The outer ionosphere

The results from ionospheric beacon observations are compared with the theory of F-layer and outer ionosphere. Near sunspot minimum the total electron content up to 1000 km at medium latitudes is 1–2 × 10¹⁷ m^-2 in the afternoon and about 0.3 × 10¹⁷ m^-2 at night. The diurnal and latitudinal variations are more irregular during summer than during winter. Near sunspot maximum the respective values are about 1.5 × 10¹⁷ m^-2 at night, 6–7 x 10¹⁷ m^-2 near winternoon, 4 × 10¹⁷ m^-2 near summernoon.The complete theory of diffusion in a multi-ion plasma gives very complicated expressions. Simpler approximations can be found respectively. Some models were calculated numerically and the dependence of the total electron on the parameters of the balance equation is investigated.Comparison with experimental results shows, that some main features of the diurnal and latitudinal variations of the total electron can be explained. The seasonal anomaly, the slow decrease in the late night hours and the large increase toward equator cannot be explained without additional assumptions.     

Latitudinal variation of solar wind velocity

Single station solar wind velocity measurements using the Ooty Radio Telescope (ORT) in India (operating at 327 MHz) are reported for the period August 1992 to August 1993. Interplanetary scintillation (IPS) observations on a large number of compact radio sources covering a latitudinal range of ±80° were used to derive solar wind velocities using the method of fitting a power law model to the observed IPS spectra. The data shows a velocity versus heliographic latitude pattern which is similar to that reported by Rickett and Coles (1991) for the 1981–1982 period. However, the average of the measured equatorial velocities are higher, being about 470 km s^–1 compared to their value of 400 km s^–1. The distribution of electron density variations (N _e ) between 50R and 90R was also determined and it was found that N _e was about 30% less at the poles as compared to the equator.     

Helium in the terrestrial atmosphere

After a brief historical review of the discovery of helium in the terrestrial atmosphere, the production mechanisms of the isotopes He⁴ and He³ are discussed. Although the radioactive production of He⁴ in the Earth is well understood, some uncertainty still exists for the degassing process leading to an atmospheric influx of (2.5 ±1.5) × 10⁶ atoms cm^–2 s^–1. Different production mechanisms are possible for He³ leading to an influx of (7.5±2.5) atoms cm^–2 s^–1. Observations of helium in the thermosphere show a great variability of this constituent. The different mechanisms proposed to explain the presence of the winter helium bulge are discussed. Since helium ions are present in the topside ionosphere and in the magnetosphere, ionization mechanisms are analyzed. Owing to possible variations and uncertainties in the solar UV flux, the photoionization coefficient is (8±4) × 10^–8 s^–1. Finally, the helium balance between production in the earth and loss into the interplanetary space is discussed with respect to the different processes which can play an effective role.     

Techniques in balloon X-ray astronomy

The first balloon observation of a cosmic X-ray source, the Crab Nebula, was made in 1965, only three years after the initial discovery of such sources by rocket observations. Since then balloon data has provided much information on the positions, spectra, time variability and pulsed nature of localized sources, and on the spectrum and isotropy of diffuse galactic and universal components. Measurements are limited to energies above about 20 keV by atmospheric attenuation at 2–3 g cm ^–2 depth and to below several hundred keV by detector sensitivity. Detectors usually consist of large area NaI or CsI scintillation counters with anticoincidence collimators for rejection of charged particles and scattered X-rays. Proportional counters are occasionally used at lower energies and solid state detectors are used where extreme energy resolution is important. The instruments require a pointing capability on the order of 1.0 to 0.1°, depending on the collimator aperture. Digital data is either recorded on board or telemetered using a PCM technique. Exploratory work in the 0.2–10 MeV -ray range is starting now, and balloon observations may be expected to make important contributions in the near future.     

In situ measurements of interstellar dust with the Ulysses and Galileo spaceprobes

Interstellar dust was first identified by the dust sensor onboard Ulysses after the Jupiter flyby in February 1992. These findings were confirmed by the Galileo experiment on its outbound orbit from Earth to Jupiter. Although modeling results show that interstellar dust is also present at the Earth orbit, a direct identification of interstellar grains from geometrical arguments is only possible outside of 2.5 AU. The flux of interstellar dust with masses greater than 6 · 10^–14 g is about 1 · 10^–4 m ^–2 s ^–1 at ecliptic latitudes and at heliocentric distances greater than 1AU. The mean mass of the interstellar particles is 3 · 10^–13 g. The flux arrives from a direction which is compatible with the influx direction of the interstellar neutral Helium of 252° longitude and 5.2° latitude but it may deviate from this direction by 15 – 20°.     

Hard X-ray balloon observations of compact galactic and extragalactic X-ray sources

A balloon program in hard X-ray astronomy (20–200 keV) is jointly pursued by the Astronomisches Institut der Universität Tübingen (AIT) and the Max Planck-Institute für Extraterrestrische Physik in Garching (MPE). Since 1973 nine successful balloon flights have been performed from Texas and Australia. Here results on Centaurus A and on several galactic binary X-ray sources are summarized. In particular the high energy photon spectrum of Hercules X-1 and the evidence for the cyclotron line feature which was discovered by us in 1976 is reviewed.     

Tailored analyses of 24 Galactic WN stars

The fundamental properties of 24 Galactic WN stars are determined from analyses of their optical, UV and IR spectra using sophisticated model atmosphere codes (Hillier, 1987, 1990). Terminal velocities, stellar luminosities, temperatures, mass loss rates and abundances of hydrogen, helium, carbon, nitrogen and oxygen are determined. Stellar parameters are derived using diagnostic lines and interstellar reddenings found from fitting theoretical continua to observed energy distributions.Our results confirm that the parameters of WN stars span a large range in temperature (T_*=30–90,000 K), luminosity (log L_*/L=4.8–5.9), mass loss (M=0.9–12×10^–5 M yr^–1) and terminal velocity (v =630–3300 km s^–1). Hydrogen abundances are determined, and found to be low in WNEw and WNEs stars (<15% by mass) and considerable in most WNL stars (1–50%). Metal abundances are also determined with the nitrogen content found to lie in the range N/He=1–5×10^–3 (by number) for all subtypes, and C/N 0.02 in broad agreement with the predictions of Maeder (1991). Enhanced O/N and O/C is found for HD 104994 (WN3p) suggesting a peculiar evolutionary history. Our results suggest that single WNL+abs stars may represent an evolutionary stage immediately after the Of phase. Since some WNE stars exist with non-negligible hydrogen contents (e.g. WR136) evolution may proceed directly from WNL+abs to WNE in some cases, circumventing the luminous blue variable (LBV) or red supergiant (RSG) stage.     

Proton phase space densities (0.5eV<Ep<5MeV) at midlatitudes from Ulysses SWICS/HI-SCALE measurements

Proton phase space densities in the solar wind frame from suprathermal velocities 10 km s^–1 to 30,000 km s^–1 (0.5 eV–5 MeV) were derived from combined SWICS and HISCALE measurements when Ulysses was at 5 AU and –24° heliolatitude. The period (19–23 January 1993) encompasses a forward/reverse shock pair (20 January, 0500 UT and 22 January, 0300 UT). Strong evidence is found for shock acceleration of pickup protons from interstellar hydrogen at all energies measured.     

The Voyager infrared spectroscopy and radiometry investigation

The infrared investigation on Voyager uses two interferometers covering the spectral ranges 60–600 cm^–1 (17–170 m) and 1000–7000 cm^–1 (1.4–10 m), and a radiometer covering the range 8000–25 000 cm^–1 (0.4–1.2 m). Two spectral resolutions (approximately 6.5 and 2.0 cm^–1) are available for each of the interferometers. In the middle of the thermal channel (far infrared interferometer) the noise level is equivalent to the signal from a target at 50 K; in the middle of the reflected sunlight channel (near infrared interferometer) the noise level is equivalent to the signal from an object of albedo 0.2 at the distance of Uranus.For planets and satellites with substantial atmospheres, the data will be used to investigate cloud and gas composition (including isotopic ratios), haze scale height, atmospheric vertical thermal structure, local and planetary circulation and dynamics, and planetary energy balance. For satellites with tenuous atmospheres, data will be gathered on surface and atmospheric composition, surface temperature and thermal properties, local and global phase functions, and surface structure. For Saturn's rings, the composition and radial structure, particle size and thermal characteristics will be investigated. Comparative studies of the planets and their satellite systems will be carried out.Paris Observatory.Cornell University.Jet Propulsion Laboratory.University of Maryland.     

The einstein observatory medium sensitivity survey

Results are presented from an X-ray survey of 50 square degrees of the high galactic latitude sky at sensitivities in the range 7·10^–14 – 5·10^–12 erg/cm² sec (0·3–3·5 keV) carried out with the Imaging Proportional Counter (IPC) aboard the Einstein Observatory. The extragalactic sample consists of 48 sources which have been used to determine the number flux relation. The content of the sample is analyzed in terms of types of sources and is found to be significantly different from the content of similar samples selected at higher fluxes.     

Skylab measurements of low energy cosmic rays

In this review the present state of our knowledge on the properties of heavy ions in low energy cosmic rays measured in the Skylab mission and in other spacecrafts is summarised and the possible mechanisms of their origin are discussed. A brief review of the general features of the galactic and solar cosmic rays is given in order to understand the special features of the low energy heavy ions of cosmic rays. The results of the cosmic ray experiment in the Skylab show that in the low energy interval of 8–30 MeV/N, the abundances of oxygen, nitrogen, and neon ions, relative to carbon are enhanced by a factor of 5 to 2 as compared to high energy cosmic rays; while Mg, Si, S, and A are depleted. In 50–150 MeV/N energy interval the abundance of nuclei of Ca-Cr relative to iron-group (Z = 25–28) is found to be highly enhanced, as compared to high energy cosmic rays. Furthermore the observations of the energy spectra of O, N, and Ne ions and their fairly large fluences in the energy interval of 8–30 MeV/N below the geomagnetic cut off energy of 50 MeV/N for fully stripped nuclei at the Skylab orbit indicate that these heavy ions are probably in partly ionised states. Thus, it is found that the Skylab results represent a new type of heavy ion population of low energy cosmic rays below 50 MeV/N, in the near Earth space and their properties are distinctly different from those of high energy cosmic rays and are similar to those of the anomalous component in the interplanetary space. The available data from the Skylab can be understood at present on the hypothesis that low energy interplanetary cosmic ray ions of oxygen etc. occur in partly ionised state such as O⁺¹,O⁺², etc. and these reach the inner magnetosphere at high latitudes where stripping process occurs near mirror points and this leads to temporarily trapped ions such as O⁺³, O⁺⁴, etc. It is noted that the origin of these low energy heavy cosmic ray ions in the magnetosphere and in interplanetary space is not yet fully understood and new type of sources or processes are responsible for their origin and these need further studies.     

Possible shock wave in the local interstellar plasma, very close to the heliosphere

We show, using the HST — GHRS data on velocity and temperature in the nearby interstellar medium, that the observed 3 – 4 km s^–1 relative velocity between the Local Interstellar Cloud (LIC) and the so-called G-cloud located in the Galactic Center hemisphere can be quite naturally explained assuming that the two clouds do interact with each other. In the proposed interpretation the two media are separated by a (quasiperpendicular) MHD shock front propagating from the LIC into the G-cloud. The LIC plasma is then nothing else but the shocked (compression 1.3 – 1.4) gas of the G-cloud. A 1-D single-fluid solution of the Rankine — Hugoniot equations can fit the most probable observed values of the relative velocity (3.75 km/s), LIC (6700 K) and G-cloud (5400 K) kinetic temperatures, if the plasma-beta of the LIC plasma is in the range 1.3 – 1.5 (Table 1). This corresponds to a super — fast magnetosonic motion of the heliosphere through the LIC, independently of LIC density. The LIC magnetic field strength is 1.9 (3.1) G for the LIC electron density n_e = 0.04 (0.10) cm^–3. In this case the shock is less than 30 000 AU away and moves at about 10 km s^–1 relative to the LIC plasma. The Sun is chasing the shock and should catch up with it in about 10⁴ years. If the heliospheric VLP emissions cutoff at 1.8 kHz is indicative of n_e (LIC) = 0.04 cm^–3 (Gurnett et al., 1993), the (pure plasma) bowshock ahead of the heliopause could be the source of quasi-continuous heliospheric 2-kHz emission band. We believe that with the expected increase in the performance of modern spectroscopic instrumentation the proposed method of magnetic field evaluation may in the future find wider application in the studies of the interstellar medium.     

LISM structure — Fragmented superbubble shell?

Small scale structure in local interstellar matter (LISM) is considered. Overall morphology of the local cloud complex is inferred from Ca II absorption lines and observations of H I in white dwarf stars. Clouds with column densities ranging from 2–100 × 10¹⁷ cm^–2 are found within 20 pc of the Sun. Cold (50 K) dense (10⁵ cm^–3) small (5–10 au) clouds could be embedded and currently undetected in the upwind gas. The Sun appears to be embedded in a filament of gas with thickness 0.7 pc, and cross-wise column density 2 × 10¹⁷ cm^–2. The local magnetic field direction is parallel to the filament, suggesting that the physical process causing the filamentation is MHD related. Enhanced abundances of refractory elements and LISM kinematics indicate outflowing gas from the Scorpius-Centaurus Association. The local flow vector and Sco data are consistent with a 4,000,000 year old superbubble shell at –22 km s^–1, which is a shock front passing through preshock gas at –12 km s^–1, and yielding cooled postshock gas at –26 km s^–1in the upwind direction. A preshock magnetic field strength of 1.6 G, and postshock field strength of 5.2 G embedded in the superbubble shell, are consistent with the data.Abbreviations LISM Local ISM - SIC Surrounding Interstellar Cloud - LIC Local Interstellar Cloud     

High latitude electrodynamics: Observations from S3-2

The high spatial-temporal resolution of instrumentation on the polar-orbiting S3-2 satellite has allowed a wide variety of measurements of the electrodynamic characteristics of both large- and small-scale structures at high latitudes. Analyses of large scale features observed by S3-2 have shown that: (i) The IMF B _ydependence of polar cap convection, first observed in June 1969 by OGO-6 persists in other seasons. During periods of northward IMF B _zextensive regions of sunward convection may be found in the sunlit polar cap. (ii) In the dawn and dusk MLT sectors >90% of the region 1 currents lie equatorward of the convection reversal line. Potentials across the ionospheric projection of the low-latitude boundary layer are typically a few kV. (iii) The location of extra field-aligned currents, near the dayside cusp and poleward of the region 1 current sheet is dependent on the IMF B _ycomponent. (iv) Simultaneous observations by TRIAD and S3-2 show that sheets of field-aligned current extend uniformly for several hours in MLT, but may have an altitude dependence in the 1000–8000 km range. (v) During magnetic storms ionospheric irregularities occur in regions of poleward density gradients and downward field-aligned currents near the equatorward boundary of diffuse auroral precipitation. In the winter polar cap, density irregularities were also found in regions of highly structured electric fields and soft electron precipitation. (vi) During an intense magnetic storm the auroral zone height-integrated Pederson conductivity was calculated to be in the range 10–30 mho and downcoming energetic electron fluxes accounted for between 50% and 70% of the upward Birkeland currents.Analysis of small-scale structures (latitudinal width < 1°), observed by S3-2, have shown that: (i) Intense meridional electric fields (50–250 mV m^-1) generated by charge separation near the inner edge of the plasma sheet drive intense subauroral convection and are associated with field-aligned currents, on the order of 1–2 A m^-2. (ii) Case studies of discrete arcs in the auroral oval have shown that arcs are associated with pairs of small-scale, field-aligned currents embedded in the large-scale region 1/region 2 field-aligned current sheets. The maximum observed field-aligned current was an upward current of 135 A m^-2, confined to a latitudinal width of 2km and carried by field-aligned accelerated electrons. Return (downward) currents associated with arcs are limited to intensities of 10–15 A m^-2. At this limit the ionospheric plasma becomes marginally stable to the onset of ion-cyclotron turbulence. Two instances of plasma vortices, characteristic of auroral curls, have been observed in the region between the paired current sheets. (iii) Sun-aligned arcs in the polar cap are found in a region of negative electric field divergence, embedded in an irregular electric field pattern. The electrons producing the arcs have a temperature of 200 eV and have been accelerated through potential drops of 1 kV along the magnetic field. Return currents may appear on both sides of polar-cap arcs.     

Composition of anomalous cosmic rays and implications for the heliosphere

We use energy spectra of anomalous cosmic rays (ACRs) measured with the Cosmic Ray instrument on the Voyager 1 and 2 spacecraft during the period 1994/157-313 to determine several parameters of interest to heliospheric studies. We estimate that the strength of the solar wind termination shock is 2.42 (–0.08, +0.04). We determine the composition of ACRs by estimating their differential energy spectra at the shock and find the following abundance ratios: H/He = 5.6 (–0.5, ⁺0.6), C/He = 0.00048 ± 0.00011, N/He = 0.011 ± 0.001, O/He = 0.075 ± 0.006, and Ne/He = 0.0050 ± 0.0004. We correlate our observations with those of pickup ions to deduce that the long-term ionization rate of neutral nitrogen at 1 AU is 8.3 × 10^–7 s^–1 and that the charge-exchange cross section for neutral N and solar wind protons is 1.0 × 10^–15 cm² at 1.1 keV. We estimate that the neutral C/He ratio in the outer heliosphere is 1.8(–0.7, +0.9) × 10^–5. We also find that heavy ions are preferentially injected into the acceleration process at the termination shock.     

The flux of interstellar dust observed by Ulysses and Galileo

Interstellar dust detected by the dust sensor onboard Ulysses was first identified after the Jupiter flyby when the spacecraft's trajectory changed dramatically (Grün et al., 1994). Here we report on two years of Ulysses post-Jupiter data covering the range of ecliptic latitudes from 0° to –54° and distances from 5.4 to 3.2 AU. We find that, over this time period, the flux of interstellar dust particles with a mean mass of 3·10^–13 g stays nearly constant at about 1·10^–4, m^–2 s^–1 ( sr)^–1, with both ecliptic latitude and heliocentric distance.Also presented are 20 months of measurements from the identical dust sensor onboard the Galileo spacecraft which moved along an in-ecliptic orbit from 1.0 to 4.2 AU. From the impact direction and speeds of the measured dust particles we conclude that Galileo almost certainly sensed interstellar dust outside 2.8 AU; interstellar particles may also account for part of the flux seen between 1 and 2.8 AU.     

Observations of the solar wind from coronal holes

The solar wind emanating from coronal holes (CH) constitutes a quasi-stationary flow whose properties change only slowly with the evolution of the hole itself. Some of the properties of the wind from coronal holes depend on whether the source is a large polar coronal hole or a small near-equatorial hole. The speed of polar CH flows is usually between 700 and 800 km/s, whereas the speed from the small equatorial CH flows is generally lower and can be <400 km/s. At 1 AU, the average particle and energy fluxes from polar CH are 2.5×10⁸ cm^–2 sec^–1 and 2.0 erg cm^–2 s^–1. This particle flux is significantly less than the 4×10⁸ cm^–2 sec^–1 observed in the slow, interstream wind, but the energy fluxes are approximately the same. Both the particle and energy fluxes from small equatorial holes are somewhat smaller than the fluxes from the large polar coronal holes.Many of the properties of the wind from coronal holes can be explained, at least qualitatively, as being the result of the effect of the large flux of outward-propagating Alfvén waves observed in CH flows. The different ion species have roughly equal thermal speeds which are also close to the Alfvén speed. The velocity of heavy ions exceeds the proton velocity by the Alfvén speed, as if the heavy ions were surfing on the waves carried by the proton fluid.The elemental composition of the CH wind is less fractionated, having a smaller enhancement of elements with low first-ionization potentials than the interstream wind, the wind from coronal mass ejections, or solar energetic particles. There is also evidence of fine-structure in the ratio of the gas and magnetic pressures which maps back to a scale size of roughly 1° at the Sun, similar to some of the fine structures in coronal holes such as plumes, macrospicules, and the supergranulation.     

Emission mechanism for low-frequency radiation in the outer heliosphere

The question of how low-frequency radio emissions in the outer heliosphere might be generated is considered. It is argued that the free energy contained in an electron beam distribution is first transformed into electrostatic Langmuir waves. The nonlinear interactions of these waves which can produce electromagnetic waves are then treated in the semi-classical formalism. Comparison of the results of the discussed model with electromagnetic radiation coming from upstream of the Earth's bow shock shows that the model adequately explains the generation of plasma waves at planetary shocks. By analogy, this model can provide a quantitative explanation of intensity of radio emissions at 2 to 3 kHz detected by the Voyager plasma wave instrument in the outer heliosphere provided that the electron beams generating Langmuir waves exist also in the postshock plasma due to secondary shocks in the compressed solar wind beyond the termination shock. The field strength of Langmuir waves required to generate the second harmonic emissions are approximately of 100–200 V m^–1 for the primary and 50–100 V m^–1 for the secondary foreshocks. However, only in the secondary foreshock the expected density is consistent with the observed frequency.     

The Jovian magnetodisk

Magnetic field measurements made by the vector helium magnetometers on board Pioneers-10 and 11 reveal the existence of a current sheet (thickness 2R _J) carrying an eastward current. Self-consistent studies of the current sheet show that the magnitude of the current is of the order of 10⁺² Am⁺¹ and that the current is carried by a hot (T>1 keV) plasma, the density of which varies between 1 cm⁺³ at 30R _J to 10⁺² cm⁺³ at 80R _J. The current sheet is warped azimuthally and parallel to the magnetic dipole equator.The existence of an azimuthal field component indicates a poloidal plasma flow transporting some 10²⁹ ions per second from Jupiter into the outer magnetosphere. It is shown that, if the outer magnetosphere is in a steady state, this plasma must be transported outward within the current sheet by a diffusion process which is faster than the one responsible for particle transport in the inner magnetosphere but slower than Bohm diffusion. It is suggested that the diffusion is due to the observed mhd turbulence in the current sheet. Such a model requires the existence of open field lines along which particles can escape freely into interplanetary space.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.