Solar Orbiter SEP Dropout during a Magnetic Cloud with Evidence for Strong Connectivity Gradients

(Solar Orbiter nugget #97 by R. Bučík¹, R. Gómez-Herrero², S. T. Hart¹, M. A. Dayeh^1,3, and N. V. Nitta⁴)

Introduction

Impulsive ³He-rich solar energetic particle (SEP) events sometimes show abrupt intensity dropouts, in which particle fluxes decrease simultaneously over a broad energy range and can recover rapidly. Two main explanations have been proposed: changes in magnetic connectivity between neighboring flux tubes with different access to the SEP source, or filamentary particle transport produced by solar wind turbulence. Distinguishing between these possibilities observationally remains difficult. Solar Orbiter has previously observed dropout signatures in impulsive SEP events at different heliocentric distances [1, 2]. The event studied here adds new constraints on the origin of the dropout by combining directional energetic-particle measurements with source-region magnetic-connectivity mapping.

Results

We investigated a pronounced dropout in a ³He-rich SEP event observed by Solar Orbiter [3] at 0.93 au on 2022 December 24 [4]. Solar Orbiter EPD/EPT [5] observed a clear velocity-dispersed SEP onset, where higher-energy (faster) ions arrive first and lower-energy ions arrive later. This appears as a slanted pattern in the top panel of Figure 1. Before the dropout, the particles formed a strongly field-aligned beam. At about 07:15 UT, this beam abruptly disappeared, and the intensity remained strongly depleted for nearly three hours.

The dropout onset was not accompanied by an abrupt change in the local magnetic field direction; accordingly, the EPT pitch-angle coverage remained essentially unchanged. The dropout is therefore unlikely to result simply from a change in the instrument viewing geometry. As the field-aligned population disappeared, a weak 100–200 keV ion component appeared at pitch angles of about 90°–180°, while no comparable component was present at MeV energies.

Figure 1. Solar Orbiter observations of the main SEP dropout. From top to bottom: EPT-Sun inverse-velocity spectrogram, IMF magnitude and components, and EPT ion pitch-angle distributions at 103–203 keV and 1.03–1.46 MeV. Adapted from Figure 1 of [4].

The event was associated with a jet from a compact source at the edge of a large sunspot in AR 13169 (Figure 2, Top). We examined Solar Orbiter's magnetic connection to the source using ballistic back-mapping, PFSS extrapolations, and a proxy for quasi-separatrix layers (QSLs; Figure 2, Bottom).

Figure 2. Top: Composite SDO/AIA 171 Å and HMI image of the jet source at the edge of a large sunspot. The arrow marks the jet. The Carrington grid is overlaid. Bottom: QSL-proxy map of the source region. Bright structures mark strong magnetic connectivity gradients. The red crosshair marks the jet-base center, the blue diamond the nominal Solar Orbiter footpoint, and the blue squares the ±10° mapping offsets. Green × and cyan + symbols mark open field footpoints connecting to the ecliptic and other latitudes, respectively. Adapted from Figures 2 and 3 of [4].

The nominal Solar Orbiter connection mapped to the jet source region, close to strong connectivity gradients. Relatively small displacements can therefore shift the connection to neighboring open flux tubes with very different access to particles released from the compact source. This magnetic topology provides a plausible basis for interpreting the dropout as a transition between adjacent open flux tubes despite the absence of an abrupt local IMF change.

The SEP event occurred during a magnetic cloud passage with reduced magnetic fluctuations. Such conditions may limit cross-field transport, helping preserve sharp contrasts in SEP intensity between neighboring flux tubes and making dropouts more apparent.

Figure 3. Top: IMF around the event, with the magnetic cloud interval shaded. Bottom: EPD/SIS heavy-ion measurements showing the main dropout and several additional dropouts later in the event. Adapted from Figure 5 of [4].

Conclusion

The Solar Orbiter observations favor an interpretation in which the SEP dropout reflects rapid changes in particle access between adjacent open flux tubes near strong connectivity gradients, even without an abrupt local IMF change. The magnetic cloud in which the SEP event occurred may have helped make the dropouts more apparent. 

Affiliations

(1) Southwest Research Institute, San Antonio, TX, USA

(2) Universidad de Alcalá, Space Research Group, Alcalá de Henares, Spain

(3) University of Texas at San Antonio, San Antonio, TX, USA

(4) Lockheed Martin Advanced Technology Center, Palo Alto, CA, USA

References

[1] Ho, G. C., Mason, G. M., Allen, R. C., et al. (2022), Front. Astron. Space Sci., 9, 939799, DOI: 10.3389/fspas.2022.939799

[2] Wimmer-Schweingruber, R. F., Berger, L., Kollhoff, A., et al. (2023), A&A, 678, A98, DOI: 10.1051/0004-6361/202346319

[3] Müller, D., St. Cyr, O. C., Zouganelis, I., et al. (2020), A&A, 642, A1, DOI: 10.1051/0004-6361/202038467

[4] Bučík, R., Gómez-Herrero, R., Hart, S. T., Dayeh, M. A., Nitta, N. V. (2026), ApJL, 1006, L48, DOI: 10.3847/2041-8213/ae8994

[5] Rodríguez-Pacheco, J., Wimmer-Schweingruber, R. F., Mason, G. M., et al. (2020), A&A, 642, A7, DOI: 10.1051/0004-6361/201935287

Acknowledgements

R.B. and M.A.D. acknowledge support by NASA grants 80NSSC22K0757, 80NSSC24K1441, and 80NSSC25K7687. R.G.H. acknowledges the financial support by project PID2023-150952OB-I00 funded by MICIU/AEI/10.13039/501100011033 and EU/FEDER and project SBPLY/24/180225/000108 funded by EU/FEDER and JCCM/INNOCAM.