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

Suprathermal population associated with stream interaction regions observed by STEREO-A: New insights