IndianJournalof Radio& SpacePhysics Vol.27, April 1998,pp. 76-81 Storm-time disturbance field and the computed equatorial Dst index S Mukherjee,S Alex& G K Rangarajan IndianInstituteof Geomagnetism, Colaba,Mumbai400005 Received 7 February1997; revised27 January1998; accepted5 February1998 The geomagnetic disturbance field (D) is a composite of both universal time component (DJ and the local time-dependent part (SD) of the field. The equatorial Dst index is an accepted measure to quantify the strength of the magnetic storm, defming the contribution" from the symmetric magnetospheric ring current. A new approach is made to compute the D'I index with a less cumbersome process using observed horizontal field intensity at low latitudes, which incorporates certain normalization factors for removing the part associated with the quiet-time ionospheric current and the elimination of the secular trend. Systematic correspondence between the conventional equatorial Dst index [D'I(ev)] and the computed D'I [Dst(ca1)] is shown to exist. Since the spacing of stations used for the calculation of D,I(ev)is non-uniform, too large and far from ideal, an attempt is made to compute the Dst index by the inclusion of an additional station Alibag (lat.18.38°N ; long.72.52°E) in the Indian longitude, covering a wide gap. Progressive change in the asymmetric component after the inclusion of the station Alibag in the computation of Dst is brought out from the analysis. 1 Introduction Large-scale disturbances in the magnetosphere originate in the solar plasma streaming towards the earth. Injection of chaf!~ed particles in the magnetosphere, under favourable interplanetary conditions, gives rise to the geomagnetic disturbance fields. Horizontal component of the magnetic field recorded on the ground at low and middle latitudes shows clear signature of the enhanced energy transfer from the interplanetary space into the inner magnetosphere. Chapmanl described this disturbance field as a combination of an axially symmetric part (D,J and an asymmetric part ( SD), i.e. D = D'I + SD. In other words D'I will be UT-dependent while the SD component will depend on local time. During the main phase of a magnetic storm, a major portion of the D field can be attributed to the symmetric ring current encircling the earth at a distance of 3-5 RE• Initially it was thought that the SD (asymmetric) component had its origin in the auroral ionosphere2, but later observations of concurrent variations in phase between geostationary altitudes and ground clearly indicated that the asymmetric component could be of magnetospheric origin3• It is now attributed to the currents in the ionosphere or magnetosphere or both4• To quantify the strength of the magnetospheric ring current in the equatorial plane, the equatorial DSI index was proposed during the International Geophysical Yea,-5·6. It is derived from the hourly observations of the H field at a network of four low latitude stations, viz. San Juan, Honolulu, Hermanus and Kakioka (see Table 1) well away from both the equatorial and auroral electrojet belts. According to Mayaud7 the equatorial DSI index is one with a direct physical significance as an indicator of the strength of the ring current. In deriving the index, care is taken to eliminate long term secular trend and the seasonal and diurnal quiet day variation through a suitable harmonic representation. The derivation of the DSI index by the method adopted by Sugiura6 involves the following steps: (i) Long term secular variation is eliminated through polynomial fitting to the annual mean values of the International Quiet (IQ) days. (ii) Hourly values of the Sq variations are synthesized from the Fourier analysis of the mean monthly hourly values of quiet days for each month for individual stations. (iii)A final correction to normalize the variations to the dipole equator by multiplying the hourly 'III I" '!' '1" J~'''' "1 ! II I ' 11-111111 II i~'1 ill l I II I Itll I I I I 'I_I