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Chromospheric and Transition Region Dynamics

Investigating nonthermal electron heating in the chromosphere and transition region, exploring energy thresholds, temperatures, and spectroscopic evolution, using space-based instruments like SOHO/CDS and RHESSI.

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Chromospheric and Transition Region Dynamics

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  1. Chromospheric and Transition Region Dynamics Peter T Gallagher L-3 Com EER Systems NASA Goddard Space Flight Center Peter T Gallagher (NASA/GSFC)

  2. Chromospheric and Transition Region Heating • How do nonthermal electrons heat the upper chromosphere and transition region? • Assume a source function of energetic electrons with a spectrum: • F(E) ~ (E/Ec)- electrons cm-2 s-1 • Nonthermal electrons supply the necessary energy to heat the plasma and hence cause flows. • Enonthermal = Ethermal + Ekinetic Peter T Gallagher (NASA/GSFC)

  3. T1: Nonthermal Electrons Nonthermal Chromospheric Heating • A large overpressure can only be met if the heating time scale is less than the hydrodynamic expansion time scale: • 3 kB T / Q < L / cs • Q = heating rate per particle • T = final temperature of the heated plasma • cs = sound speed • L = length of the heated high-pressure region • Resulting pressure gradient will drive mass flows of hot plasma from the chromosphere to the corona. T3: Vup<1000 km/s T2: Impulsive Heating Den T3: VDOWN<100 km/s Peter T Gallagher (NASA/GSFC)

  4. Gentle Vs. Explosive Evaporation • There is a nonthermal energy flux threshold between gentle and explosive evaporation. • This threshold can be determined by equating the nonthermal thick-target heating rate per particle which the peak radiative loos rate. • Gentle evaporation velocities < 100 km s-1 • Explosive evaporation velocities up to 600 km s-1 Peter T Gallagher (NASA/GSFC)

  5. Temperature Sensitivities Peter T Gallagher (NASA/GSFC)

  6. Spectral Evolution • Soft-hard-soft spectrum Peter T Gallagher (NASA/GSFC)

  7. CDS and TRACE: 26 March 2002 Flare • SOHO/CDS • He I (0.03 MK) • O V (0.25 MK) • Mg X (1.1 MK) • Fe XVI (2.5 MK) • Fe XIX (8 MK) • TRACE 17.1 nm • Fe IX/X (1.0 MK) Peter T Gallagher (NASA/GSFC)

  8. RHESSI Imaging Spectroscopy • RHESSI 6-12 keV • CDS Fe XIX (8 MK) Outflow Footpoints Peter T Gallagher (NASA/GSFC)

  9. T* Conductive Radiative t* Loop Cooling Curves Peter T Gallagher (NASA/GSFC)

  10. C3.0 Flare Properties • Total nonthermal energy: Etot(>10 keV) ~ 5.7 x 109 ergs cm-1 s-1 • Spectral index:  ~ 6.6 • Maximum upflow velocity: ~250 km/sec • Maximum downflow velocity: < 100 km/sec Peter T Gallagher (NASA/GSFC)

  11. Footpoint Downflows • Loops are not static • Maximum downflow ~110 km/sec • Loops cool via conduction, radiation, and flows. (SHOW MOVIE) Peter T Gallagher (NASA/GSFC)

  12. Future Work • What is the nonthermal energy flux threshold for explosive evaporation? • Is all the nonthermal energy in a beam lost at a particular height? What is the efficiency? • Study how the chromospheric and TR response varies with low-energy cut-off, spectral index, total flux, etc. using a large sample of data from CDS and RHESSI. • Look at energy balance between nonthermal energy and resulting thermal and kinetic energies. • General question: Impulsive v continuous energy release. Peter T Gallagher (NASA/GSFC)

  13. Prolonged Energy Release Peter T Gallagher (NASA/GSFC)

  14. T* Conductive Radiative t* Loop Cooling Curves Peter T Gallagher (NASA/GSFC)

  15. CDS Emission Measure Diagnostics • Assume an isothermal plasma at a temperature T, which corresponds to the maximum of the contribution function. • The integrated intensity of the line can be approximated by: • I = ( 1 / 4  ) A G(T) EM => EM = I 4  / A G( T ) • G( T ) from CHIANTI or ADAS. • Elemental abundances of Fludra & Schmeltz (1995) Peter T Gallagher (NASA/GSFC)

  16. CDS Density Diagnostics • Assuming a homogeneous and isothermal plasma: • Ne = SQRT( EM / V ) • This approximation does not take into account filamentary plasma. • The fraction of the emitting volume actually contributing to the total emission is termed the filling factor (f). • Estimating the filling factor (0.1-1), the electron density can then be given by: • Ne = SQRT( EM / fV ) Peter T Gallagher (NASA/GSFC)

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