High Cutoff Rigidity Research Articles

Exploring the relationship between space weather parameters and cosmic ray muons observed at high cut-off rigidity site: A correlation, artificial neural network, and spectral analysis

The objective of this study was to examine the relationship between cosmic rays (CR) and various solar activity parameters, including the interplanetary magnetic field (B), solar radio emission flux at 10.7 cm (F10.7), solar wind speed (V), solar wind density (N), solar wind pressure (P), and the variance of the interplanetary magnetic field vector (σF). The CR data collected from 2007 to 2013 were obtained using a muon detector situated at King Abdulaziz University (KAAU) in Jeddah, Saudi Arabia, with a rigidity cutoff of 14.8 GV. To explore the correlation between the CR data and the selected parameters, three distinct methods were employed: regression analysis, artificial neural network (ANN), and power spectral analysis.Regression analysis revealed significant positive correlations between CR muons and plasma pressure (R = 0.1, p < 0.05), as well as solar wind speed (R = 0.1, p < 0.05). On the other hand, negative correlations were observed between CRs and radio flux (R = 0.75, p < 0.05), as well as interplanetary magnetic field (R = 0.1, p < 0.05). However, no significant correlations were found between CRs and plasma density or the variance of the interplanetary magnetic field vector (p > 0.05). Furthermore, multivariable correlation analyses were conducted to explore the relationship between CR muons and the considered parameters, resulting in only marginal improvement.For comparison purposes, data from the King Abdulaziz City for Science and Technology (KACST) muon detector in Riyadh, Saudi Arabia (with a rigidity cutoff of 14.4 GV), as well as CR data from Oulu NM (with a rigidity cutoff of 0.3 GV), were utilized for the same period as the KAAU data. The correlation results indicated that the relationships between the CR data from the three detectors and some of the considered parameters were consistent. However, the strength of the correlations and the magnitudes of the slopes exhibited variations.ANN procedures were also employed to compare the efficiency of neural network analysis in forecasting CR muons based on solar activity parameters. The Multilayer Perceptron (MLP) Module trained using the backpropagation learning algorithm was utilized to build a neural network model and assess its accuracy. The results indicated that the ANN method provided slightly more accurate predictions compared to regression analysis.Finally, Fast Fourier Transform (FFT) analyses were conducted to examine the presence of periodicities between the CRs and the variables under consideration. The analysis revealed several shared periodicities, including cycles spanning 270–277 days, 146–158 days, 89–95 days, 66–73 days, and 21–31 days.

Cosmic ray measurements at high cutoff rigidity site - Preliminary results

In January 2007, King Abdulaziz University, Jeddah (21.5o N; 39.2o E; 14.8 GV), Saudi Arabia, installed a single channel muon detector for monitoring and studying the variations of the cosmic ray (CR) muons. Brief description of the developed detector will be presented. A few results obtained by the detector so far will be given and discussed. These include the impact of the atmospheric pressure and temperature, the daily variations, the responses of the detector to transit and long term of the solar disturbances. Performances of the detector were compared with the King Abdulaziz city for science and technology (KACST) muon detector installed in Riyadh Saudi Arabia and with the Oulu neutron monitor at Oulu and showed comparable results.

Extended Cosmic Ray Decreases with Strong Anisotropy after Passage of Interplanetary Shocks

The passage of an interplanetary shock and/or interplanetary coronal mass ejection often causes a rapid decrease in the Galactic cosmic-ray (GCR) flux, known as a Forbush decrease, followed by a recovery of the flux over some days. These local effects are of short duration and strongly rigidity dependent, with higher-rigidity particles exhibiting much weaker effects. In contrast, we present data for two events in which the cosmic-ray flux gradually decreased for about 1 week after shock passage, then recovering over the following week, with the highest anisotropy levels observed throughout Solar Cycle 24. These extended decreases have a weak rigidity dependence and are much more prominent in observations at higher cutoff rigidity, where the initial Forbush decrease is not clearly detected and other variations are generally weak, as we demonstrate using data from the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand with a cutoff rigidity of about 17 GV. We propose that these extended decrease events were initiated upon the passage of an interplanetary shock that inhibited the inflow of GCRs along the interplanetary magnetic field, possibly due to magnetic mirroring at the shock. We also discuss the general behavior of GCR anisotropy as observed at this high cutoff rigidity.

Cosmic rays detection in Saudi Arabia: Review of the facilities and preliminarily results

Solar activity modulates cosmic ray (CR) particles with different magnitudes on different time scales. CR modulations have been studied using ground-based detectors, primarily neutron monitors, distributed around the world. In 2002, CR research began in Saudi Arabia with the installation of the first CR detector at the King Abdulaziz City for Science and Technology (KACST) in Riyadh (lat. 24 43; long. 46 40; Rc ~ 14.4 Gv), the capital of Saudi Arabia. The facility is located in the central Arabian Peninsula. Because of its high cutoff rigidity, the site is ideal for monitoring CR variations and is of great significance to the research community.Different CR detectors employing various measuring techniques have been developed and installed, at this site, to record the intensity of cosmic rays on different time scales and assess their correlation with atmospheric and climatic parameters. They include scintillator detectors, single-channel and rotatable telescopes, multi-wire detectors, CARPET detectors, small mobile detector, and mini-neutron monitors. In this paper, we briefly describe these detectors and offer observations on their use in CR research.

Correlation analyses between solar activity parameters and cosmic ray muons between 2002 and 2012 at high cutoff rigidity

• The impacts of seven solar, interplanetary, and geophysical parameters on CR data have been investigated. • CR data for the period 2002–2012 from KACST muon detector located at high cutoff rigidity have been used. • CR data from Oulu neutron monitor for the same period have been used for comparisons. • Multivariable models were developed to predict CR variability at KACST and Ouu. Solar activity modulates cosmic ray (CR) flux with different magnitudes on different time scales. Several investigations from locations with different, mainly low, cutoff rigidity utilizing different solar activity parameters have been carried out to characterize their influence on the observed CR flux. In this study, the effect of seven solar, interplanetary, and geophysical parameters on the secondary CR muons has been investigated and established using correlation analyses. Daily mean values of the CR data for the period between 2002 and 2012 were obtained from the King Abdulaziz City for Science and Technology (KACST) muon detector, Riyadh, central Saudi Arabia (Rc = 14.4 GV). Considered parameters are sunspot number, radial component of the interplanetary magnetic field, Kp index, solar radio emission flux at 10.7 cm, Dst index, solar wind speed, and solar wind density. Same analyses have been carried out using data from the Oulu neutron monitor (Rc = 0.8 GV) for comparison purposes. Obtained results showed that the intensities of the secondary CRs from both stations are significantly correlated with the Dst index and plasma density, and anti-correlated with the rest of the variables. These results are in agreement with those obtained by several investigators. Magnitude and strength of the correlations between the considered variables and CRs were higher and stronger for the CR neutrons than the CR muons. These findings may be due to the greater influence of seasonal variations on the CR muons than on the CR neutrons. Additionally, the muon detector is sensitive (due to its high cutoff rigidity) to higher-energy CR particles, which are less affected by solar disturbances. Time-lag cross-correlation analyses between the monthly mean CR values from both stations and the monthly mean values of the considered variables were conducted. Comparable results with the previous investigations were found. Multivariable models using the seven parameters were developed to predict the CR variations for both sites. For the data from the KACST detector, the model was able to predict the measured data with a correlation coefficient of 0.48 and a standard deviation of 0.60%. On the other hand, the developed model for the Oulu neutron monitor has a correlations coefficient of 0.77 and a standard deviation of 3.7%.

Atmospheric effects on secondary cosmic ray muons observed by multi-wire muon detector at a high cutoff rigidity station

In October 2017, a multi-layer small multi-wire (MW) detector for cosmic ray (CR) muon measurements was installed at King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia (Rc = 14.4 GV). This detector is considered to be the first of its kind used for long term CR measurements. In this study, CR data collected from KACST-MW detection system and radiosonde measurements were used to investigate the effect of atmospheric pressure, ground temperature, height of the muon production layer, temperature at that layer, and temperature weighted by atmospheric mass, on CR muons. Regression analyses between the observed muon count rate and these variables were carried out and the corresponding coefficients were obtained and used to correct the observed muon count rate for the temporal variation of these variables. The effect of the atmospheric pressure was conducted first and used to correct the data for this effect. The pressure corrected count rate is anti-correlated with the height of muon production, ground temperature, and atmospheric mass-weighted temperature. On the other hand, the temperature at the muon production layer is positively correlated with the muon count rate. The obtained results from this study are in good agreement with those reported by several researchers. Taking advantages of the relationship between the detected CR muons and the temperature at the muon production layer (stratospheric temperature), this relationship is inverted to calculate this temperature from muon observations. The proposed technique provides excellent predictability of the stratospheric temperatures.

Distinct Pattern of Solar Modulation of Galactic Cosmic Rays above a High Geomagnetic Cutoff Rigidity

Abstract Solar modulation refers to Galactic cosmic-ray variations with the ∼11 yr sunspot cycle and ∼22 yr solar magnetic cycle and is relevant to the space radiation environment and effects on Earth’s atmosphere. Its complicated dependence on solar and heliospheric conditions is only roughly understood and has been empirically modeled in terms of a single modulation parameter. Most analyses of solar modulation use neutron monitor (NM) data from locations with relatively low geomagnetic cutoff rigidity, i.e., the threshold for cosmic rays to penetrate Earth’s magnetic field. The Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand, has the world’s highest cutoff rigidity (≈17 GV) where observations span a complete solar modulation cycle (since late 2007). The pattern of solar modulation at Doi Inthanon during 2011–2014 was qualitatively very different from that at a low geomagnetic cutoff and is not well described by the same modulation parameter. At other times, NM count rates from Doi Inthanon and McMurdo, Antarctica (cutoff ∼1 GV), were linearly correlated and confirm the observation from latitude surveys in the previous solar cycle that the slope of the correlation changes with solar magnetic polarity. Low solar magnetic tilt angles (&lt;40° at negative polarity) were well correlated with variations at both NM stations, as predicted by drift models. At a higher tilt angle, the Doi Inthanon count rate is well correlated with the interplanetary magnetic field, which is consistent with an increase in diffusion at high rigidity short-circuiting the effects of drifts and the heliospheric current sheet.

Preliminary Results of High-Energy Cosmic Ray Muons as Observed by a Small Multiwire Detector Operated at High Cutoff Rigidity

Solar disturbances modulate primary cosmic rays on different time scales. Studying cosmic ray variation is an important subject that attracts scientists from different disciplines. We have constructed and installed (in Riyadh, Saudi Arabia, Rc =14.4 GV) a three-layer small (20 × 20 cm2) MultiWire Chamber (MWC) telescope to study cosmic ray variations and investigate their influence on various atmospheric and environmental processes. Preliminary results obtained from the developed detector are given. The influence of both atmospheric pressure and temperature was studied. Both the temperature and pressure coefficients were calculated and were consistent with those previously obtained. Short-term cosmic ray periodicities, such as the 27-day period, and its two harmonics, have been identified. Sporadic variations caused by some solar activity processes have been inspected. The obtained results from this detector have been compared to the existing 1 m2 scintillator detector, as well as to some of the neutron monitors, showing comparable results.

Atmospheric Effect on Cosmic Ray Muons at High Cut-Off Rigidity Station

Cosmic ray data and radiosonde measurements from Riyadh, Saudi Arabia (Rc = 14.4 GV), for the period 2002–2012, were used to study the effect of atmospheric pressure, level of pion production, and temperature at that level, on cosmic ray muons. We found that, even if corrections were made to the detected muons using these three parameters, seasonal variations of the cosmic rays still exist. This suggests that other terrestrial and/or extraterrestrial causes may be considered. The levels of pion production and atmospheric pressure are inversely correlated with the muon rate. On the other hand, the temperature at the pion production level is correlated with muons in spring and winter and inversely correlated in fall and summer. There is no clear explanation for this behavior.

Forbush decreases observed by Daejeon neutron monitor

The neutron monitor (NM) is a ground-based detector designed to estimate the cosmic ray intensity by measuring secondary particles. In October 2011, an NM64-type NM with a vertical cutoff rigidity of 11.2GV was installed at Daejeon in Korea. It has produced reliable cosmic ray data after detector stabilization. In order to examine the reliability of cosmic ray data collected by the Daejeon NM, we select Forbush decreases (FDs) that occurred during the three years of 2012–2014. We also analyze the FDs at the Oulu NM in Finland in order to identify and compare them. We identify 37 FDs at both Daejeon and Oulu NMs. Student t-test analysis reveals that FDs at Daejeon have smaller intensity variation of main phase and shorter duration of main phase than those at the Oulu NM. Of the 37 FDs, 17 are simultaneous and 20 are non-simultaneous. The intensity variation of simultaneous FDs is larger than that of non-simultaneous FDs at both NMs with high confidence levels in the Student t-test. Most of the non-simultaneous FDs that have an onset time in the dayside hold typical properties of non-simultaneous FDs. Our study results demonstrate that the Daejeon NM can provide cosmic ray data of reliability comparable to that of the Oulu NM. As one of only a few NMs worldwide with a high vertical cutoff rigidity exceeding 10.0GV, the Daejeon NM will continue to provide important information on higher-energy cosmic ray spectra.

Altitude profile of atmospheric muons in a location with relatively high cut-off rigidity

We have investigated the differential and integrated muon fluxes, muon charge ratios and the zenith angle dependence of the integrated muon intensities at sea level and various depths in the atmosphere of Tsukuba, Japan, for muons with momenta below 400GeV/c using the Geant4 simulation package. The simulated sea level muon spectrum and charge ratio have been seen to be in good agreement, throughout the momentum interval of the interest, with those from the measurements made by the BESS-TeV spectrometer. Integrated muon intensities have been found to increase as the atmospheric depth decreases up to slightly above 200g/cm2, and then they have a trend to decrease at lower atmospheric depths. Muon charge ratios at each of the atmospheric depth of interest have been shown to have a very similar behavior with the one at sea level. Simulation results for the zenith angle dependence of muon intensity have been quite compatible with the existing information in the literature for sea level. It has also been shown that the zenith angular dependence of muon intensity has a tendency to disappear (to obey the secθ distribution) as the atmospheric depth decreases for muons with relatively low (high) momenta.

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).

Statistical validation of HZETRN as a function of vertical cutoff rigidity using ISS measurements

Measurements taken in Low Earth Orbit (LEO) onboard the International Space Station (ISS) and transit vehicles have been extensively used to validate radiation transport models. Primarily, such comparisons were done by integrating measured data over mission or trajectory segments so that individual comparisons to model results could be made. This approach has yielded considerable information but is limited in its ability to rigorously quantify and differentiate specific model errors or uncertainties. Further, as exploration moves beyond LEO and measured data become sparse, the uncertainty estimates derived from these validation cases will no longer be applicable. Recent improvements in the underlying numerical methods used in HZETRN have resulted in significant decreases in code run time. Therefore, the large number of comparisons required to express error as a function of a physical quantity, like cutoff rigidity, are now possible. Validation can be looked at in detail over any portion of a flight trajectory (e.g. minute by minute) such that a statistically significant number of comparisons can be made. This more rigorous approach to code validation will allow the errors caused by uncertainties in the geometry models, environmental models, and nuclear physics models to be differentiated and quantified. It will also give much better guidance for future model development. More importantly, it will allow a quantitative means of extrapolating uncertainties in LEO to free space. In this work, measured data taken onboard the ISS during solar maximum are compared to results obtained with the particle transport code HZETRN. Comparisons are made at a large number (∼77,000) of discrete time intervals, allowing error estimates to be given as a function of cutoff rigidity. It is shown that HZETRN systematically underestimates exposure quantities at high cutoff rigidity. The errors are likely associated with increased angular variation in the geomagnetic field near the equator, the lack of pion production in HZETRN, and errors in high energy nuclear physics models, and will be the focus of future work.

The results of cosmic radiation in-flight TEPC measurements during the CAATER flight campaign and comparison with simulation

The European-Commission-supported project DOSMAX (Dosimetry of Aircrew Exposure to Radiation During Solar Maximum) was aimed at measuring aircrew exposure to cosmic radiation on-board the aircraft during solar maximum. During a dedicated international comparison mission (Co-ordinated Access to Aircraft for Transnational Environmental Research; CAATER) different measurement techniques have been compared by six European institutes (Results of the CAATER Mission, DOSMAX Meeting, Dublin, June 2004). In this paper, we present the tissue-equivalent proportional counter (TEPC) measurements carried out by ARC Seibersdorf research (ARCS), Austria, and Institut de Radioprotection et de Sûreté Nucléaire (IRSN), France, together with a comparison with simulation results under the same conditions. The whole flight campaign consists of four different in-flight investigations performed at two different geographical positions at 12.2 km (FL 400) and 9.8 km (FL 320). One location was chosen above Rome (42 degrees North, 12 degrees East), Italy, for high cut-off rigidity (6.4 GV) and the second above Aalborg (57 degrees North, 10 degrees East), Denmark, for low cut-off rigidity (1.8 GV). The TEPC measurements are presented in terms of absorbed dose and ambient dose equivalent as well as microdosimetric spectra as a function of lineal energy. For the same conditions of the CAATER flights the response of the TEPC has also been simulated by using the Monte Carlo Transport Code FLUKA (version 2003). The results from simulations are compared with measurements and they show a reasonable agreement.

Cosmic Ray Intensity Variations in Relation with Solar Flare Index and Sunspot Numbers

From the monthly data of cosmic ray intensity (CRI), sunspot numbers (SSN) and solar flare index (SFI), an attempt has been made to study the relationship between CRI and solar activity (SA) parameters SSN and SFI. The correlation between SA parameters and CRI for different neutron monitoring stations having low, middle and high cut-off rigidity has been investigated. The anti-correlation between SA and CRI is found to exist with some time lag. Based on the method of minimizing correlation coefficient and time-delayed component method, the observed time-lag between SA parameters (SSN and SFI) and CRI has been found to be large for odd solar cycles in comparison to even solar cycles. The results of time-lag analysis between CRI and SSN and between CRI-SFI have also been compared. The findings of correlative study between CRI and SSN are in agreement with earlier results, while the CRI-SFI relationship provides new insights to understand the solar modulation of cosmic rays.

Changes of geomagnetic transmissivity in the disturbed magnetosphere: ground-based and CORONAS-F observations

We report increases of cosmic-ray intensity at lowL shells, on the ground as well as on the altitude of 500 km, during the selected intervals when magnetosphere was strongly disturbed. The geomagnetic transmissivity for cosmic ray vertical access has been computed for four events, using two geomagnetic field models. The first event, namely, dated November 20–22, 2003, led to an increase of galactic cosmic-ray intensity due to the improvement of magnetospheric transmissivity at neutron monitors with high nominal cut-off rigidity. Other two events with high-energy solar proton emissions, namely, dated October 28 and November 2, 2003, caused different responses at middle latitudes. The first one followed by the strong geomagnetic disturbance led to the shift of the penetration boundary of protons, having(50–90) MeV at CORONAS-F satellite toL<3, coinciding approximately with the cut-off reduction expected byDst depression, while the other one, without remarkableDst decrease, did not shift the outer boundary of penetration belowL ≈ 4. And the fourth of the events, on November 8, 2004, with strong geomagnetic disturbance, yielded a complicated structure of cosmic-ray time profiles: superimposed on the cosmic-ray decrease, viewed by neutron monitors and by CORONAS-F at high latitude; an increase of intensity at middle latitudes both on the ground and on the altitude of 500 km has been observed during theDst depression.

Extended scaling factors for in situ cosmogenic nuclides: New measurements at low latitude

Production rates of cosmogenic nuclides at the earth's surface are controlled by the intensity of energetic cosmic-ray nucleons, which changes rapidly with elevation. An incomplete knowledge of how nucleon fluxes vary with elevation remains a major obstacle to utilizing cosmogenic nuclides as geochronometers in applications requiring highly accurate ages. One problem is that attenuation characteristics depend on nucleon energy. Measurements of high-energy (> 50 MeV) nucleon fluxes tend to give shorter attenuation lengths than low-energy (< 1 MeV) fluxes, but these differences are not well characterized due to a lack of data at lower energies. Another problem is that the atmospheric attenuation length for nucleon fluxes varies with the geomagnetic cutoff rigidity (a parameter related to geomagnetic latitude), RC, and that there has been an incomplete mapping of nucleon fluxes at high RC (low geomagnetic latitude). We report new measurements of nucleon fluxes from altitude transects in Hawaii (RC = 12.8 GV) and Bangalore, India (RC = 17.3 GV). Our measurements in Hawaii of low-energy neutrons (median energy 1 eV) and energetic nucleons (median energy 140 MeV) confirm that nucleon scaling functions are energy-dependent in the range of energies at which cosmogenic nuclides are produced. Our measurements in southern India extend our previously reported scaling model for spallation reactions [D. Desilets, M. Zreda, Spatial and temporal distribution of secondary cosmic-ray nucleon intensity and applications to in situ cosmogenic dating. Earth Planet. Sci. Lett. 206 (2003) 21–42] from RC = 13.3 GV to RC = 17.3 GV, nearly the highest cutoff rigidity on earth. The anomalously high cutoff rigidity over India provides a geomagnetic shielding condition that is effectively the same as would be observed at the geomagnetic equator in a dipole field with an intensity 1.2 times the modern value. This makes it possible to scale low-latitude production rates to paleomagnetic fields that are stronger than the present dipole field.

Cosmic ray variability and geomagnetic activity: A statistical study

We availed hourly data of four middle and high cut-off rigidity neutron monitors recorded over 21 years and then discussed various measures of the CR variability and simplified estimates of the CR anisotropy in relation to the Dst time history. The correlation of Dst with the prehistory of CR variability at these cut-offs is shown. A different distribution of the CR indices for 24 h before the sharp Dst decreases in comparison with that for geomagnetically quiet periods is found. The relation of CR variability with the solar wind speed appeared but neither with the solar wind density nor with the Bz component of the interplanetary magnetic field. The multiple regression of Dst with the prehistory of CR variability resulted to be significant, suggesting that the parameters describing CR variability/anisotropy are potentially useful to be implemented into the schemes of forecasting of geomagnetic activity, performed with interplanetary medium data obtained before their arrival at the Earth (e.g. at the L1 point).

Galactic cosmic ray fluctuations during solar cycles 22 and 23 at high altitude neutron monitors

Abstract Cosmic ray detectors installed at high magnetic cutoff rigidity and mountain elevation are only a few. It is therefore important to have reliable data for active monitors with these characteristics. The Mexico City 6-NM64 has been in continuos operation since 1990. It is located at an altitude of 2274 m above sea level and has a vertical magnetic cutoff rigidity of 8.2 GV for the 1990 epoch (Shea and Smart, 1997). In this work we make a study of the cosmic ray intensity fluctuations observed in Mexico City and compare it with other mountain altitude neutron monitors of the american sector during the years 1990–1999. The period comprises the maximum and declining phase of solar cycle 22 and the beginning of cycle 23. Evolution of important variation periodicities compared with solar activity indicators are presented. We have also constructed a new solar activity index based on the measured hard x-rays flux.

Characteristics of Two-Way Cosmic-Ray Diurnal Anisotropy

We examine the deviation of the solar diurnal anisotropy vector from the 18 LT direction during the positive state of the solar cycle by assuming two anisotropies in free space. We use two detectors characterized by two linearly independent coupling functions. The median primary rigidity of response of these detectors covers the range 16 GV ≤ Rm ≤ 331 GV. Amplitude, direction, spectrum exponent, and the upper cut-off rigidity of each anisotropy have been calculated using the least-squares method over the time interval 1968–1988. This period covers a complete solar magnetic cycle. Only one anisotropy is dominant during each magnetic state of the solar cycle. The upper cut-off rigidity at which the dominant anisotropy vanishes varies between 50–250 GV. The direction of the dominant anisotropy vector points toward the 18 LT direction during the negative state of the solar cycle and toward earlier hours than 18 LT during the positive state. The non-dominant anisotropy is characterized by very high upper cut-off rigidity and sharper energy spectral.