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M component—In a lightning discharge, an increase in channel luminosity accompanied by a rapid electric field variation, itself called an M electric field change. The M components occur when the channel is already faintly luminous. Downward-moving leaders have not been observed to precede M components. The M components may be confused with branch components, the increases in channel luminosity that occur between each branch and the ground when the upward-propagating return stroke reaches that branch, since the higher branches are obscured by the cloud. The M component is a minor surge of current that reilluminates the channel in a negative cloud-to-ground flash. It may occur within microseconds or up to a few milliseconds of a return stroke. Evidence exists that M components are a result of the sequence of fast in-cloud negative leaders (K changes) contacting a conducting ground channel and renewed ground potential wave that reilluminate the channel.
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M-region—Name given to a region of activity on the sun when the details and nature of that activity cannot be established. This term was used in the 1940s to account for recurrent geomagnetic storms with a period the same as the period of solar rotation relative to the earth, that is, 27.3 days. Today these storms are usually associated with high-speed solar wind streams emanating from coronal holes. See magnetic storm.
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machine learning—The process by which computer systems can be directed to improve their performance over time. Examples are neural networks and genetic algorithms.
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mackerel breeze—(Also called mackerel gale.) A wind that ruffles the water, favoring the catching of mackerel.
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