Fig. 1

Locations of ground magnetic observatories whose data are used in the derivation of CHAOS-6. IAGA codes for the observatories are AAA, AAE, ABG, ABK, AIA, ALE, AMS, AMT, API, AQU, ARS, ASC, ASP, BDV, BEL, BFE, BFO, BGY, BJN, BLC, BMT, BNG, BOU, BOX, BRW, BSL, CBB, CBI, CDP, CKI, CLF, CMO, CNB, CSY, CTA, CZT, DLR, DLT, DMC, DOB, DRV, DVS, EBR, ELT, ESA, ESK, EYR, FCC, FRD, FRN, FUR, GAN, GDH, GLM, GNA, GNG, GUA, GUI, GZH, HAD, HBK, HER, HLP, HON, HRB, HRN, HTY, HUA, HYB, IPM, IQA, IRT, IZN, JCO, KAK, KDU, KEP, KHB, KIR, KIV, KMH, KNY, KNZ, KOU, KSH, KSH, LER, LIV, LMM, LNP, LOV, LRM, LRV, LVV, LYC, LZH, MAB, MAW, MBO, MCQ, MEA, MIZ, MMB, MOS, MZL, NAQ, NCK, NEW, NGK, NMP, NUR, NVS, OTT, PAF, PAG, PBQ, PET, PHU, PND, PST, QGZ, QIX, QSB, QZH, RES, SBA, SBL, SFS, SHU, SIL, SIT, SJG, SOD, SPT, SSH, STJ, SUA, TAM, TAN, TDC, TEO, TFS, THJ, THL, THY, TIR, TND, TRO, TRW, TSU, TUC, UPS, VAL, VIC, VSK, VSS, WHN, WIK, WNG, YAK, YKC