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Modeling Impulsive Radio Bursts in Solar Flares

Study on impulsive radio bursts during a solar flare on August 24, 2002. Models reveal mechanisms behind radio emissions at various frequencies. Analyzes thermal and nonthermal components in the flare's environment.

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Modeling Impulsive Radio Bursts in Solar Flares

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  1. Radio Observations And Modeling Of A Post-flare Arcade Hazel Bain1,2, Lyndsay Fletcher2(1) Space Sciences LaboratoryUC Berkeley(2)Department of Physics and AstronomyUniversity of Glasgow hbain@ssl.berkeley.edu

  2. Event Overview • Goes X3.1 class flare on the 24th August 2002. • Radio observations show 6 impulsive radio bursts. • RHESSI shows increased flux at 50 -100 keV just before night, 00:56. • SONG detector onboard CORONAS-F shows correlation between HXR (64-180 keV and 180-600 keV) and radio. 6s at 17 GHz, 8s at 34 GHz (Reznikova 09). • Associated prominence eruption/CME at 00:55.

  3. TRACE and NoRH observations 17 GHz34 GHz • 00:45 – 00:56 : coronal loops move outwards (a - c). • 00:59 – 01:10 : arcade forms from west to east i.e. right to left (d - g). • 01:24: radio emission cospatial with TRACE hot diffuse source ~107 K (h - o). • 01:24 – onwards: TRACE arcade forms at greater heights (h - o). • Second radio loop appears. 01:24 34 GHz, 01:26 17GHz (h)

  4. RHESSI 01:34 T1 = 16 MK, EM1 = 2 x 1049 cm-3 T2 = 30 MK, EM2 = 1 x 1048 cm-3 γ = 4 02:20 T1 = 12 MK, EM1 = 5 x 1049 cm-3 T2 = 18 MK, EM2 = 1 x 1048 cm-3 6 – 12 keV (blue) 12 – 25 keV (green) 25 – 50 keV (red) n = 5 x 109 – 1 x 1010 cm-3

  5. Decay phase NoRH 34 GHz (redscale) 17 GHz (contours) Brightness temperature, Tb (K) vs θ -20 -120 -70

  6. Plasma parameters Bightness temperature (K) Radio spectral index Electron spectral index Nonthermal electron density N (cm-3). (Dulk & Marsh 82)

  7. Radio Model • Melnikov 05, Tzatzakis 06, Reznikova 09 model impulsive radio bursts using Fokker-Planck approach. • Consider only a simple dipole loop. • Don’t consider thermal effects. • Our model • Dipole vs arcade magnetic field models. • Radio emission calculated for individual voxels – GS code by Dr Gregory Fleishman (Fleishman et al 09, Nita et al 09). • Code uses Petrosian-Klein approximation (Petrosian 81). • Vary input parameters for individual voxels and rotate viewing angle • Radiative transfer along line of sight.

  8. Nonthermal Gyrosynchrotron (Dipole) • Gaussian distribution of nonthermal electrons centred at the looptop. • Ratio of NLT:NFP ~ 1 order of magnitude to get both sources. • δ decreases, ratio decreases.

  9. Nonthermal Gyrosynchrotron (Arcade) BLT = 150 G, BFP = 800 G N = 104 cm-3, δ = 3 Input parameters are constant along loop Line profile along loop (green line)

  10. Thermal/Nonthermal Model • Continuous function. • Thermal and nonthermal components matched at critical point parameterised by ε.

  11. Thermal/Nonthermal Model Thermal/Nonthermal Nonthermal Gyrosynchroton Thermal Gyrosynchrotron component used for TNT model

  12. Thermal/Nonthermal Model Black Red Blue

  13. Summary • Two or three promising models… • For a dipole magnetic field model, an enhancement of nonthermal electrons is required at the looptop to produce both looptop and footpoint emission. • An arcade model increases the line of sight distance at the looptop, resulting in enhanced emission without the need for an increase in NLT. • However neither of these models are able to reproduce the steeper spectrum observed at the footpoints. • For high temperature and strong B the thermal component can become important and dominate over the nonthermal GS spectrum leading to a steeper spectrum at the footpoints. • However the addition of a thermal component results in absorption at lower frequencies and does not match the observed NoRP flux.

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