Lars P. Block

A double layer review

A review is given of the main results on electrostatic double layers (sometimes called ‘space charge layers’ or ‘sheaths’) obtained from theory and laboratory and space experiments up to the spring of 1977.The paper begins with a definition of double layers in terms of potential drop, electric field, and charge separation. Then a review is made of the theoretical results obtained so far. This covers, among other things, necessary criteria for existence and stability, quantitative estimates of charge separation and thickness, and some probable cause of DL-formation in terms of an instability.Next, experimental results obtained in the laboratory are compared with the theoretical results. Due to recent progress in experimental technique, the interior of a double layer can now be studied in much more detail than was possible before.By means of barium jets and satellite probes, double layers have now been found at the altitudes that were previously predicted theoretically. The general potential distribution above the auroral zone, suggested by inverted V-events and electric field reversals, is corroborated.

Prebreakup arcs: A comparison between theory and experiment

We have developed a model describing the structure of a prebreakup arc based on an ionospheric Cowling channel and its extension into the magnetosphere. A coupled two-circuit representation of the substorm current wedge is used which is locally superimposed on both westward and eastward electrojets. We find that brighter, more unstable prebreakup arcs are formed in the premidnight (southwest of the Harang Discontinuity) than in the postmidnight (northeast of the Harang Discontinuity) sector. This contributes to the observed prevalence of auroral activity in the premidnight sector. Also, our model predicts that the north-south dimensions of the current wedge in the ionosphere should vary from a few kilometers at an invariant latitude (Λ) of 62° to hundreds of kilometers above Λ = 68°. Comparison of the model results with the extensive observations of Marklund et al. (1983) for a specific arc observed just after onset shows good agreement, particularly for the magnitude of the polarization electric field and the arc size. We conclude that this agreement is further evidence that the substorm breakup arises from magnetosphere-ionosphere coupling in the near magnetosphere and that the steady state model developed here is descriptive of the breakup arc before inductive effects become dominant.

Acceleration of Auroral Particles by Magnetic-Field Aligned Electric Fields

Measurements on the S3-3 and Viking satellites appear to show that at least a large fraction of magnetic field-aligned potential drops are made up of multiple double layers. Solitons and double layers inU-shaped potential structures give rise to spiky electric fields also perpendicular to the magnetic field in agreement with satellite measurements. The large scale potential structures associated with invertedV-events are built up of many similar short-lived structures on a small scale. Viking measurements indicate that electric fields parallel to the magnetic field are almost always directed upward.

Potential and inductive electric fields in the magnetosphere during auroras

Abstract During quiescent auroras the large-scale electric field is essentially irrotational. The volume formed by the plasma sheet and its extension into the auroral oval is connected to an external source by electric currents, which enter and leave the volume at different electric potentials and which supply sufficient energy to support the auroral activity. The location of the actual acceleration of particles depends on the internal distribution of electric fields and currents. One important feature is the energization of the carriers of the cross-tail current and another is the acceleration of electrons precipitated through relatively low-altitude magnetic-field-aligned potential drops. Substorm auroras depend on rapid and (especially initially) localized release of energy that can only be supplied by tapping stored magnetic energy. The energy is transmitted to the charged particle via electric inductive fields. The primary electric field due to changing electric currents is redistributed in a complicated way—but never extinguished—by polarization of charges. As a consequence, any tendency of the plasma to suppress magnetic-field-aligned components of the electric fields leads to a corresponding enhancement of the transverse component.

Acceleration of Auroral Particles by Electric Double Layers

Recent observations of precipitating particles and motions of auroral forms indicate that auroral particles are accelerated by electric fields directed along the magnetic field. Such electric fields have also been observed. It is argued that large field aligned potential drops should sometimes be concentrated in thin double layers (sheaths). Properties and consequences of such sheaths are reviewed and compared with evidence from several rocket and satellite observations.

On the development of a magnetospheric substorm influenced by a storm sudden commencement: Ground, balloon, and satellite observations

Previous statistical investigations have revealed a relationship between storm sudden commencements (ssc) and magnetospheric substorm onsets. Little is known about the physical processes constituting this relationship. We used a comprehensive data set for a detailed case study. The ssc occurred on July 6, 1979, at 1930 UT. The substorm expansion phase started 5 min later. The event was preceded by a loading phase of more than 1.5 hours. The loading phase developed in three steps. During each step the cross-tail current sheet suddenly expanded earthward and intensified. The third step, at 1930 UT, coincided with the ssc. It was very likely caused by the interaction of ions with magnetohydrodynamic waves generated by the ssc. This step was followed at 1935 UT by the onset of the expansion phase accompanied by PiB magnetic pulsations, impulsive electron precipitation, and energetic ion injection at GEOS 2 orbit. The dipolarization of the geomagnetic field started 9 min after the expansion phase onset together with energetic electron injection and a decrease in the energetic ion flux. Signatures of Birkeland currents at dipolarization, and a pressure anisotropy P∥ions > P⊥ions during the 4 min before dipolarization, indicate field-aligned processes consistent with partial diversion of the cross-tail current into the ionosphere. The observations are discussed in the framework of a model in which the ballooning mode instability (BMI) developing in the near-Earth plasma sheet is regarded as the trigger process for the expansion phase. Quantitative estimates of the instability criteria show that the necessary conditions for the BMI are fulfilled during the whole loading phase. An energetic ion pressure gradient, which is needed to drive the BMI, was observed during the interval 1931 UT (ssc) to 1944 UT (dipolarization). Signatures of the instability itself, in terms of regular ion pressure gradient variations, were recorded during the same time interval. We conclude that the magnetosphere was potentially unstable for the BMI when the ssc occurred and that the interaction of the ssc with the magnetospheric particles expanded the cross-tail current sheet further earthward, thereby creating plasma conditions in which the BMI could grow. The instability started at 1931 UT, and the BMI in turn triggered the expansion phase at 1935 UT.

Rocket measurements of electric fields, electron density and temperature during different phases of auroral substorms

On 27 January 1979, three rocket payloads were launched from Kiruna, Sweden into different phases of two successive auroral substorrns. Among other experiments, the payloads carried the RIT double probe electric field experiments providing electric field, electron density and temperature data which are presented here. These data supported by rocket particle observations are discussed mainly in association with ground-based observations (magnetometer, TV) and very briefly with GEOS electric field data. The motions of the auroral forms as obtained from auroral pictures are compared with E × B/B2 drifts and the currents calculated from the rocket electric field and density measurements with the equivalent current system deduced from ground-based magnetometer data (Scandinavian Magnetometer Array).

Particle dynamics in a spatially varying electric field

For an MHD description of a plasma a distinct separation between the macroscopic and microscopic spatial and temporal scales is assumed. In this paper we solve the particle dynamics with finite first and second spatial derivatives in the electric field. We find that (1) MHD (ideal and nonideal) becomes invalid for a sufficiently strong constant electric field gradient perpendicular to the magnetic field; (2) a sufficiently large second derivative in the electric field can cause heavy ions to become chaotically untrapped; (3) for an electric field with a constant gradient the ion drift velocity is equal to (E×B)/|B|2 as long as the orbit-averaged value of E is used. There are no finite currents associated with the ion drift for such an electric field; (4) perturbation technique gives a poor approximation to the ion drift velocity even for values of the second derivative that may well occur in the magnetosphere. Results 1 and 2 provide necessary criteria for the applicability of magnetospheric MHD models of spatially varying electric fields. They also predict an asymmetry in the heavy ion fluxes, a feature that could be useful in inferring magnetospheric electric field structure. We illustrate these results by application to the Harang discontinuity. It is found that if the interplanetary magnetic field swings northward under substorm growth conditions the orbits of the equatorial O+ may dramatically change due to result 2. This effect may contribute to the substorm onset process.

Scaling considerations for magnetospheric model experiments

Abstract The scaling problems for plasma gun-terrella experiments on the interaction between the solar wind and the magnetosphere are considered. Good scaling requires a magnetized, collision-free, supersonic and super-Alfvenic plasma wind. The gyro-radii should be small compared with the size of the magnetosphere both in the plasma wind and at the magnetopause. The Debye length should be small. The mean free paths should be long to produce collision-free conditions. The magnetosphere must be much larger than the terrella. It is found that a simulation of the entire magnetosphere and the collisionless shock requires plasma temperatures in excess of 107°K and terrella fields of the order of 106 G. However, the transition region and the outer parts of the magnetosphere beyond about 3 earth radii may be simulated by a plasma gun and a terrella with a finite surface conductivity. The collisionless shock may be simulated by a theta-pinch and the inner part of the magnetosphere by a PIG-discharge with high mirror ratios.

A study of the interaction of VLF waves with equatorial electrons and its relationship to auroral X-rays in the morning sector

Abstract Precipitation of electrons in the morning sector of the auroral zone related to magnetospheric substorm activity has been observed via X-ray measurements from balloons. Coordinated observations within the IMS period both aboard balloons and on the geostationary satellite GEOS-2, provided detailed information for the study of the relation between the distribution function of energetic electrons at GEOS 2 and the precipitation at and near the magnetically conjugate point in the auroral zone. The anisotropy of the electron distribution function is determined by the injection processes and by further modifications, e.g. convection and drift. It is large enough to allow the growth of VLF waves responsible for scattering of electrons into the loss cone.