First results of plasma and neutral gas measurements from Vega-1/2 near comet Halley

Based on the ion, electron and neutral gas observations, performed by five of the six sensors comprising the PLASMAG-1 experiment on board VEGA-1 and -2, the following results are discussed: (1) the existence of the bow shock and its location at 1.1×10⁶ km for VEGA-1 inbound; (2) the existence of a cometopause and its location at 1.6×10⁵ km for VEGA-2 inbound; (3) the plasma dynamical processes occurring inside the cometosheath; (4) the phenomena taking place within the cometary plasma region including mass-spectroscopy of cometary ions at distances 1.5×10⁴ km; (5) the existence of keV electrons near closest approach to the nucleus; and (6) the radial dependence of the cometary neutral gas and the comparison with model calculations, yielding a mean ionization scale length of 2×10⁶ km and an overall production rate of 1.3×10³⁰ molecules s⁻¹ for VEGA-1 inbound. The results are also discussed in the context of the other, both remote and in-situ, observations, performed on board the VEGA- and GIOTTO-spacecraft.     

High spectral resolution Fabry-Perot interferometer measurements of comet Halley at H-alpha and 6300 Å

A 40.6 cm Newtonian telescope has been interfaced to the Fabry-Perot interferometer at the Arecibo Observatory to make high spectral resolution measurements of Comet Halley emissions at 6562.72 Å (H-alpha) and 6300.3 Å (OI). In March 1986 the H-alpha surface brightness for a 5′.9 field of view centered on the comet nucleus decreased from 39±7.8 rayleighs on 12 March to 16±3.8 rayleighs on 23 March. The atomic hydrogen production rate on 12 March 1986 was 1.62±0.5 × 10³⁰ s⁻¹, and on 23 March 1986 it was 6.76±2.3 × 10²⁹ s⁻¹. Using spectral resolution of 0.196 Å, we found the atomic hydrogen outflow velocity to be approximately 7.9±1.0 km s⁻¹. In general, the H-alpha spectra are highly structured, and indicative of a multiple component atomic hydrogen velocity distribution. An isotropic outflow of atomic hydrogen at various velocities is not adequate to explain the spectra measured at H-alpha. The 6300.3 Å emission of O(1D) had a surface brightness of 81±16 rayleighs on 15 March 1986, and 95±11 rayleighs on 17 March 1986. After adjustment for atmospheric extinction, the implied O(1D) production rate on 15 March is 6.44±3.0 × 10²⁸ s⁻¹, and the production rate on 17 March is 5.66±2.7 × 10²⁸ s⁻¹. These spectra included a feature at 6300.8 Å that we attribute to NH₂. The brightness of this emission feature was 37±11 rayleighs on 15 March.     

Study of the luminosity-dependent cyclotron resonance energies of the binary X-ray pulsar 4U 0115+63 with RXTE

We report on the luminosity dependent change of the cyclotron resonance energy obtained from a transient X-ray pulsar, 4U 0115+63. Using RXTE data observed on 1999 March, we found that the fundamental resonance energy stayed constant (11 keV) when the source luminosity was above 5 × 10³⁷ erg s⁻¹. As the luminosity decreased below 5 × 10³⁷ erg s⁻¹, the fundamental resonance energy gradually increased up to 16 keV at 0.16 × 10³⁷ erg s⁻¹. The luminosity dependence of the resonance energy can be understood by the change of the accretion column height.     

S3 and S4 absorption cross sections in the range of 340 to 600 nm and evaluation of the S3 abundance in the lower atmosphere of Venus

S₃ absorption cross section equals 6×10⁻¹⁷ cm² at 400 nm, 6 × 10⁻¹⁹ cm² at 500 nm (less by a factor of 4 than that given by Sanko), 4×10⁻²⁰ cm² at 600 nm. That of S₄ equals 1.5 × 10⁻¹⁷ cm² at 450 nm, 8 × 10⁻¹⁷ cm² at 500 nm, and 4.7 × 10⁻¹⁷ cm² at 600 nm. Preliminary evaluation of the S₃ mixing ratio in the lower atmosphere of Venus is (8±3)×10⁻¹¹ at 5 to 25km according to the Venera 14 measurements and several times lower at the locations of the Veneras-11 and -13.     

Near infrared observations of comet Halley from Vega 2

Spatial distribution of the continuum radiation in the range of 0.95–1.9 μm presumes total dust production rate of the comet of 10ρ tonne s⁻¹ (ρ is the dust material density) and its angular distribution proportional cos . Observations of the water vapor band at 1.38 μ m reveal strong jets, their time shift from the dust jet measured in situ is consistent with gas velocity of 0.82±0.1 km s⁻¹ and dust velocity of 0.55±0.08 km s⁻¹. The OH vibrational-rotational bands observed are excided directly via photolysis of water vapor. Water vapor production rate deduced from the H₂O band and OH band intensities is 8×10²⁹ s⁻¹. Intensity of the CN(0,0) band result in the CN column density of 9×10¹² cm⁻², i.e. larger by a factor of 3 than given by the violet band.     

Energy content of accelerated electrons and ions in intense solar flares

The energy content of nonthermal particles in solar flares is shared between accelerated electrons and ions. It isimportant for understanding the particle acceleration mechanism in solar flares. Yohkoh observed a few intense flares which produced both strong gamma-ray lines and electron bremsstrahlung continuum. We analyze energy spectra of X-class solar flares on October 27, 1991(X6.1), November 6, 1997 (X9.4), July 14, 2000 (X5.7) and November 24, 2000 (X2.3). The accelerated electron and proton spectra are derived from a spectral analysis of their high-energy photon emission and the energy contents in >1 MeV electrons and >10 MeV protons are estimated to be 6×l0²⁸ – 4×10³⁰ and 2×10²⁸ – 5×10²⁹ erg, respectively. We study the flare to flare variation in the energy content of >1 MeV electrons and >10 MeV protons for the four Yohkoh gamma-ray flares. Ratios of >1 MeV electron energy content to >10 MeV proton energy content are roughly within an order of magnitude.     

ROSAT PSPC observations of two starburst galaxies

We present results from ROSAT observations of NGC 1808 and NGC 2903. Exposures of 10 ksec each with the PSPC detector show X-ray sources at the central positions of both galaxies which are classified as nuclear starburst galaxies. Both targets, NGC 1808 and NGC 2903 appear slightly extended in X-ray maps in the energy band 0.1–2.4 keV. The X-ray spectrum of NGC 1808 shows almost complete absorption below 0.5 keV, indicating an extremely high hydrogen column density towards that source (N_H ≈ 8 × 10²¹cm⁻² resulting from model fits on the PSPC spectrum). In case of NGC 2903, the number of counts in the ROSAT band is significantly lower than expected from a previous EINSTEIN investigation of the source.     

Meteorites in Antarctica — Statistics on falls, concentration, recovery and alteration on ice-sheet

The Antarctic meteorites are distributed on the blue-ice area surfaces in the ablation zone of the Antarctic ice-sheet, to where meteorites have been transported by the ice-flow within the ice-sheet from the wider accumulation zone. Among the Antarctic meteorite collection H- and L-chondrites are most abundant; this is also true in the non-Antarctic meteorite collection. Meteorite showers also are involved in the collection. Several new types of stony meteorites have been discovered from the Antarctic meteorite collection. The mass and shape of Antarctic meteorites are in agreement with those of resultant fragments of high-speed impact basaltic rocks. In Antarctica, small fragments of meteorite smaller than 1 kg in weight can easily be found and collected. The solidification and the gas retention ages of Antarctic meteorites also are concentrated around 4.5×10⁹ years, but some of them are considerably younger. Their cosmic-ray exposure ages are extended up to 9×10⁶ years and their terrestrial ages are 9×10⁴-7×10⁵ years.     

Imaging the solar corona in the EUV

The SOHO (SOlar and Heliospheric Observatory) satellite was launched on December 2nd 1995. After arriving at the Earth-Sun (L1) Lagrangian point on February 14th 1996, it began to continuously observe the Sun. As one of the instruments onboard SOHO, the EIT (Extreme ultraviolet Imaging Telescope) images the Sun's corona in 4 EUV wavelengths. The He II filter at 304 Å images the chromosphere and the base of the transition region at a temperature of 5 − 8 × 10⁴ K; the Fe IX–X filter at 171 Å images the corona at a temperature of 1.3 × 10⁶ K; the Fe XII filter at 195 Å images the quiet corona outside coronal holes at a temperature of 1.6 × 10⁶ K; and the Fe XV filter at 284 Å images active regions with a temperature of 2.0 × 10⁶ K. About 5000 images have been obtained up to the present. In this paper, we describe also some aspects of the telescope and the detector performance for application in the observations. Images and movies of all the wavelengths allow a look at different phenomena present in the Sun's corona, and in particular, magnetic field reconnection.     

High resolution X-ray spectrum of AM Her

We present the analysis of archival Chandra high resolution X-ray spectra of AM Her. Emission lines from several hydrogen-like ions, helium-like ions, Fe-L shell transitions and Fe-K fluorescent are identified. Using the resonance, intercombination and forbidden lines of the few prominent helium-like ions, we infer a density greater than 2 × 10¹² cm⁻³ and a temperature of 2 MK for the oxygen and neon line emitting regions in the accretion column of AM Her.     

Validation of GOME (ERS-2) NO2 vertical column data with ground-based measurements at Issyk-Kul (Kyrgyzstan)

Here we present the results of comparison between operational NO₂ vertical column data by Global Ozone Monitoring Experiment (GOME) onboard ERS-2 satellite and ground-based measurements at Issyk-Kul station in Kyrgyzstan, northern Tien Shan. The data of GOME, taken for the period of 1996–2002, was found to be reasonably close to the results of ground-based sunrise measurements. The latter were adjusted to the time of GOME overpass nearby noon, providing direct comparison between satellite and ground-based data. According to the results, there is 0.6 × 10¹⁵ mol/cm² (18%) overestimation of NO₂ vertical column by GOME, compared to our ground-based data.     

Interplanetary dust interaction with comets: Production of x-rays

Recent results of theoretical investigations related to generation of high-energy (0.1-1 keV) photons in comets due to production of high-temperature (3×10⁵-10⁷ K) plasma clots from collisions of cometary and interplanetary grains at high relative velocities (70-700 km s⁻¹ at heliocentric distances R=0.01-1 AU) are summarized and main features of the process are marked.     

Comparison of OClO nadir measurements from SCIAMACHY and GOME

The scanning imaging absorption spectrometer for atmospheric chartography was launched successfully onboard ENVISAT on March 1, 2002. It observes the solar radiation transmitted and backscattered from the atmosphere and reflected from the ground in nadir, limb and occultation viewing modes. Chlorine dioxide (OClO), an important indicator for stratospheric chlorine activation, can be measured in the UV spectral range by differential optical absorption spectroscopy (DOAS).

First results of the DOAS retrieval of OClO slant column densities from the SCIAMACHY nadir measurements are presented and compared to measurements of the global ozone monitoring experiment (GOME), which has successfully measured OClO since 1995. While SCIAMACHY operates in the same orbit, it measures ≈30 min earlier than GOME and has an increased spatial resolution (30 × 60 km² compared to 40 × 320 km² for GOME).     

Revised upper limit on the internal magnetic moment of Venus

Over four Venus years of low altitude nightside PVO magnetometer observations are used to establish a new upper limit for the magnetic moment of Venus. Improvements over previous studies include data coverage and new instrument calibration information. The upper limit on an internal dipole moment is determined to be 8.4 × 10¹⁰ T m³.     

Mesospheric nitric oxide and ozone measurements in polar winter at 69°N

Two rocket experiments were carried out just before and after the polar night at Andoya (69°N), Norway to investigate transport of nitric oxide produced by auroral processes into the middle atmosphere and its influence on the ozone chemistry. Nitric oxide densities of (2–5) × 10⁸cm⁻³ found in the 70–90 km region are one to two orders of magnitude larger than those at middle latitudes. The influence on ozone densities in the 70–90 km region due to such enhanced nitric oxide abundance is found to be insignificant as compared to that due to transport in the middle of February. The larger ozone densities found in February (in spite of longer sunlit duration) than in November in the 40–60 km region again support predominance of transport over photochemical loss.     

Cometary ion observations at and within the cometopause-region of comet Halley

Three distinct boundaries are identified from the PICCA cometary ion observations within the innermost part of the coma of comet Halley: (1) the 'cometopause' at a cometocentric distance R_c 1.5×10⁵ km, characterized by the appearance of water-group ions well above background; (2) the 'cold cometary plasma boundary' at R_c 3×10⁴ km, characterized by a sudden and simultaneous decrease in the temperatures of all cometary ions, and (3) the 'ionopause' at R_c 6000 km, characterized by a fast decrease in the intensity of all cometary ions by a factor 3–5. Between the first two boundaries only ions with masses less than 50 amu are present, showing distinct maximum intensities at 18, 32 and 44 amu at the second boundary. Downstream of the second boundary also ions of mass 12, 64, 76, 86 and 100 amu are detected.     

Rocket observation of the ultraviolet spectrum of comet Halley

An ultraviolet sounding rocket telescope/spectrograph experiment observed Comet Halley on 26 February 1986, 17 days after perihelion. From the long-slit spectra, the production rates of O, C, and CO are calculated. The derived water production rate is a lower limit of 5.0 × 10²⁹ s⁻¹ and the volume mixing ratio of CO to H₂O is 21%. The predicted brightness distribution from a radial outflow model with H₂O and CO as parent molecules are in accordance with the measured spatial profiles of OI and CO emissions. The ratio of the production rates of CO to C is 2.7 which is consistent with the carbon source being the photodissociation of CO. However, the radial outflow model which best fits the CO data predicts significantly weaker CI emissions than was observed. A better fit to the carbon data is found when an inner coma source of C at a rate of 3% of the water production rate is included in the model.     

VEGA three-channel spectrometer experiment: Spectroscopy of comet Halley in the visible and near-ultraviolet ranges

A total of 3600 spectra of Comet Halley in the 275–710 nm were obtained on March, 8, 9, 10 and 11, 1986, from the VEGA 2 spacecraft. The emissions of OH, NH, CN, C₃, CH, C₂, NH₂ and H₂O⁺ are identified. From the OH intensity in the (0,0) band: 1.1 Megarayleigh at 5400 km from the nucleus, it can be inferred that the OH production rate was (1.4 ± 0.5)×10³⁰ molecules s⁻¹. The NH, C₃, CH and NH₂ bands became comparatively more intense at distances from the nucleus shorter than 3000km. At 06:40 U.T. when the instrument field of view was 6000×4500 km, two jets were observed. Spectra from the jets show significant differences with other spectra. Inside a jet NH, C₃ and NH₂ are comparatively more intense and the rotational distributions of OH, CN and C₂ are strongly distorted. This shows that part of the observed emissions probably comes from radicals directly produced in the excited state during the initial process of photolysis of the parent molecules.     

Energetic particles in the comet Halley environment

An overview is presented of electrons, protons and heavier ions (E > 20 keV) recorded by the energetic particle detector EPONA in the Comet Halley environment, 12–15 March, 1986. Pick-up ions were detected at distances of up to at least 7.5 × 10⁶ km from the nucleus. Estimates of the energies that typical cometary ions may be expected to acquire from the solar wind pertaining at Encounter show that the pick-up process is insufficient to account for the energies of the particles detected. An additional mechanism must thus be postulated to account for the observed particle signatures. Preliminary correlations with magnetic and plasma wave data from other instruments suggest that the presence of MHD turbulence at several million kilometers upstream of the bowshock may have contributed to the acceleration of the first pick-up ions observed. The bowshock boundary (inbound) does not appear to have constituted a location where particle acceleration to high energies took place. Downstream of the shock boundary, hardening of the energy spectrum and the development of less anisotropic particle streaming was observed to occur when the spacecraft was in a turbulent environment 1 × 10⁶ km from the nucleus. The waxing influence of mass loading as a mechanism for reducing energetic particle fluxes as well as the depletion of energetic ions due to their escape along open field lines and to charge exchange collision with neutrals in a progressively more stagnant solar wind, may be inferred in a regime (seen on the magnetometer data to be largely non-turbulent) traversed by the spacecraft from 5 × 10⁵ km from the nucleus to within the magnetic pile-up region. A major burst of ions and electrons (not yet established to be of cometary origin) occurred when the spacecraft was close to the Contact Surface. A population of high energy electrons (from 180 keV to at least 300 keV) was detected for about one hour before Closest Approach and for several hours thereafter. Also an energetic beam of electrons was identified exiting from a location at about 1 × 10⁶ km from the nucleus (outbound). Finally, differences between inbound and outbound particle signatures are described.     

The luminosity dependence of the phase-averaged X-ray spectrum of the transient 42 second pulsar EXO 2030+375

We report on the evolution in the X-ray spectrum of the transient X-ray pulsar EXO 2030+375 during part of an outburst in 1985 May–August. The overall continuum spectral shape is similar to that of other accreting pulsars and can be represented by a power-law spectrum modified at low energies by significant absorption and at high-energies either by an exponential cut-off or by the effects of cyclotron scattering. As the luminosity decreased by a factor 100, the X-ray spectrum became harder with the photon index decreasing from 1.83 ± 0.01 to 1.29 ± 0.01. In addition, the high-energy cutoff decreased from 20 to 10 keV during the same interval. If the cutoff is interpreted in terms of cyclotron resonance scattering, then this implies a magnetic field strength that decreased from 2.6 × 10¹² G to 1.3 × 10¹² G. This variation implies that the cutoff energy does not provide a reliable measure of the surface magnetic field strength in this system.