P. F. Bythrow

The UARS particle environment monitor

The overall objective of the particle environment monitor (PEM) is to provide comprehensive measurements of both local and global energy inputs into the Earths atmosphere by charged particles and Joule dissipation using a carefully integrated set of instruments. PEM consists of four instruments: the atmospheric X ray imaging spectrometer (AXIS), the high-energy particle spectrometer (HEPS), the medium-energy particle spectrometer (MEPS), and the vector magnetometer (VMAG). AXIS provides global scale images and energy spectra of 3- to 100-keV bremsstrahlung X rays produced by electron precipitation into the atmosphere. HEPS and MEPS provide in situ measurements of precipitating electrons in the energy range from 1 eV to 5 MeV and protons in the energy range from 1 eV to 150 MeV. Particles in this energy range deposit their energy in the atmosphere at altitudes extending from several hundred kilometers down to as low as ∼30 km. VMAG provides the magnetic field direction needed to indicate and interpret the locations and intensities of ionospheric and field-aligned currents as well as providing a reference for the particle measurements. This paper describes each instrument separately and also in the context of the PEM objectives which include the determination of energy deposition and ionization production rates as functions of altitude. Examples of data acquired early in the Upper Atmosphere Research Satellite (UARS) mission are presented.

Magnetic Field Experiment on the Freja Satellite

Freja is a Swedish scientific satellite mission to study fine scale auroral processes. Launch was October 6, 1992, piggyback on a Chinese Long March 2C, to the present 600 × 1750 km, 63° inclination orbit. The JHU/APL provided the Magnetic Field Experiment (MFE), which includes a custom APL-designed Forth language microprocessor. This approach has led to a truly generic and flexible design with adaptability to differing mission requirements and has resulted in the transfer of significant ground analysis to on-board processing. Special attention has been paid to the analog electronic and digital processing design in an effort to lower system noise levels, verified by inflight data showing unprecedented system noise levels for near-Earth magnetic field measurements, approaching the fluxgate sensor levels. The full dynamic range measurements are of the 3-axis Earth’s magnetic field taken at 128 vector samples s-1 and digitized to 16 bit resolution, primarily used to evaluate currents and the main magnetic field of the Earth. Additional 3-axis ‘AC channels are bandpass filtered from 1.5 to 128 Hz to remove the main field spin signal, the range is ±650 nT. These vector measurements cover Pc waves to ion gyrofrequency magnetic wave signals up to the oxygen gyrofrequency (~40 Hz). A separate, seventh channel samples the spin axis sensor with a bandpass filter of 1.5 to 256 Hz, the signal of which is fed to a software FFT. This on-board FFT processing covers the local helium gyrofrequencies (~160 Hz) and is plotted in the Freja Summary Plots (FSPs) along with disturbance fields. First data were received in the U.S. October 16 from Kiruna, Sweden via the Internet and SPAN e-mail networks, and were from an orbit a few hours earlier over Greenland and Sweden. Data files and data products, e.g., FSPs generated at the Kiruna ground station, are communicated in a similar manner through an automatic mail distribution system in Stockholm to PIs and various users. Distributed management of spacecraft operations by the science team is also achieved by this advanced communications system.

Auroral currents during the magnetic storm of November 8 and 9, 1991: Observations from the Upper Atmosphere Research Satellite Particle Environment Monitor

The magnetic storm of November 8 and 9, 1991, lead to expansion of the auroral oval below 45° magnetic latitude (MLAT) and auroral phenomena were sampled by the Upper Atmosphere Research Satellite (UARS). The attitude precision and stability of UARS allow monitoring of Birkeland and ionospheric currents via magnetic field measurements from the Particle Environment Monitor (PEM) magnetometer. This paper reports the development of the intensity and location of Birkeland currents associated with this storm. Two principle results are obtained: (1) total Birkeland currents exceed 30 MA, more than 6 times nominal values, indicating Joule heating of about 3×1012 W; (2) Birkeland currents below 50°, polar cap currents indicative of anti-sunward convection, and cusp particle signatures of southward IMF all persist at least eight hours into recovery phase of the storm.

Filamentary current structures in the postnoon sector: Observations from UARS

During an intense geomagnetic storm (Kp 7+) that began at {approximately} 1830 UT on October 1, 1991, the UARS satellite encountered the dayside postnoon auroral oval. On two consecutive crossings of the northern hemisphere between 2040 and 2240 UT, the vector magnetometer detected region 1 and 2 Birkeland and ionospheric currents in the postnoon sector. Low-energy electron events were observed near 1400 MLT within a narrow portion of the region 1 current system. Simultaneous magnetic field measurements revealed the presence of intense ({approximately} 20 {mu}A/m{sup 2}) bipolar filament current structures embedded in the auroral oval. The upward-directed currents were associated with the more concentrated region of precipitating electrons. Ions associated with the more intense flux of low-energy electrons exhibited a dispersion signature typical of an ion velocity filter. The dispersion, aligned along the orbit, exhibited higher-energy ions at lower latitudes and earlier local times. The colocation of filament currents and ion dispersion signatures at such late postnoon local times is not consistent with typical E x B {open_quotes}cusp{close_quotes} dispersions. These features more likely result from dayside boundary wave phenomena. 34 refs., 9 figs.

Field-aligned currents associated with spatially periodic X ray structures in the morningside auroral oval

On February 9, 1992, the Atmospheric X ray Imaging Spectrometer (AXIS) on the Upper Atmosphere Research Satellite (UARS) observed spatially periodic structures in X ray luminosity from the top of the atmosphere during a pass over the dawnside auroral oval. The patches of luminosity were several hundred kilometers in size and separated by a mean distance of about 700 km. As many as a dozen distinct patches could be identified extending from local times just after midnight to the noon meridian. The characteristics of the precipitating electrons responsible for the patches were determined from the measured X ray fluxes. Peak electron energy fluxes were between 4 and 10 ergs cm−2 s−1 with e-folding energies of 10–25 keV. In situ magnetic field data from the onboard magnetometer were used to model the field-aligned currents associated with two of the patches. The model that best fit the data had upward currents in the western portions of the patches and more intense downward currents confined to the eastern edges. The modeling also indicated that these small-scale currents were embedded within the large-scale region 2 field-aligned current sheet. Data from a ground-based magnetometer near the satellite ground track indicated the presence of Ps 6 pulsations. We have interpreted the observations in terms of eastward-drifting current patch systems. Under this assumption, the patches moved at 1.7 km s−1, the drift speed of ∼30-keV electrons. This motion, and the correlation with Ps 6 pulsations suggests that the X ray patches were associated with omega bands that are seen in the morning sector during geomagnetically active times.

Freja's contribution to the ISTP event of October 27, 1992. A distorted magnetosphere

A unique arrangement of satellites and ground-based magnetometers has provided the opportunity to study the distortion of the magnetosphere by conditions of elevated solar wind dynamic pressure and high geomagnetic activity on October 27, 1992. Data were acquired by the near-Earth satellites Freja, UARS, DMSP-F8 and DMSP-F11, and by IMP-8. This study will focus on patterns of large-scale Birkeland currents deduced from the APL magnetic field experiments on Freja and UARS. Because of the relatively low inclinations of their orbits (63° for Freja and 57° for UARS) these satellites skim the current systems over a wide range of local time and provide a kind of “global image” of the Birkeland currents. Energetic particle data from the DMSP satellites are used in an attempt to complete the image of the auroral region. The principal findings of this study include the following. The dayside Birkeland current system is displaced 6° to 9° equatorward of the statistical pattern determined by Iijima and Potemra [1978] for undisturbed conditions (|AL|< 100 nT) and 3° equatorward of their disturbed statistical pattern (|AL|≥ 100 nT). The densities of these daytime Birkeland currents are estimated to be 1 to 2 µA/m², comparable to, but not larger than the statistical values determined by Iijima and Potemra for disturbed conditions. The nightside Birkeland currents are located close to the Iijima and Potemra disturbed statistical pattern and they are less intense than the dayside currents.

Satellite observations of currents and waves in space plasmas

Plasma processes occurring in the magnetosphere are examined in the light of recent observations of currents and waves with satellite-born magnetic experiments. In particular, results from the Viking and AMPTE/CCE satellites indicate that geomagnetic field lines that guide stationary Birkland currents can also support resonant Alfven waves. The relationship of these waves to the current systems and their source in the magnetosphere is still under investigation. It is emphasized that Birkland currents and Alfven waves are fundamental to an understanding of the earths plasma environment.