Merging fluxgate and induction coil data to produce low-noise geomagnetic observatory data meeting the INTERMAGNET definitive 1s data standard

Abstract

For frequencies above 30mHz the instrument intrinsic noise level of typical fluxgate magnetometers used at geomagnetic observatories usually masks ambient magnetic field variations on magnetically quiet days. This is especially true for stations located at middle and low latitudes, where variations are generally smaller than at high latitudes. INTERMAGNET has set a minimum quality standard for definitive 1s data. Natural field variations referred to as pulsations (Pc-1, Pc-2, Pi-1) fall in this band. Usually their intensity is so small that they rarely surpass the instrumental noise of fluxgate magnetometers. Moreover, high-quality magnetic field observations in the band 30mHz-0.5Hz contain interesting information, e.g., for the study of ionospheric electron interactions with electromagnetic ion cyclotron plasma waves. We propose a method to improve 1Hz observatory data by merging data from the proven and tested fluxgate magnetometers currently in use with induction coil magnetometers into a single data stream. We show how measurements of both instruments can be combined without information loss or phase distortion. The result is a time series of the magnetic field vector components, combining the benefits of both instruments: long-term stability (fluxgate) and low noise at high frequencies (induction coil). This new data stream fits perfectly into the data management procedures of INTERMAGNET and meets the requirements defined in the definitive 1s data standard. We describe the applied algorithm and validate the result by comparing power spectra of the fluxgate magnetometer output with the merged signal. Daily spectrograms from the Niemegk observatory show that the resulting data series reveal information at frequencies above 30mHz that cannot be seen in raw fluxgate data.