In standard AM broadcast what mode of propagation is used for transmitting a signal?

AM broadcasting uses vertically polarised waves which propagate via the ground-wave, along the ground.

The rate at which the signal decreases with distance depends mainly on the electrical conductivity of the ground. Lower frequencies decay at a lower rate and travel further than higher frequencies.

Ground waves propagate to large distances over sea-water which is highly conductive, for example about 40 miles at 27 MHz and further at lower frequencies. Over typical grassland the range of a 2 kW AM transmitter is 20-30 miles on medium-wave frequencies around 1 MHz.