Boundary of the Slow Solar Wind

Abstract

This work argues that there are two fundamental states of the nontransient solar wind, and that these can be distinguished by a number of criteria. Here we define the states, which will be termed slow and fast, or SSW and FSW, for lack of better terms, by the level of velocity fluctuations, δv, in them, with the slow wind having systematically lower fluctuations than the fast wind. Almost all winds with speeds less than 450 km s−1 are in the slow class, and winds with speeds greater than 600 km s−1 are fast, but we argue that in between, consistent with other work, the δv classification is more fundamental than speed. We show that the fluctuation categorization coincides well with classes based on Alfvénicy, proton specific entropy, ion thermal speed, and ionic composition. This correlated behavior among these solar wind parameters exists regardless of it being associated with a heliospheric current sheet or a pseudostreamer. This work provides evidence that both the so-called SSW I and SSW II scenarios coexist for the SSW formation. In addition, that the dynamical properties (thermal, magnetic, and turbulence properties) correlate well with properties set at the inner corona (ion ionization states and FIP bias) implies that there exists a boundary layer on the Sun within which the SSW is formed. This boundary layer would set up the coronal conditions for the source and transport of the SSW.

Element Abundances: A New Diagnostic for the Solar Wind

“…Since its discovery ( Gringauz et al 1960 ; Neugebauer & Snyder 1962 ), Parker’s original concept of a wind driven by thermal pressure in a corona heated by magnetohydrodynamic (MHD) waves ( Parker 1963 ) has been slightly modified to a scenario where the MHD waves drive the wind directly (e.g., Belcher 1971 ; Isenberg & Hollweg 1982 ; Ofman 2010 ). The fast solar wind is established to emerge from coronal holes, open field regions where plasma emerges directly from the solar chromosphere into the wind, and exhibits largely unbalanced Alfvénic turbulence ( Bruno & Carbone 2013 ; Ko et al 2018 ). By contrast the slow solar wind, which shows strong chemical fractionation effects in its composition and more balanced (or lower cross-helicity) turbulence, is frequently believed to originate in closed coronal loops where the fractionation occurs (e.g., Antiochos et al 2011 ), before being released into the solar wind by interchange reconnection with the surrounding open field, as well as possibly coming directly from the open field like the fast solar wind ( Cranmer et al 2007 ).…”

Some Properties of the Solar Wind Turbulence at 1 AU Statistically Examined in the Different Types of Solar Wind Plasma

“…Note that “ejecta” here includes magnetic clouds, plus other (e.g., Cane & Richardson, ; Richardson & Cane, ) noncloud plasma. Note also that this algebraic scheme includes the separate categorization of plasma associated with streamer stalks (sector‐reversal‐region plasma; e.g., Foullon et al, ; Gosling et al, ; Ko et al, ; Suess et al, ; Susino et al, ). It is important to note that sector‐reversal‐region plasma is not synonymous with the “heliospheric plasma sheet”; the heliospheric plasma sheet is identified by high plasma density and high plasma beta (cf.…”

Extracting Inner‐Heliosphere Solar Wind Speed Information From Heliospheric Imager Observations

“…In situ observations show that the slow solar wind is typically more dense and more variable than fast solar wind, with differing turbulent profiles (Bruno & Carbone, ; Neugebauer & Snyder, ). We also note that Ko et al () recently argued that the magnitude of solar wind velocity fluctuations are a good discriminator between fast and slow solar wind, with slow solar wind typically showing smaller velocity fluctuations, although this work did not consider density fluctuations. Furthermore, in situ and remote observations demonstrate that fast solar wind originates from coronal holes, which tend toward more polar latitudes, particularly at solar cycle minimum (Schwenn, ; Zirker, ).…”