The intense geomagnetic storm of 2023 March 23-24 (SYM-H peak about -170 nT) was produced by a stealth coronal mass ejection whose interplanetary magnetic cloud was observed in near-radial alignment by Solar Orbiter (SolO; 0.5 AU), STEREO-A (0.96 AU), and WIND (0.99 AU). Between 0.5 and 1 AU the cloud's axis inclination changed from about -69 degrees to about -25/-34 degrees, a density enhancement stood at its trailing edge at all three spacecraft (driving the storm's second SYM-H peak), and its mean magnetic field decayed as Bav∝RH−1.97B_{av}\propto R_H^{-1.97} (SolO-STA) and Bav∝RH−1.53B_{av}\propto R_H^{-1.53} (SolO-WIND). The source study (ApJ 994, 3, 2025) documents these changes but does not explain them, and adjacent work on heliospheric CME rotation, magnetic-cloud density structure, and ICME field-decay statistics does not resolve them either. The problem asks for a physically grounded account of this cloud's radial evolution: the mechanism of the axis-inclination change (real rotation versus sampling effects), the origin of the trailing density enhancement (near-Sun material versus in-transit formation), and the physical origin of the field-decay exponents in this event, all quantitatively consistent with the multipoint observations.

Background

A coronal mass ejection (CME) is a large eruption of magnetized plasma from the solar corona; its heliospheric counterpart is called an interplanetary CME (ICME). A subset of ICMEs, termed magnetic clouds (MCs), exhibits enhanced magnetic-field strength, a smooth rotation of the field direction over the passage, and low proton beta (the ratio of plasma pressure to magnetic pressure) - the classic signature of a spacecraft crossing a magnetic flux rope, a tube of twisted field with a well-defined axis. 'Stealth' CMEs erupt without the usual low-coronal signatures (flares, extreme-ultraviolet brightenings, radio bursts), which makes them easy to miss and a recognized source of unpredicted 'problem' geomagnetic storms (Nitta et al. 2021). Geomagnetic storms are measured by the Dst and SYM-H indices (the disturbance storm-time index and its higher-cadence variant), and are driven mainly by sustained southward interplanetary magnetic field (the geocentric solar ecliptic BzB_z component), which reconnects with Earth's field, with storm intensity further increased by enhanced solar-wind dynamic pressure Pdyn=nVsw2P_{dyn}=nV_{sw}^2.

On 2023 March 23-24 a severe storm (SYM-H about -170 nT) occurred whose solar origin was initially ambiguous. Teng et al. (2024, Nature Communications) traced it to the faint eruption of a trans-equatorial, longitudinal flux rope with weak coronal emission, and Vemareddy & Selva Bharathi (2025, ApJ 994, 3) - the source of this problem - followed the resulting CME through heliospheric imaging to an MC encountered by Solar Orbiter (SolO, 0.5 AU), STEREO-A (STA, 0.96 AU), and WIND (0.99 AU) in close radial alignment. The MC showed right-handed magnetic helicity at all three spacecraft (an ESW, east-south-west, field configuration at SolO and STA versus SWN at WIND - a different ordering of field directions around the same twist sense, not a handedness change), expanded from 0.08 AU radial size at SolO to 0.18 AU at STA, and carried a density enhancement toward its trailing edge at all three spacecraft, which produced the storm's second SYM-H peak. The orientation of the MC axis at each spacecraft was obtained by minimum variance analysis (MVA), the standard single-spacecraft technique in which the flux-rope axis is taken along the intermediate-variance eigenvector of the magnetic-field covariance matrix; the derived inclination to the ecliptic changed from −69.5-69.5 degrees at SolO (aligned with the source filament channel) to −25.1-25.1 degrees at STA and −34.5-34.5 degrees at WIND - a rotation-like change of roughly 35-45 degrees between 0.5 and 1 AU that the authors regard as a clear signature of axis rotation during propagation, without identifying a mechanism. The mean MC magnetic field decreased with heliocentric distance RHR_H as Bav∝RH−1.97B_{av}\propto R_H^{-1.97} between SolO and STA and Bav∝RH−1.53B_{av}\propto R_H^{-1.53} between SolO and WIND (peak-field exponents -2.1 and -1.7).

The adjacent literature frames what is and is not known. Rotation of erupting flux ropes in the low corona, driven by magnetic (Lorentz) forces, is well documented; rotation during heliospheric propagation is reported only in isolated case studies. Kumar et al. (2023, ApJ 958, 103) measured a continuously increasing tilt of the 2012 October 5 stealth CME out to 58 solar radii and further at Earth; Ma et al. (2024, ApJ 976, 183) reproduced the interplanetary rotation of the 2021 December 4 CME in a Sun-to-Mars magnetohydrodynamic (MHD) simulation and attributed it to the CME's flanks moving in fast versus slow ambient solar wind; Kumar et al. (2024, ApJ 977, 57) found that only 2 of 15 Earth-impacting CMEs showed significant rotation in heliospheric-imager fields of view, so both the occurrence rate and the causes of heliospheric rotation remain unsettled. Crucially for interpretation, Davies et al. (2020, Sol. Phys. 295, 157) showed that spacecraft in near-radial alignment can infer clearly different flux-rope orientations even at small longitudinal separation, because each samples a different path through a global, distorted rope - so apparent versus real rotation is itself a live discrimination problem. For the radial field decay, statistical studies of ICMEs between about 0.3 and 1+ AU (Good & Forsyth 2015, MESSENGER and Venus Express; Salman et al. 2020, a catalog of 47 conjunction CMEs; Zhao et al. 2022, 139 MCs from Helios to Ulysses) consistently find power-law decay of the magnetic field with substantial event-to-event variability, whose controlling parameters are not established. Trailing density enhancements in MCs are known to steepen storm main phases; Gopalswamy et al. (2022, JGR 127, e2022JA030404) attributed the density increase in the second half of the 2018 August 26 MC to prominence (filament) material carried from the Sun, while compression by an ambient high-speed solar-wind stream is the competing, in-transit explanation. Vemareddy & Selva Bharathi's event - a stealth CME whose MC trailing-density enhancement already exists at 0.5 AU and whose axis apparently rotates between 0.5 and 1 AU while the field decays with unusually steep exponents - sits at the intersection of these open questions and remains unexplained.

Problem Statement

For the magnetic cloud of 2023 March 21-24 observed in near-radial alignment by Solar Orbiter (0.5 AU), STEREO-A (0.96 AU), and WIND (0.99 AU) - the interplanetary counterpart of the 2023 March 19 stealth CME - provide a physically grounded account of the cloud's radial evolution from 0.5 to 1 AU that is quantitatively consistent with the multipoint magnetic-field, plasma, and heliospheric-imaging observations, and that addresses three linked, currently unexplained aspects of that evolution: (i) the mechanism responsible for the change in flux-rope axis inclination from about −69-69 degrees at SolO to about −25-25 to −34-34 degrees at STA and WIND, discriminating between real rotation of the rope (for example differential ambient solar-wind flow or magnetic forces acting on the structure) and apparent rotation due to observational or sampling effects (for example the three spacecraft traversing different distances from the rope axis or different legs of a globally distorted rope); (ii) the origin of the density enhancement at the MC's trailing edge that is present at all three spacecraft and drives the storm's second SYM-H peak - material incorporated near the Sun (for example prominence plasma) versus formation or amplification in transit (for example compression during interaction with the ambient high-speed solar-wind stream, which in this event precedes the magnetic cloud); and (iii) the physical origin of the measured radial decay of the mean MC magnetic field, Bav∝RH−1.97B_{av}\propto R_H^{-1.97} between SolO and STA and Bav∝RH−1.53B_{av}\propto R_H^{-1.53} between SolO and WIND - how this decay relates, in this event, to the cloud's observed expansion (radial size 0.08 AU at SolO to 0.18 AU at STA), its propagation speed, and its interaction with the structured ambient solar wind. These three aspects are parts of one objective - a causal, observationally constrained description of how this stealth-CME flux rope evolved between 0.5 and 1 AU.

References

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