K. W. Ogilvie

SWE, A COMPREHENSIVE PLASMA INSTRUMENT FOR THE WIND SPACECRAFT

The Solar Wind Experiment (SWE) on the WIND spacecraft is a comprehensive, integrated set of sensors which is designed to investigate outstanding problems in solar wind physics. It consists of two Faraday cup (FC) sensors; a vector electron and ion spectrometer (VEIS); a strahl sensor, which is especially configured to study the electron ‘strahl’ close to the magnetic field direction; and an on-board calibration system. The energy/charge range of the Faraday cups is 150 V to 8 kV, and that of the VEIS is 7 V to 24.8 kV. The time resolution depends on the operational mode used, but can be of the order of a few seconds for 3-D measurements. ‘Key parameters’ which broadly characterize the solar wind positive ion velocity distribution function will be made available rapidly from the GGS Central Data Handling Facility.

Plasma Observations Near Uranus: Initial Results from Voyager 2

Extensive measurements of low-energy positive ions and electrons in the vicinity of Uranus have revealed a fully developed magnetosphere. The magnetospheric plasma has a warm component with a temperature of 4 to 50 electron volts and a peak density of roughly 2 protons per cubic centimeter, and a hot component, with a temperature of a few kiloelectron volts and a peak density of roughly 0.1 proton per cubic centimeter. The warm component is observed both inside and outside of L = 5, whereas the hot component is excluded from the region inside of that L shell. Possible sources of the plasma in the magnetosphere are the extended hydrogen corona, the solar wind, and the ionosphere. The Uranian moons do not appear to be a significant plasma source. The boundary of the hot plasma component at L = 5 may be associated either with Miranda or with the inner limit of a deeply penetrating, solar wind-driven magnetospheric convection system. The Voyager 2 spacecraft repeatedly encountered the plasma sheet in the magnetotail at locations that are consistent with a geometric model for the plasma sheet similar to that at Earth.

The Global Geospace Science Program and its investigations

The detailed study of the solar-terrestrial energy chain will be greatly enhanced with the launch and simultaneous operation of several spacecraft during the current decade. These programs are being coordinates in the United States under the umbrella of the International Solar Terrestrial Physics Program (ISTP) and include fundamental contributions from Japan (GEOTAIL Program) and Europe (SOHO and CLUSTER Programs). The principal United States contribution to this effort is the Global Geospace Science Program (GGS) described in this overview paper. Two spacecraft, WIND and POLAR, carrying an advanced complement of field, particle and imaging instruments, will conduct investigations of several key regions of ‘geospace’. This paper provides a general overview of the science objectives of the missions, the spacecraft orbits and the ground elements that have been developed to process and analyze the instrument observations.

Hydra — A 3-dimensional electron and ion hot plasma instrument for the POLAR spacecraft of the GGS mission

HYDRA is an experimental hot plasma investigation for the POLAR spacecraft of the GGS program. A consortium of institutions has designed a suite of particle analyzers that sample the velocity space of electron and ions between ≃2 keV/q – 35 keV/q in three dimensions, with a routine time resolution of 0.5 s. Routine coverage of velocity space will be accomplished with an angular homogeneity assumption of ≃16°, appropriate for subsonic plasmas, but with special ≃1.5° resolution for electrons with energies between 100 eV and 10 keV along and opposed to the local magnetic field. This instrument produces 4.9 kilobits s−1 to the telemetry, consumes on average 14 W and requires 18.7 kg for deployment including its internal shielding. The scientific objectives for the polar magnetosphere fall into four broad categories: (1) those to define the ambient kinetic regimes of ions and electrons; (2) those to elucidate the magnetohydrodynamic responses in these regimes; (3) those to assess the particle populations with high time resolution; and (4) those to determine the global topology of the magnetic field. In thefirst group are issues of identifying the origins of particles at high magnetic latitudes, their energization, the altitude dependence of the forces, including parallel electric fields they have traversed. In thesecond group are the physics of the fluid flows, regimes of current, and plasma depletion zones during quiescent and disturbed magnetic conditions. In thethird group is the exploration of the processes that accompany the rapid time variations known to occur in the auroral zone, cusp and entry layers as they affect the flow of mass, momentum and energy in the auroral region. In thefourth class of objectives are studies in conjunction with the SWE measurements of the Strahl in the solar wind that exploit the small gyroradius of thermal electrons to detect those magnetic field lines that penetrate the auroral region that are directly ‘open’ to interplanetary space where, for example, the Polar Rain is observed.

Correlation of changes in the outer‐zone relativistic‐electron population with upstream solar wind and magnetic field measurements

A study has been made of the correlation of the population of relativistic electrons in the outer-zone magnetosphere with the properties of the solar wind (speed, density, magnetic field) during a solar minimum period. The study is based upon observations made in the Spring of 1995 with sensors aboard 1994-026 and WIND. It is found that a large relativistic electron enhancement depends upon a substantial solar-wind speed increase associated with precursor solar-wind density enhancement, and, in particular, upon a southward turning of the interplanetary magnetic field.

Observations of the lunar plasma wake from the WIND spacecraft on December 27, 1994

On December 27, 1994, the WIND spacecraft crossed the lunar wake at a distance of 6.5 lunar radii ( RL ) behind the moon. The observations made were the first employing modem instruments and a high data rate. The SWE plasma instrument on WIND observed new aspects of the interaction between the solar wind and unmagnetized dielectric bodies. The plasma density decreased exponentially from the periphery of the wake towards its center as predicted by simple theory. Behind the moon two distinct cold ion beams were observed refilling the lunar cavity. The ions were accelerated along the direction of the magnetic field by an electric field of the order 2 × 10−4 volts/m. The region of plasma depletion was observed to extend beyond the light shadow, consistent with a rarefaction wave moving out from the wake into the undisturbed solar wind.

Recurrent geomagnetic storms and relativistic electron enhancements in the outer magnetosphere: ISTP coordinated measurements

New, coordinated measurements from the International Solar-Terrestrial Physics (ISTP) constellation of spacecraft are presented to show the causes and effects of recurrent geomagnetic activity during recent solar minimum conditions. It is found using WIND and POLAR data that even for modest geomagnetic storms, relativistic electron fluxes are strongly and rapidly enhanced within the outer radiation zone of the Earths magnetosphere. Solar wind data are utilized to identify the drivers of magnetospheric acceleration processes. Yohkoh solar soft X-ray data are also used to identify the solar coronal holes that produce the high-speed solar wind streams which, in turn, cause the recurrent geomagnetic activity. It is concluded that even during extremely quiet solar conditions (sunspot minimum) there are discernible coronal holes and resultant solar wind streams which can produce intense magnetospheric particle acceleration. As a practical consequence of this Sun-Earth connection, it is noted that a long-lasting E>1MeV electron event in late March 1996 appears to have contributed significantly to a major spacecraft (Anik E1) operational failure.

MAGNETIC AND THERMAL PRESSURES IN THE SOLAR WIND.

Explorer 34 solar wind data for the period June to December, 1967 show that(a) The magnetic pressure, PB≡B2/8π, and thermal pressure,Pk≡npkTp+nαkTα+nekTe,are variable and positively correlated on a scale of ≳ 2 days, but (b) changes in Pb and Pk are anticorrelated on a scale ∼ 1 hr (∼0.01 AU). Thus, dynamical hydromagnetic processes (dv/dt⊄o) must occur on the mesoscale, but the solar wind tends to be in equilibrium(PB+PK∼constant) on a smaller scale, the microscale. The 3-hr averages show that the most probable value of β≡Pk/PB is β=1.0±0.1, which implies that the most probable state of the solar wind at 1 AU is not one of equipartition between the thermal energy and magnetic energy. The average total pressure for a given bulk speed(P(V)=Pk+Pk+PB) is essentially independent of V, implying that P is not determined by the heating or acceleration mechanisms of the solar wind; the average pressure is P=(2.9±1.5)×10-10dyne/cm2.

A strong CME‐related magnetic cloud interaction with the Earth's Magnetosphere: ISTP observations of rapid relativistic electron acceleration on May 15, 1997

A geoeffective magnetic cloud impacted the Earth early on 15 May 1997. The cloud exhibited strong initial southward interplanetary magnetic field (BZ∼−25 nT), which caused intense substorm activity and an intense geomagnetic storm (Dst ∼−170 nT). SAMPEX data showed that relativistic electrons (E ≳ 1.0 MeV) appeared suddenly deep in the magnetosphere at L=3 to 4. These electrons were not directly “injected” from higher altitudes (i.e., from the magnetotail), nor did they come from an interplanetary source. The electron increase was preceded (for ∼2 hrs) by remarkably strong low-frequency wave activity as seen by CANOPUS ground stations and by the GOES-8 spacecraft at geostationary orbit. POLAR/CEPPAD measurements support the result that high-energy electrons suddenly appeared deep in the magnetosphere. Thus, these new multi-point data suggest that strong magnetospheric waves can quickly and efficiently accelerate electrons to multi-MeV energies deep in the radiation belts on timescales of tens of minutes.

THE LOW-ENERGY NEUTRAL ATOM IMAGER FOR IMAGE

The ‘Imager for Magnetosphere-to-Aurora Global Exploration’ (IMAGE) will be launched early in the year 2000. It will be the first mission dedicated to imaging, with the capability to determine how the magnetosphere changes globally in response to solar storm effects in the solar wind, on time scales as short as a few minutes. The low energy neutral atom (LENA) imager uses a new atom-to-negative ion surface conversion technology to image the neutral atom flux and measure its composition (H and O) and energy distribution (10 to 750 eV). LENA uses electrostatic optics techniques for energy (per charge) discrimination and carbon foil time-of-flight techniques for mass discrimination. It has a 90° x 8° field-of-view in 12 pixels, each nominally 8° x 8°. Spacecraft spin provides a total field-of-view of 90° x 360°, comprised of 12 x 45 pixels. LENA is designed to image fast neutral atom fluxes in its energy range, emitted by auroral ionospheres or the sun, or penetrating from the interstellar medium. It will thereby determine how superthermal plasma heating is distributed in space, how and why it varies on short time scales, and how this heating is driven by solar activity as reflected in solar wind conditions.