The fast solar wind is established to originate in coronal holes, but where and how the slow solar wind is formed remains an outstanding open question in solar physics. In the authoritative formulation of the review by Abbo et al. (2016), written by an International Space Science Institute (ISSI) team, the problem asks: (1) which coronal source regions supply the slow solar wind and what is each region's fractional contribution; (2) what role the coronal magnetic topology plays in its origin, release, and energy deposition; and (3) what mechanisms accelerate and heat it. The leading candidate pictures are the separatrix-web (S-web) topology, interchange reconnection (reconnection between open and closed magnetic field lines) at the open–closed flux boundary, streamer-tip reconnection, and Alfvénic slow wind (slow-speed streams with fast-wind-like magnetic fluctuations) from equatorial coronal holes; no quantitative, observationally validated attribution of the slow wind to its sources or a settled release/acceleration/heating mechanism is known in the literature to date.

Background

The solar wind is the continuous outflow of ionized plasma from the solar corona into interplanetary space. It has two classical states: a fast wind of roughly 700–800 km/s and a slow wind of roughly 300–500 km/s. The fast wind is securely traced to coronal holes — regions of open, unipolar magnetic field that appear dark in extreme-ultraviolet (EUV) images because their plasma is cooler and less dense. Near the ecliptic plane (where Earth and most spacecraft sit), the slow wind dominates, and its origin is the unresolved question addressed here.

The slow wind is observed to differ from fast wind in composition and variability: it shows enhanced charge-state ratios such as O7+^{7+}/O6+^{6+} (higher ions mean the plasma was hot where it originated), a stronger 'FIP effect' — fractionation by first ionization potential, in which elements that ionize easily in the cool low atmosphere are overabundant relative to their photospheric values — and large hour-to-hour variability. These fingerprints indicate that much slow-wind plasma was previously confined in closed magnetic loops, where it was processed before release, but they do not uniquely identify the release sites or mechanisms.

Several candidate source pictures structure the field. In the streamer-belt picture, slow wind is released from the tips or flanks of helmet streamers — closed-loop structures that straddle the equatorial current sheet, the magnetic sector boundary extending into the heliosphere — through intermittent reconnection that detaches quasi-periodic density 'blobs'; Wang (1994) already distinguished two types of slow wind associated with different coronal environments. In the interchange-reconnection picture of Fisk, Schwadron and Zurbuchen (1998), previously confined plasma escapes when closed field lines reconnect with open field lines, allowing closed-field plasma to enter the wind anywhere open and closed flux interact. In the S-web model of Antiochos et al. (2011), the slow wind is released along a topological 'separatrix web' — an extended network of open-field corridors and separatrix surfaces that forms where coronal holes and closed-field regions touch, so that even distant active regions connect to the wind; Crooker et al. (2012) argued the global in situ slow-wind pattern traces such a network. A key structural concept unifying these pictures is the open–closed flux boundary (OCB), the coronal interface between open and closed magnetic field, where interchange reconnection is thought to release closed-field plasma.

The observational difficulty is one of connectivity: remote-sensing instruments image the corona (EUV spectrographs and coronagraphs measure plasma temperature, density, and outflow), while in situ instruments measure the wind's velocity, density, temperature, composition, and magnetic state only after it has traveled to the spacecraft. Linking the two requires magnetic-field models — commonly the potential-field source-surface (PFSS) extrapolation or time-dependent magnetohydrodynamic (MHD) models — whose differing topologies are themselves a source of systematic uncertainty. The problem is sharpened by the discovery of Alfvénic slow wind: slow-speed streams, especially near solar minimum, whose large-amplitude Alfvénic fluctuations (correlated velocity and magnetic-field perturbations, including abrupt 'switchback' deflections) resemble fast wind and which trace to equatorial coronal holes (D'Amicis & Bruno 2015; Stansby et al. 2019), blurring the simple fast/slow source dichotomy. Two dedicated missions — NASA's Parker Solar Probe (PSP), which has flown through the young solar wind inside ~0.1 astronomical units since 2018, and ESA/NASA's Solar Orbiter, which images the corona while measuring the wind in situ — were motivated in large part by this problem. Reviews across three decades (Schwenn 2007; Poletto 2013; Cliver & von Steiger 2015; Abbo et al. 2016) concur that the question remains open, and Akasofu (2021) lists the cause of the solar wind among the four longest-standing unsolved problems of solar physics.

Problem Statement

Determine where and how the slow solar wind is formed. In the authoritative formulation of Abbo et al. (2016), the problem comprises three jointly defining questions that a complete answer must address together:

  1. Source regions and their contributions. Which coronal source regions supply the ambient slow solar wind (excluding plasma ejected transiently by coronal mass ejections, CMEs) — streamer-belt plasma released near the heliospheric current sheet, active-region upflows, narrow open-field corridors of the separatrix web (S-web), and coronal-hole boundaries or interiors (including Alfvénic slow wind) — and what is the fractional contribution of each source class to the slow-wind mass flux observed in situ, as a function of solar-cycle phase?
  2. Role of magnetic topology. What role does the coronal magnetic topology — the open–closed flux boundary, helmet streamers, pseudostreamers (streamer-like structures separating coronal holes of like polarity, lacking a current sheet), and the S-web — play in locating, releasing, and channeling this plasma into the heliosphere?
  3. Release, acceleration, and heating. By what physical mechanisms is previously confined or newly opened plasma released (e.g., interchange reconnection, streamer-tip tearing), and what processes accelerate it to the observed roughly 300–500 km/s speeds and supply its heating and compositional signatures, producing the observed variability and spatial structure?

A complete answer is a quantitative, observationally validated attribution of the slow wind to identified coronal sources together with a demonstrated release/acceleration/heating mechanism consistent with joint remote-sensing and in situ measurements from missions such as SOHO (the Solar and Heliospheric Observatory), ACE (the Advanced Composition Explorer), Ulysses, Parker Solar Probe, and Solar Orbiter across the solar activity cycle.

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