bluuebunny/crossref_metadata_2025 huggingface dataset:
A three‐dimensional model of diurnal and semidiurnal tides and tidal mixing in the North Channel of the Irish Sea
Authors: Davies Alan M. , Hall Philip
Published: February 2004

The spatial variability of the M2, S2, N2, K1 and O1 tides are examined using a high‐resolution model of the North Channel of the Irish Sea. The sensitivity of computed currents and elevations to two formulations of the vertical eddy viscosity, one involving a turbulence energy submodel, and another in which it is computed from the flow field, are examined. Tidal elevation amplitudes and phases are compared at 20 gauges and shown to be in good agreement with observations. Root mean square (rms) errors of 7.85 cm, 32° (turbulence energy model) and 7.93 cm, 41° (flow dependent viscosity model) were found for the M2 tide. Comparable errors were found for the other semidiurnal tides. Differences in the amplitude errors between the two models are below the uncertainty associated with open boundary forcing and measurement errors and are therefore not statistically significant. Computed cotidal charts showed good agreement with published cotidal charts derived from observations. Comparison with observed M2 tidal current profiles suggests that at some locations a more accurate profile is determined with the turbulence model, while at others the flow‐dependent solution is more accurate. A comparison of M2 tidal current shear in the vertical showed that the turbulence energy model had a slight bias to underestimate this shear. This bias was not found in the flow‐dependent viscosity model. For the other tidal constituents the uncertainty in the accuracy of the currents prevented such a comparison. Also, it is not possible to decide with any confidence which closure method is more accurate, due to possible errors in the data and the small number of locations where significant shear is found in the vertical. The spatial variability of tidal elevations and currents shows an amphidromic point for the semidiurnal tides but none for the diurnal tides, in the northern part of the North Channel. Significant differences in tidal current are found between the North Channel and the Clyde Sea, where tidal currents are strong and weak, respectively. This difference is related to the tidal energy flux in the region which is confined to the North Channel. The spring‐neap temporal and spatial variability of tidal turbulence shows a close correlation with tidal current magnitude and water depth. This in part explains why the simple flow‐related viscosity model gives comparable results to those from the turbulence model.
Geocoronal Hα intensity measurements using the Wisconsin Hα Mapper Fabry‐Perot facility
Authors: Nossal S. , Roesler F. L. , Bishop J. , Reynolds R. J. , Haffner M. , Tufte S. , Percival J. , Mierkiewicz E. J.
Published: February 2004

The Wisconsin Hα Mapper (WHAM), a remotely operable, semi‐automated Fabry‐Perot located at Kitt Peak Observatory, has been making an all‐sky survey of interstellar hydrogen Balmer α(Hα) emissions since 1997. Using the annular summing spectroscopy technique, WHAM has acquired ∼37,000 spectra to date, spanning almost 100 nights of observations. Since all of the galactic emission spectral data contain the terrestrial Hα (6562.7 Å) emission line, these measurements constitute a rich source of geocoronal data for investigating natural variability in the upper atmosphere. The WHAM observations also serve as a benchmark for comparison with future data. Analysis of the first year of WHAM data shows only small day‐to‐day variations after shadow altitude variations are taken into account. For example, at shadow altitudes of 2000 and 3000 km, the RMS scatter is within approximately +/− 20%; this variability is expected to be reduced with accurate accounting of the smaller‐scale effects of observational slant path, zenith angle, and azimuth on the Hα intensity. This result is consistent with past midlatitude Wisconsin data sets but different from observations made by other observers and instruments at the low‐latitude Arecibo site. The multiple viewing geometries of the observations provide stringent modeling constraints, useful in testing current modeling capabilities. Modeling of the WHAM data with a global nonisothermal resonance radiation transport code (lyao_rt) indicates that the signal‐to‐noise of the data is sufficient to determine relative variations in upper atmospheric atomic hydrogen column densities to better than 5%. This paper describes the WHAM aeronomy program and its observational scheme, analysis procedures, and results from data taken in 1997. Case study comparisons are made with past data sets and with predictions from the lyao_rt resonant radiation transport modeling code of Bishop [1999].
Gyroresonant interactions of radiation belt particles with a monochromatic electromagnetic wave
Authors: Albert J. M.
Published: February 2004

The interaction of magnetized, relativistic test particles with a monochromatic electromagnetic wave is analyzed, taking into account the passage through cyclotron resonance in the spatially varying background magnetic field in slab geometry. A resonance‐averaged Hamiltonian is used to delineate two distinct regimes. In both cases, termed “adiabatic” and “nonadiabatic,” the first adiabatic invariant of the particle is broken during a resonant interaction, leading to change in energy and pitch angle. The adiabatic case is characterized by a limited range of resonant phase and a well‐defined value of the change of the invariant. In the nonadiabatic case the phase at resonance ranges over 0 to 2π, and only the magnitude of the change of the invariant is determined; a phase factor gives the invariant change an effectively random sign. The appropriate regime is determined by a ratio of timescales which, in turn, depends on the particle and wave properties: adiabaticity is favored by large wave amplitudes and small parallel gradients of the geomagnetic field Bo. The long‐term consequences of the two regimes are explored with a Monte Carlo simulation in the form of an iterated mapping. In a particular example, while some particles pitch angle scatter into the loss cone, energy is also removed from the distribution by particles in the adiabatic regime decaying in energy owing to repeated resonant interactions with the wave. Important potential magnetospheric applications include flux levels in the inner radiation belts, observations of “pancake” pitch angle distributions, and energization of storm time “killer” electrons.
Bursty bulk flows without a near‐Earth neutral line: Generation of fast intermittent flow in a highly curved magnetic field
Authors: Liu W. William
Published: February 2004

Despite the heightened interest in bursty bulk flows (BBFs) in the magnetotail, there have been relatively few studies on how these fast intermittent flows occur in a region where slow and steady convection is expected. Conventional wisdom attributes BBFs to the formation of a near‐Earth neutral line (NENL) on the appropriate side of the flow. We demonstrate that this view is not inevitable or necessary. We develop an alternate mechanism of BBFs generation based on the magnetohydrodynamics of a plasma situated in a Harris‐type current sheet. We find that without a NENL or other extraneous assumptions, a highly curved current sheet (characterized by a field ratio |Bz/Bx| « 1) can generate high‐speed cross‐field flows of > 100 km/s when perturbed by modest impulses of ∼ 10 km/s at its boundary. The flow has an Earthward sense in a thinning current sheet. The dependence on the field ratio of the maximal speed has the form c1 + c2|Bx/Bz|2, where c1 and c2 are constants. For reasonable parameters the BBF threshold of 400 km/s is reached when Bz/Bx is approximately 0.1. The investigation also reveals attendant effects and signatures of the BBF‐generating process, and these effects and signatures are listed as our theoretical predictions, against which better resolved satellite data from upcoming missions may be compared. We conclude with discussions on the potential role fast intermittent flows might play in the excitation of the substorm and other interesting magnetotail phenomena.
A role of the second‐order cyclotron resonance effect in a self‐consistent approach to triggered VLF emissions
Authors: Trakhtengerts V. Y. , Hobara Y. , Demekhov A. G. , Hayakawa M.
Published: February 2004

A role of the second‐order cyclotron resonance effect in the self‐consistent approach to the problem of triggered ELF/VLF emissions is estimated. The self‐consistency includes the calculation of energetic electron beams produced by a quasi‐monochromatic whistler wave packet and the analysis of secondary whistler wave generation by this beam with the magnetic field inhomogeneity taken into account. The theory allows one to estimate the maximum amplification of the secondary waves, as well as the dynamic frequency spectrum of a triggered emission. A simple computer program has been realized to find dynamic spectrograms of triggered VLF signals in particular cases with realistic dependences of electron beam velocity and geomagnetic field on the spatial coordinate and time. The main and general conclusions are summarized as follows: (1) The second‐order cyclotron resonance effects give an important contribution to the formation of the dynamic frequency spectrum of triggered ELF/VLF emissions. (2) The short pump whistler wave packets (with pulse durations 0.5–0.7 s in our case) generate fallers, while the long packets (pulse durations 0.9–1.3 s) generate predominantly rising tones. (3) There are critical maximal and minimal values for the pump wave pulse duration, when triggered emissions are not generated in the absence of an external electron beam.
Quantitative modeling of modulated ion injections observed by Polar‐Thermal Ion Dynamics Experiment in the cusp region
Authors: Delcourt D. C. , Moore T. E. , Giles B. L. , Fok M.‐C.
Published: February 2004

On May 13, 1996, as the Polar spacecraft was traveling at high invariant latitudes (∼78°–79°) in the prenoon sector (∼1050 magnetic local time), the Thermal Ion Dynamics Experiment on board recorded successive injections of protons with clear energy‐time dispersion. These dispersion structures spread over several minutes and extend from several hundreds of eV down to a few tens of eV. During this pass, simultaneous measurements from the Wind spacecraft revealed little variation of the solar wind dynamical pressure but a gradual turning of the interplanetary magnetic field (IMF) from an essentially dawn‐to‐dusk orientation (i.e., predominant positive BY component and slightly negative BZ) to a north‐to‐south one (predominant negative BZ). We show that the observed injections result from magnetosheath particle entry at higher and higher latitudes in the dawn sector. Using test particle calculations in a simple model of reconnected interplanetary and magnetospheric field, we show that the injection modulation likely follows from changes in the dynamical regime experienced by the ions upon traversal of the magnetopause current sheet. That is, as the IMF gradually rotates, the time‐varying BY and BZ lead to changes in the adiabaticity parameter κ in the region of entry and affect particle access to the Polar location. In the morning sector where magnetosheath plasma accelerates downtail, such an access to the inner magnetosphere requires magnetic moment damping and is thus favored during nonadiabatic episodes. The flux variations obtained numerically are in qualitative agreement with those observed, both in terms of characteristic energy and overall time evolution. This supports our interpretation of the modulated ion injections in terms of intermittent nonadiabatic entry from the magnetosheath followed by time of flight dispersion between the magnetopause and the spacecraft.
On the influence of the initial pitch angle distribution on relativistic electron beam dynamics
Authors: Khazanov G. V. , Liemohn M. W. , Krivorutsky E. N. , Kozyra J. U. , Albert J. M. , Gilchrist B. E.
Published: February 2004

Simulations of the evolution of a relativistic electron beam injected into the Earth's magnetosphere are examined. It is found that the lifetime of beam particles strongly depends on the initial pitch angle distribution and that lifetimes similar to those found for the radiation belts are obtained for nearly equatorially mirroring injections. It is concluded that the results of our previous study on beam propagation [Khazanov et al, 1999], which only considered upper ionospheric injections, are consistent with other relativistic electron studies and should be regarded as an examination of the loss cone edge population, whether naturally occurring or artificially injected into near‐Earth space.
Modeling the diurnal tide for the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) 1 time period
Authors: Oberheide Jens , Hagan Maura E. , Ward William E. , Riese Martin , Offermann Dirk
Published: February 2004

High‐resolution stratospheric and mesospheric temperature measurements from the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment taken during the space shuttle mission STS 66 in November 1994 show large tidal signatures in the day/night temperature differences. Previous comparisons with the predictions of the global‐scale wave model (GSWM‐95) for equinox conditions have shown these differences to be in very good qualitative agreement with GSWM results for the diurnal component of the migrating solar tide, although quantitative differences in the tidal amplitudes and phases did exist. In this paper the source of these differences is examined. Background fields corresponding to the flight conditions are derived from the CRISTA data and used as input to an updated version of the GSWM. The updated GSWM includes revised tidal heating and dissipation schemes. The background fields from CRISTA include temperature, ozone, pressure, mass density, and derived geostrophic wind (in geostrophic balance with the CRISTA temperatures). These model updates significantly improve the agreement with the CRISTA observations at all latitudes and altitudes. Modeled and observed phases match to within 1 hour at the equator. The amplitudes agree within 1 K for most altitudes. Above 75 km, however, the CRISTA measurements show a decrease in the tidal amplitude which is not present in the GSWM results, suggesting that the model underestimates the equatorial gravity wave dissipation in the form of eddy diffusivity and wave stress in the upper mesosphere. The mesospheric wind field update accounts for the most profound improvement in the phase distributions of the diurnal tide. The results show the strengths of the GSWM but also indicate the need for some model improvements. Our determination of the GSWM capability for reliable predictions of phases and amplitudes of the migrating diurnal tide when realistic atmospheric background conditions are used, provides some guidance in the way such models should be used in the future. In addition, the understanding of the diurnal cycle of chemical active species requires an accurate tidal prediction, due to the temperature dependencies of the reactions and the transport processes associated with the tides.
The thermosphere of Titan simulated by a global three‐dimensional time‐dependent model
Authors: Müller‐Wodarg I. C. F. , Yelle R. V. , Mendillo M. , Young L. A. , Aylward A. D.
Published: February 2004

We present three‐dimensional numerical simulations for dynamics and energetics of Titan's thermosphere. In so doing, we distinguish between the dynamics driven by solar insolation and those driven by vertical coupling to winds in Titan's middle atmosphere. Our calculations reveal that the solar‐driven thermospheric dynamics are characterized by the balance between pressure gradients and viscosity, while the super‐rotating zonal winds detected in Titan's stratosphere set up a balance between the pressure gradients, curvature and Coriolis forces. The day to night temperature gradients in the upper thermosphere (around 1300 km) typically lie around 20 (10) K for solar maximum (minimum), with peak solar‐driven winds of around 60 (30) m/s. This difference decreases with height and virtually disappears below 1000 km as a result of dayside adiabatic cooling and nightside adiabatic heating. The model highlights unique features about the thermosphere on Titan, such as the important nighttime heating from mid‐latitudes to high‐latitudes caused by the relatively small size of the planet's shadow, leading to features in the wind profiles which are not found on Earth. Although the lack of measurement constraints prevents us from making predictions of actual wind profiles on Titan, the model does illustrate the physical processes driving the dynamics and suggests that anticipated thermospheric measurements from the Cassini spacecraft may provide constraints also for the dynamics at lower altitudes.
Ion cyclotron waves, pickup ions, and Io's neutral exosphere
Authors: Crary F. J. , Bagenal F.
Published: February 2004

During Galileo's December 1995 encounter with Io, the magnetometer observed very strong electromagnetic waves near the SO2+ gyrofrequency. The amplitude of these waves previously has been used to determine the density of SO2+, as well as the production rate near Io. In this paper we examine the details of the interaction between pickup ions and ion cyclotron weaves and use these results to constrain the production rate of O+ and lo's exospheric loss rate. In a sub‐Alfvenic flow we show that ion cyclotron waves scatter pickup ions into a filled, asymmetric distribution, rather than a shell distribution. In addition, the observed waves are strong enough to cause non‐resonant scattering of S+ ions and may result in an asymmetric core population of S+. Because of the absence of waves near the O+ gyrofrequency and the criteria for marginal stability, we find that the O+ production rate was probably under a few times 1026 ions s−1. The amplitude of the observed waves is consistent with an exospheric escape rate of 1–3.5×1027 SO2+ ions s−1, depending on the asymmetry of the loss process.
Ionospheric duct parameters from a dual‐payload radio‐frequency sounder
Authors: James H. G.
Published: February 2004

Observed characteristics of extraordinary (X) mode reflection pulses in the Observations of Electric‐field Distributions in the Ionospheric Plasma‐A Unique Strategy C (OEDIPUS‐C) topside sounding ionograms have been analyzed to yield the parameters of depletion irregularities that trap and guide (duct) high‐frequency waves. The amplitude of successive ionospheric reflection pulses fluctuates by 10 dB or more during frequency sweeps with 50 kHz steps. The pulse period is 3 ms. At the fixed frequency of 4.5 MHz the reflected amplitudes fluctuate much more slowly. The direct pulse from the OEDIPUS‐C transmitter to the receiver, consisting of an interference signal between the ordinary and extraordinary modes, is very sensitive to local density changes but shows fluctuations of 10 dB over even longer timescales of ∼1 s. An analysis based on the radiation properties of the transmitter dipoles and on ray optics in cylindrical field‐aligned density depletion ducts shows that very shallow depletions of <1% of the ambient density can explain the magnitudes and fluctuations observed in the reflected pulses. Duct density models with steep isodensity contours produce more realistic predictions of signal levels and their fluctuations with frequency than models with gently sloping contours.
Two‐satellite observations of substorm injections at geosynchronous orbit
Authors: Thomsen M. F. , Birn J. , Borovsky J. E. , Morzinski K. , McComas D. J. , Reeves G. D.
Published: February 2004

Energetic particle and plasma measurements from three fairly closely spaced geosynchronous satellites are used to examine the simultaneous behavior of substorm particle injections at spatially separated locations. A total of 43 injection events are identified as dispersionless at two or more satellites. The observations confirm that the ion and electron injection regions are spatially offset in the azimuthal direction, as previously deduced statistically from single‐satellite observations and revealed by test‐particle trajectories in MHD simulations of geomagnetic tail reconnection. The two‐spacecraft observations show further that the injection regions for both ions and electrons expand azimuthally in both the eastward and westward directions over a timescale of several minutes. The observed expansion rates are comparable to expansion rates derived from the test particle/MHD simulations, lending further support to the identification of such reconnection as the ultimate cause of substorm particle injections.
The kinetic shell model of coronal heating and acceleration by ion cyclotron waves: 1. Outward propagating waves
Authors: Isenberg Philip A. , Lee Martin A. , Hollweg Joseph V.
Published: February 2004

We introduce a new kinetic treatment of the heating of the magnetically open solar corona and the acceleration of the fast solar wind by the cyclotron resonant interaction of coronal protons with ion cyclotron waves. In this “kinetic shell” formalism we approximate the evolution of the collisionless coronal proton distribution by the assumption that the pitch angle diffusion due to the resonant ion cyclotron waves is much faster than the other processes taking place. Under this assumption the resonant protons uniformly populate velocity space surfaces, or shells, of constant energy in the frame moving with the wave phase speed. These resonant shells then evolve slowly in response to the nonresonant large‐scale forces in the system. For this initial demonstration of the kinetic shell concept, we additionally take the resonant waves to be solely outward propagating and dispersionless. In this case the resonant shells are spherical sections in velocity space which are confined to the sunward half of the proton distribution. We then calculate the radial evolution of collisionless protons in a coronal hole using this simplified system, which also includes the effects of gravity, the charge‐separation electric field, and the mirror force. We find that a fast solar wind can be generated by this process using reasonable values of the physical parameters. However, we also prove that the proton distribution generated by the interaction with only outward propagating waves will necessarily be unstable to the generation of inward propagating waves. Thus this illustrative calculation is incomplete and will have to be extended to include waves in both propagation directions.
Reconnection remnants in the magnetic cloud of October 18–19, 1995: A shock, monochromatic wave, heat flux dropout, and energetic ion beam
Authors: Collier Michael R. , Szabo A. , Farrell W. M. , Slavin J. A. , Lepping R. P. , Fitzenreiter R. , Thompson B. , Hamilton D. C. , Gloeckler G. , Ho G. C. , Bochsler P. , Larson D. , Ofman L.
Published: February 2004

Evidence is presented that the Wind spacecraft observed particle and field signatures on October 18–19, 1995, due to reconnection near the foot points of a magnetic cloud (i.e., between 1 and 5 solar radii). These signatures include (1) an internal shock traveling approximately along the axis of the magnetic cloud, (2) a simple compression of the magnetic field consistent with the foot point magnetic fields being thrust outward at speeds much greater than the solar wind speed, (3) an electron heat flux dropout occurring within minutes of the shock, indicating a topological change resulting from disconnection from the solar surface, (4) a very cold 5 keV proton beam, and (5) an associated monochromatic wave. We expect that given observations of enough magnetic clouds, Wind and other spacecraft will see signatures similar to the ones reported here indicating reconnection. However, these observations require the spacecraft to be fortuitously positioned to observe the passing shock and other signatures and will therefore be associated with only a small fraction of magnetic clouds. Consistent with this, a few magnetic clouds observed by Wind have been found to possess internal shock waves.
Coronal and interplanetary magnetic field geometry: Streak lines
Authors: Schatten Kenneth H.
Published: February 2004

This paper discusses coronal and interplanetary magnetic field models, including static and dynamical aspects. The seminal work of Parker [1958] led to an Archimedian spiral field geometry in the interplanetary medium. The early coronal and interplanetary magnetic field models were static models developed to deal with the lack of magnetic field observations throughout most of the heliosphere. Since we are still in this deplorable state, with a particular lack of observations in the inner corona, these static models are still being run since they enable the field throughout the heliosphere to be estimated with only photospheric measurements. By now extending the static models into the temporal domain, a better understanding of the field line dynamics is just beginning to emerge. This understanding is needed to calculate cosmic ray properties, as well as other dynamic events, such as coronal mass ejections. Presently, a number of poorly understood aspects of the coronal field exist: the overall large‐scale field structure, the magnetic field winding angle, and rotational properties of the corona and photosphere. This last topic relates to changing magnetic field patterns. We provide a new paradigm involving “streak lines” for a better understanding of interplanetary field line behavior. The concept of streak lines was developed for understanding fluid dynamics. Use of this concept may lead to future developments and possible prediction of coronal mass ejections, helping us understand some of the causes of “space weather.”
Long‐term variations in the magnetic fields of the Sun and the heliosphere: Their origin, effects, and implications
Authors: Lockwood M.
Published: February 2004

Recent studies of the variation of geomagnetic activity over the past 140 years have quantified the “coronal source” magnetic flux Fs that leaves the solar atmosphere and enters the heliosphere and have shown that it has risen, on average, by an estimated 34% since 1963 and by 140% since 1900. This variation of open solar flux has been reproduced by Solanki et al. [2000] using a model which demonstrates how the open flux accumulates and decays, depending on the rate of flux emergence in active regions and on the length of the solar cycle. We here use a new technique to evaluate solar cycle length and find that it does vary in association with the rate of change of Fs in the way predicted. The long‐term variation of the rate of flux emergence is found to be very similar in form to that in Fs, which may offer a potential explanation of why Fs appears to be a useful proxy for extrapolating solar total irradiance back in time. We also find that most of the variation of cosmic ray fluxes incident on Earth is explained by the strength of the heliospheric field (quantified by Fs) and use observations of the abundance of the isotope 10Be (produced by cosmic rays and deposited in ice sheets) to study the decrease in Fs during the Maunder minimum. The interior motions at the base of the convection zone, where the solar dynamo is probably located, have recently been revealed using the helioseismology technique and found to exhibit a 1.3‐year oscillation. This periodicity is here reported in observations of the interplanetary magnetic field and geomagnetic activity but is only present after 1940. When present, it shows a strong 22‐year variation, peaking near the maximum of even‐numbered sunspot cycles and showing minima at the peaks of odd‐numbered cycles. We discuss the implications of these long‐term solar and heliospheric variations for Earth's environment.
On the global structure of the heliospheric magnetic field
Authors: Fisk L. A.
Published: February 2004

The structure of the heliospheric magnetic field, an extension of the solar magnetic field, is influenced by dynamic processes that occur on the solar surface and in the solar corona. Recent work has demonstrated that the interplay between the differential rotation of the photosphere and the nonradial expansion of the solar wind in the corona can lead to large‐scale, systematic motions of the coronal magnetic field. These motions, in turn, alter the large‐scale configuration of the heliospheric field, from that of the standard spiral structure. A critical analysis is presented of the processes that drive these coronal motions and of possible effects that could limit them. The model for the heliospheric magnetic field driven by coronal motions is also contrasted with other approaches and descriptions of the coronal and heliospheric magnetic field, e.g., potential field models for the coronal magnetic field. Finally, the challenges and opportunities for observing the differences between the heliospheric magnetic field driven by coronal motions and the standard spiral field are discussed.
MHD simulation of magnetospheric waveguide modes
Authors: Keller Kristi A. , Lysak Robert L.
Published: February 2004

Ultralow frequency waves with frequencies of 1.3, 1.9, 2.6, and 3.4 mHz are often observed and associated with field line resonances (FLRs) in the inner magnetosphere of the Earth. Waveguide modes have been proposed as a mechanism for generating FLRs. Using a compressible two‐dimensional MHD simulation, we will explore the nonlinear effects on the, excitation of waveguide modes and the frequencies of the modes. We will discuss how changes in key parameters affect waveguide modes.
Two‐dimensional hybrid simulation of the dayside reconnection layer and associated ion transport
Authors: Xie H. , Lin Y.
Published: February 2004

The structure of the reconnection layer at the dayside magnetopause is studied by using a two‐dimensional (2‐D) hybrid code. The simulation domain is a rectangle in the xz plane around an X line at the magnetopause. In our previous study the guide magnetic field By was assumed to be zero. In the present simulation the effects of a finite By on the reconnection layer are studied. In addition, the influence of shear flows on the magnetic reconnection is also investigated. In the cases with a shear flow speed ΔV = 0, as near the subsolar region, a large‐amplitude rotational discontinuity is present on the magnetosheath side of the reconnection layer, across which the magnetic field changes direction from the magnetosheath to the magnetosphere. A high‐speed accelerated flow is present on the magnetospheric side of the rotational discontinuity. For a higher‐latitude reconnection in the Northern Hemisphere, where a shear flow is present across the magnetopause, the structure of the reconnection layer northward of the X line is very different from that southward. Northward of the X line, the rotational discontinuity with a larger field rotational angle exists on the magnetospheric side if the shear flow speed ΔV > 0.33(VAm − VAs), where VAm and VAs are the Alfvén speeds in the magnetosphere and the magnetosheath, respectively. Below the X line, a thin, strong rotational discontinuity is always present on the magnetosheath side. By tracing the orbits of individual ion particles, we have performed a detailed analysis of ion transmission and reflection at the magnetopause. The average transmission (reflection) rate of the magnetosheath ions is found to be ∼ 85% (15%). The reflection of the magnetosheath ions occurs mainly in the inner boundary layer.
Multispacecraft and ground observations of magnetospheric and ionospheric responses to continuous energy input from the solar wind
Authors: Lee D.‐Y. , Min K. W.
Published: February 2004

On June 28, 1997, Wind spacecraft measurement indicated a long period of mostly southward interplanetary magnetic field (IMF) Bz while other parameters remained overall quasi‐steady. This continuous energy input to the magnetosphere over nearly 6 hours has led to large auroral activities in the ionosphere as well as the magnetospheric disturbances. Polar UVI measurement showed the active auroral substorm brightening over wide area covering a deep‐duskside to local midnight, but the auroral regions with peak intensity were clearly separated into three events in longitude. They appear to have initiated separately and have developed rather independently at the early stage from the onset. These independent occurrences on three events are further evidenced by interpreting the corresponding (three) westward electrojets in the ionosphere along with the Geotail spacecraft observations at near‐tail which show the magnetic field dipolarization, its corresponding current reduction in the central current sheet, and the signature of the field‐aligned current. At later stages of time evolution, on the other hand, auroral brightenings at two events tend to merge and more interestingly spread eastward. This eastward spreading is likely due to eastward drift of precipitating auroral electrons under the enhanced convection. Also, the geosynchronous energetic particle injections though with weak magnitude reveal an opposite pattern to normal energy‐dispersion of the typical substorm, another signature of enhanced convection. In conclusion, (1) what was observed here is a spatially multiple occurrence of not fully expanded substorms under the continuous forcing by the prolonged southward IMF, and (2) this is in strong superposition with the enhanced convection under the same IMF condition.
The dependence of plasma and magnetic field correlations in the solar wind on geomagnetic activity
Authors: Jurac S. , Richardson J. D.
Published: February 2004

The solar wind structures measured by four spacecraft (ISEE 1, ISEE 3, IMP 8, and Wind) are correlated and examined in the light of their geoeffectiveness. The average plasma and magnetic field correlations range from about 0.6 to 0.7 during quiet periods and increase substantially during the geomagnetic storms. Using the Ap and Dst indices as a measure for the level of geomagnetic disturbance, we find that correlations of plasma density, |B|, Bz and By are about 0.85 for the most geoeffective events. Earth's foreshock significantly degrades the correlation when one or both spacecraft are within ∼30 RE of Earth during quiet periods, and this degradation extends beyond 50 RE during geoeffective periods. Excluding the foreshock effect, the correlations show little dependence on the spacecraft separation along the Sun‐Earth line, but depend strongly on the spacecraft separation in the direction perpendicular to the Earth‐Sun line, consistent with previous findings. The decrease in correlation in the perpendicular direction is less severe during geoeffective periods, suggesting that geoeffective features have larger scale sizes. The front normals of geoeffective features are predominantly in the propagation direction, consistent with radially outward propagating ejecta.
Generation of the narrowband electrostatic noise in the geomagnetic tail: Geotail observations
Authors: Kojima H. , Ashour‐Abdalla M. , Paterson W. R. , Matsumoto H. , Frank L. A. , Anderson R. R. , Richard R. L. , Kokubun S. , Yamamoto T.
Published: February 2004

The wave‐particle interactions involved in the generation of narrowband electrostatic noise (NEN) are discussed in the context of plasma waves and plasma measurements by Geotail spacecraft. NEN is characterized by its frequency spectrum which ranges from ƒpi to ƒpe, where ƒpi and ƒpe represent the ion and electron plasma frequencies, respectively. For this frequency range, no standard electrostatic plasma wave mode exists that propagates in the direction parallel to the ambient magnetic field. The present paper shows the early results from our examination of the correlation between the simultaneously observed NEN and plasmas. We demonstrate the strong correlation between NEN emissions and the occurrence of “structures” in the electron pitch angle distributions. Our observation results indicate that NEN is excited by the electron velocity space hole mode and support the results based on the collaboration between electron measurements and computer simulations which were presented by Yin et al. [1999].
Measurements of the solar soft X‐ray irradiance by the Student Nitric Oxide Explorer: First analysis and underflight calibrations
Authors: Bailey S. M. , Woods T. N. , Barth C. A. , Solomon S. C. , Canfield L. R. , Korde R.
Published: February 2004

Beginning on March 11, 1998, the Student Nitric Oxide Explorer (SNOE) satellite has made daily observations of the solar soft X‐ray irradiance. These measurements are carried out by a multichannel photometer system. The spectral range between 2 and 20 nm is covered by three channels with bandpasses of 2 – 7 nm, 6–19 nm, and 17 – 20 nm respectively. Absolute sensitivities were measured preflight using the Synchrotron Ultraviolet Radiation Facility of the National Institute of Standards and Technology. The results of the first 1.5 years of SNOE solar measurements are presented. During this time period the F10.7 solar index varied between 80 and 250 × 10−22W m−2 Hz−1 and the 81‐day average of the F10.7 solar index varied between 100 and 175 × 10−22W m−2 Hz−1. The solar irradiances in the 2 – 7 nm interval varied between 0.3 and 2.5 mW m−2, while the irradiances in the 6–19 and 17 – 20 nm intervals varied between 0.5 and 3.5 and 1.0 and 3.5 mW m−2, respectively. The measured irradiances are correlated with the F10.7 solar index with a correlation coefficient of ∼0.9 in all three bandpasses. For the levels of activity observed so far the SNOE measurements are typically a factor of 4.0 larger than the irradiances predicted by the Hinteregger et al. [1981] empirical model (hereafter the Hinteregger model). This fact and a long‐term trend in the ratio of SNOE measurements to Hinteregger model predictions show that the Hinteregger model underpredicts the long‐term variability in the solar soft X‐ray irradiance. It is shown that other empirical models provide a reasonable representation of the 27‐day variability but also underpredict the magnitude and long term variability. A sounding rocket measurement made on November 2, 1998, by the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Solar EUV Experiment prototype instrument using the same technique measured the solar irradiance in similar wavelength bands and produced results that are in good agreement with the SNOE measurements.
Growth and decay of relativistic electrons during a magnetic storm as seen in low‐Earth orbit
Authors: Pesnell W. Dean , Goldberg Richard A. , Chenette D. L. , Gaines E. E. , Schulz M.
Published: February 2004

Highly relativistic electron events (HREs) are periods of intense, long‐lived, energetic electron fluxes in the outer radiation zone. We are using measurements from the High Energy Particle Spectrometer (HEPS) on the Upper Atmosphere Research Satellite (UARS) to develop a database of the pitch‐angle‐resolved and energy‐resolved electron fluxes with energies between 30 keV and 5 MeV. The data acquired by HEPS have overlapped with the declining phase of solar cycle 22, making these data very important, since HREs are thought to peak in frequency and intensity during this phase of the solar cycle. We find a consistent scenario of electrons being injected into the radiation belts by a magnetic storm (deduced from Dst) and being slowly accelerated to ever higher energies over days to weeks. The energy dependence of the flux is an essential part of the analysis. Above 700 keV the most energetic electrons are the last to appear and the slowest to fade following an injection event.
Flaring time interval distribution and spatial correlation of major X‐ray solar flares
Authors: Moon Y.‐J. , Choe G. S. , Yun H. S. , Park Y. D.
Published: February 2004

A statistical study is performed on X‐ray flares stronger than C1 class that erupted during the solar maximum between 1989 and 1991. We have investigated the flaring time interval distribution (waiting‐time distribution) and the spatial correlation of successive flare pairs. The observed waiting‐time distribution for the whole data is found to be well represented by a nonstationary Poisson probability function with time‐varying mean flaring rates. The period most suitable for a constant mean flaring rate is determined to be 2–3 days by a Kolmogorov‐Smirnov test. We have also found that the waiting‐time distribution for flares in individual active regions follows a stationary Poisson probability function m exp(−mt) with a corresponding mean flaring rate. Therefore the flaring probability within a given time is given by 1 ‐ exp(‐mt), when the mean flaring rate m is properly estimated. It is also found that there are no systematic relationships between peak fluxes of flares and their waiting‐time distributions. The above findings support the idea that the solar corona is in a self‐organized critical state. A comparison of the angular distances of successively observed flare pairs with those of hypothetical flare pairs generated by random distribution shows a positive angular correlation within ∼ 10° (∼ 180 arc sec in the observing field) of angular separation, which suggests that homologous flares occurring in the same active region should outnumber sympathetic flares.
Two‐dimensional global hybrid simulation of pressure evolution and waves in the magnetosheath
Authors: Lin Y. , Denton R. E. , Lee L. C. , Chao J. K.
Published: February 2004

A two‐dimensional hybrid simulation is carried out for the global structure of the magnetosheath. Quasi‐perpendicular magnetosonic/fast mode waves with large‐amplitude in‐phase oscillations of the magnetic field and the ion density are seen near the bow shock transition. Alfvén/ion‐cyclotron waves are observed along the streamlines in the magnetosheath, and the wave power peaks in the middle magnetosheath. Antiphase oscillations in the magnetic field and density are present away from the shock transition. Transport ratio analysis suggests that these oscillations result from mirror mode waves. Since fluid simulations are currently best able to model the global magnetosphere and the pressure in the magnetosphere is inherently anisotropic (parallel pressure p‖ ≠ perpendicular pressure p⊥), it is of some interest to see if a fluid model can be used to predict the anisotropic pressure evolution of a plasma. Here the predictions of double adiabatic theory, the bounded anisotropy model, and the double polytropic model are tested using the two‐dimensional hybrid simulation of the magnetosheath. Inputs to the models from the hybrid simulation are the initial post bow shock pressures and the time‐dependent density and magnetic field strength along streamlines of the plasma. The success of the models is evaluated on the basis of how well they predict the subsequent evolution of p‖ and p⊥. The bounded anisotropy model, which encorporates a bound on p⊥/p‖ due to the effect of ion cyclotron pitch angle scattering, does a very good job of predicting the evolution of p⊥; this is evidence that local transfer of energy due to waves is occurring. Further evidence is the positive identification of ion‐cyclotron waves in the simulation. The lack of such a good prediction for the evolution of p‖ appears to be due to the model's lack of time dependence for the wave‐particle interaction and its neglect of the parallel heat flux. Estimates indicate that these effects will be less significant in the real magnetosheath, though perhaps not negligible.
Incoherent scatter radar identification of the dayside magnetic separatrix and measurement of magnetic reconnection
Authors: Blanchard G. T. , Ellington C. L. , Lyons L. R. , Rich F. J.
Published: February 2004

Incoherent scatter (IS) radar measurements of the ionospheric electron density are analyzed for the signature of the magnetic separatrix in the dayside ionosphere by comparison with separatrix locations determined using precipitating electron data from DMSP spacecraft within 1.5 hours in magnetic local time (MLT) of the Sondrestrom radar. A model of photoionization is used to remove its effects from the measured electron density to study the ionization by precipitating particles. The altitude of peak ionization and the peak ionization rate are used to determine whether a magnetic field line is open or closed. We observe that on closed field lines the peak ionization rate is high (> 2200 cm−3 s−1) or the peak is lower than 140 km in altitude, and on open field lines the peak ionization is low (< 900 cm−3 s−1) or the peak is higher than 140 km. These rules are used to identify the separatrix with an accuracy of 0.36° and a precision of ±0.39°. This signature of the separatrix is only apparent in the prenoon and noon sectors (0600 to 1300 MLT). In the postnoon sector (1630–1800 MLT) the altitude of peak ionization and the peak ionization rate do not show any systematic difference between the open and closed field line regions. Finally, an example is presented in which the data on the location and motion of the separatrix and IS radar measurements of F region plasma velocity are used to measure the magnetic reconnection rate as a function of the interplanetary magnetic field clock angle.
Evolution and dissipation of imbedded rotational discontinuities and Alfvén waves in nonuniform plasma and the resultant proton heating
Authors: Vasquez Bernard J. , Hollweg Joseph V.
Published: February 2004

We show that nonuniform Alfvén speed gradients across field lines generally arise from the evolution of Alfvén waves. The evolution of a group of nonlinear Alfvén waves with the same sign of parallel wavenumber generate small‐amplitude pressure‐balanced structures (PBSs) which cause the speed variations. This always causes refraction. In most cases, the Alfvén waves also couple to magnetosonic waves and acquire a weak compressional component and can undergo resonant absorption or transfer, wherein wave energy can propagate across field lines. At large amplitudes the waves also generate imbedded rotational discontinuities (RDs). Some of these RDs can be dissipated owing to resonant transfer. This process could partly contribute to the observed decrease of solar wind RDs with increasing distance from the Sun. Resonant transfer also triggers a cascade due to steepening, which leads to sustained proton heating. The cascade produces oblique and large wavenumber waves which travel in different directions and have associated compressions. Protons interact with these by pitch angle scattering. They gain energy from second‐order Fermi acceleration and from Landau and transit time damping. Oblique waves are inferred to be present in the dissipation range of Alfvénic fluctuations at 1 AU. We argue that the process of proton heating should proceed similarly to simulation results. We also propose a role for the wave imbedded RD in coronal heating through its formation in the chromosphere and its likely dissolution in the corona where wave amplitudes are very small.
Quasi‐linear Zakharov simulations of Langmuir turbulence at rocket altitudes in the auroral ionosphere
Authors: Sanbonmatsu K. Y. , Newman D. L. , Goldman M. V.
Published: February 2004

A quasi‐linear Zakharov simulation model has been constructed to study intense Langmuir waves and precipitating field‐aligned electrons observed in the auroral ionosphere at altitudes below 1000 km. This self‐consistent model couples the magnetized Zakharov equations to a modified quasi‐linear diffusion equation, which includes an advective term describing electrons streaming into the simulation region. Two‐dimensional beam‐driven simulations demonstrate that for the parameters of the auroral ionosphere below altitudes of 1000 km, Langmuir wave‐wave and wave‐particle interactions occur on similar timescales. The resulting reduced electron distribution has multiple shoulders and is a result of a balance between quasi‐linear beam‐flattening, wave‐wave instabilities and beam replenishment due to streaming electrons.