Plasma Injection Events Research Articles

Tracking Counterpart Signatures in Saturn's Auroras and ENA Imagery During Large‐Scale Plasma Injection Events

AbstractSaturn's morningside auroras consist mainly of rotating, transient emission patches, following periodic reconnection in the magnetotail. Simultaneous responses in global energetic neutral atom (ENA) emissions have been observed at similar local times, suggesting a link between the auroras and large‐scale injections of hot ions in the outer magnetosphere. In this study, we use Cassini's remote sensing instruments to observe multiple plasma injection signatures within coincident auroral and ENA imagery, captured during 9 April 2014. Kilometric radio emissions also indicate clear injection activity. We track the motion of rotating signatures in the auroras and ENAs to test their local time relationship. Two successive auroral signatures—separated by ~4 hr UT—form postmidnight before rotating to the dayside while moving equatorward. The first has a clear ENA counterpart, maintaining a similar local time mapping throughout ~9 hr observation. Mapping of the ionospheric equatorward motion post‐dawn indicates a factor ~5 reduction of the magnetospheric source region's radial speed at a distance of ~14‐20 RS, possibly a plasma or magnetic boundary. The second auroral signature has no clear ENA counterpart; viewing geometry was relatively unchanged, so the ENAs were likely too weak to detect by this time. A third, older injection signature, seen in both auroral and ENA imagery on the nightside, may have been sustained by field‐aligned currents linked with the southern planetary period oscillation system, or the re‐energization of ENAs around midnight local times. The ENA injection signatures form near magnetic longitudes associated with magnetotail thinning.

An SLS5 Longitude System Based on the Rotational Modulation of Saturn Radio Emissions

AbstractDespite the axisymmetry of Saturn's internal field, Saturn radio emissions like Saturn kilometric radiation (SKR) are modulated due to planetary rotation. With the completion of Cassini mission in September 2017, we now have around 14 years of observation of Saturn radio emissions, roughly from southern solstice to northern solstice. In this study, we extend the SLS4 longitude system to the end of the Cassini mission using a phase tracing method. The new Saturn longitude system (SLS5) organizes the observed SKR maxima around 0° subsolar longitude in both northern and southern hemispheres and can be used to organize other phenomena observed in Saturn's magnetosphere, for example, hot plasma injection events. SKR is modulated like a clock when the main source on the morning side is visible. To convert the observed phase to the clock phase, the phase of the morning side source, we also define a second longitude system SLS5*, which takes the spacecraft position into account based on a simple visibility model.

Plasma injection events in Saturn's magnetosphere

Periodic radio emissions, including the strong radio emissions known as the Saturn kilometric radio (SKR) emissions, originate from the northern and southern polar regions and are observed throughout Saturn's magnetosphere. Scientists have been studying these phenomena but have not been able to pin down the physical mechanism linking periodicities between the inner and outer magnetospheres.

Saturn's ring current: Local time dependence and temporal variability

Radial profiles of the azimuthal current density between similar to 3 and 20 R-S in Saturn's magnetosphere have been derived using plasma and magnetic field data from 11 near-equatorial Cassini orbits spanning a 10 month interval. The current density generally shows only modest variations with local time and from pass to pass within this region, rising rapidly near similar to 5 R-S to peak at similar to 90 pAm(-2) at similar to 9 R-S and falling more gradually to below similar to 20 pA m(-2) at 20 R-S. The pressure gradient current is overall the most important component, the dominant inertia current in the inner region being significantly canceled by the oppositely directed pressure anisotropy current. These characteristics principally reflect the properties of the warm water plasma originating from the Enceladus torus to distances of similar to 10 R-S encompassing the usual current peak, inside of which distance the plasma properties are generally unvarying within factors of less than similar to 2. Increased variability is present at larger distances where the pressure of the hot magnetospheric plasma plays the more important role. In this region the dominant pressure gradient current is found to be strongest in the dusk to midnight sector and declines modestly, by factors of similar to 2 or less, in the midnight to dawn and dawn to noon sectors. Pass-to-pass temporal variability by factors of similar to 2-3 is also present in the outer region, particularly in the dawn to noon sector, probably reflecting both hot plasma injection events as well as solar wind-induced variations.

Importance of plasma injection events for energization of relativistic electrons in the Jovian magnetosphere

[1] Effects of density decrease in plasma injection regions and a latitude-dependent wave normal angle distribution on the energization of electrons by whistler mode waves at Jupiter are investigated. Previous work showed that whistler mode waves could enhance the fluxes of a few MeV electrons by an order of magnitude on a time scale of 30 days. However, density decrease in plasma injection regions and latitude dependence of the wave normal angle distribution of waves were not considered. Because this information is difficult to obtain from available observations, we perform a sensitivity study to demonstrate that the effect of density decrease on energization of electrons becomes important when the density inside injection regions is reduced to 25% of that outside. We also investigate the effect of a latitude-dependent wave normal angle distribution on energization of electrons using a ray tracing program and demonstrate that a realistic latitude-dependent wave normal angle distribution increases the rate of pitch angle diffusion near the loss cone, and thus enhances the loss rate, of 1–3 MeV electrons. However, the flux of 10 MeV electrons is not significantly affected. Results of the work are useful for understanding energization of MeV electrons at Jupiter, especially when combined with observations from future missions.

Cassini evidence for rapid interchange transport at Saturn

During its tour Cassini has observed numerous plasma injection events in Saturn's inner magnetosphere. Here, we present a case study of one “young” plasma bubble observed when Cassini was in the equatorial plane. The bubble was observed in the equatorial plane at ∼7 Saturn radii from Saturn and had a maximum azimuthal extent of ∼0.25 Rs (Rs=Saturn radius ∼60330 km). We show that the electron density inside the event is lower by a factor ∼3 and the electron temperature higher by over an order of magnitude compared to its surroundings. The injection contains slightly increased magnetic field magnitude of 49 nT compared with a background field of 46 nT. Modelling of pitch angle distributions inside the plasma bubble and measurements of plasma drift provide a novel way to estimate that the bubble originated between 9< L<11 and had an average radial propagation speed of ∼260+60/−70 km s −1. An independent estimate of the speed of the injection following theoretical work of Pontius et al. [1986. Steady State Plasma transport in a Corotation-Dominated Magnetosphere. Geophys. Res. Lett. 13(11), 1097–1100] based on the mass per unit flux gives a maximum radial propagation speeds of 140 km s −1. These results are similar to those found by Thorne et al. [1997. Galileo evidence for rapid interchange transport in the Io torus. Geophys. Res. Lett. 24, 2131] for one event observed in Jupiter's magnetosphere near Io. We therefore suggest this is evidence of the same process operating at both planets.

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

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 (&lt;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 (&lt;1 s to a few seconds), and the overall structure of this chorus appears more similar to chorus detected at the Earth.

Strong sunward propagating flow bursts in the night sector during quiet solar wind conditions: SuperDARN and satellite observations

Abstract. High-time resolution data from the two Iceland SuperDARN HF radars show very strong nightside convection activity during a prolonged period of low geomagnetic activity and northward interplanetary magnetic field (IMF). Flows bursts with velocities ranging from 0.8 to 1.7 km/s are observed to propagate in the sunward direction with phase velocities up to 1.5 km/s. These bursts occur over several hours of MLT in the 20:00–01:00 MLT sector, in the evening-side sunward convection. Data from a simultaneous DMSP pass and POLAR UVI images show a very contracted polar cap and extended regions of auroral particle precipitation from the magnetospheric boundaries. A DMSP pass over the Iceland-West field-of-view while one of these sporadic bursts of enhanced flow is observed, indicates that the flow bursts appear within the plasma sheet and at its outward edge, which excludes Kelvin-Helmholtz instabilities at the magnetopause boundary as the generation mechanism. In the nightside region, the precipitation is more spot-like and the convection organizes itself as clockwise U-shaped structures. We interpret these flow bursts as the convective transport following plasma injection events from the tail into the night-side ionosphere. We show that during this period, where the IMF clock angle is around 70°, the dayside magnetosphere is not completely closed.Key words. Ionosphere (Auroral ionosphere; Ionospheremagnetosphere interactions; Particle precipitation)

Impulsive broadband electrostatic noise in the cleft: A signature of dayside reconnection

Magnetosheath plasma injection events are frequently observed in the dayside oval. From the time dispersion characteristics of the injected magnetosheath ions it is possible to deduce the injection time of magnetosheath electrons. It is shown by means of specific examples taken from the Swedish Viking spacecraft data set, that this electron injection time occurs together with the generation of burst of broadband electrostatic noise (BEN). The overall duration of the BEN emissions lasts typically of the order of 1 min, probably representing the total duration of the injection events. This is within the same time domain as that inferred for flux transfer events (FTE). On a shorter timescale, BEN consists of a succession of very impulsive bursts which last a few seconds; this reveals the characteristic timescales of the elementary injection process. The BEN emissions appear to be the “messengers” of the reconnection process occurring along the magnetic field lines which are connected to the reconnection sites. The information is transmitted via solitary waves, probably of electron‐acoustic type, and consisting of isolated potential wells. From the observations, the overall extension of the injection region at the magnetopause is estimated to be of the size of a large fraction of an Earth radius. However, plasma injection appears as a noncontinuous process that takes place in narrow spatially localized regions of the size of merely more than an ion gyroradius at the magnetopause.

A new type of ion injection event observed by Viking

A new type of low energy (&lt; 200 eV) plasma injection event is found near local noon several degrees equatorward of the cusp. Its V‐shaped energy‐pitch angle pattern is very similar to that of impulsive or transient magnetosheath plasma injection events reported in the past. Thus, the source region of these injected particles is not a persistent one and that it is likely of magnetosheath origin. However, there are three facts which distinguish this newly found phenomenon from the other impulsive or transient magnetosheath plasma injection events associated with solar wind pressure pulse or southward/fluctuating interplanetary magnetic field conditions. First, the magnetic field and the electric field observations do not show such a strong electromagnetic coupling between the ionosphere and the source region. Second, a high energy (&gt; 300 eV) population is absent, making the ion energy‐time dispersion very weak. Third, this event does not seem to be related to either any solar wind density increase or southward/disturbed interplanetary magnetic field conditions. These observations suggest that this is a more stagnant phenomenon, and we propose to call this a “Stagnant Plasma Injection” event.

Plasma injection events at synchronous orbit related to positive DST

Synchronous orbit measurements from the University of New Hampshire ATS 6 instrument have been compared with several other types of space and ground data in a study of the plasma effects related to SI and SC (positive excursions of Dst) magnetic perturbations. It is found that plasma injections at synchronous orbit, related to SI and SC, are very similar to plasma injections associated with substorms, except that the former occur at all local times. These injections can be interpreted as the co‐location of particle boundaries on an inward propagating front. The boundaries are those due to magnetospheric convection and precipitation losses and normally are spatially dispersed as a function of energy. The compression magnetic pulse associated with SI and SC events is apparently responsible for the co‐location of these boundaries and their inward propagation. The +Dst magnetic pulse plays a role similar to that of the magnetic tail reconfiguration pulse associated with substorms. Theories of convection in steady state magnetic fields cannot account for the dispersionless character of the injection front. The electron plasma behind the injection front is a hot Maxwellian for local evening +Dst plasma events as is the case for substorms. +Dst‐related injections observed on the dayside have enhanced low‐energy electron intensities, perhaps owing to an ionospheric component. The plasma observed behind the front appears to be hot tail plasma convected to its point of observation. We show evidence that +Dst enhances magnetospheric convection independent of interplanetary magnetic field direction. Since +Dst does not necessarily trigger substorms, enhanced convection is apparently not a sufficient condition for substorm generation.

A statistical analysis of the low-energy geosynchronous plasma environment—II. Ions

This is the second of two papers which together provide a detailed statistical analysis of the low-energy (1-eV–100-keV) geosynchronous plasma environment. In this paper, data on the ion environment between ∼1 eV and 80 keV are statistically analysed. About 50 days of 10-min averages of the first 4 moments of the ion distribution function from the University of California at San Diego low-energy plasma detectors on the ATS-5 and ATS-6 geosynchronous satellites are studied in terms of: occurrence frequency, local time variations, K p variations, injection events, and intercomparisons among the 4 moments. Variations in the 4 moments can be explained by assuming the existence of 2 dominant plasma populations (the plasmasphere and ring current) upon which are superimposed plasma injection events. In conjunction with this analysis, the accuracy of the 4-moment representation for ions was tested and, unlike that for electrons, found to be limited by the energy band-pass of the instrumentation and by a lack of knowledge of the angular anisotropies and ionic composition. Even so, these limitations are outweighed by the utility and compactness of the method. A technique for correcting the limited energy band-pass of the instrumentation is presented in the Appendix.

Kinetic energy density and diamagnetic effects during plasma injection events

A magnetoplasma in which energetic particles are injected is considered. The laws which govern the magnetic induction, the magnetic field and the kinetic energy density of the particles are given. It is shown that whereas the beta of the plasma can reach very high values, the perpendicular kinetic energy density cannot exceed a certain critical value Wc which is equal to (11.6/L)6 keV.cm−3 in the magnetosphere and which is of the order of 1 keV.cm−3 in the tail.