bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of reconnected flux tubes within the midaltitude cusp
Authors: Saflekos N. A. , Burch J. L. , Sugiura M. , Gurnett D. A. , Horwitz J. L.
Published: February 2008

Dynamics Explorer 1 observations within the midaltitude polar cusp provide indirect evidence of reconnected flux tubes (RFT) envisioned to be extensions of the flux transfer events reportedly found near the magnetopause. In this study, low‐energy plasma, high‐energy plasma, magnetic fields, and electric fields were used to identify the signatures of reconnected flux tubes in the midaltitude cusp. Inside isolated flux tubes, low‐energy plasma was observed to be transferred from the magnetosheath to the magnetosphere, and relatively hot plasma was observed to be transferred from the magnetosphere to the magnetosheath. The cool magnetosheath plasma and the relatively hot magnetospheric plasma shared the same magnetic flux tube. The RFT signature is most easily identified in electron and ion energy fluxes plotted versus time for all pitch angles. The electron signature of an RFT is characterized by a drop in energy at the upper side of the energy band. The ion energy signature is similar to the electron signature except the ions form distinct Vs within the RFTs which differ from Vs outside the RFTs by the fact that the thickness of the lines forming the sides of the Vs is thinner inside the RFTs in comparison to that outside, and the height of these Vs is shorter inside the RFTs. The characteristics of spatial scale, time duration, and frequency of occurrence between flux transfer events and midaltitude cusp reconnected flux tubes are consistent, although they differ in the direction of motion. However, the merging cell topology and the interplanetary magnetic field BY effect can explain this difference. Larger‐scale (space and time) events can be explained by motion of the cusp resulting from a quasi‐steady reconnection process. The field‐aligned currents associated with reconnected flux tubes at midaltitudes within the cusp are consistent with twisting of magnetic field lines and with closure by Pedersen currents. It is possible that what appear to be field‐aligned currents closing by Pedersen ionospheric currents may also be interpreted as currents carried by Alfvén waves.
Computer simulations of electromagnetic instabilities in the plasma sheet boundary layer
Authors: Gary S. Peter , Winske Dan
Published: February 2008

One‐dimensional hybrid computer simulations are used to study electromagnetic instabilities driven by hot, anisotropic, counterstreaming proton components similar to those observed by ISEE‐1 and ‐2 in the plasma sheet boundary layer of Earth's magnetotail. These simulations are used to study two types of growing modes: the ion cyclotron anisotropy instability driven by the temperature anisotropy of individual ion components, and two different ion/ion instabilities driven by the relative streaming of two ion components. As in previous such simulations, the ion cyclotron anisotropy instability exhibits a very low saturation amplitude and weak ion scattering. Simulations of the current‐driven kink‐like instability using more general particle‐particle codes yield similarly weak fluctuation amplitudes and very small ion scattering. If the relative streaming speed of the hot proton components is increased to greater than 2υA, the proton/proton nonresonant instability can grow to saturation at larger amplitudes and produce strong proton scattering which leads to significant heating of the two hot proton distributions. Furthermore, if a relatively large density of cool ionospheric oxygen ions is introduced in the simulations, the ion/ion right‐hand resonant instability becomes the dominant mode, and pitch angle scatters the hot proton components to produce distributions which resemble those observed from ISEE. Thus electromagnetic ion/ion instabilities are a possible mechanism by which the hot streaming proton components of the plasma sheet boundary layer become the single very hot isotropic proton component of the central plasma sheet.
Injection and diffusive transport of suprathermal through energetic solar flare protons (35 keV to 20 MeV)
Authors: Beeck J. , Mason G. M. , Marsden R. G. , Hamilton D. C. , Sanderson T. R.
Published: February 2008

We have investigated the injection and interplanetary propagation of low‐energy protons that originated in a prompt solar particle event on July 20, 1981. The event was highly unusual in that solar flare protons could be seen from suprathermal energies (35 keV) to the energetic particle range (20 MeV). The observations were made in interplanetary space using the low‐energy ion instrument (DFH) and ULEWAT instrument on ISEE 3. At the higher energies, the appearance of the time‐intensity and time‐anisotropy profiles in this event were characteristic of diffusive particle propagation; however, owing to extremely quiet interplanetary conditions, it was possible to observe solar particles down to proton energies well below previous studies. We were able to fit the observed time‐intensity and time‐anisotropy profiles over the entire energy range using a model based on the spherically symmetric Fokker‐Planck equation including convection, diffusion, and adiabatic deceleration. From our model fits we find that (1) the radial interplanetary diffusion coefficient K increases with heliocentric radius r as K = K0 rb, where b is close to 1; (2) the scattering mean free path for protons, λ, increases with particle rigidity P as λ ∝ P(0.22±0.03) over the range between 9 and 160 MV; and (3) at Earth orbit, λ values lie between 0.02 AU (for ∼0.5‐MeV electrons) and 0.17 AU (for protons, 10–20 MeV). Anisotropies in this event are long lasting (>48 hours) owing in part to an extended injection: the lower the energy, the longer the injection lasts. The extended period of injection appears to be evidence for diffusive coronal shock acceleration of protons during this event. Following the solar particle event maximum of intensity, a bidirectional streaming event was observed; during this period the particle anisotropies along with changes in the interplanetary magnetic field give evidence for the passage of a magnetic cloud by ISEE 3.
Propagation of a VLF electromagnetic wave packet in a magnetoplasma
Authors: Xu J. S. , Yeh K. C.
Published: February 2008

In this paper the propagation of a VLF electromagnetic wave packet in a magnetoplasma is studied. The wave packet is constructed by superposing a group of spectrum‐weighted characteristic vectors. Accurate to the second order, a set of analytic expressions of the complex amplitude factor and field vectors for a Gaussian packet has been obtained. Based on these expressions, effects of dispersion and anisotropy on the vector property of wave field and the amplitude envelope are simulated. Numerical results show that the polarization of wave fields is varying when the packet moves and somewhat deviates from exact circular polarization in a plane transverse to the carrier wave vector, ko. The Gaussian envelope is found to rotate around the Vg×ko direction and to spread three dimensionally as it propagates. The rotation of the ellipsoidal pulse tends toward a stable direction after a propagation of about 250 periods.
CDAW 7 revisited: Further evidence for the creation of a near‐Earth substorm neutral line
Authors: Kettmann G. , Fritz T. A. , Hones E. W.
Published: February 2008

The well‐defined CDAW 7 substorm that occurred at 1112 UT on April 24, 1979, has initially been considered one of the most unambiguous examples supporting the near‐Earth neutral line model of magnetospheric substorms. In a recent paper, this point of view has been questioned by Eastman et al. (1988), who state that the magnetic field changes observed on board the ISEE 1 and 2 spacecraft as well as plasma and energetic particle phenomena can be explained simply in terms of spatial movements of a preexisting plasma sheet boundary layer (PSBL) and its associated current sheets across the spacecraft. By contrast, reinvestigation of the ISEE 1 and 2 energetic particle measurements around substorm onset on short time scales demonstrates that the observed flux pattern requires the formation of a particle source earthward of the ISEE spacecraft, well inside the plasma sheet, associated with the substorm onset. The energetic ion pattern observed at ISEE 1 and 2 is consistent with the lobeward progression of field lines being reconnected at a near‐Earth reconnection region. About one minute after the onset of the strong tailward anisotropy of the energetic ions, the lobeward expansion ceases, leading temporarily to an almost stationary state where the outer edge of a separatrix layer is located between ISEE 1 and 2. Remote sensing methods allow us to trace the thinning of the plasma sheet before the 1112 UT substorm onset by determining the contraction speed and the normal vector of the plasma sheet boundary. While both satellites resided within a few ion gyroradii distance from the lobe in the expected PSBL region, strong flows were absent prior to substorm onset, which indicates the temporal nature of the event as opposed to an encounter with a preexisting PSBL containing large flows. At about 3 min after substorm onset, the ISEE spacecraft cross the separatrix and enter an open field line region, consistent with the tailward retreat of a recently formed plasmoid.
Solar maximum‐minimum extremes in the summer noontime polar cap F region ion composition: The measurements
Authors: Grebowsky J. M. , Hoegy W. R. , Chen T. C.
Published: February 2008

All available, computer‐readable, thermal ion composition measurements from Earth‐orbiting satellites have been put into a common computer file format for easy manipulation. The ion measurements obtained under solar maximum conditions at polar cap latitudes within a 6‐hour magnetic local time window about noon in the summer were culled from this data set for comparison to similar data for a solar minimum period. The observations show distinctive solar cycle variations in topside F region altitude distributions of O+ and H+ concentrations that reflect changes in the polar wind source region. The summer noontime polar cap data are statistically the best ordered of all seasons and magnetic local time ranges in the polar cap because of the domination of solar EUV effects over those of auroral energy inputs and convection processes. The polar cap minor ion, H+, is in charge exchange equilibrium to higher altitudes during solar maximum than solar minimum periods, reflecting the change in altitude where the light ion polar wind development begins. The concentration of the major ion, O+, increases from solar minimum to solar maximum due to changes in photoionization rates and heating of the plasma and neutral thermosphere. The data support the strong dependence of the polar ionosphere and polar wind features on the F10.7 index and are consistent with the results of the model study of the same region and similar extremes of solar activity by Cannata and Gombosi.
Correlation of whistler mode phase delay with transient hydromagnetic waves
Authors: Paschal E. W. , Lanzerotti L. J. , Maclennan C. G.
Published: February 2008

Changes in the phase delay of a ducted whistler mode signal from the Siple Station VLF transmitter to a receiver at Roberval, Quebec, are compared to variations in the horizontal components of the Earth's magnetic field measured nearby at La Tuque, Quebec. The VLF transmission contains 10.5 min of two distinct tones with 30‐Hz separation. Each tone suppresses the cyclotron‐resonance growth of the other, eliminating growth‐associated phase effects, and allows phase changes due to radial duct motion to be observed. Spectrograms of the VLF phase delay and variations in the components of the geomagnetic field show similar features, with phase features preceding magnetic features by 20 to 30 s. A cross‐correlation plot of VLF phase delay and variations in the east–west component shows a bipolar signature which is interpreted as evidence of transient hydromagnetic waves in the 20–45 s band (Pc 3 band) moving from equator to ground. Previous studies have noted correlations between VLF Doppler shifts and resonant hydromagnetic waves, but this is the first report where the VLF phase delay is used directly, and where resolution is sufficient to observe a transient hydromagnetic wave which has a parallel wavelength shorter than the length of the field line.
Gyroresonant wave‐particle interactions in a dynamic magnetosphere
Authors: Gail W. B. , Inan U. S. , Helliwell R. A. , Carpenter D. L.
Published: February 2008

Experimental and theoretical methods for studying gyroresonant wave‐particle interactions (WPI) in the magnetosphere commonly assume that the magnetic field is static over the time scales of interest. In order to investigate the effect of nonequilibrium or dynamic magnetospheric processes on WPI, the response of the interaction mechanism to time‐dependent perturbations in the magnetic field was examined using the global field compressions that occur during sudden commencements (sc). By measuring modifications of the wave spectrum and electron distribution during sc, the response of the WPI mechanism to slow variations in the magnetic field and plasma environment can be determined. This technique was used to examine the applicability of both nonlinear and quasi‐linear descriptions of gyroresonant WPI to dynamic environments. Nonlinear interactions were investigated by calculating the nonlinear parameters (interaction length, wave trapping threshold, and bunching efficiency) as a function of time for a given field line during a simulated sc compression. The results showed that the occurrence probability for discrete emissions generation increases during the compression, which is consistent with the observed triggering of discrete emissions during sc. Quasi‐linear interactions were investigated using a modified formulation of quasi‐linear theory in which a small time‐dependent perturbation is introduced in the magnetic field and the resulting change in the wave growth rate is determined. The observed growth rate of 0.3–2.7 dB/s, total growth of 12–29 dB, and growth duration of 10–20 s were found to be comparable to the values of 1.5 dB/s, 30 dB, and 20 s predicted by the theory. In addition, the modified theory predicts the occurrence of 3–4 cycles of damped oscillations with period 35–40 s in response to the sc, similar to the observed 3–4 cycles of 60–90 s period damped oscillations which were observed. The results indicate that current theories of gyroresonant WPI, when properly modified to include a time‐dependent magnetic field, can be used to predict properties of WPI in a dynamic magnetospheric environment.
Multiple ground‐based and satellite observations of global Pi 2 magnetic pulsations
Authors: Yumoto K. , Takahashi K. , Sakurai T. , Sutcliffe P. R. , Kokubun S. , Lühr H. , Saito T. , Kuwashima M. , Sato N.
Published: February 2008

Four Pi 2 magnetic pulsations, observed on the ground at L = 1.2–6.9 in the interval from 2300 UT on May 22 to 0300 UT on May 23, 1985, provide new evidence of a global nature of Pi 2 pulsations in the inner (L ≲ 7) region of the magnetosphere bounded by the plasma sheet during quiet geomagnetic conditions. In the present study, magnetic data have been collected from stations distributed widely both in local time and in latitude, including conjugate stations, and from the AMPTE/CCE spacecraft located in the magnetotail. On the basis of high time resolution magnetic field data, the following characteristics of Pi 2 have been established: horizontal components, H and D, of the Pi 2 oscillate nearly antiphase and in‐phase, respectively, between the high‐ and low‐altitude stations in the midnight southern hemisphere. Both the H and D components of the Pi 2 have nearly in‐phase relationships between the nightside and the dayside stations at low latitude. The Pi 2 amplitude is larger at the high‐latitude station and decreases toward lower latitudes. The dominant periods of the Pi 2 are nearly identical at all stations. Although a direct coincidence between spacecraft‐observed and ground‐based global Pi 2 events does not exist for these events, the Pi 2 events are believed to be a forced field line oscillation of global scale, coupled with the magnetospheric cavity resonance wave in the inner magnetosphere during the substorm expansive phase.
Simultaneous measurements of HF‐enhanced plasma waves and artificial field‐aligned irregularities at Arecibo
Authors: Noble S. T. , Djuth F. T.
Published: February 2008

Two radar systems with beams intersecting in the HF‐modified F region were used to simultaneously measure HF‐enhanced plasma lines (HFPLs) and artificial geomagnetic field‐aligned irregularities (AFAIs). These measurements were made with the high‐power HF facility located near the Arecibo Observatory, Arecibo, Puerto Rico. The Arecibo 430‐MHz radar was used for the HFPL observations, and a portable 49.92‐MHz backscatter radar was deployed on the island of Guadeloupe to monitor the AFAIs. The experiment was designed to examine the degree to which HF‐induced plasma turbulence influences the development of AFAIs. When the HF beam is stepped up in power, sustained HFPLs and AFAIs are first observed at the same HF power level, indicating that ponderomotively driven instabilities may be involved in the early time development of AFAIs. As the HF power is increased, the HFPL backscatter power begins to saturate at ∼70 MW effective radiated power (ERP). However, the backscatter from AFAIs is linearly dependent on HF power, even at the highest (120 MW ERP) HF power levels available at Arecibo. This suggests that additional processes may contribute to the development of AFAIs. For example, ponderomotively driven instabilities may give rise to weak geomagnetic field‐aligned irregularities that are subsequently driven unstable by processes excited near the upper hybrid resonance. It is also likely that AFAIs greatly impact the development of HF‐induced plasma turbulence at late times (>1 s) following HF turn‐on. Once the ionosphere is preconditioned by high‐power HF modifications, AFAIs and HFPLs can be simultaneously sustained at a much lower HF power level than that needed to originally excite them. The nature of the preconditioning process is currently not well understood. New theoretical initiatives are clearly needed to guide future experimental activity in this area.
Nonlinear mixing of beam‐driven bernstein modes and electromagnetic emissions at cyclotron harmonics in the ionosphere
Authors: Laxmi V. N. , Tripathi V. K.
Published: February 2008

An energetic electron beam propagating along the field lines in the ionosphere excites electron Bernstein waves having small group velocity across the beam. Two Bernstein waves may nonlinearly couple with each other to produce electromagnetic waves at sum and difference frequencies. A kinetic theory of this process has been developed using a guiding center formalism. Employing the nonlinear coupling coefficient, an estimate of the electromagnetic emissions at cyclotron harmonics has been made. The emitted field amplitude scales directly as the product of pump wave field strengths and varies inversely with their frequency.
Effect of plasma mantle injection on the dynamics of the distant magnetotail
Authors: Liu W. W. , Hill T. W.
Published: February 2008

We investigate the role of the plasma mantle in the dynamics of the Earth's distant magnetotail. The plasma mantle can exert a substantial influence on the current‐sheet force equilibrium by transporting both momentum and mass from the dayside magnetopause to the tail current sheet. Such influences are particularly strong when the interplanetary magnetic field contains a significant southward component for a prolonged period. We find that a number of processes characterizing a substorm growth phase can be attributed, at least in part, to the plasma mantle. Among these processes are the thinning of the plasma sheet, storage of magnetic energy in the tail, and formation of plasmoids. The rates of these processes are found to increase nonlinearly with the momentum density of the mantle plasma. The present model applies only to the distant magnetotail (x ≲ −30 RE) because of various approximations that we make to allow an analytical treatment of the governing equations.
The latitudinal gradient of the NO peak density
Authors: Fesen C. G. , Rusch D. W. , Gérard J.‐C.
Published: February 2008

The latitudinal gradients of the maximum nitric oxide densities near 110 km are presented for solstice and equinox periods from 1982 through 1985 as observed by the Solar Mesosphere Explorer satellite. The data indicate that the response of the maximum NO densities to the declining level of solar activity is latitudinally and seasonally dependent: the polar regions exhibit little sensitivity to solar activity, while the low latitude NO responds strongly. The data also reveal marked asymmetries in the latitudinal structure of the two hemispheres for each season. During June solstice periods, the latitudinal distribution is fairly flat, unlike December solstice periods which tend to show a definite minimum near 30° N. Similarly, March data show very little latitudinal variation in the NO peak density between about ±40°, while the September data show marked gradients for the later years. The SME data further indicate that the nitric oxide densities vary considerably from day to day, even during very quiet geomagnetic periods, suggesting that the concept of an “average” distribution is of limited usefulness in understanding nitric oxide. A two‐dimensional model is used to simulate the June solar cycle minimum data. The latitudinally averaged magnitudes of the observed NO peak densities are reproduced reasonably well by the model, but the shape of the latitudinal variation is not. Sensitivity studies which varied the N(²D)+ O quenching rate and the eddy diffusion coefficient are presented. The model simulations support a quenching coefficient of 0.5–1×1012 cm³ s−1.
The plasma wave signature of a “magnetic hole” in the vicinity of the magnetopause
Authors: Treumann R. A. , Brostrom L. , LaBelle J. , Sckopke N.
Published: February 2008

Magnetic holes are occasionally observed in the vicinity of the magnetopause. These holes are similar to crater flux transfer events in size and magnetic signature, but have a lower magnetic field and different wave spectra. An exemplary magnetic hole has been observed by the AMPTE IRMspacecraft on September 4, 1984, in the early evening sector of the magnetosphere. We here investigate the plasma wave spectra inside and in the immediate vicinity of the hole and their relation to the plasma properties of the hole. Inside the hole where magnetic field levels are lowest, wave emissions between a few times the ion cyclotron frequency and the plasma frequency drop to levels close to the instrumental noise level. This behavior is similar to that observed in the artificial comet and plasma cloud releases in the solar wind, magnetosheath, and magnetospheric tail produced by the same satellite. Electromagnetic waves with frequency up to several times the ion cyclotron frequency are found inside the hole with relatively strong amplitudes (δB/B ≈ 1). These could represent either whistler waves or ion cyclotron waves. Estimations of the local electron plasma frequency suggest that this hole has a plasma density as high as 200 cm−3, whereas the ambient plasma maintains a density of 10–30 cm−3. In contrast to the low activity in the parts of the hole where the magnetic field is lowest, the boundaries contain very intense broadband noise from less than 1 Hz to several kHz. These could be generated by the lower hybrid drift instability. The energy density of the boundary waves, assuming they are lower hybrid waves, is far greater than the energy density of the low‐frequency waves inside the hole; hence, it is plausible that the interior waves represent the leakage of waves from the boundary region into the hole. Some ideas about the origin of the magnetic holes are discussed.
Sound velocities in calcareous oozes and chalks from sonobuoy data: Ontong Java Plateau, western equatorial Pacific
Authors: Johnson Thomas C. , Hamilton Edwin L. , Bachman Richard T. , Berger Wolfgang H.
Published: February 2008

The Ontong Java Plateau, in the western equatorial Pacific Ocean, is blanketed by over 1 km of flatlying, highly stratified calcareous ooze, chalk, and limestone. Wide‐angle reflection data from 17 sonobuoys deployed over the plateau were analyzed for compressional wave velocities in these sediments. No particular velocity horizons stand out as prominent features that can be correlated from one sonobuoy station to the next. When the data from all stations are plotted together, a fairly consistent velocity‐depth relationship is derived: V = 1.56 + 2.97t with the standard error of estimate equal to 0.08, where V is the compressional wave velocity in kilometers per second at depth t and t is the depth below the sea floor in one‐way travel time of sound in seconds. When the velocity data are plotted against depth in kilometers below the sea floor, the velocity gradient is 1.4 s−1, which is considerably greater than that in most marine turbidites, silts, and clays that are buried to 1 km. The linear uniform velocity gradient is consistent with the type of lithology underlying the Ontong Java Plateau as determined by the Deep‐Sea Drilling Project. That is, there are gradual transitions from ooze to chalk to limestone with intervals of the different lithologies interbedded rather than a single abrupt change from ooze to chalk at one level and from chalk to limestone at a deeper level. A slight difference was seen between velocity profiles in calcareous sediments that had been deposited below and those that had been deposited above the lysocline (the depth below which calcareous ooze is markedly corroded). Sublysoclinal sediments have anomalously low acoustic velocities in the upper 200 m of strata but have normal velocities at greater burial depths. These observations suggest that calcareous sediments deposited below the lysocline lithify more slowly after burial than those deposited above the lysocline.
Upward continuation of gravity disturbance covariance functions
Authors: Jordan Stanley K. , Heller Warren G.
Published: February 2008

One‐ and two‐dimensional integrals are derived for upward continuation of gravity disturbance covariance functions. Both spherical earth and flat earth formulas are presented as well as the associated Legendre and Fourier transforms.
Mapping ocean tides with satellites: A computer simulation
Authors: Won I. J. , Kuo J. T. , Jachens R. C.
Published: February 2008

As a preliminary study for the future worldwide direct mapping of the open ocean tide with satellites equipped with precision altimeters we conducted a simulated study using sets of artificially generated altimeter data constructed from a realistic geoid and four pairs of major tides (O1‐Q1, M2‐N2, K1‐P1, and S2‐K2) in the northeastern Pacific Ocean. Recovery of the original geoid and eight tidal maps is accomplished by a space‐time, least squares harmonic analysis scheme. The resultant maps appear fairly satisfactory even when random noises up to ±100 cm are added to the altimeter data of sufficient spacetime density. The method also produces a refined geoid which is rigorously corrected for the dynamic tides.
The magnetization of Mount Shasta and implications for virtual geomagnetic poles determined from seamounts
Authors: Blakely Richard J. , Christiansen Robert L.
Published: February 2008

New aeromagnetic data over Mount Shasta were analyzed to determine the bulk magnetic properties of the volcano by an inversion technique used successfully by other researchers to interpret marine magnetic anomalies over seamounts. Assuming that the intensity of magnetization is uniform and that the base of the source is at an elevation of 2438 m, we calculate by least squares a magnetization having an intensity of 1.7 A/m, an inclination of 45°, and a declination of 51°. This solution for the direction of magnetization is unacceptable because the volcano is geologically very young and should be magnetized in the direction of the earth's present field. The spurious direction, coupled with a positive residual anomaly over the western flank of the mountain, suggests either that this part of the mountain is magnetized more intensely or that magnetized material there extends to greater depth. Apparently, either the percentage of lava flows relative to breccias is higher within the western part of the volcano, thereby increasing its magnetization, or the Curie point isotherm rises beneath the summit. Both conclusions are geologically acceptable. Results of this study indicate that care is required in using magnetic anomalies over seamounts to obtain virtual geomagnetic pole positions. Model studies show that if the intensity of magnetization of a seamount is actually nonuniform, the calculated direction will be incorrect, and more important, the goodness‐of‐fit parameters will not necessarily indicate an invalid result.
The depth of mantle convection
Authors: Elsasser Walter M. , Olson Peter , Marsh Bruce D.
Published: February 2008

In this paper we elaborate on suggestions in the recent literature that the mantle convects uniformly through its entire depth. The main novel feature introduced here, which leads to a satisfactory account of earth temperatures, it the assumption of a thermal boundary layer at the mantle. Such a layer is produced by the contrast of thermal conductivities of core and mantle if, as is now generally believed, the effects of radiative heat transfer in the mantle are small. On assuming in agreement with much current geochemical thinking that the core is mainly a solution of FeS in Fe, it is possible to estimate crudely the temperature of the core‐mantle boundary as 4000±500°K. The temperature curve in the mantle can be approximated by adding two boundary layer temperature drops that can be calculated to the adiabat in between; this is also calculable, and the sum of these three agrees roughly with the numerical value just given. It has long been known that the Rayleigh number of the mantle is so large as to make convection likely. Lately, Golitsyn has introduced a scaling analysis that allows one to express the depth of the convective zone as a function of known parameters; this yields a depth in rough agreement with the depth of the entire mantle. Finally, we discuss the likelihood that mantle convection has been going on through the entire life of the earth, beginning with the early formation of the core; this has obvious geological implications.
Comparison of the offset dipole and zonal nondipole geomagnetic field models using Icelandic paleomagnetic data
Authors: Harrison C. G. A. , Watkins N. D.
Published: February 2008

Two hypotheses have been suggested to explain the far‐sided nature of virtual geomagnetic poles from low latitude sites. One is the off‐centered axial dipole model, and the other is the zonal nondipole field model. At high latitudes the two models predict different inclination anomalies. Two sets of data from Iceland (which is at an ideal latitude to distinguish between the two models) were therefore studied to resolve the relative merit of the two hypotheses. Three tests were done on each set of data and also on the combined data. The weight of evidence lies toward favoring the axial off‐centered dipole hypothesis, at least for the period between 13 and 7 m.y.
A note on velocity, density, and attenuation models for marine sediments determined from multibounce phases
Authors: Helmberger Donald V. , Engen Gladys , Scott Pat
Published: February 2008

Seismic pulses trapped between the ocean surface and the subbottom structure commonly observed in refraction profiling are modeled synthetically. The relative amplitudes of single‐ and double‐bounce phases are used to study reflection coefficients and attenuation. The technique was applied to a 1‐km‐thick section of mature oceanic sediments located in the Bering Sea. The most surprising result is the lack of attenuation in the sediments which is estimated to have a Qα greater than 500 for pulses dominated by 10‐Hz energy.
Surface wave focal mechanisms for eastern North American earthquakes with tectonic implications
Authors: Herrmann Robert B.
Published: February 2008

Using P wave first motions together with amplitude and phase data from long‐period Love and Rayleigh waves, the focal mechanisms of 22 earthquakes occurring in eastern North America between 1962 and 1976 are determined. The techniques used yield good results for events with body wave magnitudes as small as 4.0. Eighteen of the events have depths between 5 and 16 km; 15 have significant components of strike slip motion; and for focal mechanisms having nearly horizontal pressure axes, 70% of the pressure axes trend northeast or east. Though the earthquake sample may be biased toward the central United States, it is significant that no focal depths greater than 25 km have been encountered. A confidence factor is assigned to each focal mechanism for use in future tectonic syntheses.
Comparison of gravimetric geoids with Geos 3 altimetric geoid
Authors: Chapman Michael E. , Talwani Manik
Published: February 2008

By means of 1° × 1° gravimetric geoids in the North Atlantic, Northwest Pacific, and Indian oceans, comparisons have been made with Geos 3 radar altimeter estimates of geoid height. Most commonly, there are constant offsets and long‐wavelength discrepancies between the two data sets; there are many probable causes, including radial orbit error, scale errors in the geoid, or bias errors in altitude determination. Across the Aleutian trench the 1° × 1° gravimetric geoids do not measure the entire depth of the geoid anomaly. This is due to averaging over 1° squares and subsequent aliasing of the data. Especially above trench systems in the Pacific, there can be considerable energy in the geoid for frequencies higher than are resolvable by 1° × 1° geoids; such a signal, though, is detected by the Geos 3 altimeter. After adjustment of Geos 3 data to eliminate long‐wavelength discrepancies, agreement between the altimeter geoid and gravimetric geoid is between 1.7 and 2.7 m in rms error. In a hypothetical experiment a comparison can be made of an altimeter determining the geoid and a gravimeter measuring gravity. At longer wavelengths an altimeter will have a higher signal to noise ratio, but at shorter wavelengths a gravimeter will perform better.
Oceanic geoid and tides derived from Geos 3 satellite data in the northwestern Atlantic Ocean
Authors: Won I. J. , Miller L. S.
Published: February 2008

Two sets of Geos 3 altimeter data which fall within about a 2.5° width are analyzed for ocean geoid and tides. One set covers a path from Newfoundland to Cuba, and the other a path from Puerto Rico to the North Carolina coast. Forty different analyses using various parameters are performed in order to investigate convergence. Profiles of the geoid and four tides, M2, O1, S2, and K1, are derived along the two strips. While the analyses produced convergent solutions for all 40 cases, the uncertainty caused by the linear orbital bias error of the satellite is too large to claim that the solutions represent the true ocean tides in the area. A spot check of the result with the Mode deep‐sea tide gauge data shows poor agreement. A positive conclusion of this study is that despite the uncertain orbital error the oceanic geoid obtained through this analysis can improve significantly the short‐wavelength structure over existing spherical harmonic geoid models.
The geoid spectrum from altimetry
Authors: Wagner C. A.
Published: February 2008

Satellite altimetry information from the world's major oceans has been analyzed to arrive at a geoid power spectrum. Using the equivalent of about 7 revolutions of data (mostly from Geos 3) the power spectrum of the sea surface generally follows the expected values from Kaula's rule applied to the geoid. Analysis of overlapping altimetry arcs (and oceanographic data) shows that the surface spectrum is dominated by the geoid to about 500 cycles (40km half wavelength) but that sea state departures are significant starting at about 250 cycles (80km). Estimates of geopotential variances from a derived (smooth) geoid spectrum show significantly less power than Kaula's rule to about 60 cycles but somewhat more from there to about 400 cycles. At less than 40km half wavelength the total power in the marine geoid may be negligible (≪20cm).
Ocean gravity and geoid determination
Authors: Kahn W. D. , Siry J. W. , Brown R. D. , Wells W. T.
Published: February 2008

Gravity anomalies have been recovered in the North Atlantic and the Indian Ocean regions. Comparisons of 63 2° × 2° mean free air gravity anomalies recovered in the North Atlantic area and 24 5° × 5° mean free air gravity anomalies in the Indian Ocean area with surface gravimetric measurements have shown agreement to ±8 mGal for both solutions. Geoids derived from the altimeter solutions are consistent with altimetric sea surface height data to within the precision of the data, about ±2 m.
Geos 3 ocean geoid investigation
Authors: Yionoulis S. M. , Eisner A. , Pisacane V. L. , Black H. D. , Pryor L. L.
Published: February 2008

A determination of the fine‐scale sea surface topography in the Geos 3 calibration area using the radar altimeter data is presented. Estimates of the north‐south and east‐west components of the deflections of the vertical as well as values of the geoidal heights are made. Three major stages of processing are used in obtaining the final results. The first two use pass processors; in the final stage the processor combines all the pass results to compute the final results. Comparison with a gravimetrically determined geoid for the calibration area, provided by the National Aeronautics and Space Administration, yielded a mean and standard deviation of 0.45 and 1.5 m, respectively.
A quasi‐geometric method for investigating tidal displacements of the Earth's surface using laser ranging to Geos 3
Authors: Bower D. R. , Paul M. K.
Published: February 2008

An analysis of NASA laser ranging data from three stations located at Goddard Space Flight Center, Greenbelt, Maryland, Grand Turk Island, and Bermuda, respectively, was carried out in an attempt to measure the geometric earth tide. A ‘quasi‐geometric’ method was employed that reduces the dependence on unmodelled satellite dynamics to a knowledge of only the radial position of the satellite by considering two‐station simultaneous ranging at the precise time that the satellite passes through the plane defined by the two stations and the center of mass of the earth. Using this method, a good approximation to the theoretical Love number h2 was obtained from five passes over the Geos 3 calibration area when the geocentric distance to the satellite is adjusted by an amount, the same for each pass, which yields the least sum of the squares of the errors in the stations' geocentric distances and their angular separations.
Combination solution for gravity field including altimetry
Authors: Chovitz B. H. , Koch K.‐R.
Published: February 2008

A previous global solution for the earth's gravitational field and station positions employed a combination of 21 weeks of Doppler tracking data from seven satellites, a complete set of 5° mean gravity anomalies, and geometric constraints from the worldwide satellite triangulation network on 17 of the stations. To this is now added geoid heights over the major part of the oceans obtained from Geos 3 altimeter observations. The solution has a form based on the simple density layer method and is equivalent to a solution in spherical harmonics to the tenth degree and order. The rms difference of 5° mean geoid heights of the new solution with the GEM 10 solution is 5.6 m.
Smoothing of Geos 3 satellite radar altimeter data
Authors: West Gladys B.
Published: February 2008

Filtering of a large portion of Geos 3 satellite radar altimetry data at the Naval Surface Weapons Center has been accomplished by a Wiener smoother. The statistics of the geoid for the Wiener smoother is based on a third‐order Markov process. Because of some limitations the Wiener smoother has been replaced by a Kalman smoother which satisfies more closely the conditions of the altimetry data. The Kalman smoother also uses the same third‐order Markov statistics. A discussion of the smoothers (filter models), results obtained from the models, and a comparison of results with ship survey data are given in this paper.
Frost streaks in the south polar cap of Mars
Authors: Thomas P. , Veverka J. , Campos‐Marquetti R.
Published: February 2008

Viking Orbiter images of the annual south polar cap on Mars show elongated bright features that are associated with craters and resemble wind streaks seen elsewhere on Mars. The seasonal behavior of these albedo features indicates that they are accumulations of CO2 frost. They appear in the fall, shortly after the annual cap has formed, and some survive as outliers for as many as 70 days during the cap's retreat in the spring. The frost streaks indicate a prograde circulation pattern of near‐surface winds around the pole with a definite component away from the pole. There is evidence that this prograde pattern persists throughout fall and winter. As the cap is receding in spring, and in the summer when the annual cap is absent, a retrograde circulation pattern around the pole, again with a definite component away from the pole, is indicated by dark crater‐associated streaks and other eolian features. The higher albedo of the frost streaks relative to the surrounding annual cap is probably due to a slightly more complete coverage of the ground by frost within the streaks. From their persistence during the spring recession of the main cap we estimate that some frost streaks near 60°S contain up to 120–130 g/cm2 of CO2 frost, or about 3 times the maximum amount in the surrounding cap at these latitudes. We suggest that the frost streaks represent accumulations in the lees of craters of wind‐transported frost, indicating that frost is effectively redistributed by winds within the polar caps. Threshold wind velocities required are estimated to be one half or less of those needed to initiate dust movement at equatorial and temperate latitudes.
Theory of global thermoremanent magnetization of planetary lithospheres in dipole fields of internal origin
Authors: Srnka L. J. , Mendenhall M. H.
Published: February 2008

A model is presented for the global thermoremanent magnetization of spherical lithospheres which cool in the presence of central dipole fields. Reversals and intensity variations of the field are incorporated in this model, which is applicable to bodies whose interiors have remained above the Curie point throughout their evolution. The model demonstrates that even considering Runcorn's magnetostatistics theorems for spherical shells, a nonzero magnetic permeability and a finite cooling rate in the lithosphere permit the acquisition of a sizable global remanent dipole moment, which would be detectable by external measurements after the magnetizing field has disappeared. Preliminary application of this model to Mercury, Venus, and Mars suggests that only the combination of a nonreversing ancient source field with a surface value near 1 Oe plus a sizable concentration (∼1% by volume) of ferromagnetic material in their crusts could produce remanent planetary dipole fields as large as those measured by spacecraft. On the other hand, if ancient reversing dynamos existed in these planets, it is unlikely that large planetary‐scale fields like those observed at Mercury could be due to remanence in their crusts, irrespective of their composition.
Frictional attenuation: An inherent amplitude dependence
Authors: Mavko Gerald M.
Published: February 2008

Frictional sliding on crack surfaces and grain boundaries is examined as a mechanism of wave attenuation. In contrast to previous work based on idealized elliptic crack models a general description of internal surfaces is considered which allows for irregularities and partially closed cracks. This leads to Q−1 that increases with strain amplitude. Such an amplitude dependence is often observed in large‐strain laboratory measurements. This suggests that under in situ (small strain) conditions, frictional attenuation becomes secondary to linear loss mechanisms, either disappearing or becoming masked.
Accuracy of relative oceanic geoid computations
Authors: Christodoulidis Demosthenes C.
Published: February 2008

An error analysis based on a model which requires potential coefficients and gravity anomaly information for the computation of oceanic geoidal undulations is developed. The rigorous models are presented and abandoned owing to the numerical difficulties associated with them. An approximate error model is developed in the frequency domain through which an idea is obtained as to the size and behavior of the errors involved. The use of the method is demonstrated by obtaining data requirements for the realization of a 10‐cm relative oceanic geoid. Various data sets could result in the desired accuracy. One such set, indicative of the strictness of the requirements, involves gravity profile spacings of approximately 3 km with observational noise not exceeding 0.5 mGal inside detailed data caps of 30° and potential coefficients of prescribed accuracy available to degree and order 70.
Tests for the recovery of 5° mean gravity anomalies in local areas from ATS 6/Geos 3 satellite to satellite range‐rate observations
Authors: Hajela D. P.
Published: February 2008

The prediction of 5° mean gravity anomalies is considered by collocation from ATS 6/Geos 3 satellite to satellite range‐rate observations. The auto‐ and cross‐covariances of the residual line of sight accelerations
from Geos 3 to ATS 6 are computed rigorously in the anomalous potential field. The raw residual range‐rate
values are filtered and smoothed by approximating them in a least‐squares sense by a cubic spline function, and the residual accelerations
are obtained as the first derivatives of the spline. Tests are described to fit the raw
values by a spline function with variable knots, and to remove any linear trend from
values due to residual errors in the initial state vectors. The correlation coefficients between the predicted values of neighboring anomalies are considered. The procedures are tested numerically by predicting eight 5° anomalies in the Carribean Sea area using destruct‐Doppler data from four Geos 3 revolutions. The predicted anomalies are compared with anomalies obtained from altimeter data, with terrestrial anomalies, and with anomalies implied by the (30, 28) PGS 110 gravitational model. In spite of a low density of range‐rate data directly over the anomalies, and the high noise level of the destruct‐Doppler data, the tests indicate that 5° anomalies can be predicted with a standard deviation of 6 mgals. It is found that the predicted anomalies agree more closely with the altimeter anomalies as compared to the agreement of PGS 110 anomalies with the altimeter anomalies.
Gravitational spectra from the tracking of planetary orbiters
Authors: Wagner C. A.
Published: February 2008

Two‐dimensional gravitational spectra are derived from simple harmonic analysis of range rate tracking data on planetary orbiters. The eccentricity of the orbit is arbitrary, and results are shown to vary substantially with the aspect angle of the tracking line of sight with the orbit plane. The development for arbitrary start and stop times (with respect to periapsis) uses modified eccentricity functions evaluated by quadrature. Simulations with a point‐masses model of Venus using tracking data on the Pioneer Venus orbiter show excellent predictions of the average orbiter spectrum over 1 Venus day. The Venus gravitational signal should be above the tracking noise level for arc lengths longer than 40° (in true anomaly) about periapsis and for terms as high as 55th degree. Analysis has been made of tracking residuals from a short‐arc fit to Mariner Mars 9 data over the Hellas Basin (using a complete sixth‐degree field). Results are most consistent with higher residual gravitational power than predicted from Kaula's rule for Mars.