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

Abstract

Abstract. We report important results of the ®rst 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 dierent 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 ®rst 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 con®rmed 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 T i , 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 Super-DARN radars. Enhanced ion temperature (T i ) was also observed by the Sondrestrom and EISCAT radars. These channels of high T i 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¯ow is important to de®ne 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 eect 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.

Transient production of F-region irregularities associated with TCV passage

“…Several investigations of TCV events were carried out using the Super Dual Auroral Radar Network (SuperDARN) measurements. Convection velocities of TCVs obtained by magnetometer networks and SuperDARN were found to be consistent (Lyatsky et al, 1999;Valladares et al, 1999;Kataoka et al, 2001). Kataoka et al (2001) also found that SuperDARN data show a pronounced increase in the backscatter power and the Doppler spectral width as the TCV passes through the irregularity region.…”

“…Also, Lühr et al (1993Lühr et al ( , 1996 indicated that the Hall conductivity decreased in the upward field-aligned current region of a TCV. Valladares et al (1999) showed that a section of the high-latitude plasma density was eroded by a factor of 2 during a TCV event, producing magnetic perturbations larger than 100 nT. Several investigations of TCV events were carried out using the Super Dual Auroral Radar Network (SuperDARN) measurements.…”

Motion of the dayside polar cap boundary during substorm cycles: II. Generation of poleward-moving events and polar cap patches by pulses in the magnetopause reconnection rate

“…An association between patches and transient reconnection pulses has been suggested by a number of recent studies Lockwood et al, 2000Opgenoorth et al, 2001;Moen et al, 2001). Other studies suggest patches are linked to other phenomena, such as travelling convection vortices (Valladares et al, 1994(Valladares et al, , 1999.…”

“…[T e ] 2 ( K ) be enhanced (Whitteker, 1977;Walker et al, 1999;Millward et al, 1999;Vontrat-Reberac et al, 2001) and this is indeed observed in the cusp/cleft region (Titheridge, 1976;Curtis et al, 1982;Doe et al, 2001). However, as the field lines convect poleward of the cusp/cleft into the mantle and polar cap precipitation regions, modelling studies show T e will fall rapidly as the precipitation flux decays; on the other hand N e will decay only slowly (Watermann et al, 1994;Davis and Lockwood, 1996;).…”

“…1.3 In-situ effects in the cusp Rodger et al (1994a) point out that plasma production by soft particle precipitation could be an important effect in patch production (Whitteker, 1977;Watermann et al, 1994;Davis and Lockwood, 1996;Millward et al, 1999;Walker et al, 1999;Vontrat-Reberac et al, 2001). In addition, Rodger et al (1994a) note that plasma loss rates are enhanced by strong ion drifts (Schunk et al, 1975) and these have been both predicted and observed to generate significant depletions in the cusp region (Balmforth et al, 1998Ogawa et al, 2001) within flow channel events (Pinnock et al, 1993).…”