bluuebunny/crossref_metadata_2025 huggingface dataset:
Implications of extreme thinness of pulsating auroral structures
Authors: Shepherd G.G. , Fälthammar C.‐G.
Published: February 2008

Pulsating auroras of very thin vertical extent have recently been observed. It is shown that if the primary electrons are stopped in a region as thin as that reported, extremely strong electric fields, ac or dc (or both), are implied. This consequence could be tested by in situ measurements. If the stopping is due to dc electric fields, a potential drop of the order of 10 kV or more in a thin region is required. The local field strength, several mV/m, has an order of magnitude comparable to that estimated for so‐called electric double layers. If the stopping were due to ac fields, the rms electric field strength would have to be even larger. Alternatively, the reported thinness of the luminosity distribution might result without a correspondingly abrupt stopping of the primary electrons. Two conceivable mechanisms for this are discussed and shown to be inadequate. However, there is at least one, involving excitation of the N2+ Meinel band by keV electrons, that cannot be definitely rejected on the basis of existing observations. This question could, however, be resolved by TV observations using suitably band‐limited filters.
Temperatures and collision frequency in the polar E region measured with the incoherent scatter technique
Authors: Schlegel K. , Kohl H. , Rinnert K.
Published: February 2008

Incoherent scatter measurements with a range resolution of 3.0 km have been performed at the facility at Chatanika, Alaska, between 90‐ and 120‐km altitude. From the shape of the measured autocorrelation function, values for the ion‐neutral collision frequency, the electron temperature, and the ion temperature have been deduced. During quiet conditions the obtained quantities do not significantly deviate from the Cira model, whereas during particle precipitation events the temperatures are increased. The measurements also show that, contrary to common assumptions, during disturbed conditions the electron to ion temperature ratio is significantly greater than unity between 100 and 110 km. Below about 95 km the results indicate that coherent reflections from D region electron density gradients or possibly ground clutter become important. This leads to correction factors for electron density measurements at those heights.
A statistical determination of the shape and position of the geomagnetic neutral sheet
Authors: Fairfield D.H.
Published: February 2008

The magnetotail neutral sheet is best represented by an arched surface which separates field lines of opposite polarity. The deviations of this surface from the solar magnetospheric XY plane can be represented by the equation δz = [(Ho+D) (1 ‐ Y²/Yo2)1/2 −D]sin χ when Y <Yo and δz = − Dsin χ when Y ≥Yo, where Y is an east‐west location relative to the center of an aberrated tail, χ is the tilt of the dipole, and Ho, Yo, and D are constants. Optimum values of Ho = 10.5 RE, Yo = 22.5 RE, and D = 14 RE have been determined by minimizing the number of erroneous predictions of tail magnetic field polarity in a large set of Imp 6, 7, and 8 magnetometer data. The model with D ≠ 0 is found to be superior to the D=0 model used in all previous studies of the neutral sheet. This nonzero D value implies that during northern hemisphere summer the neutral sheet is raised above the solar magnetospheric equatorial plane near the center of the tail but is depressed below this plane on the flanks. The neutral sheet tends to be slightly nearer the equatorial plane during intense magnetic activity, but there is little tendency for the neutral sheet to approach this plane at large distances down the tail.
The spatial‐temporal ambiguity in auroral modeling
Authors: Rees M.H. , Roble R.G. , Kopp J. , Abreu V.J. , Brace L.H. , Brinton H.C. , Heelis R.A. , Hoffman R.A. , Kayser D.C. , Rusch D.W.
Published: February 2008

Time‐dependent model studies of the aurora show that various ionospheric parameters respond to the onset of auroral ionization with different time histories. While satellite‐borne instruments sample the spatial morphology of these parameters, geostationary detectors are needed to follow the temporal morphology at a point in space. A pass of the Atmosphere Explorer C satellite over Poker Flat, Alaska, and ground‐based photometric and photographic observations have been used to resolve the time‐space ambiguity of a specific auroral event. The density of the O+, NO+, O2+, and N2+ ions, the electron density, and the electron temperature observed at 280‐km altitude in a 50 km wide segment of an auroral arc are predicted by the model if particle precipitation into the region commenced about 11 min prior to the overpass. Continuous photometric measurements and all‐sky photographs taken at 1‐min intervals from Poker Flat show that the arc formed and became bright approximately 11 min before the satellite measurements were made.
On ‘potential’ well treatment for atmospheric gravity waves
Authors: Yu L. , Tuan T.F. , Tai H.
Published: February 2008

The problem of gravity wave propagation in the lower atmosphere may be formally reduced into a scattering problem with an effective ‘potential.’ In such a formalism, the eigenvalues (in units of inverse square horizontal phase velocity) may be discrete, corresponding to fully guided modes (‘bound’ states) and partially guided modes (‘resonance’ states), or they may be continuous, corresponding to free modes (‘scattering’ states). A phase shift analysis reveals two ‘resonance’ states corresponding to horizontal phase velocities of 309.7 and 264.2 m/s, which may be respectively identified as the Lamb mode and the S2 mode. It appears that although there are a number of means for ducting the F region gravity wave, atmospheric structure is not likely to be one of them.
A study of concurrent riometer and magnetometer variations in the PC 4–5 pulsation band
Authors: Olson J.V. , Rostoker G. , Olchowy G.
Published: February 2008

We report the results of a study of concurrent, periodic variations in riometer and magnetometer signals in the ULF frequency band. We have found that the riometer signals show azimuthal phase variations consistent with the phase variations observed in the magnetometer signals. Prenoon pulsations show apparent phase motion toward the dawn terminator, while post‐noon pulsations show apparent phase motion toward the dusk terminator. The apparent phase motion of the riometer pulses toward dawn in the morning sector is in opposition to the direction of electron drift, indicating modulation of electron precipitation by the ULF magnetic perturbations. In addition, we find the magnitude of the riometer absorption pulsations are linearly correlated with the magnitude of the background riometer absorption level with δA = (0.22±0.04)A when measured in decibels from the quiet day level. This fact indicates modulation of the loss rate of an electron plasma which is at or near the stable trapping limit. These two observations, when combined with other information on variations in electron precipitation, imply that the variations in electron precipitation are controlled by the ULF magnetic perturbations through a ULF‐VLF‐electron interaction.
Alfven waves and drift compressional modes in multispecies plasmas
Authors: Patel V.L. , Migliuolo S.
Published: February 2008

The stability of a multispecies plasma under magnetospheric storm conditions is investigated. The dynamics of the impurity ions are incorporated in the hot plasma component response, since the response (to perturbations) of the cold component is totally dominated by the electrons. The coupling between drift compressional and shear Alfven modes is studied in weak and strong coupling limits, the coupling being weakened when a finite fraction of the hot ions belongs to the heavy species. The characteristic growth rates of the mirror instability, modified by the presence of heavy impurity ions, are determined. For the case of strong coupling, the dominant unstable mode is a compressional Alfven oscillation, driven by the finite density gradient of the plasma. The main effect of the impurity ions, in this case, is to stabilize the mode through their inertia. The instability is found to be self‐limiting with increasing plasma pressure, and the asymptotic normalized growth rate is determined.
Preliminary report of numerical simulations of intermediate wavelength collisional Rayleigh‐Taylor Instability in equatorial spread F
Authors: Keskinen M.J. , Ossakow S.L. , Chaturvedi P.K.
Published: February 2008

Computer simulations of the intermediate wavelength (100–1000 m) collisional Rayleigh‐Taylor instability in local unstable regions of the postsunset bottomside (300 km) equatorial F region ionosphere have been performed. For ambient electron density gradient scale lengths L=5, 10, 15 km we find that the linearly unstable horizontal modes saturate by nonlinear generation of linearly damped vertical modes with the result that in the nonlinear regime, power laws are observed in the horizontal P(kx) ∝ kx−n and vertical P(ky) ∝ ky−n one‐dimensional power spectra with n=2‐2.5. These results are consistent both with in situ experimental data and with theoretical prediction.
The IMF By effects observed by the IMS Alaska Meridian Chain
Authors: Akasofu S.‐I. , Romick G.J. , Kroehl H.W.
Published: February 2008

The effects of the By component of the interplanetary magnetic field on the polar daily magnetic variations are examined by using magnetic records from the International Magnetospheric Study Alaska meridian chain of observatories. A large difference (∼180°) in the direction of the disturbance vectors in the 9–13 MLT sector between 75° and 85° in magnetic latitude is clearly seen in the daily records. The results are in agreement with the earlier studies, which indicate that this difference in direction depends on the sign of the interplanetary magnetic field By component.
Some observations of long‐delay wireless echoes on the 28‐MHZ amateur band
Authors: Goodacre A.K.
Published: February 2008

Eight possible examples of long‐delay echoes have been obtained on the 28‐MHz amateur band by operating when the band is just ceasing to support ground‐to‐ground communication. The delay times of these echoes tend to be integral multiples of 138 milliseconds. Since it takes about 138 milliseconds for a radio wave to travel around the world, these signals may have been trapped in interlayer ionospheric ducts.
Observations of neutral wind profiles between 115‐ and 175‐km altitude in the dayside auroral oval
Authors: Pereira E. , Kelley M.C. , Rees D. , Mikkelsen I.S. , Jørgensen T.S. , Fuller‐Rowell T.J.
Published: February 2008

Lithium trail neutral wind measurements have been made in the morning dayside auroral oval as a function of altitude. During all four experiments the winds displayed a large altitude shear. In the most extreme case the wind at 135‐km altitude was toward the south at 210 m/s, and at 175‐km altitude toward the northwest at 170 m/s. A one‐dimensional numerical solution to the Navier Stokes equation including an initial steady state wind profile, viscosity, pressure gradient, Lorenz force (J×B), and the Coriolis force was made in order to study the effect of auroral processes on the neutral atmosphere. Since electric fields and plasma density were measured, the Lorenz force could be modeled with reasonable accuracy. The pressure gradient was determined in an iterative way as a best fit to the observation. Within the limitations of a one‐dimensional model the numerical solution is in reasonable agreement with one of the observations, which is applicable to such a modeling, and indicates that both Lorenz forces and pressure gradients due to auroral processes are required to explain the observation.
Backstreaming ions outside the Earth's bow shock and their interaction with the solar wind
Authors: Bonifazi C. , Egidi A. , Moreno G. , Orsini S.
Published: February 2008

Analysis of plasma data supplied by the European satellite ISEE 2 has confirmed the existence in the solar wind of two different ion populations (referred to as ‘reflected’ and ‘diffuse’) backstreaming from the earth's bow shock. The two populations have similar bulk velocities and flux densities but quite different temperatures. In addition, the diffuse ions strongly interact with the solar wind, producing fluctuations in the distribution of the proton velocities, while the reflected ions do not disturb significantly the incoming solar wind. The origin of the two ion populations is then discussed. Although no firm conclusion is reached, the experimental evidence seems to favor a model which explains the appearance of the diffuse population as the result of an interaction of the reflected ions with the solar wind.
Cross‐spectral analysis of Pc 1 emissions recorded at different stations
Authors: Glangeaud François , Lacoume Jean‐Louis , Fargetton Hervé , Gendrin Roger , Perraut Sylvaine , Troitskaya Valeria A.
Published: February 2008

We describe a technique for measuring time delays by using the cross spectrum of Pc 1 recorded at different stations. The method is based upon the computation of the coherency coefficient and of the phase of the cross spectrum. It is applied to data which were obtained during a campaign of observations at two pairs of conjugate stations (L=.7 and L=.5). We show that the coherency coefficient between signals recorded at conjugate points may be large (∼1) for structured Pc 1, which implies that the optical path is constant, to within a fraction of a wavelength, over long periods of time (the integration time is of the order of 16 min). Measurements of time delay between the northern and southern hemispheres show that there is generally an asymmetry, the delay being longer for the north to south propagation. This asymmetry is tentatively interpreted in terms of differences between the ionospheric characteristics above the two stations but also in terms of a difference between the two magnetospheric paths. The detailed study of the cross‐spectral power density phase versus frequency variation shows that on some occasions a reamplification process takes place at each bounce of the wave, but only on one of the two trajectories.
Nighttime enhancements in total electron content near Arecibo and their association with VHF scintillations
Authors: Kersley L. , Aarons J. , Klobuchar J.A.
Published: February 2008

From the analysis of 16 months of transionospheric propagation data taken near Arecibo it has been found that there are associations between nighttime enhancements and quasi‐periodic fluctuations in total electron content and the occurrence of amplitude scintillations. A postmidnight peak is found in the occurrence of electron content increases, maximum wave activity, and largest scintillation index, with some evidence to suggest a second premidnight peak in winter and reduced probability of the features being found at equinox. An analysis of the larger‐scale variations of the total content data for three stations has shown that the postsunset minimum in electron content propagates at high speed toward the northeast, while the premidnight minimum advances toward the south with the main postmidnight maximum having a northward component of motion. The results are discussed in terms of the known dynamical features of the ionosphere at Arecibo and in particular the effects of the neutral wind on the bottomside ionosphere. It appears that conditions conducive to the production of nighttime enhancements in total electron content may also allow the penetration of medium scale gravity waves through the lower thermosphere with an unstable breakdown process giving rise to the development of scintillation‐producing irregularities.
Measurements of solar EUV and soft X ray emission during sudden frequency deviations
Authors: Horan D.M. , Kreplin R.W.
Published: February 2008

Broadband sensors aboard the Solrad 11B satellite measured solar emission in the 0.5‐ to 3‐Å, 1‐ to 8‐Å, 8‐ to 20‐Å, 100‐ to 500‐Å, and 700‐ to 1030‐Å bands during six sudden frequency deviation (SFD) events in 1977. Simultaneous analysis of the solar measurements and the SFD sonograms led to a proposed extension of an existing model to provide a qualitative description of the influence of solar flare emission on SFD development throughout the course of an SFD event. The proposed model clearly reflects the observed greater importance of the rate of change, rather than the magnitude, of solar emission during the early phases of an SFD and then a shift of dominance to solar emission magnitude as the SFD progresses. The observations also demonstrate that solar emission at wavelengths less than 20 Å does not cause the positive frequency deviation peaks in larger SFD events.
Energization of helium ions by proton‐induced hydromagnetic waves
Authors: Gendrin Roger , Roux Alain
Published: February 2008

We consider the diffusion of He+ ions under the influence of ion cyclotron waves generated in a plasma consisting of three different ion populations: a thermal isotropic population containing both H+ and He+ ions and an energetic H+ population, with a positive anisotropy A=T⊥/T∥‐1. We compute, in the velocity space υ∥, υ⊥, the diffusion curves that He+ ions will follow in the presence of ion cyclotron waves propagating in such a medium. We show that for small concentrations of the He+ ions, of the order of 1 to ∼10%, these ions can be energized by such a process up to and above suprathermal energies (E ≳20 eV). On some occasions the He+ ions may even reach energies of the order of the Alfvén energy of the cold plasma population: Ea ≈ mp Va²/2 ≈ 5 keV. Characteristic diffusion times, in pitch angle and energy, for both ion species, are evaluated. They are of the order of 2 to 20 min. These theoretical results are discussed in the frame of recent observations by Geos experimenters showing the close association that exists between the occurrence of ion cyclotron ULF waves and the presence of thermal or supra‐thermal He+ ions in the equatorial region of the magnetosphere (Young et al., 1979).
Conditions for acceleration of energetic ions ≳30 keV associated with the Earth's bow shock
Authors: Scholer M. , Ipavich F.M. , Gloeckler G. , Hovestadt D.
Published: February 2008

We have performed a statistical analysis of particles >30 keV/charge upstream of the earth's bow shock. The rate of occurrence of upstream particle events has been determined relative to the angle between the magnetic field and the shock normal at the shock intersection point as well as relative to the angle between the magnetic field and the radial direction (i.e., the sun‐earth line). The probability for upstream particle events is highest for small angles between the magnetic field and the radial direction, indicating that the occurrence is determined by the bow shock connection time of a field line convected with the solar wind. We have therefore determined for a model bow shock the occurrence rate of upstream particle events relative to the bow shock connection time. Assuming that the particles are accelerated by multiple reflections between upstream and downstream scattering centers, we can use the observed time constants in order to derive propagation parameters. We find a diffusion coefficient depending linearly on energy per charge, the value of the mean free path of a 30‐keV proton to be ∼4 RE, and the region of strong scattering to be limited to ∼30 RE in front of the bow shock.
Auroral X ray observations at 60‐ to 30 km altitudes
Authors: Vij K.K. , Vogel J.S. , Venkatesan D.
Published: February 2008

This paper deals with auroral X ray observations extended to altitudes above those normally attained by balloons (∼30 km). Auroral X ray payloads, consisting of NaI (Tℓ) scintillation crystal cum photomultiplier assembly, are boosted to 60‐ to 65‐km altitudes by Black Brant VI rockets, the ejected payloads float down under parachute making auroral X ray observations down to balloon altitudes. The present study presents specifically the results of a rocket‐borne payload launched from Fort Churchill, Manitoba on February 27, 1978. Twenty X ray energy spectra were calculcated above balloon altitude, and each of these can easily be fitted with a unique e‐folding energy in energies above the peak. The X ray e‐folding energy is observed to vary from 13 to 19 keV and to level off at that value. The observed values of the peak energy and its increase from 26 to 66 keV with increase in depth cannot be reconciled with presently available theoretical calculations, which are considerably lower. These differences are discussed in terms of currently available Monte Carlo and Fokker‐Planck calculations. The trend of the depth dependence of peak energy appears to be closer to the Fokker‐Planck calculations.
Bimodality and the formation of Saturn's ring particles
Authors: Gehrels Tom
Published: February 2008

The F ring appears to have an outer and an inner rim, with only the latter observed by the imaging photopolarimeter (IPP) on the Pioneer Saturn spacecraft. The inside of the G ring, near 2.49 Rs, may also be seen in the optical data. 1979S1 is red as well as dark. The light scattered through the B ring is noticeably red. The A ring has a dense outer rim. The Cassini Division and the French Division (Dollfus Division) have a dark gap near their centers. The C ring becomes weaker toward the center such that outer, middle, and inner C rings can be recognized. The Pioneer and earth‐based observations are explained with a model for the B and A rings to some extent of a bimodal size distribution of particles; the larger ones may be original accretions, while small debris diffuses inward through the Cassini Division and the C ring. During the formation of the ring system, differential gravitation allowed only silicaceous grains of higher density (ρ ≳3 g cm−3) to coagulate. These serve as interstitial cores for snowy carbonaceous grains, between the times of accretion from interplanetary cometary grains and liberation by collision followed by diffusion inward to Saturn and final evaporation.
Dependence of mid‐latitude hydromagnetic energy spectra on solar wind speed and interplanetary magnetic field direction
Authors: Wolfe A.
Published: February 2008

Relationships are examined between interplanetary parameters and local daytime geomagnetic energy (at Pittsburg, New Hampshire; L ∼3.5) in three period bands. Ground‐based magnetometer data, for 336 hours during July and August 1975, are studied when good interplanetary data coverage was available from instruments on the Imp J spacecraft. Multiple linear regression analyses show that the most important interplanetary parameter controlling ground hourly magnetic energy (in the period range 60–240 s) is the solar wind speed. Multiple linear regression analyses further show that hourly magnetic energy in the 30‐ to 60‐s period range is controlled by the solar wind speed and the interplanetary magnetic field direction. The velocity dependence supports the idea of wave production at the magnetopause via the Kelvin‐Helmholtz instability. These waves are later observed as magnetic pulsations (30–240 s) on the ground. The interplanetary magnetic field direction dependence supports the idea that waves produced at or near the earth's bow shock are later observed on the ground as magnetic pulsations in the shortest period band (30–60 s).
Coupling fluxes in the presence of cross L drifts: A comparison of whistler mode and radio beacon results
Authors: Andrews M. K.
Published: February 2008

On the basis of plasmasphere column content models, Davies et al. [1979] deduced the existence of ionosphere/magnetosphere coupling fluxes of order 1 × 1013 el m−2 s−1 per hemisphere, driven by compressed tubes of plasma in the course of an E × B drift. Such large fluxes should be easily detectable on a VLF whistler mode experiment but have not been seen. Data from that experiment are presented that show inward drifts at L=2.3 accompanied at times by small upward fluxes. This is contrary to some theoretical predictions but is in agreement with recent measurements regarding the dynamics of the ionosphere at latitudes approaching the plasmapause. Calculations show column content measurements may be insensitive to compression effects even in the absence of fluxes.
Nonpermanent nighttime H Lyman alpha emissions at low and middle latitudes, detected from the D2A satellite
Authors: Cazes S. , Emerich C.
Published: February 2008

Strong nonpermanent emissions are observed from two H Ly α experiments placed on board the satellite D2A. They are superimposed to the permanent emission due to resonant scattering of the Ly α solar line by telluric atomic hydrogen. These emissions are detected both above and under the altitude of the satellite (∼500 km), in a latitude range ±40°. The correlation observed between their occurrence frequency and the Kp index strongly suggests that the source of excitation is precipitation of particles. The intensities of these emissions observed under the satellite’s altitude are of the order of a few hundred rayleighs. These intensities are in a ratio of about 10 relative to the strong Balmer α emissions previously detected on board the same satellite, and they could be attributed to precipitation of protons in the range 10–300 keV. The intensities of the order of 1 kR measured above the satellite's altitude would require particle fluxes of lower energies too large to be provided by the ring current from double charge exchange. There remains then the possibility of an interference between the experiment and precipitating particles. Even in this eventuality our measurements show evidence of large fluxes of soft particles at low and middle latitudes, even for moderate magnetic activity.
Observations of the velocity distribution of solar wind ions
Authors: Ogilvie K. W. , Bochsler P. , Geiss J. , Coplan M. A.
Published: February 2008

Measurements made by the Isee 3 ion composition experiment have been used to determine the kinetic temperatures of 3He++, 4He++, 16O6+, and 16O7+ in the solar wind. It is found that these temperatures generally obey the relation that Ti/mi = const, but fluctuations, some of which are caused by dynamical effects in the flow, are observed. Whether this relation applies to ions with masses greater than 16 requires more analysis to determine. The temperature of oxygen sometimes rises above 106°K, which is very strong evidence for heating outside the collisional region of the corona. The tendency toward equal temperatures per nucleon occurs everywhere where collisions are unimportant, suggesting that the temperatures are set up close to the sun rather than elsewhere in the interplanetary medium. The velocity distribution function of helium is observed to be non‐Maxwellian, with a pronounced high velocity tail. As this is one condition for heating by wave dissipation, this mechanism must still be considered as a heating mechanism.
Pioneer Venus plasma observations of the solar wind‐Venus interaction
Authors: Mihalov J. D. , Wolfe J. H. , Intriligator D. S.
Published: February 2008

Collection of data from the Ames plasma analyzer on the Pioneer Venus orbiter has permitted long‐term measurements of the interaction of the solar wind with Venus. This paper presents a mapping of the ionosheath flow field, as well as plasma measurements in the distant ionosheath and near the distant plasma cavity (∼10 Venus radii downstream), a summary of observations of jumps in the solar wind proton parameters across Venus' bow shock, and also the apparent detection of ionospheric O+ accelerated up to solar wind speeds downstream in Venus' ionosheath.
Model calculations of the dayside ionosphere of Venus: Energetics
Authors: Cravens T. E. , Gombosi T. I. , Kozyra J. , Nagy A. F. , Brace L. H. , Knudsen W. C.
Published: February 2008

A model of the energy balance of the dayside ionosphere of Venus is presented. Calculations of the dayside electron and ion temperature profiles are carried out and compared with data from experiments on the Pioneer Venus orbiter. The coupled heat conduction equations for electrons and ions are solved for several values of the solar zenith angle. It is shown that thermal conductivities are inhibited by the presence of a horizontal magnetic field. A realistic model of the magnetic field that includes fluctuations is employed in deriving an appropriate expression for the thermal conductivity. The contributions of photoelectrons, ion chemistry, Joule heating, and solar wind heating to the energy balance of the ionosphere are considered.
Polar cloud structure as derived from the Pioneer Venus Orbiter
Authors: Cimino J. B. , Elachi C. , Kliore A. J. , McCleese D. J. , Patel I. R.
Published: February 2008

Pioneer Venus orbiter radio occultation data taken on orbits 9, 18, and 19 at S band (13.06 cm) are used to obtain vertical absorption coefficient profiles of the Venus clouds in the north polar regions. Resulting profiles show dense cloud decks at the 1.5‐ to 4.7‐bar level in the Venus atmosphere. The cloud decks are lower in altitude and more extensive vertically than those found in the equatorial region by Mariner 10 and the Pioneer Venus probes, which have peaks at about the 1‐bar level. These lower clouds appear to be fairly uniform in absorption properties throughout the polar regions. Pioneer Venus orbiter infrared radiometer data are combined with the occultation data in interpreting the north polar cloud structure. The three occultation profiles represent three different regions of the north pole in terms of vertical temperature structure and proximity to the polar hot spots. Regions near the hot spots appear to have depressed upper cloud heights as well as increased optical density. These areas lack the temperature inversions characteristic of the north polar regions far from the hot spots. In addition, the cloud tops at the pole are about 100 mbar deeper in the atmosphere than at the equator.
Images of Venus by three‐station radar interferometry—1977 results
Authors: Jurgens R. F. , Goldstein R. M. , Rumsey H. R. , Green R. R.
Published: February 2008

During the 1977 inferior conjunction of Venus, radar observations were made using three receiving stations as a multiple interferometer. Maps of surface reflectivity and altimetry were prepared from these observations. The new altimetry maps show considerable improvement in relation to many of the earlier maps made using the two‐station interferometer. In particular, there are consistent and explainable correlations between the altimetry and reflectivity maps that did not always exist in the past. The highest‐resolution maps (about 8 km) show three isolated mountains having altitudes of approximately 2 km above their environs, a pair of ridges separated by approximately 100 km and extending 800 km, and a few anomalous reflectivity features for which little or no altitude change is observed. Other maps at slightly lower resolution show a bright irregular ringed crater, a few large low‐reflectivity regions, a shallow crater 150 km in diameter, a gently sloping mountain, and a short ridge running north‐south. Many of the later features have been seen in earlier radar maps and should be useful in refining the spin axis and further characterizing the regolith of certain areas of Venus.
A theory for the nonthermal radio continua in the terrestrial and Jovian magnetospheres
Authors: Melrose D. B.
Published: February 2008

It is assumed that the nonthermal radio continua in the terrestrial and Jovian magnetospheres result from conversion of observed upper hybrid waves into o mode waves. The only favorable conversion mechanism for the escaping terrestrial continuum is coalescence with low‐frequency waves, and the most favorable waves are ion‐cyclotron waves, which should be generated under conditions similar to those required for generation of upper hybrid waves. It is suggested that the Jovian continuum is in a steady state in which the conversion process is balanced by the inverse process, and it is shown that the implied steady state level is reasonably consistent with observations. To account for the observed spectrum of the Jovian continuum requires that the plasma pressure should be approximately constant over a range ne ≈ 10²m−3 to 105m−3. It is suggested that the terrestrial continuum may escape in bubbles of radiation enclosed by plasma downstream from the magnetotail.
Electron temperature anisotropy in the polar wind
Authors: Schunk Robert W. , Watkins David S.
Published: February 2008

We studied the steady state flow of a fully ionized H+‐O+‐electron plasma along geomagnetic field lines in the high‐latitude topside ionosphere. Our theoretical formulation for the electron gas was based on the 13‐moment system of transport equations, which allows for different electron temperatures parallel and perpendicular to the geomagnetic field and nonclassical heat flows, but for the H+ and O+ gases we used a simplified set of transport equations that did not allow for ion temperature anisotropies and that only included ordinary thermal condution in the ion heat flow equations. Solutions to the plasma transport equations were obtained over the altitude range from 1500 to 12,000 km for a range of lower boundary conditions leading to both subsonic and supersonic H+ outflows. For subsonic H+ outflows, the electron gas remains collision dominated to high altitudes; that is, the electron temperature anisotropy is small, the electron temperatures are low, the electron temperature profile is isothermal at high altitudes, and the electron heat flow is given by the classical collision‐dominated expression at all altitudes. For supersonic H+ outflows we found the following results. (1) At altitudes below 2500 km the electron gas is essentially collision dominated. (2) Above 2500 km an anisotropy in the electron temperature distribution develops such that Te⊥ >Te∥. This anisotropy increases with altitude, and at 12,000 km, Te⊥/Te∥ ∼2. (3) The magnitude of downward electron heat flux at 1500 km does not appreciably affect the temperature ratio Te⊥/Te∥ at high altitudes, but it has a dramatic effect on the individual parallel and perpendicular electron temperatures. For small lower boundary electron heat fluxes, Te∥ and Te⊥ are low at all altitudes (Te⊥ ≲7000°K), but for large electron heat fluxes at the lower boundary, Te⊥ can exceed 100,000°K at 12,000 km. Also, for large electron heat fluxes at the lower boundary the electron heat flow profile has a strong tendency to become constant with altitude above about 3000 km.
Ion waves associated with solar wind beam‐plasma interactions
Authors: Rodriguez Paul
Published: February 2008

Electrostatic ion waves with frequencies 0.5–6 kHz are often detected in the solar wind upstream of the bow shock. The ion waves are correlated with the low‐frequency magnetic field fluctuations that are driven by upstreaming protons of energies 4–7 keV. In terms of the frequency of occurrence, the ion waves occur primarily in the region connected to the bow shock by the solar wind magnetic field. The ion waves therefore appear to be driven by a beam‐plasma interaction involving the 4‐ to 7‐ keV protons streaming into the solar wind along magnetic field lines connected to the bow shock. A statistical study of the characteristic ion wave spectrum suggests that it may contain waves similar to Buneman waves with rest frame frequencies f ∼3.5fpi, where fpi is the ion plasma frequency and wavelengths λ ∼ km. We suggest that the ion waves are driven by a Landau resonant wave‐particle interaction with the 4‐ to 7‐keV protons, after initial excitation by upstreaming suprathermal electrons. The study suggests that previously detected ion acoustic waves are not the only waves associated with upstreaming protons. Bragg scattering off ion waves with wavelengths of 1–2 km is suggested to explain distant (>0 Re) source locations for terrestrial kilometric radiation. The ion waves are also suggested as a possible upstream agent for stochastically accelerating the >30‐keV protons that are correlated with the upstreaming 4‐ to 7‐keV protons. Electron plasma oscillations, associated with upstreaming electrons, often appear to be anticorrelated with the ion waves, implying some interaction that damps electron plasma oscillations.
A possible Ionospheric Wave‐Mixing Experiment
Authors: Rypdal Kristoffer
Published: February 2008

A theory is presented that describes a wave‐mixing experiment involving two moderately powerful transmitters at different locations and a sensitive radar at a third location. The two transmitters produce a plasma wave at their difference frequency, and the sensitive radar is used as a diagnostic. The scattering cross section of this coherent process is calculated and compared with the ever present incoherent cross section. In the presence of ionospheric irregularities the backscattered power may be substantially reduced. The experiment is shown to be a promising diagnostic tool.
Prompt striations observed in a barium thermite release at 335 km
Authors: Simons D. J. , Pongratz M. B. , Smith G. M. , Barasch G. E. , Fitzgerald T. J.
Published: February 2008

Three barium clouds have been released from a single rocket at altitudes of 335, 443, and 535 km. The releases were by means of barium thermite injection. Adding the different cross‐field velocity components of the rocket at each of the releases resulted in barium clouds with different kinetic velocity distributions. The barium cloud with the most peaked velocity distribution striated promptly, in good agreement with theoretical predictions based on a kinetic plasma instability. Details of the data analysis and experiment are discussed.
The detectability of ducted echoes in the magnetosphere
Authors: Calvert W.
Published: February 2008

Elongated plasma density irregularities in the magnetosphere could guide extraordinary mode waves from a satellite and produce detectable ducted echoes. The ducted waves are undamped and immune to spatial attenuation. For instance, a 10% depletion 100 km wide is sufficient for echoes 10–30 dB above nonthermal continuum, regardless of the distance. The remote measurement of plasma density and magnetic field structure in the magnetosphere would be practical using such echoes.
Coronal evolution during the sunspot cycle: Coronal holes observed with the Mauna Loa K‐Coronameters
Authors: Hundhausen A. J. , Hansen R. T. , Hansen S. F.
Published: February 2008

The white‐light corona was observed regularly at the Mauna Loa Observatory during the years 1965–1967 and 1969–1978. Display of the measured polarization brightness in the form of synoptic maps permits the identification of large coronal holes and the study of their slow evolution during the sunspot cycle. The polar coronal holes were clearly seen to shrink in size during the ascending phase of cycle 20 (1965–1967), to be absent during a two‐year period (1969–1970) just after sunspot maximum, to reappear near the end of 1970, and to remain as prominent features of the corona for the years 1971–1978. During the sunspot maximum epoch the corona was dominated by ‘mid‐latitude’ holes, elongated in the direction parallel to the solar equator. Large equatorial holes or equatorward extensions of the polar holes were observed during the ascending, maximum, and descending phases of cycle 20 and appear to be sources of solar wind streams with maximum speeds over 600 km s−1 at all of these epochs. The lifetimes of these holes and streams were greatest during the descending phase of the cycle, or in 1974–1975.
The dynamics of energetic electrons in the Earth's outer radiation belt during 1968 as observed by the Lawrence Livermore National Laboratory's Spectrometer on Ogo 5
Authors: West H. I. , Buck R. M. , Davidson G. T.
Published: February 2008

An account is given of measurements of electrons made by the LLNL magnetic electron spectrometer (60–3000 keV in seven differential energy channels) on the Ogo 5 satellite in the earth's outer‐belt regions during 1968 and early 1969. The data were analyzed to identify those features dominated by pitch angle and radial diffusion; in doing so all aspects of phase space covered by the data were studied, including pitch angle distributions and spectral features, as well as decay rates. The pitch angle distributions are reported elsewhere. The spectra observed in the weeks after a storm at L ∼3–4.5 show the evolution of a peak at ∼1.5 MeV and pronounced minima at ∼0.5 MeV. The observed pitch angle diffusion lifetimes are identified as being the shortest decays observed and are found to be highly energy and L dependent with minimum lifetimes of ∼1–2 days occurring at L ∼3–4.5. Two contiguous periods of decay, following the intense storm injection on October 31 and November 1, were analyzed in terms of radial diffusion. Significant differences were found between the derived values of DLL for the two periods; also significant energy dependence shows in the results. Although the values of DLL vary by about a factor of 10, representative values are 0.3 day−1 at L=6, 0.06 at L=4, 0.015 at L=3, and 0.001 at L=2.5. Despite the wide variation of many prior results in the literature, there is a family of results in approximate agreement with the present results. By noting the variations in DLL, as a function of the invariant quantities, we are able to order a fair body of previous results with our new results.
Electron precipitation in the midday auroral oval
Authors: Meng Ching‐I.
Published: February 2008

Simultaneous observations of auroral displays and electron precipitations by the DMSP 33 satellite provide an excellent and unique opportunity to study precipitation characteristics of the midday auroral oval. Attention is given to two topics: (1) the nature of the ‘gap’ of the midday discrete auroras which is a permanent feature of the dayside auroral oval observed by both Isis 2 and DMSP satellites and (2) the relationship of this gap with the polar cusp region. Based on 2‐month (June, July 1975) observations of the midday auroras over the southern hemisphere, it is found that inside the ‘gap’ of the discrete auroras along the dayside auroral oval, soft electron precipitations with a magnetosheathlike spectrum were invariably detected. The spatial extent of this region was about few degrees in latitude and about 2–3 hours in local time near 1130 magnetic local time meridian. No significant electron precipitation was detected poleward of the instantaneous midday auroral oval. Typical plasma sheet and discrete auroral types of electron precipitations were detected in the other parts of the midday auroral oval. Therefore it is proposed that the ionospheric projection of the polar cusp is a small region of the instantaneous dayside auroral oval near the noon meridian, coinciding with the ‘gap’ of the midday discrete auroras.
One and two direction models for VLF electromagnetic waves observed on‐board Geos 1
Authors: Buchalet L. J. , Lefeuvre F.
Published: February 2008

Single propagation models are investigated for analyzing VLF electromagnetic wave fields observed in magnetosphere. They are selected according to the values of the eigenvalues of the spectral matrices of the three magnetic field components. If there is one single nonzero eigenvalue, a one‐direction model is considered, while if there are two nonzero eigenvalues, a two‐direction model is chosen. A likelihood ratio test is performed in the doubtful cases. The two models are used to analyze VLF chorus and hiss observed onboard the Geos 1 satellite. The two‐direction model is generally the most likely, suggesting that waves are simultaneously generated at all wave normal directions.
The radial gradient of 0.1‐ to 32‐keV H+ and O+ and the azimuthal wave electric field as inferred from a large‐scale dayside pulsation
Authors: Kaye S. M. , Shelley E. G.
Published: February 2008

In this report we use 0.1‐ to 32‐keV H+ and O+ flux modulations observed during a large‐scale dayside pulsation event at ∼6 RE near the equatorial plane on April 4, 1979, to estimate the azimuthal wave electric field and spatial radial gradient of the phase space densities for the two ion species as a function of energy. We first find that the average azimuthal electric field associated with the wave has an amplitude of approximately 10 mV/m; this electric field magnitude can cause field lines to be displaced ∼1.5 RE during the course of an oscillation. Becuase of the large field displacement we can infer the ion distribution spatial radial gradients from a theoretical expression which describes the effects of a resonant hydromagnetic wave on a particle distribution. We find that for both H+ and O+, ∂f/∂R <0 for all energies. In addition, we find that the spatial scale length for changes in f for both O+ and H+ is generally of the order of several earth radii.
A two‐layer ionospheric dynamo calculation
Authors: Stening R. J.
Published: February 2008

The equivalent circuit method is extended to the case where two layers of the ionosphere, e.g., E and F regions, are treated as separate connected circuits. In this way the driving of E region currents by F region emf's can be studied. The net effect of the two layer current systems is similar to previous calculations, but some insight into current profiles is provided. It is seen that these may affect foE variations. The dependence of electrostatic field strength on sunspot number is studied. Variations of the sunset increase in field are suggested to be mainly due to phase changes in the driving winds.
The ultraviolet dayglow 4. The spectrum and excitation of singly ionized oxygen
Authors: Feldman P. D. , Anderson D. E. , Meier R. R. , Gentieu E. P.
Published: February 2008

The emission line spectrum of singly ionized atomic oxygen (O II) dominates the day airglow spectrum in the extreme ultraviolet below 834 Å. The strongest resonance line, at 834 Å, is optically thick and an analysis of height profiles obtained from rocket observations between 140 and 265 km in specific viewing directions indicates that the principal excitation source is direct photoionization of neutral atomic oxygen. Strong emission at 538–539 Å is most likely due to the quartet rather than the doublet transitions, which both occur at these wavelengths and which are not spectrally resolved in the data. The intensities of the weaker lines are consistent with recent laboratory measurements of transition branching ratios. Several strong O II lines near He I 584 produce severe contamination of low‐altitude (<400 km) measurements of geocoronal helium emission made with thin‐film broadband photometers.
Simultaneous observations of OI 7774‐Å and [OI] 6300‐Å emissions and correlative study with ionospheric parameters
Authors: Sahai Y. , Bittencourt J. A. , Teixeira N. R. , Takahashi H.
Published: February 2008

Regular measurements of the OI 7774‐Å and [OI] 6300‐Å nightglow emissions were carried out at Cachoeira Paulista (22.7°S, 45.0°W), Brasil, during the period April 1978 to March 1979. An ionosonde is also operated at Cachoeira Paulista. A correlative study of the OI 7774‐Å and [OI] 6300‐Å emission observations, with simultaneous ionosonde measurements, shows good correlations between (J7774)1/2 and F layer peak electron density (nm(e)) and between the ratio (J7774)1/2/(J6300) and the height of the F layer (hmF2), where J7774 and J6300 are the column emission rates for the OI 7774‐Å and [OI] 6300‐Å emissions, respectively. Simultaneous measurements of these two emissions would be a very useful technique for remote sensing of the ionospheric F layer dynamics.