bluuebunny/crossref_metadata_2025 huggingface dataset:
Lightning‐induced electron precipitation from the magnetosphere
Authors: Chang H. C. , Inan U. S.
Published: February 2008

Precipitation of radiation belt particles induced by whistlers that are generated by atmospheric lightning discharges and propagate over L shells of 2–4.5 is considered. Using a test particle model of the whistler‐particle interaction, the energy spectra and temporal profile of whistler‐induced fluxes as a function of L shell are quantitatively determined for a representative plasmaspheric cold plasma distribution. Results indicate that for higher energy electron precipitation (E > 40 keV) there exists an inner magnetospheric region (2 < L < 3) where the level of whistler‐induced precipitation can be expected to be comparatively high. Implications of this finding in terms of observational results are discussed.
Polar cusp dynamics
Authors: Eather R. H.
Published: February 2008

The position of dayside aurora (as measured from South Pole station in 1980) is compared with the interplanetary magnetic field Bz component and to the AE index. The results are consistent with our earlier work showing a close relationship with AE and little correlation with Bz. Two recent papers have presented data that were interpreted to indicate a dominant Bz correlation. A reconsideration of the data sets used in those papers does not support dominant Bz dependence, and in fact reaffirms a close dependence on AE. We conclude that the position of the dayside cusp is largely controlled by substorm processes internal to the magnetosphere rather than by direct merging and erosion processes with the interplanetary field.
Enhanced ion outflows measured by the DE 1 high altitude plasma instrument in the dayside plasmasphere during the recovery phase
Authors: Menietti J. D. , Winningham J. D. , Burch J. L. , Peterson W. K. , Waite J. H. , Weimer D. R.
Published: February 2008

Ion flow velocities both parallel and perpendicular to the magnetic field and including the effects of spacecraft charging and spacecraft velocity have been measured during the recovery phase of two large magnetic storms on October 14 and 21 of 1981. These measurements were made both inside and outside the plasmasphere and indicate unreported yet substantial outflows of ions within the dayside plasmasphere (October 14). Combined data from instruments on board the Dynamics Explorer satellite, including the high altitude plasma instrument (HAPI), the energetic ion composition spectrometer (EICS), the retarding ion mass spectrometer (RIMS) and the plasma wave instrument (PWI), indicate that these ions are most likely dominantly O+ at energies at least as low as 5 eV. The nightside pass (October 21), which occurred during the recovery phase of a similar storm, showed no plasmaspheric outflows. The results indicate that a large contribution to the outflux into the dayside plasmasphere during the recovery period is due to E > 5 eV ions.
Density response of neutral atmospheric layers to gravity wave perturbations
Authors: Gardner C. S. , Shelton J. D.
Published: February 2008

Expressions for the density response of neutral atmospheric layers to propagating waves are derived. The expressions include nonlinear effects and are used to compute the response of the mesospheric sodium layer to low‐frequency internal gravity waves. The results are compared with lidar observations of the sodium layer and used to deduce gravity wave parameters. It is shown that because of the large sodium density gradients and the relatively high amplitudes of gravity waves at mesospheric heights the nonlinear components of the layer density response must be considered in interpreting sodium lidar data.
A theory of satellite sweeping
Authors: Paonessa Mark , Cheng Andrew F.
Published: February 2008

A theory of energetic charged particle absorption by insulating moons is presented that includes gyrophase dependence of the absorption probability when the gyroradius is not small compared to the satellite radius. This effect increases the average lifetime against absorption by factors of approximately 2 to 6 above previous estimates at Saturn. The sweeping lifetime then becomes significantly greater than the strong diffusion lifetime. A rigorous expression for the average lifetime against absorption, appropriate for a drift phase averaged radial diffusion equation, is derived assuming (1) a uniform distribution of gyrocenter equatorial plane crossing points in the sweeping corridor and (2) randomization of gyrophase. The pitch angle and energy dependences of the sweeping lifetime are found to be significantly different from the predictions of previous analytical estimates.
Thermal ion temperatures from the retarding ion mass spectrometer on DE 1
Authors: Comfort R. H. , Waite J. H. , Chappell C. R.
Published: February 2008

Data from the retarding ion mass spectrometer (RIMS) on Dynamics Explorer 1 are analyzed by means of a thin sheath model to determine ion temperatures. A number of characteristic features of ion temperatures in and near the plasmasphere are presented. Typical H+ temperatures in the plasmasphere are found to be 4000 K to 6000 K on the morningside and 2000 K to 4000 K on the eveningside, increasing with L throughout. In the plasma trough, typical temperatures observed are a few times 104 K. In the outer plasmasphere, multiple temperature components are frequently observed. H+ and He+ ions are found to be very close to thermal equilibrium with each other in all but perhaps the outer part of the plasmasphere. Within the plasmasphere, temperature profiles observed by DE 1/RIMS rarely show small‐scale variation. During geomagnetically active times, high‐altitude temperatures between L=2 and 3 appear to be depressed over quiet time values, to the extent that they may be cooler than temperatures at low altitudes along approximately the same field line.
Radial diffusion of low‐energy ions in Saturn's radiation belts: A Combined analysis of phase space density and satellite microsignature data
Authors: Hood L. L.
Published: February 2008

Phase space densities for low‐energy (80 MeV/G) ions in Saturn's inner magnetosphere are analyzed using solutions of the time‐averaged radial diffusion equation for charged particle transport in a dipolar magnetic field. A series of distributed loss models ranging from satellite absorption only to satellite and maximum estimated Ring E absorption losses plus pitch angle scattering losses occurring at the strong diffusion limit in the inner magnetosphere are assumed. In each case the corresponding form of the magnetospheric radial diffusion coefficient (assumed to be expressible as D(L) = DoLn, where Do is a constant and n is an integer) which yields a minimum rms residual between model and data is determined. Independent constraints on the diffusion rate at specific L values derivable from satellite microsignatures in low‐energy ions and electrons are then considered. Estimates previously derived from the Dione microsignature in Voyager 1 low‐energy electrons are supplemented by additional analyses of the Tethys microsignature in Voyager 2 low‐energy ions and the Rhea microsignature in Voyager 1 low‐energy electrons. The resulting diffusion rate estimates of D(4.9) ∼4 ± 2×10−8 Rs² s−1 and D(8.7) > 6 ± 5×10−8 Rs² s−1 are consistent with those derived from the Dione microsignature if D(L) increases outward. A comparison with the phase space density modeling results shows that satellite and maximum Ring E absorption losses alone are insufficient to yield diffusion rates at Tethys and Dione that are in agreement with the microsignature estimates. Models containing weak pitch angle scattering losses of low‐energy ions in the inner magnetosphere occurring at a rate less than one‐tenth that of the strong diffusion limit produce diffusion rates that are compatible with microsignature estimates and additionally result in an improved fit to the radial variation of the experimental phase space densities. The preferred radial diffusion coefficient for these low‐energy ions is characterized by a relatively high amplitude and low‐order L dependence that is most consistent with Jovian‐type diffusion mechanisms including the centrifugal interchange instability and the ionospheric dynamo mechanism.
Heating of thermal helium in the equatorial magnetosphere: A simulation study
Authors: Omura Y. , Ashour‐Abdalla M. , Gendrin R. , Quest K.
Published: February 2008

Heating of heavy ions is observed in the equatorial magnetosphere in conjunction with ion cyclotron waves generated by anisotropic hot protons (i.e., GEOS 1 and 2 and ATS 6 results). The mechanism of the heating is studied by a numerical simulation. The plasma parameters which have been chosen are those which prevail in the dayside magnetosphere at geostationary altitudes. The plasma consists of cold and isotropic H+ and He+ ions (κT/2 ∼ 1.7 eV) with a small number of hot anisotropic protons which provide the free energy necessary to generate the waves. The code is one‐dimensional in length and three‐dimensional in velocity. It is electromagnetic and hybrid; i.e., the electrons are treated as a massless fluid. The results obtained during the linear phase (t ≤ 150ΩH−1) are in agreement with those expected from the linear theory as far as the growth rate of the wave (and the frequency of the most amplified wave) and the variation of the hot proton anisotropy are concerned. The saturation (Bwave ∼ 0.05Bo) is explained by trapping of the helium particles. But the most interesting results concern the heating of cold species. He+ ions are heated mainly in the perpendicular direction (kT⊥/2 ≃ 150 eV, kT∥/2 ≃ 80 eV), and they are heated more than cold H+ ions (kT⊥/2 ≃ 15 eV, kT∥/2 ≃ 20 eV). The heating of He+ ions is a two‐step process: first, He+ ions are set into oscillations by the growing wave (in both υ⊥ and υ∥) until some of the ions reach a parallel velocity of the order of the resonant velocity, at which time strong heating occurs. Phase‐space plots for the different particle species at different times illustrate the time evolution of this heating mechanism.
Low‐altitude field‐aligned electrons
Authors: Arnoldy R. L. , Moore T. E. , Cahill L. J.
Published: February 2008

Since some of the earliest measurements of the auroral electron spectrum at rocket altitudes were made, there has been a question about how two very distinct features of the spectrum can occur simultaneously. These features are (1) a spectrum peaked in energy and isotropic over all pitch angles directed into the atmosphere (the isotropic peak) and (2) intense fluxes of field aligned (FA) electrons which are usually observed at pitch angles of 10° or less. Recent rocket measurements with improved temporal and pitch angle resolution have added considerably to the data set on the FA component and are discussed in this paper. A summary of the characteristics of FA electrons is as follows: (1) Field‐aligned electrons are associated with evening, midnight, and cusp auroras. Field aligned electrons are a common feature, seen in 8 out of 10 University of New Hampshire evening auroral rocket flights and in half of the AE satellite orbits through the cusp over a six‐year period. About 80% of the S3‐3 passes through the statistical auroral oval recorded field aligned electron beams. (2) Rocket data associate the field aligned electrons with active auroral forms and on the edges of moving discrete arcs, predominantly the leading edge. Occasionally they are observed between bright auroral forms. (3) The spatial/temporal scale of field aligned events seen by satellites is a few seconds or tens of kilometers. The rocket data time scale extends from seconds to hundreds of seconds. When bursts of field aligned electrons appear, they may be due to incomplete energy and/or pitch angle coverage by the detectors. The intensity of field aligned electrons has been observed to oscillate at a few hertz. (4) At any given time the electrons are field aligned over a wide range of energies, often extending from the peak energy in the isotropic Maxwellian population that coexists with the field aligned electrons to very low energies. On occasion the field aligned electrons terminate at low energies in an isotropic halo. (5) Because of the high degree of field alignment and the restriction to low energies, the field aligned population does not deposit appreciable energy into the ionosphere but can be a significant current carrier. (6) At satellite altitudes of 3000–8000‐km, precipitating field aligned electrons are often associated with up streaming field aligned electrons and conic ion distributions. At satellite altitudes, field aligned electrons are also observed on the edges of ion inverted‐V structures. Recent rocket data have revealed field alignment of auroral electrons up to 15‐keV energy. The differential intensity of these electrons is very intense and gives no evidence for acceleration by static field aligned electric fields. These field aligned electrons are observed to have a distribution function very similar to that for electrons injected at synchronous orbit during substorms. Recent acceleration mechanisms involving turbulence are discussed.
Ohm's Law for a current sheet
Authors: Lyons L. R. , Speiser T. W.
Published: February 2008

We derive an Ohm's law for single‐particle motion in a current sheet, where the magnetic field reverses in direction across the sheet. The result is considerably different from the resistive Ohm's law often used in MHD studies of the geomagnetic tail. We extend our single‐particle analysis to obtain a self‐consistency relation for a current sheet which agrees with previous results. Our results are applicable to the concept of reconnection in that we obtain the electric field parallel to the current for a one‐dimensional current sheet with constant normal magnetic field. Dissipated energy goes directly into accelerating particles within the current sheet.
The cleft ion fountain
Authors: Lockwood M. , Chandler M. O. , Horwitz J. L. , Waite J. H. , Moore T. E. , Chappell C. R.
Published: February 2008

Low‐energy (below approximately 50 eV) ionospheric ions, injected into the magnetosphere at the dayside cleft, are studied using data from the retarding ion mass spectrometer (RIMS) experiment on the Dynamics Explorer 1 satellite. It is concluded that upwelling ions at the cleft form an ion fountain and are blown into the polar cap by antisunward convection. At high Kp (>4), convection is generally strong enough to fill the entire polar magnetosphere with low‐energy O+ ions, whereas at low kp (<2) they are largely restricted to the dayside half of the cap. Using a two‐dimensional kinetic ion trajectory model, the locations where RIMS detected O+ within the cap are shown to be consistent with the spatial distributions of O+ density, predicted for an upwelling ion source at the cleft and various dawn‐dusk convection electric fields. A detailed study is made of one polar pass of DE 1, during which RIMS detected He+, N+, O+, and O++ ions, the ion trajectory model being used to trace all these ions back to a common source at an observed upwelling ion event near the cleft. All observed species are deduced to be falling earthward in the nightside of the cap, as predicted from the model, indicating the dominance of gravity over upward field‐aligned acceleration (such as by the ambipolar electric field). Comparison of field‐aligned velocities observed for O+ and O++ ions defines a maximum limit to the upward electrostatic acceleration present within the cap which was only sufficient to eject ionospheric H+ ions, all heavier ions being supplied from the dayside by the cleft ion fountain.
The cleft ion fountain: A two‐dimensional kinetic model
Authors: Horwitz J. L. , Lockwood M.
Published: February 2008

The transport of ionospheric ions from a source in the polar cleft ionosphere through the polar magnetosphere is investigated using a two‐dimensional, kinetic, trajectory‐based code. The transport model includes the effects of gravitation, longitudinal magnetic gradient force, convection electric fields, and parallel electric fields. Individual ion trajectories as well as distribution functions and resulting bulk parameters of density, parallel average energy, and parallel flux for a presumed cleft ionosphere source distribution are presented for various conditions to illustrate parametrically the dependences on source energies, convection electric field strengths, ion masses, and parallel electric field strengths. The essential features of the model are consistent with the concept of a cleft‐based ion fountain supplying ionospheric ions to the polar magnetosphere, and the resulting plasma distributions and parameters are in general agreement with recent low‐energy ion measurements from the DE 1 satellite.
Solar periodicities expressed in varves from Glacial Skilak Lake, southern Alaska
Authors: Sonett C. P. , Williams G. E.
Published: February 2008

A sequence of 236 varve thickness measurements by Perkins and Sims from proglacial Skilak Lake in southern Alaska is shown to contain spectral features similar to those of the sunspot index. In general, the varve spectrum qualitatively mimics that of the sunspot index, with discrepancies perhaps arising from the misinterpretation of random summer layers as annual increments (varves). These discrepancies may be largely removed through maximization of the sunspot index‐varve cross correlation whereby a time‐scale factor increment is applied to the varve series measurements. Varve thickness is positively correlated with the sunspot index and with vanishing lag (insofar as can be resolved). As varve thickness and meteorological variables have previously been shown to be strongly correlated, a direct link between the sunspot index and varve thickness is suggested by way of climate response to solar variations.
Relationships among the harmonic coefficients of scan plane anisotropies
Authors: Northrop Theodore G.
Published: February 2008

Frequently, a detector on a spinning spacecraft measures the flux of charged particles whose velocities lie in a plane containing the magnetic field. This flux may be Fourier analyzed as a function of the spacecraft roll angle. Relationships among the Fourier coefficients are derived using the adiabatic solution of the Vlasov equation. These relationships depend only on the fact that the lowest order (in gyroradius) of the distribution function is a function of the magnetic moment and that the first‐order term is a function of the lowest order. These relationships may be used to separate the proton from the electron counts registered in Saturn's inner magnetosphere by the University of California's Cerenkov counter on Pioneer 11.
A perturbation approach to cosmic ray transients in interplanetary space
Authors: Chih Ping Ping , Lee Martin A.
Published: February 2008

A general linear response analysis is presented for investigating cosmic ray transients in interplanetary space. The diffusion equation for energetic particle transport in the solar wind is first linearized about a stationary configuration. The perturbation in the particle omnidirectional distribution function resulting from perturbations to the solar wind velocity, drift velocity, or spatial diffusion tensor corresponding to transient solar wind conditions may then be found by solving an inhomogeneous boundary value problem. As illustrative examples of the general analysis, transient perturbations in the cosmic ray number density due to variations in the spatial diffusion coefficient are computed in a one‐dimensional solar wind based on the convection‐diffusion equation, which disregards cosmic ray energy changes in the solar wind. Two variations are considered, corresponding to (1) a Forbush decrease arising from the passage of an interplanetary traveling disturbance (shock) and (2) the solar cycle variation arising from an 11‐year sinusoidal variation of the spatial diffusion coefficient at the sun which then propagates out through the heliosphere. The predicted solar cycle variation exhibits a hysteresis effect in which high‐energy particles lead low‐energy particles and a time delay which increases with heliocentric distance within the inner heliosphere. The predicted Forbush decrease profiles exhibit precursors, precipitous decreases, and gradual approximately energy‐independent recoveries arising from a decay of the propagating disturbance. The general features of both variations are in accord with observations.
In situ measurements of the plasma bulk velocity near the Io flux tube
Authors: Barnett Alan
Published: February 2008

We study the flow around the Io flux tube by analyzing the eleven spectra taken by the Voyager 1 Plasma Science (PLS) experiment in its vicinity. We determine the bulk plasma parameters using a procedure that uses the full response function of the instrument and the data in all four PLS sensors. The mass density of the plasma in the vicinity of Io is found to be 22,500±2500 amu/cm³, and its electron density is found to be 1500±200 cm−3. We determine the Alfvén speed using three independent methods; the values obtained are consistent and, taken together, yield VA = 300±50 km/s, corresponding to an Alfvén Mach number of 0.19±0.02. For the flow pattern we find good agreement with the model of Neubauer (1980) and conclude that the plasma flows around the flux tube with a pattern similar to the flow of an incompressible fluid around a long cylindrical obstacle of radius 1.26±0.1 RIo.
Two‐dimensional quasi‐neutral description of particles and fields above discrete auroral arcs
Authors: Newman A. L. , Chiu Y. T. , Cornwall J. M.
Published: February 2008

This paper reports a full two‐dimensional numerical computation of the particle distributions, electric fields, and currents associated with a standard adiabatic model of a quiet auroral arc and inverted‐V potential. Both the region of auroral electron precipitation and the return current region are modeled; for the latter we assume a normal return current dominated by ambipolar E∥, with no substantial downward E∥. We go beyond earlier treatments of the adiabatic model (which either had no coupling between different field lines or treated all field lines as having essentially identical conditions along B) by specifying latitudinal variations in particle sources and parallel potential drops, which require every field line to be treated separately, coupled to other lines by current conservation. In particular, differential pitch angle anisotropies which drive upward E∥ are varied with latitude in order to accommodate a smooth transition to the return current region. In this transition regime the model predicts density enhancements on field lines at the edge of the inverted V that have only a small potential drop. We believe this is the first work to address the return current region with a realistic adiabatic model of the magnetosphere, including particle populations of both magnetospheric and ionospheric origin.
An analysis of the role of drift waves in equatorial spread F
Authors: LaBelle J. , Kelley M. C. , Seyler C. E.
Published: February 2008

Two sounding rockets launched from Peru as part of Project Condor confirm and extend a number of previous rocket measurements of the wave number spectrum of equatorial spread F irregularities. Other papers in this series present investigations of the intermediate‐ and long‐wavelength regimes; here, we concentrate on wavelengths less than 100 m. The Condor density fluctuation spectra display a break at a wavelength near 100 m, identical to that found in the PLUMEX experiment (Kelley et al., 1982b). The Condor data also confirm a subrange in which the density and the wave potential obey the Boltzmann relation—a strong indication of the presence of low‐frequency electrostatic waves with finite wavelength parallel to the magnetic field, perhaps low‐frequency drift waves as proposed by Kelley et al. (1982b). The Condor data are also consistent with the previous conjecture that drift waves only exist above 300 km altitude. To investigate the difference in spectra observed over two altitude ranges, we have fit to the data a form for the power spectrum taken from Keskinen and Ossakow (1981). The fitted spectrum, along with empirically determined growth and dissipation rates, is next used to calculate the energy pumped into the spectrum at long wavelengths as well as the energy dissipated at shorter wavelengths. It is found that the energy is balanced by classical collisional effects in the low‐altitude case, but energy balance in the high‐altitude case requires an enhanced dissipation of about 500 times that due to classical diffusion. This result again implies the existence of electrostatic waves, which cause anomalous diffusion due to two mechanisms: nonambipolar diffusion and wave‐particle interactions. We propose that the enhanced diffusion coefficient should also be consistent with a convective transport model for anomalous diffusion. In this model, the long‐wavelength portion of the spectrum consists of coherently steepened (k−2) Rayleigh‐Taylor generated structures, while the short wavelengths are a turbulent cascade of low‐frequency drift waves which cause convective transport. We have used the measured electric field spectrum in the drift wave regime to calculate the effective diffusion due to stochastic E × B scattering. The resulting diffusion coefficient is comparable to that indicated by the observations, although its value depends on the outer scale selected for the turbulent spectrum. Furthermore, the experimental data show that the wavelength at which the drift wave cascade originates varies inversely with the power spectral density. If stochastic E × B scattering is the dominant enhanced diffusion mechanism, then in order to duplicate the observed behavior, the turbulent portion of the spectrum must extend to longer scales than the wavelength at which the drift waves are excited. Thus the model is consistent with but does not uniquely imply an inverse cascade of drift wave turbulence in equatorial spread F.
Optical interferometric measurements of nighttime equatorial thermospheric winds at Arequipa, Peru
Authors: Meriwether J. W. , Moody J. W. , Biondi M. A. , Roble R. G.
Published: February 2008

We obtained nighttime measurements of equatorial thermospheric wind dynamics at Arequipa, Peru, with an automated field‐widened Fabry‐Perot interferometer between April 1983 and August 1983 and reduced data from 62 nights. Significant seasonal variations in both zonal and meridional components of the thermospheric neutral wind vector were observed. Near the equinox, between 2000 and 2300 LT, the zonal wind component is eastward with an amplitude between 100 and 150 m/s that gradually ebbs to zero by dawn. The meridional component is generally small throughout the night. In the winter months (May–August) and at the winter solstice the zonal wind persists eastward throughout the night with speeds between 50 and 150 m/s. The meridional component is directed poleward (southward) toward the winter hemisphere with a speed of 50–75 m/s that decays to zero by midnight. Comparison with the predictions of the National Center for Atmospheric Research thermospheric general circulation model for equinoctial and solstice conditions shows good agreement. We conclude that the observed seasonal changes are caused by the changing nature of the solar forcing function. The Arequipa results found the day‐to‐day variability in the winter thermospheric winds to be less than that found for the summer equatorial observations obtained at Kwajalein. Interferometric measurements of the 630.0‐nm intensity at equinox showed a major reduction of the emission lasting 1 or 2 hours in all directions but south shortly after evening twilight; this decrease was not observed during winter.
Solar wind expansion in a polar coronal hole: Inferences from coronal white light and interplanetary Lyman alpha observations
Authors: Lallement R. , Holzer T. E. , Munro R. H.
Published: February 2008

Analysis of white light coronagraph observations of a polar coronal hole led Munro and Jackson (1977) to infer large solar wind speeds in the coronal hole and a consequent need for substantial energy addition to the outflow well above the coronal base. Recent interplanetary Lyman α observations by Kumar and Broadfoot (1979) and Lallement et al. (1985) indicate that the solar wind mass flux from well‐developed polar coronal holes may be considerably less than that typically observed in situ near the ecliptic plane. A reexamination of the Munro and Jackson analysis, with attention given both to the inferences drawn from the interplanetary Lyman α observations and to the uncertainties inherent in the coronal white light observations, leads to conclusions significantly different from those drawn by Munro and Jackson. Specifically, the coronal observations are found to be consistent not only with the high flow speeds and substantial energy addition inferred by Munro and Jackson, but also with much lower flow speeds and the absence of any significant energy addition in the region observed. This removal of definite observational confirmation of substantial energy addition to the coronal expansion between 1 Rs and 5 Rs has important implications for our understanding of energy balance in the solar wind.
Electromagnetic instabilities driven by cool heavy ion beams
Authors: Winske D. , Gary S. P.
Published: February 2008

The linear and nonlinear behavior of the right‐hand resonant and nonresonant electromagnetic ion beam instabilities driven by cool, β ∼ 1 ion beams is investigated as a function of beam mass and density. When the heavy ion beam is the less dense component, the maximum growth rate of the resonant mode and the threshold velocity of the nonresonant mode decrease with beam mass, while the maximum growth rate of the nonresonant mode and the threshold velocity of the resonant mode are nearly independent of the mass of the beam. When the heavy ion beam is the more dense component, a reversal of roles occurs, with the properties of the instabilities being determined by the sparser core ions. The nonlinear properties obtained from simulation show a similar behavior. At low beam densities the magnetic field fluctuation level increases with beam mass; at higher beam density it depends on the core mass. Very large magnetic fluctuations (δB/B0 ∼ 3), and smaller but still sizeable density fluctuations that do not correlate with the magnetic fluctuations, are observed and shown to agree well with those derived from theory. Applications to comets and Venus are suggested.
Detailed observations of the plasma sheet during a substorm on April 24, 1979
Authors: Hones E. W. , Fritz T. A. , Birn J. , Cooney J. , Bame S. J.
Published: February 2008

This paper reports a detailed study of plasma, magnetic field, and energetic particle data acquired by the ISEE 1 and 2 satellites, about 20 RE from earth in the magnetotail plasma sheet during a well‐defined substorm on April 24, 1979. The paper addresses, particularly, the plasma flows and energetic particle anisotropies that were encountered during the various stages of the substorm's evolution and compares these to predictions of the neutral line model of substorms. We find that the observations support that model in considerable detail. The interval from substorm onset at earth to plasma dropout was only 4½ min, and we stress the importance of high time resolution in all measurements if a full understanding of substorm initiation is to be developed. We found that the energetic ions (up to 125 keV) and plasma ions, although they sometimes showed somewhat different angular distributions in the plasma rest frame, typically shared a common bulk speed, i.e., the bulk flow speed of the plasma, parallel and perpendicular to the magnetic field. Thus the anisotropies of the energetic ion fluxes that have sometimes led to their characterization as earthward or tailward “beams” were found to be largely a consequence of their bulk motion with the plasma. We identify the region at the lobe‐plasma sheet interface, which has been called the plasma sheet boundary layer, with the separatrix layer in the neutral line model. This is the region of field lines including, and just equatorward of, the lobe‐plasma sheet separatrix, which should carry the most recent dynamic signatures of reconnection. Our results seem to support that expectation. Several features of our observations were inconsistent with the substorm picture, called the boundary layer dynamics model, in which a satellite's observation of fast plasma and particle flow is due primarily to its envelopment by a preexisting boundary layer of flowing plasma.
Anomalous nighttime increase of ion transition height in the equatorial ionosphere during sunspot maximum period
Authors: Goel M. K. , Rao B. C. N.
Published: February 2008

Analysis of ion composition data at 1100 km from the ISS‐b satellite revealed an anomalous nighttime increase of the transition height in the equatorial ionosphere for December solstice conditions. This feature was absent during June solstice. It appears that this anomalous behavior is observed only during high solar activity periods. Possible causes for this behavior are discussed qualitatively.
Instability of a comet ionopause: Consequences of collisions and compressibility
Authors: Ershkovich Alexander I. , Prialnik Dina , Eviatar Aharon
Published: February 2008

We have analyzed the stability of a contact surface located at the ram pressure balance position in a comet coma ionosphere in the presence of collisions between ions and neutrals and finite compressibility. We find that if the surface is taken to be a tangential discontinuity which excludes the interplanetary magnetic field from the ionosphere, this ionopause will be violently unstable to MHD modes. We therefore expect the large‐scale magnetic field to permeate the entire coma, even during periods of quiet solar activity. As a result, a stable cometary ionopause may form at a much greater distance from the nucleus.
Interaction of the solar wind with interstellar neutral hydrogen: Three‐fluid model
Authors: Isenberg Philip A.
Published: February 2008

It is commonly assumed in models of the solar wind‐interstellar neutral hydrogen interaction that the ionized interstellar particles are quickly assimilated into the solar wind proton population and “become indistinguishable” from the original solar wind. This assumption leads to the prediction that the solar wind proton temperature should increase with radius in the outer heliosphere. This temperature increase has not been observed. We point out that assimilation of the interstellar particles to the point of indistinguishability takes place on the very long Coulomb collision time scale and is not expected to occur within the heliosphere. We present results of a three‐fluid model of the solar wind which consists of comoving thermal populations of protons of solar origin, protons produced by ionization of interstellar hydrogen, and electrons. The steady‐state results yield a solar wind with a “core” proton distribution which cools adiabatically, and a “halo” of interstellar pickup protons which is maintained near 107 °K by the energy input of continued ionization and pickup. Such a distribution will not be observed to manifest the temperature increase at large heliocentric distances which is predicted from a one‐fluid analysis. Further time‐dependent calculations show a strong correlation between the densities of the solar wind and the interstellar pickup protons. We suggest that the interstellar pickup population may be observable by the Voyager plasma instruments in low resolution mode during periods of high solar wind density and low solar wind temperature.
Complementary analysis and interpretation of the shocklike electrostatic noise observed during the AMPTE solar wind lithium releases
Authors: Brinca A. L. , Moreira A. A. , Serra F. M. , Haerendel G. , Paschmann G.
Published: February 2008

The original interpretation of the electrostatic noise observed during the AMPTE solar wind lithium releases is complemented to clarify the nature of the intervening instabilities, the role of the particle populations on wave excitation, and the generation of the higher‐frequency (a few kilohertz) noise. The results suggest that the lower frequencies are created by an electron beam instability providing growth within a broad range of wave vector directions, whereas a negative‐energy, slow proton beam mode might contribute to the existence of the higher noise frequencies.
A factor of 2 reduction in theoretical F2 peak electron density due to enhanced vibrational excitation of N2 in summer at solar maximum
Authors: Richards P. G. , Torr D. G.
Published: February 2008

We have investigated the degree to which the O+ + N2 reaction rate is increased as a result of enhanced vibrational excitation of the N2 molecule in the thermosphere. We find that the reaction rate may be sufficiently elevated in summer at solar maximum to decrease the peak O+ density by a factor of 2, but there is only a small reduction in the winter peak density. Therefore the vibrational excitation of N2 acts to increase the magnitude of the seasonal anomaly. This work emphasizes the need for more laboratory work to clear up uncertainties in some of the key parameters. Uncertainties in the calculations arise from the lack of measurements of the key loss rates for O+ to O2 and N2 at the temperatures encountered at solar maximum, and also to uncertainties in the cross sections for excitation and de‐excitation of N2 vibrational quanta by electrons.
Median density altitude profiles of the major ions in the central nightside Venus ionosphere
Authors: Knudsen William C. , Miller Kent L. , Spenner Karl
Published: February 2008

Altitude profiles of median values of total ion density Ni and constituent ion densities [O+], [H+], and the molecular ion group density [O2+ + M29+] (=[O2+] + [NO+]+[N2+]+[CO+]) for the central nightside Venus ionosphere are constructed from measurements made by the Pioneer Venus (PV) orbiter retarding potential analyzer (ORPA) during the first five seasonal passages of the PV spacecraft through the nightside hemisphere. The profiles cover the altitude range 145–900 km altitude. The altitude profiles for Ni, [O+], and [O2+ + M29+] are compared with previously predicted profiles derived from numerical models in which the ionization source was O+ transported from the dayside ionosphere and electron impact produced ionization, respectively. The numerically derived profile for O+ transport reproduces the median profiles of Ni, [O+], and [O2+ + M29+] quite satisfactorily. The extent to which electron impact ionization is required remains uncertain. The temporal variability of a quantity Q from its median value Qm is characterized by a variance parameter such that two thirds of the measurements Q fall within the limits Qm/σ ≤ Q ≤ σ Qm. The value of σ for Ni, [O+] and [H+] above an altitude of 150 km is 2.5. Below 150 km the variance for Ni and [O2+ + M29+] is approximately 1.5, but the statistics from which it is derived are less reliable. ORPA‐derived median plasma density Ni and [O2+ + M29+] agree closely with the average (and median) value of the peak nightside electron density Ne,max measured by the PV radio occultation experiment when Ni and [O2+ + M29+] are extrapolated a short altitude interval to that of Ne,max. The standard deviation of Ne,max also agrees well with the variances of Ni and [O2+ + M29+] calculated for altitudes below 150 km. The ORPA median Ni profile at low altitudes is a factor of two smaller than the median Ne profile derived from published PV Langmuir probe (OLP) medians. At higher altitudes the Ni median profile is 80–85% that of Ne. The plasma densities measured by PV in situ instruments in the altitude range 200–250 km exceed those derived from Venera 9 and 10 radio occultation data by a factor of the order of 10. The Venera measurements were made at sunspot minimum and, if representative of the median density, imply an order of magnitude variation over a solar cycle in the median O+ flux transported from the dayside ionosphere.
Origin of large magnetic fluctuations in the magnetosheath of Venus
Authors: Winske D.
Published: February 2008

The origin of large‐amplitude hydromagnetic waves in the Venus magnetosheath down‐stream of the quasi‐parallel bow shock is investigated by means of numerical simulation. It is shown that the most likely source of these waves is the bow shock itself, rather than an instability involving the solar wind and oxygen ions of planetary origin. Pickup of O+ ions by these waves is also examined and shown to be in agreement with previous test particle calculations. The effect of mass loading on the structure of the shock is also discussed.
Cyclotron maser radiation from a source structure localized perpendicular to the ambient magnetic field
Authors: Pritchett P. L.
Published: February 2008

The plasma cavity in which the auroral kilometric radiation (AKR) is generated is known to contain localized density enhancements, and the cavity itself is surrounded by regions of much higher plasma density. Particle simulations are used to investigate the effects of such a source structure perpendicular to a uniform magnetic field on the generation and propagation of electromagnetic radiation produced by the cyclotron maser instability. It is found that the efficiency of radiation production is significantly increased in comparison to the uniform geometry case. The radiation is observed to consist of wave packets which travel at speeds of the order of 0.1c–0.2c, which is consistent with the X mode group velocity near cutoff. The reflection of these structures at the edge of a density enhancement is quite weak, with less than 0.1% of the incident energy being reflected in a typical case. The radiation with frequency ω ≲ Ωe produced in the cavity region and emitted perpendicular to the magnetic field is observed to penetrate through regions of high‐density cold plasma with little diminution. The mechanism for this propagation is identified as mode conversion of the original X mode waves into Z mode waves at the edge of the plasma cavity and subsequent conversion to the R‐X mode as the plasma density decreases again. The implications of these results for theories concerning the generation and propagation of AKR are discussed.
Auroral zone thermospheric dynamics: 2. Individual nights
Authors: Sica R. J. , Hernandez G. , Romick G. J. , Rees M. H. , Roble R. G.
Published: February 2008

Ground‐based Fabry‐Perot spectrometer measurements of the O I 15867 K (630.0 nm; 1 K = 1 cm−1) emission line were obtained from College, Alaska, during the winters of 1981 to 1983. Averages of the temperatures and winds, derived from these measurements, show a general flow driven by ion drag due to magnetospheric convection. This hypothesis was supported by results from a global thermospheric circulation model. Individual nights, however, show differences from the average flow pattern. During the recovery phase of an auroral substorm, when the westward electrojet is over the observing site, the equatorward meridional wind decreases in magnitude. The zonal wind is generally westward in the evening and eastward in the morning. However, the crossover from westward to eastward flow is a function of the location of the substorm onset and the arrival of the westward electrojet over College. Zenith winds can be very large (200 m/s) and show no systematic pattern, perhaps due to a superposition of wave modes from the auroral zone source region. An example of a night with little auroral activity highlights the effects of magnetospheric convection and the auroral electrojet on the normal auroral zone wind pattern. Temperature measurements occasionally show large enhancements, usually associated with type‐A red aurora.
Geocoronal imaging with Dynamics Explorer
Authors: Rairden R. L. , Frank L. A. , Craven J. D.
Published: February 2008

The ultraviolet photometer of the University of Iowa spin scan auroral imaging instrumentation on board the Dynamics Explorer 1 satellite has returned numerous images of the geocorona from altitudes of 570 km to 23,300 km. The geocoronal observations from 1981 through 1985 are compared to a spherically symmetric isothermal Chamberlain model of the exospheric density distribution. Model parameters are varied to obtain an acceptable fit. The radiative transfer equation is solved numerically. Stellar intensities are monitored for an independent calibration of the DE 1 instrument in flight. The solar Ly α flux is estimated through concurrent measurements made by the Solar Mesosphere Explorer satellite, supplemented by published values of ground‐observable solar indices. Extraterrestrial background intensities are adopted from earlier OGO 5 high‐altitude measurements. The optimum fit for 1981 imaging data utilizes a Chamberlain model of temperature T = 1050 K and exobase density nc = 44,000 atoms cm−3. The exobase is taken as rc = 1.08 RE (500 km altitude), and a critical radius for satellite atoms of rcs = 3.0 rc is adopted. This model continues to compare well with the DE 1 measurements over the entire 4‐year period studied, even though the exobase conditions are expected to have changed appreciably during this interval of declining solar activity. It is concluded that the apparently constant hydrogen density and scale height observed by DE 1 are not directly indicative of the exobase conditions through the classical Chamberlain model but rather show the effects of charge exchange with thermal ions in the plasmasphere. A readily observable departure from spherical symmetry is the geotail, an enhancement in the atomic hydrogen column densities in the antisunward direction.
Geomagnetic equatorial anomaly in zonal plasma flow
Authors: Aggson T. L. , Maynard N. C. , Herrero F. A. , Mayr H. G. , Brace L. H. , Liebrecht M. C.
Published: February 2008

We report here on the observation of a geomagnetic signature in the zonal eastward plasma flow, which is a striking feature of the equatorial ionosphere in the evening quadrant. These observations were derived from (E × B)/B² measurements made with the cylindrical double floating probe experiment carried on the Dynamics Explorer 2 (DE 2) satellite. The signature consists of a crest‐trough‐crest effect in the latitude dependence of the eastward plasma flow with the crests at ± 8° dip latitude and the trough nearly centered at the dip equator at all geographic longitudes. This phenomenon can be readily interpreted in terms of the altitude dependence of the F region dynamo electric field, and it is related to dip equator signatures in the plasma density and the magnetic declination which have been reported earlier.
Stationary Alfvénic structures
Authors: Wright A. N. , Southwood D. J.
Published: February 2008

A nonlinear, analytical model is developed to describe equilibrium Alfvén wave structures such as that associated with flow about a conducting obstacle in a flowing plasma. Firstly, in order to give physical insight into the processes involved, we discuss two worked examples one of which extends previous models of the Alfvén wave wake produced by the Jovian satellite Io. We describe in detail the fields and, in particular, the current systems associated with the model. Finally, we present a general formalism which leads to a general solution that should be applicable in modeling other incompressible space plasma flows.