bluuebunny/crossref_metadata_2025 huggingface dataset:
Short‐wavelength lower mantle seismic velocity anomalies
Authors: Machetel Philippe
Published: February 2008

The lower mantle seismic velocity anomalies display very smooth shapes and low amplitudes in apparent contradiction with the thermal anomalies obtained from chaotic convection models. An attempt to reconcile theoretical convection models with tomographic results is presented in this paper. Along with a coherent approach of lower mantle convection, the influences of radial and angular filtering on the shapes and on the amplitudes of anomalies have been monitored. A tremendous spreading of the radial anomalies into horizontal structures and a significant loss of amplitude of the anomalies result from this exercice. The level of the Root Mean Square of the thermal anomalies falls from several tens of degrees to values in close agreement with tomographic results. This study shows that the short‐wavelength of lower mantle velocity anomalies are dramatically hidden by the truncation level of seismic tomography.
On the nature and rate of resurfacing of Venus
Authors: Arvidson Raymond E. , Grimm Robert E. , Phillips Roger J. , Schaber Gerald G. , Shoemaker Eugene M.
Published: February 2008

Crater production and obliteration are modeled for the plains of Venus, using: (1 ) the observed distribution of Venus‐crossing asteroids and comets, (2) viscous relaxation of crater topography, and (3) erosion and burial by atmospheric, volcanic, and tectonic processes. Crater lifetimes are assumed to be proportional to crater depths for both classes of obliterative processes although the individual criteria vary. An average crater retention age between 0.4 to 2.0 Ga is estimated for plains under the assumption that craters are produced and not removed. The range is driven by uncertainty in identifying degraded impact as opposed to volcanic craters. On the other hand, crater retention ages greater than about 1.6 Ga are unlikely if viscous relaxation operates without loading of crater floors by burial. Our preferred model has plains subject to crater production and obliteration processes that vary over both space and time. In some areas, radar‐bright crater ejecta haloes are preserved for long periods of time because volcanism, tectonism, and weathering occurs at rates « 1 km/Ga. Viscous relaxation has probably generated numerous shallow craters in these relatively quiescent regions. In other areas, volcanism and tectonism have resurfaced the terrain at rates greater than several km/Ga. The global coverage and high resolution SAR and altimetry data expected from Magellan will allow testing of this model, based on detailed crater observations (diameter, depth distributions; morphologic criteria; surface scattering properties) and their association with volcanic and tectonic features.
Modeling core fluid motions and the drift of magnetic field patterns at the CMB by use of topography obtained by seismic inversion
Authors: Kohler Monica D. , Stevenson David. J.
Published: February 2008

The thermal wind equations, in which the Coriolis force is balanced by pressure gradients and horizontal density gradients rather than by Lorentz forces, are used to describe patterns of magnetic field drift associated with core fluid motions near the core‐mantle boundary (CMB). The advection of magnetic field may be due in part to the flow driven by such horizontal temperature gradients, just as East‐West air flow is driven by North‐South temperature gradients in the Earth's atmosphere. It is argued that this flow may be concentrated in a shell near the CMB, and the horizontal temperature gradients are expected to be directly proportional to horizontal gradients in CMB topography, the lowest harmonics of which are approximately constrained in seismology. Part of the zonal drift is then associated with the 1=2, m=0 harmonic of CMB topography, and anticyclones are attached to topographic highs (thermal highs). Comparison of our derived flow pattern with those determined purely by magnetic field observations provides tentative support for our model.
Observed envelopes of coupled seismic free oscillations
Authors: Park Jeffrey
Published: February 2008

On a rotating, aspherical earth, coupling between spheroidal and toroidal seismic free oscillations is manifested in the time domain as a slow alternation of the oscillation envelope between the transverse and the radial and/or vertical motion components. Using a narrow‐band smooth envelope estimator, we have observed this behavior in data from the MS = 8.1 Macquarie Ridge earthquake of 5/23/89. Rotational coupling is the principal cause of mixed‐type coupling for f < 4.2 mHz. However, envelope perturbations consistent with mixed‐type coupling are observed for 0S38‐0T36 and 0S37‐0T35, which do not couple through rotation, and couple only weakly through smooth upper mantle velocity models. Such envelope perturbations can bias estimates of modal Q, and suggest that scattering may play a role in the attenuation of long‐period surface waves.
Comparison of plasma wave measurements in the bow shocks at Earth, Jupiter, Saturn, Uranus and Neptune
Authors: Moses S. L. , Coroniti F. V. , Kennel C. F. , Kurth W. S. , Gurnett D. A.
Published: February 2008

We present plasma wave measurements from the Voyager 2 crossing of Neptune's bow shock and compare them with measurements from the bow shocks of Earth, Jupiter, Saturn and Uranus. The wave amplitudes above 0.01fp, when normalized to the solar wind ion thermal energy density at each planet, are significantly higher at the outer planets than at Earth. Despite the differences in amplitude the shock spectra of all the planets can be fitted to curves of similar fora in this frequency range. The total normalized electric field energy densities exhibit an exponential dependence on ion thermal Mach number. Magnetosheath wave energies are comparable at all of the planets when normalized to the downstream plasma pressure.
Site response across downtown Santa Cruz, California
Authors: Cranswick Edward , King Kenneth , Carver David , Worley David , Williams Robert , Spudich Paul , Banfill Robert
Published: February 2008

Some buildings in Santa Cruz, California, particularly those in the downtown section which is built on a flood plain, were severely damaged by the 17 October 1989 Loma Prieta Earthquake. During an aftershock study conducted in the fortnight following the mainshock, an array of seismographs was deployed across downtown to study the site response of the flood plain. Analyses of the records of eight aftershocks reveal that ground motions of the flood plain are amplified relative to those of crystalline rock sites by factors of 4–8 in the frequency band 2–8 Hz. Both resonance in the whole sedimentary column which overlies the crystalline basement and resonance/broad‐band amplification in the low‐velocity alluvium of the flood plain contribute to this site response.
The lobe to plasma sheet boundary layer transition: Theory and observations
Authors: Schriver D. , Ashour‐Abdalla M. , Treumann R. , Nakamura M. , Kistler L. M.
Published: February 2008

The lobe and the plasma sheet boundary layer in the Earth's magnetotail are regions of different plasma conditions and share a common interface. The transition from the lobe to the plasma sheet boundary layer is examined here using AMPTE/IRM data. When the satellite crossed from the lobe to the plasma sheet boundary layer, intense narrow banded wave bursts at 1 kHz were observed and then broadband electrostatic noise (BEN) immediately followed. Simultaneous with the onset of BEN, high energy earthward streaming proton beams at > 40 keV (> 2700 km/s) were detected. These results are used as input into a numerical simulation to study ion beam instabilities in the PSBL.
The fast atomic oxygen corona extent of Mars
Authors: Ip W.‐H.
Published: February 2008

The production of fast oxygen atoms in the Martian exosphere as a result of the electron dissociative recombination of

ions is re‐evaluated for the time period appropriate to the Phobos 2 mission. Because of the prevalent condition of solar maximum activities in 1989, both the upper atmosphere and ionsphere should have more extended structures than those encountered during the Viking Orbiter (VO) mission in 1976 which was near solar minimum. We compare the distributions of the hot oxygen atoms for these different levels of solar activities and find that the number density of the hot oxygen atoms during the Phobos 2 mission should be a factor of two larger than estimated for the time period of the VO mission. The non‐thermal escape rate of the exospheric oxygen atoms is thus comparable to the ionospheric escape rate measured in the Martian magnetotail. Furthermore, the flux of the oxygen ions picked up in the hot atomic corona should be more intense than previously thought.
Plasma sheet polytropic index as inferred from the FPE measurements
Authors: Zhu X. M.
Published: February 2008

Recent calculations of the plasma sheet polytropic index based on different data sets resulted in quantitatively different conclusions. Baumjohann and Paschmann [1989] (referred to as BP) used AMPTE/IRM data, obtaining an index around 5/3. Huang et al. [1989] (referred to as HCF) used ISEE LEPEDEA data, obtaining an index less than 1. Using the ISEE FPE ion data, we demonstrate in this work that 1. the above noted difference is due primarily to the different variables the authors used to estimate the index; 2. the estimated index varies with the degree of the data scattering; 3. the polytropic index of the central plasma sheet is found to be larger than 1 but less than 1.67; 4. the variation of the specific entropy among flux tubes is considerable and indicates the different origins of the plasma in different flux tubes.
Anomalous transport produced by kinetic Alfvén wave turbulence
Authors: Hasegawa Akira , Mima Kunioki
Published: February 2008

Alfvén wave turbulence is shown to produce anomalous transport phenomena if the wavelength in the direction perpendicular to the magnetic field is comparable to the ion gyroradius. Because the scale length of the Alfvén wave turbulence is much larger than that of electrostatic waves, such anomalous transport phenomena play crucial roles in a large‐scale plasma such as the plasma in space. Applications to diffusion of radiation particles and ring current, heating of plasmapause electrons, and formation of the stable auroral red arcs as well as viscous interaction between the solar wind and the magnetosphere are discussed.
Nonconvective and convective electron cyclotron harmonic instabilities
Authors: Ashour‐Abdalla M. , Kennel C. F.
Published: February 2008

We present a systematic parameter study of the spatial growth rates of electrostatic waves in an electron plasma consisting of a cold component and a hot component with a weak ‘loss cone’ perpendicular velocity distribution. The cold electron density controls which harmonic band can be excited. When the upper hybrid frequency based upon the cold electron density alone is between 1 and 2 times the electron cyclotron frequency, only the first harmonic band is unstable; when the upper hybrid frequency is between 2 and 3 times the cyclotron frequency, the first and second harmonic bands are unstable; and so on. Sufficiently large cold density eventually stabilizes the low harmonic bands. The cold electron temperature Tc controls the spatial amplification; when 0 < Tc/TH < a few times 10−2, where TH is a characteristic energy of the hot electrons, the instability is nonconvective. The first and second harmonic bands can be simultaneously nonconvective. The nonconvective property has been found for a wide range of hot and cold electron densities. It persists even when the hot electron loss cone is almost completely filled. Sufficiently large Tc/TH changes the instability from being nonconvective to being convective. Since cold electrons which come from the ionosphere would be confined to the loss cone if they were not turbulently scattered, the ionospheric source velocity distributions induce a nonconvective instability. However, cold electrons can be heated by electrostatic wave turbulence. Until Tc/TH = 5 × 10−2, nonresonant, quasilinear diffusion heats the cold electrons more rapidly than resonant diffusion heats or scatters the hot electrons into the loss cone. We propose that cold electron heating removes the nonconvective property, so that the spatial amplification of the instabilities can be reduced to a magnitude consistent with their hot and cold electron sources in steady state.
Summarized results of the Artificial Auroral Experiment, Precede
Authors: O’Neil R. R. , Bien F. , Burt D. , Sandock J. A. , Stair A. T.
Published: February 2008

A 2‐kW electron accelerator was launched in October 1974 from the White Sands Missile Range, New Mexico, as the initial launch in the Excede series of artificial auroral experiments. The launch, designated Precede, was supported by a number of ground‐based optical systems to record the electron‐induced atmospheric emissions as a remote diagnostic technique of accelerator performance in addition to recording emissions of aeronomic interest in a controlled artificial aurora. The electron source, square wave modulated at 0.5 Hz, was initiated at 95 km on payload ascent and continued through apogee (120 km) to a descent altitude of approximately 80 km, providing a total of 90 pulses of the 2.5‐kV 0.8‐A electron beam over a period of 180 s. A rocket‐borne retarding potential analyzer provided a measure of the energy distribution of electrons returning to the vehicle skin. The energy distribution of the return current electrons has been compared with laboratory measurements of the energy distribution of secondary electrons as a function of scattering angle to infer a vehicle potential due to a net positive charge buildup on the electron‐emitting payload. The steady state vehicle potential at apogee is less than 30 V, with substantially smaller values determined at lower altitudes. Langmuir probe theory is shown to model accurately the altitude dependent steady state vehicle potential. Ground‐based optical systems included an image‐intensified spectrograph and a dual channel telephotometer recording the time dependent emission profile of the N2+ (B²Σu+ → X²Σg+) first negative (0‐0) band at 3914 Å and the O(¹S → ¹D) transition at 5577 Å. The spectrograph recorded emissions in the 4200‐ to 8500‐Å wavelength range including the prominent transitions of the N2+ first negative, N2(B³Πg → A³Σu+) first positive and N2+ (A²Πu → X²Σg+) Meinel systems as well as the O(¹S) 5577‐Å line. With the exception of the N2+ first negative system the image‐intensified spectrograph indicated that these emissions are collisionally deactivated in the 80‐ to 110‐km altitude range. The ground‐based telephotometer measurements were corrected for the effects of atmospheric extinction by monitoring the apparent photon emission rates of several bright stars. An electron‐induced luminous efficiency of (4.5 ± 0.4) × 10−3 was determined for the N2+ 1 N (0‐0) transition at 3914 Å in the 80‐ to 100‐km altitude range.
On the location of auroral arcs near substorm onsets
Authors: Lui A. T. Y. , Burrows J. R.
Published: February 2008

Two fortuitous polar passes of the Isis 2 satellite are used to examine the location of auroral arcs near substorm onsets in relation to the low‐altitude magnetospheric plasma boundaries. In one case the auroral arc was brightening at substorm onset during the pass. In the other case the satellite passed the arc after its brightening but just prior to its poleward expansion. In both cases the arcs are situated near the transition between the central plasma layer and the boundary plasma layer as defined by Winningham et al. [1975]. The arc location was also near the poleward termination of the durably trapped energetic electrons (E > 40 keV). Thus these observations suggest that the substorm triggering process occurs on closed magnetic field lines well within the outer and inner edges of the plasma sheet.
Magnetic flux transfer associated with expansions and contractions of the dayside magnetosphere
Authors: Holzer Robert E. , Slavin James A.
Published: February 2008

An empirical magnetopause model is developed by using Ogo 5 magnetometer data, solar wind data from three satellites, and auroral zone magnetic indices. By employing this model it is found that the average displacements of the dayside magnetopause due to changes in solar wind dynamic pressure psw are comparable with, but greater than, the average displacements associated with the transfer of magnetic flux between the nose and the tail of the magnetosphere. After correcting for changes in psw, multiple encounters of the magnetopause occurring over distances of ≥0.5 RE on inbound or outbound legs of the satellite orbit are termed contraction or expansion events, respectively. Events occurring during periods of significant ring current, as judged by the Dst index, are excluded from the study. The remaining 24 expansion and contraction events are attributed to net changes ΔΦ in the total magnetic flux present in the dayside magnetosphere. The model dependence of the results is discussed, and for the events observed it is found that 2 × 1015 < ΔΦ < 2 × 1016 Mx. For the expansion events, ΔΦ shows a positive correlation with the time integral of the auroral zone magnetic index AL over the temporal extent of the observation. A similar correlation between the net flux losses during contraction events with the southward component of the IMF flux impinging on the magnetosphere is complicated by flux returning from the tail during the erosion process. Displacements associated with net flux transfers were observed in all regions of the dayside magnetopause accessible to the satellite, a finding suggesting that the entire dayside magnetosphere is affected by the flux transfer process. The bearing of these results on magnetic reconnection in the earth's magnetosphere is briefly discussed.
Geomagnetic field fluctuations at synchronous orbit 1. Power spectra
Authors: Arthur C. W. , McPherron R. L. , Lanzerotti L. J. , Webb D. C.
Published: February 2008

Power spectra of geomagnetic field variations at synchronous orbit were calculated for the month of August 1974. Two segments of approximately 9 hours each, centered at local noon and local midnight, were used for the analysis. Each spectrum was parameterized as a straight line in the log‐log regime. The power spectral density at 1 mHz was higher for local night (particularly for the D component) than for local day and increased with geomagnetic activity as indicated by ∑Kp for the half day. The slopes of the straight line fits to the spectra ranged mainly from −1.5 to −3.0 and showed no clear difference night to day (except for the H component) and no clear relationship to geomagnetic activity. The day‐night difference in power can be explained mainly by broadband noise caused by substorms. The average power spectrum at synchronous orbit, as represented by the ATS 6 observations, was compared with reported observations from the interplanetary medium, the magnetosheath, the region inside the magnetosphere near the magnetopause, and the ground.
Geomagnetic field fluctuations at synchronous orbit 2. Radial diffusion
Authors: Lanzerotti L. J. , Webb D. C. , Arthur C. W.
Published: February 2008

Power spectra of geomagnetic field variations measured at synchronous equatorial altitude (geomagnetic shell parameter L ∼ 6.6) in the magnetosphere are used to calculate the time dependence of the radial diffusion coefficient for particles in the radiation belts. The diffusion coefficients calculated are mainly applicable for relativistic electrons. The magnitudes of the derived diffusion coefficients using data only from local day observations are consistent with those reported from analyses of most particle observations and thus are slightly larger than those derived from magnetic sudden commencements. They are consistent with the diffusion coefficients calculated from power spectra of ground‐based geomagnetic data measured near L = 4.
The relation of open magnetic structures to solar wind flow
Authors: Levine Randolph H.
Published: February 2008

A study has been made of solar wind observations and models of the low‐coronal magnetic field of the sun during the Skylab mission. Potential sources of solar wind plasma have been identified and in most cases systematically associated with observed solar wind flow features. The study pinpoints the problems of associating significant portions of the observed solar wind with sources outside coronal holes, although such associations are strongly suggested by the models. The phenomenological relationship between observed high solar wind speed at 1 AU and small coronal field divergence near the sun is examined and found to be well established but causally inconclusive in light of models of the outer heliosphere.
Linear theory for the collisionless drift wave instability with wavelengths near the ion gyroradius
Authors: Costa Emanoel , Kelley M. C.
Published: February 2008

In a companion paper a role is suggested for drift wave modes in the theory of equatorial spread F. Here, some results of the collisionless theory of electrostatic drift waves are examined. The region of validity for a previously derived expression for the growth rate is determined. The region is then extended, and an expression for the growth rate is derived which is valid over a larger region including wavelengths much smaller than the ion gyroradius. The growth rate is maximum for k⊥ρi ≈ 1.5. These results may have applications in a variety of space plasmas wherein steep gradients in plasma density occur.
Midlatitude F region neutral winds and temperatures during the geomagnetic storm of March 26, 1976
Authors: Sipler Dwight P. , Biondi Manfred A.
Published: February 2008

A Fabry‐Perot interferometer has been used to determine midlatitude thermospheric wind velocities and temperatures during the large geomagnetic storm of March 26, 1976. These quantities were determined from the doppler shifts and broadening of the twilight and nightglow 630.0‐nm emission line observed from the Laurel Ridge Airglow Observatory (40°8′N, 790°10′W). Following the onset of the storm at ∼0300 hours UT, the wind vector increased from a twilight, quiet period value of ∼100 m/s (eastward) to a maximum of ∼600 m/s, directed generally southward. Large velocities (>200 m/s) persisted from ∼0400 hours UT to the end of the observing period (∼1100 hours UT) and were generally larger by a factor of ≥2 to the north of the observatory than to the south. A temperature rise of ≥500 K during the course of the storm was noted in all directions, with pronounced, relatively rapid fluctuations (of up to 600 K) to the north.
Modification of solar lines propagating through the interplanetary medium
Authors: Wu F. M. , Judge D. L.
Published: February 2008

The optical depths of the solar H Lyman α and the He 584‐Å lines in interplanetary space are calculated. From these the solar line profiles at Saturn and Uranus are determined. It is found that the solar H Lyman α line can be diminished strongly within a spectral region of |Δλ| <0.2 Å from the line center. On the other hand, there is no significant absorption of the He 584‐Å line except in the downwind region where absorption occurs at Δλ ∼0.04 Å. The effects of solar line modification on planetary and interplanetary glow observations are discussed.
Long‐period oceanic rayleigh wave group velocity dispersion curve from HF Doppler sounding of the ionosphere
Authors: Najita K. , Yuen P. C.
Published: February 2008

Earthquake‐generated Rayleigh waves excite shock waves at the surface of the earth which propagate upward nearly vertically through the atmosphere as infrasound. The HF Doppler sounder is a sensitive tool for detecting and recording these infrasounds at ionospheric heights. By constructing electron density distributions from ionograms the propagation ray paths and delay times for the long‐period waves can be determined. By using these data the dispersion curves for oceanic Rayleigh waves have been deduced. These curves are within 5% of the expected curve synthesized from data obtained from seismograms. Application of these data to the 1969 Kurile Island earthquake shows that it had a single source, while application to the 1968 Tokachi‐Oki earthquake shows the existence of several sources.
Sodium nightglow measurements and implications on the sodium photochemistry
Authors: Kirchhoff V. W. J. H. , Clemesha B. R. , Simonich D. M.
Published: February 2008

Sodium nightglow measurements made at São José dos Campos (23.2°S) with a tilting filter photometer over a period of 2 years show that on the average the intensities reach a minimum close to local midnight. Between midnight and dawn the intensity remains fairly constant or increases slightly. The most consistent feature is the premidnight decrease in intensity. Seasonal variations show peak intensities at the equinoxes, the stronger one occurring in autumn. In a few cases the nocturnal variation does not follow the average trend, showing rather strong peaks in intensity which have been related to dynamic effects. Calculations based on the photochemistry described require the reaction rate for Na + O3 → NaO* + O2 to be much larger than the rate for the analogous hydrogen reaction, in agreement with a recent suggestion in the literature that the above reaction proceeds through the so‐called electron jump mechanism.
Large‐scale magnetic field perturbations and particle measurements at 1400 km on the dayside
Authors: McDiarmid I. B. , Burrows J. R. , Wilson Margaret D.
Published: February 2008

Particle and magnetic field measurements made on the dayside of the magnetosphere by the Isis 2 satellite are reported. Northern hemisphere magnetic perturbations perpendicular to the main geomagnetic field are obtained by subtracting a model field from the field measured at the spacecraft. When plotted as two‐dimensional vectors, these perturbations, on the average, fit a pattern very similar to the usual two‐cell convection pattern except in the noon cleft region, where the perturbation direction tends to be eastward when the IMF By component is positive and westward when it is negative. This relatively strong dependence of the perturbation direction on the sign of By is taken as further evidence for interconnection between the IMF and the geomagnetic field. Comparing various features of the magnetic perturbations with particle measurements, it is found that, on the average, the maximum perturbation in the noon sector falls within the region of cleft precipitation lying near the poleward edge of cleft electrons and just poleward of the 40‐keV electron boundary. The location of the maximum perturbation in this sector is identified with the polar cap boundary. In the dawn and dusk sectors a marked change in slope of the magnitude of the perturbation typically occurs at the poleward edge of the plasma sheet; this is also the poleward edge of the region 1 field‐aligned currents defined by Iijima and Potemra (1976a) and identified here with the polar cap boundary.
On the average configuraton of the geomagnetic tail
Authors: Fairfield D. H.
Published: February 2008

Over 3000 hours of Imp 6 magnetic field data obtained between 20 and 33 RE in the geomagnetic tail have been used in a statistical study of the tail configuration. A distribution of 2.5‐min averages of Bz as a function of position across the tail reveals that more flux crosses the equatorial plane near the dawn and dusk flanks
than near midnight
. The tail field projected in the solar magnetospheric equatorial plane deviates from the x axis due to flaring and solar wind aberration by an angle α = −0.9 YSM−2.7, where YSM is in earth radii and α is in degrees. After removing these effects, the By component of the tail field is found to depend on interplanetary sector structure. During an ‘away’ sector the By component of the tail field is on average 0.5γ greater than that during a ‘toward’ sector, a result that is true in both tail lobes and is independent of location across the tail. This effect means the average field reversal between northern and southern lobes of the tail is more often 178° rather than the 180° that is generally supposed.
Postsunrise refilling of the low‐latitude topside ionosphere
Authors: Burke W.J. , Sagalyn R.C. , Rastogi R.G. , Ahmed M. , Rich F.J. , Donatelli D.E. , Wildman P.J.L.
Published: February 2008

Plasma density measurements from a polar‐orbiting Defense Meteorological Satellite Program satellite at 840 km have shown the existence of significant plasma depletions near the magnetic equator in the postsunrise hours. The phenomenon is observed regularly during magnetically quiet times near the equinoxes. The magnitude of the depletion depends upon longitude and the time elapsed since sunrise in the lower F region. It is significantly less pronounced or absent near the solstices. During magnetically active periods the low‐latitude topside ionosphere is modified due to vertical E × B plasma drifts in the predawn sector. The quiet time equinox observations are interpreted in terms of a diurnal flux tube filling model. It is suggested that the observed equatorial depletions result from a pair of plasma fronts that are moving upward from conjugate ionospheres and that are associated with the early stages of the postsunrise refilling process.
Possible traversals of Jupiter's distant magnetic tail by Voyager and by Saturn
Authors: Scarf Frederick L.
Published: February 2008

It is shown that in the spring and summer of 1981, Voyager 2 and Saturn will both cross the expected region of the tail or wake about 7000–8000 RJ downstream from Jupiter. If Jupiter's tail is well defined at this distance, Saturn's magnetopause may vary significantly as Saturn moves across the tail.
Observations pertaining to the dynamics of the plasma sheet
Authors: DeCoster R.J. , Frank L.A.
Published: February 2008

Plasmas on the surface of the plasma sheet at its interface with the high‐latitude lobes are studied at ∼30–40 earth radii (RE) in the tail of the earth's magnetosphere with Lepedea plasma instrumentation on board the earth‐orbiting Imp 7 and Imp 8 satellites. Protons at this interface are frequently found to be flowing toward earth with velocities greater than 400 km s−1. Proton bulk speeds measured near the plasma sheet surface are typically greater than those at locations deeper within the plasma sheet. Ten of the twenty cases studied exhibit plasma sheet cooling. Simultaneous magnetotail crossings by Imp 7 and Imp 8 occurred on October 31, 1975, during which the plasma sheet first engulfs Imp 7 and ∼74 min later is detected by Imp 8, located 7.2 RE farther from the neutral sheet. Earthward‐streaming protons are detected at the boundary of the plasma sheet by both spacecraft. Typical plasma sheet conditions are observed to prevail deep within the plasma sheet coincident with the earthward streaming at the boundary of the plasma sheet. Simultaneous magnetic field measurements with Imp 8 are used to verify that the observed proton streaming is parallel to the magnetic field. The AE index indicates that a large magnetic substorm is nearing recovery as the plasma sheet expands past the two satellites. This event is interpreted as the buildup of the plasma sheet by the plasma flowing on its surface, and the velocity of this expansion of 11 km s−1, perpendicular to the neutral sheet, is provided by our two‐point measurements. The plasma sheet cools subsequent to passage of the expanding surface beyond each of the two observing spacecraft. Characteristic anisotropic proton velocity distributions are commonly measured for the earthward‐jetting plasma. A three‐parameter model is presented that explains these velocity distributions and that places a field‐aligned acceleration region at geocentric radial distances exceeding 30–40 RE in the magnetotail.
A unified kinetic model of the tangential magnetopause structure
Authors: Lee L.C. , Kan J.R.
Published: February 2008

A self‐consistent model of the tangential magnetopause is formulated on the basis of the Vlasov‐Maxwell equations. In this model the plasmas on both sides are magnetized; the magnetic field is assumed everywhere parallel to the magnetopause (i.e., the normal component Bn=0) and is allowed to rotate through an arbitrary angle across the magnetopause. It is shown that the thickness of the magnetopause is greater than the gyroradius of the hotter species of the plasmas (the ions). The presence of a trapped particle population inside the magnetopause is shown to be required in order to allow the magnetic field to rotate more than a certain critical angle (∼90°). The observed ‘bulge‐out’ feature in B vector hodograms is found to be due to lack of overlap between plasmas from the two sides of the magnetopause. An electric field normal to the magnetopause is present in our model, with magnitudes from 0.1 to 10 mV/m, depending primarily on the relative flow speed parallel to the magnetopause between plasmas on the two sides. The magnetopause current in our model is carried predominantly by the ions and has a significant field‐aligned component. The magnetopause current is a possible source of field‐aligned currents on cusp as well as auroral oval field lines. The proposed model can reproduce the observed features of the tangential magnetopause structure by specifying boundary conditions on both sides of the magnetopause. It also provides a basis for understanding the electric and magnetic fields, the current, and the stability of the tangential magnetopause in terms of the collective particle dynamics.
Inertial limit on corotation
Authors: Hill T.W.
Published: February 2008

Corotation of a planetary magnetosphere with the rotation frequency of the planet is maintained by the viscous torque exerted by ion‐neutral collisions in the planetary atmosphere, this torque being transmitted to the magnetosphere by Birkeland currents. In a steady state this torque balances the inertial drag associated with the production and/or outward transport of magnetospheric plasma. The viscous torque in the atmosphere requires some departure from rigid corotation, i.e., some difference between the average rotation velocities of the ionospheric plasma and of the un‐ionized atmosphere. In this paper we calculate the inertial corotation lag as a function of radial distance in the magnetosphere, the solution being parameterized in terms of the Pedersen conductivity of the atmosphere and the rate at which plasma mass is produced and transported outward in the magnetosphere. Although insignificant in the case of earth's magnetosphere, the calculated inertial corotation lag is significant in the case of Jupiter’s magnetosphere, where the rotation frequency may decrease by a factor of the order of 2 between the planetary surface and the magnetopause. One interesting consequence is that the active sector of Jupiter's magnetosphere (which is associated with a longitudinally restricted sector of enhanced ionospheric conductivity) should rotate faster, at a given distance, than adjacent longitude sectors and should therefore sweep up plasma from adjacent longitudes, thus amplifying the preexisting enhancement of plasma concentration in the active longitude sector.
A possible closure relation for heat transport in the solar wind
Authors: Feldman W.C. , Asbridge J.R. , Bame S.J. , Gosling J.T. , Lemons D.S.
Published: February 2008

IMP electron data are studied with the aim of determining a closure relation describing heat transport in the solar wind. We have compared a simple model of the relative drift speed between core‐electron and proton populations, ΔVc, with measured values. In this model, core electron acceleration responds to the superposition of two opposing forces. The first results from proton‐electron Coulomb friction which is characterized by the slowing down time τcp, and tends to accelerate the core‐electron population up to the proton bulk speed. Wave‐electron collisions supplement this friction thereby increasing the core‐electron acceleration. The second force results from the macroscopic interplanetary electric and magnetic fields and tends to reduce the core speed to zero with a time constant, τσ, equal to the bounce period of a typical bound electron. At equilibrium, the two opposing forces balance yielding a relation between ΔVc and the solar wind bulk speed in a corotating reference frame, Usw: ΔVc = Usw/[1 + βτσ/τcp]. The model predictions are found to agree best with measured values if β ≃4.5. A model for the heat flux, Q = γNckTcUsw/[1 + βτσ/τcp], with γ = 10.7 and β = 4.5, implied by the above and previous results, also fits measurements at 1 AU. The results suggest that the model may be useful for closing the Boltzmann moment equations describing the solar wind expansion.
Seasonal and solar cycle variations in the thermospheric circulation observed over Millstone Hill
Authors: Babcock Richard R. , Evans John V.
Published: February 2008

Incoherent scatter measurements of the ionospheric F region electron density, electron and ion temperature, and vertical ion drift made at Millstone Hill have been used to derive estimates of the exospheric neutral temperature T∞ and the horizontal component of the neutral wind along the magnetic meridian VHn. These data then were employed in a semiempirical model for the local thermosphere (Emery, 1978a) to calculate the diurnal variation of the zonal and meridional winds for the days of the measurements. A total of 64 geomagnetically quiet days from the years 1970, near solar maximum, through 1975 were analyzed. The mean meridional and zonal winds were found to exhibit a marked seasonal variation of about ±50 and ±30 m/s, respectively. The amplitude of this oscillation did not appear to decrease with decreasing sunspot activity; however, the annual mean meridional wind was found to decrease from 25 m/s equatorward to about 0 m/s over the 6‐year period. It is believed that this is indicative of a decrease in the auroral forcing relative to the solar forcing at solar minimum. The meridional winds near equinox shifted from being generally equatorward at solar maximum to being poleward at solar minimum, also suggesting decreased auroral forcing. The diurnally averaged zonal winds maintained a fairly consistent pattern of eastward winds in winter and westward winds in summer over the 6‐year interval.
Generation of large‐scale regions of auroral currents, electric potentials, and precipitation by the divergence of the convection electric field
Authors: Lyons L.R.
Published: February 2008

It is shown that discontinuities in the magnetospheric convection electric field E with ▽ · E <0 can generate large‐scale regions (of the order of 100 km in width) of magnetic field‐aligned currents with associated field‐aligned electric potential differences and electron precipitation of the magnitudes and widths observed in auroral regions. Such an electric field discontinuity is known to exist along the evening boundary between sunward and antisunward convection. In addition, such discontinuities may also exist over the polar cap, on account of inhomogeneities in the magnetosheath flow and in regions, such as the Alfvén layer, where drifting trapped particles charge separate. The present analysis assumes that the field‐aligned current is governed by the free particle motion in dc electric and magnetic fields, and nothing is assumed to inhibit this free particle motion.
Stare and Triad observations of field‐aligned current closure and Joule heating in the vicinity of the Harang discontinuity
Authors: Greenwald R.A. , Potemra T.A. , Saflekos N.A.
Published: February 2008

By combining data from the Stare auroral radar system and the Triad satellite, it has been possible to study the nature of field‐aligned current closure in the vicinity of the Harang discontinuity. For the 2 days studied the largest electric fields were directed predominantly in the north/south directions, and field‐aligned current closure was via north/south directed Pedersen currents. The concurrent electrojets were overlapping east/west directed Hall currents with the westward electrojet to the north. From the Stare‐Triad data, latitudinal profiles of the ionospheric Joule heating rate and the height‐integrated Pedersen conductivity have been calculated. The Joule heating rate maximized to the north and south of an auroral arc and had peak values of 2–4 mW/m². The height‐integrated Pedersen conductivity outside of the auroral arc was approximately 4Ω−1, whereas in the vicinity of the arc, it was in excess of 12Ω−1. These values are in good agreement with incoherent scatter and rocket measurements. The agreement is gratifying, since our technique for the calculation of the Pedersen conductivity does not require a model of the altitude‐dependent collision frequency.
The semidiurnal and terdiurnal tides in the equatorial thermosphere from AE‐E measurements
Authors: Hedin A.E. , Spencer N.W. , Mayr H.G.
Published: February 2008

Measurements of neutral composition and temperature aboard the near equatorial AE‐E satellite are analyzed to determine the semidiurnal and terdiurnal variations from 145 to 295 km. The amplitudes and phases of the semidiurnal variation are approximately the same for N2, O, He, and Ar at the lowest altitude, and the amplitude increases with decreasing altitude. Except for He, the phase undergoes a 180° shift around 200 km. The behavior of the terdiurnal variation is in many respects similar to the behavior of the semidiurnal variation. These characteristics suggest that the semidiurnal tide and (to a lesser extent) the terdiurnal tide have an origin primarily in the lower thermosphere or below.
The orientation of birkeland current sheets in the dayside polar region and its relationship to the IMF
Authors: Saflekos N.A. , Potemra T.A.
Published: February 2008

Vector magnetic field observations made with the three‐axes magnetometer on the Triad satellite have been used to study the orientation of magnetic disturbances in the dayside polar region. These measurements were all made over the southern polar region and recorded at McMurdo, Antarctica. These disturbances are transverse to the main geomagnetic field and may be interpreted as being caused by field‐aligned Birkeland current sheets consistent with Maxwell's equations. The current sheets in the regions usually associated with the morning and afternoon auroral regions are most often aligned in the geomagnetic east‐west direction. The amplitudes of these ‘south auroral’ currents are larger in the morning than in the afternoon when the interplanetary magnetic field (IMF) is directed toward the sun (By < 0, Bx >0) and larger in the afternoon when the IMF is directed away (By > 0, Bx <0). This is the systematically reversed relationship between Birkeland current intensities and IMF found in the northern auroral zone by McDiarmid et al. (1977). The Birkeland current sheets in the region associated with the southern cusp are also aligned in the geomagnetic east‐west direction during periods of negative Bz. During periods of By <0 and Bx > 0 the Birkeland current flow in the region of the southern cusp is predominantly away from the ionosphere in contrast to the downward flow into the northern cusp as determined earlier (e.g., McDiarmid et al., 1978b; Iijima et al., 1978). The cusp Birkeland current flow directions appear to reverse for By > 0 and Bx < 0. From a search of the Triad data set, some rare examples of magnetic disturbances with a large north‐south (noon‐midnight) component have been discovered in the polar cap near noon. These always occurred during periods of northward IMF. These disturbances have been interpreted as being caused by sun‐aligned Birkeland current sheets. The intensities of these currents in a few cases studied here are correlated with the positive amplitude of Bz and are comparable to the cusp and auroral currents studied earlier (0.1–0.8 A/m). The fluxes of electrons associated with sun‐aligned polar arcs measured with the Isis 2 satellite (Ismail et al., 1977) are sufficiently intense to produce upward flowing sun‐aligned Birkeland current sheets, but the origin of the downward flowing currents is not clear. These sun‐aligned current sheets may be associated with the rare occurrence of multiple‐cell convective flow patterns in the polar cap (Burke et al., 1979; Spiro et al., 1979).
Day‐to‐day and average magnetic variations along the IMS Alaska Meridian Chain of observatories and modeling of a three‐dimensional current system
Authors: Akasofu S.‐I. , Kisabeth J. , Romick G.J. , Kroehl H.W. , Ahn Byung‐Ho
Published: February 2008

The daily magnetic variations above 60° in magnetic latitude are studied by using records from the International Magnetospheric Study Alaska meridian chain of observatories. Several days with activity varying from moderate to disturbed are chosen to study day‐to‐day variations, namely, the daily variability of polar magnetic variations. The average daily magnetic field variations and the equivalent current pattern are also obtained by using the successive records between March 9 and April 27, 1978. They represent the disturbance condition on a moderately disturbed day (the average ΣKp value during this period was 23o). The equivalent current pattern reveals clearly the large‐scale features, in particular the so‐called ‘two‐cell’ pattern. Then, the distribution of the ionospheric currents and the field‐aligned current are computed on the basis of the average daily magnetic field variations. It is shown that the computed ionospheric current distribution has little resemblance to the two‐cell pattern. It is concluded that the main part of the ionospheric currents must be fed by field‐aligned currents with a distribution similar to that inferred from Triad satellite data.
Evidence for parallel electric field particle acceleration in the dayside auroral oval
Authors: Torbert R.B. , Carlson C.W.
Published: February 2008

Electron and ion energy spectra and electron pitch angle distributions are presented for two sounding rocket flights in the dayside auroral zone. At times, effects of dc electric fields parallel to the magnetic field are evident in that (1) within precipitation features, protons are decelerated by an amount of energy consistent with that which electrons gain and (2) electrons are sometimes aligned to within 3° (full width at half maximum) of the magnetic field. A maximum altitude for the accelerating region of several thousand kilometers is deduced from the narrow width of the pitch angle distribution and also from time‐of‐flight delays between the observation of accelerated electrons and decelerated protons.
Tidal dynamics and composition variations in the thermosphere
Authors: Forbes Jeffrey M. , Hagan Maura E.
Published: February 2008

The effects of mutual diffusion between O and N2 are examined for diurnal and semidiurnal tidal oscillations in the thermosphere. A binary gas extension of a previous f plane equivalent gravity wave formalism is developed and utilized to study this effect. Compared with calculations that omit O‐N2 mutual diffusion, diffusive interaction affects the temperatures and horizontal velocities associated with the diurnal thermospheric tide excited in situ by less than 10% with negligible shifts in phase, and by roughly 20–40% the semidiurnal tide excited in situ, and the (2, 2) and (2, 4) modes propagating from the mesosphere, accompanied by 1‐ to 4‐hour shifts in phase. These results affect the interpretation of incoherent scatter measurements of thermospheric tides and the construction of synthesized tidal models that are calibrated by the measurements and suggest that the inequality of the vertical transport velocities of O and N2 be accounted for in three‐dimensional numerical models of semidiurnal thermospheric tides.