Magnetic observatories: biases over CHAMP satellite mission

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 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.

An annual proxy for the geomagnetic signal of magnetospheric currents on Earth based on observatory data from 1900–2010

“…Fig. 4 shows differences between our biases Bb i and values from three other studies, namely Langel et al (1982), Mandea & Langlais (2002) and Verbanac et al (2015). While a detailed explanation of disagreements between these biases at specific locations is beyond the scope of our study, we want to highlight a systematic trend that is consistent across all studies.…”

“…This way the constant part of the external field signal is not included in the static lithospheric contribution Bb i , which is not achieved in the calculation of the Dst and RC indices. Both these indices underestimate the ring current effect by a constant amount Verbanac et al 2015). The values originally published by Langel et al (1982) were taken from Gubbins & Bloxham (1985).…”

Verification of the geomagnetic field models using historical satellite measurements obtained in 1964 and 1970

“…For some of the considered observatories (CLF, FRD, VIC, GNA) the data was corrected for the local crustal anomalies (also known as observatory biases), which were quanti ed using recent high-quality satellite data [Verbanac et al, 2015]. Unfortunately, the biases are unknown for the rest of considered observatories (MMB, TKT, PAF, HER, PIL).…”

Recovering external contribution from the monthly mean series of a given geomagnetic observatory

“…Such "crustal biases" of annual means over 42 years period are well investigated [Verbanac et al 2007a] for 46 European geomagnetic observatories by using models based on Magsat, Ørsted, CHAMP and SAC-C satellites. Also, the temporal evolution of the observatory monthly means (over 9 years period) "crustal biases" are analyzed by Verbanac et al [2015] using the G-field model [Lesur et al 2015]. Even in this analysis, "crustal biases" are considered the differences of the magnetic components measured at the observatory with the values predicted by a model obtained from satellite data only.…”