Instability of the magnetosphere-ionosphere convection and formation of auroral arcs

In this paper we study an instability of the plasma moving towards the Earth near the inner plasma sheet boundary. We include both the interchange instability of the plasma sheet and the magnetosphere-ionosphere interaction instability caused by an effect of field-aligned currents (connected with electron precipitation) on ionospheric conductivity. The instability leads to the separation of steady-state magnetospheric convection into parallel layers. This instability may be responsible for the appearance of quiet auroral arcs inside region 2 of field-aligned currents flowing out of the ionosphere. This instability allows us to explain also the existence of crossing auroral arcs.     

Guided waves in the plasma sheet and triggering of a substorm

A guided propagation of magnetoacoustic wave in the plasma sheet located between two lobes of the magnetotail is investigated. The dispersion equation for the wave and equation connecting a disturbance of plasma pressure inside the plasma sheet and amplitude of the plasma sheet boundary oscillations are obtained. For some value of plasma pressure disturbance, the displacement of the plasma sheet boundaries becomes of order of the half-thickness of the plasma sheet. In the case of symmetrical oscillations of the boundaries (“sausage-like” mode), it creates the favorable conditions for reconnection of the magnetic field lines in the magnetotail and may lead to triggering of a substorm. The magnetoacoustic wave may be generated by sudden impulse of the solar wind plasma pressure.     

The response of the azimuthal component of the ionospheric electric field to auroral arc brightening

We have analyzed the response of azimuthal component of the ionospheric electric field to auroral arc activity. We have chosen for analysis three intervals of coordinated EISCAT and TV observations on 18 February, 1993. These intervals include three kinds of arc activity: the appearance of a new auroral arc, the gradual brightening of the existing arc and variations of the arc luminosity. The arcs were mostly east-west aligned. In all cases, the enhancement of arc luminosity is accompanied by a decrease in the westward component of the ionospheric electric field. In contrast, an increase of that component seems to be connected with arc fading. The observed response is assumed to have the same nature as the short circuit of an external electric field by the conductor. The possible consequence of this phenomenon is discussed.     

Alfven Wave Reflection model of field-aligned currents at Mercury

An Alfven Wave Reflection (AWR) model is proposed that provides closure for strong field-aligned currents (FACs) driven by the magnetopause reconnection in the magnetospheres of planets having no significant ionospheric and surface electrical conductance. The model is based on properties of the Alfven waves, generated at high altitudes and reflected from the low-conductivity surface of the planet. When magnetospheric convection is very slow, the incident and reflected Alfven waves propagate along approximately the same path. In this case, the net field-aligned currents will be small. However, as the convection speed increases, the reflected wave is displaced relatively to the incident wave so that the incident and reflected waves no longer compensate each other. In this case, the net field-aligned current may be large despite the lack of significant ionospheric and surface conductivity. Our estimate shows that for typical solar wind conditions at Mercury, the magnitude of Region 1-type FACs in Mercury’s magnetosphere may reach hundreds of kilo-Amperes. This AWR model of field-aligned currents may provide a solution to the long-standing problem of the closure of FACs in the Mercury’s magnetosphere.     

Convection in the polar cap after the interplanetary magnetic field becomes northward

As shown by Iwasaki (1971); Maezawa (1976); Kuznetsov and Troshichev (1977) and other investigators, the electric field and the plasma convection in the polar cap change their direction after an appearance of a significant northward component of the interplanetary magnetic field. Two possible mechanisms of this phenomenon may be suggested: (i) the direct penetration of the dusk-to-dawn electric field from the solar wind into the magnetosphere, and (ii) the generation of the observed electric field and convection in a process of the decay of the three-dimensional current system which existed before the appearance of the northward interplanetary field. The latter mechanism implies that the value of the electric field generated in the polar cap will decrease with time after the appearance of the northward interplanetary magnetic field. The results of the experimental investigation show such a decrease which favours the second mechanism.     

Imaging and EISCAT radar measurements of an auroral prebreakup event

The results of coordinated EISCAT and TV-camera observations of a prebreakup event on 15 November 1993 have been considered. The variations of the luminosity of two parallel auroral arcs, plasma depletion on the poleward edge of one of these arcs as well as electron and ion temperatures in front of a westward travelling surge were studied. It was found that a shortlived brightening of a weak zenith arc before an auroral breakup was accompanied by fading of an equatorial arc and, vice versa. A plasma depletion in the E region was detected by the EISCAT radar on the poleward edge of the zenith arc just before the auroral breakup. The plasma depletion was associated with an enhancement of ion (at the altitudes of 150–200 km) and electron (in E region) temperatures. During its occurrence, the electric field in the E-region was extremely large (150 mV/m). A significant increase in ion temperature was also observed 1 min before the arrival of a westward travelling surge (WTS) at the radar zenith. This was interpreted as the existence of an extended area of enhanced electric field ahead of the WTS.     

Pi-2 pulsations as a result of evolution of an Alfvén impulse originating in the ionosphere during a brightening of aurora

It is assumed that the original impulse producing Pi-2 pulsations is generated in the ionosphere at the moment of a brightening of aurora. The electric field is known to decrease in the auroral arc almost by an order of magnitude. The electric impulse that appears will be transferred along magnetic field lines and reflected from the ionosphere of the opposite hemisphere, forming the standing Alfvén wave. The electric field impulse of 100 $\text{mV}\text{m}$ is capable of causing magnetic field oscillations of order of 100 γ. Reflection of the Alfvén impulse from the ionosphere with horizontal inhomogeneities corresponding to different forms of auroras is studied. The following is found: (a) the resonance is possible only for harmonics with the rotating vector of polarization; (b) the resonance periods appear to depend essentially on the ionospheric conductivity; this may bring a significant error into determination of the magnetospheric plasma density from the pulsation periods; (c) the auroral zone exerts a screening influence on the pulsations excited at latitudes higher than the zone itself.     

On the formation of a plasma pressure anisotropy in the dayside magnetosheath

We present a numerical solution for the momentum equation of the magnetosheath particles that describes the distribution of the pressure anisotropy of the magnetosheath plasma in the midday meridian plane. The pressure anisotropy is a maximum near the magnetopause subsolar point (p_⊥/p_\Vert\cong10). The pressure anisotropy is caused by two factors: particles with small pitch angles (V_\Vert>V_⊥) which travel along the magnetic field lines away from the equatorial plane of the magnetosheath; and particles, after crossing the bowshock, which reach the bulk velocity component directed along the magnetic field lines again, away from the magnetosheath equatorial plane. This velocity increases with increasing distance from the subsolar point of the bowshock, and does not permit particles with large pitch angles (V_⊥>V_\Vert) to move toward the equatorial plane.     

Surface waves on the plasma sheet boundary

A dispersion equation for the surface waves on the inner boundary of the magnetospheric plasma sheet is obtained. The wave group velocity has both components along and across the magnetic field. For the waves with the period 1 min the transverse component is about 100 km s⁻¹, the parallel component is approximately equal to the Alfvén velocity. Pi2 pulsations, as well as east-westward motions of auroral riometer absorption bays, may be possible displays of surface waves.