Time-lag of cosmic-ray intensity

The best correlation coefficient between the monthly cosmic-ray intensity of the Inuvik Station and various kinds of solar, interplanetary, and geophysical parameters has been found. It is calculated for different time-lags of cosmic-ray intensity with respect to these parameters. The maximum of these coefficients lead us to a useful empirical model for the 11-year cosmic-ray modulation.     

Time-evolution of cosmic-ray intensity modulation

Application of analyzing time-series into trigonometric series allows the investigation of cosmic-ray intensity variations in a wide periodicity range from a few months to 10 or even more years. By this technique, the amplitude and the phase of all observed fluctuations can be given. For this purpose, cosmic-ray data of five ground-based neutron-monitor stations for the time interval 1964–1985 have been analyzed.Two kinds of periodicities appeared in these data. The first one includes occurrences at periods greater than two years, as the ones of 10.41, 8.41, and 5.50 yr, which differ very little in amplitude from station to station but are similar in phase, and the second one includes periodicities smaller than two years (24, 12, 8, and 6 months) which are similar in all stations but appeared in variable time intervals.The possible origin of each observed variation due to a contribution either of cosmic-ray interaction in the upper atmosphere or to the solar dynamics is discussed.     

VLF hiss,visual aurora and the geomagnetic activity

Observations and analyses of hiss events, recorded at College (dp. lat. 64.62°N) and Bar 1 (dp. lat. 70.20°N) during periods of varying auroral and geomagnetic activity, reveal three different types of events. These are (1) auroral substorm events with associated hiss bursts during disturbed period, (2) quiet-time hiss events accompanying stationary quiet auroral arcs and (3) hissless events at times of auroral and magnetic activity. Quiet-time observations seem to suggest that the substorm activity is not a necessary requirement for generating wideband hiss. On the other hand, examples of auroral and magnetic activity with complete absence of VLF hiss indicate that the ground reception of VLF/ELF natural emissions is largely controlled by propagation conditions in the ionosphere. There is either little or no correlation found between hiss observations at the two stations separated by about 600 km.     

Large amplitude wave-trains of cosmic-ray intensity

The large amplitude wave-trains of cosmic-ray intensity observed during June, July and August, 1973, were analysed. These events exhibit the same characteristics as the event of May, 1973. During these days the phase of the enhanced diurnal anisotropy is shifted to a point earlier than either the corotation direction or the anti-garden-hose direction. For this analysis we used data from high- and middle-latitude neutron monitors and from the satellites HEOS-2, IMP-7 and IMP-8. The diurnal variation of these days is well understood in terms of a radially outward convective vector and a field-aligned inward diffusive vector yielding a diurnal anisotropy vector along about 1600 h in space.     

Simulation of long-term cosmic-ray intensity variation

Selecting the most appropriate source functions among the various solar, interplanetary and terrestrial activity indices we have attempted to reproduce to a certain degree the long-term modulation of galactic cosmic-rays. For this study monthly cosmic-ray data from nine world-wide neutron monitor stations for the period 1975–1985 have been analysed. The empirical formula which has been used to compute the long-term cosmic-ray variations follows the observations fairly well.It is noteworthy that the residuals in the cosmic-ray intensity between that observed and that calculated by this empirical formula exhibits a still remaining short-term variation in all stations of 2.7 and 3.7 months. Possible interpretations of these observed periodicities related to galactic origin are given.     

The solar causes of geomagnetic disturbances

Geomagnetic disturbances have been identified with respect to their sources for 1977–1983. The disturbance level was found using the daily planetary index A _p. High-amplitude ( 50), mean-amplitude (24) and low-amplitude ( 12) disturbances are caused by solar flares of importance 1, coronal holes, and filament cavities, respectively. The ranges of probable amplitudes of disturbances of different nature and their relative number are found from Poisson random distributions of amplitudes.     

Power-spectrum normalization from the local abundance of rich clusters of galaxies

The number density of rich galaxy clusters still provides the most robust way of normalizing the power spectrum of dark matter perturbations on scales relevant to large-scale structure. We revisit this constraint in the light of several recent developments: (1) the availability of well-defined samples of local clusters with relatively accurate X-ray temperatures; (2) new theoretical mass functions for dark matter haloes, which provide a good fit to large numerical simulations; (3) more accurate mass–temperature relations from larger catalogues of hydrodynamical simulations; (4) the requirement to consider closed as well as open and flat cosmologies to obtain full multiparameter likelihood constraints for CMB and SNe studies. We present a new sample of clusters drawn from the literature and use this sample to obtain improved results on σ ₈, the normalization of the matter power spectrum on scales of 8  h ⁻¹ Mpc, as a function of the matter density and cosmological constant in a universe with general curvature. We discuss our differences with previous work, and the remaining major sources of uncertainty. Final results on the normalization, approximately independent of power spectrum shape, can be expressed as constraints on σ at an appropriate cluster normalization scale R _Cl. We provide fitting formulas for R _Cl and σ ( R _Cl) for general cosmologies, as well as for σ ₈ as a function of cosmology and shape parameter Γ. For flat models we find approximately σ ₈≃(0.495_−0.037^+0.034)Ω_M^−0.60 for Γ=0.23, where the error bar is dominated by uncertainty in the mass–temperature relation.     

The geomagnetic and cosmic-ray storm of May 25/26, 1967

A series of geomagnetic disturbances and cosmic ray variations caused by the McMath plage region 8818 in the latter half of May 1967 were examined. The systematic changes of the geomagnetic disturbances were observed as the relative location between the responsible flares and the earth changed during the half solar rotation period.The storm of May 25/26, 1967 was then studied in great detail on the basis of records from a number of magnetic and cosmic ray observatories. A large asymmetric main phase field in mid-and low-latitude regions (and thus an asymmetric ring current belt) grew rapidly during the first three successive polar magnetic substorms. The cosmic ray intensity variations during the storm consisted of the Forbush decrease and the ring current effect. The Forbush decrease had a marked north-south asymmetry during its developing phase.     

Solar and interplanetary disturbances causing moderate geomagnetic storms

The effect of solar and interplanetary disturbances on geomagnetospheric conditions leading to 121 moderate geomagnetic storms (MGS) have been investigated using the neutron monitor, solar geophysical and interplanetary data during the period 1978–99. Further, the duration of recovery phase has been observed to be greater than the duration of main phase in most of the cases of MGS. It has further been noted that Ap-index increases on sudden storm commencement (SSC) day than its previous day value and acquires maximum value on the day of maximum solar activity. Generally, the decrease in cosmic ray (CR) intensity and Dst begins few hours earlier than the occurrence of MGS at Earth. Furthermore, negative Bz pointing southward plays a key causal role in the occurrence of MGS and the magnitude and the duration of Bz and Bav also play a significant role in the development of MGS. The solar features Hα, X-ray solar flares and active prominences and disappearing filaments (APDFs) which have occurred within lower helio-latitudinal/helio-longitudinal zones produce larger number of MGS. Solar flares seem to be the major cause for producing MGS.     

Delay times between geoeffective solar disturbances and geomagnetic indices

We have examined delay times between solar disturbances (X-ray flares and DSFs) and storm sudden commencements(SSC) as well as between SSC and major geomagnetic storms. To carry out cross-correlation analysis of these point series data, we have introduced a new correlation measure which is defined by the ratio of the median value of the absolute residual differences between two sets of time series data to the one determined from hypothetical target series. We have confirmed from the correlation analyses that (1) the most probable traveling time of a solar disturbance from the Sun to the Earth is estimated to be about 2 days for a disturbance associated with major (X and M class) solar flares, and about 3 days for a disturbance associated with DSFs, (2) long-duration flares are better correlated with SSCs than short-duration flares, (3) travelling times of solar disturbances strongly depend on the heliolongitude where they originate, and (4) solar disturbances associated with flares and DSFs at the western limb can hardly reach the Earth. This revised version was published online in July 2006 with corrections to the Cover Date.     

A proposed method for measurement of cosmic-ray mass composition based on geomagnetic spectroscopy

The effect of the geomagnetic Lorentz force on the muon component of extensive air shower (EAS) has been studied in a Monte Carlo generated simulated data sample. This geomagnetic field affects the paths of muons in an EAS, causing a local contrast or polar asymmetry in the abundance of positive and negative muons about the shower axis. The asymmetry can be approximately expressed as a function of transverse separation between the positive and negative muons barycentric positions in the EAS through opposite quadrants across the shower core in the shower front plane. In the present study, it is found that the transverse muon barycenter separation and its maximum value obtained from the polar variation of the parameter are higher for iron primaries than protons for highly inclined showers. Hence, in principle, these parameters can be exploited to the measurement of primary cosmic-ray mass composition. Possibility of practical realization of the proposed method in a real experiment is briefly discussed.     

Statistical studies of geomagnetic pulsations with periods between 10 and 70 sec and their relationship to the plasmapause region

Geomagnetic pulsations, in the period range 10–150 sec, have been analysed from five stations; Eskdalemuir (L = 3.1), Lerwick (L = 4.0), St. Anthony (L = 4.9), Sodankyla (L = 5.3) and Tromsø (L = 6.6). The results of 12 observatory years' worth of data are presented in the form of contour maps showing the frequency of occurrence of the pulsations as a function of K_p index and of local time. The maps show that a ground based observatory is more likely to record shorter period oscillations (pc 3) when the geomagnetic field line linking the station with the southern hemisphere passes through the plasmatrough than when the observatory field line links the plasmasphere. The peak occurrence of pc 3 for the observatories considered is at 08:45 hr ± 1 hr LT and is related to the observatory L value and the average night-time K_p index by the equation, L = 8.1 ? 1.2K_p. At Eskdalemuir, the spectrum is broader band than the other stations and tends to divide into two peaks; the pc 3 (20 sec) peak tends to occur when the plasmapause has moved in close to the observatory; while the pc 4 (60 sec) peak occurs when the K_p values have been lower and the plasmapause is further away at higher latitudes.     

Influence of interplanetary interaction regions on geomagnetic disturbances and tropospheric circulation

Proposed solar wind-magnetosphere energy coupling functions are studied. An empirical formula proposed by Svalgaard (1977) is found to predict the geomagnetic activity quite well.

The influence of solar wind interaction regions on the tropospheric circulation, through a suggested cosmic ray mechanism, was investigated. The cosmic ray intensity at Earth clearly showed a decrease at the time of passage of an interaction region. It is suggested that the well-known dip in the Vorticity Area Index may be caused by an interaction-modulated decrease in cosmic ray intensity.     

Solar-cycle phenomena in cosmic-ray intensity: Differences between even and odd cycles

Cosmic-ray intensity data for the period 1964–1985 covering two solar cycles are used to investigate the solar activity behaviour in relation to cosmic-ray modulation. A detailed statistical analysis of them shows a large time-lag of about one and half years between cosmic-ray intensity and solar activity (as indicated by sunspot number, solar flares and high-speed solar-wind streams) during the 21st solar cycle appearing for a first time. This lag indicates the very high activity level of this solar cycle estimating the size of the modulating region to the unambiguous value of 180 AU. The account of the solar-wind speed in the 11-year variation significantly decreases the modulation region of cosmic-rays to the value of 40 AU.A comparison with the behaviour of the previous solar cycle establishes a distinction between even and odd solar cycles. This is explained in terms of different contributions of drift, convection and diffusion to the whole modulation mechanism during even and odd solar cycles.     

Measurements of the optical emission height profiles of medium intensity aurora

Height profiles of auroral emissions at 3914 Å, 4861 Å, and 5577 Å were obtained in two rocket flights through medium intensity stable aurora. The 3914 Å N₂⁺ integral intensity data were compared with intensity variations predicted by an auroral model for a range of primary electron energy spectra. The observed profiles for the two flights were well reproduced respectively by a 5.6 keV mono-energtic flux and by a flux with an exponential spectrum cutting off around 12 to 15 keV. The data for 5577 Å (available only above 120 km) bear a constant ratio to that for 3914 Å. The emission profiles derived for 3914 Å, peak at 115 and 107 km respectively.     

Study and effect of magnetic clouds on the transient modulation of cosmic-ray intensity

Data of cosmic-ray intensity from the Calgary Super Neutron Monitor and interplanetary plasma and field data are divided into three groups corresponding to the magnetic clouds preceded by shocks, followed by interaction region and clouds without any such association, observed during the period 1967–1982. A superposed epoch analysis of these data, in addition to the field variance data, have been performed. The results suggest the hypothesis that the Forbush decreases are caused by the scattering of particles in the region of enhanced turbulence, observed during the passage of shocked plasma (i.e., sheath) between the shock front and the magnetic cloud.     

Spectral studies of geomagnetic pulsations with periods between 20 and 120 sec,and their relationship to the plasmapause region

Digital spectrograms have been computed for 18 days of geomagnetic pulsation activity at three UK Earth current stations (L = 2.6?3.6).Three main conclusions are drawn: (1) There are days when the period of the dominant spectral amplitude is ordered according to the observatory latitude. The most frequently observed large amplitude spectral peaks are centred on 80, 60 and 45s for South Uist (L = 3.6), Eskdalemuir (L = 3.1) and East Anglia (L = 2.6). respectively. (2) There are other days when the period of the dominant spectral amplitude is the same at all the observatories. (3) When Pc 3 and 4 period waves have been detected together, the latitude dependence of the amplitudes supports the theory that the shorter period pulsation is enhanced in the plasmatrough while the longer period wave is enhanced within the plasmasphere.     

Formation of proton fluxes under the radiation belts during geomagnetic disturbances

The fluxes of energetic particles under the radiation belts are studied using data obtained in the experiments onboard the CORONAS-I and CORONAS-F satelites. The spatial structure of the distributions of proton fluxes with E _p > 1 MeV both near the geomagnetic equator on L ≤ 1.2 and at high latitudes on L ～ 3.5–6.5 as well as the particle flux variations with geomagnetic activity are analyzed. The scattering processes that lead to particle precipitation and, in particular, the scattering of protons as they interact with VLF emission and the scattering when the particle motion becomes nonadiabatic are considered. We compare the data on particle dynamics during geomagnetic disturbances of various kinds to determine whether the physical processes that lead to particle precipitation are a manifestation of the geoefficiency of a given magnetic storm or they are controlled by internal magnetospheric conditions.     

The onset time interval of geomagnetic disturbances in the conjugate areas

The onset time interval of geomagnetic disturbances, as deduced from the numerous cases of rapid changes of K-indices over a cycle in solar activity, was studied for two conjugate auroral stations, Macquarie Island and College. There is a distinct peak in the occurrence number of the disturbances for both stations at an interval 09.00–12.00 U.T., which is close to the local midnight at College but is in the pre-midnight sector at Macquarie Island. For comparison, a similar study was applied to the magnetic data obtained at two more auroral stations, Kiruna and Sodankyla. The onset time of the disturbances for these stations was most frequent at 18.00–21.00 U.T., centered at the conjugate midnight of Kiruna and Sodankyla in the Southern Hemisphere but well ahead of the local midnight of the stations themselves. The specific diurnal occurrences of the disturbances at all four auroral stations are consistent with a difference in the geometry between the southern and northern auroral ovals. It appears that the prevailing onset time of geomagnetic disturbances is associated with the time when both conjugate stations (or conjugate locations) are within the auroral oval and thus accessible to a direct particle influx from the Earth's magnetotail.     

Unusual enhancement of galactic cosmic-ray intensity near an interplanetary magnetic field annihilation region

The ground level cosmic-ray intensity enhancement on September 18, 1979 is analysed by combining neutron monitor data with simultaneous in situ interplanetary measurements. An explanation of the increase with respect to local acceleration process, alternate to that suggested by Agrawal and Venkatesan (1982), is given in connection with the effect of a magnetic field annihilation region.