MilliHertz

Millihertz (mHz) is the SI unit of frequency equal to one thousandth of a hertz, i.e., 0.001 cycles per second. It is used to describe very low-frequency phenomena where cycles occur over long timescales. Because frequency is the reciprocal of period, 1 mHz corresponds to a period of 1000 seconds (about 16.7 minutes).

In terms of the relation to higher units, milli- denotes 10^-3, so 1 mHz = 0.001 Hz and 1 Hz = 1000 mHz. Millihertz measurements are common in disciplines that study slowly varying signals or long-period processes, requiring stable instrumentation over extended observation times.

Applications and contexts for millihertz frequencies include geophysics and Earth observation, where long-period seismic waves, atmospheric tides, and oceanic fluctuations occur in the millihertz range. In solar and stellar physics, natural oscillations and pulsations can lie in the millihertz domain; for example, solar oscillations are often observed around a few millihertz. In astronomy and gravitational-wave science, millihertz frequencies are a target for space-based detectors such as the Laser Interferometer Space Antenna (LISA), which aims to detect signals from sources like white-dwarf binaries and supermassive black-hole mergers in roughly the 0.1 mHz to 1 mHz band.

Measurement considerations for millihertz signals include long-term stability and low drift in instruments, as well as careful calibration over timescales of minutes to hours. These requirements distinguish millihertz work from higher-frequency measurements.

See also: Hertz, SI prefixes, frequency, LISA.