B. V. Kozelov

Interpretation of Cluster data on chorus emissions using the backward wave oscillator model

The measurements of chorus emissions by four closely separated Cluster spacecraft provide important information concerning the chorus generation mechanism. They confirm such properties of the wave source as their strong localization near the equatorial cross section of a magnetic flux tube, an almost parallel average wave-vector direction with respect to the geomagnetic field, and an energy flux direction pointing outward from the generation region. Inside this region, Cluster discovered strong temporal and spatial variations in the amplitude with correlation scale lengths of the order of 100 km across the magnetic flux. The wave electric field reached 30 mV/m, and the maximum growth and damping rates are of the order of a few hundreds of s−1. These and other properties of the detected chorus emissions are discussed here in relation with the backward wave oscillator mechanism. According to this mechanism, a succession of whistler wave packets is generated in a small near-equatorial region with temporal an...

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

[1] 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. Citation: Macusova, E., et al. (2010), Observations of the relationship between frequency sweep rates of chorus wave packets and plasma density,

Formation of VLF chorus frequency spectrum: Cluster data and comparison with the backward wave oscillator model

[1] The dependence of the frequency spectrum of individual chorus elements on the position of the observation point in and near the generation region is analyzed using recent Cluster data obtained on two different geomagnetically active days. The source of night-side chorus is localized using multicomponent measurements of the wave electric and magnetic fields. We have revealed that the spectrum of the chorus elements lacks the lower frequencies at the center of the source region. One possible explanation of this effect is provided by applying the backward wave oscillator model of chorus generation to these data. According to this model, the chorus frequency is determined by the parallel velocity corresponding to a steplike deformation in the distribution function of resonant electrons. This velocity decreases during the generation of an element as the electrons move through the source region. Thus, only a part of a chorus element is visible inside this region. For the typical case of rising-tone chorus elements, the lower frequencies are generated downstream with respect to the chorus propagation and, hence, disappear as a receiver is moved upstream towards the center of the source region. Citation: Trakhtengerts, V. Y., A. G. Demekhov, E. E. Titova, B. V. Kozelov, O. Santolik, E. Macusova, D. Gurnett, J. S. Pickett, M. J. Rycroft, and D. Nunn (2007), Formation of VLF chorus frequency spectrum: Cluster data and comparison with the backward wave oscillator model, Geophys. Res. Lett., 34, L02104, doi:10.1029/2006GL027953.

Identification of the source of quasiperiodic VLF emissions using ground‐based and Van Allen Probes satellite observations

We report on simultaneous spacecraft and ground-based observations of quasiperiodic VLF emissions and related energetic-electron dynamics. Quasiperiodic emissions in the frequency range 2–6u2009kHz were observed during a substorm on 25 January 2013 by Van Allen Probe-A and a ground-based station in the Northern Finland. The spacecraft detected the VLF signals near the geomagnetic equator in the night sector at Lu2009=u20093.0–4.2 when it was inside the plasmasphere. During the satellite motion toward higher latitudes, the time interval between quasiperiodic elements decreased from 6u2009min to 3u2009min. We find one-to-one correspondence between the quasiperiodic elements detected by Van Allen Probe-A and on the ground, which indicates the temporal nature of the observed variation in the time interval between quasiperiodic elements. Multiсomponent measurements of the wave electric and magnetic fields by the Van Allen Probe-A show that the quasiperiodic emissions were almost circularly right-hand polarized whistler mode waves and had predominantly small (below 30°) wave vector angles with respect to the magnetic field. In the probable source region of these signals (L about 4), we observed synchronous variations of electron distribution function at energies of 10–20u2009keV and the quasiperiodic elements. In the pause between the quasiperiodic elements pitch angle distribution of these electrons had a maximum near 90°, while they become more isotropic during the development of quasiperiodic elements. The parallel energies of the electrons for which the data suggest direct evidence of the wave-particle interactions is in a reasonable agreement with the estimated cyclotron resonance energy for the observed waves.

Highly anisotropic distributions of energetic electrons and triggered VLF emissions

The work considers the highly anisotropic electron distribution functions observed by Bell and co-workers at Stanford in the magnetosphere. IOt is maintained that Bells observations provide evidence for the existence of step like deformations in electron distribution function, and that his observations may be due to such steps. The origin of such steps at the top of hiss band emissions is explained, and the implications for triggering of emissions and chorus is explained.

Properties of the magnetospheric backward wave oscillator inferred from CLUSTER measurements of VLF chorus elements

[1]xa0According to the backward wave oscillator (BWO) model, a sharp gradient (or step-like deformation) on the electron distribution function is the most important factor in chorus generation, but such a feature is very difficult to observe directly. The properties of the step in the BWO model determine the dimensionless parameterqquantifying the excess of the energetic electron flux above the absolute-instability threshold. This parameter, in turn, is related to the frequency sweep rate of chorus elements, which we obtained by using data from the WBD instrument onboard the CLUSTER satellites in the equatorial region for more than 7000 chorus elements. Then, using the CLUSTER data for the plasma density and magnetic field, we calculatedq assuming the validity of the BWO theory and found that the q values depend only weakly on the density; the average values of q ≈ 7 for the lower band chorus (f/fce 0.5). These q values constitute a large excess over the generation threshold (q > 3) resulting from numerical simulation of discrete elements with rising frequency and are thus consistent with the simulations. Another important feature of the q parameter is the significant scatter of its values during each Cluster passage of the generation region. Using the obtained q values we estimate the relative height of the step in the electron distribution function to lie in the range from 0.01 to 0.3.

Testing of the backward wave oscillator model by using the spectral characteristics of VLF chorus elements

A generation mechanism for chorus was suggested by Trakhtengerts (1995) on the basis of the backward wave oscillator (BWO) regime of the magnetospheric cyclotron maser. According to this model, step-like deformation on the electron distribution function is the most important factor of chorus generation, but such a feature is very difficult to observe. By measuring the frequency sweep rates in chorus elements detected by the Cluster spacecraft we determine the mean values and distributions of a dimensionless parameter characterizing the step feature. These values are in agreement with the results of numerical simulations of chorus elements based on the BWO model.

On statistical distribution of characteristics of chorus element generation

A generation mechanism for VLF chorus was suggested by V. Yu. Trakhtengerts [1, 2] on the basis of the backward wave oscillator (BWO) regime of magnetospheric cyclotron maser. Up to now many predictions of the BWO regime were supported by observations. In this report we discuss a statistical distribution of the dimensionless parameter q, quantifying an excess of the electron flux over the absolute-instability threshold. Typically the q parameter values deduced from VLF chorus elements observed by WBD instrument onboard the CLUSTER spacecraft exhibit significant scatter, while their mean values were ∼10. We suppose that so large excess of the instability threshold cannot be permanently supported in the magnetospheric plasma. On the basis of the discrete numerical model we demonstrate that if the noised “on-off” intermittency regime generation is realized, then the observed q values deduced from chorus elements should be extreme ones, but the average value over the entire event can be much smaller. We stress an importance of taking the noise-induced type of chorus generation into account.