The flow of plasma in the solar terrestrial environment

The overall goal of our NASA Theory Program is to study the coupling, time delays, and feedback mechanisms between the various regions of the solar-terrestrial system in a self-consistent, quantitative manner. To accomplish this goal, it will eventually be necessary to have time-dependent macroscopic models of the different regions of the solar-terrestrial system and we are continually working toward this goal. However, our immediate emphasis is on the near-earth plasma environment, including the ionosphere, the plasmasphere, and the polar wind. In this area, we have developed unique global models that allow us to study the coupling between the different regions. Another important aspect of our NASA Theory Program concerns the effect that localized structure has on the macroscopic flow in the ionosphere, plasmasphere, thermosphere, and polar wind. The localized structure can be created by structured magnetospheric inputs (i.e., structured plasma convection, particle precipitation or Birkeland current patterns) or time variations in these inputs due to storms and substorms. Also, some of the plasma flows that we predict with our macroscopic models may be unstable, and another one of our goals is to examine the stability of our predicted flows. Because time-dependent, three-dimensional numerical models of the solar-terrestrial environment generally require extensive computer resources, they are usually based on relatively simple mathematical formulations (i.e., simple MHD or hydrodynamic formulation). Therefore, another long-range goal of our NASA Theory Program is to study the conditions under which various mathematical formulations can be applied to specific solar-terrestrial regions. This may involve a detailed comparison of kinetic, semikinetic, and hydrodynamic predictions for a given polar wind scenario or it may involve the comparison of a small-scale particle-in-cell (PIC) simulation of a plasma expansion event with a similar macroscopic expansion event. The different mathematical formulations have different strengths and weaknesses and a careful comparison of model predictions for similar geophysical situations will provide insight into when the various models can be used with confidence

The flow of plasma in the solar terrestrial environment

The overall goal of our NASA Theory Program is to study the coupling, time delays, and feedback mechanisms between the various regions of the solar-terrestrial system in a self-consistent, quantitative, manner. To accomplish this goal, it will eventually be necessary to have time-dependent macroscopic models of the different regions of the solar-terrestrial system and we are continually working toward this goal. However, our immediate emphasis is on the near-earth plasma environment, including the ionosphere, the plasmasphere, and the polar wind. In this area, we have developed unique global models that allow us to study the coupling between the different regions. These results are highlighted. Another important aspect of our NASA Theory Program concerns the effect that localized structure has on the macroscopic flow in the ionosphere, plasmasphere, thermosphere and polar wind. The localized structure can be created by structured magnetospheric inputs (i.e., structured plasma convection, particle precipitation or Birkeland current patterns) or time variations in these inputs due to storms and substorms. Also, some of the plasma flows that we predict with our macroscopic models may be unstable. Another one of our goals is to examine the stability of our predicted flows. Because time-dependent three-dimensional numerical models of the solar-terrestrial environment generally require extensive computer resources, they are usually based on relatively simple mathematical formulations (i.e., simple MHD or hydrodynamic formulations). Therefore, another long-range goal of our NASA Theory Program is to study the conditions under which various mathematical formulations can be applied to specific solar-terrestrial regions. This may involve a detailed comparison of kinetic, semikinetic, and hydrodynamic predictions for a given polar wind scenario or it may involve the comparison of a small-scale particle-in-cell (PIC) simulation of a plasma expansion event with a similar macroscopic expansion event. The different mathematical formulations have different strengths and weaknesses and a careful comparison of model predictions for similar geophysical situations will provide insight into when the various models can be used with confidence

Data analysis and interpretation related to space system/environment interactions at LEO altitude

Several studies made on the interaction of active systems with the LEO space environment experienced from orbital or suborbital platforms are covered. The issue of high voltage space interaction is covered by theoretical modeling studies of the interaction of charged solar cell arrays with the ionospheric plasma. The theoretical studies were complemented by experimental measurements made in a vacuum chamber. The other active system studied was the emission of effluent from a space platform. In one study the emission of plasma into the LEO environment was studied by using initially a 2-D model, and then extending this model to 3-D to correctly take account of plasma motion parallel to the geomagnetic field. The other effluent studies related to the releases of neutral gas from an orbiting platform. One model which was extended and used determined the density, velocity, and energy of both an effluent gas and the ambient upper atmospheric gases over a large volume around the platform. This model was adapted to study both ambient and contaminant distributions around smaller objects in the orbital frame of reference with scale sizes of 1 m. The other effluent studies related to the interaction of the released neutral gas with the ambient ionospheric plasma. An electrostatic model was used to help understand anomalously high plasma densities measured at times in the vicinity of the space shuttle orbiter

Electric fields and double layers in plasmas

Various mechanisms for driving double layers in plasmas are briefly described, including applied potential drops, currents, contact potentials, and plasma expansions. Some dynamical features of the double layers are discussed. These features, as seen in simulations, laboratory experiments, and theory, indicate that double layers and the currents through them undergo slow oscillations which are determined by the ion transit time across an effective length of the system in which double layers form. It is shown that a localized potential dip forms at the low potential end of a double layer, which interrupts the electron current through it according to the Langmuir criterion, whenever the ion flux into the double is disrupted. The generation of electric fields perpendicular to the ambient magnetic field by contact potentials is also discussed. Two different situations were considered; in one, a low-density hot plasma is sandwiched between high-density cold plasmas, while in the other a high-density current sheet permeates a low-density background plasma. Perpendicular electric fields develop near the contact surfaces. In the case of the current sheet, the creation of parallel electric fields and the formation of double layers are also discussed when the current sheet thickness is varied. Finally, the generation of electric fields and double layers in an expanding plasma is discussed

Solar- Terrestrial Physics: A Space Age Birth

Solar- Terrestrial Physics, in its broadest sense, is concerned with the transport of energy, particles, and fields from the sun to the earth and their consequent effect on the terrestrial environment. Most of the solar energy eventually deposited in our atmosphere, at a rate of approximately a trillion megawatts, arrives in the form of visible light. The study of how this energy affects our environment falls within the purview of meteorology, a discipline that has experienced an independent development and that has sufficiently different problems from solar-terrestrial physics that it can be regarded as a separate but neighboring discipline. In contrast, solar-terrestrial physics is concerned with the higherenergy radiations (ultraviolet, x-ray, and gamma-ray) that carry a relatively small amount of power (approximately a million megawatts), but nevertheless have significant and highly variable effects on the terrestrial environmen

Theoretical Study of the Effect of Ionospheric Return Currents on the Electron Temperature

An electron heat flow can occur in a partially ionized plasma in response to either an electron temperature gradient (thermal conduction) or an electron current (thermoelectric heat flow). The former process has been extensively studied, while the latter process has received relatively little attention. Therefore a time-dependent three-dimensional model of the high-latitude ionosphere was used to study the effect of field-aligned ionospheric return currents on auroral electron temperatures for different seasonal and solar cycle conditions as well as for different upper boundary heat fluxes. The results of this study lead to the following conclusions: (1) The average, large-scale, return current densities, which are a few microamps per square meter, are too small to affect auroral electron temperatures. (2) Current densities greater than about 10−5 A m−2 are needed for thermoelectric heat flow to be important. (3) The thermoelectric effect displays a marked solar cycle and seasonal dependence. (4) Thermoelectric heat transport corresponds to an upward flow of electron energy. (5) This energy flow can be either a source or sink of electron energy, depending on the altitude and geophysical conditions. (6) Thermoelectric heat transport is typically a sink above 300 km and acts to lower ambient electron temperatures by as much as 2000 K for field-aligned return current densities of the order of 5 × 10−5 A m−2. For this case, the electron temperature decreases with altitude above 300 km with a gradient that can exceed 1 K km−1. Also, the electron temperature can drop below both the ion and neutral temperatures in the upper F region owing to thermoelectric cooling. (7) A downward magnetospheric heat flux in combinations with an upward thermoelectric heat flux can produce steep positive electron temperature gradients in the topside ionosphere

The flow of plasma in the solar terrestrial environment

The development of electric fields in an expanding plasma was studied. With regard to the polar wind, it was found that hot magnetospheric electrons have a pronounced effect on the polar wind. In addition, there is no O(+) charge exchange barrier and substantial fluxes of O(+) ions can escape with the polar wind. In the auroral plasma physics area, the excitation of electrostatic waves by field aligned auroral electron beams was examined. It was demonstrated that the auroral field aligned current density can be large enough to excite Buneman double layers. For situations that lead to strong double layers, it was shown that the temporal evolution of the potential profile is controlled by current fluctuations. Two dimensional particle in cell simulations were conducted, and the high frequency wave turbulence excited by an auroral electron beam of finite width perpendicular to an ambient magnetic field was investigated. The formation of V shaped auroral potential structures was studied, and numerical simulations of double layers and auroral electric fields were reviewed

Ionosphere: Past, Present and Future Problems

Global view of aeronom

Theoretical and Experimental Investigation of High-Latitude Outflow for Ions and Neutrals

The outflow of ions at high latitudes is one mechanism thought to populate the magnetosphere with ionospheric ions [H+, He+, O+]. Computer modeling can give an insight into the mechanisms and rates at which these ions can populate the magnetosphere, but for atomic oxygen the temperature is about 40% lower than measurement. This can be accounted for by the inclusion of a hot O population at a higher temperature, of about 4000K

A Hydrodynamic Model for Plasmasphere Refilling Following Geomagnetic Storms

The refilling of the plasmasphere following a geomagnetic storm remains one of the longstanding problems involving ionosphere-magnetosphere coupling. Both diffusion and hydrodynamic approximations have been adopted for the modeling and solution of this problem. The diffusion approximation neglects the nonlinear inertial term in the momentum equation and so this approximation is not rigorously valid immediately after a storm. The principle focus of this work is the formulation and development of a hydrodynamic refilling model (that includes the nonlinear inertial term) using the fluxcorrected transport method, a numerical method that is extremely well suited to handling nonlinear problems with shocks and discontinuities. In a previous study, this model has been validated against exact analytical benchmark problems and in this study, the model is used to describe plasmasphere refilling. The plasma transport equations are solved along 1-dimensional closed magnetic field lines that connect conjugate ionospheres and the model currently includes three ions (H+, O+, He+) and two neutral (O, H) species. In this study, each ion species under consideration has been modeled as two separate streams emanating from the conjugate hemispheres and the model correctly predicts supersonic ion speeds and the presence of high levels of helium during the early hours of refilling. The ultimate objective of this research is the development of a 3-dimensional model for the plasmasphere refilling problem, and with additional development, the same methodology can be applied to the study of other complex space plasma coupling problems in closed flux tube geometries