bluuebunny/crossref_metadata_2025 huggingface dataset:
Solar cycle variations of the equatorial ionospheric anomaly in total electron content in the Asian region
Authors: Huang Yinn‐Nien , Cheng Kang
Published: February 2004

Daily equatorial ionospheric anomaly (EIA) contour charts in total electron content (TEG) were obtained by receiving simultaneously two coherent radio signals transmitted from the U.S. Navy Navigation Satellite System satellite at Lunping Observatory (25.00°N, 121.17°E) from September 1985 to December 1994. The latitude, occurrence time, and strength of the most developed EIA crest obtained from daily TEC contour charts have been used to study the solar cycle variations of EIA in the Asian region. No significant solar cycle effect can be seen in the occurrence time and latitude of the most developed EIA crest. Seasonally, the winter crest appears larger and earlier than the summer crest, and the summer crest appears in lower latitude than other seasons. These seasonal changes are mainly accounted for by the effect of daytime meridional wind. The strength of EIA crest increases with the increasing of solar activity and exhibits winter anomaly with the winter strength larger than the summer one Positive correlation is found between the 12‐month smoothed EIA crest strength and sunspot number. No saturation effect can be seen during the high solar activity period. The 12‐month smoothed EIA crest strength exhibits a hysteresis variation about the solar cycle variation. Monthly mean TEC contour charts for the latitude from 15°N to 35°N are also obtained from those daily contour charts. By using linear regression analysis, the monthly mean TEC I at a given month is linearly correlated with smoothed sunspot number R by a formula I = A + BR. These results can be used to construct the monthly EIA contour chart in TEC for any given sunspot number.
Longitude structure of the ionosphere F region from TOPEX/Poseidon and ground‐based data during January 20–30, 1993, including the quasi 2‐day oscillation
Authors: Forbes J. M. , Revelle D. , Zhang X. , Markin R. E.
Published: February 2004

The global structure of the F region ionosphere during January 20–30, 1993, is examined with emphasis on longitudinal variations. The database consists of total electron content (TEC) measurements from the TOPEX/Poseidon dual‐frequency altimeter experiment from −66° to +66° geographic latitude for prenoon and premidnight conditions, and hourly ƒoF2 and hmF2 values from eight ionosonde stations between −42° and +26° geographic latitude in the Pacific sector. The 10‐day averaged daytime TEC data reveal latitude versus longitude structures whose salient features are captured (within ∼10 TEC units) by the 1990 International Reference Ionosphere (IRI‐90), and which can be explained by dynamically induced neutral composition changes due to a combination of solar and magnetically driven circulation systems. The premidnight TEC data at low latitudes reveal the double‐banded structure of the equatorial ionization anomaly with, however, pronounced variations in amplitude and position as a function of geomagnetic longitude. None of these features, which probably reflect some combination of electric field and neutral wind effects, are captured by IRI‐90. Time series analyses of the ƒoF2 and hmF2 data, and space‐time Fourier spectra of the TEC data, reveal the presence of a quasi 2‐day (QTD) oscillation in the ionosphere which is associated with QTD wind and temperature oscillations in the mesosphere and lower thermosphere previously documented for this time period. The satellite‐based QTD results are subject to significant caveats described herein. Nevertheless, the TOPEX/Poseidon TEC data set clearly represents an important resource for global studies of ionospheric structure and plasma‐neutral interactions.
Auroral emission profiles extracted from three‐dimensionally reconstructed arcs
Authors: Frey S. , Frey H. U. , Carr D. J. , Bauer O. H. , Haerendel G.
Published: February 2004

The mathematics of emission‐computed tomography is applied to the analysis of stereoscopic observations of auroral arcs. The incomplete data problem for three‐dimensional computerized tomography, due to the very limited angular range and the small number of observers, is solved by an iterative least squares method. A theoretical example with the projection of two auroral arcs can be reconstructed with projection root‐mean‐square errors of the order of 1%. Application of noise deteriorates the quality of reconstruction, but with suitable low‐pass filtering, root‐mean‐square errors of less than 5% can be obtained. The agreement between observed and calculated projections, as well as between the original and the reconstructed volume emission distribution, is quantitatively discussed for different observation geometries. With only two observation sites the range of acceptable observation geometries is limited to distances of 25 to 50 km between the observers but depends on the actual location and the morphology of the aurora. Sources of distortion of real auroral observations with intensified CCD cameras are discussed and correction procedures proposed. The complete procedure is applied to a real stereoscopic auroral observation. The correspondence of original pictures and projections of the reconstructed volume is discussed, and emission profiles along magnetic field lines are extracted from the three‐dimensionally reconstructed arc.
Structure of the magnetotail current sheet
Authors: Larson Douglas J. , Kaufmann Richard L.
Published: February 2004

An orbit tracing technique was used to generate current sheets for three magnetotail models. Groups of ions were followed to calculate the resulting cross‐tail current. Several groups then were combined to produce a current sheet. The goal is a model in which the ions and associated electrons carry the electric current distribution needed to generate the magnetic field B in which ion orbits were traced. The region −20 RE < x < −14 RE in geocentric solar magnetospheric coordinates was studied. Emphasis was placed on identifying the categories of ion orbits which contribute most to the cross‐tail current and on gaining physical insight into the manner by which the ions carry the observed current distribution. Ions that were trapped near z = 0, ions that magnetically mirrored throughout the current sheet, and ions that mirrored near the Earth all were needed. The current sheet structure was determined primarily by ion magnetization currents. Electrons of the observed energies carried relatively little cross‐tail current in these quiet time current sheets. Distribution functions were generated and integrated to evaluate fluid parameters. An earlier model in which B depended only on z produced a consistent current sheet, but it did not provide a realistic representation of the Earth's middle magnetotail. In the present study, B changed substantially in the x and z directions but only weakly in the y direction within our region of interest. Plasmas with three characteristic particle energies were used with each of the magnetic field models. A plasma was found for each model in which the density, average energy, cross‐tail current, and bulk flow velocity agreed well with satellite observations.
Model of the low‐latitude boundary layer with finite field‐aligned potential drops and nonconstant mapping factors
Authors: Wei C. Q. , Sonnerup B. U. Ö. , Lotko W.
Published: February 2004

A 2½‐dimensional, steady state model of the low‐latitude boundary layer (LLBL) on closed field lines has been developed. Coupling to the dayside auroral ionosphere via region 1 field‐aligned currents and associated finite field‐aligned potential drops is included. The boundary layer (BL) flow is influenced by viscous forces, inertia forces, pressure forces, and j × B forces. The BL approximation and kinematic treatment of the plasma motion perpendicular to the equatorial plane greatly simplify the model. The model has three modules: an equatorial LLBL module, a force‐free coupling module, and an ionospheric module. It includes nonconstant magnetosphere‐ionosphere mapping factors, describing the increasing stretching of magnetic field lines with increasing distance away from the subsolar region, as well as variations in mapping factors across the BL. In this paper, both the BL height and the ionospheric conductivities are assumed constant. The effects of finite field‐aligned potential drops and nonconstant mapping factors on the BL flow, on the plasma density and temperature, on the magnetic field line bending, and on the field‐aligned current distribution are presented. The effects of varying the magnitude of the (constant) ionospheric Pedersen conductivity are also discussed. In addition, the effects of viscosity, including varying its magnitude and allowing it to be a function of location both along and across the LLBL, are examined. A number of qualitative features of the simulation results are found to be consistent with various observations.
The 22‐year cycle of geomagnetic and solar wind activity
Authors: Cliver Edward W. , Boriakoff Valentín , Bounar Khaled H.
Published: February 2004

The 22‐year cycle in geomagnetic activity is characterized by high activity during the second half of even‐numbered solar cycles and the first half of odd‐numbered cycles. We present new evidence for this 22‐year cycle using the aa magnetic index for the years 1844–1994. Over this 150‐year interval, the 22‐year cycle can be observed through differences between the decay phases of even‐ and odd‐numbered cycles in (1) average values of a 27‐day recurrence index; (2) the results of a χ2 “event” analysis of 27‐day recurrences of both disturbed and quiet days; and (3) an apparent annual modulation of the 27‐day peak in the power spectrum of the aa index. Currently, the 22‐year variation is attributed to the Russell‐McPherron solar wind ‐ magnetosphere coupling mechanism working in conjunction with the Rosenberg‐Coleman polarity effect. Contrary to this viewpoint, we argue that an intrinsic 22‐year solar variation (other than polarity reversal), revealed in the systematic low‐high alternation of even‐odd sunspot maxima within the last six complete Hale cycles, is the dominant cause of the 22‐year cycle in geomagnetic activity. This sunspot and related coronal mass ejection variation should lead directly to higher geomagnetic activity during the first‐half of odd‐numbered solar cycles. Various lines of evidence (including 1–3 above) indicate that 27‐day recurrent wind streams are more prominent during the decline of even‐numbered solar cycles, contributing to the higher geomagnetic activity observed at those times. These stronger recurrence patterns may be related to the more rapid expansion of polar coronal holes (faster movement of the coronal streamer belt to low latitudes) observed following the maxima of recent even‐numbered cycles. The amplitudes of the 22‐year sunspot and geomagnetic activity cycles over the last 150 years are shown to be highly correlated. The 22‐year pattern of geomagnetic activity appears to be a reflection of the solar dynamo coupling of poloidal magnetic fields on the decline of one solar cycle to the toroidal fields at the maximum of the following cycle. It seems likely that the 22‐year variation in sunspot/solar wind activity plays a role in the observed 22‐year modulation of galactic cosmic ray intensity.
A possible driving source for transient field line oscillations in the postmidnight sector at geosynchronous altitudes
Authors: Saka O. , Watanabe O. , Baker D. N.
Published: February 2004

Geosynchronous orbit data from charged particle analyzer (30‐ to 300‐keV electron flux) and magnetometer instruments on board the 1984‐129, GOES 6, GOES 5, and 1982‐019 satellites distributed in the midnight‐to‐dawn sector are examined during a particle injection event that occurred at 0820 UT on May 21, 1986. The injection was detected by 1984‐129 in the midnight sector as a dispersionless and short‐lived (duration was 9 min) event. The injected electron cloud drifted eastward and was detected by 1982‐019 in the dawn sector. Meanwhile, the GOES 6 and 5 satellites were in the postmidnight sector between the satellites which measured the electron flux. They observed transient field line oscillations during the periods when the azimuthal plasma pressure arising from the electron cloud was in the postmidnight sector. It is suggested that field‐aligned currents are likely to be built up preferentially by a divergence of diamagnetic currents in the region where the azimuthal plasma pressure gradient dominates. It is argued that these currents excite an azimuthally polarized transient field line oscillation.
Particle simulations of collisionless reconnection in magnetotail configuration including electron dynamics
Authors: Dreher J. , Arendt U. , Schindler K.
Published: February 2004

The role of electron dynamics in collisionless magnetotail reconnection is investigated by using two‐dimensional full particle simulations with ion‐to‐electron mass ratios mi/me set to 1 and 10. The results indicate that the electrons have a stabilizing influence on the ion tearing instability in the sense that growth rates are reduced in runs with the initial electron Larmor radius being sufficiently small in comparison with the ion Larmor radius. A comparison of our results with those obtained by Pritchett [1994] using a significantly different numerical model shows encouraging agreement. The attempt to achieve an approach closer to realistic magnetotail conditions is inhibited by present technical restrictions.
Simulation of dynamics and structure in the black aurora
Authors: Keskinen M. J. , Ganguli G.
Published: February 2004

Simulations of the dynamics and structure associated with the black aurora have been performed. We find that the nonlinear evolution of the stratified Kelvin‐Helmholtz shear instability in the presence of a density gradient leads to structured vortices and electric fields in addition to large density depletions. Timescales for vortex generation, roll‐up, and breaking are of the order of a second with spatial scales of the order of several hundred meters to several kilometers. Good agreement is found between simulation results and recent experimental observations.
Nonclassical viscosity and resistivity of the solar wind plasma
Authors: Verma Mahendra K.
Published: February 2004

In this paper we have estimated typical viscosity and resistivity of the solar wind using turbulence phenomenology rather than Branskii's [1965] formalism. Since the solar wind is a collisionless plasma, in this paper we have assumed that the dissipation in the solar wind occurs at the proton gyroradius through wave‐particle interactions. Using this dissipation length scale and the dissipation rates calculated using MHD turbulence phenomenology [Verma et al., 1995], we estimate the viscosity and resistivity. The resisitivity has been recalculated using anomalous scattering time. We find that all of our transport quantities are several orders of magnitude higher that those calculated using classical theories of Branginskii [1965].
ISEE observations of low‐latitude boundary layer for northward interplanetary magnetic field: Implications for cusp reconnection
Authors: Le G. , Russell C. T. , Gosling J. T. , Thomsen M. F.
Published: February 2004

We present a study of the formation of the low‐latitude boundary layer for northward interplanetary magnetic field that provides evidence for the reconnection poleward of the region of the cusp. Velocity distributions as observed by the joint Los Alamos/Max‐Planck‐Institut (MPE) Garching Fast Plasma Experiment on the ISEE spacecraft reveal two types of low‐latitude boundary layer plasma: heated magnetosheath plasma with little or no hot magnetospheric component in the outer boundary layer and a mixture of magnetosheath and magnetospheric plasmas in the inner boundary layer. The observed plasma characteristics can be explained by the process of magnetic reconnection poleward of the region of the cusp. The outer boundary layer that contains heated magnetosheath plasma and little or no hot magnetospheric component appears to be formed by reconnection between magnetosheath and lobe field lines poleward of one cusp. It is identified to be on open field lines with one end in the ionosphere and the other one in the solar wind. The magnetosheath plasma entering the outer boundary layer is heated and accelerated at the cusp reconnection site and is then transported to the low latitudes along the magnetic field. The hot plasma from the magnetosphere is severely depleted but is not necessarily completely absent since they can escape to the magnetosheath on open field lines, with a finite time being required for total depletion to occur. The inner boundary layer is identified to be on closed field lines that have become closed by reconnection of the open end of the flux tube poleward of the second cusp. The inner boundary layer contains a mixture of magnetosheath and magnetospheric plasmas since the hot magnetospheric plasma can drift onto these closed field lines, even if the hot plasma was completely absent when the field line was open. Thus, when the interplanetary field is strongly northward, two boundary layers are formed, one on open field lines and one on closed field lines, by the sequential action of reconnection beyond the polar cusp.
The tearing instabilities driven by lower hybrid waves in the current sheet
Authors: Sundaram A. K. , Fairfield D. H.
Published: February 2004

We have investigated the excitation of tearing instabilities driven by lower hybrid waves in the current sheet. Considering a two‐dimensional equilibrium, we have shown that lower hybrid waves observed throughout the plasma sheet can cause large anomalous resistivity as a consequence of nonlinear interactions and significantly influence the excitation of tearing mode in the layer, where Bx (z) reverses sign. We have found that the perturbed ponderomotive force gives two new physical effects, one due to current gradients in the inner core of the current sheet (z ≪ δ ≪ λ, where λ is the current sheet width and δ is the thickness of the field reversal layer) and the other due to anomalous resistivity in the outer core (δ ≪ z ≤ λ). In the presence of these effects, we have demonstrated that global tearing modes with growth times ranging from fraction of a second to several tens of milliseconds are excited for wave electric fields of order of a few mV/m and scale lengths in the range koyLz < 1 or koyLz > 1, where koy is the wavenumber of the lower hybrid waves and Lz is the perturbation scale length. Finally, we have discussed the relevance of this wave‐driven mode to the magnetic reconnection and the substorm onset processes.
Ion dynamics in a hybrid simulation of magnetotail reconnection
Authors: Kuznetsova Masha M. , Hesse Michael , Winske Dan
Published: February 2004

The 2 1/2‐dimensional hybrid simulations are carried out to investigate self‐consistent ion dynamics in time‐dependent magnetic reconnection. The system dimensions 25 RE × 5 RE are able to accommodate large‐scale structures associated with magnetotail reconnection. The simulations are started with two ion populations: hot ions forming the thin current sheet self‐consistent with the initial equilibrium two‐dimensional tail‐like magnetic configuration and cold lobe ions with initially uniform density and temperature. The evolution of the initially stable system is triggered by superposition of a spatially localized resistivity. We find ion acceleration and formation of non‐Maxwellian multicomponent ion distributions as predicted by test particle studies. Fine features, such as layers of counterstreaming ions, are found in the distributions of the initially cold lobe ions. The hot current sheet “source,” however, gives rather smeared “output distributions” due to significant spatial overlapping of ion populations with different histories. We find that ions, which are strongly accelerated in the close vicinity of the reconnection site, contribute little to the equilibrium current before reconnection is initiated. Ions, which significantly contribute to the equilibrium current, are either ejected earthward or carried away inside the released plasmoid without significant energy gain during a single “impulsive” interaction with the reconnection fields. Multiple encounters of the acceleration region and quasi‐Fermi acceleration of ions trapped between the mirroring earthward boundary and the growing normal magnetic field are found. The simulations also show effective heating of initially cold background ions in the plasma sheet behind the released plasmoid. The tailward speed of energetic ions flowing around the plasmoid along reconnected lobe field lines exceeds the speed of plasmoid. The ions which contribute to these beams are part of the hot population and initially reside in the current sheet. These results match spacecraft observations prior to plasmoid arrival.
Reconstruction of past solar wind variations: Inversion of the geomagnetic response at Godhavn
Authors: Andreasen G. Kjœrgaard
Published: February 2004

The first quantitative inference of the past solar wind conditions is successfully attempted. Using Godhavn magnetometer data, we are able to deduce the gross temporal variations of the solar wind back to 1926. First, a set of daily perturbation quantities is defined from the hourly magnetic elements H, D, and Z characterizing different aspects of the high‐latitude current system. An extensive analysis by means of robust multiregression methods enable us for the satellite era to chart the sensitivity of the perturbation quantities to specific interplanetary parameters. Second, at an arbitrary day the derived statistical relations are used to invert the geomagnetic response to give the daily averages of the southward interplanetary magnetic field (IMF) component Bs, the azimuthal IMF component By, and the solar wind speed V. It is necessary to incorporate a priori observations of Bs, By, and V in the least squares solution but only with very low weights relative to those of the Godhavn observations. The inversion procedure is verified and optimized on solar cycle 20 and 21. Reconstruction results are given for solar cycle 16, 17, and 18 covering the interesting period with a steep long‐term increase of the solar activity. The cyclic features observed in situ are also found for the deduced solar wind conditions: a pronounced modulation of Bs and By, and the maximum in V occurring a few years before the activity minimum. A systematic trend is only observed for the solar wind speed. This occurs in the first half of cycle 16 and cycle 17, where the normal variations appear superposed on a linear increase. A distinct increase in the solar wind speed is apparent through cycle 17.
Hybrid simulations of the magnetic topology of Mars
Authors: Brecht Stephen H.
Published: February 2004

The structure of the magnetic field surrounding the planet Mars has been investigated using a fully three‐dimensional hybrid particle code. The results of these simulations are found to agree well with Phobos 2 data. However, the results also indicate that the solar wind interaction with the planet Mars produces a magnetic environment around the planet that is unique among the planets of the solar system. This paper reports the findings from a series of large‐scale, three‐dimensional simulations and many two‐dimensional simulations used to address numerical issues.
HF radar probing of the lower magnetosphere
Authors: Hysell D. L. , Kelley M. C. , Gurevich A. V. , Karashtin A. N. , Babichenko A. M. , Yampolski Y. M. , Beley V. S. , Providakes J. F.
Published: February 2004

The first observations of ion acoustic waves at altitudes of about 4000 km in the auroral zone were carried out using the powerful SURA HF radar in 1991. Here we report additional observations made with the Russian SURA radar and the Ukrainian UTR 2 receiving system in December 1992. Although echoes similar to the earlier observations were detected, we performed a number of diagnostics and have concluded that no unambiguous magnetospheric echoes were seen at SURA during the 1992 campaign. (There is some indication that the more sensitive UTR 2 system received magnetospheric echoes.) Some aspects of the coding scheme used in the experiments that might lead to spectral artifacts are discussed. A new antenna under construction at SURA will allow transmission at twice the frequency used to date and should permit more definitive observations in the future.
Electron and ion signatures of field line topology at the low‐shear magnetopause
Authors: Fuselier S. A. , Anderson B. J. , Onsager T. G.
Published: February 2004

Electrons above 50 eV are a sensitive indicator of field line topology at the magnetopause, particularly when the solar wind dynamic pressure is high and the shear across the boundary is low. AMPTE/CCE electron observations under conditions when these criteria are fulfilled indicate a clear topological transition from the magnetosheath to open field lines threading the magnetopause in the magnetosheath boundary layer (MSBL). Once across the magnetopause and in the low latitude boundary layer (LLBL), the fast moving electrons are no longer a good indicator of magnetic field topology. In particular, the counterstreaming electron observations in this region are not an indicator of a closed magnetic topology. Rather, the field topology continues to be open, and the counterstreaming occurs because electrons from the magnetopause region move rapidly enough along the LLBL magnetic field to make it to the ionosphere, mirror, and return to the observation point Slower moving ions provide important additional information on magnetic field topology in the LLBL. CCE observations discussed here indicate that two types of solar wind ion distributions are observed in this layer. One type consists of a single, heated distribution which resembles somewhat the electron distribution in the layer. The field‐aligned velocity of this distribution is near zero. The other type consists of a unidirectional streaming distribution. The field‐aligned velocity of this distribution is higher than in the adjacent magnetosheath. Combining these observations with magnetospheric ion observations (e.g., O+) in the LLBL and with electron observations in the MSBL, two distinct magnetic field topologies emerge for the low‐shear magnetopause. The first, which gives rise to single, low‐parallel‐velocity and heated solar wind ion distributions in the LLBL, is magnetic reconnection poleward of the cusp. The second, which gives rise to unidirectional streaming solar wind ion distributions in the LLBL, is magnetic reconnection equatorward of the cusp. This type of component reconnection may not be sustained in a quasi‐steady fashion.
Coronal streamer belt asymmetries and seasonal solar wind variations deduced from Wind and Ulysses data
Authors: Crooker N. U. , Lazarus A. J. , Phillips J. L. , Steinberg J. T. , Szabo A. , Lepping R. P. , Smith E. J.
Published: February 2004

Solar wind measurements from Wind during March 1995 are combined with those from Ulysses' fast latitude scan to construct a map of the streamer belt. On the timescale of coronal change, the map is nearly a snapshot view of solar wind speed contours threaded by the trace of the heliospheric current sheet (HCS) in the ±30° heliolatitude range. The combined set of HCS crossings agrees remarkably well with the neutral line on the corresponding classical source surface map. The neutral line is displaced slightly southward, with latitudinal excursions ranging from −22° to +17°. In contrast, a line running through the locus of minimum speed, although following the general trend of the neutral line, is confined to lower latitudes and displaced slightly northward, ranging from −4° to +13°. The separation between the minimum speed locus and the neutral line marking the HCS was unexpected. Possible interplanetary and solar origins are discussed. The deduced asymmetries as well as coronal change between December and March were responsible for solar wind variations at Earth that mimicked the previously reported seasonal variation in Wind data and would have masked it had the observations been taken during September equinox.
Possible plasma depletion layer ahead of an interplanetary ejecta
Authors: Farrugia C. J. , Erkaev N. V. , Biernat H. K. , Burlaga L. F. , Lepping R. P. , Osherovich V. A.
Published: February 2004

We present observations of a plasma depletion layer in front of an interplanetary magnetic cloud and analyze them quantitatively using a recent magnetohydrodynamic (MHD) theory of flow around magnetic clouds driving shocks. The magnetic field and plasma measurements were made by the ISEE 3 and IMP 8 on September 28–29, 1978. The magnetic cloud is expanding, and with a frontside boundary speed of ≈ 900 km s−1 it is one of the fastest ever seen at Earth orbit. We find a simple variation of field and plasma parameters in the sheath region behind the shock. Along the spacecraft trajectory the sum of the gas and plasma pressures is approximately constant, with a plasma beta less than unity. As the frontside boundary of the magnetic cloud is approached, the magnetic field strength increases while the density and temperature simultaneously decrease. These are defining characteristics of a plasma depletion layer, similar to that often seen in the terrestrial magnetosheath adjacent to the magnetopause. This suggests that the underlying cause is the same: the stretching of interplanetary magnetic field lines around the ejecta. The plasma depletion layer extends throughout the sheath region for which data are available, that is, for approximately 3 out of 5.7 hours. The sheath terminates at a clear discontinuity at the cloud's surface. To compare the observations with MHD theory, we model the ejecta as a magnetic flux rope of locally straight cylindrical geometry. We consider two limiting cases separately: (1) bulk motion relative to the surrounding medium but without expansion and (2) expansion into the ambient medium but without bulk motion. The predictions of both these limiting theories yield good agreement with observed variations of plasma parameters and magnetic field strength across the sheath. This paper is intended as a first attempt at modeling plasma depletion layers associated with interplanetary magnetic clouds.
Electrodynamics of midlatitude spread F 3. Electrohydrodynamic waves? A new look at the role of electric fields in thermospheric wave dynamics
Authors: Kelley Michael C. , Miller Clark A.
Published: February 2004

The study of atmospheric tides has long recognized the importance of electrodynamic or ohmic losses, in the form of
forces, on the strength of winds in the Earth's upper atmosphere. Now, recent observations of ionospheric and atmospheric phenomena (particularly, but not exclusively, at midlatitudes) are suggesting that electrodynamic processes may also be important for atmospheric buoyancy waves with periods as short as a few hours. During the daytime, passive ohmic losses may be responsible for a strong azimuthal filtering of buoyancy waves which limits their propagation in the upper atmosphere to near equatorward where there is a minimum of the
drag. At night, the active development of plasma instabilities, whose growth rate is also strongly azimuthally dependent, may not only minimize ion drag but may also feed energy into particular buoyancy waves to counteract the nighttime's enhanced molecular and thermal dissipation, thus creating a strong equatorward and westward orientation to observed traveling ionospheric disturbances. In all, the strength of the apparent coupling between buoyancy waves and ionospheric electrodynamics in the upper atmosphere may suggest the classification and study of a new form of electrohydrodynamic waves.
Modeling Jupiter's decametric modulation lanes
Authors: Imai Kazumasa , Wang Liyun , Can Thomas D.
Published: February 2004

The modulation lanes in Jupiter's decametric radio spectra were discovered by Riihimaa [1968]. We have developed a model for the mechanism responsible for their production in which the free parameters have been adjusted to provide a very close fit with the observations. In our model, a grid‐like interference screen composed of field‐aligned columns of enhanced or depleted plasma density is located near Io's orbit close to the longitude of the sub‐Earth point. The column spacing is typically about 140 km. As a band of frequency components emitted from near the foot of an excited tube of magnetic flux passes through the screen, interference patterns of slightly different orientations are produced by the different frequencies. The corotation of this set of interference patterns with Jupiter results in the sloping modulation lanes of the observed dynamic spectrum. Newly calculated results indicate that (1) the Io‐B and Io‐A radiations are emitted from the northern hemisphere, while that from Io‐C comes mainly from the southern hemisphere, (2) the half‐angle of the assumed hollow‐cone emission beam for Io‐B is typically 60°, with a variation of a few degrees, and (3) the equatorial lead angle of the radio‐emitting previously excited flux tube ahead of the flux tube through Io at the same instant is more variable, 50° being a typical value for Io‐B. Modulation lanes from Io‐unrelated emission were also successfully modeled, the assumption in this case being that some source of energy other than a direct encounter with Io excited the flux tube containing the emitting radio source. The L shell value for non‐Io‐A was less clearly defined but was definitely between 4 and 7. Sensitive measurements of the lane modulation depths for Io‐B and Io‐A indicated that the decay time for the columns of the interference screen is of the same order of magnitude as Jupiter's rotation period. Although the predominant modulation lane periodicity of about 2 s indicates an upper limit on Io‐B source width of 70 km along the direction of increasing longitude, finer structure that was present in at least one case suggests an upper limit of only 20 km.
Self‐consistent superthermal electron effects on plasmaspheric refilling
Authors: Liemohn M. W. , Khazanov G. V. , Moore T. E. , Guiter S. M.
Published: February 2004

The effects of self‐consistently including superthermal electrons in the definition of the ambipolar electric field are investigated for the case of plasmaspheric refilling after a geomagnetic storm. By using the total electron population in the hydrodynamic equations, a method for incorporating superthermal electron parameters in the electric field and electron temperature calculation is developed. Also, the ambipolar electric field is included in the kinetic equation for the superthermal electrons through a change of variables using the total energy and the first adiabatic invariant. Calculations based on these changes are performed by coupling time‐dependent models of the thermal plasma and superthermal electrons. Results from this treatment of the electric field and the self‐consistent development of the solution are discussed in detail. Specifically, there is a decreased thermal electron density in the plasmasphere during the first few minutes of refilling, a slightly accelerated proton shock front, and a decreased superthermal electron flux due to the deceleration by the electric field. The timescales of plasmaspheric refilling are discussed and determined to be somewhat shorter than previously calculated for the thermal plasma and superthermal electron population due to the effects of the field‐aligned potential.
Bathymetry, depth to magnetic basement, and sediment thickness estimates from aerogeophysical data over the western Weddell Basin
Authors: LaBrecque John L. , Ghidella Marta E.
Published: February 2004

We estimated the bathymetry and sediment thickness of a remote and difficult to access portion of the Antarctic continental margin using aerogeophysical surveying techniques. The U.S., Argentina, Chile aerogeophysical survey collected magnetic and gravity data over the basins surrounding the Antarctic Peninsula. Thirty‐seven of these flight lines were used to estimate bathymetry and depth to magnetic basement for the western Weddell Basin. A wavenumber technique was applied to individual magnetic anomaly profiles in an automated fashion to obtain estimates of the depth to magnetic basement. The bathymetric estimates were obtained by admittance inversions of the gravity field. The results were then gridded at a 40‐km interval for the region 64°W, 44°W, 73°S, and 62°S. Bathymetric estimates and depth to magnetic basement estimates were differenced at each grid point to obtain a regional estimate of the thickness of nonmagnetic overburden (assumed to be sediment). Subsequent spot measurements of topography in the estimated region of the continental margin generally agree to about 52 m. The estimated magnetic basement deepens from the Antarctic Peninsula margin eastward to a maximum of 10–12 km near 54°W. We also postulate the existence of two moderately large basins flanking the eastward continuation of the Jason Peninsula. Farther east, the basement steps upward, with a correspondent thinning of the sedimentary layer. Along the east coast of the peninsula, results agree well with seismic studies on James Ross Island and magnetotelluric studies on Marambio Island and the Larsen nunatak, as well as the British Antarctic Survey basement estimates from aeromagnetic data. This study further demonstrates the utility of combined application of airborne and satellite geophysical techniques in the study of structure and tectonic evolution of continental margins and marine basins.
Age‐dependent three‐dimensional magnetic modeling of the North Pacific and North Atlantic oceanic crust at intermediate wavelengths
Authors: Yañez Gonzalo A. , LaBrecque John L.
Published: February 2004

Three‐dimensional magnetic modeling of the North Atlantic and northeast Pacific is performed at intermediate wavelengths using three models for the acquisition of a natural remanent magnetization. It is shown that a remanent magnetization which is dependent on the crustal age is the dominant source for the intermediate‐wavelength pattern in both basins. However, a pure thermoremanent magnetization of layer 2 alone is insufficient to model the intensity and shape of the observed magnetic anomalies at satellite altitude. We conclude that the best fitting magnetization model for both basins is a combination of a chemical remanent magnetization in the altered upper crust and a thermoviscous remanent magnetization of the slowly cooling lower crust and upper mantle. The North Pacific requires a bulk magnetization which is 50% higher than that of the North Atlantic in order to fit the Magsat field. Geological processes associated with a faster spreading rate such as a faster hydrothermal alteration and the growth of a thicker gabbro layer at the expense of a weakly magnetized sheeted dike layer are plausible explanations for the higher North Pacific magnetization. The lineated positive magnetic anomaly observed over the North Atlantic spreading center is not well reproduced by our models. This anomaly is likely due to a highly magnetized body along or in the vicinity of the spreading center. This highly magnetic body could be an unstable serpentinized lens of crustal material younger than 20 m.y. which develops within the zone cooled by hydrothermal circulation.
Digital isochrons of the world's ocean floor
Authors: Müller R. Dietmar , Roest Walter R. , Royer Jean‐Yves , Gahagan Lisa M. , Sclater John G.
Published: February 2004

We have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self‐consistent set of global isochrons and associated plate reconstruction poles. The age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models. We have constructed an age grid with error estimates for each grid cell as a function of (1) the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid, (2) the distance of a given grid cell to the nearest magnetic anomaly identification, and (3) the gradient of the age grid: i.e., larger errors are associated with high age gradients at fracture zones or other age discontinuities. Future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere, the heat loss of the Earth, ridge‐push forces through time, asymmetry of spreading, and providing constraints for seismic tomography and mantle convection models.
Regional wave propagation in southern California and Nevada: Observations from a three‐component seismic array
Authors: Wagner Gregory S.
Published: February 2004

We present results from a study conducted to characterize regional wave propagation in southern California and Nevada. The data are from events whose paths sample the southern Coast Ranges, the southern Sierra Nevada Mountains and the southern Basin and Range. These regional earthquake data were recorded by a small‐aperture three‐component seismic array located at the Piñon Flat Observatory, southern California. The data are analyzed using a statistical array processing algorithm that provides estimates of the propagation direction, frequency, wavenumber, and particle motion polarization characteristics of the coherent arrivals observed at the array. Perhaps the most striking feature shared by these data is the extent to which theP,SandLgwave trains are composed of coherent, forward scattered/multipathed energy. The nature of theP,SandLgwave trains suggests that spatially anisotropic scattering plays an important role in regional wave propagation in southern California and Nevada, and that scattering sources are located not on elliptical shells whose foci are the source and the receiver but instead in a limited volume subparallel to the path of the direct arrivals. The high‐amplitude, coherent nature of numerous of thePgandLgcoda waves suggests that strong scattering in the source zone plays an important role in shaping thePgandLgwave trains. While theP,SandLgwave trains for these regional events share some general characteristics they also exhibit fairly significant differences. These differences illustrate the important role regional and source zone structure play in shaping the wave field, and the need to account for both complex scattering and three‐dimensional propagation effects in regional waveform modeling. The coda for these regional data do not begin to exhibit a character consistent with the random scattering model for coda generation until the arrival of the surface waves. The fact that much of the surface wave coda cannot be effectively modeled as plane waves suggests that it is not the product of scattering from distant heterogeneities.
An analog experiment for the Aegean to describe the contribution of gravitational potential energy
Authors: Hatzfeld Denis , Martinod Joseph , Bastet Gilles , Gautier Pierre
Published: February 2004

The southern Aegean seafloor exhibits clear evidence of internal deformation (stretching) as shown by tectonics, seismology and space geodesy. We use an analog three‐layer laboratory experiment of sand, silicone putty and honey to investigate the deformation of the southern Aegean lithosphere. The model is installed in a box and confined by a vertical wall. We open a gate in the wall and observe the deformation of the two upper layers due to buoyancy forces. The general pattern of the deformation of the southern Aegean is found in the analog model. We observe the formation of an arc spreading outward with time, the extension is radial in the inner part, but parallel to the arc in the external part and of comparable importance. At both ends of the gate we observe strike‐slip motion (dextral in the western part, sinistral in the eastern part). Rotation (clockwise in the western part, counterclockwise in the eastern part) of up to 40° is seen on both sides of the gate but is also present, with a smaller amplitude, far in the internal region, partially due to distributed shear. The spreading is associated with the thinning of the two upper layers and affects a region of dimensions comparable to the length of the free boundary. This spreading does not propagate inward with time. Some pieces of material located near the active boundary remain undeformed during the experiment.
Global geodetic signatures of the Antarctic Ice Sheet
Authors: James Thomas S. , Ivins Erik R.
Published: February 2004

Four scenarios of present day Antarctic ice sheet mass change are developed from comprehensive reviews of the available glaciological and oceanographic evidence. The gridded scenarios predict widely varying contributions to secular sea level change
ranging from −1.1 to 0.45 mm/yr, and predict polar motion
and time‐varying low‐degree gravitational coefficients
that differ significantly from earlier estimates. A reasonably linear relationship between the rate of sea level change from Antarctica
A and the predicted Antarctic
is found for the four scenarios. This linearity permits a series of forward models to be constructed that incorporate the effects of ice mass changes in Antarctica, Greenland, and distributed smaller glaciers, as well as postglacial rebound (assuming the ICE‐3G deglaciation history), with the goal of obtaining optimum reconciliation between observed constraints on
and sea level rise
. Numerous viable combinations of lower mantle viscosity and hydrologie sources are found that satisfy “observed”
in the range of 1 to 2–2.5 mm/yr and observed
for degrees 2, 3, and 4. In contrast, rates of global sea level rise above 2.5 mm/yr are inconsistent with available
observations. The successful composite models feature a pair of lower mantle viscosity solutions arising from the sensitivity of
to glacial rebound. The paired values are well separated at
 mm/yr, but move closer together as
is. increased, and, in fact, merge around
=2‐2.5 mm/yr, revealing an intimate relation between
and preferred lower mantle viscosity. This general pattern is quite robust and persists for different
solutions, for variations in source assumptions, and for different styles of lower mantle viscosity stratification. Tighter
constraints for l > 2 may allow some viscosity stratification schemes and source assumptions to be excluded in the future. For a given total observed
, the sea level rise from Antarctica
A is tightly constrained and ranges from 0 to + 1 mm/yr (corresponding to an ablating ice sheet) as estimates of
are raised from 1 to 2.5 mm/yr. However, when the degree 3 zonal harmonic constraint is removed, the solutions show little sensitivity to Antarctic mass balance, emphasizing the need for a well determined odd‐degree secular zonal harmonic for determining polar ice mass balance.