Return Current Region Research Articles

On the auroral current‐voltage relationship

A simple model of collisionless plasma kinetic theory [Knight, 1973] has been used to predict the relationship between the upward parallel current and the parallel potential drop. The DE 1/DE 2 pair offers a unique opportunity to test this relationship because the DE 1 spacecraft can measure the high altitude plasma parameters without contamination from auroral heating. The field‐aligned current density from Knight's formula is almost linear in the ratio of j∥/eΦ∥ for values of eΦ∥/kT between 1 and 10. We find, using measured values of J∥ (mapped to the surface) and Φ∥ (inferred from measurements either at high altitudes or low altitudes), the ratio of J∥/eΦ∥ varies considerably but with a mean value about 0.5∼2.2×10−9 mho/m². Using either the full Knight's formula or the linear simplification, we can predict the parallel potential drop (given the high‐altitude density, temperature, current, and magnetic mirror factor) to test against the measured potential drop, or we can predict the current given the potential drop and the high‐altitude plasma parameters. The agreement is generally quite good and the linear simplification is reasonable; however, the Knight current, since it does not include upward ionospheric electrons, is not valid in the downward return current region. We also show that suprathermal electron bursts are observed in the diffuse aurora at the same invariant latitudes, both at high and at low altitudes. Thus we suggest that these “bursts” are more properly described as a spatial rather than temporal phenomenon.

Simulations of pulsed electron beam injection during active experiments

During electron beam injection the spacecraft can become positively charged, reaching potentials as high as the beam energy, particularly at currents exceeding about 100 mA. After beam turnoff the spacecraft can become negatively charged and the current system around the spacecraft can be strongly modified. The dynamics of such an alternating current system are of particular importance in understanding results from pulsed beam injection. Two‐dimensional electromagnetic particle simulations are used to investigate the characteristics of the spacecraft charging, particle acceleration, and wave emissions during beam injection, turnoff and subsequent pulsing of the beam. It is shown that during beam injection the beam current is neutralized by a spatially separate return current region extending several tens of meters from the beam region, with the currents being closed across the field lines by the perpendicular acceleration of ambient plasma ions into the beam region. After beam turnoff this current system reverses after a time lag of about an ion plasma period. The current reversal is accompanied by (1) prolonged electron collection by the spacecraft near the beam region, (2) preferential ion collection by sections of the spacecraft magnetically connected to the initial return current regions, and (3) the creation of hot plasma extending well into the return current regions. Because of the time lag, the currents induced in the plasma during periodic beam injection can be strongly modified from the imposed beam current. The resultant wave spectrum is dominated by (1) the fundamental pulsing frequency if the pulse frequency ωp is much greater than the ion plasma frequency ωpi, (2) odd harmonics if ωp ≳ ωpi, and (3) even harmonics if ωp ≲ ωpi. This dependence on ωp/ωpi is proposed as the reason for the different spectra during VLF pulsing of an electron beam during the shuttle STS‐3 experiments.

Particle acceleration and wave emissions associated with the formation of auroral cavities and enhancements

Observations from DE 1 and electrostatic particle simulations are combined in an effort to provide a unified model for (nightside) auroral particle acceleration and wave emissions and their association with plasma cavities and enhancements. The observations show that enhanced electron precipitation during inverted‐V events is associated with broadband electrostatic bursts (BEB), increased upward field‐aligned currents, and density enhancements. These regions are flanked by return current regions where the density is depleted (i.e., by plasma cavities). Perpendicular acceleration of ambient plasma ions can occur in both upward and return current regions. It is shown through the simulations that these processes are integrally related and are not independent of each other. The free energy for the auroral particle acceleration can be provided by energetic ion beams in the plasma sheet boundary layer with nonzero perpendicular energy. The perpendicular energy allows charge separation between the beam ions and costreaming electrons to occur. The resultant space charge fields accelerate electrons on the same field lines as the costreaming electrons downward toward the ionosphere, without the beam ions actually propagating down to auroral altitudes. Ambient plasma electrons on adjacent field lines are accelerated upward, forming a return current. Because these currents are spatially separate, a perpendicular electrostatic shock develops which accelerates the plasma ions across the field lines in an effort to close the currents. This acceleration creates ion conics in velocity space and, in coordinate space, plasma cavities are formed in the return current regions and plasma enhancements in the enhanced positive current regions. Strong broadband electrostatic waves are generated with spectral maxima being generated near the local electron plasma frequency by the accelerated electrons and near the lower hybrid frequency by the accelerated ions, similar to that observed in BEB.

DE 1 observations of return current regions in the nightside auroral oval

Data from the Dynamics Explorer 1 high altitude plasma instrument (HAPI) and magnetometer (MAG‐A) are used to examine particle acceleration and Birkeland current phenomena in the mid‐altitude (10,000–20,000 km) nightside auroral region between 2000 and 2200 hours MLT. Field‐aligned current densities as derived from the two instruments are compared, generally resulting in good agreement. The latitudinal distribution of upward and downward field‐aligned currents is examined, both on a large scale with respect to regional boundaries and on a local scale with respect to individual inverted‐V events. The charge carriers of the downward currents are found to be cold ionospheric electrons accelerated into upward beams by potential drops of a few tens of volts located at altitudes near 1 Earth radius. In several instances, large potential differences below the satellite are indicated by energetic upward ion beams with no (or very small) potential drops above the satellite. In these cases, very small Birkeland currents (≲10−8 A/m²) are observed above the inverted V, suggesting the possibility that the field‐aligned current systems associated with individual inverted‐V events may close at relatively low altitudes above the auroral oval, or that field‐aligned potential differences may be generated without significant Birkeland currents.

Effects of electron heating on the current driven electrostatic ion cyclotron instability and plasma transport processes along auroral field lines

Fluid simulations of the plasma along auroral field lines in the return current region have been performed to show that the onset of electrostatic ion cyclotron (EIC) related anomalous resistivity and the consequent heating of electrons leads to much higher transverse ion temperature than the current driven EIC instability (CDICI) alone would produce. Anomalous resistivity enhances ion heating in two ways: (1) by inhibiting the growth of the critical electron‐ion drift velocity, VcH, which must be exceeded to excite the EIC instability and (2) by increasing the relative drift velocity between the electrons and the ions, VD, through the formation of density cavities due to increased ambipolar electric field. The anomalous resistivity associated with the turbulence is limited by electron heating, so that CDICI eventually saturates, but at a substantially higher transverse temperature than would be the case in the absence of resistivity. This process demonstrates a positive feedback loop in the interaction between CDICI, anomalous resistivity, and parallel large scale dynamics in the topside ionosphere.

Comparative study of cross‐field and field‐aligned electron beams in active experiments

Beam‐plasma interactions associated with the cross‐field injection of electron beams from spacecraft are investigated by means of two‐dimensional (three velocity component) electrostatic particle simulations and are compared with those for parallel injection. The degree of spacecraft charging and the nature of the plasma response depends on the ratio of the plasma response time trp to the beam stagnation time ts. In particular, when trp/ts ≲ 0.4 and the spacecraft size is comparable to the return current region, the spacecraft charging is reduced sufficiently so that the beam is able to escape from the near environment of the spacecraft. The parameters which determine the value of the ratio trp/ts are the density of the ambient plasma relative to the beam density, the beam width and energy and the spacecraft size. Thin energetic beams injected from wide spacecraft are most easily able to escape, with the spacecraft potential being proportional to the parallel beam energy rather than to the total energy. Strong space‐charge oscillations near the lower hybrid and upper hybrid frequencies are produced as the beam propagates into the plasma. These oscillations tend to cause beams injected across the field lines to lose their coherence after about one or two gyrations. The beam electrons are scattered into a hollow cylinder with radius equal to the beam electron gyroradius and thickness of about two beam Debye lengths. Parallel injected beams experience similar forces which cause the beam to expand to fill a solid cylinder of a comparable thickness. The return currents produced by both cross‐field and parallel beam injection have an extent across the field lines of the order of the gyroradius of a plasma ion accelerated up to about the spacecraft potential.

Auroral perturbation experiments

Neutral gas releases from sounding rockets into the auroral ionosphere result in a number of perturbation effects: those directly related to the release as such, including excitation of plasma waves and the enhancement or reduction of local plasma density; and those resulting from ionospheric-magnetospheric responses to the release, including stimulated particle acceleration, generation of large-amplitude electric field (current) pulse, and propagation of field-aligned current pulse in an Alfven wave mode associated with the release. A number of early barium thermite and shaped charge experiments provided valuable insights into the latter effects, despite the fact that they were not anticipated and consequently the observations were often incomplete and their interpretations tentative. These early insights and a number of theoretical models for auroral are formation suggested an important role for ionospheric conductivity and field-aligned currents in the formation of auroral arcs, and laid the foundations for subsequent auroral perturbation experiments (Project Trigger and Project Waterhole), in which the perturbation to the auroral ionosphere was intentional and used as a tool to probe the auroral acceleration mechanism. The local ionospheric plasma density was enhanced by 1–2 decades in Trigger and was depleted by that amount in Waterhole. In Trigger, energetic particle precipitation was stimulated. In Waterhole, it was impeded in return current regions (2 cases) but was enhanced in the upward current region (1 case). Together, the two experiments strongly suggest that (1) field-aligned currents are capable of driving instabilities in the topside ionosphere and thereby stimulating particle acceleration; (2) ionospheric conductivity plays an important role in auroral arc formation; and (3) increased conductivity inside the auroral arcs provides a positive feedback to the precipitating particles resulting in increased particle precipitation.

Plasma transport in the auroral return current region

The classical and anomalous transport properties of a multifluid plasma consisting of H+, O+, and electron populations in the presence of auroral field‐aligned return currents are investigated, using a multimoment fluid model with anomalous transport coefficients. This approach offers the possibility of simulating large‐scale dynamic phenomena without neglecting the important macroscopic consequences of microscopic processes such as anomalous resistivity and turbulent heating. The macroscopic effects of the electrostatic ion cyclotron (EIC) instability (perpendicular ion heating) and of an EIC‐related anomalous resistivity mechanism which heats the electrons are included in the present version of the model. The responses of the outflowing polar wind plasma to the application of current, with and without instabilities, are exhibited. The simulations show that the electron drift velocity corresponding to a return current of 0.65 µA/m² is above the threshold for EIC waves. Downward electron heat flow competes with upward convection and adiabatic effects to determine the direction of the electron temperature anisotropy. Resistive electron heating lowers the critical drift velocity for marginal EIC stability and leads to enhanced ion heating. The model predictions are compared with the experimental observations where possible.

Two‐dimensional quasi‐neutral description of particles and fields above discrete auroral arcs

This paper reports a full two‐dimensional numerical computation of the particle distributions, electric fields, and currents associated with a standard adiabatic model of a quiet auroral arc and inverted‐V potential. Both the region of auroral electron precipitation and the return current region are modeled; for the latter we assume a normal return current dominated by ambipolar E∥, with no substantial downward E∥. We go beyond earlier treatments of the adiabatic model (which either had no coupling between different field lines or treated all field lines as having essentially identical conditions along B) by specifying latitudinal variations in particle sources and parallel potential drops, which require every field line to be treated separately, coupled to other lines by current conservation. In particular, differential pitch angle anisotropies which drive upward E∥ are varied with latitude in order to accommodate a smooth transition to the return current region. In this transition regime the model predicts density enhancements on field lines at the edge of the inverted V that have only a small potential drop. We believe this is the first work to address the return current region with a realistic adiabatic model of the magnetosphere, including particle populations of both magnetospheric and ionospheric origin.

Origin of auroral electric potential structures

Careful analyses during the past 5–6 years of particle and electric field signatures observed in association with the evening discrete arc have led to major advances in the basic understanding of the structure of the auroral electric potential. The origin of this basic potenital structure, which is inherent in the principle of kinetic plasma distributions in globally inhomogeneous magnetic configurations, is reviewed and examined. It is shown that, once the discrete arc potential structure is understood, this principle of arc formation can be applied to other geomagnetic configurations. In this way, a comprehensive theory of discrete auroral arcs of general configuration, including the theta aurora, can be constructed. Furthermore, completion of the auroral field-aligned current circuit in this theory leads to the hypothesis of downward parallel electric fields in the return current region away from the central portion of the discrete arc potential. Such downward parallel electric fields accelerate ionospheric electrons upward to form the return current and trap low-energy ions at the foot of the field line. It is shown that such trapping of ionospheric ions between the magnetic mirror below and the electric reflection above plays a crucial role in the heating and acceleration of ionospheric ions, which have been found to be an important source of magnetospheric plasma. Evidence for such downward parallel electric field signatures have been found in the last year or so.

Potential structures and particle acceleration on auroral field lines

In the 1970's major advances in the understanding of auroral processes were brought about by observations of plasmas and electric fields within the regions of space responsible for auroral particle acceleration. The major contribution of these observations was the verification of the existence of electric fields with components parallel to the magnetic field over large regions of altitude (1000–20000 kilometers). These electric fields constitute potential drops of several kilovolts, accelerating magnetospheric electrons downward to form the aurora and ionospheric ions upward, where they contribute significantly to the magnetospheric hot ion population. Perpendicular spatial scales of ∼100 kilometers are most common, although finer scales have been observed embedded, and individual small amplitude double layers occur on much smaller parallel spatial scales. More recently, these same data sets have revealed the existence of ∼100 V electric potential drops directed downward in return current regions. Downward electric fields are in a direction to accelerate electrons out of the ionosphere and tend to retard the propagation of ions upward. An association between upflowing electron beams and tranversely heated ions at low altitude has been noted, and a causal relationship between downward electric fields and ion conics is suggested.

On the structures and mapping of auroral electrostatic potentials

We examine the mapping of magnetospheric and ionospheric electric fields in a kinetic model of magnetospheric‐ionospheric electrodynamic coupling proposed for the aurora by Chiu and Cornwall (1980). A new feature is the generalization of the kinetic current potential relationship to the return current region (identified as a region where the parallel potential drop from magnetosphere to ionosphere is positive); such a return current always exists unless the ionosphere is electrically charged to grossly unphysical values. We are able for the first time to give a coherent phenomenological picture of both the low‐energy return current and the high‐energy precipitation of an inverted V. The mapping between magnetospheric and ionospheric electric fields is phrased in terms of a Green’s function that acts as a filter, emphasizing magnetospheric latitudinal spatial scales of order (when mapped to the ionosphere) 50–150 km. We identify this length as roughly the overall size of the inverted V region. Roughly speaking, this scale length, when multiplied by the perpendicular electric field just above the ionosphere, gives the magnitude of the parallel potential drop between the ionosphere and equatorial magnetosphere. A precise relation between these quantities is given—for the first time—in the body of the paper; previous two‐dimensional kinetic models have taken the parallel potential drop as an assumed input.

Rapid collapse of antiparallel magnetic field system of finite width by fast reconnexion

The present computer experiment studies an explosive magnetic energy conversion in an antiparallel-fleid system of finite width where sheet current flows in the middle of the system and return current flows on both sides of the system. It is demonstrated that, initiated by local onset of anomalous resistivity, the global plasma flow grows so as to bring about the so-called fast reconnexion process that provides rapid magnetic flux transfer as well as rapid magnetic energy release. In the resulting configuration of fast reconnexion, the plasma flow drastically reverses its direction across the magnetic- field region, and the effective width of the magnetic-field region becomes rapidly reduced with time. It is found that the fast reconnexion suddenly decays when the front tip of the return-current region has just arrived at the field-reversal region.