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.     

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.     

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.     

Average high latitude magnetic field: Variation with interplanetary sector and with season—I. Disturbed conditions

High latitude magnetic field data from 16 northern observatories are averaged during periods of magnetic disturbance level Kp = 2^? to 3⁺. Within this disturbance level, variations between interplanetary magnetic field sector (toward and away from the Sun) and geomagnetic season (dipole latitude of the Sun: > 10° = summer, < ? 10° = winter) are delineated. Variations between seasons are: (1) The positive bay and polar cap disturbance is a maximum in summer and a minimum in winter for both sectors. (2) The negative bay disturbance is a maximum in summer and a minimum in winter when the interplanetary field is toward the Sun and vice versa during away sectors. Variations between sectors are: (1) During summer and equinox the negative bay disturbance is greater for toward sectors than for away sectors. The reverse occurs during winter. (2) The positive bay disturbance is greater during toward sectors than during away sectors for all seasons. (3) All diiferences in disturbance level are greater at sunlit local times than in darkness. (4) Angular differences in the direction of the horizontal disturbance of up to 75° occur between sectors in the polar cap and dayside during all seasons. (5) The polar cap-auroral belt boundary location is different for the two sectors. Compared to data from away sectors, this boundary for toward sectors is shifted northward near dawn (5–8^h) and southward between 10 and 22^h. (6) Accompanying this boundary difference there is a change in the direction of the vertical disturbance in the region between 9 and 14^h at geomagnetic latitudes 77–88°. ΔZ in this region is negative during away sectors and positive during toward sectors. Differences between sectors are attributed to changes in the ionospheric electric field configuration and in the distribution of magnetic field aligned currents.Features unrelated to sector or season also occur: (1) A significant Y component is present in both the positive and negative bays. (2) The vertical disturbance $\text{(¦ΔZ¦)}$ to the north of the auroral belt is much larger than that to the south. (3) Two distinct regions of maximum activity are present in the ΔZ accompanying the positive bay disturbance.     

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.     

Latitudinal characteristics of the geomagnetic field during the storm of 15–16 July 2000

In this paper geomagnetic disturbances at middle and low latitudes are discussed by using geomagnetic data of the magnetic storm of 15–16 July 2000. This storm is a response to the solar Bastille Day flare on 14 July. Generally, the geomagnetic disturbances at middle and low latitudes during a storm are mainly caused by three magnetospheric–ionospheric current systems, such as the ring current system (RC), the partial ring current and its associated region II field-aligned currents (PR), and the region I field-aligned currents (FA). Our results show that: (1) The northward turning of IMF-Bz started the sudden commencement of the storm, and its southward turning caused the main phase of the storm. (2) The PR- and FA-currents varied violently in the main phase. In general, the field of the FA-current was stronger than that of the PR-current. (3) In the first stage of the recovery phase, the RC-field gradually turned anti-parallel to the geomagnetic axis from a 15° deviation, and the local time (Λ) pointed by the RC-field stayed at 16:00. After that, Λ rotated with the stations, and the RC-field was not anti-parallel to the geomagnetic axis, but 5°–10° deviated. These facts suggest that the warped tailward part of the ring current decays faster than the symmetric ring current.     

Numerical analysis of equatorial enhancement of geomagnetic sudden commencement

Equatorial behaviour of a polar-originating ionospheric current is examined by solving numerically the continuity equation on a two-dimensional spherical shell with appropriate assumptions for the ionospheric conductivity and the field-aligned source currents. The results show a clear daytime equatorial enhancement of the ionospheric currents in spite of much reduced electric field due to shielding effects of the enhanced Cowling conductivity there. The results are used for interpretation of the preliminary impulse of the geomagnetic sudden commencement.     

Joint two-dimensional observations of ground magnetic and ionospheric electric fields associated with auroral zone currents: Current systems associated with local auroral break-ups

On 15 February, 1977, ground magnetic, ionospheric electric and auroral signatures of a multiple onset substorm were observed simultaneously by the Scandinavian Magnetometer Array (SMA), the Scandinavian Twin Auroral Radar Experiment (STARE) and the Finnish all-sky camera chain. Between 21:00 and 21:30 U.T., i.e. around local magnetic midnight, three consecutive local auroral break-ups were observed over Scandinavia. Each of these break-ups was preceded by a clear fading of the aurora and magnetic fields (while the electric fields remained unaffected), and occurred slightly south of the Harang discontinuity in the region of north-westward-directed electric fields. They were associated with a sudden change in direction of the electric field from north-west to south-west and the appearance of a westward equivalent current in the localized active region (about 1200 × 300 km²). These observations matched the features to be expected during the generation of a Cowling channel by a strong increase of the ionospheric conductivities due to precipitating auroral electrons. Numerical model calculations, based on the observations during the initial brightening and peak development of the second, most conspicuous break-up, show that the field-aligned currents at the northern and southern border of the active region are indeed very weak. However, highly localized and intense upward field-aligned currents at the western edge of the active region and more widespread and less intense downward currents in the eastern half preserve current continuity of the westward Cowling current and complete the substorm current wedge.     

Pi 2 pulsations: High latitude results

A review of recent experimental results from studies of high latitude Pi 2 pulsations indicates that these pulsations are fundamentally related to the initiation of the auroral breakup and substorm. At high latitudes, the Pi 2's show their peak intensities in the region where the breakup begins and appear to remain in this region after the breakup has spread poleward. In addition, the Pi 2's occur simultaneously with, or before all other ionospheric phenomena associated with the breakup. The field aligned and ionospheric currents associated with the Pi 2 resemble those of a typical substorm, but the ionospheric currents are phase shifted compared to the field aligned current. The periodic oscillations of the Pi 2's are probably caused by a reflection of the initial field aligned current pulse from the auroral ionosphere. This pulse is trapped on dipolar field lines leading to multiple reflections from North and South auroral ionospheres.     

Turbulent transport and heating in the auroral plasma of the topside ionosphere

Using plasma parameters from a typical stormtime ionospheric energy balance model, we have investigated the effects of plasma turbulence on the auroral magnetoplasma. The turbulence is assumed to be comprised of electrostatic ion cyclotron waves. These waves have been driven to a nonthermal level by a geomagnetic field-aligned, current-driven instability. The evolution of this instability is shown to proceed in two stages and indicates an anomalous increase in field-aligned electrical resistivity and cross-field ion thermal conductivity as well as a decrease in electron thermal conductivity along the geomagnetic field. In addition, this turbulence heats ions perpendicular to the geomagnetic field and hence leads to a significant ion temperature anisotropy.     

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.     

Birkeland currents in the Earth's magnetosphere

As a result of his polar expeditions at the beginning of this century, Kristian Birkeland determined that intense ionospheric currents were associated with the aurora. Birkeland suggested that these currents originated far from the Earth and that they flowed ointo and away from the polar atmosphere along the geomagnetic field lines. The existence of such field-aligned or Birkeland currents was disputed because it was not possible to unambiguously identify current systems that are field-aligned (as suggested by Alfvén, 1939, 1940) and those which are completely contained in the ionosphere (as developed by Vestine and Chapman, 1938) with surface magnetic field observations. The presence of Birkeland currents has been absolutely confirmed with satellite-borne particle and magnetic field experiments conducted over the past two decades. These satellite observations have determined the large-scale patterns, flow directions, and intensities of Birkeland currents in the auroral and polar regions, and their relationship to the orientation and magnitude of the interplanetary magnetic field. The Birkeland currents are directly associated with visible and UV auroral forms observed with satellites. The results obtained from a variety of recently launched satellites are discussed here. These include Sweden's first satellite, VIKING, which has provided evidence for resonant Alfvén waves on the same geomagnetic field lines that guide stationary Birkeland currents. These observations demonstrate the important role that these currents play in the coupling of energy between the interplanetary medium and the lower ionosphere and atmosphere.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Modelling of Pc5 pulsation structure in the magnetosphere

Magnetohydrodynamic resonance theory is used to model the structure of the magnetospheric and ionospheric electric and magnetic fields associated with Pc5 geomagnetic pulsations. In this paper the variation of the fields across the invariant latitude of the resonance are computed. The results are combined with calculations of the variation along a field line to map the fields down to the ionosphere. In one case the results are compared with measurements obtained by the STARE auroral radar and show good agreement. The relationship between the width of the resonance region and ionospheric height-integrated Pedersen conductivity is computed and it is shown how auroral radar measurements of Pc5 oscillations could be used to determine ionospheric height-integrated Pedersen conductivity. It is pointed out that from these calculations it would be possible to identify the field line on which a satellite was located by comparing a Pc5 pulsation observed by the satellite, and the same pulsation observed by STARE.     

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.     

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.     

Electric fields and currents in the ionosphere generated by field-aligned currents observed by TRIAD

On the basis of the experimental data on the ionospheric conductivities and field-aligned currents the electric fields and currents in the ionosphere generated by the field-aligned currents were computated for various magnetic activity conditions. The model of the ionospheric conductivities by Vanyan and Osipova (1975) was used taking into account the influence of the universal time seasons and magnetic activity. The field-aligned current patterns and their change with magnetic activity was set on the basis of the TRIAD data. It is shown that the calculated patterns of the ionospheric electric fields and currents are in agreement with the measured electric fields and the equivalent current systems of the magnetic disturbances in high latitudes. The conclusion is made that the magnetospheric field-aligned currents are the main sources of the presently known polar magnetic disturbances.     

Electron precipitation equatorward of the auroral oval and the mantle aurora in the midday sector

In the midday sector, the hard electron precipitation and the associated patchy aurora at geomagnetic latitude ～65° are the only auroral features (? 20 keV) located equatorward of the dayside auroral oval during intense and moderately disturbed geomagnetic conditions. We identify the patchy luminosity in the midday and late morning sectors as the active mantle aurora. The mantle aurora was found by Sanford (1964) using the IGY-IGC auroral patrol spectrographs and which was thought to be non-visual. The precipitating electrons reside mostly at energies greater than several keV with an energy flux of ? 0.1 erg cm^?2 s^?1 sr^?1 during geomagnetic active periods. This hard precipitation occurs in a region which is asymmetric in L.T. with respect to the noon meridian. The region extends from the morning sector to only early afternoon (13–14 M.L.T.) along the geomagnetic latitude circle of about 65–70°. The model calculation indicates that the mantle aurora is produced by the precipitation of the energetic electrons which drift azimuthally from the plasma sheet at the midnight sector to the dayside magnetopause during magnetospheric substorms.     

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.     

Role of field-aligned currents in generation of high-latitude magnetic disturbances

The distant effects of the field-aligned currents (FAC) observed by TRIAD are computed for conditions of low and moderate activity. The systems of total ionospheric currents (both Hall and Pedersen) generated by corresponding FAC are also examined and the contribution of the distant effects and the ionospheric currents into the total equivalent current system is estimated. The conclusions are as follows. In cases of low magnetic activity the magnetic effects produced by Pedersen currents are mainly cancelled by the FAC distant effect in accord with Fukushima's theorem. In cases of moderate activity when the zone of high ionospheric conductivity and the two-sheet FAC structure are present the FAC distant effect is too small to cancel the effect of Pedersen currents. For these conditions the system of total ionospheric currents shows the best correspondence with the experimental equivalent current system. Effects produced by the IMF azimuthal component are also analysed.     

The Auroral electrojet and field-aligned current

The location of field-aligned currents in the evening sector with respect to the auroral electrojets is examined. The tri-axial TRIAD satellite data and the simultaneous ground magnetometer data from along the Alaska meridian are analysed. It is shown that an intense upward fieldaligned current flows out from the region of the westward electrojet where discrete auroras are located. The downward flowing current exists in the region further equatorward, namely in the region of the eastward electrojet. However, the downward current is present even when there is no eastward electrojet. The boundary between the upward and the downward currents coincides, in most cases, with the boundary between the westward and the eastward auroral electrojets. Thus, the Harang discontinuity, a narrow area separating the positive and negative H bays, is the region where there is no field-aligned current.