bluuebunny/crossref_metadata_2025 huggingface dataset:
Interpretation and modeling of the high‐latitude electromagnetic energy flux
Authors: Thayer J. P. , Vickrey J. F. , Heelis R. A. , Gary J. B.
Published: February 2004

An interpretation of the electromagnetic energy flux at high latitudes under steady state conditions is presented and analyzed through modeling of the large‐scale coupling between the high‐latitude ionosphere and magnetosphere. In this paper we elucidate the steady state relationship between the electromagnetic energy flux (divergence of the dc Poynting flux), the Joule heating rate, and the mechanical energy transfer rate in the high‐latitude ionosphere. We also demonstrate the important role of the neutral wind and its conductivity‐weighted distribution with altitude in determining the resultant exchange of electromagnetic energy at high latitudes. Because the Poynting flux approach accounts for the neutral wind implicitly and describes the net electromagnetic energy flux between the magnetosphere and ionosphere, it is a fundamental measure of energy transfer in the system. A significant portion of this energy transfer results in Joule heating; however, the conversion of electromagnetic energy flux into mechanical energy of the neutrals is also considerable and can in some regions exceed the Joule heating rate. We will show that neglect of the neutral dynamics in calculations of the Joule heating rate can be misleading. To evaluate and interpret the electromagnetic energy flux at high latitudes, we employ the vector spherical harmonic model, which is based on the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model, to provide the steady state properties of the thermosphere‐ionosphere system under moderate to quiet geomagnetic activity. For the specific geophysical conditions modeled we conclude that (1) the electromagnetic energy flux is predominantly directed into the high‐latitude ionosphere with greater input in the morning sector than in the evening sector, as supported by DE 2 observations. (2) The Joule heating rate accounts for much of the electromagnetic energy deposited in the ionosphere with the conductivity‐weighted neutral wind contributing significantly to the Joule heating rate and thus affecting the net electromagnetic energy flux in the ionosphere. (3) On average, the mechanical energy transfer rate amounts to about 10% to 30% of the net electromagnetic energy flux in the auroral dawn, dusk, and polar cap regions, acting as a sink of electromagnetic energy flux in the dawn and dusk sectors and a source in the polar cap. (4) Weak regions of upward electromagnetic energy flux are found near the convection reversal boundaries where the mechanical energy transfer rate exceeds the Joule heating rate. In general, large upward electromagnetic energy fluxes may be rare, as the always positive Joule heating rate increases irrespective of the source of electromagnetic energy flux; that is, neutral dynamics contribute directly to the Joule heating rate.
A radiative transfer model using spherical geometry and partial frequency redistribution
Authors: Bush Brett C. , Chakrabarti Supriya
Published: February 2004

The proper analysis of resonance emissions requires the use of a radiative transfer model to accurately simulate the scattering mechanisms. We describe a model that provides a solution to the radiative transfer equations by incorporating spherical geometry in a nonisothermal atmosphere while also accounting for partial frequency redistribution (PFR) during the scattering process. The sensitivity of the model is demonstrated, and comparisons are made to existing radiative transport models. Specific references are made toward applying this model to studying the resonance emissions of hydrogen and helium.
Simulations of seasonal and geomagnetic activity effects at Saint Santin
Authors: Fesen C. G. , Roble R. G. , Duboin M.‐L.
Published: February 2004

Simulations from the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model constitute controlled numerical experiments which may be used to assess current understanding of Earth's upper atmosphere. Comparisons with a long term data base are particularly valuable in this regard. Accordingly model simulations of geomagnetically quiet and active periods are compared with an observational database from Saint Santin. The simulations and observations are for equinox and northern summer and winter during solar cycle minimum. The observations consist of the diurnal variation of the meridional neutral winds near 300 km; harmonic analysis yielded the mean components and the 24‐, 12‐, 8‐, and 6‐hour waves. The model/data comparisons for the diurnal variations are good to excellent: differences are generally ≤ 25 m/s with largest differences typically occurring between 1800 and 0600 UT. In the observations, the diurnal component is approximately 60 m/s in amplitude and 12 hours in phase during quiet periods. These values persist during active periods except in summer when the diurnal amplitude falls to 47 m/s. The model predicts a weaker diurnal amplitude in winter than the observations indicate; it also does not predict the observed decrease of the diurnal amplitude in summer with increasing activity. Harmonic analysis of the data indicated that (1) 12‐r and 8‐hour waves are important in summer and equinox; in winter the variation is largely diurnal; (2) the semidiurnal and terdiurnal waves vary with season during quiet periods; and (3) the largest effect of geomagnetic activity is in the 12‐ and 8‐hour waves. In the model, the semidiurnal and terdiurnal tides do not vary with season. Further, the effects of varying geomagnetic activity are predominantly in the diurnal component; the semidiurnal and terdiurnal tides in the TIGCM are relatively unaffected in marked contrast to the observations. The differences between the modeled and observed winds illustrate the pervasiveness and importance of variability in the atmosphere: in the high‐latitude energy and momentum sources, in the solar forcing, and in the waves that originate in the lower atmosphere and penetrate the thermosphere.
Hybrid simulations of interstellar pickup ion acceleration at the solar wind termination shock
Authors: Liewer P. C. , Rath Sharadini , Goldstein B. E.
Published: February 2004

Hybrid (kinetic ion/fluid electron) simulations have been used to study self‐consistently the injection and initial stages of acceleration of interstellar pickup ions at the solar wind termination shock. Results are presented from one‐dimensional simulations of high Mach number oblique (40°–50°) shocks with a 10% population of interstellar pickup hydrogen. In these simulations, the pickup ions, the solar wind ions, the shock fields and the waves are all treated self‐consistently. Pickup ions reflected by the shock excite large amplitude (ΔB/B ∼ 0.3) upstream magnetosonic waves. These waves, in turn, scatter the pickup hydrogen, as expected in the diffusive shock acceleration process. The spectrum of excited waves broadens in time. We find that, for the parameters studied, the termination shock efficiently injects and accelerates the interstellar pickup hydrogen, and thus this work represents a microscopic verification of this anomalous cosmic ray injection mechanism. Comparison of injected fluxes from the simulations with Voyager fluxes extrapolated to the termination shock suggests that injection at the termination shock is probably greater than injection by interplanetary shocks. A study of accelerated pickup ion orbits shows that the energy gain comes predominantly from shock drift acceleration in the shock front, with the upstream waves aiding the acceleration by allowing multiple encounters with the shock.
Location and characteristics of the reconnection X line deduced from low‐altitude satellite and ground‐based observations: 2. Defense Meteorological Satellite Program and European Incoherent Scatter data
Authors: Lockwood M. , Davis C. J. , Smith M. F. , Onsager T. G. , Denig W. F.
Published: February 2004

We present an analysis of a cusp ion step, observed by the Defense Meteorological Satellite Program (DMSP) F10 spacecraft, between two poleward moving events of enhanced ionospheric electron temperature, observed by the European Incoherent Scatter (EISCAT) radar. From the ions detected by the satellite, the variation of the reconnection rate is computed for assumed distances along the open‐closed field line separatrix from the satellite to the X line, do. Comparison with the onset times of the associated ionospheric events allows this distance to be estimated, but with an uncertainty due to the determination of the low‐energy cutoff of the ion velocity distribution function, ƒ(ν). Nevertheless, the reconnection site is shown to be on the dayside magnetopause, consistent with the reconnection model of the cusp during southward interplanetary magnetic field (IMF). Analysis of the time series of distribution function at constant energies, ƒ(ts), shows that the best estimate of the distance do is 14.5±2 RE. This is consistent with various magnetopause observations of the signatures of reconnection for southward IMF. The ion precipitation is used to reconstruct the field‐parallel part of the Cowley D ion distribution function injected into the open low‐latitude boundary layer in the vicinity of the X line. From this reconstruction, the field‐aligned component of the magnetosheath flow is found to be only −55±65 km s−1 near the X line, which means either that the reconnection X line is near the stagnation region at the nose of the magnetosphere, or that it is closely aligned with the magnetosheath flow streamline which is orthogonal to the magnetosheath field, or both. In addition, the sheath Alfvén speed at the X line is found to be 220±45 km s−1, and the speed with which newly opened field lines are ejected from the X line is 165±30 km s−1. We show that the inferred magnetic field, plasma density, and temperature of the sheath near the X line are consistent with a near‐subsolar reconnection site and confirm that the magnetosheath field makes a large angle (>58°) with the X line.
Location and characteristics of the reconnection X line deduced from low‐altitude satellite and ground‐based observations: 1. Theory
Authors: Lockwood M.
Published: February 2004

A method for estimating both the Alfvén speed and the field‐aligned flow of the magnetosheath at the magnetopause reconnection site is presented. The method employs low‐altitude cusp ion observations and requires the identification of a feature in the cusp ion spectra near the low‐energy cutoff which will often be present for a low‐latitude dayside reconnection site. The appearance of these features in data of limited temporal, energy, and pitch angle resolution is illustrated by using model calculations of cusp ion distribution functions. These are based on the theory of ion acceleration at the dayside magnetopause and allow for the effects on the spectrum of flight times of ions precipitating down newly opened field lines. In addition, the variation of the reconnection rate can be evaluated, and comparison with ground‐based observations of the corresponding sequence of transient events allows the field‐aligned distance from the ionosphere to the reconnection site to be estimated.
Computer simulation of electron and ion densities and temperatures in the equatorial F region and comparison with Hinotori results
Authors: Watanabe S. , Oyama K.‐I. , Abdu M. A.
Published: February 2004

A time‐dependent three‐dimensional computer simulation of equatorial F region ionosphere has been carried out to understand the electron temperature structure observed by Hinotori satellite in the low and middle ionosphere. This model provides three‐dimensional distributions of ion densities, electron temperature, and ion temperatures. The simulations showed the electron temperature enhancements around the equator in the morning, in the midlatitude in the afternoon, and around the equatorial anomaly region from afternoon to midnight. The enhancements in the morning are due to photoelectron heating. The afternoon enhancements in the midlatitude come from the balance of heating and cooling. When no meridional neutral wind is included in the simulation, the electron temperature did not show remarkable enhancements in the midlatitude in the afternoon because of strong cooling by the dense electron density. Around the equatorial anomaly region the electron temperature increased at high altitude in the evening because of the competing effects of plasma cooling and the plasma movements. Since the ionospheric plasma zonal E×B drift is eastward near the sunset (where E is ionospheric electric field and B is magnetic field) and the vertical drift is downward, the high‐altitude dayside hot plasma can enter into the topside F region in the premidnight. The computer simulations were directly compared with the Hinotori satellite data. The simulation results were consistent with the equatorial electron density and temperature observed by the Hinotori satellite.
Demagnetization of electrons in inhomogeneous E ⊥ B: Implications for electron heating in shocks
Authors: Gedalin M. , Gedalin K. , Balikhin M. , Krasnosselskikh V. , Woolliscroft L. J. C.
Published: February 2004

We describe properties of perpendicular electron heating in inhomogeneous E ⊥ B fields. The heating is due to the demagnetization of the electrons when the electric field slope becomes large enough such that e |▿E|/meΩ² > 1, Ω = eB/me. Electrons can be efficiently accelerated across the magnetic field by the electric field. Such a phenomenon may occur in the ramp of a quasi‐perpendicular collisionless shock, resulting in the effective transfer of the cross‐shock potential energy into the downstream electron gyration energy. This energization should be observed as heating. Numerical analysis of electron dynamics in a model shock structure shows that experimentally observed dependencies are reproduced quite naturally. Efficiency of the proposed mechanism depends on the electric field scale in the shock front. Available observational data do not allow unambiguous conclusions about the presence of necessary scales. On the other hand, electron measurements and comparison with the proposed mechanism can help understand the shock fine structure.
Electromagnetic proton cyclotron instability: Interactions with magnetospheric protons
Authors: Gary S. Peter , Thomsen Michelle F. , Yin Lin , Winske Dan
Published: February 2004

The temperature anisotropy of the hot (keV) protons of the outer magnetosphere can excite the electromagnetic proton cyclotron instability. Wave‐particle scattering by this instability not only maintains the hot proton anisotropy at or below an upper bound but also imparts energy to the cool (eV) proton component. This instability and its consequences are examined through the use of linear and second‐order Vlasov theory and one‐dimensional hybrid simulations in a homogeneous plasma model which represents these two protonic constituents. A different form for the upper bound on the hot proton anisotropy is derived from linear theory and the simulations; comparison against plasma observations from Los Alamos magnetospheric plasma analyzers in geosynchronous orbit shows very good agreement. Second‐order theory and simulations are used to obtain a different scaling for the late‐time apparent temperature of the cool proton component Tc which indicates that it is inversely correlated with the cool proton density nc. Observations in both the plasmasphere and the outer magnetosphere sometimes show a similar inverse correlation, suggesting that the electromagnetic proton cyclotron instability is an important contributor to cool proton heating in these regions.
Models of flux ropes embedded in a harris neutral sheet: Force‐free solutions in low and high beta plasmas
Authors: Kivelson Margaret Galland , Khurana Krishan K.
Published: February 2004

We present two models of an array of flux ropes embedded in a Harris neutral sheet. The first solution satisfies the two‐dimensional Vlasov‐Maxwell equilibrium and is electromagnetically force free. Such a solution is pertinent to a plasma in which β, the ratio of the thermal pressure to the magnetic pressure, is small. Several features of the model are in excellent agreement with observations of flux ropes in the magnetotail of the Earth. The maximum magnitude of the core field can be larger than the magnitude of the lobe field, BL; the scale length of the individual flux ropes equals the scale length of the current reversal region in the Harris neutral sheet; and the maximum value of the north‐south component is less thanBL. Magnetotail flux ropes are thought to be isolated structures. Our solution, although periodic in the down‐tail direction, is interesting as it provides a consistent description not only of the flux ropes but also of the fields in which they are embedded, a feature that is lacking in other models. We use data from the December 1990 Galileo flyby of Earth and find that the unit cells of the periodic solution are unsatisfactory for the β≈1 conditions of the tail plasma sheet. We then develop a closely related solution valid in the high β regime that incorporates particle pressure self‐consistently and find that the unit cells of this solution provide an excellent fit to the Galileo flux rope observations.
Hybrid modeling of the tearing instability in collisionless two‐dimensional current sheets: Linear theory
Authors: Kuznetsova Masha M. , Hesse Michael , Winske Dan
Published: February 2004

The linear stability of thin magnetic field reversals is analyzed using a hybrid approach. The ion resonant response to electromagnetic perturbations is calculated from the kinetic Vlasov equation, whereas the electron response is derived from fluid equations using a suitably adapted electron pressure model. Effects of off‐diagonal components of the perturbed pressure tensor on the compressional stabilization are taken into account. The results are presented with emphasis on a comparison with the linear fully kinetic theory and hybrid simulations of the same model (Hesse et al., this issue). We find the stability of perturbations with gradient scales greater than the maximum electron Larmor radius. The transition to instability with the increase of the electron mass is illustrated.
Cusp low‐energy ion cutoffs: A survey and implications for merging
Authors: Newell Patrick T. , Meng Ching‐ I.
Published: February 2004

A fundamental prediction of merging theory is that recently merged field lines should exhibit sharp low‐energy ion cutoffs: If a field line merged at a distance d and a time t removed from the observation point, only ions with velocity above d/t should be observable. Two previous cases at the magnetopause and one in the midaltitude particle cusp have shown such effects. We studied the low‐altitude cusp for 22 quasi‐randomly selected DMSP F7 passes and determined that the low‐energy cutoff indeed clearly existed in most spectra for every pass. Of the individual spectra with cutoffs, 95% had the expected dispersion sense, with cutoff energy less than or equal to the next equatorward cutoff observed. Lockwood and Smith (1992) have pointed out that it is possible to reconstruct the time history of merging by examining d/t. The contribution to the cross‐polar cap potential from cusp merging could be determined by this means for 15 cases with reasonable accuracy; the value per unit length of merging line fell in the range 17–55 V/km, with a mean value of 31.5 V/km. We consider some of the ramifications for monitoring merging variability subject to the constraints of a detector with discrete energy levels. The grainy nature of the observations reduces the frequency of observed jumps in d/t (e.g., intervals without merging) while increasing the apparent size of small jumps. Longer intervals without merging can be either underestimated or overestimated. Despite these limitations, the present survey shows unambiguously that merging rarely ceases for as long as 1 min. Finally, a comparison between what the DMSP detector should observe under steady state merging with cutoffs actually observed in apparently smooth cusp crossings suggests that the merging rate is probably always fluctuating on timescales shorter than a few minutes.
On the relationship between the solar soft X ray flux and thermospheric nitric oxide: An update with an improved photoelectron model
Authors: Siskind David E. , Strickland D. J. , Meier R. R. , Majeed T. , Eparvier F. G.
Published: February 2004

An improved photoelectron model is used to reevaluate the effect of solar soft X rays on lower thermospheric nitric oxide. The improvements include an extension of the upper bound of the energy grid from 450 to 800 eV, extension of the solar spectrum from 19 Å down to 11 Å, the incorporation of Auger ionization for wavelengths shortward of the K shell edge, and the inclusion of a non zero yield of N(²D) from dissociative ionization of N2 by electron impact. The impact of each of these upgrades is studied with a one‐dimensional photochemical model and compared with previously published nitric oxide observations. It is shown that for conditions of high solar activity the photochemical model can now produce over twice as much nitric oxide for a given level of soft X ray input. The net effect of these changes is to reduce significantly the need for extremely large 18–50 Å solar flux values in the model in order to reproduce NO observations which have peak densities of the order of 2×108 cm−3.
Nitric oxide γ band fluorescent scattering and self‐absorption in the mesosphere and lower thermosphere
Authors: Stevens Michael H.
Published: February 2004

The fluorescent scattering of UV sunlight and self‐absorption by the nitric oxide (NO) γ bands between 2000–2500 Å are quantified for the purpose of inferring NO density profiles as a function of altitude in the mesosphere and above. Rotational line emission rate factors and cross sections are calculated at a variety of temperatures. The observed variation of the solar spectrum across the γ bands and its effect on emission rate factors are explored by using irradiance measurements that resolve features down to 0.1 Å. The model also includes quenching by O2 and N2, multiple scattering, temperature effects, attenuation of the solar irradiance by O2 and ozone, and self‐absorption with the summation of adjacent rotational features. Results indicate that for resonant γ bands, the rotational structure in emission is not symmetric to that in absorption so that as self‐absorption increases the shape of the observed emission envelope changes. For γ(1, 0) this is largely characterized by an increase in the integrated emission observed longward of 2151 Å compared to shortward. It is found that solar irradiances measured at 0.1 Å resolution decrease the calculated γ(1, 0) and γ(0, 0) band emission rate factors by less than 3% compared to those measured at 2 Å resolution. However, more Fraunhofer structure included in the calculation is reflected in the relative intensities of the rotational features. It is also found that extinction of the solar irradiance by ozone and quenching by O2 rapidly reduce the γ(1, 0) emission rate factor with decreasing altitude below 60 km.
Statistical fits for auroral oval boundaries during the substorm sequence
Authors: Kauristie K.
Published: February 2004

We present statistical fits for the auroral oval boundaries during the substorm sequence. The fits describe some substorm phase‐dependent changes in the boundary locations, which have not been taken into account in the models presented earlier. We have located both the diffuse and discrete oval boundary crossings using particle precipitation data from Defense Meteorological Satellite Program satellites and categorized the locations according to the substorm phase and the level of magnetic activity determined from the AE index. Our analysis and data set are especially favorable for studying the oval boundary locations during growth and recovery phases. During expansion phase the dynamic poleward boundary of the oval is very difficult to model statistically, and in addition, the critical postmidnight data points (making the auroral bulge more visible) were missing from our data set. In our results the phase‐dependent changes are largest during the recovery phase, and they strengthen with increasing magnetic activity (up to about 5°). In the morning sector during the recovery phase the ovals are clearly wider than in the evening and midnight sectors and during other substorm phases. This effect cannot be seen in the traditional oval models because they have not been designed for use during the recovery phase when magnetic activity is low but the morning sector of the oval is still very wide.
Generation of ion acoustic waves by fan instability
Authors: Vaivads A. , Rönnmark K. , André M.
Published: February 2004

In this paper we consider a fan instability as a possible source of oblique ion acoustic waves generated by ion beams above the auroral acceleration region. Our model includes a homogeneous plasma with protons as the only ion component. The ion distribution has a suprathermal tail along the ambient magnetic field. A beam with a positive derivative and a tail without a positive derivative in the ion distribution are considered. In the first case, the beam of the ion distribution generates electrostatic ion cyclotron waves and oblique ion acoustic waves. In the second case, the ion cyclotron waves are damped, but ion acoustic waves are still generated by the fan instability. The fan instability mechanism generates waves even when the positive derivative in the ion distribution has disappeared. The paper includes an analytical description and numerical calculations of the fan instability. Particle and wave data from the Viking satellite indicate that the fan instability is important above the auroral acceleration region.
Structures in the polar solar wind: Plasma and field observations from Ulysses
Authors: McComas D. J. , Barraclough B. L. , Gosling J. T. , Hammond C. M. , Phillips J. L. , Neugebauer M. , Balogh A. , Forsyth R. J.
Published: February 2004

Ulysses has frequently observed solar wind structures lasting from several hours to several days since it passed into the region of solar wind emanating from the southern polar coronal hole at ∼36° S. In addition to previously reported Alfven waves and coronal mass ejections, two other sorts of structures are routinely evident. This paper provides the first report of these structures in the high‐latitude solar wind: (1) small‐scale compressional structures and (2) pressure balance structures. The compressional structures are driven by faster solar wind overtaking slower solar wind ahead of it and exhibit the plasma and field properties expected for compressions. Unlike large‐scale stream interaction regions observed in and near the ecliptic plane, these structures are much smaller scale and are nonrecurring from one rotation to the next. These smaller‐scale compressional structures appear to arise preferentially from a limited (∼120° of heliolongitude) region in the southern polar coronal hole. The pressure balance structures, on the other hand, are indicated by roughly equal increases in the plasma pressure and decreases in the magnetic field pressure. These structures, which are several degrees across, are more dense and have higher plasma pressures and betas than the surrounding solar wind. These pressure balance structures may be the interplanetary manifestations of “polar plumes.”
Radial profiles of ion density and parallel temperature in the Io plasma torus during the Voyager 1 encounter
Authors: Herbert Floyd , Sandel B. R.
Published: February 2004

We have analyzed a moderately large set of Voyager 1 ultraviolet spectrometer preencounter scans of the Io warm plasma torus in order to deduce its three‐dimensional spatial structure. The density distributions of the five ions O+, O++, S+, S++, and S+++ have been determined in order to deduce their scale heights and thus the ion kinetic temperature parallel to the magnetic field. We find that the parallel temperature of the ions decreases with increasing Jovicentric distance. This variation is opposite to the profile of perpendicular ion temperature derived from in situ plasma measurements but consistent with an adiabatic thermal profile. To reconcile these measurements, we propose that rapid radial transport is adiabatically cooling the torus plasma and mixing it with hot plasma from the middle magnetosphere. We also derive a plasma ionization profile that increases with Jovicentric distance, consistent with in situ measurements of the electron temperature, which also rises with distance. From these observations we conclude that (1) torus plasma transport and the associated adiabatic cooling is faster than ion heating in the outer torus; (2) outer torus electrons are heated by a process that neglibly heats the ions; and (3) this electron heating process is likely to be collisional thermalization with inwardly transported hot plasma from the middle magnetosphere, as postulated by other authors. We also propose a picture of torus dynamics that is consistent both with these conclusions and with the in situ measurements.
Dynamical‐chemical model of fluctuations in the OH airglow driven by migrating tides, stationary tides, and planetary waves
Authors: Walterscheid R. L. , Schubert G.
Published: February 2004

The theory of tidally driven fluctuations in the OH airglow, generalized to account for emission from an extended layer, is applied to airglow fluctuations due to both migrating and zonally symmetric tides and free traveling planetary (Rossby) waves. Krassovsky's ratio ηE (the ratio of normalized airglow fluctuation intensity to normalized temperature fluctuation) is evaluated for a suite of tidal modes and for the traveling planetary waves. Values of ηE are distinct enough to allow for identification of a variety of tidal modes and planetary waves in airglow observations. The theory predicts that airglow observations at very high latitude should contain prominent signatures of zonally symmetric tidal modes with periods of 6, 8, and 12 hours.
Simultaneous excitation of parametric decay cascades and of the oscillating two‐stream instability in one‐dimensional numerical simulations based on Zakharov's equations
Authors: Sprague R. A. , Fejer J. A.
Published: February 2004

The excitation of a parametric decay cascade and of the oscillating two‐stream instability (OTSI) have been predicted earlier by one‐dimensional and two‐dimensional weak turbulence approximations for observations with the 430‐MHz radar for ionospheric modification experiments at Arecibo, for heights 1–2 km below the reflection height of the pump wave. For the same height range the present numerical simulations, using a version of the one‐dimensional driven and damped Zakharov system of equations, are shown to lead over a limited range of pump powers to the simultaneous excitation of a weak turbulence decay cascade and of the OTSI or modulational instability. The linearized form of the Zakharov equations is used to derive the functional dependence of the transfer of power from one Langmuir wave to another on the parameters of the two waves. Comparison of that function with the more correct version derived from kinetic theory was then used to guide the choice of the damping coefficient of ion acoustic waves. With that choice the simulations show that for a pump power that exceeds the threshold for excitation of the parametric decay instability by a factor less than about 3, the OTSI is not excited in the steady state which only shows the excitation of the parametric decay cascade. For pump powers that exceed the threshold power by a factor greater than about 3 but are not great enough to lead to a Langmuir condensate and ultimately to strong cavitational Langmuir turbulence, the simulations show the simultaneous excitation of a weak turbulence cascade and of the OTSI. These conclusions agree qualitatively with the conclusions reached on the basis of earlier weak turbulence approximations.
On the interaction between the shocked solar wind and the planetary ions on the dayside of Venus
Authors: Shapiro V. D. , Szegö K. , Ride S. K. , Nagy A. F. , Shevchenko V. I.
Published: February 2004

Around Venus the planetary ionosphere is directly exposed to the shocked solar wind. The interaction takes place in a broad region surrounding the dayside ionosphere, called the mantle, where the shocked solar wind plasma and the plasma of planetary origin have equally important roles. In this paper both the experimentally determined characteristics and the microphysics of the mantle are discussed in detail. It is shown that as a result of the interaction between the two plasma populations, a modified two‐stream instability develops, and waves are excited with a frequency of a few times the lower hybrid frequency. The polarization of the waves is almost perpendicular to the magnetic field. The stabilization of the higher‐frequency part of the wave spectrum is the result of transverse wave convection in the particular sheet‐like geometry of the mantle. The interaction of these waves with planetary ions and electrons is described within the framework of a nonlinear model in which the saturation of the modified two‐stream instability is due to induced scattering of the waves on cold planetary ions. The effective collision frequency between the shocked solar wind protons and planetary ions is also calculated; it is shown how this leads to ion pick up and heating. Other macroscopically observable effects of these processes are electron acceleration along the magnetic field and ionospheric heating. The experimental data collected in the dayside mantle of Venus by the instruments carried onboard the Pioneer Venus Orbiter are compared to our model. It is believed that the observations support the scenario presented.
Statistical analysis of Pi 2 pulsations observed by the AMPTE CCE Spacecraft in the inner magnetosphere
Authors: Takahashi Kazue , Ohtani Shin‐ichi , Anderson Brian J.
Published: February 2004

The spatial variation of the properties of magnetospheric Pi 2 pulsations is studied using magnetic field records acquired simultaneously by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) satellite at radial distances less than 6.6 Earth radii and at geomagnetic latitudes from −16° to 16° and at the Kakioka ground station located at magnetic shell of L = 1.23. Pi 2 magnetic pulsations are identified from the Kakioka data acquired within 3 hours of midnight, but no restriction is imposed on the local time of CCE. An automated Pi 2 selection procedure resulted in 249 events from the Kakioka data. We have characterized magnetic field variations in the radial Bx, azimuthal By, and compressional Bz components at CCE in terms of their spectral density, coherence, and phase relative to those of the Pi 2 pulsation in the horizontal H component of the Kakioka data and then examined how these parameters depend on the location of CCE. It is found that high‐coherence events (coherence between CCE and Kakioka > 0.6) are observed primarily when CCE is on the nightside and at L < 4. For these events the magnetic field perturbations at CCE are dominated by the poloidal components Bx and Bz, and these components exhibit a ground‐to‐satellite cross phase of either ∼0 or ∼180°, depending on the location of the satellite. The spatial phase structure is consistent with the eigenmode structure of a compressional cavity‐mode‐type resonance excited between two reflecting boundaries. We find no evidence supporting the view that ground Pi 2 are midlatitude toroidal field line resonances excited in response to source waves on auroral zone field lines. Rather, the results imply that midlatitude (2 <L< 5) Pi 2 pulsations observed on the ground originate from a cavity‐mode‐type resonance excited in the inner magnetosphere bounded below by the ionosphere and at high altitudes by an Alfvén velocity gradient. The cavity resonance is probably excited by earthward propagating fast mode waves launched at substorm onset by the large‐scale magnetic reconfiguration associated with cross‐tail current disruption.
Nongyrotropy of heavy newborn ions at comet Grigg‐Skjellerup and corresponding instability
Authors: Cao J. B. , Mazelle C. , Belmont G. , Rème H.
Published: February 2004

The existence of the nongyrotropic velocity distribution of newborn ions is a very important feature for comet Grigg‐Skjellerup (P/G‐S); this kind of distribution may be a reservoir of free energy and play a rather important role in the generation of plasma waves observed at P/G‐S, especially close to the comet. In this paper we predict the distribution function of the newborn ion population, taking into account the source term for this population (implantation from neutrals) and the loss term (pickup, i.e., velocity diffusion leading to assimilation in the solar wind). We integrate the equation backward along the solar wind plasma flow line modified by the ion gyration motion using the observations of the Giotto spacecraft at P/G‐S on July 10, 1992. We obtain the nongyrating zero‐order nongyrotropic velocity distribution of the newborn ions which depends on the average pickup time of implanted ions and on the distance from the comet. The stability of the above equilibrium is then studied by adding linear perturbations and solving numerically the inferred dispersion equation. We show that this kind of distribution function triggers a left‐handed instability which can be called “nongyrotropic ring instability” and which arises from the coupling between the two transverse electromagnetic modes and the longitudinal electrostatic mode.
Cross‐field diffusion in corotating interaction regions
Authors: Intriligator Devrie S. , Siscoe George L.
Published: February 2004

Energetic ions associated with the forward and reverse shocks of corotating interaction regions (CIRs) are commonly observed at places within CIRs that are not magnetically connected to either shock. Examples have been documented with data from Pioneer 10 and 11 and Ulysses. They pose a problem for models that account for these shock‐associated, corotating energetic ion populations (CEIPs) in terms of ion energization at the shocks followed by ion propagation along field lines. According to these models, regions magnetically unconnected to a shock should contain no shock‐associated ions. Cross‐field diffusion has been suggested as a mechanism for populating the shock‐unconnected regions of CIRs. Here we quantitatively examine this suggestion. We use the Green's function solution to the convection‐diffusion equation applied to idealized CIR geometry, with a source at the reverse shock, to compute the ratio of the ion flux in the heart of a CIR to the flux in the shock‐connected part of the CIR and to compare the result with typical ratios observed by the Pioneer spacecraft. The computed ratio for resonant diffusion is many orders of magnitude below the observed ratio. For stochastic field line diffusion, the computed ratio is many orders of magnitude above the observed ratio if a diffusion coefficient appropriate to the free solar wind is used. It agrees with the observed ratio, however, if a reduced diffusion coefficient appropriate to CIRs is used. We conclude that the stochastic field model of cross‐field diffusion can account for the presence of energetic ions in regions of CIRs that are magnetically unconnected to shock waves.
Penetration of the interplanetary magnetic field By into Earth's plasma sheet
Authors: Hau L.‐N. , Erickson G. M.
Published: February 2004

There has been considerable recent interest in the relationship between the cross‐tail magnetic field component By and tail dynamics. The purpose of this paper is to give an overall description of the penetration of the interplanetary magnetic field (IMF) By into the near‐Earth plasma sheet. We show that plasma sheet By may be generated by the differential shear motion of field lines and enhanced by the flux tube compression. The latter mechanism leads to a By analogue of the pressure‐balance inconsistency (Erickson and Wolf, 1980) as flux tubes move from the far tail toward the Earth. The growth of By, however, may be limited by the dawn‐dusk asymmetry in the shear velocity as a result of plasma sheet tilting. By penetration into the plasma sheet implies field‐aligned currents flowing between hemispheres. These currents together with the IMF By related mantle field‐aligned currents effectively shield the lobe from the IMF By.
Substorm changes of the electrodynamic quantities in the polar ionosphere: CDAW 9
Authors: Ahn B.‐H. , Kamide Y. , Kroehl H. W. , Candidi M. , Murphree J. S.
Published: February 2004

On the basis of ground magnetometer data from 75 northern hemisphere stations and the ionospheric conductivity distribution estimated from Viking satellite observations of auroral images, various electrodynamic quantities in the polar ionosphere are calculated for the April 1, 1986, Coordinated Data Analysis Workshop (CDAW) 9 substorm. Since the Scandinavia and Russia chains of magnetometers were located in the premidnight‐midnight sector during this interval and the estimated conductivity distribution is instantaneous, our data set provides us with a unique opportunity to examine some long‐standing problems associated with the substorm expansion onset. Several important findings of this study are summarized as follows: (1) Before the expansion onset of the substorm, intensifications of ionospheric currents or the cross‐polar cap potential are very weak in this particular example. Both quantities begin to increase notably only with the initiation of the substorm expansion onset. (2) The intensified westward electrojet flows along the poleward half of the enhanced ionospheric conductivity belt in the midnight sector during the expansion phase, while its equatorward half is occupied by a weak eastward electrojet. (3) The Joule heating rate and the energy input rate of auroral particles are quite comparable preceding the expansion onset. During the expansion phase of the substorm, however, Joule heating shows a marked intensification, but the latter increases only moderately, indicating that the Joule dissipation is more effective than auroral particle energy input during substorm times. (4) The Hall currents are not completely divergence‐free. The corresponding field‐aligned currents show highly localized structures during the maximum epoch of the substorm, with the upward current being located in the region of the steepest conductivity gradient on the poleward side of the westward electrojet in the midnight sector. This is indirect evidence that the so‐called imperfect Cowling channel is effective behind the westward traveling surge.
Reconnection in a dipole‐dominated magnetosphere: A two‐dimensional model
Authors: Wiechen H. , Büchner J. , Otto A.
Published: February 2004

We study the problem of onset of reconnection and the corresponding nonlinear evolution of reconnection in the near‐Earth plasma sheet, where the influence of the Earth's dipole field is essential. This is done by the help of two‐dimensional resistive MHD simulations. The simulations start from a two‐dimensional equilibrium model for the near‐Earth plasma sheet, including the Earth's dipole field explicitly. The stability properties of this start configuration differ significantly from the stability of two‐dimensional tail equilibria, including both plasma sheet and lobe. This allows us to study onset and evolution of resistive modes that are linearly stable but non‐linearly unstable. We find qualitatively significant differences, as compared with simulations of tail configurations. For instance, we find sunward flow both tailward and earthward of the reconnection X line. The corresponding reconnection rates come out to be about an order of magnitude lower compared with reconnection as a result of a large‐scale tearing mode in tail configurations. We show that the reconnection rates can be significantly increased by suitable modifications of the start configuration.
A three‐dimensional reconnection model of the magnetosphere: Geometry and kinematics
Authors: Wang Xiaogang , Bhattacharjee A.
Published: February 2004

A fully three‐dimensional magnetic reconnection model is proposed for the merging between a spatially nonuniform interplanetary magnetic field and the geomagnetic field. The model generalizes the canonical model of Cowley (1973) and Stern (1973), replacing the two magnetic nulls of the canonical model by a web of A and B nulls and their associated null lines. It is shown that the new A‐B model helps resolve some of the difficulties of the canonical model and allows a qualitatively consistent description of magnetic reconnection that begins on the dayside and subsequently evolves to the nightside. A kinematic study based on the induction equation demonstrates that in an ideal plasma, the velocity of the magnetic field has power‐law singularities at the separatrices which are the sites of reconnection. Qualitative implications of this global model for ionospheric potential patterns and substorm signatures are discussed.
Statistical properties of mirror mode structures observed by Ulysses in the magnetosheath of Jupiter
Authors: Erdős G. , Balogh A.
Published: February 2004

During the outbound pass of the Jovian magnetosheath in February 1992, a large number of magnetic depressions, without a significant change in the direction of the magnetic field vector, were observed by the Ulysses magnetometer. These low‐frequency fluctuations, identified earlier as mirror mode waves, lasted about 1 day. The largest amplitude fluctuations (with a decrease in the field magnitude by a factor 3, on average) were observed just after the spacecraft crossed the magnetopause. The amplitude of the waves decreased as Ulysses approached the bow shock. We report here the results of a systematic study on the statistical properties of these field depressions. We have found that there is a saturation in the level of low field at about 1.3 nT. The average duration of the depressions is about 40 s, corresponding to a spatial size of about 20 gyroradii of the plasma protons. The angle between the magnetic field vector and the minimum variance direction is almost perpendicular, the average of that angle for the 350 individual field depressions we have analyzed is 83°. Constraints on the theoretical models which these observations impose are discussed. Many characteristics of the field depressions suggest the importance of nonlinear effects. The longitudinal size of the weak field seems rather large, raising the question whether the particles have enough time to fill in that space during the development of the instability. This difficulty might be resolved if energetic particles are involved.