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Plasma frequency is the frequency of the oscillation of electron charge density in some plasma. The term is used for a common model of plasma, indicating the frequency that the model predicts, or may be used for the corresponding actual (e.g., measured) frequency in some plasma. The term can also be used for other charged particles, but as electrons are often the lightest by far, they produce the largest oscillation of the position/movement of electric charge. Electrons among ionized atoms tend toward a frequency stemming from relatively massive positive particles that are comparatively fixed in position, but a plasma can also include positrons. which affect the frequency since they are positive particles that can be pushed around as easily as electrons. The oscillation results from the net electric force pushing/pulling particles, and as distances between them vary, the net force from all the surrounding particles eventually reversing, providing a restoring force. Given typical densities of material classified as plasma and assuming an insignificant number of positrons, frequency basically varies with the square root of the number density of electrons.
Radio waves are strongly attenuated when passing through plasma at below the plasma frequency. For the ionosphere, this is about 1-10 MHz, defining the low end of the radio window, affecting Earth-based radio astronomy. For the interplanetary medium (IPM), the plasma frequency is about 9 kHz and for the interstellar medium, about 1 kHz.
The term plasma frequency is commonly used for the above-described oscillation a plasma's in charge density, which is a common and prominent type of oscillation of a plasma. In addition to this, plasmas have a number of other oscillation-mechanisms (having various other resonant frequencies) and associated waves (various types of plasma waves, such as Alfvén waves and magnetosonic waves), due to the variety of particles in plasmas and the variety of effects associated with electromagnetism. The plasma waves associated with the above-described plasma frequency are termed Langmuir waves.