Hale cycle and long-term trend in variation of galactic cosmic rays related to solar rotation

Abstract

Context. Galactic cosmic ray (GCR) intensities around solar minimum times are modulated by magnetic drifts that depend on the overall solar polarity. GCR intensities reach a higher but more narrow peak during negative minima than during positive minima. However, despite these higher intensities, the variation of GCRs over timescales of solar rotation is smaller during negative minima than during positive minima. Aims. We study the variation of GCR intensity over the 27-day synodic solar rotation and over the 14-day half-rotation, in particular the long-term trend and cyclic pattern of this variation, and propose a unifying explanation for the observations. Methods. We used two high-latitude neutron monitors, Oulu and Apatity, which are most sensitive to the low-energy part of the GCR spectrum and thereby more strongly affected by the changes in the conditions of the local heliosphere. We calculated the yearly mean amplitudes of the GCR intensity variation during the full solar rotation (A 27 ) and half-rotation (A 14 ) in 1964-2016. Results. We verify that the A 27 and A 14 amplitudes exhibit a clear 22-yr Hale cycle during solar minima at both stations, with larger amplitudes in positive minima. We find that the mean amplitude of the Hale cycle is about 30-45% of the mean amplitude for A 14 , while is only about 15-30% for A 27 . We also find that all amplitudes depict a declining long-term trend, which we suggest is due to the weakening of solar polar magnetic fields during the last four solar cycles and the ensuing latitudinal widening of the heliospheric current sheet (HCS) region. An exceptionally wide HCS region during the last solar minimum, when A 14 reached its all-time minimum, is demonstrated by Ulysses probe observations. Conclusions. Our results emphasize the effect of polarity-dependent drift and the properties of the HCS in modulating the variation of GCR intensity during solar rotation in solar minimum times. The second rotation harmonic yields a larger Hale amplitude than the first because it is more probable for the Earth to be outside the HCS only once during the rotation than twice or more, which more strongly reduces A 14 during negative polarity times than A 27 . With the HCS region widening from minimum to minimum, the decrease in A 14 is relatively faster than in A 27 .

Behaviour of different periodicities in galactic cosmic particles as observed by ACE during solar cycles 23 and 24

“…During particularly solar minima with negative polarity GCR's drift preferentially inward towards the Sun along the heliospheric current sheet [7,9]. The wavy current sheet as well as the more variable structure of the solar wind during these intervals are most probably responsible for the strong observation of 7-day periodicities when A<0 in comparison to solar dipole orientations characterised by A>0.…”

“…The general conclusion from these investigations is that the origin of the higher harmonics, particularly the 14-day and 9-day periodicities is the simultaneous existence of active regions on the solar surface at different longitude positions [6] as well as the the presence of large-scale structures present in the heliosphere during solar minimum intervals. Studies by Gil and Mursula [7] determined that the amplitudes of the ~27-day variation of the GCR intensity by neutron monitors are indeed larger during the minimum epochs of solar activity for the A>0 compared to conditions when A<0. This particular behaviour of the ~ 27-day period has also been confirmed by our investigation using GCR data from the ACE satellite.…”

Quasi-periodic changes of three dimensional solar anisotropy of galactic cosmic rays for 1965-2014

“…27-day variations of the GCR intensity [e.g., [21][22][23][24][25][26] and anisotropy [e.g., [27][28] have a sporadic character. Their amplitudes significantly increase and decrease averagely during 4-6 solar rotation periods.…”

Comparing Two Intervals of Exceptionally Strong Solar Rotation Recurrence of Galactic Cosmic Rays

“…We have also calculated the amplitudes A k and phases k of the harmonic variation of the three parameters for each solar rotation in Interval A and Interval B. The Fourier series for parameter x(t) is calculated as follows (e.g., Gil & Mursula, 2017;Xue & Chen, 2008): where…”