bluuebunny/crossref_metadata_2025 huggingface dataset:
Compound streams, magnetic clouds, and major geomagnetic storms
Authors: Burlaga L. F. , Behannon K. W. , Klein L. W.
Published: February 2008

Data from ISEE 3, Helios A, and Helios B were used to identify the components of two compound streams and to determine their configurations. (A compound stream is a stream which has formed as a result of the interaction of two or more distinct fast flows.) In one case, ejecta containing a magnetic cloud associated with a disappearing quiescent filament were interacting with a corotating stream. In the second case, ejecta containing a magnetic cloud associated with a 2B flare were overtaking ejecta from a different source. Each of these compound streams produced an unusually large geomagnetic storm, on April 3, 1979, and on April 25, 1979, respectively. The largest geomagnetic storm in the period 1968–1986, which occurred on July 13, 1982, was associated with a compound stream. Thirty geomagnetic storms with Ap > 90 occurred between 1972 and 1983, and there are interplanetary magnetic field and plasma data for 17 of these events. The data suggest that most large geomagnetic storms are associated with compound streams and/or magnetic clouds.
Model calculations of minor ion populations in the plasmasphere
Authors: Chandler M. O. , Ponthieu J. J. , Cravens T. E. , Nagy A. F. , Richards P. G.
Published: February 2008

Recent observations of the density of minor ions at high altitudes in the outer plasmasphere show relative enhancements of O++ in regions of simultaneous O+ enhancements. These regions also exhibit high ion temperatures. Computer simulations of the temperature structure of the plasmasphere under conditions of electron heating in the equatorial region suggest that such heating produces large gradients in both the electron and ion temperature in the ionosphere. These gradients result in an increase in the pressure of the electrons, which increases the polarization field, and of the ions, which results in large plasma scale heights at low altitudes and increased ion densities at high altitudes. The subsequent enhanced flux of O++ from the ionosphere produced by collisional drag of O++ by O+ and the increased polarization field results in a significant increase in the O++ density above the ionosphere. At higher altitudes the O++‐O+ collisions inhibit the upward flow of O++ resulting in a high‐altitude peak in the O++ density. Above this peak, where collisions with O+ begin to become insignificant, the O++ pressure gradient pushes the O++ into the equatorial reservior. Simulations of conditions of moderate flux tube depletion result in an increase in this effect. The N+ is also affected by collisions with O+, but the increase in its density at high altitudes is primarily due to the scale height effect.
Differential cross sections for scattering of 0.5‐, 1.5‐, and 5.0‐keV oxygen atoms by He, N2, and O2
Authors: Schafer D. A. , Newman J. H. , Smith K. A. , Stebbings R. F.
Published: February 2008

This paper reports measurements of absolute scattering cross sections, differential in angle, for collisions of ground state oxygen atoms with He, N2, and O2. Data are presented for scattering of 0.5‐, 1.5‐, and 5.0‐keV oxygen atom projectiles in the range of laboratory frame angles between 0.06° and 5°. These measurements provide information relevant to calculations of the aeronomic consequences of O+ precipitation in the earth's upper atmosphere.
Cosmic ray variations and magnetic field fluctuations in the outer heliosphere
Authors: Perko J. S. , Burlaga L. F.
Published: February 2008

We have formally confirmed that galactic cosmic ray intensity variations measured by Voyager 2 during recovery from solar maximum are caused by traveling compressions and rarefactions in the mean interplanetary magnetic field. We used Voyager's magnetic field data as input to a time‐independent, spherically symmetric, cosmic ray transport equation in the force field approximation. The solutions closely followed the count rate of cosmic rays greater than 75 MeV/nucleon over 4 years, during the recovery phase of the 11‐year solar‐driven cosmic ray cycle. This strongly supports prior theoretical assertions that turbulent interaction regions traveling with the solar wind are the major cause of the solar cycle variation of galactic cosmic rays in the ecliptic region.
Low‐energy protons associated with interplanetary shocks as a coherent population
Authors: Sanahuja B. , Domingo V.
Published: February 2008

We investigate the flow pattern of low‐energy protons (35–1600 keV) associated with interplanetary shocks observed by ISEE‐3 between August 1978 and April 1980. The analysis of the shape of the distribution function in the solar wind frame and its temporal evolution indicates that the low‐energy protons can behave as a coherent, independent population of particles in the solar wind. Ahead of the shock this population propagates along the magnetic field in the same direction as the solar wind flow, while after the passage of the perturbed region associated with the shock, it propagates in the opposite sense. The behavior of the flow pattern of this population through the shock front is discussed for the 17 largest events observed in this period.
On the global mean structure of the thermosphere
Authors: Roble R. G. , Ridley E. C. , Dickinson R. E.
Published: February 2008

A self‐consistent global average model of the coupled thermosphere and ionosphere has been developed and used to examine the global mean structure of these regions for solar minimum and maximum conditions. The model uses a parameterized version of the solar EUV and UV flux measured from the Atmospheric Explorer satellite to calculate photoionization rates of the major ion species and photo‐dissociation rates for O2 and N2. The ionospheric calculation solves for the vertical distributions of O+, including ambipolar diffusion, and of NO+, O2+, N2+, and N+, assuming photochemical equilibrium with respect to O+. It also solves the electron and ion energy equations as well as the coupled minor neutral constituent diffusion equations for N(²D), N(4S), and NO. The neutral gas energy equation is solved considering heating from photoelectrons, O2 absorption in the Schumann‐Runge continuum and bands, excess energy from exothermic ion‐neutral and neutral‐neutral chemical reactions, thermal electron, ion, and neutral collisions, O3 absorption in the Hartley bands, and atomic oxygen recombination and cooling from molecular and eddy thermal conduction and NO and CO2 IR radiation. The O2 photodissociation rates are used in a model of major species diffusion to solve for the O, O2, and N2 number densities. From specified lower‐boundary conditions and arbitrary initial conditions, both the thermospheric and the ionospheric structure evolve self consistently with time for specified solar flux values. The calculated globally averaged thermospheric and ionospheric structure compares well with the globally averaged structure predicted by the thermospheric and ionospheric empirical models based on data for both solar minimum and maximum conditions by considering solar EUV and UV radiation and aurora particle precipitation and joule heat sources to represent high‐latitude processes. Total joule heating is 7 × 1010 W for geomagnetic quiet conditions for both solar minimum and maximum. The model results show that the dominant heat sources for the thermosphere are collisions between ambient electrons, ions, and neturals above about 250 km, exothermic ion‐neutral chemical reactions between 200 and 250 km, exothermic neutral‐neutral chemical reactions between 200 and 150 km, and heating in the Schumann‐Runge continuum and bands below 150 km. Dominant cooling mechanisms are downward molecular thermal conduction above 150 km and NO and CO2 radiation below 150 km. The results also show that NO cooling must be considered in the thermal energy balance if the effect of minor neutral constituent chemical heating is considered in a definition of the neutral gas solar heating efficiency. Otherwise, the heat input into the thermosphere is overestimated.
Interaction of the plasma sheet with the lobes of the Earth's magnetotail
Authors: Swift Daniel W. , Allen Clinton
Published: February 2008

Simulations with a two‐dimensional electromagnetic particle code in the Darwin approximation are used to investigate mechanisms for the generation of potential differences across magnetic field lines in the magnetotail region earthward of the neutral line. Two mechanisms are found. One is the electrostatic ion cyclotron wave generated by counterstreaming ion beams. Another and more powerful process depends on the generation of magnetic fields, By, normal to the noon‐midnight meridian plane of the magnetosphere. Currents carried primarily by electrons generate a By. This causes the magnetic field to loop out of the meridian plane in a direction such that electrons lose potential energy and gain kinetic energy in the convection electric field. Much of the kinetic energy goes into motion in the x direction in the region of the neutral sheet. The current carried by the electrons is in a direction to intensify the By field. The electrons are also compressed, resulting in a negative space charge in the region of the electron current jet. This process therefore generates electrostatic potential variations across magnetic field lines. Contrary to prior expectations, there is no evidence that tearing mode instabilities or conducting boundaries play any prominent role in the generation of the potentials.
Theory of magnetospheric standing hydromagnetic waves with large azimuthal wave number: 1. Coupled magnetosonic and Alfvén waves
Authors: Walker A. D. M.
Published: February 2008

A new hydromagnetic theory is developed for describing compressional pulsations with large azimuthal wave number. It is assumed that there are two plasmas, one hot, in which pressure effects are important, and the other cold. The equations are derived in a general set of magnetic coordinates which allow realistic calculations including geometrical effects in the magnetosphere. The equations describe the three hydromagnetic modes which are coupled by the geometry. When the azimuthal wave number is large, the fast mode is strongly evanescent. This allows an expansion in 1/m in order to decouple the fast wave. The remaining equations describe the coupled transverse Alfvén and magnetosonic modes. Some of the puzzling features of the observations of polarization are discussed.
Theory of magnetospheric standing hydromagnetic waves with large azimuthal wave number: 2. Eigenmodes of the magnetosonic and Alfvén oscillations
Authors: Taylor J. P. H. , Walker A. D. M.
Published: February 2008

When the azimuthal wave number is large, the equations describing standing hydromagnetic waves in the magnetosphere can be written as a set of coupled equations describing the coupled magnetosonic and Alfvén waves. These equations are decoupled when the field lines are straight. The eigenfrequencies of the decoupled oscillations are computed. For typical conditions in the outer magnetosphere these give periods in the Pc 4–5 band or above. The longitudinal magnetosonic wave consists of oscillations in the plasma pressure, the longitudinal plasma drift velocity and the compressional magnetic field. Higher harmonics of the standing waves have nodes quite near the equator. These higher harmonics have larger fractional pressure perturbations at high latitudes. The compressional magnetic field for all modes, however, is substantially attenuated at higher latitudes, and the theory predicts that compressional oscillations of B are only likely to be seen near the equator. Conditions can be favorable for resonance to occur between the magnetosonic mode and the transverse Alfvén mode. The computed results show periods of the right order of magnitude to explain observations of compressional pulsations. The theory has the potential to explain the polarization when coupling is fully taken into account.
Collisional interchange instability: 2. A comparison of the numerical simulations with the in situ experimental data
Authors: Kelley M. C. , Seyler C. E. , Zargham S.
Published: February 2008

A two‐dimensional model applicable to F region ionosphere plasma instabilities has been developed and described in a companion paper. Here we apply the model to equatorial F region irregularities and in particular test the model against rocket and satellite data. As a diagnostic we create simulated data sets similar to the one‐dimensional measurements of plasma density performed by space probes and take the Fourier transform of these data in the same manner and used by the space experimentors. To our knowledge this is the first attempt to directly relate a simulation to in situ data in this way, diagnostic technique which resolves the ambiguity inherent to one‐dimensional measurements. The comparison is in excellent agreement with the in situ data. In particular, we have been able to resolve the apparent differences between satellite quasi‐sinusoidal observations with the shocklike observations made on rockets. Unlike previous simulations of this phenomenon we find an inherent anisotropy in the instability development which is mirrored in the in situ data. We also present evidence that the shallow spectral slopes which often characterize spread F rocket spectra near the F peak may be due to a change in the angle between the rocket velocity vector and the characteristic directions in the medium.
Time‐weighted accumulations ap(τ) and Kp(τ)
Authors: Wrenn Gordon L.
Published: February 2008

The planetary geomagnetic indices Kp and ap are widely used in space geophysics. They provide an estimate of maximum magnetic perturbation within a 3‐hour period. Many geophysical properties are clearly related to the indices, through energy transfer from a common disturbance source, but direct correlation is often lacking because of poor matching between the frequency of sampling and the physical response functions. The index ap(τ) is a simple accumulation of the linear ap calculated with an attenuation factor τ included to take account of natural temporal relaxation. The case for ap(τ) and the related Kp(τ) is made using applications to the variability of the plasma environment in the ionosphere and inner magnetosphere. These examples of improved correlation suggest that time‐weighted integration might profitably be applied to other indices.
Dynamics of equatorial low‐energy particles inside the plasmasphere during magnetically quiet periods
Authors: Pinto O. , Mendes O. , Gonzalez W. D.
Published: February 2008

In this paper, the trajectories of equatorial low‐energy particles (electrons and protons with energies between 1 and 100 keV) inside the plasmasphere (L = 1.5–4.5) have been computed for an average quiet dynamo‐electric field model that is deduced by incoherent scatter radar observations. It is concluded that electrons tend to corotate with the Earth. On the other hand, protons show a more complex dynamical behavior, strongly depending on energy and local time, presenting open orbits similar to those of electrons, except for an L‐dependent energy interval in which closed orbits are found to exist within certain local time intervals.
Vibrational development of the N2+ Meinel Band System in the aurora
Authors: Espy P. J. , Pendleton W. R. , Sivjee G. G. , Fetrow M. P.
Published: February 2008

The relative column abundances N2+ (A²Πu, υ′) ions with υ′ = 0–2 have been examined for a wide range of auroral conditions. Statistically significant changes in the distribution were found to be strongly correlated with auroral “height indicators” such as the red‐to‐blue (R/B) ratio, the intensity ratio I{[NI]3F}/I[N2+M(0, 0)], and the effective rotational temperature of selected molecular electronic bands (N21P, N2+M). The lower‐altitude [(R/B) ≤ 0.1] distributions closely approached that observed in the laboratory using a low‐pressure N2(X¹Σg+, υ = 0) target and a 125‐eV electron beam. In sharp contrast, the higher‐altitude [(R/B) ≥ 1] distributions were characterized by a substantial excess (up to ≈ 60%) of N2+(A²Πu, υ′ = 1) ions. A plausible explanation of these observations is afforded by the long‐standing suggestion that the accidental resonance charge transfer between O+(²D) and N2(X¹Σg+, υ = 0) may provide a selective auroral source of N2+(A²Πu, υ′ = 1).
Statistical and functional representations of the pattern of auroral energy flux, number flux, and conductivity
Authors: Hardy D. A. , Gussenhoven M. S. , Raistrick R. , McNeil W. J.
Published: February 2008

The Hardy et al. (1985) global patterns of the integral energy flux and average energy of precipitating auroral electrons are used to determine the global pattern of the electron‐produced, height‐integrated Hall and Pedersen conductivities. The conductivities were determined in spatial bins in magnetic local time (MLT)‐corrected geomagnetic latitude (CGL) coordinates for all MLTs and for CGLs greater than 50° and for seven levels of activity as measured by Kp. The conductivities vary smoothly with latitude and MLT typically having a single peak in latitude within the auroral oval at any MLT. On the nightside the two conductivities increase with increasing Kp. The largest conductivities are found near midnight, where the peak value of the Pedersen (Hall) conductivity varies from 3.09 (4.05) mhos to 12.5 (25.9) mhos as Kp varies from 0 to ≥6−. The peak conductivity decreases with MLT away from midnight with the lowest peak values found postnoon. At noon and on much of the morning side of the oval the Pedersen and Hall conductivities increase for Kp up to 2 and then decrease for higher Kp. The highest ratios of the Hall to Pedersen conductivity are on the morning side of the oval and at noon. The peak conductivities on the dayside are significant compared to the conductivities produced by solar radiation at all seasons of the year. The global maps of the integral energy flux, integral number flux, and height‐integrated Hall and Pedersen conductivities at each level of Kp were fit using both spherical harmonic and Epstein functions. The Epstein functions were found to reproduce better the original maps. At Kp = 2 the distribution of differences between the Epstein function fit and the original data is roughly symmetric about zero with a full width at half maximum of 16 (20)% for the Pedersen (Hall) conductivity and 32 (40)% for the integral energy (number) flux. The distribution of differences broadens with increasing and decreasing activity.
Dependence of Pc 3 magnetic energy spectra at south pole on upstream solar wind parameters
Authors: Yumoto K. , Wolfe A. , Terasawa T. , Kamen E. L. , Lanzerotti L. J.
Published: February 2008

In order to clarify which of two principal postulated sources of broadband Pc 3 magnetic pulsations, i.e., upstream waves in the Earth's foreshock and surface waves on the dayside magnetospheric boundary, dominates at high latitudes near the cusp, we reexamined the statistical relationships between solar wind parameters and narrow‐band and broadband spectral powers in the Pc 3 frequency range (22–100 mHz) measured on the ground at the geographic south pole station. Both the broadband (20–45 s) and narrow‐band (20–30 s) magnetic powers measured at South Pole Station show good correlations with the hourly averaged solar wind speed but little (or no) correlations with the hourly averaged interplanetary magnetic field (IMF) cone angle or with the theoretically inferred reflected ion beam speed in the Earth's foreshock. We also find no relationship between Pc 3 broadband powers near local noon in the cusp region and the IMF cone angle. The correlation with the solar wind speed suggests that a Kelvin‐Helmholtz type instability at the magnetopause plays an important role both in the generation of surface waves and in the amplification of already existing waves as they are convected across and transmitted through the dayside high‐latitude magnetopause.
Cusp displacement at the magnetopause for large IMF Y component
Authors: Crooker N. U. , Berchem J. , Russell C. T.
Published: February 2008

The magnetic field orientation just earthward of the dayside magnetopause from 95 ISEE and 11 HEOS crossings with |By| > |Bz| in the adjacent magnetosheath indicate a statistically significant ∼1 RE shift of the cusps toward dawn (dusk) in the northern hemisphere and dusk (dawn) in the southern hemisphere for positive (negative) By. This small shift near the magnetopause and the contrasting large shifts of some reported ionospheric cusp signatures are explained in terms of a finite thickness magnetopause model through which the cusps shift gradually from the inner to the outer edge.
The atmosphere of Uranus: Results of radio occultation measurements with Voyager 2
Authors: Lindal G. F. , Lyons J. R. , Sweetnam D. N. , Eshleman V. R. , Hinson D. P. , Tyler G. L.
Published: February 2008

During Voyager 2's occultation by Uranus the radio link from the spacecraft probed the atmosphere of the planet at latitudes ranging from 2° to 7° south. The measurements, which were conducted at two coherently related wavelengths, namely, 13 cm (S band) and 3.6 cm (X band), did not show any clear signs of microwave absorption. However, the Doppler frequency perturbations observed on the radio link have provided new data on the equatorial radius and atmosphere of the planet. From integral inversion of the Doppler data, profiles in height of the electron number density in the ionosphere and the gas refractivity, number density, pressure, temperature, and methane abundance in the troposphere and stratosphere have been determined. The gas data were acquired in the pressure range from about 0.3 mbar to 2.3 bars over an altitude interval of approximately 250 km. At the 2.3‐bar level the nominal model has a temperature of 101 K with a 1σ uncertainty of 2 K when the uncertainty in the composition is assumed to be negligible. The corresponding temperature lapse rate is 0.95 ± 0.1 K/km. A 2‐ to 4‐km‐thick layer with a small refractivity scale height was detected during ingress and egress, which is consistent with the presence of a methane cloud layer centered at the 1.2‐bar level. For the nominal model the methane mixing ratio below the base of the cloud is 2.3% by number density. At the tropopause, which was observed near the 100‐mbar level, the temperature is 53 ± 1 K. A comparison with infrared data acquired with the infrared interferometer spectrometer instrument on board the Voyager spacecraft indicates that the gas in this region consists of 85 ± 3% hydrogen with the remainder being mostly helium. Above the tropopause the gas temperature increases with increasing altitude, reaching 114 ± 10 K near the 0.5‐mbar level. Several warm layers, which may be produced by absorption of solar radiation by hydrocarbon aerosols, were detected in the stratosphere. From the data acquired at ingress and egress the shape and size of the isobaric surfaces of Uranus have been determined. The shape indicates that the gas in the region probed by the link rotates with an average period of about 18 hours, which corresponds to a zonal wind velocity of 110 m/s relative to the magnetic field. This implies that the equatorial atmosphere rotates slower than the interior, in contrast to the situation at Jupiter and Saturn. The 1‐bar isobaric surface has an equatorial radius of 25,559 ± 4 km. Extrapolating from the equator to the south pole by using available data on the gravity field and the zonal wind velocities gives a polar radius of 24,973 ± 20 km. The corresponding oblateness, (Req ‐ Rp)/Req, is 0.02293 ± 0.00080.
Nonlinear evolution of the Kelvin‐Helmholtz instability in the high‐latitude ionosphere
Authors: Keskinen M. J. , Mitchell H. G. , Fedder J. A. , Satyanarayana P. , Zalesak S. T. , Huba J. D.
Published: February 2008

The nonlinear evolution of the electrostatic Kelvin‐Helmholtz instability, resulting from velocity‐sheared plasma flows perpendicular to an ambient magnetic field, has been studied including Pedersen conductivity effects (i.e., ion‐neutral collisions). We find that the Kelvin‐Helmholtz instability develops in a distinctly different manner in the nonlinear regime with Pedersen coupling than without it. Specifically, we show that Pedersen coupling effects, in conjunction with a neutral wind and density gradient, (1) result in an increased time scale for Kelvin‐Helmholtz instability wave growth, (2) inhibit Kelvin‐Helmholtz vortex formation, (3) lead to nonlinear structures which can be described as “breaking waves”, and (4) generate, in the nonlinear regime, small scale turbulence by means of secondary instabilities growing on the primary waves. We have also computed the spatial power spectra of the electrostatic potential and density fluctuations and find that there is a tendency for the potential and density spectra to become shallower when Pedersen conductivity effects are included. We compare our results with recent Dynamics Explorer satellite observations of velocity‐sheared plasma flows in the high‐latitude, near‐Earth space plasma and find good agreement.
Temporal evolution of the HF‐enhanced plasma line in sporadic E
Authors: Djuth F. T. , Gonzales C. A.
Published: February 2008

The high‐power, high‐frequency (HF) facility at Arecibo, Puerto Rico, has been used to study the excitation of Langmuir waves in mid‐latitude sporadic E. Measurements of the temporal evolution of the so‐called HF‐enhanced plasma line (HFPL) were made using the Arecibo 430‐MHz radar. After HF turn‐on in the plasma the HFPL exhibits a rapid growth phase followed by a quick overshoot. During periods of strong HFPL excitation the e‐folding growth time of the HFPL power is typically ≲ 20 µs, and the total overshoot period is ∼1 ms. On the basis of the current observations, mode conversion of the HF wave into Langmuir waves near HF reflection appears to be a promising mechanism for the production of Langmuir waves in sporadic E. Caviton formation at the critical layer is expected to accompany this process, and there is some evidence that the 430‐MHz radar is probing the plasma in a region where density cavities of this nature form. While no specific explanation is offered for the HFPL overshoot, it appears that this phenomenon is fundamental to the Langmuir wave excitation process.
The MHD intermediate shock interaction with an intermediate wave: Are intermediate shocks physical?
Authors: Wu C. C.
Published: February 2008

Contrary to the usual belief that MHD intermediate shocks are extraneous, we have recently shown by numerical solutions of dissipative MHD equations that intermediate shocks are admissible and can be formed through nonlinear steepening from a continuous wave. In this paper, we clarify the differences between the conventional view and our results by studying the interaction of an MHD intermediate shock with an intermediate wave. The study reaffirms our results. In addition, the study shows that there exists a larger class of shocklike solutions in the time‐dependent dissipative MHD equations than are given by the MHD Rankine‐Hugoniot relations. It also suggests a mechanism for forming rotational discontinuities through the interaction of an intermediate shock with an intermediate wave. The results are of importance not only to the MHD shock theory but also to studies such as magnetic field reconnection models.
An open magnetopause model of the Earth's distant tail based on ISEE 3 evidence
Authors: Grzedzielski S. , Macek W.
Published: February 2008

It is shown that an “open” hydromagnetic equilibrium model of the Earth's distant magnetotail can describe the gross features of the cross‐tail dimensions, the magnetic field intensity, and the average plasma density in the tail lobes, as measured by ISEE 3. The tail flaring rate changes (decreases) downstream at around X ∼ −(100–120) RE. Beyond that distance our model predicts a noncircular flattened tail whose average diameter is 2 RT = 2|yz|1/2 = 60–70 RE at down‐tail distances of X ∼ −200 RE and 2RT ∼ 70–110 RE at X ∼ −1000 RE. These values agree with measurements by ISEE 3 and Pioneer 7, respectively. The degree of flattening y/z (east‐west/north‐south dimension ratio) is related to the degree of anisotropy of the plasma pressure (p∥/p⊥ ratio) and to the degree of anisotropy of the field pressure in the solar wind. The most probable flattened tail, elongated in the east‐west direction, is obtained in our model (for p∥/p⊥ ∼ 1) with an y/z ratio of 1.4–1.7 at X ∼ −200 RE and 7–12 at X ∼ −;1000 RE. A tail cross section elongated in the north‐south direction (y/z < 1) would require a large p∥/p⊥ ratio, exceeding the threshold for the excitation of the fire hose instability in the solar wind. The magnetopause in our model is assumed to be partially open as implied by the observed electron distribution functions, by magnetic field data, and by the presence of mantle‐type plasma in the distant tail lobes. The degree of openness α, defined as the fraction of the ambient interplanetary magnetic field lines that become connected to the lobe field lines, is evaluated to be α ∼ 0.1. If the initial magnetic flux in each lobe is taken to be 0.65×109 Wb and if α = 0.1, we obtain a decrease of the average (X aberrated) component of the tail lobe field from |BT| = 7.5–8.5 nT at X ∼ −200 RE to |BT| ≲ 5 nT at X ∼ −1000 RE. The related lobe plasma density increase between these distances is from nT = 0.3–0.4 to 0.8–1.5 cm−3. The field and plasma values obtained are in reasonable agreement with the observations. The total length of the tail depends on α, and the value α = 0.1 allows the tail to reach X ∼ −(2–3) × 10³ RE as supported by the evidence of magnetospheric tail‐associated phenomena seen in Pioneer 7 data. Higher values of up to α = 0.2 cannot be ruled out on the basis of plasma data, but in that case the magnetic flux loss from the lobes becomes too fast to permit the tail to extend beyond X ∼ −(700–1000) RE.
Local spectra and wave distribution functions
Authors: Rönnmark Kjell , Larsson Jonas
Published: February 2008

Wave spectra observed by satellites in space are local, in the sense that they depend on the time and place of observation. Fourier transforms and power spectra are on the other hand global quantities, defined by integrals over infinite intervals. This discrepancy between theory and practice often causes difficulties, which can be overcome if a new definition of a local power spectrum is adopted. The proposed definition is closely related to experimental practice, yet very convenient in analytical calculations. A distribution function, and an associated continuity equation, for the wave energy density in a plasma is derived.
Calculation of escape currents of electrons emitted from negatively charged spacecraft surfaces in a magnetic field
Authors: Laframboise J. G.
Published: February 2008

In low Earth orbit, the geomagnetic field B is strong enough that secondary electrons or photoelectrons emitted from spacecraft surfaces have an average gyroradius much smaller than typical dimensions of large spacecraft. This implies that escape of these electrons will be strongly inhibited on surfaces which are nearly parallel to B, even if a repelling electric field exists outside them. This effect is likely to make an important contribution to the current balance and hence the equilibrium potential of such surfaces, making high‐voltage charging of them more likely. We present numerically‐calculated escaping electron fluxes for these conditions, based on the approximations of uniform fields and Maxwellian emission‐velocity distributions. We also present an analytic curve‐fit to the results for the important case of normal electric field (uniformly‐charged surfaces). For strong normal electric fields, escape is effectively suppressed only when a surface is parallel to B within a few degrees or less, and this leads to “sensitivity effects” in attempts to predict auroral‐zone spacecraft charging. A nonzero tangential component in the surface electric field can greatly enlarge the range of surface orientations for which escape is suppressed, and can also produce large surface currents. We also propose a simple approximate method for calculating the space‐charge‐density distribution of escaping electrons. Our results imply that on a mostly dielectric large spacecraft such as Shuttle, local charging, especially on surfaces nearly parallel to B, may occur in ionospheric conditions which do not produce overall charging.
A semiquantitative theory for the 2ƒp radiation observed upstream from the Earth's bow shock
Authors: Cairns Iver H.
Published: February 2008

A semiquantitative theory for the 2ƒp radiation observed upstream from the Earth's bow shock is presented: the radiation is produced by the process L + L → T + S, proceeding as two sequential three‐wave steps L → L′ + S and L + L′ → T, in the foreshock where nonthermal L and S waves are observed. (Here L, S, and T denote Langmuir, ion acoustic, and transverse electromagnetic waves, respectively.) This theory is consistent with all the available wave data, including the characteristics and levels of a class of low‐frequency waves identified as S wave products of the process L → L′ + S, and the brightness temperature and bandwidth of the 2ƒp radiation. Indeed the theory could account for higher 2ƒp brightness temperatures if required. Predictions of the theory suitable for observational testing include (1) the existence of two 2ƒp sources, one to each wing of the foreshock, (2) the spatial location of the source regions, and (3) the characteristics and levels of the product L′ and S waves in the source regions. The radiation should (4) have intrinsic bandwidths of the order of 1 kHz or less, (5) be less than 0.1% circularly polarized, and (6) have a limiting brightness temperature equal to the effective temperature TL of the L waves producing the radiation.
Geomagnetic response to sudden expansions of the magnetosphere
Authors: Araki Tohru , Nagano Hiroshi
Published: February 2008

The geomagnetic response to five successive sudden expansions of the magnetosphere was examined by the use of magnetic data observed on the ground and by satellites. At the geosynchronous orbit between 0800 and 1100 LT the magnetic field component parallel to Earth's rotation axis decreased successively. The amplitude and the fall time of each decrease were 20–30 nT and 2.5–3.5 min, respectively. The decrease was propagated about 10 min later to the distance of about 31 RE from Earth in the antisunward direction, indicating propagation speed of about 300 km/s. The H component of ground magnetograms from low‐latitude stations showed decreases with waveform similar to that at the geosynchronous orbit, but each decrease at the dayside equator was greatly enhanced and preceded by a short small positive impulse. Each of the corresponding geomagnetic variations at high latitude stations consisted of two successive sharp pulses of opposite sense with 2–3 min duration. The dominant component and the sense of these high‐latitude pulses were highly dependent upon local time and latitude. The distribution of equivalent ionospheric current arrows for each high‐latitude pulse showed clear twin vortices centered at 70–76° geomagnetic latitude in the dayside and was approximately symmetric with respect to the noon meridian. The current direction of the vortices was reversed from the first pulse to the second. It suggests successive appearance of a dawn‐to‐dusk and then a dusk‐to‐dawn electric field, both of which were transmitted from the magnetosphere to the polar ionosphere. The effect of ionospheric currents due to these polar electric fields was superposed on the simple magnetic decrease produced by an expansion of the whole magnetosphere and produced the complex waveform distribution on the ground.
Wavelength dependence of eddy dissipation and coriolis force in the dynamics of gravity wave driven fluctuations in the OH nightglow
Authors: Hickey M. P.
Published: February 2008

The theory of Walterscheid et al. (1987) to explain internal gravity wave induced oscillations in the emission intensity I and rotational temperature T of the OH nightglow was modified by Hickey (1988) to include the effects of eddy dissipation and Coriolis force. In the theory of Walterscheid et al. (1987) the ratio η = (δI/I0)/(δT/T0) (δ refers to a perturbation quantity, and a zero subscript refers to an average) was found to be independent of horizontal wavelength at long periods, while in the extended theory of Hickey (1988) some such dependence was inferred. In the present paper the horizontal wavelength dependence of η is examined. It is found that values of η will be dependent on both wave period and horizontal wavelength, meaning that in order to compare measurement with theory, horizontal wavelengths will need to be measured in conjunction with the OH nightglow measurements. At long periods the modifications to η by the inclusion of eddy dissipation are much larger for the shorter horizontal wavelength waves, although such modifications may be more observable for some of the longer horizontal wavelength waves. The Coriolis force is important only for waves of very large horizontal wavelength.
Resonance absorption of compressible magnetohydrodynamic waves at thin “surfaces”
Authors: Hollweg Joseph V. , Yang G.
Published: February 2008

If an MHD surface wave is supported by a true discontinuity, then the total pressure fluctuation, δPtot, is constant across the discontinuity. If the discontinuity is replaced by a thin transition layer, then δPtot will be approximately constant across the transition layer, with a value approximately the same as the value obtained for the case of a true discontinuity. We use this approximation to study the behavior of the plasma and fields in the transition layer. We regard δPtot as known, and write the relevant equations in forms in which δPtot appears as driving terms. Two resonances appear. The cusp resonance affects the density and pressure fluctuations, and the velocity and magnetic field components along the background magnetic field, B0. The Alfven resonance affects the velocity and magnetic field components normal to B0. We concentrate on the Alfven resonance, and show in a simple way how energy is pumped out of the surface wave into thin layers surrounding the resonant field lines. We consider also the effects of three types of viscosity on the Alfven resonance. Only classical shear viscosity is able to absorb the energy which is pumped into the thin resonant layer. In the steady state, the net viscous heating is independent of the viscosity coefficient, if the heating occurs in a sufficiently thin layer. We suggest that the large velocity shears which occur in the vicinity of the resonant field lines can lead to Kelvin‐Helmholtz instabilities, which can in turn lead to an effective eddy viscosity, which we estimate to be large enough in the solar corona to distribute heat throughout large portions of coronal active region loops. We show also that coronal heating by the Alfven resonance is compatible with a variety of coronal data.
Assessment of the boundary layer model of the magnetospheric substorm
Authors: Nishida A. , Bame S. J. , Baker D. N. , Gloeckler G. , Scholer M. , Smith E. J. , Terasawa T. , Tsurutani B.
Published: February 2008

Since the plasma sheet boundary layer has been identified as a site of fast streaming plasma, suggestions have been made that the features which have hitherto been taken as signatures of the formation of a neutral line in the near‐Earth tail during the expansion phase of substorms can be interpreted differently in terms of the boundary layer characteristics. In order to determine if such suggestions are viable we have analyzed the ISEE 3 data obtained during one of its tail traversais at the distance of 80 RE. It has turned out that at this distance, which is intermediate between the near tail and the distant tail, the plasma sheet boundary layer does not represent a clearly definable entity in terms of the plasma flow characteristics, and thus it is fundamentally difficult to explain substorm observations at this distance in terms of any boundary layer model. Indeed, observations are quite consistent with the formation of a near‐Earth neutral line during the expansion phase. The presence of the boundary layer at the ISEE 1 orbit (apogee: 23 RE) but not at 80 RE suggests that the neutral sheet acceleration region which extends from the distant tail terminates somewhere between 80 and 23 RE. It is further noted that the correspondence between activities in the inner magnetosphere and in the tail may not be absolutely one‐to‐one.
Properties and dynamics of the electron beams emanating from magnetized plasma double layers
Authors: Borovsky Joseph E.
Published: February 2008

The properties of the electron sheet beams that are produced by magnetized plasma double layers (which may reside in the auroral zone) are examined by means of particle‐in‐cell simulations, test particle simulations, and analytic theory. It is found that these beams are characterized by (1) a large degree of gyrophase bunching, (2) large T⊥/T∥ ratios, (3) very strong ∂f/∂υ∥ gradients, and sometimes (4) a minimum in f(υ⊥)/υ⊥ near υ⊥ = 0. The temporal modulation of the electron beams by the cyclic disruption and reformation of the magnetized double layers is quantified, and the disruption distance of the sheet‐shaped electron beams via the Langmuir waves that they drive is estimated.
Shaping of an ion cloud's velocity field by differential braking due to Alfvén wave dissipation in the ionosphere, 1, coupling with an infinite ionosphere
Authors: Nalesso G. F. , Jacobson A. R.
Published: February 2008

We study the interaction of a plasma cloud, jetting across the geomagnetic field with the surrounding ionosphere. The cloud is assumed of finite extension in the direction normal to both the direction of motion and the magnetic field, while the ionosphere is considered a collisional anisotropic magnetized plasma. It is shown that two main mechanisms contribute to the cloud's braking: momentum exchange with the ionosphere via Alfv'n waves and momentum dissipation due to resistive currents. Due to the finite size of the cloud a differential braking of the different transverse harmonics of the Alfv'n wave appears when the momentum exchange mechanism is dominant. The result is a sharpening of the cloud's velocity field.
Shaping of an Ion Cloud's velocity field by differential braking due to Alfvén wave dissipation in the ionosphere, 2, reflections from the E layer
Authors: Nalesso G. F. , Jacobson A. R.
Published: February 2008

A previously studied model of the interaction of a plasma cloud (jetting across the geomagnetic field) with the surrounding ionosphere is implemented by considering the impact on the cloud's dynamics of the Alfvén waves reflected back from the highly collisional (νi > ωci) E layer. The effect of the E layer is accounted for by terminating the ionosphere in a sharp lower boundary where a discontinuity of the conductivity is allowed. It is found that the results are strongly dependent on the (complex) reflection coefficient of the wave at the discontinuity surface. Some limiting cases are numerically analyzed and discussed in connection with previous results.
The role of metastable states in critical ionization velocity discharges
Authors: Lai Shu , McNeil William J. , Murad Edmond
Published: February 2008

Despite the possible fast formation of a hot electron tail distribution due to plasma‐wave interaction, ionization in Alfvén's critical ionization velocity (CIV) process has to come from electron impact ionization. Excited states may play an important role in this process. By solving coupled rate equations numerically, we have examined the time evolution of ionization in a CIV process, finding that metastable states of some of the gases contribute substantially to the rapidity of the process. The metastable states serve the function of storing energy from collisions between low‐energy electrons and ground state neutrals. Subsequent collisions between the metastable states with low‐energy electrons lead to pooling of the energy, such that ionization can occur rapidly. Competing processes are evaluated for their effect in quenching CIV.
Long‐term variations of the cosmic ray north‐south anisotropy and the radial cosmic ray gradient at high rigidity
Authors: Swinson Derek B.
Published: February 2008

Data from the Embudo underground cosmic ray muon telescope from 1965 to 1985 have been analyzed as a function of the polarity of the interplanetary magnetic field (IMF), B, to examine long‐term variations in the cosmic ray north‐south anisotropy, B × ▽NR, resulting from the interaction of B and ▽NR, the radial component of the heliocentric cosmic ray density gradient. The direction of the cosmic ray north‐south anisotropy is shown to be essentially constant over this period, and the extraterrestrial nature of the anisotropy is confirmed. The magnitude of the anisotropy is shown to be only weakly dependent on solar activity and to be independent of the magnetic polarity of the solar dipole which is the source of the IMF B. By inference, it is concluded that the radial heliocentric cosmic ray density gradient at high rigidity does not show a large variation with solar activity or with solar magnetic polarity.
DE/ISIS conjunction comparisons of high‐latitude electron density features
Authors: Hoegy Walter R. , Benson Robert F.
Published: February 2008

Electron number density (Ne) profiles determined remotely by ISIS 1 and 2 topside sounder measurements were compared with in situ ion and electron density measurements by the Dynamics Explorer 2 (DE 2) Langmuir probe during four high‐latitude ISIS/DE magnetic field‐aligned conjunctions. In all cases, the universal time separations between the two data sets were small (less than 12 min). The horizontal separations away from conjunction were up to several thousand kilometers. The Ne comparisons near the conjunction times represents the first cross‐calibration of in situ probe and remote topside sounding Ne measurements. The ISIS‐derived Ne values, even at the greatest distance from the sounder (near the F peak), agreed with the DE 2 Langmuir probe measurements to within about 30% over a density range of more than 2 decades on three of the comparisons. The fourth comparison, which only had data on one side of the conjunction in a region with strong Ne irregularities, showed a difference of about 60%. Away from conjunction the Ne comparisons were made by displaying Ne along the orbital tracks on invariant latitude (Λ)/magnetic local time (MLT) polar plots, or by displaying Ne versus time where the two data sets are mapped to one another either along arcs of constant Λ or along radii of constant MLT. These plots display density structure simultaneously at distinct locations in the convection controlled polar ionosphere. During one of the conjunctions a comparison was made between the DE 2 electron temperature and the temperature derived from the ISIS sounder scale height. This comparison revealed a region with Te > Ti and a Te enhancement colocated with spread F (observed on ISIS), but not correlated with weak particle precipitation.