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The IDV observations of S4 0917+624 were carried out monthly, from August2005 to January 2010, with the Urumqi 25m radio telescope at 4.8 GHz. Thequasar S4 0917+624 exhibits only very weak or no IDV during our 4.5 yearobserving interval. Prior to the year 2000, the source S4 0917+624 was one ofthe most prominent IDV sources. Our new data indicate that the previous strongIDV has ceased. We analyzed the long-term VLBI structural variability usingGaussian model-fitting. From this we obtained the flux densities and thedeconvolved sizes of core and inner-jet components of the source. We studiedthe properties such as core fraction, angular size, spectral index, andbrightness temperature of VLBI core for S4 0917+624, as well as the time delaybetween 5 and 15 GHz variations, and compared them with the IDV properties ofS4 0917+624. The source shows ejection of several jet components that aresuspected to have partially reduced the IDV amplitude of S4 0917+624. However,during 2005-2006, the VLBI core size was comparable to the size before the year2000, but no strong IDV was detected in the period, suggesting that thequenching effect due to source size changes may not be responsible for the lackof strong IDV after the year 2000. The refractive scattering properties for thestrong IDV phase of S4 0917+624 before the year 2000 are discussed. Thedisappearance of strong IDV in S4 0917+624 after the year 2000 is a mystery andcannot be explained via the quenching effect by changes in the observable VLBIstructure. However, it may be caused by changes in the interstellar medium,i.e. by interstellar weather, which induces changes in the scintillationpattern on timescales of several years. Further coordinated multi-frequencyobservations will be required to distinguish between the effect ofsource-intrinsic variability and changing properties of the interstellarmedium.

Intra-day variability observations and the VLBI structure analysis of quasar S4 0917+624