On March 23, 2023, a severe geomagnetic storm occurred unexpectedly. This study demonstrates that this storm originated from the eruption of a trans-equatorial, longitudinal, and low-density magnetic flux rope (FR) exhibiting weak coronal emission and lacking chromospheric signatures. The FR's gentle eruption resulted in a faint full-halo coronal mass ejection (CME) missed by forecasters and absent from CME catalogs. Using magnetic field modeling and in-situ measurements, the study reveals the FR's southward axial magnetic field as the primary cause of the storm. This CME is identified as the stealthiest to date to cause a severe geomagnetic storm, highlighting that erupting trans-equatorial FRs can generate major geomagnetic storms in a stealthy manner. Characteristic observational signatures of such eruptions are proposed to improve future forecasts.

Weilin Teng, Yingna Su, Haisheng Ji, Qingmin Zhang