Excitation of Saturnian ECH Waves Within Remote Plasma Injections: Cassini Observations

Liu

Geophysical Research Letters

et al. 2023

Electron cyclotron harmonic (ECH) waves, which are responsible for the diffuse aurora, are usually observed with the strongest intensity in the first band. Here, we present observations of enhanced higher‐band ECH with wave intensities above 10−3 mV2m−2 Hz−1 by Van Allen Probes. Fully thermal simulation results indicate that these waves can be locally excited by the observed electron distributions, and higher plasma density and lower magnetic strength are favorable for generating the enhanced higher‐band ECH. The pitch angle diffusion coefficient 〈〉Dαα $\left\langle {D}_{\alpha \alpha }\right\rangle $ of the higher‐band ECH can exceed 10−5 s−1 inside the loss cone for energetic electrons (300 eV–∼15 keV), greater than the momentum diffusion coefficient 〈〉Dpp $\left\langle {D}_{pp}\right\rangle $ by ∼100 times. It is suggested that the enhanced higher‐band ECH can also efficiently scatter these electrons into the loss cone and contribute to the diffuse aurora.

Section: Calculationmentioning

confidence: 99%

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

Liu

Geophysical Research Letters

et al. 2023

Geophysical Research Letters

“…Resonant interactions with plasma waves have been recognized as an important non-adiabatic mechanism responsible for the acceleration and loss of electrons in Saturn's magnetosphere (e.g., Cao et al, 2023;Shprits et al, 2012;Tripathi et al, 2018;Woodfield et al, 2018Woodfield et al, , 2019Woodfield et al, , 2022Yu et al, 2019). Electrostatic electron cyclotron harmonic (ECH) waves, occur within bands between the harmonics (n) of electron cyclotron gyrofrequency (f ce ), and are frequently observed in the magnetospheres of Saturn and Earth (e.g., Long et al, 2021Long et al, , 2023Lou et al, 2018;Menietti et al, 2008aMenietti et al, , 2008bMenietti et al, , 2012Menietti et al, , 2017Meredith et al, 2009;Ni et al, 2011aNi et al, , 2011bNi et al, , 2012Ni et al, , 2016Ni et al, , 2017Tao et al, 2010;Zhang & Angelopoulos, 2014). ECH wave-induced pitch angle scattering of ∼100 eV-10 keV electrons has been proposed to be responsible for the generation of diffuse aurora at Earth and Saturn (Lou et al, 2021(Lou et al, , 2022Ni et al, 2011aNi et al, , 2016Tripathi et al, 2018).…”

Section: Formation Of Electron Butterfly Pitch Angle Distributions In...mentioning

confidence: 99%

Formation of Electron Butterfly Pitch Angle Distributions in Saturn's Magnetosphere Due To Scattering by Equatorial ECH Waves

Long,

Cao,

et al. 2023

Self Cite

The features of electron pitch angle distributions (PADs) often imply different physical mechanisms in planetary magnetospheres. We report a simultaneous equatorial electrostatic electron cyclotron harmonic (ECH) wave event with butterfly PADs of electrons at L ∼ 7.6–9 observed by the Cassini spacecraft. Via calculating the bounce‐averaged electron diffusion rates, we found that Saturnian ECH waves can resonate with ∼10 eV to several keV electrons at <60° pitch angles at time scales from ∼10−8 to 10−4 s−1. Simulations show that the formation of ∼100 eV to ∼1 keV electron butterfly PADs are mainly caused by the pitch angle scattering of electrons at low pitch angles (<30°) and the momentum scattering at intermediate pitch angles of ∼30°–45°, though previous studies suggested the adiabatic transport is the dominated mechanism. Additionally, our results successfully reproduce the variation of peak pitch angles (αp) and phase space density ratios between α90° and αp with L‐shell.

“…The role of plasma waves in the dynamics of energetic electrons and protons at Saturn has recently been in the spotlight. Many plasma wave modes are frequently observed in Saturn's magnetosphere, including whistler mode chorus waves (Menietti et al., 2013, 2014), auroral hiss (Menietti et al., 2019, 2020), Z ‐mode waves (Menietti et al., 2016; Ye et al., 2010), electrostatic electron cyclotron harmonic waves (Long et al., 2021, 2023; Menietti et al., 2017; Tao et al., 2010) and ion cyclotron waves (Long, Cao, et al., 2022; Russell & Blanco‐Cano, 2007; Yao et al., 2021). Resonant interactions with these waves are recognized to play an important role in the dynamics of Saturn's radiation belt particles.…”

Section: Introductionmentioning

confidence: 99%

Quantifying the Absorption Loss of Radiation Belt Energetic Particles by Saturn's Inner Moons

Wang,

Si,

Long

et al. 2023

JGR Planets

Self Cite

Saturn owns various moons embedded in the magnetosphere, which results in the absorption loss of energetic particles in Saturn's radiation belts when they encounter the body of moons. In the present study, we perform a quantitative analysis of radiation belt energetic electron and proton lifetimes due to the absorption loss by five of inner moons of Saturn (Mimas, Enceladus, Tethys, Dione, and Rhea). Our results show that the resultant averaged lifetimes of energetic particles generally range from ∼0.01 days to infinity, depending strongly on particle energy, equatorial pitch angle, and moon orbit. The averaged lifetimes of energetic electrons increase with increasing electron energy at energies of <1 MeV and peak at the resonance energy, at which the electron drift velocities approach the Kepler velocities of moons. For >1 MeV electrons, the corresponding electron lifetimes decrease as electron energy increases. The averaged lifetimes of protons decrease monotonically with increasing equatorial pitch angles and kinetic energies. We find that the absorption loss by Tethys is the strongest, with the corresponding electron lifetimes shorter than 1 day and the corresponding proton lifetimes shorter than 10 days. The moon absorption effects due to Mimas are the weakest. Our results of energetic particle lifetimes due to the absorption loss by Saturn's moons can be incorporated into future modeling efforts of the radiation belt particle dynamics at Saturn.