Hugh Hudson SSL UC Berkeley and U Of Glasgow 1 The Sun and its coronawind Solar flares and CMEs Extreme events new facts Nanoflares The Sun itself 2 It is not obvious from this sketch but the very thin ID: 483401
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Slide1
What do we Know of Solar Flares?
Hugh HudsonSSL, UC Berkeley and U. Of Glasgow
1
The Sun and its corona/wind
Solar
flares and
CMEs
Extreme events: new facts
“
Nanoflares
”Slide2
The Sun itself
2It is not obvious from this sketch, but the very thin chromosphere is the most interesting layer physically,: it separates radically different physical domains.Slide3
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!Slide4
Photosphere to
chromosphere
Chromosphere
to corona
1859
1972
2001Slide5
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.Slide6
A flare/CME observed by TRACE
6Slide7
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
f
lare
or CME requires a
magnetic implosion
to release the energy:
Slide8
Flare theory in cartoons
8http://solarmuri.ssl.berkeley.edu/~hhudson/cartoons/Sturrock, 1966Slide9
How does flare energy flow?
chromosphere
electron beam
Strauss &
Papagiannis
,
ApJ
164,
369 (1971) – basically, “CSHKP”
Kane & Donnelly,
ApJ
164, 171 (1971) – basically, the “thick-target model”
(courtesy L.
Fletcher)
DSlide10
My new favorite cartoons
Russell et al. 2015
Janvier et al. 2013Slide11
The problem of the power law:
11Akabane, 1956
Crosby et al., 1993
a
break
is
required for flare energies Slide12
Can we see the break in SEPs
?12Kovaltsov & Usoskin 2014
Lingenfelter & Hudson 1980Slide13
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.
13Slide14
The Kepler “
superflares”14• “Starspots” are blamed for these superflares.
Maehara
et al., 2015Slide15
The sad fate of Kepler-438b
15• This very Earth-like planet has been found to be bombarded by “superflares” – hence, likely no atmosphere (Armstrong et al. 2015).Slide16
The Kepler “
superflares”16“Give me a big spot, and I can give you a big flare.”
Aulanier et al. 2014Slide17
Solar-stellar quandary
• 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.
Maehara et al. 2012
Willson
et al. 1971Slide18
Extreme events in tree rings
18Miyake et al. 2013Slide19
19
Usoskin & Kovaltsev 2013
Jull et al. 2014
Büntgen
et al. 2014
Liu et al. 2014Slide20
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).
20Slide21
Where would these events fit?
21Slide22
Nanoflares
22Power-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.Slide23
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