Influence of the Precipitating Energetic Particles on Atmospheric Chemistry and Climate

We evaluate the influence of the galactic cosmic rays (GCR), solar proton events (SPE), and energetic electron precipitation (EEP) on chemical composition of the atmosphere, dynamics, and climate using the chemistry-climate model SOCOL. We have carried out two 46-year long runs. The reference run is driven by a widely employed forcing set and, for the experiment run, we have included additional sources of NO _x and HO _x caused by all considered energetic particles. The results show that the effects of the GCR, SPE, and EEP fluxes on the chemical composition are most pronounced in the polar mesosphere and upper stratosphere; however, they are also detectable and statistically significant in the lower atmosphere consisting of an ozone increase up to 3?% in the troposphere and ozone depletion up to 8?% in the middle stratosphere. The thermal effect of the ozone depletion in the stratosphere propagates down, leading to a warming by up to 1?K averaged over 46?years over Europe during the winter season. Our results suggest that the energetic particles are able to affect atmospheric chemical composition, dynamics, and climate.     

The importance of energetic particle precipitation on the chemical composition of the middle atmosphere

An assessment is made of the relative contribution of certain classes of energetic particle precipitation to the chemical composition of the middle atmosphere with emphasis placed on the production of odd nitrogen and odd hydrogen species and their subsequent role in the catalytic removal of ozone. Galactic cosmic radiation is an important source of odd nitrogen in the lower stratosphere but since the peak energy deposition occurs below the region where catalytic removal of O₃ is most effective, it is questionable whether this mechanism is important in the overall terrestrial ozone budget. The precipitation of energetic solar protons can periodically produce dramatic enhancement in upper stratospheric NO. The long residence time of NO in this region of the atmosphere, where catalytic interaction with O₃ is also most effective, mandates that this mechanism be included in future modelling of the global distribution of O₃. Throughout the mesosphere the precipitation of energetic electrons from the outer radiation belt (60°70°) can sporadically act as a major local source of odd hydrogen and odd nitrogen leading to observable O₃ depletion. Future satellite studies should be directed at simultaneously measuring the precipitation flux and the concomitant atmosphere modification, and these results should be employed to develop more sophisticated models of this important coupling.     

Cooling of the mesosphere and lower thermosphere due to doubling of CO2

A new parameterization of infrared radiative transfer in the 15-m CO₂ band has been incorporated into the Spectral mesosphere/lower thermosphere model (SMLTM). The parameterization is applicable to calculations of heating rates above approximately 15 km for arbitrary vertical profiles of the CO₂ concentration corresponding to the surface mixing ratio in the range 150–720 ppm. The sensitivity of the mesosphere and lower thermosphere (MLT) to doubling of CO₂ has been studied. The thermal response in the MLT is mostly negative (cooling) and much stronger than in the lower atmosphere. An average cooling at the stratopause is about 14 K. It gradually decreases to approximately 8 K in the upper mesosphere and again increases to about 40–50 K in the thermosphere. The cooling and associated thermal shrinking result in a substantial density reduction in the MLT that reaches 40–45% in the thermosphere. Various radiative, chemical, and dynamical feedbacks potentially important for the thermal response in the MLT are discussed. It is noted that the results of simulations are strikingly similar to observations of long-term trends in the MLT. This suggests that during the last 3–4 decades the thermal structure in the real upper atmosphere has undergone substantial changes driven by forcing comparable with that due to doubling of CO₂.     

On the Use of VLF Narrowband Measurements to Study the Lower Ionosphere and the Mesosphere–Lower Thermosphere

The ionospheric D-region (~60 km up to ~95 km) and the corresponding neutral atmosphere, often referred to as the mesosphere–lower thermosphere (MLT), are challenging and costly to probe in situ. Therefore, remote sensing techniques have been developed over the years. One of these is based on very low frequency (VLF, 3–30 kHz) electromagnetic waves generated by various natural and man-made sources. VLF waves propagate within the Earth–ionosphere waveguide and are extremely sensitive to perturbations occurring in the D-region along their propagation path. Hence, measurements of these signals serve as an inexpensive remote sensing technique for probing the lower ionosphere and the MLT region. This paper reviews the use of VLF narrowband (NB) signals (generated by man-made transmitters) in the study of the D-region and the MLT for over 90 years. The fields of research span time scales from microseconds to decadal variability and incorporate lightning-induced short-term perturbations; extraterrestrial radiation bursts; energetic particle precipitation events; solar eclipses; lower atmospheric waves penetrating into the D-region; sudden stratospheric warming events; the annual oscillation; the solar cycle; and, finally, the potential use of VLF NB measurements as an anthropogenic climate change monitoring technique.     

Infrared Radiation in the Mesosphere and Lower Thermosphere: Energetic Effects and Remote Sensing

This paper discusses the formation mechanisms of infrared radiation in the mesosphere and lower thermosphere (MLT), the energetic effects of the radiative absorption/emission processes, and the retrieval of atmospheric parameters from infrared radiation measurements. In the MLT and above, the vibrational levels of the molecules involved in radiative transitions are not in local thermodynamic equilibrium (LTE) with the surrounding medium, and this then requires specific theoretical treatment. The non-LTE models for CO₂, O₃, and H₂O molecules are presented, and the radiative cooling/heating rates estimated for five typical atmospheric scenarios, from polar winter to polar summer, are shown. An optimization strategy for calculating the cooling/heating rates in general circulation models is proposed, and its accuracy is estimated for CO₂. The sensitivity of the atmospheric quantities retrieved from infrared observations made from satellites to the non-LTE model parameters is shown.     

Midlatitude mesopause region winds and waves and comparison with stratospheric variability

We present time series of January–May mean mesosphere/lower thermosphere (MLT) mean winds and planetary wave (PW) proxies over Europe together with stratospheric stationary planetary waves (SPW) at 50°N and time series of European ozone laminae occurrence. The MLT winds are connected with stratospheric PW and laminae at time scales of several years to decades. There is a tendency for increased wave activity after 1990, together with more ozone laminae and stronger MLT zonal winds. However, possible coupling processes are not straightforward. While mean MLT winds before the 1990s show similar interannual variations than stratospheric PW at 100 hPa, later a tendency towards a connection of the MLT with the middle stratosphere SPW is registered. There is also a tendency for a change in the correlation between lower and middle stratosphere SPW, indicating that coupling processes involving the European middle atmosphere from the lower stratosphere to the mesopause region have changed.     

Influence of stratospheric ozone changes on long-term trends in the meso- and lower thermosphere

As predicted by model calculations, long-term changes in the stratospheric ozone content should influence trends in the meso- and thermosphere also. These predictions have been tested by means of ionospheric reflection height data in the low-frequency (LF) range and critical frequency data series of the ionospheric E layer, foE, observed at different stations around the world. It was shown that an essential part of the derived trends in the mesosphere and in the lower thermosphere is correlated with long-term changes of the atmospheric ozone content. During the sub-interval with the strongest ozone decrease (1979–1995) the detected ionospheric trends are most pronounced. Additionally was also demonstrated that the longitudinally dependent ozone trends are related to similar variations in the foE trends.     

Simulation of the stratospheric ozone and temperature response to the solar irradiance variability during sun rotation cycle

Despite substantial progress in atmospheric modeling, the agreement of the simulated atmospheric response to decadal scale solar variability with the solar signal in different atmospheric quantities obtained from the statistical analysis of the observations cannot be qualified as successful. An alternative way to validate the simulated solar signal is to compare the sensitivity of the model to the solar irradiance variability on shorter time scales. To study atmospheric response to the 28-day solar rotation cycle, we used the chemistry–climate model SOCOL that represents the main physical–chemical processes in the atmosphere from the ground up to the mesopause. An ensemble simulation has been carried out, which is comprised of nine 1-year long runs, driven by the spectral solar irradiance prescribed on a daily basis using UARS SUSIM measurements for the year 1992. The correlation of zonal mean hydroxyl, ozone and temperature averaged over the tropics with solar irradiance time series have been analyzed. The hydroxyl has robust correlations with solar irradiance in the upper stratosphere and mesosphere, because the hydroxyl concentration is defined mostly by the photolysis. The simulated sensitivity of the hydroxyl to the solar irradiance changes is in good agreement with previous estimations. The ozone and temperature correlations are more complicated because their behavior depends on non-linear dynamics and transport in the atmosphere. The model simulates marginally significant ozone response to the solar irradiance variability during the Sun rotation cycle, but the simulated temperature response is not robust. The physical nature of this is not clear yet. It seems likely that the temperature (and partly the ozone) daily fields possess their own internal variability, which is not stable and can differ from year to year reflecting different dynamical states of the system.     

Planetary waves observed by TIMED/SABER in coupling the stratosphere–mesosphere–lower thermosphere during the winter of 2003/2004: Part 2—Altitude and latitude planetary wave structure

Part 2 of the present paper is focused on the planetary wave coupling from the stratosphere to the lower thermosphere (30–120 km) during the Arctic winter of 2003/2004. The planetary waves seen in the TIMED/SABER temperature data in the latitudinal range 50°N–50°S are studied in detail. The altitude and latitude structures of the planetary wave (stationary and travelling) clearly indicate that the stratosphere and mesosphere (30–90 km) are coupled by direct vertical propagation of the planetary waves, while the lower thermosphere (above 90–95 km altitude) is only partly connected with the lower levels probably indirectly through in-situ generation of disturbances by the dissipation and breaking of gravity waves filtered by lower atmospheric planetary waves. A peculiar feature of the thermal regime in the lower thermosphere is that it is dominated by zonally symmetric planetary waves.     

Ozone and temperature change in the winter stratosphere

Summary In an experiment to study the correlation between temperature and ozone in the disturbed winter atmosphere, two rocket-borne ozonesondes were flown during a time in which the temperature structure, in the stratosphere and lower mesosphere changed at certain altitudes from the previous day. The resulting ozone and temperature distribution, as compared to distribution on the control day at the end of the winter season, showed variations during the time the atmosphere was disturbed. These variations may be explained by considering the temperature-dependence of reaction rates for an oxygen atmosphere.     

Recent Results from Studies of Electric Discharges in the Mesosphere

The paper reviews recent advances in studies of electric discharges in the stratosphere and mesosphere above thunderstorms, and their effects on the atmosphere. The primary focus is on the sprite discharge occurring in the mesosphere, which is the most commonly observed high altitude discharge by imaging cameras from the ground, but effects on the upper atmosphere by electromagnetic radiation from lightning are also considered. During the past few years, co-ordinated observations over Southern Europe have been made of a wide range of parameters related to sprites and their causative thunderstorms. Observations have been complemented by the modelling of processes ranging from the electric discharge to perturbations of trace gas concentrations in the upper atmosphere. Observations point to significant energy deposition by sprites in the neutral atmosphere as observed by infrasound waves detected at up to 1000 km distance, whereas elves and lightning have been shown significantly to affect ionization and heating of the lower ionosphere/mesosphere. Studies of the thunderstorm systems powering high altitude discharges show the important role of intracloud (IC) lightning in sprite generation as seen by the first simultaneous observations of IC activity, sprite activity and broadband, electromagnetic radiation in the VLF range. Simulations of sprite ignition suggest that, under certain conditions, energetic electrons in the runaway regime are generated in streamer discharges. Such electrons may be the source of X- and Gamma-rays observed in lightning, thunderstorms and the so-called Terrestrial Gamma-ray Flashes (TGFs) observed from space over thunderstorm regions. Model estimates of sprite perturbations to the global atmospheric electric circuit, trace gas concentrations and atmospheric dynamics suggest significant local perturbations, and possibly significant meso-scale effects, but negligible global effects.     

Variability of the mesospheric wind field at middle and Arctic latitudes in winter and its relation to stratospheric circulation disturbances

Continuous wind observations allow detailed investigations of the upper mesosphere circulation in winter and its coupling with the lower atmosphere. During winter the mesospheric/lower thermospheric wind field is characterized by a strong variability. Causes of this behaviour are planetary wave activity and related stratospheric warming events. Reversals of the dominating eastward directed mean zonal winds in winter to summerly westward directed winds are often observed in connection with stratospheric warmings. In particular, the amplitude and duration of these wind reversals are closely related to disturbances of the dynamical regime of the upper stratosphere.The occurrence of long-period wind oscillations and wind reversals in the mesosphere and lower thermosphere in relation to planetary wave activity and circulation disturbances in the stratosphere has been studied for 12 winters covering the years 1989–2000 on the basis of MF radar wind observations at Juliusruh (55°N, since 1989) and Andenes (69°N, since 1998). Mesospheric wind oscillations with long-periods between 10 and 18 days are observed during the presence of enhanced planetary wave activity in the stratosphere and are combined with a reversal of the meridional temperature gradient of the stratosphere or with upper stratospheric warmings.     

Antarctica's role in understanding long-term change in the upper atmosphere

Within the global context, Antarctica has a key role to play in understanding long-term change in the upper atmosphere, both because of its isolation from the rest of the world and because of its unique geophysical attributes. Antarctic upper atmosphere data can provide global change observations regarding the mesosphere, thermosphere, ionosphere, plasmasphere and magnetosphere. It will not only provide trend estimates but, just as importantly, it will define the background variability which exists in the upper atmosphere and against which these trends must be resolved. Upper atmospheric change can be driven both from within the Earth's near environment primarily through changing atmospheric composition, dynamics or geomagnetic field, or it can be driven externally, predominantly by the Sun. Recent observations are discussed in the light of increasing interest in global change issues and sun-weather relationships.     

Temperature variability over the tropical middle atmosphere

A study on the variability of temperature in the tropical middle atmosphere over Thumba (8 32’ N, 76 52’ E), located at the southern part of India, has been carried out based on rocket observations for a period of 20 years, extending from 1970 to 1990. The rocketsonde-derived mean temperatures over Thumba are corrected prior to 1978 and then compared with the middle atmospheric reference model developed from satellite observations and Solar Mesosphere Explorer (SME) satellite data. Temperature variability at every 1 km interval in the 25–75 km region was analysed. The tropical stratosphere is found to be highly stable, whereas considerable variability is noted in the middle mesosphere. The effect of seasonal cycle is least in the lower stratosphere. Annual and semi-annual oscillations in temperature are the primary oscillations in the tropical middle atmosphere. Annual temperature oscillations are dominant in the mesosphere and semi-annual oscillations are strong in the stratosphere. The stratopause region is noted to be the part of the middle atmosphere least sensitive to the changes in solar activity and long-term variability.     

On the application of a proposed global system for measuring meteor winds

Summary This paper discusses the need for a global network of meteor wind stations for determining the general circulation of the upper mesosphere and lower thermosphere. Continuous observations of horizontal motions from such a network would permit resolution of planetary scale eddy winds, tides, and gravity waves, and hypotheses that such motions propagate vertically from the lower atmosphere or are generated in situ by solar activity could be examined critically with observational data. The observed mean winds from the lower stratosphere to the meteor wind level are summarized to support the hypothesis that a standing wave pattern in the winds extends into the lower thermosphere. Data on tidal meridional momentum transports from meteor wind stations suggest that tides in the lower thermosphere are important for the maintenance of mean winds. Some of the geomagnetic and photochemical processes in the lower thermosphere that could be investigated with meteor wind data are briefly reviewed.This paper is adapted from our presentation at the 1966 Fall URSI meeting at Palo Alto, California     

Solar activity forcing of the middle atmosphere

Studies on the influence of solar activity in 11-year cycle on middle atmospheric thermodynamic parameters, such as temperature, pressure and density, and zonal and meridional wind components over three meteorological rocket launching stations, located in the tropics (Thumba), mid-latitude (Volgograd) and high-latitude (Heiss Island) regions of the northern hemisphere have been carried out. The temperature in all the three regions showed a negative response in the stratosphere and positive association in the mesosphere with the changes in solar activity. The temperature decreases by 2-3% from its mean value in the stratosphere and increases by 4-6% in the mesosphere for an increase in 100 units of solar radio flux. Atmospheric pressure is found to be more sensitive to solar changes. An average solar maximum condition enhances the pressure in the stratosphere by 5% and in the upper mesosphere by 16-18% compared to the respective mean values. Density also showed strong association with the changes in solar activity. Increase in the solar radio flux tends to strengthen winter westerlies in the upper stratosphere over the mid-latitude and summer easterlies in the middle stratosphere over tropics. Larger variability in the zonal wind is noted near stratopause height. Results obtained from the study indicate that there is an external force exerted on the Earth’s atmosphere during the period of high solar activity. These results can be incorporated for further studies on the dynamics of the middle atmosphere in association with the changes in solar activity.     

Sensitivity of the Earth’s middle atmosphere to short-term solar variability and its dependence on the choice of solar irradiance data set

We simulate the time evolution of the neutral and charged species in the terrestrial middle atmosphere using a 1-D radiative-convective model with interactive neutral and ion chemistry driven by four different sets of daily spectral solar irradiance (SSI) available in the literature for the year 2000. Obtained daily time series of ozone, hydroxyl and electron densities are used to calculate their sensitivity to the short-term SSI variability at 205 nm. All applied SSI data sets possess 27-day solar rotation cycle; however, its amplitude and phase as well as the correlation between considered SSI time series differ among data sets leading to the different behavior of the atmospheric response. Contrary, the ozone and hydroxyl sensitivities to the SSI changes during solar rotation cycle are almost identical for all applied SSI data sets in the stratosphere. In the mesosphere, the difference in correlation between SSI in Herzberg continuum and Lyman-α line in considered SSI data sets leads to substantial scatter of the sensitivity estimates based on 205 nm. Our results show that for the sensitivity analysis in the stratosphere based on the SSI at 205 nm any considered SSI data sets can be applied. For the mesosphere, where the sensitivity strongly varies among applied SSI data sets more robust results can be obtained using the sensitivity calculations based on the SSI in Lyman-α line.     

The atmospheric effects of October 2003 solar proton event simulated with the chemistry–climate model SOCOL using complete and parameterized ion chemistry

October 2003 solar proton events (SPE) is rather well covered by the observations; therefore its studies represent a good way for model validation and intercomparison. Here we apply chemistry–climate model (CCM) SOCOL with complete (SOCOLⁱ) and parameterized ion chemistry to evaluate the accuracy of a commonly used ion chemistry parameterization scheme. We performed ensemble experiments with and without SPE to characterize the effect of the October 2003 SPE on the NO_x, HO_x, ClO_x and O₃ in the middle atmosphere. Preliminary comparison of the simulated effects against MIPAS observations revealed rather good general agreement for most of the species. Comparison of the results obtained with complete and parameterized ion chemistry representation showed that the model with parameterized ion chemistry underestimates the effect of SPE on chemical composition of the middle atmosphere by up to 40% for NO_x and N₂O, up to 70% for HO_x and ClO_x and up to 600% for HNO₃. The parameterization is more accurate for ozone, however the model with parameterized ion chemistry underestimates ozone depletion by up to 15% during the SPE in the mesosphere and by 10% 2 weeks later in the stratosphere, which can be important for the long-term effects of SPE on the ozone layer.     

Impact of different energies of precipitating particles on NOx generation in the middle and upper atmosphere during geomagnetic storms

Energetic particle precipitation couples the solar wind to the Earth's atmosphere and indirectly to Earth's climate. Ionisation and dissociation increases, due to particle precipitation, create odd nitrogen (NO_x) and odd hydrogen (HO_X) in the upper atmosphere, which can affect ozone chemistry. The long-lived NO_x can be transported downwards into the stratosphere, particularly during the polar winter. Thus, the impact of NO_x is determined by both the initial ionisation production, which is a function of the particle flux and energy spectrum, as well as transport rates. In this paper, we use the Sodankylä Ion and Neurtal Chemistry (SIC) model to simulate the production of NO_x from examples of the most representative particle flux and energy spectra available today of solar proton events (SPE), auroral energy electrons, and relativistic electron precipitation (REP). Large SPEs are found to produce higher initial NO_x concentrations than long-lived REP events, which themselves produce higher initial NO_x levels than auroral electron precipitation. Only REP microburst events were found to be insignificant in terms of generating NO_x. We show that the Global Ozone Monitoring by Occultation of Stars (GOMOS) observations from the Arctic winter 2003–2004 are consistent with NO_x generation by a combination of SPE, auroral altitude precipitation, and long-lived REP events.     

Mesospheric temperature trends derived from ground-based LF phase-height observations at mid-latitudes: comparison with model simulations

From indirect phase-height observations in the LF range at mid-latitudes, significant negative long-term trends of the ionospheric reflection height can be derived. The lowering of the reflection height at constant solar zenith angle can mainly be explained by a temperature decrease of the mesosphere due to increasing greenhouse gases (e.g. CO₂) and a reduction of the atmospheric ozone content. Marked seasonal differences of the temperature trends could be found with a stronger cooling of the mesosphere in summer than in winter. A comparison of experimental trend results and model calculations with the three-dimensional global circulation model COMMA-IAP shows a reasonable agreement.