  Stream interaction regions (SIRs) are often thought to be responsible for the
generation of suprathermal population in the interplanetary medium. Despite the
source being same, wide variations in spectral indices of suprathermal
populations are observed at 1 au during SIRs. This poses significant
uncertainty in understanding the generation of suprathermal ion populations by
SIRs and indicates interplay of multiple source mechanisms. In the present
work, by analyzing variations in suprathermal 4He, O, and Fe for 20 SIR events
recorded by STEREO-A during 2007 - 2014, we find that the spectral indices of
these elements vary in the range of 2.06-4.08, 1.85-4.56, and 2.11-4.04
respectively for 19 events. However, in one special case, all the three
suprathermal elements show nearly identical (1.5) spectral indices. We offer
possible mechanisms, which can cause significant variations in the spectral
indices of suprathermal particles. More importantly, we show the possible role
of merging and/or contraction of small-scale magnetic islands near 1 au in
producing nearly identical spectral indices for three different elements with
different first ionization potential and mass-to-charge ratio. The occurrence
of these magnetic islands near 1 au also supports the minimal modulation in
spectral indices of these particles. The role of a possible solar flare in
generating these magnetic islands near the heliospheric current sheet is also
suggested.
