Service International des Indices Géomagnétiques
International Service of Geomagnetic Indices

ASY/SYM indices

ASY-D, ASY-H, SYM-D, SYM-H time resolution: 1 minute (UT) interval
unit: nT
Available from 1981 onwards
Type of index Longitudinally asymmetric and symmetric horizontal component disturbances
Purpose To describe the geomagnetic disturbances at mid-latitudes in terms of longitudinally asymmetric (ASY) and symmetric (SYM) disturbances for both H and D components respectively parallel and perpendicular to the dipole axis.
SYM-H is essentially the same as the Dst index with a different time resolution.
Network Made of 6 observatories evenly distributed in longitude (in fact of 11 observatories whose data are interchangeable depending on their availability.
(see list of actual ASY magnetic observatories)
Derivation The ASY/SYM indices are derived in a four-steps procedure (*):
   (i) Subtraction of the geomagnetic main field and the solar quiet daily variation field to obtain the disturbance field component.
   (ii) Coordinate transformation to a dipole coordinate system.
   (iii) Calculation of the longitudinally symmetric component (i.e. six station average) and the asymmetric component (i.e. disturbance field minus the symmetric component).
   (iv) Derivation of the asymmetric indices (i.e. the range between the maximum and the minimum asymmetric fields).

(*) More specific and detailed information may be found on the WDC for Geomagnetism - Kyoto website devoted to ASY/SYM
ISGI Collaborating Institute WDC for Geomagnetism - Kyoto, Japan
IAGA endorsed No
Main references • Iyemori, T., M. Takeda, M. Nose, Y. Odagi and H. Toh (2010) - Mid-latitude Geomagnetic Indices ASY and SYM for 2009 (Provisional), Internal Report of Data Analysis Center for Geomagnetism and Space Magnetism, Kyoto University, Japan. Get PDF (405K)