bluuebunny/crossref_metadata_2025 huggingface dataset:
Acceleration of electrons by a nearly perpendicular curved shock wave: 2. Nonzero shock thickness
Authors: Vandas M.
Published: February 2004

Recently, we have published (Vandas, 1994) an analytical solution for electron acceleration by a curved shock approximated by a cylinder or a sphere. However, real curved shocks (like the Earth's bow shock) are far from these ideal shapes, and this matter affects electron acceleration. We present a method for the calculation of electron acceleration by a curved shock wave of a general shape and find an analytical solution in a specific case with an application for the Earth's bow shock. We compare the theory for reflected electrons in two cases, for an interplanetary shock and for the Earth's bow shock. We get comparable levels of fluxes of 2–3 keV electrons at the interplanetary shock. Accelerated electrons above 10 keV at the Earth's bow shock have harder spectrum than follows from the theory. We suggest that some additional process (like pitch angle scattering) keeps the electrons longer in the shock layer and enables them to gain more energy.
Neutral sheet oscillations at substorm onset
Authors: Bauer T. M. , Baumjohann W. , Treumann R. A.
Published: February 2004

Using magnetometer and plasma instrument data of the Active Magnetospheric Particle Tracer Explorers/Ion Release Module satellite obtained in the plasma sheet, we investigate three events characterized by compressional narrowband fluctuations of the magnetic field with spectral peaks at periods of 1–2 min. Since such events are only observed in the vicinity of the neutral sheet, we call them neutral sheet oscillations. The oscillation starts some minutes before or after the onset of a magnetospheric substorm. Exceeding 10 nT, the fluctuation of the magnetic field is comparable to the background field. The compressional component of the magnetic fluctuation is distinctly greater than the two transverse components. The proton and electron pressure exhibit fluctuations that are in antiphase with the fluctuation of the magnetic pressure. In all three events the oscillation is accompanied by an earthward directed high‐speed flow with a typical velocity of 400–800 km/s and a typical duration of 2 min. In the course of the events the elevation of the magnetic field increases by more than 40°. We interpret the neutral sheet oscillations as the transient response of the plasma sheet to the formation of the substorm current wedge at substorm onset. The formation of the current wedge causes a damped oscillation of the dawn‐to‐dusk electric field with a period of 1–2 min which is associated with plasma motions at the same period. In the vicinity of the neutral sheet, where the plasma has strong vertical gradients, those motions lead to fluctuations of the magnetic field and of the ion and electron pressure seen by the spacecraft.
Distortion of Trimpi shapes by high‐altitude winds
Authors: Dowden R. L.
Published: February 2004

Phase and amplitude perturbations (Trimpis) on man‐made VLF signals propagating in the Earth‐ionosphere waveguide, due to lightning‐induced ionization occurring off the great circle path from VLF transmitter to receiver, are sensitive to the instantaneous lateral location of this ionization. High‐altitude winds of ∼100 m/s can move this ionization fast enough during its decay to change the rate, shape, and even sign of the phase and amplitude perturbation decays.
Proton velocity distributions in the magnetotail: Theory and observations
Authors: Ashour‐Abdalla M. , Frank L. A. , Paterson W. R. , Peroomian V. , Zelenyi L. M.
Published: February 2004

The structure of a plasma sheet ion distribution is modeled by using the large‐scale kinetic approach in which the trajectories of a distribution of plasma mantle particles are followed through the magnetotail. In the plasma sheet boundary layer, beamlets that originate from localized regions in the current sheet maintain their identities even after multiple interactions with the current sheet. Because of their origin, beamlet distribution functions appear to be phase bunched and azimuthally asymmetric, even far from the current sheet. Structures in the distribution of ions manifest themselves in both configuration space and velocity space. At the outer edge of the plasma sheet the particles experiencing the greatest acceleration in the distant magnetotail form a structure with multiple peaks in velocity space. When earthward and tailward beamlets are found in the same region, there is an increase in density and a decrease in bulk velocity. At the outer edge of the plasma sheet and thus near the boundary layer, counterstreaming distributions are observed. In the plasma sheet, two populations are found: beamlets with large drift velocity and a chaotic counterpart which has been thermalized by multiple crossings of the current sheet. In the vicinity of the earthward edge of the plasma sheet, the two populations merge to become an isotropic distribution function. A detailed comparison of these results with observations from the plasma instrument (PLS) on the Galileo spacecraft during the Earth 1 flyby is presented. The principal features of the observed and computed distributions are in substantial qualitative agreement.
Upper thermospheric variability over Millstone Hill during the LTCS‐2 and LTCS‐6 Campaigns
Authors: Hagan M. E. , Salah J. E.
Published: February 2004

We report on winds and temperatures observed in the upper thermosphere over Millstone Hill during the coupling energetics and dynamics of atmospheric regions (CEDAR) Lower Thermosphere Coupling Study 2 (LTCS‐2) and LTCS‐6 campaigns conducted during December 6–11, 1988, and December 4–11, 1991, respectively. Comparable geophysical conditions which prevailed during these periods facilitate their intercomparison and our investigation of interannual and day‐to‐day variability. Our analyses suggest that variability in exospheric temperature is largely characterized by variations in both the diurnal mean and tidal harmonics. While there may be unresolved signatures of geomagnetic effects in these data, the observed variability appears to be correlated with changes in the EUV forcing as represented by the 10.7‐cm solar radio flux. Conversely, variations on shorter (< 6 hours) timescales characterize the day‐to‐day variability observed in the thermospheric winds for the periods. These signatures may be partially attributable to changes in aurorally driven circulation, but our results are not conclusive. Finally, the wind and temperature signatures that we observe confirm that upward propagating semidiurnal tides do not play a preeminent role in the energy and momentum budgets of the middle latitude upper thermosphere during solar maximum winter conditions.
Cellular structures in the high‐latitude thermosphere
Authors: Crowley Geoffrey , Schoendorf Jacqueline , Roble Raymond G. , Marcos Frank A.
Published: February 2004

An organized density (and pressure) structure was recently discovered in the neutral thermosphere at high‐latitudes. The structure consists of two to four high‐ and low‐density regions having diameters of 1000 to 2000 km. The density in each region is enhanced or depleted from the hemispheric average by up to 30%. The structure is thus a significant feature of the near‐Earth space environment at high‐latitudes. We refer to each distinct region of enhanced or depleted density as a density “cell.” The cells extend upward from about 120 km into the upper thermosphere, and once formed they remain approximately fixed with respect to the geomagnetic pole. A parametric study of the density cell morphology for different magnetic activity levels is described for equinox solar minimum using the National Center for Atmospheric Research thermosphere ionosphere general circulation model (NCAR‐TIGCM). Density data were sought to verify the existence of the structures first predicted by the NCAR model. The TIGCM simulations were used to predict the large density perturbations observed by the S85‐1 satellite in a circular sun‐synchronous orbit near 200 km altitudes. The most obvious manifestation of the cells was the presence of density peaks located near 70°Λ on the dayside and nightside, and a density minimum near the magnetic pole. Since high‐latitude densities are generally expected to increase during magnetic activity, the low densities over the pole are perhaps the most interesting feature of the cell structure discussed here. The satellite data confirm the existence of the cellular structure over a range of magnetic activity levels. The discovery of the cells is important because the structure provides a unifying framework for the analysis and interpretation of high‐latitude data from both past and future experiments. The cells result from various forms of coupling between the ionosphere and thermosphere. The cell formation is quantitatively consistent with concepts from dynamic meteorology.
Events of enhanced convection and related dayside auroral activity
Authors: Moen J. , Sandholt P. E. , Lockwood M. , Denig W. F. , Løvhaug U. P. , Lybekk B. , Egeland A. , Opsvik D. , Friis‐Christensen E.
Published: February 2004

In this paper we study the high‐latitude plasma flow variations associated with a periodic (∼8 min) sequence of auroral forms moving along the polar cap boundary, which appear to be the most regularly occuring dayside auroral phenomenon under conditions of southward directed interplanetary magnetic field. Satellite data on auroral particle precipitation and ionospheric plasma drifts from DMSP F10 and F11 are combined with ground‐based optical and ion flow measurements for January 7, 1992. Ionospheric flow measurements of 10‐s resolution over the range of invariant latitudes from 71° to 76° were obtained by operating both the European incoherent scatter (EISCAT) UHF and VHF radars simultaneously. The optical site (Ny Ålesund, Svalbard) and the EISCAT radar field of view were located in the postnoon sector during the actual observations. The West Greenland magnetometers provided information about temporal variations of high‐latitude convection in the prenoon sector. Satellite observations of polar cap convection in the northern and southern hemispheres show a standard two‐cell pattern consistent with a prevailing negative By component of the interplanetary magnetic field. The 630.0 nm auroral forms located poleward of the persistent cleft aurora and the flow reversal boundary in the ∼1440–1540 MLT sector were observed to coincide with magnetosheath‐like particle precipitation and a secondary population of higher energy ions, and they propagated eastward/tailward at speeds comparable with the convection velocity. It is shown that these optical events were accompanied by bursts of sunward (return) flow at lower latitudes in both the morning and the afternoon sectors, consistent with a modulation of Dungey cell convection. The background level of convection was low in this case (Kp =2+). The variability of the high‐latitude convection may be explained as resulting from time‐varying reconnection at the magnetopause. In that case this study indicates that time variations of the reconnection rate effectively modulates ionospheric convection.
Collisional Joule dissipation in the ionosphere of Venus: The importance of electron heat conduction
Authors: Strangeway R. J.
Published: February 2004

The ionosphere of an unmagnetized planet, such as Venus, is characterized by relatively high Pedersen conductivity in comparison to the terrestrial ionosphere because of the weak magnetic field. Collisional Joule dissipation of plasma waves might therefore be an important source of heat within the Venus ionosphere. However, any assessment of the importance of collisional Joule dissipation must take into account the cooling provided by electron heat conduction due to temperature gradients. Once heat conduction is included we find that Collisional Joule dissipation is significant only in the bottomside ionosphere; waves observed at or near the dayside ionopause, or at higher altitudes (> 150 km) within the nightside ionosphere do not cause significant heating through collisional Joule dissipation. However, lightning‐generated whistler mode waves propagate through the highly collisional bottomside ionosphere, and we have performed detailed wave propagation calculations where we self‐consistently calculate the heating due to Joule dissipation and the cooling due to heat conduction. The heat conduction always exceeds the collisional cooling from elastic collisions. Because the high collision frequency at low‐altitude results in a low thermal conductivity, a steep temperature gradient is required to provide the heat flux. However, this gradient thermally decouples the bottomside ionosphere from higher altitudes. Collisional Joule dissipation of lightning generated whistlers is not likely to have any consequences for the global ionospheric energy budget. Cooling by inelastic collisions, specifically the vibrational excitation of CO2, further reduces the bottomside temperature. It is the inelastic cooling rate that determines the atmospheric heating rate, any excess heat again being carried away through heat conduction. We find that for typical wave field amplitudes of 10 mV/m the bottomside is heated to a few eV, while intense fields (100 mV/m) result in bottomside temperatures of a few tens of eV. This high a temperature may cause electronic excitation of the neutrals, which could result in optical or ultraviolet emissions from the ionosphere due to lightning. This possibility requires further investigation but requires the incorporation of additional inelastic cooling processes, such as electronic excitation of the neutral atmosphere.
Dominant two‐dimensional solar wind turbulence with implications for cosmic ray transport
Authors: Bieber John W. , Wanner Wolfgang , Matthaeus William H.
Published: February 2004

Two new methods for distinguishing two‐dimensional (2D) turbulence from slab turbulence are applied to Helios magnetometer data. Two‐component models with varying slab and 2D ingredients are considered. Both methods indicate that solar wind magnetic turbulence possesses a dominant (∼85 % by energy) 2D component. The presence of such a large 2D component provides a natural solution to the long‐standing problem of “too small” cosmic ray mean free paths derived from quasilinear scattering theory when using the slab model.
A statistical study of flux transfer event signatures in the dayside aurora: The IMF By‐related prenoon‐postnoon symmetry
Authors: Karlson K. A. , Øieroset M. , Moen J. , Sandholt P. E.
Published: February 2004

Theoretical and experimental studies suggest four main classes of magnetopause/boundary layer processes to explain how particles, momentum and energy transport from the solar wind into the magnetosphere‐ionosphere system. Assuming that the IMF By‐related prenoon‐postnoon asymmetry of plasma convection is specific to the magnetic merging coupling mode, we determine statistically the local time distribution of transient auroral events as a function of the IMF By polarity. This is based on concurrent observations of the interplanetary magnetic field components Bz and By and optical auroral observations of cusp/cleft activities within approximately 1000–1400 magnetic local time (MLT) and 71°–81° magnetic latitudes. Intervals of negative IMF Bz and nonzero By are selected for this study. Earlier case studies have shown that transient auroral events moving eastward or westward in the midday sector, depending on the IMF By polarity, are candidate ionospheric signature of magnetopause flux transfer events (FTEs). An example of this auroral event sequence, illustrating some main characteristics of the expected ionospheric footprints of FTEs, is presented. The statistical study reported here shows an asymmetric prenoon‐postnoon auroral occurrence distribution which depends on IMF By, consistent with the predictions of the reconnection model within a 95% level of confidence. This result confirms the previously existing evidence in favor of an FTE‐related interpretation of the actual auroral phenomenon.
A statistical study of the possible effects of solar wind variability on the recurrence rate of substorms
Authors: Freeman M. P. , Farrugia C. J.
Published: February 2004

The recurrence of substorms in the terrestrial magnetosphere depends on many factors. Chief among these are (1) the mechanism(s) by which the magnetosphere stores and rids itself of excess magnetic flux accumulated in the tail (loading‐unloading behavior) and (2) the way in which the power input from the solar wind to the magnetosphere (the coupling) varies with time. In this paper we explore the possible effects of the variability of the interplanetary medium on the statistical temporal distribution of substorms, using a simple substorm model. In this model, substorms recur at fixed time intervals in response to a steady solar wind power input, regardless of its level. In our simulations the power input into the magnetosphere is measured by the rectified north‐south component, Bs, of the interplanetary magnetic field (IMF). We use IMF data at 15‐s resolution from long‐term surveys to construct three statistical model inputs: (1) the random Bs model, (2) the shortwave Bs model, and (3) the longwave Bs model. We find that all the resultant distributions of intersubstorm intervals are skewed with respect to a clear modal peak and cover a wide range of intersubstorm intervals. We also find that the temporal succession of substorm onsets is sensitive to the ratio of the timescale of IMF variation to the assumed intrinsic intersubstorm period. For small values of this ratio the mode of the distribution can be greater than the intrinsic intersubstorm interval by a factor of 2 or more. For large values of this ratio the modal substorm recurrence rate approaches the intrinsic value. We also assess the effect on the temporal distribution of substorm onsets of random (failure to identify a substorm onset) and quasi‐random (incomplete coverage) errors. We use our findings to interpret the results of a large‐scale survey of substorm recurrence rates in the terrestial magnetosphere under nonstorm conditions recently undertaken [Borovsky et al., 1993]. Both shortwave and longwave Bs models could provide an interpretation for this empirical distribution but with certain provisos, which are different for the two alternatives. In the shortwave Bs model, allowance would have to be made for about a 45% failure rate in the identification of substorms. In the longwave Bs model the solar wind power input over the >8‐month duration of the survey would have to be dominated by intervals, each longer than about 3 hours, when the power input was continuously on or off. Reasons are given for preferring the first of these alternatives. Thus, once the contribution of the variability of the solar wind power input on the substorm recurrence rate has been separated out, this would then show that the empirical distribution is not at variance with the notion that under constant solar wind power input, substorms recur periodically.
Dissipative MHD solutions for resonant Alfvén waves in two‐dimensional poloidal magnetoplasmas
Authors: Tirry W. J. , Goossens M.
Published: February 2004

The resonant excitation of Alfvén waves is considered in a resistive warm plasma embedded in a purely poloidal field. The magnetostatic equilibrium is invariant in they direction. The driven problem is studied in the asymptotic state, so we can assume that all wave fields vary as exp[i(λy ‐ ωt)]. Resistive solutions are derived in the vicinity of the resonance with the aid of an asymptotic expansion procedure. We find that the zeroth‐order functions of the flux coordinate have to satisfy differential equations analogous to those obtained bGoossens et al. [1995] for the resistive one‐dimensional systems. Applied to the two‐dimensional box model for the Earth's magnetosphere, we recover the results found by Thompson and Wright [1993] in ideal MHD, but in addition, we obtain the behavior in the dissipative layer in the asymptotic state.
Adaptive identification and characterization of polar ionization patches
Authors: Coley W. R. , Heelis R. A.
Published: February 2004

Dynamics Explorer 2 (DE 2) spacecraft data are used to detect and characterize polar cap “ionization patches”, loosely defined as large‐scale (>100 km) regions where the F region plasma density is significantly enhanced (≳ 100%) above the background level. These patches are generally believed to develop in or equatorward of the dayside cusp region and then drift in an antisunward direction over the polar cap. We have developed a flexible algorithm for the identification and characterization of these structures, as a function of scale‐size and density enhancement, using data from the retarding potential analyzer, the ion drift meter, and the langmuir probe on board the DE 2 satellite. This algorithm was used to study the structure and evolution of ionization patches as they cross the polar cap. The results indicate that in the altitude region from 240 to 950 km ion density enhancements greater than a factor of 3 above the background level are relatively rare. Further, the ionization patches show a preferred horizontal scale size of 300–400 km. There exists a clear seasonal and universal time dependence to the occurrence frequency of patches with a northern hemisphere maximum centered on the winter solstice and the 1200–2000 UT interval.
Longitudinal dependence of equatorial F region vertical plasma drifts in the dusk sector
Authors: Sastri J. Hanumath
Published: February 2004

The effect of solar activity on the postsunset peak of F region vertical plasma drift at Kodaikanal (77°28′E, dip 3°N) is determined as a function of season and for two levels of magnetic activity using extensive HF phase path observations and compared with that at Jicamarca (75° W, dip 2°N) reported in the literature. The postsunset peak vertical velocity Vzp at Kodaikanal increases linearly with 10.7‐cm solar flux (F10.7) irrespective of season and level of magnetic activity. The sensitivity of Vzp to solar flux changes is more at Jicamarca than at Kodaikanal during equinoxes and December solstice. This feature is virtually absent in June solstice. The seasonally averaged values of dusktime vertical velocity at Kodaikanal for moderate to high solar flux conditions and quiet magnetic conditions(Kp ≤ 3) are in reasonable agreement with those reported very recently for the Indian sector (80°E) from AE‐E satellite data [Fejer et al., 1995]. In June solstice, the evening upward vertical velocities at Kodaikanal are, on the average, much smaller than in the Pacific sector (160°–200°E), while the postreversal downward velocities are more or less of the same magnitude in all longitude sectors except in the western American sector (230°–310°E) where there they are higher. The present study supports the emerging view that the dusktime equatorial F region vertical plasma drifts exhibit large longitudinal variations during the June solstice.
Enhanced radar backscatter from space shuttle exhaust in the ionosphere
Authors: Bernhardt P. A. , Ganguli G. , Kelley M. C. , Swartz W. E.
Published: February 2004

Enhancements in the backscatter from the 430‐MHz radar at Arecibo were recorded during the Spacelab 2 mission when the space shuttle orbital maneuver system (OMS) engines were fired in the ionosphere. The modifications in the backscatter could have been the result of (1) compression of the electrons to produce higher densities, (2) generation of ion acoustic waves, (3) variations in the electron to ion temperature ratio, (4) enhanced scatter cross section by charging of ice particles in the exhaust, or (5) excitation of dust acoustic waves. Rapid cooling and condensation of the exhaust are important in determining the scattering properties of the modified ionosphere. A dusty plasma is formed when electrons are attached to ice particles in the exhaust plume. The calculated neutral temperature inside the exhaust plume is 120 K. Charge exchange between ambient O+ and the cold exhaust molecules yields low‐temperature ion beams that excite weakly damped, ion acoustic waves. The enhanced radar echoes are probably the result of scatter from these waves, but the effects of the dusty plasma may be important. During future experiments, the space shuttle will fire the OMS engines over radars located at Arecibo, Puerto Rico; Jicarmarca, Peru; or Kwajalein, Marshall Islands. Measurements of the spectra from these radars will provide the means to distinguish between the various backscatter processes.
Disturbance propagation times to the far tail
Authors: Kaymaz Z. , Petschek H. E. , Siscoe G. L. , Frank L. A. , Ackerson K. L. , Paterson W. R.
Published: February 2004

Simultaneous data collection by IMP 8 in the near‐Earth solar wind and Geotail in the distant tail make possible the correlation of solar wind disturbances with their effects in the distant tail. This paper discusses some of the possible propagation times between the two spacecrafts and compares them with observed times for a particular disturbance when Geotail was near the magnetopause. The best fit matches very closely with the solar wind velocity. Several other propagation mechanisms also match the observed delay times within the observational error. The conversion processes operating across the bow shock, through the magnetosheath, and through the tail tend to maintain the net propagation speed constant, which accounts for the empirical finding that the direct solar wind transit time between IMP 8 and Geotail works pretty well. There is little if any effect of a significant delay resulting from the flow slowing down in the subsonic dayside magnetosheath. The event cannot discriminate between a windsock response of the tail to the solar wind disturbance and a standing wave response without further quantitative modeling.
MHD simulations of the transition of magnetic reconnection from closed to open field lines
Authors: Hesse Michael , Birn Joachim , Baker Daniel N. , Slavin James A.
Published: February 2004

Two‐ and three‐dimensional resistive MHD simulations are used to study the behavior of a dynamic magnetotail model when reconnection proceeds from closed to open field lines. For this purpose, the initial, quasi‐static magnetotail configuration is set up for various lobe density values. Initiated by the increase or occurrence of anomalous resistivity, all simulations then exhibit the growth of an unstable tail mode, which leads to neutral line and plasmoid formation. The reconnection process starts in a plasma sheet region of high mass density and progresses to open field lines with much lower plasma density and much lower plasma β. At the time of transition, drastic changes of typical dynamical parameters occur, involving strongly enhanced electric fields and reconnection rates and the generation of field‐aligned currents. Further, typical substorm signatures such as the dipolarization of the magnetic field in the inner regions of the magnetotail appear on very short timescales, comparable to those found in spacecraft observations. It is argued that this behavior should indeed become important during substorm evolution, in support of substorm models based on this transition.
Statistical investigation of IMF Bz effects on energetic (0.1‐ to 16‐keV) magnetospheric O+ ions
Authors: Lennartsson O. W.
Published: February 2004

More than 2 years of magnetospheric O+ data from the Plasma Composition Experiment on the ISEE 1 spacecraft are compared, in a statistical manner, with concurrent data on the interplanetary magnetic field (IMF) and the solar wind plasma in an attempt to clarify the geophysical responses to varying solar wind inputs, in particular to variations in the GSM Bz component of the IMF. It is found that the energy density of 0.1‐ to 16‐keV O+ ions in the plasma sheet, inside of 23 RE, is well correlated with the average solar wind energy flux during the preceding few hours, whether that flux is measured by its electromagnetic (Poynting) component P or by its far greater kinetic component K. Although P and K are well correlated with each other, the correlation of the O+ energy density is slightly better with K than with P during times of positive Bz, while the opposite holds for negative Bz. In either case the O+ energy density is more nearly proportional to K, and, given a typical value of K, there is at most a marginal (less than a factor of 2) increase in this density associated with a negative Bz. Except for this latter effect, which can perhaps be taken as evidence that a change in polarity of Bz from positive to negative may induce “unloading” of internal tail energy, there is no evidence in these O+ data, including data from the inner magnetosphere (L < 10), that a negative, or southward, Bz is inherently favorable to the transfer of solar wind power across the magnetopause. These findings, together with the results of correlating the hourly AE index with P and K, suggest that the strong dependence of the AE on the Bz polarity is substantially exaggerated by the lack of AE magnetometers above 71° magnetic latitude.
The self‐focusing instability in the presence of density irregularities in the ionosphere
Authors: Guzdar P. N. , Chaturvedi P. K. , Papadopoulos K. , Keskinen M. J. , Ossakow S. L.
Published: February 2004

The self‐focusing instability of high‐power radio waves in the ionosphere in the presence of density irregularities is presented. The present study addresses the role of preexisting density irregularities which are always present in the instability region. The presence of ambient irregularities results in the excitation of wave numbers, which, on the basis of homogeneous theory of the self‐focusing instability, should be stable. This effect can explain the puzzling observations that indicate growth of the medium‐scale (on the order of hundreds of meters) irregularities during ionospheric heating. The computational results are compared with available experimental data and will be used to plan experiments using the upcoming High‐Frequency Active Auroral Research Program heater.
Source‐surface modeling of planetary magnetospheres
Authors: Schulz Michael , McNab Michael C.
Published: February 2004

In the source‐surface approach to field modeling, the magnetosphere is divided conceptually into inner and outer regions (called S and T) by prescribing a cross‐magnetospheric surface that marks the tail entrance. The source surface thus consists of the prescribed magnetopause and the prescribed tail‐entrance surface. In the inner region (S) enclosed by the source surface, the magnetic field B is expanded formally in a series of analytical functions (e.g., gradients of spherical harmonics) with coefficients determined by a least‐squares fit to the desired boundary conditions (namely that the field be tangential to the magnetopause and normal to the tail‐entrance surface). Field lines in the tail region (T) are constructed geometrically so as to intersect the tail‐entrance surface normally but not to intersect each other or the magnetopause. Expansion coefficients for region S are determined by minimizing a user‐specified linear combination of the mean‐square normal component of B on the magnetopause and the meansquare tangential component of B on the tail‐entrance surface. This model leads to a neutral line (contour of vanishing normal component of B) on the prescribed cross‐tail surface. The neutral line constitutes the inner edge of the neutral sheet and marks the boundary (separatrix) between closed and open magnetic field lines. A mapping of the separatrix along magnetic field lines to the planetary surface defines (in the absence of a penetrating interplanetary magnetic field, which can be added later) the poleward boundary of the auroral oval. This mapping accounts well for quiet‐time auroral ovals seen in DMSP images. The source‐surface model also yields the position and shape of the neutral sheet in the tail region (T), where the strength of B can be calculated (from the normal component of B at the tail‐entrance surface) by invoking flux conservation. The current density J in region T can be computed from ▿ × B there.
Contribution of trapped particles to the dielectric tensor of magnetospheric plasmas
Authors: Grishanov N. I. , de Azevedo C. A. , de Assis A. S.
Published: February 2004

Analytical expressions for the components of the dielectric permeability operator tensor of plasmas confined in planetary dipolar magnetospheres are derived. The kinetic Vlasov's equation of a two‐dimensional plasma model, confined by an axisymmetric mirror trap, with circular magnetic field lines has been used. The conditions for single‐particle resonances of trapped particles are discussed, taking into account the bounce resonances. It is shown that the collisionless Cherenkov damping of the Alfvén and magnetosonic waves differs substantially from the Cherenkov damping of these modes in the straight confining magnetic field case; this is true due to the presence of the trapped particles. Estimations of the damping coefficients for Langmuir and kinetic Alfvén waves are made, and some observational consequences of this study are discussed.
Lower hybrid wave cavities detected by the FREJA satellite
Authors: Pécseli H. L. , Iranpour K. , Holter Ø. , Lybekk B. , Holtet J. , Trulsen J. , Eriksson A. , Holback B.
Published: February 2004

Localized electrostatic wave packets in the frequency region of lower hybrid waves have been detected by the instruments on the FREJA satellite. These waves are often associated with local density depletions indicating that the structures can be interpreted as wave filled cavities. The basic features of the observations are discussed. On the basis of simple statistical arguments it is attempted to present some characteristics which have to be accommodated within an ultimate theory describing the observed wave phenomena. An interpretation in terms of collapse of nonlinear lower hybrid waves is discussed in particular. It is argued that such a model seems inapplicable, at least in its simplest form, by providing a timescale and a length scale which are not in agreement with observations. Alternatives to this model are presented.
The anisotropy variations of electron distributions in the terrestrial ion foreshock
Authors: Chisham G. , Schwartz S. J. , Burgess D.
Published: February 2004

Measurements of the electron distribution function in the ion foreshock, by Active Magnetospheric Particle Tracer Explorers United Kingdom Subsatellite (AMPTE UKS), reveal distributions which vary on a ULF wave timescale between anisotropic (Te‖ > Te⊥) and approximately isotropic (Te‖ ≈ Te⊥). The electron anisotropy changes quasi‐periodically in reasonable correlation with the changes in the magnetic field magnitude, so that high field regions coincide with the more isotropic distributions. Individual contours of phase space density display these correlations over a large region of velocity space. By mapping these contours through several wave cycles, we show that such changes in anisotropy can be largely explained by the conservation of the magnetic moment and energy of the electrons moving through different magnetic field regions. The wave electrostatic potential can also be inferred from Liouville mapping of the thermal electron distribution functions. The time‐averaged electron distribution is hotter than it would be in the absence of the ULF wave field, but this is due to the adiabatic behaviour of the electrons, rather than to irreversible scattering.
Numerical simulation of whistler‐triggered VLF emissions observed in Antarctica
Authors: Nunn D. , Smith A. J.
Published: February 2004

The British Antarctic Survey VLF database from Halley (L = 4.3) and Faraday (L = 2.3) stations, Antarctica, has been searched for clear examples of whistler‐triggered emissions (WTEs). Dominant events were the triggering of risers or quasi‐constant frequency emissions from the upper arm of a whistler. A fairly frequent occurrence was the triggering of steep fallers from the whistler upper arm. At Faraday most WTE events were the triggering of long steep risers from the lower whistler arm. A VHS/VLF Vlasov hybrid simulation code was run and successfully simulated the main categories of WTE: risers and fallers off the upper arm and risers from the lower arm. Agreement with observations was generally very good, although in the case of triggered fallers and risers from the lower arm, very high frequency sweep rates were not obtained. The Vlasov code is highly efficient and well suited to this problem.
Tearing instability, flux ropes, and the kinetic current sheet kink instability in the Earth's magnetotail: A three‐dimensional perspective from particle simulations
Authors: Zhu Zhongwei , Winglee R. M.
Published: February 2004

In this paper the tail current sheet is shown to be unstable to the kinetic current sheet kink instability (or simply the kinetic kink instability) in the crosstail plane (y–z plane) and under similar conditions that drive the tearing instability in the noon‐midnight meridional plane (x–z plane). The tail current sheet is assumed to be a thin Harris current sheet (ρi/Lc ∼ 1) with equal ion and electron temperature. The kinetic kink instability develops due to the bending of the tail current sheet and the resulting pressure imbalance. The development of the kinetic kink instability including its growth rate and resultant distortion of the current sheet, is first examined using two‐dimensional electromagnetic particle simulations. The coupling between the kinetic kink and tearing instabilities is then investigated via three‐dimensional electromagnetic particle simulations. The results show that the tearing instability and the kinetic kink instability occur on the same timescale as what we expected from the two‐dimensional simulations and that the projection of the field lines in the x–z plane reproduces a standard plasmoid shape. However, the three‐dimensional plasmoid produced by the tearing instability is shown to consist of a series of flux ropes where the magnetic field lines are tightly wound as they cross the center of the current sheet. The entry and exit points of the field lines of the flux ropes are displaced in the dawn‐dusk direction. Twist and displacement of the magnetic field lines arise from the magnetic field component By generated by a plasma current due to the differential motion between electrons and ions. This current and the associated flux ropes result from intrinsic particle effects. The kinetic kink instability bends the current sheet and the flux ropes along the y direction and generates large‐scale cross‐tail wavy structures. The wave fronts may eventually collide, causing a total distortion of the current sheet configuration and strong electron heating, while the tearing accounts for most of the ion heating.
Electron beam instabilities as generation mechanism of electrostatic solitary waves in the magnetotail
Authors: Omura Y. , Matsumoto H. , Miyake T. , Kojima H.
Published: February 2004

We present computer experiments of electrostatic solitary waves (ESW) observed by Geotail in the magnetotail. ESW correspond to broadband electrostatic noise, and they are excited through electron two‐stream instabilities along a static magnetic field. We performed one‐dimensional electrostatic particle simulations involving two electron beams and an ion beam traveling along the static magnetic field. We vary the density ratio of the electron beams and the thermal velocities of the electron and ion beams. The values of these parameters strongly affect diffusion processes of the electron beams, and accordingly, different types of electrostatic waves are generated. We studied four different cases: cold bistream instability, weak‐beam instability, bump‐on‐tail instability, and warm bistream instability. For these electron beam instabilities, we performed two different runs with cold and hot ions, respectively. The cold bistream instability gives ESW for hot ions and ion acoustic waves for cold ions. The weak‐beam instability gives Langmuir waves, while the bump‐on‐tail instability gives ESW. The amplitudes of the waves excited by the weak‐beam and bump‐on‐tail instabilities are small and do not induce nonlinear decay to ion acoustic waves even in the presence of cold ions. The warm bistream instability gives electron hole modes regardless of the value of the ion temperature. The electron hole mode is a normal mode in the presence of a two‐hump electron distribution, and it is regarded as narrowband electrostatic noise. It also leads to formation of ESW, if the phase velocity of the electron hole mode is much larger than the ion drift velocity. A necessary condition for ESW formation through the bump‐on‐tail instability is derived theoretically, and its significance to Geotail observations is discussed.
Coronal holes near the equatorial plane and the solar wind abundance of iron
Authors: Ogilvie K. W. , Coplan M. A. , Yellin K. A.
Published: February 2004

Composition analysis of the solar wind from two equatorial coronal holes has been carried out with the Ion Composition Instrument on the ISEE‐3 spacecraft. The abundances of oxygen, neon and iron were determined as coronal hole‐related material flowed past the spacecraft. The results show that the edges of the hole‐related flow are sharply defined with abundances closer to the abundances in the photosphere than in the slower solar wind. These results are similar to those found in flows from the southern polar coronal hole and suggest an underlying unity between the polar and equatorial regions of the sun.
Solar wind control of upstream wave frequency
Authors: Le G. , Russell C. T.
Published: February 2004

In the region upstream from the Earth's bow shock, large‐amplitude ULF waves are generated by ions backstreaming along the magnetic field and subsequently convected downstream across the bow shock. Early observations showed that the frequency of these waves is proportional to the magnitude of the interplanetary magnetic field (IMF). The large scatter in this relationship implied that there are parameters other than IMF strength that significantly affect the frequencies of these waves. In this study, we examine the frequency of upstream ULF waves observed by ISEE spacecraft under various solar wind conditions. We find that the wave frequency in the spacecraft frame depends on both the IMF strength and cone angle. The IMF strength is the most important parameter that controls the wave frequency in the spacecraft frame, consistent with previous results. However, the IMF cone angle also plays a role in determining the frequency. Increasing the cone angle lowers the wave frequency in the spacecraft frame. The effect of Alfvenic Mach number is minimal. Increasing Alfvenic Mach number tends to enhance the wave frequency in the spacecraft frame. The results are consistent qualitatively with the prediction of linear theory based on the resonant interaction between the right‐handed magnetosonic wave and backstreaming ions, suggesting that this instability is responsible for the majority of waves. However, there is still a subgroup of wave events that are not consistent with this instability. The strength of the bow shock as measured by the magnetosonic Mach number appears to have little effect on the ratio of the backstreaming ions velocity to the solar wind velocity as inferred from the resonant condition.
Plasma heating and flow in an auroral arc
Authors: Moore T. E. , Chandler M. O. , Pollock C. J. , Reasoner D. L. , Arnoldy R. L. , Austin B. , Kintner P. M. , Bonnell J.
Published: February 2004

We report direct observations of the three‐dimensional velocity distribution of selected topside ionospheric ion species in an auroral context between 500 and 550 km altitude. We find heating transverse to the local magnetic field in the core plasma, with significant heating of O+, He+, and H+, as well as tail heating events that occur independently of the core heating. The O+ velocity distribution departs from bi‐Maxwellian, at one point exhibiting an apparent ring‐like shape. However, these observations are shown to be aliased within the auroral arc by temporal variations that are not well‐resolved by the core plasma instrument. The dc electric field measurements reveal superthermal plasma drifts that are consistent with passage of the payload through a series of vortex structures or a larger scale circularly polarized hydromagnetic wave structure within the auroral arc. The dc electric field also shows that impulsive solitary structures, with a frequency spectrum in the ion cyclotron frequency range, occur in close correlation with the tail heating events. The drift and core heating observations lend support to the idea that core ion heating is driven at low altitudes by rapid convective motions imposed by the magnetosphere. Plasma wave emissions at ion frequencies and parallel heating of the low‐energy electron plasma are observed in conjunction with this auroral form; however, the conditions are much more complex than those typically invoked in previous theoretical treatments of superthermal frictional heating. The observed ion heating within the arc clearly exceeds that expected from frictional heating for the light ion species H+ and He+, and the core distributions also contain hot transverse tails, indicating an anomalous transverse heat source.
Contributions of amplitude and phase modulation to geomagnetic pulsation phase and spectrum
Authors: McDiarmid D. R. , Ziesolleck C. W. S.
Published: February 2004

The phase and spectral characteristics of long‐period geomagnetic pulsation wave trains consisting of two overlapping responses to independent excitations are investigated. The major purposes of the work are to expand diagnostic processes in pulsation analysis and to use these techniques to determine the phase and spectral characteristics of these wave trains. It is shown that the inherent phase and amplitude modulation in such wave trains can significantly alter the power spectrum and can introduce either a phase jump or a phase skip. When the two individual responses have the same underlying (carrier) frequency, the spectral peak or peaks of the composite wave train can be offset from the carrier frequency, and these offsets are associated with a phase jump or skip. These phase and spectral characteristics are dependent on the difference in phase between the two responses and on the extent of their overlap. Analytic signal representation of our modeled wave trains yields more accurate estimates of the underlying pulsation frequency than do the power spectra. When the frequencies of the two responses differ, the phase and spectral characteristics become more complex. Finally, the analytic approach is applied to a real pulsation event.