Ward et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-021-00433-8(2021) 8:47

REVIEW

Progress in Earth andPlanetary Science

Open AccessRole Of the Sun and the Middle
atmosphere/thermosphere/ionosphere In
Climate (ROSMIC): a retrospective andprospective view
William Ward1*† , Annika Seppälä2† , Erdal Yiğit3 , Takuji Nakamura4 , Claudia Stolle5 , Jan
Laštovička6 , Thomas N. Woods7 , Yoshihiro Tomikawa4 , Franz-Josef Lübken8 , Stanley C. Solomon9 , Daniel R.
Marsh9,10 , Bernd Funke11 and Duggirala Pallamraju12

Abstract
While knowledge of the energy inputs from the Sun (as it is the primary energy source) is important for understanding
the solar-terrestrial system, of equal importance is the manner in which the terrestrial part of the system organizes
itself in a quasi-equilibrium state to accommodate and re-emit this energy. The ROSMIC project (2014–2018 inclusive)
was the component of SCOSTEP’s Variability of the Sun and Its Terrestrial Impact (VarSITI) program which supported
research into the terrestrial component of this system. The four themes supported under ROSMIC are solar influenceon climate, coupling by dynamics, trends in the mesosphere lower thermosphere, and trends and solar influence inthe thermosphere. Over the course of the VarSITI program, scientific advances were made in all four themes. This
included improvements in understanding (1) the transport of photochemically produced species from thethermosphere into the lower atmosphere; (2) the manner in which waves produced in the lower atmosphere
propagate upward and influence the winds, dynamical variability, and transport of constituents in the mesosphere,
ionosphere, and thermosphere; (3) the character of the long-term trends in the mesosphere and lower thermosphere;and (4) the trends and structural changes taking place in the thermosphere. This paper reviews the progress made inthese four areas over the past 5 years and summarizes the anticipated research directions in these areas in the future.
It also provides a physical context of the elements which maintain the structure of the terrestrial component of thissystem. The effects that changes to the atmosphere (such as those currently occurring as a result of anthropogenic
influences) as well as plausible variations in solar activity may have on the solar terrestrial system need to beunderstood to support and guide future human activities on Earth.
Keywords: Climate, Atmosphere, Middle atmosphere, Thermosphere, Ionosphere, Solar irradiance, Energetic
particles, Atmospheric coupling, Trends*Correspondence: wward@unb.ca† William Ward and Annika Seppälä contributed equally to this work.
1
Department of Physics, University of New Brunswick, POBox 4400,
Fredericton, Canada
Full list of author information is available at the end of the article© The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
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Ward et al. Progress in Earth and Planetary Science(2021) 8:47

1 Introduction
The primary goal of solar terrestrial physics is to understand the physical foundations of the structures and variability associated with the coupled Sun-Earth system. The
challenges are significant, since the important physics ofthe various components of this system (Sun, Solar wind,magnetosphere, ionosphere, and atmosphere) differ anddominate in different spatial regions. Furthermore, theimportant time scales of the atmospheric responses tothe forcings become longer closer to the surface of the
Earth. Significant progress has been made in understanding the physics within each region. However, to facilitate
progress, the boundary conditions linking the components have tended to be simplified. As a result, interactions between the different regions have not been realistic,
and a full Sun to Earth system description has not beendeveloped.
Recently, progress on establishing a full physically selfconsistent understanding of the solar-terrestrial system
has been made. This is due to the development of new
observational techniques, advances in the sophisticationand computing power of models, and the generation oftools allowing processes from the Sun to the Earth tobe visualized and analyzed. This evolution is reflectedin the nature of recent Scientific Committee on SolarTerrestrial Physics (SCOSTEP) programs where investigation of the coupling between regions has been encouraged(see Shiokawa and Georgieva (2021)).
The recently completed VarSITI program (Variabilityof the Sun and Its Terrestrial Impact, 2014–2019) supported four projects: SEE (Solar Evolution and Extrema),
MiniMax24/ISEST (International Study of Earth-affecting
Solar Transients), SPeCIMEN (Specification and Prediction of the Coupled Inner-Magnetospheric Environment),
and ROSMIC (Role Of the Sun and the Middle atmosphere/thermosphere/ ionosphere In Climate). Interactions between the various projects were encouraged. In
PRESTO (PREdictability of variable Solar-Terrestrial cOupling), the 2020–2024 SCOSTEP science program, thesystem aspects of solar-terrestrial relations is further
emphasized by defining projects in terms of their temporal and spatial relationships through the full system.
The ROSMIC project was devoted to understanding theimpact of the Sun on the Earth’s middle atmosphere/lower
thermosphere/ionosphere (MALTI) and its troposphericclimate. It sought to estimate the importance of this natural forcing over time scales from minutes to centuries, ina world where anthropogenic forcing is driving large-scalechanges across all atmospheric regions. It was a continuation of the science initiated during the CAWSES (Climate
And Weather of the Sun-Earth System) programs underwhich many of the intricacies and subtleties of the solarterrestrial system started to be recognized. The essential
insight was that the terrestrial component of the system

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plays an active role in determining the character of thesystem and its response to variations in solar input. On
average, the terrestrial component of the system is in equilibrium with respect to solar inputs, i.e., the incoming andoutgoing energy fluxes are equal. However, the physicalproperties of the terrestrial system and the non-local andnon-linear processes that are involved determine the form
that this equilibrium takes and its response to variations
in solar input.
Two scientific issues stimulated work to develop insight
into the terrestrial response. The first was that while significant sensitivity to solar variability in the stratosphereand troposphere is observed, the mechanisms causing
these dependencies are difficult to determine (Gray et al.
2010). The second was the recognition that, in addition
to radiation and energetic particles, much of the communication between layers of the atmosphere is due towaves (most of which are excited in the troposphere)
and that these waves propagate to significant heights
and cause ionospheric and thermospheric variability(Oberheide et al. 2015).
Over the course of the ROSMIC project, research into
these issues has continued, and although they have notbeen settled, our understanding has advanced. The inconsistency between different observations and methods todetermine the solar spectral irradiance variations is being
resolved (Woods et al. 2015, 2018, Yeo et al. 2017).
Together with total solar irradiance, solar spectral irradiance is now being included along with energetic particle
forcing as input to model runs for the Coupled Model
Intercomparison Project Phase 6 (CMIP6) of the World
Climate Research Program (Matthes et al. 2017). Investigations to identify the subtle processes leading to thesolar influence of the lower atmosphere are progressing,and new challenges have been identified (Gordon 2020;
Dhomse et al. 2016; Thiéblemont et al. 2015; Chiodo etal. 2019). New complexities in the upward coupling fromthe troposphere and stratosphere to the mesosphere, thermosphere, and ionosphere associated with waves are now
being addressed. These include recognition of the importance of secondary gravity wave generation to wave penetration into the thermosphere (Becker and Vadas 2018)
and pole-to-pole coupling of dynamical signatures (Karlsson and Becker 2016; Smith et al. 2020) and that the presence of wave signatures in the thermosphere and ionosphere need not be due to direct propagation (Miyoshi
and Yamazaki 2020). Sudden stratospheric warmings andtheir global influences are being extensively studied (Butler et al. 2015; Pedatella et al. 2018a). They are the largest
perturbations to the middle atmosphere on record andprovide insights into the mechanisms of the coupling
between atmospheric layers.
The need for coordinated ground observations andcampaigns and associated modeling started to beWard et al. Progress in Earth and Planetary Science(2021) 8:47

addressed during ROSMIC. Examples include the Interhemispheric Coupling Study by Observations and Modeling (ICSOM) project, the Antarctic Gravity Wave Instrument Network (ANGWIN) (Moffat-Griffin et al. 2019),
and The Deep Propagating Gravity Wave Experiment(DeepWave) (Fritts et al. 2016). Modeling advanced significantly with whole atmosphere models increasing insophistication and including the ionosphere (Liu et al.
2018; Miyoshi et al. 2018; Solomon et al. 2019), and data
assimilation efforts start to include mesospheric observations (Eckermann et al. 2018).
The scientific questions which guided ROSMIC activities were as follows:• What is the impact of solar forcing on the entire
atmosphere? What is the relative importance ofvariability in solar irradiance versus energetic
particles?
• How are the solar signals transferred between thethermosphere and the troposphere?
• How does coupling within the terrestrial atmospherefunction (e.g., what are the roles of gravity waves andturbulence?).
• What is the impact of anthropogenic activities on the
MALTI?
• Why does the MALTI show varying forms oflong-term variations?
• What are the characteristics of reconstructions andpredictions of TSI and SSI?
• What are the implications of trends in theionosphere/thermosphere for technical systems suchas satellites?
During VarSITI, ROSMIC supported the activity in thecommunity through existing activities (i.e., Workshop on
Coupling in the Atmosphere-Ionosphere System, ANtartic Gravity Wave Instruments Network (ANGWIN), HighEnergy Particle Precipitation in the Atmosphere/SOLAR
Influences for SPARC (Stratospheric Processes andtheir Role in Climate) (HEPPA-SOLARIS) Workshop,
13th International Workshop on Layered Phenomenain the Mesopause Region (LPMR), and 9th IAGA ICMA/IAMAS - ROSMIC/VarSITI/SCOSTEP workshop
on Long-Term Changes and Trends in the Atmosphere)
as well as through the organization of sessions at international meeting such at COSPAR, IUGG, and the VarSITI
symposia.
This paper is organized as follows. The topic is introduced in an overview section which summarizes thephysical context underlying the atmosphere/ionospherecomponents of the Sun/Earth system. This is followed by
two sections which address the progress in the two mostsignificant pathways for modulation and propagation of
atmospheric/ionospheric responses to solar variability tosolar variability namely coupling from above and coupling

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from below. The fourth section consists of a summaryof progress in observing and understanding long-term
trends in the mesosphere and thermosphere (this includes
progress associated with themes 3 and 4). The paper concludes with a summary and thoughts on future progress inthis area.

2 Overview: general physical principles of theenergetics and organization of theatmosphere/ionosphere
Identifying the mechanisms which underlie the mean
global atmospheric and ionospheric structures and theirvariability is especially important at this time. First, we
do not fully understand the processes which couple thevarious regions of the atmosphere and ionosphere. Second, uncertainties remain in establishing the extent towhich these structures and their variability might change
in response to changes to solar inputs and/or to anthropogenically driven changes to atmospheric composition(especially those involved in radiative effects in the atmosphere). Either of these directly changes the thermal structure and circulation of the atmosphere and initiates acascade of secondary effects associated with atmosphericwaves and their filtering and the transport of radiatively
active constituents. As noted in more detail below, manyof these effects are non-linear and non-local and point tothe complex and active character of the terrestrial part ofthe solar-terrestrial system.
The zeroth-order energy balance between solar radiation incident on the Earth and outgoing radiation fromthe Earth is well known and covered in most atmospheric texts. More complex considerations of the natureof this balance for the lower atmosphere can be foundin review papers (Trenberth et al. 2009; von Schuckmann et al. 2016) where the effects of wind and current
systems and eddies are also quantified. Thermodynamic
considerations, involving the analysis of the total energyof the atmosphere (the sum of the internal and potential) relative to the energy of an isothermal reference
atmosphere, provide some constraints on the partitioningof the energy. Recent analysis along these lines (Bannon 2013) shows that the percentage of the total energyof the atmosphere (2.5 Giga Joules) available for atmospheric motion (termed available potential energy) is asmall percentage (< 1%) of the total energy although thesemotions are responsible for much of the meridional transport of energy toward the poles. This implies that muchof the absorbed solar energy is devoted to maintaining
the internal and potential energy of the atmosphere witha small amount being available for motion. In his analysis, Bannon (2013) suggests that the rate of conversion ofavailable energy to kinetic energy is ∼ 5-7 W m−2 . While
this is small relative to the solar constant, 1361 W/m2 , itplays an essential role in the terrestrial response to solar

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

radiation, as it is associated with the poleward transport
of energy and the global distribution of radiatively active
constituents. An analysis along these lines, which identifies the vertical variation of the partitioning of the incident
energy, appears not to have been undertaken to this point,
although Kwak and Richmond (2017) have described therelative contributions of momentum forcing and heating in the lower high-latitude ionosphere, albeit withoutcomplete consideration of upward coupling.
Figure 1 summarizes the relationships between thecomponents involved in establishing the large-scale structures of the middle atmosphere. The large-scale structuralelements are in green, and the processes influencing theirform are in blue. The complexity of these relationships is
illustrated by considering the effects a shift in the thermal structure might have. Through thermal wind balance,
the zonal wind structure would alter. These shifts in thethermal structure and zonal wind structure would modify
the wave propagation conditions and location where thewaves dissipate. This modification in dissipation location
would induce secondary circulations and feedback into
the thermal and wind structures. Changes to the winds
would also affect the transport of constituents and possibly their distribution. Any effects that this transport might
have on the distribution of radiatively active species would
alter the heating, further feeding back into the thermal
structure. The full impact of the initial change in thermal structure requires consideration of all these potential
adjustments simultaneously. Changes to any other element would result in a similar cycle.

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Figures 2 and 3 are overview figures which provide acontext for the processes and phenomena discussed laterin this paper. Figure 2 is a schematic of the drivers of
the coupling processes noted above. The solstice zonal
mean temperature and wind structures are backgroundfields in the figure with the temperatures being the color
background (blue being cooler and yellow warmer)and thewinds, the contouring (solid green, eastward; dashed blue,
westward). The three regions of most significant solar
radiative heating are indicated through the white boxes,
the wave processes by orange boxes and black arrows, andthe energetic particle precipitation by green boxes andarrows. The induced residual circulation is indicated by
the white arrows. This figure provides an indication of thespatial relationships between the various processes.
Figure 3 presents the phenomena important to coupling
processes in the MALTI. It includes phenomena relevantto the observation of these processes as well as driversof variability which provide the means to empiricallyinvestigate coupling processes. These are all mentioned
later in the paper. Also included is a summary of various ground-based and satellite observation types with theheight ranges over which observations are provided. The
background to the figure again is the solstice zonal mean
temperature. Drivers of atmospheric variability are identified by ovals and include sudden stratospheric warmings (SSW), the quasi-biennial oscillation (QBO), and El
Niño-Southern Oscillation (ENSO). The height range of
their influence is indicated by the vertical white arrows.
These phenomena cause variations in the large-scale wind

Fig. 1 Links in the middle atmosphere. A schematic showing the relationship between various components affecting the large-scale structure of
the middle atmosphere. The main elements appear in the green boxes, and the processes linking them appear in the blue boxes. The arrows
indicate the direction of the influence. Although thermal wind balance links the zonal mean temperature and wind structure, wave and constituent
fields affect the heating and momentum deposition leading to these structures and themselves depend non-linearly on these large-scale structures

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

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Fig. 2 The main drivers for coupling from above and below. The induced residual circulation is presented by white arrows. Black arrows represent
the wave forcing and tides, as labeled. The color background represents the zonal mean temperature structure with blue indicating the coldest
regions and yellow the warmest regions. The contour lines represent the zonal mean zonal wind structure (yellow is positive, i.e., eastward wind;dashed blue is negative, i.e. westward wind; black is the zero wind line). The radiative processes are in white, the elements of wave driving in orange,and downward particle precipitation in green

and/or temperature fields and their variability which allow
the strength and nature of the coupling processes to beinvestigated. Polar mesospheric clouds (PMC) appear in
the summer upper mesosphere and are the result of upwelling over the summer pole associated with the induced
residual circulation. The photochemistry involves thecycling of constituents from the well-mixed lower atmosphere into the thermosphere where they are dissociatedand then diffuse downward to the mesosphere where they
recombine to form the original molecular species. The
cycling between molecular and atomic oxygen is particularly important for the formation of the airglow which
provides an important means of probing the dynamics ofthe mesopause region. Ozone is a constituent whose distribution depends on photochemistry and transport. The
downward transport of NOx which affects ozone chemistry is one of the significant mechanisms of downward

coupling. The aurora is directly associated with downward
fluxes of electrons along field lines and hence a signature of downward influence. The ionospheric dynamo andplasma bubbles are ionospheric phenomena whose form
is influenced by upward propagating waves.
The main terrestrial-associated solar-terrestrial coupling components, namely energetic particle precipitation, solar irradiance absorption, and coupling associated with waves and constituent transport, are summarized briefly below. This serves as an introduction to themore detailed review of recent progress which follows.
Of importance during the ROSMIC project was facilitating work which helped clarify the role and importanceof these various processes in the atmosphere/ionosphereresponse to solar variability. Also of importance was work
being undertaken to model and assimilate space weather
effects on the ionosphere and thermosphere. This topic is

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Fig. 3 Overview of the MALTI phenomena. The schematic summarizes the phenomena/processes of importance to the coupling processes in the
MALTI as well as ground based (left side of figure) and satellite observation techniques (right side of figure) and their altitude extent. Ovals identify
significant sources of internal atmospheric variability, and squares, phenomena of importance. Vertical arrows represent the height range, andhorizontal arrows, the latitude range over which the phenomena/processes have influence

not reviewed in this paper, but the interested reader canfind a review of the current capabilities in Scherliess et al.
(2019).
2.1 Energetic particles

Energetic charged particles, mainly electrons and protons from the Sun and the Earth’s magnetosphere, deposit
energy into the atmosphere, particularly in the polar
regions (see Fig. 2), where the particles are guided by
the Earth’s magnetic field. Solar protons, ejected from theSun, typically have energies between about 1 MeV to a few
hundred MeV and have direct access to the high polar latitude middle atmosphere during solar proton events (SPEs)
as shown in Fig. 4 (Seppälä et al. 2014). This range ofenergies means that they can directly impact the altitudes
from the stratosphere to mesosphere. Energetic electrons,with auroral electrons with energies less than 10 keV, andmedium energy electrons with energies from tens of keV
up to a few MeV, originate in the magnetosphere and as aresult of magnetospheric processes, enter the atmospherenear the auroral ovals (as shown in Fig. 4). Note that
we will not explicitly cover galactic cosmic rays (GCR),

which mainly impact the troposphere; here, the reader
is directed to the recent comprehensive review article byMironova et al. (2015).
The process of all these particles interacting with theatmosphere is known as energetic particle precipitation,
or EPP. As a result of EPP, whether of protons or electrons, ionization is enhanced, leading to production of
odd-hydrogen (HOx = H + OH + HO2 ) and odd-nitrogen(NOx = NO + NO2 ) in the middle atmosphere. As
these are both known catalysts in ozone loss reactions,
solar influence via variations in EPP levels has implications to both atmospheric chemical and radiative balance
hence potentially affecting surface climate via downward
coupling of introduced wind anomalies (Seppälä et al.
2013). HOx is chemically short lived and thus so are
its atmospheric impacts. In contrast, NOx produced by
EPP (known as “EPP-NOx ”) in the middle atmosphere ismainly destroyed by photolysis and so has a long chemical lifetime during polar winter. It is therefore subject totransport by the downward circulation over the winterpole. One of the challenges with estimating EPP effects isthat, while good observations on proton fluxes and energy

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

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Fig. 4 Sources of EPP. The so-called medium energy electrons (MEE, > 10 keV to several MeV) originate in the Earth’s outer radiation belt, and theirfluxes into the atmosphere are understood to be controlled by magnetospheric wave-particle interactions. Auroral electrons (< 10 keV) originate
primarily from the plasma sheet which is located in the tail/night side of the magnetosphere, while solar protons (not included in the top image) areinjected as a result of solar storms. The map in the bottom panel illustrates that the different types of particles can influence slightly different
geographic areas when they enter the atmosphere (note that locations are only indicative and are influenced by the particle energies). While theNorthern Hemisphere is shown here, precipitation also occurs in the Southern Hemisphere. For detailed information, see Matthes et al. (2017)

spectra during the large but sporadic SPEs exist, the limitations in electron observations have long hindered goodcharacterization of both flux and energy spectrum of precipitating electrons. Unlike solar radiation, there is nota clear relation to the 11-year solar cycle, and the roleof magnetospheric processes (such as those addressed inthe VarSITI SPeCIMEN project) in electron precipitation
provides another challenge in their quantification.
2.2 Solar irradiance variability

Solar radiation is the dominant energy input to the Earthsystem with about 70% absorbed by the atmosphere, land,and ocean, and the remainder scattered and reflectedback to space (L’Ecuyer et al. 2015). This energy partiallydetermines the temperature and structure of the atmosphere and warms the Earth surface. Globally, a delicate
balance is maintained between incoming solar radiation,

the Earth’s albedo (fraction of radiation reflected backto space), and outgoing long-wave infrared radiation thatcan be altered by greenhouse gases, clouds, and aerosols.
Changes in solar irradiance have both direct and indirect effects on the Earth climate system, and the rolesof solar irradiance are evident in many climate records(e.g., Solanki et al. (2013); Ermolli et al. (2013); Lean etal. (2005)). There are two dominant direct solar heating
effects in the lower atmosphere: the “bottom-up” mechanism is the solar irradiance warming the Earth’s surface
and upward coupling, and the “top-down” mechanism
whereby solar ultraviolet radiation absorbed in the stratosphere couples downward. The winds and circulationsin the atmosphere and oceans invoke complicated feedbacks that introduce non-linear and non-local responses(e.g., Meehl et al. (2008)). The solar radiation is also critically important in the Earth’s upper atmosphere. The solar

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

vacuum ultraviolet (VUV: shorter than 200 nm) photons
originate in the Sun’s chromosphere, transition region,and corona and deposit their energy in the Earth’s ionosphere, mesosphere, and thermosphere, being regions
strongly influenced by space weather events like solar
flares.
A well-known example of the non-linear response of
the atmosphere to solar forcing (and an example of how
the atmosphere modulates solar forcing) is what has beentermed the natural “thermostat” effect (Mlynczak et al.
2003). During periods of enhanced geomagnetic effects(coronal mass ejections and particle precipitation) andvariations in solar irradiance, the production of NO andthe associated near-infra-red cooling is enhanced. As
these events are also associated with enhanced heating of
the atmosphere, the enhanced cooling serves to reduce thetemperature response of the lower thermosphere to these
inputs. The extent to which this process influences thecooling was described using 12 years of SABER observations by Mlynczak et al. (2014). Recent refinements to ourunderstanding of this cooling process appeared in studies
by Knipp et al. (2017) and Nischal et al. (2019).
The solar irradiance varies on all time scales with the
key variations being the 11-year solar activity cycle (22year magnetic cycle), the 27-day solar rotation variability,
and short-term (seconds to hours) variability during solar
flares. The solar variability is highly dependent on wavelength, which is related to the source regions in the solar
atmosphere. The total solar irradiance (TSI: integrated
over all wavelengths) and the solar spectral irradiance(SSI) in the near ultraviolet (NUV, 300–400 nm), visible (VIS, 400–800 nm), and near infrared (NIR, 800–
3000 nm) comprises the bulk of the solar radiative energy
and vary the least amount at about 0.1% over the 11-year
solar cycle and typically have less variability for shortertime scales (e.g., Woods et al. (2018); Lean et al. (2005)).
These NUV-VIS-NIR wavelengths are most important forclimate studies for assessing the influence of the Sun on
Earth and for comparison with other natural processes(such as volcanic eruptions and the El Niño Southern
Oscillation) and human activity (such as greenhouse gas
production from fossil fuel combustion). The SSI is much
more variable in the ultraviolet (UV) ranges: middle ultraviolet (MUV; 200–300 nm), far ultraviolet (FUV; 120–
200 nm), extreme ultraviolet (EUV; 10–120 nm), and soft
X-rays (SXR, 0.1–10 nm) but involves only 1.5% of thesolar radiative energy. The solar cycle variability in the UV
is less than 15% for MUV wavelengths, about a factor of
two for many EUV and FUV wavelengths, and even to factors of more than a 1000 for most SXR wavelengths (e.g.,
Woods et al. (2018); Woods and Rottman (2002)). TheseUV ranges are most important for atmospheric research
and for space weather studies and applications involving
satellite operations, communications, and navigation.

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2.3 Transport and upward coupling through waves

The details of the chemistry and dynamics underlying thezonal mean structure of the stratosphere and mesosphereand its variability have been recently reviewed by Baldwin
et al. (2018). Much of the structure of the middle atmosphere is due to a combination of radiative forcing andtransport of momentum and constituents by mean flows(i.e., the residual circulation) and mixing which are driven
in part by a broad spectrum of atmospheric waves. In
the middle atmosphere, the combined effect of the residual circulation and mixing is now starting to be termed
the Brewer-Dobson circulation (Baldwin et al. 2018). The
current consensus is that the Brewer-Dobson circulation
will be enhanced as a result of climate change. However,
because of the complexities of the linkages between the
different components identified in Fig. 1, exact drivers of
this change remain difficult to determine (Butchart 2014).
The situation in the upper mesosphere and lower thermosphere remains less studied and understood. Recent
advances in our knowledge of sudden stratospheric warmings reveal that the various regions of the atmosphere areclosely coupled. The associated wave dissipation in thepolar stratosphere not only influences the state of the middle atmosphere but drives global changes to the state of
the thermosphere and ionosphere (Pedatella et al. 2018a).
Nevertheless, details of the mechanisms which maintain the average conditions above the stratopause remain
poorly understood. Wave dissipation in the mesosphereis considered important for the cycling of air throughthe stratosphere and mesosphere and associated age of
air calculations (see for example Kovács et al. (2017)).
The cycling of constituents and associated photochemistry across the mesopause region is still being examinedin terms of global temporal means (Garcia et al. 2014;
Gardner et al. 2019; Swenson et al. 2018) as opposed tozonal means or regional averages. There is indirect evidence of a wave dissipation-driven, lower thermosphere
winter pole to summer pole circulation (opposite in direction to the better known residual circulation in the middle
atmosphere) (Qian et al. 2017; Qian and Yue 2017) which
provides some indication of zonal mean structures atthese heights. Apart from this, there is little informationon the seasonal variation and latitudinal structures of constituents and dynamics at these heights. This remains animportant topic for future research.
The source of the wave activity is predominantly inthe troposphere. As the waves propagate upwards, theyinteract with the mean wind and temperature fields,
which influence their propagation and dissipation, and in
turn, the stress associated with wave dissipation modifies the global fields. These wave processes are therefore
non-linear and non-local. The nature of the wave field
as a function of height was explored in detail in general circulation models of sufficient resolution and heightWard et al. Progress in Earth and Planetary Science(2021) 8:47

(Shepherd et al. 2000; Koshyk and Hamilton 2001; Hamilton et al. 2008). They noted that the rotational components dominated the kinetic energy spectrum lower in
the atmosphere with the divergent component becoming stronger with height and in the mesosphere becoming
as strong as the rotational component. The divergent
component is associated with atmospheric tides and gravity waves. Their increased relative importance above thestratopause is reflected in the extensive literature devoted
to exploring gravity wave effects throughout the middle
and upper atmosphere (and references therein Liu et al.
(2014b); Yiğit and Medvedev (2015); Liu et al. (2017);
Becker and Vadas (2018); Miyoshi and Yiğit (2019)). Of
particular interest in these investigations is the analysis of
SSWs as they provide the means to investigate structuraldifferences in wave forcing and propagation for conditions
which deviate significantly from normal (Manney et al.
2009; Goncharenko et al. 2013; Ern et al. 2016; Siddiqui
et al. 2017; Pedatella et al. 2018b). The character of the
waves penetrating and affecting the ionosphere and thermosphere is a topic that is of significant current interest
and continues to evolve. The waves of interest and someof their characteristics are summarized by Liu (2016), andthe density variations associated with various atmosphereand ionospheric phenomena are summarized in Liu et al.
(2017).

3 Coupling from above
This section will focus on the aspects of coupling fromabove via solar forcing on the atmosphere and climate
system. We will address EPP and solar irradiance separately. As there have been significant advances on themodeling capability that impact both, we will first address
the latest progress in modeling during the VarSITI period
and briefly discuss some remaining challenges. In thiscontext, the earlier works of Gray et al. (2010) and
Seppälä et al. (2014) outline the progress on the topic
of solar influence on climate, as a result of previous
SCOSTEP science programs. Many of the data sets important to this area are from satellite missions that are
no longer active. The lack of planned future missions,
important for validating models and new ideas on thenature of this coupling, is of considerable concern to thecommunity.
The key questions initially identified by the Working
Group on Solar Influence on Climate were as follows:
1. Drivers of solar variability: How well do we know
their magnitude and variability?
2. Mechanisms and coupling processes: How is thesolar signal transferred down to the troposphere andsurface?
3. Solar influence on climate: What are theuncertainties in establishing the long-term effect?

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3.1 Progress and challenges in modeling

Various advances have been made in both improving
modeling capability and identifying current limitations
and future requirements. Reviews that contain details andinsights to atmospheric modeling have recently been published by, e.g., Baldwin et al. (2018); Maher et al. (2019).
One issue of importance to modeling is the influenceof solar UV irradiance on climate variability. Chiodo andPolvani (2016) addressed the limitations arising from theassessments of this influence when interactive (coupled)
stratospheric ozone chemistry is not included. Model
integrations with coupled chemistry are computationallyexpensive. For example, the WACCM (Whole Atmosphere Community Climate Model) model has a throughput of 7.5 simulated years/day on the US–based Yellowstone supercomputer (Smith et al. 2014) with interactive
stratospheric chemistry. This is increased to 14.8 simulated years/day with specified (non-interactive) chemistry with the same number of CPUs (central processingunit), nearly a doubling of performance. Thus, for better computational efficiency, interactive chemistry is often
omitted. Chiodo and Polvani (2016) showed that inclusion of interactive stratospheric ozone chemistry reduces
the surface warming signal from solar irradiance significantly, by a third, when contrasted to predictions from
non-interactive (specified) chemistry. They conclude thatmodels that do not take into account the responses instratospheric chemistry are likely overestimating the surface level response to solar variability.
Another issue of importance is the credibility ofmodel representations of the downward coupling of EPPinduced chemical responses. The multi-model, multisatellite intercomparison work of Funke et al. (2017)
examined how well various medium-top (model top lid
at about ∼ 80 km) and high-top (model top lid above
120 km) models performed in this capacity. They examined the ability of these models to reproduce polar downward transport of high-altitude carbon monoxide andodd nitrogen (NOx = NO + NO2 ) as well as observed
middle atmosphere temperatures during a dynamically
perturbed Northern Hemispheric (NH) winter. The models performed reasonably well until the polar vortex was
disrupted by a SSW event. These events frequently occur
in the Northern Hemisphere causing major disturbances
to the dynamical state of the atmosphere. They lead tothe mixing of air masses and, sometimes, in the recovery phase, a reformation of the stratopause at typicallymesospheric altitudes. These latter events are known aselevated stratopause (ES) events and can result in effective transport from the mesosphere and lower thermosphere (MLT) into the stratosphere (see also discussion in
Section 4).
The descent of air masses as the newly formed
stratopause moves downwards to 50 km is challenging forWard et al. Progress in Earth and Planetary Science(2021) 8:47

models to reproduce, with most simulating a too rapid
return of the stratopause to its typical height. Funke et al.
(2017) determined that model improvements are neededto address the lack of dynamical representation of ES
events. To date, the closest representation of the downward descent following an ES event has required either
relaxing (“nudging”) the model toward assimilated meteorological fields up to about the altitude of 90 km (Siskind
et al. 2015), or the more self-consistent approach of using
data assimilation in a chemistry climate model such as
WACCM-DART (Pedatella et al. 2018b). This approachis only possible for times when meteorological observations (and thus assimilated products) are available and themodels can be constrained to observed dynamics. The
model improvements called on by Funke et al. (2017)
have not been fully implemented and remain necessary
to improve the dynamical capabilities of atmospheric andclimate models.
Meraner et al. (2016) found that the parameterizationof non-orographic gravity waves played an important rolein improved representation of the MLT to stratospheredescent following ES events in the Hamburg Model of the
Neutral and Ionized Atmosphere (HAMMONIA). Theirresults suggest that improvements to gravity wave parameterizations may need to be made to simulate realistic
downward transport brought on by ES events. As theES events can bring MLT air, rich in EPP-NOx , into thestratosphere, Randall et al. (2015) further highlighted theneed to have a realistic representation of energetic particle precipitation (EPP) to bring down sufficient amount ofNOx into the stratosphere.
The missing EPP contributions and the associatedunderestimation of EPP-NOx in the polar mesospherehave lead to two significant developments in climate modeling during VarSITI. Firstly, we now have the first longterm energetic electron precipitation (EEP) dataset for
use in climate modeling. This is now incorporated into
the recommended solar irradiance and energetic particle forcing for CMIP6 simulations (Matthes et al. 2017)
and will be addressed further later in the Section 3.2.
The second major improvement is the development of
the first fully coupled climate model with comprehensive
lower ionosphere (D-region) ion chemistry (307 reactionsof 20 positive ions and 21 negative ions) (Verronen etal. 2016). This version of the WACCM model, the wholeatmosphere component of the Community Earth System
Model (CESM), called WACCM-D enables detailed studies of the global lower ionosphere and its response toexternal (e.g., solar) and internal (e.g., dynamical) forcing.
WACCM-D includes fully interactive chemistry, radiation, and dynamics. For the purposes of EPP studies, theinclusion of detailed ion chemistry leads to improved representation of production of NOx and HOx gases, byremoving the need for parameterizations, and the re-Page 10 of 38

partitioning of nitrogen compounds (Andersson et al.
2016; Funke et al. 2017; Orsolini et al. 2018).
There has been an increased interest in the role of thestratosphere in tropospheric variability and the potential for improved predictions. While it still remains to
be determined if stratospheric influence is relevant fordaily weather forecasts (Baldwin et al. 2018), growing
evidence is supporting inclusions of stratospheric information to produce skillful seasonal forecast from a few
months to up to a year ahead (Thiéblemont et al. 2015;
Dunstone et al. 2016). There are further indications thatthe ability to simulate stratospheric variability, whether inozone, SSW occurrence or large-scale modes such as thequasi-biennial oscillation (QBO) leads to improvements
elsewhere in the atmosphere/climate system (Baldwin etal. 2018). For the top-down coupling, this would suggest that mesospheric and thermospheric chemical anddynamical processes contributing to stratospheric variability also need to be investigated—one of the questions
to be addressed in PRESTO (Shiokawa and Georgieva2021) is the predictability in sub-seasonal to decadal variability for the atmosphere and climate.
3.2 Energetic particle precipitation

The widespread shift from SPE focused, event type studies(e.g., Funke et al. (2011)), toward more general consideration of EPP impacts on the atmosphere that we saw during
the CAWSES-II program (Seppälä et al. 2014) has continued during VarSITI. The current consensus is that EPP,in the form of EEP, modulated by magnetospheric activity, is an important driver for the chemical variability ofthe polar middle atmosphere. This needs to be captured inchemistry-climate models in order to correctly represent
natural polar ozone variability. Figure 4 illustrates some of
the key differences in the sources and impact areas of SPEsand different types of EEP.
In particular, Stone et al. (2018) highlighted the need toaccount for SPEs when evaluating the recovery of stratospheric ozone due to chlorofluorocarbons (CFCs). SPEs
could have both direct (in situ) and indirect (via transported EPP-NOx ) influences on stratospheric ozone. Denton et al. (2017; 2018) analyzed ozonesonde data fromthe NH polar region and found that, once seasonal background variability was taken into account, lower stratospheric ozone (altitudes below 35 km) was reduced inexcess of 30 days from the start of the SPEs, with anaverage depletion of 5–10%. However, these results wererecently disputed by Jia et al. (2020).
As SPEs are only a fraction of the total EPP, it is likelythat further improvements on simulated ozone variability on decadal scales would arise from inclusion of EEP.
This has been partially addressed by the development of
the first EEP proxy model for inclusion of ionization fromthe so called “medium energy electrons” (with energies up

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

to 1 MeV) in climate models (van de Kamp et al. 2016;
Matthes et al. 2017; van de Kamp et al. 2018). As mentioned earlier, this is now the official input to CMIP6
model simulations going into IPCC AR6. Overall, the first
EEP proxy is an underestimation of the total electron precipitation, and recent improvements to EEP observations(Peck et al. 2015; Nesse Tyssøy et al. 2016; Clilverd etal. 2017; Oyama et al. 2017; Nesse Tyssøy et al. 2019)will likely further aid our understanding of variability inthis type of solar forcing into the atmosphere and lead toinclusion of more accurate levels of EEP in model studies(Smith-Johnsen et al. 2018).
To test the van de Kamp et al. (2016) EEP model, Andersson et al. (2018) ran simulations with the WACCM model
to investigate the impact of inclusion of electron precipitation on middle atmosphere chemistry. They foundthat, on average, mesospheric ozone was reduced by up
to 20% by inclusion of the new EEP forcing, while in thestratosphere, there was an additional 7% ozone loss whencontrasted to simulations without EEP. They further notedthat on solar cycle scales, the inclusion of EEP in WACCM
doubled the stratospheric ozone response. These results
can be contrasted to the multi-satellite observations presented by Andersson et al. (2014), who found that on
solar cycle timescales, EEP can drive mesospheric ozone
variations of up to 34%.
In their multi-satellite observational study, Damiani
et al. (2016) showed evidence that EPP in general has
influenced Antarctic upper stratospheric ozone since
1979, at a level of 10–15% O3 depletion on a monthly
basis. This is slightly more than found by Fytterer et al.
(2015), who estimated Antarctic ozone depletion of 5–
10% using observations from a different set of satellite
instruments. Using two independent chemistry-climate
models (CCMs), Arsenovic et al. (2019) and Pettit et al.
(2019) showed that only by including EEP were models
able to reproduce stratospheric ozone anomalies foundin satellite observations. A number of further studies(e.g., Zawedde et al. (2016); Smith-Johnsen et al. (2017);
Smith-Johnsen et al. (2018); Newnham et al. (2018)) havedemonstrated EEP impacts on both HOx and NOx gases.
Together, these suggest that there is need to improve
the EEP forcing (the current EEP proxies are known tounderestimate precipitating levels, see Nesse Tyssøy etal. (2019)) to ensure better representation of ozone variability in the middle atmosphere. A wider source of EEP
are also likely to influence ozone variability in the mesosphere. Seppälä et al. (2015), Turunen et al. (2016), andSeppälä et al. (2018) investigated the potential impacts on
mesospheric ozone levels from auroral substorms (which
are known to occur frequently), pulsating aurora, and themore energetic relativistic electron microbursts, respectively. They found all to influence ozone levels by up totens of percent, but thus far, this has not been verified

Page 11 of 38

from observations, nor is it clear to what extent thesetypes of electron precipitation are included in the existing
EEP proxy (Matthes et al. 2017).
To capture the descent of MLT NOx into the stratosphere, Funke et al. (2014b) analyzed 10 years of satellite observations from the Michelson Interferometer for
Passive Atmospheric Sounding (MIPAS) instrument onboard the Envisat satellite. The instrument simultaneously measured several different middle atmosphere constituents, enabling the extraction of purely EPP produced
reactive nitrogen, NOy , from the record. By the end of
the polar winter/start of spring, the descent of NOy into
the stratosphere was observed to reach altitudes of 30 km
and below in the NH and 25 km and below in the SH
during the time period studies. The results highlighted
the asymmetries between the two poles arising from thevery different dynamical conditions controlling the atmosphere in the north and the south: In the SH, Funke etal. (2014b) show a steady annual descent of EPP-NOy
from the mesosphere to the stratosphere inside the stable SH polar vortex, with the overall amount of NOy
depending on the solar cycle and level of geomagnetic
activity. In the NH, not only did the overall levels of NOy
depend on the solar and geomagnetic activity, but also
the large dynamical variability present in the polar atmosphere. SSW/ES events had a significant impact on thedescent rates as well as the amount and timing of the
NOy reaching the stratosphere. Funke et al. (2014a) use
these EPP-NOy observations to derive the total amountof NOy molecules in the polar atmosphere reaching altitudes below 70 km. They found a nearly linear correlation
between the amount of NOy and the geomagnetic Ap
index for the winter period extending to early spring, confirming earlier works by Randall et al. (2007); Seppälä etal. (2007), with the NH again showing large responses todynamical perturbations.
Based on these findings from the MIPAS data set,
Funke et al. (2016) presented a semi-empirical, a p-driven,proxy model for EPP-NOy descending though mesosphere and upper-stratosphere. The model provides aseasonal dependent flux of EPP-NOy descending throughgiven vertical levels. One of the main uses of this semiempirical model is as an upper boundary condition (UBC)
for chemistry-climate models to emulate the descent of
NOy rich air from the MLT region. For CMIP6, the recommended way of taking into account production of
EPP-NOx above the upper boundary of medium-top models is by employing the Funke et al. (2016) UBC (Matthes
et al. 2017). Figure 5 outlines the different approaches ofinclusion of sources of EPP in model simulations.
Following from the work of Funke et al. (2014b), Gordon (2020) recently showed evidence that not only can
EPP-NOx reach altitudes of 25 km and below by the endof the Antarctic polar winter, but once in the stratosphere,

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

Page 12 of 38

Fig. 5 CMIP6 recommendation for including EPP in model simulations. The different approaches for auroral energy electrons (< 10 keV), medium
energy electrons (MEE), and solar proton events (SPE). For details, see Matthes et al. (2017)it remains in the stratospheric NOx column throughout
the spring until breakup of the SH polar vortex. They
found that the role of EPP in the springtime Antarctic stratospheric NOx column is modulated by the QBO.
They proposed that as CFCs are reduced, EPP-NOx will
likely take a larger role in Antarctic springtime ozone loss
processes.
As we see from above, there is a growing body of evidence that EPP plays an important role in atmosphericchemical balance, extending to decadal scales. Including
this source of variability in medium-top and high-top
models is now possible for the first time due to development of the CMIP6 EPP forcing input and the EPP-NOx
UBC (Matthes et al. 2017), but further improvements arestill needed. Questions remain unanswered at the end of
the VarSITI program: What is the exact size of the EEPNOx source and how important is overall EPP-induced
chemistry to climate variability?

While the latest meteorological re-analysis investigations (e.g., Maliniemi et al. (2018); Salminen et al. (2019))
continue to suggest that there are important implications
to atmospheric dynamical variability on decadal scales,
the results from simulations with a range of CCMs are
somewhat inconclusive (Arsenovic et al. 2016; Meranerand Schmidt 2018; Sinnhuber et al. 2018), particularly
when it comes to relevance of EPP forcing to surface levelclimate variability. However, importantly, results from thelatest model studies by Arsenovic et al. (2016), and themulti-model study by Sinnhuber et al. (2018) are in agreement with earlier model investigations (Baumgaertner et
al. 2011), and also with earlier meteorological reanalysis
studies (Seppälä et al. 2013). They found that inclusionof EPP resulted in changes in polar winter stratospherictemperatures (Arsenovic et al. 2016) or radiative heating patterns (Sinnhuber et al. 2018). The Sinnhuber et al.
(2018) study further found that state-of the-art models

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

accounting for EPP are able to reproduce observed chemical effects. The introduced radiative forcing changes in thepolar stratosphere are of a similar order to those caused
by UV variability in the tropics, however, with alternating
sign in mid winter (polar night) and spring.
Recently, Maliniemi et al. (2020) addressed the benefitof accounting for the full description of the CMIP6 solar
forcing (both irradiance and EPP) in future projections. By
looking at the amount of NOx descending into the stratosphere, they were able to conclude that climate change will
likely increase the EEP-related atmospheric effects toward
the end of the century.
3.3 Solar irradianceThis section will outline the recent progress in theunderstanding of variability in solar irradiance, in bothPage 13 of 38

long-term (solar cycle scales) and short-term, models of
solar variability, and, finally, the atmospheric and climate
impacts.
3.3.1 Long-term variability—solar cycle

The nearly periodic 11-year solar cycle is manifested by
the regular presence of numerous large sunspots duringsolar maximum and few, if any, sunspots during solar minimum. These sunspots are the result of magnetic activity
rising up through the Sun’s photosphere. Three decadesof space-based research document dramatic increases insolar photon, particle, and plasma energy output accompanying the increase in sunspot numbers. Figure 6 showsthat in the recent solar cycles 23 and 24, the TSI increased
during the maximum by about 0.14% relative to solar minimum and that the SSI H I Lyman-α emission at 121.6 nm

Fig. 6 Solar variability over solar cycles 23 and 24. Examples of solar variability over solar cycles (SC) 23 and 24 for a the total solar irradiance (TSI) andb the bright ultraviolet H I 121.6 nm Lyman-α emission. The SORCE TSI observations shown here are being extended with TSIS-1 observations.
Important solar UV irradiance records over solar cycle (SC) 23 and 24 have been established with TIMED SEE observations, and the TIMED SEE andSDO EVE observations will be extended into the next solar minimum and cycle 25. These extensions are particularly important for overlapping with
the new GOLD and ICON missions to observe the ionosphere-thermosphere during cycle 25

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

increased by about a factor of 1.7 at maximum relative
to minimum. It is also important to note the differencein cycle maximums. In particular, the ratio of the solar
variability, defined as maximum minus minimum, at cycle
24 maximum to the variability in cycle 23 is 0.84 for TSI
and 0.65 for Lyman-α. Moreover, cycle 24 maximum has
proven to be the weakest during the past 90 years.
As described earlier, the SSI variability is very wavelength dependent. The solar cycle variability for the SSI asa function of wavelength is shown in Fig. 7. These observations are from NASA’s Solar Radiation and Climate
Experiment (SORCE (Rottman et al. 2005)), launched in2003 and also the Thermosphere Ionosphere Mesosphere
Energetics and Dynamics (TIMED (Woods et al. 2005))
spacecraft, launched in 2001. The exact amount of SSI
variability from the SORCE mission is under debate primarily in the visible where the amount of variability is

Page 14 of 38

smaller than the measurement uncertainty and in thenear infrared (NIR) where many wavelengths have outof-phase (negative) variability over the solar cycle (e.g.,Ermolli et al. (2013)). The NASA Total and Solar Spectral Irradiance Sensor (TSIS-1) observations that started
in 2018 are anticipated to address the SSI variability moreaccurately for the visible and NIR ranges.
The TSI at the 2008–2009 minimum also appears to
be lower by 0.2 W/m2 (− 140 ppm) than the previous
minimum in 1996 (Fröhlich 2009); however, the 100 ppm
uncertainty of this TSI change makes this finding lesscertain (e.g., Kopp and Lean (2011)). Observations taken
during the next solar minimum in 2019–2020 will bevery intriguing in terms of determining whether the secular trend of lower solar activity is continuing or not.
There are already some indications that the solar activity could continue to decline as indicated from studying

Fig. 7 SORCE solar spectral irradiance variability. The SORCE solar spectral irradiance variability results from Woods et al. (2015), shown in black, arecompared to the SATIRE-S (blue) and NRLSSI (red) model estimates for solar variability between February 2002 (max) to December 2008 (min). The
top panel shows the relative variability in percent, and the bottom panel shows the absolute variability in irradiance units. The dashed lines areout-of-phase (negative) solar cycle variability results. The irradiance variability (max-min) in broadbands is provided, and those numbers are in unitsof mW/m2 . The total variability is for the 115 to 1600 nm range. This figure is adapted from Woods et al. (2015)

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

the overlapping solar activity bands of the 22-year solar
magnetic cycle (McIntosh et al. 2015). Furthermore, thisnext cycle minimum in 2019–2020 will be characterizedeven better than the 2008–2009 minimum (e.g., Chamberlin et al. (2009); Woods et al. (2009)) with the continuationof the SORCE, TIMED, and SDO missions and the addition of new ionosphere-thermosphere measurements by
NASA’s Global-scale Observations of the Limb and Disk
(GOLD (Eastes et al. 2017)) and Ionospheric Connection
Explorer (ICON (Immel et al. 2018)).
3.3.2 Short-term variability—flares

Solar flares have long been an interest for sudden ionosphere disturbances and their effect on radio communication (e.g., Dellinger (1937)). Flare observations have been
made for decades in the visible, primarily in H-α (e.g., Ellison (1946)), and also in the SXR and EUV ranges fromsounding rocket and satellite experiments (e.g., Friedman (1963)). Hudson (2010, 2011), Doschek and Feldman(2010), Lang (2009), and Aschwanden et al. (2009) provide reviews of recent progress in understanding flaresfrom observations that involve the Solar and Heliospheric
Observatory (SOHO), Transition Region and Coronal
Explorer (TRACE), Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI), Hinode (Solar-B), andSolar TErrestrial RElations Observatory (STEREO) missions. These satellites include imagers in X-ray and EUV
broadbands and imaging spectrographs with high spectral
resolution, but over a limited EUV range.
The new and exciting aspects of solar irradiance observations for flare studies are with the Solar Dynamics
Observatory (SDO) EUV Variability Experiment (EVE
(Woods et al. 2012)). They include spectral coverage overthe full EUV range from 0.1 to 106 nm with 0.1 nm
resolution and the continuous monitoring of the solar
activity at a high cadence of 10 s. There is also a continuous monitoring of the solar EUV images with cadence of
12 s with the SDO Atmospheric Imaging Assembly (AIA
(Lemen et al. 2012)). The AIA image time series permit
physical interpretation of the EVE (full-disk) irradianceflare variability. The EVE flare observations have revealed
that many EUV emissions do not behave like the X-ray
variations that are often used for classifying the flare magnitude and as a proxy for EUV emissions in models such as
the Flare Irradiance Spectral Model (FISM (Chamberlin etal. 2007; Chamberlin et al. 2008)). Therefore, the EVE flareobservations are important to improve the understanding of flare energetics and their impacts on Earth’s spaceenvironment.
Flare events can be studied in detail with EVE’s full-disk
EUV spectral irradiance as long as only one major flare ishappening at a time, which happens frequently. Woods etal. (2011) provide examples of the four major phases seen
during flares with the EVE data. These phases include (1)Page 15 of 38

the impulsive phase best seen in transition region emissions such as He II 304 Å, (2) gradual phase seen in hot
coronal emissions such as the Fe XX/Fe XXIII 133 Å, (3)coronal dimming seen in cool corona emissions such as
Fe IX 171 Å, and (4) EUV late phase (ELP), which has asecond, broad peak one to 5 h after the main flare phases
and seen best in the Fe XVI 335 Å emission. The X-ray
flare classification by the Geostationary Operational Environmental Satellite (GOES) X-Ray Sensor (XRS) is basedon the peak SXR variations during the gradual phase, andthe derivative of the GOES XRS 1-8 Å emission can bea proxy for the impulsive phase, as related to the Neupert effect (Neupert 1968). Prior to EVE observations,flare observations in the SXR and radio wavelengths wereusually decomposed into an impulsive phase with significant non-thermal signatures and a gradual (slow) mostly
thermal phase that follows the impulsive phase (Donnelly
1976; Hudson 2010; Hudson 2011). The coronal dimmingand EUV late phase effects are observable primarily in
the EUV emissions. Each flare can have its own uniquebehavior; some flares have all four of these phases, andsome flares only have the gradual phase (by definition
from the X-ray flare identification by GOES/XRS). For
more detailed information, Hudson (2010) reviews flareprocesses and phases, and Hock (2012) identifies different categories of flares based on the new SDO/EVE andSDO/AIA observations of hundreds of flares. Notably,
the eruptive flares tend to have impulsive phase, gradual phase, and coronal dimming, and some eruptive flaresalso have the EUV late phase. The C8.8 flare on 2010 May
5 is a good example when all four components clearly
exist as shown in Fig. 8. The various EUV emissions haveone or more of these aspects in their time series, and thefour emissions that best highlight each component areincluded in this figure. The EUV spectral variations during
this C8.8 flare are shown in Fig. 9.
[Prior to the SDO mission, the flare irradiance models have been using the GOES X-ray signal as a proxy
for the gradual phase and the derivative of the X-ray signal as a proxy for the impulsive phase emissions. It isclear now with the SDO EVE and AIA measurementsthat at least two additional flare components—(a) coronal dimming for cool coronal emissions and (b) an EUV
late phase for warm coronal emissions—are required for
improved modeling of the EUV irradiance. [While coronal dimming and long-duration events like post-flare giant
arches have been known for some time, their impact on
EUV irradiance is now being clarified with the new SDO
observations. The new EVE results are also very important for many space weather applications as depositionof the solar EUV irradiance into the Earth’s atmospheredepends on the spectral variability and on the timingthat determines the local (regional) effects on Earth. For
example, the ionospheric F layer is expected to have an

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

Page 16 of 38

Fig. 8 Example of flare variations. Flare variations for the C8.8 flare on 2010 May 5 as adapted from Woods et al. (2011). The relative irradiance (Rel.
Irr.), being the solar irradiance spectrum minus the pre-flare spectrum, represents well the flare variations over its different phases. The transition
region He II 30.4 nm emission highlights the impulsive phase. The GOES X-ray (0.1–0.8 nm) defines the gradual phase, and the hot corona Fe XX / FeXXIII 13.3 nm emission behaves almost identically as the X-ray. The cool corona Fe IX 17.1 nm emission is the EUV emission with the largest amountof coronal dimming that often follows after the impulsive phase. The warm corona Fe XVI 33.5 nm emission has its first peak a few minutes after the
X-ray gradual phase peak and then has a second peak many minutes later. The change in slope of the GOES X-ray during the gradual phase isindicative of the late phase contribution (second Fe XVI peak). The four vertical dashed lines, left to right, are for spectra in Figure 9 of the pre-flare,main phase, coronal dimming, and EUV late phase

additional increase one to 5 h after the GOES X-ray peaks
for EUV late phase flares. These late-phase flares can besignificant because they can enhance the total EUV irradiance flare variation by a factor of 40% or more when
the EUV late-phase contribution is included. Furthermore, the occurrence of late-phase flares are clustered
before and after each solar cycle minimum and has a minimum frequency of occurrence during cycle maximum(Woods 2014). Another advance is that the coronal dimming observations can serve as proxies for the mass andvelocity of Earth-directed coronal mass ejections (CMEs)(Mason et al. 2014; Mason et al. 2016).
3.3.3 Models of solar variability

There are several models of the solar spectral irradiancevariability that provide temporal coverage when there aregaps in the observations and also for extending back totimes prior to the advent of satellite solar measurements.
The physics-based solar irradiance models, such as the
Solar Radiation Physical Model (SRPM (Fontenla et al.
2011; Fontenla et al. 2014)), can also provide insight into
the causes for the solar variability. A few of the morecommonly used models for climate studies (e.g., CMIP6)
are the Naval Research Laboratory TSI model (NRLTSI(Lean et al. 2005)), Naval Research Laboratory SSI model(NRLSSI (Lean et al. 2005; Lean et al. 2011), NRLSSI2(Coddington et al. 2016)), and Spectral and Total Irradiance Reconstruction (SATIRE (Ball et al. 2014; Krivova et
al. 2011; Wenzler et al. 2004; Fligge et al. 2000)). Thesemodels provide daily estimates for the solar variability
and also extend back to the Maunder Minimum period,a time of low solar activity in the 1600s. It should be
noted that, as pointed out by Matthes et al. (2017), someof the models show a discrepancy over the last threesolar cycles: while SATIRE predicts a significant downward trend of the baseline SSI at solar minimum, this is
not as apparent in NRLSSI2 (a combinaton of these models is used for CMIP6 recommended SSI and TSI forcing
datasets).
For space weather studies, the models need to be at highcadence of at least once a minute in order to describe flarevariations. The Flare Irradiance Spectral Model (FISM(Chamberlin et al. 2007; Chamberlin et al. 2008)) is oneof the solar irradiance models used for studying flare
effects in the ionosphere and thermosphere (e.g., Qianet al. (2019b)). FISM has also been extended to be usedat Mars (FISM-M (Thiemann et al. 2017)), and similar algorithms as used by FISM have been adopted forWard et al. Progress in Earth and Planetary Science(2021) 8:47

Page 17 of 38

Fig. 9 Flare spectral variations from the EVE MEGS Flare spectral variations from the EVE MEGS A channel (6–37 nm) for the C8.8 flare on 2010 May 5
as adapted from Woods et al. (2011). A The pre-flare spectrum. B–D The variability between the pre-flare irradiance and the main phase, coronaldimming, and EUV late phase, respectively. These results used 5-min averages taken at the times indicated in Fig. 8 as vertical dashed lines

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

NOAA’s operational solar irradiance model (Thiemann etal. 2019).
3.3.4 Influence of solar variability on the atmosphere andclimate system

Representation of variations in solar irradiance, both in
TSI and SSI, as outlined in the above sections, is of highimportance for capturing the influence the Sun has on
the atmosphere and climate system. Not only does itlink directly to radiative heating, but solar radiation (inthe UV) also modulates stratospheric ozone budget (fordetailed descriptions of the processes, see, e.g., (Gray et al.
2010; Matthes et al. 2017)).
Kodera et al. (2016) provide a recent update on understanding of the physical mechanisms for solar radiative
forcing of the Earth’s surface, through coupling of thestratosphere and troposphere, including interactions withthe ocean and sea surface temperatures (SST). They writethat “The role of the ocean as heat storage can be seen as
persistent surface temperature anomalies from winter tospring. In addition, ocean currents advect SST anomalies
to higher latitudes, which may introduce further delayed
response.” Kodera et al. (2016) highlight how zonal mean
zonal wind anomalies extend from the stratosphere into
the troposphere during winter, but these disappear in thespring as the polar-night jet dies out. This is found tocoincide with temperature anomalies diminishing overcontinents, while those over ocean basins (east of continents) are still developing. They also point to the role ofoceanic frontal zones (regions of strong temperature gradients in SSTs and surface air temperatures forming inregions of confluent cool and warm ocean currents) as oneof the key regions where solar cycle related surface level
warming takes place.
The solar minimum between cycles 23/24 was anomalous with regard to its length as well as its depth andthus could provide new insights in the longer term (secular) variations that underlie the 11-year activity cycle.
Satellite drag data indicate that the thermosphere waslower in density, and therefore cooler, during the protracted solar minimum period of 2008–2009, than at anyother time in the past 47 years (Emmert 2009). Satellite measurements indicate that solar EUV irradiance wasalso lower in 2008 than during the previous solar minimum. However, secular change due to increasing levels of CO2 and other greenhouse gases, which cool theupper atmosphere, also plays a role in thermosphericclimate, and changes in geomagnetic activity could also
contribute to the lower density. Solomon et al. (2010);
Solomon et al. (2011); Solomon et al. (2013) conclude that
CO2 and geomagnetic activity play small but significant
roles and that the primary cause of the low temperaturesand densities is the unusually low levels of solar EUV
irradiance.

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The upper atmosphere and ionosphere are influenced
by forcing from above through solar cycle variations, shortterm solar flares, or dissipation of magnetospheric energy.
Within the ROSMIC period, ESA’s Swarm satellite mission (Olsen et al. (2013)) started orbiting the Earth in2013. Due to its nature as a geomagnetic mission, its data
were mainly explored to investigate ionospheric current
systems in the E- and F-region. These were often successfully combined with data from the CHAMP mission(2000–2010) to achieve a multi-year data set.
Many of the results associated with these missions are
relevant to coupling from below (see the Section 4.3
below). However, solar and geomagnetic effects were also
addressed. These have been reviewed in several papers.
Laundal et al. (2018) developed an empirical model of thehigh-latitude E-region and auroral field-aligned currents
and their responses to the direction and strength of thesolar wind for different seasons, during geomagneticallyquiet and active times Alken et al. (2017) and Yamazaki
and Maute (2017) reviewed the current knowledge midand low-latitude currents in the F-region and E-region(the latter Sq and EEJ), respectively, derived climatologicalbehaviors from these long-term observations, and compared and assessed recent model results to simulate thesecurrents.
Maycock et al. (2016) revisited our understanding of thesolar cycle signal in stratospheric ozone, using updated
satellite datasets. They found that on annual scales, thesignal in stratospheric ozone is much smaller than previously estimated, but note potential, substantial, monthly
scale variations. Ball et al. (2019) used new ozone composite datasets to also estimate the solar signal in the upper
stratosphere. They determined a statistically significant“U-shaped” annual mean response (peaking at approximately 3% near the equator) that is likely the imprint of
seasonal response variations. Good agreement was foundwith model simulations using the SOCOL and WACCM
models.
Dhomse et al. (2016) called into question how well weunderstand the response of upper stratospheric ozone
to the 11-year solar cycle. They suggest that there may
be large uncertainties in observations, the impact fromchanging GHG emissions, recent changes in the amplitude of the 11-year solar cycle, and potential challenges
arising from meteorological reanalysis data sets. While, on
annual scales, the solar cycle signal in stratospheric ozone
may be smaller than previously estimated, accounting forthis solar cycle ozone response in global models is important. This was highlighted by Maycock et al. (2018) where
they pointed to the necessity of accounting for the ozone
response to adequately capture solar cycle impacts on theatmosphere. How this is done must be carefully considered, particularly for models that do not have interactive
chemistry.

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

The influence of the 11-year solar cycle on North
Atlantic via the North Atlantic Oscillation (NAO) has
been a topic of high interest, particularly in the past two
decades. This is not least due to the potential increased
predictability of large scale climate patterns influencing
the large metropolitan areas around the North Atlantic
and extending into central Asia. Recently Thiéblemont etal. (2015), Gray et al. (2016), and Roy (2018), found further evidence of the link between the solar cycle and theselarge scale climate patterns, pointing to the role of downward propagation of the solar signal from the stratosphereto the troposphere. However, Chiodo et al. (2019) proposed that the signal in the North Atlantic climate thatseveral previous studies have attributed to the 11-year
solar cycle is, in fact, a chance occurrence, resulting frominternal variability in the climate system. If this is the case,it would limit the usefulness of the 11-year solar cycle asa predictor for large scale circulation. The authors notethat there are several caveats that may affect their conclusions, but nonetheless, call for much needed caution wheninterpreting quasi-decadal signals, such as those from thesolar cycle, in the existing, relatively short, observational
records. In this context, studies providing insights into thedetailed physical mechanisms behind the so called “topdown” coupling are important. investigations such as thatby Lu et al. (2017b); Lu et al. (2017a) who investigate rolesof planetary wave breaking, reflection and resonance inthe propagation of the solar signal from the stratosphereto the troposphere, may be able to provide some further
understanding of the details of the solar cycle signals inclimate.
As discussed earlier, there have been some indications
that solar activity could continue to decline in the next
decades. Changes in the solar input warrant further investigation to assess the potential impact of a potential future“grand solar minimum.” Following initial work by Meehl
et al. (2013), Maycock et al. (2015) and Ineson et al.
(2015) investigated the combined effects of lower solar
activity (0.12% reduction in TSI, 0.85% reduction in UV),
reducing ozone depleting substances and increasing GHG
concentrations. They confirmed that there was negligible impact on global mean surface temperatures (around−0.1 K), which could not offset the projected impactsof anthropogenic climate change. However, the predicted
regional-scale changes were much larger, particularly formid- to high latitudes. Both the polar vortex and thetropospheric midlatitude jet were found to respond to
the reduced solar activity (indicating links to top-down
coupling) resulting in changes in large scale dynamicalpatterns in both hemispheres. These in turn could potentially drive “larger regional scale surface climate effects”(Ineson et al. 2015), highlighting the need to carefully consider changes in both anthropogenic and natural drivers
together when assessing future projections.

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3.4 Future challenges

What will the Sun do in the future? Variations in the 11year solar cycle as well as longer cycles are important for
any future projections of the state of the atmosphere andclimate. Continued observations of solar activity both forradiation and particles is vital for our understanding. At
the same time from the atmosphere side, we are relying
on aging missions to detect changes in atmospheric composition in response to forcing from above and below.
The growing unease in the scientific community has notdiminished during VarSITI: we are heading into the future
relying more and more on models that continue to needa “ground truth” from satellite observations to enable
increased accuracy.
What will be the role of solar forcing for climate in the
future? One of the big challenges we are now facing isunderstanding how the atmosphere and climate response
will adjust due to changes brought on by enhanced GHGs
and climate change, recovery of the Antarctic ozone hole(which will influence both the chemical and dynamical
state of the SH atmosphere), and even the potential future
grand solar minimum—together. Most studies thus far
have focused on the solar irradiance changes but withthe developing EPP proxies we are now seeing the firstevidence that there is benefit in accounting for the fulldescription of the CMIP6 solar forcing in future projections (Maliniemi et al. 2020). On a smaller scale, thequestion on whether there is improved seasonal prediction skill from inclusion of solar activity is showing somepromise.

4 Coupling from below
ROSMIC’s working group “Coupling by Dynamics”
focused on the dynamical and thermodynamical processes originating in the lower and middle atmosphere,
influencing the upper atmosphere. Modeling, observations, and theoretical studies have all been relevant to theinterests of this working group. In addition to the physicalprocesses impacting the atmosphere-ionosphere systemfrom above discussed in the Section 3, upward propagating lower atmospheric processes can shape the structureand evolution of the upper atmosphere. This is generally
referred to as “coupling from below.” The key science questions of the Coupling by Dynamics Working Group havebeena as follows:
1. Wave coupling: What are the influences of lower
atmospheric waves on the state and evolution of thethermosphere-ionosphere?
2. Electrodynamic coupling: How does atmospheric
dynamics constrain electrodynamics in theionosphere?
3. Small-scale dynamics: How can we characterizesignificance of small-scale structures for thelarge-scale features in the upper atmosphere?

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

These science questions have been actively studied by
the solar-terrestrial community and fostered collaborations among atmospheric and space scientists. The vast
majority of the recent studies have indicated that the primary mechanism of coupling from below is via internal
atmospheric waves. As noted in the Overview (see Figs. 1
and 2 for a schematic context), they play a role in the transfer of energy and momentum, the mixing of constituents,
and development of large-scale flows where they dissipate
and atmospheric and ionospheric variability wherever
they are present. Their role in the downward propagation of NOx and the formation of elevated stratopauses
has already been mentioned in the section on downward
coupling.
Overall, the lower atmosphere is the primary sourceof internal waves. At higher altitudes in the atmospherehigher-order waves can be found (Satomura and Sato
1999; Yasui et al. 2018; Becker and Vadas 2018), which
are produced via either primary waves or other dynamical
and thermal processes, such as instabilities and localized
heating taking place in situ. These waves span a broad
spectrum of periods (or frequencies) and horizontal andvertical scales. The key components of the spectrum of
atmospheric waves are gravity waves, tides, Kelvin waves,

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and Rossby-planetary waves, covering a range from a few
tens of kilometers to several thousand kilometers. The
influence of the upward propagating waves can be substantially modulated by lower and middle atmospherictransient processes, such as sudden stratospheric warmings (Pancheva et al. 2008; Pancheva et al. 2009; Tomikawa
et al. 2012; Nayak and Yiğit 2019; Yiğit and Medvedev
2012; Yiğit et al. 2014; Yiğit and Medvedev 2016; Pedatella
et al. 2018a; Yamazaki et al. 2020). These modulations
occur primarily due to the underlying changes in the background atmospheric circulation and temperature, which
directly effect wave propagation and dissipation.
Comprehensive reviews of how upward propagatinginternal waves influence the upper atmosphere and ionosphere have been recently provided in the work of Yiğitand Medvedev (2015) and Liu (2016). Figure 10 adopted
from Yiğit and Medvedev (2015) gives a good overview ofthe different neutral atmospheric and ionospheric layers
and coupling from below. They discuss various theoretical, numerical modeling, and general circulation modelingapproaches to internal waves, as well as observation ofinternal wave activity in the middle and upper atmosphere. In particular, the importance of SSWs has beenhighlighted in their review of wave-induced processes.

Fig. 10 Vertical coupling in the atmosphere-ionosphere system, from the troposphere to the thermosphere-ionosphere. The atmosphere is
influenced from below by internal waves (gravity waves, tides, planetary waves, Kelvin waves) and from above by solar and geomagnetic processes,
referred to as space weather processes. These mechanisms produced wave-like disturbances in the upper atmosphere, known as travelingionosphere/atmospheric disturbances (TIDs/TADs). The thermosphere and ionosphere coexist, interacting with each other in a two-way coupledmanner. Taken from Yiğit and Medvedev (2015)

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

The review by Liu (2016) concentrates on the variability inthe ionosphere and thermosphere associated with waves.
The review by Yiğit and Medvedev (2016) specifically
capitalized on the dynamical and thermal influence of
atmospheric GWs during SSWs. In this context, theyturned their attention to the assumption and limitations to
GW parameterizations in atmospheric models. Observedchanges in the upper atmosphere during SSWs were discussed in detail, emphasizing that more work needs tobe conducted in determining the effects of GWs on thecirculation and thermal budget of the upper atmosphere
during SSWs. The importance of this aspect of gravity
waves was discussed earlier (in the Section 3) in reference to the elevated stratopause and the downward transport of NOx . Modeling efforts with HAMMONIA andother GCMs have also indicated that physics of nonorographic gravity waves must be taken into account fora more realistic representation of the wave-mean flowinteractions and mixing necessary for this downward
transport.
4.1 Modeling of vertical coupling by waves

Numerical models are very powerful in the investigationof basic physical processes, i.e., generation, propagation,and dissipation, of waves and in studying how dynamical
processes associated with waves and other transient processes influence different atmospheric layers. In essence,vertical coupling is the exchange of energy, momentum,
and mass within the atmosphere in the vertical direction. In a stratified atmosphere with nearly exponentiallydecreasing background density, waves grow in amplitudewith increasing altitude in order to conserve energy, withthe growth rate modulated by background atmosphericdissipation. Therefore, models can serve as a tool to study
how waves are generated, how they propagate from theirsource, the altitude variation of their dissipation, and as
a result, their influence on the dynamical and thermal
structure of the atmosphere. In addition, they facilitate
the identification of the altitudes where the various types
of wave perturbations dominate and what their characteristics are. Wave propagation and dissipation are theprimary mechanisms for the coupling between the lower
and upper atmosphere.
While there are a number of different genres of atmospheric models, all focus on providing deeper insight into
the physical mechanisms of either already observed or
yet to be observed phenomena. In doing so, they utilize different analytical and numerical techniques. Model
predictions often motivate observers to organize targeted
campaigns and measurements and observations stimulate
model development and simulations.
One flavor of the modeling of coupling processes isbased on idealized numerical models. These generally
involve simplified background conditions and specified

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wave characteristics and provide conceptual frameworks
useful for the interpretation of observations. The majority of these numerical models focus on small-scale GWs(e.g., Hickey et al. (2009); Hickey et al. (1993); Kshevetskii and Gavrilov (2005); Gavrilov and Kshevetskii (2013)).
Such models are often either one-dimensional or twodimensional and facilitate the examination of the propagation and dissipation of GWs under various circumstances
and at very high-resolution. Earlier studies of the propagation and dissipation of acoustic-gravity waves in thethermosphere (e.g., Hickey and Cole (1988)) have provided significant motivation for recent numerical efforts.
Heale et al. (2014b) have used a nonlinear, fully compressible, two-dimensional numerical model to study theeffects of dissipation on GW packets in the thermosphere.
Such models can also be used to investigate wave propagation over large horizontal distances as a function of
direction of propagation relative to the average modeled
background winds (Heale et al. 2014a). Control simulations conducted in these studies revealed that waves
can undergo varying degrees of reflection, ducting, andtrapping, which ultimately influence the extent of theirhorizontal propagation.
Another type of numerical model is the finite-volume
anelastic model that discretizes the anelastic NavierStokes equations (Lund and Fritts 2012). One advantage
of using finite volume numerical formulation is that they
include exact numerical conservation laws. Fritts et al.
(2018) have used this anelastic model to explore GW
dynamics in the vicinity of mesospheric inversion layers(MILs). They found that MILs can cause GWs to yield
instabilities below the altitude where they would usuallyoccur.
Another type of numerical wave model was used in
the work by Gavrilov and Kshevetskii (2015) to better
investigate the details of dynamical and thermal effects
on the upper atmosphere of waves originating in Earth’s
troposphere. They focused on the nonlinear propagation of GWs, generated with a fixed forcing, to the
middle and upper atmosphere. They demonstrated that
GWs can directly propagate to the thermosphere andalso found out that shorter GWs with slower horizontal phase speeds generate relatively smaller mean heating and wave-induced velocity fluctuations in the upper
atmosphere. Shpynev et al. (2015) emphasized that during strong SSWs, the main stratospheric jet stream canamplify GWs to large amplitudes, which facilitate theirbreakdown. Hickey et al. (2015) have used a two-gas fullwave model to examine the role of various thermosphericdissipation mechanisms on the wave-induced fluctuationsof atomic oxygen and molecular nitrogen. They found thatviscosity and thermal conduction can alter the oscillation of one species relative to another and can impact therelative phases.

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

While in a realistic atmosphere, GWs are generated
primarily by weather related processes and lower atmospheric features, such as convection, orography, jets,
frontal systems, and various localized heating events, idealized numerical models usually represent GWs by a specified initial wave packet (e.g., Gavrilov and Kshevetskii(2013); Gavrilov et al. (2015); Fritts et al. (2018)). On theother hand three-dimensional general circulation models(GCMs) with coarse grid resolutions utilize GW parameterizations, which are analytical representations of GW
physics. Typically, larger scale waves are resolved in themodel provided that their generation mechanisms areaccounted for.
There are various types of gravity wave parameterizations. Some are nonlinear; others are linear schemes (fora discussion see Medvedev and Yiğit (2019)). Some aredesigned solely for the middle atmosphere; some cover
the entire atmosphere. The common purpose of GW
schemes is to represent subgrid-scale GWs, which are
not sufficiently resolved in coarse-grid GCMs. Yiğit et al.
(2014)’s simulation study using the whole atmosphere GW
parameterization (Yiğit et al. 2008) implemented in the
Coupled Middle Atmosphere Thermosphere-2 (CMAT2)
GCM, extending from the lower to the upper atmosphere,
demonstratingthat small-scale GWs produce up to 50% of
the wind variability during a minor sudden stratosphericwarming. This latter study was motivated by earlier GCM
simulation studies demonstrating direct GW propagationfrom the lower atmosphere to the middle thermosphere(Yiğit et al. 2009; Yiğit et al. 2012; Yiğit and Medvedev
2012) and by idealized numerical simulations (Hickey andCole 1988; Gavrilov and Kshevetskii 2013). The wholeatmosphere scheme takes into account the dissipation of
upward propagating GWs due to eddy diffusion, radiative
damping, nonlinear interactions (Medvedev and Klaassen
2000), molecular diffusion and thermal conduction, andion-neutral friction, known as ion-drag. Since gravity
wave processes are uniquitous features of all terrestrial
and planetary atmospheres, GW parameterizations developed for Earth models can also be tested in other planetary models. For example, this scheme has also beenused in a Martian GCM to study coupling between GWs
and ice clouds (Yiğit et al. 2018), which highlights thebroader impact of terrestrial GW studies. Meanwhile,
it is well-established that GW processes play a significant dynamical and thermodynamical role also in otherplanetary atmospheres (Yiğit and Medvedev 2019).
Medvedev et al. (2017)’s theoretical work implemented
the influence of the geomagnetic field in the physics of
GW dissipation in the whole atmosphere scheme. In addition to the effect of ion drag on the large-scale thermospheric circulation, the effect of ion friction on the propagation and dissipation of small-scale GWs has been taken

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into account in their formulation. Their numerical experiments have shown that taking into account the geometryof the geomagnetic field can increase GW drag by morethan a hundred m s−1 day−1 , GW cooling/heating rateby ∼15 K day−1 , and GW-induced temperature variance by several K in the ionosphere-thermosphere region.
Although GCMs, extending into the upper atmosphere,
often include the influence of the magnetic field on thelarge-scale atmosphere, they do not include direct magnetic field effects on GW propagation and dissipation. The
impact of this limitation is yet to be explored.
While GW parameterizations provide a practical toolin studying GWs, high-resolution GCMs are increasingly resolving GW-induced perturbations in Earth’s (e.g.,
Miyoshi and Fujiwara (2008); Watanabe and Miyahara(2009); Gardner and Schunk (2011); Miyoshi et al. (2014);
Liu et al. (2014b); Liu et al. (2014a); Becker and Vadas(2018)) atmosphere. However, these models can be computationally challenging to run for longer time scales,
especially if they extend from the lower atmosphere to
the thermosphere. Another issue is that high-resolution
models often damp small-scale GWs above the mesopause
via an artificial hyperdiffusivity, which can potentially leadto an underestimation of GW activity at higher altitudes.
Miyoshi et al. (2014)’s high-resolution GCM simulations
as well as those described in Liu (2016) confirmed that
the GW drag due to upward propagating GWs frombelow plays an important role in the general circulationof the thermosphere. This confirms predictions from previous coarse-grid GCM studies with whole atmosphere
GW parameterizations (e.g., Yiğit and Medvedev (2009);
Yiğit et al. (2009); Yiğit and Medvedev (2010)). Theirresults also provide robust evidence that mean thermospheric effects of GWs can be successfully parameterized
in lower-resolution GCMs.
Using the CMAT2-GCM coupled with the whole atmosphere GW scheme, Yiğit and Medvedev (2017) analyzed
the interaction of the small-scale GWs with the migrating
diurnal tide. Their GCM simulations have suggested theGWs significantly modulate the diurnal tide in the MLT
as well as in the thermosphere. Their work suggested that
GW parameterization should consider a broad spectrumof wave harmonics and describe wave propagation anddissipation properly in order to capture GW-tidal interactions in the atmosphere. Greer et al. (2018) have recently
used the Kühlungsborn mechanistic circulation model(KMCM) to select GW characteristics that helped to better interpret thermospheric airglow emissions. Recently,
Miyoshi and Yiğit (2019) have implemented the wholeatmosphere GW parameterization (Yiğit et al. 2008) into
a whole atmosphere GCM, which covers the region fromthe ground to the exobase. They showed that GWs
attenuate the semidiurnal solar tide (SW2) in the lower

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

thermosphere and modify its latitudinal structure above
150 km.
The vertical coupling induced by large-scale waves canbe studied with GCMs as well. Karpov et al. (2018) studied with the Global Self-Consistent Model of the Thermosphere, Ionosphere, and Protonosphere (GSCM TIP,
(Namgaladze et al. 1988)) the response of the upper atmosphere to an SSW. They have demonstrated significant
changes in thermospheric and ionospheric characteristics, such as the a shift in the maximum atomic oxygen
concentration to lower latitudes in the Southern Hemisphere. Lilienthal and Jacobi (2019) studied the generation mechanism of the terdiurnal tide with the Middle
and Upper Atmosphere Model (MUAM) model. Theirsimulations have indicated that the primary excitationof the terdiurnal tide is due to the terdiurnal component of solar radiation absorption in the troposphereand stratosphere. Secondary sources, such as nonlinear
tide–tide interactions and gravity wave–tide interactions,can also play a role. Geißler et al. (2020) have used
the same GCM to study the forcing mechanisms of the
migrating quarterdiurnal tide (QDT). Among the studied mechanisms were the absorption of solar radiationby ozone and water vapor, nonlinear tidal interactions,and gravity wave–tide interactions. Nonlinear processes
and GW forcing were found to be important for the
QDT besides the primary mechanism of solar radiation
forcing.
The nature of short-term tidal variability was identified as a significant issue for our understanding of tides
by Oberheide et al. (2015) in their summary of CAWSES
II activities for Task Group 4. Advances in this area havebeen made both in the refinement of data analysis techniques which permit shorter term variability to be diagnosed from satellite observations and the use of generalcirculation models to explore the nature of this variability.
The interesting issues being explored include resolving thestark difference in gravity wave forcing between parameterizations which do not include propagation times andhorizontal transport and those which do (Ribstein andAchatz 2016), the lack of correlation between the shortterm variability of tropospheric sources and the amplitudeof tidal components in the mesopause region (Du et al.
2014) and why tidal amplitudes exhibit short-term variability (of the order of days) (Baumgarten et al. 2018).
Recent studies addressing these questions include the statistical approach of Vitharana et al. (2019) and the Houghmode extension approach of Ortland (2017). These questions regarding tidal variability and interactions with gravity waves remain open and illustrate the complexity ofresolving interactions between waves and determining theimpact of their dissipation.
Overall, models provide a powerful methodology ofstudying all types of waves. However, observations are

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needed in order to complement, validate, or challenge thenumerical findings.
4.2 Observation of coupling processes

In recent years, the global distribution of gravity wave
activity in the middle atmosphere has been obtained bysatellite temperature measurements with high vertical or
horizontal resolution such as Microwave Limb Sounder(MLS), Sounding of the Atmosphere using Broadband
Emission Radiometry (SABER), Atmospheric InfraredSounder (AIRS), and High-Resolution Dynamics Limb
Sounder (HIRDLS). The area from the Andes Mountains to the Antarctic Peninsula has been realized as a“hot spot” of gravity wave activity through these satellite
observations (Ern et al. 2018a; Hocke et al. 2016; Meyer
et al. 2018). Nonetheless, observations of gravity waves
around the Antarctic continent are limited. Therefore, thezonal wind acceleration/deceleration induced by gravity
waves is uncertain and so are climate change projections(Geller et al. 2013). In particular, the “missing gravity wavedrag” near 60◦ S latitude causes a strengthening of thesouthern hemisphere polar vortex and a delay of its breakdown, which prevents the quantitative reproduction of the
Antarctic ozone hole (McLandress et al. 2012).
Since satellites observe only a part of the spectrumof gravity waves (i.e., “observational filter”, see Alexander (1998)), ground-based and in situ measurements
of GWs, despite their limited coverage in time andspace, have become important to obtain the entire spectrum of gravity waves. The Mesosphere-StratosphereTroposphere (MST)/Incoherent Scatter (IS) radar is able
to obtain the time-altitude section of gravity wave activity and its momentum flux in all frequency bands throughobservations at a high time cadence and vertical resolution. Sato et al. (2017) utilized the PANSY radar, theonly MST/IS radar in the Antarctic, and found that the
momentum transfer to the Antarctic summer mesosphereis mainly due to long-period gravity waves (i.e., inertia
gravity waves). On the other hand, in situ observations
with a super-pressure balloon can provide a horizontal
distribution of the GW activity and its momentum flux
for the entire (intrinsic) frequency band at a single altitude (Vincent and Hertzog 2014; Jewtoukoff et al. 2015).
Lidar observations of gravity waves are now available(both day and night) of suitable quality and resolution
for mean wave characteristics to be derived and compared with models (Baumgarten et al. 2017; Strelnikova
et al. 2020). A recent gravity wave climatology at 54◦ N
and 69◦ N based on nearly 15,000 h of lidar measurements has some similarity to reanalysis models but alsosignificant discrepancies (Strelnikova et al. 2020). It is now
generally accepted that simple gravity wave parametrization scenarios such as those being used in several
GCMs, cannot cover all aspects of the physical processes

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

related to gravity waves, i.e., their observed spatial andtemporal intermittency and their interaction with tides(Baumgarten et al. 2018). Still, satellite, ground-based, andin situ observations of global gravity wave activity are
expected to provide the means to improve representationof the middle atmosphere general circulation in climate
models. These measurements provide information on thespatial and temporal distribution of the amplitudes andspectra of gravity waves and the associated energy andmomentum transfer (see for example Ern et al. (2016);
Ern et al. (2018b)). These distributions along with theobserved background zonal mean meridional and zonal
winds provide a valuable framework for validating theassumptions underlying GW parameterizations and theirimpact in models.
The horizontal propagation and intermittency of gravity waves must be taken into account to improve theparameterization of gravity wave drag in climate models(Plougonven et al. 2020; Senf and Achatz 2011), because
they control the horizontal and vertical distribution ofgravity wave drag. The horizontal propagation is mainly
detected by satellite observations (Yamashita et al. 2013;
Fritts et al. 2016), and the intermittency is characterizedthrough balloon, radar, and lidar observations (Hertzog etal. 2012; Minamihara et al. 2020; Baumgarten et al. 2018).
Observational evidence for secondary GWs is also emerging through lidar and satellite observations (Bossert et al.
2017; Song et al. 2017; Kogure et al. 2020).
Airglow observation in the mesopause region is a usefulmeans to identify GWs propagating from below and determine dynamical vertical coupling. To date, ground-based
airglow imagers have been installed and are in operation
at various locations. The Optical Mesosphere Thermosphere Imagers (OMTIs) are located at more than twenty
sites from high to low latitudes, mainly in the northern hemisphere. The recent extension during the VarSITI
period was eight new OMTIs at around 60◦ N in magnetic
latitude which covers all longitudinal regions (Shiokawa et
al. 2017). In the high latitude Southern Hemisphere, or in
the Antarctic region, airglow imaging at more than eightstations form an observation network called ANGWIN(Antarctic Gravity Wave Imager/Instrument Network)(Moffat-Griffin 2019). A dense network of airglow imagers
covering a wide area without gaps is in operation in
the midlatitudes of China (Xu et al. 2015). The advantage of such ground-based imagers is the opportunity forlong-term monitoring of GWs (Tsuchiya et al. 2018) andsimultaneous observations with co-located instruments
such as wind measurement radars and/or temperature
measurement lidars, e.g., Taylor et al. (2019); Criddle etal. (2020). Variabilities with local time, day-to-day, seasonal and inter-annual scales have been studied, as well as
vertical propagation from the ground using backgroundwind/temperature profiles.

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Airglow imaging from satellites is also a means to identify coupling processes associated with GWs. Concentric
GWs observed by satellite imagers with large fields of view(FOV) indicate the impact of meteorological disturbancessuch as typhoons/cyclones. They excite gravity waves
which propagate up into mesosphere, thermosphere, andionosphere. IMAP/VISI on board International Space Station has shown global maps of these concentric waves
and their seasonal variations (Perwitasari et al. 2016).
Observations from the Visible Infrared Imaging Radiometer Suite (VIIRS) on board the Suomi/NPP satellite havedemonstrated that a hurricane excites concentric GWs
when the intensity is maximum (Xu et al. 2019). Effort
has been exerted on the development of analysis methods for airglow images. Matsuda et al. (2014) obtained
power spectra of airglow perturbation in the horizontal
phase velocity domain (M-transform) using 3-D Fouriertransforms. This new technique can quantitatively indicate GW interaction with the background mean wind andtemperature during their vertical propagation from below(Matsuda et al. 2017; Perwitasari et al. 2018). On the other
hand, various approaches, such as machine-learning (Laiet al. 2019), for extracting GW events have also been tried.
Meteor radars have also been used to study the dynamics in the mesopause region. Recently, a regional network
for meteor radar observations with a common scattering volume has been developed as a powerful tool forthis purpose (Stober and Chau 2015; Chau et al. 2017;
Chau et al. 2019; Stober et al. 2018; Vierinen et al. 2019).
Wind vectors can be accurately derived fora smaller areawith this new network observation technique. The three
dimensional structure of wind fields over a wide horizontal range is observed (Stober and Chau 2015). New
dynamical parameters of a wind field such as the horizontal divergence and relative vorticity can be determined, an
example of which is described by Chau et al. (2017) for
observations over northern Scandinavia. The possibilityof vertical wind velocity measurement is also indicated.
These new techniques are expected to be highly beneficial for studying mesosphere-thermosphere coupling innear future when combined with the EISCAT3D radar
being constructed in northern Scandinavia (https://eiscat.
se/eiscat3d-information/). Vierinen et al. (2019) evenstepped forward to develop a method to estimate thekinetic energy of turbulence using the anticipated observations.
A SSW is the most drastic event in the middle atmosphere, and its impact is widespread. In recent years,“elevated stratopause” events where the stratopause is reformed at an altitude of about 80 km following a SSW has
attracted strong interest (see also Section 3.1 for the roleof these events for downward coupling). Global temperature observations in the middle atmosphere by satellites
such as SABER and MLS capture the overall structure ofWard et al. Progress in Earth and Planetary Science(2021) 8:47

these events and established their occurrence after major
SSWs in the Northern Hemisphere (Manney et al. 2008;
Manney et al. 2009). Prolonged periods of westward windin the stratosphere following SSWs, enhance westward
gravity wave drag above 80 km altitude through the filtering of upward-propagating GWs. This causes a strong
downwelling over the pole and the associated adiabatic
heating results in a reformation of a stratopause (temperature maxumum) around 80 km (Tomikawa et al. 2012;
Chandran et al. 2013). The circulation changes during elevated stratopause events are observed to be associatedwith the enhanced downward transport of NOx , which, as
noted earlier, affects the ozone budget in the stratosphereand mesosphere (Randall et al. 2006; Randall et al. 2009).
In addition, radar and satellite observations have shownthat changes in the wind system and the distribution of
trace constituents in the stratosphere and mesosphere
during SSW have a significant impact on the ionospherethrough the modulation of solar and lunar atmospherictides in the thermosphere (Chau et al. 2015). Recently,
using total electron content (TEC) data from five GPS
stations combined with TIMED/SABER data, Nayak andYiğit (2019) analyzed GW activity variations in the ionosphere during the major SSW of December 2008/January
2009, demonstrating that there is significant variability
in GW activity and associated TEC variability during theSSW.
On the other hand, interhemispheric coupling is also
attracting attention as a pattern of meridional teleconnection due to SSWs. The occurrence of polar mesosphericclouds in the Antarctic mesopause region is observed todecrease following SSWs in the Northern Hemisphere(Karlsson et al. 2007). Kornich and Becker (2010) proposed a coupling mechanism between the SSW in thewinter hemisphere and the warming of the summer mesosphere due to modulation of the global gravity wave
forcing. However, another mechanism has also been suggested (Smith et al. 2020), and observational validationis required. An international collaborative observation
campaign, ICSOM (Interhemispheric Coupling Study by
Observations and Modeling; see also http://pansy.eps.s.
u-tokyo.ac.jp/icsom/), has been carried out as an official activity of SCOSTEP/VarSITI/ROSMIC since 2016.
ICSOM is composed of the global MST/IS radar networkfrom the Arctic to the Antarctic with many complementary radio and optical observations.
The resonance scattering lidar which involves scattering from the meteoric metal layers (e.g., Fe, Na, K, Ca,
and Ni) can measure temperature, wind, and the density of the metallic atoms in the MLT region with acadence of minutes and high vertical resolution. Recent
notable results include direct measurements of eddy heat
fluxes and thermal diffusion coefficients in the mesopause
region (e.g., Guo et al. (2017)) and the discovery of neutral

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metal layers up to ∼200 km altitude in the thermosphere(e.g., Chu et al. (2011)). The eddy heat fluxes and thermal diffusion coefficients in the mesopause region were
derived from the turbulence-scale perturbations in temperature and wind obtained with an Na wind/temperaturelidar. The observation suggests that vertical constituent
transport induced by gravity waves can be estimated bylidar measurement (Gardner et al. 2019). Liu et al. (2016)derived the temperature and horizontal wind profiles up
to 140 km with an Na lidar at Andes Lidar Observatory in Cerro Pachon, Chile, during a thermospheric Na
enhancement. Chu and Yu (2017) used a thermosphereionosphere Fe/Fe+ (TIFe) model to infer that a major
source of the thermospheric Fe layers observed in Antarctica between 120 and 200 km is the neutralization of
metallic ions Fe+ . The ions are transported upward by
the polar electric field from their main deposition region(∼80–110 km) into the ionosphere. Observations of Ca+ ,the only metallic ion in the MLT region observable witha ground-based lidar, provide insights into the behaviorof ions in the E and F region ionosphere. Recent highresolution Ca+ density measurements revealed various
structures and the evolution of the Ca+ layer between
80 km and 180 km altitudes. These results provide information on the coupling of the E and F regions (Ejiri etal. 2019b; Ejiri et al. 2019a; Raizada et al. 2020; Wu et al.
2020). Recently, mesospheric Ni layers were also detectedby lidar over Alaska and Germany (Collins et al. 2015;
Gerding et al. 2019). Large differences, however, in Ni
density between the two sites were observed.
Thermospheric dayglow emissions obtained using anovel daytime imaging spectrograph (Pallamraju et al.
2013) from Hyderabad (a low-latitude location in India)
and radio (digisonde) measurements from Ahmedabad(typically a region under the crest of the equatorial ionization anomaly in Indian longitudes) of several years’
duration have been used to assess gravity wave activity,
both in terms of their occurrence numbers and their vertical propagation speeds. From their study of the occurrence statistics, it was found that the percentage of days
when waves in the spectral periods of gravity wave range
occurred was greater for times of relatively higher solar
activity duration (in 2013) compared to those of low
solar activity (in 2011) (Laskar et al. 2015). Furthermore,by using a new method to extract neutral gravity wave
information from digisonde measurements (Mandal et al.
2019), vertical wave propagation speeds and vertical scale
sizes of waves were diagnosed. These revealed the presence of a systematic enhancement in these parameterswith solar flux (Mandal et al. 2020; Mandal and Pallamraju
2020)).
Overall, the community’s ability to explore the natureof coupling processes in the MALTI has advanced significantly during the period of the ROSMIC project.

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

New observation techniques, data analysis methods andapproaches to instrument networks have been developed.
These include advances to ground-based radar, lidar andairglow imagers, as well as satellite instruments. While
the associated data sets and their analysis have clarified
some aspects of coupling from the ground to the thermosphere/ionosphere, they have also generated many newquestions.
4.3 Ionospheric processes and the equatorial electrojet

Forcing from the middle and lower atmosphere also
affects the ionosphere and ionospheric currents. Thisconnection is believed to occur due to alteration of thethermospheric winds though atmospheric waves, such as,planetary waves and tides. Winds in turn drive the daytime ionospheric dynamo, which means that especially theE-region electric field is affected. At mid and low latitudes,
where the forcing from above is less direct, the forcing
from below can cause alterations in ionospheric structures of a similar magnitudes to those occurring during
geomagnetic storms.
The E-region dynamo mainly affects the daytime Eregion currents, most prominently the Equatorial electrojet (EEJ) and the mid-latitude solar quiet (Sq) currents(e.g., Yamazaki and Maute (2017)), and the low-latitude
plasma distribution. This occurs through the direct driving of the equatorial ionization anomaly by the equatorial
E-region electric field (Stolle et al. 2008). Due to high conductivities along the magnetic equator at the dayside, theEEJ, being an additional signal to the low latitude Sq system, is especially sensitive to these variabilities. Its signals
have recently been used extensively to investigate atmosphere/ionosphere interactions. Though the EEJ is typically eastward around local noon, it was found that there isa longitudinal dependence of the occurrence and strengthof the westward electrojet (often called counter electrojet(CEJ)) in the morning and afternoon hours. This is mainly
related to the well-known longitudinal wave-4 structurein the background equatorial electrojet strength resulting
from nonmigrating solar tides and stationary planetary
waves (Singh et al. 2018; Soares et al. 2018). Soares et al.
(2018) further suggested that additional longitudinal variability in CEJ can be attributed to the effect of migratingtides and regional geomagnetic field gradients. In particular during SSW events, modulation of the ionosphere(i.e. the EEJ) has often been identified. Forbes and Zhang(2012); Yamazaki et al. (2012); Fejer et al. (2010), suggested that enhanced lunar semidiurnal tides play a majorrole in altering the equatorial electrodynamics. Later, theeffects of both the semidiurnal lunar and solar tides weresuggested as contributing factors (Siddiqui et al. 2018).
Most recently, bursts coherent with the quasi-6 day wave
have been identified in the EEJ during the SH SSW ofSeptember 2019 (Yamazaki et al. 2020).

Page 26 of 38

Post-sunset equatorial plasma irregularities are a severe
threat to trans-ionospheric radio waves used for navigation, and can lead to their interruption (e.g., Xiong et al.
(2016)). The formation of these irregularities is generally
thought to be favored during the strong eastward electricfield and upward plasma density gradients present in thelower F region after sunset when conditions are suitablefor Rayleigh-Taylor instabilities to develop. Different scenarios have been proposed regarding the initiation of thegrowth of irregularities. A currently favored scenario proposes upward propagating gravity waves as the source of
the initiating perturbations in the bottom side F region(see Kelley et al. (2003); Woodman (2009)). Recent work,
based on multi-instrumental ground-based observationand novel regional analyses from satellite-based optical
data, has provided further evidence of the connection
between GWs and plasma irregularities (e.g., Sivakandan
et al. (2019); Eastes et al. (2019); Das et al. (2020)).
4.4 Future challenges

While the study of wave associated upward coupling isa relatively new topic for ionospheric and thermosphericinvestigations, it has been under way for several decades in
the middle atmosphere. The dissipation of gravity waves
in the mesosphere is now generally accepted as the cause
for the solsticial pole to pole circulation. However, theparticular waves causing this circulation are yet to be
identified observationally and models do not yet havesufficient resolution to faithfully simulate sources andpropagation conditions so that these waves can be identified. Secondary wave generation has been identified as
an additional mechanism for wave generation which further complicates our understanding of the height variationof the character of these waves. Thus, one of the major
challenges for the future remains the identification of thesources, propagation conditions, interactions and dissipation mechanisms for these waves. A related challenge
is determining the causes of constituent distributions in
the mesosphere and lower thermosphere since they are
dependent on the induced large scale flows and mixing
associated with waves of all types. This requires the further development of a detailed understanding of the wave
breaking process and any turbulence that results.
Solutions to these questions require advances in modeling capabilities (from whole atmosphere GCMs to highresolution simulations of wave propagation), the development of data assimilation techniques for the middle andupper atmosphere, instruments capable of higher spatial
and temporal resolution and more extensive international
observational networks. Local time variations in winds
and temperatures associated with tides mean that shortterm diagnoses (i.e., daily) of the dynamical conditionsin the upper atmosphere using observations from single

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

satellites cannot be cleanly achieved. In principle, a multiple satellite mission or ground based chains of suitably
selected instruments can address this problem but theissues of funding and international coordination make thisdifficult in practice. In addition, the lower mesosphereand lower thermosphere are difficult regions to observe
because of the lack of suitable natural phenomena which
can be easily and inexpensively observed. To observe theseregions, new instrument techniques need to be developed.
The upward coupling of waves into the ionosphere andlower thermosphere is now an active area of research.
Whole atmosphere GCM’s which include ionosphericphysics and ionospheric models which include realistic lower boundary conditions are currently available
to explore this coupling and are being refined. As thisresearch is at the interface between the neutral atmosphere and ionosphere, progress can best be made when
researchers familiarize themselves with the complementary concepts and analysis context which have been developed to understand each region.
The lack of observations in the lower thermosphere(noted above) remains a significant concern. Withoutthem, the models cannot be validated, and data assimilation for these heights remains a tentative venture. The
establishment of suitable observing capabilities whichallow the dynamical conditions in the middle and upper
atmosphere to be monitored is the greatest challenge for
future advances in this field.

5 Trends
Working group 3 of ROSMIC has been dealing with longterm trends in the mesosphere, lower thermosphere (MLT
region) and related part of the ionosphere (below about
120 km). Working group 4 treated long-term trends in thethermosphere and ionosphere (above about 120 km). The
key question for both working groups was as follows:• What is the impact of anthropogenic activities on themesosphere-thermosphere-ionosphere (MTI)
system?
This is a long-term strategic question and ROSMIC contributed to developing answers to this question. Some
important findings reached during the ROSMIC project,
concerning trends in the MLT region, including therelated part of the ionosphere, are briefly outlined in thissection. In addition, a few results concerning higher levels
are also mentioned, specifically those from studies dealing
with effects throughout the MTI. It is important to realizethat while long-term change in the atmospheric concentration of CO2 is the primary driver of trends, somesecondary trend drivers must also be taken into account,e.g., long-term changes of the stratospheric ozone concentration for trends in the MLT region.

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The main driver of long-term trends in the MLT region
is CO2 . The apparent discrepancy between the observed(ACE/FTS and TIMED/SABER) and modeled long-term
trend of the CO2 volume mixing ratio has been resolved(Qian et al. 2017; Rezac et al. 2018). The reason for theapparent difference was found to be the neglect of someinfluences on observational data during the derivationof concentrations from observations. The observed trendof CO2 in the MLT region now does not differ significantly from the modeled trend of about 5%/decade. Thisis approximately equal to the trend at Earth’s surface.
With the issue of CO2 mixing ratio trend resolved,
attempts to model the atmospheric response to trace
gas trends gain additional credibility. Global simulationsof temperature change due to anthropogenic trace gas
emissions, that extended from the surface, through thethermosphere and ionosphere, to the exobase were performed by Solomon et al. (2018); Solomon et al. (2019)using the Whole Atmosphere Community Climate Model– eXtended (WACCM-X). Warming in the lower atmosphere switched to cooling above the lower stratosphere.
The cooling is dramatic in the thermosphere, 2–3 K per
decade for the global mean annual mean, with the higher
rates corresponding to solar minimum conditions, and thelower rates for solar maximum. This dependence of globalchange on solar activity is due to solar-driven increases
in radiatively active gases other than carbon dioxide, suchas nitric oxide. The dependence is fairly small in comparison with previous observational and modeling studies,but is commensurate with updated satellite drag analysis
by Emmert (2015). They found that any trend magnitudedependence on solar cycle is weak, relative to the trend
uncertainties. The reason for this study was that sincesolar and geomagnetic activity have tended to decline overthe past several cycles, it has become difficult to disentangle the relative effects of increasing CO2 over the sametime period.
An ancillary result of these simulations is an estimateof the solar cycle effect on temperatures as a functionof altitude. These simulations used modest, five-member
ensembles. Observed sea surface temperatures rather
than a fully coupled ocean model was used, so any solar
cycle effects were not statistically significant in the lower
troposphere. Temperature change from solar minimum tomaximum increased from near-zero at the tropopause, toabout 1 K at the stratopause, to approximately 500 K in theupper thermosphere, commensurate with the empiricalevidence, and previous numerical models.
Changes in stratospheric ozone concentration play animportant role in trends in the MLT region. Observational
data reveal a change of temperature trends in the MLT
region that coincide with a change of ozone trends in thesecond half of the 1990s. The trends in the mesopause
region (∼90 km) temperature change from no trends

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

to negative trends, whereas trends in mesospheric (∼75km) temperatures become less negative. WACCM4 model
simulations confirm that temperature trends in the mesosphere are clearly different in 1975–1995 versus 1995–
2015. There is also a tendency that the trends of bothzonal and meridional summer prevailing winds are changing after 1995. Using a numerical circulation model withrealistic ozone and carbon dioxide fields and with tropospheric/lower stratospheric circulation taken from ERA
reanalyses, the observed trend changes can be reproduced
qualitatively (Jacobi et al. 2015).
Despite several objections (von Zahn 2003; Thomas etal. 2003), polar mesospheric clouds (PMC) are still considered to play a major role in the evaluation of trends in thesummer mesosphere at mid- to polar latitudes. Observations of PMC by ground-based observers (visual and lidar)
and by satellites now cover several decades which allows
the study of the dependence of the morphology of PMC on
various processes such as the solar cycle, volcanoes, andgreenhouse gases such as CO2 and H2 O. For example, theice water content (IWC) in bright clouds increased rapidly
in the period 1979–1990 but became almost constant in
the period 1990–2013. This pattern correlates with the
behavior of stratospheric ozone. Sensitivity analysis indicates comparable roles for the influence of temperatureand H2 O on boreal trends in IWC but mainly a temperature influence on austral trends in IWC (Hervig et al.
2016).
Recent studies using LIMA/MIMAS (Leibniz-InstituteMiddle Atmosphere model; Mesospheric Ice Microphysics And tranSport model) for a period of 138
years (1871–2008) suggest that long-term trends of PMC
parameters are most prominent after 1960 when theincrease of CO2 and H2 O accelerates (Lübken et al. 2018;
Lübken et al. 2020). Trends at polar latitudes are rather
different from those at middle latitudes. Some tendencies are caused by the increase of water vapor (due toincreasing methane abundance): for example brightness,albedo, backscatter coefficient (relevant for lidar), andoccurrence rates. On the other hand, the weak decrease of(geometric) PMC altitude is primarily due to a shrinkingof the atmosphere below PMC heights. LIMA/MIMAS
model results are consistent with long-term observations
by ground based observers (visual or lidar) and by satellite measurements (Bremer and Berger 2002; Kirkwood etal. 2008; Pertsev et al. 2014; Fiedler et al. 2017; Russell III
et al. 2015). It should be noted that the observational database is still limited and does not yet allow for a clear cut
statement regarding the role of PMC for detecting climatechange effects in the middle atmosphere (Lübken et al.
2018; Lübken et al. 2020).
The solar cycle has a direct impact on PMC as has
been demonstrated in many experimental and simulation
studies. They show that there is a clear anti-correlation

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between solar activity and PMC characteristics (brightness, occurrence, etc.) which is caused by the effects ofsolar heating and the photo-dissociation of water vapor(Lübken et al. 2009). Surprisingly, a solar cycle response is
missing in satellite observations after 2002 which is presumably caused by a not yet fully understood suppressionof the solar cycle response of water vapor (Thomas et al.
1991; Hervig et al. 2019).
A long (1979–2016) series of winds, combining low frequency and meteor radar wind measurements at Collm(51◦ N, 13◦ E), Germany reveals trends in zonal andmeridional winds. The trend in zonal wind is a gradualchange from strong eastward to weak eastward. The trend
in meridional wind is a weakening of its magnitude overtime (both northward and southward) in all months of theyear. Figure 11 illustrates this result, with fewer regions
exceeding ±8 m/s (orange and red in winter, blue andgreen in summer) in recent years.
Qian et al. (2019a) compared model simulations with
WACCM-X with the wind measurements from Collm andfound that dynamics have a significant impact on trends
in mesospheric temperatures and, in particular, in mesospheric winds, where the effects of dynamics appear to
be of primary importance. Global average mesospherictemperature trends peaked in the lower and middle mesosphere in qualitative agreement with observations. The
observed trends in winds were stronger and more statistically significant than the simulated trends.
Liu et al. (2017) analyzed latitude-time cross-sections
to determine the linear trend term of the relative GW
potential energy as a function of height during each monthfor the altitude range of 30–100 km based on SABER
data over 2002–2015. They found the monthly trends tobe both positive and negative, changing remarkably frommonth to month. This is an example of temporal variability of trends in atmospheric wave activity. Examplesof significant regional variability are also known. Thisvariability makes investigation of trends in wave activity
difficult.
Hall et al. (2016) reported some increase in the turbopause height in summer at Tromsø, (70◦ N) and astatistically significant change of the turbopause heightin Saskatoon (52◦ N) and in Tromsø in winter. From
those results they inferred a decrease of atomic oxygen
concentration in the lower thermosphere.
Calculation of ionospheric trends requires removal ofsolar cycle effects from the data. Analysis of trends in foE(heights around 110 km) shows that the solar correction(using both F10.7 and solar Lyman-α flux) changes withtime (Laštovička et al. 2016), perhaps due to changes in therelation between solar ionizing radiation and solar proxies (there are indications of such changes on the Sun incycles 23 and 24, e.g., Balogh et al. (2014)). Danilov andKonstantinova (2019) found a strong seasonal and diurnal

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

Page 29 of 38

Fig. 11 Long-term trend in meridional wind Meridional wind near 90 km, Collm, Central Europe, 1979–2016. Reproduced from Lastovicka, J., Beig, G.,
DeLand, M., Jacobi, C., Keckhut, P., Mlynczak, M., Qian, L. (2017), Progress reached by ROSMIC WG3 "Trends in Mesosphere and Lower Thermosphere".
VarSITI Newsletter, 15, 6-9

variation of trends in foE, the trends being remarkably
stronger in winter than in summer.
5.1 Future challenges

Progress in investigating long-term trends in the MLT
region, which largely reflect the global change in the atmosphere, during VarSITI was significant but many questions remain open. Future investigations of trends in MLT
should deal with filling in gaps in scenario of trends in the
MLT region and removal/explanation of controversies,
particularly with respect to the following:
1. Investigations of trends in atmospheric wave activity,
which is the key problem of trends in the MLT region
2. Further development and improvement of complex
models3. Monitoring and investigations of changes ofsecondary (= non-CO2 ) trend drivers and of theirimpacts on trends

6 Conclusions
The ROSMIC science topics covered large parts of the
middle atmosphere, thermosphere, ionosphere, and climate research. As we have seen above, significant progresson various aspects of these was made during VarSITI, withaccess to improved models and continued high-quality
observations proving critical for these advances. One of

the remaining substantial challenges is considering thepicture, presented in Fig. 1, as a whole. While this is
not a straightforward task, it will be critical for the next
SCOSTEP science program, PRESTO, PREdictability ofvariable Solar-Terrestrial cOupling, in order to assess how
different parts of the system interact, with the goal of
improving predictability in different scales, from temporalto spatial.
Evident in all topics, is the importance of the interactions between various components of the terrestrial
part of the Sun/Earth system. While the specification ofparticular parameters have been identified as targets for
future work (i.e., flux and energy spectra of electronsand constituent transport—downward coupling; the gravity wave spectrum and its dissipation—upward coupling;
wave activity—trends), they are important in terms ofallowing the full system and its interactions to be better defined. A prime example of this is the SSW which
was identified as significant in several contexts. For downward coupling, it was notable for its role in the downward
transport of constituents and changes in the large-scale
thermal structure. In upward coupling, it was implicatedin the development of global meridional teleconnections,as a modifier of wave signatures and middle atmospherecirculation and as the driver of significant changes inionospheric current systems. A significant development
during ROSMIC is the recognition that although different

Ward et al. Progress in Earth and Planetary Science(2021) 8:47

aspects of SSWs are important depending on the context,
inclusion of all aspects is necessary to fully understandthem and their impact.
Mention was made of the successful incorporation ofsatellite and ground based observations to provide a global
perspective on various phenomena during ROSMIC. Also
highlighted was the successful development of wholeatmosphere models and associated parameterizations toprovide the means to explore the interactions between thevarious components of the atmosphere/ionosphere. These
two scientific thrusts need to continue being supported
in the future. Although the recently launched GOLD
(Eastes et al. 2017) and ICON (Immel et al. 2018) missions will substantially enhance our understanding of theionosphere and thermosphere, it is concerning that plansto ensure continuation and enhancement of the satellite measurements that led to our current understandingof the MLT are not in place. They would provide an
extremely valuable complement to these new missions.
As has been emphasized throughout this paper, we aremoving past the time in solar terrestrial system science
when the study of phenomena in isolation is of primary
importance. Work which reveals the connections and linkages between components and phenomena is becoming
essential. PRESTO is organized to support this latter typeof investigation.
As the development of a Sun to Earth system description proceeds, it is important to keep in mind that muchof the solar energy flux is engaged in maintaining thelarge-scale structures of the atmosphere. Distinguishing
the underlying processes which are essential to maintaining these structures from those which manifest shortterm responses to solar variability is vital to assessing
the manners in which our atmosphere will evolve as aresult of anthropogenic influences. Establishing these distinctions also presents challenges to the solar-terrestrial
community as a whole. For example, the time scales ofthe forcings and responses in each part of the system varysignificantly which can lead to misunderstandings acrossdisciplines. The societal relevance of understanding therole of this coupling is tremendous, not least because of
the whole atmosphere impacts of anthropogenic influences and any geo-engineering solutions that might beproposed to counter them.
Abbreviations
CAWSES: Climate and Weather of the Sun-Earth System; CCM: ChemistryClimate model; CEJ: Counter electrojet; CFC: Clorofluorocarbon; CME: Coronalmass ejection; CMIP6: Coupled Model Intercomparison Project Phase 6; EEJ:
Equatorial electrojet; EEP: Energetic electron precipitation; EPP: Energetic
particle precipitation; ES: Elevated stratopause; EUV: Extreme ultraviolet; FUV:Far ultraviolet; GCM: General Circulation Model; GHG: Greenhouse gas; GOLD:
Global-scale Observations of the Limb and Disk ; GW: Gravity wave; ICON:
Ionospheric Connection Explorer ; IPCC: Intergovernmental Panel for Climate
Change; IWC: Ice Water Content; MALTI: Middle Atmosphere/Lower
Thermosphere/Ionosphere; MEE: Medium energy electrons; MIL: Mesospheric

Page 30 of 38

inversion layer; MLT: Mesosphere lower-thermosphere; MUV: Middle
ultraviolet; NASA: National Aeronautics and Space Administration; NAO: North
Atlantic Oscillation; NH: Northern Hemisphere; NIR: Near infrared; NUV: Near
ultraviolet; PMC: Polar mesospheric cloud; PRESTO: PREdictability of variable
Solar-Terrestrial cOupling; QBO: quasi-biennial oscillation; QDT: Quarterdiurnaltide; SCOSTEP: Scientific Committee on Solar-Terrestrial Physics; SH: Southern
Hemisphere; SPE: Solar proton event; SSI: Solar spectral irradiance; SSW:
Sudden stratospheric warming; SXR: Soft X-rays; TEC: Total electron content;
TSI: Total solar irradiance; UBC: Upper boundary condition; UV: Ultraviolet; VIS:
Visible (radiation); VUV: Vacuum ultraviolet
Authors’ contributions
WW and AS coordinated the writing of the manuscript. All authors contributed
to the writing or providing comments on the text. All authors read andapproved the final manuscript.
Funding
WW and AS gratefully acknowledge funding from the ISEE (Institute forSpace-Earth Environmental Research), Nagoya University, for support to attend
the VarSITI summarizing workshop where the results discussed in this review
paper were presented.

Declarations
Availability of data and materials
Not applicableCompeting interests
The authors declare that they have no competing interests.
Author details
1 Department of Physics, University of New Brunswick, POBox 4400,
Fredericton, Canada. 2 Department of Physics, University of Otago, PO Box 56,
Dunedin, New Zealand. 3 George Mason University, 4400 University Drive,
Fairfax, USA. 4 National Institute of Polar Research, 10-3, Midori-cho,Tachikawa-shi, Tokyo, Japan. 5 Geoforschungszentrum Potsdam,
Telegrafenberg, Potsdam, Germany. 6 Institute of Atmospheric Physics, Czech
Academy of Sciences, Bocni II, Prague, Czech Republic. 7 Laboratory for
Atmospheric and Space Physics (LASP), University of Colorado (CU), 3665
Discovery Drive, Boulder, USA. 8 Leibniz-Institute of Atmospheric Physics,
Schloss-Str. 6, Kühlungsborn, Germany. 9 High Altitude Observatory, National
Center for Atmospheric Research, POBox 3000, Boulder, USA. 10 Faculty ofEngineering and Physical Sciences, University of Leeds, Leeds, UK. 11 Institutode Astrofísica de Andalucía, CSIC, Granada, Spain. 12 Space and Atmospheric
Sciences Division, Physical Research Laboratory, Ahmedabad, India.
Received: 15 October 2020 Accepted: 8 June 2021

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