High-speed solar wind streams and polar mesosphere winter echoes at Troll, Antarctica

Abstract

Abstract.A small, 54 MHz wind-profiler radar, MARA, was operated at Troll, Antarctica (72 • S, 2.5 • E), continuously from November 2011 to January 2014, covering two complete Antarctic winters. Despite very low power, MARA observed echoes from heights of 55-80 km (polar mesosphere winter echoes, PMWE) on 60 % of all winter days (from March to October). This contrasts with previous reports from radars at high northern latitudes, where PWME have been reported only by very high power radars or during rare periods of unusually high electron density at PMWE heights, such as during solar proton events. Analysis shows that PWME at Troll were not related to solar proton events but were often closely related to the arrival of high-speed solar wind streams (HSS) at the Earth, with PWME appearing at heights as low as 56 km and persisting for up to 15 days following HSS arrival. This demonstrates that HSS effects penetrate directly to below 60 km height in the polar atmosphere. Using local observations of cosmic-noise absorption (CNA), a theoretical ionization/ion-chemistry model and a statistical model of precipitating energetic electrons associated with HSS, the electron density conditions during the HSS events are estimated. We find that PMWE detectability cannot be explained by these variations in electron density and molecularion chemistry alone. PWME become detectable at different thresholds depending on solar illumination and height. In darkness, PWME are detected only when the modelled electron density is above a threshold of about 1000 cm −3 , and only above 75 km height, where negative ions are few. In daylight, the electron density threshold falls by at least 2 orders of magnitude and PWME are found primarily below 75 km height, even in conditions when a large proportion of negative ions is expected. There is also a strong dawn-dusk asymmetry with PWME detected very rarely during morning twilight but often during evening twilight. This behaviour cannot be explained if PMWE are caused by small-scale structure in the neutral/molecular-ion gas alone but may be explained by the presence of charged meteoric dust.

Simultaneous Observations of Polar Mesosphere Winter Echoes and Cosmic Noise Absorptions in a Common Volume by the PANSY Radar (69.0°S, 39.6°E)

“…This result was interpreted as being due to electron‐dust particle interactions that caused charge saturation of aerosol particles and an increase in electron diffusivity ( 1/ S c ), thereby leading to a reduction in radar volume reflectivity (Cho et al, ). In addition, the reflectivity increases with increasing electron density only when the density is much less than that of charged dust (Kirkwood et al, ). Accordingly, the PMWE decay above 70 km can be explained by small S c or reduction of S c that is equal to an increase in electron diffusivity resulting from unusually high electron density.…”

“…In this study, we present simultaneous PMWE and CNA observations made in May 2013 by the PANSY radar. The PMWE and CNA data originated from the same data set of the PANSY radar in a similar way as in Kirkwood et al (). This is the first study focusing on the one‐to‐one relationship between PMWE morphology and CNA in a common volume.…”

Transient ionization of the mesosphere during auroral breakup: Arase satellite and ground-based conjugate observations at Syowa Station

“…2. Note that both the horizontal and vertical appearance and disappearance of radar echoes in the wintertime mesosphere have been known to depend on a complex interplay of both ionizing sources and possibly turbulence, in addition to nonionizing radiation, such as daylight, ion-chemistry and the charging behavior of dust or smoke layers (Kirkwood 2007;Lübken et al 2006;Kirkwood et al 2015;Nishiyama et al 2018). It is important to note that during the present spike event the deep ionization disappeared rather quickly.…”

Dynamically Unstable Strong Wind Shears Observed in the Polar Mesosphere Summer Echo Layer Associated With Geomagnetic Disturbances

“…High‐speed solar wind streams (HSS) emanate from solar coronal holes and occur more often with approaching solar minimum years (Krieger et al, ; McComas et al, ). Specifically, in near solar minimum years of 2006 and 2008 in the 23th solar cycle, PMSE were found to be closely associated with the effects of HSS‐induced geomagnetic disturbance (e.g., Kirkwood et al, ; Lee et al, , ). The PMSE were modulated at 7‐, 9‐, and 13.5‐day periodicities, of which periodic signature is inherited from the recurrent HSS.…”