Abstract
Abstract. For frequencies above 30 mHz 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 1 s 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 30 mHz–0.5 Hz contain interesting information, e.g., for the study of ionospheric electron interactions with electromagnetic ion cyclotron plasma waves. We propose a method to improve 1 Hz 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 1 s 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 30 mHz that cannot be seen in raw fluxgate data.
Highlights
Conventional fluxgate magnetometers used at geomagnetic observatories are optimized towards long-term stability as their main purpose
For frequencies above 30 mHz the instrument intrinsic noise level of typical fluxgate magnetometers used at geomagnetic observatories usually masks ambient magnetic field variations on magnetically quiet days
For each sample time t0 we look at values XFG(ti) registered by the fluxgate magnetometer at times ti varying from t0 − TsN to t0 + TsN, with the sample width Ts
Summary
Conventional fluxgate magnetometers used at geomagnetic observatories are optimized towards long-term stability as their main purpose. Even though they have excellent properties to measure the low-frequency part of the magnetic spectrum, their noise usually surpasses the natural background field variations at frequencies higher than 30 mHz at quiet days and mid- or low-latitude observatories. Variations in the band 30 mHz–0.5 Hz correspond to plasma waves, which are important for radiation belt physics. Electrons interacting with such waves can be scattered into the loss cone and reduce the energy content of the radiation belt. Pc-1 pulsations (f > 0.1 Hz) can be caused by electromagnetic ion cyclotron (EMIC) waves, which is the subject of many recent investigations (e.g., Shprits et al, 2016 or Usanova et al, 2014)
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