bluuebunny/crossref_metadata_2025 huggingface dataset:
Extraordinary mode resonance cones in the Uranian magnetosphere?
Authors: Grabbe Crockett L.
Published: February 2008

A theory is introduced for previously unidentified waveforms involving decreasing frequency with time that were picked up in the Voyager 2 flyby of Uranus. They are identified as a possible band of extraordinary waves detected on resonance cone surfaces. Because of the resonance cone phenomenon, certain frequencies are only observed at certain angles from the source, and the frequency decreases as the angle decreases. The homogeneous plasma theory is shown to predict an almost linear drop in frequency with time. In principle one can use the wave frequency with time to estimate the distance and angle of the source for the homogeneous case. Inhomogeneities in the plasma are shown to produce a substantial variation on that and give the waveforms an important fine structure. They give arches to the waveform and can explain sudden drops in the frequency and changes in the slope of the waveforms at certain times, as is observed in the data. Inhomogeneities from a plasma wave (possibly an Alfvén wave) near the spacecraft are shown to explain the modulation in one of the waveforms. A decreasing magnetic field of growing negative slope can independently account for the arching over of the waveform and the decreasing modulation frequency. The multiple peaks in the frequency observed for given times in the waveform may be caused by the multiple peaks from the resonance cone interference structure, which are predicted in the warm plasma theory of resonance cones.
Auroral studies with a chain of meridian scanning photometers, 2, Mean distributions of proton and electron aurora as a function of magnetic activity
Authors: Creutzberg F. , Gattinger R. L. , Harris F. R. , Wozniak S. , Jones A. Vallance
Published: February 2008

Data obtained during the IMS with a ground‐based chain of meridian photometers which measured the Hβ and OI 5577‐A emissions have been analyzed quantitatively to yield the mean distributions of proton and electron aurora in geomagnetic latitude and time as functions of magnetic activity. The total energy inputs from electron and proton precipitation around the nightside auroral oval are estimated as functions of AE from the 5577‐A and Hβ intensity data. Average latitude profiles for the two emissions are given as a function of AE for the four night time sectors between 1800 and 0600 local magnetic time. In agreement with previous studies it is found that the average energy flux due to protons is an order of magnitude less than that due to electrons under disturbed conditions although under very quiet conditions the two are comparable. Finally, the Hβ observations have been fitted to a quantitative empirical model giving Hβ intensity distributions as a function of AE. The peak values of Hβ intensity increase linearly with AE for all magnetic time sectors.
Stationary nonlinear Alfvén waves and solitons
Authors: Hada T. , Kennel C. F. , Buti B.
Published: February 2008

Stationary solutions of the derivative nonlinear Schroedinger equation are discussed and classified by using a pseudopotential formulation. The solutions consist of a rich family of nonlinear Alfvén waves and solitons with parallel and oblique propagation directions. Expressions for the envelope and the phase of nonlinear waves with periodic envelope modulation, and “hyperbolic” and “algebraic” solitons are given. We evaluate the propagation angle for the slightly modulated elliptic, periodic waves and for oblique solitons. Also, we present periodic stationary waves which may arise in numerical simulations using periodic boundary conditions. The parallel and oblique stationary solutions discussed here can serve as a starting point for studies of modulational and decay instabilities and for a stability analysis of the solitons.
Self‐consistent model of the low‐latitude boundary layer
Authors: Phan T. D. , Sonnerup B. U. Ö. , Lotko W.
Published: February 2008

A simple two‐dimensional, steady state, viscous model of the dawnside and duskside low‐latitude boundary layer (LLBL) has been developed. It incorporates coupling to the ionosphere via field‐aligned currents and associated field‐aligned potential drops, governed by a simple conductance law, and it describes boundary layer currents, magnetic fields, and plasma flow in a self‐consistent manner. Slab geometry is assumed, with no variations along the flow direction −x and with the layer on closed field lines. The currents in the layer are regulated by coupling to the ionosphere. The magnetic field induced by these currents leads to two effects: (1) a diamagnetic depression of the magnetic field in the equatorial region and (2) bending of the field lines into parabolas in the xz plane with their vertices in the equatorial plane, at z = 0, and pointing in the flow direction, i.e., tailward. Both effects are strongest at the magnetopause edge of the boundary layer and vanish at the magnetospheric edge. The diamagnetic depression corresponds to an excess of plasma pressure in the equatorial boundary layer near the magnetopause. This pressure drops off both with increasing distance z from the equatorial plane and with increasing distance y from the magnetopause. It reaches the magnetospheric level for z = ±H as well as for y → ∞. The boundary layer structure is governed by a fourth‐order, nonlinear, ordinary differential equation in which one nondimensional parameter, the Hartmann number M, appears. A second parameter, introduced via the boundary conditions, is a nondimensional flow velocity
at the magnetopause. It is shown that for large M values the coupling to the ionosphere is weak; in that limit, or when
is small, the model reduces to that discussed by Lotko et al. (1987) in which induced magnetic fields are neglected. Numerical results from the model are presented and the possible use of observations to determine the model parameters is discussed. The general predictions of the model in terms of region 1 currents and associated ionospheric signatures are similar to those obtained earlier by Lotko et al. and by Sonnerup (1980). Those predictions are in qualitative and approximately quantitative agreement with a number of observations. The main new contribution of the study is to provide a better description of the field and plasma configuration in the LLBL itself and to clarify in quantitative terms the circumstances in which induced magnetic fields become important. In particular, it appears that for the low values of the field‐aligned conductance expected on the duskside of the magnetosphere, these fields may remain relatively unimportant, at least in the noon to dusk segment of the LLBL.
Nonlocal theory of long‐wavelength plasma waves associated with sporadic E layers
Authors: Riggin D. , Kadish A.
Published: February 2008

In this paper we calculate the nonlocal growth rate of gradient drift plasma waves under conditions where the electron density gradient scale length changes with altitude. The results are compared with the local growth rate and discussed in the context of the kilometer‐scale waves which have been observed in the vicinity of mid‐latitude sporadic E layers. These large‐scale waves drastically violate the local approximation, kLm»1, where k is the irregularity wave number and Lm is the minimum gradient scale length on the edge of a layer. The first step in the analysis is to derive a general eigenmode equation, starting with the same assumptions usually used in the derivation of the local dispersion relation for long wavelength waves. Modeling a sporadic E layer as a slab, the nonlocal growth rate spectrum is found by solving the eigenmode equation for this profile. The solution is an algebraic dispersion relation with a growth rate spectrum which is roughly proportional to k, rather than the k2 dependence predicted by conventional local theory at long wavelengths. At short wavelengths the nonlocal growth rate determined with the slab is unbounded, in disagreement with local theory. The slab is an inadequate model for short wavelength waves and a bound on the growth rate is instead derived from a theory which can be applied to any realistic profile with nonzero Lm. At short wavelengths this bound is identical to the local growth rate expression, while at long wavelengths the bound remains proportional to k and thus is consistent with the dispersion relation for a slab. Nonlocal effects alone do not explain the dominance of kilometer scales, but they do tend to favor the excitation of long wavelengths.
Multisatellite and ground‐based observations of transient ULF waves
Authors: Potemra T. A. , Lühr H. , Zanetti L. J. , Takahashi K. , Erlandson R. E. , Marklund G. T. , Block L. P. , Blomberg L. G. , Lepping R. P.
Published: February 2008

A unique alignment of the Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE and Viking satellites with respect to the EISCAT Magnetometer Cross has provided an opportunity to study transient ULF pulsations associated with variations in solar wind plasma density observed by the IMP 8 satellite. These observations were acquired during a relatively quiet period on April 24, 1986, during the Polar Region and Outer Magnetosphere International Study (PROMIS) period. An isolated 4‐mHz (4‐min period) pulsation was detected on the ground which was associated with transverse magnetic field oscillations observed by Viking at a ∼2‐RE altitude above the auroral zone and by CCE at ∼8‐RE in the equatorial plane on nearly the same flux tube. CCE detected a compressional oscillation in the magnetic field with twice the period (∼10 min) of the transverse waves, and with a waveform nearly identical to an isolated oscillation in the solar wind plasma density measured by IMP 8. We conclude that the isolated 10‐min oscillation in solar wind plasma density produced magnetic field compression oscillations inside the magnetosphere at the same frequency which also enhanced resonant oscillations at approximately twice the frequency that were already present. The ground magnetic field variations are due to ionospheric Hall currents driven by the electric field of the standing Alfvén waves. The time delay between surface and satellite data acquired at different local times supports the conclusion that the periodic solar wind density variation excites a tailward traveling large‐scale magnetosphere wave train which excites local field line resonant oscillations. These oscillations diminish immediately after the passage of the wave train. We conclude that these transient magnetic field variations are not associated with magnetic field reconnection or flux transfer events.
Nitrogen I 3F (²P* → ²D* Emissions in the aurora
Authors: Pendleton W. , Espy P. , Hartman J. , Fetrow M. , Sivjee G.
Published: February 2008

The technique of Fourier transform infrared spectroscopy with a field‐widened Michelson interferometer has been used to realize improved auroral observations of the [N I] 3F (²P* → ²D*) multiplet. Time‐resolved spectra (spectral resolution ≈2.5 cm−1) were acquired at the rate 1 per ≈20 s. Under the extant conditions, the ²P* → ²D*3/2 and ²P* → ²D*5/2 components of this multiplet were fully resolved, and its blending with the rotational structure of the N2 1P (0,0) band was significantly reduced. Reliable component emission rates were extracted from the spectrally blended data by fitting a set of critically evaluated, high‐quality synthetic spectral components to the data in a least squares sense. These improved observations have yielded an intramultiplet intensity ratio [(²P* → ²D*5/2)/(²P* → D*3/2)] in agreement with the theoretical statistical equilibrium ratio for a wide range of auroral heights. In addition, a strong correlation of the [N I] 3F emission rate with that of the [O I] 1F multiplet (λ6300 Å) has been established, and the nascent yield of N(²P*), relative to N2+, associated with energetic e‐N2(X) collisions has been estimated using data from an unusually high altitude nightside aurora.
Solar sources of interplanetary southward Bz events responsible for major magnetic storms (1978‐1979)
Authors: Tang Frances , Tsurutani Bruce T. , Gonzalez Walter D. , Akasofu Syun I. , Smith Edward J.
Published: February 2008

Tsurutani et al. [1988] analyzed the 10 intense interplanetary southward Bz events that led to major magnetic storms (Dst<−100 nT) during the 500‐day interval, August 16, 1978, through December 28, 1979. In this paper we report solar sources of the major storms and discuss their interplanetary effects. Seven of the sources of the 10 major storms are flares and 3 are prominence eruptions. The ratio is consistent with frequency of these two types of events occurring on the Sun during this period. The flares range from M1 to X2 in peak soft X ray emissions. However, the largest of the 10 storms (Dst = −220 nT) as well as the two strongest shocks (M>3.0) are associated with prominence eruptions. For three of the five southward Bz events in which the driver gases are the causes of the intense southward field leading to magnetic storms, the photospheric fields of the solar sources have no dominant southward component, indicating the driver gas fields do not always result from a simple outward convection of solar magnetic fields. Finally we compare the solar events and their resulting interplanetary shocks and find that the standard solar parameters do not correlate with the strengths of the resulting shocks at 1 AU. The implications are discussed.
A self‐consistent picture of Jupiter's nightside magnetosphere
Authors: Zimbardo Gaetano
Published: February 2008

A self‐consistent axisymmetric steady model of Jupiter's nightside magnetosphere is presented. Magnetospheric currents are assumed to be restricted to a thin equatorial disc and to magnetopause current sheets. The plasma velocity is assumed to be purely toroidal, and the magnetic field is assumed to be purely poloidal. Data from the Voyager missions are used as input for the solution of the equation determining the self‐consistent equilibrium magnetic structure. Several numerical computations are performed, varying parameters such as the hot plasma composition and the current disc cutoff radius. We find that, with sufficiently large discs, the magnetic configuration has an X line and an O line. The X line is located between 37 and 45 Jovian radii, depending on the values chosen for the relevant parameters, while the position of the O line is less well determined. Excellent agreement between the magnetic field data in the tail and the results of the model is found. When the actual, nonaxisymmetric magnetosphere is considered, equilibrium with a purely azimuthal velocity is not possible beyond the X line, and an ordered tailward motion must develop. This could be identified either with the “magnetospheric wind” observed by the Voyager satellites or with the possible formation of plasmoids in the Jovian magnetosphere.
Theoretical model of Saturn's kilometric radiation spectrum
Authors: Galopeau Patrick , Zarka Philippe , Le Quéau Dominique
Published: February 2008

A theoretical model is developed here in order to determine an envelope for the average spectrum of the Saturnian kilometric radiation (SKR). The microscopic generation mechanism is supposed to be the so‐called synchrotron (or cyclotron) maser instability. As in recent works on the terrestrial kilometric radiation, the effect of the magnetic field inhomogeneity on the generation process must be taken into account. Then, assuming that the emission is nonlinearly saturated by trapping, our calculation allows us to put an upper limit on the SKR spectral intensity very simply: the maximum level of the wave electric field within the source region is indeed linked to a few macroscopic plasma parameters, which can be derived from the observations (the structure of the magnetospheric magnetic field, the cold plasma density, and the characteristic energy of the hot emitting electrons). We have used a dipolar magnetic field model, while the plasma distribution results from the superposition of two components: an ionospheric population and a plasma disc, whose scale heights have been roughly determined from Voyager measurements. The energetic electrons responsible for the emission are supposed to precipitate along the high‐latitude magnetic field lines where SKR emission is known to take place. The width of the source region can be self‐consistently estimated from the model. A very good agreement is obtained between the theoretical spectrum and the observational radio data. The calculated spectral intensities exceed the most intense observed intensities by up to 1 order of magnitude, suggesting that the SKR emission is only marginally saturated by nonlinear processes.
Path‐integrated growth of electrostatic waves: The generation of terrestrial myriametric radiation
Authors: Horne R. B.
Published: February 2008

It is generally accepted that electrostatic wave energy is the source of terrestrial myriametric radiation (TMR), but there are several theories to suggest how this energy is converted into TMR. The linear mode conversion “window” theory of Jones (1976, 1980), which in the past has been considered by some to be too inefficient to account for the observed wave amplitudes, is considered here. First, the ray tracing program HOTRAY is described. This program is used to trace electromagnetic and electrostatic waves in a hot magnetized plasma and to calculate the path‐integrated growth rates for a realistic unstable particle distribution function. A density model is constructed from wave observations made by DE 1 of an event where TMR was beamed to northern and southern latitudes from a source very close to the magnetic equator. Ray tracing shows that backward propagating electrostatic waves can refract into electromagnetic Z mode waves and transport energy to the so‐called radio window at the equator. At this point, mode conversion of energy into O mode radiation is assumed to take place. Ray tracing of O mode radiation from the radio window shows that TMR is beamed to northern and southern latitudes as observed and as predicted by the theory. Path‐integrated growth rates show that the electrostatic waves amplify by a factor ≥42 from the background fluctuation level before reaching the window. This is sufficient to account for the observed TMR wave amplitudes which require the waves to amplify by a factor ≥20. Increasing the depth of the loss cone, or increasing the hot plasma density, to within observed limits, increases the wave amplification up to a factor of 104. Strong Landau and cyclotron damping from the hot plasma component restricts the efficient transfer of energy to the radio window to within a few tenths of a degree in latitude about the magnetic equator. Thus the strongest TMR is emitted from the equator. In general, TMR beamed to northern latitudes from radio windows north of the equator is stronger than that beamed to southern latitudes; conversely, TMR beamed to northern latitudes from radio windows south of the equator is generally weaker than that beamed to southern latitudes. It is shown that electrostatic wave energy can still be transported efficiently to the radio window for magnetic field intensity variations of 1 or 2%. Thus the generation of TMR is not sensitive to variations in the magnetic field strength of this magnitude.
Some low‐altitude cusp dependencies on the interplanetary magnetic field
Authors: Newell Patrick T. , Meng Ching‐I. , Sibeck David G. , Lepping Ronald
Published: February 2008

Although it has become well established that the low‐altitude polar cusp moves equatorward during intervals of southward interplanetary magnetic field (IMF Bz<0), many other important aspects of the cusp's response to IMF components are not as well investigated. An algorithm for identifying the cusp proper was applied to 12,569 high‐latitude dayside passes of the DMSP F7 satellite (which is in a nearly circular polar orbit at ∼838 km altitude), and the resulting cusp positioning data were correlated with the IMF (IMF data were available for about 25% of the cases). It was found that the peak probability of observing the cusp shifts prenoon for By negative (positive) in the northern (southern) hemisphere and postnoon for By positive (negative) in the northern (southern) hemisphere. The By induced shift is much more pronounced for southward than for northward Bz, a result that appears to be consistent with elementary considerations from, for example, the antiparallel merging model. No interhemispherical latitudinal differences in cusp positions were found that could be attributed to the IMF Bx component. As expected, the cusp latitudinal position correlated reasonably well (0.70) with Bz when the IMF had a southward component; the previously much less investigated correlation for Bz northward proved to be only 0.18, suggestive of a half‐wave rectifier effect. The ratio of cusp ion number flux precipitation for Bz southward to that for Bz northward was 1.75±0.12. The statistical local time (full) width of the cusp proper was found to be 2.1 hours for Bz northward and 2.8 hours for Bz southward.
High‐voltage polar‐orbit and beam‐induced charging of a dielectric spacecraft: A wake‐induced barrier effect mechanism
Authors: Laframboise J. G. , Luo J.
Published: February 2008

We propose a mechanism which permits high‐voltage auroral zone charging to occur on a large enough mostly dielectric spacecraft, in situations where one would otherwise expect such charging to be suppressed by secondary‐electron or photoelectron escape from exposed surfaces. This extends, to lower equivalent temperatures, the range of auroral electron plasma conditions in which one can expect such a spacecraft to undergo high‐voltage charging. It should also permit such charging to occur in sunlight. Our results yield a tentative prediction that the Shuttle Orbiter is a large enough spacecraft for such charging, if one adopts the usual definition of “high‐voltage” charging as involving surface potentials at least 100 V different from space potential. The mechanism involves the suppression of electron emission by a potential barrier in the spacecraft wake. In contrast with barrier effect charging in geosynchronous orbit conditions, (1) this barrier is produced by unbalanced electron space charge and not by the presence of surfaces at larger negative potentials elsewhere on the spacecraft, (2) instead of limiting differential charging, this barrier produces it, and (3) instead of a saddle point, the potential distribution involves a minimum which migrates toward the spacecraft and almost reaches it as the spacecraft charges toward a steady state. We also apply this mechanism to propose an explanation for features of Shuttle Orbiter charging observed during the SEPAC (Space Experiments With Particle Accelerators) Spacelab 1 electron beam experiment.
A study of electron impact excitation of NO: The middle ultraviolet from 170 to 270 nm
Authors: Ajello J. M. , Pang K. D. , Franklin B. O. , Howell S. K. , Bowring N. J.
Published: February 2008

The electron impact induced fluorescence spectrum of NO at 200 eV has been studied from 170 to 270 nm in a crossed beam experiment. By modeling the laboratory spectrum, including the effects of spin‐orbit splitting of the ground state, we determined the emission cross sections of the γ (A²Σ+ → X²Πr) and ε (D²Σ+ → X²Πr) band systems. The emission cross section of the γ bands includes a large cascade cross‐section contribution from high lying Rydberg states that predominantly populate the υ′ = 0 level of the A²Σ+ state. The ε bands are modeled by excitation rates proportional to Franck‐Condon factors for direct excitation. One feature is tentatively identified as the (1, 0) δ band and permits an estimate of the (C²Πr, υ′ = 1 → X²Πr, υ′ = 0) emission cross section. Previous low‐resolution emission studies of electron impact excitation of the γ bands have not taken into account the heavy spectral blending by the δ, υ, and β′ bands. A major result of this study indicates great care has to be used if NO is taken as a target for relative calibration in the middle ultraviolet (MUV) using the molecular branching ratio technique. The identification of all features in the MUV spectrum allows aeronomers to identify emission features that may arise from electron impact excitation and other chemical processes occurring in the upper atmospheres of the inner planets where NO is a constituent.
Plasma pressure in the environment of Jupiter, inferred from Voyager 1 magnetometer observations
Authors: Caudal G. , Connerney J. E. P.
Published: February 2008

A spherical harmonic model of Jupiter's planetary magnetic field is combined with a self‐consistent model of the Jovian magnetodisc. The sets of parameters of both models are determined simultaneously by using a generalized inverse technique. Assuming that the pressure P in the middle (and outer) magnetosphere is related to the unit flux tube volume V through PVγ = const, the fit yields a value of 0.88 for γ. If the hot (30 keV) plasma is transported adiabatically inward under the interchange instability triggered by the centrifugal force of the heavy torus ions, losses are not sufficient to account for such a low value of γ beyond L = 10 (as compared to γ ≈ 5/3 that would be expected for adiabatic transport of monoatomic gas). Closer to the planet, as the outer edge of the Io plasma torus is approached (at distances between 7 and 9 RJ from Jupiter), PVγ is found to decrease inward, as expected from the particle measurements, which identified an inner boundary of the particle fluxes in that region. At the magnetic equator, our pressure estimates were compared with the ones obtained from direct particle measurements (low‐energy charged particle (LECP) experiment). Assuming a mixture of O+ and H+ at the same temperature (or with the same spectral power law exponent), consistency between those two independent determinations of the pressure would require that the pressure produced by H+ constitute 18–36% (at most) of the total pressure, at distances between 13 and 21 RJ. Finally, as concerns the internal field coefficients, despite an overall consistency, a slight variability is found between the present estimates and previous ones derived from the same data set, which puts some limitation on the accuracy with which internal coefficients can be determined from the Voyager 1 encounter alone.
Effects of irregularity anisotropy on Faraday polarization fluctuations
Authors: Lee M. C. , Nghiem S. V. , Yoo C.
Published: February 2008

The previous model [Lee et al., 1982] of the Faraday polarization fluctuations (FPF) is extended after taking into account the anisotropic nature of the commonly observed, rodlike and sheetlike ionospheric irregularities. Striking effects of irregularity anisotropy are found in the longitudinal radio propagation. However, if the wave propagation angle is not small (say, greater than 5°), the effects of irregularity anisotropy on FPF introduced by rodlike irregularities weaken significantly, while those caused by sheetlike irregularities remain prominent. Therefore, under the same ionospheric propagation conditions, sheetlike ionospheric irregularities are more effective than rodlike ionospheric irregularities in causing the FPF of radio waves. It is expected that intense FPF of VHF radio signals can be observed not only near the equatorial anomaly but also in the auroral region.
Global simulation of the solar wind‐magnetosphere interaction: The importance of its numerical validity
Authors: Watanabe K. , Sato T.
Published: February 2008

A new three‐dimensional magnetohyrodynamic (MHD) simulation code based on the fourth‐order Runge‐Kutta‐Gill time advance and the direct finite space difference is applied to the detailed study of the solar wind interaction with the Earth's dipole field with special emphasis on its numerical soundness. The dynamic formation processes of the bow shock and the magnetopause are revealed, where the transient propagation characteristics of fast waves play the leading role. The plasma sheet formation process is also revealed. The high temperature, high density, and hence high pressure in the plasma sheet are caused by the fast waves which compress the dipole field. The study of the effects of the interplanetary magnetic field (IMF) on the plasma sheet finds that when the IMF is directed south, the plasma sheet is strongly compressed by the reconnected magnetic flux which is carried from the day side. The peak plasma sheet pressure that appears around 8 RE (RE∶ Earth's radius) on the nightside is almost exactly as large as the solar wind dynamic pressure in the absence of the IMF. When the IMF is north, the nightside lobe magnetic field is peeled off, so that the plasma sheet pressure becomes weakly depressed in comparison with the no‐IMF case. This indicates that the direction of the IMF largely influences the pressure of the plasma sheet. More interestingly and importantly, no Kelvin‐Helmholtz instability occurs along the magnetopause in an ideal MHD, contrary to the conventional understanding. Furthermore, no entry of mass, momentum, and energy through the magnetopause is observed at all in our simulation. This strongly ensures enough numerical accuracy and validity of the present work. Thus the simulation results of earlier works where a considerable amount of solar wind momentum is transported into the magnetosphere must be due to a numerical inaccuracy. As further evidence showing the numerical validity of the present code, the relationship between the standoff distance and the adiabatic constant is compared with the strong shock theory, and a fairly good agreement is obtained. On the other hand, the standoff distance is found to be weakly dependent upon the dynamic pressure of the solar wind in the realistic parameter range.
Thermosphere‐ionosphere coupling: An experiment in interactive modeling
Authors: Forbes Jeffrey M. , Roble Raymond G.
Published: February 2008

The National Center for Atmospheric Research thermosphere general circulation model (TGCM) is utilized to perform a series of controlled experiments aimed at better understanding the interactive coupling between ionospheric plasma densities and thermospheric neutral winds. The experiments are simple and controlled so as to facilitate identification of governing mechanisms. The interaction is accomplished by parameterizing the F layer peak height (hmF2) in an empirical ionospheric model in terms of the meridional wind (υsouth) and forcing hmF2 and υsouth to remain mutually coupled in a dynamical calculation. Interactive computations are performed where the TGCM is driven by 30‐kV and 90‐kV cross polar cap potentials and are compared with corresponding reference simulations where hmF2 is constrained to a balance height near 280 km with a small (∼20 km) diurnal variation. Mutual coupling between hmF2 and υsouth is found to be weak during the daytime when the F layer exhibits a broad vertical structure. At night, when the F2 layer is more localized, the neutral dynamical structure is dependent on whether hmF2 is significantly above or below the altitude (approximately 275–300 km) at which ion drag effectively competes with viscosity in the neutral momentum balance. At Arecibo during both levels of high‐latitude forcing, the premidnight elevation (+75 km) of hmF2 relative to a nighttime balance height of approximately 290 km is accompanied by an increase (∼50 m s−1) in the zonal wind above 250 km relative to the noninteractive simulation. A much smaller effect, characterized by an 8‐hour periodicity spanning 24 hours, is seen in the meridional wind component. At Millstone Hill under the 90‐kV forcing, a 40‐km ascent of hmF2 to 330 km around midnight due to southward winds is accompanied by a 40–60 m s−1 increase in the westward wind above 200 km. Interestingly, the southward wind is diminished in magnitude by about 35 m s−1 compared to the noninteractive simulation, opposite to what one would expect if the ion drag presented to the meridional flow were reduced. We interpret this as a Coriolis effect induced by the increased westward wind.
Heliocentric distance and temporal dependence of the interplanetary density‐magnetic field magnitude correlation
Authors: Roberts D. A.
Published: February 2008

The Helios, IMP 8, ISEE 3, and Voyager 2 spacecraft are used to examine the solar cycle and heliocentric distance dependence of the correlation between density n and magnetic field magnitude B in the solar wind. Previous work had suggested that this correlation becomes progressively more negative with heliocentric distance out to 9.5 AU. Here we show that this evolution is not a solar cycle effect, and that the correlations become even more strongly negative at heliocentric distances larger than 9.5 AU. There is considerable variability in the distributions of the correlations at a given heliocentric distance, but this is not simply related to the solar cycle. Examination of the evolution of correlations between density and speed suggests that most of the structures responsible for evolution in the anticorrelation between n and B are not slow‐mode waves, but rather pressure balance structures. The latter consist of both coherent structures such as tangential discontinuities and the more generally pervasive “pseudosound” which may include the coherent structures as a subset.
Pressure‐Balanced structures between 1 AU and 24 AU and their implications for solar wind electrons and interstellar pickup ions
Authors: Burlaga L. F. , Scudder J. D. , Klein L. W. , Isenberg P. A.
Published: February 2008

“Pressure‐balanced structures” (PBSs) in the heliosphere are microscale phenomena with a characteristic length along the radial dimension of the order of 0.05 AU across which the sum of the thermal pressures and the magnetic pressure is constant. We have identified pressure‐balanced structures in the Voyager data from 1 AU to 24 AU. If the density of pickup ions Ni were proportional to the proton density across a PBS and if the electron pressure were negligible, then from measurements of the magnetic field and the proton density and temperature across pressure balanced structures we derive a pickup ion density which is significantly smaller than the density predicted by a model based on the same assumption. This suggests that there are processes which “smear out” the pressure of the pickup ions so that it is nearly constant on the scale of a PBS. If the pressure of the pickup ions is uniform across a PBS, then one can determine the electron temperature from the pressure balance condition. The electron temperatures derived from the PBS are consistent with the directly measured electron temperatures in the region within 5 AU, and they are consistent with the extrapolation of the polytropic relation between the electron temperature and density derived by Sittler and Scudder (1980) for densities of ≥0.03 cm−3. From 11 pressure‐balanced structures between 18.9 AU and 21.0 AU we derive an average temperature Te = (5.2±2.3) × 104 °K at ∼20 AU.
Steepening of kinetic magnetosonic waves into shocklets: Simulations and consequences for planetary shocks and comets
Authors: Omidi N. , Winske D.
Published: February 2008

Numerical simulations are used to investigate the nonlinear evolution of oblique low‐frequency electromagnetic (kinetic magnetosonic) waves, which have been observed upstream of planetary bow shocks and at comet Giacobini‐Zinner. The observations show that the waves are elliptically polarized and have a sinusoidal form when their amplitude is small, but they become steepened and linearly polarized as they grow in amplitude. These waves have been referred to as shocklets. A high‐frequency whistler wave packet is commonly seen at the steepened edge of the shocklets. To investigate the generation and the nonlinear evolution of kinetic magnetosonic waves in a self‐consistent manner, an electromagnetic hybrid code (particle ions, fluid electrons) is used in two stages. In the first stage, a large nonperiodic box is utilized to generate a series of small‐amplitude waves, one of which is then isolated for the second stage of the simulations, in which much higher resolution is employed to investigate further growth and the nonlinear evolution of the isolated wave. The results show that the original small‐amplitude elliptically polarized wave grows and steepens, such that its polarization changes and becomes somewhat linear. The steepening process is associated with the coherent generation of a broad spectrum of waves on the magnetosonic whistler branch, which propagate at various phase and group velocities. As a result, the waveform spreads in space, causing the change in polarization. Because the phase and group velocities of the whistler waves increase with frequency, the high end of the spectrum generated during the steepening propagates ahead of the steepened front and forms a high‐frequency wave packet, whose amplitude decreases with distance from the steepened edge of the magnetosonic wave. This drop in amplitude is due to the transitory nature of the wave packet. By investigating the behavior of the solar wind upstream and within the steepened wave it is shown that these waves are very similar to subcritical dispersive shock waves and can result in local heating and deceleration of the solar wind. This interpretation of the steepened waves has a number of implications for planetary bow shocks and the nature of the transition region at comet Giacobini‐Zinner. The presence of shocklets upstream of a planetary bow shock can modify its local structure by changing the solar wind Mach number and temperature, or by colliding with the shock. Further, it is suggested that the transition region at comet Giacobini‐Zinner consists of solar wind plasma which has been heated and decelerated by a series of steepened waves, until it becomes subsonic in the comet's rest frame.
On waves below the local proton gyrofrequency in auroral acceleration regions
Authors: Gustafsson Georg , André Mats , Matson Lligo , Koskinen Hannu
Published: February 2008

The Viking wave electric field and density fluctuation measurements together with simultaneous particle observations are used to study waves at frequencies below the local proton gyrofrequency. Such waves were observed during about 20% of nightside auroral field line crossings by Viking at altitudes between 2000 and 10,000 km. The observations are different from earlier spacecraft observations of similar waves in such a way that the center frequency in about one out of four of the observed events was below the gyrofrequency of singly charged helium, which has not been reported previously. The waves were well correlated with precipitating electrons of energies of a few keV and with VLF auroral hiss. Detailed investigations of simultaneously observed wave emissions, particles, and total densities strongly suggest that secondary peaks at keV energies in the distributions of downgoing electrons can cause the emissions.
Plasma structuring by the gradient drift instability at high latitudes and comparison with velocity shear driven processes
Authors: Basu Sunanda , Basu S. , MacKenzie E. , Coley W. R. , Sharber J. R. , Hoegy W. R.
Published: February 2008

Satellite in situ measurements made by the Dynamics Explorer 2 (DE 2) satellite were utilized to describe the nature of plasma structuring at high latitudes caused by the gradient drift instability process. Specifically, by using noon‐midnight and dawn‐dusk orbits of the DE 2 satellite it was found possible to study the simultaneous density and electric field spectra of convecting large‐scale (approximately hundreds of kilometers) plasma density enhancements in the polar cap known as “patches”) in directions parallel and perpendicular to their antisunward convection. Distinct differences were noted in the behavior of the ac and dc electric field structure and short‐scale (<125 m) density irregularities in these two mutually orthogonal directions perpendicular to the geomagnetic field. However, since these two orthogonal directions were not sampled simultaneously, the observed differences cannot be unequivocally related to the direction of convection. Structured plasma density enhancements in the auroral oval (known as “blobs”) were found to have considerable power spectral density at these short scales in the presence of significant Pedersen and Hall conductances in the 10‐ to 20‐mho range. While density irregularity amplitudes (ΔN/N)rms were found to be as large as 15–20% using 8‐s samples of the DE 2 data, the corresponding dc electric field fluctuation ΔE was found to be less than a few millivolts per meter for both patches and blobs. This (ΔN/N)RMS vis‐a‐vis ΔE behavior for the gradient drift process provided a fairly dramatic contrast with velocity shear driven processes where the ΔE magnitudes were found to be at least an order of magnitude larger for the same levels of density irregularities. The electric field spectra for the moderate shear category discussed by Basu et al. (1988a) were also found to have a significantly different spectral index as compared to such spectra associated with the gradient drift process. The results of this paper together with those of Basu et al. (1988a) provide fairly conclusive evidence for the existence of at least two generic classes of instabilities operating in the high‐latitude ionosphere: one driven by large‐scale density gradients in a homogeneous convection field with respect to the neutrals and the other driven by the structured convection field itself in an ambient ionosphere where density fluctuations are ubiquitous.
Storm time heavy ion outflow at mid‐latitude
Authors: Yeh H.‐C. , Foster J. C.
Published: February 2008

Local ionospheric observations with the Millstone Hill incoherent scatter radar reveal an upward ion bulk velocity in excess of 3 km s−1 at 1000 km altitude during the very large magnetic storm on February 8, 1986. The upward flux of O+ ions exceeded 3×109 cm−2s−1 at 42° geodetic latitude (55°Λ) for a 3‐hour period around 18 MLT during the event. Frictional ion heating with ion temperatures in excess of 4000°K at 500 km altitude was observed by the radar in the vicinity of the ion outflow event. Satellite observations place the ion outflow event within a region of intense ion and electron precipitation on field lines associated with the storm‐perturbed ring current. For a one‐dimensional analysis of the observed plasma profiles, continuity considerations indicate a region of intense O+ production (200 cm−3 s−1) as well as significant upward acceleration (5–10 m s−2) in the region between 600 km and 800 km altitude where the outflow approaches supersonic speed. Ionizing collisions involving fast backsplash neutral O atoms (Torr et al., 1974) produced by ring current heavy ion precipitation can provide sufficient upward momentum to account for the acceleration in the observed outflowing thermal O+ fluxes. Alternatively, the outflow event can be explained in terms of a time‐dependent diffusion process triggered by a sudden change in the frictional heating rate in the collision‐dominated F region (St.‐Maurice, 1989). The concurrence of rapid ion convection and energetic ring current precipitation is unique at mid‐latitudes during intense magnetic storms. Under these conditions, our observations indicate that the mid‐latitude ionosphere constitutes a significant source of upflowing thermal O+ fluxes to the overlying magnetosphere.
Simultaneous conjugate observations of dynamic variations in high‐latitude dayside convection due to changes in IMF By
Authors: Greenwald R. A. , Baker K. B. , Ruohoniemi J. M. , Dudeney J. R. , Pinnock M. , Mattin N. , Leonard J. M. , Lepping R. P.
Published: February 2008

Two conjugate HF radars are currently operating at Goose Bay, Labrador, and Halley Station, Antarctica, and are providing continuous, high temporal resolution measurements of plasma convection over large areas of the high‐latitude ionosphere. In this paper, data from these radars for a single 45‐min period about local noon on 22 April 1988 are examined to study near‐instantaneous, conjugate, two‐dimensional patterns of plasma convection in the vicinity of the cusp and their response to changes in the By component of the interplanetary magnetic field (IMF). Our observations indicate that under quasi‐stationary IMF conditions, the conjugate convection patterns are quite similar to the synthesized patterns of Heppner and Maynard [1987]. The patterns respond rapidly to changes in the IMF By component. The average response time was 8 min between an IMF transition at the IMP‐8 satellite and the beginning of a convection reconfiguration in the ionosphere. Typically, the newly reconfigured convection pattern filled the radar field of view (10° to 15° of invariant latitude and 2.5 h of local time) within 6 min of reconfiguration onset. For the examples studied, the reconfiguration onsets began over an extended local time sector in the invariant latitude range from 73° to 75° and proceeded by means of a poleward expansion of the reconfigured pattern into a region containing fossil convection associated with the prior IMF state. We interpret the onset region in the ionosphere as being associated with cusp field lines and the poleward region as being associated with open field lines in the plasma mantle and tail lobe.
Long‐term study of longitudinal dependence in primary particle precipitation in the north Jovian aurora
Authors: Livengood T. A. , Strobel D. F. , Moos H. W.
Published: February 2008

An analysis of wavelength‐dependent absorption in 229 FUV spectra (1150–1950 Å) of the Jovian north polar emissions obtained with the International Ultraviolet Explorer over the period 1978–1989 shows that the hydrocarbon optical depth in front of the auroral emission has a consistent dependence on the Jovian magnetic longitude. This variation is interpreted in two distinct ways: (1) the Jovian upper atmosphere is longitudinally homogeneous, and the variation in optical depth is due to a variation in penetration, and thus energy, of the primary particles; or (2) the energy of the primaries is longitudinally homogeneous, and it is aeronomic properties which change, most likely as a result of auroral heating. For case 1, the estimates of energy per particle for each of four particles which have been proposed as auroral primaries vary by a factor of ∼1.5–3.2 as a function of magnetic longitude, depending on the identity of the particles considered; the atmospheric model‐dependent energies are of the order of 14 keV for electrons, 0.4 MeV for protons, 0.4 MeV/nucleon for oxygen, and 0.3 MeV/nucleon for sulfur ions in a simple model polar atmosphere which we propose. Energy estimates for oxygen and sulfur ions fall well within limits established by Voyager measurements of the population of energetic O and S ions in the magnetosphere. For case 2, changes in atmospheric properties are modeled as a variation in the eddy diffusion coefficient K in the polar model, which covers the range from ∼3×106 to 2×107 cm² s−1, consistent with expected effects of localized auroral heating. Both interpretations imply a remarkable degree of stability in the coupling of the magnetosphere and atmosphere during the period of the observations, which covers a complete solar cycle and includes the Voyager encounters with Jupiter.