The past solar minimum activity period and the consequent minimum modulation conditions for galactic CRs was unusual. It was expected that the new activity cycle would begin early in 2008, assuming a 10.5 year periodicity. Instead, solar minimum modulation conditions had continued until the end of 2009, characterized by a much weaker HMF compared to previous cycles. The tilt angle of the wavy HCS, on the other hand, had not decreased as rapidly as the magnitude of the HMF at Earth during this period, but eventually also reached a minimum value at the end of 2009. Ahluwalia and Ygbuhay (2011), Stozhkov et al. (2009), Mewaldt et al. (2010) and Krymsky et al. (2012) all reported that CRs with high rigidity reached record setting intensities during this time (see also Heber et al., 2009; McDonald et al., 2010). It followed from observations for this period that the delay between the time for minimum sunspot numbers and maximum CR intensities was at least three times longer than during previous even numbered solar cycles (e.g., Kane, 2011). The decay phase of the sunspot cycle 23 exhibited two unusual features, it lasted quite long while the HMF at Earth reached the lowest value since in situ measurements in space began in 1963. See also Cliver et al. (2011), Zhao and Fisk (2011), Ahluwalia and Jackiewicz (2012), and the reviews by Aslam and Badruddin (2012) and Mewaldt (2012).
Since the beginning of the space age, the highest CR proton spectrum was observed by PAMELA in December 2009. This was unexpected because during previous polarity cycles, proton spectra were always lower than for cycles at kinetic energies less than a few-GeV, in full accord with drift models. The PAMELA experiment has provided important results on the antiproton (Adriani et al., 2009a) and positron galactic abundances (Adriani et al., 2009b; Boezio et al., 2009; Boezio and Mocchiutti, 2012).
The high-resolution PAMELA spectrometer also allows to perform hydrogen and helium spectral measurements up to 1.2 TV (Sparvoli, 2012), which is the highest limit achieved by this kind of experiments. Adriani et al. (2013) presented observations down to 400 MV of the absolute flux of protons from July 2006 until the end of 2009. Large proton statistics collected by the instrument allowed the measurement of the proton flux for each Carrington rotation. In Figure 25 these spectra are shown from July 2006 to the very beginning of 2010. The spectrum at the end of December 2009 was the highest recorded. In January 2010, solar activity picked up significantly so that the proton intensity had started to decrease. In order to emphasize how decreasing solar modulation conditions allowed galactic protons to increase at Earth, especially at low kinetic energies, the spectra in Figure 25 are used to calculate and plot the intensity ratios as a function of kinetic energy with respect to July 2006. This is shown in Figure 26.
The Ulysses mission produced solar modulation observations from 1990 to 2009 as reviewed by Heber and Potgieter (2006, 2008) and Heber (2011). These and other CR observations had yielded several new and surprising insights and are summarized as follows:
- It was observed by Ulysses that the galactic CR flux was not symmetric to the heliographic equator implying a North-South-asymmetry in CR modulation (McKibben et al., 1996). Surprisingly, neither the solar wind experiments nor the magnetic field investigations reported this asymmetry. Later magnetic field investigations were interpreted to give a deficit of the magnetic flux in the southern hemisphere suggesting a relatively large latitudinal offset of the location of the HCS by 10° (Smith et al., 2000). Recently, Erdős and Balogh (2010) disputed this number, arguing that the Ulysses magnetic field measurements did not give evidence for such a large displacement of the HCS, only a southward displacement of 2° – 3° could be possible. It remains thus an open question whether this observation was an occurrence of events that pertained during this rapid pole to pole passage of Ulysses or was related to an asymmetrical magnetic flux.
- Small latitudinal galactic CR gradients were observed at solar minimum, confirming that the LIS cannot be observed in the inner polar regions of the heliosphere. A particular motivation of the Ulysses mission was to explore this possibility. Drift dominated models of that time actually allowed for this (Jokipii and Kopriva, 1979). In contrast, the observed proton spectrum was highly modulated to large heliolatitudes.
- The latitudinal gradients for CR protons as a function of rigidity was observed to reach a maximum around 2 GV, to decrease significantly below these values, in sharp contrast to what drift dominated models predicted at that time, a factor of 10 at 200 MeV.
- A renowned observation was that recurrent particle events occurred at high heliolatitudes without corresponding features in the solar wind and magnetic field data. It appears that similar effects did not occur during the last polar excursion by Ulysses. Dunzlaff et al. (2008) reported that measurements in the fast solar wind show differences: In cycle 22 the recurrent cosmic ray decreases showed a clear maximum near 25° heliolatitude and were still present beyond 40°, whereas in cycle 23 neither such a pronounced maximum nor significant decreases were observed above 40°. The periodicity in the CR intensity that could be clearly seen in the slow solar wind appeared to have vanished in the fast solar wind.
- Essentially no latitudinal gradients were observed for any galactic CRs at solar maximum, indicating that drifts, mainly responsible for setting-up these gradients during solar minimum conditions, were almost absent at solar maximum.
- The observed electron to proton ratios (implicitly also containing the radial and latitudinal gradients) indicated that large particle drifts were occurring during solar minimum but diminished significantly toward solar maximum when rather diminished drifts were required in models to explain the observed values (see Figure 23).
- Jovian electrons were observed at high heliolatitudes, implying effective latitudinal transport of electrons at these energies ( 10 MeV). This still has to be explained from basic theoretical considerations, in particular if this could be the cause of extraordinary effective transport for these low-energy electrons including large polar perpendicular diffusion (see also Ferreira, 2005).
- Anomalous fluxes of oxygen, nitrogen, and neon were observed along the Ulysses trajectory and at Earth and with spatial gradients different from galactic CRs (e.g., Heber and Marsden, 2001; Cummings et al., 2009).
Several other space missions, at or near Earth, and several balloon experiments also made numerous and valuable observations of CR modulation (see the review by Mewaldt, 2012). Some of the recent PAMELA results are discussed above within a given context.
A major surprise came from the outer heliosphere when Voyager 1 crossed the TS in 2004 and found that the intensity of the ACRs did not reach a peak intensity at the shock for energies more than a few MeV/nuc. As mentioned above, the ACR intensity kept increasing far into the heliosheath implying that they are accelerated somewhere else and probably by different processes other than diffusive shock acceleration. Continued observations from the Voyagers will hopefully offer an explanation. The TS was observed to be disappointingly weak so that it is unlikely that it will reaccelerate galactic CRs to the extent that it influences the spectral shape of the CR spectra in the heliosphere.
With Voyager 1 very close to the HP it is relevant to ask what signatures of the HP in CRs can be expected? A magnetic barrier or ‘wall’, if present, should cause a significant increase of the magnetic field magnitude at the HP. It is unlikely that high energy galactic CRs will change abruptly but the effect should be strongly energy-dependent so that the low energy part of the CR spectrum should rise steeply at the HP while the ACR intensities drop sharply.
Apart from the ACRs, the galactic electrons at low energies ( MeV) showed extraordinary behaviour in the heliosheath. This is shown in Figure 27. Note the jumps in intensity and accompanying changes in the radial gradients. After crossing the TS in late 2004, the 6 – 14 MeV galactic electron intensity measured at Voyager 1 increased rapidly and irregularly. By about 2008.5 this intensity was 5 times that measured when Voyager 1 crossed the shock. The local radial intensity gradient was as high as (18.5 ± 1.5) %/AU at times, to drop to as low (8 ± 1) %/AU as shown in the figure (see also Nkosi et al., 2011). Webber et al. (2012) argued that these sudden changes in intensity and in the gradients of electrons and protons are evidence that Voyager 1 crossed into regions of significantly different propagation conditions.
Even more spectacular is what happened in August 2012 to the intensity of 0.5 MeV protons (mainly ACRs) and 6 – 14 MeV galactic electrons, and MeV proton count rates when Voyager 1 was at 121.7 AU, as displayed in Figure 28. Within a few days the intensity of the dominant energetic component above 1 – 2 MeV decreased by more than 90%. At the same time a sudden increase of a factor of 2 occurred in lower energy (6 – 100 MeV) electrons and 30 – 50% for the higher energy nuclei above 100 MeV. The magnitude of this intensity change for ACRs has not been observed in the 35 years of the Voyager mission except close to Jupiter. This simultaneous abrupt reduction of ACR (and TSPs) intensities at lower energies and increase in galactic CR intensities at somewhat higher energies was interpreted by Webber and McDonald (2013) as evidence of the crossing of the HP, or at least the crossing of a ‘heliocliff’. This is another spectacular milestone for the Voyager mission. It should be noted that this apparent crossing has not yet been confirmed by magnetic field observations. The crossing of the HP by Voyager 2 is expected to happen soon.