We take advantage of a partial and a total occultation of the jovian radio sources observed during the flyby of Io to characterize the plasma environment of this moon. We show that it presents a strong upstream / downstream asymmetry. A dense plasma structure (n > 5 × 10 4 cm −3) extends far from Io (at least 1000 km) in the wake direction whereas no extended dense region is found upstream. The dense region in the wake could correspond to a the stagnation region of the flow of the corotating plasma.
Abstract Local acceleration driven by whistler mode chorus waves largely accounts for the enhancement of radiation belt relativistic electron fluxes, whose favored region is usually considered to be the plasmatrough with magnetic local time approximately from midnight through dawn to noon. On 2 October 2013, the Van Allen Probes recorded a rarely reported event of intense duskside lower band chorus waves (with power spectral density up to 10 −3 nT 2 /Hz) in the low‐latitude region outside of L =5. Such chorus waves are found to be generated by the substorm‐injected anisotropic suprathermal electrons and have a potentially strong acceleration effect on the radiation belt energetic electrons. This event study demonstrates the possibility of broader spatial regions with effective electron acceleration by chorus waves than previously expected. For such intense duskside chorus waves, the occurrence probability, the preferential excitation conditions, the time duration, and the accurate contribution to the long‐term evolution of radiation belt electron fluxes may need further investigations in future.
Abstract Whistler mode waves are important for precipitating energetic electrons into Earth's upper atmosphere, while the quantitative effect of each type of whistler mode wave on electron precipitation is not well understood. In this letter, we evaluate energetic electron precipitation driven by three types of whistler mode waves: plume whistler mode waves, plasmaspheric hiss, and exohiss observed outside the plasmapause. By quantitatively analyzing three conjunction events between Van Allen Probes and POES/MetOp satellites, together with quasi‐linear calculation, we found that plume whistler mode waves are most effective in pitch angle scattering loss, particularly for the electrons from tens to hundreds of keV. Our new finding provides the first direct evidence of effective pitch angle scattering driven by plume whistler mode waves and is critical for understanding energetic electron loss process in the inner magnetosphere. We suggest the effect of plume whistler mode waves be accurately incorporated into future radiation belt modeling.
The measurements of electron density made by the Plasma Wave Subsystem instruments on Galileo during its pass through the torus on December 7th, 1995 are compared with a model based on Voyager 1 measurements made in March 1979. Outside Io's orbit, the plasma densities observed by Galileo are approximately a factor of two higher than the Voyager values. Shortly after crossing Io's orbit, the Galileo density profile dropped sharply and remained at low values for the rest of the inbound leg, suggesting that the ‘ribbon‧ region was either absent or much farther from Jupiter than usual. The peak density on the outbound leg is consistent with Voyager‐based predictions for the cold torus in both location (5.1 Rj) and magnitude (950 cm −3 ). Inside 5 Rj the density dropped sharply to less than 3 cm −3 .
The impulsive noise that the plasma wave and radio astronomy instruments detected during the Voyager 2 swing by Saturn was attributed to dust grains striking the spacecraft. This report presents a reanalysis of the dust impacts recorded by the plasma wave instrument using an improved model for the response of the electric antenna to dust impacts. The fundamental assumption used in this analysis is that the voltage induced on the antenna is proportional to the mass of the impacting grain. Using the above assumption and the antenna response constants used at Uranus and Neptune, the following conclusions can be reached. The primary dust distribution consists of a “disk” of particles that coincides with the equator plane and has a north‐south thickness of 2Δ z = 962 km. A less dense “halo” with a north‐south thickness of 2Δ z = 3376 km surrounds the primary distribution. The dust particle sizes are of the order of 10 µm, assuming a mass density of 1 g/cm³. The corresponding particle masses are of the order of 10 −9 g, and maximum number densities are of the order of 10 −2 m −3 . Most likely, the G ring is the dominate source since the particles were observed very close to that ring, namely at 2.86 R S . Other sources, like nearby moons, are not ruled out especially when perturbations due to electromagnetic forces are included. The calculated optical depth differs by about a factor of 2 from photometric studies. The current particle masses, radii, and the effective north‐south thickness of the particle distribution are larger than what Gurnett et al. (1983) reported by about 2, 1, and 1 orders of magnitude, respectively. This is attributed to the fact that the collection coefficient used in this study is smaller than what was used in Gurnett et al.'s earlier publication.
Abstract Juno made a close flyby of Ganymede and flew through its magnetosphere on 7 June 2021, including an outbound crossing of Ganymede's upstream magnetopause. We present plasma and magnetic field observations near the upstream magnetopause from Juno's Jovian Auroral Distributions Experiment (JADE) and magnetometer. JADE observed enhanced electron fluxes, including field‐aligned electrons accelerated up to 2–3 keV/q, some having bidirectional pitch angle distributions, as Juno crossed Ganymede's magnetopause. Energy enhancements of cold protons and heavy ions originating from Ganymede were also observed on approach to the magnetopause. We interpret the presence of accelerated, field‐aligned electrons as indicating that magnetic reconnection is occurring on magnetic field lines that connect the spacecraft to Ganymede's magnetopause at that time. Counter‐streaming electrons observed on both sides of the magnetopause suggest the presence of multiple reconnection sites, both north and south of the spacecraft.
Abstract Whistler mode chorus waves are responsible for electron acceleration in Earth's radiation belts. It is unclear, however, whether the observed acceleration is still well described by quasi‐linear theory, or if this acceleration is due to intense waves that require nonlinear treatment. Here, we perform a comprehensive statistical analysis of intense lower‐band chorus wave packets to investigate the relationships between wave frequency variations, packet length, and wave amplitude, and their temporal variability. We find that 15% of the wave power is carried by long packets, with low frequency sweep rates (linear trend in time) that agree with the nonlinear theory of chorus wave growth. Eighty‐five percent of the wave power, however, comes from short packets with large frequency variations around the linear trend. The kappa‐like probability distribution of these variations is consistent with random superposition of different waves that could result in a destruction of nonlinear resonant interaction.
