bluuebunny/crossref_metadata_2025 huggingface dataset:
Solar wind absorption by Venus
Authors: Gombosi T. I. , Cravens T. E. , Nagy A. F. , Elphic R. C. , Russell C. T.
Published: February 2008

The fraction of solar wind which interacts with the dayside ionosphere and atmosphere of Venus was calculated assuming the presence of magnetic field fluctuations in the ionosheath region and taking into consideration the interaction with the upper neutral atmosphere extending above the ionopause. The Monte Carlo calculations indicated that fluctuations with Brms/〈B〉 of 0.3, a not unreasonable value according to recent observations, implies that about 0.3% of solar wind may be absorbed on the average. Interactions with the upper neutral atmosphere due to charge exchange can decrease the shocked solar wind flux by 1–7%, depending on the ionopause altitude. Most of the fast H atoms produced by charge exchange interactions with the atmosphere escape, and only a negligible fraction can enter the atmosphere. The precipitation of solar wind ions into the atmosphere because of magnetic field fluctuations is not uniform over the dayside atmosphere but was calculated to be greatest near the terminator. The effect that these penetrating solar wind particles have on the structure of the ionosphere was calculated. It was found that while some of the energy deposition processes lead to unique and observable signatures (e.g., a narrow Lyman α emission region), these penetrating solar wind particles do not appear to play a significant role in controlling the physical and/or chemical structure of the daytime Venus ionosphere.
Vertical structure of the atmosphere of Venus from Pioneer Venus Orbiter radio occultations
Authors: Kliore Arvydas J. , Patel Indu R.
Published: February 2008

Between December 1978 and February 1979 the Pioneer Venus orbiter spacecraft returned some 140 S band and X band radio occultation measurements of the Venus atmosphere. Results from 13 measurements, covering diverse latitudes from near equatorial to polar, are presented in this paper in the form of vertical profiles of temperature. The temperature profiles show a pronounced inversion at the tropopause for latitudes higher than about 50°, with the deepest inversions occurring between the latitudes of 60° and 70°, the latitudes at which the cold collar cloud feature was observed by the Pioneer Venus Vortex instrument. A comparison of the temperature profiles derived from radio occultation measurements with the stratospheric infrared temperature soundings of the Vortex instrument and the in situ measurements of tropospheric temperature and pressure by the North, Sounder, and Day probes indicates excellent agreement.
Lightning on Venus: Orbiter detection of whistler signals
Authors: Scarf F. L. , Taylor W. W. L. , Russell C. T. , Brace L. H.
Published: February 2008

Recently Taylor et al. (1979a) presented preliminary evidence for lightning on Venus, based on Pioneer Venus orbiter detection of whistler mode signals as the spacecraft first traversed the nightside ionosphere near periapsis. The initial periapsis eclipse season for the orbiter has now been completed, and the plasma wave instrument (OEFD) obtained low‐altitude nightside data for about 100 orbits. We present here an analysis of the impulsive whistler mode signals measured during these orbits, and we discuss the connection with atmospheric lightning. It is shown that the signals are detected in the 100‐Hz channel when the local magnetic field is sufficiently strong and steady and when the field is oriented to point down below the ionosphere. These facts strongly support the previous lightning interpretation.
Pioneer Venus Radar results altimetry and surface properties
Authors: Pettengill Gordon H. , Eliason Eric , Ford Peter G. , Loriot George B. , Masursky Harold , McGill George E.
Published: February 2008

The radar altimeter carried aboard the Pioneer Venus orbiter spacecraft has yielded a topographic map covering 93% of the Venus globe, with a linear surface resolution of better than 150 km. Vertical measurement accuracy exceeds 200 m. Extremes in relief (expressed as a center‐of‐mass‐to‐surface radius) extend from a low of 6049 km to a high of 6062 km. Only about 5% of the observed surface is elevated more than 2 km above the mean radius (6051.5 ±0.1 km). Although the elevated terrain comprises a number of separated components, it is dominated by a massive equatorial region the size of South America. Of the total surface, 60% lies within 500 m, and 20% within 125 m, of the modal radius (6051.1 km). The planetary polar ellipticity is nearly zero, with an upper bound of 4 × 10−5. In addition to the surface relief, the distribution of average meter‐scale surface slopes, in the observable range from 1° to 10°, is determined for the same regions, and at the same footprint resolution, as in the altimetric observations. Elevated areas have generally higher values of average slope; most features seen in the earth‐based images are also seen in the vertical‐incidence spacecraft observations, although a few exceptions are noted. Of interest are several very long (up to nearly 5000 km, in one case), thin, and relatively straight parallel features, not hitherto reported on Venus.
Intense daytime radio wave scintillations and sporadic E layer near the dip equator
Authors: Rastogi R. G. , Mullen J.
Published: February 2008

The paper describes two cases of very intense scintillation activity (amplitude exceeding 10 dB) recorded at Huancayo during the daytime on radio beacons on 257 MHz from Marisat satellite and ATS 3 (137 MHz) and LES 6 (254 MHz) signals. Both these events are shown to be associated with an intense blanketing equatorial sporadic E layer at Huancayo. It is suggested that the bursts of strong daytime scintillations of VHF radio waves are due to densely ionized thin sheets of ionization over the equator and only normally weak (≈ 1 dB) daytime scintillations may be due to instabilities associated with the q type of equatorial Es.
The hot plasma environment and floating potentials of an electron‐beam‐emitting rocket in the ionosphere
Authors: Arnoldy Roger L. , Winckler John R.
Published: February 2008

With an extensive array of particle sensors the plasma environment surrounding the Echo III accelerator payload is studied. From measurements of the thermal ion spectrum, negative payload potentials referenced to the unperturbed ionospheric plasma are obtained. Multiple detectors determined the electron population from a fraction of an eV up to 40‐keV energy. An intense electron population extending up to the energy of injected electrons by the accelerator is produced when the acceleration is turned on. The energetic tail of this population is returned to the payload principally from directions in which the beam was fired, with the most intense fluxes coming from the atmosphere for downward injections of the beam. The atmospheric‐scattered beam and secondary electrons are called ‘quick echoes.’ Electrons of energy less than several keV down to the detector threshold (0.1 eV) are called the suprathermal component. These electrons are produced isotropically around the payload during gun firings and decay away in approximately 32 ms. The largest directional intensities of this component are observed at the higher altitudes. Quick echo electrons are also observed to produce suprathermal electrons when they encounter the payload. The mechanism by which the suprathermal electrons are produced is discussed but remains unknown at the present time. The hot electrons surrounding the accelerator payload during gun injections bring sufficient charge to the payload to neutralize it provided the loss of charge by secondary production on the payload skin is small, presumably owing to a positive payload floating potential during injection. Since the hot population exists for tens of milliseconds after the gun turn off, it results in driving the payload up to 4 volts negative during this time. Secondary production on the payload skin apparently prevents much larger negative potentials after gun turnoff when the payload is immersed in the hot population. Quick echo electrons creating suprathermal electrons around the payload also drive the payload to a few volts negative. This is an important consideration when discussing the origin of the suprathermal electrons, since electrical discharge and beam plasma discharge mechanisms do not apply to the quick echo beams.
A theoretical study of the high‐latitude winter F region at solar minimum for low magnetic activity
Authors: Sojka J. J. , Raitt W. J. , Schunk R. W.
Published: February 2008

We combined a simple plasma convection model with an ionospheric‐atmospheric composition model in order to study the high‐latitude winter F region at solar minimum for low magnetic activity. Our numerical study produced time dependent, three‐dimensional ion density distributions for the ions NO+, O2+, N2+, O+, N+, and He+. We covered the high‐latitude ionosphere above 54°N magnetic latitude and at altitudes between 160 and 800 km for a time period of one complete day. The main result we obtained was that high‐latitude ionospheric features, such as the ‘main trough,’ the ‘ionization hole,’ the ‘tongue of ionization,’ the ‘aurorally produced ionization peaks,’ and the ‘universal time effects,’ are a natural consequence of the competition between the various chemical and transport processes known to be operating in the high‐latitude ionosphere. In addition, we found that (1) the F region peak electron density at a given location and local time can vary by more than an order of magnitude, owing to the UT effect that results from the displacement between the geomagnetic and geographic poles; (2) a wide range of ion compositions can occur in the polar F region at different locations and times; (3) the minimum value for the electron density in the main trough is sensitive to nocturnal maintenance processes; (4) the depth and longitudinal extent of the main trough exhibit a significant UT dependence; (5) the way the auroral oval is positioned relative to the plasma convection pattern has an appreciable effect on the magnetic local time extent of the main trough; (6) the spatial extent, depth, and location of the polar ionization hole are UT dependent; (7) the level of ion production in the morning sector of the auroral oval has an appreciable effect on the location and spatial extent of the polar ionization hole; and (8) in the polar hole the F region peak electron density is below 300 km, and at 300 km, diffusion is a very important process for both O+ and NO+. Contrary to the suggestion based on an analysis of AE‐C satellite data obtained in the polar hole that the concentration of NO+ ions is chemically controlled, we find diffusion to be the dominant process at 300 km.
Estimation of ionospheric electric fields, ionospheric currents, and field‐aligned currents from ground magnetic records
Authors: Kamide Y. , Richmond A. D. , Matsushita S.
Published: February 2008

An approximate method of separating the effects of ionospheric currents from those of field‐aligned currents in ground magnetic perturbations observed in high latitudes is developed. The distribution of ionospheric electric fields can also be estimated. The procedure includes the following steps: (1) the calculation of the equivalent ionospheric current function on the basis of magnetic H and D component records on the earth's surface, (2) the computation of the electric potential distribution from the equivalent ionospheric current function by using a simple model of the ionospheric conductivity, (3) the derivation of ionospheric current vectors as well as electric fields and, (4) the derivation of the field‐aligned current intensity by taking the divergence of the ionospheric currents. Several examples for both quiet and disturbed conditions are utilized to demonstrate how our method is successful in estimating the intensities of the electric fields and the ionospheric and field‐aligned currents in the sense that the estimated values are in good agreement with those observed recently by radar and satellite methods. Significant portions of the H component in nightside auroral latitudes appear to result from the east‐west ionospheric currents, called the auroral electrojets, while both the north‐south ionospheric current and field‐aligned current are almost equally important in producing the D component excursions. It is found that the ionospheric and field‐aligned current distributions obtained are not very sensitive to the choice of the ionospheric conductivity model, unless the auroral enhancement is not given in an appropriate place. This indicates that even a simple conductivity distribution inferred from the distribution of the magnetic perturbations can make it possible to estimate the three‐dimensional current system with a reasonable accuracy.
On the polarization state of hydromagnetic fluctuations in the solar wind
Authors: Bavassano B. , Dobrowolny M. , Mariani F. , Ness N. F.
Published: February 2008

From presently available observations one can infer that the Alfvénic turbulence measured in the solar wind, predominantly on trailing edges of high‐speed streams, is a mixture of modes with two different polarizations, namely, Alfvénic modes and modes which are the incompressible limit of slow magnetosonic waves. Using Helios 2 magnetic data and a variance analysis, we have separated parallel (to the mean field) and perpendicular components of the fluctuations and studied the possible correlation between such components which would be predicted as a consequence of the incompressible character of the turbulence. Correlations between eigenvalues of the variance matrix are also investigated and discussed.
Anomalous heating of the polar E region by unstable plasma waves 2. Theory
Authors: St.‐Maurice J. P. , Schlegel Kristian , Banks P. M.
Published: February 2008

We have found that anomalous electron temperatures in the disturbed high‐latitude E region can be quantitatively explained in terms of heating by unstable plasma waves. The electron temperatures at 110 km have been measured to be as high as 1500°K instead of the expected value of about 300°K. We show by using quasi‐linear theory that there is an ample source of heat in the unstable waves and that the measured electron temperature profiles have a shape very similar to what is expected from plasma wave heating by the modified two‐stream instability. We find that there is even more heating going to the ion gas, but that the resulting effect on the ion temperature may be difficult to measure. Our best estimate of the wave heating rates leads us to conclude that wave heating can be as much as 50% of the Joule heating for dc electric field strengths of the order of 45 mV/m or greater.
On the drift motions of magnetospheric charged particles in the vicinity of a planetary satellite with dipole field
Authors: Ip W.‐H.
Published: February 2008

If a planetary satellite has an intrinsic dipole field, the drift motion of charged particles will be diverted, by varying degree, around the satellite. It is shown here that, if Io has a dipole field with an equatorial surface field of the order of 2000 nT, both protons and electrons with energies less than 1 MeV will be strongly absorbed, while those with higher energy may not be affected. Furthermore, if the dipole moment of Io is anti‐parallel to that of Jupiter, it would be expected that the surface sputtering effect by corotating magnetospheric ions to be most pronounced near the inner and trailing hemispheres, as indicated by some observations of the neutral sodium cloud.
Particle signature of magnetic flux transfer events at the magnetopause
Authors: Daly P. W. , Williams D. J. , Russell C. T. , Keppler E.
Published: February 2008

Energetic electron (E >20 keV) and ion (E >25 keV) enhancements have been observed using the Isee 1 and 2 spacecraft during magnetic flux transfer events in the dayside magnetosheath just outside the magnetopause. The ions are seen to be streaming along the magnetic field, filling the 90°–180° pitch angle region. The electrons are more isotropic and yet exhibit slight anisotropy in the direction opposite to that of the ions. From their intensities and spectra the ions appear to be of magnetospheric origin. With the interpretation of the flux transfer events as ‘patchy’ interconnection of magnetosheath and magnetospheric field lines, the ions are then seen to be previously trapped magnetospheric particles escaping along freshly opened field lines.
Precipitation from an asymmetric magnetic flux tube
Authors: Melrose D. B. , White S. M.
Published: February 2008

A simple model for precipitation of trapped particles from an asymmetric magnetic flux tube is formulated and analytic solutions are found in the strong diffusion and weak diffusion limits. The loss cones α <α1 and α >π ‐ α2 (α = pitch angle) are assumed unequal (α1 <α2), the pitch angle diffusion coefficient D(α) is assumed asymmetric (D(α)≠D(π‐α)), and the particles are assumed to be injected at an angle αs≠π/2. The steady state solution in the strong diffusion limit implies that the ratio of the precipitation rates at the two feet is equal to the ratio of the solid angles filled by the loss cones, R1/R2=α1²/α2². An explicit result for R1 is obtained in the weak diffusion limit. For α2 ‐ α1 ≫αD1, where αD1 is roughly the typical angle a particle is deflected in one transit across the flux tube from foot 2 to foot 1, R1 is smaller than R2 by a factor of order exp [−(α2‐α1) /αD1]. The effect of asymmetric diffusion, which is to favor precipitation at foot 1 for D(α1) >D(α2) and at foot 2 for D(α2) >D(α1), is described in terms of a factor F(d) of an asymmetry parameter d given by equations (35) and (24), respectively. Applications to the solar corona, to Jupiter's radio emission and to the terrestrial magnetosphere are discussed briefly.
Generation of whistler‐mode sidebands in the magnetosphere
Authors: Park C. G.
Published: February 2008

VLF transmitter experiments conducted at Siple, Antarctica (L ≃4) show that long (≳l s) keydown signals injected into the magnetosphere often generate sidebands as a result of nonlinear interactions with energetic particles. The spectral characteristics of observed sidebands are quite varied and complex. The sideband frequency spacing varies from ∼2 to 100 Hz, but it bears no simple relationship to the carrier amplitude, in sharp contrast to the predictions of some theories. The sideband amplitude is usually 10 dB or more below the carrier amplitude, but sometimes it can exceed the carrier amplitude and also trigger emissions. Multiple sidebands are often observed, and their frequency separations from the carrier may or may not be harmonically related. Sideband amplitudes may be symmetrical or asymmetrical about the carrier. In the asymmetrical case it is usually the upper sideband that is stronger. Various sideband generation mechanisms are discussed in the light of these new experimental data.
Energetic heavy ions with nuclear charge Z ≧4 in the equatorial radiation belts of the Earth: Magnetic storms
Authors: Spjeldvik Walther N. , Fritz Theodore A.
Published: February 2008

Magnetic storms are known to perturb the earth's radiation belts in profound ways. In this paper we present direct in situ observations of trapped energetic heavy ions with nuclear charge Z ≧4 at energies in the lower MeV range made with Explorer 45 during the period June–December 1972. All measurements were carried out in the vicinity of the geomagnetic equatorial plane (1.0 ≦B/B0 <1.4), and the data show the varying effects of four major magnetic storm periods. Orders of magnitude increases in the trapped heavy ion population were seen deep within the radiation belts (L ∼ 2.5–5) following the August 1972 solar flare and magnetic storm events. Fluxes of the Z ≧4 ions were found to decay faster than those of helium ions of comparable energies; typical decay times for Z ≧4 ions are found to be 24–40 days at L ≦4 and shorter at higher L shells. The observations are compared with the expected post‐injection long‐term behavior of atomic oxygen ions deduced from charge exchange, Coulomb collisions, and radial diffusive transport. We report a good agreement between theory and observations.
Energetic ion and electron observations of the geomagnetic plasma sheet boundary layer: Three‐dimensional results from Isee 1
Authors: Spjeldvik Walther N. , Fritz Theodore A.
Published: February 2008

We present a description of the northern boundary of the geomagnetic tail plasma sheet as observed with the medium‐energy particle experiment (NOAA/WAPS) on the spacecraft Isee 1. The energy coverage extends from 24 to 2081 keV for energetic ions and from 22.5 to 1200 keV for electrons, and essentially all spatial directions are sampled at these energies. Large‐scale motions of the plasma sheet energetic particle population were observed as an apparent result of a series of magnetospheric disturbances (substorms) that took place prior to and during April 24, 1978. The plasma sheet underwent substantial contractions and expansions (thinning and thickening) observed both in the energetic particle populations and in the cross‐tail magnetic field components. The three‐dimensional particle observations reveal a distinct plasma sheet boundary layer where the ion and electron characteristics are very different and where the ion flow is strong, with the flow velocity essentially coinciding with the kinetic velocity. During 0000–0700 UT, Isee‐1 crossed this boundary layer 5 times, and in each case the flow was seen from the tail towards the earth. The thickness of this flow layer is estimated to be less than 2 ion gyroradii for ions in our energy range. For one of the crossings the motion of the boundary layer can be interpreted as large‐amplitude boundary waves superimposed on the general plasma sheet behavior associated with the substorm process. These waves have periodicities of a few minutes.
Tearing at the dayside magnetopause
Authors: Quest Kevin B. , Coroniti Ferdinand V.
Published: February 2008

Initial observations of the dayside magnetopause thickness using UCLA magnetometers on board Isee 1 (International Sun‐Earth Explorer) and Isee 2 indicate that this boundary may only be a few ion gyroradii thick during periods of observed southward interplanetary magnetic field (IMF). Such a thin current layer can destabilize the collisionless tearing mode, a probable first step in the reconnection sequence. We have calculated the linear growth rate assuming typical magnetopause parameters and find that it is a sensitive function of IMF orientation, magnetopause thickness, and electron number density. We also show that the finite size of the dayside magnetopause necessitates the existence of a two‐dimensional tearing mode wave vector spectrum, a consequence of the inability of a thermal electron to maintain Landau resonance with the wave for a growth period. Implications regarding reconnection are discussed.
Structure of Jupiter's magnetopause: Pioneer 10 and 11 observations
Authors: Sonnerup Bengt U. Ö. , Smith E. J. , Tsurutani B. T. , Wolfe J. H.
Published: February 2008

The magnetic structure of Jupiter's magnetopause, as observed by the space probes Pioneer 10 and 11, is compared with terrestrial magnetopause structures from the OGO 5 mission. The Jovian magnetopause thickness, deduced from a rapid triple crossing, is found to lie in the range 3500–5200 km, while the adjoining plasma boundary layer was 5600–8400 km thick. Comparison with the terrestrial situation suggests that the dayside magnetopause thickness in both cases is a few times the ion gyroradius and that the dayside boundary layer is also of about the same width. The magnetopause normal vector and normal magnetic field component are determined for each crossing by use of minimum variance analysis. The results indicate a blunt and floppy mangetopause surface, for the most part with an insignificant normal magnetic field component. Only for three of the 14 crossings did this component exceed 1.5 times its error estimate. In two of these cases, the magnetic field tangential to the magnetopause displayed the characteristic features of a rotational discontinuity. Such structures are occasionally also seen at the earth's magnetopause and are presumably associated with an open (reconnecting) magnetosphere. Some of the Jovian crossings also display other features that have been observed at the earth: initial field rotation in the ‘wrong’ sense at the outer edge of the magnetopause; diamagnetic effects and field‐aligned currents at the inner edge of the plasma boundary layer.
Amplification of electrostatic noise in cyclotron resonance with an adiabatic auroral beam
Authors: Lotko W. , Maggs J. E.
Published: February 2008

The adiabatic evolution of the auroral beam in the convergent geomagnetic field gives rise to a region of ∂F/∂ν⊥ >0 in the electron distribution function. The altitude dependence of this source of free energy is examined, and its ability to drive electrostatic waves toward instability is considered. General linear properties of the possible instabilities are discussed, and a detailed numerical calculation based on a collisionless height dependent model of an auroral arc (Maggs and Lotko, this issue) is described. Of the three wave modes considered, the upper hybrid mode is optimally amplified at altitudes below about 1000 km. Oblique electron Bernstein modes may be weakly unstable at the lowest altitudes, and the whistler mode appears to be stable at all altitudes. Wave amplification is limited by a finite resonance time which is determined by refraction of parallel wavenumbers by the topside ionospheric density gradient. Slow beams originating at relatively high altitudes and nearly perpendicularly propagating waves with frequencies away from cyclotron harmonics are favored by this instability mechanism.
5–50‐m wavelength plasma instabilities in the equatorial electrojet 1. Cross‐field conditions
Authors: Hanuise C. , Crochet M.
Published: February 2008

Electron density irregularities detected with coherent radars in the equatorial electrojet have been studied mostly at wavelengths between 3 and 10 m. For low drift velocities they have been interpreted by the cross‐field instability. Longer wavelengths have been extensively observed with a multifrequency HF radar system installed in Ethiopia. Data gathered for drift velocities lower than 200 m/s are presented. From 5 to 50 m the measured phase velocity varies with wavelength and elevation angle as predicted by the linear theory. Spectral width increases with wave number but is constant with elevation angle. Ratio of spectral width to mean Doppler shift is of the order of unity and indicates that strong turbulence has to be included in nonlinear theories. These results are studied in light of recent numerical simulations and theoretical works.
Modulation of stable auroral red (SAR) arc occurrence rates
Authors: Slater D. W. , Smith L. L.
Published: February 2008

Data taken by the Pacific Northwest Laboratory photometer chain have been reviewed in a comprehensive search for stable auroral red (SAR) arc events during the period 1968–1978. Results of this search indicate both a seasonal and a solar cycle control of SAR arclike events. The occurrence rate of these events is shown to follow the solar activity cycle, though with a phase lag of 2–3 years. Further, a rather distinct summertime minimum of occurrence frequency is pointed out (northern hemisphere). Possible mechanisms for the existence of this effect are discussed.
Solar wind helium and hydrogen structure near the heliospheric current sheet: A signal of coronal streamers at 1 AU
Authors: Borrini G. , Gosling J. T. , Bame S. J. , Feldman W. C. , Wilcox J. M.
Published: February 2008

We have investigated solar wind flow properties associated with very low helium to hydrogen abundance ratios (A(He)) observed with Los Alamos instrumentation on IMP 6, 7, and 8 during the 1971 ‐ 1978 interval. A characteristic pattern has been discovered, consisting of correlated interplanetary field reversals, high plasma density, low and nearly identical H+ and He++ bulk velocities, low H+ and He++ kinetic temperatures and minimums in their ratio. A superposed epoch analysis of 74 well defined sector boundary crossings observed during the 1971–1978 interval provides impressive verification of the above correlation, featuring a pronounced minimum in A(He) at the sector boundary passage. Our epoch analysis provides the cleanest pattern of plasma characteristics yet found in the vicinity of the current sheet associated with sector boundaries. Because coronal streamers straddle the current sheet close to the sun, the pattern discovered is the ‘signal’ of a coronal streamer at 1 AU. Our results provide important constraints on models of helium dynamics in coronal streamers and the low speed solar wind.
Wave‐electron interactions in the high speed solar wind
Authors: Schwartz Steven J. , Feldman William C. , Gary S. Peter
Published: February 2008

We investigate the role of plasma wave‐particle interactions in modifying the electron velocity distributions of the high speed solar wind. Both electrostatic and electromagnetic fluctuations are considered. We find that interactions between thermal electrons and a small fraction of the observed wave spectrum provide a momentum exchange rate strong enough to compete with Coulomb collisions.
Nonducted coherent VLF waves and associated triggered emissions observed on the ISEE‐1 satellite
Authors: Bell T. F. , Inan U. S. , Helliwell R. A.
Published: February 2008

The ISEE‐1 spacecraft has been an important component of VLF wave injection experiments for studying interactions between coherent VLF waves and energetic particles. The coherent waves are injected into the magnetosphere by ground‐based transmitters such as that at Siple Station, Antarctica (76°W, 84°S geographic, L ∼4.1), and those of the worldwide Omega navigation network. During the period October 1977–August 1979, the Stanford VLF receiver on ISEE‐1 acquired data on approximately 90 separate orbits in the plasmasphere in which the satellite longitude lay within 30° of the magnetic field lines linking the North Dakota Omega transmitter (98°W, 46°N geographic, L ∼3.5). During 12 passes the nonducted Omega signals at 10.2 kHz were observed to trigger VLF emissions on magnetic shells ranging from L=1.8–3.8. The emission characteristics often differed significantly from those of emissions triggered by ducted signals and detected at ground stations. In many cases each of the triggered signals consisted of a large array of discrete rising tones whose rate of change of frequency approached 40 kHz/sec, a value much larger than that usually associated with emissions triggered by ducted signals. The emission activity occurred during periods in which signals propagated to the spacecraft on at least two separate ray paths of disparate time delay and with a received signal duration of 2 to 4 times that of the transmitted pulses. Typically, the emission activity took place only along the paths of longer time delay. Unusual emission activity involving signals from the Siple Station transmitter was also observed on two occasions outside the plasmasphere. Our results indicate that the emission generation process can take place under much more general conditions that previously believed.
Elevated electron temperatures in the auroral E layer measured with the Chatanika Radar
Authors: Wickwar Vincent B. , Lathuillere Chantal , Kofman Wlodek , Lejeune Gerard
Published: February 2008

An extensive series of spectral measurements has been made in the auroral E region with the Chatanika incoherent scatter radar. Becasue of the small scale length for variations of electron density, temperatures, and ion‐neutral collisions we used the operating mode with the best possible range resolution—9 km. About 5% of the time the data exhibited an unusual spectral shape that was most pronounced at 105 and 110 km. Instead of being almost Gaussian with only a small hint of two peaks, the spectra are much wider, with two well‐developed peaks. After carefully considering the validity of the measurements and their interpretation, we conclude that the unusual spectra are due to greatly enhanced electron temperatures. At 110 km, the electron temperature may increase from 250 K to 800 K, while the ion temperature remains near 250 K. This enhancement of the electron temperature extends from 99 km to at least 116 km. We show that the temperature increase is too large to be accounted for by auroral particle precipitation, though it coincides in time with ion temperature enhancements at altitudes above 125 km. Because these latter enhancements are believed to be due to joule heating, we deduce that electric fields of 24–40 mV/m are present and that the electrons are moving through the ions and neutrals at speeds of 500–800 m/s. Despite these velocities, we find that joule heating of the electrons also cannot account for the elevated electron temperatures. Several consequences of the elevated electron temperatures are discussed. One is that the rate constants for molecular recombination are reduced. Another is that during periods of significant joule heating, the deduced electron density profile, when fully corrected for temperatures, has a significantly lower peak altitude and greater density than that deduced under the usual assumption of equal electron and ion temperatures. Since conductivities, currents, ionization rates, and differential energy spectra are dependent upon the density profile, care must be taken to account properly for the temperature effects when deriving these quantities.
The E and F region ionospheric response to solar flares: 1. Effects of approximations of solar flare EUV fluxes
Authors: Mariska John T. , Oran Elaine S.
Published: February 2008

SOLRAD and many other satellite systems have provided a large data base showing the time‐dependent behavior of broadband solar fluxes in the X ray and EUV spectral regions. These bands are broad in the sense that one band may contain many ionospherically important spectral lines. We present results of tests performed to determine how this information can best be used to predict the effects of a solar flare on the ionosphere. Our approach has been to first adopt a model of the spectral line and continuum enhancements based on a synthesis of many types of flare observations. This detailed spectral model is used in a time‐dependent ionosphere model to calculate the response of the electron and ion density profiles. Then the spectral model is mathematically filtered to show how it would appear to the SOLRAD EUV detectors, and this degraded information is used in the ionosphere model. Comparison of the two ionosphere calculations shows that the two spectra produce changes in the total electron content in the ionosphere that differ by only a few percent. Thus, given the present uncertainty in our knowledge of solar flare EUV spectra, SOLRAD broadband EUV solar flux measurements can be used to calculate the ionospheric effects of EUV flare enhancements. Significant changes due to the flare which occur in the individual species densities are described.
On the possibility of detection of ions from Venus at 1 AU
Authors: Russell C. T. , Neugebauer M.
Published: February 2008

The possibility that singly ionized helium enhancements observed near 1 AU are due to ion pickup from Venus is examined and found to be implausible. It is still possible that Venus ions are present in the solar wind with sufficiently high fluxes to be detectable by earth‐orbiting spacecraft near the time of inferior conjunction. Because of the inclination of Venus' orbit to the ecliptic, some inferior conjunctions are more favorable for such a search than others. In particular, the inferior conjunctions on June 17, 1972, and June 15, 1980, are most favorable for detection.
Observations of differential charging effects on ATS 6
Authors: Olsen Richard Christopher , McIlwain Carl E. , Whipple Elden C.
Published: February 2008

Differential charging effects observed in the electron data of the University of California, San Diego, auroral particles experiment on Applied Technology Satellite 6 are described and analyzed. An electrostatic barrier around the environmental measurements experiment (EME) package on ATS 6 is shown to be the natural result of dielectrics around the spacecraft which are more negatively charged than the mainframe of the spacecraft. In particular, the large dish antenna on ATS 6 causes the formation of a barrier which traps particles emitted from the EME package surface and returns them to the spacecraft. The insulating surface of the rotating University of Minnesota detector on the otherwise conducting EME package is shown to be the source of accelerated fluxes of electrons during charging events.
Field line equipotentiality and ion neutral collision frequencies in the dynamo region deduced from Saint‐Santin ion drift measurements
Authors: Taieb Charley , Blanc Michel
Published: February 2008

We analyze three‐dimensional ion drift data from the Saint‐Santin incoherent scatter facility to test experimentally the theoretical description of ion transport in the ionospheric dynamo layer, and to deduce electric fields and ion neutral collision frequencies from the observed drifts. Using a geometrical representation of the ion momentum equation, we show that at middle latitudes, because horizontal neutral wind influences ion motions both parallel and orthogonal to the field lines in the ionospheric dynamo layer, the information contained in a three‐dimensional ion drift measurement is redundant, thus permitting to check the standard theoretical description of ionospheric electrodynamics in two ways. First, assuming a model ion‐neutral collision frequency profile, one can deduce the north‐south perpendicular component of the electric field function of height in the E region from Saint‐Santin drift data. We find that its altitude variations remain within the experimental uncertainty of the method, in agreement with the theoretical assumption of equipotential field lines. Second, assuming that the electric field is constant in altitude, one can determine the ion collision ratio, or ratio of the ion collision frequency to the ion gyrofrequency, from a comparison of E and F region drift measurements. Daily median values of the ion collision frequencies, thus obtained for each of the three seasons, are found to compare reasonably well with ion collision frequencies derived from the Jacchia neutral atmosphere model for the case of the equinox sample, but determinations for the other seasons are contaminated by a high level of measurement noise.
Magnetic storm associated enhanced particle precipitation in the South Atlantic Anomaly: Evidence from VLF phase measurements
Authors: Abdu M. A. , Batista I. S. , Piazza L. R. , Massambani O.
Published: February 2008

Phase recordings at Atibaia, Brazil (23°S, 46°W), of 13.6 ‐kHz signal transmitted from Golfo Nuevo, Argentina (43°S, 65°W), a trajectory confined almost completely within the South Atlantic anomaly region, show significant perturbations, indicative of the lowering of the VLF reflection level, following the onset of magnetic disturbances. Simultaneous measurements of the Es layer parameters ftEs and fbEs over Cachoeira Paulista (22°S, 45°W) also show enhancements, with some delay with respect to the magnetic disturbance onset, as was found in our earlier work (Batista and Abdu, 1977). These results show magnetic storm associated ionization enhancements taking place in a height region from approximately 110 km down to 70 km, which we interpret as having been produced by precipitation of high‐energy charged particles in the South Atlantic magnetic anomaly. The results also suggest some degree of day to day variability in the abundance of metallic species and/or in the dynamics of the E region over this region.
Character of the Jovian decametric arcs
Authors: Staelin David H.
Published: February 2008

The planetary radio astronomy (PRA) experiment on the Voyager 1 and Voyager 2 spacecraft reveals strong radiation in the form of arcs when the data are displayed in time‐frequency coordinates. The vertex frequencies of these arcs, i.e., the central frequencies at which the arcs are first or last observed, are correlated with the magnetic field strength at the foot of the L=6 shell magnetic flux tubes that emitted the arcs, provided that the emission is conical with a cone angle that varies slightly in a prescribed way. This interpretation further supports the association of the left circularly polarized arcs with the southern hemisphere, where the relation between vertex frequency and magnetic field strength is preserved. One way to produce a frequency dependent cone angle is described; it is relevant to processes where the cyclotron emission originates directly from streaming electrons with apparent cyclotron frequencies that are both relativistically depressed and Doppler shifted. This process is qualitatively consistent with the cone angles inferred from the PRA data.
Volcanic eruptions on Io
Authors: Strom Robert G. , Schneider Nicholas M. , Terrile Richard J. , Cook Allan F. , Hansen Candice
Published: February 2008

Nine eruption plumes were observed over a period of 6 1/2 days during the Voyager 1 encounter with Io. During the Voyager 2 encounter, 4 months later, eight of these eruptions were still active. The largest plume viewed by Voyager 1 became inactive sometime between the two encounters. However, a major eruption occurred at Surt sometime between the two encounters and deposited an ejecta blanket comparable in size to those associated with the largest plumes. The plumes range in height from about 60 to over 300 km with corresponding ejection velocities of about 0.5 to 1.0 km/s. Where topographic information exists (plumes 1 and 8), the plume source is located on level plains rather than topographic highs and consists of either fissures or calderas. With the exception of plume 1 (Pele), the brightness distribution monotonically decreases from the core to the top of the plume. The shape and brightness distribution together with the pattern of the surface deposit of at least plume 3 (Prometheus) can be simulated by a ballistic model in which the ejection velocity is constant (0.5 km/s) and ejection angles vary from vertical (0°) to 55°. The brightness distribution of plume 1 is probably better explained by a shock front near the top of the plume. Numerous surface deposits similar to those associated with active plumes probably mark the sites of recent eruptions. The distribution of active and recent eruptions appears to be concentrated in the equatorial regions and indicates the volcanic activity is more frequent and intense in the equatorial regions than the polar regions. This suggests that the depositional rate is greater and the surface age younger in the equatorial regions, which may account for the darker polar regions. The geologic setting of certain plume sources and the very large reservoirs of volatiles required for the active eruptions suggests that sulphur volcanism rather than silicate volcanism is the most likely driving mechanism for the eruption plumes.
Radiometric performance of the Voyager cameras
Authors: Danielson G. Edward , Kupferman Peter N. , Johnson Torrence V. , Soderblom Laurence A.
Published: February 2008

The Voyager Imaging Experiment provided high‐quality data of Jupiter and the Galilean satellites with the two flyby trajectories in March and July of 1979. Moderately accurate radio‐metric measurements have been made using these data. This paper evaluates the radiometric results and describes the inflight and ground geometric and radiometric correction factors. The radiometric quantities of intensity I and geometric albedo I/F are derived, and scaling factors for each of the filters are tabulated for correcting the ‘calibrated’ data from the Image Processing Laboratory at JPL. In addition, the key characteristics of both Voyager 1 and Voyager 2 cameras are tabulated.
Quasi‐thermal noise in a stable plasma at rest: Theory and observations from ISEE 3
Authors: Couturier P. , Hoang S. , Meyer‐Vernet N. , Steinberg J. L.
Published: February 2008

The noise voltage found at the terminals of an electric dipole antenna immersed in a hot plasma is calculated. The plasma is assumed stable and at rest. It is described as made of two maxwellian electron populations. A grid of calculated noise spectra is presented in normalized coordinates, and we give analytical expressions valid for frequencies much lower or larger than the plasma frequency fp or close to the noise peak. The antenna impedance is also calculated; in contrast to the noise spectrum, it is found to be very weakly dependent upon the hot population parameters. From these results one can calculate the response of a receiving system, thus enabling the actual measurement of the plasma parameters from the noise spectrum. Using ISEE 3 SBH experimental data, it is shown that the observations do fit the theoretical predictions over most of the frequency range. Plasma parameters are obtained and found in good agreement with results from the Los Alamos plasma analyzer on the same spacecraft. It appears that the high energy tail of the particle distribution function accounts for the peak noise voltages of several times 10−12 V²Hz−1 found at the terminals of a 45‐m half‐length dipole. The interpretation of these observations does not therefore require an unstable plasma. We discuss these results and show that more work should be done to explain an unpredicted increase of the spectrum below fp and to explain the weak spin modulation. Solving these remaining problems requires the introduction of ions and of the bulk and/or drift velocity into the calculations. Appendix A contains a partial justification of the assumption of a triangular current distribution on the antenna. The calculations are presented in appendix B.
Substorm signatures at synchronous altitude
Authors: Kokubun Susumu , McPherron Robert L.
Published: February 2008

The signatures of magnetospheric substorms near 6.6 RE are statistically examined by using data obtained on board ATS 6 by magnetic field and energetic particle measurements. It was confirmed that the configuration change toward a taillike field in the dusk‐to‐midnight sector typically begins about one hour before the onset of the expansion phase of a substorm. Configuration changes before the onsets of moderate substorms are characterized by a directional change of magnetic field at magnetic latitude of ∼10°, rather than a change in the field magnitude. During storm periods it was found that the magnetic field orientation occasionally becomes almost parallel to the magnetic equator before expansion phase onsets. In such cases, the field magnitude usually increases above background levels. These facts strongly suggest that a taillilke configuration is caused by an intensification and earthward motion of the cross‐tail current system, often close to 6.6 RE, than the development of diamagnetic ring current. The degree of taillike field distortion is statistically correlated with a subsequent field recovery during the expansion phase. Namely, a large field recovery at ATS 6 and associated large positive bays on the ground tend to follow a pronounced deviation from a quiet field level at ATS 6. The examination of ground magnetograms indicates that the development of Sqp or convection bay type current system is associated with the taillike field configuration. These results are discussed in term of a phenomenological model of magnetospheric substorms including the precursory or growth phase.