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Mapping of Ionospheric Outflows Into the Magnetosphere for Varying IMF

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Mapping of Ionospheric Outflows Into the Magnetosphere for Varying IMF Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 527±540 Mapping of ionospheric out¯ows into the magnetosphere for varying IMF conditions R.M. Winglee Geophysics Program, University of Washington, Seattle WA 98195-1650, USA Received 9 July 1999; received in revised form 24 September 1999; accepted 24 September 1999 Abstract The presence of dierent ion species and components provides important insight into the processes that determine the entry, transport and acceleration of plasma within the magnetosphere. Multi-¯uid simulations that include both ionospheric H and O and solar wind H are used to evaluate the ionospheric out¯ows in response to a range of solar wind conditions. O is particularly important as its provides a de®nitive means for determining the in¯uence of ionospheric ions in the magnetosphere. While O is gravitationally bound to the earth during quiet periods, enhancements in the cross-polar cap potential during southward IMF can produce centrifugal acceleration of the O to produce enhanced out¯ows. However the same strong convective electric ®eld causes much of this O to be con®ned to the inner magnetosphere. During subsequent northward turning of the IMF, this ionospheric-rich plasma is ejected down the length of the deep tail. The derived O density for this combination of alternate southward and northward IMF produces concentrations similar to those observed by Geotail. 7 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction Some of the earliest evidence for such strong out¯ows was provided by Shelley et al. (1972) who showed that The magnetosphere is supported by plasmas from during a magnetic storm the precipitating ¯ux of keV two sources: (1) the ionosphere, and (2) the solar wind. O ions could exceed that of H: Ionospheric out¯ows The presence of these populations in the magneto- has since been documented by a variety of spacecraft, sphere can be inferred from the energy characteristics including ISIS-1 and -2 (Klumpar, 1979), S3-3 (Sharp of the particle populations or by the presence of et al., 1977; Ghielmetti et al., 1978; Gorney et al., speci®c ion species. For example signi®cant com- 1981), DE-1 (Waite, 1985; Yau et al., 1985, 1988; ponents of He or O would be suggestive of a strong Moore et al., 1986; Collin et al., 1987; Roberts et al., ionospheric source whereas a strong solar wind com- 1987; Pollock et al., 1990) and Akebono (Yau et al., ponent would be associated with signi®cant com- 1993). Recent reviews of the importance of the iono- ponents of He 2: Understanding the presence of these spheric source are given by Yau and Andre (1997) and dierent species provides important clues as to the pro- Andre and Yau (1997). cesses governing the entry of plasma into the magneto- The calculated ion out¯ow led Chappell et al. (1987) sphere and the heating and acceleration of this plasma to propose that the ionosphere was a sucient source during disturbed periods. to populate the plasma sheet. This hypothesis lead Intense ionospheric out¯ows have been observed for Moore (1991) and Moore and Delcourt (1995) to pro- more than two decades by polar orbiting spacecraft. pose that within the magnetosphere there was a 1364-6826/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S1364-6826(00)00015-8 528 R.M. Winglee / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 527±540 boundary called the geopause, where the dynamics and solar wind sources make equal contributions to within this boundary were dominated by ionospheric the plasma density and pressure, respectively. Dier- plasma, and regions outside by plasma of solar wind ences in the position of the density and pressure geo- origin. The geopause was envisaged to extend into at pauses can provide important insight for example into least the mid-tail region as suggested by single particle the structure of the magnetotail including the merging tracking (Delcourt et al., 1989; Delcourt et al., 1993). of the low latitude boundary layer (LLBL) with the More recently, Geotail observations have shown the hotter plasma sheet. presence of O in both the mid-tail lobe/mantle These initial multi-¯uid simulations were limited to (Mukai et al., 1994) and in the distant tail (Hirahara et treating only a plasma in which all the ions were H al., 1996; Seki et al., 1996). A statistical study by Seki and the identi®cation of the dierent sources relied on et al. (1998) showed that the lobe/mantle O beams the dierent energy characteristics of the particle distri- between 8 and 210 Re have an average density during butions. In this paper the multi-¯uid simulations are solar minimum of about 1  10 3 cm3 which corre- extended to include both H and O components so sponds to about 1.2% of the proton component. In that the role of the ionospheric source can be directly addition, the oxygen ions appear to have been strongly identi®ed through ion species measurements. In being energized attaining energies up to about 3.5 keV. able to account for the O, we are also then able to in- However, there is equally compelling evidence that directly con®rm the size of the geopause and the rela- plasma of solar wind origin is also playing an import- tive importance of the ionosphere in producing mass ant role in supplying plasma to the magnetosphere. loading of the magnetosphere. Lennartsson (1987; 1992), has shown from ISEE 1 The simulation model is augmented with respect to data that the plasma sheet always has a signi®cant previous models in that it not only includes dierent population of He 2 ions (indicating a non-negligible ion species but also includes gravity. This feature is im- contribution from the solar wind source). This popu- portant because much of the O is expected to be lation was observed to be the largest during periods of gravitationally bound to the earth during quiet periods extremely weak geomagnetic activity when the interpla- and some additional process is needed to accelerate the netary magnetic ®eld (IMF) was persistently north- oxygen into the magnetosphere. ward. Recent Geotail observations also indicate the The model assumes a ®xed oxygen concentration at presence of a cold-dense stagnant ion component that the lower boundary of the simulations in order to is of magnetosheath origin and exists several RE inside evaluate changes in the ionospheric out¯ow driven of the nominal position of the magnetopause (Fuji- solely by the solar wind conditions. In this limit the moto et al., 1998a, 1998b). Observations during a oxygen out¯ows are controlled by primarily two pro- Wind perigee pass also show the presence of this cold- cesses. The ®rst is centrifugal acceleration (Cladis, dense component in the plasma sheet and that mixing 1986; Horwitz, 1987) where fast convection of mag- of ionospheric and solar wind plasma occurs across a netic ®eld lines across the polar cap can produce su- wide section of the magnetotail (Li et al., 1999). cient velocities in the O ions to overcome the The very strong entry of solar wind plasma has been gravitational force. The second process is through the further documented in recent statistical studies between buildup of large pressure gradients. This process is solar wind density observations and tail density also convection related since enhancements in the measurements by Geotail (Terasawa et al., 1997) and cross-polar cap potential convect and compress the by ISEE (Borovsky et al., 1997). In these studies, the plasma to produce increased temperatures and den- density of the plasma sheet was shown to be pro- sities in the inner magnetosphere. Such pressure portional to that in the solar wind, particularly for excesses are seen in global models when there is a sub- northward IMF. This result is surprising in the sense sequent reduction in the cross-polar cap potential as- that the magnetosphere is expected to be closed for sociated with changes in the solar wind conditions. northward IMF and open for southward IMF. These pressure excesses can drive plasma out¯ows into In order to understand the origins of the dierent the deep tail. plasma populations within the magnetosphere, a multi- In reality, the out¯ow of ionospheric ions is also ¯uid treatment of the magnetosphere was developed by controlled by a variety of processes that lead to the Winglee (1998a, 1998b). Unlike MHD which is a single parallel acceleration of ionospheric ions out of the ¯uid treatment, the new method is able to distinguish auroral zone (e.g., see the review by Andre and the ionospheric populations from the solar wind popu- Yau, 1997). For example, various wave-particle in- lations, and provides the ®rst 3D visualization of the teractions can produce large enhancements in the geopause as a function of the solar wind conditions. O out¯ow at low altitudes. Some of these inter- These simulations allowed the concept of the geopause actions appear to depend on auroral and solar ac- to be re®ned to include both the density and pressure tivity. These interactions include heating by geopauses where the contributions from the ionosphere electrostatic and electromagnetic ion-cyclotron waves R.M. Winglee / Journal of Atmospheric and Solar-Terrestrial Physics 62 (2000) 527±540 529 (e.g., Singh and Schunk, 1985; Chang et al., 1986), and thereby modify subsequent activity, including and ion-ion two stream instability (e.g., Winglee et magnetic reconnection. al., 1989). In addition, changes in the photoelectrons in the polar wind can produce changes in O out- ¯ow of the order of 2±5 between solar minimum and solar maximum (Su et al., 1998).
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