Diurnal Variation Of Cosmic Rays Research Articles

Selection of the Solar-Diurnal Anisotropy of Cosmic Rays by Local and Global Methods

According to the data of Moscow neutron monitor, using harmonic analysis, the characteristics ofthe solar-diurnal anisotropy of cosmic rays on quiet days have been obtained for a long period from 1965 to2020. It has been established that the average diurnal variation of cosmic rays at the Moscow neutron monitoris almost completely described by two harmonics of the solar-diurnal anisotropy and does not contain signsof other influences. A comparison with the average daily characteristics of the equatorial component of cosmicrays vector anisotropy, obtained from the data of the worldwide neutron monitor network using the globalsurvey method showed a good agreement between the results of the two methods. From a comparison of localand global results, estimates were obtained for the coupling coefficients of the first harmonic of the cosmicray anisotropy for the Moscow neutron monitor, and a new experimental method for calculating the couplingcoefficients of individual detectors was proposed. The limitations of the local method, as well as the possibilityof continuing and expanding this study, have been discussed and justified

Solar magnetic polarity dependency of the cosmic ray diurnal variation

AbstractCosmic rays (CRs) data observed through four neutron monitors and one muon telescope are used to study the solar diurnal variation (DV) from 1953 to 2010. The median primary rigidity of response, (Rm), for these detectors encompasses the range 16 ≤ Rm ≤ 60 GV of the galactic CRs spectrum. The phase angle observed with the lower rigidity station depends strongly on the solar magnetic field polarity reversal due to drift effect. However, the higher rigidity stations show an 11 year cycle. This cycle is evident in the shape of the cross‐correlation functions (CCFs) between the phase angle and the interplanetary magnetic field (IMF) parameters due to diffusion of galactic CRs by the IMF. The amplitude of the DV, IMF magnitude, and solar activity are larger, and the phase angle is later in hours for qA < 0 than for qA > 0. In addition, the amplitude is well correlated with the IMF magnitude. The amplitudes observed by the lower rigidity station peaks 2 years after the IMF magnitude reach maximum for qA < 0. The diurnal amplitude reaches minimum 2–3 years after the solar wind speed peak due to outward convection by solar wind along the helioequator. The phase angle of the higher rigidity station shifts toward the earliest hours, 2 years after the wind speed peaks for qA > 0. This shift is due to outward convection by solar wind that increases the radial component of the DV more than the East‐West component.

Study of the Diurnal Variation of Cosmic Rays during Different Phases of Solar Activity

The pressure-corrected hourly counting rate data of ground-based super neutron monitor stations, situated in different latitudes, have been employed to study the characteristics of the long-term variation of cosmic-ray diurnal anisotropy for a long (44-year) period (1965 – 2008). Some of these super neutron monitors are situated in low latitudes with high cutoff rigidity. Annual averages of the diurnal amplitudes and phases have been obtained for each station. It is found that the amplitude of the diurnal anisotropy varies with a period of one solar activity cycle (11 years), whereas the diurnal phase varies with a period of 22 years (one solar magnetic cycle). The average diurnal amplitudes and phases have also been calculated by grouping the days on the basis of ascending and descending periods of each solar cycle (Cycles 20, 21, 22, and 23). Systematic and significant differences are observed in the characteristics of the diurnal variation between the descending periods of the odd and even solar cycles. The overall vector averages of the descending periods of the even solar cycles (20 and 22) show significantly smaller diurnal amplitudes compared to the vector averages of the descending periods of the odd solar cycles (21 and 23). In contrast, we find a large diurnal phase shift to earlier hours only during the descending periods of even solar cycles (20 and 22), as compared to almost no shift in the diurnal phase during the descending periods of odd solar cycles. Further, the overall vector average diurnal amplitudes of the ascending period of odd and even solar cycles remain invariant from one ascending period to the other, or even between the even and odd solar cycles. However, we do find a significant diurnal phase shift to earlier hours during the ascending periods of odd solar cycles (21 and 23) in comparison to the diurnal phase in the ascending periods of even solar cycles (20 and 22).

Study of the Diurnal Variation of Cosmic Rays during Different Phases of Solar Activity

Study of the Diurnal Variation of Cosmic Rays during Different Phases of Solar Activity

Study of high and low amplitude wave trains of cosmic ray diurnal variation during solar cycle 23

A detailed study has been conducted on the long-term changes in the diurnal variation of cosmic rays in terms of high and low amplitude wave trains event (HAEs/LAEs) during the period 1996–2008 (solar cycle 23), using the neutron monitor data from Kiel neutron monitoring station. As such, 17 HAE and 48 LAE cases have been detected and analyzed. These HAEs appear quite dominantly during the declining phase as well as near the maximum of the solar activity cycle 23. In contrast, the low amplitude events (LAEs) are inversely correlated with solar activity cycle. In fact, LAEs appear quite dominantly during the minimum phase of the solar activity. When we compare our results for diurnal phase with that observed on an annual average basis, we notice no significant diurnal phase shift for HAEs as well as for LAEs. Moreover, we find that the high-speed solar wind streams (HSSWS) do not play any significant role in causing these variations. These results are discussed on the basis of that observed in earlier cycles.

The influence of the sidereal cosmic-ray anisotropies originated on the tail- and nose-boundaries of the heliomagnetosphere (HMS) upon the solar cosmic-ray anisotropy produced inside the HMS

The solar diurnal variation of cosmic rays was considered to be fully explained by the diffusion-convection theory. Recently, however, it has been found that the geographic latitude distribution of the yearly averaged diurnal variations observed with the neutron monitors and the muon telescopes on the ground does not agree with that expected from the theory. The difference between the two distributions is observed almost every year, and it is especially remarkable in the solar activity minimum period in the positive polarity state of the solar polar magnetic field, when the diurnal variation reduces its magnitude and shifts its phase towards the morning side. It is shown that such a difference is produced by the seasonal variation of the sidereal heliotail-in and helionose-in anisotropies with respective right ascensions of 6 h and 18 h according to the following process. Generally, if any sidereal anisotropy from the right ascension α is subject to the seasonal variation with its maximum and minimum at the times when the Earth is closest to and farthest from the source of the anisotropy, respectively, located through its direction at the boundary of the HMS, it produces a yearly averaged residual flux from 0 h LT in solar geographic polar coordinates regardless of its direction (α). This residual flux from 0 h LT produces the difference mentioned above.

Unusual anisotropic wave trains in cosmic rays

Several workers have attempted to find out the possible origin of the “high amplitude wave trains” of enhanced diurnal variation of cosmic rays and to develop a suitable realistic theoretical model that can explain the different harmonics in individual days. The various observed harmonics of the cosmic-ray variation may be understood on a unified basis if the free-space cosmic-ray anisotropy is non-sinusoidal in form. The major objective of this paper is to study the first three harmonics of high-amplitude wave trains of cosmic-ray intensity over the period 1981–1994 for Deep River neutron monitoring station. The main characteristic of these events is that the high-amplitude wave trains show a maximum intensity of diurnal component in a direction earlier than 18:00 h/co-rotational direction. It is noteworthy that the amplitude significantly enhanced and the phase remains in the co-rotational direction during the years close to solar-activity maximum for first harmonic. Significant deviations have been observed in the semi-diurnal amplitude after the onset of solar-activity maximum. This leads us to conclude that the amplitude as well as direction of the first harmonic and the amplitude of second harmonic are correlated with solar-activity cycle during these HAEs. The amplitude and phase of all the three harmonics (diurnal/semi-diurnal/tri-diurnal) are not found to depend on the polarity of Bz component of interplanetary magnetic field for long-term variation. The occurrence of high-amplitude events is dominant for the positive polarity of Bz component of IMF. The occurrence of HAEs is dominant during the period of average solar-wind velocity, but their occurrence during HSSWSs cannot be denied. The possibility of occurrence of these events is more during the periods of co-rotating streams. The occurrence of HAE is dominant when Dst-index remains negative and this point is not reported earlier in the litterature. All the high amplitude events occurred, when geomagnetic activity index, Ap, remains ⩽20.

The Influence of Cosmic‐Ray Modulation at High Heliospheric Latitudes on the Solar Diurnal Variation Observed at Earth

During the solar minimum period of 1954 the cosmic-ray diurnal variation as observed by neutron monitors and muon telescopes underwent a dramatic swing in its direction of maximum intensity, from the normal value between 16:00 and 18:00 local time to as early as 08:00. It is shown that this swing can be explained as being due to a negative radial density gradient of cosmic rays in the inner heliosphere and that this negative gradient is caused by large radial and latitudinal diffusion mean free paths that bring in particles from high latitudes. In principle, such large diffusion mean free paths should simultaneously cause high intensities, as were observed in 1954.

Variations in the cosmic radiation, 1890–1986, and the solar and terrestrial implications

Comparison of cosmogenic 10Be data and the records from several independent cosmic ray instruments for the period 1933–1977 demonstrates that the 10Be data provide a reliable measure of the ∼2 GeV/nucleon cosmic ray intensity at Earth, over short (∼5 years) and long (>20 years) time scales. The 10Be data show that the cosmic ray flux at sunspot minimum was anticorrelated with Z min, the sunspot number at solar minimum over the interval 1901–1986. There was a 26% enhancement in the 10Be data in the interval 1951–1954, and the data from two independent cosmic ray instruments verify that this was due an influx of low energy (<3 GeV/nucleon) cosmic radiation. It is suggested that this enhancement was associated with the very low sunspot number in 1954 (one of the lowest outside the Maunder and Dalton Minima), and with an anomalous cosmic ray diurnal variation in 1954. It is proposed that these several observations are the consequence of cosmic ray drift effects, and that the heliomagnetic field was relatively ordered in 1954. It is shown that the enhancement of 1954 is not consistent with the several models of the open solar magnetic field, unless factors other than the magnetic field strength are taken into account. Using the instrumental and 10Be data, the cosmic ray spectra are estimated for 1965, 1954, 1890–1895, 1810–1820, and 1680–1720. From these, the ionization in the Earth's polar atmosphere is estimated for each of these periods.

Characteristics of enhanced and low-amplitude cosmic-ray diurnal variation

The occurrence of a large number of high- and low-amplitude cosmic-ray diurnal wave trains during the two solar cycles (20 and 21) over the years 1965–1990 has been examined as a function of solar activity. The high-amplitude days with the time of maximum in the ∼ 18:00 hr corotation direction do not indicate any significant correlation with solar activity. But, the low-amplitude days are inversely correlated with solar activity and the time of maximum shifts to earlier hours (∼ 15:00 hr direction). The slope of the power-specrum density roughly characterized by power spectral index ‘n’ in the high-frequency range 3.5 x 10−5 Hz to 8.3 x 10−4 Hz (time scales of 20 min to 8 hr) is different for the two classes of events. A suggestion is made that the enhanced and low-amplitude cosmic-ray diurnal variations are produced by different types of interplanetary magnetic field distributions.

Enhanced Cosmic Ray Diurnal Variations in Mawson and Hobart Neutron Monitor and Underground Data Records

AbstractAnalysis of surface and underground detector data from Mawson and Hobart for the period 1982 to 1988 has revealed a number of episodes of enhanced diurnal variation lasting more than 5 days. A preliminary study of these enhancements shows that variations in the rigidity spectrum and in the upper limiting rigidity must be present to explain the phenomenon.

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.

A survey of the cosmic ray diurnal variation during 1973–1979—II. Application of diffusion-convection model to diurnal anisotropy data

We have developed a matrix approach to the application of the Diffusion-Convection Model (DCM) of cosmic ray transport to high rigidity ( R) particles detected at the Earth. The equations are treated parametrically in terms of the product of “coupled variables” in order to deduce the behavior of the heliospheric transport parameters during 1973–1979. The variables can be uncoupled only by using additional information about the heliosphere. Assuming an R 2 rigidity-dependence for K ‖ as suggested by Jokipii (1971, Rev. Geophys. Space Phys. 9, 27), we show that the resultant radial gradients ( G r ) come out to be extremely small. We conclude that the rigidity dependence for K ‖ must be considerably flatter. It appears that the diurnal variation observed at Embudo during 1976 may require a contribution from the charged particle drifts. Our results are discussed.

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.

Long-term variations in north-south asymmetry of solar activity

We present a new set of data on relative sunspot number (total, northern hemisphere, and southern hemisphere), taken for the 37-yr period 1947 to 1983; this constitutes a particularly coherent and consistent set of data, taken by the same observer (Hisako Koyama) using the same observing instrument. These data are combined with earlier data (White and Trotter, 1977) on the variation of sunspot areas for both solar hemispheres from 1874 to 1971. The combined data, covering 110 years and 10 solar cycles, are examined for periodicity in solar activity north-south asymmetry. We show that, in general, northern hemisphere activity, displayed as either An/(An + As) or Rn/(Rn + Rs), peaks about two years after sunspot minimum. This peak is greater during even cycles, pointing to a 22-yr periodicity in north-south asymmetry in solar activity, suggesting that the asymmetry is related to the 22-yr solar magnetic cycle. We demonstrate that the largest and most protracted period of northern-hemisphere activity excess in the last 110 years has occurred from 1959 to 1970; we show that there is a strong correlation between northern activity excess and a cosmic-ray density gradient perpendicular to the ecliptic plane, pointing southward, which is evident in cosmic-ray diurnal variation data from the Embudo underground cosmic-ray telescope.

Cosmic ray sidereal diurnal variation of galactic origin observed by neutron monitors

We have analyzed the sidereal diurnal variation of cosmic rays, using 620 station-years of neutron monitor data during the period 1958–1979. The sidereal variation averaged over the period for all the stations in the Northern Hemisphere is different from the corresponding variation in the Southern Hemisphere. The difference is statistically significant and can be identified with the spurious sidereal variation produced from the stationary anisotropy of solar origin, responsible for the solar semi-diurnal variation. The variation common to both hemispheres is also exceptionally significant from the statistical point of view and could be regarded as being due to a uni-directional galactic anisotropy. This variation has an amplitude of 0.0204 ± 0.0015% and a phase of 6.8 ± 0.3 h and is clearly different from that ( ∼ 0.05%, 0 ∼ 3 h) observed in the high rigidity region (500 ∼ 10 4 GV). The physical meaning of the variation is discussed from the standpoint of the heliomagnetospheric modulation of galactic anisotropy.

A unified theory of cosmic ray diurnal variations

The various observed harmonics of the cosmic ray diurnal variation may be understood on a unified basis if the free space cosmic ray anisotropy is non‐sinusoidal in form. The major objective of this report is to emphasize that such a non‐sinusoidal anisotropy is a natural result of the normal interplanetary transport processes of pitch angle scattering and adiabatic focusing in a spatially varying mean field. Interplanetary scattering parameters λ ∼ 0.5 AU and q ∼ 1.1 at 10 GV rigidity — values that are quite consistent with current knowledge of interplanetary transport processes — lead to predicted semidiurnal and tridiurnal variations that agree with observation. Thus, there is no need to invoke exotic processes to explain the higher order harmonics of the cosmic ray diurnal variation.

The power spectrum analysis of the cosmic-ray diurnal variation

We present a study of the CR diurnal variation by means of the power spectrum analysis of the neutron monitor flux as observed by a group of nonpolar stations. The power spectrum structure indicates a different behaviour for the two components—the steady and the transient—of the diurnal variation. The steady diurnal variation is found to be identical to the one obtained by averaging the diurnal vectors over a one-year period. Moreover, it appears to be made up of two parts, the first one apparently constant over a period of at least 8 years and the second one whose amplitude and phase appear randomly variable over a period comparable to a fraction of a year. The transient diurnal variation instead is correlated to the fluctuations of the interplanetary magnetic field and it is in agreement with the scintillation model of Jokipii and Owens.

Relationships between the enhanced diurnal variation of cosmic rays and the interplanetary magnetic field

Relationships between the enhanced diurnal variation of cosmic rays and the interplanetary magnetic field

Study of cosmic ray diurnal variation on a day-to-day basis

From a careful examination of the diurnal variation of cosmic ray intensity at high energies and the interplanetary field characteristics, the average characteristics of diurnal variation were recently explained by us in terms of a balance between outward convection and field aligned diffusion, the latter arising out of a positive radial density gradient. In this paper, we extend this new concept to explain the large variability observed in the diurnal variation on a day-to-day basis and further demonstrate that the measurement of diurnal anisotropy characteristic of cosmic ray particles on a day-to-day basis can be used directly to infer the nature and scale sizes of interplanetary field parameters. Comparing with the magnetic field vector, we show that this simple concept holds good on more than 80% of days. On the rest 20% of days which have a predominant morning maxima, the diurnal anisotropy characteristics seem to indicate the presence of a significant component of transverse diffusion current in addition to the normal convection and diffusion flow. Such days are found to be present in the form of trains of consecutive days and are found to be associated with abrupt changes in the interplanetary field direction having scale sizes >4 hr. The value ofK ⊥/K ‖ which is normally about ⩽0.05 is found to be ≈1.0 on non-field aligned days.