This paper summarizes developments in observing and understanding the composition and chemical reactions in the MLT over recent decades. There is little space given to the basic composition and dynamics of the MLT; instead, the paper assumes a working familiarity with those and focusses almost completely on recent or still unsolved aspects. The topic is broad and many details are mentioned only briefly but the authors provide an extensive list of references.  The paper is very clearly written. For the most part it maintains a balance in the topics covered without undo emphasis on any particular aspect of the development.

Section 2 describes the wealth and variety of observations that have facilitated the recent advances in describing the chemical composition. The focus is on constituents for which measurements were limited or nonexistent previously. Subsequent sections describe MLT variability, airglow processes, meteoritic metals, and ice clouds. The authors also provide a section with their suggestions for future directions. This section contains a reminder that the future is likely to see a diminishment in measurements by satellites as existing missions have already reached or will reach the ends of their lifetimes.

Major comments

Many of the works cited in Section 3 include explanations of the dynamically coupled or dynamically driven processes that lead to various outcomes, e.g. responses to solar variability, MJO, QBO, interhemispheric coupling, volcanic eruptions, trends. These explanations are repeated in the paper without judgement. In some cases, the statistics are poor because the temporal records are short (especially the solar cycle and volcanic eruption time series), the signals are weak and irregular, and the gravity waves that are involved in many interactions are poorly observed. It is appropriate to pass along the contents of other studies in a review such as this one. In the interest of caution, a more forceful recognition that verification of many of these mechanisms awaits additional data would be a good idea. As written, the accumulation of these reports gives the impression that the variability associated with external forcing and dynamical coupling on large spatial or temporal scales is better accepted and understood than is the case.

Minor comments

The section on future directions describes several measurement systems or global modeling advances that are still in planning stages or, even for those already collecting data, for which publications are not yet available. In order for readers to follow the progress in the months or years following initial publication, it would be useful to have web links for the individual programs.

Editorial

1. 38: “mbar”, “mbar” Consider using SI units (Pascal) or at least use the same units for the two values being compared.

1. 60-61: “Observations of NO and excited NO infrared emissions have been particularly useful in detecting the presence of atmospheric tides in the MLT.”

I think you mean the impact of tides on composition. The tides themselves are best detected from temperature and wind observations.

1. 135: “all the way to the MLT”

How about “all the way up to the MLT” or is it “all the way down”?

1. 520-521: “… the statistical significance of these trends remains limited”

This comes across as wishy-washy. Can you just say there is no significant trend?

In this paper, an overview is provided in recent developments and future directions in the science of the Mesosphere and Lower Thermosphere (MLT) region. The MLT region is a highly interesting transition region between the lower and middle atmosphere, and the near-Earth space of the thermosphere-ionosphere-magnetosphere. As it is not directly accessibly by measurement platforms apart from by sounding rocket, observations of the MLT region are difficult, and many aspects of MLT processes are still not well understood. However, a lot of progress has been made in the last two decades due to advances in observation techniques and the large number of satellite observations starting 2002, and the aim of the paper is to focus on these recent advances in a selection of topics. The paper provides a very short introduction defining the MLT region and its most important properties, and then discusses observations of the MLT region from different platforms followed by a wide area of different topics: the variability of the MLT regarding energetics, wave-driven dynamics, solar forcing, and long-term trends, airglow emissions and chemistry, the cosmic dust influx and the resulting metal chemistry and cosmic dust formation, and noctilucent clouds. It ends with a discussion of future directions. The paper thus provides an interesting overview of a highly relevant, fast developing field which I quite enjoyed reading.  

As this is an opinion paper, the selection of topics discussed is by nature somewhat subjective, depending on the choices of the authors. I have summarized a few more references and further points below (“Topical”), however would like to emphasize that these are suggestions only. A few points in my opinion should be clarified before final publication, see “Major issues”, and a few minor and technical issues are listed at the end as well.

Major issues:

The title and abstract are not very clear in the sense that they do not describe the content of the paper very well. I) the paper clearly deals with much more than just the chemistry of the MLT, covering topics like wave coupling, microphysics, and energetics, but actually not covering what I would call the photochemistry of the MLT driven by photolysis, photoionization and particle impact ionization, in great detail. You could just change the title and description in the abstract as well as in two more places listed in “Minor”. II) It is stated on the one hand that the paper reviews important advances over the past two decades, but with a focus on work during the past 10 years. This alleged focus is actually not clear in most of the subsections, which mainly discuss work done since 2000/2002, so in the last two decades.

In the introduction, a very short definition and overview is given about what is the mesosphere, thermosphere, and MLT region. As the topic of the publication is on recent developments in MLT science and work during the past two decades, it would have been helpful for the non-expert reader to provide also a short summary of the state of the art 10 years ago as a starting point. Some of the following chapters have a short summary like this at the beginning (e.g., chapter 3.1 which deals only with research prior to 2005), but not all; maybe some starting point like this could be provided clearly at the beginning of all chapters, or in the introduction.

Chapter 3.3, lines 217-220: the high-latitude MLT region is affected by particle impact ionization of precipitating protons and electrons which come from different sources. There are solar particles, mostly protons, from solar coronal mass ejections, but there are also auroral electrons -- solar wind particles accelerated in the magnetotail -- and electrons from the radiation belts and ring currents accelerated into the loss cone e.g., in geomagnetic storms. Geomagnetic storms can be initiated by solar coronal mass ejections CMEs, but also by corotating interaction regions CIRs. Reviews of this can be found, e.g., in Sinnhuber et al., Surv. Geophys., 2012; Mironova et al., Space Sci Rev., 2015; Baker et al., Space Sci Rev., 2018; Sinnhuber and Funke, in: The dynamic loss of Earth’s radiation belts, Elsevier, 2020. Precipitation of solar energetic particles in solar proton events as mentioned here can have a spectacularly large impact on the chemical composition of the middle atmosphere, but mostly in the stratosphere and lower mesosphere; in the MLT region, geomagnetic storms and auroral substorms arguably have a larger impact on, e.g., the distributions of NO, upper mesosphere OH, auroral airglow, and temperature.

Chapter 3.4: Solar coronal mass ejections and energetic particle precipitation have already been introduced, and their effects discussed to some extent, in Section 3.3, see my above comment. However, both here and in Chapter 3.3, the different sources of energetic particle precipitation are mixed up. This should be clarified; see my summary of sources in the comment above. Normally, “space weather” refers to the disturbances of the magnetosphere-ionosphere-thermosphere by high-speed solar wind, which could come from solar coronal holes, corotating interaction regions, or solar coronal mass ejections; some but by far not all solar coronal mass ejections come with high fluxes of high-energy protons > 10 MeV, and initiate solar proton events. You appear to refer to solar proton events in line 217 and lines 252-257, but than reference publications which deal with geomagnetic activity impacts by geomagnetic storms or auroral substorms without clarifying that these are different types of events with different particle sources, particle energies, and temporal evolution: e.g., Fytterer et al 2015; Hendrickx et al., 2015 in lines 221-222; and Anderssen et al., 2014; Smith-Johnsen et al., 2018 in lines 258-259 deal with geomagnetic activity, but not with solar proton event; and in lines 259-262 MEE is discussed, which are likely related mainly to geomagnetic storms.

Topical:

Lines 58-59: there has also been interesting work on O2 airglow emissions. E.g., the derivation of the O2 emissivity from SCIAMACHY observations (Zarboo et al., AMT, 2018) has enabled a more precise observation of CO2 from space (Sun et al., GRL, 2018; Bertaux et al., AMT, 2020). SCIAMACHY O2 airglow observations have also been used to simultaneously derive O2 emissivity and MLT temperatures (Sun et al., AMT, 2022).

Lines 60-63: NO number density is a clear tracer of atmospheric ionization in the upper mesosphere and lower thermosphere; the abundance of NO at low and midlatitudes above 80 km altitude clearly demonstrates the role of EUV photoionization, while at high latitudes, NO corresponds to geomagnetic forcing by electron precipitation in auroral substorms and geomagnetic storms (e.g., Marsh et al.,JGR, 2004). Observations of NO density in the upper mesosphere and lower thermosphere have been carried out since 2002, e.g., by MIPAS/ENVISAT in the MA/UA modes (Bermejo-Pantaleon et al., JGR, 2011), by SCIAMACHY/ENVISAT (Bender et al., AMT, 2015; Bender et al., AMT, 2017), by ODIN/SMR (Sheese et al., JGR, 2013), ACE/FTS (Boon et al., 2005, 2013), and SNOE/AIM (Gordley et al., 2009; Hervig et al., 2019), and have been widely used to study particle impact ionization in the MLT region, e.g., showing a clear impact of geomagnetic forcing well into the mesosphere (e.g. Kirkwood et al., Ann. Geo., 2015; Hendrickx et al., JGR, 2015; Sinnhuber et al., JGR, 2016; Hendrickx et al., GRL, 2017; Smith-Johnsen et al., 2017; Kiviranta et al., ACP, 2018; Sinnhuber et al., JGR, 2022). A similar response to geomagnetic forcing has been found for OH based on observations from MLS/AURA in the high-latitude mesosphere (Verronen et al., 2011; Andersson et al., 2012; Fytterer et al., 2015). A response of mesospheric NO to geomagnetic storms and auroral substorms has also been observed by a ground-based microwave radiometer (Newnham et al., GRL, 2011; Newnham et al., JGR, 2015) (Section 2.3).

Lines 67—74: you could add the observations of MIPAS/ENVISAT here, which also observes CO2 as well as CO, NO and T; though the full period of 10 years (2002-2012) of observations is only available for the nominal mode scanning up to 68 km, while the UA/MA modes scanning up to 170 km only started in 2005.

Section 2.1: radio occultation observations could be mentioned here as well, which provide global observations of electron density above ~80 km for the first time, e.g., from satellite instruments like FORMOSAT-3/COSMIC-1 (Wu et al., Remote Sensing, 2022), or a detailed view of sporadic E layers (e.g., Arras et al., Earth., Planets and Space, 2022).

Line 201: arguably at least as spectacular manifestations of wave coupling in the MLT are (I) Elevated Stratopause Events (ESEs), which are characterized by the re-formation of the stratopause at mesopause altitudes followed by the formation of a very strong and stable polar vortex with strong downwelling, after some SSWs (Manney et al., GRL, 2009; Orsolini et al., JGR, 2010; Siskind et al., JGR, 2010; Siskind et al., GRL, 2010; Thurairajah et al., JGR, 2010; Chandran et al., GRL, 2011; Ren et al., JGR, 2011; Limpasuvan et al., JASTP, 2012); (II) the fact that the impact of SSWs is observed even in the ionosphere (e.g., Chau et al., JGR, 2010; Goncharenko et al., GRL, 2010; Chau et al., Space Sci Rev, 2012; Goncharenko et al., GRL, 2021; Goncharenko et al., Frontiers, 2022)

Lines 261-265: there are a number of observations and model studies showing a temperature response to geomagnetic activity, mostly related to geomagnetic storms, but also to SPEs (Tyssoy et al., JGR, 2010; Sun et al., Universe, 2022; Wang et al., JGR, 2021; Zou et al, Astrophys. J., 2020; Li et al, GRL,  2018; Liu et al., Atmosphere, 2018).

Minor:

Line 40: There is also a very steep temperature gradient from the mesopause into the lower thermosphere, with changes of several hundreds of K between 80 and 120 km.

Line 85: small-scale “vertical” structures

Line 158: photodissociation and dissociative ionization in the EUV of O2 and N2

Line 174-175: The gw-driven meridional overturning circulation controls the dynamics of the upper mesosphere; however, there is a turn-around of the zonal winds (e.g., Smith et al., Surv. Geophys., 2012) and meridional winds (e.g., Wang et al.,JGR, 2022, Figure 1) around the mesopause which should separate the large-scale circulation patterns in the upper mesosphere from the lower thermosphere. Large-scale transport of tracers from the lower thermosphere to the upper mesosphere appears unlikely, turbulent mixing appears more likely.

Chapter 3.3, lines 212-216: the highly variable solar electromagnetic radiation in the EUV and soft x-ray range could be mentioned here as well, which is the source of the daytime ionospheric E-layer, and ionizes the daytime atmosphere down to around 80-90 km altitude.

Line 222: observations of the 27-day signatures in mesospheric NO are also reported in Sinnhuber et al., JGR, 2016 and shown from model experiments in Fytterer et al., JGR, 2016. It should be noted that the 27-day signature found in NO and OH at high latitudes is likely mainly related to the 27-day signature in the solar wind, not in the electromagnetic spectrum. Insofar it might be more fitting to discuss these in the next section “Space weather impacts”.

Line 231-233:…. and EUV photoionization above 90 km

Line 261: and Sinnhuber et al., JGR, 2022 (companion paper to Tyssoy et al., 2022)

Lines 381-383: … and sporadic E-layers (in Section 5.2)

Line 436-437: narrow layers of high concentrations of Fe+ and Mg+ ions and electrons

Line 549: into the composition, energetics, and dynamics of the MLT, as there is discussion of e.g., gravity waves, NLCs and Airglow as well.

Lines 565-571: you could mention here also ESAs Earth Explorer 11 candidate mission The Changing-Atmosphere IR Tomography Explorer CAIRT (cairt.eu), an IR imager capable to simultaneously observe temperature and a large number of trace gases from the UTLS to the lower thermosphere with high spatial resolution, as a long-term perspective.

Lines 582-583: atomic oxygen at 63 mym has already been observed in the lower thermosphere from the CRISTA instrument on two space shuttle flights in 1994 and 1997 (Grossmann et al., GR,L 2000). Good to see that this method may be used again in the near future.

Lines 595-601: There is also still the Network for the Detection of the Network for the Detection of Atmospheric Composition Change NDACC with some of the observations (e.g., by the microwave radiometers) reaching into the mesosphere (ndacc.larc.nasa.gov), and the Network for the Detection of Mesospheric Change NDMC (ndmc.dlr.de), which targets the mesopause region.

Line 723-724: see my comments above: the topic is much broader than chemistry, as it includes wave coupling, airglow, cosmic dust and noctilucent clouds.

Technical:

The paper would read better with using less brackets (). At least in the abstract (line 14-15, 16, 17-18), they should not be used.

Line 50: the acronym list is “at the end of the paper” – below suggests on this or the next page.

Line 54: that revealed “that” photochemistry dominates …

Line 106: or “a” combination of both

Line 308: can you clarify what “high v” means? Instead of using this, you could write “v>= 5(?)” here and in the following (e.., line 312, line 347, … ). That would be more precise.

Line 342: Panka et al. (2017) appeared in …