coronagraph
Definition: An instrument that blocks the bright solar disk so faint coronal structures and outward-moving CMEs can be imaged.
Why it matters here: A coronagraph (Unit 5) occults the bright photospheric disk with an opaque disk, allowing the much fainter corona and any propagating CMEs to be imaged in visible (Thomson-scattered) light out to several solar radii and beyond; SOHO’s LASCO instrument has continuously catalogued CMEs since 1996 in the standard CDAW catalog.
Relationship: Use this term alongside plane-of-sky speed; the lecture's quantitative model is summarized below.
\[ t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \]
Boundary or common confusion: Treating a coronagraph-measured plane-of-sky speed as the CME’s true propagation speed without qualification, especially for Earth-directed "halo" events where projection effects are largest. This lecture explicitly uses the real, published LASCO plane-of-sky speed while flagging that it is a projected, not necessarily true radial, speed -- a distinction a careful reviewer must check is stated, not silently elided.
plane-of-sky speed
Definition: The velocity component inferred from motion projected across an image plane, excluding unmeasured line-of-sight motion.
Why it matters here: The 6 September 2017 CME’s real, published SOHO/LASCO plane-of-sky linear speed was 1571 km/s -- fast enough that a naive constant-speed extrapolation predicts an Earth-arrival time of only 26.5 hours, yet the real, observed transit time was closer to 36 hours, a substantial, real, and physically meaningful discrepancy that this lecture’s deceleration physics explains.
Relationship: Use this term alongside projection effect; the lecture's quantitative model is summarized below.
\[ t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \]
Boundary or common confusion: Treating a coronagraph-measured plane-of-sky speed as the CME’s true propagation speed without qualification, especially for Earth-directed "halo" events where projection effects are largest. This lecture explicitly uses the real, published LASCO plane-of-sky speed while flagging that it is a projected, not necessarily true radial, speed -- a distinction a careful reviewer must check is stated, not silently elided.
projection effect
Definition: Bias introduced when a three-dimensional position or velocity is inferred from a two-dimensional image.
Why it matters here: Statistical studies of many CMEs (e.g., Gopalswamy et al. and others) show that fast CMEs (well above the ambient solar wind speed) systematically decelerate during their interplanetary transit due to aerodynamic-like drag against the ambient solar wind, while slow CMEs can actually accelerate toward the ambient wind speed -- both effects make a naive constant-speed extrapolation from the initial coronagraph-measured speed systematically inaccurate for fast events.
Relationship: Use this term alongside CME kinematics; the lecture's quantitative model is summarized below.
\[ t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \]
Boundary or common confusion: Treating a coronagraph-measured plane-of-sky speed as the CME’s true propagation speed without qualification, especially for Earth-directed "halo" events where projection effects are largest. This lecture explicitly uses the real, published LASCO plane-of-sky speed while flagging that it is a projected, not necessarily true radial, speed -- a distinction a careful reviewer must check is stated, not silently elided.
CME kinematics
Definition: The time-dependent position, speed, and acceleration of a coronal mass ejection from the low corona through interplanetary space.
Why it matters here: CME kinematics -- measured via coronagraphy and modified by interplanetary drag -- determine how long a real eruption takes to reach Earth, a genuinely uncertain quantity this course’s case study shows can differ from a naive estimate by tens of percent; Lecture 08 now derives the ambient solar wind that CMEs propagate through and decelerate toward.
Relationship: Use this term alongside aerodynamic drag; the lecture's quantitative model is summarized below.
\[ t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \]
Boundary or common confusion: Treating a coronagraph-measured plane-of-sky speed as the CME’s true propagation speed without qualification, especially for Earth-directed "halo" events where projection effects are largest. This lecture explicitly uses the real, published LASCO plane-of-sky speed while flagging that it is a projected, not necessarily true radial, speed -- a distinction a careful reviewer must check is stated, not silently elided.
aerodynamic drag
Definition: Momentum exchange that tends to bring a CME's speed toward the ambient solar-wind speed during propagation.
Why it matters here: Statistical studies of many CMEs (e.g., Gopalswamy et al. and others) show that fast CMEs (well above the ambient solar wind speed) systematically decelerate during their interplanetary transit due to aerodynamic-like drag against the ambient solar wind, while slow CMEs can actually accelerate toward the ambient wind speed -- both effects make a naive constant-speed extrapolation from the initial coronagraph-measured speed systematically inaccurate for fast events.
Relationship: Use this term alongside kinetic energy budget; the lecture's quantitative model is summarized below.
\[ t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \]
Boundary or common confusion: Treating a coronagraph-measured plane-of-sky speed as the CME’s true propagation speed without qualification, especially for Earth-directed "halo" events where projection effects are largest. This lecture explicitly uses the real, published LASCO plane-of-sky speed while flagging that it is a projected, not necessarily true radial, speed -- a distinction a careful reviewer must check is stated, not silently elided.
kinetic energy budget
Definition: The translational energy one-half M v squared assigned to CME bulk motion, subject to uncertainties in both mass and speed.
Why it matters here: A CME’s kinetic energy can be estimated as (1/2) M v^2 given an estimated ejected mass M (from coronagraph brightness, itself a measure of Thomson-scattered light from the CME’s electron content) and its speed v -- providing an independent energy estimate that can be compared to Lecture 05’s flare free-energy budget for the same event.
Relationship: Use this term alongside coronagraph; the lecture's quantitative model is summarized below.
\[ t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \]
Boundary or common confusion: Treating a coronagraph-measured plane-of-sky speed as the CME’s true propagation speed without qualification, especially for Earth-directed "halo" events where projection effects are largest. This lecture explicitly uses the real, published LASCO plane-of-sky speed while flagging that it is a projected, not necessarily true radial, speed -- a distinction a careful reviewer must check is stated, not silently elided.