A simple model of the interaction of the equatorial ionosphere with the eastward F region neutral wind in the presence of evolving spread F bubbles is given. A consequence of the model is that the upper portions of bubbles will take on a westward tilt, while the lower portions will tilt eastward, giving rise to the ‘fishtails’ and ‘C’s’ observed by coherent backscatter radar measurements of field‐aligned small‐scale irregularities. The essence of the model is that the plasma away from the equatorial plane (e.g., a background nighttime E region at higher latitudes) makes a finite contribution to the magnetic field line‐integrated Pedersen conductivity, causing an incomplete coupling of the plasma motion to the neutral wind. The degree of coupling is then a function of the Pedersen conductivities both near the equator in the F region and in the higher‐latitude E region, giving rise to vertical shears of east‐west plasma motion having opposing signs on either side of the equatorial Pedersen conductivity maximum. Evolving spread F bubbles are caught up in this shear as they rise vertically, resulting in the characteristic ‘C’ shape seen by backscatter radar and in the westward motion of plasma bubbles observed by satellite in situ measurements. Numerical simulations, incorporating an eastward neutral wind in the equatorial F region and E region Pedersen conductivity effects, are presented to further support the model and analysis. The simulations also show the result that it may be the eastward as well as the westward wall of a bubble which is subject to secondary instabilities in the presence of an eastward neutral wind. In addition, even without the neutral wind the numerical simulations show that E region Pedersen conductivity effects can result in a slowing down of equatorial spread F and attendant bubble evolution.
Magnetohydrodynamic simulations of the evolution of a flux tube accelerated through a stationary magnetized plasma are presented. As the flux tube moves through the external plasma, its shape becomes distorted and reconnection can take place between the flux tube and external fields. The coupling between the moving flux tube and the external plasma is generally efficient, with simulated flux tube velocities many times smaller than those expected from frictionless motion. The reconnection between the flux tube and external field takes place when there is a unidirectional external field component in the direction of flux tube propagation. The reconnection is intrinsically nonsymmetric around the flux tube boundary. The principal reconnection site is at the rear of the flux tube, where strong vortices convect the external field toward the flux tube. Drag coefficients (C'D) that parameterize this interaction have been determined. When the flux tube is continually accelerated, CD > 1 is appropriate, consistent with previously used ad hoc values. Examples of when the flux tube is accelerated for a short time but allowed to continue interacting with the external plasma are presented. It is shown that in the absence of reconnection, the coupling time is several Alfv•n wave transit times across the flux tube. However, when reconnection takes place, this coupling can cease to occur, and the flux tube may move frictiordessly (C'D m 0). The results are discussed in terms of interplanetary magnetic clouds, and it is suggested that the observations of cornoving coronal mass ejection and solar wind plasma can be accounted for by drag between the two. 1. Introduction Magnetic flux tubes are important elemental plasma structures in a wide range of astrophysical environments. Their structure and dynamics have been investigated extensively in space and solar plasmas. An extensive compendium of papers on this subject are given by Russell et al. [1990]. The structure of flux tubes has been studied in the solar convection zone, in the solar corona, in interplanetary space, and in the Earth's magnetosphere. The dynamical properties, such as eruption and subsequent propagation of flux tubes, are particularly important for understanding solar wind structures of solar origin that can influence geomagnetic conditions. The flux structures arriving at 1 AU are the product of forces responsible for their propagation and interaction with the ambient plasma medium. Thus the nature of the interaction between a moving flux tube and the ambient plasma is important in understanding This paper is not subject to U.S. copyright. Published in 1996 by the American Geophysical Union. Paper number 95JA03769. how to relate sparse in situ measurements at 1 AU to the properties of the large-scale structures. A qualitative picture of the interaction of a flux tube with the external plasma can be obtained by using knowledge obtained from fluid flows around a rigid body. It is well known [e.g., Batckelor, 1967] that at high Reynolds number, vortices form on the...
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...
Four different two‐dimensional (perpendicular to the ambient magnetic field) plasma fluid‐type numerical simulations following the nonlinear evolution of the collisional Rayleigh‐Taylor instability in the nighttime equatorial F region ionosphere have been performed. Realistic altitude dependent ion‐neutral collision frequencies, recombination rates, and ambient electron density profiles were used. In three cases (ESF 0, 1, 3) the electron density profile was kept constant, with a minimum bottomside background electron density gradient scale length L ∼ 10 km, but the altitude of the F peak was changed, with F peak altitudes at 340, 350, and 430 km. All cases resulted in bottomside growth of the instability (spread F) with dramatically different time scales for development. Plasma density depletions were produced on the bottomside with rise velocities, produced by nonlinear polarization E × B forces, of 2.5, 12, and 160 m/s and percentage depletions of 16, 40, and 85, respectively. In one case, ESF 0, the bubble did not rise to the topside, but in ESF 1 and 3, topside irregularities were produced by the bubbles (where linear theory predicts no irregularities). In these three cases, spread F could be described from weak to strong. In the fourth case (ESF 2) the altitude of the F peak was 350 km, but the minimum L on the bottomside was changed to 5 km. This resulted in a bubble rise velocity of ∼23 m/s and a 60% depletion with strong bottomside and moderate topside spread F and a time scale for development between ESF 1 and 3. Two other cases, ESF 0′ and 0″ with peaks at 330 and 300 km, respectively, and bottomside L ∼ 10 km, were investigated via linear theory. These cases resulted in extremely weak bottomside spread F and no spread F (entire bottomside linearly stable), respectively. These simulations show that under appropriate conditions, the collisional Rayleigh‐Taylor instability causes linear growth on the bottomside of the F region. This causes the formation of plasma density depletions (bubbles) which rise to the topside (under appropriate conditions) F region by polarization E × B motion. High altitude of the F peak, small bottomside electron density gradient scale lengths, and large percentage depletions yield large vertical bubble rise velocities, with the first two conditions favoring bottomside linear growth of the instability. The numerical simulation results are in good agreement with rocket and satellite in situ measurements and radar backscatter measurements, including some of the recent results from the August 1977 coordinated ground‐based measurement campaign conducted by Defense Nuclear Agency at Kwajalein.
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