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

Abstract

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

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

“…Thus, by measuring the frequency sweep rate in the source region we can obtain indirect information about the properties of a step on the distribution function. A decrease of the mean values of the frequency sweep rate with increasing mean cold plasma density was demonstrated in [4]. This result is in agreement with the BWO model if the q parameter is independent of the plasma density.…”

“…The distributions of q have an asymmetric shape with a long tail towards higher values. This scatter of the q values is related to the scatter in the frequency sweep rate of the chorus elements, which is typical for CLUSTER observations [4]. …”

Electron hybrid code simulation of whistler-mode chorus generation with real parameters in the Earth’s inner magnetosphere

“…This difference implies that the mechanism of a gap at 0.5 Ω e0 is governed by the physical process that is not described by the spatially one-dimensional system treating purely parallel propagating electromagnetic waves, such as the nonlinear damping through Landau resonance Yagitani et al 2014) and/or properties of obliquely propagating whistler-mode waves (e.g., Bell et al 2009;Katoh 2014;Nunn and Omura 2015). Macúšová et al (2010) analyzed 13 events of whistlermode chorus observed by the Cluster spacecraft during different levels of geomagnetic activity and showed that the frequency sweep rate decreases with the decreasing background plasma density. Based on the nonlinear wave growth theory, Omura et al (2015) showed that the frequency sweep rate decreases with increasing ω pe /Ω e0 in the frequency range higher than 0.1 Ω e0 .…”

New evidence for generation mechanisms of discrete and hiss‐like whistler mode waves

“…These calculated wave polarization parameters have a time resolution of~0.032 s, which is much shorter than the typical temporal duration of each discrete chorus element (a few tenths of seconds [e.g., Cully et al, 2008;Santolik et al, 2008;Macusova et al, 2010]). The high polarization ratio (R p ), defined as the ratio of polarized power to total power, indicates the high reliability of the calculated wave polarization parameters [Bortnik et al, 2007].…”