U.W., April 14, 2005 Solar flares in the new millennium H.S. Hudson SSL/UCB.

U.W., April 14, 2005

Solar flares in the new millennium

H.S. Hudson

SSL/UCB

U.W., April 14, 2005

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

http://solarmuri.ssl.berkeley.edu/~hhudson/cartoons/

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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)





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





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



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

U.W., April 14, 2005 Solar flares in the new millennium H.S. Hudson SSL/UCB.

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