Chorus Bursts Research Articles

Data‐Driven Simulation of Rapid Flux Enhancement of Energetic Electrons With an Upper‐Band Whistler Burst

AbstractThe temporal variation of the energetic electron flux distribution caused by whistler mode chorus waves through the cyclotron resonant interaction provides crucial information on how electrons are accelerated in the Earth's inner magnetosphere. This study employs a data‐driven test‐particle simulation which demonstrates that the rapid change of energetic electron distribution observed by the Arase satellite cannot be simply explained by a quasi‐linear diffusion mechanism, but is essentially caused by nonlinear scattering: the phase trapping and the phase dislocation. In response to upper‐band whistler chorus bursts, multiple nonlinear interactions finally achieve an efficient flux enhancement of electrons on a time scale of the chorus burst. A quasi‐linear diffusion model tends to underestimate the flux enhancement of energetic electrons as compared with a model based on the realistic dynamic frequency spectrum of whistler waves. It is concluded that the nonlinear phase trapping plays an important role in the rapid flux enhancement of energetic electrons observed by Arase.

Relativistic Electron Microbursts as High‐Energy Tail of Pulsating Aurora Electrons

AbstractIn this study, by simulating the wave‐particle interactions, we show that subrelativistic/relativistic electron microbursts form the high‐energy tail of pulsating aurora (PsA). Whistler‐mode chorus waves that propagate along the magnetic field lines at high latitudes cause precipitation bursts of electrons with a wide energy range from a few kiloelectron volts (PsA) to several megaelectron volts (relativistic microbursts). The rising tone elements of chorus waves cause individual microbursts of subrelativistic/relativistic electrons and the internal modulation of PsA with a frequency of a few hertz. The chorus bursts for a few seconds cause the microburst trains of subrelativistic/relativistic electrons and the main pulsations of PsA. Our simulation studies demonstrate that both PsA and relativistic electron microbursts originate simultaneously from pitch angle scattering by chorus wave‐particle interactions along the field line.

Different types of whistler mode chorus in the equatorial source region

The Time History of Events and Macroscale Interactions during Substorms-D spacecraft crossed an active equatorial source region of whistler mode chorus rising tones on 23 October 2008. Rising tones are analyzed in terms of spectral and polarization characteristics, with special emphasis on wave normal angles. The latter exhibit large variations from quasi-parallel to oblique, even within single bursts, but seem to follow a definite pattern, which enables an unambiguous classification into five different groups. Furthermore, we discuss the frequently observed splitting of chorus bursts into a lower and an upper band around one half of the local electron cyclotron frequency. At chorus frequencies close to the gap, we find significantly lowered wave planarities and a tendency of wave normal angles to approach the Gendrin angle.

Experimental evidence of the simultaneous occurrence of VLF chorus on the ground in the global azimuthal scale – from pre-midnight to the late morning

Abstract. Night-time VLF (very low frequency) chorus bursts lasting about one hour have been recorded at Finnish temporal station Kannuslehto (CGM: 64.2°; 107.9°, L = 5.3) during two VLF campaigns (on 25 February–4 March 2008 and 27 March–17 April 2011). The chorus bursts were associated with substorm development. They were accompanied by riometer absorption enhancements, which occurred simultaneously within as large longitude areas as from pre-midnight (Sodankylä, ~22:00 MLT) to the late morning (Tixie, ~03:00 MLT and Gakona, ~08:00 MLT) longitudes. It was found that the pre-midnight chorus observed on the ground occurred simultaneously with VLF chorus emissions recorded in the late morning on the low-altitude DEMETER satellite crossing the similar geomagnetic latitudes on the opposite local time sector. For the first time some evidence of simultaneous chorus burst generation in the global longitudinal scale was found (from pre-midnight to the late morning) by using direct comparison with satellite data as well as using non-direct indicator–azimuthally extended riometer absorption enhancements.

Multievent study of the correlation between pulsating aurora and whistler mode chorus emissions

[1] A multievent study was performed using conjugate measurements of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and an all-sky imager during periods of intense lower-band chorus waves. The thirteen identified cases support our previous finding, based on two events, that the intensity modulation of lower-band chorus near the magnetic equator is highly correlated with quasiperiodic pulsating auroral emissions near the spacecraft's magnetic footprint, indicating that lower-band chorus is the driver of the pulsating aurora. Furthermore, we identified a fortuitous measurement made simultaneously by two THEMIS spacecraft with small spatial separation. The two spacecraft were found to be located in a single pulsating chorus patch and the spacecraft footprints were in the same pulsating auroral patch when intense chorus bursts were measured simultaneously, whereas only one of the spacecraft's footprints was in a patch when the other spacecraft did not detect intense chorus. On the basis of this event, we can estimate the pulsating chorus patch size by mapping the pulsating auroral patches from the ionosphere toward the magnetic equator, giving a roughly circular region of ∼5000 km diameter for corresponding azimuthally elongated patches with ∼100 km size in the ionosphere. Using a ray-tracing-based calculation of the divergence of chorus raypaths from a point source, together with the corresponding resonant energies, we found that the chorus patch size is most probably not a result of ray divergence but a property of the wave excitation region.

Characteristics of short‐duration electron precipitation bursts and their relationship with VLF wave activity

Energetic (>6 keV) electron data from the SEEP payload on the low altitude (∼200 km) polar orbiting S81‐1 satellite indicate a high rate of occurrence of short duration (<0.6 s) electron precipitation bursts. Characteristics of events observed at night (2230 MLT) versus daytime (1030 MLT) and at midlatitudes (2<L<3) versus higher latitudes (L>3) were distinctly different in several ways. For 2<L<3 the daytime bursts occurred approximately uniformly in longitude and were equally distributed between the northern and southern hemispheres. The nighttime bursts in the same L shell range occurred approximately twice as often on a worldwide basis and were observed predominantly in the northern hemisphere and at longitudes of 260°E to 320°E. In a significant number of the nighttime events at 2<L<3 the median electron energy increased with time during the burst, but most of the other spectra showed no well‐defined trend. During some of the nighttime bursts broad peaks were observed in the energy spectra, but these peaks were not so evident in the daytime bursts. On higher L shells, L>3, narrow electron precipitation bursts (<0.3 s duration) were frequently observed poleward of the plasmapause, more often near noon than near midnight and much more frequently than at 2<L<3. Comparison of the electron data with simultaneous VLF wave data from Palmer (L≃2.4) and Siple (L≃4.3) stations in Antarctica indicated a varying degree of association of electron bursts with whistlers and chorus emissions. Several of the electron bursts observed at nighttime and at 2<L<3 were correlated with lightning‐generated whistlers observed at Palmer Station. When daytime bursts at higher latitudes (L>3) were observed on satellite passes within ±50° of the Siple meridian, chorus was invariably detected at Siple, but correlation of electron bursts with individual chorus spectral elements was not evident. The lack of such correlation may be due to the limited spatial extent of flux tubes excited by individual chorus elements which are possibly generated without mutual coherence in multiple high altitude magnetospheric locations. Within one hour of all four of the satellite passes within 400 km of Siple with the highest rate (≥10 individual bursts per pass) of burst occurrence, overhead electron precipitation was clearly detected by ground based sensors at Siple Station. Quasi‐periodic bursts of several seconds duration were observed on one or more of the photometer, riometer, and magnetic pulsation sensors and many of these bursts were correlated with clustered VLF chorus bursts. Hence, there is at least association of chorus with electron precipitation.

Analysis of chorus emissions at Jupiter

On the Voyager 1 inbound pass through the Jovian magnetosphere, the frequent acquisition of wide band data from the plasma wave instrument has permitted a quasi‐continuous survey of the waves which occur in the frequency band below the electron cyclotron frequency, the chorus band. Structured, rising frequency chorus was observed just outside the dense Io plasma torus but inside the low‐density middle magnetosphere. A quasi‐continuous, very narrow band emission occurred just above one‐half the electron cyclotron frequency. Power spectra, which were constructed from the wide band data, showed that the half‐cyclotron emission is unaffected by the simultaneous occurrence of chorus at lower frequencies and is separated from the chorus by a deep spectral gap just below one‐half the cyclotron frequency. Within the rising chorus band, a hisslike emission was observed which consisted of two quasi‐continuous, very narrow band tones located near the upper and lower frequency limits of the rising chorus band. Sequential power spectra showed that these twin‐frequency tones persisted during rising chorus bursts. Chorus occurs in the spatial regions where whistlers have cyclotron resonant interactions with the suprathermal population of ∼keV electrons. A possible interpretation of the twin tones is that these signals are electrostatic or resonance cone whistlers. Growth rate calculations show that the electrostatic whistler is destabilized by a loss cone distribution with the same pitch angle anisotropy which is needed to excite the electromagnetic chorus whistlers. A speculative hypothesis is presented to explain the half‐cyclotron emission. The Landau absorption of oblique chorus whistlers results in the spectral gap just below one‐half the cyclotron frequency and in the formation of a quasi‐linear plateau in the parallel velocity distribution of the suprathermal electrons. The plateau alters the Landau and cyclotron resonant interactions and permits the unstable growth of electrostatic and electromagnetic whistlers in a narrow range just above one‐half the cyclotron frequency.

Terrestrial versus Jovian VLF chorus; A comparative study

The relevant parameters of the magnetospheres of Jupiter and earth are studied from the point of view of wave‐particle resonant interactions that are believed to be responsible for the generation of VLF chorus emissions observed on Voyager‐1. Using existing models of the cold and energetic plasma distributions in the Jovian magnetosphere, expressions for the wave‐particle interaction length (LI) and the nonlinearity parameter (ρ) are derived. Values of these parameters are compared with those computed for the earth's magnetosphere. It is found that the typical interaction lengths are at least 2–5 times larger in the Jovian than in the terrestrial magnetosphere. Also, the wave intensity necessary to reach the threshold of nonlinearity in the Jovian magnetosphere was found to be up to 5–100 times lower. The Voyager 1 measurements show, however, that the inferred wave magnetic field intensities of the Jovian chorus are in the range of reported intensities for terrestrial chorus. This is attributed to fact that the fluxes of few keV resonant particles found in the Jovian magnetosphere were typically two orders of magnitude higher. In this case, it is predicted that the temporal growth rates of Jovian chorus bursts should be higher than for the earth. Growth rate measurements on Voyager 1 broadband wave data are used to confirm this hypothesis.

Plasma waves near the magnetopause

Plasma waves associated with the magnetopause, from the magnetosheath to the outer magnetosphere, are examined with an emphasis on high time resolution data and the comparison between measurements by using different antenna systems. An early ISEE crossing of the magnetopause region, including passage through two well‐defined flux transfer events, the magnetopause current layer, and boundary layer plasma, is studied in detail. The waves in these regions are compared and contrasted with the waves in the adjoining magnetosheath and outer magnetosphere. Four types of plasma wave emissions are characteristic of the nominal magnetosheath: (1) a very low frequency continuum, (2) short wavelength spikes, (3) ‘festoon‐shaped’ emissions below about 2 kHz, and (4) ‘lion roars.’ The latter two emissions are well correlated with ultra‐low frequency magnetic field fluctuations. The dominant plasma wave features during flux transfer events are (1) an intense low‐frequency continuum, which includes a substantial electromagnetic component, (2) a dramatic increase in the frequency of occurrence of the spikes, (3) quasi‐periodic electron cyclotron harmonics correlated with ∼1‐Hz magnetic field fluctuations, and (4) enhanced electron plasma oscillations. The plasma wave characteristics in the current layer and in the boundary layer are quite similar to the features in the flux transfer events. Upon entry into the outer magnetosphere, the plasma wave spectra are dominated by intense electromagnetic chorus bursts and electrostatic (n + 1/2)fg− emissions. Wavelength determinations made by comparing the various antenna responses and polarization measurements for the different waves are also presented.

Correlations between very low frequency chorus bursts and impulsive magnetic variations at L ~ 4.5

Coordinated observations of aurora, ULF, and VLF waves were made at 13 stations in Canada in January and February, 1980. The analysis of simultaneous ULF and VLF data obtained at Park Site (L = 4.4) revealed a close relationship between irregular magnetic pulsations and VLF emissions in the frequency range of 1.5–5 kHz. One-to-one correlations were observed between VLF chorus bursts and impulsive magnetic variations, called magnetic impulses, during sub-storms of Kp ≥ 4+ on the local morning-to-noon side. VLF chorus bursts consist of discrete risers of 0.1–0.3 s duration. It is found that magnetic impulses with a rise time of 0.5–1 s and with a duration of ~2 s coincide with the occurrence of VLF riser groups of a similar duration within ~2 s. This short time difference strongly suggests that magnetic impulses are caused by a conductivity enhancement due to electron precipitation induced by whistler mode waves.

Correlations between λ4278 optical emissions and VLF wave events observed at L ∼4 in the Antarctic

One‐to‐one correlations have been observed at L∼4 between bursts of ducted VLF noise in the ∼2 to 4‐kHz range and optical emissions at λ4278. The optical observations were made at Siple Station, Antarctica, in the austral winter of 1977; the wave activity was recorded both at Siple and its conjugate station Roberval, Canada. The one‐to‐one correlations were observed during 6 of 32 observing sessions. Most of the events occurred near dawn (prior to interruption of the observations by twilight interference) during or shortly following substorm events (Kp =2.5; Dst=−10–40). In most of the cases the plasmapause was equatorward of Siple, and the equatorial electron densities were low (∼10 cm−3). In one case, July 24, 1977, the equatorial density had recovered to a level of 100 cm−3 at the time of observation. The estimated precipitated energy fluxes ranged from 0.04 to 0.1 ergs cm−2 s−1. The correlated VLF activity usually consisted of clusters of discrete rising tones or chorus ranging in duration from ∼1 s to ∼10 s. In three of the six cases some or all of the chorus bursts were triggered by whistlers. Photometer peaks lagged the wave peaks by 1–2 s. In the case of July 24, 1977, a model of the interaction process was constructed using the lag time and the whistler‐derived cold plasma density. The data were found to be consistent with scattering of electrons into the loss cone over Siple by emissions that were triggered by waves propagating away from the equator after reflection in the ionosphere over Siple. This interpretation differs from that of a previous study of X‐ray bursts induced by similar VLF emissions. This difference is attributed to the occurrence of whistler echoing during the reported X‐ray event. During the same period, VLF hiss was observed in the range 2.5–5.0 kHz. Immediately following each discrete event the hiss and the λ4278 intensities were reduced below their pre‐event levels for about 10 s. Both reductions are tentatively attributed to a temporary depletion of particles near the loss cone. Recovery to pre‐event levels could be explained by refilling of the duct through east‐west gradient drift of energetic electrons.

Microburst phenomena: 3. An association between microbursts and VLF chorus

Observations made with the Injun 3 satellite of bursts of precipitating Ee ≥ 40-kev electrons and of VLF chorus emission have revealed their simultaneous occurrence. Observed microbursts are always accompanied by a group of VLF chorus emissions; chorus is not necessarily accompanied by microbursts. The maximum region of microburst occurrence from 0400 ≤ magnetic local time ≤ 1300 and 65° ≤ invariant latitude ≤ 70° lies well within the maximum region of chorus emissions from 0300 ≤ magnetic local time ≤ 1500 and 55° ≤ invariant latitude ≤ 75°. It is not generally possible to find a one-to-one (burst to burst) correspondence between individual microbursts and VLF chorus bursts.