bluuebunny/crossref_metadata_2025 huggingface dataset:
Galactic and heliotail‐in anisotropies of cosmic rays as the origin of sidereal daily variation in the energy region < 104 GeV
Authors: Nagashima K. , Fujimoto K. , Jacklyn R. M.
Published: February 2004

It is shown that the cosmic ray sidereal daily variation in the energy region <EU ∼ 104 GeV is due to two kinds of anisotropy, one is a galactic anisotropy from the direction with right ascension αG = 0 hours and declination δG = −20° and can be observed even in the energy region as low as ∼60 GeV with the same form and almost the same phase as observed by small air showers with ∼EU [Nagashima et al., 1989]. The other is a newly discovered directional excess flux confined in a narrow cone with a half opening angle of ∼68° from the direction (αT ∼ 6.0 hours; δT ∼ −24°) and observed only in the energy region less than EU with its maximum near 103 GeV. It is suggested that the excess flux is of solar origin and the direction toward it seems to coincide with the expected heliomagnetotail direction (αTP = 6.0 hours; δTP = −29.2°) opposite the proper motion of the solar system but does not coincide with the expected tail direction (αTN = 4.8 hours; δTN = 15° ∼ 17°) opposite to the relative motion of the system to the neutral gas. The flux (called the tail‐in anisotropy hereafter) shows maximum at the December solstice when the Earth is closest to the magnetotail and almost disappears at the remote side of the Earth's orbit from the tail at the June solstice. Owing to the discovery of the tail‐in anisotropy, the observed phase shift of the sidereal diurnal (24 hours) variation from 6 to 0 hours with the increase of energy, which has been one of the unsolved problems, can be explained by the distinctive contributions from the two anisotropies. Finally, it appears that the observed sidereal variations deny the existence of the Compton‐Getting effect due to the motion of the solar system at least in the energy region less than ∼EU. This implies that the solar system drags with it in its motion the surrounding interstellar magnetic field within which the cosmic rays with low energy (less than ∼EU) are isotropically confined.
Flux transfer events produced by bursty merging at a single X line
Authors: Ku Hwar C. , Sibeck David G.
Published: February 2004

We present predictions for the signatures of flux transfer events (FTEs) at the dayside magnetopause produced by bursty merging at a single extended X line. In this MHD model, the time‐dependent enhanced local resistivity turns on at a low northern latitude for a brief time and then turns off. Realistic ratios of magnetosheath to magnetosphere parameters are chosen to conform with observations and our previous simulations [Ku and Sibeck, 1997, 1998]: ρmsh/ρsph = 10, Bmsh/Bsph = 0.5, and Tmsh/Tsph = 0.175. The burst of merging produces FTEs marked by strongly asymmetric bipolar magnetic field and plasma velocities normal to the magnetopause in the magnetosheath and no significant signatures in the magnetosphere, just as in the previously studied case for the step‐function onset of merging [Ku and Sibeck, 1997]. However, the simulation produces other FTE signatures that differ strikingly from those generated by the step‐function onset of merging. After the resistivity is switched off, one or more secondary reconnection sites appear on the trailing edge of the events. Merging at these sites inflates and propels the main FTE as it moves out of the simulation domain. Merging at these sites also fills a magnetic island with hot tenuous plasma near the subsolar (symmetry) point. Transient magnetic islands form on the trailing edge of the main FTE, but these remain small and may disappear. Scatterplots of various parameters versus the plasma density reveal that steep changes for the temperature kT, magnetic field BZ, and the Alfvén velocity occur on the magnetosheath side immediately adjacent to the magnetopause and also reflect the fact that the events scarcely deform the magnetopause.
Lower bound for electron core beta in the solar wind
Authors: Gary S. Peter , Newbury Jennifer A. , Goldstein Bruce E.
Published: February 2004

Solar wind electrons, especially under conditions of relatively low speed flow, often can be represented as two bi‐Maxwellian components, a cooler, more dense core (denoted by the subscript c) and a hotter, more tenuous halo. Solar wind observations from Ulysses between 1.5 and 2 AU further indicate that the β for electron core temperatures parallel to the background magnetic field, β‖c, has a distinct lower bound near 0.1. To seek the cause of this possible constraint, numerical solutions of the full Vlasov linear dispersion equation are used for four heat flux instabilities under a core/halo model with parameters representative of the solar wind near 1 AU. In this model the whistler heat flux instability is the growing mode of lowest threshold at most observed values of β‖c. As β‖c is decreased, however, the growth of this mode is reduced, so that at sufficiently small values of this parameter the Alfvén heat flux instability or the electron/ion acoustic instability becomes the fastest growing mode. The critical condition corresponding to this transition is calculated as a function of T‖c/Tp (where Tp is the proton temperature) and approximately corresponds to the observed constraint at β‖c ≃ 0.1. The Alfvén and ion acoustic instabilities both resonate with core electrons; the hypothesis is proposed that core heating by these two modes at the critical condition establishes a lower bound on β‖c.
Extended solar wind helium distribution functions in high‐speed streams
Authors: Chotoo K. , Collier M. R. , Galvin A. B. , Hamilton D. C. , Gloeckler G.
Published: February 2004

The extended velocity distribution function for solar wind doubly charged helium (He+2) has been systematically studied for seven high‐speed streams occurring in the first 7 months of 1995, near solar minimum. The two instruments, suprathermal ion composition spectrometer (STICS) and high mass resolution spectrometer (MASS), used in this study are both part of the SMS experiment on board the Wind spacecraft. The helium distributions were fit over the speed range v/vsw ≈ 0.8–1.6 constituting ∼7.5 thermal widths and 5 orders of magnitude in phase space density, where vsw is the proton solar wind speed. The distributions are reasonably well fit by kappa functions with values of kappa ranging from 3.4 to 5.8, although there may be systematic deviations from a perfect kappa function over the 5 orders of magnitude in phase space density. This is the first study that characterizes the extended helium distribution functions using kappa functions in the high‐speed solar wind.
Proton resonant firehose instability: Temperature anisotropy and fluctuating field constraints
Authors: Gary S. Peter , Li Hui , O'Rourke Sean , Winske Dan
Published: February 2004

The electromagnetic proton firehose instability may grow in a plasma if the proton velocity distribution is approximately bi‐Maxwellian and T‖p > T⊥p, where the directional subscripts denote directions relative to the background magnetic field. Linear Vlasov dispersion theory in a homogeneous electron‐proton plasma implies an instability threshold condition at constant maximum growth rate 1 − T⊥p/T‖p = Sp/β‖pαp over 1 < β‖p ≤ 10 where
and Bo is the background magnetic field. Here Sp and αp are fitting parameters and αp ≃ 0.7. One‐ and two‐dimensional initial value hybrid simulations of this growing mode are carried out under proton cyclotron resonant conditions in a homogeneous plasma on the initial domain 2 ≲ β‖p ≤ 100. The two‐dimensional simulations show that enhanced fluctuations from this instability impose a bound on the proton temperature anisotropy of the form of the above equation with the fluid theory result αp ≃ 1.0. On this domain both one‐ and two‐dimensional simulations yield a new form for the upper bound on the fluctuating field energy density from the proton resonant firehose instability
where SB and αB are empirical parameters which are functions of the initial growth rate. This logarithmic behavior is qualitatively different from a fluid theory prediction and, like the anisotropy bound, should be subject to observational verification in any sufficiently homogeneous plasma in which the proton velocity distribution is approximately bi‐Maxwellian.
Electron heating by the cross‐shock electric potential
Authors: Ball Lewis , Galloway D.
Published: February 2004

Electrons can experience superadiabatic heating at a collisionless shock due to the presence of a cross‐shock electric potential. This heating can greatly exceed the temperature rise associated with the conservation of the magnetic moment of the electrons as they move into the stronger magnetic field downstream of the shock. The criterion for the onset of superadiabatic heating and the effects of the detailed structure of the shock are investigated for the special case where the magnetic field is perpendicular to the shock normal. A comparison of the natural length scale of the electron orbits and the shock thickness is used to derive a new criterion for the onset of superadiabatic heating. This new heating criterion facilitates an improved understanding of the role of various physical parameters in determining the degree of electron heating. It has the same functional form as the condition for the electron equations of motion to permit exponential divergence of electron orbits (an important feature of dynamical chaos) which it has previously been argued determines the onset of superadiabatic heating. However, we show that the orbits of individual electrons undergoing superadiabatic heating do not diverge significantly because of the very short time they spend within the shock, the only region in which the equations of motion are nonlinear. The techniques used to analyze dynamically chaotic systems are therefore of little assistance in understanding this potentially important process.
Electron precipitation accompanying Pc 5 pulsations observed by the DE satellites and at a ground station
Authors: Nosé M. , Iyemori T. , Sugiura M. , Slavin J. A. , Hoffman R. A. , Winningham J. D. , Sato N.
Published: February 2004

Using data from the polar orbiting Dynamic Explorer (DE) −1 and −2 satellites and a ground‐based station, we investigated electron precipitation phenomena accompanying Pc 5 pulsations. DE‐2 observed oscillatory disturbances in the magnetic and electric fields in the upper ionosphere at the geomagnetic footprint of the high altitude region in which transverse Pc 5 pulsations were detected by DE‐1. DE‐2 observed electrons precipitating into the ionosphere with energies of several keV to several tens of keV. These electrons were accelerated in the direction of the ambient magnetic field. When Pc 5 pulsations in the H‐component and periodic variations of cosmic radio noise absorption (CNA pulsations) were observed at Syowa Station, DE‐2 which was in geomagnetic conjunction with Syowa Station also observed oscillatory disturbances in the magnetic and electric fields. These oscillatory disturbances are caused by small‐scale field‐aligned currents each with width of 0.5°–1.4° invariant latitude. This suggests that Pc 5 pulsations have a small‐scale resonance structure in the radial direction. The resonance structure has a small scale comparable to the ion acoustic gyroradius, then kinetic Alfvén waves having electric fields parallel to the ambient magnetic field can arise. The parallel electric field generates a field‐aligned potential drop of about 3–5 kV. Electrons accelerated by these kinetic Alfvén waves would cause CNA pulsations, the phase of which leads that of the H‐component of the Pc 5 pulsations by 90° in the southern hemisphere. This is consistent with the observations at Syowa Station.
UARS observations of Birkeland currents and Joule heating rates for the November 4, 1993, storm
Authors: Anderson B. J. , Gary J. B. , Potemra T. A. , Frahm R. A. , Sharber J. R. , Winningham J. D.
Published: February 2004

Magnetic field and particle observations from the Upper Atmosphere Research Satellite particle environment monitor (UARS/PEM) are used to estimate field‐aligned currents, electron precipitation energy flux, ionospheric conductivities, and Joule heating rates during the main phase of the November 4, 1993, geomagnetic storm. From 0300 to 1200 UT on November 4 the auroral oval expanded equatorward of 65° magnetic latitude (MLAT), and UARS encountered the polar cap on seven consecutive passes during the storm main phase. These passes provide data appropriate to determine field‐aligned currents and estimate ionospheric Joule heating. For this storm, UARS sampled the midnight to dawn sector in the northern hemisphere and the noon to dusk sector in the southern hemisphere. The maximum net currents on the day side and nightside are comparable and reach 1 A/m for several hours. The average Joule heating rates are comparable at midnight, early morning, and noon, where they are 9.2, 6.6, and 7.7 GW/h, respectively, but have a strong peak in the late afternoon, where they are 25.6 GW/h. In contrast, the electron precipitation energy deposition is highest near midnight at 5.6 GW/h but drops to less than half this level to 2.4 GW/h and 1.9 GW/h in the early morning and at dusk, respectively, but is very small near noon, only 0.24 GW/h. The Joule to particle energy deposition rate ratio thus varies by roughly an order of magnitude with local time, being over 40 near noon, about 20 at dusk, 3 near dawn, and 2 at midnight. The hemispherical Joule and electron precipitation heating rates, HJ and Helec, are estimated to have been 290 GW and 50 GW, respectively, giving HJ/Helec = 4.5 and HJ + Helec = 340 GW. Differences between these averages and assimilative mapping of ionospheric dynamics (AMIE) results, HJ = 200 GW and Helec = 80 GW, reflect time variability during the storm and are largely resolved when AMIE results only at the times of UARS passes are considered.
Ionospheric total electron content perturbations monitored by the GPS global network during two northern hemisphere winter storms
Authors: Ho C. M. , Mannucci A. J. , Sparks L. , Pi X. , Lindqwister U. J. , Wilson B. D. , Iijima B. A. , Reyes M. J.
Published: February 2004

The global evolution of two major ionospheric storms, occurring on November 4, 1993, and November 26, 1994, respectively, is studied using measurements of total electron content (TEC) obtained from a worldwide network of ground‐based GPS receivers. The time‐dependent features of ionospheric storms are identified using TEC difference maps based on the percent change of TEC during storm time relative to quiet time. The onset of each ionospheric storm is indicated by the appearance of auroral/subauroral TEC enhancements which occur within 1 hour of the beginning of the geomagnetic storm main phase. Significant TEC enhancements (> 100%) are observed in the winter northern hemisphere. The rate at which TEC enhancements appear is found to correlate with gradients in the Dst index. The large scale ionospheric structures identified during the storms are (1) nightside auroral/subauroral enhancements which surround the auroral oval, (2) dayside (around noon) high‐latitude and middle‐latitude enhancements associated with traveling ionospheric disturbances, and (3) conjugate latitudinal enhancements. For the November 1993 storm, a short positive phase (about 15 hours) is followed by a long negative phase (∼60 hours). In the November 1994 storm, we have identified the clear signature of a traveling ionospheric disturbance (TID) which propagated at a speed of ∼460 m/s from ∼60° N to ∼40° N. The motion of this disturbance appears to conserve angular momentum.
Properties of Ganymede's magnetosphere as revealed by energetic particle observations
Authors: Williams D. J. , Mauk B. , McEntire R. W.
Published: February 2004

Energetic particle observations made during the Galileo satellite's close encounters with Jupiter's moon Ganymede provide a measure of many characteristics of Ganymede's magnetosphere, a magnetosphere larger than that of Mercury. Changes in energetic ion anisotropy signatures, caused by Jupiter's corotating magnetic field, show that Ganymede's magnetosphere significantly slows down the ambient nearly corotating Jovian plasma. The data further indicate that this convection slowdown apparently extends at least several Jupiter radii away from Ganymede along Jovian field lines connected to the moon. The locations of these anisotropy signature changes occur at the locations of magnetometer and plasma wave magnetopause identifications that match surprisingly well the predictions of a simple model comprised of an intrinsic dipole field superimposed on Jupiter's ambient field. Loss cones measured throughout Ganymede's magnetosphere provide a quantitative estimate of the moon's surface magnetic field along the subsatellite track that agrees well with model field predictions and provides verification of Ganymede's intrinsic magnetic field. On Jovian field lines connected to Ganymede, electrons are trapped between the moon and a near‐Jupiter mirror point for several bounces as they convect across Ganymede's magnetosphere. This unique geometry allows a measurement of the amount of electron pitch angle scattering occurring in a single bounce between the Ganymede and near‐Jupiter mirror points. The energy‐dependent pitch angle diffusion coefficient and scattering lifetimes have been extracted from these observations. The consistency of these results from encounter to encounter demonstrates the stability of these processes, at least over the several month interval between encounters. Evidence for loss cone signatures established one Jovian rotation (∼10 hours) earlier indicates that the entire Jovian L shell region traversed by Ganymede provides an electron scattering environment similar to that measured on Jupiter‐Ganymede field lines. Measurements from Galileo's last close encounter with Ganymede show electron pitch angle distributions characteristic of particles trapped on closed magnetic field lines in a distorted magnetospheric configuration. Thus with its distorted magnetic field and apparent radiation belt, Ganymede takes its place as a unique member of the solar system family of magnetospheres.
Interplanetary Lyman α remote sensing with the Ulysses Interstellar Neutral Gas Experiment
Authors: Pryor Wayne R. , Witte Manfred , Ajello Joseph M.
Published: February 2004

The Ulysses neutral gas instrument obtained celestial sphere maps of interplanetary Lyman α emission from neutral hydrogen in 1991–1996. These maps are unique because the spacecraft was located at a wide range of heliocentric ecliptic latitudes. Eleven of these maps are compared with the predictions of an interstellar wind hydrogen model previously used to study Galileo and Pioneer Venus Lyman α data obtained near the ecliptic plane. The model provides reasonable agreement with the Ulysses maps. The lifetime of the interstellar hydrogen atoms against charge exchange with solar wind protons is shown to be more latitudinally isotropic at solar maximum in 1991 than at solar minimum in 1994 and 1995. This result agrees with white light coronagraph data that generally show a more isotropic coronal brightness pattern at solar maximum. The transition from a more symmetric to a more asymmetric solar wind charge exchange rate also explains differences between published Galileo data from solar maximum and Solar and Heliospheric Observatory (SOHO) Solar Wind Anisotropy Experiment (SWAN) data from solar minimum.
A three‐dimensional MHD study of solar wind mass loading processes at Venus: Effects of photoionization, electron impact ionization, and charge exchange
Authors: Bauske Rainer , Nagy Andrew F. , Gombosi Tamas I. , De Zeeuw Darren L. , Powell Kenneth G. , Luhmann Janet G.
Published: February 2004

Ionization of hot and cold planetary oxygen ions caused by solar radiation, electron impact, and charge exchange with the solar wind protons lead to a heavy plasma population near Venus which is accelerated and incorporated into the solar wind plasma. lonization of hot and cold hydrogen also enhances the solar wind density and reduces its momentum. Charge exchange of the solar wind plasma with hot and cold planetary hydrogen removes a part of this heavy ion population. We include these processes in a three‐dimensional magnetohydrodynamic (MHD) model of the solar wind interaction with Venus and analyze their impact on the magnetic and flow field geometry around Venus. We find that photoionization is the most important of these processes if the change in the plasma composition is taken into account. The ionization processes lead to a strong deceleration of the flow around the planet, which in turn results in changes of the magnetic field. Because of the increasing mean mass and density of the plasma near the ionopause velocity gradients develop in the near terminator region, which lead to a local increase of the magnetic field. The inclusion of mass loading in our model moves the bow shock position outward, close to the location observed by the Pioneer Venus Orbiter magnetometer at solar maximum.
Solar semidiurnal anisotropy of galactic cosmic ray intensity observed by the two‐hemisphere network of surface‐level muon telescopes
Authors: Munakata K. , Kitawada T. , Yasue S. , Mori S. , Kato C. , Koyama M. , Akahane S. , Hall D. L. , Fujii Z. , Fujimoto K. , Humble J. E. , Fenton A. G. , Fenton K. B. , Duldig M. L.
Published: February 2004

Observations made by the two‐hemisphere network of surface‐level, multidirectional muon telescopes at Hobart (Tasmania, Australia) and Nagoya (Aichi, Japan) are used to examine the origin of the solar semidiurnal variation in cosmic ray intensity. The network allows us to precisely determine the asymmetry of the variation across both hemispheres. It is shown that the variation is consistent with the north–south (NS) symmetric distribution of cosmic ray intensity in space. The phase of the space harmonic vector responsible for the variation is consistent with both the second‐order anisotropy expected from a bidirectional latitudinal density gradient (type I) and also one arising from pitch angle scattering (type II). The network also observed a purely NS antisymmetric, antisidereal diurnal variation with the maximum phases differing by 12 hours between the two hemispheres. This is consistent with an antisidereal diurnal variation arising from annual modulation of the solar diurnal variation produced by a second‐order anisotropy. The phase of the space harmonic vector responsible for the antisidereal diurnal variation is consistent with the phases predicted from both type I and type II anisotropies. It is shown, however, that the ratio of the amplitude of the space harmonic vector of the antisidereal diurnal variation to that of the solar semidiurnal variations is consistent with the type II anisotropy but not with the type I anisotropy. This result implies that the solar semidiurnal variation and the antisidereal diurnal variation observed during the period 1992–1995 mainly arise from the type II anisotropy and cannot be explained solely as arising from the type I anisotropy.
Presunrise spread F at Fortaleza
Authors: MacDougall J. W. , Abdu M. A. , Jayachandran P. T. , Cecile J.‐F. , Batista I. S.
Published: February 2004

At Fortaleza, Brazil, in the equatorial zone about 400 km south of the magnetic equator a presunrise (secondary) maximum of spread F occurrence is observed during sunspot minimum and, in particular, during December solstice. The spread F takes the form of patches of irregularities that are convecting eastwards at ∼50 m s−1. Most of the patches are collocated with bottomside bulges of the ionosphere. Our measurements indicate that these bottomside bulges are unstable due to a gradient‐drift instability that is slowly growing and produces the spread F. The bulges themselves seem to be evidence of a Rayleigh‐Taylor instability process.
Antarctic HF radar observations of irregularities associated with polar patches and auroral blobs: A case study
Authors: Ogawa T. , Nishitani N. , Pinnock M. , Sato N. , Yamagishi H. , Yukimatu A. S.
Published: February 2004

We report a case study of decameter‐scale electron density irregularities associated with polar cap patches and auroral (boundary) blobs in the southern high‐latitude F region ionosphere. The observations were carried out on July 14, 1995, with the Antarctic Super Dual Auroral Radar Network HF radars located at Syowa Station and Halley. On that day, 17 irregularity events associated with the patches were identified in the polar cap. The time distribution of these events is consistent with previous model calculations of patch formation and transportation in the northern hemisphere for southward interplanetary magnetic field (IMF) conditions (Bz < 0). The irregularity areas observed can attain to 1000×1500 km that is comparable to a typical polar patch size, suggesting that the electron density within the patch is not uniform but structured. Somewhat surprisingly, patch‐associated irregularities appear even under long‐lasting northward IMF conditions (Bz > 0). These patches seem to have been transported into the polar cap from the dayside cusp where the patches had been generated under negative Bz conditions. The striated radar echo patterns due to a series of auroral blobs, clearly observed at Halley in the evening auroral zone, are well explained by previous simulations that calculated the time evolution and transportation of a patch initially located in the polar cap.
A statistical study of ion energization mechanisms in the auroral region
Authors: Norqvist Patrik , André Mats , Tyrland Magnus
Published: February 2004

More than one mechanism is causing ion energization perpendicular to the geomagnetic field in the auroral region. Observations by the Freja satellite obtained during 20 months at altitudes around 1700 km are used to study the most common O+ energization mechanisms. Ion energization can be due to broadband low‐frequency waves, causing resonant energization by interaction at frequencies around the ion gyrofrequency. Energization can also be caused by resonant energization by electromagnetic ion cyclotron waves at about half the proton gyrofrequency, or by emissions near the lower hybrid frequency. About 90% of all O+ heating events and about 95% of the total O+ upflow are caused by ion energization associated with broadband low‐frequency waves. The dependence of the different energization types on magnetic local time, magnetic latitude, magnetic activity, and season is studied. We find that the prenoon auroral region is a major source of O+ ions energized by broadband low‐frequency waves.
Phase skipping and Poynting flux of continuous pulsations
Authors: Chi P. J. , Russell C. T.
Published: February 2004

It is recognized that the continuous pulsations (Pc) in the magnetosphere are in general composed of many wave packets and that these wave packets are separated by phase skips. Previous observations by multiple spacecraft and ground magnetometer stations suggest that the phase skipping and packet structure of pulsation signals are most likely due to impulsive wave sources or the beating of waves. However, the magnetic field data alone have not led to an understanding of the propagation of these wave bursts. By using both the electric field and magnetic field data of the ISEE 1 spacecraft, we find that the Poynting flux of Pc3–4 pulsations in the outer magnetosphere has an impulsive nature. The direction of time‐average Poynting flux changes every several wave cycles, and the phase skips in wave signals, both in the magnetic field and electric field, are often found between two adjacent Poynting flux bursts. Furthermore, the phase difference between the electric wave and the magnetic wave indicates that Pc3–4 waves are generally not in exact resonance, counter to the traditional field line resonance paradigm. These observational results provide us with strong evidence that the “continuous” pulsations in the Pc3–4 band are in fact maintained by a series of pulses, and the phase skips in the wave signals are its natural consequence. The direction of wave energy propagation and its implications are also discussed.
Odd oxygen measurements during the Noctilucent Cloud 93 rocket campaign
Authors: Gumbel J. , Murtagh D. P. , Espy P. J. , Witt G. , Schmidlin F. J.
Published: February 2004

The NLC‐93 campaign at Esrange, Sweden, addressed the chemical, dynamical and electrical environment of the summer mesopause in the presence of noctilucent clouds (NLC). A major topic was the detailed investigation of odd oxygen abundances and their structure in the vicinity of the cloud layer. Applying independent in situ measurements, we have obtained consistent profiles of atomic oxygen, ozone, and mesospheric total density. Atomic oxygen was measured on two rocket flights by means of a newly developed probe utilizing the 1304 Å resonance fluorescence / absorption technique. Detailed Monte Carlo simulations of the radiative transfer and the instrument aerodynamics are included in the data analysis. We find relatively low atomic oxygen densities with a peak of 2 × 1011 cm−3 at 96 km. An airglow radiometer recorded the O2 IR‐atmospheric band emission, allowing the retrieval of ozone densities by means of an inversion model that considers both direct and indirect sources of O2 (a1Δg). The odd oxygen measurements are compared to a steady state photochemical model. While good agreement is found in the mesosphere, the results deviate close to the cold mesopause. Significant vertical structures are found in the atomic oxygen profiles; sharp gradients and distinct minima coincide with the observed NLC layer. Possible influences of heterogeneous chemistry on the odd oxygen abundance are discussed.
A classification of dayside auroral forms and activities as a function of interplanetary magnetic field orientation
Authors: Sandholt Per Even , Farrugia Charles J. , Moen Jøran , Noraberg Øystein , Lybekk Bjørn , Sten Torleif , Hansen Truls
Published: February 2004

We present a classification of auroral forms in the dayside high ‐ latitude ionosphere, based on ground observations from Svalbard. Having sorted the different auroral forms by magnetic local time (MLT) and morphological and optical spectral characteristics, we then study them as a function of the orientation of the interplanetary magnetic field (IMF). We find that the IMF clock angle θ is a good parameter with which to order the different dayside auroras. This is illustrated by two case examples covering the whole dayside: (1) the 4‐hour‐long passage of the sheath region of the January 10 – 11, 1997, magnetic cloud and (2) a 10‐hour‐long interval on January 12, 1997, during passage of the corotating stream overtaking the cloud. A variety of IMF conditions were realized. We identify the following three auroral configurations in the cusp region and the IMF clock angle regimes in which they occur: (1) In the clock angle range θ< 45° there are auroral bands located at high latitudes (∼ 78° – 79° magnetic latitude (MLAT)), (2) For θ within ∼ 45° – 90°, auroral bands are simultaneously present at high and low (< 75°MLAT) latitudes, (3) For θ > ∼ 90° the high ‐ latitude aurora disappears, and only the low ‐ latitude forms remain. These latter forms manifest themselves as quasiperiodic sequences of moving bands or band fragments within ∼ 73° – 78°MLAT (called poleward moving auroral forms) or quasi ‐ steady auroral bands with east ‐ west moving forms at low latitudes (< 73°MLAT). Strong asymmetries in auroral forms and motions are related to the east ‐ west component (By) of the IMF. The above auroral configurations are discussed in terms of current knowledge on particle precipitation, IMF ‐ related, field ‐ aligned currents, and corresponding modes of solar wind ‐ magnetosphere coupling. We find that the time history of the basic magnetopause coupling modes is manifested in the dayside aurora. We identify candidate auroral signatures of both quasi ‐ steady and pulsed reconnection processes occurring at either low or high magnetopause latitudes. Additional auroral forms in the dawn and dusk sectors are discussed in terms of processes in a closed magnetospheric model, such as the Kelvin ‐ Helmholtz instability.
Nitric oxide variations in the mesosphere and lower thermosphere during the November 1993 storm period
Authors: Crowley G. , Ridley A. , Winningham D. , Frahm R. , Sharber J. , Russell J.
Published: February 2004

The variability of nitric oxide has not been fully quantified. Particle precipitation is known to enhance nitric oxide; however, a detailed understanding of this process is lacking, and its importance has not been quantified. In this paper, we present nitric oxide measurements between 80 and 200 km altitudes from the Halogen Occultation Experiment (HALOE) on UARS during the November 1993 space weather special study period, near solar minimum conditions. The nitric oxide mass mixing ratio routinely varies by 3 orders of magnitude above 80 km altitudes. The variation of NO with magnetic activity is discussed, using two measures of magnetic activity level. The first is the well‐known Kp index, and the second is derived from the particle environment monitor (PEM) on UARS. The Kp index varied between 0 and 7 during the study period. A detailed comparison of NO enhancements with ionization rates derived from the PEM data indicates that the particle precipitation can account for much of the NO. In the context of the November 1993 storm period, this study is interesting because it emphasizes the potential importance of space weather for the entire atmosphere and not only regions above 100 km altitude.
Electrodynamics of the inner magnetosphere observed in the dusk sector by CRRES and DMSP during the magnetic storm of June 4–6, 1991
Authors: Burke W. J. , Maynard N. C. , Hagan M. P. , Wolf R. A. , Wilson G. R. , Gentile L. C. , Gussenhoven M. S. , Huang C. Y. , Garner T. W. , Rich F. J.
Published: February 2004

We compare equatorward/earthward boundaries of convection electric fields and auroral/plasma sheet electrons detected by the DMSP F8 and CRRES satellites during the June 1991 magnetic storm. Measurements come from the dusk magnetic local time sector where the ring current penetrates closest to the Earth. The storm was triggered by a rapid increase in the solar wind dynamic pressure accompanied by a southward turning of the interplanetary magnetic field (IMF). Satellite data show the following: (1) All particle and field boundaries moved equatorward/earthward during the initial phase, probably in response to the strong southward IMF turning. (2) Electric field boundaries were either at lower magnetic L shells or close to the inner edge of ring current ions throughout the main and early recovery phases. Penetration earthward of the ring current occurred twice as the polar cap potential increased rapidly. (3) Electric potentials at subauroral latitudes were large fractions of the total potentials in the afternoon cell, twice exceeding 60 kV. (4) The boundaries of auroral electron precipitation were more variable than those of electric fields and mapped to lower L shells than where CRRES encountered plasma sheet electrons. Observations qualitatvely agree with predictions of empirical models for auroral electron and electric field boundaries.
Plasma waves in the Earth's electron foreshock: 1. Time‐of‐flight electron distributions in a generalized Lorentzian plasma and dispersion solutions
Authors: Yin L. , Ashour‐Abdalla M. , Bosqued J. M. , El‐Alaoui M. , Bougeret J. L.
Published: February 2004

The correlated studies discussed here used high‐quality and frequent in situ measurements provided by the Wind spacecraft to investigate the properties of the foreshock plasma (measured by Wind/3DPlasma) and the enhanced plasma waves (detected by Wind/WAVES). A time‐of‐flight electron distribution model was developed in a generalized Lorentzian plasma using experimental data. The Wind/3DP results show that typical foreshock electron distributions are well approximated by this model, although discernible “bumps” in distributions that may be unstable are not often observed or resolved by the 3DP instrument. We used unstable distributions described by the model to initialize our study. In addition to verifying the interdependence of dispersion, cutoff velocity and beam density that was previously found in a Maxwellian beam‐plasma system, our dispersion solutions demonstrate how the dispersion topology and wave spectral bandwidths evolve as a consequence of wave modifications of time‐of‐flight distributions. By identifying sections of distributions that interact with each of the growing and damping branches, we analyzed the role played by nonthermal electrons in the evolution of the instability. Our results complement those from previous investigations and, more importantly, provide information about the linear behavior of the system that will enable the simulation studies discussed in our companion paper [Yin et al., this issue] to examine possible nonlinear interactions and to address the issue of whether nonlinear wave‐wave interactions in the foreshock nonthermal plasma are a significant factor in producing the observed electromagnetic emissions.
Ionospheric structure and the generation of auroral roar
Authors: Shepherd S. G. , LaBelle J. , Doe R. A. , McCready M. , Weatherwax A. T.
Published: February 2004

Ionospheric electron density data from the Sondrestrom incoherent scatter radar (ISR) have been used to characterize the structure of the F region ionosphere during ground‐based LF/MF/HF receiver observations of natural ionospheric radio emissions known as auroral roar. In five out of six cases, the F region ionosphere has significant horizontal Ne gradient scale lengths (
, measured with 23–137 km spatial resolution). In three of these cases, localized F region auroral ionospheric cavities, with horizontal scales ∼50 km, are observed. In one of six cases, the ionosphere lacks either of these features, and a laminar, mostly unstructured, F region is observed. The data suggest that auroral roar events may occur for a range of large‐scale (>30 km) ionospheric conditions. Some theories for the generation of auroral roar require that the relationship between the electron plasma frequency (ƒpe) and the electron gyrofrequency (ƒce) in the source region is

, where n is the harmonic number of the observed emission. Comparisons between observed auroral roar emission frequencies, ISR observations of electron density, and the IGRF model for the magnetic field show that this frequency‐matching condition holds somewhere in the ionosphere in 16 out of 18 cases studied and in all 3 cases of ISR elevation scans capable of measuring a source located directly overhead.
Excitation of resonant Alfvén waves in the magnetosphere by negative energy surface waves on the magnetopause
Authors: Ruderman Michael S. , Wright Andrew N.
Published: February 2004

The instability of the magnetopause caused by the resonant interaction of a negative energy surface wave with Alfvén waves localized in the vicinity of the resonant magnetic surface is considered. The most important property of this instability is that it takes place for flow velocities in the magnetosheath below the Kelvin‐Helmholtz (KH) threshold velocity, i.e., when there is no KH instability of the magnetopause. The magnetopause is modeled by an MHD tangential discontinuity with a magnetic‐free plasma on the magnetosheath side and cold plasma on the magnetospheric side. It is shown that one of the two surface waves that propagate along the discontinuity when the shear velocity is smaller than the KH threshold velocity is a negative energy wave when the shear velocity is larger than a critical velocity. This negative energy wave propagates tailward in the magnetospheric frame, although it propagates sunward in the magnetosheath frame. When, in addition, a resonant condition is satisfied, the negative energy surface wave resonantly interacts with localized Alfvén waves. This interaction results in growth of both the surface wave and the Alfvén waves. The resonant condition can only be satisfied when the plasma density increases and, consequently, the Alfvén velocity decreases in the direction toward the magnetopause in its vicinity. The increment of the resonant instability is calculated under the assumption that the plasma density changes only in a slab in the vicinity of the magnetopause with the thickness much smaller than the wavelength. The possible observational signatures of the resonant instability are discussed.
Observations of solar wind directly driven auroral electrojets and gravity waves
Authors: Huang Chao‐Song , Andre Dieter A. , Sofko George J.
Published: February 2004

One of principal objectives in studies of atmospheric gravity waves is to identify their sources. It has been hypothesized for many years that variations of the interplanetary magnetic field (IMF) can result in occurrence of the auroral electrojets in the high‐latitude ionosphere and the auroral electrojets can generate gravity waves, but identification of this relationship in experiments has been difficult since gravity waves are generally observed far from the source region. In this paper, we present simultaneous observations of the IMF, auroral electrojets, and gravity waves on December 7, 1994. It is found that when the IMF Bz component turned southward, auroral electrojet currents appeared after a time delay of about 9 min. The Saskatoon HF radar observed quasiperiodic gravity waves after each burst of the auroral electrojet with a time delay of about 60–70 min. The IMF southward turnings, the auroral electrojets, and the gravity waves had a recurrence period of about 40 min. These observations show clearly the cause and effect relationship between the IMF southward turnings, the auroral electrojets, and the gravity waves. Another important phenomenon observed in this event is that the quasiperiodic occurrence of the gravity wave pulses in far‐field points was determined by the repetitive energy input in the source region rather than by the wave dispersion. It seems that each burst of the auroral electrojet generated a single gravity wave pulse.
On VLF wave scattering in plasma with density irregularities
Authors: Shklyar David R. , Nagano Isamu
Published: February 2004

A systematic approach to the problem of wave scattering on plasma density irregularities is developed. The scattering process is treated as a wave field spatial evolution while crossing the scattering region, without dividing the wave field into a given incident wave and the field of the scattered waves. In such an approach, the wave energy conservation in the scattering process naturally follows from general formulas. We show that a spatial modulation of the wave packet amplitude caused by spectral broadening in k space is a general consequence of wave scattering. As an example of applications of theoretical consideration in this paper, we suggest an explanation of the amplitude modulation of VLF signals observed by the Akebono satellite in the upper ionosphere. We discuss the relation between amplitude modulation and the apparent spectral broadening of VLF transmitter signals. Both effects are described by the present theory in a similar manner.
Sector boundary transformation by an open magnetic cloud
Authors: Crooker N. U. , McAllister A. H. , Fitzenreiter R. J. , Linker J. A. , Larson D. E. , Lepping R. P. , Szabo A. , Steinberg J. T. , Lazarus A. J. , Mikic Z. , Lin R. P.
Published: February 2004

A magnetic cloud observed by the Wind spacecraft on February 8, 1995, was remarkable for its impact on the interplanetary sector structure. The magnetic field data imply that the cloud occurred in the middle of a sector and that the arrival of the following sector boundary on February 10 coincided with the arrival time predicted from the corresponding source surface map. The electron heat flux data, however, give incontrovertible evidence that instead the cloud brought the sector boundary, well ahead of the predicted arrival time. The electron heat flux data show little counterstreaming within the cloud, indicating predominantly open helical field lines. Under the assumption that the cloud originally had the form of a closed flux rope loop with legs rooted to the Sun, observational constraints dictate that the sector boundary was displaced not because it was pushed aside by the cloud but because reconnection in the leading leg opened field lines there, creating a topological change spanning 45° of heliographic longitude. The solar source of the cloud was deduced from an associated eruptive arcade event extending northeastward from an active region in Yohkoh soft X ray data on February 4. On February 8, the same active region was the source of impulsive energetic electron events observed at Wind during a brief counterstreaming interval, consistent with magnetic connection in the leading leg at that time. The cloud's helicity matches that predicted from the skew of the arcade fields in the February 4 X ray event, but the predicted alignment of the arcade and cloud axes was off by 35°. We use an MHD model with boundary conditions derived from solar magnetograms to illustrate the tilted arcade configuration in the corona that gave rise to the magnetic cloud and the lesser tilt of the heliospheric current sheet stemming from it.
X ray and EUV emission spectra of oxygen ions precipitating into the Jovian atmosphere
Authors: Kharchenko V. , Liu Weihong , Dalgarno A.
Published: February 2004

Spectra of soft X ray and EUV emissions of oxygen ions, precipitating into the Jovian atmosphere, are calculated, taking into account the dynamical evolution of the energy and charge distributions of the ions as they propagate. Monte Carlo simulations are performed using experimental and theoretical cross sections of ion collisions with the atmospheric gases. The numbers of X ray and EUV photons produced per precipitating oxygen ion are calculated as functions of the initial ion energy and charge. The energy and charge distribution functions are used to evaluate the intensities of characteristic X ray and EUV emission lines of oxygen ions in Jovian auroras. Radiative cascade transitions from highly excited ion states are shown to be an important source mechanism of soft X ray and ultraviolet emission lines.
Pulsating cusp aurora for northward interplanetary magnetic field
Authors: Sandholt Per Even , Farrugia Charles J. , Cowley Stanley W. H.
Published: February 2004

We report a characteristic bifurcation of the cusp aurora in association with northward transitions of the interplanetary magnetic field (IMF), leading to the coexistence of latitudinally separated forms, called types 1 and 2. In the northernmost form (type 2), which is located at ∼77 – 79° MLAT, there appears a sequence of auroral brightenings at the poleward boundary of the preexisting luminosity. Each brightening is followed by equatorward motion and fading of the individual forms. Furthermore, coherent intensity variations of the two latitudinally separated forms are often observed. This is documented by three case examples. The most active phase of the type 2 aurora is observed during intervals of 15 – 20 min duration following northward turnings of the IMF and with nonzero IMF By component, corresponding to the transition state before a new equilibrium state of solar wind‐magnetosphere coupling is established. The auroral observations are discussed in relation to theoretical models of ionospheric convection, field‐aligned currents and solar wind‐magnetosphere coupling in the polar cusp region. The type 2 auroral activity in the north is interpreted as an ionospheric signature which is directly related to reconnection between the IMF and the terrestrial lobe field at the high‐latitude magnetopause, whereas the type 1 aurora is coupled to the low ‐ latitude magnetopause. The coherent activations of the two auroral forms indicate that the corresponding current systems are strongly coupled at the ionospheric level.