bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of solar flare soft X Rays and their relation to particle events observed with IMP 8
Authors: Nonnast J. H. , Armstrong T. P. , Kohl J. W.
Published: February 2008

Data from the Johns Hopkins University Applied Physics Laboratory charged particle measurement experiment aboard IMP 8 was used to study solar flare events and find correlations between soft X rays (4–17 Å) and particle events. The results show that a greater probability exists of observing a proton event if the observed soft X ray flare has a relatively large decay time. It is also found that the Hα brightening area is a good indicator of the strength of the soft X ray burst. Results also show that the strength of the soft X ray burst can be used to estimate a probability that a proton event will occur, along with the strength of that particle event.
The effects of a conducting E layer on classical F region cross‐field plasma diffusion
Authors: Vickrey James F. , Kelley Michael C.
Published: February 2008

The rate of cross‐field plasma diffusion in the F region ionosphere is significantly increased when the magnetic field lines thread a highly conducting E region below. This reduces the lifetime of small‐scale F region electron density irregularities in the polar ionosphere where the presence of a highly conducting E region is commonplace. A simple model is developed to describe the effects of a conducting E layer on classical F region plasma diffusion. In the absence of an E region, the difference in ion and electron diffusion rates leads to a charge separation and, hence, to an electrostatic field that retards ion diffusion. When the highly conducting magnetic field lines are tied to a conducting E region, however, electrons can flow along B to reduce the ambipolar diffusion electric field, and ions can proceed perpendicular to B at a rate approaching their own (higher) diffusion velocity. It is shown that the enhanced total diffusion rate that results depends strongly on the height of the F layer and on the ratio of the E to F region Pedersen conductivities. Although the enhanced classical diffusion rate hastens the removal of irregularities once their production source is removed, it is not a strong enough damping mechanism to prevent instabilities from operating routinely in the polar ionosphere. However, the E region probably plays an important role in determining the scale size of the irregularities that are favored. E region ‘images’ may be important for low E region electron densities and small scale sizes, in which case the diffusion rate is lowered. However, if the E region conductivity is high, the presence of images only reduces the F region cross‐field plasma diffusion rate by about 25% from the ion rate. We hypothesize that the spectrum of high‐latitude plasma density irregularities is controlled at large scales (λ ≳ 10 km) by structured soft electron precipitation and classical diffusion. Smaller scale waves are produced by plasma instabilities operating on the edges of the large scale structures. The generalized
instability (including the current convective process) acts to strengthen waves in the intermediate scale size (100 m ≤ λ ≤ 10 km) in regions where the geometry is appropriate or where field‐aligned currents are significant. Universal drift waves transfer energy from the intermediate scale to smaller structures but are ineffectual at large scales. The classical diffusion process described herein is applied (in conjunction with a model of irregularity production and convection) to the problem of explaining the morphology of the large scale high‐latitude irregularities in a companion paper (Kelley et al., this issue). The anomalous diffusion due to the instabilities mentioned above is also described in more detail.
Wave heating and acceleration of solar wind ions by cyclotron resonance
Authors: Marsch E. , Goertz C. K. , Richter K.
Published: February 2008

In order to explain the ion temperature anisotropies and differential speeds, as observed in solar wind high‐speed streams, a fluid type model is presented that takes into account ion heating and acceleration by resonant wave‐particle interactions. Ion‐cyclotron and fast magnetosonic waves propagating away from the sun parallel to the interplanetary magnetic field are considered. The radial evolution of the spectral wave energy density and of the bi‐Maxwellian model ion distributions is calculated self‐consistently for a spherically symmetric solar wind geometry. Numerical results are given for the dependence on heliocentric radial distance for the ion parallel and perpendicular temperatures and ion speeds relative to their center of mass frame. The respective ion flow speeds in the inertial frame are also calculated, based on momentum equations that include the self‐consistent temperature gradients. It is shown that transfer of wave momentum to the ions can lead to a preferential acceleration of the alpha particles with respect to the protons. Owing to the combined action of the ion cyclotron and magnetosonic waves, the alphas are accelerated to a differential speed of about the local Alfvèn speed in close accord with in situ observations. By damping of wave energy the heavier ions are also preferentially heated with the result that alpha particle thermal speeds become equal or slightly larger than proton thermal speeds. Typical signatures in ion temperature anisotropies (like Tp⊥ > Tp∥) as predicted by the model agree fairly well with the observations in fast streams. The results are discussed with respect to the effects of various boundary conditions and the inhomogeneity of the expanding solar wind plasma.
The structure of perpendicular bow shocks
Authors: Leroy M. M. , Winske D. , Goodrich C. C. , Wu C. S. , Papadopoulos K.
Published: February 2008

A hybrid simulation model with kinetic ions, massless fluid electrons, and phenomenological resistivity is used to study the perpendicular configuration of the bow shocks of the earth and other planets. We investigate a wide range of parameters, including the upstream Mach number, electron and ion beta (ratios of thermal to magnetic pressure), and resistivity. Electron beta and resistivity are found to have little effect on the overall shock structure. Quasi‐stationary structures are obtained at moderately high ion beta (βi ∼ 1), whereas the shock becomes more dynamic in the low ion beta, large Mach number regime (βi ∼ 0.1, MA > 8). The simulation results are shown to be in good agreement with a number of observational features of quasi‐perpendicular bow shocks, including the morphology of the reflected ion stream, the magnetic field profile throughout the shock, and the Mach number dependence of the magnetic field overshoot.
Estimates of plasma wave amplitudes in the turbulent high‐latitude E region using electron temperature measurements
Authors: St.‐Maurice J.‐P. , Schlegel Kristian
Published: February 2008

The excitation of unstable waves by large perpendicular currents in the high‐latitude E region is a well‐known phenomenon and has been documented experimentally. It has been found recently that the unstable waves heat the E region plasma to produce easily detectable effects on the electron temperature (increases up to 1000°K have been measured). The present paper offers a refinement of previous plasma heating rate calculations by including finite Larmor radius effects in the heating rate expressions and by selecting values for the parallel wave vectors that are more appropriate for the E region plasma. As was done previously, we use a quasi‐linear theory to estimate the amount of heating going into the electron gas. Since it is based on measured electron temperature values, the amount of heating is the same as found previously. However, the heating rate is now described as a function of the saturation amplitude of the unstable waves. As a result, we are able to estimate the level of turbulence in the plasma from a measurement of the electron temperature. By using Chatanika radar measurements, we infer the presence of mean density fluctuation levels of the order of 2–4% when perpendicular currents are well above threshold values.
Numerical evaluation of the precursory increase to the Forbush decrease expected from the Diffusion Convection Model
Authors: Nishida Atsuhiro
Published: February 2008

A model calculation of the cosmic ray Forbush decrease is performed on the basis of the diffusion‐convection model. The main purpose is to evaluate the magnitude of the precursory increase expected from a model where the cosmic ray particles are swept away by traveling disturbances in which the solar wind speed is raised and/or the diffusion coefficient is reduced. It is found that the ratio of the precursory increase to the main decrease is less than ∼10% if the field‐aligned diffusion coefficient is larger than ∼3×1021 cm²/s at 1 AU and the speed of the solar wind disturbance is less than ∼1000 km/s. Existence of a small precursory increase of this magnitude does not seem to contradict the observations of the Forbush decrease. The precursory increase can be >20% of the main decrease when the speed of the disturbance is >2000 km/s, however, and August 4, 1972, event seems to represent such a case.
STARE observations of a Pc 5 pulsation with large azimuthal wave number
Authors: Allan W. , Poulter E. M. , Nielsen E.
Published: February 2008

In recent years the STARE system has been used to analyze Pc 5 pulsations of several different types. In this paper a detailed description of a new type of pulsation is given. The event described occurred during local magnetic afternoon; it had small amplitude (|E| ≲ 5 mV m−1 in the ionosphere), a period which varied from 220 to 385 s during the event, and a large azimuthal wave number (m ∼ 35) which varied such as to keep the azimuthal phase velocity approximately constant for a given geomagnetic latitude; propagation was geomagnetic westward. When mapped into the equatorial plane, the properties of the wave strongly suggest that the wave phase velocity was determined by hot proton drift motions near the equatorial plane. The observational results are compared with theoretical properties of the drift mirror instability. While the results do not contradict the predicted properties, it is felt that further theoretical development is required.
Ray tracing of ULF waves in a multicomponent magnetospheric plasma: Consequences for the generation mechanism of ion cyclotron waves
Authors: Rauch J. L. , Roux A.
Published: February 2008

Intense ultralow‐frequency waves are commonly observed on GEOS 1 and 2 spacecraft, around FHe+, the helium gyrofrequency. These waves were identified as ion cyclotron waves (ICW's) by Young et al. (1981), who showed their close connection with a sufficient amount of He+ in the magnetospheric plasma. Motivated by these observations, we present a ray tracing study of ULF waves below FH+, the proton gyrofrequency. In the presence of He+ the dispersion relation of ULF waves is split into three branches, and we have studied the ray paths for these three branches. Of particular interest is the ion cyclotron branch, which is left handed above the new crossover frequency Fcr introduced by the presence of He+ ions. This mode is amplified, in the equatorial region, by anisotropic (T⊥ > T∥) energetic protons. It is well guided along field lines, suffers a polarization reversal when F = Fcr locally, and continues to be guided up to the point where F = Fbi, the bi‐ion hybrid frequency. Then it is reflected and returns to the equator, where it is amplified again. Thus such waves undergo several bounces through the amplifying region without significant drift, either azimuthal or radial. This mirroring effect, which can only occur when minor ions, such as He+, are present, is believed to be crucial for ICW amplification. We also show that for each such equatorial crossing the parallel wave number is conserved while the perpendicular one progressively increases. As a consequence, ICW's become quasi‐electrostatic, which enables them to acquire a small but finite parallel electric field; it is suggested that this electric field can in turn accelerate thermal electrons parallel to B.
Drift and aspect sensitivity of scattering irregularities in the upper equatorial E region
Authors: Røyrvik O.
Published: February 2008

Recent observations at Jicamarca, Peru, of VHF radio signals coherently scattered from the upper equatorial E region during midday hours show that the corresponding irregularities in the electron density distribution are aligned along the earth's magnetic field. Vertical electron drift velocities in the altitude region between 120 km and 170 km show spatial and temporal variations of as much as 30%. These variations are attributed to small‐scale varying electric fields which are generated by the dynamo action of gravity waves in the neutral atmosphere.
Pioneer 11 observations of trapped particle absorption by the Jovian ring and the satellites 1979, J1, J2, and J3
Authors: Pyle K. R. , McKibben R. B. , Simpson J. A.
Published: February 2008

Pioneer 10 and 11, during their encounters with Jupiter in 1973 and 1974, penetrated to L values of 2.8 and 1.7, respectively. During these encounters, at several L values, decreases in the intensity of energetic trapped particles were observed, some of which could be explained as due to absorption by the known moons Io and Amalthea; however, some decreases inside L=4 could not be explained. The recent Voyager 1 and 2 optical discoveries of several new moons and a ring in this region has led us to reexamine our particle data, and we summarize results in this report. We report observations in three channels: protons 0.5–8.7 MeV; electrons > 3.4 MeV; and medium‐Z nuclei, probably oxygen and sulfur >70 MeV/nucleon. We find that with the additional moons and the ring, all observed intensity features in the stably trapped radiation are accounted for by satellite and ring absorption.
Winter anomalous effects of mode conversion observed in mid‐latitude VLF transmission
Authors: Muraoka Yoshikazu
Published: February 2008

Mode conversion effects were observed on a long‐distance VLF propagation path at mid‐latitudes during two winters: 1974/75 and 1975/76. The occurrence of amplitude minima as a result of interference between modal components was found to be delayed in the sunrise transition during the D region winter anomaly. At the same time the depth of amplitude fading and the magnitude of phase step decreased and, sometimes, became indiscernibly small. These anomalous effects of mode conversion are interpreted as being due to the lowering of the nighttime VLF reflection height in excellent agreement with the high absorption of HF radio waves in the daytime. The implication is that the electron density in the D region is enhanced day and night during winter anomalous days. Dynamical processes in the middle atmosphere are likely to play an important role in the day‐long enhancement.
A perpendicular ion beam instability: Solutions to the linear dispersion relation
Authors: Kintner Paul M. , Kelley Michael C.
Published: February 2008

A 200‐eV Xe+ ion beam directed perpendicular to the terrestrial magnetic field in the F region ionosphere produced very narrow band electrostatic emissions just above multiples of the hydrogen cyclotron frequency. Although the plasma conditions associated with the ion beam were undoubtedly very complex, we consider first a simple ion beam in a background ionosphere. The dispersion relation for flute mode waves and an unmagnetized perpendicular ion beam is solved for a diffuse H+ plasma and then for a combination of dense O+ and diffuse H+. These solutions account for most of the wave properties, including the observation of narrow spectral peaks separated by the hydrogen cyclotron frequency and the observation of no spectral peaks below 2000 Hz. We cannot dismiss field‐aligned currents associated with the Xe+ beam as an alternate source of free energy for the narrow band emissions. However, our intention here is to examine closely the Xe+ beam as a source for directly exciting the plasma waves.
Nuclear cascades in Saturn's rings: Cosmic ray albedo neutron decay and origins of trapped protons in the inner magnetosphere
Authors: Cooper John F.
Published: February 2008

The nearly equatorial trajectory of the Pioneer 11 spacecraft through Saturn's high energy proton radiation belts and under the main A‐B‐C rings provided a unique opportunity to study the radial dependence of the >30 MeV proton intensities in the belts in terms of models for secondary nucleon production by cosmic ray interactions in the rings, in situ proton injection in the radiation belts by neutron beta decay, magnetospheric diffusion, and absorption by planetary rings and satellites. Maximum trapped proton intensities measured by Pioneer 11 in the radiation belts are compared with calculated intensities and found consistent with trapping times ≈40 years and a radial diffusion coefficient DLL ≈ 10−15 L9 RS²/s. Differential energy spectra J(E) ∝ E−2 estimated from integral measurements of trapped protons with E > 10² MeV are consistent with the beta decay model, but an inferred turndown of the spectra toward lower energies and reported integral proton anisotropies of the form J(αeq) ∝ sin4–6 αeq both indicate the need for more realistic calculations of the neutron source from the rings and the radiation belt loss processes.
Comparison of S3‐3 polar cap potential drops with the interplanetary magnetic field and models of magnetopause reconnection
Authors: Wygant J. R. , Torbert R. B. , Mozer F. S.
Published: February 2008

Measurements of the cross polar cap electric potential, by the double probe electric field experiment aboard S3‐3, from 55 orbits in the dawn‐dusk plane are compared with the reconnection electric fields predicted by a variety of models, both theoretical and experimental. The purpose of these comparisons is to understand the extent to which nonreconnection contributions to the polar cap potential must be included, to determine the time response of the polar cap potential to time varying reconnection rates, and to determine the efficiency and saturation levels of the reconnection process. It is found that (1) After several hours of northward interplanetary magnetic field, the cross polar cap potential declines to progressively lower values than those after 1 hour of northward interplanetary magnetic field. This suggests that it requires several hours for the ionospheric polar cap potential to respond to the ‘turning off’ or ‘turning down’ of the reconnection process. (2) The decay of the polar cap potential is used to demonstrate that contributions to the polar cap potential not associated with the reconnection process can be limited to less than 20 kV. It is shown that contributions to the polar cap potential that scale with the dynamic pressure of the solar wind are limited to less than 1 kV. (3) The cross polar cap electric potential is best predicted by a weighted sum of contributions from interplanetary magnetic field parameters over the 4 hours previous to the measurement. The weighting functions have the form of an exponential decay of 2–3 hours with the strongest weight on interplanetary parameters over the 1 hour previous to the measurement. (4) For values of the dawn dusk component of the interplanetary electric field less than about 0.5 mV/m, the measured polar cap potential is consistent with reconnection of all the interplanetary magnetic flux incident on a 30 RE wide frontside magnetopause. For larger values of the dawn dusk component of the interplanetary electric field, the proportion of field lines reconnecting is less, indicating saturation of the reconnection process. The above results are obtained when the extended response time of the magnetosphere to changes in interplanetary parameters is considered. They are independent of the detailed reconnection model assumed, and they are quite different from the viscous contribution, saturation levels of reconnection, and reconnection efficiencies inferred from comparisons of polar cap potentials to single hour averages of interplanetary parameters.
Geometry of depleted plasma regions in the equatorial ionosphere
Authors: Mendillo Michael , Tyler Ann
Published: February 2008

An image intensified, all‐sky, 6300‐Å airglow imaging system operating on Ascension Island is early 1981 has been used to assemble a photographic data base of several hundred airglow depletion patterns. By mapping the airglow depletion features along geomagnetic field lines to the equatorial plane, the geometrical characteristics of plasma depleted regions may be described over the 400‐ to 1200‐km altitude region above the equator. Such an analysis revealed that single or linear depletions tilted toward the west with an average skewness of 0.6° longitude west/100‐km altitude, corresponding to a zenith angle tilt of ∼40° at a height of 800 km. The tilts increased with local time during the evening hours, showing that an effective velocity shear of ∼0.05 m/s/km is required to account for the observed evolution. In several cases, the depletion geometry showed a ‘wishbone’ or bifurcated pattern. Such features had a mean height of bifurcation of 740 km and tilt factors very similar to those found for linear depletions observed during the same local time periods.
Interaction of ring current ions with the magnetopause
Authors: Scholer M. , Ipavich F. M.
Published: February 2008

We have analyzed phase space density observations of ring current protons (30–130 keV) during a magnetopause crossing of ISEE 1 on July 5, 1978. Plasma and magnetic field data obtained during this crossing have been reported to be consistent with quasi‐steady reconnection and a reconnection line south of the satellite crossing (Sonnerup et al., 1981). We find that within the magnetosphere the phase space densities of ∼30‐ to ∼65‐keV protons are higher for particles streaming parallel to the field (i.e., inflowing particles) than antiparallel to the field (i.e., outward flowing particles). In the magnetosheath the phase space densities parallel to the field are higher than the densities of both outward and inward flowing particles parallel to the field within the magnetosphere. The observations are discussed in terms of acceleration of the reflected and transmitted ring current protons in the magnetopause current layer.
Triggering of expansive phase intensifications of magnetospheric substorms by northward turnings of the interplanetary magnetic field
Authors: Rostoker Gordon
Published: February 2008

Over the past decade evidence has arisen suggesting that northward turnings of the interplanetary magnetic field after an interval of sustained southward IMF can trigger the onset of major substorm expansive phase effects. However, there has been some question in the past as to just how many substorms are triggered and whether, indeed, the correlation of substorm onsets and northward turnings was just a matter of chance. One reason for doubt was that the examples presented in the literature appeared during disturbed periods where the IMS was fluctuating significantly on a time scale of less than one hour and many substorm expansive phase intensifications were observable in the time frame where the correlation between the IMF and ground data was reported. The purpose of this paper is to present three clear‐cut examples of isolated substorms where the reader may be able to convince himself of the validity of the reported triggering effect. For these events, it will be clear that the substorm current wedge triggered by the northward turning of the IMF carries substantially larger currents than the driven system associated with the overall substorm event. The role of the IMF in externally triggering episodes of Joule heating and particle precipitation within the substorm process will be discussed.
Earth's magnetic field as a radiator to detect cosmic ray electrons of energy >1012eV
Authors: Stephens S. A. , Balasubrahmanyan V. K.
Published: February 2008

We have examined in detail the synchrotron emission by electrons of energy greater than a few TeV in the earth's magnetic field. The photon spectrum lies in the X ray and γ ray region. As the emission takes place in a narrow cone along the direction of the electron, the photons would be incident nearly along a straight line on a detector. This unique feature provides the signature to identify the electron unambiguously. The mean energy of the photons being proportional to the square of the electron energy allows us to determine the energy accurately. Though it may appear that one needs to know the arrival direction of electrons to obtain its energy, we have shown that an omnidirectional detector can be satisfactorily used to estimate the energy. We also show that the collecting power of the detector is a sensitive function of the area of the detectorA, the energy of electronE0, and the number of photons required to identify an electronnγ; asymptotically the collecting power is proportional toA1.43E0nγ−1.8. An instrument, with an energy threshold for the detection of photons, can be used to measure reliably the integral flux of electrons, even if it has limited energy resolution. We have calculated the event rate expected by using an ideal balloon‐borne detector capable of detecting above 20 keV at 4 g cm−2of atmospheric depth over Palestine, Texas, and compared with the expected rates using instruments based on currently available techniques of detection.
Polarization patterns of Pi 2 magnetic pulsations and the substorm current wedge
Authors: Lester Mark , Hughes Jeffrey W. , Singer Howard J.
Published: February 2008

The results of an analysis of 16 Pi 2 pulsations observed with an extended east–west chain of mid‐latitude, ground‐based magnetometers are reported. The events were chosen such that the center of the substorm current system, defined by using the midlatitude bays associated with these pulsations, was within the longitudinal extent of the station chain. A current wedge model is also used to interpret the observed polarization pattern of the Pi 2 pulsations. This pattern is used to locate the center of the Pi 2 current system. The centers of the Pi 2 and substorm current systems occur at the same meridian for only ∼65% of the events. This result suggests that the Pi 2 current system and the substorm current wedge are not always the same. The longitudinal extent of the Pi 2 current system estimated from the Pi 2 polarization pattern averages ∼90° or ∼6 hours local time for the events in the study. The sense of ellipticity of the waves, anticlockwise looking down the field line in the northern hemisphere, agrees with previous results from these latitudes. Estimates of the wave‐phase difference between stations show that, in general, the eastern station of a station pair leads the western one at all local times for both H and D components. When plotted in a substorm coordinate system based on the mid‐latitude bay, the phase difference per degree of longitude shows a tendency to decrease in the eastern portion of the current wedge. This longitudinal pattern of phase difference is consistent with the eastern, downward, field‐aligned currents being less localized than the western, upward, field‐aligned currents.
Modeling the total electron content observations above Ascension Island
Authors: Anderson D. N. , Klobuchar J. A.
Published: February 2008

Recent Faraday rotation (136 MHz) measurements between Ascension Island (8° S geographic latitude; 345° E geographic longitude; 15° S. dip latitude) and the Sirio satellite (azimuth 0°, elevation 80°) reveal two characteristic features associated with the undisturbed, ambient ionosphere: a late afternoon decrease in TEC followed by a postsunset enhancement that lasts for two to three hours. We theoretically investigate the ambient ionosphere above Ascension Island by numerically solving the time‐dependent plasma continuity equation, including the effects of ionization production by solar ultraviolet radiation, loss through charge exchange and transport by diffusion,
drift and neutral wind (both zonal and meridional components) appropriate for an equinoctial, solar cycle maximum period. It is found that the postsunset enhancement in upward
drift, which is a characteristic feature observed by the Jicamarca incoherent scatter radar facility during solar cycle maximum periods, is primarily responsible for the postsunset increase in TEC observed at Ascension Island between 1900 and 2300 LT. The late afternoon decrease in TEC is caused by an increase in the poleward neutral wind velocity, which lowers the F layer into a higher loss rate region. Inclusion of the meridional wind not only reproduces the observed decrease but also modulates the postsunset peak in TEC so that it is in much better agreement with the observed values. Calculated north‐south asymmetries in TEC caused by a zonal wind component are also discussed.
Comparison of heliospheric current sheet structure obtained from potential magnetic field computations and from observed polarization coronal brightness
Authors: Wilcox John M. , Hundhausen A. J.
Published: February 2008

We compare the structure of the heliospheric current sheet early in sunspot cycle 21 as computed from the observed photospheric magnetic field with a potential field approximation and as inferred from synoptic maps of the observed coronal polarization brightness. On most of the solar rotations compared, the two methods give essentially the same results; the basic shape of the warped current sheet and the amplitude (in solar latitude) of the displacements of the sheet from the solar equator are similar. On one rotation the current sheet computed with the potential field approximation appears to be distorted by a large photospheric region of unbalanced magnetic flux.
Survey of low‐energy plasma electrons in Saturn's magnetosphere: Voyagers 1 and 2
Authors: Sittler E. C. , Ogilvie K. W. , Scudder J. D.
Published: February 2008

This paper is a survey of the low‐energy plasma electron environment within Saturn's magnetosphere made by the plasma science experiment (PLS) during the Voyager encounters with Saturn. Over the full energy range of the PLS instrument (10 eV to 6 keV) the electron distribution functions are clearly non‐Maxwellian in character; they are composed of a cold (thermal) component with Maxwellian shape and a hot (suprathermal) non‐Maxwellian component. A large‐scale positive radial gradient in electron temperature is observed, increasing from less than 1 eV in the inner magnetosphere to as high as 800 eV in the outer magnetosphere. This increase in electron temperature explains the observed order of magnitude increase in plasma sheet thickness with increasing radial distance from Saturn. Scale heights of the cold heavy ion plasma can be as small as 0.2 Rs in the inner magnetosphere and as much as 3 Rs in the outer magnetosphere. Many of the observed density variations can be attributed to changes in density scale height without a change in plasma flux tube content. Three fundamentally different plasma regimes have been identified from the measurements: (1) the hot outer magnetosphere, (2) the extended plasma sheet, and (3) the inner plasma torus. The hot outer magnetosphere is a region within which the suprathermal electrons are the dominant contributors to the electron pressure, and at times to the electron density. Near the noon meridian, the electrons display a highly time dependent behavior with order of magnitude changes in density and temperature, which can occur in less than 96 s. Sudden density enhancements of cold plasma occur, which are thought to be either “plumes” associated with Titan (Eviatar et al., 1982) or plasma “blobs”, (Goertz, 1983). The extended plasma sheet, with mean inner and outer boundaries of 7 and 15 Rs, respectively, has enhanced levels of cold plasma relative to that in the hot outer magnetosphere, although the suprathermals continue to dominate the electron pressure. Plasma with energy less than 6 keV from this region is an important contributor to the ring current, and a significant current system is probably present between 6 and 8 Rs. The inner plasma torus is a region of reduced electron temperature (as low as 1 eV), enhanced equatorial densities (as high as 100/cm³), and reduced scale height (as small as 0.2 Rs). Localized reductions in electron temperature are observed near the L shells of Tethys, Dione, and possibly Rhea. The suprathermal electrons are observed to be severely depleted within the inner plasma torus, relative to that in the extended plasma sheet and hot outer magnetosphere. The energy dependence of the depletions indicate an interaction with dust or plasma waves at times; interactions with neutral gas or plasma ions may also contribute to the depletions. The data also indicate an association between the appearance of suprathermal electrons and the observed emission of whistler mode waves reported by Gurnett et al. (1981).
General characteristics of hot plasma and energetic particles in the Saturnian magnetosphere: Results from the Voyager spacecraft
Authors: Krimigis S. M. , Carbary J. F. , Keath E. P. , Armstrong T. P. , Lanzerotti L. J. , Gloeckler G.
Published: February 2008

The low energy charged particle (LECP) experiment on the Voyager 1 and 2 spacecraft made measurements of the intensity, energy spectra, and spatial distributions of ions (30 keV ≲ E ≲ 150 MeV) and electrons (22 keV ≲ E ≲ 20 MeV) during encounters with the Saturnian magnetosphere in November 1980 and August 1981, respectively. Detailed analysis of the data has revealed the following: (1) Energetic ions are present in the interplanetary medium both upstream (to ∼200 Rs) and off the dawn bow shock (to ∼400 Rs) of the magnetosphere, with maximum energies ∼100 keV. (2) Low‐energy (≳22 keV) electrons are generally depleted inward of L ∼ 10 Rs, while low‐energy (≳30 keV) ions are greatly enhanced in the same region. (3) The composition of low‐energy ions is most likely dominated by protons in the outer magnetosphere but is consistent with oxygen in the inner (L ≲ 9) magnetosphere. (4) The ion spectrum is described well by the κ distribution with characteristic temperatures kTH ranging from ∼15 to ∼55 keV; the hot plasma region is generally confined between the L shells of Tethys and Rhea but exhibits substantial variability. (5) The electron energy spectrum at L ≲ 10 develops a secondary peak at E ≳ 200 keV that shifts to higher (∼1 MeV) energies inside the orbits of Enceladus and Mimas, indicative of electron resonance interactions with the planetary satellites. (6) There is a noon‐dawn asymmetry in ion and electron intensities with peak fluxes near the Rhea‐Dione L shells at local morning; this is the region in local time where Saturn kilometric radiation is modulated by the presence of Dione. (7) The ion energy density (≳30 keV) represents a significant fraction of the field energy density in the outer magnetosphere of the planet (L ≳ 13 Rs), with values of β ranging from 0.1 up to ∼4, when projected to the equator. (8) Comparison of electron and ion intensities measured by Voyagers 1 and 2 in the inner (L ≲ 6) magnetosphere at common points in B, L space shows that the radiation belts are substantially stable over periods of ∼9 months; both ion and electron intensities compared well with Pioneer 11 observations in 1979. It is evident from the results that the inner satellites of Saturn play a dominant role in the determination of intensity and spectral features of energetic particles at L ≲ 10. These aspects of the data are discussed in the context of proposed physical mechanisms expected to be operating within the magnetosphere of Saturn.
Voyager observations of Saturnian ion and electron phase space densities
Authors: Armstrong T. P. , Paonessa M. T. , Bell E. V. , Krimigis S. M.
Published: February 2008

Voyager 1 and 2 low‐energy charged particle (LECP) observations of 30‐keV to 2‐MeV electron and ion energy spectra and angular distributions have been used to calculate phase space densities at constant first and second adiabatic invariants in the Saturnian magnetosphere. The results are generally consistent with inward radial diffusion from an external source. The data obtained also indicate a source of ions located within the orbital distance of Enceladus capable of producing 10‐ to 40‐MeV/Gauss ions as well as a source of electrons at about 3.5 Rs which produces particles at 100 to 200 MeV/Gauss. Higher magnetic moment (200–400 MeV/Gauss) ions extend from the sunward boundary between a plasma mantle and the region of durable trapping at Rs; the behavior of the phase space density suggests inward diffusion of these particles from a source at the boundary. The identification of sources of low (10 to 200 MeV/Gauss) magnetic moment particles deep in the Saturnian magnetosphere is a new result of this work. Several analyses of the observed phase space densities in terms of time‐independent radial diffusion are presented.
Voyager plasma wave measurements at Saturn
Authors: Scarf F. L. , Gurnett D. A. , Kurth W. S. , Poynter R. L.
Published: February 2008

The entire set of Voyager plasma wave observations at Saturn is summarized and attention is focused on several specific problem areas. The topics that are emphasized include (1) the size and location of the Saturn foreshock, (2) possible detection of the Jovian magnetic tail after the Saturn encounter, (3) pitch angle scattering of electrons by equatorial whistler mode emissions, and (4) the detection of dust impact signals near the ring plane and at other locations in the magnetosphere.
The structure of the heliospheric current sheet: 1978–1982
Authors: Hoeksema J. Todd , Wilcox John M. , Scherrer Philip H.
Published: February 2008

The structure of the heliospheric magnetic field changes substantially during the 11‐year sunspot cycle. We have calculated its configuration for the period 1976–1982 by using a potential field model, continuing our earlier study near solar minimum in 1976–1977 (Hoeksema et al., 1982). In this paper we concentrate on the structure during the rising phase, maximum, and early decline of sunspot cycle 21, from 1978 to 1982. Early in this interval there are four warps in the current sheet (the boundary between interplanetary magnetic field toward and away from the sun) giving rise to a four‐sector structure in the interplanetary magnetic field observed at earth. The location of the current sheet changes slowly and extends to a heliographic latitude of approximately 50°. Near maximum the structure is much more complex, with the current sheet extending nearly to the poles. Often there are multiple current sheets. As solar activity decreases, the structure simplifies until, in most of 1982, there is a single, simply shaped current sheet corresponding to a two‐sector interplanetary magnetic field structure in the ecliptic plane. The sun's polar fields, not fully measured by magnetographs such as that at the Stanford Solar Observatory, substantially influence the calculated position of the current sheet near sunspot minimum. We have determined the strength of the polar field correction throughout this period and include it in our model calculations. The lower latitude magnetic fields become much stronger as the polar fields weaken and reverse polarity near maximum, decreasing the influence of the polar field correction. The major model parameter is the radius of the source surface, the spherical surface at which the field lines become radial. Correlations of interplanetary magnetic field polarity observed by spacecraft with that predicted by the model calculated at various source surface radii indicate that the optimum source surface radius is not significantly different from 2.5 RS during this part of the solar cycle.
Wave‐particle interactions in the Venus wake and tail
Authors: Intriligator D. S. , Scarf F. L.
Published: February 2008

We present the first analysis of Pioneer Venus Orbiter plasma, electric field, and magnetic field observations in the Venus tail. We have studied the first season of Pioneer Venus Orbiter tail passage (approximately June 1979) in order to determine the main plasma and field configuration in this region at this time and to ascertain some of the basic physical processes. Our analysis shows that the boundary of the Venusian tail is often characterized by changing plasma distributions and enhanced plasma wave activity. We use summary plasma probe parameters to argue that the waves are Doppler‐shifted ion acoustic oscillations. In the magnetotail region there is generally an exclusion of high‐density plasma, but when plasma is detected, the distributions often appear to be non‐Maxwellian at the highest time resolution, and these distorted distribution functions are generally accompanied by enhanced plasma wave signals and magnetic field reversals indicative of electric currents. On the basis of our analyses of the high‐resolution plasma and wave observations for orbit 189, we identify a different time from that previously defined on the basis of the magnetic field data for the spacecraft entrance into the magnetotail. The wave activity in the Venus tail appears similar to the broadband noise identified in the earth's tail, but at Venus the levels are usually higher.
Comparison of an analytical approximation for particle motion in a current sheet with precise numerical calculations
Authors: Speiser T. W. , Lyons L. R.
Published: February 2008

Approximate analytic solutions exist for particle motion in a one‐dimensional current sheet with a constant normal magnetic field component. These solutions are tested against precise numerical calculations, and a range of validity of the analytic solutions is inferred. For example, in the geomagnetic tail neutral sheet, for a dawn‐dusk electric field of 0.1–1 mV/m, lobe field of 10–40 nT, and sheet thickness of 1000 km, the analytic solutions serve as a good predictor of particle motion when the normal magnetic field component is less than 3 or 4 nT. By using the analytic solutions, initial distribution functions are mapped into final (accelerated) distributions, and the analytic mappings are compared with numerical mappings.
On the equatorial confinement of thermal plasma generated in the vicinity of the rings of Saturn
Authors: Ip W.‐H.
Published: February 2008

Consideration of the force balance of the ring plasma generated by meteoroid impact in the rotating ionosphere of Saturn is extended to include the magnetic mirroring effect. It is found that there are modifications to the so‐called siphon flow limit derived for charged particles with zero magnetic moment if the ionospheric plasma has a thermal temperature exceeding a few electron volts. The nature of the force balance is such that, instead of a sharp division between complete loss and complete reabsorption, a transition zone of partial loss appears. If part of the ions near the rings are produced by ionization of the neutral atmosphere in the vicinity of the ring system (also generated by meteoroid impact vaporization), another theoretical limit dividing the upward flow from the equatorially confined motion is located near the observed boundary between the B ring and the C ring. This new limit is very sharp, as required to explain the B‐C ring boundary, which has not yet been explained by gravitational theory.
Observations of plasma bulk parameters and the energy balance of the solar wind between 1 and 10 AU
Authors: Gazis Paul R.
Published: February 2008

Observations are made of the radial evolution of various solar wind bulk parameters between 1 and 10 AU by using data from Voyagers 1 and 2 and the IMP 8 spacecraft. The solar wind bulk velocity, the product of the mass flux and the square of the heliocentric distance, and the product of density and the square of the heliocentric distance are all found not to vary with heliocentric distance. The proton temperature varies with heliocentric distance as R0.7±0.2. At 1 AU, the density is anticorrelated with velocity. This anticorrelation vanishes at larger distances from the sun. The solar wind bulk velocity and thermal speed are correlated over the entire region between 1 and 10 AU. An attempt is made to explain the radial variation of the solar wind proton temperature between 1 and 10 AU, and the global energy balance in the solar wind. Heating by thermal processes, conversion of bulk kinetic energy to thermal energy, and electron heat conduction are considered. It is found that the radial variation of the solar wind proton temperature between 1 and 10 AU can be explained by the addition of a heat flux due to heat conduction. This heat flux is comparable to the proton and electron thermal energy fluxes but decreases faster with heliocentric distance, varying as R−0.44.
The nonlinear gyroresonance interaction between energetic electrons and coherent VLF waves propagating at an arbitrary angle with respect to the Earth's magnetic field
Authors: Bell T. F.
Published: February 2008

A theory is presented of the nonlinear gyroresonance interaction that takes place in the magnetosphere between energetic electrons and coherent VLF waves propagating in the whistler mode at an arbitrary angle ψ with respect to the earth's magnetic field B0. In particular, we examine the phase trapping (PT) mechanism believed by some to be responsible for the generation of VLF emissions. The phase considered in this study is the phase γ between the right‐hand circularly polarized component of the wave magnetic field perpendicular to B0 and the component of the energetic particle velocity vector perpendicular to B0. This model is an extension of one developed in earlier work [Bell, 1965; Dysthe, 1971; Nunn, 1974] involving the special case where ψ = 0. The extended theory predicts that for any finite value of ψ there is a range of resonant particle pitch angle α for which γ is not bounded within the range 0 ≤ |γ| < π, and thus PT in the usual sense is not possible. However, PT in an average sense can still exist and long term energy transfer between the gyroresonant electrons and the wave can still take place. It is found that for given values of ψ and wave frequency, the trapping frequency ωτ has a countably infinite set of zeros when plotted as a function of α. Regions between zeros alternate between regions of normal PT and regions of anomalous PT. The average phase angle of anomalously phase trapped electrons differs by approximately 180° from the average phase of the normal phase trapped electrons. However, the average stimulated radiation from the two groups of particles tends to add in phase. Resonant particles for which ωτ⇒0 are not phase trapped. Near the geomagnetic equatorial plane, the threshold value of wave amplitude necessary to produce PT is directly proportional to the gradient of ψ along B0. A simple model predicts threshold values greater than 10 mγ for ψ > 37°. For interactions far from the magnetic equator, ψ variations appear to be less important than gradients in B0 and in this case we calculate the wave power increase necessary to produce the same PT efficiency for typical nonducted waves as that appropriate to ducted waves. The component of the wave electric field parallel to B0 generally enhances the strength of the PT process and tends to dominate the process for moderate to high ψ at midfrequency. It is concluded that near the magnetic equatorial plane gradients of ψ may play a very important part in the PT process for nonducted waves. Predictions of a higher threshold value for PT for nonducted waves generally agree with experimental data concerning VLF emission triggering by nonducted waves.