bluuebunny/crossref_metadata_2025 huggingface dataset:
Ponderomotive upward acceleration of ions by ion cyclotron and Alfvén waves over the polar regions
Authors: Guglielmi A. , Lundin R.
Published: February 2004

A ponderomotive theory that describes the acceleration of magnetospheric ions along the geomagnetic field lines is developed and compared to Viking and Freja satellite observations showing that low‐frequency electric field fluctuations correlate with the upward escape of ions. From the general expressions of the averaged field‐aligned wave forces, we develop a theory for the nonresonance acceleration of the polar wind and resonant acceleration of a small admixture of heavy ions under the action of Alfvén and ion cyclotron waves. Using a hydrodynamic polar wind model, we show analytically and numerically that the critical point of supersonic transition shifts toward the Earth under the ponderomotive action of the waves. Furthermore, the ion velocity, as well as the acceleration of the flow, increases as the wave amplitude increases. It is shown that the critical velocity always equals the local sound velocity, independent of whether Alfvén waves are present or not. The asymptotic behavior of the flow (far above the critical point) is also modified substantially under the action of Alfvén waves. In particular, the mere existence of waves in the flow causes an increasing upward acceleration with increasing distance from the Earth. We emphasize that the subsonic polar breeze is not subject to a qualitative modification under the action of Alfvén waves, as opposed to the supersonic polar wind. In addition, we present a ponderomotive mechanism for the generation of field‐aligned small‐scale plasma density depletions which have been discovered by polar‐orbiting satellites. The acceleration of heavy ions has been examined from the viewpoint of single‐particle motion. The emphasis is on a phenomenon termed ponderomotive resonance. It is found that the energy gain of heavy ions increases as the first power of the wave amplitude provided the ion intersects the resonance region from the bottom upward, despite the fact that the ponderomotive force (“magnetic moment pumping”) is proportional to the second power of the wave amplitude. This leads to fast acceleration of heavy ions provided that high‐amplitude waves propagate along geomagnetic field lines.
Reply [to “Solar cycle lengths and climate: A reference revisited”]
Authors: Lassen K. , Friis‐Christensen E.
Published: February 2004

In a critical assessment of the results regarding a possible association between solar activity variations and climate by Friis‐Christensen and Lassen [1991] and Lassen and Friis‐Christensen [1995], Laut and Gundermann [this issue] conclude that the correlation between the solar cycle length parameter and the Northern Hemisphere land temperature is weak. In this short reply we confirm our earlier conclusion, on the basis of new and independent temperature data, that during 400 years, from 1570 to 1970, there is a significant statistical correlation between these parameters, indicating that solar forcing constitutes an important contribution to the natural temperature fluctuations. This does not exclude, as also stated previously, that other climate forcings may have an effect on global temperature, including the effect of man‐made greenhouse gases, in particular, after 1970.
Three‐dimensional hybrid simulation of solar wind interaction with unmagnetized planets
Authors: Shimazu Hironori
Published: February 2004

The interaction between the solar wind and unmagnetized planets was investigated using computer simulation and a three‐dimensional hybrid code that treats kinetic ions and massless fluid electrons. In the simulation the planet was treated as an ionized gaseous body with uniform and constant supply of plasma. The results given in this paper showed that the shock size and magnetic barrier intensity are asymmetrical in the direction of the convection electric field because of oxygen ions escaping from the side of the planet to which the electric field was pointing. The calculated asymmetry in the magnetic barrier intensity was consistent with observations, although the direction of the calculated asymmetry in the shock size did not match the asymmetry observed near either Venus or Mars. When the planet size was comparable to the Larmor radius of protons, a multiple‐shock structure was observed in the simulation. It is suggested that this structure is due to dispersion of the magnetosonic waves caused by the finite Larmor radius of protons. The simulation results also showed that ions escaped to the magnetotail through the tail rays and that the tail rays were connected with the plasma sheet.
The interplanetary shock of September 24, 1998: Arrival at Earth
Authors: Russell C. T. , Wang Y. L. , Raeder J. , Tokar R. L. , Smith C. W. , Ogilvie K. W. , Lazarus A. J. , Lepping R. P. , Szabo A. , Kawano H. , Mukai T. , Savin S. , Yermolaev Y. I. , Zhou X.‐Y. , Tsurutani B. T.
Published: February 2004

At close to 2345 UT on September 24, 1998, the magnetosphere was suddenly compressed by the passage of an interplanetary shock. In order to properly interpret the magnetospheric events triggered by the arrival of this shock, we calculate the orientation of the shock, its velocity, and its estimated time of arrival at the nose of the magnetosphere. Our best fit shock normal has an orientation of (−0.981 −0.157 −0.112) in solar ecliptic coordinates, a speed of 769 km/s, and an arrival time of 2344:19 at the magnetopause at 10 RE. Since measurements of the solar wind and interplanetary magnetic field are available from multiple spacecraft, we can compare several different techniques of shock‐normal determination. Of the single spacecraft techniques the magnetic coplanarity solution is most accurate and the mixed mode solution is of lesser accuracy. Uncertainty in the timing and location of the IMP 8 spacecraft limits the accuracy of solutions using the time of arrival at the position of IMP 8.
Automatic identification of large‐scale field‐aligned current structures
Authors: Higuchi Tomoyuki , Ohtani Shin‐ichi
Published: February 2004

The study of large‐scale field‐aligned currents (LSFACs) has depended on visual inspection of data plots, which is often subjective and time‐consuming, and therefore limits the efficiency of analyses. The present paper reports a new procedure to automatically identify the spatial structure of LSFACs from satellite magnetic field measurements. The procedure is based on the concept of the first‐order B spline fitting with variable node positions, which may be envisioned as fitting line segments to a line plot. The fitting is made for the maximum variance component of magnetic variations in the plane perpendicular to the background magnetic field. If the distribution of LSFACs can be approximated as an infinite sheet, each slope of the plot corresponds to the crossing of a FAC sheet. The number of node points, which determines the number of FAC sheets, is one of the fitting parameters and is optimized for each orbit so that the Akaike information criterion (AIC) is minimized. Whereas other methods, such as a spherical harmonic fitting, seek to obtain two‐dimensional distributions of FACs from assembled data, the present method is basically the automation of the way we visually examine a plot of satellite magnetic field data, and it can be applied even to a single satellite pass. Therefore the procedure can be implemented to real‐time data processing and now‐casting. The procedure should also provide a powerful tool for data mining. Magnetic field data from the Defense Meteorological Satellite Program‐F7 (DMSP‐F7) were used for demonstration, and the present paper reports the initial results.
Four‐sheet structures of dayside field‐aligned currents: Statistical study
Authors: Ohtani S. , Higuchi T.
Published: February 2004

The present study investigates the statistical characteristics of four‐sheet structures of large‐scale field‐aligned currents (FACs) observed by the Defense Meteorological Satellite Program‐F7 (DMSP‐F7) satellite in the dayside high‐latitude region. A total of 495 four‐FAC‐sheet events were selected from the entire set of DMSP‐F7 magnetometer data. With respect to the polarity of FACs, the selected events are classified into two groups, which are named type M and type W after the shape of a plot of the azimuthal magnetic component. The polarity of the equatorward most FAC is downward for the type M structure, whereas it is upward for the type W structure. Type W and type M events tend to take place in the prenoon and postnoon sectors, respectively, but the transition takes place slightly before noon. The type W structure tends to be observed for negative and positive interplanetary magnetic field (IMF) BY in the Northern and Southern Hemispheres, respectively. For a given hemisphere, the favorable sign of IMF BY for the type M structure is opposite to that for the type W structure. It is also found that both types occur more often when the IMF BZ is northward than when it is southward. The four‐sheet structure can be interpreted in terms of two zonal ionospheric convection flows separated in latitude. The equatorward one is a part of the viscous cell and, possibly, also the merging cell, whereas the poleward one is a part of the lobe cell. The direction (of the major cell) of the lobe convection is determined by the sign of IMF BY and is different in different hemispheres, explaining the discovered IMF BY dependence. The preference for northward IMF BZ is consistent with the required development of the lobe convection. It is inferred from the expected polar distribution of FACs that the four‐sheet structure is a superposition of a pair of midday region 0 and region 1 currents and a pair of conventional region 1 and region 2 currents.
Linear magnetohydrodynamic response of the magnetopause to magnetosheath fluctuations
Authors: De Keyser J.
Published: February 2004

We study the response of the magnetopause to incident magnetosheath ULF fluctuations by combining the linear magnetohydrodynamics (MHD) approach with harmonic analysis. We consider plane wave fronts arriving at the magnetopause with various tangential wavelengths. The fluctuations in the magnetosheath are pulse trains or waves with a broadband ULF spectrum. We obtain the magnetopause response to these fluctuations based on the computation of the transmission, reflection, and absorption characteristics of monochromatic linear MHD waves at different frequencies in a given equilibrium magnetopause configuration. Particular attention is given to MHD wave mode conversion as an ingredient for explaining the enhanced electromagnetic fluctuation level near the magnetopause.
Atmospheric Ca and Ca+ layers: Midlatitude observations and modeling
Authors: Gerding M. , Alpers M. , von Zahn U. , Rollason R. J. , Plane J. M. C.
Published: February 2004

We report on a comprehensive set of observations of the upper atmospheric Ca and Ca+ layers. The observations were obtained by ground‐based lidars at Kühlungsborn, Germany (54°N, 12°E), between December 1996 and December 1998. During this period, 112 nights of Ca soundings and 58 nights of Ca+ soundings were realized. The Ca layer has an average column abundance of 2.1·107 cm−2, centered around 90.3 km with a mean peak density of 22 cm−3 at 89.9 km altitude. The Ca+ dominates the total Ca amount above 90 km and has an average column abundance of 4.9·107 cm−2. Because the vaporization of cosmic dust is the most probable source of atmospheric metals, the column densities of the metals within the atmosphere are often compared with the abundance in chondritic CI meteorites. We show that the atmospheric Ca is severely depleted with respect to other metals such as Na and Fe, compared with their relative abundances in CI chondrites. We present a one‐dimensional steady state chemistry model of the nighttime Ca and Ca+ layers, based on new laboratory studies of CaO reaction kinetics. This model is able to reproduce satisfactorily the characteristic features of the annual mean layers and to provide a possible explanation for the unusual seasonal variation of the Ca layer which exhibits a pronounced summertime enhancement around 87 km.
Convection enhancements in the low‐latitude lobe for substorm onset
Authors: Taguchi S. , Kiyohara M. , Mukai T. , Nosé M. , Saito Y.
Published: February 2004

Characteristics of the lobe plasma convection enhancements around Pi2 substorm onset have been clarified with Geotail data. We identified 92 lobe substorm onset events within 5 RE from an estimated neutral sheet from X = −10 to −35 RE, and we found 27 convection enhancement events. These events were categorized into two types depending on whether the magnetic field tilts northward or southward in coincidence with intensity decrease. The former type was identified in 12 of the 27 and found to be generally at shorter distances from the Earth than the latter. The frequencies of the lobe observations in the above Z width are small (≤11%); that is, the plasma sheet usually occurs. In this situation, 80% of the 27 are identified only within 2.5 RE from the estimated neutral sheet. This indicates that the convection enhancement events occur in association with the lobe appearance in the vicinity of the neutral sheet. This is also recognized in the X‐Y distribution of the low‐latitude lobe observation; the relatively high frequencies of ≥4% which have a dawn‐dusk asymmetry roughly correspond to the event location. Examination of the convection speed shows that the peak lies in 0.04VA to 0.1VA (VA is the local Alfvén velocity) and that the duration is 1 – 3 min for most cases. Identification probability of the convection enhancement events was also examined. Both types of events have very similar occurrence probability in Y=0–10 RE, which is consistent with an interpretation that both are created by a common process, i.e., lobe reconnection in the near‐Earth neutral line (NENL). For the dawnside of this Y region, however, the southward field tilting type more often appears than the other, implying that the convection enhancements occur in an asymmetric manner with respect to the NENL.
Structure of the magnetopause reconnection layer and of flux transfer events: Ion kinetic effects
Authors: Nakamura Masao , Scholer Manfred
Published: February 2004

We have performed large‐scale two‐dimensional hybrid simulations of reconnection in an asymmetric current sheet applicable to the magnetopause. The ions are treated as macroparticles, and the electrons are a massless charge‐neutralizing fluid. We have recovered the essential features of the equivalent MHD simulations, that is, a moving and expanding bulge and a reconnection layer with a high‐speed flow of magnetosheath ions on the magnetospheric side. The bulk speed of the magnetosheath ions is twice as high as the speed of the bulge; that is, high‐speed flows are predicted on the magnetospheric side of the bulge. A quasi‐steady reconnection layer develops behind the bulge in the reconnection outflow region. In the coplanar case the change of the magnetic field from the magnetosheath side to the magnetospheric side takes place at a discontinuity with an electron sense of rotation. This rotational sense is a consequence of the Hall effect near the reconnection line. Magnetosheath ions exhibit a higher perpendicular temperature on the downstream side of the discontinuity; the discontinuity is therefore most likely an intermediate shock (IS). The gradient scale length of the IS is of the order of a few magnetosheath ion inertial lengths. In the noncoplanar case (including a By interplanetary magnetic field (IMF) component) a rotational discontinuity occurs, which is the minimum shear solution. No other discontinuities, such as slow mode shocks or slow mode waves, have been identified. The high‐speed layer is bound to the magnetospheric side by a magnetic boundary with unconnected field lines. Magnetospheric and magnetosheath ions exhibit the expected velocity filter effects. The two populations penetrate each other without interaction. In the noncoplanar case there are important differences in the structure of the magnetic field and the flow pattern in the high‐speed layer. The sense of the rotational discontinuities is determined by the superposition of the magnetic field produced by the Hall current loop and the IMF By component. This leads to different rotational senses of the rotational discontinuities on the two sides of the reconnection line.
A plasma vortex revisited: The importance of including ionospheric conductivity measurements
Authors: Kosch M. J. , Amm O. , Scourfield M. W. J.
Published: February 2004

In an earlier paper [Kosch et al., 1998], simultaneous all‐sky TV imager and Scandinavian Twin Auroral Radar Experiment (STARE) observations of an ionospheric plasma vortex located poleward of an auroral arc were presented. The vortex is associated with a sudden brightening of the arc and corresponds to an ionospheric region of diverging horizontal electric fields, which is equivalent to a downward field‐aligned current (FAC), i.e., the closure current for the upward current above the arc. This event has been revisited because of the subsequent availability of data from the Scandinavian Magnetometer Array. These data, combined with STARE electric fields, have been used to determine the real ionospheric conductance distribution throughout the field of view. As a result, a more realistic, quantitative picture of the current system associated with the arc is obtained than was possible in an earlier model based on an assumed constant conductance. In particular, a complete macroscopic electrodynamic description of a plasma vortex, composed of ionospheric conductances, true horizontal currents, and FACs, is obtained for the first time. It is shown that the plasma vortex corresponds to an area of decreased conductance, thus broadening the FAC distribution and reducing the current density compared to the earlier results. The study illustrates that horizontal conductance gradients should not be neglected when computing FACs.
Plasma density suppression process around the cusp revealed by simultaneous CUTLASS and EISCAT Svalbard radar observations
Authors: Ogawa T. , Buchert S. C. , Nishitani N. , Sato N. , Lester M.
Published: February 2004

Simultaneous CUTLASS and EISCAT Svalbard radar (ESR) observations on February 1, 1998, are used to study the generation of plasma density suppression, which may ultimately result in polar cap patch formation, occurring around the cusp region under IMF Bz negative and By positive conditions. The CUTLASS HF radars in Iceland and Finland observed F region plasma drifts in a wide area including the ESR field of view while the ESR monitored electron density, electron temperature, ion temperature, and ion motion along the geomagnetic field. We focus on two events for which the density suppressions (30–60%) formed in harmony with strong plasma drifts (> 1500 m s−1) lasting for 5–12 min. In one event, the suppression is mainly caused by enhanced chemical reactions in the F region due to the intensified convection flows which also raise ion temperature by 1500–2000 K through frictional heating. This process can chop preexisting high‐density region produced by energetic particle precipitation, maybe giving rise to polar patches. In the other event, the density suppression is related to an appearance of eastward directed high‐speed plasma jets in a limited region with a latitudinal width of > 100 km and a longitudinal extension of > 500 km. The ESR data, however, show no ion temperature increase, suggesting that the suppression may not be caused by enhanced chemical reactions. We tentatively propose that the eastward plasma jets transported less dense plasma from earlier local times over the ESR. Rapid change of IMF By polarity is another candidate for producing the density suppression. A role of HF radar wave refraction in explaining this event is discussed.
Role of the driver in the dynamics of a coupled‐map model of the magnetotail: Does the magnetosphere act as a low‐pass filter?
Authors: Takalo J. , Mursula K. , Timonen J.
Published: February 2004

A coupled‐map model of the magnetotail is used to study the dynamics of the magnetosphere. Using different types of colored noise as an input to the model, it is shown that the high‐frequency part of the behavior is intrinsic to the model, while the low‐frequency part reflects the dynamics of the input time series. It is also shown that the relation of the power spectra of the magnetospheric input (vBS) and output (AE) time series is very similar to the relation of the power spectra of the model input and output. It is then argued that the magnetosphere acts, similarly to the model, as a low‐pass filter. It is also suggested that the power law distributions of the AE data may be partly due to the power law distribution of polarity changes in the interplanetary magnetic field Bz.
On the transition from collisionless to collisional magnetohydrodynamics
Authors: Strangeway R. J. , Raeder J.
Published: February 2004

Magnetosphere‐ionosphere coupling entails the interaction of two quite different plasmas. The magnetosphere is to a large extent a collisionless magnetohydrodynamic (MHD) fluid, while the ionosphere is strongly collisional. As such the relationship between the electric currents and electric field appear to be fundamentally different in the two regimes. For the magnetosphere the currents are determined by the forces within the plasma, while in the ionosphere the current and electric field are related through an anisotropic Ohm's law. Here we explore the transition between these two regimes and show that there is a clear ordering of the governing equations, with a single “collisional Ohm's law” that contains both the collision‐frequency‐dependent Pedersen and Hall conductivities and collisionless MHD terms, without making any prior assumptions concerning the ordering of the collision frequencies. The generalized Ohm's law of MHD is also present, but this equation reduces to the statement that within the ionosphere the magnetic field is “frozen” to the electron fluid, unless electron collision frequencies become comparable to the electron gyrofrequency. It is the freezing‐in of the electron fluid that leads to the direct equivalence of mechanical and electromagnetic loads. This equivalence of loads also indicates that Poynting flux traveling upward out of the ionosphere can only occur if there is a convergence of horizontal Poynting flux in excess of the ionospheric Joule dissipation.
Observation of anomalous temperatures in the daytime O(1D) 6300 Å thermospheric emission: A possible signature of nonthermal atoms
Authors: Hubert B. , Gérard J.‐C. , Killeen T. L. , Wu Q. , Bisikalo D. V. , Shematovich V. I.
Published: February 2004

A study of the effect of a thermospheric population of nonthermal O(1D) atoms on the 6300 Å emission is undertaken based on a comparison between daytime observations from space and theoretical simulations. Vertical temperature profiles deduced from 6300 Å airglow emission measurements using the Fabry‐Perot interferometer (FPI) instrument onboard the Dynamics Explorer 2 (DE 2) satellite are compared to the MSIS‐90 model. Metastable O(1D) temperatures about 150 K larger than the MSIS neutral kinetic temperature are deduced from the 6300 Å line profiles observed during daytime, when the satellite altitude is higher than 400 km. We propose a theoretical explanation for this difference, based on the presence of a nonthermal O(1D) population in the line‐of‐sight of the instrument. Monte Carlo simulations of the nonthermal O(1D) energy distribution function are used to calculate the red line emission and to simulate the FPI behavior, including the line‐of‐sight integration. The comparison between the simulated data and the FPI ones as well as sensitivity tests allow us to conclude that the presence of nonthermal atoms in the instrument field of view is the most likely explanation of the observed discrepancy.
Ap time variations and interplanetary magnetic field intensity
Authors: Ahluwalia H. S.
Published: February 2004

We have studied the data strings for the planetary index Ap, the interplanetary magnetic field intensity (B), and the solar wind bulk velocity (V) for the period 1963 to 1998. The study covers three sunspot number cycles (20, 21, 22). We report the presence of a long‐term trend in the available B data (1963 to 1998) that corresponds in time with the three‐cycle quasiperiodicity discovered by us earlier in Ap data for the 1932 to 1996 period. A set of regression calculations has been carried out correlating V and B with Ap. Our analysis brings out some of the steady state characteristics of the long‐term changes in the parameters and their interrelationships. Among other things, our phenomenological study indicates that solar polar field reversals may be responsible for the existence of the Gnevyshev gap in Ap data. A function proportional to BV2 describes the time variations of Ap quite well, as does the function proportional to the product BV. The latter represents the interplanetary electric field responsible for the magnetosphere‐solar wind coupling. We ascribe the Ap time variations to this fluctuating electric field. We suggest that B is a more fundamental parameter than V.
Shock drift acceleration of electrons: A parametric study
Authors: Vandas M.
Published: February 2004

Spikes of energetic (1–100 keV) electrons have been observed at the nearly perpendicular Earth's bow shock and at traveling interplanetary shocks and have been explained as solar wind electrons accelerated by the shock drift mechanism. We present spectral and angular distributions of the accelerated electrons using the theory of shock drift acceleration at curved shocks. The influence of various shock parameters on these distributions is extensively discussed here. The theoretical fluxes of accelerated electrons are highly anisotropic, and their spectra cannot be fitted by a power law. The spectra strongly depends on the location near the shock, and they are harder for upstream electrons: the highest fluxes can be found in the regions of the shock with θBn ≈ 86°–88°. The fluxes of reflected electrons increase with decreasing shock curvature or shock thickness, while fluxes of transmitted electrons do not depend significantly on these parameters. The magnetic field profile shapes through the shock have no large effect on the acceleration. A higher upstream plasma velocity (Mach number) results in larger electron fluxes. These basic properties of electron distributions are then qualitatively compared with observational facts. These comparisons lead us to the conclusion that the shock drift acceleration is strongly modified by some additional processes, which could be pitch angle scattering of electrons in the shock layer, or spatial/temporal changes of the shock front.
A self‐consistent model of the Jovian auroral thermal structure
Authors: Grodent Denis , Waite J. Hunter , Gérard Jean‐Claude
Published: February 2004

A one‐dimensional (1‐D) model coupling a two‐stream electron transport model of energy deposition with a 1‐D thermal conduction model has been developed. It is applied to investigate the links between auroral heat input and the vertical temperature of Jupiter's upper atmosphere. Two energy distributions meant to reproduce the emissions of a diffuse and a discrete aurora are used to evaluate the importance of the energy spectrum of the incident electrons for the thermal balance of Jupiter's auroral thermosphere. The values of observable quantities such as the altitude of the H2 emission peak, thermal infrared (IR), ultraviolet (UV) emissions, and temperatures associated with various optical signatures are used to constrain the parameters of these distributions. It is shown that the high‐energy component of these energy distributions heats a region of the homosphere between 10−4 and 10−6 bar and mainly controls the H2 temperature and the far‐UV (FUV) emission. A 3‐keV soft electron component is necessary to heat the region directly above the homopause, between 10−6 and 10−9 bar. It has a large influence on the H2 and H3+ temperatures and on the H3+ near‐IR (NIR) emission. It is used in conjunction with a weak 100 eV component which is responsible for heating the thermosphere, from 10−9 to 10−12 bar and exerts a control on the exospheric temperature. The calculated temperatures, UV, and IR emissions suggest that the model probably misses a nonparticle heat source in the 10−5 bar region, that is expected to balance the strong hydrocarbon cooling. Sensitivity tests are performed to evaluate the importance of the parameters of the energy distributions. They show that the FUV color ratio increases with the characteristic energy (or high‐energy cutoff) of the high‐energy component, while the H2 rovibrational temperature varies inversely. A trade‐off is therefore necessary for these two parameters to simultaneously meet their observational constraints. Further tests demonstrate the essential thermostatic role played by H3+, which regulates the net heating in the thermosphere. An increased eddy diffusion reproduces the effect of a possible auroral upwelling of methane but gives rise to an H2 temperature smaller than the observed value.
A new instrument for space plasma exploration: The current density coil
Authors: Meyer A. , Rezeau L. , Mottez F. , de Feraudy H. , Roux A.
Published: February 2004

This paper presents an instrument aimed at measuring current densities in space plasmas: a current density coil. Such an instrument already exists for the estimation of currents in the laboratory. A special design has been developed and tested for use on board spacecraft. The characteristics of the instrument are explained in details and many tests performed on the ground are presented. It is shown that the current density coil is sensitive enough to measure ionospheric currents.
A case study test of Araki's physical model of geomagnetic sudden commencement
Authors: Lam M. M. , Rodger A. S.
Published: February 2004

This paper tests the Araki [1994] computational model of the Earth‐ionosphere system during geomagnetic sudden commencement (sc). In particular, we test the model's ability to predict the signs of the preliminary and main impulses, given the latitude and the magnetic local time (MLT), using a case study of an sc which occurred at 0949 UT on November 22, 1997. Data from a global network of magnetometer stations and from satellites are used. Model predictions compare well with the case study data at high latitudes (above ∼ 72°N Altitude Adjusted Corrected Geomagnetic Coordinates (AACGM)), less well for lower latitudes, particularly on the nightside. In addition, the position of the footprints of the field‐aligned currents (FACs) associated with the sc ground signature varies with MLT, contrary to the model. Data from satellites in polar and geostationary orbits suggest that the FACs for both the preliminary and the main impulses map to gradients in magnetospheric plasma concentration, such as the outer radiation belt and the plasmapause.
A neural network‐based local model for prediction of geomagnetic disturbances
Authors: Gleisner Hans , Lundstedt Henrik
Published: February 2004

This study shows how locally observed geomagnetic disturbances can be predicted from solar wind data with artificial neural network (ANN) techniques. After subtraction of a secularly varying base level, the horizontal components XSq and YSq of the quiet time daily variations are modeled with radial basis function networks taking into account seasonal and solar activity modulations. The remaining horizontal disturbance components ΔX and ΔY are modeled with gated time delay networks taking local time and solar wind data as input. The observed geomagnetic field is not used as input to the networks, which thus constitute explicit nonlinear mappings from the solar wind to the locally observed geomagnetic disturbances. The ANNs are applied to data from Sodankylä Geomagnetic Observatory located near the peak of the auroral zone. It is shown that 73% of the ΔX variance, but only 34% of the ΔY variance, is predicted from a sequence of solar wind data. The corresponding results for prediction of all transient variations XSq + ΔX and YSq + ΔY are 74% and 51%, respectively. The local time modulations of the prediction accuracies are shown, and the qualitative agreement between observed and predicted values are discussed. If driven by real‐time data measured upstream in the solar wind, the ANNs here developed can be used for short‐term forecasting of the locally observed geomagnetic activity.
Production of the low‐latitude night E layer
Authors: Titheridge J. E.
Published: February 2004

A recent study of data from Arecibo concluded that the night E layer must consist primarily of metallic ions produced by meteors. Full model calculations show that this is not necessary. Known EUV sources (from starlight, scattered sunlight, and F region recombination) will produce an E region that agrees well with Arecibo results throughout the night. Direct rocket observations confirm that metallic ions are insignificant in the E region at all times. Model and data also agree on the absence of any appreciable changes with season or solar activity in the night values of NmE. The large solar cycle change in the IRI‐95 model seems to come from an incorrect application of F region theory and should be deleted.
The early phase of spread F development studied in situ with impedance and Langmuir probes
Authors: Steigies Christian T. , Block Dietmar , Hirt Markus , Piel Alexander
Published: February 2004

The postsunset equatorial ionosphere is studied with rocket‐borne plasma diagnostic instruments within the Dynamics of the Equatorial Ionosphere Over Shriharikota Range (DEOS) campaign. Data from an impedance probe and a Langmuir probe allow for a critical comparison between the instruments, with ionosonde data, and with the International Reference Ionosphere model. During flight F05, which was launched before the onset of equatorial spread F (ESF), a generally smooth density profile was found. Density depletions (bubbles) are only found when the rocket crosses the lower edge of the F layer during the downleg of the trajectory. In flight F06, launched shortly after onset of ESF, a large‐scale depletion is found on the upleg of the trajectory below 360‐km altitude, where a transition to an unperturbed density profile occurs. Impedance probe and Langmuir probe data show close agreement of the density depletions and their fine structure with a resolution of 100 m. The morphology of the plasma depletion is compared with models and previous investigations. Electron temperature measurements with Langmuir probes show that the temperature in the depleted region is lower than that in the ambient plasma.
Observations of the seasonal dependence of the thermal plasma density in the Southern Hemisphere auroral zone and polar cap at 1 RE
Authors: Johnson M. T. , Wygant J. R. , Cattell C. , Mozer F. S. , Temerin M. , Scudder J.
Published: February 2004

Synoptic maps of the thermal plasma density in the auroral zone and polar cap around 1 RE altitude are produced from measurements of Polar spacecraft floating potential during 1 year of perigee passes. The densities are accurate to around a factor of 2 in this region of space. These measurements, which provide the first comprehensive maps of the thermal plasma density over the polar cap and auroral oval, show clear variations in plasma density due to solar illumination of the ionosphere. Number density increases by a factor of 5 for illuminated verses nonilluminated ionospheric conditions over the entire auroral oval and polar cap. The maps delineate the global extent of the large‐scale auroral density cavity near 70° ± 5° invariant latitude. These maps further show that the depth of the density cavity is strongly influenced by solar illumination. The minimum average density (∼0.1 cm−3) occurs in darkness near 1900 magnetic local time.
A new particle population near the high‐latitude plasma sheet
Authors: Wilber M. , Li Q. , Winglee R. M. , Parks G. K. , McCarthy M. , Lin R. P.
Published: February 2004

We present observations of a new, intermediate‐energy particle population between the lobe and the plasma sheet. These were seen during two equatorial magnetosphere transits by the Wind spacecraft, when it was 15 RE distant from the Earth, south of the current sheet and near 0300 local time, and tended to overlap tail‐thinning episodes. The observed plasma bulk parameters exhibited a continuous transition from characteristics of the plasma sheet to those of the lobe, suggesting either a mixing region for particles from the two regions, or a transport layer where some processes condition the plasma as it moves from one region to the other. While temperature and density are typically shown to be anticorrelated throughout equatorial transits which span the plasma sheet and the magnetosheath, these two parameters were seen to be positively correlated during the intervals when the new population was observed. Particle distributions obtained by the 3‐D Plasma Experiment (3DP) electrostatic analyzers (0.08–27 keV ions; 0.02–28 keV electrons) during these periods often include a kidney‐shaped low‐energy ion component that appears like the conies commonly seen at low altitudes. These components indicate a likely transverse acceleration mechanism which can extend nearly out to the orbit of Wind.
Contribution of the Solar Constant (SOLCON) program to the long‐term total solar irradiance observations
Authors: Dewitte S. , Joukoff A. , Crommelynck D. , Lee R. B. , Helizon R. , Wilson R. S.
Published: February 2004

On long timescales the variation of the total solar irradiance (TSI) received by the Earth is believed to be one of the climate change drivers. Therefore accurate and time‐stable measurements of the total solar irradiance are necessary. The Solar Constant (SOLCON) instrument made TSI measurements in April 1992 and during the International Extreme Ultraviolet Hitchhiker (IEH) 3 STS 95 shuttle flight in autumn 1998. We assume that the SOLCON instrument remained stable within 0.01 % in between those measurements, and we verify this assumption as well as possible. From the SOLCON measurements we conclude the following: (1) The 1998 Space Absolute Radiometric Reference (SARR) adjustment coefficient applicable to the Active Cavity Radiometer Irradiance Monitor II (ACRIM II) during the IEH 3 period is 1.000438 with a one sigma uncertainty of 18 ppm, compared to the 1993 SARR adjustment coefficient of 1.000258. (2) The solar monitor on the Earth Radiation Budget Satellite (ERBS), with a 1993 SARR adjustment coefficient of 1.000453, has not aged within a one sigma uncertainty level of 130 ppm; and (3) the 1998 SARR adjustment coefficients for the Variability of Solar Irradiance and Gravity Oscillations (VIRGO) radiometers have been determined with a one sigma uncertainty of 10 ppm: They are 1.000025 for the Differential Absolute Radiometer left channel (DIARAD‐L) and 1.000279 for the version 1.2 data from the PMO6‐VA radiometer.
Electromagnetic ion cyclotron waves in the high‐altitude cusp: Polar observations
Authors: Le G. , Blanco‐Cano X. , Russell C. T. , Zhou X.‐W. , Mozer F. , Trattner K. J. , Fuselier S. A. , Anderson B. J.
Published: February 2004

High‐resolution magnetic field data from the Polar Magnetic Field Experiment (MFE) show that narrowband waves at frequencies ∼0.2–3 Hz are a permanent feature in the vicinity of the polar cusp. The waves have been found in the magnetosphere adjacent to the cusp (both poleward and equatorward of the cusp) and in the cusp itself. The occurrence of waves is coincident with depression of magnetic field strength associated with enhanced plasma density, indicating the entry of magnetosheath plasma into the cusp region. The wave frequencies are generally scaled by the local proton cyclotron frequency and vary between 0.2 and 1.7 times local proton cyclotron frequency. This suggests that the waves are generated in the cusp region by the precipitating magnetosheath plasma. The properties of the waves are highly variable. The waves exhibit both left‐handed and right‐handed polarization in the spacecraft frame. The propagation angles vary from nearly parallel to nearly perpendicular to the magnetic field. We find no correlation among wave frequency, propagation angle, and polarization. Combined magnetic field and electric field data for the waves indicate that the energy flux of the waves is guided by the background magnetic field and points downward toward the ionosphere.
First tomographic estimate of volume distribution of HF‐pump enhanced airglow emission
Authors: Gustavsson B. , Sergienko T. , Rietveld M. T. , Honary F. , Steen A. , Brändström B. U. E. , Leyser T. B. , Aruliah A. L. , Aso T. , Ejiri M. , Marple S.
Published: February 2004

This report presents the first estimates of the three‐dimensional volume emission rate of enhanced O(1D) 6300 Å airglow caused by HF radio wave pumping in the ionosphere. Images of the excitation show how the initially speckled spatial structure of excitation changes to a simpler shape with a smaller region that contains most of the excitation. A region of enhanced airglow was imaged by three stations in the Auroral Large Imaging System (ALIS) in northern Scandinavia. These images allowed for a tomography‐like inversion of the volume emission of the airglow. The altitude of maximum emission was found to be around 235 ± 5 km with typical horizontal and vertical scale sizes of 20 km. The shape of the O(1D) excitation rate varied from flatish to elongated along the magnetic field. The altitude of maximum emission is found to be approximately 10 km below the altitude of the enhanced ion line and 15 km above the altitude of maximum electron temperature. Comparisons of the measured altitude and temporal variations of the 6300 Å emission with modelled emission caused by O(1D) excitation from the high energy tail of a Maxwellian electron distribution show significant deviations. The 6300 Å emission from excitation of the high energy tail is about a factor of 4 too large compared with what is observed. This shows that the source of O(1D) excitation is electrons from a “sub‐thermal” distribution function, i.e. the electron distribution is Maxwellian at low energies and at energies above 1.96 eV there is a depletion.
Statistical study of auroral spirals
Authors: Partamies N. , Kauristie K. , Pulkkinen T. I. , Brittnacher M.
Published: February 2004

We present results of a statistical study of 189 auroral spiral forms. The data set was collected by a systematic examination of the images recorded at five all‐sky camera (ASC) stations in northern Fennoscandia and Svalbard during the winters 1996–1997 and 1997–1998. For every spiral we defined the time of occurrence, diameter, direction of motion, arid distance between the adjacent spirals (wavelength) in cases of spiral streets. The magnetic activity level was determined by using the magnetic north component (Bx) recorded at the ASC station and the local auroral electrojet index (IL index) computed by using the data from the IMAGE magnetometer network. The typical values of the diameter, wavelength, and ratio between diameter and wavelength were 25–75 km, 125‐175 km, and 2–6, respectively. According to our ASC data set, spirals are more common in the magnetic postmidnight sector than previous satellite observations suggest. Spirals drift predominantly in the same direction as the large‐scale ionospheric convection. Our observations of the average drift speed of spirals of 4 km/s are comparable to the corresponding values of other auroral small‐scale structures. Statistical analysis revealed that spirals occur most frequently under magnetically quiet (IL > −200 nT) conditions, and thus they cannot always be considered as precursors of substorm activity.
Ionization state and magnetic topology of coronal mass ejections
Authors: Henke T. , Woch J. , Schwenn R. , Mall U. , Gloeckler G. , von Steiger R. , Forsyth R. J. , Balogh A.
Published: February 2004

Charge state distributions of heavy solar wind ions measured in interplanetary space can be used to probe the physical conditions in the solar corona. This paper presents a study of the charge state distributions and the magnetic topology of 56 coronal mass ejections (CMEs) observed in interplanetary space by the Ulysses spacecraft. The analysis of the data from the Solar Wind Ion Composition Spectrometer (SWICS) instrument and the Vector Helium Magnetometer (VHM) experiment onboard Ulysses shows a clear correlation between the charge state distributions and the magnetic topology of CMEs. Almost all CMEs whose charge state distributions are shifted to higher charge states with respect to the ambient solar wind have the structure of magnetic clouds, whereas CMEs with the same charge state distributions as the surrounding solar wind do not show magnetic cloud structure. This correlation is found for CMEs observed at low, mid, and high solar latitudes. On the basis of the numerical solution of the ionization/recombination equations for oxygen and silicon, it is investigated which changes of the electron temperature, electron density, and the speed of the ions in the source region of the CMEs can reproduce the observations. It is shown that the main reason for the observed enhancement of higher charge states in the cloud CMEs is an increased electron temperature. However, the evolution of the density and velocity of the CMEs before the charge states freeze in cannot be neglected.
Evidence from GPS measurements for contemporary interplate coupling on the southern Hikurangi subduction thrust and for partitioning of strain in the upper plate
Authors: Darby Desmond , Beavan John
Published: February 2004

The spatial extent and strength of contemporary coupling between the Pacific and Australian plates on the Hikurangi subduction interface beneath the Wellington region of New Zealand are not well known. Consequently, the potential for the plate interface to produce great thrust earthquakes, or to load the major Wellington and Wairarapa faults in the overlying plate, is also unknown. To address these issues, a GPS network across the southern North Island has been partially or wholly surveyed six times between 1992 and 1999 and yields well‐determined relative velocities between 82 points. The relative velocity of the east coast with respect to the west coast across the 80‐km width of the southern end of the North Island is ∼14 mm yr−1 at an azimuth of 258°. This is about one third of, and closely parallel to, the NUVEL‐1A relative velocity of the Pacific with respect to the Australian Plate (39.4 mm yr−1 at 261°). Strain rate solutions indicate partitioning of contemporary strain, with a 35° change in principal axis orientations across the network. Using elastic dislocation theory, an inversion of the observed point velocities and their full variance‐covariance matrix determines the variation with depth of contemporary coupling at the subduction interface. If uniform coupling is assumed, then 100% coupling occurs from very shallow depths down to ∼29±1 km. However, if variable coupling is assumed, it reaches 100% at depths between 16 and 24 km. At greater depths, there is a transition zone at least 20 km wide, down to a depth of at least 40 km, over which contemporary coupling diminishes to zero. These depth ranges correlate moderately well with low‐angle thrust focal mechanisms recently obtained by other workers and indicate that the downdip width of the interface, over which a great thrust earthquake could occur, exceeds 100 km. Elastic dislocation models also explain the observed partitioning of strain, with Coulomb stress calculations indicating that the Wairarapa fault is being loaded at seismogenic depths.