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We've heard that space plasma is "neutral." Doesn't that mean it can't conduct currents?


Short Answer

No. Plasma is quasi-neutral, meaning it has approximately the same number of positive charges (protons or +ions) in a given volume as it has unbound negative charges (electrons) in the same volume. Since its charges are free to move independently, unlike in neutral atoms, plasma is referred to as quasi-neutral to reduce confusion.

However, quasi-neutral and nonconductive are not synonymous. Neutrality has to do with the relative proportion of positive and negative charges in a given volume. Conductivity has to do with the freedom of the charge carriers in a medium and the ease with which an electric current can flow through it.

In a plasma, the charge carriers are able to move freely and thus the conductivity of plasmas can be extremely high.


When it comes to a discussion of plasmas, the language of high school electrostatics can create confusion. To properly differentiate the behaviors of gases and plasmas within the cosmos, it's essential that we first clarify the language used to describe electric charge and electric currents in plasmas and other materials. It's particularly important to understand that, unlike most solids and gases that we are familiar with, a neutral plasma can conduct electricity.

Since conductivity and relative charge are independent variables, there can be four scenarios where they co-exist:
Objects that:
  • are neutral and don't conduct a current;
  • are neutral but do conduct a current;
  • are non-neutral but don't conduct a current;
  • are non-neutral and do conduct a current;
Within the context of high school electrostatics, one might use the example of a wooden pith ball hanging on a string from the ceiling. If we add several electrons to the pith ball (causing it to become charged), then it's no longer electrically neutral. That is to say, it has  more charged particles of one sign (negative charges) than particles of the opposite sign (positive charges). Additionally, since it is made of wood, it is an insulator and an electric current cannot flow through it. A pith ball, therefore, is non-neutral and does not conduct electricity.

However, if an object with a sufficient positive charge were brought into close proximity, a discharge (spark) might occur in the space between the oppositely charged objects. Such a discharge may redistribute charges between the objects, sufficient to return them to  a state close to charge neutrality (approximately equal proportions of positive and negative charges in and on each object). This is the traditional context in which non-neutrality is discussed in high  school physics classes.

A plasma may be considered "quasi-neutral" if it possesses an equivalent number of (positively charged) ions and (negatively charged) electrons. Plasma is also oftentimes described as an "ionized gas", which is to say that some number of its ions and/or outer valence electrons are freed from bondage (dissociated). This freedom of the charge carriers makes plasma highly conductive. Therefore plasma is quasi-neutral and does conduct an electric current.

A plasma can exist in different degrees of ionization. For example, we can clearly observe the Sun's corona. At the temperatures achieved in the corona, we know that the plasma there will be 100% dissociated. In other words, there are no "gases" (composed of neutral atoms) within the Sun's corona.

Within the interplanetary medium (the plasma contained within the heliosphere) the degree of dissociation is still around 100%, but the density is very low. Nevertheless, where it exists, the plasma's temperature is very high (~100,000K) and the conductivity is  still very high (on the order of metals).

In the interstellar medium (the plasma pervading the space between neighboring stars), there are different regions whose degree of dissociation varies from less than 1% to nearly 70%. Even "gases" in which the degree of ionization is as low as 0.00001%, may still be influenced so much by the tiny ionized proportion that the partially ionized gas as a whole still behaves as a plasma and may still be significantly electrically conductive.

Now, suppose that the excitation (from heat, current, etc.) is removed from the plasma and the positive and negative charges get back together (they "recombine" into atoms). We would then have a non-ionized cloud of gas rather than a plasma. This cloud of gas is also well described by the word neutral and it does not conduct an electric current.

A non-neutral plasma, on the other hand, is one that contains an unequal number of ions and electrons. But, it is still a plasma because it is still completely or partially ionized. Therefore, it's non-neutral but does conduct electricity.

We should now understand, from the above examples, that conductivity is related to the freedom of the charge carriers and not to the relative proportions of positive and negative charges in a given volume.
  • Atmospheric gases usually act as an insulator, that is to say they are neutral and non-conductive.
  • A pith ball is a piece of wood (also an insulator) to which excess charges can be added, making it non-neutral but still non-conductive.
  • A non-neutral plasma is an "ionized gas" with an uneven number of positive and negative charges that are free to move independently, thus it is non-neutral and does conduct.
  • A typical plasma has approximately equal numbers of positive and negative charges and those charges are likewise free to move, thus it is quasi-neutral and does conduct.
The outer layer(s) of the sun are composed largely of plasma. Interplanetary space is filled with low-density plasma. Interstellar and intergalactic space is filled with extremely low-density plasma. In fact, up to 99.999% of the visible universe is composed of matter in the plasma state.

To argue that the quasi-neutrality of space plasma precludes that plasma from conducting currents would be erroneous. In light of this revelation, we suggest that astronomers and astrophysicists should be re-evaluate many current quandaries and prior assumptions in light of over a century of low-density plasma discharge experiments by such pioneering individuals as Kristian Birkeland, Irving Langmuir and Hannes Alfvén (among others).
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