bluuebunny/crossref_metadata_2025 huggingface dataset:
Predictions of global and regional sea‐level rise using AOGCMs with and without flux adjustment
Authors: Gregory J. M. , Lowe J. A.
Published: February 2004

Future global and regional sea‐level changes have been calculated using two versions (HadCM2 and HadCM3) of the Hadley Centre coupled atmosphere‐ocean general circulation model forced by the IS92a scenario for emissions of greenhouse gases. HadCM3 is a newly developed model which does not require flux adjustment to maintain a stable climatology. Global‐average sea‐level rise from 1990 to 2100 is predicted to be 0.48 m in HadCM2 and 0.44 m in HadCMS, 60% resulting from thermal expansion of sea‐water and the rest from loss of mass of glaciers and small ice‐caps. Sea‐level rise is smaller in HadCM3 principally because the radiative forcing is slightly less, giving reduced ocean heat uptake and thermal expansion. However, the heat penetrates less deeply in HadCMS; consequently the surface warming is nearly the same. There is marked geographical variation of sea‐level change, which is generally similar in the two models; local values range between zero and twice the global average.
The Extended Canadian Middle Atmosphere Model
Authors: Beagley Stephen R. , McLandress Charles , Fomichev Victor I. , Ward William E.
Published: February 2004

First results from the extended Canadian middle atmosphere model are presented. The model extends from the surface to the middle thermosphere and includes relevant physical parameterizations. To simulate unresolved gravity waves a non‐orographic gravity wave drag parameterization has also been implemented. Results from a two‐year simulation are discussed and are shown to be in good qualitative agreement with upper atmosphere research satellite wind observations. The realistic features which are reproduced include the reversal of the mesospheric jets and the semi‐annual variation of the migrating diurnal tide in the low latitude mesosphere.
Anomalously high porosities in the proto‐decollement zone of the Barbados Accretionary Complex: Do they indicate overpressures?
Authors: Screaton Elizabeth , Ge Shemin
Published: February 2004

Elevated pore pressures have been hypothesized to play a key role in the development of the decollement zones of accretionary complexes. Drilling within the proto‐decollement zone of the Barbados accretionary complex revealed anomalously high porosities up to 6 km seaward of the deformation front. Defining whether the high porosities are a reflection of overpressures is critical for understanding the seaward propagation of the decollement zone. In this investigation, we used numerical modeling to assess whether the high porosities reflect overpressures due to sedimentation. From this work, we concluded that reported permeabilities are not low enough to produce overpressures, even if additional fluid were contributed from areas of greater sedimentation. Furthermore, overpressures within the proto‐decollement zone are demonstrated to be inconsistent with normally compacted underlying sediments. Locally anomalous sediment strength provides a more likely explanation for the observed high porosities.
Capabilities of 3‐D wavelet transforms to detect plume‐like structures from seismic tomography
Authors: Bergeron Stephen Y. , Yuen David A. , Vincent Alain P.
Published: February 2004

The wavelet transform methods have been applied to viewing 3‐D seismic tomography by casting the transformed quantities into two proxy distributions, E‐max, the maximum of the magnitude of the local spectra about a local point and the associated local wavenumber, k‐max. Using a stochastic background noise, we test the capability of this procedure in picking up the coherent structures of upper‐mantle plumes. Plumes with a Gaussian shape and a characteristic width up to 2250 km have been tested for various amounts of the signal‐to‐noise ratios (SNR). We have found that plumes can be picked out for SNR as low as 0.08 db and that the optimal plume width for detection is around 1500 km. For plume width ranging between 700 km and 2000 km, the SNR can be lower than 1 db. This length‐scale falls within the range for plume‐detection based on the signal‐to‐noise levels associated with the current global tomographical models.
Validating the NSCAT winds in the vicinity of the Pacific Intertropical Convergence Zone
Authors: Yu Zuojun , Moore Dennis W.
Published: February 2004

It is known that the NSCAT winds are influenced by precipitation, and so are potentially in error in regions of high rainfall, such as the Pacific ITCZ. We assess this potential error by comparing NSCAT winds to other products and by determining the possible impact on modeling the Pacific NECC. In the latitude band of the ITCZ, there are large differences between wind products from the ECMWF and NSCAT. A comparison with TAO buoy winds shows a bias of NSCAT data towards a higher zonal wind stress at 8°N. The spatial distribution of this bias distorts the Ekman pumping velocity across the ITCZ, and results in a NECC that is 0.1–0.2 m/s weaker than that forced by the ECMWF winds. This is not a trivial amount during the spring, when the strength of the NECC in the eastern Pacific is of the same magnitude.
Doppler profiler observations of Hurricane Georges at landfall
Authors: Knupp Kevin R. , Walters Justin , McCaul Eugene W.
Published: February 2004

The boundary layer (BL) within the western eyewall of Hurricane Georges, during its landfall on the Mississippi coastline, is described using measurements from a mobile Doppler profiling system. The turbulent layer depth, roughly proportional to the BL wind magnitude, attained a height of about 2 km above ground level (AGL) during maximum measured winds of 40–45 m s−1 near 0.5 km altitude. The BL contains updrafts and downdrafts of 1–3 m s−1 magnitude that may be associated with the vertical branches of horizontal convective rolls or wind streaks recently identified within landfalling hurricanes. These turbulent eddies, and the large vertical shear of horizontal winds within the BL above and below the jet maximum within the eyewall, appear to produce a uniform mass concentration of relatively small raindrops within the BL.
Active fault zones and groundwater flow
Authors: Gudmundsson Agust
Published: February 2004

Boundary‐element studies of an active strike‐slip fault zone subject to fault‐parallel loading of 6 MPa show tensile stress concentration in large areas around the fault‐zone tips. In these areas, tensile stress exceeds typical in situ tensile strengths of rocks, resulting in the formation or reactivation of tensile fractures. These fractures curve toward the tips of the fault zone, and if interconnected they increase the rock permeability. Fault slip also increases the temporary permeability of the fault zone, by as much as many orders of a magnitude. Its effects on the surrounding groundwater flow, however, is normally small if the fault trends at a high angle to the groundwater flow but gradually increases as the angle between the flow and the fault decreases. When the trend of the fault zone and the groundwater flow coincide, the upstream part collects groundwater whereas the downstream part expels it. It follows that the yield of springs decreases in the upstream part, but increases in the downstream part.
Variations in tropical tropopause observed with radiosondes in Indonesia
Authors: Shimizu Atsushi , Tsuda Toshitaka
Published: February 2004

Characteristics of the tropical tropopause are described using radiosonde observations in Indonesia. Annual cycles of tropopause height are clearly seen and year‐to‐year variations are also recognized, especially the in the rainy seasons. Over Indonesia, the tropopause is colder and located higher than that over western Pacific. Cross spectral analyses indicates that the minimum temperature and zonal wind velocity at 16.5 km have good coherency in the period of 10–25 days, which suggests that the structure of temperature around the tropopause is mainly controlled by Kelvin waves passing through. On the other hand, the minimum temperature fluctuation with a period of 25–50 days shows a large correlation with precipitable water vapor in the troposphere.
Static‐stress impact of the 1992 Landers earthquake sequence on nucleation and slip at the site of the 1999 M=7.1 Hector Mine earthquake, southern California
Authors: Parsons Tom , Dreger Douglas S.
Published: February 2004

The proximity in time (∼7 years) and space (∼20 km) between the 1992 M=7.3 Landers earthquake and the 1999 M=7.1 Hector Mine event suggests a possible link between the quakes. We thus calculated the static stress changes following the 1992 Joshua Tree/Landers/Big Bear earthquake sequence on the 1999 M=7.1 Hector Mine rupture plane in southern California. Resolving the stress tensor into rake‐parallel and fault‐normal components and comparing with changes in the post‐Landers seismicity rate allows us to estimate a coefficient of friction on the Hector Mine plane. Seismicity following the 1992 sequence increased at Hector Mine where the fault was unclamped. This increase occurred despite a calculated reduction in right‐lateral shear stress. The dependence of seismicity change primarily on normal stress change implies a high coefficient of static friction (µ≥0.8). We calculated the Coulomb stress change using µ=0.8 and found that the Hector Mine hypocenter was mildly encouraged (0.5 bars) by the 1992 earthquake sequence. In addition, the region of peak slip during the Hector Mine quake occurred where Coulomb stress is calculated to have increased by 0.5–1.5 bars. In general, slip was more limited where Coulomb stress was reduced, though there was some slip where the strongest stress decrease was calculated. Interestingly, many smaller earthquakes nucleated at or near the 1999 Hector Mine hypocenter after 1992, but only in 1999 did an event spread to become a M=7.1 earthquake.
Seismic evidence for a thinner mantle transition zone beneath the South Pacific Superswell
Authors: Niu Fenglin , Inoue Hiroshi , Suetsugu Daisuke , Kanjo Kenji
Published: February 2004

Broadband seismograms recorded by the seismic stations deployed on oceanic islands in the South Pacific for two deep earthquakes in 1998 are used to investigate the mantle transition zone structure beneath the South Pacific, where a large‐scale hot plume might ascend from the core‐mantle boundary (CMB). By stacking S waves reflected at the mantle transition zone discontinuities, we find clear signals associated with the 410‐km and 660‐km discontinuities. The transition zone thickness, which is constrained from the travel time difference between the reflected waves from the 410‐km and 660‐km discontinuities, is observed to be approximately 15 km thinner beneath the South Pacific Superswell than that beneath other regions. The result suggests that the transition zone temperature beneath French Polynesia is approximately 100∼200 K higher than the surrounding mantle.
Ground displacements around the fault of the September 20th, 1999, Chi‐Chi Taiwan Earthquake
Authors: Shin T. C. , Wu F. T. , Chung J. K. , Chen R. Y. , Wu Y. M. , Chang C. S. , Teng T. L.
Published: February 2004

Ground displacements in the meizoseismal area of the September 20, 1999, Chi‐chi earthquake are derived from accelerograms at 39 stations. On the hanging wall side a maximum horizontal, NW‐directed motion of 8 meters and a vertical rise of 3.7 meters are found near the northern end of the N‐S trending surface trace of the fault. On the footwall side and next to the fault a maximum horizontal motion of 1.3 meters and a maximum downward vertical motion of 1 meter are observed. The displacement field describes the source as a thrust fault with left‐lateral motion. The displacements increase from south to north along the fault, and so does the left‐lateral component of the fault motion, from about 10% to 60% of the total horizontal displacement. The recorded motions, especially ground velocities and accelerations, are noticeably stronger on the hanging wall side than on the footwall side.
Release of NOx from sunlight‐irradiated midlatitude snow
Authors: Honrath R. E. , Peterson M. C. , Dziobak M. P. , Dibb J. E. , Arsenault M. A. , Green S. A.
Published: February 2004

Photochemical production and release of gas‐phase NOx (NO + NO2) from the natural snowpack at a remote site in northern Michigan were investigated during the Snow Nitrogen and Oxidants in Winter study in January 1999. Snow was collected in an open 34 L chamber, which was then sealed with a transparent Teflon cover and used as an outdoor flow and reaction chamber. Significant increases in NOx mixing ratio were observed in synthetic and ambient air pulled through the sunlit chamber. [NOx] enhancements were correlated to ultraviolet sunlight intensity, reaching ∼300 pptv under partially overcast midday, midwinter conditions. These findings are consistent with NOx production from photolysis of snowpack NO3−; the observed NOx release implies production of significant amounts of OH within the snow. Snowpack NO3− photolysis may therefore significantly alter boundary layer levels of both NOx and oxidized compounds over wide regions of the atmosphere.
An investigation of the interaction of carbonyl compounds with the snowpack
Authors: Couch Tara Lin , Sumner Ann Louise , Dassau Terra M. , Shepson Paul B. , Honrath Richard E.
Published: February 2004

Measurements of formaldehyde, acetaldehyde, and acetone in ambient and snowpack air were conducted as a part of the SNOW99 study in northern Michigan. Vertical profiles of ambient and snowpack air illustrate large concentration gradients through the top ∼10 cm of the snowpack, implying a positive flux of these species from the surface. Snow chamber experiments that involved flushing a snow‐filled 34L Teflon‐lined chamber with zero air at 20 slpm indicated that release from the snow followed first order kinetics, with decay constants of 0.19, 0.44, and 0.34 hr−1 for formaldehyde, acetaldehyde, and acetone, respectively. Although it is likely that temperature dependent adsorption/desorption processes play a role, the data are not inconsistent with loss from the snowpack via snow grain metamorphism. The data also imply that formaldehyde is not hydrated in the snow grain surface layer.
Space‐time analysis of TIMED Doppler Interferometer (TIDI) measurements
Authors: Azeem S. M. I. , Killeen T. L. , Johnson R. M. , Wu Q. , Gell D. A.
Published: February 2004

A technique is developed for reducing the amount of aliasing in the spectral analysis of TIDI observations, by ingestion of ground‐based data into the satellite data set. A multi‐dimensional (space‐time) least squares fitting approach is applied to the satellite and ground‐based data to determine the aliasing spectra. The addition of ground‐based data to the TIDI data set reduces the aliased components in the aliasing spectrum. For example, at 20° latitude, the combined ground‐based and TIDI data set of a sampled input semidiurnal (frequency of 2 days−1) signal with zonal wavenumber 2 results in a factor of 2 reduction in the amount of power aliasing into a signal with zonal wavenumber 0 and frequency 0 days−1.
2.5‐D surface deformation of M6.1 earthquake near Mt Iwate detected by SAR interferometry
Authors: Fujiwara Satoshi , Nishimura Takuya , Murakami Makoto , Nakagawa Hiroyuki , Tobita Mikio , Rosen Paul A.
Published: February 2004

The two‐dimensional displacement vector field on the Earth's surface associated with a large earthquake near Iwate volcano in Japan, was mapped by combining satellite radar interferometry images from two different track directions. The displacement reveals not only large main ruptures, but details of many small faults movement of aftershocks and volcanic earthquakes. The displacements of the aftershock faults coincide with a stress field reorganized by the main earthquake that accelerated the aftershock fault movements. The displacements mimic the topography, suggesting that repeated movements of the same faults in the Quaternary have led to the presently existing topography.
On the influence of topography in the induction of coastal upwelling along the Chilean Coast
Authors: Figueroa Dante , Moffat Carlos
Published: February 2004

The contribution of the winds and the topography in the induction of upwelling along the Chilean coastal zone is calculated. The topographic‐induced upwelling is estimated taking into account the effect of the meridional changes of the coastline orientation. The wind‐induced upwelling was calculated for the austral summer using NSCAT scatterometer data, resulting a total amount of 2.1 × 106 m³s−1 of water transported offshore for the whole Chilean upwelling zone. It is shown that the topographic effect dominates the wind‐induced upwelling in several parts of the Chilean coast, particularly at 23°S (Antofagasta), 25.5°S, 30°S (Coquimbo), 33°S (Valparaíso) and 37°S (Concepción). Antofagasta, Coquimbo, Valparaíso and Concepción are well known zones of intense upwelling. The results show that the topography plays a key role in the upwelling of the Chilean coast, specially in its northern part.
GPS observation of the sea level variation in Lützow‐Holm Bay, Antarctica
Authors: Aoki Shigeru , Ozawa Taku , Doi Koichiro , Shibuya Kazuo
Published: February 2004

Differential GPS observations were conducted on the fast ice in Lützow‐Holm Bay, Antarctica. The observations were carried out intermittently from April to December in 1998. The vertical displacement, which is a combination of oceanic tidal variation and high frequency variations of 3–6 minutes period, was clearly detected. Eight hours' comparison of the height variation derived from GPS with that from a video monitoring system showed good agreement with a standard deviation of 2.2 cm. The high frequency fluctuations of about 1 cm were obvious in both observations and had significant correlation with each other. The low‐pass filtered variations agreed well with a standard deviation of 1.7 cm. Tidal constituents derived from the GPS observations for the entire period showed good agreement with those from the seven years of pressure gauge observations. Thus, GPS has demonstrated the ability to observe 2 cm accuracy sea level variations.
El Niño and La Niña—equatorial Pacific thermocline depth and sea surface temperature anomalies, 1986–98
Authors: Harrison D. E. , Vecchi Gabriel A.
Published: February 2004

Simple models of the El Niño‐Southern Oscillation (ENSO) phenomenon have provided many of our basic ideas about ENSO mechanisms. These models exhibit a range of correlation patterns between thermocline depth anomaly (Z20A) and sea surface temperature anomaly (SSTA). We use 13 years of Pacific equatorial waveguide observations to explore the relationships between Z20A and SSTA. We find significant correlation in the eastern Pacific, and in the east‐central Pacific when the east‐central Pacific is normal or cooler than normal. We find no correlation in the western, west‐central and east‐central (when warmer than normal) Pacific. It is inappropriate to attribute SSTA changes to Z20A changes. Coupled ENSO models should be reexamined in light of these observed Z20A/SSTA relationships. Analysis of ocean general circulation models suggests that progress in understanding ENSO may depend as much on understanding SSTA/wind/near‐surface current relationships and processes, as upon thermocline change processes.
Anomalous temperature and salinity variations in the tropical Atlantic: Possible causes and implications for the use of altimeter data
Authors: Segschneider J. , Balmaseda M. , Anderson D. L. T.
Published: February 2004

Near real time subsurface observations at a PIRATA mooring in the western equatorial Atlantic show a temperature and salinity drop in boreal spring 1999 which is consistent with a 60 m upward shift of the thermocline. At the same time, sea level observations from TOPEX/Poseidon and ERS‐1/2 show anomalously low values over a large part of the tropical Atlantic north of the equator. The sea level observations are also used to correct for errors in upper ocean heat content in a global ocean analysis, which is used to initialize seasonal forecasts. It is shown that the ability of altimeter data to reproduce observed temperature changes is limited in the western equatorial Atlantic. This may partly be because large temperature changes occur very rapidly, but is mainly because strong salinity variations compensate up to 10 cm of the sea level signal from temperature.
Changes in oceanic precipitation during the 1997–98 El Niño
Authors: Quartly Graham D. , Srokosz Meric A. , Guymer Trevor H.
Published: February 2004

The recent El Niño has been by most measures one of the most extreme, and there have been several papers on its thermal signature and associated wind field. There has also been wide coverage of the changes in terrestrial precipitation, with torrential rains in California and devastating fires in Borneo in response to the prolonged drought. Here we complete the picture by examining oceanic precipitation data derived from novel processing of dual‐frequency altimetry. An increased area of precipitation, with more frequent and slightly more intense rainfall, is found to mirror the expansion of the western warm pool.
Determination of Persian Gulf Water Transport and oxygen utilisation rates using SF6 as a novel transient tracer
Authors: Law Cliff S. , Watson Andrew J.
Published: February 2004

Sulphur hexafluoride (SF6) has potential as a transient tracer of recently ventilated water masses, as its atmospheric burden continues to increase. Northern Arabian Sea hydrography was examined using measurements of atmospheric and dissolved SF6, CFC‐11, CFC‐12 and CFC‐113. Persian Gulf Water (PGW) was characterised by its SF6 signal, and the time elapsed since its formation was evaluated by two approaches. Four ventilation age estimates were derived from SF6/CFC‐11, SF6/CFC‐12, CFC‐113/CFC‐11 and CFC‐113/CFC‐12, and their agreement at the oceanic stations confirms the validity of SF6 as a transient tracer. A second approach, of correcting SF6 partial pressure for PGW dilution by an optimal mixing model and referencing to the atmospheric SF6 chronology, provided a relative tracer age. This indicated a PGW flow of 0.016 (+/−0.003) m/s across the northern Arabian Sea, with an associated oxygen consumption of 10.1 µmol/l p.a. that exceeds tracer‐derived estimates but confirms rates derived from export flux.
On the interpretation of AGCMs response to prescribed time‐varying SST anomalies
Authors: Czaja Arnaud , Marshall John
Published: February 2004

Recently, Bretherton and Battisti (1999) have presented an interesting interpretation of ensemble experiments with atmospheric general circulation models (AGCMs) forced by observed sea surface temperature (SST) whose mean successfully simulates the decadal evolution of the observed North Atlantic Oscillation (NAO) index. Using a linear model of atmosphere/ocean interaction, they plausibly argue that this hind‐cast skill, as measured by lowpass correlations between observed and simulated indices, is consistent with the ocean mixed layer merely integrating stochastic surface heat flux forcing governed by the natural variability of the atmosphere. They go on to suggest, however, that predictability associated with middle‐latitude SST anomalies is limited to timescales associated with the thermal inertia of the oceanic mixed layer (perhaps a year). Here, we include ocean circulation in a simple coupled ocean‐atmosphere model and also consider hypothetical limits in which the coupled system is highly predictable at low frequencies. We find that low pass correlations between observed and simulated NAO indices, obtained from ensembles of SST‐forced AGCMs, are insensitive to the predictability of the system. Thus inferences about predictability of the atmosphere‐ocean system cannot be made on the basis of this measure of the hindcast skill of atmosphere‐only simulations.
Impacts of the ocean lateral diffusion on the El Niño/Southern Oscillation‐like variability of a global coupled general circulation model
Authors: Raynaud S. , Speich S. , Guilyardi E. , Madec G.
Published: February 2004

A global coupled General Circulation Model is used to investigate effects of the lateral ocean mixing on the amplitude and spectral characteristics of the ENSO‐like variability. Three 40‐year coupled simulations are analysed. They only differ by the lateral ocean physics parametrized: a horizontal diffusion scheme; an isopycnal scheme; isopycnal scheme with an Eddy Induced Velocity (EIV) term.

The Multichannel Singular Spectrum Analysis is used to extract linear modes of variability from each simulation and from observations. The strength of the variability, close to that observed in the horizontal and only‐isopycnal simulations, is stronger in the two isopycnal configurations, and more especially when the EIV is added. The dominant periods of the variability, also close to those observed, are also affected by the lateral diffusion: the time scale becomes slightly longer as the complexity of the lateral physics increases.
Resolving ambiguities in gravity wave propagation directions inherent in satellite observations: A simulation study
Authors: Hickey Michael P. , Brown Jason S.
Published: February 2004

We simulate space‐based, sub‐limb viewing observations of airglow brightness fluctuations caused by atmospheric gravity wave interactions with the O2 atmospheric airglow, and we demonstrate that, due to the geometry associated with such observations, the brightness fluctuations observed for the optically thick 0–0 band emission will always appear stronger for waves traveling towards the observer (satellite). The effect should be most noticeable for waves having relatively small vertical wavelengths (∼10 km) and horizontal wavelengths of 50 km or greater. For waves of short (∼100 km) horizontal wavelength, the brightness fluctuation anisotropy with respect to viewing direction may also be evident in the optically thin 0–1 band emission. Therefore, the 180° ambiguity in wave propagation direction associated with space‐based observations may be eliminated for waves dissipating in the upper mesosphere and lower thermosphere.
A modified pool permutation procedure for short regional climate model simulations nested within analyzed fields
Authors: MacKay Murray D.
Published: February 2004

Short regional climate model simulations are routinely compared with observations, though because of extremely small sample sizes (often only a single season or year is simulated) it is generally difficult to establish whether any model‐reality differences are statistically significant. In the following, a permutation technique is proposed to estimate the statistical significance of similarities between spatial fields as simulated by a regional climate model, and observations, when the climate model is nested within operational analyses (i.e. “perfect” lateral boundary conditions). To illustrate the technique, we consider a short simulation over the Mackenzie River Basin of northwestern Canada made using the Canadian Regional Climate Model, and focus our attention on accumulated monthly precipitation and monthly average screen temperature. For comparison we have a 45 year gridded, monthly climate dataset produced by the Meteorological Service of Canada over the same region, based on adjusted operational climate station data.
A method for mapping apparent stress and energy radiation applied to the 1994 Northridge Earthquake Fault Zone
Authors: McGarr A. , Fletcher J. B.
Published: February 2004

Using the Northridge earthquake as an example, we demonstrate a new technique able to resolve apparent stress within subfaults of a larger fault plane. From the model of Wald et al. (1996), we estimated apparent stress for each subfault using
where G is the modulus of rigidity, β is the shear wave speed, and
is the average slip rate. The image of apparent stress mapped over the Northridge fault plane supports the idea that the stresses causing fault slip are inhomogeneous, but limited by the strength of the crust. Indeed, over the depth range 5 to 17 km, maximum values of apparent stress for a given depth interval agree with τa(max)=0.06S(z), where S is the laboratory estimate of crustal strength as a function of depth z. The seismic energy from each subfault was estimated from the product τaDA, where A is subfault area and D its slip. Over the fault zone, we found that the radiated energy is quite variable spatially, with more than 50% of the total coming from just 15% of the subfaults.
The onset and association of CMEs with sigmoidal active regions
Authors: Glover Alexi , Ranns Neale D. R. , Harra Louise K. , Culhane J. Leonard
Published: February 2004

Previous studies of active regions characterised by Soft X‐ray S or inverse‐S morphology [Canfield et al., 1999], have found these regions to possess a higher probability of eruption. In such cases, CME launch has been inferred using X‐ray proxies to indicate eruption. Active regions observed during 1997, previously categorised as both sigmoidal and eruptive [Canfield, 1999], have been selected for further study, incorporating SoHO‐LASCO, SoHO‐EIT and ground based H‐alpha data. Our results allow re‐classification into three main categories; sigmoidal, non‐sigmoidal and active regions appearing sigmoidal due to the projection of many loops. Although the reduced dataset size prevents a statistical measure of significance, we note that regions comprising a single S (or inverse‐S) shaped structure are more frequently associated with a CME than those classed as non‐sigmoidal. This motivates the study of a larger dataset and highlights the need for a quantitative observational definition of the term “sigmoid”.
Coherent and incoherent scatter radar observations during intense mid‐latitude spread F
Authors: Swartz Wesley E. , Kelley Michael C. , Makela Jonathan J. , Collins Stephen C. , Kudeki Erhan , Franke Steve , Urbina Julio , Aponte Nestor , Sulzer Michael P. , González Sixto A.
Published: February 2004

An intense mid‐latitude spread‐F event occurred over Puerto Rico during the night of February 17, 1998. Simultaneous observations were made with the Cornell University Portable Radar Interferometer (CUPRI) located near Isabela, PR, the University of Illinois VHF radar located at Salinas, PR, GPS receivers at Isabela and St. Croix, measuring total electron content, the Arecibo incoherent scatter radar, and the Cornell All‐Sky imager located at the Arecibo Observatory. This was the first time that such a broad range of complementary instrumentation captured a mid‐latitude spread‐F space weather event. It was the first (and still only) time that a spread‐F event over the Caribbean exhibited large Doppler shifts in the VHF spectra. This event was characterized with multiple filaments that initially produced receding Doppler velocities exceeding 300 m/s as seen by CUPRI and the Illinois radar. The Arecibo incoherent scatter radar recorded line‐of‐sight velocities exceeding 100 m/s that moved the F‐layer peak to over 400‐km altitude. Airglow images of 630.0 nm emissions from F‐region heights showed depleted structures oriented southeast to northwest. The large velocities observed with the radars suggest that we caught this event in a stage of explosive development. It is interesting that the first fully documented Caribbean event occurred during a magnetically active period.
Intercomparisons of total electron content measurements using the Arecibo Incoherent Scatter Radar and GPS
Authors: Makela Jonathan J. , González Sixto A. , MacPherson Bryan , Pi Xiaoqing , Kelley Michael C. , Sultan Peter J.
Published: February 2004

We compare total electron content measurements made using the Arecibo Incoherent Scatter Radar (ISR) with those made using the Global Positioning System (GPS). The ISR measurements have a limited range for these observations, up to 1500 km. We extend these profiles to GPS heights of 20,200 km with the aid of a numerical model. We use a GPS receiver on St. Croix, which has been calibrated using JPL's Global Ionospheric Mapping (GIM) technique. In addition, we also use the ISR to calibrate GPS measurements made at Isabela, PR and see how the calibration holds up on the next day. The GIM technique gives very good results on both a quiet night and a night with a severe ionospheric depletion. Normalizing the Isabela receiver to the ISR also gives good results and shows promise as a way to independently calibrate nearby GPS receivers in the future. Finally, we give evidence that the severe depletion observed by the ISR on the night of June 25/26, 1998 was associated with an elongated TEC depletion. The structure may be related to a disturbance originating in the southern hemisphere.
Advection of the equatorial anomaly over Arecibo by small‐storm related disturbance dynamo electric fields
Authors: Aponte Néstor , González Sixto A. , Kelley Michael C. , Tepley Craig A. , Pi Xiaoqing , Iijima Byron
Published: February 2004

During highly disturbed geomagnetic periods, both the Arecibo radar and the Ramey ionosonde have recorded impressive nighttime ionospheric enhancements in which the peak electron density exceeded 1 × 1012 m−3 and the F2 peak height went above 400 km. In the past it has been suggested that these events could be caused by either a downward plasmaspheric flux that increases the density in the F region or by an eastward electric field that pushes the equatorial anomaly poleward to the Caribbean sector. On February 17–18, 1999 the Arecibo radar made observations during an event in which the electron density again rose to daytime values near midnight. For this event, the peaks in density were observed predominantly southeast of Arecibo while the ions sustained a northward‐eastward motion due to an eastward‐southward storm dynamo electric field. TEC maps from GPS for this night confirmed that the density enhancements were due to a poleward expansion of the equatorial anomaly.
Improving the Abel inversion by adding ground GPS data to LEO radio occultations in ionospheric sounding
Authors: Hernández‐Pajares M. , Juan J. M. , Sanz J.
Published: February 2004

GPS radio occultations allow the sounding of the Earth's atmosphere (i.e. troposphere and ionosphere). The basic observable of this technique is the additional delay, due to the refractivity index, of a radio signal when passing through the atmosphere. This additional delay is proportional to the integrated refractivity, in such a way that we can obtain an estimation of the vertical refractivity profiles using observations at different elevation angles by solving an inverse problem. Traditionally, the solution of this inverse problem is obtained by using the Abel inversion algorithm assuming a refractivity index that only depends on the altitude. In this paper we present a modified Abel inversion algorithm for ionospheric sounding that overcomes the spherical symmetry assumption of the traditional Abel inversion algorithm. Processing a set of simulated data and 1 day of real data with this algorithm, a clear improvement over the traditional one can be obtained when comparing the derived critical frequencies with the ionosonde measurements. It is also shown that this improvement is sufficient to measure critical frequencies associated with the ionospheric E layer.
Telescopic imaging of sprites
Authors: Gerken E. A. , Inan U. S. , Barrington‐Leigh C. P.
Published: February 2004

Telescopic images of sprites show a wide variety of generally vertical but also slanted fine structure, including branching tree‐like shapes and well defined but isolated columns, with transverse spatial scales ranging from tens of meters to a few hundred meters at ∼60–85 km altitude. Simultaneous analysis of radio atmospheric and lightning data indicates that specific columnar regions are selectively excited by successive discharges.
Low‐charge‐state heavy ions upstream of Earth's bow shock and sunward flux of ionospheric O+1, N+1, and O+2 ions: Geotail observations
Authors: Christon S. P. , Desai M. I. , Eastman T. E. , Gloeckler G. , Kokubun S. , Lui A. T. Y. , McEntire R. W. , Roelof E. C. , Williams D. J.
Published: February 2004

Energetic ∼10–210 keV/e low‐charge‐state heavy ions (LCSHI) were measured sunward of Earth's bow shock (to X ∼ 30 RE) during numerous intervals lasting from minutes to hours using Geotail/STICS in 1995–1998. LCSHI fluxes are strong and continuous in a few tens of intervals, during which a strong component of ionospheric origin O+1, N+1, and O+2 streams sunward on nearly radial IMF. Most often though only ‘trace’ levels are present. LCSHI flux is typically, but not exclusively, observed during diffuse upstream ion events. LCSHI are accompanied by sunward energetic electron bursts in three of the four cases shown and twice by enhanced IMF fluctuations. As O+1 streams sunward in the spacecraft frame during the strongest case, H+, He+2, He+1, and O+6 fluxes have weak anisotropies and the H+ energy spectrum is kappa‐like. The strongest LCSHI fluxes tend to occur duskward of local noon during disturbed geomagnetic conditions. On average, LCSHI flux is more uniformly distributed across the dayside.
Evidence of ENA precipitation in the EUV dayglow
Authors: Stephan Andrew W. , Chakrabarti Supriya , Cotton Daniel M.
Published: February 2004

Observations from the STP 78‐1 satellite, at 600 km altitude, of the OI 989 and 1304 Å, and OII 834 Å dayglow emissions between March 20–29, 1979 show brightening at low and mid‐latitudes (<30° geomagnetic latitude) during enhanced geomagnetic activity, as determined by the Dst index. We attribute this increased dayglow, which varies by up to 20% from average quiet time emissions, to increased energetic neutral atom production in the ring current. These particles precipitate to lower altitudes where they collisionally excite ambient atmospheric and ionospheric oxygen, that manifest in enhanced airglow intensities. We present our results and the evidence from which we conclude we have detected the first evidence of extreme ultraviolet dayglow excited by low‐latitude precipitation of energetic neutral atoms.
Evolution of the Sun's Spectral Irradiance Since the Maunder Minimum
Authors: Lean Judith
Published: February 2004

Because of the dependence of the Sun's irradiance on solar activity, reductions from contemporary levels are expected during the seventeenth century Maunder Minimum. New reconstructions of spectral irradiance are developed since 1600 with absolute scales traceable to space‐based observations. The long‐term variations track the envelope of group sunspot numbers and have amplitudes consistent with the range of Ca II brightness in Sun‐like stars. Estimated increases since 1675 are 0.7%, 0.2% and 0.07% in broad ultraviolet, visible/near infrared and infrared spectral bands, with a total irradiance increase of 0.2%.
Periodic perturbations in Saturn's magnetic field
Authors: Espinosa Stéphane A. , Dougherty Michele K.
Published: February 2004

Analysis of all the existing magnetic field data from Saturn's magnetosphere is carried out. These data arise from the three spacecraft encounters with Saturn (Pioneer 11, Voyager 1 and 2). In order to carry out a direct comparison between the three magnetic field data sets, they are all expressed in the same planetocentric spherical polar coordinate system. In the cases of Pioneer 11 (whole encounter) and Voyager 2 (inbound) a surprising perturbation of a few nanoteslas is observed (with a period close to Saturn's rotation period) in the radial and/or azimuthal components of the magnetic field. It is shown that a dipole tilt and/or current sheet crossings are unlikely causes for such periodic perturbations. This is in agreement with the negligible magnetic dipole tilt resulting from the existing models of Saturn's magnetic field.
Combined Ionospheric Campaign 1: Ionospheric tomography and GPS total electron count (TEC) depletions
Authors: Bust Gary S. , Coco David , Makela Jonathan J.
Published: February 2004

Results from the June 1998 combined ionospheric campaign (CIC) are presented. The CIC represents an attempt to focus a large number of different instruments on one interesting geophysical region. The Center for Ionospheric Research (CIR) at Applied Research Laboratories, the University of Texas at Austin (ARL:UT), has had several computerized ionospheric tomography (CIT) receivers deployed in the Caribbean region since July 1997. In this paper we compare CIT data, GPS TEC data and data from the incoherent scatter radar at Arecibo to try to obtain an understanding of the temporal and spatial distribution of ionospheric structure observed during the campaign. We use the three data sets as inputs to the 3DVAR tomography algorithm developed at CIR and present results of the 3DVAR “objectively analyzed” electron density field. An ionization wall was found near 40° latitude in agreement with previous Millstone Hill and DMSP observations in high Kp. Several elongated density depletions were also detected.
Electromagnetic proton cyclotron anisotropy instability: Wave‐particle scattering rate
Authors: Gary S. Peter , Yin Lin , Winske Dan
Published: February 2004

The electromagnetic proton cyclotron instability is driven by a proton temperature anisotropy T⟂p/T∥p>1 where the directional subscripts denote directions relative to the background magnetic field. Enhanced field fluctuations from instability growth lead to wave‐particle scattering which reduces the anisotropy; previous research has demonstrated that the anisotropy is constrained by a power‐law function of β∥p. Here two‐dimensional hybrid simulations in a homogeneous, magnetized, collisionless plasma are used to derive a scaling relation for the maximum scattering rate of proton anisotropy reduction. The result is a function only of the anisotropy and β∥p and so is appropriate for use in fluid models of anisotropic plasmas such as the magnetosheath.
Dayside/nightside auroral X ray emission differences‐Implications for ionospheric conductance
Authors: Petrinec S. M. , Imhof W. L. , Chenette D. L. , Mobilia J. , Rosenberg T. J.
Published: February 2004

In this study we examine the occurrence probability of intense X ray emissions (>2 keV) from the auroral ionosphere in daylight and in darkness. The Polar Ionospheric X ray Imaging Experiment (PIXIE) on board the NASA/GGS Polar spacecraft remotely images the Earth's ionosphere from high altitude (apogee of about 9 Earth radii). This instrument is capable of observing the entire northern auroral region at once, and is normally unaffected by scattered sunlight; resulting in an unbiased study of differences in auroral emissions between daylight and darkness. The differences in the occurrence probability between daylight and darkness are interpreted as being due to differences in the local ionospheric conductivity, as described in previous studies using in situ particle populations and lower energy ionospheric emissions. There is also a clear dependence upon season which is in accordance with the Russell‐McPherron effect. Under similar geomagnetic conditions, comparisons of occurrence probability separated by whether each region is in daylight or in darkness illustrate that intense auroral emissions are consistently more likely when a given region is in darkness than when the same region is in daylight.
Solar cycle effects in planetary geomagnetic activity: Analysis of 36‐year long OMNI dataset
Authors: Papitashvili V. O. , Papitashvili N. E. , King J. H.
Published: February 2004

NSSDC's OMNI dataset, which now spans 1963–1999, contains a collection of hourly means of interplanetary magnetic field (IMF) and solar wind (SW) plasma parameters measured near the Earth's orbit, as well as some auxiliary data. We report a study of solar cycle effects in planetary geomagnetic activity in which 27‐day averages of several OMNI parameters are compared with equivalent Kp and Dst averages. Some established trends in these parameters over solar cycles are confirmed; for example, it is concluded that changes in the magnitude (rather than in direction) constitute the primary solar cycle variation in the IMF. However, this study also reveals that long‐term changes in planetary geomagnetic activity are driven more actively by solar wind‐magnetosphere coupling of an electrodynamic nature rather than by plasma transport into the magnetosphere. This suggests that ambient (background) interplanetary “electric” environment (in which the Earth's magnetosphere is immersed over the solar cycles) may play a more significant role in causing changes in the frequency of geomagnetic storms and substorms than previously realized.
A gyrophase‐bunched electron signature upstream of the Earth's bow shock
Authors: Gurgiolo C. , Larson D. , Lin R. P. , Wong H. K.
Published: February 2004

A possible sustained signature of a gyrophase‐bunched electron distribution just upstream of the Earth's bow shock is presented. The data, obtained by the ISTP WIND 3‐D Plasma and Energetic Particle Experiment, show several clear examples of a non‐gyrotropic, energetic electron distribution moving into the upstream. The signature is probably that of a locally phase‐trapped electron distribution.
The solar spectral irradiance since 1700
Authors: Fligge M. , Solanki S. K.
Published: February 2004

The change in the irradiance spectrum of the Sun from 1700 to the last solar minimum is determined and compared to the change in the spectrum between activity minimum and maximum. For this purpose we have used detailed model flux spectra of solar magnetic features. Also, time‐series of the solar spectral irradiance since 1700 in different wavelength bands are reconstructed. We expect that these reconstructions are more accurate than previously published ones, although they suffer (like all reconstructions of solar irradiance on such time‐scales) from uncertainties in our knowledge of the evolution of the solar network with time.
Solar‐cycle variation of low density solar wind during more than three solar cycles
Authors: Richardson I. G. , Berdichevsky D. , Desch M. D. , Farrugia C. J.
Published: February 2004

The May, 1999 low density (<1 cm−3) solar wind interval is one of a series of intervals of low density solar wind which have been detected since in‐situ, near‐Earth observations began. Examining the NSSDC OMNI database since 1965, covering solar Cycles 20–23, we show that such intervals, which are also periods of unusually low mass flux and low dynamic pressure, occur most frequently around sunspot maximum and are rarer at solar minimum. The occurrence rate of low‐density plasma may be higher in weaker sunspot cycles (Cycle 20 and the current Cycle 23). Around two‐thirds of periods with densities ≤1 cm−3 are associated with transient solar wind structures, in particular with ejecta and post‐shock flows. The majority of other events are associated with corotating streams. The May 1999 event is unusual because it is not associated with an ejecta or stream. A similar period was observed in July–August 1979.
Normal modes modelling of post‐seismic ionospheric oscillations
Authors: Artru Juliette , Lognonné Philippe , Blanc Elisabeth
Published: February 2004

Since 1960, several experiments have shown strong perturbations of the ionosphere after earthquakes. For the biggest quakes, Doppler ionospheric soundings have revealed displacements of several tens of meter of the ionized layers E and F. An accurate modelling of these phenomena was however never described.

We present here synthetic displacements of such oscillations and compare it with experimental data. Realistic models of the atmosphere including viscosity are used, as well as realistic Earth and seismic source model. These ionospheric oscillations are computed by normal modes summation taking explicitly into account the coupling between the solid Earth and the open atmosphere. They are associated to the Rayleigh fundamental surface waves. We obtain a good agreement between synthetics and observations, so it should be possible to use this method to study the ionospheric post‐seismic perturbations and possibly the high altitude profile of the density and the viscosity of the atmosphere.