SM51C-01
Weak Bipolar Fields in the Solar Wind and Cusp: A Theoretical Interpretation*
Localized bipolar electrostatic field structures have now
been measured in Earth's auroral ionosphere,1 in
conjunction with magnetic reconnection in the
magnetotail,2 in the vicinity of the magnetopause and
cusp,3 and in the solar wind.4 In most of these venues
the associated measured potentials, φ, often have
extremely low amplitudes with ψm≡
eφmax/Te≪ 1 (sometimes with ψm 10-3
or less). A systematic analytic theory of
stationary bipolar fields (i.e., as viewed in their
co-moving frame) is developed based on the smallness
parameter ψm. The theory appears to explain a number
of features of measured weak bipolar structures in terms of
electron phase space holes, and predicts relatively simple
relationships between bipolar waveforms and the
corresponding distributions of passing and trapped
electrons. The analytic potential obtained from theory
takes the form of a hyperbolic secant raised to the fourth
power for the simplest trapped electron distribution. This
potential yields a good fit to bipolar waveforms
measured in the solar wind and Earth's cusp region. For
measurements of hundreds of weak bipolar field events in the
cusp,3 the theory is shown to be consistent with the most
frequently observed half-width (defined as half the
distance between bipolar field peaks, which is ~2 Debye
lengths). The theory is also consistent with various other
features of the measured distribution of hole velocities vs
hole half-widths. It is demonstrated using Vlasov
simulations that a variety of the weak bipolar structures
are stable over long times in one dimension, provided that
they moving sufficiently fast relative to the mean velocity
of the ions.
* Research supported by NSF, NASA, and DOE.
1 Ergun, R.E., et al., Phys. Rev. Lett., 81, 826 (1998).
2 Cattell, C., et al., J. Geophys. Res., 110, A01211 (2005).
3 Franz, J.R., et al., J. Geophys. Res., 110, A09212 (2005).
4 Mangeney, A., private communication.
SM51C-02
Propagation of solar wind and IMF disturbances from L1 to Earth's bowshock: Data analysis and MHD modeling
In recent years, significant progress has been achieved in understanding solar wind propagation from the Sun to the Earth's orbit and its interaction with the terrestrial magnetosphere by using global massively parallel MHD models with adaptive grids. However, most global magnetospheric models require knowledge of the solar wind parameters (speed, density, and IMF) at about 35 Re upstream from the Earth where the inflow boundary conditions are imposed. In reality, the solar wind conditions are typically measured measured in the vicinity of the L1 point approximately 200 Re upstream. To date, only limited attention has yet been given to the actual propagation of the varying solar wind from L1 to the Earth's bow shock. If the plasma flow were homogeneous and steady, then this 200-Re distance would be covered by solar wind in about an hour, and the flow parameters measured near the bow shock would be the same as at L1. There are numerous techniques developed for describing the transport of solar wind plasma through this domain - from simple ballistic propagation to the methods involving minimum-variance analyses. None of these methods, however, provides a clear understanding what might happen with the solar wind in this domain when slower and faster flows are interspersed, and various SW and IMF discontinuities interact while moving through the slower ambient plasma. In this study, we present some results of the MHD modeling of steady and varying (slowing/accelerating) solar wind flow from L1 to the Earth's bowshock and compare these results with real events of the ACE-Wind data comparisons for intervals of minimal spacecraft transverse separations.
SM51C-03
MHD Simulations of Solar Wind-Geospace Coupling
We present a study of transpolar potential using global MHD simulations of the solar wind magnetosphere interaction. The role of Region 1 currents in producing saturated transpolar potential is discussed in terms of the dissipation of solar wind mechanical energy before the solar wind actually reaches the magnetopause. The simulations show that the bow shock has a current that closes with Region 1 type currents flowing on open magnetic field lines. During strongly southward IMF, a large current must flow on the bow shock to satisfy the jump conditions. The bow shock current exerts a JxB force on the solar wind that is larger than the Chapman- Ferraro current, dissipating most of the solar wind mechanical energy far from the magnetopause. Under these conditions, the bow shock closure currents become a major factor in determining the ionospheric potential, and the force balance at the bow shock controls the magnetosheath parameters, and hence the reconnection rate. This results in a reduced electric field applied at the magnetopause and a smaller potential due to magnetopause reconnection. Since the primary driver of the polar cap potential is determined by the electric field applied the magnetopause, the polar cap potential is reduced, leading to the saturation effect. However, the ionospheric potential can also be modulated by the current required to close the bow shock current.
SM51C-04
What can we learn about geoffectiveness of solar wind turbulence?
Solar wind fluctuations exhibit multi-scale intermittent properties. There exist increasing evidence that these features attributed to turbulence influence the level of the solar wind-magnetosphere coupling. In this work we present a comparison of the intermittent properties of the solar wind fluctuations and the mean values of magnetic field. The main goal is to investigate the correlations between the fluctuations and the mean values of some geo-effective parameters and interplanetary magnetic field components to understand better the relative contribution of intermittence to the efficiency of solar wind-magnetosphere coupling.
SM51C-05
Motion of Solar Wind Ions in an Analytical Field-Line Model of the Magnetosheath
To investigate the transport of representative solar-wind ions within the Earth's magnetosheath, we trace their trajectories in an analytical model of the magnetosheath, based on the draping of plasma streamlines and magnetic field lines around a conducting magnetosphere that consists of a prolate ellipsoid, extending ~ 11 R_E upstream and ~ 65 R_E downstream from Earth, matched to a cylinder of radius ~ 28.4 R_E. The bow shock is well represented in our model by an axisymmetric paraboloid that intersects the Sun-Earth line about 26% farther upstream from the point dipole than the nose of the magnetopause does. For a uniform but arbitrarily directed interplanetary magnetic field (IMF), the magnetosheath magnetic field can be obtained by superposition of results for special cases in which the IMF is respectively parallel to and perpendicular to the solar-wind velocity. In this study we consider only the simplest case, in which the interplanetary magnetic field (IMF) is parallel to the solar-wind velocity. Under this condition magnetosheath magnetic-field lines are the same as plasma streamlines, and the magnetosheath magnetic field is derivable from Euler potentials. We express the magnetosheath magnetic field analytically as a function of ellipsoidal or cylindrical coordinates that scale inversely with the sixth root of solar-wind pressure, and we qualitatively compare the resulting field lines and streamlines with those obtained by Spreiter and Alksne [Rev. Geophys., 7, 11-50, 1969] from their hydrodynamic calculation. As an application of our analytical model magnetosheath, we trace the motion (gyration and gradient-curvature drift) of representative solar-wind ions through it. (There is no interplanetary or magnetosheath electric field for IMF purely in the B_x direction.) We identify regions in which the first adiabatic invariant breaks down for ions of various energies. This study constitutes a first step toward understanding the motion of solar-wind ions through the magnetosheath and their penetration into the magnetosphere for more general IMF configurations.
SM51C-06
Comparison of local energy transfer estimates from Cluster with global MHD simulations
We have used several magnetopause crossings by the multi-spacecraft mission Cluster to make observational estimates of the local energy transfer across the magnetopause. During one occasion we could identify a load region on the dayside high-latitude magnetopause, where the energy was found to be transferred from the magnetic field to the particles as a probable consequence of the reconnection process during southward IMF. During another crossing further towards the dawn flank of the magnetosphere we could identify a generator region, where solar wind kinetic energy was transferred to the magnetic field as a result of magnetic stresses along the magnetopause during dominantly duskward IMF. These results from real spacecraft crossings have at the same locations been compared to the BATS-R-US global MHD simulation results, using the actual ACE solar wind plasma and magnetic field measurements from the individual events as model input. The local characteristics of the global model results correctly predict the regions crossed by Cluster during the two different cases as a load and a generator region, correspondingly. Furthermore, the magnitude of the energy transfer across the magnetopause as deduced from Cluster and the model are in reasonable agreement. These results may help to validate the use of global MHD simulations to estimate the total energy input to the magnetosphere, which is of considerable importance for Space Weather considerations. Also, it appears that any local Cluster observations at the magnetopause can be used to scale the MHD model results to obtain better global estimates.
SM51C-07
Geotail Observations of Flux Transfer Events
We present the results of a statistical study of 500 flux transfer events (FTEs) observed by the Geotail spacecraft. We study the motion of events within the magnetosheath that move faster or slower relative to the magnetosheath flow itself and the signatures that they produce in plasma parameters. Event boundaries can be distinguished by sharp reversals in flow directions. We made plots of velocity distribution function for FTEs observed in the sheath and sphere to determine direction of event motion in dependence on sense of polarity of Bn component of FTEs. We determine the region where events originate and their mode of event generation as a function of solar wind conditions. We determine whether events originate along a tilted subsolar merging line during periods of southward IMF orientation, but at high-latitudes during periods of northward IMF orientation. We test the solar wind conditions to determine whether events generated by solar wind/foreshock pressure pulses move across the magnetopause in a direction determined by the orientation of solar wind discontinuities striking the magnetosphere.
SM51C-08
Energetic Particle Behavior in the Magnetospheric Cusp Observed by ISEE-1
Observations on 30 October 1978 show the ISEE-1 spacecraft passing though the high altitude dayside northern magnetospheric cusp region from roughly 16:00 to 18:30 UT, during a slow solar wind period (~380 km/s). More than two orders of magnitude enhancements of the cusp energetic particle (CEP) fluxes were observed along with a depressed and turbulent local magnetic field. The observed variations of the pitch angle distributions (PAD) provide a unique opportunity to determine the structure of the cusp and the origin of the CEP fluxes. Through a boundary sounding technique, the location and orientation of the cusp poleward (or backside) boundary was observed for almost 10 minutes during which time it appeared initially to be stationary in the GSM/GSE X-direction and then move sunward about 0.12 Earth radii (RE). The orientation remained approximately perpendicular to the GSM/GSE X-axis until it was observed to rotate by 60 degrees in ~3 minutes before ISEE-1 was fully inside the cusp cavity. The cavity itself was filled with CEP fluxes displaying large anisotropies, indicative of their source being located below (Earthward) of the satellite location. The spacecraft entered from the backside of the cusp, then traveled ~4 RE through the cavity, and exited through the "top" of the cavity leaving a region of energetic ions below. The PADs demonstrate that the bow shock cannot be the main source of the observed CEPs. The CEP fluxes were measured at about 8.5 hours MLT when the IMF had both an 8-10 nT duskward and southward component.