Intense Auroral Kilometric Radiation Research Articles

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.

Perpendicular electron heating by absorption of auroral kilometric radiation

We investigate the possibility of perpendicular heating of electrons and the generation of quasiperpendicular (type 2) electron conics by particle diffusion in velocity space due to wave–particle interaction with intense auroral kilometric radiation. By introducing a ring distribution and a warm plasma dispersion relation, we conduct a basic simulation of conditions near an auroral kilometric radiation (AKR) source. We then solve the diffusion equation using a finite difference algorithm. The results show significant perpendicular electron heating as might exist adjacent to the AKR source region and indicate that the main characteristics of a quasiperpendicular (type 2) electron conic distribution can be reproduced under these conditions.

Observations of the upper frequency cutoffs of the auroral kilometric radiation

Abstract. Intense auroral kilometric radiation (AKR) is being frequently observed with POLRAD from the Auroral Probe (Interball-2). Observations of the abrupt upper frequency cutoffs (UFCs) in the spectra of AKR are reported. The UFCs can be observed at a frequency range from 300 to 700 kHz, corresponding to AKR generation altitudes from approximately 4800 to 2100 km, and are distributed in magnetic local time (MLT) hours similarly to the AKR events, with a maximum at 1 h MLT. The observed frequency extent of the UFCs is ≤12 kHz, and is often determined by the instrumental resolution (4 kHz). It is suggested that the UFC may be associated with an abrupt switching on of the generation mechanism, when the electron density becomes sufficiently low inside a plasma depletion at an altitude where the ratio of fpe/fce crosses some threshold value. The steepness of the UFCs can imply a non-linear process of generation. The estimated distance of the e-folding field aligned wave amplification is between 3 and 8 km. The UFCs are sometimes, though very seldom (<10%), accompanied by narrow band (less than 4 kHz) "ridges" of radiation observed at the cutoff frequency. They are smoothly drifting in frequncy for several minutes. The power density of radiation in the "ridge" can be up to 2 orders of magnitude stronger than in the accompanying wide band emission of AKR. The "ridge" at UFC can imply either energy concentration at the source bottom, or focusing, if specific conditions for the escape of the radiation are assumed.Key words. Magnetospheric physics (auroral phenomena; plasma waves and instabilities) · Radio science (magnetospheric physics)

Observations of the upper frequency cutoffs of the auroral kilometric radiation

Intense auroral kilometric radiation (AKR) is being frequently observed with POLRAD from the Auroral Probe (Interball-2). Observations of the abrupt upper frequency cutoffs (UFCs) in the spectra of AKR are reported. The UFCs can be observed at a frequency range from 300 to 700 kHz, corresponding to AKR generation altitudes from approximately 4800 to 2100 km, and are distributed in magnetic local time (MLT) hours similarly to the AKR events, with a maximum at 1 h MLT. The observed frequency extent of the UFCs is ≤12 kHz, and is often determined by the instrumental resolution (4 kHz). It is suggested that the UFC may be associated with an abrupt switching on of the generation mechanism, when the electron density becomes sufficiently low inside a plasma depletion at an altitude where the ratio of fpe/fce crosses some threshold value. The steepness of the UFCs can imply a non-linear process of generation. The estimated distance of the e-folding field aligned wave amplification is between 3 and 8 km. The UFCs are sometimes, though very seldom (<10%), accompanied by narrow band (less than 4 kHz) "ridges" of radiation observed at the cutoff frequency. They are smoothly drifting in frequncy for several minutes. The power density of radiation in the "ridge" can be up to 2 orders of magnitude stronger than in the accompanying wide band emission of AKR. The "ridge" at UFC can imply either energy concentration at the source bottom, or focusing, if specific conditions for the escape of the radiation are assumed.Key words. Magnetospheric physics (auroral phenomena; plasma waves and instabilities) · Radio science (magnetospheric physics)

Discrete, stimulated auroral kilometric radiation observed in the Galileo and DE 1 wideband data

The Galileo spacecraft observed intense auroral kilometric radiation during the second Earth encounter in 1992. High‐resolution frequency‐versus‐time spectrograms obtained by the wideband receiver of the plasma wave instrument on board the spacecraft often show discrete, negative‐slope features each extending over a period of several seconds. These features may be due to impulsive wave generation and intrinsic velocity dispersion and/or a source stimulation by plasma waves traveling up the magnetic field line through the source region. We present several examples of these signatures, seen also on the Dynamics Explorer 1 satellite, and examine scenarios for their generation.

Correlation between terrestrial myriametric and kilometric radio bursts observed with Galileo

Data from the Galileo plasma wave system recorded during an investigation of the Earth magnetotail on December 8, 1990, are analyzed in both the myriametric and the kilometric wavelength range. In the far tail, between 80 and 30 Earth radii, two components are observed in the myriametric range (frequencies between 6 and about 30 kHz): (1) the “classical” trapped continuum radiation with a smoothly varying intensity in time and (2), superposed on this component, bursts of waves with typical spectral densities of 10−15 V²m−2Hz−1. These bursts of myriametric waves are shown B to correlate with bursts of intense auroral kilometric radiation (AKR) with typical spectral density of 10−12 V²m−2Hz−1. This bursty myriametric radiation is the analog of the low frequency radiation (LFR) reported by Filbert and Kellog (1989). Its close correlation with the kilometric radiation suggests the existence of a direct relationship between these two radiations. A generation mechanism that explains this correlation and the frequency gap between the AKR and the LFR is proposed.

Natural and man‐made emissions at 1.0–5.6 MHz measured between 10 and 18 RE

A survey has been undertaken of the 1.0‐ to 5.6‐MHz electric field spectrum measured with the Active Magnetosphere Particle Tracer Explorers/Ion Release Module spacecraft at distances of 15–18 RE(100,000–120,000 km) from the Earth. Data from one full year of operation were considered in order to obtain complete local time coverage at the ecliptic plane. The data were further binned according to the presence or absence of intense auroral kilometric radiation (AKR) emissions at lower frequencies, in order to resolve emissions processes or interference sources that might be related to the level of AKR. For the times of intense AKR, the principal features found in this survey are (1) the occurrence statistics of the intense AKR with local time generally confirm previous AKR surveys using, for example, the ISEE satellite; and (2) the only 1‐ to 5.6‐MHz emissions unique to the times of intense AKR are the occasional occurrences of signals at the first harmonic of the AKR. The latter observation seems likely to be natural rather than artificial, and it is interesting because so few examples of harmonic AKR have previously been published that the effect has been controversial. For the times of weak or absent AKR, the results of this survey may be summarized as follows: for times of roughly 0800–1600 LT (dayside), a nearly constant level was recorded which at 1–3.2 MHz was equal to the galactic background level, and at 3.2–5.6 MHz was above the galactic backgound level and probably was due to spacecraft‐generated noise; at other local times, the same nearly constant level was sometimes recorded, but frequently bursts of noise at 3–5.6 MHz were observed. These bursts seem more likely to represent ionospheric breakthrough of man‐made radiation rather than natural emissions. A comparison with previous satellite observations implies that the terrestrial background level in this frequency range may have increased considerably between 1970 and 1985.

Snapshots of high‐latitude electrodynamics using Viking and DMSP F7 observations

Simultaneous observations by the Viking and the DMSP F7 satellites have been used as input to a new method to obtain snapshot pictures of the auroral electrodynamics. In particular, an “instantaneous” global equipotential (or convection) pattern is calculated from distributions of field‐aligned current and conductivity which are qualitatively consistent with the Viking auroral imager data and quantitatively consistent with magnetic field and particle data from the two satellites. This convection pattern, which is of the normal two‐cell type, with a weak dusk cell and a strong, elongated crescent‐shaped dawn cell (consistent with positive interplanetary magnetic field By), agrees well with the Viking electric field data. The model and the observed potential profiles agree nicely along the entire Viking orbit except for two intervals above acceleration regions where deviations are to be expected (due to parallel electric fields). These regions are characterized by U‐shaped potential minima, upward field‐aligned currents, upgoing ion beams, and relatively intense auroral kilometric radiation. Thus, the model results are consistent with the Viking observations not only on a global scale but also on the scale of the auroral acceleration regions. The corresponding convection in the magnetosphere is obtained from a simple projection to the equatorial plane of the deduced two‐cell convection pattern. From this the location of the plasmapause is inferred.

Auroral kilometric radiation: Wave modes, harmonics, and source region electron density structures

A number of dramatic changes are observed in the characteristics of auroral kilometric radiation (AKR) as the source region plasma to gyro‐frequency ratio fN/fH varies from 0.1 to 1.3. Most notable of these is a change from right hand polarized extraordinary (X) mode AKR dominance to left hand ordinary (O) mode AKR dominance as fN/fH varies from smaller to larger values. In addition to X and O mode AKR, Z (the slow branch of the X mode) and whistler (W) mode are also observed. The Z mode is observed over all fN/fH values, is often confined to the frequency region between fN and fH, and is typically slightly less intense than the O mode. The W mode is confined to frequencies less than fH/2, suggesting that it is the result of field‐aligned ducted signals reaching the satellite from a source at lower altitudes. Harmonic AKR bands, which appear to be of natural rather than instrumental origin and appear to be associated with the O as well as the X mode, are commonly observed. The second harmonic X mode appears to be due to propagating signals, whereas the higher harmonics appear to be confined to low‐density source regions. The second harmonic associated with the O mode is observed mainly at frequencies above the ambient 2fH value when fN/fH is large and below 2fH when fN/fH is small. The observations were obtained from some 200 ionograms from 12 passes of the ISIS 1 satellite through AKR source regions. Inferences concerning wave mode identification are based on comparisons of the observed AKR frequencies with sounder‐induced plasma resonances and wave cutoffs of ionospheric reflection traces. Electron density contours from the satellite altitude down to the altitude of the F layer ionization peak were obtained from the sounder data on each pass. Source encounters were made on nearly all of these passes, and both the minimum AKR source altitudes and the corresponding fN/fH values (which correspond to maximum values) were determined. The deduced (fN/fH) max is always less than 0.4 and is typically less than 0.2 during the generation of X mode AKR. For O mode AKR, on the other hand, (fN/fH) max approaches 0.9. The corresponding AKR source region minimum altitudes extend down to 2400 km. The (fN/fH) max values are in excellent agreement with previously published results of the maximum instability temporal growth rates, obtained from the AKR Doppler‐shifted cyclotron mechanism, as a function of fN/fH. The latitudinal extent of the low‐density, i.e., Ne &lt; 100 cm−3, AKR source regions ranged from a few degrees to more than 20° along the satellite orbit (88° inclination). Within the wide density depletions, i.e., those that extended over many degrees, there were no large enhancements of the ambient fN/fH value. These results, which were limited by the ∼100 km spatial resolution corresponding to the spacing between topside sounder ionograms, were substantiated on a much finer spatial scale (∼10 km for Ne or 1/10 km for ΔNe) using in situ data from the on‐board Langmuir probe which were available for 10 of the 12 passes. The observed lack of large density enhancements within AKR source region density cavities provides additional confidence that the observed intense AKR is cyclotron X mode radiation rather than plasma frequency O mode radiation. The observed Ne enhancements, however, were larger than those required to support a feedback mechanism in the source region.

Auroral kilometric radiation/aurora correlation

Auroral kilometric radiation (AKR) observations from the ISIS 1 topside sounder receiver were compared with visual auroral observations from the network of Alaskan all‐sky camera stations. The goal was to relate AKR source region encounters to specific auroral forms on the same magnetic field line. Thirty‐eight simultaneous data sets were identified and analyzed. In general, intense AKR was associated with bright auroral arcs, and conditions of weak or no AKR corresponded to times when either no aurora or only a faint arc or weak diffuse aurora was observed. Five cases, when both intense AKR and bright visual aurora were present, were analyzed in detail. Complete electron density Ne contours, from the satellite altitude down to the F region ionization peak, were obtained along N‐S traversals of the AKR source region. In addition, the ISIS 1 orbital tracks were projected down the magnetic field lines to the auroral altitude and compared to auroral features on a map derived from the all‐sky camera images. Density cavities (regions where Ne &lt; 100 cm−3) were encountered on each of these passes. In one case the cavity extended in altitude down to just below 2000 km. The latitudinal extent of these cavities ranged from 3° to almost 30°. A variety of situations relating AKR to the visual aurora were identified, and both spatial (within about 100 km at the satellite altitude) and temporal (within about l min) correlations were identified. In addition to confirming that a density cavity is a necessary but not sufficient condition for the generation of AKR, the present results indicate that even the combination of a density cavity on an auroral‐arc field line is not always sufficient for AKR generation. Thus additional constraints must be satisfied in order to generate intense AKR from density cavities. These constraints most likely involve the characteristics of the energetic electron distribution function and possibly also latitudinal Ne gradients.

Ordinary mode auroral kilometric radiation, with harmonics, observed by ISIS 1

ISIS 1 topside sounder receiver observations that reveal examples of O mode auroral kilometric radiation (AKR) are presented. They correspond to locations outside the low‐density source region of intense AKR X mode emission. The propagation modes are identified by comparing the natural radiation wave cutoffs with the local resonant and wave cutoff phenomena stimulated by the sounder transmitter. The O mode AKR is the dominant emission in these regions of relatively high electron density, but it is considerably weaker than the intense X mode AKR observed to emanate from low‐density cavities above the auroral regions. In addition to the fundamental O mode, second and third harmonic bands of radiation have also been detected. These harmonics associated with O mode AKR are less intense than the harmonics associated with X mode AKR. It is difficult to explain the variety of harmonic AKR observations (X as well as O mode) on the basis of present AKR theories.

A correlation between auroral kilometric radiation and inverted V electron precipitation

Simultaneous observations of energetic electron precipitations and auroral kilometric radiation (AKR) were obtained from the polar orbiting satellites AE‐D and Hawkeye. The Hawkeye observations were restricted to periods when the satellite was in the AKR emission cone in the northern hemisphere and at radial distances ≳7 RE to avoid local propagation cutoff effects. In addition, the AE‐D measurements were restricted to complete passes across the auroral oval in the evening to midnight local time sector (from 20 to 01 hours magnetic local time). This is the local time region where the most intense bursts of AKR are believed to originate. A qualitative survey of AKR and electron particle precipitation shows that AKR is more closely associated with inverted V electron precipitation than with plasma sheet precipitation. Quantitatively, a good correlation is found between the AKR intensity and the peak energy of inverted V events. In addition, in the tail of the most field‐aligned portion (∼0° pitch angle) of the distribution functions of the inverted V events, systematic changes are indicated as the associated AKR intensity increases. When the AKR power flux is weak (&lt;10−17 W/(m² Hz)), the effective tail temperature (T∥) in F(V∥) of the inverted V events is less than 1.8 × 108°K, while for T∥ greater than 1.8 × 108°K the associated AKR power flux is moderate (10−17 to 10−15 W/(m² Hz)) to intense (&gt;10−15 W/(m² Hz)). From a determination of the simultaneous power in the inverted V events and the AKR bursts, the efficiency of converting the charge particle energy into EM radiation increases to a maximum of about 1% for the most intense AKR bursts. However, conversion efficiencies as low as 10−5% are also found. There is some evidence which suggests that the tail temperature, T∥ in F(V∥) of the inverted V events, may play an important role in the efficient generation or amplification of auroral kilometric radiation.

On the polarization and origin of auroral kilometric radiation

Observations of radio emissions by the Hawkeye 1 satellite at low altitudes over the southern hemisphere have now provided measurements at radial distances from about 1.5 to 2.5 RE along the auroral field lines, in the region where the intense nightside auroral kilometric radiation is believed to be generated. These measurements provide new evidence on the mode of propagation and origin of the auroral kilometric radiation. At low altitudes the auroral kilometric radiation is consistently observed to have a low‐frequency cutoff at the local electron gyrofrequency fg−. Since the electron plasma frequency fp− is usually much smaller than fg− in the region where these observations are obtained, this cutoff corresponds closely with the propagation cutoff for the right‐hand mode of propagation. These observations therefore provide a strong indication that the auroral kilometric radiation is right‐hand polarized in agreement with previous conclusions made on the basis of the angular distribution of this radiation. In the local evening region, where the intense auroral kilometric radiation is believed to be generated, a few events have been detected for which no low‐frequency cutoff is evident. In these cases the auroral kilometric radiation appears to merge essentially continuously into a band of intense auroral hiss which extends downward to frequencies as low as 1 kHz. These observations suggest that the generation of the whistler mode auroral hiss and the escaping auroral kilometric radiation are very closely related. Possible mechanisms which could produce strong coupling between the whistler mode and the escaping free space electromagnetic modes are discussed.