Auroral Cavity Research Articles

Cluster Multi‐Probing of the Aurora During Two Decades

AbstractThe Cluster spacecraft, launched in the year 2000, were pioneers using multi‐point measurements to explore the dynamic space plasma environment of Earth. This capability offered a new, powerful approach to study in situ fundamental space plasma processes of the solar‐terrestrial interaction. Although not a major science objective of the mission, the aurora and associated processes proved to be a very fruitful target for exploration by the Cluster fleet. Here, results are presented from a selection of Cluster auroral studies using the multi‐point measurements capability to further our understanding on various open issues on the aurora. These include the nature and properties of the auroral acceleration region, in particular the altitude distribution of the quasi‐static parallel electric field and potential, the auroral density cavity and its relation to the acceleration region, the role of and relative contribution by quasi‐static and Alfvénic acceleration processes in producing aurora. A related topic addressed by Cluster is acceleration processes and dynamics of the downward current region and its relation to black aurora. To get a broader perspective on these phenomena, event and statistical studies were used, supported by, in a few cases, numerical simulation studies.

Nonlinear radiation generation processes in the auroral acceleration region

Abstract. It is known from laboratory plasma experiments that double layers (DLs) radiate in the electromagnetic spectrum; but this is only known qualitatively. In these experiments, it was shown that the electron beam created on the high-potential side of a DL generates nonlinear structures which couple to electromagnetic waves and act as a sender antenna. In the Earth auroral region, observations performed by auroral spacecraft have shown that DLs occur naturally in the source region of intense radio emissions called auroral kilometric radiation (AKR). Very high time-, spatial-, and temporal-resolution measurements are needed in order to characterize waves and particle distributions in the vicinity of DLs, which are moving transient structures. We report observations from the FAST satellite of a localized large-amplitude parallel electric field (∼ 300 mV m−1) recorded at the edges of the auroral density cavity. In agreement with laboratory experiments, on the high-potential side of the DL, elementary radiation events are detected. They occur substantially above the local electron gyrofrequency and are associated with the presence of electron holes. The velocity of these nonlinear structures can be derived from the measurement of the Doppler-shifted AKR frequency spectrum above the electron gyrofrequency. The generated electron holes appear as the nonlinear evolution of electrostatic waves generated by the electron–electron two-stream instability because they propagate at about half the beam velocity. It is pointed out that, in the vicinity of a DL, the shape of the electron distribution gives rise to a significant power recorded in the left-hand polarized ordinary (LO) mode.

Beaming of intense AKR seen from the Interball‐2 spacecraft

AbstractWe present results of intense auroral kilometric radiation (AKR) sources direction finding based on single‐spacecraft vector source location performed in the frame of Interball‐2 mission (POLRAD experiment on board Auroral Probe). With our swept frequency analyzer we are not able to work with single AKR bursts generated in small, elementary sources, but we improve our signal‐to‐noise (s/n) ratio and determine direction to the AKR source region averaging data over 10 consecutive 4 kHz frequency steps. Measurements of directions to the AKR sources confirm recent Mutel et al. () findings based on Cluster Very Long Baseline Interferometry (VLBI) data—AKR rays are mostly confined to the direction tangent to the auroral oval as measured in Mutel's tangent plane (TP) coordinates. In this paper we use additional coordinate system rotated with respect to TP coordinates in order to determine azimuths of AKR rays with respect to the auroral oval. We see cases of AKR propagation significantly deflected from the tangent plane. Additional information concerning geometry of auroral arc at the AKR source can help to distinguish between propagation along and propagation across the auroral cavity. Examples of instantaneous AKR visibility maps defined in this paper for both coordinate systems are shown and discussed. Using such map (valid for our spacecraft for relatively short observational periods of the order of 10 min), it is possible for known positions of the AKR sources in invariant latitude‐magnetic local time coordinates to visualize direction angles of AKR beams reaching the observer.

Formation of the small-scale structure of auroral electron precipitations

This paper is aimed at physical causes of the small-scale transverse structure in the flows of auroral electrons, generating the corresponding small-scale structure of discrete auroras. The parallel electric field existing in the lower part of the auroral magnetosphere, in the auroral cavity region, in the presence of a strong upward field-aligned current, accelerates magnetospheric electrons to energies of ∼1−10 keV. The flow of these particles while maintaining the high density of the field-aligned current, produces a current-driven instability, which generates Alfvénic turbulence at short perpendicular wavelengths ≤1 km. These short-wavelength inertial Alfvén disturbances possess a nonzero parallell electric field, which modulates the electron flow velocity. The modulation occurring at high altitudes ≥104 km leads to a nonlinear effect of formation of strong density peaks at low altitudes of electron precipitation. The transverse, horizontal scales of the corresponding electron flow structure coincide with the small scales of the Alfvénic turbulence; and this structuring leads to non-uniformities in the auroral luminosity on the same scales, i.e., to small-scale structure of discrete auroras.

Dynamics of density cavities generated by frictional heating: Formation, distortion, and instability

AbstractA simulation study of the generation and evolution of mesoscale density cavities in the polar ionosphere is conducted using a time‐dependent, nonlinear, quasi‐electrostatic model. The model demonstrates that density cavities, generated by frictional heating, can form in as little as 90 s due to strong electric fields of ∼120 mV/m, which are sometimes observed near auroral zone and polar cap arcs. Asymmetric density cavity features and strong plasma density gradients perpendicular to the geomagnetic field are naturally generated as a consequence of the strong convection and finite extent of the auroral feature. The walls of the auroral density cavities are shown to be susceptible to large‐scale distortion and gradient‐drift instability, hence indicating that arc‐related regions of frictional heating may be a source of polar ionospheric density irregularities.

Electron density and parallel electric field distribution of the auroral density cavity

AbstractWe present an event study in which Cluster satellites C1 and C3 encounters the flux tube of a stable auroral arc in the premidnight sector. C1 observes the midcavity, while C3 enters the flux tube of the auroral arc at an altitude which is below the acceleration region, before crossing into the top half of the acceleration region. This allows us to study the boundary between the ionosphere and the density cavity, as well as large portion of the upper density cavity. The position of the two satellites, in relation to the acceleration region, is described using a pseudo altitude derived from the distribution of the parallel potential drop above and below the satellites. The electron density exhibits an anticorrelation with the pseudo altitude, indicating that the lowest electron densities are found near the top of the density cavity. Over the entire pseudo altitude range, the electron density distribution is similar to a planar sheath, formed out of a plasma sheet dominated electron distribution, in response to the parallel electric field of the acceleration region. This indicates that the parallel electric fields on the ionosphere‐cavity boundary, as well as the midcavity parallel electric fields, are part of one unified structure rather than two discrete entities. The results highlight the strong connection between the auroral density cavity and auroral acceleration as well as the necessity of studying them in a unified fashion.

Statistical altitude distribution of the auroral density cavity

AbstractThe statistical altitude distribution of auroral density cavities located between 3.0 and 6.5 RE is investigated using in situ observations from flux tubes exhibiting auroral acceleration. The locations of the observations are described using a pseudo altitude derived from the distribution of the parallel potential drop above and below the satellite. The upper edge of the auroral acceleration region is observed between 4.375 and 5.625 RE. Above 6.125 RE, none of the events exhibit precipitating inverted V electrons, though the upward ion beam can be observed. This indicates that the satellites are located inside the same flux tube as, but above, the auroral acceleration region. The electron density decreases as we move higher into the acceleration region. The spacecraft potential continues to decrease once above the acceleration region, indicating that the density cavity extends above the acceleration region. From 3.0 to 4.375 RE the pseudo altitude increases by 0.20 per RE, consistent with a distributed parallel electric field. Between 4.375 and 5.625 RE the pseudo altitude increases weakly, by 0.01 per RE, due to an increasing number of events per altitude bin, which are occurring above the acceleration region. Above 5.625 RE the pseudo altitude increases by 0.28 per RE, due to a rapid increase in the number of events per altitude bin occurring above the acceleration region, indicating that the remaining parallel potential drop is concentrated in a narrow region at the upper edge of the acceleration region, rather than in a distributed parallel electric field.

Density cavity formation through nonlinear interaction of 3‐D inertial Alfvén wave and ion acoustic wave

AbstractDensity cavities having transverse‐scale size of the order of electron inertial length have been observed in auroral ionosphere by Freja spacecraft. In the present study, density cavity formation in auroral regions has been investigated using numerical simulation techniques. Nonlinear interaction of inertial Alfvén waves and ion acoustic waves has been suggested as a possible mechanism to apprehend density cavity formation in auroral regions. In the proposed mechanism, strong modification in the density profile takes place due to the ponderomotive force of inertial Alfvén waves and accounts for these density depleted regions. In literature also, it has been demonstrated that these cavities are associated with the ponderomotive forces of inertial Alfvén wave. Our presented model also attempts to understand the salient features of auroral density cavities which are reported in literature from the analysis of data available from FAST spacecraft. Relevance of obtained results with observations in auroral ionosphere has also been discussed.

In situ observations of density cavities extending above the auroral acceleration region

AbstractThe uppermost part of a stable potential structure in the auroral acceleration region was studied using simultaneous observations of Cluster satellites C1 and C3. Both satellites observe a monotonically decreasing electron density as they ascend through the auroral acceleration region. As C1 exits the top of the auroral acceleration region, the electron densities continue to decrease, and the minimum electron density is reached 14 km above the upper edge of the auroral acceleration region. The electron density does not return to noncavity values until the spacecraft exits the potential structure's flux tube. The data indicate that the auroral density cavity is not confined by the potential structure and may extend above the auroral acceleration region.

Radiation in the neighbourhood of a double layer

Abstract. In the auroral kilometric radiation (AKR) source region, acceleration layers narrow in altitude and associated with parallel field-aligned potential drops of several kV can be identified by using both particles and wave-field high time-resolution measurements from the Fast Auroral SnapshoT explorer spacecraft (FAST). These so-called double layers (DLs) are recorded around density enhancements in the auroral cavity, where the enhancement can be at the edge of the cavity or even within the cavity at a small scale. Once immersed in the plasma, DLs necessarily accelerate particles along the magnetic field lines, thereby generating locally strong turbulent processes leading to the formation of nonlinear phase space holes. The FAST data reveal the asymmetric character of the turbulence: the regions located on the high-potential side of the DLs are characterized by the presence of electron holes, while on the low-potential side, ion holes are recorded. The existence of these nonlinear phase space holes may affect the AKR radiation pattern in the neighbourhood of a DL where the electron distribution function is drastically different from a horseshoe shape. We present some observations which illustrate the systematic generation of elementary radiation events occurring significantly above the local electron gyrofrequency in the presence of electron holes. These fine-scale AKR radiators are associated with a local electron distribution which presents a pronounced beam-like shape.

Scaled Experiment to Investigate Auroral Kilometric Radiation Mechanisms in the Presence of Background Electrons

Auroral Kilometric Radiation (AKR) emissions occur at frequencies ~300kHz polarised in the X-mode with efficiencies ~1-2% [1,2] in the auroral density cavity in the polar regions of the Earth's magnetosphere, a region of low density plasma ~3200km above the Earth's surface, where electrons are accelerated down towards the Earth whilst undergoing magnetic compression. As a result of this magnetic compression the electrons acquire a horseshoe distribution function in velocity space. Previous theoretical studies have predicted that this distribution is capable of driving the cyclotron maser instability. To test this theory a scaled laboratory experiment was constructed to replicate this phenomenon in a controlled environment, [3-5] whilst 2D and 3D simulations are also being conducted to predict the experimental radiation power and mode, [6-9]. The experiment operates in the microwave frequency regime and incorporates a region of increasing magnetic field as found at the Earth's pole using magnet solenoids to encase the cylindrical interaction waveguide through which an initially rectilinear electron beam (12A) was accelerated by a 75keV pulse. Experimental results showed evidence of the formation of the horseshoe distribution function. The radiation was produced in the near cut-off TE01 mode, comparable with X-mode characteristics, at 4.42GHz. Peak microwave output power was measured ~35kW and peak efficiency of emission ~2%, [3]. A Penning trap was constructed and inserted into the interaction waveguide to enable generation of a background plasma which would lead to closer comparisons with the magnetospheric conditions. Initial design and measurements are presented showing the principle features of the new geometry.

Sudden change in the resonance structure in the electromagnetic noise spectrum in the 0.1–10 Hz range during a substorm

The data of the geophysical observation complex at Barentsburg observatory on Spitsbergen archipelago, together with the data from other stations and satellite observations, were used to interpret a sharp increase in the frequency interval in the electromagnetic noise spectral resonance structure (SRS) in the 0.1–10 Hz range that took place during a substorm that occurred on December 24, 2005. It has been shown that such a change in SRS is related to a decrease in the electron density in the ionospheric F region, which agrees with the ionospheric Alfven resonator theory. In turn, a decrease in electron density is probably related to the fact that the station was in the auroral cavity region related to the field-aligned current flowing into the ionosphere.

The evolution of the U‐shaped double layer at the auroral cavity‐ionosphere boundary in Earth's upward current region

The evolution of the ionosphere‐auroral cavity boundary is studied using a two‐dimensional electrostatic particle‐in‐cell simulation. The boundary is modeled as a double layer which forms by imposing a density cavity as part of the initial conditions. The density cavity consists of a contact discontinuity which separates the cold, dense ionosphere on one side and the hot, tenuous auroral cavity on the other side. The simulation consists of hot electrons, H+ and O+ antiearthward traveling ion beams, hot magnetospheric H+and cold ionospheric electrons. A U‐shaped double layer is formed by initializing the ion beams with a perpendicular shear in the parallel drift velocity. We show that the strength and obliqueness angle of the evolved double layer depends most strongly on the ratio of the ion beam's gyrofrequency to the plasma frequency, and that the U‐shaped double layer is quasi‐stable on simulation time scales. Finally, we show that the oblique double layer causes perpendicular ion heating in the ion beams and that a ring distribution forms in a weakly magnetized species. However, we find that at high altitudes above the double layer, the dominant effect on the shape of ion phase space and the ion temperature is the turbulence in the auroral cavity, leading to the conclusion that oblique double layers have little lasting effect on the shape and temperature of the ion beams.

Pseudo altitude: A new perspective on the auroral density cavity

Studying the density distribution inside the auroral density cavity is complicated by the difficulties in achieving simultaneous measurements within the same flux tube at different altitudes. Comparisons between different events are complicated by variations in both the location of the density cavity and the location of the related potential structure. Describing the spacecraft's location inside the density cavity relative to the potential structure instead of the Earth offers a more practical and consistent frame of reference, a pseudo altitude. The pseudo altitude is determined by comparing the potential drop above the spacecraft, as determined from the characteristic energy of the downward electrons, with the parallel potential drop below the spacecraft, determined from the characteristic energy of the upward ions. A pseudo altitude of 0 corresponds to the bottom of the potential structure and a pseudo altitude of 1 to the top of the structure. Seven events from 2008 were selected, each of which corresponds to a Cluster crossing of a mainly quasi‐static potential structure. All of the events exhibit a consistent anticorrelation between the pseudo altitude and the electron density. No upper limit of the density cavity can be observed, while all cavities have a lower limit above a pseudo altitude of 0.33. These observations show that the auroral density cavity is predominately concentrated to the upper parts of the quasi‐static potential structure.

The formation and evolution of double layers inside the auroral cavity

The formation and dynamic evolution of double layers which form inside the auroral cavity are studied using one‐ and two‐dimensional electrostatic particle‐in‐cell simulations. Both the one‐ and two‐dimensional simulations are confined to processes that occur in the auroral cavity and include four plasma populations: hot electrons, H+ and O+ anti‐earthward ion beams and a hot H+background population. We show that double layers inside the auroral cavity can evolve nonlinearly from ion phase space holes and are supported by the cold, anti‐earthward traveling O+beam. We then present two‐dimensional particle‐in‐cell results which show that double layers in the interior of the auroral cavity can form in higher dimensions. The electric field structure in 2‐D is verified as a double layer through simultaneous analysis of the parallel electric field and H+ and O+ phase space.

A simulation study of the current‐voltage relationship of the Io tail aurora

The Io tail aurora extends for approximately 100 degrees downstream in longitude from the Io footprint aurora. Observations indicate that the brightness of the Io tail aurora continuously decreases along the footpath while its peak altitude remains constant. According to the quasi‐steady theoretical frame, this suggests that the field‐aligned voltage is constant while the parallel current density decreases in the downstream direction. The mechanism that realizes the current‐voltage relationship of the Io tail aurora remains unresolved. In this paper, we apply a new multimagnetofluid code to the Io‐Jupiter system to clarify the origin of the current‐voltage relationship. The code solves a set of equations that includes the electron convection term in Ohm's law, which enables us to simulate the current‐driven ion acoustic instability in the fluid frame. The instability forms a transition layer at a high altitude, which accelerates the magnetospheric electrons and blocks the magnetospheric ions, leading to the formation of a density depleted region called an auroral cavity. We find that if the ionospheric proton density decreases at the same rate as the parallel current density, the timescale on which the transition layer disappears is consistent with the longitudinal extent of the tail aurora, and the potential gap is constant all along the tail. We discuss the possibility that the fringe, wideband repetitive bursts of the Io‐related Jovian decametric radiation, is excited in the auroral cavity.

Ion acoustic solitons in Earth’s upward current region

The formation and evolution of ion acoustic solitons in Earth’s auroral upward current region are studied using one- and two-dimensional (2D) electrostatic particle-in-cell simulations. The one-dimensional simulations are confined to processes that occur in the auroral cavity and include four plasma populations: hot electrons, H+ and O+ anti-earthward ion beams, and a hot H+ background population. Ion acoustic solitons are found to form for auroral-cavity ion beams consistent with acceleration through double-layer (DL) potentials measured by FAST. A simplified one-dimensional model simulation is then presented in order to isolate the mechanisms that lead to the formation of the ion acoustic soliton. Results of a two-dimensional simulation, which include both the ionosphere and the auroral cavity, separated by a low-altitude DL, are then presented in order to confirm that the soliton forms in a more realistic 2D geometry. The 2D simulation is initialized with a U-shaped potential structure that mimics the inferred shape of the low altitude transition region based on observations. In this simulation, a soliton localized perpendicular to the geomagnetic field is observed to form and reside next to the DL. Finally, the 2D simulation results are compared with FAST data and it is found that certain aspects of the data can be explained by assuming the presence of an ion acoustic soliton.

Strong magnetic field fluctuations within filamentary auroral density cavities interpreted as VLF saucer sources

The Geoelectrodynamics and Electro‐Optical Detection of Electron and Suprathermal Ion Currents (GEODESIC) sounding rocket encountered more than 100 filamentary density cavities associated with enhanced plasma waves at ELF (<3 kHz) and VLF (3–10 kHz) frequencies and at altitudes of 800–990 km during an auroral substorm. These cavities were similar in size (∼20 m diameter in most cases) to so‐called lower‐hybrid cavities (LHCs) observed by previous sounding rockets and satellites; however, in contrast, many of the GEODESIC cavities exhibited up to tenfold enhancements in magnetic wave power throughout the VLF band. GEODESIC also observed enhancements of ELF and VLF electric fields both parallel and perpendicular to the geomagnetic fieldB0within cavities, though the VLF E field increases were often not as large proportionally as seen in the magnetic fields. This behavior is opposite to that predicted by previously published theories of LHCs based on passive scattering of externally incident auroral hiss. We argue that the GEODESIC cavities are active wave generation sites capable of radiating VLF waves into the surrounding plasma and producing VLF saucers, with energy supplied by cold, upward flowing electron beams composing the auroral return current. This interpretation is supported by the observation that the most intense waves, both inside and outside cavities, occurred in regions where energetic electron precipitation was largely inhibited or absent altogether. We suggest that the wave‐enhanced cavities encountered by GEODESIC were qualitatively different from those observed by earlier spacecraft because of the fortuitous timing of the GEODESIC launch, which placed the payload at apogee within a substorm‐related return current during its most intense phase, lasting only a few minutes.

Magnetosphere‐ionosphere coupling at Jupiter: A parameter space study

Jupiter's main auroral emission is a signature of the current system that transfers angular momentum from the planet to radially outward moving Iogenic plasma. Ray et al. (2010) developed a steady state model of this current system which self‐consistently included the effects of a field‐aligned potential, Φ∥, and an ionospheric conductance modified by precipitating electrons. The presented parameter space study extends their model to explore how variations in the auroral cavity density and temperature, magnetospheric mass loading rate, and background ionospheric Pedersen conductance affect the current system and resulting auroral emission. We show that while the solutions found by Ray et al. (2010) vary with changes in the system parameters, the gross general trends remain similar to the original solutions. We find that, for an outer constraint of I100 = 86 MA, the high‐latitude electron temperature and density have a lower limit of ∼1.5 keV and an upper limit of ∼0.01 cm−3, respectively, in order for solutions to be consistent with observations of Jupiter's auroral emission. For increases in the radial mass transport rate and an outer constraint of = 75 kV the auroral emission brightness increases.

Characterization of a Penning discharge for investigation of auroral radio wave generation mechanisms

Auroral Kilometric Radiation (AKR), observed by satellites in the Earth's magnetosphere, is naturally generated in regions of partial plasma depletion (auroral density cavity) in the polar magnetosphere at approximately 3200 km altitude. As an electron descends through these regions of partial plasma depletion along magnetic field lines towards the Earth's ionosphere, the field lines increases and, through conservation of the magnetic moment, the electron gives up axial velocity in favour of perpendicular velocity. This results in a horseshoe-shaped distribution function in parallel/perpendicular-velocity space which is unstable to X-mode radiation, near the cyclotron frequency. Power levels as high as GW levels have been recorded with frequencies around 300 kHz. The background plasma frequency within the auroral density cavity is approximately 9 kHz corresponding to a plasma density 1 cm−3. A laboratory experiment scaled from auroral frequency to microwave frequency has previously been reported. Here, the addition of a Penning trap to simulate the background plasma of the density cavity is reported, with measurements ne ∼ 2 × 1014–2.17 × 1015 m−3, fpe ∼ 128–418 MHz and fce ∼ 5.21 GHz giving a ratio of ωce/ωpe comparable to the magnetospheric AKR source region.