Jiannan Tu

On the concept of penetration electric field

The concept of penetration electric field is discussed in the context of magnetosphere and ionosphere convection. We first calculate orbits of a pair of electron and ion in the presence of an external electric field and magnetic field. It is illustrated that the usual E×B motion for single particle does not apply when the mutual interaction (Coulomb force) between two particles is considered. We then perform a full particle-in-cell simulation for a magnetized plasma subjected to an externally applied potential drop. It is shown that the potential drop (equivalent to an electric field) applied to the plasma is greatly reduced in the plasma due to the plasma sheaths at the boundaries. The plasma sheaths tend to shield the potential (or electric field) from penetrating into the plasma. The resulting plasma bulk flow speed is thus much smaller than the expected E×B drift speed. Those results suggest that the externally applied electric field cannot penetrate into the plasma.The concept of penetration electric field is discussed in the context of magnetosphere and ionosphere convection. We first calculate orbits of a pair of electron and ion in the presence of an external electric field and magnetic field. It is illustrated that the usual E×B motion for single particle does not apply when the mutual interaction (Coulomb force) between two particles is considered. We then perform a full particle-in-cell simulation for a magnetized plasma subjected to an externally applied potential drop. It is shown that the potential drop (equivalent to an electric field) applied to the plasma is greatly reduced in the plasma due to the plasma sheaths at the boundaries. The plasma sheaths tend to shield the potential (or electric field) from penetrating into the plasma. The resulting plasma bulk flow speed is thus much smaller than the expected E×B drift speed. Those results suggest that the externally applied electric field cannot penetrate into the plasma.

Inductive-dynamic magnetosphere-ionosphere coupling via MHD waves

In the present study, we investigate magnetosphere-ionosphere/thermosphere (M-IT) coupling via MHD waves by numerically solving time-dependent continuity, momentum, and energy equations for ions and neutrals, together with Maxwells equations (Amperes and Faradays laws) and with photochemistry included. This inductive-dynamic approach we use is fundamentally different from those in previous magnetosphere-ionosphere (M-I) coupling models: all MHD wave modes are retained, and energy and momentum exchange between waves and plasma are incorporated into the governing equations, allowing a self-consistent examination of dynamic M-I coupling. Simulations, using an implicit numerical scheme, of the 1-D ionosphere/thermosphere system responding to an imposed convection velocity at the top boundary are presented to show how magnetosphere and ionosphere are coupled through Alfven waves during the transient stage when the IT system changes from one quasi steady state to another. Wave reflection from the low-altitude ionosphere plays an essential role, causing overshoots and oscillations of ionospheric perturbations, and the dynamical Hall effect is an inherent aspect of the M-I coupling. The simulations demonstrate that the ionosphere/thermosphere responds to magnetospheric driving forces as a damped oscillator.

Characteristic ion distributions in the dynamic auroral transition region

A Dynamic Fluid Kinetic (DyFK) simulation is conducted to study the H + /O + flows and distribution functions in the high-latitude dynamic transition region, specifically from 1000 km to about 4000 km altitude. Here, the collisional-to-collisionless transition region is that region where Coulomb collisions have significant but not dominant effects on the ion distributions. In this study, a simulation flux tube, which extends from 120 km to 3 R E altitude, is assumed to experience a pulse of auroral effects for approximately 20 minutes, including both soft electron precipitation and transverse wave heating, and then according to different geophysical circumstances, either to relax following the cessation of such auroral effects or to be heated further continuously by waves with power at higher frequencies. Our principal purpose in this investigation is to elicit the characteristic ion distribution functions in the auroral transition region, where both collisions and kinetic processes play significant roles. The characteristics of the simulated O + and H + velocity distributions, such as kidney bean shaped H + distributions, and O + distributions having cold cores with upward folded conic wings, resemble those observed by satellites at similar altitudes and geographic conditions. From the simulated distribution function results under different geophysical conditions, we find that O + -O + and O + -H + collisions, in conjunction with the kinetic and auroral processes, are key factors in the velocity distributions up to 4000 km altitude, especially for the low speed portions, for both O + and H + ions.

Plasma sheath structures around a radio frequency antenna

[1] A one-dimensional particle-in-cell (PIC) simulation code is developed to investigate plasma sheath structures around a high-voltage transmitting antenna in the inner magnetosphere. We consider an electrically short dipole antenna assumed to be bare and perfectly conducting. The oscillation frequency of the antenna current is chosen to be well below the electron plasma frequency but higher than the ion plasma frequency. The magnetic field effects are neglected in the present simulations. Simulations are conducted for the cases without and with ion dynamics. In both cases, there is an initial period, about one-fourth of an oscillation cycle, of antenna charging because of attraction of electrons to the antenna and the formation of an ion plasma sheath around the antenna. With the ion dynamics neglected, the antenna is charged completely negatively so that no more electrons in the plasma can reach the antenna after the formation of the sheath. When the ion dynamics are included, the electrons impulsively impinge upon the antenna while the ions reach the antenna in a continuous manner. In such a case, the antenna charge density and electric field have a brief excursion of slightly positive values during which there is an electron sheath. The electron and ion currents collected by the antenna are weak and balance each other over each oscillation cycle. The sheath-plasma boundary is a transition layer with fine structures in electron density, charge density, and electric field distributions. The sheath radius oscillates at the antenna current frequency. The calculated antenna reactance is improved from the theoretical value by 10%, demonstrating the advantage of including the plasma sheath effects self-consistently using the PIC simulations. The sheath tends to shield the electric field from penetrating into the plasma. There is, however, leakage of an electric field component with significant amplitude into the plasma, implying the applicability of the high-voltage antennas in whistler wave transmission in the inner magnetosphere.

Evaluating the diffusive equilibrium models: Comparison with the IMAGE RPI field‐aligned electron density measurements

The diffusive equilibrium models that are widely used by the space physics community to describe the plasma densities in the plasmasphere are evaluated with field-aligned electron density measurements from the radio plasma imager (RPI) instrument onboard the IMAGE satellite. The original mathematical form of the diffusive equilibrium model was based on the hydrostatic equilibrium along the magnetic field line with the centrifugal force and the field-aligned electrostatic force as well as a large number of simplifying approximations. Six free parameters in the mathematical form have been conventionally determined from observations. We evaluate four sets of the parameters that have been reported in the literature. The evaluation is made according to the equatorial radial distance dependence, latitudinal dependence at a given radial distance, and the combined radial and latitudinal dependences. We find that the mathematical form given in the diffusive equilibrium model is intrinsically incompatible with the measurements unless another large number of free parameters are artificially introduced, which essentially changes the nature of a theoretical model to an empirical model.

A two-dimensional global simulation study of inductive-dynamic magnetosphere-ionosphere coupling

We present the numerical methods and results of a global two-dimensional multifluid-collisional-Hall magnetohydrodynamic (MHD) simulation model of the ionosphere-thermosphere system, an extension of our one-dimensional three-fluid MHD model. The model solves, self-consistently, Maxwells equations, continuity, momentum, and energy equations for multiple ion and neutral species incorporating photochemistry, collisions among the electron, ion and neutral species, and various heating sources in the energy equations. The inductive-dynamic approach (solving self-consistently Faradays law and retaining inertia terms in the plasma momentum equations) used in the model retains all possible MHD waves, thus providing faithful physical explanation (not merely description) of the magnetosphere-ionosphere/ thermosphere (M-IT) coupling. In the present study, we simulate the dawn-dusk cross-polar cap dynamic responses of the ionosphere to imposed magnetospheric convection. It is shown that the convection velocity at the top boundary launches velocity, magnetic, and electric perturbations propagating with the Alfven speed toward the bottom of the ionosphere. Within the system, the waves experience reflection, penetration, and re-reflection because of the inhomogeneity of the plasma conditions. The reflection of the Alfven waves may cause overshoot (stronger than the imposed magnetospheric convection) of the plasma velocity in some regions. The simulation demonstrates dynamic propagation of the field-aligned currents and ionospheric electric field carried by the Alfven waves, as well as formation of closure horizontal currents (Pedersen currents in the E-region), indicating that in the dynamic stage the M-I coupling is via the Alfven waves instead of field-aligned currents or electric field mapping as described in convectional M-I coupling models.