Co-spatial White Light and Hard X-ray Flare Footpoints seen above the Solar Limb:

RHESSI and HMI observations

Säm KruckerSpace Sciences Laboratory, UC Berkeley

University of Applied Sciences Northwestern Switzerland

Implications for flare energetics and chromospheric evaporation

HXR bremsstrahlung

• Flare loop T, V, n

• Spectrum of acc. electrons

thermal bremsstrahlung

T ~ 30 MK

non-thermal bremsstrahlungaccelerated electrons with typical energies

above ~10 keV

Flare footpoints in WL and HXR

Carrington, R. C., 1859

Close connection in space, time, and intensity.

2 arcsec

Flare ribbons in WL and HXR

Carrington, R. C., 1859

Close connection in space, time, and intensity.

25-100 keVSOT G-band

2 arcsec

Krucker et al. 2011

Heating of flare ribbons

SDO/HMI(6173 A)

Significant fraction of flare energy is radiated away in optical range

SDO/AIA 171 A(~1 MK)

Flares larger than GOES M5 can be generally detected with HMI, for smaller events non-flare related time variations are hiding flare mission.

(Kuhar et al. 2015, TBS)

HM

I 61

73 A

incr

ease

RHESSI 30 keV flux

Statistical study HMI/RHESSI: all flares have WL footpoints

Good correlation strongly suggests that flare-accelerated electrons are involved in

the production of the WL emission

Kyoko Watanabe et al. 2010

Assuming:thick target (HXR)black body (WL)

E0=low energy cutoff in electron spectrum

40-100 keVSOT G-band

IRIS continuum observations from flare ribbon (GOES X1)

Heinzel & Kleint 2014Kleint et al. 2015 (to be submitted)

IRIS continuum

RHESSI30-100 keV

IRIS, HMI, FIRS continuum

HMI

FIRSIRIS

Kleint et al. 2015 (to be submitted)

IRIS, HMI, FIRS continuum

HMI

FIRSIRIS

Kleint et al. 2015 (to be submitted)

Energy in >27 keV is equal to radiative losses in optical range

First attempt: Thick target modelAbbett et al. 2015 (to be submitted)

Next step: compare to modeling

Heating & exponential decay (~ 10 s)

Penn et al. 2015 (to be submitted)

Heating of flare ribbons to ~MK

SDO/AIA 171 A(~1 MK)

Heated ribbon can evaporate hot plasma into corona to form flare loopThermal conduction from hot coronal loop can also drive evaporation

De-saturated AIA 171A(Schwartz et al. 2014)

Heating of flare ribbons to ~MK

SDO/AIA 171 A(~1 MK)

Hot ribbons, but colder than post flare loops. XRT to constrain higher temperatures?

Where do flare accelerated electrons heat the chromosphere?

Thick target beam model gives altitudes of HXR source of ~800-3000 km (see Battaglia et al. 2012):

• Density • Energy of electrons• Pitch angle• Ionization level• Field line tilt

Since these parameters are mostly unknown, there is no unique prediction.

photosphere

flare-acceleratedelectrons

HXR source in chromosphere due to bremsstrahlung

Range for low density models800 – 1500 km

Stereoscopic observations

photosphere

flare-acceleratedelectrons

HXR source in chromosphere due to bremsstrahlung

Range for low density models800 – 1500 km

Martinez-Oliveros et al. 2012:RHESSI/HMI/STEREO• Use STEREO EUV ribbon

as proxy• Single event

Absolute source height at 305+-170 km195+-70 km

This is surprisingly low:•Very, very low density•Source within Wilson depression•Not thick target beam model

indirectly observed

Stereoscopic observations

photosphere

flare-acceleratedelectrons

HXR source in chromosphere due to bremsstrahlung

800 – 1500 km

Martinez-Oliveros et al. 2012:RHESSI/HMI/STEREO• Use STEREO EUV ribbon

as proxy• Single event

Absolute source height at 305+-170 km195+-70 km

This is surprisingly low:•Very, very low density•Source within Wilson depression•Not thick target beam model

Altitude of WL source?Optical emission is thought to be thermal emission at low temperatures (~10 000 K)

•Heated by electrons: co-spatial source with HXRs•Backwarming would predict a lower altitude.

This talk: look at flares that occur within one degree of limb passage (Krucker et al. 2015).

HMI (617.3 nm):•resolution: 1.1”•placing: <0.1”

RHESSI hard X-rays:•resolution: 2.3”•placing: <0.1”

photosphere

flare-acceleratedelectrons

HXR source in chromosphere due to bremsstrahlung

Range for low density models800 – 1500 km

?

?

Emission from the limb is influenced by the opacity of the atmosphere

radiation cannot escape

disk

~350 km

STEREO is used to get flare location relative to limb

Projection effects estimated to be less than 100 km for derived altitudes for selected events.

Image+Image+

Co-spatial HXR and WL footpoints

Image: HMI with pre-flare image subtracted. Black is enhanced emission.30-80 keV

617.3 nm

thermal loop top

footpoints

Non-thermalabove the loop top

Image+Image+

Co-spatial HXR and WL footpoints

Altitude above photosphere:WL: 810+-70 kmHXR: 722+-122 kmLow values for TTBM

30-80 keV617.3 nm

footpoint

flare

pre-flarepre-flare derivative pre-flare

Image+Image+

Two similar events

30-80 keV617.3 nm

30-80 keV617.3 nm

Synchronous source motion in HXR and WL

Time evolution of fluxes and altitudes

617.3 nm

30-80 keV

GOES

617.3 nm

30-80 keV

Consistent results: co-spatial emission below ~1000 km

Implications of co-spatial sources• HXR emission comes from

relative cold plasma• HXR producing electrons (>30

keV) do not heat chromosphere to millions of degrees

• Energy of >30 keV electrons are lost by radiation in the optical range

• >30 keV electrons are not responsible for evaporation!

• Heating to MK by low energy electrons at higher altitudes?

electron energy

flux

>30 keV

energy is lost to WL radiation

lower energies?

energy goes into evaporated plasma?

observation of footpoints at lower energies (<20 keV) very difficult due to limited dynamic range of RHESSI.

Low-energy (thermal) emission from footpoints is lost in limited dynamic range

Upper limits of footpoint emission at low-energies

Spectra of footpoint as inferred from images

?

Low-energy (thermal) emission from footpoints is lost in limited dynamic range

Upper limits of footpoint emission at low-energies

Spectra of footpoint as inferred from images

?HXR focusing optics can overcome this limitation

HINODE XRT and SOT observations

•XRT hot filters: – constrain high temperatures in footpoints– Time evolution: conduction vs beam heating– Locate GOES fast time variations– is a 2 second cadence to match GOES feasible, at

least for some time intervals during the flare?

•SOT– Fast time cadence to observe decay on second

time scale– Is ~2 second cadence possible in a single filter?

HINODE SOT RGB •Small source sizes•Footpoint motion?•fast decay

mismatch between spatial and time resolution

Proposition to occasionally run flare mode with higher time cadence, maybe only one filter. Summing over pixel to save telemetry.

t=0 t=19 s

t=3 s t=22 s

t=6 s t=25 s

Summary• HXR source altitude is low

<1000 km– TTBM works only with very low

density models, strongly beamed case

• Co-spatial WL and HXR sources: – energy of >30 keV electrons is

radiated in optical range

– >30 keV electrons are not responsible for evaporation!

• Unexpected results with implication on our standard picture

photosphere

flare-acceleratedelectrons

Co-spatial HXR and WL sources

<1000 Mm

Additional source?