Speaker
Description
The partial solar eclipse on 29th March 2025 had become an opportunity to conduct radio observations from a stratospheric balloon platform, simultaneously and as an aid to the ground-based observations by the LOFAR radiotelescope station in Borówiec near Poznań. The stratospheric flight conditions were chosen as a mean to mitigate the influence of the man-made radio noise; the experiments’ primary objectives were to register potential stray high-frequency radio signals reflected from any occurring ionospheric irregularities, as well as to monitor the level of the background noise of the chosen radio frequency channel. The prepared balloon platform was equipped with a HF/VHF beacon, transmitting using WSPR protocol and a horizontal cross antenna, and a narrow-bandwidth analog receiver with monopole antenna, operating on 23 MHz with simultaneous data saving. The mission was launched from EPPZ airfield (Warsaw University of Technology Aerospace Research Centre "OBLot") on 12.26 local time, with end of the eclipse at 13.06 local time. The 23-MHz experiment did not register any stray radio emissions, but the recorded radio frequency noise levels did correlate with the dynamics of the eclipse, slowly rising and reaching an approximately constant amplitude after the end of the eclipse, indicating the rise of the ionosphere’s reflection coefficient and electron density. The reduction of man-made noise was well-correlated with the balloon launch. The maximum altitude of the mission reached 33608 m a.s.l. at 13.45 local time, with the constant ascent velocity. The WSPR beacons were received twice after the end of the eclipse, 190 km away to the south-west from the balloon’s trajectory – this indicated the reach of the f0 F2 at 28.1261 MHz. The rising radio frequency noise levels, recorded at 23 MHz, have been re-calculated to the difference in total electron density between the nearly-constant state after the end of the eclipse and the changing state during the eclipse; this difference slowly decreased with the eclipse’s fading away. Its low value, well below 1 TECU, indicates that in the end of the relatively weak eclipse the dominating mechanism was the photoionisation increasing the electron density, primarily in the F2 layer. The overall performance of the experiments and the balloon mission was more than satisfactory and shall serve as a basis for the years-spanning campaign of scheduled stratospheric flights with improved instrumentation, aiding terrestrial radio observations during solar phenomena.
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