Indices 


Available  from 1981 onwards  
Type of index  Longitudinally asymmetric and symmetric horizontal component disturbances  
Purpose  To describe the geomagnetic disturbances at midlatitudes in terms of
longitudinally asymmetric (ASY) and symmetric (SYM) disturbances for both H and D components
respectively parallel and perpendicular to the dipole axis. SYMH 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 foursteps 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)  Midlatitude Geomagnetic Indices ASY and SYM for 2009 (Provisional),
Internal Report of Data Analysis Center for Geomagnetism and Space Magnetism, Kyoto University, Japan.
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