bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Titan on trapped particles in Saturn's magnetosphere
Authors: Maclennan C. G. , Lanzerotti L. J. , Krimigis S. M. , Lepping R. P. , Ness N. F.
Published: February 2008

The close fly‐by of the Voyager 1 spacecraft to the Saturnian satellite Titan provided an opportunity for the low energy charged particle (LECP) experiment to investigate the influences of Titan on the magnetosphere energetic particle distributions. Magnetic field data from the magnetometer (MAG) experiment are used with angular distribution data from LECP to study the changes in the pitch angle distributions of ions (40 < Ei < 220 keV), and electrons (26 < Ee < 61 ReV). Before and after the Titan encounter the ions are observed to exhibit a bulk motion in the direction expected of corotation of the particles with the planetary magnetic field. The magnetosphere corotation velocity outside the influence of Titan is deduced to be ∼100 to 150 km/s, less than the ∼200 km/s expected for rigid corotation. Titan appears to disrupt the corotation motion of the ions: the fluxes of ions in the corotation direction are appreciably diminished in intensity, while those observed in the equatorial plane perpendicular to this direction are relatively unaffected. The resulting distribution and trajectory modeling conclusions suggest that the convection electric field is effectively absent in the wake of Titan. Before and after the Titan encounter the electrons are observed to exhibit pitch angle distributions peaked at ∼90°, representative of trapped particles. The presence of Titan produces a reduction in the fluxes of the electrons and flatter distributions with pitch angle.
Experimental verification of an S‐shaped potential structure
Authors: Mizera P. F. , Gorney D. J. , Fennell J. F.
Published: February 2008

S3‐3 plasma and ac‐dc electric field data are used to construct a schematic picture of electrostatic potential structures for an afternoon auroral zone transversal. The signatures impressed on the charged particle distribution functions were used to place constraints on the latitude and altitude extent of the structures, while the electric field data provided precise definition of the structures at the satellite altitude. Upflowing field‐aligned ions, ion conics, accelerated electrons, and the associated ULF‐VLF waves are located consistently with respect to the inferred potential structures. The potential distribution is found to be V‐shaped at high altitudes and S‐shaped at low altitudes.
Threshold criterion for a space simulation beam‐plasma discharge
Authors: Szuszczewicz E. P. , Papadopoulos K. , Bernstein W. , Lin C. S. , Walker D. N.
Published: February 2008

We have conducted an experimental and theoretical study of the threshold characteristics of a space simulation beam‐plasma discharge with emphasis on density profiles and a density‐dependent ignition criterion. The study included various beam‐plasma conditions covering beam currents from 8 to 85 ma, beam energies from 0.8 to 2.0 keV, and magnetic fields at 0.9 and 1.5 G. The study included experimental determinations of radial profiles of electron density for each of the selected conditions extending from a low‐density, pre‐beam‐plasma discharge state to a strong beam‐plasma discharge condition. At beam‐plasma discharge threshold it was determined that 〈ωp/ωc〉 = 5.4 was the density‐dependent ignition criterion. The experimental results are shown to agree with detailed model calculations, which consider the beam‐plasma discharge to be produced by large‐amplitude electron plasma waves resulting from the beam‐plasma interaction.
High temporal resolution energetic particle soundings at the magnetopause on November 8, 1977, using ISEE 2
Authors: Fritz Theodore A. , Fahnenstiel Steven C.
Published: February 2008

We present a detailed analysis of >24 keV ion data obtained from the ISEE 2 satellite on an inbound crossing of the magnetopause at 1130 LT on November 8, 1977, from 0200 to 0330 UT. Based on the technique presented by Williams (1979) of sounding the position of the magnetopause using energetic particle azimuthal asymmetries, we exploit the four second time resolution available on the ISEE 2 satellite to determine the location, structure, orientation, and temporal variation of the magnetopause region. We find that the trapping boundary for energetic ions is sharp and well defined for ∼35 keV ions and that it corresponds most of the time to the earthward edge of the plasma boundary layer. Usually magnetosheath plasma penetrated the trapping boundary only up to distances approximately that of the plasma (1 keV) ion gyroradius (∼100 km). On some occasions magnetosheath‐like plasma was observed up to 800 km inside the trapping boundary but these occurrences were usually associated with rapid trapping boundary movement with velocities exceeding 50 km/s. If the trapping boundary determines the position of the last closed field line, the occasional occurrence of magnetosheath plasma deep inside the trapping boundary is inconsistent with accepted merging theories. The determination of the position of the trapping boundary using five separate ion energy channels from 24 to 70 keV was internally consistent for the lowest three channels although the higher energy channels consistently indicated somewhat smaller values. Radial motion was present affecting the position of the trapping boundary on two scales; a wave‐like oscillation with a period of ∼105 s superimposed on a larger scale irregular ‘breathing’ motion. We argue that the wave nature of the trapping boundary was the cause of the slight difference between the higher and lower energy ion trapping boundary locations.
Energetic ion composition in the subsolar magnetopause and boundary layer
Authors: Peterson W. K. , Shelley E. G. , Haerendel G. , Paschmann G.
Published: February 2008

Energetic ion mass spectrometer data obtained on ISEE 1 have shown that the plasma in the subsolar magnetospheric boundary layer, magnetopause, and adjacent magnetosheath have an ionospheric component (He+ and O+) in addition to the solar wind component (H+ and He++). We have examined in detail nine intervals where the location of the subsolar magnetopause and boundary layer are well defined by the ISEE 1 fast plasma and magnetic filed measurements. In five of the identified boundary layer intervals, keV He+ ions were observed; energetic O+ ions were seen, above background, in two of the boundary layers where He+ was observed. The temporal resolution of the ion mass spectrometer was too coarse to rule out the possibility of He+ or O+ ions being present in the four remaining intervals. Ionospheric He+ ions were also observed in the magnetosheath in two (and O+ in one) of the nine intervals studied.
Conical pitch angle distributions of very low‐energy ion fluxes observed by ISEE 1
Authors: Horwitz J. L. , Baugher C. R. , Chappell C. R. , Shelley E. G. , Young D. T.
Published: February 2008

Observations of low‐energy ionospheric ions by the plasma composition experiment aboard ISEE 1 often show conical pitch angle distributions, that is, peak fluxes between 0° and 90° to the directions parallel or antiparallel to the magnetic field. Frequently, all three primary ionospheric ion species (H+, He+, and O+) simultaneously exhibit conical distributions with peak fluxes at essentially the same pitch angle. A distinction is made here between unidirectional, or streaming, distributions, in which ions are traveling essentially from only one hemisphere, and symmetrical distributions, in which significant fluxes are observed traveling from both hemispheres. The orbital coverage for this survey was largely restricted to the night sector, approximately 2100–0600 LT, and moderate geomagnetic latitudes of 20°–40°. Also, lack of complete pitch angle coverage at all times may have reduced detection for conies with small cone angles. However, we may conclude that the unidirectional conical distributions observed in the northern hemisphere are always observed to be traveling from the northern hemisphere and that they exhibit the following characteristics relative to the symmetric distributions, in that they (1) are typically observed on higher L shells (that is, higher geomagnetic latitudes or larger geocentric distances or both), (2) tend to have significantly larger cone angles, and (3) are associated with higher magnetic activity levels.
The interpretation of ground‐based observations of energetic electron fluxes modulated by Pc 5 hydromagnetic oscillations
Authors: Poulter E. M. , Nielsen E.
Published: February 2008

Mauk et al. (1981) have presented data on modulated precipitation observed by a new X ray camera, capable of imaging an 80‐km field of view. There is some uncertainty, however, over the nature of the modulation and whether it represents a standing or traveling wave structure. A similar event showing a periodic modulation of the precipitating electron flux observed with a multi‐narrow‐beam riometer and concurrent Scandinavian twin auroral radar experiment radar data is considered. These data suggest that the observations of Mauk et al. are consistent with the apparent poleward motion of tracers associated with resonant Pc 5 events.
An evaluation of three predictors of geomagnetic activity
Authors: Holzer Robert E. , Slavin James A.
Published: February 2008

Three functions, BsV, BsV², and Bs²V where Bs is the southward component of the IMF and V, the solar wind speed, have been tested for their ability to predict geomagnetic activity as measured by the AL index. The data were selected from a period within 1 month of the June solstice for the years 1967–1974 inclusive, when AL appears to be most nearly a linear measure of magnetic activity. After correcting the index for processes not similar to substorms, separate comparisons of ΣBsmVn with ΣAL were made during isolated magnetic intervals so that the flux eroded from the forward magnetosphere could return, thus avoiding time delay estimates. Intervals during which there was evidence that the westward electrojet was displaced above or below the auroral observatory chain were eliminated. Under these carefully specified conditions, regression studies between AL and each of the three functions yielded correlation coefficients of 0.97 for BsV², 0.92 for BsV, and 0.82 for Bs²V. A study of the variation of each of three functions of the type ΣAL/ΣBsmVn with Bs and with V has indicated that m ≃ 0.85 and n = 2. Some suggestions are offered to account for the diverse forms of the empirical prediction function used by various investigators.
A practical method of predicting geomagnetic storms by using precursory enhancements of solar wind density
Authors: Rosenberg Ronald L.
Published: February 2008

The solar wind density begins to become enhanced at the same time that azimuthal flow removes material from compressions built up by stream‐stream interaction. This lateral flow delays the formation of a shock and the resulting onset of enhancement in the magnetic field. The density increases typically six hours before the field and velocity increase. Density enhancements in the ISEE 3 data are identified here by use of an algorithm that is based on both the three‐hour averages of the density and daily averages of the density. In the ISEE 3 data from the first twenty weeks following launch on August 12, 1978, 25 density enhancements were picked out by the algorithm. The times at which these enhancements started were used for superposed epoch analyses. The variables examined were the geomagnetic activity index (Kp), the ring current index (Dst), the field magnitude, the absolute value of the north‐south field component (BN), the solar wind velocity and the composite standard deviation of the field components. These variables became enhanced six or more hours after the time when the density enhancement condition was first satisfied. The distribution of current sheet crossings showed no systematic behavior in a superposed epoch treatment. The results of the superposed epoch analyses indicate that density enhancements can be used to anticipate the occurrence of enhanced geomagnetic activity.
Further evidence for the dependence of fast atmospheric light pulsations on solar activity
Authors: Tümer O. Tümay
Published: February 2008

The millisecond time scale diffuse atmospheric light emissions showing 10‐kHz damped oscillations were studied for a period of 4 years from June 12, 1972, to April 1, 1976. The experimental setup consisted of a wide‐angle photomultiplier system. New evidence was found relating these light pulses to solar activity. They were concentrated in the autumn and winter months.
VLF wave growth and discrete emission triggering in the magnetosphere: A feedback model
Authors: Helliwell R. A. , Inan U. S.
Published: February 2008

Doppler‐shifted cyclotron resonance between electrons and narrowband whistler mode waves is employed in a new feedback model to account for observed signal growth and emission triggering in controlled experiments using the Siple Station, Antarctica, VLF transmitter. The interaction region (IR) is centered on the magnetic equator and is treated like an unstable feedback amplifier with a delay line. The temporal growth rate is given by γ = (G−1)/T, where G is the loop gain and T is the effective loop delay. For G<1 the system acts like an amplifier, while for G>1 the system is unstable and can generate self‐excited oscillations. The self‐excited oscillations reach saturation when G falls to unity, which occurs when the electron transit time through the interaction region is comparable with the phase bunching time. At signal levels well below saturation the model predicts exponential temporal growth. Saturation fields just inside and outside the plasmapause are estimated to be 1 mγ and 12 mγ, respectively, consistent with measurements. Because of a progressive loop phase shift the stimulated radiation can cancel the input signal at a certain point (located near the input to the IR) in time and space, giving rise to transient conditions resembling those at the tail‐end of the triggering signal. It is postulated that this temporary null may be the cause of emission triggering, and hence the region around this point is called the ‘triggering window.’
An analysis of shock wave disturbances observed at 1 AU from 1971 through 1978
Authors: Borrini G. , Gosling J. T. , Bame S. J. , Feldman W. C.
Published: February 2008

An analysis has been made of all the interplanetary shock wave disturbances detected with Los Alamos plasma instruments aboard IMP 6, 7, and 8 from 1971 through 1978. The study of the 103 forward shocks observed reveals that shocks occur preferentially during conditions of low proton temperature and speed and that the shell of shocked gas following shock passage is typically ∼0.14 AU thick. Helium enrichments (helium/hydrogen flux ratio ≥ 8%) are observed in association with 46% of the shocks. Shocks followed by helium enrichments (He shocks) are on the average the strongest shocks observed, in the sense that they exhibit the largest jumps in flow speed, temperature, and magnetic field strength and induce the largest geomagnetic response. The geometry of sampling the disturbances may account for the difference between He and non‐He shocks. There is a tendency for interplanetary shocks and helium enrichments to be part of complex, multiple events. This may result from multiple outbursts from the same solar active region. Usually the helium enrichment is found in plasma of higher than average field strength. Since helium enrichments are thought to identify the coronal material driving the shock, our observations support the conclusion that coronal ejecta originate in solar regions where the field strength is high.
Auroral studies with a chain of meridian‐scanning photometers: 1. Observations of proton and electron aurora in magnetospheric substorms
Authors: Jones A. Vallance , Creutzberg F. , Gattinger R. L. , Harris F. R.
Published: February 2008

A chain of sensitive meridian‐scanning photometers and all‐sky cameras was operated during two new moon periods in 1978 to observe auroral proton and electron precipitation patterns with high time resolution (30 s). The latitude range was from 59.5° to 73.3° N (invariant) with Churchill, Manitoba, as the northernmost of three stations. Intensity plots on a latitude‐time scale were prepared, and from these, 14 auroral substorms over a broad range of local times were selected for analysis. Data from a widely spaced array of auroral zone and low‐latitude magnetometers were used to determine substorm onset times and longitude sectors. Representative meridian intensity profiles of Hβλ4861 and OI λ5577 were then assembled as a function of invariant latitude and local magnetic time to obtain a synthetic model of a typical substorm. It is found that the data are better ordered when the local magnetic time scale for the individual substorms is shifted so as to place the substorm origins at local magnetic midnight. While the results are in general agreement with earlier observations, the quantitative nature of the present data base leads to a more objective proton substorm model.
Numerical modelling of the thermospheric heat budget
Authors: Gordiets B. F. , Kulikov Yu. N. , Markov M. N. , Marov M. Ya.
Published: February 2008

Numerical models of the thermal budget of the earth's upper atmosphere in the height range of 90–500 km are developed. The main sources and sinks of energy, including infrared radiative cooling in vibrational‐rotational bands of NO, CO2, OH, and O3 as well as heating and cooling arising from dissipation of turbulent energy and eddy heat transport, are taken into account. The calculated temperature and density height profiles are in good agreement with the respective profiles from the Cira (1972) and Jacchia (1977) models. It is found that thermal collisions between CO2 and atomic oxygen play an important role in the formation of the lower thermospheric 15µ radiation. It is also found that the maximum value of the time average eddy diffusion coefficient in the thermosphere cannot exceed 3×106 cm²s−1. The calculations show that for the statistically average thermospheric condition infrared radiative cooling by CO2 and NO in the 15µ and 5.3µ bands, but not eddy turbulence or molecular conduction provides the major loss of heat in the height range of 90–180 km.
The vertical wave number spectrum of topside equatorial spread F: Estimates of backscatter levels and implications for a unified theory
Authors: Kelley M. C. , Livingston R. C. , Rino C. L. , Tsunoda R. T.
Published: February 2008

A wave number spectrum for plasma density fluctuations in decaying equatorial spread F covering 5 orders of magnitude in spatial scale and 12 orders of magnitude in spectral power is presented. The outer scale is found, as is the spectral transition between a shallow spectral form (slope < 2) at intermediate scales and a steep spectrum at smaller scales. The characteristic features of the spectrum are discussed and related to the physical processes operating in several regimes. A unified model is discussed involving aeronomic effects and neutral atmospheric dynamics at the largest scales, a generalized Rayleigh‐Taylor plasma instability at intermediate scales, and a cascade of wave energy involving drift waves at transitional wavelengths. Open questions still remain concerning free energy sources near 1 km wavelength and at wavelengths less than a meter. Estimates of backscatter levels using the plasma density fluctuation spectra are consistent with Jicamarca and Altair radar observations except in topside regions of low plasma density, where in situ data at very short wavelengths are needed.
Atmospheric tides: 1. Model description and results for the solar diurnal component
Authors: Forbes Jeffrey M.
Published: February 2008

The equations and boundary conditions governing tidal oscillations in a viscous, rotating, spherical atmosphere from the surface to 400 km are formulated, including model parameterizations of background winds, temperature, composition, hydromagnetic coupling, Newtonian cooling, eddy and molecular diffusion, and tidal forcing mechanisms. Excitation of tidal oscillations occurs via absorption of EUV and UV radiation in the thermosphere, H2O insolation absorption in the troposphere and lower stratosphere, O3 insolation absorption in the mesosphere, ion‐neutral momentum coupling in the F region, and lunar gravitational forcing. The method of solution for the equations is outlined, and numerical convergence of the viscid model for certain test cases is verified by comparison with analytic calculations below 100 km from classical (inviscid) tidal theory. Calculated results for the solar diurnal tide from the surface to 400 km are presented and compared with rocket and radar observations. Considering the day‐to‐day variations that are clearly evident in the data, observations of tides by various rocket and radar techniques are in good agreement with the ‘mean tidal structures’ represented by the model. The greatest inconsistencies occur in the 80‐ to 100‐km height region where the diurnal propagating tide and the in situ trapped tide are of comparable importance at mid‐latitudes. The (1, 1) propagating tide above 70 km is sensitively coupled to the intensity of turbulence in the tropical mesosphere as parameterized by the eddy coefficients for diffusion of momentum and heat, undoubtedly contributing to its vagarious nature. Other factors causing inconsistencies with individual observations include possible ‘mode distortions’ induced by background mean winds and the attempted interpretation of global data sets taken during non‐overlapping time periods. Forthcoming analyses of CTOP observational periods should alleviate this latter problem and contribute to a clearer understanding of mesospheric and lower thermospheric tides.
A dynamic model of the magnetospheric magnetic and electric fields for July 29, 1977
Authors: Olson W. P. , Pfitzer K. A.
Published: February 2008

A quantitative vector representation of the magnetospheric field was developed specifically for the complex event of July 29, 1977. The magnetic field associated with each of the three major magnetospheric current systems was ‘controlled’ by solar wind and interplanetary field parameters and a magnetic index. This dynamic field model was compared with observed field data and found to be in good agreement. The procedures developed for quantitatively modeling time varying magnetospheric phenomena are general, and eventually it should be possible to use them to ‘predict’ some aspects of magnetospheric behavior.
Computer simulation of inner magnetospheric dynamics for the magnetic storm of July 29, 1977
Authors: Wolf R. A. , Harel M. , Spiro R. W. , Voigt G.‐H. , Reiff P. H. , Chen C.‐K.
Published: February 2008

We present preliminary results of applying the Rice convection model to the early main phase of the magnetic storm of July 29, 1977. The computer model self‐consistently computes electric fields and currents, as well as plasma distributions and velocities, in the inner‐magnetosphere/ionosphere system. In the equatorial plane, the region modeled includes geocentric distances less than about the magnetopause standoff distance. Particle loss, parallel electric fields, and neutral winds are neglected. On the basis of solar wind parameters and the AL index as input, the model predicts the injection of plasma‐sheet plasma to form a substantial storm time ring current. The total strength of the model‐predicted ring current agrees accurately with the observed Dst index. Comparison of the model results with electric fields and Birkeland currents measured by S3‐3 shows qualitative agreement but interesting quantitative discrepancies. During this event, region 1 currents, which in standard convection theory would connect to the outer magnetosphere, are observed as low as 60° invariant latitude at dawn and dusk. We examine the possibility that the magnetic field might be so highly inflated that 60° field lines extend to the outer magnetosphere. In the model, distortion of the inner edge of the plasma sheet by the magnetospheric compression associated with the sudden commencement temporarily disturbs the normal Birkeland‐current pattern. The normal tendency for the plasma sheet's inner edge to shield low L values from the convection electric field is also temporarily disrupted. Normal Birkeland currents and shielding reassert themselves after about an hour. Time‐integrated Joule heating in the model ionosphere over the first 5.5 hours of the storm main phase is about half the increase in model ring‐current energy.
Geomagnetic and solar wind cycles, 1900–1975
Authors: Feynman Joan
Published: February 2008

Recent research has established the reality of the 80 to 100‐year cycle (long cycle) in solar‐terrestrial phenomena that for many years has been suspected to occur. This paper investigates some of the systematic changes that took place during the most recent long cycle. The variations of one hundred years of data on the aa index of geomagnetic activity are reinvestigated. Since the solar wind drives geomagnetic activity the results of the study are interpreted in terms of the solar wind. It is shown that the 11‐year solar cycle as expressed by the number of sunspots (the sunspot cycle) is very different from the 11‐year solar cycle as expressed by the solar wind and geomagnetics (the solar wind cycle). With an objective technique, using only sunspot numbers and not involving any judgement as to the transient or recurrent nature of any geomagnetic activity, the solar wind cycle is decomposed into the sun of two equally strong periodic variations, each having the period of the sunspot cycle, but differing in phase. One term in the decomposition, the R component, is chosen to have the phase and relative amplitude of the sunspot cycle. The other term in the decomposition, the I component, is found to be almost, but not quite, 180° out of the phase with R and to have very closely related amplitudes. The source of the R component is shown to be sporadic or short‐lived solar events and that of the I component is shown to be long lived solar features such as coronal holes. The amplitudes are interpreted as being due to solar wind parameters, probably 1BZ1V². The solar wind observed at earth oscillates rhythmically between the two sources. The amplitude of both the oscillations increased smoothly and steadily from 1900 to 1960, corresponding to the ascending branch of the long cycle of solar activity which has been established elsewhere as occuring in the solar wind with an average period since the Middle Ages of 87 years. From these results it is concluded that there is a very strong relationship between coronal conditions at the sites of origin of long lived streams of solar wind and the corresponding sites of sporadic and/or short lived events 5 or 6 years later. Moreover, there is a remarkable and orderly evolution of these conditions over at least 60 years. The interruption of the smooth evolution in 1960 suggests we are now in the declining phase of the long cycle, which is to be expected since the average cycle period is 87 years and the last minimum was in the first decade of this century.
Simultaneous Chatanika radar and S3‐2 satellite measurements of ionospheric electrodynamics in the diffuse aurora
Authors: Vondrak Richard R. , Rich F. J.
Published: February 2008

Simultaneous Chatanika radar and S3‐2 satellite measurements have been used to investigate the spatial variation of the auroral zone ionosphere near the equatorward edge of the diffuse aurora. During early evening satellite passes on 2 days in February 1976, the equatorward boundary of the diffuse aurora was nearly overhead at Chatanika. The configurations of E region ionization and conductivity were nearly identical in both passes, but the latitudinal variations of F region ionization were quite dissimilar. During both passes, the ionospheric electric field in the diffuse aurora was directed northward. As a result, the closing current of the field‐aligned current system was primarily a Pedersen current, and the eastward electrojet was larger than the northward ionospheric current. Although the morphology of ionospheric conductivity was very similar on the 2 days, different patterns were observed for the latitudinal distribution of electric field and field‐aligned current.
Wave normal directions and wave distribution functions for ground‐based transmitter signals observed on GEOS 1
Authors: Lefeuvre F. , Neubert T. , Parrot M.
Published: February 2008

VLF whistler mode pulses of ∼1‐s duration emitted by the omega navigation system transmitter located in Norway have been observed on‐board the GEOS 1 satellite. The wave normal directions of 7 pulses have been determined by three different methods of analysis: the cross product method, Means' method, and the maximum entropy method of determination of the wave distribution function (WDF). The first method is based on the interpretation of measurements of the three magnetic wave field components in a 300‐Hz band, including the omega frequency and the second and the third method on the interpretation of the spectral matrix elements of the three magnetic wave field components at the Omega frequency. While the cross product method and Means' method assume that the electromagnetic wave field is that of a single plane wave propagated in a fixed direction with respect to the ambient magnetic field, the WDF method assumes the wave field to be random. The three methods and their solutions are compared and discussed in terms of the statistical properties of the signals (signal to noise ratio, stationarity in time, degree of polarization, ellipticity), and in terms of sensitivity to inconsistencies in the data. Some recommendations are made concerning the best choice of method in given situations. The wave normals are found to have angles ∼130° with the earth's magnetic field direction, pointing slightly eastward of the local magnetic meridian. They exhibit a time variation with a 0.2‐ to 0.4‐s periodicity.
A transient response model of Pi 2 pulsations
Authors: Kan Joseph R. , Longenecker David U. , Olson John V.
Published: February 2008

A transient response model of Pi 2 pulsations due to an enhanced convection in the plasma sheet is presented. A nonlinear Alfvén wave is launched by the enhanced convection electric field. The reflection coefficient at the ionosphere is derived by matching the field‐aligned current of the Alfvén wave to the field‐aligned current from the divergence of the ionospheric Hall and Pedersen currents. The resulting transient magnetic perturbation on the ground exhibits the characteristics of the Pi 2 pulsation signature accompanying a substorm onset. The predicted fundamental period of the magnetic perturbation is 4 times the one‐way Alfvén travel time which increases toward higher latitude. The predicted Pi 2 source regions in the ionosphere are in reasonable agreement with observations.
Seasonal dependence of the distribution of large‐scale plasma depletions in the low‐latitude F region
Authors: Dachev Ts. P. , Walker James C. G.
Published: February 2008

From several years of Atmosphere Explorer data we show that large‐scale plasma density depletions (horizontal scale greater than 1000 km) in the nocturnal F region occur mainly during the first half of the night. The geomagnetic latitude of these depletions depends on season: at solstice periods the occurrence is mainly in the summer hemisphere and at equinoxes mainly on the geomagnetic equator. Large‐scale depletions occur most often at longitudes characterized by a large magnetic declination. We interpret the large‐scale plasma depletions to be a consequence of imposed upward drift which raises the F region above the satellite trajectory. We have tested this interpretation by calculating the vertical ion drift velocity imposed by zonal and meridional winds and electrostatic fields, with allowance for the effects of diffusion. The calculations reveal a correspondence between the locations of maxima in the calculated vertical ion drift velocity and the large‐scale plasma depletions.
Injection and loss mechanisms for energetic ions in the inner Jovian magnetosphere
Authors: Thorne Richard M.
Published: February 2008

The pronounced depletion of energetic ions observed by Voyager outside the orbit of Io can be understood as a steady state balance between particle loss and inward diffusion. For ions above 1 MeV/nuc (nucleon) G sweeping loss by Io itself is unimportant; rather the removal mechanisms involve processes associated with the extensive torus of neutral and ionized gas emanating from the satellite. There is compelling evidence to suggest that losses throughout an extended region in the outer torus, exterior to the orbit of Io, occur at a rate comparable to the upper limit imposed by strong pitch angle diffusion; although the plasma waves responsible for such scattering have yet to be detected. This is by far the dominant process for removing energy from the inner Jovian magnetosphere, and it also provides a viable energy source for exciting intense Jovian auroral emissions. Charge exchange removal of equatorially trapped lower‐energy (≲10 MeV/nuc G) heavy ions (e.g., O+ or S+) can become significant over a limited region near and just inside the orbit of Io, but this process appears to have little overall effect on the energetics of the Jovian system. The trapped particle distribution function in fact exhibits little evidence for effective loss inside the orbit of Io, presumably due to the absence of intense plasma waves. An equilibrium solution for the ion phase space density requires a pronounced increase in the rate of radial diffusion between the orbit of Io and the outer torus (L > 7). This suggests a change in the dominant mechanism for cross‐L transport consistent with the anticipated onset of rapid centrifugally driven interchange diffusion with DLL ≈ 5×10−6 (L/8)4 RJ²/s in the outer torus. In contrast, the diffusion rate near Io is substantially slower, DLL (L ≃ 6) ≈ 2×10−7 RJ²/s. This is comparable to the value deduced earlier from Pioneer 11 data by Thomsen et al., who suggested neutral winds in the Jovian ionosphere as a driving mechanism.
Field line distortion and Joule heating by the corotation enforcement current in the Jovian magneto‐ionosphere
Authors: Watanabe Yukio , Nishida Atsuhiro
Published: February 2008

When radial flow is generated in the rapidly rotating magnetosphere of Jupiter, angular velocity of the magnetospheric plasma departs from the corotational value on account of the conservation of the angular momentum. This causes twisting of magnetic field lines in the magnetosphere and Joule heating in the ionosphere. We have estimated the energy storage and dissipation rates by using a model in which finite transit time of the signals between the equatorial plasma disc and the ionosphere as well as the reflection of the signal at the ionosphere are taken into account. In the case of the inward flow produced by the magnetospheric compression the energy stored in the magnetic field and dissipated in the ionosphere while the compression is in progress both amount to 1017∼18 J.
Magnetic field rotation through the magnetopause: ISEE 1 and 2 observations
Authors: Berchem Jean , Russell C. T.
Published: February 2008

ISEE 1 and 2 magnetic field data obtained during dayside magnetopause crossings have been analyzed using a minimum variance technique in order to determine the characteristics of the rotation of the magnetic field across the magnetopause. Only a small fraction of the magnetopause crossings examined present a sufficiently coherent structure that they can be compared with theoretical predictions, and only a few of the hodograms of the magnetic field at the crossings show the ideal rotation expected for a rotational discontinuity structure. Observed senses of rotation are not consistent with the predictions of the electron whistler polarization theory. Rotations greater than 180° are very seldom observed; they usually occur across multiple crossings, either as 360° rotation about the minimum variance direction or, occasionally, in more complicated patterns associated with ‘S’‐shaped hodograms. These observations suggest that the sense of the magnetic field rotation through the magnetopause is controlled by the relative orientation of the magnetosheath and magnetospheric field and that the angular rotation is minimized when the magnetic field changes from one orientation to the other.
Ps 6 spatial and temporal structure from STARE and riometer observations
Authors: Nielsen E. , Sofko G.
Published: February 2008

At the time of ground‐based observations of Ps 6 events, the two‐dimensional electron drift velocity maps (or the corresponding electric field maps) deduced from the Scandinavian twin auroral radar experiment (STARE) measurements exhibit a characteristic wavelike oscillation pattern. This pattern extends 3–4° in latitude and moves eastward roughly along L shells at a speed typically in the range 800–1300 m/s. The pattern has a precursor, a core, and a recovery zone. In the core, the electron drift attains some of the largest values ever seen by STARE (>2000 m/s). The electric field divergence indicates the presence of an east‐west field‐aligned current (FAC) pair, the more spatially confined leading FAC being downward, the more broadly spread following FAC being upward. A multiple narrow beam 51.4‐MHz riometer at Tromsö indicates a broad absorption zone associated with the trailing upward FAC. By using the Biot‐Savart Law to integrate the magnetic flux density due to the STARE velocity vectors (elemental currents) and by comparing the result with the ground‐based magnetometer records, it can be inferred that the closure current for the FAC pair has both Pedersen and Hall components, thereby indicating the presence of conductivity gradients. Furthermore, the STARE flows predict the observed ground D component variation very well and indicate integrated Hall conductivity of ∼50 mhos.
Convergence and approximation of auroral ion velocity distribution function
Authors: Hubert D.
Published: February 2008

The exact solution of the auroral ion velocity distribution for the Bhatnagar Gross and Krook (B.G.K.) collision model is revised in order to obtain insights on convergence and approximation of the generalized orthogonal polynomial solution of the Boltzmann equation. Starting from a Maxwellian weighting function we show that the accuracy of the expansion is very sensitive to the assigned ‘temperature’. This leads us to propose a generalized Grad's expansion solution of the Boltzmann equation that, without increasing either the number of calculated coefficients or the complexity of their equations, significantly enhances the accuracy of the limited expansion. This method when applied to the relaxation model increases the range of electric fields and enlarges the region of the velocity space where the limited expansion is valid.
On the energy dependence of the radial diffusion coefficient and spectra of inner radiation belt particles: Analytic solutions and comparison with numerical results
Authors: Westphalen Hartwig , Spjeldvik Walther N.
Published: February 2008

We present a theoretical method by which the energy dependence of the radial diffusion coefficient DLL may be deduced from spectral observations of the particle population at the inner edge of the earth's radiation belts. This region has previously been analyzed with numerical techniques (for a review see, e.g., Walt (1971)); in this report we give an analytical treatment that illustrates characteristic limiting cases in the L shell range where the time scale of Coulomb losses is substantially shorter than that of radial diffusion (L ∼ 1–2). We demonstrate both analytically and numerically that the particle spectra there are shaped by the energy dependence of DLL regardless of the spectral shapes of the particle populations diffusing inward from the outer radiation zone, so that from observed spectra the energy dependence of the diffusion coefficient can be determined. To insure realistic simulations, inner zone data obtained from experiments on the DIAL, AZUR, and ESRO 2 spacecraft have been used as boundary conditions. We report excellent agreement between analytic and numerical results.
Correlations of magnetospheric ion composition with geomagnetic and solar activity
Authors: Young D. T. , Balsiger H. , Geiss J.
Published: February 2008

A large ion composition data set consisting of 1‐month averages has been assembled for the energy per charge range 0.9–15.9 keV/e. It includes 48 months of data taken by the Ion Composition Experiments on the ESA/GEOS 1 and 2 satellites at or near geostationary orbit. Data were obtained during the rising and maximum phases of the current solar cycle from May 1977 through November 1981 inclusive. Five ion species are routinely identifiable: H+, He++, He+, O++, and O+, above a limiting density ∼10−3 ions cm−3. Ion densities exhibit a number of very striking statistical correlations with one another and with both Kp and solar EUV as measured by F10.7. One principal result is that increases in the densities of magnetospheric He+, O++, and O+ are observed that are apparently due entirely to increased solar EUV fluxes associated with the rising phase of the current solar cycle. There is a marked rise in O+ density by a factor of ∼8 with increasing geomagnetic activity, but no corresponding increase in either He+ or O++ and only a small increase in H+. The He++/H+ ratio is found to be remarkably constant at ≈0.01. Contrary to ion density results, little or no variation is found in mean energy. These observations are interpreted in terms of the composition and dynamics of two sources of magnetospheric plasma: the solar wind and the high‐latitude topside ionosphere.
The properties of ionospheric O+ ions as observed in the magnetotail boundary layer and northern plasma lobe
Authors: Candidi M. , Orsini S. , Formisano V.
Published: February 2008

The ISEE 2 plasma experiment data are used to investigate the properties of the O+ ions streams flowing in the earth's magnetotail low latitude boundary layer and northern plasma lobe. The E × B filter is seen to be the organizer of the flow of these ions, after their acceleration in the topside ionosphere. Between XSE=−15 RE and −23 RE the velocity of the O+ ions is a function of the distance from the center magnetotail axis, the slower flowing closer to this and the faster closer to the magnetopause. The O+ stream density is observed to vary with this distance as well. The temperature of the streaming ions is shown to be anisotropic with respect to the ambient magnetic field; a generally unresolved narrow conic pitch angle distribution of the O+ ions is suggested to be the cause of this anisotropy. The properties of the O+ ions are shown to be consistent with an acceleration mechanism, or a combination of mechanisms, that act orthogonally and parallel to the magnetic field.
Observations of field‐aligned energetic electron and ion distributions near the magnetopause at geosynchronous orbit
Authors: Korth A. , Kremser G. , Daly P. W. , Amata E.
Published: February 2008

On August 28, 1978, the dayside magnetopause crossed the geosynchronous satellite GEOS 2 several times during a geomagnetically disturbed period, and clear signatures of the interconnection of field lines through the magnetopause were observed. The MPAE particle spectrometer provided high time resolution observations of the distribution of energetic electrons (E > 22 keV) and ions (E > 27 keV). Magnetometer data were used to determine the location of GEOS 2 relative to the magnetopause. The pitch angle distributions of ions and electrons were found to be strongly asymmetric with respect to 90°, and the asymmetries have been interpreted in terms of field‐aligned particle streaming. Evidence is provided for the first time for electron streaming inside the magnetopause which continues for many bounce periods. It is concluded that magnetospheric field lines opened, at least for brief time intervals, as a consequence of interconnection with magnetosheath field lines. Comparisons of electron spectra provide evidence that the streaming electrons observed in the magnetosheath originate in the magnetosphere.
Observations of parallel ion energization in the equatorial region
Authors: Quinn J. M. , Southwood D. J.
Published: February 2008

In this paper we examined in detail an example of bounce‐phase‐clustered ions such as earlier reported by McIlwain (1976), Quinn (1978) and Quinn and McIlwain (1979). The cluster has small pitch angle, but investigation of the energy time dispersion properties of the cluster reveals that it was formed at the equator. The acceleration region extended over about 2.4 RE along the field direction. We suggest that the ion cluster is set up by acceleration during a transient convection surge, and we discuss the properties of such a process.
Magnetospheric convection effects at mid‐latitudes: 2. A coordinated Chatanika/Saint‐Santin Study of the April 10–14, 1978, magnetic storm
Authors: Blanc Michel , Alcaydé Denis , Kelly John D.
Published: February 2008

We analyze simultaneous measurements by the incoherent scatter radars at Saint‐Santin (45° latitude) and Chatanika (65° latitude) during a major magnetic storm in April 1978 to examine several disturbance mechanisms operating on the mid‐latitude ionosphere during periods of strong magnetospheric and auroral activity. The first type of mechanism, the extension of magnetospheric convection electric fields to mid‐latitudes, is illustrated by two large localized departures of the E×B plasma drifts over Saint‐Santin that appeared in conjunction with the two storm negative phases. In both cases they were associated with large electric fields in the afternoon sector, within the eastward electrojet region, over Chatanika. The first event can be very clearly interpreted as a global short term enhancement of magnetospheric convection electric fields, since simultaneous plasma flux intensifications were identified over the polar cap, in the afternoon eastward electrojet, and at 45° latitude. The IMF and ring current signatures of these two events confirm our previous findings: large southward values of BƵ and large values of the rate of energy injection into the ring current system, are associated with the extension of magnetospheric convection electric fields to midlatitudes. In addition, a marked equatorward shift of the auroral oval was observed during both events. During the night of the first event, the mid‐latitude drift perturbation magnitudes remained moderate (100 m/s) and did not seem to induce any significant ionospheric perturbation. The joint increase in ion and electron temperatures by 100 to 200° K above the quiet day reference level, which preserved the thermal equilibrium of the plasma, can be simply interpreted in terms of the storm‐induced global heating of the thermospheric gas. Conversely, on the night of the second event, when drift magnitudes reached several hundreds of m/s, large‐amplitude fluctuations in the F layer height and density, and very large increases (by more than 500° K) in the ion and electron temperatures were observed. Whereas collisional heating can account for a large part of the ion temperature increase, an additional energy source, heating preferentially the electron gas, is needed to explain the maintenance of the electron temperature 200° K above the ion temperature observed from the onset of the Dst decrease onward during that night. The identification of this source suffers from the lack of additional data, in particular photometric ones, in the Saint‐Santin sector. However, the unusually large fields observed at Saint‐Santin and the equatorward shift of the auroral zone observed at Chatanika in the early afternoon suggest that the plasmapause may have moved close to Saint‐Santin and that the enhanced electron temperatures observed there might have been produced by a SAR arc.