Solar Wind-magnetosphere Dynamo Research Articles

Magnetic Signatures of Large-Scale Electric Currents in the Earth’s Environment at Middle and Low Latitudes

The purpose of space weather is the systemic study of the Sun–Earth system, in order to determine the impact of solar events on the electromagnetic environment of the Earth. This article proposes a new transdisciplinary approach of the Sun–Earth system based on the universal physical process of the dynamo. The dynamo process is based on two important parameters of the different plasmas of the Sun–Earth system, the motion and the magnetic field. There are four permanent dynamos in the Sun–Earth system: the solar dynamo, the Earth dynamo, the solar wind-magnetosphere dynamo, and the ionospheric dynamo. These four permanent dynamos are part of different scientific disciplines. This transdisciplinary approach links all of these dynamos in order to understand the variations in the Earth’s magnetic field. During a magnetic disturbed period, other dynamos exist. We focused on the ionospheric disturbed dynamo generated by Joule energy dissipated in the high latitude ionosphere during magnetic storms. Joule heating disrupts the circulation of thermospheric winds and in turn generates disturbances in the Earth’s magnetic field. This systemic approach makes it possible to understand magnetic disturbances previously not well understood.

Electric Current Approach Studying Both Auroral Substorms and Solar Flares Together

Auroral substorms and solar flares are basically various manifestations of electromagnetic energy dissipation, so that it is useful to consider both phenomena in terms of a chain of processes, consisting of power supply (dynamo), transmission (currents/circuits) and dissipation (auroral substorms, solar flares), the electric current approach. In this short review, we briefly describe both phenomena together on the basis of the chain process. It is shown that the introduction of a dynamo process in this consideration provides a step-by-step way of studying both phenomena. It is shown: (a) Both the solar wind-magnetosphere dynamo and a photospheric dynamo proposed by Lee et al. (1995) have enough power to accumulate enough energy for the explosive features of both phenomena, respectively. (b) For substorms, the power is accumulated in the inner magnetosphere and inflates it, and for flares the power is likely to be accumulated in a loop current along and above two-ribbon flares. (c) For substorms, the energy release (unloading) process deflates the inner magnetosphere, resulting in an earthward electric field, and for flares the disruption of the loop current suggested by Alfven (1950) may be responsible for the energy release. (d) The processes in (c) are responsible for the explosive features of both phenomena.

The Explosive Characteristics of the Aurora: The Electric Current Line Approach

The aurora shows explosive activities a few times in 24 hours on a moderately active day. This specific phenomenon is called the auroral substorm, which consists of the growth, expansion and recovery phases; the explosive activities occur during the expansion phase. As an introduction, the explosive activities of the aurora are morphologically described first on the basis of ground-based all-sky and satellite images. In terms of theoretical understanding, the processes for the explosive activities have been considered almost exclusively in terms of “the magnetic field line approach” in the past, including the process of magnetic reconnection. Instead, in this paper, we consider the processes in terms of “the electric current line approach”. This approach requires that the whole process of auroral substorms should be considered as a chain of processes, which consists of power supply (dynamo), transmission (currents/circuits) and dissipation (auroral substorms). An increased power of the solar wind-magnetosphere dynamo (≈ 5x1018 erg/s or 1011w) intensifies the electric currents mainly in the main body (just outside of the ring current) of the magnetosphere, resulting in accumulating energy in its inductive circuit and inflation of the magnetosphere. When the accumulated energy reaches about 5x1022 ergs (5x1015 J), the magnetosphere tends to become unstable (because of current instabilities). As the current intensity is reduced as a result, the magnetosphere is deflated. It is suggested that it is in this deflation process, during which the accumulated energy is unloaded, and an earthward electric field (5-50 mV/m) is produced on the equatorial plane, establishing the current system (the UL current system), which is responsible for the expansion phase, including the most characteristic features of the expansion phase, such as the poleward advance of the aurora and the development of the auroral electrojet. The electric current approach is rather new and needs much more effort to develop.

The relationship between the magnetosphere and magnetospheric/auroral substorms

Abstract. On the basis of auroral and polar magnetic substorm studies, the relationship between the solar wind-magnetosphere dynamo (the DD dynamo) current and the substorm dynamo (the UL dynamo) current is studied. The characteristics of both the DD and UL currents reveal why auroral substorms consist of the three distinct phases after the input power ε is increased above 1018 erg s−1. (a) The growth phase; the magnetosphere can accumulate magnetic energy for auroral substorms, when the ionosphere cannot dissipate the power before the expansion phase. (b) The expansion phase; the magnetosphere releases the accumulated magnetic energy during the growth phase in a pulse-like manner in a few hours, because it tries to stabilize itself when the accumulated energy reaches to about 1023 erg s−1. (c) The recovery phase; the magnetosphere becomes an ordinary dissipative system after the expansion phase, because the ionosphere becomes capable of dissipating the power with the rate of 1018 ~ 1019 erg s−1. On the basis of the above conclusion, it is suggested that the magnetosphere accomplishes the pulse-like release process (resulting in spectacular auroral activities) by producing plasma instabilities in the current sheet, thus reducing the current. The resulting contraction of the magnetic field lines (expending the accumulated magnetic energy), together with break down of the "frozen-in" field condition at distances of less than 10 RE, establishes the substorm dynamo that generates an earthward electric field (Lui and Kamide, 2003; Akasofu, 2011). It is this electric field which manifests as the expansion phase. A recent satellite observation at a distance of as close as 8.1 RE by Lui (2011) seems to support strongly the occurrence of the chain of processes suggested in the above. It is hoped that although the concept presented here is very crude, it will serve in providing one way of studying the three phases of auroral substorms. In turn, a better understanding of auroral substorms will also be useful in studying the magnetosphere, because various auroral activities can be the visible guide for this endeavor.

Apposite of atmospheric electric parameters with the energy coupling function ( ε) during geomagnetic storms at high latitude

In this paper we are concerned with the variation of the atmospheric electric field and the air–earth current due to the excessive power generated by the solar wind-magnetosphere dynamo during geomagnetic storms, recorded at Maitri in Antarctica during 2004. A major part of the power generated by the solar wind-magnetosphere dynamo is used in the formation of the ring current and the rest is utilized for Joule heating and auroral particle precipitation. The method adopted by Frank-Kamenetsky et al. [Frank-Kamenetsky A.V., Troshichev O.A., Burns G.B., Papitashvili V.O., 2001. Variations of atmospheric electric field in the near-pole region related to the interplanetary magnetic field. J. Geophys. Res. 106, A1, 179–190.] was utilized to delineate the variations due to the signatures of tropical thunderstorm activity from the geoelectric data; while statistical methods used in our earlier studies were used to delimit variation due to the constant buffeting of the solar wind. We find that the solar wind-magnetosphere energy coupling function ( ε) to be well correlated with the atmospheric electric parameters during the onset of geomagnetic disturbances. However the correlation breaks down during minor storms and sub-storm events.

An Overview of Earth’s Global Electric Circuit and Atmospheric Conductivity

The Earth’s global atmospheric electric circuit depends on the upper and lower atmospheric boundaries formed by the ionosphere and the planetary surface. Thunderstorms and electrified rain clouds drive a DC current (∼1 kA) around the circuit, with the current carried by molecular cluster ions; lightning phenomena drive the AC global circuit. The Earth’s near-surface conductivity ranges from 10−7 S m−1 (for poorly conducting rocks) to 10−2 S m−1 (for clay or wet limestone), with a mean value of 3.2 S m−1 for the ocean. Air conductivity inside a thundercloud, and in fair weather regions, depends on location (especially geomagnetic latitude), aerosol pollution and height, and varies from ∼10−14 S m−1 just above the surface to 10−7 S m−1 in the ionosphere at ∼80 km altitude. Ionospheric conductivity is a tensor quantity due to the geomagnetic field, and is determined by parameters such as electron density and electron–neutral particle collision frequency. In the current source regions, point discharge (coronal) currents play an important role below electrified clouds; the solar wind-magnetosphere dynamo and the unipolar dynamo due to the terrestrial rotating dipole moment also apply atmospheric potential differences.

Detailed dayside auroral morphology as a function of local time for southeast IMF orientation: implications for solar wind-magnetosphere coupling

Abstract. In two case studies we elaborate on spatial and temporal structures of the dayside aurora within 08:00-16:00 magnetic local time (MLT) and discuss the relationship of this structure to solar wind-magnetosphere interconnection topology and the different stages of evolution of open field lines in the Dungey convection cycle. The detailed 2-D auroral morphology is obtained from continuous ground observations at Ny Ålesund (76° magnetic latitude (MLAT)), Svalbard during two days when the interplanetary magnetic field (IMF) is directed southeast (By>0; Bz<0). The auroral activity consists of the successive activations of the following forms: (i) latitudinally separated, sunward moving, arcs/bands of dayside boundary plasma sheet (BPS) origin, in the prenoon (08:00-11:00 MLT) and postnoon (12:00-16:00 MLT) sectors, within 70-75° MLAT, (ii) poleward moving auroral forms (PMAFs) emanating from the pre- and postnoon brightening events, and (iii) a specific activity appearing in the 07:00-10:00 MLT/75-80° MLAT during the prevailing IMF By>0 conditions. The pre- and postnoon activations are separated by a region of strongly attenuated auroral activity/intensity within the 11:00-12:00 MLT sector, often referred to as the midday gap aurora. The latter aurora is attributed to the presence of component reconnection at the subsolar magnetopause where the stagnant magnetosheath flow lead to field-aligned currents (FACs) which are of only moderate intensity. The much more active and intense aurorae in the prenoon (07:00-11:00 MLT) and postnoon (12:00-16:00 MLT) sectors originate in magnetopause reconnection events that are initiated well away from the subsolar point. The high-latitude auroral activity in the prenoon sector (feature iii) is found to be accompanied by a convection channel at the polar cap boundary. The associated ground magnetic deflection (DPY) is a Svalgaard-Mansurov effect. The convection channel is attributed to effective momentum transfer from the solar wind-magnetosphere dynamo in the high-latitude boundary layer (HBL), on the downstream side of the cusp.

Neutral wind influence on the electrodynamic coupling between the ionosphere and the magnetosphere

The influence of the ionospheric wind dynamo on the steady state electrodynamic interaction between the ionosphere and the inner magnetosphere is evaluated with the Magnetosphere‐Thermosphere‐Ionosphere Electrodynamics General Circulation Model (MTIE‐GCM) of Peymirat et al. [1998]. Two types of interaction between the solar wind–magnetosphere (SWM) dynamo and the ionospheric dynamo are considered: either the SWM dynamo is a voltage generator imposing a fixed electric potential in the polar cap or it is a hybrid current/voltage generator such that the electric potential in the polar cap results from the combined action of the SWM and ionospheric dynamos. The following results are found. (1) The dynamo effect of winds that are accelerated by the high‐latitude ion convection increases the meridional electric field in the auroral zone but reduces the potential variation around the Auroral Zone Equatorial boundary (AZEQ) and the plasma pressure in the nightside magnetosphere close to the Earth, corresponding to an enhancement of the shielding effect produced by Region 2 field‐aligned currents. The magnitudes of the wind‐induced effects are on the order of 10% for the shielding potential and 20% for the plasma pressure reduction. (2) The wind‐induced reduction of magnetospheric plasma pressure near the Earth is ∼4 times larger for a doubling of the plasma source density in the tail. (3) The relative influence of winds on shielding is similar for polar cap potential drops of 30 kV and 70 kV. (4) When the wind is allowed to influence the polar cap potential, it increases the potential drop along the polar cap boundary by ∼10% but does not modify the net influence on the shielding potential along AZEQ. (5) The influence of the neutral winds set up by the high‐latitude convection is restricted to the auroral zone.

Electrodynamics of solar wind-magnetosphere-ionosphere interactions

The authors present a coherent picture of fundamental physical processes in three basic elements of the SW-I (solar wind-magnetosphere-ionosphere) coupling system: (i) the field-aligned potential structure which leads to the formation of auroral arcs; (ii) the magnetosphere coupling which leads to the onset of magnetospheric substorms; and (iii) the solar wind-magnetosphere dynamo which supplies the power for driving various magnetospheric processes. The field-aligned potential structure on auroral lines is forced into existence by the loss-cone constriction effect when the upward field-aligned current density exceeds the loss-cone thermal flux limit. The substorm onset occurs when the ionosphere responds fully to the enhanced magnetospheric convection driven by the solar wind. The energy is transferred from the solar wind to the magnetosphere by a dynamo process primarily on open field lines. >

A first generation numerical geomagnetic storm prediction scheme

Geomagnetic and auroral disturbances cause significant interference on many electrical systems. Therefore, it is essential to develop a reliable geomagnetic and auroral storm prediction scheme. Present geomagnetic storm prediction schemes rely entirely on statistical results, so that they can hardly provide quantitative information on the intensity of a geomagnetic storm caused by a particular solar event. It is for this reason that we have been developing a first generation numerical prediction scheme. The scheme consists of two major computer codes which in turn consist of a large number of subroutine codes and of empirical relationships. First of all, when a solar flare occurs, six flare parameters are determined as the input data set for the first code which is devised to show the simulated propagation of solar wind disturbances in the heliosphere to a distance of 2 a.u. Thus, one can determine the relative location of the propagating disturbances with the Earth's position. The solar wind speed ( V) and the three interplanetary magnetic field (IMF) components ( B x , B y , B z , or B, 0, φ) are then computed as a function of time at the Earth location or any other desired (space probe) locations. These quantities in turn become the input parameters for the second major code which computes first the power (ϵ) of the solar wind-magnetosphere dynamo as a function of time. The power thus obtained and the three IMF components can be used to compute or infer: (i), the predicted geometry of the auroral oval; (ii), the cross-polar cap potential; (iii), the two geomagnetic indices AE and Dst; (iv), the total energy injection rate into the polar ionosphere; (v), the atmospheric temperature, etc. At the present time, the weakest part of the scheme arises from the fact that causes of changes of the IMF B z (or θ) component are not well understood. Further, several empirical relationships are incorporated in the present scheme in the second major code. Thus, there are still a number of solar and geomagnetic storm features which require theoretical studies and proper modeling efforts to eventually replace the empirical relationships by theoretical ones, as our understanding of solar-terrestrial physics advances.

Explosive magnetic reconnection: Puzzle to be solved as the energy supply process for magnetospheric substorms?

During the last two decades, magnetospheric substorms have almost exclusively been discussed in terms of hypothetical explosive magnetic reconnection, a process which requires the initial accumulation and the subsequent sudden conversion of magnetic energy. However, it is only recently that the input‐output relationship of the magnetosphere has been examined, enabling us to test the validity of this hypothesis. The results are at variance with what we would expect from explosive magnetic reconnection. Instead, magnetospheric substorms can be understood primarily as a direct consequence of an increased energy input rate (power) of the solar wind—magnetosphere dynamo.

A comprehensive view of solar-terrestrial relationships in terms of a chain of four dynamo-powered plasma acceleration processes

This paper emphasizes an effort to link processes which relate solar activity and magnetospheric disturbances in terms of energy transfer through a chain of four elements. In this view, each element is explicitly thought to be powered by a dynamo, namely the solar wind generation dynamo, the solar flare dynamo, the solar wind-magnetosphere dynamo and the auroral dynamo, respectively. Each dynamo powers a plasma acceleration process by the Lorentz force and the plasma flows thus generated are the solar wind, the flare-generated solar wind disturbance, the magnetospheric plasma convection and the ionospheric convection, respectively. Each plasma flow conveys the energy from one element to the next in the chain. Some of the kinetic energy of the photospheric plasma is eventually deposited in the polar ionosphere as heat energy.

Magnetospheric convection at a low level power ϵ

It is demonstrated that the magnetospheric convection becomes evident in terms of the AE index only when the power ϵ of the solar wind-magnetosphere dynamo becomes greater than ∼ 10 18 erg s −1 or a slightly lower value. An enhanced conductivity is a crucial factor for the magnetospheric convection to manifest even in a low-level increase of the AE index of ∼ 50–100 γ.

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

It is shown that the power ε 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.

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

A solar wind parameter ε, 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 ε 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.

Dependence of the amount of open magnetic flux on the direction of the interplanetary magnetic field

The power generated by the solar wind—magnetosphere dynamo is proportional to the amount of the open magnetic flux Φ. It is difficult to use this fact in determining observationally the dependence of Φ on the orientation of the interplanetary magnetic field vector. It is shown that, for a simple vacuum superposition of the earth's dipole field and a uniform magnetic field, Φ is very closely proportional to sin θ/2) for a wide range of the intensity of the uniform field, where θ denotes the polar angle of the interplanetary magnetic field vector in the Y- Z plane of solar-magnetospheric coordinates.

Interplanetary shock waves and magnetospheric substorms

It is shown that substorm activity after a storm sudden commencement (S.S.C.) depends on whether or not an interplanetary shock wave is accompanied by a large increase of the solar wind-magnetosphere energy coupling function ε. It has long been thought that substorm activity associated with an S.S.C. results from sudden conversion of magnetic energy stored in the magnetotail and that this conversion is triggered by the shock wave. However, the present result implies that the magnetospheric substorm is not a sudden conversion of stored magnetic energy, but is a direct consequence of increased efficiency of the solar wind-magnetosphere dynamo.

Magnetic energy conversion, magnetospheric substorms and solar flares

An explosive conversion of magnetic energy has long been suggested as the cause of various electrodynamic processes in astrophysical conditions, such as auroral magnetospheric substorms, solar flares, stellar flares and some galactic scale processes1–6. It is generally believed that the magnetic energy is stored in a certain region before the onset of those phenomena and that the conversion takes place spontaneously or by triggering. Here it is suggested that the magnetospheric substorm is a direct consequence of increasing power generated by the solar wind–magnetosphere dynamo, rather than of a hypothetical sudden conversion of the stored magnetic energy.

The aurora: an electrical discharge process around the earth

The aurora is a large-scale electrical discharge phenomenon around the earth, which is powered by the solar wind-magnetosphere dynamo. This review shows how our understanding of the auroral phenomenon has been substantially improved by intensive satellite, rocket, and ground-based observations during the last two decades.