D. A. Hardy

A statistical model of auroral ion precipitation: 2. Functional representation of the average patterns

In the study of Hardy et al. (1989) the average pattern of auroral ion precipitation was determined as a function of corrected geomagnetic latitude, magnetic local time (MLT), and Kp using data from the SSJ/4 detectors on the satellites of the Defense Meteorological Satellite Program (DMSP). One product of this study was a set of hemispheric maps of the integral number flux and integral energy flux of the precipitating ions for each of seven levels of activity as defined by Kp. In order to facilitate the use of these maps in magnetospheric, ionospheric, and thermospheric models, they have been fit to a functional form. To accomplish this, the latitudinal variation in the integral energy and number flux in each of 48 half-hour MLT bins of the original statistical study, for each Kp level, was fit to a general form of the Epstein function. This functional form was chosen since it fits well the asymmetric shape of the latitudinal profile of the integral energy and number flux. The Epstein function fits to the latitudinal profiles require six variables in each MLT zone. To further reduce the number of variables needed to specify a map, the MLT variation of each of the six variables was expanded as a sixth-order Fourier series with 13 coefficients. The resulting fit reproduces well the original statistical data. The distribution of differences between the original data and the fit normalized to the original data shows agreement that is generally better than 25% for both the integral energy and number flux.

A statistical study on the effects of IMF Bz and solar wind speed on auroral ion and electron precipitation

The variation in the average particle number and energy flux in the high-latitude region is determined as a function of the solar wind velocity, Vsw, and the z component of the interplanetary magnetic field, Bz. The study is made using the data from the SSJ/4 detectors on the satellites of the Defense Meteorological Satellite Program. In the study the high-latitude region is divided into spatial elements in magnetic local time and corrected geomagnetic latitude. One such matrix of spatial divisions is constructed for each of 30 paired ranges of values in Vsw and Bz. There are six divisions in Bz covering the range from −10 to +10 nT and five divisions in velocity covering the range from 200 to 800 km/s. Using approximately 34 million SSJ/4 spectra, the average electron and ion spectra are determined in each spatial element for each of the 30 paired values of Vsw and Bz. From the average spectra, the average integral energy and number fluxes for electrons and ions are calculated in each spatial bin. These values are then spatially integrated to give the average hemispheric inputs of the integral particle energy and number flux. Both quantities are found to vary in a simple and consistent manner with both Vsw and Bz. For both electrons and ions the variation with Bz tends to a minimum value for weak to moderately strong Bz positive. For decreasing values of Bz from the minimum, both quantities increase at a rate greater than linear. For increasing values of Bz from the minimum, both quantities tend to increase, least for the hemispheric electron energy flux and most for the hemispheric ion number flux. The variation in both quantities with Vsw for both electrons and ions is generally linear with the steepest slopes for the electron hemispheric energy flux. The variation with Bz and Vsw for all four quantities can be well fit by a simple quadratic equation either of the form ƒ(Bz, Vsw) = a(Bz − b)² + cVsw + d or ƒ(Bz, Vsw) = [a(Bz − b)² + 1][cVsw + d]. Either form indicates that the variation is no greater than quadratic in Bz and no more than linear in Vsw and that the variation is approximately symmetric for values above and below the minimum value of Bz, namely, b. This result is significantly different than either the half-wave rectifier or epsilon function for describing the solar-wind magnetospheric interaction.

Correlator measurements of megahertz wave‐particle interactions during electron beam operations on STS

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 (<3×10 6 m s −1 ) where beam-generated waves are in resonance with suprathermal electrons.

Banded electron structures in the plasmasphere

The low-energy plasma analyzer on CRRES has detected significant fluxes of 10-eV to 30-keV electrons trapped on plasmaspheric field lines. On energy-versus-time spectrograms these electrons appear as banded structures that can span the 2 50 to <20 kV. Subsequent encounters with the lower-energy cloud on alternating CRRES orbits over the next 2 days showed a progressive, earthward movement of the electrons’ inner boundary. Whistler and electron cyclotron harmonic emissions accompanied the most intense manifestations of cloud electrons. The simplest explanation of these measurements is that after initial injection, the Alfven boundary moved outward, leaving the cloud electrons on closed drift paths. Subsequent fluctuations of the convective electric field penetrated the plasmasphere, transporting cloud elements inward. The magnetic shell distribution of electron temperatures in one of the banded structures suggests that radiative energy losses may be comparable in magnitude to gains due to adiabatic compression.

Average Height-Integrated Joule Heating Rates and Magnetic Deflection Vectors Due to Field-Aligned Currents during Sunspot Minimum

Abstract Height-integrated Joule heating has been calculated from simultaneous observations of field-aligned currents and precipitating electrons made by the Defense Meterorological Satellite Program (DMSP) satellite F7. The DMSP/F7 satellite provided nearly continuous data from January 1984 to October 1987. In this paper we use data from January 1984 to December 1985, a period of nearly uniform, low solar EUV flux near the minimum of the sunspot cycle. By assuming that the majority of the observed field-aligned currents connect through the ionosphere via local Pedersen currents, we have calculated the local Joule heating rates. By combining Joule heating observations from multiple orbits, a survey of Joule heating vs magnetic latitude, magnetic local time, magnetic activity level and season has been compiled. In addition, the average magnetic deflection vector has been compiled. Our survey of the distribution of Joule heating has finer resolution than previous surveys, and is comparable with previous case studies. We have found evidence that the source of the field-aligned current into/out of the dayside cusp is on open field lines and that the source of the field-aligned current into/out of the auroral zones is on closed field lines.

High latitude electrodynamics: Observations from S3-2

The high spatial-temporal resolution of instrumentation on the polar-orbiting S3-2 satellite has allowed a wide variety of measurements of the electrodynamic characteristics of both large- and small-scale structures at high latitudes. Analyses of large scale features observed by S3-2 have shown that: (i) The IMF Bydependence of polar cap convection, first observed in June 1969 by OGO-6 persists in other seasons. During periods of northward IMF Bzextensive regions of sunward convection may be found in the sunlit polar cap. (ii) In the dawn and dusk MLT sectors >90% of the region 1 currents lie equatorward of the convection reversal line. Potentials across the ionospheric projection of the low-latitude boundary layer are typically a few kV. (iii) The location of ‘extra’ field-aligned currents, near the dayside cusp and poleward of the region 1 current sheet is dependent on the IMF Bycomponent. (iv) Simultaneous observations by TRIAD and S3-2 show that sheets of field-aligned current extend uniformly for several hours in MLT, but may have an altitude dependence in the 1000–8000 km range. (v) During magnetic storms ionospheric irregularities occur in regions of poleward density gradients and downward field-aligned currents near the equatorward boundary of diffuse auroral precipitation. In the winter polar cap, density irregularities were also found in regions of highly structured electric fields and soft electron precipitation. (vi) During an intense magnetic storm the auroral zone height-integrated Pederson conductivity was calculated to be in the range 10–30 mho and downcoming energetic electron fluxes accounted for between 50% and 70% of the upward Birkeland currents.Analysis of small-scale structures (latitudinal width < 1°), observed by S3-2, have shown that: (i) Intense meridional electric fields (50–250 mV m-1) generated by charge separation near the inner edge of the plasma sheet drive intense subauroral convection and are associated with field-aligned currents, on the order of 1–2 μA m-2. (ii) Case studies of discrete arcs in the auroral oval have shown that arcs are associated with pairs of small-scale, field-aligned currents embedded in the large-scale region 1/region 2 field-aligned current sheets. The maximum observed field-aligned current was an upward current of 135 μA m-2, confined to a latitudinal width of ∼ 2km and carried by field-aligned accelerated electrons. Return (downward) currents associated with arcs are limited to intensities of 10–15 μA m-2. At this limit the ionospheric plasma becomes marginally stable to the onset of ion-cyclotron turbulence. Two instances of plasma vortices, characteristic of auroral curls, have been observed in the region between the paired current sheets. (iii) Sun-aligned arcs in the polar cap are found in a region of negative electric field divergence, embedded in an irregular electric field pattern. The electrons producing the arcs have a temperature of ∼ 200 eV and have been accelerated through potential drops of ∼ 1 kV along the magnetic field. Return currents may appear on both sides of polar-cap arcs.

Observations of electron beam propagation perpendicular to the Earth's magnetic field during the TSS 1 mission

We report on measurements by the Shuttle Potential and Return Electron Experiment (SPREE), acquired during a period of the Electrodynamic Tethered Satellite mission when the fast pulsed electron generator (FPEG) injected a 1-keV electron beam nearly perpendicular to the Earths magnetic field. Using multiangular electrostatic analyzers mounted on rotary tables, SPREE was capable of determining the flux of electrons and ions in the energy range from 10 eV to 10 keV and over a solid angle of 2π sr. SPREE was located in the shuttle bay where it could observe beam electrons after they had completed ∼1 gyrocycle when fired nearly perpendicular to the local magnetic field. For the case presented here, the beams intensity decreased from ∼100 mA cm−2 at FPEGs aperture to ∼0.18 nA cm−2 at the location of SPREE. The spectrum of the return electrons displays a sharp peak at the beam energy with an intensity at the peak of approximately 2×1010 electrons cm−2 s−1 sr−1. The distribution of the electrons around the peak has a half width of several hundred eV, with observed energies as high as 1850 eV. For energies between 10 and a few hundred eV, intense fluxes of electrons are seen at all look angles. For angles where the beam is observed the spectrum in this energy range has a power law shape. At angles away from the direction of beam return, the spectrum in this energy range can display a more thermal shape with a peak at energies up to 50 eV. In general, the flux intensity in the lower-energy portion of the spectrum is isotropic with an average integral flux of 0.5 to 2×1012 electrons cm−2 s−1 sr−1. Integrating over energy and pitch angle gives number densities of ∼ 5×104 electrons cm−3. The return current density of 0.5 to 2µA cm−2 s−1 sr−1 carried by this isotropic component is sufficient to balance that emitted by FPEG and keep the shuttle at a low potential. We find that both scattering and spreading of the beam near FPEG are necessary for primary electrons to reach the locations of the SPREE detectors.

Electron acceleration by megahertz waves during OEDIPUS C

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 flights 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.

Energy distributions of thruster pickup ions detected by the Shuttle Potential and Return Electron Experiment during TSS 1

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 shuttles velocity vector was nearly perpendicular to the Earths 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.

Heating and low-frequency modulation of electrons observed during electron beam operations on TSS 1

We have studied electron responses measured by two electrostatic analyzers (ESA A and B) that comprise the Shuttle Potential and Return Electron Experiment (SPREE) to 60 prolonged beam emissions by the fast pulsed electron generator (FPEG) during the first flight of the Tethered Satellite System (TSS 1). When the beam cleanly escaped into space, responses depended on whether the pitch angle of the beam, α B , was less than or greater than 90°. Beam-like structures were detected by SPREE when α B 90°. Secondary electron fluxes measured by SPREE peaked at pitch angles a between 65° and 75° when α B 90°. At other pitch angles the distributions of electrons returning to the shuttle had repeatable thermal and power law shapes. The distinctive distribution functions are attributed qualitatively to the different regions in and near the beam traversed by electrons reaching SPREE under the two α B conditions. A large fraction of the trajectories of electrons reaching SPREE ESA A with α B )90° lie inside (outside) beam flux tubes. Measurements by a particle correlator in the SPREE data processor show that in 25 cases some of the returning-electron distributions fe were modulated at frequencies in the low kilohertz range. The modulations appeared in portions of the distributions where ∂ƒ e /∂υ < 0 and at frequencies that correspond to none of the plasmas normal modes. In light of previously reported wave measurements taken near the shuttle during electron beam emissions, we suggest that the modulated electrons were bunched by large-amplitude, ion acoustic waves propagating nearly perpendicular to the Earths magnetic field. The waves were generated as plasma responses to negative space charges in the electron-beam flux tubes moving at orbital speed across the ionosphere.