51�������

Abstract:

<jats:p>The May 2024 Mother’s Day superstorm, the strongest since November 2003, triggered significant ionospheric disturbances. Indeed, during the superstorm, the ionosphere above Europe was transformed into a severely depleted, super-high-altitude plasma structure extending beyond 1500 km above Earth’s surface. We highlight the unique contributions of the LOw Frequency ARray (LOFAR) to ionospheric weather research on this extreme event. Originally designed for radio astronomy, LOFAR’s extensive European network of 52 stations and wide-band capabilities enable high-resolution ionospheric monitoring at both local and regional scales. Leveraging LOFAR measurements, we characterise a plethora of ionospheric effects that occurred during the main and early recovery phases of the storm. We did this by observing significant signal fading associated with the equatorward expansion of the auroral oval during the main phase and by capturing high-speed moving ionospheric irregularities (up to ~800 m/s), and quantifying extreme ionospheric uplift (up to 1500 km) under conditions where conventional HF instruments were not usable due to the occurrence of D-layer absorption, ionospheric G-conditions and uplift above ionosonde altitude range. In this investigation, we harness the unique capabilities of LOFAR to direct view into the structure and dynamics of the ionosphere under the most extreme space weather conditions. Our results confirm LOFAR as an insightful instrument for ionospheric research, offering critical capabilities for advancing storm impact assessment, forecasting, and mitigation strategies.</jats:p>

Abstract:

We present the discovery of radio emission from the Be/X-ray binary A0538−66 with the Australian Square Kilometre Array Pathfinder, and results from a subsequent weekly monitoring campaign with the MeerKAT radio telescope. A0538−66, located in the Large Magellanic Cloud, hosts a neutron star with a short spin period ( ms) in a highly eccentric -d orbit . Its rare episodes of super-Eddington accretion, rapid optical and X-ray flares, and other peculiar properties make it an interesting system among high-mass X-ray binaries. Our MeerKAT data reveal that it is also one of the most radio-luminous neutron star X-ray binaries observed to date, reaching (at 1.28 GHz), with radio emission that appears to be orbitally modulated. We consider several possible mechanisms for the radio emission, and place A0538−66 in context by comparing it to similar systems.

Abstract:

The Low Frequency Array (LOFAR) is one of the most advanced radio telescopes in the world. When radio waves from a distant astronomical source traverse the ionosphere, structures in this plasma affect the signal. Results are presented from a statistical study using 2,810 hours of observations of Cassiopeia A from a LOFAR station located in the Netherlands (station CS032, located at 52.9° N; 6.9° E) between 28th June 2014 and 27th November 2016. Ionospheric structures were identified in 469 (~17 %) of these observations. A comparison with proxies for geomagnetic activity (the K p index) and solar activity (the F10.7 cm solar radio flux) showed that geomagnetic or solar effects were not the primary driver of these ionospheric structures. Ionospheric structures were more common in summer and between ~21 LT and 02 LT. These patterns in season and local time showed similarities to the occurrence of lightning strikes. When ionospheric structures were present, the mean number of lightning strikes in a spatial region close to the LOFAR observations (51.9° – 56.5° N; 3.9° – 9.9° E) two hours prior to the LOFAR observations was (70 ± 25) per hour. This was substantially larger than the mean value of (19 ± 5) per hour when the ionospheric structures were absent. This suggests that quasi-upward propagating Atmospheric Gravity Waves (AGWs) launched by thunderstorm activity could be one of the sources of the ionospheric structures. Collectively, these observations show that LOFAR can be used to infer ionospheric signatures of vertical coupling processes in the mid-latitude atmosphere. Such observations have the potential to be used to develop or validate existing Global Circulation Models (GCMs) or new models of vertical coupling within the terrestrial atmosphere.

Abstract: