SM11C-01 INVITED
Magnetospheric Convection as a Global Force Phenomenon
Since 1959 when Thomas Gold showed that motions in the magnetosphere were possible despite plasma being frozen to the magnetic field, magnetospheric convection as a subject of study has gone through several stages (to be reviewed) leading to a recent one that integrates convection into a global system of balance of forces. This area of research has opened by focusing on the region 1 current system as a carrier of force between the solar wind and the ionosphere/thermosphere fluid. An important result to emerge from it is the realization that the force that the solar wind delivers to the magnetosphere in being transferred by the region 1 current system to the ionosphere/thermosphere fluid is amplified by about an order of magnitude. (Vasyliunas refers to this as "leveraging.") The apparent violation of Newton's Third Law results from the main participants in the force balance being not the solar wind force but the JxB force on the ionosphere/thermosphere fluid and the mu-dot-grad-B force on the Earth's dipole. This talk extends the study by considering the global force-balance problem separately for the Pedersen current (a completion of the region 1 problem), the Hall current (thus introducing the region 2 current system), and the Cowling current (bringing in the substorm current wedge). The approach is through representing the ionosphere/thermosphere fluid by the shallow water equations. Novelties that result include force balance by means of tidal bulges and tidal bores.
SM11C-02
Magnetosphere-Ionosphere/thermosphere Coupling: Self-consistent Solutions for a 1-D Stratified Ionosphere in 3-fluid Theory
We continue our three-fluid (electrons, ions, and neutrals) approach to describing the dynamic processes of solar wind-magnetosphere-ionosphere/thermosphere coupling based on the three-fluid generalized Ohm's law, the plasma momentum equation, and the neutral momentum equation, as well as the Maxwell equations, which include the electromagnetic coupling among the charged species and collisions among the three species. We study the responses of the ionosphere to a change in the magnetospheric convection assuming a one- dimensional stratified ionosphere. The driver is added only at the top boundary of the ionosphere. The whole system starts moving in a self-consistent manner. The system is not coupled via the electric field and/or field- aligned currents vertically as conventional models are. We examine the time dependence and height dependence of the plasma flow, the current, the electric field, and the neutral wind velocity.
SM11C-03
A Theory of Solar-Wind/Magnetosphere Coupling Derived from First Principles
A formula that expresses the dayside reconnection rate in terms of upstream solar wind parameters is derived and tested. The derivation is based on the principle that dayside reconnection is governed by local plasma parameters and that whatever controls those parameters controls the reconnection rate. The starting point of the derivation is the Cassak-Shay formula, which expresses the dayside reconnection rate in terms of four parameters: the magnetic-field strengths in the magnetosphere and magnetosheath and the plasma mass densities in the magnetosphere and magnetosheath. Using the Rankine-Hugoniot relations at the bow shock and an analysis of the magnetosheath flow, three of these parameters are expressed in terms of upstream-solar- wind parameters. These three expressions are then used in the Cassak-Shay formula to obtain the "solar-wind control function". The interpretation of the control function is that solar-wind pressure largely sets the reconnection rate. The solar-wind magnetic field enters into the control function because of a bow-shock Mach-number dependence. The "plasmasphere effect", wherein the magnetosphere begins to exert control over solar-wind/magnetosphere coupling, is captured by the formula. Using the OMNI2 data set and seven geomagnetic indices, the solar-wind control function is tested on its ability to describe the variance in the geomagnetic indices. The control function is found to be very successful, statistically as good as the best "solar-wind driver function" in the literature.
SM11C-04 INVITED
Solar Wind-Magnetosphere-Ionosphere Coupling in the Context of the Expanding/Contracting Polar Cap, or the ECPC Made Easy-Peasy
The expanding/contracting polar cap (ECPC) paradigm provides a theoretical framework for understanding the role of magnetic reconnection at the magnetopause and in the magnetotail in determining the form, structure, and dynamics of the magnetosphere, including the excitation of ionospheric convection and the occurrence of substorms. This talk will review (a) the basic concepts behind the ECPC paradigm, (b) techniques for measuring the open flux content of the magnetosphere, and (c) recent advances in our understanding of solar wind- magnetosphere-ionosphere coupling and substorms in the context of the ECPC.
SM11C-05 INVITED
Analysis of the Response Function Relating the Solar Wind Electric Field to the Dayside Magnetic Reconnection Rate
Recent advances in ground-based radar monitoring of the ionosphere have enable remote sensing of the reconnection rate. We present a statistical analysis of the response of the magnetic reconnection rate to the interplanetary electric field using the method of linear prediction filtering. The magnetic reconnection rate is calculated from Super Dual Auroral Radar Network (SuperDARN) measurements. The magnetic separatrix is identified as the offset circle (3° toward midnight from the magnetic pole) that best separates high spectral width (greater than 150 m/s) backscatter, indicative of open magnetic field lines, from low spectral width backscatter. The electric field on the separatrix is determined from the best fit of the line-of-sight F-region plasma velocity to an eighth-order spherical harmonic function of ionospheric electrical potential. The reconnection rate is determined from the electric field component along the separatrix in the separatrix rest frame. We conclude that the magnetospheric response to a 1 mV/m step increase(decrease) in the y component of the interplanetary electric field (-VBz) is as follows: there is a transient increase(decrease) in the reconnection potential due to the separatrix motion that reaches 2 kV after 12 min then returns to 0 kV after 28 min, and there is a sustained increase(decrease) in the reconnection potential due to ionospheric convection that reaches 5 kV after 25 min. While the nightside reconnection rate is not directly measured by this method, the reconnection due to separatrix motion implies unbalanced day-night reconnection, while reconnection due to ionospheric convection implies balanced reconnection.
SM11C-06
Dependence of Earth's Magnetopause Reconnection Rate on Solar Wind Drivers
Combined measurements of polar cap boundary and ionospheric convection have been used to calculate merging rate at the Earth's night-side magnetopause. The algorithm that quantifies merging rate combines the location, orientation and rate of displacement of the polar cap boundary, identified from all-sky images, with high latitude plasma convection determined with incoherent scatter radars. The algorithm was applied to 46 night-side intervals in the winter seasons between 1999 and 2005, totaling 1966 individual measurements of reconnection under diverse geomagnetic conditions. It is found that the median reconnection rate (MErec) increases with increasing strength of the VBz driver, from 11.8 mV/m for VBz > 0 mV/m to 35.2 mV/m for -8 mV/m < VBz < - 4 mV/m. However, the standard deviation of the distribution (SErec), does not show an appreciable change relative to the strength of the driver, staying at a value of ~29 mV/m. Therefore, the variability of the reconnection rate, measured as SErec/MErec, is three times higher for a weakly driven magnetosphere than it is for a strongly driven magnetosphere. The median reconnection rate also increases with increasing solar wind ram pressure, from 15.3 mV/m for Psw < 2 nPa, to 27.4 mV/m for Psw > 4 nPa, and the standard deviation does not have an appreciable change (30.1 mV/m for the lowest pressure to 29.8 mV/m for the highest pressure). Several periods of extended southward IMF were observed, some of them consisting of Steady Magnetospheric Convection (SMC). The latter cases occurred for a weak and relatively stable driver (MVBz ~ -1.1 mV/m, SVBz ~ 0.5 mV/m) and for low and stable solar wind ram pressure (MPsw ~ 1.9 nPa, SPsw ~ 0.5 nPa). In those cases the variability in the reconnection rate is approximately twice as high as the median rate (MErec ~ -14.9 mV/m, SErec ~ 31.7 mV/m). Periods of a strong steady VBz driver (MVBz -3.1 mV/m, SVBz 1.5 mV/m and MPsw 2.6 nPa, SPsw 1.2 nPa) show a comparatively less variable reconnection rate (MErec -20.9 mV/m, SErec 25.1 mV/m). A stepwise increase in solar wind ram pressure was observed during the main phase of a storm. It produced a rapid poleward excursion of the polar cap boundary, as previously reported for this type of forcing, but the concomitant increase in reconnection rate occurred during the relaxation of the polar cap boundary to its original state, not during its expansion.
SM11C-07 INVITED
Comparison of dayside and nightside reconnection changes resulting from a sudden enhancement in solar wind dynamic pressure
Magnetic reconnection at the dayside magnetopause is the main process by which mass, energy, and momentum from the solar wind enter the terrestrial magnetosphere. Magnetic reconnection at the nightside energizes magnetotail plasma and closes the lobe open flux, thus completing the cycle that initiates and sustains magnetospheric convection. Understanding the drivers of reconnection and convection in the magnetosphere is one of the primary goals of magnetospheric physics. It has long been recognized that the Interplanetary Magnetic Field (IMF) is the most influential factor in initiation of reconnection and convection in the magnetosphere. Recent evidence has shown that the solar wind dynamic pressure plays also an important role in enhancing both dayside and nightside reconnection, and driving enhanced ionospheric convection. Super Dual Auroral Radar Network (SuperDARN) observations show that solar wind pressure fronts induce significantly enhanced ionospheric convection in the dayside ionosphere. In parallel, Defense Meteorological Satellite Program (DMSP) precipitating particle measurements and POLAR Ultra-Violet Imager (UVI) images have demonstrated that sudden solar wind pressure increases also significantly affect the size of the polar cap. The polar cap is observed to shrink after an increase in solar wind pressure, especially on the nightside, suggesting an enhancement of magnetotail reconnection. MHD models of the interaction of the magnetosphere with solar wind pressure fronts have managed to reproduce the enhancement of dayside reconnection, but have failed so far to account for the observed closing of the polar cap on the nightside and the suggested magnetotail reconnection increase. We use SuperDARN observations of ionospheric convection within both the dayside and nightside polar ionosphere, including near the magnetic separatrix, to evaluate the relative strengths of the observed dayside and nightside reconnection enhancements after an abrupt increase in solar wind dynamic pressure. We show that enhancements of both dayside and nightside convection occur after an increase in pressure, suggesting an increased reconnection rate on both sides of the ionosphere. We discuss these results in terms of a competition between dayside and nightside reconnection in the determination of the size of the polar cap and possibly their effect on the transpolar potential.