bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of anomalous underground electric field variations associated with a volcanic eruption
Authors: Fujinawa Yukio , Kumagai Teiji , Takahashi Kozo
Published: February 2008

Continuous measurements of vertical subsurface geoelectric fields in the frequency range dc – 0.6 Hz and 1 – 3 kHz (VLF) are reported. Anomalous field variations of the ULF band are observed a few days prior to, and a month after, a minor volcanic eruption of Mt. Mihara. The variations displayed essentially distorted pulses, rising sharply initially, gradually decreasing, and then suddenly falling off. The electric field variations also seem to extend to the VLF band. It is felt that the anomalous electric field variations were caused by electrokinetic phenomena induced by variations of hydrothermal circulation around the crater, or by variations of crack density, due to volcanic activity.
Winter atmospheric circulation in the Arctic Basin and possible relationships to the great salinity anomaly in the northern North Atlantic
Authors: Serreze Mark C. , Maslanik James A. , Barry Roger G. , Demaria Ted L.
Published: February 2008

Variations in synoptic activity are assessed in terms of potential influences on ice transport and indicators of shifts in the Arctic atmospheric circulation related to the “Great Salinity Anomaly” of the northern North Atlantic. Winter pressure anomalies suggest an increased contribution to the Fram Strait sea ice flux by multiyear ice from along the northern coasts of Greenland and Ellesmere Island. This may be related to a teleconnection involving changes in anticyclonicity over the central Arctic Ocean and Canadian Arctic Archipelago.
Stationary MHD waves generated by a source in a moving plasma
Authors: McKenzie J. F.
Published: February 2008

The inductive interaction between a conducting body and a magnetized plasma in relative motion is formulated in terms of a source driving stationary magnetohydrodynamic waves in the frame of the body. Of particular interest is the existence of analytic solutions which describe the stationary wave pattern associated with a point source current. Such solutions are significant because not only do they, in principle, facilitate the construction of more general solutions by serving as Green's functions but also they provide an asymptotic description of the far field of the stationary wave pattern generated from each point of a source of finite size. The total pressure perturbation exhibits a Laplacian type behavior in the case of “subsonic” flow and a Mach cone like disturbance for “supersonic” flow. On the other hand, the associated electric potential response is mixed in nature in that it consists of compressive magnetoacoustic perturbations as well as the well‐known Alfvén wings corresponding to the shear Alfvén mode. In the subsonic case it is shown that although there is a strong disturbance in the potential on the Alfvén wings it is not narrowly confined to these wings by virtue of the two‐dimensional dipolelike contributions from the fast magnetoacoustic mode. In supersonic flow the potential disturbance is swept downstream and confined to the fast Mach cone, on which it is singular and within which lies the other singularity on the Alfvén wings.
Dusty plasmas in the vicinity of a large dielectric object in space
Authors: Murphy D. L. , Chiu Y. T.
Published: February 2008

The mutual interaction of plasma and dust is an important feature of the extraterrestrial environment. We investigate modifications of this interaction in the presence of a large material body. In our study we assume the dust density to be low, with a random, Maxwellian velocity distribution and a dynamically determined dust density distribution. We have also assumed spherical symmetry to simplify the problem and find that the magnitudes of both the charge on the body and its potential are reduced by the presence of dust. We also find a significant reduction of the dust density near the body due to electrostatic repulsion. Both the reduced magnitude of charge on the body and the repulsion of dust are sensitive to the mean dust velocity; more slowly moving dust is more effectively repelled and also results in a greater magnitude of charge on the body. In addition, some effects depend strongly on the size of the body with respect to the Debye length. The dust density as a function of radial distance from the body approaches its asymptotic value most slowly when the size of the body is of the order of 1–10 Debye lengths.
Viking 1 electron observations at Mars
Authors: Johnson Francis S. , Hanson William B.
Published: February 2008

The Viking retarding potential analyzer (RPA) flown to Mars in 1976 had negative retarding potential sweeps in addition to the positive ion sweeps. An analysis of the electron mode sweeps made in Viking 1 above the ionosphere is presented here. A spectrum covering the range 0 to −78 V was recorded in 1 s, and electron observations were made at intervals of 4 or 8 s. The derived electron concentrations and temperatures in the solar wind are lower than the generally accepted values, but the maximum electron pressures derived above the ionosphere are about as high as can be expected for generally accepted solar wind values. Except in the highly variable or turbulent region from about 2500 km down to 500 km, the scans show a high degree of consistency from scan to scan. The initial concentrations near 15,000 km were slightly less than 1 cm−3 characterized by a temperature of about 40,000 K, plus what is interpreted as a backstreaming component from the planetary shock of about 0.1 cm−3 at about 250,000 K. The shock appeared to be quasi‐parallel, as there was no evidence of a sharp transition anywhere near the expected shock position; it appears that the shock transition region extended from about 3500 down to 850 km. The hottest region was near 850 km, where there is clear evidence of an electron component too hot for the RPA to measure; it produced abundant secondary emission and is assumed to be characterized by energies of about 200 eV so as to be efficient in the production of secondaries. The electron concentrations there averaged about 30 cm−3.
Wave mode identification of electrostatic noise observed with ISEE 3 in the deep tail boundary layer
Authors: Tsutsui M. , Strangeway R. J. , Tsurutani B. T. , Matsumoto H. , Phillips J. L. , Ashour‐Abdalla M.
Published: February 2008

Characteristics of VLF electrostatic noise observed with ISEE 3 in the low‐latitude boundary layer of distant geomagnetic tail (∼210Re) have been examined in detail. Gray tone dynamic spectra with high time resolution of 16 frequency channels of electric field exhibit successive spiky bursts with duration less than the wave sampling period (0.5 s) and with a wide frequency extent. A set of noise parameters such as peak amplitude, its frequency, and bandwidth has been determined from a Gaussian distribution function derived for each spiky noise spectrum by using a least squares fitting routine. In order to find a possible energy source for the noise generation, available correlations between the noise parameters and plasma electron parameters have been examined for a 2‐hour data interval when ISEE 3 was in the low‐latitude boundary layer. The analysis has been conducted on the basis of a hypothesis that the wide frequency extent of the noise spectra is composed of Doppler effects of waves propagating nearly omnidirectionally within the plasma rest frame, which is moving with the electron bulk speed. On the basis of the hypothesis the wavelength of the observed waves has been determined from the width of the frequency extent and the measured electron bulk speed. Although no obvious free energy source has been found from the analysis, the obtained dispersion relation for the electrostatic noise has demonstrated the validity of the hypothesis. The obtained dispersion relation is consistent with theoretical ion acoustic dispersions for the measured electron parameters. The result shows that the wavelength ranges from 2 to 8 times the plasma Debye length. The dispersion also indicates that the ion temperature is in a range from 1 to 4 times the electron temperature.
Forced magnetic reconnection in a plasma sheet with localized resistivity profile excited by lower hybrid drift type instability
Authors: Hoshino M.
Published: February 2008

A forced magnetic reconnection process with a temporal evolution of resistivity is studied for a plasma sheet with a nonuniform resistivity profile based on the nonlocal mode structure of the lower hybrid drift type instability. The growth rate of the mode found is almost independent of the resistivity at the neutral sheet, but depends on the resistivity of the region of maximum density gradient away from the neutral sheet. We study this by using both a nonlinear numerical MHD simulation and a linear theory. The mode may be relevant to the prevalent theoretical concept of MHD reconnection and the localized anomalous resistivity profile based on the lower hybrid drift instability.
Charge‐coupled device spectral images of spatially resolved regions of Jupiter in the 6190‐ and 8900‐Å methane and 6450‐Å ammonia bands during the 1989 Opposition
Authors: Moreno F. , Molina A. , Lara L. M.
Published: February 2008

High‐quality charge‐coupled device spectroscopic observations at different regions of Jupiter in the spectral regions λλ 6000–6600 and 8700–9200 Å at a resolution of 1.2 Å have been obtained. These spectra allowed us to obtain the center‐to‐limb curves of the equivalent widths of the CH4 6190‐Å band, the NH3 6450‐Å band, and the band depth of the 8900‐Å band, as well as their latitudinal distribution. Some asymmetries in the center‐to‐limb distribution of the equivalent widths of the CH4 6190‐Å and NH3 6450‐Å bands were detected, as well as a very prominent decrease toward the planetary limbs for the ammonia band. The latitudinal variations of the CH4 6190‐Å and NH3 6450‐Å equivalent widths were strongly correlated for fractional radii less than 0.4, with a maximum of absorption at the North Equatorial Belt. The South Equatorial Belt, undetectable at visible and near‐infrared continuum wavelengths, was discernable at λ = 8900 Å. Model computations for the Equatorial Region assuming three aerosol layer models gave some constraints on tropospheric atmospheric parameters during the 1989 Jupiter opposition date.
The O++ CO2 reaction: New results and atmospheric implications
Authors: Hunton D. E. , Viggiano A. A. , Morris R. A. , Paulson J. F. , Smith D. , Adams N. G.
Published: February 2008

The vibrational distribution of the O2+ product ion in the reaction of O+ with CO2 has been measured in a selected ion flow tube using the monitor ion technique. At 300 K the measured vibrational distribution is 45% of the ions in υ = 1, 32% in υ ≥ 2 and 23% in υ = 0. However, it is possible that vibrational quenching by the monitor gas may affect the distribution, and the nascent population may be entirely in υ > 0. The branching ratio between the O2++CO and the CO2++O product channels has also been measured as a function of ion drift velocity. The implications of these experiments for terrestrial releases of CO2 into the ionosphere and for the ionospheres of Mars and Venus are considered.
Energy dissipation in structured electrodynamic environments
Authors: Heelis R. A. , Vickrey J. F.
Published: February 2008

The coupling of electromagnetic energy into the ionosphere and thermosphere is an essential consideration for understanding the thermal structure and dynamics of the neutral and charged particles at high latitudes. Often the dissipated electromagnetic energy exceeds that deposited by precipitating energetic particles at high latitudes. It is usually assumed that the profile of the ion Pedersen conductivity determines the altitude dependence of the energy dissipation rate. Herein we point out the strong altitude dependence of the energy dissipation rate on the spatial scale size of the imposed electric field. To illustrate the importance of such considerations, we show examples of the ubiquity of electric field structure in the high‐latitude ionosphere; this is particularly prominent when the interplanetary magnetic field has a northward component. We then show quantitatively how the existence of electric field structure with scale sizes of 10 km or less strongly impacts both the altitude extent over which the electromagnetic energy is dissipated and its partitioning between current systems perpendicular and parallel to the magnetic field.
On the nonlinear response of airglow to linear gravity waves
Authors: Isler J. R. , Tuan T. F. , Picard R. H. , Makhlouf U.
Published: February 2008

The question of whether a linear gravity wave will give rise to nonlinear effects in ground‐based airglow observations is important for the proper interpretation of gravity wave dynamics. In this paper we obtain a closed form solution for the integrated airglow response to a linear gravity wave, containing all the higher‐order nonlinear response terms. A comparison is made to the linear response, and the higher orders are seen to be significant. In addition, the wave‐induced airglow intensity fluctuations are shown to be much greater than the corresponding major species density fluctuations.
On ultralow frequency waves in the lobes of the Earth's magnetotail
Authors: Chen Sheng‐Hsien , Kivelson Margaret G.
Published: February 2008

Motivated by Thermal Catastrophe Model (TCM) of Goertz and Smith (1989), we turn attention to ultralow frequency (ULF) waves in the lobes, where waves are usually of relatively low amplitude. Remarkably little attention has been paid to characterizing the lobe spectra and the relation of lobe wave power to geophysical processes. In this paper we examine ULF waves (0.26 to 5 mHz or 3.2‐ to 64‐min periods) in the Earth's magneto tail lobe region (−10 ≲ XGSM ≲ −23 RE) using data from the ISEE 2 flux gate magnetometer. The studies used all the tail data from the year 1978. Data from the ISEE 2 Fast Plasma Experiment (FPE) enabled us to exclude from the analysis all intervals not in the tail lobe region. The relation between the substorm activity, characterized by the AE index, and the wave power is also described. We find that the wave amplitudes at periods of ∼3 to 16 min (∼1 to 5 mHz) are below 0.8 nT for the transverse magnetic field components and below 0.6 nT for the compressional components ∼90% of the time. The amplitudes of lower frequency waves are somewhat larger. For waves with periods 16 to 64 min (∼0.26 to 1 mHz) the amplitudes are below 1 nT for both the compressional and transverse components ∼90% of the time. The wave amplitudes and the AE index, which characterizes substorm activity, are weakly correlated, the correlation coefficients being ∼0.55 and ∼0.35, for the compressional and transverse components respectively. The correlation coefficients between the transverse wave amplitudes and the AE index change little when AE is taken for 1 hour prior to or subsequent to the hour in which the wave amplitude is measured. For the compressional component the correlation coefficient remains unchanged when AE is measured for the subsequent or simultaneous hour but decreases when AE is taken for the previous hour. A primarily statistical study of frequency‐weighted power of ULF waves at frequencies of 1 to 5 mHz is provided. The probability is 50% (5%) of observing frequency‐weighted power for the frequency range 1 to 5 mHz in excess of 10−4 nT² Hz (10−3 nT² Hz). Three extended intervals in the lobe were selected for case studies. The largest wave amplitude we observed was about 5 nT which occurred when the AE index was higher than 1500 nT. The period corresponding to the largest wave amplitudes was ∼35 min (∼0.48 mHz). Power spectra from three different substorm intervals are provided as examples of wave activity in the lobes. The power spectral density of all components falls off as ƒ−2 or faster. Statistical study by itself does not provide an appropriate test of the requirement on wave power for the TCM. Additional case studies using ground magnetometers to relate wave power to substorm phases will be needed to test the assumptions of the TCM.
Representations of currents and magnetic fields in anisotropic magnetohydrostatic plasma: 2. General theory and examples
Authors: Heinemann M. , Pontius D. H.
Published: February 2008

We develop a general treatment of field‐aligned currents in quasi‐static adiabatic plasma. The formalism is an extension of an an earlier analysis (Heinemann, 1990) to include electric and gravitational fields. The assumption that the particle motions are adiabatic along the magnetic field leads to an expression for the total current density that is a generalization of expressions given by Grad (1964) and Vasyliunas (1970). The current density is a vector function of the gradients of the field line constants characterizing the plasma and the gradients of field line integrals of the partial derivatives of the parallel pressure with respect to the constants. The use of the expression as the current source in Ampere's law leads to an equation governing the equilibrium of the system of plasma and magnetic field. Examples based on bi‐Maxwellian distribution functions suggest that the effects of thermal anisotropy can be about as large as the currents due to isotropic plasma and that the effects of parallel electric fields are of the same order of magnitude.
On the limit of field‐aligned current intensity in the polar magnetosphere
Authors: Cole Keith D.
Published: February 2008

Field‐aligned current (FAC) is here defined by 4πj = αB, where α is constant along a magnetic field line. The upper limit value of α in the polar magnetosphere, possible source regions of the strongest FAC and the relationship of them to some auroral and ionospheric irregularity cross‐field scale sizes are discussed. Cross‐field dimensions of the strongest FAC are related to the gyroradii of source particles (O+, He++, He+, H+, e) in the current‐generating region. It is suggested that experimental determination, and mapping of the values of α, may assist with the search for the generators of such currents in near‐Earth space including in the nearby solar wind. We associate the upper limit of α with the breakup of FAC systems.
Quasi‐linear evolution of ULF waves excited by cometary ion pickup
Authors: Lee Martin A. , Gary S. Peter
Published: February 2008

An analytical model is presented which describes the quasi‐linear evolution of both ultralow‐frequency (ULF) magnetic fluctuations and cometary ion flux in the solar wind due to continuous ion pickup. The derived solution yields both the initial exponential growth of the total wave intensity and the time‐asymptotic growth proportional to time obtained in numerical simulations using a one‐dimensional hybrid code with a constant rate of cometary ion injection (Gary et al., 1988, 1989). For pickup rates appropriate to cometary environments, the derived fluctuation intensity and pickup ion flux are in general accord with ULF wave observations and water group ion pitch angle distributions upstream of the bow waves at both comet Giacobini‐Zinner and comet Halley.
Riometer observations of whistler‐induced electron precipitation at Sanae (L = 4.0)
Authors: Serfontein D. E. , Stoker P. H.
Published: February 2008

Strong and well‐defined single‐hop whistlers have been observed on L = 4.0 at the mid‐latitude subauroral station Sanae, Antarctica, between 1000 and 1100 UT (or LT) on June 16, 1986, when Kp ∼ 2− amid a magnetically quiet period extending over several weeks. These single‐hop whistlers were followed often by diffused echo whistlers 6 ± 1 s later. A significant correlation has been found between the arrival times of the single‐hop whistlers and 30‐MHz riometer absorption events, using point statistics. A superposed epoch analysis of the cross‐correlated events, using the instants of maxima of the integrated VLF signals as zeroth time, yields a maximum absorption of 30‐MHz cosmic radio noise of ∼0.04 dB with a delay of 3±2 s and decay time to half maximum of ∼10 s. This correlation is evaluated by calculations of the energy spectrum of electrons precipitated into the atmosphere by gyroresonance interaction of whistler mode waves with radiation belt electrons, using the results of the computer‐simulated test particle program of Chang and Inan (1985). The temporal variation of the electron density in the upper atmosphere due to the whistler‐induced electron precipitation spectrum is derived as a function of altitude, taking into account electron‐ion production and recombination rates. The resulting enhanced riometer absorption maximizes at ∼7×10−3 dB, based on the average power spectral density of the input wave packet and the average plasmaspheric density distribution used by Chang and Inan (1985). Variations in the magnitude of the equatorial wave magnetic field intensities, and approximations made in calculating the riometer absorptions, impose an order of magnitude uncertainty on the calculated value and hence consistency with the observed 0.04‐dB‐averaged absorption maximum, which is just above the sensitivity limit of a riometer with a wide‐angle antenna. Furthermore, these calculations depend on the trapped electron energy spectrum and pitch angle distribution accepted for the calculations and on features of whistlers related to convection electric field variations, which could not be identified from available recordings. In the evaluation of the observed correlations between riometer absorptions and whistlers it must also be taken into account that the VLF receiver, having a wide viewing window, is receiving VLF emissions via many ducts. Hence the electron precipitation region associated with a particular VLF signal may not necessarily coincide with the field of view of the wide‐angle riometer antenna.
The boundary value problem of magnetotail equilibrium
Authors: Birn Joachim
Published: February 2008

The equilibrium problem for the Earth's magnetotail is discussed under the assumption that the boundary of the tail can be prescribed or derived from the force balance with the solar wind. A general solution of this problem is presented for the two‐dimensional case, where the dependence on the y coordinate and the presence of By are neglected. These solutions are further generalized to include the y dependence (but no By) and an open magnetopause. In this formulation, a solution can be obtained by integration when the magnetopause boundary a(x, y), the total pressure function
and the magnetic flux distribution Ab(x, y) at the magnetopause are prescribed. Certain restrictions, however, may limit the free choice of these functions to yield physically reasonable, real solutions. When the interaction with the solar wind is included, the boundary location can no longer be chosen freely but follows from the force balance of the magnetotail with the solar wind. For a simplified description of this force balance a differential equation for the boundary location is derived, which generalizes an earlier result by Coroniti and Kennel (1972). It is shown that solutions of this differential equation are bounded by a maximum tail width if the plasma sheet thickness is limited. Several explicit solutions are presented, illustrating cases with and without tail flaring in the z direction, and including the restrictions of the force balance with the solar wind and of the conservation laws of adiabatic convection in a steady configuration.
The lower subsidiary diffuse plasma resonances and the classification of radio emissions below the plasma frequency
Authors: Osherovich Vladimir A. , Benson Robert F.
Published: February 2008

The diffuse ionospheric resonances Dn, stimulated by topside sounders, have been studied for over 2 decades. These resonances are observed below the plasma frequency ƒN between the harmonics of the electron gyrofrequency ƒH. The Dn resonances are often accompanied by upper and lower subsidiary branches. The present paper concentrates on the classification of diffuse resonances, one motivation being the possible application to the interpretation of naturally occurring radio emissions in the magnetosphere. Osherovich has shown that the Dn resonances are characterized by a nonequally spaced spectrum of frequencies ƒDn = ƒDsn1/2 (n = 1, 2, 3, and 4), where ƒDs = 0.95(ƒNƒH)1/2, and that the upper subsidiary resonances Dn+ can be considered as upper hybrid resonances of Dn with frequencies given by ƒDn+ = (ƒDn² + ƒH²)1/2. This classification is here extended to include the lower subsidiary resonances Dn− (n = 1, 2, 3, and 4), and it is shown that their frequencies are related to ƒDn and ƒH by the expression ƒDn− = (ƒDn² − ƒH²)1/2. This result is based on a combination of previously published data and newly scaled ionograms from the Alouette 2 and ISIS 1 topside sounder experiments.
Filament disappearances and associated shocks of May 1979
Authors: Cane H. V. , Richardson I. G. , Harvey K.
Published: February 2008

In late May 1979 a shock was seen at each of two locations separated by about 90° in heliolongitude and at about the same time. A shock was detected near the Earth, and a shock was detected at Helios 1 when it was off the Sun's west limb. Some authors have considered that these shocks were part of the same event, which originated with a filament disappearance near 65°W. We discuss the observations which show that a large complex filament, extending from 20°W to 70°W, disappeared in several places, probably generating more than one shock. The shock detected near Earth was probably associated with a filament section which disappeared near the central meridian. If this is the case, then the interplanetary scintillation observations of this time period can be explained without invoking highly anisotropic shock propagation.
DMSP F7 observations of a substorm field‐aligned current
Authors: Lopez R. E. , Spence H. E. , Meng C.‐I.
Published: February 2008

In this paper we present observations of a substorm field‐aligned current (FAC) system that DMSP F7 traversed just after 0300 UT on April 25, 1985. Ground magnetometer data show that a major substorm was in progress at that time and that DMSP F7 flew through a region of predominantly upward FAC. The DMSP F7 magnetic field data are consistent with this interpretation. The precipitating particle data suggest that there were three distinct large‐scale FAC systems. In ascending latitude these were a downward current, an upward current, and a paired upward/downward current system. We identify the first current, which was coincident with the diffuse aurora, as region 2. The next (upward) FAC was coincident with a spatially unstructured region of energetic (∼12 keV) electron precipitation. This was the substorm‐associated FAC that made up part of the current wedge. The upward/downward current pair was coincident with a region of highly structured precipitation. We suggest that these currents may have been the duskside region 1 and, poleward of that, the extension of the dawnside region 1. The particle data show that the upward substorm current lay well equatorward of the boundary between open and closed field lines. In fact, using a model field, the equatorward boundary of the substorm FAC maps to the neutral sheet at 6.9 RE. While one should be cautious in stressing results obtained by mapping model field lines, our result is consistent with scenarios for substorms which postulate a disruption and diversion of the near‐Earth cross‐tail current.
Scattering of electromagnetic waves by a distribution of charged dust particles in space plasmas
Authors: de Angelis U. , Forlani A. , Tsytovich V. N. , Bingham R.
Published: February 2008

We analyze the scattering of electromagnetic waves by a distribution of charged dust particles, a common component of many space plasmas. For the case when the dust grains cannot be considered as independent scatterers (that is, when the intergrain separation is of the order or less than the Debye length), the scattering cross section is calculated for a statistical distribution of dust particles correlated via their electrostatic coupling. The nonneutrality of the plasma is taken into account, due to the charging of the grains by plasma currents, secondary emission, and photoelectrons (the Sun being the source of photons). The grain charges and corresponding scattering cross section are calculated for values of the plasma dust parameters relevant to many space plasma environments in the solar system, showing that enhancement with respect to scattering by free electrons is possible.
HF sounding of the auroral magnetosphere
Authors: Gurevich A. V. , Babichenko A. M. , Karashtin A. N. , Rapoport V. O.
Published: February 2008

First results of HF radar investigations of the auroral magnetosphere are presented. It is shown that the spectral characteristics of the signal correspond to scattering off ion acoustic turbulence in the field‐aligned current region at altitudes about 4000 km.
Magnetospheric waves and the atmosphere‐ionosphere layer
Authors: Hameiri E. , Kivelson M. G.
Published: February 2008

Hydromagnetic waves in the magnetosphere are affected by the presence of the adjacent conducting ionosphere and insulating atmosphere. Boundary conditions at the ionosphere are derived under the assumption that the magnetospheric wavelength is much larger than the height of the ionosphere or atmosphere. Unlike earlier works, our boundary conditions are expressed in a form that applies both to pure Alfven or magnetosonic waves and to fully coupled waves. The use of these new conditions is demonstrated by giving an exact solution, within the box model, to a special case of excitation of magnetospheric micropulsations. It is shown that, consistent with well‐known observations, the amplitude of resonantly excited micropulsations is much larger in the dayside magnetosphere than in the nightside because of the larger dayside ionospheric conductivity. We argue that the efficiency of wave damping by the low‐conductivity nightside ionosphere is not a convincing explanation of this phenomenon. Our alternative interpretation is that the Alfven resonances are not efficiently excited where the ionospheric conductivity is low because boundary conditions impose different field‐aligned structure on the compressional and transverse parts of a signal. The differences diminish as the conductivity increases, allowing resonant coupling to become efficient.
Quasi‐linear pitch angle and energy diffusion of pickup ions near comet Halley
Authors: Huddleston D. E. , Johnstone A. D. , Coates A. J. , Neubauer F. M.
Published: February 2008

The process of ion pickup in the environment of Halley's comet is studied in order to see if velocity diffusion driven by the observed level of turbulence can explain the observed development of the implanted ion distribution. The theoretical description used is based on a quasi‐linear approach and considers the implantation and transport of cometary ions along solar wind flow lines. To make such a study requires some way of extrapolating our measurements on the Giotto trajectory into the upstream region; models for mass loading and turbulence are used. A simplified kinetic equation describing the source, convection, and quasi‐linear velocity diffusion of the heavy cometary ions is solved numerically along flow lines parallel to the Sun‐comet line. Full two‐dimensional (pitch angle and velocity) distributions are obtained at positions along the Giotto trajectory, which are compared with measurements. This study finds that quasi‐linear theory, with our empirical model for the observed turbulence level, produces the right order of pitch angle diffusion.
Ionospheric simulation compared with Dynamics Explorer observations for November 22, 1981
Authors: Sojka J. J. , Bowline M. , Schunk R. W. , Craven J. D. , Frank L. A. , Sharber J. R. , Winningham J. D. , Brace L. H.
Published: February 2008

Dynamics Explorer (DE) 2 electric field and particle data have been used to constrain the inputs of a time‐dependent ionospheric model (TDIM) for a simulation of the ionosphere on November 22, 1981. The simulated densities have then been critically compared with the DE 2 electron density observations. This comparison uncovers a model‐data disagreement in the morning sector trough, generally good agreement of the background density in the polar cap and evening sector trough, and a difficulty in modelling the observed polar F layer patches. From this comparison, the consequences of structure in the electric field and precipitation inputs can be seen. This is further highlighted during a substorm period for which DE 1 auroral images were available. Using these images, a revised dynamic particle precipitation pattern was used in the ionospheric model; the resulting densities were different from the original simulation. With this revised dynamic precipitation model, improved density agreement is obtained in the auroral/polar regions where the plasma convection is not stagnant. However, the dynamic study also reveals a difficulty of matching dynamic auroral patterns with static empirical convection patterns. In this case, the matching of the models produced intense auroral precipitation in a stagnation region, which, in turn, led to exceedingly large TDIM densities.
Comparison of Doppler scintillation and in situ spacecraft plasma measurements of interplanetary disturbances
Authors: Woo Richard , Schwenn Rainer
Published: February 2008

Since 1979 the radio path of the Pioneer Venus orbiter (PVO) spacecraft has spent considerable time probing the near‐Sun solar wind off the limbs of the Sun. On occasion, Helios 1 has also been in position above the Sun's limb at the same solar longitude to simultaneously observe in situ the solar wind plasma. These fortuitous circumstances, along with the availability of near‐continuous measurements, have made it possible for the first time to carry out detailed comparisons between Doppler scintillation and in situ plasma measurements and to improve our understanding of Doppler scintillation transients. During a combined observing period of nearly 3 months in 1981–1982 near solar maximum, 22 transients were observed by PVO and 23 shocks were observed by Helios 1. On the basis of a comparison of mass flux density and rms Doppler scintillations, we find that at least 84% of the transients are shocks, while at least 90% of the shocks are transients. Hence there is a near one‐to‐one correspondence between transients and interplanetary shocks. Although the temporal profiles of Doppler scintillation and mass flux density are similar, the magnitudes of the Doppler scintillation transients may not simply reflect those of mass flux density. Only one pronounced solar wind event that was observed in the mass flux density measurements showed no signature in the scintillation data; field and particle measurements by Helios 1 suggest that it is a noncompressive density enhancement and/or a magnetic cloud. One of the scintillation transients that is not a fast‐mode shock appears to correspond to a slow shock. However, when scintillations alone are available, slow shocks may be difficult to identify. Shock speeds based on transit times between the PVO radio scintillation path and the Helios 1 spacecraft are consistent with those from the in situ plasma measurements and indicate shock deceleration in essentially all cases. A significant consequence of this investigation is that Doppler scintillation measurements can now be used by themselves to detect and locate interplanetary shocks near the Sun with a relatively high degree of confidence and hence can be used to conduct useful correlative studies in the future with other solar and interplanetary observations.
Combined optical and radar wind measurements in the F region over Millstone Hill
Authors: Sipler D. P. , Hagan M. E. , Zipf M. E. , Biondi M. A.
Published: February 2008

A Fabry‐Perot interferometer has been gathering F region wind data at Millstone Hill routinely since mid‐1988. A total of 27 nights of optical data have been collected with simultaneous incoherent scatter radar data. Data for 22 nights (of which 10 were geomagnetically quiet and 12 were unsettled) have been used to evaluate the O+‐O collision frequency. Our data show that the collision frequency based on Dalgarno's adjusted theoretical value (Dalgarno, 1964) that was used at Millstone Hill Observatory before 1988 should be increased by a factor of 1.9±0.15.
Current collection in an anisotropic collisionless plasma
Authors: Li Wei‐Wei
Published: February 2008

A general method is given to derive the current‐potential relations for collectors in anisotropic collisionless plasmas. The Debye length is much larger than the probe dimensions. Orbit‐limited current is assumed. The effects of space charge are ignored. The only effect of magnetic fields is to indicate the direction of the anisotropy; otherwise, it is neglected. The collector is a conductive sphere or an infinite cylinder. Any distribution which is an arbitrary function of the velocity vector can be considered as a superposition of many monoenergetic beams whose current‐potential relations are known. The results for two typical pitch angle distributions are derived and discussed in detail. The general properties of the current‐potential relations are very similar to those for a Maxwellian plasma except for the effective temperature, which varies with the angle between the magnetic field and the charging surface. The conclusions provide insight into the situation for generalized geometries.
Electron energy distribution from intense electron beams in the upper mesosphere and lower thermosphere
Authors: Martinez‐Sanchez Manuel , Cheng Wai , Dvore David , Zahniser Mark S.
Published: February 2008

A model was developed to calculate the electron energy spectrum created by an electron beam in the upper atmosphere. A significant feature of the model is the inclusion of the effects of electron‐electron collisions which are important at high beam intensity when the ratio of the electron to ambient gas density is high. Comparing the calculated results for a 2.6‐kV, 20‐A beam at 110‐km altitude from models with and without the electron‐electron collision term, the electron‐electron collisions have the effect of smoothing out the electron spectrum in the low‐energy region (<10 eV). In particular, the “bite‐out” features of the spectrum induced by the vibrational excitation of N2 and O2 are filled in, resulting in an increase in the calculated production rate of these species compared with model calculations that neglect this effect.
The polarization electric field and its effects in an anisotropic rotating magnetospheric plasma
Authors: Huang T. S. , Birmingham T. J.
Published: February 2008

Spatial variations of density and temperature along a magnetic field line are evaluated for a plasma undergoing adiabatic motion in a rotating magnetosphere. The effects of centrifugal and gravitational forces are accounted for, as is anisotropy in the pitch angle distribution functions of individual species. A polarization electric field is invoked to eliminate the net electric charge density resulting from the aforementioned mass dependent forces and different anisotropies. The position of maximum density in a two‐component, electron‐ion plasma is determined both in the absence and in the presence of the polarization effect and compared. A scale height, generalized to include anisotropies, is derived for the density fall‐off. The polarization electric field is also included in the parallel guiding center equation; equilibrium points are determined and compared in both individual and average senses with the position of density maximum. Finally a transverse (to magnetic field lines) electric component is deduced as a consequence of dissimilar charge neutralization on adjacent field lines. The E × B velocity resultant from such a “fringing” electric field is calculated and compared with the magnitude of other drifts.
Three‐dimensional MHD modeling of magnetotail dynamics for different polytropic indices
Authors: Hesse Michael , Birn Joachim
Published: February 2008

The evolution of the three‐dimensional resistive tearing instability in a magnetotail configuration is analyzed by means of a resistive MHD code for various forms of the energy equation including ohmic and compressional heating. As an introduction we present a complete derivation of the set of MHD equations from a kinetic formalism noting all simplifying assumptions. The following study emphasizes the changes in magnetotail evolution brought about by a variation of the assumed value of the polytropic exponent γ, representing the ratio of specific heats in the energy equation, which closes the set of MHD equations. We find that within the range of 2/3 ≤ γ ≤ 4, a tearing mode develops in all cases. Certain properties, such as the formation and ejection of a plasmoid, associated with the formation of the reversal region of the north‐south component of the magnetic field, and strong earthward and tailward flow fields prevail as qualitative features independent of the value of γ chosen. Quantitatively, however, magnetotail evolution can be quite different for different values of γ. This is found primarily in the evolution of the temperature, where we find heating by a factor of order two with higher temperatures for higher values of γ. This is combined with a density reduction by a similar factor. Both features are consistent with recent observational results from a superposed epoch analysis of 39 substorm events in the near tail (Baumjohann et al., 1991). Ohmic dissipation is found to be the major source of the heating, while adiabatic or nonadiabatic compression may contribute primarily in the closed field region earthward of the reconnection site. Furthermore, quantitative values of typical flow speeds and evolution time scales show some dependence on the polytropic index. These results suggest that the qualitative features of the nonlinear evolution of the resistive tearing mode are not strongly influenced by the energy equation, whereas details can show strong dependence.
Chaotic scattering of pitch angles in the current sheet of the magnetotail
Authors: Burkhart G. R. , Chen J.
Published: February 2008

The process of pitch angle scattering by a current sheet is studied using the modified Harris field model. The relationship between the incoming asymptotic pitch angle αin and the outgoing asymptotic pitch angle αout is studied from first principles by numerically integrating the equation of motion. We give evidence that charged particles undergo chaotic scattering by the current sheet. For fixed αin, it is shown that αout exhibits sensitive dependence on the energy parameter Ĥ in certain energy ranges. For fixed Ĥ in the same energy ranges, αout sensitively depends on αin. For other energy values, αout does not show sensitive dependence on αin for most phase angles. The latter energy values occur at the resonance values given by the previously identified relationship Ĥ1/4 ∼ N + 0.6. Thus αout exhibits no significant sensitivity near these resonance energies. This conclusion applies to a range of αin centered at αin = 0 (i.e., field aligned in the asymptotic region). A distribution of αin is mapped from the asymptotic region to the midplane, and it is found that the resulting particle distribution should have beam structures with well‐collimated pitch angles near each resonance energy value. Implications for the particle distribution functions in the Earth's magnetotail are discussed.
Chaotic Alfvén waves in multispecies plasmas
Authors: Buti B.
Published: February 2008

By means of the Hamiltonian formalism, it is shown that the nonlinear Alfvén waves in a multispecies plasma in the presence of a driver can be chaotic provided the amplitude of the driver exceeds a certain threshold. This threshold is larger in the presence of heavier ions, e.g., oxygen in the case of comets and helium in the case of solar wind, than for plasma with only two species. This clearly shows that heavier ions tend to reduce the chaos.
Finite geometry effects of field‐aligned currents
Authors: Fung Shing F. , Hoffman R. A.
Published: February 2008

Traditionally, field‐aligned current densities calculated from gradients in the perturbation magnetic field measured from low‐altitude spacecraft have been based on the assumption that the current regions are infinite in length. However, many measurements clearly indicate that this assumption is invalid. We have performed model calculations of the magnetic field produced by finite current regions that would be measured by a spacecraft‐borne magnetometer. These calculations serve two purposes: (1) to assess the extent to which an infinite current sheet approximation is applicable in calculating field‐aligned current densities by comparing the current density computed from the model perturbation magnetic field assuming an infinite length current region to the model current density; and (2) to assist in the interpretation of the magnetometer signals when the infinite sheet current approximation is clearly inapplicable. For current regions with various aspect (length to width) ratios, we find that the perturbation magnetic fields within the current region more than one current element width from an end produce a calculated current density with less than a 10% departure from the model current density. Certain nonlinear patterns (e.g., V‐shaped signatures) in the correlation plots of the two horizontal components of the perturbation magnetic field can be explained in terms of the end field effects of field‐aligned current regions. Such signatures can be used to determine the configuration of the satellite track through the current region.
Plasma wave profiles of Earth's bow shock at low Mach numbers: ISEE 3 observations on the far flankTtethered Satellite System
Authors: Greenstadt E. W. , Coroniti F. V. , Moses S. L. , Smith E. J.
Published: February 2008

The Earth's bow shock is weak along its distant flanks where the projected component of solar wind velocity normal to the hyperboloidal surface is only a fraction of the total free stream velocity, severely reducing the local Mach number. We present a survey of selected crossings far downstream from the subsolar shock, delineating the overall plasma wave (pw) behavior of a selected set of nearly perpendicular crossings and another set of limited Mach number but broad geometry; we include their immediate upstream regions. The result is a generalizable pw signature, or signatures, of low Mach number shocks and some likely implications of those signatures for the weak shock's plasma physical processes on the flank. We find the data consistent with the presence of ion beam interactions producing noise ahead of the shock in the ion acoustic frequency range. One subcritical case was found whose pw noise was presumably related to a reflected ion population just as in stronger events. The presence or absence, and the amplitudes, of pw activity are explainable by the presence or absence of a population of upstream ions controlled by the component of interplanetary magnetic field normal to the solar wind flow.
A determination of twisting of the Earth's magnetotail at distances 115‐220 RE: ISEE 3
Authors: Macwan Snehavadan E.
Published: February 2008

Observations from magnetic field and plasma experiments on ISEE 3 are used to identify and to analyze 16 crossings of the distant magnetotail neutral sheet. Four methods of analyses have been employed to measure the extent of the magnetotail twisting, which is found to be much greater than what was previously observed or, the theoretically inferred twist. A twist of more than 90° has frequently been observed on both open and closed magnetic field lines. In addition, the location of the magnetic neutral line of the magnetotail has been found to move over the distance interval of 115 to 220 Earth radii. Some features of these results suggest that a modification of the present solar wind‐geomagnetic field interaction model may be needed.