Energetic neutral atoms detected in the large solar energetic particle event of February 2022

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Nugget
Number: 486
1st Author: Christina COHEN
2nd Author:
Published: 20 January 2025
Next Nugget: Evaporation and rain on the Sun
Previous Nugget: Magnetic topology of quiet-Sun Ellerman bombs and associated ultraviolet brightenings



Energetic Neutral Atoms

We study the Sun via a wide variety of "messengers," not limited just to electromagnetic radiation. These include MeV-energy neutral atoms (hydrogen), as shown in a previous case (Ref. [1]). This Nugget reports a second case. We don't have instrumentation specifically designed to detect these unusual particles, which accounts for their rarity. But the physics is important.

Energetic neutral atoms (ENAs) are expected to be created from solar energetic particles (SEPs) via "charge exchange" interactions near the Sun. For ENAs to escape the Sun and be observed in-situ, they need to be generated in a region where the ambient densities are high enough for the original SEP to be ionized but quickly move to a region where the ambient densities are not so high that they are likely to be re-ionized before escaping. Once the ENAs have left the Sun, they are not influenced by the interplanetary magnetic field as SEP ions are. This property of neutral charge makes solar ENAs a direct probe of the acceleration processes and conditions at work close to the Sun.

Unfortunately, the lack of dedicated solar ENA instruments on current spacecraft means it is very difficult to detect solar ENAs in-situ. SEP instruments with the appropriate configurations can observe them, but as the ENAs enter such instrument they are stripped of their electrons and thus appear as ions in the detectors. Distinguishing ENAs from SEP ions relies on differences in arrival time and arrival direction; ENAs will travel radially outward from their source region while SEPs will generally follow the interplanetary magnetic spiral. For the same energy, ENAs will arrive sooner and from a more radial direction than SEP ions.

The Low-Energy Telescope (LET)

The LET instrument on the STEREO spacecraft measures SEPs from ~1 to tens of MeV/nuc. It is capable of measuring particle arrival direction including along the spacecraft-Sun direction. Ref. [1] used LET to discover the first solar ENA event from STEREO, via SEPs associated with the major flare SOL2005-12-05. Indirect observations of solar ENAs have also been reported (see the earlier Nugget), but no other direct observations had been reported until a clear signature was observed by STEREO-A/LET associated with the SOL2022-02-15 coronal mass ejection (Ref. [2]), which we describe here.

The ENAs were clearly identifiable and distinct from the SEPs as they arrived well before the initial rise of the SEP event intensities (Figure 1, left); their arrival directions were strongly peaked in the direction of the Sun and not along the interplanetary magnetic field direction (Figure 1, right). The length of their trajectories from the Sun (derived from their observed arrival time and energies) was consistent with the radial distance of STEREO-A and not the longer length calculated for the SEPs (Figure 2, left panel).

Figure 1: Left, Intensities of 1.83.6 MeV protons as a function of time for three sectors. Ranges of angles (relative to the spacecraft-Sun line) for each sector are given in the legend; note that sector 7 includes directions radially from the Sun. Right, Angular distribution of the ENAs and protons for the ENA time period (blue line) and an earlier time period (red line). To increase statistical accuracy, the selected earlier time is 5 times longer; thus, for quantitative comparison, the red histogram is scaled by a factor of 5. The range of the interplanetary magnetic field angles for the ENA time period, as indicated by the black arrow, was 21.878 (and primarily varying between 40 and 70).

Interpretation

Comparison of the calculated ENA energy spectrum to that expected from models (Ref. [3]) suggests that the ENAs were generated near the shock driven by the CME, rather than in large post-flare loops on the Sun. This would naturally explain the high kinetic energies of the neutral particles, which originated as charged SEPs that had been accelerated by the large-scale shock driven by the CME. Such a CME-related source was also surmised to be the source of the original event (Ref. [1]). Comparison of the observed height of the CME at the calculated time the ENAs escape indicates the CME was at ~2-4 solar radii (Figure 2, right panel).

Figure 2: Left, inverse velocity vs. time for all protons (red points) and the ENAs (blue). Dashed lines show expected velocity dispersion for path lengths of 0.967 AU (the distance to STEREO-A) and 6 AU (the latter also has a later release time to better correspond to the apparent SEP onset). Right, Height as a function of time for the leading edges of the CME (black points) and the prominence (red points). For comparison, the release time for each ENA is overplotted. The time for light to travel from the Sun to STEREO-A is added to enable direct comparison to remote-sensing data.


ENAs such as we have observed probably occur with every CME and shock, and of course a dedicated instrument would become a powerful tool for investigating the physics of particle acceleration in the near-Sun solar wind.


References

[1] "STEREO Observations of Energetic Neutral Hydrogen Atoms During the 2006 December 5 Solar Flare"

[2] "Energetic Neutral Atoms Detected in the 2022 February 15 Solar Energetic Particle Event"

[3] "Modeling Solar Energetic Neutral Atoms from Solar Flares and CME-driven Shocks"