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Recent photometric studies have revealed that surface spots that produce fluxvariations are present on virtually all L and T dwarfs. Their likely magneticor dusty nature has been a much-debated problem, the resolution to which hasbeen hindered by paucity of diagnostic multi-wavelength observations. To testfor a correlation between magnetic activity and photometric variability, wesearched for H$\alpha$ emission among eight L3$-$T2 ultra-cool dwarfs withextensive previous photometric monitoring, some of which are known to bevariable at 3.6 $\mu$m or 4.5 $\mu$m. We detected H$\alpha$ only in thenon-variable T2 dwarf 2MASS J12545393$-$0122474. The remaining seven objects donot show H$\alpha$ emission, even though six of them are known to varyphotometrically. Combining our results with those for 86 other L and T dwarfsfrom the literature show that the detection rate of H$\alpha$ emission is veryhigh (94$\%$) for spectral types between L0 and L3.5 and much smaller (20$\%$)for spectral types $\ge$L4, while the detection rate of photometric variabilityis approximately constant (30$\%-$55$\%$) from L0 to T8 dwarfs. We concludethat chromospheric activity, as evidenced by H$\alpha$ emission, andlarge-amplitude photometric variability are not correlated. Consequently, dustclouds are the dominant driver of the observed variability of ultra-cool dwarfsat spectral types at least as early as L0.

Weather on Other Worlds. IV. H$\alpha$ emission and photometric variability are not correlated in L0$-$T8 dwarfs