1. Emission Measure Distributions: A Tutorial Nancy S. Brickhouse Harvard-Smithsonian Center for Astrophysics From Spectral Lines to Emission Measures Emission Measure Distributions of Different Stars Hot Research Questions Coronal Structure Coronal Abundances Coronal heating: flares and nanoflares New England Space Science Cambridge, MA March 1, 2006

2. We use UV, EUV, and X-ray Lines from log T = 4.0 to 8.0 Chandra From Spectral Lines to Emission Measure

3. Line Flux = ∑ ε (T) EM(T) / (4 π R 2 ), where R is the distance, EM (T) = ∫ N e N H dV is the emission measure, and ε (T) depends on a lot of atomic physics, e.g. ionization and recombination rates collisional excitation rates radiative decay rates AND we make a number of assumptions, such as negligible optical depth collisional ionization equilibrium

4. Emission Measure Distributions for Different Stars

5. I.Understanding Coronal Structure Rotation and Activity Evolution Sun (G2 V) Yohkoh Image Capella (G8 III + G1 III); Expanding Loops? Hot Research Questions

6. Chandra Gratings Capella Electron Density Determination Compelling evidence for high density, small emitting region(s) Multiple pressures in the system Lower Pressure; L=.02 R * High Pressure; L=.003 R *

7. II. Abundances: Continuum and Line Modeling HR 1099 High Neon Abundance An Inverse FIP Effect? Hot Research Questions

8. III. Coronal heating: flares > 1 day flare with exponential decay Eclipse gives the extent of the flare loop Algol

9. Hot Research Topics III. Coronal heating: nanoflares Discrepancies are not explained by: atomic rate uncertainties calibration uncertainties absorption time variability I EUV Ω EUV [T e ] —— = ———— exp (-ΔE/kT e ) I X-ray Ω X-ray [T e ] 6 MK EMD peak

10. Breaking the Assumptions of Emission Measure Distribution Analysis We consider episodic heating (nanoflares) with: - heat input to the chromosphere - adiabatic expansion with rapid cooling. We calculate the time-dependent ionization state and obtain the resulting line emission. Chromosphere B T e (0) = 12 MK N e (0) = 4 x 10 12 cm -3 ~c s Loop Footpoint Energetic Beam T e (t) = 5 MK N e (t) = 10 12 cm -3 Δt~ 1 sec Chromosphere