Pekka T. Verronen

Missing driver in the Sun-Earth connection from energetic electron precipitation impacts mesospheric ozone.

Energetic electron precipitation (EEP) from the Earth’s outer radiation belt continuously affects the chemical composition of the polar mesosphere. EEP can contribute to catalytic ozone loss in the mesosphere through ionization and enhanced production of odd hydrogen. However, the long-term mesospheric ozone variability caused by EEP has not been quantified or confirmed to date. Here we show, using observations from three different satellite instruments, that EEP events strongly affect ozone at 60–80 km, leading to extremely large (up to 90%) short-term ozone depletion. This impact is comparable to that of large, but much less frequent, solar proton events. On solar cycle timescales, we find that EEP causes ozone variations of up to 34% at 70–80 km. With such a magnitude, it is reasonable to suspect that EEP could be an important part of solar influence on the atmosphere and climate system.

Arctic and Antarctic polar winter NOx and energetic particle precipitation in 2002-2006

Received 19 February 2007; revised 8 May 2007; accepted 16 May 2007; published 26 June 2007. [1] We report GOMOS nighttime observations of middle atmosphere NO2 and O3 profiles during eight recent polar winters in the Arctic and Antarctic. The NO2 measurements are used to study the effects of energetic particle precipitation and further downward transport of polar NOx. During seven of the eight observed winters NOx enhancements occur in goodcorrelation withlevelsofenhancedhigh-energyparticle precipitation and/or geomagnetic activity as indicated by the Ap index. We find a nearly linear relationship between the average winter time Ap index and upper stratospheric polar winterNO2columndensityinbothhemispheres.IntheArctic winter 2005–2006 the NOx enhancement is higher than expected from the geomagnetic conditions, indicating the importance of changing meteorological conditions.

Nighttime ozone profiles in the stratosphere and mesosphere by the Global Ozone Monitoring by Occultation of Stars on Envisat

[1] The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European Space Agency’s Envisat satellite measures ozone and a few other trace gases using the stellar occultation method. Global coverage, good vertical resolution and the self-calibrating measurement method make GOMOS observations a promising data set for building various climatologies. In this paper we present the nighttime stratospheric ozone distribution measured by GOMOS in 2003. We show monthly latitudinal distributions of the ozone number density and mixing ratio profiles, as well as the seasonal variations of profiles at several latitudes. The stratospheric profiles are compared with the Fortuin-Kelder daytime ozone climatology. Large differences are found in polar areas and they can be shown to be correlated with large increases of NO2. In the upper stratosphere, ozone values from GOMOS are systematically larger than in the Fortuin-Kelder climatology, which can be explained by the diurnal variation. In the middle and lower stratosphere, GOMOS finds a few percent less ozone than Fortuin-Kelder. In the equatorial area, at heights of around 15–22 km, GOMOS finds much less ozone than Fortuin-Kelder. For the mesosphere and lower thermosphere, there has previously been no comprehensive nighttime ozone climatology. GOMOS is one of the first new instruments able to contribute to such a climatology. We concentrate on the characterization of the ozone distribution in this region. The monthly latitudinal and seasonal distributions of ozone profiles in this altitude region are shown. The altitude of the mesospheric ozone peak and the semiannual oscillation of the number density are determined. GOMOS is also able to determine the magnitude of the ozone minimum around 80 km. The lowest seasonal mean mixing ratio values are around 0.13 ppm. The faint tertiary ozone peak at 72 km in polar regions during wintertime is observed.

The effects of hard‐spectra solar proton events on the middle atmosphere

The stratospheric and mesospheric impacts of the solar proton events of January 2005 are studied here using ion and neutral chemistry modeling and subionospheric radio wave propagation observations and modeling. This period includes three SPEs, among them an extraordinary solar proton storm on 20 January, during which the >100 MeV proton fluxes were unusually high, making this event the hardest in solar cycle 23. The radio wave results show a significant impact to the lower ionosphere/middle atmosphere from the hard spectrum event of 20 January with a sudden radio wave amplitude decrease of about 10 dB. Results from the Sodankyla Ion and Neutral Chemistry model predict large impacts on the mesospheric NOx (400-500%) and ozone (-30 to -40% NH, -15% SH) in both the northern (winter) and the southern (summer) polar regions. The direct stratospheric effects, however, are only about 10- 20% enhancement in NOx, which result in -1% change in O-3. Imposing a much larger extreme SPE lasting 24 hours rather than just 1 hour produced only about 5% ozone depletion in the stratosphere. Only a massive hard-spectra SPE with high-energy fluxes over ten times larger than observed here (>30 MeV fluence of 1.0 x 10(9) protons/cm(2)), as, e. g., the Carrington event of 1859 (>30 MeV fluence of 1.9 x 10(10) protons/cm(2)), could presumably produce significant in situ impacts on stratospheric ozone.

Nitric acid enhancements in the mesosphere during the January 2005 and December 2006 solar proton events

We investigate enhancements of mesospheric nitric acid (HNO(3)) in the Northern Hemisphere polar night regions during the January 2005 and December 2006 solar proton events (SPEs). The enhancements are caused by ionization due to proton precipitation, followed by ionic reactions that convert NO and NO(2) to HNO(3). We utilize mesospheric observations of HNO(3) from the Microwave Limb Sounder (MLS/Aura). Although in general MLS HNO(3) data above 50 km (1.5 hPa) are outside the standard recommended altitude range, we show that in these special conditions, when SPEs produce order-of-magnitude enhancements in HNO(3), it is possible to monitor altitudes up to 70 km (0.0464 hPa) reliably. MLS observations show HNO(3) enhancements of about 4 ppbv and 2 ppbv around 60 km in January 2005 and December 2006, respectively. The highest mixing ratios are observed inside the polar vortex north of 75 degrees N latitude, right after the main peak of SPE forcing. These measurements are compared with results from the one-dimensional Sodankyla Ion and Neutral Chemistry (SIC) model. The model has been recently revised in terms of rate coefficients of ionic reactions, so that at 50-80 km it produces about 40% less HNO(3) during SPEs compared to the earlier version. This is a significant improvement that results in better agreement with the MLS observations. By a few days after the SPEs, HNO(3) is heavily influenced by horizontal transport and mixing, leading to its redistribution and decrease of the SPE-enhanced mixing ratios in the polar regions.

Global measurement of the mesospheric sodium layer by the star occultation instrument GOMOS

We present the first global measurement of the sodium mesospheric layer obtained from the processing of about 100 000 star occultations by the GOMOS instrument onboard the ENVISAT satellite. The retrieval method is developed on the basis of a simple DOAS retrieval applied to averaged transmittances. The vertical inversion of the sodium slant path optical thickness is performed by using a modified Gaussian extinction profile. A global climatology is derived by using monthly bins of 20 degrees in latitude. The high variability of the sodium layer is confirmed on a global scale as well as the presence of an important modulation in the annual cycle. Also, we present some evidence for the existence of a diurnal cycle characterized by an increase of the sodium concentration in daylight.

Substorm-induced energetic electron precipitation: Impact on atmospheric chemistry

Magnetospheric substorms drive energetic electron precipitation into the Earths atmosphere. We use the output from a substorm model to describe electron precipitation forcing of the atmosphere during an active substorm period in April–May 2007. We provide the first estimate of substorm impact on the neutral composition of the polar middle atmosphere. Model simulations show that the enhanced ionization from a series of substorms leads to an estimated ozone loss of 5–50% in the mesospheric column depending on season. This is similar in scale to small to medium solar proton events (SPEs). This effect on polar ozone balance is potentially more important on long time scales (months to years) than the impulsive but sporadic (few SPE/year versus three to four substorms/day) effect of SPEs. Our results suggest that substorms should be considered an important source of energetic particle precipitation into the atmosphere and included in high-top chemistry-climate models.

WACCM-D—Whole Atmosphere Community Climate Model with D-region ion chemistry

Energetic particle precipitation (EPP) and ion chemistry affect the neutral composition of the polar middle atmosphere. For example, production of odd nitrogen and odd hydrogen during strong events can decrease ozone by tens of percent. However, the standard ion chemistry parameterization used in atmospheric models neglects the effects on some important species, such as nitric acid. We present WACCM-D, a variant of the Whole Atmosphere Community Climate Model, which includes a set of lower ionosphere (D-region) chemistry: 307 reactions of 20 positive ions and 21 negative ions. We consider realistic ionization scenarios and compare the WACCM-D results to those from the Sodankyla Ion and Neutral Chemistry (SIC), a state-of-the-art 1-D model of the D-region chemistry. We show that WACCM-D produces well the main characteristics of the D-region ionosphere, as well as the overall proportion of important ion groups, in agreement with SIC. Comparison of ion concentrations shows that the WACCM-D bias is typically within ±10% or less below 70 km. At 70–90 km, when strong altitude gradients in ionization rates and/or ion concentrations exist, the bias can be larger for some groups but is still within tens of percent. Based on the good agreement overall and the fact that part of the differences are caused by different model setups, WACCM-D provides a state-of-the-art global representation of D-region ion chemistry and is therefore expected to improve EPP modeling considerably. These improvements are demonstrated in a companion paper by Andersson et al.

Description and validation of a limb scatter retrieval method for Odin/OSIRIS

In this paper we present the Modified Onion Peeling (MOP) inversion method, which is for the first time used to retrieve vertical profiles of stratospheric trace gases from Odin/OSIRIS limb scatter measurements. Since the original publication of the method in 2002, the method has undergone major modifications discussed here. The MOP method now uses a spectral microwindow for the NO 2 retrieval, instead of the wide UV-visible band used for the ozone, air, and aerosol retrievals. We give a brief description of the algorithm itself and show its performance with both simulated and real data. Retrieved ozone and NO 2 profiles from the OSIRIS measurements were compared with data from the GOMOS and HALOE instruments. No more than 5% difference was found between OSIRIS daytime and GOMOS nighttime ozone profiles between 21 and 45 km. The difference between OSIRIS and HALOE sunset NO 2 mixing ratio profiles was at most 25% between 20 and 40 km. The neutral air density was compared with the ECMWF analyzed data and around 5% difference was found at altitudes from 20 to 55 km. However, OSIRIS observations yield as much as 80% greater aerosols number density than GOMOS observations between 15 and 35 km. These validation results indicate that the quality of MOP ozone, NO 2 , and neutral air is good. The new version of the method introduced here is also easily expanded to retrieve additional species of interest.

Effects of meteoric smoke particles on the D-region ion-chemistry

This study focuses on meteor smoke particle (MSP) induced effects on the D region ion chemistry. Hereby, MSPs, represented with an 11 bin size distribution, have been included as an active component into the Sodankya Ion and Neutral Chemistry model. By doing that, we model the diurnal variation of the negatively and positively charged MSPs as well as ions and the electron density under quiet ionospheric conditions. Two distinct points in time are studied in more detail, i.e., one for sunlit conditions (Solar zenith angle is 72∘) and one for dark conditions (Solar zenith angle is 103∘). We find nightly decrease of free electrons and negative ions, the positive ion density is enhanced at altitudes above 80 km and reduced below. During sunlit conditions the electron density is enhanced between 60 and 70 km altitude, while there is a reduction in negative and positive ions densities.