Solar Diurnal Variation Research Articles

Galactic cosmic ray transport in the heliosphere: Diffusive anisotropy 1965–1994

Preliminary information pertaining to the diffusive anisotropy of the galactic cosmic rays (GCR) in the inner heliosphere is derived from the study of the steady state solar diurnal variation (SDV) for the period 1965 to 1994. This time period spans three solar activity cycles. Data obtained with the neutron monitors (NM) at Deep River and Huancayo and the underground vertical muon telescopes (UT) at Embudo, are used in our analysis.

Magnetic Cycle Dependence of the Cosmic-Ray Diurnal Anisotropy

The two components of the solar diurnal variation observed with two detectors characterized by linearly independent coupling functions have been used to estimate the free space anisotropy vector during the period 1968–1995 using the least-squares method (LSM). The values of Rcshow ∼20-year magnetic cycle with the lowest values at solar activity minima for positive polarity (qA>0). A good correlation is obtained between Rcand the IMF magnitude. The amplitude of the radial anisotropy (AR) shows ∼20-year magnetic cycle with the highest values around solar activity minima for qA>0 (1975–1976 and 1995), whereas that of the east-west (Aφ) is minimum. This results in shifting the anisotropy vector to the earliest hours. The amplitude of the anisotropy is high around solar maxima and low around solar minima. It is also enhanced during the declining phase of solar activity (1971, 1984–1985, and 1991). Our results of the anisotropy have been used to calculate the cosmic-ray radial and transverse gradients. The value of the radial gradient exhibits a magnetic polarity dependence as well, with larger value during qA 0.

Solar semidiurnal anisotropy of galactic cosmic ray intensity observed by the two‐hemisphere network of surface‐level muon telescopes

Observations made by the two‐hemisphere network of surface‐level, multidirectional muon telescopes at Hobart (Tasmania, Australia) and Nagoya (Aichi, Japan) are used to examine the origin of the solar semidiurnal variation in cosmic ray intensity. The network allows us to precisely determine the asymmetry of the variation across both hemispheres. It is shown that the variation is consistent with the north–south (NS) symmetric distribution of cosmic ray intensity in space. The phase of the space harmonic vector responsible for the variation is consistent with both the second‐order anisotropy expected from a bidirectional latitudinal density gradient (type I) and also one arising from pitch angle scattering (type II). The network also observed a purely NS antisymmetric, antisidereal diurnal variation with the maximum phases differing by 12 hours between the two hemispheres. This is consistent with an antisidereal diurnal variation arising from annual modulation of the solar diurnal variation produced by a second‐order anisotropy. The phase of the space harmonic vector responsible for the antisidereal diurnal variation is consistent with the phases predicted from both type I and type II anisotropies. It is shown, however, that the ratio of the amplitude of the space harmonic vector of the antisidereal diurnal variation to that of the solar semidiurnal variations is consistent with the type II anisotropy but not with the type I anisotropy. This result implies that the solar semidiurnal variation and the antisidereal diurnal variation observed during the period 1992–1995 mainly arise from the type II anisotropy and cannot be explained solely as arising from the type I anisotropy.

Galactic cosmic ray modulation and the passage of the heliospheric current sheet at Earth

We have examined the effect of heliospheric current sheet (HCS) crossings on cosmic ray intensity (CRI), solar wind speed, interplanetary magnetic field (IMF) and the cosmic ray solar diurnal anisotropy in free space. Data from eight neutron monitors and three muon telescopes, covering a wide energy and latitude range, were separated into two epochs, 1971–1979 (A > 0) and 1981–1990 (A < 0) according to the solar magnetic polarity. The CRI is more strongly affected during the A > 0 than during the A < 0 epoch. The increase in CRI associated with A-T HSC crossings was larger in magnitude than that of T-A crossings, and the magnitude was dependent on the rigidity of the particles. A symmetric negative cosmic ray gradient was observed with respect to the HCS. HCS crossings have no observable effect on the CRI observed by shallow underground detectors. Changes in solar wind speed and IMF magnitude, near HCS crossings, are related to the distance of the Earth from the sheet. We found no consistent changes in the solar diurnal variation related to HCS crossings.

Cosmic‐Ray Modulation Parameters Derived from the Solar Diurnal Variation

From an analysis of the solar diurnal variation in Galactic cosmic rays with rigidities between 17 and 195 GV from 1957 to 1990, we have derived the product of the parallel mean free path and the radial density gradient () and the bidirectional latitudinal gradient indicator G|z|. We find the rigidity spectrum of the solar diurnal anisotropy averaged over the entire time span to have a spectral index of -0.1 ± 0.2 and an upper limiting rigidity of 100 ± 25 GV. The yearly averaged spectrum may depend on the heliospheric polarity state, having a slightly positive spectral index during the A > 0 epoch. We find that the parameter depends inversely on rigidity, more so during the A > 0 polarity state. Around solar minimum of this epoch, is also much lower than for corresponding years in the A < 0 polarity state. Combining the derived values of with results from previous studies of the radial gradient, we find evidence for the values of λ (at all the rigidities examined) having a variation related to the magnetic polarity state of the heliosphere. G|z| indicates that at rigidities up to 185 GV a bidirectional latitudinal gradient exists and changes direction following solar magnetic polarity reversals, in agreement with drift theories. This gradient attains its largest magnitudes around times of solar minimum and also appears to be larger during A < 0 polarity states.

Features of the magnetic field of the equatorial electrojet determined from experimental data for the South-Eastern Asia region

Abstract Experimental data, obtained in Vietnam, have been used for studying the solar-diurnal variations of the magnetic field in the equatorial electrojet region and for comparing them with the worldwide part of Sq-variations. The features of equatorial electrojet variations were investigated at different stages, from their appearance in the morning, to their decline in the evening and their disappearance at night. The character of dip equatorial variations on days of a reverse jet electric current is considered. It is shown that eastward electrojet currents in the daytime ionosphere gradually decay far into the night. The question of a shift of the electrojet axis during the day is examined.

Analyses of sidereal and solar anisotropies in cosmic rays

The Sun's interplanetary magnetic field and the solar wind modulate the distribution of galactic cosmic-ray particles in the heliosphere. The particles diffuse inward, convert outward and have drifts in the motion of their gyro-centres. Irregularities in the IMF also scatter particles from their gyro-orbits. These processes are the components of solar modulation and produce streaming (and higher-order anisotropies) of particles in the heliosphere. The anisotropies can be investigated at the Earth by examining the count rates of cosmic-ray detectors. The anisotropic streams appear as diurnal variations in solar and sidereal time in the count rates. Higher-order anisotropies produce generally much smaller semi-diurnal and higher-order variations. Theoretical models of solar modulation predict effects that depend on the polarity of the Sun's magnetic dipole. The solar diurnal and north-south anisotropies can be used to test these predictions. This paper is a short review of analyses of 60 years of cosmic-ray data collected at the Earth for the solar and sidereal diurnal variations present. Past analyses have yielded interesting and controversial results regarding the rigidity spectra and components of these anisotropies. Some of the controversy remains today. Analyses of these anisotropies have also yielded quantitative information about parameters important to solar modulation, such as latitudinal and radial density gradients. The relatively new techniques used for these determinations are explained here. Calculations of these modulation parameters from Earth-based cosmic-ray detectors are reviewed and compared to spaceprobe measurements and theoretical predictions of their values. Recently, investigations of the sidereal and solar diurnal anisotropies have been combined to calculate mean-free-paths of cosmic rays in the heliosphere. The latest conclusions from these analyses are that the parallel mean-free-paths of cosmic rays may depend on the polarity of the Sun's magnetic field. The results of these investigations are included in this paper to indicate the present state of knowledge concerning this facet of cosmic-ray research.

Long-term behavior of latitudinal cosmic-ray gradients

We have used neutron monitor data covering a wide range of energy over a period of 22 years (1966–1987), as well as sea-level multidirectional meson telescope data from Nagoya to examine the latitude effect of solar diurnal vectors and its dependence on the polarity of interplanetary magnetic field (IMF). By sorting the daily cosmic-ray data according to whether the IMF is toward (T) or away (A) from the Sun, the annual mean solar diurnal variations (amplitude and phase) for the T and A days were determined separately. Results showed a northward-pointing latitudinal gradient from neutron monitors of the most northerly latitudes, and a predominant southward gradient at high southerly latitudes. The resultant latitudinal cosmic-ray gradients are the sum of two gradients: a north-south symmetry gradient (occurring in minimum and maximum solar activity years), and a north-south asymmetry gradient (occurring during different phases of solar activity cycles). The difference vector (T - A) between the solar diurnal vector for two groups was calculated, which represents a good indicator for the resultant perpendicular gradient relative to the Earth. This difference vector shows a considerable change in phase for detectors located in the northern hemisphere of the Earth. On the other hand, there exists much less change in phase for detectors located in the southern hemisphere.

Diurnal variations of cosmic rays during a solar magnetic cycle

AbstractWe have used data from five neutron monitor stations with primary rigidity (Rm) ranging from 16 GeV to 33 GeV to study the diurnal variations of cosmic rays over the period: 1965–1986 covering one 22‐year solar magnetic cycle. The heliosphere interplanetary magnetic field (IMF) and plasma hourly measurements taken near Earth orbit, by a variety of spacecraft, are also used to compare with the results of solar diurnal variation. The local time of maximum of solar diurnal diurnal variations displays a 22‐year cycle due to the solar polar magnetic field polarities. In general, the annual mean of solar diurnal amplitudes, magnitude of IMF and plasma parameters are found to show separte solar cycle variations. Moreover, during the declining period of the twenty and twenty‐ne solar cycles, large solar diurnal amplitudes are observed which associated with high values of solar wind speed, plasma temperature and interplanetary magnetic field magnitude B3.

Galactic cosmic-ray modulation effect by solar-wind streams

Data, from the worldwide network of neutron monitors, recorded at Deep River, Hermanus, Rome, Tokyo, and Huancayo, over two solar cycles (Nos. 20 and 21) are analyzed to study the long-term variations of the solar diurnal variations as they relate to solar-wind speed. The median primary rigidities of response (Rm) for these detectors cover the range 16 GV ≤ Rm ≤ 33 GV. We discuss the solar diurnal variations (amplitude and phase) of cosmic rays as a function of solar activity. The behavior of solar diurnal phases is completely different for the two epochs of high-wind speed. Data of solar-wind speed from 1966–1986 are classified according to the state of the daily mean values. Variation in the amplitudes of the diurnal variations, as functions of the median primary rigidity of cosmic rays, for the two selected periods (1973–1975 and 1979–1981) of high and low solar-wind speeds were determined at the selected stations. The rigidity dependence of the averaged solar diurnal variations of cosmic rays related to the high solar-wind speed was studied. The most sensitive rigidity of modulation is around 20 and 30 GV during the 1973–1975 and 1979–1981 periods, respectively. Our results also show that there is a significant correlation in the solar diurnal amplitudes between the two divisions of high and low solar-wind speed days.

Solar Diurnal and Semi-Diurnal Variations of Cosmic Rays Observed by the North-South Network or Surface-Level Multi-Directional Muon Telescopes.

A scintillator muon telescope was installed at the Hobart Campus of the University of Tasmania (42.85°S, 147.42°E), Australia in December 1991. It is part of a North-South Network of Multi-Directional Cosmic Ray Muon Surface Telescopes, in conjunction with the Nagoya Cosmic Ray Muon Telescope (35.12°N, 136.97°E). Using two years of data from the North-South Network, we have investigated the average features of the solar diurnal and semi-diurnal variations. Preliminary results of the analysis show that two years averages of the observed diurnal variations are composed of a north-south symmetric and an asymmetric variations. The north-south asymmetric diurnal variation is consistent with that expected from the semi-diurnal anisotropy which produces the observed semi-diurnal variation. It is shown that the reference axis of the anisotropy is in the direction of about 7°S sunward from the ecliptic.

Radial and Latitudinal Gradients in Galactic Cosmic Rays

AbstractWe have deduced the yearly averaged value of the solar diurnal variation as observed by a surface muon telescope and three underground muon telescopes over the years 1957 to 1985. This has allowed us to examine the temporal variation in both the latitudinal gradient Gz and the product of the parallel mean free path and the radial gradient of galactic cosmic rays during three consecutive solar cycles. The median rigidities of the primary particles being detected by the telescopes are 50 GV in the case of the surface muon telescope and greater than 150 GV in the case of the underground muon telescopes. We have compared our results with those of a similar study made from observations of the solar diurnal variation by neutron monitors and an ion chamber, which have median rigidities of response between 17 and 70 GV (Bieber and Chen 1991a). The product has a solar magnetic cycle dependence and our values are lower than those observed by neutron monitors, in agreement with the Bieber and Chen observation that reverses after a solar magnetic field reversal, in accordance with drift theories.

Geomagnetic quiet daily variations in the Australian region information from a new station at Charters Towers (20.1°s)

The establishment of the new magnetic observatory at Charters Towers is described. Hourly values of the magnetic elements are analysed from two data sets (1984–1985 and 1986–1987) to provide solar diurnal variations during different months. These are compared with other stations in Australia and Papua New Guinea to study the behaviour of the focus of the Sq current systems. The unar geomagnetic variations are consistent with other Australian stations. An oceanic lunar tide is detected in the vertical element Z.

Observation of Earth's Orbital Motion Using Cosmic-Ray Compton-Getting Effect at Matsushiro Underground Station.

The Earth's motion Compton-Getting effect on the solar diurnal variation of cosmic ray intensity, has been examined by using observed data at Matsushiro underground station located at the effective vertical depth of 220m.w.e. underground (threshold energy -7.1010eV, median primary energy≥7.1011eV) for the period 1980-1990. In the analysis, to eliminate the atmospheric effects on the muon intensity, the differences of the observed intensity between two-directional telescopes; the east-pointing (E-) and the west-pointing (W-) telescope, are used. Also the analysis is made for two different periods of the solar activity; the quiet period for averaged sunspot numbers Z 60 (1980-1984 and 1988-1989). It is shown that for the quiet period, the difference E-W diurnal vector has the phase of 22.5±1.3hr LT and the amplitude of 0.030±0.007%. With a clockwise rotation of this E-W vector by the right angle, we may compare it with that expected from the Earth's motion Compton-Getting effect. It is found that the observed (0.030±0.007% in 4.5±1.3hr LT) and the expected vector (-0.021% in -6hr LT) are in fair agreement with each other. It is also found that for the active period, the observed E-W diurnal vector seems likely to show some contribution from the solar diurnal anisotropy, in addition to the Compton-Getting effect on it. This suggests that for the last solar active period the upper cut-off energy of solar diurnal anisotropy might be as high as that (several 1011eV) for the threshold (-7.1010eV) of 220m.w.e. depth underground.

Long-term variation of solar diurnal variation of cosmic ray nucleonic components.

Les donnees d'un reseau mondial de detecteurs neutroniques sont utilisees pour analyser la variation a long terme de la variation diurne solaire de la composante nucleonique du rayonnement cosmique afin de souligner l'existence d'accroissements significatifs de l'amplitude diurne solaire de la composante nucleonique du rayonnement cosmique dans les periodes decroissantes de l'activite solaire

A survey of the cosmic ray diurnal variation during 1973–1979—I. Persistence of solar diurnal variation

We find that the solar diurnal variation in cosmic ray intensity is a persistent phenomenon over the years 1973–1979. That is, even during solar minimum, conditions in the heliosphere lead to a net flux of cosmic rays from a particular (time-varying) direction in space. If we regard the daily fluctuations in the amplitude and phase of the diurnal variation as random perturbations about the mean vector, we are able to quantify the relative magnitude of the random component.

The diurnal anisotropy of cosmic rays and the heliospheric transport parameters

We review the data on long-term changes in the solar diurnal variation of cosmic rays, observed at Deep River with a neutron monitor and at Embudo with an underground muon telescope, during the period 1965–1976. We show that the model proposed by Erdos and Kota (1979, Sixteenth ICCR, Kyoto. Conference Papers 4, 45). cannot explain the systematic changes seen in our data during 1971–1976. An insight gained from the diffusion-convection model enables us to obtain a unique solution for K ‖ G r in terms of the parameters applicable to the solar wind and the diurnal anisotropy. This helps us calculate the rigidity ( R) dependence of the diffusion coefficient K ‖. We find that K ‖ = 1.37 × 10 18 R 0.6 m 2 s −1, where R is in gigavolts, and K ⊥ = 1.08 × 10 18 m 2 s −1. Our results are discussed.

Recurrence of periods of enhanced cosmic ray solar diurnal variation associated with a two‐sector structure

Swinson et al. (1981) showed that in 1974 the two‐sector structure of the interplanetary magnetic field gave rise to conditions in which there were extended trains of enhanced diurnal variation in cosmic ray intensity which were organized according to the sector structure. In this paper we show that the heliomagnetosphere, in its continuing evolution, has again produced a distinct two‐sector structure in 1984. We demonstrate, using neutron monitor data and data from underground muon telescopes, that these conditions once more give rise to extended periods of enhanced cosmic ray solar diurnal variation, again organized by the sector structure. These field‐dependent anisotropies are consistent with the observations and expectations described in the earlier paper.

Simulation of solar tides in the Canadian Climate Centre general circulation model

Two 60‐day simulations of the global atmospheric circulation produced by the Canadian Climate Centre general circulation model were analyzed to determine the solar diurnal and semidiurnal variations in the wind, pressure, and precipitation fields. In these simulations the radiative transfer calculations were performed every hour in order to resolve adequately the diurnal cycle. One simulation was for boreal summer, and the other was for winter conditions. The diurnal and semidiurnal surface pressure variations in the model were found to be quite realistic, although somewhat smaller than actually observed. Even some fairly subtle details of the geographical and seasonal variability of the solar tidal pressure oscillation were well simulated. The daily cycle of the wind in the model also appears to be reasonably consistent with the limited observations available. By contrast, the diurnal and semidiurnal variations of precipitation were not successfully simulated by the model. Linear tidal theory calculations were performed with thermal forcings, vertical resolution, and boundary conditions similar to those in the full general circulation model. These calculations allowed an evaluation of the effects of the different numerical approximations in the general circulation model on the simulated tide. One striking result of these linear computations was the indication of a tendency for the effects of the “rigid‐lid” upper boundary condition to compensate for the omission of the thermal tidal forcing above the top level of the model.

On the determination of K-indices by computer

At a magnetic observatory, a 'K-index' is assigned to each 3 h interval of recording to be a measure of whether conditions were quiet or active during this time. A K-index measures the maximum amplitude or disturbance variations, independent of the regular daily (or solar diurnal) variation. The numbers used for K-indices are the digits from 0 to 9, with higher numbers corresponding to greater magnetic activity. The K-index scale is quasi-logarithmic, and may be thought of as being like the magnitude scale for earthquakes in seismology. Thus K = 0 means very quiet magnetic conditions and K = 9 means a very strong magnetic storm. More information on K-index scales is given in Parkinson (1983).