H.S. Ahluwalia

Present status of the recovery phase of cosmic ray 11‐year modulation

The recovery from cosmic ray 11-year modulation follows two distinct repetitive patterns [Ahluwalia, 1994]. When the magnetic polarity of the Sun in the northern hemisphere is negative (qA 0) the recovery period is reduced to less than half as much. These repetitive patterns have been observed for the last five solar activity cycles (17 to 21). The recovery for cycle 22 is still in progress. By the end of 1994, the intensity monitored with the nuetron detectors at Deep River, Climax, and Huancayo continues to be significantly below the cosmic ray maxium observed in March 1987. The physical significance of our results is discussed.

Galactic cosmic ray intensity variations at a high latitude sea level site 1937-1994

We have created an extended cosmic ray data string (1937–1994) by combining data obtained with ion chambers at Cheltenham/Fredericksburg (1937–1972) and Yakutsk (1953–1994). Both represent high-latitude, sea level sites, with an atmospheric cutoff of about 4 GV. Their common median rigidity of response to galactic cosmic ray spectrum is 67 GV. This data string represents the longest continuous cosmic ray intensity variations record yet. Therefore it is useful for the cosmic ray modulation studies over a longer time period. We show that there is no striking quantitative correspondence between the amplitudes of solar activity cycles and cosmic ray modulation over six cycles. However, an inverse correlation is apparent between the two. Moreover, the results on the durations of the two are ambiguous. Flat-topped recovery pattern predicted for even cycles (A > 0), at lower rigidities, is not observed in the ion chamber data. We speculate that three solar activity cycle quasi-periodicity may be present in the extended data string. Its origin is not identified yet. The quasi-periodicity is shown to exist also in the planetary index Ap data. As such, it may represent a characteristic feature of the solar wind source region on the Sun as yet unexplored. The odd cycles, with a negative polarity in the solar northern hemisphere, yield a better linear correlation between galactic cosmic ray intensity decrease and Ap than even cycles and A > 0 epochs.

Measurements of transverse cosmic ray particle density gradient at high rigidities in the Ulysses era

Transverse cosmic ray gradient at high rigidities can only be determined from diurnal anisotropy data obtained with the neutron monitors and muon detectors based on a unique spinning platform at 1AU, namely, the Earth. This fortunate circumstance has several distinct advantages. Use of high counting rate detectors ensures excellent statistics. The detector performance is checked periodically to ensure stability which makes it possible to obtain long data strings with 100% coverage and reliability, over a wide range of primary rigidities. Very surprisingly, there is an impressive qualitative agreement between measurements made on Earth in the inner heliosphere and those made on board the various spacecrafts in the outer heliosphere, at much lower rigidities, at different short time intervals. This argues for the existence of long lived, spatially extensive, electromagnetic structures in the heliosphere, which affect a very large rigidity range in the galactic cosmic ray spectrum. We describe the results of our analysis of the data obtained with detectors with median rigidity of response (Rm) spanning the range, 16 GV ≤ Rm < 134 GV, over nearly three decades encompassing three epochs each of solar activity maxima, minima, and polar field reversals, including the era of pole-to-pole scan of Sun by the spacecraft Ulysses.

Cosmic ray solar semidiurnal anisotropy: 2. Heliospheric relationships of anisotropy parameters

We have used the corrected experimental semidiurnal variation observations described at length in Paper 1, to compute the amplitude and direction of semidiurnal anisotropy in free space for the period 1965 to 1992. The time interval covers several epochs of solar activity maxima, minima, and polar field reversals. We find that the amplitude of anisotropy fluctuates between 0.05% and 0.10%, except for the period 1971 to 1977 when it acquires a value as large as 0.25% at high primary rigidities (Rm > 16 GV). The physical cause of this anomalous behavior is not identified yet. The asymptotic time stays close to 0300 LT, independent of the primary rigidity (R). This direction is perpendicular to the nominal direction of interplanetary magnetic field (IMF) at Earths orbit. The polarity of IMF has no effect on the semidiurnal anisotropy parameters. The physical significance of our results is discussed in the context of the theoretical ideas available in the literature. The contributions from the symmetric transverse gradients to semidiurnal anisotropy appear to be minimal, but those from pitch angle scattering may be appreciable.

Sunspot Cycle 24 and the Advent of Dalton-Like Minimum

Ahluwalia and Jackiewicz (2011) have predicted that sunspot cycle 24 will be only half as active as cycle 23, reaching its peak in May 2013±6 months. Here, we discuss the timeline for cycle 24 since its onset in December, 2008 and compare it to the timelines for the last ten cycles (14 to 23) of the 20th century; cycle 24 is rising the slowest. We speculate that cycle 24 may herald the onset of a Dalton-like minimum in the 21st century. The implications of this outcome on global temperature change and ensuing socioeconomic and political scenarios are discussed, on the basis of the historical record.

IMF intensity and galactic cosmic ray modulation

Abstract We have studied the data strings for the interplanetary magnetic field intensity (B), and the neutron monitor (NM) annual mean hourly rates for the period 1964 to 1999, at different global sites. The median rigidity of detector response (Rm) to galactic cosmic ray (GCR) spectrum lies in the range of 10 GV to 33 GV. We relate the observed features of 11 year modulation to the time variations of B and the long term trend in its baseline which are not related to SSN. For example, the minimum GCR intensity for cycle 21 (maximum modulation) occurs in 1982 when B has a high value, 3 years away from SSN maximum. We confirm that the long term trend in the IMF time variations plays a significant role in controlling the recovery from 11 year GCR modulation.

Temporal variations of the cosmic ray intensity and the magnetic configurations of the heliosphere

Abstract Data from worldwide network of neutron and muon detectors are used to study 11-year variations of cosmic rays, over four solar activity cycles. We find that the recovery of the cosmic ray intensity follows one of the two distinct modes. During odd cycles recovery is completed in 6 to 8 years, but during even cycles complete recovery occurs in 2 to 3 years. Two model magnetic configurations of the heliosphere are proposed to understand these recovery modes. Implications of these models are also discussed.

Current Forecast for Sunspot Cycle 24 Parameters

Our prediction for the development of sunspot cycle 23 activity came true; one of the very few to have attained this status. We use the 3‐cycle quasi‐periodicity observed in the planetary index Ap. We improve our method by including data for 150 years and draw inferences as to what to expect for the development phase of cycle 24. Our forecast for the smoothed sunspot number at cycle 24 peak 78±5 in June 2013; the possibility that next three cycles may be progressively less active cannot be ruled out; the trend may possibly continue for the rest of the 21st century.

Cosmic ray heliospheric transport study with neutron monitor data

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, 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 helio-equatorial 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 (a.k.a 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 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.

Prediction of galactic cosmic ray modulation by CME at solar cycle 23 maximum

Abstract The results of a preliminary study of the high rigidity galactic cosmic rays (GCR) and sunspot numbers (SSN) data strings for six solar cycles are presented. A scheme is developed for predicting the size of the solar cycle modulation of GCR well in advance. We apply it to cycle 23 which started in May 1996.