Cassini Observation of Relativistic Electron Butterfly Distributions in Saturn’s Inner Radiation Belts: Evidence for Acceleration by Local Processes

Yuan, Chenfeng; Roussos, E.; Wei, Yong; Krupp, N.; Sun, Yixin; Hao, Yixin

doi:10.1029/2021gl092690

Cited by 8 publications

(14 citation statements)

References 38 publications

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“…In the present study, we will investigate if this transition also exists for >1 MeV electron PADs. A similar regime transition of electron PADs was observed by Cassini at Saturn, with electron PADs mostly field‐aligned or isotropic outward of M = 11 and pancake inward of this limit, but with butterfly distributions also present at M ∼ 8–11 and M ∼ 3 (Carbary et al., 2011; Clark et al., 2014; Yuan et al., 2021). Finally, Katoh et al.…”

Section: State Of the Art Of Energetic Electron Pads In The Jovian Ma...supporting

confidence: 63%

Pitch Angle Distribution of MeV Electrons in the Magnetosphere of Jupiter

Nénon

Nénon²,

Kollmann

et al. 2022

JGR Space Physics

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The magnetosphere of Jupiter harbors the most extreme fluxes of MeV electrons in the solar system and therefore provides a testbed of choice to understand the origin, transport, acceleration, and loss of energetic electrons in planetary magnetospheres. Along this objective, the Pitch Angle Distribution (PAD) of energetic electrons may reveal signatures of the dominant physical processes. Here, we analyze for the first time the PAD of MeV electrons observed by the Galileo‐Energetic Particle Detector (EPD) experiment in orbit around Jupiter from 1995 to 2003. We find that the MeV electron PADs observed by the integral channels of EPD appear relatively isotropic with a flux anisotropy lower than a factor of 3. Due to the relatively large angular apertures of the EPD telescopes, the actual anisotropy may be larger than the observed one. The fine anisotropy observed by Galileo‐EPD reveals persistent pancake distributions at the M‐shell of M = 9. Outward of this distance, at M = 15 and M = 20–60, MeV electron distributions have pancake, isotropic, and scattered beam field‐aligned distributions. The scattered beam distributions can either be evidence of outward adiabatic transport or may suggest that high‐latitude auroral acceleration can transiently supply as much trapped MeV electrons to the middle magnetosphere as the inward adiabatic transport of electrons from an outer equatorial reservoir.

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Section: State Of the Art Of Energetic Electron Pads In The Jovian Ma...supporting

confidence: 63%

Pitch Angle Distribution of MeV Electrons in the Magnetosphere of Jupiter

Nénon

Nénon²,

Kollmann

et al. 2022

JGR Space Physics

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“…This raises the question, if radial transport episodes are intense enough to occasionally fill the moon corridors (in the L-shell profiles of ultrarelativistic electrons like E7) at low latitudes, why do we not see that happening at high latitudes, during the proximal orbits? Yuan et al (2021) found that the same energetic electron measurements discussed here, including the 7-20 MeV ones measured by E7, have butterfly equatorial pitch angle distributions (PADs) outward of the main rings, peaking at an equatorial pitch angle around 55°, whereas proton PADs are peaked at 90°. They additionally showed that fluxes at equatorial pitch angles of ∼15°, i.e., the maximum value that could be measured at Cassiniʼs high-latitude proximal orbits in the L > 2.27 radiation belts, are more than 20 times below the peak values at ∼55°, indicating that the E7 signal falls rapidly with latitude at those distances.…”

Section: Grand Finale Orbitssupporting

confidence: 68%

The Electric Field outward of Saturn's Main Rings

Paranicas¹,

Roussos

Dialynas

et al. 2022

ApJ

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Cassini data are consistent with a global electric field in Saturn's magnetosphere that points approximately antisunward. The inner radial extent of this field was initially established using Saturn orbit insertion data but measurements of ultrarelativistic electrons from that pass cast some doubt on whether the electric field reaches all the way to the A ring. It was not until the so-called ring-grazing and proximal orbits near the end of the mission in 2017 that relevant data were again obtained on magnetic field lines that connect to the region just outward of the main rings. Here we report on the energetic charged particle data during those orbits, showing that electron observations at a wide range of energies are consistent with an electric field that influences charged particle drift paths near the outer edge of the A ring. We include a very detailed analysis of Cassini's ultrarelativistic electron measurements (channel E7 in the text) and argue they provide no information about the electric field. This result further strengthens the case of several studies that have used the presence of the electric field to explain signatures of acceleration in the data.

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“…At L‐shells close to the planet it has recently been demonstrated that butterfly PADs are observed inside of L = 3.5 at MeV energies (Yuan et al., 2021 ). This is in line with predictions from previous analysis of Z‐mode and chorus interactions with electrons (Woodfield et al., 2018 , 2019 ).…”

Section: Discussionmentioning

confidence: 99%

“…Yuan et al. ( 2021 ) show that the electron MeV PADs just inside of the orbit of Enceladus (L = 3.9) are isotropic or pancake shaped but then rapidly become butterfly PADs inside of approximately 3.5 R S . This is the opposite behavior we would expect from both radial diffusion and rapid adiabatic transport further indicating the importance of the wave‐particle interactions inside of 3.5 R S .…”

Section: Discussionmentioning

confidence: 99%

“…Both these processes will increase the equatorial pitch angle of the electrons as they are transported inwards but only by a few degrees at constant first and second adiabatic invariants between 4 and 2.5R S (Schulz & Lanzerotti, 1974). Yuan et al (2021) show that the electron MeV PADs just inside of the orbit of Enceladus (L = 3.9) are isotropic or pancake shaped but then rapidly become butterfly PADs inside of approximately 3.5R S . This is the opposite behavior we would expect from both radial diffusion and rapid adiabatic transport further indicating the importance of the wave-particle interactions inside of 3.5R S .…”

Section: Between the Main Rings At 227r S And 4r Smentioning

confidence: 95%

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Acceleration of Electrons by Whistler‐Mode Hiss Waves at Saturn

Woodfield

Glauert

Menietti

et al. 2022

Geophysical Research Letters

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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,

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Cassini Observation of Relativistic Electron Butterfly Distributions in Saturn’s Inner Radiation Belts: Evidence for Acceleration by Local Processes

Cited by 8 publications

References 38 publications

Pitch Angle Distribution of MeV Electrons in the Magnetosphere of Jupiter

Pitch Angle Distribution of MeV Electrons in the Magnetosphere of Jupiter

The Electric Field outward of Saturn's Main Rings

Acceleration of Electrons by Whistler‐Mode Hiss Waves at Saturn

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