The THEMIS Constellation

The Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission is the fifth NASA Medium-class Explorer (MIDEX), launched on February 17, 2007 to determine the trigger and large-scale evolution of substorms. The mission employs five identical micro-probes (termed “probes”), which have orbit periods of one, two and four days. Each of the Probes carries five instruments to measure electric and magnetic fields as well as ions and electrons. Each probe weighs 134 kg including 49 kg of hydrazine fuel and measures approximately 0.8×0.8×1.0 meters (L×W×H) and operates on an average power budget of 40 watts. For launch, the Probes were integrated to a Probe Carrier and separated via a launch vehicle provided pyrotechnic signal. Attitude data are obtained from a sun sensor, inertial reference unit and the instrument Fluxgate Magnetometer. Orbit and attitude control use a RCS system having two radial and two axial thrusters for roll and thrust maneuvers. Its two fuel tanks and pressurant system yield 960 meters/sec of delta-V, sufficient to allow Probe replacement strategies. Command and telemetry communications use an S-band 5 watt transponder through a cylindrical omni antenna with a toroidal gain pattern. This paper provides the key requirements of the probe, an overview of the probe design and how they were integrated and tested. It includes considerations and lessons learned from the experience of building NASA’s largest constellation.     

The visible imaging system (VIS) for the polar spacecraft

The Visible Imaging System (VIS) is a set of three low-light-level cameras to be flown on the POLAR spacecraft of the Global Geospace Science (GGS) program which is an element of the International Solar-Terrestrial Physics (ISTP) campaign. Two of these cameras share primary and some secondary optics and are designed to provide images of the nighttime auroral oval at visible wavelengths. A third camera is used to monitor the directions of the fields-of-view of these sensitive auroral cameras with respect to sunlit Earth. The auroral emissions of interest include those from N ₂ ⁺ at 391.4 nm, Oi at 557.7 and 630.0 nm, Hi at 656.3 nm, and Oii at 732.0 nm. The two auroral cameras have different spatial resolutions. These resolutions are about 10 and 20 km from a spacecraft altitude of 8R _e . The time to acquire and telemeter a 256×256-pixel image is about 12 s. The primary scientific objectives of this imaging instrumentation, together with thein-situ observations from the ensemble of ISTP spacecraft, are (1) quantitative assessment of the dissipation of magnetospheric energy into the auroral ionosphere, (2) an instantaneous reference system for thein-situ measurements, (3) development of a substantial model for energy flow within the magnetosphere, (4) investigation of the topology of the magnetosphere, and (5) delineation of the responses of the magnetosphere to substorms and variable solar wind conditions.     

How to See the Forest for the Trees (The Requirements Definition Process)

This paper addresses the avionics requirements definition process, at the conceptual level, in light of changing threats, technology, and business environments. The objective is to provide a perspective of total integrated system performance which illuminates broad requirements issues rather than specific subsystem specifications. The fundamental premise is that avionics requirements are driven by four factors; Information and Data Sources, Control Opportunities and Information Needs, Concepts and Algorithmic Techniques, and Realization Technologies. These four factors are set in a systems structure which shows their inter-relationships and provides the framework for conceptualizing avionics system solutions to meet particular mission needs. Three generic mission areas are considered and an examination is made of the issues of fielding and affording the solution with specific emphasis on architecture, fusion, production, and support.     

Observations of coronal structure during sunspot maximum

This paper presents some of the results that have been obtained from the Kitt Peak observations of coronal holes and the NRL observations of coronal transients during the recent years near sunspot maximum (1979–1981). On the average, low-latitude coronal holes of comparable size contained 3 times more flux near sunspot maximum than near the previous minimum. In the outer corona, transients occurred at the observed rate of at least 2 per day, and quiet conditions persisted during less than 15 % of the observed days. We describe a sample of the more than 800 events that we have observed so far, including the observation of a comet apparently colliding with the Sun.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.Visiting Astronomer, KPNO.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

The Cluster Spatio-Temporal Analysis of Field Fluctuations (STAFF) Experiment

The Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment is one of five experiments which together comprise the Wave Experiment Consortium (WEC). STAFF consists of a three-axis search coil magnetometer to measure magnetic fluctuations at frequencies up to 4 kHz, and a spectrum analyser to calculate in near-real time aboard the spacecraft, the complete auto- and cross-spectral matrices using the three magnetic and two electric components of the electromagnetic field. The magnetic waveform at frequencies below either 10 Hz or 180 Hz is also transmitted. The sensitivity of the search coil is adapted to the phenomena theo be studied: the values 3 × 10^-3 nT Hz^-1/2 and 3 × 10^-5 nT Hz^-1/2 are achieved respectively at 1 Hz and 100 Hz. The dynamic range of the STAFF instruments is about 96 dB in both waveform and spectral power, so as to allow the study of waves near plasma boundaries. Scientific objectives of the STAFF investigations, particularly those requiring four point measurements, are discussed. Methods by which the wave data will be characterised are described with emphasis on those specific to four-point measurements, including the use of the Field Energy Distribution function.     

Mission Overview for the Radiation Belt Storm Probes Mission

Provided here is an overview of Radiation Belt Storm Probes (RBSP) mission design. The driving mission and science requirements are presented, and the unique engineering challenges of operating in Earth’s radiation belts are discussed in detail. The implementation of both the space and ground segments are presented, including a discussion of the challenges inherent with operating multiple observatories concurrently and working with a distributed network of science operation centers. An overview of the launch vehicle and the overall mission design will be presented, and the plan for space weather data broadcast will be introduced.     

Planning for the international space year

Preparations and proposals for the 1992 International Space Year are being made which cover a wide range of activities, organizations and nations; the momentum is building. There is an urgent need for an informal and independent ISY Steering Committee to legitimize the ISY and catalyse activities. With creativity and international cooperation, the International Space Year could be the most inspiring and transforming activity of the space age.     

Coronal holes and solar magnetic fields

Since 1972, nearly continuous observations of coronal holes and their associated photospheric magnetic fields have been made using a variety of satellite and ground-based equipment. The results of comparisons of these observations are reviewed and it is demonstrated that the structure and evolution of coronal holes is basically governed by the large-scale distribution of photospheric magnetic flux. Non-polar holes form in the decaying remnants of bipolar magnetic regions in areas with a large-scale flux imbalance. There is strong indirect evidence that the magnetic field in coronal holes is always open to interplanetary space but not all open-field regions have associated coronal holes. The well-observed declining phase of the last solar cycle was characterized by stable magnetic field and coronal hole patterns which were associated with recurrent, high-speed wind streams and interplanetary magnetic field patterns at the Earth. The ascending phase of the current cycle has been characterized by transient magnetic field and coronal hole patterns which tend to occur at high solar latitudes. This shift in magnetic field and coronal hole patterns has resulted in a less obvious and more complicated association with high-speed wind streams at the Earth.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Visiting Scientist, Kitt Peak National Observatory.