bluuebunny/crossref_metadata_2025 huggingface dataset:
Temporal fluctuations in the luminosity of diffuse aurora
Authors: Berkey F. T.
Published: February 2008

Using low light level television observations of the diffuse aurora, frequency spectra have been derived from time series data obtained by digitizing segments of a recorded video signal. The results suggest that temporal fluctuations having periods in the range 2–40 s are common to both quiescent and pulsating forms of the diffuse aurora and are the dominant fluctuation period. A representation of the data in the form of dynamic spectra, which shows the evolution of the temporal fluctuations at intervals of 20–30 s, has been derived by using maximum entropy spectral analysis. In one of the examples the 2 to 4‐Hz modulation often observed during pulsating aurora is evident in the dynamic spectrum.
Effects of high‐latitude conductivity on observed convection electric fields and Birkeland currents
Authors: Smiddy M. , Burke W. J. , Kelley M. C. , Saflekos N. A. , Gussenhoven M. S. , Hardy D. A. , Rich F. J.
Published: February 2008

Relationships between convective electric fields, Birkeland currents, and ionospheric conductivity at high latitudes are investigated by using simultaneous measurements of electric and magnetic fields and electron fluxes from the S3‐2 satellite. Cases in which the polar cap is in sunlight and darkness are studied to assess the importance of ionospheric conductivity. We find that when the ionospheric conductivity is high, as it normally is in the auroral zones and the sunlit polar cap, Birkeland current sheets exist that deflect the magnetic field approximately in the direction of the convection, typically antisunward in the polar caps and sunward in the auroral zones. Under winter conditions (a dark polar ionosphere), no large‐scale antisunward deflection of magnetic field lines is found in the polar cap. Magnetic field deflections in the auroral zone are the result of the region 1/region 2 current systems discussed by Iijima and Potemra (1976). In the cases studied, region 1 currents are not observed to extend significantly poleward of the electric field reversal. We interpret this result as indicating the existence of strong conductivity gradients near the poleward boundary of the auroral oval, even in sunlight conditions. Deflections of magnetic field lines in the summer polar cap, first reported by McDiarmid et al. (1978), result from currents which serve to link the high‐latitude generator to the polar ionospheric load. Two types of linkage appear in the data; (1) Region 1 currents are greater than region 2 currents, with the excess region 1 current flowing across the polar cap from dawn to dusk. (2) An extra set of field‐aligned currents exist which are clearly detached from those of region 1 and are entirely poleward of the convective electric field reversal. We find that the evidence for antisunward convection on closed field lines threading the boundary layer, cited by McDiarmid et al. (1978), is inconclusive. With our more complete data set there is some evidence at ionospheric altitudes for a boundary layer, but in such cases there is only a modest (a few kilovolts) potential drop across the layer and there are insignificant Birkeland currents.
Observations of the dayside ionopause and ionosphere of Venus
Authors: Elphic R. C. , Russell C. T. , Slavin J. A. , Brace L. H.
Published: February 2008

Some of the principal features of the dayside solar wind ionosphere interaction at Venus are presented. The dayside ionopause and ionosphere are observed to respond dramatically to solar wind pressure variations. The ram pressure of the solar wind is manifested mainly as magnetic pressure just external to the subsolar ionopause, and the ionopause location is controlled principally by this pressure. The ionosheath field is observed to drape across the dayside ionosphere, and ionopause currents over most of the dayside usually act to exclude the high ionosheath field from the generally low‐field ionosphere. Tenuous, warm ionospheric plasma is sometimes observed on field lines outside the ionopause current sheet, suggesting that this dayside ionospheric plasma can be transported to the nightside and into the Venus wake. Some of the magnetic and thermal plasma features of the dayside ionosphere are shown, and modeling and distribution of flux ropes, small scale helical magnetic structures, are discussed in the context of thermal plasma observations.
Model calculations of the dayside ionosphere of Venus: Ionic composition
Authors: Nagy A. F. , Cravens T. E. , Smith S. G. , Taylor H. A. , Brinton H. C.
Published: February 2008

Comprehensive model calculations of the dayside ion density distributions were carried out and compared with results from the Pioneer Venus ion mass spectrometer. The coupled continuity and momentum equations were solved for O2+, O+, CO2+, C+, N+, He+, and H+ densities for altitudes well away from the ionopause, where the horizontal transport terms are negligible. Chemical equilibrium solutions, describing conditions below about 200 km, were also obtained for N2+, NO+, and CO+. The agreement between the model calculations of ion density and the measurements is good for some species, such as O+, and rather poor for others, such as CO+, indicating that while a basic understanding of the major chemical and physical processes controlling the composition and vertical distribution of the dayside Venus ionosphere, well below the ionopause, has been achieved, there are many important details requiring further investigations.
Pioneer Venus gas chromatography of the lower atmosphere of Venus
Authors: Oyama V. I. , Carle G. C. , Woeller F. , Pollack J. B. , Reynolds R. T. , Craig R. A.
Published: February 2008

A gas Chromatograph mounted in the Pioneer Venus sounder probe measured the chemical composition of the atmosphere of Venus at three altitudes. Ne, N2, O2, Ar, CO, H2O, SO2, and CO2 were measured, and upper limits set for H2, COS, H2S, CH4, Kr, N2O, C2H4, C2H6, and C3H8. Simulation studies have provided indirect evidence for sulfuric acid‐like droplets and support the possibility of water vapor at altitudes of 42 and 24 km. The paper discusses the implications of these results for the origin, evolution, and present state of Venus' atmosphere.
Radio occultation measurements of turbulence in the Venus atmosphere by Pioneer Venus
Authors: Woo Richard , Armstrong J. W. , Ishimaru Akira
Published: February 2008

As in the cases of Mariner 5 and 10 and Venera 9, the Pioneer Venus radio occultation measurements of Venus show an upper region of turbulence located in the vicinity of 60 km. Estimates of the deduced intensity of turbulence are consistent with the upper bound obtained earlier from the complementary Pioneer Venus probe measurements. Comparison with the Pioneer Venus in situ temperature measurements of 1 km scale size also shows very good agreement. It is clear that for scale sizes smaller than the Fresnel size (∼1 km), the refractive index irregularities are represented by a continuum of scale sizes having a spatial wave number spectrum that is power law and close to Kolmogorov. These irregularities are elongated in the horizontal direction (axial ratios are large, ≳10). The fact that the upper region of turbulence coincides with a region of high stability marked by one or more peaks in the stability profile suggests that the turbulence is associated with trapped gravity waves. The amplitude scintillations appear to increase poleward of about 70° which is interpreted as an increase in turbulence level with latitude.
The structure of the clouds of Venus: Results of the Pioneer Venus Nephelometer Experiment
Authors: Ragent Boris , Blamont Jacques
Published: February 2008

The results of the nephelometer experiments conducted aboard the four probes of the Pioneer Venus mission are presented. The vertical structure of the clouds exhibits an upper haze region leading to three more or less clearly differentiated layers within the main cloud bank. Below the main cloud banks are variable series of strata followed by a lower atmosphere haze region with occasional larger concentrations of particulates at lower altitudes. The general structures, especially the middle cloud layers, are remarkably constant planetary features but do exhibit some local variation from one site to another. Concentrated sulfuric acid appears to be the principal constituent of most of the particulate matter in all of the cloud structure with the possible exceptions of the largest particles in the lower cloud layers, whose composition is still uncertain, and the hazes and particulate matter in the lower atmosphere. Near‐UV radiation is absorbed throughout the cloud structure, primarily in the highest layers and, perhaps, below it. Absorbing species are not yet identified, but a combination of SO2 and an unspecified absorber look promising. Particles ejected from the surface of the planet on probe impact appear to be in the 10‐ to 100‐µm size range. The source of a faint glow detected by the night and north probes in the lower atmosphere is as yet uncertain.
Discrete aurora as the direct result of an inferred high‐altitude generating potential distribution
Authors: Lyons L. R.
Published: February 2008

In a previous paper it was shown that spatial changes in the high‐altitude magnetospheric electric field E with ▽ · E <0 can result in the generation of large‐scale ‘inverted V’ regions of auroral electron precipitation. In the present paper it is shown that smaller‐scale precipitation regions, as are required to account for discrete aurora, result from the basic analysis in the previous paper if appropriate structure is introduced in the high‐altitude electric field distribution. Using observations of electric fields and precipitating electrons from a rocket flight over discrete aurora the required structure in the electric field is inferred to exit at high altitudes along magnetic field lines connected to the aurora. By using this inferred high‐altitude electric potential distribution the ionospheric current continuity equation is solved for the ionospheric potential. The analysis assumes that the field‐aligned current is governed by single‐particle motion along field lines. The solution gives ionospheric potentials and precipitating electron energy fluxes in good quantitative agreement with those observed throughout the auroral rocket flight. The results in this and the previous paper imply that the ‘inverted V’ scale size (order of 200 km in width) is a natural result of the current versus electric potential relations along auroral field lines and in the ionosphere. This scale size need not be imposed by structure in the high‐altitude electric field distribution. Smaller‐scale (tens of kilometers) discrete auroral precipitation regions require the same current versus electric potential relations, but the scale size is imposed by structure in the high‐altitude electric potential distribution. The cause of this structure is not considered in the present analysis.
Seasonal and magnetic activity variations of ionospheric electric fields over Millstone Hill
Authors: Wand Ronald H. , Evans John V.
Published: February 2008

Incoherent scatter observations of E and F region drifts over Millstone Hill (42.6°N) were made routinely from May 1976 to November 1977 at monthly intervals. Some 667 hours of data gathered over this 19‐month period have been analyzed to yield the ionospheric polarization electric field in the magnetic south and magnetic east directions for three seasons during quiet magnetic conditions, as well as the annual average during magnetically disturbed conditions (Kp ⩾3o). The results for the quiet days are compared with earlier results obtained at Millstone Hill [Kirchhoff and Carpenter, 1976] and with those reported from St. Santin (44.6°N) by Blanc and Amayenc [1979]. We find that there is best agreement between Millstone and St. Santin for the east‐west component of the field which exhibits a roughly 12‐hour period variation during the daytime in all seasons. The north‐south field exhibits a predominantly diurnal variation that, in winter and equinox, is almost identically opposite at the two stations. This component shows substantial changes in summer at both stations and, at Millstone, appears to exhibit a significant (∼1mV/m) average northward component not seen at St. Santin. We attribute the differences in the quiet time behavior observed at Millstone and St. Santin to the very large difference in the latitude of their conjugate points where, presumably, the tides (and hence the dynamo effects) are dissimilar. There are marked differences in the electric field patterns between disturbed and quiet times which mainly are evident in the nighttime period. Commencing after noon, there is a gradual enhancement of both the northward and eastward fields on disturbed days. The eastward field reverses near midnight and becomes westward until dawn, while the other component remains northward for most of the night. These changes are thought to result from the penetration of auroral electric fields inside the plasmasphere.
Nonthermal scattering of radio waves near 150 km above Jicamarca, Peru
Authors: Røyrvik O. , Miller K. L.
Published: February 2008

Ten hours of coherent scatter radar data were collected near 150‐km altitude above Jicamarca, Peru, on July 27, 1977. The antenna system probed three separate volumes horizontally separated by approximately 9 km at 150 km. Measured vertical velocity varied between 10 and 20 m/s and is similar to the vertical ionization drift at these altitudes. However, the vertical velocity may change as much as 6 m/s over a 4‐km height interval. Ten‐minute amplitude modulations in the scattered power are seen throughout the observation period and are probably related to evanescent gravity waves. Several layers of enhanced scattering are observed at times. The half width of the measured spectra varies between 1 and 3 Hz with the vertically pointing antenna showing a consistently broader spectrum than the other two antennas. This narrow spectral width indicates a nonthermal scattering owing to electron density fluctuations.
Phase space variations of near equatorially mirroring ring current ions
Authors: Williams D. J.
Published: February 2008

We present Isee 1 observations of near equatorially mirroring ring current ions before and after the magnetic storm of November 25–26, 1977. The data are presented as phase space densities, f (s³/cm6), versus the first adiabatic invariant, µ (MeV/G), for the L range ∼2.7–8 RE. The µ range covered varies from ∼50–1000 MeV/G at L=8 to ∼1–100 MeV/G at L=2.7. The prestorm phase space densities show an intensity peak at a µ value which varies with L as µpeak ∼38 MeV/G for 5 ≲L ≲8 and µpeak ∼ 10e0.7(L−3) for 2.7 ≲ L ≲5. Phase space densities remain nearly constant throughout the storm for µ values greater than µpeak and are enhanced for µ values less than µpeak. Thus high‐energy ions respond adiabatically to the magnetic field changes caused by the low‐energy ion enhancements. This result agrees with earlier Explorer 45 results (Lyons and Williams, 1976). The Isee 1 data are compared directly with the Explorer 45 data and are found to agree very well. The time difference of ∼6 years and local time separation of ∼12 hours between the two data sets lead to the conclusion that the ring current ion behavior presented here is a characteristic feature of geomagnetic storms.
Thermal response of the F region ionosphere in artificial modification experiments by HF radio waves
Authors: Mantas George P. , Carlson Herbert C. , LaHoz Caesar H.
Published: February 2008

The thermal response of the nighttime F region ionosphere to local heating by HF radio waves has been observed with the incoherent scatter radar at Arecibo, Puerto Rico. The observations consist of high‐resolution space and time variation of the electron temperature as a high‐power HF transmitter is switched on and off with a period 240 s. As soon as the HF transmitter is turned on, the electron temperature begins to rise rapidly in a narrow altitude region near 300 km, below the F2 layer peak. The electron temperature perturbation subsequently spreads over a broader altitude region. The observations are compared with the anticipated thermal response of the ionosphere based on numerical solutions of the coupled time‐dependent heat conduction equations for the electron and composite ion gases and are found to be in good agreement over the entire altitude region covered by the observations. The calculations show that heat conduction is responsible for the spreading of the electron temperature enhancement outside the region where the radio wave energy is deposited. They also show that a smaller, but experimentally observable, ion temperature enhancement takes place while electron temperatures are enhanced by 40% or more of their equilibrium values. For the data presented they also show ionospheric absorption of roughly half the radiated HF energy, with anomalous absorption roughly equal to deviation absorption heating.
Ionization, luminosity, and heating of the upper atmosphere of Venus
Authors: Fox J. L. , Dalgarno A.
Published: February 2008

A model based upon Pioneer Venus data is constructed of the Venus atmosphere. A comprehensive discussion is given of the ultraviolet dayglow and a detailed assessment is made of the heating of the neutral and ionized components of the atmosphere arising from the absorption of solar ultraviolet radiation.
Enhancing whistler wave‐Electron interaction by the use of specially modulated VLF wave injection
Authors: Brinca A. L.
Published: February 2008

It is shown that interactions between artificially injected whistler mode waves and electrons can be strongly enhanced if the frequency and amplitude of the injected signal are appropriately programmed. The frequency variation produces efficient and steady changes in the particles energy and pitch angle through an extension of the cyclotron‐resonance interval. The amplitude modulation might prove necessary in certain interactions to avoid instability of the phase trapping mechanism. Sixteen interactions evolving in a duct centered at a geomagnetic field line with L=4 are studied by following the (test particle) evolution of the equatorial pitch angle and energy, and obtaining the required (ground) transmitter spectrogram format. Transmission using the proposed modulation with sufficient accuracy to match the real magnetosphere appears feasible and could be attempted in the near future.
VLF emissions from a modulated electron beam in the auroral ionosphere
Authors: Holzworth R. H. , Koons H. C.
Published: February 2008

A discrete VLF frequency of 3 kHz was successfully radiated by a modulated electron beam on a rocket launched into an active aurora. Instrumentation on this flight included a programable electron accelerator on the aft section with various particle and field detectors on the aft section as well as the ejected forward payload. The accelerator programmer included a current modulation period at fixed electron energy for 0.45‐s duration approximately every 11 s throughout the flight. In each of these program steps, 4‐kV electrons are current modulated at a 3‐kHz rate between Imin=0 or 10 mA and Imax ≅80 mA. The forward payload, which was ejected at about 10 m/s, included a pair of spherical double probes separated by 2.75 m and connected to a VLF receiver operating between 30 Hz and 18 kHz. Both this broadband receiver output as well as various narrow band channel outputs were directly telemetered to ground. Post flight spectrum analysis of the broadband VLF data clearly indicates that signals during the 3‐kHz accelerator modulation periods were propagated to the forward payload. A detailed analysis of these modulated pulses detected by the VLF receiver is presented. A time‐delay analysis between the start of the modulation and detection at the forward payload indicates time delays up to 0.2 s. The electron beam is believed to have produced a beam‐plasma discharge making a radiation efficiency calculation difficult. However, absolute received signal strength was about 1 mV/m at 1.4‐km separation.
Gradient drift irregularities in mid‐latitude sporadic E
Authors: Ecklund W. L. , Carter D. A. , Balsley B. B.
Published: February 2008

Three‐meter E region irregularities observed by a portable 50‐MHz radar on the island of Guadeloupe in June 1977 are shown to have the same properties as 3‐m irregularities generated by the gradient drift instability at the equator and in the auroral zones. The irregularities occur when intense sporadic E patches are known to occur, suggesting that they are generated by the gradient drift instability acting on the large vertical gradients in sporadic E patches. Large, rapid E region drift variations observed on two occasions suggest either that the E region electric fields are structured and less than 450 km in horizontal extent, or that E and F region fields couple and decouple intermittently during ionospheric sunset.
The influence of geomagnetic activity on the radial variation of the magnetospheric electric field between L=4 and 10
Authors: Kaye Stanley M. , Kivelson Margaret G.
Published: February 2008

In this report we use electric field data measured aboard the low‐altitude Ogo 6 spacecraft [Heppner, 1972] to determine the gross radial variation of the equatorial convection electric field. To transform the electric field from low altitudes to the equatorial magnetosphere, we assume that the earth's magnetic field is dipolar and that E · B=0. To determine the radial variation of the electric field, we fit the data between L=4 and 10 to a power law function of the form Φ=cLκ sin ϕ where E=−▽Φ and ϕ is azimuth. We find that in general the convection electric field magnitude decreases with decreasing radial distance (κ >1), implying that the field is partially shielded from the inner magnetosphere. This result is consistent with prior theoretical and experimental work. We find, in addition, that the magnitude of the radial variation of the electric field is a function of geomagnetic activity such that κ varies inversely with Kp for Kp ≲3, and κ ≃1 for Kp ≳3. We argue that this κ‐Kp relation probably reflects the fact that the electric field is shielded only within the ring current which moves radially inward as Kp increases. Only for Kp ≲3 does the region of measurement (L=4 to 10) include the region of shielded electric field in and near the ring current inner edge. For Kp ≳3, the measurements sample the electric field only outside the region of shielding.
SAS 2 observations of the Earth albedo gamma radiation above 35 MeV
Authors: Thompson D. J. , Simpson G. A. , Özel M. E.
Published: February 2008

Observations with the SAS 2 high‐energy γ ray telescope have been used in a study of the earth albedo γ radiation above 35 MeV in the equatorial region. The γ ray intensity has a peak toward the horizon which is about a factor of 10 more intense than the radiation coming up from the nadir. The horizon is about 2 orders of magnitude more intense than the cosmic γ radiation from most parts of the sky. The radiation from the western horizon is about 4 times as intense as the radiation from the eastern horizon and about 3 times as intense as that from the northern or southern directions. This azimuthal variation results from geomagnetic effects on the incident cosmic rays which interact in the atmosphere to produce the albedo γ rays. The upcoming high‐energy γ ray intensity seen with SAS 2 has a variation with vertical cutoff rigidity consistent with the empirical relationship of R−1.13 obtained by Gur'yan et al. (1979). The integral photon number spectrum from the horizon has an energy dependence of E−(1.0±0.1), which is flatter than the spectrum seen from the nadir, E−(1.4−0.2+0.3).
A cause of solar wind speed variations observed at 1 A.U.
Authors: Hakamada K. , Akasofu S.‐I.
Published: February 2008

An attempt is made to interpret solar wind variations observed at the earth's distance, namely the solar cycle variations, the semi‐annual variations, and the 27‐day variations, as well as the polarity changes of the interplanetary magnetic field, mainly in terms of two effects, a positive latitudinal gradient of the solar wind speed and a wobbling solar dipole, combined with the annual (heliospheric) latitudinal excursion of the earth. It is shown that a significant part of the solar wind variations observed at the earth's distance and the changes of polarity pattern of the interplanetary magnetic field can be reasonably well reproduced by the two effects.
Observations of transient H+ and O+ bursts in the equatorial magnetosphere
Authors: Kaye S. M. , Johnson R. G. , Sharp R. D. , Shelley E. G.
Published: February 2008

Twenty‐two days of data from the Lochkeed ion mass spectrometer aboard the Scatha satellite were used to perform a statistical study of short‐lived H+ and O+ bursts observed in the equatorial magnetosphere. The results of the study indicate that the ion bursts were transient phenomena occurring primarily in the nighttime sector during periods of enhanced geomagnetic activity. The average energy of the bursts, 〈W〉, was 1 keV, although the bursts were found to occur over any portion of the instrument's 100‐eV to 32‐keV energy range. Over one third of the observed bursts were associated with field‐aligned electrons flowing from the same hemisphere as the bursts. The energy width (ΔW/〈W〉 ≃1) and the pitch angle width (as great as 30°) of the bursts suggest that the ions had undergone substantial velocity space diffusion close to the geomagnetic equator.
Observed signatures of reconnection in the magnetotail
Authors: Nishida A. , Hayakawa H. , Hones E. W.
Published: February 2008

Plasma and magnetic field data from Imp 6 are examined statistically to study the occurrence of reconnection in the near‐earth region (within ∼20 RE) of the magnetotail. It is confirmed that the southward polarity of the magnetic field observed in the low‐latitude magnetotail (within a few RE of the estimated neutral sheet position) is strongly coupled to a tailward flow of plasma, as expected from the near‐earth reconnection model. It is noted, however, that reconnection is not the unique source of the tailward flow, and plasma is seen to be flowing back and forth regardless of the occurrence of near‐earth reconnection. It is fast tailward flow having speeds greater than about 400 km/s that is mostly due to near‐earth reconnection. There is an indication that detectability of an associated signature in the auroral zone magnetic field is related to the reconnection rate.
Effects of the August 1972 solar particle events on stratospheric ozone
Authors: Reagan J. B. , Meyerott R. E. , Nightingale R. W. , Gunton R. C. , Johnson R. G. , Evans J. E. , Imhof W. L. , Heath D. F. , Krueger A. J.
Published: February 2008

The spatial extent and the ionization profiles within the extended oval‐shaped regions irradiated by the intense solar particle events (SPE) of August 1972 have been derived from high‐energy proton data obtained with the 1971‐089A polar‐orbiting satellite and from several balloon flights. The particle ionization during the most intense 10‐hour period of the event on August 4 greatly enhanced the concentrations of short‐lived HOx and long‐lived NOx constituents, which in turn were responsible for the creation of a polar ozone cavity (POC) that has been identified and tracked with the backscattered ultraviolet (BUV) ozone sensors on the Nimbus 4 satellite. At the end of the peak irradiation period the ozone concentrations within the northern hemisphere POC were reduced by 46, 16, and 4% at altitudes of 49.5, 41, and 32 km, respectively. The total columnar ozone is estimated to have been reduced by ∼2% at this time. Above ∼45 km the ozone recovered on the time scale of several days. At 38.7 km in the northern hemisphere, however, the POC persisted and rotated as a semirigid mass in an east‐to‐west direction for some 53 days until the autumnal changes in wind patterns finally prevented further tracking. Time‐dependent chemistry calculations have been performed to explain the cause, magnitude, and temporal features of the ozone reductions. Using the calculated diurnal and particle‐induced behavior of the ozone during the SPE, the changes in heating rate and temperature expected in the stratosphere have been estimated. As a result of the initial large HOx‐caused ozone reduction, the temperature at 45 km should have decreased by ∼4°K several days after the event. Attempts to verify the predicted temperature changes have been unsuccessful due to limitations in the temperature measurement techniques.
Neutral winds and electric fields in the dusk auroral oval 2. Theory and model
Authors: Mikkelsen I. S. , Jørgensen T. S. , Kelley M. C. , Larsen M. F. , Pereira E.
Published: February 2008

In this paper we apply a two‐dimensional numerical model of the thermosphere in order to gain a better understanding of the observations of neutral winds and electric fields in the auroral zone presented in a companion paper. The model is zonally invariant with a resolution of 1/2° in the meridional direction and 1/2 scale height in the vertical. This allows inclusion of the sharp gradients in ionization density in and near the auroral oval. The model integration is carried out with the two‐step Lax‐Wendroff method, which sustains the shorter spatial scales in the solution with only slight damping. This is an improvement over other numerical methods in which it is necessary to filter out the smaller‐scale components to avoid numerical instabilities. The model integration as well as analysis of the observed wind profiles shows that the Lorentz force and viscosity provide the dominant effects on the zonal wind component. The shape of the zonal wind profile is well reproduced by the model in spite of zonal invariance. The meridional wind component is affected by a complex interplay of Joule heating, the Lorentz force, and the background solar heating pressure gradient force. Neither the direction of the meridional wind nor the shape of the vertical profile could be consistently explained without a combination of these three forces.
Mercury's magnetosphere and magnetotail revisited
Authors: Bergan Sandra , Engle Irene M.
Published: February 2008

Magnetic observations which are not complicated by currents of trapped plasma are a good test of geomagnetopause and geomagnetotail predictions. Recent attempts to model the Hermean magnetospheric field based on a planet‐centered magnetic multipole field with a quadrupole moment in addition to the planetary dipole field or a dipole field linearly displaced from planet center and no quadrupole moment have produced reasonably good fits to the Mercury magnetic field measurements. In this work we find a better fit for a dipole displacement from the planet center by making use of an improved representation of the magnetic field in the magnetotail, where many of the Mercury measurements were made. The rms deviation of the data was reduced from 10. or 11. γ to 9.3 γ by employing this new tail field representation. Also, by making use of this new tail field representation, we find a best fit for a dipole displacement of −0.0285 RM (earlier, 0.026 RM) toward the dawn in the magnetic equatorial plane and 0.17 RM (earlier, 0.189 RM (earlier, 0.189 RM) northward along the magnetic dipole axis, where RM is the planet radius. Thus with only minor adjustments in the displacement vector of the dipole from the planet center we achieve a measurable improvement in the fit of the data by using the improved magnetotail field representation.
Ionospheric convection and currents in the midnight sector on November 8, 1979
Authors: Foster J. C. , Doupnik J. R. , Stiles G. S.
Published: February 2008

Chatanika Radar observations of ionospheric convection velocities spanning auroral latitudes (58°–75°Λ), and contours of electrostatic potential derived from them, are described for a period of moderately strong geomagnetic activity in the midnight sector. Digitized H component magnetograms and contours of magnetic perturbation, ΔH, derived from them are presented in order to compare the convection pattern and radar‐deduced currents with the observed ground response to the auroral electrojets. In spite of midnight sector substorm activity, the convection pattern maintains a two cell character. The latitude of greatest negative potential in the evening sector, determined from the radar data, accurately denotes the position of the Harang discontinuity in the ionospheric currents.

Premidnight, substorm enhancements in the westward electrojet current are accompanied by precipitation‐enhanced Hall conductivity but little change in the convection electric field. At lower latitudes, the region of strong sunward convection (and northward electric field) is displaced equatorward during the substorm. As a result, the electric field increases as the eastward current increases at previously subauroral latitudes during the substorm. After midnight, enhancements in the electric field magnitude and the Hall conductivity alternate in supporting the substorm enhanced currents.
Quantitative simulation of a magnetospheric substorm 3. Plasmaspheric electric fields and evolution of the plasmapause
Authors: Spiro R. W. , Harel M. , Wolf R. A. , Reiff P. H.
Published: February 2008

Results of the Rice University substorm simulation have been used to investigate the penetration of substorm‐associated electric fields into the plasmasphere. Near 4 RE in the equatorial plane, our time dependent electric field model is characterized by eastward components in the dusk‐midnight local time sector and westward components after midnight. Except for a small region just before dusk, the model predicts eastward electric field components throughout the daytime sector. The characteristic radial component is directed inward at all local times except for a small region just after dawn. These results compare favorably with available whistler and incoherent‐scatter radar measurements obtained during magnetically disturbed periods. By assuming an initial plasmapause shape and by following the computed
drift trajectories of plasma flux tubes from that initial boundary we have examined the short term evolution of the plasmapause during the substorm‐like event of September 19, 1976. We find that narrow filamentary tails can be drawn out from the plasmasphere near dusk within hours of substorm onset. These tail‐like appendages to the plasmasphere subsequently drift rapidly from the dusk sector toward the daytime magnetopause. Investigation of the large‐scale time dependent flow of plasma in the evening sector indicates that some mid‐latitude plasma flux tubes that drift eastward past the dusk terminator reverse their motion between dusk and midnight and begin to drift westward toward dusk. Such time dependent changes in flow trajectories may be related to the formation of F region ionization troughs.
Numerical simulation of the generation of electrostatic turbulence in the magnetotail
Authors: Swift Daniel W.
Published: February 2008

A two‐dimensional plasma model is used to investigate the development of electrostatic turbulence in a magnetized plasma from plasma instabilities. The simultaion consists of following the motion of 105 ions in their self‐consistent electrostatic field. The electrons are treated as a constant neutralizing background. The instabilities modeled are driven by a ring‐type velocity distribution and by interpenetrating ion beams in a time variable magnetic field. Instability growth times are of the order of an ion gyroperiod in the case of the ring distribution and of the order of an ion plasma period in the case of the beam simulation. Maximum potential differences generated are of the order of the ion kinetic energies. These simulations demonstrate the cascade of wave energy to long wavelengths, thus showing that E × B turbulence can be generated from plasma microinstabilities. After the free energy feeding, the instabilities are exhausted, and wave energy at wavelengths less than an ion gyrodiameter decays quickly to equilibrium levels, while longer wavelength modes persist for much longer times. In one model with a time dependent, but spatially uniform, magnetic field the electric field energy at long wavelengths appeared to increase as a result of the increase of the magnetic field.
Near‐equatorial, high‐resolution measurements of electron precipitation at L ≃6.6
Authors: Baker D. N. , Stauning P. , Hones E. W. , Higbie P. R. , Belian R. D.
Published: February 2008

Previous studies of precipitating electrons at low altitudes in the auroral zone have shown that such precipitation is impulsive and highly fluctuating (time scales of seconds). It is also generally supposed that energetic electron precipitation results from wave‐particle scattering that occurs primarily near the outer zone (L=5‐8) equatorial plane. In contrast to prior low‐altitude measurements, however, nearly all previously reported measurements made in the near‐equatorial regions have been quite slow, with loss cone sampling times of minutes. This paper presents correlated measurements of Technical University of Denmark auroral zone riometer data (20‐s sampling times) and Los Alamos Scientific Laboratory high‐resolution (8‐ms sampling, 10‐s rotation) >30‐keV electron measurements made at geostationary orbit during the course of magnetospheric substorms. Prior to substorm expansion onset, substantial evidence is found to support the concept of a substorm growth phase. Following the expansion phase onset and the concomitant injection of newly energized electrons into the outer magnetosphere, it is possible to examine equatorial electron fluxes in the loss cone for cases of weak pitch angle diffusion as well as for cases of strong pitch angle diffusion. During strong diffusion events, it is found that precipitating electron fluxes are highly fluctuating and impulsive at the minimum loss cone sampling time of 10 s. For such event periods the pitch angle distribution outside the loss cone is nearly flat (isotropic flux), the absolute electron intensity (>30 keV) is near ‘saturation’ (j ∼5 × 10 7 cm −2 s −1 sr −1), and the 20‐MHz conjugate riometer intensity is very high (5–15 dB). These results demonstrate that equatorial wave‐particle interactions do not build up steadily and gradually but rather that they develop in an impulsive or sporadic way. Thus these observations are quite consistent with low‐altitude results and probably relate equatorial phenomena to such ionspheric phenomena as pulsating aurora and fluctuating auroral zone bremsstrahlung X rays.
Wave distribution functions estimation of VLF electromagnetic waves observed onboard Geos 1
Authors: Lefeuvre F. , Parrot M. , Delannoy C.
Published: February 2008

Two methods to determine the electromagnetic wave distribution function are presented. The first is based on the use of the dirichlet kernels and provides us with a local average. It has the disadvantage, however, of a nonsystematic approach to positive solutions. The second uses the maximum entropy concept. It leads to particular solutions that are smooth and positive everywhere. The two methods are shown to be complementary. Applications to VLF electromagnetic waves observed onboard Geos 1 are discussed. One of the most striking results is that the wave energy of the natural VLF emissions is generally concentrated within two wave packets whose wave normals are approximately in the same off‐meridian plane and oriented in the same way relative to the direction of the earth's magnetic field. It is suggested that those two wave packets have a common source.
Fine‐scale characteristics of interplanetary sector boundaries
Authors: Behannon K. W. , Neubauer F. M. , Barnstorf H.
Published: February 2008

This investigation has studied the structure of the interplanetary sector boundaries observed by Helios 1 within sector transition regions during the time interval from December 1974 to April 1975. A sector transition region is the region of variable magnetic field magnitude and direction observed in or near the ecliptic plane between magnetic sectors in the interplanetary magnetic field (IMF). Such regions are found generally to be complex in character, consisting of intermediate (nonspiral) average field orientations in some cases, as well as a number of large‐angle (120° ≤ω ≤180°) directional discontinuities (DD's) on the fine scale (time scales <1 h). Such DD's are found to be more similar to tangential than rotational discontinuities, to be oriented on average more nearly perpendicular than parallel to the ecliptic plane (the mean tilt is ∼58°), to be accompanied usually by a large dip (≳80%) in B (≡|B|), and, with a most probable thickness of 3 × 104 km, significantly thicker structures on average (by a factor of 10 or more) than ordinary DD's previously studied in the IMF. It is hypothesized that the observed structures represent multiple traversals of the global heliospheric current sheet caused by local fluctuations in the position of the sheet. There is evidence that such fluctuations are sometimes produced by wavelike motions or surface corrugations of scale length 0.05–0.1 AU, superimposed on the large‐scale structure. The observed steep inclinations could be produced by a combination of current sheet warping at the sun, a global wave structure, and the observed small‐scale fluctuations. No radial distance dependence is found for the current sheet properties studied.
Ion composition of zipper events
Authors: Kaye S. M. , Shelley E. G. , Sharp R. D. , Johnson R. G.
Published: February 2008

A class of ion distributions has recently been identified by Fennell et al. (this issue). The distributions are composed of two components, a low‐energy component with peak fluxes directed along the field line and a high‐energy component with peak fluxes in the perpendicular direction. The transition between the two components occurs over a very narrow range of energies but can occur anywhere between approximately several hundred electron volts and 20 keV. Because of the appearance of this distribution on an energy versus time spectrogram, the ion events have been called zippers. The purpose of this report is to examine the mass composition of the zipper events. We find that the low‐energy and parallel component is composed primarily of O+, with, to a lesser degree, H+ and a trace of He+. The high‐energy and perpendicular component is predominantly H+, with the relative abundances of O+ and He+ down from those of the low‐energy component by a factor of ∼10. These results suggest that whereas the low‐energy component is probably ionospheric in origin, the source of the high‐energy component is most probably the plasmasheet.
Large‐scale current systems in the dayside Venus ionosphere
Authors: Luhmann J. G. , Elphic R. C. , Brace L. H.
Published: February 2008

The occasional observation of large‐scale horizontal magnetic fields within the dayside ionosphere of Venus by the flux gate magnetometer on the Pioneer Venus orbiter suggests the presence of large‐scale current systems. Using the measured altitude profiles of the magnetic field and the electron density and temperature, together with the previously reported neutral atmosphere density and composition, it is found that the local ionosphere can be described at these times by a simple steady state model which treats the unobserved quantities, such as the electric field, as parameters. When the model is appropriate, the altitude profiles of the ion and electron velocities and the currents along the satellite trajectory can be inferred. These results elucidate the configurations and sources of the ionospheric current systems which produce the observed large‐scale magnetic fields, and in particular illustrate the effect of ion‐neutral coupling in the determination of the current system at low altitudes.
Effect of geomagnetic activity on equatorial radio VHF scintillations and spread F
Authors: Rastogi R. G. , Mullen J. P. , MacKenzie E.
Published: February 2008

The paper discusses the occurrence of scintillations of ATS 3 (137 MHz) beacons recorded at Huancayo on geomagnetically quiet and disturbed days during the years 1969–1976 and compares the results with the corresponding occurrence of range and frequency spread F at Huancayo. During the equinoctial months and the December solsticial months the geomagnetic activity reduces the equatorial scintillations during premidnight hours but increases their occurrence during the postmidnight hours. These features are very similar to the effect of geomagnetic activity on the occurrence of the range type of equatorial spread F rather than on the occurrence of frequency spread, which decreases for any hour of the night during geomagnetic active periods. During the June solsticial months, the occurrence of both scintillations and spread F is very much reduced; however, both the phenomena are more frequent on disturbed than on quiet days for any of the hours of the night. These effects are consistently the same for any of the years within the solar cycle. It is suggested that the equatorial radio scintillations at 137 MHz during the nighttime are produced primarily by the occurrence of the range type of spread F. The geomagnetic effects are due to the modifications of the equatorial electric field by the geomagnetic disturbance and thereby affect the development of F region irregularities causing scintillations.
Microinstabilities upstream of the Earth's bow shock: A brief review
Authors: Gary S. Peter
Published: February 2008

This is a brief review of short wavelength fluctuations and their associated phenomena upstream of the earth's bow shock. Sources of free energy, the most likely instabilities, and the associated nonlinear consequences that may be observed are discussed.
The 2fp circumterrestrial radio radiation as seen from ISEE 3
Authors: Hoang S. , Fainberg J. , Steinberg J. L. , Stone R. G. , Zwickl R. H.
Published: February 2008

Narrow band radio emissions are observed with the radio astronomy experiment on ISEE 3 from a position 0.01 AU upstream of the earth. These are interpreted as radio line emission generated at twice the local plasma frequency in the vicinity of the earth's bow shock by energetic particles. The frequency of the 2fp radio emission is well correlated to large scale density changes observed by both the LASL plasma experiment and the radio experiment at ISEE 3, about one hour earlier in accordance with the transit time of the solar wind to the bow shock region. The bandwidth of the 2fp emission varies from less than 3 kHz to more than 20 kHz and is likely broadened by solar wind density variations across the source region. The 2fp source is visible about 50% of the time and the position of its centroid is usually located within 30 RE of the subsolar point, but at times is observed at a distance of up to 60 RE. However, these positions may have systematic errors if significant linear polarization is present in the observed radiation or if refraction takes place along the ray path. Intensity variations of 20 db and apparent motions of 40 RE can occur with time scales of minutes.
Production and transport of O++ in the ionosphere and plasmasphere
Authors: Geiss J. , Young D. T.
Published: February 2008

The abundance ratio of O++/O+ has been observed to reach values of unity and above in the equatorial plasmasphere at L ∼3. This is two to three orders of magnitude greater than the relative O++ abundance at corresponding latitudes in the topside ionosphere. In this paper we pursue our earlier suggestion that thermal diffusion of O++, driven by the temperature gradient between the ionosphere and equatorial plasmasphere, is responsible. We have carried out this study by numerically integrating the time dependent diffusion equation for equinox conditions. Recent plasmaspheric data from the GEOS‐1 spacecraft have been incorporated into a realistic model of the temperature and densities of major ion species extending from the ionosphere to the equatorial plane. Convection as well as local time variations and details of ion chemistry have been included in the model. Our main conclusion is that only thermal diffusion can account for observations of O++/O+ ∼1 in the equatorial plasmasphere. Furthermore, we have found that there is a threshold temperature gradient between ionosphere and equatorial plasmasphere, corresponding to an equatorial temperature at L = 3 of ≃5000 K, below which thermal diffusion becomes ineffective.