Pancake Distribution Research Articles

Relationship between ULF waves and radiation belt electrons during the March 10, 1998, storm

Acceleration of electrons due to ULF oscillations is considered to be an explanation for the radiation electron flux enhancement during the storm. In this study, we examine the relationship between the ULF wave activity observed by the Equator-S satellite and the relativistic electron flux variation observed by POLAR satellite during a magnetic storm which commenced on March 10, 1998. The storm was associated with a 5-day high speed solar wind interval. Rapid enhancement of MeV electrons was observed in the outer radiation belt within the first half day after the storm commencement with strong pancake distribution in equatorial pitch angle, followed by a more gradual flux increase for the next several days with a more isotropic distribution. During this interval, when both rapid and gradual acceleration of electrons took place, enhanced ULF wave activity was observed in the dawnside magnetosphere extending down to L < 6 region. We compare the observations with different models considering the ULF waves. The observation generally supports the mechanisms of the electron acceleration due to interaction with ULF waves, although different mechanisms are possibly involved for acceleration during the early recovery phase and the later recovery phase.

The temporal evolution of electron distributions and associated wave activity following substorm injections in the inner magnetosphere

The temporal evolution of electron distributions and associated wave activity following substorm injections in the inner magnetosphere are investigated using data from the CRRES satellite. Equatorial electron distributions and concomitant wave spectra outside the plasmapause on the nightside of the Earth are studied as a function of time since injection determined from the auroral‐electrojet index (AE). The electron cyclotron harmonic (ECH) wave amplitudes are shown to be very sensitive to small modeling errors in the location of the magnetic equator. They are best understood at the ECH equator, defined by the local maximum in the ECH wave activity in the vicinity of the nominal magnetic equator, suggesting that the ECH equator is a better measure of the location of the true equator. Strong ECH and whistler mode wave amplitudes are associated with the injected distributions and at the ECH equator, in the region 6.0 ≤ L &lt; 7.0, exponential fits reveal wave amplitude decay time constants of 6.3±1.2 and 4.6±0.7 hours, respectively. Pancake electron distributions are seen to develop from injected distributions that are nearly isotropic in velocity space and, in this region, are seen to form on a similar timescale of approximately 4 hours suggesting that both wave types are involved in their production. The timescale for pancake production and wave decay is comparable with the average time interval between substorm events so that the wave‐particle interactions are almost continually present in this region leading to a continual supply of electrons to power the diffuse aurora. In the region 3.8 ≤ L &lt; 6.0 the timescale for wave decay at the ECH equator is 2.3 ± 0.6 and 1.1 ± 0.2 hours for ECH waves and whistler mode waves respectively, although the pancakes in this region show no clear evolution as a function of time.

Electron pitch angle diffusion by electrostatic electron cyclotron harmonic waves: The origin of pancake distributions

It has been suggested that highly anisotropic electron pancake distributions are the result of pitch angle diffusion by electrostatic electron cyclotron harmonic (ECH) and whistler mode waves in the equatorial region. Here we present pitch angle diffusion rates for ECH wave spectra centered at different frequencies with respect to the electron gyrofrequency Ωe corresponding to spacecraft observations. The wave spectra are carefully mapped to the correct resonant electron velocities. We show that previous diffusion calculations of ECH waves at 1.5Ωe, driven by the loss cone instability, result in large diffusion rates confined to a small range of pitch angles near the loss cone and therefore cannot account for pancake distributions. However, when the wave spectrum is centered at higher frequencies in the band (&gt; 1.6Ωe), the diffusion rates become very small inside the loss cone, peak just outside, and remain large over a wide range of pitch angles up to 60° or more. When the upper hybrid resonance frequency ωUHR is several times Ωe, ECH waves excited in higher bands also contribute significantly to pitch angle diffusion outside the loss cone up to very large pitch angles. We suggest that ECH waves driven by a loss cone could form pancake distributions as they grow if the wave spectrum extends from the middle to the upper part of the first (and higher) gyroharmonic bands. Alternatively, we suggest that pancake distributions can be formed by outward propagation in a nonhomogeneous medium, so that resonant absorption occurs at higher frequencies between (n + ½) and (n + 1)Ωe in regions where waves are also growing locally at &lt; 1.5Ωe. The calculated diffusion rates suggest that ECH waves with amplitudes of the order of l mV m−1 can form pancake distributions from an initially isotropic distribution on a timescale of a few hours. This is consistent with recent CRRES observations of ECH wave amplitudes following substorm injections near geostationary orbit and the timescales for pancake formation. Persistent but much weaker ECH waves can further intensify and maintain pancake distributions during magnetically quiet periods.

An investigation into the roles of ECH and whistler mode waves in the formation of “pancake” electron distributions using data from the CRRES satellite

Electron pitch angle distributions sharply peaked at 90° pitch angle were first recorded in the energy range 50 eV < E < 500 eV by the GEOS-1 and GEOS-2 spacecraft in 1977/8, from the plasmapause out to geostationary orbit. At the time they were explained as the remnants of pitch angle diffusion driven solely by Electron Cyclotron Harmonic (ECH) waves. Here we use observations by instruments on board the CRRES spacecraft to study these distributions in more detail. The pancake distributions are now seen to develop from injected distributions that are nearly isotropic in velocity space, on a time scale that is greater than 2 hours. The freshly injected distributions are associated with strong ECH and whistler mode waves suggesting that the pancake distributions are likely to be caused by a combination of both wave types. Our results suggest that whistler mode waves play a dominant role in the formation of pancake distributions outside L = 6.0, whereas inside L = 6.0 and, in particular, in the vicinity of the plasmapause, the ECH waves also play a significant role. Consequently both types of waves should be considered in any attempt to explain the diffuse aurora and the variation with L taken into account.

Plasma characteristics of high-altitude cusp for steady southward-dawnward IMF

High-altitude observations of plasma flow across the magnetopause are made by the Interball-Tail satellite near the polar cusp for steady Bz < 0, By < 0 of the interplanetary magnetic field. Three-dimensional proton distribution functions measured in this region contain the features predicted by existing reconnection models. The evolution of the distribution function in magnetospheric parts of field lines tailward from the cusp from D-shaped distribution in the vicinity of the magnetopause to the pancake and torus distributions deeper in the magnetosphere can be interpreted in terms of a gradually diminishing ion injection. A simulation that uses the Toffoletto-Hill model of the open magnetosphere to trace the particle transport from the magnetosheath to magnetospheric locations of Interball is reasonably consistent with the observations. The results show that for the steady southward interplanetary magnetic field with a large By component the entry of magnetosheath plasma into the magnetosphere can be described in terms of open topology of field lines near the high-altitude cusp.

Energetic electron butterfly distributions near Io

Pronounced variations in the energetic electron distribution observed by the Energetic Particle Detector during the Galileo flyby of Io are described as a quasi‐adiabatic response to the changing electric and magnetic field environment near the satellite. The energetic particle signatures can therefore be used to remotely sense the spatial distribution of electric and magnetic fields in the vicinity of Io. Electron pitch angle distributions evolve from a normal pancake distribution (peaked at 90° pitch angle) in the undisturbed torus to a butterfly distribution in the strong field depression near Io. The strongest flux depletions at 90° pitch angle result from a reduction in kinetic energy due to conservation of the first adiabatic invariant, as electrons are transported into the vicinity of Io. The magnitude of the flux depletion is related to the spectral index n of the electron energy spectrum (J ∼ E−n). Since the value of n tends to increase with increasing energy, the largest flux drop occurs at higher energy. In the low‐speed wake region downstream of Io, electrons exhibit an abrupt transition to a population which is consistent with trapping on bounce orbits within the magnetic depression near Io. This trapped population, which appears in the same spatial region as intense field‐aligned beams, is not a result of adiabatic transport from a source region upstream of Io. The phase space density of the “trapped” electron population is reduced, compared to the background torus, and particle tracing calculations in a realistic model environment near Io suggest that such electrons must be scattered into the region sampled by Galileo. Torus electrons with energies well above an MeV are excluded from a broad spatial region surrounding Io. This leads to a pronounced drop in the flux of penetrating particles near Io which allows the modest “trapped” electron population to be detected above the background level for energies up to 200 keV.

“Pancake” electron distributions in the outer radiation belts

Electron pitch angle distributions sharply peaked at 90° pitch angle were first recorded in the energy range 50 eV &lt; E &lt; 500 eV by the GEOSl and GEOS2 spacecraft in 1977/1978, from the plasmapause out to geostationary orbit. At the time they were explained as the remnants of pitch angle diffusion driven solely by electron cyclotron harmonic (ECH) waves. Here we report new observations by the Low Energy Plasma Analyser on board the Combined Release and Radiation Effects Satellite, which measured the complete pitch angle distribution over the energy range 100 eV &lt; E &lt; 30 keV. The pancake distributions are seen to develop from injected distributions that are nearly isotropic in velocity space, on a timescale that is greater than 2 hours. The freshly injected distributions are associated with strong ECH and whistler mode waves suggesting that the pancake distributions are likely to be caused by a combination of both wave types. Outside L = 6.0 the fitting analysis at energies in the range 100 eV &lt; E &lt; 1 keV shows that in the marginally stable state the phase space density contours lie approximately along the characteristic curves for diffusion by whistler mode waves determined independently from the plasma wave data. However, inside L = 6.0, significant departures are observed. Our results suggest that whistler mode waves play a dominant role in the formation of pancake distributions outside L = 6.0, whereas inside L = 6.0 and, in particular, in the vicinity of the plasmapause, the ECH waves also play a significant role. Consequently, both types of waves should be considered in any attempt to explain the diffuse aurora and the variation with L taken into account.

Suspected wave-particle interactions coincident with a pancake distribution as seen by the CRRES spacecraft

On 1990 October 13 the Sussex Particle Correlator Experiment on the USAF/NASA Combined Release and Radiation Effects Satellite (CRRES) observed modulation in the onboard wave-particle correlation functions at frequencies between one third and one half of the electron gyrofrequency during the period from 14:45 UT to 15:45 UT. At this time the Iowa plasma wave experiment measured chorus emissions in the region of one third of the local electron gyrofrequency, and the Lockheed IMS-LO electron instrument observed an apparent velocity dispersion event in electrons between 770eV and 20 keV, beginning at about 15:12 UT. We describe the particle correlation technique, briefly present this event, and then describe an approach to testing the hypothesis that wave-particle interactions were responsible for the large modulation seen in the particle correlator. Recent work on the statistical analysis of such data is presented.

Mapping the Auroral Oval into the Magnetotail Using Dynamics Explorer Plasma Data.

The mapping of the nightside auroral oval into the magnetotail is investigated using DE-1 and -2 plasma data in combination with the Tsyganenko '89 and the Hilmer-Voigt'91 magnetic field models. In an attempt to clarify the relationship between magnetospheric regions and the BPS and CPS, data from the Low Altitude Plasma Instrument (LAPI) are used to determine the invariant latitude of the polar cap boundary, the poleward edge of the discrete aurora, the discrete-diffuse (BPS/CPS) auroral boundary, and the equatorward edge of the diffuse aurora. When these field lines are traced into the magnetotail, both models indicate that the region of discrete aurora (BPS) maps to an extended region (usually>20 RE in width) of the main portion of the plasma sheet outside the more dipolar, inner plasma sheet (CPS). Since BPS field lines map to such a wide range of neutral sheet crossing distances, we propose a less confusing and more descriptive terminology in which the term “BPS” is divided into two regions: the Plasma Sheet Boundary Layer (PSBL) and the “IPS”, for Isotropic (Boundary) Plasma Sheet.Auroral zone-magnetotail topology is further examined by using data from the High Altitude Plasma Instrument (HAPI) to determine values of electron number density and temperatures at 1° intervals across the auroral zone. The corresponding field lines are traced into the magnetotail to a distance of-15RE and compared with magnetotail (10-25RE) mean plasma sheet ion densities and temperatures for low- and mid-activity levels from HUANG and FRANK (1986). We find a very good correspondence between the density and temperature measurements mapped from DE-1 altitudes and those measured in the mid-tail, suggesting that both models successfully reproduce the average magnetospheric configuration at this distance. The mean of the ion temperatures mapped with the Hilmer-Voigt model were slightly higher than those reported from the ISEE data, indicating that the model may be slightly too stretched. A significant region (between-5-7 RE) of overlap between the hotter ring current and the cooler, fresher plasma sheet is observable at DE-1; these hotter electrons are so anisotropic (with a trapped pancake distribution) that they are not observable at the 1000km altitude of DE-2.

Rocket measurements of relativistic electrons: New features in fluxes, spectra and pitch angle distributions

We report new features of precipitating relativistic electron fluxes measured on a spinning sounding rocket payload at midday between altitudes of 70 and 130 km in the auroral region (Poker Flat, Alaska, 65.1°N, 147.5°W, and L=5.5). The sounding rocket (NASA 33.059) was launched at 21:29 UT on May 13, 1990 during a relativistic electron enhancement event of modest intensity. Electron fluxes were measured for a total of about 210 seconds at energies from 0.1 to 3.8 MeV, while pitch angle was sampled from 0° to 90° every spin cycle. Flux levels during the initial 90 seconds were about 5 to 8 times higher than in the next 120 seconds, revealing a time scale of more than 100 seconds for large amplitude intensity variations. A shorter time scale appeared for downward electron bursts lasting 10 to 20 seconds. Electrons with energies below about 0.2 MeV showed isotropic pitch angle distributions during most of the first 90 seconds of data, while at higher energies the electrons had highest fluxes near the mirroring angle (90°); when they occurred, the noted downward bursts were seen at all energies. Data obtained during the second half of the flight showed little variation in the shape of the pitch angle distribution for energies greater than 0.5 MeV; the flux at 90° was about 100 times the flux at 0°. We have compared the low altitude fluxes with those measured at geostationary orbit (L=6.6), and find that the low altitude fluxes are much higher than expected from a simple mapping of a pancake distribution at high altitudes (at the equator). Energy deposition of this modest event is estimated to increase rapidly above 45 km, already exceeding the cosmic ray background at 45 km.

Recent findings on angular distributions of dayside ring current energetic ions

Angular distributions of dayside ring current energetic ions are studied with the Medium Energy Particle Analyzer (MEPA) on the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer spacecraft. The anisotropy index n for pancake distribution during 10 geomagnetically quiet days is examined for the first time in the intermediate energies (50–1000 keV) for which compositional information is now available. It is shown that n for protons and helium ions increases with L whereas n for CNO ions appears to be independent of L. The empirical formula n(E) = no+k log10 (E(MeV)), for the energy dependence given by Fritz and Spjeldvik (1982), is extended to these energies and ion species. The parameter no is found to increase generally with ion mass and L, except for CNO ions which show no noticeable L dependence. The parameter k appears to be independent of L and ion mass. We have also surveyed the varieties of pitch angle distributions. Two unusual distributions were detected during the September 4–7, 1984, geomagnetic storm. The first is a net field‐aligned transport of ring current energetic ions at 6 ≲ L ≲ 9 and 1030–1300 MLT. Such a signature is shown most prominently by the heavy ions (mostly oxygen; E &gt; 137 keV), to a lesser extent by helium ions (E &gt; 72 keV), and only slightly by protons (E &gt; 56 keV) which are the dominant ionic species. The second new feature is a sharp intensity enhancement over a pitch angle range centered at 90° which occurs in all ionic species and energy passbands monitored by MEPA at 3 ≲ L ≲ 4 and 1700–1800 MLT. Its characteristics suggest that it represents the addition of a freshly injected ring current population in the main phase development of the storm, and not due to L shell splitting or wave‐particle interaction.

Electron Fermi acceleration in collapsing magnetic traps: Computational and analytical models

We consider the heating and acceleration of electrons trapped on magnetic field lines between approaching magnetic mirrors. Such a collapsing magnetic trap and consequent electron energization can occur whenever a curved (or straight) flux tube drifts into a relatively straight (or curved) perpendicular shock. Our relativistic, three‐dimensional, collisionless test particle simulations show that an initial thermal electron distribution is bulk heated while a few individual electrons are accelerated to many times their original energy before they escape the trap. Upstream field‐aligned beams and downstream pancake distributions perpendicular to the field are predicted. In the appropriate limit the simulation results agree well with a nonrelativistic analytic model of the distribution of escaping electrons which is based on the first adiabatic invariant and energy conservation between collisions with the mirrors. Space science and astrophysical applications are discussed.

A model of the energetic ion environment of Mars

Because Mars has a weak intrinsic magnetic field and a substantial atmosphere, instruments on orbiting spacecraft should detect a population of energetic heavy planetary ions which result from comet‐like ion pickup in the solar wind and magnetosheath convection electric fields, in addition to those that might result from processes internal to a Martian “magnetosphere.” Although this ion exosphere has been previously discussed in the literature, detailed predictions that might be directly applied to the interpretation of data are not available. Here a test particle model is used to construct a global picture of Martian pickup ions in the Mars environment. The model makes use of the recent Nagy and Cravens [1988] model of the Martian exosphere and Spreiter and Stahara's [1980] gas dynamic model of the magnetosheath. The pickup of ions originating at Phobos is also considered. Notable properties of the resulting ion distributions include their near‐monoenergetic spectra, pancake pitch angle distributions, and large gyroradii compared to the planetary scale.

Pitch angle distributions of low‐energy ions in the near‐Earth magnetosphere

Low‐energy (&lt;1 keV) ions have been frequently observed to have some field alignment in their pitch angle distributions in the magnetosphere at geosynchronous orbit or near the equatorial plane. Using data from the energetic ion composition spectrometer on Dynamics Explorer 1, acquired from September 1981 to March 1985, we have examined the occurrence probabilities of various forms of ion pitch angle distributions, including field‐aligned, conical, and equatorially trapped ions. For both H+ and O+, bidirectional field‐aligned distributions at low energy (&lt;1 keV), with pitch angle peaks parallel and antiparallel to the field line direction, were most common at L shells less than about 6 in local times from postmidnight to dawn, except for H+ near the equator, where the equatorially trapped (pancake) distribution had a comparable or dominant occurrence probability. The presence of O+ suggests that these bidirectional ions have a terrestrial origin. Upflowing ion events, which are believed to be a major source of magnetospheric hot ions, were observed most frequently at higher L shells and earlier local times than the bidirectional ions. Bidirectional H+ ions with local minima near the field line (bidirectional conies) were observed to dominate their field‐aligned counterparts in the morning local times. This is attributed to the charge exchange loss of the field‐aligned ions with atmospheric hydrogen in their course of eastward drift. Bidirectional ions were rarely observed in the afternoon local times. This is interpreted as a loss of ions whose drift paths crossed the magnetopause into the magnetosheath.

Weak electrostatic waves near the upper hybrid frequency: A comparison between theory and experiment

Two types of generating mechanisms have been proposed to explain the observations of weak electrostatic waves near the local upper hybrid frequency fuh. In this paper data from the wave and plasma diagnostic experiments on GEOS 1 are presented with simultaneously observed particle data in an attempt to identify the generating mechanism. The electron data contain two sources of free energy: a highly anisotropic "pancake" distribution in the warm (20 eV to ∼1 keV) plasma and a loss cone type distribution in the hot (&gt;few keV) plasma. This leads us to consider a coherent generating mechanism. A model of the plasma distribution, with cold, warm and hot components is constructed and linear instability calculations are presented. It is shown that the relatively large resonant perpendicular velocities in the loss cone distribution generate instabilities at small perpendicular wave numbers preferentially in the upper hybrid band since the wave frequency can lie midway between the gyroharmonics where cyclotron damping is reduced. The calculations are able to reproduce the spectral characteristics observed by previous spacecraft over a range of cold plasma densities. The temporal growth rates are comparable to those obtained previously for strong electrostatic emissions near fuh, but it is shown that the unstable wave number region is an order of magnitude smaller. Thus it is argued that the waves remain weak since they are quickly refracted out of resonance. It is demonstrated that when there is a significant amount of warm plasma present the strongest natural emission can be used to derive the total plasma density, cold plus warm plus hot, and not the cold plasma density, and that the warm plasma introduces new regions of small group Velocities which result in fine structure in the wave spectrum. The effects of the warm plasma and particularly the pancake distribution are discussed in connection With the ray propagation of these and other classes of much stronger emissions.

EXOS‐B/Siple Station VLF wave‐particle interaction experiments: 1. General description and wave‐particle correlations

In situ measurements of both energetic particles and VLF waves have been carried out in a joint program involving the Japanese satellite EXOS‐B and the Siple Station VLF transmitter. A general description of the experiment is given as well as some results concerning wave‐particle correlations. Detailed analysis of the observed wave characteristics is given in a companion paper. Correlations of electron flux and pitch angle anisotropy in the energy range from 85 eV to 6.9 keV with waves in a range from 300 Hz to 9 kHz are examined. These electrons sometimes have a pitch angle distribution with a peak flux at 90° pitch angle (so‐called pancake distribution). On five passes out of a total of 50 during the summer campaign in 1979, the energy of the electrons that showed a high pitch angle anisotropy shifted upward as the satellite moved into the plasmasphere, crossing the plasmapause in the equatorial region. In two cases out of five, strong Siple signals were observed in the geomagnetic equatorial region just outside the plasmapause, in association with such a pancake pitch angle distribution of electrons. The Siple signals are most likely amplified by the cyclotron instability due to the high pitch angle anisotropy (HPAA), although the flux of resonant electrons was relatively small. For three other cases of HPAA, the satellite location was so far away from the Siple meridian that the signal level, even if amplified, was too weak to be detected by the satellite. Emissions associated with Siple signals were detected on five (two equatorial and three high latitude) passes, which were all confined on 6 days after a large magnetic storm. On the days when the Siple triggered emissions were observed, the pitch angle anisotropy was low, but the electron flux at resonant energies in the equatorial region was four or five times larger than that on other non‐triggering days in all energy channels from 85 eV to 6.9 keV.

ISEE 1 observations of thermal plasma in the vicinity of the plasmasphere during periods of quieting magnetic activity

Thermal (≲100 electron volts) ion observations made with the plasma composition experiment on ISEE 1 are combined with plasma density profiles obtained from plasma frequency measurements made with the plasma wave experiment to conduct an investigation of thermal plasma behavior in the vicinity of the plasmasphere during periods of quieting magnetic activity. Normally, the principal thermal ion population in the plasmasphere consists of cold (kT ≲ 1 eV), isotropic distributions with ion species in the order of dominance H+ :He+ :O+, while outside the plasmapause, the observed E ≲ 100 eV ion distributions usually are field‐aligned in structure, have characteristic energies E ≲ 10 eV and H+ :O+ :He+ order of dominance in fluxes. During periods in which the magnetic activity quiets, the above two regions are separated by a new region in which, at times, low‐energy (∼1–2 eV) H+ and He+ are found flowing along the magnetic field lines. On other occasions following quieting magnetic activity, pancake distributions (peak fluxes at 90° pitch angle) are observed in this region. Other complex distributions have been seen, and these complexities and the limitations of the data coverage preclude a satisfactory simple interpretation. It seems plausible to identify this region as the site of plasmasphere refilling. However, the data presumably also contain evidence of the quiet time rotation of the plasmasphere bulge region into the morning sector.

Pancake pitch angle distributions in warm ions observed with ISEE 1

Observations of pancake (peak flux near 90° pitch angle) distributions of low‐energy (≲100 eV) ions are reported. Pancake distributions occur often in H+ and He+ simultaneously while O+ fluxes are either undetectable or field‐aligned. These H+ and He+ pancake distributions typically display characteristic energies of the order of 10 eV and are frequently mixed with higher density, colder (κT≲3 eV), isotropic, quasi‐Maxwellian components. They appear often within the outer regions of the plasmasphere, and seem to occur most frequently on the dayside and near the magnetic equator.

Neutral atom precipitation—a review

The production of energetic neutral atoms by charge exchange of ring current ions with neutral hydrogen in the geocorona was predicted many years ago, and there are now a number of measurements of the effect of the impact of these energetic atoms on the thermosphere. Theoretical models of the process have been developed. The latitude variation of the precipitating flux depends very much on the pitch angle distribution of the ions in the ring current, and on the L shell on which they are located. The production of a belt of trapped particles at low altitude near the magnetic equator may occur when neutral atoms re-ionize and become trapped on impacting the thermosphere, and this belt has been found in particle measurements near the equator and is enhanced during periods of magnetic activity. A region of enhanced optical emission due to precipitating neutrals is found in the thermosphere near the magnetic equator in both disturbed and quiet times, implying a low L value and/or pancake pitch angle distribution for the ring current particles that give rise to these neutrals. An isotropic pitch angle distribution is present in parts of the ring current at time during magnetic storms. This gives rise to neutral atom precipitation at all latitudes, and particularly of particles near 90° pitch angle in the region of SAR arc occurrence, about 10° in dip latitude equatorward of the isotropic region. The rate of energy deposition and the rate of production of ionization in the thermosphere depend on the ion species present in the ring current; their energy spectra, and on the distributions of the ions with L value and pitch angle. The rate of energy deposition may at times reach 10 −2 to 10 −1 mWm −2, sufficient for significant heating and wind generation. The rate of production of ionization in the thermosphere at night may be much greater than that of other low latitude night-time ionization sources.

Charged particle anisotropies in Saturn's magnetosphere

We report observations of anisotropies and pitch angle distributions for 0.5–1.8 MeV protons, 7–17 MeV electrons and ⪞3.4 MeV electrons in Saturn's magnetosphere made with the University of Chicago experiments on Pioneer 11. In the outer magnetosphere (L &gt;6) there is clear evidence for corotation of the proton flux, and the proton pitch angle distribution shows maximum flux perpendicular to the magnetic field (‘pancake’ distribution). Observed changes in the amplitude and shape of the pitch angle distributions suggest the existence of significant temporal variations in the outer magnetosphere. From L=6 to L=4, the proton intensity decreased by more than two orders of magnitude, and the pitch angle distribution shifted to a ‘dumbbell’ form (maximum flux parallel to magnetic field). The shift in pitch angle distribution most likely results from preferential absorption of large pitch angle particles by the tenuous E‐ ring found in the equatorial plane out to at least R=5 Rs. For L &lt;4, the proton intensity increased inwards, implying an inner edge for the E ring at R ≈4 Rs. Except for regions where the flux was reduced by satellite absorption, the pitch angle distributions remained dumbbell. In absorption regions, pancake distributions were found. The observations are consistent with the suggestion by McKibben and Simpson (this issue) that inward diffusion and acceleration at Saturn may proceed primarily via large, infrequent disturbances. Electron anisotropies were measurable only for L ⪝4.5, and the pitch angle distributions were found to be pancake for the entire region L &lt;4.5, suggesting that the absorbing particles in the E ring have radii that lie between the range of ∼1 MeV proton and ∼10 MeV electrons, or of the order of millimeters.