UCL Department of Space and Climate Physics
Mullard Space Science Laboratory

Energy flow from the photosphere to thecorona
Louise Harra & UV/EUV sub-WG

How is energy transferred from the photosphere through thechromosphere and transition region into the large scalecoronal structures? What is the morphology of the transition
region?
•Relationship between different layers of the atmosphere – both inclosed and open field lines.

•Flows in active regions and coronal holes – where does the energy
come from and where does it go?

•Emerging flux – responses in the atmosphere

The solar transition region•

There does not seem to be a continuous transition between thechromosphere and the corona that is measured (e.g. Feldman et al.,1999). SUMER found many small-scale loop-like structures•

EIS also shows low temperature loop probably arise at supergranularcell size, whereas hot loops connect supergranular cells or open fields.

Matsuzaki et al., 2007

Photospheric footpoints of quiet Sun transition region loops•Small cool loop-like structures fill the quiet Sun.
•Photospheric bright points are associated with TR structures, but
not the brightest and not the strongest red-shifts.
•Explosive events not associated.

Sanchez-Almeidaet al., 2007

Linking the chromosphere to corona
The weak emission in coronal and TR lines
have a B-R asymmetry at a range oftemperatures.
Explanation is that the chromosphere isheating the corona (De Pontieu et al., 2009,
McIntosh and De Pontieu, 2009).

Fast solar wind outflow observed by EIS•

The outflow seems to startin the TR and becomesprominent with increasing
temperature (Tian et al.,2010).

•Patches of significant
outflow are small in TR but
merge in the corona –consistent with the solar
wind being guided byexpanding magneticfunnels.

•Plasma accelerated in thefunnel above 5Mm, but
originates from below theneighbouring loops (Tu etal., 2005).

Linking magnetic field and corona in the poles

X-ray activity inthe poles
appears aroundminority polarities
near the kG
patches (Shimojoand Tsuneta,2009)

Relationship between magnetic field and beyond.
•The total area and net fluxof CHs are directly related tothe solar wind speed.
•The filling factor is found to
decrease below 2Mm.
•This suggests a multi-fractal
structure and highlyintermittent burst like energy
release.

Abramenko et al., 2009

Slow solar wind
Sakao et al. 2007, showed
continuous ‘spurting’ streamsof plasma from the edges ofactive regions.
Outflows of up to 50 km/s areseen in the region of outflow in
XRT, Harra et al. (2008).
Marsch et al. (2008) describe thisas ‘coronal circulation’.
Doschek et al. Del Zanna etal., Hara et al., Baker et al.

‘Unipolar’ footpoint –has many small
bipolar fragments
appear and disappear.
(Hara et al., 2009)Wind speed at ACE andbackmapped windspeed at the Sun(black)
BUT has the charge state ratio expected from coronal hole.
Synoptic map (MDI)

Synoptic map (EUVI)

Response of the atmosphere to small emerging flux

Guglielmino et al.,
2008•

Emerging flux in the photosphere is associated with chromosphericbrightenings (e.g. Ca II H).

•

Even these small-scale flux emergences heat the chromosphere andcorona (e.g. Li et al., 2007, Guglielmino et al., 2008).

•The Ca II heated before the coronal emission.

Response to emerging flux in the corona
1-Dec 16:00-2-Dec 16:00

Flux emergence

Magara, 2008

Harra et al., 2010

Coronal enhancements are rarely seen in
the ‘fragmented’ magnetic region – but
chromospheric enhancements frequentlyseen.

Active region flow onset

EF region

Progression of flux
emergence over 27hours produces
enhanced outflows
at the edges of theactive region(Harra et al., 2010).

Simulations of emerging flux
•3-D MHD simulation of rise of
buoyant magnetic flux throughconvection zone into photosphere.
•Interaction of convection
downflows and the rising magnetic
flux tube undulates it to form
serpentine field lines.

Cheung et al., 2008

What is the relationship between magnetic field evolution, flux
emergence, and solar eruptions? What is the initiation
mechanism for flares and CMEs?

•Pre-flare/CME triggers.

•Response to eruptions

FlareBuild-up to flares••With Hinode we can measure thehelicity build-up following flux
emergence (Magara andTsuneta, 2008).
We can follow the response in
the corona (Harra et al., 2009).

Sharp increase in‘turbulence’ of the corona

Peak of helicity

Precursors also seen in small events•Few hours prior to CME, outflow velocity intensifies on western side of AR in vicinity of filament
along PIL
•Magnetic flux falls 10% per day during AR expansion and eruption suggesting flux cancellation
destabilizes the tied-down field-> increasing twist in and expansion of flux rope -> CME•Pre-eruption expansion of flux rope containing filament provides required increase incompressive forces causing intensification of outflows?
Baker et al 2007 and Murray et al 2010

Arcade to flux rope transition•The soft X-ray arcade develops high shear over 2 days and transitions
to sigmoidal before the eruption.

•The centre of the sigmoid makes an inverse crossing of the polarity
inversion line 50 mins before the eruption.

•The formation of the flux rope is driven by fragmentation of the mainpolarities, motion due to supergranular flows and cancellation at thepolarity inversion line.

Green,Wallace& Kliem, 2010

Exploring the source of a CME•

Spectroscopy allows us to lookat both the core dimming region
and the distant dimming region(Harra et al., 2007).

•This allows a more accurate
determination of the source of anICME.

•The relationship withtemperature has been derived by
Imada et al. (2007) – providing
boundary conditions for models.

Harra et al., 2010

Summary•

The formation of outflow from active regions and its link to the solar
wind is still confused.

•We need to observe the pre-flare build-up – which requires observingvery small scale phenomena (such as flux cancellation).

•

We need fast enough cadence to see the active region with the core‘dimmings’ and the outside region to see the ‘secondary’ dimmings.

•The solar wind starts ‘outflowing’ between 5-20 Mm. We have a few
snapshots of change in altitude.

•There is a clear change between the chromosphere and above - thisneeds to be understood in terms of plasma outflow and heating.

We need to observe spectroscopically through theatmosphere with a high time cadence and large FOV.
EUVS covers from 104K to 107K plasma, 10 times effective
area, spatial resolution of ~0.3”, FOV 400”X400”