Quiescent current sheets in the solar wind and origins of slow wind

Abstract

[1] Solar wind near the heliospheric current sheet is investigated using Ulysses and ACE data in a superposed epoch analysis for several days on either side of the current sheets. Only data near sunspot minima are used, minimizing the influence of transients. New results are shown for composition and ionization state. Existing results showing a $2 day wide depletion in He/H (He ++ /H + ) at the current sheet are confirmed, although the depletion is generally more narrow. A recent finding of a broad 5-10 day wide reduction in He/H around the current sheet is also confirmed. An important result is that the narrow depletion is not a real phenomenon but is instead a statistical consequence of the superposition of transient depletions that also create the broad reduction in the averages. These transient depletions last from a few hours up to several days, come from the core of streamers, and are embedded in a quasi-steady flow from streamers' legs. Most depletions contain a current sheet just inside one edge, leading to the apparent narrow depletion at the current sheet in the superposed epoch analysis. These results lead us to a hypothesis for how the He/H depletions form with a current sheet just inside one edge. Fe/O fluctuations associated with the He/H fluctuations further show that mixing of plasma from coronal holes adjacent to streamer brightness boundaries into outflow inside the brightness boundary is not an important process.

Plasmoid Releases in the Heliospheric Current Sheet and Associated Coronal Hole Boundary Layer Evolution

“…They reached the conclusion that the He/H depletions originate in the core, or the helmet of the streamer, based on (1) a correlation between He/H and O/H depletions over a two-month interval, which indicate a common region of origin, and (2) depletion in O occuring in the core of streamers relative to the legs according to results obtained with the SOHO/Ultraviolet Coronagraph Spectrometer (Raymond et al 1997;Raymond 2004). Suess et al (2009) attribute the in situ signatures to transient blobs flowing out from the streamer belt and explain the asymmetry across the sector boundary by a difference in velocity.…”

“…In this context, ion composition and charge state distributions are of great importance to help discriminate between plasma origins (chromospheric and coronal, respectively). Recently, Suess et al (2009) showed that depletions in He 2+ with respect to H + abundance (density) are preferentially located on one side of the "quiescent" HCS. They reached the conclusion that the He/H depletions originate in the core, or the helmet of the streamer, based on (1) a correlation between He/H and O/H depletions over a two-month interval, which indicate a common region of origin, and (2) depletion in O occuring in the core of streamers relative to the legs according to results obtained with the SOHO/Ultraviolet Coronagraph Spectrometer (Raymond et al 1997;Raymond 2004).…”

“…It is reasonable to envisage, following Suess et al (2009), that the flow shear can cause the properties of transients inside the HCS to differ substantially from those of, e.g., magnetotail transients (Sharma et al 2008). To our knowledge, however, the only reported observation so far of a flux-rope transient "in the HCS" by Crooker & Intriligator (1996) indicates an ICME forming a highly distended flux rope "occlusion."…”

“…In the most recent coupled MHD solar corona-heliosphere models, the empirical velocity relationship parameters that impact the solar wind speed predictions are the divergence of coronal holes (through a coronal flux tube expansion factor) and the distance from the coronal hole boundary (or angular depth inside a coronal hole; Arge et al 2004; Lee et al 2009). Many theories have explored whether the KH instability could also occur in these regions (Korzhov et al 1984;Joarder et al 1997;Suess et al 2009). In general, the KH instability generated by shear flows across tangential discontinuities (Neugebauer et al 1986; Suess et al 1995) and other forms of shear-driven turbulence (Roberts et al 1992) could be responsible for Alfvénic fluctuations in the solar wind (Belcher & Davis 1971;Roberts et al 1987;Tsurutani et al 1994).…”

Solar wind ion trends and signatures: STEREO PLASTIC observations approaching solar minimum

“…Other variations might be linked to non-uniform compositional signatures at HCS crossings (see Suess et al, 2009). These are items for future study.…”