Source of magnetospheric plasma in ultracool dwarf auroral systems

Identify the source of magnetospheric plasma in ultracool dwarf (spectral types later than M7) magnetospheres that exhibit auroral electron cyclotron maser radio emission, determining whether the plasma originates from exoplanet companions (analogous to Io supplying Jupiter’s magnetosphere) or from intrinsic magnetospheric processes such as magnetic reconnection, in order to explain the low radio detection rate of these objects and to constrain planetary dynamo and magnetosphere models.

Background

Ultracool dwarfs have been observed to produce bright, highly polarized radio bursts attributed to electron cyclotron maser emission, indicating auroral processes analogous to those in planetary magnetospheres. Despite extensive searches, the overall radio detection rate for ultracool dwarfs remains low (around 10%), and understanding the magnetospheric conditions that enable or inhibit radio emission is crucial.

The authors propose that exoplanet companions could seed the magnetosphere with plasma, as Io does for Jupiter, offering a potential explanation for observed auroral signatures. Alternatively, internal magnetospheric processes such as magnetic reconnection could provide the necessary plasma. Establishing the dominant plasma source will clarify the mechanisms driving ultracool dwarf aurorae and inform strategies for companion searches and magnetospheric modeling.