An intense radio flare on the dMe star AD Leo , observed with the Effelsberg radio telescope and spectrally resolved in a band of 480 MHz centred at 4.85 GHz is analysed .
A lower limit of the brightness temperature of the totally right handed polarized emission is estimated as T _ { b } \sim 5 \times 10 ^ { 10 } K ( with values T _ { b } \ga 3 \times 10 ^ { 13 } K considered to be more probable ) , which requires a coherent radio emission process .
In the interpretation we favour fundamental plasma radiation by mildly relativistic electrons trapped in a hot and dense coronal loop above electron cyclotron maser emission .
This leads to densities and magnetic field strengths in the radio source of n \sim 2 \times 10 ^ { 11 } cm ^ { -3 } and B \sim 800 G. Quasi-periodic pulsations during the decay phase of the event suggest a loop radius of r \sim 7 \times 10 ^ { 8 } cm .
A filamentary corona is implied in which the dense radio source is embedded in hot thin plasma with temperature T \geq 2 \times 10 ^ { 7 } K and density n _ { ext } \leq 10 ^ { -2 } n .
Runaway acceleration by sub-Dreicer electric fields in a magnetic loop is found to supply a sufficient number of energetic electrons .