  We present mid-infrared (MIR) spectral-timing measurements of the
prototypical Galactic microquasar GRS 1915+105. The source was observed with
the Mid-Infrared Instrument (MIRI) onboard JWST in June 2023 at a MIR
luminosity L(MIR)~10^{36} erg/s exceeding past IR levels by about a factor of
10. By contrast, the X-ray flux is much fainter than the historical average, in
the source's now-persistent 'obscured' state. The MIRI low-resolution spectrum
shows a plethora of emission lines, the strongest of which are consistent with
recombination in the hydrogen Pfund (Pf) series and higher. Low amplitude (~1%)
but highly significant peak-to-peak photometric variability is found on
timescales of ~1,000 s. The brightest Pf(6-5) emission line lags the continuum.
Though difficult to constrain accurately, this lag is commensurate with
light-travel timescales across the outer accretion disc or with expected
recombination timescales inferred from emission line diagnostics. Using the
emission line as a bolometric indicator suggests a moderate (~5-30% Eddington)
intrinsic accretion rate. Multiwavelength monitoring shows that JWST caught the
source close in-time to unprecedentedly bright MIR and radio long-term flaring.
Assuming a thermal bremsstrahlung origin for the MIRI continuum suggests an
unsustainably high mass-loss rate during this time unless the wind remains
bound, though other possible origins cannot be ruled out. PAH features
previously detected with Spitzer are now less clear in the MIRI data, arguing
for possible destruction of dust in the interim. These results provide a
preview of new parameter space for exploring MIR spectral-timing in XRBs and
other variable cosmic sources on rapid timescales.
