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The evolution of a weakly ionized magnetized sheet plasma generated in 100-200 mtorr air has been investigated. A 1.2×50 cm hollow cathode imbedded in a 250 Gauss magnetic field was used to produce a 50×60 ×2-cm planar plasma sheet with an electron density ne≅1012 cm-3. Such plasmas have applications in microwave beam steering and plasma processing. Time-resolved measurements of the discharge parameters, local electric field, electron density and temperature, optical emissions, and microwave transmission show the effects of neutral gas heating on formation of the plasma. A beam-generated negative glow discharge was formed at low neutral gas density and high beam voltage. At higher gas densities and lower beam voltages, a positive column discharge sustained by avalanche ionization formed in the region near the anode. For pulsed discharges with t>150 μs, a transition from the positive column to the negative glow mode was observed due to heating of the neutral gas.