Observations of recurrent cosmic ray decreases during solar cycles 22 and 23

Abstract

Abstract. During solar cycle 22, the modulation of several hundred MeV galactic cosmic rays (GCRs) by recurrent and transient cosmic ray decreases was observed by the Ulysses spacecraft on its descent towards the solar south pole. In solar cycle 23, Ulysses repeated this trajectory segment during a similar phase of the solar cycle, but with opposite heliospheric magnetic field polarity. Since cosmic ray propagation in the heliosphere should depend on drift effects, we determine in this study the latitudinal distribution of the amplitude of recurrent cosmic ray decreases in solar cycles 22 and 23. As long as we measure the recurrent plasma structures in situ, we find that these decreases behave nearly the same in both cycles. Measurements in the fast solar wind, however, show differences: in cycle 22 (A>0) the recurrent cosmic ray decreases show a clear maximum near 25 • and are still present beyond 40 • , whereas we see in cycle 23 (A<0) neither such a pronounced maximum nor significant decreases above 40 • . In other words: the periodicity in the cosmic ray intensity, which can be clearly seen in the slow solar wind, appears to vanish there. Theoretical models for drift effects, however, predict quite the opposite behaviour for the two solar cycles. To closer investigate this apparent contradiction, we first put the visual inspection of the data onto a more solid basis by performing a detailed Lomb (spectral) analysis. The next step consists of an analysis of the resulting periodicities at 1 AU in order to distinguish between spatial and temporal variations, so that we can obtain statements about the question in how far there is a correlation between the in-situ data at 1 AU and those measured by Ulysses at larger latitudes. We find a good correlation being present during cycle 22, but not for cycle 23. As one potential explanation for this behaviour, we suggest the difference in the coronal hole structures between the cycles 22 and 23 due to a large, stable coronal hole structure, which is present during cycle 22, Correspondence to: P. Dunzlaff (dunzlaff@physik.uni-kiel.de) but not in cycle 23. We support this possibility by comparing Yohkoh SXT and SOHO EIT maps.

Variability of Low Energy Cosmic Rays Near Earth

“…The double-peak structure, namely with maxima at ~ 27 days and another at ~ 31 days is found around solar activity maxima, similar to (Dunzlaff et al, 2008) based on data of GCR, EPHIN on SOHO. The structure is more complex during the long time period.…”

The 27-Day Cosmic Ray Intensity Variations During Solar Minimum 23/24

“…In contrast to this, during the third (2005 -2007) southern excursion of Ulysses, which also took place in low-activity conditions but when A < 0, observations showed a more variable structure in the solar wind stream. Dunzlaff et al (2008) suggested that this might be caused by the difference in the coronal hole structures between cycles 22 (1986 -1996) and 23 (1996 -2008); an extended, stable coronal hole structure was present during cycle 22, but not in cycle 23. The polar coronal hole disappeared during the declining phase of cycle 23 (Kirk et al, 2009) and a part of the coronal hole structure existed in the equatorial region (Abramenko et al, 2010).…”

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

“…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. The authors suggested that the absence of large, stable, coronal-hole structures in SC 23 might be a reason for this dissimilarity (e.g.…”

“…The recurrent variation of the galactic cosmic-ray intensity was described for the (Forbush, 1938) and is still of interest. Among recent publications devoted to this issue one can mention, e.g., Richardson (2004); Dunzlaff et al (2008) ;Gil, Modzelewska, and Alania (2012); Guo and Florinski (2014). However, changes of the amplitudes of the recurrent variation of the GCR intensity versus the rigidity [R = T 2 + 2T T r , where T is kinetic energy and T r = 0.938 GeV is a proton's rest-mass energy] were studied very rarely until the work of Gil and Alania (2008).…”