Author: 阿白特尔
Reviewed by: 一毫秒的永恒

  When we think of states of matter, the first three that probably come to mind are solids, liquids, and gases. In a solid, particles are confined near fixed positions, where they vibrate randomly. The material therefore has a fixed volume and shape as a whole. As the kinetic energy of the particles’ random thermal motion increases, a point eventually comes when they break free from those fixed positions and begin to flow. The particles no longer occupy fixed locations, but attraction from their neighbors still prevents them from escaping the group. At the macroscopic scale, the material has a relatively fixed volume but no fixed shape: the familiar liquid state. Raise the temperature further and the particles’ thermal motion grows so intense that they almost completely overcome their mutual attraction and disperse freely. Matter with neither a fixed volume nor a fixed shape is called a gas. The essential distinction among solids, liquids, and gases, then, lies in the increasing extent to which thermal motion overcomes—or disrupts—interactions among particles.

  Once we know these three forms of matter, it is natural to ask whether there might be others. What happens if we raise the temperature still further? A gas has already escaped almost all the interactions among the particles, such as molecules, that directly make up the material, so further heating can disrupt only stronger and tighter bonds. In a molecular gas, the added heat may first break covalent or coordinate bonds, causing the atoms or atomic groups within each molecule to separate and gradually turning the molecular gas into an atomic one. Continue heating, and even the sturdy atom can no longer hold together: electrons continually escape the attraction of the atomic nucleus and become free particles independent of the atom. A formerly neutral atom loses negative charge and becomes positively charged, turning into an ion. When the density of electrons and ions in the gas reaches a level at which their collective effects dominate its properties, a fourth state of matter—plasma—emerges.

  The process just described is one way to create plasma: thermal ionization. Heating intensifies the thermal motion of electrons until they escape their atoms. Plasma produced this way is often in thermal equilibrium, meaning that its electrons and ions have the same temperature. The plasma in the Sun and in nuclear-fusion reactors is produced by this process and is in thermal equilibrium. Plasma in lightning and fluorescent tubes, by contrast, forms through electrical discharge. During a discharge, electrons emitted from an electrode strike gas atoms and knock electrons out of them, producing electron–ion pairs. Unlike heating, a discharge injects kinetic energy only into the electrons, so the electron temperature in the resulting plasma is far higher than the ion temperature.

  Having described how plasma forms, let us examine two of its unusual collective effects, neither of which occurs in ordinary matter. The first is called plasma oscillation. Plasma consists of large numbers of positive and negative charges. Under Coulomb’s law, each charge is continually acted on by all the others: like charges repel and unlike charges attract. Because a plasma contains roughly equal amounts of positive and negative charge, however, its attractive and repulsive forces are approximately equal, leaving the particles in statistical equilibrium. Any equilibrium can be disturbed. If all charges of one kind in a plasma shift some distance from their equilibrium positions, that balance is broken.

  Imagine an electron–ion plasma in which some disturbance makes the electrons shift together to the right. Their “departure” leaves behind a region where the ion density exceeds the electron density, giving it a net positive charge. Because electrons are negative and opposite charges attract, the displaced electrons feel a backward force trying to pull them home. But the story does not end there. When the electrons return, the attractive force disappears, yet they have accelerated on the way and cannot “brake” at the equilibrium position. Inertia carries them past it, so they depart again in the opposite direction. Once more, the positive charge attracts and pulls them home. A cycle emerges: electrons depart → ions pull them back → electrons depart in the opposite direction → ions pull them back again. Over time, the electrons display a collective oscillation, which we call a plasma oscillation. An ion has nearly 2,000 times the mass of an electron, so under an equal reaction force its acceleration is far smaller. While the electrons oscillate, the ions remain almost where they began; before they have time to move, the electrons are already on the other side.

  Plasma oscillations have many applications. One—arising from the difference between plasma and ordinary solid, liquid, and gaseous states—is a sound explanation for disruptions to shortwave radio communications during solar storms. Every oscillator has a natural frequency, and plasma is no exception. The natural frequency of plasma oscillation increases with the density of positive and negative charges. We also know that when the frequency of an external force approaches an oscillator’s natural frequency, resonance occurs. The oscillator absorbs as much energy as possible from the force and reaches its greatest amplitude. The electromagnetic waves used in radio communication are, in essence, continuously varying electric and magnetic fields. Charged particles such as electrons experience a force in an electric field. An electromagnetic wave passing through plasma is therefore equivalent to applying a periodic external force to its electrons. When this force has a frequency far above the natural frequency of the plasma oscillation, the electrons’ motion and the changes in the force are badly out of step. The force does almost no work on the electrons, allowing the wave to cross the plasma with almost no loss and carry communications, as it ordinarily does on Earth.

  Once a solar storm erupts, however, large numbers of charged particles enter Earth’s atmosphere and increase the density of positive and negative charges. As noted above, the plasma’s natural oscillation frequency rises with that density. When it reaches roughly the frequency of the electromagnetic waves used for shortwave communication, resonance occurs and as much wave energy as possible is transferred to the plasma. The electromagnetic wave then struggles to propagate through the plasma, and communication is interrupted. Because high-voltage alternating-current transmission also operates through periodically changing electric fields, a solar storm can in some cases cause widespread power outages as well.

Diagram of a solar storm and Earth's magnetic field | Credit: SOHO (ESA & NASA)

  Debye shielding is another curious story about charge. We might call it the way a charge becomes “invisible” in a plasma. Picture a mass of electron–ion plasma, where disturbances occasionally produce plasma oscillations like waves rolling across an ocean. Now throw a positively charged sphere into that ocean, and it is quickly submerged. Something remarkable happens. The positive sphere attracts negative electrons and repels positive ions, so the electron density around it exceeds the ion density. From a distance, the sphere looks like a steamed bun wrapped in an electron “skin.” Because the surrounding shell of electrons is negatively charged, its effect cancels that of the positive sphere, and the bun as a whole has only a weak net electrical effect on its surroundings. A positively charged sphere went in, and its positive charge seems to vanish in the plasma. This is the plasma’s “invisibility trick” for charge: Debye shielding.

  These effects are only the tip of the iceberg. Keep exploring, and you will discover many more of plasma’s secrets and wonders.