Auroral Kilometric Radiation Source Region Research Articles

Quantification of magnetosphere–ionosphere coupling timescales using mutual information: response of terrestrial radio emissions and ionospheric–magnetospheric currents

Abstract. Auroral kilometric radiation (AKR) is a terrestrial radio emission excited by the same accelerated electrons which excite auroral emissions. Although it is well correlated with auroral and geomagnetic activity, the coupling timescales between AKR and different magnetospheric or ionospheric regions have yet to be determined. Estimation of these coupling timescales is non-trivial as a result of complex, non-linear processes which rarely occur in isolation. In this study, the mutual information between AKR intensity and different geomagnetic indices is used to assess the correlation between variables. Indices are shifted to different temporal lags relative to AKR intensity, and the lag at which the variables have the most shared information is found. This lag is interpreted as the coupling timescale. The AKR source region receives the effects of a shared driver before the auroral ionosphere. Conversely, the polar ionosphere reacts to a shared driver before the AKR source region. Bow shock interplanetary magnetic field BZ is excited about 1 h before AKR enhancements. This work provides quantitatively determined temporal context to the coupling timelines at Earth. The results suggest that there is a sequence of excitation following the onset of a shared driver: first, the polar ionosphere feels the effects, followed by the AKR source region and then the auroral ionosphere.

Quasiperiodic Saturn Auroral Hiss Observed During a Cassini Proximal Orbit

AbstractSaturn auroral hiss is intense whistler mode emission similar in morphology to terrestrial auroral hiss, and is observed at high latitude very often in quasiperiodic episodes with a period of approximately 1 hr. Bader et al. (2019) report auroral pulsations that may be due to duskside magnetodisk reconnection. The source of the 1‐hr period is not definitively known but has been purported to be due to second harmonic Alfven waves standing along near planet magnetic field lines (Yates et al., 2016). Observations of auroral hiss at high latitude along Cassini proximal orbits are often excellent, and we have focused on an event for which we have concurrent ultraviolet auroral images as well as electron flux data. A series of repeating auroral hiss episodes is observed to initiate near the magnetic field line that traverses a Saturn kilometric radiation source region in each hemisphere, with periodic episodes of hiss recurring at higher L‐shells. Magnetic field lines centered on individual hiss episodes have auroral footprints that lie near and within a region of intense auroral ultraviolet emissions. These observations have a parallel in terrestrial return current electron beam‐generated auroral hiss seen near magnetic field lines supporting auroral kilometric radiation source regions. Recent findings link periodic plasma injections with Saturn reconnection sites observed preferentially on the duskside. These injections may spawn Saturn kilometric radiation source regions and periodic auroral hiss emission in nearby return current regions.

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.

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.

Electron-cylotron maser radiation from electron holes: upward current region

Abstract. Electron holes are suggested to be an important source for generation of electron-cyclotron maser radiation. We demonstrate that electron holes generated in a ring-horseshoe distribution in the auroral-kilometric radiation source region have the capacity to emit band-limited radiation. The radiation is calculated in the proper frame of a circular model hole and shown to be strictly perpendicular in this frame. Its bandwidth under auroral conditions is of the order of ~1 kHz, which is a reasonable value. It is also shown that much of the drift of fine structure in the radiation can be interpreted as Doppler shift. Estimates based on data are in good agreement with theory. Growth and absorption rates have been obtained for the emitted radiation. However, the growth rate of a single hole obtained under conservative conditions is small, too small for reproducing the observed fine structure flux. Trapping of radiation inside the hole for the hole's lifetime helps amplifying the radiation additionally but introduces other problems. This entire set of questions is discussed at length and compared to radiation from the global horseshoe distribution. The interior of the hole produces a weak absorption at slightly higher frequency than emission. The absorptivity is roughly two orders of magnitude below the growth rate of the radiation thus being weak even when the emission and absorption bands overlap. Transforming to the stationary observer's frame it is found that the radiation becomes oblique against the magnetic field. For approaching holes the radiated frequencies may even exceed the local electron cyclotron frequency.

Particle-in-cell simulation study of the impact of ion cyclotron waves on auroral kilometric radiation

[1] We will present results of 2 1/2-dimensional particle-in-cell simulations where the impact of ion cyclotron waves on auroral kilometric radiation (AKR) has been studied. A horseshoe electron distribution function in a strong magnetic field is unstable to modes with frequencies slightly below the electron cyclotron frequency and can generate AKR. In the presence of an electron beam superimposed on the horseshoe distribution, the system is unstable to ion cyclotron waves with phase velocity parallel to the magnetic field equal to the electron beam velocity. Ion cyclotron waves are often observed near the source region of AKR and have been predicted to have an influence on AKR. We have found that the parallel electric field in the ion cyclotron wave modulates the energy density of the electrons at the ion cyclotron frequency. Forced oscillation of the parallel electric field with the ion cyclotron frequency has been added in the simulations to see the influence on AKR. The growth rate of AKR decreases significantly when the ion cyclotron wave has the parallel phase velocity in the same direction as the electron beam. We have found that the intensity of the AKR as well as the electron energy density is also modulated by the ion cyclotron wave. We will discuss how the modulation of the electron energy affects the growth of AKR and how this modulation may generate negative frequency drifting fine structure of AKR.

Striated drifting auroral kilometric radiation bursts: Possible stimulation by upward traveling EMIC waves

We investigate waves observed by the Cluster wideband instrument (WBD) during anorbital conjunction with the Polar spacecraft. During this perigee pass, Polar was at theupper extent of the auroral kilometric radiation (AKR) source region in the SouthernHemisphere nightside auroral region. Cluster was located at higher altitude above thisregion and observed AKR with clear signatures of ordered fine structure striations (rain).Using electron particle data observed by HYDRA on board Polar, we have modeled theelectron distribution function within the AKR source region. This distribution function isunstable to a number of low-frequency wave modes and supports EMIC wavespropagating along the magnetic field line. These waves appear to be feasible to stimulatethe growth of AKR, producing the ordered fine structure observed by WBD on boardCluster, but further analysis will be important, especially plasma simulations.

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.

Auroral plasma turbulence and the cause of auroral kilometric radiation fine structure

Electron holes excited in the auroral kilometric radiation (AKR) source region in presence of a very dilute cool electron background are interpreted as causing the observed fine structure in AKR radiation. Using high time and frequency resolution measurements of the FAST wave tracker, we demonstrate that a substantial part of the AKR emission consists of a large number of elementary radiation events that we interpret as traveling electron holes that may have resulted from the nonlinear evolution of electron acoustic waves and have the properties of Bernstein‐Green‐Kruskal modes. Estimates of the propagation velocity of these structures are in good agreement with theory. Power estimates show that each elementary radiation event may contribute ≃ 103–4 W of power to AKR implying that a moderately large number of elementary radiators is required to reproduce the total AKR emission. The elementary radiation structures are sometimes reflected from the acceleration potential or become trapped in larger structures like ion acoustic waves or ion holes. The observations indicate that the radiation efficiency is highest at the turning point where the velocity of the elementary radiators with respect to the reflector system vanishes. Monitoring the time variation of the frequency drift of the elementary radiators allows to qualitatively infer about the mesoscale motion of the AKR source region and the spatial extension of the mesoscale field‐aligned electric potential drops.

Accelerated electrons as the source of Auroral Kilometric Radiation

Data from the Fast Auroral Snapshot Explorer (FAST) have confirmed that Auroral Kilometric Radiation (AKR) is generated by the accelerated electrons in the auroral density cavity. FAST observations of the electron distribution within the AKR source region show a characteristic horse-shoe distribution expected from the parallel electric field acceleration and magnetic mirroring of the electrons, the precipitating portion of which constitute the auroral electrons. Within the density cavity it is likely that the hot electrons carry most, if not all, of the plasma density. The AKR wave characteristics within the source region show that the lower frequency cut-off is below the cold electron gyro-frequency, and near the cut-off, the wave electric and magnetic field polarization is consistent with perpendicularly propagating X-mode waves. Taken together these observations are consistent with the cyclotron maser instability, but with the inclusion of relativistic effects in the wave dispersion, as well as the resonance condition. This allows the accelerated electrons to generate AKR, and further the replenishment of the distribution by the parallel electric field ensures that AKR can be generated over a large altitude range on auroral zone field lines

Auroral kilometric radiation at substorm onset

The temporal relationship of substorm signatures that occur in different regions of space is an important part of substorm research. In the present study we investigate the relative timing between sharp enhancement of auroral kilometric radiation (AKR) and auroral breakups. It is found, on the basis of 136 isolated substorm events identified with global auroral images from the Polar ultraviolet imager (UVI), that (1) 70% (83%) of the time AKR enhancements were detected within ±1 (±2) min of the auroral breakups; (2) AKR onset tends to occur, on an average, slightly later (0.36 min) than the corresponding auroral breakup; and (3) similar to previous study results, substorm‐associated AKR has a forbidden area at Polar altitude in the noon sector. These results suggest that when the satellite is suitably located, the enhancement of AKR is a good substorm onset indicator and it can be used to time substorm onsets adequately within the ∼1 min of uncertainties from the UVI observations, especially if confirmed by other substorm onset identifiers. The average frequency for the enhancement of AKR at substorm onset is found to be ∼300 kHz, corresponding to an upper limit of the AKR source altitude of ∼4700 km. The entire AKR frequency band expands to ∼64–650 kHz within a few minutes after onset, corresponding to a wider source altitude of ∼2100–12,000 km, a typical zone for auroral electron acceleration. We suggest that the fast expansion of the AKR source region along the local magnetic field lines, equivalent to the expansion of the auroral acceleration region, should be considered as one of the fundamental signatures in the substorm expansion phase.

Electron‐Cyclotron Maser Driven by Charged‐Particle Acceleration from Magnetic Field–aligned Electric Fields

We present a detailed description of the auroral kilometric radiation (AKR) source region based on observations from the Fast Auroral SnapshoT (FAST) satellite and discuss how these new results may pertain to solar and stellar radio sources. FAST satellite observations are directly within the AKR source region and have unprecedented spatial and temporal resolution. They confirm many of the fundamental elements of the electron-cyclotron maser mechanism but with substantial modification. The most important modification is that the emissions do not draw their energy from a loss-cone instability; rather, the radiation results from an unstable horseshoe or distribution. The most far-reaching implication is that the electron-cyclotron maser is directly associated with a particular type of charged particle acceleration, a magnetic field-aligned (parallel) electric field in a dipole magnetic field. These findings change several of the characteristics of the electron-cyclotron maser mechanism and may necessitate reanalysis of some astrophysical radio sources. Under the shell instability, radio emissions with brightness temperatures ~1014 K, the steady state limit of the loss-cone instability, may be continuous. Through observations, we demonstrate that source brightness may be as high as 1020 K in steady state. A moderately or strongly relativistic beam may result in broadband emissions. A loss cone is not required, so the radiation source may be high above the stellar or planetary surface. Although the generation is in the X mode with k|| = 0, we suggest that the radiation, guided by a density cavity that is created by the parallel electric field, efficiently converts to the R mode, which experiences substantially lower absorption at higher harmonics. These findings also suggest that parallel electric fields may be a fundamental particle acceleration mechanism in astrophysical plasmas.

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.

Free energy sources and frequency bandwidth for the auroral kilometric radiation

Electron distributions obtained in the source regions of auroral kilometric radiation (AKR) by the Fast Auroral SnapshoT (FAST) satellite have revealed several free energy sources with positive gradients with respect to υ⊥ superimposed on a broad plateau with a radius close to the primary incident electron acceleration energy and covering pitch angles from near field‐aligned all the way to the (upgoing) loss cone. Two‐dimensional electromagnetic particle simulations are used to demonstrate that such a distribution arises as a quasisteady feature of a process in which the increase of the perpendicular velocity of the electrons as they propagate into a region of increasing magnetic field strength is balanced by the diffusion to lower υ⊥ caused by the electron‐cyclotron maser instability. The maser radiation is emitted nearly perpendicular to the ambient magnetic field at frequencies between the relativistic and nonrelativistic cyclotron frequencies. In these circumstances, the entire primary auroral electron distribution can contribute to the resonant wave‐particle interaction, leading to electric field intensities of the order of 500 mV/m. In contrast, a pure loss cone distribution is shown to produce much weaker electric fields, leads to emission at angles ≳ 10° away from perpendicular, and cannot produce the broad plateau observed in the electron distribution. The simulations and linear theory indicate that the maser instability in a uniform system produces an intrinsic bandwidth of the order of a few tenths of 1% of the cyclotron frequency (∼0.5–1.0 kHz in the AKR source region). Any narrower spectra would appear to require some nonuniform or time‐dependent feature in the source region.

A theory for AKR fine frequency structure

An alternative explanation of the fine frequency structure (≤1 kHz) associated with auroral kilometric radiation (AKR) is presented. It is based on a realistic model of the electron distribution function inside the AKR source region. The resulting cyclotron maser instability growth rate is shown to possess an extremely narrow bandwidth, of the order Δω/ω ∼ 10−3, which may naturally account for the AKR fine frequency structure. The results differ from many previous calculations and simulations which relied on simpler electron distribution function models such as a loss‐cone, ring‐beam or DGH distribution.

FAST observations of electron distributions within AKR source regions

Observations of high‐time resolution 3‐D electron distributions within the source regions of Auroral Kilometric Radiation (AKR) are reported. In general, the electron data display a broad plateau over a wide range of pitch angles indicating that these distributions have been rapidly stabilized by AKR wave growth. The source of the electron instability appears to come from several features in the distribution, including an isotropic beam feature and its mirroring components, occasional electrons in the “trapped” region, as well as steep gradients present in the atmospheric loss cone. Taken together these features may provide a nearly continuous region of ∂f/∂v⟂ which could contribute to the relativistic cyclotron maser instability. Computer simulations of the evolution of the electron distribution which assume plasma conditions similar to the parameters measured by FAST show similar results to the observed electron distributions. The FAST observations also show that relativistic corrections to the AKR dispersion relation may enable a small k|| mode with a resonance condition that is able to take maximum advantage of the initial instability in the mono‐energetic electron distributions within the auroral acceleration regions.

The Fast Auroral SnapshoT (FAST) Mission

The FAST satellite mission investigates plasma processes occurring in the low altitude auroral acceleration region, where magnetic field‐aligned currents couple global magnetospheric current systems to the high latitude ionosphere. In the transition region between the hot tenuous magnetospheric plasma and the cold, dense ionosphere, these currents give rise to parallel electric fields, particle beams, plasma heating, and a host of wave‐particle interactions. FAST instruments provide observations of plasma particles and fields in this region, with excellent temporal and spatial resolution combined with high quantitative accuracy. The spacecraft data system performs on‐board evaluation of the measurements to select data “snapshots” that are stored for later transmission to the ground. New measurements from FAST show that upward and downward current regions in the auroral zone have complementary field and particle features defined by upward and downward directed parallel electric field structures and corresponding electron and ion beams. Direct measurements of wave particle interactions have led to several discoveries, including Debye‐scale electric solitary waves associated with the acceleration of upgoing electron beams and ion heating, and the identification of electrons modulated by ion cyclotron waves as the source of flickering aurora. Detailed quantitative measurements of plasma density, plasma waves, and electron distributions associated with auroral kilometric radiation source regions yield a consistent explanation for AKR wave generation.

DE 1 and Viking observations associated with electron conical distributions

Data from the electron detectors on board the Swedish Viking satellite launched during a period of low solar activity and from the DE 1 satellite launched during active solar conditions have been examined for the occurrence and location of electron conical distributions and several conclusions can be drawn. First, we note that most of the best examples of electron conics observed by the V‐3 experiment onboard Viking occurred in the afternoon sector in the range of magnetic local time 14 hours < MLT < 18 hours, at midaltitudes in the range 10,000 km < h < 13,500 km, with few occurring in the nightside auroral region, a region poorly sampled at altitudes greater than 5000 km. For the Viking data there is an association of electron conics with upper hybrid waves. DE 1 observations made by the high‐altitude plasma instrument (HAPI) indicate that electron conics were observed in the midmorning sector and the late evening sector, and as has been reported earlier, the correlation with upper hybrid waves was good. The HAPI did not sample the afternoon sector. The electron conics observed on both satellites occurred in the presence of at least a modest (several kilovolts) potential difference beneath the satellite with a maximum energy that was usually, but not always, equal to or greater than the maximum energy of the electron conics. Two independent sets of observations by DE 1 suggest two distinct production mechanisms for electron conics. Examination of DE 1 electric field measurements from the plasma wave instrument during the observation of electron conics show simultaneous parallel oscillations in the frequency range 0.2 Hz < f < 0.5 Hz during one and perhaps two of four events examined, and upper hybrid waves were observed on all four events. In addition, recent observations of “90‐deg” electron conics associated with auroral kilometric radiation source regions suggest a perpendicular heating mechanism produced by wave‐particle interaction. Such distributions may be observed as electron conics at higher altitudes. These results suggest more than one possible source mechanism may be responsible for electron conics.

DE 1 particle and wave observations in auroral kilometric radiation (AKR) source regions

The high‐altitude plasma instrument on board the DE 1 satellite was operating during several near crossings of the auroral kilometric radiation (AKR) source in the nightside auroral region. Observations of electron distributions indicate a region of perpendicular heating (T⊥/T∥ > 10) adjacent to and within the source region. Loss cones, trapped particles, beams, and electron conical distributions are also observed near and within the source region, which extends perpendicular to the magnetic field line for at least 20 km. Near the AKR source region wave‐particle interactions appear to have modified the observed electron distributions. We compare the observations to those predicted by recently published numerical simulations.

Generation of auroral kilometric radiation in upper hybrid wave‐lower hybrid soliton interaction

Sporadic bursts of auroral kilometric radiation (AKR) associated with strong bursty electrostatic turbulence in the vicinity of the lower hybrid frequency have been frequently recorded in the AKR source region by the Viking satellite. The variation time scale of these emissions is typically 1 s, long enough for lower hybrid waves to grow to amplitudes of several hundred millivolts per meter and to evolve nonlinearly into solitons. On the basis on these observations it is suggested that formation of lower hybrid solitons may play a role in the generation of AKR. A theoretical model is proposed which is based on the direct acceleration of electrons in the combined lower hybrid soliton and upper hybrid wave fields. The solitons act as sporadic, localized antennas allowing for efficient conversion of the electrostatic energy stored in upper hybrid waves into electromagnetic radiation at a frequency above the X mode cutoff. Excitation of lower hybrid waves is due to the presence of energetic electron beams in the auroral zone found to be associated with steep plasma density gradients. Upper hybrid waves can be excited by a population of energetic electrons with loss cone distributions. The power of the electromagnetic radiation obtained is only noticeable in regions where the plasma frequency is less than the electron gyrofrequency. The theory predicts spectral power densities of the order of 10−11 to 10−9 W m−2 Hz−1 in the source region, in good agreement with the Viking observations. The sporadic nature of the radiation derives from lower hybrid soliton collapses which occur on ∼1‐s time scales.