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

The solar origins of some geomagnetic storms are ambiguous, which makes the prediction difficult. On March 23, 2023, a severe geomagnetic storm occurred; however, forecasts based on remote-sensing observations failed to predict it. Here, we demonstrate that this storm originates from the faint eruption of a trans-equatorial, longitudinal and low-density magnetic flux rope (FR). Before the eruption, the FR appears as a lengthy strip with weaker coronal emission and no chromospheric signs. Then, the FR’s gentle eruption results in a faint full-halo coronal mass ejection (CME), which is missed by forecasters and not identified in CME catalogs. Clear evidence from both remote-sensing and in-situ observations shows that this FR-containing CME propagates to Earth and causes the geomagnetic storm. Combining magnetic field modeling and in-situ measurements, we reveal that the FR’s southward axial magnetic field is the main cause of the storm. This CME is the stealthiest one reported causing a severe geomagnetic storm, and our study highlights that erupting trans-equatorial FRs can generate major geomagnetic storms in a stealthy way. Characteristic observational signatures of similar eruptions are proposed to help in future forecasts.