Plasma

Plasma is called the fourth state of matter after solid, liquid, and gas. It consists of a gas of ions – atoms or molecules which have one or more orbital electrons stripped, and free electrons. Excluding dark matter and dark energy, plasma is the most abundant form of ordinary matter in the universe, making up over 99% of baryonic matter.

Plasmas occur naturally or can be human-made:

Natural
Plasma is mostly associated with stars, including our Sun which is a ball of plasma and gas. The solar wind is a stream of plasma released from the upper atmosphere of the Sun, called the corona. When that wind collides with the protective, plasma-rich magnetic cushion that envelopes Earth, the interactions produce rivers of light seen in the aurora borealis and aurora australis. It is also found around the Earth's magnetosphere, ionosphere and plasmasphere, and the interplanetary medium (space between planets), interstellar medium (space between star systems) and intergalactic medium (space between galaxies).

St. Elmo’s fire is a natural phenomenon that has ignited superstitions for millennia. Often described as a ghostly blue-white dancing flame, this is actually an electrical weather phenomenon in which a glowing plasma emits light. The plasma is created by atmospheric electricity, and the eerie glow often appears at the tips of pointed objects such as church towers or the masts of ships during stormy weather. St. Elmo was the patron saint of Mediterranean sailors, who regarded St. Elmo’s fire as a good omen because the glow was often brightest near the end of a storm. Sharp pointed objects encourage the formation of the “fire” because electric fields are more concentrated in areas of high curvature. Pointed surfaces discharge at a lower voltage level than surfaces without points. The color of the fire arises from the nitrogen and oxygen composition of the air and their associated fluorescence. If the atmosphere were neon, the fire would be orange, just like a neon light.

When produced by high temperatures, such as in the sun or by lightning, it’s called hot plasma. A flame is the visible portion of a fire that consists mainly of carbon dioxide, water vapor, oxygen and nitrogen. If hot enough, the gases may become ionized to produce plasma.

Artificial
Plasma created in a plasma lamp and other room-temperature, low-pressure environments is called cold plasma. Plasma lamps or balls are filled with a gaseous mixture that includes one of the very stable, noble gases, like argon, xenon, neon or krypton. Plasma makes up those glowing tendrils that reach out from the center. The high-frequency current excites electrons that then separate from the atoms of gas. The pixels in plasma televisions contain gas that forms a plasma, sealed inside tiny cells sandwiched between two glass plates, and neon signs and fluorescent lights glow because of plasma.

An ion thruster, ion drive, or ion engine is used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.