Syun-Ichi Akasofu

The development of the auroral substorm

Abstract A working model of simultaneous auroral activity over the entire polar region is presented in terms of the auroral substorm. The substorm has two characteristic phases, an expansive phase and a recovery phase. Each phase is divided into three stages, and characteristic auroral displays over the entire polar region during each stage are described in detail. Further, all the major features seen at a single station are combined into a consistent picture of large-scale auroral activity.

Energy coupling between the solar wind and the magnetosphere

This paper describes in detail how we are led to the first approximation expression for the solar wind-magnetosphere energy coupling function ɛ, which correlates well with the total energy consumption rate UTof the magnetosphere. It is shown that ɛ is the primary factor which controls the time development of magnetospheric substorms and storms. The finding of this particular expression ɛ indicates how the solar wind couples its energy to the magnetosphere; the solar wind and the magnetosphere constitute a dynamo. In fact, the power P generated by the dynamo can be identified as ɛ by using a dimensional analysis. Furthermore, the finding of ɛ indicates that the magnetosphere is closer to a directly driven system than to an unloading system which stores the generated energy before converting it to substorm and storm energies. Therefore, the finding of ɛ and its implications have considerably advanced and improved our understanding of magnetospheric processes. The finding of ɛ has also led us to a few specific future problems in understanding relationships between solar activity and magnetospheric disturbances, such as a study of distortion of the solar current disk and the accompanying changes of ɛ. It is also pointed out that one of the first tasks in the energy coupling study is an improvement of the total energy consumption rate UTof the magnetosphere. Specific steps to be taken in this study are suggested.

Polar and Magnetospheric Substorms.

Book on polar and magnetospheric substorms including daily variation data, satellite observations, etc

The polar electrojet.

Abstract It is shown that the polar electrojet which causes intense ‘negative bays’ or polar magnetic substorms in the high latitudes, flows usually westward in all longitudes along a closed oval curve. This substorm oval is also the site along which at the instant the active aurora is situated. Further, it appears to be the line of intersection of the outer boundary of the outer radiation belt with the ionosphere. The center of the oval is appreciably displaced from the earths magnetic axis pole, along the midnight meridian. The oval coincides approximately with the statistically determined auroral zone only in the midnight sector. The strength of the polar electrojet is not uniform along the substorm oval; some of the excess current in the most intense part of the electrojet (near the midnight meridian) returns across the polar cap, encircling a focus in this cap; the remainder departs from the afternoon sector and crosses the midnight meridian to complete its circuit by rejoining the main westward flow in the morning sector, with its focus in the midnight sector not far outside the oval. This latter flow produces positive bays everywhere south of the focus; these bays can be particularly intense in the afternoon sector along the auroral zone, and were formerly interpreted as indicating the existence of an eastward polar electrojet. The changing geometry of the polar eleetrojet is brought into relation with the dynamical morphology of the simultaneous auroral substorm, which has lately been determined by study of simultaneous all-sky camera records from the entire polar region.

Field-aligned and ionospheric currents

Abstract 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 ▽ . I = j ∥ 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, Bostrom 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.

Mathematical separation of directly driven and unloading components in the ionospheric equivalent currents during substorms

This paper attempts to separate objectively the directly driven and unloading components in substorm processes by applying the method of natural orthogonal components (MNOC). A time series of the ionospheric equivalent current function with time resolution of 5 min during March 17–19, 1978 is calculated on the basis of six meridian chains magnetometer data during the International Magnetospheric Study in order to obtain the fundamental orthogonal basis set. The first and second natural components of the set thus obtained dominate over the rest of the natural components. The first natural component is found to have a two-cell pattern, which is well known to be associated with global plasma convection in the magnetosphere. It is enhanced during the growth phase and expansion phase of substorms and decays during the recovery phase of substorms. Further, it is in fair correlation to the ϵ parameter with time lag of 20–25 min. This can be identified as the directly driven component. The second natural component reveals itself as an impulsive enhancement of the westward electrojet around midnight between 65° and 70° latitude during the expansion phase only. It is much less correlated with the ϵ parameter than the first one. Thus, as a first approximation, we identify it as the unloading component. It is shown that the directly driven component tends to dominate over the unloading component except for a brief period soon after substorm onset. This is the first clear determination of the time profile of the unloading component.

Dynamic morphology of auroras

Simultaneous changes of auroral forms, brightness, and motions over the whole polar region are studied, using IGY all-sky camera records from widely distributed stations in eastern Siberia, Alaska, Canada and the northern United States. It is found that the auroral system centered in the midnight sector in the auroral zone repeatedly undergoes an expansion and subsequent contraction; during the maximum stage of the activity, the whole auroral system extends over a substantial portion of the darkened polar region. Such extensive auroral activity as a whole may be regarded as a single event, and is described in terms of the auroral substorm. The substorm has two characteristic phases, an expansive phase and a recovery phase. Characteristic auroral displays over the entire polar region during the substorm are described in detail. The basic physical processes involved for the auroral substorm are also discussed.Geomagnetic disturbances associated with the auroral substorm are also described in detail in terms of the polar magnetic substorm, and it is shown that both the auroral substorm and the polar magnetic substorm are different aspects of the manifestation of a large-scale plasma motion in the magnetosphere.The distribution of the aurora for different degrees of the geomagnetic activity is also discussed in terms of the auroral belt. It is shown that the center line of the auroral belt moves greatly with respect to its average location (namely the auroral zone), depending on the degree of the magnetic activity.

The diffuse aurora

Abstract A study of ground-based all-sky photographs substantiates the presence of the diffuse auroral belt as seen by the ISIS-2 (polar orbiting satellite) scanning auroral photometer. The intensity of the diffuse aurora increases when discrete auroras become active; in particular the diffuse aurora is most clearly seen equatorward of westward travelling surges. However, in the morning sector, it may or may not be detectable near eastward drifting patches in all-sky photographs. Some of what has been previously identified visually and in all-sky photographs as the proton aurora probably is a part of what we identify here as the diffuse aurora. The diffuse aurora appears sometimes to branch out into two, one along the auroral oval and the other along a constant geomagnetic latitude circle. The latter probably corresponds to the mantle aurora and the drizzle zone precipitation.

Magnetospheric Substorms: A Model

An attempt is made to integrate and interpret various polar and magnetospheric substorm phenomena as consequences of the acceleration process of auroral particles which could result from the diversion of a part of the magnetotail current to the night side of the auroral oval. Thus, this paper is not intended to be a literature survey, but to present a way along which magnetospheric substorms might be studied in the future.

Electrodynamics of the magnetosphere: Geomagnetic storms

Geomagnetic and auroral storms provide a great deal of detailed information on the interaction between the solar plasma flows and the magnetosphere. Vast numbers of observations have been accumulated, and many theories have been developed to explain them. However, many of the most vital features of the interaction remain unsolved. The purpose of this paper is to provide the background for future work by summarizing fundamental morphological data and by reviewing critically the proposed theories.The paper consists of four sections. In the first section, the structure of the solar plasma flows and the magnetosphere are briefly discussed. Effects of the direct impact of the plasma flows on the magnetosphere are described in Section 2. Both Sections 3 and 4 are devoted to the discussion of the major phase of geomagnetic storms, namely the formation of the asymmetric ring current belt and the development of the auroral and polar magnetic substorms, respectively.