In 2009 Harrison, Davis and Davies reported, in images from the Heliospheric Imagers on board the twin STEREO (Solar Terrestrial Relations Observatory) spacecraft, narrow rays of successively outward-propagating plasma blobs ('pre-CME fuses') forming near the edge of the streamer belt in the period before many coronal mass ejection (CME) eruptions, and asked whether these fuses trace the CME onset process. Their pilot study left the phenomenon unquantified: prevalence and the statistical association with CME onset were established only for a small visually identified sample, and the physical origin was undetermined — enhanced production of ordinary streamer blobs versus direct signatures of interchange reconnection between closed streamer-belt fields and adjacent open field lines, a reading that would have implications for the end-of-life of CMEs. Later work established that streamer blobs in general are released by magnetic reconnection at helmet-streamer tips, in direct association with falling-back 'raining inflows', and that blob production rises when a streamer is activated by underlying flux emergence; but no study has statistically validated the pre-eruption fuse phenomenon or discriminated its magnetic topology. Whether pre-CME fuses are a genuine, physically understood and potentially predictive signature of CME onset remains open.

Background

A coronal mass ejection (CME) is a large-scale eruption of magnetized plasma from the solar corona; identifying reliable observational signatures of the approach to eruption ('onset') is a long-standing goal of space-weather research. The quiet corona is organized by helmet streamers — bright closed magnetic arcades overlying photospheric polarity-inversion regions (lines where the surface magnetic field reverses direction) which, seen in white light, form the encircling streamer belt, with the heliospheric current sheet (the magnetic surface separating the Sun's oppositely directed polar fields) extending outward from their tops. White-light coronagraphs and heliospheric imagers see density structures through Thomson scattering, the elastic scattering of sunlight by coronal electrons, so image brightness traces electron density integrated along the line of sight. Since Sheeley et al. (1997), small quasi-periodic density enhancements ('streamer blobs' or 'Sheeley blobs') have been observed propagating outward along the streamer belt at slow-solar-wind speeds, and are widely used as tracers of the slow solar wind; Sheeley and Rouillard (2010) tracked them into the heliosphere. Subsequent work established that blobs are released by magnetic reconnection at the tips of helmet streamers: Sánchez-Díaz et al. (2017) obtained the first direct association between outflowing blobs and falling-back 'raining inflows' — the expected reconnection signature — and Wang and Hess (2018) showed that the formation height and visibility of blobs and inflows depend on the underlying photospheric field, with the blob production rate tending to increase when a streamer is 'activated' by underlying flux emergence. A second key concept is interchange reconnection: reconnection between closed coronal loops and open field lines that exchanges magnetic connectivity. It is central to models of slow-wind release at the boundaries between streamers and coronal holes — persistent dark regions of open magnetic field (the 'SWeb' or slow-wind web picture of Antiochos et al., 2011) — and to the evolution of the magnetic legs by which an interplanetary CME (ICME) remains connected to the Sun (Crooker and Horbury, 2006). Against this background, Harrison, Davis and Davies (2009) inspected Heliospheric Imager (HI-1) data and identified, in the hours before many CME eruptions, narrow rays at the streamer-belt edge composed of series of outward-propagating blobs, which they named pre-CME 'fuses' (the burning fuse leading to an explosion). They proposed two readings: (i) merely enhanced release of ordinary streamer blobs as the corona approaches eruption, or (ii) a direct signature of interchange reconnection between closed field lines in the streamer belt and adjacent, open field lines — a reading that would tie the fuse to the onset mechanism itself and, per the authors, have implications for the end-of-life of CMEs (how eruptions eventually lose magnetic connection to the Sun). The study was explicitly a pilot: fuses were visually identified in a limited data set, and how often fuses occur, how they are timed and located with respect to CME launch sites, and which mechanism produces them were left unresolved. If validated, fuses could serve as an early-warning signature of CME onset; Chen's (2011) review placed the observation among proposed CME precursors while stressing that no known precursor is a necessary or sufficient condition for eruption.

Problem Statement

Determine whether pre-CME 'fuses' — narrow, spatially localized chains of outward-propagating plasma blobs observed at the streamer-belt edge in white-light heliospheric images before CME eruptions, identified in the STEREO Heliospheric Imager data by Harrison, Davis and Davies (2009) in an explicitly pilot study of a limited data set — are a genuine, physically explicable signature of the approach to CME onset. Concretely, an answer must establish, with reproducible methods and beyond the pilot sample: (1) Statistical reality and phenomenology: whether fuses are genuinely associated with CME onset — how often fuses occur, the timing and location of fuses with respect to CME launch sites, the strength and lead time of the fuse–CME association, and the speed and topology of the fuses. These quantities together characterize the phenomenology of (1) and are not four separately mandatory deliverables; (1) is resolved by a reproducible determination, whether positive or negative, of whether fuses are significantly associated with CME onset. A demonstrated absence of significant association would refute the phenomenon's onset-diagnostic value and is an admissible outcome. (2) Physical origin: whether the enhanced blob expulsion represents an increase in the release of outward-propagating blobs from the streamers themselves (streamer reconnection at helmet-streamer tips), evidence for interchange reconnection between closed field lines in the streamer belt and adjacent, open field lines in the period leading to CME onset — the reading that, per the source paper, would have implications for the end-of-life of CMEs — or another process. Both sub-questions are required, with the following fallback: if a reproducible analysis resolves (1) negatively — no significant fuse–CME association — then the pre-eruption enhancement presupposed by (2) does not obtain, so (2) is moot and that negative resolution of (1) alone completes the task.

References

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