bluuebunny/crossref_metadata_2025 huggingface dataset:
Tropospheric aerosol measurements in the Arctic by FTIR‐emission and star photometer extinction spectroscopy
Authors: Becker E. , Notholt J. , Herber A.
Published: September 2002

FTIR‐emission measurements of the polar Arctic atmosphere were made in Ny‐Ålesund (79°N, 11°E) with commercially available instruments. The infrared emission spectra were analysed with the FASCOD3 algorithm and home made analysis software. In order to get an agreement between experiment and simulation and to retrieve the correct column amounts of trace gases present in the atmosphere, the contribution of aerosols had to be included in subsequent radiative transfer calculations. We varied the aerosol optical depth by the parameter of the visibility in FASCOD3 in selected spectral microwindows, which are not superimposed by molecular emission bands. Furthermore, the optical depth was measured with a sun (under daylight conditions) or a star photometer (during polar night) in the UV/Visible‐range. The comparison of the UV/Visible‐data with the results from the FTIR‐emission measurements shows a strong decay in the optical depth of the tropospheric aerosols over the two spectral regions.
Observations of downwelling infrared spectral radiance at Mauna Loa, Hawaii during the 1997–1998 ENSO event
Authors: Shaw Joseph A. , Bates John J. , Zorn Heather M. , Churnside James H.
Published: September 2002

Measurements of downwelling infrared spectral radiance at Mauna Loa, Hawaii during April and May of 1998 indicate unusually high infrared atmospheric transmittance. This is linked to large‐scale drying of the upper troposphere and stratosphere in the subtropical Pacific, which appears to be associated with the extreme El Niño‐Southern Oscillation (ENSO) event of 1997–1998. During the driest periods, transmission windows opened in the wavenumber range of 500–600 cm−1, a water vapor rotational band that normally has high opacity. These data demonstrate the potential value of long‐term mountain‐top measurements for monitoring upper tropospheric water vapor and for validating satellite measurements of upper‐tropospheric water vapor.
Evidence of a solar origin for pressure balance structures in the high‐latitude solar wind
Authors: Reisenfeld D. B. , McComas D. J. , Steinberg J. T.
Published: September 2002

Ulysses observations of the high‐latitude solar wind have shown that on time scales of < 1 day, the polar wind is dominated by pressure balance structures (PBSs). Fluctuations of the plasma beta within PBSs appear to be strongly correlated with fluctuations in the helium abundance. The correlation occurs in both the northern and southern hemispheres. In addition, a mechanism is apparently at work in the high‐latitude solar wind that dissipates the beta/He correlation over a distance of a few AU. Solar wind composition is established at the base of the corona; thus, the He abundance signature strongly suggests the observed solar wind PBSs are associated with structures low in the solar atmosphere. In particular, high‐beta structures appear to originate in locations of enhanced He abundance. We suggest an interpretation of the high‐beta portion of PBSs as the solar wind extensions of polar plumes.
GPS‐meteorology: Impact of predicted orbits on precipitable water estimates
Authors: Peter Kruse Lars , Sierk Bernd , Springer Tim , Cocard Marc
Published: September 2002

Studies of atmospheric effects on Global Positioning System (GPS) signals have proven the possibility of deriving the total water vapor content from estimates of tropospheric path delays. The accuracy of GPS derived Precipitable Water (PW) depends (besides other parameters) on the quality of satellite orbits used in the analysis. High precision orbits provided by the International GPS Service (IGS) yield PW estimates with an accuracy of about 1 mm. While these orbits are provided with a delay of several days, weather forecasting requires near real‐time determination of PW. Therefore operational meteorological GPS analysis would have to rely on orbit predictions. We investigate the impact of introducing predicted orbit information on the accuracy of GPS water vapor retrievals. The presented data were acquired during a 14‐day field experiment carried out in the north‐west region of Madrid, Spain using GPS and a Water Vapor Radiometer (WVR). The comparison of WVR measurements with estimated time series of PW using both 24 and 48 hour predicted orbits and final precise IGS orbits shows that the accuracy of PW decreases by a factor of about 2 from precise to predicted orbit data.
Carbonatite alkaline magmatism associated with continental flood basalts at stratigraphic boundaries: Cause for mass extinctions
Authors: Ray Jyotiranjan S. , Pande Kanchan
Published: September 2002

The debate between the impact scenario and the hypothesis of the Deccan volcanism leading to Cretaceous/Tertiary (K/T) extinctions is far from settled. We add a new dimension to this debate by introducing an overlooked aspect of the Deccan volcanism, the emplacement and eruption of associated carbonatite‐alkaline complexes, which are capable of rapidly injecting catastrophic amounts of CO2 and SO2 into the atmosphere. 40Ar‐39Ar dating of one of the several coeval late‐Deccan alkaline pulses has revealed that this alkaline volcanism took place at the K/T boundary. A conservative estimate shows that these activities can indeed emit a substantial amount of CO2 and SO2, comparable to that of an impact scenario, which could have enhanced the catastrophic effects of the ongoing basaltic eruptions. Other major Phanerozoic extinctions also show contemporaneity with the late carbonatite‐alkaline pulses of associated flood basalt provinces, suggesting a causal relationship between such carbonatite‐alkaline magmatisms and the mass extinctions.
Dynamic modelling of stress accumulation in central Italy
Authors: Negredo A. M. , Carminati E. , Barba S. , Sabadini R.
Published: September 2002

We develop a viscoelastic dynamic model to calculate the stress distribution in central Italy induced by different geodynamic processes likely active in the region: Africa‐Eurasia convergence, Adriatic underthrusting/subduction and asthenospheric upwelling underneath Tuscany. Model results are compared with the hypocentral distribution of earthquakes and with the orientation of P and T axes available for the region. The first order characteristics of the seismotectonic observations in central Italy are: subcrustal seismicity, the pair extension‐compression in Tuscany and in the outer Apennines, and the flexural behaviour evident in long wavelength orientation of T axes. These features are reproduced by the combination of two processes: underthrusting of the Adriatic plate under the Apennines and asthenospheric upwelling underneath Tuscany. The comparison with available data does not give insights about slab pull as an ongoing process.
Néel temperatures of synthetic substituted goethites and their rapid determination using low‐field susceptibility curves
Authors: Mathé Pierre‐Etienne , Rochette Pierre , Vandamme Didier , Fillion Gérard
Published: September 2002

The Néel temperatures (TN) of synthetic goethites (α‐FeOOH), substituted with diamagnetic elements (Al, Ga) have been determined from the temperature dependence of their low‐field susceptibility using a Kappabrige AC bridge. These temperatures proved to be identical to those derived from more conventional measurements of high‐field or remanent magnetization versus temperature, and in good agreement with those inferred from Mössbauer models. The low‐field susceptibility based‐technique appears to offer a precise and rapid method of determining TN. While Néel temperature coincides with maximum unblocking temperature in Gagoethite s, supporting the assumption that antiferromagnetism and parasitic ferromagnetism have a similar origin in goethite, the lack of clear correlation between saturation remanent magnetization and increasing diamagnetic substitution suggests that the weak ferromagnetism of goethite is due to substitution independent defects induced during crystal growth.
Surface recombination of O and H2 on meteoric dust as a source of mesospheric water vapor
Authors: Summers Michael E. , Siskind David E.
Published: September 2002

One of the most provocative recent developments in mesospheric science is the discovery of a narrow layer of water vapor located near 70 km altitude where the H2O mixing ratio reaches the highest values observed in the middle atmosphere [Summers et al., 1997a]. This layer can only be explained by a local source of H2O, which is contrary to conventional gas phase chemistry that predicts net H2O destruction in that part of the mesosphere. Although suggestions have been made that this layer is produced by an influx of small comets releasing water in the upper atmosphere, we propose that the reaction O + H2 → H2O on the surface of meteoric dust can account for the observed H2O layer. Using a chemical‐transport model we find that the inclusion of this reaction with a reaction probability of ∼0.01 yields a model H2O layer with the observed characteristics. We suggest that the key to understanding the origin of the H2O layer lies in coincident observations of mesospheric H2 and H2O.
The last spreading episode of the West Philippine Basin revisited
Authors: Deschamps Anne , Lallemand Serge , Dominguez Stéphane
Published: September 2002

In 1996, bathymetric data and backscattering imagery were gathered along a 20 km‐wide NW‐SE transect across the West Philippine Basin (WPB), between Taiwan and the Palau‐Kyushu Ridge. These data reveal fine‐scaled structures of the fossil spreading axis from which we infer episodes of oblique deformation and diminished magmatic supply resulting from cessation of spreading. An original NE‐SW seafloor fabric is observed in the basin northeast of the Benham volcanic plateau. Since this trend is oblique to the more common E‐W and NW‐SE seafloor fabrics known in the WPB, it could be the result of overlap of two segments of the spreading axis. Several rhomboidal structures exist in the vicinity of the rift valley, suggesting that dextral shear occurred along the central basin rift axis during the last spreading phase. The E‐W spreading segments associated with N‐S nontransform discontinuities that occur between 127°E and 132°E were probably cross‐cut during a final, largely amagmatic, extensional phase to produce a N130° ‐trending deep rift valley.
Ozone production in the upper troposphere and the influence of aircraft during SONEX: approach of NOx‐saturated conditions
Authors: Jaeglé L. , Jacob D. J. , Brune W. H. , Faloona I. C. , Tan D. , Kondo Y. , Sachse G. W. , Anderson B. , Gregory G. L. , Vay S. , Singh H. B. , Blake D. R. , Shetter R.
Published: September 2002

During October/November 1997, simultaneous observations of NO, HO2 and other species were obtained as part of the SONEX campaign in the upper troposphere. We use these observations, over the North Atlantic (40–60°N), to derive ozone production rates, P(O3), and to examine the relationship between P(O3) and the concentrations of NOx (= NO + NO2) and HOx (= OH + peroxy) radicals. A positive correlation is found between P(O3) and NOx over the entire data set, which reflects the association of elevated HOx with elevated NOx injected by deep convection and lightning. By filtering out this association we find that for NOx>70 pptv, P(O3) is nearly independent of NOx, showing the approach of NOx‐saturated conditions. Predicted doubling of aircraft emissions in the future will result in less than doubling of the aircraft contribution to ozone over the North Atlantic in the fall. Greater sensitivity to aircraft emissions would be expected in the summer.
Deep structure of a fault discontinuity
Authors: Felzer Karen R. , Beroza Gregory C.
Published: September 2002

Relocated aftershocks at a fault discontinuity that ruptured in the 1992 Landers earthquake show that there is no simple connection between the Homestead Valley and Emerson faults at depth. Rather, faulting within the discontinuity is as complex as observed at the surface. Both aftershocks and mainshock slip on the far side of the discontinuity, that is on the Emerson fault, were confined primarily to depths above 10 km, whereas slip and aftershock activity on the near side of the discontinuity extended to 17 km. Similar behavior was also observed at a fault discontinuity in the 1979 Coyote Lake earthquake and may be attributable to the greater effectiveness of dynamic stress in inducing fault rupture at shallow depths.
Local seismicity following the 1998 eruption of Axial Volcano
Authors: Sohn Robert A. , Crawford Wayne C. , Webb Spahr C.
Published: September 2002

A local network of seven ocean bottom hydrophones was deployed on Axial volcano 14 days after the onset of seismic activity from the January 1998 diking/eruption episode. Eighty‐one locatable microearthquakes were observed during a 166 day deployment. The first group of events, observed immediately after the eruption, cluster around the rim of the central caldera. These events are believed to be aftershocks from the major caldera collapse at the beginning of the diking event. The second group of events, observed 2–5 months after the eruption, are primarily located on a line striking 125° from the caldera onto the southeast shoulder of the volcano, with a few events extending into Helium basin to the north. These events are interpreted to result from horizontal shear associated with the CoAxial and Vance ridge segment offset, or extension across the Juan de Fuca/Pacific plate boundary underlying the eastern flank of Axial volcano.
Evidence for acceleration of ions to ∼ 1 Mev by adiabatic‐like reflection at the quasi‐perpendicular Earth's bow shock
Authors: Meziane K. , Lin R. P. , Parks G. K. , Larson D. E. , Bale S. D. , Mason G. M. , Dwyer J. R. , Lepping R. P.
Published: September 2002

On December 6, 1994, during a CIR (Corotating Interaction Region) event, the WIND 3D‐Plasma and energetic particle experiment observed a burst (≤24 s) of 238–676 keV protons close to the electron foreshock boundary, followed by a 156–236 keV, 101–156 keV and 33–101 keV proton bursts about ∼40 s, 65 s and 85 s later, respectively. Similar dispersed bursts of helium with energies between ∼50 keV and ∼1 MeV followed the proton bursts. During this time, the IMF direction varied slowly with an almost monotonic decrease in θBn. The proton energy spectrum is initially peaked at ∼350 keV, and progresses to lower energies with time. The proton 3D angular distributions are peaked at ∼30° pitch‐angle, propagating away from the shock, but they are nongyrotropic. Finally, we show that the proton energy‐spectra as well as the pitch angle distribution agree quantitatively with a model of a single adiabatic reflection of the incident energetic interplanetary ions by the quasi‐perpendicular shock.
Basic modes of cumulus heating and drying observed during TOGA‐COARE IOP
Authors: Tung Wen‐wen , Lin Chichung , Chen Baode , Yanai Michio , Arakawa Akio
Published: September 2002

With the CPC Merged Analysis of Precipitation (CMAP) and a gridded dataset based on the TOGA‐COARE IOP in‐situ soundings, we study the convective activities in the Indian Ocean‐western Pacific sector. Three basic modes of cumulus heating and drying, extracted by a Rotated Principal Component Analysis (RPCA), are identified to be associated with convective types that appear under different large‐scale conditions. The result underlines the importance of considering various modes of cumulus heating and drying in developing a cumulus parameterization scheme.
Influence of the North Atlantic SST on the atmospheric circulation
Authors: Czaja Arnaud , Frankignoul Claude
Published: September 2002

Using the monthly COADS dataset and NMC‐NCAR archives we show that significant anomalies of the atmospheric circulation are related to previous SST anomalies in the North Atlantic. A signal over the northwest Labrador Sea in late spring is associated with the dominant mode of SST variability during the preceeding winter. It is more clearly seen in the mid‐troposhere than at sea level and appears to be related to the anomalous surface heat exchanges that slowly damp the SST anomalies. In addition, a NAO‐like signal in early winter is associated with SST anomalies east of Newfoundland and in the eastern subtropical North Atlantic during the preceeding summer.
A stratospheric ozone trends data set for global modeling studies
Authors: Randel William J. , Wu Fei
Published: September 2002

A global stratospheric ozone trends data set is described, providing monthly profile trend estimates derived for the period 1979–1997. SAGE I/II profile trends are used above 20 km outside of polar regions, and results between the tropopause and 20 km are derived by differencing SAGE and TOMS column ozone trends. In polar regions trends are derived from ozone sonde data up to 27 km, using the near‐complete profile records at Syowa (69°S) and Resolute (75°N). While these polar ozonesonde trends agree well with TOMS outside of polar night, the Arctic data furthermore indicate significant ozone losses beginning in midwinter (January) that are not observable in TOMS data.
Three‐dimensional numerical simulations of particle injection and acceleration at quasi‐perpendicular shocks
Authors: Giacalone Joe , Ellison Donald C.
Published: September 2002

We present results from three‐dimensional numerical simulations of quasi‐perpendicular collisionless shocks in order to determine whether cross‐field diffusion, which is otherwise artificially suppressed in simulations which contain at least one ignorable spatial coordinate, is large enough to efficiently accelerate thermal particles and/or pickup ions. We find that although the simulated downstream distribution functions are quite steep, a fraction of the particles are accelerated to energies well above the thermal energy. This occurs only if the system contains fluctuations with wavelengths that are considerably larger than the gyroradii of the particles of interest (the high‐energy ones), We find that this is due to the fact that the transport of the particles normal to the shock, against the downstream convection, is mostly influenced by the meandering of field lines on large scales. We also show that if the system does not contain these long‐wavelength waves, the scattering is not sufficient to accelerate thermal particles or pickup ions. We use two different types of simulations to demonstrate this. In the first set of simulations we use the well‐known hybrid simulation which treats the interaction between the particles and fields self‐consistently. For computational tractability these simulations use very small spatial domains, and the effect of the large‐scale field‐line random walk is suppressed. However, this approximation accurately addresses the physics of the scattering at resonant wavelengths. Test‐particle simulations, which are more computationally tractable than the hybrid simulations, are also performed for larger systems and to illustrate the effect of the long‐wavelength waves.
An incoherent scatter radar technique for determining two‐dimensional horizontal ionization structure in polar cap F region patches
Authors: Pedersen T. R. , Fejer B. G. , Doe R. A. , Weber E. J.
Published: September 2002

We present a technique which combines time series of line‐of‐sight (LOS) velocity and electron density measurements from the Sondrestrom incoherent scatter radar (74.5° invariant latitude) to reconstruct the large‐scale horizontal structure of the F region ionosphere during polar cap patch events. This reconstruction technique provides a new density‐based means of examining patch morphology. Its wide region of coverage also facilitates comparison of radar measurements with other observational data sets. For two periods when patches were present and convection conditions in the nightside polar cap could be adequately approximated by the simple velocity model used in this initial implementation of the technique, we compare reconstructed radar data montages with in situ ion density data from DMSP satellites and 630.0 nm all‐sky images taken at the radar site. The satellite data agree well with the radar reconstructions near the observation site, and show general agreement well beyond the radar field of view (FOV). Data from satellite passes to the east and west of the radar coverage suggest high densities were present over many hours of magnetic local time in the nightside polar cap. Many of the patches observed in the radar data were elongated perpendicular to the convective flow and may have extended well beyond the radar FOV. The characteristics of density enhancements observed by the satellites in the nightside auroral zone were found to be consistent with the creation of auroral zone blobs by distortion of patches seen exiting the polar cap in the radar data. Radar data reconstructions also show reasonable qualitative agreement with instantaneous 630.0 nm all‐sky images regarding the location, size, and shape of patch features. In addition to validating the streamline‐mapping technique, this result also lends support to previous optical studies of patch occurrence and morphology.
Ionospheric convection response to changes of interplanetary magnetic field Bz component during strong By component
Authors: Huang Chao‐Song , Murr David , Sofko George J. , Hughes W. Jeffrey , Moretto Therese
Published: September 2002

It is well established that ionospheric convection patterns are strongly controlled by the interplanetary magnetic field (IMF). There has been an increased interest in ionospheric convection response to IMF changes. Some studies found that ionospheric flows change first near noon and later near the dawn‐dusk meridian plane, which is interpreted as propagation or expansion of newly generated convection cells in the cusp region. Other studies showed that the change in convection pattern in response to IMF reorientations is spatially fixed. In this paper, we investigate the ionospheric convection response to IMF Bz changes during strong IMF By. On March 23, 1995, Bx was small, By was strongly positive (7–11 nT), and the Bz polarity changed several times after 1300 UT. The dayside ionospheric convection is dominated by a large clockwise convection cell. The cell focus (the “eye” of the convection pattern) is located in the prenoon sector for northward Bz and in the postnoon sector for southward Bz. It is found that the cell focus shifts from the prenoon sector to the postnoon sector following a southward Bz turning and vice versa for a northward Bz turning. However, the motion of the convection cell, or the largest change in the convection pattern, is limited roughly to the region between the previous cell focus and the new cell focus. Outside this region, the ionospheric flows could be greatly enhanced or weakened, while the convection pattern shape changes very little. When By is strong enough, the Bz reorientation causes changes in the flow intensity but not in the shape of the convection pattern. The results show the characteristics of ionospheric convection response during strong By and suggest that the convection reconfiguration is not only determined by the changing Bz but also significantly influenced by the stable and large By.
Self‐consistent model of the solar wind interaction with two‐component circumsolar interstellar cloud: Mutual influence of thermal plasma and galactic cosmic rays
Authors: Myasnikov Artyom V. , Izmodenov Vladislav V. , Alexashov Dmitrii B. , Chalov Sergei V.
Published: September 2002

The interaction of the solar wind with the two‐component interstellar medium consisting of thermal plasma and galactic cosmic rays is studied numerically using a soft fitting technique based on the high‐resolution Godunov scheme and the physical process splitting method. Mutual influence of plasma and cosmic ray components on the solar wind termination is considered. It is shown that cosmic rays can considerably modify the shape and structure of the solar wind termination shock and the bow shock, and change the positions of the heliopause and bow shock if no H atoms are taken into account.
High‐temperature, high‐resolution cross sections of O2 in the 83.4 nm region
Authors: Wu C. Y. Robert , Hung T. , Judge D. L. , Matsui Takashi
Published: September 2002

The high‐resolution absorption cross‐section measurements of O2 in the 83.0 – 83.5 nm spectral region have been carried out with a resolution of 0.0008 nm and at temperatures of 295 K and 535 K. These are apparently the first such high‐temperature cross‐section measurements of O2 in this wavelength band. The absorption features of the O2 I‐X (4,0) band systems are observed to be quite diffuse with a line width of full width at half maximum (FWHM) equal to 0.03 nm. Therefore the cross‐section values of O2 in this spectral region will not critically depend upon the resolution as long as it is better than 0.03 nm, as is evident in the reported data. As the temperature increases from 295 K to 535 K we observed an increase of 30% in the cross‐section values on both sides of the triplet features of the I‐X (4,0) band, whereas a 20% decrease occurs over the peak region.
Connecting the Sun and the solar wind: Source regions of the fast wind observed in interplanetary space
Authors: Woo Richard , Habbal Shadia Rifai
Published: September 2002

Highly sensitive radio occultation and white light measurements of path‐integrated density have shown that the solar corona comprises three distinct morphological regions, streamer, quiet Sun, and polar coronal hole, which except for the streamer region, extend radially into interplanetary space from 1.15 Rs to at least 30 Rs. In this paper we build on these results by comparing solar wind flow speeds observed at the same time as path‐integrated density. Flow speeds are inferred from the Doppler dimming of O VI lines with the ultraviolet coronagraph spectrometer on the Solar and Heliospheric Observatory, while the simultaneous polarized brightness measurements of path‐integrated density are from the High Altitude Observatory Mauna Loa Mk III K‐coronameter. The comparison of global flow speed and density observations in 1997 produces three new results. First, it shows, that the three distinct morphological regions, identified earlier in density measurements, are present in the latitudinal profile of the flow speed in the corona. In particular, the flow speed measurements provide evidence for the quiet Sun as an additional source of fast wind. Second, the comparison shows that flow speed and density are anticorrelated in the solar corona. Third, it demonstrates that the Mk III pB measurements can readily serve as a proxy for velocity distribution in the outer corona. The extensive Mk III data set that spans nearly two solar cycles therefore provides the framework of near‐Sun measurements with which connections with solar wind measurements in interplanetary space can be made. Specifically, we show that fast wind regions in the heliosphere, observed directly by Ulysses and Wind plasma measurements and remotely by Nagoya interplanetary scintillation measurements, map radially back to fast wind regions at the Sun identified by the Mk III data.
Alfvén waves as a source of lower‐hybrid activity in the ring current region
Authors: Khazanov G. V. , Gamayunov K. V. , Liemohn M. W.
Published: September 2002

A coupling mechanism between lower‐hybrid waves and low‐frequency electromagnetic activity in the Earth's ring current region is analyzed. In the ring current region the observed amplitudes of Alfvén and/or fast magnetosonic waves, as a rule, do not exceed the threshold of flux instability of lower‐hybrid waves in an electromagnetic field of a low‐frequency wave. Even for these conditions, however, a mechanism of parametric generation of lower‐hybrid waves from low‐frequency Alfvén or fast magnetosonic waves is possible, and this theory is presented. In the studied case, lower‐hybrid waves are saturated because of induced scattering with plasma electrons. For typical plasma and low‐frequency wave parameters in the Earth's ring current region the saturation level of lower‐hybrid waves, excited by the proposed mechanism, is in good agreement with experimental data.
Field line resonances in a nonaxisymmetric magnetic field
Authors: Lee Dong‐Hun , Lysak Robert L. , Song Yan
Published: September 2002

In the near‐Earth magnetotail region the magnetic geometry becomes nonaxisymmetric. Field line resonances (FLR) are studied in such a system. The effects of nonaxisymmetry imply that the azimuthal wavenumber is no longer invariant on each magnetic shell, unlike in an axisymmetric geometry. In this numerical simulation study we investigate how geometric asymmetry affects the ansatz of FLR theory that any radial oscillations are asymptotically damped and transfer wave energy to shear standing modes. It is found in our numerical results that the FLR persist in such nonaxisymmetric cases. The oscillations corresponding to these shear modes are tangential to the ambient Alfven speed surface. It is suggested that the Alfven speed surface can be very useful in establishing a local coordinate system to define the wave polarization. The direction normal to the local Alfven speed gradient is found to be similar to the radial direction in an axisymmetric case, while the tangential direction is similar to the azimuthal direction. In addition, we examine the effects of the finite ionospheric dissipation on the formation of FLR and field‐aligned currents.
Optical remote sensing of the thermosphere with HF pumped artificial airglow
Authors: Bernhardt P. A. , Wong M. , Huba J. D. , Fejer B. G. , Wagner L. S. , Goldstein J. A. , Selcher C. A. , Frolov V. L. , Sergeev E. N.
Published: September 2002

Optical emissions excited by high‐power radio waves in the ionosphere can be used to measure a wide variety of parameters in the thermosphere. Powerful high‐frequency (HF) radio waves produce energetic electrons in the region where the waves reflect in the F region. These hot or suprathermal electrons collide with atomic oxygen atoms to produce localized regions of metastable O(1D) and O(1S) atoms. These metastables subsequently radiate 630.0 and 557.7 nm, respectively, to produce clouds of HF pumped artificial airglow (HPAA). The shapes of the HPAA clouds are determined by the structure of large‐scale (≈10 km) plasma irregularities that occur naturally or that develop during ionospheric heating. When the HF wave is operated continuously, the motion of the airglow clouds follows the E × B drift of the plasma. When the HF wave is turned off, the airglow clouds decay by collisional quenching and radiation, expand by neutral diffusion, and drift in response to neutral winds. Images of HPAA clouds, obtained using both continuous and stepped radio wave transmissions, are processed to yield the electric fields, neutral wind vectors, and diffusion coefficients in the upper atmosphere. This technique is illustrated using data that were obtained in March 1993 and 1995 at the ionospheric modification facility near Nizhny Novgorod, Russia. Analysis of HPAA clouds yields zonal plasma drifts of 70 m s−1 eastward at night. On the basis of artificial airglow from energetic electrons generated at 260 km the zonal neutral wind speed was estimated to be 96 m s−1 and the O(1D) diffusion coefficient was determined to be between 0.8 and 1.4 × 1011 cm2 s−1. The quenched lifetime of the O(1D) was determined to be 29.4 s. The diffusion and quenching rates are directly related to the atomic and molecular concentrations in the thermosphere. Improvements in the remote‐sensing technique may be obtained if the intensity of the artificial airglow emissions is increased. High‐power radio transmissions employing pulse sequences and tuning near electron cyclotron harmonics were attempted to increase the optical emissions. Both of these, however, produced reduced intensity, and consequently, continuous transmission at frequencies away from electron gyro harmonics is the preferred heating regime.
Solar wind control of the tail lobe magnetic field as deduced from Geotail, AMPTE/IRM, and ISEE 2 data
Authors: Tsyganenko N. A.
Published: September 2002

A statistical study was made of the tail lobe field response to the dynamical pressure of the incoming solar wind and to the interplanetary magnetic field, as well as its relation to the concurrent level of the Dst field. The study covers a wide range of distances between 10 and 60RE and is based on a large set of tail lobe magnetic field data, compiled from three sources: (1) Geotail magnetometer and low‐energy plasma instrument data for 1993–1997, (2) AMPTE/IRM magnetometer and plasma instrument data for 1985–1986, and (3) ISEE 2 magnetometer and fast plasma experiment data for 1978–1980. The tailward variation of the tail lobe field and of its response to the solar wind and interplanetary magnetic field (IMF) conditions was studied using a regression relationship, including various combinations of the interplanetary quantities, measured by IMP 8 and Wind spacecraft. An investigation of the role of the time lag effects was made by tagging each lobe field measurement by a “trail” of 12 consecutive 5‐min average values of the solar wind parameters and finding best fit distributions of the lagged response amplitudes in the solar wind and IMF‐related regression terms. The goal of the work is to find a set of input variables providing the highest correlation between the observed and predicted magnitude of the tail lobe field, so that their combination could be used for parameterizing the strength of the cross‐tail current in the data‐based magnetospheric models.
North‐south flows at 47 AU: A heliospheric vortex street?
Authors: Burlaga L. F. , Richardson J. D.
Published: September 2002

A north–south flow pattern with a recurrence period approximately equal to the solar rotation period was observed by Voyager 2 during 1995 at 47 AU, nearly a solar cycle after the north–south flows were first observed near 21 AU during 1986. The north–south flow deflections observed during 1995 were less regular than those observed in 1986. One finds the same qualitative associations between the north–south flows and bulk speed and temperature that were observed in 1986, but the correlations are weaker at the larger distances and higher latitudes. Thus it appears that the flow observed during 1995 is basically the same kind of flow observed in 1986, but it is much less regular owing to evolutionary effects as the flow moves from 21 AU to 47 AU. The observations are consistent with the several predictions of the vortex street model. They fail to support some predictions of both the interaction region model and the vortex street models. It is possible that both a vortex street and interaction regions are significant near solar minimum between 20 and 50 AU. Further development of the models is needed. The interaction models must be extended to include time variations, and the effects of pickup protons should be included in future models of the north–south flows.
Deriving solar X ray irradiance from Yohkoh observations
Authors: Acton Loren W. , Weston David C. , Bruner Marilyn E.
Published: September 2002

The soft X ray telescope (SXT) on Yohkoh produces calibrated images which are used to derive X ray irradiance and coronal temperature. In this paper we compute the mean temperature and spectral irradiance for 25 coronal differential emission measure (DEM) models ranging from very quiet solar conditions to the main phase of large flares. These results are compared to what would have been derived from an SXT observation of the solar conditions represented by the DEM models. After applying an empirical correction algorithm, derived as part of this work, 24 of 25 SXT filter‐ratio temperatures fall within 0.1 in log10(T) of the mean DEM temperatures. The irradiance error depends both on the chosen spectral band and the spectrum, here represented by a single plasma temperature. As expected, the irradiance is more accurate and less sensitive to spectrum for bands that match the band pass of the sensor. Even a crude color temperature approximation to the coronal X ray spectrum greatly improves the accuracy of derived radiance over values derived for an assumed temperature of 3 × 106 K: the mean temperature of all DEM. An RMS improvement of a factor of 12–17 was found for the cases studied. For two bands, which match the sensor band pass, the RMS accuracy is better than 10%. Yohkoh X ray irradiance data, converted with SXT color temperatures, will be placed in the National Geophysical Data Center for use by the solar and atmospheric physics communities.
Solar wind magnetic field control of magnetospheric response delay and expansion phase onset timing
Authors: Bargatze L. F. , Ogino T. , McPherron R. L. , Walker R. J.
Published: September 2002

Variability in the response of the magnetosphere under different interplanetary magnetic field (IMF) conditions is investigated by utilizing a high‐resolution, three‐dimensional, global magnetohydrodynamic (MHD) model. Changes in the MHD model response time delay are found by performing a parameter search based on the strength of the IMF Bz component, which is held constant for 4 hours prior to and following southward turnings. The parameter search reveals that variations in the growth phase duration and, thus in the timing of expansion phase onset, follow a simple pattern governed by the relative strengths of IMF Bz before and after the southward turnings. If the IMF northward and southward values are equal in magnitude, the expansion phase onset occurs ∼60 min after dayside merging begins. However, if the north IMF setting is smaller (larger) than the subsequent southward field intensity, the expansion phase onset can occur after a much shorter (longer) time delay. The range of onset delay times found from the simulation is ∼60±30 min. The MHD results are tested by means of an empirical analysis, which is based on the assumption that the solar wind response of the magnetosphere, as represented by the AL index, is bimodal, owing to the different solar wind response timescales of the convection current system and the substorm current wedge. The empirical analysis utilizes short duration time intervals including solar wind input time series and corresponding AL index data. The empirical results agree favorably with the results found via MHD simulation.
Shocks in the distant heliosphere
Authors: Whang Y. C. , Burlaga L. F.
Published: September 2002

Voyager data are used to study two aspects of pickup proton shocks in the distant heliosphere: (1) Two shocks having similar speed jump of ∼50 km/s observed at 5 AU and at 43 AU demonstrate the effects of pickup protons on weakening interplanetary shocks at increasing heliocentric distance. (2) On the basis of three shocks identified outside 34 AU from high‐resolution plasma and magnetic field data we develop an empirical model to study the jump conditions of pickup protons across solar wind shocks. Making use of the empirical model one can study the postshock conditions of pickup protons for the global merged interaction region (GMIR) shock and the termination shock; this will enable us to calculate more accurately the propagation of the GMIR shock and the interaction of GMIR shock with the termination shock. We also find that the variation of the pickup proton temperature TI across the shock is very close to the adiabatic relation that TI is proportional to the magnetic field strength B.
Radar and satellite global equatorial F region vertical drift model
Authors: Scherliess L. , Fejer B. G.
Published: September 2002

We present the first global empirical model for the quiet time F region equatorial vertical drifts based on combined incoherent scatter radar observations at Jicamarca and Ion Drift Meter observations on board the Atmospheric Explorer E satellite. This analytical model, based on products of cubic‐B splines and with nearly conservative electric fields, describes the diurnal and seasonal variations of the equatorial vertical drifts for a continuous range of all longitudes and solar flux values. Our results indicate that during solar minimum, the evening prereversal velocity enhancement exhibits only small longitudinal variations during equinox with amplitudes of about 15–20 m/s, is observed only in the American sector during December solstice with amplitudes of about 5–10 m/s, and is absent at all longitudes during June solstice. The solar minimum evening reversal times are fairly independent of longitude except during December solstice. During solar maximum, the evening upward vertical drifts and reversal times exhibit large longitudinal variations, particularly during the solstices. In this case, for a solar flux index of 180, the June solstice evening peak drifts maximize in the Pacific region with drift amplitudes of up to 35 m/s, whereas the December solstice velocities maximize in the American sector with comparable magnitudes. The equinoctial peak velocities vary between about 35 and 45 m/s. The morning reversal times and the daytime drifts exhibit only small variations with the phase of the solar cycle. The daytime drifts have largest amplitudes between about 0900 and 1100 LT with typical values of 25–30 m/s. We also show that our model results are in good agreement with other equatorial ground‐based observations over India, Brazil, and Kwajalein.
Simultaneous observations of the July 1996 2‐day wave event using the Super Dual Auroral Radar Network and the High Resolution Doppler Imager
Authors: Bristow W. A. , Yee J.‐H. , Zhu X. , Greenwald R. A.
Published: September 2002

Simultaneous observations of a 2‐day wave using the Super Dual Auroral Radar Network (SuperDARN) and the Upper Atmospheric Research Satellite‐based (UARS) High Resolution Doppler Imager (HRDI) are presented. Recent comparisons of SuperDARN data with data from the Saskatoon MF radar [Hall et al., 1997] demonstrate that SuperDARN data at ranges close to the radar is most likely due to scatter from meteor trails and can be used to trace neutral winds in the meteor region. This study exploits this near‐range scatter to observe the winds during July 1996. During the study period a clear 2‐day wave event was observed in both the radar data and in the HRDI data. Comparisons between the two data sets show similarities and distinct differences. Both instruments observed a wave of similar amplitude in the same time interval. Both instruments indicated a westward propagating wave with zonal wave number 3. HRDI observed the wave to form and diminish at earlier times than SuperDARN observed. In addition, there were distinct differences between the winds observed by HRDI at the radar longitudes and the winds observed by the radars. Many of the differences may be explained by the latitude difference between the radars and the northern most latitude observed by HRDI.
Solar cycle variation of nitric oxide in the thermosphere
Authors: Kumar Cidambi K. , Batra Anand P. , Klein Lewis , Batra Arun
Published: September 2002

The measurements of trace gases in the atmosphere made by the Halogen Occultation Experiment instrument on UARS have been used to investigate the solar cycle variation of nitric oxide. The data covers the period from near solar activity maximum in late 1991 to its near minimum in late 1996. Systematic errors in the NO measurements were corrected, and the results were correlated with the F10.7 index. Regression coefficients derived from this analysis are presented. It is found that nitric oxide in the equatorial thermosphere varies significantly with F10.7, but its variation in polar thermosphere is very small.
Role of overshoots in the formation of the downstream distribution of adiabatic electrons
Authors: Gedalin M. , Griv E.
Published: September 2002

We analyze the influence of the magnetic overshoot on the downstream electron distributions formed as a result of the collisionless adiabatic motion of the electrons in the quasi‐stationary electric field in shock front. We show that a substantial overshoot can result in a significant distortion of the downstream distribution due to cutting out the electrons with high perpendicular velocities. We calculate numerically the electron distribution in the υ‖ – v⊥ plane, as well as expected cuts through the distribution depending on the angle with respect to the magnetic field direction, also taking into account the averaging procedure of the ISEE type apparatus. These distorted distributions and cuts should be clearly observed in the downstream region just behind the shock transition layer. Absence of such observations would indicate that the above picture of adiabatic electron dynamics in the static field is significantly incomplete, and estimates based on these assumptions should be considered with caution.
Propagation of electromagnetic waves through the Martian ionosphere
Authors: Melnik Olga , Parrot Michel
Published: September 2002

This paper is related to the study of ELF‐VLF wave propagation through the Martian ionosphere using a WKB method. It is expected that waves coming from the atmosphere could be observed by a probe in orbit around Mars. Characteristics of the wave propagation have been determined from Maxwell's equations. The ionospheric conductivities have been calculated using known parameters of the Martian ionosphere in different cases: nightside and dayside, low and high solar activity, and low and high magnetic field. It is shown that ELF‐VLF waves with a maximum frequency ∼ 4000 Hz can only propagate if we consider a strong magnetic field on the nightside and a low solar activity.
A search for small comets with the Naval Space Command radar
Authors: Knowles S. , Meier R. R. , Gustafson B. Å. S. , Giovane F. J.
Published: September 2002

We have searched for the hypothetical small comets proposed by Frank et al. [1986a, b] and Frank and Sigwarth [1993] using the world's most powerful radar in terms of gain‐aperture product. The Naval Space Surveillance System can detect most space objects in low Earth orbit with radar cross sections (RCSs) of 0.1 m2 or larger; at higher altitudes of the order of 10,000–20,000 km the radar can detect objects with RCSs of 1 m2. We carried out detailed first‐principle calculations of the RCS of spherical comet using the properties proposed by Frank and Sigwarth [1993]. We find that 8–12 m diameter comets have an average cross section of 0.4 m2 at the radar frequency (217 MHz), with peaks reaching 1 m2. Therefore the Naval radar system has sufficient sensitivity to detect many small comets, especially as they approach low Earth orbit. We estimate that at least 800–5000 small comets should have been detected by the radar during the 37 day search period during fall 1997. None of the more than 12,000 unidentified detections can be explained by small comets. The lack of detection of small comets by the radar can be explained only if small comets have RCSs <0.1% of their assumed physical size (which is unrealistic, given that human technology can match this value only by tailoring a design for a specific radar) or if their impact rate with Earth is some 4 orders of magnitude less than proposed by Frank et al. [1986a] and Frank and Sigwarth [1993].