Introduction Arcs are the most typical auroral forms in the premidnight sector auroral oval . They belong to the group of discrete auroras for which field-aligned acceleration plays an important role. Because of this, the spectrum of the precipitating electrons includes a monoenergetic component, and the resulting ionospheric ionisation profile has a sharper E layer peak than other auroral types. The average energy of the precipitating electrons is higher than in diffuse aurora, but lower than in pulsating aurora.
Auroral arcs are luminous bands elongated in east-west direction.
Arcs drift often in north-south direction. During increased geomagnetic activity arcs tend to get deformed . It's direction is opposite to local convection electric field, and thus there is an anticorrelation between optical intensity and strength of this electric field. The other type of field reletes to the field-aligned currents . The electron precipitation within the arc creates an upward flowing current.