Catastrophic fragmentation as a stochastic process: sizes and shapes of fragments

It is rather difficult to understand theoretically and to analyse the experimental data concerning the mass and shape distributions of fragments created by catastrophic collisions. The fragmentation process is discussed as being a purely stochastical phenomenon; the size and shape distributions obtained in this way are compared with the results of laboratory experiments. The results are presented of some computer simulations of random volume fragmentation processes; they are a 3-D generalization of the numerical experiments described in Grady and Kipp (J. Appl. Phys. 58(3), 1210–1222, 1985). The features of the size distribution are discussed, comparing it with the expectations of the Mott-Linfoot and Grady-Kipp theories. In the literature the shape of fragments is defined in terms of the ratios B/A and C/A, where A, B, C are defined as the sizes of a fragment along three orthogonal axes. The definition of the shape of a fragment cannot be considered unique, since it is not obvious in which order to define the three axes when the fragments are not ellipsoidal. A few possible methods are introduced explicity, and the resulting differences are discussed. In this light, the shape results (the mean values and the distribution of the axial ratios) obtained in recent laboratory experiments are rediscussed and critically reviewed. For what concerns the stochastical modelling, the results of various simulations, corresponding to different assumptions regarding fragmentation properties are presented. It is shown that the main features of the shape distributions from laboratory experiments cannot be satisfactorily reproduced. Comparison of the results with the outcomes of the semiempirical fragmentation model by Paolicchi et al. (Icarus 121, 126–157, 1996), as well as with some results coming out from hydrodynamical simulations, shows how only a “global” and physical model, not a purely statistical one (neither global nor “local”), can afford to reproduce the observed data.     

Charged boson stars and vacuum instabilities

We consider charged boson stars and study their effect on the structure of the vacuum. For very compact particle like “stars”, with constituent mass m_* close to the Planck mass m_Pl, i.e. m²_* = O(m²_Pl), we argue that there is electric charge Z_c, which, primarily, is due to the formation of a pion condensate (Z_c 0.5⁻¹e, where is the fine structure constant and e is the electric charge of the positron). If the charge of the “star” is larger than Z_c we find numerical evidence for a complete screening indicating a limiting charge for a very compact object. There is also a less efficient competing charge screening mechanism due to spontaneous electron-positron pair creation in which case Z_c ⁻¹e. Astrophysical and cosmological abundances of charged compact boson stars are briefly discussed in terms of dark matter.     

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 quantitative interpretation of solar X-ray images

We examine the interpretation of plasma parameters derived by quantitative analysis of solar X-ray photographs obtained through broad band filters. We find some of the recent criticisms of the filter ratio method to be unfounded. Using active region and quiet Sun emission measures derived mostly from spectroscopic observations we find that effective emission measures and temperatures derived from S-054 data by filter ratio analysis are within better than 20% of the total emission measure and average temperature, respectively. The uncertainties associated with filtered flux determination are found to produce an error of about 10% in the derived effective temperature. We thus conclusively demonstrate that parameters derived from S-054 data by filter ratio analysis are representative of the observed active region and quiet Sun material.     

Some aspects of Sporadic-E at mid-latitudes

Evidence is presented to suggest that the ƒ₀E_s value tends to be high at positions in the E_s-layer where trough slope-lines and crest slope-lines of F2-layer irregularities meet the E_s-layer. These slope-lines are drawn through the troughs and crests, respectively, of the characteristic kinks in the F2-layer ionization contours, which are associated with F2-layer irregularities.

A rotating-loop direction-finding system has allowed an estimation of the distribution of ionization, which gives rise to Sporadic-E echoes. Analysis of Sporadic-E occurrence, on two occasions, suggests that the reflecting surfaces are frontal in nature, the fronts having a separation from each other of some tens of kilometres, and probably existing as closed curves, with diameters of the order of several hundreds of kilometres. A possible association between these structures and the occurrence of the green line of the airglow, is discussed.

A distribution of ionization, which will give contours showing “clouds” of ionization at some frequencies and a ripple structure at other frequencies, is proposed, in an endeavour to explain the apparent dual nature of Sporadic-E occurrence.

The evidence seems to indicate that the mechanism operating at the E_s-layer level, producing the phenomenon of Sporadic-E, is the same as that which produces the F2-layer irregularities which are responsible for “Spread-F”.     

Geminid Meteor shower activity 2003–2005 as observed by Gadanki radar

The distribution of meteor signals reflected from a backscatter radar is considered according to their duration. This duration time (T) is used to classify the meteor echoes and to calculate the mass index (S) of different meteoroids of shower plus sporadic background. Observational data on particle size distribution of the Geminid meteor shower are very scarce, particularly at low latitudes. In this paper the observational data from Gadanki radar (13.46°N, 79.18°E) have been used to determine the particle size distribution and the number density of meteoroids inside the stream of the Geminid meteor shower. The mean variation of meteor number density across the stream has been determined for three echo duration classes, T<0.4, T=0.4–1 and T>1 s. We are more interested in the appearance of echoes of various durations and therefore meteors of various masses in order to understand more on the filamentary structure of the stream. It is observed that the faint particle flux peaks earlier than the larger particles. We found a decreasing trend in the mass index values from the day of peak activity to the next observation days. The mass index profile was found to be U-shaped with a minimum value near the time of peak activity. The observed minimum s values are 1.64±0.05 and 1.65±0.04 in the years 2003 and 2005, respectively. The activity of the shower indicates the mass segregation of meteoroids inside the stream. Our results are best comparable with the “scissors” structure model of the meteoroid stream formation of Ryabova [2007. Mathematical modeling of the Geminid meteoroid stream. Mon. Not. R. Astron. Soc. 375, 1371–1380] by considering the asteroid 3200 Phaethon as an extinct comet.     

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.     

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.     

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.     

The measurement of the decorrelation function in underground muon pairs as a probe of primary cosmic ray interactions

We discuss the measurement of the average angle within pairs of muons detected in underground experiments versus their relative distance, as a tool for studying primary Cosmic Ray interactions. We propose to call Φ(d) “decorrelation” function. Under simplifying approximation, the decorrelation function can be computed analytically to show its dependence on details of cosmic ray interactions as well as on the propagation in the rock. We argue that this new measurement is useful in a real experiment, complementing traditional ones like the decoherence function.     

时间序列数字滤波后自由度的确定--应用于日长变化与南方涛动指数的相关分析

原始观测时间序列进行数字滤波后，其自由度将大大降低，对各种数字滤波器都可以通过Monte Carlo方法模拟获得对应置信水平的相关系数临界值，从而得到时间序列滤波后的自由度，如果滤波后的频段宽度与Nyquist频率之比为Z，那么对于理想的单边和双边滤波器，时间序列滤波后的自由度为原始值的Z／2和Z倍，非理想滤波器一般不符合此规律。     

Acoustic-Power Maps of Solar Active Regions with Direction Filters and Phase-Velocity Filters

We study the properties of power maps of solar acoustic waves filtered with direction filters and phase-velocity filters. A direction filter is used to isolate acoustic waves propagating in a narrow range of directions. The acoustic-power map of the waves filtered with a direction filter shows extended reduced-power features behind magnetic regions with respect to the wave direction. A phase-velocity filter is further applied to isolate waves with similar wave paths. In the power maps of the waves filtered with both a direction filter and a phase-velocity filter, a reduced-power image of a sunspot appears behind the sunspot with respect to the wave direction. The distance between the sunspot and the secondary image is consistent with the one-skip travel distance of the wave packet associated with the phase-velocity filter. The waves filtered with direction and phase-velocity filters at the location of the secondary image could be used to probe the sunspot. In the quiet Sun, spatial fluctuations exist in any acoustic-power map. These fluctuations are mainly caused by interference among modes with the same frequency. The fluctuations are random with two properties: They change rapidly with time, and their magnitude decreases with the square root of the number of frames used in computing the acoustic-power map.     

Measurement of the cosmic ray composition at the knee with the SPASE-2/AMANDA-B10 detectors

The mass composition of high-energy cosmic rays at energies above 10¹⁵ eV can provide crucial information for the understanding of their origin. Air showers were measured simultaneously with the SPASE-2 air shower array and the AMANDA-B10 Cherenkov telescope at the South Pole. This combination has the advantage to sample almost all high-energy shower muons and is thus a new approach to the determination of the cosmic ray composition. The change in the cosmic ray mass composition was measured versus existing data from direct measurements at low energies. Our data show an increase of the mean log atomic mass lnA by about 0.8 between 500 TeV and 5 PeV. This trend of an increasing mass through the “knee” region is robust against a variety of systematic effects.     

On the existence of a low-dimensional chaotic attractor in the short-term solar UV time series

A mixture of periodic and chaotic components makes the detection of chaos difficult. The periodic components are sought in the solar UV time series by the Maximum Entropy Method and are removed by a digital notch filter. The filtered output is subjected to investigation for chaotic behavior by three different techniques. (1) Fixed-size method for attractor dimension determination; (2) sensitive initial dependence via prediction error; (3) trajectory direction estimation. All the investigation points to the existence of a chaotic attractor of fractional dimension.     

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.     

Triple splitting with the F2-region of the ionosphere at high and mid-latitudes

The occurrence of the third (z-ray) component of the F2-trace on ionograms is investigated at high- and mid-latitudes. Diurnal variations show a systematic shift, with magnetic inclination, of the time of maximum occurrence. Seasonal variations show a winter maximum, and an inverse sunspot-cycle relationship exists. Maximum occurrence appears between a magnetic inclination of 70° and 80° with a fall-off either side.

Evidence is presented to suggest a z-ray association with “Spread-F” fronts, and a possible mechanism for the recording of the z-ray trace at the transmitter site is described. This involves longitudinal propagation of the o-mode at its normal reflection level, coupling at this point, and ultimate reflection for the z-ray mode as a result of sloping ionization contours belonging to “Spread-F” fronts extending in directions perpendicular to the magnetic meridian.

An association with V.L.F. emissions (“dawn-chorus”) is discussed.     

Boundary layer plasmas as a source for high-latitude, early afternoon, auroral arcs

Simultaneous measurements of hot boundary layer plasma from PROGNOZ-7 and particle precipitation from the TIROS/NOAA satellite in nearly magnetically conjugate regions have been used to study the dynamo process responsible for the formation of high latitude, early afternoon, auroral arcs.

Characteristic for the PROGNOZ-7 observations in the dayside boundary layer at high latitudes is the frequent occurrence of regions with injected magnetosheath plasma embedded in a “halo” of antisunward flowing magnetosphere plasma. The injected magnetosheath plasma have several features which indicate that it also acts as a local source of EMF in the boundary layer. The process resembles that of a local MHD dynamo driven by the excess drift velocity of the injected magnetosheath plasma relative to the background magnetospheric plasma.

The dynamo region is capable of driving field-aligned currents that couple to the ionosphere, where the upward current is associated with the high latitude auroral arcs.

We demonstrate that the large-scale morphology as well as the detailed data intercomparison between PROGNOZ-7 and TIROS-N both agree well with a local injection of magnetosheath plasma into the dayside boundary layer as the main dynamo process powering the high-latitude, early afternoon auroral arcs.     

Fluctuations in tidal (24-, 12-h) characteristics and oscillations (8-h-5-d) in the mesosphere and lower thermosphere (70–110 km): Saskatoon (52°N, 107°W), 1979–1981

An M.F. radar (2.2 MHz) operating at Saskatoon, Canada (52°N, 107°W) has been used to produce continuous wind data ( 80–110km) from September 1978–April 1981. The 24-, 12-h tidal oscillations reveal regular summer-winter transitions; in particular the semi-diurnal tide demonstrates strikingly regular and rapid equinoctial changes over the three years. The vernal and autumnal equinox changes are clearly different in morphology. Shorter term tidal fluctuations (2d τ 10d) are compared with mean winds and gravity wave amplitudes, as well as with satellite-derived stratospheric temperatures.

Spectral analysis of monthly data sets for 1980, from 90–105 km, reveal oscillations of the expected 8-, 12-, 24-h periods, but also of 10-, 16- and 2-, 5/6d. A modulation of the “2-d” wave by the 12-h wave is suggested as a possible cause of these surprisingly regular oscillations.     

Contribution of nuclei accelerated by gamma-ray pulsars to cosmic rays in the Galaxy

We consider the galactic population of gamma-ray pulsars as possible sources of cosmic rays at and just above the “knee” in the observed cosmic ray spectrum at 10¹⁵–10¹⁶ eV. We suggest that iron nuclei may be accelerated in the outer gaps of pulsars, and then suffer partial photo-disintegration in the non-thermal radiation fields of the outer gaps. As a result, protons, neutrons, and surviving heavier nuclei are injected into the expanding supernova remnant. We compute the spectra of nuclei escaping from supernova remnants into the interstellar medium, taking into account the observed population of radio pulsars.

Our calculations, which include a realistic model for acceleration and propagation of nuclei in pulsar magnetospheres and supernova remnants, predict that heavy nuclei accelerated directly by gamma-ray pulsars could contribute about 20% of the observed cosmic rays in the knee region. Such a contribution of heavy nuclei to the cosmic ray spectrum at the knee can significantly increase the average value of lnA with increasing energy as is suggested by recent observations.     

Evaluation of the effect of equal power spectrum filtering in image restoration

The effectiveness of equal power spectrum filtering in image restoration is analyzed via digital simulation. We start with a gray-scale map of concentric rings with different grey values. To obtain a dirty image, the original image is first transformed via a linear space-invariant system. Then, white noise is mixed into the image. The equal power spectrum filter is then used for the image restoration. The signal to noise ratio (SNR) and a defined steepness of the steps, together with the visual impression, are accepted to characterize the performance of the image restoration. It is concluded that a satisfactory restoration can be realized when the SNR > 15. When SNR < 5, the image is completely un-restorable. The effect of system's transfer function on image restoration is also discussed. The effectiveness of the presented model of equal power spectrum filter is confirmed by applying it to the restoration of an actual astronomical image. Finally, a typical application of the equal power spectrum filter is given.