An improved algorithm for separating clock delays from ionospheric effects in radio astronomy

Abstract

Context: Low-frequency radio observations are heavily impacted by the ionosphere, where dispersive delays can outpace even instrumental clock offsets, posing a serious calibration challenge. Especially below 100 MHz, phase unwrapping difficulties and higher-order dispersion effects can complicate the separation of ionospheric and clock delays. Aims: We address this challenge by introducing a method for reliably separating clock delays from ionospheric effects, even under mediocre to poor ionospheric conditions encountered near solar maximum. Methods: The approach employs a key technique: we modelled our likelihood space using the circular Gaussian distribution (von Mises random variable) rather than non-circular distributions that suffer from $2\pi$ phase ambiguities. This ensures that noisier data are weighted less heavily than cleaner data during the fitting process. Results: The method reliably separates clock delays and ionospheric terms that vary smoothly in time whilst providing a good fit to the data. A comparison with the clock-ionosphere separation approach used in standard LOFAR data processing shows that our technique achieves significant improvements. In contrast to the old algorithm, which often fails to return reliable results below 100 MHz even under good ionospheric conditions, the new algorithm consistently provides reliable solutions across a wider range of conditions. Conclusions: This new algorithm represents a significant advance for large-scale surveys, offering a more dependable way to study ionospheric effects and furthering research in ionospheric science and low-frequency radio astronomy.

9 pages, 7 figures

References in corpus (21)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Array Programming with NumPy
• LOFAR: The LOw-Frequency ARray
• Systematic effects in LOFAR data: A unified calibration strategy
• Probing Ionospheric Structures using the LOFAR radio telescope
• Ultra-Wideband Detection of 22 Coherent Radio Bursts on M Dwarfs
• The 21 cm Power Spectrum from the Cosmic Dawn: First Results from the OVRO-LWA
• The first power spectrum limit on the 21-cm signal of neutral hydrogen during the Cosmic Dawn at z=20-25 from LOFAR
• Gentle re-energisation of electrons in merging galaxy clusters
• The population of M dwarfs observed at low radio frequencies
• High-redshift radio galaxies: a potential new source of 21-cm fluctuations
• The effect of the ionosphere on ultra-low frequency radio-interferometric observations
• Discovery of a radio galaxy at z = 5.72
• LOFAR discovery of a 700-kpc remnant radio galaxy at low redshift
• FR II radio galaxies at low frequencies I: morphology, magnetic field strength and energetics
• The detectability of radio emission from exoplanets
• Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies
• The AARTFAAC Cosmic Explorer: observations of the 21-cm power spectrum in the EDGES absorption trough
• Hunting for exoplanets via magnetic star-planet interactions: geometrical considerations for radio emission
• The milliarcsecond-scale radio structure of AB Dor A
• Estimating the concentration parameter of a von Mises distribution: a systematic simulation benchmark