bluuebunny/crossref_metadata_2025 huggingface dataset:
Modelled climate sensitivity of the mass balance of Morteratschgletscher and its dependence on albedo parameterization
Authors: Klok E. J. , Oerlemans J.
Published: February 2004

This paper presents a study of the climate sensitivity of the mass balance of Morteratschgletscher in Switzerland, estimated from a two‐dimensional mass balance model. Since the albedo scheme chosen is often the largest error source in mass balance models, we investigated the impact of using different albedo parameterizations on the climate sensitivity of the mass balance. We first ran the mass balance model for the period 1980 to 2002 and calculated the mass balance sensitivity by perturbing the air temperature by values ranging from −2 to +2 K and precipsitation by values ranging from −20 to +20%. The albedo parameterization that we used for these simulations relates the snow albedo to snow age and snow depth, whereas it considers the ice albedo constant in time and space. The mass balance sensitivity to temperature and precipitation was estimated at −59 cmWE year−1 K−1 and at 17 cmWE year−1 per 10% respectively (cmWE: centimetres water equivalent). Then, we used three other albedo parameterizations. One parameterization uses ice albedo values that vary in space and were acquired from a Landsat image. Another parameterization calculates the snow albedo from accumulated daily maximum temperatures since snowfall. The third parameterization is the simplest and uses two constant albedo values, one for snow and one for ice. The differences between sensitivities calculated from the different albedo parameterizations were generally small. However, when the simplest parameterization was used, the mass balance sensitivity to temperature decreased to −52 cmWE year−1 K−1. This is mainly due to the constant snow albedo, which implies that the albedo feedback is not fully considered. For an accurate estimate of the mass balance sensitivity, the albedo parameterization should capture the process of a decreasing snow albedo when a snow pack gets older or thinner. Copyright © 2004 Royal Meteorological Society
A method for defining monsoon onset and demise in the southwestern USA
Authors: Ellis Andrew W. , Saffell Erinanne M. , Hawkins Timothy W.
Published: February 2004

The southwestern USA is subject annually to an inflow of atmospheric moisture in association with the North American monsoon, which is a summertime shift in the atmospheric circulation stretching from the Caribbean Sea, to Mexico, and into the southwestern USA. There are currently no regional criteria used to define the temporal aspect of the annual monsoon season in the southwestern USA, and only a single local definition. A regionalized definition of the annual timing of the monsoon season would seem to be a greater representation of the synoptic‐scale effects of the monsoon rather than individual definitions at point locations across the region. The research presented here outlines a method for defining the annual onset and demise of the regional monsoon season of the southwestern USA, and assesses the results through comparisons with a method currently used at one location within the region and with atmospheric composites from historic data.

Using hourly dew‐point temperature data at five surface stations and daily precipitation data for 193 stations across the southwestern USA for June through to October for a 52 year period, daily humidity and precipitation threshold values indicative of monsoon moisture are established. The first and last occurrences of both humidity and precipitation sustained for a synoptic period of 3 days mark the annual onset and demise of the monsoon season, whereas all days in between that meet the humidity and precipitation thresholds are categorized as ‘monsoon days’. Results show good agreement with sample historic data for the sole local definition within the region, and atmospheric composites indicate a likelihood of accurate representation of monsoon onset and demise across the region. The significance of accurate onset and demise dates is apparent in the intimate relationship between seasonal precipitation across the region and the length of the monsoon season. Based upon the procedure, a historic monsoon season database was constructed for the southwestern USA for use in climatological, meteorological, and case‐study analyses. Copyright © 2004 Royal Meteorological Society
A statistical downscaling method for monthly total precipitation over Turkey
Authors: Tatli Hasan , Nüzhet Dalfes H. , Sibel Menteş Ş
Published: February 2004

Researchers are aware of certain types of problems that arise when modelling interconnections between general circulation and regional processes, such as prediction of regional, local‐scale climate variables from large‐scale processes, e.g. by means of general circulation model (GCM) outputs. The problem solution is called downscaling. In this paper, a statistical downscaling approach to monthly total precipitation over Turkey, which is an integral part of system identification for analysis of local‐scale climate variables, is investigated. Based on perfect prognosis, a new computationally effective working method is introduced by the proper predictors selected from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data sets, which are simulated as perfectly as possible by GCMs during the period of 1961–98. The Sampson correlation ratio is used to determine the relationships between the monthly total precipitation series and the set of large‐scale processes (namely 500 hPa geopotential heights, 700 hPa geopotential heights, sea‐level pressures, 500 hPa vertical pressure velocities and 500–1000 hPa geopotential thicknesses). In the study, statistical preprocessing is implemented by independent component analysis rather than principal component analysis or principal factor analysis. The proposed downscaling method originates from a recurrent neural network model of Jordan that uses not only large‐scale predictors, but also the previous states of the relevant local‐scale variables. Finally, some possible improvements and suggestions for further study are mentioned. Copyright © 2004 Royal Meteorological Society
Seasonal and universal time distribution of patches in the northern and southern polar caps
Authors: Coley W. R. , Heelis R. A.
Published: February 2004

Ion density measurements from the Dynamics Explorer 2 and Defense Meteorological Satellite Program F8 and F9 satellites are used to examine hemispherical differences in the occurrence patterns of polar ionization patches as a function of season, universal time (UT), and interplanetary magnetic field (IMF). When Bz<0, the greatest frequency of patch occurrence in the northern hemisphere is in the winter in the 1000–2200 UT range. This time corresponds to the interval when the northern magnetic pole (and hence the cusp) lies the farthermost toward the dayside. This fact is often used to explain the creation of patches in terms of the entrainment of dayside plasma into the cusp by high‐latitude convection. In the southern hemisphere we see that the occurrence frequencies peak at over twice the northern hemisphere values in the same general UT region (1000–2300 UT). However, the southern hemisphere cusp is most dayward at approximately 0300 UT, a time of minimum patch formation. Examination of the relationship of the terminator to the polar cap boundaries in each hemisphere leads to a simple explanation in terms of the differing offset of the magnetic poles (and the ionospheric convection pattern) from the geographic poles.
Mean free paths of solar electrons (Ee≳45 keV) in the interplanetary medium
Authors: Van Allen James A.
Published: February 2004

This paper reports intensity‐time signatures of eight impulsive (nearly) pure solar electron (Ee≳45 keV) events observed in interplanetary space in the heliocentric radial range 0.7<r<1.0 AU, seven by Mariner V (July–October 1967) and one by Mariner II (November 1962). In six cases, simultaneous solar X ray and Earth‐based optical data provide reasonably certain identification of the causative flares. Complementary data on X ray emission and electron angular distributions from Explorer 33 (in a loose Earth orbit) and Explorer 35 (in a lunar orbit) are also cited. In all eight cases the electron signatures exhibit a two‐phase structure, an early scatter‐free peak [Lin, 1970] having a duration of ≲20 min and a subsequent main phase having a full width at half maximum of several hours. A detailed analysis of each intensity‐time signature is described, and the corresponding parameters are tabulated. All main phases are well represented by a three‐dimensional, isotropic diffusion equation, although it is puzzling that such a representation can be so empirically successful. The principal results are apparent values of the interplanetary diffusion coefficient D and corresponding values of the mean free path λ between scattering centers for the main phases. Values of λ range from 0.035 to 0.154 AU. The mean of these values is 0.091 (±0.013) AU and the standard deviation of individual values from the mean is 0.040 AU. This standard deviation exceeds the estimated uncertainty of each determination by a factor of about three and is plausibly attributed to variability of the properties of the interplanetary magnetic field.
SuperDARN observations of quasi‐stationary mesoscale convection vortices in the dayside high‐latitude ionosphere
Authors: Huang Chao‐Song , Sofko George J. , McWilliams Kathryn A. , Bristow William A. , Greenwald Ray A. , Kelley Michael C.
Published: February 2004

During the past few years of SuperDARN observations, mesoscale (500–1000 km diameter) clockwise‐sense convection vortices which do not show significant movement in any direction have been observed in the postnoon sector. These convection vortices or small convection cells are herein named quasi‐stationary convection vortices (QSCVs). In this paper, a statistical study of 28 QSCVs is presented, using Saskatoon‐Kapuskasing radar data from the period October 1993 to March 1997. These QSCVs occur in the magnetic latitude interval 72°–81° between 1430 and 1630 magnetic local time (MLT) and are centered near the western end of the afternoon convection cell. The QSCVs occur mainly during southward interplanetary magnetic field (IMF) but sometimes for northward IMF. They appear at lower latitudes as the IMF Bz becomes more negative; they occur at higher latitudes and earlier local times for positive IMF By and at lower latitudes and later local times for negative IMF By. All the QSCVs appear to be associated with large‐amplitude fluctuations either in the IMF or in the solar wind pressure, or in both. It is found that the time of the ionospheric convection change in response to the fluctuations in the IMF and solar wind pressure at the magnetopause is about 6–9 min. The observations have shown some evidence that the termination of the vortex is caused also by large changes in the IMF and/or solar wind pressure. The lifetime of the QSCVs is typically 10–20 min, but it can be as long as 50 min. The QSCVs have clockwise rotational sense and are closely associated with the region 1 field‐aligned currents. Unlike traveling convection vortices, the QSCVs do not appear to be related to any instability at the magnetopause or low‐latitude boundary layer. The large fluctuations in the solar wind pressure and IMF seem to be likely initiating mechanisms for the QSCVs.
Evolution of mechanical state in Carrara marble during deformation at 400° to 700°C
Authors: Covey‐Crump S. J.
Published: February 2004

The evolution of the mechanical state of Carrara marble during deformation up to strains of 0.2 at 400° to 700°C and strain rates of 10−4 to 10−6 s−1 has been investigated by applying Hart's [1976] state variable description of inelastic deformation to the results of isothermal, constant displacement rate experiments performed at these conditions. At T⪕600°C the magnitude of the mechanical state increases over the full range of strains investigated even when the deformation occurs at almost constant stress and strain rate, although the rate of increase at given strain decreases with increasing temperature and decreasing strain rate. The deformation microstructures suggest that under these conditions mechanical twinning controls the deformation behavior, with twin boundary migration becoming significant at T⪖450°C. As a dynamic recovery process, twin boundary migration is as efficient as strain‐induced grain boundary migration. At T>600°C the magnitude of the mechanical states attained is difficult to evaluate reliably using the method employed, but the continuing evolution of the deformation microstructures suggests that a steady state is not attained by strains of 0.2. At these conditions, mechanical twinning is replaced by other intracrystalline slip processes and by grain boundary bulge nucleated dynamic recrystallization. Hart's description of inelastic deformation, as previously evaluated for Carrara marble at T⪕400°C, may be extrapolated to 600°C provided the mechanical state evolution equation is modified to accommodate twin boundary migration and the influence of annealing‐type recovery processes. Extrapolation to higher temperatures may require modifications to the equation of state in that description. Although at the conditions investigated steady state appears to be a poor approximation, steady state flow laws of exponential and power law type do provide a good empirical description of the data. However, the observation that steady state is not attained indicates that any microphysical interpretation of the magnitude of the material parameters in such steady state flow law fits should be treated with caution.
Deformation and dewatering of the subducting plate beneath the lower slope of the northern Barbados accretionary prism
Authors: Zhao Zhiyong , Moore Gregory F. , Shipley Thomas H.
Published: February 2004

Detailed structural and stratigraphic analysis of a three‐dimensional seismic data set reveals two scales of structural fabric in the westward dipping oceanic basement under the toe of the northern Barbados accretionary prism. The large‐scale fabric consists of three NNE trending structural highs with relief of about 300 m and spacing of 5–6 km. The small‐scale structure is characterized by a series of NNE trending normal faults, resulting in horsts and graben with relief of 50–200 m and spacing of 0.2–2 km. The structural fabric at both scales is preexisting. Structures in the underthrust sediment are mostly secondary, having formed by differential compaction. Three distinctive seismic stratigraphic units are defined in the underthrust sedimentary section. The upper Cretaceous to lower Eocene unit mostly fills in basement lows, resulting in great thickness variations. The thickness of the Oligocene and middle to upper Eocene units is less variable. We estimate that the upper two sedimentary units have lost 25% of their total initial void space owing to the load of the accretionary prism within the first 3.5 km landward of the thrust front. This suggests a current fluid expulsion rate of 1008 m3/yr per kilometer of strike length from the toe. The fluid discharge is expected to increase to 1092 m3/yr within 64,000 years as a thicker sediment section is subducted. Our results also suggest that the basement indirectly controls fluid movement in the underthrust Oligocene unit by creating secondary normal faults that act as major fluid conduits between the overlying décollement and the underlying, more permeable middle to upper Eocene turbidite‐bearing section. Ponded sediments between basement horst blocks are not faulted and may compact more slowly.
Spectra of cometary X rays induced by solar wind ions
Authors: Kharchenko V. , Dalgarno A.
Published: February 2004

The X‐ray and extreme ultraviolet spectra produced by solar wind ions undergoing change transfer collisions with the atoms and molecules present in the atmospheres of comets are calculated. The calculations take account of the cascading transitions that follow electron capture into highly excited states. It is shown that sequential collisions cause the X‐ray spectrum from the cometary center to be softer than that of the peripheral regions. Some of the brightest lines arise from forbidden transitions of helium‐like oxygen and carbon ions. The X‐ray luminosities and averaged spectra reflect the composition of the solar wind. The calculated luminosities and spectra corresponding to a typical distribution of impinging solar wind ions are consistent with observations of comets Hyakutake and Hale‐Bopp. The spectral resolution required to establish charge transfer as the source of cometary X rays is discussed; an energy resolution of 10 eV should be adequate.
Low‐energy upflowing ion events at the poleward boundary of the nightside auroral oval: High‐altitude Interball‐Auroral probe observations
Authors: Malingre M. , Dubouloz N. , Berthelier J. J. , Galperin Y. , Chugunin D. , Perraut S. , Sauvaud J. A. , Delcourt D. , Stepanov V.
Published: February 2004

Ion data acquired by the Interball‐Auroral satellite during crossings of the poleward boundary of the auroral oval in the 2200‐0300 MLT sector at altitudes of ∼2.5–3 Earth's radii reveal the frequent occurrence of thermal and superthermal H+ ion outflows. These events are strongly correlated with suprathermal electron fluxes and broadband electromagnetic ULF waves. The pitch angle distributions give evidence of transverse heating occurring in a latitudinally narrow layer at the boundary between the polar cap and the plasma sheet boundary layer, over a broad altitude range extending up to the satellite altitude. The distributions evolve with latitude, exhibiting fluxes maximizing at pitch angles close to 90° at the poleward edge of the outflow structure and at pitch angles closer to the upward field‐aligned direction at lower latitudes. The data analysis suggests that ion cyclotron resonance interaction with ULF electromagnetic turbulence can account for the observed heating, even if we cannot totally exclude that transverse velocity shears and nonresonant stochastic transverse acceleration sometimes contribute to the ion energization in view of the dc electric field fluctuations commonly observed at the same times. During the expansion phase of substorms the region of transverse heating at the poleward boundary of the discrete auroral oval exhibits a latitudinal structure characterized by an alternate occurrence of latitudinally narrow regions of intense and weak ion fluxes. These latitudinal variations are associated with magnetic fluctuations at a frequency of ∼2×10−2 Hz, interpreted in terms of hydromagnetic Alfvén waves. Equatorward of the heating region, the energy spectrograms recorded during the same events exhibit an energy‐latitude dispersion signature with energy decreasing as latitude decreases. This dispersion is the result of the velocity filter effect due to the large‐scale convection and of the poleward motion of the ion heating source associated with the poleward motion of the high‐latitude edge of the active auroral region. The poleward edge of the low‐energy ion structure marked by a sharp latitudinal gradient of the ion flux appears as a reliable midaltitude criterion for identifying the poleward boundary of the soft electron layer lying at the high‐latitude edge of the plasma sheet boundary layer.
MHD simulation of the magnetotail during the December 10, 1996, substorm
Authors: Wiltberger M. , Pulkkinen T. I. , Lyon J. G. , Goodrich C. C.
Published: February 2004

This paper presents results of a global MHD simulation of a substorm that occurred on December 10, 1996. We concentrate on the relationship between the simulation results and the magnetotail observations during the growth and expansion phases of the substorm. In general, we find excellent agreement between the single point observations made by various spacecraft in both the geosynchronous and mid‐tail regions: the simulation accurately represented the energy loading (lobe field increase), small‐scale activations (partial dipolarizations), and a global substorm onset (large dipolarizations and fast flows). The global view presented by the simulation shows complex series of discrete flow channels during the expansion phase prior to the onset of global reconnection. It is these flows channels that disrupt the thin current sheets present during the expansion phase of the substorm.
Current disruption during November 24, 1996, substorm
Authors: El‐Alaoui Mostafa
Published: February 2004

This study uses global magnetohydrodynamic (MHD) simulations driven by solar wind data along with Geotail, Interball, and IMP 8 observations of the magnetotail to investigate the dynamics of the near‐Earth plasma sheet during a substorm that occurred on November 24, 1996. The MHD simulation shows that prior to the onset of the substorm, the magnitude of the current density decreases in a small region in the near‐Earth plasma sheet. During and after the substorm onset this region of weak current becomes larger and more pronounced, expanding dawnward, duskward, upward, downward, and tailward. Part of the cross‐tail current is redirected to the ionosphere via an earthward field‐aligned current on the dawnside and a tailward return current on the duskside. The simulation showed that the field‐aligned current was associated with velocity shear and flow vortices.
Models and numerical simulation of velocity distribution of pickup ions at low‐Earth orbit
Authors: Zhang Yinghui , Raitt W. J. , Thompson D. C.
Published: February 2004

Large manned spacecraft release and carry up huge amounts of water in their orbital motion. Some of the neutral water molecules undergo charge exchange with the ambient ionospheric plasma and become what are called pickup ions at low‐Earth orbit (LEO) altitudes. In this paper the velocity distribution of the pickup ions is studied on the basis of the analysis of the trajectories of the pickup ions, the influence of thermal movement, and the influence of collisions between the pickup ions and other particles. The integral models are developed to calculate the velocity distribution of the pickup ions in the frame moving with the orbiter. The comparison between the simulation result and actual measurement data from the spherical retarding potential analyzer (SRPA) on mission Tethered Satellite System ‐ 1 Reflight (TSS‐1R) and earlier missions on which the SRPA was flown shows that the models obtained are correct for the special environment of a large space platform at LEO altitudes and are useful in analyzing the data of the ion probes mounted on similar spacecraft. With the models obtained the influence of some factors on the speed distribution of the ions collected by the payload SRPA of a large spacecraft orbiter is analyzed.
Electron acceleration by megahertz waves during OEDIPUS C
Authors: Huang C. Y. , Burke W. J. , Hardy D. A. , Gough M. P. , James H. G. , Villalón E. , Gentile L. C.
Published: February 2004

Observations of Electric Field Distributions in the Ionospheric Plasma ‐ A Unique Strategy (OEDIPUS C) was a tethered mother‐son experiment that was launched northward from the Poker Flat rocket range at 0638 UT on November 7, 1995, across a sequence of auroral structures. During the flight's upleg the magnetically aligned tether was deployed to a separation of ∼1.2 km and then cut at both ends. The forward payload contained a 50‐kHz to 8‐MHz stepped‐frequency transmitter. Receivers were carried on both forward and aft payloads. The transmitter swept through the frequency range every 0.5 s. During each of the 3‐ms steps the transmitter emitted only for the first 0.3 ms. The scientific complement also included multiangular electrostatic analyzers on both payloads that were sensitive to fluxes of electrons with energies from 20 eV to 20 keV. The durations of sampling and frequency steps were matched. During the flight the electron gyrofrequency was approximately twice the plasma frequency. When the transmitter swept through the local gyrofrequency, the particle detectors on both payloads detected sounder‐accelerated electrons (SAEs) independent of the energy steps being sampled. In addition, SAEs were detected at the aft payload out to separations of several hundred meters for wave emissions at harmonics of the electron gyrofrequency as well as in the upper hybrid and whistler bands. As the vehicle separation increased, significant time differences developed between the wave‐emission pulses and the onsets/durations of SAE detections. The data indicate that electrons were heated through strong wave‐particle interactions. However, a simple resonant‐interaction explanation appears inadequate. We outline requirements for any models purporting to explain OEDIPUS C measurements.
Kelvin‐Helmholtz instability on the magnetospheric flanks: An absolute and convective instability approach
Authors: Mills Katharine J. , Longbottom Aaron W. , Wright Andrew N. , Ruderman Michael S.
Published: February 2004

In this paper, an absolute and convective instability approach to the Kelvin‐Helmholtz instability of magnetospheric surface and body/waveguide modes is presented. Rather than considering normal modes individually, the development of localized wavepackets is considered. It is shown that the time asymptotic behavior of such a wavepacket is determined by the double roots of the dispersion relation and that each double root may be identified with a normal mode, for our particular system. The dominant behavior in any reference frame is given by the double root with the largest growth rate. We consider the absolute or convective nature of the instability in the rest frame of the magnetosphere and deduce that in this reference frame wavepackets may only be absolutely unstable (growing at any fixed point in space at large time) close to the nose of the magnetosphere and are convectively unstable (moving away so that at large time there is no disturbance at any fixed point) elsewhere. The e‐folding lengths of fast surface mode wavepackets are found to be small but increase around the flanks (these will become nonlinear and lead to the broadening of the low‐latitude boundary layer in agreement with previous studies) [Manuel and Samson, 1993]. Fast body modes will e‐fold only once as they convect around the flanks, so would be expected to remain linear in this region. Slow‐mode wavepackets are also studied and are found to have very large e‐folding lengths, so that their growth on the flanks will be negligible. The results are compared to a numerical simulation and excellent agreement is obtained.
A shear instability seeding mechanism for quasiperiodic radar echoes
Authors: Larsen M. F.
Published: February 2004

The quasiperiodic echoes with periods in the range of ∼ 1–10 minutes originally discovered at the Middle and Upper Atmosphere (MU) radar in Japan in the early 1990s are now known to be a common feature of midlatitude sporadic E. A possible role of shear instabilities and the associated Kelvin‐Helmholtz structure in seeding the plasma instabilities that produce the radar echoes is suggested in this paper. The proposed mechanism has the advantage that the seed structure is generated in situ as part of the same neutral wind structure that serves to initiate the generation of the sporadic E layer. In addition, the range of observed periods can easily be explained. Since the Kelvin‐Helmholtz structure is embedded in the shear flow and moves with the mean flow in the shear layer, the period observed in the ground‐based frame is simply a product of the horizontal wavelength and the mean flow velocity at the center of the disturbance. Finally, a typical feature of the radar observations has been that the quasiperiodic structure often ebbs and wanes in intensity over a period of 30 to perhaps 60 min. Recent simulations of unstable neutral shears show that the timescale for the build up and eventual restabilization of the shear flow is 30–40 min, comparable to the timescale found in the observations.
Kinetics and thermalization of hot N(4S) atoms in the upper atmosphere
Authors: Balakrishnan N. , Sergueeva E. , Kharchenko V. , Dalgarno A.
Published: February 2004

The production of energetic N(4S) atoms and their subsequent thermalization by collisions with the ambient gas play an important role in the heat and energy balance of the thermosphère. It has been suggested that nitric oxide formation by the reaction of energetic nitrogen atoms with O2 may account for the NO deficiency in atmospheric models at altitudes of 105–110 km. Using calculated differential scattering cross sections for thermalization of N(4S) atoms by elastic and inelastic collisions with atmospheric bath gases, we determine the energy distribution function of N(4S) atoms and evaluate the importance of the N(4S)+O2 reaction as a source of NO in the thermosphere. We show that the nonthermal component produces large increases in the effective rate coefficient at low altitudes. However, we find that at altitudes below 130 km, where the production of NO by energetic N(4S) atoms substantially exceeds the production by the thermal N(4S) component, there is still a larger contribution from N(2D) atoms, whereas at higher altitudes, where the N(2D) atom source is smaller than the N(4S) atom source, it is the thermal component of the N(4S) distribution that dominates. Thus the production of NO by the reactions of energetic N(4S) atoms cannot be invoked to explain the high abundance of NO.
The relation of temporal variations of soft X‐ray emission from comet Hyakutake to variations of ion fluxes in the solar wind
Authors: Neugebauer M. , Cravens T. E. , Lisse C. M. , Ipavich F. M. , Christian D. , von Steiger R. , Bochsler P. , Shah P. D. , Armstrong T. P.
Published: February 2004

Both the Röntgen X‐Ray Satellite (ROSAT) and the Extreme Ultraviolet Explorer (EUVE) have detected soft X‐ray emission from comet C/Hyakutake 1996 B2. This emission varied by a factor of about 2 over a few hours and by a factor of 4 from day to day. One explanation for the excitation of cometary X rays is the charge transfer mechanism suggested by Cravens. This process involves charge exchange collisions between highly charged heavy ions in the solar wind and neutral gas in the cometary coma. Oxygen ion fluxes observed by the Charge, Element, and Isotope Analysis System (CELIAS) Mass Time‐of‐Flight (MTOF) instrument on the SOHO spacecraft and proton fluxes measured by near‐Earth spacecraft are mapped to the location of the comet to demonstrate that the comet X‐ray variability can be explained on the basis of variability in the solar wind. There is a good correlation between cometary X‐ray emission and oxygen ion fluxes and a poorer correlation with proton flux. The correlation between the solar wind oxygen flux and cometary X rays degrades with increasing latitudinal separation of SOHO from the comet. Cometary X‐ray emission is not sensitive to variations in solar X‐ray fluxes and is unlikely to be caused by crossing of the heliospheric current sheet. The charge transfer mechanism appears to be supported by all the data examined to date. Cometary X rays have some shortcomings as remote sensors of the solar wind, however, because of variations in cometary gas production rates and in the charge states and abundances of heavy solar wind ions.
An explanation of ground observations of auroral hiss: Role of density depletions and meter‐scale irregularities
Authors: Sonwalkar Vikas S. , Harikumar Jayashree
Published: February 2004

Auroral hiss is one of the most intense whistler mode plasma wave phenomena observed both on the ground at high latitudes and on spacecraft in the auroral zone. Propagation of auroral hiss from its source region to the ground is poorly understood. The standard whistler mode propagation in a smooth magnetosphere predicts that auroral hiss generated at large wave‐normal angles along the auroral field lines by Cerenkov resonance cannot penetrate to the ground. We show that the presence of density depletions along the field lines in the auroral zone and meter‐scale density irregularities at altitudes < 5000 km at high latitude permits the auroral hiss propagation to the ground. In our mechanism the auroral hiss generated at high altitudes (> 5000 – 20, 000 km) propagates to lower altitudes (< 3000 – 5000 km) in two modes: (1) a ducted mode guided by field‐aligned density depletions and (2) a nonducted mode. The hiss with large wave‐normal angle arriving at < 5000 km altitude is scattered by meter‐scale irregularities, and about 0.1% to 10% of the scattered hiss has small wave‐normal angles which can penetrate to the ground. Our mechanism explains the following features of auroral hiss observed on the ground: (1) the characteristic spectra of continuous and impulsive auroral hiss, (2) the upper and lower frequency cutoffs, (3) the dispersion of impulsive auroral hiss, (4) the location of ionospheric exit points of auroral hiss with respect to visible aurora, and (5) the 2–5 order of magnitude intensity decrease of auroral hiss observed on the ground relative to that observed on spacecraft. Based on the model presented here, we provide methods to infer parameters of density depletions and intensity of lower hybrid waves stimulated by auroral hiss from the ground measurements of auroral hiss together with optical and radar measurements.
Dynamical properties of self‐consistent magnetotail configurations
Authors: Peroomian Vahé , Ashour‐Abdalla Maha , Zelenyi Lev M.
Published: February 2004

This study of the magnetotail employs a large‐scale kinetic ion model specifically developed to consider the region's approach to equilibrium and its dynamics under the influence of an imposed electric field. Results from the self‐consistent, two‐dimensional model indicate that the magnetotail achieves equilibrium by maintaining a delicate balance between the influx of plasma from a mantle source and the accelerated loss of these particles resulting from nonadiabatic particle dynamics. The quasi‐steady oscillations that result from instantaneous imbalances between sources and losses occur independent of solar wind variations. This variability occurs even for steady solar wind conditions and is caused by the rapid nonadiabatic energization of ions incident on the current sheet and their loss through the flanks of the magnetotail faster than they can be replenished. To determine the influence of external parameters on the dynamical state of the magnetotail, we performed a parameter search in which we varied the magnitude of the convection electric field and the influx of plasma mantle ions. This parameter search showed that the magnetotail configuration can adjust itself to compensate for the various combinations of solar wind parameters. As a rule, this self‐adjustment occurs in the form of quasi‐steady magnetotail states, where only the average characteristics of the configuration are steady but pronounced cyclic variations occur in the magnetotail topology. For cases in which the magnetotail is underpopulated, the current sheet is depleted via the flanks. In overpopulation cases a flaring of magnetotail field lines caused by the additional current in the neural sheet limits access of additional mantle particles to the region. The magnetotail system is flexible enough to self‐consistently regulate its own population. However, the resulting configuration becomes dynamic and only a quasi‐steady state is achieved.
Kelvin‐Helmholtz instability at the magnetotail boundary: MHD simulation and comparison with Geotail observations
Authors: Otto A. , Fairfield D. H.
Published: February 2004

On March 24, 1995, the Geotail spacecraft observed large fluctuations of the magnetic field and plasma properties in the low‐latitude boundary layer about 15 RE tailward of the dusk meridian. Although the magnetospheric and magnetosheath magnetic fields were strongly northward, the Bz component showed strong short‐duration fluctuations in which Bz could even reach negative values. We have used two‐dimensional magnetohydrodynamic simulations with magnetospheric and magnetosheath input parameters specifically chosen for this Geotail event to identify the processes which cause the observed boundary properties. It is shown that these fluctuations can be explained by the Kelvin‐Helmholtz instability if the k vector of the instability has a component along the magnetic field direction. The simulation results show many of the characteristic properties of the Geotail observations. In particular, the quasi‐periodic strong fluctuations are well explained by satellite crossings through the Kelvin‐Helmholtz vortices. It is illustrated how the interior structure of the Kelvin‐Helmholtz vortices leads to the rapid fluctuations in the Geotail observations. Our results suggest an average Kelvin‐Helmholtz wavelength of about 5 RE, with a vortex size of close to 2 RE for an average repetition time of 2.5 min. The growth time for these waves implies a source region of about 10–16 RE upstream from the location of the Geotail spacecraft (i.e., near the dusk meridian). The results also indicate a considerable mass transport of magnetosheath material into the magnetosphere by magnetic reconnection in the Kelvin‐Helmholtz vortices.
Three‐dimensional and twisted: An MHD interpretation of on‐disk observational characteristics of coronal mass ejections
Authors: Gibson S. E. , Low B. C.
Published: February 2004

A physical interpretation of observed coronal “on‐disk” manifestations of an Earth‐directed coronal mass ejection (CME) is presented. The fundamental question of how the CME's magnetic field and its plasma distribution are related is largely unanswered, because a crucial piece of the puzzle, that is the three‐dimensional (3‐D) morphology of the CME, remains difficult to ascertain so long as coronal observations are limited to projections onto a single plane of the sky. In order to understand the relationship between observations of CMEs projected at the solar limb and those projected on the solar disk, some sort of model of the 3‐D CME is required. In this paper we address both the question of the 3‐D morphology of the CME and the more fundamental question of the nature of the plasma‐magnetic field relationship, by comparing the limb and on‐disk CME representations of an analytic 3‐D MHD model based on a spheromak‐type flux rope magnetic field configuration. In particular, we show that the morphology of twin dimmings (also referred to as transient coronal holes) observed in X ray and EUV can be reproduced by the CME model as the on‐disk projection of the prominence cavity modeled for limb CMEs. Moreover, the bright core of a limb CME, generally corresponding to the material in an erupting prominence, may be interpreted to be the S‐shaped central core of the modeled on‐disk CME, splitting the cavity into twin dimmings when observed head‐on without obstruction. The magnetic field structure of this central core exhibits many of a filament's magnetic field features required to match observations. Finally, we consider the nature of S‐shaped filaments and X‐ray “sigmoids” in the context of the model, in terms of localized heating and cooling acting on the modeled CME magnetic field structure.
Self‐organized criticality in the substorm phenomenon and its relation to localized reconnection in the magnetospheric plasma sheet
Authors: Klimas A. J. , Valdivia J. A. , Vassiliadis D. , Baker D. N. , Hesse M. , Takalo J.
Published: February 2004

Evidence is presented that suggests that there is a significant self‐organized criticality (SOC) component in the dynamics of substorms in the magnetosphere. We assume that observations of bursty bulk flows, fast flows, localized dipolarizations, plasma turbulence, etc. show that multiple localized reconnection sites provide the basic avalanche phenomenon in the establishment of SOC in the plasma sheet. First results are presented from a study of this avalanche process based on this working assumption. A magnetic field reversal model is discussed. Resistivity, in this model, is self‐consistently generated in response to the excitation of an idealized currentdriven instability. When forced by convection of magnetic flux into the field reversal region, the model yields rapid magnetic field annihilation through a dynamic behavior that is shown to exhibit many of the characteristics of SOC. Over a large range of forcing strengths, the annihilation rate is shown to self‐adjust to balance the rate at which flux is convected into the reversal region. Several analogies to magnetotail dynamics are discussed: (1) It is shown that the presence of a localized criticality in the model produces a remarkable stability in the global configuration of the field reversal while simultaneously exciting extraordinarily dynamic internal evolution. (2) Under steady forcing it is shown that a loading‐unloading cycle may arise that, as a consequence of the global stability, is quasi‐periodic and, therefore, predictable despite the presence of internal turbulence in the field distribution. Indeed, it is shown that the global loading‐unloading cycle is a consequence of the internal turbulence. (3) It is shown that under steady, strong forcing the loading‐unloading cycle vanishes. Instead, a recovery from a single unloading persists indefinitely. The field reversal is globally very steady while internally it is very dynamic as field annihilation goes on at the rate necessary to match the strong forcing. From this result we speculate that steady magnetospheric convection events result when the plasma sheet has been driven close to criticality over an extended spatial domain. During these events we would expect to find localized reconnection sites distributed over the spatial domain of near criticality, and we would expect to find plasma sheet transport in that domain to be closely related to that of BBF and fast flow events.
Auroral observations in Finland: Results from all‐sky cameras, 1973–1997
Authors: Nevanlinna Heikki , Pulkkinen Tuija I.
Published: February 2004

Analyses of time‐space behavior of auroral occurrence were carried out using quick‐look data of Finnish all‐sky cameras operated in 1973–1997 using seven stations in Finland and one in Svalbard. This unique data set consists of 25 years of continuous auroral observations, or about 100,000 hours of all‐sky pictures. The basic parameter derived from quick‐look data is the auroral occurrence index (AO), which gives the probability (in percentage) of the occurrence of auroras during clear nights in the dark season (January to April and August to December for each year). We have analyzed the time variations of AO in diurnal, semiannual, and solar cycle timescales. The daily and semiannual occurrence of auroral frequency in Finland are consistent with the results of earlier studies carried out in other countries. There is no clear correlation between annual values of sunspot numbers and auroral occurrence within a solar cycle, but the AO values show a rather systematic increase for 1973–1993. A comparison between AO data with synoptic auroral observations made in Finland and worldwide from 1881 to 1979 reveals that both data series exhibit an increasing trend of auroral activity since the beginning of the twentieth century, similar to the global magnetic activity index series aa and sunspot number. We interpret that the increase of AO values is associated with the long‐term increase of solar magnetic activity, as has been suggested recently.
The nightside poleward boundary of the auroral oval as seen by DMSP and the Ultraviolet Imager
Authors: Baker J. B. , Clauer C. R. , Ridley A. J. , Papitashvili V. O. , Brittnacher M. J. , Newell P. T.
Published: February 2004

A lack of reliable error estimates for poleward auroral emission (PAE) boundaries derived from satellite‐borne auroral imagers has hampered the application of these instruments in quantitative magnetospheric energy balance and substorm analysis. In this study, PAE boundaries from Polar Ultraviolet Imager (UVI) images are compared with precipitation boundaries from Defense Meteorological Satellite Program (DMSP) satellite spectrograms. In particular, the study quantifies the accuracy with which UVI images can be used to reproduce the DMSP poleward auroral oval (b5e) boundary. Most of the DMSP b5e boundaries were obtained in the evening sector. It has been determined that a UVI PAE boundary defined by a fixed ratio to the maximum in the auroral oval at each magnetic local time correlates better with the DMSP b5e boundary than one defined by a constant brightness threshold (0.90 versus 0.80 maximum correlation). The optimal threshold and ratio values are found to be 4.3 photons cm−2S−1 and 0.30 normalized flux, respectively. The study also reveals a systematic latitudinal offset between UVI and DMSP in the evening sector with a magnitude of approximately 1°. This offset might represent a real height‐dependent geomagnetic influence (e.g., active magnetic topology) or result from systematic errors in the analysis (e.g., removal of the UVI platform wobble). It is demonstrated that the offset can be partially removed with a linear calibration model, allowing the reproduction of DMSP b5e boundaries from UVI images with a standard deviation error of approximately 1°.
Ionospheric ion and electron heating at the poleward boundary of a poleward expanding substorm‐disturbed region
Authors: Fox N. J. , Cowley S. W. H. , Davies J. A. , Greenwald R. A. , Lester M. , Lockwood M. , Lühr H.
Published: February 2004

We present observations of a poleward propagating substorm‐disturbed region which was observed by the European Incoherent SCATter (EISCAT) radar and the Svalbard International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers in the post‐midnight sector. The expansion of the disturbance was launched by a substorm intensification which started ∼25 min after the initial onset, and ∼10 min before the disturbance arrived over Svalbard. In association with the magnetic disturbance, a poleward expanding enduring enhancement in the F region electron temperature was observed, indicative of soft electron precipitation, with a narrow band of enhanced ion temperature straddling its poleward edge, indicative of fast ion flows and ion‐neutral collisional heating. This electron temperature boundary was coincident with the poleward propagating electrojet current system detected by the high‐latitude IMAGE magnetometer stations and is taken to be a proxy for the observation of a substorm auroral bulge. The electron temperature boundary is inferred to have a width comparable or less than one radar range gate (∼60 km transverse to the magnetic field), while the region of high ion temperature was found to be approximately three gates wide, extending approximately two gates (∼120 km) poleward of the electron temperature boundary, and approximately one gate (∼60 km) equatorward. The two‐beam radar line‐of‐sight velocity data are found to be consistent with the existence of a layer of high‐speed flow in the boundary, peaking at values ∼1.5–3 km s−1, roughly consistent with the ion temperature data. The flow is directed either east or west along the boundary depending on the direction of the flow in the poleward region. We infer that the flow is deflected along and around the boundary of the substorm‐disturbed region due to the high conductivity of the latter. Variations in the flow poleward of the boundary produced no discernible magnetic effects on the ground, confirming the low conductivity of the preboundary ionosphere.