Solar Flare

1. CSI 769-001/PHYS 590-001 Solar Atmosphere Fall 2004 Lecture 11 Nov. 10, 2004 Solar Flare

2. Flare Properties and Models • Temporal Properties • Spectral Properties • Spatial Properties

3. Different phases of a Flare • A flare may have three phases in its temporal evolution • Preflare phase: e.g., 4 min from 13:50 UT – 13:56 UT • Impulsive phase: e.g., 10 min from 13:56 UT – 14:06 UT • Gradual phase: e.g., many hours after 14:06 UT

4. Different Phases of a Flare (Cont’d) • Pre-flare phase: flare trigger phase leading to the major energy release. It shows slow increase of soft X-ray flux • Impulsive phase: the flare main energy release phase. It is most evident in hard X-ray, γ-ray emission and radio microwave emission. The soft X-ray flux rises rapidly during this phase • “Neupert effect” • Gradual phase: no further emission in hard X-ray, and the soft X-ray flux starts to decrease gradually. • Loop arcade (or arch) starts to appear in this phase

5. “Neupert Effect” • Definition: in flare impulsive phase, it is found that the soft X-ray profile roughly matches the time integral of the hard X-ray profile (or microwave profile); in other words, the time rate of change of the soft X-ray emission should be equal to the hard X-ray emission. • An Explanation of “Neupert Effect” • A downward-moving electron beam of non-thermal energetic electrons causes hard X-ray emission, when they hit the dense chromosphere and instantaneously lose energy • The footpoint portion of chromosphere is heated up by deposited non-thermal electron energy, and evaporated into the corona. • Soft X-ray emission originates from the evaporated hot plasma that fills up coronal loops

6. Flare Spectrum (cont’d) • Bremsstrahlung emission (German word meaning "braking radiation") • the radiation is produced as the electrons are deflected in the Coulomb field of the ions. Bremsstrahlung emission

7. Flare Spectrum • The EM emission spectrum during flare’s impulsive phase

8. Flare Spectrum (cont’d) • Flare Spectrum: distribution of photon number versus photon energy • A full flare spectrum may have three components: • Exponential distribution in Soft X-ray energy range (e.g., 1 keV to 10 keV) • Power-law distribution in hard X-ray energy range (e.g., 10 keV to 100 keV) • Power-law plus spectral line distribution in Gamma-ray energy range (e.g., 100 keV to 100 MeV)

9. Flare Spectrum (cont’d) • Exponential distribution in soft X-ray • dF(E)/dE = A e-E/E0 Photons cm-2 s-1 keV-1 • Where F(E) the photon flux in unit of Photons cm-2 s-1, E the photon energy in unit of keV, E0 the e-folding energy, and A a fitting constant • Exponential distribution indicates a thermal origin • The exponential component is produced by thermal electrons in a hot plasma at a temperature of about 10 MK through Bremmstrahlung emission • e.g., 1 keV photon is equivalent to 10 MK thermal electron in terms of energy

10. Flare Spectrum (cont’d) • Power-law distribution in hard X-ray • dF(E)/dE = AE–γ Photons cm-2 s-1 keV-1 • Where γ is the power-law index • Power-law distribution indicates a non-thermal origin • The power-law component in hard X-ray range is produced by non-thermal electrons (at 10 keV to 100 keV) through Bremsstrahlung emission • The source electron energy distribution should also be a power-law.

11. Flare Spectrum (cont’d) • Assuming γ the power-law index of hard X-ray, and δ the power-law index of source non-thermal electrons. There are two cases (textbook P. 293) • Thick target: • γ= δ-1 • particle loss energy instantaneously, e.g., particle hits dense chromosphere • Thin target: • γ= δ+1 • particle loss energy slowly, e.g, in the thin corona

12. Flare Spectrum (cont’d) • Power-law plus spectral line distribution in Gamma-ray range • The power-law component is produced by non-thermal electrons (> 100 keV) through Bremsstrahlung emission • Many γ-ray lines superposed on the continuous power-law distribution is produced by nuclear interaction between energetic protons and many nuclei in ambient atmosphere • The prominent spectral line at 511 keV is due to positron-electron annihilation; positron is produced by certain nuclear reactions.

13. Flare Morphology • Loop structure of soft X-ray emission • Compact hard X-ray sources appear at two foot-points of soft X-ray loop • Hard X-ray sources appear at top of soft X-ray loops

14. Flare Model • Basic elements of a flare model from energy argument (e.g., “the standard model” in textbook P.282, Figure 9.3) • Magnetic free energy is stored in the corona, due either to motions of the photospheric footpoints or to the emergence of current-carrying field from below the photosphere • The field evolves slowly through equilibrium states, finally reaching a non-equilibrium state that leads to reconnection • The reconnection provides particle acceleration and plasma heating that we call the flare

15. Flare Model (cont’d) • Flare models are constantly evolving • One model by Shibata et al. (Figure 9.8, P. 298 in text book), showing • HXR loop top source • HXR footpoint sources • SXR loop • Reconnection • Reconnection inflow • Reconnection jet • Fast shock • Plasmoid/Filament ejection

16. Flare Model (cont’d) A cartoon model by Gurman