Reply to comment by P. Riley and J. T. Gosling on “Are high‐latitude forward‐reverse shock pairs driven by overexpansion?”

[1] During its passage through the high-latitude heliosphere, Ulysses observed interplanetary coronal mass ejections (ICMEs) bounded by forward-reverse shock pairs. Gosling et al. [1995] originally proposed the shock pairs form as a result of CME overexpansion into the ambient solar wind. Manchester and Zurbuchen [2006] suggested an alternative explanation for forward-reverse shock pairs in which the reverse shock forms as a result of deflections of the solar wind caused by the passage of the CME. In this model, fast solar wind overtakes slower plasma forming a reverse shock at high-latitude poleward of the ejected flux rope. Apparent signatures of ICMEs such as enhanced magnetic field strength, low plasma beta, and field direction rotation are produced by the plasma flows outside of the flux rope. The salient difference between these two models is that in the case of Gosling et al. [1995], the shocks surround the ejecta while in the model ofManchester and Zurbuchen [2006], the shocks extend laterally beyond the ejecta. [2] It is possible that both overexpansion and flowdeflection are responsible for forming high-latitude forward-reverse shock pairs in the solar wind. However, shock pairs formed in these two ways are expected to have different observational signatures particularly with respect to their plasma composition. In the case of overexpansion, Ulysses will have passed directly through the ejecta, while in the case of Manchester and Zurbuchen [2006], Ulysses will pass only through perturbed fast solar wind. The ambient solar wind and CME ejected plasma have different charge state composition for the following reasons. The ambient fast solar wind originates in open, coronal-hole associated field whereas CMEs originate from closed line regions in the corona where the temperatures are higher and the plasma is more highly ionized. Consequently, CMEs have elevated amounts of highly charged ions such as O [Richardson and Cane, 2004; Zurbuchen and Richardson, 2006] Second, coronal hole and topologically closed regions are also associated with distinct differences with respect to their elemental composition [von Steiger et al., 2000]. This dichotomy in elemental and ionic composition and their relation to magnetic topology has also been found through spectroscopic analysis close to the Sun [Feldman et al., 2005]. [3] Considering these compositional signatures, the highlatitude events under discussion by Gosling et al. [1995] fall in two categories. In some events, the composition of the high-latitude events look just like low-latitude CMEs, with all the signatures one would expect, and additional bounding shock pairs which are not found at low-latitudes, as pointed out by Gosling et al. [1995]. However, a portion of the so-called high-latitude events look indistinguishable from coronal hole associated wind with respect to their elemental and ionic composition [von Steiger and Richardson, 2006]. However, in this study, it was found that the ratio of He to H is found to be much more erratic and less useful for identification as only half of CMEs show an elevation. He enhancements occur with strong correlation with ionic charge state enhancements, but not all ionic charge state enhancements lead to He enhancement [von Steiger et al., 2006] These compositional signatures are complemented by observations of counter streaming electrons [Gosling et al., 1988], which have been taken to indicated closed field lines and CME ejected plasma at high latitude. Events possessing both solar wind composition and counterstreaming electrons therefore present a contradiction with conclusions regarding the closed magnetic topology and plasma that originates on open field lines. In the work of Manchester and Zurbuchen [2006], we speculated that counterstreaming electrons might exist on open field lines associated with high-latitude forward-reverse shock pairs. In support of this view, we now point out that counterstreaming electrons are in fact observed leaking out of corotating interaction regions (CIRs) where they are energized at shocks bounding the CIR [Steinberg et al., 2005]. These energized electrons stream sunward along open field lines opposite to the normal electron flux that streams away from the Sun. As pointed out by Steinberg et al. [2005], the properties of these counterstreaming electrons is quite similar to those observed on closed field lines associated with ICMEs. This counterstreaming mechanism can explain bidirectional electrons on the open field lines of our model of CME-driven high-latitude forward-reverse shock pairs JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, A07103, doi:10.1029/2007JA012272, 2007