Comment on “Comment on the abundances of rotational and tangential discontinuities in the solar wind” by M. Neugebauer

[1] Neugebauer [2006] has very nicely reviewed the current status of work done on identifying the abundance of rotational discontinuities (RDs) and tangential discontinuities (TDs) occurring in interplanetary space. This has been a topic of great interest and heated debate since the 1970s [Smith, 1973; Belcher and Solodyna, 1975; Burlaga et al., 1977; Lepping and Behannon, 1980] (see also discussion by Neugebauer et al. [1984]). Neugebauer [2006] has also reexamined jump conditions across discontinuities and has ended up with inconclusive answers. [2] We wish to make some suggestions that may help clarify the apparently conflicting results of the RD/TD occurrence ratio existing in the literature. We will argue that in many cases discontinuities are ‘‘contaminated’’ by overlying plasma and induced magnetic fields (see also discussion by Sonnerup and Scheible [1998], Horbury et al. [2001], and Knetter et al. [2004] concerning contamination by electromagnetic plasma waves), leading to errors (in interpretation) of results using the Sonnerup and Cahill [1967] minimum variance method (MVA). We also will argue that the establishment of pure (or nearly pure) solar wind convection of discontinuities does not necessarily lead to the conclusion that they are TDs. [3] Tsurutani et al. [1994] have argued that interplanetary discontinuities are (often) the phase steepened edges of nonlinear Alfvén waves as Neugebauer [2006] notes. Another feature detected in interplanetary space are decreases in the interplanetary magnetic field magnitude. These have been given the name magnetic holes (MHs), magnetic decreases (MDs) and other descriptive names in the literature [Turner et al., 1977; Winterhalter et al., 1994, 2000; Tsurutani and Ho, 1999]. These magnetic field magnitude (pressure) decreases are supplanted by enhanced, anisotropic plasma [Fränz et al., 2000; Neugebauer et al., 2001]. The total pressure is constant across these structures, to first order [Winterhalter et al., 1994]. It has recently been shown that these MHs/MDs are (often) collocated with the discontinuities/phase steepened edges of Alfvén waves [Tsurutani et al., 2002a, 2002b]. Dasgupta et al. [2003] and Tsurutani et al. [2002b, 2005a] have argued that the ponderomotive force associated with the steepened Alfvén wave edges (the discontinuities) accelerate solar wind ions (and electrons) perpendicular to the ambient magnetic field and thus create the MHs/MDs by plasma diamagnetic effects. In this scenario, the plasma blobs and their resultant magnetic decreases are external features to the discontinuities and not parts of the discontinuities/Alfvén waves themselves. From this viewpoint, MHs/MDs can thus be thought of as byproducts of the Alfvén wave dissipation process. [4] We view MH/MD plasma and induced field decreases which are collocated with the discontinuities as possible contaminants to the intrinsic discontinuity structures. In some cases the MHs/MDs can appear to be bounded by a pair of TD-like structures as well [Tsurutani and Ho, 1999]. Minimum variance analysis results of this very complex region of multiple discontinuities will be difficult, if not impossible to interpret. [5] Tsurutani et al. [2005b] have examined several (7) events from the Knetter [2005] Cluster discontinuity data set where the discontinuities were collocated with MHs/MDs. All of the discontinuities were associated with Alfvén waves. The same discontinuities were identified at ACE, 0.01 AU upstream of Cluster, by their similar field rotational characteristics. The time delay from detection at ACE to that at Cluster was measured. It was found that the discontinuities/MHs/MDs propagated at almost the solar wind convection speed (determined by plasma measurements), within measurement uncertainties. Tsurutani et al. [2005b] speculated that the low wave propagation speed relative to the ambient solar wind was due to a ‘‘slowing’’ of the wave phase speed through the high-density plasma (the MHs/MDs), oblique wave propagation, or a combination of both factors. [6] As to why most directional discontinuities (DDs) in the solar wind have small values of BN [Knetter et al., 2004; JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, A03101, doi:10.1029/2006JA011973, 2007