Magnetic observatories: Biases over CHAMP satellite mission

Abstract

Taking advantage of 9 years of the CHAMP (CHAllenging Minisatellite Payload) satellite mission (June 2000-August 2009), we investigate the temporal evolution of the observatory monthly magnetic biases. To determine these biases we compute X (northward), Y(eastward) and Z(vertically downward) monthly means from 42 observatory 1 min values or hourly values, and compare them to synthetic monthly means obtained from a G field model (Lesur et al., 2015). Afterwards, the average of biases at all observatories over 9 years is calculated and analyzed. Both the long-term trends and short-period variations (hereafter ∈ variations) around these averages are then investigated. The simple oscillatory pattern of ∈, found at all observatories and in each component over the entire considered period, indicates that the crustal field has not changed. A comparison with both MAGSAT and Ørsted biases given for epochs 1979.92 and 1999.92 which are based on 2 single months (November and December) of MAGSAT and Ørsted satellite data, respectively, further shows that the crustal field has probably remained constant over last 3 decades. The long-trend seen in ∈ reflects the changes within the solar cycle 23. The short period variations observed in the ∈ time series are related to the external field. The amplitudes of these variations are found to be in phase with solar cycle periods, being remarkably larger over 2000-2005 than 2006-2009. Furthermore, clear semi-annual variations are observed in ∈, with larger extremes appearing mostly around October and November, and around May and June of each year in X, and vice versa in Y and Z. A common external field pattern is found for the European monthly biases. The dependence of the bias monthly variations on geomagnetic latitudes is not found for non-European observatories. The results from this study represent a base to further exploit the magnetic biases computed for observatories and repeat station locations together by using data from the new satellite mission Swarm.