#8. Positive Ions

Matter is made of atoms. Each atom, in its turn, is made of electrically charged components:

a positive central nucleus, where most of the atom's mass is concentrated, and
one or more negative electrons.

Nucleus and electrons are held together by the electric attraction between positive (+) and negative (-) charges. In any atom, the two charges are exactly balanced, so that to the outside world the atom is electrically neutral.

Ions help gases conduct electricity

When an atom is hit by a fast-moving particle, like those emitted by radioactive materials, or absorbs light, an electron may be torn off. What is left is an electrically charged atom or "ion," carrying a positive charge, and the process is known as "ionization."

When such processes occur in air, they produce there free ions and electrons, which can move and carry an electric current, something neutral atoms cannot do. Air is usually an excellent electrical insulator, but with ionization present, electric charges can leak through it.

The Electroscope

This leakage was used, around 1900, to detect radioactive emissions and measure their intensity. The drawing below shows a simple instrument for performing such measurements. It is called an electroscope and contains two parallel leaves of metal foil, protected from wind inside a metal box with transparent windows and attached to a metal rod insulated from the box and leading outside (drawing).

When the plate at the end of the rod is electrically charged (e.g. by rubbing it with a dry cloth), the leaves spread wide apart, since both carry electric charges of the same sign and repel each other. However, when a radioactive substance is brought close, the electric charge leaks to the box and the leaves gradually drop down again.

Ion types

Hydrogen, the simplest atom, has one electron. When that electron is removed, we get the simplest positive ion, the "proton"; like the electron, it is a fundamental particle, but 1836 times heavier. The chemical symbol for hydrogen is H, but for the proton it is H+.

The next heavier atom is that of helium (chemical symbol He) and it contains two electrons. Its nucleus consists of two protons and also two neutrons, particles similar to the proton but with no electric charge. The Sun gets its energy by combining protons (some of which convert to neutrons in the process) into helium, deep in the Sun's core; since the helium nucleus is an unusually stable combination of particles, energy is released in the process.

The completely ionized helium atom He++, missing both electrons, is also known as the "alpha particle" (see history section). Just as in the Sun and in most stars, hydrogen is the most abundant element with helium next, so the solar wind consist mostly of protons, with 5% alpha particles and small numbers of heavier ions.

It may be mentioned that in addition to such atomic ions, there also exist molecular ions of either sign, formed when intact molecules lose or gain an electron. Such ions occur in ionospheric processes.

Clouds of barium ions

An atom can become ionized by the absorption of light. The atom of barium is particularly easy to ionize, because its outermost electron is very loosely bound. If a mass of barium is vaporized in space, producing a barium cloud, much of the barium becomes ionized by sunlight within less than a minute. The cloud then moves in response to electric forces in space, and can be used to study the electrical field in space.

In practice the barium is packed into canisters with copper oxide, and these are released from rockets or satellites and ignited. The resulting chemical reaction produces great heat, but more barium is packed into the canister than can combine chemically, and some the excess is vaporized to form a large spherical greenish cloud.

Typically the release is done after sunset or before sunrise, so that while the canisters explode in full sunlight, observers on the ground can watch the cloud against the dark sky: soon a bluish ion cloud separates from the green one, usually elongated or striped in the direction of the magnetic field lines, which guide the ions.

Some barium releases are conducted far from Earth and are tracked by telescopes. The AMPTE mission (Active Magnetospheric Particle Tracer Experiment), launched in 1984, released barium clouds near the "nose" of the magnetosphere and in the magnetospheric tail.

The AMPTE mission included three spacecraft, shown here stacked up during launch. Click here for a full size version of this image.

In addition it released a barium cloud in the solar wind to produce an "artificial comet". Soon after the cloud formed, the magnetic field embedded in the solar wind picked it and made it share the wind's flow, a process similar to the one which creates the ion tails of comets (see solar wind, history).

The AMPTE Charge Composition
Explorer (CCE) satellite

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Next Stop: #8H. Positive Ions--History

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Authors and Curators:

David P. Stern - NASA/GSFC Code 695 (u5dps@lepvax.gsfc.nasa.gov)
Mauricio Peredo - Raytheon STX Corporation (peredo@istp1.gsfc.nasa.gov)

Last updated March 13, 1999