photosphere
Definition: The thin visible layer from which most solar optical photons escape, conventionally associated with continuum optical depth near unity.
Why it matters here: Moving outward from the photosphere (T_eff=5772 K), the temperature first falls to a minimum of roughly 4100 K, then rises through the chromosphere to about 2e+04 K, and then jumps abruptly, across a transition region only a few hundred kilometers thick, to the corona’s million-plus-kelvin temperature (1.5e+06 K) -- a temperature increase of more than two orders of magnitude in a region with no additional nearby heat source as obvious as the solar interior below the photosphere.
Relationship: Use this term alongside chromosphere; the lecture's quantitative model is summarized below.
\[ T_{\rm corona} \gg T_{\rm chromosphere} > T_{\rm eff} \ \text{despite increasing distance from the nuclear energy source (requires non-radiative heating)} \]
Boundary or common confusion: Assuming higher temperature always means more visually or energetically dominant. The corona’s temperature vastly exceeds the photosphere’s, yet the photosphere emits far more visible light per unit area (Stefan-Boltzmann, proportional to T^4, is not directly applicable to the extremely tenuous corona, which is optically thin and not a blackbody emitter) -- do not conflate "hotter" with "brighter in visible light."
chromosphere
Definition: The structured atmospheric layer above the photosphere in which temperature rises again and hydrogen and calcium lines become prominent.
Why it matters here: Moving outward from the photosphere (T_eff=5772 K), the temperature first falls to a minimum of roughly 4100 K, then rises through the chromosphere to about 2e+04 K, and then jumps abruptly, across a transition region only a few hundred kilometers thick, to the corona’s million-plus-kelvin temperature (1.5e+06 K) -- a temperature increase of more than two orders of magnitude in a region with no additional nearby heat source as obvious as the solar interior below the photosphere.
Relationship: Use this term alongside transition region; the lecture's quantitative model is summarized below.
\[ T_{\rm corona} \gg T_{\rm chromosphere} > T_{\rm eff} \ \text{despite increasing distance from the nuclear energy source (requires non-radiative heating)} \]
Boundary or common confusion: Assuming higher temperature always means more visually or energetically dominant. The corona’s temperature vastly exceeds the photosphere’s, yet the photosphere emits far more visible light per unit area (Stefan-Boltzmann, proportional to T^4, is not directly applicable to the extremely tenuous corona, which is optically thin and not a blackbody emitter) -- do not conflate "hotter" with "brighter in visible light."
transition region
Definition: A narrow interface where solar-atmosphere temperature rises from chromospheric to coronal values while density falls sharply.
Why it matters here: Moving outward from the photosphere (T_eff=5772 K), the temperature first falls to a minimum of roughly 4100 K, then rises through the chromosphere to about 2e+04 K, and then jumps abruptly, across a transition region only a few hundred kilometers thick, to the corona’s million-plus-kelvin temperature (1.5e+06 K) -- a temperature increase of more than two orders of magnitude in a region with no additional nearby heat source as obvious as the solar interior below the photosphere.
Relationship: Use this term alongside corona; the lecture's quantitative model is summarized below.
\[ T_{\rm corona} \gg T_{\rm chromosphere} > T_{\rm eff} \ \text{despite increasing distance from the nuclear energy source (requires non-radiative heating)} \]
Boundary or common confusion: Assuming higher temperature always means more visually or energetically dominant. The corona’s temperature vastly exceeds the photosphere’s, yet the photosphere emits far more visible light per unit area (Stefan-Boltzmann, proportional to T^4, is not directly applicable to the extremely tenuous corona, which is optically thin and not a blackbody emitter) -- do not conflate "hotter" with "brighter in visible light."
corona
Definition: The Sun's hot, tenuous, magnetically structured outer atmosphere, visible in extreme ultraviolet, X-rays, and during total eclipses.
Why it matters here: Moving outward from the photosphere (T_eff=5772 K), the temperature first falls to a minimum of roughly 4100 K, then rises through the chromosphere to about 2e+04 K, and then jumps abruptly, across a transition region only a few hundred kilometers thick, to the corona’s million-plus-kelvin temperature (1.5e+06 K) -- a temperature increase of more than two orders of magnitude in a region with no additional nearby heat source as obvious as the solar interior below the photosphere.
Relationship: Use this term alongside temperature minimum; the lecture's quantitative model is summarized below.
\[ T_{\rm corona} \gg T_{\rm chromosphere} > T_{\rm eff} \ \text{despite increasing distance from the nuclear energy source (requires non-radiative heating)} \]
Boundary or common confusion: Assuming higher temperature always means more visually or energetically dominant. The corona’s temperature vastly exceeds the photosphere’s, yet the photosphere emits far more visible light per unit area (Stefan-Boltzmann, proportional to T^4, is not directly applicable to the extremely tenuous corona, which is optically thin and not a blackbody emitter) -- do not conflate "hotter" with "brighter in visible light."
temperature minimum
Definition: The atmospheric height above the photosphere at which the mean temperature reaches its lowest value before chromospheric heating dominates.
Why it matters here: Moving outward from the photosphere (T_eff=5772 K), the temperature first falls to a minimum of roughly 4100 K, then rises through the chromosphere to about 2e+04 K, and then jumps abruptly, across a transition region only a few hundred kilometers thick, to the corona’s million-plus-kelvin temperature (1.5e+06 K) -- a temperature increase of more than two orders of magnitude in a region with no additional nearby heat source as obvious as the solar interior below the photosphere.
Relationship: Use this term alongside coronal heating problem; the lecture's quantitative model is summarized below.
\[ T_{\rm corona} \gg T_{\rm chromosphere} > T_{\rm eff} \ \text{despite increasing distance from the nuclear energy source (requires non-radiative heating)} \]
Boundary or common confusion: Assuming higher temperature always means more visually or energetically dominant. The corona’s temperature vastly exceeds the photosphere’s, yet the photosphere emits far more visible light per unit area (Stefan-Boltzmann, proportional to T^4, is not directly applicable to the extremely tenuous corona, which is optically thin and not a blackbody emitter) -- do not conflate "hotter" with "brighter in visible light."
coronal heating problem
Definition: The unresolved question of how magnetic and wave energy is converted into enough heat to maintain million-kelvin coronal plasma.
Why it matters here: Parker Solar Probe (Unit 5) and other missions have directly measured Alfven-wave activity and small-scale magnetic reconnection events ("nanoflares," a mechanism proposed by Eugene Parker, the mission’s namesake) in the near-Sun corona, both leading candidate mechanisms for solving the coronal heating problem.
Relationship: Use this term alongside photosphere; the lecture's quantitative model is summarized below.
\[ T_{\rm corona} \gg T_{\rm chromosphere} > T_{\rm eff} \ \text{despite increasing distance from the nuclear energy source (requires non-radiative heating)} \]
Boundary or common confusion: Assuming higher temperature always means more visually or energetically dominant. The corona’s temperature vastly exceeds the photosphere’s, yet the photosphere emits far more visible light per unit area (Stefan-Boltzmann, proportional to T^4, is not directly applicable to the extremely tenuous corona, which is optically thin and not a blackbody emitter) -- do not conflate "hotter" with "brighter in visible light."