Unusually long path length for a nearly scatter-free solar particle event observed by Solar Orbiter at 0.43 au

(Solar Orbiter nugget #17 by R.F. Wimmer-Schweingruber1, L. Berger1, A. Kollhoff1, P. Kühl1, B. Heber1, et al.) 


On the 9th of April 2022 Solar Orbiter observed a virtually scatter-free solar particle event with an unusually long magnetic connection while immersed in a magnetic cloud. It was observed both in energetic electrons and ions while Solar Orbiter was at 0.43 au from the Sun. The length of the magnetic field line was inferred to be about three times longer by analyzing the velocity dispersion of electrons and ions.



Solar particle events originate in association with solar flares and/or coronal shocks driven by coronal mass ejections (CMEs). A spacecraft that is magnetically well connected to the acceleration region will first register the fastest escaping particles, and then increasingly slower ones. By measuring the arrival times and speeds of the registered particles one can thus determine both the onset time of the particle event as well as the path length along which the particles traveled.

On the 9th of April 2022, the Energetic Particle Detector (EPD, [1]) on Solar Orbiter detected electrons and ions which showed clear velocity dispersion. Remote-sensing observations showed that they were released from the same solar source region in the western limb region of the Sun; the source event was observed by the Spectrometer/ Telescope for Imaging X-rays (STIX, [2]), as well as the Extreme Ultraviolet Imager (EUI; [3]), see Fig. 1. Using data from EPD, the magnetometer (MAG, [4]) and the Electrostatic Analyser System (EAS) of the Solar Wind Analyzer (SWA; [5]), Wimmer-Schweingruber et al.  determined that the energetic electrons and ions traveled along an unusually long magnetic field line. While Solar Orbiter was located at 0.43 astronomical units form the Sun, they inferred a path length s ≈ 1.35 au or longer.


Figure 1. EUI Observations of the source region of the 9th April 2022 solar particle event show a small jet indicated by a red arrow. 



Energetic electrons measured by EPD show two injections, as can be seen in Fig. 2. The two electron events show clear velocity dispersion with higher energy electrons arriving before electrons with lower energy. X-ray observations shown in the top two panels have been time-shifted (by 215 s) for the light travel time from Sun to Solar Orbiter, the gray curve in the second panel from the top illustrates an un-shifted time profile. Horizontal bars indicated by s/c = 675 s in the third panel show the inferred travel time to Solar Orbiter of highly relativistic (v c) electrons released at the Sun. Their left-hand ends thus indicate the solar release times of highly relativistic electrons. The first event is delayed by approximately three minutes with respect to the small peak in low-energy (10 – 15 and 15 – 25 keV) X-rays, as would be expected based on previous studies [6, 7, 8]. The second event, however, appears to coincide with increased X-ray emission at 25 – 50 and 50 – 84 keV. This remarkable observation was very likely only made possible by the small variability of the interplanetary magnetic field (IMF), as seen in the bottom panel of Fig. 2 which illustrates the low level of fluctuations. The variance of the 1s-averaged pitch angle during this time period is 2.8 degrees while EPT’s half opening angle is 15 degrees. Inferred onset times and path lengths are given in Table 1.



Onset time

Inferred path length

Electron 1

11:25:14 ± 00:33

1.231 ± 0.038 au

Electron 2

11:34:29 ± 00:28

1.456 ± 0.028 au

Proton 1

11:45:16 ± 02:58

1.374 ± 0.011 au

Proton 2

12:06:53 ± 05:16

1.678 ± 0.021 au

Table 1: Inferred onset times and path lengths of the two electron and ion events observed by Solar Orbiter EPD on 9thApril 2022.


Figure 2. STIX light curves (top two panels) and EPD measurements of energetic electrons agree remarkably well when allowing for a travel path length of 1.35 au. 


Solar Orbiter remained magnetically connected to the solar source region for many more hours and so EPD also observed the ions accelerated by this solar eruption, as can be seen in Fig. 3 which shows EPD’s electron and ion observations for all four telescopes. The two electron injections discussed above are clearly seen in EPD’s north telescope while the other telescopes measure elevated electron intensities later than the north telescope. Similarly, the north ion telescope sees two ion injections. It is obvious that the ion event is confined to the north-pointing telescope (as was the electron event) with a very weak counterpart in the sunward-pointing telescope. The dash-dotted lines in all panels of Fig. 3 show the central pitch angle of the various telescopes. Comparison of ion intensities in EPD’s north- and sunward-pointing telescopes shows that the particle beam had a 1/e-width of approximately 30 degrees which is comparable to the full opening angle of one of EPD’s EPT telescopes. The solar release times inferred from velocity dispersion analysis of the ions are also reported in Tab. 1 and are only slightly delayed with respect to the electrons.


Figure 3. EPD observations in the sun, anti-sun, north and south telescopes of electrons (top 4 panels) and ions (next 4 panels) during the 9 April 2022  particle event. The bottom two panels show EPD/SIS measurements of the helium isotopes in the sunward/anti-sunward directions


Interpretation & Conclusions

The solar electron and ion events reported here are unusual not only because of their long magnetic connection back to the Sun, but also because the timing of these two homologous pairs of (electron and ion) events appear to agree so well with the X-ray observations. Typically, ions are expected to be accelerated higher up in the corona than the electrons which results in a delay in the ion onsets [9]. The 9 April 2022 event discussed in this paper, however, shows a reasonable agreement with the timing of the X-ray flares for electrons and a shorter delay for ions than usual, and very similar path lengths for electrons and ions. While the ions are injected after the electrons in this event, their time lag is small compared to the Krucker & Lin (2000) results, especially if the ions were accelerated in conjunction with the second, larger, and more energetic flare or electron acceleration event.


Remarkably, the IMF pointed in the negative N direction (in Radial, Tangential, Normal (RTN) coordinates) throughout the event, an unusual orientation, and thus particles were observed primarily by the EPT north telescope. This unusual orientation, likely due to Solar Orbiter being immersed inside an ICME flux rope, provided the unusually long path length along which the particles traveled. The low level of fluctuations in the IMF throughout this time period resulted in less scattering than is usually observed and thus maintained the strongly focused electron and ion beams. Together, these observations provide strong limitations on the properties of energetic particle transport inside an ICME (and the associated flux rope).

This work has been published in: Wimmer-Schweingruber et al., 2023, A&A, https://doi.org/10.1051/0004-6361/202346319



1 Institute of Experimental and Applied Physics, Christian-Albrechts-University Kiel, Leibnizstraße 11, 24118 Kiel, Germany


[1] Rodríguez-Pacheco, J., Wimmer-Schweingruber, R. F., Mason, G. M., et al. 2020, A&A, 642, A7

[2] Krucker, S., Hurford, G. J., Grimm, O., et al. 2020, A&A, 642, A15

[3] Rochus, P., Auchère, F., Berghmans, D., et al. 2020, A&A, 642, A8

[4] Horbury, T. S., O’Brien, H., Carrasco Blazquez, I., et al. 2020, A&A, 642, A9

[5] Owen, C. J., Bruno, R., Livi, S., et al. 2020, A&A, 642, A16

[6] Krucker, S., Larson, D. E., Lin, R. P., & Thompson, B. J. 1999, ApJ, 519, 864

[7] Haggerty, D. K., & Roelof, E. C. 2002, ApJ, 579, 841

[8] Simnett, G. M., Roelof, E. C., & Haggerty, D. K. 2002, ApJ, 579, 854

[9] Krucker, S., & Lin, R. P. 2000, ApJ, 542, L61

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