  Pulses from 16 previously known rotating radio transients (RRAT) have been
searched at the 110 MHz daily monitor program for 4 to 5.5 years by using the
Large-Phased-Array (LPA) at Pushchino. The total number of pulses detected in
such a long observation interval is only 90 pulses for RRAT J0640+07 or is as
high as 10,751 pulses for RRAT J0302+22. The number and amplitude of pulses
varies at a time-scales from six to twenty months for RRATs J1336+33, J1404+11,
J1848+15, J2051+12, J2105+22, and the pulse number can increase by one or two
orders of magnitude in active phases. The long-term trends are found for RRATs
J0139+33 and J0302+22, showing a 2-3 times increase in detected pulse number
over 1,959 days. Some RRATs show the annual variations on both pulse number and
pulse amplitude. It is hard to explain all these variation time scales by
refractive scintillation on the interstellar medium. The annual and semi-annual
variations are likely caused by scintillations of the inhomogeneous
interplanetary plasma. Our data show that the number of observational sessions
with no pulse detection over the threshold decreases exponentially with the
length of pulse silence.
