bluuebunny/crossref_metadata_2025 huggingface dataset:
Superevents and cosmic ray modulation, 1974‐1985
Authors: Cliver E. W. , Dröge W. , Müller‐Mellin R.
Published: February 2008

Superevents are long‐lived (∼40 days at 1 AU) ∼10‐MeV proton events that originate in episodes of intense solar activity characterized by major coronal mass ejections (CMEs) and individual solar energetic particle (SEP) events. Superevents exhibit only weak intensity variation with heliolongitude. They propagate to the outer heliosphere at speeds above that of the average solar wind, and, at Pioneers 10 and 11, prominent superevents are generally associated with strong interplanetary shocks. For the period from 1974 to 1985, we find that superevents are not reliable signatures of the onsets of long‐term steps in the modulation record of >1‐GV galactic cosmic rays (GCRs) at 1 AU. Of six intense superevents during this interval, one occurred coincident with the onset of a step (June‐July 1982), two occurred midway through step decreases, and three occurred at the ends of step decreases. Two step decreases during this period began in conjunction with relatively weak SEP activity. Thus the largest superevents occurring from 1974 to 1985 were neither necessary nor sufficient conditions for long‐term GCR intensity steps and therefore were not closely related to the global merged interaction regions that have been identified with such steps. In terms of convection/diffusion models of cosmic ray modulation, the poor association of the largest superevents with long‐term GCR intensity decreases during this period suggests that the background level of more frequently occurring, and less energetic, CMEs is more important for establishing the 11‐year cycle than are the sporadic, and relatively short‐lived, outbreaks of major CME activity that give rise to superevents.
Whistlers and plasmaspheric hiss: Wave directions and three‐dimensional propagation
Authors: Draganov A. B. , Inan U. S. , Sonwalkar V. S. , Bell T. F.
Published: February 2008

Wave data from the DE 1 satellite showing simultaneously nonducted whistlers and hiss are analyzed to determine wave propagation directions. At L = 3.8 and a geographic latitude of λg = 12°S, the average wave normal directions of discrete whistlers are measured to be ∼51° for fnof;= 4.5 kHz and ∼60° for fnof;= 3.5 kHz, forming a small (<20°) angle with the magnetic meridional plane. Hiss wave normal angles are determined as ∼70° and ∼77° for ƒ = 3.5 kHz and fnof; = 2.5 kHz, respectively, with the wave vector being almost perpendicular to the meridional plane. While the measured wave normal angles of whistlers and hiss are consistent with generation of hiss by magnetospheric whistlers, existence of a significant azimuthal component indicates that further assessment of this connection must be based on three‐dimensional ray tracing. A new approximate analytical formulation of three‐dimensional propagation of whistler waves is developed and used to model the drift of magnetospherically reflected whistlers in azimuth. The results show that depending on initial parameters, the time of arrival of whistler rays at a fixed observation point can differ by 10 ‐ 20 s, with signals from different magnetospherically reflected whistlers overlapping to evolve into a hiss like signal. The total azimuthal drift of whistler rays is found to not exceed ∼30°, so that plasmaspheric hiss may be produced by nonducted whistlers at longitudes correlated with the location of thunderstorm activity.
Magnetosheath density fluctuations from a simulation of auroral kilometric radiation radio propagation
Authors: Steinberg J. ‐L. , Hoang S.
Published: February 2008

Using ray‐tracing techniques in an inhomogeneous plasma, we calculate the direction and size of the scattered image of the low‐frequency auroral kilometric radiation (AKR) radio source seen through the magnetosheath and the shock by an observer in the 0200 to 0800 LT sector at geocentric distances of 50 to 250 RE. The geometry of the limiting surfaces and the density distribution are those derived from in situ measurements. A good fit to three sets of ISEE‐3 observations is obtained at all frequencies assuming that the magnetosheath density inhomogeneities are weak, their autocorrelation function Gaussian and their scattering power at twice the local fnof;p is 4 × 10−9 rad² m−1. Variations of the model show to what extent observations of the AKR scattered image could, in principle, yield information on the density jump rs across the shock, its variation with X (GSE) and as a function of time.
Instantaneous mapping of high‐latitude convection with coherent HF radars
Authors: Hanuise C. , Senior C. , Cerisier J. ‐C. , Villain J. ‐P. , Greenwald R. A. , Ruohoniemi J. M. , Baker K. B.
Published: February 2008

Coherent HF radars at Goose Bay (Labrador) and Schefferville (Quebec) are used to study plasma convection in the high‐latitude ionosphere. Maps of the two‐dimensional flow pattern are obtained by merging simultaneous sets of radial velocity data, each with a time resolution of a few minutes. From a time sequence of such maps we have separated the changes in flow due to magnetic local time (MLT) variations over the region of observation, from those due to UT time variations. We study in detail the convection in the early morning sector observed on October 15, 1989, when the interplanetary magnetic field (IMF) reversed from southward to northward. This IMF reversal was not associated with a clear response in the nightside convection but rather with several sudden changes, some of which anticipated the Bz reversal. We suggest that these changes are associated with delayed and superposed ionospheric responses to previous IMF perturbations, or to local effects. After the IMF reversal from south to north our observations of westward and southwestward velocities in the 71°‐77° invariant latitude range are consistent with the earlier simulations for Bz > 0 and By < 0. During the period of steady northward IMF after the reversal the convection pattern was observed to reconfigure slowly: a region of large westward velocities progressively moved poleward, while convection in the low‐latitude part of the field of view faded away. The time constant of this slow reconfiguration was about 1 hour and varied with MLT, such that it was larger closer to midnight. These data, combined with particle data from successive passes of the DMSP satellites, provide information on the contraction of the polar cap after the IMF Bz reversal and on the MLT dependency of the velocity at which this contraction occurs. They show that the polar cap contracts more rapidly in the daytime than in the nightime and more rapidly in the postmidnight sector than in the premidnight sector.
Reevaluation of the O+(²P) reaction rate coefficients derived from Atmosphere Explorer C observations
Authors: Chang T. , Torr D. G. , Richards P. G. , Solomon S. C.
Published: February 2008

O+(²P) is an important species for studies of the ionosphere and thermosphere: its emission at 7320 Å can be used as a diagnostic of the thermospheric atomic oxygen density. Unfortunately, there are no laboratory measurements of the O and N2 reaction rates which are needed to determine the major sinks of O+(2P). The reaction rates that are generally used were determined from aeronomic data by Rusch et al. (1977) but there is evidence that several important inputs that they used should be changed. We have recalculated the O and N2 reaction rates for O+(2P) using recent improvements in the solar EUV flux, cross sections, and photoelectron fluxes. For the standard solar EUV flux, the new N2 reaction rate of 3.4 ± 1.5 × 10−10 cm3 s−1 is close to the value obtained by Rusch et al. (1977), but the new O reaction rate of 4.0±1.9 × 10−10 cm3s−1 is about 8 times larger. These new reaction rates are derived using neutral densities, electron density, and solar EUV fluxes measured by Atmosphere Explorer C in 1974 during solar minimum. The new theoretical emission rates are in good agreement with the data for the two orbits studied by Rusch et al. and they are in reasonable agreement with data from five additional orbits that are used in this study. We have also examined the effect of uncertainties in the solar EUV flux on the derived reaction rates and found that 15 % uncertainties in the solar flux could cause additional uncertainties of up to a factor of 1.5 in the O quenching rate.
On the development of a magnetospheric substorm influenced by a storm sudden commencement: Ground, balloon, and satellite observations
Authors: Ullaland S. , Kremser G. , Tanskanen P. , Korth A. , Roux A. , Torkar K. , Block L. P. , Iversen I. B.
Published: February 2008

Previous statistical investigations have revealed a relationship between storm sudden commencements (ssc) and magnetospheric substorm onsets. Little is known about the physical processes constituting this relationship. We used a comprehensive data set for a detailed case study. The ssc occurred on July 6, 1979, at 1930 UT. The substorm expansion phase started 5 min later. The event was preceded by a loading phase of more than 1.5 hours. The loading phase developed in three steps. During each step the cross‐tail current sheet suddenly expanded earthward and intensified. The third step, at 1930 UT, coincided with the ssc. It was very likely caused by the interaction of ions with magnetohydrodynamic waves generated by the ssc. This step was followed at 1935 UT by the onset of the expansion phase accompanied by PiB magnetic pulsations, impulsive electron precipitation, and energetic ion injection at GEOS 2 orbit. The dipolarization of the geomagnetic field started 9 min after the expansion phase onset together with energetic electron injection and a decrease in the energetic ion flux. Signatures of Birkeland currents at dipolarization, and a pressure anisotropy P∥ions > P⊥ions during the 4 min before dipolarization, indicate field‐aligned processes consistent with partial diversion of the cross‐tail current into the ionosphere. The observations are discussed in the framework of a model in which the ballooning mode instability (BMI) developing in the near‐Earth plasma sheet is regarded as the trigger process for the expansion phase. Quantitative estimates of the instability criteria show that the necessary conditions for the BMI are fulfilled during the whole loading phase. An energetic ion pressure gradient, which is needed to drive the BMI, was observed during the interval 1931 UT (ssc) to 1944 UT (dipolarization). Signatures of the instability itself, in terms of regular ion pressure gradient variations, were recorded during the same time interval. We conclude that the magnetosphere was potentially unstable for the BMI when the ssc occurred and that the interaction of the ssc with the magnetospheric particles expanded the cross‐tail current sheet further earthward, thereby creating plasma conditions in which the BMI could grow. The instability started at 1931 UT, and the BMI in turn triggered the expansion phase at 1935 UT.
Flow downstream of the heliospheric terminal shock: The magnetic field on the heliopause
Authors: Nerney Steven , Suess S. T. , Schmahl E. J.
Published: February 2008

Modeling the kinematic magnetic field in the solar wind beyond the terminal shock shows that a ridge of magnetic pressure is produced just inside the heliopause. This ridge has its maximum amplitude in the plane defined by the solar rotation axis and the heliotail and decreases to zero amplitude in the solar equatorial plane. The ridge is sufficiently large that it will cause the layer immediately inside the heliopause to thicken, pushing the heliopause outward and slightly affecting its position relative to the terminal shock. However, the ridge is far too thin to cause an important change in the distance of the terminal shock from the Sun. The kinematic assumption prevents us from estimating the actual magnitude of the ridge, but we show that these conclusions are a simple consequence of geometrical arguments for incompressible, steady, laminar flows. Moreover, the heliopause magnetic field originates on the terminal shock near the substagnation point. Consequently, the heliospheric current sheet field reversals are painted onto the inside surface of the heliopause. Alternate magnetic polarity strips will be oppositely directed relative to the interstellar magnetic field, implying that reconnection inevitably occurs on a fine scale near the nose of the heliosphere. This suggests that the heliopause is a leaky, diffuse surface.
Solar cycle and seasonal variations in F region electrodynamics at Millstone Hill
Authors: Buonsanto M. J. , Hagan M. E. , Salah J. E. , Fejer B. G.
Published: February 2008

Incoherent scatter radar observations of ion drifts taken at Millstone Hill (42.6°N, 288.5°E) during 73 experiments in the period February 1984 to February 1992 are used to construct, for the first time at this station, average quiet‐time E×B drift patterns for both solar cycle maximum and minimum, for the summer, winter, and equinox seasons. The daily variation of V⊥N shows a reversal from northward to southward drifts near noon, and a return to northward drifts in the premidnight hours. The weaker southward drift in the afternoon in summer noted by Wand and Evans (1981) is shown to occur only at sunspot minimum. The daily variation of V⊥E shows daytime eastward drifts and nighttime westward drifts, except in summer when the usual daytime eastward maximum near 1200 LT is suppressed. The daily mean drift is westward for all seasons, and is largest in summer. The daytime eastward drift and nighttime westward drift tend to be stronger at solar maximum than at solar minimum. Average drift patterns are also constructed for equinox for both extremely quiet and geomagnetically disturbed periods. V⊥N is appreciably more northward under extremely quiet than under disturbed conditions in the postmidnight and morning periods. During extremely quiet periods, V⊥E turns slightly eastward in the evening hours, while it is strongly westward for disturbed conditions. This result contrasts with the strong eastward drifts in the evening in summer reported for extremely quiet conditions at Millstone Hill by Gonzales et al. (1978). A strong anticorrelation is seen at Millstone Hill between V⊥N and V∥, as is found at lower latitude stations. The quiet‐time patterns are discussed in terms of the causative E and F region dynamo mechanisms. At Millstone Hill, conjugate point electric fields are also important in winter when the conjugate ionosphere is sunlit for much of the night.
Thermospheric molecular oxygen measurements using the ultraviolet spectrometer on the Solar Maximum Mission Spacecraft
Authors: Aikin A. C. , Hedin A. E. , Kendig D. J. , Drake S.
Published: February 2008

The technique of solar occultation has been utilized to measure molecular oxygen densities in the 140‐ to 220‐ km altitude region. The ultraviolet spectrometer/polarimeter on the Solar Maximum Mission (SMM) spacecraft was operated in the occultation mode between 1985 and the end of 1989, at wavelengths of 137.46 and 139.06 nm. This covered the period from the solar sunspot minimum in 1986 to December 1989 when solar activity was near the sunspot maximum. The observed solar area was 10×10 arc s2, which yields O2 concentrations with an altitude resolution of 0.17 km. These molecular oxygen data are compared with O2 data obtained under similar conditions of local time, location, and solar activity using other techniques. SMM data are also compared with the mass spectrometer/incoherent scatter (MSIS) ‐ 86 model. Good agreement is found near 150 km for several O2 temporal variations including the annual cycle, local time, solar activity, and magnetic activity. Unlike the model, the measured density profiles exhibit no significant increase with increasing solar activity. The disagreement of the SMM O2 data with the MSIS‐86 model increases with increasing altitude.
The spatial development of the low‐latitude boundary layer
Authors: Manuel John R. , Samson J. C.
Published: February 2008

The low‐latitude boundary layer (LLBL) comprises a large fraction of the magnetospheric boundary layer making it a potentially important site for the transport of mass, momentum, and energy from the magnetosheath into the magnetosphere. We have used an ideal magnetohydrodynamic computer simulation with nonperiodic inflow and outflow boundary conditions to investigate the processes responsible for the spatial development of a 6.4 Earth radii ( RE) long section of the dayside LLBL. Initializing the simulation with a realistic boundary layer configuration, slightly modified for computational efficiency, leads to the development of a LLBL which broadens with downstream distance from a thin (0.12 RE thick) and laminar boundary layer to a broad (∼0.7 RE thick) and turbulent one capable of significant transport. Broadening occurs through the action of Reynolds and Maxwell stresses that develop during the roll‐up and merging of vortices generated by the Kelvin‐Helmholtz instability in the simulated boundary layer and indicates the transport of mass, momentum, and energy into the LLBL. The steep gradients that develop at the magnetopause between the vortices may further aid the transport by encouraging wave‐particle diffusion processes capable of transporting magnetosheath plasma into the LLBL. The presence of a flow‐aligned component of the magnetic field, on the other hand, can hinder downstream development of the boundary layer by slowing and then stopping boundary layer vortices through transfer of their rotational kinetic energy into deformations of the magnetic field.
Contributions of the low‐latitude boundary layer to the finite width magnetotail convection model
Authors: Spence Harlan E. , Kivelson Margaret G.
Published: February 2008

Convection of plasma within the terrestrial nightside plasma sheet contributes to the structure and, possibly, the dynamical evolution of the magnetotail. In order to characterize the steady state convection process, we have extended the finite tail width model of magnetotail plasma sheet convection. The model assumes uniform plasma sources and accounts for both the duskward gradient/curvature drift and the earthward E × B drift of ions in a two‐dimensional magnetic geometry. During periods of slow convection (i.e., when the cross‐tail electric potential energy is small relative to the source plasma's thermal energy), there is a significant net duskward displacement of the pressure‐bearing ions. The electrons are assumed to be cold, and we argue that this assumption is appropriate for plasma sheet parameters. We generalize solutions previously obtained along the midnight meridian to describe the variation of the plasma pressure and number density across the width of the tail. For a uniform deep‐tail source of particles, the plasma pressure and number density are unrealistically low along the near‐tail dawn flank. We therefore add a secondary source of plasma originating from the dawnside low‐latitude boundary layer (LLBL). The dual plasma sources contribute to the plasma pressure and number density throughout the magnetic equatorial plane. Model results indicate that the LLBL may be a significant source of near‐tail central plasma sheet plasma during periods of weak convection. The model predicts a cross‐tail pressure gradient from dawn to dusk in the near magnetotail. We suggest that the plasma pressure gradient is balanced in part by an oppositely directed magnetic pressure gradient for which there is observational evidence. Finally, the pressure to number density ratio is used to define the plasma “temperature.” We stress that such quantities as temperature and polytropic index must be interpreted with care as they lose their nominal physical significance in regions where the two‐source plasmas intermix appreciably and the distributions become non‐Maxwellian.
Turbulence generated by a gas of electron acoustic solitons
Authors: Dubouloz N. , Treumann R. A. , Pottelette R. , Malingre M.
Published: February 2008

The characteristics of the electrostatic turbulence generated by a gas of electron acoustic solitons are investigated. Electron acoustic solitons are shown to propagate in magnetized plasmas up to about 30° off the parallel direction without significant changes in their properties relative to the nonmagnetized case. Using the conservation properties of the Korteweg‐deVries equation, the velocity distribution function of the soliton gas is calculated in the small‐amplitude limit. For low cold to hot electron density ratios and high‐mean‐square electric fields, a significant number of solitons with high velocities and amplitudes is produced, implying the generation of an intense broadband electrostatic turbulence. These theoretical results are compared with the properties of the broadband electrostatic noise (BEN) emissions observed by the Viking satellite in the dayside auroral zone. The electric spectra generated by the electron acoustic soliton gases which can be derived from Viking plasma and wave observations are calculated numerically. These spectra are shown to agree with experimental data and in particular to explain the high‐frequency part of BEN emissions, which lies in a range forbidden for linear electrostatic waves.
Three‐dimensional magnetic field annihilation
Authors: Jardine M. , Allen H. R. , Grundy R. E.
Published: February 2008

We present a family of three‐dimensional nonlinear solutions for magnetic field annihilation in a current sheet, including the effects of resistivity and viscosity. The different members of the family are characterized by the imposed vorticity of the flow that brings the field lines together. Since in a three‐dimensional flow the vorticity can be increased by the stretching of vortex lines (an effect that is absent in two dimensions), we find some striking differences to our previous two‐dimensional analysis. In both the two‐dimensional and three‐dimensional analyses, above a certain critical imposed vorticity ωcrit, the flow breaks up into cells with current sheets at their boundaries. The nature of the original central current sheet is completely altered. In the two‐dimensional analysis, ωcrit is a steeply increasing function of the viscous Reynolds number R, whereas in the three‐dimensional case, it quickly asymptotes to only ωcrit = 2υ0/L where υ0 and L are the characteristic velocity and length scale of the flow, respectively. The width of the current sheet, which depends on the speed at which field lines are carried into it, also responds differently to an increase in R. In two dimensions, the current sheet narrows for all vorticities, but in three dimensions, it narrows when the imposed vorticity is negative and widens when it is positive. Also we find that the current density within the current sheet varies as the nature of the flow is changed, rather than being constant as in the two‐dimensional case. Finally, we find that there is a minimum value of the plasma beta βmin below which the plasma pressure is negative. For the nonsheared (neutral current sheet) case, βmin increases rapidly with the magnetic Reynolds number Rm such that this type of annihilation is only possible for a high‐beta plasma. For a sheared magnetic field, however, βmin is much lower, making this type of annihilation more relevant to the solar corona.
Plasma waves downstream of weak collisionless shocks
Authors: Coroniti F. V. , Greenstadt E. W. , Moses S. L. , Smith E. J. , Tsurutani B. T.
Published: February 2008

In September 1983 the ISEE 3/ICE spacecraft made a long traversal of the distant dawnside flank region of the Earth's magnetosphere and had many encounters with the low Mach number bow shock. These weak shocks excite plasma wave electric field turbulence with amplitudes comparable to those detected in the much stronger bow shock near the nose region. Downstream of quasi‐perpendicular (quasi‐parallel) shocks, the E field spectra exhibit a strong peak (plateau) at midfrequencies (1‐3 kHz); the plateau shape is produced by a low‐frequency (100‐300 Hz) emission which is more intense behind quasi‐parallel shocks. Polarization measurements made in the very steady magnetic field conditions downstream of two quasi‐perpendicular shocks show that the low frequency signals are polarized parallel to the magnetic field, whereas the midfrequency emissions are unpolarized or only weakly polarized. A new high frequency (10‐30 kHz) emission which is above the maximum Doppler shift frequency is clearly identified as a separate wave component. High time resolution spectra often exhibit a distinct peak at high frequencies; this peak is often blurred by the large amplitude fluctuations of the midfrequency waves. The high‐frequency component is strongly polarized along the magnetic field and varies independently of the lower‐frequency waves.
Magnetopause shape as a bivariate function of interplanetary magnetic field Bz and solar wind dynamic pressure
Authors: Roelof Edmond C. , Sibeck David G.
Published: February 2008

We present a new method for determining the shape of the magnetopause as a bivariate function of the hourly averaged solar wind dynamic pressure (p) and the north‐south component of the interplanetary magnetic field (IMF) Bz. We represent the magnetopause (for XGSE>−40RE) as an ellipsoid of revolution in solar‐wind‐aberrated coordinates and express the (p, Bz) dependence of each of the three ellipsoid parameters as a second‐order (6‐term) bivariate expansion in lnp and Bz. We define 12 overlapping bins in a normalized dimensionless (p, Bz) “control space” and fit an ellipsoid to those magnetopause crossings having (p, Bz) values within each bin. We also calculate the bivariate (lnp, Bz) moments to second order over each bin in control space. We can then calculate the six control‐space expansion coefficients for each of the three ellipsoid parameters in configuration space. From these coefficients we can derive useful diagnostics of the magnetopause shape as joint functions of p and Bz: the aspect ratio of the ellipsoid's minor‐to‐major axes; the flank distance, radius of curvature, and flaring angle (at XGSE = 0); and the subsolar distance and radius of curvature. We confirm and quantify previous results that during periods of southward Bz the subsolar magnetopause moves inward, while at XGSE = 0 the flank magnetopause moves outward and the flaring angle increases. These changes are most pronounced during periods of low pressure, wherein all have a dependence on Bz that is stronger and functionally different for Bz southward as compared to Bz northward (i.e., the behavior of a “half‐wave rectifier”). In contrast, all these changes are much less sensitive to IMF Bz at the highest pressures. As an application of these new results, we use a pressure balance relationship to estimate the difference between the magnetic field strength just inside the subsolar magnetopause and that of the dipole field, and we find that this difference decreases rapidly as Bz becomes more negative (although it is relatively insensitive to northward changes in Bz). Quantitative comparison shows that Region 1 Birkeland currents could make the dominant contribution to this depression in the inferred magnetic field at the subsolar point.
A survey of low frequency waves at Jupiter: The Ulysses encounter
Authors: Tsurutani Bruce T. , Southwood David J. , Smith Edward J. , Balogh Andre
Published: February 2008

We report the results of a survey of low‐frequency (LF) plasma waves detected during the Ulysses Jupiter flyby. In the Jovian foreshock, two predominant wave periods are detected: 102‐s and 5‐s, as measured in the spacecraft frame. The 102‐s waves are highly nonlinear propagate at large angles to(typically 50°), are steepened, and sometimes have attached whistler packets. For the interval analyzed the 102‐s waves had mixed right‐and left‐hand polarizations. We argue that these are all consistent with being right‐hand magnetosonic waves in the solar wind frame. The 102‐s waves with attached whistlers are similar to cometary waves. The trailing portions are linearly polarized and the whistler portions circularly polarized with amplitudes decreasing linearly with time. The emissions are generated by ∼2‐keV protons flowing from the Jovian bow shock/magnetosheath into the upstream region. The instability is the ion beam instability. Higher Z ions were considered as a source of the waves but have been ruled out because of the low sunward velocities needed for their resonance. The 5‐swaves have, are compressive and are left‐hand polarized in the spacecraft frame. Local generation by three different resonant interactions were considered and have been ruled out. One possibility is that these waves are whistler mode by‐products of the steepened lower‐frequency magnetosonic waves. Mirror mode structures were detected throughout the outbound magnetosheath passes. For these structures, the θkBvalues were consistently in the range of 80° to 90°, exceptionally high values. Assuming 1‐keV protons, the spacing between the magnetic decreases is ∼10‐20rp. Close to the magnetopause, small amplitudes, transverse waves were detected. Within the wave packet, right, left‐ and linear polarizations were found. These may be similar to waves detected in the Earth's plasma depletion layer. Within the Jovian low‐latitude boundary layer, enhanced transverse spectral power was detected at frequencies just below the proton cyclotron frequency. A total magnetic power of 10−1nT2was determined. Cross‐field diffusion of ∼1‐keV protons yields a calculated boundary layer thickness of 0.15RJ. This is in excellent agreement with previous measurements. Transverse left‐hand (spacecraft frame) waves were detected within the Jovian magnetosphere. These waves have periods of 5‐8 min, are elliptically to circularly polarized and propagate in a range of 10° to 43° relative to. It is most likely that these waves are generated by an ion beam instability. Assuming S+, we get a resonant parallel energy of 60 keV.
An interpretation of the broadband VLF waves near the Io torus as observed by Ulysses
Authors: Farrell W. M. , MacDowall R. J. , Hess R. A. , Kaiser M. L. , Desch M. D. , Stone R. G.
Published: February 2008

The requirements for the Ulysses trajectory to attain high ecliptic latitudes using a Jovian gravitational assist resulted in a fortuitous passage through the Io torus region. Specifically, the spacecraft spent many hours at latitudes just above the torus. During this time the low‐frequency cutoff of an ordinary mode (O mode) emission allowed a determination of the local electron plasma frequency (i.e., electron density) along the northern flank of the torus. Also, near a Jovian System III longitude of 100°, the spacecraft flew past a set of active field lines that have been previously identified to be associated with the hectometric generation region. During the passage, Ulysses observed a newly discovered O mode component and a whistler mode emission similar to that observed by Voyager 1 13 years previously. All of the broadband VLF emissions imply the presence of a particular population of electrons. We suggest that broadband VLF emissions can be used as a “particle detector” to qualitatively measure the electron plasma conditions in the torus region and identify active regions.
ULF waves in the Io torus: Ulysses observations
Authors: Lin Naiguo , Kellogg P. J. , MacDowall R. J. , Mei Y. , Cornilleau‐Wehrlin N. , Canu P. , de Villedary C. , Rezeau L. , Balogh A. , Forsyth R. J.
Published: February 2008

Throughout the Io torus, Ulysses has observed intense ULF wave activity in both electric and magnetic components. Such ULF waves have been previously suggested as the source of ion precipitation leading to Jovian aurorae. The peaks of the wave spectra are closely related to the ion cyclotron frequencies, which is evidence of the waves being ion cyclotron waves (ICWs). Analysis of the dispersion relation using a multicomponent density model shows that at high latitudes (∼ 30°), peak frequencies of the waves fall into L mode branches of guided or unguided ICWs. Near the equator, in addition to the ICWs below fnof;cO++, there are strong signals at ∼10 Hz which require an unexpectedly large energetic ion temperature anisotropy to be explained by the excitation of either convective or nonconvective ion cyclotron instabilities. Their generation mechanism remains open for the future study. Evaluation of the Poynting vector and the dispersion relation analysis suggest that the waves near the equator had a small wave angle relative to the magnetic field, while those observed at high latitudes were more oblique. The polarization of the waves below fnof;cH+ is more random than that of the whistler mode waves, but left‐hand‐polarized components of the waves can still be seen. The intensity of the ICWs both near the equator and at high latitudes are strong enough to meet the requirement for producing strong pitch angle scattering of energetic ions.
MHD modeling applied to Giotto encounter with comet P/Grigg‐Skjellerup
Authors: Schmidt H. U. , Wegmann R. , Neubauer F. M.
Published: February 2008

First from a one‐dimensional MHD model we derive estimates for position and shape of the bow shock applicable for arbitrary magnetosonic Mach numbers > 1 in the solar wind. This generalization is important in the case of the Giotto encounter with P/Grigg‐Skjellerup with the unusual interplanetary magnetic field (IMF) of 18 nT. We compare three‐dimensional model calculations with observations from six cometary missions and derive new corrected estimates for the gas production of P/Grigg‐Skjellerup, which turn out to be substantially lower than those hitherto proposed assuming a large magnetosonic Mach number in the solar wind. Furthermore, we use global MHD models for this comet to determine from the measured magnetic field the position of the Giotto trajectory with respect to the comet. As a measure of the goodness of fit we use the mean square of the difference of the observed and the model field. We fit segments of the measured data of three different lengths centered near closest approach and for three different values of the unperturbed IMF. For all parameters considered, we find a unique best approximating trajectory which depends only little on the parameters. In any case, our best fitting trajectory passes through the comet at the side of the nucleus nearly straightly eastward (within ±15°) at a distance of 400 km. It has closest approach at about 1518∶59 space craft event time (SCET).
Variation in cross‐sectional area of the axial ridge along the East Pacific Rise: Evidence for the magmatic budget of a fast spreading center
Authors: Scheirer Daniel S. , Macdonald Ken C.
Published: February 2008

Along the fast and ultrafast spreading East Pacific Rise, the cross‐sectional area of the axial ridge varies significantly over length scales similar to its morphologic segmentation. Using an automated method, we measure the ridge area (volume per kilometer along axis) where there is complete bathymetric coverage. Along 3500 km of the northern and southern East Pacific Rise, our two study areas encompass numerous transform faults, large overlapping spreading centers, small overlapping spreading centers, and smaller discontinuities (first‐, second‐, third‐, and fourth‐order discontinuities). The cross‐sectional area variation mimics the undulation of the ridge crest depth; local area maxima occur toward the middle of segments, and the axial area decreases by 40% or more at first‐ and second‐order discontinuities. Third‐order discontinuities are generally marked by smaller disruptions in the ridge area, and fourth‐order offsets do not systematically correspond with features of the cross‐sectional area profile. The correlation between shallower ridges and larger ridge areas breaks down at some locations because axial cross‐sectional area represents a longer term average of the ridge's magmatic state than axial depth. A correlation between large ridge areas and negative residual gravity anomalies indicates that inflated ridges are underlain by low‐density crust and mantle. Also, a correlation between larger area and higher MgO content of axial basalts suggests that inflated areas generally erupt hotter magmas which are presumably supplied more rapidly to the neovolcanic zone. The cross‐sectional area of the axial ridge appears to correlate with the width of the axis‐centered low‐velocity zone in the crust. These observations, as well as the absence of large, relict axial ridges off‐axis, indicate that the axial ridge originates from buoyancy due to thermal expansion and the presence of melt in the crust and mantle within about 10 km of the rise axis. Portions of the northern East Pacific Rise underlain by a magma chamber reflector generally occur where the ridge cross‐sectional area is greatest; this supports the connection between processes which inflate the axial ridge and those which heat the crust and upper mantle and produce melt. Thus, while the axial high on fast spreading centers resembles a constructional volcano in cross section, it is more like a long, narrow balloon whose cross‐sectional area is a sensitive indicator of magma supply. Using this relationship, we predict with 75% confidence that at least 45% of the unsurveyed northern East Pacific Rise (18°N to 13°N and 9°N to 5°N) and at least 60% of the southern East Pacific Rise (4°S to 23°S) is underlain by a magma chamber reflector.
Structure of Archean crust and passive margin of southwest Greenland from seismic wide‐angle data
Authors: Gohl Karsten , Smithson Scott B.
Published: February 2008

To investigate the structure and composition of the Archean crust beneath the shelf of southwest Greenland parallel to the passive margin, we conducted an onshore/offshore seismic wide‐angle experiment in 1989. One of our goals was to obtain a velocity/depth profile along the thinned continental or transitional crust to place constraints on the magmatic rifting history in the northern Labrador Sea. Seismic recording instruments at four locations along the coast recorded energy from an air gun array up to 400 km distance. Due to the dense shot spacing of 100 to 150 m we were able to identify continuous reflected and refracted phases from the upper to lower crust. Results from a one‐dimensional extremal inversion indicated a high‐velocity zone (HVZ) with P wave velocities of more than 7.2 km/s at the bottom of the transitional crust. Combined two‐dimensional travel time forward modeling and inversion produced a velocity/depth model that contains midcrustal and lower crustal discontinuities between 12 and 25 km depth and a Moho dipping northward at 30 to 42 km depth. An average crustal velocity of about 6.6 km/s indicates a high portion of mafic components in the crust. The lowermost crust consists of a HVZ with a maximum thickness of 8 km and temperature‐corrected velocities of 7.5 to 7.85 km/s. The HVZ may be explained by an underplated igneous complex often observed under passive margins worldwide. Metamorphism to garnet‐pyroxene‐granulite during postrift isobaric cooling of the 30‐ to 40‐km‐deep, thinned continental crust could explain the velocities that are higher than on other margins. A large volume of magmatic material must have been added to the crust but did not produce extrusions. Magmatic underplating of the thinned continental or transitional crust of SW Greenland is likely associated with a possible hot spot magmatism in the Davis Strait and Baffin Bay region. Magmatism associated with continental rifting was found to reach farther south along the northern and central part of the Archean block than previously thought.
The April 22, 1991, Valle de la Estrella, Costa Rica (Mw = 7.7) earthquake and its tectonic implications: A broadband seismic study
Authors: Goes Saskia D. B. , Velasco Aaron A. , Schwartz Susan Y. , Lay Thorne
Published: February 2008

The rapture process of a large back arc thrusting earthquake, the April 22, 1991, Valle de la Estrella, Costa Rica (Mw = 7.7), earthquake, is investigated using broadband body waves and long‐period surface waves. We find the source process to be relatively simple, with the source models separately obtained from body and surface waves being very consistent. The event occurred on a shallow, southwest dipping rapture plane on which most energy is released updip of the hypocentral location (10–20 km deep). High‐frequency radiation appears to have been released over a relatively small source area. Our preferred model has a focal mechanism with strike 102 ± 10°, dip 17 ± 14° and rake 63 ± 17°, a seismic moment of 3.8 ± 1.5 × 1020 N m, and a total rupture duration of 40 ± 6 s. The earthquake appears to be associated with the North Panama Deformed Belt (NPDB), a thrust and fold complex that has accommodated the oroclinal deformation of Panama. This event, along with previous large events north of Panama in 1882 and 1916, indicates that there is substantial convergence along the NPDB, marking the NPDB as a probable emerging plate boundary. It remains difficult to gauge the earthquake hazard in the region because of the tectonic complexity.
Thermal infrared emission spectroscopy of natural surfaces: Application to desert varnish coatings on rocks
Authors: Christensen Philip R. , Harrison Stephanie Thliveris
Published: February 2008

Thermal infrared Spectroscopy has become an increasingly important tool for remote compositional analysis and geologic mapping. Most published laboratory measurements have been obtained in bidirectional reflection or transmission, whereas remotely sensed thermal infrared data are obtained by measuring the emitted energy. Section 2 of this paper describes a laboratory technique for determining calibrated emissivities of natural surfaces. Equations are developed to account for the energy reflected from the environment and to determine directly the sample temperature from measurements of hot and cold blackbody targets. Two methods for determining emissivity are developed: one in which only a hot sample measurement is made and the reflected background energy is removed by modeling, and a second in which the sample is cooled and the reflected energy is measured directly. Relative emissivity can be obtained to approximately 1% and absolute emissivities can be obtained to 2–15%, depending on the validity of the assumption that the emissivity of the sample is unity at some wavelength. The emission data agree well with the hemispherically integrated reflection data but point out problems associated with bidirectional reflectance measurements. Section 3 applies emissivity measurements to the study of layered surfaces consisting of desert varnish coatings on granite and granodiorite rock suites. Two linear models are developed: the first assumes linear mixing of independent emission from the substrate and varnish (checkerboard model); the second models transmission through an absorbing/emitting medium. Regardless of whether the varnish is or is not relatively transparent and strongly absorptive, the spectral effect of varnish increases linearly with varnish thickness, indicating that thick patches of varnish dominate the spectral properties. As a result, median varnish thickness can be determined from spectral measurements. In addition, the composition of a substrate can be estimated through varnish layers up to 40–50 μm in median thickness, and the composition of the varnish material can be determined if the substrate material is known. The varnishes studied are composed primarily of clay materials, consistent with previous studies of varnish composition.
Chemical variation trends at fast and slow spreading mid‐ocean ridges
Authors: Niu Yaoling , Batiza Rodey
Published: February 2008

We examined an expanded global data set of mid‐ocean ridge basalt (MORB) major element analyses. In agreement with previous results, we show that slow spreading ridges tend to have more primitive (high Mg/Fe) lavas than fast spreading ridges. Fractionation‐corrected values of Na(8) and Ca(8)/Al(8) (indices of the extent of melting) and Si(8)/Fe(8) (an index of the pressure of melting) do not vary systematically with spreading rate. Assuming a mantle that is generally homogeneous in major elements, we conclude that average mantle temperature in the region of melting below mid‐ocean ridges is independent of spreading rate. Using data for 32 best sampled ridge segments of variable length, we show that the so‐called global and local trends of chemical variation (Klein and Langmuir, 1989) are systematically distributed with spreading rate. The global trend (positive correlation between extent of melting and melting pressure) occurs at fast spreading ridges (> 60 mm/yr), while the local trend (negative correlation between extent of melting and melting pressure) occurs at slow spreading ridges (< 50 mm/yr). This distribution is independent of geographic length scale. Among the 32 ridge systems we examined, the slopes of the two trends on chemical diagrams show some variability, but no regular pattern, such as fanning. The global trend is well‐explained by differences in average mantle temperature occurring at several length scales within mantle rising passively in response to plate separation. We propose that the local trend arises from processes occurring in buoyant diapirs undergoing melting and melt‐solid reequilibration. Several lines of geophysical and geological evidence point to the importance of buoyant, three‐dimensional mantle upwelling beneath slow spreading ridges. Petrologic modeling presented here is consistent with this hypothesis, as is the existence of the local trend at seamounts on the flanks of the East Pacific Rise.
On the frequency‐length distribution of the San Andreas Fault System
Authors: Davy P.
Published: February 2008

The frequency‐length distribution of the San Andreas fault system was analyzed and compared with theoretical distributions. Both density and cumulative distributions were calculated, and errors were estimated. Neither exponential functions nor power laws are consistent with the calculated distributions over the range of studied lengths. The best fit on both density and cumulative distributions was achieved with a gamma function which mixes a power law and an exponential function. At small lengths, the gamma function behaves as a power law with an exponent of −1.3±0.3. At large lengths (above 10km), the distribution is a mixed exponential‐power law function with a characteristic length scale of about 23±6 km. The gamma distribution is proposed to result from a length‐dependent segmentation of a fractal fault pattern. This study shows the importance of comparing both cumulative and density distributions. It also shows that the studied range of lengths (1–100 km) is not appropriate for measuring power law exponents.