Could negative ion production explain the polar mesosphere winter echo (PMWE) modulation in active HF heating experiments?

Abstract

The mechanism behind the modulation of polar mesospheric winter echoes (PMWE) observed in active HF heating experiments is considered. We propose that negative ion chemistry plays a role in the heater‐induced modulation of the PMWEs. In the mesosphere, an increased electron temperature leads to a higher rate of electron to neutral attachment, and therefore to a decreased electron density. To a first‐order approximation, scattering from Bragg‐scale gradients caused by turbulence can be considered to be proportional to the electron density squared. Under this assumption, the electron density variation calculated by the detailed Sodankylä Ion Chemistry model (SIC) predicts correctly both the magnitude and the timescale of the modulation observed with the EISCAT VHF radar.

Statistical characteristics of PMWE observations by the EISCAT VHF radar

“…Lübken et al, 2006Lübken et al, , 2007Rapp et al, 2011;Morris et al, 2011). However, measurements with lidar and radar (Kirkwood et al, 2002;Stebel et al, 2004;Belova et al, 2005;Kero et al, 2008;La Hoz and Havnes, 2008) have triggered the hypothesis that small charged aerosol particles (probably of meteoric origin) play a similar role as in the case of PMSE (e.g. Rapp and Lübken, 2004).…”

Estimate of size distribution of charged MSPs measured in situ in winter during the WADIS-2 sounding rocket campaign

“…Typical values for Sc found in PMSEs are several hundred on average and can often be several thousands (Lübken et al, 1994;Li et al, 2010;Rapp et al, 2011;Strelnikov and Rapp, 2011). The same mechanism but involving MSPs was proposed to explain the PMWEs in the case when neutral turbulence is not sufficiently large (e.g., Kavanagh et al, 2006;Lübken et al, 2007;Kero et al, 2008;La Hoz and Havnes, 2008;Havnes and Kassa, 2009;Havnes et al, 2011;Strelnikova and Rapp, 2013;Stebel et al, 2004;Belova et al, 2008). The latter has not yet been confirmed by direct measurement.…”

“…The latter is only possible when MSPs produce secondary effects by influencing properties of ionospheric plasma which leads to radar echoes (PMWEs). There are also active radar experiments that heat free electrons at MLT heights by HF waves emitted from the ground and, in parallel, examine the behavior of PMWEs in VHF band (Kero et al, 2008;Havnes and Kassa, 2009;La Hoz and Havnes, 2008;Belova et al, 2008;Havnes et al, 2011). The so-called heating experiments gave a strong indication that dust is involved in the PMWE formation and that there are large charged particles on the order of ∼ 3 nm at lower altitudes (Havnes et al, 2011) which also substantiate the increase in the number of large particles with decreasing altitude.…”

“…The MLT ice particles in summer reveal sizes of tens of nanometers to 100 nm diameter (e.g., Berger and von Zahn, 2002;Bailey et al, 2015) and consist of water ice. In winter the sizes of MSPs are significantly smaller, i.e., up to some nanometers (e.g., Strelnikova et al, 2007;Kero et al, 2008;La Hoz and Havnes, 2008;Belova et al, 2008;Havnes and Kassa, 2009;Fentzke et al, 2009;Robertson et al, 2014;Rapp et al, 2010Rapp et al, , 2012Strelnikov et al, 2012). Hence, in winter Sc will be much smaller as it is connected to the radius by Sc ∼ r 2 (Lübken et al, 1998, and Sect.…”

“…It has been shown that MSPs are important for the D-region charge balance (Friedrich et al, 2011Baumann et al, 2013Baumann et al, , 2015Robertson et al, 2014;Asmus et al, 2015). Moving with the background flow and bound to neutral air turbulence, charged MSPs are also suggested to be potentially involved in the formation of so-called polar mesospheric winter echoes (PMWEs) (e.g., Kavanagh et al, 2006;Lübken et al, 2007;Kero et al, 2008;La Hoz and Havnes, 2008;Havnes and Kassa, 2009;Havnes et al, 2011;Strelnikova and Rapp, 2013;Stebel et al, 2004;Belova et al, 2008). There are also theories explaining the formation of PMWEs by neutral turbulence and an increased electron density (e.g., Lübken et al, 2006).…”