bluuebunny/crossref_metadata_2025 huggingface dataset:
Density profile modifications by electromagnetic ion cyclotron wave pressures near the dayside magnetospheric boundary
Authors: Pokhotelov O. A. , Feygin F. Z. , Stenflo L. , Shukla P. K.
Published: February 2004

It is shown that the existence of the so‐called “magnetic holes” (magnetic field minima) along the field lines play a significant role in the plasma pressure balance in the Earth's magnetosphere. Ponderomotive forces induced by the Pc 1 geomagnetic pulsations propagating through these regions can lead to a significant plasma accumulation there. A numerical analysis of this effect based on an analytical model for the geomagnetic field is carried out. The variation of the redistributed plasma density versus radial distance, amplitude, and ratio of wave to ion cyclotron frequencies is presented. It turns out that density enhancements occur off the equator at high latitudes and that they increase when the McIlwain parameter L increases. The specific features of the Pc 1 propagation along the lines of forces in the presence of “magnetic holes” are discussed. It is suggested that the existence of regions with decreasing magnetic fields at high‐altitude field lines can lead to resonance absorption of ion cyclotron waves (the “magnetic beach” effect).
The relationship between recurring cosmic ray depressions and corotating solar wind streams at ≤1 AU: IMP 8 and Helios 1 and 2 anticoincidence guard rate observations
Authors: Richardson I. G. , Wibberenz G. , Cane H. V.
Published: February 2004

We examine the detailed relationship between recurrent cosmic ray depressions and corotating high‐speed streams in the inner heliosphere near the ecliptic plane using counting rates from anticoincidence guards of instruments on the IMP 8, Helios 1, and Helios 2 spacecraft. These rates indicate the density of >60 MeV/amu ions with reasonable time resolution (∼15 min) and high counting statistics. Essentially all corotating streams are accompanied by significant particle depressions. The particle decrease commences most frequently (∼63% of events) at the leading edge of the stream which is typically colocated with the stream interface inside the corotating interaction region (CIR). If the depression starts ahead of the interface, there is usually an additional abrupt decrease at the interface. In ∼61% of events the onset of the depression is closely associated with the onset of enhanced field turbulence which typically occurs near the stream leading edge. Minimum particle densities are generally found in the vicinity of the maximum solar wind speed in the high‐speed stream. The density recovers during the declining phase of the stream. The observations are most consistent with modulation of the cosmic ray density in high‐speed streams by the increase in solar wind speed. Enhanced scattering by turbulence in the CIR may also contribute near the onset of the depression, in particular in cases where the decrease commences ahead of the stream interface. The absence of a consistent relationship between the depressions and magnetic field enhancements suggests that localized particle drifts in the enhanced magnetic fields of CIRs do not produce the particle depressions. The observation of a depression is not dependent on the presence of a heliospheric current sheet crossing in the CIR, in contrast to previous reports.
Emission and propagation of auroral kilometric radiation in the density depletions of the auroral region
Authors: de H. Cavalcanti C. J. , Schneider R. S. , Ziebell L. F.
Published: February 2004

We investigate emission and propagation of auroral kilometric radiation in the auroral zones of the Earth, using the geometries optics approximation in order to describe the ray trajectory and the transfer equation to obtain the intensity of the wave along the trajectory. Both the cyclotron emissivity and the components of the dielectric tensor are evaluated by using the locally homogeneous plasma approximation, taking into account thermal effects and considering both the energetic and the background electron populations. The amplification level predicted by the transfer equation is similar to the value obtained from simple integration of the amplification coefficient along the wave trajectory. Using model profiles which incorporate repeated series of density depletions inside large‐scale auroral cavities, we show that these depletions may contribute to increase the wave amplification, as compared with the case of auroral cavities without internal structure.
Dynamics of MHD wave propagation in the low‐latitude magnetosphere
Authors: Lee Dong‐Hun
Published: February 2004

The propagation of MHD waves depends on a local Alfven speed and ambient geometry. The dynamical properties of MHD waves in the plasmasphere and magnetosphere are investigated by assuming a realistic Alfven speed profile in a dipole field. The WKB approximation is used to determine the cutoff boundaries and estimate the wave dispersion over a whole meridional plane. It is found that most wave energy may be transmitted effectively into the inner magnetosphere near the equatorial region, since the reflection of incoming waves at the plasmapause becomes weakest at the equator. It is also examined how the transmission from the outer magnetosphere to the inner magnetosphere depends on wave frequencies and azimuthal wavenumbers. The cutoff boundaries of wave propagation are quantitatively determined for each mode and wavenumber, which show various structures on the meridian. The results suggest that the propagation region may consist of two separable domains of the inner and outer magnetosphere for a relatively low‐frequency wave. The inner region of propagation appears to be a distorted torus around the dipole axis. Such spatial separation of the two regions becomes weak and gradually interconnected for the waves with a relatively small azimuthal wavenumber or high frequency. The wave spectra and energy distribution are also investigated for different azimuthal wavenumbers. The numerical results show that the cavity modes may exist in the plasmasphere even at the absence of the outer magnetospheric boundary, which is found to be strongly associated with the characteristics of wave parameters. In particular, it is suggested that the plasmaspheric cavity modes, in the nightside region, may play a crucial role in producing Pi 2 pulsations. In addition, theories of waveguide and cavity modes based on the dipole model are discussed in detail.
Formation of arc‐shaped Alfvén waves and rotational discontinuities from oblique linearly polarized wave trains
Authors: Vasquez Bernard J. , Hollweg Joseph V.
Published: February 2004

The forms of Alfvénic fluctuations in the solar wind sometimes possess nearly constant magnetic intensities but have an approximate arc rather than circular polarization. They are also associated with layers of abrupt field rotation called rotational discontinuities (RDs) where the field changes direction by < 180°. Ion‐sense and electron‐sense rotations are observed in approximately equal numbers. To explore the origin of this form, we conduct a one‐and‐one‐half‐dimensional hybrid numerical simulation study of the evolution of obliquely propagating, low‐frequency (≪ion cyclotron) Alfvén wave trains. Starting from a linearly polarized wave train, an approximate arc polarization evolves rapidly where the magnetic field moves to and fro on a less than semicircular arc. Large‐amplitude (|δB|/B ∼ 1) wave trains steepen and produce RDs which always rotate the field by < 180° with no preference for ion or electron sense of rotation. These properties correspond to those of Alfvénic fluctuations in the solar wind, and our model is the first which offers an explanation of the observed arc‐shaped waves and imbedded RDs. At early times, a large density signal is also generated. For large plasma β, the signal rapidly damps, and the waveform varies little with time. For small plasma β, the generated constant‐B Alfvén wave is parametrically unstable and causes the density signal to grow further before the instability saturates. The wave train and density signal beat strongly giving a periodic time variation of the wave amplitude and waveform. Ion heating from steepening, RD formation, relaxation to constant B, and parametric processes occurs mainly parallel to the background magnetic field and cannot explain the perpendicular heating of ions observed in the solar wind.
Altitudinal comparison of dayside field‐aligned current signatures by Viking and DMSP‐F7: Intermediate‐scale field‐aligned current systems
Authors: Ohtani S. , Blomberg L. G. , Newell P. T. , Yamauchi M. , Potemra T. A. , Zanetti L. J.
Published: February 2004

Dayside large‐scale and intermediate‐scale field‐aligned current (FAC) signatures are examined with multi‐instrument measurements from Viking and DMSP‐F7 at magnetic conjunctions. The present paper reports four such conjunction events, with an emphasis on an event that occurred on October 13, 1986. In these four events both Viking and DMSP‐F7 crossed prenoon FAC systems approximately along meridians. The altitude of DMSP‐F7 was 835 km, whereas that of Viking ranged from 8500 to 12,000 km. The electric to magnetic field ratio measured by Viking indicates that intermediate‐scale FAC systems, as well as large‐scale FAC systems, are often quasi‐stationary. This is also supported by the comparison between the Viking and DMSP‐F7 magnetic measurements. The only obvious exception is the equatorward part of the October 13 event, in which the Viking and DMSP‐F7 measurements are better explained in terms of Alfvén waves. In two other events the Viking signature projected to the DMSP‐F7 altitude is significantly more structured than the DMSP‐F7 signature, although the electric to magnetic field ratio observed by Viking suggests that the associated FACs were quasistationary. This apparent discrepancy is possibly ascribed to the fact that Viking stays longer in FAC systems and therefore has more chance to observe temporal changes in FACs. However, such temporal effects must operate longer than the Alfvén transit time so that FAC systems become quasi‐stationary. Although the generation mechanism(s) of intermediate‐scale FAC systems remains an open question, possibilities include a localized shear of plasma convection and a localized merging between the solar wind and magnetospheric field lines.
Elucidation of the physics of the gravity wave‐TID relationship with the aid of theoretical simulations
Authors: Kirchengast Gottfried
Published: February 2004

The physics of the relationship between atmospheric gravity waves (AGWs) and traveling ionospheric disturbances (TIDs) is thoroughly investigated with emphasis on large‐scale AGW/TIDs at F region heights at middle and high latitudes (provided field‐perpendicular drifts are small). In support, simulations using a realistic AGW model (“Clark‐based AGW model”) in combination with a realistic ionospheric model (“Graz Ionospheric Flux Tube Simulation (GIFTS) model”) were performed. All fundamental AGW/TID quantities are treated consistently, i.e., perturbations in neutral densities, wind, and temperature as well as disturbances in electron density, ion drift, and ion and electron temperature. The AGW‐induced ionospheric response is inspected for all TlD quantities, based on their governing conservation equations. The results are discussed by means of detailed and approximative formulae as well as instructive figures which provide a firm quantitative understanding significantly beyond the current state of knowledge. Especially the physics of the electron temperature disturbance (Te‐TID), up to now not yet quantitatively inspected, is thoroughly explored. A major finding is that the disturbance in specific terms of the electron energy equation is an order of magnitude more pronounced than the net disturbance determining the strength of the Te‐TID. Furthermore, simulation results illustrating the natural variability of the AGW/TID quantities (1) due to varying AGW properties and (2) due to changing thermosphere/ionosphere background conditions are discussed. Features observed include the following: AGW period and magnetic field line‐induced south‐north asymmetries are major causes of variability. Change from high/moderate to low solar activity enhances amplitudes of most AGW/TID quantities, electron density and temperature being likely exceptions; nighttime conditions tend to lower amplitudes versus daytime. The insight gained is valuable from a basic research point of view and also for suitable AGW/TID descriptions for thermosphere/ionosphere weather modeling.
Characteristics of midlatitude coherent backscatter from the ionospheric E region obtained with Sporadic E Scatter experiment
Authors: Haldoupis C. , Schlegel K.
Published: February 2004

Sporadic E Scatter (SESCAT) experiment is a continuous wave radio Doppler experiment in Crete, which operates at 50 MHz and observes coherent backscatter from field‐aligned irregularities in the midlatitude E region ionosphere (L = 1.35). In this paper a large database of high‐resolution Doppler spectrum observations from over one and a half years of radar operation is utilized to investigate the properties of coherent radio wave scattering. The statistics of the spectral moments show that echo intensities are in the 5‐ to 15‐dB range, Doppler velocities point northward and upward having magnitudes usually less than 100 m/s, whereas spectrum widths are narrow taking values mostly in the range from 50 to 150 m/s. On the basis of the data, we postulate that 50‐MHz midlatitude echoes are largely due to secondary irregularities generated during conditions of weak plasma turbulence; A striking feature is the seasonal and diurnal echo occurrence. The echoes are overwhelmingly present in the summer months and appear only during nighttime, mostly in the premidnight local time sector. Knowing the close connection with the midlatitude sporadic layers, Es, the echo occurrence can be understood in terms of the well‐known morphology of intense Es layers. On the other hand, the absence of daytime echoes means that the gradient drift instability is inoperable during the day, probably because of electron density gradient smoothing due to strong solar photoionization production. Finally, the mean Doppler velocities are larger in amplitude but vary approximately in phase with the ambient northward and upward E × B drifts, infeired from past incoherent scatter radar measurements, and model predictions of meridional neutral winds.
Scintillations, plasma drifts, and neutral winds in the equatorial ionosphere after sunset
Authors: Basu S. , Kudeki E. , Basu Su. , Valladares C. E. , Weber E. J. , Zengingonul H. P. , Bhattacharyya S. , Sheehan R. , Meriwether J. W. , Biondi M. A. , Kuenzler H. , Espinoza J.
Published: February 2004

An equatorial campaign was conducted during September 25 to October 7, 1994, to investigate the neutral and plasma dynamics in the equatorial ionosphere after sunset in relation to the day‐to‐day variability of the occurrence of equatorial spread F (ESF). The campaign was organized under the auspices of National Science Foundation's Multi‐Instrumented Studies of the Equatorial Thermosphere Aeronomy program (MISETA), which included the Jicamarca radar, spaced‐antenna satellite scintillation, digisonde, all‐sky imager, and Fabry‐Perot interferometer (FPI) measurements near the magnetic equator in Peru. During a part of the period September 27 to October 3, the Geophysics Directorate of Phillips Laboratory performed measurements away from the magnetic equator at Aguaverde, Chile (magnetic latitude: 11°S) located 800 km to the east of the Jicamarca meridian using geostationary and GPS satellite scintillation, digisonde and all‐sky imager systems. The incoherent scatter radar results indicate that the postsunset enhancement of upward plasma drift, even though of the order of only 20 m s−1 during the solar minimum period, is a necessary condition for the generation of ESF. In view of the extreme difficulty of determining the neutral wind speed during the early evening hours by the FPI due to low airglow intensity, it was not possible to unequivocally associate the observed postsunset enhancements with strong eastward neutral winds. However, considering a few observations contiguous to the campaign period, it appears that such a causal relationship may exist. The scintillation drift measurements in Peru and Chile indicated that the zonal irregularity drift was smaller away from the magnetic equator, implying a variation of neutral wind with latitude. This is reproduced in the altitude variation of zonal drift observed by the Jicamarca radar. During a magnetic storm, scintillation measurements indicated that eastward drifts near the magnetic equator are accompanied by westward drifts near the anomaly peak, which is consistent with the effects of a disturbance dynamo. The campaign results indicate that in order to resolve the variability of ESF, a careful probing of neutral dynamics as a function of latitude needs to be undertaken during the postsunset period.
Flow angle effects in E region 398‐MHz auroral backscatter at small aspect angle
Authors: Moorcroft D. R.
Published: February 2004

A study of flow angle effects on small aspect angle backscatter from the auroral E region has been made using data obtained in the 1970s with the 398‐MHz radar at Homer, Alaska. Several findings differ from previous results obtained at 140 MHz with the Scandinavian Twin Auroral Radar Experiment and the Sweden and Britain Radar‐Auroral Experiment radars: (1) backscatter power is virtually independent of flow angle; (2) there is much less difference between type 1 and type 2 echoes; (3) as found in previous UHF studies, Doppler velocities of type 2 (low Doppler velocity) echoes vary with flow angle more rapidly than predicted by the “cosine law”. These differences are considered in the light of the recent strong turbulence theory developed by Hamza and St‐Maurice [1993a, b].
Plasma waves and field‐aligned currents in the Venus plasma mantle
Authors: Strangeway R. J. , Russell C. T.
Published: February 2004

Plasma waves, observed above the dayside Venus ionosphere by the Pioneer Venus Orbiter (PVO) plasma wave instrument, have been attributed to whistler mode waves, lower hybrid waves, or ion acoustic waves. In order to clarify the nature of the waves, we have performed both case study and statistical analyses of the plasma wave and magnetic field data. We find that the plasma wave data are well ordered by altitude with respect to the location where the PVO Langmuir probe, or orbiter electron temperature probe (OETP), measures a density of 100 cm−3, known as the OETP ionopause. The dominant signature in the wave data appears to be a change in the instrument noise level because of changes in the plasma Debye length. However, there is a burst of wave activity near the OETP ionopause. Also, we find that there is a rotation in the magnetic field at or near this location. By casting the magnetic field data into a coordinate system ordered by the presumed magnetosheath flow, we find that the rotation of the field tends to orient the field in a more flow‐aligned direction at lower altitudes. We attribute this to mass loading at lower altitudes. We further suggest that the field‐aligned current associated with the field rotation corresponds to a shear Alfvén wave standing in the magnetosheath flow. The field‐aligned currents are present because of boundary conditions imposed on the flow, and it is not clear if the waves are actually associated with the field‐aligned currents or are simply coincidental. Since the waves are observed at the OETP ionopause, further progress in understanding these waves will be made though determining what underlying plasma structure, if any, is related to the OETP ionopause, which is defined by a specific instrument threshold. Nevertheless, our study confirms that the wave activity, field‐aligned currents, and OETP ionopause all occur within the plasma mantle above the ionosphere. As such, the plasma waves are not an energy source for the dayside ionosphere.
GPS phase fluctuations in the equatorial region during the MISETA 1994 campaign
Authors: Aarons J. , Mendillo M. , Yantosca R. , Kudeki E.
Published: February 2004

In this paper we present the first coordinated use of Global Positioning System (GPS) multisite and multisatellite observations with ground based radar and optical diagnostics to investigate equatorial irregularity patterns. Thirty second samples of total electron content (TEC) obtained from GPS phase differences between 1.2‐ and 1.6‐GHz signals are used to study phase fluctuations at several stations. Comparisons were made with various types of ground measurements during the multi‐instrument studies of the equatorial thermosphere aeronomy (MISETA) period. Depletions of 6300Å airglow emission from Arequipa, Peru, correlated with phase fluctuations recorded at the same site. Phase fluctuations at Arequipa occurred at the times when the Jicamarca radar backscatter returns from plumes were noted but were also seen on other nights when there were no radar returns from plumes. Levels of phase fluctuations noted at Arequipa varied considerably on nights when only thin layers of irregularities were observed by the Jicamarca radar. Differences of ionospheric conditions between the two sites, separated by only 5.5° geographic longitude, may account for the different behavior patterns of irregularities noted. Similar differences in the general behavior pattern of phase fluctuations were shown when data from Arequipa and Fortaleza, Brazil, were compared. These stations, 33° apart, but at the same dip latitude had different patterns for some days. During a magnetic storm, a very high altitude plume was observed by the radar and by phase fluctuations noted at Santiago at 18° dip latitude. This correlation of high plume altitude during some periods of magnetic activity was validated by additional examples of phase fluctuations from three other magnetic storms in the solar minimum years of 1994 and 1995.
Surface waves and magnetic reconnection at a magnetopause
Authors: Uberoi C. , Lanzerotti L. J. , Wolfe A.
Published: February 2004

It is argued that surface waves with wavelengths larger than the thickness of a neutral layer that separates two different plasma regimes can induce the tearing mode instability. Magnetic reconnection is shown to occur on the tearing mode timescale.
Three‐dimensional multiscale MHD model of cometary plasma environments
Authors: Gombosi Tamas I. , De Zeeuw Darren L. , Häberli Roman M. , Powell Kenneth G.
Published: February 2004

First results of a three‐dimensional multiscale MHD model of the interaction of an expanding cometary atmosphere with the magnetized solar wind are presented. The model starts with a supersonic and super‐Alfvènic solar wind far upstream of the comet (25 Gm upstream of the nucleus) with arbitrary interplanetary magnetic field orientation. The solar wind is continuously mass loaded with cometary ions originating from a 10‐km size nucleus. The effects of photoionization, electron impact ionization, recombination, and ion‐neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our new multiscale upwind scalar conservation laws‐type numerical technique (MUSCL). We have named this the multiscale adaptive upwind scheme for MHD (MAUS‐MHD). The combination of the adaptive refinement with the MUSCL‐scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. The main findings are the following: (1) Mass loading decelerates the solar wind flow upstream of the weak cometary shock wave (M ≈ 2, MA ≈ 2), which forms at a subsolar standoff distance of about 0.35 Gm. (2) A cometary plasma cavity is formed at around 3 × 103 km from the nucleus. Inside this cavity the plasma expands outward due to the frictional interaction between ions and neutrals. On the nightside this plasma cavity considerably narrows and a relatively fast and dense cometary plasma beam is ejected into the tail. (3) Inside the plasma cavity a teardrop‐shaped inner shock is formed, which is terminated by a Mach disk on the nightside. Only the region inside the inner shock is the “true” diamagnetic cavity. (4) The model predicts four distinct current systems in the inner coma: the density peak current, the cavity boundary current, the inner shock current, and finally the cross‐tail current. (5) The calculated plasma parameters (magnetic field, plasma density, speed, and temperature) are in very good agreement with published Giotto observations.
Variation of plasmatrough density derived from magnetospheric field line resonances
Authors: Waters C. L. , Samson J. C. , Donovan E. F.
Published: February 2004

The diurnal variation of ULF field line resonant frequencies has been calculated using the cross phase of data from the north‐south components recorded at seven latitudinally spaced ground magnetometers in the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) array. CANOPUS magnetometers range in latitude from Rankin Inlet (L = 12.4) south to Pinawa (L =4.3). Using cold plasma MHD theory, an R−4 plasma density function, and the T87 magnetic field model, the variation of plasma density in the equatorial region has been calculated from the experimentally determined resonant frequencies. Consecutive, adjacent magnetometer pairs provide six local daytime spatial estimates of the variation in plasma mass density between 4 and 11 RE. Typical values are 1–20 H+cm−3 for the plasmatrough and 50–200 H+cm−3 for the plasmasphere. The data recorded on June 7, 1990, shows an afternoon increase in density near geosynchronous orbit in agreement with convection models of the magnetosphere. The ground‐based measurements of plasma mass density have been compared with data from the Los Alamos Magnetospheric Plasma Analyser on board the 1989‐046 geosynchronous spacecraft. These comparisons show that the ground‐based technique should allow a robust procedure for calculating dayside, time‐dependent mappings of the equatorial plasma mass density from the plasmapause to the magnetopause in near real time.
Growth and damping of oblique electromagnetic ion cyclotron waves in the Earth's magnetosphere
Authors: Xue Song , Thome Richard M. , Summers Danny
Published: February 2004

The dispersive properties of oblique electromagnetic ion cyclotron (EMIC) waves are examined for conditions in the Earth's outer magnetosphere (L = 7) when the energetic particle distribution has a high‐energy tail modeled by a generalized Lorentzian distribution. For small wave normal angles ψ to the ambient magnetic field, the wave growth and damping rates for a generalized Lorentzian distribution are smaller than those for a Maxwellian distribution (typically by a factor of 2), while for larger wave normal angles (ψ ≳ 60°) the corresponding differences in the growth and damping rates are relatively small. For both the generalized Lorentzian and Maxwellian distributions, maximum wave growth due to hot proton temperature anisotropy occurs for parallel propagation, but significant wave growth can occur for wave normal angles |ψ| ≲ 30°. Unstable waves produced near the magnetic equator are expected to be damped at higher latitudes as a result of cyclotron damping by thermal helium (He+) ions near the bi‐ion frequency or near the second harmonic of the helium gyrofrequency. A new physical process identified in this study is the excitation of high‐frequency oblique (ψ ∼ 50°–60°) EMIC waves due to second harmonic resonance with hot anisotropic protons. This leads to significant wave growth at frequencies above the maximum unstable frequency for parallel propagating waves.
Structure, phase motion, and heating within Alfvén resonances
Authors: Wright Andrew N. , Allan W.
Published: February 2004

An analytical investigation of Alfvén resonances is presented for the cases when energy is dissipated by (1) resistivity (η) within the body of the plasma, and (2) finite Pedersen conductivity (∑p) within the ionospheric boundaries. Universal functions for both solutions are compared, and we find that the structure and phase motion of the resonant fields is fairly insensitive to the details of dissipation. We also identify a curious property of resonances: the spatially integrated (but not time‐averaged) dissipation rate is independent of time for whichever dissipation process we consider. It appears that resonances have a remarkably robust structure.
Heating of the thermal electrons in the coma of comet P/Halley
Authors: Häberli Roman M. , Altwegg Kathrin , Balsiger Hans , Geiss Johannes
Published: February 2004

In the inner coma of a comet the total ion (or electron) density results from an equilibrium between ion production and ion destruction. The ions are mainly produced by photoionization of cometary gas, whereas the main destruction process is dissociative recombination of molecular ions with electrons. The reaction rate for the recombination depends on the temperature of the thermal electrons and decreases with increasing temperature. The electron temperature therefore determines (together with the photoionization rate and the gas density) the total ion density. During the flyby of the European spaceprobe Giotto at comet P/Halley an unexpected peak of the ion density at a cometocentric distance of 12,000 km was observed [Balsiger et al., 1986]. The ion density at the position of the maximum was even higher than at the nearest recorded spectrum at 1300 km from the nucleus. The observed peak of the ion density is explained by a steep increase of the electron temperature with distance [Häberli et al., 1995; Eberhardt and Krankowsky, 1995]. We solved the coupled equations for the electron temperature and density in local energy balance. We further assumed that local photochemical equilibrium holds for the electron density, which is indeed justified in the region of interest. If one assumes the excess energy from photoionization as the only energy source for the electrons, the results are inconsistent with the measurements. The resulting increase of the electron temperature with distance is not sufficiently steep to produce a peak of the ion density around 12,000 km from the comet. The observed peak of the ion density can only be explained with this model if an additional energy source for the electrons is taken into account which locally even dominates over heating by photoionization. As an additional energy source, a population of suprathermal electrons with a typical energy of 10 to 15 eV is suggested. We present a mechanism which may lead to an enhanced flux of solar wind electrons in the vicinity of the nucleus due to the compression of the interplanetary magnetic field. The expected flux of suprathermal electrons at about 15,000 km from the nucleus is in good agreement with the electron measurements in the coma of P/Halley aboard the VEGA 2 spacecraft. This flux is also sufficient to heat the thermal electrons to the required temperature in order to explain the peak of the ion density.
Daytime high‐latitude pulsations associated with solar wind dynamic pressure impulses and flux transfer events
Authors: Yagodkina Oksana I. , Vorobjev Vyacheslav G.
Published: February 2004

Spectral and morphological characteristics of dayside high‐latitude magnetic pulsations within the 3‐to 100‐s period range were examined both during magnetic impulses related to solar wind dynamic pressure impulses (DPIs) and specific quasi‐periodic optical events associated with flux transfer events (FTEs). Pulsation features are compared with the structure of the traveling convection twin vortices and with the signature of dayside aurorae as well. It has been found that both phenomena were accompanied by enhancements of geomagnetic pulsation power. The power of the DPI‐associated pulsations was increased within the 10‐ to 30‐s and 30‐ to 60 s period ranges for 10–15 min and 2–5 min, respectively. The rise in the 30‐ to 60‐s period pulsation power occurred during the passage of twin vortex centers over the station, and the oscillation regimes in H and D components were closely connected with the twin vortex structure. The spectra of the FTE‐associated pulsations have two clear maxima with 4‐ to 8‐s and 30‐ to 60‐s periods. The 4‐ to 8‐s pulsations were generated as bursts of 0.1‐ to 0.2‐nT amplitude and 2‐ to 5‐min duration during the appearance of the poleward moving aurora. Simultaneously, there were sharp increases in the 30‐ to 60‐s period pulsations being registered almost permanently in the dayside high‐latitude region.
Assimilative mapping of ionospheric electrodynamics in the thermosphere‐ionosphere general circulation model comparisons with global ionospheric and thermospheric observations during the GEM/SUNDIAL period of March 28–29, 1992
Authors: Emery B. A. , Lu G. , Szuszczewicz E. P. , Richmond A. D. , Roble R. G. , Richards P. G. , Miller K. L. , Niciejewski R. , Evans D. S. , Rich F. J. , Denig W. F. , Chenette D. L. , Wilkinson P. , Pulinets S. , O'Loughlin K. F. , Hanbaba R. , Abdu M. , Jiao P. , Igarashi K. , Reddy B. M.
Published: February 2004

Satellite and ground‐based observations from March 28 to 29, 1992, were combined in the assimilative mapping of ionospheric electrodynamics (AMIE) procedure to derive realistic global distributions of the auroral precipitation and ionospheric convection which were used as inputs to the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere general circulation model (TIGCM). Comparisons of neutral model winds were made with Fabry‐Perot measurements and meridional winds derived from ionosondes. The peak equatorward winds occurred 1–2 hours later in the model. Gravity waves launched from high‐latitude Joule heating sources reached the equator in about 2 hours and agreed with observed variations in the height of the maximum electron density (hmF2) and in the meridional winds. Joule heating events produced minima in the O/N2 ratio that moved equatorward and usually westward in longitudinal strips which lasted about a day. Changes in the O/N2 ratio and in the peak electron density (NmF2) were strongly correlated so the observed daytime NmF2 values for stations near 50° magnetic latitude were generally reproduced by AMIE‐TIGCM on the second day of the simulation. The AMIE‐TIGCM underestimated the electron density after midnight by up to a factor of 2 in midlatitudes, while the modeled F2 layer was about 35 km lower than the observations at midnight. Shifting the model winds 2 hours earlier at night could double the NmF2 at 0400 LT and increase hmF2 by 20 km. NmF2 could also be increased at night by realistically increasing the TIGCM nighttime downward fluxes of O+ at the upper boundary.
Ionospheric drift similarities at magnetic conjugate and nonconjugate locations
Authors: Takami T. , Oliver W. L. , Richmond A. D. , Fukao S.
Published: February 2004

Suggestions in the literature indicate that the observed similarity in ionospheric drifts between Arecibo‐summer and Shigaraki‐winter and again between Arecibo‐winter and Shigaraki‐summer are due to conjugate effects owing to the circumstance that the Shigaraki magnetic conjugate point has the same latitude as does Arecibo (in the opposite hemisphere). Here we develop this theory further from one of association to one of cause and effect. We base our explanation on the control of drifts at conjugate locations resting with the location of higher electrical conductance and suggest that this usually results in practice in summer hemisphere control of the winter hemisphere at night, with the daytime case being more complicated. Consequences of our theory are that the Arecibo‐summer Shigaraki‐winter similarity is, indeed, due to conjugate effects but that the Arecibo‐winter Shigaraki‐summer similarity has little to do with conjugacy. We combine empirical models of thermosphère and ionosphere structure and dynamics to test our conceptual ideas. We make predictions for such reverse‐season similarities for other pairs of incoherent scatter radar locations.
A two‐dimensional model of the Venus ionosphere: 2. Magnetized ionosphere
Authors: Shinagawa H.
Published: February 2004

A time‐dependent two‐dimensional MHD model of the ionosphere of Venus has been developed in order to study the magnetized ionosphere of Venus. The time‐dependent continuity, momentum, and magnetic induction equations are solved self‐consistently. Using this model, two‐dimensional profiles of magnetic fields, electron densities, and velocities are obtained. It is found that the calculated horizontal velocities in the magnetic barrier are of the order of 10 km/s, which is consistent with the Pioneer Venus Orbiter (PVO) observations. Horizontal and vertical variations of the electron density, and the magnetic field are reproduced reasonably well. It is shown that above 240 km, both vertical and horizontal flows play an important role in transporting magnetic flux as well as in determining electron density profiles. Below 240 km, on the other hand, the behavior of the magnetic field is controlled primarily by the vertical motion because the horizontal transport is inhibited by the ion‐neutral drag in the lower region. The overall magnetic field profiles in the model are quite similar to those of one‐dimensional MHD models. The calculated electron densities of the nightside ionosphere are smaller than those for unmagnetized conditions because of less plasma transport from the dayside.
Saturation of the Farley‐Buneman instability via nonlinear electron E×B drifts
Authors: Oppenheim Meers , Otani Niels , Ronchi Corrado
Published: February 2004

The Farley‐Buneman instability is a collisional two‐stream instability observed in the E region ionosphere at altitudes in the range 90–120 km. While linear theory predicts the dominant wavelengths, it cannot fully describe the behavior of this nonlinearly saturated instability, as observed by radar and rocket measurements. This paper explores the nonlinear behavior of this phenomenon and the resulting waves through simulations and theory. Our two‐dimensional simulations model wave behavior in the plane perpendicular to the Earth's magnetic field, applying a fluid model to describe the electron dynamics and either a particle or a fluid model to describe ion behavior. The results show the growth, saturation, and nonlinear behavior of the instability for a much longer period of time than was possible with the pure particle codes used in previous studies. These simulations show (1) growth of Farley‐Buneman waves, (2) the development of secondary waves which propagate along the extrema and perpendicular to the Farley‐Buneman waves, (3) turning of the primary waves away from the electron drift direction, (4) a saturated wave phase velocity below the one predicted by linear theory but above the acoustic speed and (5) nonlinear electron E×Bo drifting dominates the behavior of the saturated waves. This paper describes both the simulation techniques and fundamental results. Additionally, this paper outlines a theory explaining the dominant nonlinear process seen in this instability.
A numerical study of the wind field effect on the growth and observability of equatorial spread F
Authors: Chou S. Y. , Kuo F. S.
Published: February 2004

The neutral wind field effects on the development of the equatorial plasma bubbles have been simulated by a two‐dimensional time‐dependent model similar to that developed by Zalessak and Ossakow. The results indicate that when there exists no neutral wind, any perturbation on the bottom‐side of the ionosphere density profile will be amplified by the gravitational Rayleigh‐Taylor (GRT) instability and an upwelling bubble will form as expected. When a zonal neutral wind field exists, the uplift velocity of the bubble will be enhanced by a uniform neutral wind, but suppressed by vertical shear of the wind field. Secondary structures called plumes will grow out from the side walls of the primary bubble if some secondary perturbation signal with vertical structure is seeded. We notice that patches and multiple plume structures have been observed in the mid‐latitude and low‐latitude ionosphere. So we have developed a two‐dimensional local theory of the generalized GRT instability to calculate the growth rate of a small‐scale perturbation signal in the vicinity of the primary bubble and found that the growth rate increases with time along with the growth of the bubble. It is proved that the irregularities generated by some of our simulations should be able to cascade into observable (e.g., by radar) turbulence in a reasonable period of time after their generation. Similar to the effect on the growth of the primary bubble, a strong uniform wind field may enhance the growth rate of a perturbation signal in the vicinity of the bubble, while a strong vertical wind shear will suppress the growth rate. The main point of this theory is that the growth rate of the GRT instability is controlled by the density gradients and the magnitudes of the velocity of the plasma motion relative to the neutral wind in both directions. In addition, our simulations have proved that a seeding wave with phase velocity matching the background wind speed will generate the fastest growing bubble, disapproving the theory of spatial resonance.
Ulysses observations of differential alpha‐proton streaming in the solar wind
Authors: Neugebauer M. , Goldstein B. E. , Smith E. J. , Feldman W. C.
Published: February 2004

Data from the solar wind spectrometer on the Ulysses spacecraft are used to study the differential streaming between the alpha particles and protons in the solar wind over the heliographic distance range of 1.3 to 5.4 AU and latitudes from 0° to ±80° during the period December 1990 through September 1995. The study is based on 6‐hour averages of the parameter Vαp = |Vα ‐ Vp| where Vα and Vp are the vector velocities of the alpha particles and protons, respectively. It is found that Vαp decreases with increasing distance from the Sun and with decreasing solar wind speed. The distance and velocity dependencies can be combined into a single dependence on travel time Tfrom the Sun to the point of observation, with Vαp declining, on the average, as T−0.70±0.07. After normalization by this travel time factor, there is no residual dependence of Vαp, on heliographic latitude thus ruling out any rotational effects on either the acceleration or deceleration of the alphas relative to the protons. There is also no significant difference in the normalized values of Vαp between quasi‐stationary and transient (coronal mass ejection) flows. The ratios Vαp/VA, where VA is the Alfvén speed, and Vαp/Vwave, where Vwave is the observed propagation speed of Alfvénic fluctuations, both decline with increasing distance from the Sun, but Vαp/Vwave remains in the range of 1.0 to 1.5 out to a travel time of 5 or 10 days. There are weak correlations between the normalized value of Vαp and the amplitudes of fluctuations in both the magnitude and the direction of the interplanetary magnetic field. Although Vαp anticorrelates strongly with the ratio of the Coulomb collision time to the solar wind expansion time, it is believed that the correlation is not evidence of a cause and effect relation between those two parameters over much of the solar wind regime observed by Ulysses. Where comparisons are possible, the Ulysses data closely agree with extrapolations of the Helios data to greater solar distances.
Statistical patterns of high‐latitude convection obtained from Goose Bay HF radar observations
Authors: Ruohoniemi J. M. , Greenwald R. A.
Published: February 2004

We have derived patterns that describe the statistical interplanetary magnetic field (IMF) dependencies of ionospheric convection in the high‐latitude region of the northern hemisphere. The observations of plasma motion were made with the HF coherent backscatter radar located at Goose Bay, Labrador, over the period September 1987 to June 1993. The area covered by the measurements extended poleward of 65°Λ to a working limit of about 85°Λ. Distributions of electrostatic potential have been derived and expressed as series expansions in spherical harmonics. The patterns are the first derived from direct ground‐based observations of ionospheric convection that approach in completeness and level of detail the patterns derived in recent satellite studies [Rich and Hairston, 1994; Weimer, 1995]. We show the dependence of the convection on IMF angle in the GSM y–z plane for three intervals of IMF magnitude in this plane. Except for predominantly northward IMF, the convection is primarily two‐cell. The dusk cell is larger in terms of both spatial extent and potential variation/The effect of IMF By is apparent in the global shaping of the cells and the orientation of the overall pattern in MLT; for By + (By−) the dusk (dawn) cell is more round (crescent‐shaped) and the pattern more rotated toward earlier MLTs. The By effect on the nightside convection is pronounced and is hemispherically antisymmetric, like the well‐known day side By effect. For IMF increasingly northward, the convection trajectories on the dayside become increasingly distorted, evolving through a three‐cell to a four‐cell circulation. The additional cells appear on either side of the noon meridian and result in sunward flow. The overall agreement with the results of the satellite studies is good and extends to quite fine detail in the case of the comparison with Weimer [1995]. There are significant differences with the statistical patterns derived from magnetometer measurements, which tend to show domination by the dawn rather than the dusk cell.
O+–O collision cross section and long‐term F region O density variations deduced from the ionospheric energy budget
Authors: Oliver W. L. , Glotfelty K.
Published: February 2004

Recent estimates of the O+–O collision cross section QO+–O have suggested that the commonly used value of Banks [1966b] is too low by a factor of 1.2–1.9. On the other hand, past deductions of the F region neutral atomic oxygen density [O] from incoherent scatter (IS) radar studies have been quite compatible with the original Banks value, and any serious adjustment of that value would cause serious disagreement between these radar [O] results and those of the mass spectrometer incoherent scatter (MSIS) model. We have derived the mean daytime value for the product of QO+–O and [O] for 153 days during the period 1970–1975 from Millstone Hill IS radar measurements of ionospheric temperature and density. If we assume that MSIS model [O] is correct on average over this period, we find that QO+–O is only 75% of the Banks value and 40 to 60% lower than other values recently proposed. We show that all previous derivations of [O] from IS radar energy‐budget studies are compatible with our value, and thus a discrepancy exists in QO+–O determination by different methods. We could make our QO+–O results compatible with the larger values noted above by allowing a neutral temperature gradient to exist at high altitudes, where none is currently expected, or by increasing the heat input to the ion gas. We note that both of these effects could be produced by a sufficient reservoir of hot oxygen in the topside of the ionosphere. Independent of the way in which we choose a value of QO+–O to make the radar and MSIS [O] values agree on average over our 6‐year span of data, the radar record shows [O] to have episodic departures from MSIS predictions reaching amplitudes of up to 50% and lasting for periods of several months. This argues for caution in analyses which depend crucially upon the validity of MSIS densities during campaign‐like experimental studies.