Anomalous cosmic rays in the distant heliosphere and the reversal of the Sun's magnetic polarity in Cycle 23

Abstract

[1] Beginning in early June 2001 and extending over a $3 month period the Cosmic Ray Subsystem (CRS) experiment on Voyager 1 (80.3 AU, 34°N) observed a rapid transition in the energy of the peak intensity of anomalous cosmic ray (ACR) He from $11 MeV/n to $25 MeV/n. One month later there began a similar spectral shift at Voyager 2 (64.7 AU, 24°S). When these ACR transition times are extrapolated back from the estimated location of the heliospheric termination shock (TS) to the Sun ($1 year) there is reasonable agreement with the range of times obtained for the polarity reversal of the interplanetary magnetic field (IPB) in the northern hemisphere near the Sun and the later reversal in the south. We interpret these changes in the ACR spectra as giving a global view of the interplanetary field reversal at the TS and confirmation of the important role of the TS in controlling the ACRs. Citation:

Temporal and spectral variations of anomalous oxygen nuclei measured by Voyager 1 and Voyager 2 in the outer heliosphere

“…We note that similar, although not exactly identical, changes are also observed during this transition time period between ∼2001.5 and 2002.0 for 10–21 MeV/nuc and 30–56 MeV/nuc He* nuclei [ McDonald et al , 2006]. In this case, most of the change to a new regression line between the higher and lower energy He* particles at V1 appeared to occur during the transition time period.…”

“…This decrease is shown in Figure 2 on an expanded timescale for 4.0–7.1 MeV/nuc O* nuclei using 52‐day averages. The two main decreases, 1 and 2, occurring at approximately 2000.4 and 2001.1 for V2 and 2000.6 and 2001.3 for V1, coincide with much larger decreases of H* [ Webber et al , 2006] and He* [ Webber et al , 2005; McDonald et al , 2006], as well as decreases of higher‐energy galactic cosmic rays [ Webber and Lockwood , 2004; McDonald et al , 2005]. The time period (1) shown in Figure 2 is a period of nearly constant intensity where the spectra of O* can be obtained, likewise for the time period labeled “min” from about 2001.6–2002.0.…”

“…It is a time period when significant changes in the shape of the O* spectra are observed. In Figure 2, we show as a shaded region the approximate times at which the solar magnetic field changed from positive to negative in 2000–2001 in the north and south polar regions, as derived from solar magnetograph and chronograph observations and source surface extrapolations by Wang et al [2002], delayed by 1.0 year because of the estimated solar wind traveltime from the Sun to the HTS [e.g., McDonald et al , 2006].…”

“…However, even during the period of minimum cosmic‐ray intensities in 1990–1991, only O* was clearly observable at Voyager 1 (V1) and 2 (V2) at 38 AU and 33 AU, respectively; the H* and He* components were lost under the galactic particle background or background from solar/interplanetary cosmic‐ray events propagating outward in the heliosphere. During the most recent intensity minimum in 2001–2004, H* completely vanished at both V1 and V2 at ∼74 and 57 AU, respectively [ Webber et al , 2006]; He* was still present but with a greatly modified spectrum [ Webber et al , 2005; McDonald et al , 2006]. This was also the case for O*, which could be followed most easily because of its distinctive spectrum.…”

Stochastic acceleration and adiabatic heating of anomalous cosmic rays in the inner heliosheath

“…In this work we assumed K^ | for protons as calculated by Teufel and Schhckeiser [19] (damping model) for the inner heliosphere. In comparison the Voyager 1 spectra observed at the termination shock are shown as the triangles [10,21]. For simplicity we will assume that Krr and K00 scale as K\\.…”