High latitude equivalent current systems during extremely quiet times

The magnetic perturbation patterns in the polar cap and auroral zone regions are obtained for extremely quiet days using two different techniques. It is shown that the form of the equivalent current flow pattern is extremely sensitive to the level of quietness, and that even so-called quiet days are at times disturbed by substorm activity. Certain characteristic equivalent flow not typically observed during substorms is noted in the polar cap, and this flow appears to be associated with effects associated with polar cap perturbations discussed by Svalgaard (1973). As well a region of equatorward flow appears at high latitudes near the dawn meridian, which appears to be Hall current driven by an eastward electric field. The dayside sub-auroral zone is dominated by the S_q-current system, while the nightside shows no significant current flow in the absence of substorm activity.     

The development of three-dimensional current system during a magnetospheric substorm

It is suggested that the pattern of three-dimensional substorm current circuit varies significantly even during the lifetime of a single substorm. This gives rise to quite complex time variations of the magnetic field at low latitude stations even for relatively isolated substorms. To verify this, three-dimensional current models with time dependent spatial variations are used to simulate one type of complex low-latitude “substorm signature”. It is shown that the utmost care should be exercised in determining different substorm phases on the basis of such a signature. The results indicate also that, in certain longitudes in the evening sector, one should expect distinct differences in characteristics between positive bays observed on the ground and at the synchronous distance.     

The current systems of the magnetic substorm growth and explosive phases

On the basis of the geomagnetic data of highlatitude arctic stations the development of polar magnetic substorms is examined. It is shown that there exist two current systems of the magnetic substorm: DP₁₁ and DP₁₂. ₁₁ is a current system with one westelectrojet in the nighttime auroral zone. That system is peculiar to the break-up phase of a substorm. DP₁₂ is a two-vortex current system in the polar cap with two auroral electrojets, eastward and westward, of about equal intensity. The DP₁₂ system is typical for growth and recovery phases.There are two different types of substorm development. The first type is characterized by the DP₁₂ system during the growth phase. The intensity of this current system increases until the explosive phase begins. The other type does not seem to be characterized by any distinct current system during the growth phase. The commencement of such a substorm is associated with a rapid explosive development of the DP₁₂ system.A conclusion about the the different origins of the DP₁₁ and DP₁₂ current systems is made.     

Ionospheric conductivities, electric fields and currents associated with auroral substorms measured by the EISCAT radar

E-region electron density profiles with high resolution in time and altitude (5 s and 2 km, respectively) measured by the EISCAT incoherent scatter radar are used to examine the conductivity changes during substorm growth, onset and expansion phases for seven substorms occurring in the local evening sector. The measurements are related to electric fields and neutral winds measured by the radar, to ground magnetometer and riometer records, and to optical features, including the westward-travelling surge and auroral bulge. Auroral features are identified using all-sky camera photographs and images from the Viking satellite. Conductances and electric fields in the zone of diffuse aurora corresponding to the westward substorm electrojet are found to be consistent with existing models. Conductances in the discrete auroral arcs marking the expanding edge of the substorm are found to be much higher, and electric fields rather lower, than previously assumed. The magnetic signatures of the discrete arcs are found to be best explained by Hall and Pedersen currents driven by a southward neutral wind, as is observed by the radar. The highest conductances observed, with Hall and Pedersen conductances reaching 120 and 48 S, respectively, are found to be associated with arcs appearing at the southern edge of activity in the vicinity of a westward-travelling surge.     

A solar flare model including the formation and destruction of the current sheet in the corona

A numerical simulation method is used to show the possibility of forming a current sheet in the solar corona in an active region with four magnetic poles. The evolution of the quasi-stationary current sheet can lead to its transfer to an unsteady state. The MHD instability of this sheet causes its decay, accompanied by a set of events which characterizes the solar flare. The electrodynamical model of a solar flare includes a system of field-aligned currents typical of a magnetospheric substorm. Several events in substorms and solar flares are explained by the generation of field-aligned currents.     

The Hall effect as the producer of current sheets in astrophysical plasmas

Our solution of the MHD equations with the Hall effect shows that this effect can produce thin current sheets in stellar atmospheres at heights where the plasma is tenuous and the Hall effect can profoundly influence the magnetic field variations. The current in the sheets is directed oppositely to the local plasma density gradient. In partiuclar, such a phenomenon is possible on the Sun near the base of the corona.     

Effects of Hall current and rotation

Effects of Hall current and rotation on the flow of electrically conducting rarefied gas due to combined buoyant effects of thermal and mass diffusion, past an infinite porous plate in the presence of transverse magnetic have been investigated. The equations governing the flow problem have been solved and the profiles are shown on graphs. Effects ofm (Hall parameter) andE (Ekman number) on velocity are discussed.     

Three-dimensional current system in different phases of a substorm

It is assumed that the three-dimensional current system of a substorm passes three successive stages. (1) When a dawn-to-dusk magnetospheric electric field appears, a current system with field-aligned currents at the poleward boundary of the auroral zone arises. An equivalent ionospheric current system calculated, taking into account a day-night asymmetry of ionospheric conductivity, looks like the well-known DP-2 system including an eastward low-latitude current and a greater magnitude of the dusk vortex in comparison with the dawn one. (2) An electric drift of plasma towards the Earth leads to the appearance of a westward partial ring current increasing in time. This current is closed by field-aligned currents at the equatorward boundary of the auroral zone. The calculated equivalent current system is similar to the well-known one of the precursory phase. (3) An increase of the auroral ionospheric conductivity during the expansive phase produces an increase of all currents and a turning of field-aligned currents at the equatorward boundary of the auroral zone relative to those at the poleward one. The calculated equivalent current system is similar to the DP-1 system.     

A model current system for the magnetospheric substorm

We propose a model three-dimensional current system for the magnetospheric substorm, which can account for the new findings of the field-aligned and ionospheric currents obtained during the last few years by using new techniques. They include (1) the ionospheric currents at the auroral latitude deduced from the Chatanika incoherent scatter radar data, (2) the field-aligned currents inferred from the vector magnetic field observations by the TRIAD satellite and (3) the global distribution of auroras with respect to the auroral electrojets appearing in DMSP satellite photographs. The model current system is also tested by a computer model calculation of the ionospheric current pattern. It is shown that the auroral electrojets have a strong asymmetry with respect to the midnight meridian. The westward electrojet flows along the discrete aurora in the evening sector, as well as along the diffuse aurora in the morning sector. The eastward electrojet flows equatorward of the westward electrojet in the evening sector. It has a northward component and joins the westward electrojet by turning westward across the Harang discontinuity. Thus, the latitudinal width of the westward electrojet in the morning sector is much larger than that in the evening sector. The field-aligned currents, consisting of two pairs of upward and inward currents (one is located in the morning sector and the other in the evening sector), are closed neither simply by the east-west ionospheric currents nor by the north-south currents, but by a complicated combination of the north-south and east-west paths in the ionosphere. The magnetospheric extension of the current system is also briefly discussed.     

A numerical study of the unsteady MHD free convection with hall current

Effect of Hall current on the hydromagnetic free-convection flow of an electrically-conducting viscous incompressible fluid past an impulsively accelerated vertical porous plate in the presence of a uniform transverse magnetic field subjected to a constant transpiration velocity is analyzed for the case of small magnetic Reynolds number. Numberical solutions are obtained for the axial and transverse components of the velocity as well as the skin-friction by employing the Crank-Nicolson implicit finite-difference method for all probable values of the Prandtl number. The results are discussed with the effects of the Grashof number Gr, the transpiration velocity parameter , the Hall current parameterm, and the magnetic field parameterM for the Prandtl number Pr=0.71 which represents air at 20° C.     

On current continuity at the Harang discontinuity

Although the Harang discontinuity has so far been identified in terms of various phenomena (such as ground magnetic fields, ionospheric currents, auroral features, and electric fields), the loci defined by those different phenomena do not always coincide. It is suggested that the Harang discontinuity may not be a line boundary across which the electric field changes its direction simply from poleward to equatorward, but that the field gradually rotates counterclockwise in a narrow region; thus the westward electric field dominates there. In such a case, no field-aligned current is necessarily required to flow from or into the discontinuity region. This view may be contrasted with the conventional view that an intense upward field-aligned current should flow from the Harang discontinuity. A model is presented in which the poleward ionospheric current (the Hall current resulting from the westward electric field) in the Harang discontinuity region connects the eastward electrojet and the westward electrojet.     

The transient response mechanism and Pi2 pulsations at substorm onset—Review and outlook

Pi2 pulsations are nowadays thought to be transient hydromagnetic signals associated with the build-up of a new stationary magnetosphere-ionosphere coupling system after the sudden formation of the substorm current wedge. To illustrate this transient response mechanism, we will first briefly describe the substorm current circuit. Subsequently, we will demonstrate that the gross characteristics of high-latitude Pi2 can be explained by the sudden switch-on of this current wedge during substorm onset if its westward expansion is taken into account. We will conclude by discussing some additional phenomena and processes (like conductivity and electron density gradients, kinetic Alfvén waves, ionospheric polarization electric fields, and mode coupling) which have to be included into a realistic model for Pi2 pulsations and thus timedependent magnetosphere-ionosphere coupling.     

Event study on pre-substorm phases and their relation to the energy coupling between solar wind and magnetosphere

On 11 November 1976, after a magnetically quiet period with the interplanetary magnetic field (IMF) directed northward, a sudden southward turning of the IMF immediately led to a world-wide intensification of convection which was observed to start almost simultaneously at stations within the auroral zone and polar cap. The two-dimensional equivalent current system over the northern hemisphere had a typical two-cell convection pattern with a maximum disturbance of ΔH = ?300 nT observed on the morningside in the westward electrojet region. This enhancement of activity ended after 35 min in a localized substorm onset in the midnight sector over Scandinavia.The recordings made in this area indicate large fluctuations of various ionospheric parameters starting several minutes before the substorm onset. Two subsequent stages can be resolved: (1) high-energy particle precipitation recorded by balloon X-ray detectors and maximum ionospheric current density increase, while the electrojet halfwidth shrinks and the total electrojet current becomes weaker; (2) the maximum ionospheric current density stays constant and the high-energy particle precipitation decreases, while the auroral brightness increases and the total electrojet current and its half-width show a growing trend prior to the final breakup. A suggestion is made that the time interval of these two stages should be called “trigger phase”. A short discussion explains the trigger phase observations in a magnetospheric scale. The energy coupling between solar wind and magnetosphere during the pre-substorm phases is discussed by utilizing the energy coupling function ? defined by Perreault and Akasofu (Geophys. J. R. Astr. Soc.54, 547, 1978). The ? values appear to be on substorm level during the period of enhanced convection. A good correlation between ? and the growth of the Joule heating rate (estimated from the AE data) is found in the beginning, but during the last 20 min before substorm triggering ? is high while the Joule heating rate decreases. The behaviour of ? during the two stages of the trigger phase suggests that the start of the trigger phase is purely internally controlled while the length of the trigger phase and the final substorm onset may be influenced by the variation in ?.     

A modeling of the magnetospheric substorm

This paper reports some results of an attempt to simulate the large-scale changes of the internal structure of the magnetosphere during the magnetospheric substorm by assuming the growth of two current systems, one in the nightside and the other in the day-side.     

Major auroral substorm features in the dark sector observed by a USAF DMSP satellite

Synoptic auroral photographs acquired by a United States Air Force DMSP satellite have provided the data necessary for an improved understanding of the common auroral phenomena that occur during the substorm expansive phase. Specifically, it is recognized that diffuse auroras (patches, large scale wave structures and Omega bands) form an integral part of the auroral substorm and that their morphological features should be added in the substorm pattern proposed by Akasofu (1964). Several other important auroral substorm features have been added to or corrected in the original pattern. A revised substorm pattern at about the maximum epoch of the substorm is presented.     

Ground-based observations of an isolated substorm

An isolated substorm occurred in Northern Scandinavia on 1 March, 1977 around magnetic midnight. The ionospheric phenomena associated with this substorm were studied by ground magnetometers, the Scandinavian Twin Auroral Radar Experiment (STARE), riometers and an all-sky camera. The physical properties of the auroral electrojet are determined from the ground magnetic field and the ionospheric electric field data. Mid and low latitude magnetic field data show evidence of field-aligned current flow. It is shown that the enhancement of the electrojet's current density is essentially determined by an increase in the ionospheric conductivity. The current system derived from the data of this study corresponds to a model of Yasuhara et al. (1975a).     

VLF emissions and substorm activity

The resonant interaction between the whistler mode waves and the energetic electrons near the plasmapause boundary has been studied in the presence of field aligned currents which seem to exist during substorm activity. It is shown that the electrons which carry the current along the direction of the magnetic field enhance the whistler mode growth considerably if the streaming velocity is small compared to the phase velocity of the wave. It is likely that this is one of the mechanisms explaining the intense VLF emissions observed near the plasmapause during substorm activity.     

Latitude variation of the spectral components of auroral zone Pi2

Evidence is presented from spectral analysis of Pi2 pulsations detected during a substorm by the University of Alberta meridian chain of magnetometers to support the conclusion that at auroral latitudes there is no apparent correlation between the principal spectral components of Pi2 pulsations and the latitude of the observations. From these data we infer that the Pi2 magnetic variations observed at the Earth's surface are not generated by simple MHD eigenoscillations of magnetospheric field. As well, the data show clear contributions to the Pi2 pulsation spectrum by ionospheric currents. These observations lead to the suggestion that Pi2 pulsation spectra are produced by the sudden changes in magnetospheric and ionospheric current systems which take place at the beginning of a substorm.     

Inductive electric fields in the magnetosphere

A quantitative estimate of the electric fields induced by the time dependent ring current is made incorporating the drifts and induced electric fields in a self-consistent manner. It has been shown that in the ring current region, the results of the self-consistent calculations deviate substantially from the first order estimates hitherto obtained. Since for a rapidly varying ring current, the induced electric field can be of the same order as the convection electric field in the magnetosphere, these deviations have to be taken into account in substorm studies.     

The mechanism of magnetospheric substorm and the MHD waves of the solar wind

A mechanism of the Earth's magnetospheric substorm is proposed. It is suggested that the MHD waves may propagate across the magnetopause from the magnetosheath into the magnetotail and will be dissipated in the plasma sheet, heating the plasma and accelerating the particles. When the solar wind parameters change, the Poynting flux of the waves transferred from the magnetosheath into the tail, may be greater than 10¹⁸ erg s^?1. The heated plasma and accelerated particles in the plasma sheet will be injected into the inner magnetosphere, and this may explain the process of the ring current formation and auroral substorm.The Alfvén wave can only propagate along the magnetic force line into the magnetosphere in the open magnetosphere, but the magnetosonic wave can propagate in both the open and closed magnetosphere. When the IMF turns southward, the configuration of the magnetosphere will change from a nearly closed model into some kind of open one. The energy flux of Alfvén waves is generally larger than that of the magnetosonic wave. This implies that it is easy to produce substorms when the interplanetary magnetic field (IMF) has a large southward component, but the substorm can also be produced even if the IMF is directed northward.