bluuebunny/crossref_metadata_2025 huggingface dataset:
Coordinated measurements of F region dynamics related to the thermospheric midnight temperature maximum
Authors: Colerico M. , Mendillo M. , Nottingham D. , Baumgardner J. , Meriwether J. , Mirick J. , Reinisch B. W. , Scali J. L. , Fesen C. G. , Biondi M. A.
Published: February 2004

As part of the NSF/CEDAR program (Coupling Energetics and Dynamics of Atmospheric Regions) in Multi‐Instrumented Studies of Equatorial Thermospheric Aeronomy (MISETA), an all‐sky CCD airglow imaging system has been in operation in Arequipa, Peru, since October 1993. Here we report on the first such use of a wide‐field imager to document the optical signature and variability of a brightness feature associated with the so‐called midnight temperature maximum (MTM). While theo observational driver of this study is a “brightness wave” (BW) seen in 6300 Å and 5577 Å airglow images, detailed case studies are conducted during two campaign periods when Fabry‐Perot interferometer (FPI) and digital ionosonde data were also available. During the passage of a BW, the FPI observed enhancements in thermospheric temperatures, reversals (from equatorward to poleward) of the meridional neutral winds, and local minima in the zonal neutral winds. The ionosonde recorded decreases in the height of the F‐layer during BW events. This lends support to the concept that the poleward winds generated by the MTM pressure bulge cause the lowering of the F‐layer to regions of enhanced loss (h < 300 km) and corresponding airglow production. The two‐dimensional field‐of‐view of the imager allows identification of the geographical orientation of the BW pattern. We use the orientation angle of the BW as an indicator of the geographical orientation of the MTM. Significant day‐to‐day variability in these patterns suggests a complex mix of tidal mode interactions that lead to the overall MTM phenomena.
Solar wind composition changes across the Earth's magnetopause
Authors: Fuselier S. A. , Shelley E. G. , Lennartsson O. W.
Published: February 2004

ISEE 1 and Active Magnetosphcric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) composition measurements are used to investigate the change in solar wind composition across the Earth's magnetopause. The combined data set of 59 magnetopause crossings covers 0500 to 1900 LT and −20° to +40° magnetic latitude. For each magnetopause crossing, the ratio of the He2+/H+ density in the magnetosheath and in the low latitude boundary layer (LLBL) were determined. In addition, some crossings on the duskside flank of the magnetosphere were sufficiently long to determine this density ratio in the current layer. Averaged over all local time, the He2+/H+ density ratio is observed to decrease by 40% from the magnetosheath to the low latitude boundary layer. The density ratio decreases by its greatest amount (47%) in the dawnside LLBL and decreases by its smallest amount (16%±14%) in the duskside LLBL. In addition, there is essentially no reduction in the density ratio (7%±12%) in the duskside current layer. Finally, even those events that show clear signatures of magnetic reconnection at the magnetopause exhibit an average decrease in the density ratio that is comparable to the decrease seen in the entire data set. This decrease in the He2+/H+ density ratio across the magnetopause is not due to the presence of a significant magnetospheric H+ population in the LLBL. One possible explanation for this composition change is a mass dependent reflection coefficient at the magnetopause. To be consistent with the observations, the reflection coefficient for He2+ at the magnetopause must be 40–50% higher than that for H+. If true, then the He2+ concentration must increase by about 20% in the magnetosheath boundary layer (MSBL) just sunward of the magnetopause.
Substorm expansion phase caused by an intense localized convection imposed on the ionosphere
Authors: Kan J. R. , Sun W.
Published: February 2004

It is shown that the substorm expansion phase requires an intense localized convection in the midnight sector. The earlier magnetosphere‐ionosphere coupling (MIC) model showed that the enhanced global convection imposed on the ionosphere can only produce the growth phase. By imposing the intense localized convection on the ionosphere at the end of the growth phase, the new MIC model presented in the present paper can quantitatively produce the signatures of all substorm phases. The pattern of the localized convection is consistent with the wedge‐like field‐aligned current obtained from the solution to the Alfvén wave “field‐current” equation. During the expansion phase in the new MIC model, the ionospheric convection field increases to ∼40 mV/m, the Hall conductance is enhanced to ∼33 mho, the upward field‐aligned current increases to ∼1.8 μA/m2, and the auroral electrojet current increases to ∼0.8 × 106 A. The results of the MIC model are in good quantitative agreement with substorm observations. We suggest that the intense localized nightside convection is driven by localized reconnection of closed field lines in the plasma sheet. A remaining great challenge in substorm research is to uncover the cause of the localized plasma sheet reconnection.
On the acceleration of positive ions by high‐latitude, large‐amplitude electric field fluctuations
Authors: Hultqvist Bengt
Published: February 2004

The effect on cold ions of ionospheric origin of the low‐frequency, large‐amplitude electric field fluctuations (LEF) observed by Viking on auroral latitude magnetic field lines is calculated directly with the use of an observed time series of the electric field. The perpendicular acceleration of O+ ions is found to be more effective than that of protons. Energies in keV and tens of keV ranges are reached within a few tens of seconds by both O+ and H+ ions, but O+ ions obtain higher energies. Outflow velocities along the field line of hundreds of kilometers per second are generally acquired. Similar results can be found with a completely random variation of the electric field, but with less favoring of O+ ions over protons. The amplitude dependence of the perpendicular acceleration is roughly quadratic. The efficiency of LEF in bringing ionospheric ions of all kinds, but particularly heavier ions, into the magnetosphere is very high.
Heliospheric modulation of galactic electrons: Consequences of new calculations for the mean free path of electrons between 1 MeV and ∼10 GeV
Authors: Potgieter M. S.
Published: February 2004

With several new incentives, revisiting electron modulation using a comprehensive numerical model is appropriate. First, one has better confidence in the galactic electron spectrum at energies ≤ 300 MeV, second, several good independent observations of modulated electron spectra at 1 AU at solar minimum and along the Ulysses trajectory have been made, and third, progress has been made about theoretical predictions for mean free paths in the heliosphere. In this work, as a first approach, an assessment of the various new predictions for electron parallel mean free paths at 1 AU, especially their rigidity dependencies, is made by looking at the effects of these predictions on heliospheric modulation of galactic electron at energies below ∼300 MeV. Electron modulation is most suitable at these energies because adiabatic energy changes are still negligible and drift effects become less important. Several spectra are shown together with the radial dependence in the equatorial plane of the electron fluxes associated with each approach that was used. It was found that a damping turbulence model with composite slab, two‐dimensional geometry gave parallel mean free paths that resulted in reasonable electron modulation at these energies. However, several other important modulating parameters have to be considered of which diffusion perpendicular to the averaged magnetic field is arguably the most important.
The tilts of corotating interaction regions at midheliographic latitudes
Authors: Riley Pete , Gosling J. T. , Weiss L. A. , Pizzo V. J.
Published: February 2004

Previous analysis of midlatitude corotating interaction regions (CIRs) observed by Ulysses in the southern heliosphere has revealed that the flow downstream of the forward (F) shock (or wave) on the leading edge of a CIR generally turns northward and into the direction of planetary motion (westward), while the flow downstream of the reverse (R) shock (or wave) on the trailing edge generally turns southward and eastward. These systematic flow deflections are a natural consequence of large‐scale pressure gradients associated with the CIRs and indicate that the F shocks tend to propagate toward and across the equator with increasing heliocentric distance, while the R shocks tend to propagate toward the pole. Numerical simulations indicate that these effects are a natural consequence of the tilt of the solar magnetic‐dipole axis relative to the solar rotation axis. The present work utilizes a variety of techniques to analyze the flow deflections observed within midlatitude CIRs from which we can infer the overall orientations of the CIRs and the speeds and directions of propagation of the waves. Notable results include the following: (1) On the whole, F shocks do propagate equatorward and westward, and R shocks propagate poleward and eastward; (2) shock parameters show a modulation in amplitude, peaking at latitudes roughly equivalent to the inferred dipole‐tilt angle; (3) R shocks tend to propagate faster (in the upstream solar wind frame) than their counterpart F shocks; and (4) meridional deflections tend to be larger than azimuth deflections for both F and R shocks. While it is reassuring that most of the CIRs analyzed fit the paradigm, there are a significant number of anomalies. We discuss several mechanisms which could, in principle, cause these apparent contradictions. For example, the present analysis necessarily emphasizes the local structure at the shock front. The agreement improves significantly, however, when the large‐scale flow deflections throughout the CIR are taken into account.
Spontaneous substorm onset during a prolonged period of steady, southward interplanetary magnetic field
Authors: Angelopoulos V. , Sergeev V. A. , Mozer F. S. , Tsuruda K. , Kokubun S. , Yamamoto T. , Lepping R. , Reeves G. , Friis‐Christensen E.
Published: February 2004

We document a clear case of spontaneous substorm onset under conditions of steady southward interplanetary magnetic field on January 22, 1993. The substorm occurred at 0415 UT with an intensification at 0455 UT during an interval of convection bay activity but had most of the features associated with substorms. These features include high‐latitude and midlatitude magnetic bays at ground magnetometers, and magnetic field compression and energetic particle injection at geosynchronous altitude. In addition, a plasmoid release was observed in the distant tail plasma sheet by the Geotail spacecraft at a GSM position (−96, 1, −6) RE. We conclude that spontaneous‐onset substorms occur occasionally even during steady convection bay events. Such substorms and “classical” substorms progress in a very similar manner, irrespective of possible differences in their onset mechanism.
Observations of ion and electron velocity distributions associated with slow‐mode shocks in Earth's distant magnetotail
Authors: Seon J. , Frank L. A. , Paterson W. R. , Scudder J. D. , Coroniti F. V. , Kokubun S. , Yamamoto T.
Published: February 2004

Three‐dimensional observations of ion and electron velocity distributions with the Geotail spacecraft for three crossings of slow‐mode shocks encountered in Earth's distant magnetotail are presented. Important findings from the observations are (1) the ion velocity distributions in the shock layer and the downstream region of the shocks are highly anisotropic because of the presence of multiple ion components in velocity space and (2) the pressure tensors in the downstream regions as determined from the measured velocity distributions can be diagonalized in magnetic field coordinates. A modification to the procedure for the identification of the slow‐mode shocks is developed with an assumption of anisotropic, gyrotropic plasmas in order to take these observations into account. The predictions for the downstream plasma moments and magnetic fields from the modified procedure are in sufficient agreement with the measured parameters to provide considerably greater confidence in the identifications of the slow‐mode shocks. The possibility that the boundaries are tangential discontinuities is eliminated with observations of upstream plasma mantle ions that penetrate the shock layer and enter the downstream hot plasmas. Also observed in the upstream ion velocity distributions is the presence of accelerated ions escaping from the downstream region which may play an important role in the dynamics of the slow‐mode shock. Observations of the electron velocity distributions allow identification of the separatrix in Earth's magnetotail. The separatrix in the upstream region is identified with the first appearance of hot electrons streaming away from the shock. Flat‐topped electron velocity distributions are found in the downstream region.
Perpendicular ion acceleration in field‐aligned density striations
Authors: Reitzel K. J. , Morales G. J.
Published: February 2004

A basic process similar to transit‐time absorption is evaluated as a possible source of fast perpendicular ions under conditions typically encountered during sounding rocket flights in the auroral ionosphere. Electric fields localized within density striations aligned with the geomagnetic field have a nonuniform perpendicular amplitude that results in irreversible energy absorption. The efficiency of the process depends on the ratio of the ion Larmor radius to the perpendicular scale length of the amplitude gradient and on the scaled frequency ν ≅ ω/Ωi. This absorption mechanism does not depend on a Landau resonance and does not require the electric field to be larger than a threshold value, because it is active in the linear regime. Theory, numerics, and particle‐in‐cell simulations demonstrate the process for electrostatic fields with frequencies a few times the ion gyrofrequency, but the process does not depend explicitly on the fluctuations being lower hybrid waves. Calculations of the evolving ion distribution function due to coherent electric fields suggest that the effect of spatial localization should be considered in the analysis of fast ions of geophysical origin.
Structure instability and nonlinear evolution of magnetic islands at the magnetopause
Authors: Pu Z. Y. , Wang X. M.
Published: February 2004

Structure instability and nonlinear evolution of magnetic islands produced by time‐dependent magnetic reconnection at the magnetopause are studied. The analytic expression for the width of magnetic islands and critical conditions for the occurrence of bifurcation and chaos axe derived. It is shown that the magnetic islands are unstable against the interference from external disturbances. Their structure can be destroyed by medium and small‐scale solar wind turbulences, leading to stochastic magnetic reconnection and the formation of irregular small‐scale structures in magnetospheric boundary regions.
The eigenmode of solitary kinetic Alfvén waves observed by Freja satellite
Authors: Huang Guang‐Li , Wang De‐Yu , Wu De‐Jin , de Féraudy H. , Le Quéau D. , Volwerk M. , Holback Bengt
Published: February 2004

The properties of the solitary kinetic Alfvén waves (SKAWs) observed by Freja satellite are studied statistically for a typical event on March 7, 1994. The eigenmode of the SKAWs observed is still recognized by their anisotropic structure (1000 km × 1 km), their waveform which is similar to that of a wave packet and their frequency close to the local oxygen ion cyclotron frequency. If these spatial structures really exist, they may have resulted from the evolution of the SKAWs or some envelope solitons of ion cyclotron and magnetosonic waves, which is supported by four statistical results: (1) The phase velocity is directly proportional to the Alfvén velocity. (2) The amplitudes of the electric filed, magnetic field and density fluctuations are directly proportional to the timescales of the pulsations. (3) The amplitudes as well as the timescales are directly proportional to the phase velocity. (4) The polarization sense is both of right circular and left circular mode with different phase relation among the electric field, magnetic field, and density fluctuations. Moreover, the SKAWs are often superposed by a strong electrostatic fluctuations at several hundred hertz, the causal relation between the SKAWs and superposed waves is still unknown.
Theory of prominence eruption and propagation: Interplanetary consequences
Authors: Chen James
Published: February 2004

The dynamics of solar magnetic flux ropes are investigated. The model starts with an equilibrium flux rope which contains a low‐density hot component and a denser cold component, qualitatively resembling a prominence cavity with an embedded prominence. The “eruption” of the entire flux rope is triggered by an increase in the poloidal magnetic flux of the structure. The subsequent expansion of the flux rope through a model corona and solar wind is analyzed using macroscopic quantities, including the forces, apex speed and height from the Sun, and magnetic field. The computed flux rope properties are in good agreement with those of observed magnetic clouds. On the basis of these results, magnetic clouds are identified as the interplanetary counterpart of the magnetic field and plasma of the initial cavity. The dense cold component, although drained out near the Sun, significantly influences the resulting magnetic structure in the heliosphere. After a cloud passes 1 AU, it is found that the magnetic field strength of the flux rope at 1 AU continues to decrease and that the magnetic energy in any finite heliocentric sphere vanishes as ∼t−1, even with zero resistivity, where t is time. The cumulative magnetic energy inside the 1 AU sphere associated with repeated eruptions of flux ropes is calculated.
Three‐dimensional stability of thin quasi‐neutral current sheets
Authors: Pritchett P. L. , Coroniti F. V. , Decyk V. K.
Published: February 2004

In a thin current sheet (ρi0/L ≲ 1, where ρi0 is the ion gyroradius in the lobe field and L is the current sheet half thickness) of the generalized Harris type, the relative ion‐electron cross‐field drift is comparable to the ion thermal velocity. The three‐dimensional stability properties of such a thin current sheet are investigated by means of nonlocal two‐fluid theory and two‐dimensional and three‐dimensional full particle simulations. As was suggested originally by Zhu et al. [1992], the drift kink mode is found to be of critical importance. For the simple case of no initial Bz field, the fluid theory demonstrates that the drift kink mode is a non‐MHD mode with a polarization structure such that E1y is an antisymmetric function of z while E1z is a symmetric function with E1z(0) ≠ 0. Two‐dimensional (y,z) particle simulations indicate that the nonlinear behavior of this mode is dominated by long‐wavelength modes with kyL ∼ 1 and frequency ωr ∼ Ωi0, where Ωi0 is the ion gyrofrequency in the lobe field. Three‐dimensional particle simulations performed on a massively parallel computer show that while the growth rates for the drift kink mode are reduced by the finite Bz, they can still be appreciable (γ/Ωi0 ≲ 0.05–0.10). The kyL ∼ 1 drift kink modes are always the first to grow in the simulations; subsequently, tearing‐like modes with a dominant kx wave vector also become unstable. Implications of these results for the triggering of substorms are discussed.
MHD waves generated by the Kelvin‐Helmholtz instability in a nonuniform magnetosphere
Authors: Fujita S. , Glassmeier K.‐H. , Kamide K.
Published: February 2004

A linear analysis of the Kelvin‐Helmholtz instability is performed by assuming that one of the plasma regimes (magnetosphere) separated by a velocity shear layer of zero thickness is spatially nonuniform. The previous theories based on the semi‐infinite uniform model with infinitesimally thin magnetopause showed that wave growth due to the Kelvin‐Helmholtz instability exhibits monotonic increase for shorter frequency. This finding implies that no monochromatic waves whose spatial scales are as large as the magnetospheric size are generated by the Kelvin‐Helmholtz instability. In contrast, the present analysis reveals that the Kelvin‐Helmholtz instability generates global‐scale monochromatic waves when the magnetosphere is nonuniform and the magnetosheath flow speed is quite fast. The wave number is given through the dispersion relation of the Kelvin‐Helmholtz unstable wave.
Collisional degradation of the proton‐H atom fluxes in the atmosphere: A comparison of theoretical techniques
Authors: Decker Dwight T. , Kozelov Boris V. , Basu B. , Jasperse J. R. , Ivanov V. E.
Published: February 2004

Three methods for calculating the transport of energetic protons and hydrogen atoms within the Earth's atmosphere are compared. The methods are (1) a Monte Carlo (MC) simulation, (2) a discrete energy loss solution to the linear transport equations, and (3) a continuous slowing‐down approximation (CSDA). In the calculations performed, all three models use the same cross sections, three‐component (N2, O2, O) neutral atmosphere, and incident isotropic Maxwellian proton fluxes of various characteristic energies (1–20 keV). To ensure that all three methods include the same physical processess, the effects of magnetic mirroring and the lateral spreading of particles are “turned off” in the MC simulations, as these processes are not included in the present linear transport or CSDA models. A variety of quantities are calculated and compared, including energy deposition rates, eV/ion pair, hemispherically averaged differential fluxes of protons and H atoms, energy‐integrated differential fluxes, and total proton and H atom fluxes. The agreement between all three models is excellent, except at the lowest altitudes. Apart from these altitudes, the differences that do exist are small compared to the errors that generally result from poorly known inputs and compared to the typical errors quoted for geophysical observations. The altitudes where the results do differ significantly are where the proton and H atom fluxes are severely attenuated and are below the altitudes where the bulk of the energy deposition and ionization takes place. The success of these comparisons suggests that our ability to model actual observations is presently limited by uncertainties in cross sections and the lack of suitable observations rather than our ability to solve the equations that describe the known physics of proton‐H atom transport.
Modeling the global magnetic field of the large‐scale Birkeland current systems
Authors: Tsyganenko Nikolai A. , Stern David P.
Published: February 2004

Quantitative models are developed for representing the global distribution of the average magnetic field produced by the region 1 and 2 Birkeland current systems. The problem is solved in four following steps: (1) constructing a realistic tilt‐dependent model of the Birkeland current sheets, based on the formalism of Euler potentials, (2) numerically computing their field at a large number of points within the modeling region, (3) finding a best‐fit analytical approximation for that field, and (4) adding a current‐free shielding field which confines the Birkeland field within the model magnetopause. At low altitudes, the model field‐aligned currents reach the ionosphere along eccentric ovals, which fit the observed region 1 and 2 zones of Iijima and Potemra, and they continue there as horizontal currents. At larger distances, the nightside region 1 currents map to the plasma sheet boundary layer and are then diverted toward the tail flanks, while currents in the dawn‐dusk and dayside sectors connect directly to the higher‐latitude magnetopause. The region 2 current closes azimuthally near the equator, forming a spread‐out partial ring current system. The described approach allows a great flexibility in the geometry of the Birkeland currents, making it feasible to infer their properties from spacecraft data.
Quasiperiodic ∼5–60 s fluctuations of VLF signals propagating in the Earth‐ionosphere waveguide: A result of pulsating auroral particle precipitation?
Authors: Carpenter D. L. , Galand M. , Bell T. F. , Sonwalkar V. S. , Inan U. S. , LaBelle J. , Smith A. J. , Clark T. D. G. , Rosenberg T. J.
Published: February 2004

Subionospheric very low frequency and low‐frequency (VLF/LF) transmitter signals received at middle‐latitude ground stations at nighttime were found to exhibit pulsating behavior with periods that were typically in the ∼5–60 s range but sometimes reached ∼100 s. The amplitude versus time shape of the pulsations was often triangular or zigzag‐like, hence the term “zigzag effect.” Variations in the envelope shape were usually in the direction of faster development than recovery. Episodes of zigzag activity at Siple, Antarctica (L ∼4.3), and Saskatoon, Canada (L ∼4.2), were found to occur widely during the predawn hours and were not observed during geomagnetically quiet periods. The fluctuations appeared to be caused by ionospheric perturbations at the ∼ 85 km nighttime VLF reflection height in regions poleward of the plasmapause. We infer that in the case of the Saskatoon and Siple data, the perturbations were centered within ∼500 km of the stations and within ∼ 100–200 km of the affected signal paths. Their horizontal extent is inferred to have been in the range ∼50–200 km. The assembled evidence, supported by Corcuffs [1996] recent research at Kerguelen (L ∼3.7), suggests that the underlying cause of the effect was pulsating auroral precipitation. The means by which that precipitation produces ionospheric perturbations at 85 km is not yet clear. Candidate mechanisms include (1) acoustic waves that propagate downward from precipitation regions above the ∼ 85 km VLF reflection level; (2) quasi‐static perturbation electric fields that give rise to E×B drifts of the bottomside ionosphere; (3) secondary ionization production and subsequent decay at or below 85 km. Those zigzag fluctuations exhibiting notably faster development than recovery probably originated in secondary ionization produced near 85 km by the more energetic (E >40 keV) electrons in the incident electron spectrum.
Latitudinal solar wind velocity variations from polar coronal holes: A self‐consistent MHD model
Authors: Stewart G. A. , Bravo S.
Published: February 2004

Coronal holes are regions of the solar corona magnetically open to the interplanetary medium where the plasma flows outward to form the solar wind. An axisymmetric magnetohydrodynamic model has been developed that describes both the magnetic structure of the solar corona and the outward flow of coronal plasma in a self‐consistent manner, considering a dipolar field at the solar surface. Equilibrium solutions were found for a field at the solar pole of 10 G and a variety of plasma boundary conditions at 1 AU in order to explore the latitudinal dependence of the solar wind velocity from polar coronal holes. As expected, two large polar coronal holes were found, extending down to 32° latitude, separated by a belt of closed coronal helmets. The coronal helmet belt extended to 1.7 Rs, and above this height all flux lines were open and connected to one of the polar coronal holes, from which all solar wind flowed. Three cases of latitudinal velocity variation at 1 AU were considered: (1) a sharp transition between a slow wind at low latitudes and a fast wind at high latitude (in accordance with Ulysses' observations), (2) a linear latitudinal increase of velocity from slow wind at the equator to fast wind over the pole, and (3) a constant velocity irrespective of latitude. In all cases a fast wind flowed from the poles, but there always existed a low‐latitude band of slow solar wind close to the Sun. This slow wind flowed out from the boundary of both coronal holes and formed a band of slow wind on either side of the heliospheric current sheet. The transition between slow and fast wind close to the Sun was sharply defined in all cases.
Magnetic field control of the dayside ion temperatures in the ionosphere of Venus
Authors: Nagy Andrew F. , Sinkovics Annamaria , Cravens Thomas E. , Knudsen William C.
Published: February 2004

Ion temperatures measured by the retarding potential analyzer carried aboard the Pioneer Venus Orbiter were sorted according to the ionospheric magnetic field conditions. It was found that the dayside ion temperatures measured during the first couple of years of operations, corresponding to solar maximum conditions and while the periapsis was held at low altitudes (∼150 km), were basically independent of the magnetic conditions. This contrasts with earlier studies, which showed that electron temperatures are significantly elevated when the ionosphere is magnetized. Theoretical studies by Cravens et al. [1980] predicted that electron temperatures will depend on fluctuating magnetic fields, but not the ion temperatures. This is consistent with these findings.
Field line resonant pulsations associated with a strong dayside ionospheric shear convection flow reversal
Authors: Clauer C. R. , Ridley A. J. , Sitar R. J. , Singer H. J. , Rodger A. S. , Friis‐Christensen E. , Papitashvili V. O.
Published: February 2004

We discuss a set of coordinated observations from the arrays of Greenland ground magnetometers, Sondrestrom incoherent scatter radar, and the DMSP, GOES 7, and IMP 8 satellites during 1000–1400 UT on August 4, 1991. The work presented here follows work presented by Clauer and Ridley [1995]. In the previous work we show that this particular interval is characterized by a large positive interplanetary magnetic field (IMF) By and near‐zero IMF Bz components. Associated with these conditions is a very strong ionospheric convection shear reversal boundary in the dayside noon and prenoon sector in the northern hemisphere. The convection reversal boundary is observed to be very dynamic, showing wave‐like displacements of several degrees in invariant latitude with a tailward phase propagation. These variations are associated with magnetic pulsations with a period of about 34 min. We have suggested that these waves are produced by a Kelvin‐Helmholtz instability at the shear convection reversal boundary. We present here a more detailed analysis of the magnetic variations from the Greenland arrays of magnetometers. Our new findings show that equatorward of the convection reversal boundary, there is power in the pulsation spectra in bands with periods of about 34, 17, 12, and 8 min. Examination of the northward component of the pulsations along a meridional chain of stations ranging from 76.23° to 66.86° invariant latitude shows high coherence between stations and a change in relative phase of about 160° over the observed latitude range for the 8‐min. pulsations. These observations are consistent with the expected signature of a field line resonance. We suggest that the source of the resonance is the disturbance generated at the shear convection reversal boundary which then produces resonances on nearby, equatorward closed field lines. In this case we suggest that the source is the Kelvin‐Helmholtz wave established at the east‐west convection shear which develops near local noon and prenoon regions during large IMF By positive conditions. This is possibly the first observation of the field line resonance associated with such a source.
Negative pickup ions detected during the TSS 1 mission
Authors: Rubin A. G. , Burke W. J. , Hardy D. A. , Huang C. Y. , Gentile L. C. , Hunton D. E.
Published: February 2004

We report the occasional detection of negative pickup ions by the Shuttle Potential and Return Electron Experiment (SPREE) in the shuttle payload bay during the first Tethered Satellite System (TSS 1) mission. These ions appear as arch‐like structures of enhanced fluxes in energy versus time spectrograms for the SPREE electron zone looking in the plane perpendicular to the Earth's magnetic field. Maximum energies of ∼56 eV were measured when the sensor looked closest to the shuttle ram direction. Empirically, the energy of associated spectral peaks decreases as cos2 ξ, where ξ is the angle between the sensor look direction and the projection of the shuttle's velocity onto the plane perpendicular to the magnetic field. These signatures may be found under day or night conditions and always in conjunction with positively charged pickup ions generated as a result of firings by the L5D and/or R5D vernier thrusters. We show that positive ion contamination and accelerated electrons cannot be the responsible agents. Observed spectral characteristics are most easily explained as either NO2− or MMH− ions created near the shuttle and accelerated by the motional electric field to energies detectable by SPREE.
Properties of the magnetospheric hot plasma distribution deduced from whistler mode wave injection at 2400 Hz: Ground‐based detection of azimuthal structure in magnetospheric hot plasmas
Authors: Sonwalkar Vikas S. , Carpenter D. L. , Helliwell R. A. , Walt M. , Inan U. S. , Caudle D. L. , Ikeda M.
Published: February 2004

Siple station VLF wave injection experiments aimed at finding the properties of the magnetospheric hot plasma were conducted for a 9‐hour period between 1705 and 0210 UT on January 23–24, 1988. A special frequency versus time format, lasting l min and transmitted every 5 min, consisted of a sequence of pulses, frequency ramps, and parabolas, all in a 1‐kHz range centered on 2400 Hz. The transmitted signals, after propagating along a geomagnetic field‐aligned duct, were recorded at Lake Mistissini, Canada. At various times during the 9‐hour interval the Siple signals showed features characteristic of wave‐particle interactions, including wave growth, sidebands, and triggered emissions. Our observations, primarily at 2400 Hz, show that (1) there were no correlations between the initial levels, the growth rates, and the saturation levels of constant‐frequency pulses; (2) in general, the values of growth rate and saturation level of two pulses injected within 30 s were nearly the same; (3) the initial level, growth rate, and saturation level showed temporal variations over 5–15 min and 1–2 hour timescales; (4) the leading edges of constant‐frequency signals underwent spatial amplification; and (5) under conditions of saturation the received signal bandwidth (∼ 20 Hz) remained constant over a 1‐hour period, although the saturation level and growth rate varied during the same period. On the assumption that gyroresonant interactions were responsible for the observed wave growth and saturation, the timescales over which those phenomena varied provide constraints on the possible energetic electron population within the duct. In the reference frame of the duct (L ∼ 5.1, Ne ∼ 280 cm−3) the particle fluxes showed no variation over a 30‐s timescale but varied over 5–15 min and 1–2 hour timescales. The 5–15 min timescale variations indicate longitudinal structures ranging from ∼ 0.2° or ∼100 km (in the equatorial plane) for electrons with energy E = 0.6 keV and pitch angle α = 40°, to ∼ 5° or ∼2800 km for electrons with energy E = 11 keV and pitch angle α = 80°. The hour‐long time variations indicate longitudinal structures ranging from ∼ 2° or ∼1100 km (in the equatorial plane) for electrons with energy E = 0.6 keV and pitch angle α = 40°, to ∼ 45° or ∼25,000 km for electrons with energy E = 11 keV and pitch angle α = 80°. We conclude that ground‐based active and passive wave experiments have substantial potential for investigating properties of the hot plasma of the magnetosphere.
Oxygen cyclotron harmonic waves in the deep plasmasphere during magnetic storms
Authors: Pokhotelov O. A. , Pokhotelov D. O. , Feygin F. Z. , Gladychev V. A. , Parrot M. , Hayashi K. , Kangas J. , Mursula K.
Published: February 2004

A new approach to the generation of storm‐associated ULF waves with discrete spectra (ion cyclotron harmonic waves) observed in the equatorial plasmasphere is presented. It is proposed that the appearance of waves with phase velocities smaller than the Alfvén velocity is connected with a strong dispersion of magnetosonic waves near the bi‐ion frequency occurring in the presence of oxygen ions of ionospheric origin. The waves are generated by an instability involving hot oxygen ions with loss cone or ring like distributions. Such ions are found in the magnetosphere during magnetic storms. A simple analytical model of this instability is elaborated. It is shown that ULF wave observations on board Akebono satellite are in a reasonable agreement with the present theoretical approach.
On the thermal electron energy balance in the ionosphere in January 1993 and June 1990
Authors: Richards Phil G. , Khazanov G. V.
Published: February 2004

This paper examines the electron energy balance in the F region of the ionosphere at Millstone Hill in January 1993 by comparing model electron temperatures with measurements from the incoherent scatter radar. Initial modeling indicated three problem areas for the electron temperature at the height of the peak electron density. First, the nighttime temperature was much too low. Second, the afternoon temperature was a little lower than the measured temperature. Third, the model temperature was too high in the morning just after sunrise. A new algorithm is presented for using the measured topside electron temperature to determine the amount of plasmaspheric heating. It was found that the plasmaspheric heating from photoelectrons escaping the ionosphere must be doubled in order for the model to reproduce the observed topside temperature. With this increased plasmaspheric heat flux, the model temperature at the peak height was also much closer to the measured temperature at night. The low temperature in the afternoon could be easily explained by nominal errors in the model inputs or the radar measurements, but the high model temperatures in the morning remain unexplained. Examination of the cooling rates showed that ion cooling is the main cooling rate and is clearly dominant in the afternoon, while O fine structure cooling is equally important in the morning and at night in January 1993. In order to provide a reference, model‐data comparisons were also for June 1990. In this case the agreement between model and data was excellent at all times without the need to adjust local heating and cooling rates. For these summer conditions, vibrationally excited N2 and ion cooling were the two main cooling mechanisms. As was the case in January 1993, doubling of the plasmaspheric heating rate was also needed for the model to match the topside electron temperatures in June 1990. The daytime peak plasmaspheric heat flow deduced from the topside electron temperature showed little variation between June 1990 and January 1993 despite the different seasons and levels of solar activity.
Observations of field‐aligned density microstructure near the Sun
Authors: Grall R. R. , Coles W. A. , Spangler S. R. , Sakurai T. , Harmon J. K.
Published: February 2004

Radio scattering observations made with multiple antennas provide a direct measure of the two‐dimensional microstructure of the solar wind. Previous multiple antenna observations have shown that the microstructure at scales of the order of 10 km becomes highly field‐aligned inside of 6 RS [e.g., Armstrong et al., 199O]. Single antenna observations, which can measure only a radial cut through the microstructure, have shown that scales larger than 1000 km have a Kolmogorov spectrum, whereas the smaller‐scale structure has a flatter spectrum and is considerably enhanced above the Kolmogorov “background” [e.g., Coles et al., 1991]. Here we present new multiple antenna “angular broadening” observations made in 1990 and 1992. These confirm that the microstructure is highly field‐aligned near the Sun, they show that it has elliptical symmetry, and they show that the axial ratio changes quite abruptly near 6 RS. We also present simultaneous measurements at 9 RS of the anisotropy on scales of 1 to 30 km and on scales of 200 to 3000 km. Significant anisotropy was seen on the smaller scales but not on the larger scales. This suggests that the process responsible for the anisotropic microstructure is distinct from the larger‐scale, more isotropic structure.
Plasma waves observed inside plasma bubbles in the equatorial F region
Authors: Koons H. C. , Roeder J. L. , Rodriguez P.
Published: February 2004

Plasma waves have been detected within and around density depletions in the topside equatorial F region by the electric and magnetic field sensors of the Extremely Low Frequency Wave Analyzer (ELFWA) instrument which is part of the Low‐Altitude Satellite Studies of Ionospheric Irregularities experiment on the Combined Release and Radiation Effects Satellite. The plasma waves include both electrostatic waves that have a small magnetic field component and electromagnetic waves propagating in the extraordinary mode. Thus they are not simply zero‐frequency irregularities as generally assumed by previous investigators who were working without the benefit of high‐sensitivity ac magnetic field measurements. The waves exhibit no resonances or cutoffs at characteristic frequencies of the plasma within the range of the ELFWA which is 2–125 Hz. They occur from late evening to early morning primarily at altitudes around 400–500 km. An example has been observed as high as 1500 km. The waves are associated with plasma depletions from a factor of 2 to a factor of 80 less than the surrounding density. The observations indicate that the waves are related to equatorial spread F.
Physics of the explosive growth phase: Ballooning instability revisited
Authors: Liu W. William
Published: February 2004

In situ observations have led to the notion of the explosive growth phase when the crosstail current sheet inside L∼10 suddenly thins in a time of ∼1 min. A theoretical explanation of the current intensification envisages the ballooning instability of magnetospheric plasma. In this paper, this theoretical notion is reanalyzed mathematically. In contrast to some previous work, we find that for a high‐plasma, the ballooning instability can be excited easily by an earthward pressure gradient of any magnitude and for perturbations having parallel wavenumbers comparable to the field line curvature. The instability is compounded by its own development, leading ultimately to the extreme condition of a thin current sheet of a few ion gyroradius thick.
Kinetic electrons in high‐speed solar wind streams: Formation of high‐energy tails
Authors: Lie‐Svendsen Øystein , Hansteen Viggo H. , Leer Egil
Published: February 2004

We study the evolution of the electron velocity distribution function in high‐speed solar wind streams from the collision‐dominated corona and into the collisionless interplanetary space. The model we employ solves the kinetic transport equation with the Fokker‐Planck collision operator to describe Coulomb collisions between electrons. We use a test particle approach, where test electrons are injected into a prescribed solar wind background. The density, temperature, and electric field associated with the background are computed from fluid models. The test electrons are in thermal equilibrium with the background at the base of the corona, and we study the evolution of the velocity distribution of the test electrons as a function of altitude. We find that velocity filtration, due to the energy dependence of the Coulomb cross section, is a small effect and is not capable of producing significant beams in the distribution or a temperature moment that increases with altitude. The distribution function is mainly determined by the electric field and the expanding geometry and consists of a population with an almost isotropic core which is bound in the electrostatic potential and a beam‐like high‐energy tail which escapes. The trapped electrons contribute significantly to the even moments of the distribution function but almost nothing to the odd moments; the drift speed and energy flux moments are carried solely by the tail. In order to describe the high‐speed solar wind observed near 0.3 AU by the Helios spacecraft, we use a multifluid model where ions are heated preferentially. The resulting test electron distribution at 0.3 AU, in this background, is in very good agreement with the velocity distributions observed by the Helios spacecraft.
The effect of a distorted interplanetary magnetic field configuration on the December 7–8, 1982, ground level enhancement
Authors: Cramp J. L. , Duldig M. L. , Humble J. E.
Published: February 2004

The cosmic ray ground level enhancement (GLE) of December 7–8, 1982, occurred when the interplanetary magnetic field (IMF) direction measured near Earth indicated significant distortion from the nominal form. We have modeled the ground level response to this event to derive rigidity spectra and pitch angle distributions centered on apparent arrival directions. A deficit region associated with a near‐Earth magnetic cloud‐like structure has been modeled during the initial phase of the GLE. Using this modified model, we find agreement between the derived particle arrival directions and the measured IMF directions. We find a significant contribution from pitch angles greater than 90° throughout the event. This is probably due to scattering in the turbulent region behind the cloud‐like structure.
VLF remote sensing of high‐energy auroral particle precipitation
Authors: Cummer S. A. , Bell T. F. , Inan U. S. , Chenette D. L.
Published: February 2004

Ground‐based measurements of VLF transmitter signals propagating in the Earth‐ionosphere waveguide can be used to determine the location of nighttime high‐energy (≳100 keV) auroral particle precipitation. When the region of auroral particle precipitation passes over a VLF propagation path, disturbances in the D region of the ionosphere created by the high‐energy particles perturb the amplitude of VLF signals propagating below in a characteristic manner. Continuous nighttime observations of the amplitude of the signal from the NLK transmitter (24.8 kHz, Jim Creek, Washington) were made in Gander, Newfoundland, during November 1993 and January 1994. Simultaneous images of atmospheric X rays created by auroral particle precipitation taken by the AXIS instrument on the UARS satellite were examined for times when large‐scale auroral particle precipitation extended over the NLK‐Gander propagation path. Quantitative characteristics of the precipitation‐associated NLK signal perturbations are established from days which clearly exhibit good correlation between the AXIS images and VLF data, and a larger data set from the months of November 1993 and January 1994 is examined statistically to determine the effectiveness of the VLF technique in capturing particle precipitation events. The number of particle precipitation onsets seen in the AXIS images that can be readily identified in the VLF amplitude data is found to be almost 94%. VLF propagation model calculations show that the observed VLF amplitude decreases are consistent with propagation under conditions of enhanced lower ionosphere electron density caused by auroral electron precipitation and suggest that electrons with energies greater than 100 keV are responsible for the VLF amplitude perturbations.