Abstract
Abstract. We report important results of the first campaign specially designed to observe the formation and the initial convection of polar cap patches. The principal instrumentation used in the experiments comprised the EISCAT, the Sondrestrom, and the Super DARN network of radars. The experiment was conducted on February 18, 1996 and was complemented with additional sensors such as the Greenland chain of magnetometers and the WIND and IMP-8 satellites. Two different types of events were seen on this day, and in both events the Sondrestrom radar registered the formation and evolution of large-scale density structures. The first event consisted of the passage of traveling convection vortices (TCV). The other event occurred in association with the development of large plasma jets (LPJ) embedded in the sunward convection part of the dusk cell. TCVs were measured, principally, with the magnetometers located in Greenland, but were also confirmed by the line-of-sight velocities from the Sondrestrom and SuperDARN radars. We found that when the magnetic perturbations associated with the TCVs were larger than 100 nT, then a section of the high-latitude plasma density was eroded by a factor of 2. We suggest that the number density reduction was caused by an enhancement in the O+ recombination due to an elevated Ti, which was produced by the much higher frictional heating inside the vortex. The large plasma jets had a considerable (>1000 km) longitudinal extension and were 200-300 km in width. They were seen principally with the Sondrestrom, and SuperDARN radars. Enhanced ion temperature (Ti) was also observed by the Sondrestrom and EISCAT radars. These channels of high Ti were exactly collocated with the LPJs and some of them with regions of eroded plasma number density. We suggest that the LPJs bring less dense plasma from later local times. However, the recent time history of the plasma flow is important to define the depth of the density depletion. Systematic changes in the latitudinal location and in the intensity of the LPJs were observed in the 2 min time resolution data of the SuperDARN radars. The effect of the abrupt changes in the LPJs location is to create regions containing dayside plasma almost detached from the rest of the oval density. One of these density features was seen by the Sondrestrom radar at 1542 UT. The data presented here suggest that two plasma structuring mechanisms (TCVs and LPJs) can act tens of minutes apart to produce higher levels of density structures in the near noon F-region ionosphere.Key words. Ionosphere (ionospheric irregularities) · Magnetospheric physics (electric fields; polar cap phenomena)
Highlights
Since the ®rst observations of plasma density were conducted at very high latitudes, it became evident that large-scale (>100 km) plasma density structures were always present within the polar cap (Hill, 1963; Sato and Rourke, 1964)
We present the line-of-sight velocities from the SuperDARN, the Sondrestrom, and the EISCAT radars to imply that a dierent process, channels containing large plasma jets (LPJs), developed at high latitudes between 1530 and 1630 UT
We repeated the same procedure for the overlapping line-of-sight velocities measured with the Pykkvibaer and EISCAT radars. In this analysis only two components of the full vector velocity are calculated; the third component, the one along the magnetic ®eld, is assumed to be zero (Valladares and Carlson, 1991). Results of this analysis are presented in Fig. 12a±d, where we show the horizontal velocities for four periods around the time of the poleward phase motion of the LPJ
Summary
Since the ®rst observations of plasma density were conducted at very high latitudes, it became evident that large-scale (>100 km) plasma density structures were always present within the polar cap (Hill, 1963; Sato and Rourke, 1964). The works by Lassen (1972), Berkey et al (1976) and Ismail et al (1977) had previously demonstrated that the sun-aligned F-region arcs existed inside the polar cap preferentially when the magnetic conditions were quiet or equivalently when the Bz component of the IMF was directed northward. In analogy to this discovery, Buchau et al (1983) noted that the polar cap patches. The patches move in the antisunward direction (Fukui et al, 1994), lack polar cap particle precipitation (excluding polar rain) (Weber et al, 1986), have sizes between 100 and 1000 km, and possess a patchy signature in the 630.0 nm airglow emissions
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