|ULF waves (geomagnetic pulsations). Space Physics Textbook of Oulu.||"Geomagnetic pulsations; i.e., ultra-low-frequency (ULF) waves cover roughly the frequency range from 1 mHz to 1 Hz."||1011–108 m|
|Gravity and Magnetics Committee. Gravity and Terms for the 4th Edition of the SEG Encyclopedic Dictionary. Society of Exploration Geophysicists (SEG).||"micropulsations: Small amplitude fluctuations in the Earth's magnetic field, usually in the frequency range from 0.01 to 3 Hz and usually with amplitudes less than 10 nT."||1010–108 m|
|McGreevy, Stephen P. More about Natural ELF/VLF Radio. Natural Radio.||"… ULF is below 300 Hz radio-freqs., (though opinion differs when it comes to the terms ULF and ELF -- most agree that ULF refers to below 300 Hz toward below 1 Hz -- really LOW frequencies!)"||106–108 m|
|Heirtzler, James R. "The Longest Electromagnetic Wave." Scientific American. March 1962.||"As an example, at one end of the spectrum, electromagnetic waves with frequencies of 10−2 Hz (which corresponds to a period of 100 s and to a wavelength of about 5000 Earth radii) have been detected at the Earth's center."||1010 m|
|Military Acronyms, Initialisms, and Abbreviations. Federation of American Scientists.||"ULF: ultra low frequency (300-0 Hz)"||106 m–infinity|
The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. All electromagnetic waves travel at the speed of light (c = 3.0 × 108 m/s) in a vacuum. There seem to be no upper and lower limits to the frequency or wavelength of electromagnetic waves and no gaps in the spectrum. However, electromagnetic waves have been observed with incredibly long wavelengths -- these waves are known as ultra low frequency (ULF) waves, or micropulsations. Since frequency and wavelength are inversely proportional (v = fλ, for electromagnetic waves c = fλ), the name "ultra low frequency"is equivalent to "ultra long wavelength" -- although nobody refers to them as such. The range of wavelengths which refer to ULF waves is disputable, and different sources cite different ranges. The consensus seems to be that the wavelength of the longest electromagnetic wave is in the range from 106 to 1011 M. However, it is not impossible to discover a wave with a wavelength approaching infinity.
ULF waves seem to have extraterrestrial sources (they seem to "result from interactions between plasma emitted from the sun (solar wind) and the Earth's [magnetic] field"). Geomagnetic pulsations were first observed by Balfour Stewart in 1859, and he published his findings in 1861. Some people are interested in the sounds produced by ULF waves, VLF waves (very low frequency), and ELF waves (extremely low frequency). There is also speculation and research into the possibility that micropulsations may have an affect on people's health and on women's menstrual cycles.
Rachel Shapiro -- 2001