Auroral Plasma Cavity Research Articles

Electron acceleration by an Alfvénic pulse propagating in an auroral plasma cavity

[1] With the help of a 2.5-D particle-in-cell simulation code, we investigate the physics of the acceleration of auroral electrons, through the interaction of an isolated Alfven wave packet with a plasma density cavity. The cavity is edged by density gradients perpendicular to the magnetic field. We show that a single passing of an isolated wave packet over a (infinite) cavity creates an electron beam. It triggers local current and beam-plasma instabilities and small-scale coherent electric structures. The energy flux of downgoing electrons is significantly increased, whereas upgoing electrons are also accelerated, even if no beam is formed. Accelerated electrons remain after the passage of the Alfvenic pulse, allowing the observation of energetic particles without any significant electromagnetic perturbation. The dependence of this process on the electron to ion mass ratio is consistent with its control by inertial effects.

Suppression of convective cell generation by Alfvén waves in the ionospheric auroral cavity

A theory for the parametric interaction of inertial Alfvén wave (IAW) packets with large-scale convective cells accounting for the nonzero ratio of the Alfvén and light velocities is presented. It is shown that in the auroral plasma cavity where the Alfvén velocity has its maximum value, the parametric interaction of IAWs with convective cells is substantially reduced. In this region the Alfvén velocity is comparable to the speed of light and thus the displacement current cannot be neglected. This current plays a stabilizing role and results in a decline of the parametric instability growth rate. On the contrary, the incorporation of the finite width of the IAW packet leads to increase in the convective cell growth rate. A compact expression for the instability growth rate and the optimal conditions for the fastest growing convective mode are derived. The relevance of the theory to recent spacecraft measurements in the Earth’s ionosphere is pointed out.

Seasonal and solar cycle dynamics of the auroral kilometric radiation source region

Several year's worth of observations from the plasma wave instruments on both Magnetopause‐to‐Aurora Global Exploration (IMAGE) and Polar spacecraft are used to study the seasonal and solar cycle variations in the spectrum of auroral kilometric radiation (AKR). Only AKR observations when the spacecraft were in the Northern Hemisphere emission cones were used. The results from the seasonal analysis show significant changes in the AKR spectrum as a function of dipole tilt. The average AKR spectral peak for positive dipole tilt is ∼150 kHz but is ∼300 kHz during times of negative dipole tilt. In addition, the average emission spectrum for positive tilt is up to two orders of magnitude weaker over the 200–500 kHz frequency range when compared with the average emission spectrum for negative tilt. Assuming the cyclotron maser mechanism for AKR, these results imply that the AKR source region (the auroral density cavity) moves to higher altitudes during the summer and to lower altitudes during the winter. Using data from the DE‐1 plasma wave instrument, the magnetic local time of average AKR source region is also investigated with dipole tilt. From these observations it is found that for negative dipole tilt a broad AKR source region exists, ranging from ∼18 to ∼24 MLT, with peak emission coming from ∼20 MLT. In comparison, under positive dipole tilt the source region narrows (∼20 to ∼24 MLT) with peak emission at ∼22 MLT. Taking into account the above seasonal effect, a comparison of the average spectra from IMAGE and Polar plasma wave data also demonstrates a solar cycle effect. The average AKR spectrum at solar maximum has the same structure with dipole tilt as at solar minimum but is typically lower (by as much as 1–2 orders of magnitude). The observations presented support the concept that the expected increases in ionospheric densities (with positive dipole tilt for the Northern Hemisphere and solar EUV flux increases during solar maximum) play a significant role in magnetospheric‐ionospheric coupling by: (1) shortening the altitude range of the auroral plasma cavity, (2) confining the cavity to a smaller range of MLT and closer to midnight, and (3) decreasing the overall intensity of AKR by lessening the density depth of the auroral density cavity. The results of this study should be taken into account in future studies of using AKR as a substorm index and other statistical emission cone studies at both low and high frequencies.

Particle acceleration linked to Alfvén wave propagation on small scale density gradients

We study how Alfvén waves propagate in the presence of sharp density gradients in the direction perpendicular to the ambient magnetic field. A fully electromagnetic electron guiding centre code is used for the simulation. During the propagation, initially parallel (k∥ = 0), transverse scales of the order of c/ωpe are quickly reached which contributes to the creation of a significant parallel component of the electric field in the region of density inhomogeneity. The effects of this field on the velocity distribution functions are then discussed. In particular, we show that they can present a strong deviation from their initial Gaussian shape (global shift in energy) due to the action of the parallel electric field. Evidences are then given for a net energy gain of the electrons, to the expense of the wave, during this process. This energy transfer mechanism may be relevant in order to explain the particle acceleration in the auroral plasma cavities.

Electron acceleration by Alfvén waves in density cavities

A new electromagnetic two‐dimensional guiding center particle in cell (PIC) code is used to investigate the propagation of an Alfvén wave in the perpendicular density gradients that characterize the edges of the auroral density cavities. It is shown that the wave planes are strongly distorted by the inhomogeneities of the plasma and that a significant electric field develops in the direction parallel to the background magnetic field during the propagation. This field efficiently accelerates the electrons in the parallel direction, and the incident wave is thus strongly absorbed. The associated dissipation rate is sufficiently strong to explain a complete wave absorption on the density gradients over a fraction of wavelength. The electron parallel acceleration is also characterized. It corresponds to a global parallel acceleration of the electron population. These PIC simulations suggest that the perpendicular density gradients corresponding to the auroral plasma cavities play an important role in the auroral particle acceleration.

Source of auroral myriametric radiation observed with Geotail

The Geotail plasma wave instrument has found the auroral myriametric radiation (AMR). Its frequency range is about 1–30 kHz similar to the trapped continuum radiation, and its occurrence shows a good correlation with auroral kilometric radiation (AKR). This suggests the source location of AMR is similar to that of AKR. Ray tracing is performed for L‐O mode waves radiated just above the local plasma frequency in the auroral plasma cavity. The waves are guided in the cavity, propagate far from the Earth, arid are observed as AMR. In the source region these guided waves are observed by the DE 1 satellite and reported as the trapped L‐O mode waves within the cavity. A limitation on initial conditions of wave normal angles for the ray tracing derived from a comparison with these observations suggests that its generation mechanism is similar to that of the nonthermal continuum. It is, however, generated in the auroral plasma cavity and it is not mode‐converted from the upper hybrid mode waves. The ray tracing results are consistent with AMR observations by Geotail.

Observations of mesoscale auroral plasma cavity crossings with the Freja satellite

We have used data from the Freja satellite, with an apogee of 1700 km over the auroral oval, to make a statistical analysis of mesoscale auroral plasma density cavities. Such cavities are defined to be regions in the auroral zone with size scales between discrete arcs and inverted‐V events in which the plasma density abruptly drops to less than 50% of the ambient plasma density. We present occurrence distributions by magnetic activity, local time, and magnetic latitude. It is found that cavity occurrence does not depend on magnetic activity. In contrast to earlier studies, we find cavities at all local times; however, a dawn‐dusk asymmetry is found in the distribution. There is likewise a weak dawn‐dusk asymmetry in the latitudinal distribution. Since our data cover a full year, seasonal dependence of cavity occurrence can also be investigated. We find a small seasonal trend, with cavities found more commonly during the dark season, but the trend is not quite statistically significant. We also study several plasma depletion processes that could be responsible for cavities at this altitude. The results suggest that more than one process is compatible with the distributions we find.

Dynamics of the generation of magnetic‐field‐aligned electric fields by drift currents

A study is made of the process of generation and evolution of parallel electric fields on closed geomagnetic field lines. Charge separation that is due to the different character of the drift of energetic electrons and protons of inhomogeneous magnetospheric plasma in the geomagnetic field is considered to be the factor responsible for the generation of the electric field. In a magnetic flux tube that drifts together with the cold plasma, this separation of charges is manifested as the injection of energetic electrons or protons. A numerical self‐consistent calculation of the evolution of the distribution functions of particles and of the parallel electric field is carried out for the injection of energetic electrons into the geomagnetic flux tube. An analytical investigation of the macroscopic and microscopic processes accompanying such injection is carried out. The calculations take into account cold ionospheric particles and energetic magnetospheric protons and electrons. It was found that an intense parallel electric field is produced in the form of two collisionless rarefaction shock waves that separate from the equatorial plane in the northward and southward directions. Below the front of each shock wave, there is mainly the ionospheric plasma, while behind the front there are the upward accelerated ionospheric ions and the hot magnetospheric plasma. Microscopic structure behind the shock front is formed by electrostatic oscillations of a large amplitude. Generation conditions for intense longitudinal fields are formulated: The difference in densities of injected energetic electrons and protons near the equatorial plane must exceed the initial density of cold particles, and the current‐associated velocity of cold electrons must be larger than the ion sound velocity. It is shown that these conditions can be satisfied, owing to a drift mechanism of charge separation. A rarefaction shock wave is excited with substantially smaller field‐aligned currents as compared with anomalous resistance or double layers. The observed manifestations of the rarefaction shock waves are auroral plasma cavities.

Effect of parallel electric fields on the propagation paths of auroral kilometric radiation

The effect of parallel electric fields on the angular distribution of auroral kilometric radiation at frequencies between 56.2 and 500 kHz is examined by ray tracing computations in polar wind and auroral plasma cavity models. The results show that the effect is not significant at frequencies between 100 and 500 kHz in both models. The presence of an electric field does affect the angular distribution at 56.2 kHz where the ray paths representing the boundary of the emission cone in the dayside and nightside sectors of the noon‐midnight meridian are tilted equatorward and poleward, respectively, by about 20° (i.e., the cone axis is redirected toward the evening sector by about 20°). An attempt to match the results with the observations shows that the initial wave normal angles for these ray paths are 262° and 85.5°, respectively. The ray paths computed in the plasma cavity model also yield the same result except for some minor adjustment with the initial wave normal angles.

Evidence of auroral plasma cavities at Uranus and Neptune from radio burst observations

Radio bursts originating from the stronger magnetic polar regions of both Uranus and Neptune were detected by the planetary radio astronomy experiment during the Voyager 2 encounters with the planets. It has previously been demonstrated that these bursts are beamed into a broad, hollow emission pattern from their auroral sources. We now show that the bursts at both planets also manifest similar detailed patterns, with the waves beamed into two separate and distinct radiation cones at intermediate wave frequencies. This double‐cone emission pattern is predicted by relativistic cyclotron resonance theory, and application of this theory to the observed emission pattern yields the plasma density structure within the radio source region. Calculations indicate that at both Uranus and Neptune the plasma‐to‐cyclotron frequency ratio can drop well below 0.01 within the active region. Such low values indicate that the southern auroral zones at both planets contain an auroral plasma cavity that is similar to that found in Earth's nightside auroral zone.

The magnetoionic modes and propagation properties of auroral radio emissions

The different magnetoionic wave modes which accompany the aurora are identified using DE 1 not only by their appearance on satellite radio spectrograms, but also by concurrent measurements of their wave polarization and arrival directions, and by ray‐tracing models of their expected propagation behavior. Of the four possible propagation modes, designated O, X, W, and Z for the ordinary, extraordinary, whistler, and Z modes, respectively, all four are found to occur in the auroral zone, as follows: The most intense, of course, is the well‐known auroral kilometric radiation (AKR), which originates primarily in the X mode near the electron cyclotron frequency, but which is frequently also accompanied by a weaker O‐mode component from the same location. The next most prominent auroral emission is the W‐mode auroral hiss originating from altitudes always well below the DE 1 satellite at frequencies below the local cyclotron frequency. For a particular case which was studied in detail, both of these components were found to originate from approximately the same source field lines near the poleward edge of the auroral plasma cavity, with the latter exhibiting a funnel shape on the radio spectrograms, centered on the source field lines but having different shapes inside and outside the cavity because of the differing plasma densities of these two regions. Equatorward of the cavity and at frequencies above the minimum plasma frequency within the cavity, this upcoming W mode exhibits a shadow zone which is attributed to the blockage of W‐mode propagation at the plasma frequency. The previously reported Z‐mode auroral radiation was also detected, but from sources also below the satellite and at the poleward edge of the cavity, and not from the expected AKR source at the cyclotron frequency. A weaker O‐mode component seems to accompany these emissions also, both within the polar cap poleward of the source and inside the cavity, the latter seemingly being guided upward by the cavity's lower plasma densities. Finally, exactly on the source field lines at the poleward edge of the cavity, there also occasionally seems to be localized Z‐mode emissions extending from the Z‐mode cutoff at quite low frequencies up to and above the plasma frequency.

Observation of the Z mode with DE 1 and its analysis by three‐dimensional ray tracing

Certain Z‐mode wave emissions in the Earth's magnetosphere have been identified using the wave spectra and polarization measurements of the DE 1 satellite. Although such emissions accompany the aurora, and thus presumably originate from the evening‐sector auroral zone, they are found to occur over much wider ranges of latitude and longitude. Since the predicted cyclotron maser emission at the cyclotron frequency could not have produced waves which travel such great distances, as we have shown by three‐dimensional ray tracing, it is proposed instead that these emissions must originate from lower altitudes within the auroral zone and probably from near the plasma frequency inside the auroral plasma cavity.

Generation and propagation of kilometric radiation in the auroral plasma cavity

Two‐dimensional particle simulations and three‐dimensional ray‐tracing calculations are used to investigate the generation and propagation of electromagnetic radiation produced by the electron cyclotron maser instability under conditions that prevail in the auroral plasma cavity. The simulations incorporate a continual flow of primary energetic electrons along the magnetic field. In such a driven system the maser instability leads to a quasi‐steady state in which as much as 7% of the incident flux of electron energy is converted into radiation. The maximum radiation intensity occurs within 20‐30 km from the injection point of the primary electrons, and the radiation consists of discrete wave packets with a spatial extent of 4‐6 km parallel to the field. The magnetic field gradient is shown to be unimportant for these quasi‐local properties of the maser instability. The ray path calculations are performed in a dipolar field geometry and retain the relativistic corrections to the wave dispersion. For the case where the energetic electrons are the dominant species, amplifications of the order of e10 are achieved within path lengths of less than 100 km. This indicates that the maser instability can produce the observed amplification of auroral kilometric radiation above cosmic background levels within a distance smaller than the dimensions of the auroral cavity.

Temporal features of the outflow of heavy ionospheric ions in response to a high altitude plasma cavity

Using a hydrodynamic model for the plasma, it is demonstrated that the auroral plasma cavity is capable of drawing an appreciably large flux of oxygen ions, which are normally gravitationally bound. This escape mechanism of O+ does not involve any additional heating or acceleration of the plasma in the ionosphere. The temporal evolution of the outflow shows that it starts near the cavity and penetrates into the ionosphere with a flux front moving down with the ion‐acoustic speed. A steady outflow is reached in a few hours for a cavity at a height of one Re if the cavity is maintained. This time reduces to < 1 hour for cavity heights < 3000 km. During the transient state (< 1 hour) the outflux can appreciably exceed that in the steady state. In view of the horizontal convection, the transient outflow is of special significance. As the cavity descends, the ionospheric flux of O+ (Γo+) increases; for the lower edge of the cavity in the altitude range 3 ‐ 10 × 10³ km, the steady flux is found to be in the range 109 > Γo+ (cm−2s−1) > 107. An analysis for the steady state shows that outflow is controlled by the density scale height in the cavity.

Generation of ordinary mode auroral kilometric radiation from extraordinary mode waves

The ratio of x to o mode radiation produced when an x mode wave strikes a steep density gradient is determined for wave and plasma parameters typical of those observed in the auroral kilometric radiation source region. We find that the waves reflected from or transmitted through the density gradient generally have an o mode component 20 dB or more weaker than the x mode component. This agrees with the ratio of x to o mode intensity observed in auroral kilometric radiation signals near the auroral plasma cavity by both the DE 1 and ISIS 1 spacecraft.

The auroral kilometric radiation: DE 1 direction finding studies

We have determined the directions of arrival of auroral kilometric radiation during three separate intervals using data from the DE 1 plasma wave instrument. In the case of the dominant extraordinary mode component, these directions were consistent with generation at the local electron cyclotron frequency on nightside auroral field lines. The ordinary mode component appeared to have a similar source in one case, but in other cases came from different directions. These other cases were consistent with reflection at the plasmapause and the wall of the auroral plasma cavity.

A reconciliation of propagation modes of auroral kilometric radiation

The propagation modes of auroral kilometric radiation (AKR) are theoretically investigated by three‐dimensional ray tracing using an electron density model based on the auroral plasma cavity. Both right‐hand extraordinary (R‐X) mode and left‐hand ordinary (L‐O) mode observations are explained in a consistent manner. The following assumptions, which are based on observations near the source, are used for the ray tracing. (1) The radiation is generated in both R‐X and L‐O modes. (2) The ratios of the wave frequency to the R‐X mode cutoff frequency f/fR range from 1.01 to 1.1. (3) AKR waves are generated at wave normal angles almost perpendicular to the geomagnetic field. (4) AKR is generated in the auroral plasma cavity along a nightside field line with an invariant latitude of 70°. L‐O mode waves can propagate within a wider region than R‐X mode waves. Observed characteristics of AKR can be well explained under the following two additional assumptions, which are consistent with the cyclotron maser mechanism. (1) The R‐X mode is stronger than the L‐O mode. (2) Predominantly upgoing waves are generated at the source. The R‐X mode is dominant at high latitudes because it is stronger. The L‐O mode is received at lower latitudes which the R‐X mode waves cannot reach because of refraction and reflection mainly in the vicinity of the source and the plasmapause. Previous papers which claim the R‐X mode radiation are reconsidered. The present interpretation consistently explains the results of their observations without contradiction with the existence of the L‐O mode. It is clarified that they observed both modes.

The auroral plasma cavity

A region of diminished plasma density has been found to occur at the source of auroral kilometric radiation (AKR). The density within this auroral plasma cavity, determined from limited Hawkeye wave data, was less than 1 cm−3 from 1.8 RE to 3 RE geocentric, at 70° ± 3° invariant magnetic latitude. The altitude variation of the magnetic field produces a minimum in the ratio of plasma frequency to cyclotron frequency within the cavity which accounts for the observed spectrum of AKR.