Energetic proton bursts downstream of an interplanetary shock

(Solar Orbiter Nugget #62 by L. Yang¹, X.-Y. Li^2,1, V. Heidrich-Meisner¹, R. F. Wimmer-Schweingruber¹, L. Wang² , A. Kollhoff¹, X. Zhu³, G. Nicolaou⁴, Z. Ding¹, L. Berger¹, H. Liu², J. Rodríguez-Pacheco⁵, G. M. Mason⁶, and G. C. Ho⁷)

1. Introduction

Interplanetary shocks are well-established sources of energetic charged particles from tens of keV to several MeV (e.g., [1]). Despite extensive research, detailed mechanisms and processes of proton acceleration at interplanetary shocks continue to pose challenges. Recent high-resolution measurements from the Energetic Particle Detector (EPD; [2]) on board the Solar Orbiter spacecraft ([3]) have significantly advanced our ability to investigate proton dynamics in the vicinity of interplanetary shocks (e.g., [4, 5]). In this study, we combine data from two EPD sensors—the SupraThermal Electron Proton (STEP) sensor and the Electron-Proton Telescope (EPT)—to perform a comprehensive analysis of energetic proton bursts observed downstream of an interplanetary shock. This combined dataset enables the investigation of proton behavior across a wide energy range with broad angular coverage and high angular resolution. We examine the temporal flux profile, pitch-angle distribution (PAD) in the solar wind frame, and energy spectrum of these proton bursts, and discuss their potential acceleration mechanisms and physical implications.

2. Data and Method 

The EPD suite on board Solar Orbiter provides electron and ion measurements spanning energies from a few keV to several hundred MeV [2]. The EPT sensor consists of two pairs of telescopes: one pair points sunward and anti-sunward along the orbit-averaged nominal Parker spiral, while the other pair points northward and southward. Each telescope has a circular field of view (FOV) of ~30° and measures ions from ~50 to ~6000 keV in 64 energy bins with a cadence of one second. The STEP sensor consists of a 3×5 multipixel detector array with a FOV of ~30×60° around the orbit-averaged nominal Parker spiral. Each pixel measures ions from ~6 keV to ~60 keV in 32 energy bins with a cadence of one second. Each pixel also has an individual FOV of ~10×12°, which enables resolving the angular distribution within STEP’s FOV. Notably, STEP’s central 9 pixels share almost the same FOV with EPT’s sunward telescope. In addition, we use data from the Suprathermal Ion Spectrograph (SIS) sensor, which measures the ion composition from ~0.1-10 MeV/nucleon with sunward and anti-sunward telescopes.
In order to correct for the Compton-Getting effect, we reconstructed the PAD in the solar wind proton bulk velocity frame, under the assumption that energetic ions measured near the shock are predominantly protons. In the reconstructed PAD, EPT’s four telescopes cover most of the 0-180° pitch angles, while STEP’s multipixel array provides finer angular resolution within the FOV of EPT’s sunward telescope.

Fig. 1. Overview plot of the 2021 November 3 shock event. (a)–(b) Spectrograms of dynamic energy spectra measured by EPT’s sunward telescope (a) and STEP’s central 9 pixels (b). The horizontal dashed red line indicates the time interval during which proton bursts are observed. (c)–(d) Line profiles corresponding to the spectrograms in (a)–(b), respectively. (e) Temporal flux profiles of ~268 keV protons measured by SIS’s sunward telescope (black) and ~293 keV protons measured by EPT’s sunward telescope (pink). (f)–(h) Magnitude |B|, elevation angle θ_B, and azimuthal angle φ_B of the IMF. (i) Solar wind proton bulk speed |V_sw|.

3. Observations

On 2021 November 3 at 14:04:26, Solar Orbiter encountered an interplanetary shock with a θ_Bn of 66°±12° at a heliocentric distance of 0.84 au. Fig. 1 presents an overview of the shock event. After the current sheet that appeared ~10 minutes downstream of the shock (marked by the dashed yellow line), multiple bursts of ~20-1000 keV protons are clearly evident in the dynamic energy spectra (Figs. 1a–b, indicated by the horizontal dashed red line) and the temporal flux profiles (Figs. 1c–d). These bursts are also evident in the SIS ~268 keV proton measurements (Fig. 1e), which agree very well with the EPT ~297 keV measurements. Such joint observations by STEP, EPT, and SIS rule out instrumental effects as the cause of these observed bursts and can confirm the dominance of protons during the time period.

Fig. 2. Zoom-in plot in the vicinity of the proton bursts. (a) Dynamic energy spectrum measured by EPT’s sunward telescope. (b) Dynamic energy spectrum scaled by energy squared, measured by STEP’s central 9 pixels. (c)–(d) PAD in the solar wind frame of ~38 keV protons measured by STEP (c) and ~60 keV protons measured by EPT (d). The color indicates the normalized differential flux, which is normalized to the flux averaged over all available pitch angles for each time bin. Beamed distributions show higher values (yellowish) in the beaming direction and lower values (deep blue) in other directions. (e) Trace PSD of the magnetic field fluctuations in units of nT2/Hz. (f)–(h) Magnitude |B|, elevation angle θ_B, and azimuthal angle φ_B of the IMF. The blue curves in (g)–(h) show the absolute values of the variations in θ_B and φ_B, respectively. The vertical dashed lines bound several time intervals with labels at the top, which are analyzed in Fig. 3.

Fig. 2 shows a zoom-in plot in the vicinity of the proton bursts. After the current sheet, multiple proton bursts are clearly evident at energies ranging from ~20-30 keV to ~1000 keV (Figs. 2a–b). At ~14:23, a noticeable flux enhancement appears in a narrow energy range around ~200 keV. Moreover, most of the proton bursts last for ~10 to 20 s. None of them exhibit velocity dispersion feature. Based on the dynamic energy spectra and temporal flux profiles, we selected several time intervals of clear proton bursts, labeled B1 to B5 and S1 to S2, to analyze their spectral features in more detail in Fig. 3.
Figs. 2c–d present the PADs in the solar wind frame of ~38 keV protons measured by STEP and ~60 keV protons measured by EPT, respectively. EPT’s PAD show that most of the proton bursts exhibit strong anisotropy in the anti-parallel direction (e.g., B3 to B5 in Fig. 2d), suggesting that they are strong beams traveling anti-parallel to the IMF. Moreover, STEP’s proton PADs reveal that these proton beams have a narrow width of less than 30° (Fig. 2c). Fig. 2g shows that the IMF is pointing southward, substantially out of the ecliptic plane, during these proton bursts. Fig. 2e shows that the power spectral density (PSD) of 0.1-4 Hz magnetic field fluctuations exhibits no noticeable enhancements during these proton bursts. In addition, these proton bursts do not coincide with significant changes in the IMF direction (blue curves in Figs. 2g–h).

Fig. 3. Energy spectra averaged over the time intervals defined in Fig. 2. The black horizontal bars at the top indicate the energy ranges covered by STEP and EPT. Other horizontal bars mark the valley and peak of the spectral bumps in corresponding colors.

Fig. 3 presents the joint energy spectra from STEP and EPT for the time intervals defined in Fig. 2. The STEP spectra transition smoothly into the EPT spectra, exhibiting strong agreement in the overlapping energy range. The B1 and B2 spectra show prominent bumps in the ~20-80 keV range, with a valley at ~20-30 keV, a peak at ~40-50 keV, and a full width at half maximum (FWHM) of ~30 keV (see Table 1). Both spectra exhibit a positive spectral slope between the valley and peak. In contrast, the S1 and S2 spectra are smoother, with no distinct bumps. They resemble the B1 and B2 spectra at energies above ~50 keV but have significantly higher intensities in the ~10-50 keV range. The B5 spectrum exhibits two bumps: a low-energy bump in the ~30-100 keV range and an additional high-energy bump in the ~100-250 keV range with a peak at ~180 keV and an FWHM of ~50 keV (Fig. 3b). This high-energy bump is similar to the monoenergetic proton event reported by [6].

Table 1. Parameters of the spectral bumps

4. Discussion and conclusions

We investigated the energetic proton bursts observed downstream of the 2021 November 3 shock by combining data from EPD/STEP and EPT on board Solar Orbiter. These bursts typically last for ~10-20 s and span a wide energy range from ~20 to ~1000 keV. Their energy spectra typically exhibit an evident bump in the ~20-100 keV range with a peak at ~40-50 keV.
The anti-parallel motion of these proton bursts along the substantially southward IMF suggests their source region below the ecliptic plane. One candidate source could be the part of the shock front situated below the ecliptic plane. These proton bursts exhibit no velocity dispersion feature, suggesting that they continuously fill the flux tubes that they are traveling in. Moreover, their intermittent occurrence could result from the spacecraft traversing flux tubes connected to different parts of the source region with efficient or inefficient proton acceleration. Their dynamic variations may also suggest that they could be accelerated by mechanisms sensitive to small-scale structures at or near the shock, such as shock-drift acceleration or shock-surfing acceleration.
Compared with the S1 and S2 spectra, the B1 to B5 spectra exhibit noticeably lower intensities in the ~10–50 keV range, while showing comparable intensities at energies above ~50 keV. This contrast suggests that the observed spectral bumps may result from the deficit of ~10–50 keV protons, possibly caused by strong scattering during their transport from the source to the spacecraft due to their relatively short mean free paths [7].

For further details, see Yang et al., A&A, 695, A270 (2025) https://doi.org/10.1051/0004-6361/202453103
 

Affiliations

(1) Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, 24118 Kiel, Germany
(2) School of Earth and Space Sciences, Peking University, 100871 Beijing, PR China
(3) Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Huntsville, AL 35805, USA
(4) Department of Space and Climate Physics, Mullard Space Science Laboratory, University College London, Dorking RH5          6NT, UK
(5) Universidad de Alcalá, Space Research Group, 28805 Alcalá de Henares, Spain
(6) Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
(7) Southwest Research Institute, San Antonio, TX 78238, USA
 

References

[1] Lario, D., Sanahuja, B., and Heras, A. M. 1998, ApJ, 509, 415 https://doi.org/10.1086/306461
[2] Rodríguez-Pacheco, J., Wimmer-Schweingruber, R. F., Mason, G. M., et al. 2020, A&A, 642, A7 https://doi.org/10.1051/0004-6361/201935287
[3] Müller, D., Cyr, O. C. St., Zouganelis, I., et al. 2020, A&A, 642, A1 https://doi.org/10.1051/0004-6361/202038467
[4] Trotta, D., Horbury, T. S., Lario, D., et al. 2023, ApJ, 957, L13 https://doi.org/10.3847/2041-8213/ad03f6
[5] Yang, L., Heidrich-Meisner, V., Berger, L., et al. 2023, A&A, 673, A73 https://doi.org/10.1051/0004-6361/202245681
[6] Klassen, A., Gómez-Herrero, R., Müller-Mellin, R., et al. 2009, Ann. Geophys., 27, 2077 http://dx.doi.org/10.5194/angeo-27-2077-2009
[7] Mason, G. M., Von Steiger, R., Decker, R. B. et al. 1999, Proceedings of an ISSI Workshop 6–13 June 1998, Bern, Switzerland, Springer, 327 http://dx.doi.org/10.1007/978-94-017-1179-1_16