bluuebunny/crossref_metadata_2025 huggingface dataset:
Interhemispheric comparison of the development of the stratospheric polar vortex during fall: A 3‐dimensional perspective for 1991–1992
Authors: Manney Gloria L. , Zurek Richard W.
Published: February 2008

The development of the stratospheric polar vortex during fall and early winter in the Northern Hemisphere (NH) during 1991–1992, and the Southern Hemisphere (SH) during 1992 is examined using National Meteorological Center data. Compared to the NH, the polar vortex in the SH developed with less variability on short time scales, deepened more rapidly and continued to expand well into winter. Daily minimum temperatures in the lower stratosphere were lowest at equivalent seasonal dates in both hemispheres, but values below the condensation temperatures of polar stratospheric clouds occurred earlier, persisted much longer, and occupied a larger volume of air in the SH. These interhemispheric meteorological differences can account for some of the key features of the chlorine monoxide distributions observed by the Microwave Limb Sounder on the Upper Atmosphere Research Satellite.
On the relationship between stratospheric aerosols and nitrogen dioxide
Authors: Mills M. J. , Langford A. O. , O'Leary T. J. , Arpag K. , Miller H. L. , Proffitt M. H. , Sanders R. W. , Solomon S.
Published: February 2008

Observations of the stratospheric NO2 column amount over the Colorado mountains during January, February, and March, 1992 are compared to concurrent lidar measurements. During this period, large aerosol amounts due to the volcanic material injected into the stratosphere by Mt. Pinatubo were observed over Colorado. The observed NO2 column displays a strong anti‐correlation with increasing amounts of stratospheric aerosol near 25–30 km for relatively low aerosol loadings. At higher aerosol loadings, the NO2 response appears to saturate, displaying little further reduction. The reasons for the altitude dependence of this relationship are probed with a one‐dimensional stratospheric model, and it is shown that such a saturation response is to be expected for conditions where the heterogeneous reaction between N2O5 and H2O dominates the NOx chemistry, as in Prather [1992]. The results therefore provide evidence that heterogeneous reactions influence the chemical composition of the mid‐latitude stratosphere, and in the general manner predicted by theory.
A long‐term decrease in Arctic haze at Barrow, Alaska
Authors: Bodhaine Barry A. , Dutton Ellsworth G.
Published: February 2008

Surface aerosol scattering measurements have been conducted at Barrow, Alaska, from May 1976 to the present using a four‐wavelength nephelometer. Total column aerosol optical depth measurements have been obtained over the same time period. Both data sets show a maximum in 1982 and then a decreasing trend to the present. This decreasing trend is apparent, and statistically significant, only in March–April. Arctic haze, caused by long‐range transport from midlatitude industrial regions, is most evident in the vicinity of Barrow during this time of year. We suggest that the decrease in Arctic haze at Barrow, as observed in the aerosol light scattering and optical depth records, is due to decreased anthropogenic pollution emissions in Europe and the former Soviet Union, the primary source regions for the springtime aerosol at Barrow. Volcanic effects in the stratosphere have been subtracted from the optical depth data, and are not believed to be significant in the surface‐based data.
Morphostructure and magnetic fabric of the northwestern North Fiji Basin
Authors: Pelletier Bernard , Lafoy Yves , Missegue François
Published: February 2008

Four successive spreading phases are distinguished in the northwestern part of the North Fiji Basin. After an initial NE‐SW opening, a N‐S spreading phase took place, up to the northwesternmost tip of the basin, along the South Pandora, Tikopia and 9°30 Ridges. The N‐S spreading phase in the northern North Fiji Basin was followed by an E‐W opening phase along the central North Fiji Basin axis. A triple junction was probably active during an intermediate stage between the two phases. E‐W spreading underwent a reorganisation that induced the functioning of the 16°40′S triple junction and the development of the E‐W trending Hazel Holme Extensional Zone from the active central spreading axis to the southern tip of the New Hebrides Back‐Arc Troughs. Active extension also occurs along the E‐W Santa Cruz Trough which crosscuts the arc platform at the northern end of the N‐S trending Back‐Arc Troughs. The existence of the Back‐Arc Troughs is mainly due to the construction of the 400 km‐long volcanic Duff Ridge which trapped a piece of the old North Fiji Basin oceanic crust.
Instabilities from phase transitions and the timescales of mantle thermal evolution
Authors: Steinbach Volker , Yuen David A. , Zhao Wuling
Published: February 2008

We have investigated the effects of mantle phase‐transitions on the Earth's thermal evolution by numerical modelling. Our results show that the tendency of layering grows with increasing vigor of convection. A self‐consistent model which couples the convective dynamics of the mantle to the secular cooling of the core is constructed. For a temperature‐dependent effective viscosity, we found that the thermal history of the Earth is characterized by two timescales, one is associated with layered convection in the early period, which is followed by a transition period with dramatic overturns. As the Rayleigh number decreases sufficiently low, whole mantle convection prevails and a faster cooling timescale ensues.
Measurement of expansion velocity of an impact‐generated vapor cloud
Authors: Kadono Toshihiko , Fujiwara Akira , Nakamura Akiko M.
Published: February 2008

The expansion velocity of an impact‐generated vapor cloud was determined. In the impact of a 7mm nylon sphere into a basaltic cube at velocity 3.8km/s, the front velocity of a luminous vapor(plasma) cloud expanding hemispherically was constant at about 4km/s within at least 5cm distance from the impact point. A simplified model calculation used to estimate the distribution of mass in the cloud suggests that most of the mass should lie near the front of the expanding cloud.
Winds and waves from the Canadian prairies (CNSR) Triple‐MF Radar System
Authors: Manson A. H. , Meek C. E. , Hall G. , McMurray R.
Published: February 2008

Two Medium Frequency radars have been developed 500 km from the Saskatoon MF Radar (52.2N, 107.1W) to form a triangle within the Canadian Prairies. The western system is at Sylvan Lake (52.4N, 114.1W) in the lee of the Rockies and the southern system is at Robsart (49.2N, 109.3W). In this preliminary assessment of winds data, daily mean winds and solar tides (24‐, 12‐hour) are shown to be very similar, with short‐term variations of several days being seen at all three radars. This shows that such short term changes are of larger scale than 500 km. Variations of the wind field with periods ranging from minutes to several hours are seen at all radars, presumably associated with Internal Gravity Waves. Of these only the longer periods appear to be correlated over this spatial scale. Spectral analysis techniques developed for the smaller GRAVNET system are applied to these oscillations, as are hodograph techniques, to determine the IGW characteristics.
Climate implications of observed changes in ozone vertical distributions at middle and high latitudes of the northern hemisphere
Authors: Wang Wei‐Chyung , Zhuang Yi‐Cheng , Bojkov Rumen D.
Published: February 2008

Ozone soundings at middle and high latitudes of the Northern Hemisphere during recent decades indicate a decreasing trend in the lower stratosphere and an increasing trend in the troposphere especially at the upper troposphere. Changes in the O3 vertical distribution can change the radiative forcing (solar and longwave radiation) to the troposphere‐surface system with climate implications. Here, we use the O3 soundings at seven middle and high latitudes stations to study its variations in the last few decades and discuss the climate implications in the context of increasing other greenhouse gases CO2, CH4, CFC‐11, CFC‐12 and N2O. The results suggest that changes in O3 vertical distribution can perturb substantially the radiative forcing, in particular at the middle latitudes where the tropospheric increases can induce a positive radiative forcing (warming) with magnitude comparable to that due to increasing these other greenhouse gases.
Long‐lived artificial ion clouds in the Earth's ionosphere
Authors: Milinevsky G. P. , Kashirin A. I. , Romanovsky Yu. A. , Stenbaek‐Nielsen H. C. , Kelley M. C.
Published: February 2008

In two barium releases made over the Caribbean during the summer of 1991, the ion cloud was observed to last more than 10 hours. Observations were made with a low light level TV imager and a TV spectrograph operated onboard a Russian research vessel conducting ionospheric research and participating in the NASA‐CRRES mission. The two clouds were observed near the release region for 15 to 25 minutes during the twilight following the initial release. One of the releases was made during evening twilight and the ion cloud reappeared the following morning, while the other was a morning release and the late‐time observations made the next evening. The observations are highly surprising since, generally, it has been assumed that the ion cloud would dissipate within a few hours. However, the long life time appears consistant with current knowledge about dissipative processes. The long lifetime of barium ion clouds may provide for the most sensitive experimental testing of ionospheric convection models, a possibility with clear implications for future barium release experiments.
Reformation of a coronal helmet streamer by magnetic reconnection after a coronal mass ejection
Authors: Hiei E. , Hundhausen A. J. , Sime D. G.
Published: February 2008

A bright feature observed on Jan. 24–26, 1992 with the soft X‐ray telescope on the YOHKOH spacecraft and with the coronameter at the Mauna Loa Solar Observatory assumed the appearance of a coronal helmet streamer as it slowly expanded. Mauna Loa observations from Jan. 22–24 indicate that a prominence eruption and coronal mass ejection occurred before this feature was seen. We interpret the Jan. 24–26 observations as evidence for “reformation” of a magnetically closed helmet structure as a consequence of magnetic reconnection that proceeded along a vertical magnetic neutral sheet formed by the mass ejection.
Energetics of the dayside ionosphere of Venus
Authors: Dobe Zoltan , Nagy Andrew F. , Brace Larry H. , Cravens Thomas E. , Luhmann Janet G.
Published: February 2008

A reanalysis of the Pioneer Venus electron temperature data base showed a strong correlation between elevated electron temperatures and induced magnetic fields in the day side ionosphere above about 200 km. These results suggest, although not conclusively, that the elevated temperatures are the result of reduced vertical conductivities caused by the horizontal, induced fields with a possible contribution from energy deposition by magnetosheath electrons moving along the field from the tail region.
Seasonal variation of N2O fluxes at a tropical savannah site: Soil consumption of N2O during the dry season
Authors: Donoso Loreto , Santana Rafael , Sanhueza Eugenio
Published: February 2008

Soil fluxes of N2O were measured during the dry season in a scrub‐grass savannah and in a nearby semideciduous forest in the savannah climatic region of Venezuela. In contrast with a significant emission recorded previously in the rainy season, during the dry season a net consumption of −2.6±2.7×109N2O molecules cm−2s−1was recorded at the savannah site. After watering a net emission of N2O was observed. The forest soil produced signicantly less N2O compared with the rainy season. As in the rainy season the addition of NH4Cl did not affect the fluxes. Larger emissions were recorded after the application of nitrate. However, the increase was lower than the enhancement produced during the rainy season. The comparison between both seasons indicates that the N2O fluxes strongly depend on the rain pattern of the region, and that during the dry season the bacterial activity or bacterial population responsible for the production of N2O must be low. At present we do not know the mechanism by which dry savannah soil consumes atmospheric N2O. This potential sink is of the same order as the production of N2O due to savannah vegetation burning.
Monitoring of strain release in central and northern California using broadband data
Authors: Romanowicz Barbara , Dreger Douglas , Pasyanos Michael , Uhrhammer Robert
Published: February 2008

Systematic cataloguing of seismic moment, depth and mechanism of regional earthquakes down to magnitude M∼4 can now be achieved in close to real‐time using data from sparse networks of digital broadband stations. The procedure we have developed relies on 3 independent methods for the determination of moment tensor, providing confidence limits on the results. We show the results of application to the seismicity in central and northern California for a one year period starting in March 1992 and illustrate how patterns of stress and strain release can now be monitored systematically and reliably in a timely fashion.
AASE‐II Observations of trace carbon species distributions in the mid to upper troposphere
Authors: Anderson B. E. , Collins J. E. , Sachse G. W. , Whiting G. W. , Blake D. R. , Rowland F. S.
Published: February 2008

We report tropospheric (altitudes > 5 km) observations of CO2, CO, CH4, and light hydrocarbons (C2 ‐ C4) over the latitude range from 90°N to 23°S recorded onboard the NASA DC‐8 aircraft during the winter 1992 Second Airborne Arctic Stratospheric Expedition (AASE‐II). Mixing ratios for these species exhibited significant north‐south gradients with maximum values in subpolar and arctic regions and minima over the southern tropics. At latitudes > 40°N, the mixing ratios of most species increased significantly over the course of the 3‐month measurement period. Also at high northern latitudes, the variations of all relatively long‐lived reactive carbon species were linearly correlated with fluctuations of CO2 with CO, CH4, C2H6, C2H2, C3H8, and n‐C4H10 exhibiting average enhancement ratios in terms of ppbV(X)/ppmv(CO2) of 13.8, 8.4, 0.21, 0.075, 0.085, and 0.037, respectively.
Repeat temperature measurements in Borehole GC‐1, northwestern Utah: Towards isolating a climate‐change signal in borehole temperature profiles
Authors: Chapman David S. , Harris Robert N.
Published: February 2008

Temperature‐depth profiles in borehole GC‐1, northwestern Utah, were measured in 1978, 1990, and 1992. Borehole temperatures below 80 m depth are highly reproducible over the 14 year period indicating long term thermal stability. A slowly changing temperature field above 80 m depth has similiar characteristics to synthetic temperature profiles computed from a 100 year record of air temperature changes at Park Valley weather station 50 km northeast of the borehole site.
Balloon‐borne in situ measurements of CLO and ozone: Implications for heterogeneous chemistry and mid‐latitude ozone loss
Authors: Avallone L. M. , Toohey D. W. , Brune W. H. , Salawitch R. J. , Dessler A. E. , Anderson J. G.
Published: February 2008

In situ measurements of chlorine oxide (ClO) obtained on 31 March 1991 with a new balloon‐borne instrument are compared to results from a photochemical model which incorporates hydrolysis of N2O5 on sulfate aerosols. With the addition of this process, there is better agreement between calculation and measurement over most of the profile, except below 20 km where observed ClO is greater by as much as a factor of four. In a model which is constrained to reproduce the observed ClO below 20 km, ozone loss by catalytic cycles involving halogen oxides becomes larger than that from NOx, which would dominate under gas‐phase or standard heterogeneous conditions.
In situ measurements of the ClO/HCl ratio: Heterogeneous processing on sulfate aerosols and polar stratospheric clouds
Authors: Webster C. R. , May R. D. , Trohey D. W. , Avallone L. M. , Anderson J. G. , Solomon S.
Published: February 2008

Simultaneous in situ measurements of stratospheric ClO and HCl have been made for the first time during numerous flights of the ER‐2 aircraft covering latitudes 24–90°N from October 1991 through March 1992. The ClO/HCl ratio is identified as a key indicator of heterogeneous processing both outside and within the Arctic polar vortex. For ClO mixing ratios below about 120 pptv, remarkably constant ClO/HCl values of about 15% characterize the lower stratosphere. The observed values are significantly higher than those derived from a 2‐D model using either gas phase photochemistry alone (2%), or including heterogeneous sulfate chemistry (5–10%). During the Arctic early spring, after conversion of HCl into reactive chlorine has taken place, the vortex edge is poorly defined by ClO levels. Loss of HCl and its slow recovery following low‐temperature polar heterogeneous chemistry distinguishes HCl as a new and unique dynamical tracer of PSC‐processed air.
Nonlinear unstable auroral‐Arc driven thermospheric winds in an ionosphere‐magnetosphere coupled model
Authors: Keskinen M. J. , Satyanarayana P.
Published: February 2008

The nonlinear evolution of thermospheric winds in an ionosphere‐magnetosphere coupled model has been studied for the first time for a dynamic unstable auroral‐arc environment. We treat the problem using a multi‐layer, quasi‐three‐dimensional model which averages in altitude the thermospheric dynamics over each layer. For the upper thermosphere, we find that (1) the thermosphere can respond to the ionospheric Kelvin‐Helmholtz (KH) instability on temporal scales on the order of an hour, depending on ambient conditions, and on spatial scales of tens to hundreds of kilometers, (2) strong thermospheric meridional and zonal vortical flows with embedded nonlinear jet‐like structures can be generated by the ionospheric/magnetospheric KH instability and (3) neutral thermospheric winds, vortices, and associated power spectra develop in a distinctly different manner in the presence of magnetospheric coupling effects. Comparison with recent observations is made.
Paleomagnetic and 40Ar/39Ar evidence for remagnetization of Mesozoic oceanic rocks on the Vizcaino Peninsula, Baja California Sur, Mexico
Authors: Hagstrum Jonathan T. , Martínez Margarita López , York Derek
Published: February 2008

Previously published paleomagnetic data for Upper Jurassic pillow lavas of the Vizcaino Peninsula indicate that they were deposited near a paleolatitude of 14°N or S. Whether or not this result implies northward transport with respect to the continental interior has been controversial due to the lack of reliable Jurassic reference poles for the North American plate. Available paleomagnetic data for nearby Upper Triassic pillow basalts and overlying pelagic sediments at Punta San Hipólito, however, fail a fold test indicating that these rocks were remagnetized post‐folding. Indistinguishable in‐situ paleomagnetic directions and perturbed 40Ar/39Ar age spectra for the Triassic and Jurassic pillow lavas are consistent with resetting of their magnetic and isotopic systems in the middle Cretaceous, probably during burial by the overlap Valle Formation (>10 km thick). Resetting apparently occurred post‐deformation so the paleolatitude of remagnetization is unknown. High‐coercivity directions from a few samples of the Triassic rocks might represent an older magnetization acquired during deposition at paleolatitudes near 6°N or S.
Fast likelihood computation of epidemic type aftershock‐sequence model
Authors: Ogata Yosihiko , Matsu'ura Ritsuko S. , Katsura Koichi
Published: February 2008

The Epidemic Type Aftershock‐Sequences (ETAS) model is a point process representing the temporal activity of earthquakes in a certain geophysical region. A method for fast and accurate computation of the likelihood is suggested for the ETAS model. Double sums of variables in the logarithm of the likelihood function with respect to the earthquake occurrence data is reduce to a single sum by using the recursive relation of the integrands in the Gamma transformation of the modified Omori functions. Then the integral is computed in unusually high accuracy by using the Double Exponential transformation followed by an application of the trapezoidal rule with an equal mesh size. The computing run time is greatly improved to be proportional to the number of recorded earthquakesNcompared to the order ofN² for the naive computation method. The present numerical procedures put the parameter estimation of the ETAS model to practical use in large number of occurrence records.
Ion measurements during Pioneer Venus reentry: Implications for solar cycle variation of ion composition and dynamics
Authors: Grebowsky J. M. , Hartle R. E. , Kar J. , Cloutier P. A. , Taylor H. A. , Brace L. H.
Published: February 2008

During the final, low solar activity phase of the Pioneer Venus mission, the Orbiter Ion Mass Spectrometer measurements found all ion species, in the midnight‐dusk sector, reduced in concentration relative to that observed at solar maximum. Molecular ion species comprised a greater part of the total ion concentration as O+ and H+ had the greatest depletions. The nightside ionospheric states were strikingly similar to the isolated solar maximum “disappearing” ionospheres. Both are very dynamic states characterized by a rapidly drifting plasma and 30–100 eV superthermal O+ ions.
First southern hemisphere intercomparison of measured solar UV spectra
Authors: McKenzie R. L. , Kotkamp M. , Seckmeyer G. , Erb R. , Roy C. R. , Gies H. P. , Toomey S. J.
Published: February 2008

Three UV spectroradiometers from the National Institute of Water and Atmospheric Research (NIWA) New Zealand, the Fraunhofer Institute (IFU) Germany, and the Australian Radiation Laboratory (ARL) Australia were intercompared at Lauder NZ on 23 February 1993. Over the spectral range 290–400 nm, the agreement between the IFU and NIWA instruments was better than 5%. At noon on this day, the irradiances measured by all three instruments agreed within ±10%, except at wavelengths shorter than 300 nm, where the ARL instrument gave higher readings. At larger solar zenith angles (SZA) the differences at short wavelengths were more pronounced, and at wavelengths above 300 nm the ARL measurements were systematically lower. The reasons for these differences are discussed. Having established the differences between the sets of instrumentation, spectra of maximum clear sky UV irradiances observed by these groups in New Zealand, Australia, and Europe are compared. The erythemally weighted irradiance observed in Melbourne Australia was the highest (0.35 W m−2). Respective maxima for Lauder NZ and for Neuherberg Germany were 85% and 66% of that in Australia. Differences are larger for DNA‐weighted UV.
The atmospheric SO2 budget for Pinatubo derived from Noaa‐11 SBUV/2 spectral data
Authors: McPeters Richard D.
Published: February 2008

Spectral scan data from the NOAA‐11 SBUV/2 instrument were used to derive SO2 for three days following the eruption of Mt. Pinatubo ‐ June 19, July 1, and July 17, 1991. Band structure between 300 and 310 nm observed in the backscattered albedo uniquely identifies the presence of SO2. Band ratios are used to infer SO2 amounts to better accuracy (∼10% to 20%) and sensitivity (about 0.5 milli‐atm‐cm of SO2) than the TOMS retrieval, but with relatively poor spatial coverage because the measurement is nadir only. Only 7 scans showed detectable SO2 on June 19 when the cloud was still very localized. On July 1 there were 29 scans between 35°N and 12°S with SO2, with the highest concentration detected over the Atlantic, and on July 17 SO2 was detected in 30 scans around the world, but in decreased concentration. Estimates of the total SO2 budget made after the cloud had spread sufficiently for the sparse SBUV/2 sampling to be adequate indicated that there were 8.4 million metric tons (MMT) of SO2 in the stratosphere on July 1, 1991, and 4.1 MMT remaining on July 17. This corresponds to an e‐folding time of about 24 days for the conversion of SO2 to aerosol, and is consistent with an initial injection into the stratosphere of 12–15 MMT of SO2.
In situ measurements of CLO at mid‐latitudes: Is there an effect from Mt. Pinatubo?
Authors: Avallone L. M. , Toohey D. W. , Proffitt M. H. , Margitan J. J. , Chan K. R. , Anderson J. G.
Published: February 2008

Observations of ClO from 20 to 60°N made before and after the eruption of Mt. Pinatubo are compared for changes which may result from increased sulfate aerosol surface area. Using ozone as a vertical coordinate and examining data at similar latitudes and seasons, elevated abundances of ClO are found at low latitudes (20 to 30°N), an effect which decreases with increasing latitude. For the flights compared, there appear to be no differences, within the uncertainty of the measurements, at latitudes poleward of 40°N. These results are consistent with the idea that the hydrolysis of N2O5 on sulfate aerosols becomes saturated at moderate aerosol loadings.
Model calculations of the dayside ionosphere of Venus at solar minimum
Authors: Shinagawa H.
Published: February 2008

The dayside ionosphere of Venus at solar minimum was studied using one‐dimensional MHD model. The observed electron density profiles were reproduced reasonably well, assuming that the ionosphere of Venus at solar minimum is magnetized by the solar wind. A bump or a ledge structure of the electron density profile at the altitude range of 170–200 km observed by the Pioneer Venus radio occultation measurements were also reproduced under magnetized conditions. It was found that the bump structure is more apparent and more realistic for lower electron temperatures, suggesting that electron temperatures at low altitudes (<200 km) significantly decrease from a solar maximum period to a solar minimum period.
Initial PVO evidence of electron depletion signatures downstream of Venus
Authors: Intriligator D. S. , Hartle R. E. , Perez‐de‐Tejada H. , Siscoe G. L.
Published: February 2008

This first analysis of Pioneer Venus Orbiter (PVO) plasma analyzer electron measurements obtained in early 1992 during the PVO entry phase of the mission indicates the presence downstream from the terminator of a depletion or “bite out” of energetic ionosheath electrons similar to that observed on Mariner 10. There is more than one possible explanation for this energetic electron depletion. If it is due to atmospheric scattering, the electrons traveling along draped magnetic flux tubes that thread through the Venus neutral atmosphere would lose energy from impact ionization with oxygen. The cross‐section for such electron impact ionization of oxygen has a peak near 100 eV, and it remains high above this energy, so atmospheric loss could provide a natural process for electrons at these energies to be selectively removed. In this case, our results are consistent with the Kar et al. (1994) study of PVO atmospheric entry ion mass spectrometer data, which indicates that electron impact plays a significant role in maintaining the nightside ionosphere. Although it is appealing to interpret the energetic electron depletion in terms of direct atmospheric scattering, alternatively it could result from strong draping which connects the depletion region magnetically to the weak downstream bow shock and thereby reduces the electron source strength.
The nightside ionosphere of Venus under varying levels of solar Euv flux
Authors: Ho C. M. , Strangeway R. J. , Russell C. T. , Luhmann J. G. , Brace L. H.
Published: February 2008

Solar activity varied widely over the 14 year lifetime of the Pioneer Venus Orbiter, and these variations directly affected the properties of the nightside ionosphere. At solar maximum, when solar EUV was largest, the Venus ionosphere was found to extend to highest altitudes and nightward ion transport was the main source of the nightside ionosphere. At solar minimum, nightward ion transport was reduced, and electron precipitation was thought to be the main source. In this study, we have attempted a separation of spatial variations from temporal variations by examining the altitude profiles of the magnetic field, and electron density and temperature for three different solar EUV flux ranges. In the upper ionosphere and near‐planet magnetotail (h > 1800 km), the solar EUV effects are significant. The electron density decreases about an order of magnitude from high to low EUV flux, while the electron temperature at least doubles. The magnetic field also increases 2–3 nT. In the lower ionosphere (200 – 600 km), lower EUV fluxes are associated with slightly reduced density, and higher temperature. These results are in accord with recent entry phase observations, where the electron density measured above the ionospheric density peak is lower than that observed at solar maximum during the early Pioneer Venus mission.
Spatial and temporal characterization of Hoar Formation in central Greenland using SSM/I brightness temperatures
Authors: Shuman Christopher A. , Alley Richard B.
Published: February 2008

The summertime formation and burial of low‐density, coarse‐grained, hoar layers in the snow of central Greenland can be mapped using satellite passive‐microwave data. Variations in a signal derived from Special Sensor Microwave/Imager (SSM/I) brightness temperature trends correlate temporally and spatially with hoar complex evolution over ≈120,000 km² of the Greenland ice sheet's dry firn facies. Observations at the Greenland Ice Sheet Project II (GISP2) site indicate that changes in surface conditions and brightness temperatures correspond to four hoar events over two summers, as expected from theory. Following a snowfall, the smooth, high‐density surface reflects some emitted, 37 GHz, horizontally‐polarized (H) radiation but little vertically‐polarized (V) radiation. Progressive surface roughness increases and density decreases during hoar formation cause a gradual decrease in H reflection. Formation and burial of a hoar layer thus causes a slow decrease followed by an abrupt increase in the V/H ratio. Hoar layers have been used to date the GISP2 ice core through the entire Holocene; archived microwave data now can be used to assess the timing and frequency of the formation of these stratigraphic markers in central Greenland.
Anomalous amplification of the Earth's normal modes near the epicenter due to lateral heterogeneity
Authors: Tsuboi Seiji , Um Junho
Published: February 2008

The broadband seismograms recorded at stations near the epicenter of the Landers, California earthquake of 28 June 1992 exhibit observed amplitudes of the spheroidal modes of the Earth that are nearly one order of magnitude larger than those calculated for a spherically symmetric Earth model. We show that these anomalous observations at stations close to the epicenter can be explained by coupling between the normal modes due to the earth's asphericity. We calculate and sum fully coupled normal modes for three‐dimensional shear‐wave velocity models of the Earth's mantle to calculate synthetic seismograms. Our coupled‐mode synthetic seismograms explain the observed anomalously large amplitude of spheroidal modes and indicate that the coupling between the normal modes due to laterally heterogeneous elastic structure of the Earth causes this anomalous observation. Our results also show that the present three‐dimensional Earth models give qualitative fits to this observation.
Excitation of Jovian normal modes by an impact source
Authors: Kanamori Hiroo
Published: February 2008

We estimated the amplitudes of Jovian normal modes excited by a near‐surface impact. The planet is modeled as a self‐gravitating fluid sphere, and the impact source is approximated by a single force or a point pressure source. The effects of rotation are not included. An impact of a comet with a radius of 3 km and momentum Mν=6.8×1023 g‐cm/s is modeled as a point source 100 km below the Jovian surface, assumed to be at 1 bar. The peak‐to‐peak amplitudes of displacement and velocity of disturbance with a period of 700 s are approximately 2.1 m and 1.9 cm/s, respectively, at a distance of 90° from the impact. An amplification of a factor of about 25 occurs near the antipode. This estimate is not sensitive to the assumptions on the density distribution above the surface. The amplitude in the higher altitude can be significantly higher. At shorter periods (100 to 200 s), the amplitudes depend on the source depth. For an impact with Mν=1023 g‐cm/s, and for a source depth of about 20 to 50 km, the amplitude and the particle velocity at a distance of 90° are of the order of 100 m, and 3 m/s, respectively. The pressure change at a depth of 20 to 50 km is of the order of 20 dyne/cm², or about 2×10−6 of the ambient pressure at a depth of 40 km.
An overview of NLC‐91: A rocket/radar study of the polar summer mesosphere
Authors: Goldberg R. A. , Kopp E. , Witt G. , Swartz W. E.
Published: February 2008

In late July and early August of 1991, a major suborbital scientific campaign (NLC‐91) involving scientists from eight countries was conducted at ESRANGE, Kiruna, Sweden and at Heiss Island, Russia. The purpose of the program was to investigate the chemical, dynamical, and electrodynamical properties of the polar summer mesosphere. Thirty one rocket flights were coordinated with two coherent radar facilities, EISCAT and CUPRI, and with other ground‐based observatories and facilities. This permitted direct comparison between the in situ measurements and those obtained by remote sensing of the mesosphere via continuous ground‐based monitoring. The primary objectives of the campaign were to study noctilucent clouds (NLCs) and polar mesospheric summer echoes (PMSEs), including their possible relationship to local aerosols and/or small scale turbulence. This overview describes the scientific program, discusses the geophysical conditions during launch activities, and reviews some of the preliminary results. More detailed results can be found in the papers which follow.
Ion mixing in the plasma sheet boundary layer by drift instabilities
Authors: Horton W. , Dong J. Q. , Su X. N. , Tajima T.
Published: February 2008

The linear stability properties of collisionless drift instabilities are analyzed in a Harris equilibrium model B0x tanh (z/L) of the plasma sheet boundary layer (PSBL). The streaming ions with drift type instabilities driven by υ0∥(z) ≃ υ(z) in the PSBL are considered. The fluid approximation leads to growth from ∂T/∂z and ∂υ0∥/∂z but predicts that the mode width Δz approaches the gyroradius ρi of the energetic ions. Thus an integral equation theory for the modes is developed taking into account that in the PSBL the curvature drift is weak compared with the grad‐B drift υD B. The exact wave particle resonance ω = kx υx + ky υD B (υ⊥²) is kept in the nonlocal response functions. Plasma density, temperature, and magnetic gradient drift motions are taken into account. The drift modes driven by ∂υx/∂z produce an anomalous cross‐field momentum transport mixing the PSBL ions on the time scale of tens of seconds. A nonlinear simulation is performed which shows the coalescence of the small scale, fast growing modes into large‐scale vortices. The relation between these collective modes and plasma sheet transport phenomena is discussed including the comparison with the competing plasma mixing from single‐particle stochasticity.
Incoherent scatter radar spectrum distortions from intense auroral turbulence
Authors: Knudsen D. J. , Haerendel G. , Buchert S. , Kelley M. C. , Steen Å , Brändström U.
Published: February 2008

We present EISCAT (European Incoherent Scatter Radar Facility) measurements of > 2000 K enhancements of the apparent ion temperature which occurred simultaneously over a latitude range of at least 100 km for brief periods (less than 1 min) in the auroral F region. One event occurred during a substorm onset and a second during passage of a westward‐traveling surge. The apparent Ti increases showed significant anisotropy, with measurements oriented less parallel to B0 exhibiting the largest amount of apparent Ti increase. In these examples the vector electric fields measured by EISCAT were much too low to account for the temperature increases via frictional heating, and also too low to generate non‐Maxwellian ion velocity distributions, which can cause errors in ion temperature estimates. We argue that the measured Ti increases are not real, and that both their magnitude and their anisotropy with respect to B0 can be satisfactorily explained by turbulent plasma flows with peak amplitudes of ∼ 2 km/s but which could not be directly resolved by EISCAT, because they varied with a time scale less than the 10‐s integration period, or possibly because their scale size was smaller than the ∼ 3 ‐ 5 km antenna beam width. While such unresolved but inferred turbulent flows can themselves cause ion frictional heating, we show that an equally important cause of high Ti estimates in our case was the distortion of the measured spectra by strongly varying Doppler shifts. We also present a counter example which shows a bright auroral arc in the radar common volume but with no large increases in the radar‐measured Ti, indicating that not all auroral structures are associated with electric fields which vary with sufficient intensity to distort incoherent scatter radar spectra.
Simulations of ion cyclotron anisotropy instabilities in the terrestrial magnetosheath
Authors: Gary S. Peter , Winske Dan
Published: February 2008

Enhanced transverse magnetic fluctuations observed below the proton cyclotron frequency in the terrestrial magnetosheath have been identified as due to the proton cyclotron and helium cyclotron instabilities driven by the T⊥ > T∥ condition of the sheath ions. One‐dimensional hybrid computer simulations are used here to examine the nonlinear properties of these two growing modes at relatively weak fluctuation energies and for wave vectors parallel to the background magnetic field. Second‐order theory predicts fluctuating magnetic field energies at saturation of the proton cyclotron anisotropy instability in semiquantitative agreement with the simulation results. In particular, introduction of the helium component enhances the wave‐particle exchange rate for proton anisotropy reduction by that instability, thereby reducing the saturation energy of that mode. The simulations demonstrate that wave‐particle interactions by the proton cyclotron and helium cyclotron instabilities lead to the anticorrelation of T⊥p/T∥p and β∥p, a result observed by Anderson and Fuselier (1993).
Solar dynamics and its impact on solar irradiance and the terrestrial climate
Authors: Nesme ‐Ribes E. , Ferreira E. N. , Sadourny R. , Le Treut H. , Li Z. X.
Published: February 2008

Among the various uncertainties present in climate modeling, the variability of total solar irradiance is not one of the least. For lack of any direct measure of the solar irradiance in the past, substitutes are needed. However, the difficulties are twofold: (1) the reliability of the proxies and (2) the need for some physical mechanism relating these proxies to the solar luminosity. On the basis of a better understanding of the solar machinery we can now propose a plausible scenario connecting the exchanges of energy between the various reservoirs: magnetic, thermal, gravitational, and kinetic. In the present paper we discuss the available proxies and suggest a way to reconstruct total solar irradiance over the past four centuries. The response of the Laboratoire de Météorologie Dynamique atmospheric general circulation model to magnetoconvective solar forcing during the Maunder minimum is discussed. The simulated cooling appears to be compatible with temperature data from the Little Ice Age; in addition, it is found that variations of globally homogeneous external forcing parameters, like incoming solar flux or greenhouse gas loading, yield climate responses with very similar geographical patterns.
Accelerated motions of the magnetopause as a trigger of the Kelvin‐Helmholtz instability
Authors: Mishin V. V.
Published: February 2008

We consider the effect of accelerated magnetopause motions arising from the arrival of a modified‐pressure region of the solar wind with a sharp forefront upon the growth rate of the velocity shear layer instability. The range of values of relative pressure variation on the front was taken to lie within 1.25‐2, and the front thickness ranged from 200 km to 1RE (6400 km). At different phases of motion of the boundary, the instability growth rate can change in either direction as a consequence of the Rayleigh‐Taylor effect, in accordance with a change of sign of acceleration g of the boundary. The typical time during which of the boundary remains inside the new equilibrium position (≤ 1 min) is sufficient for the instability to reach the nonlinear regime. It is possible that plasma flutes on the boundary penetrate the magnetosphere. Generation of disturbances can be modulated by boundary oscillations inside the new equilibrium position. We discuss the interplanetary magnetic field influence upon the growth rate of the MHD instability under consideration. An analysis is made of a number of daytime geophysical conditions which cannot be understood when interpreted in terms of the instability on a stationary boundary.
Electron velocity space hole modes
Authors: Coroniti Ferdinand V. , Ashour‐Abdalla Maha , Richard Robert L.
Published: February 2008

In space plasmas the local electron distribution is formed by ballistic motion from different source regions. Since electrons with positive and negative parallel (υ∥) velocities have different sources and experience different interactions with the large‐scale electromagnetic fields, the electron distribution function near υ∥ = 0 should possess considerable structure, including the possibility of velocity space holes, i.e., regions where the phase space density is zero, and sharp discontinuities. A particular hole distribution, which may occur upstream of slow shocks in the distant geomagnetic tail, is shown to support a nonstandard plasma wave mode with frequencies between the ion and electron plasma frequencies and parallel phase speed above the ion, but well below the electron, thermal speed. The hole mode can be destabilized by a positive slope in the low parallel velocity electron distribution, and the growth rate is strongly enhanced if the electron parallel temperature exceeds the perpendicular temperature; the mode can also be excited by a hot, fast ion beam. The hole modes exist only for a narrow range of propagation angles about the magnetic field direction. A discontinuity in the integrated parallel distribution function also supports a nonstandard wave mode which is unstable if the value of the distribution function increases across the discontinuity.
Theory and observations of electromagnetic ion cyclotron waves in Saturn's inner magnetosphere
Authors: Barbosa D. D.
Published: February 2008

High‐resolution Voyager 1 magnetic field observations of Saturn's inner magnetosphere are examined for the presence of ultralow frequency waves. Quasi‐circular left‐hand polarized transverse oscillations are found in the near‐equatorial region of 5‐7 RS with a wave period ∼10 s and peak amplitude ∼2 nT. The wave is identified as the electromagnetic oxygen cyclotron mode occurring at a frequency just below the O+ ion cyclotron frequency. The properties of the waves are very similar to those found by Smith and Tsurutani (1983) using Pioneer 11 data. A theoretical model of wave excitation based on gyroresonant coupling through a temperature anisotropy of O+ pickup ions is developed which accounts for the principal features of the wave spectrum. The theory and observations form the basis for the hypothesis that wave‐particle interactions provide a level of scattering commensurate with the weak pitch angle diffusion regime but nonetheless one that regulates and maintains a constant thermal anisotropy of ions along the magnetic field. Arguments are also presented that O+ was the dominant thermal ion of the Dione‐Tethys plasma torus at the time of the Pioneer 11 encounter the year previous to the Voyager 1 measurements.
On the generation of plasma waves in Saturn's inner magnetosphere
Authors: Barbosa D. D. , Kurth W. S.
Published: February 2008

Voyager 1 plasma wave measurements of Saturn's inner magnetosphere are reviewed with regard to interpretative aspects of the wave spectrum. A comparison of the wave emission profile with the electron plasma frequency obtained from in situ measurements of the thermal ion density shows good agreement with various features in the wave data identified as electrostatic modes and electromagnetic radio waves. Theoretical calculations of the critical flux of superthermal electrons able to generate whistler‐mode waves and electrostatic electron cyclotron harmonic waves through a loss‐cone instability are presented. The comparison of model results with electron measurements shows excellent agreement lending support to the conclusion that a moderate perpendicular anisotropy in the hot electron distribution is present in the equatorial region of L = 5‐8.
Kinetic simulations of the Kelvin‐Helmholtz instability at the magnetopause
Authors: Thomas V. A. , Winske D.
Published: February 2008

Two‐dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the kinetic evolution of the Kelvin‐Helmholtz instability for a magnetopauselike configuration. The unidirectional magnetic field is essentially transverse to the plasma flow velocity, which is the most unstable case according to linear theory and models the flow dynamics in the subsolar region of the magnetopause for northward interplanetary magnetic field. We recover effects analogous to those found in MHD simulations, including a mode cascade to longer wavelengths. The boundary layer consists of coherent structure and is not well described by a diffusive process. Isolated structures on the order of the ion gyroradius are formed which can cross the boundary in either direction. We describe how the time evolution of these structures represents transport across boundary layers, and we consider the possible connection of these entities to flux transfer events and other structure seen in the low‐latitude boundary layer at the Earth's magnetopause as well as to flux ropes commonly observed near the ionopause of Venus. We also discuss the relation of the hybrid calculations to previous MHD simulations and to observations.
Multiple heliospheric current sheets and coronal streamer belt dynamics
Authors: Crooker N. U. , Siscoe G. L. , Shodhan S. , Webb D. F. , Gosling J. T. , Smith E. J.
Published: February 2008

The occurrence of multiple directional discontinuities in the coronal streamer belt at sector boundary crossings in the heliosphere, often ascribed to waves or kinks in the heliospheric current sheet, may alternatively be attributed to a network of extended current sheets from multiple helmet streamers with a hierarchy of sizes at the base of the corona. Frequent transient outflows from these helmets can account for a variety of signatures observed at sector boundaries, including ordered field rotations, planar magnetic structure, and sandwichlike plasma structure.