Royal Observatory of Belgium - Meridian Room
The effect of the ionosphere on astronomical observations below 100 MHz
The Low-Frequency Array (LOFAR) is a hierarchical collection of thousands of dipole antennas which make it the most sensitive radio interferometer operating at low-frequencies ($10-240$ MHz). His main goal is low-frequency radio astronomy. LOFAR antennas are grouped into stations (aperture arrays capable of multi-beam forming) each about the size of a football field. LOFAR has 38 stations in the Netherlands and 13 international stations spread across seven European countries. LOFAR uses two antenna types: the High Band Antenna (HBA, sensitive between $110-240$ MHz) and the Low Band Antenna (LBA, sensitive between $10-90$ MHz). Here I will show how we used the LOFAR LBA system to obtain information on the ionosphere differential total electron content (TEC) with milliTEC precision, in hundreds of directions, on timescales of few seconds, and spatial scales of arc-minutes. I will also show the effect of phase and amplitude scintillations on our data and how we can reconstruct them. Finally, I will explore the higher order ionospheric effects that LOFAR can measure with high precision such as Faraday rotation.