ELF Noise Test in the Sz´ echenyi Istv´ an Geophysical Observatory Tam´ as Boz´ oki 1,2* , J´ ozsef B´ or 1 , D´ aniel Piri 1 , Attila Nov´ ak 1 and Csaba Moln´ ar 1 1 Institute of Earth Physics and Space Science (ELKH EPSS), Sopron, Hungary 2 Doctoral School of Environmental Sciences, University of Szeged, Hungary Abstract On the 9 th of July, 2020, a detailed noise test was carried out in the Sz´ echenyi Istv´ an Geophysical Observatory in order to survey the electromagnetic noises generated in the very low frequency (VLF; 3-30 kHz) and extremely low frequency (ELF; 3 Hz - 3 kHz) bands by different electrical devices operating in the observatory. During a ∼ 3 hour long period, the measurements of the observatory, the power supply of buildings as well as solar panels and charging of the batteries were shut down one-by-one and finally the main power supply from the nearby village Fert˝oboz was interrupted for a few minutes. A pair of induction coil magnetometers run from a battery had been installed for this test period and provided information on the changes in the ELF noise environment during the test. Although the ELF-band noise contamination in the measurements reduced in connection to the elimination of internal noise sources in the observatory, it seems that external noise sources outside the observatory are too powerful and prohibit local monitoring of the Schu- mann resonances in the atmospheric magnetic field. Therefore, a new location for a permanent magnetic SR station is needed. Keywords: Extremely low frequency, Schumann resonance, ELF noise. Motivation Measurements of the atmospheric magnetic field by induction coils in the Sz´ echenyi Istv´ an Geophysical Observatory (NCK) are contaminated by different artificial noises which hinder the detection of Schumann resonances (SRs) (S´ atori et al., 2013; B´ or et al., 2020). We usually attribute this noise contamination to the rail- way lines which run near the observatory and have been electrified well after the establishment of the observatory. In the summer of 2020 a survey of electromag- netic noises was carried out in connection to the re-installation of the renovated VLF antenna. This opportunity enabled us to revisit the question on the origin of ELF noise. The setup of the experiment allowed the separation of ELF noises of internal or external origin in the observatory. Description of the test measurements A pair of induction coil magnetometers were installed temporarily near the obser- vatory in the time period from 07/07/2020 to 13/07/2020. The sampling frequency * Corresponding author: Tam´as Boz´oki (bozoki.tamas@epss.hu) Citation: Boz´ oki, T., B´ or, J., Piri, D., Nov´ak, A., and Moln´ar, Cs. (2021): ELF Noise Test in the Sz´ echenyi Istv´an Geophysical Observatory. Geophysical Observatory Reports 2020, 14-17. https://doi.org/10.55855/gor2020.2