Co-spatial White Light and Hard X-ray Flare Footpoints seen above the Solar Limb: RHESSI and HMI observations Säm Krucker Space 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 bremsstrahlung accelerated 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 keV SOT 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) HMI 6173 A increase 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 Assuming: thick target (HXR) black body (WL) E 0 =low energy cutoff in electron spectrum keV SOT G-band

IRIS continuum observations from flare ribbon (GOES X1) Heinzel & Kleint 2014 Kleint et al (to be submitted) IRIS continuum RHESSI keV

IRIS, HMI, FIRS continuum HMI FIRS IRIS Kleint et al (to be submitted)

IRIS, HMI, FIRS continuum HMI FIRS IRIS Kleint et al (to be submitted) Energy in >27 keV is equal to radiative losses in optical range

First attempt: Thick target model Abbett et al (to be submitted) Next step: compare to modeling

Compare energy deposition with radiative losses from the same area RHESSI: Energy deposition of electrons above 20 keV within IRIS slit: ~8 x erg/cm 2 /s Integrated radiative losses: ~5 x erg/cm 2 /s  Energy in >27 keV is equal to radiative losses in optical range

Heating & exponential decay (  ~ 10 s) Penn et al (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 loop Thermal 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 ~ 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-accelerated electrons HXR source in chromosphere due to bremsstrahlung Range for low density models 800 – 1500 km

Stereoscopic observations photosphere flare-accelerated electrons HXR source in chromosphere due to bremsstrahlung Range for low density models 800 – 1500 km Martinez-Oliveros et al. 2012: RHESSI/HMI/STEREO Use STEREO EUV ribbon as proxy Single event Absolute source height at km km This is surprisingly low: Very, very low density Source within Wilson depression Not thick target beam model indirectly observed

Stereoscopic observations photosphere flare-accelerated electrons 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 km 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 (~ 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-accelerated electrons HXR source in chromosphere due to bremsstrahlung Range for low density models 800 – 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+ Co-spatial HXR and WL footpoints Image: HMI with pre-flare image subtracted. Black is enhanced emission keV nm thermal loop top footpoints Non- thermal above the loop top

Image+ Co-spatial HXR and WL footpoints Altitude above photosphere: WL: km HXR: km Low values for TTBM keV nm footpoint flare pre-flare derivative pre-flare

Image+ Two similar events keV nm keV nm

Synchronous source motion in HXR and WL

Time evolution of fluxes and altitudes nm keV GOES nm 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 st=22 s t=6 st=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-accelerated electrons Co-spatial HXR and WL sources <1000 Mm Additional source?