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5 Conclusions

Our comparisons between VLBI and TFD data show that there is a clear connection between TFD outbursts of blazars and structural changes in their jets. For every new VLBI component emerging into the jet, there is a coincident TFD flare, and also the extrapolated zero epoch of the VLBI component agrees well with the beginning of the TFD flare. At a later stage, there is a clear correspondence between the flux density of the VLBI component and that of the modelled exponential TFD flare. We conclude that one can - for most events - use the beginning of a major TFD flare as an indicator of the time of ejection of a VLBI component.

The most pronounced TFD outbursts seem to occur within the innermost few tenths of a milliarcsecond of the core, which is comparable to the maximum resolution of present-day VLBI. The flares are associated with the production of new superluminal components, commonly interpreted as shocks. However, with the present observations, we cannot determine definitively how much of the flare is contained in the shock and how much is due to changes in the flux of the core itself as it reacts to the disturbance that creates the shock wave.

Due to insufficient resolution even at 43 GHz, we usually see only "core flares'' in our sources. An example of the effects of limited resolution is BLO 1749+096. Despite violent variability in TFD, this object is virtually unresolved even with a beamsize of 0.6 mas along the jet in global 22 GHz VLBI observations (Wiik et al. 2001). Only global 86 GHz VLBI, with a 0.22 mas beam, has been able to resolve the jet (Lobanov et al. 2000).

Based on the results obtained in Sect. 4, we suggest that every large TFD outburst results from a new shock being created in the jet, and not just from a change in the parameters of the ambient jet such as the bulk Lorentz factor or the electron energy spectrum. The essential difference between these two models is that, in the jet-parameter model, luminosity changes in the underlying flow cause the observed flux variability, whereas in the shock model the flares represent an evolving shock wave. The core itself can also become brighter if the shock wave disturbs it when passing through the nozzle of the jet. However, this does not change our suggestion that there is a shock associated with every large TFD flare. This has important implications for models of $\gamma$-ray emission from blazars (see, e.g., Valtaoja et al. 2002; L $\ddot{\rm a}$hteenm $\ddot{\rm a}$ki & Valtaoja 2002; Jorstad et al. 2001b).

Given the possible blending of a new shock component with the core on VLBI images, one has to be very careful when drawing conclusions based on the behaviour of the core. For example, Lobanov & Zensus (1999) have studied the spectral evolution of the jet components in quasar 3C 345. They find two time intervals during which they could not reproduce the observed spectral changes (frequency of the spectral maximum was rising while the flux density at this frequency was constant or even decreasing slightly) of the core component in terms of the relativistic shock model. However, their spectral fits included only a single component in the core region. According to our results, there is likely to be a second, variable component blending with the core that can cause the observed spectral changes. We suggest that it may be possible to explain the behaviour of the core in a way which is consistent with the relativistic shock model.

The results presented in this article indicate that VLBI observations at 90 GHz are needed in order to study new moving components during the stage when they are still growing, and still greater resolution is required to see the actual formation of the shock. This type of study would benefit greatly from future space interferometry missions such as the proposed VSOP-2 and iARISE missions. In addition to finer resolution, we also need better sampled VLBI component flux curves, which can be obtained from intensive VLBI monitoring.

Acknowledgements
We thank Fredrik Rantakyr $\ddot{\rm o}$ for providing us the VLBA observations of CTA 102 prior to publication. This work was supported in part by NASA through CGRO Guest Investigator grants NAG5-7323 and NAG5-2508, and by U.S. National Science Foundation grants AST-9802941 and AST-0098579. The VLBA is a facility of the National Radio Astronomy Observatory, operated by Associated Universities, Inc., under cooperative agreement with the U.S. National Science Foundation.


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