Small scale ionospheric disturbances as seen by the LOFAR

The LOFAR radio telescope consists of a large network of stations distributed across Europe, with a dense core in the east of the Netherlands. Each station consists of multiple antenna fields operating at frequencies between 20 and 80MHz and 110 and 240MHz. At these frequencies the ionosphere has a major impact on the astronomical observations, that needs to be corrected for.  This ionospheric calibration, at first order mainly a phase effect, provides valuable information about the ionospheric density variations above the telescope. We report on the use of the calibration solutions to extract the ionospheric information on structure, dominant direction and variability. In particular, we investigated the data recorded with the Dutch array, consisting of a core of 48 stations all within a 3 km diameter circle and another 14 remote stations with baselines up to 100 km, operating between 110 and 170 MHz.  The different baselines give access to different scales in the ionosphere. Furthermore, we present, for the same data, an imaging technique that allows direct imaging of larger scale gradients in the ionosphere.