Daily Variation Of Cosmic Ray Intensity Research Articles

PERIODICITY IN DAILY VARIATION OF COSMIC RAY INTENSITY AS AN EFFECT OF SOLAR POLOIDAL MAGNETIC FIELD ORIENTATION

A detailed analysis of the Deep River neutron monitor (NM) data for four different phases of solar activity cycle and for four groups of days chosen according to their different geomagnetic conditions is being carried out. It is found that the 60 quiet day (QD) in a year serve a better purpose for investigating the short/long term variation in cosmic ray (CR) intensity. Furthermore, data has been harmonically analysed for the period 1964–95 to investigate the effect of solar poloidal magnetic field (SPMF) orientation in daily variation (diurnal/semi-diurnal) of CR on geomagnetically QD. The phase of the diurnal and semi-diurnal anisotropy vectors on QD has shown a significant shift to early hours when the SPMF in the northern hemisphere (NH) is positive during the periods 1971–79 and 1992–95 as compared to that during the periods 1964–70 and 1981–90 when the SPMF in NH is negative, showing a periodic nature of daily variation in CR intensity with SPMF.

The enhanced daily variation in cosmic-ray intensity

The study of the super neutron monitor data from Calgary and Sulphur Mountain during January 1964 to May 1967 shows the importance of transient changes of intensity occurring over a few hours and its contribution to the daily variation. Trains of enhanced daily variation lasting a few days occur during recovery periods of fast Forbush and gradual decreases. The diurnal hour of maximum for a total of 250 days of enhanced daily variation during the period occurs at 17–18 h LT, which is the same as that for the rest of the period. The semidiurnal hour of maximum is distributed reasonably well over all hours, which points to its origin from transient changes in intensity.

Daily variation of cosmic rays during quiet and disturbed periods in 1965 and 1966

Data from super neutron monitors at Deep River, Churchill, Resolute, and Alert have been used to study the daily variation of cosmic-ray intensity during 1965 and 1966. Intensities have been examined on a daily, weekly, and monthly basis as a function of the asymptotic direction of vertically incident 7.5-BeV particles. The data have been analyzed in an earth-centered solar-ecliptic coordinate system in which daily (due to the earth's rotation) and seasonal (due to the inclination of the earth's axis to the ecliptic plane) variations of the asymptotic directions are considered.During undisturbed periods the daily variation has been examined by applying a digital filter to the pressure-corrected data and also to the data after subtraction of a variable-amplitude Parker–Axford theoretical diurnal variation. Particular attention has been paid to the dependence of the observed daily variation on the solar-ecliptic latitude of the asymptotic direction.Seventy-three percent of the weeks considered in 1965 and 1966 give the phase of the first harmonic in a direction 85° ± 35 °E. Sixty percent of the weekly periods show a daily variation as a function of solar-ecliptic latitude which is in agreement with the Parker–Axford "streaming-velocity" theory. During Forbush decreases the diurnal phase shifts towards earlier hours and the amplitude increases to two to three times the predecrease level.

Anisotropy in cosmic-ray intensity associated with Forbush decreases

A study of the Forbush decrease of 23 September 1966 shows that the predecrease anisotropy developed from a direction 85° to the west of the sun–earth line. The rate of development of the anisotropy suggests that the "turbulent" plasma producing the Forbush decrease occupied a volume of diameter ≈0.2–0.3 AU. If the plasma clouds away from the earth produced the anisotropy at the earth, then it is reasonable to attribute a part of the highly variable daily variation in cosmic-ray intensity to such distant scattering centers.

Daily variation of cosmic-ray intensity during active and quiet periods of solar activity

Characteristics of daily variation are studied by examining several parametersviz. amplitudes and phases of the various harmonics, individual hourly deviations from the daily mean, positions of hours of maxima and minima as also the actual profile on individual days. Averaged data from several neutron monitors of high counting rate reveal that the daily variation consists mostly oflinear gradients (up or down) culminating into sharp (lasting for an hour or two) or broad (lasting for several hours) maxima or minima. Usually, one maximum and one minimum is obtained per day and their magnitudes and positions are partly interrelated, partly independent. Theaverage directions are 18 h and 6 h in outer space but variations of ± 2 h (occassionally much larger) are observed from day to day. The daily range also varies from 0 to about 1.0%. Groups of days are observed on which the range is very small (about 0.2%); also others on which the minima lie on a steady level and maxima on a changing level or vice versa or both levels changing. Several other characteristics are discussed. Phase characteristics are found to be sharper in Quiet-Sun period 1964 as compared to I.G.Y.

Day-to-day changes in the energy spectrum of the daily variation of cosmic-ray intensity

Day-to-day changes in the ratios of amplitudes and differences between the hours of maxima of the first harmonics of the daily variations of the nucleonic component at middle latitudes and at the equator are studied for the I.G.Y. period. It is concluded that apart from the unique sources of daily variation outside geomagnetic influence and local sources within the geomagnetic field, other sources effective predominantly at middle latitudes only or at equatorial stations only seem to exist for the first harmonic. The average second harmonic seems to be a predominantly equatorial phenomenon due to either a local source or alternatively a source in directions about 120° W or 60° E of the Earth-Sun line and most probably unconnected with the first harmonic.

The apparent sidereal daily variation of cosmic-ray intensity at √ 40 m w.e. at Hobart, Tasmania

Changes in the characteristics of the daily variation of vertical cosmic-ray intensity underground at Hobart since 1958 have been such as to confirm the view that a genuine sidereal component exists. The essential additional evidence since 1958 has beena) that a substantial apparent sidereal effect continues to be observed even when seasonal modulation responsible for sideband phenomena is at a low level, andb) that the total daily variation towards the end of 1961 exhibited changes in phase that were consistent with the dominating influence of a sidereal component. It is shown that the phase anomaly was to be expected on the basis of the observed long-term changes in the amplitudes of the solar and apparent sidereal daily variations. The time of maximum of the first harmonie of the observed sidereal effect, averaged over the five years 1958 to 1962, is 0608±0015 local sidereal time. There is also a significant second harmonic which appears to be in phase with the first. Making provisional allowance for the deflection of primaries in the earth’s magnetic field it seems that the Eight Ascension of the direction of maximum intensity of the primaries responsible for the first harmonic observed at Hobart would not be more than about 0700, with a year to year variability of about one hour.

Energy spectrum of daily variation of cosmic-ray intensity

Using neutron monitor data obtained during I.G.Y., the energy spectrum of the 18 monthly average daily variation (first harmonicr1) is studied, using Webber and Quenby’s coupling constants. No good fit is obtained for any value of the exponentm of the spectrumE−m, the best fit amongst these being form=0.Raoet al.’s conclusion thatm=0 is the best answer is critically examined. Here again, the correlation coefficient between expected and observed amplitudes is only about +0.6. To check whether this could be due to mixing of differenttypes of diurnal variations, data for days of lowr1 values (⩽0.3%) were studied separately. The study showed large scatter in amplitudes which, though partly explainable as due to larger statistical errors, perhaps indicates that there is not much of a quiet day diurnal variation of cosmicray intensity. A similar study for days of high amplitudes (⩾0.7%) shows a correlation of about +0.6 only. A possibility ofm≠0 is slightly indicated. However, instrumental troubles seem to predominate. The second harmonic (r2) shows very erratic behaviour and divergent phasesϕ2.

The apparent sidereal daily variation of cosmic-ray intensity at 42 m w. e. underground at Hobart, Tasmania

The apparent sidereal daily variation of cosmic ray intensity underground at Hobart during 1958 is examined. There is a large annual mean apparent sidereal effect and a comparable anti-sidereal effect, associated with pronounced month by month changes in phase and amplitude of the solar daily variation. A technique is devised which permits the daily variation (d.v.) in sidereal time to be examined month by month, uncomplicated by the presence of certain background components, including for instance, a solar daily variation of constant phase and amplitude. An attempt is made to interpret the apparent sidereal effect on the assumption:a) that it is entirely spurious, arising from a solartime modulation of the solar daily variation;b) that there is a genuine part to the effect, consisting of a sidereal d.v. of constant phase and amplitude, and a spurious part, arising from an annual variation of amplitude of the solar d.v., produced by a solar-time mechanism;c) that the effect is entirely genuine, arising from a sidereal-time modulated sidereal daily variation. The theory for each case is tested against the observations by means of models. When various simple possibilities are considered, it is found that the evidence clearly favours assumptionc). The hypothesis which gives best agreement with the observations is that a solar d.v. of constant amplitude and phase is accompanied by a sidereal d.v. with semi-annual variation of amplitude and phase. In that case, the unmodulated sidereal d.v. would have amplitude 0.09% and RA∼0700 h, suggesting that the source of the anisotropy may be in the direction looking outward along the proposed spiral arm magnetic field.

Effects of short-term world-wide modulation of the primary cosmic radiation on observed daily intensity variations

The possible magnitude of contributions to the observed solar daily variation of cosmic-ray intensity made by irregular short-term fluctuations of the primary intensity is discussed. It is shown that such fluctuations of an average type and range not inconsistent with observed patterns of change in bihourly intensity values could contribute important spurious universal time components to short-period mean daily variations.

Directional Measurements of the Diurnal Variation of Cosmic-Ray Intensity

Measurements of the solar daily variation in cosmic-ray intensity have been made for a total of 360 days using two arrays of Geiger-Muller counters inclined at 45° to the vertical in the North and South directions. Both the diurnal and semi-diurnal variations are found to be significantly different for the two directions, and since the particles recorded by the two sets have passed through the same amount of atmosphere under similar conditions, it must be supposed that at least a part of the variation is due to causes which lie outside the earth's atmosphere. It is found that both the amplitude and phase of the daily variation for the South-pointing recorder vary with season, whereas in the North direction the amplitude only shows any seasonal variation. The amplitude is greatest in summer in both directions. The experimental results are compared with those of earlier workers and are discussed in relation to some of the interpretations of the daily variation which have been put forward. The difference between the variations in the two directions is interpreted as evidence for a non-isotropic distribution of the primary cosmic rays in space. A variation with sidereal and not with solar time would be expected under these conditions, but it is pointed out that the magnetic field of the sun may modify the direction of arrival of the primary rays at the earth so as to produce a resultant solar time variation.

The Semi-Diurnal Variation in Cosmic Ray Intensity

The experimental data on the daily variation in cosmic-ray intensity are first surveyed. It appears to be established by Rau's experiments that the energetic meson component at sea level shows a marked semi-diurnal variation which is in phase with the semi-diurnal variation shown by the barometric pressure. Several authors have suggested that this phenomenon is explicable in terms of the Pekeris theory of atmospheric oscillations. The implications of this explanation on the process of meson formation are here examined quantitatively. It is concluded that the explanation is only possible if mesons arise mainly at about 60 or 70 km. above sea level, which is highly unlikely since the corresponding cross-section for meson production would be much larger than appears plausible.