bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction of ring current and radiation belt protons with ducted plasmaspheric hiss: 1. Diffusion coefficients and timescales
Authors: Kozyra J. U. , Rasmussen C. E. , Miller R. H. , Lyons L. R.
Published: February 2004

Protons that are converted into the inner magnetosphere in response to enhanced magnetic activity can resonate with ducted plasmaspheric hiss in the outer plasmasphere via an anomalous Doppler‐shifted cyclotron resonance. Plasmaspheric hiss is a right‐hand‐polarized electromagnetic emission that is observed to fill the plasmasphere on a routine basis. When plasmaspheric hiss is confined within field‐aligned ducts or guided along density gradients, wave normal angles remain largely below 45°. This allows resonant interactions with ions at typical ring current and radiation belt energies to take place. Such field‐aligned ducts have been observed both within the plasmasphere (Kozyra et al., 1987a; Koons, 1989) and in regions outside of the plasmasphere (Chan and Holzer, 1976). Wave intensities are estimated using statistical information from studies of detached plasma regions (Chan et al., 1974). Diffusion coefficients are presented for a range of L shells and proton energies for a fixed wave distribution. Harmonic resonances in the range n = ±100 are considered in order to include interactions between hiss at 100 Hz to 2 kHz frequencies, and protons in the energy range between ∼10 keV and 1000 keV. Diffusion timescales are estimated to be of the order or tens of days and comparable to or shorter than lifetimes for Coulomb decay and charge exchange losses over most of the energy and spatial ranges of interest.
Electron acceleration at nearly perpendicular collisionless shocks: 3. Downstream distributions
Authors: Krauss‐Varban D.
Published: February 2004

Spacecraft observations at the Earth's bow shock and at interplanetary shocks have established that the largest fluxes of accelerated suprathermal electrons occur in so‐called shock spike events immediately downstream of the shock ramp. Previous theoretical efforts have mainly focused on explaining upstream energetic electron beams. Here we investigate the general motion and acceleration of energetic electrons in a curved, nearly perpendicular shock by numerically integrating the orbits of solar wind halo electrons in shock fields generated by a hybrid simulation (core electron fluid and kinetic ions). Close to the angle θBn = 90° between the upstream magnetic field and shock normal, the calculations result in a (perpendicular) temperature increase proportional to the magnetic field ratio and give the highest phase space densities in the overshoot. For a steep distribution, the temperature change can correspond to an enhancement of the distribution by several orders of magnitude. These results are in agreement with predictions from adiabatic mapping. With smaller angles θBn, the overshoot and downstream densities fall off quickly, because the adiabatic energy gain is less and fewer electrons transmit. The shock curvature also leads to an accumulation of electrons close to 90°. Without pitch angle scattering, energization is only significant within a few (∼ 5° to 10°) degrees of the point of tangency. However, shock spike events appear to be observed more easily and farther away from 90°. Given that over a region of several degrees around 90° the theory gives enhancements of up to ∼ 4 orders of magnitude, such electrons could in principle account for the typically observed enhancements of 1 to 2 orders of magnitude, if they were distributed over θBn. To test the idea that scattering could efficiently redistribute the energetic electrons, we have conducted test particle simulations in which artificial pitch angle scattering is included. We find that such a process can indeed be very efficient and can explain observations of shock spike events far away from θBn ∼ 90°. The scattering naturally leads to much higher phase space densities at smaller θBn than what a local one‐dimensional mapping would predict and thus can account for an observed discrepancy with adiabatic theory stated in the literature.
A mechanism of formation of pulsating aurorae
Authors: Demekhov A. G. , Trakhtengerts V. Yu.
Published: February 2004

We consider a mechanism of pulsating aurora formation relevant for the substorm recovery phase. It is based on the spikelike regime of whistler cyclotron instability in auroral ducts with enhanced plasma density, elongated with geomagnetic field lines. We present the rigorously derived self‐consistent quasi‐linear theory of flow cyclotron maser operation, which describes spikelike regimes of whistler wave generation and energetic particle precipitation in a duct. We show that pulsations are formed due to broadening of the resonance region in velocity space in the course of instability development. We have obtained theoretical estimations for temporal characteristics of pulsating auroral patches. Pulsating regime is realized when the energetic electron number density is below some critical value depending on background plasma density and particle source parameters. Pulsation “on” time is determined by the nonlinear instability dynamics whereas “off” time is nearly equal to the time of reaching the wave excitation threshold. We compare our analytical and numerical results with known experimental data and show that the model explains reasonably most observations concerning pulsation time pattern, electron precipitation fluxes, VLF wave spectrum dynamics, latitudinal and longitudinal dependencies of pulsation characteristics, etc. We have made also preliminary estimations of nonlinear particle trapping effects during the maximum of the on phase that can give rise to a fine temporal structure of pulsations.
Investigation of S3‐2 satellite data for local time variation of energetic electron precipitation
Authors: Robbe S. , Sheldon W. R. , Benbrook J. R. , Bering E. A. , Vampola A. L.
Published: February 2004

Data on precipitating electrons from the S3‐2 satellite were investigated for local time variation at four L = 4 stations in the southern hemisphere. The equatorial pitch angles of electrons mirroring at 100 km, assumed to be the edge of the bounce loss cone, are calculated for L = 4 using the International Geomagnetic Reference Field for the epoch of the S3‐2 data, along with the variation in mirror altitude per degree of equatorial pitch angle. The largest obstacle to the investigation was uneven sampling in terms of local time for all of the stations. However, this situation was improved upon by the use of S3‐2 measurements at the conjugate locations of the four stations which provided additional data on electrons in the southern hemisphere bounce loss cone. Evidence for an effect of the dawn‐to‐dusk geoelectric field was found at two of the stations, Halley Bay and Siple, in the form of a minimum in electron precipitation at dusk. However, the present study does not completely resolve the question of local time modulation of electron precipitation at L = 4 in the southern hemisphere. Furthermore, while the average precipitation was lowest at the Kerguelen site, as would be expected on the basis of drift loss cone (DLC) theories, the intensity at that site exceeds the level that is expected on the basis of these DLC theories.
Imaging the effect of dipole tilt on magnetotail boundaries
Authors: Hammond C. Max , Kivelson Margaret G. , Walker Raymond J.
Published: February 2004

Eight years of IMP 8, four years of ISEE 2, and one year of IMP 7 magnetometer data have been combined to produce an "image" of the average magnetic field for a YZ cross section (aberrated GSM) of the magnetotail at a downtail distance of 25 RE. The shape of the neutral sheet and magnetopause boundaries can be observed directly from the images. A fitting function that qualitatively matches the observed boundary shape can then be chosen. This approach improves on previous fits to possibly unsuitable functional forms specified independently of the data. In addition, as a refinement of previous studies, we have corrected for varying solar wind dynamic pressure and the effects of tail flaring. We find the magnetopause is displaced above the XY plane with increasing dipole tilt. The neutral sheet is found to curve slightly more than the model of Fairfield (1980) during times of large dipole tilt and near the flanks appears to differ substantially from the neutral sheet shape given by the analytic model of Voigt [1984], the more recent neutral sheet model of Dandouras (1988) based on the Voigt model, and the semi‐empirical model of Tsyganenko (1989).
A mechanism of magnetic flux rope formation in the ionosphere of Venus
Authors: Kleeorin N. , Rogachevskii I. , Eviatar A.
Published: February 2004

Magnetic field observations in the dayside ionosphere of Venus revealed the existence of magnetic flux ropes. The general properties of these small‐scale magnetic field structures can be explained by a theory of magnetic fluctuations excited by random magnetohydrodynamic flows of ionospheric plasma. The local spatial distribution of the magnetic field is random: the field is concentrated inside flux tubes separated by regions with weak fields. A mechanism of amplification of magnetic fluctuations in the presence of zero mean field, proposed by Zeldovich, is applied to the nonlinear theory of flux rope formation by means of a nonlinear equation derived from the induction equation; the nonlinearity is associated with the Hall effect. The equation describes the evolution of the correlation function of the magnetic field and resembles the Schrödinger equation except for a variable mass and the absence of the imaginary unit in the time‐derivative term. In the limit of large Reynolds number the formulation is amenable to treatment by a modified WKB method. On the basis of this theory it is possible to explain why flux ropes are not observed if there is a strong regular large‐scale magnetic field caused by the lowering of the ionopause. The theory predicts correctly the cross section of the flux ropes in the ionosphere of Venus and the maximum value of the magnetic field inside the flux rope.
Ionospheric footprint of magnetosheathlike particle precipitation observed by an incoherent scatter radar
Authors: Watermann Jurgen , Lummerzheim Dirk , de la Beaujardière Odile , Newell Patrick T. , Rich Frederic J.
Published: February 2004

We have examined Sondrestrom incoherent scatter radar observations of ionospheric plasma density and temperature distributions and measurements of F region ion drifts that were made during a prenoon pass of the DMSP‐F7 satellite through the radar field of view. The spacecraft traversed a region of intense electron precipitation with a characteristic energy below approximately 200 eV. Particles with such low characteristic energies are believed to be directly or indirectly of magnetosheath origin. The precipitation region had a width of about 2° invariant latitude and covered the low‐latitude boundary layer (LLBL), the cusp, and the equatorward section of the plasma mantle (PM). The corotating radar observed a patch of enhanced electron density and elevated electron temperature in the F2 region between about 10.5 and 12 magnetic local time in the same invariant latitude range where DMSP‐F7 detected the soft‐electron flux. The ion drift pattern, also obtained by radar, shows that it is unlikely that the plasma patch was produced by solar radiation and advected into the radar field of view. We suggest that the radar observed modifications of the ionospheric plasma distribution, which resulted from direct entry of magnetosheath electrons into the magnetosphere and down to ionospheric altitudes. Model calculations of the ionospheric response to the observed electron precipitation support our interpretation. The spectral characteristics of the electron flux in the LLBL, cusp, and equatorward section of the PM were in this case too similar to allow to distinguish between them by using incoherent scatter radar measurements only.
Dispersion and wave coupling in inhomogeneous MHD waveguides
Authors: Wright Andrew N.
Published: February 2004

The propagation of the fast mode is considered in nonuniform waveguides. We show how the natural dispersion inherent in a waveguide will select waveguide modes with a small wavenumber (ky) along the guide to remain near a localized source region of fast mode energy. It is these modes that are shown to have a coherent periodic time dependence over many cycles that are suitable for driving observable Alfvén resonances (magnetic pulsations). We expect the frequencies of Alfvén resonances to be very close to the eigenfrequencies of waveguide modes with ky = 0.
Particle and field signatures during pseudobreakup and major expansion onset
Authors: Nakamura Rumi , Baker Daniel N. , Yamamoto Tatsundo , Belian Richard D. , Bering Edgar A. , Benbrook James R. , Theall Jeffrey R.
Published: February 2004

The temporal and spatial scales of the onset of two types of substorm events are investigated. These substorms were cases where the expansion onset had precursor localized auroral activation without significant negative bay enhancement, that is, “pseudobreakup”. High‐resolution energetic particle and magnetic field data at synchronous orbit are used for the analysis together with auroral and magnetic field data simultaneously taken from ground‐based instrumentation. The auroral structure following the pseudobreakup significantly resembled the major expansion aurora, except in its spatial scale. Typical magnetospheric onset signatures such as tail current diversion, dipolarization, and injection were observed associated with some of the pseudobreakups. The major expansion, on the other hand, consisted of a number of rather localized injections and expansions, each of which had timescales of 2‐8 min, a comparable timescale to that of pseudobreakups. This study shows that there does not appear to be any phenomenological differences between pseudobreakups and major expansion onsets. The major difference between pseudobreakups and major expansion onsets would be the number of occurrences, as well as the intensity and the scale size of the magnetospheric source.
Sudden impulses at low‐latitude stations: Steady state response for northward interplanetary magnetic field
Authors: Russell C. T. , Ginskey M. , Petrinec S. M.
Published: February 2004

An examination of the response of the low‐latitude H component of the Earth's magnetic field during the passage of interplanetary shocks when the interplanetary magnetic field is northward reveals that this response can be understood quantitatively in terms of the compression of a simple vacuum magnetospheric model. The compression at the surface of the Earth at 20° latitude at noon in the absence of equatorial electrojet effects is found to be 18.4 nT/(nPa)1/2. Stations below 15° latitude and above 40° appear to have additional but variable sources of current which magnify this effect. The diurnal variation of the compression is larger than expected from the simple vacuum magnetosphere, ±20% about the mean instead of ±10%. We interpret this difference to indicate that tail currents, not in the vacuum model, are as important as the magnetopause currents in determining the diurnal variation of the field at the surface of the Earth.
On the origin of narrow non‐ion‐acoustic coherent radar spectra in the high‐latitude E region
Authors: St.‐Maurice J. ‐P. , Prikryl P. , Danskin D. W. , Hamza A. M. , Sofko G. J. , Koehler J. A. , Kustov A. , Chen J.
Published: February 2004

Many types of coherent radar spectra have a width in Doppler velocity units that is less than the ion acoustic speed of the medium. In spectra labeled as type 1 the mean Doppler shift of these narrow spectra matches the ion acoustic speed of the medium. There also exist narrow high‐latitude spectra for which the mean Doppler shift is either markedly less or markedly more than the ion acoustic speed. We propose that electron density gradients with scale lengths as small as 100 m are at the origin of a large fraction of these narrow spectra near 50 MHz. The sharp density gradients in that case are created in regions of discrete auroral precipitation associated either with multiple narrow arcs or with sharp edges of broader features. Using the same principle at radar frequencies in the 10‐ to 20‐MHz range, we find that gradient scales from 20 to 30 km in size create a combination of fast and slow phase velocities closely resembling the spectral characteristics expected from NO+ ion cyclotron waves. However, gradients are not always responsible for slow narrow spectra; a detailed analysis of available observations has led us to conclude that the high‐latitude E region cannot always be considered as fully turbulent even when appreciable coherent echo returns are registered by the radars. In particular, slow narrow spectra at 50 MHz are at times produced under gradient‐free weakly turbulent conditions. In addition, at lower radar frequencies (10 to 20 MHz) the narrow spectral width of slowly moving waves and the morphology of these waves both suggest that the irregularities are generated indirectly via mode coupling of linearly unstable modes and that these “secondary” waves are themselves not coupling efficiently. This implies that processes other than mode coupling are contributing to the overall wave energy budget. In that case we suggest that the convective properties of the slowly growing modes are an important factor in removing wave energy, even for waves as small as a few meters in wavelength. We also propose that there may be two distinct generation mechanisms for secondary waves at 10 MHz, each with its own mean Doppler shift behavior.
High‐resolution radar measurements of turbulent structure in the equatorial electrojet
Authors: Swartz Wesley E. , Farley Donald T.
Published: February 2004

Observations of the equatorial electrojet were made with unprecedented altitude resolution at the Jicamarca Radio Observatory in January 1990. We obtained high‐quality Doppler spectra with 250‐m and sometimes better altitude resolution at heights from 95 to 120 km during the daytime and early evening with the large 50‐MHz radar. This resolution and coverage revealed a richness of dynamics and structure not previously appreciated, a structure that is sometimes reminiscent of VHF radar maps of equatorial spread F, albeit at smaller primary scale sizes. Individual scattering centers sometimes appear to drift in large swirls over many kilometers in altitude, maintaining their identity for times occasionally as long as a minute or two. On the other hand, we also see rapid variations with altitude (a kilometer or less) and time (a few seconds or less) in the Doppler spectra and signal power.
High‐resolution radar observations of daytime kilometer‐scale wave structure in the equatorial electrojet
Authors: Farley D. T. , Swartz W. E. , Hysell D. L. , Ronchi C.
Published: February 2004

Using a new antenna configuration and new data processing hardware, we probed the electrojet with unprecedented spatial resolution in January 1991 at Jicamarca. The altitude resolution was 125 m, and the east‐west antenna beamwidth was about 0.6°, or about 1 km at electrojet altitudes. The high spatial resolution allowed us to observe, at certain altitudes and times, pronounced wavelike structures with vertical wavelengths of the order of 1 km and periods of tens of seconds. These observations closely resemble the predictions of recent nonlocal, nonlinear numerical simulations of large‐scale turbulence in the equatorial electrojet (Ronchi et al., 1991). The comparison demonstrated the validity of the theory and confirmed the idea that the Doppler power spectra of type 2 echoes at 50 MHz (due to 3‐m plasma waves generated by the gradient‐drift instability and a nonlinear cascade of energy from long‐wavelength modes to short) are controlled primarily by the large‐scale dynamics and not by the nonlinear local growth and decay rates of the 3‐m waves themselves.
On the sources of interplanetary shocks at 0.72 AU
Authors: Lindsay G. M. , Russell C. T. , Luhmann J. G. , Gazis P.
Published: February 2004

In order to understand the solar cycle variation of interplanetary shocks and their driving source at 0.72 AU, a survey of Pioneer Venus Orbiter (PVO) magnetometer and plasma data from 1979‐1988 has been conducted. Known shock drivers at 1.0 AU include coronal mass ejections (CMEs) and fast/slow stream interactions. In our analysis, CMEs were identified by a decrease in plasma temperature to background or below accompanied by an increase in plasma density and dynamic pressure. It was also required that the magnetic field exhibit a coherent rotation over about a day and an increase and decline in magnitude on a timescale of hours to days. Stream interactions were identified by a characteristic increase in ion temperature and velocity coincident with a decrease in density and a coincident increase in the total magnetic field magnitude. These signatures were usually preceded within 24 hours by a change in flow angle. In all, 45 shocks were identified: 36 driven by CMEs, 6 resulting from fast/slow stream interactions, and 3 with sources that could not be defined. The shocks driven by CMEs show a solar cycle variation that roughly follows the sunspot number. These shocks all have normals consistent with radial propagation of the shock fronts from the sun. In contrast, the few stream interaction related shocks show a tendency to occur later in the solar cycle and have a broader distribution of shock normals.
F region meridional wind spectral behavior
Authors: Giraldez A. , Canziani P.
Published: February 2004

Analyses of power spectral density slope and integrated power (energy) of upper thermospheric meridional wind for eight ionospheric stations in both hemispheres during a full year, have been performed. The observed spectral slope ranges between 1.2 and 3, depending on station location and epoch of the year, presenting levels and variability mostly related with geomagnetic latitude. However, the associated spectral energy seems to depend on geomagnetic latitude as well as on season at some latitudes, but in a different way from the spectral slope. Both parameters seem to be uncorrelated, suggesting that their evolution depends on different physical mechanisms. There is not significant correlation between the spectral slope and energy parameters with total meridional wind, indicating that they are associated with small‐scale phenomena rather than with total wind.
By‐controlled convection and field‐aligned currents near midnight auroral oval for northward interplanetary magnetic field
Authors: Taguchi S. , Sugiura M. , Iyemori T. , Winningham J. D. , Slavin J. A.
Published: February 2004

Using the Dynamics Explorer (DE) 2 magnetic and electric field and plasma data, By‐controlled convection and field‐aligned currents in the midnight sector for northward interplanetary magnetic field (IMF) are examined. The results of an analysis of the electric field data show that when IMF is stable and when its magnitude is large, a coherent By‐controlled convection exists near the midnight auroral oval in the ionosphere having adequate conductivities. When By is negative, the convection consists of a westward (eastward) plasma flow at the lower latitudes and an eastward (westward) plasma flow at the higher latitudes in the midnight sector in the northern (southern) ionosphere. When By is positive, the flow directions are reversed. The distribution of the field‐aligned currents associated with the By‐controlled convection, in most cases, shows a three‐sheet structure. In accordance with the convection the directions of the three sheets are dependent on the sign of By. The location of disappearance of the precipitating intense electrons having energies of a few keV is close to the convection reversal surface. However, the more detailed relationship between the electron precipitation boundary and the convection reversal surface depends on the case. In some cases the precipitating electrons extend beyond the convection reversal surface, and in others the poleward boundary terminates at a latitude lower than the reversal surface. Previous studies suggest that the poleward boundary of the electrons having energies of a few keV is not necessarily coincident with an open/closed boundary. Thus the open/closed boundary may be at a latitude higher than the poleward boundary of the electron precipitation, or it may be at a latitude lower than the poleward boundary of the electron precipitation. We discuss relationships between the open/closed boundary and the convection reversal surface. When as a possible choice we adopt a view that the open/closed boundary agrees with the convection reversal surface, we can explain qualitatively the configuration of the By‐controlled convection on the open and close field line regions by proposing a mapping modified in accordance with IMF By.
One‐dimensional multispecies magnetohydrodynamic models of the ramside ionosphere of Titan
Authors: Keller C. N. , Cravens T. E. , Gan L.
Published: February 2004

The interaction of the Saturnian magnetosphere with the ionosphere of Titan has much in common with the solar wind interaction with nonmagnetic bodies in general. A one‐dimensional magnetohydrodynamic model was developed, for radial distances of 1.3 ‐ 2 Titan radii (RT), to study the dynamics of H2CN+, CnHm+ (n,m > 2), C2H5+ and CH5+, in the ramside ionosphere of Titan. For the case of a dayside ramside ionosphere (solar zenith angle of 60°) we found that the external magnetospheric plasma interacting with Titan's ionosphere produced a very broad region of ionospheric magnetic field whose peak field strength was ≈ 26 nT. The ionosphere was magnetized well below the ionospheric peak, allowing ionizing Saturnian magnetospheric electrons access to this region. The sources of ionization and the electron temperature profiles incorporated into the model depend on the relative locations of Titan, Saturn, and the Sun. The calculated electron and ion density profiles as well as the magnetic field profile also depend on the relative positions of Titan, Saturn, and the Sun.
Anomalous ion mixing within an MHD scale Kelvin‐Helmholtz vortex
Authors: Fujimoto M. , Terasawa T.
Published: February 2004

Hybrid code simulations (ion particles, charge neutralizing electron fluid) of the MHD scale transverse Kelvin‐Helmholtz (K‐H) instability in a uniform plasma have been conducted. On a macroscopic scale the saturation states are found to be similar to the MHD result, even if the initial shear layer half thickness is as thin as twice the ion thermal Larmor radius. However, as opposed to the conventional belief based on the frozen‐in concept that a large‐scale mixing will be prohibited, an enhanced mixing of ions across the shear layer is found to occur inside the vortex. The temporal/spatial scales of the mixing process are determined by the fluid dynamical parameters and are anomalously quick/large compared to what can result from the mixing due to the finite Larmor radius overlapping at the interface. We propose that the low latitude boundary layer just inside the magnetopause is the mixing layer formed possibly by this mechanism.
The solar cycle variation of coronal mass ejections and the solar wind mass flux
Authors: Webb David F. , Howard Russell A.
Published: February 2004

Coronal mass ejections (CMEs) are an important aspect of coronal physics and a potentially significant contributor to perturbations of the solar wind, such as its mass flux. Sufficient data on CMEs are now available to permit study of their longer‐term occurrence patterns. Here we present the results of a study of CME occurrence rates over more than a complete 11‐year solar sunspot cycle and a comparison of these rates with those of other activity related to CMEs and with the solar wind particle flux at 1 AU. The study includes an evaluation of corrections to the CME rates, which include instrument duty cycles, visibility functions, mass detection thresholds, and geometrical considerations. The main results are as follows: (1) The frequency of occurrence of CMEs tends to track the solar activity cycle in both amplitude and phase; (2) the CME rates from different instruments, when corrected for both duty cycles and visibility functions, are reasonably consistent; (3) considering only longer‐term averages, no one class of solar activity is better correlated with CME rate than any other; (4) the ratio of the annualized CME to solar wind mass flux tends to track the solar cycle; and (5) near solar maximum, CMEs can provide a significant fraction (i.e., ≈ 15%) of the average mass flux to the near‐ecliptic solar wind.
Comparison between Liouville's theorem and observed latitudinal distributions of trapped ions in the plasmapause region
Authors: Olsen R. C. , Scott L. J. , Boardsen S. A.
Published: February 2004

The presence of anisotropic plasma distributions, trapped at the Earth's magnetic equator, has consequences for the electric field structure which must exist in equilibrium along the magnetic field line. Data from SCATHA and Dynamics Explorer 1 indicate that the core ion distributions at the magnetic equator can be well described as bi‐Maxwellian distributions, with a perpendicular temperature an order of magnitude larger than the parallel temperature. A collisionless model is developed for the variation in plasma parameters, following the forms developed by Whipple (1977). If the core electron anisotropy is low, the resulting electric field of ∼0.1 µV m−1 is pointed away from the equator. Under these conditions the self‐consistent electric field will not overcome the effects of magnetic trapping. The resulting potential distribution results in a local maximum in total plasma density at the equator. Only when the electron distribution is primarily field‐aligned can there be a density minimum at the equator. Comparisons are made between this model and the observed variations in DE 1 plasma parameters with latitude.
Thermodynamic effect of the ion sound instability in the ionosphere
Authors: Khazanov G. V. , Gombosi T. I. , Gorbachev O. A. , Trukhan A. A. , Miller R. H.
Published: February 2004

During geomagnetic disturbances when the ring current interacts intensely with the plasmasphere, the plasma of this region undergoes a strong heating due to an ion cyclotron instability. This is followed by the transfer of heat along geomagnetic field lines from the heating region to the ionosphere. One of the results of this process is the formation of a non isothermal region (in which Te >3.4 Ti at ionospheric heights) caused by a rapid cooling the H+ ions due to their resonant charge exchange with neutral hydrogen. Heat transfer from the top of the flux tube to the ionosphere is investigated using a hydrodynamic model for the ionosphere‐plasmasphere coupling. Field‐aligned currents, present in the topside ionosphere, are often accompanied by ion sound turbulence. The turbulence scatters electrons, increasing the total electron collision frequency through wave‐particle effects. The influence of wave‐particle interactions introduces an anomalous component to the total collision frequency, which modifies substantially the heat conduction coefficient of the plasma. As a result, the plasma is heated more intensely above than below this region of ion sound turbulence.
Observational differences between flux transfer events and surface waves at the magnetopause
Authors: Song P. , Le G. , Russell C. T.
Published: February 2004

Whether the phenomenon known as a flux transfer event (FTE) is caused by a special process acting at the magnetopause such as transient reconnection or whether it is simply a double crossing of the magnetopause caused by a pressure pulse has become controversial. Theoretical investigations have shown that there are some similarities in the flow and field perturbation between a magnetopause surface wave and a converting cylindrical structure. This result by itself does not distinguish the two possibilities. We investigate the observed differences between FTEs and surface waves. In general, a realistic magnetopause surface wave has a much longer wavelength than the displacement, which is different from some models of surface waves. The scales of an FTE along the convection direction and normal to the magnetopause are of the same order. Spectral analysis of the soalr wind and upstream waves shows no evidence of pressure variations of the duration and intensity to generate the magnetopause surface wave of the required wavelength of few Earth radii and a displacement of an Earth radius. The difference between a surface wave and a series of FTEs can be easily recognized in the profiles of the magnetic field and plasma parameters. The FTEs look like a series of pulses, whereas multiple crossings of the magnetopause occur with a changing ratio of the duration of magnetosheath and magnetospheric entry. The timing difference between two spacecraft during magnetopause crossings is significantly greater than the timing difference of FTEs. For typical magnetopause crossings the perturbations in the normal component and magnitude of the magnetic field is not more than background magnetosheath fluctuations; but those for FTEs are often much greater. The reinterpretation of FTEs as surface waves arose without a quantitative assessment of the observed perturbations of the magnetopause and quantitative effects of proposed sources. When such a quantitative assessment is performed, it is clear that FTEs are not simply magnetopause crossings.
Observation of the modified two‐stream plasma instability in the midlatitude E region ionosphere
Authors: Schlegel K. , Haldoupis C.
Published: February 2004

We present the first clear evidence of the occurrence of the modified two stream (Farley‐Buneman) instability and excitation of pure type 1 irregularities in the midlatitude ionospheric E region. The observations are made with a bistatic 50‐MHz Doppler radio experiment set up recently in Crete, Greece. The system can perform high‐frequency resolution coherent backscatter measurements along a fixed direction, from 3‐m magnetic aspect sensitive irregularities inside a limited ionospheric volume in the E layer at the invariant geomagnetic latitude of 30.8° (L = 1.35). The observations presented here are from an event of backscatter characterized by large Doppler motions caused, presumably, by an impulsive electric field reaching a magnitude at least 14 mV/m. Apparently, the unusually high electron drifts along the radar viewing direction were sufficient in this case to excite pure Farley‐Buneman waves. This had been manifested convincingly by the measured power Doppler spectra, which are reminiscent of the typical spectral signature of type 1 echoes observed regularly in the equatorial 50‐MHz backscatter. Further, the spectral data confirmed the anticipation that in the midlatitude E region plasma exist two irregularity types corresponding to those of type 1 and type 2 echoes in the equatorial electrojet. The important difference with the equatorial results, however, is that the threshold conditions for the two‐stream instability are seldom met at moderate latitudes, thus the medium is highly suitable for studying secondarily generated short‐scale plasma turbulence.
Interaction between Alfvén waves and a multicomponent plasma with differential ion streaming
Authors: McKenzie J. F.
Published: February 2004

In this paper we describe the mutual interaction between Alfvén waves and a multicomponent plasma (such as the solar wind) consisting of various ion species streaming differentially with speeds ui along the ambient radial magnetic field. It is shown that the wave adiabatic invariant discovered by Isenberg and Hollweg (1982) for this multi‐ion system can be cast in the form of the conservation of wave action flux in which the “intrinsic” frequency ωm say, is simply the ratio between the mean squared Doppler‐shifted frequency and the mean Doppler‐shifted frequency both of which being weighted according to the mass density abundance of the various streaming ions. The existence of the invariant permits the self‐consistent formulation of steady flows in a multi‐ion system (such as the solar wind in which alpha particles are placed on the same footing as protons and not treated simply as test particles) subject not only to the coupling effect of an electric field or Coulomb friction but also to Alfvén wave forces, which also bind the system together. This type of coupling arises essentially because variations in the Alfvén wave pressure pω, or wave energy density Eω, are related to variations in the local hydrodynamic properties of the multi‐ion plasma flow in such a way as to conserve the wave action flux. Furthermore, if the composite multi‐ion system were subject to slow modulations of the medium in time as well as gradual variations in space, the Alfvén wave amplitude would be governed by a conservation equation for the wave action variable. This wave action equation enables us to investigate the properties of long‐wavelength/low‐frequency sound waves in a multi‐ion system subject to short‐wavelength/high‐frequency Alfvén wave forces, as well as the usual electrical coupling. It is important to carry out such an investigation since it is the condition for stationary sound waves in such a system that yields the appropriate generalization of the idea of a sonic point for a multi‐ion flow, thereby providing a physical framework within which the sonic critical curves may be discussed and analyzed. We have carried this out in the simplest case of extremely sub‐Alfvénic flow in order to illustrate its importance in the context of the initial acceleration of the solar wind plasma and to clarify the nature of possible streaming instabilities. Finally, in the ethos of the JWKB analysis developed here or by using the variational approach (Dewar, 1970; Whitham, 1974), it follows that similar wave action conservation equations hold for magnetoacoustic waves in multi‐ion plasmas.
Steepened structures in equatorial spread F: 2. Theory
Authors: Hysell D. L. , Seyler C. E. , Kelley M. C.
Published: February 2004

This paper investigates the properties of a one‐dimensional fluid model of plasma convection in the equatorial F region ionosphere. The model equations are similar in form to Burgers equation except for additional higher‐order spatial derivatives. Like Burgers equation, solutions to the model have the form of propagating, shocklike structures. Numerical simulations of the model closely resemble the steepened structures observed by sounding rocket plasma density probes within equatorial spread F. Simulated density power spectra, like the spectra computed from in situ data, seem to possess power law forms with a break at wavelengths of about 100 m. The precise wavenumber of the spectral break is determined by the ambipolar diffusion coefficient. The model predicts that electric field fluctuations perpendicular to the direction of plasma steepening should be proportional to the plasma density fluctuations. Electric field fluctuations parallel to the steepening will be due primarily to the ambipolar field and have a Boltzmann relationship with density (|δE|² ∼ k²|δn/n|²). At wavelengths less than about 300 m, the ambipolar field should be the dominant component of the total field intensity.
Comparison of hydrodynamic and semikinetic treatments for a plasma flow along closed field lines
Authors: Singh Nagendra , Wilson G. R. , Horwitz J. L.
Published: February 2004

Hydrodynamic and semikinetic treatments of plasma flow along closed geomagnetic field lines are compared. The hydrodynamic treatment is based on a simplified 16‐moment set of transport equations as the equations for the heat flows are not solved; the hear flows are treated heuristically. The semikinetic treatment is based on a particle code. The comparison deals with the distributions of the plasma density, flow velocity, and parallel and perpendicular temperatures as obtained from the two treatments during the various stages of the flow subject to certain assumed boundary conditions. In the kinetic treatment, the appropriate boundary condition is the prescription of the velocity distribution functions for the particles entering the flux tubes at the ionospheric boundaries; those particles leaving the system are determined by the processes occurring in the flux tube. The prescribed distributions are half‐Maxwellian with temperature To and density no. In the hydrodynamic model, the prescribed boundary conditions are placed on density (no), flow velocity (Vo), and temperature (To). We found that results from the hydrodynamic treatment critically depend on Vo; for early stages of the flow this treatment yields results in good agreement with those from the kinetic treatment, when Vo = (kTo/2πm)1/2, which is the average velocity of particles moving in a given direction for a Maxwellian distribution. During this early stage, the flows developing from the conjugate ionospheres show some distinct transitions. For the first hour or so, the flows are highly supersonic and penetrate deep into the opposite hemispheres, and both hydrodynamics and kinetic treatments yield almost similar features. It is found that during this period heat flow effects are negligibly small. When a flow penetrates deep into the opposite hemisphere, the kinetic treatment predicts reflection and setting up of counterstreaming. In contrast, the hydrodynamic treatment yields a shock in the flow. The reasons for this difference in the two treatments is discussed, showing that in view of the relatively warm ions, the coupling of ion beams and the consequent shock formation in the off‐equatorial region are not likely due to the enhancements in the beam temperatures. The counterstreaming in the kinetic treatment and the shock in the hydrodynamic treatment first advance upward to the equator and then downward to the ionospheric boundary from where the flow originated. The transit time for this advancement is found to be about 1 hour for the perspective models. After 2 hours or so, both models predict that the flows from the ionospheric boundaries are generally subsonic with respect to the local ion‐sound speed. At late stages of the flow, when a substantial fraction of ions entering the flux tube begin to return back in the kinetic treatment, the hydrodynamic treatment with the boundary condition Vo = (kTo/2πm)1/2 yields an overrefilling, and the choice of Vo becomes uncertain.
Properties of single‐particle dynamics in a parabolic magnetic reversal with general time dependence
Authors: Chapman Sandra C.
Published: February 2004

We investigate single charged particle dynamics in the earth's magnetotail by examining a reversal with simple spatial dependence (the field lines are parabolic) but with general time dependence, which includes the associated induction electric field. The parabolic spatial dependence has, in static models, been shown by previous authors to imply that various regular and stochastic regimes of behavior exist, with particles remaining in a given regime for all time t. Here we show that, in general, time dependence yields transitions in behavior between the various regimes of regular and stochastic behavior. We identify three independent parametric coordinates which are functions of the field spatial and temporal scales, and the particle gyroscales, and time, which will indicate the regime of particle behavior at any given t. For specific time dependent field models these parametric coordinates can be inverted to directly give the timescales for transitions between one regime of behavior and another in terms of the field and particle spatial and temporal scales. These parametric coordinates have no direct analogue in static models. For a general thinning and folding sheet (representing the magnetotail magnetic field close to the center plane just before substorm onset) this characterization implies that stochastic dynamics may only occur over a finite time period and may not occur under certain circumstances. This may have implications for our understanding of the role played by single particle dynamics in the destabilization or reconfiguration of the magnetotail current sheet associated with substorms.
Coordinated stable auroral red arc observations: Relationship to plasma convection
Authors: Foster J. C. , Buonsanto M. J. , Mendillo M. , Nottingham D. , Rich F. J. , Denig W.
Published: February 2004

During the March 20‐22, 1990, magnetic storm, Millstone Hill radar and DMSP satellite observations detailed the conditions surrounding the occurrence of a SAR are which was observed continuously through an 8‐hour interval from dusk till past midnight in the North American sector. All‐sky imaging with a 630.0‐nm imager continually monitored the two‐dimensional position and magnitude of the SAR arc emission while radar scans and satellite overflights measured magnetospheric inputs and ionospheric response. The arc Was colocated with a deep, narrow plasma trough and a region of enhanced westward plasma convection of similar width situated immediately equatorward of the low‐latitude extent of plasma sheet particle precipitation. A region of low‐energy ion precipitation was observed at the equatorward edge of the SAR arc during a period of spatial/temporal coincident satellite/radar observations near the Millstone Hill longitude. The width of the SAR arc and related phenomena was of the order of 2°, and the ∼200‐R emission was associated with an electron temperature of ∼3500°K and a 10x reduction of plasma density at an altitude of 450 km. The best‐fit model for the emission intensities of both the SAR arc and the background airglow suggests that either the electron temperature at the center of the SAR arc was somewhat higher than observed by the radar (∼4000°K), or the neutral densities, [O2] and [O], were increased by factors of 2 and 4, respectively, with respect to the MSIS values. The ionospheric trough and a colocated region of enhanced sunward convection (500 ‐ 1700 m s−1) were observed in conjugate hemispheres throughout the local time range 18 ‐ 02 MLT. The convection feature seen in association with the SAR arc had many of the characteristics of a subauroral ion drifts (SAID) event; we report here the first long‐duration observations of a colocated SAID/SAR arc event. A narrow ionospheric trough developed during the interval when the SAID velocity was >1000 m s−1 and was accompanied by a weak (100 R) 630.0‐nm emission. As the velocity fell to ∼700 m s−1, the density in the trough recovered somewhat, and the arc intensity rose to ∼300 R above background. This brighter period of the SAR arc occurred within a fossil trough/SAID. We conclude that there is a close spatial and temporal association among these three types of subauroral low‐altitude phenomena ‐ the SAR arc, the SAID event, and the fossil (convection‐related) trough ‐ and that this is indicative of the interrelationship of the magnetospheric processes and boundaries which are involved in their formation.
Plasma observations in the ring plane of Saturn
Authors: Gan‐Baruch Zameret , Eviatar Aharon , Richardson John D. , McNutt Ralph L.
Published: February 2004

We present the first detailed analysis of the Voyager 2 plasma science (PLS) observations in the vicinity of the ring plane of Saturn. We find three types of plasma in the region between z = 0.9 RS and z = −0.7 RS, namely, a tenuous and isotropic plasma derived from the large icy satellites, a dense and anisotropic plasma, and an anomalously anisotropic plasma in the ring plane region itself. We interpret these results to confirm a previously published hypothesis, according to which the natural Kronian inner magnetosphere plasma is derived from two sources, a tenuous plasma from the large icy satellites and a dense plasma from the atmosphere of the main rings. The anomalously anisotropic plasma is interpreted as the result of impacts of G ring ice motes. The in situ plasma measurements, thus, provide an additional delineation of the poorly observed G ring.