An Unusual Pattern of Cosmic-Ray Modulation During Solar Cycles 23 and 24

Abstract

By means of an analysis of data from eight neutron monitor (NM) stations with different geomagnetic cutoff rigidities, we found an unusual latitudinal effect observed in the cosmic-ray (CR) modulation during the last solar cycles. Since the beginning of the ground-based cosmic-ray monitoring, it is known that the solar-cycle modulation is more evident in data from high latitude than from the medium and low latitudes, showing an expected geomagnetic cutoff rigidity effect. However, a more detailed look shows a new latitudinal effect in cycle 24: while the magnitude of the solar modulation in the low-latitude data remains the same for the last three solar minima, the last solar minimum caused a more intense peak in the polar NM data than in the previous cycles. After correcting the data for the geomagnetic changes of the period, we found an anomalous solar modulation in the last cycle. This suggests a weaker heliospheric modulation at low-energy particles (responsible for the NM counting in polar sites) now than in the previous cycles, while there is no significant difference of the modulation for the more energetic part of the CR spectrum. Our result can be associated with changes of the solar wind turbulence, which would corroborate some recent studies about the last solar minimum phase, and indicates that this new solar modulation feature is still present in the current solar maximum stage.

The effects of magnetic field modifications on the solar modulation of cosmic rays with a SDE-based model

“…The parameters to obtain these results are summarised in Table 1 and it is clear that: 1) the diffusion was required to continually increase from 2006 to 2009, with the value of κ ||,0 increasing from 16.0 in 2006 to 17.1, 17.5, and 19.4 in 2007, 2008, and 2009 respectively; 2) the spectra became progressively softer towards 2009, so that the rigidity dependence of the diffusion coefficient below ∼ 3 GeV was required to change from P 0.85 in 2006 to P 0.81 , P 0.78 , and P 0.73 2007, 2008, and 2009. These type of changes are in agreement with those found by various other authors, including Bazilevskaya et al (2012), Ndiitwani et al (2013), Potgieter et al (2014), and Pacini and Usoskin (2015). Except for providing evidence for the applicability of the SBM in particular, this qualitative correspondence in modulation results over the entire solar minimum period of 2006 to 2009 also illustrates the already well established credibility of the SDE-based model.…”

Magnitudes and timescales of total solar irradiance variability

“…Solomon et al (2011) attribute the Earth's lower thermospheric density during the 2008 minimum, as evidenced by drag measurements of Earth-orbiting spacecraft, to be due to lower levels of extreme-ultraviolet solar-irradiance. Similarly, Pacini and Usoskin (2015) use neutron-monitor records from eight Earth-based stations to show an increase in polar cosmic-ray intensities during the 2008 solar minimum from the prior three, which they attribute to changes in the solar wind. While such independent observations are indicative of lower solar activity during this recent solar minimum and may be corroborative of possibly-similar secular trends in TSI-variations during the 2008 solar minimum.…”

Cosmic rays: bringing messages from the sky to the Earth's surface

“…It is, e.g., suggested that during periods of very weak solar activity (like the Maunder Minimum), the relation between solar activity and cosmic rays may be inverted [16]. The new features found at the heliospheric modulation of GCR measured by ground-based detectors are discussed in [17] and references therein.…”