Acceleration of Electrons by Whistler‐Mode Hiss Waves at Saturn

Abstract

The importance of whistler-mode chorus in locally accelerating electrons is well known at the Earth (Horne et al., 2005;Reeves et al., 2013) and has recently been found to be important at Jupiter and Saturn (Horne et al., 2008;Woodfield et al., 2014Woodfield et al., , 2019. Whilst chorus is a plasma wave observed in discrete rising or falling tones observed in two frequency bands separated by half the electron gyrofrequency (Li et al., 2011), whistler-mode hiss is a broadband emission at the lower end of the whistler-mode frequency range (10 Hz to several kilohertz; Li et al., 2015) which occurs in the high density plasmasphere (Meredith et al., 2018;Hartley et al., 2018). At the Earth hiss waves are well known for scattering electrons from the radiation belts into the atmosphere and are thought to be a source of diffuse aurora (Thorne, 2010;Li et al., 2015). Since hiss does not accelerate electrons in the terrestrial environment it has never been considered as a candidate for acceleration in planetary magnetospheres. However, the presence of very low density regions at Saturn coincident with observed hiss waves are highly likely to produce significant electron acceleration in a similar manner to chorus waves (Woodfield et al., 2019). This work assesses the effect of hiss on electrons at Saturn for the first time.Hiss is observed at Saturn (Menietti et al., 2019(Menietti et al., , 2020 in the form of diffuse emissions and also as the funnel shaped auroral hiss. Auroral hiss can be found near the footprints of field aligned currents, plasma injections, reconnected field lines, and field lines that connect to the moons, for example,

Space Plasma Physics: A Review

“…Plasmaspheric hiss has been observed at the magnetized planets. At Saturn, studies show that there are regions where the plasma density is sufficiently small that hiss can actually accelerate electrons and contribute to the Saturnian radiation belt [287].…”

“…At Jupiter, the chorus has been suggested as providing the missing acceleration needed to produce 50-MeV electrons that emit synchrotron radiation from the planet [332]. At Saturn, similar wave acceleration processes involving chorus [333], Z mode waves [334], and hiss [287] are now suggested to play a key role in the origin of the Saturnian radiation belt.…”

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

“…Besides ECH waves, other wave modes including whistler‐mode chorus waves (Menietti et al., 2013), hiss waves (Menietti et al., 2019; Woodfield et al., 2022), Z‐mode waves (Menietti et al., 2016; Wu et al., 2021; Ye et al., 2009, 2010), and ion cyclotron waves (e.g., Leisner et al., 2005, 2006, 2011; Long et al., 2022; Russell et al., 2006) are also frequently observed in Saturn's magnetosphere. These plasma waves are recognized to significantly impact the local acceleration and loss processes of Saturn's radiation belt electrons by wave‐particle interactions (e.g., Cao et al., 2023; Gu et al., 2013; Kollmann et al., 2018; Menietti et al., 2012; Woodfield et al., 2018, 2019, 2022; Yu et al., 2019). It is also reported that ECH waves could interact with electrons in the energy range from ∼10 eV to several keV, leading to the generation of diffuse aurora (Tripathi et al., 2018).…”

Resonant Scattering of Radiation Belt Electrons at Saturn by Ion Cyclotron Waves

“…A large variety of physical processes have been proposed to be responsible for the dynamic evolution of radiation belt electrons at Saturn (Kollmann et al., 2017; Roussos & Kollmann, 2021; Shprits et al., 2012), among which, resonant interactions with plasma waves play an important role (Yao et al., 2021). Various plasma wave modes, for example, whistler‐mode chorus waves (Menietti et al., 2013, 2014; Woodfield et al., 2019; Yu et al., 2019a), auroral hiss (Woodfield et al., 2022) and Z‐mode waves (Gu et al., 2013; Menietti et al., 2016; Woodfield et al., 2018; Ye et al., 2010), contribute significantly to the local acceleration and loss of Saturn's radiation belt electrons. Ion cyclotron waves, which are frequently observed inside the region of Saturn's radiation belt with strong averaged amplitudes, are likewise expected to resonate effectively with radiation belt electrons.…”