bluuebunny/crossref_metadata_2025 huggingface dataset:
Cusp energetic particle events: Implications for a major acceleration region of the magnetosphere
Authors: Chen Jiasheng , Fritz Theodore A. , Sheldon Robert B. , Spence Harlan E. , Spjeldvik Walther N. , Fennell Joseph F. , Livi Stefano , Russell Christopher T. , Pickett Jolene S. , Gurnett Donald A.
Published: February 2004

The Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) on board the Polar spacecraft observed 75 energetic particle events in 1996 while the satellite was at apogee. All of these events were associated with a decrease in the magnitude of the local magnetic field measured by the Magnetic Field Experiment (MFE) on Polar. These new events showed several unusual features: (1) They were detected in the dayside polar cusp near the apogee of Polar with about 79% of the total events in the afternoonside and 21% in the morningside; (2) an individual event could last for hours; (3) the measured helium ion had energies up to and many times in excess of 2.4 MeV; (4) the intensity of 1–200 KeV/e helium was anticorrelated with the magnitude of the local geomagnetic field but correlated with the turbulent magnetic energy density; (5) the events were associated with an enhancement of the low‐frequency magnetic noise, the spectrum of which typically extends from a few hertz to a few hundreds of hertz as measured by the Plasma Wave Instrument (PWI) on Polar; and (6) a seasonal variation was found for the occurrence rate of the events with a maximum in September. These characterized a new phenomenon which we are calling cusp energetic particle (CEP) events. The observed high charge state of helium and oxygen ions in the CEP events indicates a solar source for these particles. Furthermore, the measured 0.52–1.15 MeV helium flux was proportional to the difference between the maximum and the minimum magnetic field in the event. A possible explanation is that the energetic helium ions are energized from lower energy helium by a local acceleration mechanism associated with the high‐altitude dayside cusp. These observations represent a potential discovery of a major acceleration region of the magnetosphere.
Ion composition and charge state of energetic particles in flux ropes/plasmoids
Authors: Lui A. T. Y. , Williams D. J. , McEntire R. W. , Christon S. P. , Eastman T. E. , Yamamoto T. , Kokubun S.
Published: February 2004

Measurements from the energetic particles and ion composition (EPIC) instrument on Geotail are used to investigate the relative abundance of different ion species at energies ∼10 keV to 3 MeV in flux ropes or plasmoids detected at various downstream distances from ∼23 RE to ∼190 RE in the tail. In terms of charge state, we find that during encounters with these structures most of the oxygen ions are singly charged, i.e., O+ ions of ionospheric origin, while most of the helium ions are doubly charged, i.e., He++ ions of solar wind origin. Therefore the ion species from the solar wind and the ionosphere appear to be thoroughly mixed inside flux ropes or plasmoids, just like the ion composition in the tail outside these structures. This study also reveals a variety of changes in the abundance of ionospheric oxygen ions within flux ropes or plasmoids. These range from simultaneous enhancements of ionospheric oxygen ions within the structure to delayed enhancements or no significant change of ionospheric oxygen ions throughout the entire structure. The amount of ionospheric oxygen ions present in flux ropes or plasmoids increases with higher geomagnetic activity as gauged by the Kp index, but it shows no apparent trend with the downstream distance. The lack of consistent enhancement of ionospheric oxygen ions inside these structures suggests that oxygen ions may not play a significant role in their formation.
Laws for electron pressure variations across a collisionless shock
Authors: Guéret B. , Lembège B. , Belmont G.
Published: February 2004

In order to characterize the electron pressure variations across an oblique collisionless shock, statistical methods are applied to the results of two‐dimensional (2‐D) full particle electromagnetic simulations. Local correlations are looked for between the spatial variations of the pressures p‖ and p⊥ (parallel and perpendicular to the local magnetic field) throughout the shock profile and the corresponding variations of the density n and the magnetic field modulus B at the same location. Different orders in regression laws are successively analyzed, including the most general 4‐D regressions p⊥ p‖−un−vB−2w =constant, which test the degree of invariance of the quantities p⊥ p‖−un−vB−2w, the reduced 3‐D laws p⊥ n−a⊥ B−2b⊥ = cst, p‖ n−a‖ B−2b‖ = cst (as in CGL theory), and the reduced 2‐D correlations laws p⊥ n−γ⊥ = cst, p‖ n−γ‖ = cst, nB−Cp = cst (polytropic forms). Coefficients are determined quantitatively for each law. The use of these different regressions laws allows to check (1) where local correlations between these four quantities do exist at a given scale and, when verified, what is their effective forms; (2) when these general closure laws can be reduced to simpler ones, in particular to polytropic forms and with which polytropic indexes (γ=5/3?). This last result may have consequences concerning the fluid plasma modelizations for collisionless shocks or other nonlinear configurations; a comparison with the existing theories about the closure of fluid equations is briefly presented.
Electrostatic broadband ELF wave emission by Alfvén wave breaking
Authors: Seyler C. E. , Clark A. E. , Bonnell J. , Wahlund J.‐E.
Published: February 2004

The linear and nonlinear kinetic properties of electrostatic oblique waves below the lower hybrid frequency are investigated. For propagation angles the waves are damped by either parallel electron Landau or ion cyclotron damping. For Ti/Te ≫ 1 the waves are only weakly damped and can propagate. These waves are called slow ion cyclotron (SIC) and slow ion acoustic (SIA) Waves. A fluid‐kinetic model, comprised of hot linear kinetic ions and cold nonlinear fluid electrons, is proposed to describe a nonlinear wave breaking process of small‐scale Alfvén waves resulting in broadband extremely low‐frequency (ELF) wave emission. Numerical solutions of the fluid‐kinetic model are compared to the electric and magnetic fields of solitary kinetic Alfvén waves and broadband ELF waves observed by the Freja satellite within a hot ion environment. The agreement in waveform morphology and amplitude between the fluid‐kinetic simulations and the observed waves provides support for the theory that observed SIA waves are the result of a nonlinear emission process from SIC waves.
On the structure of coronal streamer belts
Authors: Eselevich V. G.
Published: February 2004

In an attempt to separate the temporal and spatial variations in polarization brightness (plasma density) along the coronal streamer belt a study is made of the parts of the belt which are longitudinally aligned with the west or east limb of the Sun. It is shown that the brightness distribution along streamer belts is inhomogeneous with relative brightness variations ≈ 1.1–2.0. Streamer belts consist of a sequence of coronal rays (streamers), each of which at distances R > 3Ro from the solar center has a cross section with a typical angular size of ≈10°–70° along the belt and ≈ 10°–30° across the belt with neighboring rays separated by L ≈ 10°–70°. Examples from CRs 1591 and 1592 show that in the absence of coronal mass ejection influence the distribution of streamer rays along the streamer belt can be stable for nearly two complete Carrington rotations.
October 1995 magnetic cloud and accompanying storm activity: Ring current evolution
Authors: Jordanova V. K. , Farrugia C. J. , Janoo L. , Quinn J. M. , Torbert R. B. , Ogilvie K. W. , Lepping R. P. , Steinberg J. T. , McComas D. J. , Belian R. D.
Published: February 2004

The passage at Earth of the October 1995 magnetic cloud and the high‐speed corotating stream overtaking it, monitored by the Global Geospace Science (GGS) spacecraft Wind, caused two consecutive geomagnetic storms: a major one during the strongBz< 0 nT phase of cloud passage and a moderate one during the intermittentBz< 0 activity in the fast corotating stream. Large dynamic pressure changes were observed in the sheath region ahead of the cloud and in the cloud‐stream interface region at its rear, resulting in substantial corrections to the measuredDstindex. A burst of superdense plasma sheet extending over ∼2 hours in local time was observed at geostationary orbit during the second storm. We simulate the ring current development during this storm period using our kinetic model and calculate the magnetic field perturbation caused by the ring current. The plasma inflow on the nightside is modeled throughout the investigated period using data measured at geosynchronous orbit. The modeledDstindex is compared with the observedDstvalues corrected for magnetopause and telluric currents. The temporal evolution of the ring current H+and O+distribution functions is computed, considering losses due to charge exchange, Coulomb collisions, and ion precipitation. We find that (1) the storm time enhancement of the plasma sheet ion population contributed significantly to the ring current buildup; (2) an additional ∼12 nT decrease inDstis achieved when the symmetry line of the plasma convection paths is rotated eastward from the dawn‐dusk direction with 3 hours during the first storm; (3) the major loss process is charge exchange, followed by Coulomb collisions and ion precipitation; (4) however, the energy losses due to ion precipitation increase monotonically during the more active periods, reaching the level of Coulomb losses at peak storm intensity. We argue that the losses due to ion precipitation considered in this study are closely related to the enhanced convection electric field, which in our model is parameterized with the planetaryKpindex. Correspondingly, we find that (5) there is a very good correlation between the variations in time of this index and the magnitude of the ion precipitation losses.
Cascade excitation in the geocoronal hydrogen Balmer α line
Authors: Nossal S. , Roesler F. L. , Coakley M. M.
Published: February 2004

This paper reports high‐accuracy measurements of geocoronal Balmer α line profiles and demonstrates that the profiles are well fit with a model which includes cascade excitation by solar Lyman series radiation from n > 3 in addition to the direct excitation of n = 3 by solar Lyman β. The increase in the signal‐to‐noise of our data is made possible by the use of the Fabry‐Perot annular summing technique implemented at our Fabry‐Perot facility at the University of Wisconsin's Pine Bluff Observatory. The new sensitivity has allowed us to make a detailed examination of line profile asymmetries and to conclude that they are compatible with predictions that of the order of 10% of the geocoronal Balmer α emission is caused by the cascade process. Cascade excitation alters the observed profile because it produces Balmer α emission along fine structure paths yielding slightly shifted wavelengths not present in direct Lyman β excitation, which is the predominant excitation mechanism for geocoronal Balmer α. We discuss how fine structure excitation affects studies of non‐Maxwellian exospheric hydrogen velocity distributions and effective temperatures through Balmer α line profile measurements. In a broader context, we consider how inclusion of the cascade excited emission in future radiation models can enhance their accuracy and their potential for assisting in the isolation in the data of shorter‐term solar geophysical effects and longer timescale changes in exospheric hydrogen densities.
Comparison of the high‐latitude ionospheric electrodynamics inferred from global simulations and semiempirical models for the January 1992 GEM campaign
Authors: Winglee R. M. , Papitashvili V. O. , Weimar D. R.
Published: February 2004

The global characteristics of the auroral oval during the Geospace Environment Modeling (GEM) campaign of January 1992 are investigated through four different models: the assimilative mapping of ionospheric electrodynamics (AMIE) technique, the IZMIRAN electrodynamic model (IZMEM), the Weimer ionospheric convection model, and three‐dimensional global fluid simulations. It is shown that all four models predict essentially the same key features with respect to the position, shape, and extent of the auroral convection cells, including the position of the separatrix between open and closed field lines. The relative change in the magnitude of the cross‐polar cap potential is about the same for the different models, being between 20% and 50% in the case studies examined. However, there is some discrepancy by a factor of about 2 in predicting the absolute value. The global simulation potential is highest because it includes convection at low to middle latitudes, which appears to add about 50 kV to the total potential. It is shown that the auroral field‐aligned currents are very sensitive to changes in Bz interplanetary magnetic field, with changes of the order of 200–300% in the total integrated current being inferred for the above changes in potential. The increase, particularly for the highest activity period, is due to an increase in the area of the oval rather than an increase in intensity.
Upstream ULF fluctuations near Mars
Authors: Delva M. , Dubinin E.
Published: February 2004

Magnetic field data from the Phobos 2 spacecraft near Mars (March 1989) are investigated for ULF fluctuations in the upstream region of the solar wind plasma onstreaming onto the planet. In general, ULF fluctuations of the magnetic field are expected when the spacecraft is located in the ion foreshock, which is defined through the actual magnetic field direction. A statistical study of the occurrence of such ULF fluctuations for all circular orbits around Mars is presented. The upstream boundary of the foreshock is found at ∂BN ≈ 61° for moderate values of the angle between the solar wind velocity vector vsw and the magnetic field vector B; for more perpendicular magnetic field configurations, the boundary lies at approximately the same value of ∂BN ≈ 60°. At Mars these boundaries occur at higher ∂BN values than at Earth. Higher turbulence is found deeper in the foreshock. Therefore the situation is qualitatively in accordance with the situation at Earth or Venus. One difference occurs in the region upstream of the foreshock, where Mars seems to have more wave activity. The reason for this difference is unclear but may be related to the role of planetary ions in the foreshock or to the contribution of ion kinetic effects to the solar wind interaction with Mars.
Evolutionary process of 10 kilometer scale irregularities in the nighttime midlatitude ionosphere
Authors: Saito A. , Iyemori T. , Takeda M.
Published: February 2004

The nighttime midlatitude ionosphere is numerically simulated to study the evolutionary process of the irregularities that generate the electric field fluctuations observed by satellites. Ionospheres in both hemispheres are included in the simulation, and the evolution of irregularities is studied with given neutral winds in these regions. In each ionosphere, an equation of the field line integrated Pedersen conductivity is derived by integration of the ion momentum equation along the geomagnetic field line. Given initial perturbations are reformed to a sheet‐like structure at the first stage of the simulation. After this reformation, the irregularities start to grow through plasma instability. The sheet‐like structures are also formed in the conjugate ionosphere, where no initial irregularities are given, when the irregularities are sufficiently developed. In a case of an intense neutral wind applied only in the source hemisphere, the growth rate is comparable to that predicted by the linear analysis which includes the electrical coupling between the conjugate ionospheres. When the intense neutral wind is applied in both the conjugate ionospheres, the growth of irregularities is affected by the electric currents in both hemispheres even when the conjugate ionosphere is in winter. The electric current of the conjugate ionosphere can grow or damp the irregularities depending on its direction.
Heliospheric tomography using interplanetary scintillation observations: 1. Combined Nagoya and Cambridge data
Authors: Jackson B. V. , Hick P. L. , Kojima M. , Yokobe A.
Published: February 2004

We have produced a computer assisted tomography program that optimizes a three‐dimensional model to fit observational data. We have used this program with interplanetary scintillation data from Nagoya, Japan, and Cambridge, England. The program iterates to a least squares solution fit of observed data using solar rotation and solar wind motion to provide perspective views of each point in space accessible to the observations. We plot the optimized model as Carrington maps in velocity V and density Ne for the two data sets with resolutions of 10° in heliographic longitude and latitude. We map the model to 1 AU and compare this to in situ observations from the IMP spacecraft. From this comparison we find ΔNe ∝ Ne0.3. We plot Carrington maps extrapolated to the solar surface to compare with Yohkoh Soft X ray Telescope (SXT), Sacramento Peak green line, and Mark III K‐coronameter observations. High velocities modeled at the solar surface for individual rotations trace coronal holes (including polar ones) observed in SXT data. Regions of high density modeled from the Cambridge scintillation level data generally show a high correlation with regions of high solar activity observed as bright in Yohkoh SXT and green line observations. There is also a general correspondence of the regions of high density and the areas which are bright in K‐coronameter observations.
Galileo energetic particle detector observations of geomagnetically trapped protons
Authors: Alinejad Naser , Armstrong Thomas P.
Published: February 2004

The Galileo spacecraft encountered the Earth once on December 8, 1990 (Earth I), and again on December 8, 1992 (Earth II). These flybys provided excellent opportunities to evaluate the performance of the energetic particle detector (EPD) and establish analysis procedures in a relatively well‐known environment. Further, because Galileo's Earth flyby trajectories were very rapid and nearly radial, the radiation belt measurements provided an excellent “snapshot” of trapped radiation. The EPD data agree with and extend the familiar radiation belt empirical models established by the National Space Science Data Center. Because of the rapid flyby and the 20‐s spin period of Galileo, great care had to be taken to remove time aliasing from the pitch angle distributions. Large anisotropies were also present due to spatial density gradients. Spherical harmonics were fitted to the pitch and phase distributions in order to remove the effects of time aliasing and density gradient anisotropies and obtain fluxes from which phase space densities could be computed. The phase space density was calculated at pitch angles and energies that conserve the first and second adiabatic invariants. The extracted phase space density was compared with that computed from the AP8 model of the stably trapped proton fluxes. Earth I and Earth II observations and the AP8 model were compared for values of the first adiabatic invariant at 10.0, 15.0, 20.0, 25.0 30.0, 35.0, 40.0, 45.0, and 50.0 MeV/G and of the second adiabatic invariant at 0.10, 0.15, 0.20, 0.25, and 0.30 G0.5 RE. Results show that for lower values of the first and second adiabatic invariants the AP8 values slightly exceed the observations, and for higher values of the first two adiabatic invariants the AP8 values are slightly lower than the observations; and in the middle range of invariants there is relatively good agreement between AP8 and observations.
Comparison of topside and bottomside irregularities in equatorial F region ionosphere
Authors: Kuo F. S. , Chou S. Y. , Shan S. J.
Published: February 2004

Numerical simulations have been carried out to study and compare the mechanisms of the formations of the topside and bottomside ionospheric irregularities. As is well‐known, the topside ionospheric irregularities have always originated from the bottomside ionosphere due to Gravitational Rayleigh‐Taylor (GRT) instability. From the penetration process we found that the primary bubble with a horizontal scale of tens of kilometers always has a smooth shape in the bottomside ionosphere and breaks into much smaller‐scale structures after penetrating into deep topside ionosphere. Since the plasma bubble is unlikely to break into irregularities in the bottomside ionosphere, the bottomside irregularities probably result from the turbulence in the neutral atmosphere. Examples of simulations are presented to demonstrate that bottomside irregularities may exist in a region with a strong neutral wind jet and seeding gravity wave motion. A strong neutral jet stream always has a strong vertical wind shear, which provides a broad height range of dynamic instability. When the atmospheric gravity wave approaches its critical level, its amplitude will continually grow to break into smaller‐scale waves and turbulence. Then all the waves of different scales as well as the turbulence will become the seeds of the bottomside irregularities.
Cosmic ray recovery and solar minimum phase of solar cycle 22: An interim report
Authors: McDonald Frank B. , Lal Nand , McGuire Robert E.
Published: February 2004

Cosmic ray observations in space now cover four successive solar minima and extend to heliocentric distances beyond 65 AU. In the outer heliosphere, comparisons between the 1996 and 1987 data show a reversal in the sign of the latitudinal gradients, smaller radial intensity gradients, and the peak in the energy spectra of anomalous H and He+ is at much lower energies but accompanied by a large reduction in their intensity at higher energies. There is also a marked difference in the cosmic ray time history over successive solar minima. Qualitatively, all of these observations are consistent with the expected effects of gradient and curvature drifts on these particles. At 1 AU the H and He spectra returned to their 1977 levels in mid‐1996, and the relative comparison of the four solar minima spectra clearly show the imprint of a 22‐year modulation cycle. In the outer heliosphere in 1997 the galactic and anomalous cosmic ray intensities at 51 AU are well below their peak intensities in 1987 at 42 AU (except for low‐energy anomalous cosmic rays), and in fact, are comparable to the Pioneer 10 intensities at 15 AU in 1977/1978. The data over successive solar minima suggest that the turbulence over the solar poles plays an important role in qA > 0 epochs. In qA < 0 periods the neutral current sheet provides ready access to cosmic ray ions in the outer heliosphere, but this path becomes increasingly difficult inside ∼20 AU. At the present time, solar minimum conditions still prevail, and there may be further increases in these energetic particle populations in the outer heliosphere.
Compressibility of the poloidal mode
Authors: Denton Richard E.
Published: February 2004

The poloidal mode is a coupled Alfvén and fast magnetosonic mode which has Alfvén‐like dispersion in the limit of large azimuthal wave number. In this brief report we discuss the assumption of incompressibility. From the point of view of ideal MHD, incompressibility is, in general, a bad assumption for the poloidal mode when the magnetic field is curved. Assuming an adiabatic equation of state for the pressure p and that it is acceptable to drop the contribution of υ‖ (a low beta assumption), an approximate condition for the incompressibility assumption to be valid in the magnetosphere is that the equilibrium pressure scale length
, where LRE is the geocentric distance from the Earth, γ is the ratio of specific heats, and the plasma beta β≡2p0/B02. However, the results of kinetic theory (only discussed here) indicate that the most applicable equation of state for an odd magnetospheric poloidal mode is an incompressible equation for p. This is not because the divergence of the bulk velocity equals zero. The high‐temperature hot particles which supply the plasma pressure have motion which is dominated by their gradient and curvature drifts and bounce motion; this motion is different from that of the bulk plasma which moves only because of the E × B drift.
Kinetic quasi‐viscous and bulk flow inertia effects in collisionless magnetotail reconnection
Authors: Kuznetsova Masha M. , Hesse Michael , Winske Dan
Published: February 2004

Both electron inertia and nongyrotropic effects were analysed using 2 1/2‐dimensional hybrid simulations of collisionless magnetic reconnection. The traditional hybrid approach which treats ions as particles and electrons as an isotropic massless fluid was modified. In the new model the electron mass dependence was introduced both in the expression for the electric field and in the evolution equation of the full electron pressure tensor. This comprehensive hybrid model includes full ion kinetics, incorporates Hall effects, describes the leading terms in electron dynamics responsible for breaking the frozen magnetic flux constraint and allows to consider arbitrary ion/electron temperature and mass ratios. We demonstrate that the kinetic quasi‐viscous electron inertia associated with nongyrotropic pressure effects dominates over the electron bulk flow inertia in controlling the structure of the dissipation region around the neutral X line. The reconnection electric field based on the nongyrotropic pressure tends to reduce the current density and to relax gradients in the vicinity of the X line (similar to the localized anomalous viscosity). On the other hand, the reconnection electric field based on electron bulk flow inertia tends to require an increased current density, with gradient scales comparable with the electron skin depth. The dependence on the ion/electron temperature ratio and on the current carrier also is discussed. An analytical analysis which supports the results of the numerical simulations is also presented.
Magnetic clouds at sector boundaries
Authors: Crooker N. U. , Gosling J. T. , Kahler S. W.
Published: February 2004

Eight of the 14 magnetic clouds in the period August 1978 to February 1982, previously identified in solar wind data by their smooth magnetic field rotations, were encountered at sector boundaries. Most of these differed from those not encountered at sector boundaries in two ways. First, the accompanying counterstreaming electrons, indicating closed magnetic topology, on average spanned intervals more than twice as long as the identified clouds. Second, the field rotations within the identified clouds appear to form part of larger‐scale rotations beyond the cloud boundaries that roughly coincide with the counterstreaming electrons. The results suggest that the documented clouds were parts of larger transient structures that are best observed in their entirety at sector boundaries. The data are consistent with spacecraft passages through both the leading and trailing sections of kinematically distorted flux rope loops rooted to the Sun.
An estimate for the size of cycle 23 based on near minimum conditions
Authors: Wilson Robert M. , Hathaway David H. , Reichmann Edwin J.
Published: February 2004

The first occurrence of a high‐latitude, new cycle spot group for cycle 23 was in May 1996, in conjunction with a minimum in the smoothed monthly mean sunspot number. Since then, new cycle spot groups have become more predominant, and the smoothed monthly mean sunspot number has slowly risen. Such behavior indicates that new cycle 23 probably had its minimum annual average sunspot number, R(min), equal to 8.7, in 1996. Because this value is larger than the average for R(min), cycle 23 is expected to have a maximum amplitude, R(max), that, likewise, will be larger than average, suggesting further that it probably will be both fast rising (i.e., peaking before May 2000) and of shorter than average length (i.e., ending before May 2007). Another parameter well correlated with R(max) is the minimum amplitude of the aa geomagnetic index, aa(mm), which usually occurs either in the year of R(min) occurrence or, more often, in the following year. For 1996 the annual average of aa measured 18.6. Presuming this value to be aa(min) for cycle 23, we calculate cycle 23's R(max) to be about 171.0 ± 17.6 (i.e., the 90% prediction interval), based on the stronger (r = 0.98) bivariate fit of R(max) versus both R(min) and aa(min). Comparison of this estimate with others, using various combinations of parameters, yields an overlap in the prediction intervals for R(max) of about 168 ± 15, a range that is within the consensus recently reported by Joselyn et al. [1997] (= 160 ± 30). Thus this study supports the view that cycle 23 will have an R(max) that will be larger than average but smaller than was seen for cycle 19, the largest cycle on record with R(max) = 190.2.
The acceleration time of anomalous cosmic rays: Observational constraints from Pioneer 10 data
Authors: Scherer K. , Fichtner H. , Fahr H. J.
Published: February 2004

Crosscorrelations of solar Lyman α and cosmic ray data from 1981 until 1990 recorded by instruments on Pioneer 10 during its interplanetary cruise are presented. Using the Lyman α radiation as a proxy for the long‐term solar activity, a comparison of the correlation functions of the anomalous and galactic cosmic ray data reveals a significantly greater phase shift in the time variation of the former. This additional time lag is increasing with increasing particle energy and is attributed to the time required to energize pickup ions at the heliospheric termination shock and thus allows a determination of the characteristic acceleration time of anomalous cosmic rays. This, in turn, results in an estimate of the efficiency of spatial diffusion of energetic particles perpendicular to the magnetic field just upstream of the heliospheric termination shock at low heliographic latitudes.
Temporal development of irregular magnetic pulsations occurring during low‐latitude auroras
Authors: Higuchi Yoshihiro
Published: February 2004

Temporal characteristics of irregular magnetic pulsations and intervals of pulsations of diminishing periods (IPDP) plasma wave events observed in association with low‐latitude auroras are reported. The two most significant issues are the particle spectrum associated with low‐latitude auroras and convective growth rate modeling for the IPDP plasma wave events. In particular, it is shown that the growth rate is sensitive to variations in oxygen‐ion kinetic energy in addition to movements of the source region. The frequency increase in the dynamic spectrum of a low‐latitude IPDP plasma wave event can be explained in terms of the variation of the kinetic energy of the precipitating oxygen‐ion stream during low‐latitude auroras.
Hydromagnetic wave propagation and coupling in a magnetotail waveguide
Authors: Allan W. , Wright Andrew N.
Published: February 2004

For some time the magnetotail has been considered as a possible region where hydromagnetic waves can propagate as waveguide modes. Recently, attention has turned to the magnetospheric flanks as waveguides, and much useful insight has been gained into propagation of fast waveguide modes there, and the structure of the field line resonances they can drive. We return to the magnetotail and investigate hydromagnetic wave propagation and coupling in a magnetotail waveguide. This problem is significantly different from the flank waveguide as the ambient magnetic field is directed along the waveguide rather than across. Field line resonances of the flank type are not possible in the lobe waveguide. We describe a numerical simulation of a model waveguide in which the Alfvén speed decreases across the waveguide to the central plasma sheet. The waveguide is stimulated by a short compressional perturbation located in the far tail. The cross‐tail spatial structure is chosen to give relatively weak coupling between fast and Alfvén modes so that phase and group velocities of uncoupled fast modes can be used to interpret the results. We find that the perturbation propagates dispersively down the waveguide in the form of fast waveguide modes. Fourier components with small parallel wavenumber contain most of the energy, and propagate relatively slowly toward the “Earth.” These act as moving sources which launch Alfvén waves continuously earthward. The wave dispersion relations are such that the waveguide modes couple with Alfvén waves only in a limited region of the transverse Alfvén speed gradient. The Alfvén waves travel at the local Alfvén speed along each field line, so that as they travel the wave on a given field line becomes increasingly out of phase with waves on adjacent field lines. The phase mixing in our model is novel in that it includes the effects of transverse gradients in both Alfvén frequency and parallel wavenumber which tend to cancel each other out. Nevertheless, the phase‐mixing process leads to increasingly fine transverse structure as the waves progress down the waveguide. The results are likely to be applicable in regions such as the plasma sheet boundary layer and the plasma mantle.
Dayside moving auroral forms and bursty proton auroral events in relation to particle boundaries observed by NOAA 12
Authors: Moen J. , Lorentzen D. A. , Sigernes F.
Published: February 2004

This paper consider two sequences of moving auroral forms that were observed in the early postnoon sector above Svalbard. The series of events observed on January 12, 1992, moved westward (noonward) under unknown IMF conditions. The events observed on December 17, 1992, moved eastward while interplanetary magnetic field BY was strongly negative. The auroral luminosity for these events was dominated by the 630.0‐nm emission line but correlated in space and time with subvisual Hß intensifications. NOAA 12 particle characteristics relate the moving auroral forms to low‐latitude boundary layer precipitation located poleward of the electron trapping boundary, i.e., on open magnetic field lines. The auroral activity and associated particle signatures are both attributed to magnetopause reconnection. Since magnetic reconnection is the only known mechanism capable of bulk injecting magnetosheath ions and electrons on the dayside, we suggest that simultaneous occurrence of proton and electron auroral activity is a unique footprint of dayside magnetic reconnection.
Inclusion of shielded Birkeland currents in a model magnetosphere
Authors: Peroomian Vahé , Lyons Larry R. , Schulz Michael
Published: February 2004

We have developed a technique for including large‐scale magnetospheric current systems in magnetic field models. In this paper, we incorporate the Region I and Region II Birkeland current systems into the Source Surface Model (SSM) of the terrestrial magnetosphere. The original SSM was a prescribed‐magnetopause model that used a spherical harmonic expansion and a variational principle to obtain the magnetic field in the near‐Earth region. Field lines in the tail were constructed geometrically. This model is ideal for studying the effects of Birkeland currents since it has a well‐defined boundary between open and closed field lines, magnetopause shielding currents separate the geomagnetic and interplanetary magnetic fields in the absence of a specified connection between the two, and Birkeland currents are not included in the model either implicitly or explicitly. The Region I and II Birkeland currents are added to the model using a series of field‐aligned, infinitely thin wire segments. The magnetic field produced by these currents is calculated from the Biot‐Savart law and its normal component on the surface of the magnetopause is shielded (minimized) by image currents carried on wires placed outside the magnetosphere. The result of these added currents is a sunward shift of the separatrix between open and closed field lines. Since Region I currents are regarded here as flowing on the separatrix, this shift necessitates an iteration of our procedure. Three iterations were required to reach a satisfactory solution in our case. We find that inclusion of Birkeland currents in the SSM results in a northward Bz in the near‐midnight tail region, and a southward Bz at the flanks. The closure of previously open flux near midnight accounts for the sunward shift of the separatrix. Moreover, the sunward shift is found to increase with increasing Birkeland current strength by an amount that agrees with observations of the auroral oval.
Electron scattering loss in Earth's inner magnetosphere: 2. Sensitivity to model parameters
Authors: Abel Bob , Thorne Richard M.
Published: February 2004

The sensitivity of the rate of energetic electron pitch angle scattering and precipitation loss in the Earth's magnetosphere due to Coulomb interactions with thermal plasma and resonant wave‐particle interactions with plasmaspheric hiss, lightning‐generated whistlers and VLF transmitter signals is computed for a realistic range of plasma and wave parameters. The computed scattering rates are most sensitive to the choice of mean wave frequency and the average angle of propagation; the frequency bandwidth and angular spread in propagation direction are relatively unimportant. Variations in plasma density can also strongly modulate the effectiveness of wave‐particle interactions. The average intensity of plasma waves controls electron lifetimes, but has little effect on the shape of the equilibrium pitch angle distribution function. All three classes of waves must be included together with Coulomb collisions in any realistic analysis of inner magnetospheric electron scattering. The long‐term energetic electron population beyond the inner belt is largely controlled by plasmaspheric hiss and, to a lesser extent, by lightning‐generated whistlers, while VLF transmitter signals constitute the principal loss mechanism over the range 1.3 ≤ L ≤ 2.4. The region below L = 1.3 is dominated by Coulomb collisions.
Excitation of X rays in comet Hyakutake (C/1996 B2)
Authors: Krasnopolsky Vladimir A.
Published: February 2004

There is great disagreement between current evaluations of the role of bremsstrahlung and line excitation by electron impact and recombination in cometary X rays. To study the problem in detail, we use the exact values of the Roentgen satellite (ROSAT) high‐resolution imager and wide‐field camera instrument response functions and apply them to all nine exposure intervals in the ROSAT observation of comet Hyakutake to determine X ray two‐parameter power law and thermal bremsstrahlung spectra. Thermal bremsstrahlung is a convenient form for two‐parameter spectrum and not a physical reality in comets. Then spectral index is α = 1.97 ± 0.18 ≈ 2, spectral temperature is kT = 360 eV, and the exposure‐mean photon emission is 3 × 1025 ph s−1 and 1.3 × 1016 ergs s−1 in a range of 90–2000 eV and in an aperture radius of 1.2 × 105 km. The exposure‐mean ROSAT spectrum is in excellent agreement with the Extreme Ultarviolet Explorer (EUVE) observation of 7.5 × 1024 ph s−1 in a range of 100–165 eV from comet Hyakutake. Then we follow the same methods as Northrop et al. [1997] to calculate a volume emission rate at the comet brightness maximum and to calculate that from bremsstrahlung with the Vega 2 electron flux. Yet we obtain a much lower emission than Northrop et al., at a level of 0.5% of the observed emission in accordance with results of Krasnopolsky [1996, 1997a] and Bingham et al. [1997]. Electron impacts of molecular, atomic, and ion species in comets produce mostly excitation of the X ray lines C 277 eV and O 525 eV at a level of 2% of the total X ray emission from comet Hyakutake. This value is calculated for the Vega 2 electron flux. The value for the Giotto electron flux is 3 × 10−6 of the measured emission. Processes which are responsible for impurity radiation in fusion plasma either are inefficient or produce ultraviolet photons in cometary plasma, and the method of Bingham et al. is inapplicable to excitation of X ray lines in comets. The calculations show that the expected effects of bremsstrahlung and electron impact are very low in cometary X rays. The obtained spectral index α = 2 differs significantly from α ∼ 5 for scattering of solar X rays by attogram dust. These facts favor charge transfer of heavy solar wind ions as the dominant process of X ray excitation in comet Hyakutake. Charge transfer is not efficient in production of X rays on the Moon and other solid bodies.
Wavelength dependence of Doppler spectrum broadening in midlatitude E region coherent backscatter
Authors: Haldoupis C. , Bourdillon A. , Delloue J. , Hussey G.
Published: February 2004

We use observations from a high‐frequency Doppler radar experiment at midlatitudes to study the wavelength dependence of spectrum broadening of backscatter from field‐aligned E region irregularities. The radar made multifrequency measurements of high‐resolution Doppler spectra simultaneously at four frequencies, that is, 9.23, 11.03, 12.71, and 16.09 MHz, which correspond to backscatter from irregularities with wavelengths of 16.3, 13.6, 11.8, and 9.3 m, respectively. In the analysis we deal with secondary irregularities (type 2 echoes) characterized by small mean velocities that show no dependence on radar frequency and have mean spectrum width to velocity ratios well above unity. The spectrum width is found to increase monotonically with wavenumber k in the range from 0.38 to 0.67 m−1 covered in the experiment. By postulating that the width is determined mainly by the nonlinear growth rate of the secondary short‐scale plasma turbulence, we compare the results to the Sudan [1983] theory. Although there is some general agreement, on the average, the measured mean spectrum width follows approximately a k4/3 power law dependence which is considerably stronger than the theoretical k2/3 dependence. This discrepancy may be reduced somewhat but cannot be fully accounted for when additional spectrum broadening effects caused by the irregularity velocity distribution are also included.
Charged dust dynamics above the surface of a comet far from the Sun
Authors: Juhász A. , Szegő K.
Published: February 2004

We investigate the dynamics of small dust particles ejected from the sunlit side of the nucleus of a comet far from the Sun. The motion of these particles is determined by gravity, electromagnetic forces, and solar radiation pressure. The nucleus is not shielded from either the solar wind or the solar UV radiation; therefore the surface of the nucleus absorbs electrons and protons and emits photoelectrons. As a result, the surface gets positively charged. Above the nuclear surface a photoelectron plasma sheath forms in which the ejected dust grains collect electrostatic charges and become responsive to the electric field in the sheath. We show that most of the dust cloud created by an impact (e.g., a lander) will settle after a few (∼4) hours elsewhere and not in the close vicinity of the impact site (i.e., on the lander itself).
Cross‐helicity and residual energy in solar wind turbulence: Radial evolution and latitudinal dependence in the region from 1 to 5 AU
Authors: Bavassano B. , Pietropaolo E. , Bruno R.
Published: February 2004

Solar wind plasma and magnetic field measurements by Ulysses have been used to study magnetohydrodynamic turbulence in different heliospheric regions. Four intervals of six solar rotations have been analyzed. Two of them are on the ecliptic around 2 and 5 AU, respectively, one is at midlatitude near 5 AU, and the last one is at high latitude around 3 AU. Conditions on the ecliptic are those typical of high solar activity periods. The midlatitude interval is characterized by very strong gradients in the wind speed, due to an intermittent appearance of the wind coming from the polar coronal hole. In the high‐latitude interval, fully inside the polar wind, the speed is steadily high. We investigated at three different scales (1, 4, and 12 hours) the level of correlation between velocity and magnetic field fluctuations, as given by the normalized cross‐helicity, and the sharing of the fluctuation energy between its kinetic and magnetic component, as measured by the normalized residual energy. The observations on the ecliptic, while confirming previous findings based on Voyagers data, clearly indicate that the normalized cross‐helicity is well different from zero also at distances as large as 5 AU. The midlatitude turbulence, when compared to that at low and high heliographic latitudes, appears much more evolved, with a remarkably lower normalized cross‐helicity (in absolute value). This unambiguously highlights that processes at velocity gradients are an important factor in the turbulence evolution. For all the analyzed intervals the residual energy values indicate an imbalance in favor of magnetic fluctuations, in agreement with previous results. The strongest imbalance is observed for the high‐latitude sample, where the turbulence is comparatively the least evolved. This is a quite unexpected result, probably related to the presence of interstellar pickup ion populations. In conclusion, our analysis indicates that (1) velocity gradients play a dominant role in driving the turbulence evolution in the solar wind and (2) pickup ion effects might be significant.
Shape of the heliospheric termination shock: Effects of latitude variations of solar wind dynamic pressure
Authors: Barnes Aaron
Published: February 2004

We investigate the consequences of internal solar wind latitude variations on the heliospheric termination shock and the flow of the gas beyond the shock. We have developed a simple gasdynamic model, assuming the solar wind to be a steady, axially symmetric radial outflow of gas that passes through a termination shock and flows incompressibly beyond the shock. We ignore any latitude variations external to the heliosphere (i.e., due to the local interstellar medium) by requiring that the stagnation pressure infinitely far away must be spherically symmetric. Analysis of the model leads to three broad conclusions: (1) The shape of the heliospheric shock is qualitatively similar to what one would predict using the “naive” assumption that the heliocentric distance of the shock is proportional to the square root of the scaled solar wind dynamic pressure ρvr2. (2) However, the existence of an internal latitude dependence of the scaled dynamic pressure requires that the shock must be oblique at some latitudes, and this obliquity produces an outward “bulge” in the shape of the termination shock; this conclusion is completely general, and in particular is true for the case of an oblate shock. (3) For a prolate termination shock the far‐down‐stream flow is deflected toward the equator, and solar wind originating in the poleward half of (say) the northern hemisphere would fill more than half of the volume of the same hemisphere beyond the termination shock; the deflection would be in the opposite sense (poleward) for an oblate termination shock.