The regimes of the east-west and the radial anisotropies of cosmic rays in the heliosphere

Ahluwalia, H. S.

doi:10.1016/0032-0633(88)90010-4

Cited by 33 publications

(37 citation statements)

References 34 publications

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“…Currently it is believed that the 4SD is a superposition of two anisotropies (E-W and radial anisotropies) at 90 degrees to each other (Ahluwalia 1988a(Ahluwalia , 1988bSwinson et al 1990) contrary to previous conclusions. It is perhaps surprising that Bieber and Chen (1991a) have shown that the solar diurnal variation varies with a period of 22 years around an axis aligned with the IMF; and this axis is very close to the direction of one of the components proposed by Forbush (1967).…”

Section: Characteristics Of the Solar Diurnal Anisotropymentioning

confidence: 59%

“…The variation could not be explained solely in terms of a varying rigidity spectrum. Ahluwalia (1988aAhluwalia ( ,1988b disputed the existence of the two independent components proposed by Forbush (1967) and Duggal et al (1969), but conceded that two independent components are present in the anisotropy during the IMF polarity configuration A>0. One of these is aligned with the E-W direction (i.e.…”

Section: Characteristics Of the Solar Diurnal Anisotropymentioning

confidence: 97%

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Modulation of high‐energy cosmic rays in the heliosphere

Hall¹,

Humble²,

Duldig³

1994

J. Geophys. Res.

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DECLARATIONSI certify that this thesis does not incorporate without acknowledgment any material previously submitted for a degree or diploma in any university; and to the best of my knowledge and belief it does not contain any material previously published or written by another person where due reference is not made in the text. Damian Lindsay Hallll This thesis may be made available for loan and limited copying in accordance with the Copyright Act 1968. Damian Lindsay Hall Ill ABSTRACTThe distribution of galactic cosmic ray particles in the heliosphere is influenced (modulated) by the Sun's interplanetary magnetic field (IMF) and the solar wind. The particles diffuse inward, convect outward and have drifts in the motion of their gyro-centres. They are also scattered from their gyro-orbits by irregularities in the IMF. These processes are the components of solar modulation and produce streaming (anisotropies) of particles in the heliosphere. The anisotropies can be investigated at Earth by examining the count rates of cosmic ray detectors. The anisotropic streams appear as diurnal and semi-diurnal variations in the count rates of cosmic ray recorders in solar and sidereal time. Theoretical models of solar modulation predict effects which are dependent on the polarity of the Sun's magnetic dipole (A >0 or A <0). The solar diurnal and North-South anisotropy can be used to test these predictions.The yearly averaged solar and sidereal diurnal variations in data recorded by seven neutron monitors and ten muon telescopes for the period 1957 to 1990 have been deduced by Fourier analysis methods. The rigidities of the galactic cosmic rays to which these instruments respond encompass the range 10 to 1400 Giga volts (GV). The rigidity spectrum of the solar diurnal anisotropy has been inferred to have a mean spectral index extremely close to zero and an idealised upper limiting rigidity of 100± 25 GV. This is in good agreement with previous determinations. It is shown that this upper limit has a temporal variation between 50 GV and 180 GV and is correlated with the magnitude of the IMF. The rigidity spectrum is likely to be dependent on the polarity of the Sun's magnetic dipole, the spectral index being determined as positive in the A >0 magnetic polarity state and negative in the A <0 polarity state. It is also shown that the amplitude of the anisotropy varies with an 11-year variation and the time of maximum varies with 22-year variation. Both of these variations are shown to be independent of any change in the rigidity spectrum.The solar diurnal anisotropy is also used as a tool to calculate the modulation parameters ?Lip, (the product of the parallel mean-free path and radial density gradient) and Gtzl (an indicator of the symmetric latitudinal density gradient). X G r is found to have a 22-year II variation at all rigidities studied and furthermore to only have rigidity dependence when the heliosphere is in the A >0 magnetic polarity state. It is unlikely that X IIG r has any rigidity dependence in the A <0 polarity s...

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Section: Characteristics Of the Solar Diurnal Anisotropymentioning

confidence: 59%

Section: Characteristics Of the Solar Diurnal Anisotropymentioning

confidence: 97%

Modulation of high‐energy cosmic rays in the heliosphere

Hall¹,

Humble²,

Duldig³

1994

J. Geophys. Res.

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“…It is known that the SMF field reverses at around the solar maximum activity (Ahluwalia, 1988). In this case the SMF reverses from positive to negative polarity in 2001 and from negative to positive in 2013.…”

Section: Diurnal Anisotropy During the Solar Cyclementioning

confidence: 95%

“…As a result, the intensity of GCR recorded by ground-based neutron monitors (NMs) shows periodic and abrupt changes as a function of space, time, and energy (Oh et al, 2010). This phenomenon, which is known as the diurnal anisotropy of CR intensity, is a localtime short-term variation (Pomerantz and Duggal, 1971;Ahluwalia, 1988).…”

Section: Introductionmentioning

confidence: 99%

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Latitudinal and longitudinal dependence of the cosmic ray diurnal anisotropy during 2001–2014

et al. 2016

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Abstract. The diurnal anisotropy of cosmic ray intensity for the time period 2001 to 2014 is studied, covering the maximum and the descending phase of solar cycle 23, the minimum between solar cycles 23 and 24, and the ascending phase and maximum of solar cycle 24. Cosmic ray intensity data from 11 neutron monitor stations located at different places around the Northern Hemisphere obtained from the high-resolution Neutron Monitor Database (NMDB) were used. Special software was developed for the calculations of the amplitude and the phase of the diurnal anisotropy vectors on annual and monthly basis using Fourier analysis and for the creation of the harmonic dial diagrams. The geomagnetic bending for each station was taken into account in our calculations determined from the asymptotic cones of each station via the Tsyganenko96 (Tsyganenko and Stern, 1996) magnetospheric model. From our analysis, it was resulted that there is a different behavior of the diurnal anisotropy vectors during the different phases of the solar cycles depending on the solar magnetic field polarity. The latitudinal and longitudinal distribution of the cosmic ray diurnal anisotropy was also examined by grouping the stations according to their geographic coordinates, and it was shown that diurnal variation is modulated not only by the latitude but also by the longitude of the stations. The diurnal anisotropy during strong events of solar and/or cosmic ray activity is discussed.

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Cosmic ray heliospheric transport study with neutron monitor data

et al. 2015

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Determining transport coefficients for galactic cosmic ray (GCR) propagation in the turbulent interplanetary magnetic field (IMF) poses a fundamental challenge in modeling cosmic ray modulation processes. GCR scattering in the solar wind involves wave‐particle interaction, the waves being Alfven waves which propagate along the ambient field (B). Empirical values at 1 AU are determined for the components of the diffusion tensor for GCR propagation in the heliosphere using neutron monitor (NM) data. At high rigidities, particle density gradients and mean free paths at 1 AU in B can only be computed from the solar diurnal anisotropy (SDA) represented by a vector A (components Ar, Aϕ, and Aθ) in a heliospherical polar coordinate system. Long‐term changes in SDA components of NMs (with long track record and the median rigidity of response Rm ~ 20 GV) are used to compute yearly values of the transport coefficients for 1963–2013. We confirm the previously reported result that the product of the parallel (to B) mean free path (λ||) and radial density gradient (Gr) computed from NM data exhibits a weak Schwabe cycle (11y) but strong Hale magnetic cycle (22y) dependence. Its value is most depressed in solar activity minima for positive (p) polarity intervals (solar magnetic field in the Northern Hemisphere points outward from the Sun) when GCRs drift from the polar regions toward the helioequatorial plane and out along the heliospheric current sheet (HCS), setting up a symmetric gradient Gθs pointing away from HCS. Gr drives all SDA components and λ|| Gr contributes to the diffusive component (Ad) of the ecliptic plane anisotropy (A). GCR transport is commonly discussed in terms of an isotropic hard sphere scattering (also known as billiard‐ball scattering) in the solar wind plasma. We use it with a flat HCS model and the Ahluwalia‐Dorman master equations to compute the coefficients α (=λ⊥/λ∥) and ωτ (a measure of turbulence in the solar wind) and transport parameters λ||, λ⊥, Gr, Gθs, and an asymmetric gradient Gθa normal to the ecliptic plane. We study their dependence on rigidity (R), p/n intervals, sunspot numbers (SSNs), and solar wind parameters at 1 AU. λ|| exhibits a strong 22y dependence but Gr does not, explaining solar polarity dependence of λ|| Gr. The computed Gr values are an order of magnitude greater than those reported by our colleagues making an ad hoc assumption that α is low (0.01). At high rigidities, the drift contribution at 1 AU is small and unsteady. A new methodology is outlined to compute yearly GCR north‐south anisotropy (Aθ) from the data for a single detector sorted for p/n intervals. We show that Gθa is the main contributor to Aθ in the steady state, and Gθa is shown not correlated with the north‐south excess SSNs.

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The regimes of the east-west and the radial anisotropies of cosmic rays in the heliosphere

Cited by 33 publications

References 34 publications

Modulation of high‐energy cosmic rays in the heliosphere

Modulation of high‐energy cosmic rays in the heliosphere

Latitudinal and longitudinal dependence of the cosmic ray diurnal anisotropy during 2001–2014

Cosmic ray heliospheric transport study with neutron monitor data

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