Maximum Usable Frequency
This is a statistical prediction of the highest frequency for HF sky wave propagation. If the prediction is correct, this frequency should be refracted by the ionosphere 50% of the time. See Optimum Working Frequency and Upper Decile MUF.

M Class Flare
Solar flares which have a particular range of X ray energy output. An M class flare will usually cause a short wave fadeout on HF circuits with daylight sectors. M class flares are less intense than X class flares but more intense than C class flares.

Median
The middle value when all values are ordered. When there is an even number of values, it is the mean of the two middle values.

Meteor
A body that enters the Earth's atmosphere and becomes incandescent by friction. A 'shooting star'.

Mid Latitude
That geographic region of the Earth that lies between about 20 and 60 degrees.

Mid Latitude Trough
A band of decreased ionisation around 60 degrees geomagnetic latitude, occurring during night hours. It appears to be related to the auroral oval since an equatorward expansion of the oval results in a decrease in width and movement of the trough towards the equator. Similarly, a poleward movement of the oval sees a corresponding expansion and movement of the trough towards the pole. Possibly the result of plasma rising in this area. The region poleward of the trough is most likely replenished by ionisation due to precipitating particles.

Mixed Mode
Sometimes more than one mode of propagation is possible on an HF circuit. There will be small time delays between the different modes which may cause multipath fading. It may be possible to resolve the different modes by careful design of the antennas used.

Mode
The path followed by a radio wave between transmitter and receiver. The so-called first propagation mode is the mode with the least number of hops for a circuit. The second propagation mode for a circuit will have one extra hop.

Molecule
Smallest part of an element or compound that exhibits the properties of the specific element or compound. A molecule is normally considered a group of atoms chemically bonded together.

MUF
See Maximum Usable Frequency.

Multipath Fading
Small time delays can occur in radio signals travelling by a single mode (due to irregularities in the ionosphere) or by mixed modes. The superposition of these multiple echoes will degrade the quality of the received signal and cause the signal level to fluctuate.