Unexpected major geomagnetic storm caused by faint eruption of a solar trans-equatorial flux rope

Geomagnetic storms are major space weather events driven primarily by interplanetary coronal mass ejections (ICMEs) and co-rotating interaction regions (CIRs). Prolonged southward interplanetary magnetic field (IMF Bz) and enhanced solar wind speed increase storm likelihood and intensity via magnetopause reconnection and ring current enhancement. Forecasting CME-driven storms remains challenging due to CME deflection, rotation, deformation, and deceleration during propagation, and the difficulty of locating and characterizing the CME source region and magnetic structure. A particularly problematic subtype are stealth CMEs, which lack clear low-coronal signatures (e.g., flares, filament eruptions, strong dimmings), leading to “problem geomagnetic storms” whose solar sources are unclear. Although ICME rotation occurs, near-Earth flux ropes with high inclination (>55°) often reflect their initial solar orientation, implying some longitudinal flux ropes originate as such on the Sun. Trans-equatorial magnetic structures can erupt with inherent Bz components and have been statistically linked to many Earth-directed CMEs, suggesting they can be highly geo-effective. On March 23, 2023, an unexpected major geomagnetic storm (Dst_min = -163 nT; Kp up to 8) occurred without an identified Earth-directed CME in forecasts. This study aims to identify its solar origin, track the Sun-to-Earth propagation, reconstruct the source-region magnetic configuration, and explain how a stealth, trans-equatorial longitudinal flux rope produced the strong southward IMF and the severe storm.

Prior work established that most major storms are CME-driven, with difficulty in predicting arrival and magnetic vectors due to CME evolution en route. Stealth CMEs—events without clear low-coronal signatures—have been shown to cause problem geomagnetic storms and complicate prediction of southward IMF because source-region magnetic structure is uncertain. Empirical schemes (e.g., helicity-based inference) and studies comparing ICME magnetic structure with source regions show partial preservation of helicity and flux, though rotations can occur. Simulations of stealth CMEs driven by long-term differential rotation have reproduced gentle eruptions but generally produced latitudinal, single-hemispheric flux ropes with low inclination relative to the ecliptic. Observations indicate about one-fifth of near-Earth flux ropes have high inclination, and inclination often decreases with heliocentric distance, arguing that highly inclined (longitudinal) near-Earth ropes likely originate longitudinally. Trans-equatorial loops and filaments are frequently associated with Earth-directed CMEs and intense storms (e.g., Bastille Day event), yet comprehensive Sun-to-Earth linkage for eruptive trans-equatorial structures to geo-effective longitudinal flux ropes has been lacking. This work addresses that gap using combined multi-perspective imaging, in-situ measurements, and coronal magnetic modeling.

- Event identification and in-situ context: Used OMNI/WIND 1-min IMF and solar wind parameters time-shifted to Earth’s bow shock to characterize the March 23–24, 2023 event. Identified sheath onset (Mar 23, 07:40 UT), magnetic interface after sheath (14:20 UT), flux-rope start (17:40 UT), and end (Mar 24, ~08:20 UT). Applied minimum variance analysis (MVA) to derive flux-rope inclination near Earth (GSE) = -74°, indicating a highly inclined, near-longitudinal magnetic cloud without a preceding shock (ICME slower than preceding wind). - Remote sensing of source region and eruption: Analyzed SDO/AIA (171, 193, 211, 94 Å), Hinode/XRT, Solar Orbiter/EUI, and STEREO-A/EUVI images. Identified a long (~1000″), longitudinal, trans-equatorial low-EUV emission “EUV channel” present for >4 days, with fragmented dark threads across multiple EUV bands and in soft X-rays. After ~17:00 UT Mar 19, faint brightening bands developed on both sides of the channel (best in warm EUV), with subtle dimmings and large-scale faint dimming near the south end, indicating gentle eruption of a low-density, trans-equatorial magnetic structure. - CME detection and 3D reconstruction: Examined SOHO/LASCO C2–C3 and STEREO-A/cor2 running-difference images. Identified a very faint, intermittent full-halo CME connected to a clearer southward-propagating component starting ~20:40 UT Mar 19 (cor2). Constructed graduated cylindrical shell (GCS) models using dual-perspective images (gcs-python). Retrieved parameters across epochs: tilt ~ -70° to -75°; apex latitude ~ -45° to -40°; apex longitude 20°→13° (eastward deflection); edge-on angular width increased from ~32.6° to ~38.6°; half-angle ~90°. Derived apex radial speeds: 195 km/s (20:39–22:38 UT Mar 19), 422 km/s (22:38–00:28 UT), 420 km/s (00:28–06:54 UT Mar 20). The CME geometry and timing matched the EUV channel and brightening bands, identifying the channel as the source. - Discrimination from a following CME: Documented a subsequent, partial eruption of a nearby southwestern sigmoidal filament (AIA 304 Å; no Hα signature; weak post-eruption arcades in warm EUV/soft X-ray; no GOES flare), producing a clearer CME toward the southwest. This CME propagated south of the ecliptic and could only graze Earth’s northern flank; forecasts predicted minimal geomagnetic impact (Kp 2.0–4.5). HI imaging showed it catching up with the southern flank of the halo CME, potentially compressing it off the ecliptic. - Heliospheric imaging and kinematics: Used STEREO-A HI1/HI2 J-maps to track the halo CME toward the ecliptic Earth line; quadratic fitting of the time–elongation track predicted Earth arrival ~13:00 UT Mar 23, consistent with near-Earth sheath and density enhancements. - Upstream in-situ linkage: Compared Solar Orbiter magnetometer and SWA data at ~0.5 AU. Detected a flux rope starting ~09:30 UT Mar 21 with By drop ~20 nT, elevation ~ -50°, smooth azimuthal rotation from ~ -100° to ~ +80°, low temperature and linearly decreasing speed—consistent with an expanding, highly inclined rope. MVA inclination ~ -82°. The IMF component trends and angles closely resembled near-Earth measurements, indicating the same ICME. Geometry using last GCS parameters and HI elongation (~27°) confirmed the CME front matching Solar Orbiter encounter. - Coronal magnetic field reconstruction: Built a non-linear force-free field (NLFFF) model via the flux rope insertion method using SDO/HMI line-of-sight magnetogram (16:30 UT Mar 19) as boundary. Inserted a thin axial flux bundle along the EUV channel path, applied magneto-frictional relaxation to a force-free state. The best-fit model reproduced a trans-equatorial flux rope (Twist number Tw > 1 near axis) co-spatial with the EUV channel and a separate structure for the southwestern filament. - Overlying field topology and stability: Performed PFSS extrapolation (pfsspy) from the surface to 2.5 R⊙, revealing trans-equatorial arcades overlying a magnetic cavity containing the flux rope. Polarity inversion lines at heights had ecliptic inclination ~ -70°, consistent with CME tilt (~ -75°). Computed decay index n = dlog(Bh)/dlog(R); a northern “ceiling” region of negative n would impede the northern portion, explaining the observed southward-leaning CME propagation. - Sun-to-Earth IMF and geomagnetic prediction: Fitted the NLFFF model along a line-of-sight to reproduce Solar Orbiter IMF time series by optimizing location (φsolo = -2.06°, θsolo = 10.60°), start time (t0 = Mar 21 05:30 UT), and magnetic amplification factor (a = 0.0166 R⊙^-1), assuming self-similar radial expansion, transverse over-expansion (coronal width 10.7° to CME width 38.6°), and B ∝ r^-2 scaling with additional amplification. Using Solar Orbiter–Earth separation and over-expansion scaling, predicted the Earth-crossing line in the model and extracted a synthetic IMF time series converted to GSM. Employed Burton’s equations with modeled Bz and estimated solar wind parameters (speed ~420 km/s; density scaled from Solar Orbiter) to predict SYM-H. - Validation: The modeled near-Earth IMF components reproduced the observed sign and variation trends, including a prolonged strong southward Bz. The predicted SYM-H tracked observations, reproducing a major storm. Similarity of STEREO-A IMF to Solar Orbiter supported minimal post-0.5 AU axis rotation.

- A severe, unexpected geomagnetic storm on Mar 23–24, 2023 reached Kp = 8 (3 hours), Dst_min = -163 nT (SYM-H_min ~ -170 nT), with >10 hours of strong southward IMF (Bz ~ -15 nT) during the ICME flux-rope passage. - The responsible solar driver was a faint, intermittent full-halo CME originating from the gentle eruption of a low-density, longitudinal, trans-equatorial flux rope observed as a large-scale EUV channel with weak or absent chromospheric signatures. It was missed by catalogs/forecasters (CACTus: none; CDAW: partial “Poor Event”; SEEDS: misidentified as three narrow CMEs) and excluded from CCMC CME Scoreboard forecasts. - Near-Earth ICME showed a cool, low-density magnetic cloud with smooth field rotation and no shock; MVA inclination ~ -74° (near-longitudinal). The sheath began ~07:40 UT Mar 23; magnetic interface ~14:20 UT; flux rope ~17:40 UT to ~08:20 UT Mar 24. - GCS modeling from SOHO/LASCO and STEREO-A constrained CME geometry: tilt ~ -70° to -75°; apex latitude ~ -45° → -40°; longitude 20° → 13° (eastward deflection); edge-on angular width increased ~32.6° → ~38.6°; half-angle ~90°. Radial speeds: 195, 422, 420 km/s over successive intervals, with early acceleration. The CME cross-section in the ecliptic pointed essentially toward Earth (central longitude ~3.2° at ~06:50 UT Mar 20). - Solar Orbiter in-situ at ~0.5 AU detected a highly inclined flux rope (MVA ~ -82°) with IMF component trends and smooth angle rotations matching those near Earth, confirming Sun-to-Earth connectivity of the same ICME. HI J-maps supported continuous propagation toward Earth and predicted arrival consistent with near-Earth sheath/density signatures. - NLFFF (flux rope insertion) modeling reproduced a trans-equatorial flux rope co-spatial with the EUV channel (Tw > 1), embedded beneath PFSS trans-equatorial arcades forming a magnetic cavity. PFSS decay index mapping revealed a northern negative-n “ceiling,” explaining the CME’s southward-leaning geometry. - Transforming the coronal model and fitting Solar Orbiter data enabled prediction of near-Earth IMF components, including sustained southward Bz arising from the rope’s southward axial field; Burton’s-equation-based SYM-H predictions reproduced a major storm. - A subsequent partial eruption of a nearby sigmoidal filament produced a clearer, southward-propagating CME that interacted with the halo CME’s southern flank but was too far south to cause significant geomagnetic impact at Earth (predicted Kp 2.0–4.5).

The study resolves the solar origin of an unforecast severe geomagnetic storm by linking it to a stealth, trans-equatorial, longitudinal flux rope eruption. The event shows that even slow, low-density CMEs can drive major storms if they carry a sustained southward axial field aligned closely with Earth’s north–south direction. The longitudinal orientation leads to minimal axial-field rotation during passage, producing prolonged southward IMF, consistent with in-situ observations at Solar Orbiter, STEREO-A, and near Earth. The coronal model and PFSS context explain the observed CME tilt and southward-leaning propagation via overlying arcade geometry and a northern region of stabilizing (negative) decay index. Reproducing the near-Earth IMF and SYM-H with a coronal flux-rope-insertion NLFFF model, constrained by upstream in-situ data, demonstrates a viable framework for forecasting the geoeffectiveness of similarly stealthy eruptions. Comparison with the August 2018 stealth event underscores that longitudinal flux ropes with southward axial fields are particularly geo-effective due to the persistence of Bz during Earth crossing. The work emphasizes the need to identify characteristic pre-/post-eruption signatures (long trans-equatorial EUV channel; faint, expanding warm-EUV brightening bands; faint, lengthy full-halo fronts) to reduce forecast failures for such events.

This work conclusively identifies the source of the March 23–24, 2023 severe geomagnetic storm as the gentle eruption of a low-density, longitudinal, trans-equatorial flux rope, producing a faint, intermittent full-halo CME that was largely missed by operational cataloging and forecasting. Multi-perspective coronagraphy and heliospheric imaging linked the CME to an ICME consistently observed by Solar Orbiter and near Earth, while NLFFF flux-rope-insertion modeling of the source region, combined with PFSS context, reproduced the flux-rope geometry and explained its southward-leaning propagation. Using the modeled structure fitted to Solar Orbiter data, the study predicted near-Earth IMF (including prolonged southward Bz) and SYM-H evolution of a major storm. The authors propose diagnostic observational features and a practical forecasting workflow to improve detection and impact prediction for similar stealth, trans-equatorial, longitudinal eruptions. Future work should broaden event statistics, refine data-driven MHD modeling of trans-equatorial structures, reduce reliance on upstream in-situ constraints, and improve automated detection of faint, lengthy full-halo CME fronts across combined coronagraph fields of view.

- Vector photospheric magnetic fields around the EUV channel were weak and uncertain, precluding direct NLFFF extrapolation from vector boundary data; the flux-rope-insertion method, while physically consistent, is an approximation reliant on line-of-sight magnetograms and relaxation assumptions. - The EUV channel formed before disk passage, limiting availability of bottom-boundary data during formation; a fully data-driven MHD simulation with higher-resolution temporal boundary conditions was not feasible. - IMF prediction assumed self-similar radial expansion, transverse over-expansion scaling, B ∝ r^-2 with an empirical amplification factor, fixed-latitude/longitude cuts, and negligible post-0.5 AU rope-axis rotation; local deformations likely contributed to component-wise discrepancies (e.g., Bx underestimation, |Bz| overestimation at STEREO-A). - Forecasting approach leveraged Solar Orbiter upstream constraints; in typical operations without upstream detections, predictive skill would be reduced, heightening reliance on remote-sensing signature recognition. - The interaction with a following southward CME may have introduced modest compression off the ecliptic plane that was not fully quantified for the near-Earth impact.