Coronal mass ejections (CMEs) that erupt with almost no low-coronal signatures ('stealth' CMEs) cause unforecast geomagnetic storms, and it remains unestablished whether they are merely the quiet end of one reconnection-driven eruption process that also produces streamer-top plasmoids and fast active-region CMEs. This problem, drawn from the open questions of Lynch et al. (2016, JGR Space Physics 121, 10677), asks for three linked determinations: (1) whether a single physical continuum of reconnection-driven plasmoid ejection connects streamer plasmoids, stealth CMEs, and fast CMEs; (2) observational criteria that separate smooth ideal expansion of sheared flux from reconnection-driven CME eruption, including whether pure ideal opening would even be recognized as a CME; and (3) whether multi-viewpoint monitoring such as from the Sun-Earth L5 Lagrange point can operationally detect and forecast Earth-directed stealth CMEs.

Background

A coronal mass ejection (CME) is a large-scale eruption of magnetized plasma from the Sun's outer atmosphere (the corona) into interplanetary space. Most CMEs announce themselves in low-corona images — extreme-ultraviolet (EUV) flares, erupting filaments (dense, cool plasma suspended in the corona), flare ribbons (bright elongated surface emission where newly reconnected field lines meet the photosphere), or coronal dimmings (transient dark EUV regions marking evacuated magnetic flux) — but a class of events dubbed 'stealth' CMEs shows almost none of these signatures while still producing a well-resolved, slow flux-rope CME in coronagraph images (a coronagraph occults the solar disk so the faint corona can be imaged). Robbrecht, Patsourakos & Vourlidas (2009) called the archetype, the 1–2 June 2008 event, 'the CME from nowhere'. Howard & Harrison (2013) argued that stealth CMEs are not a new physical class but simply the lowest-energy range of the slow CME distribution. A coronal 'helmet streamer' is a closed-field belt of magnetic loops straddling the solar equator that confines the plasma sheet; 'streamer-blowout' CMEs are eruptions in which this streamer slowly swells over hours to days and is then evacuated, and they are the dominant CME population at solar minimum. Lynch, Masson, Li, DeVore, Luhmann, Antiochos & Fisher (2016) reproduced the 1–2 June 2008 event with a three-dimensional magnetohydrodynamic (MHD) simulation: a potential-field source surface (PFSS, a standard extrapolation of the photospheric magnetic field into the corona) reconstruction of the Carrington Rotation 2070 synoptic magnetogram, an isothermal solar wind, and large-scale shearing flows near the streamer's polarity inversion line (the line separating opposite magnetic polarities) equivalent to about two weeks of differential rotation (the Sun's equator rotating faster than its poles). In their model the energized arcade expands slowly, forms a radial current sheet, and the onset of 'expansion-induced' flare reconnection — magnetic reconnection, i.e. the breaking and topological reconnection of field lines that converts magnetic energy to kinetic and thermal energy, at a vertical current sheet beneath the erupting flux rope — initiates the stealth CME and builds its flux-rope (twisted, rope-like magnetic) structure. The released magnetic energy is only of order 103010^{30} erg (about 1% of the pre-eruption total), and the mean energy flux it delivers into the eruption's current sheet (∼4×103\sim4\times10^{3} erg cm−2^{-2} s−1^{-1}) is roughly two orders of magnitude below the flux needed to heat the ambient corona, explaining the absence of observable flare emission. Magnetic reconnection in the model is mediated by numerical diffusion at the grid scale, not by an explicit resistivity.

The paper's discussion raises three substantive open questions. First, a 'unified' continuum conjecture: small plasmoids (magnetic bubbles) continuously released from streamer tops — observed as quasi-periodic density structures in the slow solar wind (Viall & Vourlidas 2015) — stealth CMEs, and fast active-region CMEs 'may well constitute a continuum of manifestations of a single process: the ejection of magnetic stress into the heliosphere (the Sun's magnetized plasma bubble, extending past the planetary orbits) via reconnection', with the stress injected on the highest-lying flux (plasmoids), intermediate heights by differential rotation (stealth CMEs), or the lowest-lying flux near polarity inversion lines (fast CMEs). If correct, this would greatly simplify the understanding and modeling of coronal evolution, and detailed studies of stealth CMEs — relatively easy to observe accurately because of their large spatial and temporal scales — may provide key insights into phenomena at smaller and/or much faster scales; whether the conjecture in fact holds across all these regimes has not been established. The 'breakout' model of Antiochos, DeVore & Klimchuk (1999) — in which reconnection at a null point above the sheared core field removes restraining overlying flux and allows eruption — supplies one established member of this reconnection-driven family at the fast end.

Second, an observability ambiguity: without reconnection, the sheared streamer flux could open by smooth 'ideal' expansion (motion governed by ideal MHD, in which field lines are frozen into the plasma), but that open state would leave a current sheet extending nearly to the surface, and both the flux rope structure and the classic three-part white-light CME morphology (bright leading edge, dark cavity, bright core) seen in coronagraphs are consequences of the flare reconnection. It is, in the authors' words, unclear whether smooth flux opening by ideal expansion would be classified as a CME by an observer; criteria that observationally separate ideal expansion from reconnection-driven eruption do not yet exist.

Third, an operational forecasting gap: stealth CMEs lack the on-disk EUV and X-ray signatures that give 2–5 days of warning for Earth-directed events, and their halo signatures are nearly invisible to a single coronagraph near Earth (such as at the L1 Lagrange point on the Sun-Earth line). The L5 Lagrange point, 60 degrees ahead of Earth in its orbit, would view Earth-directed eruptions against the plane of the sky, and the source paper argues it is ideally suited for identifying and observing such events; whether multi-viewpoint monitoring from L5 in fact enables robust operational detection and forecasting of stealth CMEs has not been established. Later work has sharpened rather than closed these questions: advanced image processing finds faint low-coronal signatures for most 'stealth' events (Alzate & Morgan 2017; O'Kane et al. 2019; Palmerio et al. 2021), streamer-blowout statistics are consistent with reconnection release of differential-rotation-accumulated energy (Vourlidas & Webb 2018), magnetofrictional simulations (a quasi-static method that evolves the coronal magnetic field under imposed photospheric flows, bypassing full MHD dynamics) driven by observed large-scale flows reproduce the June 2008 event and generate repeated stealth eruptions (Bhowmik & Yeates 2021; Bhowmik, Yeates & Rice 2022), yet the March 2023 severe geomagnetic storm was still caused by a CME too faint for operational forecasts (Teng et al. 2024), and ESA's L5 mission (Vigil) is still in development (Eastwood et al. 2024).

Problem Statement

Determine the physical nature and observability of stealth coronal mass ejections — the lowest-energy end of the solar eruption distribution — by answering three linked questions that share one objective: establishing whether the ejection of magnetic stress into the heliosphere via reconnection is a single process spanning all eruption scales, and what that implies for detecting Earth-directed stealth events.

(1) Continuum physics. Establish or refute the conjecture that streamer-top plasmoids, slow stealth streamer-blowout CMEs, and fast active-region CMEs constitute a continuum of manifestations of one reconnection-driven plasmoid-ejection process, with the magnetic stress injected at different heights (highest-lying flux near closed-field boundaries for plasmoids, intermediate heights by large-scale differential rotation for stealth CMEs, lowest-lying flux near polarity inversion lines for fast CMEs) and released in all cases through the formation and ejection of a reconnection plasmoid. The source simulation itself acknowledges limitations relevant to any such test: its reconnection is mediated by grid-scale numerical diffusion rather than an explicit resistivity, and its energy budget rests on an isothermal approximation with artificially accelerated driving (~20 h of shearing standing in for ~15 days of differential rotation).

(2) Observational discriminants. Develop and validate criteria that observationally separate smooth ideal expansion of sheared flux from reconnection-driven CME eruption, answering the source's explicit question of whether pure ideal flux opening would be classified as a CME by an observer at all — including which measurable signatures (current-sheet, reconnection-flux, dimming, ribbon, or morphological diagnostics) carry that dynamical information for faint stealth events.

(3) Multi-viewpoint detectability. Determine whether observations from additional viewpoints — in particular the Sun-Earth L5 Lagrange point 60 degrees from the Sun-Earth line, as planned for ESA's Vigil mission — enable robust operational detection, tracking, and arrival forecasting of Earth-directed stealth CMEs that are effectively invisible to L1 monitoring alone.

The three components are scientifically linked: (1) supplies the physical mechanism whose observable consequences (2) must discriminate, and (2)'s discriminants are precisely what an operational multi-viewpoint strategy (3) must exploit. A complete answer establishes whether 'stealth' is a physically meaningful class or an observational limitation of the low-energy end of one unified eruption process, and what observing architecture is required to forecast it. All three determinations — the continuum conjecture (1), validated observational criteria separating smooth ideal expansion from reconnection-driven eruption (2), and the multi-viewpoint detectability assessment (3) — are jointly required; any one determination alone is a partial result.

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