  LS I +61303 is a gamma-ray binary with periodic radio outbursts coincident
with the orbital period of P=26.5 d. The origin of the radio emission is
unclear,it could be due either to a jet, as in microquasars, or to the shock
boundary between the Be star and a possible pulsar wind. We here analyze the
radio spectral index over 6.7 yr from Green Bank Interferometer data at 2.2 GHz
and 8.3 GHz. We find two new characteristics in the radio emission. The first
characteristic is that the periodic outbursts indeed consist of two consecutive
outbursts; the first outburst is optically thick, whereas the second outburst
is optically thin. The spectrum of LS I +61 303 is well reproduced by the
shock-in-jet model commonly used in the context of microquasars and AGNs: the
optically thin spectrum is due to shocks caused by relativistic plasma
("transient jet") traveling through a pre-existing much slower steady flow
("steady jet"). This steady flow is responsible for the preceding optically
thick spectrum. The second characteristic we find is that the observed spectral
evolution, from optically thick to optically thin emission, occurs twice during
the orbital period. We observed this occurrence at the orbital phase of the
main 26.5 d outburst and also at an earlier phase, shifted by $\Delta \Phi
\sim$ 0.3 (i.e almost 8 days before). We show that this result qualitatively
and quantitatively agrees with the two-peak accretion/ejection model proposed
in the past for LS I +61303. We conclude that the radio emission in LS I +61303
originates from a jet and suggest that the variable TeV emission comes from the
usual Compton losses expected as an important by-product in the shock-in-jet
theory.
