bluuebunny/crossref_metadata_2025 huggingface dataset:
Three‐dimensional structure of coronal mass ejections
Authors: Steinolfson R. S.
Published: February 2008

Numerical solutions of the MHD equations are used to examine the three‐dimensional structure of coronal mass ejections (CMEs). The initial atmosphere consists of a streamer belt at the equator, which is characteristic of coronal conditions near solar minimum. A finite azimuthal portion of the streamer belt is then disrupted by the emergence of new magnetic flux. The CME forms as the corona adjusts to accommodate the new flux. The numerical solution in a meridional plane centered over the region of newly emerged flux has features similar to those computed in previous two‐dimensional simulations. The primary result of this study is that the CME is shaped more like an arcade with an axis parallel to the equator than like a bubble. This characteristic arcade shape occurs both in the density distribution in meridional and azimuthal planes and in the integrated white‐light brightness in meridional and polar views. By examining the brightness change due to the CME it is found that the CME in a meridional projection as viewed from the Earth has the same loop shape even if the CME originates almost directly in front of or behind the Sun. It appears dimmer as it occurs further from the limb with a reduction in the maximum brightness of less than 30% as the CME moves from the limb to 45° away from it. The peak brightness in a polar view is about 65% less than that in a meridional projection when the CME is at the limb.
O I 630.0‐nm dayglow in the region of equatorial ionization anomaly: Temporal variability and its causative mechanism
Authors: Sridharan R. , Haider S. A. , Gurubaran S. , Sekar R. , Narayanan R.
Published: February 2008

Results of the continuous measurements of O I 630.0‐nm dayglow from Mount Abu (20° 20′N dip latitude), a station in the region of equatorial ionization anomaly (EIA) by means of a recently developed ground‐based dayglow photometer are presented. The measurements on magnetically quiet and moderately active days reveal that there are strong noontime bite‐outs in 630.0‐nm intensities. The temporal variabilities in the intensities show a good correlation with those in electron densities in the lower F region over Ahmedabad (18° 23′N dip latitude) with a time delay of 1 hour. Theoretical estimation of the intensities indicates that the dissociative recombination of O2+ with ambient electrons is mainly responsible for the observed temporal variability in the dayglow emission. The time delay between the theoretically evaluated and the measured noontime bite‐outs is discussed in the context of the development of the EIA.
A new aspect of magnetic declination control of equatorial spread F and F region dynamo
Authors: Abdu M. A. , Batista I. S. , Sobral J. H. A.
Published: February 2008

Range spread F (RSF) data for a 12‐year period (1978–1990) over the Brazilian low‐latitude station, Cachoeira Paulista, are analyzed to determine the fine structure of the seasonal pattern of the irregularity occurrence, which appears consistent throughout an entire solar cycle. The RSF occurrence for one of these years is compared also with that over the equatorial station, Fortaleza, to show that the seasonal pattern discussed here corresponds to that of plasma bubble irregularity developments. A striking result that has come out of the present analysis concerns two secondary minima, during the epoch of broad annual maximum, in the RSF that occur in association with the two nodal points at the magnetically conjugate E layer sunset local times, that is, at the perfect alignment of the sunset terminator with the magnetic meridian of the station. The occurrence of these minima, seen in individual solar minimum and solar maximum years (as also in the overall mean behavior), is interpreted on the basis of a simplified F region dynamo development model that considers also asymmetric conjugate E layer decay conditions. Competing roles of a prereversal zonal electric field enhancement and of its height gradient seem to determine the occurrences of these secondary minima in RSF.
A new method for detecting the projection of magnetospheric oscillations into the ionosphere
Authors: Rajaram R. , Ruohoniemi J. M. , Greenwald R. A. , Baker K. B.
Published: February 2008

In situ magnetospheric observations and the corresponding data collected at the foot of the field line in the ionosphere can together form an invaluable tool for studying the processes involved in magnetosphere‐ionosphere coupling provided the mappings between various regions in the magnetosphere and ionosphere are precisely known. A multivariate analysis technique is used to identify spatially localized modes of oscillations in the ionospheric plasma velocities monitored at F region heights by the Goose Bay HF radar. The temporal variations of these modes are compared with the magnetic fluctuations recorded by the AMPTE CCE satellite which was within the magnetic latitude‐longitude region monitored by the radar. It is possible to clearly identify spatially localized modes in the ionosphere with temporal variations that track the azimuthal oscillations observed in the magnetosphere. This analysis approach offers a powerful method for establishing mappings between different regions of the magnetosphere and ionosphere under widely varying background conditions. The method, which is applied here exclusively to the mapping problem, can also be used to identify the natural modes of magnetospheric oscillation.
Interference patterns in the Spacelab 2 plasma wave data: Oblique electrostatic waves generated by the electron beam
Authors: Feng Wei , Gurnett Donald A. , Cairns Iver H.
Published: February 2008

During the Spacelab 2 mission the University of Iowa's Plasma Diagnostics Package (PDP) explored the plasma environment around the shuttle. Wideband spectrograms of plasma waves were obtained from the PDP at frequencies of 0–30 kHz and at distances up to 400 m from the shuttle. Strong low‐frequency (below 10 kHz) electric field noise was observed in the wideband data during two periods in which an electron beam was ejected from the shuttle. This noise shows clear evidence of interference patterns caused by the finite (3.89 m) antenna length. The low‐frequency noise was the most dominant type of noise produced by the ejected electron beam. Analysis of antenna interference patterns generated by these waves permits a determination of the wavelength, the direction of propagation, and the location of the source region. The observed waves have a linear dispersion relation very similar to that of ion acoustic waves. The waves are believed to be oblique ion acoustic or high‐order ion cyclotron waves generated by a current of ambient electrons returning to the shuttle in response to the ejected electron beam.
Banded electrostatic emissions observed by the CRRES Plasma Wave Experiment
Authors: Paranicas C. , Hughes W. J. , Singer H. J. , Anderson R. R.
Published: February 2008

A survey of banded electrostatic emissions observed by the Plasma Wave Experiment on the recently launched spacecraft “CRRES” is presented with an emphasis on the relation of the frequency of the highest intensity waves to multiples of the local electron cyclotron frequency. Because one of its goals is a first look at the CRRES high‐frequency electric field data, all the data discussed in this paper were taken in the first 8 months after launch. During this time, emissions are detected chiefly in the postmidnight quadrant of the magnetosphere between L=3.5 and L=7. We confirm that high‐intensity emissions are confined to within a few degrees of the magnetic equator. The emissions presented here are all measured below the electron plasma frequency, and we find they generally favor the upper part of the gyroharmonic interval. When ECH emissions are detected both near and far from the equator on the same orbit, it is generally true that a broad band of the same relative frequency emissions are excited in both locations. The difference is that near the equator, a few frequency channels measure much higher intensities. When we compare with one model for convective growth, we find that the observed spectra may not be consistent with predicted ones.
Whistler analysis in the time‐frequency plane using chirplets
Authors: Mihovilović D. , Bracewell R. N.
Published: February 2008

This paper shows the benefits of using chirplet analysis of multipath magnetospheric whistlers. Higher resolution can be achieved than with the customary dynamic spectra (Gabor diagram), in which the cells characterizing resolution in time and frequency are the same over the whole diagram. Here we not only allow the aspect ratio of a resolution cell to vary within the one diagram but also introduce oblique cells corresponding to a new kind of elementary signal not discovered by Gabor. Chirplet analysis is also applicable to arbitrary signals and is most striking where certain kinds of finely detailed structure are present. We also describe a user interactive system that allows regions of interest to be selected in the time‐frequency plane and enhanced by selecting chirplets appropriate to the local structure.
Dayside ionospheric convection changes in response to long‐period interplanetary Magnetic field oscillations: Determination of the ionospheric phase velocity
Authors: Saunders M. A. , Freeman M. P. , Southwood D. J. , Cowley S. W. H. , Lockwood M. , Samson J. C. , Farrugia C. J. , Hughes T. J.
Published: February 2008

Ground magnetic field perturbations recorded by the CANOPUS magnetometer network in the 7 to 13 MLT sector are used to examine how reconfigurations of the dayside polar ionospheric flow take place in response to north‐south changes of the IMF. During the 6‐hour interval in question IMF Bz oscillates between ±7 nT with about a 1‐hour period. Corresponding variations in the ground magnetic disturbance are observed which we infer are due to changes in ionospheric flow. Cross correlation of the data obtained from two ground stations at 73.5° magnetic latitude, but separated by ∼2 hours in MLT, shows that changes in the flow are initiated in the prenoon sector (∼10 MLT) and then spread outward toward dawn and dusk with a phase speed of ∼5 km s−1 over the longitude range ∼8 to 12 MLT, slowing to ∼2 km s−1 outside this range. Cross correlating the data from these ground stations with IMP 8 IMF Bz records produces a MLT variation in the ground response delay relative to the IMF which is compatible with these deduced phase speeds. We interpret these observations in terms of the ionospheric response to the onset, expansion and decay of magnetic reconnection at the dayside magnetopause.
Electric fields and field‐aligned current generation in the magnetosphere
Authors: Alexeev I. I. , Belenkaya E. S. , Kalegaev V. V. , Lyutov Yu. G.
Published: February 2008

We present a calculation of the electric potential, field‐aligned currents, and plasma convection caused by the penetration of the solar wind electric field into the magnetosphere. Ohm's law and the continuity equation of ionospheric currents are used. It is shown that the large‐scale convection system is reversed in the plasma sheet flanks. In this region the plasma flow is antisunward earthward of the neutral line and sunward tailward of it. The interplanetary magnetic field (IMF) Bz dependences on the dimension of the magnetopause “windows” which are intersected by open field lines, on the potential drop across the polar cap, and on the distance to the neutral line are determined. Because of the IMF effect and the effect of seasonal or daily variations of the geomagnetic field which violate its symmetry relative to the equatorial plane, there may arise a potential drop along field lines which causes field‐aligned currents. The values and directions of these currents, the field‐aligned potential drop, and a self‐consistent solution for the potential at the ionosphere level for high field‐aligned conductivity have been determined.
Numerical simulations of bursty radio emissions from planetary magnetospheres
Authors: Winglee R. M. , Menietti J. D. , Wong H. K.
Published: February 2008

The Voyager spacecraft observed both smooth and bursty radio emissions from Uranus and Neptune. These emissions are known to be freely propagating primarily in the right‐hand circularly polarized (RCP) mode with the bursty emissions having burst periods as short as a few tenths of a second and the smooth emissions being observed over periods of a few hours. While the smooth emission is probably due to the electron cyclotron maser instability, some other processes must be at work to produce the bursty emissions. It is proposed that one important difference in mechanisms is that the smooth emissions are associated with continuous injection of electrons while the bursty emissions are associated with impulsive injection. In the latter case, the electron distribution can develop a beam feature with a temperature anisotropy. It is shown via one‐dimensional (three velocity) relativistic particle simulations that while the beam may be initially unstable to an electrostatic instability, this instability quickly saturates and eventually the beam is unstable to a strong electromagnetic beam instability which utilizes the temperature anisotropy as free energy; a beam feature is not explicitly needed for the growth of this electromagnetic instability. The radiation generated by the instability is able to convert particle energy to wave energy at similar levels as the maser instability. For ωpe/Ωe ≳ 1, most of the wave energy is LCP and is trapped. However, for ωpe/Ωe ≲ 1 the dominant mode is RCP. The induced waves are in the form of modified whistlers when the beam speed is low and ωpe/Ωe ≃ 1 but are in the freely propagating x mode branch when ωpe/Ωe ≃ 0.4. The amount of radiation with frequencies above the local x mode cutoff increases with beam speed. It is also postulated that some of the radiation generated below the local x mode cutoff may also be able to escape the plasma and be detected remotely via mode conversion between regions where field‐aligned currents produce local perturbations in the magnetic field.
Transverse ion acceleration by localized lower hybrid waves in the topside auroral ionosphere
Authors: Vago J. L. , Kintner P. M. , Chesney S. W. , Arnoldy R. L. , Lynch K. A. , Moore T. E. , Pollock C. J.
Published: February 2008

Up to now, observations had been unable to show conclusively a one‐to‐one correspondence between perpendicular ion acceleration and a particular type of plasma wave within the O+ source region below 2000 km. In this paper we demonstrate that intense (100–300 mV/m) lower hybrid waves are responsible for transversely accelerating H+ and O+ ions to characteristic energies of up to 6 eV. This wave‐particle interaction takes place in thin filamentary density cavities oriented along geomagnetic field lines. The measurements we discuss were conducted in the nightside auroral zone at altitudes between 500 km and 1100 km. Our results are consistent with theories of lower hybrid wave condensation and collapse.
Quasi‐steady current sheet structures with field‐aligned flow
Authors: Birn J.
Published: February 2008

General properties of field‐aligned plasma flow in quasi‐steady ideal MHD configurations are discussed, and explicit solutions, modeling compressible flow around a plasmoid in the distant magnetotail, are presented. The explicit solutions are based on the assumption that plasma and field gradients along the x and y directions parallel to the tail current sheet are typically much smaller than those in the z direction perpendicular to the current sheet. In contrast to the incompressible case, two different flow solutions exist in the compressible case in the subalfvénic regime. The different regimes are distinguished by whether the flow speed is below or above a critical speed υo, which represents the group velocity of a slow magnetosonic wave (aligned with the magnetic field) in the limit of phase propagation perpendicular to the magnetic field, where the phase speed vanishes. In the incompressible case, only the subcritical flow solution is realized in the subalfvénic regime. In the compressible case, both regimes may exist simultaneously in the flow around the plasmoid. They are separated by a tangential discontinuity which may be considered as the limit of a slow shock for vanishing normal flow and normal magnetic field. The subcritical flow region surrounding the plasmoid inside of a supercritical region possibly corresponds to the layers of tailward streaming ions observed adjacent to the plasmoid proper. The signatures of a passage from the supercritical through the subcritical flow region in the vicinity of the plasmoid can be similar to those of a passage through the plasmoid itself: a reduction of Bx followed by a local enhancement in the middle of the encounter, as observed, for instance, by Hones et al. (1984) and Slavin et al. (1989). This enhancement is found for a simple loop structure within the plasmoid and hence need not be associated with multiple loops or a tail flapping as suggested by Hones et al. Field signatures outside the plasmoid are typically those of encounters of a traveling compression region: an enhancement of Bx accompanying a north‐south signature of Bz. In certain parameter regimes, however, unusual signatures are also possible: a reduction of Bx with a north‐south Bz signature for a close plasmoid encounter (without penetration) and a south‐north signature of Bz accompanying a Bx enhancement for a distant plasmoid encounter at high latitudes.
Observations of diffuse resonance emissions from low‐current electron beam injections in the auroral zone ionosphere
Authors: Goerke R. T. , Kellogg P. J. , Monson S. J.
Published: February 2008

We have observed narrowband plasma waves (≥0.1mV/m(Hz)1/2) at frequencies between the electron cyclotron frequency and the ambient plasma frequency. These waves are produced by the injection of a low‐current electron beam (>3 mA). The frequency of these emissions is consistent with the diffuse resonances that have been observed by satellite‐borne sounding experiments (Osherovich and Benson, 1991). These results were obtained from the rocket‐borne experiment SCEX 3 (Several Compatible Experiments using a rocket‐bome accelerator), NASA flight 39.002 UE, launched February 1, 1990, from Poker Flat Research Range (65.10°N, 147.50°W). These emissions are observed to propagate as far as 270 m across the ambient magnetic field with apparently little attenuation. These emissions are also observed up to 700 m from the accelerator along the electron beam.
Ionization and expansion of barium clouds in the ionosphere
Authors: Ma T.‐Z. , Schunk R. W.
Published: February 2008

A three‐dimensional time‐dependent model was used to study the ionization and plasma expansion characteristics of barium clouds released in the ionosphere. Neutral gas clouds with a total mass of 1 kg were released with a spherical Gaussian density distribution and the subsequent photoionization by solar UV radiation and ion cloud expansion were modeled. The results from three simulations are presented: a cloud without a directional velocity; a cloud with an initial velocity of 5 km/s across the B field; and a cloud with initial velocity components of 2 km/s both along and across the B field. For the cloud without a directional velocity, the ionization occurs in a spherical volume. The resulting Ba+ cloud expands along the B field and the Ba+ density distribution gradually becomes ellipsoidal from the inner to the outer parts of the Ba+ cloud. The electrostatic snowplow effect associated with the expanding Ba+ cloud creates a hole in the O+ background at the center of the Ba+ cloud and creates two O+ density bumps on the two sides of the Ba+ cloud. For the neutral gas release with an initial velocity across the B field, the resulting Ba+ cloud has a cometlike density distribution at early times. Eventually, because of the expansion along the B field, the Ba+ cloud becomes sheetlike. Again, there are two O+ density enhanced regions and one O+ density depletion region in the background ionosphere. For these two cases, although there are O+ density depletions, there are no electron density (total plasma) depletions. When the neutral gas cloud has initial velocity components both along and across the B field, the situation is quite different. The resulting Ba+ cloud has a complicated density distribution. The Ba+ snowplow effect creates an O+ density hole on one side and an O+ density bump on the other side of the expanding Ba+ cloud. There is a net plasma depletion on the side opposite to the Ba+ cloud motion along the B field.
An observational limit to the amplitude of Alfvén waves in the solar wind and comparison with an acceleration model
Authors: Coles W. A. , Esser Ruth
Published: February 2008

Measurements of angular scattering of cosmic radio sources observed near the solar limb indicate that the density microstructure can be highly anisotropic (Armstrong et al., 1990). Here we point out that Alfvén waves, which are known to be present in the solar wind, will tend to isotropize the angular scattering measurements. Thus such observations set an upper bound on the amplitude of Alfvén waves in the scattering region. Recently measurements of velocity versus distance were made near the Sun (using interplanetary scintillations). In about half of the observed streams the velocity profiles were found to be consistent with an MHD model of the solar wind in which Alfvén waves are used to increase the flow velocities (Coles et al., 1991). In this paper we demonstrate that the Alfvén wave amplitudes necessary to match the measured velocities are consistent with the bounds derived here from angular scattering observations.
Large‐scale HF‐induced ionospheric modifications: Theory and modeling
Authors: Hansen J. D. , Morales G. J. , Maggs J. E.
Published: February 2008

A quantitative explanation is given for the observed (Hansen et al., 1992) large‐scale (several kilometers in extent, δTe/Te0 ∼ 3, |δne|/ne0 ∼ 25%) ionospheric modifications produced with the Arecibo HF heating facility. The process of nonlinear refraction is introduced and shown to be the key nonlinearity which makes possible the generation of the observed effects. Nonlinear refraction consists of the self‐consistent reorientation of the HF reflection surface by heating‐induced density modifications from a gravitationally aligned density gradient to a geometry with ∇n perpendicular to the geomagnetic field. It causes the O mode radiation to refract across the geomagnetic field and results in focused heating along a narrow flux tube colinear with the modified reflection surface. A two‐dimensional transport model that incorporates the effects of nonlinear refraction on F region modification is used to establish a detailed comparison with experimental results.
Quasi‐linear velocity space diffusion of heavy cometary pickup ions on bispherical diffusion characteristics
Authors: Huddleston D. E. , Coates A. J. , Johnstone A. D.
Published: February 2008

We consider an ion transport equation describing the source, convection, adiabatic acceleration and quasi‐linear velocity diffusion of cometary pickup ions. The equation is solved numerically along solar wind plasma flow lines for the environment of comet Halley, to obtain distributions at positions on the Giotto spacecraft trajectory which may be compared with observations. We use realistic input models for the observed spectrum of upstream and downstream propagating wave turbulence and for the ion source and mass loading of the solar wind. The ion resonance condition is not approximated in the diffusion term and we obtain full two‐dimensional (pitch angle and velocity) numerical distributions which show pitch angle scattering about the wave scattering centers at ± VA where VA is the Alfvén wave speed in the solar wind frame. Peak ion phase‐space densities approximately follow the bispherical shell geometry. The energy distributions F(υ) agree well with Giotto observations at lower υ but in the high‐energy tail there is evidence possibly for the first‐order Fermi acceleration mechanism taking place in the comet Halley foreshock region.
Effect of meridional winds and neutral temperatures on the F layer heights over low latitudes
Authors: Gurubaran S. , Sridharan R.
Published: February 2008

Results of coordinated measurements of thermosphere and F region parameters with an aim to study the extent of the prevailing coupling between these two regions at low latitudes (Mount Abu/Ahmedabad, India) are presented through individual case studies. Spectroscopically measured neutral temperature and meridional winds are used in the servo equations to estimate the balance height of the F region ionization maximum and compared with independently measured Hmax by means of ground‐based ionosonde. The comparison reveals fairly good agreement, reproducing most of the observed features and providing direct experimental evidence for these two regions to behave as a closely coupled system. The electric fields, though they appear to be less significant at these latitudes, could still be inferred once the thermospheric data become available. The important conclusion is that the effect of the neutral temperature and its variabilities should be properly accounted for in the determination of meridional winds from the existing ground‐based ionosonde data.
Large‐scale field‐aligned plasma flows
Authors: Hruška A.
Published: February 2008

Stationary, large scale plasma flows along the field lines are analyzed using the first three moments of the collisionless Boltzmann equation. The number of integrals of motion which can be found is restricted not only by the cutoff of the hierarchy of the moment equations but also by the forms of the available first‐order partial differential equations which often do not satisfy Cauchy condition of integrability. Only three integrals of motion exist for the plasma continuum described by plasma density, velocity, two components of the pressure tensor and two components of the heat flow tensor. The equation for the field‐aligned current, which is algebraic in j∥, is derived. It expresses the field‐aligned current as function of the perpendicular electric field and perpendicular gradients of the pressure components and of the magnetic field strength determined at the investigated point of the field line.
Pickup protons and water ions at comet Halley: Comparisons with Giotto observations
Authors: Ye G. , Cravens T. E. , Gombosi T. I.
Published: February 2008

A quasi‐linear diffusion model including both pitch angle and energy diffusion, adiabatic compression and convective motion with the solar wind flow has been used to investigate the cometary ion pickup process along the Sun‐comet line at comet Halley. The total pickup ion densities and magnetic turbulence spectrum levels observed by Giotto were used to constrain the quasi‐linear model. Comparisons of the model results were made with energetic ion distributions observed by instruments onboard the Giotto spacecraft. The observed power spectrum index of magnetic turbulence γ is about 2–2.5. However, our simulation shows that when γ was 2, the calculated proton distributions were much more isotropic than the observed ones; hence we have chosen γ = 2.5 in our study. Furthermore, we assumed that only about 5% of the total low frequency wave power propagates away from the comet. The numerical solutions of the quasi‐linear diffusion equations show that the isotropization of the pickup ion distribution, particularly at the pickup velocity, is not complete even close to the bow shock (but upstream), which agrees with the observations. The calculated and observed proton distributions are somewhat more isotropic than the water group ion distributions in the region far upstream of the shock, but the opposite is true near the bow shock. We find that given the observed turbulence level, quasi‐linear theory gives pickup ion energy distributions that agree with the observed ones quite well and easily produces energetic ions with energies up to hundreds of keV.
Magnetic pulsations from 0.1 to 4.0 Hz and associated plasma properties in the Earth's subsolar magnetosheath and plasma depletion layer
Authors: Anderson B. J. , Fuselier S. A.
Published: February 2008

Dynamic spectral analysis covering frequencies 0.1 to 4.0 Hz has been applied to analyze Active Magnetospheric Particle Tracer Explorers CCE magnetic field data during periods of magnetospheric compressions when the spacecraft sampled the magnetosheath. Ion (H+, He2+) and electron data are used to identify regions, to monitor the upstream shock geometry, and to evaluate moments of the proton distribution. A total of 14 encounters representing 46 hours of observations are analyzed. For quasi‐perpendicular shock conditions, two distinctive band limited spectral types are found to characterize the majority of magnetosheath observations: (1) purely compressional fluctuations with F < 0.2 FH+ which are attributed to mirror waves (F and FH+ denote the wave frequency and proton gyrofrequency, respectively); and (2) 0.25–1.0 FH+ frequency purely transverse fluctuations that are often left hand polarized from 0.5–1.0 FH+ and are attributed to electromagnetic ion cyclotron (EMIC) waves. The plasma characteristics for these two pulsations are different. Mirror waves are associated with βp>5 and T⊥p/T∥p of 1.5 whereas EMIC waves occur for βp≤1 and T⊥p/T∥p = 2.5 ‐ 5 (the subscript p denotes quantities evaluated for protons). EMIC waves also occur for higher field strength and lower plasma densities than mirror waves, indicative of EMIC wave occurrence in the plasma depletion layer (PDL). The increased T⊥p/T∥p during EMIC waves relative to periods of mirror waves is due to a decrease in T∥p suggesting that the EMIC wave region has suffered a loss of particles in the parallel direction. A compilation of the temporal sequence of spectral structures shows that EMIC waves occur next to the magnetopause while mirror waves occur outside the region of EMIC waves. We conclude that there is a close relationship between EMIC waves in the sheath and the PDL. Both the density depletion and the increase in temperature anisotropy, Ap = T⊥p/T∥p ‐ 1, are attributed to extrusion of plasma along the draped field lines as plasma flows into the PDL. For quasi‐parallel upstream conditions, the magnetic field power spectra often exhibit no band limited structures but only broadband noise. The absence of EMIC wave emissions during quasi‐parallel conditions is consistent with predictions that the PDL does not form for radial IMF conditions.
ULF turbulence in the Neptunian polar cusp
Authors: Farrell W. M. , Lepping R. P. , Smith C. W.
Published: February 2008

One of the most fortuitous events occurring during the Voyager 2 Neptune encounter was the passage of the spacecraft through the southern magnetic cusp region prior to entry into the magnetosphere. This region was previously identified by a slow, steady change in the magnetic field direction to a dipolar configuration and a slow, steady decrease in electron and ion fluxes. Such observations are similar to those of the terrestrial cusp. Besides these characteristics, the region also possesses enhanced ULF magnetic turbulence at frequencies less than 0.5 Hz as measured by the Voyager 2 magnetometer (MAG) experiment. The activity is particularly strong at the frontside magnetopause boundary and at the backside cusp/magnetosphere boundary. Specifically, in these regions the fluctuation level, ΔB/B, is as large as 6%. The enhanced ULF turbulence extends from 0.01 Hz continuously up to nearly 0.5 Hz, and is consistent with the whistler mode. Calculations indicate that such turbulence should resonate with electrons having energies in the tens of kiloelectron volts. Observations indicate a very strong correlation of the ULF turbulence with the energetic electrons between 22 and 35 keV measured by Voyager's low‐energy charged particle (LECP) experiment. This result suggests a vigorous interaction between the two. Based on an assumed cusp geometry and measured wave levels, the ULF turbulence could generate as much as 107 W of power that would be absorbed by the surrounding magnetosphere. This power input represents about 1% of the total auroral power required to drive the UV aurora and radio emissions. Thus the ULF wave turbulence in the cusp may represent a significant but not complete power source for the magnetosphere.
Plasma heating by collisionless magnetic reconnection: Analysis and computation
Authors: Moses R. W. , Finn J. M. , Ling K. M.
Published: February 2008

This paper presents analytic and numerical results on particle acceleration in two‐dimensional collisionless magnetic reconnection. The magnetic field is taken to be a vacuum quadrupole field in the x‐y plane with no variation in the z direction. The electric field is uniform and in the z direction. Plasma particles are introduced with their guiding centers on a magnetic flux surface. Particles then execute E × B drift motion under which their guiding centers approach the separatrix. In the numerical simulations and in the analytic modeling presented, the particles are followed until they reach an outgoing flux surface at the same distance from the origin as the starting surface. The magnetic moment is not conserved for particles passing through the unmagnetized region around the X line at the origin. Other particles cross the separatrix without passing near the X line. The magnetic moment of the first class of outgoing particles is randomized, whereas it can be conserved for the second class. There is a consequent net change of particle kinetic energy for the first class of trajectories, which are accelerated by the electric field along the X line. The energy of the accelerated particles can have a “fractal” like dependence on trajectory initial conditions, characteristic of chaotic scattering, depending on the value of the electric field. By following the evolution of monoenergetic components of the input distribution function, it is possible to describe analytically this plasma thermalization process. The analytic model is based upon the observation of the final kinetic energy as a function of the initial conditions. Analytic results are shown which predict a Maxwellian tail for the distribution function in the perpendicular kinetic energy K⊥ with K⊥ ≫ K∥, the parallel kinetic energy. Numerical results are also presented, showing that the predicted tail temperature agrees with the numerically computed temperature to within 10% over 4 orders of magnitude in the electric field. These results provide a detailed understanding of particle acceleration and heating produced by collisionless magnetic reconnection.
On Vasyliunas's equivalent conductivity formalism
Authors: Pontius D. H.
Published: February 2008

The coupling of the ionosphere and magnetosphere can be understood by examining the field‐aligned currents passing between the two. In his treatment of the shielding of the inner magnetosphere, Vasyliunas [1972] developed a mathematical formalism for representing this coupling by a pseudo Ohm's law. All the effects of the magnetospheric plasma are comprised in an equivalent Hall conductivity that is determined by the assumed ion density distribution. In this paper, an alternate derivation of this formalism is presented in which, unlike the original derivation, the kinematics of the ions do not enter at all. This underscores the importance of a strong assumption upon which the method rests; that is, the ion distribution is assumed to be time stationary. However, there is nothing in the model to insure that the physical drifts of the ions do indeed satisfy that requirement. This places severe restrictions on the proper use of the formalism and implies that its results cannot be assumed to be correct in the absence of extensive a posteriori analysis.
On rotational discontinuities propagating in an isotropic plasma
Authors: Pesses Mark E.
Published: February 2008

The relationships between the plasma parameters downstream and upstream of plane wave rotational discontinuities (RDs) are reanalyzed for the case of an isotropic plasma. The analysis is carried out in the rest frame of the RD using a Cartesian coordinate system with axes labeled n, j, and z. The NJZ system is oriented in such a way that the n axis is parallel to the RD surface normal vector and the j component of B is the same on both sides of the RD. In this NJZ system the magnitude of each of the downstream components of B, U, and E is equal to the magnitude of the respective upstream components, and the sign of Bz, Uz, and En changes across the discontinuity. In the NJZ system the angle through which the tangential component of the magnetic field (Bt) rotates is not arbitrary. It depends explicitly on the angle between the upstream Bt and the j axis. The above jump relations can be used to simplify initial conditions used in theoretical and simulation studies of RD structure. The jump relations can also be used, in principle, to provide an additional method for determining the direction of the RD surface normal vector from spacecraft data.
Modulational and decay instabilities of Alfvén waves: Effects of streaming He++
Authors: Hollweg Joseph V. , Esser R. , Jayanti V.
Published: February 2008

We consider parametric instabilities of a circularly polarized Alfvén wave propagating along the background magnetic field. The perturbations too are assumed to propagate along the background field. The new feature of this work is the presence of a second ion species (He++) which drifts relative to the protons. Even though its abundance is small, the He++ modifies the dispersion relation of the “pump” Alfvén wave and introduces a new sound wave (alpha sound) in addition to the usual sound wave carried primarily by the electrons and protons. These features modify the wave couplings leading to instability and introduce new wave couplings which lead to several new instabilities which may be of interest in the solar wind. In particular, we will find instabilities which are close to the He++ gyroresonance. This may provide a means of directly transferring Alfvén wave energy to the alpha particles, if the alphas are able to resonantly extract energy from the unstable waves, without quenching the instability altogether. We will also find instabilities which are close to the alpha particle sound speed. The alpha particles in this case will tend to absorb energy via Landau damping, and this may again represent a mechanism of transferring Alfvén wave energy to the alpha particles, if the Landau damping does not suppress the instability. Finally, we will find new instabilities close to the proton cyclotron resonance, which may serve as a new mechanism for transferring wave energy into the protons. Some similar results have been recently obtained independently by Goldstein (1990).
Optically thin H Lyman alpha production on outer planets: Low‐energy proton Acceleration in parallel electric fields and neutral H atom precipitation from ring current
Authors: Bhardwaj A. , Singhal R. P.
Published: February 2008

A Monte Carlo model has been constructed to describe the energization and energy degradation of low‐energy protons in an H2 atmosphere in the presence of parallel electric field. The energy spectrum of Ly α collision events is employed to determine the volume emission rate (VER) of H Ly α emissions. Numerical experiments have been performed to study the effect of initial proton energy, electric field, neutral number density, initial pitch angle, and cutoff limit on H Ly α VER. The present study demonstrates that energization of solar EUV‐generated low‐energy protons by parallel electric fields is incapable of producing optically thin Ly α emissions on Uranus through direct collision with H2. However, nonthermal H atoms, produced through acceleration of protons in parallel electric fields, play an important role in enhancing the Ly α intensity through resonant scattering of solar Ly α flux: resulting emissions are consistent with the broadening of the Ly α line observed on Jupiter by IUE. The Monte Carlo model is also applied to the problem of Doppler‐shifted H Ly α emissions from the auroral atmosphere of Jupiter. Energetic neutral H precipitation from the ring current is discussed as a possible source of optically thin H Ly α emissions on outer planets. Estimates are also given for Hα, Hβ, and H2 band emissions resulting from the impact of fast H (of ring current origin) on H2. The input energies and fluxes of precipitating ring current particles are, however, too uncertain to permit definite conclusions.
The outer boundary of the Earth's electron radiation belt: Dependence upon L, energy, and equatorial pitch angle
Authors: Imhof W. L. , Robinson R. M. , Nightingale R. W. , Gaines E. E. , Vondrak R. R.
Published: February 2008

From near‐equatorial pitch angle measurements made with fine angular resolution on the CRRES and SCATHA satellites an investigation has been made of the location of the nightside outer boundary of the Earth's electron radiation belt and its dependence upon energy and mirror point altitude. The energy versus L dependence of the position of the trapping boundary is important for comparisons with models of the mechanisms to explain the loss of trapping at the outer edge of the radiation belt. Often the satellites passed through the trapping boundary very slowly, permitting detailed fine‐scale measurements. With fine spatial resolution the cutoff L values were frequently found to be almost independent of energy for E ≤ 300 keV and were nearly independent of equatorial pitch angle up to 30°. In the radiation belt near the outer edge sometimes the flux dropped to only intermediate values or to background level just for short times or equivalently narrow L intervals, suggesting time variations or the irregular onset of the mechanism(s) responsible for the trapping boundary.
Model of superthermal ions in the Venus ionosphere
Authors: Kramer Leonard , Cloutier Paul A. , Taylor Harry A.
Published: February 2008

A model is presented that simulates the behavior of superthermal ions previously reported in the dayside ionosphere of Venus. The model considers effects of E×B and gradient drifts, charge exchange and collisions with the ambient neutral atmosphere and the possible effects of a wave‐particle (anomalous) scattering process. Results indicate that scattering processes are required if superthermal ions are the explanation for the observed “missing pressure” component in the day‐side Venus ionosphere. The scattering scale length required to match the “missing pressure” distribution is similar to the scale length previously predicted for growth of a lower hybrid beam instability.
Middle and upper atmosphere radar observations of ionospheric horizontal gradients produced by gravity waves
Authors: Fukao S. , Yamamoto Y. , Oliver W. L. , Takami T. , Yamanaka M. D. , Yamamoto M. , Nakamura T. , Tsuda T.
Published: February 2008

By observing simultaneously in multiple directions with the middle and upper atmosphere phased‐array radar, we have been able to measure the horizontal structure produced by waves in the ionospheric density. We find that our data are most clearly ordered on a contour plot of the horizontal gradient of the electron density versus time and altitude. These plots show a periodic oscillation of the gradient with a downward phase progression below 400 km altitude but very nearly vertical wave fronts above that altitude. A simulation of this effect with a simple wave formulation provides a strikingly good comparison with the measurements.
Total photoabsorption cross section of molecular nitrogen near 83.4 nm
Authors: Morgan Harry D. , Seyoum Hailemichael M. , Fortna J. D. E. , Humm David C. , Asfaw Araya , Cleary David D.
Published: February 2008

The photoabsorption cross section of molecular nitrogen near 83.4 nm has been measured at room temperature. The cross sections at the wavelengths of the 2s2p4 4P5/2, 4P3/2, and 4P1/2 (all ‐ 2s²2p³ 4So3/2) O+ emission lines are measured to be 10.1 ± 1.6 Mb, 0.29 ± 0.02 Mb, and 0.049 ± 0.008 Mb respectively. These measurements were made with an O+ emission discharge source, and repeated using a synchrotron radiation source. Both measurements were made with the 0.008‐nm resolution 6.65‐m spectrometer at the SURF II storage ring of the National Institute for Standards and Technology.
The structure of mass‐loading shocks
Authors: Zank G. P. , Kh. Khabibrakhmanov I. , Story T.
Published: February 2008

A new two‐fluid model which describes mass loading in the solar wind (e.g., the interaction of the solar wind with a cometary coma or the local interstellar medium) is presented. The self‐consistent back‐reaction of the mass‐loaded ions is included through their effective scattering in low‐frequency MHD turbulence and the invocation of a diffusive approximation. Such an approximation has the advantage of introducing self‐consistent dissipation coefficients into the governing equations, thereby facilitating the investigation of the internal structure of shocks in mass‐loading environments. To illustrate the utility of the new model, we consider the structure of cometary shocks in the hypersonic one‐dimensional limit, finding that the incoming solar wind is slowed by both mass loading and the development of a large cometary ion pressure gradient. The shock is broadened and smoothed by the cometary ions with a thickness of the order of the cometary ion diffusion scale.
Polar arcs: New results from Viking UV images
Authors: Austin J. B. , Murphree J. S. , Woch J.
Published: February 2008

The magnetic field topology associated with high‐latitude polar arcs has been an interesting problem since the advent of satellite auroral imagery. These large‐scale polar arcs suggest that the topology of the magnetosphere is modified by either bifurcation of closed field lines into the magnetotail lobe or else expansion of closed field lines up to quite near the magnetic pole. The former view recognizes the generally low level of emission of both sides of such high‐latitude features. Recently, new insight has been gained by examining Viking UV images and in situ plasma measurements for cases where polar arcs could be quantitatively analyzed. A statistical analysis of the UV emissions in the vicinity of polar arcs has indicated that the regions on opposite sides of the arcs are consistently asymmetric, not occasionally asymmetric as previously reported in the literature. Simultaneous particle observations confirm the presence of precipitating electrons throughout the region of enhanced emission which have characteristics consistent with a closed field line source for the particles. Further, our results show for the first time that the extent of emission associated with these large‐scale polar arcs extends hundreds of kilometers poleward (i.e., in the direction of the presumed polar cap).
The role of nonlinear beating currents in the theory of parametric instabilities
Authors: Kuo S. P. , Lee M. C. , Fejer J. A.
Published: February 2008

It is shown that there are two contributions to the nonlinear beating current density arising from the interaction of a high‐frequency pump wave with a Langmuir wave. One of these has its origin in the product of the charge density of the Langmuir wave with the velocity vector of an electron due to the uniform high‐frequency pump field. The other one has not been given in explicit form before; its direction is that of the wave vector of the Langmuir wave for the case of an unmagnetized plasma and a dipole pump. If the wave vector of the Langmuir wave is parallel to the pump electric field and if the frequency ω2 of the Langmuir wave is only slightly smaller than the pump frequency ω1, then the two contributions almost cancel each other. However, in the more complicated case of collisional and magnetized plasmas the wave vector of the Langmuir wave is generally not parallel to the pump electric field. The current result makes it plausible that the two contributions to the nonlinear beating current may not cancel each other even for ω2 = ω1.