Short Periodic VLF Emissions Observed Simultaneously by Van Allen Probes and on the Ground

Abstract

Very low frequency (VLF) emissions in the magnetosphere are often observed in the form of periodic or quasiperiodic sequence of bursts. Helliwell (1965) termed periodic emissions (PE) as the bursts having repetition periods of 3-10 s consistent with individual wave packet propagation between conjugate ionospheres. This period can be very stable in time for long time intervals. Quasi-periodic (QP) emissions have longer periods (usually from 20-30 to 300 s), and the inter-element interval can vary smoothly due to several factors (Manninen et al., 2014).Such events can last tens of minutes and even several hours (Manninen et al., 2014), which require the amplification of whistler mode waves in the magnetosphere to compensate for the losses due to nonideal ionospheric reflection and refractive spreading of wave energy. The commonly accepted amplification mechanism of whistler mode waves in the inner magnetosphere is cyclotron resonant interaction with energetic electrons having anisotropic velocity distribution (Trakhtengerts & Rycroft, 2008).Periodic emissions are divided into two subtypes, which Helliwell (1965) termed as dispersive and nondispersive. The period of dispersive PE has a systematic frequency-time dispersion typical of multi-hop

Quasiperiodic Emissions: Fine Structure Corresponding to a Bouncing Wave

“…This pattern resembles the bouncing‐element event reported by Demekhov et al. (2021), and some, probably nonlinear, mechanism seems necessary to compensate for the group velocity dispersion. Such a mechanism has been suggested by Bespalov (1984) and recalled by Bespalov et al.…”

“…Surprisingly, this bouncing is not accompanied by dispersion, as one might expect for whistler-mode waves. This pattern resembles the bouncing-element event reported by Demekhov et al (2021), and some, probably nonlinear, mechanism seems necessary to compensate for the group velocity dispersion. Such a mechanism has been suggested by Bespalov (1984) and recalled by Bespalov et al (2010), see also Trakhtengerts and Rycroft (2008).…”

Very Low Frequency Whistler Mode Wave Events Observed Simultaneously by the Kannuslehto Station and Van Allen Probes

“…A powerful tool for the wave propagation analysis is a comparison of simultaneous measurements, best provided by a ground‐based station and a spacecraft. In such a case, it is possible to study both the temporal and spatial evolution of the waves (e.g., Martinez‐Calderon, Němec, et al., 2020; Titova et al., 2015) and to obtain important pieces of information about the wave propagation (Demekhov et al., 2021; Martinez‐Calderon et al., 2016). Moreover, statistical studies of such wave events can provide an overall picture of the favorable conditions for the wave occurrence and penetration through the ionosphere.…”

“…Moreover, statistical studies of such wave events can provide an overall picture of the favorable conditions for the wave occurrence and penetration through the ionosphere. However, it is challenging to obtain a sufficiently large data set to perform such a statistical analysis, and hence only case studies are often reported (e.g., Demekhov et al., 2017, 2021; Martinez‐Calderon, Katoh, et al., 2020; Němec et al., 2016). These case studies focus on a concrete kind of whistler mode waves, quasiperiodic emissions (Carson et al., 1965; Sato & Kokubun, 1980), and a detailed analysis of available multipoint observations.…”

“…Sometimes more instruments than two detect the same emissions, which can naturally lead to a more comprehensive study. An analysis of periodic VLF emissions detected simultaneously by the Van Allen Probes spacecraft and two ground-based stations, Kannuslehto and Lovozero, was presented by Demekhov et al (2021). An event detected in March 2019 with wave normal angles measured by Van Allen Probes below 40° for the whole duration was analyzed.…”