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Phase-resolved HST/STIS ultraviolet spectroscopy of the high-field polar ARUMa confirms that the WD photospheric Ly alpha Zeeman features are formed in amagnetic field of ~200 MG. In addition to the Ly alpha pi and sigma+components, we detect the forbidden hydrogen 1s0->2s0 transition, which becomes``enabled'' in the presence of both strong magnetic and electric fields. Ourattempt in fitting the overall optical+UV low state spectrum with singletemperature magnetic WD models remains rather unsatisfactory, indicating eithera shortcoming in the present models or a new physical process acting in AR UMa.As a result, our estimate of the WD temperature remains somewhat uncertain,Twd=20000+-5000K. We detect a broad emission bump centered at ~1445A andpresent throughout the entire binary orbit, and a second bump near ~1650A,which appears only near the inferior conjunction of the secondary star. Theseare suggestive of low harmonic cyclotron emission produced by low-level(M-dot~1e-13 Msun/yr) accretion onto both magnetic poles. However, there is noevidence in the power spectrum of light variations for accretion in gas blobs.The observed Ly alpha emission line shows a strong phase dependence withmaximum flux and redshift near orbital phase phi~0.3, strongly indicating anorigin on the trailing hemisphere of the secondary star. An additional Ly alphaabsorption feature with similar phasing as the Ly alpha emission, but a\~700km/s blueshift could tentatively be ascribed to absorption of WD emissionin a moderately fast wind. We derive a column density of neutral hydrogen ofNH=(1.1+-1.0)1e18 cm**-2, the lowest of any known polar.Comment: 26 pages, 10 figures, AAS TeX 5.0, accepted for publication in the  Astrophysical Journa

Phase‐resolvedHubble Space Telescope/STIS Spectroscopy of the Exposed White Dwarf in the High‐Field Polar AR Ursae Majoris