## Cone angle

The IMF cone angle acos(Bx/|B|) is the angle between the IMF direction and the sun-Earth line. Statistically typical IMF orientation is 45 degrees in ecliptic (tailward and duskward). It has been shown that the cone angle controls the formation of some Pc 3 pulsations types.

## Dispersion relation

Relates the wave number **k** to the wave frequency. Contains all the information about the propagation of a given plasma wave mode.

## Fukushima-Bostrom theorem

Of any ionospheric current system **I**, only the source-free part **I**sf can be detected below the ionosphere.

## Gyration

From the **equation of motion**, m d**v**/dt = q(**v** x **B**) one can calculate that, in a presence of magnetic field **B**, a charged particle is accelerated in direction perpendicular to both **v**(perp) and **B**. This forms a circular motion about a imaginative **guiding center** of the particle. The sense of rotation is opposite for positive and negative charges, being clockwise for negative particles when viewed in the direction of **B**. The electrons producing auroras are thus rotating counterclockwise when viewed from the ground in the norther hemisphere. They are also in **helicoidal** trajectories because of a additional velocity component parallel to **B**.

## Gyrofrequency

Angular frequency of gyration, cyclotron frequency, or Larmor frequency. The magnitude of the angular velocity of a charged particle gyrating around its guiding center, w = |q|B/m (in radians/s). The smaller the particle mass, the larger its gyrofrequency will be, and the higher the magnetic field, the higher the gyrofrequency, also. See also gyroradius.

## Gyroradius

Radius of gyration, or cyclotron radius. The radius of the circular orbit of a charged particle gyrating around its guiding center, r = v(perp)/w = mv(perp) /(|q|B). The smaller the particle mass, the smaller its gyroradius will be, and the higher the magnetic field, the smaller the gyroradius, also. See also gyrofrequency (w).

## Heliopause

Boundary where the outgoing solar wind meets the incoming plasma of the local interstellar medium (LISM). Believed to be at least 120 astromnomical units away.

## Langevin equation

Equation of motion for a weakly ionized cold plasma.

## Linear perturbation theory

Assumption that the variations in the plasma parameters, due to the presence of waves, are small (to the first order) as compared to the undisturbed parameters. This makes it possible to linearize equations by dropping out second order (and higher) nonlinear terms.

## Lorenz gas

The type of plasma in which only the electron motion is important. A valid approximation with high frequency phenomena, when the much heavier ions don't have time to respond. Used succesfully when studying the propagation of electromagnetic waves in cold magnetoplasmas.

## Magnetic moment

To the circular motion of a charged particle in a magnetic **B** field there is associated a circular electric current I which, in turn, has an associated magnetic field. This field is parallel to the external field outside the gyroradius of the particle, and opposite inside. The magnetic moment of the particle points thus in opposite direction to **B**, and has magnitude |**m**| = IA, if the particle orbit covers an area A. It can be shown that |**m**| = W(perp)/B, where W(perp) is the part of the particle kinetic energy associated with the transverse velocity V(perp).

## Magnetic rigidity

The magnitude of B times the gyroradius of a charged particle equals to its momentum per unit charge, called also magnetic rigidity; Br = mv(perp)/|q|.

## Magnetoionic theory

Theory of wave propagation in a cold homogenous, magnetized electron gas.

## Pitch angle

The angle *a* between magnetic field **B** and velocity vector of a charged particle, **v**., i.e., sin*a* = v(perp) / v(total), where v(perp) refers to the velocity component perpendicular to **B**.

## Pitch Angle Distribution (PAD)

A form of presenting particle fluxes at a given energy as a function of pitch angle.

## Plasma frequency

Space charge oscillations at the natural frequency of the plasma. For the electrons also known as Langmuir oscillations. In cold plasma approximation, and when ion motion is neglected, these oscillations are stationary (no wave propagation), longitudinal (electron velocity is in the same direction as the electric field), and electrostatic (there is no magnetic field associated with the oscillations). In the warm plasma model, these oscillations become propagating disturbances known as space charge waves or Langmuir waves. Since plasma frequencies are proportional to the square root of the density of the particles, one can estimate densities by measuring them.

## Plasma instability

Waves in some mode growing exponentially or faster.

## Wave packet

Superposition of waves with different values of k and f.