Strong ambipolar‐driven ion upflow within the cleft ion fountain during low geomagnetic activity

Abstract

We investigate low‐energy (<10 eV) ion upflows (mainly O+) within the cleft ion fountain (CIF) using conjunctions of the Enhanced Polar Outflow Probe (e‐POP) satellite, the DMSP F16 satellite, the SuperDARN radar, and the Resolute Bay Incoherent Scatter Radar North (RISR‐N). The SEI instrument on board e‐POP enables us to derive ion upflow velocities from the 2‐D images of ion distribution functions with a frame rate of 100 images per second, and with a velocity resolution of the order of 25 m/s. We identify three cleft ion fountain events with very intense (>1.6 km/s) ion upflow velocities near 1000 km altitude during quiet geomagnetic activity (Kp < 3). Such large ion upflow velocities have been reported previously at or below 1000 km, but only during active periods. Analysis of the core ion distribution images allows us to demonstrate that the ion temperature within the CIF does not rise by more than 0.3 eV relative to background values, which is consistent with RISR‐N observations in the F region. The presence of soft electron precipitation seen by DMSP and lack of significant ion heating indicate that the ion upflows we observe near 1000 km altitude are primarily driven by ambipolar electric fields. DC field‐aligned currents (FACs) and convection velocity gradients accompany these events. The strongest ion upflows are associated with downward current regions, which is consistent with some (although not all) previously published results. The moderate correlation coefficient (0.51) between upflow velocities and currents implies that FACs serve as indirect energy inputs to the ion upflow process.

Ion Heating in the Polar Cap Under Northwards IMF Bz

“…The May 2014 event has been previously discussed by Shen et al. (2016), and it was also a large ion upflow event observed by the enhanced Polar Outflow Probe (e‐POP). Both of these events occurred during while the IMF had a northwards component and both were observed using topside modes at RISR‐N.…”

“…During these events, RISR-N observed strong temperature enhancements, the SuperDARN backscatter coverage in the vicinity of RISR-N was reasonable, and the convection pattern exhibited dayside reverse convection cells resulting in sunwards flows. The May 2014 event has been previously discussed by Shen et al (2016), and it was also a large ion upflow event observed by the enhanced Polar Outflow Probe (e-POP). Both of these events occurred during while the IMF had a northwards component and both were observed using topside modes at RISR-N.…”

Birkeland Current Boundary Flows Associated With Field Line Resonances

“…Loranc et al (1991) found 100-3,000 m/s upward ion flows in the high-latitude F-region and associated them with sudden increases in ion temperature due to frictional heating from rapid increases in convection velocity. Shen et al (2016) observed quite time intense low-energy ion upflows in the cleft associated with downward field aligned currents and convection velocity gradients. In the midnight sector, Archer et al (2015), ( 2017) consistently observed ion upflows averaging 150 m/s collocated with perpendicular ion heating within the BCBF regions.…”

Statistical Study of the Relationship Between Ion Upflow and Field‐Aligned Current in the Topside Ionosphere for Both Hemispheres During Geomagnetic Disturbed and Quiet Time

“…However, to our knowledge, no study has yet documented a statistical relationship between FACs and ion upflow. Despite many statistical and event studies on either ion upflow or FACs (Shen et al, 2016), there is still a knowledge gap regarding the occurrence of ion upflow in relation to FACs during magnetic quiet and disturbed times. Nevertheless, the relationship between FACs and ion upflow is essential for a deeper understanding of magnetosphere‐ionosphere coupling.…”