U.W., April 14, 2005
Solar flares in the new millennium
H.S. Hudson
SSL/UCB
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Outline
• Physical and historical background
• The nature of the corona
• Current problems in flare research
• Flares, especially as viewed by RHESSI
• Conclusions
• Miscellaneous RHESSI things
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Conclusions
• New things this millennium: CMEs, helicity, -rays
• RHESSI is allowing us to understand the dominant role of accelerated particles
• Theoretically, we are forced to go beyond MHD theory and magnetic reconnection
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Two missing links:Heaviside and Rontgen
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“Sudden Flare Effect”
• Compass deflections result from enhanced ionospheric current system
• Recent suggestion that the ionization causing this results from -rays, not soft X-rays
• Echos of the physics involved in magnetar and lightning -ray behavior
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TRACE EUVobservations
Issues…
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TRACE 1600A
TRACE 195A
Shrinkage, dimming, oscillation
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G. A. Gary, Solar Phys. 203, 71 (2001)
(vA ~ 200 -1/2 km/s)
CH
Distribution of coronal plasma
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Field and energy are concentrated in active regions
• Active-region magnetic fields via Roumeliotis-Wheatland technique (McTiernan)
• Mass loading via empirical law (Lundquist/Fisher)
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NOAA 10486, Haleakala IVM data, cube
Roumeliotis-Wheatland-McTiernan method64x64x64x ~3000 km
Scaled Not scaled
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Lundquist et al., SPD 2004
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Summing up the corona
• It’s like a spherical condenser filled with a low- dielectric (about 700 F?)
• The upper boundary is the solar wind, which is massive
• The lower boundary (the “transition layer”) is extraordinarily complex and not at all understood yet
• Mysterious things happen: flares and CMEs
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Coronal Mass Ejection
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What is the mass of the solar wind?How big is a CME?
• Mcorona = 1.3 x 1018 g (1-10 Rsun) • Msw = 0.7 x 1018 g (10-100 Rsun) • Mheliosphere = 7 x 1018 g (100-1000 Rsun) • Minfinity = infinite g (universe)
• CME = 0.13 sr (40o FWHM)
Withbroe (1988) “quiet corona” modelwith 1/r2 extension
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Current problem areas
• How does magnetic energy penetrate the corona/photosphere boundary?
• Why does coronal magnetic reconnection not readily happen?
• How does particle acceleration work in solar flares?
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Emslie et al., JGR (2004)
Hudson 2005?
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Major breakthrough!(Woods et al., GRL 2004)
• First bolometric observations of a solar flare (SORCE satellite)
• Detection of the impulsive phase
• Background noise essentially from the p-modes
~ 300 mag
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RHESSI
• Particle acceleration is key to understanding
• RHESSI can image not only hard X-ray sources, but -rays as well
• RHESSI has extraordinary sensitivity
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Reuven Ramaty1937 – 2001
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SPECTROSCOPY
π0 Decay
Nonthermal Bremsstrahlung
Thermal Bremsstrahlung
Composite Solar Flare Spectrum
Positron and NuclearGamma-Ray lines
T = 2 x 107 K
T = 4 x 107 K
Fe
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10-100 keV electrons
• The Neupert effect
• The soft-hard-soft spectral pattern
• “Escape” into the heliosphere
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The Neupert effect
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The soft-hard-soft pattern
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Flare image morphology
• Ribbons and footpoints in hard X-rays
• Conjugacy
• Gamma-ray sources
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EUV flare ribbons and hard X-ray footpoint sources
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Gamma-ray imaging too…
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Problems
• Why are the hard X-ray footpoints so compact, when the ribbons are extended?
• Why are the -ray sources displaced from the X-ray sources?
…prosaic reasons? Interesting reasons?
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A major problem: bremsstrahlung is inefficient, and in a major event we need as many as 1036 e-/s.
But the footpoint areas are small!
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IR 1.56m observations (Xu et al., 2004;should show the opacity minimum height)
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Hard X-ray footpoint behavior (S. Krucker)
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Velocity vs. rate of energy loss of electrons
Nine intervals, nine spectra;thick target model energy deposition(rate of energy loss by non-thermal electrons in footpoints)
2
Reconnection rate d/dt= B v av= velocityB= magnetic field strengtha=footpoint diameter
B hard to observe for near limb flareB~1000 G; a~2000km
d/dt ~ 2e18 Mx/s E ~ 5 kV/m
Higher rate of reconnection produces more energetic electrons per unit time
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E
timeRHESSI Soft X-rays
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Status of flare theory
• Standard model of large-scale magnetic reconnection…
• Cartoons illustrating this (and other) models http://solarmuri.ssl.berkeley.edu/~hhudson/cartoons/
• Have major uncertainties for - Coupling of scales - Particle acceleration - Role of helicity
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‘Somewhat analogous outbursts often happen on the Sun, in explosive events called "solar flares." During a solar flare, magnetic field lines near the Sun's surface change the pattern by which they connect to each other, a process called "magnetic reconnection" which releases pure magnetic energy. This happens in magnetars too.’
http://solomon.as.utexas.edu/~duncan/magnetar.html
Introduction to flare theory via the physics of magnetars
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RHESSI magnetar response
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Middle-aged star
decadeslowly
solar flare
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Magnetic reconnection
• “Reconnection” is only descriptive and does not describe the physics
• Magnetic restructuring is necessary for flare energy release from the magnetic field
• Clear unambiguous evidence is hard to find
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Gosling et al., JGR 110,A01107, 2005
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Conclusions
• New things this millennium: CMEs, helicity, -rays
• RHESSI is allowing us to understand the dominant role of accelerated particles
• Theoretically, we are forced to go beyond MHD theory and magnetic reconnection
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END
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Movie of dimming (Aug 28, 1992)
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Coronal Dimming
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Moreton-Ramsey waveand EIT wave
Thompson et al., 1998
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Type III (“fast drift”)
Type II (“slow drift”),harmonic
“Ignition”
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Inference of source motion in drifting radio bursts
• Assume a density model (z) with height z, normally empirical (e.g. “fourfold Newkirk”)
• Determine the drift rate in MHz/s• Convert to height from plasma-frequency
assumption; fp = 9000 ne0.5 Hz
• Typically, assume radial motion
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Cartoons illustrating wave origins?
cf. http://solarmuri.ssl.berkeley.edu/~hhudson/cartoons
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There doesn’t seem to be a satisfactorycartoon!
Sturrock CME
Hudson flare
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The CME-driven shock in the corona
• The CME involves outward plasma motions perpendicular to the field
• We see the result of these motions as dimmings, but the data are not good enough to follow the flows nor to see a bow wave
• There is an Alfven-speed “hole” in the middle corona in which Mach numbers could be larger
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SUMMARY
• Coronal shock waves (metric type II) are blast waves (Uchida) launched by compact structures at flare onset. These propagate in an undisturbed corona
• The CME eruption restructures the corona and pushes a bow wave ahead of it into the solar wind. This creates a type II burst at long wavelengths
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Where do “Solar cosmic rays” come from?
• Consensus holds that CME-driven shocks are responsible for most SEPs, but that something else is also happening
• Shock geometry and Mach numbers in the high corona are crucial factors: quasi-perpendicular fronts and large Mach numbers preferred
• The theory is incomplete but PIC simulations are appearing for the planetary bow shocks, at least
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Imaging of coronal shocks: good news and bad news
• A shock wave should provide a sharp density gradient, easy to detect in images
• We can observe motions in two dimensions• The medium is optically thin => confusion• The wave may not be bright compared with other
flare components• The corona generally has low plasma beta, so the
observed mass may not be structurally important
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…
• Only imaging can properly characterize the large-scale structure
• The solar corona isn’t really accessible any other way
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Imaging of coronal shocks
• Type II bursts (plasma radiation)• Moreton waves (H in the chromosphere)• New modalities: EIT, X-rays1, microwaves,
He 10830, meter waves (thermal), meter waves (nonthermal)
1Three events: Khan & Aurass (2002); Narukage et al. (2002); Hudson et al. (2003)
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Direct X-ray observation
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Uchida 1968
Yohkoh 1998
EIT
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Why X-ray waves are hard to observe directly
Pre-flare transect Flare transect
The wave - just ripples on the scattering halo!
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Heliospheric shocks in images?
• Maia et al., ApJ 528, L49 (2000)
• Vourlidas et al., ApJ 598, 1392 (2003)
• SOHO/UVCS
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Vourlidas et al., ApJ 598, 1392 (2003)
Where is the bow shock?
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Inferring the Mach number
Method: Estimate temperature jump from soft X-ray imagesand apply Rankine-Hugoniot condition
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X-ray signal S ~ ne2f(T)
f(T) ~ T2
ln(S)/ln(n) ~ 2
Mach number estimate for 6 May 1998 event
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