The Stratospheric and Mesospheric NOy in the 2002–2004 Polar Winters as measured by MIPAS/ENVISAT

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board ENVISAT, provided global (pole-to-pole, the polar night winter regions) measurements of nearly all constituents of the NO_y family (including NO, NO₂, HNO₃ and H₂O₅) from July 2002 to the end of March 2004 from the upper stratosphere up to the middle mesosphere. The inter-annual variability of the NO₂ and HNO₃ abundances in the Arctic and Antarctic winters from September 2002 through March 2004 was enormous with tremendous hemispheric asymmetry and extraordinary values in two winters. The origin of these variations and of the extreme measured values has been analyzed on the basis of the changing atmospheric dynamics (using the CH₄ tracer) and solar activity, including the extraordinary solar protons events of Oct–Nov 2003.     

Stratosphere NO y Species Measured by MIPAS and GOMOS Onboard ENVISAT During 2002–2010: Influence of Plasma Processes onto the Observed Distribution and Variability

We present observations of stratosphere NO _y species from 2002 to 2010 taken by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and by the Global Ozone Monitoring by Occultation of Stars (GOMOS) instruments on board ENVISAT. We jointly used observations of MIPAS NO₂, HNO₃, N₂O₅, ClONO₂ and N₂O, and GOMOS NO₂ and NO₃. MIPAS results are part of the MIPAS2D database retrieved adopting a full 2D tomographic approach. We describe the mean distribution and variability of NO _y species in the stratosphere, identifying changes induced by plasma processes. Beside enhancements due to sporadic solar proton events, we show that winter polar NO₂ has an almost linear relationship with the geomagnetic activity index Ap down to about 10?hPa. This indicates a dominant role of energetic precipitating particles in the production of upper atmosphere NO _y . The correlation has clear signatures extending to mid latitudes. Partitioning of the NO _y reservoir species are also traced, with HNO₃ and N₂O₅ showing a correlation with Ap extending to lower altitude within the polar regions. We found no large signatures of an impact of thunderstorm-induced plasma processes onto monthly means of NO _y species in the stratosphere.     

Impact of Solar Activity on Stratospheric Ozone and NO2 Observed by Gomos/Envisat

Energetic proton precipitation occurring during solar events can increase the production of odd nitrogen in the upper stratosphere and mesosphere. A very intense solar proton event (SPE) occurred on 28 October 2003. Its impact on the composition of the middle atmosphere was observed in details due to the availability of several space instruments. Here we present GOMOS observations of a strong NO₂increase and a related ozone decrease in the upper stratosphere at north polar latitude. The perturbation of the chemical composition of the stratosphere was observed until the middle of December 2003. A strong NO₂ increase was also observed in the south polar vortex in June-July 2003. It is tentatively attributed to the effect of an SPE with protons of moderate energy occurring on 29 May 2003. If this hypothesis is confirmed, it will imply that the global effect of SPEs on the composition of the stratosphere is underestimated when only strong energy SPEs are considered.     

Recent Space Data

This paper summarises the workshop session on recent space data. Most presentations addressed the intense solar storm in October–November 2003. Large perturbations of atmospheric trace gas concentrations, notably NO₂ and HNO₃, were found over extended areas around the magnetic poles in the mesosphere and stratosphere, extending over many weeks in the stratosphere. The impact on total ozone seems to be very limited although some more subtle investigations are still to be done. Several new space instruments with many innovative data products have been introduced. Very good coverage in vertically resolved observations of many chemical species is reached for stratospheric chemistry and dynamics research. Data have already been used to improve stratospheric models. Data continuity is an issue. However, the greatest concern is the lack of any suitable future space instrumentation for tropospheric research (air quality and climate forcing/carbon cycle) as well as UTLS problems (climate/chemistry interaction, stratosphere/troposphere exchange).     

Aeronomic aspects of the polarD-region

The polarD-region has been little studied. A major part of this review concerns observations performed during the 1969, November, 2–5, solar proton event. These extensive measurements and subsequent interpretations constitute a major source of polarD-region knowledge. The aeronomic concepts garnered from the analysis of this event are discussed and compared with results from other events and other polar mesospheric data. Some mid-latitude data are resorted to for supplementary arguments. Ionization rates due to precipitating protons and alpha-particles are reviewed, particularly in view of the fact that some workers have suggested that the more intense events may influence stratospheric ozone chemistry.     

Ignition enhancement of ethylene/air by NOx addition

Recently, non-equilibrium plasma assisted combustion (PAC) has been found to be promising in reducing the ignition delay time in hypersonic propulsion system. NO x produced by non-equilibrium plasma can react with intermediates during the fuel oxidation process and thereby has influence on the combustion process. In this study, the effects of NO x addition on the ignition process of both the homogeneous ethylene/air mixtures and the non-premixed diffusion layer are examined numerically. The detailed chemistry for ethylene oxidization together with the NO x sub-mechanism is included in the simulation. Reaction path analysis and sensitivity analysis are conducted to give a mechanistic interpretation for the ignition enhancement by NO x addition. It is found that for both the homogenous and non-premixed ignition processes at normal and elevated pressures, NO 2 addition has little influence on the ignition delay time while NO addition can significantly promote the ignition process. The ignition enhancement is found to be caused by the promotion in hydroxyl radical production which quickly oxidizes ethylene. The promotion in hydroxyl radical production by NO addition is achieved in two ways:one is the direct production of OH through the reaction HO2+NO = NO2+OH, and the other is the indirect production of OH through the reactions NO+O2=NO2+O and C2H4+O = C2H3+OH. Moreover, it is found that similar to the homogeneous ignition process, the acceleration of the diffusion layer ignition is also controlled by the reaction HO2+NO = NO2+OH.     

Radio Frequency Fluctuation Spectra During the Solar Conjunctions of the Ulysses and Galileo Spacecraft

Temporal power spectra have been computed from recordings of the downlink frequency fluctuations of the Galileo and Ulysses radio signals during their solar conjunctions. Both the equatorial streamer belt and the polar coronal holes were investigated over a range of ray path solar offset distances from 4 to 80 R_⊙. By combining gapless data from successive tracking passes, Doppler scintillation power spectra could be computed down to extremely low frequencies. Some spectra feature a low-frequency turnover at frequencies around 0.1 mHz that could be interpreted as an outer scale of density turbulence in the coronal plasma. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Middle Atmospheric Ozone Response to the 11-Year Solar Cycle

Because of its chemical and radiative properties, atmospheric ozone constitutes a key element of the Earth’s climate system. Absorption of sunlight by ozone in the ultraviolet wavelength range is responsible for stratospheric heating, and determines the temperature structure of the middle atmosphere. Changes in middle atmospheric ozone concentrations result in an altered radiative input to the troposphere and to the Earth’s surface, with implications on the energy balance and the chemical composition of the lower atmosphere. Although a wide range of ground- and satellite-based measurements of its integrated content and of its vertical distribution have been performed since several decades, a number of uncertainties still remain as to the response of middle atmospheric ozone to changes in solar irradiance over decadal time scales. This paper presents an overview of achieved findings, including a discussion of commonly applied data analysis methods and of their implication for the obtained results. We suggest that because it does not imply least-squares fitting of prescribed periodic or proxy data functions into the considered times series, time-domain analysis provides a more reliable method than multiple regression analysis for extracting decadal-scale signals from observational ozone datasets. Applied to decadal ground-based observations, time-domain analysis indicates an average middle atmospheric ozone increase of the order of 2% from solar minimum to solar maximum, which is in reasonable agreement with model results.     

Simultaneous Observations of Solar Energetic Particle Events by imp 8 and the Ulysses Cospin High Energy Telescope at High Solar Latitudes

With Ulysses approaching the south solar polar latitudes during a period of high solar activity, it is for the first time possible to study the distribution of solar energetic particles (SEPs) in solar latitude as well as in radius and longitude. From July 1997 to August 2000, Ulysses moved from near the solar equator at ∼5 AU to ∼67° S latitude at ∼3 AU. Using observations of >∼30 MeV protons from Ulysses and IMP-8 at Earth we find good correlation between large SEP increases observed at IMP and Ulysses, almost regardless of the relative locations of the spacecraft. The observations show that within a few days after injection of SEPs, the flux in the inner heliosphere is often almost uniform, depending only weakly on the position of the observer. No clear effect of the increasing solar latitude of Ulysses is evident. Since the typical latitudinal extent of CMEs, which most likely accelerate the SEPs, is only ∼30°, this suggests that the enhanced cross-field propagation for cosmic rays and CIR-accelerated particles deduced from Ulysses’ high latitude studies near solar minimum is also true for SEPs near solar maximum. This revised version was published online in August 2006 with corrections to the Cover Date.     

Observations of the Acceleration of Polar Solar Wind Near Solar Maximum

In a previous paper (Tappin et al., 1999) we used cross-correlation analysis of high-cadence observations with the LASCO coronagraphs to trace the acceleration of the solar wind at low latitudes. In this paper we present a similar analysis carried out over the North pole of the Sun. The observations which were made in March 2000 with the C3 coronagraph show low bulk flow speeds (comparable to or lower than those seen at the equator in early 1998). We observe the acceleration continuing to the edge of the C3 field of view at about 30 R _⊙. We also observe, as at low latitude, a high-speed tail but now reaching out well beyond 2000 km s⁻¹. We do not see a clear signature of a fast polar bulk flow. We therefore conclude that at this phase of the solar cycle, any fast bulk flow occupies only a small part of the line of sight and is therefore overwhelmed by the denser slow solar wind in these observations. We also show that the fast component is consistent with observed solar wind speeds at 1 AU. This revised version was published online in August 2006 with corrections to the Cover Date.     

Amplification of the influence of solar flux variations on the winter stratosphere by planetary waves

Recent observational evidence has suggested that variations in solar activity may affect winter stratospheric polar ozone and temperature levels. The paucity of direct sunlight available during this season points strongly to a dynamical mechanism. We have carried out several large ensemble experiments within the middle atmosphere and the coupled middle atmosphere and lower thermosphere to simulate the radiative/dynamical coupling via planetary waves for a range of solar fluxes. In the former case, the model response in the winter stratosphere was linear and of the order of the summer stratopause forcing, whilst in the latter, the level of correlation in the winter stratosphere remained high, but was diluted over a wider volume. The inclusion of the upper atmosphere enhanced the winter polar stratospheric response by a factor of three.     

The Solar System d/h Ratio: Observations and Theories

The measured D/H ratios in interstellar environments and in the solar system are reviewed. The two extreme D/H ratios in solar system water - (720±120)×10⁻⁶ in clay minerals and (88±11)×10⁻⁶ in chondrules, both from LL3 chondritic meteorites - are interpreted as the result of a progressive isotopic exchange in the solar nebula between deuterium-rich interstellar water and protosolar H₂. According to a turbulent model describing the evolution of the nebula (Drouart et al., 1999), water in the solar system cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking 720×10⁻⁶ as a face value for the isotopic composition of the interstellar water that predates the formation of the solar nebula, numerical simulations show that the water D/H ratio decreases via an isotopic exchange with H₂. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and the isotopic homogenization of the solar nebula was completed in 10⁶ years, reaching a D/H ratio of 88×10⁻⁶. In this model, cometary water should have also suffered a partial isotopic re-equilibration with H₂. The isotopic heterogeneity observed in chondrites result from the turbulent mixing of grains, condensed at different epochs and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in agreement with these chondritic data and their interpretation (Texeira et al., 1999). This revised version was published online in June 2006 with corrections to the Cover Date.     

3He-Rich Solar Energetic Particle Events

³He-rich solar energetic particle (SEP) events show huge enrichments of ³He and association with kilovolt electrons and Type-III radio bursts. Observations from a new generation of high resolution instruments launched on the Wind, ACE, Yohkoh, SOHO, TRACE, and RHESSI spacecraft have revealed many new properties of these events: the particle energy spectra are found to be either power-law or curved in shape, with the ³He spectrum often being distinctly different from other species. Ultra-heavy nuclei up to >200 amu are found to be routinely present at average enrichments of >200 times solar-system abundances. The high ionization states previously observed near ∼1 MeV/nucleon have been found to decrease towards normal solar coronal values in these events. The source regions have been identified for many events, and are associated with X-ray jets and EUV flares that are associated with magnetic reconnection sites near active regions. This paper reviews the current experimental picture and theoretical models, with emphasis on the new insights found in the last few years.     

The Effect of Solar UV Irradiance Variations on the Earth's Atmosphere

The response of the lower and middle atmosphere to variations in solar irradiance typical of those observed to take place over the 11-year activity cycle has been investigated. The effects on radiative heating rates of changing total solar irradiance, solar spectral irradiance and two different assumptions concerning stratospheric ozone have been studied with a radiative transfer code. The response in the stratosphere depends on the changes specified in the ozone distribution which is not well known. A general circulation model (GCM) of the atmosphere up to 0.1 mbar (about 65 km) has been used to study the impacts of these changes on the thermodynamical structure. The results in the troposphere are very similar to those reported by Haigh99 using a quite different GCM. In the middle atmosphere the model is able to reproduce quite well the observed seasonal evolution of temperature and wind anomalies. Calculations of radiative forcing due to solar variation are presented. These show that the thermal infrared component of the forcing, due to warming of the stratosphere, is important but suggest a near balance between the longwave and shortwave effects of the increased ozone so that ozone change may not be important for net radiative forcing. However, the structure of the ozone change does affect the detailed temperature response and the spectral composition of the radiation entering the troposphere.     

Interplanetary Shocks and Solar Wind Structure Approaching Solar Maximum: Helios,IMP-8 and Voyager Observations

We studied solar wind observations of five different spacecraft: Helios 1, Helios 2, IMP-8, Voyager 1 and Voyager 2, from November 1977 to February 1978. In this period the large-scale dynamics of the solar wind near of the ecliptic plane was characterized by transient forward shocks (TFSs), ejecta, unstable corotating interaction regions (CIRs), and complex and variable magnetic sector structures. We identified 12 forward shock events of different origin. We did not find any clear tendency of the shock parameters with heliocentric distance nor longitudinal angle, but comparing the observations of each shock event we found local variations in the shock strength and the mean propagation velocities from one spacecraft to another. These unsystematic variations indicate that there were local deformations of the shock fronts, which we attribute to the inhomogenuos solar wind structure that affects the shock propagation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Energy Spectra, Composition, and Other Properties of Ground-Level Events During Solar Cycle 23

We report spacecraft measurements of the energy spectra of solar protons and other solar energetic particle properties during the 16 Ground Level Events (GLEs) of Solar Cycle 23. The measurements were made by eight instruments on the ACE, GOES, SAMPEX, and STEREO spacecraft and extend from ～0.1 to ～500–700?MeV. All of the proton spectra exhibit spectral breaks at energies ranging from ～2 to ～46?MeV and all are well fit by a double power-law shape. A comparison of GLE events with a larger sample of other solar energetic particle (SEP) events shows that the typical spectral indices are harder in GLE events, with a mean slope of ?3.18 at >40?MeV/nuc. In the energy range 45 to 80?MeV/nucleon about ～50?% of GLE events have properties in common with impulsive ³He-rich SEP events, including enrichments in Ne/O, Fe/O, ²²Ne/²⁰Ne, and elevated mean charge states of Fe. These ³He-rich events contribute to the seed population accelerated by CME-driven shocks. An analysis is presented of whether highly-ionized Fe ions observed in five events could be due to electron stripping during shock acceleration in the low corona. Making use of stripping calculations by others and a coronal density model, we can account for events with mean Fe charge states of 〈Q _Fe〉≈+20 if the acceleration starts at ～1.24–1.6 solar radii, consistent with recent comparisons of CME trajectories and type-II radio bursts. In addition, we suggest that gradual stripping of remnant ions from earlier large SEP events may also contribute a highly-ionized suprathermal seed population. We also discuss how observed SEP spectral slopes relate to the energetics of particle acceleration in GLE and other large SEP events.     

Space Weather and the Ground-Level Solar Proton Events of the 23rd Solar Cycle

Solar proton events can adversely affect space and ground-based systems. Ground-level events are a subset of solar proton events that have a harder spectrum than average solar proton events and are detectable on Earth’s surface by cosmic radiation ionization chambers, muon detectors, and neutron monitors. This paper summarizes the space weather effects associated with ground-level solar proton events during the 23rd solar cycle. These effects include communication and navigation systems, spacecraft electronics and operations, space power systems, manned space missions, and commercial aircraft operations. The major effect of ground-level events that affect manned spacecraft operations is increased radiation exposure. The primary effect on commercial aircraft operations is the loss of high frequency communication and, at extreme polar latitudes, an increase in the radiation exposure above that experienced from the background galactic cosmic radiation. Calculations of the maximum potential aircraft polar route exposure for each ground-level event of the 23rd solar cycle are presented. The space weather effects in October and November 2003 are highlighted together with on-going efforts to utilize cosmic ray neutron monitors to predict high energy solar proton events, thus providing an alert so that system operators can possibly make adjustments to vulnerable spacecraft operations and polar aircraft routes.     

An Update on Ultra-Heavy Elements in Solar Energetic Particles above 10 MeV/Nucleon

Measurements below several MeV/nucleon from Wind/LEMT and ACE/ULEIS show that elements heavier than Zn (Z=30) can be enhanced by factors of ∼100 to 1000, depending on species, in ³He-rich solar energetic particle (SEP) events. Using the Solar Isotope Spectrometer (SIS) on ACE we find that even large SEP (LSEP) shock-accelerated events at energies from ∼10 to >100 MeV/nucleon are often very iron rich and might contain admixtures of flare seed material. Studies of ultra-heavy (UH) SEPs (with Z>30) above 10 MeV/nucleon can be used to test models of acceleration and abundance enhancements in both LSEP and ³He-rich events. We find that the long-term average composition for elements from Z=30 to 40 is similar to standard solar system values, but there is considerable event-to-event variability. Although most of the UH fluence arrives during LSEP events, UH abundances are relatively more enhanced in ³He-rich events, with the (34<Z<40)/O ratio on average more than 50 times higher in ³He-rich events than in LSEP events. At energies >10 MeV/nucleon, the most extreme event in terms of UH composition detected so far took place on 23 July 2004 and had a (34<Z<40)/O enhancement of ∼250–300 times the standard solar value.     

The Influence of Total Solar Irradiance on Climate

To estimate the effect of the solar variability on the climate, two estimates of the solar intensity variations during the last three centuries have been used as forcing in numerical simulations. The model employed to carry out the experiments was the same coupled global ocean-atmosphere model used in a number of studies to assess the effect of the anthropogenic greenhouse gases on climate. The near surface temperature and the tropospheric temperature distribution shows a clear response to the variability of the solar input. Even the thermohaline circulation reacts on the large amplitudes in the forcing. In the stratosphere, the response pattern is similar as in the observations, however, the 11-year cycle found in the forcing data does not excite an appreciable response. This might be due to the missing parameterisation of the increase in the UV-radiation at the solar cycle maximum and the connected increase of the stratospheric ozone concentration.