bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of a lightning discharge detected by the DE 2 satellite over Hurricane Debbie
Authors: Burke William J. , Aggson Thomas L. , Maynard Nelson C. , Hoegy Walter R. , Hoffman Robert A. , Candy Robert M. , Liebrecht Carmen , Rodgers E.
Published: February 2008

We report the satellite observation of a large, ∼40 mV m−1, transient electric field disturbance over Hurricane Debbie in September 1982. The event lasted less than a second and correlated closely with a burst of highly field‐aligned, upward moving electrons with nearly 1 keV of energy. The electric field event is viewed as a spheric disturbance from a lightning discharge in the active weather system located beneath the satellite. A spheric interpretation of the observed electric field transient is consistent with a subsequent observation of energetic electrons precipitating from the radiation belts. Measured quasi‐dc electric fields and cold plasma density variations are only roughly consistent with model predictions for ULF wave propagation from a storm system to the ionosphere. To understand this first observation of upward moving electrons in the ionosphere associated with a lightning event, we compare the several mechanisms for electron acceleration by electric fields with components (E∥) along the magnetic field. In our scenario, “runaway” electrons were accelerated in ∼1 ms by a downward directed E∥ pulse of ∼1 V m−1 magnitude. Such fields can result from rapidly exposed, negative space charges near the tops of clouds during positive cloud‐to‐ground discharges. High‐frequency Fourier components of the E∥ pulse must propagate through the low‐conducting nighttime atmosphere to the ionosphere with little dissipation.
Simultaneous emissions at nωc and (n+1/2)ωc in space and laboratory plasmas
Authors: Krafft C. , Matthieussent G. , Thevenet P. , Belmont G.
Published: February 2008

Experiments done in space and laboratory on the interaction of an electron beam and a magnetized plasma exhibit the simultaneous excitation of waves near harmonics and half harmonics of the electron gyrofrequency. The dispersion equation for electrostatic modes has been solved for an anisotropic Maxwellian plasma and a suprathermal population (“beam” in the text) presenting anisotropy and regions of ∂ƒ0/∂v⊥>0 in its distribution function. For oblique propagation to the magnetic field, results similar to those observed in experiments are obtained. The hydrodynamic/kinetic nature of the instabilities is discussed.
Excitation of Alfvén waves and local turbulence by energetic ion beams in solar flares
Authors: Meytlis V. P. , Strauss H. R.
Published: February 2008

Solar flares release a large part of their energy in the form of beams of high‐energy particles. Energetic ion beams can excite Alfvén waves when the beam velocity is greater than the Alfvén speed. In turn, the Alfvén waves can be unstable with respect to secondary Kelvin‐Helmholtz instabilities, which produce turbulence and limit the Alfvén wave amplitude. In this way, the ion beam leads to the production of fluid turbulence. The turbulence is strong enough to limit the waves to an energy density which is small compared to the beam energy density. However, the turbulent resistivity produced by the waves and secondary instability is many orders of magnitude larger than molecular resistivity. The turbulence may enhance reconnection, triggering more flares.
Energetic and magnetosheath energy particle signatures of the low‐latitude boundary layer at low altitudes near noon
Authors: Roeder J. L. , Lyons L. R.
Published: February 2008

The low‐latitude boundary layer (LBL) and its separation from the cusp have previously been identified using observations of particle precipitation at magnetosheath energies. Using S3‐3 satellite observations, we have determined that these identifications can also be made from energetic particle observations on polar‐orbiting satellites. It is found that the equatorward boundary of the LBL is identifiable as an approximately discontinuous decrease in 33‐keV electron fluxes from low to high latitudes. Both the energetic ion and electron fluxes decrease discontinuously at the boundary between the LBL and the cusp or polar cap. A distinct LBL is nearly always identifiable in energetic particle measurements in the 10–14 MLT region when counting rates are statistically significant. The identifications obtained using the energetic particle measurements have been compared to those obtained using criteria developed by Newell and Meng (1988, 1989) for magnetosheath energy particle precipitation. In this way, we have evaluated the accuracy of both techniques and used the energetic particle measurements to supplement the identifications obtained using the Newell and Meng criteria. We propose that the Newell and Meng threshold on ion energy flux can be reduced by a factor of 6. This modification provides identification of the LBL for lower ion intensity levels than has previously been thought possible. Source, acceleration, and scattering processes have also been studied within and in the vicinity of the LBL. Observed trapped pitch angle distributions of energetic electrons imply that the LBL is at least partially on closed field lines. Strong scattering of energetic protons is found within and equatorward of the LBL and thus must occur at least partially along closed field lines. Field‐aligned electron acceleration by parallel electric fields can be discerned within and poleward of the LBL, but a more detailed analysis is necessary for a statistical study. Conical ion acceleration was seen relatively frequently within the LBL and about half as often poleward of the LBL. Neither acceleration process could be identified anywhere equatorward of the LBL.
Global F2 layer ionization and geomagnetic activity
Authors: Lal Chaman
Published: February 2008

A global or planetary index value for the ionization density of the F2 layer can be found by suitably averaging the diurnal critical frequency values as observed at different stations along all parallels of latitude and longitude. Monthly planetary values of F2 layer ionization show marked maxima around the equinoxes. It is shown that the seasonal trends of the EUV index F10.7 and the geomagnetic index Ap, when combined in the ratio of 4:1, reproduce faithfully the observed seasonal trend of planetary F2 layer ionization. Detailed analysis shows that the seasonal enhancement of the planetary F2 ionization is not limited to the planetary scale but is also applicable to the averaged diurnal ionization along any pair of conjugate dip latitudes when taken together.
Lower hybrid turbulence and tearing mode instability in magnetospheric plasma
Authors: Das A. C.
Published: February 2008

The growth rate of the tearing mode instability excited by the lower hybrid and/or ion cyclotron turbulence at the Earth's magnetopause has been reinvestigated on the basis of recent observations. The growth time of the tearing mode is found to be of the order of 10 s in the case of lower hybrid turbulence, whereas it becomes 2 s in case of ion cyclotron background turbulence. Thus the background turbulence, as an external driving source, can efficiently produce rapid and variable reconnection rates.
Magnetic ropes in the solar wind
Authors: Ruzmaikin Alexander , Shukurov Anvar , Sokoloff Dmitry
Published: February 2008

The generation of magnetic fluctuations by MHD turbulence in the solar wind is discussed. An estimate of the effective magnetic Reynolds number in the solar wind based on a width of the inertial range of the turbulence is proposed. Dynamo activity is predicted to lead to generation of magnetic ropes whose length is of the order of the energy range scale, 2.5 × 1011 cm, and whose thickness can be as low as 5 × 108 cm. Experimental detection of the ropes would require determination of two‐point and higher correlation functions for the magnetic fluctuations in the solar wind.
Plasma Wave Field Generation by the Tethered Satellite System
Authors: vom Stein R. , Neubauer F. M.
Published: February 2008

A main objective of the Italo‐American tethered satellite project TSS 1 is the investigation of induced plasma waves in the Earth's ionosphere. Therefore many theoretical models have been developed. In this paper we extend the model of Estes (1988) by including evanescent wave modes and multi‐ion effects. We use a plasma composition dominated by oxygen ions with an admixture of 10% protons to characterize the ionospheric F layer. Thus the highest frequencies in our model are far beyond the proton gyrofrequency. Because of this modification, the first band of frequencies where propagating waves are generated (Barnett and Olbert, 1986) splits into two parts, and waves in the proton whistler regime also contribute. In addition to the MHD‐Alfvén wing, a second wing, here called the proton whistler wing, is generated. The evanescent wave modes contribute to the wave field up to 4–5 km in the direction of the background magnetic field. These waves depend strongly on the initial source current distribution of the system and may affect the streaming ionospheric plasma. The impedance of the system in the frequency band considered is changed as a result of the change in the Alfvén speed, but it does not exceed the value of 2 ohms.
Seasonal and latitudinal variations of gravity wave‐driven fluctuations in OH nightglow
Authors: Hickey M. P. , Schubert G. , Walterscheid R. L.
Published: February 2008

The seasonal and latitudinal variations of the gravity wave‐driven fluctuations in the OH nightglow are investigated theoretically using a model that accounts for emission from an extended OH layer and includes the effects of eddy diffusivities in the gravity wave dynamics. The mean (unperturbed) state is obtained from a two‐dimensional, nighttime model so that mean‐state number densities, temperatures and eddy diffusivities are all self‐consistent. Seasonal and latitudinal variations in the background OH nightglow emission and in the propagation and dissipation characteristics of the gravity waves influence how the OH nightglow modulations due to gravity waves depend on season and latitude. At intermediate gravity wave periods (i.e., periods between 0.5 and 3 hours for λx = 100 km; between 3 and 10 hours for λx = 500 km; and greater than about 4 hours for λx = 1000 km) possible trends in the behavior of <η> (equal to
, where I is the airglow intensity, T is intensity‐weighted temperature, an overbar denotes the mean state, a prime denotes a perturbation about that mean state and the brackets indicate vertical integration over all emission levels) with latitude and season could be masked by interference effects. At long periods (i.e., periods greater than 3 hours and 10 hours for λx = 100 and 500 km, respectively) trends are complicated by the competing effects of eddy diffusivities, which directly modify both local values of and the altitudes of maximum wave amplitude, and the vertical distributions of the minor species. Only at periods of a few hours or less (depending on horizontal wavelength) are any seasonal trends seen in <η>, and these are more noticeable in the magnitude of <η> than in its phase. These seasonal variations in <η> are primarily due to seasonal variations in the undisturbed temperature which affect the temperature‐dependent chemical rate constants involved in the OH nightglow. A specific case showing latitudinal trends is presented, but the result is not representative of all of our results obtained involving latitudinal variations.
Viking magnetic and electric field observations of periodic Pc 1 waves: Pearl pulsations
Authors: Erlandson R. E. , Anderson B. J. , Zanetti L. J.
Published: February 2008

Pearl pulsations, with an average repetition period of 60 s, were recorded using the magnetic and electric field experiments on the polar‐orbiting Viking satellite. The wave event occurred on September 30, 1986, during Viking orbit 1212 at 1030 MLT, from L = 3.6 to L = 4.1, and at an altitude of 13,500 km. Electron density observations obtained from Viking show that the waves were generated at the plasmapause and at lower amplitudes in the plasmasphere. The wave Poynting flux, calculated using the magnetic and electric field, indicated that the waves generally were propagating downward toward the ionosphere, although upward Poynting fluxes were observed. Clear evidence of upward propagating waves, associated with downward propagating waves reflected at the ionosphere, was not observed. Linear convective growth rates suggest that the anisotropic ions which provide the free energy have a perpendicular temperature around 15 keV. The repetition period, calculated using the measured electron density and magnetic field strength at Viking, is consistent with the double‐hop transit time for ion cyclotron waves which propagate along field lines from one hemisphere to the other. However, the absence of upward propagating wave packets implies that the upper limit of the wave ionospheric reflection coefficient is on the order of 10 to 20%. Alternative mechanisms for producing the observed repetition are also investigated and include a periodic generation model of pearl pulsations at the ion bounce period.
A new model for plasma transport and chemistry at Saturn
Authors: Richardson John D.
Published: February 2008

A model of plasma transport and chemistry is described which calculates the evolution of a plasma population in latitude and radial distance. The integrated quantity NL² is used in the diffusion equation to calculate plasma transport, then the density of each ion species in radial distance‐latitude space is calculated to determine changes in the plasma due to chemistry. This model can use as input arbitrary neutral density and ion source profiles and correctly handles the effects of anisotropies and the ambipolar electric field. This model is applied to the magnetosphere of Saturn, where it is used to fit the density profile of the heavy ions assuming both satellite and ring sources of plasma. Use of an extended source region is found to significantly alter the resulting plasma profile. Water ions cannot fit the observed density profile inside L=6 even with a large ring source. Oxygen ions can fit the density profile throughout the region inside L=12 given a suitable profile of neutral hydrogen; a suitable profile contains up to 5 H cm−3 outside L=4 with the number increasing inside this. Preferred values of K are 1‐3×10−10 Rs²s−1, but any value K less than 10−9 RS²s−1 can be accommodated. The temperature profile is shown to favor models invoking in situ plasma formation and loss as opposed to models where transport is important.
Hydromagnetic background of the magnetosphere and gyroresonance swinging of a “giant” Pc 2 wave event
Authors: Alpert Ya. , Lanzerotti L. J. , Thomson D. J. , Maclennan C. G. , Wolfe A. , Erlandson R. E.
Published: February 2008

The spectral and temporal characteristics of a “giant” Pc 2 (τ ∼ 7 s) hydromagnetic wave event of duration ∼110 s have been reanalyzed, as have the electromagnetic oscillations generated in the magnetosphere in an interval of about 45 min preceding and following the event. The event, which was recorded near local midnight at two mid‐latitude locations, is, to the best of our knowledge, the largest such wave event reported to date. Previous analyses and interpretation of the event have centered on the bounce resonance generation of this packet of oscillations (Lanzerotti et al., 1986). The reexamination of the event, as well as other oscillations of the magnetoplasma, has caused us to identify six distinguishable spectral frequency bands which appear to encompass the Pc 2 wave packet and its evolution in time as well as to look at the development of the process as a whole. We present the details of these temporal and spectral studies and provide evidence that the Pc 2 event is probably produced by the gyroresonance swinging: amplification of the oscillatory magnetoplasma background in the frequency band ∼0.1‐0.2 Hz. The analysis suggests that the oscillatory background could be produced by impulse‐like disturbances. The possible source(s) of such disturbances and their nature require further study.
Effect of parallel electric fields on the propagation paths of auroral kilometric radiation
Authors: Garg Rajeev , Singh Birbal
Published: February 2008

The effect of parallel electric fields on the angular distribution of auroral kilometric radiation at frequencies between 56.2 and 500 kHz is examined by ray tracing computations in polar wind and auroral plasma cavity models. The results show that the effect is not significant at frequencies between 100 and 500 kHz in both models. The presence of an electric field does affect the angular distribution at 56.2 kHz where the ray paths representing the boundary of the emission cone in the dayside and nightside sectors of the noon‐midnight meridian are tilted equatorward and poleward, respectively, by about 20° (i.e., the cone axis is redirected toward the evening sector by about 20°). An attempt to match the results with the observations shows that the initial wave normal angles for these ray paths are 262° and 85.5°, respectively. The ray paths computed in the plasma cavity model also yield the same result except for some minor adjustment with the initial wave normal angles.
Self‐absorption theory applied to rocket measurements of the nitric oxide (1, 0) γ band in the daytime thermosphere
Authors: Eparvier F. G. , Barth C. A.
Published: February 2008

Sounding rocket observations of the ultraviolet fluorescent emissions of the nitric oxide molecule in the lower thermospheric dayglow are described and analyzed. The rocket experiment was an ultraviolet spectrometer which took limb‐viewing spectra of the dayglow between 90‐ and 185‐km altitude in the spectral region from 2120 to 2505 Å with a resolution of 2.0 Å. The flight occurred at local noon on March 7, 1989, from Poker Flat, Alaska. Several NO γ bands were visible at all altitudes of the flight, along with emission features of N2, O+, and N+. The data for the NO (1, 0) and (0, 1) γ bands were modeled with optically thin synthetic spectra and used as diagnostics of nitric oxide concentrations. The resonant NO (1, 0) γ band emissions were shown to be attenuated at low altitudes relative to the expected emission rates predicted from comparison with the nonresonant (0, 1) γ band. Inversion of the optically thin data resulted in a peak nitric oxide concentration of 3.1 × 108 cm−3 at an altitude of 100 km. A self‐absorption model using Holstein transmission functions was developed and applied to the (1, 0) γ band observations. The model results agree with the measured attenuation of the band, indicating the necessity of including self‐absorption theory in the analysis of satellite and rocket limb data of NO. The success of the model also confirms the value adopted for the absorption oscillator strength of the (1, 0) γ band transition and the instrument calibration.
Particle motion in x‐dependent Harris‐like magnetotail models
Authors: Burkhart Grant R. , Chen James
Published: February 2008

The dynamics of charged particles in x‐dependent magnetotail models is examined, where x is along the Sun‐Earth direction. An earlier paper showed that particle motion in a class of x‐dependent Harris‐like equilibrium models can be significantly different from the motion in x‐independent magnetotail field models. In the present paper, it is shown that these Harris‐like equilibrium models have “bulb‐shaped” field lines and that this property leads to the reported differences. Furthermore, it is shown that the scale length of variation of the Harris‐like models in the x direction, Lx, is comparable to the typical excursion distance, Δ, of particles in the x direction and to ρn which is the gyroradius based on the magnetic field component Bn normal to the plane of the current sheet. It is argued that neither bulb‐shaped field lines nor scale lengths Lx comparable to Δ or ρn apply to the Earth's magnetotail. It is suggested that a key criterion for applicability of x‐dependent models to the terrestrial magnetotail is Δ, ρn ≪ Lx.
Interaction of fast steady flow with slow transient flow: A new cause of shock pair and interplanetary Bz event
Authors: Zhao Xuepu
Published: February 2008

The occurrence of the nonspiral magnetic field, high helium density, “cold magnetic enhancement” and counterstreaming suprathermal electron flux in the slow flow around a forward shock indicates that the slow flow is the interplanetary counterpart of the coronal mass ejection (ICME). The characteristics of the field and plasma in the fast flow around the reverse shock are typical for a fast steady flow. Thus the shock pair here appears to be caused by interaction of a fast steady flow with a slow ICME. The fact that the slow ICME possesses a planar magnetic structure and a large −Bz component suggests that the slow ICME may be disconnected from the Sun. It is shown that compression alone appears to be adequate to explain the large southward interplanetary magnetic field component within the shocked slow plasma because of the large southward field component present in the ICME ahead of the forward shock. In addition, a new method to infer the shock angle and Mach number from the observed upstream plasma β and the jump ratios of proton density and total magnetic flux density across a shock is suggested.
Lower hybrid turbulence at cometary bow wave and acceleration of cometary protons
Authors: Shapiro V. D. , Shevchenko V. I. , Sharma A. S. , Papadopoulos K. , Sadgeev R. Z. , Lebedev V. B.
Published: February 2008

The wave measurements at the spacecraft encounters with comet Halley have shown intense wave activity at the lower hybrid frequency. The excitation of the lower hybrid instability by the pickup cometary ions (protons and water group) in the bow wave region and the quasi‐linear diffusion of the ions in these fluctuations are discussed. The quasi‐linear diffusion of the pickup protons takes place over a scale length shorter than that of the heavier water group ions. This enhances damping of the waves by protons, and when the pickup proton density is large enough, it can result in the saturation of the instability as this damping balances the heavy ion driven growth. The observed electric field amplitude and the scale length of proton relaxation are in agreement with the theory. For small pickup proton density the instability can saturate due to the wave energy cascade arising from the modulation instability of the large‐amplitude lower hybrid waves. This saturation mecahnism leads to electron acceleration and suprathermal tail formation.
Electrostatic ion cyclotron velocity shear instability
Authors: Lemons D. S. , Winske D. , Gary S. P.
Published: February 2008

An electrostatic ion cyclotron instability driven by sheared velocity flow perpendicular to a uniform magnetic field is investigated in the local approximation. The dispersion equation, which includes all kinetic effects and involves only one important parameter, is cast in the form of Gordeyev integrals and solved numerically. The instability occurs roughly at multiples of the ion cyclotron frequency (but modified by the shear) with the growth rate of the individual harmonics overlapping in wavenumber. At small values of the shear parameter, the instability exists in two branches, one at long wavelength, kρi ∼ 0.5, and one at short wavelength, kρi > 1.5 (kρi is the wavenumber normalized to the ion gyroradius). At larger values of the shear parameter only the longer wavelength branch persists. The growth rate of the long wavelength mode, maximized over wavenumber and frequency, increases monotonically with the shear parameter. Properties of the instability are compared to those of Ganguli et al. (1985b, 1988b) obtained in the nonlocal limit.
Upstream waves, shocklets, short large‐amplitude magnetic structures and the cyclic behavior of oblique quasi‐parallel collisionless shocks
Authors: Scholer Manfred
Published: February 2008

The re‐formation process of more oblique quasi‐parallel shocks is investigated using one‐dimensional hybrid simulations. Several types of simulations have been performed. The simulation of a shock with a magnetic field‐shock normal angle of 30° shows that a more oblique quasi‐parallel shock exhibits reformation cycles with a larger length scale, that is of about 20 ion inertial lengths. This is considerably larger than the distance specularly reflected ions are able to propagate upstream before they are deflected so that their velocity in the shock normal direction is close to zero. These cycles are due to steepening and growth of upstream waves into pulsationlike structures when they are convected into the region of strongly increasing diffuse ion density immediately upstream of the shock. When the steepening wave packet crashes into the shock, the shock ramp dispersively radiates whistler waves into the region between the shock ramp and the approaching wave, while the steepening of the pulsation leads to phase standing whistler waves on the upstream side. Entropy production occurs either at the shock ramp or at the upstream edge of the pulsation when the steepening process has produced a large kink in the magnetic field and is due to nonadiabatic motion of the incident solar wind ions. In order to analyze the wave steepening, upstream waves have been isolated, and their subsequent interaction with a hot, tenuous ion beam representing the diffuse backstreaming ions has been studied. When an upstream wave is convected into or a region with increasing hot beam density, the wave steepens and becomes a pulsationlike wave packet. In order for the wave to grow to a pulsationlike structure the characteristic scale length of the density increase has to be of the same order as the wavelength of the original magnetosonic wave. Similar results are obtained when counterstreaming beam of hot ions is injected into a solar wind which does not initially contain a wave field. In this case the polarization of the pulsations depends on the hot beam temperature. The strong density increase of hot beam ions in these simulations is due to the steady injection of beam ions in a solar wind with an embedded field which is inclined relative to the solar wind direction. In the shock simulation the shock itself is the steady source of the hot backstreaming ions. These simulations suggest that upstream waves, shocklets, and short large‐amplitude magnetic structures are all the same entity in different stages of their development and play a crucial role in re‐forming oblique quasi‐parallel shocks.
Electron density profile in the magnetosphere deduced from in situ electron density and wave normal directions of omega signals observed by the Akebono (EXOS D) satellite
Authors: Sawada Akira , Nobata Tadashi , Kishi Youji , Kimura Iwane , Oya Hiroshi
Published: February 2008

We describe a new method to construct the global plasma distribution in the plasmasphere using the electron density measured along the trajectory of the Akebono (EXOS D) satellite and the wave normal direction of Omega signals simultaneously observed by the same spacecraft. In this method the plasma density distribution is described by using the diffusive equilibrium model, with eight parameters, that is, die relative abundances and temperatures of ions, and the electron density at a reference altitude. This model is first fit in a least squares sense to the electron density observed along the spacecraft trajectory. The fit solution is not always unique. In such cases it is necessary to constrain the possible solutions further. Therefore we ray trace Omega signals from their source location and check if the wave normal directions at the spacecraft location are consistent with those observed. We present here three different examples of fitting methods in which fixed, freely fitted, and constrained fitted parameters yield reasonable plasma distributions.
Geographic asymmetries of the Viking auroral distribution: Implications for ionospheric coordinate systems
Authors: Hearn D. J. , Elphinstone R. D. , Murphree J. S. , Cogger L. L.
Published: February 2008

Viking images of the auroral distribution have been used to investigate the relevance of various ionospheric coordinate systems. An important aspect of the large‐scale auroral shape is its dependence on the asymmetries of the Earth's internal field. Model predictions of where the aurora occurs, using the equatorial plane's volume current density, agree with observations and imply that the internal field plays a more important role than generally believed. Historically, the belief that the internal field has only small effects seems to stem from the widespread use of the corrected geomagnetic and invariant coordinate systems. These systems involve the mapping of field lines and have advantages in statistical studies and comparisons; less sophisticated systems such as the eccentric dipole coordinate system should be used in individual studies and in studies involving the differentiation or integration of some observational parameters. Observations of the auroral distribution are given to illustrate the universal time, tilt angle, and Kp variability in different coordinate systems and demonstrate that the dominant variability of the aurora is due to internal field asymmetries. A new set of coordinate systems are briefly developed as examples of how to incorporate external field models into studies of auroral images. It is proposed that one of these coordinate systems can be used as a test of how well an external field model can match observed auroral distributions.
Effect of magnetic field distortions on gyroresonance at low L values
Authors: Friedel R. H. W. , Hughes A. R. W.
Published: February 2008

From fully relativistic test particle simulations of the gyroresonance interaction between energetic electrons and whistler mode waves in the magnetosphere it is known that pitch angle diffusion due to this interaction becomes less and less effective at low L values, requiring very large wave amplitudes (∼100 nT) for significant pitch angle changes (Δα>1°). Such pitch angle changes are needed to produce the wave‐induced electron precipitation as observed through the Trimpi events which have been reported at low latitudes. Our computer simulation has been extended to model background field distortions which may alter the curvature of the field lines in such a way as to enhance the interaction efficiency. In general, “flattening” of the field line in the region near resonance enhances the interaction considerably, with a possible best case threefold increase in interaction time when compared to a undisturbed (dipole) field configuration. Our results indicate that pitch angle diffusion due to gyroresonance can only be significant at low L during periods of field line distortion, such as dayside compression or in the presence of Pc 4 or Pc 5 pulsations and various cavity mode oscillations.
Auroral event sequences at the dayside polar cap boundary for positive and negative interplanetary magnetic field BY
Authors: Sandholt P. E. , Moen J. , Rudland A. , Opsvik D. , Denig W. F. , Hansen T.
Published: February 2008

The quasi‐periodic sequence of moving auroral forms at the dayside polar cap boundary observed during southward directed interplanetary magnetic field (IMF) is one of the most interesting features of dayside auroral activity. Examples from the prenoon and postnoon sectors, corresponding to positive and negative IMF BY, respectively, are presented. Dual‐site ground‐based optical observations of one of the auroral events on January 12, 1991, are combined with “snapshot” information on the particle precipitation environment and ionospheric ion flow obtained from the polar‐orbiting satellite DMSP F9. A characteristic sequence of auroral forms observed within 08‐10 MLT expanded northwestward in a region of strong westward ion flow component, during intervals of strongly negative IMF BZ and positive BY. The optical events faded out near the 08 MLT meridian, within the regime of mantle precipitation (magnetosheath origin). The fading phase of the optical event traversed by the satellite along the 09 MLT meridian is characterized by structured mantle precipitation and associated ion flow shears within 72°‐78° magnetic latitude (MLAT). The integrated potential drop across this event in the north‐south direction, corresponding to the westward ion drift component within 72°‐78° MLAT, is 25 kV. IMP 8 observations of solar wind dynamic pressure outside the bow shock do not show good correlation between pressure variations and major optical event occurrence in the present case. Negative IMF BY events observed near the noon meridian and in the postnoon sector are moving eastward, in the same direction as the ionospheric convection. The optical, particle, and ion drift observations in combination with IMF and solar wind plasma data indicate that the events represent dynamical structures of merging cell convection over the polar magnetosphere, possibly initiated by pulses of enhanced merging rate (Bn ≠ 0) at the dayside magnetopause. The fading of the optical signature and the structured ionospheric ion flow/field‐aligned current at mantle latitudes are consistent with a high degree of spatial structure in the high‐latitude boundary layer and of the associated solar wind‐magnetosphere dynamo action (E · j < 0), provided the field line mapping implied by the present understanding of the particle precipitation characteristics is correct. The contribution from these events to the transpolar voltage (ϕpc) may be estimated on the basis of luminosity area and event repetition time (≃10 min). A wide range of spatial scales of optical events as well as their internal structure is observed. This study indicates that a nonnegligible fraction of the transpolar voltage may at times be generated by pulsed magnetopause merging. Discrete auroral activity and irregular magnetic pulsations with long periods are observed at cleft latitudes in the vicinity of the prenoon convection reversal and further north during the event sequence taking place.
Wave properties near the subsolar magnetopause: Pc 1 waves in the sheath transition layer
Authors: Song P. , Russell C. T. , Huang C. Y.
Published: February 2008

We study the waves in the frequency range of Pc 1 in the sheath transition layer of the magnetopause from the ISEE 1 and 2 observations. The waves are enhanced in the sheath transition layer, although they are scattered into the magnetosheath when the outer edge of the sheath transition layer is not sharp. A statistical study of 16 cases shows that the wave frequency is proportional to and equal to about 44% of the ion gyrofrequency. The wave amplitude is much greater when the interplanetary magnetic field (IMF) is southward than when the IMF is northward. The waves are left‐handed polarized when the IMF is southward but are linearly polarized for northward IMF. The direction of maximum variation is perpendicular to both the background field and the gradients of the field and density when the IMF is northward. For southward IMF, the waves are more turbulent. The wave generation mechanisms apparently depend on the IMF orientations rather than the shock geometry. To investigate the free energy to generate the waves for northward IMF, a method is developed combining the measurements from the fast plasma experiment and Lepedea to obtain a high time resolution estimate of the temperature anisotropy when the IMF is strongly northward. The estimated ion temperature anisotropy is enhanced, up to a factor of 2, within the sheath transition layer for northward IMF.
Imaging riometer and HF radar measurements of drifting F region electron density structures in the polar cap
Authors: Rosenberg T. J. , Wang Z. , Rodger A. S. , Dudeney J. R. , Baker K. B.
Published: February 2008

A study of riometer, photometer, and ionosonde data from South Pole Station has revealed a class of unusual dayside absorption events which occur near local magnetic noon when South Pole is located poleward of the dayside cusp. These events, which can reach values in excess of 1 dB in narrow‐beam 38.2‐MHz riometer measurements, are accompanied by significant enhancement of the O(¹D2) 630.0‐nm emission. However, because there is no corresponding change in the N2+ 427.8‐nm emission, it is unlikely that these events are caused by the conventional D or lower E region ionization increases usually attributed to the precipitation of auroral electrons of keV energy. Rather, the comparison of South Pole imaging riometer and ionosonde data with simultaneous Halley PACE HF radar data suggests that these unusual absorption events are related to F region electron density structures drifting from the dayside oval into the polar cap. If further work sustains this interpretation, then new prospects will be opened up for using imaging riometers to examine aspects of polar cap convection.
Rapid ray motions in barium plasma clouds and auroras
Authors: Wescott E. M. , Hallinan T. J. , Stenbaek‐Nielsen H. C. , Swift D. W. , Wallis D. D.
Published: February 2008

Barium plasma clouds released at high latitudes characteristically become striated with many field‐aligned rays. The rays which often resemble auroral features usually drift as a whole with the E × B/B2 drift of the cloud and alter position only slowly (order of tens of seconds). On two evenings in 1968, in released from Andøya, Norway, anomalous field‐aligned brightenings or emission enhancements of up to 3X were observed to move rapidly (10‐20 km/s) through three different Ba+ clouds. Similar effects were observed in Ba+ clouds released from rockets launched from Poker Flat, Alaska: On March 21, 1973, in two Ba thermite releases and on March 22, 1980, in the Ba‐shaped charge experiment “Miss Peggy.” On these occasions, auroras on or near the Ba+ L shell, also exhibited active rapid ray motions. This leads us to the assumption that the two phenomena are related and the expectation that an explanation of the rapid ray motions in the Ba+ clouds would lead to a better understanding of the physics of auroral ray motions and the auroral ionosphere. Seven possible mechanisms to produce the observed moving emission enhancements are discussed. Direct excitation by energetic electron bombardment is ruled out on the basis of energy fluxes required for realistic cross sections. Direct motion of an isolated Ba+ ray past the other rays by E × B/B2 motion seems very unlikely due to the observed variations in the enhancements and the large E field required (>500 mV/m). Compressional waves do not seem to be of sufficient amplitude or velocity. Formation of folds or curls in a sheet beam instability require moving radial fields of 500 mV/m but would explain the auroral features as well as the Ba+ by the same mechanism. Qualitatively, the redistribution of Ba ions as a flux tube encounters a higher electron density in the ionosphere may result in enhancements. Absorption or radiation of Doppler shifted Ba+ emissions by ions gyrating or moving at a few kilometers per second seems to be the most promising mechanism for producing the enhancements. The observations provide compelling evidence for the existence of transient electric fields of order 100 mV/m at altitudes as low as 200 km during active aurora with rapid ray motions. The affected regions have dimensions of order a few kilometers across B and move eastward at 10‐20 km/s.
Collision cross sections and diffusion parameters for H and D in atomic oxygen
Authors: Hodges R. Richard
Published: February 2008

Modeling the behavior of H and D in planetary exospheres requires detailed knowledge of the differential scattering cross sections for all of the important neutral‐neutral and ion‐neutral collision processes affecting these species over their entire ranges of interaction energies. In the upper atmospheres of Earth, Venus, and other planets as well, the interactions of H and D with atomic oxygen determine the rates of diffusion of escaping hydrogen isotopes through the thermosphere, the velocity distributions of exospheric atoms that encounter the upper thermosphere, the lifetimes of exospheric orbiters with periapsides near the exobase, and the transfer of momentum in collisions with hot O. The nature of H‐O and D‐O collisions and the derivation of a data base consisting of phase shifts and the differential, total, and momentum transfer cross sections for these interactions in the energy range 0.001 ‐ 10 eV are discussed. Coefficients of mutual diffusion and thermal diffusion factors are calculated for temperatures of planetary interest.
Stormtime transport of ring current and radiation belt ions
Authors: Chen Margaret W. , Schulz Michael , Lyons Larry R. , Gorney David J.
Published: February 2008

This is an investigation of stormtime particle transport that leads to formation of the ring current. Our method is to trace the guiding‐center motion of representative ions (having selected first adiabatic invariants µ) in response to model substorm‐associated impulses in the convection electric field. We compare our simulation results qualitatively with existing analytically tractable idealizations of particle transport (direct convective access and radial diffusion) in order to assess the limits of validity of these approximations. For µ ≲ 10 MeV/G (E ≲ 110 keV at L ≈ 3) the ion drift period on the final (ring‐current) drift shell of interest (L ≈ 3) exceeds the duration of the main phase of our model storm, and we find that the transport of ions to this drift shell is appropriately idealized as direct convective access, typically from open drift paths. Ion transport to a final closed drift path from an open (plasma‐sheet) drift trajectory is possible for those portions of that drift path that lie outside the mean stormtime separatrix between closed and open drift trajectories. For µ ∼ 10‐25 MeV/G (110 keV ≲ E ≲ 280 keV at L ≈ 3) the drift period at L ≈ 3 is comparable to the postulated 3‐hr duration of the storm, and the mode of transport is transitional between direct convective access and transport that resembles radial diffusion. (This particle population is transitional between the ring current and radiation belt). For µ ≳ 25 MeV/G (radiation‐belt ions having E ≳ 280 keV at L ≈ 3) the ion drift period is considerably shorter than the main phase of a typical storm, and ions gain access to the ring‐current region essentially via radial diffusion. By computing the mean and mean‐square cumulative changes in 1/L among (in this case) 12 representative ions equally spaced in drift time around the steady‐state drift shell of interest (L ≈ 3), we have estimated (from both our forward and our time‐reversed simulations) the time‐integrated radial‐diffusion coefficients DLLsim for particles having selected values of µ ≳ 15 MeV/G. The results agree surprisingly well with the predictions (DLLql) of quasilinear radial‐diffusion theory, despite the rather brief duration (≈ 3 hr) of our model storm and despite the extreme variability (with frequency) of the spectral‐density function that characterizes the applied electric field during our model storm. As expected, the values of DLLsim deduced (respectively) from our forward and time‐reversed simulations agree even better with each other and with DLLsim when the impulse amplitudes which characterize the individual substorms of our model storm are systematically reduced.
The Uranian corona as a charge exchange cascade of plasma sheet protons
Authors: Herbert Floyd
Published: February 2008

Models of magnetospheric convection and interparticle interactions are used in a study of the collisional interactions between atmospheric neutral hydrogen and magnetospheric charged particles observed by Voyager to be convecting through the Uranian magnetosphere. The most important reactions are e−‐H collisional ionization and p+‐H charge exchange with the atmosphere. The e−‐H collisional ionization process, continually reenergized by compressional heating of the electrons as they drift toward Uranus, produces a cascade of new plasma. This process has been suggested by many authors as the source of the warm (10 eV at L = 5) plasma and is found by this work to continue in a cascade to even cooler and more abundant plasma. This newly created plasma consists almost entirely of electrons and protons because He and H2 are nearly absent from the very upper layers of the atmosphere. The densities computed for the warm plasma are within an order of magnitude of the values observed by Voyager. The cold plasma should occur only near or sunward from Uranus, where it is too cold to have been detected by Voyager on its inbound trajectory. Low‐altitude p+‐H charge exchange between the atmosphere and the new plasma produces a significant flux of hot H. This hot H may comprise the Uranian H corona, and its production serves as a high‐altitude thermospheric heat source, both of which are implied by Voyager UVS observations (although this heat source is apparently inadequate to produce the high thermospheric temperature that was observed). The generation of significant new plasma near Uranus leads to the following speculation: If this plasma crosses the dayside magnetopause and mixes with magnetopause boundary layers such as the plasma mantle, there to be swept back along the magnetotail, reincorporated into the magnetotail by the same processes postulated for solar wind plasma entry, and reenergized in the magnetotail current sheet, it would constitute an important source for the hot plasma (1 keV at L = 5) observed by Voyager.
Review of the decadal oscillation in the stratosphere of the northern hemisphere
Authors: van Loon H. , Labitzke K.
Published: February 2008

The decadal oscillation of the temperature and geopotential height in the lower stratosphere of the northern hemisphere can be followed back to the early 1950s. During this time it was in phase with the 11‐year sunspot cycle. The correlation with the solar cycle is positive and largest in the stratospheric geopotential heights of the subtropics below 10 mbar (∼31 km, which is as high as the grid point data reach), especially on the western, ocean‐dominated side of the hemisphere. As expected from the hydrostatic equation, it is also evident in the temperatures of the upper troposphere in the same region. There is no large correlation at high latitudes. The correlation with the sunspot cycle is weakest in January‐February, but if the data in these months are grouped according to the wind direction in the quasi‐biennial oscillation (QBO) of the lower equatorial stratosphere, the positive subtropical correlations in the east years are as high as in all other months. There are, in addition large negative correlations in the Arctic in agreement with the strong teleconnection (negative correlation) between lower and higher latitudes in winter. There is no consistent sign in the weak correlations at middle and lower latitudes in the west years, but in the Arctic the correlation with the solar cycle is highly positive, because those major midwinter warmings that occur in west years of the QBO take place in solar maxima, whereas the years without major warmings are found in solar minima. There is not yet an explanation of the 10‐12 year oscillation in the stratosphere.
Longitudinal variation of sudden commencement of geomagnetic storm at equatorial stations
Authors: Rastogi R. G.
Published: February 2008

It is shown that the amplitude of storm sudden conmencement (SSC) at a station within the equatorial electrojet belt experiences daytime enhancement, latitudinal enhancement, and longitudinal enhancement just as do the similar characteristics of the strength of the equatorial electrojet current itself. It is concluded that the sudden commencements of geomagnetic storms are associated with an increase of electric fields in the dynamo region modifying primarily the equatorial rather than Sq currents. The V × Bz electric field generated at the magnetopause due to the impact of the solar plasma is transmitted along the magnetic lines of force to polar latitude as dusk to dawn electric field which is instantaneously transmitted to the equatorial latitude as transverse magnetic waveguide mode between conducting ionosphere and Earth. The enhancements of SSC amplitude over the magnetic equator seem to be related to the basic ionospheric conductivity at the longitude concerned.