bluuebunny/crossref_metadata_2025 huggingface dataset:
Convergence feedback and unstable low frequency oscillations in a simple coupled ocean‐atmosphere model
Authors: Goswami B. N. , Selvarajan Sudha
Published: February 2008

The role of convergence feedback on the stability of a coupled ocean‐atmosphere system is studied using model III of Hirst (1986). It is shown that the unstable coupled mode found by Hirst is greatly modified by the convergence feedback. If the convergence feedback strength exceeds a critical value, several new unstable intraseasonal modes are also introduced. These modes have very weak dependence on the wave number. These results may explain the behaviour of some coupled models and to some extent provide a mechanism for the observed aperiodicity of the El‐Nino and Southern Oscillation (ENSO) events.
Low‐frequency waves in the solar wind near Neptune
Authors: Zhang Ming , Belcher J. W. , Richardson J. D. , Vasyliunas V. M. , Lepping R. P. , Ness N. F. , Smith C. W.
Published: February 2008

Plasma and magnetic field observations from the Voyager 2 spacecraft when it was outbound from Neptune reveal low‐frequency waves in the solar wind which are clearly associated with the planet. The waves have frequencies below the proton cyclotron frequency fcp, which is about 10−3 Hz during the periods waves are observed. The waves are present when the interplanetary magnetic field is oriented such that the spacecraft is connected to the bow shock by the magnetic field lines. We have identified the waves to be Alfvénic waves propagating at ∼140° to the ambient magnetic field and away from the bow shock. As at the other planets, these downstream waves are thought to be generated in the upstream region, where energetic protons created near the nose of the bow shock excite waves as they stream along solar wind magnetic field lines.
Interaction of plumes with a compositional boundary at 670 km
Authors: Kellogg L. H.
Published: February 2008

The seismic discontinuity observed at a depth of 670 km may result from a phase change, a compositional boundary, or both; likewise, it may mark a change in density, viscosity, or both, between the upper and lower mantle. This paper addresses whether upwelling plumes in the lower mantle penetrate into the upper mantle, and what effect lower mantle plumes have on convection in the upper mantle if there is a density jump at 670 km due to a change in composition with depth. A finite element model of double‐diffusive convection is used to model flow in a spherical, axisymmetric shell heated from below with a Rayleigh number of 106. The ability of plumes to rise into the upper mantle is governed by the buoyancy number, B=Δρc/ραΔT, which is the ratio of the compositional to thermal buoyancy. When B < 1 plumes readily cross the 670 km discontinuity. When B ≥ 1 plumes do not enter the upper layer. When B > 1 flow in the upper layer is shear‐coupled to flow in the lower layer; that is, a plume impinging on the compositional boundary will induce a down‐welling in the overlying layer. When B ≈ 1 the coupling is more complex; strong plumes in the lower layer are shear‐coupled to flow in the overlying layer, but as plumes evolve and weaken the flow in the upper layer switches to thermal coupling with entrainment of lower mantle material.
Chaotic particle motion in Coulomb electric and uniform magnetic fields
Authors: Sonmor L. J. , Laframboise J. G.
Published: February 2008

In an attractive Coulomb electric field and a uniform magnetic field B, some particle orbits originating at infinite distance undergo many reflections, repeatedly approaching the Coulomb centre. The chaotic nature of these orbits is discussed and illustrated by numerical calculations. The divergence of neighbouring trajectories is shown to occur mostly in discrete jumps when they pass near the Coulomb centre. The appearance of chaotic behaviour in this simplified situation suggests that it will also occur when space‐charge effects significantly perturb the potential distribution around a current‐collecting electrode in a space magnetoplasma.
951 gaspra: A pre‐Galileo estimate of its surface evolution
Authors: Namiki Noriyuki , Binzel Richard P.
Published: February 2008

We attempt to quantitatively model the collisional environment and surface evolution of asteroid 951 Gaspra prior to the flyby of the Galileo spacecraft later this year. We compute Gaspra's average intrinsic collisional probability to be 5.4 × 10−18 km−2 yr−1 with a root mean square impact velocity of 5.0 km sec−1. The projectile population and impact strength for Gaspra are highly uncertain and we consider a range of values. Based on these models, Gaspra is unlikely to be a primordial object surviving from the beginning of the solar system. It is most likely a second or multiple generation fragment forming ∼108 yr ago. For a low impact flux, the crater size distribution directly reveals the size distribution of the impacting population. For a non‐equilibrium surface, we present model‐dependent isochrons for estimating the age of the surface. Our model sets lower limits for Gaspra's age (∼109 yr) and flux for the case of a high impact flux producing an equilibrium surface.
Marine dispersion of caesium 137 released from Sellafield and Chernobyl
Authors: Prandle D. , Beechey J.
Published: February 2008

This modelling study examines the dispersion within the shelf seas surrounding the UK, of 137Cs discharged from Sellafield between 1969 and 1988 together with the atmospheric deposition following Chernobyl (April 1986). The close agreement obtained between computed and observed distributions lends confidence to estimates of flushing times‐fundamental parameters in determining pollutant concentrations. Moreover, this study confirms, to first‐order, the estimates of Chernobyl fall‐out provided by atmospheric dispersion models.
M2 World Ocean tide from tide gauge measurements
Authors: Francis O. , Mazzega P.
Published: February 2008

An empirical model of the M2 oceanic tide has been computed from the harmonic constants of a subset of deep sea and coastal tide gauge measurements. The optimal interpolation of these data based on “inverse theory” uses a priori covariance functions deduced from a global hydrodynamical model. The inverse solution, produced with its associated error maps and samples of error spectra, is surprisingly good when compared to in situ data and to a hydrodynamical model.
Paleomagnetism of the Early Miocene Kani Group in southwest Japan and its implication for the opening of the Japan Sea
Authors: Hayashida Akira , Fukui Takako , Torii Masayuki
Published: February 2008

Paleomagnetic directions of a volcanic and sedimentary sequence of the Kani Basin, ranging from 22 to 16 Ma, show a clockwise declination shift of about 50°. This result implies that detectable rotation of Southwest Japan did not occur in this period, but after 16 Ma. The initial opening of the Japan Sea before 16 Ma might have caused a southeastward translation of Southwest Japan, which was then followed by clockwise rotation between 16 and 14 Ma.
Seasonal snowline instability in a climate model with realistic geography: Application to carboniferous (∼300 MA) glaciation
Authors: Baum Steven K. , Crowley Thomas J.
Published: February 2008

Instabilities in the climate system are possible mechanisms for shifting the climate from one stable mode to another. One such instability involves the nonlinearity associated with snow‐albedo feedback. Previous studies have demonstrated the existence of a snowline instability in zero‐ and one‐dimensional energy balance models (EBMs) as well as two‐dimensional seasonal EBMs with idealized geography. In this paper we identify a snowline instability that has been found in a two‐dimensional EBM utilizing a realistic geographic reconstruction of Gondwanaland for the Carboniferous (∼305 Ma), at the time of a major glacial period in Earth history. Several experiments were conducted, all with solar luminosity reduced 3% to reflect lower solar output. A baseline simulation for a “hot summer orbit” configuration yielded a South Polar temperature of 31°C. As the orbit was changed to a “cold summer orbit” and luminosity was gradually decreased, ice area linearly increased from 0.0 to 7.5×106 km2 (equivalent to about two‐thirds the size of the present East Antarctic Ice Sheet). Additional small decreases in luminosity for the cold summer orbit configuration resulted in an abrupt increase in summer ice area to 26.0×106 km2. The transition occurs due to a perturbation of only 0.002 W/m2‐a perturbation much smaller than those due to Milankovitch variations or volcanic eruptions. The calculated area of expanded ice cover compares favorably with the distribution of glacial deposits in the Carboniferous. These results provide further support for the existence of the snowline instability mechanism and suggest a possible explanation for both initiation of glaciation and glacial oscillations during the Carboniferous.
An intercomparison of model and observed global precipitation climatologies
Authors: Hulme Mike
Published: February 2008

Three independent observed precipitation climatologies are used to evaluate the performance of six different General Circulation Models (GCM) control runs. Two of the observed climatologies consist of mean monthly global fields (JAEGER and LEGATES), while the third is a new land‐only time series of monthly precipitation from 1951 to 1980 (CRU5180). The six models evaluated are from the GFDL, GISS, LLNL, OSU and UKMO.

Comparisons are undertaken of hemispheric means and mean seasonal cycles, and of the spatial pattern of mean monthly precipitation. A distinction is made between landonly and land and ocean fields. Of the models examined here, observed monthly precipitation over both ocean and land areas is best simulated by the high resolution UKMO model (UKMO‐H) and least well by the OSU model. Of the low resolution models UKMO performs demonstrably better than GISS, GFDL and OSU, especially in its reproduction of hemispheric seasonal cycles.
P‐wave residuals at stations in nepal: Evidence for a high velocity region beneath the Karakorum
Authors: Pandey M. R. , Roecker Steven W. , Molnar Peter
Published: February 2008

P‐wave residuals recorded at stations in Nepal from events to the northwest and closer than about 20° are consistently earlier than those from other directions by about 2.5 s. These early arrivals are associated with paths confined to the upper 300 km of the Earth and suggest that cold material occupies the uppermost mantle beneath the Karakorum, northwest Himalaya, and western Kunlun. Thus, these data suggest that convective downwelling occurs more vigorously in this region than beneath the rest of the Himalaya, Tibet, and their surroundings.
Ion acceleration at the equatorward edge of the cusp: Low altitude observations of patchy merging
Authors: Newell Patrick T. , Meng Ching‐I.
Published: February 2008

Controversy continues unabated concerning the detection of signatures of merging at low altitudes. One phenomenon required by nearly all models of merging is the acceleration of ions in the merging region by about the Alfvén velocity. We report the observation of spatially discrete patches of precipitating accelerated ions at the equatorward edge of the cusp. These regions of accelerated bulk flow velocity, which occur at or near the transition between boundary layer and cusp proper, are spatially distinct from the overall energy‐latitude dispersion in the main cusp and are best fitted by Maxwellian distributions with convection velocities of 450‐640 km/s; such velocities are 200–300 km/s faster than in the main cusp ion distribution. The acceleration cannot be attributed to electrostatic potentials, for the electron distributions in the patchy ion regions are also accelerated. Based on an initial survey of 13 such events, occurrence of distinctly separate accelerated ion patches appears to be primarily or exclusively a southward Bz phenomenon.
New observations of the low frequency interplanetary radio emissions
Authors: Kurth W. S. , Gurnett D. A.
Published: February 2008

Recent Voyager 1 observations reveal reoccurrences of the low frequency interplanetary radio emissions. Three of the new events are weak transient events which rise in frequency from the range of 2–2.3 kHz to about 3 kHz with drift rates of ∼1.5 kHz/year. The first of the transient events begins in mid‐1989 and the more recent pair of events both were first detected in late 1990. In addition, there is an apparent onset of a 2‐kHz component of the emission beginning near day 70 of 1991. The new transient emissions are barely detectable on Voyager 1 and are below the threshold of detectability on Voyager 2, which is less sensitive than Voyager 1. The new activity provides new opportunities to test various theories of the triggering, generation, and propagation of the outer heliospheric radio emissions and may signal a response of the source of the radio emissions to the increased solar activity associated with the recent peak in the solar cycle.
Tsunami from the Mariana Earthquake of April 5, 1990: Its abnormal propagation and implications for tsunami potential from outer‐rise earthquakes
Authors: Satake Kenji , Yoshida Yasuhiro , Abe Katsuyuki
Published: February 2008

The tsunami generated by the Mariana earthquake of April 5, 1990 was observed on the Japanese and Pacific islands as far as Hawaii. The observed tsunami amplitudes are not a simple function of distance from the source but vary with large‐scale bathymetry. Numerical computations of tsunami propagation are made for actual bathymetry and the computed amplitudes are compared with the observations. From the comparisons, the seismic moment is estimated to be 1.4 × 1020Nm, very similar to that from seismic waves, and indicates that the seismic waves and tsunami are equally excited. The numerical computations also show that the tsunami propagated toward the Japanese coast through two different paths: one is through the trench system with high velocity and small amplitude, the other along the ridge system with low velocity but large amplitude. The two kinds of tsunami are identified on recently‐installed ocean bottom pressure gauges. Since the Mariana earthquake was an outer‐rise earthquake in a weakly coupled subduction zone, where the size of outer‐rise event is larger than underthrusting events, it is possible that even larger earthquakes occur in this region. The tsunami potential from such events must be considered, including the unusual tsunami propagation.
A possible meteor shower on the Moon
Authors: Hunten D. M. , Kozlowski R. W. H. , Sprague A. L.
Published: February 2008

Over the 3‐day period from 12 to 14 October, 1990, the sodium abundance in the lunar atmosphere at 80° South increased by 60%, while interspersed measurements at the equator showed no substantial change. The source is suggested to be an unknown meteor shower with a radiant near the south ecliptic pole. A low relative velocity of ∼20 km/sec, combined with small particle masses, would keep the shower below the detectability threshold of radar. The stream could evolve from a reasonable asteroidal or cometary orbit with perihelion somewhat greater than 1 astronomical unit (AU) and a major axis of a few AU. The short residence time of lunar sodium makes it much more favorable than the terrestrial sodium layer for detection of such an event.
Duct‐model explanation of the broad component of plasma‐line spectra observed at Arecibo
Authors: Muldrew D. B.
Published: February 2008

The plasma‐line spectrum observed during on‐off cycling of the HF heater at Arecibo contains a broad component extending about 50 kHz from the first decay line. Shortly after HF turn‐on this spectrum can have a maximum intensity 10–20 kHz from the heater frequency (broad bump) or the intensity can decrease monotonically with frequency separation from the first decay line. One possibility is that Langmuir waves (L waves) generated by parametric‐decay of the heater wave are trapped in small‐scale field‐aligned ducts and are responsible for the first decay line. The heater electromagnetic wave can also become ducted near reflection in large‐scale field‐aligned ducts. If the heater wave is not ducted near reflection, its electric field is very nearly parallel to the earth's magnetic field B. The L waves then have wavenormals restricted to a range of angles about B. When these L waves decay parametrically into other L waves that can be detected by the radar, a broad‐bump spectrum results. If the heater wave becomes ducted near reflection (which is probable during spread‐F conditions), its electric field can make a considerable angle to B near reflection and the unstable L waves can propagate at all angles to B (including perpendicular). When these L waves decay parametrically into other L waves that can be detected by the radar, a monotonic spectrum results.
Impact of heterogeneous chemistry on model‐calculated ozone change due to high speed civil transport aircraft
Authors: Weisenstein Debra K. , Ko Malcolm K. W. , Rodriguez José M. , Sze Nien‐Dak
Published: February 2008

Heterogeneous chemistry could have a very significant effect on the predicted impact of engine exhaust from high speed civil transport (HSCT) aircraft on atmospheric ozone. Two‐dimensional models including only gas phase chemistry indicate that deposition of nitrogen oxides from aircraft exhaust in the lower stratosphere would significantly perturb the natural nitrogen budget, most likely resulting in ozone depletion. The model calculates that an injection of 1 megaton of NO2 per year at 17–20 km would decrease the column ozone by 3–6% at northern mid latitudes using gas phase chemistry only. Inclusion of the reaction acting on the global sulfate aerosol layer greatly decreases the sensitivity of ozone to injection of nitrogen oxides, with the calculated ozone column increasing by up to 0.4% for the same emission scenario. Additional simulations showed that this response is only slightly sensitivity to changes in chlorine content and aerosol loading.
VLF‐HF HEATING OF THE LOWER IONOSPHERE AND ELF WAVE GENERATION
Authors: Taranenko Y. N. , Inan U. S. , Bell T. F.
Published: February 2008

For incident wave power densities of 10−6 – 10−2 W/m2 (at 30 km altitude), VLF heating of the D‐region (< 90 km) is found to be 2–10 times more effective (depending on power) than HF heating, resulting in comparable perturbations of subionospheric VLF probe waves in spite of up to 103 times larger power density utilized in HF heating and at least as efficient in ELF wave generation. In view of generally larger (100×100 km) areas of the ionosphere illuminated by VLF transmitters, ELF wave generation by modulated VLF heating is estimated to produce ELF power levels of ∼100 mW, comparable with or larger than those produced in typical HF heating experiments. ELF wave generation in a typical midlatitude ambient ionosphere occurs primarily via the modulation of Pedersen current whereas in a typical auroral ionosphere Hall current is dominant for pump wave frequencies up to ∼6 MHz. For 10–30 MHz and power densities > 10−4 W/m2, Pedersen current modulation is again dominant, potentially providing up to 2–15 times higher ELF dipole moment than those found in recent experiments using 3–5 MHz heaters.
Observations of Pinatubo ejecta over Boulder, Colorado by lidars of three different wavelengths
Authors: Post Madison J. , Grund Christian J. , Langford Andrew O. , Proffitt Michael H.
Published: February 2008

NOAA lidars at wavelengths of 0.574, 0.694, and 10.591 μm have observed ejecta from the eruption of Mt. Pinatubo both in the troposphere and stratosphere over Boulder, Colorado, since July 27, 1991. Multilayered clouds have been highly variable on time scales of minutes to days. Measurements at multiple wavelengths provide valuable information on size distribution and the wavelength dependence of backscatter and optical depth.
Viscosity and thermal fields associated with strongly chaotic non‐Newtonian thermal convection
Authors: Malevsky A. V. , Yuen D. A. , Weyer L. M.
Published: February 2008

We have investigated the thermomechanical structure in strongly chaotic non‐Newtonian thermal convection for both base‐heated and internally‐heated systems. Temperature can build up in stagnant regions and a non‐Newtonian mantle can tolerate less internal‐heating. Viscosity fields of strongly chaotic regime show a granular structure. The horizontal spectra of viscosity fluctuations obey a power‐law and yield a fractal dimension of 1.6 to 1.8 for the isoviscosity lines, providing evidence for two‐dimensional turbulence. Long wavelength viscosity variations are smoothed out by the turbulent non‐Newtonian flows.
The role of cross helicity in solar modulation of cosmic rays and its charge sign dependence
Authors: Dung R. , Schlickeiser R.
Published: February 2008

The purpose of this paper is to demonstrate that solar cycle variations of the cross helicity of the Alfvénic turbulence can be the cause of the solar modulation of the galactic cosmic rays and its charge dependence if combined with nonvanishing magnetic helicity values of the turbulence. It is argued that the ratio of forward propagating Alfvén waves originating on the Sun to the total intensity is high at solar maxima and low at solar minima, whereas the intensity of backward moving waves probably originating by the pickup ion instability stays constant on time scales long compared to the 11‐year periodicity. Our explanation fulfills the observational constraints that the average solar wind speed stays constant and that the total level of magnetic fluctuation power remains constant within 25% during the solar cycle.
The source location and beaming of terrestrial continuum radiation
Authors: Morgan D. D. , Gurnett D. A.
Published: February 2008

Plasma wave data from the spacecraft Dynamics Explorer 1 were used to study the source location and beaming of terrestrial continuum radiation. This study shows that the radiation usually is generated near the magnetic equator at radial distances ranging from 2.0 to 4.0 RE. The radiation is beamed outward in a broad beam directed along the magnetic equator with a beam width of about 100°. The overall frequency of occurrence of continuum radiation was found to be 60% with a sharp increase near the midnight meridian, increasing in the dawnward direction. Several case studies are presented to illustrate various characteristics of continuum radiation. The observed characteristics are then compared with the radio window model of Jones. Several characteristics not predicted by Jones' model are observed. These include large latitudinal asymmetries, an absence of a minimum in the occurrence and intensity at the equator, a tendency for higher frequency bands to come from larger and more diffuse source regions, and the emergence of several distinct beams from a single region. In particular the absence of a minimum in the occurrence and intensity at the equator and the emergence of beams of radiation at sharply distinct angles from one source region constitute evidence against the radio window model. Also, the basic equation for the beaming angle with respect to the magnetic equatorial plane is found to be a poor predictor of the observed beaming angle. The evidence against the radio window model is partially mitigated by the facts that the source of radiation is probably extended in space and frequency and that sometimes radiation from the Sun interferes with the direction finding. Overall, these observations imply that the radio window model does not fully explain the beaming pattern of continuum radiation.
Prompt arrival of solar energetic particles from Far Eastern Events: The role of large‐scale interplanetary magnetic field structure
Authors: Richardson I. G. , Cane H. V. , von Rosenvinge T. T.
Published: February 2008

Intensity‐time profiles of solar energetic particle enhancements generally show an asymmetry with respect to the heliolongitude of the associated solar event. Particles arrive promptly from events to the west of an observer because of good magnetic connection whereas particle enhancements from poorly connected eastern source regions usually show much slower onsets. However, some 15% of eastern events do show prompt onsets. Two prompt particle enhancements associated with eastern solar events are studied using data from the Goddard Space Flight Center instruments on the ISEE 3 and IMP 8 spacecraft. In both events the prompt particle onset was observed when the spacecraft were in a postshock plasma region, apparently within a magnetic bottle. We suggest that the magnetic bottle extended back to the Sun and served as a channel for fast particle propagation to the spacecraft. Particles accelerated at an expanding coronal shock initiated by the eastern event could be injected onto field lines in the foot of the bottle.
Simulation of a gravity wave over the middle and upper atmosphere radar
Authors: Oliver W. L. , Hagan Maura E.
Published: February 2008

We previously reported the observation of the passage of a gravity wave over the middle and upper atmosphere (MU) radar at nighttime during the strong magnetic storm of February 6–8, 1986, and suggested that the electron density changes seen in the F layer may have been due to the existence of an influx of ionization from above and the modulation of the assimilation of this flux into the ambient ionosphere by the wave‐modulated neutral atmosphere. We use a simple, one‐dimensional ionospheric model to investigate this possibility. We impose a flux into the top of the ionosphere and a temporal wavelike modulation of the neutral densities and compute the resulting equilibrium nighttime F layer density. We find that our mechanism reproduces qualitatively the wave behavior observed with the MU radar. To match the amplitude of the waves observed, however, we were compelled to impose a combination of large changes in the background level of the neutral molecular densities and in the wave amplitudes of the neutral densities. This process should be routinely considered in the interpretation of wave structure in the nighttime ionosphere.
Electric and magnetic field measurements inside a high‐velocity neutral beam undergoing ionization
Authors: Kelley M. C. , Swenson C. M. , Brenning N. , Baker K. , Pfaff R.
Published: February 2008

Vector electric field measurements have been made inside two ionizing, high‐velocity streams of barium atoms in the Earth's ionosphere. A variety of electrical phenomena were observed across the frequency spectrum and are presented in this paper, which emphasizes the experimental results. Comparisons with a theoretical model for the interactions of the stream with the magnetic field and the ionosphere are presented in a companion paper (Brenning et al., this issue (a)). A most startling result is that a very large quasi‐dc electric field was detected antiparallel to the beam velocity. This by itself is not unreasonable since newly ionized barium ions with their large gyroradii are expected to create such a field. But since the beam had roughly a 45° angle with the magnetic field, Bo, we find a very large (≳500 mV/m) component of E parallel to Bo. The fluctuating electric fields were also quite large, in fact, of the same order of magnitude as the quasi‐dc pulse. The wave energy was found to maximize at frequencies below the barium lower hybrid frequency and included strong signatures of the oxygen cyclotron frequency. Measurements made on a subpayload separated across Bo by several hundred meters and along Bo by several kilometers do not show the large pulse, although a variety of wave emissions were seen. In addition, very large amplitude magnetic field fluctuations were detected in both bursts. Although we have no clear explanation, they appear to be a real phenomenon and worthy of future study. Finally, we note that even though the critical ionization velocity effect did not go into a discharge mode in this experiment, remarkable electromagnetic effects were seen in the neutral beam‐plasma interaction.
Cusp latitude magnetic impulse events: 1. Occurrence statistics
Authors: Lanzerotti L. J. , Konik R. M. , Wolfe A. , Venkatesan D. , Maclennan C. G.
Published: February 2008

Magnetic impulse events were selected by a computer algorithm procedure from magnetic records obtained at the near cusp latitude conjugate stations Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica. The algorithm was constructed to select large (≳ 50 nT in the vertical component of the magnetic field), short lived (6 to 12 min) events. These events were found to be highly localized in the 06 to 18 LT sector at the two stations. A strong minimum in occurrence was found during hour 11 LT. The field changes associated with these events can be interpreted as due to an approximately half‐cycle, odd‐mode, Alfvén wave along a near‐magnetopause flux tube. From the vertical magnetic deflections of the impulse events the directions of field‐aligned currents into the conjugate ionospheres were inferred. In the morning LT sector, field‐aligned currents were directed into the ionospheres, while in the afternoon LT sector, field‐aligned currents were directed out of the ionospheres. These findings are comparable with the statistical results for quasi‐stationary field‐aligned currents and suggest that at the times of these events, Iqaluit and South Pole are at a higher effective magnetic latitude. The average deflection in the vertical component for the events was measured to be ∼ 95 nT. From this the magnitude of the average field‐aligned currents was calculated to be J∥ ∼ 2 × 10−7 A/m².
Lower hybrid waves excited through linear mode coupling and the heating of ions in the auroral and subauroral magnetosphere
Authors: Bell T. F. , Helliwell R. A. , Hudson M. K.
Published: February 2008

Experimental observations on the ISIS 1, ISIS 2, ISEE 1, and DE 1 spacecraft demonstrate that strong lower hybrid (LH) waves can be excited by VLF electromagnetic (em) whistler mode waves as the em waves propagate through regions of the ionosphere and magnetosphere where small‐scale magnetic‐field‐aligned irregularities exist in the mean plasma density. There is strong evidence that the LH waves are excited by linear mode coupling as the em waves scatter from the irregularities. The present paper considers two related aspects of the linear mode conversion mechanism: (1) the controlled heating of suprathermal ions in the ionosphere and magnetosphere over a powerful VLF/ELF transmitter using LH waves excited by the em transmitter signals through linear mode conversion; (2) the excitation of intense LH waves in the auroral regions through the linear conversion of VLF/ELF em auroral hiss, and the subsequent heating of ions by the excited LH waves. In addressing both aspects a critical feature is the behavior of the mode coupling mechanism at frequencies less than 10.2 kHz, the lowest value observed in earlier experiments. Using the results of controlled experiments carried out at Siple Station, Antarctica, it is demonstrated that strong LH waves can be excited by em waves down to frequencies as low as 2 kHz in the subauroral low‐altitude magnetosphere. Extrapolating from these observations and making use of ISEE 1 satellite wave amplitude data, we conclude that a ∼250 kW VLF/ELF transmitter operating in the subauroral region at 2 kHz could excite sufficiently intense LH waves to heat suprathermal 1 eV H+ ions and 16 eV O+ ions to roughly 50 eV in a region extending in altitude from 1000 to 5000 km above the transmitter with a horizontal scale of ∼500 km. Furthermore, if coherent wave stochastic heating occurs, the energy gain of O+ ions could be as large as 200 eV. This effect would be readily measurable with available satellite instrumentation. Using a recently developed model of the linear mode coupling mechanism, we furthermore conclude that a significant portion of the LH waves observed in the auroral zone in regions of ion conic development may be excited by em VLF/ELF auroral hiss as the hiss propagates through the irregular background plasma commonly observed in the auroral regions.
A new look at type 4 echoes of radar aurora
Authors: Haldoupis C. , Sofko G. J. , Koehler J. A. , Danskin D. W.
Published: February 2008

Type 4 echoes represent a rare but distinct spectral signature of auroral electrojet backscatter during times of large electric fields and strong two‐stream‐generated ion acoustic turbulence. On the basis of many bistatic CW Doppler radar observations made over the past 15 years the great majority of type 4 echoes have a composite spectrum showing two main features, a broadened type 1 component peaking at elevated ion acoustic velocities and the type 4 narrow component at roughly double the ion acoustic speed. The most important property is the large phase velocities, mostly seen in the 900 to 1100 m/s range, associated with irregularities propagating in a narrow azimuthal cone along the destabilizing Hall current. In the present paper it is argued that the existing interpretation, in which it is hypothesized that the large Doppler velocities are simply from two stream waves originating in regions of highly elevated electron temperatures, cannot explain why the observed phase velocities of type 4 waves are mostly seen at preferential values near 1000 m/s and why these waves are highly directional. On the basis of the existing evidence we conclude that there is not yet satisfactory explanation for the type 4 echoes. The fact that type 4 waves have frequencies of about two times the frequency of the simultaneously observed ion acoustic waves led us to suggest the possibility of a resonant three‐wave coupling interaction process which might be in operation under some appropriate conditions in the plasma. At present this is only a preliminary proposition which would be investigated further in a future study.
Global hybrid simulation of unmagnetized planets: Comparison of Venus and Mars
Authors: Brecht Stephen H. , Ferrante John R.
Published: February 2008

Results from three‐dimensional hybrid particle simulations of the solar wind interaction with the planets Mars and Venus are presented. The simulations produce shocks and magnetic barriers which are asymmetric. These results are qualitatively in agreement with data. In the absence of an ionosphere the subsolar shock standoff distance was found to agree with the observations if the Hall current is limited. It was also found that the solar wind interaction with Mars and Venus was substantially different. The interaction with Venus can be generally viewed as a magnetized interaction. The Mars interaction is very kinetic in nature and appears not to have a shock in the classic sense.
Simultaneous observations of 46.8‐MHz and 430‐MHz radar backscatter from HF‐induced ionospheric Langmuir turbulence
Authors: Sulzer M. P. , Fejer J. A.
Published: February 2008

Simultaneous high‐resolution spectra of the upshifted enhanced plasma line were obtained with the 46.8‐MHz and 430‐MHz Arecibo radars in the presence of HF transmissions at two closely spaced frequencies. The spectra obtained with the 46.8‐MHz radar showed two narrow features with positive Doppler shifts equal to the two closely spaced frequencies of the HF transmissions; all the spectral power was contained in those two narrow features. The spectra obtained with the 430‐MHz radar showed a single narrow feature with a positive Doppler shift equal to the arithmetic mean of the same two closely spaced frequencies; the spectral power in that narrow feature contained about 3% of the total spectral power. The present results broadly confirm the tentative interpretation of earlier observations with a 46.8‐MHz radar at Arecibo. The results also show the fundamental difference in the physical processes leading to the enhanced plasma line spectra observed with the two radars.
The dynamics of submicron‐sized dust particles lost from Phobos
Authors: Horányi M. , Tátrallyay M. , Juhász A. , Luhmann J. G.
Published: February 2008

The dynamics of submicron‐sized dielectric particles lost from the Martian moon Phobos are studied in connection with the possible detection of dust by the Phobos 2 spacecraft. The motion of these small dust grains is influenced not only by gravity but also by solar radiation pressure and electromagnetic forces. The plasma environment of Mars is described by applying a hybrid gasdynamic‐cometary model. Some of the submicron‐sized grains ejected at speeds on the order of a few tens meters per second can stay in orbit around Mars for several months forming a nonuniform and time‐dependent dust halo. The lifetime of the particles depends on their size, on the actual interplanetary parameters (constant or varying with a periodicity of 28 days) and also on the orbital position of Mars at the time of ejection since there is a 24° obliquity between the orbit of Phobos and that of Mars.
The composition and plasma signature of a large dust impact on the Giotto spacecraft
Authors: Goldstein R. , Goldstein B. E. , Balsiger H. , Coates A. J. , Curdt W. , Keller H. U. , Neubauer F. M. , Perry C. , Zarnecki J.
Published: February 2008

At about 14,800 km from the comet Halley nucleus, on the inbound leg, at least six of the sensors onboard the Giotto spacecraft observed an unusual, brief (∼30 to 500 ms) event: The ion mass spectrometer data show a brief flow of energetic (up to several hundred electron volts) plasma consisting of protons, water group, and heavier ions. The Johnstone plasma analyzer data show a short burst of plasma, while the dust impact detector system data show an impact event in four of its detectors. The magnetometer signature of the event shows two brief dips in the field. The sudden change in the spacecraft attitude and spin rate observed by the camera at that same time has been interpreted as the result of a large (≥5 mg) dust particle impact on the front bumper shield of the spacecraft. In addition, at about the same time the spacecraft star‐tracker suffered damage. Here we give the first report combining direct measurements of the composition and dynamics of a dust impact plasma cloud, the dust particle mass and the location of the impact on the spacecraft. Analysis of the data indicate that the impacting particle was water or ice‐bearing, possibly loosely compacted, and was composed of one or more of: carbon, nitrogen, and silicon.
Venus ionospheric “clouds”: relationship to the magnetosheath field geometry
Authors: Ong M. , Luhmann J. G. , Russell C. T. , Strangeway R. J. , Brace L. H.
Published: February 2008

Venus plasma “clouds,” apparently detached regions of ionospheric plasma above the main ionosphere of Venus observed by the Langmuir probe on the Pioneer Venus orbiter, are thought to be evidence for bulk removal of ionospheric ions by the solar wind interaction. In addition to the primary feature of an electron density enhancement above the ionopause, the characteristic signature of a cloud may include a change in the sign of the Bx (sunward/antisunward) component of the surrounding magnetosheath magnetic field. The plasma wave instrument also sometimes observed enhanced electric field noise in the clouds. In an effort to learn more about clouds and their cause(s), we analyzed the magnetic field control of cloud positions relative to the planet by rotating their observed locations into a coordinate system in which all of the transverse upstream (interplanetary) magnetic fields were aligned. The results indicate that clouds are scattered around the periphery of the planet in the terminator plane. There is no evidence of a concentration of clouds where the magnetosheath magnetic field is most strongly “draped,” as suggested by some earlier studies of selected occurrences. On the other hand, statistics show that the change in the orientation of the transverse upstream magnetic field between the inbound and outbound bow shock crossings, for the orbits where clouds are seen, is approximately 30° greater than the average upstream change at Venus over the same time intervals. The Spreiter and Stahara gasdynamic model of the magnetosheath provides further support for the idea that a rotation in the upstream transverse magnetic field accompanies cloud formation. Modeling of the magnetic field time series in the magnetosheath for a “cloud” orbit is possible only when a sequence of upstream fields is used to reconstruct the observed time series. Thus, clouds are associated not simply with normal magnetosheath field draping but with interplanetary field orientation changes.
Magnetic field draping at the low‐latitude magnetopause
Authors: Richard R. L. , Lotko W.
Published: February 2008

The structure of the low‐latitude magnetopause is investigated for interplanetary magnetic field conditions with a dominant southward component using magnetohydrodynamic simulations. The structure is self‐consistently calculated as an initial‐value problem in which the system is allowed to evolve to a quasi steady state. All components of the three‐dimensional velocity and magnetic field as well as compressibility, resistivity, and viscosity are included in the two‐dimensional calculation. The boundary conditions sustain inflow from the magnetosheath and the magnetosphere, as well as a flow tangential to the magnetopause, sunward in the magnetosphere and tailward in the magnetosheath. The tangential flow leads to the draping of magnetic field lines in the magnetopause region. No draping (tailward or sunward) was included initially, while as the system evolved this component of the magnetic field became comparable in magnitude to the other two components. The resulting draping structure is found to be dependent sensitively on the ratio of the viscous and magnetic diffusivities.
A simulation study of impulsive penetration of solar wind irregularities into the magnetosphere at the dayside magnetopause
Authors: Ma Z. W. , Hawkins J. G. , Lee L. C.
Published: February 2008

A two‐dimensional magnetohydrodynamic code is used to study impulsive penetration processes that occur when a plasma irregularity in the magnetosheath, modeled as a field‐aligned filament, impinges on the day side magnetopause. If the magnetic fields in the magnetosheath and magnetosphere are parallel or antiparallel, then a filament in the magnetosheath can always penetrate into the magnetosphere. However, if the fields in the magnetosheath and magnetosphere are not aligned, then a filament can only penetrate into the magnetosphere when its initial kinetic energy density exceeds the magnetic energy density attributed to the transverse component of the magnetic field by a factor of 50. In this case, the magnetospheric field lines reconnect behind the filament, thereby trapping it within the magnetosphere. Otherwise, the increasing magnetic stress in front of the filament will eventually stop the filament from further penetration. For typical parameters found at the dayside magnetopause, the threshold condition obtained from this two‐dimensional model predicts that penetration is possible only when the angle between the fields is within approximately 5° of parallel or antiparallel. During the penetration process, velocity vortices are observed both inside the filament and in the external plasma. Either increased β within the magnetosphere, or the larger plasma density at the magnetopause associated with antiparallel magnetic fields, will act to reduce the penetration velocity.
The incoherent scattering of radio waves in a non‐Maxwellian plasma: The effects of Coulomb collisions
Authors: Tereshchenko V. D. , Tereshchenko E. D. , Kohl H.
Published: February 2008

In this paper the formulas for the ion distribution as well as the spectrum of radio waves scattered in a magnetized plasma with a strong electric field are derived. It is shown that the presence of the electric field in the ionosphere leads to an anisotropic ion velocity distribution and, therefore, to untypical incoherent scatter spectra for the F region of the polar ionosphere which are caused by ion‐neutral together with ion‐ion collisions. The effect of ion‐ion collisions, which has not been taken into account so far, is to reduce the anisotropy of the ion velocity distribution. Estimates of the ion‐ion collision frequency derived from EISCAT measurements show that this may happen above about 300 km.
Equatorial thermospheric wind changes during the solar cycle: Measurements at Arequipa, Peru, from 1983 to 1990
Authors: Biondi M. A. , Meriwether J. W. , Fejer B. G. , Gonzalez S. A. , Hallenbeck D. C.
Published: February 2008

Fabry‐Perot interferometer measurements of Doppler shifts in the nightglow 630‐nm emission line have been used to determine near‐equatorial thermospheric wind velocities at Arequipa, Peru, over ∼2/3 of a solar cycle. Monthly‐average nocturnal variations in the meridional and zonal wind components were calculated from the nightly data to remove short term (day‐to‐day) variability, facilitating display of seasonal changes in the wind patterns, as well as any additional changes introduced by the progression of the solar cycle. The measured seasonal variations in the wind patterns are more pronounced than the solar cycle variations and are more readily understandable in terms of the expected, underlying forcing and damping processes. For most of the years, at the winter solstice, there is a weak (≤100 m/s) transequatorial flow from the summer to the winter hemisphere in the early and the late night, with essentially zero velocities in between. At the equinoxes, an early‐night poleward (southward) flow at solar minimum (1986) is replaced by an equatorward (northward) flow at solar maximum (1989–1990). The zonal flows are predominantly eastward throughout the night, except for the solar minimum equinoxes, where brief westward flows appear in the early and the late night. The peak eastward velocities increase toward solar maximum; at the winter solstice, they are ∼100–130 m/s in 1983, 1984 and 1986, reaching ∼200 m/s in 1988, 1989 and 1990. The present equatorial thermospheric wind determinations agree in some respects with the satellite‐data‐based horizontal wind model HWM‐87 and the vector spherical harmonic form of the thermospheric general circulation model. They also are generally consistent with earlier studies at Arecibo, Puerto Rico, and at Kwajalein, Marshall Islands, when compared at similar parts of the solar cycle.
An S3‐3 satellite study of the effects of the solar cycle on the auroral acceleration process
Authors: Cattell Cynthia A. , Chari Sharad , Temerin Michael A.
Published: February 2008

A statistical study of the effects of the solar cycle, as indicated by the value of F10.7, has been made utilizing a data base of electrostatic shocks and the associated ion beams, ion conics, and electron distributions obtained by the S3‐3 satellite during the rising phase of the solar cycle. Ion composition was not included since the ion detectors utilized for this study did not determine mass. The acceleration of ions in association with electrostatic shocks is strongly dependent on F10.7 at altitudes of ∼2000–8000 km. For low F10.7 (<80), ∼43% of the shocks were associated with conics and ∼43% with beams, whereas for high F10.7 (100–225), ∼18% of the shocks were associated with beams and ∼64% with conics. This difference was observed for essentially all magnetic local times and altitudes except the period from 0000–0600 MLT. These results suggest that parallel potential drops are less common at altitudes below 8000 km during solar maximum than during solar minimum. However, for the events where ion beams were observed, the average parallel potential drop below the satellite (as determined from the peak ion energy) had very similar values, and approximately half of the potential occurred below the satellite altitude for both high and low F10.7. The observations described herein are consistent with the increased occurrence frequency of O+ conics during high F10.7 observed by Yau et al. (1985) in the DE‐1 data.