Chromospheric flares in the modern era H. Hudson Space Sciences Lab, UC Berkeley

2/21 Chronology of flare physics 19th century. The photosphere (Carrington; Trouvelot) Early 20th century. The chromosphere (spectroscopic observations, H  ) Current. The corona (X-rays, CMEs). Major theoretical ideas formulated

3/21 Hirayama 1974 Reference model of coronal magnetic reconnection

4/21 Three areas of current interest for chromospheric flares 1.Energetics: Can the chromosphere play a significant role in the energetics of a flare? No 2.Footpoints & Ribbons: The impulsive-phase excitation of the lower solar atmosphere remains opaque (to us) 3.Structure: Can the chromosphere provide clues to the important physics? Yes; Kopp & Poletto (1984) et seq

5/21 Topic 1: Energetics

6/21 Can the chromospheric provide the energy for a solar flare? Mass g Gravitational energy10 31 ergs (to infinity) Gravitational energy ergs (2” altitude) Magnetic energy10 30 ergs Ionization energy10 29 ergs Thermal energy10 29 ergs Energy in flows10 26 ergs No. There is insufficient energy or stress in the chromosphere to power a major flare Enough for a CME…

7/21 Distribution of magnetic energy in the corona

8/21 Topic 2: Ribbons and footpoints

9/21 How do we understand the flaring chromosphere? Observationally, via imaging spectroscopy Theoretically, in steps (don’t need to pay attention if you heard Carlsson’s talk) - Structure in a stratified atmosphere - Non-LTE radiative transfer - Fluid approximations (“radiation hydrodynamics”) - Fluid approximation for the fields (MHD) - Correct plasma microphysics

10/21 Flare gradual phase Accepted scenario of radiative/conductive cooling of coronal loops with initially high gas pressures Ablation (“evaporation”) of the chromosphere directly observed by XUV imaging spectroscopy A lot of the physics is well developed Kane & Anderson, 1970 Long-Decay Event (LDE) The Neupert effect Loop prominence systems Bruzek 1964

11/21 Flare impulsive phase The chromospheric and UV observations show intense intermittent excitation of unresolved features The existing imaging spectroscopy is totally inadequate The physics is completely unclear

12/21 The “thick target” model The ionospheric response showed us that the lower solar atmosphere - the UV source - dominated flare energetics After ( = 147 years), we still don’t know how the Sun concentrates so much energy in such a trivial layer Hence the “thick target” model of particle beams Najita & Orrall, 1971Svestka, 1970

13/21 Model spectra of WL/UV continuum (Fletcher et al. 2006) TRACE 1700 TRACE WL Allred et al. 2005

14/21 The biggest problem in flare chromospheres is understanding the visible/UV spectrum formed in the impulsive phase

15/21 Topic 3: Structure

16/21 Intermittency: consecutive TRACE images 24 July 2004, C x 68 arc s frames

17/21 Flare radiant energy appears in the form of compact, intense patches The high-energy footpoint excitation moves rapidly, with a crossing time of order <30 sec (Schrijver et al. 2006) M9.1 flare, time in sec,TRACE WL X10 flare, 1.56  (Xu et al., 2004)

18/21 What is not understood? How the bulk of flare energy can be focused into these tiny chromospheric structures How important flare effects can appear as deep as the opacity minimum

19/21 Coronal restructuring inferred from footpoint behavior (Kopp & Poletto 1984 et seq.) Fletcher & Hudson 2001 Footpoint motions measure magnetic flux transfer rate Hard X-rays identify site of energy release

20/21 A modern “chromosphere” problem: 511 keV line formation (e + + e - -> 2  ) Share et al., ApJ 615, L169 (2004) A column depth of a few gm/cm2 at transition-region temperatures, stable for many tens of seconds, appears to be required

21/21 Conclusions The concept of a static chromosphere is inappropriate for flare conditions Concentration of flare energy into compact flare elements is puzzling There are many theoretical issues to resolve De Pontieu et al., 2003

22/21 A chromospheric contribution to the problem of flare energy? Gravitational potential energy in partially ionized flux tubes can stress the coronal field (energy buildup) Downward motions can provide energy (flare energy release) “Dipped” flux tubes should happen naturally near sunspots and in regions of flux emergence There is little relevant theoretical work at present

23/21 Does the chromosphere have enough gravitational potential energy to make a flare? No Calculation based on Maltby et al. (1986) model E says… Sturrock et al. (1996) Chromospheric mass stresses the field, driving coronal current systems* *poorly illustrated here

24/21 Loop-top WL emission seen by TRACE Event of 2002 Nov. 12, 17:58 UT

25/21 WL versus UV (1700A) TRACE WLWL difference1700 A

26/21 Kane & Donnelly, ApJ 154, 171 (1971)