bluuebunny/crossref_metadata_2025 huggingface dataset:
The December 28, 1908, Messina Straits, southern Italy, earthquake: Waveform modeling of regional seismograms
Authors: Pino Nicola Alessandro , Giardini Domenico , Boschi Enzo
Published: February 2004

The 1908 Messina Straits earthquake is one of the most catastrophic events in history. There were 60,000 to more than 100,000 deaths, and the cities of Messina and Reggio Calabria, on the opposite sides of the straits, were almost completely destroyed. During the last decades only low magnitude events occurred in the area. The 1908 earthquake is then crucial for understanding the mode of stress release in the area. We collected and digitized several regional seismograms of this event recorded in central Europe with the aim of studying the source characteristics. In order to separate the path effects, we analyzed recent events with simple and known sources. Owing to the small azimuth range spanned by the available stations, we could not determine a fault plane solution for the Messina Straits event, nor discriminate between the published focal mechanisms. However, the normal fault character of the rupture is confirmed. By inverting the historical P waveforms we derived source time functions, and obtained a seismic moment of 5.38(±2.16)×1019 N m (Mw; = 7.1). This is in good agreement with the results obtained by several authors from the inversion of the historical leveling data. This value is confirmed by the SH wave modeling, which also allowed the assessment of the unilateral northward character of the rupture propagation along an ∼43 km fault. Finally, we applied a simple rupture model in order to derive the slip distribution along the fault. The resulting function is in good correspondence with the geodetic inversions performed by Boschi et al. [1989] and De Natale and Pingue [1991]. In particular, a maximum slip of ∼4 m is located in proximity of the center of the slipped area. The high dislocation patch beneath the Messina harbor, as depicted by De Natale and Pingue, is not confirmed by our analysis and is probably connected to surface collapse of some of the bench marks.
A renewed argument for crystal size control of ice sheet strain rates
Authors: Cuffey K. M. , Thorsteinsson T. , Waddington E. D.
Published: February 2004

At present, it is generally believed that crystal size has no direct influence on strain rate in the ice sheets and that the fraction of strain rate enhancement there which is not ascribable to c axis fabric is due to impurity content. Here we challenge this view because it is not consistent with recent results from analyses of deformation at Meserve Glacier and instead ascribe residual enhancement in the ice sheets to variations in crystal size. We resurrect the idea that variations of crystal size can be an important part of the total shear enhancement in the ice sheets, though agree with Paterson that this effect is generally dominated by variations of crystal fabric. We propose that the enhanced shear strain rate of ice age ice in southern Greenland, as inferred from tilt of the Dye 3 borehole, can be explained as a result of combined fabric variations and crystal size variations, with these two ice properties accounting for roughly 70% and 30% of the average enhancement, respectively. Permitting a grain size dependence of ice viscosity also resolves the quandary concerning closure and tilt of the Agassiz Ice Cap borehole.
Geocentric sea level trend using GPS and >100‐year tide gauge record on a postglacial rebound nodal line
Authors: Sanli D. Ugur , Blewitt Geoffrey
Published: February 2004

For studies of global sea level change, 103 years of tide gauge data were analyzed from North Shields on the North Sea coast of Great Britain, near a predicted nodal line of postglacial rebound (PGR). Simultaneous estimation of statistically significant tidal components gives a relative sea level rate of 1.8±0.1 mm/yr with no significant acceleration. Analysis of a Global Positioning System (GPS) station, which we installed directly on the tide gauge structure, shows the tide gauge rising at 1.4±1.5 mm/yr relative to the international terrestrial reference frame (ITRF96). Leveling shows the tide gauge rising with statistical significance at 0.8±0.2 mm/yr relative to a nearby lighthouse (0.5 km away) situated on bedrock. Thus our geodetic estimate of geocentric crustal rise is 0.6±1.5 mm/yr, which is consistent with the PGR nodal hypothesis. Our estimate of geocentric sea level rise using local leveling and assuming the PGR nodal hypothesis (i.e., zero crustal rise with no model uncertainty) is therefore 2.6±0.2 mm/yr. Introducing an upper bound error based on observational evidence and PGR model differences modifies this to 2.6±1.0 mm/yr. This is to be compared with our model‐independent GPS‐based estimate of 3.2±1.5 mm/yr. We conclude that PGR and geodesy corroborate a geocentric sea level rise of 2.6±1.0 mm/yr (upper bound), which is larger than the tide gauge records alone (1.8 mm/yr) would indicate.
Baroclinic instability of density current along a sloping bottom and the associated transport process
Authors: Tanaka Kiyoshi , Akitomo Kazunori
Published: February 2004

Numerical experiments with a three‐dimensional nonhydrostatic ocean model have been carried out to investigate the dynamic processes of downslope density current and associated transport of dense water, focusing on the effects of bottom slope for the range of slope inclination S from 0 to 0.04. Bottom slope has two effects on baroclinic instability of the flow. One is the stabilizing effect due to the topographic β effect and to offshoreward increase in water depth. The other is the destabilizing effect due to steepened inclination of isopycnal surface, which increases available potential energy. Since the destabilizing effect over the upper slope overcomes the stabilizing one in the early stage, unstable waves develop more rapidly with increasing S. Nonlinear energy conversion due to Reynolds stresses also affects the flow stability and then the growth rate of instability increases linearly with S in this stage. After the unstable waves have finite amplitudes in the mature stage the stabilizing effect of bottom slope becomes dominant, especially over the lower slope. Thus vigorous unstable eddies are confined to the upper slope region in steep slope cases, while they cover the whole slope region in gentle slope cases, and this difference in the eddy field causes a drastic change in downslope transport of dense water. In gentle slope cases, efficiency of the transport becomes higher with increasing S since the destabilizing effect overcomes the stabilizing one as a whole. On the contrary, the efficiency becomes lower with increasing S in steep slope cases since the stabilizing effect surpasses the destabilizing one. As a result, the most effective transport is realized in the case with S = 0.005. Although the transport due to eddies in steep slope cases becomes weak over the lower slope, it is still larger than that due to the bottom Ekman current since the eddy transport occurs in the bottom layer with a thickness of 100∼200 m, which is 4∼8 times the Ekman layer thickness.
Modeling the three‐dimensional upper ocean heat budget and subduction rate during the Subduction Experiment
Authors: Spall Michael A. , Weller Robert A. , Furey Peter W.
Published: February 2004

A global three‐dimensional primitive equation general circulation model is used to estimate the upper ocean heat budget and subduction rates in the eastern North Atlantic during the period of the Subduction Experiment, June 1991 through July 1993. The seasonal cycle is dominated by one‐dimensional processes throughout the subtropical gyre, i.e., a local balance between the net heat flux into the ocean through the surface and a vertical redistribution due to vertical mixing. However, closure of the heat budget on long timescales involves redistribution by Ekman transports (both horizonal and vertical), geostrophic advection, and vertical diffusion. Subgridscale parameterizations of eddy processes weakly restratify the upper ocean. The annual rate at which waters are subducted from the mixed layer into the permanent thermocline is estimated using both kinematic and thermodynamic methods. These subduction rates and patterns are generally consistent with each other and also with previous estimates of the large‐scale subduction rate based on climatologies and models. Consideration of a finite thickness Ekman layer is shown to reduce the thermodynamic estimate of the subduction rate at low latitudes by O(50%), in much better agreement with the kinematic method. The effects of convergent eddy fluxes in the mixed layer and a diabatic thermocline on the subduction rate are also calculated and found to be small but not negligible. The seasonal and interannual evolution of the sea surface temperature and mixed layer depth in the model compare well with in situ measurements at five mooring locations taken as part of the Subduction Experiment. These results demonstrate that a global, low‐resolution general circulation model forced with surface fluxes of heat and freshwater can accurately reproduce the evolution of the upper ocean thermal structure and provide a useful tool for the analysis of air‐sea interaction and climate variability on seasonal to interannual timescales.
Modes and mechanisms of ocean color variability in the Santa Barbara Channel
Authors: Toole Dierdre A. , Siegel David A.
Published: February 2004

Characterizing the substances, processes, and mechanisms that regulate ocean color variability is crucial for assessing marine resources and impacts of land‐ocean interactions with remote sensing data. Bimonthly optical and in situ water column observations from a 3‐year field program within the Santa Barbara Channel, California, are used to assess sources of ocean color variability in a semiarid coastal region. Correlation analyses demonstrate that remote sensing reflectance variability is tightly coupled to biologically and terrestrially derived particles throughout the visible spectrum. An empirical orthogonal function analysis indicates that nearly two thirds of the observed variability in remote sensing reflectance is contained in a backscattering mode, whereas phytoplankton‐driven absorption processes contribute only ∼30% of the observed variance. Observations from the Santa Clara River outflow during a period of high discharge demonstrate that under extreme conditions, ocean color spectra are regulated almost entirely by backscattering processes. A mechanistic partitioning of ocean color variability confirms the dominance of backscattering processes and a strong coupling between the roles of backscattering and absorption at this site. A similar analysis of data from the Sargasso Sea demonstrates a lack of coupling between the role of absorption and backscattering. Application of the Seaviewing Wide Field‐of‐view Sensor (SeaWiFS) operational chlorophyll algorithm highlights the degraded predictive power of band ratio algorithms in coastal settings such as the Santa Barbara Channel. For this case II environment, backscattering is the dominant driver of ocean color variability and must be correctly incorporated into future ocean color modeling efforts.
Numerical simulation of the transient response of the Kuroshio leading to the large meander formation south of Japan
Authors: Endoh Takahiro , Hibiya Toshiyuki
Published: February 2004

Using a three‐dimensional, primitive equation numerical model that takes realistic topography into account, we successfully reproduce the observed transient response of the Kuroshio south of Japan during the transition from the nonlarge meander path to the large meander path. The transient response is triggered by the generation of what is called the “trigger meander” off the southeastern coast of Kyushu resulting from the supply of cyclonic vorticity through vertical stretching caused by the interaction between the Kuroshio and the anticyclonic mesoscale eddy approaching the Tokara Strait. The trigger meander thus generated propagates eastward south of Shikoku while inducing an anticyclone‐cyclone pair in the lower ocean. After the trigger meander passes Cape Shiono‐misaki it slows down and rapidly amplifies so that the Kuroshio loops back west of the Izu‐Ogasawara Ridge. Then the sharpness of the meander trough gradually relaxes, and the large meander path is attained. During the rapid amplification of the trigger meander off Cape Shiono‐misaki the abyssal anticyclone develops while being trapped by the local topographic feature, Koshu Seamount, located ∼200 km to the south of Cape Shiono‐misaki. This abyssal anticyclone plays a crucial role in intensifying the trigger meander trough in the upper ocean via cross‐frontal advection; the intensified trigger meander trough then further amplifies the abyssal anticyclone over Koshu Seamount. This joint evolution of the upper ocean meander trough and the abyssal anticyclone suggests that baroclinic instability enhanced by Koshu Seamount is the dominant mechanism for the rapid amplification of the trigger meander leading to the large meander formation south of Japan.
Effects of wind variability on scatterometry at low wind speeds
Authors: Plant William J.
Published: February 2004

The dependence of the normalized radar cross section of the sea on wind variability within the resolution cell is examined by considering probability distributions of cross sections and wind vectors. If a threshold wind speed exists below which backscatter is negligible for steady winds, variability of the wind over the resolution cell is shown to cause significant backscatter at mean wind speeds below the threshold. In fact, if the variability is sufficiently high, cross sections become essentially constant at very low wind speeds. The viability of this model is tested by comparing its predictions based on the NASA scatterometer 2 (NSCAT2) model function with probability distributions obtained from NSCAT cross sections that are collocated with buoy measurements. Both the overall probability distribution of cross sections and the probability of negative cross sections obtained from the NSCAT data are shown to be in good agreement with the predictions. A means of improving the accuracy of low wind speed scatterometer measurements is suggested when wind variability is not too high.
On the sensitivity of Southern Ocean sea ice to the surface freshwater flux: A model study
Authors: Marsland S. J. , Wolff J.‐O.
Published: February 2004

The Hamburg Ocean Primitive Equation model is used to study the response of the Southern Ocean's vertical stability and sea ice cover to variations in the prescribed surface freshwater flux (SFWF). The model is used to investigate the response of the coupled ocean‐sea ice system to a number of SFWF climatologies and to changes in the mean surface air temperature of the Southern Hemisphere. The modeled sea ice cover is very sensitive to the SFWF. In particular, a large‐scale open ocean polynya develops in the Weddell Sea when the SFWF in that region falls below a critical value of ∼35 cm yr−1. In terms of the oceanic heat flux (OHF) to the base of the sea ice, decreasing the SFWF by 10 cm yr−1 has roughly the same effect as an increase of 2°C in the surface air temperature, with both of these changes acting to increase the Southern Ocean's mean annual OHF of ∼23 W m−2 by ∼10%. Coupled ocean‐atmosphere models of transient climate change due to greenhouse warming predict an increase in both surface air temperature and SFWF over the Southern Ocean sea ice zone. Because the sensitivity of the sea ice extent and volume, and of the OHF, to increasing surface air temperature is opposite to that of increasing SFWF, these effects can be expected, at least partially, to offset each other.
Circulations and eddies over the South China Sea derived from TOPEX/Poseidon altimetry
Authors: Hwang Cheinway , Chen Sung‐An
Published: February 2004

TOPEX/Poseidon (T/P) altimeter data were used to derive time‐varying circulation and eddies over the South China Sea (SCS) for 1993–1999. Large variabilities of sea surface heights and dense distribution of eddies were found to occur along a band across the SCS basin. A cross‐over method is introduced to compute pointwise velocity components. The T/P‐derived circulations and eddies are consistent with the drifter results from the World Ocean Circulation Experiment data center. (1) Because of monsoons, the SCS circulation is largely cyclonic in winter, is largely anticyclonic in summer, and is modulated interannually by El Niño – Southern Oscillation. (2) Both cold‐ and warm‐core eddies were found in the waters east of Vietnam, west of Luzon, and west of the Luzon Strait, but their times of occurrence and the way they evolved vary interannually. (3) The reversal of the alongshore currents east of Vietnam was in complete accordance with wind stress. The locations and kinematic properties of eddies over SCS were estimated by the least squares method. The averaged vorticities of the cold‐ and warm‐core eddies are 1.684×10−6 and −1.738×10−6 rad s−1, respectively, and the shearing and stretching deformations are one thousandth of the vorticities. The angular velocity of eddy decreases with increasing radius. Coherence analysis suggests that the interannual and seasonal variations of circulation are largely due to wind stress, and the semi‐annual variation is largely due to wind stress curl. The angular velocities of eddies over the central SCS basin are coherent with the magnitudes and signs of wind stress curl. The seasonal steric anomaly was also computed, and its spatial pattern and scale are quite different from those of circulation and eddy.
Meridional overturning and large‐scale circulation of the Indian Ocean
Authors: Ganachaud Alexandre , Wunsch Carl , Marotzke Jochem , Toole John
Published: February 2004

The large scale Indian Ocean circulation is estimated from a global hydrographic inverse geostrophic box model with a focus on the meridional overturning circulation (MOC). The global model is based on selected recent World Ocean Circulation Experiment (WOCE) sections which in the Indian Basin consist of zonal sections at 32°S, 20°S and 8°S, and a section between Bali and Australia from the Java‐Australia Dynamic Experiment (JADE). The circulation is required to conserve mass, salinity, heat, silica and “PO” (170PO4+O2). Near‐conservation is imposed within layers bounded by neutral surfaces, while permitting advective and diffusive exchanges between the layers. Conceptually, the derived circulation is an estimate of the average circulation for the period 1987–1995. A deep inflow into the Indian Basin of 11±4 Sv is found, which is in the lower range of previous estimates, but consistent with conservation requirements and the global data set. The Indonesian Throughflow (ITF) is estimated at 15±5 Sv. The flow in the Mozambique Channel is of the same magnitude, implying a weak net flow between Madagascar and Australia. A net evaporation of −0.6±0.4 Sv is found between 32°S and 8°S, consistent with independent estimates. No net heat gain is found over the Indian Basin (0.1 ± 0.2PW north of 32°S) as a consequence of the large warm water influx from the ITF. Through the use of anomaly equations, the average dianeutral upwelling and diffusion between the sections are required and resolved, with values in the range 1–3×10−5 cm s−1 for the upwelling and 2–10 cm2 s−1 for the diffusivity.
On the solar wind control of Pc5 ULF pulsation power at mid‐latitudes: Implications for MeV electron acceleration in the outer radiation belt
Authors: Mathie R. A. , Mann I. R.
Published: January 2004

On the basis of observations made at six sites (L shell range 3.75–6.79) belonging to the International Monitor for Auroral Geomagnetic Events (IMAGE) magnetometer array, we provide a statistical study of the relationship of solar wind speed to daily ULF pulsation power for 1995, 1996, and 1997, an interval corresponding to the declining phase of solar cycle 22. We find a very strong positive correlation between solar wind speed and dayside magnetospheric pulsation power across the entire L shell range studied. In addition, we identify a number of geomagnetic storm onset days on the basis of Dst variations and demonstrate that on these days large‐amplitude ULF pulsation activity can be driven during only moderate solar wind speed conditions. Removing the onset day population improves the correlation of pulsation power to solar wind speed and demonstrates how an improved solar wind speed proxy based on pulsation power can be determined by considering Dst. We also demonstrate an exponential decay of 1–10 mHz Pc5 wave power with decreasing L shell, the decay rate increasing with solar wind speed, thus indicating a stronger dependence of pulsation power on solar wind speed at higher L shells in the range L=3.75–6.79. Further, the L shell range studied is consistent with that over which storm‐time enhancements of MeV electron fluxes have been observed in the outer radiation belt, and we discuss the implications of the dependence of PcS ULF wave power on both solar wind speed and L shell for mechanisms which implicate large‐amplitude ULF pulsations in the acceleration of storm‐time MeV energy electrons.
Coexistence of ionospheric positive and negative storm phases under northern winter conditions: A case study
Authors: Lu G. , Richmond A. D. , Roble R. G. , Emery B. A.
Published: January 2004

The response of the thermosphere and ionosphere to the famous January 10, 1997, geomagnetic storm is simulated using the thermosphere‐ionosphere‐electrodynamics general circulation model with realistic, time‐dependent distributions of ionospheric convection and auroral precipitation as inputs. The simulation results show a dominant positive storm phase of increased F layer electron density over much of the northern winter hemisphere, but a negative storm phase with reduced electron density at middle and low latitudes is also evident in the simulation. The coexistence of both positive and negative storm phases is a result of the complex dynamical and chemical interactions between charged particles and neutral gases. The impulsive magnetospheric energy inputs via auroral precipitation and Joule heating generate traveling atmospheric and ionospheric disturbances (TADs and TIDs) which propagate from the northern auroral zone to lower latitudes and penetrate well into the Southern Hemisphere. The simulation results demonstrate that positive storm phases are caused primarily by enhanced auroral precipitation over high latitudes and by TIDs at middle and low latitudes. Globally speaking, composition changes in terms of enhancements in the N2/O ratio are mainly responsible for negative storm effects. However, although there is some correlation between increases in N2/O and decreases in the F layer critical frequency ƒoF2 in the winter hemisphere during the storm main phase and early recovery phase, the overall changes in ƒoF2 are also determined by other processes, such as the ionization production associated with enhanced auroral precipitation and the variations associated with TIDs. In the low to middle‐latitude region changes in ƒoF2 approximately anticorrelate with changes at the height of the F layer electron density peak (e.g., hmF2) at 70°W during the storm main phase as well as its early recovery phase. This is attributed in part to the relation that exists between meridional wind velocity and vertical shear of that velocity for aurorally produced TADs.
Hot carbon densities in the exosphere of Mars
Authors: Nagy Andrew F. , Liemohn Michael W. , Fox J. L. , Kim Jhoon
Published: January 2004

Theoretical results of hot carbon densities in the exosphere of Mars are presented. The calculation is a two‐step process: First a two‐stream transport code is used to solve for the distribution function at the exobase, and then these results are used in a Liouville equation solution above the exobase. It is found that photodissociation of carbon monoxide is the largest source of hot carbon atoms in the upper atmosphere of Mars, larger than dissociative recombination of CO+ and much larger than the creation of hot carbon through collisions with hot oxygen atoms. It is also found that the high solar activity densities are about an order of magnitude larger than those for the low solar activity case.
Energetic ion observations in the middle Jovian magnetosphere
Authors: Cohen C. M. S. , Stone E. C. , Selesnick R. S.
Published: January 2004

The middle magnetosphere of Jupiter has not been well studied prior to the Galileo mission. This region (10–30 RJ) has been traversed by Galileo numerous times allowing spatial and temporal features to be differentiated. We present observations of energetic ions (E ∼4.5–18.4 MeV nucleon−1) in the middle Jovian magnetosphere obtained with the Heavy Ion Counter (HIC) on Galileo. The radial dependence of the intensity, spectrum, and composition of these particles indicates that solar material dominates the 15 to 30 RJ region and diffuses without loss toward Jupiter. Near 10 RJ the composition is more Iogenic and shows evidence of strong losses. These observations can be understood in terms of a common energy per charge spectrum for all heavy ions. Due to the substantially different charge to mass ratios of the Iogenic and solar ions, the O/C and S/C abundance ratios vary with magnetic moment and therefore radial distance when observed over the fixed energy interval of the HIC instrument. Near 25 RJ there is evidence of acceleration of ions to MeV nucleon−1 energies. The location of this acceleration has been suggested previously but not directly observed before the Galileo mission.
Statistical and case studies of 2fce auroral roar observed with a medium frequency interferometer
Authors: Hughes J. M. , LaBelle J. , Watermann J.
Published: January 2004

The medium frequency interferometer (MFI) is the first direction‐sensitive instrument to observe auroral radio emissions in the frequency range 2500–3000 kHz. The MFI has observed many auroral roar events since operations began at the Sondrestrom Research Facility near Kangerlussuaq, Greenland, in April 1999. The MFI data reveal that individual auroral roar events often propagate to the MFI from multiple directions and that the source locations show interesting time‐variable features such as drifting and oscillations with a period of a few minutes in both azimuth and zenith angles. Most events have a direction of arrival from the local magnetic south, with the latitude distribution of the apparent source regions peaked near the poleward edge of the statistical auroral oval. The altitude distribution of apparent source locations has a bimodal form, which may be interpreted as evidence for two distinct well‐separated sources, one on the topside F region and one on the bottomside.
Coherence of charged particle oscillations in the heliosphere (ƒ ≈ 5 μHz): Implications for a solar modulation source
Authors: Thomson D. J. , Lanzerotti L. J. , Maclennan C. G. , Heber B. , Kunow H. , Gold R. E.
Published: January 2004

Time series analysis of low‐energy electron and high‐energy proton cosmic ray fluxes, both measured on the Ulysses spacecraft, shows that they are quite coherent in some frequency bands in the energy range 0–6 μHz, especially near 5 μHz. Interplanetary particle fluxes with very different kinematic properties (e.g., gyrofrequencies) are modulated coherently in the heliosphere at discrete frequencies; one specific example discussed here has a period of ∼2.2 days. The analyzed data span the interval January 1, 1993, to April 20, 1994, when Ulysses traveled between 22° and 60° south heliographic latitude and was ∼4 AU from the Sun. Frequencies with high coherence appear to be associated either with solar rotation or with discrete solar modal frequencies. The frequency at 5.26 μHz is tentatively identified as a zonal harmonic mode withl= 2 andm= 0.
Collisionless magnetic reconnection in a three‐dimensional open system
Authors: Pritchett P. L.
Published: January 2004

Two‐dimensional (2‐D) simulations of collisionless magnetic reconnection have demonstrated the importance of the Hall term for the structure of the diffusion region and the existence of a very thin (∼ c/ωpe) electron current layer and sharp density/pressure gradients on c/ωpi scales. The present work explores the effects of the third dimension (along the direction of the equilibrium current) and of an open geometry along the magnetic field direction on these 2‐D structures within a fully kinetic treatment based on particle‐in‐cell simulations. The thin electron current layer is found to remain a 2‐D structure. The electric field components Ez and E‖ in the diffusion region each possess a complex structure on the c/ωpi and c/ωpe spatial scales. The Ez fields are needed to maintain pressure balance on the ion and electron species in the current sheet, while E‖ results in nearly Alfvénic (for the ions) and supra‐Alfvénic (for the electrons) flows out of the diffusion region. The Hall‐generated quadrupolar By field extends to large distances away from the neutral line and is a characteristic signature of collisionless reconnection. No evidence is found for the development of small‐scale turbulent modes; the electron and ion flows are highly ballistic, and the currents and density remain sharply defined. The outflow region, however, can become unstable to an ideal pressure gradient instability of relatively long wavelength (kyw ∼ 0.25, where w is the half thickness of the current sheet) with the character of a kink or interchange mode. Attempts to induce localized reconnection by imposing a localized (in y) convection electric field at the asymptotic (lobe) boundaries do not alter appreciably the basic 2‐D configuration.
Ion acoustic HF radar echoes at high latitudes and far ranges
Authors: Lacroix P. J. , Moorcroft D. R.
Published: January 2004

Using data taken over 18 months with the Iceland East (CUT‐LASS/Iceland) Super Dual Auroral Radar Network (SuperDARN) HF radar we have made a statistical study of a class of echoes which occur at ranges typically associated with F region echoes, but which have Doppler speeds near the ion acoustic speed Cs typical of E region echoes [Milan et al., 1997]. Comparison of the seasonal, diurnal, and range distributions of these echoes with the predictions of propagation models show that these are, indeed, E region echoes, differing in morphology from similar echoes at nearer ranges mainly because of the propagation conditions which are required to observe them. For the particular radar geometry of this study, conventional theory predicts that the effects of ionospheric gradients will result in phase velocities (radar Doppler velocities) which differ significantly from Cs. in disagreement with these observations. However, the observations are consistent with a new nonlinear theory of St.‐Maurice and Hamza [2001].
New evidence for the origin of giant pulsations
Authors: Thompson S. M. , Kivelson M. G.
Published: January 2004

We report on an extended interval of giant pulsation activity in the magnetosphere observed at multiple ground stations and in geosynchronous spacecraft particle data. The pulsations display wave properties typical of previously reported giant pulsations, including reversal of phase across the latitude of maximum amplitude as expected for field line resonances, comparable H (geomagnetic northward) and D (geomagnetic eastward) component amplitudes, and strong localization in latitude to within a few degrees, The recording of pulsation activity at numerous ground sites provides an opportunity to examine the local time dependence of the frequency of giant pulsations. We find that throughout a long‐lasting event, the frequencies of giant pulsations are fixed at particular local times but decrease systematically as local time increases between midnight and noon. Geosynchronous proton fluxes between 100 and 150 keV oscillated at the same frequency as pulsations observed on the ground in the same local time sector. Nearly conjugate observations indicate the observed giant pulsations have odd spatial symmetry in the magnetosphere. Calculations of the azimuthal wave number using three Northern Hemisphere stations and one Southern Hemisphere station also suggest the observed giant pulsations have odd symmetry. Numerical calculations of resonant particle energies and the combined ground and spacecraft observations presented support the view that giant pulsations have odd spatial symmetry and are driven by a population of particles in drift resonance.
Anisotropic magnetosheath: Comparison of theory with Wind observations near the stagnation streamline
Authors: Farrugia C. J. , Erkaev N. V. , Vogl D. F. , Biernat H. K. , Oieroset M. , Lin R. P. , Lepping R. P.
Published: January 2004

We carry out a first comparison with spacecraft measurements of our recent three‐dimensional, one‐fluid magnetohydrodynamic (MHD) model for the anisotropic magnetosheath [Erkaev et al., 1999], using data acquired by the Wind spacecraft on an inbound magnetosheath pass on December 24, 1994. The spacecraft trajectory was very close to the stagnation streamline, being displaced by less than 1/2 hour from noon and passing at low southern magnetic latitudes (∼4.5°). All quantities downstream of the bow shock are obtained by solving the Rankine‐Hugoniot equations taking the pressure anisotropy into account. In this application of our model we close the MHD equations by a “bounded anisotropy” ansatz using for this purpose the inverse correlation between the proton temperature anisotropy, Ap (≡ Tp⊥/Tp‖− 1), and the proton plasma beta parallel to the magnetic field βp‖ observed on this pass when conditions are steady. In the model the total perpendicular pressure is prescribed and not obtained self‐consistently. For all quantities studied we find very good agreement between the predicted and the observed profiles, indicating that the bounded anisotropy method of closing the magnetosheath equations, first suggested by Denton et al. [1994], is valid and reflects the physics of the magnetosheath well. We assess how sensitive our model results are to different parameters in the Ap = α0βp‖−a1 (a1 > 0) relation, taking for a1 the two limiting values (0.4, 0.5) resulting from the two‐dimensional hybrid simulations of Gary et al. [1997], and varying a0 in the range 0.6 – 0.8. Input solar wind conditions are as measured on this pass. In general, the model profiles depend more strongly on a0 than on a1. In particular, decreasing a0 narrows the width of the plasma depletion layer (PDL) and widens the mirror stable region. For the lowest value of a0, the mirror stable region extends sunward of the outer edge of the PDL. For the other two values of a0, and regardless of the value of a1, it is contained within the PDL. Finally, we also study phenomenological double‐poly tropic laws and find poly tropic indices γ⊥ ≈ 1 and γ‖ ≈ 1.5. These results agree well with those of Hau et al. [1993] inferred from Active Magnetospheric Particle Tracer Explorers/Ion Release Module data on a crossing of the near‐subsolar magnetosheath.
Acoustic wave heating of the thermosphere
Authors: Hickey M. P. , Schubert G. , Walterscheid R. L.
Published: January 2004

A numerical model is used to study the dissipation in the thermosphere of upward propagating acoustic waves. Whereas dissipating gravity waves can cool the upper atmosphere through the effects of sensible heat flux divergence, it is found that acoustic waves mainly heat the thermosphere by viscous dissipation. Though the amplitudes of acoustic waves in the atmosphere are poorly constrained, the calculations suggest that dissipating acoustic waves can locally heat the thermosphere at rates of tens of kelvins per day and thereby contribute to the thermospheric energy balance. It is shown that viscous heating cannot be calculated from the divergence of the wave mechanical energy flux. Acoustic waves that are barely detectable at mesopause heights can become significant heaters of the atmosphere high in the thermosphere. We suggest that acoustic waves might be responsible for heating the equatorial F region to produce the hot spot observed in the O I 630 nm airglow over the Andes Mountains.
A transitory corotating stream, a short‐lived coronal hole, and related magnetic fields
Authors: Burlaga L. F. , Harvey K. L. , Sheeley N. R.
Published: January 2004

We discuss some implications of the finite lifetime of a short‐lived near‐equatorial coronal hole for the structure of a corresponding transitory corotating stream and magnetic field in the heliosphere. The discussion is based on the illustrative example of a corotating stream that was observed by the Wind spacecraft in the ecliptic at 1 AU in early 1998 and the associated solar observations. The transitory corotating stream was associated with a near‐equatorial coronal hole that formed, persisted for at least two solar rotations, and disappeared, all in response to the magnetic flux in evolving active regions. The corotating stream recurred for two solar rotations, and its emission time, based on the coronal hole observations, was ≈66 ± 6 days. The intersection of the resulting corotating stream with the ecliptic plane has the form of a spiral‐shaped object with a radial extent on the order of 19 ± 2 AU. The magnetic field configurations in transitory corotating streams are not known. One possibility is that the transitory corotating stream originates from a localized near‐equatorial coronal hole, possibly a member of a pair of coronal holes, joined by a flux tube, which evolve together. Such a transitory corotating stream could carry the flux tube through the corona and heliosphere. Another possibility is that the transitory corotating stream is the result of the latitudinal motion and deformation of the boundary of a polar coronal hole and the associated redistribution of a constant amount of open magnetic flux. Models and further observations of transitory corotating streams and their magnetic fields are needed in order to better understand them.
Effects of inner magnetospheric convection on ring current dynamics: March 10–12, 1998
Authors: Jordanova V. K. , Kistler L. M. , Farrugia C. J. , Torbert R. B.
Published: January 2004

A stream‐stream interaction region observed by the Wind spacecraft from ∼1200 UT to ∼2400 UT, March 10, 1998, triggered a major geomagnetic storm which peaked within a few hours at Dst≈−130 nT and Kp=7+. During the main phase of this storm the north–south component of the interplanetary magnetic field (IMF) was large and negative (Bz≈−l5 nT), the solar wind velocity was ∼550 km/s, and the solar wind dynamic pressure increased to ∼10 nPa. We simulated the storm time injection and trapping of H+, O+, and He+ ring current ions using our global drift‐loss model with initial and boundary conditions as specified by measurements from the Equator‐S ion composition (ESIC) instrument, the HYDRA instrument on Polar, and the hot plasma instruments on geosynchronous spacecraft. We demonstrated effects of magnetospheric convection, comparing results derived from two inner magnetospheric convection models: (1) the 3 hour averaged Kp‐dependent Volland‐Stern model and (2) the Weimer [1996] IMF‐driven model, where we input interplanetary data from the Magnetic Field Investigation (MFI) and Solar Wind Experiment (SWE) instruments on Wind at 30 min resolution. During the main phase of the storm the Volland‐Stern model predicted a large‐scale electric field of ∼1 mV/m at L=2 to L=5, whereas the Weimer model predicted a maximum electric field of ∼3.5 mV/m localized near dusk at L≈3. We found that both ring current simulations show reasonable agreement with ESIC and HYDRA data at larger L shells and on the nightside. However, the simulation using a Volland‐Stern model predicted wider dips in the ion energy spectra than observed at low L shells in the postnoon local time sector. Ions followed paths at larger distances from Earth and experienced less collisional losses in the Weimer convection model. As a result, the agreement between model and data was significantly improved on the dayside when the IMF‐driven convection model of Weimer was used.
A tale of two theories: How the adiabatic response and ULF waves affect relativistic electrons
Authors: Green J. C. , Kivelson M. G.
Published: January 2004

Using data from the Comprehensive Energetic Particle and Pitch Angle Distribution (CEPPAD)‐High Sensitivity Telescope (HIST) instrument on the Polar spacecraft and ground magnetometer data from the 210 meridian magnetometer chain, we test the ULF wave drift resonance theory proposed to explain relativistic electron phase space density enhancements. We begin by investigating changes in electron flux due to the “Dst effect”. The Dst effect refers to the adiabatic response of relativistic electrons to changes in the magnetic field characterized by the Dst index. The Dst effect, assuming no loss or addition of new electrons, produces reversible order of magnitude changes in relativistic electrons flux measured at fixed energy, but it cannot account for the flux enhancement that occurs in the recovery phase of most storms. Liouville's theorem states that phase space density expressed in terms of constant adiabatic invariants is unaffected by adiabatic field changes and thus is insensitive to the Dst effect. It is therefore useful to express flux measurements in terms of phase space densities at constant first, second and third adiabatic invariants. The phase space density is determined from the CEPPAD‐HIST electron detector that measures differential directional flux of electrons from 0.7 to 9 MeV and the Tsyganenko 96 field model. The analysis is done for January to June 1997. The ULF wave drift resonance theory that we test proposes that relativistic electrons are accelerated by an m=2 toroidal or poloidal mode wave whose frequency equals the drift frequency of the electron. The theory is tested by comparing the relativistic electron phase space densities to wave power determined at three ground stations with L* values of 4.0, 5.7 and 6.2. Comparison of the wave data to the phase space densities shows that five out of nine storm events are consistent with the ULF wave drift resonance mechanism, three out of nine give ambiguous support to the model, and one event has high ULF wave power at the drift frequency of the electrons but no corresponding phase space density enhancement suggesting that ULF wave power alone is not sufficient to cause an electron response. Two explanations of the anomalous event are investigated including excessive loss of electrons to the magnetopause and wave duration.
Modeling diurnal tidal variability with the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model
Authors: Hagan M. E. , Roble R. G.
Published: January 2004

We used the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) to calculate the variability of the migrating diurnal tide in a January through December 1993 simulation. While TIME‐GCM captures the salient features of the latitudinal, altitudinal, and seasonal variability of the migrating diurnal tide, upper mesospheric meridional wind amplitudes are somewhat smaller than those that have been observed from the ground and space. The discrepancies may be attributable to unresolved uncertainties in tidal forcing and/or dissipation in the TIME‐GCM. However, our diagnostic simulation suggests that the nonlinear interactions between the migrating diurnal tide and stationary planetary wave 1 produce measurable nonmigrating diurnal tidal components that modulate migrating diurnal tidal amplitudes and account for significant variability in the upper mesosphere and lower thermosphere.
Lightning‐induced energetic electron flux enhancements in the drift loss cone
Authors: Blake J. B. , Inan U. S. , Walt M. , Bell T. F. , Bortnik J. , Chenette D. L. , Christian H. J.
Published: January 2004

With its relatively low altitude (520 × 670 km) orbit, SAMPEX is mostly below the stable trapping region where charged particles repeatedly drift around the Earth, especially at midlatitudes. Recent analyses of SAMPEX data have revealed a surprisingly common set of observations of enhanced energetic (>150 keV) electron fluxes at L < 3, during times when SAMPEX was located such that any electrons that it observed were in the drift loss cone (and were thus destined to be precipitated upon reaching the longitude of the South Atlantic Magnetic Anomaly in the course of their eastward drift). The data were acquired with the Heavy Ion Large Telescope on board SAMPEX, which provides high time resolution measurements (30 ms sample rate) of electrons above energy thresholds of 150 keV and 1 MeV. Preliminary examination of 1995 SAMPEX data (when the >150 keV detector was available) revealed hundreds of cases of newly enhanced drift loss cone fluxes in localized L shell regions often associated with individual thunderstorms. In one case, SAMPEX data from three consecutive days (95/196, 95/197, and 95/198) were analyzed as the satellite proceeded northbound over the same ground track from west of New Zealand and Hawaii toward Alaska, with the underlying lightning activity documented by the Optical Transient Detector on board the OrbView‐1 satellite (750 km, 70° inclination circular orbit). Enhanced fluxes observed on SAMPEX during day 95/197 were directly associated with an oceanic storm just to the west of the SAMPEX ground track, which was well placed to generate the observed drift loss cone flux enhancements. The drift loss cone electron flux enhancements were also observed 20 min later as SAMPEX crossed the same L shells in the north and in subsequent orbits, indicating that lightning‐induced precipitation of electrons into the drift loss cone persisted at least for a few hours. Data from UARS satellite, passing through the same region within the same hour, also confirmed the presence of L‐dependent structure and further allowed the determination of the electron energy spectra, which exhibited a general shape and range strikingly similar to previously documented spectral characteristics of lightning‐induced electron precipitation (LEP) events in the bounce loss cone [Voss et al., 1998]. This similarity lends support to the argument that the observed drift loss cone features are produced by the LEP process. In summary, SAMPEX data indicate that globally distributed thunderstorms may continually precipitate energetic electrons from the radiation belts, producing transient enhancements in the drift loss cone that are detected within the few hour periods as they drift around the Earth and precipitate in the South Atlantic Magnetic Anomaly.
Solar activity variations of the Venus thermosphere/ionosphere
Authors: Fox J. L. , Sung K. Y.
Published: January 2004

We present models of the low and high solar activity thermospheres and ionospheres of Venus for a background atmosphere based largely on the VTS3 model of Hedin et al. [1983]. Our background model consists of 12 neutral species, and we compute the density profiles of 13 ions and 7 minor neutrals. We find that the peak production rates of some ions, such as CO2+ and N2+, vary approximately as the solar flux and that some, whose parent neutrals are photochemically produced, such as O+, N+, and C+, show variations that are amplified over that of the solar flux. The solar cycle variation of the O2+ density at its peak is about a factor of 1.6, in good agreement with the radio occultation measurements and previous models. The peak density of N2+ varies by a factor of ∼ 2, but that of CO+ varies by a larger factor because the mixing ratio of CO is also correlated with solar activity. The atomic ions O+, N+, and C+, which peak at high altitudes, exhibit larger density enhancements at high solar activity of factors of 5–18. Thus there is a solar cycle variation in the overall composition of the ionosphere, with a relatively larger proportion of atomic ions at high solar activity. In both measurements and models the solar activity variation of the electron density profile is altitude‐dependent, with variations of only ∼60% near the peak but up to an order of magnitude near 300 km. The high solar activity electron density profile exhibits an F2 “shoulder,” which is rarely seen in the radio occultation data and may indicate that the VTS3 atomic O mixing ratios are too large at high solar activity. We also discuss the solar activity variations of N, NO, C, O(1D), and O(1S).