Energetic neutral atoms detected in the large solar energetic particle event of February 2022
Nugget | |
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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).

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).

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"