Lag-correlated rising tones of electron cyclotron harmonic and whistler-mode upper-band chorus waves

Abstract

Electron cyclotron harmonic (ECH) and whistler-mode chorus waves can contribute significantly to the magnetospheric dynamics. In the frequency-time spectrogram, ECH usually appears as a series of harmonic structureless bands, while chorus often exhibits successive discrete elements. Here, we present surprising observations by Van Allen Probes of lag-correlated rising tones of ECH and upper-band chorus waves. The time lags of ECH elements with respect to chorus elements range from 0.05 to 0.28 s, negatively correlated with the chorus peak amplitudes. The ECH elements seemingly emerge only when the chorus elements are sufficiently intense (peak amplitude >3 mV m−1), and their peak amplitudes are positively correlated. Our data and modeling suggest that upper-band chorus may promote the generation of ECH through rapidly precipitating the ∼keV electrons near the loss cone. This phenomenon implies that ECH and chorus may not grow independently but competitively or collaboratively gain energy from hot electrons.

Periodic Rising and Falling Tone ECH Waves From Van Allen Probes Observations

“…It is noteworthy that most of these events do not show coexistence of chorus waves, which are required for the generation of ∼1 s periodic rising and falling tone ECH waves reported by Gao et al (2018) and Gao et al (2020), indicating that new mechanisms are needed to explain the observed rising and falling tone ECH waves with a period of ∼1 min.…”

“…(2018) and Gao et al. (2020), indicating that new mechanisms are needed to explain the observed rising and falling tone ECH waves with a period of ∼1 min.…”

“…A recent study shows that ECH wave amplitude can be modulated by ultra‐low frequency (ULF) waves with a period of a few minutes through periodically modulating hot electron populations (X. J. Zhang et al., 2019), although these modulated ECH waves do not show rising or falling tone features. ECH waves also show modulations of ∼1 s period with rising or falling tone features, observed by the burst mode electric field data from Van Allen Probes (Gao et al., 2018, 2020). The ∼1 s modulation of ECH waves (or ECH sidebands) is proposed to be caused by wave‐wave interactions between chorus and ECH waves (Gao et al., 2018) or the loss cone electron distribution modulated by chorus waves via cyclotron resonance or Landau resonance (Gao et al., 2020).…”

“…ECH waves also show modulations of ∼1 s period with rising or falling tone features, observed by the burst mode electric field data from Van Allen Probes (Gao et al., 2018, 2020). The ∼1 s modulation of ECH waves (or ECH sidebands) is proposed to be caused by wave‐wave interactions between chorus and ECH waves (Gao et al., 2018) or the loss cone electron distribution modulated by chorus waves via cyclotron resonance or Landau resonance (Gao et al., 2020). Both mechanisms require the co‐existence of chorus and ECH waves.…”

Efficient Scattering Loss of Energetic Electrons by Enhanced Higher‐Band ECH Waves Observed by Van Allen Probes

“…where n i denotes the electron density of component i, θ ‖i and θ ⊥i denote parallel and perpendicular thermal velocities with respect to the background magnetic field, respectively. Δ i and β i are dimensionless characteristic parameters to describe the depth and width of the loss cone (e.g., Ashour-Abdalla & Kennel, 1978;Gao et al, 2021;Gao, Shang, et al, 2020;Summers & Thorne, 1995;Zhou et al, 2017). The parameters Δ and β are used to represent the loss-cone feature of the electron distribution, which is different from the "loss cone angle" determined by the background magnetic field configuration.…”