Solar and Stellar Flares Hugh S. Hudson SSL, UC Berkeley 1 May 20111

Outline Background Basic physics New developments Outstanding problems 1 May 20112

Three kinds of flares 1 May Stellar (Kowalski et al. 2010) Solar (Woods et al. 2004) Some planet (HST)

1 May Impulsive phase and gradual phase of a solar flare Impulsive phase – primary energy release hard X-rays (10s of keV) white light, UV,  waves - broad spectrum duration < few minutes intermittent and bursty time profile, 100ms largely from the chromosphere Gradual phase - response to input thermal emission (kT ~0.1-1 keV) rise time ~ minutes coronal reservoir at work Impulsive phase: > few tenths of the total flare energy released (up to ergs) Significant role for non-thermal electrons CME acceleration

The solar astronomer’s view 1 May A 1D model with no flows or plasma physics Full RT analysis links each emission to a height range cf. the new Asplund et al. abundances, justifying better models

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AIA on SDO 1 May

Outline Background Basic physics New developments Outstanding problems 1 May

Basic Physics The transition between the low-beta corona and the high-beta photosphere Radiative transfer structurally important Most flare theory is in the MHD framework RHESSI and particle acceleration 1 May

Coronal view of a flare 1 May

1 May

Chromospheric view of a flare 1 May Inverted colors showing continuum emission from the chromosphere in a 32 arc s domain From other data, we understand that this is the main flare luminosity It is very intense, and mostly unresolved in space and time

The world of cartoons 1 May

The world of cartoons 1 May

Four environments In a wind: Dungey Magnetar: Duncan In a static structure: Gold-Hoyle With disk: Shu 1 May

The flare problem How does one extract energy suddenly from a stressed low-beta plasma? - In solar flares, we see hard X-rays resulting from ~30 keV electrons that appear to contain most of the converted energy - We also, in major events, see  -rays implying comparable energy in accelerated ions - In events with coronal mass ejections, the interplanetary particle acceleration can have 10% of the flare energy (Mewaldt), hence collisionless shocks are also important 1 May

Outline Background Basic physics New developments – two paradigms in doubt Outstanding problems 1 May

New Tools RHESSI – hard X-ray and  -ray imaging STEREO – true stereoscopic views of solar plasma SDO – comprehensive imaging and Sun-as-a-star spectroscopy through the EUV 1 May New Insights The chromosphere is after all the source of the main physics of a flare Stellar flares provide excellent tests of the ideas Concepts from the terrestrial aurora are important

Stellar flares 1 May YZ Cmi flare (Kowalski et al. 2010) Optical spectroscopy suggests that “a small A0 star appears briefly in the M star atmosphere” Also powerful Balmer lines and continuum; an optically thin source Solar spectroscopy is not this good II Peg flare (Osten et al. 2007) Hard X-ray signature captured Link to solar-type mechanisms

Stellar flares 1 May YZ Cmi flare (Kowalski et al. 2010) Optical spectroscpy suggests that “a small A0 star appears briefly in the M star atmosphere” Also powerful Balmer lines and continuum Solar spectroscopy is not this good II Peg flare (Osten et al. 2007) Hard X-ray signature captured Link to solar-type mechanisms

Auroral physics 1 May The coronal magnetic energy, at low plasma beta, must be released as field distortions (waves) The Alfvenic Poynting flux is therefore a likely transport mechanism (Fletcher & Hudson 2008) The chromosphere is a likely place for flux transfer (Haerendel 2011)

Alfven waves or particle beams? 1 May Properties of the low corona above a sunspot (ref. Allen’s Astrophysical Quantities) At 10 Mm, find 10 3 G, V A =c/3, and beta < Alfvén speed and plasma beta are similar to values in the geotail, but B is much larger

Paradigms at peril 1 May The thick-target model - Energy too intense (Krucker et al. 2011) - No hard X-ray directivity - No hard X-ray polarization 2.The standard reconnection scenario - seldom seen - no chromosphere - stability problem chromosphere electron beam Kopp-Pneuman 1976

Hard X-ray and  -ray imaging 1 May Flare SOL , TRACE background image/ RHESSI contours Hard X-rays (red contours) do not match  -rays (blue contours) (Hurford et al. 2006) These phenomena correlate in occurrence (Shih et al 2009) No reasonable mechanisms yet proposed

Outline Background – the solar atmosphere Basic physics New developments Outstanding problems 1 May

Outstanding problems Formation of small A0 star (Kowalski) Acceleration of high-energy ions and electrons Unexplained 511 keV line width * Sunquake excitation * * sorry, no time to discuss... 1 May

Conclusions The physics of solar and stellar flares has strong commonalities We should study the terrestrial aurora to learn about the microphysics NuSTAR will solve some solar problems * * sorry, no time to discuss them here 1 May