Maximum likelihood estimation of the mean parallax and kinematic parameters of the Pleiades

A simultaneous, maximum-likelihood determination of the distance and kinematic parameters of the Pleiades is made. The results are: distance of the cluster d = 135.56 ± 0.72 pc, dispersion σ_d = 7.66 ± 0.80 pc; space velocity V = 25.94 ± 0.13 km/s, dispersion σ_v = 0.58 ± 0.09 km/s coordinates of the convergent point A = 101.95° ± 0.47°, D = −41.36° ± 0.29°.     

Tianjin photographic zenith tube observations

In this paper we analyse the observational data obtained by the Chinese-made PZT in the two periods 1979 Feb – 1980 May and 1981 Dec – 1983 March. The internal accuracy of single star is found to be m_u = ±13.0 ms, m_φ = ±0. “144 for the first period, and m_u = ±14.6 ms, m_φ = ±0.” 152 for the second. Correction of star position is found by the chain method. The systematic error caused by the sealed window of the evacuated chamber and the temperature effect of the plate scale are investigated. Monthly means of time and latitude are given.     

Galactic kinematics derived from classical cepheids

On the basis of radial velocity and Hipparcos proper motion data, we have analyzed the galactic kinematics of classical Cepheids. Using the 3-D Ogorodnikov-Milne model we have determined the rotational velocity of the Galaxy to be V₀ = 240.5 ± 10.2 km/s, on assuming a glactocentric distance of the Sun of R₀ = 8.5 kpc. The results clearly indicate a contracting motion in the solar neighbourhood of (∂V_θ∂θ)/R = −2.60 ± 1.07 km s⁻¹ kpc⁻¹, along the direction of galactic rotation. Possible reason for this motion is discussed. The solar motion found here is S = 18.78 ± 0.86 km/s in the direction l = 54.4° ± 2.9° and b = +26.6° ± 2.6°.     

Changes in thermospheric molecular oxygen abundance inferred from twilight 6300 A airglow

When the local solar zenith angle, χ_L, is < 105° the 6300 A line is much stronger than expected on the basis of F region ionic recombination alone. Between 95 and 105° the additional intensity is quantitatively explained by production of O(¹D) from photolysis of O₂ in the Schumann-Runge continuum, (λλ 1300–1750 A) using current values for solar flux, atmospheric composition and quenching of O(¹D) by N₂. The Schumann-Runge (SR) component exhibits a large seasonal variation with a maximum in summer. We interpret this variation as implying a seasonal change in thermospheric O₂ abundance; the change seems largely to reflect a variation in O₂ density at the base of the diffusive regime although some contribution may come from changes in thermospheric temperature structure. Large changes in the SR component exist from day to day and with a 27 day period following a major magnetic storm. The photodissociation source becomes inadequate when x_l < 95°; at 90° more than half of the intensity comes from still another source which we identify as local photoelectron excitation of O atoms.     

Gamma ray spectrum of the crab nebula in the multi TeV region

The THEMISTOCLE array of 18 Cherenkov detectors which has a 3 TeV gamma energy threshold, has detected a signal from the Crab nebula at a 5.8 standard deviation level. Information on the energy spectrum is obtained in the range 3–15 TeV. The integrated flux can be fitted with the form, Φ (> E) = (3.7 ± 0.5) × 10^-12 (E/5)-^{−1.5 ± 0.20} cm⁻² s⁻¹ (E in TeV) compatible with the extrapolation of results at lower energies. The Crab signal is used to measure the angular resolution of the multi-telescope technique. The value obtained is 2.3 mr (0.15°) in agreement with the results of simulations, and confirms the interest of this new method for multi-TeV gamma-ray detection.     

The cosmic ray energy spectrum between 10 and 10 eV

The energy spectrum of cosmic rays with primary energies between 10¹⁴ eV and 10¹⁶ eV has been studied with the CASA-MIA air shower array. The measured differential energy spectrum is a power law (dj/dE ∝ E^−y) with spectral indices γ of 2.66±0.02 below approximately 10¹⁵ eV and 3.00±0.05 above. A new method is used for measuring primary energy derived from ground-based data in a compositionally insensitive way. In contrast with some previous reports, the “knee” of the energy spectrum does not appear sharp, but rather a smooth transition over energies from 10¹⁵ eV to 3.0 × 10¹⁵ eV.     

The troposphere-stratosphere radiation field at twilight: A spherical model

Time-dependent calculations of trace constituent distributions require as input the dissociating radiation field as a function of altitude and solar zenith angle. An isotropic, spherical, multiple scattering model of the radiation field has been developed to determine the radiation field at twilight. Comparison of the spherical model with a plane parallel model at twilight shows that: (1) for solar zenith angles less than 92°, plane parallel solutions for the source function are suitable if the initial deposition of solar energy is calculated for a spherical atmosphere; (2) for solar zenith angles greater than 92°, the plane parallel radiation field can be several orders of magnitude smaller than that calculated with the spherical model; (3) at altitudes above 40 km and at all solar zenith angles, the spherical model predicts 10–20% less radiation than the radiation field calculated with the plane parallel model. Calculations of the rate of photodissociation of NO₂ in the troposphere and stratosphere show that the spherical model yields significantly higher values at solar zenith angles greater than 92°.     

Mangeto-optical effects in sunspots and their vector magnetic field

The system of transfer equations of the four Stokes parameters I, Q, U, V under the action of the magneto-optical effect (i.e. the Unno-Beckers equations) are numerically solved in this paper for the magneto-sensitive lines FeI λλ 6302.499 and 5324.191 using an appropriate sunspot model. The errors in the expressions for the coefficients _r and _W in Beckers' paper [2] have been corrected for. From the results of calculations, features of the profiles of the Stokes parameters dependent on the magnetic vector have been isolated. Our computations also show that the magneto-optical effect should be taken into consideration in the measurement of the vector magnetic fields.

In the fourth section of this paper we have established a simple and convenient method for obtaining-information on the magnetic vector (including the field strength B, its inclination to the line of sight γ and its azimuth χ) from the profiles of the Stokes parameters. It consists of three steps: (1) The value of B is determined from the distance of the highest point in the V-profile from the central line. (2) γ is then found from V_max, i.e maximum value of V. (3) Lastly, the angle χ is found from Q₀, i.e. the value of Q at line centre.     

The potential of the ground based arrays of imaging atmospheric Cherenkov telescopes. II. Gamma ray flux sensitivities

We discuss the capability of ‘100 GeV’ class imaging atmospheric Cherenkov telescope (IACT) arrays as future powerful instruments of ground-based gamma-ray astronomy. It is assumed that the array is gathered from individually triggered quadrangular 4-IACT ‘cells’ with a linear size of about 100 m. The multi-cell concept allows coverage of large detection areas economically, and at the same time the effective exploitation of the stereoscopic approach of determination of the shower parameters using information obtained by several IACTs simultaneously. Determination of arrival directions of γ-ray primaries on an event-by-event basis with accuracy δθ ≤ 0.1° combined with high suppression efficiency (at both the hardware and software levels) of the background hadronic showers by a factor of ≈ 10³, and large, up to 1 km² collection areas, can provide minimum detectable energy fluxes of ≥ 100 GeV γ-rays from point sources down to 10⁻¹³ erg/cm² s which is about 3 orders of magnitude lower than the current sensitivities achieved by the satellite-borne detectors at MeV and GeV energies. High sensitivities of multi-IACT arrays would partially compensate the limited efficiency of the technique for all-sky surveys, as well as allow study of moderately extended (≤ 1°) γ-ray sources. IACT arrays with minimum detectable fluence of ≥ 100 GeV γ-rays S_γ < 10⁻⁸ erg/cm² are well suited for effective exploration of highly sporadic nonthermal phenomena from different classes of astrophysical objects on time-scales from ≤ 1 s to several minutes.     

Analysis of the variation in inclination of Intercosmos 13 Rocket, 1975-22B

The orbit of Intercosmos 13 rocket (1975-22B) has been determined at 103 epochs between 30 April 1975 and 10 April 1980 from almost 7000 observations. One hundred and three values of inclination have been determined and corrections incoporated for the effects due to zonal harmonic, lunisolar and tesseral harmonic perturbations, precession, and solid Earth tides. The modified data have been analysed to yield values of the atmospheric rotation rate, Λ rev day⁻¹, viz. Λ = 0.94 ± 0.10 at an average height of 322 ± 6 km and Λ = 1.27 ± 0.02 at 288 km. Analysis of the inclination near 14th-order resonance has indicated lumped harmonic values 10^{9 1.0}_1.4 = − 76.13 ± 12.47, 10^{9 1,0}₁₄ = − 29.89 ± 32.64, 10^{9 −1.2}₁₄ = − 63.11 ± 15.44 10^{9 −1.2}₁₄ = − 32.52 ± 26.96, for inclination 82.952°.     

Galactic cosmic-ray anisotropy and its heliospheric modulation, inferred from the sidereal semidiurnal variations observed in the rigidity range 300–600 GV with multidirectional muon telescope at Sakashita underground station

The existence of sidereal semidiurnal variation of cosmic-ray intensity in a rigidity region 10²-10³ GV has been reported by many researchers, but there is no consensus of opinion on its origin. In this paper, using the observed semidiurnal variations in a rigidity range (300–600 GV) with 10 directional muon telescopes at Sakashita underground station (geog. lat. = 36°, long. = 138°E, DEPTH = 80 m.w.e.), the authors determine the magnitudes (η₁, η₂) and directions (a₁, a₂) of the first- and second-order anisotropies in the following galactic cosmic-ray intensity distribution (j)

jdp = j₀{1 + η₁P₁(cos χ₁) + η₂P₂(cos χ₂)}dp

, where P_nis the nth order spherical function and χ_n is the pitch angle of cosmic rays with respect to a_n. For the determination, the influence of cosmic-ray's heliomagnetospheric modulation, geomagnetic deflection and nuclear interaction with the terrestrial material and also of the geometric configuration of the telescopes are taken into account. Usually, the semidiurnal variation is produced by the second-order anisotropy. The present observation, however, requires also the first-order anisotropy which usually produces only the diurnal variation, but can produce also the semidiurnal variation as a result of the heliospheric modulation. The first- and second-order anisotropies are characterized with η₁) > 0 and η₂ < 0 have almost the same direction (a₁ a₂) specified by the right ascension ( 0.75 h) and declination (δ 50°S) and, therefore, they can be expressed, as a whole, by an axis-symmetric anisotropy of loss-cone type (i.e. deficit intensities in a cone). It is noteworthy that this anisotropy approximately coincides with that inferred from the air shower observation at Mt Norikura in the rigidity region 10⁴ GV.     

Limits on the TeV flux of diffuse gamma rays as measured with the HEGRA air shower array

Using data from the HEGRA air shower array, taken in the period from April 1998 to March 2000, upper limits on the ratio I_γ/I_CR of the diffuse photon flux I_γ to the hadronic cosmic ray flux I_CR are determined for the energy region 20–100 TeV. The analysis uses a gamma–hadron discrimination which is based on differences in the development of photon- and hadron-induced air showers after the shower maximum. A method which is sensitive only to the non-isotropic component of the diffuse photon flux yields an upper limit of I_γ/I_CR (at 54 TeV) <2.0×10⁻³ (at the 90% confidence level) for a sky region near the inner galaxy (20°< galactic longitude <60° and |galactic latitude |<5°). A method which is sensitive to both the isotropic and the non-isotropic component yields global upper limits of I_γ/I_CR (at 31 TeV) <1.2×10⁻² and I_γ/I_CR (at 53 TeV) <1.4×10⁻² (at the 90% confidence level).     

Optical identification and observational study of unidentified IRAS LRS sources. 2. Observations of sources with the silicate feature in the 13–24northern sky region

JHK photometry was carried out for some of the sources with the silicate feature in the sky region 13^h < < 24^h, 0° < δ < 66° and a discussion on several peculiar objects was made.     

Response time of the high-latitude dayside ionosphere to sudden changes in the north-south component of the IMF

The time scale of the response of the high-latitude dayside ionospheric flow to changes in the North-South component of the interplanetary magnetic field (IMF) has been investigated by examining the time delays between corresponding sudden changes. Approximately 40 h of simultaneous IMF and ionospheric flow data have been examined, obtained by the AMPTE-UKS and -IRM spacecraft and the EISCAT “Polar” experiment, respectively, in which 20 corresponding sudden changes have been identified. Ten of these changes were associated with southward turnings of the IMF, and 10 with northward turnings. It has been found that the corresponding flow changes occurred simultaneously over the whole of the “Polar” field-of-view, extending more than 2° in invariant latitude, and that the ionospheric response delay following northward turnings is the same as that following southward turnings, though the form of the response is different in the two cases. The shortest response time, 5.5 ± 3.2 min, is found in the early- to mid-afternoon sector, increasing to 9.5 ± 3.0 min in the mid-morning sector, and to 9.5 ± 3.1 min near to dusk. These times represent the delays in the appearance of perturbed flows in the “Polar” field-of-view following the arrival of IMF changes at the subsolar magnetopause. Overall, the results agree very well with those derived by Etemadi et al. (1988, Planet. Space Sci. 36, 471) from a general cross-correlation analysis of the IMF B_z and “Polar” beam-swinging vector flow data.     

Neutrino-induced and atmospheric single-muon fluxes measured over five decades of intensity by LVD at Gran Sasso Laboratory

We report data taken by the LVD Experiment during a live-time period of 11 556 h. We have measured the muon intensity at slant depths of standard rock from about 3000 hg/cm² to about 20 000 hg/cm². This is an exclusive study, namely our data include only events containing single muons. This interval of slant depth extends into the region where the dominant source of underground muons seen by LVD is the interaction of atmospheric neutrinos with the rock surrounding LVD. The interesting result is that this flux is independent of slant depth beyond a slant depth of about 14 000 hg/cm² of standard rock. Due to the unique topology of the Gran Sasso Laboratory the muons beyond about 14 000 hg/cm² of standard rock are at a zenithal angle near 90°. Hence we have, for this fixed angle, a muon flux which is independent of slant depth. This is direct evidence that this flux is due to atmospheric neutrinos interacting in the rock surrounding LVD. The value of this flux near 90° is (8.3 ± 2.6) × 10⁻¹³ cm⁻² s⁻¹ sr⁻¹, which is the first reported measurement at a zenithal angle near 90° and for slant depths between 14 000 and 20 000 hg/cm². Our data cover over five decades of vertical intensity, and can be fit with just three parameters over the full range of our experiment. This is the first time a single experiment reports the parameters of a fit made to the vertical intensity over such a large range of standard rock slant depth. The results are compared with a Monte Carlo simulation which has as one of the two free parameters γ_πκ, the power index of the differential energy spectrum of the pions and kaons in the atmosphere. This comparison yields a value of 2.75 ± 0.03 for γ_πκ, where the error includes the systematic uncertainties. Our data are compared to other measurements made in our slant depth interval. We also report the value of the muon flux in Gran Sasso at θ = 90° as a function of the azimuthal angle.     

Generation mechanism of Pc3 magnetic pulsations at very low latitudes

Polarization properties of Pc3 magnetic pulsations at very low latitudes cannot be explained by existing theories which are based on the field line resonance model, because magnetic field lines at ¦Φ¦ < 22° are almost entirely in the ionosphere. In order to interpret Pc3 polarization characteristics observed at very low latitudes (¦Φ¦ < 20°), I would like to propose a possible, new qualitative model in which two superimposed ionospheric eddy currents, oscillating with slight differences in frequency in the Pc3 range and in azimuthal wave number, move azimuthally at very low latitudes. The equatorial ionospheric Pedersen eddy currents are believed to be predominantly caused by inductive electric fields of compressional Pc3 source waves which may possibly arrive in the equatorial ionosphere from the outer magnetosphere.     

Average high latitude magnetic field: Variation with interplanetary sector and with season—II : Comparison of disturbance levels and discussion of ionospheric currents

Average high latitude magnetic field data from northern observatories are examined for three ranges of magnetic disturbance level, Kp = 1− to 1+,2− to 3+ and ≥ 4−. Except for 0–8^h MLT, 55–78° invariant latitude, during away interplanetary magnetic field sectors, the variations between season and sector have the same characteristics at all Kp ranges. Because the amplitude of sector differences is much larger at sunlit local times than in the midnight sector, it is concluded that the current system of Svalgaard (1973) is not adequate to describe the sector variations in magnetic disturbance. Other current systems are discussed briefly. The disturbance morphology and seasonal variation at all Kp levels confirms the results of previous studies which indicate that latitudinally broad current systems, like S_q^p and nonionospheric sources are present in addition to latitudinally narrow electrojet currents. Comparison of data between Kp levels indicates that the Harang discontinuity shifts toward earlier MLT with increasing Kp level.     

Measurement of the absolute proper motion of M3 and its space motion

We have measured the absolute proper motions of 534 stars in a 1.5° × 1.5° region around the cluster M3, using 14 plates taken with a 40-cm refractor spanning 80 years. 24 stars in the ACT catalogue were used to define the reference frame and the reduction was made using the central overlapping technique. Using the new data, the membership probabilities were redetermined. The mean absolute motion of the cluster was found to be −0.3 ± 0.3 mas/yr in R.A., and −3.1 ± 0.3 mas/yr in declination. Combining the present data with the known distance and radial velocity, the three-dimensional galactic orbit of M3 was calculated for Allen's galactic potential.     

A theoretical study of the direction and polarization of solar radio millisecond spike radiation

We investigated the angular direction and polarization of the solar radio millisecond spike emission in the model in which the spike emission is due to the second harmonic instability modes driven by electron cyclotron maser of loss cone distributed electrons during the propagation of a nonlinear plasma density wave near the magnetic mirror. We found that, when the angle θ between the wave vector and the magnetic field is > 60 °, the emission is in 100% X-mode polarization; when 40 ° < θ<60 °, the emission is in 100% O-mode polarization provided the amplitude of the density wave is below a certain limit; above that limit, the polarization will fall from 100% O-mode to even the X-mode. We also found that only 0.1% of the free energy of energy carrying electrons in the source region is converted into radiation wave energy.     

Statistical analysis of correlation between the positions of quasars and the uppsala galaxies

The question of the association of quasars with galaxies is re-examined using 785 quasars at |b| 30, δ − 02°30′ in the new Hewitt and Burbidge Catalog (1980) and all galaxies in the “Uppsala General Catalogue of Galaxies” (Nilson, 1973). The results of the two-point cross-correlation function are presented and they show that there are on the average 0.153 ± 0.011 more galaxies within 10'.0 of a QSO than would be expected if the QSOs were distributed randomly. We find the marginal significance of the tendency for correlation between QSOs and galaxies to increase with increasing redshifts z or apparent magnitudes V. The nearest neighbor test has also been taken to analyse the same data as well as get further evidence for the apparent association between QSOs and galaxies.