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On the Relationship of the Plasmapause to the Equatorward JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. All, PAGES 9059-9069, NOVEMBER l, 1982 On the Relationship of the Plasmapauseto the Equatorward Boundary of the Auroral Oval and to the Inner Edge of the Plasma Sheet J. L. HORWlTZ,1 W. K. Co• 2 C R BAUGHER,3 C R. CHAPPELL3 L A FRANK,4 T. E. EASTMAN,4 R. R. ANDERSON,4 E.G. SHELLEY,5 AND D. T. YOUNG6 ISEE 1 observations of the plasmapause are compared with simultaneous observations of the electron plasma sheet and also the auroral oval observed in DMSP photographs. Only a limited amount of appropriate data was available for the comparisons: the plasmapause/plasmasheet inner edge comparisonswere restricted to the early and late morning sectors, while there were two satisfactory comparisons of the plasmapauseand the equatorward boundary of the auroral oval in the evening sector. However, these examples indicate that the plasmapauselocation often coincides to within zXL- 0.1-0.2 with both the plasma sheet inner boundary and the field line threading the equatorward boundary of the auroral oval. This co-location of the plasmapauseand the plasma sheet inner edge may be due to shielding of the magnetosphericconvection electric field by an Alfv6n layer located at the inner edge of the plasma sheet as discussedby Jaggi and Wolf (1973) and others. INTRODUCTION sheet population and inhomogeneous ionospheric electrical The locations of both the plasmapauseand the earthward conductivities together cause a modification of the magneto- edge of the plasma sheet are sensitive indicators of large- spheric electric field distribution in which the earthward scale magnetospheric convection electric fields [Axford, edge of the plasma sheet coincides with an 'Alfv6n layer' 1969]. In steady state, the conceptual definition of the that tends to shield most or all of the externally induced plasmapause is that it is the boundary dividing flux tube convection electric field from penetrating into the inner trajectories that encircle the earth, owing to the dominant magnetosphere. Inside this layer the plasma motion may be influence of the earth's corotation electric field, and outside entirely dominated by corotation. Measurements of electric trajectories dominated by the cross-tail electric field. The fields in the ionosphere indicate high electric fields within earth-encirclingflux tubes are filled by plasma flow from the the region of diffuse particle precipitation, with a decrease to ionosphere, while plasma loss at the magnetopauseis approximately zero typically (but not always) seen at the thought to keep the flux tubes outside the plasmapauseat equatorward boundary of this precipitation region (e.g., low density levels. The plasma sheet inner boundary is Gurnett and Frank, 1973]. Identifying the precipitation re- consideredthe most earthward penetration of plasma con- gions with the feet of plasma sheet field lines, the electric vected in from the nightside magnetotail; this plasma is field change would coincide with the inner edge of the diverted around the earth, by corotation and magnetic field plasma sheet. In this case, the abrupt transition from a gradient and curvature drifts, in its transit toward the convection-dominated region to a corotation-dominated re- dayside magnetopause. gion, at the inner edge of the plasma sheet, would corre- Except for zero-energy particles, the simple dawn-dusk spond to the (steady state) plasmapauselocation as well. uniform convection electric field does not lead to coinci- There are surprisingly few published measurements of dence between the plasma sheet inner edge and the plasma- simultaneoussignatures of the plasma sheet inner edge and pause. However, calculations by many authors [Wolf, 1970; the plasmapause, whose location is generally identified Swift, 1971; Taylor and Perkins, 1971; Vasyliunas, 1970, operationally as that of a sharp plasma density gradient 1972; Jaggi and Wolf, 1973; Mal'tsev, 1974; Wolf, 1974; [Chappell, 1972]. Schield and Frank [1970] consistently Harel and Wolf, 1976; Harel et al., 1979, 1981a, b], in observed a separation of 1-5 Re between the plasmapause modeling magnetosphere-ionospherecoupling effects on and the plasma sheet inner edge near local midnight during electric fields, have shown that the presence of the plasma relatively quiet periods. Frank [1971], using a narrow maxi- mum in --•100eV electron fluxes to locate the plasmapause position, observed a coincidence in the plasmapause and • Departmentof Physics,The Universityof Alabamain Hunts- plasma sheet inner edge locations in the midnight-dawn ville, Huntsville, Alabama 35899. 2 Departmentof Physics,Brandeis University, Waltham, Massa- sector, whereas in the evening sector, an 'electron trough' or chusetts 02154. gap was often seen to separatethe two boundaries. Foster et 3 NASA SpaceSciences Laboratory, Marshall Space Flight Cen- al. [1978], in comparing the field line locations for the ter, Alabama 35812. whistler-observed plasmapause density gradient with the 4 Departmentof Physicsand Astronomy, The Universityof Iowa, ISIS 2 observationsof the equatorward edge of the trapped Iowa City, Iowa 52242. 5 LockheedPalo Alto ResearchLaboratory, Palo Alto, California (peak at 90ø pitch angle) and precipitating plasma sheet 94304. electron fluxes, as well as ionospheric densities and other 6 Los AlamosNational Laboratory,Los Alamos, New Mexico parameters, claimed that the equatorward boundary of the 87545. trapped electron fluxes at both dawn and dusk agreed with the whistler-deducedplasmapause. Linscott and Scourfield This paper is not subjectto U.S. copyright.Published in 1982by the American GeophysicalUnion. [1976], examining measurements made near •1ocat dusk, indicated that one plasmapausecoincided with the equator- Paper number 2A1154. ward boundary of the diffuse aurora to within AL --• 0.25. 9059 9060 HORX,VITZET AL' PLASMAPAUSE,PLASMA SHEET, AND AURORAL OVAL 104 _ /• BASEDON ENHANCED •. J- ) NOISE AT UPPER HYBRID • RESONANCEFREQUENCY 18 ...... I ...... •0 ø 14 • H+ .• 1612 H+ 60ø• _' ' ' g r • / • / 1 Z BAS•ON / :.o6[ • V ]•o• LOWERCUTOFF o/ i 1ø2 .1 ................. J Z BEINGATELECTRON o[ ........ 10ø •, = 39.37ø, R - 3.10R E RAM ANGLE (DEGREES) 0055UT,L'5.05, L. T.-22.87, -180 180 lO 1 JANUARY 31, 1978 OUTBOUND : 0033-0O47 UT • UT, L - 3.•, L. T. = 22.01, R = 2.09 - 2.71 Re - • = •.22 ø, R= 2.• RE GM. LAT. = 36.540 - 39.36 ø -180 0 L.T. = 21:23-22:34 lO0 i I I I 2 3 4 5 L --SHELL Fig. 1. Plot of electron density profiles, observed by the plasma wave experiment, versus L shell for the plasmapausecrossing on the January 31, 1978, outboundpass. Estimation of the plasma density was difficult for this case, and two estimates are shown, based on lower cutoff at the plasma frequency and on noise enhancement at the upper hybrid frequency. Roman numeralsrefer to locationsof insetsthat are plots of selectedangular distributions of 0- to 100-eV ion count rates versus spacecraft spin angle about the ram direction. Times and locations of each angular distribution are start times and start locations (- 3-min accumulation intervals). Two separate distributions were seen during the time interval associatedwith II. Drapery-like curve refers to acute angle between magnetic field line and instrument view direction; N and S refer to ions coming from northern and southern hemispheres, respectively. The purpose of this report is to examine the relationship the auroral oval. Both examples in this section occurred in between the locations of the plasmapause and the plasma the evening sector. In Figure 1 are plotted profiles of sheet inner edge by comparing these boundaries observed in electron density based on two different methods by the data acquired by the ISEE 1 spacecraft; comparisonsof the plasma wave experiment on January 31, 1978 in the evening locations of the plasmapauseand the equatorward boundary local time sector. The angular distributions of 0- to 100-eV of the auroral oval observed in DMSP photographs are also ion fluxes from the plasma composition experiment are also made. ISEE 1 was launched in October 1977 into a highly shown for selected locations along the profile as marked by elliptical orbit with a 30ø inclination to the equator and a 22.5 roman numerals. The decrease in density between L - 3 and Re apogee. The plasmapauseis determined from thermal (0- 4 locates the plasmapausein this range. At about 0040 UT 100 eV) ion measurements made by the plasma composition (R = 2.48 Re, L - 3.84), a transition is observed in the experiment [Shelley et al., 1978] as well as electron density thermal ion distributions in Figure 1, from a broad umbrella- profiles from the plasma wave experiment [Gurnett et al., like shape peaked around the satellite ram direction to a 1978]. In this paper, the plasmapause location is identified doubly peaked distribution with peaks near the maximum from the thermal ion data as the transition from cold, and minimum pitch angles. The broad umbrella curve is a rammed plasma to warm, anisotropic ion distributions. From signature of cold, approximately isotropic plasma being previous reports [Baugher et al., 1980; Horwitz et al., rammed due principally to the spacecraftorbital motion [cf. 1981b], this transition appears to occur along the density Baugher et al., 1980; Horwitz et al., 1981b]; this plasma we gradient traditionally identified with the plasmapause and identify with the plasmasphere. The doubly peaked (field permits a more precise spatially resolved locator of the aligned) distributions are typically observed outside the plasmapause. It is not certain, however, that this operational plasmasphere [Horwitz et al., 1981b]. As discussedabove, definition results in the same location as the conceptual we take the transition between these two types of thermal definition above associated with the boundary of flux tube ion distributions as locating the plasmapause at approxi- trajectories. The inner edge of the plasma sheet is deter- mately 0040 UT, R • 2.48, L • 3.84. mined as a sharp gradient in -1 keV electron fluxes from A DMSP auroral photograph taken over --•15 min centered energetic electron measurements made with the ISEE 1 at 0031 UT on January 31, 1978, is shown in Figure 2 LEPEDEA [Frank et al., 1978].
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