World-wide cosmic ray variations, 1937-1952

Forbush, Scott E.

doi:10.1029/jz059i004p00525

Cited by 364 publications

(119 citation statements)

References 5 publications

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“…It was Forbush (1938) who first noted that following large solar flares the cosmic ray intensity at earth would drop suddenly. It was also Forbush (1954) who noted an anti-correlation of cosmic ray intensity with the 11-year solar activity cycle. This led to the interpretation that there was a component of the cosmic rays of galactic origin which was being modulated by the interplanetary medium.…”

Section: Introductionmentioning

confidence: 99%

Modulation of low-energy cosmic rays

Sari¹

1975

J. Geophys. Res.

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Recent theories have related the diffusion of cosmic rays in the solar wind to the power spectrum of interplanetary magnetic field fluctuations. In this study the power spectrum‐diffusion coefficient relation for low‐energy protons (T < 80 MeV) is tested directly, and the relative contribution to the scattering from directional discontinuities in the magnetic field is examined. Modulation parameters Vw/Krr, the solar wind speed divided by the radial diffusion coefficient, are calculated from Pioneer 6 magnetic field and solar wind data and are compared with simultaneous Imp 3 observations of the 20‐ to 80‐MeV proton flux. Daily variations in the 40‐ to 80‐MeV proton flux are found to respond to changes in Vw/Krr, as is predicted by a perturbation solution of the Fokker‐Planck equation once the contribution from directional discontinuities in the magnetic field is subtracted from the diffusion coefficients. This finding is interpreted to imply that a large percentage of the identified discontinuities are basically tangential. A lack of correlation between the 20‐ to 40‐MeV flux and the parameter changes is interpreted as being due to either the invalidity of the theory at lower energies or the presence of a continuous flux of high‐energy solar protons. A well‐defined time lag between changes in the 60‐ to 80‐MeV proton flux and corresponding changes in Vw/Krr is observed during a period when the interplanetary magnetic field is predominantly radial and the Imp 3‐sun‐Pioneer 6 angle is greater than 10°. This lag is consistent with the interpretation of little or no cross‐field diffusion or a perpendicular diffusion coefficient less than 3 × 1019 cm²/s.

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Section: Introductionmentioning

confidence: 99%

Modulation of low-energy cosmic rays

Sari¹

1975

J. Geophys. Res.

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“…It is widely known that Galactic cosmic rays (CRs) are modulated by solar activity (see the pioneering works by Forbush 1954Forbush , 1966. In the 100 years since the discovery of CRs (Hess 1912), the topic of CR modulation has been widely investigated (see Potgieter 2013b for an extended review).…”

Section: Introductionmentioning

confidence: 99%

Drift Effects on the Galactic Cosmic Ray Modulation

Laurenza

Vecchio

Storini

et al. 2014

ApJ

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Cosmic ray (CR) modulation is driven by both solar activity and drift effects in the heliosphere, although their role is only qualitatively understood as it is difficult to connect the CR variations to their sources. In order to address this problem, the Empirical Mode Decomposition technique has been applied to the CR intensity, recorded by three neutron monitors at different rigidities (Climax, Rome, and Huancayo-Haleakala (HH)), the sunspot area, as a proxy for solar activity, the heliospheric magnetic field magnitude, directly related to CR propagation, and the tilt angle (TA) of the heliospheric current sheet (HCS), which characterizes drift effects on CRs. A prominent periodicity at ∼six years is detected in all the analyzed CR data sets and it is found to be highly correlated with changes in the HCS inclination at the same timescale. In addition, this variation is found to be responsible for the main features of the CR modulation during periods of low solar activity, such as the flat (peaked) maximum in even (odd) solar cycles. The contribution of the drift effects to the global Galactic CR modulation has been estimated to be between 30% and 35%, depending on the CR particle energy. Nevertheless, the importance of the drift contribution is generally reduced in periods nearing the sunspot maximum. Finally, threshold values of ∼40 • , ∼45 • , and >55 • have been derived for the TA, critical for the CR modulation at the Climax, Rome, and HH rigidity thresholds, respectively.

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“…The fractional modulation of cosmic rays by the heliosphere is greater at lower energies and hence by selecting this high-latitude station we detect the larger variation of the lower energy particles (Bieber et al, 2004). Schwabe (1843) was the first to recognise the 11-year solar cycle variation using the periodicity in sunspot number records and the signature of this variation in cosmic rays was detected using ionisation chambers by Forbush (1954). Evidence for the 22-year solar cycle variation was first reported on by Ellis (1899), who observed high counts of geomagnetically quiet days during alternate minima in the 1850s and 1870s.…”

Section: Introductionmentioning

confidence: 99%

The 22-Year Hale Cycle in Cosmic Ray Flux – Evidence for Direct Heliospheric Modulation

2013

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The ability to predict times of greater galactic cosmic ray (GCR) fluxes is important for reducing the hazards caused by these particles to satellite communications, aviation, or astronauts. The 11-year solar cycle variation in cosmic rays is highly correlated with the strength of the heliospheric magnetic field. Differences in GCR flux during alternate solar cycles yield a 22-year cycle, known as the Hale Cycle, which is thought to be due to different particle drift patterns when the northern solar pole has predominantly positive (denoted a qA>0 cycle) or negative (qA<0) polarities. This results in the onset of the peak cosmic ray flux at Earth occurring earlier during qA>0 cycles than for qA<0 cycles and hence the peak being more domed for qA>0 and more sharply peaked for qA<0. In this study, we demonstrate that properties of the large-scale heliospheric magnetic field are different during the declining phase of the qA<0 and qA>0 solar cycles, when the difference in GCR flux is most apparent. This suggests that particle drifts may not be the sole mechanism responsible for the Hale Cycle in GCR flux at Earth. However, it is also demonstrated that these polarity-dependent heliospheric differences are evident during the space-age but much less clear in earlier data: using geomagnetic reconstructions, it is shown that for the period of 1905 -1965, alternate polarities do not give as significant a difference during the declining phase of the solar cycle. Thus we suggest that the 22-year cycle in cosmic ray flux is at least partly the result of direct modulation by the heliospheric magnetic field and that this effect may be primarily limited to the grand solar maximum of the space-age.

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World-wide cosmic ray variations, 1937-1952

Cited by 364 publications

References 5 publications

Modulation of low-energy cosmic rays

Modulation of low-energy cosmic rays

Drift Effects on the Galactic Cosmic Ray Modulation

The 22-Year Hale Cycle in Cosmic Ray Flux – Evidence for Direct Heliospheric Modulation

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