bluuebunny/crossref_metadata_2025 huggingface dataset:
The duskside plasmapause/ring current interface: Convection and plasma wave observations
Authors: LaBelle J. , Treumann R. A. , Baumjohann W. , Haerendel G. , Sckopke N. , Paschmann G. , Lühr H.
Published: February 2008

Three time intervals have been selected during which the Active Magnetospheric Particle Tracer Explorers (AMPTE) IRM satellite traversed the plasmapause around 2130, 1900, and 1730 LT. The cold plasma density is monitored with the wave experiment whose frequency range includes the upper hybrid and the plasma frequencies; in this manner, the plasmapause is easily observed as a reduction in the plasma density from 10–100 cm−3 to 0–5 cm−3. The AMPTE IRM three‐dimensional plasma instrument detects ions in the energy range 20 eV to 40 keV and allows the determination of the inner edge of the ring current ions, which penetrate the duskside plasmapause by 0.5–2 RE. This boundary is characterized by termination of the plasmaspheric hiss and an outward pointing gradient in the energetic ions. Another signature of this boundary is an eastward current which is detectable by the plasma instrument as well as the magnetometer; this current is consistent in direction and magnitude with the diamagnetic current associated with the observed gradient in the ring current ions. In two of the three cases, current‐driven lower hybrid waves occur. On all three days, the ring current overlap with the plasmasphere is marked by sunward convection (opposite to corotation) and by a plateau formation in the density profile, both of which indicate that the ring current has a significant effect on the electric field. On one of the three days (June 6, 1985), the region where the ring current overlaps the plasmasphere is marked by electromagnetic waves with frequencies below the local helium gyrofrequency. These waves appear to be about an equal mix of the left‐hand‐polarized L mode and the right‐hand‐polarized R mode ion cyclotron waves and seem to support the general explanation of such low‐frequency waves based on GEOS and ATS observations. On all three days, the overlap region is populated by magnetic pulsations with periods of the order of minutes. These are accompanied by fluctuations in the cold plasma density which are in phase with the compressional part of the pulsations and seem roughly consistent with adiabatic variation in the cold plasma density in response to the changes in the field.
Simulation of electron distributions within auroral acceleration regions
Authors: Gurgiolo C. , Burch J. L.
Published: February 2008

Plasma data from the Dynamics Explorer 1 satellite are used to investigate the electron distributions found within regions of parallel potential drops. A computer simulation was developed to predict major features of the observed electron distributions found within these regions. The simulation results are discussed and compared with observations. Good agreement is observed only when the potential distribution along the field line is split into separate potential drops, one above and one below the satellite. The simulated electron distribution exhibits a trapped particle region, the formation of the electron hole, and the evacuation of the loss cone.
Evolution of recurrent solar wind structures between 14 AU and the termination shock
Authors: Whang Y. C. , Burlaga L. F.
Published: February 2008

An MHD simulation model is used to extrapolate the solar wind conditions observed from Voyager 2 at approximately 14 AU to the region of the outer heliosphere bounded by the termination shock. The paper presents two simulation results. In 1984, Voyager 2 observed two sets of nearly recurrent solar wind interaction regions, each with a period of approximately 26 days. The first set was observed between days 45 and 120 and the second set between days 290 and 365. Initial conditions for the two simulation studies representing recurrent solar wind structures were generated by repeating the pattern of plasma and magnetic field variations respectively observed during one solar rotation in March 1984 at 13.8 AU and a similar pattern observed in November 1984 at 15.4 AU. Each simulation result describes an idealized recurrent solar wind structure in the supersonic region of the outer heliosphere out to the termination shock far beyond the present reaches of the Pioneer and Voyager spacecraft. The solar wind plasma is heated and compressed by a pair of forward‐reverse interplanetary shocks. The shocks may accelerate or decelerate as they move through the nonuniform medium. A collision between the forward shock and the reverse shock occurs approximately every 40 AU. The shocks are weakened after each collision. The termination shock is weakened and moves outward when a forward shock interacts with it, and the termination shock is strengthened and moves inward when a reverse shock interacts with it.
Solar and interplanetary control of the location of the Venus bow shock
Authors: Russell C. T. , Chou E. , Luhmann J. G. , Gazis P. , Brace L. H. , Hoegy W. R.
Published: February 2008

The Venus bow shock location has been measured at nearly 2000 shock crossings, and its dependence on solar EUV, solar wind conditions, and the interplanetary magnetic field determined. The shock position at the terminator varies from about 2.14 Venus radii at solar minimum to 2.40 Venus radii at solar maximum. The location of the shock varies little with solar wind dynamic pressure but strongly with solar wind Mach number. The shock is farthest from Venus on the side of the planet in which newly created ions gyrate away from the ionosphere. When the interplanetary magnetic field is perpendicular to the flow, the cross section of the shock is quite elliptical. This effect appears to be due to the anisotropic propagation of the fast magnetosonic wave. When the interplanetary magnetic field is aligned with the flow, the bow shock cross section is circular and only weakly sensitive to changing EUV flux.
A theoretical foundation of general magnetic reconnection
Authors: Hesse M. , Schindler K.
Published: February 2008

This paper deals with the theoretical background of magnetic reconnection including systems where the magnetic field does not vanish (general magnetic reconnection). The magnetic field is represented by Euler potentials. The time evolution of the Euler potentials as seen by the moving plasma provides an appropriate formulation of the properties of general magnetic reconnection. A critical review of the concept of magnetic merging is given, and it is shown that in certain situations there may be ideal plasma flow across a separatrix which does not involve nonideal effects. It is shown that such problems do not arise within the concept of general magnetic reconnection. It is demonstrated that general magnetic reconnection with nonvanishing magnetic field crucially depends on properties of the parallel electric field.
Derivation of ion composition from a combined ion line/plasma line incoherent scatter experiment
Authors: Bjørnå N. , Kirkwood S.
Published: February 2008

A method is established for deriving ion composition from a combined ion line/plasma line incoherent scatter experiment in the ionospheric F1 region. The new method resolves the temperature/composition ambiguity in the ion line autocorrelation function by incorporating the measured plasma line offset in the least squares fitting procedure. The method has the advantage that rather short pulse lengths and integration times can be used, giving reasonably good height and time resolution. We apply the method to data from an European incoherent scatter UHF experiment carried out in July 1985 which involves an event of soft particle precipitation. The result indicates that soft energy particles depositing their energies in the F1 region tend to increase the percentage of atomic ions at these heights.
Quasi‐static theory of the Earth's magnetotail, including the far tail
Authors: Wiechen H. , Schindler K.
Published: February 2008

In the framework of the self‐consistent quasi‐static theory for the time‐dependent evolution of the Earth's magnetotail in quiet times we present a two‐dimensional model that describes the tail in the whole region in which it exists as a well‐ordered structure. The starting point is the theory by Schindler and Birn (1982) into which we incorporate a description of the distant tail. We present two models, one with a neutral line at infinity, the other with a neutral line at a finite distance. In the latter case, the field structure near the neutral line is Y‐shaped, and the neutral line and the point where tail flaring ends have the same x coordinate. The occurrence of large plasma bulk flow velocities at the plasma sheet boundary and their relationship with magnetic reconnection at the distant neutral line is also briefly discussed.
Influence of HF radio waves on ion‐cyclotron modes in the auroral ionosphere
Authors: Laxmi V. N. , Tripathi V. K.
Published: February 2008

High‐power radio waves in the auroral ionosphere tend to further destabilize the linearly unstable electrostatic ion‐cyclotron modes responsible for the long wavelength irregularities. The process is a four‐wave parametric interaction involving a finite wave number pump, the ion‐cyclotron modes, and two electrostatic sidebands. The nonlinear coupling arising through the ponderomotive force is sensitive to the wave vector of the pump k0. The growth rate falls off rapidly with increasing value of the low‐frequency wave number k.
Mars mesosphere and thermosphere: 1. Global mean heat budget and thermal structure
Authors: Bougher S. W. , Dickinson R. E.
Published: February 2008

The range of observed Mars exospheric temperatures obtained by Mariner, Mars, and Viking instruments is not highly correlated with solar EUV input. Various physical processes that influence Mars temperatures are now examined using parameterizations previously employed for Venus in order to investigate the plausibility of solar EUV as the most important thermospheric heat source. We demonstrate, using a one‐dimensional NLTE radiative transfer code, that observed long‐term variations in Mars exospheric temperatures can be largely reproduced (∼210 to 360 K) using strong 15‐µm cooling and moderate eddy conduction to balance 16% efficient EUV heating. A wider range of calculated exospheric temperatures (150 to 410 K) is achieved under the influence of variable eddy conduction and uncertain thermospheric atomic O. We observe that CO2 cooling is relatively less important for Mars thermal balance in comparison with Venus owing to the larger Mars scale heights which shift maximum heating to lower pressures where molecular thermal conduction is dominant, and because Mars atomic O is depleted. As a result, our calculated Mars global mean energy balance more closely resembles that of the Earth than that of Venus. We also find that it is not possible to derive realistic heat balance models for both Venus and Mars using identical parameters. Venus's closer proximity to the sun requires additional cooling (possibly very efficient eddy conduction) if ≥16% efficient EUV heating is to be used to obtain observed temperatures. However, strong eddy conduction may be inconsistent with accompanying Venus model eddy effects on composition. These Mars calculations of reasonable global average temperatures provide the first step in the simulation of the dynamics and observed characteristics of the Mars upper atmosphere.
Streaming sausage, kink and tearing instabilities in a current sheet with applications to the Earth's magnetotail
Authors: Lee L. C. , Wang S. , Wei C. Q. , Tsurutani B. T.
Published: February 2008

In this paper, the growth rate and eigenmode structures of the streaming sausage and kink instabilities in a current sheet with a super‐Alfvénic flow are studied. Based on the linearized compressible MHD equations, we employ an initial‐value method to obtain the growth rate and eigenmode profiles of the fastest growing mode which is either the streaming sausage mode or kink mode. The streaming sausage and kink instabilities, similar to the Kelvin‐Helmholtz instability, are caused by the sheared plasma flow. The results show that forV0m= 2VA∞the sausage mode grows faster than the kink mode when β∞< 1.5. When β∞> 1.5, the streaming kink instability has a higher growth rate. HereVA∞is the Alfvén velocity and β∞is the ratio between the plasma and magnetic pressures far away from the current layer, andV0mis the maximum velocity of plasma flow at the current sheet. In addition, an analytical dispersion equation is obtained for an ideal four‐layer model of the current sheet in the incompressible limit. In the presence of a finite resistivity, the mixed sausage‐tearing mode or the streaming tearing mode may be excited, which leads to the formation of plasmoids in the magnetotail. As an application to the Earth's magnetotail, where super‐Alfvénic plasma flows are observed in the plasma sheet and β∞≃ 0.1–0.3 in the lobes, it is suggested that the streaming sausage and streaming tearing instabilities may occur in the magnetotail. Some of the north‐then‐south or south‐then‐northBzsignatures observed in the distant magnetotail may be associated with the streaming sausage mode or the mixed sausage‐tearing mode. It is also pointed out that kink modes are unlikely to occur in the magnetotail.
The helium components of energetic terrestrial ion upflows: Their occurrence, morphology, and intensity
Authors: Collin H. L. , Peterson W. K. , Drake J. F. , Yau A. W.
Published: February 2008

A statistical survey of the occurrence of upflowing He+ seen by the energetic ion mass spectrometer on Dynamics Explorer −1 above the high‐latitude ionosphere shows that the spatial and temporal distribution of upflowing He+ in the 10‐eV to 17‐keV energy range follows a pattern which is broadly similar to those of H+ and O+ in this energy range. It also shows that energetic He+ is a minor part of the high‐latitude ion outflow. Upflowing He+ is detected only half as often as either H+ or O+, and typically the intensities of upflowing He+ are much less than those of H+ or O+. However, the intensity of the He+ outflow quite often reaches the maximum outflow of cool He+ predicted by steady state ionospheric models and occasionally reaches substantially higher levels. This suggests that probably much of the time the major part of the outflowing ionospheric plasma is accelerated to energies of at least 10 eV and that there may be additional, non‐steady state processes which can sometimes extract unusually large amounts of ionospheric He+.
Analytic formulation and quantitative solutions of the coupled ULf wave problem
Authors: Zhu Xiaoming , Kivelson Margaret G.
Published: February 2008

In the terrestrial magnetosphere, the inhomogeneous magnetic field and plasma density give rise to a continuous spectrum of field line resonant frequencies. Compressional disturbances with characteristic frequencies lying within the range of the spectrum may couple to transverse oscillations of resonant field lines. The coupling is of particular interest for global compressional modes trapped in the magnetic cavity. These modes decay in time through the coupling, even in the absence of dissipation. The importance of the process is that, through the damping of the global modes, large‐scale motion can drive localized field line resonances. In this study, we investigate the mode coupling and examine the parameter dependence of the damping rate of the global mode. The problem is discussed as an initial value problem in the box model which retains most of the significant physics yet remains mathematically tractable. To treat the coupling, we use the analogy of Landau damping in a homogeneous plasma. From the Laplace transform approach, we obtain the complex frequencies of the compressional wave by finding the singularities of the associated Green's function. Once the complex frequency has been found numerically, we obtain the corresponding waveforms in the box. Many observed wave properties can then be obtained. The calculations agree well with other simulation work and correspond to results obtained for the reflection of radio waves from the ionosphere and for plasma heating by absorption of radiation.
Characteristics of the 22‐year modulation of cosmic rays as seen by neutron monitors
Authors: Webber W. R. , Lockwood J. A.
Published: February 2008

Using neutron monitor data from 1952 through the end of 1987, including the maximum intensity in early 1987, we have observed several new features which illustrate the importance of a 22‐year modulation cycle in addition to the well‐known 11‐year cycle. These features include a sharply peaked intensity maximum in 1987, similar to that observed 22 years earlier in 1965, in contrast to the flatter maximum observed between 1972 and 1977 and earlier in 1952–1954. We also find that the neutron monitor intensity is ∼1.5% higher at the time of the 1987 maximum and also in 1965 as compared to the maximum in the 1972–1977 period. Systematic differences in the overall shape of successive 11‐year modulation cycles and similarities in the shape of alternate 11‐year cycles are observed that seem to be related to the 22‐year magnetic cycle, to predictions of drift models, and to the effects of the equatorial current sheet. A very high correlation is observed between the tilt of this current sheet and the neutron monitor intensity over the entire 1984–1987 period. An abrupt increase in the current sheet tilt that occurs in May–June 1987 seems to be directly related to the intensity decreases observed by neutron monitors in the latter part of 1987.
Forward‐reverse shock pairs associated with transient disturbances in the solar wind at 1 AU
Authors: Gosling J. T. , Bame S. J. , Smith E. J. , Burton M. E.
Published: February 2008

While relatively common in the outer heliosphere, forward‐reverse shock pairs occur rarely in the solar wind at 1 AU. Using the combined solar wind plasma and magnetic field data from ISEE 3 for the interval from August 1978 through February 1980 we have identified several examples of such shock pairs at 1 AU. Two of these shock pair events were clearly associated with transient disturbances rather than with corotating streams. Strong bidirectional electron heat flux events, which we identify as the coronal mass ejecta (CME) driving the shocks, were detected during both of these disturbances. In the stronger of the two disturbances the reverse shock was found within the CME and was separated from the forward shock by ∼0.2 AU; the pressure was nearly constant between the two shocks. On the other hand, in the weaker of the two disturbances the reverse shock had propagated entirely through the CME and trailed the forward shock by ∼0.3–0.4 AU; the pressure declined substantially and monotonically between the two shocks. The overall disturbance profiles for these two events compare favorably with the results of previously published numerical simulations. On the basis of those simulations we suggest that these two events are prototypes of “driven” and “blast” disturbances, wherein the time scale of the initial velocity perturbation near the Sun is, or is not, short compared to the transit time to 1 AU. Our observations support the notion that reverse shocks in transient disturbances are most likely to be found near the central axes of such disturbances.
The auroral 6300 Å emission: Observations and modeling
Authors: Solomon Stanley C. , Hays Paul B. , Abreu Vincent J.
Published: February 2008

Measurements of the auroral atomic oxygen (³P‐¹D) emission line at 6300 Å made by the Atmosphere Explorer visible airglow experiment are analyzed using a tomographic inversion. Emission altitude profiles are compared to the results from an electron transport and chemical reaction model. The model incorporates measurements of the energetic electron flux, neutral composition, ion composition, and electron density. Reasonable correspondence is obtained using primarily the “classical” sources of O(¹D) excitation: electron impact on atomic oxygen and dissociative recombination of O2+. The reaction of N(²D) with O2 is considered to be a minor source. Small contributions are also calculated for cascade from O(¹S), electron impact dissociation of O2, reaction of N+ with O2, and energy transfer from O+(²D) and thermal electrons to O(³P). A possible minor source from the quenching of N(²D) or N(²P) by O(³P) is also discussed.
Average ion moments in the plasma sheet boundary layer
Authors: Baumjohann W. , Paschmann G. , Sckopke N. , Cattell C. A. , Carlson C. W.
Published: February 2008

Using four months of tail data obtained by the three‐dimensional plasma instrument on board the AMPTE/IRM satellite in 1986, we have done a statistical survey on the behavior of ion moments and related plasma parameters in the plasma sheet boundary layer and the immediately adjacent outer central plasma sheet. The more than 40,000 spin averages of ion density, ion bulk velocity, ion temperature, ion temperature anisotropy and ion β were analyzed with respect to differences between their values in plasma sheet boundary layer and adjacent central plasma sheet. We also looked into their dependence on magnetic activity, and possible interrelations among them. As a primary result, we find that the plasma sheet boundary layer is not just a transition region where plasma parameters are gradually changing from plasma sheet to lobe values, but is indeed a unique layer where crucial parameters like ion density and ion β remain on nearly constant levels which are different from those in the central plasma sheet. Accordingly, there is a sharp gradient in ion density and ion β at the interface between the plasma sheet boundary layer and the central plasma sheet. A second important finding is that the average flow velocities in the plasma sheet boundary layer are surprisingly low, typically below 100 km/s. High‐speed flows do occur quite often and under all levels of magnetic activity, but in bursts of often less than one minute duration with intermittent intervals of nearly stagnant plasma. Their rate of occurrence is higher during high AE and, in particular, during plasma sheet expansions. All high‐speed flows are field‐aligned and directed toward the Earth.
Transverse ion heating in multicomponent plasmas along auroral zone field lines
Authors: Ashour‐Abdalla Maha , Schriver David , Okuda Hideo
Published: February 2008

In this paper we consider plasma modes that can occur in a multicomponent plasma that includes heavy ions, as well as the effect these modes have on ion acceleration. It is found that, when in the presence of an auroral electron beam, the addition of heavy ions excites an ion‐ion hybrid mode and a modified lower hybrid mode. For a given electron beam, the dominant instability is dependent on Ωe/ωpe and nH/no, and it therefore varies between different regions of the auroral field lines [Schriver and Ashour‐Abdalla, 1988]. In an attempt to gain an understanding of ion heating, we have undertaken a parametric simulation study for changing plasma conditions observed in the auroral zone. The study revealed that for Ωe/ωpe < 10 and no > nH, hydrogen ions were bulk heated, whereas for lower oxygen concentrations, no ≪ nH, hydrogen distribution developed a high‐energy tail. With Ωe ≫ ωpe, a substantial heating of the oxygen distribution was observed; this heating was found to be independent of the oxygen density concentration. This paper also presents a classification system for the different types of heating as functions of altitude along auroral field lines.
Auroral energy deposition rate, characteristic electron energy, and ionospheric parameters derived from Dynamics Explorer 1 images
Authors: Rees M. H. , Lummerzheim D. , Roble R. G. , Winningham J. D. , Craven J. D. , Frank L. A.
Published: February 2008

Auroral images acquired by the Spin Scan Auroral Imager (SAI) on board the DE 1 satellite are converted to geophysical parameters by applying the physical processes that produce the images from a precipitating electron flux to the response characteristics of the instrument. The energy flux and the characteristic energy are the basic derived parameters from which several ionospheric properties may be computed. Two spectral emissions are required to specify the characteristic energy and the energy flux. With the complement of available filters and photometers the combination of an ultraviolet channel and one emission in the visible wavelength region is necessary and sufficient for the purpose. The images provide global parameters at 12‐min temporal resolution; however, the spatial resolution is limited by the field of view of a pixel. Thus, images acquired from satellites at several Earth radii are useful for specifying auroral parameters to be used in time dependent global models of the ionosphere and thermosphere. In view of the variable spatial structure in the auroral oval the analysis of 12‐min images presented in this paper yields a better representation of ionospheric parameters than empirical models based on local measurements averaged over long periods of time.
Superthermal electron processes in the upper atmosphere of Uranus: Aurora and electroglow
Authors: Waite J. H. , Chandler M. O. , Yelle R. V. , Sandel B. R. , Cravens T. E.
Published: February 2008

Strong ultraviolet emissions from the upper atmosphere of Uranus suggest that both auroral and electroglow phenomena are of significant aeronomical consequences in the structure of the upper atmosphere. Combined modeling and data analysis have been carried out to determine the effect of electroglow and auroral phenomena on the global heat and atomic hydrogen budgets in the Uranus upper atmosphere. The results indicate that the auroral and electroglow heat sources are not adequate to explain the high exospheric temperature observed at Uranus, but that the atomic hydrogen supplied by these processes is more than sufficient to explain the observations. The various superthermal electron distributions modeled have significantly different efficiencies for the various processes such as UV emission, heating, ionization, and atomic hydrogen production and produce quite different H2 band spectra. We suggest that solar fluorescence scattering of H2 rather than dynamo‐induced photoelectron acceleration is the most likely candidate for the observed electroglow (dayglow) phenomena.
Simultaneous energetic particle observations at geostationary orbit and in the upstream solar wind: Evidence for leakage during the magnetospheric compression event of November 1, 1984
Authors: Baker D. N. , Belian R. D. , Fritz T. A. , Higbie P. R. , Krimigis S. M. , Sibeck D. G. , Zwickl R. D.
Published: February 2008

Energetic ion (E ≳ 70 keV) and electron (E ≳ 30 keV) observations are presented for three fortuitously positioned geostationary spacecraft during a strong magnetospheric compression event which brought the subsolar magnetopause inward of the synchronous orbit (r = 6.6 RE) on November 1, 1984. Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) field and particle data show that the CCE spacecraft (at r = 8.8 RE) was in the upstream solar wind during the compression event and several upstream particle bursts were observed between 0650 and 0930 UT. Spacecraft 1984‐037 near 2200 LT observed multiple, sharp energetic particle injection events indicative of substorm onsets occurring at ∼0650, 0720, and 0750 UT on November 1. Spacecraft 1981‐025 near local noon observed some brief magnetopause encounters as early as 0635 UT, and after 0650 UT it remained near (and often outside) the magnetopause for most of the ensuing 3 hours. Observations of energetic particle fluxes and anisotropies near the noon magnetopause suggest that magnetospheric particles escaped into the dayside magnetosheath. Spacecraft 1982‐019 (near local dawn) observed strong energetic electron flux enhancements consistent with azimuthal eastward drift following the substorm injections near local midnight. The flux of ions at spacecraft 1982‐019, however, was depleted, particularly at ∼90° pitch angles. This suggests that in their westward drift from local midnight the energetic ions failed to transit the outer dayside magnetosphere in the subsolar region but were instead lost to the magnetosheath, where their drift paths intercepted the postnoon magnetopause surface. Thus the pattern of magnetopause escape established by the synchronous orbit spacecraft together with excellent time association and spectral similarity with the CCE bursts upstream suggests that the process of magnetospheric particle escape is the most likely source for energetic particles in the magnetosheath and the upstream solar wind in this favorably observed case.
Duct propagation of hydromagnetic waves in the upper ionosphere, 2, Dispersion characteristics and loss mechanism
Authors: Fujita Shigeru
Published: February 2008

Ducted propagation of short‐period hydromagnetic waves in the upper ionosphere are studied based on a layered model including a ducting upper ionospheric layer and an anisotropic conducting sheet (E layer) under vertical, uniform ambient magnetic fields. Energy of the ducted wave is dissipated by the ionospheric Joule loss, the Poynting loss due to the shear Alfvén wave converted from the fast magnetosonic wave and the Joule loss in the conducting Earth. The ionospheric Joule loss mainly contributes to energy dissipation of the ducted wave as far as the Pedersen conductivity is not much smaller than the Hall conductivity. Spatial attenuation of a ducted wave is obtained by total energy loss divided by horizontal transportation of the ducted wave energy. Thus when the wave frequency is near to the lower cutoff, the minimum attenuation is taking place, particularly, for the fundamental ducted wave because horizontal energy transportation is enhanced by an evanescent boundary wave in the magnetosphere. Electric field intensity of the vertically standing fast magnetosonic wave at the conducting layer is essential to the energy loss. As far as using the simplified ionosphere model, when the frequency increases the ducted wave is less attenuated owing to a decrease in the ionospheric Joule loss because the electric field disturbance tends to have a node at the ionosphere. There appears a quasi‐sinusoidal variation in the frequency dependence of the attenuation when the Hall conductivity is larger than the Pedersen conductivity. This variation is caused by the modulation of the mode conversion from the fast magnetosonic wave to the Alfvén wave associated with the standing wave pattern of the Alfvén wave in the ducting layer.
Spectral characteristics of low‐frequency plasma turbulence upstream of comet P/Halley
Authors: Glassmeier K.‐H. , Coates A. J. , Acuña M. H. , Goldstein M. L. , Johnstone A. D. , Neubauer F. M. , Rème H.
Published: February 2008

Spectral characteristics of low‐frequency plasma turbulence in a cometary environment have been determined using cross‐spectral analyses of magnetic field and plasma measurements made on board the Giotto spacecraft upstream of comet P/Halley. Two upstream regions have been identified, where the mean solar wind magnetic field was approximately parallel and perpendicular to the solar wind flow velocity direction, respectively. The two regions also differ with respect to magnetic field lines having been disconnected (connected) to the cometary bow shock in the quasi‐parallel (quasi‐perpendicular) regions. In the quasi‐perpendicular region the mean magnetic field and solar wind flow direction as well as the Giotto velocity relative to the comet are approximately mutually perpendicular to each other. Thus there is no Doppler shift between the observations made in the spacecraft and plasma frame of reference for parallel propagating waves in the quasi‐perpendicular region. In both regions the turbulence spectrum can be described by a power law with a spectral index steeper than that of a Kolmogoroff‐Obukov spectrum. In the quasi‐parallel region the observed turbulence clearly exhibits an Alfvénic character with dynamic alignment between magnetic field and flow found throughout the frequency range (1‐100 mHz) analyzed. Wave propagation is sunward for all spectral components, at least in the quasi‐parallel region. The turbulence cascade observed is probably driven by right‐hand resonant pickup water group ion instabilities in the quasi‐parallel region, while in the quasi‐perpendicular region, left‐hand ion cyclotron ring or Alfvén/ion anisotropy instabilities might be a possible driving mechanism. The polarization of the pump wave in the quasi‐parallel region is almost linear, while at higher frequencies almost linear as well as right‐ and left‐handed polarization is found. In the quasi‐parallel, region evidence for water group ion cyclotron harmonics has been found with the first harmonic exhibiting a slow mode character and all harmonics almost propagating along the mean magnetic field. In the quasi‐perpendicular connected region, no clear evidence has been found for the existence of backstreaming protons, reflected off the cometary bow shock, and associated upstream wave activity.
Whistler‐triggered emissions observed by ISIS satellites
Authors: Nakamura Y. , Ondoh T.
Published: February 2008

VLF emissions triggered by whistlers are often observed at middle and high latitudes in the topside ionosphere by ISIS satellites. Most of them are so‐called LHR emissions lasting for a few seconds. Latitudinal distributions of the occurrence rate for the whistler‐triggered emissions in the topside ionosphere have been obtained by using VLF electric field data (50 Hz to 30 kHz) received from the ISIS 1 and 2 satellites at Kashima station, Communications Research Laboratory, Japan. These VLF emissions are classified into two groups according to the type of whistlers, i.e., ducted whistlers with a continuous trace over the full frequency range of the spectrum and nonducted whistlers without a complete trace below fLHR. The latitudinal distribution of the occurrence rate for emissions triggered by ducted whistlers is considerably different from that for emissions triggered by nonducted whistlers, especially at high latitudes. The occurrence rate for the emissions by nonducted whistlers is distributed rather randomly in latitude between L = 2.0 and L = 4.2. The occurrence rate for emissions by ducted whistlers increases with latitudes between L = 1.5 and L = 2.9, and it attains a maximum of 0.33 at L = 2.7. It then abruptly drops to 0.1 at L = 3.0, and it remains below 0.1 between L = 3.0 and L = 4.0. The decrease of the occurrence rate for emissions by ducted whistlers at L = 3.0 seems to be caused by the decrease of the radiation belt electron flux near the slot region. These results suggest that the VLF emissions triggered by ducted whistlers in the topside ionosphere are generated by the cyclotron resonant interaction of ducted whistlers with the magnetospheric electrons near the geomagnetic equatorial plane. Most VLF emissions triggered by nonducted whistlers seem to be observed as the LHR hiss produced by nonducted whistlers in the vicinity of the satellite in the topside ionosphere.
Characteristics of interplanetary type II radio emission and the relationship to shock and plasma properties
Authors: Lengyel‐Frey D. , Stone R. G.
Published: February 2008

We present the first detailed study of the properties of the radio emission of interplanetary type II bursts, based on a large sample of type II events. From an analysis of 33 bursts, a data base of more than 600 flux density versus frequency spectra has been developed and used to determine statistical properties of type II bursts, as well as to study the evolution of the emission of individual events. From this analysis the following results have been obtained: (1) The type II spectra appear to be composed of fundamental and harmonic components of plasma emission. The intensity of the fundamental component increases relative to the harmonic as the burst evolves with heliocentric distance. (2) The average flux density of a type II burst increases as a power of the average velocity of the associated shock. (3) As the shock propagates outward, the observed flux density at the peak of a type II spectrum remains relatively constant. The flux density integrated over frequency, however, generally decreases with distance because of a decrease in the bandwidth of the type II emission. (4) It is likely that solar wind density structures has a significant influence on type II bandwidths, and it may be responsible for differences between coronal and interplanetary type II spectral features.
Particle distributions in a two‐dimensional reconnection field geometry
Authors: Curran D. B. , Goertz C. K.
Published: February 2008

Particle trajectories in a two‐dimensional steady state reconnection field geometry are studied in order to understand phase space density variations near the magnetopause and in the geomagnetic tail. Charged particles are traced back in time from a spacecraft position until they are in the source or inflow region for an entire gyroorbit. The inflow regions for the dayside magnetopause are the magnetosheath and the closed field lines inside the magnetosphere. The inflow regions for the magnetotail are the lobes north and south of the central plasma sheet. Using Liouville's theorem and known distribution functions in the source regions, the distribution function at the spacecraft can be calculated. Unlike previous studies of “remote sensing,” this one is in two dimensions and includes a perpendicular electric field. The question of whether “trapped” distribution functions can exist on “open” field lines on the dayside magnetopause is examined as well as the signatures of reconnection in the geomagnetic tail. We find that “trapped” distributions which have a peak at or near 90° pitch angle can exist on topologically “open” field lines. Thus the observations of such distributions in the low‐latitude boundary layer do not establish that this layer is on closed field lines. In the tail the omnidirectional flux of keV ions should show a minimum in the center of the plasma sheet at distances up to about 25 RE earthward of the neutral line. Since no such minimum is apparent in the ISEE data, we conclude that the neutral line in the tail is generally tailward of 50 RE. We also find that adiabatic theory cannot be used as a means for visualizing or predicting the orbits of charged particles in these complex magnetic field topologies. In fact, we find that some orbits are chaotic in the sense that particles which start at nearly the same place in phase space follow entirely different and exponentially (in time) diverging orbits. These rather unintuitive results suggest that there is no simple or general method for interpreting pitch angle distributions observed near magnetospheric boundary layers which are magnetically connected to a neutral line.
Numerical simulations of the heating of electrons by beam‐driven electrostatic waves
Authors: Menietti J. D. , Borovsky Joseph E.
Published: February 2008

The role of oblique double layers in the generation of ion conical distributions has been discussed by Borovsky (1984). We extend this analysis to the investigation of possible electron heating processes. We have performed plasma simulations of oblique double layers with parameters characteristic of the midaltitude nightside auroral zone. The double layers are observed to generate electron beams which, in turn, excite electrostatic waves with frequencies less than the local plasma frequency (strongly magnetized Langmuir waves). These beam‐driven electrostatic waves heat the ambient electrons primarily in the parallel direction. The electrostatic waves may be responsible for thermalizing the distributions of energetic electrons that produce auroral arcs.
Oxygen acoustic solitary waves in a magnetized plasma
Authors: Qian S. , Lotko W. , Hudson M. K.
Published: February 2008

Ion‐acoustic solitary waves in a magnetized plasma containing an arbitrary mixture of H+ and O+ ions are studied. A nonlinear wave equation has been derived from the Poisson‐Vlasov equations, including a uniform magnetic field and dissipation due to reflected electrons. When dissipation is ignored, the equation has soliton solutions associated with both oxygen and hydrogen acoustic modes, which can be either rarefactive or compressive depending on the ion concentrations and the electron/ion temperature ratio and, more weakly, on the bulk drifts of the species. If electron reflection is included, the solitary wave can be intensified. Under somewhat restrictive conditions the oxygen solitary wave is rarefactive and propagates with a velocity comparable to that observed by the Viking satellite. The three‐dimensional solitons obey a relation of scales parallel to the magnetic field and in the transverse direction. Computer simulations of one‐dimensional versions of the nonlinear wave equation are represented.
Observations of nonthermal plasmas at different aspect angles
Authors: Winser K. J. , Lockwood M. , Jones G. O. L. , Suvanto K.
Published: February 2008

Data are presented from the EISCAT (European Incoherent Scatter (Facility)) CP‐3‐E experiment which show large increases in the auroral zone convection velocities (>2 km s−1) over a wide range of latitudes. These are larger than the estimated neutral thermal speed and allow a study of the plasma in a nonthermal state over a range of observing angles. Spectra are presented which show a well‐defined central peak, consistent with an ion velocity distribution function which significantly departs from a Maxwellian form. As the aspect angle decreases, the central peak becomes less obvious. Simulated spectra, derived using theoretical expressions for the O+ ion velocity distribution function based on the generalized relaxation collision model, are compared with the observations and show good first‐order, qualitative agreement. It is shown that ion temperatures derived from the observations, with the assumption of a Maxwellian distribution function, are an overestimate of the true ion temperature at large aspect angles and an underestimate at low aspect angles. The theoretical distribution functions have been included in the “standard” incoherent scatter radar analysis procedure, and attempts have been made to derive realistic ionospheric parameters from nonthermal plasma observations. If the expressions for the distribution function are extended to include mixed ion composition, a significant improvement is found in fitting some of the observed spectra, and estimates of the ion composition can be made. The non‐Maxwellian analysis of the data revealed that the spectral shape distortion parameter, D*, was significantly higher in this case for molecular ions than for atomic ions in a thin height slab roughly 40 km thick. This would seem unlikely if the main molecular ions present were NO+. We therefore suggest that N2+ formed a significant proportion of the molecular ions present during these observations.
Interplanetary cosmic ray radial gradients with steady state modulation models
Authors: Potgieter M. S. , Le Roux J. A. , Burger R. A.
Published: February 2008

We have used steady state modulation models of increasing complexity, with emphasis on drift models, to establish to what extent these models can simulate the observed cosmic ray integral radial gradient (energy ≥60–70 MeV/nucleon) in the heliosphere from 1977 to 1986. Special attention has been given to the apparent asymmetric behavior of the radial gradient with respect to the recent interplanetary magnetic field polarity reversal, and the remarkable constant radial gradient for the years 1977–1982. Instead of using differential intensities at specific energies, we presented integral radial gradients calculated from the computed integral intensities which made comparison with observations more realistic. We found that nondrift models had difficulties producing constant radial gradients over several years of increasing solar activity, because these models depend primarily on changes of the radial diffusion coefficient Krr to simulate an 11‐year cycle and therefore produce, in general, radial gradients symmetric with respect to solar maximum activity. Making these models independent of changes in Krr needs, in our opinion, unrealistic changes in the conventional modulation parameters. Drift models, on the other hand, could produce a constant radial gradient for the period 1977‐1982 and account for the asymmetric behavior of the radial gradient. But because of the inherent insensitivity of these models to changes in modulation parameters during this period, the radial gradient remained less than 1% per AU, even with the effects of a wavy neutral sheet incorporated. In an attempt to increase it, we scaled drift effects down by a factor of 10 over the entire heliosphere and found a radial gradient remarkably compatible with observations. This reduction of drift has the advantage that the models produce drift effects closer to what has actually been observed, and it retains effects which apparently need drift as an explanation, for example, charge‐dependent modulation. With diminished drift effects a polarity reversal (as in 1981) produced an increase in the radial gradient in the inner heliosphere but a decrease in the outer heliosphere (r ≥ 25 AU). For the period after the polarity reversal, drift models accounted remarkably well for the behavior of the integral radial gradient; the magnitude and change in the radial dependence could be obtained, even without scaling drift down. With the effects of a wavy neutral sheet incorporated, the gradient responded, at first, slowly to changes in the waviness of the neutral sheet with the tilt angle α ≥ 45°, but significantly and nonlinearly when α dropped below this value.
The magnetospheric response to 8‐minute period strong‐amplitude upstream pressure variations
Authors: Sibeck D. G. , Baumjohann W. , Elphic R. C. , Fairfield D. H. , Fennell J. F. , Gail W. B. , Lanzerotti L. J. , Lopez R. E. , Luehr H. , Lui A. T. Y. , Maclennan C. G. , McEntire R. W. , Potemra T. A. , Rosenberg T. J. , Takahashi K.
Published: February 2008

This paper documents a series of brief, strong (Δp/p = 1), dynamic pressure oscillations that occurred in the region upstream of the Earth's bow shock during a period of radial interplanetary magnetic field (IMF). The analyzed set of oscillations, which may be either intrinsic solar wind or bow shock‐related phenomena, recur approximately every 8–10 min, and their magnetic field signatures occur nearly simultaneously over great distances transverse to the Earth‐Sun line. The pressure oscillations appear to drive tailward‐moving magnetopause surface wavelets. In turn, the surface wavelets can be identified as hydromagnetic waves with strong compressional components in the outer magnetosphere and as quasi‐periodic variations in electron precipitation and high‐latitude ground pulsations. We use observations by spacecraft in the outer dayside magnetosphere to predict geosynchronous and subsolar magnetic field strengths, the location of the subsolar magnetopause, the solar wind dynamic pressure, and variations in the energetic magnetospheric ion flux.
Atmosphere Explorer observations of the geocoronal H Balmer α nightglow
Authors: Burrage M. D. , Yee J. H. , Abreu V. J.
Published: February 2008

The Balmer α nightglow emission of geocoronal atomic hydrogen has been investigated using photometric data obtained by the Visible Airglow Experiment on board the Atmosphere Explorer E satellite. The measured H Balmer α intensities are presented as a function of solar depression angle, observation zenith angle, and azimuth relative to the Sun for solar minimum conditions. The results are at variance with the earlier D2A satellite observations but in better agreement with previous ground‐based studies. As a result of a comparison with theoretical predictions, the present observations reaffirm the findings of the ground‐based investigations in implying that values for the Lyman β line center flux are a factor of 2 greater than those derived from direct measurements.