Because of its distinctive compositional properties and variability, low-speed (450 km s −1 ) solar wind is widely believed to originate from coronal streamers, unlike high-speed wind, which comes from coronal holes. An alternative scenario is that the bulk of the slow wind (excluding that in the immediate vicinity of the heliospheric current sheet) originates from rapidly diverging flux tubes rooted inside small coronal holes or just within the boundaries of large holes. This viewpoint is based largely on photospheric field extrapolations, which are subject to considerable uncertainties and do not include dynamical effects, making it difficult to be certain whether a source is located just inside or outside a hole boundary, or whether a high-latitude hole will be connected to Earth. To minimize the dependence on field-line extrapolations, we have searched for cases where equatorial coronal holes at central meridian are followed by low-speed streams at Earth. We describe 14 examples from the period 2014-2017, involving Fe XIV 21.1 nm coronal holes located near active regions and having equatorial widths of ∼3°-10°. The associated in situ wind was characterized by speeds v∼300-450 km s −1 and by O 7+ /O 6+ ratios of ∼0.05-0.15, with v showing the usual correlation with proton temperature. In addition, consistent with other recent studies, this slow wind had remarkably high Alfvénicity, similar to that in high-speed streams. We conclude that small coronal holes are a major contributor to the slow solar wind during the maximum and early post-maximum phases of the solar cycle.

Conclusively determining the states of the solar wind will aid in tracing the origins of those states to the Sun, and in the process help to find the wind’s origin and acceleration mechanism(s). Prior studies have characterized the various states of the wind, making lists that are only partially based on objective criteria; different approaches obtain substantially different results. To uncover the unbiased states of the solar wind, we use “k-means clustering”—an unsupervised machine learning method—including constructed multipoint variables. The method allows exploration of different descriptive state variables and numbers of fundamental states (clusters). We show that the clusters reveal structures similar to those found by more ad hoc means, including coronal hole wind, interplanetary coronal mass ejections, “slow wind” (better: noncoronal hole flow), “pseudostreamers,” and stream interaction regions, but with differences that should be useful in refining our previous ideas. These results demonstrate the viability of the approach and warrant further study to understand the origin of remaining discrepancies. Complexity in k-means characterization of the wind may ultimately point to complexity at the source; studies closer to the Sun with Parker Solar Probe will help. We confirm the utility of a set of variables that can serve as a proxy for composition measurements. This proxy permits studies at high time resolution and where composition is not available. This and our recently developed unsupervised multivariate clustering technique are expected to be beneficial in the automated identification of structures and events in a variety of studies.

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.

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We review the history of ultraviolet and extreme ultraviolet spectroscopy with a specific focus on such activities at the Naval Research Laboratory and on studies of the extended solar corona and solar-wind source regions. We describe the problem of forecasting solar energetic particle events and discuss an observational technique designed to solve this problem by detecting supra-thermal seed particles as extended wings on spectral lines. Such seed particles are believed to be a necessary prerequisite for particle acceleration by heliospheric shock waves driven by a coronal mass ejection.