Discrete Chorus Research Articles

Spatial Evolution of Wave‐Particle Interaction Region Deduced From Flash‐Type Auroras and Chorus‐Ray Tracing

AbstractIn‐situ observations of spatial variations of the wave‐particle interaction region require a large number of satellite probes. As an alternative, flash‐type auroras, a kind of pulsating aurora, driven by discrete chorus elements, can be used to investigate the interaction region with a high spatial resolution. We estimated the spatial extent of wave‐particle interaction region from ground‐based observations of flash aurora at Gakona (62.39°N, 214.78°E), Alaska at subauroral latitudes, and found that the auroral expansion was predominantly to the low‐latitude side. The spatial displacement is thought to be caused by the propagation effects of chorus waves in the magnetosphere. Using ray tracing analysis to take into account chorus wave propagation, we reconstructed the spatiotemporal evolution of the volume emission rate and confirmed that the predominant expansion is toward the lower‐latitude side in the ionosphere. This study shows that chorus wave propagation in the magnetosphere gives new insight for characterizing the transverse size (across the geomagnetic field line) of wave‐particle interaction regions. The calculated spatial scale of the column auroral emission shows a correlation with the magnetic latitude of the resonance region at magnetic latitudes within 10° of the equatorial plane of the magnetosphere. Our results suggest that the spatial scale of a flash aurora is indirectly related to the chorus amplitude because the latitudinal range of the wave‐particle interaction is important for the growth of wave amplitude.

First Direct Observations of Propagation of Discrete Chorus Elements From the Equatorial Source to Higher Latitudes, Using the Van Allen Probes and Arase Satellites

AbstractWhistler mode chorus waves have recently been established as the most likely candidate for scattering relativistic electrons to produce the electron microbursts observed by low altitude satellites and balloons. These waves would have to propagate from the equatorial source region to significantly higher magnetic latitude in order to scatter electrons of these relativistic energies. This theoretically proposed propagation has never been directly observed. We present the first direct observations of the same discrete rising tone chorus elements propagating from a near equatorial (Van Allen Probes) to an off‐equatorial (Arase) satellite. The chorus is observed first on the more equatorial satellite and is found to be more oblique and significantly attenuated at the off‐equatorial satellite. This is consistent with the prevailing theory of chorus propagation and with the idea that chorus must propagate from the equatorial source region to higher latitudes. Ray tracing of chorus at the observed frequencies confirms that these elements could be generated parallel to the field at the equator, and propagate through the medium unducted to Van Allen Probes A and then to Arase with the observed time delay, and have the observed obliquity and intensity at each satellite.

Visualization of rapid electron precipitation via chorus element wave\u2013particle interactions

Chorus waves, among the most intense electromagnetic emissions in the Earth’s magnetosphere, magnetized planets, and laboratory plasmas, play an important role in the acceleration and loss of energetic electrons in the plasma universe through resonant interactions with electrons. However, the spatial evolution of the electron resonant interactions with electromagnetic waves remains poorly understood owing to imaging difficulties. Here we provide a compelling visualization of chorus element wave–particle interactions in the Earth’s magnetosphere. Through in-situ measurements of chorus waveforms with the Arase satellite and transient auroral flashes from electron precipitation events as detected by 100-Hz video sampling from the ground, Earth’s aurora becomes a display for the resonant interactions. Our observations capture an asymmetric spatial development, correlated strongly with the amplitude variation of discrete chorus elements. This finding is not theoretically predicted but helps in understanding the rapid scattering processes of energetic electrons near the Earth and other magnetized planets.

An excitation mechanism for discrete chorus elements in the magnetosphere

Abstract. A beam pulsed amplifier mechanism responsible for effective amplification of short very low frequency (VLF) electromagnetic pulses is proposed. Effective amplification near the magnetic equator outside the plasmasphere is considered. A conditional growth rate of short electromagnetic pulses is calculated. Obtained results can explain some important features of the oblique electromagnetic chorus emissions without hiss-like radiation background.

A direct link between chorus emissions and pulsating aurora on timescales from milliseconds to minutes: A case study at subauroral latitudes

AbstractA correlation was observed between chorus emissions and pulsating aurora (PA) from observations at Athabasca (L≈4.3) in Canada at 9:00–9:20 UT on 7 February 2013, using an electron multiplying charge‐coupled device camera and a VLF loop antenna with sampling rates of 110 Hz and 100 kHz, respectively. Pulsating aurora having a quasiperiodic variation in luminosity and a few hertz modulation was observed together with chorus emissions consisting of a group of successive rising‐tone elements. The repetition period and modulation frequency of the PA are in good agreement with those of the modulated chorus. After 9:11 UT, the temporal features of the aurora became aperiodic PA of indistinct modulation. Simultaneously, the rising‐tone chorus turned into chorus emissions consisting of numerous rising‐tone elements. The equatorial geomagnetic field inhomogeneity calculated using the Tsyganenko 2002 model shows a decreasing trend during the period. This result is consistent with nonlinear wave growth theory having a small geomagnetic field inhomogeneity, which contributes to a decrease in the threshold amplitude to trigger discrete chorus elements. These observations show a close connection between chorus emissions and PA on timescales from milliseconds for generation of discrete chorus elements on the microphysics of wave‐particle interaction to minutes for the variations of the geomagnetic field inhomogeneity related with the substorm activity.

Observation of discrete VLF emissions at Indian low-latitude ground station Srinagar (L = 1.28)

During the routine analysis of recorded very low-frequency data during the period November, 2012 to February, 2013, at an Indian low-latitude ground station Srinagar (geomag. lat., 24°10′N; L = 1.28), we observed some interesting discrete emissions during quiet period on February 5, 2013, which has been presented in the present study. The spectral analysis of the observed discrete very low-frequency emissions consisting of risers, fallers, hooks and oscillating tones has been presented. To explain the observed dynamic spectra of the discrete chorus emissions, a possible generation mechanism has been presented based on the recent nonlinear theory. On the basis of this theory, frequency sweep rate of discrete chorus emissions has been computed and compared with those of our experimentally observed values.

Simultaneous Observations of Daytime Whistlers and VLF-ELF Emissions at a Low Latitude Indian Ground Station

This paper reports an unexpected simultaneous observation of whistlers and different types of ELF/VLF emissions during daytime at a low latitude Indian ground station Jammu (geomag. lat., 22o 26/ N; L=1.17), which are (a) pulses of atmospheric bursts, (b) discrete chorus riser ELF emissions triggered from the bottom of the atmospherics, (c) whistlers, (d) whistler-triggered discrete chorus riser VLF emissions triggered from the lower end of the whistler, (e) long enduring ELF/VLF hiss emissions, (f) band limited quasi-periodic pulsing ELF/VLF hiss emissions, (g) hook and inverted hook ELF emissions, (h) oscillating tone discrete chorus riser ELF emissions. The data shown in this paper indicate that there is a strong possibility that lightning is an important source of different types of VLF/ELF emissions, at least in the embryonic sense, recorded at Jammu. The present observation is in fact the first simultaneous occurrence of different types of VLF/ELF emissions alongwith whistlers recorded on a single day during daytime at a low latitude ground station.

A numerical study of chorus generation and the related variation of wave intensity using the DAWN code

AbstractChorus waves play an important role in energetic electron dynamics in the inner magnetosphere. In this work, we present a new hybrid code, DAWN, to simulate the generation of chorus waves. The DAWN code is unique in that it models cold electrons using linearized fluid equations and hot electrons using particle‐in‐cell techniques. The simplified fluid equations can be solved with robust and simple algorithms. We demonstrate that discrete chorus elements can be generated using the code. Waveforms of the generated element show amplitude modulation or “subpackets,” and the frequency sweep rate of the generated element is found to be consistent with that of observed chorus waves. Using the DAWN code, we then investigate the variation of wave intensity () with respect to linear growth rates on the equatorial plane. Previous observations showed that the change in linear growth rates of whistler waves modulated by external processes such as density modulations is usually small (), while the variation of the wave intensity is large (). Using a chosen set of background plasma parameters, we demonstrate that a small change () in linear growth rates can lead to significant variation () of wave intensity only in the transition from the broadband whistler wave generation regime to the chorus wave generation regime. Our results demonstrate the importance of including nonlinear dynamics of chorus generation in understanding the whistler wave intensity modulation process in the inner magnetosphere.

Spatial spreading of magnetospherically reflected chorus elements in the inner magnetosphere

Abstract. Chorus-type whistler waves are known to be generated in the vicinity of the magnetic equator, in the low-density plasma trough region. These wave packets propagate towards the magnetic poles, deviating from the magnetic field lines, before being eventually reflected at higher latitudes. Magnetospheric reflection of whistler waves results in bounce oscillations of these waves through the equator. Our study is devoted to the problem of geometrical spreading of these whistler-mode waves after their first magnetospheric reflection, which is crucial to determine where wave–particle interactions occur. Recently, experimental studies stated that the relative intensity of the reflected signal was generally between 0.005 and 0.05 of the source signal. We model such wave packets by means of ray tracing technique, using a warm plasma dispersion function along their trajectory and a realistic model of the inner magnetosphere. We reproduce the topology of the reflected energy distribution in the equatorial plane by modeling discrete chorus elements generated at the equator. Our calculations show that the spatial spreading is large and strongly dependent upon initial wave parameters, especially the chorus wave frequency. Thus, the divergence of each element ray trajectories can result in the filling of a large region (about 4 Earth radii around the source) of the magnetosphere and a reflected intensity of 0.005–0.06 of the source signal in the equatorial plane. These results are in good agreement with previous Cluster and THEMIS observations.

Effect of the background magnetic field inhomogeneity on generation processes of whistler‐mode chorus and broadband hiss‐like emissions

By a series of self‐consistent electron hybrid code simulations, we study the effect of the background magnetic field inhomogeneity on the generation process of whistler‐mode chorus emissions. Chorus with rising tones are generated through nonlinear wave‐particle interactions occurring around the magnetic equator. The mirror force plays an important role in the nonlinear interactions, and the spatial inhomogeneity of the background magnetic field is a key parameter of the chorus generation process. We have conducted numerical experiments with different spatial inhomogeneities to understand properties of the chorus generation process. We assume the same initial condition of energetic electrons at the magnetic equator in all simulation runs. The simulation results reveal that the spectral characteristics of chorus significantly vary depending on the magnetic field inhomogeneity. Whistler‐mode emissions are generated and propagate away from the equator in all simulation runs, but distinct chorus elements with rising tones are only reproduced in the cases of small inhomogeneities. In the simulation that had the smallest inhomogeneity, we find excitation of broadband hiss‐like emission (BHE) whose amplitudes are comparable to discrete chorus elements found in other simulation runs. The BHE consists of many wave elements with rising tones nonlinearly triggered in the region close to the magnetic equator. We show that the small spatial inhomogeneity of the background magnetic field results in the small threshold amplitude for the nonlinear wave growth and allows the triggering process of rising tone elements to emerge easily in the equatorial region of the magnetosphere.

The relations between magnetospheric chorus and hiss inside and outside the plasmasphere boundary layer: Cluster observation

[1] Magnetospheric whistler mode emissions (chorus and hiss) can play an important role in acceleration and loss of energetic electrons by wave-particle interaction. Here we present a first report of two events in the simultaneous observations of chorus and hiss by the Cluster constellation. During the event of 30 August 2001, one spacecraft observed chorus in the plasmasphere boundary layer (PBL), and three spacecraft observed hiss in the plasmasphere. In the event of 18 November 2002, one Cluster spacecraft observed hiss inside the plasmasphere, and the other three spacecraft observed chorus outside the plasmasphere. The correlation between chorus and hiss waves in both events are analyzed via intensity comparison and cross-correlation analysis. It is found that chorus and hiss waves have significant correlation in the first event but have no correlation in the second event. Furthermore, we perform a ray-tracing study to investigate the correlation by adopting two typical density models: the MTotal density model associated with the PBL structure and the global core density model. Simulation results demonstrate that chorus waves outside the plasmapause can propagate into the plasmasphere and possibly evolve into hiss waves under the MTotal density model, supporting the previous finding that the plasmaspheric hiss may originate from discrete chorus emission. However, chorus cannot travel into the plasmasphere under the global core density model. This paper presents a first result that chorus has more opportunity to propagate into the plasmasphere through the PBL than through the plasmapause. The results suggest that the gradual density variation of the PBL appears to favor the inward propagation of whistler mode waves.

Observations of the relationship between frequency sweep rates of chorus wave packets and plasma density

Chorus emissions are generated by a nonlinear mechanism involving wave‐particle interactions with energetic electrons. Discrete chorus wave packets are narrowband tones usually rising (sometimes falling) in frequency. We investigate frequency sweep rates of chorus wave packets measured by the Wideband data (WBD) instrument onboard the Cluster spacecraft. In particular, we study the relationship between the sweep rates and the plasma density measured by the WHISPER active sounder. We have observed increasing values of the sweep rate for decreasing plasma densities. We have compared our results with results of simulations of triggered emissions as well as with estimates based on the backward wave oscillator model for chorus emissions. We demonstrate a reasonable agreement of our experimental results with theoretical ones.

On the observations of unique low latitude whistler-triggered VLF/ELF emissions

A detailed analysis of the VLF/ELF wave data obtained during a whistler campaign under All India Coordinated Program of Ionosphere Thermosphere Studies (AICPITS) at our low latitude Indian ground station Jammu (geomag. lat. = 22°26′ N, L=1.17) has yielded two types of unusual and unique whistler-triggered VLF/ELF emissions. These include (1) whistler-triggered hook emissions and (2) whistler-triggered long enduring discrete chorus riser emissions in VLF/ELF frequency range during night time. Such types of whistler-triggered emissions have not been reported earlier from any of the ground observations at low latitudes. In the present study, the observed characteristics of these emissions are described and interpreted. Dispersion analysis of these emissions show that the whistlers as well as emissions have propagated along a higher geomagnetic field line path with L-values lying ∼L=4, suggesting that these triggered emissions are to be regarded as mid-latitude emissions. These waves could have propagated along the geomagnetic field lines either in a ducted mode or in a pro-longitudinal (PL) mode. The measured intensity of the triggered emissions is almost equal to that of the source waves and does not vary throughout the period of observation on that day. It is speculated that these emissions may have been generated through a process of resonant interaction of the whistler waves with energetic electrons. Parameters related to this interaction are computed for different values of L and wave amplitude. The proposed mechanism explains some aspects of the dynamic spectra.

Ground‐based observations of diffuse auroral frequencies in the context of whistler mode chorus

We present a case study of an active auroral event containing diffuse auroral structures. These data were collected on 04 February 2009 during a ground‐based observational campaign at Poker Flat, Alaska (L ∼ 6), from January through April 2009. They were used to extract the frequencies of pulsating auroral structures that lasted many hours after substorm breakup. There are two main frequency ranges reported, coincident with the spatial resolution capabilities of the two different fields of view of the optical imagers used. The all‐sky imager clearly revealed the pulsations of the large patches, ranging from approximately 50 to 500 mHz. The narrowfield imager measured the pulsations within the finer structure to be in the range of 0.5 to 15 Hz. Both observations are consistent with the frequencies expected from theoretical, modeling and observational work on whistler mode chorus and ECH waves. In particular, the repetition frequencies observed in the visual aurora closely match the observed repetition rate of discrete chorus elements previously described in the literature. There are three significant additional insights gained through these data. The frequency of the pulsations correlates to the intensity of the aurora, with the brighter aurora containing higher frequencies. The scale size of the structures relates to the frequency of the pulsations, with the smaller structures occurring with higher frequency pulsations. The intensity profile of the pulsating structures is not symmetric, with the intensity increasing at a slower rate than it decreases.

Simultaneous observation of whistlers and emissions during a geomagnetically quiet period at low latitude

A unique night-time natural electromagnetic disturbances in the VLF/ELF range received during a magnetically quite period at a low latitude Indian ground station, Jammu (geomag. lat. 19°26′ N, L=1.17) has been reported. During the routine observation of VLF waves at Jammu, whistlers and different types of VLF/ELF emissions such as whistlers of varying dispersion confined to a small band limited frequency range, hisslers, pulsing hiss, discrete chorus emissions of rising and falling tones with multiple bands, oscillating tone discrete emission, whistler-triggered hook and discrete chorus risers emissions, etc. have been observed simultaneously during the quiet period on a single night. Such type of unique simultaneous observations has never been reported from any of the low latitude ground stations and this is the first observation of its kind. The results are discussed in the light of recorded features of whistlers and emissions. Generation and propagation mechanism are discussed briefly. Plasma parameters are further derived from the dispersion analysis of nighttime whistlers and emissions recorded simultaneously during magnetically quiet periods.

Wave‐particle interactions in the equatorial source region of whistler‐mode emissions

Wave‐particle interactions can play a key role in the process of transfer of energy between different electron populations in the outer Van Allen radiation belt. We present a case study of wave‐particle interactions in the equatorial source region of whistler‐mode emissions. We select measurements of the Cluster spacecraft when these emissions are observed in the form of random hiss with only occasional discrete chorus wave packets, and where the wave propagation properties are very similar to previously analyzed cases of whistler‐mode chorus. We observe a positive divergence of the Poynting flux at minima of the magnetic field modulus along the magnetic field lines, indicating the central position of the source. In this region we perform a linear stability analysis based on the locally measured electron phase space densities. We find two unstable electron populations. The first of them consists of energy‐dispersed and highly anisotropic injected electrons at energies of a few hundreds eV to a few keV, with the perpendicular temperature more than 10 times higher than the parallel temperature with respect to the magnetic field line. Another unstable population is formed by trapped electrons at energies above 10 keV. We show that the injected electrons at lower energies can be responsible for a part of the waves that propagate obliquely at frequencies above one half of the electron cyclotron frequency. Our model of the trapped electrons at higher energies gives insufficient growth of the waves below one half of the electron cyclotron frequency and a nonlinear generation mechanism might be necessary to explain their presence even in this simple case.

Chorus source region localization in the Earth's outer magnetosphere using THEMIS measurements

Abstract. Discrete ELF/VLF chorus emissions, the most intense electromagnetic plasma waves observed in the Earth's radiation belts and outer magnetosphere, are thought to propagate roughly along magnetic field lines from a localized source region near the magnetic equator towards the magnetic poles. THEMIS project Electric Field Instrument (EFI) and Search Coil Magnetometer (SCM) measurements were used to determine the spatial scale of the chorus source localization region on the day side of the Earth's outer magnetosphere. We present simultaneous observations of the same chorus elements registered onboard several THEMIS spacecraft in 2007 when all the spacecraft were in the same orbit. Discrete chorus elements were observed at 0.15–0.25 of the local electron gyrofrequency, which is typical for the outer magnetosphere. We evaluated the Poynting flux and wave vector distribution and obtained chorus wave packet quasi-parallel propagation to the local magnetic field. Amplitude and phase correlation data analysis allowed us to estimate the characteristic spatial correlation scale transverse to the local magnetic field to be in the 2800–3200 km range.

Daytime hiss-triggered chorus emissions observed at low latitude ground station Srinagar, India

The first observation of hiss-triggered discrete chorus riser emissions recorded during daytime at our newly setup low latitude ground station Srinagar (geomag. lat., 24° 10′ N; L = 1.28), India on March 30, 2009 are reported. From the spectral analysis of these emissions, it is found that the chorus is hiss-triggered and each chorus element has the tendency to originate from the hiss band. A possible generation mechanism of these hiss-triggered chorus riser emissions is proposed.

Microburst precipitation of energetic electrons associated with chorus wave generation

Electron microbursts, which are short‐duration (<1 sec) bursts of energetic electrons that precipitate into the Earth's atmosphere, comprise an important loss process from the outer radiation belt. By means of a self‐consistent full‐particle simulation, we show that microburst precipitation of electrons of energies 10 keV–100 keV accompanies the generation of discrete bursty chorus wave emissions. Specifically, we demonstrate a one‐to‐one correspondence between the electron microbursts and the generation of discrete chorus elements. This simulation study is the first to establish such an exact correlation between electron microbursts and the generation of chorus elements.

A generation mechanism of chorus emissions using BWO theory

In this paper, discrete VLF chorus emissions recorded at low latitude ground station Jammu (geomag. Lat. = 22° 26/ N, L = 1.17) are reported and their characteristics based on complete spectral analysis have been carried out. These discrete chorus emissions are generated during a strong geomagnetic storm period of 2-7 May, 1998. We have computed the sweep rate, repetition period, source region, and drift rate of the individual chorus elements. It is observed that the sweep rate increases with time. To explain the various temporal and spectral features of these emissions, a possible generation mechanism has been presented based on the backward wave oscillator (BWO) regime in the magnetospheric cyclotron maser. On the basis of this model, we have computed some discrete chorus emission parameters as well as magnetospheric parameters relevant to the generation process. A comparison of the computed and observed magnetospheric parameters has been presented. These results show a good agreement with the BWO model.