Effects of the interplanetary magnetic field on the magnetotail structure: Large-scale changes of the plasma sheet during magnetospheric substorms

The effects of the orientation of the interplanetary magnetic field (IMF) on the structure of the distant magnetotail are studied by superposing a uniform magnetic field on a magnetospheric model. It is shown that a southward component of the IMF alone can reduce the closed field region in the magnetotail, while a northward turning of the IMF can produce a new closed field region. It is suggested that these two effects can explain thinning and thickening, respectively, of the plasma sheet during magnetospheric substorms without invoking internal instabilities.     

Enhancements of the auroral zone ionization during substorms

During winter nights the ionization of the auroral zone increases and moves southward into the main ionospheric trough with increasing magnetic activity. The southern boundary of the ionization coincides with the southern boundary of the auroral oval. The ionization thus created remains in the ionosphere after midnight, although the precipitation of soft electrons, which is the possible source, has moved north of the auroral zone.     

Equatorward shift of the cusp during magnetospheric substorms

It is shown that the magnetic field of an enhanced dynamo current in the dayside boundary layer and of the connected circuit can quantitatively account for the equatorward shift of the cusp region which is observed during the expansive phase of magnetospheric substorms.     

Magnetic-field observations in the low-latitude magnetotail during substorms

Explorer 34 observations of the low-latitude tail field beyond 25 R_E are critically examined to see if the signature of the neutral-line formation is always visible during substorm expansion phases. Cases are found where a clear signature cannot be recognized. However, comparison of the simultaneous tail observations by multiple satellites suggests that the absence of a clear signature can largely be due to the spatial effect, namely due to the presence of the satellite outside the region where the local magnetic field condition is influenced by the neutral-line formation. On the other hand, evidences supporting the close association between the neutral-line and the expansion phase are found for substorm events having double expansion-phase onsets.     

Ion energization during substorms at Mercury

We investigate the dynamics of magnetospheric ions during transient reconfigurations of Mercury's magnetotail. At Earth, numerous observations during similar events reveal a prominent energization (up to the hundreds of keV range) of heavy ions $({\text{O}}^{+})$ originating from the topside ionosphere. This energization likely results from a resonant nonadiabatic interaction with the electric field that is induced by dipolarization of the magnetic field lines, the time scale of this reconfiguration being comparable to the heavy ion cyclotron period. The question then arises whether such an energization may occur at Mercury. Using single-particle simulations in time-varying electric and magnetic fields, we show that prominent nonadiabatic heating is obtained for ions with small mass-to-charge ratios (e.g., ${\text{H}}^{+},{\text{He}}^{+}$). As for heavy ions (e.g., ${\text{Na}}^{+},{\text{Ca}}^{+}$) that have cyclotron periods well above the time scale of the magnetotail reconfiguration (several seconds), a weaker energization is obtained. The resonant heating mechanism that we examine here may be of importance for solar wind protons that gain access to the inner hermean magnetotail as well as for light ions of planetary origin that directly feed the near-Mercury plasma sheet.     

The behavior of midday auroras during substorms

The behavior of midday auroras during auroral substorms is examined on the basis of all-sky photographs taken from the South Pole station. It is indicated that the model of auroral substorms constructed by Starkov and Feldstein (1967) needs some modifications.     

Regularities of variation of the heliospheric current sheet orientation during the solar activity cycle

Evolution of spatial orientation of the heliospheric current sheet (HCS) has been studied in detail using synoptic maps of the HCS configuration over the period 1971–1989. Analysis involves all phases of the sunspot cycle except for two years of maximum solar activity. The helmet-like coronal streamers are confirmed to be structural elements of the HCS. The r.m.s. deviation of a real HCS configuration from a plane does not exceed about 10° during most of the sunspot cycle length. Hence, minimum-type corona should be observed every time the HCS is oriented parallel to the line-of-sight, independent of the cycle phase. Such occasions have been observed apart from the sunspot minimum epochs at the solar eclipses of 31 August, 1932 and 11 July, 1991.Regularities of variation of the two following parameters of the HCS orientation have been revealed: obliquity to the solar equator plane (heel or tilt) and longitudinal orientation (yawing). Behaviour of the above parameters is repeated in different cycles. However, heeling and yawing occur probably not synchronous but rather independent of one another.     

Absence of the plasma sheet at lunar distance during geomagnetically quiet times

Plasma data from the Apollo XIV Charged Particle Lunar Environment Experiment (CPLEE) are presented to show that contrary to previously published analyses the plasma sheet does not extend to the lunar orbit with a thickness of 8 R_H. Two electron spectral types are observed: (1) low energy photoelectrons with no statistically significant medium and high energy fluxes, and (2) double peaked medium and high energy electrons. The second type is observed either coincident with auroral substorms or at the center of the tail during quiet times. These spectra are one to several orders of magnitude less intense than plasma sheet spectra measured near 20 R_E.     

Dynamics of day and night aurora during substorms

The motion of auroral forms on the day- and nightside of the Earth has been studied during different substorm phases by means of all-sky camera films. A substorm is characterized by a shift of the luminescence region towards the equator at noon and mainly towards the pole at midnight. However, individual forms drift predominantly toward the pole on the dayside and towards the equator on the nightside. The velocity of the poleward motion at noon is largest during the expansive phase of a substorm and amounts on the average to 330 $\text{m}\text{sec}$ but even during relatively quiet magnetic conditions a poleward motion is observed.     

Characteristics of plasma temperature variation during two-current-loop collisions

Characteristics of plasma temperature variations during two-current-loop collisions are described. It is revealed that plasma temperature has an oscillatory feature with damping amplitude and growing quasi-period in the case of anI-type collision. In the case of aY-type collision, if the initial current becomes strong enough, there also occur pulsations of the temperature. However, the temperature profile of anX-type collision is characterized by a single pulse only.     

Adiabatic plasma convection in the tail plasma sheet

We investigate the transport process of electrons in the tail plasma sheet by convection electric fields, under the assumption of conservation of the first two adiabatic invariants. The variation of the electron distribution function, and hence the bulk parameters with distance from the Earth are calculated. The results show that the electron distribution has a pressure anisotropy with p/p< 1 in the plasma sheet. Finally, the effects of the pressure anisotropy are qualitatively considered in terms of the modification of the geomagnetic field structure in the tail plasma sheet and instabilities due to wave-particle interactions.     

The ground signatures of the expansion phase during multiple onset substorms

The ground signatures of multiple onset substorms have been investigated in night-side magnetograms from low to high latitudes and in observations of auroral-zone electron precipitation. Pi 2 onsets at three widely spaced stations are used for accurate timing of each onset. It is found that an evening auroral arc brightens at the onset of each Pi 2 train, also in the case of weak pulsations before the first low-latitude positive bay onset. The latter onset is, on the other hand, associated with the initiation of a westward travelling surge, and field-aligned currents moving with the surge cause a similar westward movement of the magnetic signatures in subauroral and low-latitude magnetograms. At the arrival of a surge at an evening side observatory, the westward electrojet is displaced rapidly poleward, with a sharp increase in local bay activity and high-energy electron precipitation. The westward expansion of new activity appears as a continuous motion along the oval and is associated with a local poleward displacement of the westward electrojet. Consecutive surge initiation and low-latitude onsets do not, however, always occur progressively farther west. Thus, the development of the expansion phase consists of a series of intensifications and auroral surge formations at 10–20 min intervals. Near the time of maximum auroral-zone bay activity and apparently also when maximum westward extent is reached, the whole nighttime oval seems to be shifted poleward. Our findings are thus not consistent with the Wiens and Rostoker (1975) northward-westward stepping model. An alternative model is therefore presented based on the fundamental role of the westward travelling surge in carrying substorm activity westward along the oval. The associated field-aligned current system will perturb the pre-existing magnetospheric current wedge and cause positive bay increases at low latitudes and westward moving magnetic signatures at subauroral stations.     

A modeling of magnetic field variations during magnetospheric substorms

Magnetic field variations in the noon-midnight plane during the magnetospheric substorm are studied in terms of changes of three current systems: the dynamo-driven current on the magnetopause, the cross-tail current and the field-aligned current-auroral electrojet system. The field-aligned current is assumed to be generated as a result of interruption and subsequent diversion of the cross-tail current to the ionosphere. It is concluded that the available observations are consistent with a large increase of the three currents.     

Non-adiabatic processes in a two-fluid plasma and energization of the plasma sheet plasma

The change of energy of a collisionless, two-fluid plasma consists of the adiabatic gain or loss of energy, which is due to the work done by the electromagnetic forces, and of the non-adiabatic change associated with the presence of the “rest” field $\text{E}{}^1\text{=}\text{E}\text{+ (}\text{1}\text{c}\text{)}\text{V}\text{×}\text{B}$. The non-adiabatic gain or loss of energy per unit ti may be expressed by the relation

$\text{Q=}\text{E}{}_{\mathrm{\parallel }}\text{·}\text{i}{}_{\mathrm{\parallel }}\text{+}\text{c}\text{eNB}{}^2\text{B·}\text{f}\text{?}\text{×f}$

where i is the density of conductive current, N the ion number-density, and f (f?) the sum of inertia and pressure divergence of ions (electrons). Symbols of parallelism refer to the direction of B.A special case of non-adiabatic energization of a slowly convecting plasma sheet plasma is discussed in some detail. Regardless of the value of V, the non-adiabatic energization may significantly exceed any conceivable energization associated with the electric field $\text{?(}\text{1}\text{c}\text{)}\text{V}\text{×}\text{B}$.     

Slow convection of a magnetized plasma and the earth plasma sheet

Stationary convection of an isotropic, infinitely conducting plasma in a magnetic field with non-trivial geometry is discussed under the assumption that the inertial term in the equation of motion may be ignored. The energy gained or lost by a volume element of plasma per unit time does not vary along the field-lines. Simple relations between the components of the current density, depending on the field-line geometry, exist. Similar relations hold for the components of the plasma velocity.The theoretical analysis is applied to the geomagnetically-quiet plasma sheet and a qualitative physical picture of the sheet is suggested. The observed structure of the sheet is compatible with Axford-Hines type of convection perhaps combined with a low-speed flow from a distant neutral point. The magnetic-field-aligned currents are driven by the deformations of the closed field-lines which are enforced by the solar wind.     

Dipolarization fronts in the magnetotail plasma sheet

We present a THEMIS study of a dipolarization front associated with a bursty bulk flow (BBF) that was observed in the central plasma sheet sequentially at X=−20.1, −16.7, and −11.0R_E. Simultaneously, the THEMIS ground network observed the formation of a north-south auroral form and intensification of westward auroral zone currents. Timing of the signatures in space suggests earthward propagation of the front at a velocity of 300 km/s. Spatial profiles of current and electron density on the front reveal a spatial scale of 500 km, comparable to an ion inertial length and an ion thermal gyroradius. This kinetic-scale structure traveled a macroscale distance of 10R_E in about 4 min without loss of coherence. The dipolarization front, therefore, is an example of space plasma cross-scale coupling. THEMIS observations at different geocentric distances are similar to recent particle-in-cell simulations demonstrating the appearance of dipolarization fronts on the leading edge of plasma fast flows in the vicinity of a reconnection site. Dipolarization fronts, therefore, may be interpreted as remote signatures of transient reconnection.     

Surface waves on the plasma sheet boundary

A dispersion equation for the surface waves on the inner boundary of the magnetospheric plasma sheet is obtained. The wave group velocity has both components along and across the magnetic field. For the waves with the period 1 min the transverse component is about 100 km s⁻¹, the parallel component is approximately equal to the Alfvén velocity. Pi2 pulsations, as well as east-westward motions of auroral riometer absorption bays, may be possible displays of surface waves.     

A model for thinning of the plasma sheet

A one-dimensional model for thinning of the plasma sheet is developed on the basis of launching a fast mode MHD rarefaction wave propagating in the tailward direction along the plasma sheet. Behind the rarefaction wave the pressure is reduced, leading to thinning of the plasma sheet and also to an Earthward plasma flow with a speed on the order of the sound speed a₀. The plasma sheet thickness is reduced by a factor of 2 if an Earthward plasma flow speed of 0.8a₀ is induced. The predictions of the model are in reasonable agreement with observations.     

A self-consistent model of the jovian plasma sheet

An axially-symmetric, rapidly-rotating magnetosphere containing low-energy plasma is considered. The resulting plasma sheet is presumed isothermal and thin compared with the radius of the sheet. Solutions of the model equations are found which include the effects of centrifugal, pressure and electro-magnetic forces. These solutions show that the sheet has a constant thickness and that the pressure decays exponentially with distance from the equatorial plane. The calculated curves for the magnetic induction field are compared with the observed field of Jupiter.     

The sources of the polar cap and low latitude bay-like disturbances during substorms

Examination of the polar cap and low-latitude bays during substorms shows that there are two types of disturbances, DP₁₁ and DP₁₂, which are different not only in their morphological features, but in their origin as well. The DP₁₂ disturbances are associated with pure ionospheric currents, whereas the DP₁₁ are though to be generated by the Birkeland type current system. This conclusion is based on examination of the following characteristics: (1) the seasonal changes of the DP₁₁ and DP₁₂ disturbances in the polar cap, (2) the seasonal variations of the low-latitude bay intensity at the conjugate points in the cases of the DP₁₁ and DP₁₂ disturbances, (3) the distribution of the intensity of the DP₁₁ and DP₁₂ disturbances in both northern and southern hemispheres along the midnight meridian.