A numerical simulation study of the thermospheric winds produced by auroral heating during magnetic storms, and of their global dynamo effects, establishes the main features of the ionospheric disturbance dynamo. Driven by auroral heating, a Hadley cell is created with equatorward winds blowing above about 120 km at mid-latitudes. The transport of angular momentum by these winds produces a subrotation of the mid-latitude thermosphere or westward motion with respect to the earth. The westward winds in turn drive equatorward Pedersen currents which accumulate charge toward the equator, resulting in the generation of a poleward electric field, a westward E × B drift, and an eastward current. When realistic local time conductivity variations are simulated, the eastward mid-latitude current is found to close partly via lower latitudes, resulting in an ‘anti-Sq’ type of current vortex. Both electric field and current at low latitudes thus vary in opposition to their normal quiet-day behavior. This total pattern of disturbance winds, electric fields, and currents is superimposed upon the background quiet-day pattern. When the neutral winds are artificially confined on the nightside, the basic pattern of predominantly westward E × B plasma drifts still prevails on the nightside but no longer extends into the dayside. Considerable observational evidence exists, suggesting that the ionospheric disturbance dynamo has an appreciable influence on storm-time ionospheric electric fields at middle and low latitudes.

The ionospheric disturbance dynamo

Characteristics of field-aligned currents have been determined during a large number of substorms from the magnetic field observations acquired with the Triad satellite. The statistical features of field-aligned currents include the following: (1) The large-scale regions of field-aligned currents determined previously by the authors (Iijima and Potemra, 1976a) persist during all phases of substorm activity, namely, region 1, located near the poleward boundary of the field-aligned current region, and region 2, located near the equatorward boundary. Field-aligned currents flow into region 1 on the morningside and away from region 1 on the eveningside. The current flow in region 2 is reversed to region 1 at any given local time except in the Harang discontinuity region (∼2000–2400 MLT), where the flow patterns are more complicated. (2) During active periods (|AL| ≥ 100 γ) the average latitude width of regions 1 and 2 increases by 20–30%, and the centers of these regions shift equatorward by 2°–3° with respect to the quiet time values. (3) The current density in region 1 is statistically larger than the current density in region 2 at all local times except during active periods and in the midnight to morning local time sector. In this region, where the westward electrojet is most active, the current density in region 2 can exceed the current density in region 1. (4) During relatively quiet conditions (|AL| < 100 γ) the largest field-aligned current densities occur in two areas of region 1 near noon (near ∼ 1030 MLT and ∼ 1300 MLT) with an average value of ∼1.6µA/m². During active periods (|AL| ≥ 100 γ) the regions of peak current density shift toward the nightside (the region near 1030 MLT shifts to ∼0730 MLT, and the region near ∼1300 MLT shifts to ∼1430 MLT), and the average current density increases to ∼2.2 µA/m². (5) The average total amount of field-aligned current flowing into the ionosphere always equals the current flow away from the ionosphere during a wide range of quiet and disturbed conditions. The average total current during quiet periods is ∼2.7 × 106 A and during disturbed periods is ∼5.2 × 106 A. (6) A three-region pattern of field-aligned current flow persists in the Harang discontinuity region (∼2000–2400 MLT) during undisturbed and disturbed periods, when the westward auroral electrojet does not intrude into this sector. This flow pattern consists of an upward flowing field-aligned current surrounded to the north and south by downward flowing currents. During periods when the westward auroral electrojet has intruded deeply into the evening sector the Triad magnetometer data exhibit complicated and fine-structured variations indicating the presence of complex field-aligned currents in this sector. (7) The alignment of current sheets is generally along the boundary of the auroral oval (rather than in the east-west direction), but noticeable distortions of this alignment occur during very disturbed periods. The alignment of field-aligned currents is different in region 1 and region 2 during active periods. The different behavior of field-aligned currents in region 1 and 2 during substorms actively suggests that they are controlled by different source regions in the magnetosphere or ionosphere. The region 1 field-aligned currents map to the outermost part of the magnetosphere and magnetotail region, whereas the region 2 currents map to regions of the plasma sheet closer to the earth.

Large‐scale characteristics of field‐aligned currents associated with substorms

This paper describes a novel procedure for mapping high-latitude electric fields and currents and their associated magnetic variations, using sets of localized observational data derived from different types of measurements. The technique provides a formalism for incorporating simultaneously such different classes of data as electric fields from radars and satellites, electric currents from radars, and magnetic perturbations at the ground and at satellite heights; the technique also uses available statistical information on the averages and variances of electrodynamic fields. The technique provides a more rigorous way of quantitatively estimating high-latitude electric field and current patterns than other methods and has a capability to quantify the errors in the mapped fields, based on the distribution of available data, their errors, and the statistical variances of the fields. The technique is illustrated by an application to a substorm which was analyzed by Kamide et al. (1982) by an earlier technique.

Mapping electrodynamic features of the high-latitude ionosphere from localized observations: technique

An approximate method of separating the effects of ionospheric currents from those of field-aligned currents in ground magnetic perturbations observed in high latitudes is developed. The distribution of ionospheric electric fields can also be estimated. The procedure includes the following steps: (1) the calculation of the equivalent ionospheric current function on the basis of magnetic H and D component records on the earth's surface, (2) the computation of the electric potential distribution from the equivalent ionospheric current function by using a simple model of the ionospheric conductivity, (3) the derivation of ionospheric current vectors as well as electric fields and, (4) the derivation of the field-aligned current intensity by taking the divergence of the ionospheric currents. Several examples for both quiet and disturbed conditions are utilized to demonstrate how our method is successful in estimating the intensities of the electric fields and the ionospheric and field-aligned currents in the sense that the estimated values are in good agreement with those observed recently by radar and satellite methods. Significant portions of the H component in nightside auroral latitudes appear to result from the east-west ionospheric currents, called the auroral electrojets, while both the north-south ionospheric current and field-aligned current are almost equally important in producing the D component excursions. It is found that the ionospheric and field-aligned current distributions obtained are not very sensitive to the choice of the ionospheric conductivity model, unless the auroral enhancement is not given in an appropriate place. This indicates that even a simple conductivity distribution inferred from the distribution of the magnetic perturbations can make it possible to estimate the three-dimensional current system with a reasonable accuracy.

Estimation of ionospheric electric fields, ionospheric currents, and field-aligned currents from ground magnetic records

The atmosphere above 80 km exhibits a wide range of dynamical phenomena. These are largely hydrodynamic in nature, and their sources of energy are likely to lie principally at lower levels. The regions are ionized, however, and hydromagnetically controlled motions also exist. They are generated from above as well as below.

The observational evidence is reviewed broadly here, in groupings determined by the physical processes that are thought to be operative, and the corresponding theories are measured against it. Many shortcomings are disclosed and assessed, and profitable avenues for future research are tentatively explored. Several areas are indicated in which the dynamical processes modify other features of the upper atmosphere, but these are not analysed in detail.

The upper atmosphere in motion

Generalized theorem for no ground magnetic effect of vertical currents connected with Pedersen currents in the uniform-conductivity ionosphere

A new method is proposed for calculation of electric charge distribution in the ionosphere associated with the dynamo action for any given air motion If the dynamo action is asymmetric in the northern and southern hemispheres, the electric potential difference of the order of 1kV will be produced (if the field-aligned current is prohibited) at the conjugate pair of stations in middle latitudes although the potential difference will be almost cancelled by the field-aligned currents in the magnetosphere flowing from the winter hemisphere to the summer hemisphere It is also shown that the height-gradient of the ionospheric conductivity plays a very important role for charge separation in the ionosphere, the effect of which intensifies the eastward equatorial electrojet on the dayside

Electric Potential Difference between Conjugate Points in Middle Latitudes Caused by Asymmetric Dynamo in the Ionosphere

In order to trace the secular variation of the geomagnetic field in historic time, the documents of ancient aurorae can be utilized. In China, Korea and Japan, there are a number of valuable records, which can be reasonably supposed to describe the events of auroral appearance. After a comparison of these descriptions with those in Europe, the archaeo-secular variation in the geomagnetic field can be inferred. A preliminary examination of the auroral appearance on the same day in the Occident and Orient suggests that the geomagnetic dipole axis might have been inclined towards China around 11-12th centuries AD.

https://www.jstage.jst.go.jp/article/jgg1949/20/1/20_1_45/_pdf

Naoshi Fukushima

Papers

Generalized theorem for no ground magnetic effect of vertical currents connected with Pedersen currents in the uniform-conductivity ionosphere

Electric Potential Difference between Conjugate Points in Middle Latitudes Caused by Asymmetric Dynamo in the Ionosphere

Archaeo-Aurora and Geomagnetic Secular Variation in Historic Time

Polar magnetic substorms

Morphology of magnetic disturbance.