Quasi-static Parallel Electric Fields Research Articles

Spatiotemporal features of the auroral acceleration region as observed by Cluster

The two major agents for producing aurora are generally believed to be the quasi-static parallel electric fields, accelerating electrons in the auroral acceleration region (AAR), and Alfven waves. The Cluster spacecraft quartet has made multi-spacecraft measurements in the AAR possible for the first time. Four event studies are included and discussed in this thesis, using Cluster data inside and at the top of the AAR, to address various open issues regarding the nature of the quasistatic electric potential structures, such as their altitude distribution, temporal and spatial variability, as well as their interactions with regions of Alfven wave activity.In Paper 1, Cluster data from the upper and lower parts of the AAR were used to determine the altitude and latitude distribution of the acceleration potential above the aurora and to address its stability in space and time. The acceleration potential pattern derived consisted of two equally broad and intense U-shaped electric potential structures and a narrower S-shaped potential structure located below. The acceleration potential distribution was found to be stable, both in intensity and width, over five minutes. Furthermore, the perpendicular (to the magnetic field) spatial scale of the electric field was found to be much smaller than that of the current in the lower AAR, but roughly equal in the upper AAR. Revealing of these features was possible only by combining data from the two Cluster spacecraft.In Paper 2, the spatial and temporal characteristics and development of two AAR structures were studied, benefiting from a magnetic conjunction between two of the Cluster spacecraft and a short time difference (~1 min) between the spacecraft crossings. The configuration allowed for estimating the characteristic times of development for the two structures and the parallel electric field and potential drop for the more stable one. Potential structure 1, having a perpendicular width of ~80 km was short-lived, developing in less than 40 seconds and decaying in one minute. The parallel potential drop between 1.13 and 1.3 RE altitudes increased, whereas that above 1.3 RE remained almost unchanged during this time. For potential structure 2, having a width of ~50 km, the parallel potential drop increased by a factor of 3 below 1.3 RE during ~40 seconds, after which it remained stable for at least a minute. Also here, the parallel potential drop above 1.3 RE remained roughly unchanged. An average parallel electric field was estimated to be 0.56 mV/m.In the auroral zone, the quasi-static properties are dominant in the plasma sheet (PS) and especially in the central plasma sheet (CPS), while the Alfvenic properties are generally strongest at the plasma sheet boundary layer (PSBL). Therefore it is of special interest to study the PSBL/CPS boundary, regarding how the two processes interact. In Papers 3 and 4 we addressed this matter. In each event, data from the Cluster fleet making an equatorward crossing of the highaltitude AAR were used together with a DMSP UV image of the oval. The particle and field data were used to infer the acceleration potentials of the observed arcs and their distribution in altitude and latitude. In Paper 3, observations demonstrate a quasi-static potential structure extending into the Alfvenic region of the polar cap boundary (PCB). The associated density cavity did not extend into the Alfvenic region, suggesting that the Alfvenic activity observed within the PCB region prevents the cavity formation. The results show that Alfvenic and quasi-static acceleration operate jointly in the PCB region. In Paper 4, Cluster passed over a system of East-West aligned auroral arcs. The upper extent of the AAR was found to be at least 0.5 RE higher in altitude than generally expected. Overlapping Alfvenic and quasi-static regions were found within the PSBL and inside Region 2 of downward currents. Growth of small-scale potential structures was observed in the PSBL and in the middle of Region 2, during the ~1.5 minutes between the Cluster 4 and Cluster 3 passages. During this period, the Alfvenic regions retreated both at the poleward and equatorward oval boundaries, while the quasi-static potentials intensified.

Parallel electric fields in the upward current region of the aurora: Indirect and direct observations

In this article we present electric field, magnetic field, and charged particle observations from the upward current region of the aurora focusing on the structure of electric fields at the boundary between the auroral cavity and the ionosphere. Over 100 high-resolution measurements of the auroral cavity that were taken by the Fast Auroral Snapshot (FAST) satellite are included in this study. The observations support earlier models of the auroral zone that held that quasi-static parallel electric fields are the primary acceleration mechanism. In addition to the statistical study, several examples of direct observations of the parallel electric fields at the low-altitude boundary of the auroral cavity are put forth. These observations suggest that the parallel electric fields at the boundary between the auroral cavity and the ionosphere are self-consistently supported as oblique double layers.

Formation of electrostatic potential drops in the auroral zone

In order to examine the self-consistent formation of large-scale quasi-static parallel electric fields in the auroral zone on a micro/meso scale, a particle in cell simulation has been developed. The code resolves electron Debye length scales so that electron micro-processes are included and a variable grid scheme is used such that the overall length scale of the simulation is of the order of an Earth radii along the magnetic field. The simulation is electrostatic and includes the magnetic mirror force, as well as two types of plasmas, a cold dense ionospheric plasma and a warm tenuous magnetospheric plasma. In order to study the formation of parallel electric fields in the auroral zone, different magnetospheric ion and electron inflow boundary conditions are used to drive the system. It has been found that for conditions in the primary (upward) current region an upward directed quasi-static electric field can form across the system due to magnetic mirroring of the magnetospheric ions and electrons at different altitudes. For conditions in the return (downward) current region it is shown that a quasi-static parallel electric field in the opposite sense of that in the primary current region is formed, i.e., the parallel electric field is directed earthward. The conditions for how these different electric fields can be formed are discussed using satellite observations and numerical simulations.

Weak double layers in the auroral ionosphere

Previous work on the evolution of weak double layers in a hydrogen plasma has been extended to include H+ and O+ with relative drift. It has been shown (Bergmann & Lotko 1986) that the relative drift between hydrogen and oxygen ions due to a quasistatic parallel electric field gives rise to a strong linear fluid instability which dominates the ion acoustic mode at the bottom of the auroral acceleration region. This ion–ion instability can modify ion distributions at lower altitudes and the subsequent nonlinear evolution of weak double layers at higher altitudes in the ion acoustic regime. We have found that ion hole formation can occur for smaller relative electron-ion drifts than seen in previous simulations, due to the hydrogen-oxygen two-stream instability. This results in local modification of the ion distributions in phase space, and a partial filling of the valley between the hydrogen and oxygen peaks, which would be expected at higher altitudes on auroral field lines. It is shown that the observed velocity diffusion does not necessarily preclude ion hole and double layer formation in hydrogen in the ion acoustic regime. These simulation results are consistent with the experimentally measured persistence of separate hydrogen and oxygen peaks, and the observation of weak double layers above an altitude of 3000 km on auroral field lines.

VLF emission stimulated by parallel electric fields

We study the influence of a weak quasi-static parallel electric field on the stability of electromagnetic plasma waves. Using an operator calculus to solve the Boltzmann-Maxwell equations we derive a dispersion relation for the electromagnetic waves. Assuming that the electrons have a loss-cone distribution, the real frequency of waves in the whistler band is not changed by the presence of the electric field. Resonant interaction damps the HF waves for propagation parallel to the electric field. In the case of opposite propagation, a new HF excitation is found at frequencies ω ≲ ωce The width of the excitation region depends on the width of the loss cone, field strength and collision frequency. This result is applied to observations of the splitting of VLF emissions under natural conditions in the magnetosphere. It is found that the observed splitting could have been caused by the presence of the weak parallel electric field of a kinetic (shear) Alfvén wave in the emission region, which is quasi-stationary compared with the growth of the observed VLF emission.