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

Solar Flare

Flare Properties and Models

• Temporal Properties

• Spectral Properties

• Spatial Properties

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

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

“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

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

Flare Spectrum

• The EM emission spectrum during flare’s impulsive phase

Flare Spectrum (cont’d)

• Flare Spectrum: distribution of photon number versus photon energy

• A full flare spectrum may have three components:

1. Exponential distribution in Soft X-ray energy range (e.g., 1 keV to 10 keV)

2. Power-law distribution in hard X-ray energy range (e.g., 10 keV to 100 keV)

3. Power-law plus spectral line distribution in Gamma-ray energy range (e.g., 100 keV to 100 MeV)

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

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.

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

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.

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

Flare Model

• Basic elements of a flare model from energy argument (e.g., “the standard model” in textbook P.282, Figure 9.3)

1. 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

2. The field evolves slowly through equilibrium states, finally reaching a non-equilibrium state that leads to reconnection

3. The reconnection provides particle acceleration and plasma heating that we call the flare

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

Flare Model (cont’d)

A cartoon model by Gurman