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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.

Measuring Ultra-Low Frequencies

The attohertz (aHz) is an extremely small unit of frequency equal to 10⁻¹⁸ hertz, or one cycle per 10¹⁸ seconds (about 31.7 billion years). This unit is used to describe ultra-low frequency phenomena that occur on cosmic or geological timescales, far beyond everyday human experience.

Attohertz frequencies are relevant in cosmology, astrophysics, and geophysics, where they help scientists study processes that evolve over billions of years. For example, gravitational waves generated by massive cosmic events or the oscillations of the Earth’s magnetic field can be characterized by frequencies in the attohertz range. These waves have enormous wavelengths, often spanning millions or billions of kilometres.

Because the attohertz corresponds to such a long period between cycles, it is mostly used in theoretical research rather than practical applications. Understanding phenomena at this scale gives insight into the fundamental workings of the universe, including the slow evolution of cosmic structures, the expansion of space-time, and the early conditions following the Big Bang.