bluuebunny/crossref_metadata_2025 huggingface dataset:
Quiet ionospheric currents of the southern hemisphere derived from geomagnetic records
Authors: Campbell Wallace H. , Schiffmacher Edward R.
Published: February 2008

This work describes the month‐by‐month behavior of the equivalent ionospheric current systems derived from spherical harmonic analyses of the quiet time geomagnetic field daily variations in 1965 at selected observatories representing the three southern global half‐sector regions separately encompassing South America, Africa, and Australia. These external Sq current patterns were vortices having mid‐latitude foci with midday summertime amplitudes reaching 16.2×104 A above the midnight level. The wintertime amplitudes were about 10×104 A smaller. At low latitudes there was a large intrusion of the opposite hemisphere external Sq current system into the wintertime hemisphere at prenoon hours, displacing the primary current vortex to later postnoon hours. The behavior of the southern hemisphere external currents were found to be seasonally similar to those of the northern hemisphere for the same year. The quiet year behavior was compared to the results for the 1958 active year determined earlier by Matsushita. The winter‐to‐summertime increase in focus current was found to be similar in amplitude for the 2 years. The active year summertime and equinoctial current focus amplitudes were about 2.3 times the amplitudes of corresponding months in the quiet year.
Satellite observations of a new type of discrete VLF emission at L < 4
Authors: Poulsen W. L. , Inan U. S.
Published: February 2008

A new type of discrete whistler mode emission has been observed in the magnetosphere at L < 4. The emission elements are confined to a bandwidth of 1–5 kHz, with the lower cutoff frequency of the band varying with L shell, being equal to ∼0.2–0.5fHeq, where fHeq is the equatorial electron gyrofrequency. The discrete and burstlike nature of the emissions is similar to that of chorus emissions typically observed at higher L; however, dispersion of individual elements is often different from typical chorus, and the emissions are observed inside as well as outside the plasmapause. The phenomenon seems to occur mainly in the early morning local time sector (0400–0800 MLT) and is well correlated with geomagnetic activity, occurring mostly when ΣKp > 30. The analysis of data from the low‐altitude ISIS 2 and the high‐altitude DE 1 satellites indicates that the emissions may be generated near the equatorial plane at frequencies of ∼0.2fHeq inside and ∼0.35fHeq outside the plasmapause. The parallel energy of electrons for gyroresonance with the observed waves is found to be ∼20–50 keV in both cases. Observations in the vicinity of low‐altitude crossings of the plasmapause also indicate the presence of a second emission band which is interpreted to result from the reflection of the equatorially generated emissions from the lower ionosphere.
Growth of electron plasma waves above and below ƒp in the electron foreshock
Authors: Cairns Iver H. , Fung Shing F.
Published: February 2008

With increasing penetration into the electron foreshock the characteristics of the electrostatic waves driven by streaming electrons change continuously from the familiar intense waves near the electron plasma frequency ƒp to weak bursts of broadband waves initially significantly above ƒp and then well below fp. Growth well below ƒp has been demonstrated theoretically for slow, cold electron beams, and the broadband waves below ƒp in the foreshock have been interpreted in terms of the very cold or sharp “cutoff” feature of a cutoff distribution for small cutoff speeds. However, an approximate theoretical criterion indicates that the electron beams studied hitherto are unstable to reactive rather than kinetic growth, thereby favoring very narrow‐band growth contrary to the observed broadband growth. In this paper we determine conditions for kinetic growth well above and below ƒp for both cold and warm beams over a wide range of beam densities and speeds. We verify that kinetic growth below ƒp is possible for cold, slow beams and for warm, dense beams (over a wide range of beam velocities). We then argue that the observed broadband waves below fp are naturally explained in terms of slow, dilute electron beams with cold features but that an explanation involving warm, dense beams is inconsistent with the available data. Broadband growth above ƒp is explained in terms of faster, warmer beams. A unified qualitative theory for the narrow‐band and broadband waves is proposed involving (1) cutoff distributions, (2) the spatial variation in the cutoff velocity throughout the foreshock, (3) initial reactive growth with modification of the particle distribution and subsequent kinetic growth for cutoff velocities appropriate for the waves near and above ƒp, (4) initial kinetic growth and preservation of the cutoff feature for cutoff velocities appropriate to growth below ƒp, and (5) limitation of kinetic growth by inhomogeneous quasi‐linear relaxation. This theory can account qualitatively for the spatial variations in the intensity, center frequency, and bandwidth of the electron plasma waves in the foreshock. However, further work is required to understand the preservation of the cold features required for growth below ƒp and the bursty nature of the broadband waves.
Mapping the merging potential from the magnetopause to the ionosphere through the dayside cusp
Authors: Crooker N. U.
Published: February 2008

The pattern in which the potential drop generated at the magnetopause is applied across the polar cap is constructed with the use of two models: the new antiparallel merging model of the dayside magnetopause and the expanding polar cap convection model. The results yield a new set of model convection patterns. Flow into the polar cap begins at a distended, wedge‐shaped cusp, which rotates around the Chapman‐Ferarro cusp position in response to rotation of the IMF in the plane perpendicular to the Earth‐sun line. The length of the wedge base, and the potential drop across it, mapped from the merging line on the magnetopause, decrease as the IMF turns northward. For large IMF y component, rotational flow into the polar cap is distributed along one side of the wedge and feeds into a region of strong east–west flow, some of which turns sharply into the polar cap at the base of the other side of the wedge, at noon. Two convection cells driven by merging with closed geomagnetic field lines exist for all IMF orientations. The cells are confined to a circular polar cap and become distorted with rotation of the cusp, into patterns deduced independently from observations.
On the average shape and position of the geomagnetic neutral sheet and its influence on plasma sheet statistical studies
Authors: Dandouras J.
Published: February 2008

A geometric model of the average geomagnetic neutral sheet is presented. The average neutral sheet is best fitted by a smooth warped surface that crosses the solar magnetospheric equatorial plane near the tail flanks and that has no slope discontinuities. The geometry is defined to fit a magnetohydrostatic equilibrium model of the magnetotail, and the values of the parameters are determined by minimizing the Rms deviation from zero of the satellite distance to the model position of the neutral sheet when BX polarity reversals are observed. The average neutral sheet position given here is optimized for geocentric distances of the order of 20–22 RE (ISEE orbit). Comparison with the Fairfield neutral sheet model, which is optimum for R = 30 ‐ 40 RE, shows the overall geometric stability of the average neutral sheet and its alignment parallel to the solar wind flow. A small flattening that is observed as a function of the increasing geocentric distance is due to the greater magnetopause radius and the reduced influence of the tilted geomagnetic dipole. The model presented here is used to examine the spatial distribution of the particle flux dropouts observed in the plasma sheet, and it is confirmed that in the central magnetotail (|Y| < 10 RE) the plasma sheet can be completely pinched during substorms.
Ordinary mode auroral kilometric radiation fine structure observed by DE 1
Authors: Benson Robert F. , Mellott Mary M. , Huff Richard L. , Gurnett Donald A.
Published: February 2008

The fine structure observed with intense right‐hand extraordinary (R‐X) mode auroral kilometric radiation (AKR) has received major theoretical attention. Data from the Dynamics Explorer 1 plasma wave instrument indicate that left‐hand ordinary (L‐O) mode AKR possesses similar fine structure. Several theories have been proposed to explain the fine structure of the R‐X mode AKR. In order to account for the L‐O mode fine structure, these theories will have to be modified to produce the L‐O mode directly or will have to rely on mode conversion processes from the R‐X to the L‐O mode.
Duct model explanation of the plasma line overshoot observed at Arecibo
Authors: Muldrew D. B.
Published: February 2008

The plasma line overshoot, which is observed at Arecibo during ionospheric‐modification experiments, can be explained by propagation of Langmuir waves (L waves) trapped in magnetic field‐aligned ducts. The miniovershoot, i.e., the early phase of the overshoot, results from the growth of L waves to saturation within a few milliseconds of transmitter turn‐on in ducts developed during previous transmitter‐on cycles. The ponderomotive force of these trapped L waves causes the electron density in the duct to decrease rapidly from about 30 to 200 ms after transmitter turn‐on. An increase in plasma line (PL) intensity is assumed to result from this density decrease and this corresponds to the start of the main overshoot. After a few hundred milliseconds heating of the electrons begins to cause a further slow decrease in density. The heating is due to collisional and collisionless damping of the L waves. After a few seconds the duct density becomes sufficiently low that collisionless damping of the L waves responsible for the PL becomes severe and the PL intensity decreases. The ponderomotive force is maintained by other trapped L waves. Once the transmitter has been turned off the duct at first decays rapidly due to the removal of the ponderomotive force and then more slowly (10 to tens of seconds) due to both parallel and inward radial (i.e., perpendicular to the magnetic field) diffusion of plasma. The duct model can explain nearly all the observed characteristics of the PL obseved at Arecibo; other theories and models cannot.
Shock structure in classical magnetohydrodynamics
Authors: Kennel Charles F.
Published: February 2008

We examine how the structure of coplanar MHD fast and slow shocks depends upon the relative magnitudes of the dissipation coefficients contained in classical magnetohydrodynamics. We will use an asymptotic method in which the scale lengths for resistivity, thermal conduction, and viscosity are widely separated and will study the qualitative dependence of the shock solutions upon the ordering of the scales. This method may also be called that of nested subshocks. We define upper limit Mach numbers, for both fast and slow shocks, at which resistivity and thermal conduction taken together can provide all the shock dissipation and at which viscosity is not needed.
Magnetospheric convection in the nondipolar magnetic field of Uranus
Authors: Selesnick Richard S.
Published: February 2008

A method for determining the magnetospheric convection electric field using simple analytic approximations under the assumption of uniform ionospheric conductivity is described, and applied to Uranus. Magnetic field models including quadrupole and octupole moments are used to determine the shape of the polar caps and the mapping of the electric field and parallel currents between ionosphere and magnetosphere. The asymmetry in the magnetic field models between the northern and southern hemispheres leads to the inclusion of currents between the hemispheres in order to satisfy the assumption of equipotential magnetic field lines. The results show that the quadrupole moment of the Uranian magnetic field strongly influences magnetospheric convection, and that a significant octupole moment will further alter the flow pattern. Even with these modifications the basic flow is sunward in the inner magnetosphere as inferred previously. The total current which flows along field lines between the two hemispheres due to the asymmetry of the magnetic field is comparable in magnitude to that of the region 1 current system. Time dependent calculations including a self‐consistent electric field show that ring current shielding of the electric field is important and may have formed the most prominent features in the plasma observations made by Voyager 2. The effectiveness of the shielding can be influenced by the magnetic field model. Other features in the data are characteristic of substorm injection, and the model has been used to show that a combination of plasma injection and electric field shielding may be applicable to the interpretation of the Voyager 2 data.
Three‐dimensional plasma structures with anomalous flow directions near the Earth's bow shock
Authors: Paschmann G. , Haerendel G. , Sckopke N. , Möbius E. , Lühr H. , Carlson C. W.
Published: February 2008

We have examined AMPTE IRM data obtained in the solar wind near the Earth's bow shock and found 16 well‐defined cases where a region of hot subsonic plasma is embedded in the solar wind. Such structures had been observed first with instruments on ISEE 1 and 2 and later on AMPTE UKS and distinguished from bow shock crossings. Our observations confirm some of the earlier findings, notably the event profile, showing a hot, low‐density core flanked by narrow regions of high density and strong magnetic field. We also find the low (∼200 km/s) flow velocities, strongly deflected from the solar wind, and we substantially strengthen the local time dependence of the flow which invariably is directed dawnward for prenoon events and duskward for postnoon events. Our results differ from the reported ISEE results in two respects. First, the flows we observe tend to have larger angles relative to the solar wind, and they often even have a sunward component. Second, the events we have selected cannot be described as diamagnetic cavities. On the contrary, the magnetic fields are usually significantly enhanced. This apparent discrepancy may simply result from different event selection criteria. A quantitative analysis of the regions flanking the hot core shows they consist of fast, nearly perpendicular, supercritical shocks on the outside and tangential discontinuities on the inside. We find a systematic difference between the orientations of the leading and trailing edge boundary normals. While the former are directed largely transverse to the solar wind flow, the latter are more nearly aligned with the solar wind. Another new finding concerns the presence of enhanced fluxes of >70‐keV electrons which appear to be of magnetospheric origin. The majority of events are associated with directional discontinuities in the interplanetary magnetic field. We have also found events which are not embedded in the solar wind but occur between the solar wind and the magnetosheath, at times replacing the regular bow shock. Among the mechanisms discussed as causes for these events are the formation of sunward directed plasma jets from magnetopause reconnection, or from amplification of magnetic stresses associated with rotational discontinuities in the interplanetary medium; sudden and localized enhancements of bow shock reflection; and the interaction of the bow shock with tangential discontinuities having a specific internal structure.
On the origin of hot diamagnetic cavities near the Earth's bow shock
Authors: Thomsen M. F. , Gosling J. T. , Bame S. J. , Quest K. B. , Russell C. T. , Fuselier S. A.
Published: February 2008

On occasion, hot diamagnetic cavities have been observed upstream from the Earth's bow shock. The origin of these cavities has been a mystery ever since their discovery several years ago. In this paper, we note that, although only a small number of such events have so far been identified (corresponding to an average of about one event per month of observing time), for of these previously identified events occurred on a single day (November 16, 1977). Additional plasma and field data from that day are examined in considerable detail for further clues to the possible source of these events. A remarkable characteristic of the interplanetary medium on November 16 was the presence throughout that day and the next of large‐amplitude, very low‐frequency, nonmonochromatic, and noncompressive magnetic field variations (i.e., rotations), which resulted in intermittent magnetic connections between the observing spacecraft and the bow shock. The previously identified hot diamagnetic cavity events (HDCs), as well as several other intervals of HDC‐like properties observed very near the shock itself, tend to occur near transitions between quasi‐perpendicular and quasi‐parallel shock geometries. Furthermore, several of the HDC‐like episodes seen near and within the shock are closely associated with intervals of dense, nearly specularly reflected ions. These observations lead us to suggest that HDCs may form from an unusually strong interaction between shock‐reflected ions and the incoming solar wind. It is proposed that this interaction stems from a temporary and fairly localized reflection of a larger‐than‐normal fraction of the incident ions, which is stimulated by sudden changes in the upstream field orientation. The consequences of such a temporary overreflection are examined and found to be consistent with many of the observed features of HDCs, including the strong slowing, deflection, and heating of the flow as well as the localization, internal recoveries, and occasional formation upstream from the shock itself. This hypothesis also appears capable of explaining the considerable diversity of signatures seen at the leading edge of the various observed HDCs, ranging from the presence of a fast forward shock to the absence of any leading magnetic compression at all.
Formation of the wave compressional boundary in the Earth's foreshock
Authors: Skadron George , Holdaway Robert D. , Lee Martin A.
Published: February 2008

We analyze the interaction between energetic protons and hydromagnetic waves in the Earth's ion foreshock and locate compressional wave boundaries corresponding to interplanetary magnetic field (IMF) inclinations to the solar wind of θBV equal to 45° and 25°. Protons injected into the solar wind at the bow shock interact with MHD waves traveling along the IMF lines intersecting the shock. Starting with the quasi‐linear pitch angle diffusion equation, we obtain fluid equations for the densities and mean velocities of outward and inward streaming energetic protons. The excitation and damping of waves by these protons are described by linear growth rates for parallel propagation and evaluated using a model proton distribution function controlled by the local fluid variables. The coupled equations for the evolution of the wave intensities, proton densities, and mean velocities are solved numerically assuming a prescribed proton injection rate at the shock. The seed interplanetary waves are assumed to be unpolarized on average and to propagate predominantly away from the Sun, in the solar wind frame. We find that in the solar wind frame, (1) the dominant wave‐particle interaction in the outer foreshock is the damping of inward propagating (toward the shock) left‐polarized waves, producing a magnetically quiet region immediately downstream of the foreshock boundary; (2) excitation of outward propagating right‐polarized waves farther downstream leads to the recovery of δ|B| and to an upstream boundary for enhanced compressional wave activity; (3) at θBV = 45°, the calculated compressional boundary has a mean inclination of 78° from the Earth‐Sun axis, compared with the observed range of 85°±3°; and (4) at θBV = 25°, the calculated compressional boundary has a mean inclination of 42° from the Earth‐Sun axis, compared with the observed range of 42°±2°.
Three‐dimensional magnetosheath plasma ion distributions from 200 eV to 2 MeV
Authors: Williams D. J. , Mitchell D. G. , Frank L. A. , Eastman T. E.
Published: February 2008

We present initial measurements of the three‐dimensional magnetosheath plasma ion flow and spectrum over the energy range of 200 eV to 2 MeV. These observations are from two magnetosheath traversals, one each on the dawn (December 19, 1977) and dusk (July 7, 1978) flanks of the magnetosphere. Data are from three‐dimensional plasma and energetic ion observations from the ISEE 1 satellite. The data show the continuous presence of a high‐energy (≳5 keV) tail in the magnetosheath ion population during these two traversals. Comparisons with plasma sheet data lead us to conclude that for the two cases studied the magnetosheath plasma often has at least two major components, the shocked solar wind seen at energies ≲5 keV and magnetospheric (plasma sheet) particles seen at energies ≳5 keV. At times, however, magnetosheath spectra are observed that indicate the presence of only a shocked solar wind component that includes a high‐energy tail possibly produced either by ambient energetic solar particles or by acceleration processes at the bow shock. The shocked solar wind component at energies ≲5 keV behaves independently of the magnetic field direction, indicating that the frozen‐in field approximation applies. The high‐energy tail at energies ≳5 keV is highly modulated by the magnetic field. In the strongly varying magnetic field case of December 19, 1977, energetic ion transport in the plasma rest frame occurs by diffusive processes which result in an isotropic energetic ion distribution in that frame. The imposition of the plasma bulk velocity on this distribution results in the energetic ion anisotropies observed in the satellite frame. In the much more ordered field of July 7, 1978, energetic ion transport occurs via single‐particle motion along magnetic flux tubes in the plasma rest frame. The transport of flux tubes past the satellite as they change direction and connect/disconnect to remote particle sources causes the variations in energetic ion intensities and anisotropies observed in the satellite frame.
Photometric Imaging Etalon Spectrometer
Authors: Mende S. B. , Swenson G. R. , Llewellyn E. J. , Denig W. F. , Kendall D. J. W. , Slanger T. G.
Published: February 2008

A Fabry‐Perot imager that can give spectral image information for the airglow is briefly described. Such an instrument was flown on the shuttle on the STS 41‐G mission. Several spectral images of the limb view atmosphere were obtained, and rotational temperatures are derived for two of these images. The rotational temperature for the O2 A band on the limb at the emission intensity maximum was measured as 200°K. Viewing the airglow layer at a downward viewing slant path, the temperature was found to be higher. The rotational temperature was also obtained from images of the airglow at the wavelength of the OH Meinel (8‐3) band. The properties of the available filter and Fabry‐Perot combination were not ideal for this measurement. From the ratio of the line intensity of the P1(3) and the superimposition of the P2(3) and the Q(2) line the rotational temperature was found to be 220°K. These temperature measurements demonstrate the validity of the technique. Several new experiments are planned where systematic measurements of the atmospheric airglow rotational temperatures will be carried out. Such measurements will provide new insight into the global temperature structure of the mesosphere.
Density and white light brightness in looplike coronal mass ejections: Temporal evolution
Authors: Steinolfson R. S. , Hundhausen A. J.
Published: February 2008

The preceding paper (Steinolfson, this issue) describes three ambient coronal models suitable for the study of time‐dependent phenomena: (1) a static corona with a dipole magnetic field, (2) a steady polytropic corona with an equatorial coronal streamer, and (3) a heated corona with an equatorial coronal streamer. We now consider the propagation of coronal mass ejections initiated in each atmosphere by an identical energy source. A localized thermal energy input at the base of closed field regions produces mass ejections that are simulated by numerical integration of the MHD equations. We show that the computed ejection in the first atmosphere listed above does not adequately represent the general characteristics of observed looplike mass ejections. The simulated ejection in a polytropic corona with a streamer does simulate some of the observed features. It is only when a heating term is added to the initial atmosphere with a streamer that all of these are reproduced. A detailed comparison between a computed mass ejection and a representative observation demonstrates that the simulation can also duplicate some of the quantitative properties (e.g., brightness level) of individual looplike ejections.
The magnetosphere as a sufficient source for upstream ions on November 1, 1984
Authors: Sibeck D. G. , McEntire R. W. , Krimigis S. M. , Baker D. N.
Published: February 2008

We consider 10 tests which distinguish the source of energetic particles in upstream ion events. Some tests are definitive, and others inadequate. In particular, we note that the Fermi model requires a finite time of magnetic field line connection to the bow shock before energetic ions appear and inverse velocity dispersion when the interplanetary magnetic field rotates to a more radial direction but that these features are also expected for the leakage model under certain conditions. We present simultaneous magnetospheric, magnetosheath, and upstream energetic particle observations during two well‐documented upstream events on November 1, 1984, and apply the 10 tests. The events are unusual in that they occurred during a period when the magnetosphere was greatly compressed, providing for an increase in the energetic magnetospheric particle population and a decrease in the magnetosheath thickness. Magnetospheric leakage satisfactorily explains our observations on this day, but in situ Fermi acceleration does not. Consequently, we consider magnetospheric leakage a sufficient source for upstream particles during these two events.
UV resonance line dayglow emissions on Earth and Jupiter
Authors: Gladstone G. Randall
Published: February 2008

Atomic resonance line emissions have been used to probe the structure and composition of the upper regions of planetary atmospheres. In this paper the similarities and differences for some emissions of importance in the atmospheres of Earth and Jupiter are examined. For each planet both a conservative resonance line and a line for which there is a considerable branching ratio are considered. On Earth examples of important conservative and nonconservative resonance emissions are OI 1304 Å and OI 989 Å, respectively. Both of these features are excited by photoelectron impact on atomic oxygen. The OI 1304‐Å triplet has an additional source due to resonant scattering of the solar OI 1304‐Å emission feature. These emissions are typically observed from a much different perspective than corresponding emissions from other planets, usually from inside the atmosphere, at substantial line‐center optical depths. On Jupiter the brightest conservative resonance line is HI 1216Å (Lyman α), while HI 1026 Å (Lyman β) is an example of an important nonconservative resonance line. In this paper, models are presented for the intensities of these features as would be seen from a distance of several planetary radii, using the same geometry of observation for both Earth and Jupiter. The variations of the line‐integrated emissions across the disk and near the limb of each planet are examined in detail. Line profiles for the emissions near the limb and at disk‐center are also examined. The models reveal the importance of including inhomogeneities and temperature variations of the atmospheres in scattering models, and indicate that outer planet emissions previously interpreted as “electroglow” may be solely due to resonant scattering of solar emissions.
Time‐dependent modeling of field‐aligned current‐generated ion transients in the polar wind
Authors: Gombosi T. I. , Nagy A. F.
Published: February 2008

The time evolution of field‐aligned current‐generated transient features in the high‐latitude ionosphere is investigated. Ionospheric return currents generate significant downward heavy ion flows in the topside ionosphere with peak values well exceeding 108 cm−2s−1. When the return current ceases the polar ionosphere rapidly returns to its previous equilibrium state. During the recovery phase of the return current event an upward propagating heavy ion transient is formed, which is mainly characterized by a relatively short O+ upwelling event. On the other hand, the H+ escape flux remains relatively constant (within 10‐20%) during field‐aligned current events. It is also found that upward currents generate a transient heavy ion outflow, which exceeds the ambient H+ escape flux by a factor of 3 to 5, depending on the duration and strength of the field‐aligned current event.
Relaxation of magnetotail plasmas with field‐aligned currents
Authors: Paranicas C. , Bhattacharjee A.
Published: February 2008

Recently, equilibria of low free energy have been constructed for magnetotail plasmas (Bhattacharjee, 1987). This model is extended to obtain axisymmetric states which have the structure of flux ropes containing field‐aligned currents. An interesting feature of these equilibria is a chain of magnetic islands along the tail axis. Theoretical results are compared with observations from ISEE 1 and ISEE 2 on flux ropes (Elphic et al., 1986). A sufficient condition for stability is formulated.
A nonlocal linear theory of the gradient drift instability in the equatorial electrojet
Authors: Ronchi C. , Similon P. L. , Sudan R. N.
Published: February 2008

The linear global eigenmodes of the gradient drift instability in the daytime equatorial electrojet are investigated. A main feature of the analysis is the inclusion of ion‐neutral and electron‐neutral collision frequencies dependent on altitude. It is found that the basic characteristics and localization of the unstable modes are determined mainly by the profiles of the Pedersen and Hall mobilities, which are derived from the Cowling conductivity model and experimental data. The equilibrium density profile is parabolic, which is fairly representative of the actual measurements. The unstable modes are sensitive not to the details of this profile, but only to the average value of the gradient. The results are obtained from a direct numerical integration of the nonlocal linearized equations. They are further analyzed through an eikonal analysis, which provides both an interpretation of the transient modes observed by Fu et al. (1986) and some additional physical insight into the linear evolution of the global unstable modes. Finally, it is shown that the previously reported short‐wavelength stabilization effect due to velocity shear may be overshadowed by the presence of regions in which the transient modes can develop into absolute instabilities.
Tool kit for antennae and thermal noise near the plasma frequency
Authors: Meyer‐Vernet Nicole , Perche Claude
Published: February 2008

This paper provides the essential tools for deriving quickly the quasi‐thermal noise spectrum or the impedance of a given electric antenna near the plasma frequency, for calibration or diagnosis in space plasmas. We give simple analytical expressions and numerical results for either wire or sphere dipoles in an isotropic plasma with one or two Maxwellian electron populations. We include the contribution of the particles collected and/or emitted by the antenna surface. We also indicate some modifications brought about by using more complicated antenna geometries, and a drifting or a magnetized plasma. Finally, we give some conclusions for antenna design or data interpretation in plasma wave experiments.
Analytical study of the relativistic dispersion: Application to the generation of the auroral kilometric radiation
Authors: Le Queau Dominique , Louarn Philippe
Published: February 2008

The measurements recently performed by the Viking spacecraft have shown that, in addition to being cold plasma depleted, the source regions of the Auroral Kilometric Radiation (A.K.R.) are characterized by a relatively denser, more energetic electron component (a few perticle by cm−3, 〈E〉 ≈ 5 KeV). In order to properly study the Cyclotron Maser Instability (C.M.I.) which is thought to be responsible for the A.K.R. generation, it is thus necessary to include relativistic corrections in both the hermitian and the antihermitian parts of the dielectric tensor characterizing the linear properties of the plasma. Here one presents an analytical study of the corresponding dispersion equation which aims to describe stable and unstable waves having frequencies lying very close to the electronic gyrofrequency and propagating across the geomagnetic field with a perpendicular refractive index less than a few units (n⊥<5). The electronic population of the auroral plasma is supposed to have two components. The distribution function of the non‐thermal, energetic component is modelled by a Dory‐Guest‐Harris (DGH) distribution function and those of the tenuous thermal one by either a Dirac's function or a maxwellian. The dispersion equation thus depends on four parameters:

i) the normalized peak energy of the DGH distribution: δ = 1/2 (v0/c)² which is a small number and scales both the temporal growth rate and the bandwidth of the CMI.

ii) a parameter characterising the density of the energetic population: P = 1/δ(ωph/ωc)²(ωph being the plasma frequency of the energetic component).

iii) the proportion of thermal plasma : χ = nc/nh where nc and nh are the density of the thermal and the hot plasma components respectively.

iv) the ratio τ between the peak energy of the DGH and the mean energy of the maxwellian population.

Our calculations allow us to simply perform a parametric study of the various regimes presented by the CMI. For the range of parameters inferred from measurements of the Swedish spacecraft Viking: (P>1 and χ small), the growth rate could maximize at the cut‐off frequency of the relativistic X mode. It can be quite large: Im(ω)max ≈ δωc (10−2 ωc for electrons of 5 Kev). Moreover, for small χ, the relativistic X mode is connected to freely propagating modes which guarantees an easy access of the electromagnetic energy to free space.
Magnetosphere‐thermosphere coupling: An experiment in interactive modeling
Authors: Forbes Jeffrey M. , Harel Moshe
Published: February 2008

The Rice convection model (RCM) is utilized to investigate the electrodynamic coupling between the inner magnetosphere and the thermosphere including the effects of EUV‐ and convection‐driven neutral winds under quasi‐equilibrium conditions. It is shown that the parameters determining the coupling are the Pedersen and Hall “effective winds”, which are the height integrals of the respective conductivity‐weighted wind profiles divided by the respective layer conductivities. Their appearance in the RCM is equivalent to a two‐slab formulation whereby the integrated Hall conductivity originates in the lower slab, the integrated Pedersen conductivity originates in the upper slab, and the height dependence of the neutral wind is accounted for by assuming different wind vectors for the lower and upper slab. A unique aspect of the study is that the convection‐driven winds are included self‐consistently and interactively; that is, a steady state wind parameterization is written analytically in terms of the electrostatic potential, which is in turn included in a closed‐loop calculation for the electric potential itself. Simulations are performed from 1400 UT to 1600 UT during the CDAW‐6 interval on March 22, 1979, when the cross‐cap electric potential attains values of order 140–180 kV. During the early phases of the disturbance when the normal shielding from high latitudes breaks down, the neutral winds do not modify appreciably the disturbance electric fields at middle and low latitudes. As the system approaches a quasi‐equilibrium state, the neutral winds play a much more significant role. By comparison with the “no‐wind” simulation, the fields driven by EUV winds counteract the fields of magnetospheric origin and give the appearance of a shielding effect. The convection driven component of the neutral wind similarly acts to reduce the southward field in the noon sector, but gives rise to an enhancement in the dusk sector field extending to middle latitudes. The parameterized Pedersen effective winds are of order 300 ms−1 and reflect the familiar two‐cell pattern with antisunward flow over the polar cap and return flows in the dawn and dusk sectors. These amplitudes and similarity with the ion drift motions reflect the relatively large contribution to the Pedersen effective winds originating in the upper E region and lower F region of the ionosphere. Possibilities for introducing further sophistication into the wind parameterization are discussed, as well as ramifications of the present study on the possible merging of the RCM with the NCAR TGCM to attain a higher degree of self‐consistency and reality in modelling efforts.
Absolute differential and integral electron excitation cross sections for atomic oxygen, 8, The ³P → 5So transition (1356 Å) from 13.9 to 30 eV
Authors: Doering J. P. , Gulcicek E. E.
Published: February 2008

The absolute direct excitation cross section of the 9.14 eV O I ³P → 5So transition (1356 Å) has been measured from 13.9 to 30.0 eV incident energy. The measurement was carried out by comparing the 9.14 eV 5So signal to the 9.51 eV ³So signal and using the known ³P → ³So transition cross section for normalization. The differential cross sections (DCS) peak near 60° scattering angle and generally decrease both at very small and large angles although the 13.9 eV DCS is nearly isotropic. The DCS values were integrated to give integral cross sections (ICS) at the various incident energies. The result for the peak of the cross section near 15 eV was 3 × 10−18 cm² (±40%), in excellent agreement with the calculations of Julienne and Davis (1976). However, the ICS was significantly more sharply peaked in energy than the theoretical result. The results along with previous measurements of the ³P → 5P transition direct excitation cross section and total emission cross section experiments are consistent with a maximum value of the total 1356 Å emission cross section at the peak of 9–12 × 10−18 cm².
Energetic (0.1‐ to 16‐keV/e) magnetospheric ion composition at different levels of solar F10.7
Authors: Lennartsson W.
Published: February 2008

Data obtained in the near‐equatorial magnetosphere, between L = 3 and R = 23 RE, by the plasma composition experiment on ISEE‐1 are examined for possible effects of varying solar activity, as measured by the daily F10.7 index. The data consist of velocity moments for H+, He++, He+, and O+ ions, primarily number densities and mean energies, integrated over the 0.1‐ to 16‐keV/e energy range. These are grouped into four ranges of F10.7, less than 100, 100 to 150, 150 to 200, and greater than 200, using two methods. In one the data are averaged over geomagnetic activity, in the other the data are restricted to times of AE<200 γ. In both cases, the strongest effect is found in the number densities of the He+ and the O+, which increase by factors of about 3–5 and 5–10, respectively, over the full range of the F10.7, the rate of increase varying somewhat with location. The peak density of the O+ is about 20 times that of the He+ and is the highest at L ∼ 3–5 (>1 cm−3). Both species show a decreasing energy with increasing F10.7 at R<10 RE, from about 4–5 keV at low F10.7 to about 2–3 keV at high F10.7. The O+ shows a similar but weaker trend at greater distances as well. These effects are presumably all caused by the increase in the solar EUV irradiation of the Earth' atmosphere over the rising phase of the solar cycle (cycle 21), in ways that have been discussed elsewhere in the literature in conjunction with similar effects in other data sets. A third species that shows an increasing density over the time period of these data is the He++ (by about a factor of 3). It is argued, based on published solar wind data, that this effect is due to a variation of the helium abundance in the solar wind. The fourth species, the H+, is generally the dominant one at L>7 and appears to have both solar and terrestrial origins. It does not show a strong increase in density anywhere, however. In the plasma sheet it actually decreases slightly with increasing F10.7 (about 25% over the full range of the F10.7), an effect that is ascribed in part to a known variation of the solar wind proton density and in part to the apparent tendency of the plasma sheet to maintain a constant total pressure (including all ion species). At low L the H+ density appears to be dominated by a terrestrial source, but shows little or no increase with the F10.7, suggesting that this source is essentially saturated with respect to the EUV (F10.7>80).
Polar cap deflation during magnetospheric substorms
Authors: Moses J. J. , Siscoe G. L. , Heelis R. A. , Winningham J. D.
Published: February 2008

The expanding/contracting polar cap model has been used to simulate DE 2 ion drift data during substorms as determined using the AL index. Of the 39 cases modeled, 57% required the opening of a nightside gap which maps to where reconnection occurs in the tail; 75% of the 16 recovery phase cases required a nightside gap while only 29% of the 17 expansion phase cases required a nightside gap. On the basis of this result, we conclude the following. If a nightside gap implies tail reconnection, then reconnection probably occurs after expansion phase onset and continues throughout most of the recovery phase of a substorm.
Relativistic electrons in Saturn's inner magnetosphere and an estimate of their synchrotron emission
Authors: Van Allen J. A. , Grosskreutz C. L.
Published: February 2008

Energetic electron data obtained by the University of Iowa instrument during Pioneer 11's two‐way traversal of Saturn's inner magnetosphere in September 1979 are reviewed. There were substantial differences between inbound and outbound observations. It is argued that the inbound data are more likely to represent the time‐stationary state. Adopting these inbound data, we develop a quantitative model for the spatial and spectral distributions of relativistic electrons. The principal features of this model are as follows: (1) The radial dependence of omnidirectional intensity J in the equatorial plane is given by J = k′ exp(−1.05 x)[1 − exp(−6.5 x)] where k′ = 1.28 × 107 (cm² s)−1 and x = (r−2.30) with r the radial distance in units of the planet's radius 60,000 km. This distribution is applicable to the range 2.30 ≤ r ≤ 3.60. (2) The latitudinal dependence of J is derived from observed pitch‐angle distributions. These distributions are reasonably well represented by j(α) = j(90°)sin1.5 α (where j is the unidirectional intensity and α is the pitch angle), if one ignores the relatively small depletion of intensity near α = 90°, such depletion being significant in other contexts. (3) As shown previously, the energy spectrum is a relatively narrow one with characteristic energies E in MeV at various r as follows: 0.69 at 5.0; 1.10 at 4.0; 1.89 at 3.0; 2.62 at 2.5; and 3.05 at 2.30. The synchrotron emission of the entire population of relativistic electrons in Saturn's inner magnetosphere is estimated to be about 1 kW with a spectral maximum at 720 kHz. It is shown that this radiation will be very difficult, if not impossible, to observe, even in the near vicinity of the planet. The paper includes a crude but instructive explication of the truly enormous differences between the synchrotron emissions of the inner magnetospheres of Jupiter and Saturn. Indeed, this comparison was an underlying motive for undertaking the present study.
Electromagnetic waves and instabilities from cometary ion velocity shell distributions
Authors: Gary S. Peter , Sinha Raji
Published: February 2008

This paper considers the linear theory of electromagnetic waves and instabilities in the solar wind in the presence of cometary ions with complete velocity shell distribution functions. A collisionless, homogeneous plasma, propagation parallel or antiparallel to a uniform magnetic field and frequencies below the proton cyclotron frequency are considered. If there is no relative drift speed between the cometary ions and the solar wind protons, both right‐ and left‐hand polarized modes are stable, with ion cyclotron damping at wavenumbers of the order of the inverse cometary ion gyroradius. If there is a relative drift between the cometary ions and the solar wind protons greater than the Alfvén speed, both the ion/ion right‐hand and left‐hand resonant instabilities grow with maximum growth rates which increase as the shell distributions become narrower in velocity space.
A measurement of lunar tides in the mesosphere at Adelaide, South Australia
Authors: Stening R. J. , Vincent R. A.
Published: February 2008

Wind measurements at Adelaide (35°S) at heights from 76 to 98 km are analyzed for semidiurnal (or semimonthly) lunar tides. Two data sets are used, one of about 2 years (1984–1985), and one of 1 year (1986). The lunar tide amplitude generally has its largest values at the equinoxes, though this pattern changes in 1984–1985 at the greatest heights to solstice maxima, especially in winter (July). Diurnal average amplitudes reach 4 m s−1 while the largest value obtained was 8 m s−1. Inferred vertical wavelengths of the propagating tides were rather variable, and there was evidence of possible mode coupling and interference. In summer, wavelengths of 25 km occurred below 92 km and were evanescent above, while in winter, long wavelengths were found in the 78 to 90 km range with shorter 38‐km wavelengths above that height. Comparisons of phases with results from Saskatoon and Kyoto suggest that around 95 km antisymmetric modes may be dominant in June and December and symmetric modes in February. Diurnal variations in the lunar tide were evident, and year‐to‐year changes were seen in some aspects. Comparison with currently available models shows several points of disagreement.
Damping of magnetospheric cavity modes: A discussion
Authors: Allan W. , Poulter E. M.
Published: February 2008

Impulsive changes in the solar wind can deposit energy into magnetospheric cavity modes. We discuss the coupling of cavity modes to the field line continuum, and show that the time scales for dissipative phase mixing and mode conversion to kinetic Alfvén waves in the magnetosphere are long compared with lifetimes of MHD wave events with periods that vary continuously with radial distance. Therefore the ultimate dissipative sink for cavity mode energy should be the ionosphere. For model magnetospheres in which all wave components have a complex field aligned wave number kz = kz0 (1+iκ) to simulate a Poynting flux into the ionosphere, we use simple arguments to show that the coupled cavity mode damping decrement is given approximately by γ/ω0 = (γ/ω0)c + (kz0/k)² κ. Here (γ/ω0)c is the damping via coupling to the field line continuum alone, and k is the total wave number magnitude. Detailed numerical calculations in a cylindrical magnetospheric model support this, although significant departures from the approximate expression can occur for small κ. Following earlier work by other authors, we emphasize that complex kz may not simulate the appropriate boundary condition for the coupled cavity mode outside resonance regions.