Vol. 4, No. 5 Geophysical Research Letters May 1977 HEAV• ION CIRCULATION IN THE EARTH'S MAGNETOSPHERE J. W. Freeman, H. K. Hills, T. W. Hill, and P. H. Reiff Department of Space Physics and Astronomy, Rice University, Houston, Texas 77001 D. A. Hardy Air Force Geophysics Laboratory, HanscornAFB, Massachusetts 01731 Abstract. 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 odter region of the plasmasphere is known to be earth's ionosphere. The question thus arises as to how ions of ionospheric origin come to co-exist with solar wind ions in the tail lobes. The Circulation Mechanism Freeman [1968] first reported the sporadic flow of cold ions toward the dayside magnetopause stripped off during periods of enhancement of the near the equator. These intense ion flows were convection electric field. These cold found to occur during the early main phases of plasmaspheric ions E•x B•drift toward the dayside magnetic storms. They were understood to 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 represent the detachment of the outer regions of the plasmasphere as the convection electric field increases and the outer plasmasphere particles find themselves on open drift paths [Kavana•h• Freeman• and Chen, 1968; Chappell, 1972]. These regions have since become known as detached plasma regions [Chappell, 1974]. What happens to these detached plasma regions as they approach the magnetopause? ßFreeman• Warre. n • and Ma•uire [1968] found evidence during one storm that, in the equatorial plane, the cold ions were turned aside to flow parallel to the magnetopause away from the sun. The plasmaspheric ions thus become part of the distant boundary layer as observed by Frank et el. [1977]. They plasma sheet some of the heavy ions may flow back ultimately reach the plasma sheet along the flanks toward the earth and re-enter the magnetosphere as energetic ions. Introduction Frank• Ackerson, and Yeager [1977] have reported evidencefor O '+ ions in the boundary layer of the magnetospheric tail. This evidence is based on a secondary high-energy (4 several keV) peak in the flowing plasma. Recently Hard2•.. Freeman• and Hills [1977] have presented similar evidence for singly charged ions of mass 14-16 AMU in the plasma in the lobes of the magnetospheric tail. In this case the energies of the secondary peak range from 1 to 2.7 keV. [e.g. Hones and WinninKh. am, 1976]. On the other hand, if the plasmaspheric ions approach the magnetopause in a region where magnetic merging is taking place, they will be drawn into the merging region by E• x B• drift and then ejected orthogonally along magnetic field lines. At this point, the plasmaspheric ions will mix with magnetosheath plasma that has approached the merging region from the opposite side. The poleward-streaming ions will enter the cusp and be mirrored back up to form the transpolar plasma mantle [Reiff• Hill• and Burch, 1977; Hill and.. Reiff, 1977]. The convection electric field will E x B drift the ions toward the center of the ta•l w•ere they may become part The lobe plasma is generally believed to be an of the plasma sheet. Figure 1 illustrates the extension of the plasma mantle formed by the •E x B• convection of magnetosheath plasma into the tail lobes [Hardy; Hills; and Fr..eeman, 1975; Rosenbauer; Gr•nwaldt, Montgomery,Paschmann, and Sckopke, 1975; Hardy; Freeman• and Hills, 1976; Hil!s • Hardy• and Fre.e.man, 1976; Pilipp and Motfill, 1976; Crooker, 1976]. If the interpretation of the secondary peak as heavy ions is correct, their singly charged state indicates that the heavy ions originate in the Copyright 1977 by the American Geophysical Union. proposed flow pattern. This mechanism will lead to the heavyions seenin th e lobes by Hardy et el. [1977]. Pilipp andMorfii1 [1976] have shown that the plasma mantl e (and hence the lobe plasma) can provide an adequate particle source to populate the plasma sheet. The plasma sheet frequently exibits strong flows toward the earth, especially during magnetically active times [e.g. Hones• Ashridge, Berne, and Singer, 1973]. Thus heavy plasmaspheri c ions may ultimately be transported back to the inner magnetosphere where they may Paper number 7L0208. 195