bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of electrostatic ion cyclotron wave instability on plasma flow during early stage plasmaspheric refilling
Authors: Singh Nagendra
Published: February 2004

Mesoscale models for the plasmasphere refilling reveal that the expansion of the ionospheric plasma into empty flux tubes during the early stage refilling produces highly supersonic counterstreaming ion beams. Such beams are found to meet the conditions for the instability of the electrostatic ion cyclotron (eic) waves in the equatorial plasmasphere. Since the spatial scale of the eic wave instability is determined by the ion Larmor radius and the corresponding timescale is determined by the ion cyclotron period, it is unresolvable by the crude spatial and temporal resolutions afforded by the mesoscale models. Thus the eic waves and the associated microprocesses are left out by the existing mesoscale refilling models based on the fluid and semikinetic approaches. I propose here an algorithm for incorporating the effects of eic waves on mesoscale plasma flow; the algorithm is based on the critical conditions for the eic instability. When the relative drift of the counterstreamig ion beams exceeds a critical value, depending on the beam temperatures, the excess momentum and energy are dissipated into thermal motion of the ions, resulting in ion heating in the degree of freedom perpendicular to the ambient magnetic field at the expense of the parallel ion drift. This leads to the creation of two distinct ion populations consisting of field‐aligned counterstreaming ions and perpendicularly heated warm anisotropic ions trapped in the flux tube. The latter ions are created during the first few hours of the refilling process primarily in the equatorial region, but eventually they spread out all over the flux tube. Highly anisotropic ions are seen near the equator by virtue of being locally created and trapped there. Estimates of the Coulomb collision time show that the warm anisotropic population (WAP) has a lifetime ranging from about 10 hours in the topside ionosphere to several days in the equatorial region. Thus once initially created during early stage refilling, WAP ions become a common feature of the outer plasmasphere. The coexisting multiple ion populations, consisting of anisotropic warm ions and the field‐aligned counterstreaming flows, have been observed from DE 1 on flux tubes undergoing refilling.
Application of the empirically derived polytropic index for the solar wind to models of solar wind propagation
Authors: Totten T. L. , Freeman J. W. , Arya S.
Published: February 2004

An empirically derived polytropic index for the solar wind is applied to two magnetohydrodynamic models of solar wind propagation. The first of these models is time‐independent and can be used to define an initial solar wind state for the second, time‐dependent model. The time‐dependent model simulates the propagation of disturbances, such as shocks that originate at the Sun, through the interplanetary medium to Earth as a space weather forecasting tool. Both models assume adiabatic flow of the solar wind. Changes are made to these models to include the effects of heating of the solar wind by applying the empirically derived polytropic index. The adjustments made to the time‐independent model produce a moderate solar wind state that compares well with Helios 1 data. Data concerning several shocks observed at the Sun and corresponding storm sudden commencements at Earth are used to investigate the predictions for shock arrival times made by the time‐dependent model. Changes in the predictions for shock arrival times resulting from the application of the empirically derived polytropic index for the solar wind are discussed.
Growth of short wavelength lower hybrid modes in a density depletion and transverse acceleration of ionospheric ions
Authors: Andrulis R. S. , Seyler C. E. , Otani N. F.
Published: February 2004

This paper proposes a possible means for ion acceleration in the topside auroral ionosphere. Specifically, it postulates that the interaction of lower hybrid waves with a small‐amplitude (1 – 2%) density cavity leads to an energy transfer to shorter‐wavelength modes. Such a mechanism may help explain the transverse acceleration of positive ions observed in this region. Both analytic and computer simulation results on the growth of these slow phase velocity waves will be presented. The resulting acceleration of test particle ions is consistent with the theory of quasi‐linear diffusion of the test particle distribution function. The results of the test particle simulations indicate that this effect may be strong enough to account for the observed ion acceleration. This model is consistent with the results from the Marie and TOPAZ II and III sounding rockets which indicate a strong correlation between transverse ion acceleration, depletions in the background density, and lower hybrid waves.
Electrostatic waves near the field reversal region in the Earth's magnetotail
Authors: Burinskaya T. , Pivovarov V. , Indenbom E. , Schriver D. , Ashour‐Abdalla M.
Published: February 2004

The general stability properties of ion distributions with velocity space holes of the type expected in the Earth's central plasma sheet as a consequence of chaotic ion acceleration is investigated. It is shown that such distributions are unstable to electrostatic waves with wave vectors located in a narrow angular region about the Sun‐Earth line. These waves can be generated in the magnetotail midplane even if the transition layer in velocity space between the background plasma and an empty hole region is of the order of the ion thermal velocity. Therefore these electrostatic waves would form a robust and persistent component of the electrostatic wave activity in the presence of the background ion hole distributions. The solution of the quasi‐linear equation shows that the saturation of the ion hole instability occurs because of velocity space trapping of the low energy background ions and results in a flattening of the reduced distribution function in the region of the original velocity space hole. The energy level of the excited waves in the vicinity of the magnetotail field reversal region is obtained as a function of the ion hole parameters.
Observations of field line resonances, auroral arcs, and auroral vortex structures
Authors: Samson J. C. , Cogger L. L. , Pao Q.
Published: February 2004

Ultralow frequency (1–4 mHz) shear Alfvén, field line resonances are often found in the nightside magnetosphere on field lines threading the equatorward region of the auroral zone. The morphologies of the electric fields and field‐aligned currents in these field line resonances is very similar to those in discrete auroral arcs. This leads to the possibility that some types of auroral arcs might be produced by field line resonances. The field line resonances would produce arcs with latitudinal scale sizes of the order of 10 km, longitudinal scale sizes of many time zones, and with periodic restructuring on timescales of the order of minutes. The standing wave nature of the field line resonance, with electric field nodes in the northern and southern ionosphere and electric field antinodes in the equatorial magnetosphere, leads to very large radial shears in azimuthal plasma flows in the equatorial plane. These shear flows might be nonlinearly unstable to the Kelvin‐Helmholtz instability leading to vortex structures which map to longitudinal scale sizes of the order of hundreds of kilometers in the auroral ionosphere. In this study we present detailed observations from one night of data from the Canadian CANOPUS array of magnetometers, meridian scanning photometers, and digital all‐sky imager showing strong evidence for auroral arcs and auroral vortex structures formed by field line resonances. The structure of the discrete arcs within the field line resonances is particularly clear in the all‐sky imager data, showing latitudinal scale sizes of the order of 10–15 km. Meridian scans of the all‐sky imager data give clear evidence for a periodic restructuring of the arc system, with a characteristic frequency of about 2.8 mHz. Measurements of 6300/5577‐Å ratios indicate that the arc has an “inverted V” energy structure, with maximum energies ranging from several hundred eV to one or two keV. The all‐sky imager data also show clear evidence of large scale (hundreds of kilometers) vortex formation evolving from the arcs associated with the field line resonances, giving relatively strong evidence for nonlinear effects in the resonances.
Observational test of local proton cyclotron instability in the Earth's magnetosphere
Authors: Anderson B. J. , Denton R. E. , Ho G. , Hamilton D. C. , Fuselier S. A. , Strangeway R. J.
Published: February 2004

We present a study of the proton cyclotron instability in the Earth's outer magnetosphere, L > 7, using Active Magnetosphere Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetic field, ion, and plasma wave data. The analysis addresses the energy of protons that generate the waves, the ability of linear theory to predict both instability and stability, comparison of the predicted wave properties with the observed wave polarization and frequency, and the temperature anisotropy/parallel beta relation. The data were obtained during 24 intervals of electromagnetic ion cyclotron (EMIC) wave activity (active) and 24 intervals from orbits without EMIC waves (quiet). This is the same set of events used by Anderson and Fuselier [1994]. The active events are drawn from noon and dawn local times for which the wave properties are significantly different. For instability analysis, magnetospheric hot proton distributions required the use of multiple populations to analytically represent the data. Cyclotron waves are expected to limit the proton temperature anisotropy, Ap = T⊥p/T‖p − 1, according to Ap < aβ‖pc with a ∼ 1 and c ∼ 0.5, where T⊥p, T‖p, and β‖p are the perpendicular and parallel proton temperatures and the proton parallel beta, respectively. During cyclotron wave events, Ap should be close to aβ‖pc whereas in the absence of waves Ap should be below aβ‖pc. The active dawn cases yielded instability in 9 of 12 cases using the measured plasma data with an average growth rate γ/Ωp = 0.025 and followed the relation Ap = 0.85β‖p−0.52. The active noon events gave instability in 10 of 12 cases, but only when an additional ∼2 cm−3 cold plasma was assumed. The noon wave events fell well below the dawn events in Ap‐β‖p space, slightly above the Ap = 0.2β‖p−0.5 curve. The lower Ap limit for the noon cases is attributed to the presence of unmeasured cold plasma. The quiet events were all stable even for additional assumed cold ion densities of up to 10 cm−3, the upper limit implied by the plasma wave data. The quiet events gave Ap < 0.2β‖p−0.5. At noon, the unstable component has T⊥p ∼ 20 keV and Ap ∼0.8. At dawn the unstable component has T⊥p ∼ 4 keV and Ap ∼ 2.3. Observed wave frequencies agree with the frequencies of positive growth, and the difference in frequency between noon and dawn is attributable to the combined effects of the different hot proton T⊥p and Ap and the inferred higher cold plasma density at noon. The dawn events had significant growth for highly oblique waves, suggesting that the linear polarization of the dawn waves may be due to domination of the wave spectrum by waves generated with oblique wave vectors.
Low‐energy particle oscillations and correlations with hydromagnetic waves in the Jovian magnetosphere: Ulysses measurements
Authors: Krupp N. , Tsurutani B. T. , Lanzerotti L. J. , Maclennan C. G.
Published: February 2004

We report on measurements of energetic particle modulations (ions and electrons with energies E > 50 keV) observed by the heliosphere instrument for spectra, composition, and anisotropy at low energies (HI‐SCALE) aboard the Ulysses spacecraft that were associated with the only hydromagnetic (HM) wave event measured inside the Jovian magnetosphere by the Ulysses magnetometer investigation. This HM‐wave event occurred during the high latitude outbound pass of the spacecraft in the dusk sector of the planet at distances between 65 and 68 Jovian radii (RJ) and at southern magnetic latitudes between 27° and 32°. We show that the approximately 5‐min oscillations that occurred for about an hour in the north–south component of the magnetic field are well correlated with variations of the energetic particle fluxes at small pitch angles. The detection of the narrow angle ion beams by the HI‐SCALE instrument occurs at the intersection between two angular spin sectors. The small‐amplitude waves vary the magnetic field direction sufficiently that the beam is directed primarily toward one sector and then the other, and back again. Possible mechanisms for the HM‐wave generation are explained. We show that the HM waves are most likely to be generated by the presence of magnetospheric heavy ions (i.e., sulfur or oxygen) with energies in the range of a few keV.
Direct penetration of the polar electric field to the equator during a DP 2 event as detected by the auroral and equatorial magnetometer chains and the EISCAT radar
Authors: Kikuchi T. , Lühr H. , Kitamura T. , Saka O. , Schlegel K.
Published: February 2004

The quasi‐periodic DP 2 magnetic fluctuations (period of 30–40 min) appearing coherently at the auroral and equatorial latitudes during the day are analyzed based on the high time resolution magnetometer data recorded at the International Monitor for Auroral Geomagnetic Effects (IMAGE) stations in Scandinavia and at the Brazilian and African equatorial stations. It is shown that the correlation between the DP 2 magnetic fluctuations at both latitudes is excellent (correlation coefficient of 0.9). No discernible time shift has been found within the resolution of 25 s. The European incoherent scatter (EISCAT) radar observations in Scandinavia show that the DP 2 fluctuations at auroral latitudes are caused by an ionospheric Hall current which is controled by the convection electric field. The DP 2 fluctuations exhibit a strong decrease in magnitude with decreasing latitude, however, it is enhanced considerably at the dip equator with an amplitude comparable to that at the subauroral latitude. The considerable equatorial enhancement of the magnitude of the DP 2 fluctuations with an enhancement ratio of 4 is due to the concentration of the electric current along the highly conductive dayside equatorial ionosphere. These observational facts can be explained in terms of an ionospheric current which is generated by the magnetospheric electric field at the high latitude and extends to the equatorial ionosphere almost instantaneously. From the viewpoint of the electric field penetration, we conclude that the magnetospheric electric field penetrates to the equatorial ionosphere through the polar ionosphere almost instantaneously within the time resolution of 25 s. The nearly instantaneous propagation of the electric field to the equator can be explained primarily by a parallel plane transmission line model composed of the conductive Earth and ionosphere. In addition to our finding of the fast propagation of the DP 2 electric field, it is found that an impulsive magnetic change with a timescale of 100 s appears at the dayside dip equator with a time delay of about 10 s, which requires to include the effect of the high conductivity of the dayside equatorial ionosphere in future studies of the propagation model.
On the possibility of the MHD‐ballooning instability in the magnetotail‐like field reversal.
Authors: Lee Dae‐Young , Min K. W.
Published: February 2004

A stability analysis of the antisymmetric ballooning mode is presented for the tail like magnetic models within ideal MHD. The analysis particularly emphasizes the potential importance of the field‐aligned variation of the local field curvature. First, it is found that the tail field models of Kan [1973] are stable against the antisymmetric ballooning mode because of the strong stabilizing effect of the field line bending. In such models, the field curvature is significant only in the narrowly limited region, namely, very near the equatorial plane from which it then rapidly declines, more rapidly than 1/B, along the field line. Then, the antisymmetric ballooning mode is studied in a magnetic field model in which the field curvature is assumed to vary as 1/B over some small portion from the equatorial point along the field line. For such a model, two criteria for the instability are proposed, one as a necessary condition and the other as a sufficient condition. Not only a sufficient pressure gradient and/or a high‐plasma beta, but also a sufficient field‐aligned portion of a substantial curvature, seem necessary for a tail‐like field line to become unstable to the antisymmetric ballooning mode. As a specific example, it is shown that a high‐beta, tail field line can suffer from an antisymmetric ballooning instability if the field line has a geometrical roundness over a distance of Rce the equatorial radius of the field curvature, from the equatorial position.
Longitudinal variations of the topside ionosphere at low latitudes: Satellite measurements and mathematical modelings
Authors: Su Y. Z. , Oyama K.‐I. , Bailey G. J. , Fukao S. , Takahashi T. , Oya H.
Published: February 2004

The longitudinal variations of the topside ionosphere at low latitudes observed by the Hinotori satellite during equinoctial periods at high solar activity are studied using the Sheffield University plasmasphere‐ionosphere model (SUPIM). The model values show that both the neutral wind and E × B drift velocities make important contributions to the observed longitudinal variations in the topside ionosphere. The displacement of the geographic and magnetic equators and the magnetic declination angle, which give rise to conjugate‐hemisphere differences in the neutral wind in the magnetic meridian, are the principal causes of the observed north‐south asymmetries in the electron density about the magnetic equator. A comparison of the modeled and observed electron densities shows that the modeled longitudinal variations are, in general, in qualitative agreement with the observations when the neutral winds are taken from the HWM90 thermospheric wind model. Improved agreement in the magnitudes is achieved if HWM90 is modified so that at low latitudes (1) the eastward component of the zonal wind is increased during the day and decreased at night and (2) the diurnal variations of the meridional wind in the northern hemisphere at eastern longitudes and the equatorward wind at around midnight at western longitudes are reduced. The model reproduces the observed longitudinal variations in the development of the equatorial peak electron density during the day and in the equatorial trough and associated crests during the afternoon and postsunset periods. The trough and crests are most prominent at around 2000 LT, where the crest‐to‐trough ratio varies from about 1.15 at eastern longitudes to about 2.0 at western longitudes. Model calculations show that the longitudinal differences of these features can arise from a longitudinal variation in the vertical E × B drift velocity.
Heating and activity of the solar corona: 2. Kink instability in a flux tube
Authors: Galsgaard Klaus , Nordlund Åke
Published: February 2004

The development of kink instability in a flux tube is investigated numerically, by solving the resistive MHD equations in three dimensions for a setup where a flux tube is stressed by rotating both ends in opposite directions. Two cases are investigated: one where the tube is initially isolated and in pressure equilibrium with surrounding plasma (external kink) and another with an initially uniform magnetic field, where only a smaller part of the boundaries are used to twist the field (internal kink). The twist angle at the onset of the kink instability depends on several parameters, such as rotation velocity, tube diameter, field strength, and magnetic resistivity, but is generally in the range 4π–8π. Both sets of experiments are followed beyond the point where they become kink unstable into the regime of nonlinear evolution. Of particular interest is the topological evolution. As magnetic dissipation becomes significant, the connectivity between the two boundaries changes from ordered to chaotic, and small‐scale current sheets develop. Even though the gross features of the external kink appear to saturate, the total magnetic energy continues to grow, by a steady increase of the free energy in the chaotic region that develops as a result of the kink and by a secular spreading of the magnetic field into the initially field‐free region. The internal kink is confined to the cylinder defined by the boundary driving and has only limited influence on the external magnetic field. After the kink, the twist of the magnetic field is reduced, and the internal kink settles into a quasi‐steady state where the dissipation on the average balances the Poynting flux input. The average Poynting flux is similar in the external and internal kinks, with a magnitude that corresponds to local winding numbers of the order of unity. Scaling of these results to values characteristic of the solar corona indicate that systematic rotation or shear of the endpoints could be a source of quasi‐steady heating in coronal loops.
Evolution of the low‐latitude geomagnetic storm field and the importance of turbulent diffusion for ring current particle losses
Authors: Grafe A. , Trakhtengerts V. Y. , Bespalov P. A. , Demekhov A. G.
Published: February 2004

An investigation of the temporal evolution of the low‐latitude disturbance field at different magnetic local times for several storms of different activity showed characteristic features for strong, moderate, and weak storms. Strong storms are characterized by a fairly symmietric disturbance field at the beginning of the recovery phase and by about the same recovery decay rate at all magnetic local times (τ ∼ 4–7 hours). For moderate and weak storms the disturbance is strongly asymmetric at the recovery phase beginning, and the recovery decay rates are quite different at different magnetic local times, the fastest recovery (τ ∼ 1–2 hours) being‐recorded in the afternoon/evening sector. We explain our observations assuming that gyroresonant interaction of ring current ions with Alfven ion cyclotron waves is an important loss mechanism for ring current. For strong storms the injection is deep, and the plasmasphere becomes small and symmetric during the main phase, so recovery starts similarly at all longitudes. During the storms of middle and small intensity an eveningside plasmasphere bulge persists at latitudes of the main ring current location, so fast losses of westward drifting ions due to turbulent diffusion occur in that sector; they provide azimuthal asymmetry of magnetic disturbance magnitude and of recovery timescale.
High‐order modes of left‐handed electromagnetic waves in a solar‐wind‐like plasma
Authors: Astudillo H. F.
Published: February 2004

A study of the linear dispersion relation for left‐handed transverse electromagnetic waves in a solar‐wind‐like magnetized ion background is presented. In this analysis the dispersion characteristics of the high‐order modes of wave propagation are included. These modes exist below but also above the proton gyrofrequency, and its dispersion relation can be approximated by an acoustic‐like one. It is shown that a discontinuity arises in the dispersion relation for a proton alpha particles magnetized plasma, when the fraction of alpha particles is increased by virtue of coupling between the normal ion cyclotron mode and high‐order modes. It is also shown that the discontinuity can disappear by changing the anisotropy of the alpha particles or their drift velocity. In that case the normal ion cyclotron mode is restored. Some implications of these modes in heating the plasma are also considered.
Coordinated global radar observations of tidal and planetary waves in the mesosphere and lower thermosphere during January 20–30, 1993
Authors: Deng W. , Salah J. E. , Clark R. R. , Franke S. J. , Fritts D. C. , Hoffmann P. , Kuerschner D. , Manson A. H. , Meek C. E. , Murphy D. , Nakamura T. , Palo S. E. , Riggin D. M. , Roble R. G. , Schminder R. , Singer W. , Tsuda T. , Vincent R. A. , Zhou Q.
Published: February 2004

A multi‐instrument global campaign involving incoherent scatter, medium frequency, and meteor wind radars was conducted during the period of January 20–30, 1993, to study the dynamics of the mesosphere and lower thermosphere. Data obtained from 15 radar stations covering a wide latitude range have been used to determine the global distribution of planetary and tidal waves during this 10‐day campaign. Spectral analysis of the neutral winds measured by the radars in the altitude range from 80 to 130 km indicates the existence of a strong 48‐hour wave near 90 km at latitudes between 40°N and 40°S that is present up to 108 km at 18°N. The semidiurnal tide is large at middle and high latitudes near 90 km and is predominant above 110 km, while the diurnal tide is observed to be particularly important in the upper mesosphere near 40° latitude. A least squares fit to the radar data is performed to obtain the amplitudes and phases of the tidal and 48‐hour waves. Comparison with National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere general circulation model shows that the predictions from the model agree reasonably well with the observed global morphology of tidal wave amplitudes.
Collision‐induced transitions between the a1Πg, a′ 1Σu−, and w1Δu states of N2: Can they affect auroral N2 Lyman‐Birge‐Hopfield band emissions?
Authors: Eastes R. W. , Dentamaro A. V.
Published: February 2004

In the Earth's atmosphere bright ultraviolet (UV) emissions from the N2 a 1Πg state (Lyman‐Birge‐Hopfield bands) are seen, but the w 1Δu and a′ 1Σu− states, which have similar excitation cross sections, emit weakly if at all at UV wavelengths. Models of the singlet system (a, a′, and w states) have used radiative cascade, which produces transitions between excited states, and quenching, which removes molecules from them, to explain the lack of emission from the a′ and w states. Another process, which has never been properly included in such calculations, is collision‐induced electronic transitions (CIET) between the singlet states. Recent laboratory measurements of CIET involving N2 indicate that the rate constants are comparable to those for radiation, and recent measurements have yielded enough information to allow a preliminary calculation of CIET between the N2 a 1Πg, a′ 1Σu− and w 1Δu states. These calculations show that CIET can be more important than radiation for transitions between the a, a′, and w states in typical aurorae or up to the peak of typical dayglow emission profiles (∼130 km). The inclusion of CIET may allow self‐consistent modeling of observations of the Lyman‐Birge‐Hopfield (LBH) bands, one of the most prominent ultraviolet molecular emissions from the Earth's atmosphere.
Mesoscale dayside convection vortices and their relation to substorm phase
Authors: Greenwald R. A. , Ruohoniemi J. M. , Bristow W. A. , Sofko G. J. , Villain J.‐P. , Huuskonen A. , Kokubun S. , Frank L. A.
Published: February 2004

Measurements made with the first two pairs of the northern hemisphere component of the Super Dual Auroral Radar Network (SuperDARN) have revealed the intermittent existence of a mesoscale convection vortex in the high‐latitude postnoon ionosphere. The vortex is a feature of the substorm growth phase and is typically centered between 1430 and 1530 MLT and 75° and 80° invariant latitude. It has a diameter ranging from a few hundred to ∼1000 km and an associated potential drop of 5–10 kV. The vortex is centered on a filamentary upward field‐aligned current with an estimated magnitude approaching 3 μA/m2. The vortex is centered near the sunward end of the dusk convection cell just poleward of the sunward convecting plasma and just duskward of the region where the sunward convecting plasma rotates sharply poleward and enters the polar cap. As the plasma convects poleward, it passes through an irregularity zone that has been associated with the ionospheric footprint of the cusp. A remarkable feature of the vortex is that it disappears concurrently with the onset of a substorm expansion phase in the midnight sector. Several magnetospheric source mechanisms, including the Kelvin‐Helmholtz and tearing mode instabilities, flux transfer events, and macroscale current systems, have been considered for the vortex. The best explanation appears to be that the vortices are associated with filamentary field‐aligned currents that are driven by the cross polar cap potential and close as Pedersen currents through the cusp region. The disappearance of the vortex following the onset of an expansion phase is attributed to a redirection of magnetospheric closure currents as a consequence of the significant increase in nightside conductivity during a substorm expansion.
Magnetic traps in the interplanetary medium associated with magnetic clouds
Authors: Vandas M. , Fischer S. , Geranios A. , Dryer M. , Smith Z. , Detman T.
Published: February 2004

MHD simulations of the propagation of magnetic clouds in the interplanetary medium show that interplanetary magnetic field (IMF) lines, draping around the cloud, are often bent in a complicated way. The magnetic field along these field lines (even on nonbent sections) is not smoothly decreasing with increasing distance from the Sun but usually exhibits several extreme values (minima and maxima). Depressions in the IMF strength may trap energetic particles with suitable energies and pitch angles. These particles may remain trapped (in the expanding region) until the IMF configuration changes. Possible locations of magnetic traps are shown in this paper.
A theory for generation of the paired region 1 and region 2 field‐aligned currents
Authors: Yamamoto Takashi , Inoue S. , Nishitani N. , Ozaki M. , Meng C.‐I.
Published: February 2004

We present a new theoretical model for generation of a pair of region 1 and region 2 field‐aligned currents (FACs) under the condition of a southward interplanetary magnetic field. On the basis of the satellite observations it is assumed that the hot (≳1 keV) plasma particles are distributed in a magnetic shell connected to two ovals of diffuse auroras on the northern and southern polar ionospheres. The hot plasma population contained in this magnetic shell having several degrees of latitude in width is called the hot plasma torus (HPT). It is proposed that the region 1/region 2 FACs can be generated as a result of natural distortion of the HPT due to the solar wind convection. When the interplanetary magnetic field has a southward component, i.e., the IMF Bz is negative, the solar wind flow across open geomagnetic field lines gives rise to electric field convection patterns over the polar caps, which are modeled as twin vortex cells with antisunward flows in the center of the polar caps. The convection thus driven by the solar wind is referred to as the solar wind convection. If it were not for an E × B convection flow, the HPT would be shaped such that the HPT particles are contained in the “magnetic drift shells,” which are tangent to the averaged total magnetic drift velocity. In the presence of the solar wind convection, the configuration of the HPT will be deformed from the magnetic drift shells. Because of the distortion of the HPT, the pressure gradient in the HPT gains a component parallel to the magnetic drift. Therefore the HPT can be polarized because of oppositely directed magnetic drifts of the HPT electrons and protons: the high‐latitude and low‐latitude sides of the HPT on the eveningside are negative and positive, respectively, and the polarity is reversed on the morningside. The resulting pattern of large‐scale field‐aligned currents due to the polarization of the HPT is consistent with the observations of region 1 and region 2 FACs. Moreover, provided that the solar wind acts as a voltage generator in the interaction with the open field lines, as a long‐term characteristic of the paired region 1 and region 2 FACs we can obtain the relationship between the FAC intensity and the ionospheric conductivity: both the region 1 and region 2 intensities increase linearly with the Pedersen conductivity, while the proportionality constant for the region 2 FAC is smaller than that for the region 1 FAC. Our predicted relation for geomagnetic quiet conditions quantitatively agrees with the regression lines between the current intensities and the Pedersen conductivities obtained on the basis of Magsat satellite observations by Fujii and Iijima [1987].
Quasi 2‐day oscillation of the ionosphere during summer 1992
Authors: Forbes J. M. , Guffee R. , Zhang X. , Fritts D. , Riggin D. , Manson A. , Meek C. , Vincent R. A.
Published: February 2004

The relationship between quasi 2‐day (QTD) oscillations in the neutral wind near 90 km altitude, and in the critical plasma frequency (ƒoF2) of the ionospheric F region, is investigated for the June–September 1992 period. Wind data are analyzed from MF radar stations at Christmas Island (1.9°N, 202.7°E), Hawaii (20.8°N, 203.5°E), and Saskatoon (52.1°N, 253.4°E) with emphasis on characterizing temporal evolutions of the amplitude and period of the QTD oscillation; four identifiable “events” are found, one each in the months of June, July, August, and September. Evidence for corresponding ±0.5–1.0 MHz QTD oscillations in ƒoF2 is found in data from about half of the 24 ionosonde stations examined. Identification of such concurrent occurrences is complicated by the lack of consistency between QTD “burst” characteristics between station locations (for both wind and ƒoF2) and between the meridional and zonal components of the QTD wind variations. These differences are probably due to the sensitivity of the QTD wind oscillation to the background wind structure, which contains latitude and longitude dependences. Evidence is presented for a zonal wavenumber one (s = 1) longitude dependence for the middle‐latitude ƒoF2 oscillation, which contradicts the expected s = 3 dependence of the mesosphere/lower thermosphere (MLT) neutral winds, but which is consistent with the “first type” of QTD oscillation in ƒoF2 at middle latitudes described by Apostolov et al. [1995]. The data presented here are not able to shed light on which of several potential mechanisms lead to the QTD oscillation in ƒoF2.
A numerical study of the nonlinear cascade of energy in magnetohydrodynamic turbulence
Authors: Verma M. K. , Roberts D. A. , Goldstein M. L. , Ghosh S. , Stribling W. T.
Published: February 2004

Power spectra of solar wind magnetic field and velocity fluctuations more closely resemble those of turbulent fluids (spectral index of −5/3) than they do predictions for magnetofluid turbulence (a −3/2 index). Furthermore, the amount the solar wind is heated by turbulence is uncertain. To aid in the study of both of these issues, we report numerically derived energy cascade rates in magnetohydrodynamic (MHD) turbulence and compare them with predictions of MHD turbulence phenomenologies. Either of the commonly predicted spectral indices of 5/3 and 3/2 are consistent with the simulations. Explicit calculation of inertial range energy cascade rates in the simulations show that for unequal levels of fluctuations propagating parallel and antiparallel to the magnetic field, the majority species always cascades faster than does the minority species, and the cascade rates are in better agreement with a Kolmogoroff‐like MHD turbulence phenomenology than with a generalized Kraichnan phenomenology even in situations where the fluctuations are much smaller than the mean magnetic field. The “Kolmogoroff constant” for MHD turbulence for small normalized cross helicity is roughly 6.7 in two dimensions and 3.6 for one calculation in three dimensions. For large normalized cross helicity, however, none of the existing models can account for the numerical results, although the Kolmogoroff‐like case still works somewhat better than the Kraichnan‐like. In particular, the applied magnetic field has much less influence than expected, and Alfvénicity is more important than predicted. These results imply the need for better phenomenological models to make clear predictions about the solar wind.
Comparison of the rigidity dependence of the 11‐year cosmic ray variation at the earth in two solar cycles of opposite magnetic polarity
Authors: Lockwood J. A. , Webber W. R.
Published: February 2004

We have found that there is a significant difference in the rigidity dependence of the 11‐year variation of the galactic cosmic rays in the rigidity interval from 0.6 to 100 GV between the A > 0 modulation cycle from 1977 to 1981 and the A < 0 cycle from 1987 to 1990. The overall intensity decrease from 1977–1981 is found to have a rigidity dependence given by F(P) = exp (−1.46 × P−1.22±0.05), where F(P) is the modulation function and P is the rigidity [GV] and for 1987–1990 F(P) = exp (−1.75 × P−1.02±0.05) For the conventional diffusion‐convection theory of modulation the rigidity dependence of the modulation is equal to the rigidity dependence of the interplanetary diffusion coefficient. For a selected number of Forbush decreases we found that the rigidity dependence of the decreases occurring near the period of minimum solar modulation was steeper than for those occurring later in the solar cycle. In no case was the rigidity dependence of any transient decrease much different from that of the 11‐year variation, however. The crossover in the 1977 and 1987 primary proton energy spectra at E ∼ 400 MeV observed near sunspot minimum shifts to ∼650 MeV at the time of the maximum solar activity or minimum cosmic ray intensity.
Global simulations and observations of O(1S), O2(1Σ) and OH mesospheric nightglow emissions
Authors: Yee Jeng‐Hwa , Crowley G. , Roble R. G. , Skinner W. R. , Burrage M. D. , Hays P. B.
Published: February 2004

Despite a large number of observations of mesospheric nightglow emissions in the past, the quantitative comparison between theoretical and experimental brightnesses is rather poor, owing primarily to the short duration of the observations, the strong variability of the tides, and the influence of short‐timescale gravity waves. The high‐resolution Doppler imager (HRDI) instrument onboard the upper atmosphere research satellite (UARS) provides nearly simultaneous, near‐global observations of O(1S) green line, O2(0–1) atmospheric band, and OH Meinel band nightglow emissions. Three days of these observations near the September equinox of 1993 are presented to show the general characteristics of the three emissions, including the emission brightness, peak emission altitude, and their temporal and spatial variabilities. The global distribution of these emissions is simulated on the basis of atmospheric parameters from the recently developed National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM). The most striking features revealed by the global simulation are the structuring of the mesospheric nightglow by the diurnal tides and enhancements of the airglow at high latitudes. The model reproduces the inverse relationship observed by HRDI between the nightglow brightness and peak emission altitude. Analysis of our model results shows that the large‐scale latitudinal/tidal nightglow brightness variations are a direct result of a complex interplay between mesospheric and lower thermospheric diffusive and advective processes, acting mainly on the atomic oxygen concentrations. The inclination of the UARS spacecraft precluded observations of high latitude nightglow emissions by HRDI. However, our predicted high‐latitude brightness enhancements confirm previous limited groundbased observations in the polar region. This work provides an initial validation of the NCAR‐TIMEGCM using airglow data.
High‐latitude plasma transport through the Martian tail: Polar wind
Authors: Haider S. A.
Published: February 2004

Flux, density, velocity, and temperature (parallel and perpendicular) are calculated by using the polar wind acceleration process in the dayside and nightside ion exosphere of Mars at exospheric plasma temperatures 1000 K, 1500 K, and 2000 K. The results are compared with the measurements made by the solar wind plasma instrument (TAUS) and automatic space plasma experiment with a rotating analyzer (ASPERA) experiments in the magnetotail of Mars. For these calculations the ionospheric plasma is transported into the Martian tail along the magnetic field line in the presence of charge separation electric fields, which are calculated by using a quasi‐neutrality condition such that both flux and density of ions are equal at every point in the exosphere to flux and density of electrons. It has been found that the ions O+,
, and NO+ are present in the distant tail at 2000 K. The ion seems to be absent at high altitude because of the polar wind acceleration mechanism. Among these ions, O+ is the dominant species in the Martian tail. The exospheric plasma temperature is the most important parameter in the polar ion exosphere. As it increases from 1000 K to 2000 K, the results are found to be changed by several orders of magnitude.
A cyclotron resonance model of VLF chorus emissions detected during electron microburst precipitation
Authors: Skoug R. M. , Datta S. , McCarthy M. P. , Parks G. K.
Published: February 2004

VLF chorus, consisting of narrowband rising frequency emissions, has often been observed in association with microburst electron precipitation. We present the first simultaneous rocket observations of these two phenomena, with emphasis on understanding the source of the VLF emissions. The rocket experiment was launched on May 6, 1993, from Poker Flat, Alaska (L = 5.6). In this work, the observed 1–4 kHz chorus emissions are interpreted in terms of a cyclotron resonance interaction. The frequency range of the risers and the observed electron energy range agree with those required for this interaction. Using a criterion derived from the conservation of energy during an interaction, it is shown that a cold plasma cyclotron resonance interaction can produce the lower‐frequency portions of the observed chorus risers, from ≈1000 Hz to ≈2500 Hz, while a warm plasma model is required to produce frequencies > 2500 Hz. The warm plasma model assumes a two‐component plasma, with an isotropic cold component and a bi‐Maxwellian warm component. The effect of the warm component is to change the wave dispersion relation, allowing the production of the higher‐frequency risers. A portion of the anisotropy required to produce the high‐frequency emissions can also be provided by a loss cone distribution. The chorus source is estimated from this cyclotron resonance theory to be located near the equatorial plane.
Asteroid interaction with solar wind
Authors: Wang Zhi , Kivelson Margaret Galland
Published: February 2004

Motivated by the Galileo observations of asteroid‐associated perturbations of the solar wind magnetic field, we have conducted analytical and numerical studies on the solar wind interaction with an asteroid. As asteroid dimensions are small compared with solar wind ion scale lengths, the interaction cannot be described in terms of magnetohydrodynamic (MHD) theory. We have obtained the solutions for the whistler mode mediated standing waves for the asteroid wake in a general geometry. The wake wave may contain many magnetic field rotations. In the special geometry with the interplanetary field perpendicular to the solar wind flow velocity, the constant phase lines for the wake wave are parabolic. Our analytical analysis and numerical simulations show that nonlinear effects do not modify the linear theory significantly even when the perturbed field is of the order of the background field. Magnetohydrodynamic shock waves are absent in the interaction of solar wind with the small object. Kinetic effects for a β = 1 plasma modify the wake wave only slightly by damping the short wavelength waves. Numerical simulations also show that the interaction of the solar wind with a small dipole field forms a draped magnetic field structure similar to that observed by Galileo during the encounter with the asteroid Gaspra.
Multisatellite study of nightside transient toroidal waves
Authors: Takahashi Kazue , Anderson Brian J. , Ohtani Shin‐ichi
Published: February 2004

Transverse magnetic pulsations polarized in the east‐west direction (toroidal waves) are a common feature in the dayside magnetosphere, but they are also observed in the nightside magnetosphere in the form of transient oscillations typically lasting 10–20 min. Until now, little attention has been paid to these transient toroidal waves (TTWs). To get insight into the propagation mode and generation mechanism of TTWs, we have examined magnetic field records from the elliptically orbiting AMPTE CCE, geostationary GOES 5 (located at a geographic longitude of ∼75° W), and GOES 6 (∼ 100°W) satellites. We find that TTW onset times are often simultaneous among the satellites within an accuracy of 1–2 min, but that the wave periods differ from one satellite to another. The latter feature is evidence that TTWs are due to standing Alfvén waves on individual magnetic field lines. Additional evidence of the standing waves is provided from the phase relationship between oscillations in magnetic field and ion flux anisotropy. We show by a numerical calculation of the ionospheric damping rate that toroidal mode standing waves can last for a few wave periods once they are excited. Thus, the general lack of continuous toroidal waves on the nightside can be attributed to the absence of sources rather than to strong ionospheric damping. Using CCE data and ground magnetic field data, we confirm that TTWs can be observed immediately following ∼30% (∼2%) of substorm onsets when the satellite is on the nightside (dayside) at 4 < L < 7. Substorms and pseudo substorm onsets produce magnetohydrodynamic disturbances in the near‐Earth magnetotail, and these disturbances appear to be a major source of TTWs.
Numerical simulation of asymmetric particle precipitation by pitch angle diffusion
Authors: Thorne Richard M. , Abel Robert W. , Summers Danny
Published: February 2004

A numerical simulation code is developed to evaluate the loss rate of particles trapped in a mirror magnetic field geometry with asymmetric loss cones. The one‐dimensional model can accommodate particle diffusion at any prescribed rate and loss cones of any prescribed sizes, and it incorporates the important effect of atmospheric backscattering. Numerical solutions for the loss cone particle distribution function calculated for the case of equal loss cones provide an acceptable simulation of the well‐known modified Bessel function solution. The code provides the first quantitative solutions for any specified rate of pitch angle scattering for the general case of arbitrary asymmetry in loss cone size. In the case of weak or moderate diffusion the ratio of particle precipitation fluxes into the two loss cones can provide a sensitive measurement of the rate of particle scattering, but to utilize this important diagnostic property, one must also have information on the fraction of particles that are backscattered from the atmosphere.
The cosmic radiation in the heliosphere at successive solar minima: 3. Steady state drift solutions of the transport equation
Authors: Reinecke J. P. L. , Moraal H. , McDonald F. B.
Published: February 2004

Cosmic ray intensities and density gradients, as observed by the Pioneer 10 and 11, Voyager 1 and 2, and IMP 8 spacecraft during the 1977 and 1987 solar minimum periods, are interpreted in terms of a two‐dimensional version of the cosmic ray transport equation that includes drifts. This paper is a follow‐up of an earlier no‐drift version to elucidate the necessity and limitations of drift effects as demanded by actual observations.