bluuebunny/crossref_metadata_2025 huggingface dataset:
Memories, maxims, and motives
Authors: Dungey J. W.
Published: February 2004

This account displays continuing development flowing from my Ph.D. project. MHD was sufficient for the discovery of reconnection and observations on spacecraft provided supporting evidence. If the rate of reconnection is to be predicted, more sophisticated plasma theory is needed. Extensive analysis of wave‐particle interactions is described. They were thought to be important in the neutral sheet, but another mechanism, electron viscosity, is described and no conclusion is reached.
Coronal mass ejections and major solar flares: The great active center of March 1989
Authors: Feynman Joan , Hundhausen Arthur J.
Published: February 2004

The solar flare and coronal mass ejection (CME) events associated with the large and complex March 1989 active region are discussed. This active region gave us a chance to study the relation of CME with truly major solar flares. The work concentrates on questions of the relation of CMEs and flares to one another and to other types of activity on the Sun. As expected, some major (X‐3B class) flares had associated CMEs. However, an unexpected finding is that others did not. In fact, there is strong evidence that the X4‐4B flare of March 9th had no CME. This lack of a CME for such an outstanding flare event has important implications to theories of CME causation. Apparently, not all major flares cause CMEs or are caused by CMEs. The relations between CMEs and other types of solar activity are also discussed. No filament disappearances are reported for major CMEs studied here. Comparing these results with other studies, CMEs occur in association with flares and with erupting prominences, but neither are required for a CME. The relation between solar structures showing flaring without filament eruptions and structures showing filament eruptions without flares becomes important. The evolutionary relation between an active flaring sunspot region and extensive filaments without sunspots is reviewed, and the concept of an “evolving magnetic structure” (EMS) is introduced. It is suggested that all CMEs arise in EMSs and that CMEs provide a major path through which azimuthal magnetic fields escape from the Sun during the solar cycle.
Low latitude geomagnetic field line resonance: Experiment and modeling
Authors: Waters C. L. , Menk F. W. , Fraser B. J.
Published: February 2004

Geomagnetic field line resonances may be identified in ground magnetometer data by comparing the difference in amplitude and phase of signals recorded at two closely spaced sites or by examining the latitudinal variation in polarization properties across a more extended array. These two methods give comparable results for values of the resonant frequency and width at low latitudes (L < 3). We have also found an upper limit for the damping factor, γ∼0.07 at L=1.8, by applying a damped simple harmonic oscillator model. The field line resonance structure observed in 5 weeks of data showed only one resonant frequency at L=1.8 but up to four harmonies concurrently at L=2.8. An early local morning decrease in eigenfrequency was usually present at L=1.8. This is attributed to dynamic heavy ion mass loading effects in the ionosphere where the plasma density increases around dawn. The observed eigenfrequencies were used to evaluate two plasma density models. Calculations using a combined IRI‐90 and diffusive equilibrium (DE) model gave eigenfrequencies which are considerably smaller than the experimentally observed values at both L=1.8 and L=2.8. Furthermore, the calculated harmonic spacings at L=2.8 do not agree with the experimental values, although the diurnal trends were successfully modeled using the IRI‐DE plasma description. The low‐latitude plasma density model described by Bailey (1983) yields eigenfrequencies which show good agreement with the experimentally observed values at both latitudes.
Energetic electrons in the magnetosphere of Saturn
Authors: Randall B. A.
Published: February 2004

The energy spectra and angular distributions of electrons observed by Pioneer 11 as a function of radial distance in the inner magnetosphere of Saturn are reanalyzed and phase space densities are then calculated. The radial dependence of phase space density requires a distributed loss process. The loss is greatest in the region of the E ring (5.5 < L < 8.5) and is attributed to collisions with the ring particles in agreement with earlier work by Van Allen et al. (1980b). Quantitative analysis yields the following properties of the E ring: the particle radii are in the range of 4 × 10−5 to 3.2 × 10−4 cm and the thickness of the ring is approximately 3 Rs. Between the inner edge of the E ring (5.5 Rs) and the outer edge of the A ring (2.3 Rs) there are more energetic electrons than can be supplied by radial diffusion from an external source. Detailed calculations show that a cosmic ray albedo neutron decay (CRAND) source in the A and B rings is a plausible source for this excess. The radial diffusion coefficient required to explain the E ring absorption and CRAND source for electrons is 1 × 10−12 > D0 > 3 × 10−12 Rs²/s, assuming that DLL = D0L³. As part of the reanalysis program, a method for the deconvolution of pitch angle distributions observed by simple detectors on a rotating spacecraft is developed. This process removes the instrumental response and rotational smear due to finite sampling periods and yields true angular distributions.
A large magnetosphere magnetic field database
Authors: Fairfield D. H. , Tsyganenko N. A. , Usmanov A. V. , Malkov M. V.
Published: February 2004

A large magnetosphere magnetic field database compiled from the measurements of 11 Earth‐orbiting spacecraft during 1966‐1986 is described. The 11 contributing spacecraft (Explorer 33 and 35, IMP 4, 5, 6, 7 and 8, Heos 1 and 2, and ISEE 1 and 2) provide data between 4RE and 60RE, which have been edited in order to exclude data outside the magnetosphere. Currently, the database totals more than 79,000 records, which sum to a total of more than 1600 spacecraft days in the magnetosphere. Each of these records contains the average total field vector in geocentric solar magnetospheric coordinates, disturbance field vector (measured field minus Earth's internal field), time information, spacecraft position, AE, AL, Kp, and Dst geomagnetic indices, and associated solar wind and interplanetary magnetic field parameters when available. These data have constituted and will undoubtedly continue to constitute the standard database for magnetospheric modeling studies. They are also appropriate for statistical studies of the magnetic field and its dependence on solar wind quantities. Plans are to update this database in the future with data from additional spacecraft.
Bounded anisotropy fluid model for ion temperatures
Authors: Denton Richard E. , Anderson Brian J. , Gary S. Peter , Fuselier Stephen A.
Published: February 2004

A bounded anisotropy fluid model is developed which describes the temperature evolution of a collisionless plasma including the effect of pitch angle scattering due to ion cyclotron waves. The model equations accurately describe the proton temperature evolution in the plasma depletion layer, a magnetosheath regime of decreasing plasma density near the magnetopause. As in double adiabatic theory, changes in T⊥ are driven by changes in flux tube area A (∝ 1/B), while changes in T∥ are driven by changes in field line scale length L. In the bounded anisotropy model, if the proton temperature ratio T⊥/T∥ rises above the value 1 + 0.85 β∥p−0.48 (Anderson et al., 1994), where β∥p is the proton parallel beta, energy is transferred from the perpendicular to parallel temperature until this equation is satisfied. This energy exchange represents the effect of ion cyclotron wave pitch angle scattering, which keeps the plasma state near to marginal stability. Equations of the same form, employing bounded anisotropy expressions appropriate to different species, are also applied to He2+ and to electrons. These equations well describe the evolution of the He2+ but do not describe the evolution of the electron temperature, apparently due to the high electron thermal conduction which the model does not include. These results indicate that an energy exchange term may be incorporated into anisotropic fluid equations to simulate the effect of ion cyclotron wave pitch angle scattering in global fluid equations.
Convective generation of “giant” undulations on the evening diffuse auroral boundary
Authors: Yamamoto Takashi , Ozaki M. , Inoue S. , Makita K. , Meng C. ‐I.
Published: February 2004

Convective generation of “giant” undulations on the equatorward boundary of an evening diffuse aurora is numerically simulated. A giant undulation is defined as a waveform having the crest‐to‐trough amplitude comparable to the wavelength. The two‐dimensional electrostatic particle code is used for studying the motion of magnetospheric plasma perpendicular to the geomagnetic field. According to the simulation results by Yamamoto et al. [1993], the giant undulation is a manifestation of the Kelvin‐Helmholtz (K‐H) waves arising from the polarization of “an arc sheet” (dense plasma population of ionospheric origin) in the magnetosphere, which is assumed to be located just equatorward of the region of proton diffuse aurora. In this previous simulation, initially, irregularities are given evenly over the entire azimuthal length of the arc sheet so that the resulting undulations are periodic. The present simulation deals with a different situation, that a K‐H wave starts growing from local irregularities on the polarized arc sheet. The simulation results show that the disturbance propagates both westward and eastward (relative to the background flow), forming a series of K‐H waves along the arc sheet. As a consequence, giant undulations with spatially varying amplitudes are developed on the equatorward boundary of a diffuse aurora, which is located just poleward of the arc sheet. A series of giant undulations convectively produced in the simulation is remarkably similar to some auroral images photographed from the Defense Meteorological Satellite Program (DMSP) satellites. In addition, we show the direct evidence for the presence of an arc sheet, probably associated with the giant undulations, which is provided by the measurements of precipitating particles from the DMSP F7 satellite crossing the equatorward boundary of the diffuse aurora; the high‐density (∼10 cm−3) ions with energy of a few hundred electron volts are detected near the edge of the region of energetic (∼10 keV) ion precipitation. The primary cause of the formation of that arc sheet is thought to be escape of the oxygen and hydrogen ions from the topside ionosphere, due to the transverse acceleration by the ion‐cyclotron waves.
Tearing mode instability in a multiple current sheet system
Authors: Yan M. , Otto A. , Muzzell D. , Lee L. C.
Published: February 2004

The tearing mode and magnetic reconnection are studied for multiple current sheet systems by two‐dimensional magnetohydrodynamic (MHD) simulations. Both the linear and nonlinear evolution of this process are analysed for laminar perturbations. The results illustrate the existence of a linear regime with a symmetric and an antisymmetric mode and agree with previous analytic results (Otto and Birk, 1992). The nonlinear evolution shows a number of interesting new features and may explain some properties in corresponding studies of turbulent reconnection. For wavelengths larger than twice the current sheet separation the evolution of antisymmetric modes leads to an entire reconfiguration of the magnetic field and converts a major portion of the magnetic energy into kinetic energy. Antisymmetric modes with smaller wavelengths and symmetric modes are found to saturate. The influence of the value of the resistivity on the reconnection rate decreases in the nonlinear evolution, and the ratio of current sheet separation to wavelength seems to be of major importance. A comparison of the dynamics of periodic current sheets with the evolution of only two current sheets indicates that some of the results for the periodic system also apply to the evolution of only two interacting current sheets. The results are discussed with respect to observations of large‐scale plasma and magnetic field reconfigurations in the magnetosheath and near the Earth's bow shock.
Standing shocks in a two‐fluid solar wind
Authors: Habbal Shadia R. , Hu You Qiu , Esser Ruth
Published: February 2004

We present a numerical study of the formation of standing shocks in the solar wind using a two‐fluid time‐dependent model in the presence of Alfvén waves. Included in this model is the adiabatic cooling and thermal conduction of both electrons and protons. In this study, standing shocks develop in the flow when additional critical points form as a result of either localized momentum addition or rapid expansion of the flow tube below the existing sonic point. While the flow speed and density exhibit the same characteristics as found in earlier studies of the formation of standing shocks, the inclusion of electron and proton heat conduction produces different signatures in the electron and proton temperature profiles across the shock layer. Owing to the strong heat conduction, the electron temperature is nearly continuous across the shock, but its gradient has a negative jump across it, thus producing a net heat flux out of the shock layer. The proton temperature exhibits the same characteristics for shocks produced by momentum addition but behaves differently when the shock is formed by the rapid divergence of the flow tube. The adiabatic cooling in a rapidly diverging flow tube reduces the proton temperature so substantially that the proton heat conduction becomes negligible in the vicinity of the shock. As a result, protons experience a positive jump in temperature across the shock. While Alfvén waves do not affect the formation of standing shocks, they contribute to the change of the momentum and energy balance across them. We also find that for this solar wind model the inclusion of thermal conduction and adiabatic cooling for the electrons and protons increases significantly the range of parameters characterizing the formation of standing shocks over those previously found for isothermal and polytropic models.
Modeling ionospheric convection during a major geomagnetic storm on October 22‐23, 1981
Authors: Moses J. J. , Slavin J. A. , Aggson T. L. , Heelis R. A. , Winningham J. D.
Published: February 2004

Following the passage of an interplanetary shock at ∼0500 UT, a major geomagnetic storm developed on October 22‐23, 1981. Numerous auroral substorms occurred during this storm leading to an AE index greater than 1000 nT. We have used the expanding/contracting polar cap (ECPC) model (Moses et al., 1989) and data from the Dynamics Explorer 2 spacecraft to study the ionospheric electric fields for 12 consecutive traversals of the polar regions. The ECPC model can determine the voltage drops across the dayside merging and nightside reconnection gaps. We determined the relationship of the AL index (i.e., the intensity of the westward electrojet) to the nightside reconnection potential drop. An excellent linear correlation was found between the nightside reconnection gap voltage drop and the AL index. These results show that the solar wind strongly drives the magnetosphere‐ionosphere system throughout the geomagnetic storm. A substantial level of dayside merging seems to occur throughout the event. Nightside reconnection varies from satellite pass to satellite pass and within the substorm recovery phase. We find that tail reconnection is an important feature of the recovery phase of substorms.
Statistical analysis of short large‐amplitude magnetic field structures in the vicinity of the quasi‐parallel bow shock
Authors: Mann G. , Lühr H. , Baumjohann W.
Published: February 2004

Short large‐amplitude magnetic field structures (SLAMS) appear in the vicinity of the quasi‐parallel region of the Earth's bow shock. Using the magnetic field and plasma data of the AMPTE/IRM and UKS satellites, 18 of such structures in the upstream region are statistically investigated. These events occurred on one day. They have typical magnetic amplitudes of 2.6 times the ambient field and are associated with density enhancements of 1.3 times the unperturbed value. Their velocities are roughly 4 times the Alfvén speed in the solar wind frame. A relationship between the amplitude of the magnetic field and the density enhancement as well as the propagation velocity of the SLAMS has been derived from the data. This relationship is well reproduced by describing SLAMS in terms of simple nonlinear magnetohydrodynamic waves. Within the SLAMS the magnetic field rotates from a quasi‐parallel into a quasi‐perpendicular geometry.
Combined measurements of EISCAT and the EISCAT magnetometer cross to study Ω bands
Authors: Lühr H. , Schlegel K.
Published: February 2004

A combination of incoherent scatter radar data and data from a magnetometer array complement each other in an ideal way. The magnetometer results help to distinguish between temporal and spatial fluctuations in the radar data, and the radar results provide a means to separate magnetometer‐inferred equivalent currents into their Hall and Pedersen components. The technique is used to study Ω bands and leads to a better understanding of the ionospheric current structure associated with this phenomenon. In extending previous models we propose a pair of counterrotating source free ionospheric current vortices driven by field‐aligned currents, an upward current centered in the luminous part of the Ω band and a downward current in the dark part with its center about 400 km west of the upward current.
Effects of spacecraft potential on three‐dimensional electron measurements in the solar wind
Authors: Scime Earl E. , Phillips John L. , Bame Samuel J.
Published: February 2004

Using the three‐dimensional, low‐energy electron spectrometer aboard the Ulysses spacecraft, we have measured the gyrotropicity of electron distributions in the solar wind. In order to make these observations, we have developed a new technique for correcting spacecraft charging effects in three‐dimensional, low‐energy particle measurements. Comparisons of ion and electron number and current densities, and the alignment of electron temperature anisotropies with the local magnetic field, are presented as evidence of the improvement in the accuracy of the electron moments resulting from the spacecraft charging corrections. The implications of our charging correction technique go beyond simple scalar corrections to the Ulysses measurements. We discuss the effects of our charging correction upon the measurements of temporal and radial gradients in a plasma environment and for two‐dimensionally obtained low‐energy particle data.
EUVAC: A solar EUV Flux Model for aeronomic calculations
Authors: Richards P. G. , Fennelly J. A. , Torr D. G.
Published: February 2004

This paper presents a new solar EUV flux model for aeronomic calculations (EUVAC), which is based on the measured F74113 solar EUV reference spectrum. The model provides fluxes in the 37 wavelength bins that are in widespread use. This paper also presents cross sections to be used with the EUVAC flux model to calculate photoionization rates. The flux scaling for solar activity is accomplished using a proxy based on the F10.7 index and its 81‐day average together with the measured solar flux variation from the EUVS instrument on the Atmosphere Explorer E satellite. This new model produces 50‐575 Å integrated EUV fluxes in good agreement with rocket observations. The solar cycle variation of the chromospheric fluxes agrees well with the measured variation of the Lyman α flux between 1982 and 1988. In addition, the theoretical photoelectron fluxes, calculated using the new EUV flux model, are in good agreement with the solar minimum photoelectron fluxes from the Atmosphere Explorer E satellite and also with the solar maximum photoelectron fluxes from the Dynamics Explorer satellite. Its relative simplicity coupled with its ability to reproduce the 50‐575 Å solar EUV flux as well as the measured photoelectron spectrum makes the model well suited for aeronomic applications. However, EUVAC is not designed to accurately predict the solar flux variability for numerous individual lines.
Termination shock response to large‐scale solar wind fluctuations
Authors: Steinolfson R. S.
Published: February 2004

The analysis of data recorded by the Voyager 2 spacecraft indicates the presence of large‐scale fluctuations in the solar wind ram pressure on the time scale of tens of days. The amplitude of the fluctuations is highly variable but often lies within a factor of 5 to 10 change from an average or mean value of the ram pressure. Since the spacecraft has presumably not encountered the termination shock yet, these fluctuations should eventually interact with the shock and thereby play a role in determining the shock location. Numerical solutions of the time‐dependent gasdynamic equations are used to simulate the response of the termination shock to fluctuations in the solar wind ram pressure comparable to those observed. The primary result of this study is that the maximum shock excursion due to the fluctuations is of the order of 1 AU, which is much smaller than that predicted by other studies. Additional simulations show that the limited movement is due to the fact that the time scale for the termination shock response is substantially larger than the time scale of the fluctuations. It is also shown that the heliopause acts as a barrier for the fluctuations and confines them to the heliosphere.
Cusp/cleft auroral activity in relation to solar wind dynamic pressure, interplanetary magnetic field Bz and By
Authors: Sandholt P. E. , Farrugia C. J. , Burlaga L. F. , Holtet J. A. , Moen J. , Lybekk B. , Jacobsen B. , Opsvik D. , Egeland A. , Lepping R. , Lazarus A. J. , Hansen T. , Brekke A. , Friis‐Christensen E.
Published: February 2004

Continuous optical observations of cusp/cleft auroral activities within ≈ 09‐15 MLT and 70‐76° magnetic latitude are studied in relation to changes in solar wind dynamic pressure and interplanetary magnetic field (IMF) variability. The observed latitudinal movements of the cusp/cleft aurora in response to IMF Bz changes may be explained as an effect of a variable magnetic field intensity in the outer dayside magnetosphere associated with the changing intensity of region 1 field‐aligned currents and associated closure currents. Ground magnetic signatures related to such currents were observed in the present case (January 10, 1993). Strong, isolated enhancements in solar wind dynamic pressure (Δp/p ≥ 0.5) gave rise to equatorward shifts of the cusp/cleft aurora, characteristic auroral transients, and distinct ground magnetic signatures of enhanced convection at cleft latitudes. A sequence of auroral events of ≈ 5‐10 min recurrence time, moving eastward along the poleward boundary of the persistent cusp/cleft aurora in the ≈ 10‐14 MLT sector, during negative IMF Bz and By, conditions, were found to be correlated with brief pulses in solar wind dynamic pressure (0.1 < Δp/p < 0.5). Simultaneous photometer observations from Ny Ålesund, Svalbard, and Danmarkshavn, Greenland, show that the events often appeared on the prenoon side (≈ 10‐12 MLT), before moving into the postnoon sector in the case we study here, when IMF By < 0. In other cases, similar auroral event sequences have been observed to move westward in the prenoon sector, during intervals of positive By. Thus a strong prenoon/postnoon asymmetry of event occurence and motion pattern related to the IMF By polarity is observed. We find that this category of auroral event sequence is stimulated bursts of electron precipitation that originate from magnetosheath plasma that has accessed the dayside magnetosphere in the noon or near‐noon sector, possibly at high latitudes, partly governed by the IMF orientation as well as by solar wind dynamic pressure pulses.
Nonlinear effects involving whistler wave propagation in the magnetosphere
Authors: Shapiro V. D. , Ride S. K.
Published: February 2004

We consider nonlinear effects associated with the propagation of a quasi‐monochromatic whistler wave packet along the inhomogeneous geomagnetic field. We show that under the specific conditions of the Siple experiment the whistler packet is subject to the modulational instability, resulting in longitudinal and transverse modulation of the envelope amplitude and the excitation of quasi‐neutral density perturbations akin to ion acoustic waves. In particular, we examine the behavior of the wave packet in the vicinity of the region where the group velocity of the packet is a maximum. In general, the wave packet is subject to the competing effects of dispersive broadening and the nonlinear tendency to steepen. In the vicinity of the region of interest the dispersive broadening is small, and the tendency for steepening prevails. This results in the creation of anomolously narrow solitary waves. We analyze the situation analytically using a modification of the nonlinear Schrodinger equation. We evaluate the conditions required for the onset of the modulational instability and find its typical growth rate. We then construct nonlinear traveling wave solutions. These take the form of solitons, which move toward the rear edge of the wave packet. Because the dispersive effects are small, these solitons are narrow structures with steep boundaries. We then analyze the possibility that these solitons act as drivers for triggered emission. The interaction of resonant particles with the wave at the soliton leads to particle bunching in velocity. The resulting current serves as a source term for monochromatic radiation, the frequency of which changes with time as the bunches move in the inhomogeneous geomagnetic field.
Concerning flux erosion from the dayside magnetosphere
Authors: Tsyganenko N. A. , Sibeck D. G.
Published: February 2004

The dayside magnetopause moves inward during periods of southward interplanetary magnetic field in response to decreases in the outer magnetospheric magnetic field strength. We consider possible causes for the magnetic field strength decreases and demonstrate that they are consistent with increases in the region 1 Birkeland and cross‐tail currents. We reexamine the well‐known series of magnetopause crossings by OGO 5 on March 27,1968, to demonstrate that they provide evidence for two intervals of gradual inward magnetopause motion associated with magnetic flux erosion and also for two intervals of inward magnetopause motion associated with sharp increases in the solar wind dynamic pressure.
The magnetosheath of Neptune: Models and observations
Authors: Richardson John D. , Stahara Steve S. , Siscoe George L. , Spreiter John R. , Szabo Adam
Published: February 2004

This paper gives a complete overview of the plasma and magnetic field in the magnetosheath of Neptune. A more sophisticated method is used to reanalyze the plasma data. This method provides better values of plasma density, velocity, and temperature in the magnetosheath both inbound and outbound from the planet than published previously. The data are compared to results obtained from a gasdynamic model of flow around a planetary magnetosphere. Agreement of theory and data is good for some parameters and poor for others. We show that some discrepancies between the model predictions and observations may be due to changes in the solar wind parameters. The model results and data are used to check the hypothesis that Voyager encountered the high‐latitude plasma mantle downstream from Neptune. The deviations of the data from the model results in this region are consistent with those expected for a mantle crossing, with the exception of the plasma temperature which decreases in one crossing and increases in the other. This may indicate that our knowledge of the nature of the mantle‐magnetosheath boundary is not complete.
The structure of mass‐loading shocks: 1. Comets
Authors: Zank G. P. , Story T. R. , Neubauer F. M.
Published: February 2004

A new multifluid model to describe the solar wind‐cometary ion plasma in the outer cometary coma is derived. This model is distinguished from previous multifluid models in that wave‐particle effects are included explicitly. By considering hydrodynamic timescales, self‐consistent dissipative terms (which correspond to the spatial diffusion of cometary ions) are derived, thereby allowing the structure of a cometary bow shock to be resolved. In the hypersonic limit, our diffusive multifluid model reveals an attractive connection to the one‐fluid model formulated by Biermann et al. (1967), a connection which serves as an important guide when investigating our more complicated nonhypersonic diffusive model. The detailed model consists of cometary ions diffusing in a warm solar wind which is heated by the dissipation of Alfvénic turbulence (in situ and generated by the initial ring beam distribution). The quasi‐parallel cometary shock is found to be at least an order of magnitude thicker than a quasi‐perpendicular cometary shock. It is furthermore found that for undisturbed solar wind (sonic) Mach numbers in excess of ∼6, the cometary bow shock (for both perpendicular and parallel cases) is smoothed completely by the cometary ions and no “proton” or thermal subshock is necessary. Finally, a detailed comparison of the model with the plasma observations made at Halley by Giotto is undertaken. It is found that the observed shock structure, thickness, location, and plasma parameters compare well with the predictions of the theoretical model, particularly in the case of the quasi‐parallel shock.
Observations of the O2 atmospheric band nightglow by the high resolution Doppler imager
Authors: Burrage M. D. , Arvin N. , Skinner W. R. , Hays P. B.
Published: February 2004

During nighttime operation the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) measures both the horizontal wind field at about 94 km altitude and the limb brightness of the O2(b¹Σg+ − X³Σg+) (0,0) atmospheric band airglow. The dominant feature of the observed emission is a latitudinal and local time dependence which is consistent with the (1,1) diurnal tidal mode. A survey of the available data set from November 1991 to July 1993 reveals a semiannual variation in the peak brightness observed at the equator, with maxima observed at the equinoxes and minima at the solstices. These results are consistent with the long‐term variations in the diurnal tidal amplitudes detected in HRDI wind measurements.
Statistical survey of pitch angle distributions in core (0‐50 eV) ions from Dynamics Explorer, 1: Outflow in the auroral zone, polar cap, and cusp
Authors: Giles B. L. , Chappell C. R. , Moore T. E. , Comfort R. H. , Waite J. H.
Published: February 2004

Core (0‐50 eV) ion pitch angle measurements from the retarding ion mass spectrometer on Dynamics Explorer 1 are examined with respect to magnetic disturbance, invariant latitude, magnetic local time, and altitude for ions H+, He+, O+, M/Z=2 (D+ or He++), and O++. Included are outflow events in the auroral zone, polar cap, and cusp, separated into altitude regions below and above 3 RE. In addition to the customary division into beam, conic, and upwelling distributions, the high‐latitude observations fall into three categories corresponding to ion bulk speeds that are (1) less than, (2) comparable to, or (3) faster than that of the spacecraft. This separation, along with the altitude partition, serves to identify conditions under which ionospheric source ions are gravitationally bound and when they are more energetic and able to escape to the outer magnetosphere. Features of the cleft ion fountain inferred from single event studies are clearly identifiable in the statistical results. In addition, it is found that the dayside pre‐noon cleft is a consistent source of escape velocity low‐energy ions regardless of species or activity level and the dayside afternoon cleft, or auroral zone, becomes an additional source for increased activity. The auroral oval as a whole appears to be a steady source of escape velocity H+, a steady source of escape velocity He+ ions for the dusk sector, and a source of escape velocity heavy ions for dusk local times primarily during increased activity. The polar cap above the auroral zone is a consistent source of low‐energy ions, although only the lighter mass particles appear to have sufficient velocity, on average, to escape to higher altitudes. The observations support two concepts for outflow: (1) The cleft ion fountain consists of ionospheric plasma of 1‐20 eV energy streaming upward into the magnetosphere where high‐latitude convection electric fields cause poleward dispersion. (2) The auroral ion fountain involves field‐aligned beams which flow out along auroral latitude field lines; and, in addition, for late afternoon local times, they experience additional acceleration such that the ion energy distribution tends to exceed the detection range of the instrument (>50‐60 eV).
Effects of the intense solar activity of March/June 1991 observed in the outer heliosphere
Authors: McDonald F. B. , Barnes A. , Burlaga L. F. , Gazis P. , Mihalov J. , Selesnick R. S.
Published: February 2004

The properties of the large‐scale global merged interaction region (GMIR) generated by the intense solar events of March and June 1991 are studied using the available solar wind, interplanetary magnetic field, and energetic particle data from the observing network of Pioneer 10 and Voyagers 1 and 2 in the outer heliosphere. At heliocentric distances extending to 55 AU the delayed effects of this enhanced solar activity are observed in the form of large increases in the solar wind velocity and interplanetary magnetic field and significant decreases in the galactic cosmic ray intensity. For low‐energy ions (5‐MeV protons) there was a single long‐lived event extending over a period of some 6 months. Near the strongest interplanetary disturbances the H and He spectra are best represented by similar exponentials in momentum/nucleon (i.e., particle velocity at these energies). Over the rest of the event the characteristic momentum for He, (P0)He is generally ∼0.66 for hydrogen. These spectra and the consistently low H/He ratio (25.3) at 2 MeV/nucleon closely resemble that observed in corrotating interaction regions events. Despite the strong north/south asymmetry in the solar activity, the interplanetary disturbances produced the same net decrease in the galactic cosmic ray intensity of ions >70 MeV at the three widely separated spacecraft when the effects of the long‐term recovery are taken into account. A comparison of the relative intensity of MeV ions at these three spacecraft suggest that the most intense solar events occurred on the back side of the Sun in time periods adjacent to the March and June episodes of solar activity. It is argued that this GMIR as a system is responsible for the low‐frequency radio emission observed by the Voyager Plasma Wave experiment some 1.46 years after the onset of the March 1991 activity.
Determining the standoff distance of the bow shock: Mach number dependence and use of models
Authors: Farris M. H. , Russell C. T.
Published: February 2004

We explore the factors that determine the bow shock standoff distance. These factors include the parameters of the solar wind, such as the magnetosonic Mach number, plasma beta, and magnetic field orientation, as well as the size and shape of the obstacle. In this report we develop a semiempirical Mach number relation for the bow shock standoff distance in order to take into account the shock's behavior at low Mach numbers. This is done by determining which properties of the shock are most important in controlling the standoff distance and using this knowledge to modify the current Mach number relation. While the present relation has proven useful at higher Mach numbers, it has lacked effectiveness at the low Mach number limit. We also analyze the bow shock dependence upon the size and shape of the obstacle, noting that it is most appropriate to compare the standoff distance of the bow shock to the radius of curvature of the obstacle, as opposed to the distance from the focus of the object to the nose. Last, we focus our attention on the use of bow shock models in determining the standoff distance. We note that the physical behavior of the shock must correctly be taken into account, specifically the behavior as a function of solar wind dynamic pressure; otherwise, erroneous results can be obtained for the bow shock standoff distance.
Excitation and parametric decay of electromagnetic ion cyclotron waves in high‐speed solar wind streams
Authors: Gomberoff L. , Gratton F. , Gnavi G.
Published: February 2004

In a recent paper, Hollweg et al. (1993) studied the parametric decay of Alfvén waves in high‐speed solar wind streams. Following this analysis, we consider the nonlinear decay of left‐hand‐polarized ion cyclotron waves. It is shown that in a solar wind type plasma composed of electrons, protons, and alpha particles drifting relative to the protons, both branches of the dispersion relation of the circularly polarized waves can be excited by observed thermal anisotropies (Gomberoff and Elgueta, 1991). Guided by this analysis, the parametric decay of each branch of the dispersion relation is discussed. It is shown that the presence of drifting alpha particles introduces new wave couplings in the system that lead to new instabilities. Some of these instabilities involve sound waves supported essentially by the alpha particles, which, due to Landau damping, can be very efficient in the energization of alpha particles. Other instabilities involve ordinary sound waves that can lead to proton heating. A modulational instability that involves two electromagnetic daughters is also found. We have also found that a strong pump can force decays of modes that do not satisfy the resonance conditions when the pump intensity is vanishingly small. Finally, it is shown that both branches of the dispersion relation, particularly the branch close to the Doppler‐shifted alpha particle resonance, are highly unstable even for small intensities of the pump wave.
A semikinetic model for ionospheric Langmuir turbulence
Authors: Helmersen Erlend , Mjølhus Einar
Published: February 2004

Langmuir turbulence during ionospheric heating experiments is studied by a model which treats the low‐frequency ion dynamics from a linearized kinetic description, where the usual high frequency Zakharov equation describing the electron dynamics is retained. The main objective is to compare the predictions from this model with results previously obtained from quasi‐fluid Zakharov models. Our findings are that the predictions from the two types of models are in good qualitative agreement. At high values of the frequency mismatch and moderate values of the driving electric field, the saturated dynamics is cascade‐dominated, while at lower‐frequency mismatch, that is, nearer to the reflection height, or at higher drive, the dynamics is of the cavitation type. In cascade dominated cases the perturbation of the ion velocity distribution is mainly around the ion acoustic velocity, while in cavitation‐dominated cases the whole velocity range is perturbed.
Ion injection and acceleration at quasi‐perpendicular shocks
Authors: Giacalone J. , Jokipii J. R. , Kóta J.
Published: February 2004

We present and discuss results of a new model for ion injection and acceleration at quasi‐perpendicular collisionless shocks. We use the one‐dimensional hybrid simulation (kinetic ions/fluid electrons) and impose an assumption on the ion motion so that diffusion across the magnetic field (normal to the shock front) is possible. These motions are otherwise suppressed by both one‐ and two‐dimensional simulations. We find that, even in strictly perpendicular shocks, when scattering normal to the field is included, a fraction of the incident ions are accelerated to suprathermal energies. When reasonable scattering times are considered, only pickup ions are injected, whereas thermal solar wind ions are not. The acceleration of these ions is very rapid. We have found that a few of the initially low‐energy pickup ions can reach many tens to a few hundred times the plasma ramming energy in less than 100 gyroperiods. Furthermore, highly field aligned energetic ions are found to exist upstream of the slightly oblique shock. The most direct application of this study is toward the interpretation of observations of both solar wind and interstellar pickup ion distributions in the vicinity of interplanetary shocks which are most often quasi‐perpendicular. This work also directly addresses a fundamental issue with regard to our current understanding of the anomalous component of cosmic rays.
Response of thermal ions to electromagnetic ion cyclotron waves
Authors: Anderson B. J. , Fuselier S. A.
Published: February 2004

Electromagnetic ion cyclotron waves generated by 10‐50 keV protons in the Earth's equatorial magnetosphere will interact with the ambient low‐energy ions also found in this region. We examine H+ and He+ distribution functions from ≈ 1 to 160 eV using the Hot Plasma Composition Experiment instrument on AMPTE/CCE to investigate the thermal ion response to the waves. A total of 48 intervals were chosen on the basis of electromagnetic ion cyclotron (EMIC) wave activity: 24 with prevalent EMIC waves and 24 with no EMIC waves observed on the orbit. There is a close correlation between EMIC waves and perpendicularly heated ion distributions. For protons the perpendicular temperature increase is modest, about 5 eV, and is always observed at 90° pitch angles. This is consistent with a nonresonant interaction near the equator. By contrast, He+ temperatures during EMIC wave events averaged 35 eV and sometimes exceeded 100 eV, indicating stronger interaction with the waves. Furthermore, heated He+ ions have X‐type distributions with maximum fluxes occurring at pitch angles intermediate between field‐aligned and perpendicular directions. The X‐type He+ distributions are consistent with a gyroresonant interaction off the equator. The concentration of He+ relative to H+ is found to correlate with EMIC wave activity, but it is suggested that the preferential heating of He+ accounts for the apparent increase in relative He+ concentration by increasing the proportion of He+ detected by the ion instrument.
Pc 3‐4 geomagnetic pulsations observed at very low latitude conjugate stations
Authors: Feng Qiuju , Fraser B. J. , Menk F. W. , Ziesolleck C. W. S. , Saka O. , Kitamura T.
Published: February 2004

In order to investigate the properties of Pc 3‐4 geomagnetic pulsations at very low latitudes, a conjugate study at 23° geomagnetic latitude was conducted over one month in 1988. The results show that highly polarized Pc 3‐4 pulsations occur mainly in the morning from 0400 to 1000 LT. This occurrence distribution appears to be a consequence of hydromagnetic wave source and/or propagation characteristics rather than ionospheric effects. Most of the observed signals have small ellipticities, and the east‐west component is enhanced near local sunrise. The polarization major axes show mirror symmetry in opposite hemispheres. Calculations of the toroidal mode eigenfrequency using actual ionospheric data to fit plasma density models show that the observed signal frequencies at 23° geomagnetic latitude are similar to the eigenfrequencies at 40°–50° latitude. Events observed simultaneously between 23° and 45° latitude suggest that Pc 3‐4 at the very low latitude conjugate stations may be associated with field line resonances near 45° latitude. Possible source mechanisms of these pulsations are global mode resonances or trapped fast mode waves in the plasmasphere driving forced field line oscillations at very low latitudes.
Statistical characteristics of bursty bulk flow events
Authors: Angelopoulos V. , Kennel C. F. , Coroniti F. V. , Pellat R. , Kivelson M. G. , Walker R. J. , Russell C. T. , Baumjohann W. , Feldman W. C. , Gosling J. T.
Published: February 2004

Using a common methodology to analyze data from the AMPTE/IRM and ISEE 2 satellites we report on the statistical properties of bursty bulk flow events (BBFs) in the inner plasma sheet (IPS). A positive correlation between BBFs and the AE index suggests that BBFs are predominantly geomagnetically active time phenomena. Earthward BBFs are more frequent close to midnight and away from Earth, up to a distance of ∼19 RE. Tailward BBFs are very infrequent in the IRM data set and somewhat less infrequent in the ISEE 2 data set in the region of the satellites' spatial overlap, possibly due to the more active conditions prevailing during the ISEE 2 mission in that region. However, in both data sets the ratio of tailward to earthward BBFs increases with distance from Earth; more than 20% of all BBFs are anti‐sunward tailward of X = −19 RE in the ISEE 2 data set. BBFs are responsible for 60‐100% of the measured earthward transport of mass, energy and magnetic flux past the satellite in the regions of maximum occurrence rate, even though they last approximately 10‐15% of the IPS observation time there. Thus BBFs represent the primary transport mechanism at those regions. The one‐to‐one correspondence between BBFs and substorm phase, as well as the relative contribution of BBFs to the total transport observed during substorms are questions that await further investigation based on multi instrument studies of individual events.
Solar wind electrons as tracers of the Martian magnetotail topology
Authors: Dubinin E. , Lundin R. , Schwingenschuh K.
Published: February 2004

Simultaneous measurements of electrons and magnetic field are used to study the topology of the Martian magnetosphere. Halo solar wind electrons streaming along magnetic field lines replicate variations of the Bx component, thus tracing the interplanetary magnetic field draping in most parts of the Martian tail. This suggests that the magnetic field observed near the equatorial plane of Mars is mainly induced. However, there are regions where this perfect tracing is violated. This could be considered as evidence for an intrinsic planetary field. An alternative interpretation involves the relationship between these regions and ionospheric holes.
Effect of small‐scale plasma turbulence on altitude profiles of electron drift velocity in the equatorial electrojet: An experimental study
Authors: Murthy B. V. Krishna , Ravindran Sudha
Published: February 2004

Recent theoretical developments on the irregularities in the equatorial electrojet (EEJ) brought out the effect of small‐scale turbulence on the large‐scale dynamics of the EEJ. We have studied this aspect experimentally using VHF backscatter radar observations. It is found that the effect of turbulence is to shift the altitude of maximum of the electron drift velocity upward. The ratio of anomalous (manifestation of turbulence) to normal collision frequency of electrons has been derived from the observations. The experimental findings on the effect of turbulence on EEJ structure are, in general, in accordance with theoretical results.
Asymmetries in the Io plasma torus
Authors: Desch M. D. , Farrell W. M. , Kaiser M. L.
Published: February 2004

Using Ulysses radio wave data taken during the 1992 Jupiter encounter, we conclude that there are significant large and small spatial scale azimuthal asymmetries at high latitudes in the Io plasma torus. During a period of time near perijove when the spacecraft motion was predominantly in the azimuthal direction and was relatively fixed in both latitude and radial distance, inferred electron densities depart significantly from the common assumption of longitudinal symmetry. Specifically, electron plasma concentrations near 0° system III longitude (and 0400 LT) are greater than those near 180° (and 0000 LT). Superposed on this large‐scale variation are regularly spaced density depletions, 30‐50% in magnitude, and having a spatial periodicity of about 17°. Some of these depletions may drive various known radio and plasma wave sources by means of large B parallel electric potentials. The observations are compared with recent models and with the in situ Voyager observations.