1. Extreme solar particle events: what Is the worst case scenario?
Ilya Usoskin
University of Oulu, Finland

2. Spectra of SEP events

3. SPE occurrence rate
Extrapolations: Lingenfelter & Hudson (1980); Miroschnichenko & Nymmik (2012)

4. Cosmogenic radionuclides 14C and 10Be: last 11 millennia

5. Atmospheric cascade
10Be in ice cores
14C in tree rings

6. Potential signature in annual 10Be
NGRIP series: peaks > 1.3*104 at/g
1436, 1460, 1650, 1719, 1810, 1816, 1965
Dye3 series: peaks > 0.6*104 at/g
1462, 1479, 1505, 1512, 1603
Cross-check performed

7. Some candidates in IntCal13 series
Miyake et al. Radiocarbon, (2017)

8. Statistical results
Kovaltsov & Usoskin, Sol.Phys, 2014

9. Specific event of 775 AD

10. 14C signal

11. 10Be signal
Sukhodolov, Usoskin, Rozanov et al., Sci. Rep., 2017

12. Energy spectrum of 775 AD event
30 MeV –
a factor of 2;
200 MeV – a factor of 40
Mekhaldi et al. (2015)

13. Hardness of GLE events Asvestari et al., ASR, 2017 Weak Moderate
Strong events
Asvestari et al., ASR, 2017

14. The worst case scenario?

15. Events to look for in Δ14C

16. Summary
Four potential candidates with F30=(1÷1:5)*1010 cm-2 and no events with F30>2*1010 cm-2 identified since 1400 AD in the annually resolved 10Be data.
For the Holocene, 20 SPE candidates with F30=(1÷5)*1010 cm-2 are found in the 14C and 10Be data and clearly no event with F30>5*1010 cm-2.
The greatest event was ca. 775 AD F30~5*1010 cm-2. It may serve as the worst case scenario.
On average, extreme SPEs contribute about 10% to the total SEP flux.
Practical limits are: F30≈1, 2÷3, and 5*1010 cm-2 for the occurrence probability ≈ 10-2, 10-3 and 10-4 year-1, respectively.

17. case studies of all events.
Plan
From candidates 
case studies of all events.

19. Lunar/meteoritic samples
14C activity in a lunar sample (Jull et al., 1998).