The 7 and 25 June 1985 Neptune occultations: Constraints on the putative Neptune “arc”

Stellar occultations by Neptune on 7 and 25 June 1985 were observed in the K band from Sutherland (SAAO) to search for conforming evidence of the ring-like “arc” reported by Hubbard et al. (W.B. Hubbard, A. Brahic, B. Sicardy, L.-R. Elicer, F. Roques, and F. Vilas (1986). Nature 319, 636–640). A binary star was occulted on 7 June 1985, and since both components were occulted by the planet, their relative positions could be precisely determined. A single sharp dip, of high signal-to-noise ratio, was observed in the post-emersion occultation trace. If this feature were caused by material near Neptune, its corresponding projected equatorial plane radius is either 62,600 ± 160 km or 63,760 ± 120 km, depending on which of the binary star pair was occulted. The equatorial radius, width, and optical depth of the 7 June feature are similar to those described by Hubbard et al. The absence of a corresponding post-emersion dip due to the occultation of the companion star suggests that the ring-like material is discontinuous over a scale of several thousand kilometers in ring circumference. No ring-like features were observed during pre-immersion. The 25 June 1985 occultation was also successfully observed, including atmospheric occultation profiles for both immersion and emersion. No evidence for ring-like material was found in the region probed by this occultation during post-emersion, which included the entire range of equatorial radii over which “arc” events have been previously reported.     

Dominant acceleration processes of ambient energetic protons (E ⩾ 50 keV) at the Bow Shock: Conditions and limitations

Energetic proton (E_p ? 50 keV) and magnetic field observations during crossings of the Earth's Bow Shock by the IMP-7 and 8 spacecraft are incorporated in this work in order to examine the effect of the Bow Shock on a pre-existing proton population under different “interplanetary magnetic field-Bow Shock” configurations, as well as the conditions for the presence of the Bow Shock associated energetic proton intensity enhancements. The presented observations indicate that the dominant process for the efficient acceleration of ambient energetic particles to energies exceeding ~ 50 keV is by “gradient-B” drifting parallel to the induced electric field at quasi-perpendicular Bow Shocks under certain well defined limitations deriving from the finite and curved Bow Shock surface. It is shown that the proton acceleration at the Bow Shock is most efficient for high values of the upstream magnetic field (in general B₁ > 8γ), high upstream plasma speed and expanded Bow Shock fronts, as well as for directions of the induced electric field oriented almost parallel to the flanks of the Bow Shock, i.e. when the drift distance of protons parallel to the electric field at the shock front is considerably smaller than the local radius of curvature of the Bow Shock. The implications of the presented observations of Bow Shock crossings as to the source of the energetic proton intensity enhancements are discussed.     

Intensity variations and ratios of (9-4) and (7-3) hydroxyl bands in nightglow at Poona

The observed types of nocturnal intensity variations for the OH (9-4) and OH (7-3) bands during IQSY at this Station are further analysed using the theoretical band intensity distribution of Evans and Llewellyn (1972). A considerable agreement is noticed between observed and theoretical intensity ratios, $\text{I(9-4)}\text{I(7-3)}$, for a major portion of the data (～70%), which has a “continuous decrease” type of noctural intensity variation. This data is thereby satisfactorily explained on the basis of available information.For the remaining portion of the data (～30%), which has “an increase followed by decrease” type of intensity variation and higher intensities, the observed ratios are also systematically higher than the above. A satisfactory explanation is offered, by postulating a second layer of emission, by examining closely several aspects of the other observational results.     

Survival probabilities for interstellar hydrogen flowing into the interplanetary system from far regions of the heliosphere

The expressions for “survival” probabilities are presented for an atomic hydrogen particle moving on a trajectory from far regions of the heliosphere to the vicinity of the Sun. Three “destroying” processes have been considered; photoionization, charge transfer and electron ionization. The solar wind has been assumed to be a two-flux steady stream radially expanding with constant flow velocity. Recent profiles of solar-wind electron temperature have been used. The results can be useful for theoretical analyses as well as for analysis of spaceflight observations.     

The influence of thermospheric winds on exospheric hydrogen on Venus

Monte Carlo models of the distribution of atomic hydrogen in the exosphere of Venus were computed which simulate the effects of thermospheric winds and the production of a “hot” hydrogen component by charge exchange of H⁺ and H and O in the exosphere, as well as classic exospheric processes. A thermosphere wind system that is approximated by a retrograde rotating component with equatorial speed of 100 m/sec superimposed on a diurnal solar tide with cross-terminator day-to-night winds of 200 m/sec is shown to be compatible with the thermospheric hydrogen distribution deduced from Pioneer Venus orbiter measurements.     

On the E-W asymmetry and the generation of ESP events

Observations of energetic-ion intensity enhancements (E 290 keV) associated with solar flare generated shock waves (solar flare ESP events), obtained during nearly a decade by the APL/JHU instruments on board the Earth orbiters IMP-7 and 8, are incorporated in this work in order to examine the role of the heliolongitude depended large scale shock morphology with relation to the upstream interplanetary magnetic field in the formation of these ESP events. It is shown that a clear east-west solar hemisphere asymmetry is present in the distribution of the ESP relative intensity enhancements with respect to the heliolongitudes of the shock wave source-flare sites. The large ion-intensity enhancements superimposed on the ambient solar flare ion population are preferentially associated with solar flare sites located to the east of the spacecraft meridian, whereas on the average only weak ESP events are associated with solar flare sites to the west of the spacecraft meridian. The observed asymmetry and its implications on the dominant processes for the generation of the solar flare ESP events are discussed on the basis of the presented extensive survey.     

Thermal protons in the morning magnetosphere: Filling and heating near the equatorial plasmapause

Thermal H⁺ distributions have been measured as the European Space Agency GEOS-1 satellite passed through the late morning equatorial magnetosphere, plasmapause and plasmasphere. The unique capabilities of the on-board Supralhermal Plasma Analysers (SPA) have been used to overcome the retarding floating potential of the satellite and measure the velocity distribution of the cold protons. In the magnetosphere an enhanced source cone of such ions with a temperature of ~ 0.5 eV is a signature of the filling process occurring outside the plasmapause where flux tubes are relatively empty. In the plasmasphere the thermal H⁺ is essentially isotropic with a temperature less than 0.5 eV but the motion of the satellite introduces apparent drift.These measurements of cold proton velocity distribution now permit a reappraisal of the definition of the “plasmapause”. It becomes inappropriate to use an arbitrary empirical density, e.g. the conventional 10 cm ^?3, in order to establish a boundary. It is now possible to identify a plasmapause interaction region where the two cold proton populations co-exist. This region generally lies Earthward of the 10 cm ^?3 density level, has a width which is strongly dependent on magnetic activity and the temperature is typically between 0.5 and 1.5 eV. The change from “filled” to “unfilled” flux tubes relates to the physical processes which are occurring and the controlling electric field configuration; in particular, the last closed equipotential. Throughout this region, in going from the plasmasphere to the magnetosphere, the plasma drift motion is expected to change from corotation to a convection which is controlled by E ×B, and is predominantly Sunward due to the dawn-dusk electric field. Crossing the plasmapause on the morning side, little change in drift direction should occur but subtle variations in the ionic velocity distribution do reflect the change in the degree of flux tube density equilibrium.Our first direct measurement of the magnetospheric E × B drift has been reported previously but here measurements from a selected six day period show how the plasma in the plasmapause region responds to changing magnetospheric activity. The drift velocities cannot he derived with high accuracy but the analysis shows that the technique can provide a valid mapping of the magnelospheric electric field. In addition, since the magnetospheric cold plasma distribution is observed after it has come from the ionosphere, a distance of many Earth radii, the scattering and accelerating mechanisms along the flux tube can be studied. For this particular data-set taken in the late morning, the maximum potential drops along the flux tubes were less than a volt. The ionospheric proton source cone is observed to be broad, pitch angle scattering persists up to 40 or even 70°.Although these results throw new light on the plasmaspheric filling process one must recognise that, however the plasmapause is defined, it is not a simple matter to map this boundary from the equatorial plane down to low altitudes and the mid-latitude trough.     

Latitudinal profile of UV nightglow and electron precipitations

We studied experimental data on ultra-violet (UV) nightglow in the wavelength range 300-400 nm, and energetic electron fluxes measured by low-altitude polar satellite Universitetskii-Tatiana. From statistical analysis we have found three latitudinal regions of enhanced UV emission at low, middle and high latitudes. Modeling the electron precipitations to the atmosphere gave numerical estimation of the generated UV radiation. We found that the stable and quasi-stable fluxes of electrons precipitating at middle and low latitudes are too weak to explain the observed intensities of UV radiation. The high-latitude UV nightglow with intensity of several kiloRayleighs results from particle precipitation in the regions of aurora and outer radiation belt. The low-latitude UV enhancements of several hundreds Rayleighs can be related to the emission of mesospheric atomic oxygen whose concentration increases substantially at latitudes from 20° to 40°. A mechanism of the mid-latitude UV enhancements is still unknown and requires further investigations.     

Initial Fe/O Enhancements in Large, Gradual, Solar Energetic Particle Events: Observations from Wind and Ulysses

Shocks driven by fast coronal mass ejections (CMEs) are the dominant particle accelerators in large, “gradual” solar energetic particle (SEP) events. In these events, the event-integrated value of the iron-to-oxygen ratio (Fe/O) is typically ～?0.1, at least at energies of a few MeV/nucleon. However, at the start of some gradual events, when intensities are low and growing, initially Fe/O is ～?1. This value is also characteristic of small, “impulsive” SEP events, in which particle acceleration is due to magnetic reconnection. These observations suggested that SEPs in gradual events also include a direct contribution from the flare that accompanied the CME launch. If correct, this interpretation is of critical importance: it indicates a clear path to interplanetary space for particles from the reconnection region beneath the CME. A key issue for the flare origin is “magnetic connectedness”, i.e., proximity of the flare site to the solar footpoint of the observer’s magnetic field line. We present two large gradual events observed in 2001 by Wind at L1 and by Ulysses, when it was located at >?60^° heliolatitude and beyond 1.6 AU. In these events, transient Fe/O enhancements at 5?–?10 MeV/nucleon were seen at both spacecraft, even though one or both is not “well-connected” to the flare. These observations demonstrate that an initial Fe/O enhancement cannot be cited as evidence for a direct flare component. Instead, initial Fe/O enhancements are better understood as a transport effect, driven by the different mass-to-charge ratios of Fe and O. We further demonstrate that the time-constant of the roughly exponential decay of the Fe/O ratio scales as R ², where R is the observer’s radial distance from the Sun. This behavior is consistent with radial diffusion. These observations thus also provide a potential constraint on models in which SEPs reach high heliolatitudes by cross-field diffusion.     

A Science Mission for QSAT Project: Study of FACs in the Polar and Equatorial Regions

Kyushu University, Kyushu Institute of Technology and Fukuoka Institute of Technology are now designing, developing and building a micro-satellite called “QSAT”. The primary objective of QSAT is understanding the mechanism of spacecraft charging, which can be achieved with the onboard magnetometer, high-frequency probe (HP) and Langmuir probe (LP). The magnetometer measures the magnetic field variations caused by field-aligned currents (FACs) in the polar and equatorial regions. Polar FACs are well understood, while equatorial FACs are not. The science goals are as follows: (1) to better understand FACs in the polar region, (2) to compare the FACs observed in orbit with ground-based MAGDAS observations, (3) to investigate spatial distribution of FACs in the equatorial region. FACs play a crucial role in the coupling between solar wind, magnetosphere and ionosphere in terms of energy transfer. Also if we understand the relationship between the space and ground-based FACs data, then we can conduct long-term study on the solar wind–magnetosphere–ionosphere coupling in the future by mainly using data from ground-based magnetometer arrays.     

Ion temperature troughs in the equatorial topside ionosphere

The Retarding Potential Analyzer aboard OGO-6 sometimes recorded marked depressions of ion temperature as the satellite crossed the equatorial region. These “T_i troughs” occur at heights between about 700 km and the satellite apogee at 1100 km. At the centre of a trough, close to the dip equator, T_i is frequently 500–1000 K below its value at the northern and southern edges, which are usually 15°–20° in latitude from the centre of the trough. At a given season and local time, the occurrence, symmetry, depth and position of the troughs often vary markedly with longitude. The troughs have no particular association with equatorial troughs of ion concentration N_i.As suggested by Hanson, Nagy and Moffett, the T_i troughs appear to be caused by transequatorial winds that drive F region plasma along geomagnetic field lines. The plasma is adiabatically cooled as it is driven upwards on the “upwind” side of the dip equator, and heated as it descends on the “downwind” side. The available data on the occurrence of troughs at different longitudes, local times and seasons are reasonably consistent with wind directions deduced from Jacchia's model and the OGO-6 thermospheric model of Hedin et al., and with the north-south asymmetries of the tropical 630 nm airglow observed by OGO-4 and OGO-6. Factors determining the latitudinal extent of the troughs are discussed and some questions for further study are listed.     

Polar 5—an electron accelerator experiment within an aurora. 4. Measurements of the 391.4 nm light produced by an artificial electron beam in the upper atmosphere

The POLAR 5 sounding rocket, launched from Andøya, Norway, on February 1, 1976, was of the “mother-daughter” configuration.A rocket-borne electron accelerator, mounted on the “daughter,” produced a pulsed electron beam with a maximum current of 130 mA and electron energies up to 10 kev.Using a photometer the luminescence at 391.4nm produced by electrons colliding with ambient nitrogen molecules was studied. The observed light at 391.4 nm consisted of low background, with occasional flashes due to the natural auroral excitations, and intense sparkles when the electron beam was emitted.Below 130 km the light observed during beam injection can be explained by excitations of ambient N₂ due to high energy beam electrons.In the altitude range from 150 km to apogee at 220 km, the observed light level during beam emission is fairly constant and much larger than that produced by the high energy beam electrons. A possible source of this light is the excitation of ambient N₂ by an enhanced population of low energy electrons, created by the presence of a beam plasma discharge in the vicinity of the “daughter” payload.     

X-RADIOGRAPHY OF SLABS OF THE ALLENDE METEORITE

X-radiographs of eight slabs of the Allende meteorite, each about 8 mm thick, reveal significant chemical and textural heterogeneities of our Allende fragment at the 100–1000 g level. They also reveal systematic arrangements of chondrules (“strings”). Our preliminary interpretation is that the observed features are probably all due to regolith processes on Allende's parent body, and that these processes included “mini” slope slides along dry, “sandy” slopes.     

Local plasma processes and enhanced electron densities in the lower ionosphere in magnetic cusp regions on Mars

Both the MARSIS ionospheric sounder and the charged particle instrument package ASPERA-3 are experiments on board the Mars Express spacecraft. Joint observations have shown that events of intense ionospheric electron density enhancements occur in the lower ionosphere of magnetic cusp regions, and that these enhancements are not associated with precipitation of charged particles above a few hundred electron volts (<300 eV). To account for the enhancement by particle precipitation, electron fluxes are required with mean energy between 1 and 10 keV. No ionizing radiation, neither energetic particles nor X-rays, could be identified, which could produce the observed density enhancement only in the spatially limited cusp regions. Actually, no increase in ionizing radiation, localized or not, was observed during these events. It is argued that the process causing the increase in density is controlled mainly by convection of ionosphere plasma driven by the interaction between the solar wind and crustal magnetic field lines leading to excitation of two-stream plasma waves in the cusp ionosphere. The result is to heat the plasma, reduce the electron–ion recombination coefficient and thereby increase the equilibrium electron density.     

Accretional and alterational differences in a carbonaceous chondrite parent body: Evidence from the NWA 5491 CV3 meteorite

The NWA 5491 CV3 meteorite is a CVoxA subtype, and composed of two substantially different units (titled “upper” and “lower” units) in the cm size range with original accreted material and also subsequent alteration produced features. Based on the large chondrules in the “upper” unit and the small chondrules plus CAIs in the “lower” unit, they possibly accreted material from different parts of the solar nebula and/or at different times, whereas substantial changes happened in the nebula's composition. Differences are observed in the level of early fragmentation too, which was stronger in the upper units. During later alteration oxidizing fluids possibly circulated only in the upper unit, mechanical fragmentation and resorption were also stronger there. In the last phase of the geological history these two rock units came into physical contact, but impact‐driven shock effects were not observed. The characteristics of this meteorite provide evidence that the same parent body might accrete substantially different material and also the later processes could differ spatially in the parent body.     

Role of “Core” and “Halo” solar electrons in ionization of the interstellar medium

The probability of the interstellar wind atoms (H and He) to survive ionization by solar wind electrons is presented. For the first time a dual temperature electron distribution is used to model the effects of “core” (10 eV) and “halo” (60 eV) solar electrons on the probabilities. Survival probability distributions as a function of helicocentric distance were calculated for variations in the electron temperature, solar radiation force, and the interstellar wind flow velocity. These probabilities are important in determining the radial density distributions of the interstellar atoms. It has been found that the interstellar wind has a distinctively higher probability of surviving “halo” rather than “core” electron ionization only at heliocentric distances, ρ, smaller than about 0.5 a.u. For distances larger than 0.5 a.u., the probabilities of surviving “halo” electrons are close to the probabilities of surviving “core” electrons. Also, the probabilities for both “core” and “halo” electrons are relatively insensitive to changes in u_∞ (interstellar wind velocity at infinity), μ (the solar ratio of radiation to gravitational force) and α (a model parameter for solar electron temperature) for ρ > 0.5. For distances smaller than that, the sensitivity increases significantly.     

POLAR 5—an electron accelerator experiment within an aurora. 3. Evidence for significant spacecraft charging by an electron accelerator at ionospheric altitudes

The POLAR 5 rocket experiment carried an electron accelerator on a “daughter” payload which injected a 0,1 A beam of 10 keV electrons in a pulsed mode every 410ms. With spin and precession, injections were made over a wide range of pitch angles. Measurements from a double probe electric field instrument and from particle detectors on the “mother” payload and from a crude R.P.A. on the “daughter” payload are interpreted to indicate that the “daughter” charges to a potential between several hundred volts and 1 kV. The neutralizing return current to the “daughter” is shown to be assymetrically distributed with the majority being collected from the direction of the beam. The additional electrons necessary to neutralize the daughter are thought to be produced and heated through beam-plasma interactions postulated by Maehlum et al. (1980b) and Grandal et al. (1980) to explain the particle and optical measurements. Significant electric fields emanating from the charged “daughter” and the beam are seen at distances exceeding 100 m at the “mother” payload.     

A family of ionospheric models for different uses

Empirical models of three dimensional electron density distributions in the ionosphere have been constructed for global as well as regional use. The models differ by their degree of complexity and calculation time and therefore have different uses. All are based on “ionogram parameter” (critical frequencies foE, foF1, foF2 and the F2 region transfer parameter M(3000)F2). The models allow the use of global or regional maps for foF2 and M(3000)F2 and use built-in formulations for foE and foF1. Update (instantaneous mapping / nowcasting) versions exist which take foF2 and M(3000)F2 or F2 region peak height and electron density as input. The ground to F2 layer peak part of the profile is identical for all three models and is based on an Epstein formulation. The “quick calculationr” model NeQuick uses a simple formulation for the topside F layer, which is essentially a semi-Epstein layer with a thickness parameter which increases linearly with height. The “ionospheric model” COSTprof is the model which was adopted by COST 251 in its regional “monthly median” form. Its topside F layer is based on O⁺-H⁺ diffusive equilibrium with built-in maps for three parameters, namely the oxygen scale height at the F2 peak, its height gradient and the O⁺-H⁺ transition height. The “ionosphere-plasmasphere” model NeUoG-plas uses a magnetic field aligned “plasmasphere” above COSTprof Typical uses of the models and comparison among them are discussed.