We present the unsaturated peak profile of SGR 1900+14 giant flare on 1998 August 27 .
This was obtained by particle counters of the Low Energy Particle instrument onboard the GEOTAIL spacecraft .
The observed peak profile revealed four characteristic structures : initial steep rise , intermediate rise to the peak , exponential decay and small hump in the decay phase .
From this light curve , we found that the isotropic peak luminosity was 2.3 \times 10 ^ { 46 } erg s ^ { -1 } and the total energy was 4.3 \times 10 ^ { 44 } erg s ^ { -1 } ( E \gtrsim 50 keV ) , assuming that the distance to SGR 1900+14 is 15 kpc and that the spectrum is optically thin thermal bremsstrahlung with kT = 240 keV .
These are consistent with the previously reported lower limits derived from Ulysses and Konus-Wind observations .
A comparative study of the initial spikes of SGR 1900+14 giant flare in 1998 and SGR 1806-20 in 2004 is also presented .
The timescale of the initial steep rise shows the magnetospheric origin , while the timescale of the intermediate rise to the peak indicates that it originates from the crustal fracturing .
Finally , we argue that the four structures and their corresponding timescales provide a clue to identify extragalactic SGR giant flares among short GRBs .