  Solar flare emission at X-ray and extreme ultraviolet (EUV) energies can
cause substantial enhancements in the electron density in the Earth's lower
ionosphere. It is now become clear that flares exhibit quasi-periodic
pulsations with timescales of minutes at X-ray energies, but to date, it has
not been known if the ionosphere is sensitive to this variability. Here, using
a combination of Very Low Frequency (24 kHz) measurement together with
space-based X-ray and EUV observations, we report pulsations of the ionospheric
D-region, which are synchronized with a set of pulsating flare loops. Modeling
of the ionosphere show that the D-region electron density varies by up to an
order of magnitude over the timescale of the pulsations ($\sim$20 mins). Our
results reveal that the Earth's ionosphere is more sensitive to small-scale
changes in solar soft X-ray flux than previously thought, and implies that
planetary ionospheres are closely coupled to small-scale changes in
solar/stellar activity.
