Mapping the Heliosphere: 3-D Trajectory of Solar Electrons In following solar corpuscular emissions away from the sun, simultaneous radio triangulation between the WIND and ULYSSES spacecraft was used for the first time to determine remotely the 3-D trajectory in interplanetary space of fast moving solar electrons. These energetic electrons ejected from the sun follow the interplanetary magnetic field lines and generate radio emissions as they travel. The radio trajectory indirectly measures both the interplanetary magnetic field topology and the speed of the electron beam. Measurements of the exact wavelengths of the radio emissions also provide the interplanetary plasma densities along the path of the electron beam. The dual spacecraft observations of the exciter electron speeds, radio beaming geometry and emission brightness temperature, permit tight constraints on possible emission mechanisms responsible for the solar radio bursts.
Lunar Wake Physics WIND investigators have vigorously exploited the spacecraft’s traversals of the lunar wake with the result that the prevailing MHD picture of the wake has been greatly modified. Instead of a trailing shock wave 2 or 3 RL behind a fill-in region, the wake appears far more complicated with its great extension, surprisingly stable ion beams driven by flanking ambipolar electric fields, and trailing electrostatic wave activity. Motivated by these new observations, it has become clear that a kinetic approach to modeling the region is needed. Further studies will only enhance our understanding of this interesting plasma laboratory and, in addition, clarify the wake physics of small bodies, for example, Deimos, Phobos, asteroids etc., and their interaction with the solar wind. Such studies will increase the scientific yield of planned missions to such bodies.	Figure 2-5. WIND Unique Accomplishments: Tracing out the spiral structure of the IMF remotely from type III radio bursts. 3-D measurements of the solar wind distribution functions and energetic particles, including composition. Radical improvement in our understanding of the lunar wake, with its extended electrostatic tail, plasma wings, surprisingly stable ion beams. Multipoint observations of the heliospheric current sheet structure. Isotopic composition of solar wind and energetic particles. Measurements of ionosphere modification by radio waves, transmitted from the HAARP facility in Alaska. High resolution observations of gamma ray transients. Observations of galactic center radiation - potential signature of a massive black hole.
Geoeffectiveness of Solar Events With the importance of magnetic clouds established, the question of the "geoeffectiveness" of different magnetic clouds has become important. Data from the fleet of ISTP/GGS spacecraft were used in a comparison of the effects of three clouds. They show that geoeffectiveness depends on the amplitude, duration and abruptness of change of interplanetary parameters. However, the total energy input into the magnetosphere in the form of Poynting flux integrated over time seems to show a larger range than the various parameters used to measure the effect. During this period of low speed CMEs, the plasma density plays the major role in compressing the magnetopause. Studies of the effects of a fast stream overtaking a cloud can better be studied during solar minimum, since clouds during solar maximum tend to be isolated. Initial analyses of data from WIND, POLAR, SAMPEX and several other spacecraft have been used to determine the geoeffectiveness of magnetic clouds in driving relativistic electron acceleration within the magnetosphere. A two-step process has been suggested, whereby strong substorm activity caused by a large negative interplanetary magnetic field in the leading portion of the cloud produces a seed population of magnetospheric electrons. A high level and long duration of large-amplitude, low-frequency wave activity, which can occur concurrently, is an indication of a global wave field in which time varying magnetic fields are large. The induced electric fields could be very effective in accelerating electrons throughout the outer radiation zone. When these phenomena have been observed, the relativistic electrons display a global, coherent and intense acceleration.	Figure 2-7.
Bursty Bulk Flows Figure 2-8 An example of the utility of special spacecraft configurations is illustrated by the event on March 27, 1996, when WIND was in the dusk magnetotail near perigee and very close to GEOTAIL (4 RE apart), while IMP-8 monitored the solar wind. The IMP 8 measurements document the gradual input of magnetic flux to the magnetosphere (see figure) that is in contrast to the bursty conversion of stored energy into heated flowing plasmas seen by WIND and GEOTAIL. Furthermore, the flux transport associated with these bursts can be very different at the two spacecraft (especially at 1430 UT) demonstrating that the spatial scale of the bursts is a small fraction of the magnetotail dimension. Preliminary indications are that the brightenings seen in the global auroral images are well associated with the flow bursts.	GEOTAIL Unique Accomplishments: First fully 3-D mapping of the global magnetotail structure. Typical location of the nightside reconnection line established at 25 RE. Established the spatial distributions of low energy ions and wideband plasma waves in the distant tail. Discovered new kinetic effects of nightside reconnection - two electron and two ion populations. Detailed study of bursty bulk flows, first sampled by ISEE, as a key mechanism for redistribution of flux within the magnetosphere. Realization that ionospheric ions are a component of the tailward-flowing plasma, even in the distant tail. Realization that the 2-D plasmoid picture from ISEE-3 must be replaced by one of fully 3-D flux rope structures which have now been mapped by Geotail and modeled theoretically. Detection of abrupt jumps in electric field waveform measurements associated with electrostatic "noise". Extended and multiple crossings of bow shock and magnetosheath boundaries.
Magnetic Reconnection The process of magnetic reconnection is of fundamental importance to the magnetosphere and to astrophysical plasmas because it converts magnetic energy into kinetic energy. The ISTP/GGS spacecraft have observed reconnection or its effects at the equatorial magnetopause, over the polar cap and in the geomagnetic tail. On May 29, 1996, under northward IMF conditions, WIND detected an unusually large density enhancement which compressed the magnetopause sufficiently that POLAR near its 8 RE apogee at high latitude encountered the heart of the reconnection process between the strong northward magnetic field of the solar wind and the magnetic field from the Earth. For the first time a spacecraft passed from one ejection region through the region of interconnection (the "diffusion" region) and out the other ejection region. With POLAR the microphysics of this poorly understood mechanism which determines so much of the global rearrangement of magnetic flux in the magnetosphere has been probed.	Figure 2-9. Magnetic flux input (IMP-8) and transport (WIND and GEOTAIL) on March 27, 1996. Figure 2-10. POLAR UVI observations on March 27, 1996.
Sources of Magnetospheric Plasmas The answer to the important question of the sources of magnetospheric plasmas is emerging from GGS measurements. First, GEOTAIL detected the frequent occurrence of cool, tailward flowing H+, O+ (and sometimes even He+) ions along field lines of the tail lobes, with surprisingly high densities (up to 1/cc) that even exceed those of the plasma sheet. The ionosphere is the only possible source of O+ and He+ ions, where such ions have previously been observed moving upward during geomagnetically active times. POLAR has now detected these ions streaming along magnetic field lines at high latitudes over the polar cusp, thus connecting the ionospheric and magnetotail measurements. These ions form an important source of particles for the nightside plasma sheet as they drift slowly across field lines and toward the equator in the expected dawn to dusk tail electric field. Many of these first time measurements have been made possible by the incorporation aboard POLAR of a spacecraft potential control drive (PSI) which neutralizes the charge acquired by the spacecraft. GEOTAIL sampling of the He++ ions near the magnetotail boundary has also confirmed that solar wind/magnetosheath plasma is a contributing source to the plasma sheet population.	POLAR Unique Accomplishments: First tri-spectral global imaging of aurora simultaneously in visible, ultraviolet and x-ray wavelengths. Confirmed role of parallel electric fields in the acceleration of electrons into the ionosphere. Detection of thermal ion plasmas at high altitudes made possible by spacecraft charge neutralization. Found spatially extensive density cavities due to parallel electric fields over the auroral oval. Found new region of large parallel electric fields at distances >6 Re. Discovered major acceleration region in dayside cusp containing ions as energetic as 2 MeV. Found surprising appearances of energetic ions over the polar cap. Derived time-dependent ionospheric conductivity maps from auroral imaging at two wavelengths. Co-discovered a third tail of sodium atoms in Comet Hale-Bopp. Confirmed existence of atmospheric holes. Discovered large objects containing water streaking towards the Earth and dissipating in the atmosphere.
Ring Current The ring current development and decay are very important steps in the flow of energy through geospace. From its vantage point at high latitudes, POLAR has a unique panoramic view of the entire ring current that has allowed the first, time-dependent, global observations of the asymmetric properties of the ring current. In regions devoid of ion fluxes the energetic neutral atoms (ENAs) from charge exchange between energetic ring current ions and the ambient geocorona have been detected. When integrated globally these measurements show a remarkable measure of the strength of the ring current. This relationship, shown in Figure 2-11 for fifty days at the beginning of 1997, clearly identifies the magnetic storm resulting from the January 6 CME and a recurrent storm in February. In addition, energetic neutrals have been observed from substorm injection events on the nightside of the Earth.	Figure 2-11.
Theta Aurora A puzzling auroral form called the theta aurora, a double bar across the polar cap, has been interpreted to indicate a highly distorted magnetic tail, a bifurcation of the two open lobe cells. Now data from WIND, POLAR and SuperDARN have been used to develop a model that successfully explains this configuration. It predicts the form of the theta auroras after a southward turning of the interplanetary magnetic field or a large jump in the By component after a prolonged period of northward field. The transpolar arc lies on closed magnetic field lines and maps to the plasma sheet boundary layer. The analysis of in-situ plasma data placed with respect to the auroral configuration by auroral images indicates common electron distributions and ion composition in the theta aurora and the poleward zone of the nightside auroral oval. Plasma convection derived from the radar data exhibits a two-cell pattern that evolves into a turbulent state having antisunward flow in the transpolar arc.
Global Convection Patterns Another new technique developed within ISTP/GGS combines data from WIND and the ground-based SuperDARN radar network with model convection patterns to provide time-dependent high-latitude electrical potential patterns. These global "images" provide a dynamic view of the manner in which electric fields in the magnetosphere and high-latitude ionosphere respond to changes in the solar wind. Recently, this capability has played an integral role in many international campaigns supported by extended real time coverage of the WIND spacecraft.
Hemispherical energy flux The integrated hemispherical energy flux carried by auroral electron precipitation into the atmosphere is a measurable parameter of the rate of energy dissipation from the magnetosphere associated with auroral activity. This hemispherical power has been used for several years in time-dependent global models of the thermosphere and ionosphere. It has been derived from electron precipitation measurements made by polar orbiting satellites with extrapolations through models to the entire auroral oval. Now auroral images of the entire oval at ultraviolet wavelengths have been shown to provide substantial improvement in the temporal resolution of this parameter which is now consistent with the time scale of global auroral variability. Ultraviolet auroral images from POLAR are also being used as quantitative remote diagnostics of the auroral regions, yielding estimates of incident energy characteristics.
Theory and Modeling Modelers are now able to run global simulations, using measured solar wind parameters as input, that can be compared with GGS observations. A major step toward quantitative capability was realized in the MHD codes when it became computationally possible to move the tailward boundary to great distances (X=-300 RE), well beyond the point where the plasma flow becomes supersonic. This extension of the modeling domain allowed modelers to eliminate unwanted feedback from reflections at the back boundary. The simulation codes were checked for numerical accuracy and physics content against textbook cases, generic IMF orientations, and with code intercomparison. On a parallel basis, mission specific diagnostics and visualization tools were developed to cast the simulation results and observations in compatible terms. Furthermore, a complete end-to-end case study (the May 19-20, 1996 ISTP event), led by the theory group resulted in quantitative comparison between the ionospheric responses predicted by the MHD and MAMI simulations, and observations. This study has led to a better characterization of magnetosphere-ionosphere coupling that will be used to improve the models. Simulations and GEOTAIL data together have confirmed the great degree to which the IMF, particularly its north-south component, controls magnetotail structure. Long periods of steady northward IMF lead to a balloon-like tail that is virtually closed within 200 Re; under alternative IMF orientations, the magnetotail is usually very long and flaps in response to changes in the solar wind direction in the so called "wind-sock effect." The By component of the IMF twists the tail through angles that in the distant tail can probably approach 90 degrees. Coupling of fluid and kinetic models has allowed tracking of magnetotail ions, and shed some light on what fractions originate from the ionosphere, mantle and low latitude boundary layer. Another exciting result emerging from the theory effort is the suggestion that Poyting flux transferred from the solar wind into the magnetosphere becomes focused in the region around X=-10 RE resulting in strong adiabatic heating during the substorm growth phase. Observations from the March 9, 1995 ISTP event are consistent with this notion. Another hybrid model, one combining an MHD-radiation belt simulation, was used to study the Sudden Storm Commencement of March 24, 1991; the resulting dynamics and timing of the fluxes in the simulation were in excellent agreement with those measured by the CRRES satellite.	Theory and Modeling Unique Accomplishments Global movies of the response of the magnetosphere to solar wind variations of duration of days. Detailed maps of the origin of plasmas in the magnetosphere. Detailed coupling of magnetospheric input into a global atmospheric circulation model. Movies of global atmospheric response to substorms. Simulation of the birth of a radiation belt owing to strong changes in the solar wind. Detailed prediction of activity indices for the magnetosphere. Ground-Based Unique Accomplishments Ground-truth for interpretation of space based auroral imaging and resulting estimates of energy flux of electron auroral precipitation. Measurement of time-dependent electrical convection pattern Detailed tracking of ionospheric signatures of the dynamic boundaries of electron precipitation regions.