S.-I. Akasofu

Electric conductivities, electric fields and auroral particle energy injection rate in the auroral ionosphere and their empirical relations to the horizontal magnetic disturbances

Abstract Recent progress in modeling ionospheric current systems requires global conductivity models which can reflect substorm conditions on an instantaneous basis. For this purpose, empirical relations of the North-South component ( ΔH ) of the magnetic disturbance field observed at College with the Pedersen ( Σ p ) and Hall ( Σ H ) conductivities deduced from the Chatanika radar data and their ratio ( Σ H Σ p ) are examined. These empirical formulas allow us to construct approximate distribution patterns of Σ p and Σ > H over the entire polar region on the basis of the distribution of ΔH at given instants by devising an appropriate weighting function for both the polar cap and the subauroral region. The global conductivity distributions thus obtained are compared with those employed by Kamide et al. (1981) and Spiro et al . (1982). The comparisons show that the gross features are similar among them. In addition, we also examine the relationship of ΔH with the North-South component of the electric field with the particle energy injection rate ( u A ) estimated from the Chatanika radar data. Based on the empirical relation between Σ H and u A the global distribution of the latter over the entire polar region at particular instants can also be obtained.

A dynamo theory of solar flares

It is proposed that the solar flare phenomenon can be understood as a manifestation of the electrodynamic coupling process of the photosphere-chromosphere-corona system as a whole. The system is coupled by electric currents, flowing along (both upward and downward) and across the magnetic field lines, powered by the dynamo process driven by the neutral wind in the photosphere and the lower chromosphere. A self-consistent formulation of the proposed coupling system is given. It is shown in particular that the coupling system can generate and dissipate the power of 1029 erg s#X2212;1 and the total energy of 1032 erg during a typical life time (103 s) of solar flares. The energy consumptions include Joule heat production, acceleration of current-carrying particles along field lines, magnetic energy storage and kinetic energy of plasma convection. The particle acceleration arises from the development of field-aligned potential drops of 10–150 kV due to the loss-cone constriction effect along the upward field-aligned currents, causing optical, X-ray and radio emissions. The total number of precipitating electrons during a flare is shown to be of order 1037–1038.

The energy coupling function and the power generated by the solar wind-magnetosphere dynamo

Abstract A solar wind parameter e , known as the energy coupling function, has been shown to correlate with the power consumption in the magnetosphere. It is shown in the present paper that the parameter e can be identified semi-quantitatively as the dynamo power delivered from the solar wind to an open magnetosphere. This identification not only provides a theoretical basis for the energy coupling function, but also constitutes an observational verification of the solar wind-magnetosphere dynamo along the magnetotail. Moreover, one can now conclude that a substorm results when the dynamo power exceeds 10 18 ergs −1 .

The interplanetary and solar causes of geomagnetic activity

Abstract We present a review of recent work done on the topic of interplanetary and solar causes of geomagnetic activity. During solar maximum (1978–1979), 90% of the major magnetic storms ( D ST ⩽ − 100 nT) are caused by large southward B z events associated with interplanetary shocks. Of these, roughly half of the B z events are located in the sheath and half associated with the driver gas. These two sources of southward IMFs often give magnetic storms a two-step profile. The sheath field events are generated in the interplanetary medium between the outer corona and the Earth from the “shocking” of the slow solar wind upstream of the high speed stream. In contrast, the driver gas events are fields which come from the solar source region. A correlation between the field orientation at the solar source and that at 1 a.u. was sought, but none was found. Thus, quantitative predictions of storm intensities from solar observations appear to be very difficult. Prominence eruptions are shown to be an important cause of the high speed solar wind streams that lead to magnetic storms. The other 10% of the magnetic storms arc not related to interplanetary shocks or high speed streams, but to high density “non-compressional density enhancements”. Following magnetic storms arc “high-intensity long-duration AE activity events” (HILDCAAs) that are series of continuous auroral substorms that last from days to weeks during or after the storms recovery phase. HILDCAAs can also occur independently of magnetic storms. This continuous auroral activity is caused by the southward component of the magnetic field of interplanetary Alfven waves, presumably through the process of magnetic reconnection with the Earths field. These Alfven wave trains arc often observed in the trailing portions of high speed streams. From an analysis of a years data during solar maximum, it is found that the interplanetary medium is “Alfvenic” approx. 60% of the time. There appear to be no substantial differences in magnetusphcric response to Alfvenic or non-Alfvenic interplanetary intervals. The magnetopause boundary layer is shown to contain broad-band ELF/VLF plasma waves at least 85% of the time at all daysidc local times. These waves have sufficient amplitude to cause cross-field diffusion of magnetosheath plasma to form the low latitude boundary layer. Pitch angle scattering of the low latitude boundary layer particles is adequate to account for the dayside aurora. The only interplanetary /magnetosheath parameter that appears to affect the wave intensities is the IMF B z . Although the waves arc present at almost all times, they are intensified during southward IMF B z intervals.

Power transmission from the solar wind-magnetosphere dynamo to the magnetosphere and to the ionosphere: Analysis of the IMS Alaska meridian chain data

Abstract It is shown that the power e generated by the solar wind-magnetosphere dynamo is transmitted to the convective motion of magnetospheric plasma. This convective motion generates what we may call the “Pedersen counterpart currents” in the magnetosphere and drives a large part of the “region 1 and 2” field-aligned currents which are closed by the Pedersen currents in the ionosphere. These results are based on a self-consistent set of the ionospheric current and potential distribution patterns obtained from a study of the International Magnetosphere Study Alaska meridian chain data.

On the ring current energy injection rate

Abstract Assuming that the formation of the ring current belt is a direct consequence of an enhanced crosstail electric field and hence of an enhanced convection, we calculate the total ring current kinetic energy (KR) and the ring current energy injection rate (UR) as a function of the cross-tail electric field (ECT); the cross-tail electric field is assumed to have a step function-like increase. The loss of ring current particles due to recombination and charge-exchange is assumed to be distributed over the whole ring current region. It is found that: (1) the steady-state ring current energy KR is approximately linearly proportional to ECT; (2) the characteristic time tc for KR to reach the saturation level is 3–4 h; (3) the injection rate UR is proportional to ECTβ where β ≅ 1.33−1.52; and (4) the characteristic time tp for UR to reach the peak value is 1–2 h and the peak UR value is 50% higher than the steady-state value. Since β is now determined specifically for an enhanced convection, an observational determination of the relationship between ECT(or φCT) and UR is essential to a better understanding of ring current formation processes. If the observed β is greater than 1.5, additional processes (e.g. an injection of heavy ions from the ionosphere to the plasma sheet and subsequently to the ring current region) may be required.

Assessing the magnetic reconnection paradigm

The hypothesis of magnetic reconnection was originally proposed as an attempt to explain solar flares. Solar flares dissipate as much as 1029 erg/s in about 30 min, the total consumed energy being ∼1032 erg. Since there was no obvious phenomenon that could be identified as the power supply process, it was assumed that the necessary energy was stored prior to flare onset and was then released and dissipated very rapidly. Since solar flares tend to occur near complex sunspot groups, it was hypothesized that the necessary energy was stored as magnetic energy. The hypothesis of magnetic reconnection was introduced as the process that spontaneously converts the stored magnetic energy for flare processes. Thus it was hypothesized that to provide as much as 1029 erg/s, the process must also be explosive.

A mechanism for the formation of plasmoids and kink waves in the heliospheric current sheet

Satellite observations of the heliospheric current sheet indicate that the plasma flow velocity is low at the center of the current sheet and high on the two sides of current sheet. In this paper, we investigate the growth rates and eigenmodes of the sausage, kind, and tearing instabilities in the heliospheric current sheet with the observed sheared flow. These instabilities may lead to the formation of the plasmoids and kink waves in the solar wind. The results show that both the sausage and kink modes can be excited in the heliospheric current sheet with a growth time ∼ 0.05–5 day. Therefore, these modes can grow during the transit of the solar wind from the Sun to the Earth. The sausage mode grows faster than the kink mode for β∞ < 1.5, while the streaming kink instability has a higher growth rate for β∞ > 1.5. Here β∞ is the ratio between the plasma and magnetic pressures away from the current layer. If a finite resistivity is considered, the streaming sausage mode evolves into the streaming tearing mode with the formation of magnetic islands. We suggest that some of the magnetic clouds and plasmoids observed in the solar wind may be associated with the streaming sausage instability. Furthermore, it is found that a large-scale kink wave may develop in the region with a radial distance greater than 0.5–1.5 AU.

Temperature variation of the plasma sheet during substorms

Abstract The temperature and density of the plasma in the Earths distant plasma sheet at the downstream distances of about 20–25 Re are examined during a high geomagnetic disturbance period. It is shown that the plasma sheet cools when magnetospheric substorm expansion is indicated by the AE index. During cooling, the plasma sheet temperature, T, and the number density, N, are related by T ∝ N 2 3 (adiabatic process) in some instances, while by T ∝ N−1 (isobaric process) in other cases. The total plasma and magnetic pressure decreases when T ∝ N 2 3 and increases when T ∝ N−1. Observation also indicates that the dawn-dusk component of plasma flow is frequently large and comparable to the sunward-tailward flow component near the central plasma sheet during substorms.

Total current of the auroral electrojet estimated from the IMS Alaska meridian chain of magnetic observatories

Abstract Based on magnetic data from the IMS Alaska meridian chain of observatories, the total current of the westward auroral electrojet flowing across the meridian is estimated by using two independent methods. The first one is a simple integration of the north-south component of magnetic perturbations along the meridian, providing the quantity F in units of nT·km. The other is to use the forward method, providing the total current I in units of A. It is shown that F and I have nearly identical time variations. Thus, by normalizing the two quantities and taking the numerical value of F in units of nT·km, it is possible to estimate the total electrojet current flowing across a magnetic meridian, with an accuracy of 90%, by using the latitudinal profile of the H component, namely I (A) = 2.0 F (nT·km).