  The first appearance of radio type II burst emission at decameter-hectometer
(DH) waves typically occurs in connection, and often simultaneously, with other
types of radio emissions. As type II bursts are signatures of propagating shock
waves that are associated with flares and coronal mass ejections (CMEs), a rich
variety of radio emissions can be expected. However, sometimes DH type II
bursts appear in the dynamic spectra without other or earlier radio signatures.
One explanation for them could be that the flare-CME launch happens on the far
side of the Sun, and the emission is observed only when the source gets high
enough in the solar atmosphere. In this study we have analysed 26 radio type II
bursts that started at DH waves and were well-separated ('isolated') from other
radio emission features. These bursts were identified from all DH type II
bursts observed in 1998-2016, and for 12 events we had observations from at
least two different viewing angles with the instruments onboard Wind and STEREO
satellites. We found that only 30% of the type II bursts had their source
origin on the far side of the Sun, but also that no bursts originated from the
central region of the Sun (longitudes E30 - W40). Almost all of the isolated DH
type II bursts could be associated with a shock near the CME leading front, and
only few were determined to be shocks near the CME flank regions. In this
respect our result differs from earlier findings. Our analysis, which included
inspection of various CME and radio emission characteristics, suggests that the
isolated DH type II bursts could be a special subgroup within DH type II
bursts, where the radio emission requires particular coronal conditions to form
and to die out.
