A method of analysing the emission lines of solar prominences

A new method of analysing the emission spectrum of solar prominences is presented, in which the source function is allowed to vary with optical depth. Least-squares fitting of the observed profile determines simultaneously the optical depth τ₀, the Doppler width Δλ_D and the factor characterising the variation of the source function. This method is applied to the early Balmer lines in ten prominences of Ref. [1]. The results show that the source function of the self-reversed H line increases towards the centre of the prominence, the value at the centre is 1.2–2.5 times the value at the edge. Neglect of this variation will give too large values of τ₀. The degree of attenuation by selfabsorption also depends on this variation. Discussion of the variation gives support to the view that the main exciting mechanism in solar prominences is the scattering of the incident radiation.     

Constraining the power spectrum using clusters

We analyze an extended redshift sample of Abell/ACO clusters and compare the results with those coming from numerical simulations of the cluster distribution, based on the truncated Zel'dovich approximation (TZA), for a list of eleven dark matter (DM) models. For each model we run several realizations, so that we generate a set of 48 independent mock Abell/ACO cluster samples per model, on which we estimate cosmic variance effects. Other than the standard CDM model, we consider (a) Ω₀ = 1 CDM models based on lowering the Hubble parameter and/or on tilting the primordial spectrum; (b) Ω₀ = 1 Cold + Hot DM models with 0.1 ≤Ω_ν ≤0.5; (c) low-density flat ΛCDM models with 0.3 ≤Ω₀ ≤0.5. We compare real and simulated cluster distributions by analysing correlation statistics, the probability density function, and supercluster properties from percolation analysis. We introduce a generalized definition of the spectrum shape parameter Γ in terms of σ₂₅/σ₈, where σ_ris the rms fluctuation amplitude within a sphere of radius r. As a general result, we find that the distribution of galaxy clusters provides a constraint only on the shape of the power spectrum, but not on its amplitude: a shape parameter 0.18 Γ 0.25 and an effective spectral index at the 20 h⁻¹ Mpc scale −1.1 n_eff −0.9 are required by the Abell/ACO data. In order to obtain complementary constraints on the spectrum amplitude, we consider the cluster abundance as estimated using the Press-Schechter approach, whose reliability is explicitly tested against N-body simulations. By combining results from the analysis of the distribution and the abundance of clusters we conclude that, of the cosmological models considered here, the only viable models are either Cold + Hot DM ones with 0.2 Ω_ν 0.3, better if shared between two massive ν species, and ΛCDM ones with 0.3 Ω₀0.5.     

Discussion of trignometric parallaxes determined along right ascension and declination

We discuss the data of trignometric parallaxes determined along and δ published by the US Naval Observatory and Van Vleck Observatory. We found that the absolute difference δπ = π_x−π_y shows a very similar periodic variation with for both observations. This variation is shown to be due to errors in π_y and to be related to the current practice of maximising the parallax factor for parallax determination in right ascension. Therefore, if possible, we should increase observations where the parallax factor in declination is maximised.     

A correct algorithm for the proper motion of cluster members

In this paper we give, for the case where the proper motions of stars are unknown, a method of calculating the correct relative proper motion, the linear model being assumed always. We also show that the proper motion usually found on assuming Σxμ_x = Σyμ_x = 0, Σxμ_y = Σy_y = 0, Σμ_x = 0, Σμ_y = 0, is not the relative proper motion, and that the difference between the two depends on the positions of the reference stars on the plate.     

Cyclotron amplification of geomagnetic micropulsations Pc1 in the magnetosphere

A numerical analysis of cyclotron instabilities is carried out by computing the dispersion relation for a three component cold plasma-beam system. Rates of growth and damping for various values of the stream density are calculated from the dispersion relation. The rates of growth and damping increase monotonically as the number density of the proton stream increases. It is found that the frequencies at the rates of maximum growth and the damping decrease slightly to lower frequencies and a sharp peak at these frequencies becomes blunt. The minimum e-folding times of an ion cyclotron wave for (a) σ_s = 10⁻⁴, σ_i = 10⁻² and (b) σ_s = 10⁻¹, σ_i = 10⁻² are about 3·84 and 0·16 sec respectively in the vicinity of the equatorial plane at 6 Re, where σ_s and σ_i are the ratios of the beam density N_s and the helium ion (H₆⁺) density N_i to the total positive ions in the plasma-beam system.     

Determination of physical parameters of NGC 2023 using the hyperfine NH3 spectrum

An improved approach is given for deriving the physical parameters of a molecular cloud by its NH₃ rotation-inversion hyperfine spectra. The optical depth τ₀(1, 1) of NH₃ (1,1) is obtained by considering the blending effect of the magnetic hyperfine spectral lines, the (1-1) excitation temperature T_ex (1, 1) is calculated by a two level model, the effect of different collision rate and the thermalization of the (1, 1) inversion lines are discussed, the rotation temperature T_R ( 2, 1) between the NH₃ levels (2, 2) and (1, 1) and the column density N( 1, 1) of the NH₃ (1, 1) inversion level are derived, the results of the total column density from different assumptions for the abundance ratio of ortho-NH₃ and para-NH₃ are also discussed.

This approach is used for the molecular cloud NGC 2023 by using the relevant observed data and its optical depth and other physical parameters are obtained.     

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.     

An Observational Study of the Physical Properties of Star-forming Molecular Cores

We used the 13.7 m millimeter-wave telescope of the Qinghai Station of Purple Mountain Observatory at Delingha to carry out simultaneous mapping observations in the ¹²CO(J = 1 − 0), ¹³CO(J = 1 − 0) and CO¹⁸ (J = 1 − 0) lines towards 11 isolated star-forming molecular cores selected from the sample of Spitzer's c2d program. For all three lines, these sources were all observed to the positions of half the peak intensity of the ¹³CO(J = 1 − 0) line. The volume density, local thermodynamic equilibrium mass M_LTE and virial mass M_VIR were derived for each of the molecular cores. The obtained ratio of M_VIR to M_LTE is 0.85 ± 0.40 for the ¹³CO(J = 1 − 0) cores and 0.77 ± 0.35 for the CO¹⁸ (J = 1 − 0) cores. The density profiles of the cloud cores were also calculated.     

Rotation measurement of 18 B-type binaries

Using the HeI 5857 A line observed with a Reticon at the conde focus of the 2.1-metre reflector of McDonald Observatory, we measured the Vsini value for 18 binaries of spectral type B by fitting the observed profile to that of a standard star convolved with Gray's rotational broadening function.

Our results for the two components of ξ² CrB and the primary of 25 Ser are less than half the catalogue values. The profiles of U CrB and v Sco have a narrow central absorption and very wide wings, and our measured value of 75 km/s for ν Sco, based on the central absorption, is much smaller than the catalogue value of 210 km/s. The profile of AR Cas was found to change with phase.

Our values agree with Slettebak's new system better than with his old system.     

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).     

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.     

Separation of gamma and hadron initiated air showers with energies between 20 and 500 TeV

The discrimination between air showers initiated by γ rays and by hadrons is one of the fundamental problems in experimental cosmic-ray physics. The physics of this ‘γ/hadron separation’ is discussed in this paper. We restrict ourselves to the energy range from about 20 to 500 TeV, and take only the information contained in the lateral Čerenkov light distribution and the number of electrons at the detector level into consideration. An understanding of the differences between air showers generated by γ rays and those due to hadrons leads us to formulate suitable observables for the separation process. Angle integrating Čerenkov arrays (AICA) offer a promising new approach to ground-based γ-ray astronomy in the energy region from about 20 to 500 TeV. In order to establish this technique, an efficient suppression of the overwhelming hadronic background radiation is required. As an example for our general discussion, we present one method for γ/hadron separation in AICAs called ‘LES’. It is based on the simultaneous determination of the shower size and some characteristic parameters of the lateral distribution of the Čerenkov light. The potential inherent within this technique is demonstrated in quantitative detail for the existing ‘AIROBICC’ AICA. We also propose an objective measure of the intrinsic sensitivity of a detection scheme in ground-based γ-ray astronomy, the ‘reduced quality factor’. It is shown that AICAs may reach a sensitivity to γ-ray point sources in the high VHE range similar to that of the Čerenkov-telescope imaging technique in the low VHE region.     

Density profiles of dark matter halos with anisotropic velocity tensors

We present density profiles, that are solutions of the spherical Jeans equation, derived under the following two assumptions: (i) the coarse grained phase-density follows a power-law of radius, ρ/σ³ ∝ r⁻, and (ii) the velocity anisotropy parameter is given by the relation β_a(r)=β₁+2β₂ (r/r_*)/[1+(r/r_*)²] where β₁, β₂ are parameters and r_* equals twice the virial radius, r_vir, of the system. These assumptions are well motivated by the results of N-body simulations. Density profiles have increasing logarithmic slopes γ, defined by γ=−d ln ρ/d ln r. The values of γ at r=10^−2.5r_vir, a distance where the systems could be resolved by large N-body simulations, lie in the range 1.0–1.6. These inner values of γ increase for increasing β₁ and for increasing concentration of the system. On the other hand, slopes at r=r_vir lie in the range 2.42–3.82. A model density profile that fits well the results at radial distances between 10⁻³r_vir and r_vir and connects kinematic and structural characteristics of spherical systems is described.     

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.     

Generalized limits to the number of light particle degrees of freedom from big bang nucleosynthesis

We compute the big bang nucleosynthesis limit on the number of light neutrino degrees of freedom in a model-independent likelihood analysis based on the abundances of ⁴He and ⁷Li. We use the two-dimensional likelihood functions to simultaneously constrain the baryon-to-photon ratio and the number of light neutrinos for a range of ⁴He abundances Y_p = 0.225–0.250, as well as a range in primordial ⁷Li abundances from (1.6 to 4.1) ×10⁻¹⁰. For (⁷Li/H)_p = 1.6 × 10⁻¹⁰, as can be inferred from the ⁷Li data from Population II halo stars, the upper limit to N_ν based on the current best estimate of the primordial ⁴He abundance of Y_p = 0.238 is N_ν < 4.3 and varies from N_ν < 3.3 (at 95% C.L.) when Y_p = 0.225 to N_ν < 5.3 when Y_p = 0.250. If ⁷Li is depleted in these stars the upper limit to N_ν is relaxed. Taking (⁷Li/H)_p = 4.1 × 10⁻¹⁰, the limit varies from N_ν < 3.9 when Y_p = 0.225 to N_ν 6 when Y_p = 0.250. We also consider the consequences on the upper limit to N_ν if recent observations of deuterium in high-redshift quasar absorption-line systems are confirmed.     

Properties of the infrared dark cloud G79.2+0.38

The MSX infrared dark cloud G79.2+0.38 has been observed over a 11′×′ region simultaneously in the J=1-0 rotational transition lines of the ¹²CO and its isotopic molecules ¹³CO and ¹⁸CO. The dense molecular cores defined by the C¹⁸O line are found to be associated with the two high-extinction patches shown in the MSX A-band image. The two dense cores have the column density N (H₂) (5 – 12) × 10²² cm⁻² and the mean number density n (3 ± 1) × 10⁴ cm⁻³. Their sizes are 1.7 and 1.2 pc in ¹³CO(1-0) line, 1.2 and 0.6 pc in C¹⁸O(1-0) line, respectively. The masses of these cloud cores are estimated to be in the range from 2 × 10² to 2 × 10³ M. The profile of radial mean density of the cloud core can be described by the exponential function ¯n(p) p^{−0.34±0.02}. Compared with the cases of typical optical dark clouds, the abundances of the CO isotopic molecules ¹³CO and C¹⁸O in this MSX infrared dark cloud appear to be depleted by a factor of 4–11, but at present there is no evidence for any obvious variation of the relative abundance ratio X_13/18 between ¹³CO and C¹⁸O with the column density.     

Constraining cosmological models with cluster power spectra

Using extensive N-body simulations we estimate redshift space power spectra of clusters of galaxies for different cosmological models (SCDM, TCDM, CHDM, ΛCDM, OCDM, BSI, τCDM) and compare the results with observational data for Abell–ACO clusters. Our mock samples of galaxy clusters have the same geometry and selection functions as the observational sample which contains 417 clusters of galaxies in a double cone of galactic latitude |b|>30° up to a depth of 240 h⁻¹ Mpc. The power spectrum has been estimated for wave numbers k in the range 0.03k0.2 h Mpc⁻¹. For k>k_max0.05 h Mpc⁻¹ the power spectrum of the Abell–ACO clusters has a power-law shape, P(k)∝kⁿ, with n≈−1.9, while it changes sharply to a positive slope at k<k_max. By comparison with the mock catalogues SCDM, TCDM (n=0.9), and also OCDM with Ω₀=0.35 are rejected. Better agreement with observation can be found for the ΛCDM model with Ω₀=0.35 and h=0.7 and the CHDM model with two degenerate neutrinos and Ω_HDM=0.2 as well as for a CDM model with broken scale invariance (BSI) and the τCDM model. As for the peak in the Abell–ACO cluster power spectrum, we find that it does not represent a very unusual finding within the set of mock samples extracted from our simulations.     

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.     

Gamma–hadron separation methods for the VERITAS array of four imaging atmospheric Cherenkov telescopes

Ground-based arrays of imaging atmospheric Cherenkov telescopes have emerged as the most sensitive γ-ray detectors in the energy range of about 100 GeV and above. The strengths of these arrays are a very large effective collection area on the order of 10⁵ m², combined with excellent single photon angular and energy resolutions. The sensitivity of such detectors is limited by statistical fluctuations in the number of Cosmic-ray initiated air showers that resemble γ-ray air showers in many ways. In this paper, we study the performance of simple event reconstruction methods when applied to simulated data of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) experiment. We review methods for reconstructing the arrival direction and the energy of the primary photons, and examine means to improve on their performance. For a software threshold energy of 300 GeV (100 GeV), the methods achieve point source angular and energy resolutions of σ_63% = 0.1° (0.2°) and σ_68% = 15% (22%), respectively. The main emphasis of the paper is the discussion of γ–hadron separation methods for the VERITAS experiment. We find that the information from several methods can be combined based on a likelihood ratio approach and the resulting algorithm achieves a γ–hadron suppression with a quality factor that is substantially higher than that achieved with the standard methods used so far.     

Small-scale auroral arc distortions

Small-scale spatially periodic distortions of auroral forms have been studied utilizing low-light level television observations made at various locations in the Northern and Southern Hemispheres. The most commonly observed features were folds and vortex-like curl formations. The curls, identified here with the Kelvin-Helmholtz instability due to fluid shear, invariably had a counterclockwise rotational shape and motion when viewed in a direction anti-parallel to the Earth's magnetic field. The typical measured wavelength (5 km) and measured growth rate (4.2 sec⁻¹) were used to evaluate the Kelvin-Helmholtz dispersion relation for the apparent shear ω_s = ∂ ν_x/ ∂y (28 sec⁻¹). The apparent horizontal velocities of both folds (0–5 km/sec) and curls (0–22 km/sec) were invariably observed to be counterclockwise with respect to the multiple arc centre when viewed antiparallel to B. Consistent agreement between rotational shape and rotational motion suggests that the apparent growth rate and the apparent horizontal velocities closely approximate the actual values. If the shear results from E×B drifts in a space charge field, the calculated value for ω_s, implies an unneutralized electron density 0–1 cm⁻³ and a ΔE across the arc element 500mV/m. The velocity measurements indicate that the ΔE values for individual elements can combine to produce transient electric fields at the edges of multiple arcs as high as 1000 mV/m.