Harmonic quiet-day curves as magnetometer baselines for ionospheric current analyses

Abstract. This paper presents a novel method to determine a baseline for magnetometer data. This baseline consists of all magnetic field components not related to ionospheric and magnetospheric disturbances, i.e. all field components due to solar quiet variations and other background variations, as well as equipment effects. Extraction of this baseline is useful when the disturbance magnetic field is analysed. The full baseline is composed of two main constituents: the diurnal baseline and the long-term baseline. For the diurnal baseline, first "templates" are derived, based on the lowest few harmonics of the daily curves from the quietest days. The diurnal variation of the baseline is obtained by interpolating between these templates. This method ensures a smooth baseline at all times, avoiding any discontinuities at transitions between days. The long-term baseline is obtained by interpolating between the daily median values. This way, the baseline is ensured to follow long-term trends, such as seasonal and tidal variations, as well as equipment drift. The daily median values are calculated for all days except the most disturbed ones; a procedure is included to ensure that the median values used are unaffected by disturbances. This procedure avoids many problems associated with other existing baseline procedures, and makes magnetometer data suitable, for instance, for the calculation of ionospheric equivalent currents related to geomagnetic storms. Even data from remote unmanned magnetometers, which exhibit significant equipment effects, can be made suitable this way, which can give valuable contributions to the equivalent current database.