plasmapy.formulary.distribution.Maxwellian_speed_1D(v, T, particle: str | int | integer | Particle | CustomParticle | Quantity = 'e-', v_drift=0, vTh=nan, units='units', *, mass_numb=None, Z=None)[source]

Probability distribution function of speed for a Maxwellian distribution in 1D.

Return the probability density function for finding a particle with speed v in m/s in an equilibrium plasma of temperature T which follows the Maxwellian distribution function.

  • v (Quantity) – The speed in units convertible to m/s.

  • T (Quantity) – The temperature, preferably in kelvin.

  • particle (str, optional) – Representation of the particle species [e.g., 'p+' for protons, 'D+' for deuterium, or 'He-4 +1' for \(He_4^{+1}\) (singly ionized helium-4)], which defaults to electrons.

  • v_drift (Quantity) – The drift speed in units convertible to m/s.

  • vTh (Quantity, optional) – Thermal velocity (most probable) in m/s. This is used for optimization purposes to avoid re-calculating vTh, for example when integrating over velocity-space.

  • units (str, optional) – Selects whether to run function with units and unit checks (when equal to “units”) or to run as unitless (when equal to “unitless”). The unitless version is substantially faster for intensive computations.

  • mass_numb (integer, keyword-only, optional) – The mass number associated with particle.

  • Z (real number, keyword-only, optional) – The charge number associated with particle.


f – Probability density in speed-1, normalized so that \(\int_{0}^∞ f(v) dv = 1\).

Return type:


  • TypeError – The parameter arguments are not Quantities and cannot be converted into Quantities.

  • UnitConversionError – If the parameters is not in appropriate units.

  • ValueError – If the temperature is negative, or the particle mass or charge state cannot be found.


In one dimension, the Maxwellian speed distribution function describing the distribution of particles with speed \(v\) in a plasma with temperature \(T\) is given by:

\[f(v) = 2 \frac{1}{(π v_{Th}^2)^{1/2}} \exp(-(v - V_{drift})^2 / v_{Th}^2 )\]

where \(v_{Th} = \sqrt{2 k_B T / m}\) is the thermal speed.


>>> import astropy.units as u
>>> v = 1 * u.m / u.s
>>> Maxwellian_speed_1D(v=v, T=30000 * u.K, particle="e-", v_drift=0 * u.m / u.s)
<Quantity 1.1832...e-06 s / m>