bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction between ELF‐VLF emissions and magnetic pulsations: Regular period, ELF‐VLF pulsations and their geomagnetic conjugacy
Authors: Sato Natsuo , Kokubun Susumu
Published: February 2008

The characteristics of intensity modulations of quasi‐periodic (QP) VLF emissions and their relationships to magnetic variations are examined by using data obtained from Syowa and Mizuho Stations in Antarctica and at Husafell in Iceland, which is located near the geomagnetic conjugate point of Syowa. As shown in the previous studies, QP emissions can be classified into two types, according to whether emissions are closely associated with magnetic pulsations or not. It is found that the frequency of intensity modulations of type 2 QP's, without corresponding magnetic pulsations, is more stable that of type 1 QP's, associated with magnetic pulsations. In most cases magnetic variations during type 2 QP events, which occur under magnetically quiet condition, have no spectral peak corresponding to the peak in QP's spectrum. However, a small but significant peak in pulsation spectrum is occasionally noted, when the type 2 QP event occurs in a moderately disturbed condition. The peak value of magnetic spectrum, in such a case, is generally two orders of magnitude less than the maximum power of background magnetic fluctuations. The amplitude of magnetic pulsations is in the order of 0.01 nT/s. The H component of magnetic pulsations tends to be more correlated with the VLF intensity variations than the D component. These properties are observed at conjugate pair stations with good conjugacy. It is suggested that weak magnetic pulsations, correlated with the type 2 QP, are produced by ionospheric conductivity changes associated with wave induced particle precipitations.
Self‐focusing of radio waves in an underdense ionosphere
Authors: Perkins F. W. , Goldman M. V.
Published: February 2008

The theory of self‐focusing instabilities in the ionosphere is developed emphasizing the critical parameters required to obtain sufficiently fast temporal and spatial growth rates so that the instability may be observed. It is shown that self‐focusing will not occur unless 2cf/πfp²l <1, where f is the radio wave frequency, fp a typical ionospheric plasma frequency, and l the spatial growth length. (In the F region, l ≈ 25 km is used, while in the E region l ≈ 1 km.) In the F region, the threshold power flux Pc is Pc,F ≈ (1.5 µW/m²) (f/15 MHz)³ (Te/1000 °K)4 × (106 cm−3/ne)³CF, where ne and Te are typical electron densities and temperatures, and CF ∼1 depends on spatial and temporal growth rates. In the E region, the result is Pc,E ≈ (1 mW/m²) (f/15 MHz)³ (105 cm−3/ne)CE, where CE ≈ 1 again depends on growth rates. Dimensional analysis indicates nonlinear saturation will set in when variations of order unity occur in the radio wave intensity. The corresponding relative electron density fluctuations are given by δn/n ∼πcf/fp²l. Applications to planned ionospheric heating experiments and ionospheric modification by the microwave beam from a satellite power station are discussed.
Precipitation of inner zone electrons by whistler mode waves from the VLF transmitters UMS and NWC
Authors: Koons H. C. , Edgar B. C. , Vampola A. L.
Published: February 2008

The precipitation of energetic electrons which are commonly observed in the drift loss cone east of 60° east longitude between L ∼1.6 and L ∼1.8 can be accounted for by a Doppler‐shifted cyclotron resonance between the electrons and nonducted whistler mode waves from high‐power, ground‐based VLF transmitters. A ray‐tracing analysis using a diffusive‐equilibrium model shows that 17.1‐kHz waves starting with vertical wave normals between 23° and 31° magnetic latitude cross the magnetic equator between L ∼1.6 and f L ∼1.8 with wave normals of approximately 63°. A relativistic cyclotron‐resonance analysis for the same model plasmasphere using the ray‐tracing results gives an energy versus L shell dependence for the precipitated electrons which is in excellent agreement with the observed dependence. The primary VLF transmitter is most probably the UMS transmitter located near Gorki, USSR. It transmits on 17.1 kHz. VLF records covering this frequency band were available for only three of the time periods when electrons were observed. In two cases UMS was transmitting at the time required to account for the observations. In the third case a higher frequency is required to fit the data. At the time, the NWC transmitter at North West Cape, Australia was operating at 22.3 kHz. These data are consistent with a model in which weak pitch angle scattering by whistler mode waves from NWC does not completely fill the drift loss cone at the longitude of NWC.
Radar interferometry: A new technique for studying plasma turbulence in the ionosphere
Authors: Farley D. T. , Ierkic H. M. , Fejer B. G.
Published: February 2008

A new radar interferometer technique has been developed and used successfully at the Jicamarca Radio Observatory in Peru to study the strong nighttime plasma turbulence in the equatorial electrojet. The technique represents a major step forward in radar probing of turbulent irregularities such as (but not limited to) those in the electrojet. In many situations it provides far more information than previous Doppler measurements. We form the cross spectrum of the backscattered signals received from approximately overhead on two antennas, separated in this case along an east‐west baseline, as well as the individual power spectra. From the phase of the cross spectrum at different Doppler frequencies we can determine the individual positions of plasma wave packets propagating vertically with different velocities, and we find, for example, that oppositely propagating waves always come from distinctly separated regions. The data allow us to study the eddy structure within the electrojet in far more detail than hitherto possible, and by using the irregularity patches as tracers and following their east‐west motion, we can obtain a vertical profile of drift velocity. Our first observations of this sort have shown that at night the vertical Doppler velocity at times may substantially exceed the mean horizontal velocity of the patch and the small horizontal velocity near the top and bottom of the layer may actually be westward when the main motion is eastward.
Photodissociation of N2: A significant source of thermospheric atomic nitrogen
Authors: Richards P. G. , Torr D. G. , Torr M. R.
Published: February 2008

In this paper the production rate of atomic nitrogen by photodissociation of N2 is reevaluated taking into account predissociation of the numerous ′Πu and ′Σu valence and Rydberg states of N2 under optically thick conditions. We find that if it is assumed that most of the solar energy absorbed in the 800‐ to 1000‐Å spectral range results in dissociation of N2, the production of N(²D) and N(4S) will be significantly enhanced. Inclusion of this source also requires an upward adjustment of the rate coefficient for quenching of N(²D) by atomic oxygen by ∼50%. This rate coefficient was previously determined aeronomically to be (3.9–4.5) × 10−13 cm³ s−1. Our calculations yield ∼6 × 10−13 cm³ s−1. An accurate knowledge of the rate is essential in calculations of the [N(4S)]/[NO] ratio which is nonlinearly sensitive to O quenching of N(²D).
A three‐dimensional general circulation model of the thermosphere
Authors: Dickinson Robert E. , Ridley E. C. , Roble R. G.
Published: February 2008

The global neutral gas temperature distribution and circulation of the thermosphere are calculated for equinox and solstice conditions by using NCAR's thermospheric general circulation model (TGCM). The variables are determined on a 5° grid in latitude and longitude at 24 constant pressure surface layers in the vertical from about 90 to 500 km. Global empirical models of electron density and neutral composition are used to specify ion drag and the neutral gas background properties. The energy sources that drive the thermosphere include heating caused by the absorption of solar EUV and UV radiation and a high‐latitude heat source associated with auroral processes. Starting from a global average state, the TGCM reached a diurnal reproducible pattern in approximately 5 days and showed no apparent instabilities in the basic flow. The calculated motion patterns indicate that the observed day‐to‐day variability in the thermosphere is likely because of variations in the heat and momentum sources such as the result of neutral plasma interactions and waves coming from the lower atmosphere. The calculated horizontal structure of wind and temperature is qualitatively different between the upper and lower thermosphere. This difference occurs because the diurnal and semidiurnal forcing are in phase in the optically thick lower thermosphere but out of phase in the optically thin upper thermosphere. Using the latest calculated heating efficiencies and measured fluxes of EUV, the calculated diurnal temperature amplitude is 250 K, compared to 320 K predicted by the MSIS empirical model, indicating that solar heating is primarily responsible for maintaining the structure of the thermosphere. The zonally averaged properties of the TGCM essentially agree with previous results obtained from a two‐dimensional model of the zonally symmetric thermosphere. However, for reasons discussed, the latitudinal variations of temperatures is somewhat larger than previously calculated. Our previously estimated auroral heating terms need to be reduced by 30%–40% to get best agreement with observations.
Tidal effects on D and E region ion chemistries
Authors: Forbes Jeffrey M.
Published: February 2008

Numerical simulations are presented which demonstrate that diurnal and semidiurnal oscillations in temperature and O, O2, and N2 densities can produce asymmetries of D and E region electron concentrations about noon similar to those observed experimentally. In the D region it is assumed that NO+ is the precursor ion in a chain which involves three‐body formation of the intermediary cluster ions NO+(H2O)m−1 (X) (m=1–3), where X can be N2, O2, H2O, or CO2, switching reactions which convert these weakly bound clusters to hydrates of NO+, and reaction of the third hydrate of NO+ with H2O to initiate the chain to form H+(H2O)n (n=1–7). The rates of three‐body and thermal breakup reactions are affected by tidal oscillations in the ambient temperature and total density. For instance, lower (higher) temperatures enhance (inhibit) the formation of clusters and inhibit (enhance) their thermal breakup, thus reducing (increasing) the electron concentration, since the recombination coefficients of cluster ions increase with cluster size and are all large in comparison with that of NO+. In the E region, ion production is affected by variations in the attenuation of solar flux due to optical depth changes of the overlying atmosphere, as well as local changes in O, O2, and N2 densities. A noon bite‐out in D region electron concentrations, which is sometimes observed experimentally, could not be simulated with the present model. Further, a noon bite‐out at mid‐latitudes cannot be produced via modulation of the electron concentrations by any reasonable tidal variation in electron‐neutral collision frequency (it is the product of these quantities which is actually inferred from the measurements). It is suggested that such an effect might be produced by temperature oscillations associated with a gravity wave propagating through the region, or a time variation in NO concentrations due to vertical transport by tides, gravity waves, or changing eddy mixing rates.
Observations of multiple, narrow energy peaks in electrons precipitating from the inner radiation belt and their implications for wave‐particle interactions
Authors: Imhof W. L. , Gaines E. E. , Reagan J. B.
Published: February 2008

With a high‐resolution (∼20 keV full width at half maximum (FWHM)) electron spectrometer providing contiguous energy coverage from 68 to 1120 keV, a new study has been made of the previously discovered peaks in the energy spectra of electrons precipitating from the inner radiation belt. The measurements were performed from the low‐altitude polar orbiting satellite P78‐1. In some cases the spectral peaks that decrease in energy with increasing L shell on a satellite pass are extremely narrow, being of the order of 20 keV wide after the instrument resolution is unfolded. Such narrow widths require that the bandwidth of the responsible waves be less than ∼1 kHz if the electron peaks result from first‐order cyclotron resonance. A significant fraction of the interactions with nearly monochromatic waves must also take place within a narrow range of latitudes, ±5° if near the equator, or an even narrower interval if off the equator. The peak energy values and the behavior with L shell are generally consistent with cyclotron resonance near the equator involving nearly monochromatic waves in the frequency range 10–20 kHz radiating over the interval 1.5 ≲L ≲1.85, but significant deviations from this simple pattern are observed. The energy spectra sometimes contain additional higher‐energy peaks with similar L dependences and with comparable widths, but in some cases the peaks are much broader. Peak energies of the dual set on a given orbit do not generally correspond exactly to first and second harmonic resonances with waves of the same frequency, and it is concluded that another group of narrow band waves may be involved.
An analytic treatment of the structure of the bow shock and magnetosheath
Authors: Zhuang H. C. , Russell C. T.
Published: February 2008

A detailed theoretical examination of the jump conditions of the bow shock in an ideal magneto‐hydrodynamic flow pattern is used to obtain two sets of approximate analytic representations of the parameters behind the bow shock. From these analytic representations contour maps can be drawn to show clearly the influence of the solar wind magnetic fields, especially their directions on these parameters. We next use these results to examine the expected thickness of the magnetosheath. In order to obtain an analytical solution that is tractable we assume that the average values of the parameters along the radial direction in the magnetosheath are simply equal to their values just behind the bow shock. We emphasize that this assumption allows variations of these parameters along the radial direction. A very simple approximate formula for the thickness of the magnetosheath is obtained which satisfies the boundary conditions and conservation laws of mass and momentum flux. Two years magnetometer data from the Isee satellite crossing the magnetosheath during the period between October 1977 to October 1979 are used to determine empirically the thickness of the magnetosheath and to compare these observed thickness with theoretical estimates. Because of the sensitive dependence of the formula for the magnetosheath thickness on the value of γ, the polytropic exponent of the plasma gas, we can use this comparison to determine the optimum value of γ. A value of γ=2 gives much better agreement than 3, 5/3 or 7/5.
Latitudinal structure and extension of the polar atmospheric disturbance
Authors: Prölss G. W.
Published: February 2008

The dissipation of solar wind energy in the upper atmosphere generates a disturbance whose morphology depends on local time, season, and magnetic activity. A statistical analysis of ESRO 4 data reveals that (1) in the afternoon/evening sector the disturbance boundary coincides with the region of electric current dissipation along the auroral oval; (2) in the midnight/early morning sector, dynamical effects extend the disturbance zone to lower latitudes, and this expansion is strongly dependent on season and magnetic activity; and (3) in the late morning sector, direct heating effects along the auroral oval are superimposed on the residuals of the early morning disturbance. These results are consistent with previous observations and provide new boundary conditions for upper atmospheric models.
On the two‐stream approach to electron transport and thermalization
Authors: Stamnes Knut
Published: February 2008

An explicit solution to the electron transport and energy degradation problem is presented in the two‐stream approximation. The validity of this simple approach is discussed, and it is shown that it can be extended to high electron energies (appropriate for applications to auroras) provided the coupling between the two streams, described by the backscatter ratio, is correctly estimated. A simple formula for the backscatter ratio which can be used at all energies is derived.
On the effects of atmospheric gravity waves on profiles of H, O3, and OH emission
Authors: Hatfield R. , Tuan T. F. , Silverman S. M.
Published: February 2008

The effect of gravity waves on H and O3 profiles is investigated using a gravity wave model, valid for an inhomogeneous atmosphere. The consequent effect on OH emission is also considered using Good's data for the ‘undisturbed’ profiles. The results show that (1) the magnitude of the response depends on the relative magnitudes and signatures of the vertical and horizontal wave‐induced flux. Since in some regions the horizontal velocity field of the gravity wave can increase much more rapidly with height than the vertical field, horizontal flux can in some instances overcome the vertical even when there is a relatively sharp layer gradient. (2) The structures in the OH profile produced by the gravity wave are in general, but not always, more pronounced below the intensity peak than above. The observed dark areas on OH emission can be produced by gravity waves with vertical wavelength small compared with the half width of the intensity profile.
A two‐temperature plasma distribution in the magnetosheath at lunar distances
Authors: Sanders G. D. , Freeman J. W. , Maher L. J.
Published: February 2008

Least squares Maxwellian fits have been made to the data from the Apollo 15 suprathermal ion detector experiment (SIDE) while the dusk magnetosheath at lunar orbit. It is found that the data are best fit by superimposing two co‐moving proton populations whose temperatures differ radically: kT ∼10 eV and kT ∼100 eV. This two‐temperature distribution cannot be explained by alpha particles. The higher temperature population is typical of that expected for the fully shocked magnetosheath at lunar distances. The colder distribution could arise from cold plasma from within the magnetosphere brought up to magnetosheath flow speeds, but it is more probably due to unshocked solar wind.
Magnetic pumping of particles in the outer Jovian magnetosphere
Authors: Borovsky J. E. , Goertz C. K. , Joyce G.
Published: February 2008

The mechanism of magnetic pumping consists of two processes, the adiabatic motion of charged particles in a time‐varying magnetic field and their pitch angle diffusion. The result is a systematic increase in the energy of charged particles trapped in mirror (and particularly, magnetospheric) magnetic fields. A numerical model of the mechanism is constructed, compared with analytic theory where possible, and, through elementary exercises, is used to predict the consequences of the process for cases that are not tractable by analytical means. For energy dependent pitch angle diffusion rates characteristic ‘two‐temperature’ distributions are produced. The model is applied to the outer Jovian magnetosphere for two purposes; to find magnetospheric regions in which the mechanism may energize trapped particles, and to generate distribution functions involving pitch angle diffusion caused by wave‐particle interactions. We find that beyond 20 RJ in the outer magnetosphere particles may be magnetically pumped to energies of the order of 1–2 MeV and we predict two‐temperature distribution functions with ‘break points’ at 1–4 keV for electrons and 8–35 keV for ions.
Measured branching ratios for O II ²D and ²P Transitions in the wavelength range 530 to 800 Å
Authors: Morrison D. , Christensen A. B. , Cunningham A. J.
Published: February 2008

Branching ratios for four sets of extreme ultraviolet transitions terminating on the ²D0 and ²P0 metastable levels of ionized oxygen have been measured. The emissions were excited in both an open window hollow cathode and a capillary discharge lamp, and the branching ratios were derived from the observed intensity ratios of the multiplet pairs. The results are in good agreement with theoretical values and compare favorably, within experimental uncertainties, with line ratios obtained by EUV spectroscopy of the airglow.
The Kelvin‐Helmholtz Instability in inhomogeneous plasmas
Authors: Huba J. D.
Published: February 2008

The influence of plasma and magnetic field gradients on the Kelvin‐Helmholtz instability is investigated. The instability is driven by a sheared ion velocity flow parallel to the magnetic field. The modes considered satisfy k²L² ≫1, where k is the wave number and L is the scale length of the inhomogeneity. It is found that a density gradient has a stabilizing effect, while a temperature gradient has a destabilizing effect on the fastest growing mode. However, both these influences are relatively weak. More important, it is found that a magnetic field gradient is strongly stabilizing in finite β plasmas because of an ion ▽B drift wave resonance.
Collisionless shocks and upstream waves and particles: Introductory remarks
Authors: Kennel C. F.
Published: February 2008

We discuss more aspects of collisionless shock theory that might be pertinent to the problem of upstream waves and particles. It is hoped that our qualitative remarks may be a useful guide for the general reader as he goes through the detailed papers to come.
A statistical survey of ions observed upstream of the Earth's bow shock: Energy spectra, composition, and spatial variation
Authors: Ipavich F. M. , Galvin A. B. , Gloeckler G. , Scholer M. , Hovestadt D.
Published: February 2008

The characteristics of 33 diffuse particle events in the energy range from ∼30 to ∼130 keV/Q observed upstream of the earth's bow shock have been determined. The measurements were made with the Ultra Low Energy Charge Analyzer (ULECA) sensor of the Max‐Planck‐Institut/University of Maryland instrument on ISEE‐1. The energy spectra of these events are clearly steeper than power law and are generally well described by either an exponential or Maxwellian in energy per charge, with average e folding values of ∼20 keV/Q and ∼15 keV/Q, respectively. The composition in these events is remarkably constant as a function of energy per charge and is similar (within a factor of ∼2) to that of the solar wind. The particle flux is found to decay exponentially with distance from the bow shock, with an e folding distance of ∼7 RE for H and He at 30 keV/Q. Inverse velocity dispersion was observed in each event, with an average delay time of ∼40 min between the 30 keV and 130 keV proton equilibrium intensity levels. Our measurements are consistent with a Fermi acceleration mechanism and the presence of a free escape boundary upstream of the bow shock. Although existing Fermi‐escape boundary models differ somewhat in detail, our present analysis is unable to differentiate among them.
Spectral characteristics of magnetic Pi 2 pulsations in the auroral region and lower latitudes
Authors: Kuwashima Masayuki , Saito Takao
Published: February 2008

The spectral characteristics of magnetic Pi 2 pulsations are studied in detail using data obtained simultaneously over a wide latitudinal range from the auroral region (L=7.5) through low latitudes (L=1.8). Pi 2 is revealed to be observed almost simultaneously with the onset of a substorm expansion over the wide latitudinal range with the common predominant period. The Pi 2 amplitude shows a primary maximum at the auroral oval and an additional secondary maximum near the plasmapause. The Pi 2 period generally becomes shorter (longer) when the associated auroral breakup starts at lower (higher) latitudes. Pi 2 is also observed simultaneously at a pair of conjugate stations in the auroral region with similar waveforms. These observational results support the torsional oscillation model which suggests that Pi 2 is caused by the hydromagnetic torsional oscillation of the geomagnetic field line anchored on the auroral oval.
Observations of particle precipitation, electric field, and optical morphology of an artificially perturbed auroral arc: Project Waterhole
Authors: Yau A. W. , Whalen B. A. , Creutzberg F. , Pongratz M. B. , Smith G.
Published: February 2008

A sounding rocket carrying 88 kg of high explosives and plasma diagnostic instrumentation was launched from Churchill Research Range on April 6, 1980, over a premidnight auroral arc, as part of Project Waterhole, a joint NRC/LASL experiment. The object of the experiment was to produce an ionospheric hole, or plasma density depletion, at about 300‐km altitude on field lines connected to an auroral arc. The plasma depletion is produced when the explosive by‐products (mostly water) charge exchange with the ambient O+ ions to produce molecular ions (mostly H2O+) which then rapidly recombine with ambient electrons. It was speculated that the presence of the ‘hole’ would interfere with the field‐aligned current systems associated with the arc and would in turn perturb the auroral source mechanism. The release occurred about 10 km poleward of the auroral arc field lines. As expected, a large ionospheric hole was detected by the rocket‐borne plasma sensors. Within a few seconds following the release (1) the energetic electron precipitation observed in the hole dropped to background levels, (2) the luminosity of the auroral arc observed by a ground‐based auroral scanning photometer decreased by a factor of 2, and (3) the ionospheric E region density below the hole decayed at a rate consistent with a sudden reduction in particle precipitation. The simultaneous onset of these gross changes in electron precipitation coincident with the release strongly suggests a cause and effect relationship. Relevant observations of the particle precipitation, electric field, and optical morphology of the auroral arc system in the experiment are presented and their interpretations discussed in terms of current theories of auroral field‐aligned currents and auroral arc formation. Our observations suggest that the ionospheric plasma and the field‐aligned current systems play a crucial role in the auroral acceleration process.
Height dependence in the power spectrum of diffuse radar aurora
Authors: Unwin R. S. , Johnston P. V.
Published: February 2008

Observations on the 53.5‐MHz auroral radar at Slope Point, New Zealand, show that the power spectrum of the afternoon/evening diffuse radar aurora associated with the eastward electrojet consists of two different component spectral distributions, which are designated type A and type B. The type A distribution has a tendency to double peak and a width at half power corresponding to a velocity of ∼300 m s−1; the type B distribution is single‐peaked with a half‐power width ∼600 m s−1. The irregularities producing the type A distribution are centered at a height of about 106 km, those producing type B at 112 km. When viewed on an azimuth of about 25° east of the local magnetic meridian, the mean Doppler velocity of each spectral distribution can differ by up to several hundred meters per second on different occasions but that of type B always exceeds the mean velocity of type A. On the generally accepted hypothesis that the radar observes secondary waves generated by non linear processes from the primary irregularities in the electrojet, this difference indicates a real difference in the velocity of the primaries and/or that they propagate in different directions.
Three‐dimensional computer modeling of dynamic reconnection in the geomagnetic tail
Authors: Birn Joachim , Hones Edward W.
Published: February 2008

The unstable dynamic evolution of the magnetotail in response to a sudden occurrence or enhancement of resistivity is studied by means of a three‐dimensional, time‐dependent, nonlinear, resistive MHD code. We start from a realistic three‐dimensional self‐consistent tail model, including flaring in y and z and plasma sheet thickening toward the flanks. Many features that are believed to characterize the expansion phase of substorms developed without any driving mechanism at the boundary. Confirming earlier two‐dimensional results, we found plasma sheet thinning, occurrence of strong flow and negative Bz, induced electric fields and formation of neutral lines, and a tailward‐moving ‘magnetic bubble’ or ‘plasmoid.’ In addition, the results show that the occurrence of Bz <0 and the shape of the neutral lines are more complex than in 2‐D models, with the negative Bz generally more restricted in time and space than strong tailward flow. The increase in plasma sheet thickness and Bz toward the flanks of the tail causes the reconnection and acceleration process to be limited in the y direction. Earthward from the main x line, a diversion of the cross‐tail current is found, being earthward on the dawn side of the tail and tailward on the dusk side. Although the magnetic field turns so as to stay nearly perpendicular to the diverted current, net field‐aligned components of the current remain inside the plasma sheet. These have the signatures of region 2 currents (Iijima and Potemra, 1976) flowing tailward on the dawn side and earthward on the dusk side.
Some general characteristics of upflowing ion beams over the auroral zone and their relationship to auroral electrons
Authors: Collin H. L. , Sharp R. D. , Shelley E. G. , Johnson R. G.
Published: February 2008

Data from the Lockheed ion mass spectrometer and electron spectrometer on the polar‐orbiting satellite S3‐3 were used to determine some of the general characteristics of upflowing beams of protons and oxygen ions at an altitude of about one earth radius above the auroral regions and also their relationship to auroral electrons. Ions of both species were found with energies throughout the range of the instrument, 0.5–16 keV. The energies of the two ion species were well correlated with one another. The oxygen ion energy almost always exceeded the proton energy and was, on the average, 1.7 times higher. The average energy of the accompanying electrons was also correlated with the ion energies and was usually intermediate in value. The fluxes of ions and electrons appeared to behave quite independently of one another, but the fluxes were found to be limited in their range. Proton fluxes did not exceed 109 (cm² s sr)−1, and oxygen ion fluxes did not exceed 5 × 107 (cm² s sr)−1, while the fluxes of electrons always exceeded 3 × 107 (cm² s sr)−1. The results of this study suggest that the protons are principally energized by a quasi‐static parallel electric field and that on a statistical basis the O+ ions derive about half of their energy from this mechanism and about half from some mass‐dependent process.
Nonlinear simulations of type 1 irregularities in the equatorial electrojet
Authors: Newman Alice L. , Ott Edward
Published: February 2008

A nonlinear two‐dimensional computational model has been developed to examine the nonlinear state of the two‐stream plasma instability in the equatorial electrojet. The model employed the two‐fluid equations for electrons and ions in the vicinity of a constant northward magnetic field perpendicular to an applied electric field. Collisions with neutral species were included, along with a viscous representation of the Landau damping of short wavelength modes. The unperturbed plasma was assumed to be homogeneous and isothermal. The simulation confirmed radar observations of isolated Type 1 irregularities (Crochet et al., 1979): (1) horizontally directed waves were dominant and propagated at the acoustic speed, and (2) obliquely directed waves had spectral heights several orders of magnitude lower and propagated at speeds lower than the acoustic speed. The instability appeared to saturate by a rotation of the electric field vector until the component perpendicular to the current density field vector was reduced to a value lower than the critical value for marginal stability. In one dimension this would appear as a ‘velocity reduction' mechanism reducing the instability to a marginally stable state.
Interplanetary Alfvénic fluctuations: A stochastic model
Authors: Barnes Aaron
Published: February 2008

The strong alignment of the average directions of minimum magnetic variance and mean magnetic field in interplanetary Alfvénic fluctuations is inconsistent with the usual wave‐propagation models. We investigate the concept of minimum variance for nonplanar Alfvénic fluctuations in which the field direction varies stochastically. It is found that the tendency of the minimum variance and mean field directions to be aligned may be purely a consequence of the randomness of the field direction. In particular, a well‐defined direction of minimum variance does not imply that the fluctuations are necessarily planar. The fluctuation power spectrum is a power law for frequencies much higher than the inverse of the correlation time. The probability distribution of directions of a randomly fluctuating field of constant magnitude is calculated. A new approach for observational studies of interplanetary fluctuations is suggested.
Relationship between ionospheric electric field and ground magnetic pulsations in the PC 3 domain at midlatitude
Authors: Lathuillere C. , Glangeaud F. , Lacoume J. L. , Lejeune G.
Published: February 2008

At middle latitudes, measurements of electric field fluctuations in the F region, in the Pc 3 frequency range is difficult. In France, with the incoherent scatter velocity measurements, three experiments have been performed. Technical performances and difficulties are described. Specific signal processing methods are presented, and pulsations in the ionosphere are described in time and in frequency form. They are identified by comparison with ground magnetic measurements through the use of autospectrum and coherency coefficient estimations. In the ionosphere we have found few events with amplitude waves higher than the detection level. When there was correlation between ionosphere and ground it was possible to infer the impedance of the ionosphere plus ground for the transfer between the electric field in the F region and the magnetic field on the ground. The deduced value of this impedance is approximatively 10 Ω. According to Hughes and Southwood's theory this means that the signals have a very short horizontal spatial scale.
Centrifugally driven diffusion of iogenic plasma
Authors: Siscoe George L. , Summers Danny
Published: February 2008

The plasma distribution around Io as measured by Voyager 1 displays an asymmetric discontinuity at Io's orbit that has been suggested to be the signature of centrifugally driven interchange diffusion fed by plasma derived from Io. We explore this hypothesis here further and find it to be valid. The particular form for the diffusion coefficient appropriate to centrifugally driven turbulence is derived. The nonlinear character of this kind of diffusion is thereby made explicit. Solutions to the nonlinear, time‐independent and linearized, time‐dependent diffusion equations are given. These display a markedly conservative behavior. The nonlinear, steady state solutions are identical in form to the solutions of the previously studied equation of linear, atmospherically driven diffusion. The linearized, time‐dependent solutions exhibit a negative feed‐back quality that buffers the response of the density to changes in the source strength. Estimates of the source strength, the diffusion coefficient, and the signal propagation speed are also given.
The Jovian decametric arcs as an interference pattern
Authors: Boischot André , Aubier Monique
Published: February 2008

It is proposed that the nested arc structures, observed in the decametric Jovian radio emission by the planetary radio astronomy experiment, are due to the interference of two rays from a single source at each frequency, the relative phase changing with rotation of the planet. This idea can explain at the same time the shape of the arcs and their repetitivity. But the nature of the two interfering rays is not understood yet.
Radio occultation of Jupiter's ring: Bounds on optical depth and particle size and a comparison with infrared and optical results
Authors: Tyler G. L. , Marouf E. A. , Wood G. E.
Published: February 2008

The Jovian ring is not detectable in Voyager 1 radio occultation data, setting the bounds on its optical thickness of 2 × 10 −4 and 5 × 10−4 at 13‐ and 3.6‐cm wavelengths, respectively. Comparison of results at radio, infrared, and optical wavelengths suggests a population density that either falls more rapidly than the inverse square of the linear size or is sharply bounded in maximum particle size. A fragmentation power law, ∼ap for amin <a <amax, where a is the particle radius, and power index p between −3 and −4 lead to a minimum size estimate amin ≃1–2 µm. A specific model which is consistent with all observations consists of a power law distribution of lossless or very slightly absorbent Mie scatterers with refractive index near 1.62, power law index p approximately −3.5, and a minimum particle size or about 1.5 µm. In this model the upper size limit amax is not critical, provided that it is greater than about 4 µm.
The coronal and interplanetary current sheet in early 1976
Authors: Burlaga L. F. , Hundhausen A. J. , Zhao Xue‐pu
Published: February 2008

A comparison of Helios 1 and 2 observations of the interplanetary sector pattern in early 1976 with the maximum brightness curves in the K coronameter data at 1.5RS shows that the latter may be identified with the footprints of the sector boundary surface to an accuracy of ≈ 10°. The neutral line computed by Wilcox et al (1980) from a potential field model, using Mt. Wilson photospheric magnetic field measurements and a source surface at 2.6RS, is similar in shape to the K coronameter maximum brightness curves but extends to higher latitudes. The Helios observations give better agreement with the K coronameter curves for the one solar rotation on which a test of the latitutude extent of the neutral line was possible. The K coronameter results and the Helios data show that the sector boundary surface probably extended to ≈ 15° in the northern hemisphere and to ≈ 30° in the southern hemisphere, with little change between 1.5 RS and 1 AU. The surface was warped appreciably from a single tilted plane (a dipole configuration) suggesting a significant magnetic quadrupole contribution.
Production and Dynamics of High‐Latitude Irregularities During Magnetic Storms
Authors: Houminer Z. , Aarons J. , Rich F.
Published: February 2008

Scintillation observations at VHF and UHF frequencies at auroral and subauroral stations, together with in situ measurements by ISIS 2 in 1971–1972 and S3‐2 in 1975–1976, show two areas of irregularities during magnetic storms. The two regions are related to the plasmapause and to the auroral oval and straddle the minimum electron density region of the trough. The plasmapause irregularities are located on the equatorward edge of the trough and may extend several degrees in latitude. The electron density trough itself shows very low activity of irregularities, while the poleward wall of the trough is the beginning of the region where auroral oval F layer irregularities are present. By use of S3‐2 observations of electron and ion density, electron temperature, magnetic fields, and precipitating particles, it is suggested that the production mechanism of the plasmapause irregularities is the temperature gradient drift instability. It is also shown that auroral irregularities are closely related to field‐aligned currents.
Evidence for magnetic field reconnection at the Earth's magnetopause
Authors: Sonnerup B. U. Ö. , Paschmann G. , Papamastorakis I. , Sckopke N. , Haerendel G. , Bame S. J. , Asbridge J. R. , Gosling J. T. , Russell C. T.
Published: February 2008

Eleven passes of the ISEE satellites through the frontside terrestrial magnetopause (local time 0900–1700; GSM latitude 2–43°N) have been identified, where the plasma velocity in the magnetopause and boundary layer was substantially larger than it was in the magnetosheath. This paper examines the nature of the plasma flow, magnetic field, and energetic particle fluxes in these regions, with a view to determining whether the velocity enhancements can be explained by magnetic‐field reconnection. The principal question is whether the observed difference in tangential plasma velocity Δv between a point in the magnetopause or boundary layer and a reference point in the adjacent magnetosheath, had the direction and magnitude Δvtheory produced by the Maxwell stresses in the magnetopause, assuming that the magnetosheath plasma moved across that boundary. Except for its sign, Δvtheory is shown to be independent of the normal magnetic field component Bn and flow component υn. For the 11 cases, the average ratio |Δv|/|Δv|theory was in the range 0.6–1.2, with a composite average of 0.8. The average angular error was <25°, with a composite average of 10°. The plasma results would require 10 of the crossings to have been located north of the reconnection line (Bn < 0), and one (at 2.4° N latitude) south of it (Bn > 0). The Bn values obtained from minimum variance analysis of the magnetic data were mostly poorly determined, but in general their signs were consistent with the plasma results. The flow velocity across the magnetopause was also poorly determined but it had a negative (inward) composite average as expected. In several cases energetic magnetospheric particles with the proper flow anisotropy and, in one case, reflected magnetosheath particles, were observed outside but adjacent to the magnetopause. All of these results support the reconnection hypothesis. The energetic particles were also used to identify the outer separatrix surface. In one case it was possible to conclude from its location relative to the magnetopause that the reconnection site was in the vicinity of the equatorial plane and not in the cusp. The electric field tangential to the magnetopause is inferred to be in the range 0.4–2.8 mV/m.
Temporal development of composition, spectra, and anisotropies during upstream particle events
Authors: Ipavich F. M. , Scholer M. , Gloeckler G.
Published: February 2008

We present observations of the detailed temporal development of particle intensities, composition, and density gradient anisotropies for protons and heavier ions during several diffuse ion events observed upstream of the earth's bow shock. The measurements, covering the energy range ∼16 keV/Q to ∼130 keV/Q, were made with the ultra‐low energy charge analyzer sensor of the Max‐Planck‐Institut/University of Maryland experiment on ISEE 1. Composition ratios are found to be constant as a function of energy per charge not only during the equilibrium intensity levels of these events, but also during their onset and decay phases. We interpret our measurements in terms of temporal and spatial structures in the upstream region produced by a time dependent first‐order Fermi acceleration mechanism.
Bi‐directional electron pitch angle anisotropy in the plasma sheet
Authors: Hada T. , Nishida A. , Teresawa T. , Hones E. W.
Published: February 2008

Analysis of low‐energy particle measurement on Imp 6 satellite have revealed the occurrence of bi‐directional electron pitch angle anisotropy (enhanced flux in field‐aligned directions) in the magnetotail at the energy range of several hundreds to several keV. The bi‐directional anisotropy was observed most commonly near the neutral sheet, with a parallel and perpendicular temperature ratio of 2–3, and they appeared also just inside of the magnetopause and in the boundary layer. The anisotropy of this extent was not an unusual phenomenon in the plasma sheet; in about 10% of the total of ∼200‐hour interval that we examined electrons had bi‐directional anisotropy. On the other hand, more sharply field‐aligned electron flux enhancements, which cannot be represented by a Maxwellian distribution, are sometimes observed at radial distances of ∼10 RE, near the inner edge of the plasma sheet. The small pitch angle (less than ∼10°) observed in such cases suggests coupling of the enhanced field‐aligned component with the ionosphere. In this paper we describe the observed characteristics of these anisotropic distributions and attempt to explain their formation by Fermi acceleration process in the plasma sheet. Owing to this process, electrons are more accelerated in parallel than perpendicular directions to the magnetic field as they traverse the neutral sheet. The effect of static electric field along the field line caused by the difference in pitch angle anisotropy of protons and electrons is also discussed.
Magnetic declination control of the equatorial F region dynamo electric field development and spread F
Authors: Abdu M. A. , Bittencourt J. A. , Batista I. S.
Published: February 2008

We have carried out a comparative study of the evening prereversal enhancements in the equatorial F region vertical ionization drift velocities (VZ) over Fortaleza (4°S, 38°W), Brazil, and Jicamarca (12°S, 77°W), Peru, two magnetic equatorial stations in the American zone. The results show profound dissimilarities in the seasonal trends in the times and widths of the VZ prereversal peak, which reflect in the spread F characteristics as well, at the two stations. The dissimilarities are shown to be arising mainly from the difference in the magnetic field declination angles that causes differences in the conjugate E region sunset durations and, hence, in the F region polarization electric field development rates at the two stations.
Holes in the nightside ionosphere of Venus
Authors: Brace L. H. , Theis R. F. , Mayr H. G. , Curtis S. A. , Luhmann J. G.
Published: February 2008

Measurements of electron density and temperature by the Pioneer Venus orbiter electron temperature probe have been employed to examine the characteristics and morphology of ionospheric holes in the antisolar ionosphere of Venus. The holes apparently exist as north‐south pairs which penetrate the ionosphere vertically down to altitudes as low as 160 km. Magnetic field measurements show that the holes are permeated by strong radial fields whose pressure is sufficient to balance the plasma pressure of the surrounding ionosphere. The electron temperature in the holes is substantially cooler than the surrounding ionosphere, except in the lowest density regions of the holes where the temperatures greatly exceed the ionosphere temperature. The low temperatures and the low densities of the holes are consistent with the strong radial magnetic fields which inhibit horizontal transport of plasma and thermal energy from the surrounding ionosphere. Plasma depletion processes associated with magnetotail electric fields may be important in the formation of the holes.