The LOFAR Decametre Sky Survey (LoDeSS) I. Survey description and first data release

The decametre radio sky (~10–30 MHz) remains largely unexplored because of strong ionospheric distortions and severe radio-frequency interference. Despite its scientific potential—including access to steep-spectrum synchrotron emission from aged electron populations and coherent radio emission from planetary and stellar magnetospheres—sub-arcminute imaging at these frequencies has only recently become feasible with instruments such as the Low Frequency Array (LOFAR).

This work presents the first data release (DR1) of the LOFAR Decametre Sky Survey (LoDeSS), which maps the northern sky above a declination of 20° between 15 and 30 MHz. Observations were obtained with the LOFAR Low Band Antenna (LBA) system using a multi-beam strategy, with typical integration times of ~5 hours per pointing, and were carried out at night to reduce the impact of ionospherically reflected radio-frequency interference. LoDeSS reaches an angular resolution of ~45 arcsec and typical noise levels of ~11.5 mJy beam⁻¹ at a central frequency of 23 MHz. Data quality varies across fields as a result of ionospheric conditions and the fraction of data flagged due to radio-frequency interference.

This first release covers ~15% of the full survey area and consists of fields originally selected around exoplanet candidates for radio auroral emission. The products released to the community include primary-beam-corrected images in Stokes I and V, and a source catalogue for fields with reliable flux density scales.

This release demonstrates that LOFAR enables sensitive, wide-field imaging in the decametre regime with unprecedented resolution, opening new opportunities for studies of pulsars, exoplanetary systems, galaxy clusters, and other low-frequency phenomena.