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- W2064754576 abstract "[1] P. M. Bellan has recently published a paper [Bellan, 2008] (hereafter B08) in which he claims that electrons in a metallic layer formed on the surface of mesospheric ice particles are the main source for strong radar echoes, which were first detected by Czechowsky et al. [1979]. These echoes are commonly called PMSE (polar mesosphere summer echoes). We argue that instead electrons in the gas phase are causing PMSE. The difference is potentially important for the interpretation of PMSE and related background atmosphere conditions. There are basically two main arguments against the metal electron theory of B08: (1) there is a wealth of experimental proof that free electrons cause PMSE (and not electrons in a metal layer), and (2) it is extremely unlikely that a metallic layer can form on the surface of ice particles. In our comment we expatiate on these arguments in more detail. [2] To simplify matters we distinguish these two cases by the terms “metallic electrons” and “free electrons,” respectively. The difference is potentially important for the interpretation of PMSE related to the background atmosphere. [3] It is important to notice that the main idea about explaining PMSE as reviewed by Rapp and Lübken [2004], namely neutral turbulence in combination with charged ice particles, is not questioned by B08. The turbulent PMSE theory is indeed in agreement with an impressive set of ground-based (radar, lidar) and in situ observations and is highly compelling, different from the statement by B08 (we will later comment on the bite-out case discussed by B08). [4] Before we start to explore our arguments we would like to comment on the terminology of ice cloud related phenomena. The term “noctilucent clouds” (NLC) originally stems from visual observations of ice clouds and was later introduced for lidar observations of ice particles. On the other hand, large radar echoes from the summer mesosphere are commonly called PMSE. It is important to note that NLC require a minimum ice particle size (radii typically larger than 20 nm) whereas PMSE can also be created by much smaller particles (down to a few nm) which are optically not detectable. The difference between NLC and PMSE is important for this reply and the statement by B08 that “NLC… exhibit high radar reflectivity” is misleading. We strongly suggest using the standard nomenclature summarized above for mesospheric ice layers and their effect on radar backscatter. [5] There are basically two main arguments against the metal electron theory of B08: (1) there is a wealth of experimental proof that free electrons cause PMSE, and (2) it is extremely unlikely that a metallic layer can form on the surface of ice particles. In the following we will expatiate on these arguments in more detail. [6] A direct proof of the importance of electrons in the gas phase comes from the active modulation of PMSE using HF-heating. Chilson et al. [2000] were the first to use a powerful HF heating radar in order to enhance the electron temperature to ∼3000 K at altitudes where PMSE was simultaneously observed by the EISCAT VHF radar. Stunningly, the PMSE disappeared within milliseconds when the electron gas was heated and immediately reappeared when the heater was switched off [Belova et al., 2003]. This behavior was explained by Rapp and Lübken [2000, 2003b] as a consequence of the diffusion characteristics of plasma in PMSE consisting of charged ice particles, electrons, and ions. When electrons in the gas phase are heated the PMSE signal disappears because of increased electron diffusion. When the heater is switched off, electrons thermalize quickly and return to a “normal” diffusion state thereby leading to PMSE. Note that neither the ice particle size nor the spatial inhomogeneities in the ice cloud are affected by heating. The mechanism outlined above works only for electrons in the gas phase. The metal coating can certainly not disappear and recondensate within milliseconds. [7] The PMSE heating experiment was developed further by Havnes [2004] who predicted an enhanced particle charging when the electrons are “hot” which in turn leads to an enhanced PMSE signal when heating is turned off (“overshoot effect”). This effect was later confirmed in surprising detail by observations which precisely reproduced the time behavior predicted for the PMSE signal [Havnes et al., 2003; Kassa et al., 2005; Havnes et al., 2006; Biebricher et al., 2006; Naesheim et al., 2008]. We reproduce the observations and the theoretical prediction from Havnes et al. [2003] in Figure 1. The excellent agreement between theory and observations reinforces the crucial role of free electrons for PMSE and excludes the metal electron hypothesis. In summary, the PMSE heating experiment can only be explained by free electrons and not by metallic electrons. [8] There is further strong evidence from a wealth of independent observations that the scattering of radar waves comes from gas-phase electrons and not from a conducting metal film on the ice particles. For example, Rapp et al. [2002] found clear evidence for a lower (free) electron number density limit for the existence of PMSE based on a comparison of PMSE occurrence with in situ electron density measurements. Furthermore, the fact that ice-particle related radar echoes at midlatitudes (MSE, mesosphere summer echoes [see Gerding et al., 2007]) disappear after sunset is readily explained by missing electrons due to missing solar Ly-α radiation being the major source of free electrons in the midlatitude D-region [Rapp et al., 2002; Zecha et al., 2003]. Likewise, Bremer et al. [2000] and Bremer et al. [2001] found a clear dependence of PMSE on free electron density enhancements, caused by geomagnetic and solar activity, respectively. Last but not least, comparisons of absolute radar volume reflectivity (i.e., the radar scattering cross section per unit volume) from calibrated radars to in situ electron number density measurements with high spatial resolution show an excellent quantitative agreement of reflectivities [e.g., Inhester et al., 1990]. Similarly, PMSE volume reflectivities simultaneously observed at 53 MHz, 224 MHz and 930 MHz are quantitatively explained on the basis of incoherent scatter electron density measurements and estimates of the turbulence strength from corresponding spectral observations [Rapp et al., 2008]. All these explanations nicely work for electrons in the gas phase but not for electrons in a metal coating. [9] We argue that it is extremely unlikely that a metal monolayer exists because the flux of water molecules to the ice particle is orders of magnitude larger compared to metals. Unlike the meteoric metal flux, the flux of water vapor into the NLC region is from below owing to the residual circulation and turbulent transport [e.g., Körner and Sonnemann, 2001]. Taking only the vertical component of the residual circulation into account (which is a lower estimate of the actual flux) results in a minimum flux of ∼3 × 1013m−2s−1, i.e., more than 5 orders of magnitude larger than the metal atom flux. Using the same assumptions and simplifications as B08, this implies that the formation of a water molecule monolayer is about 100 times faster than the formation of a metal atom monolayer. This is elaborated in more detail below. [11] Figure 2 shows that for normal water vapor concentrations (mixing ratios ∼3–5 ppm) the growth rate is typically ∼5 nm/h implying that a particle would grow by approximately 20–150 nm in radius during the time needed to form a monolayer of metal atoms. The ice particle growth corresponds to about 50–300 monolayers of water molecules [Buch et al., 2004]; that is, the metal atoms are embedded in the interior of the water ice crystal. Even under dry conditions caused by “freeze drying,” typical growth rates are ∼1 nm/h which corresponds to many monolayers of water molecules. Note that these simple estimates of particle growth are consistent with results from full microphysical/chemical models of mesospheric ice clouds and their water vapor environment [Rapp and Thomas, 2006; Berger and von Zahn, 2002]. [12] In this context it is also instructive to consider an earlier proposal by Eidhammer and Havnes [2001], who discussed a scenario in which an elevated ice particle temperature inhibits the further collection of water molecules while metals may still be accreted on the particle surface. We note, however, that this scenario requires a near-constant temperature (i.e., the temperature where S ≈ 0 and hence dr/dt ≈ 0; see equation (1) and Figure 2 for quantitative details) over an extended time period. While a situation with S ≈ 0 might occasionally occur, it is, however, very unlikely that the temperature could be constant at this value over 5–33 h as required by B08. Lidar and rocket observations show that the temperature at altitudes between 80 and 90 km is highly variable with a typical RMS variation of 10 K [Lübken, 1999; Rapp et al., 2002; Lübken et al., 2009]. Hence, the following two cases are much more likely: Temperature is either below the frost point of water ice (S >1) such that the flux of water molecules largely outnumbers the flux of metal atoms. This is presumably the standard situation inside a mesospheric ice cloud and was hence discussed above. Alternatively, temperature is larger than the frost point of water ice (S < 1) in which case the ice particles will quickly sublimate and disappear leaving no time for efficient accretion of metal atoms. In order to underline the latter point, Figure 2 also shows evaporation rates for the cases where S < 1. This demonstrates the very strong temperature dependence of dr/dt: Assuming a typical water vapor mixing ratio of 5 ppm, a temperature change by 10 K from 145 K to 155 K charges dr/dt from +5 nm/h to −100 nm/h. Hence, a 10-nm particle will disappear at a temperature of 155 k in about 6 min! This is certainly far too short to cover the particle surface by a metal coating. [13] The arguments above clearly demonstrate that the scenario assumed by B08 is highly unrealistic. The laboratory and field studies of Plane et al. [2004] and Murray and Plane [2005] undoubtedly show that metal atoms are efficiently captured by ice particles such that the metal layers basically disappear in the presence of mesospheric ice clouds. However, this capture process is far too slow and too seldom to allow for the formation of a metal film on the ice particle surface. Instead, the metal atoms will “disappear” within a voluminous ice matrix. [14] We note that our analysis does not contradict the (controversial) detection of positively charged ice particles by Havnes et al. [1996] and Smiley et al. [2006] and the recent laboratory analysis of Vondrak et al. [2006b]. These authors recently investigated the effect of sodium impurities on the photoemission from ice in the laboratory and found that a deposition of only 0.02 monolayers of sodium led to a dramatic increase of the photoemission from the ice film. However, they also found that the photoemission rate decayed rapidly in time and estimated a decay time of ∼20 s for a typical mesospheric temperature of 135 K [Vondrak et al., 2006a]. Because of this rather short lifetime and because of the generally reduced concentration of metal atoms in the presence of ice clouds, Vondrak et al. [2006b] concluded that a rather large fresh meteoroid (500 μg or larger) would be needed to make photoemission from ice a dominant charging process. They further estimated that the probability for such an event would be on the order of only 2%. Hence, the detection of positively charged particles is no evidence for the existence of metal monolayers on ice particles. The very seldom (and controversial) observation of positively charged ice is in line with Vondrak et al.'s [2006b] arguments indicating that positive charging (resulting from as little as 0.02 monolayers) can only occur in the unlikely event of a rather large fresh meteoroid. [15] The B08 scenario is particulary unrealistic at the lower edge of PMSE. As is known from lidar and radar observations PMSE exist down to the lowest heights of ice particle existence (proven by lidar detection of NLC) where undisturbed metal densities are low and water mixing ratios are high [Lübken et al., 2004]. We further note the PMSE and metal atoms can coexist; that is, they are not mutually exclusive: Lübken et al. [2009] have recently compared hundreds of hours of simultaneous measurements of potassium lidar temperatures with PMSE at Spitsbergen. This implies that only a fraction of the ambient metal atoms is absorbed by the ice particles if their size is too small. [16] B08 argues several times in his paper that some in situ observations show a “complete” removal of free electrons (“bite-out”) which is suggested to be in disagreement with the standard PMSE theory. [17] Depletions of the electron density are indeed a common feature of a PMSE environment as revealed by a considerable number of past sounding rocket measurements [e.g., Pedersen et al., 1969; Ulwick et al., 1988; Lübken et al., 1998; Havnes et al., 2001; Blix et al., 2003; Croskey et al., 2004; Smiley et al., 2006]. It is now well accepted that these depletions are direct evidence for the attachment of electrons to mesospheric ice particles [Reid, 1990; Rapp and Lübken, 2001]. These particles hence charge negatively and influence the diffusion characteristics of the remaining free gas-phase electrons and consequently allow for the formation of small-scale free electron structures giving rise to PMSE [Cho et al., 1992; Rapp and Lübken, 2003a]. It should be noted, however, that very large depletions of the electron density (“large” meaning here that the reduction is by an order of magnitude or more) are relatively rare (<15%) as summarized in a preliminary statistical study by Blix et al. [2003] and confirmed by several later rocket results (see references above). Also, the term “complete” depletion of electron density should be used with care since a measured “zero” electron density can certainly only mean that the signal was below the detection threshold of the particular instrument. Furthermore, it should be noted that “large” depletions have so far only been observed with electrostatic probes which might be severely compromised in the presence of charged ice particles. The latter have been shown to have a dramatic impact on the charging of the rocket vehicle itself hence rendering the absolute accuracy of any electrostatic measurement from the rocket questionable [e.g., Holzworth et al., 2001; Zadorozhny et al., 1997]. In contrast, published electron density profiles obtained with a Faraday rotation experiment (which is not affected by payload charging issues) show electron densities of at least several hundreds/cm3 at PMSE altitudes [e.g., Croskey et al., 2004, Figure 1c]. [18] Finally, we note that Rapp et al. [2003] offered a straightforward and likely explanation for the apparent dilemma of a large electron depletion in the presence of PMSE, namely spatial inhomogeneities within the radar volume. Whereas the radar receives backscatter from a diameter of approximately 10 km, the sounding rocket probes a volume of only few centimeters in diameter. It is well known from noctilucent cloud photography, from dual beam lidar measurements, and from recent satellite imaging that mesospheric structures exist on horizontal spatial scales of few kilometers and smaller [e.g., Witt, 1962; Baumgarten et al., 2002; Russell et al., 2009]. It is therefore not surprising to find minor differences in the morphology of localized in situ observations and the comparably large-scale radar measurements. [19] Combining ground-based and in situ observations with theoretical considerations, we conclude that there is compelling evidence that scattering in PMSE results from gas-phase electrons and not from metal coatings of ice particles. We repeat that the standard PMSE theory (turbulence induced plasma fluctuations) is not affected by this discussion." @default.
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- W2064754576 title "Comment on “Ice iron/sodium film as cause for high noctilucent cloud radar reflectivity” by P. M. Bellan" @default.
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- W2064754576 doi "https://doi.org/10.1029/2008jd011323" @default.
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