A modeling of the magnetospheric substorm

This paper reports some results of an attempt to simulate the large-scale changes of the internal structure of the magnetosphere during the magnetospheric substorm by assuming the growth of two current systems, one in the nightside and the other in the day-side.     

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.     

The solar wind-magnetosphere dynamo and the magnetospheric substorm

In a quiet condition, the solar wind kinetic energy is converted into electrical energy. A small part of this energy is dissipated as heat energy in the polar ionosphere. We identify at least three types of magnetospheric disturbances which are not associated with an increase of the heat production and call them reversible disturbances, while the magnetospheric substorm is an irreversible disturbance which is associated with a large increase of the heat production.The magnetosphere appears to have an inherent internal instability by which a large amount of heat energy is sporadically produced in the polar upper atmosphere at the expense of the magnetic energy in the magnetotail. A positive feed-back process may be responsible for the growth of the instability and for the expansive phase, while the recovery phase sets in when some process begins to suppress the positive feed-back process.     

The development of three-dimensional current system during a magnetospheric substorm

It is suggested that the pattern of three-dimensional substorm current circuit varies significantly even during the lifetime of a single substorm. This gives rise to quite complex time variations of the magnetic field at low latitude stations even for relatively isolated substorms. To verify this, three-dimensional current models with time dependent spatial variations are used to simulate one type of complex low-latitude “substorm signature”. It is shown that the utmost care should be exercised in determining different substorm phases on the basis of such a signature. The results indicate also that, in certain longitudes in the evening sector, one should expect distinct differences in characteristics between positive bays observed on the ground and at the synchronous distance.     

The day-sector polar F-layer during a magnetospheric substorm

Ionogram and all-sky camera data have been recorded on the Air Force Cambridge Research Laboratories' Flying Ionospheric Laboratory in the day sector of the auroral oval under conditions of darkness. The airborne measurements show that the polar F-layer irregularity zone, which is characterized on ionograms by a generally non-retarded and spread F type echo, exhibits meridional motions similar to the day-sector auroras. The polar F-layer irregularity zone and the day-sector auroras move equatorward and then move poleward in harmony with the development and decay of a magnetospheric substorm. We suggest that the polar cusp also moves in essentially the same fashion.     

Magnetic pulsation Pi2 as a sensitive indicator of magnetospheric substorm

The identification of substorm onset is quite important in studying the mechanism of excitation of substorm disturbances. At present, one of the most practical methods to identify accurately the onset of substorms is to use low-latitude Pi2, which is sensitively related to the plasma instability in the magnetosphere, that triggers the substorm disturbances. This method is applied to examine the onset times of three substorm events which were already defined by various methods other than the low-latitude Pi2 method. Preceding the onset times, other evident substorm onsets are clearly determined with Pi2 onsets for all the three events by examining only the H-component of rapid-run magnetogram from a single low-latitude station in the dark hemisphere. Cooperative monitoring of Pi2 at three low-latitude stations on three well-separated meridians, therefore, is really effective in detecting most substorms.     

The mechanism of magnetospheric substorm and the MHD waves of the solar wind

A mechanism of the Earth's magnetospheric substorm is proposed. It is suggested that the MHD waves may propagate across the magnetopause from the magnetosheath into the magnetotail and will be dissipated in the plasma sheet, heating the plasma and accelerating the particles. When the solar wind parameters change, the Poynting flux of the waves transferred from the magnetosheath into the tail, may be greater than 10¹⁸ erg s^?1. The heated plasma and accelerated particles in the plasma sheet will be injected into the inner magnetosphere, and this may explain the process of the ring current formation and auroral substorm.The Alfvén wave can only propagate along the magnetic force line into the magnetosphere in the open magnetosphere, but the magnetosonic wave can propagate in both the open and closed magnetosphere. When the IMF turns southward, the configuration of the magnetosphere will change from a nearly closed model into some kind of open one. The energy flux of Alfvén waves is generally larger than that of the magnetosonic wave. This implies that it is easy to produce substorms when the interplanetary magnetic field (IMF) has a large southward component, but the substorm can also be produced even if the IMF is directed northward.     

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.     

Thermodynamics of rare events and impulsive relaxation events in the magnetospheric substorm dynamics

The magnetosphere dynamics shows fast relaxation events following power-law distribution for many observable quantities during magnetic substorms. The emergence of such power-law distributions has been widely discussed in the framework of self-organized criticality and/or turbulence. Here, a different approach to the statistical features of these impulsive dynamical events is proposed in the framework of the thermodynamics of rare events [Lavenda, B.H., Florio, A., 1992. Thermodynamics of rare events, Int. J. Theor. Phys. 31, 1455–1475; Lavenda, B.H., 1995. Thermodynamics of Extremes. Albion]. In detail, an application of such a novel approach to the magnetospheric substorm avalanching dynamics as monitored by the auroral electroject index is discussed.     

Magnetic pulsation Pi2 and substorm onset

Coincidence between the onset of sudden brightening of the auroral arc in the auroral oval and the onset of Pi2 magnetic pulsation in low latitudes is examined based on the auroral data obtained at a chain of stations in Alaska and the Pi2 data obtained at the low-latitude station Onagawa. The result shows that the low-latitude Pi2 occurs almost simultaneously with the sudden brightening of the auroral arc, i.e. the onset of an auroral substorm (T = 0). It is concluded that the onset of substorms can be identified quite well with the onset of the low-latitude Pi2.     

Importance of initial ionospheric conductivity on substorm onset

A substorm after a prolonged quiet period may differ from the typical one, as reported recently by Pellinen et al. (1982). The present knowledge on imperfect coupling between the magnetosphere and the ionosphere can explain qualitatively such a difference in terms of the role played by the initial conductivity of the ionosphere on substorm onset.     

On the longitudinal localization of the substorm active region and its changes during the substorm

During the expansive phase of a substorm the active region frequently changes its situation by a jump, both to the evening and morning sectors. The typical interval between successive activizations is often ? 10–20 min. It is emphasized that in such events the definition of a substorm phase for all the magnetosphere is ambiguous, because the conditions characteristic for different phases can be realized in different parts of the magnetosphere simultaneously.     

Implications of a steady-state magnetospheric convection

Fundamental to the development of many magnetospheric theories is the assumption of a steady-state magnetospheric convection. It is shown for the first time that this basic assumption is implicitly related to the source of magnetospheric plasma. The existence of a steady-state electric field is consistent with plasma entry into the magnetosphere from the North—South direction. If plasma entry is through the flanks of the magnetosphere, the large-scale magnetospheric electric field is intrinsically time-dependent. Thus, magnetospheric theories with the underlying assumption that the large-scale electric field is derivable from a scalar potential cannot deal with such a situation. Therefore, the origin of magnetospheric plasma and the time-variability of magnetospheric convection are directly related.     

The transient response mechanism and Pi2 pulsations at substorm onset—Review and outlook

Pi2 pulsations are nowadays thought to be transient hydromagnetic signals associated with the build-up of a new stationary magnetosphere-ionosphere coupling system after the sudden formation of the substorm current wedge. To illustrate this transient response mechanism, we will first briefly describe the substorm current circuit. Subsequently, we will demonstrate that the gross characteristics of high-latitude Pi2 can be explained by the sudden switch-on of this current wedge during substorm onset if its westward expansion is taken into account. We will conclude by discussing some additional phenomena and processes (like conductivity and electron density gradients, kinetic Alfvén waves, ionospheric polarization electric fields, and mode coupling) which have to be included into a realistic model for Pi2 pulsations and thus timedependent magnetosphere-ionosphere coupling.     

Development of the auroral absorption substorm: Studies of pre-onset phase and sharp onset using an extensive riometer network

The development of an auroral absorption substorm has been studied using riometer measurements in the northern hemisphere. In the events studied, the onset is preceded by an absorption bay which begins to develop $\text{1?1}\text{1}\text{2}\text{h}$ before the onset. The bay may occur between L-values 3–19 and can cover as much as 150° of geomagnetic longitude, generally in the same longitudinal sector where the substorm breaks up and to the west of it. Whereas the substorm breaks up at or near the midnight meridian, the preceding bay may, in some geophysical conditions, appear in the afternoon sector. The preceding bay moves southward with a velocity between 60 and 600 ms^?1, intensifying during the movement. This equatorward movement is consistent with an E × B drift in a cross-magnetotail electric field of between 0.5 and 1 mV m^?1. The absorption at the onset exceeds that in the bay, and in the sector of break up the absorption shows a minimum just before the onset; to the west-of the break up the preceding bay continues its southward movement. In 14 cases studied, the sharp onset moved to the west with a velocity of 1–31 km s^?1, median 6 km s^?1. The onset was seen at higher L-values to the west than in the break-up sector. This applied also to the preceding bay. Whereas most onsets showed westward movement, in only about half of the cases studied was there movement towards the east. The injection area affected during the first minute of the onset was typically 1–2 L-value units, but as much as 30° of geomagnetic longitude. The onset later spread to cover 1–10 L-value units, and up to 130° of longitude. The contouring method used in the analysis of the data from the riometer is described in the Appendix.     

A mechanism for the growth phase of magnetospheric substorms

A mechanism is presented whereby the rate of energy dissipation in the magnetosphere is controlled by the particle density in the plasma sheet in the near geomagnetic tail. The mechanism is based on a model in which the plasma sheet is sustained by injection of solar-wind particles into the dayside magnetosphere. The efficiency of the injection is controlled by solarwind parameters, in particular, the north-south component of the interplanetary magnetic field; the maximum injection rate occurs when the interplanetary field is northward. During geomagnetically quiet times, this source balances the loss of particles from the edges of the tail current sheet. If the dayside source rate is reduced (e.g. by a southward-turning interplanetary magnetic field), then the plasma sheet is depleted and the rate of magnetic merging is enhanced in the earthward portion of the tail current sheet. This period of steadily-enhanced merging is associated with the growth phase, i.e. the period of enhanced magnetospheric convection for about one hour preceding the breakup of a polar magnetic or auroral substorm. The breakup can be understood as the result of the collapse of a portion of the tail current sheet following the local depletion of the plasma sheet.     

The magnetospheric plasma as a source of energy for space instrumentation

It is proposed to convert the thermal motion of a plasma into electrical power: energetic electrons collected by a plate dissipate their energy into a load, and are re-injected into the medium by means of an electron source. This concept may find applications in the magnetospheres of the outer planets, but present knowledge does not allow one to assess whether the energy fluxes are sufficient for practical applications. It is therefore neccessary to perform in situ preliminary investigations with electron emitters. It is pointed out that electron sources can be simultaneously used for additional tasks: spacecraft potential clamping, plasma diagnostics and detection of electromagnetic and electrostatic waves.