What do we Know of Solar Flares Hugh

What do we Know of Solar Flares? Hugh Hudson SSL, UC Berkeley and U. Of Glasgow 1) 2) 3) 4) The Sun and its corona/wind Solar flares and CMEs Extreme events: new facts “Nanoflares” 1

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. 2

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!

Photosphere to chromosphere 1972 1859 2001 Chromosphere to corona

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. 5

A flare/CME observed by TRACE 6

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: 7

Flare theory in cartoons Sturrock, 1966 http: //solarmuri. ssl. berkeley. edu/~hhudson/cartoons/ 8

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

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

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

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

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 12 C events in tree rings. 13

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

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

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

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.

Extreme events in tree rings Miyake et al. 2013 18

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

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

Where would these events fit? 21

Nanoflares Power-law d = 1. 8 Power-law d = 4 • 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. 22

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. 23