[Paper Review] Lithium-6 from Solar Flares

R. Ramaty, V. Tatischeff+3 more|ArXiv.org|Mar 23, 2000

Solar and Space Plasma Dynamics18 citations

TL;DR

This paper proposes that accelerated 3He in solar flares produces significantly more 6Li than 7Li via the 4He(3He,p)6Li reaction, explaining the solar wind's measured 6Li/7Li ratio of 0.032 ± 0.004. Using gamma-ray data to normalize flare particle spectra, the model shows that flare-produced 6Li, combined with photospheric 7Li, accounts for the observed solar wind isotopic ratio, provided most 6Li is ejected by the solar wind.

ABSTRACT

By introducing a hitherto ignored Li-6 producing process, due to accelerated He-3 reactions with He-4, we show that accelerated particle interactions in solar flares produce much more Li-6 than Li-7. By normalizing our calculations to gamma-ray data we demonstrate that the Li-6 produced in solar flares, combined with photospheric Li-7, can account for the recently determined solar wind lithium isotopic ratio, obtained from measurements in lunar soil, provided that the bulk of the flare produced lithium is evacuated by the solar wind. Further research in this area could provide unique information on a variety of problems, including solar atmospheric transport and mixing, solar convection and the lithium depletion issue, and solar wind and solar particle acceleration.

Motivation & Objective

• To resolve the discrepancy between the low photospheric 6Li/7Li ratio and the higher solar wind ratio measured in lunar soil.
• To investigate whether solar flares can produce sufficient 6Li to account for the observed solar wind isotopic composition.
• To assess the role of accelerated 3He in enhancing 6Li production through nuclear reactions in solar flares.
• To evaluate the contribution of flare-produced lithium to the solar wind, considering particle acceleration and atmospheric transport.

Proposed method

• The study uses a nuclear reaction code to calculate 6Li and 7Li production from accelerated particles in solar flares, including the 4He(3He,p)6Li reaction.
• Cross sections for the 4He(3He,p)6Li reaction are derived using detailed balance from the inverse 6Li(p,3He)4He reaction and include contributions from the 3.56 MeV excited state.
• Thick-target models are applied, assuming particles slow down in the chromosphere, with particle energy spectra modeled as power laws with index s.
• Gamma-ray data from 20 flares are used to constrain the average proton flux and 3He/4He ratios, with 3He/4He ratios ranging from 0.1 to 1.
• The model normalizes 6Li production to observed gamma-ray emissions and compares the resulting 6Li yield to the solar wind 6Li/7Li ratio.
• The solar wind 6Li/7Li ratio is calculated using the formula (6Li/7Li)sw = [Np(>30) × Q(6Li)] / [(7Li/H)ph × Fsw], with Fsw ≈ 6×10^35 s⁻¹.

Experimental results

Research questions

• RQ1 Can accelerated 3He in solar flares produce enough 6Li to explain the observed solar wind 6Li/7Li ratio of 0.032 ± 0.004?
• RQ2 What is the contribution of the 4He(3He,p)6Li reaction to 6Li production compared to other reactions like αα interactions?
• RQ3 How do variations in 3He/4He and α/p ratios affect the predicted 6Li yield in solar flares?
• RQ4 To what extent does flare-produced 6Li survive in the solar atmosphere, and what fraction is evacuated by the solar wind?
• RQ5 Can the observed gamma-ray emission from flares be used to reliably constrain the production of lithium isotopes?

Key findings

• The 4He(3He,p)6Li reaction significantly enhances 6Li production, especially at steep particle energy spectra and high 3He/4He ratios.
• With 3He/4He > 0.1 and α/p > 0.1, the model predicts (6Li/7Li)sw between 0.007 and 0.06, consistent with the observed value of 0.032 ± 0.004.
• The average 6Li production rate from flares is estimated at 1×10^22 to 7×10^22 atoms s⁻¹, depending on 3He/4He ratios.
• Only about 15% of the total 6Li production is attributed to the 19 largest SMM flares, indicating that smaller, unobserved flares may contribute significantly.
• The model supports that flare-produced 6Li is likely ejected by the solar wind, as the photospheric 6Li/7Li ratio is too low to explain the solar wind value.
• The results suggest that direct measurements of lithium isotopes in the solar wind and spectroscopic detection of 6Li in the photosphere could resolve key uncertainties in solar atmospheric transport and particle acceleration.