Annika Seppälä

Geomagnetic activity and polar surface air temperature variability

Here we use the ERA-40 and ECMWF operational surface level air temperature data sets from 1957 to 2006 to examine polar temperature variations during years with different levels of geomagnetic activity, as defined by the A(p) index. Previous modeling work has suggested that NOx produced at high latitudes by energetic particle precipitation can eventually lead to detectable changes in surface air temperatures (SATs). We find that during winter months, polar SATs in years with high A(p) index are different than in years with low A(p) index; the differences are statistically significant at the 2-sigma level and range up to about +/- 4.5 K, depending on location. The temperature differences are larger when years with wintertime Sudden Stratospheric Warmings (SSWs) are excluded. We take into account solar irradiance variations, unlike previous analyses of geomagnetic effects in ERA-40 and operational data. Although we cannot conclusively show that the polar SAT patterns are physically linked by geomagnetic activity, we conclude that geomagnetic activity likely plays a role in modulating wintertime surface air temperatures. We tested our SAT results against variation in the Quasi Biennial Oscillation, the El Nino Southern Oscillation and the Southern Annular Mode. The results suggested that these were not driving the observed polar SAT variability. However, significant uncertainty is introduced by the Northern Annular Mode, and we cannot robustly exclude a chance linkage between sea surface temperature variability and geomagnetic activity.

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.

Polar vortex evolution during the 2002 Antarctic major warming as observed by the Odin satellite

In September 2002 the Antarctic polar vortex split in two under the influence of a sudden warming. During this event, the Odin satellite was able to measure both ozone (O3) and chlorine monoxide (ClO), a key constituent responsible for the so-called “ozone hole”, together with nitrous oxide (N2O), a dynamical tracer, and nitric acid (HNO3) and nitrogen dioxide (NO2), tracers of denitrification. The submillimeter radiometer (SMR) microwave instrument and the Optical Spectrograph and Infrared Imager System (OSIRIS) UV-visible light spectrometer (VIS) and IR instrument on board Odin have sounded the polar vortex during three different periods: before (19–20 September), during (24–25 September), and after (1–2 and 4–5 October) the vortex split. Odin observations coupled with the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model at and above 500 K isentropic surfaces (heights above 18 km) reveal that on 19–20 September the Antarctic vortex was dynamically stable and chemically nominal: denitrified, with a nearly complete chlorine activation, and a 70% O3 loss at 500 K. On 25–26 September the unusual morphology of the vortex is monitored by the N2O observations. The measured ClO decay is consistent with other observations performed in 2002 and in the past. The vortex split episode is followed by a nearly complete deactivation of the ClO radicals on 1–2 October, leading to the end of the chemical O3 loss, while HNO3 and NO2 fields start increasing. This acceleration of the chlorine deactivation results from the warming of the Antarctic vortex in 2002, putting an early end to the polar stratospheric cloud season. The model simulation suggests that the vortex elongation toward regions of strong solar irradiance also favored the rapid reformation of ClONO2. The observed dynamical and chemical evolution of the 2002 polar vortex is qualitatively well reproduced by REPROBUS. Quantitative differences are mainly attributable to the too weak amounts of HNO3 in the model, which do not produce enough NO2 in presence of sunlight to deactivate chlorine as fast as observed by Odin.

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.

Energetic electron precipitation during substorm injection events: high-latitude fluxes and an unexpected midlatitude signature

[1] Geosynchronous Los Alamos National Laboratory (LANL-97A) satellite particle data, riometer data, and radio wave data recorded at high geomagnetic latitudes in the region south of Australia and New Zealand are used to perform the first complete modeling study of the effect of substorm electron precipitation fluxes on low-frequency radio wave propagation conditions associated with dispersionless substorm injection events. We find that the precipitated electron energy spectrum is consistent with an e-folding energy of 50 keV for energies <400 keV but also contains higher fluxes of electrons from 400 to 2000 keV. To reproduce the peak subionospheric radio wave absorption signatures seen at Casey (Australian Antarctic Division), and the peak riometer absorption observed at Macquarie Island, requires the precipitation of 50–90% of the peak fluxes observed by LANL-97A. Additionally, there is a concurrent and previously unreported substorm signature at L < 2.8, observed as a substorm-associated phase advance on radio waves propagating between Australia and New Zealand. Two mechanisms are discussed to explain the phase advances. We find that the most likely mechanism is the triggering of wave-induced electron precipitation caused by waves enhanced in the plasmasphere during the substorm and that either plasmaspheric hiss waves or electromagnetic ion cyclotron waves are a potential source capable of precipitating the type of high-energy electron spectrum required. However, the presence of these waves at such low L shells has not been confirmed in this study.

Temporal variability of the descent of high-altitude NOx inferred from ionospheric data

In this study we investigate periods of enhanced ionization in the mesosphere during Northern Hemisphere wintertime. Long-lasting ionization enhancements (days) are typically produced by solar proton events or by the descent of thermospheric NOX during periods of sustained downward vertical transport associated with a strong underlying polar vortex. Using a new application of ground-based low-frequency radio wave remote sensing, we study the mesospheric ionization conditions during the Northern Hemisphere winters spanning 2003-2004, 2004-2005, and 2005-2006. The winter 2003-2004 subionospheric radio wave propagation data from a transmitter in Iceland shows signatures of the descent of NOX through 80 km altitude starting on 13 January 2004, during the occurrence of a strong polar vortex, indicating a thermospheric source for the NOX. Similar analysis of radio wave propagation data in the Northern Hemisphere winter of 2004-2005 does not show a NOX descent event passing through the mesosphere, due to a lack of downward vertical transport as a result of a weak underlying polar vortex, despite the occurrence of significant solar proton ionization during January 2005. In 2005-2006 there were no significant ionization events and also no descent of significant amounts of thermospheric NOX, despite a strong polar vortex and strong vertical transport. We model the signature of the descent of NOX seen in the radio wave propagation data using the Sodankyla Ion Chemistry model, confirming that the levels of NOX in the mesosphere are similar to 100 times the usual background levels. The combination of strong NOX sources in the thermosphere and also a strong polar vortex is required for NOX to descend into the stratosphere with significant concentration levels.

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.

Radiation belt electron precipitation due to geomagnetic storms: Significance to middle atmosphere ozone chemistry

[1] Geomagnetic storms triggered by coronal mass ejections and high‐speed solar wind streams can lead to enhanced losses of energetic electrons from the radiation belts into the atmosphere, both during the storm itself and also through the poststorm relaxation of enhanced radiation belt fluxes. In this study we have analyzed the impact of electron precipitation on atmospheric chemistry (30–90 km altitudes) as a result of a single geomagnetic storm. The study conditions were chosen such that there was no influence of solar proton precipitation, and thus we were able to determine the storm‐induced outer radiation belt electron precipitation fluxes. We use ground‐based subionospheric radio wave observations to infer the electron precipitation fluxes at L = 3.2 during a geomagnetic disturbance which occurred in September 2005. Through application of the Sodankyla Ion and Neutral Chemistry model, we examine the significance of this particular period of electron precipitation to neutral atmospheric chemistry. Building on an earlier study, we refine the quantification of the electron precipitation flux into the atmosphere by using a time‐varying energy spectrum determined from the DEMETER satellite. We show that the large increases in odd nitrogen (NO x) and odd hydrogen (HO x) caused by the electron precipitation do not lead to significant in situ ozone depletion in September in the Northern Hemisphere. However, had the same precipitation been deposited into the polar winter atmosphere, it would have led to >20% in situ decreases in O 3 at 65–80 km altitudes through catalytic HO x cycles, with possible additional stratospheric O 3 depletion from descending NO x beyond the model simulation period. Citation: Rodger, C. J., M. A. Clilverd, A. Seppala, N. R. Thomson, R. J. Gamble, M. Parrot, J.‐A. Sauvaud, and T. Ulich (2010), Radiation belt electron precipitation due to geomagnetic storms: Significance to middle atmosphere ozone chemistry,