Energetic Neutral Hydrogen from Large Solar Flares

Nugget
Number:	435
1st Author:	Glenn MASON
2nd Author:
Published:	6 August 2022
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Previous Nugget:	Fifty-year Anniversary of the First Detection of Gamma rays from a Solar Flare

Introduction

Neutral radiations from the Sun include photons, of course, but also the much harder-to-detect neutrons and neutral atoms. Free neutrons decay with a half-life of about 10.3 minutes, and so must have relativistic energies to survive the trip from Sun to Earth. Neutral atoms require very special instrumentation, given their propensity to ionize and become charged. All solar neutral radiations have the special property that their propagation does not depend upon the ill-understood coronal magnetic field, so they provide direct views of their sources.

Energetic particle instruments on the STEREO-A and -B spacecraft discovered energetic neutral hydrogen atoms emitted during the flare SOL2006-12-05 (X9) [Ref. 1]. A small pulse of few-MeV particles was observed from the solar direction, wth arrival times coinciding with the 1-8 Â GOES x-ray profile from the same event. Though this event was followed by many large solar flares in the next activity maximum, no further cases were reported, leaving open the question regarding the rarity of such a case.

Several years earlier, using data from the SAMPEX satellite, Ref. [2] had reported increases in equatorial low altitude few-MeV particles quasi-trapped near the Earth's magnetic equator, from 1 to 4 hours after major X-ray flare events but before the onsets of their associated geomagnetic storms. The authors were unable to describe a mechanism by which low energy protons from the flares could rapidly penetrate to the equatorial region where the geomagnetic cutoff rigidity is many GV.

Observations

While low energy solar flare protons cannot penetrate to the low-altitude equatorial region, energetic neutral hydrogen atoms can, and if some of them then lose their electron through interaction with the upper atmosphere they can become trapped. This mechanism is similar to the trapping of low altitude equatorial particles or anomalous cosmic rays in the magnetosphere [Ref. 3]. We examined SAMPEX data for the SOL2006-12-05 and found a similar increase, roughly coinciding with the previously reported ENA arrival times from STEREO [Ref. 4]. Figure 1 shows this result.

Figure 1: SAMPEX detector count rates versus time during SOL2005-12-06. Left, several latitude scans, with the orange line at top showing interval during which ENAs were detected on STEREO (Ref. [1]). Right: count rates during the equatorial pass with maximum signal, showing a decrease as the spacecraft maneuvers near the equator, scanning different pitch angles.

The equatorial increase seen by SAMPEX was organized by pitch angle, indicating trapped particles whose altitude would lead to loss when they drifted into the South Atlantic Anomaly after a few hours. Surveying the SAMPEX data, a total of 10 events were found between 1997-2006, with timing following large X-ray flare sources by typically about 3 hours. An additional 9 events were identified whose proton increases were smaller but which still stood out clearly in the data. This survey gives strong evidence that ENAs can be routinely emitted in large solar flares. Because of the geomagnetic field shielding, such studies are ideally carried out in low Earth orbit since the comparatively huge increase in SEP protons is swept away and doesn't bury the relatively small signal of the ENAs.

Significance

While ENAs would be expected to be copiously produced as SEP protons interact with the lower corona, they would become stripped as they moved through the corona towards interplanetary space. However, if produced at high altitudes in association with the CME-driven shock in such events, they could then escape as neutrals and be detected [Ref. 1] Their arrival direction at Earth would point to their source since they are unaffected by the interplanetary magnetic lield. This opens a surprising new technique for imaging sources of shock-associated SEPs, revealing details of the source location and timing in regions too close to the Sun to be visited by spacecraft, and giving the opportunity to probe many unanswered questions about the acceleration mechanisms for SEPs.