Role of で in 勢いがさかんで、かがやかしいようす。


I stumbled on NHK Easy Japanese upon an explanation of 華{はな}やかな ~ splendid,
gorgeous, glorious in Japanese, which is:

> 勢{いきお}いがさかん **で** 、かがやかしいようす。

Is the で in this case the copula or the particle?




This で is part of the te-form of the na-adjective 盛ん(な). 勢い is a noun that
means "momentum", "energy", or by extension, "power", "dominance", etc. 勢いが盛んだ
is a simple sentence in the form of "[noun] is [na-adjective]".




This で is part of the te-form of the na-adjective 盛ん(な). 勢い is a noun that
means "momentum", "energy", or by extension, "power", "dominance", etc. 勢いが盛んだ
is a simple sentence in the form of "[noun] is [na-adjective]".

