The Earth's near-space environment is dynamic, and space weather events such as geomagnetic storms significantly impact technological infrastructures. Accurate prediction of these storms is crucial. Geomagnetic storms are primarily driven by interplanetary coronal mass ejections (ICMEs) and co-rotating interaction regions (CIRs). ICMEs, originating from coronal mass ejections (CMEs), involve the expulsion of solar mass and magnetic fields into interplanetary space. CIRs, on the other hand, result from interactions between slow and fast solar wind streams. A southward-aligned interplanetary magnetic field (IMF) component (Bz) is key to triggering geomagnetic storms, facilitating magnetic reconnection at the magnetopause. While CIR-related storms are relatively predictable, CME-related storms pose a greater challenge due to the complex dynamics of CME propagation (deflection, rotation, deformation, deceleration) and the need for accurate source region localization and magnetic structure modeling. Stealthy CMEs, characterized by indistinct lower coronal source regions and lack of correlated solar flares or filament eruptions, are especially difficult to predict. These often lead to "problem geomagnetic storms" (PGSs) with ambiguous origins, complicating the prediction of southward IMF strength and storm intensity. While simulations have explored single-hemispheric and latitudinal erupting flux ropes, about one-fifth of near-Earth flux ropes show high inclinations (>55°), suggesting the importance of longitudinal, trans-equatorial flux ropes. Intense PGSs often occur after longitudinal trans-equatorial loops traverse the solar disk center, implying a significant potential for such structures to generate intense geomagnetic storms. However, a comprehensive study linking eruptive solar trans-equatorial structures with geo-effective longitudinal flux ropes near Earth was lacking before this research. The March 23, 2023, major geomagnetic storm, unpredicted by remote-sensing observations, provides an ideal case study to address this gap.

The literature extensively documents the causes and effects of geomagnetic storms, highlighting the role of ICMEs and CIRs. Previous studies have emphasized the importance of southward IMF Bz in triggering storms, and the challenges in predicting CME-related storms due to CME propagation complexities. The concept of stealthy CMEs, lacking clear low-coronal signatures, has been established, and their unpredictability is well-documented. Research efforts have focused on modeling stealthy CMEs through magneto-frictional and MHD simulations, primarily exploring single-hemispheric and latitudinal flux ropes. Studies have also noted the high inclination of a significant portion of near-Earth flux ropes and the correlation between intense PGSs and the transit of longitudinal trans-equatorial loops. While some studies suggest the role of trans-equatorial filaments and arcades in extreme geo-effectiveness, a comprehensive study linking these solar features to geo-effective longitudinal flux ropes near Earth had been missing, until this research.

This study identifies the solar origin of the March 23, 2023, geomagnetic storm (named the "Dragon Day Event") using data from STEREO-A and Solar Orbiter. The methodology involved several key steps: 1. **Analysis of the Geomagnetic Storm and Near-Earth ICME:** The study examined WIND spacecraft data to characterize the storm, identifying the sheath region, magnetic interface, and flux rope within the ICME. Minimum variance analysis (MVA) was used to determine the ICME flux rope's inclination angle. 2. **Tracing the CME to its Solar Source:** SOHO/LASCO and STEREO-A/cor2 images were analyzed to identify the associated CME. The graduated cylindrical shell (GCS) model was employed to fit the CME images from different perspectives and determine its three-dimensional structure and propagation characteristics. 3. **Analysis of the Solar Eruption:** SDO/AIA and SolO/EUI images were used to analyze the solar eruption associated with the CME, focusing on the pre-eruption EUV channel (a strip of low EUV emission), the faint brightening bands that developed after the eruption, and the associated dimmings. AIA 304 Å and CHASE Hα images were used to examine a nearby filament eruption, determining its contribution to the geomagnetic storm. 4. **Modeling of the Coronal Magnetic Field:** Due to weak photospheric magnetic fields around the EUV channel, the researchers used a flux rope insertion method with SDO/HMI line-of-sight magnetograms. This approach involved calculating the potential field of the source region, inserting a flux bundle, and using magneto-frictional relaxation to create a force-free field model. The model was used to assess the twist of the flux rope and its stability. 5. **PFSS Extrapolation:** A potential field source surface (PFSS) extrapolation was performed to extend the magnetic field model from the solar surface to 2.5R⊙. This allowed the examination of the overlying arcades, the trans-equatorial magnetic cavity, and the relationship between the flux rope eruption and the subsequent filament eruption. The decay index was calculated to analyze the stability of the flux rope. 6. **Prediction of Near-Earth IMF and Geomagnetic Index:** The coronal magnetic field model was used to predict the near-Earth IMF components and the geomagnetic SYM-H index. This involved fitting the model to Solar Orbiter observations to determine the best-fit location and starting time, considering the CME's deflection and propagation characteristics. The Burton's equations were used to predict the SYM-H index based on the modeled Bz and observed solar wind parameters. STEREO-A data was used to validate the assumption of minimal flux rope rotation after Solar Orbiter detection.

The study's key findings are: 1. **Identification of the Stealthy CME:** The Dragon Day Event's cause was identified as a stealthy full-halo CME originating from the eruption of a trans-equatorial flux rope. This CME was previously overlooked due to its faint and intermittent appearance and lack of classic low-coronal signatures. 2. **Characterization of the Solar Source:** The solar source was characterized as a trans-equatorial EUV channel, a longitudinal strip of low EUV emission, containing a twisted flux rope. The eruption involved faint brightening bands in warm EUV wavelengths and associated dimmings. 3. **Confirmation of Earthward Propagation:** Dual-perspective observations from STEREO-A and SOHO confirmed the CME's earthward propagation. The GCS model provided details of the CME's three-dimensional structure and propagation. 4. **Connection to Near-Earth ICME:** In-situ measurements from Solar Orbiter provided evidence linking the full-halo CME to the ICME detected near Earth that caused the geomagnetic storm. The similarity of magnetic field characteristics between Solar Orbiter and near-Earth observations supported this connection. 5. **Successful Magnetic Field Modeling:** A non-linear force-free magnetic field model, using the flux rope insertion method, successfully reproduced the observed near-Earth IMF components and geomagnetic SYM-H index after fitting to Solar Orbiter data. This model provided insights into the flux rope's twist, stability, and the possible interaction with a nearby filament. The model's prediction of the southward IMF highlights its crucial role in the geomagnetic storm. 6. **Proposed Characteristic Features:** The study proposes characteristic features for identifying similar eruptions. These features include pre-eruption trans-equatorial EUV channels, post-eruption faint brightening bands in warm EUV passbands, lengthy and longitudinal full-halo CMEs with faint and intermittent fronts, and potential subsequent eruptions of other structures beneath overlying arcades.

This study successfully linked a major geomagnetic storm to the eruption of a stealthy trans-equatorial flux rope. The unexpected intensity of the storm, despite the CME's faintness, highlights the significant geo-effectiveness of such events. The successful prediction of near-Earth IMF and geomagnetic indices based on the magnetic field model confirms the crucial role of the flux rope's southward axial magnetic field. The comparison with the August 2018 event, another stealthy CME causing a strong geomagnetic storm, further supports the notion that longitudinal flux ropes with southward axial flux are highly geo-effective due to their sustained southward IMF. The study also suggests the potential impact of the CME's southward-leaning propagation and the low density of the flux rope. The results emphasize the need to revise space weather forecasting strategies to account for the stealthy nature and geo-effectiveness of trans-equatorial flux rope eruptions.

The Dragon Day Event demonstrates the potential of erupting trans-equatorial flux ropes to generate unexpectedly intense geomagnetic storms. The event’s stealthy nature underscores the need for improved detection and forecasting methods. The proposed observational characteristics for such events should be incorporated into space weather forecasting models. Future research should focus on refining the magnetic field modeling techniques for more accurate prediction of the near-Earth IMF and storm intensity. Improved understanding of the complex dynamics of trans-equatorial flux rope eruptions will enhance the accuracy of space weather forecasting and mitigate potential risks.

The study relies on several simplifying assumptions in the magnetic field modeling and near-Earth IMF prediction. The assumption of self-similar eruption, minimal flux rope rotation, and idealized transverse over-expansion might affect the accuracy of the prediction. The use of line-of-sight magnetogram data for the model reconstruction could also lead to some uncertainties. Furthermore, the study focuses on a single event, and further research involving a larger dataset is needed to generalize the findings and enhance the robustness of the proposed characteristic features for similar eruptions.