The ASY (Asymmetric) index was originally introduced by Iyemori in 1992 as a measure of longitudinally asymmetric component of geomagnetic variations at mid-latitude. The index is derived from geomagnetic variations in the horizontal (H) and its orthogonal (D, east-west) components observed at 6 observatories in an absolute geomagnetic latitude range of 10°-50°. Detailed explanation is shown in the WWW page of WDC for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/aeasy/index.html).

The ASY-H/ASY-D indices show variations corresponding to mid-latitude positive bays at substorm onset [Iyemori et al., 1992]. It has also been shown that the variation of the asymmetric H component correlate well with the AE index [Crooker, 1972; Clauer and McPherron, 1980]. Substorm can be identified from the ASY index by finding gradual increases.

References
Clauer, C. R., and R. L. McPherron, The relative importance of the interplanetary electric field and magnetospheric substorms on the partial ring current development, J. Geophys. Res., 85, 6747-6759, 1980.
Crooker, N. C., High-time resolution of the low-latitude asymmetric disturbance in the geomagnetic field, J. Geophys. Res., 77, 773-775, 1972.
Iyemori, T., T. Araki, T. Kamei, and M. Takeda, Mid-latitude geomagnetic indices ASY and SYM (Provisional) No. 1, 1989, Data Anal. Center for Geomagn. and Space Magn., Kyoto Univ., Kyoto, Japan, 1992.