  Energy conversion via reconnecting current sheets is common in space and
astrophysical plasmas. Frequently, current sheets disrupt at multiple
reconnection sites, leading to the formation of plasmoid structures between
sites, which might affect energy conversion. We present in situ evidence of the
firehose instability in multiple reconnection in the Earth's magnetotail. The
observed proton beams accelerated in the direction parallel to magnetic field
and ion-scale fluctuations of whistler type imply the development of firehose
instability between two active reconnection sites. The linear wave dispersion
relation, estimated for the measured plasma parameters, indicates a positive
growth rate of firehose-related electromagnetic fluctuations. Simulations of
temporal evolution of the observed multiple reconnection by using a 2.5D
implicit particle-in-cell code show that, as the plasmoid formed between two
reconnection sites evolves, the plasma at its edge becomes anisotropic and
overcomes the firehose marginal stability threshold, leading to the generation
of magnetic field fluctuations. The combined results of observations and
simulations suggest that the firehose instability, operating between
reconnection sites, converts plasma kinetic energy into energy of magnetic
field fluctuations, counteracting the conversion of magnetic energy into plasma
energy occurring at reconnection sites. This suggests that magnetic energy
conversion in multiple reconnection can be less efficient than in the case of
the single-site reconnection.
