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We report important results of the firstcampaign specially designed to observe the formation and the initial convectionof polar cap patches. The principal instrumentation used in the experimentscomprised the EISCAT, the Sondrestrom, and the Super DARN network of radars. Theexperiment was conducted on February 18, 1996 and was complemented withadditional sensors such as the Greenland chain of magnetometers and the WIND andIMP-8 satellites. Two different types of events were seen on this day, and inboth events the Sondrestrom radar registered the formation and evolution oflarge-scale density structures. The first event consisted of the passage oftraveling convection vortices (TCV). The other event occurred in associationwith the development of large plasma jets (LPJ) embedded in the sunwardconvection part of the dusk cell. TCVs were measured, principally, with themagnetometers located in Greenland, but were also confirmed by the line-of-sightvelocities from the Sondrestrom and SuperDARN radars. We found that when themagnetic perturbations associated with the TCVs were larger than 100 nT, then asection of the high-latitude plasma density was eroded by a factor of 2. Wesuggest that the number density reduction was caused by an enhancement in the O+recombination due to an elevated Ti, which was produced by the muchhigher frictional heating inside the vortex. The large plasma jets had aconsiderable (>1000 km) longitudinal extension and were 200-300 km in width.They were seen principally with the Sondrestrom, and SuperDARN radars. Enhancedion temperature (Ti) was also observed by the Sondrestrom and EISCATradars. These channels of high Ti were exactly collocated with theLPJs and some of them with regions of eroded plasma number density. We suggestthat the LPJs bring less dense plasma from later local times. However, therecent time history of the plasma flow is important to define the depth of thedensity depletion. Systematic changes in the latitudinal location and in theintensity of the LPJs were observed in the 2 min time resolution data of theSuperDARN radars. The effect of the abrupt changes in the LPJs location is tocreate regions containing dayside plasma almost detached from the rest of theoval density. One of these density features was seen by the Sondrestrom radar at1542 UT. The data presented here suggest that two plasma structuring mechanisms(TCVs and LPJs) can act tens of minutes apart to produce higher levels ofdensity structures in the near noon F-region ionosphere. Key words. Ionosphere (ionospheric irregularities) ·Magnetospheric physics (electric fields; polar cap phenomena

Observations of plasma density structures in association with the passage of traveling convection vortices and the occurrence of large plasma jets