Observations of stellar prominences on young rapidly rotating stars provide unique probes of their magnetic fields out to many stellar radii .
We compare two independently obtained datasets of the K3 dwarf Speedy Mic ( BO Mic , HD 197890 ) using the Anglo-Australian Telescope ( AAT ) and the ESO Very Large Telescope ( VLT ) .
Taken more than a fortnight apart they provide the first insight into the evolution of the prominence system on such a young rapidly rotating star .
The largest prominences observed transiting the stellar disc are found at very similar rotational phases between the epochs .
This suggests that the magnetic structures supporting the prominences retain their identity on a two to three week timescale .
By taking advantage of the high signal-to-noise and large wavelength range of the VLT observations we identify prominences as transient absorption features in all lines of the Hydrogen Balmer series down to H _ { 10 } .
We use the ratios of the prominence EWs in these lines to determine their column densities in the first excited state of hydrogen .
We determine the optical depths , finding prominences to be rather optically thick ( \tau \approx 20 ) in the H _ { \alpha } line .
The total hydrogen column density and thus the prominence masses are determined via observations of the CaII H & K lines .
We find typical masses for four of the largest prominences to be in the range 0.5 - 2.3 \mathrm { x } 10 ^ { 14 } \mathrm { kg } , slightly larger than giant solar prominence masses .
Rotationally modulated emission is seen outside of the H _ { \alpha } line .
These loops of emission are shown to be caused by prominences seen off the stellar disc .
We find that all of the large emission loops can be associated with prominences we see transiting the stellar disc .
This combined with the fact that many prominences appear to eclipse the off-disc emission of others , strongly suggests that the prominence system is highly flattened and likely confined to low stellar latitudes .