Three-dimensional properties of a coronal shock and the longitudinal distribution of its related solar energetic particles

TL;DR

The paper tackles how the 3D evolution of a CME-driven shock governs the longitudinal spread of solar energetic particles by reconstructing the shock surface from multi-view observations using a mask-fitting method and deriving upstream parameters with a data-driven AWSoM background solar wind model. By linking cobpoints on the evolving shock to in situ proton measurements, the study finds the shock nose exhibits the strongest acceleration (high normal speed, compression ratio, and $M_A$) while flanks accelerate less efficiently, and confirms that the observed proton spectra largely align with relativistic diffusive shock acceleration predictions. The SEP onset and spectral variations across spacecraft reflect the combined influence of 3D shock geometry, magnetic connectivity, and transport effects, with STB showing the earliest and most rapid enhancement due to earlier connectivity. The results bolster DSA as the mechanism for this event's early-phase acceleration and demonstrate the value of 3D shock reconstruction and magnetic connectivity modeling, while acknowledging limitations from using a steady background wind and the role of transport processes; future work will couple CME-driven shocks with particle transport models for improved interpretation.

Abstract

This study aims to investigate the relationship between the spatial-temporal evolution of shock properties and the longitudinal dependence of SEP intensities and spectra. The shock parameters, including the normal speed, oblique angles, compression ratio, and Alfven Mach number, were derived by combining a steady-state solar-wind simulation with the three-dimensional (3D) reconstruction of the shock surface based on multi-view observations. We compared the local shock parameters at the magnetic connecting points with in situ proton intensities and peak spectra to establish the link between shock evolution and SEP characteristics. The shock nose consistently exhibited higher particle-acceleration efficiency with the largest normal speed, compression ratio, and supercritical Alfven Mach number, while the flanks showed delayed transition to supercritical Alfven Mach number with weaker efficiency. The earliest and most rapid proton enhancement of STEREO-B correlated with efficient shock acceleration and prompt magnetic connectivity to the shock. Spectral analysis revealed that proton energy spectra were consistent with the relativistic diffusive shock acceleration (DSA) estimations. The initial shock acceleration began at about 1.4-5 Rsun and caused the widespread longitudinal SEP distribution. The longitudinal dependence of SEP intensity and spectral variations arise from the combined influence of 3D shock properties, magnetic connectivity, and particle transport processes. The agreement between in situ proton indices and relativistic DSA estimations supports DSA in this SEP event and provides insights into the early-stage acceleration at the source region.

Figures (5)

• Figure 1: Comparison of solar-wind velocity, proton density, proton temperature, and magnetic fields observed by OMNI (left panel) and STA (right panel) with the steady-state solar-wind model for Carrington rotation 2121. The blue shading represents the passage of ICMEs.
• Figure 2: Spatio-temporal evolution of shock-normal speed, oblique angle, compression ratio, and Alfvén Mach number from top to bottom, respectively (see Movie 1 for the dynamic evolution). The gray sphere represents the Sun. Coordinates are defined in the HGC frame, with the negative X-axis directed toward Earth at 18:00 UT on 2012 March 18. The axes are in units of the solar radius.
• Figure 3: (a) Positions of STA, STB, and Earth at 00:00 UT on 2012 March 7. The black arrow represents the direction of the CME propagation. (b) 1-hour averaged proton intensities measured by SOHO/ERNE. (c) 1-hour averaged proton intensities measured by STA/HET. (d) 1-hour averaged proton intensities measured by STB/HET. The black plus signs represent the peak flux of each energy range. The inset panels display the power-law fitting of peak flux, and the fit spectral indices are indicated. The shaded region is the time interval used for the fluence spectrum.
• Figure 4: (a) Polar velocity in ecliptic plane from the SC and IH components during Carrington rotation 2121. The positions of STA, STB, and SOHO on 2012 March 7 are marked by pink, brown, and black dots, respectively. The black arrow represents the propagating direction of the associated CME. The solid lines represent the magnetic-field lines connecting each observer with the Sun. (b)-(d) Compression ratios of the reconstructed shock at 00:20:20 UT, 00:39:00 UT, and 00:54:00 UT are shown. The solid yellow, green, and purple lines are the magnetic-field lines connecting SOHO, STA, and STB to the SC component, respectively. The sphere represents the Sun. The coordinate system and axis scales are the same as those in Figure \ref{['fig:fig2']}.
• Figure 5: (a) Longitudinal distribution of shock parameters on ecliptic plane at 00:54 UT. Vertical dashed lines indicate the longitudinal locations of each observer. (b) Temporal evolution of shock parameters at 00:20:20 UT, 00:25:40 UT, 00:39 UT, and 00:54 UT.