Active Magnetospheric Particle Tracer Explorers Research Articles

AMPTE/UKS Ion Experiment observations of lithium releases in the solar wind

The two Active Magnetospheric Particle Tracer Explorers (AMPTE) lithium releases outside the bow shock were made in quite different conditions in the solar wind. The first took place in a high speed stream in the presence of a diffuse distribution of upstream ions which disturbed the solar wind. Lithium ions were only detected for 15 seconds. During the second release the solar wind flow was much smoother and slower. The lithium ions were observed for 4 minutes. In that time the energy of the detected ions increased from 10 eV up to almost 3 keV. This is consistent with a shell‐like expansion of the neutral lithium cloud and acceleration of the ions in the convection electric field. The solar wind slowed down and was deflected as it passed through the ion cloud.

Analysis and interpretation of the shocklike electrostatic noise observed during the AMPTE Solar Wind Lithium Releases

During the Active Magnetospheric Particle Tracer Explorers (AMPTE) solar wind lithium release on September 11, 1984, and again on September 20, 1984, an intense burst of electrostatic noise was observed near the upstream edge of the ion cloud. Comparisons with measurements by the IMP 6 and ISEE 1 spacecraft show that the spectrum and overall features of this noise are very similar to electrostatic noise observed at the earth's bow shock. A stability analysis using realistic parameters shows that the electrostatic noise can be accounted for by an ion beam‐plasma instability caused by the solar wind proton beam streaming through the nearly stationary lithium cloud. The growth rate of this instability is largest when the ion density and solar wind proton density are similar, which explains why the noise only occurs near the outer edge of the ion cloud. The similarity to the noise in the earth's bow shock suggests that a shock may exist in the solar wind plasma flow upstream of the ion cloud. If the noise is associated with a shock, then it must be an electrostatic shock, since the ion cyclotron radii are too small for the existence of a MHD shock. Since the electrostatic instability occurs at phase velocities near the lithium thermal velocity, the electrostatic turbulence may play a role in heating the lithium ions and transferring momentum from the solar wind to the ion cloud. The noise may also play a role in the pitch angle scattering and diffusion of energetic electrons observed in the vicinity of the ion cloud. Because of the similarity to the solar wind interaction with the gaseous envelope of a comet, it is expected that similar types of wave‐particle effects may occur upstream of comets.

Ion release experiments in the solar wind

In a new departure in experimental Space Plasma Physics, lithium and barium plasmas were injected into the solar wind approximately 110000 kms above the Earth's surface. The effects, monitored in-situ by two spacecraft of the AMPTE (Active Magnetospheric Particle Tracer Explorers) mission, included the formation of diamagnetic cavities, a slowing down locally of the solar wind, generation of plasma waves and heating of solar-wind electrons. These comet-like interactions between the solar wind and obstacles presented by the injected plasmas are discussed and compared with the effect of the natural obstacle of the Earth's magnetosphere first encountered by the solar wind at the bow shock. Particular reference is made to the 'artificial comet' created by a barium-ion release on 27 December 1984.

Ionospheric response to changes in the interplanetary magnetic field observed by EISCAT and AMPTE–UKS

A major question in solar–terrestrial physics concerns the nature of the interaction between the solar wind and the Earth's magnetosphere and ionosphere, and how this depends on the direction of the interplanetary magnetic field (IMF). We report here simultaneous observations of the IMF by the AMPTE–UKS (Active Magnetospheric Particle Tracer Explorers–UK Satellite), and of plasma flow in the high-latitude ionosphere by the EISCAT radar with better time resolution than has been achieved hitherto. We discuss one event in which a sharp southwards turning of the IMF is followed by the onset of rapid ionospheric plasma flow near local noon, demonstrating the close control of ionospheric flows by the IMF. We discuss the causes of the observed 13-min delay, and study the ionospheric heating that accompanies the acceleration.

An active current sheet in the solar wind

The supersonic solar wind possesses a variety of simple small scale (<10-min observation duration) phenomena, including discontinuities1–3 and magnetic holes4,5. We report here recent AMPTE-UKS6 (Active Magnetospheric Particle Tracer Explorer–UK Satellite) observations, however, which reveal a dramatic new complex structure within the solar wind flow. In the plasma and field observations of this magnetic hole/current sheet system, the layers surrounding the strong rotation in the magnetic field contain compressed and heated solar wind, an enhanced field strength and considerable wave activity. The interior region shows strong ion heating but with ambient densities and a fluctuating magnetic field which dips to low values. The bulk flow velocity inside is deflected sharply from the anti-sunward direction, but not in the direction predicted by magnetic stresses across the boundary. The structure is expanding in thickness and is relatively young. The source of momentum and energy for this event remains unclear, and could even be magnetospheric.

AMPTE completes first phase

Early on July 18, 1985, the Ion Release Module (IRM) spacecraft of the Active Magnetospheric Particle Tracer Explorers (AMPTE) mission released its last two cannisters of barium‐thermite mixture into the rapidly flowing solar wind plasma in the dusk‐side magnetosheath, about 110,000 km above the earth. The result was a glowing barium ion “comet,” high in the night sky, marking the last experiment in the first, or active plasma release, phase of the mission.

Results of the early orbit manoeuvres of the ampte-uks

The Active Magnetospheric Particle Tracer Explorers (AMPTE) program consists of three satellites which were launched on 16th August 1984. The scientific aim of the mission is to inject lithium and barium tracer ions inside and outside the Earth's magnetosphere and to detect and monitor these ions as they diffuse through the inner magnetosphere. The first of these three satellites, the U.S. Charge Composition Explorer (CCE) was launched into an elliptical orbit of apogee 8 R e . The other two satellites are the West German Ion Release Module (IRM) and the U.K. Subsatellite (UKS), both of which were launched on the same vehicle into a highly elliptical orbit of apogee 18 R e . At discreet intervals during the mission the IRM will release ions into the solar wind, and the movement of these ions will be monitored by the UKS. Depending on the particular scientific requirement, the UKS has to be positioned accurately at a given distance behind the IRM. Initially the UKS has to be located 100 km behind the IRM, and held there for ∼9 months. It will then be moved a distance of ∼1 R e behind the IRM. In order to manoeuvre the UKS around its orbit, a cold gas jet system is incorporated on the satellite, allowing impulses to be applied both along and perpendicular to the orbit velocity vector. The orbit control system also has to cater for relative orbit changes due to air drag at perigee, as the IRM and the UKS have different area mass ratios. This paper presents an account of the orbit control system implemented on the UKS, together with the mathematical approach adopted, and results from manoeuvres made in the first weeks of the mission.

The Ampte charge Composition Explorer and the 4-7 September 1984 geomagnetic storm

The orbit and payload of the AMPTE (active magnetospheric particle tracer explorer) Charge Composition Explorer (CCE) satellite, which provide an exellent opportunity for observing the geomagnetically trapped particle population in general and the earth's ring current in particular, are described. A global view of the September 4, 1984 geomagnetic storm is presented and the initial findings of the CCE investigators on the earth's ring current are given. It is shown that the CCE ring-current results pertain to an unusual asymmetric ring current and may not be applicable to a more typical ring-current development and decay.

The Fluxgate Magnetometer for the AMPTE UK Subsatellite

The fluxgate magnetometer flown on the Active Magnetospheric Particle Tracer Explorers United Kingdom Subsatellite (AMPTE UKS) spacecraft and its operation are described. The instrument is a modified flightspare from the ISEE 1/2 mission and shares many features with the ISEE instruments. During routine operations data are returned to the United Kingdom Control Centre and displayed in real time in a geophysical coordinate system. Instrument and data system are all operating successfully.

The AMPTE CCE Spacecraft

The flight segment of the Active Magnetospheric Particle Tracer Explorers (AMPTE) Program consisted of three separate spacecraft which were launched "piggyback" into orbit aboard a Delta 3924 launch vehicle, from Cape Canaveral, FL, on August 16, 1984. The three spacecraft are the Charge Composition Explorer (CCE), built for NASA by the Applied Physics Laboratory of The Johns Hopkins University (APL/ JHU); the Ion Release Module (IRM), built in the Federal Republic of Germany; and the United Kingdom Subsatellite (UKS), built in the United Kingdom. This paper describes the CCE Spacecraft design, development, and early performance in orbit.

Outline of the Active Magnetospheric Particle Tracer Explorers (AMPTE) Mission

This paper is intended as an introduction to a series of papers describing the three satellites of the AMPTE mission and their instrumentation. The aims and scientific context of the program are given together with a comparison of the general characteristics of the three spacecraft and their orbits. There is a brief resume of the studies performed so far, and a statement of future plans, together with a Calendar of completed and planned experiments and measurements.

Space Physics: UK part of AMPTE fails

After five months of faultless operation, the UK satellite (UKS) of the Active Magnetospheric Particle Tracer Explorers (AMPTE) mission (see Physics Bulletin November 1984 p461) seems to have come prematurely to the end of its useful life. On 16 January, the Operations Control Centre at the Rutherford Appleton Laboratory failed to detect a signal from the satellite as it came over the horizon.

Coupling of newborn ions to the solar wind by electromagnetic instabilities and their interaction with the bow shock

The process by which the solar wind assimilates newly ionized atoms is important for understanding the presence of planetary or interstellar helium in the solar wind, the dynamics of the Active Magnetospheric Particle Tracer Explorers (AMPTE) lithium releases in front of the earth's bow shock, and the formation of cometary tails. In this paper we examine how newborn ions can be coupled to the solar wind in the direction parallel to the magnetic field by means of electromagnetic instabilities driven by the distribution of newborn ions. The linear properties of three instabilities are analyzed and compared with numerical solutions of the linear dispersion equation, while their nonlinear behavior is followed by means of computer simulation to obtain the characteristic time for the pickup process. With a primary emphasis on the AMPTE lithium releases, various degrees of realism are introduced into the calculations to model the upstream conditions and the intersection of the lithium with the bow shock. It is shown that a time‐dependent shock model is needed to correctly reproduce the amount of lithium which is transmitted through the shock and that the resulting lithium ion distribution is still likely to be subject to the same type of instabilities in the magnetosheath. Application of these results to comets, in particular the artificial comet expected to be generated by the AMPTE barium release in the magnetosheath, is also briefly discussed.

The plasma physics of the Active Magnetospheric Particle Tracer Explorers (AMPTE) mission

The three-spacecraft AMPTE Mission was due to commence in August 1984. The purpose of these 'active' experiments was firstly to release lithium into the solar wind and to use the lithium ions, formed by photoionisation, as 'test particles' to investigate their crossing of the bow shock, their entry into the magnetosphere and subsequent energisation. Secondly, it was to release barium in the dawn magnetosheath to create an artificial comet. Thirdly, it was to inject both lithium and barium into the tail of the magnetosphere to study plasma convection. Observations were to be made both on the spacecraft and from the ground. These would provide information on the plasma physics of the natural plasma environment of the Earth and also of the formation and evolution of an expanding diamagnetic cavity. Particular attention was to be paid to the various plasma instabilities and wave phenomena anticipated in these experiments, and to the natural waves which would be observed.

The entry of AMPTE lithium ions into a magnetically closed magnetosphere

In a magnetically closed magnetosphere, none of the geomagnetic field lines intersects the magnetopause. Solar wind particles incident upon such a field topology are normally thought to be specularly reflected and therefore none gains entry to the magnetosphere. Strictly speaking, however, specular reflection occurs only when the incident charged particle encounters a uniform magnetic field. Although the geomagnetic field at the magnetopause does appear quite uniform to incident particles (its scale size is much greater than the gyroradii of solar wind particles), it possesses a small gradient parallel to the magnetopause. When this gradient is taken into account, a fraction of the incident particles can gain access to the magnetosphere. This gradient drift entry (GDE) process is a feature of the basic structure of the geomagnetic field and should occur at all times. It is therefore expected that the Active Magnetospheric Particle Tracer Explorers (AMPTE) lithium ion releases will penetrate certain regions of the magnetopause. An excellent test of the GDE process could be made by initiating a lithium release during a northward interplanetary magnetic field (when the magnetosphere is thought to be magnetically closed).

Ion propagation in the magnetosheath

The transport of test ions through the magnetosheath depends upon the macroscopic structure defined by the average magnetic and flow fields, B0 and U, and the fluctuating fields, δE and δB, encountered by the particles. In this paper an approximate model is used to generate the macroscopic configuration: it satisfies the jump conditions at the bow shock, treats the magnetopause as a tangential discontinuity (B0 and U tangent to the magnetospheric boundary), and uses intermediate field amplitudes and orientations that resemble the more precise results of gas dynamic simulations. Equations of motion for a fictitious ensemble average particle describe the ion propagation, with the effects of the inhomogeneous turbulence phenomenologically incorporated into a friction tensor whose components reflect the coupling of the ions to the bulk plasma along and across the magnetic field B0. Application to the solar wind releases of lithium ions by the Active Magnetospheric Particle Tracer Explorers mission illustrates the approach: we follow the guiding center trajectory of the ensemble average “particle” for several upstream solar wind and interplanetary magnetic field conditions, different locations of the release site, and quiet and disturbed magnetosheath turbulence regimes. Implications of the results for the release strategy are discussed.

Temporal evolution of whistler growth in a cold plasma injection experiment

Using a simple time dependent cold plasma density model and assuming a typical ambient radiation belt environment, we study the evolution of the whistler mode turbulence and particle precipitation in a cold plasma release experiment similar to one that may be conducted as part of the Active Magnetospheric Particle Tracer Explorers (AMPTE) program. It is known from earlier work that the release of cold lithium ions can significantly lower the critical energy Ec above which the resonant radiation belt electrons can pitch angle scatter. We study the time evolution of the one pass gain factor for a whistler wave packet and find that for parameters accessible to AMPTE type experiments the gain factor is large enough to ensure strong whistler turbulence and strong pitch angle diffusion of radiation belt particles with energies in the range between the ambient value of Ec and the reduced value of Ec. Estimates for the total power input to the ionospheric footprint of the release are of the order of an erg/cm²/s. This precipitated energy should produce a patch of visible aurora. This effect should also persist for many hours.

Acceleration of lithium test ions in the quiet time geomagnetic tail

One of the active experiments to be performed as part of the Active Magnetospheric Particle Tracer Explorers mission involves the release of lithium ions in the geomagnetic tail and their subsequent detection after earthward convection into the nightside outer ring current region. In this paper we have used the guiding center approximation to integrate ion trajectories in a simple two‐dimensional model of the quiet time nightside magnetosphere in order to estimate expected ion properties at the latter location. Our principal conclusion is that under typical quiet time conditions these ions will predominantly form a field‐aligned beam (∼5°–20° pitch angle) at energies from a few hundred eV to ∼1 keV.

Modification of pitching stability by an atmospheric wave

References G., Execution Phase Project Plan for Magnetospheric Particle Tracer Explorer (AMPTE), NASA, Aug. 1980. Thompson, R., and Neal, G., Active Magnetospheric Particle Tracer Explorer (AMPTE) Engineering Data Simulator Description and User's Guide, Computer Sciences Corporation, CSC/TM81/6069, April 1981. Neal, G., and Thompson, R., Inflight Estimation of Magnetometer Biases for the AMPTE Mission, Computer Sciences Corporation, CS/TM-81/6070, May 1981. Wertz, J.R. (ed.), Spacecraft Attitude Determination and Control, D. Reidel Publishing Company, Dordrecht, The Netherlands, 1978, pp. 329-330. Werking, R.D., A Generalized Technique for Using Cones and Dthedral Angles in Attitude Determination, NASA X-581-73-292, Sept. 1973. Shuster, M.D., Efficient Algorithms for Spin-Axis Attitude Estimation, Flight Mechanics/Estimation Theory Symposium, NASA Goddard Space Flight Center, Greenbelt, Md., Oct. 1981.

Modeling of interaction of artificially released lithium with the Earth's bow shock

A numerical simulation is used to predict the interaction between the earth’s bow shock and lithium to be released in the solar wind during the Active Magnetospheric Particle Tracer Explorers program in 1984. Based on the simulation results for a release near the subsolar point, it is recommended that a favorable release condition is when the garden‐hose angle of the interplanetary magnetic field exceeds about 60 to 70° so that the transmission of Li+ ions through the bow shock is optimized. The model also predicts that the Li+ is “shock‐drift” accelerated at the bow shock. The energy gain of Li+ ions is on the average less for the transmitted particles (below a factor of about 4) than for the reflected particles (below a factor of about 16).