bluuebunny/crossref_metadata_2025 huggingface dataset:
Thin low‐velocity zone within the krafla caldera, ne‐Iceland attributed to a small magma chamber
Authors: Brandsdóttir Bryndís , Menke William H.
Published: February 2008

We examine seismograms from microearthquakes recorded at Krafla central volcano, in the Northern Volcanic Zone of Iceland, during a short inflation period in 1988. We produce record sections for propagation both to the north and to the south of the center of the caldera by combining seismograms from many earthquakes. The northern record section contains clear evidence of a low velocity zone (LVZ), including a low amplitude diffraction of the P wave as it impinges on top of the LVZ and high amplitude reflections from beneath the LVZ. The absence of clear shear waves indicates that the LVZ is at least partially molten and should be interpretated as a magma chamber. The magma chamber is less than 1 km thick, with a top approximately 3 km beneath the surface. We find no evidence for a LVZ in the southern part of the caldera.
Interaction of seismic and air waves recorded at Stromboli Volcano
Authors: Braun T. , Ripepe M.
Published: February 2008

Explosion‐quake seismograms recorded at Stromboli show that seismic phases with a high‐amplitude and high‐frequency content propagate with a velocity of approximately 330 m/s ‐ the sound speed. The analysis of seismograms, recorded at a distance of 500 m from one of the three active vents, shows for the first onset a low‐frequency and particle motion characteristic of a p‐wave, which loses its longitudinal polarization with the onset of the air‐wave. Recording the explosion‐quakes simultaneously with a microphone we could ascertain that the high‐frequency onset coincided with the air‐wave's. In order to better understand the seismic wavefield generated by the atmospheric pressure, we performed a controlled source experiment at Stromboli using a seismic gun. Seismograms with the same two phases and particle motions comparable with the volcano‐seismic data were obtained. A second experiment demonstrated, that the air‐wave propagates at least in the uppermost 1 m of the ground. We suggest that the seismic source of the corresponding seismograms is an explosion at the top of the magma column and conclude that the p‐ and air‐waves are both generated in the same point and at the same time.
Preliminary measurements of mesospheric OH X2II by ISO on ATLAS 1
Authors: Morgan M. F. , Torr D. G. , Torr M. R.
Published: February 2008

Resonance fluorescence of the OH radical was observed in the mesosphere by the Imaging Spectrometric Observatory (ISO) on ATLAS 1. A preliminary determination of the OH density profile from 70 to 80 km has been made from these observations. This marks the first measurement of ground state OH in the mesosphere since Anderson's [1971 a,b] sounding rocket measurements, and the first from space. ISO imaged resonance scattered sunlight in the OH A2Σ–X2II(0, 0) band during limb scans at tangent heights between 60 and 85 km, at 1.6 km spatial resolution, using an f/3.5 diffraction grating spectrometer with spectral resolution of 0.5 Å. OH observations were conducted throughout most of the dayside passes during the mission, covering much of the northern hemisphere to 57°N latitude. Here we report results from an observation at 39°N, local solar time 13:15, on March 30, 1992; we find OH densities on the order of 8 × 106 cm−3 from 70 to 80 km, decreasing rapidly above 80 km.
Detection of upper atmospheric negatively charged microclusters by a rocket‐borne mass spectrometer
Authors: Schulte P. , Arnold F.
Published: February 2008

The detection of a layer of massive negative ions (masses up to 400 u and above) by means of rocket borne quadrupole mass spectrometry in the polar summer mesopause region is reported. Taking into account the geophysical and atmospheric conditions encountered during the measurement different possible explanations for these ions are evaluated. We arrive at the conclusion that they are best explained as being the negatively charged fraction of the so called meteor smoke particles.
SUSIM/UARS observations of the 120 to 300 nm flux variations during the maximum of the solar cycle: Inferences for the 11‐year cycle
Authors: Lean Judith , VanHoosier Michael , Brueckner Guenter , Prinz Dianne , Floyd Linton , Edlow Kenneth
Published: February 2008

Since October 1991, the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on board the Upper Atmosphere Research Satellite (UARS) has been measuring the solar spectral irradiance from 120 to 400 nm with low (5 nm) and moderate (1.1 nm) resolution on a daily basis. By scaling the modulation measured over four solar rotations to proxy indicators of solar activity, we estimate the amplitude of the solar ultraviolet irradiance variation during the 11‐year activity cycle, extending the wavelength coverage of existing empirical variability models to 120 nm and distinguishing the variability of the emission lines from that of the underlying solar continuum. These estimates are compared with results from another empirical variability model and from direct measurements.
Asteroidal collision probabilities
Authors: Bottke William F. , Greenberg Richard
Published: February 2008

Several past calculations of collision probabilities between pairs of bodies on independent orbits have yielded inconsistent results. We review the methodologies and identify their various problems. Greenberg's (1982) collision probability formalism (now with a corrected symmetry assumption) is equivalent to Wetherill's (1967) approach, except that it includes a way to avoid singularities near apsides. That method shows that the procedure by Namiki and Binzel (1991) was accurate for those cases where singularities did not arise.
Sensitivity of crustal deformation instruments to changes in secular rate
Authors: Langbein John , Quilty Eddie , Breckenridge Katherine
Published: February 2008

A variety of instruments (including borehole strainmeters, water wells, creepmeters, laser ranging and differential magnetometers) are used to monitor crustal deformation in areas that are prone to geologic hazards such as earthquakes and volcanic eruptions. In monitoring the deformation, one typically examines the data for either a change in rate, or a simple offset in the record. However, one needs to place a statistical confidence level that the detected signal differs from the background “noise”. Calculation of the statistical confidence level may be done using the formalism of the matched filter, whose output is the signal‐to‐noise ratio, ρ. Two ingredients are needed to form a matched filter: 1) The power density spectrum of the instrument and 2) the functional form of the signal that we desire to detect. Using the available crustal deformation data from the Parkfield, California network, the background noise for individual instruments as a function of frequency, f, is estimated using the traditional method of the power density spectra. Except for two‐color laser distance‐ranging data, the power spectra for most of the instruments have a frequency dependence of f−n where 2≤n≤3. The confidence level with which a hypothesized signal is present is determined directly from the signal‐to‐noise ratio, with the numerator being a function of the signal and the denominator being a function of the power spectrum. Using a creepmeter as an example, a 0.04‐mm change occurring over 1 hour, a 0.06‐mm occurring over 10 hours, or 0.20‐mm over 100 hours are all signals for which ρ=2 and therefore have only a 5% confidence that these signals could be background noise.
Sea‐ice interaction with the thermohaline circulation
Authors: Yang Jiayan , Neelin J. David
Published: February 2008

Linkages have been suggested between observed interdecadal variability of sea‐ice and salinity in the North Atlantic. A plausible mechanism for generating such variability through the interaction of sea‐ice and the thermohaline circulation (THC) is examined in a zonally‐averaged THC model coupled to a thermodynamic ice model. A self‐sustaining interdecadal oscillation arises through the feedbacks between salinity anomalies induced by the sea‐ice melting‐freezing process and anomalous meridional heat transport associated with the THC. The oscillation time scale is not associated with any oceanic time scales but fundamentally depends on ice‐THC coupling. The period is quite robust to stochastic forcing, although the regularity is strongly affected.
Ozone depletion over the polar caps caused by solar protons
Authors: Stephenson Judy A. E. , Scourfield Malcolm W. J.
Published: February 2008

Energetic solar protons are a natural source of ozone depletion due to nitric oxides they produce in the earth's atmosphere. In March 1989, following a period of intense solar activity, the TOMS instrument aboard the Nimbus 7 satellite recorded very similar ozone losses over both polar caps for areas extending from 90° to 70°. Ozone depletions of 7.4 × 109 kg for the south polar cap and 8.0 × 109 kg for the north polar cap indicate the degree of symmetry over the polar caps.
Absence of upstream energetic ions under turbulent radial interplanetary magnetic field
Authors: Sarris E. T. , Anagnostopoulos G. C. , Krimigis S. M.
Published: February 2008

According to Fermi models as applied to the Earth's bow shock energetic particles are accelerated most efficiently under radial interplanetary magnetic field (IMF) conditions. In an earlier paper (Sarris and Krimigis, 1988) we tested the Fermi mechanism for cases of radial IMF when no detectable ambient energetic particle fluxes were present. We concluded that the above mechanism could not account for the observations in the vicinity of the bow shock. In this work we extend the previous test by examining cases observed by the IMP 8 spacecraft where, in addition to the radial IMF, the following particle and field conditions were present: (1) in situ cyclotron‐resonant wave activity, (2) a seed energetic particle population, and (3) small (≲25°) θBn at the points of connection of the spacecraft to the bow shock. Examination of data from days 67, 1979, and 303, 1980, show that despite the fact that all of the above conditions were satisfied, no ion enhancements (≳50 keV) attributable to the Fermi process could be discerned. We conclude that, even with the addition of criteria much more stringent than those applied in all previously published upstream events, which provided the observational underpinning for the development of the Fermi model, the effects of the Fermi process in accelerating ions to energies greater than ∼50 keV are essentially undetectable (ϵ < 10−3) in the region upstream of Earth's bow shock.
Calculated ionization rates, ion densities, and airglow emission rates due to precipitating electrons in the nightside ionosphere of Mars
Authors: Haider S. A. , Kim J. , Nagy A. F. , Keller C. N. , Verigin M. I. , Gringauz K. I. , Shutte N. M. , Szego K. , Kiraly P.
Published: February 2008

The calculations presented in this paper clearly establish that the electron fluxes measured by the HARP instrument, carried on board Phobos 2, could cause significant electron impact ionization and excitation in the nightside atmosphere of Mars, if these electrons actually do precipitate. The calculated peak electron densities were found to be about a factor of 2 larger than the mean observed nightside densities, indicating that if a significant fraction of the measured electrons actually precipitate, they could be the dominant mechanism responsible for maintaining the nightside ionosphere. The calculated zenith column emission rates of the O I 5577‐Å and 6300‐Å and CO Cameron band emissions, due to electron impact and dissociative recombination mechanisms, were found to be significant.
Electron density in the Martian foreshock as a by‐product of the electron plasma oscillation observations
Authors: Trotignon Jean Gabriel , Skalsky Alexandre , Grard Réjean , Nairn Carol , Klimov Stanislas
Published: February 2008

Electron plasma oscillations have been observed in the solar wind upstream of the Martian bow shock by the plasma wave system (PWS) on the Phobos 2 spacecraft. These waves occurred when the interplanetary magnetic field line through the spacecraft position was connected to the shock, i.e., when the spacecraft went through the electron foreshock region in which suprathermal electrons accelerated at the bow shock were streaming back from it. The PWS observations are compared to the expectations one might have in light of the results obtained in the Earth's electron foreshock, which are briefly reviewed. The spectrum of the electron plasma oscillations peaks at or near the ambient solar wind plasma frequency. We then have an estimate of the solar wind plasma density upstream of the Martian bow shock. The 12‐min averages of the plasma density are shown to vary between less than 0.5 cm−3 and 19 cm−3 from February 1 to March 26, 1989. This corresponds to an electron plasma frequency range from 6.4 to 39 kHz. Moreover, some structures were observed in the electron foreshock region of Mars. They are presented and tentatively interpreted.
Finite grid radius and thickness effects on retarding potential analyzer measured suprathermal electron density and temperature
Authors: Knudsen William C.
Published: February 2008

The effect of finite grid radius and thickness on the electron current measured by planar retarding potential analyzers (RPAs) is analyzed numerically. Depending on the plasma environment, the current is significantly reduced below that which is calculated using a theoretical equation derived for an idealized RPA having grids with infinite radius and vanishingly small thickness. A correction factor to the idealized theoretical equation is derived for the Pioneer Venus (PV) orbiter RPA (ORPA) for electron gasses consisting of one or more components obeying Maxwell statistics. The error in density and temperature of Maxwellian electron distributions previously derived from ORPA data using the theoretical expression for the idealized ORPA is evaluated by comparing the densities and temperatures derived from a sample of PV ORPA data using the theoretical expression with and without the correction factor.
Current collection by probes and electrodes in space magnetoplasmas: A review
Authors: Laframboise J. G. , Sonmor L. J.
Published: February 2008

We present a survey of a very incomplete subject. The best‐developed and simplest theories for current collection in magnetic fields are steady‐state collisionless theories, and these must be understood before departures from them can be analyzed usefully, so we begin with a review of them. Recent numerical results indicate that steady‐state collisionless Laplace‐limit currents remain substantially below the Parker‐Murphy (1967) canonical upper bound out to very large electrode potentials and approach it as a limit only very slowly if at all. Attempts to correct this theory for space‐charge effects lead to potential disturbances which extend to infinite distance along the electrode's magnetic shadow, unless collisional effects are also taken into account. However, even a small amount of relative plasma drift motion, such as that involved in a typical rocket experiment, can change this conclusion fundamentally. It is widely believed that time‐averaged current collection may be increased by effects of plasma turbulence, and we review the available evidence for and against this contention. Steady‐state collisionless particle dynamics predicts the existence of a toroidal region of trapped orbits which surrounds the electrode. Light emissions from this region have been photographed, indicating that collisional ionization may also occur there, and this, and/or scattering by collisions or possibly turbulent fluctuations in this region, may also increase current collection by the electrode. We also discuss effects on particle motions near the electrode, associated with “breakdown of magnetic insulation” in the region of large electric fields near it.
Multiple switch‐off slow shock solutions
Authors: Vu H. X. , Brackbill J. U. , Winske D.
Published: February 2008

Numerical simulations of the slow‐mode switch‐off shock with a time‐dependent, implicit hybrid plasma simulation code in a bounded domain produce more than one type of internal structure for the same physical parameters, depending on the downstream boundary conditions. Time‐steady switch‐off slow shocks with large‐amplitude trailing waves are obtained with downstream boundaries that either allow incoming ion cyclotron waves or set the magnetic field component transverse to the shock normal equal to zero. Time‐steady switch‐off slow shocks with no trailing waves are obtained with downstream boundaries that set he electric field according to Ed = −ud × Bd (ud and Bd are the downstream plasma fluid velocity and the downstream magnetic field, respectively, and are obtained for given upstream conditions by solving the Rankine‐Hugoniot relations); for example, the electric field is set to zero for simulations performed in the rest frame of the downstream plasma. Both switch‐off slow shocks with and without trailing waves satisfy the Rankine‐Hugoniot conditions. Shocks with trailing wave trains are stable to changes in the downstream boundary conditions. Although both shocks with and without the trailing wave trains are linearly stable with respect to changes in the upstream magnetic field, only shocks with no trailing wave trains are nonlinearly stable. These results may explain observations of slow shocks in the magnetotail in which the expected trailing wave train is absent.
Energetic magnetospheric protons in the plasma depletion layer
Authors: Fuselier Stephen A.
Published: February 2008

Interplanetary magnetic field draping against the Earth's dayside subsolar magnetopause creates a region of reduced plasma density and increased magnetic field called the plasma depletion layer. In this region, leakage of energetic ions from the Earth's magnetosphere onto magnetic field lines in the plasma depletion layer can be studied without interference from ions accelerated at the Earth's quasi‐parallel bow shock. Active Magnetospheric Particle Tracer Experiment/Charge Composition Explorer (AMPTE/CCE) observations for 13 plasma depletion layer events are used to determine the characteristics of energetic protons between a few keV/e and ∼ 100 keV/e leaked from the magnetosphere. Results indicate that the leaked proton distributions resemble those in the magnetosphere except that they have lower densities and temperatures and much higher velocities parallel (or antiparallel) and perpendicular to the magnetic field. Compared to the low‐energy magnetosheath proton distributions present in the depletion layer, the leaked energetic proton distributions typically have substantially higher flow velocities along the magnetic field but similar flow velocities perpendicular to the magnetic field. The higher flow velocities along the magnetic field indicate that the leaked energetic proton distributions do contribute to the energetic proton population seen upstream and downstream from the quasi‐parallel bow shock. However, their contribution is small compared to the contribution from acceleration of protons at the bow shock because the leaked proton densities are on the order of 10 times smaller than the energetic proton densities typically observed in the vicinity of the quasi‐parallel bow shock.
Ionospheric disturbances observed during the period of Mount Pinatubo eruptions in June 1991
Authors: Cheng Kang , Huang Yinn‐Nien
Published: February 2008

A series of climactic eruptions of Mount Pinatubo occurred in June 1991. Many quasi‐periodic ionospheric disturbances were observed in the virtual height variations of the ionosphere at Chungli, in the total electron content variations at Lunping, Kaohsiung, and Chihpen, and in the differential Doppler shift data recorded at Lunping. These ionospheric disturbances were characterized by quasi‐periodic wavelike trains lasting for several cycles with periods around 16 to 30 min, wavelengths between 160 and 435 km, and propagation speeds ranging from 131 to 259 m/s from the direction of Mount Pinatubo to the observation sites. The oscillation periods, downward movement of distortion, and inclined phase fronts all show that these ionospheric disturbances were manifested by the gravity waves that arrived from the direction of Mount Pinatubo. By use of a simplified isothermal atmosphere model, the propagation mode is interpreted as the freely propagating gravity waves.
Reverse convection
Authors: Crooker N. U.
Published: February 2008

A model of magnetospheric topology for periods when the interplanetary magnetic field (IMF) points northward indicates that reverse convection can readily occur when Earth's dipole or the IMF tilt toward or away from the Sun. In either case, all or part of one tail lobe drapes over the dayside, and the magnetopause merging voltage is applied directly to the ionosphere in that lobe only, to the center or nightside of the polar cap. The overdraped lobe forms layers both earthward and sunward of the magnetopause, similar to layers observed during periods of northward IMF. The model predicts two quasi‐steady state patterns of reverse convection: (1) lobe cells in the polar cap of the overdraped lobe, generated by open‐to‐open flux transfer at the magnetopause and (2) merging cells in both polar caps, generated by closed‐to‐open flux transfer at the magnetopause and balanced open‐to‐closed transfer at an internal reconnection site, comparable to the tail merging site for southward IMF. The model also predicts transitional patterns of reverse convection for growing and shrinking polar caps in response to changes in dipole tilt and Bx and in response to a sudden transition from southward to northward IMF. The model predicts theta aurora bar formation on the dawnside polar cap boundary when activation of the internal reconnection site ends polar cap growth. In this view the theta aurora becomes the northward IMF counterpart to substorms.
Phenomenology of Neptune's radio emissions observed by the Voyager Planetary Radio Astronomy Experiment
Authors: Pedersen B. M. , Lecacheux A. , Zarka P. , Aubier M. G. , Kaiser M. L. , Desch M. D.
Published: February 2008

The Neptune flyby in 1989 added a new planet to the known number of magnetized planets generating nonthermal radio emissions. We review the Neptunian radio emission morphology as observed by the planetary radio astronomy experiment on board Voyager 2 during a few weeks before and after closest approach. We present the characteristics of the two observed recurrent main components of the Neptunian kilometric radiation, i.e., the “smooth” and the “bursty” emissions, and we describe the many specific features of the radio spectrum during closest approach.
On the formation and evolution of plasmoids: A survey of ISEE 3 geotail data
Authors: Moldwin Mark B. , Hughes W. Jeffrey
Published: February 2008

ISEE 3 magnetometer and electron plasma measurements from the 1983 Geotail Mission were surveyed to determine the magnetic and plasma properties of plasmoids and their evolution with distance downtail. Events were selected on the basis of a bipolar magnetic signature in either the geocentric solar magnetospheric Bz and/or By component; most had Bz bipolar signatures. We found 366 events consistent with this signature while ISEE 3 was in the plasma sheet. ISEE 3 observed plasmoids all along its trajectory whenever it was in the plasma sheet. Plasmoids are characterized by high‐speed plasma flow. Plasmoid length was determined using both the magnetometer and the electron plasma velocity data. We found the average length of plasmoids is 16.7 ± 13.0 RE, significantly smaller than previous estimates. Many plasmoids have a well‐defined magnetic core field, characterized by a field strength maximum at the center of the pass through the structure. Plasmoids appear to be relatively stable structures once their formation process is complete. The size, velocity, magnetic core strength, and Bz field amplitude of plasmoids do not depend on distance beyond 100 RE downtail. The average electron temperature inside plasmoids drops by a factor of 2 and the electron density increases by a factor of 2 as plasmoids propagate from near Earth distances (within 100 RE of the Earth) to the deep tail. We conclude that the stable size of the plasmoids, the density increase and the temperature decrease are consistent with a flux of cold electrons into the plasmoid. The strong correlation of interplanetary magnetic field By an hour before the event with the strength and direction of By observed inside plasmoids, the existence of events with the bipolar signature in both the By and Bz components, and the possible mass flux all are consistent with plasmoids being “open” magnetic structures.
Acceleration and stochastic heating of ions drifting through an auroral arc
Authors: Rothwell P. L. , Silevitch M. B. , Block L. P. , Fälthammar C. ‐G.
Published: February 2008

We find that ions E × B drifting through an auroral arc can undergo transverse acceleration and stochastic heating. This result is very analogous to recent work regarding similar phenomena in the magnetotail (Büchner and Zelenyi, 1990; Chen and Palmadesso, 1986; Brittnacher and Whipple, 1991). An analytic expression for the maximum arc width for which chaotic behavior is present is derived and numerically verified. We find, for example, that a 1.5‐km‐thick arc at Λ = 65° requires a minimum potential drop of 3 kV for transverse ion acceleration and heating to occur. Thicker arcs require higher potential drops for stochasticity to occur. This mechanism could be a source for conic ions.
Shaping of the magnetotail from the mantle: Global and local structuring
Authors: Ashour‐Abdalla Maha , Berchem Jean P. , Büchner Jörg , Zelenyi Lev M.
Published: February 2008

This paper discusses kinetic modeling of the properties of magnetotail formation from a plasma mantle source and develops a unified view of the structure of the central part of the magnetotail plasma sheet as well as the structure of its boundary layer. Trajectories of mantle protons in the presence of a uniform dawn‐dusk electric field have been traced using the Tsyganenko magnetic field model for quiet periods of magnetospheric activity. The most important portion of the particle trajectories is each particle's first interaction with the sharp reversal of the magnetic field in the tail midplane, because this interaction results in particle energization and chaotic scattering. The closer this interaction takes place to the X line, the larger a particle's energy will become. The intensity of the chaotic scattering in the tail midplane depends also on the position in the tail at which it occurs. The energization and scattering result in a significant restructuring of the tail ion distributions, both in space and in velocity coordinates. Our model shows the evolution of the global structure of the tail with a clearly defined central plasma sheet and plasma sheet boundary layer developing from its beginnings as a plasma “nucleus” in the distant tail current sheet. This large‐scale restructuring is accompanied by the creation of small‐scale features in the particle distribution functions. For example, the model not only correctly reproduces the spatial distribution and velocity dispersion of the fast ion beams moving both earthward and tailward in the plasma sheet boundary layer, but also indicates that these beams should be highly structured spatially into 5‐6 smaller beamlets with distinct velocities. In addition the model shows that complementary ring distribution structures should also exist in the central plasma sheet. Our analysis indicates that the ion distribution functions in the central plasma sheet should take a rather specific form in velocity space with loss regions oriented predominantly orthogonal to the magnetic field. Our results also emphasize the importance of counterstreaming populations, not only in the boundary layer, but also in the central part of the plasma sheet. Analytical calculations indicate that the properties of chaotic scattering in the magnetotail under realistic conditions (x dependence of the normal magnetic field and dawn‐dusk electric field) are quite different from those predicted by earlier simple xindependent models. Finally the model results are compared with recent observations of ion distribution functions and their moments for various regions of the magnetotail, and quantitative estimates from the model are shown to be in good agreement with observations. Small‐scale structuring and the presence of counterstreaming are also discussed, as well as their possible importance in explaining the observed intermittency in the plasma sheet bulk flows.
Nonthermal ions and associated magnetic field behavior at a quasi‐parallel Earth's bow shock
Authors: Wilkinson W. P. , Pardaens A. K. , Schwartz S. J. , Burgess D. , Lühr H. , Kessel R. L. , Dunlop M. , Farrugia C. J.
Published: February 2008

Ion and magnetic field measurements at Earth's bow shock from the AMPTE‐UKS and ‐IRM spacecraft, are examined in high time resolution during a 45‐min interval when the field remained closely aligned with the model bow shock normal (θBn ∼ 0°). Dense (> 1% of the solar wind phase space density) ion beams are detected almost exclusively in the midst of short‐duration (≤ 30 s) periods of turbulent magnetic field wave activity. The maximum energy of the beams, which is comparable to the solar wind's energy, and their azimuthal location suggest that these ions may originate from specular reflection off the shock. However, we discover many examples of propagation at large elevation angles relative to the ecliptic plane, which is inconsistent with reflection in the standard model shock configuration. The associated waves span the frequency range between ∼ 0.3 and 3 Hz. They are elliptically polarized, are preferentially left‐handed in the observer's frame of reference, yet are less confined to the maximum variance plane than other previously studied foreshock waves. The angles of propagation of the waves with respect to the solar wind magnetic field (and shock normal) span a wide range of values but are typically ∼ 30°. The considerable evolution in the appearance of the waves from one spacecraft to the other precludes us from estimating any of their properties in the plasma rest frame. Nevertheless, the association of the wave activity with the ion beams suggests that the former may be triggered by an ion‐driven instability, and possible candidates are discussed.
The neutral circulation in the vicinity of a stable auroral arc
Authors: Walterscheid R. L. , Lyons L. R.
Published: February 2008

We have performed new simulations with realistic potential structures and with an initial state which had been warmed and accelerated by preexisting aurora. Our earlier simulations of the neutral response to an intense symmetric stable arc showed antisymmetrically distributed counterstreaming zonal winds with the strongest winds outside and on the flanks of the arc. The electric field was taken to be zero in the center of the arc; thus the ion drag acceleration and Joule heating were small within the arc despite tight coupling between ions and neutrals. Also, these simulations did not include atmospheric acceleration or heating associated with the diffuse aurora preceding arc formation. Later theoretical work and observations indicate that the electric field decreases in the arc relative to the field southward of the arc but does not necessarily go to zero or even become small. In our new simulations, strong zonal winds are generated within the arc, and the winds outside of the arc are stronger on the equatorward side by a factor of ∼2‐3, depending on the model of arc potential structure. In addition, the temperature in the upper part of the model domain (upper E region and lower F region) is cooler by a few degrees within the arc than outside, in qualitative agreement with DE observations. Our earlier work also did not include the effects of arc‐zonal wind feedback or of large‐scale cross‐arc winds. A subsequent theoretical model of coupled auroral and neutral dynamics showed that feedback causes the electric field to increase so as to remain approximately constant in the frame of the neutrals at altitudes near the peak of the Pedersen conductivity. This maintains an approximately constant ion drag as the neutrals are accelerated. Simulations with the electric field adjusted to maintain such a constant ion drag force indicate that feedback effects are not strong over the duration of a stable arc. Cross‐arc winds decrease zonal winds by decreasing the time that air parcels are subject to strong zonal acceleration. Simulations with cross‐arc winds representing large‐scale flow indicate that strong winds are required to substantially reduce the zonal wind associated with wide stable discrete arcs (∼50 km in width).
Cosmic ray modulation and the distant heliospheric magnetic field: Voyager 1 and 2 observations from 1986 to 1989
Authors: Burlaga L. F. , McDonald F. B. , Ness N. F.
Published: February 2008

The changes in the cosmic ray intensity >70 MeV/nucleon observed by Voyager 1 (hereinafter called V1) near 30° heliographic latitude and by Voyager 2 (hereinafter called V2) near 0° heliographic latitude from 1986 to 1990 were closely related to the large‐scale fluctuations in the magnetic field strength at V1 and V2, respectively. The decreases in the cosmic ray intensity observed by V1 and V2 from 1986 to 1990 were generally related to merged interaction regions (MIRs). An exception to this relation was observed from about day 45 to day 110, 1987 at V1. The MIRs are of three general types: “global MIRs” (GMIRs), which extend around the Sun and to high latitudes, whose effects persist over several solar rotations; “corotating MIRs” (CMIRs), which are quasi‐periodic structures generated by the interaction of corotating streams lasting several solar rotations; and “local MIRs” (LMIRs), which are nonperiodic and have limited latitudinal and/or longitudinal extent, whose effects last less than one solar rotation. The cosmic ray intensity shows a net increase with time when the MIRs are weak and the magnetic field strength is relatively low, for example, from ∼day 120 to ∼day 45, 1987 at V1 and from ∼120, 1986 to ∼195, 1987 at V2. The cosmic ray intensity fluctuates about a constant level when CMIRs and rarefaction regions are dominant. This is the case from ∼230, 1987, to ∼275, 1988, at V1 and during some intervals from ∼245, 1987, to ∼60, 1989, at V2. The cosmic ray intensity decreases locally during the passage of a LMIR. Such a local decrease was observed at both V1 and V2 near day 100, 1986. Local decreases in the cosmic ray intensity were observed at V2 but not V1 beginning near days 240 and 320, 1987, and near day 275, 1988. A global, broad, steplike change in the cosmic ray intensity was observed at all of the spacecraft during the passage of a GMIR, beginning approximately near day 125, 1989, at V1 and near day 65, 1989, at V2 and extending for approximately 100 to 150 days, respectively.
Latitudinal structures of nightside field‐aligned currents and their relationships to the plasma sheet regions
Authors: Fukunishi H. , Takahashi Y. , Nagatsuma T. , Mukai T. , Machida S.
Published: February 2008

Using the magnetic field and low‐energy particle data from the Akebono (EXOS D) satellite, the latitudinal structures of field‐aligned currents and auroral particles in the nightside auroral oval region have been studied in detail for 12 northern hemisphere passes in the 20‐04 MLT sector and at altitudes of 5000‐10,000 km. It is found that the nightside field‐aligned current system consists of the latitudinally narrow (∼1°) current system located at the poleward boundary of the auroral oval and the latitudinally wide current system which occupies the main portion of the oval. The former has been designated as “the boundary current system” and the latter as “the central current system.” Both current systems are suggested to be located in the closed field line region, since the polar rain is observed just poleward of the boundary current system. The boundary current system is suggested to be independent of the traditional region 1 current system associated with inverted V electron precipitation, since suprathermal electrons with field‐aligned pitch angle distributions are found to be the main charge carrier of this current system. Another important finding is that energy‐dispersed ion precipitation events, which are interpreted to be the result of the E × B drift of ions flowing earthward in the plasma sheet boundary layer, appear in this boundary current region. The central current system consists of the two regions, the inverted V electron precipitation region on the poleward side and the unstructured electron and ion precipitation region on the equatorward side. It is suggested therefore that this central current system corresponds to the traditional region 1 and region 2 current system in the nightside sector. It is further found that the diffuse band of plasma sheet kilovolt electrons is continuously observed form the polar cap boundary defined as the polar rain terminator to the equatorward boundary of the unstructured precipitation region through the inverted V electron precipitation region. Consequently, inverted V structures and diffuse plasma sheet electron precipitation always coexist. From these spatial relationships, it is strongly suggested that the boundary current system is mapped to the plasma sheet boundary layer, while the central current system is mapped to the main or central plasma sheet.
Quasi‐linear analysis of ion Weibel instability in the Earth's neutral sheet
Authors: Lui Anthony T. Y. , Yoon Peter H. , Chang Chia‐Lie
Published: February 2008

The development of the ion Weibel instability in the Earth's neutral sheet is examined by quasi‐linear analysis for waves propagating along the ambient magnetic field. For ion drift speed reaching a sizable fraction (∼0.5 to 1) of the ion thermal speed, numerical solution of the quasi‐linear kinetic equations shows the wave growth reaching the nonlinear stage in less than one ion gyroperiod. The saturation level is attained with the ion drift speed reducing by ∼15 to 28% of its initial value, wave amplitudes ranging from ∼0.2 to 0.8 of the ambient magnetic field, and the ion temperature along the magnetic field increasing by ∼25 to 90%. This instability thus provides a means for non‐adiabatic heating of ions during substorms. The resulting anomalous resistivity is estimated to be ∼1×10−7 to 1×10−6 s, about 11 to 12 orders of magnitude above the classical Coulomb resistivity. These predictions may be regarded as lower limits on the effects of the nonlinear stage of this instability since only parallel propagating waves are considered. Nevertheless, the calculated reduction in the ion drift speed compares reasonably well with the inferred amount of current reduction from observations of current disruption events, and the anomalous resistivity is within the range of values estimated for active magnetotail regions during substorm expansion. The difference in the nonlinear evolution with different initial ion drift speeds suggests that a current system in the same sense as the substorm current wedge is expected to be established in the midnight region as a result of this instability occurring in the nightside neutral sheet. For a typical amount of current reduction, it is estimated that the plasma in this region is subjected to a substantial net force injecting it earthward.
A global simulation of the magnetosphere with a long tail: Southward and northward interplanetary magnetic field
Authors: Usadi A. , Kageyama A. , Watanabe K. , Sato T.
Published: February 2008

Through a global three‐dimensional MHD simulation, which makes it possible to reveal the physical processes and causalities of the global interaction between the solar wind and the magnetosphere (Watanabe and Sato, 1990; Kageyama et al., 1992), the effect of the interplanetary magnetic field (IMF), both northward and southward, on Earth's magnetosphere has been investigated. A southward IMF, upon reconnecting at the dayside magnetopause, sweeps the Earth's magnetic field toward the nightside and drapes the magnetotail. This gives rise to a plasma sheet cross‐tail current increase and explosive magnetic reconnection at around 15 RE from the Earth. This reconnection results in the formation of a large plasmoid which grows much faster than for the simulation with no IMF, thus supporting previous notions that a southward IMF is a driving mechanism of plasma sheet reconnection. A northward IMF is observed to reconnect with the magnetosphere along the cusp shoulder, stripping magnetic field lines away and weakening compression of the plasma sheet, thus inhibiting plasma sheet reconnection. In order to accommodate a balance of magnetic to dynamic pressure along the magnetopause, the magnetosphere changes from its usual cometlike shape to that resembling a tadpole as it attempts to return to a dipolar structure. Plasma sheet reconnection is thereby inhibited. A case when the direction of the IMF turns from south to north is also studied.
Three‐dimensional simulations of the solar wind interaction with Mars
Authors: Brecht Stephen H. , Ferrante John R. , Luhmann J. G.
Published: February 2008

Three‐dimensional hybrid particle simulations of the dayside portion of the Mars/solar wind interaction have been performed. These simulations are compared with the in situ measurements taken during the elliptical orbits of Phobos 2. The comparisons show considerable agreement between the magnetic field data and the simulations results. The results of the simulations bring into question the type of structures created in the subsolar region of Mars. It appears that the large larmor radius of the solar wind ions prevents the formation of a traditional collisionless shock in the subsolar region of the interaction.
Gravity wave‐driven fluctuations in the O2 atmospheric (0‐1) nightglow from an extended, dissipative emission region
Authors: Hickey Michael P. , Schubert G. , Walterscheid R. L.
Published: February 2008

The omission fluctuations of the O2 atmospheric nightglow due to gravity waves, evanescent waves, and acoustic waves are calculated using the Eulerian model formalism of Schubert et al. (1991) from which the complex parameter <η> is derived:
where I is the nightglow intensity, TI is intensity‐weighted temperature, the brackets denote an altitude integration over all emitting layers, an overbar denotes an unperturbed state and a prime denotes a perturbation about the unperturbed state. The state is assumed to form directly by the three‐body association reaction involving atomic oxygen and also via quenching of the intermediate state . Our calculated values of <η> are compared with those derived from the observations of Zhang (1991) and Zhang et al. (1992a). These observations, which were obtained during the AIDA (Arecibo Initiative in Dynamics of the Atmosphere) Act ′89 campaign at Arecibo (18°N) during the period covering April 5 to May 9, 1989, were made with the MORTI (mesopause oxygen rotational temperature imager) instrument and provide us with values of both <η> and the horizontal wavelengths of the gravity waves. For all of our results the undisturbed mesosphere is defined by the model output of Garcia and Solomon (1985) for a latitude of 18° and for the months of March and June, these being most relevant to the observations of Zhang. For the evanescent waves, there is essentially good agreement between the theory and observations, while for the internal waves the agreement is best for |<η>|. However, the comparatively larger errors in the observed values of |<η>| make it impossible to decide which chemical scheme used in the model provides the best results. The errors associated with the observed phases of <η> are apparently minimal, and cannot account for the differences between the observed and modeled phases of <η>. The sensitivity of our modeled values of <η> to certain rate constants and the quenching parameters is investigated and discussed. In particular, we demonstrate that our modeled values of <η> are sensitive to the quenching effects of atomic oxygen and hence also to the production mechanism of the
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The Earth's magnetosphere under continued forcing: Substorm activity during the passage of an interplanetary magnetic cloud
Authors: Farrugia C. J. , Freeman M. P. , Burlaga L. F. , Lepping R. P. , Takahashi K.
Published: February 2008

In this third paper on magnetic clouds and their interaction with the terrestrial magnetosphere we focus on substorm activity attending the 30‐hour‐long passage of the magnetic cloud seen at 1 AU on January 14/15, 1988. Our main aim is to describe high resolution field and plasma data from six spacecraft at geostationary orbit and beyond (up to 9 RE), supplemented by ground magnetograms, in order to estimate the number of substorm onsets that occurred during this passage. This analysis, attempted here for the first time over such an extended data interval, enables us to obtain a comprehensive view of the magnetospheric behavior. We then relate the interplanetary field and plasma input to the magnetospheric and ionospheric output. The paper is a case study of the response of the nightside magnetosphere to particularly well‐behaved interplanetary conditions: the variations with time of the field and plasma parameters in the magnetic cloud are simple; and the north‐south (Bz) and east‐west (By) components of the magnetic field range slowly over ∼20 nT on both sides of zero. From a synopsis of the substorm observations we find that during the earlier 11‐h interval when Bz was continuously positive, the magnetosphere was quiescent, whereas in the later 18‐h interval, when Bz was uninterruptedly negative, a large magnetic storm was set off. In this latter interval the substorm onsets recur on average every 50 min. Their average recurrence frequency remains relatively undiminished even when the magnetic cloud Bz and other measures of the interplanetary energy input have decreased considerably. We find these results to concur with the current models of magnetospheric substorms based on deterministic nonlinear dynamics. Using ancillary information from HF radar observations of ionospheric convection, we investigate the magnetospheric conditions preceding the first substorm onset in the long substorm sequence. We find that it occurred when the cloud's magnetic field had a persistent northward component but was predominantly westward pointing.
Particle orbits in model current sheets with a nonzero By component
Authors: Zhu Zhongwei , Parks George
Published: February 2008

The problem of charged particle motions in magnetotaillike model current sheets (Speiser, 1965) is revisited with the inclusion of a nonzero dawn‐dusk magnetic field component By. In a zero By current sheet, particle orbits in the phase space (ż, z) are symmetric about the z = 0 plane for most models as long as Bz is z‐independent and antisymmetry of Bx about z = 0 is assumed, i.e., Bx(z) = −Bx(−z). This can be clearly seen from the fact that the equations of motion for charged particles are unchanged when z is changed to −z. In a nonzero By current sheet, this symmetric character in the phase space does not exist. Assuming a small By (same order as Bz), three cases are examined in this paper: case 1: Bx ≠ 0, By ≠ 0, Bz = 0, and Ey = 0; case 2: Bx ≠ 0, By ≠ 0, Bz = 0, and Ey ≠ 0 for “trapped” particles; and case 3: Bx ≠ 0, By ≠ 0, Bz ≠ 0, and Ey ≠ 0 for “escaped” particles. Our study shows that a nonzero By component can disturb particle orbits, change their bounce frequency, and destroy the symmetry of the orbits about the z = 0 plane. For the escaped particles, acceleration and ejection of particles from the current sheet are mainly controlled by the zero By current sheet system. However, with the injection of low energy particles, positively (negatively) charged particles, after acceleration by the current sheet, tend to be ejected toward the northern (southern) boundary of the current sheet for a positive By component (i.e., dawn to dusk), and toward the opposite direction for a negative By component. We present several examples of test particle computations for a parabolic model.