Suppression of ion heating in the cusp during plasma flow burst

Abstract Previous studies have shown the occurrence of short-duration plasma flow bursts, typically lasting a few minutes, within the cusp region, often accompanied by the poleward motion of the 630-nm aurora. In this study, we investigated the ion heating characteristics associated with a large-scale plasma flow burst, which coincides with a poleward expansion of the 630-nm aurora. We analyzed data from simultaneous observations using an all-sky imager at Svalbard, the EISCAT Svalbard Radar (ESR), and the SuperDARN radar at Hankasalmi. Additionally, we examined magnetic perturbation data from the Swarm satellites as complementary data to assess the presence of a significant east–west component in the plasma flow. The plasma flow burst was detected by the ESR at approximately 10:40 UT on 8 December 2016, indicated by an increase in the F -region ion temperature ( $${T}_{i}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>i</mml:mi> </mml:msub> </mml:math> ) due to frictional heating arising from ion–neutral velocity differences. Coinciding with the arrival of the front of the plasma flow burst within the ESR’s field of view (accurate to within 1 min), the poleward boundary of the cusp electron precipitation, as observed in the 630-nm aurora, passed through the ESR’s field of view by moving poleward, seemingly drawn by the front of the plasma flow burst. The altitude profiles of $${T}_{i}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>i</mml:mi> </mml:msub> </mml:math> revealed a local maximum at an altitude of approximately 150 km, where the temperature increased from ~ 1200 to ~ 1880 K within 4 min. At altitudes between 190 and 300 km $${T}_{i}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>i</mml:mi> </mml:msub> </mml:math> was lower than this local maximum $${T}_{i}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>i</mml:mi> </mml:msub> </mml:math> prior to the plasma flow burst, but rapidly increased to approximately match the local maximum $${T}_{i}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>i</mml:mi> </mml:msub> </mml:math> . However, following this rapid increase, the $${T}_{i}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>i</mml:mi> </mml:msub> </mml:math> enhancement at these altitudes appeared to be suppressed, with the estimated rate of suppression being −80 and −220 K/min. Based on the investigation through two-dimensional simplified numerical simulations, we suggest that the suppression of $${T}_{i}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>i</mml:mi> </mml:msub> </mml:math> is likely due to the formation of a relatively fast neutral flow, driven by increased ion drag associated with ionization enhancement resulting from low-energy electron precipitation. Graphical Abstract