The three Rice Suprathermal Ion Detector Experiments have detected an extensive region of low energy plasma flow antisunward along the ordered field lines in the lobes of the geomagnetic tail at lunar distances. This particle regime resembles the “boundary layer” and “plasma mantle” observed at smaller geocentric distances and an interior flow region parallel to the magnetopause in the dayside magnetosphere. It possesses plasma characteristics uniquely different from the plasma sheet and magnetosheath. Spatially the particle regime is found exterior to the plasma sheet across essentially the entire tail and adjacent to the magnetopause on both the dawn and dusk sides of the magnetosphere. The integral flux varies from 105 to 108 ions/cm² sec ster with the differential flux peak between 50 and 250 eV/q. Temperatures range from 4 × 104 to 5 × 105 °K and number densities from .1 to 5/cm³.

We propose a mechanism whereby singly charged heavy ions from the plasmasphere are convected intermittently to the dayside magnetopause, accelerated there, swept into the distant tail lobes and boundary layer and convected earthward in the plasma sheet to re‐enter the inner magnetosphere. The outer region of the plasmasphere is known to be stripped off during periods of enhancement of the convection electric field. These cold plasmaspheric ions drift toward the dayside magnetopause where they may become accelerated and co‐mingled with magnetosheath plasma either through magnetic merging or some viscous process operating at the magnetopause. If the ion motion at the magnetopause is poleward, the ions may become part of the entry layer, entering the cusp and exiting as part of the plasma mantle. The cross‐tail electric field will cause them to drift into the lobes toward the center of the tail, ultimately reaching the plasma sheet in the distant tail. If the ion motion at the dayside magnetopause is equatorial, the plasmaspheric ions become part of the boundary layer, again ultimately reaching the plasma sheet in the distant tail. Once in the plasma sheet some of the heavy ions may flow back toward the earth and re‐enter the magnetosphere as energetic ions.

The United States/Canadian ISIS‐1 and ISIS‐2 satellites collected several million topside ionograms in the 1960s and 1970s with a multinational network of ground stations that provided good global coverage. However, processing of these ionograms into electron density profiles required time‐consuming manual scaling of the traces from the analog ionograms, and as a result, only a few percent of the ionograms had been processed into electron density profiles. In recent years an effort began to digitize the analog recordings to prepare the ionograms for computerized analysis. As of November 2002, approximately 390,000 ISIS‐1 and ISIS‐2 digital topside‐sounder ionograms have been produced. The Topside Ionogram Scaler With True Height Algorithm (TOPIST) program was developed for the automated scaling of the echo traces and for the inversion of these traces into topside electron density profiles. The program is based on the techniques that have been successfully applied in the analysis of ground‐based Digisonde ionograms. The TOPIST software also includes an “editing option” for manual scaling of the more difficult ionograms, which could not be scaled during the automated TOPIST run. TOPIST is now successfully scaling ∼60% of the ISIS ionograms, and the electron density profiles are available through the online archive of the National Space Science Data Center at ftp://nssdcftp.gsfc.nasa.gov/spacecraft_data/isis/topside_sounder. This data restoration effort is producing a unique global database of topside electron densities over more than one solar cycle, which will be of particular importance for improvements of topside ionosphere models, especially the International Reference Ionosphere.

Recent analysis has confirmed and expanded the characterization of the lobe plasma, the extension of the ‘boundary layer’ and ‘plasma mantle’ to lunar distances. Careful statistical analysis has verified that the observation of the ‘lobe plasma’ is strongly correlated with the y component of the Interplanetary Magnetic Field (IMF). When the moon is in the dawnside of the northern lobe or duskside of the southern lobe, the probability for observation of the lobe plasma is greatly increased when, in the hour preceding, the IMF has had a positive y component. Conversely, when the moon is in the duskside of the northern lobe or dawnside of the southern lobe, the probability for observation is much increased when the IMF has a negative y component. Analysis of lobe plasma data in conjunction with high time resolution IMF data has shown the probability of observation also is greater with a southward pointing IMF. The observed correlations with the y and z components the IMF reflect the fact that the asymmetry and changes in magnitude of the polar cap electric field induced by the IMF extends to lunar distances and determines the depth into the tail to which the ions can drift. Generally, the lobe plasma is observed sporadically for a full day after the moon has entered the tail and a full day before the last magnetopause crossing as it exits the tail. An average extent of ∼8–10 RE inward from the magnetopause is inferred; however, the lobe plasma has been seen all across the tail.

Key Points:• Soft X-ray imaging infers total lunar limb column density • The SWCX signal is dominated by exospheric species of solar wind origin • SWCX with the lunar exosphere was observed by ROSAT Abstract We analyze the Röntgen satellite (ROSAT) position sensitive proportional counter soft X-ray image of the Moon taken on 29 June 1990 by examining the radial profile of the surface brightness in three wedges: two 19 • wedges (one north and one south) 13-32 • off the terminator toward the dark side and one wedge 38 • wide centered on the antisolar direction. The radial profiles of both the north and the south wedges show significant limb brightening that is absent in the 38 • wide antisolar wedge. An analysis of the soft X-ray intensity increase associated with the limb brightening shows that its magnitude is consistent with that expected due to solar wind charge exchange (SWCX) with the tenuous lunar atmosphere based on lunar exospheric models and hybrid simulation results of solar wind access beyond the terminator. Soft X-ray imaging thus can independently infer the total lunar limb column density including all species, a property that before now has not been measured, and provide a large-scale picture of the solar wind-lunar interaction. Because the SWCX signal appears to be dominated by exospheric species arising from solar wind implantation, this technique can also determine how the exosphere varies with solar wind conditions. Now, along with Mars, Venus, and Earth, the Moon represents another solar system body at which SWCX has been observed.