Iron deposit effect observed in Kiruna geomagnetic fluctuations: Indications for an improved approach of magnetotellurics searching methods

Geomagnetic data at Kiruna station (KIR) in Sweden has been expected to be affected by the iron ore mine because high conductance underground generally means depressed ΔZor dZ/dt compared to ΔH or dH/dt, where Z and H are downward and horizontal components, respectively, Δ indicates deviation, and we took 1-min average of 1-sec resolution data when defining d/dt values. We examined the 1-sec resolution magnetometer data using both frequency-domain (i.e., standard magnetotellurics) and time-domain analyses, and compared general behaviours with the other high-latitude stations on the same longitude (Hornsund: HRN, Abisko: ABK, Lycksele: LYC, Uppsala: UPS, Nurmijärvi: NUR) for the same period (September 2014-2020).

Surprisingly, we found KIR anomaly only in time-domain derivative dB/dt and not in the frequency domain spectrum. To quantify this anomaly, we examined the standard deviation of each parameter (1-min average of 1-sec resolution values) over 3 hours. With this quantification, the level of anomaly was about the same between old magnetometer until 2019 and new magnetometer from 2020. The anomaly is somewhat present in both dZ/dt and dH/dt but is the clearest in the ratio of dZ/dt to dH/dt.  On the other hand, neither ΔZ nor ΔH showed anomaly. Furthermore, no anomaly is recognized in the inclination I (=atan(Z/H)), i.e., ΔI nor dI/dt. From all of these, we believe that the observed anomaly is caused by underground iron ore deposit and not by the magnetometer filtering setting. The reason why the anomaly is found only in d/dt values is not clear, but we suspect that the iron ore deposit might cause time delay between dZ/dt and dH/dt when step-like variation dominates as the input variation, which is often the case with auroral activity. In such variation, neither the frequency domain analyses, nor simple time domain analyses (ΔB) show any anomaly.

We applied this method to the other meridians (three meridians in North America). We could not find any anomaly similar to what KIR data showed. However, we found another type of anomaly (on dI/dt) in Barrow, Alaska. It can be related to its location, surrounded by the arctic sea in both east and west, but we have not yet found an appropriate interpretation.

[Acknowledgements:  This work resulted from a 2021 summer internship study at the Swedish Institute of Space Physics, Kiruna.   The 1-sec resolution geomagnetic data are obtained from INTERMAGNET and are originally provided by SGU (Sweden: UPS, LYC, KIR, ABK), FMI (Finland: NUR), PAS (Poland: BEL, HLP, HRN), GSC (Canada: BLC, CBB, FCC, IQA, OTT, RES, STJ, YKC), USGS (USA: BRW, CMO, FRD, SHU, SIT), IPGP (France: CLF), ZAMF (Austria, WIC), and ASCR (Czech: BDV).]