Space and Solar Definitions - P

Packet - A quantity of data used as the basis for multiplexing, for example in accordance with CCSDS.

PAM - Payload Assist Module upper stage.

Parallels - Circles in parallel planes to that of the equator defining north-south measurements, also called lines of latitude.

Pathfinder - The Mars Environmental Survey (MESUR) engineering prototype later named Mars Pathfinder.

PDS - Planetary Data System.

PDT - Pacific Daylight Time.

PE - Post Encounter phase in flyby mission operations.

Penumbra The Sunspot Area That May Surround The Darker Umbra Or Umbrae - It consists of linear bright and dark elements radial from the sunspot umbra.

Periapsis - The point in an orbit closest to the body being orbited.

Perigee - Periapsis in Earth orbit.

Perihelion - Periapsis in solar orbit.

Perijove - Periapsis in Jupiter orbit.

Perilune - Periapsis in lunar orbit.

Periselene - Periapsis in lunar orbit.

Persistence - Continuation of existing conditions. When a physical parameter varies slowly, the best prediction is often persistence.

Phase

(1) The angular distance between peaks or troughs of two waveforms of similar frequency.

(2) The particular appearance of a body's state of illumination, such as the full or crescent phases of the Moon.

(3) Any one of several predefined periods in a mission or other activity.

Photosphere - The lowest layer of the solar atmosphere; corresponds to the solar surface viewed in white light. Sunspots and faculae are observed in the photosphere.

Photovoltaic - Materials that convert light into electric current.

PHz - Petahertz (1015 Hz).

PI - Principal Investigator, scientist in charge of an experiment.

Picometer - 10-12 meter.

PIO - JPL's Public Information Office.

Plage - An extended emission feature of an active region that exists from the emergence of the first magnetic flux until the widely scattered remnant magnetic fields merge with the background.

Plage Corridor - A space in chromospheric (see chromosphere) plage lacking plage intensity, coinciding with polarity inversion line.

Plagenil - Spotless disc free of calcium plage.

Plasma - Electrically conductive fourth state of matter (other than solid, liquid, or gas), consisting of ions and electrons.

PLL - Phase-lock-loop circuitry in telecommunications technology.

Pluto - Ninth planet from the sun, sometimes classified as a small terrestrial planet.

pm - Picometer (10-12 m).

PN10 - Pioneer 10 spacecraft.

PN11 - Pioneer 11 spacecraft.

Polar Cap Absorption (Pca) - An anomalous condition of the polar ionosphere whereby hf and vhf (3 - 300 mhz) radiowaves are absorbed, and lf and vlf (3 - 300 khz) radiowaves are reflected at lower altitudes than normal. In practice, the absorption is inferred from the proton flux at energies greater than 10 mev, so that pcas and proton events are simultaneous. Transpolar radio paths may still be disturbed for days, up to weeks, following the end of a proton event.

Post-Flare Loops - A loop prominence system often seen after a major two-ribbon flare, which bridges the ribbons.

Prograde - Orbit in which the spacecraft moves in the same direction as the planet rotates. See retrograde.

Prominence - A term identifying cloud-like features in the solar atmosphere. The features appear as bright structures in the corona above the solar limb and as dark filaments when seen projected against the solar disk.

Proton - Solar activity levels with at least on high energy event (class x flares)

Proton Event - By definition, the measurement of at least 10 protons/sq.cm/sec/steradian at energies greater than 10 mev.

Proton Flare - Any flare producing significant fluxes of greater-than-10 mev protons in the vicinity of the earth.

PST - Pacific Standard Time.

PSU - Pyrotechnic Switching Unit onboard a spacecraft.