  We report the results of our HST snapshot survey with the ACS HRC PR200L
prism, designed to measure the near-UV emission in a sample of nearby M dwarfs.
33 stars were observed, spanning the mass range from 0.1 - 0.6 solar masses
(T_eff ~ 2200K - 4000K) where the UV energy distributions vary widely between
active and inactive stars. These observations provide much-needed constraints
on models of the habitability zone and the atmospheres of possible terrestrial
planets orbiting M dwarf hosts, and will be useful in refining the target
selection for future space missions such as TPF. We compare our data with a new
generation of M dwarf atmospheric models and discuss their implication for the
chromospheric energy budget. These NUV data will also be valuable in
conjunction with existing optical, FUV and X-ray data to explore unanswered
questions regarding the dynamo generation and magnetic heating in low-mass
stars.
