Diurnal Anisotropy Research Articles

Cosmic ray diurnal anisotropy 1937-1972

A previous investigation by Forbush (1969) showed that annual means of the cosmic ray diurnal anisotropy from 1937 to 1967 resulted from the addition of two distinct diurnal components. One, w, has its maximum (or minimum) in the asymptotic direction 128° east of the sun and is well approximated by a wave W with a period of 2 solar cycles. Wave W passes through zero in 1958, when the sun's poloidal field reversed. The remaining component with W eliminated has its maximum in the asymptotic direction 90° east of the sun. Annual means of this component, with its maximum at 18.0 hours local asymptotic time, are well correlated (r = +0.75) with magnetic activity and determine a solar cycle variation, the minimum being near sunspot minimum and the amplitude about two-thirds that of W. During the interval 1937–1967 or so, the ‘period’ of W was 20 years (that of the sunspot cycle was 10 years). If W were strictly periodic, its next change of sign after 1958 would have occurred in 1968. The present analysis shows this reversal of sign was delayed until 1971, near the time found for the latest reversal of the sun's polar magnetic field by Dr. Robert Howard of the Hale Observatories. These results derive from a statistical investigation of the variability of annual means of the diurnal variation from ion chamber data at Cheltenham-Fredericksburg, Huancayo, and Christchurch. The absolute, or total, diurnal anisotropy and the atmospheric diurnal temperature effect obtained are in reasonable agreement with those derived independently through a comparison of the diurnal anisotropy from ion chamber data and that from Simpson's 1953–1966 IGY neutron monitor data at Huancayo.

Diurnal anisotropies of the cosmic ray intensity underground during maximum solar activity

The diurnal variations of underground cosmic ray intensities are analyzed on a day-to-day basis. At the depths of the London and Torino stations an anisotropy in the direction of the interplanetary magnetic field is observed in addition to the corotation effect, which varies in relation to the perturbations produced by the sun. This effect is not recorded in the Hobart data during the same period of observation.

Long-term variation in the cosmic-ray diurnal anisotropy

The long-term modulation of the diurnal variation of the cosmic-ray intensity, observed in quiet times during the ascending phase of solar cycle number 20, is fully explained by the change in the interplanetary sector pattern. The sector structure seems to generate an upper rigidity cut-off for the diurnal variation which increases from ∼50 GV during 1965 to ∼150 GV during 1968. Moreover, the amplitude of the diurnal variation is reduced by a factor ∼2 in periods of mixed polarities of the interplanetary magnetic field. As a consequence the North-South anisotropy seems to be completely generated by the cosmic-ray radial gradient. Estimates of the time change of this gradient from 1965 to 1968 are in agreement with those calculated from changes in the interplanetary magnetic structure.

Characteristics of quiet as well as enhanced diurnal anisotropy of cosmic radiation

It is shown that the model which has been successful in explaining the anisotropy of low energy cosmic radiation of solar origin in terms of simple convection and field aligned diffusion can also be extended to explain both the quiet and enhanced diurnal variation of cosmic radiation at higher energies. The enhanced diurnal variation which shows a maximum around ∼ 2000 hr is shown to be caused by the superposition of normal convection and enhanced field aligned diffusion due to an enhanced positive density gradient of approximately ∼10 per cent/AU. The enhanced gradient in the early part of the event is caused by a depletion of cosmic ray intensity along the garden-hose direction and later in the event (T ⩾ 4 days) is caused by an excess flux from the anti-garden-hose direction. The quiet day average diurnal anisotropy of cosmic radiation can be understood as due to an equilibrium between the convective and diffusive flow, resulting in a corotation vector along the 1800 hr direction. The observed amplitude of the annual mean diffusive vector is consistent with a positive radial density gradient of ∼4.5 per cent/AU, which agrees with other radial gradient measurements. The diffusive vector during both quiet and enhanced anisotropy periods is shown to be along the interplanetary field direction. The ratio of perpendicular to parallel diffusion coefficient under normal conditions is derived to be ⩽ 0.05 indicating that the transverse gradients, in general, are negligible.

Characteristics of Cosmic Ray Diurnal Variation from Deep River Neutron and Meson Data and Temperature Effects

From an extensive analysis of Deep River meson data for over 5 years, new temperature coefficients have been derived for correcting the meson data. It is shown that meson data corrected using new coefficients yield values of diurnal and semidiurnal anisotropy consistent with those obtained from neutron monitors. Using the temperature-corrected meson and neutron data, the upper cutoff rigidity beyond which the diurnal variation ceases is shown to vary with solar cycle showing a minimum of about 35 GV during the Quiet Solar Year of 1965, and a maximum of ≈ 125 GV during 1968–1969.

Quiet-time electron increases: A measure of conditions in the outer solar system

One possible explanation for the increases in the intensity range of 3- to 12-Mev interplanetary electrons that McDonald et al. (1972) have labeled as ‘quiet-time electron increases’ is discussed. It is argued that the electrons in quiet-time increases are galactic in origin but that the observed increases are not the result of any variation in the modulation of these particles in the inner solar system. It is suggested instead that quiet-time increases may occur when more electrons than normal penetrate a modulating region that lies far beyond the orbit of the earth. The number of electrons penetrating this region may increase when field lines that have experienced an unusually large random walk in the photosphere are carried by the solar wind out to the region. As evidence of this increased random walk, it is shown that five solar rotations before most of the quiet-time increases occur there is an extended period when the amplitude of the diurnal anisotropy (measured by the Deep River neutron monitor) is relatively low. A delay time of five rotations implies that the proposed modulating region lies at ∼30 AU from the sun if the average solar-wind speed is assumed to be constant at ∼400 km/sec over this distance. The implications for the correlation between periods of low-amplitude diurnal anisotropy and quiet-time increases on interplanetary conditions out to ∼30 AU and some possible models for the proposed modulating region are also considered.

Solar modulation of galactic cosmic radiation

In this review an attempt is made to present an integrated view of the solar modulation process that cause time variation of cosmic ray particles. After briefly surveying the relevant large and small scale properties of the interplanetary magnetic fields and plasma, the motion of cosmic ray particles in the disordered interplanetary magnetic fields is discussed. The experimentally observed long term variations of different species of cosmic ray particles are summarised and compared with the theoretical predictions from the diffusion-convection model. The effect of the energy losses due to decelaration in the expanding solar wind are clearly brought out. The radial density gradient, the modulation parameter and their long term variation are discussed to understand the dynamics of the modulating region. The cosmic ray anisotropy measurements at different energies are summarised. At high energies (E≳ 1 GeV), the average diurnal anisotropy is shown to be energy independent and along the 18.00 h direction consistent with their undergoing partial corotation with the sun. The average semi-diurnal anisotropy seems to vary with energy as E +1 and incident from a direction perpendicular to the interplanetary field line, consistent with the semi-diurnal component being produced by latitudinal gradients. Both the diurnal and semi-diurnal components are shown to be practically time invariant. On a day to day basis, however, the anisotropy characteristics such as the exponent of variation, the amplitude and the phase show very high variability which are interpreted in terms of convection and variable field aligned diffusion due to the redistribution of the galactic cosmic ray density following transient changes in the interplanetary medium. The anisotropy observation at low energies (E≲ 100 MeV) are, however, not explained by the theory. The rigidity dependence and the anisotropies during short term variations such as Forbush decreases are discussed in terms of the proposed field models for the interplanetary field structure and are compared with the observed rigidity dependence of long term variations. The data pertaining to the 27 day corotating Forbush decreases and their association with enhanced diurnal variation are also presented. The relationship between the energetic storm particle events which are caused by the acceleration of particles in the shock fronts and the Forbush decreases which are caused by the exclusion of galactic particles by the enhanced field structure in the same fronts are clearly brought out. Thus the recurrent increases at low energies and recurrent decreases at high energies may both be caused by the field structure in the shock front. In conclusion, the properties of the very short period fluctuations (18–25 cph) are summarised.

Streaming of galactic cosmic rays in the interplanetary magnetic field

We have studied in detail the correlation between the 24 hr average anisotropy of galactic cosmic rays as measured by a ring of high latitude neutron monitors and the direction of the 24 hr average interplanetary magnetic field. From the two year data period 1967–68, we have selected events showing instances of marked non-equilibrium anisotropy (i.e. large departures from the average ‘corotational’ anisotropy). These include trains of spontaneously arising enhanced diurnal variation as well as Forbush decrease events showing large anisotropy in the pre-decrease, main or recovery phase. We find that the observed anisotropies are well fitted by the assumption that the streaming of galactic cosmic rays is composed of two components (1) radial streaming away from the Sun with a velocity of ≈400 km/s and (2) diffusive streaming along the direction of the interplanetary magnetic field. The diffusive streaming is mostly towards the Sun. This description is also applicable to periods characteristic of the quiet time diurnal anisotropy. These results are analogous to those obtained by McCracken, Rao and Ness for 7–45 MeV solar flare particles.

The cosmic ray solar diurnal anisotropy

This paper reviews the present state of knowledge concerning the diurnal and semidiurnal variations in the galactic cosmic ray intensity. The analytical procedures that are required for extracting from the original data the desired information concerning the characteristics of the anisotropies outside the magnetosphere are described. These include corrections for atmospheric fluctuations, the determination of the amplitudes and phases of the daily variations, and their interpretation in terms of the free space anisotropies that give rise to them. The experimental results concerning the 24-h wave, including its long-term variations with periods of one and two solar cycles, and the characteristics of the total diurnal anisotropy are then discussed. The semidiurnal anisotropy is considered next. Transient anisotropies which manifest themselves as day-to-day variations, recurrence tendencies, cosmic ray storms, and diurnal variation trains, and which can introduce appreciable changes in the spectral parameters, are also of interest. The evolution of theoretical models developed to account for the diurnal and semidiurnal anisotropies, and fluctuations thereof, are then discussed in the light of the experimental results. Finally, the general features that are now well established, and the nature of the remaining problems, are summarized.

Amplitude of diurnal anisotropy of cosmic ray intensity

Cosmic ray intensity mean diurnal variation, taking into account angle between solar corotational velocity and earth equatorial plane

Cosmic Ray Intensity Decrease of 23 September, 1966

Earlier studies have interpreted the Forbush decrease of 23 September 1966 in terms of two phases; an initial predecrease and a later worldwide decrease. This interpretation precluded the possibility of correlation with a concurrent magnetic storm and led to an explanation of the predecrease (Mathews et al. 1968) in terms of a shadow cast by a distant plasma cloud approaching from a direction 85° to the west of the sun–earth line.In the present study, particle and magnetic field data from satellite-borne detectors and ground-based neutron monitors clearly show the onset of the Forbush decrease coincident with the SSC magnetic storm. It is pointed out that the Forbush decrease arises from a corotating shock front approaching from the east of the sun–earth line and is not associated with any solar flare effect. Further, the increases observed by the various neutron monitors 9 h after the onset of the Forbush decrease are interpreted to be an enhancement of the diurnal anisotropy. An example of an increase in intensity in the IMP 3 detector arising from electron contributions is also pointed out.

Diurnal anisotropy of cosmic ray intensity

Diurnal anisotropy of cosmic ray intensity

The variation with a period of two solar cycles in the cosmic ray diurnal anisotropy for the nucleonic component

It has been shown previously that annual means of the diurnal anisotropy determined from 30 years of ionization chamber data result from the superposition of two distinct independent components. One component, W, has its maximum (or minimum) in the asymptotic direction 128° east of the sun-earth line, with amplitude that varies sinusoidally with a period of two solar cycles. The other, V, has its maximum in the asymptotic direction 90° east of the sun-earth line; it is well correlated with magnetic activity and varies with the sunspot cycle. For the observed component of the total diurnal anisotropy 90° east of the sun (which contains all of V and 79% of W), a correlation coefficient of 0.94 was found between annual means from the Huancayo neutron monitor and from ionization chambers. The resulting implication that V and W were each about 1.35 times greater in the neutron monitor data is explicitly established. Analysis of the diurnal anisotropy, from a geographical distribution of neutron monitors normalized to Churchill, shows that there V and W are each about twice the value for ionization chambers. These results reveal that the variational spectrum is the same for V and for W within the experimental uncertainties. Thus, it is valid to derive variational spectra from total vectors.

Periodic solar time variations in the cosmic-ray muon component near sea level

Eight scintillation counters, each about 1.5 m2, have been assembled in a muon telescope array at Winnipeg, Canada. Six telescopes (geometric factor about 750 cm2sr) have their axes approximately in the equatorial plane. For these, the asymptotic cones of acceptance are quite narrow with respect to asymptotic longitude, and they are therefore well suited for time variation studies. Two nearly vertical telescopes (geometric factor about 350 cm2sr) complete the array. Experimental results obtained between 13th November 1966 and 5th May 1968 are presented. It is shown that the phase of the average diurnal anisotropy is consistent with the Parker (1964, 1967) and Axford (1965) theories alone, and its amplitude is dependent on the time in the solar cycle. Further, a semi-diurnal anisotropy is found, extra- terrestrial in origin, with intensity maxima at right angles to the interplanetary magnetic field.

Direction of the nearby galactic magnetic field inferred from a cosmic-ray diurnal anisotropy

Twenty year wave in diurnal anisotropy component of galactic cosmic rays arriving at earth from asymptotic direction east of sun interpreted as magnetic reconnection

Solar Diurnal Anisotropy of Cosmic Rays

THE solar diurnal anisotropy of cosmic rays is attributed to the bulk streaming of the cosmic ray gas caused by the corotating interplanetary magnetic field that is “rigidly attached to the Sun”1,2. By analogy with the classical Compton–Getting effect3, a(R), the amplitude of the daily variation at magnetic rigidity R is related to the ratio of particle speed v and the bulk streaming velocity u where γ is the exponent of the differential spectrum. Because, at the orbit of the Earth, u≈450 km s−1 a neutron monitor with mean rigidity of response about 10 GeV and atmospheric threshold 1 GeV should observe an amplitude extrapolated through the atmosphere to free space of about 0.7 per cent.

Variation with a period of two solar cycles in the cosmic-ray diurnal anisotropy and the superposed variations correlated with magnetic activity

Annual means of the diurnal anisotropy from 1937 to 1967 are shown to result from the addition of two distinct diurnal components. One component, with maximum in the asymptotic direction 128° east of the sun, contains a well-determined wave, W, with a period of two solar cycles. W passes through zero in 1958 when the sun’s poloidal field reversed. The remaining component with W eliminated, has its maximum in the asymptotic direction 90° east of the sun. Annual means of this component, with maximum at 18.0 hours local asymptotic time, are well correlated (r = +0.75) with magnetic activity and determine a solar cycle variation with minimum near sunspot minimum and amplitude about two-thirds that of W. These results derive from a statistical investigation of the variability of annual means of the diurnal variation from ion-chamber data at Cheltenham-Fredericksburg, Huancayo, and Christchurch. The absolute, or total, diurnal anisotropy and the atmospheric diurnal temperature effect are in reasonable agreement with those derived independently through a comparison between the diurnal anisotropy from ion-chamber data and from Simpson's 1953–1966, IGY neutron monitor data at Huancayo.

Diurnal cosmic-ray variation with the inclusion of the geometrical effects and the asymptotic cones of approach

Employing the data from cosmic-ray neutron monitors at high latitude, the spatial distribution of the axis of the diurnal anisotropy is determined. The effects of the earth's revolution around the sun on the diurnal intensity variation is investigated. A new method for further investigation of the spatial distribution of the anisotropy and for the determination of its spectra in various directions has been proposed.

A variation, with a period of two solar cycles, in the cosmic-ray diurnal anisotropy

Annual means, 1937-1965, of the cosmic-ray diurnal anisotropy component, in the asymptotic direction 128°E of the sun, are well-fitted by a wave with a period of 20 years, which is twice the solar cycle period of 10 years for the interval 1937–1965. This variation is independent of magnetic activity, and when removed the residual variations combine with those in the asymptotic component 387deg;E of the sun (in which there is no 20-year wave) to give resultant variations, which are principally in the asymptotic component 90°E of the sun. Yearly means of this resultant component are well correlated (r = +0.75) with magnetic activity, U0, and, on the average, vanish for U0 = 0. U0 is the absolute value of the southward geomagnetic component of the so-called equatorial ring current [Forbush, 1966].

Cosmic-ray propagation processes: 3. The diurnal anisotropy in the vicinity of 10 Mev/nucleon

The results of a measurement of the anisotropic character of the cosmic radiation in the vicinity of 10 Mev/nucleon during 1965–1966 are presented. The periods considered were those during which solar flare cosmic-ray flare effects were not discernible in the data; that is, they are representative of the anisotropy that is responsible for the diurnal variation observed in the energy range 1–100-bev by detectors situated on the surface of the earth. The anisotropy in the vicinity of 10-Mev/nucleon exhibits maximum cosmic-ray flux from the approximate direction of the sun, this being at quadrature to that observed in the 1- to 100-bev energy range. This observation suggests that in the vicinity of 10 Mev/nucleon (a) the cosmic radiation exhibits a density gradient directed toward the sun, and (b) the 10-Mev/nucleon cosmic radiation was primarily of solar origin during the period under consideration. Consideration of these and earlier results suggests that (a) the direction of the cosmic-ray density gradient reversed between 107 and 109 ev during the period of sunspot minimum, and (b) the cosmic radiation in the range 107–1011 ev corotates with the sun.