We are interested in ionospheric dynamics not only because of the physical phenomena involved, but also because of the growing dependence on trans-ionospheric radio signals for radio astronomy, as well as for communication and navigation. Current methods for probing the ionosphere include line-of-sight TEC (total electron content) measurements from ground- or space-based global positioning system receivers, ultraviolet limb scans of recombination emission, radar, and ionosondes. Incorporated into a Global Assimilative Ionospheric Model, these methods have been successful in measuring the TEC of the ionosphere to an accuracy of about 0.1 TECU (1 TECU = 10^l2 electrons/cm^2) at a resolution of a few hundreds of km. Structure on smaller scales, such as scintillation, cannot be measured directly through current techniques, although they can cause major disruptions to trans-ionospheric radio communication. Astronomers have recently begun using low-frequency radio interferometers to obtain high-resolution images of celestial radio emitters. Calibration of such instruments requires the accurate determination and removal of the ionospheric phase difference between each pair of antennas in the array. In doing so, the TEC differences between antenna elements can be determined to an accuracy of better than 0.001 TECU on time scales of approximately 1 minute. In current arrays, antennas are spaced between about 50 m to 35 km apart and there are a number of emerging large high frequency/very high frequency (HF/VHF) arrays—enabled by the NRL program in HF/VHF adaptive optics—that may allow ionospheric structure to be probed at much higher sensitivity, resolution, or both than current methods. Consequently, far more advanced techniques will have to be developed to perform the calibration necessary and to understand the ionospheric physics and phenomenology in much greater detail. Such an understanding opens a new area of ionospheric physics, modeling, and prediction. To help open this new field of study we are (1) developing algorithms and automated processes for retrieving the ionospheric TEC differences from measured interferometer phase fluctuations; (2) characterizing the spatial and temporal resolution and precision of the radio astronomical data bases; (3) exploring the physics of the transient ionospheric phenomena, which are detected with this new technique; and (4) exploiting the new capabilities of emerging new large arrays as they are developed./p>