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We present a study of the variation timescales of the chromospheric activityindicator H$\alpha$ on a sample of 13 fully-convective, active mid-to-late Mstars with masses between 0.1--0.3 solar masses. Our goal was to determine thedominant variability timescale and, by inference, a possible mechanismresponsible for the variation. We gathered 10 or more high-resolution spectraeach of 10 stars using the TRES spectrograph at times chosen to span all phasesof stellar rotation, as determined from photometric data from the MEarthObservatories. All stars varied in their H$\alpha$ emission. For 9 of thesestars, we found no correlation between H$\alpha$ and rotational phase,indicating that constant emission from fixed magnetic structures, such asstarspots and plage, are unlikely to be the dominant source of H$\alpha$emission variability. In contrast, one star, G 7-34, shows a clear relationshipbetween H$\alpha$ and stellar rotational phase. Intriguingly, we found thatthis star is a member of the AB Doradus moving group and hence has the youngage of 149 Myr. High-cadence spectroscopic observations of three additionalstars revealed that they are variable on timescales ranging from 20--45minutes, which we posit may be due to flaring behavior. For one star, GJ 1111,simultaneous TESS photometry and spectroscopic monitoring show an increase inH$\alpha$ emission with increased photometric brightness. We conclude thatlow-energy flares are able to produce variation in H$\alpha$ on the timescaleswe observe and thus may be the dominant source of H$\alpha$ variability onactive fully-convective M dwarfs.

Variability Timescales of H$\alpha$ on Active Mid-to-Late M dwarfs