Total electron content
Total electron content (TEC) is an important descriptive quantity for the ionosphere of the Earth. TEC is the total number of electrons integrated between two points, along a tube of one meter squared cross section, i.e., the electron columnar number density. It is often reported in multiples of the so-called "TEC unit", defined as TECU10el/m. TEC is significant in determining the scintillation and group and phase delays of a radio wave through a medium. Ionospheric TEC is characterized by observing carrier phase delays of received radio signals transmitted from satellites located above the ionosphere, often  using Global Positioning System satellites. TEC is strongly affected by solar activity. Formulation. The TEC is path-dependent. By definition, it can be calculated by integrating along the path "ds" through the ionosphere with the location-dependent electron density "n(s)": The "vertical" TEC ("VTEC") is determined by integration of the electron density on a perpendicular to the ground standing route, the "slant" TEC ("STEC") is obtained by integrating over any straight path. Propagation delay. To first order, the ionospheric radio propagation effect is proportional to TEC and inversely proportional to the radio frequency "f". The ionospheric phase delay compared to  propagation in vacuum reads: while the ionospheric group delay has the same magnitude but opposite sign: The ionospheric delay is normally expressed in units of length (meters), assuming a delay duration (in seconds) multiplied by the vacuum speed of light (in m/s). The proportionality constant "κ" reads: where "q", "m", r are the electron charge, mass, and radius, respectively; "c" is the vacuum speed of light and "ϵ" is the vacuum permittivity. The value of the constant is approximately "κ" ≈ 40.308193 m·s; the units can be expressed equivalently as m·m·Hz to highlight the cancellation involved in yielding delays  τ in meters, given "f" in Hz and TEC in m.