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Exploring Extremely Low Frequency Phenomena

The femtohertz (fHz) is a unit of frequency equal to 10⁻¹⁵ hertz, representing one cycle per 1,000,000,000,000,000 seconds—which is about 31.7 million years. This incredibly low frequency scale is used primarily in astrophysics, cosmology, and geophysics to describe ultra-slow oscillations and waves occurring over vast cosmic timescales.

Frequencies in the femtohertz range are associated with phenomena such as primordial gravitational waves, oscillations in the cosmic microwave background radiation, and long-term magnetic or seismic cycles on Earth. These waves have correspondingly immense wavelengths, stretching over billions of kilometres or even larger cosmic distances.

Because femtohertz frequencies are far beyond everyday human experience, they are mostly relevant for understanding the deep-time evolution of the universe and large-scale cosmic processes. Studying such slow oscillations helps scientists learn about the formation of galaxies, the behavior of space-time, and fundamental physical laws governing the cosmos.

Using femtohertz as a measurement allows researchers to quantify these vast time periods and wavelengths, connecting tiny frequency values with the immense scale of astrophysical phenomena and Earth’s geological history.

The Scale of Ultra-Low Frequency Waves

A megametre (Mm) equals 1,000,000 metres (10⁶ m) and is used to describe extraordinarily long wavelengths found in the ultra-low frequency (ULF) and extremely low frequency (ELF) bands of the electromagnetic spectrum. These wavelengths correspond to frequencies less than a few hertz, often in the range of millihertz to a few hertz. At this scale, wavelengths span hundreds to thousands of kilometres, extending into the megametre range.

Waves with megametre-scale wavelengths are critical for studying natural phenomena such as Earth’s magnetospheric oscillations, geomagnetic pulsations, and seismic electromagnetic signals. These frequencies and wavelengths are also important in geophysical research, allowing scientists to monitor changes in the Earth’s magnetic field and space weather effects. For example, a frequency of 0.1 Hz corresponds to a wavelength of about 3,000,000 metres, or 3 Mm.

Because of their immense scale, megametre wavelengths are not used for typical communication systems but are crucial in understanding planetary and space environments. Using the megametre unit helps researchers conceptualize and quantify these gigantic waves, linking electromagnetic theory with geophysical observations and space science.