Polar mesospheric clouds (PMCs), also called noctilucent clouds (NLCs), occur in the coldest regions of the atmosphere near the summer polar mesopause. PMCs normally form a layer extending vertically for several kilometres, peaking near 83 km, located at latitudes polewards of 50∘. In this region the temperature frequently drops below the frost point which, for mesospheric pressures and humidities, is about 150 K. They mainly consist of water ice particles with radii ranging from a few nm to about 100 nm .

PMCs modify the ambient plasma of the D-region and gives rise to intense radar echoes, the so-called PMSE (polar mesospheric summer echoes) . It is now generally accepted that larger ice particles are located near the bottom of the layer, while smaller ones are more likely to be near the top of the layer .

PMCs have been intensively studied using ground, rocket, and space observations using SNOE/UVS (Student Nitric Oxide Explorer/Ultraviolet Spectrometer, SBUV (Solar Backscatter Ultraviolet), Odin/OSIRIS (Optical Spectrograph and InfraRed Imager System), Envisat/SCIAMACHY (SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY), GOMOS (Global Ozone Monitoring by Occultation of Stars), AIM/SOFIE (Aeronomy of Ice in the Mesosphere/Solar Occultation For Ice Experiment), and AIM/CIPS (Cloud Imaging and Particle Size). e.g.; as well as sophisticated models e.g.. A good review of our knowledge on PMCs up until 2006 was compiled by . A more recent review, including a comparison with mesospheric clouds on Mars, was conducted by .

PMCs are being discussed as potential early indicators of global climate change in the middle atmosphere because they are very sensitive to temperature and water vapour concentration. Since enhanced CO2 amounts see, e.g. are expected to lead to an eventual cooler upper mesosphere/lower thermosphere, and higher CH4 amounts to enhanced H2O near the mesopause , they may both lead to an increase of PMC occurrence, which might be interpreted as an effect of climate change in the middle atmosphere. There is not, however, a consensus in the scientific community about this aspect . The recent study of SBUV data from 1979 to 2013 by has shown, in addition to the clear solar cycle signal, a good correlation with stratospheric ozone variations. Also, they have found that PMC ice water content in bright clouds increased rapidly from 1979 to the late 1990s and has been approximately constant from the late 1990s to 2013. Similar results were found by using SBUV data and a different method for calculating the ice water content (IWC). These authors also compared SBUV and SOFIE data and found good agreement in average IWC if an appropriate threshold was applied to the SOFIE data set and consistent day-to-day and year-to-year variations between both data sets were used.

looked at trends in the northern midlatitude noctilucent cloud occurrences using satellite data and model simulations and found a significant increase in the PMC occurrences at midlatitudes from 2002 to 2011. This result differs somewhat from the insignificant trend found by for a similar period but at higher latitudes.

analysed trends in mesospheric ice layers in the high-latitude Northern Hemisphere for the 1961–2013 period with model simulations. They reported a generally good agreement between long-term PMC variations from the MIMAS model and the SBUV satellite observations. They found that the modelled trends in ice water content are latitudinally dependent with no clear trend at midlatitudes (50–60∘ N) but with a clear positive trend at high latitudes (74–82∘ N) and also in extreme PMC events.

studied the solar-induced 27-day variations in polar mesospheric clouds using 15 seasons of SOFIE data and suggested that the 27-day variations in the PMCs are due to 27-day variations of vertical winds.

As described above, a large fraction of the observations taken so far were performed by measuring the scattered light, in the visible or UV, of the solar radiation (in the case of instruments from space) or lidar light (in case of ground instruments). This technique usually observes the ice particles with radii larger than about 20 nm but lacks sensitivity for smaller particles see, e.g.. A different technique, however, has been developed recently by the AIM/SOFIE instrument. These measurements have provided key characteristics of PMCs such as their frequency, mass density, particle shape, and size distribution, as well as their seasonal evolution and altitude dependence see, e.g.. Furthermore these satellite data have supplied critical information about the relationship of the ice density distribution with mesopause temperature and water vapour concentration see, e.g..

While PMCs emit thermal radiation, their infrared (IR) emissions are very difficult to observe due to the low ice particle volume density and the very cold polar summer mesopause temperatures. In fact, only three IR emission observations have been reported to date: those taken by CRISTA , by SPIRIT and those taken by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) . This technique has the advantages of being able to measure PMCs in dark conditions, thus providing a better spatial and temporal coverage, and of being sensitive to the total ice volume density, regardless of particle size, and hence include the very small particles.

In a previous paper reported the detection of infrared emissions from PMCs taken by the MIPAS instrument on board Envisat (Environmental Satellite), and provided further evidence of the water ice nature of the PMC particles. We also described the retrieval of the ice particle volume density and reported the analysis of the retrieved densities for 19–21 July 2005. In this paper we present the global distribution (by altitude, latitude, and longitude) of ice volume density measured by MIPAS for several days in each of the Northern (NH) and Southern Hemisphere (SH) seasons from 2005 to early 2012. We also analyse several aspects of the PMCs such as (i) the mean altitude of the layer, the ice water content and its hemispheric dependence; (ii) the correlation of ice volume density with the frost-point temperature and the water vapour concentration; and (iii) the diurnal variation of the ice volume density. MIPAS, as well as SOFIE, has the advantage of measuring the whole content of ice particles (all sizes) in the mesosphere. Hence, a comparison with SOFIE observations, version 1.3, is also shown.

MIPAS is a high-resolution limb sounder on board the Envisat satellite, launched on 1 March 2002. It took measurements until 8 April 2012, when the Envisat satellite failed. MIPAS measurements covered a wide spectral range with a high spectral resolution, operating at 0.025 cm-1 from 2002 to 2004 and 0.0625 cm-1 from 2005 to the end of the mission. It also operated with a high sensitivity, allowing measurements of most of the atmospheric emissions in the mid-infrared over a large altitude range . MIPAS operated with a global latitudinal coverage (pole-to-pole) and performed measurements irrespective of day- or night-time. The instrument carried out most of its observations in the 6–68 km altitude range (the nominal mode), but it also regularly made observations at higher altitudes in its middle-atmosphere (MA), noctilucent cloud (NLC), and upper-atmosphere (UA) modes .

In the MA mode, the spectra are available at limb tangent heights from about 20 up to 102 km with a vertical sampling of 3 km. The UA mode ranges from about 42 to 172 km, and has a vertical sampling of 3 up to 102 km, and 5 km above this altitude. The NLC mode is a variant of the MA mode specifically tailored for measuring the PMCs during the summer seasons . In this mode the spectra cover tangent heights from 39 up to 78 km at 3 km steps; then from 78 up to 87 km at 1.5 km steps, and from 87 up to 102 km again in 3 km steps. The horizontal field of view (FOV) of MIPAS is approximately 30 km. The days of PMC measurements in the different observation modes are listed in Table , and a summary of the distribution of these days within the different seasons is shown in Table .

The method used for the retrieval of PMC ice volume density from the MIPAS spectra has been described by . A brief excerpt is included here. The spectra analysed in this work were all taken with the optimized spectral resolution of 0.0625 cm-1. The ice volume density was retrieved from the radiance profiles obtained by integrating the spectra from 770 to 920 cm-1. The profiles were corrected for an offset variable in altitude, latitude, and time. The noise equivalent spectral radiance in this spectral region is about 20 nW/(cm2 sr cm-1), and the corresponding noise in the integrated radiances of a single scan is ∼60 nW/(cm2 sr).

The ice volume density was retrieved from the spectrally integrated radiance profiles using a linearly constrained least squares fitting, where the Jacobians were calculated using the Karlsruhe Optimized and Precise Radiative transfer Algorithm (KOPRA) radiative transfer algorithm . The inversion was constrained by a Tikhonov-type scheme using a squared first-order differences matrix to obtain a reasonably smoothed vertical profile of volume densities. The ice refractive indices were taken from .

In this analysis we have included the following improvements and updates with respect to : (i) The more recent version 5 (5.02/5.06) of MIPAS L1b spectra has been used ; (ii) an updated version of the temperature is used for the retrieval of ice density (see below); (iii) the altitude registration of the L1b spectra has been improved using the information from the retrieved temperature and LOS (line of sight) instead of the engineering information included in the L1b files ; (iv) the offset correction of the integrated radiance profiles was improved by taking into account its altitude and latitudinal variations; (v) the ice density profiles were retrieved only for scans with converged pressure-temperature profiles (no interpolation over latitude/longitude was done); and (vi) due to a mistake in the calculation of the volume of the particles distribution, the volume densities presented here are nearly double those previously reported by .

The temperature and LOS required to retrieve the ice density have been retrieved from the CO2 emission near 15 µm, recorded in the same MIPAS band A as the PMC emission. Non-local thermodynamic equilibrium (non-LTE) emissions were taken into account. The detailed description of the method and the characterization of the inverted temperature profiles are described by . The upgrades in the retrieval of the temperature used here (version vM21) and a validation of the results are reported by . Briefly, these authors include an updated version of the calibrated L1b spectra in the 15 µm region (versions 5.02/5.06); the HITRAN (HIgh-resolution TRANsmission molecular absorption) 2008 database for CO2 spectroscopic data; the use of a different climatology of atomic oxygen and carbon dioxide concentrations; the improvement of several aspects of the retrieval set-up (temperature gradient along the line of sight, offset regularization, and the spectral apodization); and some minor corrections to the CO2 non-LTE modelling as detailed by . This version of MIPAS temperatures corrects the main systematic errors of the previous version and show, in general, a remarkable agreement with the measurements taken by ACE-FTS Fourier transform spectrometer, MLS (Microwave Limb Sounder), Odin-OSIRIS, TIMED-SABER (Thermosphere Ionosphere Mesosphere Energetics Dynamics - Sounding of the Atmosphere using Broadband Emission Radiometry), AIM-SOFIE, and the Rayleigh lidars at Mauna Loa and Table Mountain . In the region of interest here, however, there are still significant differences, with MIPAS polar summer mesopause temperatures differing by 5–10 K from the other instruments, being warmer than SABER, MLS, and OSIRIS and colder than ACE-FTS and SOFIE.

Since MIPAS measures PMCs in IR emission, knowledge of the temperature of the ice particles is crucial. There is still disagreement on the temperature of the particles, particularly on whether they are warmer or colder than the ambient atmosphere. Using SOFIE measurements, have found that the ice particles are about 5–20 K cooler than the ambient atmosphere. They suggested, however, that the V1.022 SOFIE CO2 temperatures they used might have a warm bias of 5–10 K near the polar summer mesopause. , using infrared solar occultation measurements from the ACE instrument, also found that the ice particles are cooler than the ambient temperature. They argue that this might be caused by inhomogeneities in the temperature along the instrument field of view, with the ice particles sensing only the cold(er) parcels where they are present, while the gas temperature is representative of the whole (warmer) air mass along the FOV. Physical considerations, however, would suggest that the particles are warmer than the surrounding gas because they are heated by absorption of radiation . For example, for a particle distribution with a mean radius between 30 and 50 nm and an accommodation coefficient of 0.5, found that the ice particles are warmer than the ambient gas by about 1 K at 80 km and 2 K at 90 km. Analogously, the model calculation of , when applied to a normal distribution of ice particle size with a mean radius varying from 40 nm at 80 km to 15 nm at 90 km, gives a temperature increase of 0.7 K at 80 km and 2.7 K at 90 km. As suggested by these models, we applied a temperature correction of the emitting particles that varies linearly from 1 K at 80 km to 2 K at 90 km. In principle, MIPAS measurements should also be affected by the problem pointed out by . However, our observations do not support that finding. If we assume that ice particles are cooler than the retrieved gas temperature we obtain very high (and unreasonable) concentrations of ice particles (see Sect. ).

The vertical resolution of the ice density profiles, in terms of the half-width of the columns of the averaging kernel matrix, depends on the observational mode. For the over-sampled NLC mode, it varies from ∼2.5 km at 81–82 km to ∼3 km at 86 km, and to 3.5–4 km at 90 km. For the middle- and upper-atmosphere modes (MA and UA, or together MUA), it is coarser, with values ranging from 3.5 to 4 km. The error in the absolute pointing is about 200 m. The averaging kernels shown in are for the NLC mode measurements that have a sampling step (i.e. tangent altitude increment) of 1.5 km. For the MA and UA modes the averaging kernels are wider because of the coarser sampling of 3 km.

The random single-profile error of the retrieved ice volume density is about 60 %, including both the instrumental noise and the temperature noise error. The systematic error is about 25–30 % and is mainly due to the temperature error in the summer mesopause region . More details of the retrieval of the ice volume density can be found in .

Figure  shows typical daily zonal means of ice volume density retrieved from MIPAS for four days in SH and NH summer seasons in different observation modes. The thick solid red line is the frost-point temperature contour, and the red dashed line is the mesopause altitude. The solid black line is an estimated altitude of the PMC layer (i.e. the altitude weighted with the 4th power of the density). Note that MIPAS is sensitive to all ice particles, including those with a small radius. Noise errors in these plots, estimated by the standard error of the mean, are about 0.3 × 10-14 cm3 cm-3. The PMCs are generally located at regions colder than the frost-point temperature for almost all conditions. Note also the large variability in latitude and altitude of the ice density, particularly on 6 July 2009 (bottom right panel), where the PMCs reach latitudes as low as 60∘ N. Weak PMCs located at latitudes equatorwards of about 60∘ and outside of the frost-point temperature contour are likely false detections caused by instrumental (most likely offset) errors.

Anomalous low-altitude detection of weak PMCs (i.e. below ∼80 km and outside of the Tfrost region) could be due to the limb nature of the measurements. Emissions from isolated clouds located in the LOS far away from the tangent point are reported at abnormally low tangent altitudes (see, e.g. , their Fig. 11). Also, the FOV can affect the height of the lower and upper boundaries of the layer. have shown that the bottom and top altitudes as measured by SOFIE, which has a FOV of 1.5 km, can be smeared out in about 1–1.5 km. These two effects, along with the temperature error, can explain why MIPAS observes occasional ice volume concentration at the bottom of the layer at temperatures warmer than the frost-point temperature.

The latitudinal and longitudinal distributions of ice volume density at 84 km for corresponding days are shown in Fig. . As shown before for the zonal means, the PMC layer is almost always confined to regions with temperature below the frost-point temperature. The variability of the latitudinal and longitudinal spread is also large. Although the PMCs are generally around the pole, they are sometimes far away (see top right panel in Fig. ) and their distribution could be controlled by 2-day and/or 5-day planetary waves . In particular the distributions of the days 12 January 2005 (top/left), 16 January 2009 (top/right), and 6 July 2009 (bottom/right) seem to be affected by wavenumber-1 planetary waves.

Figure  shows the zonal mean ice volume density averaged for all measured days in the Southern (left) and Northern (right) hemispheres for the NLC (top panels) and for the MA+UA (MUA) (lower panels) MIPAS modes (see Table ). These distributions are analysed in detail later, but we describe the main features briefly here: (1) PMCs are confined to altitudes between around 81 km and 89 km with maximum concentrations around 84 km; (2) PMCs are confined to latitudes polewards of about 60∘, with increasing concentration towards the poles; and (3) from these figures it is evident that the ice particles occur in higher concentrations in the NH and that the ice layer is located at slightly lower altitudes in the NH. These figures also show an apparent higher concentration for the measurements taken in the NLC mode than in the MUA mode. The NLC mode has a better vertical resolution, which leads to sharper temperature profiles see and hence to sharper ice particle profiles and larger ice particle densities. However, not all the differences between the NLC and the MUA modes can be attributed to the better vertical resolution of the former because observations in different modes occurred on different days, with observations in the NLC mode generally occurring closer in time to the peak of the PMC season than observations in the other modes.

Figure  shows the mean altitude of the PMC layer for the SH (left) and the NH (right) seasons for all measurements. The altitude of the PMC layer has been calculated as the altitude weighted with the 4th power of the volume ice density. We observe that the mean altitude in the NH for the NLC mode is located around 83.5–84 km, while in the SH it is about 1 km higher (84.5–85 km). The fact that the mean altitude is higher (in ∼1 km) for the MA + UA modes is attributed to the coarser sampling and to the broader vertical resolution in the retrieved temperature from these modes. The different temporal sampling of the NLC and MUA modes might also have an effect. have shown that PMCs are located higher at the beginning and end of the season, and lower in the middle of the season. This coincides with our results since the NLC-mode measurements are usually taken in the middle of the PMC season while MUA are taken earlier and later in the season. We should also note from Fig.  that PMCs tend to be located at lower altitudes near the poles, and at higher altitudes towards midlatitudes (both in NH and SH but more clearly in the latter).

reported an average value for the mean altitude of the PMC layer of 83.5 km for NH and 84.7 km for SH in SOFIE measurements, and of 83.3 km for the NH from concurrent lidar ALOMAR measurements in northern Norway (69∘ N). The MIPAS mean values for the mean altitude obtained here for the NH is very close to both measurements. Also, it is very much in line with SOFIE, locating the maximum of the layer about 1 km higher in the SH than in the NH.

carried out a multi-year analysis of the Odin/OSIRIS, SNOE/UVS, AIM/SOFIE, and TIMED/SABER data sets in the polar regions northwards and southwards of 65∘ N (∘ S) and found that the mean PMC height is located 3.5 km ± 0.5 km below the mean mesopause height. In the case of SOFIE measurements, however, this difference is significantly smaller, by ∼1 km, for most of the season, except around the middle of the season . We also looked at the difference between the mean PMC height and the mean mesopause height in the MIPAS PMC measurements (see Fig. ). In general MIPAS observations are more in line with SOFIE observations than with the other instruments. For the case of NLC and MUA MIPAS observation modes in the NH near 70∘ N, the difference is about 2.5 km, smaller than the mean value of 3.5 km obtained for all instruments and closer to the SOFIE value obtained by . It is worth noting that this altitude difference increases towards the North Pole, more clearly in the case of the NLC mode (taken around the middle of the season) and reaching about 4 km. In the Southern Hemisphere the difference between the mesopause and mean ice layer altitudes is even smaller than for NH, with values ranging between 2 and 2.8 km; again in better agreement with SOFIE than with the other instruments.

Figure  shows the latitudinal variation of the ice water content of the PMC layer for the SH (left) and the NH (right) seasons for all measurements. The figure shows clearly that PMCs are more abundant in the NH than in the SH, extending to lower latitudes in the NH. The main reason for this is the warmer polar upper mesosphere in the SH than in the NH, about a 10 K difference as measured by MIPAS . Figure  is consistent with the zonal mean ice volume density shown in Fig. , which shows that ice mass density increases towards the poles. Large variability from season to season is also clearly visible, which in the case of MIPAS is attributable not only to the yearly changes but also to the daily variation because of the infrequent temporal sampling of MIPAS. The ice column is large for the NLC mode (not shown) in correspondence with the zonal mean fields shown in Fig. . As mentioned before, this is probably due to the fact that the NLC measurements are taken around the middle of the season (see Table ). The NH / SH ratio of the ice water content varies with latitude (not shown), ranging from about a factor of 2 near 60∘ to 1.4 near the poles, with a value of 1.7 near 70∘, which is in very good agreement with the factor of 65 % reported by from SOFIE measurements.

Figure  shows the correlation between the ice water content and the altitude of the lower branch of the frost-point temperature contour (see Fig. ) for the data taken in the NLC and MUA observation modes in the SH and NH PMCs seasons. The correlation is significant and shows that the PMC layers contain more ice when the frost-point temperature occurs at lower altitudes. We have done the analysis for each hemisphere and mode separately (not shown) and found a very similar correlation for all cases except for the NLC mode in the NH. The reason for this exception could be the smaller sample size of this case or that the altitude range of the frost temperature in NH for this mode is very small and hardly reaches altitudes higher than 82 km.

We have also found that the ice volume density is also anti-correlated with the mean altitude of the PMC layer (not shown), that is, that the denser PMC layers are located at lower altitudes and the thinner ones at higher altitudes. This is consistent with the behaviour shown in Figs.  and where the denser layers are usually found near the poles and at lower mean altitudes.

suggest that, as opposed to HALOE and MLS water vapour measurements, the SOFIE vertical resolution is well suited for the study of correlations between water ice and water vapour. This is also the case for MIPAS. Given the good latitudinal coverage of MIPAS (covering the whole polar region) and the fact that the instrument is able to measure the ice water content and the water vapour concentration simultaneously, we have looked at the zonal mean as well as latitudinal and longitudinal distribution of both quantities in the polar summer region.

The water vapour concentrations used here have been derived from MIPAS high-resolution spectra in the region around 6.3 µm. We used version v5r_h2o_M22 retrievals. The retrieval baseline is an extension to the lower mesosphere of the set-up described by with the updates described by . The main difference of this extension is the inclusion of non-LTE emissions from the H2O vibrational levels, which are important above around 50 km . Additional microwindows, covering stronger H2O v2 spectral lines, are also included in order to increase the sensitivity in the upper mesosphere .

Figure  shows a typical case (21 July 2005) of the zonal mean cross sections of the ice volume density (left) and the H2O concentration anomaly (right). We can clearly distinguish three distinct altitude zones near the polar region: a region centred near the peak of the PMC layer (∼83 km), where the ice volume density is largest; a hydrated region a few kilometres below, where H2O presents a relative maximum at latitudes northwards of 70∘ N, more markedly seen in the bottom panel of Fig. ; and a dehydrated region above the ice layer around ∼ 90 km, where H2O exhibits a clear relative minimum (see top panel of Fig. ). This global behaviour fits very well with the current picture we have of the PMCs, where sequestration of H2O in the gas phase to form ice leads to a drier atmosphere just above the ice layer, and where the sedimentation of ice and its subsequent sublimation enhances the H2O gas-phase abundance at ∼80 km. The MIPAS water vapour layered structure gets sharper towards the pole. This is in contrast to findings from , who located the maximum at about 70∘ N.

These features are more clearly observed in the latitude/longitude maps (Fig. ), where the dry region at 90 km (top), the water ice layer at 83 km (middle), and the wetter H2O region at 80 km (bottom) are all well confined to the polar region. This topic has been recently studied quantitatively by using SOFIE observation of ice content, water vapour, and temperature at latitudes near 70∘. They found that, in both hemispheres, the altitude of the peak of the dehydration regions is ∼ 1.8 km above the height of peak ice mass density, and the altitude of the peak of the hydration region is ∼ 0.3 km above the observed bottom of the ice layer. Although no general conclusion can be drawn from the single day of MIPAS data shown here, we have found different results. In MIPAS the peak altitude of the hydration region is about 1 km below the bottom altitude of the PMC layer, and the dehydration region is found to be significantly (about 2–3 km) higher than in SOFIE (see right panel in Fig. ).

also found that the column abundance of H2O in the gas phase is roughly equal in the dehydration and hydration regions, but less than that contained in the ice layer. From the day of MIPAS data analysed here we find that the excess of H2O gas-phase column in the hydration region ranges from 5.5 to 9 ppmv × km for 70–90∘ N, while the column of the upper drier region is significantly smaller, ranging from -1 to -4.5 ppmv × km. We should note, however, that we use the mean H2O gas profile averaged over all latitudes as the “background”, which could probably partially explain the differences with SOFIE. MIPAS and SOFIE, however, agree very well in that the excess and deficit H2O gas-phase concentrations are significantly much smaller than those contained in the ice cloud. Figure  shows MIPAS-enhanced values of about 1.5 ppmv in the hydration layer and a decrease of 0.5 ppmv in the dehydration region, while the Qice peak is about 6 ppmv. A more comprehensive study using all MIPAS data should, however, be carried out to confirm these findings.

The diurnal variation of PMCs is an important factor to be taken into account when comparing data sets with different temporal sampling. Several studies have shown that the IWC may have a significant diurnal variation at latitudes close to and equatorwards of 70∘, mainly driven by tidal effects in temperature and in meridional advection at subpolar latitudes . MIPAS measures PMCs at two local times, 10:00 and 22:00, and hence easily allows for the inspection of variations due to the diurnal migrating tide see. Figure  shows the absolute (upper panels) and relative (middle panels) zonal mean differences (morning–evening) of MIPAS ice volume density averaged over all measurements in the SH (left panels) and NH (right panels).

The morning–evening absolute differences are larger in the NH, partially due to the larger concentrations in this hemisphere. The relative differences in the NH are larger at 60–80∘ N, and reach a maximum value of 0.75 × 10-14 cm3 cm-3. The morning enhancement is in line with the predictions of but not as large as their calculated factor of 4–5 in the IWC at 69∘ N. At this latitude, we find a maximum enhancement of about 80 % in the volume density and 40–50 % in the IWC (not shown). Note however that simulations by correspond only to June 2007. The changes in the ice volume density at 65–75∘ N shown in Fig.  result in the MIPAS NH evening clouds being on average at slightly lower altitudes, also in agreement with .

The morning–evening difference of ice volume density at 50–60∘ N is 0.25–0.5 × 10-14 cm3 cm-3 at 81–87 km, i.e. morning / evening ratios lying between 1.5 at 86 km and 7 at 82 km (Fig. ). These changes result in narrower and thinner evening clouds, on average, that mainly disappear below 84 km, in agreement with findings at subpolar latitudes from and . The IWC morning / evening ratio increases rapidly towards these lower latitudes and varies in the range of 1.5 to 2.8 at 50–60∘ N (not shown).

The bottom panels of Fig.  correspond to 10:00–22:00 differences in the kinetic temperature measured by MIPAS simultaneously with the ice volume densities. These are a good measure of the temperature perturbations due to the diurnal migrating tide . The morning–evening ice volume density differences in the NH (right panels in Fig. ) are generally anti-correlated with the corresponding morning–evening kinetic temperature differences. For example, the positive ice differences at 80–85 km equatorward of 80∘ N correspond to negative temperature differences. The temperature differences tend to be positive above 87 km northward of 65∘ N, which is reflected in the ice volume density differences. Nevertheless, it is not possible to infer from these anti-correlations alone the extent to which diurnal temperature perturbations affect the ice volume density. Direct influence from other factors, like tidal variation of meridional advection see, e.g. or non-linear behaviour of phase transitions, cannot be ruled out.

Indeed, the anti-correlation between diurnal variation of the ice density (upper and middle left panels in Fig. ) and that of temperature (lower left panel) in the SH is not so clear. In this hemisphere, the negative morning–evening temperature difference at 50–60∘ S and 80–84 km is weaker and located at lower altitudes than in the NH. The corresponding absolute diurnal ice change is small but the ice volume density at these latitudes is also very small (less than 5 × 10-15 cm3 cm-3). The negative ice concentration difference at 84–88 km corresponds to a positive temperature difference but only at 60–80∘ S. And most strikingly, the morning–evening temperature perturbation around 80–84 km at 65–80∘ S is positive but so is the ice variation. This indicates that a diurnally varying driver other than temperature more significantly affects the diurnal ice variation at those latitudes, at least below 84 km. The overall effect on ice density results in vertically alternating positive and negative changes that lead to lower SH morning mean cloud altitude. The impact of that driver most likely depends on altitude and latitude. A deeper analysis involving wind fields is beyond the scope of this paper and will be the focus of a future study.

We have analysed MIPAS IR measurements of PMCs for the NH and SH summer seasons from 2005 to 2012. PMCs were measured in the middle IR in emissions where, due to the small particle size, the signal is only affected by absorption and not by scattering. MIPAS is therefore sensitive to the total ice volume, including the very small ice particles that UV–VIS scattering observations are generally not sensitive to. The measurements cover only a few days of the PMC season (varying from 3 to 15) but have global pole-to-pole coverage. In this way, MIPAS measurements show, for the first time, global latitudinal coverage (from 50∘ to the pole) of the total ice volume density.

MIPAS measurements indicate mesospheric ice existing as a continuous layer extending from about ∼81 km up to about 88–89 km on average and from the poles to about 55–60∘ in each hemisphere. These altitudes are in very good agreement with SOFIE measurements, with the lowest altitude being slightly lower (∼ 0.7 km) in MIPAS, and the uppermost altitude slightly higher (1.7 km), probably caused by the wider MIPAS field of view. This bottom altitude is also slightly lower than that derived from lidar measurements but the uppermost altitude is significantly higher (4–5 km on average). This indicates that both MIPAS and SOFIE instruments are sensing the small ice particles in the upper part of the PMC layer. This has also been proved recently by the concurrent observations from the ALOMAR wind (ALWIN) radar and measurements from SOFIE .

PMCs are very variable, both in space and time. On average, MIPAS measurements show that ice mass density increases towards the poles. The IWC measured by MIPAS at latitudes where SOFIE measurements are available show a good overall agreement, being in general slightly larger (∼ 10 %), and also exhibiting a larger variability, probably caused by the smaller sensitivity of MIPAS. A distinctive feature, however, is that in general MIPAS shows larger ice volume densities than SOFIE in the region above ∼ 85 km and smaller below.

The ice concentration is larger in the Northern Hemisphere than in the Southern Hemisphere. The ratio between the IWC in both hemispheres is also latitude-dependent, varying from a NH / SH ratio of 1.4 close to the poles to a factor of 2.1 around 60∘. This also implies that PMCs extend to lower latitudes in the NH.

We have found that the mean altitude of the PMC layer in the NH for the NLC mode of MIPAS observations is located around 83.5–84 km, while in the SH it is about 1 km higher (84.5–85 km). This hemispheric asymmetry is in very good agreement with SOFIE observations . For those MIPAS observations taken in the middle- and upper- atmosphere modes, the mean altitude is higher (by ∼ 1 km). This difference is attributed to the coarser sampling and to the broader vertical resolution (particularly in the retrieved temperature) and also to the different temporal sampling of the modes since the NLC-mode measurements are usually taken in the middle of the PMC season while MUA-mode observations are taken earlier and later in the season. A very clear feature in MIPAS observations is that PMCs tend to be at higher mean altitudes towards lower latitudes (in both hemispheres), particularly equatorwards of 70∘.

MIPAS observations show that the difference between the mean PMC height and the mean mesopause height is about 2.5 km in the NH near 70∘ N. This is smaller than the mean value of 3.5 km obtained from several satellite instruments by and closer to the SOFIE value . MIPAS also shows that this altitude difference increases towards the North Pole, reaching a value close to 4 km. In the Southern Hemisphere this difference is smaller than for the NH, with values ranging between 2 and 2.8 km; again the agreement with SOFIE is better than that with other instruments.

The anti-correlation between the ice water content and the altitude of the lower branch of the frost-point temperature contour is significant in MIPAS observations and shows that the PMC layers have larger ice mass densities when the frost-point temperature occurs at lower altitudes. The simultaneous observations of MIPAS PMCs and water vapour have confirmed that PMC layers are surrounded by a hydrated region below and a dehydrated region above. These regions are more pronounced towards the poles, particularly at latitudes poleward of 70∘ N. This global behaviour fits very well with the current picture we have of the PMCs, where sequestration of H2O in the gas phase to form ice leads to a drier atmosphere just above the ice layer and where the sedimentation of ice and its subsequent sublimation enhances the H2O gas-phase abundance at ∼80 km. The analysis of a single day of water vapour and PMCs measurements of MIPAS has shown different results than for SOFIE. The peak altitude of the hydration region is about 1 km below the bottom altitude of the PMC layer in MIPAS, while in SOFIE it is ∼0.3 km above , and the dehydration region is found to be at ∼2–3 km above the height of peak ice mass density in MIPAS but ∼1.8 km in SOFIE. Further, MIPAS shows that the column abundance of water vapour excess in the hydration layer is about twice than the deficit in the dehydration layer near 70∘ N, while they are very similar in SOFIE. However, they both agree that both quantities are much smaller than the water content in the form of ice.

Finally, MIPAS observations, which are taken at 10:00 and 22:00, also show a diurnal variation in the ice volume density. The IWC is larger at in the morning than in the evening in the NH, in line with the model predictions of . This diurnal variation is anti-correlated with corresponding differences in temperature in the NH, suggesting that it is driven by the temperature-migrating diurnal tide, but effects from other factors cannot be ruled out. In the SH, the lack of a clear anti-correlation with temperature points to a significant impact of an additional driver.