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Observations of Electric Field Distributions in the Ionospheric Plasma - A Unique Strategy (OEDIPUS C) was a tethered mother-son experiment that was launched northward from the Poker Flat rocket range at 0638 UT on November 7, 1995, across a sequence of auroral structures. During the flight's upleg the magnetically aligned tether was deployed to a separation of ∼1.2 km and then cut at both ends. The forward payload contained a 50-kHz to 8-MHz stepped-frequency transmitter. Receivers were carried on both forward and aft payloads. The transmitter swept through the frequency range every 0.5 s. During each of the 3-ms steps the transmitter emitted only for the first 0.3 ms. The scientific complement also included multiangular electrostatic analyzers on both payloads that were sensitive to fluxes of electrons with energies from 20 eV to 20 keV. The durations of sampling and frequency steps were matched. During the flight the electron gyrofrequency was approximately twice the plasma frequency. When the transmitter swept through the local gyrofrequency, the particle detectors on both payloads detected sounder-accelerated electrons (SAEs) independent of the energy steps being sampled. In addition, SAEs were detected at the aft payload out to separations of several hundred meters for wave emissions at harmonics of the electron gyrofrequency as well as in the upper hybrid and whistler bands. As the vehicle separation increased, significant time differences developed between the wave-emission pulses and the onsets/durations of SAE detections. The data indicate that electrons were heated through strong wave-particle interactions. However, a simple resonant-interaction explanation appears inadequate. We outline requirements for any models purporting to explain OEDIPUS C measurements.

We report on modulations of electron fluxes at megahertz frequencies measured by the Shuttle Potential and Return Electron Experiment (SPREE) during fast pulsed electron gun (FPEG) beam experiments conducted after the tether break event of the Tethered Satellite System Reflight. Six intervals of sustained modulations were identified while FPEG emitted a 100 mA beam of 1 kev electrons. During five events the beam pitch angle α B was near 90° and the modulations were near even or odd half harmonics of the electron gyrofrequency f ce . In the sixth event with 60° > α B > 45°, electron modulations were near estimated values of the electron plasma frequency f pe and 2f pe . Whenever SPREE detected beam electrons modulated at a given frequency, secondary electrons were also modulated at the same frequency over a broad range of energies. Occasionally, some secondary electrons were modulated simultaneously at a second frequency. Multiple frequencies were related as ratios of low integers. In one case the beam electrons were simultaneously modulated at 0.8 MHz and 1.25 kHz. SPREE measurements suggest that the beam electrons propagate in cylindrical shells whose inner edge is marked by steep spatial gradients in fluxes at 1 keV [Hardy et at., 1995]. Inside the shell, electron distribution functions have positive slopes ∂f/∂v⊥ > 0 at velocities near that of the beam. Velocity space gradients act as free-energy sources to drive cavity modes that alter the instantaneous guiding centers of electrons causing SPREE to sample alternating parts of the beam cylinder's inner edge. Associated time-varying electric fields also modulated the fluxes of secondary electrons reaching SPREE. Other cavity modes may be excited through nonlinear processes [Calvert, 1982]. With α B far from 90°, electrons in the beam cylinder evolved toward bump-on-tail distributions to excite large-amplitude Langmuir modulations at f pe and its harmonics [Klimas, 1983]. Low-frequency modulations are attributed to electron interactions with ion acoustic-like waves generated as the beam moved across magnetic field lines in the ionosphere at supersonic speeds.

During the Tethered Satellite System reflight the Spacecraft Particle Correlation Experiment detected fluxes of energetic electrons and ions that were simultaneously modulated at low frequencies during firings of both the fast pulsed electron gun (FPEG) and the electron generator assembly (EGA). The modulations have been interpreted as signatures of large-amplitude, ion acoustic-like waves excited in Cerenkov interactions between electron beams and ambient plasmas as the shuttle moved at supersonic speeds across the ionospheric magnetic field. We present examples of particle modulations observed during steady beam emissions. Measurements show that (1) most electron modulations were at frequencies of several hundred Hertz and (2) ions modulated at similar frequencies appeared at spectral energy peaks during shuttle negative charging events. Detection of modulated ion fluxes confirms the Cerenkov emission hypothesis. Observed frequency variations indicate that the EGA beam underwent more spatial spreading than the FPEG beam.

During the reflight of the tethered satellite system, the electron generator assembly (EGA) emitted steady electron beams six times with the satellite deployed between 6.2 and 16.1 km. Maximum beam energies and currents were 1.65 keV and 0.4 A, respectively. Data presented here show that: (1) emissions create local electron clouds that can charge the shuttle, (2) ionization of thruster gas by beam electrons in the sheath did not reduce the shuttle potential, (3) the EGA utilizes more than half the induced potential, and (4) ionospheric density gradients and magnetic field orientations affect the circuit's potential distribution.

We present environmental responses ob- served by the Shuttle Potential and Return Electron Experiment during 13 prolonged electron beam emis- sions from the Fast Pulsed Electron Generator (FPEG) during the TSS 1R deployment. As the tether length- ened from 0.18 to 2.5 kin, FPEG fired nine times at a nominal current of 100 mA at 1 keV. The motional potential induced across the system increased from 24 to 225 V. With a 15 fl resistor connecting the tether to shuttle ground, current was higher and the degree of negative shuttle charging lower when FPEG was off than when it was on. While most FPEG firings left the shuttle uncharged, two created significant poten- tials: one to +70 V in a nighttime, equatorial plasma depletion and one to-60 V, with the tether -15 km. the tether end and shuttle ground. The second, the Fast Pulsed Electron Generator (FPEG), emitted electrons at a fixed energy and current of 1 keV and 100 mA. This paper presents ionospheric plasma responses to extended FPEG emissions detected in the payload bay by the Shuttle Potential and Return Electron Exper- iment (SPREE). We briefly describe SPREE, FPEG and the TSS 1R circuit during FPEG operations, then present SPREE spectral data for 13 FPEG firings. Two events illustrate positive and negative shuttle charging. The last section discusses the shuttle's electrical cou

We studied 21 intervals during the TSS 1R deployment with a 15 Ω or 25 kΩ resistor connecting the tether to shuttle ground. Ion spectral peaks detected by the Shuttle Potential and Return Electron Experiment indicate that the shuttle consistently charged negatively with respect to the local plasma. With the 15 Ω shunt in the circuit, shuttle potential, Φs, decreased from −17 to −245 V as tether length, L, increased to 2.6 km. Current in the circuit depended strongly on ionospheric density. With the 25 kΩ resistor in place, Φs ≈ −300 V in the low density, nightside ionosphere with L = 5.1 km. Near local noon Φs ≈ −80 V with L = 17.2 km. The shuttle charged to ∼−600 V during two dawn terminator crossings, one with and one without thruster firings. As on TSS 1, firings of two aft vernier thrusters significantly increased |Φs|. In the case without thruster firings, simultaneous variations of Φs, tether current, and the inferred satellite potential are consistent with strong azimuthal and vertical ionospheric density gradients. These are the first known direct measurements of strong negative shuttle charging.

Experiments were conducted during the Tethered Satellite System Reflight in which a 1 keV, 100 mA electron beam was emitted from the shuttle at pitch angles near 90°. Rapid plasma responses measured by the Shuttle Potential and Return Electron Experiment (SPREE) show time-modulated electron fluxes within the beam flux tube. Megahertz modulations fall into two classes: (1) narrow-bands close to harmonics of the electron gyrofrequency fce and (2) broad-bands at interharmonic frequencies in which electrons of different energies had different modulation frequencies. When SPREE intercepted beam electrons after a single gyroturn they, too, were modulated at similar frequencies. Data suggest that beam electrons were modulated by strong plasma interactions near the emission aperture, generating time-varying electric fields that modulated other electrons near the beam cylinder. This is analogous to the electron cyclotron maser responsible for auroral kilometric radiation.

In a recent one-dimensional self-consistent hybrid code simulation of the local dipolarisation of the initially distended magnetic field in the near-earth nightside region, (Richardson and Chapman, 1994), initially isotropic ions were accelerated up to twice the Alfven speed along the direction of the magnetic field. A bistreaming ion population was generated in the weak field region at the centre of the reversing field, and monostreaming ions were found to move away from the region of dipolarisation. Moreover, a bipolar signature in the cross tail component of the field and fast magnetosonic waves propagating away from the reversal region are predicted by the simulation. We test these predictions by examining particle and magnetic field data from the Low Energy Plasma Analyser (LEPA) experiment and fluxgate magnetometer instrument onboard the Combined Release and Radiation Effects Satellite (CRRES). We examine a set of events that were found to be consistent with some of the features of the simulation results.

The Oedipus C rocket that was launched from Poker Flats, Alaska in November 1995 has detected significant local increases in the observed auroral electron precipitation flux during an auroral substorm which were not accompanied by a corresponding increase in the electric potential of the auroral electron acceleration region. The energy of the electrons which contributed to these increases in flux were also found to extend well below the electric potential energy of these electrons at the top of the acceleration region, thereby requiring a related loss in energy inside the acceleration region. This loss in energy must therefore be attributed to a wave instability which extracts energy from the electrons which are scattered into the loss cone inside the electron acceleration region to cause the discrete aurora during a substorm.

We report the occasional detection of negative pickup ions by the Shuttle Potential and Return Electron Experiment (SPREE) in the shuttle payload bay during the first Tethered Satellite System (TSS 1) mission. These ions appear as arch-like structures of enhanced fluxes in energy versus time spectrograms for the SPREE electron zone looking in the plane perpendicular to the Earth's magnetic field. Maximum energies of ∼56 eV were measured when the sensor looked closest to the shuttle ram direction. Empirically, the energy of associated spectral peaks decreases as cos 2 ξ, where ξ is the angle between the sensor look direction and the projection of the shuttle's velocity onto the plane perpendicular to the magnetic field. These signatures may be found under day or night conditions and always in conjunction with positively charged pickup ions generated as a result of firings by the L5D and/or R5D vernier thrusters. We show that positive ion contamination and accelerated electrons cannot be the responsible agents. Observed spectral characteristics are most easily explained as either NO 2 - or MMH - ions created near the shuttle and accelerated by the motional electric field to energies detectable by SPREE.

Data from the Shuttle Potential and Return Electron Experiment (SPREE) flown as part of the Tethered Satellite System (TSS 1) are used to determine the detailed characteristics of beam arc distributions of pickup ions due to molecules outgassed or ejected from the shuttle. These ion distributions are only detected near the plane perpendicular to the magnetic field direction in an angular range of ±45° about the minimum angle to ram. Their flux is largest when the angle between this plane and the shuttle ram direction is smallest. Generally, ion spectra peak in the range 19 to 25 eV at the minimum angle between the perpendicular plane and the ram direction. The peak energy decreases smoothly as this angle increases. Weak fluxes are measured above the peak, to energies as high as 150 eV. Within the SPREE energy range, two-dimensional distribution functions of beam arc ions in the perpendicular plane have teardrop shapes, symmetric about the minimum angle to shuttle ram with deep minima in the centers. Variations in the peak energies of differential number fluxes agree with collisionless trajectory analysis, assuming that the ions are H2O+ and allowing for different initial velocities before charge exchange. The lowest densities for beam arc ions occur during periods of purely residual outgassing from the shuttle. Ion densities increase by a factor of 5 during waste water dumps. The highest densities occur during operation of the flash evaporator system when the pickup ion densities in daylight can exceed 105 ions cm−3, about 30% of the estimated ambient plasma density. We also present a nonlinear numerical analysis to study the stability of beam arc generated plasma waves and explain electrostatic spectra measured during previous shuttle flights.

We have surveyed measurements taken by the Shuttle Potential and Return Electron Experiment (SPREE) in the shuttle payload bay during the satellite deployment phase of the Tethered Satellite System (TSS 1) mission. The shuttle was flying in an inverse airplane mode with its engine bells facing the ram direction to facilitate ion current collection. SPREE ion spectral measurements indicate that with no thrusters firing, the shuttle charged to, at most, a few tens of volts negative. We identified 13 intervals during which the tether was electrically connected to the shuttle by a low-impedance shunt and thruster firings occurred. In previously reported examples of neutral gas releases from charged spacecraft the magnitude of the vehicle potential decreased. Thruster firings during our events either left the TSS circuit unchanged or caused the current measured in the tether to diminish and the shuttle to become more negatively charged. In the latter case it appears that thruster gas impeded access of ambient ions to the current collecting surfaces of the shuttle. The changes in the distribution of the motionally induced potential between the tether and sheath calculated using Ohm's law are in agreement with SPREE measurements.

The electrodynamics of the inner magnetosphere near times of substorm onsets have been investigated using CRRES measurements of magnetic and electric fields, energetic electron fluxes, in conjunction with ground-based observations. Six events were studied in detail, spanning the 2100 to 0000 MLT sector and L values from 5 to 7. In each case the dawn-dusk electric field was enhanced over typical background electric fields, and significant, low-frequency pulsation activity was observed. The amplitudes of the pulsations were larger than the background electric fields. Dusk-dawn excursions of the cross-tail electric field often correlated with changes in currents and particle energies at CRRES and with ULF wave activity observed on the ground. Variations of the electric field and Poynting vectors with periods in the Pi 2 range are consistent with bouncing AlfVen waves that provide electromagnetic communication between the ionosphere and plasma sheet. Magnetic signatures of field-aligned current filaments directed away from the ionosphere, presumably associated with the substorm current wedge, were observed during three orbits. In all cases, ground signatures of substorm expansion were observed at least 5 min before the injection of electrons at CRRES. Field-aligned fluxes of counter-streaming, low-energy electrons were detected after three of the injections. We develop an empirical scenario for substorm onset. The process grows from ripples at the inner edge of the plasma sheet associated with dusk-dawn excursions of the electric field, prior to the beginning of dipolarization. Energy derived from the braking of the inward plasma convection flows into the ionosphere in the form of Poynting flux. Subsequently reflected Poynting flux plays a crucial role in the magnetosphere-ionosphere coupling. Substorms develop when significant energy (positive feedback?) flows in both directions, with the second cycle stronger than the initial. Pseudobreakups occur when energy flow in both directions is weak (negative feedback?). “Explosive-growth-phase” signatures occur after onset, early in the substorm expansion phase. Heated electrons arrive at the spacecraft while convection is earthward, during or at the end of electromagnetic energy flow away from the ionosphere.

We report on the analysis of megahertz modulation of electrons as measured by the Shuttle Potential and Return Electron Experiment (SPREE) during dc firing of the shuttle electrodynamic tether system (SETS) fast pulsed electron generator (FPEG). The SPREE and FPEG were flown aboard the space shuttle Atlantis flight STS 46 as part of the Tethered Satellite System (TSS 1) mission. The principal data reported here are from the SPREE multiangular electrostatic analyzers (ESAs) and Space Particle Correlator Experiment (SPACE). The ESAs, mounted on rotary tables, measured electrons and ions in the energy range from 10 eV to 10 keV over a solid angle of 2π sr. The SPACE is a signal processing system that analyzes the pulse stream from the SPREE ESAs to identify bunching of the electrons and ions produced by coherent wave-particle interactions (WPIs). The SPACE detects modulations in the electron fluxes in frequency range 0- to 10-MHz. This paper concerns 2- to 4-MHz modulations of the electron flux detected by the SPACE when the FPEG was firing in a dc mode at pitch angles close to 90°. During such operations, FPEG emitted a current of 100 mA at an energy of 1 keV. For these times, electrons with energies from 10 to 1850 eV were measured by the SPREE. For energies between ∼10 and 100 eV the electron flux is basically isotropic. At higher energies the flux increases for pitch angles near 90°. The electron distribution functions generally decrease monotonically with increasing energy up to 100 eV. At energies >100 eV the distributions either monotonically decrease or exhibit a peak or plateau at energies near the beam emission energy. Megahertz modulations were observed for electrons with energies from 10 to 1180 eV, on both positive and negative slopes in the distribution function and throughout the 2π sr sampled by the ESAs. The occurrence and strength of the modulations exhibit no clear dependence on the pitch angle at which the electrons are measured. However, they appear to be limited to low parallel velocities (

The authors look at the question of precipitating electrons from the radiation belts. The common argument has been that pitch angle scattering of trapped electrons on whistler mode waves was the source of the precipitating electrons. As part of this theory there has been a threshold energy E[sub C] = B[sup 2]/2[mu][sub 0]N, the magnetic energy per particle which has been a threshold for this process. This energy is typically 10 keV. The observation of diffuse aurorae, as distinguished from distinct aurorae, was made in the 1970's. This is a rather weak and structureless ion and electron precipitation which extends around the auroral oval. The intensities are not inconsistent with pitch angle diffusion of plasma sheet electrons into the loss cone as being the source. The authors argue that there is no fundamental energy threshold for the whistler mode interactions, and therefore such scattering can account for the diffuse aurorae. They show theoretically that there is no reason why the characteristic energy E[sub C] should limit the energy of electrons participating in pitch angle diffusion from whistler waves. An additional consequence of this work is that experimental releases of lithium in the outer magnetosphere to stimulate increased precipitation or initiatemore » a magnetic substorm are not likely to succeed, consistent with CRRES lithium releases.« less

[1] We have integrated more than 600 million energetic electron spectra measured by the Special Sensor for Precipitating Particles, version 4 (SSJ4) sensor on nine Defense Meteorological Satellite Program (DMSP) spacecraft to obtain total number fluxes (Jtot [#/cm2 s sr]) and energy fluxes (JEtot [keV/cm2 s sr]). These quantities were first separated into bins of 1° in magnetic latitude (MLat), 1 h in magnetic local time (MLT) and unit steps of Kp and then into 26 × 26 logarithmically-spaced matrices covering the ranges 104 to 1010.25 for JEtot and Jtot and 10−3 to 103.25 for average energy EAVE. Joint probability densities were then calculated as ratios of the number of samples within a matrix element to the total samples in each MLat-MLT-Kp bin. Our analysis shows that (1) in all bins probabilities for detecting any of the three parameters were lognormally distributed, and (2) in most bins distributions were multimodal. Measured multimodal probability distributions are well represented as superpositions of contributions from as many as four lognormal populations. These distributions have inherent positive skews with significant separations between their mean and most probable values. Average values of JEtot and EAVE now widely used to model distributions of auroral conductance at different levels of Kp are verified in the large DMSP data set. However, the lognormal and multimodal characters of their realizations in nature indicate that probabilities of detecting them at a given time in a MLat-MLT-Kp bin never exceed 10%.

The characteristics of polar cap arc occurrence are reviewed to show that the assumption of a closed magnetospheric magnetic field topology at very high latitudes when the IMF B sub z is strongly northward is difficult to reconcile with a wide variety of observational and theoretical considerations. In particular, we consider the implications of observations of particle entry for high and low energy electrons, magnetic flux conservation between the near and far tail, the time sequencing in polar cap arcs events, and the hemispherical differences in polar cap arc observations. These points can be explained either by excluding the need for a major topological magnetic field change from explanations of polar cap arc dynamics, or by assuming a long-tailed magnetosphere for all IMF orientations in which magnetic field lines eventually merge with solar wind field lines in either a smooth or a patchy fashion.

Instrumentation to monitor the shuttle environment during the Tethered Satellite System (TSS 1) mission included the Shuttle Potential and Return Electron Experiment (SPREE) and the Quadrupole Ion-Neutral Mass Spectrometer (QINMS). SPREE measured fluxes of electrons and ions with energies between 10 eV and 10 keV; QINMS monitored neutral and ion species close to the shuttle. We report on energy distributions of pickup ions detected during and after thruster emissions while the shuttle's velocity vector was nearly perpendicular to the Earth's magnetic field. With SPREE looking in the ram direction and almost perpendicular to the magnetic field, fluxes >1011 ions cm−2 s−1 sr−1 were detected in the 10-to-60-eV energy range. Both prompt and prolonged ion flux enhancements were recorded. The prolonged fluxes lasted many seconds after the initiating thruster turned off. Only when thrusters fired close to the magnetic field direction were no pickup-ion flux enhancements detected. The SPREE measurements are compared with the predictions of a simple two-dimensional model of collisionless pickup-ion trajectories. Using the distribution of ion species recorded by QINMS, the model explains the main features of the ion energy spectra measured by SPREE. Comparison of the model with data also indicates that strong scattering of ejected materials must occur almost immediately after thruster emission and later between the time of pickup-ion creation and detection of the increased ion flux by SPREE.

The authors show how often a simple convection model can explain the structure seen in 120-eV to 28-keV particles at the inner boundary of the Earth`s plasma sheet. A total of 1346 samples of the boundary were made over a 14-month period using the low-energy plasma analyzer on the CRRES satellite. The cutoff energy below which enhanced fluxes of electrons are seen during the passage into the boundary increased with L shell; the opposite was observed for ions. Forty to 50 percent of the cutoffs sampled on the nightside of the Earth were consistent with those derived from a convection model with one variable parameter. The model was normally inconsistent with cutoffs measured during increasing activity and with those made on the day side. For consistent cases a correlation of {approximately}0.9 was found between the zero energy Alfven layer at midnight and the midnight equatorward boundary measured by the DMSP satellite. 36 refs., 8 figs., 3 tabs.

On February 7, 1986, during a major geomagnetic storm the Bz component of the interplanetary magnetic field (IMF) turned strongly northward for several hours. Data from the Defense Meteorological Satellite Program F6 and F7 satellites and the HILAT satellite were used to study the evolution of the pattern of high-latitude precipitation and field-aligned currents in response to this change. Prior to the northward IMF period, the auroral zone was observed down to mid-latitudes and was very wide in latitude, and strong, large-scale “region 1/region2” currents were clearly present (ΔB ∼ 1700 nanoteslas). Following the northward turning, the equatorward boundary of the auroral zone on the nightside contracted sharply poleward and polar cap arcs were observed. The strength of the region 1/region 2 currents decreased markedly and became immeasurably small at the time of the maximum contraction of the auroral oval. An NBZ current system was observed to grow and expand in the southern (summer) high latitude region over a period of more than 2 hours. Simultaneously, an irregular pattern of field-aligned currents was observed in the northern (winter) hemisphere. During the contraction, the latitudinal width of the auroral region mapping to the central plasma sheet (CPS) decreased dramatically while the width of the area mapping to the boundary plasma regions (BPR) in the magnetosphere increased greatly. At the time of the maximum contraction the BPR extended up to a latitude of at least 87.1°. The NBZ currents expanded with and were entirely located within the BPR precipitation. Polar cap arcs were observed in both regions of BPR precipitation and polar cap precipitation and were not correlated with the location of the large-scale field-aligned currents. There was no indication of the CPS intruding to high latitudes, and thus no evidence for bifurcation of the magnetotail. The boundary between the CPS and the BPR showed little change. If this implies that the boundary between open and closed field lines contracted slowly or not at all, then a significant portion of the observed BPR precipitation was observed along open field lines. We interpret the pattern of particle precipitation and field-aligned current and the implied plasma convection in terms of a distorted, two-cell convection pattern driven by merging of closed field lines with the IMF as proposed by Crooker [1988]. When the IMF turned southward again, the pattern quickly reversed. The BPR contracted; the CPS precipitation regions expanded; the equatorward boundary of the auroral oval moved to lower latitudes; the NBZ currents disappeared in less than 30 min; and the region 1/region2 currents reappeared. Again, the BPR/CPS boundary did not move as rapidly and thus may indicate that the changes are due more to a reconfiguration within the magnetosphere than a change in the portion of the magnetosphere that is open or closed.

Quasi-periodic structures of small-scale Birkeland currents, energetic electrons, ion drifts, and auroral forms in the dawn sector have been observed simultaneously with the HILAT spacecraft. These structures map to the Low Latitude Boundary Layer using a model geomagnetic field. This mapping is supported by the observed characteristics of the energetic electrons and the relationship between precipitating electrons, Region 1 Birkeland currents, and plasma drift. These observations are interpreted as the result of quasi-periodic variations of the Low Latitude Boundary Layer. Values of potential, estimated from assumed characteristics of a large-scale wave propagating in this layer, agree with those determined from the HILAT particle and drift measurements. These results support the view that wave propagation at the Low Latitude Boundary Layer/Plasma Sheet interface can be a source of multiple auroral forms at low altitudes. We suggest that the Kelvin-Helmholtz instability may be the ultimate source of these waves.

A new statistical study of the total power input to high latitude regions from precipitating ions, making primary data sorts by the IMF Bz and solar wind speed, shows that a) the minimum power input occurs for low, positive values of Bz and low values of Vsw, b) the power input increases proportionately more with increasing Vsw than with increasing Bz negative.

Recent analysis has confirmed the existence of the lobe plasma, the extension of the 'boundary layer' and 'plasma mantle' to lunar distances. The observation of the lobe plasma is strongly correlated with the y component of the IMF. Generally, the lobe plasma is observed sporadically for a full day after the moon has entered the tail and a full day before the last magnetopause crossing as it exits the tail. An average extent of approximately 8-10 earth radii inward from the magnetopause is inferred; however, the lobe plasma has been seen all across the tail.

High-resolution magnetic field and charged-particle data acquired on March 25, 1986 by the Viking and DMSP-F7 satellites, as they traversed the dawn sector auroral zone on nearly antiparallel trajectories within 40 min of each oher, are analyzed. Magnetic field measurements by Viking at 0850 MLT and by DMSP at 0630 MLT indicate the presence of a large-scale earthward-directed region 1 Birkeland current and an upward-flowing region 2 current. Both satellites also observed a third Birkeland current adjacent to and poleward of the region 1 system with opposite flow. This poleward system is about 0.5 deg invariant latitude wide and has a current density comparable to the region 1 and 2 systems. The highest-latitude current is identified as region 0. Its charged-particle signatures were used to infer field line mapping to the equatorial plane.

Detailed observations from a late evening sector pass of S3-2 over the northern auroral zone are presented and compared with nearly simultaneous DMSP imagery. High time resolution measurements are presented from electric field, magnetic field, energetic electron (80 eV ≤E ≤17 keV), and thermal plasma detectors. The region of diffuse aurorae was characterized by nearly isotropic electron fluxes and slowly varying (≤20 mV/m) electric fields. The Birkeland currents in this region were predominantly into the ionosphere with embedded, narrow sheets of upward currents. Simultaneous magnetometer and satellite potential fluctuations show that the upward current sheets are real and that these upward currents are probably carried by precipitating, low-energy plasma sheet electrons. Poleward of the diffuse aurorae, the covective electric field varied rapidly in magnitude, anticorrelating with the intensity of precipitating electrons. An inverted-V structure was observed above a visible arc. An intense downward current sheet (>10 µA/m²) was found near the equatorward edge of the arc. At this place the ionospheric thermal plasma was near marginal stability for the onset of O+ ion cyclotron turbulence. An electric field component parallel to B of ∼10 mV/m was measured in this region of strong downward current. The plasma and electric field distributions in the vicinity of the arc suggest that the auroral electrojet was distributed over the region of diffuse auroral but was most intense near the equatorward boundary of the auroral arc.

Suprathermal Ion Detector Experiments (SIDE) have suggested the presence of a significant secondary peak in the ion distribution function during geomagnetic active periods when the moon is within the lobe plasma. It is observed that: (1) an increase in the primary peak bulk velocity is reflected in an increase in secondary peak bulk velocity, (2) both spectra are narrow in the instrument-look direction (assumed parallel to the flow) and peak temperatures are usually less than about 10 eV, (3) periods for double peak observations comprise about 10% of the total lobe plasma observation time, (4) assuming that peaks are caused by protons, and without correcting for lunar surface potential, primary peak bulk velocity is between about 70 and 160 km/s, and secondary peak bulk velocity is between about 360 and 840 km/s, and (5) assuming equal flow velocities outside the influence of the lunar surface potential are equal for the ions of the two peaks, and making corrections for this potential, secondary peak ions are consistent with N(+) or O(+), and the ratio of O(+) to proton number densities is usually less than about 2 x 10 to the -3rd.

Results are presented from the Polar Ionospheric Irregularities Experiment (PIIE), conducted from Sondrestrom, Greenland, on March 15, 1985, designed for an investigation of processes which lead to the generation of small-scale (less than 1 km) ionospheric irregularities within polar-cap F-layer auroras. An instrumented rocket was launched into a polar cap F layer aurora to measure energetic electron flux, plasma, and electric circuit parameters of a sun-aligned arc, coordinated with simultaneous measurements from the Sondrestrom incoherent scatter radar and the AFGL Airborne Ionospheric Observatory. Results indicated the existence of two different generation mechanisms on the dawnside and duskside of the arc. On the duskside, parameters are suggestive of an interchange process, while on the dawnside, fluctuation parameters are consistent with a velocity shear instability.

Observations are reported of an extensive region of low-energy plasma particles (LEP) flowing antisunward along the ordered field lines in the lobes of the geomagnetic tail at lunar distances. The flow was detected by three suprathermal ion detectors deployed on the lunar surface during the Apollo 12, 14, and 15 missions. This particle regime is found to be similar to the 'boundary layer' and 'plasma mantle' observed at smaller geocentric distances and to an interior flow region parallel to the magnetopause in the dayside magnetosphere. It is located exterior to the plasma sheet across essentially the entire tail and adjacent to the magnetopause on both the dawn and dusk sides of the magnetosphere. Variations in the integral flux, temperature, and number density are described. It is suggested that this flow and the three similar regimes are simply connected along the inner surface of the magnetopause and are, in fact, the same phenomenon.

By using continuous ground-based observations of polar cusp auroras and electromagnetic parameters obtained from satellites in polar orbit, electrodynamic processes in the polar cusp have been investigated. HILAT satellite measurements of magnetic field, precipitating electrons and ion drift components above auroral structures observed from the ground permit detailed investigations of the cusp ionosphere. Information on Birkeland currents, Poynting flux, height integrated Pedersen conductivity and Joule heat dissipation rate has been derived. Similar latitudinal structures were observed from HILAT (electron precipitation/ion drift) and in the simultaneous photometer profiles obtained from the ground ∼200 km to the east of the satellite pass. Intermediate scale (tens of kilometers) structures of enhanced electron precipitation (peak energy flux ∼10 ergs/cm²s (0.01 W/m²) and average energy ∼0.1 keV), strong northward electric fields (peak values ∼200 mV/m), and Birkeland current densities ∼10 µA/m² were observed. Rather complex patterns of Birkeland currents were connected to these structures. From correlated latitudinal variations in electric and magnetic field components the downward directed Poynting flux and the ionospheric Joule heat dissipation rate within one of the auroral forms could be estimated. The dissipation rate in the center of this structure was found to be a factor of 2 higher than the electron energy input rate. The satellite pass occurred within an interval (interplanetary magnetic field (IMF) Bz < 0) characterized by a series of poleward moving auroral structures (east-west motion not resolved by the present technique), each appearing at the cusp equatorward boundary. The auroral forms were dominated by red oxygen emission (I630.0 nm/I557.7 nm ≃5 and I630.0 nm ≃5 kR), which is typical in the cusp region.

A mechanism for heavy ion circulation in the magnetosphere is proposed. Singly charged ions heavy ions from the plasmasphere are convected intermittently to the dayside magnetopause, accelerated there, swept into the distant tail lobes and boundary layer, and convected earthward in the plasma sheet to reenter the magnetosphere.

The first sounding rocket flight into the dayside cusp with dark ground and northward interplanetary magnetic field (IMF) conditions was launched from the new SvalRak range at Ny-Alesund in the Svalbard archipelago in early December 1997. Extensive ground observations of auroral emissions and radar backscatter provided contexts for in situ rocket measurements. Real-time interplanetary measurements from the Wind satellite aided launch selection with foreknowledge of impending conditions. NASA rocket flight 36.153 was launched near local magnetic noon while the IMF was dominated by positive B X and had lesser northward B Z and negative B Y components. The rocket's westward trajectory carried it toward auroral forms associated with morningside boundary layers. The rich set of vector dc electric and magnetic fields, energetic particles, thermal plasma, plasma waves, and optical emissions gathered by the rocket reveal a complex electrodynamic picture of the cusp/boundary-layer region. Four factors were important in separating temporal and spatial effects: (1) Near the winter solstice the Earth's north magnetic pole tilts away from the Sun, (2) at the UT of the flight the dipole axis was rotated toward dawn, (3) the variability of solar wind driving was low, and (4) B X was the dominant IMF component. We conclude that no signatures of dayside merging in the Northern Hemisphere were detected in either the rocket or ground sensors. Electric field variations in the interplanetary medium directly correlate with those observed by the sounding rocket, with significantly shorter lag times than estimated for simple propagation between Wind and the ionosphere. The correlation requires that the observed Northern Hemisphere convection structures were stirred in part by merging of the IMF with closed field lines in the Southern Hemisphere, thereby adding open flux to the northern polar cap. Subsequent motions of adiaroic polar cap boundaries were detected in the rocket electric field measurements. The observations indicate that IMF B X significantly affected the location and timing of merging interactions.

Data from the charging hazards and wake studies (CHAWS) flight experiments on board space shuttle missions STS-60 and STS-69, during which a negatively biased, high-voltage (0–5 kV) probe was placed in a plasma wake in low Earth orbit, are presented. For these experiments the source of the wake was the 4-m-diameter Wake Shield Facility (WSF), which was operated both as a free-flying spacecraft and attached to the shuttle orbiter's robot arm. Current collection by the biased probe is investigated as a function of the density and temperature of the ambient plasma and the probe's location in the plasma wake. Current collection behavior is determined by the expansion of the high-voltage sheath into the ambient plasma stream. Consistent with preflight predictions, current collection on the probe is highly nonuniform, varying by more than 5 orders of magnitude across the surface of the probe. The onset of current collection, however, begins at voltages that are an order of magnitude lower than anticipated. This is likely due to the low-energy, turbulent plasma (typically 2–5% of the ambient density and up to 40% on occasion) observed in the ambient environment. This important minority constituent of the plasma was observed in the vicinity of the shuttle Orbiter and observed while the WSF was free-flying.

Approximately 2500 equatorward boundaries of auroral electron precipitation were determined for times when hourly averages of the interplanetary magnetic field and solar wind velocity V were available. The equatorward boundaries were determined from data returned by the SSJ/3 electron detector on board the DMSP/F2 satellite in magnetic local sectors between 0400 and 1100 hours on the morningside of the oval and between 1500 and 2300 hours on the eveningside of the oval. The boundary data were separated into 1-hour zones in magnetic local time. Within each zone the boundary locations were studied as functions of Bz and Bz², VBz and VBz². Significant results were obtained when the boundaries were correlated with hourly average of Bz and VBz for the hour preceding the one in which the boundary was measured and when the linear regression was performed separately for data points where Bz ≤ 1 nT and Bz > 1 nT. For points where Bz ≤ 1 nT, the correlation with Bz and VBz gave slopes generally between 0.8° and 1.2°/nT and between 1.5° and 2.5°/mV/m, respectively, with correlation coefficients of ∼0.7. The latitudes of the intercepts tend to decrease with increasing local time in the evening sector and with decreasing local time in the morning sector. In the range above 1 nT, the slope of the best fit line for correlation with Bz, and VBz changes sign. Slopes fall in the range −0.1° to −0.4°/nT and 0.21° to 1°/mV/m. Correlation coefficients are typically worse than −0.4. Correlations of the boundary with Bz² and VBz² for both ranges of Bz are uniformly worse than those for Bz and VBz. The trend in slopes and intercepts with magnetic local time for Bz ≤ 1 nT is found to be similar to that previously found for the correlation of the boundaries with Kp (Gussenhoven et al., 1981). In this range of Bz, Kp and VBz are found to be related by the equation Kp=2.01–0.91 VBz (mV/m). From the work of Ejiri et al. (1978) an equation relating the magnetospheric electric potential to VBz is derived. The results are in agreement with the general features of the half-wave rectifier model of the magnetosphere and measurements of polar cap potential.

Simultaneous observations by the Viking and the DMSP F7 satellites have been used as input to a new method to obtain snapshot pictures of the auroral electrodynamics. In particular, an “instantaneous” global equipotential (or convection) pattern is calculated from distributions of field-aligned current and conductivity which are qualitatively consistent with the Viking auroral imager data and quantitatively consistent with magnetic field and particle data from the two satellites. This convection pattern, which is of the normal two-cell type, with a weak dusk cell and a strong, elongated crescent-shaped dawn cell (consistent with positive interplanetary magnetic field By), agrees well with the Viking electric field data. The model and the observed potential profiles agree nicely along the entire Viking orbit except for two intervals above acceleration regions where deviations are to be expected (due to parallel electric fields). These regions are characterized by U-shaped potential minima, upward field-aligned currents, upgoing ion beams, and relatively intense auroral kilometric radiation. Thus, the model results are consistent with the Viking observations not only on a global scale but also on the scale of the auroral acceleration regions. The corresponding convection in the magnetosphere is obtained from a simple projection to the equatorial plane of the deduced two-cell convection pattern. From this the location of the plasmapause is inferred.

Regions of intense keV electron precipitation, such as inverted-V structures, at times colocate with ionospheric plasma depletion regions in the high-latitude polar ionosphere. When Defense Meteorological Satellite Program (DMSP) F6 and F7 satellites, at 840 km, enter these regions in darkness, ion signatures of high spacecraft-to-ambient plasma potential differences (several hundred volts negative) are observed with the new SSJ/4 ion detectors. A systematic survey of charging events and the environment in which they occur was made using the DMSP F6 and F7 precipitating ion and electron detectors, the SSIE thermal plasma probes, and the SSM (F7 only) vector magnetometer. The charging events of November 26, 1983, are analyzed in detail since they occurred on both satellites. Critical levels of number flux and average energy for the precipitating electrons, and the threshold density of the thermal ionospheric ions are defined for different levels of spacecraft charging.

The HILAT satellite is scheduled to be launched June 1983 into a circular, 830-km-altitude orbit with an 82.2° inclination. This spacecraft is a modified U.S. Navy TRANSIT navigation satellite and will conduct a unique combination of experiments: (1) transmissions of radio signals from the spacecraft to surface stations for measurements of ionospheric scintillations and total electron content (TEC), (2) in situ measurements of plasma density, electric fields, magnetic fields, and soft electron fluxes, and (3) visible photometric measurement and UV imaging of aurora and airglow. The objective of this mission is to provide definition information on the formation, development, transport, and decay of plasma density irregularities in the high-latitude ionosphere; the relationship of these plasma processes to auroral and polar cap current systems, electric fields, convective flow patterns, and low-energy electron distributions; and the relationship of these plasma processes to the global distribution of auroral phenomena during quiet and substorm periods and during various interplanetary conditions.

It is argued that a series of two-satellite observations leads to a clarification of substorm plasma injection, in which boundary motion plays a major role. Emphasis is put on a type of event characterized by abrupt, dispersionless changes in electron intensity and a coincident perturbation that consists of both a field magnitude increase and a small rotation toward more dipolar orientation. Comparing plasma observations at two points, it is found that in active, preinjection conditions the two most important features of the plasma sheet are: (1) the low-energy convection boundary for near-zero energy particles, determined by the magnitude of the large-scale convection electric field; and (2) the precipitation-flow boundary layer between the hot plasma sheet and the atmospherically contaminated inner plasma sheet.

Three late evening passes of the DMSP/F2 satellite over the northern auroral zone have been studied using simultaneous visible emissions and measurements from a zenith-looking, 16-channel, electron spectrometer. All three orbits occurred during periods of substorm activity; the second and third passes were consecutive. The auroral oval is conveniently divided into three latitudinal segments: a most poleward bright arc; a region of structured, inverted-V precipitation; and the diffuse auroral region. The most poleward bright arc is characterized by directional fluxes occasionally in excess of 1010 (cm² s sr)−1 carried by two populations with temperatures of ∼100 eV and ∼600 eV. These fluxes, which apparently continue well into substorm recovery, are not easily reconciled with a simple model for pressure balance in the magnetotail. Presumably they reflect not well understood dynamic processes in the distant magnetotail. In the second regions precipitating electrons had average energies in excess of 1 keV and had spectral shapes that were either quasi-thermal or monoenergetic peaks superimposed on a secondary-electron background. Beams of low-energy (

From an analysis of particle and field data obtined on January 19 and 20, 1973, we have determined several important characteristics of the plasma populations in the geomagnetic tail at lunar distance and their relationships to conditions in the solar wind and interplanetary magnetic field (IMF). During an extended period of low geomagnetic activity with the IMF Z component predominantly northward the plasma sheet at lunar distance is observed to cool significantly. Ion temperatures are observed in the range kT/sub i/approx.200--400 eV with electron temperatures of approx.100 eV. Number densities are approximately 1 cm/sup -3/. During this period the plasma mantle is also present as an extensive but tenuous region of plasma with densities n/sub i/=n/sub e/approx.10/sup -2/ cm/sup -3/, bulk velocity V/sub B/approx.100 km/s, and ion temperature kT/sub i/approx.3 eV. Following a sharp southward turning of the IMF Z component, there is a net transfer of magnetic flux to the tail. At lunar distance the magnetic field in the lobes increases in magnitude, and the density of the plasma mantle increases sharply. The time delay between the southward turning and the observed changes at lunar distance is consistent with the southward IMF being the cause of the increase ofmore » the magnetic flux and the plasma mantle density. The electrons in the plasma mantle display a core and halo distribution indicating a solar wind origin. The plasma sheet is observed to become hotter with increasing geomagnetic activity, reaching ion temperatures in excess of 1 keV and electron temperatures up to 900 eV.« less

We have completed an extensive analysis of the electron and optical data from the DMSP satellites for an external period of polar cap arc occurrences on December 12, 1977. The polar cap arcs are observed in three distinct intervals in a period of quieting after a time of intense substorm activity. The observation of polar cap arcs is associated with the admittance of large and variable fluxes of low-energy electrons into a major portion of both the northern and southern hemisphere polar caps. These fluxes fall into the following categories: First, nearly Maxwellian distributions of electrons with temperatures between 50 eV and 200 eV and number densities varying from 0.03/cm/sup 3/ to 4/cm/sup 3/. The highest densities are found at the poleward boundary of the diffuse aurorae and near the visible polar cap arcs. The lowest densities are associated with the polar rain. Second, distributions of electrons peaked between 50 eV and 200 eV. These distributions result from accelertion of the cold Maxwellian distribution through a potential of 50 to 200 V without any heating of the electrons. Third, distributions of electrons displaying two populations; an intense low-energy component with a temperature of approx.20 eV and a much weaker high-energymore » component with a temperature of 180 eV. We interpret such distributions as evidence of direct admittance of magnetosheath electrons into the polar cap. Fourth,, distributions of electrons peaked at approx.1 keV. These distributions produce the visible arcs. They result from the acceleration of a two-component electron population with temperatures of 100 and 350 eV through a potential drop of approx.750 V.« less

Results of a statistical study of the occurrence and characteristics of the polar rain (Winningham and Heikkila, 1974) are reported. Precipitating electron data in the energy range from 50 eV to 20 keV from the SSJ/3 sensor on the polar-orbiting DMSP/F2 satellite were used for the study. Intervals of clear polar rain were identified in all orbits in the 1-year interval September 1977 to August 1978. The spectra from the intervals were binned in a two-dimensional spatial array in geomagnetic latitude and magnetic local time. Separate arrays were maintained for different levels of magnetic activity, values of the components of the interplanetary magnetic field (IMF), and season. The average spectra were determined in each bin and were used to calculate the integral flux and average energy. Two-dimensional maps of average energy and integral flux show significant large-scale spatial variations that are roughly symmetric about an axis running prenoon to premidnight. Integral flux (average energy) decreases (increases) from the dayside to the nightside along this axis by a factor of almost 15 (3). This basic variation was maintained in all separations by magnetic activity, season, intensity of the polar rain, and sector structure, although small rotations in magnetic local time of the symmetry axis could be seen in some cases. The second most prominent variation is one that has been noted previously (Yeager and Frank, 1976; Meng and Kroehl, 1977); polar rain occurs preferentially in the northern (southern) cap for away (toward) IMF sectors. In the preferred cap the precipitation is stronger in the morning. Approximately 70% of the spectra occurred for Bz negative. The overall intensity of the polar rain increases and cools with increasing magnetic activity. The occurrence of polar rain falls within a circular region whose center is offset toward premidnight and whose radius increases with magnetic activity.

The question of the mechanism producing the premidnight region 2 currents is investigated using magnetic field and precipitating particle data from the Defense Meteorological Satellite Program F7 satellite. In this study, the variations in the extent and intensity of the region 2 currents on the nightside, in the darkened hemisphere, are determined for a small, isolated substorm on January 18, 1984, and the variations related to the characteristics of the latitudinal profile in electron and ion precipitation. The region 2 currents are found to collocate with a steep slope in the ion energy density. The ion energy density decreases with decreasing latitude. The region 2 current intensity increases proportionally to the increase in the ion energy density gradient. In the premidnight sector the equatorward boundary of auroral ion precipitation generally extends to lower latitudes than that for electrons, suggesting charge imbalance in the right sense to drive region 2 currents. We find no correlation between the extent of the latitudinal separation of the two boundaries and the strength of the region 2 currents. In fact there was one pass with a clear region 2 current where this boundary behavior reversed. The observations are consistent with an azimuthal gradient in themore » ion energy density near the inner edge of the plasma sheet being an important factor in the generation of the premidnight region 2 currents if the increase in the azimuthal gradient is proportional to the increase in the radial gradient there. {copyright} American Geophysical Union 1989« less

Data recorded by Defense Meteorological Satellite Program, Tiros and P-78-1 satellites for the CDAW 6 event on March 22, 1979, have been compared with a statistical model of precipitating electron fluxes. Comparisons have been made on both an orbit-by-orbit basis and on a global basis by sorting and binning the data by AE index, invariant latitude, and magnetic local time in a manner similar to which the model was generated. It is concluded that the model flux agrees with the data to within a factor of two, although small features and the exact locations of features are not consistently reproduced. In addition, the latitude of highest electron precipitation usually occurs about 3 deg more poleward in the model than in the data. This discrepancy is attributed to ring current inflation of the storm time magnetosphere.

The statistical occurrence of spacecraft charging at near-geosynchronous orbit in daylight is studied with reference to results of an experiment conducted on the SCATHA satellite. In particular, it is found that: (1) the external current that creates high negative satellite frame potentials is the high-energy electron current from the electron population with energies greater than about 30 keV; (2) the electron current to the satellite from particles with energies less than about 30 keV neither drives the frame potential nor provides the current to balance the high-energy populations; and (3) the ion current provided from the entire range of measured ions is also not the primary source of the balancing current.

In November 1985 an experimental campaign was conducted at Sondre Stromfjord, Greenland, to study the electrodynamics of auroral arcs at very high latitudes. The experiments involved coordinated observations by the Sondrestrom radar and two polar orbiting spacecraft. For several hours around local dawn on November 17 a system of auroral arcs was observed over Sondrestrom. These arcs varied in intensity and width but were all extended generally in the magnetic east-west direction. A Defense Meteorological Satellite Program photograph obtained during the observations showed that two types of arcs were present. In the south the arcs were localized enhancements or striations within a broad diffuse aurora. Poleward of the diffuse aurora was a system of arc segments that extended over about 5° of latitude. The radar measured enhanced ionospheric electron densities associated with each of these arcs, although the altitudes of the enhancements were different for the two types. In the diffuse aurora the ionization peaked at altitudes near 115 km, whereas in the arc segments the ionization peaked at altitudes above 180 km. Examination of electric field data from the radar indicated that the boundary in ionization associated with the two types of aurora occurred where the southward electric field maximized. The HILAT satellite passed in the vicinity of Sondrestrom during the experiment and measured precipitating electron fluxes and field-aligned currents. The electron flux data showed that the low-altitude ionization observed by the radar was produced by electrons with energies near 5 keV. The higher-altitude ionization was produced by precipitation associated with enhancements in the fluxes of low-energy electrons. The magnetometer data indicated that the boundary between the region 1 (downward) and region 2 (upward) current sheets in the morning sector is just poleward of the boundary separating the hard and soft precipitation regions. The latitudinal variation in field-aligned current is consistent with the measured variations in electric field and conductivity.

An intense (94 µA/m²) earthward directed Birkeland current was detected by the HILAT satellite on July 23, 1983, less than one month after launch. It was located at the equatorward edge of a large-scale earthward flowing current in the late evening sector. This current is the most intense ever to be reported that is consistent with upward flowing thermal electrons. Simultaneous vacuum ultraviolet images from the auroral imager mapper show that the large-scale current is embedded within the diffuse aurora. Energetic electron measurements in the range 20 eV to 20 keV indicate an increase in downward flux from ∼5×107 to ∼7×109 (cm² sec sr)−1 in ∼0.75 s. This increase in flux results in a gradient of ∼2 mhos/km in the height-integrated Pedersen conductivity (Σp). In the presence of a typical dc electric field (∼40 mV/m), this sharp gradient in the ionospheric conductivity is consistent with the inferred high-density current. Measurement of horizontal ion drift from the ion drift meter (IDM) reveals strong turbulence present for ∼10 km on both sides of the region of intense Birkeland current. The IDM also measured an ion density (ni) of ∼2×104 cm−3; thus, if ne ≃ ni, the parallel electron drift velocity derived from J = ne Vd q is ∼30 km/s, resulting in a net electron flux of ∼6×1010 cm−2 s−1. A flux of electrons of this magnitude is sufficient to destabilize O+ ion cyclotron waves. The velocity of 30 km/s coupled with an F region ion density gradient scale length of ∼3 km is also comparable to criteria required to drive the current convective instability.

The position and extent of the region in which electrons with energies less than 40-keV are durably trapped in the nightside magnetosphere is found for both normal and disturbed geomagnetic conditions by using data from the P78-2 (SCATHA) satellite. The region of the magnetosphere from 5.3 to 7.9 RE was studied. In this region neither solar-magnetic nor geocentric-solar magnetospheric coordinates order the data satisfactorily. A new coordinate system called composite coordinates is introduced. It takes account of the fact that this region of the magnetosphere is strongly influenced by both the earth's dipole field and the direction of the solar wind. In composite coordinates when Kp≤4+, 40-keV electron fluxes were almost continuously present in a region centered on the equatorial plane and 1.2 RE in half width. At larger composite coordinate latitudes there is another region more than 1 RE thick within which 40-keV electron fluxes routinely appear and disappear on time scales of one hour as the trapping boundary actively moves over the satellite. We have no evidence that SCATHA ever entered the tail lobes where no particles are trapped. When Kp≥6− the region in which 40-keV electron fluxes were always present moved earthward and/or thinned but remained ordered in composite coordinates. We suggest that the new coordinate system will be useful for ordering other data sets taken in this region of the magnetosphere.

DMSP/F2 and DMSP/F4 precipitating electron data are used to determine statistically the systematic variations of the equatorward diffuse auroral boundary with Kp as a function of local time. This work extends a previous study of dawn and dusk boundaries to the noon and midnight regions. The boundaries are well-ordered by Kp in the night sector but show increasingly greater scatter from dawn to postnoon. In the noon sector it is often the case that no diffuse aurora is discernible within detector sensitivity. The equatorward diffuse auroral boundary is well fit by a circle at each activity level. The center of the circle is offset from the geomagnetic pole, and the radius of the circle increases with increasing magnetic activity. The circular fits are projected to the ecliptic plane by using the Mead-Fairfield magnetic field model where they are identified with the inner edge of the plasma sheet in order to make comparison with Volland-Stern type convection electric field predictions. Previously found variations of the electric field with Kp are confirmed, including a rotation of the axis of symmetry, away from the dawn-dusk meridian. All equatorward auroral boundaries from DMSP for 1978 were compiled. Using the equations for boundary variations with Kp, each evening sector boundary was projected to a midnight boundary. The projected midnight boundary serves as an index of auroral activity and an indicator of the strength of the large-scale magnetospheric convection electric field. The auroral boundary index is given in monthly plots for 1978.

Energetic electron measurements from a particle detector on the DMSP/F2 satellite have been used to determine the corrected geomagnetic latitude ΛCGM of the equatorward boundary of the auroral oval. The satellite was launched into a nearly sun-synchronous, polar orbit centered on the 0700–1900 MLT meridian. Because of the wobble of the dipole and a very slow precessional motion of the orbit more than 6000 boundary crossings could be studied in the 1600–2300 and 0400–1000 MLT sectors. The detectors, which have large geometric factors, look radially away from the earth and detect precipitating electrons with energies between 50 eV and 20 keV. In the evening sector the equatorward boundaries are precisely determined from the rise in the total electron flux. The morningside boundary cannot always be clearly delineated. Data were divided into 1-hour magnetic local time bins with ∼400 samples per bin. In each of the MLT bins, ΛCGM was found to be linearly correlated to the KP index. Regression values of ΛCGM were projected to the magnetic equatorial plane by using the Mead-Fairfield magnetic field model. The projected boundaries are not in good agreement with the injection boundaries of Mauk and McIlwain (1974). They are best fit to boundaries derived by using Volland-Stern type electric fields with the axis of symmetry rotated into the evening sector for KP