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
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
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.
The plasma science (PLS) Instrument on the Galileo spacecraft (orbiting Jupiter from December 1995 to September 2003) measured properties of the ions that were trapped in the magnetic field. The PLS ...data provide a survey of the plasma properties between approx. 5 and 30 Jupiter radii R(sub J) in the equatorial region. We present plasma properties derived via two analysis methods: numerical moments and forward modeling. We find that the density decreases with radial distance by nearly 5 orders of magnitude from approx. 2 to 3000 cm(exp.-3) at 6R(sub j) to approx. 0.05cm(sub -3) at 30 R(sub j). The density profile did not show major changes from orbit to orbit, suggesting that the plasma production and transport remained constant within about a factor of 2. The radial profile of ion temperature increased with distance which implied that contrary to the concept of adiabatic cooling on expansion, the plasma heats up as it expands out from Io's orbit (where TI is approx.60-80 eV) at approx. 6R(sub j) to a few keV at 30R(sub j).There does not seem to be a long-term, systematic variation in ion temperature with either local time or longitude. This latter finding differs from earlier analysis of Galileo PLS data from a selection of orbits. Further examination of all data from all Galileo orbits suggests that System Ill variations are transitory on timescales of weeks, consistent with the modeling of Cassini Ultraviolet Imaging Spectrograph observations. The plasma flow is dominated by azimuthal flow that is between 80% and 100% of corotation out to 25 R(sub j).
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
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.
The Radiation Belt Storm Probes (RBSP) and the Arase satellites have different inclinations and sometimes they fly both near the equator and off the equator on the same magnetic field line ...simultaneously. Such conjunction events give us opportunities to compare the electron density at different latitudes. In this study, we analyzed the plasma waves observed by Arase and RBSP during the three conjunction events during and after the September 7, 2017 storm. The electron number density at the satellite positions was estimated from frequencies of the Upper Hybrid Resonance emissions obtained by the High Frequency Analyzer of the Plasma Wave Experiment onboard the Arase and the Waves instrument onboard the RBSP, respectively. During the three conjunction events, the satellites passed through the plume, inner trough (the narrow region with low electron density between the main body of the plasmasphere and the plume), plasmatrough with variable electron density, and partially refilled plasmasphere. The power‐law index m for the inner trough and plume was inferred to be 4–7 and ∼0, respectively. This is interpreted to mean that the trough was close to collisionless and the plume was relatively near diffusive equilibrium. In the plasmatrough with the varying density, both the high‐density and low‐density regions had m ∼ 0. The low‐density portion of this region may have a different origin from the inner trough, because of the different m indices. For the partially refilled plasmasphere in the storm recovery phase, the power‐law index m showed negative values, meaning that the density in the equatorial plane was higher than at higher latitudes.
Plain Language Summary
The plasmasphere is a region filled with cold, dense ionized gas in geospace. The ionized gas mainly consists of protons, helium ions, oxygen ions, and electrons, which come from Earth's ionosphere and fill in magnetic flux tubes. The density distribution of the ionized gas along the flux tube provides important information to understand how the ions and electrons have been supplied from the ionosphere. Many satellites fly in the equatorial plane; hence, they do not provide information on the electron density along the field. The Radiation Belt Storm Probes and the Arase satellites have different inclinations and sometimes they simultaneously fly near the equator and off the equator on the same magnetic field line. Using electron densities observed by these satellites during the September 7, 2017 storm, we successfully estimated the electron density distribution along the field lines inside the partially refilled plasmasphere, outside of the plasmasphere, and in the tail‐like structure called a plume.
Key Points
Using in situ measurements of the electron density from Arase and Radiation Belt Storm Probes, we estimated the density distribution along the magnetic field lines
The power‐law index of the electron density distribution was 4∼7, ∼0, and −2∼−1 for the trough, plume, and partially refilled plasmasphere
This is the first estimation of the power‐law index using the data from different spacecraft projects
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