  The accretion flow / jet correlation in neutron star (NS) low-mass X-ray
binaries (LMXBs) is far less understood when compared to black hole (BH) LMXBs.
In this paper we will present the results of a dense multi-wavelength
observational campaign on the NS LMXB 4U 1820-30, including X-ray (Nicer,
NuSTAR and AstroSAT) and quasi-simultaneous radio (ATCA) observations in 2022.
4U 1820-30 shows a peculiar 170 day super-orbital accretion modulation, during
which the system evolves between "modes" of high and low X-ray flux. During our
monitoring, the source did not show any transition to a full hard state. X-ray
spectra were well described using a disc blackbody, a Comptonisation spectrum
along with a Fe K emission line at 6.6 keV. Our results show that the observed
X-ray flux modulation is almost entirely produced by changes in the size of the
region providing seed photons for the Comptonisation spectrum. This region is
large (about 15 km) in the high mode and likely coincides with the whole
boundary layer, while it shrinks significantly (<10 km) in low mode. The
electron temperature of the corona and the observed RMS variability in the hard
X-rays also exhibit a slight increase in low mode. As the source moves from
high to low mode, the radio emission due to the jet becomes about 5 fainter.
These radio changes appear not to be strongly connected to the hard-to-soft
transitions as in BH systems, while they seem to be connected mostly to
variations observed in the boundary layer.
