Electron excitation and auroral emission parameters

Auroral luminosities of the main emission lines in the aurora have been calculated for excitation by an isotopic primary electron flux with spectra of the form J(E) = AE exp (−E/E₁) + B(E₂)E exp (−E/E₁). The variation of emissions from O and N₂⁺ with height are shown, as are the variations of column integrated intensities and pertinent intensity ratios with the characteristic energy E₂, this leading to a method of estimating the electron spectrum from ground observation.     

Deactivation of N2A Σumolecules in the aurora

Recent rocket observations of the N₂ V-K (Vegard-Kaplan) system in the aurora have been reinterpreted using an atmospheric model based on mass spectrometer measurements in an aurora of similar intensity at the same time of year. In contrast to the original interpretation, we find that population by cascade from the C³Π_u and B³Π_g states in the A³Σ_u⁺v=0,1 levels, as calculated using recently measured electron excitation cross sections, accurately accounts for the observed relative emission rates (IV-K/12PG_0.0). In addition there is no need to change the production rate of A ³ Σ _u⁺ molecules relative to that of C³Π_uv=0 as a function of altitude in order to fit the profile of the deactivation probability to the atmospheric model. Quenching of A ³ Σ _u⁺ molecules at high altitudes is dominated by atomic oxygen. The rate constants for the v=0 and v=1 levels are 8 × 10⁻¹¹ cm³ sec⁻¹ and 1.7 × 10⁻¹⁰ cm³ sec⁻¹ respectively, as determined using the model atmosphere mentioned above. Recent observations with a helium cooled mass spectrometer suggest that conventional mass spectrometer measurements tend to underestimate the atomic oxygen relative concentration. The rate coefficients may therefore be too large by as much as a factor of 3. Below 130 Km we find that it is possible to account for the deactivation in bright auroras by invoking large nitric oxide concentrations, similar to those recently observed mass spectrometrically and using a rate constant of 8 × 10⁻¹¹ cm³ sec⁻¹ for both the v=1 levels. This rate constant is very nearly the same as that measured in the laboratory (7 × 10⁻¹¹ cm³ sec⁻¹). Molecular oxygen appears not to play a significant role in deactivating the lower A ³ Σ _u⁺ levels.     

Rotational discontinuities in an anisotropic plasma

A general theory of rotational discontinuities is developed and the changes in the components of the plasma pressure, p_| and p, and in the magnetic induction, B, are found. For a given value of λ=(p_| − p) 4πμ/B² upstream only a limited range of downstream anisotropies are possible. If λ>0.6 upstream then isotropy is not possible downstream. Some special solutions are analysed and the identification of rotational discontinuities is the solar wind is discussed.     

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 mass ratio of Nova Muscae 1991

We report intermediate resolution H spectroscopy of the black hole candidate Nova Muscae 1991 during quiescence. We classify the companion star as a K3-K4V which contributes 85±6 percent to the total flux from the binary. The photospheric absorption lines are broadened by 106±13 kms⁻¹ with respect to template field stars, leading to a system mass ratio of q =M₁/M₂ = 7.8_−2.0^+3.4. Doppler imaging of the H line shows strong emission coming from the secondary star (EW=3.1±0.6Å) which we associate with chromospheric activity. However, the hot-spot is not detected and this may indicate a lower mass transfer rate than in other X-ray transients of comparable orbital periods. The surface brightness distribution of the accretion disk in H follows a relation I∝R^−1.1, less steep than typically observed in cataclysmic variables. Updated system parameters are also presented.     

Interchange stability of a rapidly rotating magnetosphere

A rotation-dominated magnetosphere is unstable to magnetic flux-tube interchange motions if and only if the plasma content of a unit magnetic flux tube is a decreasing function of distance from the spin axis. For a spin-aligned dipole field the marginally stable distribution is approximately ρr^9/2 = constant, where ρ is the plasma mass density at the radial distance r in the equatorial plane. Plasma filling the Jovian magnetosphere from internal sources would initially violate this stability criterion so that interchange motions would act to establish the marginally stable distribution.     

Chemistry of the nightside ionosphere of Venus

We have constructed a one-dimensional model of the nightside ionosphere of Venus in which it is assumed that the ionization is maintained by day-to-night transport of atomic ions. Downward fluxes of O⁺, C⁺ and N⁺ in the ratios measured on the dayside at high altitudes are imposed at the upper boundary of the model (about 235 km). We discuss the resulting sources and sinks of the molecular ions NO⁺,CO⁺,N₂⁺,CO₂⁺ and O₂⁺. As the O⁺ flux is increased, the peak density of O⁺ increases proportionally and the altitude of the peak decreases. The O₂⁺ peak density is approximately proportional to the square root of the O⁺ flux and the peak rises as the O⁺ flux increases. NO⁺ densities near the peak are relatively unaffected by changes in the O⁺ flux. If the ionosphere is maintained mostly by transport, the ratio of the peak densities of O⁺ and O₂⁺ indicates the downward flux ofO⁺, independent of the absolute magnitudes of the densities. The densities of mass-28 ions are, however, still considered to be the most sensitive indicator of the importance of electron precipitation. We examine here the inbound and outbound portions of six early nightside orbits with low periapsis and use data from the Pioneer Venus orbiter ion mass spectrometer, the retarding potential analyzer and the electron temperature probe to determine the relative importance of ion transport and electron precipitation. For most of the orbits, precipitation is inferred to be of low to moderate importance. Only for orbit 65, which was the first nightside orbit published by Taylor et al. [J. geophys. Res. 85, 7765 (1980)] and for the inbound portion of orbit 73 does the ionization structure appear to be greatly affected by electron precipitation.     

Galactic gamma halo by heavy neutrino annihilations?

The diffused gamma halo around our Galaxy recently discovered by EGRET could be produced by annihilations of heavy relic neutrinos N (of fourth generation), whose mass is within a narrow range (M_Z/2<m_N<M_Z). Neutrino annihilation in the halo may lead to either ultrarelativistic electron pairs whose Inverse Compton Scattering on infrared and optical galactic photons could be the source of observed GeV gamma rays, or prompt 100 MeV–1 GeV photons (due to neutral pion secondaries) born by reactions. The consequent gamma flux (10⁻⁷–10⁻⁶ cm⁻² s⁻¹ sr⁻¹) is well comparable to the EGRET observed one, and it is also compatible with the narrow window of neutrino mass 45 GeV <m_N<50 GeV, recently required to explain the underground DAMA signals.The presence of heavy neutrinos of fourth generation do not contribute much to solve the dark matter problem of the Universe, but may be easily detectable by outcoming LEP II data.     

Spectrum of the relic neutrino background from past supernovae and cosmological models

It is greatly expected that the relic neutrino background from past supernovae will be detected by Superkamiokande (SK) which is now under construction. We calculate the spectrum and the event rate at SK systematically by using the results of simulations of a supernova explosion and reasonable supernova rates. We also investigate the effect of a cosmological constant, Λ, on the spectrum, since some recent cosmological observations strongly suggest the existence of Λ. We find following results. (1) The spectrum has a peak at about 3 MeV, which is much lower than that of previous estimates (6–10 MeV). (2) The event rate at SK in the range from 10 MeV to 50 MeV, where the relic neutrinos from past supernovae are dominant, is about 25h₅₀²(R_SN/0.1 yr⁻¹)(n_Gh₅₀⁻³/0.02 Mpc⁻³) events per year, where R_SN is the supernova rate in a galaxy, n_G is the number density of galaxies, and h₅₀ = H₀/(50 km/s Mpc), where H₀ is the Hubble constant. (3) The event rate is almost insensitive to Λ. The flux increases in the low energy side (< 10 MeV) with increasing Λ, but decreases in the high energy side (> 10 MeV) in models in which the integrated number of supernovae in one galaxy is fixed.     

Comparison of high-energy galactic and atmospheric tau neutrino flux

We compare the tau neutrino flux arising from the galaxy and the earth atmosphere for 10³E/GeV10¹¹. The intrinsic and oscillated tau neutrino fluxes from both sources are calculated. The intrinsic galactic ν_τ flux (E10³ GeV) is calculated by considering the interactions of high-energy cosmic-rays with the matter present in our galaxy, whereas the oscillated galactic ν_τ flux is coming from the oscillation of the galactic ν_μ flux. For the intrinsic atmospheric ν_τ flux, we extend the validity of a previous calculation from E10⁶ GeV up to E10¹¹ GeV. The oscillated atmospheric ν_τ flux is, on the other hand, rather suppressed. We find that, for 10³E/GeV5×10⁷, the oscillated ν_τ flux along the galactic plane dominates over the maximal intrinsic atmospheric ν_τ flux, i.e., the flux along the horizontal direction. We also briefly mention the presently envisaged prospects for observing these high-energy tau neutrinos.     

The aries auroral modelling campaign: characterization and modelling of an evening auroral arc observed from a rocket and a ground-based line of meridian scanners

An auroral arc system excited by soft electrons was studied with a combination of in situ rocket measurements and optical tomographic techniques, using data from a photometer on a horizontal, spinning rocket and a line of three meridian scanning photometers. The ground-based scanner data at 4709, 5577, 8446 and 6300 Å were successfully inverted to provide a set of volume emission rate distributions in the plane of the rocket trajectory, with a basic time resolution of 24 s. Volume emission rate profiles, derived from these distributions peaked at about 150 km for 5577 and 4709 Å, while the 8446 Å emission peaked at about 170 km with a more extended height distribution. The rocket photometer gave comparable volume emission rate distributions for the 3914 Å emission as reported in a separate paper by McDade et al. (1991, Planet. Space Sci. 39, 895). Instruments on the rocket measured the primary electron flux during the flight and, in particular, the flux precipitating into the auroral arc overflown at apogee (McEwen et al., 1991; in preparation). The local electron density and temperature were measured by probes on the rocket (Margot and McNamara (1991; Can. J. Phys. 69, 950). The electron density measurements on the downleg were modelled using ion production rate data derived from the optical results. Model calculations of the emission height profile based on the measured electron flux agree with the observed profiles. The height distribution of the N₂⁺ emission in the equatorward band, through which the rocket passed during the descent, was measured by both the rocket and the ground-based tomographic techniques and the results are in good agreement. Comparison of these profiles with model profiles indicates that the exciting primary spectrum may be represented by an accelerated Maxwellian or a Gaussian distribution centered at about 3 keV. This distribution is close to what would be obtained if the electron flux exciting the poleward form were accelerated by a 1–2 kV upward potential drop. The relative height profiles for the volume emission rate of the 5577 Å OI emission and the 4709 Å N₂⁺ emission were almost indistinguishable from each other for both the forms measured, with ratios in the range 38–50; this is equivalent to I(5577)/I(4278) ratios of 8–10. The auroral intensities and intensity ratios measured in the magnetic zenith from the ground during the period before and during the rocket flight are consistent with the primary electron fluxes and height distributions measured from the rocket. Values of I(5577)/I(4278) in the range 8–10 were also measured directly by the zenith ground photometers over which the arc system passed. These values are slightly higher than those reported by Gattinger and Vallance-Jones (1972) and this may possibly indicate an enhancement of the atomic oxygen concentration at the time of the flight. Such an enhancement would be consistent with our result, that the observed values of I(5577) and I(8446) are also significantly higher than those modelled on the basis of the electron flux spectrum measured at apogee.     

On the joint robust estimation of parameters and variance

A method of joint estimation of the parameters and the variance σ² in the linear model is presented in this paper. The M-estimators with finite rejection points are adopted for estimating the parameters. In order to estimate the variance σ², the Grubbs statistics and the Kurtosis test statistics are adopted to test the residual sequence {;r_i};. And the sample variance of {;r_i}; after discarding the outliers is taken as the estimation of σ². This method of estimating σ² is less computational demanding and more accurate, compared with the well-known method which takes 1.483 med_i |r_i| as the estimation of σ. The breakdown point of the above estimation of σ² is more than 20%, while the brekdown point is less than 10%, if only the Grubbs statistics is used.     

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.     

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.     

Measurements of radio noise at 0.700 mc and 2.200 mc from a high-altitude rocket probe

Radio noise observations at frequencies of 0·700 Mc and 2·200 Mc were made at altitudes between 3000 and 11,000 km from a Blue Scout Jr. high-altitude rocket probe on 30 July 1963. A steady background flux of (7·5₋₃⁺⁶) × 10⁻¹⁹ W m⁻²)(c/s)⁻¹ at 0·700 Mc and (1·8^+1.0_−0.5 × 10⁻¹⁹ W m⁻² (c/s)⁻¹ at 2·200 Mc was observed. Assuming a galactic origin of the observed fluxes at both frequencies, the averaged sky brightnesses are b(0·700 Mc) = (6₋₃⁺⁵) × 10⁻²⁰ W m⁻² (c/s)⁻¹ sr⁻¹b(2·200 Mc) = (1.4^+1.0_−0.5 × 10⁻²⁰ W m⁻² (c/s)⁻¹ sr⁻¹ The observed brightness at 2·200 Mc is in reasonable agreement with the results of other observers. The apparent brightness at 0·700 Mc is, however, greater than was expected from previous observations. An alternative source of the 0·700 Mc flux in the terrestrial exosphere, as well as characteristics of several noise bursts observed during the flight, is briefly discussed.     

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.     

Space–time foam and cosmic-ray interactions

It has been proposed that propagation of cosmic-rays at extreme-energy may be sensitive to Lorentz-violating metric fluctuations (“foam”). We investigate the changes in interaction thresholds for cosmic-rays and gamma-rays interacting on the CMB and IR backgrounds, for a class of stochastic models of space–time foam. The strength of the foam is characterized by the factor (E/M_P)^a, where a is a phenomenological suppression parameter. We find that there exists a critical value of a (dependent on the particular reaction: a_crit3 for cosmic-rays, 1 for gamma-rays), below which the threshold energy can only be lowered, and above which the threshold energy may be raised, but at most by a factor of two. Thus, it does not appear possible in this class of models to extend cosmic-ray spectra significantly beyond their classical absorption energies. However, the lower thresholds resulting from foam may have signatures in the cosmic-ray spectrum. In the context of this foam model, we find that cosmic-ray energies cannot exceed the fundamental Planck scale, and so set a lower bound of 10⁸ TeV for the scale of gravity. We also find that suppression of p→pπ⁰ and γ→e⁻e⁺ “decays” favors values aa_crit. Finally, we comment on the apparent non-conservation of particle energy–momentum, and speculate on its re-emergence as dark energy in the foamy vacuum.     

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.     

A prediction for three neutrino masses and mixings and similarity between quark and lepton mixings

If neutrinos have mass, we give reasons for a possible pattern of three (squaed) mass eigenvalues: m₁² (2.8−5.8) (eV)², m₂² 0.01 (eV)², m₃² (1.5−1) × 10⁻⁴ (eV)². The flavor states ν_μ and ν_e are mixtures of the eigenstates with m₂ and m₃ with a significant mixing, corresponding to an effective mixing angle of about 0.45. The ν_τ is nearly the state with m₁; the other two effective mixing angles are about an order of magnitude smaller than 0.45. There is a marked similarity to mixing in the quark sector.     

Rocket measurements of the 6300 Å and 3914 Å dayglow features

Rocket results are presented on the OI 6300 Å line and on the N₂⁺ 3914 Å band in the dayglow. An altitude range of 78–335 km is covered. Theoretical interpretations are given, using results of simultaneous measurements of electron density and electron temperature. The apparent brightness of the 6300 Å line at the base of the emitting region is found to be 13 kR, of which 5.5 kR are ascribed to excitation through the Schumann-Runge dissociation of O₂ by the solar UV radiations, 0.55 kR to the dissociative recombination of O₂⁺ and NO⁺ ions, and 0.03 kR to the excitation of O by thermal electrons. An additional source of excitation above 280 km is suggested. The deactivation of O(¹D) by O₂(X³Σ_g⁻) is found to be appreciable below 200 km, and its rate coefficient is estimated to be 2 × 10⁻¹⁰ cm³/sec. The apparent brightness of the 3914 Å band at the base of the emitting region is found to be 6.5 kR, decreasing to 3.2 kR at 330 km. Assuming that fluorescent scattering of solar radiation is the mechanism involved the distribution of N₂⁺ ions is calculated. The rate coefficients for the loss of these ions are hence calculated.