bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of O+ ions in the plasmasphere
Authors: Guiter S. M. , Moore T. E. , Khazanov G. V.
Published: February 2004

Heavy ion (O+, O++, and N+) density enhancements in the outer plasmasphere have been observed using the retarding ion mass spectrometer instrument on the DE 1 satellite. These are seen at L shells from 2 to 5, with most occurrences in the L = 3 to 4 region; the maximum L shell at which these enhancements occur varies inversely with Dst. It is also known that enhancements of O+ and O++ overlie ionospheric electron temperature peaks. It is thought that these enhancements are related to heating of plasmaspheric particles through interactions with ring current ions. This was investigated using a time‐dependent one‐stream hydrodynamic model for plasmaspheric flows, in which the model flux tube is connected to the ionosphere. The model simultaneously solves the coupled continuity, momentum, and energy equations of a two‐ion (H+ and O+) quasi‐neutral, currentless plasma. This model is fully interhemispheric and diffusive equilibrium is not assumed; it includes a corotating tilted dipole magnetic field and neutral winds. First, diurnally reproducible results were found assuming only photoelectron heating of thermal electrons. For this case the modeled equatorial O+ density was below 1 cm−3 throughout the day. The O+ results also show significant diurnal variability, with standing shocks developing when production stops and O+ flows downward under the influence of gravity. Numerical tests were done with different levels of electron heating in the plasmasphere; these show that the equatorial O+ density is highly dependent on the assumed electron heating rates. Over the range of integrated plasmaspheric electron heating (along the flux tube) from 8.7 to 280×109 eV/s, the equatorial O+ density goes like the heating raised to the power 2.3.
Numerical simulations of mass loading in the solar wind interaction with Venus
Authors: Murawski K. , Steinolfson R. S.
Published: February 2004

Numerical simulations are performed in the framework of nonlinear two ‐ dimensional magnetohydrodynamics to investigate the influence of mass loading on the solar wind interaction with Venus. The principal physical features of the interaction of the solar wind with the atmosphere of Venus are presented. The formation of the bow shock, the magnetic barrier, and the magnetotail are some typical features of the interaction. The deceleration of the solar wind due to the mass loading near Venus is an additional feature. The effect of the mass loading is to push the shock farther outward from the planet. The influence of different values of the magnetic field strength on plasma evolution is considered.
Electron trajectories in a model magnetosphere: Simulation and observation under active conditions
Authors: Lin Zhongmin , Bieber John W. , Evenson Paul
Published: February 2004

Trajectories of electrons with energies of 10–2000 MeV were studied using Tsyganenko's global quantitative model of the magnetosphere. Results were compared with balloon observations of atmospheric, return albedo, and cosmic electrons made at Prince Albert on August 26, 1987. Active geomagnetic conditions were associated with unusual variations in the flux of electrons on that date, which we interpret as a manifestation of an anomalously low and highly variable geomagnetic cutoff. Increasing the ring current intensity in the Tsyganenko model reduces the calculated effective cutoff rigidity somewhat, but to explain all of our observations, we must postulate that trajectories traditionally assumed to be forbidden are sometimes systematically converted into allowed trajectories. Within this model we can also identify field lines on which bursts of auroral X rays were generated.
Characterizing the quiet time magnetic field at geostationary orbit
Authors: Skone S. H. , Donovan E. F. , Rostoker G.
Published: February 2004

There is now an increasing body of evidence which suggests that the onset of a substorm expansive phase is triggered in the near‐Earth tail region, inside X ∼ −12 RE [Samson et al., 1992]. For this reason, it is necessary to characterize the magnetic field in the near‐Earth region, particularly the quiet time reference field. We have therefore carried out a detailed study of the magnetic field at geostationary orbit, using data acquired during quiet times from the GOES 5 and GOES 6 satellites which operated simultaneously during 1986. Diurnal and seasonal variations of the total field, and the perturbation magnetic field with the IGRF removed, have been identified and attributed to variations in the position of the neutral sheet with respect to the satellite. We have processed the data to remove seasonal effects by using the semiempirical expression developed by Lopez [1990] to express the displacement of the neutral sheet with respect to the magnetic equatorial plane. We then use our magnetic field model, recently developed by Donovan [1993a], to model the observed quiet time magnetic field at geostationary orbit, and also determine the quiet time current configuration.
Mantle crossings at Geotail: Comparison with MHD model
Authors: Shodhan S. , Siscoe G. L. , Frank L. A. , Ackerson K. L. , Paterson W. R. , Fairfield D. , Kokubun S. , Yamamoto T.
Published: February 2004

From 2000 UT on March 17 to 0400 UT on March 18, 1993, when Geotail was in the distant tail, it underwent a series of mantle crossings. These mantle crossings are manifested in the magnetic field and plasma data as smooth excursions between magnetosheath values and lobe values. We test the one‐dimensional self‐similar MHD slow mode expansion fan model of the mantle by comparing the modeled predictions with the data, including, for the first time, magnetic field data. We use the measured speed, density, magnetic field strength, ion and two different electron temperatures to initialize the mantle flow. Using this range of upstream parameters, we show that the model reasonably well brackets the observed variations of both magnetic field and plasma parameters through the mantle. The total pressure through the modeled fan remains constant as the thermal pressure at the outer edge converts into magnetic field pressure at the inner edge. Using the modeled fit, we estimate the mantle to be 10–12 RE thick.
Three‐dimensional magnetic reconnection without null points: 1. Basic theory of magnetic flipping
Authors: Priest E. R. , Démoulin P.
Published: February 2004

In two or three dimensions, magnetic reconnection may occur at neutral points and is accompanied by the transport of magnetic field lines across separatrices, the field lines (or flux surfaces in three dimensions) at which the mapping of field lines is discontinuous. Here we show that reconnection may also occur in three dimensions in the absence of neutral points at so‐called “quasi‐separatrix layers,” where there is a steep gradient in field line linkage. Reconnection is a global property, and so, in order to determine where it can occur, the first step is to enclose the volume being considered by a boundary (such as a spherical surface). Then the mapping of field lines from one part of the boundary to another is determined, and quasi‐separatrix layers may be identified as regions where the gradient of the mapping or its inverse is very much larger than normal. The most effective measure of the presence of such layers is the norm of the displacement gradient tensor; their qualitative location is robust and insensitive to the particular surface that is chosen. Reconnection itself occurs when there is a breakdown of ideal MHD and a change of connectivity of plasma elements, where the field line velocity becomes larger than the plasma velocity, so that the field lines slip through the plasma. This breakdown can occur in the quasi‐separatrix layers with an electric field component parallel to the magnetic field. In three dimensions the electric field E (and therefore the field line velocity v⊥) depends partly on the imposed values of E (or v⊥) at the boundary and partly on the gradients of the inverse mapping function. We show that the inverse mapping determines the location of the narrow layers where the breakdown of ideal MHD can occur, while the imposed boundary values of v⊥ determine mainly the detailed flow pattern inside the layers. Thus, in general, E (and therefore v⊥) becomes much larger than its boundary values at locations where the gradients of the inverse mapping function are large. An example is given of a sheared X field, where a slow smooth continuous shear flow imposed on the boundary across one quasi‐separatrix produces a flipping of magnetic field lines as they slip rapidly through the plasma in the other quasi‐separatrix layer. It results in a strong plasma jetting localized in, and parallel to, the separatrix layers.
Observations of magnetic structure at the dayside ionopause of Venus
Authors: Law Colin C. , Cloutier Paul A.
Published: February 2004

The solar wind interaction with the nonmagnetic Venerean ionospheric obstacle is unique. Ionospheric models of this interaction have primarily been in two dimensions that do not allow for changes in the orientation of the solar wind magnetic field near the obstacle. Analysis of high‐resolution magnetic field data from the Pioneer Venus Orbiter spacecraft has revealed field rotations that are observed to occur in conjunction with the dayside ionopause. These rotations are a result of the velocity shear at the ionopause and indicate the alignment (“weathervaning”) of the magnetic field with the radial day to night flow of ionospheric plasma. A new configuration of the dayside magnetic field draping has been derived from these results. Assuming Mars also represents a nonmagnetic obstacle to the flow, as past experimental observations indicate, the field diagnostics discovered here make it possible to probe the structure of the Martian ionosphere using magnetometer data in the absence of ion mass spectrometer data. These results would have had direct application to the Mars Observer spacecraft data and could play a major role in future Mars missions such as Mars Surveyor.
Field/field spatial correlation function: Electromagnetic proton cyclotron instability
Authors: Gary S. Peter , Winske Dan
Published: February 2004

One‐dimensional initial‐value hybrid simulations are used to study the electromagnetic proton cyclotron anisotropy instability under the condition k × Bo =0 where Bo is the uniform background magnetic field. The spatial correlation function of the fluctuating magnetic field is calculated at post‐saturation times for aggregations of simulations corresponding to various initial values of the proton parallel β and the maximum growth rate of the instability. A statistical analysis of the results shows that the characteristic wavenumber of the correlation function may be approximately determined by linear theory; that is, by using the instantaneous values of the plasma parameters in the linear dispersion equation. In contrast the wavenumber bandwidth of the correlation function is an intrinsically nonlinear quantity and appears to depend on a weak inverse power of the fluctuating magnetic field energy density. These results predict properties of correlation functions of the enhanced proton‐cyclotron‐like fluctuations which may be compared against observations by the multi‐spacecraft Cluster mission in the outer magnetosphere and under low flow speed conditions in the magnetosheath.
Is the magnetopause Rayleigh‐Taylor unstable sometimes?
Authors: Gratton F. T. , Farrugia C. J. , Cowley S. W. H.
Published: February 2004

At the terrestrial magnetopause the dense magnetosheath flowing around the magnetosphere interfaces with the normally much more tenuous magnetospheric plasma. Temporal variations in the solar wind dynamic pressure cause the magnetopause to be in continuous motion. When it decelerates and rebounds sunward after a strong and rapid drop in the solar wind dynamic pressure, we have a situation of a heavy‐over‐light plasma in an effective gravitational field. In this paper we investigate theoretically the conditions under which this configuration may become Rayleigh‐Taylor (RT) unstable. The motion of the magnetopause is studied using a gas‐dynamic model which takes into account forces arising from hypersonic flow past an object of prescribed shape. Thus the effect of the magnetic field near the magnetopause is not included. For sufficiently large and rapid dynamic pressure variations we obtain accelerations ≥ 1kms−2 lasting for periods of the order of 1–2 min. We discuss the possible instability of both global (λ ≫ Δ, thickness of the magnetopause) and internal (λ < Δ) modes, taking into account the stabilizing effects of (1) magnetic shear across the magnetopause and (2) viscosity of the magnetosheath plasma. We find that both modes are stable when the magnetic shear across the magnetopause is large. When the magnetosheath field points strongly northward, however, we find that both modes may be RT unstable. Among the effects of RT instability on the magnetopause and its environs are (1) large distortions of the magnetic field lines, (2) localized compressions of the magnetospheric magnetic field, (3) increased magnetopause thickness, and (4) reduced average density gradient across the magnetopause. (Since we only concentrate here on the linear stage of the development of the instability, no mass transfer into the magnetosphere is included.) These are all, in principle, observable effects, though the rapid motion of the magnetopause would require a fortuitous spacecraft‐magnetopause configuration for them to be actually seen in single‐spacecraft data. This work may be considered äs a contribution to the current research on the structure and physics of the low‐shear magnetopause.
Simulation of high‐frequency solar wind power spectra using Hall magnetohydrodynamics
Authors: Ghosh S. , Siregar E. , Roberts D. A. , Goldstein M. L.
Published: February 2004

Solar wind frequency spectra show a distinct steepening of the ƒ−5/3 power law inertial range spectrum at frequencies above the Doppler‐shifted ion cyclotron frequency. This is commonly attributed to dissipation due to wave‐particle interactions. We consider the extent to which this steepening can be described, using a magnetohydrodynamic formulation that includes the Hall term. An important characteristic of Hall MHD is that although the ion cyclotron resonance is included, there is no wave‐particle dissipation of energy. In this study we use a compressible Hall MHD code with a constant magnetic field and a polytropic equation of state. Artificial dissipation in the form of a bi‐Laplacian operator is used to suppress numerical instabilities, allowing for a clear separation of the dissipative scales from the ion cyclotron scales. A distinct steepening appears in the simulation power spectra near the cyclotron resonance for certain types of initial conditions. This steepening is associated with the appearance of right circularly polarized fluctuations at frequencies above the ion cyclotron resonance. Similar steepenings and polarization enhancements are observed in solar wind magnetic field data.
Structure of the heliospheric MHD bow shock: Effects of ion‐atom drifts
Authors: Mullan D. J. , Arge C. N.
Published: February 2004

We report on the structure of a perpendicular MHD shock in the partially ionized gas of the local interstellar medium (LISM). In situations where the flow speed is sub‐Alfvenic in the ion gas and supersonic in the atom gas, the ions and atoms behave differently in the shock. Ion‐atom drifts provide a dissipation mechanism which permits computation of the steady state structure of the transition between upstream and downstream conditions. Using parameter ranges which are appropriate for the LISM, we find that the heliospheric bow shock may be many hundreds of AU thick.
Multiple charge exchange and ionization collisions within the ring current‐geocorona‐plasmasphere system: Generation of a secondary ring current on inner L shells
Authors: Bishop James
Published: February 2004

Charge exchange (CE) collisions between ring current ions and geocoronal atoms erode the ring current and give rise to a pervasive flux of energetic neutral atoms (ENAs). ENAs generated in the main ring current traversing the inner magnetosphere can be reionized in several ways, converting ENAs back into ring current ions albeit on new L shells (provided the angle between the direction of ENA travel and the local magnetic field direction at the point of collision corresponds to a pitch angle outside the local loss cone). These displaced ring current ions in turn undergo subsequent CE collisions with geocoronal atoms, generating secondary ENA fluxes that can participate in further ionizing collisions. A modeling scheme for exploring the effectiveness of this mechanism in establishing and maintaining a secondary ring current at L ≲ 3 has been developed taking full account of multiple CE and ionizing collisions. It is based on a matrix solution technique for an integral form of the Boltzmann transport equation and relies on two simplifications: (1) Since gyroperiods ≪ bounce periods, gyroradii ≪ scale lengths for variations in geocoronal and plasmaspheric densities, and single‐bounce collisional depths along ion gyropaths ≪ 1, it is adequate to work in terms of quantities integrated along bounce paths and averaged in gyrophase. (2) The thermal populations are not directly perturbed by the energetic populations, while the sources of the main ring current are external to the charge‐exchange‐coupled systems, so that the transport equation for the secondary flux is linear. The results are steady state fluxes of trapped ions as functions of L shell, equatorial pitch angle, and energy. This model is described in full, and an example of steady state secondary H+ ring current fluxes for quiet conditions is presented using the quiet time empirical model of Sheldon and Hamilton [1993] to prescribe the main proton ring current.
Nonlocal ion‐Weibel instability in the geomagnetic tail
Authors: Yoon Peter H. , Lui Anthony T. Y.
Published: February 2004

The nonlocal stability of a thin current sheet against the electromagnetic ion‐Weibel mode perturbation is investigated. The analysis is carried out for physical parameters typical of the near‐Earth plasma sheet condition just prior to the substorm expansion onset. It is found that the ion‐Weibel mode is stabilized in a Harris‐like equilibrium profile with no velocity shear associated with the cross‐field current, but positive growth of the instability is recovered once a finite velocity shear is incorporated into the equilibrium current profile and a sufficient, concomittant reduction of the current sheet thickness is imposed. This result is analogous to the case of the nonlocal modified‐two‐stream mode analysis which also indicates the importance of a velocity shear in the current sheet profile and the raising of the instability onset threshold from that predicted by the local theory. The parallelism suggests that, in general, nonlocal consideration of the stability of a thin current sheet predicts a higher onset threshold than the local stability analysis for a class of cross‐field current instabilities.
Interrelation of noise‐like and discrete ELF/VLF emissions generated by cyclotron interactions
Authors: Trakhtengerts V. Y. , Rycroft M. J. , Demekhov A. G.
Published: February 2004

It is shown that quasi‐monochromatic whistler waves (wavelets) can be caused by the strong cyclotron instability, stimulated by hiss emissions. The hiss, generated by the cyclotron instability of an anisotropic smooth energetic electron distribution, creates a step‐like deformation of the distribution function at the boundary between resonant and nonresonant electrons. This deformation leads to the strong amplification of the wavelet whose frequency corresponds to that for cyclotron resonance with electrons at the step. Analytical calculations for this amplification have been made using the rigorous theory of the cyclotron instability in an inhomogeneous magnetic field. The wave amplification can be 2 orders of magnitude greater than that for a smooth distribution function. A self‐consistent computational analysis of the cyclotron instability is developed on the basis of quasi‐linear theory. This confirms both the formation of the step‐like deformation of the distribution function and the wavelet generation.
Temporal and spatial variations in upper atmospheric Mg+
Authors: Joiner J. , Aikin A. C.
Published: February 2004

Long‐term behavior of magnesium ions in the upper atmosphere has been observed at wavelengths near 2800 Å using the Nimbus 7 solar backscattered ultraviolet (SBUV) instrument. Observations were made in the continuous spectral scan mode that operated approximately once a month from 1979 to 1986. Total column abundance of Mg+ near local noon is the quantity derived from the nadirviewing SBUV observations. Mg+ total column abundances vary as a function of magnetic activity, solar sunspot activity, season, dip latitude, and longitude, as the result of changes in chemistry and ion transport. Increased Mg+ column abundances are observed during periods of high magnetic activity at all latitudes. Total Mg+ column abundances are observed to increase with solar sunspot activity at low and middle latitudes. Seasonal variation in the midlatitude Mg+ column abundance is observed with a maximum near summer solstice. The total column abundances derived here are complementary to previous metallic ion studies in the upper E and F regions.
Variability of the tail lobe field strength
Authors: Fairfield D. H. , Jones J.
Published: February 2004

A large magnetosphere database has been used to study spatial and temporal variations of the geomagnetic tail lobe field strength. The study uses 12,388 average tail lobe measurements from the region −15 > X > −70 RE obtained from 11 different spacecraft over a 20‐year period. The average radial gradient is found to be represented by B = 1659.2/R1.46 + 7.47 which can be used to eliminate the dependence of individual measurements on distance down the tail. A set of 5562 of the resulting normalized data points is associated with simultaneously measured solar wind dynamic pressure and interplanetary magnetic field (IMF) values which permit the study of relationships of Blobe with geomagnetic activity and with solar wind parameters on various timescales. Variations of annual averages of Blobe and AE exhibit a solar cycle dependence and are very similar to each other and to solar wind dynamic pressure variations. On an hourly time scale, tail lobe field strength normalized to a distance of 30 RE is found to depend on both solar wind dynamic pressure, Psw, and IMF Bz. This dependence can be represented as
where θ varies from 0° for northward to 180° for southward. The ability to quantitatively predict average Blobe given solar wind pressure and IMF B provides a baseline which should be useful in the study of more rapid substorm variations which are due to internal magnetospheric dynamics.
Macroparticle simulation of collisionless parallel shocks generated by solar wind and planetary plasma interactions
Authors: Shimazu H. , Machida S. , Tanaka M.
Published: February 2004

An implicit‐particle simulation of the collisionless parallel shock created at the interface between an injected beam and a stationary plasma is performed in one‐dimensional geometry. The solar wind plasma, which consists of ions and electrons, is injected into a stationary dense plasma that corresponds to the planetary ionosphere. Electromagnetic waves with right‐hand circular polarization that propagate upstream (R− waves) are generated at the interface of the two plasmas, which decelerate the solar wind to form a shock. The shock transition region is not monotonic but consists of two distinct regions, a pedestal and a shock ramp. The transition region, which contains the ionopause, is a few thousand electron skin depths long. The parallel shock varies in time and periodically collapses and re‐forms. The right‐hand circularly polarized electromagnetic waves that propagate downstream (R+ waves) are excited at the shock ramp. Nonlinear wave‐particle interaction between the solar wind and the R+ waves causes wave condensation and density modulation. These R+ waves may be sweeping away the downstream plasma to suppress its thermal diffusion across the shock. The electrons at the shock ramp exhibit a flat‐topped velocity distribution along the magnetic field owing to the ion acoustic‐like electrostatic waves.
Search for the “unknown” quantity in the solar wind: A personal account
Authors: Akasofu S.‐I.
Published: February 2004

In 1964 when S. Chapman and I concluded that there is some “unknown” quantity in the solar wind controlling the development of the main phase of geomagnetic storms. This statement was based on our study of the variety of the development of geomagnetic storms and on the resulting conclusion that the Chapman‐Ferraro's solar wind flows around the Earth and confines the Earth in a cavity (the magnetosphere) but does not transfer the energy to the magnetosphere. It was thought to be an outrageous statement, since it had been firmly believed in the 1960s that a strong solar wind was all that was needed to cause geomagnetic storms. Our search for the unknown quantity in the solar wind eventually led us to a study of auroral/magnetospheric substorms, which, in turn, led us to the conclusion that the unknown quantity is the north–south component of the IMF, or more accurately, a specific combination of three parameters: the solar wind speed V, the IMF magnitude B, and its polar angle θ. This paper describes a personal account of the history of this search for the unknown quantity. One important lesson I learned during this research is that a scientific field is often in trouble when the majority of researchers agree on one particular theory or one particular interpretation of observations. In general, one particular theory in a scientific field, however popular it may be, is eventually bound to fail. Thus, when the majority of researchers believe in that particular theory for a very long time, it means that the advance in that field is temporarily stalled.
Electron and ion temperature gradients and suprathermal tail strengths at Parker's solar wind sonic critical point
Authors: Scudder J. D.
Published: February 2004

New constraints are placed on the electron and ion temperature gradients at Parker's sonic point, demonstrated here as the location for an ion where the force of gravity and the electric field parallel to the magnetic field are balanced. The equal‐temperature, one‐fluid versions of these results precisely recover Parker's necessary and sufficient conditions for the supersonic expansion. The two species constraints are largely within the Parker envelope. For all possible critical points, the electron partial pressure must exceed 0.2437 of the plasma pressure at this point. The location of the critical point relative to the isothermal critical point is determined by the partial pressure average of the electron and ion logarithmic derivatives of temperature at the critical point. The bulk acceleration at the critical point is controlled by (1) the ion (as distinct from the electron) temperature gradient at this point and (2) the ratio of the ion to electron pressure at the critical point. The sonic point is a velocity space deLaval nozzle rather than a traditional configuration space nozzle. Critical points are most frequently accompanied by positive radial ion and electron temperature gradients. The electron and ion temperature gradients required for a critical point are coupled and not arbitrary. Parker's one‐fluid discussion requires, by this analysis, both electron and ion temperature gradients to have positive radial exponents β = 3/4 near the critical point and a local effective gas polytrope behavior of γ = 10/13 < 1. For equal electron and ion temperatures, all possible critical points require positive ion temperature gradients at the critical point. Nonthermal electron and ion distributions are demonstrated to be generally required near the sonic critical point of a spherically symmetric solar wind. Effective polytrope exponents near the critical point range from γi(r*) < 1 and 1.5 > γe(r*) > 1/3. The allowed critical points with γ < 1 are used to infer a range of nonthermal κ distributions that permit this unusual γ < 1 behavior; ion nonthermal suprathermal tail indices ranging between 2 < κi < 5 are required, reflective of decidedly nonthermal distributions at the critical point and consistent with previous inferences of their size [Scudder, 1992b]. Even when unequal electron and ion temperatures are considered, the vast preponderance of the critical points delineated also is found with 1/3 < γe < 1, with the attendant range of electron nonthermal indices, κe, ranging from 2 to the Maxwellian limit of ∞. Critical points with maximal acceleration invariably occur for a positive electron temperature power law exponent of βe ≃ 1.12 that corresponds to κe ≃ 3.3, whether or not the ions and electrons have a common temperature at the critical point.
Low‐energy He+ and H+ distributions and proton cyclotron waves in the afternoon equatorial magnetosphere
Authors: Fuselier S. A. , Anderson B. J.
Published: February 2004

Characteristics of low energy (∼10 to 50 eV) H+ and He+ distributions in the afternoon magnetosphere from L = 4 to 9 are studied using Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) data. He+ distributions in this study routinely display features indicating resonant heating by proton cyclotron waves in the outer magnetosphere (L > 7) whereas protons exhibit minimal evidence of heating. Low energy H+ distributions have T⊥ ≫ T‖ for L < 5 but T⊥ ≪ T‖ for L >7. The change is due to a decrease in T⊥ with increasing L and is consistent with adiabatic cooling in the perpendicular direction as the ions convect sunward. By contrast, low‐energy He+ distributions have T⊥ ≫ T‖ independent of L. The failure of the He+ to cool adiabatically indicates that He+ experiences transverse heating as it convects sunward through the outer magnetosphere. The occurrence of proton cyclotron waves in the afternoon magnetosphere is evaluated using AMPTE/CCE magnetometer data and shows that low energy ions convecting sunward from the afternoon plasmasphere have a 20% chance of being exposed to cyclotron waves by the time they reach L = 5, 50% by L = 6 and > 95% by L = 7. The majority of cyclotron waves in the early afternoon outer magnetosphere occur above the local He+ gyrofrequency and will interact resonantly with He+ but nonresonantly with H+. The combined effects of adiabatic sunward convection and interactions between the low‐energy ions and proton cyclotron waves therefore account for the observed characteristics of both the H+ and He+ distributions in the afternoon magnetosphere.
Thruster effects on the shuttle potential during TSS 1
Authors: Machuzak J. S. , Burke W. J. , Gentile L. C. , Davis V. A. , Hardy D. A. , Huang C. Y.
Published: February 2004

We have surveyed measurements taken by the Shuttle Potential and Return Electron Experiment (SPREE) in the shuttle payload bay during the satellite deployment phase of the Tethered Satellite System (TSS 1) mission. The shuttle was flying in an inverse airplane mode with its engine bells facing the ram direction to facilitate ion current collection. SPREE ion spectral measurements indicate that with no thrusters firing, the shuttle charged to, at most, a few tens of volts negative. We identified 13 intervals during which the tether was electrically connected to the shuttle by a low‐impedance shunt and thruster firings occurred. In previously reported examples of neutral gas releases from charged spacecraft the magnitude of the vehicle potential decreased. Thruster firings during our events either left the TSS circuit unchanged or caused the current measured in the tether to diminish and the shuttle to become more negatively charged. In the latter case it appears that thruster gas impeded access of ambient ions to the current collecting surfaces of the shuttle. The changes in the distribution of the motionally induced potential between the tether and sheath calculated using Ohm's law are in agreement with SPREE measurements.
Zonal/meridional wind and disturbance dynamo electric field control of the low‐latitude ionosphere based on the SUNDIAL/ATLAS 1 Campaign
Authors: Abdu M. A. , Sobral J. H. A. , Richards P. , de Gonzalez Marta M. , Huang Y. N. , Reddy B. M. , Cheng K. , Szuszczewicz E. P. , Batista I. S.
Published: February 2004

We present an evaluation of global‐scale equatorial and low latitude ionospheric disturbances in response to the weak‐to‐moderate disturbed conditions that marked the SUNDIAL/ATLAS 1 twelve‐day campaign of March 22 to April 2, 1992. Ionosonde data from South American and Indian‐Asian longitudes are analyzed to examine first the climatological (the average) pattern of the criticalFregion parameters (the layer peak density and height) in comparison with the empirical International Reference Ionospheric model, and then to characterize the day‐to‐day variabilities aiming at identifying potential causal contributions from either disturbance zonal and meridional winds or magnetospheric and disturbance dynamo electric fields. Included in this analysis are data from South American midlatitude locations which are used to determine meridional winds using an adaptation of the servo analysis technique in the Field Line Integrated Plasma (FLIP) model. We have made an assessment of the causal mechanism of the day‐to‐day variabilities as arising from latitude dependent disturbance meridional winds, and from electric fields produced by disturbance zonal winds and disturbance dynamo. While the contribution from disturbance meridional winds decreases from middle to equatorial latitudes, that of the electric fields maximizes around the equator. In particular, first‐time evidence based on ionosonde data is provided for a disturbance dynamo electric field in the equatorial ionosphere. It is found that there are two time intervals of maximum ionospheric variability resulting from the weak to moderate magnetospheric disturbance conditions that prevailed during the campaign: one near the evening/postsunset hours and the other in the postmidnight‐sunrise hours over low latitudes. At midlatitudes, a broad maximum of the response occurs from premidnight to morning hours. We provide a comparison of results for the South American and Indian‐Asian longitudes and a discussion of the competing roles of the disturbance zonal and meridional winds, and magnetospheric and disturbance dynamo electric fields as a function of latitude.
Whistler instability: Electron anisotropy upper bound
Authors: Gary S. Peter , Wang Joseph
Published: February 2004

If the electron distribution function is approximately bi‐Maxwellian with T⊥/T‖ > 1, where the subscript symbols denote directions perpendicular and parallel to the background magnetic field, and if this temperature anisotropy is sufficiently large, the whistler anisotropy instability is excited. This mode is studied using two‐dimensional particle‐in‐cell simulations in a spatially homogeneous plasma model. Theory predicts a threshold electron anisotropy for this instability which depends inversely on the electron parallel β. The simulations show that wave‐particle scattering by enhanced whistler fluctuations maintain the initially bi‐Maxwellian character of the electron distribution, and that this scattering imposes an upper bound on the electron T⊥/T‖ commensurate with that predicted by linear theory.
Substorm onsets observed by CRRES: Determination of energetic particle source regions
Authors: Friedel R. H. W. , Korth A. , Kremser G.
Published: February 2004

We examine data from the Electron/Proton Angle Spectrometer instrument, also known as Medium Energy B, flown on the CRRES satellite (July 1990 to August 1991). Because of its strongly eccentric orbit, CRRES can detect substorm onsets in the region L = 4 to L = 8, covering the time sectors 1600–0800 MLT. Substorm onsets which are virtually dispersionless in energy have commonly been associated with acceleration processes which occur in the immediate vicinity of the satellite. Data from CRRES show that such virtually dispersionless onsets can occur far in the inner magnetosphere down to L = 4.3 and are distributed up to ±5 hours around local magnetic midnight. Results from a statistical study are presented. We develop a method of using single‐satellite measurements of near‐dispersionless substorm onsets, together with the observed “drift echoes” of these injections, to establish a “source region” for the energetic particles observed at onset. This method is here applied to electrons only. A database of model drift paths has been set up and is used to calculate the possible interceptions between drift paths of various energies from a source region with the satellite trajectory. Our results indicate that the source region can have a sharp boundary in MLT and can extend over several RE. For some events it is possible to deduce an outer radial limit for the injection region, favoring a more local substorm onset mechanism. Our modeling further shows that the injection and drift echo signature observed at the satellite is quite insensitive to the actual shape of the injection boundary.
Transfer of magnetosheath momentum and energy to the ionosphere along open field lines
Authors: Wright Andrew N.
Published: February 2004

A framework is established for describing the transfer of magnetosheath energy along open field lines to the ionosphere in terms of MHD (magnetohydrodynamic) waves. The evolution of field lines crossing an initial velocity shear is shown to extract momentum and energy from the faster flow (magnetosheath) and deposit it in the slower flow. If the slower plasma is taken to be at rest and have density ρ0 and Alfvén speed VA, while the faster plasma has flow speed V0 and Alfvén speed VA′, then the rate of energy density exchange is ρ0V02VA3/(VA + VA′)2 while that of momentum density is ρ0V0VA2/(VA + VA′). The effect of an ionosphere is included by introducing a height‐integrated Pedersen conductivity (Σp), and we find that field lines slip with a velocity V0/(1 + μ0ΣpVA) just above the ionosphere. By considering the accelerated flows associated with newly opened field lines, it may be possible to understand transient features seen in data such as initial azimuthal surges.
Hydrodynamic instability of the heliopause driven by plasma‐neutral charge‐exchange interactions
Authors: Liewer Paulett C. , Karmesin S. Roy , Brackbill J. U.
Published: February 2004

Results from time‐dependent two‐dimensional hydrodynamic simulations of the global heliosphere suggest that drag between the plasma ions and the interstellar neutrals, caused by charge‐exchange collisions, may cause the heliopause to be hydrodynamically unstable. Both ions and neutrals are treated as fluids coupled by charge‐exchange collisions. The neutral‐ion drag is proportional to the plasma density and introduces an effective gravity in the direction of the neutral flow, which, because the interstellar plasma is much denser than the heliosheath plasma, causes a Rayleigh‐Taylor‐like instability to develop. The heliopause is unstable only near the stagnation point at the “nose” of the heliosphere. In the simulations, the heliopause is seen to oscillate nonlinearly about its equilibrium position with a timescale of the order of a hundred years and amplitudes of tens of AUs. Growth rates from the simulations are in reasonable agreement with theoretical estimates. The possible stabilizing influence of energetic solar wind neutrals, neglected in the present model, is discussed. Implications of this instability on the interpretation of the Voyager 2–3 kHz emissions are also discussed.
Interplanetary shock waves and large‐scale structures: Ulysses' observations in and out of the ecliptic plane
Authors: González‐Esparza J. A. , Balogh A. , Forsyth R. J. , Neugebauer M. , Smith E. J. , Phillips J. L.
Published: February 2004

We present a study of 153 fast shock waves and their relation to other large‐scale features in the solar wind: corotating interaction regions (CIRs), interplanetary counterparts of coronal mass ejections (ICMEs), and the magnetic sector structure, observed by Ulysses from October 1990 to the south solar polar pass in the summer of 1994. This is a comprehensive statistical study of interplanetary shock waves and their possible causes between 1 and 5.4 AU, in particular, out of the ecliptic. We identify six different heliographic intervals with distinct dynamic characteristics and shock wave populations (transient and corotating shocks). We present maps of large‐scale features, which provide a general context to studies of particular events observed by Ulysses and a comparison of Ulysses observations with results from other missions. From our analysis of the associations between interplanetary shocks and their possible causes we find that the strongest in‐ecliptic shock waves were leading CIRs about 4–5 AU. The strongest out‐of‐ecliptic shock waves were attributed to diverse causes at about 20° south. We observed many quasi‐parallel (θBn < 45°) corotating shocks; in fact, most of the corotating reverse shocks detected during the in‐ecliptic trajectory were quasi‐parallel. The correlation between transient forward shocks and ICMEs (ejecta signatures) is similar to previous results within 1 AU: during the in‐ecliptic trajectory Ulysses detected 25 ICMEs and 31 transient forward shocks, 13 of which were associated with ICMEs. The out‐of‐ecliptic results show an analogous correlation. After the Jupiter flyby we observed a significant number of nonrecurrent reverse shocks that do not show any association with ICMEs. This type of shock, instead of being driven by supermagnetosonic plasma clouds, might be produced by a different mechanism.
Details of current disruption and diversion in simulations of magnetotail dynamics
Authors: Birn Joachim , Hesse Michael
Published: February 2004

Properties of current disruption and diversion are investigated on the basis of resistive MHD simulations which include the inner tail region with the transition toward a dipole field and the dynamic formation of a thin current sheet in the inner tail. As in earlier simulations, magnetotail instability leads to reconnection and plasmoid formation and ejection, accompanied by features of the substorm current wedge. In contrast to earlier simulations of the breakup of a thick current sheet, the presence of the thin current sheet leads to a much faster evolution on the timescale of few minutes and to larger electric fields (up to 20 mV/m), concentrated mostly in the dipolarizing region earthward of the reconnection site. The fast growth is relatively insensitive to the resistivity model used. The current “disruption” is largely a consequence of an expansion of the near‐Earth current sheet, associated with the dipolarization. It starts well earthward of the reconnection site in association with a pileup of magnetic flux and propagates tailward. Field‐aligned currents of region 1 type (toward the Earth on the dawnside and away on the duskside) at the near‐Earth boundary are found to increase both during the driven phase of the thin current sheet formation and during the energy release and current disruption phase. They result from a diversion of perpendicular into parallel currents, which is associated with a gradient of the magnetic flux tube volume directed away from midnight at the flank edges of the dipolarization region. Locally, the current diversion takes place earthward of the reconnection site, predominantly in shear layers above and below the neutral sheet rather than close to the neutral sheet. The peaks of the field‐aligned current densities associated with the substorm wedge do not follow field lines very closely, because of the presence of strong perpendicular currents, so that care must be taken in using these features for a mapping between the Earth and the equatorial plane.
Transmitted ions and ion heating in nearly perpendicular low‐Mach number shocks
Authors: Gedalin M.
Published: February 2004

Nonadiabatic ion heating in low‐Mach number shocks is only partially due to reflected ions. Directly transmitted ions contribute significantly into the downstream ion temperature and can be responsible for the whole heating even in the absence of reflected ions, due to insufficient and inhomogeneous deceleration in the cross‐shock potential. As a result, the average ion velocity at the downstream edge of the shock ramp is significantly greater than the velocity required by the Rankine‐Hugoniot relations, and the ion distribution gyrates as a whole. Because of the nonlinear dependence of the deceleration on the cross‐shock potential and initial ion velocity, the gyrating ion distribution is also much more dispersed than the upstream distribution. Additional dispersion is caused by the increase of the vector potential across the shock ramp. The heating depends not only on the bulk shock parameters, as Mach number and β, but also on the field profile. The ion distribution which leaves the ramp is gyrophase‐bunched. Further downstream, strong spatially periodic heating occurs, because the initially gyrophase bunched ions become periodically gyrophase dispersed due to nonlinear dependence of the ion gyrophase on its coordinate and velocity.
Helium on Mars: EUVE and PHOBOS data and implications for Mars' evolution
Authors: Krasnopolsky V. A. , Gladstone G. R.
Published: February 2004

The EUVAC model of the solar EUV flux [Richards et al., 1994] and a recent analysis of terrestrial airglow at the He 584‐Å line by Bush and Chakrabarti [1995] imply an intensity in the center of the solar 584‐Å line which is lower by a factor of 1.8 than that estimated from the model of Tobiska [1991] and used in an earlier analysis of an EUVE observation of the He 584‐Á airglow on Mars [Krasnopolsky et al., 1994]. Two different methods used to select signal from noise in the EUVE observation give Martian effective disk average airglow intensities of 44 and 70 R. To reproduce these intensities with the new solar flux, the helium mixing ratio in the lower atmosphere is required to be 4 ± 2 ppm. The main escape process from the Martian atmosphere for helium is ionization above the ionopause followed by sweeping out by the solar wind. Our new result for the total escape rate is larger by a factor of 2.8 than the previous EUVE‐derived value and is equal to (7.2 ± 3.6)×1023 s−1. This escape rate agrees fairly well with the (1.2 ± 0.6)×1024 s−1 value determined from Phobos 2 measurements by Earabash et al. [1995], taking into account the difference in solar activity. Helium is a minor component of the Martian exosphere reaching a fractional abund3/6/2009 4:08:30 PMance of a few percent at some heights. Our models for the formation of 4He and 40Ar by radioactive decay of U, Th, and K and their outgassing from Mars' interior confirm the low degassing from Mars' interior and require a helium outgassing rate which is smaller by a factor 2–4 than the loss rate. The difference may be due to uncertainties of both values, to a possible enhanced outgassing during the last 5×104 years, to a higher outgassing coefficient of helium than that of argon, and to the contribution of solar wind α particles to the production of helium. Our models show that loss of 40Ar by impact erosion and sputtering was substantial and constituted 75% of the current atmospheric abundance.