in-situ measurement
Definition: Direct sampling of local plasma, fields, particles, or dust by an instrument located within the environment being studied.
Why it matters here: In-situ measurement directly samples the plasma, fields, and particles at the spacecraft’s own location, in contrast to remote sensing’s reliance on radiation received from a distance; the two techniques are complementary, not competing, and heliophysics missions are frequently coordinated (e.g., Parker Solar Probe and Solar Orbiter observing simultaneously from different vantage points, as both missions have done).
Relationship: Use this term alongside perihelion; the lecture's quantitative model is summarized below.
\[ \text{In-situ: measure plasma/field properties directly at the spacecraft.}\quad \text{Remote sensing: infer properties from received radiation (Lecture 09).} \]
Boundary or common confusion: Confusing Parker Solar Probe’s own orbital (heliocentric) speed with the solar wind’s outflow speed that it measures. The spacecraft’s record-breaking 191 km/s speed at perihelion is a consequence of its own orbital mechanics (falling deep into the Sun’s gravitational well), not a measurement of how fast the solar wind itself is flowing at that location -- these are two entirely distinct velocities that a careless reading of headlines can easily conflate.
perihelion
Definition: The point in a spacecraft or planet's orbit at which its distance from the Sun is smallest.
Why it matters here: Parker Solar Probe, launched 2018-08-12, uses repeated gravity assists from Venus to reach a perihelion of just 9.86 solar radii (6.9e+06 km from the Sun’s center) -- deep inside the corona, well within Lecture 08’s computed critical radius (4.62 R_sun) -- reaching a record heliocentric speed of 191 km/s (690,000 km/h) at its closest approach, making it the fastest human-made object ever built.
Relationship: Use this term alongside gravity assist; the lecture's quantitative model is summarized below.
\[ \text{In-situ: measure plasma/field properties directly at the spacecraft.}\quad \text{Remote sensing: infer properties from received radiation (Lecture 09).} \]
Boundary or common confusion: Confusing Parker Solar Probe’s own orbital (heliocentric) speed with the solar wind’s outflow speed that it measures. The spacecraft’s record-breaking 191 km/s speed at perihelion is a consequence of its own orbital mechanics (falling deep into the Sun’s gravitational well), not a measurement of how fast the solar wind itself is flowing at that location -- these are two entirely distinct velocities that a careless reading of headlines can easily conflate.
gravity assist
Definition: An exchange of energy and angular momentum with a moving planet that changes a spacecraft's heliocentric orbit without equivalent propellant use.
Why it matters here: Parker Solar Probe, launched 2018-08-12, uses repeated gravity assists from Venus to reach a perihelion of just 9.86 solar radii (6.9e+06 km from the Sun’s center) -- deep inside the corona, well within Lecture 08’s computed critical radius (4.62 R_sun) -- reaching a record heliocentric speed of 191 km/s (690,000 km/h) at its closest approach, making it the fastest human-made object ever built.
Relationship: Use this term alongside Alfven surface; the lecture's quantitative model is summarized below.
\[ \text{In-situ: measure plasma/field properties directly at the spacecraft.}\quad \text{Remote sensing: infer properties from received radiation (Lecture 09).} \]
Boundary or common confusion: Confusing Parker Solar Probe’s own orbital (heliocentric) speed with the solar wind’s outflow speed that it measures. The spacecraft’s record-breaking 191 km/s speed at perihelion is a consequence of its own orbital mechanics (falling deep into the Sun’s gravitational well), not a measurement of how fast the solar wind itself is flowing at that location -- these are two entirely distinct velocities that a careless reading of headlines can easily conflate.
Alfven surface
Definition: The boundary where outward solar-wind speed first exceeds the inward communication speed of Alfven waves along the magnetic field.
Why it matters here: In 2021, Parker Solar Probe crossed the Alfven surface (the boundary within which the solar wind’s outward speed is slower than the local Alfven wave speed, so that magnetic disturbances can in principle propagate back toward the Sun) for the first time, entering the solar atmosphere in a meaningful physical sense -- described by NASA as the mission "touching the Sun."
Relationship: Use this term alongside magnetic switchback; the lecture's quantitative model is summarized below.
\[ \text{In-situ: measure plasma/field properties directly at the spacecraft.}\quad \text{Remote sensing: infer properties from received radiation (Lecture 09).} \]
Boundary or common confusion: Confusing Parker Solar Probe’s own orbital (heliocentric) speed with the solar wind’s outflow speed that it measures. The spacecraft’s record-breaking 191 km/s speed at perihelion is a consequence of its own orbital mechanics (falling deep into the Sun’s gravitational well), not a measurement of how fast the solar wind itself is flowing at that location -- these are two entirely distinct velocities that a careless reading of headlines can easily conflate.
magnetic switchback
Definition: A localized, often sharp reversal or fold in the heliospheric magnetic-field direction observed in the young solar wind.
Why it matters here: Parker Solar Probe has directly detected numerous "magnetic switchbacks" (sudden, localized reversals in the solar wind’s magnetic field direction) in the near-Sun solar wind, a real, still-being-investigated phenomenon relevant to the coronal-heating and wind-acceleration problems introduced in Lecture 02 and Lecture 08.
Relationship: Use this term alongside Faraday cup; the lecture's quantitative model is summarized below.
\[ \text{In-situ: measure plasma/field properties directly at the spacecraft.}\quad \text{Remote sensing: infer properties from received radiation (Lecture 09).} \]
Boundary or common confusion: Confusing Parker Solar Probe’s own orbital (heliocentric) speed with the solar wind’s outflow speed that it measures. The spacecraft’s record-breaking 191 km/s speed at perihelion is a consequence of its own orbital mechanics (falling deep into the Sun’s gravitational well), not a measurement of how fast the solar wind itself is flowing at that location -- these are two entirely distinct velocities that a careless reading of headlines can easily conflate.
Faraday cup
Definition: A particle detector that collects charged particles and uses the resulting current-versus-voltage response to infer plasma density, speed, and temperature.
Why it matters here: Parker Solar Probe carries direct-sampling ("in-situ") instruments -- a Faraday cup and electrostatic analyzers (SWEAP) to directly measure solar wind ion/electron velocity, density, and temperature; magnetometers (FIELDS) to directly measure the local magnetic field; and energetic-particle detectors (IS(sun)IS) -- rather than remotely imaging the Sun from a distance as SDO and SOHO do (Lecture 09).
Relationship: Use this term alongside in-situ measurement; the lecture's quantitative model is summarized below.
\[ \text{In-situ: measure plasma/field properties directly at the spacecraft.}\quad \text{Remote sensing: infer properties from received radiation (Lecture 09).} \]
Boundary or common confusion: Confusing Parker Solar Probe’s own orbital (heliocentric) speed with the solar wind’s outflow speed that it measures. The spacecraft’s record-breaking 191 km/s speed at perihelion is a consequence of its own orbital mechanics (falling deep into the Sun’s gravitational well), not a measurement of how fast the solar wind itself is flowing at that location -- these are two entirely distinct velocities that a careless reading of headlines can easily conflate.