Observations Of Auroral Kilometric Radiation Research Articles

Wind/WAVES Observations of Auroral Kilometric Radiation: Automated Burst Detection and Terrestrial Solar Wind ‐ Magnetosphere Coupling Effects

AbstractAuroral Kilometric Radiation (AKR) is the strongest terrestrial radio emission, and emanates from the same electron acceleration regions from which particles precipitate into the ionosphere, exciting the aurorae and other phenomena. As such, AKR is a barometer for the state of solar wind ‐ magnetosphere ‐ ionosphere coupling. AKR is anisotropically beamed in a hollow cone from a source region generally found at nightside local times, meaning that a single source region cannot be viewed from all local times in the magnetosphere. In radio data such as dynamic spectra, AKR is frequently observed simultaneously to other radio emissions which can have a similar intensity and frequency range, making it difficult to automatically detect. Building on a previously published pipeline to extract AKR emissions from Wind/WAVES data, in this paper a novel automated AKR burst detection technique is presented and applied to Wind/WAVES data. Over a five year interval, about 5000 AKR bursts are detected with median burst length ranging from about 30 to 60 min. During detected burst windows, higher solar wind velocity is observed, and the interplanetary magnetic field clock angle is observed to tend toward BZ < 0, BY < 0, when compared with the entire statistical interval. Additionally, higher geomagnetic activity is observed during burst windows at polar, high and equatorial latitudes.

A Review of Cluster Wideband Data Multi‐Spacecraft Observations of Auroral Kilometric Radiation

AbstractWe review important advances in the understanding of auroral kilometric radiation (AKR) resulting from observations by the Wideband Data instruments on the four Cluster spacecraft. AKR is an intense radio emission originating in the Earth's auroral regions with frequencies typically in the range 50–700 kHz, usually observed from space. It is now widely accepted that AKR is generated by the cyclotron maser instability (CMI) in density cavities in the auroral acceleration region. Multi‐point observations by the Cluster spacecraft with a time delay of arrival technique have allowed the source locations of many individual AKR bursts to be determined. The position uncertainty is around 500 km at the source region or about 200 km when mapped on to the auroral zone. AKR is emitted in a narrow beam close to the tangent to the magnetic field vector in the source region. This has important implications for the possible generation mechanisms, being incompatible with filled or hollow cone beaming models. It also implies that an observer at a given location can only see AKR from a fraction of possibly active source regions. The complex frequency time structure of AKR sometimes shows regular striations or pulsations. Cluster observations of these phenomena have been interpreted as modulation of the CMI by disturbances propagating through the generation region. Exceptionally, AKR can sometimes be observed from low altitude spacecraft or even on the ground. Recent work has involved simultaneous observations of AKR on the Cluster spacecraft and on the ground at the South Pole.

AKR‐like emissions observed at low altitude by the DEMETER satellite

This paper reports observations of Auroral Kilometric Radiation (AKR) by the low altitude satellite DEMETER (700 km) during the super magnetic storm of November 2004. AKR is generated along auroral field lines at altitudes above ∼3000 km and escapes from the Earth so that most observations have been made close to the source regions or at large distances from the Earth. However, EM waves with AKR‐like frequency spectra detected by the low altitude EXOS‐C satellite were interpreted by Oya et al. (1985) as AKR emissions propagating toward the Earth in the whistler mode. During the large magnetic storm of November 8–10, 2004, numerous events have been observed by DEMETER and, in one case, the emissions were recorded when DEMETER was flying over an intense aurora observed by the UVI (Ultra Violet Imager) of the POLAR spacecraft. During this event, density depletions have been observed at the satellite altitude. Similar AKR‐like EM waves were also recorded by DEMETER at other times showing that these emissions are a common phenomenon at low altitudes in the auroral regions during periods of sustained magnetic activity.

Simultaneous radio and optical observations of auroral structures: Implications for AKR beaming

[1] Polar spacecraft observations of auroral kilometric radiation (AKR) spectra, combined with simultaneous optical images of the auroral oval at the source foot points, show a strong correlation with discrete auroral features. We discuss several examples of distinctive “V”-shaped profiles in time-frequency AKR spectra for which the time of nadir passage (bottom of V) coincides with the spacecraft crossing field lines connected to discrete auroral arcs. These results support models of AKR emission originating in density cavities with radiation initially confined to a “tangent plane” but strongly refracted upward in a frequency-dependent manner. The inferred spatial distributions of AKR sources are closely coincident with field lines connecting discrete auroral arcs, indicating that discrete arcs are highly correlated with robust AKR emission on the same field lines, as first suggested by Gurnett (1974) several decades ago.

On the simultaneity of substorm onset between two hemispheres

Simultaneous observations of auroral kilometric radiation from the Northern and Southern Hemispheres showed some cases in which the buildup of field-aligned acceleration occurred only in one hemisphere at the substorm onset. This indicates that a substorm does not always complete the current system by connecting the cross-tail current with both northern and southern ionospheric currents. Conjugate auroral observations showed that in one case, the auroral breakup in the Northern and Southern Hemispheres was not simultaneous; rather, they occurred a few minutes apart. This time difference in the breakup between two hemispheres suggests that the local auroral ionosphere controls auroral breakup in each hemisphere independently. The evidence in this study may indicate that the buildup of the field-aligned acceleration region at the auroral breakup does not result only from the magnetospheric process and that the auroral ionosphere finally controls and/or ignites the substorm onset, that is, the auroral breakup.

Auroral kilometric radiation diurnal, semidiurnal, and shorter‐term modulations disentangled by Cassini

During the flyby of the Earth by Cassini in 1999, the Radio and Plasma Wave Science (RPWS) instrument recorded a month of quasi‐continuous observations of auroral kilometric radiation (AKR). Analyzing the full polarization of incoming radio waves, we found AKR to be 100% circular left‐handed (LH) or right‐handed (RH). As AKR is emitted predominantly on the extraordinary mode, we analyzed separately the northern (RH) emission, from the southern (LH) one with respect to the magnetic equator. AKR power variations reveal (1) a log‐normal distribution at time scales of minutes, (2) bursts of emission at time scales of a few hours, and (3) a clear overall modulation at 24 h, together with a weaker modulation at 12 h (especially visible for LH emissions). The prominent 24 h modulation is found to modulate LH and RH AKR in phase opposition. This is interpreted as being due to visibility effects related to the precession of the terrestrial magnetic dipole, making Cassini oscillate relative to the average AKR beaming pattern from each hemisphere. We accordingly quantified the AKR beaming as a function of explored latitudes. On time scales shorter than a few hours, LH and RH emissions are found to be correlated. This is attributed to the actual conjugacy of the corresponding sources, simultaneously turned on by substorm occurrence. The geometrical anticorrelation (at 24 h) dominates close to Earth, while the short‐term correlation (substorms) dominates far from Earth, where the detection threshold makes the visibility less important than the occurrence of substorms. Finally, the 12 h modulation is detected when it is not masked by strong visibility effects, namely, for the LH emission, which is observed mostly near the magnetic equator along the path of Cassini. As a 12 h modulation of some geomagnetic indices was also observed, we suggest that the AKR semidiurnal modulation may have a physical cause, for which we favor the variable geometry and efficiency of the reconnection between interplanetary and geomagnetic fields, or magnetotail oscillations.

Auroral kilometric radiation and magnetosphere‐ionosphere coupling process during magnetic storms

This paper presents an investigation of a magnetosphere‐ionosphere (M‐I) coupling process during magnetic storms, using observations of auroral kilometric radiation (AKR) in the field‐aligned acceleration region and plasma density variation in the plasma sheet. The main phase electron precipitation, which AKR does not accompany, was confirmed as not characterized by inverted‐V type precipitation but rather by precipitation with Maxwellian type distribution. This indicates the absence of the field‐aligned potential in the M‐I coupling region during the early phase of the magnetic storm. We found that the superdense plasma in the plasma sheet is closely concerned with the AKR activity; that is, the appearance of the superdense plasma sheet seems to suppress the AKR activity. On the basis of these observations we discuss the current‐voltage relation between the magnetosphere and ionosphere. The occurrence of the superdense plasma sheet accompanied with the AKR disappearance was found to be caused by coronal mass ejection (CME) with magnetic cloud. The overall story from the arrival of CME at the Earth to the AKR disappearance in the M‐I coupling region is presented.

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.

Propagation characteristics of auroral kilometric radiation observed by the MEMO experiment on Interball 2

The MEMO experiment is a part of the Interball 2 wave consortium. It is connected to a total of six electric and nine magnetic independent sensors. It provides waveforms associated with the measurement of two to five components in the three frequency bands: ELF (5–1000 Hz), VLF (1–20 kHz), and LF (20–250 kHz). Waveforms of three magnetic components and one electric component recorded during observations of auroral kilometric radiation (AKR) allow a detailed study of the characteristics of these emissions. In particular, the wave normal directions of AKR relative to the Earth's magnetic field are determined using several methods: the classical methods based on the plane wave approximation [Means, 1972] and the wave distribution function method which represents the evaluation of the wave energy density distribution with respect to the angular frequency and the wave normal direction(s). One event is fully analyzed in this paper. It is shown that AKR propagates with a polarization quasi‐circular (ellipticity value ∼ 0.9), a right polarization (i.e., R‐X mode), and wave normals weakly oblique (∼30°).

Viking observations of auroral kilometric radiation from the plasmasphere to night auroral oval source regions

For two VIKING orbits crossing the night oval source region of Auroral Kilometric Radiation (AKR), we derive the AKR polarization from the cutoff‐frequencies and from the spin modulated pattern of the emission. The R‐X mode is predominant, but can be accompanied by L‐O mode and Z mode band. The Z mode is observed continuously from inside the plasmasphere to the AKR source region. An offset in the spin modulation pattern near the AKR source region during one of the orbits shows that steep gradients of the plasma density at the edge of the source region can play an important role for the orientation of the wave polarization plane or for the mode conversion.

The detailed intensity distribution of the AKR emission cone

Auroral kilometric radiation (AKR) observations in the northern hemisphere from 3.5 years of IMP 6 data and 4 years of Hawkeye data, at large radial distances, are combined to produce detailed intensity distributions of the AKR emission pattern below 200 kHz. All Hawkeye and IMP 6 AKR observations are normalized to a radial distance of 7 earth radii (RE) from the average source region reported by Gallagher and Gurnett (1979). The normalization is done to compensate for the expected radial variations in the intensity. Unlike previous emission pattern studies which showed the AKR frequency of occurrence above a preset power flux threshold, intensity contours of spectral power flux are presented. The AKR observations are well ordered in the source‐centered coordinates which provides further support for auroral zone generation of this emission at those altitudes. AKR is found to be emitted into a large emission cone pattern whose size decreases with frequency from the AKR spectral peak to low frequencies, consistent with previous studies. The emission cones have solid angles of 4.6, 3.3, and 2.7 steradians at 178, 100, and 56.2 kHz, respectively. Unexpected in these results is that there is less than a 10 dB variation in the illumination intensity across nearly the entire time‐averaged AKR emission patterns, at 178 and 100 kHz. In addition, simultaneous observations by Hawkeye and IMP 6 reveal that individual bursts of AKR can easily illuminate the northern emission cone uniformly. No systematic AKR intensity difference in longitude or latitude, within the cone at 178 kHz, is found during the simultaneous two spacecraft observations. This is not consistent with the hollow emission cone pattern reported by AKR at low frequencies (see Calvert, 1981c) near the source region.

DE‐1 observations of ordinary mode and extraordinary mode auroral kilometric radiation

Observations of auroral kilometric radiation (AKR) made with DE‐1 indicate the presence of both ordinary and extraordinary wave modes. Although the two modes usually occur separately, they are sometimes observed together. When both modes are present, the ordinary‐mode component tends to occur at lower frequencies and with lower amplitudes than those of the accompanying extraordinary‐mode component. On the other hand, the local electron gyrofrequency is an absolute lower frequency cutoff for both modes. Ordinary mode intensities are proportional to extraordinary mode intensities but less by roughly a factor of 50. Extraordinary mode ray paths are generally confined to a cone within 50° of the source magnetic field direction and ordinary mode emissions are typically observed outside of this cone. This behavior suggests that both components are produced within the same source region but are then refracted differently as they escape.

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.

Plasma waves in planetary magnetospheres

The studies of magnetospheric plasma waves in the 1979–1982 quadrennium have included not only intensive studies of plasma waves in the Earth's magnetosphere but also, for the first time, in situ observations and detailed analyses of plasma waves in the magnetospheres of Jupiter and Saturn. While much of the research has involved the continuing identification and classification of plasma wave phenomena in the magnetospheres, increasingly greater emphasis has been placed on understanding the sources of the plasma waves and the interactions of plasma waves with the particles. Many studies have also shown that plasma wave measurements may be used to discern numerous geophysical boundaries and parameters.At the earth, significant progress in plasma wave research has been made that has increased our understanding of wave‐particle interactions and their effects, throughout the magnetosphere from the auroral ionosphere out to the bow shock and back into the tail. Observations of auroral kilometric radiation both in the source region and far from it have provided much new information about this intense emission. Details of the fine structure of the radiation and its polarization and propagation characteristics along with particle distribution and other plasma wave measurements in the source region have provided valuable inputs to various generation theories. Similar progress has also been made in understanding electron cyclotron harmonic emissions and nonthermal continuum radiation. Detailed characterizations of plasma wave phenomena reported during this quadrennium in the plasmasphere, outer magnetosphere, magnetopause, magnetosheath and tail regions have been important in assessing the impact of the waves on the particle distributions and vise versa.

General concepts on the generation of auroral kilometric radiation

Auroral kilometric radiation (AKR) has a peak intensity at 250 kHz and is associated with discrete aurora in regions where ωp/ωc < 0.2. There are two high‐frequency electromagnetic wave modes in which AKR could propagate, the L mode and the R mode. Observations of AKR are inconsistent with the characteristics of L mode radiation. However, the characteristics of the upper frequency branch of the R‐X mode are consistent with AKR observations. We therefore look at the following free‐energy sources in the auroral electron distribution functions as possibilities for exciting the R‐X mode: (1) the accelerated, precipitating electrons, and (2) the upgoing loss cone associated with the electrostatically trapped electrons. Solving the relativistic resonance condition exactly gives contours in velocity space that are elliptical. Diffusion of precipitating electrons, which develop ∂f/∂υ⊥ > 0 because of the magnetic mirroring force, and diffusion of trapped electrons in the upgoing loss cone with R‐X waves are toward decreasing energies, so that wave growth is possible. To determine which frequency and wave vector (k, θ) is associated with each resonant contour, we solve the cold plasma dispersion relation for ωR, k, and θ, where ωR > ωx, the right‐hand cutoff frequency. Growing waves will be associated with those contours that pass through regions of velocity space where ∂f/∂υ⊥ > 0 is large. A simple criterion is given to show which R‐X waves will have large positive growth rates. Finally, we calculate the group velocity of R‐X waves and show that R‐X waves with large, positive growth rates also have small group velocities (Vg/c ≪ 1), implying a very small convective growth length ∼10 m. The intense wave generation should occur at wave frequencies just above the right‐hand cutoff frequency and have wave normal angles 75° < θ ≲ 105°. Wave growth maximizes in regions where ωp/ωc is small (≲0.06) and where there exists sufficient free energy (∂f/∂υ⊥ > 0 is large) associated with either the upgoing loss cone or the downgoing precipitating electrons that are undergoing mirroring.

ISIS 1 observations at the source of auroral kilometric radiation

Observations of auroral kilometric radiation (AKR) are made by ISIS 1 in the source region. The radiation is found to be generated in the extraordinary mode just above the local cut‐off frequency and to emanate nearly perpendicular to the magnetic field. It occurs within local depletions of electron density, where the ratio of plasma frequency to cyclotron frequency is less than 0.2. The density depletion is restricted to altitudes above about 2000 km, and the upper AKR frequency limit corresponds to the extraordinary cutoff frequency at this altitude. AKR is observed from ISIS 1 above the nighttime auroral zone over a wider extent in longitude than in latitude with an intense source region observed most often near 2200 LMT and 70° invariant latitude. It is directly related to inverted V electron precipitation events with an electron‐to‐wave energy conversion efficiency of the order of 0.1 to 1%.