The high frequency limit (HFL) of the Saturnian Kilometric Radiation (SKR) can probe the deepest SKR sources, closest to Saturn's ionosphere. In this study, we determined and analyzed the SKR HFL ...throughout the entire Cassini Saturn orbital tour. The maximum frequency of the northern SKR, whose distribution peaks at ∼625 kHz, is shifted by +100 to +200 kHz from the distribution of southern SKR HFL, consistent with the magnetic field offset toward the northern hemisphere at Saturn. The uniformly observed SKR HFL in the vicinity of Saturn suggests a broad extent and beaming of the SKR source. When the observer is confined to certain locations, the rotational modulation of the SKR HFL is clearly observed. This modulation feature of the SKR HFL is statistically established and analyzed in this study. The modulation of HFL is best observed at mid‐latitudes between 10° and 40° and at almost all local times. We perform a simulation that suggests that the modulation of HFL requires the superposition of a “clock” like and a rotating source behavior. By comparing the derived HFL modulation using different longitudes with variable and fixed rotation periods, we can exclude the existence of a magnetic anomaly that was proposed in a previous study based on the Voyager data. The calculation of the least‐square periodogram confirms that the modulation observed in HFL is similar to the ones previously detected at Saturn.
Plain Language Summary
Auroral radio emission from Saturn, namely the Saturn Kilometric Radiation (SKR), is generated along high latitude magnetic field lines via the resonance between energetic electrons and a wave's electric field. The first work on the high frequency limit (HFL) of SKR dates back to 1991. Using data from the Voyager Saturn fly‐by, scientists found an asymmetry when the HFL is organized by the longitude of the Sun. Based on this asymmetry, a hypothesis about the existence of a magnetic anomaly in Saturn's magnetic field was proposed, which was a novel and breakthrough discovery at that time, but the later Cassini measurements did not confirm this magnetic anomaly. Cassini's expedition around Saturn with 13‐yr continuous measurements provided an opportunity to re‐study the HFL of SKR. The long‐term statistics allow us to exclude the magnetic anomaly hypothesis and instead attribute the asymmetry to a modulation which is introduced by an ionospheric/magnetospheric current system at Saturn. A simulation suggests that both temporal and spatial effects play a role to a certain degree. The average frequency and visibility of the HFL are also discussed. These new results provide new insights into the studies of cyclotron maser‐related radio emissions.
Key Points
The high frequency limit (HFL) of Saturn Kilometric Radiation is obtained during the 13‐yr Cassini mission
The average HFL is found to be above and below 600 kHz in the northern and southern hemisphere, respectively
A rotational modulation of HFL is verified statistically and by simulation, which excludes a magnetic field anomaly
Naturally occurring chorus emissions are a class of electromagnetic waves found in the space environments of the Earth and other magnetized planets. They play an essential role in accelerating ...high-energy electrons forming the hazardous radiation belt environment. Chorus typically occurs in two distinct frequency bands separated by a gap. The origin of this two-band structure remains a 50-year old question. Here we report, using NASA's Van Allen Probe measurements, that banded chorus waves are commonly accompanied by two separate anisotropic electron components. Using numerical simulations, we show that the initially excited single-band chorus waves alter the electron distribution immediately via Landau resonance, and suppress the electron anisotropy at medium energies. This naturally divides the electron anisotropy into a low and a high energy components which excite the upper-band and lower-band chorus waves, respectively. This mechanism may also apply to the generation of chorus waves in other magnetized planetary magnetospheres.
Whistler mode wave properties inside the plasmasphere and plumes are systematically investigated using 5‐year data from Van Allen Probes. The occurrence and intensity of whistler mode waves in the ...plasmasphere and plumes exhibit dependences on magnetic local time, L, and AE. Based on the dependence of the wave normal angle and Poynting flux direction on L shell and normalized wave frequency to electron cyclotron frequency (fce), whistler mode waves are categorized into four types. Type I: ~0.5 fce with oblique wave normal angles mostly in plumes; Type II: 0.01–0.5 fce with small wave normal angles in the outer plasmasphere or inside plumes; Type III: <0.01 fce with oblique wave normal angles mostly within the plasmasphere or plumes; Type IV: 0.05–0.5 fce with oblique wave normal angles deep inside the plasmasphere. The Poynting fluxes of Type I and II waves are mostly directed away from the equator, suggesting local amplification, whereas the Poynting fluxes of Type III and IV are directed either away from or toward the equator, and may originate from other source regions. Whistler mode waves in plumes have relatively small wave normal angles with Poynting flux mostly directed away from the equator and are associated with high electron fluxes from ~30 keV to hundreds of keV, all of which support local amplification. Whistler mode wave amplitudes in plumes can be stronger than typical plasmaspheric hiss, particularly during active times. Our results provide critical insights into understanding whistler mode wave generation inside the plasmasphere and plumes.
Key Points
Whistler mode waves are statistically analyzed both inside the plasmasphere and in the plumes based on Van Allen Probes observations
The occurrence rate and amplitudes of whistler mode waves inside the plasmasphere and plumes show dependence on L, MLT, and geomagnetic activity
The majority of whistler mode waves in plumes are suggested to be locally amplified due to energetic electron injection
Abstract
The spatial distribution and polarization of Saturn narrowband (NB) emissions have been studied by using Cassini Radio and Plasma Wave Sciences data and goniopolarimetric data obtained ...through an inversion algorithm with a preset source located at the center of Saturn. From 2004 January 1 to 2017 September 12, NB emissions were selected automatically by a computer program and rechecked manually. The spatial distribution shows a preference for high latitude and intensity peaks in the region within 6 Saturn radii (
R
s
) for both 5 and 20 kHz NB emissions. 5 kHz NB emissions also show a local time preference roughly in the 18:00−22:00 sector. The Enceladus plasma torus makes it difficult for NB emissions to propagate to the low latitude regions outside the plasma torus. The extent of the low latitude regions where 5 and 20 kHz NB emissions were never observed is consistent with the corresponding plasma torus density contour in the meridional plane. 20 kHz NB emissions show a high circular polarization while 5 kHz NB emissions are less circularly polarized with
V
<
0.6
for majority of the cases. And cases of 5 kHz NB emissions with high circular polarization are more frequently observed at high latitude especially at the northern and southern edges of the Enceladus plasma torus.
Our knowledge about the fine structure of lightning processes at Jupiter was substantially limited by the time resolution of previous measurements. Recent observations of the Juno mission revealed ...electromagnetic signals of Jovian rapid whistlers at a cadence of a few lightning discharges per second, comparable to observations of return strokes at Earth. The duration of these discharges was below a few milliseconds and below one millisecond in the case of Jovian dispersed pulses, which were also discovered by Juno. However, it was still uncertain if Jovian lightning processes have the fine structure of steps corresponding to phenomena known from thunderstorms at Earth. Here we show results collected by the Juno Waves instrument during 5 years of measurements at 125-microsecond resolution. We identify radio pulses with typical time separations of one millisecond, which suggest step-like extensions of lightning channels and indicate that Jovian lightning initiation processes are similar to the initiation of intracloud lightning at Earth.
Measurements made by the Galileo Energetic Particles Detector and the plasma wave/radio instrument are analyzed to establish relationships between dynamic processes observed independently in the ...distant and the inner Jovian disk (at 80–120 Jovian radii (RJ) and 10–25 RJ, respectively). It is first shown that global magnetospheric disturbances identified from the radio emissions (the “energetic events” are well correlated with reconnection/reconfiguration events observed at the outer edge of the disk. Then, considering all Galileo perijoves, it is also demonstrated that the energetic events occurring as Galileo was at less than ~25 RJ are systematically associated with new injections of energetic particles seen from ~10 to 25 RJ. This demonstrates that major disturbances commonly affect the whole magnetodisk, from 10 to 80–120 RJ. Overall, their phenomenology involves simultaneous auroral activation, formation of new sources of radio emission (narrow‐band kilometric radiation) and particle injections in the Io torus, magnetic reconfigurations, and radial flow bursts in the distant disk, over time scale of a few hours.
Key Points
Link between particle injections and large‐scale disk disturbances at Jupiter
Global‐scale dynamics of Jovian magnetodisk
Combined analysis of EPD and PWS Galileo data
Clear first harmonic emissions of Saturn Kilometric Radiation are discovered during the Cassini Grand Finale orbits. Both ordinary (O) and extraordinary (X) mode fundamental emissions accompanied by ...X mode harmonics are observed. Analysis shows that the frequency ratio between the fundamental and harmonic emissions is 2.01 ± 0.08, and the harmonic emissions display weaker intensities than the fundamental, by 30–40 dB for the X‐X (fundamental‐harmonic) type harmonic and 10–30 dB for the O‐X type harmonic. The intensity relations between the two types of harmonics, that is, O‐X and X‐X show different patterns that we attribute to different conditions of emission at the source. Direction‐finding results shows that the fundamental and harmonic emissions are plausibly generated in the same source region. In agreement with previous studies at Earth, the generation of the two types of harmonics can be attributed to the cyclotron maser instability operating with different plasma density and electron energy distributions in the source region.
Plain Language Summary
Auroral radio emission from Saturn, namely the Saturn Kilometric Radiation (SKR), is generated along high latitude magnetic field lines via the resonance between energetic electrons and wave's electric field. This resonance mechanism is called the cyclotron maser instability. Theoretical and observational studies of the same emission at Earth, called the Auroral Kilometric Radiation (AKR), have shown that the emissions near the harmonic frequency could be generated simultaneously with the fundamental AKR emission. However, no study of SKR harmonic emissions has been reported to date. This paper focuses on searching for the possible harmonic emissions of SKR by using the data measured by the radio experiment onboard the Cassini Spacecraft. Several clear harmonic emissions are found, and their characteristics are analyzed. Based on the circular polarization, two different types of harmonic emissions are identified. We find that the harmonic emissions have a frequency two times that of the fundamental emissions, but a much weaker intensity. The analysis of the source location of simultaneous fundamental and harmonic emissions suggests that they originate from the same source region. These new features of SKR observed in Saturn's magnetosphere provide new insights to the studies of cyclotron maser‐related radio emissions.
Key Points
X mode first harmonics of Saturn Kilometric Radiation (SKR) associated with X/O mode fundamental emissions are identified
SKR 1st harmonic occurs at frequencies very close to twice the fundamental emissions and, display weaker intensities
Direction‐finding analysis is consistent with harmonic and fundamental emissions from the same source but affected by large uncertainties
We present a statistical survey of the latitudinal structure of the fast magnetosonic wave mode detected by the Van Allen Probes spanning the time interval of 21 September 2012 to 1 August 2014. We ...show that statistically, the latitudinal occurrence of the wave frequency (f) normalized by the local proton cyclotron frequency (f(sub cP)) has a distinct funnel-shaped appearance in latitude about the magnetic equator similar to that found in case studies. By comparing the observed E/B ratios with the model E/B ratio, using the observed plasma density and background magnetic field magnitude as input to the model E/B ratio, we show that this mode is consistent with the extra-ordinary (whistler) mode at wave normal angles (theta(sub k)) near 90 deg. Performing polarization analysis on synthetic waveforms composed from a superposition of extra-ordinary mode plane waves with theta(sub k) randomly chosen between 87 and 90 deg, we show that the uncertainty in the derived wave normal is substantially broadened, with a tail extending down to theta(sub k) of 60 deg, suggesting that another approach is necessary to estimate the true distribution of theta(sub k). We find that the histograms of the synthetically derived ellipticities and theta(sub k) are consistent with the observations of ellipticities and theta(sub k) derived using polarization analysis.We make estimates of the median equatorial theta(sub k) by comparing observed and model ray tracing frequency-dependent probability occurrence with latitude and give preliminary frequency dependent estimates of the equatorial theta(sub k) distribution around noon and 4 R(sub E), with the median of approximately 4 to 7 deg from 90 deg at f/f(sub cP) = 2 and dropping to approximately 0.5 deg from 90 deg at f/f(sub cP) = 30. The occurrence of waves in this mode peaks around noon near the equator at all radial distances, and we find that the overall intensity of these waves increases with AE*, similar to findings of other studies.
At Jupiter, tail reconnection is thought to be driven by an internal mass loading and release process called the Vasyliunas cycle. Galileo data have shown hundreds of reconnection events occurring in ...Jupiter's magnetotail. Here we present a survey of reconnection events observed by Juno during its first 16 orbits of Jupiter (July 2016-October 2018). The events are identified using Juno magnetic field data, which facilitates comparison to the Vogt et al. (2010, https://doi.org/10.1029/2009JA015098) survey of reconnection events from Galileo magnetometer data, but we present data from Juno's other particle and fields instruments for context. We searched for field dipolarizations or reversals and found 232 reconnection events in the Juno data, most of which featured an increase in |
|, the magnetic field meridional component, by a factor of 3 over background values. We found that most properties of the Juno reconnection events, like their spatial distribution and duration, are comparable to Galileo, including the presence of a ~3-day quasi-periodicity in the recurrence of Juno tail reconnection events and in Juno JEDI, JADE, and Waves data. However, unlike with Galileo we were unable to clearly define a statistical x-line separating planetward and tailward Juno events. A preliminary analysis of plasma velocities during five magnetic field reconnection events showed that the events were accompanied by fast radial flows, confirming our interpretation of these magnetic signatures as reconnection events. We anticipate that a future survey covering other Juno datasets will provide additional insight into the nature of tail reconnection at Jupiter.
Suprathermal electrons (~0.1–10 keV) in the inner magnetosphere are usually observed in a 90°‐peaked pitch angle distribution, formed due to the conservation of the first and second adiabatic ...invariants as they are transported from the plasma sheet. We report a peculiar field‐aligned suprathermal electron (FASE) distribution measured by Van Allen Probes, where parallel fluxes are 1 order of magnitude higher than perpendicular fluxes. Those FASEs are found to be closely correlated with large‐amplitude hiss waves and are observed around the Landau resonant energies. We demonstrate, using quasilinear diffusion simulations, that hiss waves can rapidly accelerate suprathermal electrons through Landau resonance and create the observed FASE population. The proposed mechanism potentially has broad implications for suprathermal electron dynamics as well as whistler mode waves in the Earth's magnetosphere and has been demonstrated in the Jovian magnetosphere.
Plain Language Summary
Hiss waves are structureless and incoherent “hissy” emissions found in the magnetized near‐Earth space, typically in a frequency range from 0.1 to 2 kHz. Hiss waves have traditionally been treated as an energetic electron removal mechanism, because they can precipitate energetic electrons through resonant interactions and cause electron loss into the atmosphere. Here we show, by presenting observations from NASA's Van Allen Probes, that intense hiss waves are accompanied by enhancements of field‐aligned suprathermal electrons. We propose that hiss waves can accelerate suprathermal electrons as they travel at the same speed in the direction along the magnetic field. Computer simulations successfully reproduce the rapid enhancement of field‐aligned suprathermal electrons under the impact of hiss waves, with detailed features similar to observations.
Key Points
Pronounced field‐aligned suprathermal electron enhancements are observed in correlation with intense hiss waves
Numerical simulations reproduce field‐aligned electron distributions similar to observations
Intense hiss waves can accelerate field‐aligned suprathermal electrons via Landau resonance on a timescale of several minutes
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