Global MHD simulations of Saturns's magnetosphere at the time of Cassini approach

We present the results of a 3D global magnetohydrodynamic simulation of the magnetosphere of Saturn for the period of Cassini's initial approach and entry into the magnetosphere. We compare calculated bow shock and magnetopause locations with the Cassini measurements. In order to match the measured locations we use a substantial mass source due to the icy satellites (\sim1 x 10^{28} s^{-1} of water product ions). We find that the location of bow shock and magnetopause crossings are consistent with previous spacecraft measurements, although Cassini encountered the surfaces further from Saturn than the previously determined average location. In addition, we find that the shape of the model bow shock and magnetopause have smaller flaring angles than previous models and are asymmetric dawn-to-dusk. Finally, we find that tilt of Saturn's dipole and rotation axes results in asymmetries in the bow shock and magnetopause and in the magnetotail being hinged near Titan's orbit (\sim20 R _S)

Auroral hiss, electron beams and standing Alfven wave currents near Saturn's moon Enceladus

Observations from the Cassini spacecraft have shown that Saturn's small icy moon Enceladus ejects a plume of water vapor and small ice particles into Saturn's rapidly co-rotating magnetosphere. In this paper we show that the interaction of the moon with the magnetospheric plasma produces a number of electrodynamics effects that are remarkably similar to those observed in Earth's auroral regions and near Jupiter's moon Io. These include whistler-mode emissions similar to terrestrial auroral hiss, magnetic-field-aligned electron beams, and currents associated with a standing Alfven wave excited by the moon. Ray path analyses of the auroral hiss show that the electron beams responsible for the emissions are accelerated very close to the moon, most likely by parallel electric fields associated with the Alfven wave. However, other possibilities such as electric fields due to electrostatic charging of the moon's surface or of particles in the water vapor plume should be considered. Citation: Gurnett, D. A., et al. (2011), Auroral hiss, electron beams and standing Alfven wave currents near Saturn's moon Enceladus, Geophys. Res. Lett., 38, L06102, doi:10.1029/2011GL046854

Electron scattering by magnetosonic waves in the inner magnetosphere

We investigate the importance of electron scattering by magnetosonic waves in the Earth's inner magnetosphere. A statistical survey of the magnetosonic wave amplitude and wave frequency spectrum, as a function of geomagnetic activity, is performed using the Van Allen Probes wave measurements and is found to be generally consistent with the wave distribution obtained from previous spacecraft missions. Outside the plasmapause the statistical frequency distribution of magnetosonic waves follows the variation of the lower hybrid resonance frequency, but this trend is not observed inside the plasmasphere. Drift and bounce averaged electron diffusion rates due to magnetosonic waves are calculated using a recently developed analytical formula. The resulting timescale of electron energization during disturbed conditions (when AE∗ > 300 nT) is more than 10 days. We perform a 2-D simulation of the electron phase space density evolution due to magnetosonic wave scattering during disturbed conditions. Outside the plasmapause, the waves accelerate electrons with pitch angles between 50° and 70° and form butterfly pitch angle distributions at energies from ∼100 keV to a few MeV over a timescale of several days; whereas inside the plasmapause, the electron acceleration is very weak. Our study suggests that intense magnetosonic waves may cause the butterfly distribution of radiation belt electrons especially outside the plasmapause, but electron acceleration due to magnetosonic waves is generally not as effective as chorus wave acceleration

Electron scattering by magnetosonic waves in the inner magnetosphere

We investigate the importance of electron scattering by magnetosonic waves in the Earth's inner magnetosphere. A statistical survey of the magnetosonic wave amplitude and wave frequency spectrum, as a function of geomagnetic activity, is performed using the Van Allen Probes wave measurements and is found to be generally consistent with the wave distribution obtained from previous spacecraft missions. Outside the plasmapause the statistical frequency distribution of magnetosonic waves follows the variation of the lower hybrid resonance frequency, but this trend is not observed inside the plasmasphere. Drift and bounce averaged electron diffusion rates due to magnetosonic waves are calculated using a recently developed analytical formula. The resulting timescale of electron energization during disturbed conditions (when AE∗ > 300 nT) is more than 10 days. We perform a 2-D simulation of the electron phase space density evolution due to magnetosonic wave scattering during disturbed conditions. Outside the plasmapause, the waves accelerate electrons with pitch angles between 50° and 70° and form butterfly pitch angle distributions at energies from ∼100 keV to a few MeV over a timescale of several days; whereas inside the plasmapause, the electron acceleration is very weak. Our study suggests that intense magnetosonic waves may cause the butterfly distribution of radiation belt electrons especially outside the plasmapause, but electron acceleration due to magnetosonic waves is generally not as effective as chorus wave acceleration

First evidence for chorus at a large geocentric distance as a source of plasmaspheric hiss: Coordinated THEMIS and Van Allen Probes observation

Recent ray tracing suggests that plasmaspheric hiss can originate from chorus observed outside of the plasmapause. Although a few individual events have been reported to support this mechanism, the number of reported conjugate events is still very limited. Using coordinated observations between Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Van Allen Probes, we report on an interesting event, where chorus was observed at a large L shell (~9.8), different from previously reported events at L<6, but still exhibited a remarkable correlation with hiss observed in the outer plasmasphere (L~5.5). Ray tracing indicates that a subset of chorus can propagate into the observed location of hiss on a timescale of ~5-6s, in excellent agreement with the observed time lag between chorus and hiss. This provides quantitative support that chorus from large L shells, where it was previously considered unable to propagate into the plasmasphere, can in fact be the source of hiss

First evidence for chorus at a large geocentric distance as a source of plasmaspheric hiss: Coordinated THEMIS and Van Allen Probes observation

Recent ray tracing suggests that plasmaspheric hiss can originate from chorus observed outside of the plasmapause. Although a few individual events have been reported to support this mechanism, the number of reported conjugate events is still very limited. Using coordinated observations between Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Van Allen Probes, we report on an interesting event, where chorus was observed at a large L shell (~9.8), different from previously reported events at L<6, but still exhibited a remarkable correlation with hiss observed in the outer plasmasphere (L~5.5). Ray tracing indicates that a subset of chorus can propagate into the observed location of hiss on a timescale of ~5-6s, in excellent agreement with the observed time lag between chorus and hiss. This provides quantitative support that chorus from large L shells, where it was previously considered unable to propagate into the plasmasphere, can in fact be the source of hiss

Statistical properties of plasmaspheric hiss derived from Van Allen Probes data and their Effects on radiation belt electron dynamics

©2015. American Geophysical Union. Plasmaspheric hiss is known to play an important role in controlling the overall structure and dynamics of radiation belt electrons inside the plasmasphere. Using newly available Van Allen Probes wave data, which provide excellent coverage in the entire inner magnetosphere, we evaluate the global distribution of the hiss wave frequency spectrum and wave intensity for different levels of substorm activity. Our statistical results show that observed hiss peak frequencies are generally lower than the commonly adopted value (~550Hz), which was in frequent use, and that the hiss wave power frequently extends below 100Hz, particularly at larger L shells ( > ~3) on the dayside during enhanced levels of substorm activity. We also compare electron pitch angle scattering rates caused by hiss using the new statistical frequency spectrum and the previously adopted Gaussian spectrum and find that the differences are up to a factor of ~5 and are dependent on energy and L shell. Moreover, the new statistical hiss wave frequency spectrum including wave power below 100Hz leads to increased pitch angle scattering rates by a factor of ~1.5 for electrons above ~100keV at L~5, although their effect is negligible at L≤3. Consequently, we suggest that the new realistic hiss wave frequency spectrum should be incorporated into future modeling of radiation belt electron dynamics

Statistical properties of plasmaspheric hiss derived from Van Allen Probes data and their Effects on radiation belt electron dynamics

©2015. American Geophysical Union. Plasmaspheric hiss is known to play an important role in controlling the overall structure and dynamics of radiation belt electrons inside the plasmasphere. Using newly available Van Allen Probes wave data, which provide excellent coverage in the entire inner magnetosphere, we evaluate the global distribution of the hiss wave frequency spectrum and wave intensity for different levels of substorm activity. Our statistical results show that observed hiss peak frequencies are generally lower than the commonly adopted value (~550Hz), which was in frequent use, and that the hiss wave power frequently extends below 100Hz, particularly at larger L shells ( > ~3) on the dayside during enhanced levels of substorm activity. We also compare electron pitch angle scattering rates caused by hiss using the new statistical frequency spectrum and the previously adopted Gaussian spectrum and find that the differences are up to a factor of ~5 and are dependent on energy and L shell. Moreover, the new statistical hiss wave frequency spectrum including wave power below 100Hz leads to increased pitch angle scattering rates by a factor of ~1.5 for electrons above ~100keV at L~5, although their effect is negligible at L≤3. Consequently, we suggest that the new realistic hiss wave frequency spectrum should be incorporated into future modeling of radiation belt electron dynamics