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Physics 681: Solar Physics and Instrumentation – Lecture 10

Physics 681: Solar Physics and Instrumentation – Lecture 10. Carsten Denker NJIT Physics Department Center for Solar–Terrestrial Research. The Atmosphere. Radiative Transfer – LTE Equation of Transfer Equlibria Absorption Lines in LTE Radiative Transfer – Statistical Equilibrium (SE)

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Physics 681: Solar Physics and Instrumentation – Lecture 10

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  1. Physics 681: Solar Physics and Instrumentation – Lecture 10 Carsten Denker NJIT Physics Department Center for Solar–Terrestrial Research

  2. The Atmosphere • Radiative Transfer – LTE • Equation of Transfer • Equlibria • Absorption Lines in LTE • Radiative Transfer – Statistical Equilibrium (SE) • Model Assumptions • Line Radiation and Einstein Coefficients • Continuum Radiation • Collisions • Source Function • Equations of Statistical Equilibrium Center for Solar-Terrestrial Research

  3. Equation of Transfer • Optical depth • Radiative transfer equation • Formal solution • Total emergent intensity • How can we derive the source function from the absolute intensity? Inversion! Center for Solar-Terrestrial Research

  4. Equilibria • Thermodynamic equilibrium • A single temperature T describes the state of the atmosphere everywhere • Maxwellian velocity distribution • Ionization and excitation according to the Saha and Boltzmann equations • Homogeneous and isotropic black-body radiation field • No temperature gradient! • Local thermodynamic equilibrium (LTE) • Locally, a single temperature T describes the atmosphere • Important simplification: Sν = Bν(T ) • Thermalization length has to be shorter than the distance over which the temperature changes • LTE might not apply to all species of particles • Good approximation for visible and IR continua, line wings, and weak line profiles • Non-LTE (a single temperature T is insufficient) • Radiative interactions are too rare • Thermalization length is too long Center for Solar-Terrestrial Research

  5. Absorption Lines in LTE • Continuum and line absorption coefficients • Intensity at disk center • Doppler and collisional line broadening (Voigt profile) Center for Solar-Terrestrial Research

  6. Radiative Transfer – SE • Statistical Equlibrium (SE) • Electrons are still described by a Maxwell distribution (single electron temperature Te) • However, population depends on radiative processes • Einstein coefficients • Spontaneous emission • Induced emission • Absorption Center for Solar-Terrestrial Research

  7. Continuum Radiation and Collisions • Photoionization • Radiative recombination • Collisional transitions between two bound states • Collisional transition from and to the continuum Center for Solar-Terrestrial Research

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