Climax Neutron Monitor Research Articles

New Insights on Cosmic Ray Modulation through a Joint Use of Nonstationary Data-Processing Methods

The time variability of the cosmic ray (CR) intensity, recorded by the Climax neutron monitor and covering the period 1953–2004, has been analyzed by the joint application of the wavelet and the empirical mode decomposition (EMD) analyses. Dominant time scales of variability are found at~11 yr,~22 yr,~6 yr and in the range of the quasi-biennial oscillations (QBOs). The combination of the 11 yr cycle and QBOs explains the Gnevychev Gap (GG) phenomenon and many step-like decreases characterizing the CR modulation. The additional scales of variability at~22 yr and~6 yr are responsible for other features of the long-term CR trend, such as the intensity flat-topped profile, following the maxima of even-numbered cycles during positive polarity state of the heliosphere (). Comparison with basic time scales of variability derived from the sunspot area (SA) allows the association of the 11 yr cycle and QBOs with solar activity variations, whereas the other two modes with the drift effects govern the CR entrance in the heliosphere.

A correlation study of high‐altitude and midaltitude clouds and galactic cosmic rays by MIPAS‐Envisat

The cloud index (CI), the cloud occurrence frequency (Occ), and the extinction data (Ext) of the Michelson Interferometer for Passive Atmospheric Sounding instrument on board Envisat (MIPAS‐E) from July 2002 to March 2004 are used to investigate a possible link between galactic cosmic rays (GCRs) and midaltitude and high‐altitude clouds (CI, Occ: 9–18 km, Ext: 12–24 km). The zonally averaged data with 3 km vertical resolution are averaged over six major Forbush decrease (FD) events and subsequently correlated with the Climax neutron monitor data (CNM). In order to allow for nucleation and growth of ice particles, time lags from 0 to 5 days are applied. We find several weak but statistically significant correlations with an excess of positive cloud‐GCR correlations. Introducing a time lag does not enhance the correlations significantly. Subdividing the data in a global grid with 30° × 90° × 3 km resolution shows higher correlations in some regions. The investigation of the individual FD events yields a heterogeneous picture. Overall, there is a weak tendency toward a positive cloud‐CNM correlation. A Kolmogorov‐Smirnov test shows that for time shifts from 0 to +5 days a weak GCR‐cloud effect is evident in the MIPAS‐E measurements. An estimation of the impact of this effect delivers that a 15% increase in CNM would result in a small decrease in CI (corresponding to an increase in cloud opacity) which is most pronounced at 9 km altitude (−9% to +0.5%). For log(Ext) a decrease of −5 to 0% is calculated at 12 km altitude which shifts toward weak positive values at higher altitudes.

Two intense Forbush decreases of solar activity cycle 22

We study two intense Forbush decreases that occurred during two adjacent SOLTIP (Solar connection of Transient Interplanetary Processes) intervals; namely SOLTIP 1 (22–27 March 1991) and SOLTIP 2 (1–17 June 1991); galactic cosmic ray intensity at the depth of the second Forbush decrease was the lowest ever recorded since continuous monitoring by Climax neutron monitor began in 1951 (58% below the solar minimum value of 1954), indicating extreme conditions in the heliosphere that prevented galactic cosmic rays from reaching the Earth. These decreases were seen propagating in outer heliosphere by the deep space missions Voyagers 1, 2 and Pioneer 10, 11, with suitable time delays. We analyze hourly, pressure corrected, neutron monitor data from the global sites in both hemispheres, and muon telescopes located underground; they respond to 10–300 GV range of the galactic cosmic ray spectrum. This circumstance provides us an ideal opportunity to study the rigidity dependence of the amplitudes of the two Forbush decreases. In both cases the amplitude is found to be a power law in rigidity, with negative exponents.

Mean energy of response to galactic cosmic ray spectrum: IMP 8 and Climax neutron monitor

Solar modulations of galactic cosmic ray (GCR) intensity contain a wealth of information about their transport in the heliosphere. To extract this information from the data one studies the dependence of the observed modulations on the mean energy of response of detectors providing data for the analyses. There is a great deal of confusion about the detector energy response to GCR spectrum in the literature. We present a preliminary report on the computations of the mean energy of response for the Climax neutron monitor (CL/NM) and IMP 8 cosmic ray nuclear composition instrument to GCR protons for 1973–1998, covering the solar cycles 21 and 22. We find that for penetrating proton channel on IMP 8 the mean energy changes by a factor of over two whereas for the neutron monitor the change is only 21%. However, the corresponding change for the computed modulation function is a factor of about 3.5.

On the role of the active-region latitudes in the processes of cosmic-ray modulation

The time dependence of galactic cosmic rays is described using the solar activity indices ηαexp[−(ϕ/ϕ0)β] and ηα/(k + ϕβ), where η and ϕ are the number of groups of sunspots and their heliolatitude. The data of measurements of fluxes of galactic cosmic rays in the maximum of the high-altitude run in the polar atmosphere (Murmansk) and the Climax neutron monitor data were used. Beginning with 1957, four periods between the inversions of the polar magnetic field of the Sun are considered, during which the field sign remained invariable for several years. For each of these periods, the values of α, β, ϕ0, and k were determined by the least-squares method. The best description of the experimental data on cosmic rays was found by enumeration of the values of these parameters. The reported solar activity indices quite adequately describe the long-term modulation of galactic cosmic rays over the ∼50-year observation period.

High frequency of occurrence of large solar energetic particle events prior to 1958 and a possible repetition in the near future

It is shown that the >4 GeV fluence of large solar energetic particle events was a factor of 10 greater and the frequency of occurrence a factor of four greater prior to 1958 than during the space era. There were two events in 1946 and 1949 with >4 GeV fluences similar to that of 23 February 1956, suggesting that the >4 GeV fluence of the largest probable event for a similar period would be ∼3 times greater. The historic cosmic ray and glaciological records indicate that the fluences and probability of occurrence of such large events at both high and low energies are greatest in periods of low long‐term solar activity and anticorrelated with the estimated strength of the heliospheric magnetic field. A working model is proposed where the factors controlling the occurrence of large SEP events are (1) the occurrence of a large solar flare or CME and (2) the heliomagnetic field being <6.5 nT near Earth. A theoretical basis for this model is discussed. It is proposed that the next Gleissberg minimum of solar activity will lead to a repetition of the pre‐1958 era of high‐frequency, high‐fluence GLE and SEP events. Two predictive indicators of higher SEP activity are proposed: (1) the Climax neutron monitor rate at solar minimum rises >5% compared to 1954 and (2) the measured heliomagnetic field near Earth remains <6.5 nT for ≥2 years into a new solar cycle, or alternatively it is substantially less than 5 nT at sunspot minimum.

A temporal forecast of radiation environments for future space exploration missions

The understanding of future space radiation environments is an important goal for space mission operations, design, and risk assessment. We have developed a solar cycle statistical model in which sunspot number is coupled to space-related quantities, such as the galactic cosmic radiation (GCR) deceleration potential (phi) and the mean occurrence frequency of solar particle events (SPEs). Future GCR fluxes were derived from a predictive model, in which the temporal dependence represented by phi was derived from GCR flux and ground-based Climax neutron monitor rate measurements over the last four decades. These results showed that the point dose equivalent inside a typical spacecraft in interplanetary space was influenced by solar modulation by up to a factor of three. It also has been shown that a strong relationship exists between large SPE occurrences and phi. For future space exploration missions, cumulative probabilities of SPEs at various integral fluence levels during short-period missions were defined using a database of proton fluences of past SPEs. Analytic energy spectra of SPEs at different ranks of the integral fluences for energies greater than 30 MeV were constructed over broad energy ranges extending out to GeV for the analysis of representative exposure levels at those fluences. Results will guide the design of protection systems for astronauts during future space exploration missions.

Coronal Mass Ejections and Galactic Cosmic‐Ray Modulation

We present a study of the long-term evolution of coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronograph (LASCO) on board SOHO during the ascending, maximum, and part of the descending phases of solar cycle 23 and their relation with the modulation of galactic cosmic-ray (GCR) intensity observed at 1 AU by the Climax neutron monitor and IMP-8 spacecraft. We compare the long-term GCR modulation with the CME occurrence rate at all, low, and high latitudes, as well as the observed CME parameters (width and speed). Twenty-seven day averages of CME occurrence rates and CME properties from 1996 January to 2003 December are presented in the Appendix. The general anticorrelation between GCR intensity and the CME rate is relatively high (~-0.88). However, when we divide the CME rate into low- and high-latitude rates and compare them with the GCR intensity during the ascending phase of solar cycle 23, we find a lower anticorrelation between the low-latitude the CME rate and GCR intensity (~-0.71) and a very high anticorrelation between the high-latitude CME rate and GCR intensity (~-0.94). This suggests that, in general, CMEs could cause the decrease in the GCR flux in the inner heliosphere, as stated by the global merged interaction region (GMIR) theory. In particular, during the ascending phase of cycle 23 (qA > 0), this flux comes mainly from heliospheric polar regions. Thus, high-latitude CMEs may play a central role in the long-term cosmic-ray modulation during this phase of the cycle by blocking the polar entrance of GCRs to the inner heliosphere. This study supports the scenario in which CMEs, among other structures, are the building blocks of GMIRs, although we propose that the spherical shells (GMIRs) are closed separately at polar and equatorial regions by CMEs of different latitudes. Our results suggest that all CME properties show some correlation with the GCR intensity, although there is no specific property (width, speed, or a proxy of energy) that definitely has a higher correlation with GCR intensity.

Low- and high-frequency spectral behavior of cosmic-ray intensity for the period 1953–1996

Abstract. A study of the cosmic-ray intensity power spectrum using the Climax Neutron Monitor data in the frequency range from 10-9 Hz to 10-7 Hz (which corresponds to periodicities from 11 years to a few months) during the period 1953–1996, was carried out by means of the successive approximations method of analysis and was compared against the power spectrum and the maximum entropy methods. The contributions of the time evolution of several peaks to the global one were obtained. Except for the well-known 11-year and the 1-year variations, peaks at 7.7, 5.5, 2 and 1.7 years are found. Several peaks with periods less than 10 months have appeared in our analysis, while the occurrence of 5.1 months is obtained in all the examined solar cycles with a strong signature in cycle 21. Transitions of these quasi-periodicities are seen in power spectra plots. Some of them can be attributed to the modulation of the cosmic ray intensity by solar activity. Others are sporadic and have been previously attributed to the interplanetary magnetic field. The results obtained support once again the argument regarding the difference in the solar activity between odd and even solar cycles.Key words. Interplanetary physics (Cosmic rays, Interplanetary magnetic fields)

Time evolution of cosmic-ray intensity and solar flare index at the maximum phase of cycles 21 and 22

Analysis of the time series into trigonometric series allows the investigation of cosmic-ray (CR) intensity variations in a range of periodicities from a few days to 1 year. By this technique the amplitude and the phase of all observed fluctuations can be given. For this purpose, daily CR intensity values recorded at Climax Neutron Monitor station for the time intervals 1979–1982 and 1989–1991, which correspond to the epochs of maximum activity for solar cycles 21 and 22, respectively, have been studied. The data analysis revealed the occurrence of new periodicities, common or not, in the two solar maxima. A search of our results was done by a power spectral analysis determining independently possible systematic periodic or quasi-periodic variations. Based on the fact that during these maxima the CR intensity tracks the solar flare index better than the sunspot number, the same analysis was performed on these data, which are equivalent to the total energy emitted by the solar flares. Both analyses result in periodicities with different probability of occurrence in different epochs. Occurrence at peaks of 70, 56, 35, 27, 21 and 14- days were observed in all time series, while the periods of 140–154 and 105 days are reported only in the 21st solar maximum and are of particular importance. All of the short-term periods except of those at 27 and 154-days are recorded for first time in CR data, but they had already been observed in the solar activity parameters. Moreover, each parameter studied here has a very different power spectrum distribution in periods larger than 154 days. The possible origin of the observed variations in terms of the CR interaction in the upper atmosphere and the solar cavity dynamics is also discussed here.

Fall, classification and cosmogenic records of the Sabrum (LL6) chondrite

Abstract— The petrographic and chemical characteristics of a fresh Indian meteorite fall at Sabrum are described. Its mean mineral composition is defined by olivine (Fa31.4), orthopyroxene (Fs25.1,Wo2.0), clinopyroxene (Wo45En45.6Fs9.4) and plagioclase (An10.6Ab83.6Or5.8). The meteorite shows moderate shock features, which indicate that it belongs to the S4 category. Based on mineralogical and chemical criteria the meteorite is classified as an LL6 brecciated veined chondrite. Several cosmogenic radioisotopes (46Sc, 7Be, 54Mn, 22Na and 26Al), noble gas (He, Ne, Ar, Kr and Xe), nitrogen isotopes, and particle tracks density have been measured. Concentrations of cosmogenic 21Ne and 38Ar indicate that its cosmic‐ray exposure age is 24.8 Ma. Small amounts of trapped Kr and Xe, consistent with petrologic class 5/6, are present. The track density in olivines is found to be (1.3 ± 0.3) × 106/cm2. Activities of most of the short‐lived isotopes are lower than those expected from solar cycle variation. 22Na/26Al (1.12 ± 0.02) is found to be significantly anomalous, being ˜25% lower than expected from the Climax neutron monitor data. These results indicate that the cosmic‐ray flux during the terminal segment of the meteoroid orbit was low. The activities of 26Al and 60Co and the track density indicate small meteoroid size with a radius ˜15 cm.

Simulation of the temporal variations of the galactic cosmic-ray intensity at neutron monitor energies

We show that the temporal variations of the integrated galactic cosmic-ray intensity at neutron monitor energies (approximately above 3 GeV) can be reproduced applying a semi-empirical 1-D diffusion-convection model for the cosmic-ray transport in interplanetary space. We divide the interplanetary region into `magnetic shells' and find the relative reduction that each shell causes to the cosmic-ray intensity. Then the cosmic-ray intensity at the Earth is reproduced by the successive influence of all shells between the Earth and the heliospheric termination shock. We find that the position of the termination shock does not significantly affect the cosmic-ray intensity although there are some differences between the results for a constant and a variable termination shock radius. We also reproduce the cosmic-ray intensity applying the analytical solution of the force-field approximation (Perko, 1987) and find that the results cannot fit the observed data. Our results are compared with the Climax (geomagnetic cut-off ∼ 3 GV) and Huancayo (geomagnetic cut-off ∼ 13 GV) neutron monitor measurements for almost two solar cycles (1976–1996).

Observation of galactic cosmic ray particles by the HEP-HI telescope on the GEOTAIL satellite

The energy spectra (40 to 300 MeV/n) of galactic cosmic ray particles (C, N, O, Ne, Mg, Si, and Fe) observed by the HEP-HI silicon detector telescope onboard the GEOTAIL satellite were investigated over the period from January 1993 to May 1996, from the viewpoint of solar modulation. From the GEOTAIL data and those of relativistic heavy particles observed by HEAO-3, the modulation potential φ was obtained by using a solution of the Fokker-Plank equation with the “force-field” approximation. In the equation the diffusion coefficient was used as adjustable parameter, under the assumptions of a fixed velocity of solar wind (400km/sec) and the radius of the heliosphere of 50AU. The time variation of the modulation potential thus obtained is compared with that obtained from the CLIMAX neutron monitor. A reasonable correlation between both time variations is found when five months delay is applied to the results obtained by the HEP-HI telescope.

Energetic Neutrons, Protons, and Gamma Rays during the 1990 May 24 Solar Cosmic‐Ray Event

The solar cosmic-ray event on 1990 May 24 can be divided into three phases: a first impulsive production of γ-rays and greater than 200 MeV neutrons; a second slower phase during which there were high-energy protons at the Sun for ~20 minutes producing pions and high-energy neutrons; and a third phase when the protons observed by the IMP 8 and GOES spacecraft and by neutron monitors were injected into interplanetary space. This third phase started after the onset of the event but before the second phase had ceased. We found that high-energy neutron production occurred during the last 60 s of the impulsive phase and at least the first 19 minutes of the second phase. During the second phase the high energy-neutron and γ-ray emissions decayed more slowly than either the 2.2 MeV or 4-7 MeV γ-ray line emissions. A two-component neutron energy spectrum that changes between the first and second phases gives a reasonable fit to the count rate increase recorded by the Climax neutron monitor. From the fit we infer that the integrated neutron emissivity at the Sun was ~3.5 × 1030 sr-1 for E > 100 MeV. The maximum intensity of P > 1.5 GV solar protons near the Earth was 4.5 × 103 (m2 sr s)-1. The differential solar proton energy flux (dJ/dE) as a function of rigidity at the Sun can be described by an evolving power-law spectrum. We estimate that the number of escaping protons with E > 30 MeV in the third phase was 7%-14% of the number of protons required to produce the solar neutron increase at the Earth. Although it is attractive to assume that the interplanetary solar protons simply leaked out from the trapping region at the Sun, the data suggest that the interplanetary solar protons were not from the population of energetic particles that produced the neutron and γ-ray emissions but were freshly accelerated during the third phase of the solar flare.

Hale-cycle effects in cosmic-ray intensity during the last four cycles

Monthly cosmic-ray data from Inuvik (0.16 GV) and Climax (2.96 GV) Neutron Monitor stations has been studied with the aid of solar activity parameters for the time period 1947–1995. Systematic differences in the overall shape of successive 11-year modulation cycles and similarities in the alternate 11-year cycles seem to be related to the polarity reversals of the polar magnetic field of the Sun. This suggests a possible effectiveness of the Hale cycle during even and odd solar activity cycles. Our results can be understood in terms of open and closed models of the heliosphere. Positive north pole of the Sun leads to open heliosphere where particles reach the Earth more easily when their access route is by the heliospheric oolar regions (even cycles) than when they gain access along the current sheet (odd cycles). In this case as the route of access becomes longer due to the waviness of the neutral sheet, the hysteresis effect of cosmic-rays is also longer. This interpretation is explained in terms of different contributions of convection, diffusion and drift mechanisms to the whole modulation process influencing cosmic-ray transport in the heliosphere.

Neutron and electromagnetic emissions during the 1990 May 24 solar flare

In this paper, we are primarily concerened with the solar neutron emission during the 1990 May 24 flare, utilizing the counting rate of the Climax neutron monitor and the time profiles of hard X-rays and gamma-rays obtained with the GRANAT satellite (Pelaez et al., 1992; Talon et al., 1993; Terekhov et al., 1993). We compare the derived neutron injection function with macroscopic parameters of the flare region as obtained from the H-alpha and microwave observations made at the Big Bear Solar Observatory (BBSO) and the Owens Valley Radio Observatory (OVRO) respectively. Our results are summarized as folows: (1) to explain the neutron monitor counting rate and 57.5-110 MeV and 2.2 MeV gamma-ray time profiles, we consider a two-component neutron injection function, Q(E, t). (2) From the H-alpha observations, we find a relatively small loop of length approximattely equal to 2 x 10(exp 4) km, which may be regarded as the source for the fast-decaying component of gamma-rays (57.5-110 MeV) and for the first component of neutron emission. From microwave visibility and the microwave total power spectrum we postulate the presence of a rather big loop (approximately equal to 2 x 10(exp 5) km), which we regard as being responsible for the slow-decaying component of the high-energy emission. We show how the neutron and gamma-ray emission data can be explained in terms of the macroscopic parameters derived from the H-alpha and microwave observations. (3) The H-alpha observations also reveal the presence of a fast mode MHD shock (the Moreton wave) which precedes the microwave peak by 20-30 s and the peak of gamma-ray intensity by 40-50 s. From this relative timing and the single-pulsed time profiles of both radiations, we can attribute the whole event as due to a prompt acceleration of both electrons and protons by the shock and subsequent deceleration of the trapped particles while they propagate inside the magnetic loops.

Long-term modulation of galactic cosmic radiation and its model for space exploration

As the human exploration of space has received new attention in the United States, studies find that exposure to space radiation could adversely impact the mission design. Galactic Cosmic Radiation (GCR), with its very wide range of charges and energies, is particularly important for a mission to Mars, because it imposes a stiff mass penalty for spacecraft shielding. Dose equivalent versus shielding thickness calculations, show a rapid initial drop in exposure with thickness, but an asymptotic behavior at a higher shielding thickness. Uncertainties in the radiobiology are largely unknown. For a fixed radiation risk, this leads to large uncertainties in shielding thickness for small uncertainties in estimated dose. In this paper we investigate the application of steady-state, spherically-symmetric diffusion-convection theory of solar modulation to individual measurements of differential energy spectra from 1954 to 1989 in order to estimate the diffusion coefficient, κ(r,t), as a function of time. We have correlated the diffusion coefficient to the Climax neutron monitor rates and show that, if the diffusion coefficient can be separated into independent functions of space and time: κ(r,t) = K(t)κ 0βPκ 1(r), where β is the particle velocity and P the rigidity, then (i) The time dependent quantity l/K(t), which is proportional to the deceleration potential, φ(r,t), is linearly related to the Climax neutron monitor counting rate. (ii) The coefficients obtained from hydrogen or helium intensity measurements are the same. (iii) There are different correlation functions for odd and even solar cycles. (iv) The correlation function for the Climax neutron monitor counting rate for given time, t, can be used to estimate mean deceleration parameter φ(t) to within ±15% with 90% confidence. We have shown that κ(r,t) determined from hydrogen and/or helium data, can be used to fit the oxygen and iron differential energy spectra with a root mean square error of about ±10%, and essentially independent of the particle charge or energy. We have also examined the ion chamber and 14C measurements which allow the analysis to be extended from the year 1906 to 1990. Using this model we have defined reference GCR spectra at solar minimum and solar maximum. These can be used for space exploration studies and provide a quantitative estimate of the error in dose due to changes in GCR intensities.

Direct solar neutrons detected by neutron monitors on 24 May 1990

The ground‐level energetic solar particle event on 24 May 1990 has an extremely impulsive onset in the particle intensity that we attribute to a significant flux of direct solar neutrons impacting the top of the atmosphere. This is the largest solar neutron event reported to date with a 25.3% increase recorded by the Climax neutron monitor in the time interval 2050–2055 UT. At least 7 neutron monitors in North America recorded this increase simultaneously, the magnitude of which is ordered by the air mass along the line of sight to the sun at the time of the event. The initial solar neutrons arrive within the same minute as the maximum in both the soft X‐ray and the H‐alpha emission observed at the earth, whereas the onset of the world‐wide increase of solar protons occurred approximately 15 minutes later.

Dependence of cosmic-ray transport on the orientation of the solar magnetic dipole

In this paper we discuss the dependence of cosmic-ray transport in the interplanetary space on the polarity configuration of the solar and interplanetary magnetic fields. The diffusion mechanism is not only related to the irregularities present in the interplanetary magnetic field, but also to its regular structure, because of the curvature and gradient drifts of the particles. Hence diffusion is affected by the polarity reversals of the interplanetary magnetic dipole which are related to the reversals of the solar dipole. As a consequence a phase reversal of both the annual and semi-annual variations in the cosmic-ray intensity occurs. This effect has been studied throughout the period 1954–1976, by analysing the Climax neutron monitor data. In addition, the latitudinal distribution of the average solar activity itself in this period changes simultaneously to the polarity reversals of the solar polar fields.

Stochastic Variations of Cosmic Rays in the Solar System

view Abstract Citations (11) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stochastic Variations of Cosmic Rays in the Solar System Jokipii, J. R. Abstract The response of the cosmic-ray intensity in the solar system to turbulent fluctuations in the solar wind is calculated. The cosmic-ray diffusion coefficient K and the wind velocity V vary stochastically about their mean values, and the variations are assumed to be frozen into the solar plasma. The resulting fluc- tuations in the cosmic-ray intensity I are determined to first order in the fluctuations of K and V by solving the full time-dependent Fokker-Planck equation and regarding all three fluctuations as stationary random functions of time. The results, expressed in terms of autospectra and cross-spectra of I, V, and ,c (at a given heliocentric radius r) depend on distant as well as local features of the solar-wind turbulence and may therefore serve as a sensitive probe of the struc'ture of plasma turbulence in the solar wind. This technique may permit an estimate of the distance to the boundary of the modulating region (that helio- centric radius beyond which there is little turbulence). Power spectra which were computed from counting rates obtained by the Climax neutron monitor in 1965 show some evidence of enhanced fluctuations at a frequency of 0 15 cycles per day. The theory would predict a peak at this frequency if the boundary of the modulating region were at 2.7 ± 0.5 a u. This tentative estimate of the size of the modulating region is consistent with, and independent of, previous estimates. I. INTRODUCTION The intensity of galactic cosmic rays observed in the inner solar system changes with the time because of varying modulation by the solar wind. The well-known eleven-year cyclic variation, for example, is due to the eleven-year cycle of solar activity. Shorter- term, irregular fluctuations in the intensity also occur due to a variety of causes, such as blast waves from solar flares (Forbush decreases). The interpretation of these phenomena in terms of Parker's (1958, 1965) diffusion-convection theory and in terms of the Fourier spectrum of the magnetic field (Jokipii 1966, 1967) has considerably deepened our insight into the turbulent structure of the solar wind. For example, analysis of the phase lag between the solar-cycle variations of the cosmic-ray intensity and appropriate solar ac- tivity indices (Forbush 1954) permits an estimate of the size of the modulating region (that heliocentric distance beyond which turbulence in the wind becomes too weak to exclude cosmic rays effectively; e.g., Simpson and Wang 1967). This paper concerns the theory of irregular, short-term variations in the cosmic-ray intensity which are produced by the day-to-day variations in the turbulent solar wind. It has been demonstrated observationally that the fluctuating cosmic-ray intensity dur- ing quiet times shows a significant correlation with the measured particle-diffusion co- efficient (Jokipii and Coleman 1968). Using measurements carried out on board Mariner 4 in 1965, Jokipii and Coleman found that periods when the diffusion coefficient was smaller than normal (high magnetic-field variance) tended statistically to be followed by decreased counting rates, as the region of high magnetic variance was carried out beyond the point of observation so that it shielded Mariner 4 from incoming galactic particles. It is shown in the present paper that variations in the cosmic-ray intensity and in the solar wind may be related statistically by using Parker's modulation theory. The obser- vations may then be used to analyze the structure of solar-wind turbulence. The plan of this paper is as follows: In the next section Parker's diffusion-convection theory with adiabatic deceleration is discussed, and appropriate time-dependent solu- tions are presented for small variations in solar-wind parameters. In § III these results are used to obtain the statistical properties (power spectra) of the cosmic-ray intensity 110 Publication: The Astrophysical Journal Pub Date: June 1969 DOI: 10.1086/150038 Bibcode: 1969ApJ...156.1107J full text sources ADS |