L. J. Zanetti

Freja observations of electromagnetic ion cyclotron ELF waves and transverse oxygen ion acceleration on auroral field lines

Extremely low-frequency (ELF) magnetic and electric field plasma wave emissions were recorded on 2 October 1993 on auroral field lines by the Magnetic Field Experiment during Freja orbit 4770. The ELF wave frequencies were below the local oxygen gyrofrequency (25 Hz) and between the helium and proton gyrofrequencies (100 to 400 Hz). The ELF waves, interpreted as electromagnetic ion cyclotron (EMIC) waves, were observed in a region of inverted-V-type electron precipitation. The EMIC waves were correlated over time with auroral and lower energy ({approximately} 100 eV) electrons, which are both possible sources of free energy, and also with transversely accelerated oxygen ions. The waves above the helium gyrofrequency were more closely correlated with the transverse oxygen ion acceleration than the waves below the oxygen gyrofrequency. These observations are consistent with a scenario in which electron beams generate EMIC waves, which then produce transverse oxygen ion acceleration through a gyroresonant interaction. 16 refs., 4 figs.

The low‐latitude boundary layer at mid‐altitudes: Relation to large‐scale Birkeland currents

In this work the authors seek to test a projected relationship between the low latitude boundary layer (LLBL) and field aligned currents (FAC), or Birkeland currents. They use the procedure developed by Woch and Lundin for identifying LLBL boundaries. They look for correlations between properties of the FAC and properties of the LLBL. Their results show that in most cases the FAC observed are totally inside the region which exhibits LLBL plasma precipitation. The authors argue that within the biases to their data because of its source, and relative sensitivities, their conclusions support earlier work which argues for the LLBL acting as a source region for FAC features.

Geomagnetic storm forecasts and the power industry

There is a well-recognized link between solar activity, geomagnetic disturbances, and disruptions to man-made systems such as power grids, satellites, communications, and defense systems. As technology evolves, these systems become more susceptible to magnetic disturbances than their counterparts of previous solar cycles. Analysis suggests that these vulnerabilities will continue and perhaps even increase as these systems continue to evolve. Geomagnetic disturbances can cause geomagnetically induced currents (GIC) to flow through the power system, entering and exiting the many grounding points on a transmission network. This is generally of most concern at the latitudes of the northern United States, Canada, and Scandinavia, for example, but regions much farther south are also affected during intense magnetic storms.

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.

In-flight calibration of the NEAR magnetometer

The science objectives for the Near Earth Asteroid Rendezvous (NEAR) Magnetometer Experiment (MAG) are to measure a possible magnetic field of 433 Eros to 5 nT accuracy and secondarily to detect asteroid-solar wind interaction signatures. Because the MAG sensor is body mounted, achieving this accuracy required detailed analysis of spacecraft magnetic fields during cruise. Sources of magnetic contamination identified prior to launch and during cruise are: propulsion latch valves, fixed 190 nT residual held, solar arrays and power harness, variable 15 to 60 nT field, power distribution terminal board, /spl sim/30 nT field with 5 nT variations, power shunting circuitry, 1-5 nT variations, the MAG sensor survival heater, 6 nT steps and attitude control momentum wheels, and 1 nT amplitude at 0.5 to 10 Hz from each of four wheels. Analysis of cruise data was used to create accurate /spl plusmn/1 nT models for the fixed and variable fields with signals below 0.5 Hz to provide correction of the raw data. The spacecraft field corrections and MAG calibration were validated with data from the Earth swing-by of January 1997. Comparison of solar wind magnetic held measurements from the WIND spacecraft and NEAR from January 22-24, 1997, before and after the Earth swing-by maneuver, confirm that the resulting NEAR magnetic field measurements are accurate to 1-2 nT.

Near Magnetic Field Investigation, Instrumentation, Spacecraft Magnetics and Data Access

The primary objective of the investigation is the search for a body-wide magnetic field of the near Earth asteroid Eros. The Near Earth Asteroid Rendezvous (NEAR) 3-axis fluxgate magnetometer includes a sensor mounted on the high-gain antenna feed structure. The NEAR Magnetic Facility Instrument (MFI) is a joint hardware effort between GSFC and APL. The design and magnetics approach achieved by the NEAR MFI effort entailed low-cost, up-front attention to engineering solutions which did not impact the schedule. The goal of the magnetometer is reliable magnetic field measurements within 5 nT, which necessitates the use of an extensive spacecraft magnetic interference model but is achievable with the full years orbital data set. Such a goal has been shown viable with recent in-flight calibration data and comparisons to the WIND magnetometer data. The NEAR MFI effort has succeeded in providing magnetic field measurements for the first flight in NASAs Discovery line.

Ionospheric currents correlated with geomagnetic induced currents; Freja magnetic field measurements and the Sunburst Monitor System

Temporal and spatial fluctuations of large-scale electric currents totaling millions of amperes in the ionosphere have long been known to induce current and voltage in the Earths surface and on large manmade conductive structures. A strong, persistent solar wind can interact with the geomagnetic environment and produce auroral zone currents at mid latitudes. Presented here is a specific correlation of ionospheric auroral currents inferred from magnetic field measurements from the Swedish Freja satellite and induced current in the electric power distribution system on the east coast of the United States, in particular the Chalk Point, Maryland, transformer station. The event discussed here registered a Kp level of 6 to 8 and occurred on April 4–5, 1993. An important aspect is that information from both of these space- and ground-based monitoring systems is available automatically and in real time. In addition, satellite data from earlier local times, e.g., data monitored from European ground stations, can give precursor information of later North American geomagnetic activity.

Periodic longitudinal structure of field‐aligned currents in the dawn sector: Large‐scale meandering of an auroral electrojet

The azimuthal structure of field-aligned current (FAC) systems is one of the target subjects of the Freja satellites magnetometer observation. In the event of May 28, 1993, the satellite observed large-amplitude fluctuations of the latitudinal, as well as the longitudinal, magnetic field components along a postmidnight-to-morning orbit. By combining radar and ground magnetometer data, we inferred that the associated FAC system has a periodic structure consisting of multiple FAC sheets inclined significantly from the azimuthal (east-west) direction and moving westward and possibly poleward. Such a structure may be interpreted in terms of the meandering of an auroral electrojet, and perhaps as the ionospheric projection of boundary waves.

First Viking results - Magnetic field measurements

The VIKING spacecraft carries a high-resolution Magnetic Field Experiment for the operational purpose of determining spacecraft attitude and to fulfill the scientific objectives of providing magnetic-field measurements necessary for the determination of particle pitch angles, identification of geospace boundaries, and the study of magnetospheric current systems and plasma processes. This experiment includes a fluxgate magnetometer system with the sensors mounted on a 2-m boom. It has 4 automatically switchable ranges from ± 1024 nT to ± 65 536 nT (full scale) and resolutions commensurate with a 13-bit A/D converter in each range (± 0.125 nT to ± 8 nT). Approximately 53 vector samples/sec are acquired. The fast sampling rate of this instrument provides a spatial resolution of 12 m in the lower ionosphere when the measurements are mapped down from the apogee altitude. Transverse magnetic field perturbations are readily observed with this instrument which identify the large-scale auroral and cusp-region Birkeland current systems. A sharp gradient was detected in the dayside region near apogee and 08:40 MLT during a pass on March 25, 1986 which has been interpreted as an earthward flowing Birkeland current of 8 μA/m2. When projected to ionospheric altitudes, it is estimated that this current density is 200 μA/m2. This unusually intense current may be similar to the intense earthward flowing Birkeland current associated with a conductivity gradient studied by Bythrow et al. [1].