Record-Setting Cosmic-Ray Intensities in 2009 and 2010

Abstract

We report measurements of record-setting intensities of cosmic-ray nuclei from C to Fe, made with the Cosmic Ray Isotope Spectrometer carried on the Advanced Composition Explorer in orbit about the inner Sun-Earth Lagrangian point. In the energy interval from ∼70 to ∼450 MeV nucleon −1 , near the peak in the near-Earth cosmic-ray spectrum, the measured intensities of major species from C to Fe were each 20%-26% greater in late 2009 than in the 1997-1998 minimum and previous solar minima of the space age (1957-1997). The elevated intensities reported here and also at neutron monitor energies were undoubtedly due to several unusual aspects of the solar cycle 23/24 minimum, including record-low interplanetary magnetic field (IMF) intensities, an extended period of reduced IMF turbulence, reduced solar-wind dynamic pressure, and extremely low solar activity during an extended solar minimum. The estimated parallel diffusion coefficient for cosmic-ray transport based on measured solar-wind properties was 44% greater in 2009 than in the 1997-1998 solar-minimum period. In addition, the weaker IMF should result in higher cosmic-ray drift velocities. Cosmic-ray intensity variations at 1 AU are found to lag IMF variations by 2-3 solar rotations, indicating that significant solar modulation occurs inside ∼20 AU, consistent with earlier galactic cosmic-ray radial-gradient measurements. In 2010, the intensities suddenly decreased to 1997 levels following increases in solar activity and in the inclination of the heliospheric current sheet. We describe the conditions that gave cosmic rays greater access to the inner solar system and discuss some of their implications.

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

“…Mason et al (2009) found that during the recent Minimum 23/24 in comparison with the previous one (22/23) there were many more co-rotating interaction regions observed, connected with the existence of more high-speed streams of solar wind. Moreover, a low solar-wind dynamic pressure, ≈ 40 % in comparison with the previous minimum, was observed (Mewaldt et al, 2010) and this was caused by the drop in the average proton density, which was ≈ 5.5 cm −3 , being less dense in comparison with the previous three minima by ≈ 70 % (Jian, Russell, and Luhmann, 2011). All of the phenomena and processes mentioned caused diminishing of the solar-modulation influence on GCR intensity and its variations.…”

“…All of the phenomena and processes mentioned caused diminishing of the solar-modulation influence on GCR intensity and its variations. McComas et al (2008) recognized a shrinkage of the size of the heliosphere (caused by the drop of the solar-wind dynamic pressure), which should result in an increase in the GCR intensity at 1 AU (Mewaldt et al, 2010). Indeed, the GCR intensity values measured by neutron monitors were the highest ever recorded (e.g.…”

“…Vos and Potgieter, 2015) since gradually more protons with lower energy were able to reach the Earth (Potgieter et al, 2014). Mewaldt et al (2010) found ≈ 44 % growth of the parallel mean free path of GCR particles [λ ] from the minimum in 1997 to 2009 and its rapid reduction in 2010. Cliver, Richardson, and Ling (2013) stated that particle drifts were not the main processes in the recent minimum, but the GCR modulation was determined by diffusion and convection, contradicting the results obtained by Alania, Modzelewska, and Wawrzynczak (2014), i.e.…”

Heliosheath Magnetic Field and Plasma Observed by Voyager 2 During 2012 in the Rising Phase of Solar Cycle 24

“…The observations of B at 1 au (http://cdaweb.gsfc.nasa.gov/) show that B reached the minimum value of 4 nT in mid-2009. This minimum was associated with a historically low minimum of solar activity (McDonald et al 2010;Mewaldt et al 2010;Ahluwalia & Ygbuhay 2011;Ahluwalia & Jackiewicz 2012;McComas et al 2013). Solar activity during solar cycle 23 decreased to minimum value in August 2009, when the monthly mean sunspot number was zero.…”

Characterization and Physical Explanation of Energetic Particles on Planck HFI Instrument

“…Cosmic rays (CRs) 7,8 at Lagrangian point L2 are essentially composed by massive particles: about 89% of protons, 10% of alpha particles, 1% are the nuclei of heavier elements and less than 1% of electrons like beta particles. The total flux of CRs peaks around 200 MeV, giving a total proton flux of 3000 -4000 particles m −2 sr −1 s −1 GeV −1 .…”