_______________________________________________________ R&DUPDATES STAMATIOS M. KRIMIGIS, GERHARD HAERENDEL, RICHARD W. McENTIRE, GOTZ PASCHMANN, and DUNCAN A. BRYANT THE ACTIVE MAGNETOSPHERIC PARTICLE TRACER EXPLORERS PROGRAM The Active Magnetospheric Particle Tracer Explorers (AM PTE) mission will release and monitor tracer ions (lithium and barium) in the solar wind and within the distant magnetosphere in order to study access of solar wind ions to the magnetosphere and the processes that transport and accelerate magnetospheric particles. In addition, a single massive release of barium in the dawn magnetosheath will create a visible artificial comet in the flowing solar wind plasma within which studies of a num­ ber of different plasma effects will be made. The AM PTE will also obtain comprehensive measure­ ments of the elemental composition and dynamics of the natural magnetospheric particle popula­ tions. INTRODUCTION AND BACKGROUND medium nuclei (carbon, nitrogen, and oxygen ions)6,7 did not resolve the source question, because the The magnetosphere is that large volume of space observed proton-to-helium and helium-to-medium around the earth dominated by the earth's magnetic energetic particle ratios were dissimilar to ratios in field. The hot solar wind plasma flowing outward either solar wind or ionospheric source plasmas. This from the sun is diverted around this obstacle at the fact, however, led to the recognition that source magnetopause (the magneto hydrodynamic boundary elemental abundances are subject to modification by between the earth's field and the solar wind), and dis­ transport and energization processes within the mag­ tinctly separate plasma populations and processes netosphere. Extensive plasma composition studies characterize the space environment inward of that over the last decade have revealed that the relative surface. Ever since the discovery of the intense Van contributions of the ionospheric and solar wind Allen radiation belts around the earth, there has been sources are extremely variable and that either can 8 great interest in the physics of magnetospheric dominate at any given time. ,9 Because of the com­ plasma populations-including their sources, their plexity of the natural system, injection of artificial interactions with the solar wind and the rotating "tracer" ions at a known location and time offers a earth, and the processes that accelerate particles to promising tool for attacking the issues of plasma en­ high energy. The solar wind was early thought to be a try, acceleration, transport, and loss within the possible source,I -3 and Nakada et al. 4 proposed that magnetosphere. the region of access was the magnetospheric boun­ In June 1971, one of us (SMK) wrote to NASA dary, with subsequent inward diffusion and energiza­ Headquarters suggesting the idea of a tracer ion tion. The limited data available in the mid-60's were release in the solar wind, combined with an appropri­ thought to support this hypothesis, and the dif­ ately instrumented spacecraft inside the magneto­ fusion-convection model has so far provided the sphere to detect the released ions as they were trans­ main theoretical framework for explaining particle ported through the system. A feasibility study was energization in the near-earth magnetosphere. undertaken by APL and the Max Planck Institute for Nevertheless, increasing appreciation of the com­ Extraterrestrial Physics, with support from NASA plexity of the magnetospheric system has resulted in a and the Bundesministerium fur Forschung und Tech­ continuing search for ways of identifying the sources nologie. The AMPTE project was selected for inclu­ of magnetospheric plasma and of testing particle ac­ sion in the NASA Explorer program in 1977 and, celeration and transport models. The discovery that a after an extended mission definition study, the hard­ significant component of the near-earth hot magne­ ware phase began in 1981. The program has two pri­ tospheric plasma was of ionospheric originS further mary spacecraft - the Ion Release Module (lRM) and complicated the situation. Spacecraft measurements the Charge Composition Explorer (CCE), which will of geomagnetically trapped alpha particles and be launched into elliptical orbits of apogee 18.7 and Volume 4, Number 1, 1983 3 S. Krimigis et al. - AMPTE Program 9.0 earth radii, respectively. A supporting instru­ lective interactions with the ambient plasma. Specifi­ mented spacecraft, the United Kingdom Sub satellite cally, the barium release in the magnetosheath (and (UKS), positioned a few hundred kilometers from the to a lesser extent, the releases in the magnetotail) will IRM, will provide two-point in situ plasma diagnos­ create an artificial comet. The formation and evolu­ tic measurements. All three spacecraft are currently tion of the diamagnetic "coma," or head, of our ar­ scheduled for launch on a Delta 3924 vehicle from tificial comet and the erosion of plasma to form a the Kennedy Space Center in early August 1984. comet "tail" will be monitored at the point of release AMPTE represents a unique international collabora­ by the IRM and UKS instrumentation, and remotely tive effort-it is the first program in which three na­ by ground optical observations. An artist's concept tions have each provided a spacecraft to carry out a of the releases is shown in Fig. 1. coordinated scientific mission. The measurement of the detailed elemental and charge composition of the naturally occurring ener­ getic particles (-0.05 kiloelectronvolt (keV) per charge to ~ 6 megaelectronvolts (MeV» within the AMPTE - Active Magnetospheric Particle Tracer magnetosphere makes possible the study of the de­ Explorers pendence of ion acceleration and transport processes CCE - Charge Composition Explorer built by The on the ion nuclear mass and charge. 10 An immediate Johns Hopkins University Applied Physics and important by-product of such measurements will Laboratory be the determination of the heretofore unmeasured II IRM - Ion Release Module built by the Max ion composition of the quiet and storm-time ring cur­ Planck Institute for Extraterrestrial Physics rent distributions at energies greater than 30 ke V. In UKS - United Kingdom Subsatellite built by the addition, measurements of the naturally occurring Rutherford Appleton Laboratory and Mullard tracer elements and element ratios in the magneto­ Space Science Laboratory sphere eHII H, CIO, 4He/2°Ne, etc.) will allow sources of radiation-belt particles to be inferred. Selection of an element as a plasma tracer involves a variety of both physical and practical considera­ SCIENTIFIC RATIONALE tions. For example, the seeded element should be rare AND OBJECTIVES in the natural environment in order to be distinguish­ The broad scientific objectives of the AMPTE pro­ able. It must have a sufficiently large charge-to-mass gram may be summarized as follows: ratio to maximize the diffusion coefficient. 12 It must have a time scale for photoionization long enough to 1. To investigate the transfer of mass from the allow the neutral gas to expand over a substantial solar wind to the magnetosphere and its further volume of space before ionization occurs, so that transport and acceleration within the magneto­ perturbation of the local medium via a diamagnetic sphere; cavity can be kept small, yet short enough for plasma 2. To study the interaction between artificially in­ to be injected at reasonable densities. The element jected and natural space plasmas; must have a sufficiently low evaporation temperature 3. To establish the elemental and charge composi­ for it to be readily vaporized by standard chemical tion and dynamics of the charged-particle pop­ methods. These considerations led to the selection of ulation in the magnetosphere over a broad en­ the element lithium, with a photoionization time con­ ergy range (a few electron volts to a few mega­ stant of about 1 hour. electronvolts) ; On the other hand, the criteria for interaction 4. To explore further the structure and dynamics studies between an injected and a natural plasma dif­ of ambient plasmas in the magnetosphere, par­ fer in some important aspects from those established ticularly in the boundary regions. for tracer element releases. Where an artificial comet Objectives 1 and 2, the principal scientific objec­ or an extensive diamagnetic cavity is the objective, it tives of the program, are to be carried out by actively is necessary to have a short ionization time constant. releasing tracer elements (lithium and barium) into The element most suitable for this aspect of the sci­ the solar wind, into the magnetosheath, and into the ence objectives is barium, with a photoionization distant magnetotail, and by detecting and monitoring time constant of 28 seconds. the resulting tracer ions at both the point of release Thus, the elements selected for the AM PTE and much closer to the earth within the magneto­ releases are lithium and barium, the former for use as sphere. These measurements will provide informa­ a tracer in the solar wind and magnetotail releases tion on the access of solar wind ions to the magneto­ and the latter for use both in the creation of the arti­ sphere, the mechanisms and points of entry into the ficial comet and in magnetotail plasma interaction magnetosphere, the degree of energization and loss and tracer releases. of solar wind particles, and convective motions with­ in the magneto tail. Although the tracer ions will act EXPERIMENT CONCEPT as individual test particles, the density will be