ANNALS OF GEOPHYSICS, 57, 2, 2014, G0211; doi:10.4401/ag-6299 G0211 Electromagnetic background noise at L’Aquila Geomagnetic Observatory Lucia Santarelli 1,* , Paolo Palangio 1 , Marcello De Lauretis 2 1 Istituto Nazionale di Geofisica e Vulcanologia, L’Aquila, Italy 2 Università degli Studi dell’Aquila, Dipartimento di Scienze Fisiche e Chimiche, L’Aquila, Italy ABSTRACT In this paper we analyze the electromagnetic background noise at L’Aquila Geomagnetic Observatory during 2006 and 2007 in the fre- quency band 1-100 mHz. In this band a pronounced daily variation is ob- served both in the natural signals as well as in the artificial ones, giving rise to the problem of separating different contributions of very similar morphology. We analyzed periods when the local K index was approxi- mately zero, in correspondence with minimum of the magnetospheric and ionospheric activity. We found that in our area the main source of artifi- cial noise is constituted by the DC electrified railways. 1. Introduction In the frequency band 10 -3 -10 5 Hz a wide variety of electromagnetic natural phenomena are observed without significant changes during last millions years. All living organisms appeared and evolved into this background radiation. The natural electromagnetic soupe (Figures 1 and 2) can be observed at any point on the terrestrial surface and is the result of an enormous number of signals pro- duced by different sources in the magnetosphere and its interaction with the solar wind, the ionosphere, the Earth-ionosphere cavity and the Earth’s interior. This background permeates the near Earth environment in a complex manner involving, for example: (1) free oscillation modes of the magnetospheric cavity (0.001 Hz-0.01 Hz) [Campbell 1997]. (2) resonance modes of the alfvenic cavity (0.1 Hz- 10 Hz) [Belyaev et al. 1990]. (3) emissions associated with various instability processes in the magnetospheric and ionospheric plasma (0.001 Hz-100 kHz) [Campbell 1997]. (4) emissions associated with tectonic and volcanic phenomena (0.001 Hz-1000 Hz) [Palangio et al. 2007]. (5) magnetic storms and associated phenomena (0.0001 Hz-1 Hz) [Campbell 1997]. Some aspects of magnetospheric dynamics such as wave propagation, resonance phenomena, global oscil- lation modes, request simultaneous measurements from large magnetometer networks, often installed in highly populated regions. As a consequence, a careful evalua- tion of the possible contamination of man-made distur- bances on ULF measurements represents an important topic in a wide scientific context. The main sources of artificial EM noise in the fre- quency bands of interest are railways transport systems on rails (underground trains, trams, trolley buses), elec- trical energy distribution plants, industrial plants etc. These sources, owing to their spatial extent, affect vast, heavily populated areas. There are also local sources of smaller scale signals as, for example, technological de- vices daily used worldwide. The artificial emissions contributed to the primordial broth only during the last century, and their influence on ULF measurements have been discussed by several authors. Fraser and Ellyett [1964] found that the amplitude of small fluctuations of the Earth’s magnetic field (the geomagnetic micropulsations [Jacobs 1970]) was influ- enced by magnetic signatures associated with the elec- trical network up to a distance of about 15 miles from the city (practically all micropulsation activity under observing conditions strongly decreased on Sunday, providing a simple test for man-made influences). An isolated dc electric train system, although dozens of miles away, is able to produce pulsations of large am- plitude on a wide-band micropulsation recorder, which can be confused with natural signals. During the recording of the ULF signals Fraser- Smith and Coates [1978] observed the simultaneous occurrence of large-amplitude ULF signals and the ar- Article history Received February 8, 2013; accepted December 19, 2013. Subject classification: Electromagnetic noise by railways, Man-made electromagnetic noise, Quiet days noise analysis, Man-made disturbances on ULF measurements.