[1] In this paper, for steady-state northward interplanetary magnetic field (IMF) with jB Y j $ B Z , we describe a new merging sequence that results in polar cap bifurcation and accompanying paired ''exchange cells'' in the ionospheric convection pattern. Although the IMF is northward, it reconnects with the closed geomagnetic field on the dayside highlatitude magnetopause, creating two types of open geomagnetic field lines. For the first type the neutral point and the foot point are in the same hemisphere; for the second type the neutral point and the foot point are in opposite hemispheres. The latter type of field lines slips on the magnetopause in the azimuthal direction opposite to the normal B Yassociated flux transport and forms an overdraped tail lobe. The ionospheric signature of this overdraped lobe is the appearance of an open magnetic flux island inside the dawn/ dusk plasma sheet (i.e., polar cap bifurcation). For B Y > 0 the island emerges in the duskside (dawnside) plasma sheet in the northern (southern) ionosphere and conversely for B Y < 0. The overdraped field lines which have slipped on the magnetopause then reconnect with closed geomagnetic field lines in the opposite hemisphere to the foot points, thereby transferring the open magnetic flux to the nightside convection system and maintaining the steady-state magnetic flux circulation. As a result, paired ionospheric convection cells form which exchange magnetic flux. For B Y > 0 the pair is located in the noon-dusk and midnight-dawn (dawn-noon and dusk-midnight) quadrants of the northern (southern) ionosphere; for B Y < 0 a mirror image with respect to the noon-midnight meridian applies to the convection pattern. We demonstrate observational evidence that supports this model.

Abstract. Several factors are known to control the HF echo occurrence rate, including electron density distribution in the ionosphere (affecting the propagation path of the radar wave), D-region radio wave absorption, and ionospheric irregularity intensity. In this study, we consider 4 days of CUTLASS Finland radar observations over an area where the EISCAT incoherent scatter radar has continuously monitored ionospheric parameters. We illustrate that for the event under consideration, the D-region absorption was not the major factor affecting the echo appearance. We show that the electron density distribution and the radar frequency selection were much more significant factors. The electron density magnitude affects the echo occurrence in two different ways. For small F-region densities, a minimum value of 1 × 10 11 m −3 is required to have sufficient radio wave refraction so that the orthogonality (with the magnetic field lines) condition is met. For too large densities, radio wave strong "over-refraction" leads to the ionospheric echo disappearance. We estimate that the over-refraction is important for densities greater than 4×10 11 m −3 . We also investigated the backscatter power and the electric field magnitude relationship and found no obvious relationship contrary to the expectation that the gradientdrift plasma instability would lead to stronger irregularity intensity/echo power for larger electric fields.

We compare measurements of polar cap ionospheric plasma flow over Resolute Bay, Canada, made by a digital ionosonde using the Doppler drift technique with simultaneous measurements at the same location made by the first operational pair of SuperDARN HF radars. During the 3‐hour comparison interval the flow varied widely in direction and from 100 to 600 m/s in speed. The two measurement techniques show very good agreement for both the speed and direction of flow for nearly all of the samples in the interval. The difference between the velocities determined by the two techniques has a scatter of about ±35° in direction and ±30% in speed, with no systematic difference above the level of the scatter. The few samples which strongly disagreed were usually associated with strong spatial structure in the flow pattern measured by SuperDARN in the vicinity of the comparison point. The drift speed measured by the ionosonde was independently verified by observing the time taken for polar cap F layer ionization patches to drift between ionosondes sited at Eureka and Resolute Bay. These results confirm that the speed and direction of the polar cap ionospheric convection can be reliably monitored by the ionosonde Doppler drift technique.

[1] Magnetic reconnection that involves overdraped lobe field lines is called internal reconnection since it occurs inside the magnetopause. When the interplanetary magnetic field (IMF) is due northward and the Earth's dipole is tilted significantly, internal reconnection occurs in the winter hemisphere, not only between a summer lobe field line and a winter lobe field line but also between a summer lobe field line and a closed field line. The latter internal reconnection drives ''reciprocal cells'' in the winter ionosphere that circulate exclusively in the closed field line region. The reciprocal cells are intimately related to the lobe cells in the summer ionosphere in that in the steady state, the reconnection voltage associated with merging of IMF and open field lines is equal to the sum of the lobe cell potential and the reciprocal cell potential. In this paper we present observations of convection patterns consistent with those expected for reciprocal cells, using ionospheric radar and low-altitude satellite data. We also show the concurrence of lobe cells and reciprocal cells. The observations of reciprocal cells provide support for the internal reconnection between a summer lobe field line and a closed field line. In addition, we show that equatorward of the polar cap boundary, magnetosheath-like ions are drifting from noon toward the flankside in both hemispheres. We suggest that these ions are of magnetosheath origin and that they entered the closed region of the magnetosphere through the rotational discontinuity associated with internal reconnection. These magnetosheath-like ion observations strongly support the occurrence of internal reconnection.

[1] Simultaneous two-dimensional observations of airglow enhancement and radar backscatter from field-aligned irregularities (FAIs) associated with polar cap patches were conducted. The spatial structure of 630 nm airglow from polar cap patches was imaged using an all-sky airglow imager at Resolute Bay, Canada, while backscatter echoes from decameter-scale FAIs were observed using the newly constructed HF Polar Dual Auroral Radar Network (PolarDARN) radar at Rankin Inlet, Canada. Both the airglow enhancement and the radar backscatter appeared within a structured region with the spatial extent of about 500-1000 km. The decameter-scale FAIs were found to extend over the entire region of airglow enhancement associated with polar cap patches, indicating that the polar patch plasma became almost fully structured soon after initiation (within approximately 20-25 min). These findings imply that some rapid structuring process of the entire patch area is involved in addition to the primary gradient-drift instabilities.