bluuebunny/crossref_metadata_2025 huggingface dataset:
Pickup protons and pressure‐balanced structures from 39 to 43 AU: Voyager 2 observations during 1993 and 1994
Authors: Burlaga L. F. , Ness N. F. , Belcher J. W. , Whang Y. C.
Published: February 2004

The pressure of interstellar pickup protons in the distant heliosphere can be determined by analyzing pressure‐balanced structures, observed on a scale of a few hundredths of an AU. This paper extends the earlier work of L. F. Burlaga et al. (Journal of Geophysical Research, 99, 21,511, 1994) by analyzing pressure‐balanced structures observed by Voyager 2 from 39.3 to 40.6 AU in 1993 and from 42.6 to 43.2 AU during 1994. The pickup proton temperature is high in the region of the distant heliosphere that we considered: (5.4 ± 0.1) × 106K at 39–41 AU and (6.0 ± 0.4) × 106K at ≈43 AU. The density of the pickup protons is (1.6 ± 0.3) × 10−4cm−3at 39–41 AU and (1.2 ± 0.2) × 10−4cm−3at ≈43 AU. The ratio of the pickup proton density to the solar wind proton density (Ni/N) is small, only 0.03 ± 0.01 during both 1993 and 1994. Nevertheless, the pickup proton pressures are relatively high because of their high temperatures. The pickup proton pressure was (11 ± 2) × 10−14erg cm−3at 39–41 AU and (9 ± 1) × 10−14erg cm−3at 43 AU. There is a possible decrease inPiwith increasing distance from the Sun. The pickup proton pressure is an order of magnitude greater than the solar wind proton pressure:Pi/Pswp= 10 ± 2 at 39–41 AU and 8 ± 2 at 43 AU. Our results support the hypothesis of Burlaga et al. that the pickup proton pressure is more important than the solar wind thermal pressure in the dynamics of the distant heliosphere. The ratio of the pickup ion pressure to the magnetic pressure isPi/(B2/8π) = 1.7 ± 0.3 at 39–41 AU and 1.7 ± 0.72 at ≈43 AU. These results are compared with a model.
Coarse‐grained texture of ion distributions in the magnetotail: A fractal‐like approach
Authors: Ashour‐Abdalla M. , Zelenyi L. M. , Peroomian V. , Frank L. A. , Paterson W. R.
Published: February 2004

Experimental results from Geotail and Galileo, as well as results from large‐scale kinetic modeling, demonstrate the characteristic structuring of ion distribution functions, especially in the distant and middle parts of the magnetotail. This structuring of the distributions is associated with nonadiabatic acceleration processes in the magnetotail. This paper suggests that the complexity of ion distributions can be quantified by using the analog of the fractal dimension D of the isodensity contours of the ion velocity distributions. It is found that D is usually larger than 1 for various locations within the tail and that as a result of the smearing of the structuring, the value gradually approaches the topological dimension D = 1 as the Earth is approached. The use of the parameter D may be helpful in comparing the degree of granularity of the distribution functions obtained in different regions of the magnetotail, as well as in comparing experimental and theoretical results.
Simultaneous VHF and transequatorial HF observations in the presence of bottomside equatorial spread F
Authors: Flaherty J. P. , Kelley M. C. , Seyler C. E. , Fitzgerald T. J.
Published: February 2004

Simultaneous transequatorial HF and VHF incoherent scatter measurements were conducted in Peru in January 1994. On the evening of January 13, conditions as measured at VHF by the Jicamarca Radio Observatory were comparatively mild. The spread F observed on this evening was confined to the bottomside of the F layer and relatively weak. Yet dramatic propagation effects were seen in the HF. Strong off great circle signals with more than 10 Hz in Doppler spread were measured. Large spreads in angle of arrival, as much as 20 deg, were also seen. The strength of the off great circle returns are comparable to the strength of signals arriving from the direction of the great circle and are believed to result from total reflections from horizontal gradients in the ionosphere. Jicamarca VHF measurements substantiate this hypothesis. We present data from a single evening during which two separate events occurred. From the combined VHF and HF measurements, we conclude that the source of the off great circle reflections consists of a longitudinal density gradient which is immediately followed by a region of unstable plasma. The unstable plasma affecting the HF signals is shown to consist of a sequence of finger‐like structures having a nominal separation of 15 km which remain below the height of maximum F region electron density.
On the rotational distribution of the 5.3‐μm “thermal” emission from nitric oxide in the nighttime terrestrial thermosphere
Authors: Sharma R. D. , Dothe Hoang , von Esse F.
Published: February 2004

The rotational distribution of the “thermal” emission from the v = 1 vibrational level of NO, resulting from impacts of NO in the ground vibrational level (v=0) with oxygen atoms, is examined using the cryogenic infrared radiance for shuttle (CIRRIS‐1A) database. A block of NO quiescent (nonauroral) nighttime limb radiances observed by the CIRRIS ‐1A interferometer and radiometer around 5.3 μm are inverted to obtain the local rotational envelopes of the 1→0 vibrational transition as functions of altitude for both spin components. It is found possible to describe these local rotational envelopes by Maxwell‐Boltzmann distributions and to obtain rotational temperatures for each spin component of the vibrationally excited NO. The two spin components, within the accuracy of the measurements, are described by the same rotational temperature, which differs, however, from the mass spectrometer incoherent scatter (MSIS) model temperature at most altitudes. At low altitudes (≤110 km), the rotational temperature and the temperature describing the relative population of the spin states of the vibrationally excited NO approach each other, indicating the onset of thermodynamic equilibrium for the spin and the rotational degrees of freedom.
Observations of auroral substorms occurring together with preexisting “quiet time” auroral patterns
Authors: Henderson M. G. , Murphree J. S. , Weygand J. M.
Published: February 2004

Although horse collar configurations are commonly associated with relatively quiescent magnetospheric conditions, observations made with the Viking UV imager are presented which clearly demonstrate that classical auroral substorms can coexist with such patterns. Thus the magnetospheric conditions that lead to the production of these “quiet time” auroral patterns are not necessarily stable to the growth of the instability that leads to substorm onset. As well, the horse collar patterns provide useful natural indicators of the underlying magnetospheric topology and can be used to determine (in a relative sense) the source location of the substorm activity. We conclude that the horse collar patterns have a distant tail source location and that the substorm onsets have a source location on closed field lines with much lower L values. The observations are therefore inconsistent with the onset mechanisms proposed in either the thermal catastrophe model or the boundary layer dynamics model. In addition, we have shown that substorm‐related plasma sheet dropouts are not always produced by a neutral line induced thinning of the plasma sheet but could also be caused by the large‐scale motion of the open field line region. Plasma data from DMSP F6, DMSP F7, ISEE 1, and Viking are consistent with the interpretation that the regions between the horse collar arcs are threaded by open lobe field lines and that open field lines can also partially intrude into the space between the transpolar arcs and the lower latitude main oval. The significance of these results for substorm activity which occurs together with quiet time auroral patterns is discussed in terms of the flux rope plasmoid version of the near‐Earth neutral line model. Depending upon the location and extent of the open field line regions relative to the developing plasmoid, it is suggested that several possibilities exist for the subsequent evolution of the substorm activity in the tail and that these different scenarios can be related to specific types of auroral morphologies in the ionosphere.
Evolution of turbulent magnetic fluctuation power with heliospheric distance
Authors: Zank G. P. , Matthaeus W. H. , Smith C. W.
Published: February 2004

On the basis of transport theories appropriate to a radially expanding solar wind, new results for the evolution of the energy density in solar wind fluctuations at MHD scales are derived. The models, which represent a departure from the well‐known WKB description, include the effects of “mixing”, driving by stream‐stream interactions (compression and shear) and interstellar pick‐up ions as well as non‐isotropic MHD turbulence. Magnetometer data from Voyager 1 and 2 and Pioneer 11 are compared to the turbulence‐based models and close agreement is found between theory and data for a reasonable choice of parameters.
Interaction of a nonuniform solar wind with the local interstellar medium
Authors: Pauls H. L. , Zank G. P.
Published: February 2004

Results from a fully time dependent three‐dimensional gasdynamic model of the interaction of the solar wind with the local interstellar medium are presented. Both subsonic and supersonic interstellar winds are considered, while the mediating effects of interstellar neutrals, magnetic fields, and cosmic rays are ignored. In accord with solar minimum observations by Ulysses, the solar wind properties are assumed to depend on heliolatitude. Two large, long‐lived polar coronal holes, one in the northern and the other in the southern hemisphere, are assumed to produce a hot, low‐density, high‐speed wind which bounds a cooler, higher‐density, low‐speed ecliptic wind. The solar wind boundary conditions for the simulation are drawn directly from published Ulysses data [Phillips et al., 1995, 1996]. Results from these calculations are compared to simulations which adopt isotropic solar wind conditions. For the parameters used in these simulations (which correspond to a solar wind ram pressure increase of 1.5 from the ecliptic plane to solar pole), the termination shock is found to be elongated along the solar polar axis and weakly time dependent. The elongation results in an increased flow in the ecliptic plane compared to that over the solar poles. With the increased flow in the ecliptic plane comes enhanced pressure and density gradients which leads to the generation and amplification of turbulent vortices in the heliotail. The rotational axes of the heliotail vortices are perpendicular to the ecliptic plane. Overall, it is found that a modest increase in solar wind ram pressure with heliolatitude has a pronounced effect on the global structure of the termination shock and heliopause.
A two‐dimensional model of the Venus ionosphere: 1. Unmagnetized ionosphere
Authors: Shinagawa H.
Published: February 2004

A two‐dimensional model of the ionosphere of Venus under unmagnetized conditions has been developed in order to study the processes of horizontal plasma transport. The continuity and momentum equations are solved self‐consistently, using the observed plasma temperatures. It is found that the vertical velocity is upward (∼10 m/s) at high altitudes (z > 240 km) in the dayside ionosphere. Below 240 km, on the other hand, the vertical velocity is downward and is basically the same as that obtained by one‐dimensional models. The results suggest that the nightside ionosphere can be maintained by day‐to‐night plasma transport, as was suggested by previous studies. It is also shown that the horizontal pressure gradient forces are sufficient to produce the observed acceleration of ionospheric plasma at least under some conditions. However, the results do not rule out the possibility that the solar wind momentum is transported to the ionosphere, which drives the day‐to‐night flow in the Venus ionosphere.
Mirror instability II: The mechanism of nonlinear saturation
Authors: Kivelson Margaret Galland , Southwood David J.
Published: February 2004

Mirror mode disturbances have been reported in many different space plasma environments. We suggest that these structures are fully evolved mirror mode waves that have achieved the condition for marginal stability against further growth. The limiting form is nonlinear, with field variations of the order of 50% of the average field. We argue that as an initially unstable plasma in a uniform field approaches stability, the particle distributions must separate into trapped and untrapped components that respond differently to the changing field. Most of the trapped particles are excluded from the mirror region. Exclusion sufficient to create marginal stability in the vicinity of the magnetic mirrors can be achieved by relatively small field intensifications. The trapped part of the distribution cannot achieve marginal stability without cooling. We envisage the cooling process as a Fermi deceleration achieved as the magnetic wells become deep and the mirror points move apart. Our analysis is both nonlinear and nonquantitative, but it provides an explanation for various aspects of the observations including the commonly reported feature that the mirror waves look like magnetic holes in the ambient field. We describe the pitch angle dependence of the plasma distribution that results from the processes discussed and note that the predicted distributions compare well with the forms observed in plasma data.
Low‐altitude energetic neutral atoms imaging of the inner magnetosphere: A geometrical method to identify the energetic neutral atoms contributions from different magnetospheric regions
Authors: Milillo A. , Orsini S. , Daglis I. A. , Bellucci G.
Published: February 2004

Energetic neutral atoms (ENA) imaging is an innovative technique of global magnetospheric observations, useful for studying geomagnetic storm and substorm processes. An intrinsic limitation of this technique is the difficulty of spatial distinction of the various magnetospheric regions contributing to the generation of the detected ENA fluxes. Here we apply a geometrical method able to distinguish the contribution to the ENA fluxes given by the inner radiation belt ions and by the ring current ion distributions. The ENA energy spectra are simulated by using the Active Magnetospheric Particle Tracer Explorers CCE charge‐energy‐mass energetic ion distributions, averaged over 2 years. The calculations are made for a specific vantage point (altitude is 550 km, MLT is 1930 and magnetic latitude is 0°); however, the technique can be easily extended to all locations on the equatorial plane, at similar altitudes. The simulated ENA spectra, taken at +30° and at −30° with respect to the midnight direction are used to reconstruct the inner radiation belt/ring current ENA spectra integrated along a general radial direction within the considered angular interval. For estimating the two contributions, we assume that the ion distributions are isotropic (within 17%) over the 4‐hour LT interval considered and that the functional form of the inner radiation belt ion distributions versus distance is not dependent on energy nor geomagnetic activity. The results show that there is a good agreement (within ∼5%) between the simulated ENA spectra along the radial direction (separately generated in the inner radiation belt and in the ring current) and those reconstructed through the proposed method. This technique can be applied to the real data for a zero‐order analysis.
Gompressive fluctuations generated by time evolution of Alfvénic perturbations in the solar wind current sheet
Authors: Malara Francesco , Primavera Leonardo , Veltri Pierluigi
Published: February 2004

The velocity‐magnetic field correlation in the solar wind low‐frequency turbulence decreases with increasing distance from the sun. A 2½‐dimensional MHD numerical simulation has been set up in order to investigate the interaction between the solar wind current sheet and Alfvén waves, as a possible decorrelation mechanism. The equilibrium structure models the magnetic field rotation in a sector boundary. On such an equilibrium we have superposed a perturbation intended to represent Alfvénic fluctuations converging with opposite correlation on the two sides of the heliospheric current sheet. In consequence of the opposite correlation a source of compressive fluctuations, related to a locally nonsolenoidal velocity field, is localized inside the current sheet. Moreover, the relation B2 = const is initially verified. During the time evolution the Alfvénic correlation is destroyed inside the current sheet arid compressive fluctuations are generated. Two kinds of such fluctuations have been identified, according to the sign of the density‐magnetic field intensity correlation. For β < 1, anticorrelated fluctuations (slow mode) remain mainly confined inside the neutral sheet, where they form pressure‐balanced structures (tangential discontinuities). Positive correlated fluctuations (fast mode), which also propagate perpendicular to the background magnetic field, also spread in the homogeneus region. The dominance of positive (negative) correlated fluctuations at frequencies lower (higher) than ∼ 0.1 hour−1 observed in the solar wind is found when β < 1. This seems to be due to the prevalence of anticorrelated fluctuations in regions where stronger nonlinear interactions transfer the fluctuation energy to small scales. Negative correlations at all scales are found for β > 1. A different dynamical mechanism is at work in the homogeneous region, producing spectra different from those found in the current sheet; this mechanism has been identified as a parametric decay. Spectra of physical quantities show features similar to those observed in the solar wind turbulence, both in the region where the Alfvénic correlation is destroyed and in the region where it is conserved.
Tailward progression of magnetotail acceleration centers: Relationship to substorm current wedge
Authors: Angelopoulos V. , Mitchell D. G. , McEntire R. W. , Williams D. J. , Lui A. T. Y. , Krimigis S. M. , Decker R. B. , Christon S. P. , Kokubun S. , Yamamoto T. , Saito Y. , Mukai T. , Mozer F. S. , Tsuruda K. , Reeves G. D. , Hughes W. J. , Friis‐Christensen E. , Troshichev O.
Published: February 2004

During a fortuitous near‐alignment of the IMP 8 and Geotail spacecraft along the magnetotail axis, geomagnetic activity monitors on the ground and at geosynchronous altitude detected the evolution of a magnetospheric substorm. We searched for evidence of the neutral line formation and evolution during this substorm. There exists evidence for multiple, time‐varying, localized (<9 RE) acceleration regions in the magnetotail at the time. Late in the expansion phase of the substorm a reversal of the predominant direction of the energetic particle anisotropy as well as the plasma flow velocity from tailward to Earthward was observed at Geotail at X=−61 RE. The tailward‐to‐Earthward anisotropy reversal is consistent with a tailward progression of the acceleration centers at Geotail. This phenomenon could be identified as the tailward retreat of the neutral line. However, the tailward progression of the activity is in reality the re appearance of new acceleration regions at gradually more tailward sites. Although the large temporal and spatial development is consistent with that of a reconnection geometry, the local Geotail acceleration is bursty and quite possibly localized. An ensuing increase of the northward magnetic field component seen at IMP 8 at X =−32 RE is consistent with a tailward expansion of the substorm current wedge out to that distance. Current wedge formation at IMP 8 was delayed by several minutes relative to the anisotropy reversal at Geotail. We interpret this as evidence that the location of current wedge and the location of magnetotail particle acceleration are separated by at least 30 RE during the late expansion phase of this substorm. Given the ∼9 RE cross‐tail separation of IMP 8 and Geotail, there still exists some uncertainty as to the three dimensional evolution of the acceleration and dipolarization regions in the late expansion phase of this substorm. Given the scarcity of similar IMP 8‐Geotail conjunctions during the distant tail Geotail phase, a full understanding of that evolution has to await a future multiprobe mission.
Electromagnetic ion cyclotron waves in the terrestrial plasma depletion layer: Effects of possible differential speeds between thermal H+ and He2+ ions
Authors: Gratton Fausto T. , Farrugia Charles J.
Published: February 2004

The terrestrial plasma depletion layer (PDL) is characterized by electromagnetic ion cyclotron waves (EICWs) [Anderson et al., 1991, 1994; Anderson and Fuselier, 1993]. These are of two types, continuous (CON), where the wave power transverse to the local mean field decreases continuously with frequency, and bifurcated (BIF), where a diminution in wave activity occurs between two activity peaks [Anderson et al., 1994]. Linear kinetic theory of plasma waves has been applied successfully to explain many features of the observed spectra [Denton et al., 1993; Gary et al., 1993]. In this paper we complement these theoretical studies by examining the effects a possible relative motion between the thermal H+ and He2+ populations has on the ion cyclotron activity, illustrating theoretical predictions by comparison with observed CON and BIF spectra. We find that even a modest differential speed (Uα ≡ | vα| − | vp |∼ 0.1 vA, where vA is the local Alfvén speed) can regulate the presence, location, and strength of the diminution in wave power often observed between the upper (∼ 0.55fp < f < fp) and lower (0.2 fp < f <∼ 0.40 fp) local maxima in wave activity. Using parameters appropriate to the CON spectrum, we find that when Uα = 0, there is a frequency interval centered around 0.45 fp, where the waves are damped, in disagreement with CON observations. With Uα, negative (i.e., the alpha particles are slower), the damping is enhanced and the damped frequency interval broadens, accentuating the discrepancy with the observations. When, however, Uα is small but positive, the previously damped frequency interval becomes an unstable region where wave growth rates are smaller with respect to peak values. Thus, although the activity is reduced there, it is not fully damped. By raising the positive values of Uα further, the bifurcation disappears and the spectrum becomes continuous. This may thus explain CON observations. For the BIF observation we find that a linear theory which does not incorporate inter‐ion species drift predicts a frequency gap (i. e., a region where there are no waves, damped or otherwise) centered at ∼ 0.45 fp, in addition to further intervals where the waves are damped. A small positive Uα (∼ 0. 07 vA) eliminates the frequency gap. A further, small increase in Uα eliminates the damped frequency intervals. We also compare the effects of the relative positive drift between H+ and He2+ ions with those obtained by raising the proton beta. In summary, we suggest that an H+ – He2+ relative drift may further help explain features of the bifurcated and continuous wave spectra observed by the CCE spacecraft in the PDL.
Parametric instabilities of Langmuir waves observed by Freja
Authors: Stasiewicz K. , Holback B. , Krasnoselskikh V. , Boehm M. , Boström R. , Kintner P. M.
Published: February 2004

We have analyzed complete spectrum of waves (0–4 MHz) and electron distributions during events of modulated Langmuir waves observed by Freja in the topside polar ionosphere. Modulated Langmuir waves are observed with amplitudes 1–1000 mV/m in association with beams of electrons with energies 50–5000 eV streaming through cold ionospheric plasma. We find overwhelming evidence for interactions of Langmuir (L) waves with lower hybrid (LH) waves. The interactions are consistent with parametric decay of beam induced L waves (L ↝ L′ + LH) as well as with scattering of L waves on preexisting LH waves (L + LH ↝ L′). These interactions are observed at all wave amplitudes ∼ 1 – 1000 mV/m but are sensitive to the k vectors of the primary waves (i.e., electron beam energy). In association with modulated Langmuir waves we also observe ion acoustic‐like signals which appear to be produced by rectification of the high‐frequency waves in the probe sheath through nonlinearities in the current‐voltage characteristic. The Langmuir wave events occur frequently in localized regions which are usually related to the phase of magnetic perturbation of an Alfvén wave. The observations show that Langmuir waves with amplitudes > 50 mV/m are very common in the auroral zone, and they represent the strongest electrostatic wave mode at altitudes of ∼ 1500 km. Langmuir waves appear to be an important element in the energy exchange chain in which part of the energy carried by Alfvén waves is transferred to energetic electrons and finally deposited to thermal plasma in a sequence: Alfvén wave ↝ electron beam ↝ Langmuir wave ↝ LH decays ↝ heating of thermal plasma.
Particle precipitation in auroral breakups and westward traveling surges
Authors: Olsson A. , Persson M. A. L. , Opgenoorth H. J. , Kirkwood S.
Published: February 2004

We have used data from the European Incoherent Scatter (EISCAT) facility to determine the characteristics of particle precipitation before and within auroral breakups and westward traveling surges. The EISCAT data of electron density versus altitude were transformed into energy spectra of the precipitating electrons [Kirkwood and Osepian, 1995]. We have used other ground‐based data such as auroral images and magnetic recordings to put the EISCAT observations into the context of the substorm development. In particular, we tried to determine the exact location of EISCAT with respect to the initial auroral forms of an onset. Characteristic differences are found in the deduced energy spectra of the precipitating electrons for the onset and the WTS events. Both the WTS and onset spectra showed a tendency of superposed spectral peaks at 10 keV and 20–30 keV respectively, which could be related to a near Earth acceleration mechanism. The conductance ratio in the onset events was twice as high as in the WTS events and also the measured particle fluxes were higher. We will discuss the implementations of our results for our understanding of particle acceleration mechanisms during the very onset of the substorm.
Field‐aligned currents associated with Alfven waves in the poleward boundary region of the nightside auroral oval
Authors: Nagatsuma T. , Fukunishi H. , Hayakawa H. , Mukai T. , Matsuoka A.
Published: February 2004

We have examined characteristics of the field‐aligned currents in the poleward boundary region of the nightside auroral oval, using magnetic field, electric field, and particle data obtained from the Akebono satellite. We found that large‐amplitude, wave‐like fluctuations of magnetic and electric fields frequently occur at mid‐altitudes confined in this latitudinally narrow region. We examined the relationship between electric and magnetic field fluctuations, examined using the complex impedance function defined as
, where
and
are the X component of the electric field and the Y component of the magnetic field, respectively. The most important result obtained from this analysis is that the features of the impedance functions in the frequency range between 2 × 10−3 and 8 × 10−2 Hz are well explained by a time‐dependent magnetosphere‐ionosphere coupling model. Further, the calculation of the field‐aligned Poynting flux showed that most of the Alfven wave energy is reflected at the ionosphere rather than dissipated in the ionosphere. These facts suggest that the majority of the field‐aligned current fluctuations observed at mid‐altitudes in the poleward boundary region of the nightside auroral oval are due to the superposition of incident and reflected Alfven waves. It is speculated that the breaking of reflected Alfven waves in the PSBL may cause the heating of plasma sheet electrons and ions.
Is there lower hybrid wave collapse at auroral latitudes?: Theory versus observations
Authors: Robinson P. A. , Melatos A. , Rozmus W.
Published: February 2004

Observations of lower hybrid wave packets in density depletions at altitudes of 500 to 13,000 km are compared with predictions from the theory of lower hybrid wave nucleation and collapse. It is shown that a widely accepted interpretation of these cavities as collapsing lower hybrid wave packets localized in self‐consistent density wells produced by their ponderomotive force is only partly consistent with observations. Observed potentials match or exceed those required for collapse and most observed length scales and aspect ratios lie in the theoretically predicted range. However, associated density perturbations appear to be almost always negative, contrary to theory based on ponderomotive force balance, which predicts them to be positive and negative equally often; moreover, they are typically 100 times greater in magnitude than this theory predicts. It is concluded that the electric‐field and length‐scale data are consistent with the occurrence of lower hybrid collapse, but that the lower hybrid waves either accumulate in preexisting density depressions, or produce the depressions ponderomotively via multiple collapses at a single site, or by means other than their ponderomotive force, as suggested by some previous authors. This work also suggests a possible resolution of conflicting results relating to the dominance of electron or ion heating by collapsing lower hybrid wave packets, with electron heating dominating for large ratios of electron temperature to ion temperature and ion heating prevailing in the opposite limit.
Quasi‐neutral sheet tearing instability induced by electron preferential acceleration from stochasticity
Authors: Sitnov M. I. , Malova H. V. , Lui A. T. Y.
Published: February 2004

The role of tearing instability in substorm expansion onset is examined. From a consideration of observational constraints on the onset mechanism we conclude that this instability must have a considerable initial stage when the equilibrium magnetic field topology (without neutral lines) is still conserved. Moreover, this instability is shown to have no linear stage. Instead, either explicitly nonlinear or pseudolinear instability of negative energy eigenmodes can develop. The latter possibility may arise from electron preferential acceleration. This acceleration occurs for the electrons near the equilibrium chaos/adiabatic boundary that interact with the perturbation electric field and provide a means of dissipation to restore positive feedback for the instability development. We compute a set of single‐particle orbits with realistic pitch angle distributions in a given tearing field model and evaluate statistical averages over the ensemble of particles and wave phases to verify the preferential acceleration of electrons. The resulting instability is analyzed by using a quasi‐hydrodynamical multifluid model. Its amplitude can be less than that required for neutral line formation. It has a fast growth rate well within the timescales for substorm expansion onset. Its excitation leads to earthward acceleration of electrons and ions as well as local current amplification in the near‐Earth region for excitation of cross‐field current instability. In the theme of substorm as a low‐dimension catastrophe for the current sheet, this nonlinear tearing instability approximates a fold catastrophe near the upper energy state of the system, while the unstable whistler waves from the cross‐field current instability correspond to positive energy oscillations near a new lower‐energy state of the current sheet achieved in the catastrophe. The nonlinear saturation of these whistler waves provides the irreversible relaxation of this state.
Martian planetopause as seen by the plasma wave system onboard Phobos 2
Authors: Trotignon Jean Gabriel , Dubinin Edik , Grard Réjean , Barabash Stanislav , Lundin Rickard
Published: February 2004

The existence of a plasma boundary between the bow shock and the atmosphere of Mars has been confirmed by the Phobos 2 observations. This boundary is called planetopause, magnetopause, ion‐composition boundary or protonopause, depending upon the authors. A careful examination of plasma wave system (PWS) data has revealed that planetopause signatures are generally clear and not questionable, enabling the calculation of the mean planetopause position up to 16 Martian radii in the distant tail. The planetopause model derived from PWS measurements is compared with models calculated from data sets obtained with other instruments on Phobos 2. With the exception of the transition plasma layer, called mass‐loading boundary by the Automatic Space Plasma Experiment with a rotating analyzer (ASPERA) investigators, the published planetopause, magnetopause/areomagnetopause and ion‐composition boundary locations are similar to the planetopause locations deduced from PWS data. This would suggest that, despite their different names, these boundaries correspond to the same plasma transition region. The PWS observations show that the planetopause position varies in the same way as does the bow shock position and is not significantly affected by the solar wind ram pressure. The absence of correlation between the planetopause position and solar wind ram pressure raises serious questions about the role that possible intrinsic magnetic field may play at the Martian planetopause. Finally, the planetopause is believed to be an ion‐composition discontinuity rather than the “obstacle” predicted by gasdynamic models.
Absolute differential and integral electron excitation cross sections for atomic nitrogen: 3. The 4S0 → 2D (λ5200 Å) transition from 5 to 30 eV
Authors: Yang J. , Doering J. P.
Published: February 2004

The absolute direct differential and integral electron excitation cross sections for the forbidden N I 4S0 → 2D (λ5200 Å) transition have been measured at incident energies from 5 to 30 eV. The differential cross section (DCS) was found to be peaked in the backward direction at 30 eV but isotropic at the lowest energies measured. The DCS measured versus scattering angle was integrated to give the integral cross section (ICS) at each incident energy. The maximum value of the ICS was 96 ± 49% × 10−18 cm2 at 7–8 eV. Comparisons with several theoretical calculations of the ICS showed excellent agreement in absolute magnitude with the experimental results. However, the peak of the experimental ICS was at 7‐8 eV, while the peak in the calculations was approximately 2.5 eV lower in energy. The experimental confirmation of the large peak value of the ICS for this multiply forbidden transition suggests that direct excitation of nitrogen atoms by electron impact may be the a significant source of 5200‐Å emission in dayside cusp precipitation events.
Are two‐fluid effects relevant to ULF pulsations?
Authors: Wright Andrew N. , Allan W.
Published: February 2004

It has been suggested recently that the traditional single‐fluid MHD description of ULF pulsations is wrong and that a two‐fluid model (which includes electron inertia) is required [Bellan, 1994]. If this claim is correct, it suggests most previous studies are inadequate and of questionable value. We have examined the appropriate equations and find that two‐fluid effects are not important for typical ULF pulsations. Specifically, the time taken to develop spatial scales similar to the electron inertia length far exceeds the lifetime of the waves. We argue that the singular normal modes of single‐fluid MHD are important properties of the magnetospheric system that should be studied. They may be employed to calculate the evolution of waves and are also a useful mathematical limit of the more realistic dissipative normal modes.
Ion acceleration at both the interior and exterior Alfvén waves associated with the magnetopause reconnection site: Signatures in cusp precipitation
Authors: Lockwood M. , Cowley S. W. H. , Onsager T. G.
Published: February 2004

We present evidence for the acceleration of magnetospheric ions by reflection off two Alfvén waves, launched by the reconnection site into the inflow regions on both sides of the reconnecting magnetopause. The “exterior” wave stands in the inflow from the magnetosheath and is the magnetopause, in the sense that the majority of the field rotation occurs there. The other, “interior” wave stands in the inflow region on the magnetospheric side of the boundary. The population reflected by the interior wave is the more highly energized of the two and appears at low altitudes on open field lines, immediately equatorward of the cusp precipitation. In addition, we identify the population of magnetosheath ions transmitted across the exterior Alfvén wave, as well as a population of magnetospheric ions which are accelerated, after transmission through the interior wave, by reflection off the exterior wave. The ion populations near the X line are modeled and, with allowance for time‐of‐flight effects, are also derived from observations in the dayside auroral ionosphere. Agreement between observed and theoretical spectra is very good and the theory also explains the observed total fluxes and average energies of the precipitations poleward of the open/closed field line boundary. The results offer a physical interpretation of all the various classifications of precipitation into the dayside ionosphere (central plasma sheet, dayside boundary plasma sheet, void, low‐latitude boundary layer, cusp, and mantle) and allow the conditions in both the magnetosphere and the magnetosheath adjacent to the X line to be studied.
On the origin of the high‐altitude electric field fluctuations in the auroral zone
Authors: Aikio A. T. , Blomberg L. G. , Marklund G. T. , Yamauchi M.
Published: February 2004

Intense fluctuations in the electric field at high altitudes in the auroral zone are frequently measured by the Viking satellite. We have made an analysis of the origin of electric and magnetic fluctuations in the frequency range of 0.1 – 1 Hz by assuming four different sources for the signals: (1) spatial structures, (2) spatial structures with a parallel potential drop below the satellite, (3) traveling shear Alfvén waves, and (4) interfering shear Alfvén waves. We will show that these different sources of the signals may produce similar amplitude ratios and phase differences between the perpendicular electric and magnetic fields. Since the different sources have different frequency dependencies, this can be used as an additional test if the signals are broadband. In other cases, additional information is needed, for example, satellite particle measurements or ground magnetic measurements.

The ideas presented in the theory were tested for one Viking eveningside pass over Scandinavia, where ground‐based magnetometer and EISCAT radar measurements were available. The magnetic conditions were active during this pass and several interfering shear Alfvén waves were found. Also, a spatial structure with a parallel potential drop below the satellite was identified. The magnitude of the 10‐km‐wide potential drop was at least 2 kV and the upward field‐aligned current 26 μA m−2 (value mapped to the ionospheric level). The field‐aligned conductance was estimated as 1.3 − 2.2 × 10−8 S m−2.
Plasma characteristics of the boundary layer in the Martian magnetosphere
Authors: Dubinin E. , Sauer K. , Lundin R. , Norberg O. , Trotignon J‐G. , Schwingenschuh K. , Delva M. , Riedler W.
Published: February 2004

Plasma and magnetic field data from circular orbits of the Phobos 2 spacecraft near Mars are examined to provide a description of the plasma properties of inner regions of the Mars magnetosheath and the boundary layer/plasma mantle. The data are analyzed in the VB coordinate system, which is reasonable for draping magnetospheres of nonmagnetized planets and comets. It is shown that a boundary almost impermeable for protons is formed. The ion composition changes at this boundary, and a transition layer dominated by planetary ions is observed. The characteristics of the magnetosheath plasma is drastically changed near this ion composition boundary. A strong drop of the proton bulk velocity is accompanied by an increase of the proton temperature and intense fluxes of planetary ions. In dependence of the solar wind dynamic pressure and other factors, radius of the “magnetospheric cavity”, virtually void of solar wind plasma varies from 4500 to 9500 km. During time intervals of very high solar wind dynamic pressure, the cavity, divided up in lobe cells, is almost degenerated. The comparison of positions of different magnetospheric boundaries identified earlier from single‐instrument measurements shows their collocation.
Kinetic thinning in one‐dimensional self‐consistent current sheets
Authors: Harold J. B. , Chen J.
Published: February 2004

The plasma that enters the region of the magnetotail current sheet can consist of multiple populations, each having a different spatial and temporal history. Processes remote from the current sheet can affect the distribution function of each component. In turn, these populations determine the structure of the current sheet. This interaction provides a purely kinetic mechanism by which “remote” processes can influence the current sheet structure. We investigate the effects of such multicomponent distributions on the current sheet, using self‐consistent, one‐dimensional test particle simulations. We model the system by separating the incoming plasma distribution into two components: a slowly drifting Maxwellian primary population and a low‐density secondary population. We then examine the influence of the secondary component by varying the energy and maximum pitch angle of this population. We find that changes in a small fraction of the incoming particle distribution can strongly modify the current sheet structure. In particular, increasing the field alignment or energy of 10% of the incoming particles can lead to significant thinning of the current sheet. This “kinetic thinning” is distinct from forces related to the large‐scale electric and magnetic fields and suggests that remote processes can have a strong influence on the structure of the magnetotail.
Solar wind proton flux latitudinal variations: Comparison between Ulysses in situ data and indirect measurements from interstellar Lyman α mapping
Authors: Summanen Tuula , Lallement Rosine , Quémerais Eric
Published: February 2004

We compare the solar wind proton flux latitude dependence derived in the past from the interstellar neutral H distribution in the inner heliosphere from Lyman α observations with recent “in situ” solar wind observations by Ulysses [Goldstein et al., 1995, Phillips et al., 1995, 1996, J.L. Phillips, private communication, 1996]. We find common features, such as a significant proton flux decrease with increasing heliographic latitude (about 30%) in the low‐latitude regions and broad “plateaus” of low particle fluxes (2.0 – 2.5 × 108 protons cm−2 s−1) around the poles. We use our model of interstellar H distribution under the influence of a multiparameter, latitude dependent solar wind to investigate the effects of a solar wind distribution matching as closely as possible the south heliographic Ulysses observations. For the first time, multiple scattering is included in such an anisotropic model. The Ulysses‐type wind is found to produce a secondary minimum of Lyman α intensity in the upwind direction, something already observed near solar minimum of activity. However, the modeled feature has a larger amplitude than the observed one, probably an indication of smoothing due to the combination of solar rotation and waviness of the neutral sheet. The total solar wind particle flux and the full Sun‐averaged ionization rate of the interstellar neutral H are estimated in various cases. For identical equatorial and polar fluxes, the existence of broad plateaus results in a significant reduction of the average neutral H ionization (and then of the ionization cavity) when comparing with models using a classical “harmonic” dependence with latitude. As a result, the downwind cavity is less depleted. This may partially explain some discrepancies between the expected and observed Lyman α emissions from the interplanetary hydrogen cavity, in particular, the excess of emission from the downwind cavity compared with the classical model.
Structure of electron and ion distribution functions near a reconnection line
Authors: Smets R. , Delcourt D. , Fontaine D. , Martin R. F.
Published: February 2004

We examine the transport of electrons and ions in the vicinity of an X line in the magnetotail, using single‐particle trajectory codes. It is shown that the particle distribution functions downstream of the reconnection region exhibit two distinct density domains separated by a narrow channel of enhanced densities. This narrow structure, which was first reported for ions by Martin and Speiser [1988] and referred to as a ridge, is due to particles which are essentially E × B convected from the lobes and experience large energy gains upon interaction with the neutral line. This structure weakly depends upon the characteristics of the inflowing population. As for the density domains on either side, they result from mixing of the particle populations during the reconnection process, high and low densities corresponding to particles originating earthward and tailward from the neutral line, respectively. The simulations also reveal fine low‐density structures embedded in the overall pattern. It is shown that these fine structures result from prominent gradient drift effects upon approach of the field reversal. It is demonstrated that, at a given observation point downstream of the reconnection region, the computed distribution functions strongly depend upon the azimuthal direction of view. An analytical estimate is proposed for the ridge location, which provides information on the neutral line characteristics.
Effect of centrifugal acceleration on the polar wind
Authors: Demars H. G. , Barakat A. R. , Schunk R. W.
Published: February 2004

The ionospheric convection electric fields that occur at high latitudes cause plasma to drift across the cusp region and the polar cap. Since the magnetic field at high latitudes is close to vertical, pointing downward (upward) in the northern (southern) hemisphere, the convecting plasma experiences a centrifugal acceleration as it crosses the polar region because of the diverging magnetic field geometry. The centrifugal force is directly proportional to the mass of the plasma particles, and it is reasonable to ask whether this force has an effect on polar plasma outflow, particularly for the more massive ion O+. To date, a number of studies have addressed this question, but the theoretical models used in these studies were either overly simplified (i.e., neglected processes known to be important in the polar ionosphere) or else did not use appropriate boundary conditions or take account of the time variability of the problem. The results of these prior investigations were often contradictory. In order to overcome the limitations of these earlier studies, we have used a macroscopic particle‐in‐cell (PIC) code, which is sophisticated in the sense that a broad range of physical processes are incorporated in its description, in conjunction with time‐varying boundary conditions obtained from a time‐dependent, three‐dimensional, hydrodynamic model of the polar ionosphere. This enables us to properly account for the variation of boundary conditions along a flux tube trajectory. Initially, our macroscopic PIC model was solved for steady state conditions. This allowed us to compare results from our code with those of a prior study of centrifugal acceleration that uses a PIC formulation. Also, by obtaining steady state solutions for both low and high electron temperatures, we have been able to directly compare the effects of electron temperature and centrifugal force on the polar plasma outflow, a comparison that a time‐dependent simulation might obscure. Then time‐dependent PIC solutions were obtained for the plasma in a convecting flux tube, using solutions to a time‐dependent, three‐dimensional, hydrodynamic model to provide realistic boundary values for the electron and ion temperatures and the H+ and O+ densities and drift velocities along a flux tube trajectory. Both steady state and time‐dependent solutions indicate that centrifugal acceleration does not significantly contribute to the loss of plasma from the polar ionosphere.