bluuebunny/crossref_metadata_2025 huggingface dataset:
A statistical study of accelerated flow events at the dayside magnetopause
Authors: Scurry L. , Russell C. T. , Gosling J. T.
Published: February 2004

Accelerated plasma flows in the magnetopause current layer and low‐latitude boundary layer are examined using the ISEE 2 spacecraft. Accelerated flow of magnetosheath plasma inside the magnetopause is an indication that reconnection is occurring or has recently occurred. Over 180 inbound and outbound magnetopause crossings of the ISEE 2 spacecraft have been examined for such evidence of reconnection. Of those orbits for which a good determination could be made, a total of 58 statistically independent accelerated flow events were found where only one event is chosen per inbound or outbound magnetopause traversal. Upstream solar wind data are available for 42 of these 58 events. These 42 traversals are examined in relation to the upstream solar wind and magnetosheath parameters. The occurrence of accelerated flows is found to be dependent upon the direction of the upstream interplanetary magnetic field and the local magnetic shear across the magnetopause. The occurrence rate of accelerated flows is greatest when the interplanetary magnetic field has a southward component in solar magnetospheric coordinates and the local magnetic shear across the magnetopause is large. As the magnetosheath beta increases, the occurrence of accelerated flows decreases. Events in which the magnetic shear is small occur only when the beta is low. This study also provides a statistical estimate of the length and tilt of the merging line. The location and direction of the accelerated flows indicate that merging occurs most often near the subsolar point along a line of length ∼6 RE that tilts in response to the direction of the interplanetary magnetic field.
Geomagnetic activity and the beta dependence of the dayside reconnection rate
Authors: Scurry L. , Russell C. T. , Gosling J. T.
Published: February 2004

In previous work we have used geomagnetic activity as a proxy measure of the reconnection rate to determine how the upstream solar wind parameters control reconnection. Herein we extend that work to show how the solar wind magnetosonic Mach number, upstream solar wind beta, and the cone angle of the interplanetary magnetic field are correlated with a proxy measure of the efficiency of the reconnection process. Each of these three parameters controls the plasma beta in the magnetosheath downstream of the Earth's bow shock. The dependence of the efficiency of reconnection on the above parameters is in agreement with the expectation that large magnetosheath betas reduce the rate of or hinder the onset of dayside reconnection.
Electron beam excitation of upstream waves in the whistler mode frequency range
Authors: Wong Hung K. , Smith Charles W.
Published: February 2004

We examine whistler mode instabilities arising from electron beams in interplanetary space at 1 AU. Both parallel and obliquely propagating solutions are considered. We demonstrate that the generation of two simultaneous whistler mode waves is possible, and even reasonably likely, for beam parameters frequently encountered upstream of the Earth's bow shock and at interplanetary shocks. We also explore the generation of left‐hand polarized waves at whistler mode frequencies under these same conditions. We offer both parametric variations derived from numerical solutions of the various instabilities as well as an analytical treatment of the problem which succeeds in unifying the various numerical results.
On the nature of compressive fluctuations in the solar wind
Authors: Tu C. ‐Y. , Marsch E.
Published: February 2004

A statistical analysis of the amplitudes of the thermal pressure and total pressure of the solar wind and of several related cross correlations between different compressive parameters has been performed with the plasma and magnetic field data obtained by Helios 1 and 2 in their primary missions. The statistical analysis is based on small‐band averages of the relevant spectra over the frequency range (2‐5) × 10−4 Hz, corresponding to hourly timescales. The analysis shows that the theoretical values, given by the relation between the fluctuation of total pressure and density for perpendicular fast magnetoacoustic waves, present an upper limit for the observed amplitudes of the normalized total pressure fluctuations. With decreasing ratio between the fluctuation amplitudes of the total pressure and the density, we found a systematical decrease of the correlation coefficient between density and total pressure and of the correlation coefficient between density and magnetic field magnitude. Decreases of the correlation coefficients between temperature and density and between thermal and magnetic pressure are also found with decreasing ratio of the normalized amplitudes of the fluctuations of the thermal pressure and the temperature. For high‐speed wind data the pattern of the data distribution in plots of one correlation coefficient versus the other correlation coefficient shows some systematical changes. Most of these results can be explained qualitatively by a model based on a superposition of small‐amplitude perpendicular fast magnetosonic waves and small‐amplitude pressure‐balanced structures. We have found a class of data points which seem to represent fluctuations dominated by fast magnetosonic waves. In many cases in low‐speed wind the correlations between density and total pressure and between temperature and density are both negative, while the correlation between temperature and magnetic magnitude is about zero. The nature of this phenomenon has not yet been clearified. Some possible explanations are suggested. The applicability and relevance of the nearly incompressible magnetohydrodynamics theory for the compressive fluctuations in the inner heliosphere are also discussed.
Low‐latitude boundary layer near noon: An open field line model
Authors: Lyons L. R. , Schulz M. , Pridmore‐Brown D. C. , Roeder J. L.
Published: February 2004

We propose that many features of the cusp and low‐latitude boundary layer (LLBL) observed near noon MLT can be explained by interpreting the LLBL as being on open lines with an inner boundary at the separatrix between open and closed magnetic field lines. This interpretation places the poleward boundary of the LLBL and equatorward boundary of the cusp along the field line that bifurcates at the cusp neutral point. The interpretation accounts for the abrupt boundary of magnetosheath particles at the inner edge of the LLBL, a feature that is inconsistent with LLBL formation by diffusion onto closed field lines, and for the distribution of magnetosheath particles appearing more as one continuous region than as two distinct regions across the noon cusp/LLBL boundary. Furthermore, we can explain the existence of energetic radiation belt electrons and protons with differing pitch angle distributions within the LLBL and their abrupt cutoff at the poleward boundary of the LLBL. By modeling the LLBL and cusp region quantitatively, we can account for a hemispherical difference in the location of the equatorial boundary of the cusp that is observed to be dependent on the dipole tilt angle but not on the interplanetary magnetic field (IMF) x component. We also find important variations and hemispherical differences in that the size of the LLBL that should depend strongly upon the x component of the IMF. This prediction is observationally testable. Finally, we find that when the IMF is strongly northward, the LLBL may include a narrow region adjacent to the magnetopause where field lines are detached (i.e., have both ends connected to the IMF).
Correlation between terrestrial myriametric and kilometric radio bursts observed with Galileo
Authors: Louarn P. , Hilgers A. , Roux A. , de Villedary C. , Gurnett D. A. , Kurth W. S.
Published: February 2004

Data from the Galileo plasma wave system recorded during an investigation of the Earth magnetotail on December 8, 1990, are analyzed in both the myriametric and the kilometric wavelength range. In the far tail, between 80 and 30 Earth radii, two components are observed in the myriametric range (frequencies between 6 and about 30 kHz): (1) the “classical” trapped continuum radiation with a smoothly varying intensity in time and (2), superposed on this component, bursts of waves with typical spectral densities of 10−15 V²m−2Hz−1. These bursts of myriametric waves are shown B to correlate with bursts of intense auroral kilometric radiation (AKR) with typical spectral density of 10−12 V²m−2Hz−1. This bursty myriametric radiation is the analog of the low frequency radiation (LFR) reported by Filbert and Kellog (1989). Its close correlation with the kilometric radiation suggests the existence of a direct relationship between these two radiations. A generation mechanism that explains this correlation and the frequency gap between the AKR and the LFR is proposed.
Auroral radar backscatter at off‐perpendicular aspect angles due to enhanced ionospheric refraction
Authors: Uspensky M. V. , Williams P. J. S. , Romanov V. I. , Pivovarov V. G. , Sofko G. J. , Koehler J. A.
Published: February 2004

This paper studies the effect of ionospheric refraction upon auroral radar backscatter under conditions where the aspect angle appears far from ideal, i.e., when the unrefracted ray path trajectory is at least a few degrees from the perpendicular to the Earth’s magnetic field. It is found that wave trapping by curved electron density layers can cause ionospheric refraction as large as 20o, even at 150 MHz. This suggests that many so‐called off‐orthogonal VHF echoes are in reality due to backscattering at near‐orthogonal aspect angles, the discrepancy arising from increased ionospheric refraction by curved or tilted layers.
Auroral activity associated with Kelvin‐Helmholtz instability at the inner edge of the low‐latitude boundary layer
Authors: Farrugia C. J. , Sandholt P. E. , Burlaga L. F.
Published: February 2004

Auroral activity occurred in the late afternoon sector (∼16 MLT) in the northern hemisphere during the passage at Earth of an interplanetary magnetic cloud on January 14, 1988. The auroral activity consisted of a very dynamic display which was preceded and followed by quiet auroral displays. During the quiet displays, discrete rayed arcs aligned along the geomagnetic L shells were observed. In the active stage, rapidly evolving spiral forms centered on magnetic zenith were evident. The activity persisted for many minutes and was characterized by the absence of directed motion. They were strongly suggestive of intense filaments of upward field‐aligned currents embedded in the large‐scale region 1 current system. Distortions of the flux ropes as they connect from the equatorial magnetosphere to the ionosphere were witnessed. We assess as possible generating mechanisms three nonlocal sources known to be associated with field‐aligned currents. Of these, partial compressions of the magnetosphere due to variations of solar wind dynamic pressure seem an unlikely source. The possibility that the auroral forms are due to reconnection is investigated but is excluded because the active aurora were observed on the closed field line region just equatorward of the convection reversal boundary. To support this conclusion further, we apply recent results on the mapping of ionospheric regions to the equatorial plane based on the Tsyganenko 1989 model (Kaufmann et al., 1993). We find that for comparable magnetic activity the aurora map to the equatorial plane at XGSM = ∼3 RE and ∼2 RE inward of the magnetopause, that is, the inner edge of the boundary layer close to dusk. Since the auroral forms are manifestly associated with magnetic field shear, a vortical motion at the equatorial end of the flux rope is indicated, making the Kelvin‐Helmholtz instability acting at the inner edge of the low‐latitude boundary layer the most probable generating source. Our proposed auroral signature of viscous‐type solar wind‐magnetosphere interaction is easily distinguished from the well‐documented sequence of moving auroral forms in the near‐noon sector, which are observed to be associated with a time‐dependent structure of merging‐cell convection.
Coupling of IMF BY variations into the polar ionospheres through interplanetary field‐aligned currents
Authors: Stauning P.
Published: February 2004

Studies of the correlation between the geomagnetic fields recorded at high‐latitude observatories and the interplanetary magnetic field (IMF) observed from the IMP 8 satellite in the solar wind have indicated that variations in the IMF BY component may generate perturbations of similar appearance in the geomagnetic north (H) components observed in the noon sector of the dayside polar cap. During southward interplanetary field conditions (IMF BZ < 0) these perturbations are observed to progress poleward from the cusp region across a large fraction of the dayside polar cap, changing only slightly in shapes and amplitudes. These progressing geomagnetic variations are interpreted as the footprints of the variable IMF BY component present in the solar wind volume which is magnetically connected to the polar cap ionosphere. It is suggested in this work that both the IMF BY variations and the corresponding variations in the H(ground) components are related to interplanetary field‐aligned currents. The “open” magnetospheric topology, resulting from the merging of the geomagnetic field and the interplanetary field during southward interplanetary field conditions, allows these interplanetary currents to flow along the merged field lines to the northern and southern polar cap ionospheres. Here they generate the convection‐related Hall currents responsible for the observed geomagnetic perturbations. The closure from one polar cap to the other can be partly in the form of ionospheric Pedersen currents and partly in the form of field‐aligned currents along closed field lines in the inner magnetosphere.
Alfvénic disturbances in the equatorial solar wind with a spiral magnetic field
Authors: Lou Yu‐Qing
Published: February 2004

We study a two‐dimensional propagation of Alfvénic perturbations within the solar equatorial plane (SEP) in the solar wind with a spiral magnetic field. Analytical solutions within the entire frequency range are derived at large radial distance r. Numerical solutions which smoothly pass through the Alfvén critical point rA are also obtained. Since the spiral magnetic field caused by the solar rotation scales as ∼r−1 at large r, the velocity and magnetic field perturbations, which are perpendicular to the SEP, remain small relative to the wind parameters in the entire radial range. The frequency criterion for the non‐WKB radial scalings of the perturbation variables to appear is (f + mf⊙)² < fc², where f is the perturbation frequency, f⊙ is the solar rotation frequency, m is the azimuthal wavenumber, and fc is the well‐known characteristic frequency determined by the wind parameters. We demonstrate that the process of continuous reflection in a spiral magnetic field and the effect of superposing Alfvénic perturbations with various f and m can lead to variations of the relative magnetic field fluctuation, the normalized cross helicity σc, the Alfvén ratio ra, the magnetic perturbation energy density Eb, and the kinetic perturbation energy density Eυ, which differ from the predictions of the conventional WKB theory. In particular, the non‐WKB radial scalings of the perturbation variables can appear at any timescales with appropriate values of m. The relevance of these results to data analyses on interplanetary fluctuations is discussed, and we emphasize the importance of obtaining fluctuation spectrum with respect to both f and m in the outer heliosphere.
Landau damping of magnetospherically reflected whistlers
Authors: Thorne Richard M. , Horne Richard B.
Published: February 2004

Unducted VLF signals produced by lightning activity can form a population of magnetospherically reflected (MR) whistlers in the inner magnetosphere. It has been suggested recently that in the absence of significant attenuation such waves could merge into a broadband continuum with sufficient intensity to account for plasmaspheric hiss. To test this conjecture we have evaluated the path‐integrated attenuation of MR whistlers along representative ray paths using the HOTRAY code. Using a realistic plasma distribution modeled on in situ data, we find that the majority of MR waves experience significant damping after a few transits across the equator. This is primarily due to Landau resonance with suprathermal (0.1‐1 keV) electrons. The attenuation is most pronounced for waves that propagate through the outer plasmasphere; this can readily account for the infrequent occurrence of multiple‐hop MR waves for L ≥ 3.5. Selected waves that originate at intermediate latitudes (15° ≤ λ ≤ 35°) and whose ray paths are confined to the inner plasmasphere may experience up to 10 magnetospheric reflections before substantial attenuation occurs. These waves should form the population of observed MR waves. Wave attenuation becomes more pronounced at higher frequencies; this can account for the absence of multiple‐hop waves above 5 kHz. Weakly attenuated MR waves tend to migrate outward to the L shell, where their frequency is comparable to the equatorial lower hybrid frequency. The enhanced concentration of waves due to a merging of ray paths would produce a spectral feature that rises in frequency at lower L. This is quite distinct from the reported properties of plasmaspheric hiss, which maintains a constant frequency band throughout the entire plasmasphere. Furthermore, in the absence of mode conversion, waves below 500 Hz, which often form an important if not dominant part of the spectral properties of hiss, are unable to escape from the topside ionosphere in the whistler mode. Consequently, we conclude that unducted lightning signals cannot account for the origin of plasmaspheric hiss.
Dynamics of the CRRES barium releases in the magnetosphere
Authors: Fuselier S. A. , Mende S. B. , Geller S. P. , Miller M. , Hoffman R. A. , Wygant J. R. , Pongratz M. , Meredith N. P. , Anderson R. R.
Published: February 2004

The Combined Release and Radiation Effects Satellite (CRRES) G‐2, G‐3, and G‐4 ionized and neutral barium cloud positions are triangulated from ground‐based optical data. From the time history of the ionized cloud motion perpendicular to the magnetic field, the late time coupling of the ionized cloud with the collisionless ambient plasma in the magnetosphere is investigated for each of the releases. The coupling of the ionized clouds with the ambient medium is qualitatively consistent with predictions from theory in that the coupling time increases with increasing distance from the Earth. Quantitative comparison with simple theory for the coupling time also yields reasonable agreement. Other effects not predicted by the theory are discussed in the context of the observations.
The June 1991 thermospheric storm observed in the southern hemisphere
Authors: Smith R. W. , Hernandez G. , Price K. , Fraser G. , Clark K. C. , Schulz W. J. , Smith S. , Clark M.
Published: February 2004

Neutral upper thermospheric wind and temperature measurements obtained at South Pole, Antarctica (90°S, 75° invariant latitude (INV)), and Mt John, New Zealand (44°S, 171°E, 52° INV) during the storm period June 11‐13 1991 are presented. Winds peaking at about 800 m/s and typical temperatures reaching up to 2000 K were found in the polar cap. Peak winds above Mount John reached 300 m/s in response to the strong high‐latitude forcing which had spread equatorward to midlatitudes. The temperature predictions of the MSIS 86 model were in broad agreement with the observations but were overestimates by several hundred degrees near 0800 UT on June 12 and underestimates by a similar amount near 0800 UT on June 13. The observed meridional winds at South Pole were less uniform and a few hours different in phase than indicated by the vector spherical harmonic (VSH) model predictions. Although the wind magnitudes were similar to VSH at most times, there was a 12 hour difference in the phase in the zonal component. For the Mount John observations the VSH model predictions exaggerated the equatorward penetration of the polar thermospheric circulation relative to the observations. Predicted zonal winds exceeded observations at almost all times. The observed wind pattern at Mount John differed from normal midlatitude quiescent behavior according to the usual pattern but not in simple proportionality with the variations of geomagnetic activity, expressed as Ap or Kp. It is suggested that the sign of the interplanetary magnetic field Y component is an important factor which determines how momentum is coupled between high and middle latitudes during storms and that it will be needed in the specifications for models such as VSH.
Solar activity variations in midlatitude thermospheric meridional winds
Authors: Hedin A. E. , Buonsanto M. J. , Codrescu M. , Duboin M. ‐L. , Fesen C. G. , Hagan M. E. , Miller K. L. , Sipler D. P.
Published: February 2004

Upper thermospheric meridional wind data at midlatitudes and for low magnetic activity are examined for solar activity variations following an analysis scheme suggested by a Coordinated Analysis of the Thermosphere workshop. Wind data from incoherent scatter, Fabry‐Perot, and F2 peak heights show decreasing diurnal amplitudes with increasing solar activity during all seasons, except for Saint Santin data, which show a slight increase in summer. Equivalent winds from F2 peak height data have strong decreases in diurnal amplitude in all seasons. The coupled thermosphere ionosphere model and thermosphere ionosphere global circulation model predictions of diurnal amplitude, while differing considerably in magnitude, also show decreasing amplitudes during all seasons except summer, while the HWM90 empirical model amplitudes increase slightly with solar activity during all seasons. The diurnal mean wind trends with solar activity are fairly weak, except for Millstone Hill incoherent scatter radar, which shows a shift from strong southward to near zero or northward wind with increasing activity. Model results for the mean generally fall within the band of measurements. Near midnight, most of the data also show that the typically southward winds weaken with increasing solar activity in all seasons except summer, when results are mixed. There are significant differences between the trends and between absolute values for the various data sets and models which need further investigation.
Observations of a solar‐wind‐driven modulation of the dayside ionospheric DPY current system
Authors: Clauer C. R. , Stauning P. , Rosenberg T. J. , Friis‐Christensen E. , Miller P. M. , Sitar R. J.
Published: February 2004

We report here a collection of complementary measurements near local magnetic noon of a unique ionospheric convection variation observed by the Sondrestrom radar which is related to a nearly periodic variation (≈ 25 ‐ 30 min period) in the interplanetary magnetic field By component. We observe also a poleward phase propagation of magnetic pulsations over a region limited in longitude and latitude near the dayside polar cusp. A series of poleward propagating radio absorption enhancements are observed in the Greenland and South Pole imaging riometer data. The pulsations and absorption enhancements are associated with a latitudinally narrow and longitudinally limited intensification of the westward convection and associated eastward Hall current, which propagates poleward over the magnetometers and radar field of view. For the cases presented the interplanetary Bz is strongly negative, while the ionospheric variations are associated with the low‐frequency component of variations in the interplanetary By component. In contrast to the previously discovered traveling convection vortices, these features exhibit a poleward phase motion rather than one along lines of invariant latitude. The propagation velocity is slower (≈ 0.5 ‐ 1.0 km/s) and the structures cover 2 to 3 hours of local time. We interpret the observations as a poleward propagation of the DPY current system intensification associated with enhancements in the IMF By component. Our observations indicate that the DPY field‐aligned current system is propagating poleward and may be moving independent of the convection motion of the plasma and associated field lines.
Interplanetary magnetic field collimated cosmic ray flow across magnetic shock from inside of Forbush decrease, observed as local‐time‐dependent precursory decrease on the ground
Authors: Nagashima K. , Fujimoto K. , Morishita I.
Published: February 2004

In the previous papers (Nagashima et al., 1992, 1993) the authors pointed out the existence of the local‐time‐dependent precursory decrease of cosmic rays in front of the shock wave of the interplanetary magnetic field (IMF) and interpreted it as being due to the IMF‐collimated outward flow of the low‐density cosmic rays across the shock from the inside of the Forbush decrease. In those papers, however, the physical properties of the collimated flow such as the direction, the collimation angle, and the rigidity spectrum of the constituent cosmic rays were only estimated qualitatively owing to the lack of the simulation of the precursory decrease produced by the flow. In the present paper these properties are quantitatively obtained by analyzing the hourly data of the precursory decreases observed on January 25 and 26, 1968, at the worldwide neutron monitor stations. The obtained direction and collimation angle of the flow approximately coincide respectively with those expected from the IMF. This fact gives evidence for the reconfirmation of the existence of the IMF‐guided collimated flow, responsible for the precursory decrease.
Interactions of solar wind streams and related small structures
Authors: Odstrčil D.
Published: February 2004

Coronal holes produce high‐speed, low‐density, and high‐temperature streams that propagate into the interplanetary space. These streams interact with the slow‐speed, high‐density, and low‐temperature stream of the ambient solar wind. We investigate this problem using the time‐dependent, two‐dimensional hydrodynamic model in the spherical‐equatorial coordinate system. More accurate numerical methods and finer difference meshes used enable us to track the evolution of detailed features of the fast and slow stream interaction. An analysis of formation of shock pairs (forward and reverse shocks) is presented for both erupting and corotating parts of fast streams. Further, it is shown that the process of interaction of fast and slow solar wind streams may contain richer structures. Such structures may originate during the reconfinement process (internal shocks), spatial substructures (flux tubes), and small temporal modulations (shock wings). They may influence the global shape of stream interfaces and heating of the plasma. Finally, conclusion can be made that boundaries between the fast and slow coronal streams seem to be stable against small random fluctuations and against small introduced disturbances and the Kelvin‐Helmholtz instability is not initiated.
Evolution of energy‐containing turbulent eddies in the solar wind
Authors: Matthaeus William H. , Oughton Sean , Pontius Duane H. , Zhou Ye
Published: February 2004

Previous theoretical treatments of fluid‐scale turbulence in the solar wind have concentrated on describing the state and dynamical evolution of fluctuations in the inertial range, which are characterized by power law energy spectra. In the present paper a model for the evolution of somewhat larger, more energetic magnetohydrodynamic (MHD) fluctuations is developed by analogy with classical hydrodynamic turbulence in the quasi‐equilibrium range. The model is constructed by assembling and extending existing phenomenologies of homogeneous MHD turbulence, as well as simple two‐length‐scale models for transport of MHD turbulence in a weakly inhomogeneous medium. A set of equations is presented for the evolution of the turbulence, including the transport and nonlinear evolution of magnetic and kinetic energy, cross helicity, and their correlation scales. Two versions of the model are derived, depending on whether the fluctuations are distributed isotropically in three dimensions or restricted to the two‐dimensional plane perpendicular to the mean magnetic field. This model includes a number of potentially important physical effects that have been neglected in previous discussions of transport of solar wind turbulence. Numerical solutions are shown for several cases of interest that demonstrate the advantages of this approach. We suggest that this model may prove useful in studies of solar wind heating and acceleration, as well as in describing the response of interplanetary turbulence to wave energy injected by pickup ions and planetary upstream waves.
Substorm recurrence during steady and variable solar wind driving: Evidence for a normal mode in the unloading dynamics of the magnetosphere
Authors: Klimas A. J. , Baker D. N. , Vassiliadis D. , Roberts D. A.
Published: February 2004

Farrugia et al. (1993) have recently studied substorm activity driven by the passage of an interplanetary magnetic cloud during which the interplanetary magnetic field turned southward for approximately 18 hours. It was shown that both the ϵ and the V Bs parameters varied slowly on the timescale of a substorm but changed considerably over the interval as a whole. The substorm occurrence rate did not reflect the variation in magnetospheric energy loading rate as measured by these parameters but, rather, remained roughly constant with a 50‐min average period. Klimas et al. (1992) showed that the Faraday loop analogue model of geomagnetic activity predicts this single unloading rate under various constant loading rates. However, various model parameters were adjusted to yield a 1‐hour unloading period in agreement with the Bargatze et al. (1985) linear prediction filters and in approximate agreement with the Farrugia et al. (1993) results. It has since been found necessary to add a slow relaxation mechanism to the Faraday loop model to allow for its approach to a ground state during long periods of inactivity. It is proposed that the relaxation mechanism is provided by slow convection of magnetic flux out of the magnetotail to the dayside magnetosphere. In addition, a rudimentary representation of magnetotail‐ionosphere coupling has been added to enable comparison of model output to measured AL. The present study is of the modified Faraday loop model response to solar wind input from the Bargatze et al. data set with comparison of its output to concurrent AL. This study has removed the degree of freedom in parameter choice that had earlier allowed adjustments toward the 1‐hour unloading period and has, instead, yielded the 1‐hour unloading period under various constant loading rates. It is demonstrated that the second peak of the bimodal Bargatze et al. linear prediction filters at ≈ 1‐hour lag and the approximately constant substorm recurrence rate observed by Farrugia et al. can be interpreted as both being due to the existence of a normal unloading recurrence period in the dynamics of the magnetosphere.
Convective instabilities of electromagnetic ion cyclotron waves in the outer magnetosphere
Authors: Horne Richard B. , Thorne Richard M.
Published: February 2004

The path‐integrated linear growth of electromagnetic ion cyclotron waves in the outer (L ≥ 7) magnetosphere is investigated using a realistic thermal plasma distribution with an additional anisotropic energetic ring current H+ to provide free energy for instability. The results provide a realistic simulation of the recent AMPTE observations. For conditions typical of the dayside magnetosphere, high plasma beta effects reduce the group velocity and significantly increase the spatial growth rates for left‐hand polarized instabilities just below the helium gyrofrequency ΩHe+, and on the unguided mode above Ω>He+ but below the cross over frequency ωcr. Relatively high densities, typical of the afternoon local time sector, favor these low group velocity effects for predominantly field‐aligned waves. Lower densities, typical of those found in the early morning local time sector, increase the group velocity but allow strong convective instabilities at high normalized frequencies well above ΩHe+. These waves are reflected in the magnetosphere and can exist for several equatorial transits without significant damping. They are left‐hand polarized only on the first equatorial crossing and become linearly polarized for the remainder of the ray path. Consequently, these waves should be observed with basically linear polarization at all frequencies and all latitudes in the early morning local time sector. Wave growth below ΩHe+ is severely limited owing to the narrow bandwidth for instability and the small resonant path lengths. In the afternoon sector, where plasma densities can exceed 107 m−3, intense convective amplification is possible both above and below ΩHe+. Waves below ΩHe+ are not subject to reflection when the O+ concentration is small and therefore should be observed with left‐hand polarization near the equator and essentially linear polarization at higher latitudes. Since the He+ concentration is usually large in the afternoon sector, guided mode waves above ΩHe+ reflect to form a background distribution with basically linear polarization. We suggest that the strong left‐hand polarized emissions observed by AMPTE in the afternoon sector near the equator are probably due to strongly growing low group velocity waves at frequencies just below ΩHe+, and on the unguided mode above ΩHe+.
Modeling studies of ionospheric variations during an intense solar cycle
Authors: Balan N. , Bailey G. J. , Moffett R. J.
Published: February 2004

Modeling studies are carried out using the Sheffield University plasmasphere‐ionosphere model to investigate the relative importance of neutral winds, neutral densities, and solar EUV fluxes in leading to the saturation of ionospheric ionization observed during the intense solar cycle 21. Values of mean daytime (1100‐1700 LT) ionospheric electron content, peak electron density (Nmax), and peak height (hmax) computed for four midlatitude stations for the month of October 1980‐1985, when the 10.7‐cm solar activity index (F10.7) varied from 66 to 303, increase nonlinearly with F10.7 with saturation for high values of F10.7. Neutral winds, neutral densities, and solar EUV fluxes (obtained from empirical models based on observed data) used in the model computations also undergo similar nonlinear increase with F10.7. The study reveals that (1) the nonlinear increase of neutral winds and neutral densities has no net effect on the saturation of ionospheric ionization, and (2) the saturation of ionization is caused by the saturated production of ionization due to the nonlinear increase of the solar EUV fluxes. The model values of ionospheric height saturate mainly due to the nonlinear increase of the neutral densities determined by solar EUV and UV radiations. The study concludes that the ionosphere (and atmosphere) responds linearly to the solar EUV (and UV) inputs and nonlinearly to F10.7 because the expected linear relationship between the EUV (and UV) fluxes and F10.7 breaks down during solar maximum. Thus it is recommended that the F10.7 proxy, which has conventionally been used as an index of solar activity, must be replaced or be used carefully, particularly during solar maximum.
Relativistic electron flux comparisons at low and high altitudes with fast time resolution and broad spatial coverage
Authors: Imhof W. L. , Gaines E. E. , McGlennon J. P. , Baker D. N. , Reeves G. D. , Belian R. D.
Published: February 2004

Analyses are presented for the first high‐time resolution multisatellite study of the spatial and temporal characteristics of a relativistic electron enhancement event with a rapid onset. Measurements of MeV electrons were made from two low‐altitude polar orbiting satellites and three spacecraft at synchronous altitude. The electron fluxes observed by the low‐altitude satellites include precipitating electrons in both the bounce and drift loss cones as well as electrons that are stably trapped, whereas the observations at geosynchronous altitude are dominated by the trapped population. The fluxes of > 1 MeV electrons at low‐satellite altitude over a wide range of L shells tracked very well the fluxes >0.93 MeV at synchronous altitude.
Magnetic mapping and Birkeland currents in the Toffoletto‐Hill and Tsyganenko Magnetosphere Models
Authors: Ding Cheng , Hill T. W. , Toffoletto F. R.
Published: February 2004

We investigate the mapping of magnetic flux tubes between the high‐latitude ionosphere and the equatorial magnetosphere, using the Toffoletto and Hill (1989) theoretical model and the Tsyganenko (1987, 1989) empirical models. In all models, strong distortion of flux tubes occurs near the magnetopause and in the distant tail. These effects are illustrated for various interplanetary magnetic field directions in the Toffoletto and Hill (1989) model and for various Kp values in the Tsyganenko (1987, 1989) model. We calculate the Birkeland current that is actually present in each model (from Ampere's law), as well as the Birkeland current that would be implied if the model were in magnetostatic equilibrium with isotropic plasma pressure (from the Vasyliunas equation). Comparison of the actual and implied Birkeland currents indicates that no physically significant Birkeland current is included in any of the models tested. Each model contains a “noise” component of Birkeland current that is, however, much smaller in magnitude than the “real” Birkeland currents that remain to be included.
Steady linear X‐point magnetic reconnection
Authors: Priest E. R. , Titov V. S. , Vekestein G. E. , Rickard G. J.
Published: February 2004

In the main part of this paper a model for linear reconnection is developed with a current spike around the X‐point and vortex current sheets along the separatrices, which are resolved by the effects of viscosity and magnetic diffusivity. The model contains three regions. In the external ideal region, diffusion effects are negligible, and the flow is purely radial but becomes singular both along the separatrices and at the X‐point. Near the separatrix there is a self‐similar boundary layer with strong electric current and vorticity, where resistivity and viscosity resolve the singularity and allow the flow to cross the separatrix. A composite solution is set up that matches the external and separatrix solutions. Near the origin diffusion also resolves the singularity and is described approximately by a biharmonic solution. A classification of steady two‐dimensional reconnection regimes is proposed into viscous reconnection (Me > Re), extra slow (linear) reconnection (Me < Rme−1), slow reconnection (Rme−1 < Me Rme−½), and fast reconnection (Rme−½ < Me), where Me is the dimensionless reconnection rate, Rme the magnetic Reynolds number, and Re the Reynolds number, all based on the Alfven speed far from the reconnection point. Also, an antireconnection theorem is proved, which has profound effects on the nature of linear reconnection. It states that steady two‐dimensional MHD reconnection with plasma flow across the separatrices is impossible in a plasma which is inviscid, highly sub‐Alfvenic, and has uniform magnetic diffusivity.
Kinetic and thermodynamic properties of a convecting plasma in a two‐dimensional dipole field
Authors: Huang T. S. , Birmingham T. J.
Published: February 2004

Charged particle guiding center motion is considered in the magnetic field of a two‐dimensional (“line”) dipole on which is superimposed a small, static, perpendicular electric field. The parallel equation of motion is that of a simple harmonic oscillator for cos θ, the cosine of magnetic colatitude θ. Equations for the perpendicular electric and magnetic drifts are derived as well as their bounce‐averaged forms. The latter are solved to yield a bounce‐averaged guiding center trajectory, which is the same as that obtained from conservation of magnetic moment µ, longitudinal invariant J, and total (kinetic plus electrostatic) energy K. The algebraic simplicity of the trajectory equations is also manifest in the forms of the invariants. An interesting result is that guiding centers drift in such a way that they preserve the values of their equatorial pitch angles and (equivalently) mirror latitudes. The most general Maxwellian form of the equilibrium one‐particle distribution function f is constructed from the invariants, and spatially varying density and pressure moments, parallel and perpendicular to the magnetic field, are identified. Much of the paper deals with the more restricted problem in which f is specified as a bi‐Maxwellian over a straight line of finite length in the equatorial plane of the dipole and perpendicular to field lines. This might be thought of as specifying a cross‐tail ion injection source; our formalism then describes the subsequent spatial development. The distribution away from the source is a scaled bi‐Maxwellian but one that is cut off at large and small kinetic energies, which depend on position. Density and pressure components are reduced from the values they would have if the total content of individual flux tubes convected intact. The equatorial and meridional variations of density and pressure components are examined and compared systematically for the isotropic and highly anisotropic situations. There appears to be little qualitative difference due to anisotropy. An anisotropy measure is defined, and its spatial variation determined as a signature of possible MHD instability. Extreme values are found, larger than at the source, but the plasma beta in such regions is probably so low as to render the effect inconsequential energetically. Finally, the possible consequence of “nonadiabatic” pressure profiles on electrostatic interchanges is considered, and a boundary delineating stabilizing and destabilizing regions determined.
Diffusion of radiation belt protons by whistler waves
Authors: Villalón Elena , Burke William J.
Published: February 2004

Whistler waves propagating near the quasi‐electrostatic limit can interact with energetic protons (∼80 ‐ 500 keV) that are transported into the radiation belts. The waves may be launched from either the ground or generated in the magnetosphere as a result of the resonant interactions with trapped electrons. The wave frequencies are significant fractions of the equatorial electron gyrofrequency, and they propagate obliquely to the geomagnetic field. A finite spectrum of waves compensates for the inhomogeneity of the geomagnetic field allowing the protons to stay in gyroresonance with the waves over long distances along magnetic field lines. The Fokker‐Planck equation is integrated along the flux tube considering the contributions of multiple‐resonance crossings. The quasi‐linear diffusion coefficients in energy, cross energy/ pitch angle, and pitch angle are obtained for second‐order resonant interactions. They are shown to be proportional to the electric fields amplitudes. Numerical calculations for the second‐order interactions show that diffusion dominates near the edge of the loss cone. For small pitch angles the largest diffusion coefficient is in energy, although the cross energy/ pitch angle term is also important. This may explain the induced proton precipitation observed in active space experiments.
Axisymmetric modeling of cometary mass loading on an adaptively refined grid: MHD results
Authors: Gombosi Tamas I. , Powell Kenneth G. , De Zeeuw Darren L.
Published: February 2004

The first results of an axisymmetric MHD model of the interaction of an expanding cometary atmosphere with the solar wind are presented. The model assumes that far upstream the plasma flow lines are parallel to the magnetic field vector. The effects of mass loading and ion‐neutral friction are taken into account by the governing equations, which are solved on an adaptively refined unstructured grid using a MUSCL‐type numerical technique. The combination of the adaptive refinement with the MUSCL‐scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the near nucleus of the comet. The main findings are the following: (1) A shock is formed ≈ 0.45 Mkm upstream of the comet (its location is controlled by the sonic and Alfvenic Mach numbers of the ambient solar wind flow and by the cometary mass addition rate). (2) A contact surface is formed ≈ 5,600 km upstream of the nucleus separating an outward expanding cometary ionosphere from the nearly stagnating solar wind flow. The location of the contact surface is controlled by the upstream flow conditions, the mass loading rate and the ion‐neutral drag. The contact surface is also the boundary of the diamagnetic cavity. (3) A closed inner shock terminates the supersonic expansion of the cometary ionosphere. This inner shock is closer to the nucleus on dayside than on the nightside.
A 1957 survey of cosmic ray intensity, 0 to 25 km altitude and 86°N to 73°S geomagnetic latitude
Authors: Van Allen James A.
Published: February 2004

During 1957, as part of the program of the International Geophysical Year, the University of Iowa conducted a survey of cosmic ray intensity with equipment on balloon‐launched rockets (“rockoons”) over the range of geomagnetic latitude λ from 86°N to 73°S. This paper reports the results obtained up to altitudes of about 25 km during the flights of 26 balloons. The epoch of the observations was near the time of maximum solar activity. A comparison is made with similar observations at λ = 52° in 1953 at a time near minimum solar activity. The mean solar modulation factor is 0.69 ± 0.04 at altitudes ranging from 10 to 20 km altitude.
Database of theoretical thermosphere/ionosphere predictions
Authors: Fesen C. G. , Roble R. G.
Published: February 2004

The National Center for Atmospheric Research thermosphere‐ionosphere general circulation model has performed a series of self‐consistent calculations of the thermosphere/ionosphere structure for a variety of geophysical conditions. The model simulates March, June, and December periods for solar cycle minimum and maximum for three levels of geomagnetic activity representing quiet, moderate, and disturbed conditions. The calculations include predictions of N2, O2, O, N(4S), N(²D), NO, N2+, O2+, O+, NO+, and N+, along with the ion, electron, and neutral temperatures and the three components of the neutral winds. The model simulations are available from the authors or through the Coupling, Energetics, and Dynamics of Atmospheric Regions database at NCAR.
Longitudinal ELF to LF electromagnetic oscillations and waves generated in the ionosphere under the influence of strong high‐frequency electric field
Authors: Alpert Yakov L.
Published: February 2004

Results of detailed numerical calculations of some parametric decay effects, arising in a magnetoplasma under the influence of a HF, sufficiently strong electric field
, are given in this paper. The resonance branches and the VLF parametric resonances are calculated in the ionosphere at altitudes Z = 200, 300, and 400 km. Calculations in the resonance regions ωE = sω0 (ω0 is the electron Langmuir frequency, s= 1,2,…) were done in the cold palsma and also in the kinetic approximations. It is shown that the angle dependence ω1(Θ, Ep ≠ 0) of the ELF (0 < ω ≤ ΩB) resonance branch is close to the cosine law. This is in contrast with the earlier published results and with the angle dependence ω1(Θ, E = 0). This important effect and the other dependencies given in the paper may be used for the search of the parametric instabilities and of the electric field in the ionosphere and magnetosphere, especially by experiments in situ on satellites.