J. D. Menietti

Latitudinal density dependence of magnetic field lines inferred from Polar plasma wave data

Using observations of the electron density ne based on measurement of the upper hybrid resonance frequency by the Polar spacecraft Plasma Wave Instrument, we have examined the radial density dependence along field lines in the outer plasmasphere and the near plasma trough. Our technique depends on the fact that Polar crosses particular L values at two different points with different radial distance R. Sampled L values range from 2.5 to 6.3. In our plasmaspheric data set (ne ≥ 100 cm−3) we find that, on average, ne is flat along field lines from the equator up to the latitudes sampled by Polar (R ≳ 2 RE). In the plasma trough data set (ne < 100 cm−3) there is, on average, a mild radial dependence ne ∝ R−1.7.

Cusp energetic ions: A bow shock source

Recent interpretations of cusp energetic ions observed by the POLAR spacecraft have suggested a new energization process in the cusp [Chen et al., 1997; 1998]. Simultaneous enhancement of H+, He+2, and O>+2 fluxes indicates that they are of solar wind origin. In the present study, we examine H+ and He+2 energy spectra from 20 eV to several 100 keV measured by the Hydra, Toroidal Imaging Mass-Angle Spectrograph (TIMAS), and Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) on POLAR. The combined spectrum for each species is shown to be continuous with a thermal distribution below 10 keV/e and an energetic component above 20 keV/e. Energetic ions with comparable fluxes and a similar spectral shape are commonly observed downstream from the Earths quasi-parallel (Q∥) bow shock. In addition to the similarity in the ion spectra, electric and magnetic field noise and turbulence detected in the cusp by the Plasma Wave Instrument (PWI) and Magnetic Field Experiment (MFE) onboard POLAR are similar to the previously reported observations at the bow shock. The waves appear to be coincidental to the cusp energetic ions rather than causal. We suggest that these ions are not accelerated locally in the cusp. Rather, they are accelerated at the Q∥ bow shock and enter the cusp along open magnetic field lines connecting both regions.

Plasma waves observed in the cusp turbulent boundary layer: An analysis of high time resolution wave and particle measurements from the Polar spacecraft

The boundary layer located in the cusp and adjacent to the magnetopause is a region that is quite turbulent and abundant with waves. The Polar spacecrafts orbit and sophisticated instrumentation are ideal for studying this region of space. Our analysis of the waveform data obtained in this turbulent boundary layer shows broadband magnetic noise extending up to a few kilohertz (but less than the electron cyclotron frequency); sinusoidal bursts (a few tenths of a second) of whistler mode waves at around a few tens of hertz, a few hundreds of hertz, and just below the electron cyclotron frequency; and bipolar pulses, interpreted as electron phase-space holes. In addition, bursts of electron cyclotron harmonic waves are occasionally observed with magnetic components. We show evidence of broadband electrostatic bursts covering a range of ∼3 to ∼25 kHz (near but less than the plasma frequency) occurring in packets modulated at the frequency of some of the whistler mode waves. On the basis of high time resolution particle data from the Polar HYDRA instrument, we show that these bursts are consistent with generation by the resistive medium instability. The most likely source of the whistler mode waves is the magnetic reconnection site closest to the spacecraft, since the waves are observed propagating both toward and away from the Earth, are bursty, which is often the case with reconnection, and do not fit on the theoretical cold plasma dispersion relation curve.

Observations of chorus at Saturn using the Cassini Radio and Plasma Wave Science instrument

[1] Observations at Saturn of whistler mode chorus emissions have been obtained by the Cassini Radio and Plasma Wave Science instrument. Data from the first 45 orbits are analyzed, and the characteristics of the chorus emissions are discussed. Wave normal and Poynting vector measurements from the five-channel waveform receiver are used to examine the propagation characteristics of the chorus, and high-resolution measurements from the wideband receiver are used to examine the fine structure. At Saturn, two different types of chorus are detected. The most common observations are of chorus propagating away from Saturn’s magnetic equator, suggesting a source near the magnetic equator. This chorus is usually detected for many hours, is only observed below half the electron cyclotron frequency, and occurs primarily from L shells of about 5 to 8, and the occurrence of the emission shows no obvious correlation with Saturn magnetic latitude, longitude, or local time. The high-resolution measurements show that the fine structure of this chorus typically consists of larger time scale features (many seconds to minutes) than detected at the Earth (<1 s). The second region of chorus detected at Saturn is in association with local plasma injections. For many of the plasma injection events, chorus emissions are detected both above and below half the electron cyclotron frequency, with a gap in the emission at half the cyclotron frequency. This chorus also shows fine structure at smaller time scales (<1 s to a few seconds), and the overall structure of this chorus appears more similar to chorus detected at the Earth.

Gyroresonant interactions between the radiation belt electrons and whistler mode chorus waves in the radiation environments of Earth, Jupiter, and Saturn: A comparative study

In the current study we perform a comparative analysis of the gyroresonant interactions of whistler mode waves with radiation belt electrons in the magnetospheres of Earth, Jupiter, and Saturn. Our primary goal is to evaluate the effect of resonant wave-particle interactions with chorus waves and determine whether chorus waves can produce net acceleration or net loss of radiation belt electrons on the outer planets. The ratio of plasma frequency to gyrofrequency is a key parameter that determines the efficiency of the pitch angle and energy resonant scattering. We present a comparison of statistical maps of the ratio of plasma frequency to gyrofrequency for Jupiter, Saturn and Earth in terms of radial distance and latitude. Preliminary maps of the plasma frequency to gyrofrequency ratio and 2D simulations of pitch angle and energy diffusion using the Versatile Electron Radiation Belt (VERB) indicate that the Kronian plasma environment is not likely to support as efficient gyroresonant interactions with whistler mode chorus waves as in the Terrestrial or Jovian environments. Inefficiency of the local acceleration by whistler mode waves in the Kronian environment raises important questions about the origin of the relativistic electrons in the Saturns radiation belts. Two-dimensional diffusive simulations of local acceleration and loss to the atmosphere using the VERB code confirm previous suggestions that the acceleration of electrons may be very efficient in the outer radiation belt of Jupiter. However, sensitivity simulations also show that the result of the competition between acceleration and loss in the Jupiters magnetosphere strongly depends on the currently unknown latitudinal distribution of chorus waves that will be provided by the upcoming Juno mission. If waves extend to high latitudes, it is likely that the loss rates due to whistler mode waves will exceed energization rates.

Auroral electron distributions within and close to the Saturn kilometric radiation source region

On 17 October 2008, Cassini observed for the first time the electron populations associated with the crossing of a Saturn kilometric radiation source region and its surroundings. These observations allow for the first time the constraint and quantification of the high-latitude acceleration processes, the current systems, and the origin of the low-frequency electromagnetic waves. Enhanced fluxes of field-aligned energetic electrons were measured by the Cassini electron plasma spectrometer in conjunction with unusual intense field-aligned current systems identified using the magnetometer instrument. In the region where downward field-aligned currents were measured, electron data show evidence of two types of upward accelerated electron beams: a broadband energetic (1-100 keV) electron population that is observed throughout the region and a narrow-banded (0.1-1 keV) electron population that is observed sporadically. In the regions where the magnetic field signatures showed evidence for upward field-aligned currents, we observe electron loss cone distributions and some evidence of shell-like distributions. Such nonthermal electron populations are commonly known as a potential free energy source to drive plasma instabilities. In the downward current region, the low-energy and energetic beams are likely the source of the very low frequency emissions. In the upward current region, the shell distribution is identified as a potential source for Saturn kilometric radiation generation via the cyclotron maser instability.

The origin of Jupiter's outer radiation belt

The intense inner radiation belt at Jupiter (>50 MeV at 1.5 RJ [1]) is generally accepted to be created by radial diffusion of electrons from further away from the planet [2]. However, this requires a source with energies that exceed 1 MeV outside the orbit of the moon Io at 5.9 RJ, which has never been explained satisfactorily. Here we test the hypothesis that this source population could be formed from a very soft energy spectrum, by particle injection processes and resonant electron acceleration via whistler mode chorus waves. Using the first simulations at Jupiter combining wave particle interactions and radial diffusion, we calculate the change in the electron flux between 6.5 and 15 RJ with the BAS Radiation Belt Model starting from a very soft spectrum. The electron flux after 30 days at 100 keV and 1 MeV lies very close to the Galileo Interim Radiation Electron (GIRE) model spectrum [3] after 1 and 10 days respectively. The primary driver for the increase in the flux is cyclotron resonant acceleration by chorus waves, which causes an increase in the flux by a factor of 106 from the soft spectrum. The radial diffusion does not affect the magnitude of this increase to any great extent, but acts to smooth out variations in the phase space density with L. The variation of chorus wave power with radial distance from Jupiter results in a peak in phase space density such that inside L≈9 radial diffusion transports electrons towards Jupiter, but outside L≈9 radial diffusion acts away from the planet. The results are insensitive to the softness of the initial energy spectrum but do depend on the value of the flux at the minimum energy boundary. The overall shape of the flux after 30 days remains very similar but the magnitude of the flux at all energies is dependent on the flux at the minimum energy boundary. We show that individual injections of particles at a few tens of keV would have a cumulative effect on the increases in flux at energies of a few MeV by varying the flux at the minimum energy boundary in a time dependent manner based on the injections reported in [4]. We conclude by suggesting that the source population for the inner radiation belt at Jupiter could indeed by formed by wave-particle interactions.

Electron beams as the source of whistler‐mode auroral hiss at Saturn

Over the last three years, the Cassini spacecraft has been in a series of high inclination orbits, allowing investigation and measurements of Saturnian auroral phenomena. During this time, the Radio and Plasma Wave Science (RPWS) Investigation on Cassini detected low frequency whistler mode emissions propagating upward along the auroral field lines, much like terrestrial auroral hiss. Comparisons of RPWS data with Cassini Plasma Spectrometer (CAPS) plasma measurements during a high-latitude pass on 17 October 2008, show that intense upward moving electron beams with energies of a few hundred eV were associated with auroral hiss emissions. In this paper we show that these beams produce large growth rates for whistler-mode waves propagating along the resonance cone, similar to the generation of auroral hiss at Earth. Citation: Kopf, A. J., et al. (2010), Electron beams as the source of whistler-mode auroral hiss at Saturn, Geophys. Res. Lett., 37, L09102, doi: 10.1029/2010GL042980.

Influence of Saturnian moons on Saturn kilometric radiation

[1] Similar to past studies at Jupiter, we conduct an investigation of possible associations of radio emission occurrence probability with the orbital phases of Saturnian moons. We use a new definition of the Saturn longitude system (SLS) based on the results of Kurth et al. (2007). This paper presents results of our findings to date, sampling a large portion of the Radio and Plasma Wave Science (RPWS) instrument data over the frequency range 12 kHz < f < 16 MHz. We also investigate the intensity of Saturn kilometric radiation (SKR) as a function of the local time and subsolar longitude. When Titan is near a local time of midnight, there is a significant increase in the occurrence probability of SKR and a diminution of SKR when Titan is near local noon and afternoon. This indicates Titan may play a role in the process of substorm generation at Saturn perhaps due to its large plasma wake. Rhea displays a marginal orbital phase ‘‘control’’ of a subset of the SKR. In the past there have been conflicting reports of the absence of SKR emission at particular local times of Dione. We find no long-term, statistical influence of Dione on SKR occurrence probability. In addition, we find no significant statistical influence of Enceladus or Tethys on SKR occurrence probability.

Numerical simulations of bursty radio emissions from planetary magnetospheres

The Voyager spacecraft observed both smooth and bursty radio emissions from Uranus and Neptune. These emissions are known to be freely propagating primarily in the right-hand circularly polarized (RCP) mode with the bursty emissions having burst periods as short as a few tenths of a second and the smooth emissions being observed over periods of a few hours. While the smooth emission is probably due to the electron cyclotron maser instability, some other processes must be at work to produce the bursty emissions. It is proposed that one important difference in mechanisms is that the smooth emissions are associated with continuous injection of electrons while the bursty emissions are associated with impulsive injection. In the latter case, the electron distribution can develop a beam feature with a temperature anisotropy. It is shown via one-dimensional (three velocity) relativistic particle simulations that while the beam may be initially unstable to an electrostatic instability, this instability quickly saturates and eventually the beam is unstable to a strong electromagnetic beam instability which utilizes the temperature anisotropy as free energy; a beam feature is not explicitly needed for the growth of this electromagnetic instability. The radiation generated by the instability is able to convert particle energy to wave energy at similar levels as the maser instability. For ωpe/Ωe ≳ 1, most of the wave energy is LCP and is trapped. However, for ωpe/Ωe ≲ 1 the dominant mode is RCP. The induced waves are in the form of modified whistlers when the beam speed is low and ωpe/Ωe ≃ 1 but are in the freely propagating x mode branch when ωpe/Ωe ≃ 0.4. The amount of radiation with frequencies above the local x mode cutoff increases with beam speed. It is also postulated that some of the radiation generated below the local x mode cutoff may also be able to escape the plasma and be detected remotely via mode conversion between regions where field-aligned currents produce local perturbations in the magnetic field.