Working Group 2: From Flares to Microflares A Critique of the Standard Models. 8 th RHESSI Workshop Potsdam, Germany 2 6 Sept 2008

Transcription

1 Working Group 2: From Flares to Microflares A Critique of the Standard Models 8 th RHESSI Workshop Potsdam, Germany 2 6 Sept iain@astro.gla.ac.uk

2 Who Talked... Name Affiliation Weiqun Gan Purple Mountain Astrid Veronig graz.at Observatory University of Graz Hugh Hudson UC Berkeley Iain Hannah University of Glasgow John Brown University of Glasgow Steven Christe UC Berkeley Claire Raftery Trinity College Dublin Arnold Benz ETH Zurich Hazel Bain University of Glasgow Valentina Zharkova University of Bradford Lyndsay Fletcher University of Glasgow

3 Astrid Veronig: Plenary Two standard models: Eruptive Flare/CME event (CSHKP/Standard Model) Energy Release & Magnetic Configuration Thick target model (TTM) Energy Transport & Atmospheric Response Plenty of evidence to support the models as well as evidence that causes problems Getting some parts of models correct but not complete picture RHESSI data is making us think hard! Can t do justice to Astrid s great plenary so will just focus on some thoughts and what was presented in our group

4 Diversion to some overall issues Particle Acceleration is a Black Box Some accelerated population is assumed Magnetic reconnection facilitates the release of energy and that energises the particles via DC electric fields, stochastic acceleration, shock acceleration, betatron acceleration.? See Group 4 s plenary Don t blame the model, blame the scientist? The models aren t that standard but designed for specific purposes and we are trying to use them to explain everything CSHKP was originally developed for late phase, used for impulsive, and only for eruptive events TTM is for an impulsive electron beam transporting the energy from a coronal acceleration site depositing it in the dense chromosphere, with the resulting atmospheric response

5 More Diversions Aspects of a Standard Model Acceleration/Energy Release Mechanism Transport of the Energy Emission Mechanism Each can be changed & might not be separate processes Should there be a standard model? Just standard/required physics which we then use in standard/typical scenarios for different events? Don t want a rare event to become popular because we can make a nice cartoon for it...

6 Back to Astrid Veronig: Plenary Lot s of Issues: BOTH: Neither deal with details of acceleration/heating BOTH: Number problem CSHKP: Some observed feature agree but how common? TT: Fast e signature only (no ions or direct heating) TT: Neupert Effect: heating and/or evaporation? TT: Low energy cut off and microflare energy What did our speakers come up with then: I ll be summarising...

7 John Brown: Death of Thick Target We use collisional TTM as it is relatively easy Main TTM problems: Directivity and the albedo bump issue If HXR source size < 1 2, beam density too high for coronal No Number problem but a Closure problem SXR EM(t) vs T(t) do not agree with TTM for either monolothic OR filamented loops IP versus HXR electron spectra & numbers Alternatives Process/Components: To reduce beam density Nonthermal recombination HXRs from hot plasma threads Local Reacceleration Alfven waves?

8 John Brown: Death of Thick Target Thick Target Model in death throes? It s only a flesh wound

9 Arnold Benz Why Don t we have a Standard Model? 1D Intermediate Thin Thick Target model a majority of observations, but is too simple to serve as a standard. Partially fixed with assumption of electric field/return current Return currents are standard in every particle accelerating flare. They may be unstable and involve a large electric field. Heating of coronal plasma is standard in every flare if particle acceleration occurs stochastically. Heat is transferred by conduction (low mfp /L loop ) => contention in the group Many relatively independent processes, that interact nonlinearly but there is a finite number of processes Each flare appears different so don t want a standard model but standard processes (study the latter)

10 Hugh Hudson RHESSI and global models of flares and CMEs: What is the status of the implosion conjecture? Magnetic Implosion still seems to make sense for flare/cme excitation Need better measurements of B > FASR This is not inconsistent with large scale reconnection flows, and modelling shows the Poynting flux to have attractive properties (J. Birn 2008 simulations) The source energy is at low altitudes and may be associated with simple large scale currents To solve a crime, follow the money ; to solve a flare, follow the energy!

11 Valentina Zharkova Fokker Plank approach to electron beam distributions and their resulting photon spectra and polarization Return currents are VERY important put as much physics in as you can You get a return current even from just the convergence of B Ohmic heating dominates lower corona/upper Chromosphere and for high initial beam fluxes (10 9 erg/cm 2 /s) Coulomb collisions dominate for lower initial beam fluxes (<10 9 ) Calculating the X ray (of both the downward and upwards electrons) and Microwave spectra

12 Lyndsay Fletcher: Waves White Light Footpoints UV/WL/IR is where the flare energy ends up > 5 x10 7 cm PRIMARY energy transport is by an Alfvénic wave pulse Before flare energy is stored in twisted B Field reconfigures, twist redistributes, Poynting flux generated Wave can get to chromosphere <1 sec What happens when the wave pulse reaches the chromosphere Damp in Chromosphere > heating > WL/UV production? Turbulent cascade > stochastic acceleration in chromosphere? magnetic stresses drive multiple chromospheric current sheets? Issues: A non coronal acceleration model needs to replicate: TOF observations HXR footpoint sources synchronised to ~0.1s

13 Weiqun Gan Standard Models & RHESSI Observations Shrinkage before upward motion Need more simulations to explain the motion irregular temperature distribution of looptop source during the shrinkage (Shen J. et al. 2008), suggests that reconnection happens between highly sheared loops Footpoint motions : how to explain parallel motion? Neupert Effect peak time difference of SXR from HXR depends on the loop size, indicating a role of chromospheric evaporation > why a timeshift? Electron Number: influxed total not enough to supply the X ray producing electrons JCB => Not a number problem but a Closure Problem

14 Hazel Bain: RHESSI HXR from jet 6 12keV 12 20keV 20 30keV 30 50keV

15 Claire Raftery The Hydrodynamics of Cooling Solar Flare Plasma Cooling in short duration event with RHESSI, TRACE, GOES and CDS Using EBTEL model fit Conductive then radiative losses dominate Using RHESSI & Messenger XRS SAX to get DEM of cooling plasma Compare this to EBTEL hydro model Analyse cooling mechanisms, suppressed conduction? Radiation dominating?

16 With the RHESSI microflare statistics (Christe et al 2008, Hannah et al 2008) have lot s of information including the centroid positions of >25000 events Use Matsushita (1992) technique and MC simulations to recover height distributions Difference between centroid location of the thermal and nonthermal sources in microflares is small. The observed distribution not fully captured by the assumed height distribution model. Steve Christe: Flare Height Distributions

17 Iain Hannah: Microflares Low energy cutoff in microflares is a problem as it gives too high a non thermal energy Assumptions for using model (cold target, distinct thermal/non thermal populations) not valid for microflares Solution: stop fitting power laws and try more valid models. Problem: Not easy and difficult to place new model in context with previous work for us and non HXR people Trying to do statistics with both RHESSI and Hinode/XRT to investigate HXR and SXR response and Neupert Effect behaviour.

18 Final Thoughts For standard model really just mean framework in which we understand the processes of Energy Release, Transport & Emission Which processes are possible How do they relate to each other When might one dominate over the other... More of us working in F(E) instead of I(ε) might help to reveal what s going on I need to learn my plasma physics not just how to make how to make nice RHESSI images. Thanks to all the speakers