Glossary:
O and P

N.. O.. P.. Q.. R.. S.. T.. U.. V.. W.. X.. Y.. Z..

O

Orbit

The path a body takes around another object or point in space under the influence of various physical forces, including gravity.

Order of magnitude

Two quantities are of the same order of magnitude if one is less than 10 times as large as the other. Each increase of one power of 10 is an increase in magnitude of 1. For example, 10⁶ is 4 orders of magnitude above 10².

P

Parker spiral

As the Sun's magnetic field is carried out through the solar system, the Sun is rotating. Its rotation winds up the magnetic field into a large rotating spiral, known as the "Parker spiral", named after the scientist who first described it. The Parker spiral magnetic field is indicated by the yellow arrows.

The magnetic field is primarily directed outward from the Sun in one of its hemispheres, and inward in the other. This causes opposite magnetic field directions in the Parker spiral. The thin layer between the different field directions is described as the "neutral current sheet". Since this dividing line between the outward and inward field directions is not exactly on the solar equator, the rotation of the Sun causes the current sheet to become "wavy", and this waviness is carried out into interplanetary space by the solar wind.

Image courtesy J. Jokipii, University of Arizona

Photo-ionization

The loss of an electron by an atomic particle by a solar ultraviolet photon. This can occur when neutral interstellar material leaks into the heliosphere and creates pick-up ions.

Photon

A unit or "particle" of electromagnetic energy, or light.

Photosphere

The visible surface of the Sun. Above it are the chromosphere, then the corona.

Physics

The science dealing with matter and energy and their interaction.

Pick-up ions

The forerunners of ACRs. Neutrally charged interstellar material leaks into the heliosphere, and as it does it loses an electron. The Sun's magnetic field then picks up these newly charged particles, and carries them outward to the solar wind termination shock. These particles are called pick-up ions during this time.

Planet

A body that orbits a star in a solar system and which shines only by reflected light. This image of Saturn is courtesy of NASA GSFC.

More about the planets...

Plasma

A fourth state of matter -- not a solid, liquid or gas. In a plasma, the electrons are pulled free from the atoms and can move independently. The individual atoms are charged, even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality.

More about plasma...

Post-acceleration

An instrument that re-accelerates a previously slowed particle to determine its mass. Post-acceleration is used in the SWICS instrument on the ACE spacecraft.

Proton

The positively charged part of an atom. The number of protons determines which element the particle is. It is part of the nucleus.

More about protons...