bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental evidence for solitary waves in the middle atmosphere
Authors: Widdel H.‐U.
Published: February 2008

Certain types of transient irregularities seen in results of foil cloud measurements in the mesosphere are consistent with the properties of solitary waves.
Analysis of ion densities in the vicinity of space vehicles: Ion‐neutral chemical kinetics
Authors: Dressler Rainer A. , Gardner James A. , Cooke David L. , Murad Edmond
Published: February 2008

Recent laboratory studies of ion‐molecule reactions relevant to the analysis of ion densities in the vicinity of space vehicles in low‐Earth orbit are discussed. The studies include new cross‐section measurements of the important O++ H2O charge transfer collisions and the H3O+ ‐producing H2O++ H2O collisions, as well as product kinetic energy analysis of the reaction products. The charge transfer reaction rate is found to be less collision energy dependent than previously assumed, whereas the H3O+ ‐producting reaction rate is highest at thermal energies and decreases rapidly with increasing collision energy. Implications of the new results on previous interpretations and recent space observations are discussed.
High‐latitude Pc 1 bursts arising in the dayside boundary layer region
Authors: Hansen H. J. , Fraser B. J. , Menk F. W. , Hu Y. ‐D. , Newell P. T. , Meng C. ‐I. , Morris R. J.
Published: February 2008

Dayside Pc 1 geomagnetic pulsation bursts have been studied using a three‐station array of induction magnetometers located at high latitudes (geomagnetic latitude (MLAT) of −70° to −81°). The data set reveals that when these emissions are continually recorded at Davis (MLAT = −74.5°; L = 14), the low‐latitude cleft, observed by the DMSP F7 satellite, is located at the Davis latitude. Associated magnetic variations in the form of solitary pulses often lead the Pc 1 bursts by 1 to 2 min. These pulses are typically associated with riometer absorption events and consequently the precipitation of fluxes of keV electrons. The Pc 1 bursts are interpreted as resulting from ion cyclotron waves which have propagated to the ionosphere from the equatorial boundary layer region. The associated boundary layer ions, identified by the low‐altitude DMSP F7 satellite, range between 1 and 5 keV in energy. These particles are considered to be the most likely free energy source for the ion cyclotron waves. It is considered that such resonant ions enter the magnetosphere via the cleft and cusp because this enables a prenoon time of occurrence of most of the observations to be explained. Measured time delays of 40 to 120 s between the associated riometer absorption and Pc 1 bursts are consistent with an ion cyclotron wave generation region located in the equatorial magnetosphere.
A semikinetic model for early stage plasmasphere refilling: 1, Effects of Coulomb collisions
Authors: Wilson G. R. , Horwitz J. L. , Lin J.
Published: February 2008

Recently we have modified our collisionless, time‐dependent, kinetic plasma model (Wilson et al., 1990) to include the effects of Coulomb self‐collisions in a theoretically rigorous fashion. The algorithm employed (Takizuka and Abe, 1977) faithfully mimics a Fokker‐Planck operator and conserves both energy and momentum. This collisional kinetic model has been applied to the problem of baseline plasmasphere refilling of an initially depleted flux tube, neglecting the effects of wave‐particle interactions. The companion report by Lin et al. (this issue) examines the effects of wave‐particle interactions on refilling. We have performed refilling calculations for various flux tubes (L = 3, 4, 5, 6) and for different ionospheric plasma fluxes and temperatures. In each case considered, the same set of events occurs. Initially, two polar wind outflows develop from each hemisphere and set up counterstreaming beams. With time the vacant phase space region between these beams fills, primarily because of to collision‐induced particle diffusion but also because of lowering ambipolax potential drops resulting from the increasing density in the plasmasphere. In contrast to all previous hydrodynamic approaches, we find no formation of shocks. The plasma first evolves an isotropic nearly Maxwellian velocity distribution in a region that starts near the ionosphere and moves outward toward the equator. We find that for reasonable topside ionospheric temperatures and fluxes the thermal plasma all along an L shell will become nearly isotropic in 6 to 30 hours (longer times for smaller fluxes or larger L shells), consistent with the observations of Horwitz et al. (1984).
Direct experimental measurement of electron impact ionization‐excitation branching ratios: 1. Results for N2 at 100 eV
Authors: Doering J. P. , Goembel L.
Published: February 2008

A new laboratory experiment has been developed to measure electron impact ionization‐excitation branching ratios for formation of positive ions of atmospheric species in the ground and excited electronic states of the ion. In the experiments described here the distribution of N2+ ions produced in the X ²Σg+, A ²Πu, and B ²Σu+ electronic states by 100‐eV electron impact ionization of N2 has been measured directly for the first time. The experiment is a modification of general purpose (e,2e) electron coincidence ionization experiments which have been developed in recent years to study ionization processes. We have added a second electron energy analyzer to an existing electron energy loss spectrometer so that both the scattered primary and ejected secondary electrons can be detected in coincidence. The final ion state is determined from the energy of the ejected electron and the energy lost by the incident electron. Spectra of coincidence rate versus primary electron energy loss have been obtained at fixed ejected secondary electron energies of 2.5, 5.0, 10.0, and 15.0 eV. Peaks in these spectra are observed corresponding to the three final states of the N2+ ion. The relative peak areas are proportional to the partial branching ratios at each secondary electron energy. Branching ratios at 100 eV integrated over angle and energy variables have been computed from the data. They are 0.50, 0.40, and 0.10 for the X ²Σg+, A ²Πu, and B ²Σu+ electronic states respectively. These values are in excellent agreement with branching ratios obtained by other techniques.
Magnetospheric substorms and nighttime height changes of the F2 region at middle and low latitudes
Authors: Reddy C. A. , Nishida A.
Published: February 2008

Using the published plots of 15‐min interval values of h′F2, the minimum virtual height of the ionospheric F2 region, at five Japanese stations spread over 26°–45°N (15°–35°N geomagnetic) and 128°–142°E, it is shown that there are frequent rapid increases and less frequent decreases of h′F2 by 100–200 km in about 2 hours with near‐simultaneous start and end at all five stations in the postmidnight hours and at three to five stations in the premidnight hours. In the months of September and December 1988 and March 1989, about 80 h′F2 perturbations of the above type are identified. The near simultaneity of the h′F2 disturbances at the five stations suggests that an
plasma drift due to an east‐west electric field is the cause of the h′F2 disturbances. This interpretation is validated by showing that there is a simultaneous presence of bay disturbances in auroral latitude magnetograms over a wide longitude range during almost all the h′F2 increases. It is shown that neutral wave disturbances propagating equatorward even with a speed of 750 m/s cannot account for the h′F2 disturbances, The study reveals two categories of h′F2 perturbations: (1) h′F2 increases starting near the beginning of the geomagnetic negative bays at auroral latitudes and (2) rapid h′F2 increases starting near the maximum intensity phase of the auroral bays and preceded by h′F2 decreases starting at the onset time of the bay disturbance or a little earlier. Using the vast amount of h′F2 data collected all over the globe, it is possible to quantify and characterize the substorm‐related electric fields and their variabilities. Such characterization is very useful in understanding the magnetospheric and the magnetosphere‐ionosphere interaction processes involved in the generation of magnetospheric substorms. It is also pointed out that the understanding of F2 region changes during magnetic storms may improve greatly if the large F2 region changes caused by electric fields on the substorm time scale are taken into account. The h′F2 disturbances reported in this study are similar in time scale and appearance to the h′F2 disturbances caused by long‐period atmospheric waves; and this suggests the possibility of a mixup of the two effects and the consequent wrong identification on some occasions.
A case study of plasma structure in the dusk sector associated with enhanced magnetospheric convection
Authors: Carpenter D. L. , Smith A. J. , Giles B. L. , Chappell C. R. , Décréau P. M. E.
Published: February 2008

In a case study from June 8–9, 1982, data from ground whistler stations Siple and Halley, Antarctica, located at L ∼4.3 and spaced by ∼2 hours in MLT, and from satellites DE 1 and GEOS 2, have provided confirming evidence that the bulge region of the magnetosphere can exhibit an abrupt westward “edge,” as reported earlier from whistlers. The present data and previous MHD modeling work suggest that this distinctive feature develops during periods of steady or declining substorm activity, when dense plasma previously carried sunward under the influence of enhanced convection activity begins to rotate with the Earth at angular velocities that decrease with increasing L value and becomes spirallike in form. For the first time, whistler data have been used to identify a narrow dense plasma feature, separated from the main plasmasphere and extending sunward into the late afternoon sector at L values near the outer observed limits of the main plasmasphere bulge. The westward edge of the main bulge, found by both whistler stations to be at ∼1800 MLT, appeared to be quasi‐stationary in Sun‐Earth coordinates during the prevailing conditions of gradually declining geomagnetic agitation. It is possible that outlying dense plasma features such as the one observed develop as part of the process leading to the occurrence of the more readily detectable abrupt westward edge of the bulge. It was not possible in this case to determine the extent to which the outlying feature was smoothly attached to or isolated from the main bulge region.
An almost two‐dimensional approach to type 2 irregularities in the equatorial electrojet
Authors: Albert J. M. , Similon P. L. , Sudan R. N.
Published: February 2008

A recently developed theory of “almost two‐dimensional” turbulence (Albert et al., 1990) is applied to E×B fluctuations in the E region equatorial electrojet. This theory of plasma turbulence in an external magnetic field extends strictly two‐dimensional theory (e.g., Sudan and Keskinen, 1979) to include weak phase variations along the magnetic field. Based on the direct interaction approximation of Kraichnan (1959), it describes the tendency of spectral energy to transfer nonlinearly towards modes with high k∥ despite strong linear damping of these modes. Here, we discuss application of this theory to the aspect angle k∥/k of fluctuations at 3‐m scales, for which radar backscatter measurements have been made (Kudeki and Farley, 1989). Allowing for uncertainty in characterizing the plasma turbulence, we find good agreement except at the lowest altitudes of the electrojet.
Measurement of atmospheric and diffuse radiation using a time‐of‐flight telescope
Authors: da Costa Ferreira Neri Jose Angelo , Agrinier Bernard , Lavigne Jean Michel
Published: February 2008

A time‐of‐flight telescope constructed by the Commissariat à l'Energie Atomique (Saclay) and the Centre d'Etude Spatiale des Rayonnements (Toulouse) was used for gamma ray flux measurements. The atmospheric component was studied from balloon as a function of the pressure and the energy. Gamma ray fluxes were also measured with the telescope axis oriented in various directions and with the experiment inverted. The atmospheric and diffuse components were determined in various energy ranges.
Measurements of ionospheric and thermospheric temperatures and densities with the middle and upper atmosphere radar
Authors: Oliver William L. , Takami Tomoyuki , Fukao Shoichiro , Sato Toru , Yamamoto Mamoru , Tsuda Toshitaka , Nakamura Takuji , Kato Susumu
Published: February 2008

The first incoherent scatter radar local time/seasonal/solar cycle averages of F region electron, ion, and neutral temperatures for Asian longitudes are presented. These measurements were made with the four‐pulse experiment by the middle and upper atmosphere (MU) radar in Japan over the period August 1986 to April 1990. The neutral temperature and density results are compared with MSIS‐86 model predictions. We divide our data into low and high solar activity levels, four seasons, and 24 one‐hour times bins. While the seasonal and solar activity behavior of the ionospheric density, and of the anticorrelation of the electron temperature with this density, shows many aspects in common with those at other locations, a few particular differences are evident. At low solar activity the height of the F layer varies from 240 km during the day to 320 km at night, regardless of season. At high solar activity the maximum density of the F layer varies little with season. These facts seem to indicate little seasonal change in F region neutral atmospheric composition at this geographic location. At solar maximum the F layer is established at a higher altitude than at low solar activity, and the summer F layer in particular is formed at such a high altitude that its decay at night is very slow, its diurnal variation is hence weak, and the density remains so high at sunrise that little sunrise effect in the electron temperature is produced. The neutral temperatures in the F region have the same basic diurnal pattern as does the MSIS‐1986 model temperatures, but the radar temperatures are consistently lower by an amount ranging up to 160 K for summer conditions at high solar activity. The measured and model temperatures are closest in the autumn and winter. The neutral density results are of low quality and do not provide any evidence of need to modify the MSIS model densities.
Plasma diffusion at the magnetopause: The case of lower hybrid drift waves
Authors: Treumann R. A. , LaBelle J. , Pottelette R.
Published: February 2008

We have recalculated the diffusion expected from the quasi‐linear theory of the lower hybrid drift instability at the Earth's magnetopause. The resulting diffusion coefficient is marginally large enough to explain the thickness of the boundary layer under quiet conditions, based on observational upper limits for the wave intensities. Thus one possible model for the boundary layer could involve equilibrium between the diffusion arising from lower hybrid waves and various loss processes.
Initial survey of the wave distribution functions for plasmaspheric hiss observed by ISEE 1
Authors: Storey L. R. O. , Lefeuvre F. , Parrot M. , Cairó L. , Anderson R. R.
Published: February 2008

Multicomponent ELF/VLF wave data from the ISEE 1 satellite have been analyzed with the aim of identifying the generation mechanism of plasmaspheric hiss, and especially of determining whether it involves wave propagation on cyclic trajectories. The data were taken from four passes of the satellite, of which two were close to the geomagnetic equatorial plane and two were farther from it; all four occurred during magnetically quiet periods. The principal method of analysis was calculation of the wave distribution functions. The waves appear to have been generated over a wide range of altitudes within the plasmasphere, and most, though not all, of them were propagating obliquely with respect to the Earth's magnetic field. On one of the passes near the equator, some wave energy was observed at small wave normal angles, and these waves may have been propagating on cyclic trajectories. Even here, however, obliquely propagating waves were predominant, a finding that is difficult to reconcile with the classical quasi‐linear generation mechanism or its variants. The conclusion is that another mechanism, probably nonlinear, must have been generating most of the hiss observed on these four passes.
Effects of equatorially trapped ions on refilling of the plasmasphere
Authors: Singh Nagendra , Chan C. Bun
Published: February 2008

The observation of trapped ions in the equatorial region of the outer plasmaspheric flux tubes suggests interesting plasma processes, which can significantly affect the refilling of such tubes after geomagnetic storms. The trapped ions are created by perpendicular ion heating by wave‐particle interactions. During the early stage of the refilling, the heated ions come from the field‐aligned flows of thermal plasma from the conjugate ionospheres. If the heating is sufficiently strong, the mirror force on the heated ion decouples the interhemispheric plasma exchange, and each hemisphere refills with the plasma supplied by its own ionosphere. The generation of the trapped ions and the refilling processes are modeled by solving time‐dependent plasma transport equations. The refilling is found to occur in two stages. The first (initial) stage involves (1) development of supersonic flows from the conjugate ionospheres, (2) perpendicular ion heating at the equator, (3) retardation of the interhemispheric flow by the mirror force as the heated ions tend to flow downward from the equator, and (4) formation of shocks in the supersonic flows at the equator and their downward propagation to the conjugate ionospheres. This entire phase lasts only a few hours. Since shock formation in a hydrodynamic model is questionable, it is verified kinetically by means of a numerical simulation which shows that shocks form at the mirror points of the equatorially heated ions. The kinetic simulation deals with small‐scale counterstreaming plasma expansion in a mirror type of magnetic field with minimum magnetic field at the center. The second stage of refilling starts when the temperature anisotropy created by the passage of the shocks begins to relax as a result of Coulomb collisions at relatively high latitudes where plasma density is relatively high. During this stage the flow from the conjugate ionospheres is generally subsonic, and the associated refilling is relatively slow. During the course of refilling in this subsonic stage, density and temperature in the flux tube evolve through a variety of structures. The temperature anisotropy T⊥ > T∥ is found to persist in an extended equatorial region over several days, but near the equator itself an approximate temperature isotropy (T⊥ ∼ T∥) is achieved in about a day after the start of refilling. When equatorial isotropy is achieved, the perpendicular temperature shows a double‐peak structure with a peak on either side of the equator lasting over several days.
Multipoint reconnection in the near‐Earth magnetotail: CDAW 6 observations of energetic particles and magnetic field
Authors: Lin Naiguo , McPherron R. L. , Kivelson M. G. , Walker R. J.
Published: February 2008

The Coordinated Data Analysis Workshop (CDAW 6) substorm event at 1054 UT on March 22, 1979, is studied using energetic particle (>20 keV) data from the medium‐energy particle experiment of ISEE 1 as well as high‐resolution magnetometer data of ISEE 1 and 2. These analyses cast new light on the structure and temporal evolution of the plasma sheet near 15 RE during the substorm. These changes are generally consistent with predictions from the near‐Earth reconnection model of substorms, modified to include subsidiary plasma and field structure on scales of a few Earth radii. In particular, the onset of streaming flow, nearly aligned with magnetic field lines, of energetic ions and electrons in all available energy channels and its reversal and subsidence are closely correlated with substorm phases. The energetic particles (which are isotropic during the growth phase) begin streaming tailward right after the expansion onset. They reverse their sense of flow in the midexpansion phase. The magnetic field variations suggest that two small magnetic island structures embedded within the plasma sheet moved tailward past the spacecraft shortly after the substorm expansion onset. These structures may be signatures of small‐scale multisite reconnection associated with the formation of the near‐Earth neutral line. Some magnetic and particle signatures not anticipated in the description of the substorm based on a two‐dimensional model of the neutral line are discussed and shown to be reasonable for plausible extensions of the model.
On dipolarization and its relation to the substorm current wedge
Authors: Hesse Michael , Birn Joachim
Published: February 2008

Within the framework of our three‐dimensional self‐consistent MHD model of magnetotail activity we investigate near‐Earth properties of magnetospheric substorms. Particular emphasis is on the generation mechanism and the three‐dimensional aspects of the Bz increase which is commonly called magnetic field dipolarization, and its spatial and temporal relation to the field‐aligned currents of the substorm current wedge and the current reduction and diversion. We find that the dipolarization can be understood as the flux pileup due to the slowing down of the fast earthward flows carrying reconnected flux from the reconnection region. The gradients of Bz at the dawn and dusk flanks of the dipolarization region contribute to the region 1 field‐aligned current system of the substorm current wedge. The major contribution, however, is found to be associated with the shear between the y and x components of the magnetic field. We also find that the region where the cross‐tail current is reduced (“disrupted”) and diverted is located tailward from the dipolarization region. Our results indicate that the apparent tailward propagation of dipolarization effects does not support the conclusion that the underlying process is initiated further earthward.
Mesospheric temperature inferred from daytime observation of the O2 atmospheric (0, 0) band system
Authors: Heller J. W. , Christensen A. B. , Yee J. H. , Sharp W. E.
Published: February 2008

A rocket‐borne Ebert‐Fastie spectrometer was used to measure the dayglow O2(b¹Σg+) atmospheric (0,0) band centered at 761.9 nm. The tangent altitude for the observations ranged from 63 to 101 km. Analysis of the (0,0) band observations is presented. The peak intensity of the (0,0) band was 3.7±0.2 megarayleigh at 87 km. The results were compared to a multiple‐scattering radiative transfer model that includes a detailed photochemical calculation of the sources and sinks of the O2 excited state. The kinetic temperature was determined by analysis of the rotational development of the O2 (0,0) band. The average temperature between 94 and 101 km was 182±7 K.
Preliminary study of the CRRES magnetospheric barium releases
Authors: Huba J. D. , Bernhardt P. A. , Lyon J. G.
Published: February 2008

Preliminary theoretical and computational analyses of the Combined Release and Radiation Effects Satellite (CRRES) magnetospheric barium releases are presented. The focus of the studies is on the evolution of the diamagnetic cavity which is formed by the barium ions as they expand outward, and on the structuring of the density and magnetic field during the expansion phase of the releases. Two sets of simulation studies are discussed. The first set is based upon a two‐dimensional ideal MHD code and provides estimates of the time and length scales associated with the formation and collapse of the diamagnetic cavity. The second set uses a nonideal MHD code; specifically, the Hall term is included. This additional term is critical to the dynamics of sub‐Alfvénic plasma expansions, such as the CRRES barium releases, because it leads to instability of the expanding plasma. We performed detailed simulations of the G4 and G10 releases. In both cases the expanding plasma rapidly structured: the G4 release structured at time t ≲ 3 s and developed scale sizes ∼ 1‐2 km, while the G10 release structured at time t ≲ 22 s and developed scale sizes ∼ 10‐15 km. We also find that the diamagnetic cavity size is reduced from those obtained from the ideal MHD results because of the structure. On the other hand, the structuring allows the formation of plasma blobs which appear to free stream across the magnetic field; thus, the barium plasma can propagate to larger distances transverse to the magnetic field than the case where no structuring occurs. Finally, we also discovered a new normal mode of the system which may be excited at the leading edge of the expanding barium plasma. This mode is a magnetic drift wave which propagates azimuthally around the barium cloud in the frequency range Ωi ≪ ω ≪ Ωe.
Heating of protons by resonant absorption in a multicomponent plasma: 2. Theoretical model
Authors: Le Quéau Dominique , Roux Alain , Rauch Jean Louis , Lefeuvre François , Bosqued Jean Michael
Published: February 2008

A theory is proposed to explain the selective heating of protons in a direction transverse to the geomagnetic field lines, which has been observed on board the AUREOL 3 satellite, as shown in a companion paper. This process, which occurs only in multicomponent plasma, ultimately leads to the formation of a conical distribution, inside and above the heating region. It is shown that magnetosonic Alfvén waves generated at higher altitudes can undergo a resonant mode conversion process as they propagate downward. The efficiency of the subsequent resonant absorption depends rather strongly on the propagation angle of the waves and on the relative abundance of H+ ions: up to 25% of the incident energy flux can be transferred to protons. The present theory moreover foresees that a strong parallel electric field component has to occur at the altitude at which the wave frequency matches the proton gyrofrequency. The corresponding spectral signature has been unambiguously identified in the AUREOL 3 data, together with an intensification of the upward flux of suprathermal protons, which indicates the presence of proton conics. We have verified that within the uncertainties of the measurements the energy balance between the incoming electromagnetic power and the outgoing flux of kinetic energy is well explained by the theory.
The role of the equatorial electrojet in the evening ionosphere
Authors: Haerendel G. , Eccles J. V.
Published: February 2008

This paper focuses on the role of the equatorial E region in the electrodynamics of the evening ionosphere. The influence and reaction of the electrojet current on the equatorial ionosphere at sunset is investigated using a field line integrated, one‐dimensional, electrodynamic model. The one‐dimensional, time‐varying model predicts the divergence of the horizontal current of the equatorial electrojet for a given time variation of the horizontal electric field. The negative divergence of the horizontal current during the evening hours provides a net upward current out of the equatorial E region into the integrated ionosphere of higher equatorial altitudes (and equivalent latitudes). This upward current affects the vertical electric field magnitudes and subsequent horizontal plasma drifts of the overlying ionosphere. The model allows for chemical recombination and dynamic redistribution of ionization within the electrojet region under the assumption that the profile of the ionization density along a field line is proportional to the chemical equilibrium profile. The eastward horizontal electric field and the net upward current during the 2 hours after sunset combine to lift the ionization out of the E region resulting in ionization densities less than the equilibrium values. As the ionization densities (conductivities) are reduced, the electrodynamics of the equatorial ionosphere is altered. This model of the equatorial electrojet current divergence can be used as a lower boundary to global, two‐dimensional models of the equatorial electric fields.

Finally, it is proposed that the equatorial electrojet current near sunset has a significant role in the determination of the postsunset enhancement of the horizontal electric field. The electrojet region provides the best avenue for current to be channeled from the dayside to meet the vertical current demands of the F region neutral wind dynamo after sunset. The conductivity reduction in the E region due to the recombination of ionization and the plasma uplift enhances the horizontal (eastward) electric field and thereby increases the speed of the uplift. Thus the dynamic adjustment of ionization has an unstable, feedback relationship with the electric fields which may explain the night to night variability of the horizontal electric field enhancement.
Structure and evolution of compound streams at ≤1 AU
Authors: Behannon K. W. , Burlaga L. F. , Hewish A.
Published: February 2008

Two cases of compound stream formation and evolution at heliocentric distances of ≤1 AU have been studied using simultaneous data from both multipoint in situ and global sources. The sequences of events analyzed occurred around December 16, 1978, and February 19, 1979, respectively. The sources of the multipoint observations were Helios A, Helios B, IMP 8, and ISEE 3. The global observations were obtained by interplanetary scintillation analysis of radio sources. It was found that the radio and in situ data complemented each other and that both were needed for a unique interpretation of complex, time‐dependent flows. The cases studied show that a single corotating stream can interact with different transient flows at different times, and a single transient flow can interact with different corotating streams at different times. In each instance, a compound stream is formed. This work further shows that the cosmic ray intensity at a point depends on the global compound stream configuration in the vicinity of the point.
Absolute state‐selected total cross sections for the O+(²D,²P) + N2 reactions
Authors: Flesch G. D. , Ng C. Y.
Published: February 2008

Absolute state‐selected total cross sections for the reactions O+(²D) (or O+(²P)) + N2 → N2+ O, NO+ + N, and N+ + NO (or N + O) have been measured in the center‐of‐mass (c.m.) collision energy (Ec.m.) range of 1.3–25 eV. Charge transfer is overwhelmingly the major product channel. The ratios of the charge transfer cross section due to O+(²P) to that associated with O+(²D) at the corresponding Ec.m. are found to be in the range of 0.71‐0.88. The cross sections for N+ resulting from the O+(²D,²P) + N2 reactions are significantly greater than those formed by the O+(4S) + N2 reaction. The Ec.m. dependences observed for the N+ cross section indicate that the formation of N+ + NO from the collision of O+(²P) + N2 is a viable process, whereas N+ formed in the O+(²D) + N2 reaction is mainly associated with the N+ + N + O channel.
Model of the formation of the low‐latitude boundary layer for strongly northward interplanetary magnetic field
Authors: Song P. , Russell C. T.
Published: February 2008

Observations of the low‐latitude boundary layer in the subsolar region when the interplanetary magnetic field (IMF) is strongly northward indicate that the boundary layer consists of steplike multiple layers rather than a single diffusive layer. These sublayers can be formed by spatially limited, temporally varying reconnection near the polar cusp. In this model when the interplanetary magnetic field is strongly northward a magnetosheath flux tube reconnects in the north and south beyond the cusp. The tube shortens itself and reorients to align itself with other magnetospheric field lines and eventually be assimilated with other magnetospheric field lines. Energy from the shortening of the flux tube and the reduction of magnetic energy is transferred into the magnetosphere and increases the pressure above its initial equilibrium value while the reconnected flux tube sinks into the magnetosphere. The interchange instability is one of the possible mechanisms to disperse and expand the newly captured magnetosheath flux tube azimuthally along the magnetopause current layer and to lead the system back to equilibrium. It is stable to radial interchanges and unstable to azimuthal interchanges. Thus the newly formed flux tube becomes a boundary layer. The interchange instability converts the thermal energy of the plasma into dynamic energy. The interchange front moves with a fraction of the sound speed and accelerates toward the terminator. Subsequent reconnection forms a new layer before the last one is completely dispersed. In this model, different sublayers represent different ages after reconnection.
Atomic oxygen in the Martian thermosphere
Authors: Stewart A. I. F. , Alexander M. J. , Meier R. R. , Paxton L. J. , Bougher S. W. , Fesen C. G.
Published: February 2008

Modern models of thermospheric composition and temperature and of excitation and radiative transfer processes are used to simulate the O I 130‐nm emission from Mars measured by the Mariner 9 ultraviolet spectrometer. We use the Mars thermospheric general circulation model calculations (MTGCM) of Bougher et al. (1988a) and the Monte Carlo partial frequency redistribution multiple scattering code of Meier and Lee (1982). We find that the decline in atomic oxygen through the daylight hours predicted by the MTGCM cannot be reconciled with the excess afternoon brightness seen in the data. Oxygen concentrations inferred from the data show a positive gradient through the day, in agreement with the original analysis by Strickland et al. (1973), although the absolute amounts are somewhat less because we use a larger photoelectron impact excitation and a somewhat larger solar flux in the 130‐nm triplet. In addition, the data suggest that the oxygen abundance increases toward high southerly latitudes, in contrast with the MTGCM prediction of high values in the northern (winter) hemisphere. It appears that solar forcing alone cannot account for the observed characteristics of the Martian thermosphere and that wave and tidal effects may profoundly affect the structure, winds, and composition.
Decay instability of incoherent Alfvén waves in solar wind
Authors: Umeki H. , Terasawa T.
Published: February 2008

The nonlinear evolution of a large‐amplitude incoherent Alfvén wave is studied via one‐dimensional magnetohydrodynamic simulation. The initial wave magnetic field is given as a superposition of circularly polarized Alfvén waves with the same helicity and propagation direction but with different wave numbers. What we observe is contrary to the previous belief that incoherent Alfvén waves are stable against decay: In a low β plasma (β = 0.2) we can clearly see the growth of backscattered Alfvén waves, which are opposite in helicity and propagation direction from the original Alfvén waves. In a high β plasma (β = 2.0), on the other hand, no backscattered Alfvén waves are observed. These results are consistent with the expectation from the theory of the parametric decay instability developed for a coherent Alfvén wave. We also show that incoherent Alfvén waves in the solar wind can decay parametrically in the region of 4–20 RS where β is sufficiently low. For the decay process to work, the period of Alfvén waves should be less than several minutes.
Ground and satellite observations of an auroral event at the cusp/cleft equatorward boundary
Authors: Sandholt Per Even , Newell Patrick T.
Published: February 2008

Detailed observations by ground‐based and satellite instruments of a transient auroral event in the ∼10–11 MLT sector during negative IMF BZ are reported. The optical phenomenon belongs to a category of dayside auroral event sequences that is considered a candidate signature of impulsive magnetopause reconnection. The present event is characterized by the brightening of a discrete arc (at least 600 km in east‐west extent) at the cusp/cleft equatorward boundary, subsequently moving poleward and increasing in latitudinal extent. This was followed 6 min later by enhanced 630.0‐nm auroral emission ∼3 degrees (300 km) further north, at the cleft/mantle boundary. The satellite (DMSP F7) passed through the event at ∼1000 MLT, in its early phase, revealing a discrete electron precipitation structure with the spectral peak at ∼1 keV, immediately equatorward of a zone of cleft/low‐latitude boundary layer particles. The multiple structures of field‐aligned current near the cleft equatorward boundary included an upward directed (out of the ionosphere) component at the latitude of the discrete electron precipitation structure and a downward current and accelerated ions (1–3 keV) further north. A continuously decreasing electron energy flux was observed along the satellite track from the cleft equatorward edge to the polar cap, as well as clear energy versus latitude dispersion of the precipitating ions. These low‐altitude observations are found to be consistent with essential features of unsteady Petschek‐type reconnection at the magnetopause and modeled mapping of flux transfer events to the ionosphere.
Filamentation of plasma in the auroral region by an ion‐ion instability: A process for the formation of bidimensional potential structures
Authors: Mottez F. , Chanteur G. , Roux A.
Published: February 2008

A two‐dimensional, explicit, electrostatic particle code is used to investigate the nonlinear behavior of electrostatic ion waves generated by an ion beam flowing through a thermal ion and electron background in a strongly magnetized plasma (ωce ≫ ωpe where ωce and ωpe are the electron gyrofrequency and the plasma frequency). To follow the nonlinear evolution of these ion waves, a long‐lasting simulation is run with a large simulation grid: 128 × 512λd. Beam ions are shown to generate oblique waves. The nonlinear beatings between these oblique waves produce purely transverse waves, which leads to a strong modulation of the density and of the electric potential in a direction transverse to the magnetic field. The transverse scale of these essentially field‐aligned filaments is L⊥ = 10ρi where ρi is the ion Larmor radius of beam ions. Within these filaments, relatively stable field‐aligned density and potential structures develop. The typical size, along the magnetic field, of these structures is L∥ = 10λd, the density is modulated by 30%, and the electric potential is as large as Te within these structures. Unlike the potential structures that develop in a two‐component plasma with downgoing electrons, these structures move upward. These characteristics are in good agreement with the weak double layers recently detected by Viking.
Precise ground‐based solar photometry and variations of total irradiance
Authors: Chapman G. A. , Herzog A. D. , Lawrence J. K. , Walton S. R. , Hudson H. S. , Fisher B. M.
Published: February 2008

Variations in the total solar irradiance measured by the active cavity radiometer irradiance monitor (ACRIM) on SMM have been correlated with measures of magnetic activity on the solar disk. Quantitative indices of magnetic activity were derived from ground‐based, full‐disk, photometric images of the Sun at red (6723 Å) and violet (3934‐Å K line) wavelengths. The red images have been obtained on a daily basis at the San Fernando Observatory since 1985, and the K line images since 1988. Sunspot irradiance deficits are calculated directly from the red images while proxy measures of facular irradiance excesses are derived from the K line images. The images analyzed here were made during 21 days between June 20 and July 14, 1988, a period centered on the disk passage of a large sunspot group. The best two‐parameter multiple correlation coefficient between the ACRIM data and the photometric data is R² = 0.97 (21 data points, 18 degrees of freedom). The zero point S0 = 1367.27 W m−2 agrees well with the solar irradiance measured by ACRIM/SMM during the 1986 activity minimum; the residual standard deviation was 0.13 W m−2 (about 100 ppm). The multiple correlations were extended to include measures of the irradiance contribution of “network” magnetic fields, unassociated with active regions. NOAA 9 spacecraft observations of UV Mg II lines at 2800 Å gave R² = 0.99 (17 degrees of freedom) with S0 = 1366.68 + 0.08 W m−2. The index of 10.7‐cm microwave flux gave R² = 0.98, with S0 = 1366.43 + 0.11 W m−2. We can thus model short‐term irradiance changes to within 100 ppm relative precision from ground‐based data.
The effects of pressure anisotropy on birkeland currents in dipole and stretched magnetospheres
Authors: Birmingham Thomas J.
Published: February 2008

The expression for the Birkeland current density in an anisotropic P⊥, P∥ plasma derived in a companion paper (Birmingham, this issue) is simplified by linearization: for purposes of computing j∥ the magnetic configuration is assumed to be axisymmetric, either a dipole or a dipole stretched by the field arising from an azimuthal current sheet. The anisotropy is that of a bi‐Maxwellian and characterized by equatorial values P⊥ (s0) and r, the equatorial temperature (pressure) ratio. Parameter j∥ is proportional to the azimuthal derivatives of P⊥ (s0) and r, with coefficients that are integral functions along the background B of the pressure and field strength from the equator s0, where there is (by assumption) no Birkeland current, to the point of reference. Values of j∥ at the ionosphere are compared between the two magnetic models for 0.1 < r < 10., a range that assuredly spans anisotropy ratios likely to be encountered in planetary magnetospheres. The effect of the anisotropy is modest, amounting to a factor of roughly 10 (larger or smaller depending on whether r < 1 or vice versa) for a stretched field line crossing the equator at r0 = 20, less than half this amount for the comparable dipole line. Parameter j∥ grows, owing to the diversion of perpendicular currents, rapidly away from the equator, significantly more so for a pressure anisotropy favoring P⊥ (r > 1) than for one favoring P∥(r < 1). The growth away from the equator is particularly steep in the stretched model because of the strong curvature of the field lines, especially at large r0: plasma is trapped in the weak equatorial field, confined on either side by the strong B which arises from the azimuthal current sheet.
Aspect angle variations in intensity, phase velocity, and altitude for high‐latitude 34‐cm E region irregularities
Authors: Foster J. C. , Tetenbaum D. , del Pozo C. F. , St‐Maurice J. ‐P. , Moorcroft D. R.
Published: February 2008

The sensitivity of the Millstone 440‐MHz radar system is such that coherent echoes from E region irregularities can be observed over a 90‐dB dynamic range above the incoherent scatter background. At antenna elevation angles between 4° and 20°, aspect angles between 0° and 10° (from perpendicularity with the magnetic field) are viewed at E region heights at invariant latitudes between 61°Λ and 57°Λ. During disturbed conditions, when convection electric fields in excess of 15 mV/m and E region irregularities span this range of latitudes, antenna scanning experiments have been performed to determine the aspect angle sensitivity with high precision. Our measurements are unique in that they provide a clear high‐frequency description of the variation in both power and Doppler shift as functions of aspect angle, all the way from a region where the waves are known to be linearly unstable, in a direction perpendicular to the geomagnetic field, to as much as 10° away from perpendicularity. We find that the 440‐MHz aspect sensitivity is about −15 dB deg−1 for aspect angles between 0° and 3°, −10 dB deg−1 for aspect angles between 3° and 6°, and −7 dB deg−1 for aspect angles between 6° and 9°. The magnitude of the phase velocity is at an approximate ion acoustic level (350 m/s) for aspect angles <2° and decreases to <200 m/s as the aspect angle increases to >3°. For highly disturbed conditions the magnitude of the velocity can increase to >700 m/s for aspect angles <2°. The tendency for the altitude of the most intense return to decrease by ∼5 km as the aspect angle increases beyond 2° can be explained as a consequence of the variation of aspect angle with height.
Absolute differential and integral electron excitation cross sections for atomic nitrogen: 2. The 4So → 2p4 4P (λ1135 Å) transition from 30 to 100 eV
Authors: Doering J. P. , Goembel L.
Published: February 2008

The absolute direct differential and integral electron excitation cross sections for the atomic nitrogen 4So → 2p4 4P (λ1135 Å) transition have been measured at incident energies of 30, 50, and 100 eV. The differential cross sections measured versus scattering angle were integrated to give the integral cross section (ICS) as a function of incident energy. The ICS has a maximum value of 0.43×10−16 cm² (±30%) near 50 eV. Above 50 eV, the direct excitation cross section is within 20% of the fluorescence cross section of Stone and Zipf (1973) when the latter is adjusted downward to take account of the recent revision of the H2 dissociative excitation benchmark cross section. At 30 eV, the direct cross section is ∼1/10 of the fluorescence cross section. These results show that as for the λ1200‐Å N I transition cross section, there is a strong cascade contribution to the fluorescence cross section from forbidden transitions to higher states excited by low‐energy electron impact near threshold; but above 50 eV, unlike the λ1200‐Å N I transition cross section, there is only a small cascade contribution. The optical oscillator strength of the N I (λ1135 Å) multiplet obtained from the present data is 0.1 ± 0.005.
Oligocene basaltic volcanism of the Northern Rio Grande Rift: San Luis Hills, Colorado
Authors: Thompson Ren A. , Johnson Clark M. , Mehnert Harald H.
Published: February 2008

The inception of the Rio Grande rift in northern New Mexico and southern Colorado was accompanied by voluminous mafic volcanism preserved in part as erosional remnants on an intrarift horst within the current axial rift graben of the San Luis Valley. Oligocene (∼26 Ma) volcanic rocks of the Hinsdale Formation at San Luis Hills range from 49 to 57 wt % SiO2 and include nepheline and hypersthene normative lavas. A mildly alkalic series consisting of trachybasalt, basaltic trachyandesite, and trachyandesite is volumetrically dominant, olivine tholeiites are subordinate, and xenocrystic trachyandesites containing abundant quartz and plagioclase xenocrysts occur only locally. Relative to the San Luis Hills olivine tholeiites which have La/Smn ∼ 2, the more alkaline series are enriched in light rare earth elements (LREE) and have La/Sm ratios that increase in the trachybasalt‐basaltic trachyandesite suite (La/Smn ∼ 3) to xenocrystic trachyandesites that are the most LREE enriched (La/Smn ∼ 4). Chondrite‐normalized, trace element patterns for the lavas in the San Luis Hills are similar in shape within the mildly alkaline to transitional series; they have characteristic Nb and Ta depletions and high K and Th relative to Ta, Nb, and LREE. Major and trace element constraints support a petrogenetic model of fractionation plus lower crustal assimilation for petrologic suites within the San Luis Hills rocks, although the model cannot relate lavas for the entire series to a common parent. Most mafic lavas of the San Luis Hills were evolved (Mg # <60) and contaminated by LREE‐enriched silicic partial melts of granulitic lower crust depleted in Rb, Th, and U. Pb isotopes are the most sensitive indicators of crustal contamination, whereas shifts in Nd and Sr isotope ratios are associated with large amounts of assimilation. However, relatively noncontaminated lavas can be identified and indicate at least two mantle source regions were involved.