Frequency-time-resolved Imaging Spectroscopy of Fine Structures in a Solar Radio Noise Storm

Solar radio noise storms are common phenomena, composed of broadband continuum emission embedded with diverse fine structures, yet their unusually compact apparent sizes remain unexplained. We present frequency-time-resolved imaging spectroscopy of a near-disk-center noise storm observed by LOFAR between 30--40 MHz, together with anisotropic radio-wave scattering simulations. The continuum forms a bright, spatially compact source that drifts across the solar disk over tens of minutes. Across the band, the measured apparent major axis is $\sim8.0^\prime$ to $\sim4.3^\prime$ between 31.3 and 38.4~MHz, less than half the size of typical type III burst sources at comparable frequencies. Embedded type I bursts, S-bursts, and spikes exhibit a range of dynamic spectra appearances, yet share nearly identical apparent sizes within uncertainties, suggesting a common size-determining mechanism. Using anisotropic scattering simulations, we show that compact apparent source sizes naturally arise for emission embedded within closed magnetic field structures, where anisotropic turbulence directs radiation away from the observer's line of sight. Additional modifications arise from enhanced coronal densities, steeper density gradients, reduced turbulence levels, and strong fluctuation anisotropy, but these exert secondary influence. Our results provide a unified explanation for the similar apparent sizes of diverse fine structures in noise storms, and demonstrate that the compactness of type I storm sources is governed primarily by the large-scale coronal environment rather than intrinsic differences in emission processes, where the magnetic topology plays a crucial role in determining the observed source size.