Injection, Interchange And Reconnection:
Energetic Particle Observations In Saturn's Magnetotail
D.G. Mitchell1, P. C. Brandt1, J.F. Carbary1, W.S. Kurth4, S.M. Krimigis1, C. Paranicas1, N.
Krupp2, D.C. Hamilton3, B.H. Mauk1, G.B. Hospodarsky4, M.K. Dougherty5, W. R. Pryor6
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Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
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Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany
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University of Maryland, College Park, MD, USA
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Department of Physics and Astronomy, University of Iowa, Iowa City, IA, USA
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Blackett Laboratory, Imperial College, London, UK
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Central Arizona College, Coolidge, AZ, USA
Submitted to Journal of Geophysical Research, August, 2013
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Abstract
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Saturn’s and Jupiter’s magnetotails comprise regions where most of the plasma from
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internal sources ultimately escapes from the systems. The primary active plasma processes
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involved in transport of plasma from where it is formed to the outer magnetosphere and
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ultimately to the solar wind are flux tube interchange and reconnection. Both processes likely
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produce phenomena that are labeled as “injections” because of their associated abrupt onsets in
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increased intensities of energetic particles and in plasma heating. In Saturn’s magnetosphere
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these processes may be important for transport and energization of plasma ions and electrons but
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their signatures in the data are not always easily interpreted. We discuss how and where these
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transport and energization processes may be recognized in energetic particle and ENA
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observations in Saturn’s magnetosphere.
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Introduction
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There have been many papers written on the topic of particle injection at Saturn, at
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Jupiter, and of course at Earth. In the following, we will not discuss the Earth at all, and in fact
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will only go into detail regarding the phenomenology at Saturn, where the Cassini fields and
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particles data, with support from UV and IR auroral imaging data, have provided a number of
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well correlated measurements of injections. While we are not discussing Jupiter, many of
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Saturn’s observations of injections likely inform the plasma and energetic particle observations
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and phenomena at Jupiter as well.
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The term “injection” has probably been used too loosely, which has led to some
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confusion regarding what physics and which plasma dynamics are being described. In this paper
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we will focus on two phenomena for which the term has been used, although even the two
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described here may have multiple interpretations. These two flavors of injections are 1) those
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interpreted to be the plasma heating manifestation of inward moving flux tubes associated with
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the interchange instability, and 2) those interpreted as particle acceleration associated with the
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planetward transport and heating of plasma in the a night side post-reconnection plasma
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sheet/current disk. It may be argued that these two categorizations are not distinct from one
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another because some regard the planetward transport and heating of plasma in flux tubes
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associated with the post-reconnection plasma sheet as just another example of flux tube
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interchange. We would argue against that interpretation, since in that case the transition to an
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anomalous resistivity regime as the current sheet collapses is accompanied by development of
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cross-sheet electric field that non-adiabatically energizes, heats, and transports the plasma
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particles via ExB drift in the planetward direction. This is quite different from flux tube
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interchange, in which the primary forces are buoyancy and field line tension, and the plasma is
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heated primarily adiabatically. Of course, an electric field is associated with the motion of the
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plasma planetward in this instance as well, but because the particles are for the most part
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adiabatic, they do not gain energy directly from that electric field.
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So, what do these two types of injections look like, both in energetic particles, and when
imaged remotely, in energetic neutral atoms (ENA)?
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Observations
We will investigate the two types of injection through measurements of energetic ions
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and electrons by the magnetospheric imaging instrument (MIMI) ion and neutral camera
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(INCA), the MIMI low energy magnetospheric measurement system (LEMMS), and the MIMI
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charge, energy and mass sensor (CHEMS); ENAs from INCA; magnetic fields from the fluxgate
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magnetometer (MAG); and plasma waves from the radio and plasma wave science instrument
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(RPWS). Descriptions of these instruments are given by Krimigis et al. (2004), Dougherty et al.
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(2004), and Gurnett et al. (2004). These injections are readily identified in the thermal plasma
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observed by the Cassini Plasma Sensor (CAPS) as well, but this paper is motivated by the
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characterization of injections of energetic particles, and including the CAPS data is beyond the
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scope of this work, and it will not be emphasized here. A complete treatment of particle
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injections at Saturn would include CAPS data. Many papers have been written on interchange
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injections at Saturn, and the signatures of such injections are very clear and well understood
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[André et al., 2005; Burch et al., 2005; Hill et al., 2005; Chen and Hill, 2008; Kennelly et al.,
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2013]. Fewer papers have appeared on plasma observations associated with current sheet
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collapse, with most of the focus being on the tailward side of the reconnection region. One
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exception in this latter category was described by Bunce et al., (2005). They discussed a plasma
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heating event observed just after Cassini Saturn Orbit Injection (SOI) in July 2004.
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Current sheet events, charged particles:
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Figure 1 provides an overview, primarily of the energetic particle characteristics of the
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SOI event. This event is typical of a current sheet event. Cassini encounters the injection fairly
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far out at dipole L values between about 17 and 20. The bursty structure internal to the event
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shows no significant energy dispersion, a common characteristic of the magnetosphere beyond
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L~12-15 Rs, and energetic particles were accelerated to energies of several hundred keV. The
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measured intensities at those energies were much higher than is typical for the outer
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magnetosphere suprathermal particle population. Furthermore, the event’s particles show a
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harder energy spectrum than even the inner magnetospheric suprathermal particle population,
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excluding the durably trapped radiation belt ions and electrons.
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The composition of the ions in this event is the same as that of the thermal plasma, that is,
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a mix of protons and oxygen. These particles were energized by a dynamic event that occurred
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on the night side of the planet, possibly still taking place as it corotates over Cassini at about
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0400LT. As suggested by Bunce et al. (2005), such events are probably the consequence of
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nightside reconnection and plasmoid release into the tail, with the subsequent dipolarization and
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collapse of the current disk central current sheet after the plasmoid was released and field line
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tension was no longer balanced by the centrifugal force that had been exerted by the rotating cold
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plasma. During this reconfiguration of the magnetic field an electric field is generated and the
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particles are quickly energized. The energization is more efficient for the higher energy and
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higher mass ions because their first adiabatic invariant is not conserved. This mechanism only
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applies to the ions; the electrons are also energized, but because they ought to remain adiabatic
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their energization must be through a different mechanism. Simple compression into a smaller
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volume, as well as betatron acceleration, may account for at least some of the electron heating.
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Plasmoid release and reconnection leaves behind the reconnected flux tubes containing hot but
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very low density plasma. This entire, rather large region is left occupied by depleted flux tubes,
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for which the corotational centrifugal forces are now much smaller than they were before the
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plasmoid release.
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Interchange injection events, charged particles:
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Interchange events have been the subject of a great many studies, at Earth, Jupiter, and
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especially at Saturn where they are thought to play the dominant role in radial plasma transport
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in Saturn’s middle magnetosphere [e.g., Mauk et al., 2009]. Many others have written about
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such events in considerable detail, and it is not the purpose of this work to repeat or review those
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results [Burch et al., 2005; Hill et al., 2005; André et al., 2005, 2008; Leisner et al., 2005; Chen
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and Hill, 2008; Rymer et al. 2009; Kennelly et al., 2013]. Mauk et al., (2005), Paranicas et al.,
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(2007), and Muller et al., (2010) discussed the dispersion of energetic particles as these events
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age and disperse via gradient and curvature drifts relative to the cold sub-corotating plasma. We
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will not extend those detailed treatments here, but rather we will attempt to distinguish these
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events from the current sheet collapse injections introduced above. Figure 2 details several large
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scale interchange events as they appear in energetic ions and electrons as well as the magnetic
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field.
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The events highlighted in Figure 2 are very recent flux tube interchange events. Older
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events are characterized by dispersed energies quite easy to spot in an energetic particle
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spectrogram—the sweeping contours for which the peak intensity decreases in energy with
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increasing time seen in each of the 4 lower panels of Figure 2 are just such “old” interchange
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events. The vertical features with sharply defined boundaries are good examples of “new”
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interchange events. In the bottom panel, it is also evident that there is very little in the way of
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dispersed injection events beyond L=15. Examination of other periapsis passes by Cassini
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reveals this boundary to move around a bit; it can be encountered anywhere between about L =
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10 and L = 18, with a typical value being about L=12.
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These new events, unlike the old, dispersed events, have very well defined boundaries
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both in particle intensities and in magnetic field strength. The magnetic field signature is as
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expected for a near-equatorial interchange flux tube, in that the field interior to the event is larger
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in magnitude and visibly quieter as well, with much less variability in strength than the field in
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the surrounding medium. This magnetic field behavior is in direct contrast to the
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diamagnetically depressed, highly variable field characteristic of the current sheet collapse event
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in Figure 1. Furthermore, whereas the field external to the interchange flux tubes clearly shows
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the radial gradient in the field magnitude as the spacecraft moves outward along its orbit, the
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field internal to each interchange flux tube is basically flat, with no indication of a radial
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gradient.
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Inside the interchange flux tubes, the energetic particles have been energized far more
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efficiently in the direction perpendicular to the magnetic field than in the parallel direction. The
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protons accelerated perpendicular to the magnetic field in the events reach energies of about 200
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keV, whereas those in directions closer to parallel reach only about 30 keV. More curiously, as
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noted by Paranicas et al., Fall AGU, 2008 the dispersed energetic protons from older events
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apparently have direct, unimpeded access to these newly interchanged flux tubes. Examining
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either the 35° or the 145° pitch angle particles in the three events between 0430 and 0530, it
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would appear that these gradient and curvature drifting ions are virtually unaffected by the flux
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tubes’ presence. This may be a gyroradius dependent effect, explained in Figure 3. The faint
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drifting electron event in the lower panel of Figure 3, most prominent in the energetic electrons
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between 200 and 300keV, seems to have moved to slightly higher energy within the flux tubes.
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This would indicate that these “new” flux tubes have actually been at the L-shell of the drifting
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ions for sufficient time that they have drifted onto the flux tube field. However, if that is the
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case, it is difficult to understand how the ions accelerated at near 90° pitch angle have remained
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confined to the flux tube as effectively as they seem to be. So, this is a bit of a mystery.
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The ion intensities measured by LEMMS between 40 keV and 100 keV perpendicular to
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the magnetic field in these interchange flux tubes ranges between 103 and 104 (cm2-s-sr-keV)-1,
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as compared with 102 to 103 (cm2-s-sr-keV)-1 for the ion intensity in the current sheet collapse
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injection in Figure 1. However outside the interchange flux tubes, the ambient energetic ion
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intensity in the same energy range falls to 1 to 2 orders of magnitude lower such that a volume
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average intensity in the region may be either higher or lower than the average intensity in the
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current sheet collapse injection, for which the accelerated ions fill the volume fairly uniformly.
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Discussion:
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These two kinds of injection events (current sheet collapse and flux tube interchange)
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both accelerate charged particles to high energies, both transport magnetic flux tubes containing
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low plasma density radially inward, and both are consequences of the requirement for Saturn’s
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rotating magnetosphere to rid itself of the stresses induced by cold plasma whose source is near
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Enceladus. In the inner to middle magnetosphere, the magnetic field is sufficiently strong that
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the centrifugal force of the rotating cold plasma does not distort it greatly, the azimuthal current
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associated with the cold plasma angular momentum is weak, and the field remains nearly dipolar.
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Still, the rotating plasma exerts a radial force, and unloaded flux tubes outside this region are
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“light” (the magnetic field tension is not offset by the cold plasma centrifugal force) and so
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conditions are ripe for flux tube interchange. The process results in a slightly more dipolar field
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on the light flux tubes that have participated in interchange, and the outward displacement of the
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loaded flux tubes. Again, much has been written on this topic, and it is not our intention to
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expand upon it here.
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Current sheet collapse and the accompanying processes that accelerate plasma both
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adiabatically and non-adiabatically are expected responses to plasmoid release in the tail. As the
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loaded flux tubes move farther out in the magnetosphere through interchange motion, the field
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magnitude decreases until a point is reached where the dipolar field gives way to a stretched field
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dominated by the azimuthal equatorial current sheet. This current sheet is continuous from about
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L = 7 out to the magnetopause (e.g., Connerney et al. 1983; Bunce et al., 2007) and both the
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centrifugal (inertial) current and particle pressure gradients contribute to the total current (Mauk
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et al., 2009; Sergis et al. 2007; Kellet et al., 2010). According to the mechanism put forward by
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Vasyliunas (1983), as the rotating plasma moves farther from the planet a point is reached when
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the field tension can no longer balance the inertial force of the plasma. The field will then
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reconnect across the current disk current sheet, and the cold plasma will be released tailward in a
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plasmoid (this does not occur on the dayside, because the solar wind pressure helps confine the
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plasma within the magnetopause in the dayside outer magnetosphere).
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When the plasmoid has been released, the tension in the reconnected field lines (still
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stretched) is no longer balanced by plasma centrifugal force, and they begin to snap back toward
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the planet. Their plasma content has been greatly reduced by the plasmoid release, but there will
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remain whatever plasma they contained in their off-equatorial extensions before reconnection
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took place. The current that separates the field reversal between the northern and southern
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hemispheres must collapse in this dipolarization process, and as it does so a self-consistent
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electric field is generated in the current sheet region that can accelerate the plasma that remains
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on the planetward retreating flux tubes. It is this heated plasma that we associate with the first
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type of event discussed above, namely, the current sheet collapse events.
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As an aside, we would like to point out that the plasmoid released during this process
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may or may not retreat quickly tailward. Following reconnection of the last closed field line,
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conditions for continued reconnection involving open lobe magnetic flux may or may not
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prevail. If reconnection of open flux does proceed, then the field line tension associated with the
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newly reconnected open flux whose other connection is to the solar wind will exert tension in the
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tailward direction on the plasmoid, accelerating it tailward (the classical concept of plasmoid
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behavior). However, if lobe reconnection does not proceed, then the plasmoid (which, after all,
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was released because of the centrifugal force exerted by the rotation of the cold heavy plasma
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contained within it) as a whole will not experience a tailward force. The planetward portion may
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move tailward in response to the stretched fields there, but the bulk of the plasma should
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experience no new forces on it, and so will not accelerate tailward, but rather maintain constant
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velocity in the direction it was already moving, which is primarily tangent to the azimuthal
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trajectory it was recently describing. This being the case, the signature of such a recently
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released plasmoid will be difficult to distinguish from the general flow of the plasma throughout
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the tail. Eventually such a plasmoid will move tailward; if it was released near the dusk flank,
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then by the time it travels across the tail it will have acquired a significant tailward component to
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its velocity. A plasmoid released post midnight would presumably travel into the dawn side
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magnetopause and eventually be picked up in the low latitude boundary and/or the sheath flow.
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The two types of events (current sheet collapse and interchange) are thus generally
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located in different regions. Flux tube interchange takes place primarily in the inner and middle
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magnetosphere where the magnetic field is quasi-dipolar, and buoyancy drives the instability.
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Current sheet collapse takes place primarily in the outer magnetosphere, and the unopposed field
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line tension that results after a plasmoid is released through reconnection drives the instability,
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likely similar to the bursty bulk flow process in Earth’s magnetosphere.
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However, the two types of events are very likely connected. When a plasmoid is released
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in the tail and the current sheet collapses, the flux tubes returning planetward contain only a
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tenuous, hot plasma. This planetward propagation will stop when the dipolarizing flux tubes
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reach the already existing dipolar region inside about 12 Rs (in much the same way the bursty
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bulk flows stop at the dipolar region of Earth’s field). This situation is a perfect set-up for flux
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tube interchange. The rather large region of reconnected, light flux tubes now sits just radially
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outward from the inner region, which is still loaded with cold, dense plasma. Buoyancy forces
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will be therefore strongest just at the longitude where the plasmoid release and current sheet
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collapse have occurred (of course, everything will continue to rotate azimuthally, driven by the
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ionosphere). Flux tube interchange is therefore most likely to happen in a (rotating) longitudinal
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sector where plasmoid release has just taken place, and the two types of events will therefore be,
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at least to some extent, correlated. This is not to say that one will not happen without the other,
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but there should be a greater probability for flux tube interchange following current sheet
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collapse.
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Conversely, if flux tube interchange is already proceeding in the inner to middle
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magnetosphere at a particular (rotating) longitude, then cold plasma is being efficiently
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transported radially outward at that longitude. This could lead directly to plasmoid release at that
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longitude, since the enhanced outward transport of cold plasma will more quickly load the outer,
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stretched field with even more cold plasma, leading more imminently to reconnection and
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plasmoid release at that longitude.
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Energetic Neutral Atom (ENA) Injections:
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A primary reason for this contribution to the conference is to use it as a platform for
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clearing up confusion regarding ENA observations of Saturn’s magnetosphere. Again, in ENA
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observations abrupt brightenings are often referred to as injections, and bright intensifications in
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general as “blobs”. Here, we will relate these observations to the two injections types discussed
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above.
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As the current sheet collapses, injections of charged particles are generally encountered
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in the outer magnetosphere, so it is in the outer magnetosphere that we would expect to see the
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ENA emissions from such events. Indeed, there is such a class of event seen in ENA, with quite
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repeatable characteristics that fit very well the current sheet collapse scenario.
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Figure 4 presents ENA images from January 22, 2009. Cassini was at about 14 Rs just
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post noon, and at 62 degrees latitude. This vantage point provides a good view of the night side
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of Saturn’s magnetosphere out to beyond the orbit of Titan (the outermost dashed contour). The
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large image shows a time integration of the ENA flux over an 8 minute span from 0624 to 0632
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UT for neutral hydrogen energies between 55 and 90 keV. The image shows a localized bright
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emission region very close to midnight local time. This is a fairly typical location for the onset
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of current sheet collapse events as seen in ENA, but they can initially appear any place between
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this local time and about the dawn meridian, and from about 15 Rs to just beyond Titan’s orbit.
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Regardless of where they first appear, they always rotate in the direction of the general
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magnetospheric corotational flow.
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Above the large image in Figure 4 we have included 6 smaller images of the ENA
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emission at slightly lower energy, between 24 and 55 keV, spanning the time between about
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0350 and 0810UT. This sequence shows the progression from a weak but increasing emission
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just post midnight and centered near 10 Rs in the first frame to a more well-developed, spread
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out emission at 10 Rs that is rotating toward dawn. By the third upper panel a new brightening at
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midnight and beyond 20 Rs appears in juxtaposition with the 10 Rs emission. The fourth panel
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is centered very close to the time of the main large image. From this we can see that the relative
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intensity of the new midnight enhancement beyond 20 Rs and the rotating event centered near 10
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Rs is quite different at the two energies. At 55-90 keV, the 10 Rs emission is much weaker than
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the outer midnight feature, whereas at 24-55 keV the two are comparable in intensity. To
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understand this difference, it is important to bear in mind how the ENA observation comes
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about. It is the line of sight integration of the charge exchange product between the local neutral
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gas density and the energetic ion intensity, at the particular pitch angle that corresponds to the
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intersection of the line of sight with the local magnetic field. Given the high latitude of Cassini’s
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location during this sequence, the ENA emission is generated by ions with pitch angles closer to
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field aligned than field perpendicular. From Figure 2 we can see that the ion acceleration for
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pitch angles within 30° or 40° of the magnetic field extends only to roughly 30 keV, whereas
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perpendicular to the field it extends to over 200 keV. So with our closer-to-parallel pitch angle
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vantage point for the images in Figure 4, flux tube interchange events would be expected to drop
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to relatively lower intensities at higher energies than would current sheet collapse events, for
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which the acceleration (or at least the observed distribution) is relatively independent of pitch
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angle, presumably as a consequence of pitch angle scattering in the weak, fluctuating magnetic
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field. And that is what is observed—the relative brightness of the two areas of emission at high
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energies favors the outer region (current sheet collapse), while they are more nearly equal at the
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lower energies.
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This is not to say that the in situ ion intensities are equal for the two types of events, for the two
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regions. The outer region would be expected to have higher volume-averaged ion intensity by
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virtue of the fact that the neutral gas density is lower at greater radial distances, so for the line of
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sight integral of the ENA flux to be comparable in value the ion intensity must be higher.
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However, from Cassini’s distance the ENA imager does not resolve structure at the angular size
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of an interchange flux tube and so the ENA brightness again reflects the average ENA intensity
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from the region. So the ion intensities within the interchange flux tubes could easily be
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comparable to or higher than the ion intensities in the current sheet collapse events (at about 25
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to 40 keV), and still the volume average over the ENA imager’s resolution element could be
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lower. A lower average ion intensity combined with a higher neutral gas density can result in
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nearly equal ENA intensities from the two regions.
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A clear example of a current sheet collapse event in relative isolation was presented in
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Mitchell et al. (2005, their Figures 2 and 3), who attributed it to current sheet disruption and
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showed a close correspondence with the onset and development of a particularly intense Saturn
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kilometric radiation (SKR) event. That event was characteristically dispersionless in energy, and
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showed a fast rise in both hydrogen and oxygen over a range of energies. In Hill et al. (2008) a
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similar event was closely associated with the release of a fast tailward propagating plasmoid
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observed by Cassini at 44 Rs and 0300 LT in the magnetotail. Another example from Mitchell et
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al. (2009, their Figure 8) documents the association between an injection event (probably
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involving both current sheet collapse and interchange) and a solar wind pressure enhancement,
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SKR enhancement, and dawn auroral brightening. Another event from that same work (their
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Figure 7) again associates an event that likely includes both current sheet collapse and
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interchange with a strong SKR enhancement and a well associated, rotating dawn side auroral
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bulge.
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Examples of especially well localized versions of the inner, interchange types of events
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have been associated with discrete, rotating auroral features (Radioti et al., 2013). Figure 5
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shows a keogram comparing of the auroral observations of UVIS to the ENA observations of
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INCA, showing that the auroral features and the associated ENA emissions track each other very
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closely in local time over
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Of the two types of injections discussed above, the inner-to-middle magnetosphere
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interchange type of injection is by far the more common. It is rare that an ENA imaging
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sequence of a full rotation of Saturn goes by without at least one such event at some intensity
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occurring. During more active times, it is not uncommon for up to three such regions to be active
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simultaneously (for example, in Figure 5, above), rotating in the corotation direction and
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typically intensifying as they rotate through the dusk meridian, through midnight, and dimming
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again at dawn. The current sheet collapse events in the outer magnetosphere are infrequent, and
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are often (although based on scanty statistics) associated with solar wind compression events.
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There is some evidence that the interchange events are also more common and more intense
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during solar wind compressions, although that relationship has not been well established.
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Although the association between ENA observations of rotating features and interchange
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events above is made through inference, we do have an example for which the ENA images of
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such an event are validated by direct observation of the same interchange events that produce the
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imaged ENA emission. In Figure 6a we show another example of a series of three interchange
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events, seen on day 301, 2004 during an inbound equatorial Cassini orbit near noon local time.
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Although later in the mission INCA was typically run in ion mode for this orbital geometry, for
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this pass INCA was in ENA mode, and was able to obtain a sequence of ENA images beginning
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about 6 hours before the events passed over the spacecraft, and extending until 4 hours after the
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events had rotated past Cassini (see the inset in 6a for the orbital geometry). In Figure 6b we
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present the INCA ENA images. As the text annotation of the individual images indicates, each
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of the ENA images corresponds quite naturally with the motion of the observed interchange
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events from an initial position on Saturn’s night side, through dawn and over the spacecraft just
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post noon, finally retreating through dusk back to the night side. The time progression of the
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INCA images is just what would be expected for a high energetic ion intensity set of flux tubes
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rotating about Saturn at close to corotation speed at about 10Rs, and gives us confidence in
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making the association between similar interchange events and the high inclination image
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sequences of rotating features seen near 10 Rs.
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Relationship with Saturn kilometric radiation.
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The current sheet collapse events of the outer magnetosphere are usually accompanied by
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some enhancement in Saturn kilometric radiation (SKR), which is radio emission from ~10 kHz
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to ~1500 kHz (Lamy et al., 2008). In Figure 7 we display RPWS electric field plasma wave
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power spectrograms from 3 to 3000 kHz for two period of interest. The upper panel is for 11
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through 13 January of 2009, the lower panel from a period 10 days later, 21 through 22 January
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2009. Both were high latitude intervals with good ENA imaging from Cassini, the second of
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which was the subject of Figure 4 and its discussion. This frequency band fully contains the
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SKR emission, and it also contains periodic bursts of narrow band emission, generally below but
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overlapping in frequency with the SKR at times. Horizontal yellow boxes identify the narrow
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band emission in the range between about 4 and 7 kHz in each interval.
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For the ENA image emissions previously discussed in Figure 4, as well as for images
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from a very similar event viewed by INCA in ENA from January 11 through 13, 2009, Figure 8
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compares integrated ENA intensity at 25-55 keV neutral hydrogen (red) and at 55-90keV neutral
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hydrogen (blue) to the frequency-integrated SKR power from Figure 7 between 10 and 30 kHz.
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The ENA fluxes have been corrected for slant viewing and integrated between 14-25 RS, which
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is intended to capture the outer, current sheet collapse events. The RPWS power has been
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normalized to 20 RS to correct for spacecraft range changes during the time interval.
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One notices a rough correspondence in comparing the ENA and RPWS curves. During
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the 11-13 period (top) the strongest ENA enhancements at both energies correspond quite closely
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with the abrupt increases in 10-30kHz SKR power (the dashed lines correspond to interpolated
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intervals during which the 10-30 kHz SKR was not present, but the band was bright from the
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narrow band wave power). Likewise the largest ENA enhancement between 22.2 days and 22.5
383
days agrees with the largest SKR enhancement during the same approximate interval (this
384
interval did not have significant narrow band power within the 10-30 kHz band). The ENA and
385
SKR profiles do not exactly match, of course, and this should not be expected owing to the rather
386
crude way in which the integrations are performed.
387
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In Figure 9 we compare estimates of the RPWS narrow band power within the yellow
389
boxes drawn in the spectrograms in Figure 7 with the ENA intensity between 25 and 55 keV,
390
integrated over the distance range from 5 to 14 Rs. Here we can see that on both days, the
391
narrow band emission power estimate quite closely follows the ENA intensity in this radial
18
392
distance range. This suggests that the two are closely related, and presumably through a
393
common cause of the interchange instability.
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395
Conclusions
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We have demonstrated through several case studies that there exist two types of energetic
398
particle injection mechanisms at Saturn. The first type consists of the centrifugal interchange
399
instability in which hot tenuous plasma radially interchanges with cold dense plasma as a result
400
of centrifugal forces, and the second type consists of current sheet collapse, probably following
401
plasmoid formation and release caused by stretching of the field in the magnetotail by centrifugal
402
force exerted by corotating cold plasma. The first type of injection occurs in the inner
403
magnetosphere (L<12-15) and can occur at any local time, although in ENA it is typically
404
stronger on the night side, while the second type occurs in the outer magnetosphere and occurs
405
primarily in the midnight-to-dawn sector. Each type of injection can be recognized by its
406
signature in energetic particles and energetic neutrals, and can be corroborated by corresponding
407
signatures in magnetic fields and characrteristic radio emissions.
408
19
409
Acknowledgements
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This research was supported in part by the NASA Office of Space Science under Task
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Order 003 of contract NAS5-97271 between NASA Goddard Space flight Center and the Johns
413
Hopkins University. The research at The University of Iowa is supported by the National
414
Aeronautics and Space Administration through Contract 1279973 with the Jet Propulsion
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Laboratory.
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Figure Captions
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Figure 1. “Typical” current sheet collapse injection event. The lower two panels show MIMI
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LEMMS electron (upper) and ion (lower) spectrograms from just after Cassini SOI. More detail
526
on this event regarding suprathermal particle populations appears above these lower panels. The
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top panel shows magnetic field magnitude, which exhibits irregular variations, and is generally
528
diamagnetically depressed relative to the surrounding medium. The second panel shows electron
529
energy flux from 100 eV to 500 keV, derived by combining data from the CAPS ELS and the
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MIMI LEMMS sensors. Note that the energy density peaks in the energy range between 10 and
531
100 keV. There is evidence for remnants of the cold plasma population in filaments where the
532
electron energy density extends down to 100eV or less. The third panel from the top shows
533
energetic ion energy flux from LEMMS. Ion pitch angle anisotropies (insets) are displayed for
534
specific times at the top of the spectrogram. The open, egg-shaped patterns are characteristic of
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plasma convection in the usual, azimuthal (corotational) direction. The diagonal shapes are the
536
characteristic of fast planetward convective flow. The energetic ions are a mix of energetic
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protons and O+, as determined by the MIMI CHEMS sensor. For brevity we show only O+
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between 50 and 230 keV as measured by telescope 3 from CHEMS, the telescope that for this
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spacecraft attitude records planetward flowing ions. The plasma flows as determined by the
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LEMMS and CHEMS ion anisotropy measurements are primarily in the azimuthal (corotation)
541
direction, with a couple of short intervals of strong planetward flow (especially between 0630
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and 0640).
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Figure 2. “Typical” flux tube interchange injections in energetic particles. The bottom panel
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covers from 0000UT on day 80, 2006 to 1800UT on day 81, featuring many ion injections, both
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new (vertical, non-dispersed features) and older, dispersed injections (leaning to the left at higher
26
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energies). In the panels above, the interval between 0400 and 0800 on day 80 is expanded,
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revealing several interesting features. The top panel shows the Bz (dominant) component of the
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magnetic field, the second panel shows energetic electrons. In that panel, the high intensities at
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low energies decreasing between 0400 and 0500 result from sunlight background in the LEMMS
551
electron telescope, and should be ignored. The following three panels display proton intensities
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at 3 different pitch angles. From this it can be readily seen that the energetic particle
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energization inside the interchange flux tubes is strongly pitch angle dependent.
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Figure 3. Energetic ions (top panel) and electrons (bottom panel, energy scale inverted) as seen
556
by LEMMS, which is situated very close to 90° in pitch angle throughout this period. The high
557
intensity at low energy in the left corner is sunlight interference.
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Figure 4. ENA image in 55-90 keV Hydrogen at the onset of a current sheet collapse event. In
560
the row of smaller images at the top, the progression of the event is shown in 24-55 keV H.
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There is ENA emission in the midnight to pre-morning LT region at about 10Rs (the orbits of
562
Titan at 20 Rs, Rhea at 8.7 Rs, and Dione at 6.25 Rs are included for reference), with much
563
stronger emission centered near midnight, a bit beyond Titan’s orbit, beginning at about 0630
564
UT.
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Figure 5. Keogram of UVIS auroral intensity between 16° and 20° colatitude (upper panels) and
567
INCA ENA intensity between 5 and 15 Rs (lower panels). The colored diagonal lines are
568
repeated above and below to guide the eye. The slope of the diagonal reflects the rotation
569
angular velocity for each feature. The slopes are not identical, indicating different rotation
570
angular velocities for different pairs of features. Overall, the ENA and UV features are quite
571
closely linked by common local times, as concluded in Radioti et al. [2013].
27
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Figure 6a. Example of three consecutive interchange events seen on day 301, 2004. The
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CHEMS sensor measured the energetic protons in these events, which show some energy
575
dispersion consistent with the fact that they have aged several hours since they were freshly
576
injected. The inset in the upper left shows Cassini’s orbital geometry. The three interchange
577
events are shown schematically at three different locations as they rotate about Saturn near 10Rs,
578
from the night side through noon and back through dusk. The orbits of Rhea and Dione are
579
included for reference. Cassini’s orbit is close to Saturn’s equatorial plane, and during the data
580
interval shown was inbound between about 12 and 10 Saturn radii, moving from near noon to
581
about 1300LT.
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583
Figure 6b. ENA images from Cassini acquired before, during and after the measurement of the
584
three interchange events shown in Figure 6a. This sequence demonstrates directly the
585
correspondence between ENA images of rotating features near 10 Rs and observed interchange
586
events.
587
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Fig. 7. Comparisons of integrated INCA ENA fluxes for 14-25 RS with RPWS power integrated
589
over the 10-30 kHz SKR waveband for the events discussed previously for 11-14 January 2009
590
(top) and 21-23 January 2009 (bottom). The red traces represent integrations of 25-55 keV ENA
591
fluxes, while the blue traces represent 55-90 keV ENA fluxes. The SKR power has been
592
normalized to 20 RS after correction for spacecraft range.
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28
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Figures
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Fig. 1
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Fig. 2.
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Fig. 3.
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Fig. 4.
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Fig. 5.
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615
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Fig. 6
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Fig. 7.
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Fig 8.
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Fig. 9
37