rhessi observations: a new flare pattern and a new model for the old pattern h. s. hudson (ssl...

RHESSI OBSERVATIONS:A new flare pattern and a

new model for the old pattern

H. S. Hudson (SSL Berkeley)

Bozeman, April 10 2007

The new pattern• The usual scheme for flare development features footpoint

hard X-ray sources and the Neupert effect (the “thick target model”)

• The new pattern, mainly from RHESSI via the work of Säm Krucker, features:

- Hard X-rays and -rays from high in the corona

- Minimal footpoint emission

- Strong association with the biggest events (ie, CMEs and SEPs)


A new explanation ofthe old pattern

• Modern data show that the thick-target model has insuperable problems

• Our proposal (Fletcher & Hudson 2007) replaces the particle beam with wave energy transport and places the electron acceleration in the chromosphere


RHESSI vital statistics

• FOV whole Sun, 3 keV - 15 MeV

• Angular scales in 9 discrete collimators, 2.15 arc sec through 2.9 arc min (FWHM)

• Spectral resolution 1 keV for hard X-rays, somewhat larger for -rays

• Launch February 2002 into middle-inclination low Earth orbit via a Pegasus rocket

• Many “firsts” from these observations, which continue

• RHESSI is the only high-energy solar observatory existing or planned


Reuven Ramaty, 1967-2001


The RHESSI “iris diaphragm” innovationand its front/rear segmentation

Lin et al. 2002


The RHESSI scheme

RHESSI was built at SSL (Berkeley),NASA/GSFC and the Paul-Scherrer Institut (Switzerland).

The RHESSI team is led by Bob Lin (Berkeley), Brian Dennis(NASA), and Arnold Benz (Zurich)


QuickTime™ and aGIF decompressor

are needed to see this picture.

Thanks to Gordon Hurford


New improved science nuggets


Topic 1: the new patternfor solar hard X-rays

• There’s some history to this, some brief comments about March 30, 1969

• Then on to Säm’s new wonderful RHESSI material


Frost & Dennis 1971

Enome & Tanaka 1971(3.5 GHz)

March 30, 1969:X-rays and Microwaves

No H flare, ~W105


Palmer & Smerd, 1972

March 30, 1969: meter waves (Culgoora)


RHESSI Coronal X-ray imaging

Two-ribbon flare with both hard X-ray footpoints (blue) and thermal loop (red)


Exponential decay reveals coronal source at 250 keV!


Imaging spectroscopyCoronal source has HARDER/FLATTER spectrum than footpoints!

spatially integrated

~ 1.5 +- 0.3

~ 2.9 +- 0.1


Another high coronal event:2002 Oct 27

MARS: GRS

RHESSI 15-25 keV

• -ray flare seen by GRS (MARS)• GOES soft X-ray (thermal): tiny flare (B2) • HXR emission up to 80 keV• simultaneous onset• exponential decay (~135 s)

Earth


Very large (>200 arcsec),expanding and rising,CME velocity ~2000 km/s

October 27, 2002

~400 km/s

~800 km/s

size

motion

300”


Coronal source summary

• RHESSI sees many coronal sources, but no Masuda flares

• The large-scale sources have spectra that are extremely hard - recall Kramers ~ 1/E, hence relativistic electrons

- stable trapping

- large non-thermal pressure

• Sources can move or be stable


Topic 2: the new explanationfor the impulsive phase

• Wave transport of energy, not electron beams

• Translation of magnetospheric ideas

• Thanks to many magnetospheric physicists in Berkeley and elsewhere for explanations


QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Death to the thick target!


Basic ideas

• The electron beams of the thick-target model have become untenable

• Alfvén speeds in the flaring corona has been systematically underestimated

• Magnetic reconnection in a sheared field ought to have Alfvén-mode exhaust flows

• There are ways to get parallel electric fields, hence fast electrons, from dispersive Alfvén waves


Collisional thick-target model

(e.g. Kane & Anderson ‘69, Brown ’71, Hudson’72)

Hard X-ray emission is primarily electron-proton bremsstrahlung from energetic electron beam in a cold, collisional chromosphere

Beam also heats chromosphere producing white light and UV

Coronal accelerator

Coronal electron transport (generally no treatment of plasma collective effects)

Bremsstrahlung HXRs and collisional heating

HXRs, UV, WL

chromosphere


‘Volumetric’ acceleration:

Wave-particle turbulence (e.g. Larosa et al, Miller et al)

Stochastic current sheets (e.g. Turkmani et al)

Betatron acceleration (e.g. Brown & Hoyng, Karlicky et al)

Diffusive shock or shock drift acceleration (e.g. Tsuneta & Naito, Mann et al)

Reconnecting X-line or current-sheet acceleration

Multiple X-lines/islands (e.g. Kliem, Drake)

Single macroscopic current sheet (e.g. Litvinenko & Somov, Somov & Kosugi)

Acceleration in the corona requires a high fraction of a large volume of electrons to be accelerated to high energies


Orange=25-50keV

Blue=WL

RHESSI HXR and TRACE White Light

White light footpoint area ~ 1017 cm2

From area and WL power, calculate beam number and energy flux.

RHESSI 25-50keV

1 px = 0.5” ~ 300km


Is there any evidence for a beam?

By studying the photospheric albedo contribution to the HXR spectrum it is possible to discover the ratio of downward-going and upward-going electrons in the chromosphere.

Ratio of downward/upward-going electrons is < 1

This is not consistent with a beam from the corona.

Aug 20 2002 Jan 17 2005

Kontar & Brown 2007


Challenges for the impulsive phase “beam”

(1) HXRs imply electron flux can be in excess of 21036 e/s typical flare ‘volume’ is emptied of electrons in ~10s

but HXRs continue for several minutes.

(2) Small WL & HXR footpoints imply electron beam density 0.1 of background plasma density unstable beam propagation in corona

(3) If corona is dense enough for stable propagation no HXR footpoints are formed below ~ 50keV.

(4) HXR albedo observations are not consistent with a downward-directed beam entering the chromosphere from the corona.


The propagation time for these waves to the chromosphere is shorter than their damping time in the corona.

Coronal B in core of active region ~ 500-1000G from microwave measurements.

Pre-flare coronal n ~ 109cm-3

Alfven speed ~ 35,000 km/s

fast-mode -like

The relaxation of a twisted, 3D field is an almighty magnetic ‘convulsion’, with a fast / Alfvénic character.

Alfven-like Melrose 1992


Coronal propagation

B = 500 G B = 1000 G

Use expression for damping time by phase-mixing (e.g. Roberts 2000) – A < damp for long wavelengths

Cascade to short parallel wavelengths will not happen but cascade to short perpendicular wavelengths proceeds (Kinney & McWilliams 1998).


Implications

Wave Poynting flux dissipated in chromosphere produces local heating (white-light and UV?) and may cause local electron acceleration.

Wave propagating to chromosphere and below will re-orient magnetic field – changes in line-of-sight B.

In corona, parallel field will accelerate electrons to ~ 2vA, or ~ 20-40 keV. (NB this is the self-consistent electron current in 2-fluid MHD)

Wave partially reflects in non-uniform chromosphere, draws electrons back into corona for re-acceleration.

Process intimately connected to reconnection, so signatures related to magnetic topology changes are preserved.


End

Thanks for discussion and input:Lyndsay Fletcher, Säm Krucker

rhessi observations: a new flare pattern and a new model for the old pattern h. s. hudson (ssl...

Documents

planned slide

seps slide

chromosphere slide

w105 slide

rhessi scheme rhessi

s earth slide

new pattern

grs rhessi