Coronal mass ejections (CMEs) that exhibit no obvious low-coronal signatures (LCS) - no flare, filament eruption, coronal dimming, or EUV wave - are called stealth CMEs, and their interplanetary counterparts are documented causes of geomagnetic storms that operational forecasting misses. The problem, framed by Nitta et al. (2021) and the primary literature on the field's open questions - with Bhatt, Srivastava and Jadhav (2018) contributing a five-event case study of stealth CMEs and their associated ICMEs - asks three linked things about this population: (1) which coronal magnetic configurations and triggering processes initiate stealth CMEs, and whether they form a physically distinct eruption class or merely the faint, low-energy tail of ordinary CMEs hidden by observational limits; (2) what observational diagnostics and detection or forecast procedures can identify Earth-impacting stealth CMEs in routine operations before arrival; (3) what the statistical prevalence, kinematic and magnetic parameter distributions, and geoeffectiveness conditions of stealth-CME-driven interplanetary CMEs (ICMEs) are. Despite progress - dual-viewpoint source identification, image processing that recovers faint signatures for many events, magnetohydrodynamic and magnetofrictional models of streamer-blowout eruptions, and case studies of storms up to severe intensity (SYM-H of about -170 nT in March 2023, unforecast) - the initiation mechanism remains debated, no operational detection capability exists, and population-level statistics remain limited to a single storm-census count of six stealth-linked intense storms (1996-2025). The problem is open.

Background

A coronal mass ejection (CME) is a large eruption of plasma and magnetic field from the Sun's outer atmosphere into interplanetary space, observed primarily with white-light coronagraphs such as LASCO on the SOHO spacecraft and the SECCHI coronagraphs on the two STEREO spacecraft. Ordinary CMEs are usually accompanied in the low corona by identifiable low-coronal signatures (LCS): solar flares, erupting filaments (dense, cool plasma suspended in the corona), coronal dimmings (transient dark regions in extreme-ultraviolet, EUV, images marking evacuated magnetic flux), EUV waves, or post-eruption arcades (loop systems formed by magnetic reconnection after an eruption). Because these signatures are routinely observed by EUV imagers such as SDO/AIA, they anchor the identification of a CME's source region and provide early warning.

A minority of CMEs show none of these signatures. Robbrecht, Patsourakos and Vourlidas (2009) presented the first well-studied case (2008 June 1-2), a slow flux-rope CME (a CME whose magnetic structure is a twisted rope of field lines) erupting from the quiet Sun above a helmet-streamer belt arcade (a so-called streamer-blowout eruption); the source could be located only because the two STEREO spacecraft viewed the Sun from different sides, and the authors attributed the absent signatures to the unusually large height from which the flux rope lifted off. Ma et al. (2010) made the first statistical comparison of CMEs with and without distinct LCS, and D'Huys et al. (2014) analyzed 40 CMEs without LCS from 2012, finding them characteristically slow. Whether 'stealth' CMEs are a distinct physical class or an observational artifact is itself contested: Alzate and Morgan (2017), applying new multiscale image-processing techniques to the same 40 events, found at least faint LCS for all of them and concluded that stealthiness arises from observational and processing limitations, while O'Kane et al. (2019) found two stealth CMEs originating in active regions (magnetically strong sunspot regions) and argued they are the low-energy, low-velocity tail of the ordinary CME distribution. On the modeling side, Lynch et al. (2016) reproduced the 2008 June event with a 3D magnetohydrodynamic (MHD) simulation in which differential-rotation-like shear slowly energizes a helmet streamer until expansion-induced reconnection releases a slow CME with almost no low-coronal imprint, and Bhowmik, Yeates and Rice (2022) showed that magnetofrictional models (an approximate evolution method for the coronal magnetic field driven by observed photospheric magnetograms) naturally produce such eruptions, with eruption frequency controlled by footpoint shearing.

The space-weather relevance is that stealth CMEs and their interplanetary counterparts (ICMEs) can arrive at Earth with little or no warning. Nitta and Mulligan (2017) studied Earth-affecting CMEs without obvious LCS and found that coronal dimmings are recoverable in only several cases even with long-baseline difference imaging, and that their source regions cluster near coronal holes (regions of open, unipolar magnetic field that appear dark in EUV images) and other open-field regions. Nitta et al. (2021) reviewed 'problem geomagnetic storms' of solar cycle 24 - storms whose solar and CME precursors are elusive - and Palmerio et al. (2021) showed that remote-sensing techniques can recover weak eruptive signatures for well-observed case events, but with effectiveness strongly dependent on viewing geometry. Storm intensity is measured by the Dst or SYM-H indices (hourly/minute geomagnetic disturbance indices, in nT, negative during storms), driven mainly by the southward component of the ICME magnetic field. The problem is current: the severe storm of 2023 March 23-24 (SYM-H about -170 nT) was not forecast, and both Teng et al. (2024) and Vemareddy and Selva Bharathi (2025) trace it to a faint, stealthy eruption - while disagreeing on which eruption was the parent. The initiation region lies largely in the 'middle corona' (roughly 1.5-6 solar radii), a height range poorly covered by EUV imagers from below and coronagraphs from above, which is one reason identification is hard.

Problem Statement

The problem is to characterize, at the level of the event population rather than individual case studies, the class of coronal mass ejections that lack observable low-coronal signatures ('stealth CMEs') and the geoeffectiveness of their interplanetary counterparts, so that they cease to be a source of unforecast geomagnetic storms. It comprises three linked questions that share this one objective. A complete solution must address all three of the following questions together; settling any one of them, however thoroughly, does not by itself resolve the problem. (1) Origins: determine the coronal source regions, magnetic configurations, and triggering or initiation processes that produce eruptions with no observable LCS - in particular whether stealth CMEs are a physically distinct class (for example high-altitude streamer-blowout eruptions driven by slow, weak energy release) or the faint, low-energy tail of ordinary CMEs hidden by observational and processing limitations. (1) is resolved only once the source regions, magnetic configurations, and initiation processes themselves have been determined; the distinct-class-versus-faint-tail verdict must follow from that determination rather than substitute for it. (2) Detection and forecasting: establish observational diagnostics and detection or forecast procedures that identify Earth-impacting stealth CMEs in routine operations before their arrival at Earth, including, for example, image-processing techniques, viewing geometries, and instrument coverage of the poorly observed middle corona (roughly 1.5-6 solar radii) relevant to such identification. (2) is resolved only by a procedure formulated for advance warning and validated against past events; a method that identifies a stealth CME's source only after the storm has passed does not by itself resolve it. (3) Statistics and geoeffectiveness: from systematic, larger, multi-event datasets combining remote-sensing and in situ observations, quantify how often stealth CMEs produce geoeffective ICMEs, the distributions of their kinematic and magnetic parameters, and the conditions - such as a strong southward interplanetary magnetic field component, sheath compression (the compressed solar-wind region ahead of an arriving ICME), or preconditioning of the upstream solar wind - under which a slow, faint stealth CME produces a moderate-to-severe geomagnetic storm. Geoeffectiveness here means a geomagnetic storm of at least moderate intensity (minimum Dst below -50 nT). As framed in the cited literature (Nitta et al. 2021), progress on (1) supplies the physical basis for the predictive capability sought in (2), so that stealth CMEs are routinely detected well before they reach Earth and the geomagnetic storms they produce are no longer considered 'problem' storms, while (3) measures the hazard these efforts are meant to close.

References

  1. Bhatt, Miral; Srivastava, Nandita; Jadhav, Ravindra (2018). Study of Stealth CMEs and associated ICMEs. Proceedings of the International Astronomical Union 13(S340), 89-90. doi:10.1017/S1743921318002053, https://doi.org/10.1017/S1743921318002053
  2. Robbrecht, E.; Patsourakos, S.; Vourlidas, A. (2009). No Trace Left Behind: STEREO Observation of a Coronal Mass Ejection Without Low Coronal Signatures. The Astrophysical Journal 701, 283-291. doi:10.1088/0004-637X/701/1/283, https://doi.org/10.1088/0004-637X/701/1/283
  3. Ma, S.; Attrill, G. D. R.; Golub, L.; Lin, J. (2010). Statistical Study of Coronal Mass Ejections with and without Distinct Low Coronal Signatures. The Astrophysical Journal 722, 289-301. doi:10.1088/0004-637X/722/1/289, https://doi.org/10.1088/0004-637X/722/1/289
  4. D'Huys, E.; Seaton, D.; Poedts, S.; Berghmans, D. (2014). Observational Characteristics of Coronal Mass Ejections without Low-Coronal Signatures. The Astrophysical Journal 795, 49. doi:10.1088/0004-637X/795/1/49, https://doi.org/10.1088/0004-637X/795/1/49
  5. Lynch, B. J.; Masson, S.; Li, Y.; DeVore, C. R.; Luhmann, J. G.; Antiochos, S. K.; Fisher, G. H. (2016). A model for stealth coronal mass ejections. Journal of Geophysical Research: Space Physics 121. doi:10.1002/2016JA023432, https://doi.org/10.1002/2016JA023432
  6. Nitta, Nariaki V.; Mulligan, Tamitha (2017). Earth-Affecting Coronal Mass Ejections Without Obvious Low Coronal Signatures. Solar Physics 292, 125. doi:10.1007/s11207-017-1147-7, https://doi.org/10.1007/s11207-017-1147-7
  7. Alzate, Nathalia; Morgan, Huw (2017). Identification of Low Coronal Sources of "Stealth" Coronal Mass Ejections Using New Image Processing Techniques. The Astrophysical Journal 840, 103. doi:10.3847/1538-4357/aa6caa, https://doi.org/10.3847/1538-4357/aa6caa
  8. O'Kane, J.; Green, L.; Long, D. M.; Reid, H. (2019). Stealth Coronal Mass Ejections from Active Regions. The Astrophysical Journal 882, 85. doi:10.3847/1538-4357/ab371b, https://doi.org/10.3847/1538-4357/ab371b
  9. Yardley, S.; Pagano, P.; Mackay, D.; Upton, L. (2021). Determining the source and eruption dynamics of a stealth CME using NLFFF modelling and MHD simulations. Astronomy & Astrophysics 652, A160. doi:10.1051/0004-6361/202141142, https://doi.org/10.1051/0004-6361/202141142
  10. Palmerio, E.; Nitta, N.; Mulligan, T.; Mierla, M.; O'Kane, J.; Richardson, I.; Sinha, S.; Srivastava, N.; Yardley, S.; Zhukov, A. (2021). Investigating Remote-Sensing Techniques to Reveal Stealth Coronal Mass Ejections. Frontiers in Astronomy and Space Sciences 8, 695966. doi:10.3389/fspas.2021.695966, https://doi.org/10.3389/fspas.2021.695966
  11. Nitta, N.; Mulligan, T.; Kilpua, E.; Lynch, B.; Mierla, M.; O'Kane, J.; Pagano, P.; Palmerio, E.; Pomoell, J.; Richardson, I.; Rodriguez, L.; Rouillard, A.; Sinha, S.; Srivastava, N.; Talpeanu, D.; Yardley, S.; Zhukov, A. (2021). Understanding the Origins of Problem Geomagnetic Storms Associated with "Stealth" Coronal Mass Ejections. Space Science Reviews 217, 82. doi:10.1007/s11214-021-00857-0, https://doi.org/10.1007/s11214-021-00857-0
  12. Bhowmik, P.; Yeates, A.; Rice, O. (2022). Exploring the Origin of Stealth Coronal Mass Ejections with Magnetofrictional Simulations. Solar Physics 297, 41. doi:10.1007/s11207-022-01974-x, https://doi.org/10.1007/s11207-022-01974-x
  13. Ying, B.; Bemporad, A.; Feng, L.; Nitta, N.; Gan, W. (2022). Is There a Dynamic Difference between Stealthy and Standard Coronal Mass Ejections? The Astrophysical Journal 942, 3. doi:10.3847/1538-4357/aca52c, https://doi.org/10.3847/1538-4357/aca52c
  14. Pal, S.; Lynch, B.; Good, S.; Palmerio, E.; Asvestari, E.; Pomoell, J.; Stevens, M.; Kilpua, E. (2022). Eruption and Interplanetary Evolution of a Stealthy Streamer-Blowout CME Observed by PSP at ~0.5 AU. Frontiers in Astronomy and Space Sciences 9, 903676. doi:10.3389/fspas.2022.903676, https://doi.org/10.3389/fspas.2022.903676
  15. Teng, Weilin; Su, Yingna; Ji, Haisheng; Zhang, Qingmin (2024). Unexpected major geomagnetic storm caused by faint eruption of a solar trans-equatorial flux rope. Nature Communications 15, 9198. doi:10.1038/s41467-024-53538-1, https://doi.org/10.1038/s41467-024-53538-1
  16. Vemareddy, P.; Selva Bharathi, K. (2025). An Intense Geomagnetic Storm Originated from Stealth Coronal Mass Ejection: Remote and In Situ Observations by Near Radially Aligned Spacecraft. The Astrophysical Journal 994, 3. doi:10.3847/1538-4357/ae0992, https://doi.org/10.3847/1538-4357/ae0992
  17. D'Awarave, Arron Augustine; Panditi, Vemareddy (2026). Statistical Study of Intense Geomagnetic Storms and Their Solar Origins over the Period 1996-2025. Solar Physics 301. doi:10.1007/s11207-026-02666-6, https://doi.org/10.1007/s11207-026-02666-6