnetic field, plasma flow and magnetic and electric noise: the interior and exterior cusps. The observations show highly variable flows in the exterior portion of the cusp and constantly strong dawn-dusk flows in the interior portion during periods of strong IMF By component. Results of a minimum variance analysis of the magnetic field at each cusp interface crossing provides evidence that the magnetopause surface normal deviated highly from empirical models. During intervals of relatively steady solar wind dynamic pressure, the motion of the cusp relative to the slow moving spacecraft was modulated by the varying IMF clock angle as observed by IMP 8 in the upstream solar wind. The motion did not show a correlation with internal processes monitored by the AE index. We propose that observed plasma flow patterns and cusp motion are results of reconnection between the IMF and the magnetospheric magnetic field. Flow velocity observed in the interior cusp is consistent with stress balance for a reconnection process. This unique interval provides an opportunity for detailed studies of the plasma, magnetic field, and plasma wave properties in both the exterior and interior cusp.

Ion temperature anisotropies in the Earth's high‐latitude magnetosheath: Hawkeye observations

We present here for the first time observations of the inverse correlation between the ion temperature anisotropy and plasma beta in the Earths high-latitude magnetosheath. Hot proton data with energies of 0.3–8 keV were obtained from magnetosheath passages by the Hawkeye spacecraft which had a polar orbit with an apogee of 20–21 RE. A newly developed technique has been used to calculate the distribution functions of protons in their non-streaming frame in which their first-order anisotropy is absent. The ion-energy dependence of distribution functions indicates the existence of two hot ion components. Thus the correlation has been examined for each hot ion component separately. We have analyzed three Hawkeye magnetosheath passes during which the magnetosheaths magnetic field was close to the spacecraft spin plane, so that the two-dimensional Hawkeye sensor can adequately sample temperature anisotropies. Results of our analyses are consistent with the theoretical prediction given by Gary et al. [1994; 1995] that a universal inverse-correlation relationship exists between the temperature anisotropy and plasma beta of hot ions.

Radio Plasma Imager Simulations and Measurements

The Radio Plasma Imager (RPI) will be the first-of-its kind instrument designed to use radio wave sounding techniques to perform repetitive remote sensing measurements of electron number density (Ne) structures and the dynamics of the magnetosphere and plasmasphere. RPI will fly on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission to be launched early in the year 2000. The design of the RPI is based on recent advances in radio transmitter and receiver design and modern digital processing techniques perfected for ground-based ionospheric sounding over the last two decades. Free-space electromagnetic waves transmitted by the RPI located in the low-density magnetospheric cavity will be reflected at distant plasma cutoffs. The location and characteristics of the plasma at those remote reflection points can then be derived from measurements of the echo amplitude, phase, delay time, frequency, polarization, Doppler shift, and echo direction. The 500 m tip-to-tip X and Y (spin plane) antennas and 20 m Z axis antenna on RPI will be used to measures echoes coming from distances of several RE. RPI will operate at frequencies between 3 kHz to 3 MHz and will provide quantitative Ne values from 10−1 to 105 cm−3. Ray tracing calculations, combined with specific radio imager instrument characteristics, enables simulations of RPI measurements. These simulations have been performed throughout an IMAGE orbit and under different model magnetospheric conditions. They dramatically show that radio sounding can be used quite successfully to measure a wealth of magnetospheric phenomena such as magnetopause boundary motions and plasmapause dynamics. The radio imaging technique will provide a truly exciting opportunity to study global magnetospheric dynamics in a way that was never before possible.

OBSERVATIONS OF MAGNETOSPHERIC PLASMAS BY THE RADIO PLASMA IMAGER (RPI) ON THE IMAGE MISSION

The Imager for Magnetopause-to-Aurora Global Exploration satellite (IMAGE, see http://image.gsfc.nasa.gov), launched on March 25, 2000, is the first mission dedicated to observing the global-scale structure and dynamics of the magnetosphere. Remote-sensing instntments on IMAGE are designed to observe the magnetopause, ring current, plasmasphere, polar cusp, and the auroral region, in order to reveal the morphologies and interactions among these inter-connected regions. In particular, the RPI, a digital radio sounder with direction-finding

Spatial distributions of the inner radiation belt electrons: A comparison between observations and radial diffusion theory predictions

Abstract It is well known that the quiet-time electron radiation belts exhibit a two-zone radial structure. Although the inner radiation belt does show dynamic variations during geomagnetically active periods, the stably trapped electrons found in this region are thought to be populated primarily by diffusive transport processes. Recent analyses of the long-term (1984–1987) energetic electron (0.19-3.2 MeV) observations taken at 350–850 km altitude by the OHZORA satellite indicate that the trapped electrons also show local-time variations in pitch angle distributions. We here report the observed L-shell distributions of the energetic electrons and show that they are in good agreement with the azimuthally averaged predictions of the radial diffusion theory (the Salammbo model) for inner zone electron fluxes. We also compare the OHZORA observations with earlier observations as compiled in the NASA empirical radiation belt models.

Radio imaging of the magnetosphere

Radio sounding can be used to produce “images” of magnetospheric electron density distributions that could revolutionize research into the magnetosphere and its plasma content, especially when combined with other techniques. Based on more than a half-century heritage of ionospheric sounding combined with digital techniques, the magnetospheric radio sounder is yielding measurements that were once impossible to obtain. A magnetospheric radio sounder can provide unprecedented global magnetospheric information by providing quantitative electron density profiles simultaneously in different directions. From a sequence of these, a contour plot of the density structure in the orbital plane can be constructed, with some out of plane information as well.