S. Fujita

A numerical simulation of the geomagnetic sudden commencement: 1. Generation of the field‐aligned current associated with the preliminary impulse

[1]xa0The magnetospheric response to a solar wind impulse, a geomagnetic sudden commencement, is studied using an MHD model of the coupled solar wind-magnetosphere-ionosphere system. This paper discusses propagation of the first signal launched by the impulse and generation of the field-aligned current that causes the ground magnetic signal detected as the preliminary impulse (PI). It is revealed that the PI current is first excited as an enhanced Chapman-Ferraro current in the magnetopause and next turns to the magnetosphere along the wavefront of the compressional signal launched by the impulse. It is finally converted to a field-aligned current via mode coupling due to plasma nonuniformity. The current in the wavefront region is an inertia current. We present a quantitative model of the PI model presented by Araki [1994] by using a numerical simulation.

A numerical simulation of the geomagnetic sudden commencement: 2. Plasma processes in the main impulse

[1]xa0A geomagnetic sudden commencement (SC) is studied numerically based on a model of buffeting the magnetosphere by a solar wind density impulse. This paper treats two successive current systems in the main impulse (MI) phase. The two current systems have different current generating mechanisms. The first current generator appears behind the wavefront of a compressional disturbance launched by the impulse. The inertia current of the compressional mode is generated by free energy due to deceleration of plasma flows. A field-aligned current (FAC) is excited through mode conversion from the compressional wave in a VA gradient region. The magnetospheric flows and the ionospheric flows are not connected self-consistently to each other. The second generator is located in the tailward side of the cusp. It is the same as the generator of the region 1 current system. The current generated there is connected with the FAC with the region 1 sense via a diamagnetic current flowing around an isolated enhancement of pressure in the nightside equatorial magnetosphere. The pressure enhancement is induced through compression of the magnetospheric flank due to the solar wind impulse. In this period, plasma convection vortices appear both in the magnetosphere and in the ionosphere, which are correspondent to each other. This is a peculiar convection confined within the magnetosphere (the SC transient cell convection). This convection is driven though compression of the magnetospheric flank due to the solar wind impulse.

A numerical simulation of the Pi2 pulsations associated with the substorm current wedge

[1]xa0The present paper deals with the transient behavior of MHD perturbations in the inner magnetosphere induced by an impulsive localized eastward current (source current) as a model of Pi2 pulsations in the magnetosphere. The magnetospheric model consists of a dipole magnetic field, plasmasphere, ionosphere with Pedersen conductivity, and a free outer boundary. The source current is an impulsive magnetospheric current at the onset of the substorm current wedge and is distributed around the equatorial plane of L = 10 with ±2 hour longitudinal extent around midnight. The numerical results allow us to track variation in the expected Pi2 pulsation signals in both local time and L. The poloidal-mode wave exhibits plasmasphere virtual resonance, resulting in large amplitudes around midnight, weakening toward dayside. The toroidal-mode wave is excited as a field line resonance immediately after the wave front of the poloidal-mode wave crosses regions where the radial gradient of VA is steep. The toroidal-mode wave has largest amplitude at the local time of the east/west edge of the source current. The duration of this wave is ∼5 min. In the middle plasmasphere where the radial gradient of the VA is smaller, the poloidal-mode wave tends to predominate over the toroidal-mode wave. These numerical results are consistent with satellite observations, in so far as the day-night asymmetry of Pi2 pulsations and the observation of transient toroidal waves.

Relationship between the Pi2 pulsations and the localized impulsive current associated with the current disruption in the magnetosphere

Behavior of fast-magnetosonic-mode MHD signals in the inner magnetosphere that are driven by the impulsive eastward current is investigated as a model of the Pi2 signal at midnight. The magnetosphere is treated as an axisymmetric cold MHD regime with dipole magnetic fields and has the plasmaspheric structure of the Alfvén speed distribution. MHD perturbation is assumed to be axisymmetric. Numerical calculation revealed the following: 1) The impulsive current induces the plasmasphere virtual resonance oscillation; 2) The compressional magnetic field perturbation is confined near the equator; 3) The waveform of the compressional magnetic perturbation in the plasmasphere depends on the spatial extent of the source current, its temporal variation, as well as its location; 4) The typical Pi2 waveform in the plasmasphere is obtained when the source current is located near the plasmapause (L ≤10); 5) When the source current is not located on the equator, the compressional component and the poloidal component have different waveforms.

A plasmaspheric cavity resonance in a longitudinally non-uniform plasmasphere

Power spectra of a plasmaspheric cavity resonance (strictly, a plasmaspheric virtual resonance) in a longitudinally non-uniform plasmasphere are calculated. It is shown that the spectra depend on longitude. Therefore, a cavity resonance mode can have local time depending spectra when the plasmasphere is non-uniform in a longitudinal direction. This fact concludes that the local time dependent peak frequencies of the mid- and low-latitude Pi2 pulsations discussed by Kosaka et al. (2002) are also explained by the cavity resonance model. We also discuss that the surface eigenmode can be a possible generation mechanism for Pi2 pulsations localized in a longitudinal direction.

Transient response of the Earth's magnetosphere to a localized density pulse in the solar wind: Simulation of traveling convection vortices

[1]xa0We investigate the transient response of the Earths magnetosphere-ionosphere system to a localized density pulse in the solar wind using a magnetohydrodynamic simulation. The simulated ionospheric disturbances exhibit many properties similar to those observed during traveling convection vortices (TCVs). The simulated TCVs are driven by pairs of field-aligned currents. The generation mechanisms of the field-aligned currents in the magnetosphere are evaluated in terms of a wave equation of field-aligned currents. The source current is the inertial current associated with in-out plasma acceleration in the magnetosphere near the impact region. The inertial current is converted into the field-aligned current off from the equatorial region via the curvilinear effect. The inhomogeneous effect also generates the field-aligned current near the inner equatorial region where the sharp gradient of Alfven speed exists.

Ground magnetic perturbations associated with the standing toroidal mode oscillations in the magnetosphere-ionosphere system

The behavior of toroidal mode oscillations of standing Alfvén waves (refer to as standing Alfvén oscillations) in the coupled magnetosphere-ionosphere system is investigated using a trapezoid-shape magnetosphere model. It is found that the magnetic perturbation is transmitted across the ionosphere differently in the two cases where the ionospheric electric field perturbation is static (Pedersen conductivity > Hall conductivity) and where it is inductive (Pedersen conductivity < Hall conductivity). It is noted that the ionospheric Hall current for the inductive condition shields the magnetic field perturbation. The north-south asymmetry of the conjugate ground magnetic perturbations is calculated by using a trapezoid model with the ionospheric and magnetospheric parameters based on the IGRF and IRI. It is revealed that the ionospheric electric field is almost static for the fundamental mode oscillation, whereas inductive for the higher harmonic ones. It is also found that the north-south asymmetry of the ground magnetic perturbations depends not only on the L-value but also on magnetic longitude; this is because the ionosphere and magnetic field conditions are not uniform as a function of longitude.

A numerical simulation of a negative sudden impulse

A numerical experiment of magnetospheric response to a negative pressure impulse in the solar wind is carried out by using a MHD model of the solar wind-magnetosphere-ionosphere coupled system. The numerical simulation confirms mirror-image relationship of the ionospheric and magnetospheric signatures between the negative and positive impulses, which has been suggested by previous observations. The plasma processes associated with the negative impulse are again divided into the three phases—the preliminary impulse phase, and the first and second main impulse phases in terms of the ionosphere-magnetosphere coupling. The SC transient cell convection in the second main impulse phase is related to the Region 2 current in the case of the negative impulse. In the last, we discuss a possible model for the auroral brightening at the onset of the negative impulse.

Propagation property of transient MHD impulses in the Magnetosphere‐Ionosphere System: The 2D model of the Pi2 pulsation

Propagation property of the 2D-coupled MHD wave induced from an impulsive current is studied by using a magnetospheric model having the dipole magnetic field, the ionosphere with Pedersen conductivity, and a free outer boundary. This study is the first step toward realistic modeling of Pi2 pulsation generated by the current wedge model. Numerical calculation reveals that the ionosphere controls not only damping of the field-line resonance oscillation but also its frequency. The field-line resonance oscillation exhibits the fundamental mode structure along the field line in the plasmasphere. In high latitudes, there are waves bouncing between the ionospheres. This suggests that the waveform in the middle- and low-latitudes is different from that in high-latitudes.

A wave equation describing the generation of field-aligned current in the magnetosphere

A wave equation describing the generation of field-aligned current (FAC) in the magnetosphere is derived. The equation has four source terms. The first and second terms represent the effects of inhomogeneous Alfvén speed (VA) and curvilinear magnetic field line, respectively. The perpendicular perturbation inertial current produces the perturbation FAC via these effects. Around the magnetic equator in the region of dipolar magnetic field where VA is inversely proportional to the power of the radial distance from the Earth’s center, the first and second terms have magnitudes of the same order and their signs are identical. The first term dominates over the second one around the region where the gradient of VA is sharp and vice versa around the position where the stretched field line intersects the magnetic equator. The third and fourth terms are related to the diamagnetic current. When the unperturbed magnetic pressure has an inhomogeneous distribution, the perpendicular diamagnetic current due to the perturbation of the plasma pressure yields the perturbation FAC (third term). When the perpendicular diamagnetic current flows in the unperturbed state, the perturbations of the magnetic and plasma pressures also bring about the perturbation FAC (fourth term). In the case of β ∼ 1, the third and fourth terms have magnitudes of the same order. If the disturbance bears a diamagnetic property, this would be especially the case. However, if the disturbance propagates perpendicularly to the ambient magnetic field, the perturbation FAC would be little generated by the fourth term.