bluuebunny/crossref_metadata_2025 huggingface dataset:
Cloud transitions and decoupling in shear‐free stratocumulus‐topped boundary layers
Authors: Stevens Bjorn
Published: February 2004

Large‐eddy simulations of shear‐free stratocumulus‐topped layers tend strongly toward a two‐layer structure as the negative area in the buoyancy flux profile that would be necessary to maintain a mixed layer increases to more than 10% of the positive area. In addition to identifying a threshold for what amounts to a convective transition, the simulations provide further evidence that a commonly used entrainment closure is ill‐founded. Lastly the simulations demonstrate that attempts to formulate a similarity theory for convective layers must account for the geometry of the buoyancy‐flux profile, and not simply its integral.
Detailed distribution of microearthquakes along the northern Reykjanes Ridge, off SW‐Iceland
Authors: Mochizuki M. , Brandsdóttir B. , Shiobara H. , Gudmundsson G. , Stefánsson R. , Shimamura H.
Published: February 2004

We have obtained a snapshot image of the earthquake activity along the northern Reykjanes Ridge (RR) using a dense ocean bottom seismometer (OBS) array. The earthquakes are narrowly concentrated along the NE‐SW trending ridge crest. The seismicity is segmented into clusters by the en‐echelon NNE‐SSW trending axial volcanic systems which characterize the obliquely spreading ridge. The hypocenters are concentrated at somewhat greater depths (3–7 km) than on the Reykjanes Peninsula (RP), where the crust is thicker. Normal faulting dominates at the ridge crest. The focal planes are oriented more NE‐SW than NNE‐SSW, i.e., parallel to the ridge crest and its peripheral faults rather than the trend of individual axial volcanic systems and associated faults.
Interdecadal changes in eastern Pacific ITCZ variability and its influence on the Atlantic ITCZ
Authors: Chiang John C. H. , Kushnir Yochanan , Zebiak Stephen E.
Published: February 2004

The eastern Pacific and Atlantic Intertropical Convergence Zones (ITCZ) exhibit the largest year‐to‐year variations in boreal spring. We show evidence suggesting that Atlantic ITCZ April–May variability is linked to that for the eastern Pacific through the Walker circulation as it respond to changes in equatorial Pacific convection. Analysis of ITCZ proxy indices shows the link appears to be strong in the 1980–90's and 1920–30's but virtually nonexistent in the 1950–60's. We argue that this apparent nonstationarity results from the nonlinear relationship between sea surface temperature (SST) and convection in the eastern equatorial Pacific and its consequent effect on the Walker circulation and the Atlantic ITCZ. This mechanism was modulated over 1856–1998 by interdecadal changes in peak SST attained in the eastern equatorial Pacific during boreal spring.
Upper limit for the UV absorption cross sections of H2SO4
Authors: Burkholder James B. , Mills Mike , McKeen Stuart
Published: February 2004

An upper limit for the gas phase absorption cross sections for H2SO4 over the wavelength range 195 to 330 nm has been determined. Absorption measurements were made using a diode array spectrometer and a 100 cm long heated absorption cell. Gas phase H2SO4 was generated in a gas flow by titrating (equilibrating) known amounts of SO3 via the reaction at elevated temperatures, ∼ 500 K. Upper limits to the H2SO4 absorption cross sections were obtained, <1 × 10−21 cm² molecule−1, and provide an upper limit to the atmospheric photolysis rate of H2SO4. The implications of this cross section limit to stratospheric sulfur chemistry are discussed.
A tropical Atlantic Paradox: Shipboard and satellite views of a tropospheric ozone maximum and wave‐one in January–February 1999
Authors: Thompson Anne M. , Doddridge Bruce G. , Witte Jacquelyn C. , Hudson Robert D. , Luke Winston T. , Johnson James E. , Johnson Bryan J. , Oltmans Samuel J. , Weller Rolf
Published: February 2004

During the Aerosols99 trans‐Atlantic cruise from Norfolk, VA, to Cape Town, South Africa, daily ozonesondes were launched from the R/V Ronald H Brown between 17 January and 6 February 1999. A composite of tropospheric ozone profiles along the latitudinal transect shows 4 zones, nearly identical to the ozone distribution during a January‐February 1993 trans‐Atlantic cruise [Weller et al., 1996]. Sondes from the cruise and Ascension Island (8S, 14.5W), as well as the Earth‐Probe (EP)/TOMS satellite instrument, show elevated tropospheric ozone (>35 Dobson Units) throughout the south Atlantic in January 1999. Ozone layers associated with biomass burning north of the ITCZ (Intertropical Convergence Zone) are prominent at 0–5 km from 10–0N, but even higher ozone (100 ppbv, 5–15 km) occurred south of the ITCZ, where it was not burning‐an ozone “paradox” that contributes to a wave‐one zonal pattern in tropospheric ozone. Back trajectories, satellite observations and shipboard tracers suggest that the south Atlantic ozone results from a combination of interhemispheric transport, aged stratospheric‐upper tropospheric air, and possibly from ozone supplied by lightning nitric oxide.
Abrupt shift of the ITCZ over West Africa and intra‐seasonal variability
Authors: Sultan Benjamin , Janicot Serge
Published: February 2004

The onset of the monsoon system over West Africa is linked to the northward migration of the Inter‐Tropical Convergence Zone (ITCZ) during the northern spring and summer. By using daily gridded rainfall data and NCEP/NCAR wind reanalyses over the period 1968–1990, we show that this migration is characterised by an abrupt latitudinal shift of the ITCZ in late June from a quasi‐stationary location at 5N in May–June to another quasi‐stationary location at 10N in July–August. A composite analysis based on the shift dates shows that this northward shift is associated with the occurrence of a westward‐travelling monsoon depression pattern over the Sahel with characteristic periodicities of 20–40 days.
Uniform slip‐rate along the Kunlun Fault: Implications for seismic behaviour and large‐scale tectonics
Authors: Van der Woerd J. , Ryerson F. J. , Tapponnier P. , Meriaux A. ‐S. , Gaudemer Y. , Meyer B. , Finkel R. C. , Caffee M. W. , Guoguang Zhao , Zhiqin Xu
Published: February 2004

A long‐term slip‐rate is derived from concordant 10Be, 26Al and 14C dating of cumulative offsets along much of the length of the Kunlun Fault. Values at 6 sites indicate uniform slip (11.5±2.0 mm/yr) since ∼40 kyr BP. This relatively high slip rate corresponds to a first‐order discontinuity in the Asian crustal velocity field. M∼8 and M∼7.5 earthquakes on 2 segments of the fault recur with characteristic slip (∼10±2 m and 4.4±0.4 m) every ∼850 and ∼420 yrs, respectively.
Volcanic eruption prediction: Magma chamber physics from gravity and deformation measurements
Authors: Rymer Hazel , Williams‐Jones Glyn
Published: February 2004

One of the greatest remaining problems in modern volcanology is the process by which volcanic eruptions are triggered. It is generally accepted that eruptions are preceded by magma intrusion [Sigurdsson and Sparks, 1978]. The degree of interaction between previously ponded magma in a chamber and newly intruded magma determines the nature and rate of eruption and also the chemistry of erupted lavas and shallow dykes. Here, we investigate the physics of this interaction. Volcano monitoring at its most effective is a synergy between basic science and risk assessment, while hazard mitigation depends on reliable interpretation of eruption precursors. The simple and much used Mogi model relates ground deformation (Δh) to changes in magma chamber volume. Gravity changes (Δg) combined with ground deformation provide information on magma chamber mass changes. Our new models predict how the Δg/Δh gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity. Thus by simultaneous measurement of deformation and gravity at a few key stations, magma chamber processes can be identified prior to the onset of conventional eruption precursors.
Seismic reflectivity and magmatic underplating beneath the Kenya Rift
Authors: Thybo H. , Maguire P. K. H. , Birt C. , Perchuć E.
Published: February 2004

The lower crust around the Kenya Rift is generally reflective in wide‐angle seismic sections. Remarkably, high amplitude reflections of low frequency originate from underneath the rift, whereas weaker reflections of high frequency prevail from outside the rift. This indicates thicker layering and larger reflection coefficients in the lower crust beneath the rift than outside it. Petrologically, magmatic intrusions are compatible with the thick layering beneath the rift axis, and the associated large reflection coefficients are indicative of their cumulate layering and fractionation. Hence, the observed thinning of the crust below the rift may be substantially less than the real mechanical thinning due to the addition of intrusive or underplated material.
Geodetic network optimization for geophysical parameters
Authors: Blewitt Geoffrey
Published: February 2004

The first order design problem in geodesy is generalized here, to seek the network configuration that optimizes the precision of geophysical parameters. An optimal network design that satisfies intuitively appropriate criteria corresponds to minimizing the sum of logarithmic variances of eigenparameters. This is equivalent to maximizing the determinant of the design matrix, allowing for closed‐form analysis. An equivalent expression is also given specifically for square root information filtering, to facilitate numerical solution. Appropriate seeding of numerical solutions can be provided by exact analytical solutions to idealized models. For example, for an ideal transform fault, simultaneous resolution of both the locking depth Dand location of the fault is optimized by placing stations at ±D/√3 (∼9 km) from the a priori fault plane. In a two‐fault system, the resolution of slip partitioning is optimized by including a station midway between faults; however resolution is fundamentally limited for fault separation <2D(∼30 km).
FES98: A new global tide finite element solution independent of altimetry
Authors: Lefèvre Fabien , Lyard Florent H. , Le Provost Christian
Published: February 2004

A new version of the FES hydrodynamic finite element tide model combined with a revised data assimilation procedure is now available. The CEFMO hydrodynamic model previously used to obtain the solution FES94.1 was improved and the assimilation method based on the representer approach was revised. A careful selection of in situ tide gauge data from different data banks allowed us to build a collection of about 700 data values for each of the eight computed waves (M2, S2, N2, K2, 2N2, K1, O1 and Q1), which were assimilated. Eighteen other constituents of the tide spectrum were derived by admittance and three long period waves were added. Referred to as FES98, these new solutions are available on a 0.25° × 0.25° gridded version of the full finite element solutions. This paper presents these new FES solutions and proposes a brief analysis of the improvements of the new approach with respect to the former FES94.1 and FES95.2.1 solutions.
Reversing bottom circulation in the Somali Basin
Authors: Beal Lisa M. , Molinari Robert L. , Chereskin Teresa K. , Robbins Paul E.
Published: February 2004

Two sets of direct velocity measurements were taken, concurrent with hydrographic data, in the bottom waters of the northern Somali Basin in June and September, 1995. The velocities indicate a temporal flow reversal in the bottom circulation, which is consistent with the changing density structure between the sections. In June, there is evidence of a southward Deep Western Boundary Current with a transport of 5 Sv. By September, flow close to the boundary is northward, with a transport of 2.6 Sv. Furthermore, the deep density gradient across the interior of the Somali Basin also changes between occupations, implying a cyclonic circulation in June and anticyclonic flow in September. Rossby wave activity is high in this region during the southwest monsoon, yet there is also evidence of a strong barotropic component to the Great Whirl in September, which may cause the reversal in the abyssal circulation.
Pacific interdecadal variability in this century's sea surface temperatures
Authors: Chao Yi , Ghil Michael , McWilliams James C.
Published: February 2004

Analysis of this century's sea surface temperatures over the Pacific Ocean reveals an interdecadal oscillation with a period of 15–20 years. Our results show that the well‐known 1976–77 climate regime shift is not unique, but represents one of several phase transitions associated with this interdecadal oscillation, also found around 1924–25, 1941–42, and 1957–58. The oscillation's striking north‐south symmetry across the equator implies strong interactions between tropics and extratropics. A mode with a period of approximately 70 years and an apparently different spatial pattern is also identified tentatively but has to be evaluated further using longer time series.
Variations in the tropical uplift following the Pinatubo eruption studied by infrared solar absorption spectrometry
Authors: Notholt J. , Toon G. C. , Sen B. , Jones N. B. , Rinsland C. P. , Lehmann R. , Rex M.
Published: February 2004

Infrared solar absorption spectra of atmospheric trace gases were measured during a ship cruise in October 1996 between 57°N and 45°S. The abundances of stratospheric species are compared with those derived from NASA DC‐8 aircraft spectra from spring 1992, recorded seven months after the Pinatubo eruption. We examine column abundances of HCl, HF and HNO3 to study variations in the tropical uplift, following the enhanced aerosol loading. The comparison yields a vertical shift in the mixing ratio profiles of HCl and HF of less than 1 km at 25 km altitude (Post Pinatubo versus 1996), which is of the same order of magnitude as the known seasonal, longitudinal and year‐to‐year variabilities of the tropical uplift. The results suggest that long term pertubations following the eruption, like the observed long term ozone reduction at midlatitudes, are mainly chemical in nature rather than the result of sustained dynamical pertubations.
A model estimate of cooling in the mesosphere and lower thermosphere due to the CO2 Increase over the last 3–4 decades
Authors: Akmaev R. A. , Fomichev V. I.
Published: February 2004

Long‐term observations indicate a substantial cooling in the mesosphere and lower thermosphere (MLT) over the last 3–4 decades. Available model studies have primarily considered the effects of CO2 doubling expected to occur in the future. We present a benchmark estimate of radiative forcing in the MLT due to the increase of CO2 mixing ratio from 313 ppm to about 360 ppm (or by 15%) observed over the last four decades. The Spectral Mesosphere/Lower Thermosphere Model is employed for “retrocasting” the atmospheric response. As expected, the thermal response is predominantly negative. As a function of altitude, the cooling maximizes in the mesosphere at about 3 K, practically vanishes at 100–120 km, and grows to 10–15 K in the thermosphere. Although this vertical shape is remarkably consistent with various sets of observations, the magnitude of the cooling rate is smaller by about a factor of 2–10. This suggests that other mechanisms, e.g., the ozone depletion, might have contributed substantially to the negative temperature trend.
Evolution of newly formed aerosol particles in the continental boundary layer: A case study including OH and H2SO4 measurements
Authors: Birmili W. , Wiedensohler A. , Plass‐Dülmer C. , Berresheim H.
Published: February 2004

An event of new particle formation is presented, based on simultaneous measurements of aerosol number size distributions, relevant gaseous components including H2SO4 and OH, and meteorological parameters. Measurements were conducted at Hohenpeissenberg, a rural continental mountain site in southern Germany. The event was observed under intense solar radiation, with total particle number concentrations increasing from 6000 to 25000 cm−3 within one hour, and ultrafine particles (3–11 nm) accounting for more than 50% of total number. Observed OH and H2SO4 concentrations reached maximum levels around 107 cm−3. A lower limit of the particle nucleation rate was estimated to be 3 cm−3·s−1, consistent with present models of ternary nucleation involving the H2SO4‐H2O‐NH3 system. Roughly 80% of the subsequent drop in ultrafine mode particle number concentration could be explained by coagulation. The observed particle growth rate of 2.1±0.1 nm/h was largely attributed to the condensation of measured H2SO4, assuming neutralization by ammonia.
Meteoric smoke production in the atmosphere
Authors: Kalashnikova O. , Horányi M. , Thomas G. E. , Toon O. B.
Published: February 2004

Nanometer‐sized smoke particles produced during ablation of incoming micrometeorites influence the ion chemistry in the 80–120km region and are believed to play an important role in the nucleation of mesospheric ice particles. We present a new model of cosmic dust ablation and smoke production in the Earth's atmosphere using realistic size and velocity distributions of the incoming dust particles. We show that, while the seasonal changes in our atmosphere can be safely neglected, the shape of micro‐meteoroids strongly affects their ablation.
An SST anomaly dipole in the northern subtropical Pacific and its relationships with ENSO
Authors: Yu Jin‐Yi , Liu W. Timothy , Mechoso Carlos R.
Published: February 2004

This study examines the links between tropical and subtropical sea surface temperature (SST) anomalies in the Pacific Ocean during ENSO (El Niño‐Southern Oscillation) events. A long‐term simulation by the UCLA coupled atmosphere‐ocean general circulation model is used. It is found that a zonally‐oriented SST anomaly dipole in the subtropical Pacific develops almost simultaneously with and is closely related to tropical ENSO events. The dipole is located east of the dateline between 20°N and 40°N and consists of an anomaly center off the coast of the North America and another anomaly center with opposite sign further to the west. It is demonstrated that this dipole feature is primarily driven by anomalous surface heat fluxes associated with the altered atmospheric circulation during ENSO events.
The retrieval of O3 profiles from limb scatter measurements: Results from the Shuttle Ozone Limb Sounding Experiment
Authors: McPeters Richard D. , Janz Scott J. , Hilsenrath Ernest , Brown Tammy L. , Flittner David E. , Heath Donald F.
Published: February 2004

Two instruments were flown on Shuttle flight STS‐87 to test a new technique for inferring the ozone vertical profile using measurements of scattered sunlight from the Earth's limb. The instruments were an ultraviolet imaging spectrometer designed to measure ozone between 30 and 50 km, and a multi‐filter imaging photometer that uses 600 nm radiances to measure ozone between 15 km and 35 km. Two orbits of limb data were obtained on December 2, 1997. For the scans analyzed, the ozone profile was measured from 15 km to 45 km with approximately 3 km vertical resolution. Comparisons with a profile from an ozonesonde launched from Ascension Island showed agreement mostly within ±5%. The tropopause at 15 km appears to have been detected in this comparison. A comparison with two HALOE ozone profiles showed that on average ozone measured by SOLSE was lower by 9%±5% in the 30 km to 45 km range.
O3 profiles retrieved from limb scatter measurements: Theory
Authors: Flittner D. E. , Bhartia P. K. , Herman B. M.
Published: February 2004

An algorithm is presented for retrieving vertical profiles of O3 concentration using measurements of UV and visible light scattered from the limb of the atmosphere. The UV measurements provide information about the O3 profile in the upper and middle stratosphere, while only visible wavelengths are capable of probing the lower stratospheric O3 profile. Sensitivity to the underlying scene reflectance is greatly reduced by normalizing measurements at a tangent height high in the atmosphere (∼55 km), and relating measurements taken at lower altitudes to this normalization point. To decrease the effect of scattering by optically thin aerosols/clouds that may be present in the field of view, these normalized measurements are then combined by pairing wavelengths with strong and weak O3 absorption. Excluding pointing errors, we conclude that limb scatter can be used to measure O3 between 15 km and 50 km with 2–3 km vertical resolution and better than 10% accuracy.
Sustainability of least cost policies for meeting Mexico City's future water demand
Authors: Downs Timothy J. , Mazari‐Hiriart Marisa , Domínguez‐Mora Ramón , Suffet I. H.
Published: February 2004

Meeting future water demand without degrading ecosystems is one important indicator of sustainable development. Using simulations, we showed that compared to existing policy, more sustainable water supply options are similar or cheaper in cost. We probabilistically forecasted the Mexico City metropolitan zone population for the year 2015 to be 23.5 million and total required water supply to be 106 m3 s−1. We optimized existing and potential supply sources from aquifers, surface water, treatment/reuse, and efficiency/demand management by cost to meet future supply needs; the applied source supply limits determined the degree of sustainability. In two scenarios to supply 106 m3 s−1, the business‐as‐usual scenario (zero sustainability) had an average relative unit cost of 1.133; while for the most sustainable scenario (it includes reducing potential supply basins' exploitation limits by 50%), the value was 1.121. One extreme scenario to supply the forecast's 95% confidence value (124 m3 s−1) showed little unit cost change (1.106). The simulation shows sustainable policies can be cost‐effective.
Influence of net freshwater supply on salinity in Florida Bay
Authors: Nuttle William K. , Fourqurean James W. , Cosby Bernard J. , Zieman Joseph C. , Robblee Michael B.
Published: February 2004

An annual water budget for Florida Bay, the large, seasonally hypersaline estuary in the Everglades National Park, was constructed using physically based models and long‐term (31 years) data on salinity, hydrology, and climate. Effects of seasonal and interannual variations of the net freshwater supply (runoff plus rainfall minus evaporation) on salinity variation within the bay were also examined. Particular attention was paid to the effects of runoff, which are the focus of ambitious plans to restore and conserve the Florida Bay ecosystem. From 1965 to 1995 the annual runoff from the Everglades into the bay was less than one tenth of the annual direct rainfall onto the bay, while estimated annual evaporation slightly exceeded annual rainfall. The average net freshwater supply to the bay over a year was thus approximately zero, and interannual variations in salinity appeared to be affected primarily by interannual fluctuations in rainfall. At the annual scale, runoff apparently had little effect on the bay as a whole during this period. On a seasonal basis, variations in rainfall, evaporation, and runoff were not in phase, and the net freshwater supply to the bay varied between positive and negative values, contributing to a strong seasonal pattern in salinity, especially in regions of the bay relatively isolated from exchanges with the Gulf of Mexico and Atlantic Ocean. Changes in runoff could have a greater effect on salinity in the bay if the seasonal patterns of rainfall and evaporation and the timing of the runoff are considered. One model was also used to simulate spatial and temporal patterns of salinity responses expected to result from changes in net freshwater supply. Simulations in which runoff was increased by a factor of 2 (but with no change in spatial pattern) indicated that increased runoff will lower salinity values in eastern Florida Bay, increase the variability of salinity in the South Region, but have little effect on salinity in the Central and West Regions.
A statistical study of the magnetosphere boundary crossings by the Geotail satellite
Authors: Ivchenko N. V. , Sibeck D. G. , Takahashi K. , Kokubun S.
Published: February 2004

Geotail magnetometer observations from 1995–97 were used to identify 1369 dayside magnetospheric boundary crossings. Statistical properties of this set of multiple crossings are presented and discussed in terms of models for magnetopause motion. Our results indicate that equatorial boundary crossings are caused by north‐south aligned ridges propagating azimuthally around the magnetosphere. Perpendicular orientations of the magnetospheric and magnetosheath magnetic fields inhibit the boundary motion. For the garden hose orientations of the interplanetary magnetic field (IMF), magnetospheric boundary motion is favoured in the dawn sector, while orthospiral orientations favour motion in the dusk sector. These results suggest that pressure pulses generated at the quasi‐parallel bow shock drive the boundary motion.
Caribbean Ionosphere Campaign, year one: Airglow and plasma observations during two intense mid‐latitude spread‐F events
Authors: Kelley Michael C. , Makela Jonathan J. , Swartz Wesley E. , Collins Stephen C. , Thonnard Stefan , Aponte Nestor , Tepley Craig A.
Published: February 2004

A series of campaigns has been carried out in the Caribbean over a one‐year period to study intense mid‐latitude spread‐F events using a cluster of diversified instrumentation. These events are relatively rare but a number of them have now been captured and will be discussed in this and several companion papers. This paper focuses on 630 nm airglow images obtained by the Cornell All‐Sky Imager for two of the more spectacular cases that began on February 17, 1998 and February 17, 1999. In the latter case, and for the first time, a poleward surge of depletion/enhancement airglow zones was captured by radar as well as an airglow imager. In the former case structures grew in place overhead and produced strong VHF F‐region backscatter as observed by the CUPRI and University of Illinois radars; the other event, exactly one year later, did not result in detectable 3‐m backscatter. The two data sets show quantitatively that the low airglow region is elevated in height and depleted in plasma density and Pedersen conductivity. We suggest an enhanced eastward electric field inside the low conductivity zone may be responsible for the surge. The data also suggest small scale turbulence can only be observed in developing structures.
50 MHz radar observations of mid‐latitude E‐region irregularities at Camp Santiago, Puerto Rico
Authors: Urbina Julio , Kudeki Erhan , Franke Steven J. , Gonzalez Sixto , Zhou Qihou , Collins Stephen C.
Published: February 2004

A 50 MHz radar interferometer was used near Salinas, Puerto Rico, to probe the meter‐scale E‐region plasma density irregularities during two campaigns conducted in 1998. During the February–April period E‐region echoing layers were primarily observed between 90 and 100 km heights. The layers were typically thin (∼1 km) and unstructured, although in several cases short period (∼90 s) layer oscillations were observed. During the June–July period E‐region echoes showed more varied characteristics. In addition to low altitude layers, quasi‐periodic structures with descending echoing layers were observed at altitudes above 100 km. Zonal motions detected during descending layer events were at times variable and oscillated between westward and eastward directions while the layer descent rates remained fixed.
Shear AlfvéN waves on stretched magnetic field lines near midnight in Earth's magnetosphere
Authors: Rankin R. , Fenrich F. , Tikhonchuk V. T.
Published: February 2004

The spectrum of toroidal standing shear Alfvén waves (SAWs) is derived for a stretched magnetic field line geometry that is approximated by the Tsyganenko 96 magnetic field model. The model is applied to field lines near the midnight meridional plane. It is demonstrated that the fundamental mode frequency for typical ambient parameters of Earth's magnetosphere is in the 1–4 mHz range. This frequency is in agreement with the measured frequency of magnetospheric field line resonances, and is up to an order of magnitude smaller than for a dipolar magnetic field. We conclude that stretching of Earth's magnetic field offers a possible explanation for anomalously low frequency SAWs observed within diffuse auroral emission (Hβ) produced by energetic proton precipitation in the inner plasma sheet.
Neutral winds in the lower thermosphere observed by WINDII during the April 4–5th, 1993 storm
Authors: Zhang Shengpan P. , Shepherd Gordon G.
Published: February 2004

Data from a major geomagnetic storm in April 1993 were selected for the first WINDII study of storm‐induced neutral winds in the lower thermosphere between 90 and 200 km. The storm commenced at ∼1200 UT April 4th, when the Kp index increased from 1.3 to 7.7. The geomagnetic latitude range of WINDII measurements from the daytime O(¹S) emission reaches the southern geomagnetic pole. The wind exhibits a dramatic change during the storm. In the polar region the maximum velocity of ∼200 ms−1 at 140 km altitude on April 2nd increases to ∼650 ms−1 at 200 km on April 5th. Winds plotted in geomagnetic coordinates show the familiar two‐cell pattern in that winds are much stronger in the dusk sector than in the dawn sector. This pattern penetrates down to ∼130 km. During the recovery period on April 8th, winds at higher altitudes return to normal while the two‐cell pattern persists at lower altitudes. The thermospheric O(¹S) emission rate is severely depleted by the storm to half of its normal value.
A comparison of optical and radar measurements of mesospheric winds and tides
Authors: Greet P. A. , Murphy D. J. , Vincent R. , Dyson P. L.
Published: February 2004

Optical measurements of mesospheric winds by Fabry‐Perot spectrometers, FPSs, at Mawson, 67.6°S 62.9°E, and Davis, 68.6°S 78.0°E, Antarctica are compared with similar measurements obtained using a spaced‐antenna MF radar at Davis. The FPSs observed the OH emission. Different analysis procedures, used to determine the mean wind, and amplitude and phase of the semidiurnal tide, have been compared. At these latitudes the diurnal tide is weak and the semi‐diurnal tide, although highly variable in amplitude, is usually the dominant periodicity. When comparing the amplitude and phase of the semidiurnal tide good agreement is obtained between measurements by the two instruments.
Pi2 magnetic pulsations as response on spatio‐temporal oscillations of auroral arc current system
Authors: Solovyev S. I. , Baishev D. G. , Barkova E. S. , Molochushkin N. E. , Yumoto K.
Published: February 2004

Pi2 magnetic pulsations in the period range from 40 to 150 s are excited both at the magnetospheric substorm onset and during pseudobreakups. Using observations at the 190–210° MM we study the association between the characteristics of high‐latitude Pi2 pulsations and parameters of wave‐like and vortex auroral structures of 50–200 km scale in the brightening auroral arc during short‐term (∼5–10 min) activations. We found that the maximum Pi2 amplitude is observed at the same latitude where the auroral arc is located. The period of high‐latitude Pi2 pulsations is found to be consistent with the ratio of the luminosity wavelength to their propagation velocity and/or coincides with the lifetime of luminosity bright spots. The Pi2 polarization patterns are defined by the direction of luminosity bright spot rotation. These findings indicate that high‐latitude Pi2 pulsations are the response of spatio‐temporal current system oscillations of the auroral arc.
The onset of turbulence in collisionless magnetic reconnection
Authors: Rogers B. N. , Drake J. F. , Shay M. A.
Published: February 2004

Two dimensional simulations of collisionless magnetic reconnection produce strong gradients in the current and plasma profiles near the magnetic X‐line and along the separatrix during the nonlinear phase. Here we present 3D two‐fluid MHD simulations of reconnection that show these sharp gradients break up due to the onset of secondary instabilities, leading to a strongly turbulent configuration in the full 3D system. Two main secondary modes are observed, an electron shear flow instability and the lower‐hybrid drift instability. The fluctuations driven by the former mode produce an effective anomalous viscosity (but no resistivity) near the X‐line that breaks the frozen‐in condition and broadens the main current profile. The lower‐hybrid drift fluctuations cause a substantial relaxation of the gradients near the separatrix through a new mechanism, based not on direct particle transport (which is negligible) but rather on the turbulent diffusion of the strong electron flows near the separatrix.
Dual spacecraft determinations of magnetopause motion
Authors: Bauer T. M. , Dunlop M. W. , Ö Sonnerup B. U. , Sckopke N. , Fazakerley A. N. , Khrabrov A. V.
Published: February 2004

We examine the motion of Earth's magnetopause for 16 dawnside traversals of this boundary by the sister spacecraft AMPTE/UKS and IRM in December, 1984, when their separation was 400–900 km. We compare magnetopause normal vectors, n, and speeds of motion, un, obtained separately from each spacecraft by use of three different methods, and also compare those un to corresponding speeds, un*, obtained from observed time lags between the two spacecraft. Agreement between un values and n vectors determined from the three methods ranges from poor to excellent. Comparing un* and un values, we find a clear tendency for |un*| to be larger than |un|: While slightly less than half of the results show reasonable agreement (0.5 < un/un* < 2), there are about as many results in the range 0 < un/un* < 0.5, and a few cases give the wrong sign of un.
Simultaneous observations of E‐region coherent backscatter and electric field amplitude at F‐region heights with the Millstone Hill UHF Radar
Authors: Foster J. C. , Erickson P. J.
Published: February 2004

A combined coherent backscatter‐incoherent scatter experiment with the Millstone Hill UHF radar provided simultaneous observations of electric field magnitude and coherent backscatter parameters on the same L shell. A carefully‐designed geometry used sidelobe coherent contamination from two‐stream irregularities at 110 km altitude, appearing at ranges corresponding to F‐region altitudes in the main beam, in conjunction with simultaneous uncontaminated F‐region observations of the
drift velocity in adjacent range gates. Both logarithmic coherent power and the magnitude of the coherent phase velocity Vph vary linearly with
. With the assumption that the coherent phase velocity is approximately the perturbed ion sound speed in the heated E region, we find an excellent agreement between the electron temperature inferred from Vph and previous incoherent‐scatter results relating E‐region Te to E. A maximum value of ∼3100 K has been found for such wave‐induced E‐region heating.
Initiation of a coronal mass ejection
Authors: Zhang Jun , Wang Jingxiu
Published: February 2004

There are by now only one or two examples to show the initiation of coronal mass ejections (CMEs). In this paper we present a clear instance of a CME initiation. The enhanced disturbance was seen as the slow upward motion in segments of a prominence. Four hours later, the prominence was accelerated to about 60 km s−1. It greatly expanded, showing twisted structure, in later one hour. The CME onset was characterized as the disrupting and opening of the prominence. During the onset phase of the CME, a nearby small prominence erupted. It appeared that the global instability responsible for the CME was triggered by the initial mass ejection of the first prominence.
Auroral electrojet activity associated with fast plasma flows in the magnetotail
Authors: Lui A. T. Y.
Published: February 2004

We report here a case study of simultaneous observations from Geotail in the mid‐tail region and from ground CANOPUS stations to determine the auroral electrojet activity associated with the detection of fast plasma flows in the magnetotail. Ground observations from the meridian scanning photometers indicate the occurrence of a detached auroral feature in association with a transient fast plasma flow in the mid‐tail. Ground magnetometer observations suggest that the detached auroral feature and the transient fast flow in the mid‐tail correspond to a new segment of the auroral electrojet (probably limited in the east‐west extent) moving equatorward, with the original electrojet anchoring at the poleward boundary of the auroral bulge. Based on this result, we propose a possible resolution to the controversy on whether or not transient fast flows in the tail or north‐south auroral structures are substorm features.
Solar cycle dependence of the geomagnetically trapped anomalous cosmic rays
Authors: Selesnick R. S. , Cummings A. C. , Leske R. A. , Mewaldt R. A. , Stone E. C. , Cummings J. R.
Published: February 2004

Solar cycle variations in the intensity of geomagnetically trapped anomalous cosmic ray (ACR) oxygen are demonstrated by ∼7 years of data from the SAMPEX satellite. They correspond to similar variations in the interplanetary ACR source. Trapped particle losses during the approach to solar maximum are not associated with geomagnetic activity and may be consistent with atmospheric energy degradation. The average trapped particle pitch angle distribution and energy spectra support this interpretation in which both source and loss rates are proportional to atmospheric density.
Disruption of parallel current at substorm breakup
Authors: Perraut S. , Le Contel O. , Roux A. , Pellat R. , Korth A. , Holter Ø. , Pedersen A.
Published: February 2004

We study the development of substorm breakups characterized by dispersionless injections of energetic particles at the geostationary orbit. The corresponding magnetic signature is a fast change from tail‐like to dipole‐like configuration with transient superimposed low frequency oscillations (T∼1 mn). We show that intense waves (δB ≈ 1 nT) with shorter periods (1 s) systematically develop at breakup, and that their intensification is strongly related to the dipolarization and to the fast increase of energetic electrons. These “higher frequency” (F ∼1 Hz) waves appear as short lasting bursts, strongly confined across the magnetic field. Hence they look like kinetic Alfvén waves and are likely to have finite parallel electric fields, thereby resonating with electrons. We compute the diffusion coefficient and show that electrons are heated along the parallel direction and can gain up to 5 keV in a few tens of seconds. This fast parallel diffusion of electrons leads to cancellation of the parallel current and therefore to a complete modification of the current system.
Slowdown of the Solar Wind in the Outer Heliosphere and the Interstellar Neutral Hydrogen Density
Authors: Wang C. , Richardson J. D. , Gosling J. T.
Published: February 2004

Voyager 2 and Ulysses were at nearly the same latitude in the southern heliosphere at the beginning of 1999 and their radial separation was about 55 AU. The effect of pickup ions should be profound at Voyager's position, especially, the deceleration of the solar wind flow. This unique opportunity allows us to investigate the effect of pickup ions based on direct multi‐spacecraft observations. The Ulysses observations were used as input into a 1‐dimensional MHD numerical model. We run cases with and without pickup ions in an attempt to estimate the slowdown of the solar wind and the interstellar neutral hydrogen density. The theoretical predictions at the location of Voyager 2 were compared with the observations. For the purpose of this study, we focus on the statistical characteristics rather than detailed structures of the solar wind in the outer heliosphere. We found a decrease of ∼40 km s−1 or ∼10% in the radial velocity near 60 AU. This speed decrease implies an interstellar neutral density of 0.05 cm−3.
Simulation of the pre‐reversal enhancement in the low latitude vertical ion drifts
Authors: Fesen C. G. , Crowley G. , Roble R. G. , Richmond A. D. , Fejer B. G.
Published: February 2004

Low latitude F region ion motions exhibit strong seasonal and solar cycle dependences. The pre‐reversal enhancement (PRE) in the vertical ion drifts is a particularly well‐known low latitude electrodynamic feature, exhibited as a sharp upward spike in the velocity shortly after local sunset, which remains poorly understood theoretically. The PRE has been successfully simulated for the first time by a general circulation model, the National Center for Atmospheric Research thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM). The TIEGCM reproduces the zonal and vertical plasma drifts for equinox, June, and December for low, medium, and high solar activity. The crucial parameter in the model to produce the PRE is the nighttime E region electron densities: densities ≥ 104 cm−3 preclude the PRE development by short‐circuiting the F region dynamo. The E region semidiurnal 2,2 tidal wave largely determines the magnitude and phase of the daytime F region drifts.
Fluctuations of the heliospheric magnetic field and the anisotropy of pickup ions
Authors: Németh Z. , Erdős G. , Balogh A.
Published: February 2004

Pitch angle distribution of interstellar pickup ions at high heliographic latitude is studied. Based on high time resolution magnetic field observations by Ulysses during the south polar pass in 1994, we have determined the angular distribution of particles at the moment of ionization. Due to large deviations of the field from Parker's spiral, it was found that 13% of the freshly ionized particles stream away of the Sun in the solar wind frame. We have solved the transport equation of the pickup particles which included an anisotropic source term. Our model calculation shows that the effect of the source on the particle anisotropy is the dominant one, as opposed to the effect of the density gradient. Matching the results of the model with particle anisotropy observation by SWIGS aboard Ulysses, we have estimated a scattering mean free path of interstellar pickup protons λ = 0.25AU.
Hot O and nighttime ionospheric temperatures
Authors: Litvin A. , Oliver W. L. , Amory‐Mazaudier C.
Published: February 2004

By analysing a large set of St. Santin F region Incoherent Scatter (IS) data for low solar activity we find nighttime ion temperature (Ti) on average to be apparently slightly higher than electron temperature (Te). We show that this difference is a bias incurred by the IS spectral analysis procedure owing to the neglect of H+ ions in the 300–500 km altitude region. Further, we show Te and Ti to depart from the neutral temperature Tn with increasing altitude. We perform a simulation of ion and electron temperatures and show that the difference between Ti, Te and Tn can be caused by the presence of a small amount of hot oxygen. Using our simulation we estimate the ratio of hot oxygen to cold oxygen density to be 0.13% on average at 400‐km altitude for low solar activity.
An altitude‐normalized magnetic map of Mars and its interpretation
Authors: Purucker M. , Ravat D. , Frey H. , Voorhies C. , Sabaka T. , Acuña M.
Published: February 2004

Techniques developed for the reduction and analysis of terrestrial satellite magnetic field data are used to better understand the magnetic field observations made by Mars Global Surveyor. A global distribution of radial (Br) magnetic field observations and associated uncertainties is inverted for an equivalent source magnetization distribution and then used to generate an altitude‐normalized map of Br at 200 km. The observations are well‐represented by a potential function of crustal origin, consistent with a remanent origin for the Martian magnetic features. The correlation between the 40546 Br observations and Br calculated from the magnetization solution at observation locations is 0.978. For a magnetization distribution confined to a 50 km layer, calculated magnetizations range from ‐22 to +17 A/m. We see correlations with tectonics that were only hinted at in earlier maps. Magnetic features appear to be truncated against Valles Marineris and Ganges Chasma, suggestive of a major change in crustal properties associated with faulting.
EISCAT observation of a high‐latitude ionization trough associated with a reversed westward plasma flow
Authors: Ma S. Y. , Liu P. , Schlegel K.
Published: February 2004

Trough‐like plasma structures in the high‐latitude ionosphere near the polar cap boundary are investigated with EISCAT CP3 data and in‐situ satellite observations. Emphasis is placed on one interesting event occurring in the morning sector during the recovery phase of a moderate magnetic storm. The event features are probably ionospheric signatures of cusp processes caused by magnetic reconnection on the dayside magnetopause.
Global X‐ray observations of magnetospheric convection‐driven auroral disturbances
Authors: Anderson P. C. , McKenzie D. L. , Lyons L. R. , Hairston M.
Published: February 2004

Global observations of the auroral x‐ray emissions during three large magnetic storms in 1998 are presented from the Polar Ionospheric X‐ray Imaging Experiment (PIXIE). The cross‐polar‐cap potential drop (ΔΦPC) remained large (>100 kV) throughout most of the stormtime periods, and most of the x‐ray emissions observed were associated with enhanced earthward convection and dawnward drift of electrons caused by substantial cross‐tail electric fields. Large increases in ΔΦPC drove impulsive disturbances termed convection enhancements that showed signatures inconsistent with substorm activity, including substantial auroral emissions in the morning sector and little activity in the premidnight sector.
On the electron distribution function in the F region and airglow enhancements during HF modification experiments
Authors: Mishin Evgeny , Carlson Herbert C. , Hagfors Tor
Published: February 2004

The variation with height and plasma frequency of the distribution function of ambient electrons (EDF) in the F region ionosphere during high‐power, HF radio wave modification experiments is discussed. It is shown that the deviation of the EDF from a Maxwellian distribution (MD) in the high energy, >2 eV, range may be quite significant (decreasing by a factor of 2 to 5) depending on the background parameters. As a result, the cooling and excitation rates are reduced with respect to a MD calculation. This, for example, can improve agreement of the model based on excitation by heated thermal electrons with the observations of 630.0 nm airglow during HF modification experiments.
Lidar observations of elevated temperatures in bright chemiluminescent meteor trails during the 1998 Leonid Shower
Authors: Chu Xinzhao , Liu Alan Z. , Papen George , Gardner Chester S. , Kelley Michael , Drummond Jack , Fugate Robert
Published: February 2004

Seven persistent trails associated with bright fireballs were probed with a steerable Na wind/temperature lidar at Starfire Optical Range, NM during the 17/18 Nov peak of the 1998 Leonid meteor shower. These chemiluminescence trails were especially rich in Na. The average Na abundance within the trails was 52% of the background Na layer abundance, which suggests that the corresponding masses of the meteors were from l g up to 1 kg. CCD images show that the chemiluminescent emissions (including Na and OH) are confined to the walls of a tube, which expands with time by molecular diffusion. Lidar profiles within the trails show that the temperatures are highest at the edges of the tube where the airglow emissions are brightest. Approximately 3 min after ablation, temperatures at the tube walls are 20–50 K warmer than the tube core and background atmosphere. Neither chemical nor frictional heating provides a satisfactory explanation for the observations.