Terms Developed in Context
coronagraph
An instrument that blocks the bright solar disk so faint coronal structures and outward-moving CMEs can be imaged. In this lecture, the term is needed to interpret the following evidence or model claim: 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.
Its relationship to plane-of-sky speed is part of the lecture's reasoning chain rather than a synonym: A coronagraph measures only the CME’s speed projected onto the plane of the sky (perpendicular to the observer’s line of sight); a CME propagating close to the Sun-Earth line (as an Earth-directed "halo" CME does) has its true radial speed systematically underestimated by simple plane-of-sky measurements unless a full 3D reconstruction (using multiple viewpoints, e.g., STEREO plus SOHO) is performed. The working relation \( t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \) makes the relevant observable or model dependence explicit.
Limitation and usage: 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. This caution sets the boundary on what can be inferred when using coronagraph in an explanation.
plane-of-sky speed
The velocity component inferred from motion projected across an image plane, excluding unmeasured line-of-sight motion. In this lecture, the term is needed to interpret the following evidence or model claim: 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.
Its relationship to projection effect is part of the lecture's reasoning chain rather than a synonym: A coronagraph measures only the CME’s speed projected onto the plane of the sky (perpendicular to the observer’s line of sight); a CME propagating close to the Sun-Earth line (as an Earth-directed "halo" CME does) has its true radial speed systematically underestimated by simple plane-of-sky measurements unless a full 3D reconstruction (using multiple viewpoints, e.g., STEREO plus SOHO) is performed. The working relation \( t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \) makes the relevant observable or model dependence explicit.
Limitation and usage: 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. This caution sets the boundary on what can be inferred when using plane-of-sky speed in an explanation.
projection effect
Bias introduced when a three-dimensional position or velocity is inferred from a two-dimensional image. In this lecture, the term is needed to interpret the following evidence or model claim: 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.
Its relationship to CME kinematics is part of the lecture's reasoning chain rather than a synonym: 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. The working relation \( t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \) makes the relevant observable or model dependence explicit.
Limitation and usage: 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. This caution sets the boundary on what can be inferred when using projection effect in an explanation.
CME kinematics
The time-dependent position, speed, and acceleration of a coronal mass ejection from the low corona through interplanetary space. In this lecture, the term is needed to interpret the following evidence or model claim: 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.
Its relationship to aerodynamic drag is part of the lecture's reasoning chain rather than a synonym: 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. The working relation \( t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \) makes the relevant observable or model dependence explicit.
Limitation and usage: 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. This caution sets the boundary on what can be inferred when using CME kinematics in an explanation.
aerodynamic drag
Momentum exchange that tends to bring a CME's speed toward the ambient solar-wind speed during propagation. In this lecture, the term is needed to interpret the following evidence or model claim: 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.
Its relationship to kinetic energy budget is part of the lecture's reasoning chain rather than a synonym: A CME experiences a drag-like force from momentum exchange with the ambient solar wind, often modeled (in simplified "drag-based models" used operationally for space-weather forecasting) as proportional to the relative velocity squared between the CME and the ambient solar wind, causing a fast CME to decelerate toward the ambient wind speed over its transit and a slow CME to accelerate toward it. The working relation \( t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \) makes the relevant observable or model dependence explicit.
Limitation and usage: 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. This caution sets the boundary on what can be inferred when using aerodynamic drag in an explanation.
kinetic energy budget
The translational energy one-half M v squared assigned to CME bulk motion, subject to uncertainties in both mass and speed. In this lecture, the term is needed to interpret the following evidence or model claim: 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.
Its relationship to coronagraph is part of the lecture's reasoning chain rather than a synonym: 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. The working relation \( t_{\rm naive} = \frac{d}{v_{\rm CME}}\ \text{(constant speed)}, \qquad E_{\rm kinetic} = \frac{1}{2}Mv^2\ \text{(order-of-magnitude CME energy)} \) makes the relevant observable or model dependence explicit.
Limitation and usage: 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. This caution sets the boundary on what can be inferred when using kinetic energy budget in an explanation.