Characteristics of Fragmented Aurora-like Emissions (FAEs)

<p>We present observations of a new type of small-scale aurora-like feature, which is further referred to as fragmented aurora-like emission(s) (FAEs).</p><p>They seem to appear in two categories – randomly occurring individual FAEs and wave-like structures with regular spacing between FAEs alongside auroral arcs. FAEs show horizontal sizes typically below 20 km, a lack of field-aligned emission extent, and short lifetimes of less than a minute. Emissions were observed at the 557.7 nm line of atomic oxygen and at 673.0 nm (<span>N<sub>2</sub></span>; first positive band system) but not at the 427.8 nm emission of <span><span tabindex="0" role="presentation" data-mathml='<math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn><mo>+</mo></msubsup></mrow></math>'><span><span><span><span><span>N</span><sub><span>2</span></sub><span><sup>+</sup></span></span></span></span></span></span></span> or the 777.4 nm line of atomic oxygen. This suggests an upper limit to the energy that can be produced by the generating mechanism. Their lack of field-aligned extent and 777.4 nm emissions indicates a different generation mechanism than for aurorae, which are caused by particle precipitation. Possible sources are Farley–Buneman instabilities or electrostatic ion cyclotron waves.</p>