Observation of Birkeland currents with the TRIAD satellite

The intimate connection between geomagnetic storms and the aurora was appreciated by many early scientists including Edmund Halley and Anders Celsius, but the first serious study of this phenomena was made by Kristian Birkeland who, during his polar expeditions of 1902–1903, determined that large-scale ionospheric current were associated with the aurora. Birkeland was also the first to suggest that these currents originated far from the Earth and that they flowed into and away from the polar atmosphere along the geomagnetic field lines. The existence of such field-aligned orBirkeland currents was widely disputed because it was not possible to unambiguously identify current systems that are field-aligned (Alfvén, 1939, 1940) and those which are completely contained in the ionosphere (Vestine and Chapman, 1938) only from a study of surface magnetic field measurements. During the last decade, the presence of Birkeland currents has been absolutely confirmed with particle and magnetic field observations acquired from a variety of rocket and satellite instruments. The vector magnetometer on the low-altitude (800 km) polar orbiting TRIAD satellite has been used to determine for the first time the flow direction, spatial distribution, and intensities of Birkeland currents in the north and south auroral regions. These observations support the mechanism originally proposed by Alfvén (1939, 1940)-later expanded by Shieldet al. (1969)-to drive Birkeland currents in the auroral regions, and they demonstrate the important role that these intense currents (ranging between 10⁶ and 10⁷ amperes) play in the coupling of energy between the magnetosphere and the lower ionosphere and atmosphere.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978.     

Auroral and magnetic variations in the polar cusp and cleft — Signatures of magnetopause boundary-layer dynamics

By combining continuous ground-based observations of polar cleft/cusp auroras and local magnetic variations with electromagnetic parameters obtained from satellites in polar orbit (low-altitude cleft/cusp) and in the magnetosheath/interplanetary space, different electrodynamic processes in the polar cleft/cusp have been investigated. One of the more controversial questions in this field is related to the observed shifts in latitude of cleft/cusp auroras and the relationship with the interplanetary magnetic field (IMF) orientation, local magnetic disturbances (DP2 and DPY modes) and magnetospheric substorms. A new approach which may contribute to clarifying these complicated relationships — simultaneous ground-based observations of the midday and evening-midnight sectors of the auroral oval—is illustrated. A related topic is the spatial relationship between the cleft/cusp auroras and the ionospheric convection currents. A characteristic feature of the polar cusp and cleft regions during negative IMFB _Z is repeated occurrence of certain short-lived auroral structures which seem to move in accordance with the local convection pattern. Satellite measurements of particle precipitation, magnetic field and ion drift components permit detailed investigations of the electrodynamics of these cusp/cleft structures. Information on electric field components, Birkeland currents, Poynting flux, height-integrated Pedersen conductivity, and Joule heat dissipation rate has been derived. These observations are discussed in relation to existing models of temporal plasma injections from the magnetosheath.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Meridional ionospheric electric field caused by curvature current in the magnetosphere

The bending of geomagnetic field lines towards the geotail produces a curvature drift of charged particles parallel to the geomagnetic axis. The divergence of the current so produced forms Birkeland current to the ionosphere where a meridional electric field is created. This field would drive ionospheric currents to form a negative magnetic bay in the dawn sector of the auroral zone and a positive one in the dusk sector. Also it would cause a dawn-dusk field across the polar cap.     

Effects of interplanetary magnetic field and magnetospheric substorm variations on the dayside aurora

Photometric observations of dayside auroras are compared with simultaneous measurements of geomagnetic disturbances from meridian chains of stations on the dayside and on the nightside to document the dynamics of dayside auroras in relation to local and global disturbances. These observations are related to measurements of the interplanetary magnetic field (IMF) from the satellites ISEE-1 and 3. It is shown that the dayside auroral zone shifts equatorward and poleward with the growth and decay of the circum-oval/polar cap geomagnetic disturbance and with negative and positive changes in the north-south component of the interplanetary magnetic field (B_z). The geomagnetic disturbance associated with the auroral shift is identified as the DP2 mode. In the post-noon sector the horizontal disturbance vector of the geomagnetic field changes from southward to northward with decreasing latitude, thereby changing sign near the center of the oval precipitation region. Discrete auroral forms are observed close to or equatorward of the ΔH = 0 line which separates positive and negative H-component deflections. This reversal moves in latitude with the aurora and it probably reflects a transition of the electric field direction at the polar cap boundary. Thus, the discrete auroral forms observed on the dayside are in the region of sunward-convecting field lines. A model is proposed to explain the equatorward and poleward movement of the dayside oval in terms of a dayside current system which is intensified by a southward movement of the IMF vector. According to this model, the Pedersen component of the ionospheric current is connected with the magnetopause boundary layer via field-aligned current (FAC) sheets. Enhanced current intensity, corresponding to southward auroral shift, is consistent with increased energy extraction from the solar wind. In this way the observed association of DP2 current system variations and auroral oval expansion/contraction is explained as an effect of a global, ‘direct’ response of the electromagnetic state of the magnetosphere due to the influence of the solar wind magnetic field. Estimates of electric field, current, and the rate of Joule heat dissipation in the polar cap ionosphere are obtained from the model.     

Polar magnetic substorms

From the world distribution of geomagnetic disturbance, the connection between the electric current in the ionosphere, the field-aligned current and asymmetric equatorial ringcurrent in the magnetosphere is discussed. The partial ring-current in the afternoon-evening region, whose intensity is closely correlated with the AE-index, usually develops and decays earlier than the symmetric ring-current in the course of magnetic storms. The partial ringcurrent seems to have a direct connection with the positive geomagnetic bay in high latitudes in the evening hours through the ionizing effect of the particles leaking from the partial ringcurrent. The dawn-to-dusk electric field in the magnetospheric tail is transferred to the polar ionosphere, producing there the twin vortex Hall current responsible for polar cap geomagnetic variation. The magnetic effect of the associated Pedersen current in the ionosphere is shown to be small but still worth considering. The electrojet near midnight along the auroral oval is thought to appear when the electric conductivity of the ionosphere is locally increased under the presence of large scale dawn-to-dusk electric field. The occasional appearance of a localized abnormal geomagnetic disturbance with reversed direction near the geomagnetic pole seems to suggest the occasional reversal of electric field near the outer surface of the magnetospheric tail, especially when the interplanetary magnetic field is northward.     

Configuration of the auroral oval and the interplanetary magnetic field

The poleward boundary of the auroral oval, whose footline forms the periphery of the polar cap, is calculated, based on a model in which the geomagnetic field is interpermeated with the interplanetary field. It is shown that the calculated auroral oval size varies with the strength and direction of the interplanetary magnetic field, in agreement with recent observations of the location of large-scale nightside auroras.     

Magnetosphere-ionosphere interactions —near-Earth manifestations of the plasma Universe

As the Universe consists almost entirely of plasma, the understanding of astrophysical phenomena must depend critically on our understanding of how matter behaves in the plasma state.In situ observations in the near-Earth cosmical plasma offer an excellent opportunity of gaining such understanding. The near-Earth cosmical plasma not only covers vast ranges of density and temperature, but is the site of a rich variety of complex plasma physical processes which are activated as a result of the interactions between the magnetosphere and the ionosphere.The geomagnetic field connects the ionosphere, tied by friction to the Earth, and the magnetosphere, dynamically coupled to the solar wind. This causes an exchange of energy and momentum between the two regions. The exchange is executed by magnetic-field aligned electric currents, the so-called Birkeland currents. Both directly and indirectly (through instabilities and particle acceleration) these also lead to an exchange of plasma, which is selective and therefore causes chemical separation. Another essential aspect of the coupling is the role of electric fields, especially magnetic-field aligned (parallel) electric fields, which have important consequences both for the dynamics of the coupling and, especially, for energization of charged particles.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.Copyright 1986 IEEE. Reprinted with permission from IEEE Transactions on Plasma Science, Vol. PS-14, No. 6.     

A possible current system associated with the Sqp variation

It is suggested that the quiet day daily magnetic variation in the polar cap region, S_q^p, results partly from the short-circuit effect of the magnetotail current by the polar ionosphere. This implies that there is an inward field-aligned current from the dawnside magnetopause to the forenoon sector of the auroral oval (positively charged) and an outward field-aligned current to the duskside magnetopause from the afternoon sector of the oval (negatively charged), together with the ionospheric (Pedersen and Hall) currents. The distribution of the magnetic field vectors of both combined current systems agrees with the observed S_q^pvector distribution. The space charges provide an electric field distribution which is similar to that which has been observed by polar orbiting satellites.     

Equivalent ionospheric current systems representing IMF sector effects on the polar geomagnetic field

Equivalent ionospheric current systems representing IMF sector effects on the geomagnetic field in high latitudes are examined for each of the twelve calendar months by spherical harmonic analyses of geomagnetic hourly data at 13 northern polar stations for seven years. The main feature of obtained equivalent current systems includes circular currents at about 80° invariant latitude mostly in the daytime in summer and reversed circular currents at about 70° invariant latitude mainly at night in winter. Field-aligned current distributions responsible for equivalent currents, as well as vector distributions of electric fields and ionospheric currents, are approximated numerically from current functions of equivalent current systems by taking assumed distributions of the ionospheric conductivity. Two sets of upward and downward field-aligned current pairs in the auroral region, and also a field-aligned current region near the pole show seasonal variations. Also, ionospheric electric-field propagation along geomagnetic field lines from the summer hemisphere to the winter hemisphere with auroral Hall-conductivity effects may provide an explanation for the winter reversal of sector effects.     

Field-aligned currents between 400 and 3000 km in auroral and polar latitudes

The polar orbiting magnetically stabilized satellite AZUR measured transverse magnetic variations in auroral and polar latitudes by means of a two component flux-gate magnetometer. Simultaneous measurements of $\text{λ2972}\text{A}\text{?}$ and $\text{λ3914}\text{A}\text{?}$ auroral emissions are related to low-energy zero-pitch-angle electron fluxes, which cause the transverse magnetic disturbances. Power spectra of the magnetic field variations are consistent with those of geomagnetic micropulsations.The sources of the field-aligned currents can be located in the Alfvén layer and in the magnetotail.     

A cometary ionosphere model for Io

The ionosphere of Jupiter's satellite Io, discovered by the Pioneer 10 radio-occultation experiment, cannot easily be understood in terms of a model of a gravitationally bound, Earth-like ionosphere. Io's gravitational field is so weak that a gravitationally bound ionosphere would probably be blown away by the ram force of the Jovian magnetospheric wind — i.e., the plasma corotating in the Jovian magnetosphere. We propose here a model in which the material for Io's atmosphere and ionosphere is drawn from the ionosphere of Jupiter through a Birkeland current system that is driven by the potential induced across Io by the Jovian corotation electric field. We argue that the ionization near Io is caused by a comet-like interaction between the corotating plasma and Io's atmosphere. The initial interaction employs the critical velocity phenomenon proposed many years ago by Alfvén. Further ionization is produced by the impact of Jovian trapped energetic electrons, and the ionization thus created is swept out ahead of Io in its orbit. Thus, we suggest that what has been reported as a day-night ionospheric asymmetry is in fact an upstream-downstream asymmetry caused by the Jovian magnetospheric wind.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30th May, 1978.     

Anomalous magnetic diffusion in coronal current layers

The current-driven kinetic Alfvén instability is proposed as an anomalous transport mechanism for regions of concentrated, field-aligned currents in the solar corona. Anomalous magnetic diffusivity ( _{e f f} 10⁹cm²s^–1), produced by kinetic Alfvén turbulence in the vicinity of the saturation level, provides fast magnetic energy release with a local inflow Alfvén Mach numberM _in 0.1.     

The interplanetary electric field, cleft currents and plasma convection in the polar caps

This report investigates the suggestion that the pattern of plasma convection in the polar cleft region is directly determined by the interplanetary electric field (IEF). Owing to the geometrical properties of the magnetosphere, the East-West component of the IEF will drive field-aligned currents which connect to the ionosphere at points lying on either side of noon, while currents associated with the North-South component of the IEF will connect the two polar caps as sheet currents centered at noon. The effects of the hypothesized IEF driven cleft current systems on polar cap ionospheric plasma convection are investigated through a series of numerical simulations. The simulations demonstrate that this simple electrodynamic model can account for the narrow “throats” of strong dayside antisunward convection observed during periods of southward interplanetary magnetic field (IMF) as well as the sunward convection observed during periods of strongly northward IMF. Thedawn-dusk shift of polar cap convection which is related to the B_y component of the IMF is also accounted for by the model.     

A dawn-to-dusk electric field in the Jovian magnetosphere

We show that if Io-injected plasma is lost via a planetary wind a sun-fixed Birkeland current system may result. This is due to the fact that a current flows across a density gradient produced by the loss of plasma through the planetary wind in the tail. The divergent current is connected to field-aligned Birkeland currents which flow into the ionosphere at dawn and out of it at dusk. The closure currents in the ionosphere require a dawn-to-dusk electric field which at the orbit of Io is estimated to have a strength of a few mV m^?1. Independent estimates derived from the local time asymmetry of the torus u.v. emission indicate a field of 1.5mVm^?1.     

Short-wavelength drift dissipative instability in the presence of field-aligned currents

Sharp density gradients coupled with field-aligned currents can give rise to short wavelength (?15 m) drift waves due to collisional effects in the F-region of the auroral ionosphere. In this wavelength range, ion-ion collisions at altitudes of 300–450 km render the ions unmagnetized and a field-aligned current can drive a drift wave, propagating almost transverse to the magnetic field, unstable due to the resistance in electron parallel motion arising from electron collisions.     

Field-aligned and ionospheric currents

Zmuda and Armstrong (1974) showed that the field-aligned currents consist of two pairs; one is located in the morning sector and the other in the evening sector. Our analysis of magnetic records from the TRIAD satellite suggests that in each pair the poleward field-aligned current is more intense than the equatorward current, a typical ratio being 2:1. This difference has a fundamental importance in understanding the coupling between the magnetosphere and the ionosphere. We demonstrate this importance by computing the ionospheric current distribution by solving the continuity equation $\text{▽ .}\text{I}\text{= j}{}_{\mathrm{\parallel }}$ using the “observed” distribution of j_∥ for several models of the ionosphere with a high conductive annular ring (simulating the auroral oval).It is shown that the actual field-aligned and ionospheric current system is neither a simple Birkeland type, Boström type nor Zmuda-Armstrong type, but is a complicated combination of them. The relative importance among them varies considerably, depending on the conductivity distribution, the location of the peak of the field-aligned currents, etc. Further, it is found that the north-south segment of ionospheric current which connects the pair of the field-aligned currents in the morning sector does not close in the same meridian and has a large westward deflection. Thus, it has an appreciable contribution to the westward electrojet. One of the model calculations shows that the entire north-south closure current contributes to the westward electrojet.     

The equatorial latitude of auroral activity during 1972–1977

The equatorial latitude of auroral activity has been derived from both electron and optical observations with the DMSP satellites. Virtually all of the observations that were obtained during the 5-year interval June 1972-September 1977 have been used to construct a nearly continuous plot of invariant geomagnetic latitude versus time.This plot has two main characteristics: (1) A diurnal variation of approximately ± 5° which is associated with the precession of the Earth's magnetic dipole axis about the Earth's rotation axis; (2) an irregular variation of roughly 5–10° for intervals of one to several days associated with the occurrence of solar flares and coronal holes.With the help of a condensed, Bartels-type display of these measurements, we conclude that: (a) Modest auroral expansions (to ~ 60°) occur during the main body of high-speed streams from coronal holes; (b) great expansions (to < 55°) occur only during intervals of intense interplanetary magnetic fields such as may occur at the leading edge of a high-speed stream or at a flare-produced interplanetary shock.     

Outline of a theory of solar wind interaction with the magnetosphere

Some new ideas on the interaction of the solar wind with the magnetosphere are brought forward. The mechanism of reflection of charged particles at the magnetopause is examined. It is shown that in general the reflection is not specular but that a component of momentum of the particle parallel to the magnetopause changes. A critical angle is derived such that particles whose trajectories make an angle less than it with the magnetopause enter the magnetosphere freely, so transferring their forward momentum to it. Spatially or temporally non-uniform entry of charged particles into the magnetosphere causes electric fields parallel to the magnetopause which either allow the free passage of solar wind across it or vacuum reconnection to the interplanetary magnetic field depending on the direction of the latter. These electric fields can be discharged in the ionosphere and so account qualitatively for the dayside agitation of the geomagnetic field observed on the polar caps. The solar wind wind plasma which enters the magnetosphere creates (1) a dawn-dusk electric field across the tail (2) enough force to account for the geomagnetic tail and (3) enough current during disturbed times to account for the auroral electrojets. The entry of solar wind plasma across the magnetosphere and connection of the geomagnetic to interplanetary field can be assisted by wind generated electric field in the ionosphere transferred by the good conductivity along the geomagnetic field to the magnetopause. This may account for some of the observed correlations between phenomena in the lower atmosphere and a component of magnetic disturbance.     

The propagation of Alfvén waves and their directional anisotropy in the solar wind

The propagation of solar Alfvén waves in interplanetary space is studied in the approximation of geometrical optics. Ray paths and the change of wave vectors and amplitudes along the rays are determined assuming an Archimedean-spiral interplanetary magnetic field. In particular, the Alfvénic fluctuations in the 2 directions perpendicular to the magnetic field direction are calculated under the assumption that the Alfvén waves are produced at the Sun and emitted with an isotropic directional distribution from a reference level close to the Sun. It turns out that due to the combined effect of spherical expansion of the solar wind flow and the spiralling of the interplanetary field the magnetic fluctuations in the direction perpendicular both to the unperturbed field and the radial direction have much more power than in the other directions (directional anisotropy).Our results are compared with spacecraft observations made by Belcher and Davis (1971), that show an anisotropy of a similar character. It is argued that under average conditions the physical process leading to an anisotropy is not selective coupling of Alfvén waves into compressional waves, as suggested by Belcher and Davis, but rather the above mentioned dissipationfree effect of geometrical optics. Finally, arguments are presented to explain the discrepancy between the calculated high anisotropy and the measured low anisotropy in terms of finite amplitude effects and wavescattering.     

Global Simulation of Magnetospheric Space Weather Effects of the Bastille Day Storm

We present results from a global simulation of the interaction of the solar wind with Earth's magnetosphere, ionosphere, and thermosphere for the Bastille Day geomagnetic storm and compare the results with data. We find that during this event the magnetosphere becomes extremely compressed and eroded, causing 3 geosynchronous GOES satellites to enter the magnetosheath for an extended time period. At its extreme, the magnetopause moves at local noon as close as 4.9 R _E to Earth which is interpreted as the consequence of the combined action of enhanced dynamic pressure and strong dayside reconnection due to the strong southward interplanetary magnetic field component B _z, which at one time reaches a value of −60 nT. The lobes bulge sunward and shield the dayside reconnection region, thereby limiting the reconnection rate and thus the cross polar cap potential. Modeled ground magnetic perturbations are compared with data from 37 sub-auroral, auroral, and polar cap magnetometer stations. While the model can not yet predict the perturbations and fluctuations at individual ground stations, its predictions of the fluctuation spectrum in the 0–3 mHz range for the sub-auroral and high-latitude regions are remarkably good. However, at auroral latitudes (63° to 70° magnetic latitude) the predicted fluctuations are slightly too high. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014228230714