bluuebunny/crossref_metadata_2025 huggingface dataset:
Interpretation of pulse‐like electric field distortions observed by Exos D satellite
Authors: Amano Kyoko , Tsuruda Koichiro , Hayakawa Hajime , Matsuoka Ayako , Nakamura Tadas K.
Published: February 2004

The 32‐Hz electric field data measured by the electric field detector EFD‐P on board the EXOS D satellite show pulse‐like distortions (PLDs) in nearly 10% of 1000 paths selected at random from April 1989 to June 1990. The purpose of the present paper is to analyze these distortions as a basis for future measurements of the parallel electric field in space. The PLDs are observed at altitudes higher than about 3000 km, with a typical duration of a few tens of minutes. Comparing with data obtained by other detectors on board EXOS D, it is suggested that the necessary conditions for the appearance of the distortions are (1) low plasma density, which implies a large Debye sphere around the spacecraft, (2) low ambient electron flux, which is necessary to keep the spacecraft potential high enough to attract photoelectrons from the probes, (3) small angle between the magnetic field and the wire probe, which makes the photoelectrons emitted from the probe accessible to the satellite, and (4) asymmetry in the photoelectron current from a pair of probes. When all four of these conditions are satisfied simultaneously, then the photoelectrons emitted from the probes reach the spacecraft body and produce spurious current flows in the measurement circuit to generate PLDs. A generation mechanism proposed here can explain well all the observed properties of PLDs.
Magnetic reconnection: Observations on October 29, 1979, and model results
Authors: Biernat H. K. , Semenov V. S. , Drobysh O. A. , Heyn M. F.
Published: February 2004

We discuss the structure of the reconnection layer as recorded by ISEE 2 during an outbound crossing of the dayside, northern magnetopause on October 29, 1979. Besides a rotational discontinuity, this crossing shows slow mode shock structure. We interpret this event theoretically in terms of magnetic field line reconnection based on a pulse‐like Petschek‐type model, rather than in terms of steady state reconnection on a moving magnetopause. Our model is generalized through the introduction of a space‐and time‐varying reconnection rate. Furthermore, the magnetic fields and velocities on either side of the magnetopause current sheet may have arbitrary strength and orientation with respect to each other; additionally, the densities on either side of the current sheet may be different in general. Using boundary/initial conditions from the spacecraft data, we calculate the temporal/spatial behavior of all reconnect ion‐associated structures. In particular, having chosen a suitable trajectory through the magnetopause, we consider the temporal behavior of the magnetic field and plasma parameters as these reconnection‐associated events pass by. The results reproduce the behavior of the data reasonably well.
Evidence for resonant mode coupling in Saturn's magnetosphere
Authors: Cramm R. , Glassmeier K.‐H. , Stellmacher M. , Othmer C.
Published: February 2004

Ultralow‐frequency (ULF) pulsations are an important class of phenomena characterizing the dynamics of the terrestrial magnetosphere and are usually interpreted as standing magnetohydrodynamic waves in the magnetospheric plasma of planet Earth. Spacecraft observations indicate that the Hermean and Jovian magnetospheres also support such eigenoscillations. Whether ULF waves exist in the magnetosphere of the planet Saturn is the topic of this work. On the basis of a simple plasma mass distribution model, periods of Kronian eigenoscillations are estimated. Values comparable to the planetary rotation period are found, which makes the existence of standing waves improbable. Despite this significant difference in Earth's magnetosphere it is shown that this does not inhibit the resonant coupling between fast mode type magnetopause surface waves and local propagating Alfvé modes. Against this background we examine a pattern in the magnetic field data recorded by Voyager 1 during its encounter with Saturn. The analysis of the pattern and a modeled satellite crossing give indications that resonant mode coupling was detected.
Short‐term delays (hours) of ionospheric spread F occurrence at a range of latitudes, following geomagnetic activity
Authors: Bowman G. G.
Published: February 2004

The analyses have investigated the short‐term responses of spread F occurrence (as identified on ionograms) at low‐and middle‐latitude stations to geomagnetic activity changes so that comparisons can be made with results from equatorial stations reported earlier [Bowman, 1995]. Using superposed‐epoch methods, it is found that the AE index is either enhanced or depressed a few hours prior to spread F occurrence depending on whether or not low or high (respectively) spread F controls are used. The geomagnetic activity related to this inverse relationship is centered mainly around the local times of 1200 and 1800. A direct relationship is also found for geomagnetic activity which occurs in the night hours. The results for these other latitude stations are the same as those reported in the earlier paper for equatorial stations. This paper also considers for the equatorial station Huancayo extreme responses of spread F occurrence to geomagnetic activity, involving occasions of high spread F occurrence when there is a sudden drop in the level to zero for isolated days when a few hours earlier at the favored times mentioned above enhanced geomagnetic activity occurs. It is suggested that the inverse relationship involves the control of medium‐scale traveling ionospheric disturbance (MS‐TID) wave amplitudes by neutral‐density changes, produced by large‐scale traveling ionospheric disturbances (LS‐TIDs) generated by geomagnetic activity concentrated mainly around the local times of 1200 and 1800. The LS‐TIDs generated by geomagnetic activity at night behave differently. Height rises are produced, and as a result of the lower neutral‐density levels, spread F is recorded particularly in the presunrise period. Extending the comparison of spread F characteristics at different latitudes, some comments are made on recent observations using data from satellite recordings, concerning the similarities between electron‐density depletions detected for equatorial regions and those for other regions (e.g., midlatitude regions.)
Velocity‐shear‐driven ion‐cyclotron waves and associated transverse ion heating
Authors: Amatucci W. E. , Walker D. N. , Ganguli G. , Duncan D. , Antoniades J. A. , Bowles J. H. , Gavrishchaka V. , Koepke M. E.
Published: February 2004

Recent sounding rocket experiments, such as SCIFER, AMICIST, and ARCS‐4, and satellite data from FAST, Freja, DE‐2, and HILAT, provide compelling evidence of a correlation between small‐scale spatial plasma inhomogeneities, broadband low‐frequency waves, and transversely heated ions. These naturally arising, localized inhomogeneities can lead to sheared cross‐magnetic‐field plasma flows, a situation that has been shown to have potential for instability growth. Experiments performed in the Naval Research Laboratory's Space Physics Simulation Chamber demonstrate that broadband waves in the ion‐cyclotron frequency range can be driven solely by a transverse, localized electric field, without the dissipation of a field‐aligned current. Significant perpendicular ion energization resulting from these waves has been measured. Detailed comparisons with both theoretical predictions and space observations of electrostatic waves found in the presence of sheared cross‐magnetic‐field plasma flow are made.
Electromagnetic instabilities in a gyrotropic and nongyrotropic hybrid solar‐wind‐like plasma
Authors: Astudillo H. F. , Gomberoff L. , Hernández R. M.
Published: February 2004

We study the stability of electromagnetic waves for nongyrotropic velocity distribution functions in a solar‐wind‐like plasma. The dispersion relation is solved numerically in closed form. On the basis of recently reported observations of such plasma distribution functions in the solar wind at 1.3 AU, we study the instabilities of transverse electromagnetic waves propagating in the direction of an external magnetic field driven by ion beams and thermal anisotropies. We show that electromagnetic waves have three instability bands in the frequency range, 0 < ω < Ωp, Ωp < ω < 2Ωp, and 2Ωp < ω < 3Ωp. Our results are illustrated by using parameters consistent with observed nongyrotropic plasma distribution function in the solar wind.
Analysis of time‐dependent reconnection in compressible plasmas
Authors: Semenov Vladimir S. , Drobysh Oxana A. , Heyn Martin F.
Published: February 2004

Time‐dependent Petschek‐type reconnection including moving shock waves and discontinuities in a compressible plasma is studied. Reconnection is initiated by a dissipative electric field pulse along a reconnection line and generates nonlinear perturbations propagating along an initial current sheet. The induced linear wave perturbations in the adjacent half‐spaces are obtained analytically in the form of a convolution integral. The results are evaluated for typical dayside magnetopause parameters and presented for comparison with observations of dayside reconnection events.
Tomographic inversion for ALIS noise and resolution
Authors: Gustavsson Björn
Published: February 2004

In this report the problems of resolution and noise sensitivity of tomographic reconstructions from ground‐based multistation imaging of aurora with the auroral large imaging system (ALIS) are considered. ALIS is a ground‐based grid of high‐performance CCD‐imaging stations at high latitudes. For evaluation of the resolution and noise sensitivity of current tomographic reconstruction procedures a full model simulation of the ALIS system is presented. The results show that relative errors are typically in the range 0.05 – 0.1 for typical noise levels in measurements of aurora. A general method to estimate resolution in a tomographic imaging system is developed and used to give estimates of the horizontal and vertical resolution. Its current limitations and future perspective are briefly discussed. A method to retrieve feasible tomographic reconstructions from a few image projections with variable noise level are outlined.
The solar energetic particle event of April 14, 1994, as a probe of shock formation and particle acceleration
Authors: Kahler S. W. , Cane H. V. , Hudson H. S. , Kurt V. G. , Gotselyuk Y. V. , MacDowall R. J. , Bothmer V.
Published: February 2004

Gradual solar energetic particle (SEP) events observed at 1 AU are associated with coronal mass ejections (CME) that drive shocks which are presumed to accelerate the ions and electrons to suprathermal energies. However, high‐energy (> 30 MeV) proton and (> 1 MeV) electron events are nearly always associated with both CMEs and flares, suggesting that the acceleration of those particles, particularly the electrons, could be attributed to the associated flares. Only one clear example of a high‐energy SEP event without an active region flare association has been reported previously. We discuss a second such SEP event, on April 14, 1994, associated with a well‐observed solar X ray arcade structure spanning ∼ 150° of solar longitude. The SEP event, observed by detectors on the IMP 8 and Koronas I spacecraft, began ∼ 10 hrs after the beginning of the X ray event and was temporally and spatially associated with the last of three weak interplanetary type III radio bursts observed by the Ulysses low‐frequency radio experiment. The delayed onset and rapid rise of the SEP intensities preclude a recent interpretation in which SEPs were presumed to be accelerated by a shock driven by a CME which erupted at the onset of the X ray event. Yohkoh soft X ray subtracted images show a large‐scale arcade brightening west of ∼ E10° beginning about 8 hours after the initial brightening near the east limb. We suggest that the April 14 SEP event at Earth was produced by a shock driven by a CME associated with the later brightening near central meridian. The initial X ray brightening may also have been associated with an earlier CME.
Open geometry of the magnetotail cross section
Authors: Kaymaz Zerefsan , Siscoe George
Published: February 2004

We present a picture showing the magnetic topology and geometry of Earth's magnetotail and surrounding magnetosheath. The picture is constructed from a 4‐year composite of magnetometer data from IMP 8. It represent a cross‐sectional plane around 30 Re tailward from Earth. It depicts the situation that a quasi‐equatorial interplanetary magnetic field produces, which is the common situation. It shows that the separatrix between open and closed magnetic field lines lies in the boundary, as expected. It shows that the tail is open at high latitudes, also as expected. However, it shows further that the tail is open along the equatorial flanks, contrary to a standard assumption. That is, the picture in which permanent, continuous boundary segments run along both flanks separating closed field lines from fully disconnected field lines is inconsistent with the data.
Geoeffectiveness of three Wind magnetic clouds: A comparative study
Authors: Farrugia C. J. , Scudder J. D. , Freeman M. P. , Janoo L. , Lu G. , Quinn J. M. , Arnoldy R. L. , Torbert R. B. , Burlaga L. F. , Ogilvie K. W. , Lepping R. P. , Lazarus A. J. , Steinberg J. T. , Gratton F. T. , Rostoker G.
Published: February 2004

We compare the large‐scale goemagnetic response to the three magnetic clouds observed by Wind in October 1995 (OCT95), May 1996 (MAY96), and January 1997 (JAN97), studying specifically storm and substorm activity, and other global effects due to untypically large and variable solar wind dynamic pressures. Since the temporal profiles of the interplanetary parameters of the three clouds resemble one another closely, the comparison is meaningful. Using the integrated Poynting flux into the magnetosphere as a rough measure of energy input into the magnetosphere, we find relative energy inputs to be OCT95: JAN97:MAY96 = 22:11:4, with most of the accumulation in the 3‐day periods occurring during passage of the Bz < 0 cloud phase. The peak Dst ring current indices, corrected for magnetopause currents, were in the ratio −138:−87:−38, and hence OCT95 caused a major, JAN97 a moderate, and MAY96 a weak storm. The empirical criterion derived from studies near solar maximum that a solar wind dawn‐dusk electric field ≥ 5mV m−1 lasting for at least 3 hours is necessary and sufficient to generate major storms does not hold for JAN97. Storm main phase onset coincides with cloud arrival in all three cases. The number of substorm onsets during the cloud periods were OCT95: JAN97:MAY96 = 5:3:2, with peak AL values in the ratio −1180:−1750:−570. The dayside magnetosphere was variably compressed, the largest amplitude of variation being on JAN97, where the dynamic pressure change spanned 2 orders of magnitude. MAY96 showed the least variation. The interaction of the individual clouds with the faster trailing flows had two major effects on the magnetosphere: (1) a compression of the cavity during passage of the Bz > 0 cloud phase and the leading edge of the fast stream; and (2) a weakening of the control of the cloud field on magnetosheath flow during the Bz > 0 cloud phase. In summary we find that under most of the aspects considered, OCT95 is the most geoeffective. The buffetting of the magnetospheric cavity by dynamic pressure changes was, however, strongest on JAN97. The profound differences in the magnetospheric response elicited by the clouds is found to be due to the amplitude, duration and rapidity of change of the relevant interplanetary parameters. At present, interplanetary monitors are indispensable for understanding the geomagnetic response to interplanetary structures.
Theoretical model for long time stimulated electromagnetic emission generation in ionospheric radio modification experiments
Authors: Mjølhus E.
Published: February 2004

Experimental evidence points toward magnetic‐field‐aligned small‐scale plasma density striations as responsible for the long time features of stimulated electromagnetic emissions (SEE) in ionospheric radio modification experiments. This paper emphasizes that such striations act as resonators for the generation of trapped upper hybrid oscillations. The sidebands observed in long time SEE experiments are associated with nonstationary behavior, such as parametric instabilities, of these trapped oscillations. A theoretical model for this is formulated and studied numerically. Source spectra for SEE generation, consistent with the main properties of observed downshifted maximum and broad continuum features, will be presented. The first is associated with a trapped oscillations decay instability, while the latter is associated with cavitation in the pre‐formed striations.
Electron temperature and density at high latitude
Authors: Kletzing C. A. , Mozer F. S. , Torbert R. B.
Published: February 2004

The background electron temperature and density at altitudes between 1000 and 8000 km at invariant latitudes > 60° have been determined from swept Langmuir probe measurements from the S3‐3 satellite. These plasma parameters are determined by fitting the measured probe current‐voltage relation to the expected theoretical response. Statistically acceptable fits are found for ∼20% of all measurements and do not include measurements within the auroral density cavity. The results indicate that the density varies as an inverse power law with increasing altitude which has a typical value of 10 cm−3 at 8000 km in altitude. The electron temperature shows a slight increase with altitude but is < 5 eV for almost all measurements. These results suggest that the background plasma outside of auroral density cavities on high‐latitude field lines below 8000 km is dominated by cold plasma of ionospheric origin which is at least an order of magnitude more dense than hotter magnetospheric components.
Subionospheric VLF signal perturbations possibly related to earthquakes
Authors: Molchanov O. A. , Hayakawa M.
Published: February 2004

A likely VLF subionospheric signal effect related to seismic activity was first reported by Hayakawa et al. [1996a, b] in association with the great Kobe earthquake. We have analyzed similar data during periods around 10 other great earthquakes (magnitude M>6) in order to understand the main features of such an effect. The following characteristics emerged from our analysis: The effect appears as a transient oscillation with a 5‐ to 10‐day period, which is initiated a few days before a large earthquake and decays over a few days to weeks after it. It is mainly related to crustal earthquakes. It appears when resonant atmospheric oscillations with periods in a range of 5–11 days exist before the earthquake. The seismic influence on the VLF signal is probably explained by the generation of long‐period gravity waves during the earthquake process and their intensification at heights of 70–90 km.
Identification of solar drivers: The November 3–4, 1993, geomagnetic storm
Authors: McAllister A. H. , Knipp D. J. , Crocker N. U. , Mukai T. , Kokubun S.
Published: February 2004

Solar wind data at Earth and coronal soft X ray images are used to identify the probable solar sources for the November 3–4, 1993, geomagnetic storm. This storm falls in a recurrent sequence associated with the leading edge of a high speed stream from a large coronal hole. We have found convincing, though not certain, evidence that there were also two coronal mass ejections (CMEs) that contributed to the storm. The solar wind transients to which they gave rise arrived at Earth after passage of the heliospheric current sheet and in association with the corotating interaction region (CIR) at the leading edge of the high‐speed stream. This combination of transients with a CIR at the leading edge of a new sector is an example of the dominant pattern for significant geomagnetic storms in the declining phase of the solar cycle. We suggest that global analysis, as used in this paper, can contribute to understanding the relative geoeffectiveness of different CMEs.
The latitude dependence of auroral roar
Authors: Hughes J. M. , LaBelle J.
Published: February 2004

LF/MF/HF radio receivers at many sites which extend from 66.5° to 79.9° invariant allow a statistical study of the variation of 2ƒce and 3ƒce auroral roar with latitude. At each of the sites, the frequency distribution is found to be approximately Gaussian, and the center frequencies of the distributions increase with the magnitude of the local geomagnetic field, as expected if the waves are associated with electron gyroharmonics. Furthermore, peak occurrence rates of 2ƒce auroral roar are found at 75°–76° invariant and show a strong correlation with Kp, including a 27‐day periodicity. These data establish that auroral roar is associated with electron gyrofrequency harmonics and suggest that 2ƒce auroral roar originates near 275 km if it is generated at the second gyroharmonic.
UARS particle environment monitor observations during the November 1993 storm: Auroral morphology, spectral characterization, and energy deposition
Authors: Sharber J. R. , Frahm R. A. , Link R. , Crowley G. , Winningham J. D. , Gaines E. E. , Nightingale R. W. , Chenette D. L. , Anderson B. J. , Gurgiolo C. A.
Published: February 2004

During the major storm of November 3–11, 1993, the UARS particle environment monitor (PEM), in a 57° inclination, 585‐km circular orbit, made measurements of energetic particles, X rays, and magnetic field perturbations in the midnight‐dawn magnetic local time sector of the northern hemisphere and in the noon‐evening sector of the southern hemisphere. X‐ray images provide a global picture (∼20‐min resolution) of the electron precipitation regions twice per 90‐min orbit, which were used to study morphology during the main and recovery phases of the storm. Detailed examination of the precipitating electrons measured between 5 eV and 5 MeV show that in the several hundred eV to several MeV energy range the spectra may often be represented by the kappa distribution, which has a thermal character in the keV range and a power law distribution at higher energies. Selected spectral measurements were used to calculate ionization production rates as functions of altitude. The profiles peak between 95 and 120 km, each having a secondary ledge at lower altitudes produced by the bremsstrahlung X rays. We show that the ionization thus produced can be significant at altitudes as low as the upper stratosphere. Because of the length of time over which this ionization is produced globally, it is expected to have a substantial impact on atmospheric chemistry. Global power inputs to the auroral regions due to precipitating particles have been computed for each day of the storm using PEM X‐ray and particle observations. On November 4, 1993, the day that includes the main and early recovery phases of the storm, the northern hemispheric power calculated by in situ measurement of the electron fluxes was 56.8 GW; that determined by using the X‐ray images was 47.4 GW, These values may be compared with the northern hemisphere Joule heating rate of 290 GW on the same day computed using the PEM magnetometer and electron flux measurements.
Electron temperatures within magnetic clouds between 2 and 4 AU: Voyager 2 observations
Authors: Sittler E. C. , Burlaga L. F.
Published: February 2004

We have performed an analysis of Voyager 2 plasma electron observations within magnetic clouds between 2 and 4 AU identified by Burlaga and Behannon [1982]. The analysis has been confined to three of the magnetic clouds identified by Burlaga and Behannon that had high‐quality data. The general properties of the plasma electrons within a magnetic cloud are that (1) the moment electron temperature anticorrelates with the electron density within the cloud, (2) the ratio Te/Tp tends to be >1, and (3) on average, Te/Tp ≈7.0. All three results are consistent with previous electron observations within magnetic clouds. Detailed analyses of the core and halo populations within the magnetic clouds show no evidence of either an anticorrelation between the core temperature TC and the electron density Ne or an anticorrelation between the halo temperature TH and the electron density. Within the magnetic clouds the halo component can contribute more than 50% of the electron pressure. The anticorrelation of Te relative to Ne can be traced to the density of the halo component relative to the density of the core component. The core electrons dominate the electron density. When the density goes up, the halo electrons contribute less to the electron pressure, so we get a lower Te. When the electron density goes down, the halo electrons contribute more to the electron pressure, and Te goes up. We find a relation between the electron pressure and density of the form Pe = aNγi with γ ≈ 0.5.
Parametric excitation of lower hybrid waves by Z‐mode waves near electron cyclotron harmonics at Tromso
Authors: Kuo S. P. , Koretzky E. , Lee M. C.
Published: February 2004

Z‐mode wave converted from O‐mode heater wave is considered as the pump wave in Tromso HF heating experiments for the parametric excitation of lower hybrid decay mode together with electron Bernstein sidebands in the region above the O‐mode reflection layer. This is the process, suggested by Mishin et al. [1997], which generates superthermal electrons by the excited lower hybrid waves. These superthermal electrons produce plasma waves in the region above the O‐mode reflection layer, with frequencies much greater than the heater wave frequency, as observed by Isham et al. [1990, 1996]. Our detailed analyses of this process are carried out for two experimental cases wherein the pump wave frequencies are near the third and fifth harmonic electron cyclotron frequencies, respectively. The results show that the minimum threshold field of the instability in either case is less than 0.2 V m−1. However, because the wavelength of the instability is also short, the collisionless dampings restrict the instability occurrence to narrow wavelength and frequency ranges. For 1 V m−1 pump field intensity, the instability can be excited in less than 1 ms. Since the excited lower hybrid waves are strongly Landau damped, superthermal electrons are efficiently generated by them.
Climatology of neutral exospheric temperature above Millstone Hill
Authors: Buonsanto M. J. , Pohlman L. M.
Published: February 2004

We present results of a comprehensive study of exospheric temperature T∞ at Millstone Hill. T∞ was calculated using zenith measurements of ion and electron temperature taken by the Millstone Hill incoherent scatter radar and a heat balance calculation during 201 experiments between 1981 and 1997. Fourier decomposition of the data is carried out to investigate how diurnal means and diurnal amplitudes and phases vary with season, solar, and magnetic activity and to search for a long‐term trend in the diurnal mean. The results show generally good agreement with the mass spectrometer/incoherent scatter‐86 model, and the data will be included in the next update to the model. It is found that the diurnal mean T∞ increases with increasing solar and magnetic activity and depends on season, with largest values in summer. No significant long‐term trend is found in this data set. If there is saturation of T∞ at high levels of solar activity, it is not statistically significant in these data. The diurnal amplitude increases with increasing solar activity, but no other statistically significant dependencies are found. The diurnal phase has an earlier phase maximum in summer. Significant semidiurnal and terdiurnal components are found during some times but not others.
Periodic and quasiperiodic ELF/VLF emissions observed by an array of Antarctic stations
Authors: Smith A. J. , Engebretson M. J. , Klatt E. M. , Inan U. S. , Arnoldy R. L. , Fukunishi H.
Published: February 2004

This paper describes amplitude modulations in the frequency range 0–500 mHz of ELF/VLF (0.5–4.0 kHz) radio wave power recorded throughout 1993 and 1995 at Halley and South Pole stations, Antarctica, which lie in approximately the same magnetic meridian and at geomagnetic latitudes (Λ) of 61° and 74°, respectively. Data from the intermediate automatic geophysical observatories P2 and P3 (Λ = 70° and 72°, respectively) were also analyzed where available. In agreement with earlier work, spectrograms have revealed the frequent day‐time (typically 0700‐1700 MLT) occurrence of modulations lying almost entirely within the two period ranges: 10–60 s and 4–6 s. The first range corresponds to quasiperiodic (QP) emissions, while the latter is typical of the two‐hop whistler mode echo period in the plasmatrough, and the events are termed periodic emissions (PEs). QP occurrence rates higher than some earlier studies (335 station‐days out of 667 examined) may be attributable to the sensitive spectral analysis technique. The type I QPs (i.e., those correlated with geomagnetic pulsations observed at South Pole and/or P2/P3) were consistent with an upstream wave driver, controlled by the IMF cone angle. Type II QPs (uncorrelated with magnetic pulsations) were always accompanied by PEs, suggesting a link between the two, reinforced by a frequently observed steady increase in period in both phenomena, especially during the morning, possibly associated with increasing densities due to upward flow of photoionized plasma from the ionosphere after dawn. Here we propose that type II QPs are driven by field line resonant ULF waves which in turn are generated by field‐aligned currents arising from PE induced electron precipitation.
Impact of the Ulysses velocity on the diagnosis of the electron density by the Unified Radio and Plasma Wave sounder in the outskirts of the Io torus
Authors: Le Sager Philippe , Canu Patrick , Cornilleau‐Wehrlin Nicole
Published: February 2004

The resonance spectra collected on February 8, 1992, in the outskirts of the Io plasma torus by the Unified Radio and Plasma wave (URAP) relaxation sounder on board the Ulysses spacecraft present significant differences from the active spectra gathered by earthbound spacecraft in similar plasma conditions. The most striking anomaly is the lack of resonances at the harmonics of the electron gyrofrequency, one of the most common signatures found in usual sounder spectra. These differences are interpreted, when computing the dispersion relation of the Bernstein modes triggered by the sounder, by taking into account the large frequency shift induced on the resonances by the high speed of the Ulysses spacecraft in the plasma. The frequency of the shifted resonances depends on the ratio Vsat/p/Vth of the spacecraft speed relative to the plasma and the thermal speed of the electrons. The observed resonances are all found to be in excellent agreement with the computed frequencies obtained in this way. Some predicted resonances are not observed on the Ulysses spectra, but it is suggested that they have too long a wavelength to be easily detected by the URAP antenna. As a by‐product of the electron density diagnosis allowed by this analysis a rough estimate of the electron temperature can be deduced.
An extended analysis of the September 1989 cosmic ray ground level enhancement
Authors: Lovell J. L. , Duldig M. L. , Humble J. E.
Published: February 2004

The September 29, 1989, cosmic ray ground level enhancement had a complex intensity‐time profile with two distinct peaks observed at some neutron monitors. The event was detected by surface and some underground muon telescopes indicating the presence of particles up to ∼30 GV. We have modeled the response of neutron monitors and surface muon telescopes at the times of both intensity peaks and during the decay phase. We have determined particle rigidity spectra and pitch angle distributions centered on apparent arrival directions. Some evidence of bidirectional flow is present during the second peak. This may be due to backscattering of particles from beyond Earth. There was a high degree of scattering from all directions resulting in a significant isotropic component. Spectra derived from ground level responses are consistent with satellite observations. The spectrum between a few MeV and a few tens of GeV is consistent with shock acceleration.
Waves, structures, and the appearance of two‐component turbulence in the solar wind
Authors: Ghosh S. , Matthaeus W. H. , Roberts D. A. , Goldstein M. L.
Published: February 2004

Spacecraft observations of magnetic field fluctuations in the solar wind reveals a “Maltese Cross” pattern in the two‐dimensional correlation function measurements of solar wind fluctuations [Matthaeus et al., 1990]. This pattern suggests the presence of two components: fluctuations with their (Fourier) wave vector approximately parallel to the ambient magnetic field (e.g., slab turbulence) and fluctuations with their (Fourier) wave vector approximately perpendicular to the ambient magnetic field (e.g., quasi two‐dimensional turbulence). To date, the appearance of such a pattern has never been reproduced from numerical simulation studies. Here we present results of several MHD simulations that address this issue using both two‐and‐one‐half dimensional and three‐dimensional compressible models and a wide variety of initial states and plasma parameters. Slab turbulence and quasi two‐dimensional turbulence appear in various runs; however, their simultaneous appearance is difficult to achieve and seems to rely upon their separate existence in the initial data. In contrast, the presence of transverse pressure‐balanced magnetic structures causes slab turbulence to evolve in such a manner that a two‐component correlation function emerges through time averaging. We suggest that the Maltese Cross and similar observations may be a consequence of either the initial data or of averaging over different parcels of solar wind.
Identification of low‐frequency magnetosheath waves
Authors: Denton Richard E. , Lessard Marc R. , LaBelle James W. , Gary S. Peter
Published: February 2004

Four low‐frequency modes may propagate in a high‐beta, nearly bi‐Maxwellian plasma, the magnetosonic, Alfvén, ion acoustic, and mirror modes. This study uses a procedure based on linear Vlasov theory for the identification of these modes by use of transport ratios, dimensionless ratios of the fluctuating field and plasma quantities. A single parameter, the mode deviation, is calculated which characterizes the difference between the theoretical transport ratios of a particular mode and the observed ratios. As well as determining the mode that best describes the observed fluctuations, it gives us a measure of whether or not the resulting identification is unique. Unfortunately, a unique identification is not always possible because of problems discussed herein. One problem is that the parallel phase ratio (related to the phase angle between density and parallel magnetic fluctuations) is often not well defined. Using the plasma and magnetic field data gathered by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module spacecraft, we calculate the mode deviations and identify the modes observed in the magnetosheath. The quasi‐perpendicular (with wave vector at a large angle to the background magnetic field) mirror mode is clearly identified in the inner (close to the magnetopause) and middle magnetosheath. The quasi‐parallel mirror mode may be observed in the inner magnetosheath, but that identification is not certain. Alfven‐like modes are observed for one event in the outer magnetosheath (near the bow shock) and are probably observed in two others.
Statistics of the lower hybrid wave cavities detected by the FREJA satellite
Authors: Kjus S. H. , Pécseli H. L. , Lybekk B. , Holtet J. , Trulsen J. , Lühr H. , Eriksson A.
Published: February 2004

Localized electrostatic wave packets in the frequency region of lower hybrid waves have been detected by the instruments on the FREJA satellite and also earlier by instrumented rocket payloads. These waves are often associated with local density depletions, indicating that the structures can be interpreted as wave‐filled cavities which are strongly elongated along the magnetic field. The basic features of the observations are discussed, providing a survey of the conditions for occurrence and spatial distributions. The basic properties of the individual cavities and associated fluctuating electric fields are discussed as well, and typical densities, widths, and other characteristics are obtained for selected orbits.
Geotail observations of a fast tailward flow at XGSM = −15 RE
Authors: Nagai T. , Fujimoto M. , Nakamura R. , Saito Y. , Mukai T. , Yamamoto T. , Nishida A. , Kokubun S. , Reeves G. D. , Lepping R. P.
Published: February 2004

Geotail observed a fast tailward flow burst (a speed of >600 km/s) with southward Bz in the Earth's magnetotail at a radial distance of 15.5 RE on March 30, 1995. Ions in this burst consisted of a single‐component plasma showing convection motion, and these ions were confined near the neutral sheet. This flow burst was likely associated with a pseudobreakup rather than a major substorm onset. In this event, magnetic reconnection appeared to take place only for the field lines near the neutral sheet. The magnetic reconnection was quenched soon and resulted in a short‐lived tailward flow burst embedded in the plasma sheet.
Modeling of twisted/warped magnetospheric configurations using the general deformation method
Authors: Tsyganenko N. A.
Published: February 2004

A new method is developed for modeling the effects of the planetary dipole tilt and of the interplanetary magnetic field (IMF) related twisting of the cross‐tail current sheet. The method extends the field deformation technique of Stern [1987] and makes it possible to easily represent a wide variety of warped magnetospheric configurations, starting from simple models with an axially symmetric magnetopause and a planar tail current sheet. The proposed transformations do not violate the condition ∇ · B = 0 and allow one to retain a desired distribution of the normal component of the total field at the magnetospheric boundary. Furthermore, the method makes it possible to add flexibility to the model magnetopause, so that the effects of the dipole tilt and of the IMF upon its shape can be reproduced. In particular, the transformation with a radially dependent rotation of the X and Z axes, while providing the desired tilt‐related bending of the cross‐tail current sheet, can also deform the magnetopause and reproduce its tilt‐related asymmetry, indicated by observations and reported here for the first time. The deformation technique also allows algorithms that are more compact and faster than the currently used ones. Because of the general nature of the proposed approach it should be possible to extend it to the modeling of other (e.g., Jovian) planetary magnetospheres.
Plasma waves in the Earth's electron foreshock: 2. Simulations using time‐of‐flight electron distributions in a generalized Lorentzian plasma
Authors: Yin L. , Ashour‐Abdalla M. , El‐Alaoui M. , Bosqued J. M. , Bougeret J. L.
Published: February 2004

This study considers the level of occurrence of nonlinear wave‐wave interactions in foreshock nonthermal plasma and whether such interactions are significant in producing observed electromagnetic emissions. To this end, we performed two‐dimensional electromagnetic particle‐in‐cell (PIC) simulations using the time‐of‐flight electron distribution model described in the paper proceeding this [Yin et al., this issue], and input parameters based on two sets of Wind/3DP distribution measurements made when narrowband and broadband waves were detected by the Wind/WAVES instruments. In the simulations we limited the wave energy density to W≲10−3, a value inferred from the largest wave amplitude detected by Wind. We consider the dynamics of narrowband and broadband wave instabilities, focusing on wave‐coupling processes in the presence of foreshock hot ions as well as the most intense narrowband waves, which are the most likely candidates for producing the detected electromagnetic emissions. Results are compared with recent Wind measurements. We showed that when foreshock conditions exist, quasilinear diffusion dominates the instabilities, that weak wave coupling does not prevent beam flattening, and that no discernible wave backscattering occurs. The study indicates that the weak nonlinearity of foreshock electrostatic waves in a homogeneous plasma and in the presence of foreshock nonthermal electrons and moderate beam parameters is not responsible for the generation of electromagnetic emissions at either the fundamental or the second harmonic of the plasma frequency.
On the relation of the AE and PC indices
Authors: Takalo J. , Timonen J.
Published: February 2004

It is shown that although the power spectra of the AE and PC data are quite similar, they appear different in the structure function (SF) analysis. While the AE time series has a clear drop in the slope of the SF after the first 2 hours, the slope of the SF of the PC data only decreases gradually, and at a somewhat longer timescale. It is also shown by using 15‐min averaged data, that both SFs are periodic with a clear diurnal variation. The PC time series seems to have a more pronounced periodicity, probably because it is measured at a single station at Thule. The relation between the PC and AE indices has been analyzed in detail. The cross‐correlation time between these data is shown to be 2–2.5 hours. It appears that the correlation function has two different scaling regions, one at the timescale within 2.5 hours and another between 2.5 and 12 hours. It also seems that a neural network prediction of the AE index from the PC index is only possible for time scales shorter than the cross‐correlation time of these systems. The AE index has been derived from the PC index for 7.5 min ahead. These predictions gave normalized mean square errors (NMSE) in the range 0.18–0.25 during wintertime. The corresponding correlation coefficient was 0.91 at the best. This shows that well within the cross‐correlation time, the AE time series can be derived quite accurately from the PC data, at least during wintertime when the field‐aligned currents are the main source of the PC index.
Analysis of the waves associated with the unusual interplanetary shock observed on day 109, 1992
Authors: Lucek E. A. , Balogh A.
Published: February 2004

On day 109 of 1992 Ulysses observed an extraordinary interplanetary shock at a time of extremely unusual interplanetary magnetic field conditions. The field was closely aligned with the radial direction for nearly a day ahead of the shock, suggesting that it was most likely to be a quasi‐parallel shock. Unlike most interplanetary shocks, this shock was associated with substantial upstream and downstream wave fields, which is consistent with the shock being quasi‐parallel in nature. There were several separate intervals of wave activity in the upstream region, the first occurring on day 108 and the second two intervals occurring on day 109 leading up to the shock. In addition, just ahead of the shock the background field magnitude decreased to a value close to zero, and superimposed on this decrease were a number of field magnitude enhancements. The downstream region was populated by large‐amplitude compressive waves. A combination of spectral and wavelet analysis techniques was used to characterize the different intervals of wave activity. The waves on day 108 and the first interval of wave activity on day 109 are consistent with generation by an ion beam instability, but the waves on day 109 increased in amplitude as the shock was approached, suggesting that these waves were directly produced by the shock.
Comparison of macroscopic particle‐in‐cell and semikinetic models of the polar wind
Authors: Barakat A. R. , Thiemann H. , Schunk R. W.
Published: February 2004

The outflow of the polar wind along diverging geomagnetic field lines has been the subject of many modeling studies for the past 25 years. As the plasma drifts up and out of the topside ionosphere, it undergoes several transitions; for instance, its velocity changes from subsonic to supersonic and its velocity distribution changes from Maxwellian to non‐Maxwellian. The complexity of the flow led to the development of several modeling approaches, such as the generalized moment, the kinetic, and the semikinetic models. Recently, a “macroscopic” particle‐in‐cell (PIC) model was adopted to study the polar wind. However, because one is always restricted to a finite number of particles, the validity of the approach must be established when it is applied to macroscopic flows. In this study the polar wind predictions obtained from a macroscopic PIC simulation were compared to those obtained from the more rigorous semikinetic model for steady state conditions. The study also shows the sensitivity of the PIC simulation to the adopted modeling parameters for both time‐dependent and steady state conditions, including the number of simulation particles, the time step, the spatial bin size, etc. The study indicates that (1) the PIC model can be a powerful simulation tool if special attention is given to its potential pitfalls; (2) because of the finite number of particles, the PIC technique is subject to a considerable amount of noise; (3) the noise level is higher for the higher‐order moments, such as heat flow, and for the velocity distribution function; (4) the use of a time step that is too large leads to a modulation of the results; (5) an insufficient number of spatial bins yields a poor spatial resolution, while too many spatial bins leads to more noise; (6) the noise level can be reduced by averaging over time and/or space, but this affects the spatial and/or temporal resolution; (7) the bin size in velocity space must be carefully chosen to balance numerical noise and velocity space resolution; (8) in the steady state the PIC technique can achieve the same accuracy as the semikinetic model if all of the PIC modeling parameters are optimized; and (9) a few hundred thousand simulation particles, as used in some previous studies, are not adequate to resolve the tail of the velocity distribution, even in the steady state when time averaging is possible. For 106 particles the noise in the tail is still appreciable; and (10) when poor spatial resolution is used, important features can be missed, as was the case in some previous studies.