  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 $2.3\times10^{46}$ 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.
