Modelling of particle charging in the polar summer mesosphere: Part 2—Application to measurements

Abstract

A model describing the charging of aerosol particles in the vicinity of the polar summer mesosphere is applied to in situ measurements of electron and ion number densities. More general results from this model are presented in a companion paper (Rapp and Lübken, 2001). Here we apply this model to various in situ measurements. Charging of the aerosols causes disturbances in the electron and ion number density profiles which depend on various parameters, such as the electron/ion production rate Q, the electron/ion recombination coefficient α, the aerosol size, r A, and the aerosol number density N A. We have analyzed measurements from rocket flights where at least two of these parameters are known (in most cases Q and α). Under most circumstances the disturbance in the electron density profile Δ n e (‘biteout’) depends on r A and N A in a different way than the disturbance in the positive ion density profile Δ n i. In many cases this allows to unambiguously determine r A and N A from measurements of Δ n e and Δ n i. From a total of 11 flights found in the literature we have analyzed in detail four cases which are characterized by simultaneous observations of noctilucent clouds (NLC) and/or polar mesosphere summer echoes (PMSE) by ground based or in situ techniques. In all cases our model successfully explains the observed plasma disturbances in terms of aerosol charging. During NLC conditions we arrive at particle radii of 32– 60 nm and number densities of 110– 850 cm −3 . Assuming that the particles consist of water ice the amount of ice in the particles corresponds to gas phase water vapor concentrations of 4– 14 ppm v which is rather large compared to ‘standard’ model values. These large values support the idea that water vapor is collected by the aerosols in a larger horizontal area and/or extended altitude range when they sediment from the mesopause region (88 km) to typical NLC altitudes (82 km) . In one flight we arrive at much smaller (1 nm) and much more abundant (several 100,000 cm −3 ) aerosol particles.