Long‐term neutron monitor observations and the 2009 cosmic ray maximum

Abstract

[1] The solar minimum of 2009 was characterized by a prolonged increase toward the maximum cosmic ray intensity, which was higher than it was during the maxima of 22 and 44 years ago. In the previous two so-called qA <0 (solar dipole moment facing South) magnetic cycles, these increases were more sharply peaked than in 2009. The observations of the Sanae, Hermanus, Potchefstroom, and Tsumeb neutron monitors are used to investigate this behavior in terms of propagation conditions due to solar activity, the heliospheric magnetic field, and the profile of the wavy current sheet in the field. This 2009 cosmic ray maximum can only be understood after an investigation of the long-term cosmic ray record. This study is augmented by observations of eight other neutron monitors. During 2009, solar activity parameters were significantly different from previous solar minima: The sun was much quieter, and the the heliospheric magnetic field was more than 20% weaker than during other recent minima. Both of these parameters imply a higher cosmic ray diffusion coefficient, which provides a natural explanation for both the higher galactic cosmic ray intensities that were observed and the absence of such an effect for anomalous cosmic rays.

A history of solar activity over millennia

“…An interesting feature is related to the recent decade. The CR flux in 2009 was the highest ever recorded by NMs (Moraal and Stoker 2010), as caused by the favorable heliospheric conditions (unusually weak heliospheric magnetic field and the flat heliospheric current sheet) (McDonald et al 2010). On the other hand, the sunspot minimum was comparable to other minima.…”

Energy Spectrum of the Recurrent Variation of Galactic Cosmic Rays During the Solar Minimum of Cycles 23/24

“…Indeed, the GCR intensity values measured by neutron monitors were the highest ever recorded (e.g. Moraal and Stoker, 2010;Oh et al, 2013). In 2007, neutron monitors (NMs) recorded values last measured during the last A < 0 solar minimum, 1985 -1987, and the values continued to grow, exceeding the previous levels by a few percent, throughout the end of 2009.…”

“…It was ascribed to the anomaly increasing the diffusion of cosmic rays (Moraal and Stoker, 2010) during the Minimum 23/24. Dunzlaff et al (2008) reported, based on Ulysses observational data, that in SC 22 the decrease in the component of the recurrent GCR arising from the fast solar wind showed a visible maximum from 25 • to 40 • and beyond, but in SC 23 there was no such substantial change.…”

Solar change and climate: an update in the light of the current exceptional solar minimum

“…stations are showing GCR fluxes that are equalling the previous record high (Moraal et al 2009). In addition, the long sequence of BiSON helioseismology data is revealing larger changes in the solar interior than seen in previous minima (Broomhall et al 2009).…”