A generation mechanism for chorus emission

A chorus generation mechanism is discussed, which is based on interrelation of ELF/VLF noise-like and discrete emissions under the cyclotron wave-particle interactions. A natural ELF/VLF noise radiation is excited by the cyclotron instability mechanism in ducts with enhanced cold plasma density or at the plasmapause. This process is accompanied by a step-like deformation of the energetic electron distribution function in the velocity space, which is situated at the boundary between resonant and nonresonant particles. The step leads to the strong phase correlation of interacting particles and waves and to a new backward wave oscillator (BWO) regime of wave generation, when an absolute cyclotron instability arises at the central cross section of the geomagnetic trap, in the form of a succession of discrete signals with growing frequency inside each element. The dynamical spectrum of a separate element is formed similar to triggered ELF/VLF emission, when the strong wavelet starts from the equatorial plane. The comparison is given of the model developed using some satellite and ground-based data. In particular, the appearance of separate groups of chorus signals with a duration 2–10 s can be connected with the preliminary stage of the step formation. BWO regime gives a succession period smaller than the bounce period of energetic electrons between the magnetic mirrors and can explain the observed intervals between chorus elements.     

Two types of ELF hiss observed at Varanasi, India

The morphology of ELF hiss events observed at low-latitude ground station Varanasi (L = 1.07, geomagnetic latitude 14°55′N) are reported, which consist of two types: (1) events which propagated in ducted mode along the geomagnetic field line corresponding to observing station Varanasi and (2) events which propagated in ducted mode along higher L-values (L = 4–6), after reaching the lower edge of ionosphere excite the Earth-ionosphere wave guide and propagate towards equator to be received at Varanasi. To understand the generation mechanism of ELF hiss, incoherent Cerenkov radiated power from the low latitude and middle latitude plasmasphere are evaluated. Considering this estimated power as an input for wave amplification through wave-particle interaction, the growth rate and amplification factor is evaluated which is too small to explain the observed wave intensity. It is suggested that some non-linear mechanism is responsible for the generation of ELF hiss.     

Whistler–electron interactions in the magnetosphere: new results and novel approaches

Attention is focused here on the quasilinear and nonlinear physics of cyclotron interactions between magnetospheric whistler mode waves and energetic electrons on dipolar geomagnetic flux tubes. These interactions can lead to the generation of noise-like emissions or phase-coherent discrete signals in the frequency-time domain. In the magnetosphere noise-like emissions called hiss are accompanied by a smooth electron precipitation pattern. Examples of discrete emissions are ELF/VLF chorus or VLF emissions triggered by whistlers from lightning or by radio transmitters on the ground. The rapid temporal variations of these signals are associated with fine structure of the distribution function of the radiation belt electrons, such as a transient step-like deformation or a well-organized beam, which are prepared by initial noise-like emissions or by a quasimonochromatic whistler–wave packet, respectively. These cause the properties of the electrons, which may be observed on a satellite, to evolve rapidly in time and on relatively short spatial scales. Bursts of precipitating electrons occur, and can contribute significantly to depleting the radiation belts. Recent results on improvements in the theoretical understanding of such processes and on new observations of magnetospheric electrons and whistler-mode waves are presented.     

Plasmaspheric hiss overview and relation to chorus

A brief overview is given of the history of plasmaspheric hiss research, particularly in the context of the recent work by Bortnik et al. (2008) indicating that chorus could be the likely source of plasmaspheric hiss. Previous suggestions given in the literature for this theory are reviewed and then the mechanism itself is outlined, focusing on the characteristic cyclical trajectories executed by typical ray paths that enter into the plasmasphere. A number of directional propagation studies performed in the past are then discussed as well as other work which bears relevance to the present mechanism.     

张衡一号卫星记录的空间电磁波传播特征分析方法及算法实现

本研究主要利用张衡一号电磁卫星(CSES)记录的电磁场波形数据,研发了电磁波波矢量分析工具,该工具包提供电磁波形频谱变换、奇异值分解(SVD)方法以及坡印廷能流计算功能.选取了张衡一号卫星在ULF/ELF/VLF频段观测到的哨声波、准周期辐射、电离层嘶声波等典型波动事件,开展了频谱分析和波矢量分析.通过与DEMETER卫星的对比观测研究,验证了算法的正确性,并且初步探讨了这些波动的传播特征,其结果也验证了张衡一号卫星在电磁场观测方面具有良好的性能.本研究研发的波矢量分析工具包可直接运用到张衡一号卫星的常规数据处理和科学分析应用中.     

Amplitude–frequency characteristics of ion–cyclotron and whistler-mode waves from Van Allen Probes data

Using two-hour (from 2300 UT January 25, 2013 to 0100 UT January 26, 2013) measurement data from Van Allen Probes on fluxes of energetic particles, cold plasma density, and magnetic field magnitude, we have calculated the local growth rate of electromagnetic ion–cyclotron and whistler-mode waves for field-aligned propagation. The results of these calculations have been compared with wave spectra observed by the same Van Allen Probe spacecraft. The time intervals when the calculated wave increments are sufficiently large, and the frequency ranges corresponding to the enhancement peak agree with the frequency–time characteristics of observed electromagnetic waves. We have analyzed the influence of variations in the density and ionic composition of cold plasma, fluxes of energetic particles, and their pitch-angle distribution on the wave generation. The ducted propagation of waves plays an important role in their generation during the given event. The chorus VLF emissions observed in this event cannot be explained by kinetic cyclotron instability, and their generation requires much sharper changes (“steps”) for velocity distributions than those measured by energetic particle detectors on Van Allen Probes satellites.     

The “fan” instability as a possible generating mechanism of vlf quarter-gyrofrequency emissions during the recovery phase of a magnetospheric storm

Summary VLF quarter-gyrofrequency emissions, whose experimental characteristics differ from those of discrete plasmaspheric emissions usual during active times, have been observed by low-altitude Intercosmos 24 and Magion 2 satellites within the plasmasphere during periods of quieting geomagnetic activity. A new kind of instability is proposed which could lead to the production of these emissions. It is shown that quasi-electrostatic whistler waves can be generated in the eqquatorial region due to the fan instability, with maximum growth rate and spectral energy density in a frequency band below one half of the equatorial electron gyrofrequency. The observations of low-energy electrons and plasma waves in the equatorial region within or in the vicinity of the plasmapause, which could support our hypothesis experimentally, are discussed.     

VLF quarter-gyrofrequency plasmaspheric emissions as observed by intercosmos 24 and magion 2 satellites

Summary Quarter-gyrofrequency plasmaspheric emissions with spectral properties differing from those of discrete plasmaspheric emissions, usual in active intervals, have been observed by low-altitude Intercosmos 24 and Magion 2 satellites during periods in which geomagnetic activity decreases. Their occurrence in satellite records shows very good correlation with simultaneously observed subauroral electron temperature enhancements and increase of electron temperature anisotropyT _e being larger than T _e . An analysis of the observed wave characteristics is given. Propagation of the emissions within the plasmasphere is discussed. It is shown that the region where they are observed at low altitudes can be closely connected along geomagnetic field lines with the equatorial region of their origin.     

Magnotospheric imaging of high latitude ion outflows

High latitude ion outflows mostly consist of upward streaming O⁺ and He⁺ emanating from the ionosphere. At heights above 1000 km, these flows consist of cold and hot components which resonantly scatter solar extreme ultraviolet (EUV) light, however, the ion populations respond differently to Doppler shifting resulting from the large relative velocities between the ions and the Sun. The possibility of optical detection of the Doppler effect on the scattering rate will be discussed for the O⁺ (83.4 nm) ions. We have contrasted the EUV solar resonance images of these outflows by simulations of the 30.4 nm He⁺ and 83.4 nm O⁺ emissions for both quiet and disturbed geomagnetic conditions. Input data for the 1000 km level has been obtained from the EICS instrument aboard the Dynamics Explorer satellite. Our results show emission rates of 50 and 56 milli-Rayleighs at 30.4 nm for quiet and disturbed conditions and 65 and 75 milli-Rayleighs at 83.4 nm for quiet and disturbed conditions, respectively, obtained for a polar orbiting satellite and viewing radially outward. We also find that an imager at an equatorial distance of 9 R_E or more is in a favorable position for detecting ion outflows, particularly when the plasmapause is depressed in latitude. However, an occultation disk is necessary to obscure the bright plasmaspheric emissions.     

Oxygen cyclotron harmonic waves observed using Van Allen Probes

Fine structured multiple-harmonic electromagnetic emissions at frequencies around the equatorial oxygen cyclotron harmonics are observed by Van Allen Probe A outside the core plasmasphere(L~5) off the magnetic equator(MLAT~.7.5°)during a geomagnetic storm. We find that the multiple-harmonic emissions have power spectrum density(PSD) peaks during 2–8equatorial oxygen gyroharmonics( f ~ n fO+, n=2–8), while the fundamental mode(n=1) is absent, implying that the harmonic waves are generated near the equator and propagate into the observation region. Additionally, these electromagnetic emissions are linearly polarized. Different from the equatorial noise emission that propagates considerably obliquely, these emissions have moderate wave normal angles(approximately 40°–60°), which predominately increase as the harmonic number increases.Considering their frequency and wave normal angle characteristics, it is suggested that these multiple-harmonic emissions play an important role in the dynamic variation of radiation belt electrons.     

Energetic proton precipitation related to ion–cyclotron waves

This brief review summarizes recent findings related to particle precipitation associated with electromagnetic ion–cyclotron (EMIC) waves seen on the ground as geomagnetic Pc1 and IPDP pulsations.Particle precipitation signatures of ion–cyclotron interaction are described as revealed from low-altitude satellite measurements of the energetic proton fluxes as well as from observations of the proton aurora. As a result, localized proton precipitation patterns situated equatorward of the isotropy boundary are disclosed. One of the patterns is a proton precipitation spot in the morning sector, presumably mapped onto plasmapause; another one is an elongated region of the precipitation, presumably mapped onto the plasmaspheric plume.Clear evidence of the pitch-angle scattering associated with the ion–cyclotron wave activity is found near the equatorial plane in the region conjugated with the localized proton precipitation at low altitude.Thus, the revealed precipitation patterns determine the location of the region of intense pitch-angle scattering of energetic protons, and, therefore, their observations can be used to monitor the region of the ion–cyclotron interaction and to study its origin and properties. Some examples of such application of the low-altitude observations of energetic particles are described.     

Auroral pulsations and accompanying VLF emissions

Results of simultaneous TV observations of pulsating auroral patches and ELF-VLF-emissions in the morning sector carried out in Sodankylä (Finland) on February 15, 1991 are presented. Auroral pulsating activity was typical having pulsating patches with characteristic periods of about 7 s. Narrow-band hiss emissions and chorus elements at intervals of 0.3–0.4 s formed the main ELF-VLF activity in the frequency range 1.0–2.5 kHz at the same time. The analysis of auroral images with time resolution of 0.04 s allowed perfectly separate analysis of spatial and temporal variations in the auroral luminosity. Mutual correspondence between the behaviour of the luminous auroral patches and the appearance of ELF noise type hiss emissions and VLF chorus trains was found in two intervals chosen for analysis. While the hiss emissions were associated with the appearance of luminosity inside a limited area close to the zenith, the structured VLF emissions were accompanied by rapid motion of luminosity inside the area. The spatial dimension of the pulsating area was about 45–50 km and luminosity propagated inside it with velocity of about 10–12 kms. We discuss a new approach to explain the 5–15 s auroral pulsation based on the theory of flowing cyclotron maser and relaxation characteristics of ionosphere.     

高压输电系统引起的空间电磁扰动

随着全球工业化进程的大力推进,高压输电系统逐渐成为电磁环境监测的主要人为干扰源之一。本文总结了以法国DEMETER卫星电磁场数据为主开展的高压电力线辐射研究的最新成果,对电力线产生的各类空间电磁扰动现象及其时空分布特征进行了归纳分析,其中包括电力线谐振辐射(PLHR)、磁层线辐射(MLR)、ELF/VLF辐射、高能粒子沉降等,并对与其耦合机制相关的电磁波传播及波粒相互作用模型进行了总结讨论,旨在对中国电磁监测试验卫星(CSES-1)数据处理方法、干扰识别和地震弱信号提取分析提供更多的参考信息。     

Eigenmode analysis of pitch-angle diffusion of energetic electrons in the outer zone

We demonstrate a new method of analyzing observed storm-time pitch-angle distributions to obtain information regarding the appropriate choice of the pitch-angle diffusion coefficients. We apply this method to MeV electrons in the outer zone as a diagnostic of the relative contribution of electromagnetic ion-cyclotron (EMIC) waves and whistler-mode hiss and chorus. We assume EMIC and hiss are confined to a plasmaspheric plume (hence, “plume waves”), with chorus prevalent over large portions of the day and night side. First, we determine the eigenmodes and eigenvalues of the pitch-angle diffusion operator predicted by quasilinear diffusion theory and approximate chorus, hiss, or EMIC plasma wave parameters for energetic electrons in the outer zone. Then, by projecting pitch-angle distributions observed by CRRES into the eigenmodes, we determine whether the pitch-angle distributions are consistent with the assumed diffusion process for various relative weighting of chorus and plume waves. Eigenmodes with shorter decay times (i.e., larger negative eigenvalues) ought to represent a comparatively smaller portion of the total flux in the pitch-angle distribution. We show that several observed pitch-angle distributions are consistent with predominantly chorus-driven pitch-angle diffusion, with at most a minor contribution from plume waves.     

Preliminary results from the MEMO multicomponent measurements of waves on-board INTERBALL 2

The MEMO (MEsure Multicomposante des Ondes) experiment is a part of theINTERBALL 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 three frequency bands: ELF (5–1000 Hz), VLF (1–20 kHz), LF (20–250 kHz). Preliminary analyses of low and high resolution data are presented. The emphasis is put on the estimation of the propagation characteristics of the observed waves. VLF hiss emissions are shown to be mainly whistler mode emissions, but other modes are present. An accurate estimation of the local plasma frequency is proposed when the low L = 0 cutoff frequency is identified. AKR emissions observed just above source regions are studied. R-X and L-O modes are found: the first at the lowest frequencies and the second at the highest. Both propagate with wave normal directions weakly oblique or quasi-parallel to the Earths magnetic field direction. Propagation characteristics are also determined for a (non-drifting) fine structure of AKR. There is no fundamental difference with structurless events. Night-side and dayside bursts of ELF electromagnetic emissions are presented. It is not clear whether the two emissions belong to the lion roar emissions or not.     

Proton interaction with quasi-electrostatic whistler mode waves in an inhomogeneous plasma (magnetosphere)

We study the interaction between energetic protons of the Earth’s radiation belts and quasi-electrostatic whistler mode waves. The nature of these waves is well known: whistler waves, which are excited in the magnetosphere due to cyclotron instability, enter the resonant regime of propagation and become quasielectrostatic, while their amplitude significantly increases. Far enough from the equator where proton gyrofrequency and transversal velocity increase the nonlinear interaction between these waves and energetic protons becomes possible. We show that plasma inhomogeneity may destroy cyclotron resonance between wave and proton on the time scale of the order of particle gyroperiod which in fact means the absence of cyclotron resonance; nevertheless, the interaction between waves and energetic particles remains nonlinear. In this case, particle dynamics in the phase space has the character of diffusion; however, the diffusion coefficients are determined by the averaged amplitude of the wave field, but not by its resonant harmonics. For real parameters of the waves and magnetospheric plasma, proton pitch-angle diffusion leading to their precipitation from the magnetosphere becomes essential.     

Quasi-periodic very low frequency emissions, very low frequency chorus, and geomagnetic Pc4 pulsations (Event on April 3, 2011)

The results of an analysis of ground-based observations of very low frequency (VLF) emissions in Scandinavia (L ∼ 5) in April 2011 are discussed. A detailed study is conducted of an non-typical event (April 3, 2011) of simultaneous generation of VLF chorus at frequencies below 3 kHz and quasi-periodic VLF emissions (QP) in the band of 4–6 kHz, which were not discrete emissions but consisted of separate short (about 20 s) bursts of hiss. It is shown that these emissions were mainly characterized by right-hand polarization, which indicates the location of the exit point of waves from the ionosphere near the point of ground observations. Based on an analysis of the spectral characteristics of emissions, it is concluded that the generation regions of chorus and QP emissions were located at different L shells. The appearance of QP emissions coincided with the excitation of resonance geomagnetic pulsations of the Pc4 range in the magnetosphere with a period that was close to the quasi-period of repetition of spectral forms in QP emissions. However, based on the available data, it is not possible to conclude that these geomagnetic pulsations caused the quasi-periodic generation of bursts of VLF hiss. The time shift between the peaks of QP and geomagnetic pulsations was inconsistent and varied from one burst of hiss to another. It is suggested that the discussed QP emissions were a result of the development of self-oscillations in the Earth’s radiation belts.     

地磁扰动期间等离子体层顶结构的模拟研究

本文选取2001年6月8-10日的一个亚暴事件,模拟了在这期间等离子体层的结构演化过程.选取Weimer(2001模式)电场和Tsyganenko(1996模式)磁场作为背景电磁场,基于E×B的漂移运动计算磁赤道面内的带电粒子分布,模拟磁扰期间的等离子体层变化.模拟了等离子体层顶的结构和形状,结果有羽状、肩状和通道状结构,与同一时间点的EUV/IMAGE探测结果一致.