Abstract
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 ε variations) around these averages are then investigated. The simple oscillatory pattern of ε, 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 ε reflects the changes within the solar cycle 23. The short period variations observed in the ε 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 ε, 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.
Highlights
Measured at any point on Earth’s surface the magnetic field is a combination of several magnetic contributions generated by various sources
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
When using the observatory data for the core field modeling, it is crucial that the observatory biases are accurately determined and thereafter applied to the raw data
Summary
Measured at any point on Earth’s surface the magnetic field is a combination of several magnetic contributions generated by various sources. A dominant part of Earth’s magnetic field, the core field (Jacobs, 1987; Merrill et al, 1998), is internal in origin and is due to the electric currents in Earth’s outer fluid core. This part of the field varies over timescales of some months to decades and longer. The external fields are produced by ionospheric and magnetospheric current systems (e.g., Campbell, 2001) The values of those fields at Earth’s surface are of a few tens of nanoteslas and even of a few hundred to thousands of nanoteslas during magnetic storms. Contributions of other sources have to be determined and taken into
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