1. What do we Know of Solar Flares?
Hugh Hudson
SSL, UC Berkeley and U. Of Glasgow
The Sun and its corona/wind
Solar flares and CMEs
Extreme events: new facts
“Nanoflares”

2. The Sun itself
It is not obvious from this sketch, but the very thin chromosphere is the most interesting layer physically,: it separates radically different physical domains.

3. The photosphere-corona interface region
• Ion-neutral physics
• Transition of beta
• Collisionality horizon
• Optical depth unity
• Big temperature jump
• Convection threshold
• Flare energy appears
Inexplicably, this physics-laden domain
(the chromosphere/TR) is often taken as a boundary for numerical simulations!

4. Photosphere to chromosphere
1972
1859
2001
Chromosphere to corona

5. Definitions
• A solar flare is the sudden electromagnetic radiation associated with a (coronal) magnetic energy release.
• A coronal mass ejection (CME) is a catastrophic expansion of a part of the coronal magnetic field into the heliosphere.
And implicitly…
• Both aspects of major activity involve complex physical processes and cannot be understood simplistically.
• In particular, highly non-thermal particles dominate the energetics of these events.

6. A flare/CME observed by TRACE

7. Notes on theories
• The energy that drives a flare/CME comes from parallel current systems in the corona, driven from below.
• The most-developed theory is MHD and requires liberal use of magnetic reconnection.
• The system is so complicated that the physics typically is dealt with in the domain of numerical simulation.
• A flare or CME requires a magnetic implosion to release the energy:

8. Flare theory in cartoons
Sturrock, 1966

9. How does flare energy flow?
electron beam
chromosphere
Kane & Donnelly, ApJ 164, 171 (1971) – basically, the “thick-target model” (courtesy L. Fletcher)
Strauss & Papagiannis, ApJ 164,
369 (1971) – basically, “CSHKP”

10. My new favorite cartoons
Russell et al. 2015
Janvier et al. 2013

11. The problem of the power law:
a break is required for flare energies
Akabane, 1956
Crosby et al., 1993

12. Can we see the break in SEPs?
Lingenfelter & Hudson 1980
Kovaltsov & Usoskin 2014

13. Implications of the power law
Superflares could cause horrendous effects on the Earth.
There’s a “Black Swan” twist to the statistics (see N. Taleb’s interesting 2007 book)
We cannot know the extent of the power law because of infrequent occurrence, but two new proxy possibilities have recently appeared: Kepler “superflares” and actual 12C events in tree rings.

14. The Kepler “superflares”
• “Starspots” are blamed for these superflares.
Maehara et al., 2015

15. The sad fate of Kepler-438b
• This very Earth-like planet has been found to be bombarded by “superflares” – hence, likely no atmosphere (Armstrong et al. 2015).

16. The Kepler “superflares”
Aulanier et al. 2014
“Give me a big spot, and I can give you a big flare.”

17. Solar-stellar quandary
Willson et al. 1971
Maehara et al. 2012
• These light curves could not be more different. The solar paradigm does not work!
• Because of this failure of the paradigm, it is premature to use this proxy to extend our solar statistics.

18. Extreme events in tree rings
Miyake et al. 2013

19. Usoskin & Kovaltsev 2013
Jull et al. 2014
Büntgen et al
Liu et al

20. Extreme events
The Kepler superflares and the radiosotope events suggest that powerful solar flares might occur.
- The proxy is not understood.
Discrete 14C events have been found.
- The weight of evidence suggests that these were solar (Mekhaldi et al. 2015).

21. Where would these events fit?

22. Nanoflares Power-law d = 4 Power-law d = 1.8
• Toy model of shot noise distinguishes flares and nanoflares (Hudson, 1991).
• The noise-like component of weak stellar variability may well conceal the presence of episodic heating.
• Many searches for solar nanoflare signatures continue, without compelling evidence but still great anticipation.

23. Conclusions
New Kepler photometry reveals “extreme events” on other stars.
Tree rings may extend our knowledge of solar CME occurrence patterns.
Parker’s nanoflares may be lurking in the quiescent solar/stellar variability.