A new low-latitude ionospheric model has been developed at the Naval Research Laboratory: Sami2 is Another Model of the Ionosphere (SAMI2). SAMI2 treats the dynamic plasma and chemical evolution of seven ion species (H +, He +, N +, O +, N• +, NO +, and O• +) in the altitude range • 100 km to several thousand kilometers. The ion continuity and momentum equations are solved for all seven species; the temperature equation is solved for H +, He +, O +, and the electrons. SAMI2 models the plasma along the Earth's dipole field from hemisphere to hemisphere, includes the E x B drift of a flux tube (both in altitude and in longitude), and includes ion inertia in the ion momentum equation for motion along the dipole field line. The final point is relevant for plasma dynamics at very high altitudes where ion inertia can be important. For example, we have found that ion sound waves, which are supported by ion inertia, may be generated in the topside ionosphere (> 1000 kin) at sunrise and sunset [Huba et al., 2000b]. The neutral species are specified using the Mass Spectrometer Incoherent Scatter model (MSIS86) and the Horizontal Wind Model (HWM93). In this paper we describe in detail the SAMI2 model and present representative results from the model. 1. Introduction Over the past two decades a number of computational models of the ionosphere have been developed. An excellent overview of the most widely used models is given in STEP: Handbook of Ionospheric Models [Schunk, 1996] and by Anderson et al. [1998]. In general, ionospheric models treat the global ionosphere in three parts: low latitude, midlatitude, and high latitude. Low-latitude models (e.g., the Phillips Laboratory global theoretical ionosphere model (GTIM) [Anderson, 1971; Anderson et al., 1996], the University of Alabama field line interhemispheric plasma model (FLIP) [Richards and Tort, 1996], and the Sheffield University plasmasphere-ionosphere model (SUPIM) [Bailey and Balan, 1996]) consider the plasma dynamics along an entire field line from hemisphere to hemisphere. Mid-latitude models (e.g., GTIM [Decker et al., 1994] and the Utah State University time-dependent ionosphere model (TDIM) [Schunk, 1988; Schunk and Sojka, 1996]) typically have the upper boundary •< 1000 kin; additional boundary conditions at the upper boundary (e.g., particle flux, heat flux) must be imposed that are generally not consistent with interhemispheric transport. Finally, high-latitude models (e.g., TDIM, GTIM) also have an upper boundary typically set at 1000 km. However, an important aspect of high latitude models is that magnetospheric effects need to be included: for example, the magnetospheric electric field and auroral precipitation effects. A common feature of these models is that they use empirical neutral atmosphere models such as the Mass Spectrometer Incoherent Scatter model (MSIS) [Hedin, 1987] and the Horizontal Wind Model (HWM) [Hedin et al., 1991] or observed data to specify neutral atmosphere densities and winds. There are also two global ionospheric models that sol...

No abstract

We report the first global magnetohydrodynamic (MHD) simulation of an actual magnetospheric substorm, which was recorded by the Viking spacecraft on October 19, 1986. The simulation is driven by IMP 8 solar wind parameters measured upstream of the Earth's bow shock. The substorm, which had expansion onset at 1132 UT, was caused by a brief period of southward interplanetary magnetic field (IMF) and two weak solar wind shocks. The simulation model includes a self-consistent auroral ionospheric conductance depending directly on the MHD magnetospheric plasma parameters and magnetic field. Synthetic auroral emissions, derived from simulation results, are compared to the Viking images, which show considerable dayside activity preceding the substorm. We also compare model-derived synthetic A U and AL indices to geomagnetic measurements. The simulation results are seen to be in reasonable agreement with the observations throughout the growth phase and expansion onset. Moreover, the results allow us to form conclusions concerning which essential processes were responsible for the substorm occurence. These results are a highly encouraging first step leading toward development of a space weather forecasting methodology based on the directly measured solar input.

The nonlinear evolution of the electrostatic Kelvin-Helmholtz instability, resulting from velocitysheared plasma flows perpendicular to an ambient magnetic field, has been studied including Pedersen conductivity effects (i.e., ion-neutral collisions). We find that the Kelvin-Helmholtz instability develops in a distinctly different manner in the nonlinear regime with Pedersen coupling than without it. Specifically, we show that Pedersen coupling effects, in conjunction with a neutral wind and density gradient, (1) result in an increased time scale for Kelvin-Helmholtz instability wave growth, (2) inhibit Kelvin-Helmholtz vortex formation, (3) lead to nonlinear structures which can be described as "breaking waves," and (4) generate, in the nonlinear regime, small scale turbulence by means of secondary instabilities growing on the primary waves. We have also computed the spatial power spectra of the electrostatic potential and density fluctuations and find that there is a tendency for the potential and density spectra to become shallower when Pedersen conductivity effects are included. We compare our results with recent Dynamics Explorer satellite observations of velocity-sheared plasma flows in the high-latitude, near-Earth space plasma and find good agreement. Recently, much experimental [Basu et al., 1988; Weber and Buchau, 1981; Bythrow et al., 1984; Cerisier et al., 1985; Rodriquez and Szuszczewicz, 1984; Curtis et al., 1982; Baker et al., 1986; Vickrey et al., 1980] and theoretical (for recent reviews, see Keskinen and Ossakow [1983] and Kintner and $eyler [1985] and references therein) attention has been given to the origin of high-latitude ionospheric and magnetospheric plasma turbulence. The Kelvin-Helmholtz or velocity-shear driven instability can lead to both electric field and density fluctuations in the high-latitude near-Earth space plasma [see, for example, Kintner and Seyler, 1985]. Studies of velocitysheared flows in space plasmas can be divided into two groups depending upon whether plasma flow velocities are either parallel [Paper number 7A9077. 0148-0227/88/007A-9077505.00 Mishin, 1981; Lee et al., 1981; Walker, 1981; Keskinen and Huba, 1983] or perpendicular [Hallinan and Davis, 1970; Miura and Sato, 1978; Miura and Pritchett, 1982; Pritchett and Coroniti, 1984; Thompson, 1983] to the ambient magnetic field. Both cases have been studied in the MHD [Mikhailovskii, 1974; Sen, 1964; Southwood, 1968] and electrostatic [D'An•lelo, 1965; Smith and yon Goeler, 1968] limits. Furthermore, the velocity, in both cases, is usually taken to vary spatially transverse to the magnetic field in the electrostatic limit. In this study we restrict ourselves to sheared flows perpendicular to the geomagnetic field. Hallinan and Davis [1970] and Webster and Hallinan [1973] have attributed the small scale vortex configurations often seen near auroral arcs [Hallinan and Davis, 1970; Oquti, 1974] to be driven by a transverse Kelvin-Helmholtz or velocity shear driven instability. Kintner [1976] and Kelley and Carlson [197...

The nature of the solar wind‐magnetosphere‐ionosphere (SW‐M‐I) coupling has been a subject of intense study and scientific interest. We report results from a numerical simulation of the SW‐M‐I system which shed light on the physics and behavior of the controlling processes. The current‐voltage relationship is characteristic of a magnetohydrodynamic dynamo with a load operating near short circuit conditions. We discuss the operation of the dynamo, its location with respect to the magnetosphere, and important implications of the results for both the earth and other planets with intrinsic magnetic fields.