Abstract
Abstract During the 24th solar cycle, the Fermi Large Area Telescope (LAT) has observed a total of 27 solar flares possessing delayed γ-ray emission, including the exceptionally well-observed flare and coronal mass ejection (CME) on 2017 September 10. Based on the Fermi/LAT data, we plot, for the first time, maps of possible sources of the delayed >100 MeV γ-ray emission of the 2017 September 10 event. The long-lasting γ-ray emission is localized under the CME core. The γ-ray spectrum exhibits intermittent changes in time, implying that more than one source of high-energy protons was formed during the flare–CME eruption. We find a good statistical correlation between the γ-ray fluences of the Fermi/LAT-observed delayed events and the products of corresponding CME speed and the square root of the soft X-ray flare magnitude. Data support the idea that both flares and CMEs jointly contribute to the production of subrelativistic and relativistic protons near the Sun.
Highlights
Impulsive–gradual classification of solar energetic particle (SEP) events was formulated for electrons and ions with energies well below 100 MeV (Cane et al 1986; Cliver 1996, and references therein)
The partial correlation analysis indicated that the only significant quantities of the eruptions were the coronal mass ejection (CME) speed and the soft X-ray fluence, suggesting that both the solar flare and the CME shock contribute to the SEP production in large events
The deduced location of the delayed γ-ray source is within ≈20° of the heliocentric angle around the flare site, not what was expected in the CME–bow shock backprecipitation model by Kouloumvakos et al (2020)
Summary
Impulsive–gradual classification of solar energetic particle (SEP) events was formulated for electrons and ions with energies well below 100 MeV (Cane et al 1986; Cliver 1996, and references therein). In the context of extreme SEP events affecting the Earth’s atmosphere and systems in space and on the ground, solar protons of much higher energies, 300 MeV (e.g., Cliver et al 2020), are essential Protons of such high energies, if they precipitate back to the Sun, can produce, via nuclear interactions, the >100 MeV γ-rays detected by the Fermi Large Area Telescope (LAT; Atwood et al 2009). An alternative hypothesis reads in application to the γ-ray flare and the ground level enhancement (GLE) event on 2017 September 10 that most of the high-energy protons, including even protons interacting during the flare’s impulsive phase, were accelerated by the CME-driven shock outside the flaring active region (Kouloumvakos et al 2020). We show that such an understanding is not correct for events like 2017 September 10, in which the γ-ray emission’s centroid is located close to the solar limb
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