The mode structure of a low-frequency instability is investigated in a magnetized low-β linear plasma with cylindrical geometry. A cathodoluminescent phosphor plasma imaging system [A. Liebscher et al., Rev. Sci. Instrum. 72(1), 953 (2001)] is used to obtain two-dimensional (2-D) plasma density images in the poloidal plane using a Zn doped ZnO (ZnO:Zn) phosphor coated aluminum disk. Short persistence time of the phosphor surface (τ _1/e = 0.4 μs) and low threshold electron energy (E_ec = 3 eV) of the cathodoluminescence produced by incident electrons results in visible light emission which contains local information on the radial and poloidal structure of plasma density fluctuations over a 2-D region. A fast-gated intensified charge coupled device (ICCD) camera fitted with interference filters to isolate the cathodoluminescent phosphor light is used to capture high-resolution 2-D plasma electron density images at 1 μs exposure time. Analysis of these images together with time series data from specialized probes reveals the spatio-temporal evolution of electron density fluctuations. The fluctuating phosphor light was monitored at discrete radial positions (2 x 2 mm² region) using a high-bandwidth photodetector system and compared with time series of ion saturation current fluctuations from axially displaced radial scanning multi-tip Langmuir probes to assess the temporal response of the phosphor imaging diagnostic. The radial profile of the cross-correlation between phosphor light and Langmuir probe signals indicates excellent agreement (high coherence) when both diagnostics monitor electron density fluctuations within the same flux tube region. These results independently confirmed the temporal response of the phosphor imaging system while verifying the observed radial mode structure of the coherent fluctuations. Furthermore, relative position of the Langmuir probe together with cross-field coherence data has shown the modes have a small but finite parallel wave number[special characters omitted], propagate perpendicular to the axial magnetic field, and have the maximum density fluctuation amplitude [special characters omitted] located near the maximum in radial density gradient. The experimental frequencies associated with the poloidal dynamics of the mode structures are analogous to that expected from the collisional drift wave instability using the two-fluid plasma description. Typically, 2-D electron density image topographies indicate organized large-scale structure with well defined m ≈ 1-2 poloidal mode numbers mixed with small-scale turbulence.