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Modeling Impacts of Geomagnetic Field Variations on Middle Atmospheric

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Modeling Impacts of Geomagnetic Field Variations on Middle Atmospheric JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D02302, doi:10.1029/2007JD008574, 2008 Modeling impacts of geomagnetic field variations on middle atmospheric ozone responses to solar proton events on long timescales Holger Winkler,1 Miriam Sinnhuber,1 Justus Notholt,1 May-Britt Kallenrode,2 Friedhelm Steinhilber,2,3 Joachim Vogt,4 Bertalan Zieger,4,5,6 Karl-Heinz Glassmeier,7 and Anja Stadelmann7 Received 21 February 2007; revised 17 August 2007; accepted 20 September 2007; published 17 January 2008. [1] Strength and structure of the Earth’s magnetic field control the deflection of energetic charged particles of solar and cosmic origin. Therefore variations of the geomagnetic field occurring on geological timescales affect the penetration of charged particles into the atmosphere. During solar proton events (SPEs) the flux of high-energy protons from the Sun is markedly increased. In order to investigate the impact of SPEs on the middle atmospheric ozone on longer timescales, two-dimensional atmospheric chemistry and transport simulations have been performed using simulated time series of SPEs covering 200 years. Monte Carlo calculations were used to obtain ionization rates, which were then applied to the atmosphere under the consideration of different shielding properties of the geomagnetic field. The present-day magnetic field configuration and four other scenarios were analyzed. For the first time, field configurations representing possible realistic situations during reversals have been investigated with respect to SPE-caused ozone losses. With decreasing magnetic field strength the impacts on the ozone are found to significantly increase especially in the Southern Hemisphere, and subsequently, the flux of harmful ultraviolet radiation increases at the Earth’s surface. The ozone destructions are most pronounced in the polar regions, and for some field configurations they exceed the values of ozone hole situations after large SPEs. In contrast to ozone holes the depletions due to SPEs are not restricted to winter and spring times but persist into polar summer. Citation: Winkler, H., M. Sinnhuber, J. Notholt, M.-B. Kallenrode, F. Steinhilber, J. Vogt, B. Zieger, K.-H. Glassmeier, and A. Stadelmann (2008), Modeling impacts of geomagnetic field variations on middle atmospheric ozone responses to solar proton events on long timescales, J. Geophys. Res., 113, D02302, doi:10.1029/2007JD008574. 1. Introduction air molecules the precipitating energetic protons lose kinetic energy and produce a number of partly ionized atomic and [2] It is well established that solar proton events (SPEs) molecular fragments, and energetic secondary electrons are sources of distinct chemical disturbances in the middle cause further dissociations and ionizations. Protons occur- atmosphere (stratosphere and mesosphere). The pioneering ring in SPEs typically have kinetic energies between several works were those of Swider and Keneshea [1973] and tens of keV and many hundred MeV. Most important in Crutzen et al. [1975]. Through collisional interactions with terms of middle atmospheric impacts are the protons in the energy range from 10 to 500 MeV, of which the most 1 energetic ones can reach the tropopause. For characteristic Institute of Environmental Physics, University of Bremen, Bremen, protons’ energy spectra the bulk of ion production takes Germany. 2Department of Physics, Universita¨t Osnabru¨ck, Osnabru¨ck, Germany. place in the upper part of the middle atmosphere. Because 3Now at Swiss Federal Institute of Aquatic Science and Technology of the large atmospheric abundance of molecular nitrogen (Eawag), Dubendorf, Switzerland. and oxygen, initially, mainly N+,O+,N+,O+, and atomic 4 2 2 School of Engineering and Science, Jacobs University Bremen, oxygen and nitrogen are produced. Subsequent rapid ion- Bremen, Germany. 5Geodetic and Geophysical Research Institute, Hungarian Academy of neutral reactions and recombinations lead to the formation Sciences, Sopron, Hungary. of NOx (= N, NO, NO2)[Crutzen et al., 1975; Porter et al., 6 Now at Department of Atmospheric, Oceanic and Space Sciences, 1976; Rusch et al., 1981]. Additionally, HOx (= H, OH, University of Michigan, Ann Arbor, Michigan, USA. HO2) is being produced by reactions involving ionic water 7Institut fu¨r Geophysik und Meteorologie, Technische Universita¨t Braunschweig, Brunswick, Germany. clusters [Swider and Keneshea, 1973; Solomon et al., 1981]. Both NOx and HOx are well known for their ability to Copyright 2008 by the American Geophysical Union. destroy the ozone in catalytic cycles. Furthermore, there is 0148-0227/08/2007JD008574 D02302 1of11 D02302 WINKLER ET AL.: GEOMAGNETIC FIELD IMPACT ON OZONE RESPONSES D02302 atomic oxygen released [Porter et al., 1976] which directly depressed by geomagnetic disturbances which are often affects the odd oxygen chemistry. As a result the ozone associated with Earth’s directed SPEs. For the large SPE chemistry can be significantly disturbed by SPEs. The HOx- in August 1972 a value of 58° for lc was determined by caused O3 destruction is very effective in the upper part of Reagan et al. [1981]. Such values correspond to a rather the middle atmosphere (above about 40 km) [Nicolet, 1970; strong coupling between the solar wind and the magneto- Lary, 1997]. Because of their high reactivity the produced sphere [Siscoe and Chen, 1975] which are indeed not HOx species have a relatively short lifetime, and transport unrealistic during and right after large SPEs. More recently, processes are of minor importance for them. In contrast to Birch et al. [2005] have investigated variations in lc during that, enhanced NOx concentrations can remain at high levels three SPEs in the year 2001, exploiting proton-counting for several weeks or even months after a SPE, provided that rates from polar-orbiting satellites. Their finding was that destruction through solar radiation is small. This is espe- the cutoff latitude varies around 60° and decreases by up to cially the case under polar night conditions which at the 10° during the observed SPEs. They also report small same time cause pronounced downward transport by diurnal variation. Nevertheless, on average, lc =60° seems descending air masses. These dynamical issues in combi- to be a reasonable assumption for the current magnetic field. nation with the longer lifetimes at high solar zenith angles [6] The terrestrial magnetic field is not static. As men- enable NOx to be transported downward quite effectively tioned above, solar wind variations have an influence on its [Jackman et al., 1990; Randall et al., 2001] and to cause structure on timescales from hours up to years. Of greater significant upper stratospheric ozone depletions months interest for our study are the slower variations of the Earth’s after an event [Jackman et al., 2000]. In terms of transport internal magnetic field which can greatly alter the shape of it is useful to consider the odd nitrogen NOy =NOx +NO3 + the magnetosphere and with that its shielding properties HNO3 +HO2NO2 + ClONO2 +2Â N2O5 instead of NOx as against charged extraterrestrial particles. Within the last it contains basically all reactive nitrogen species. In various 400 years the magnitude of the Earth’s magnetic dipole studies the impacts of several large SPEs on the middle term has decreased quite significantly by about 39% atmosphere have been analyzed. The formation of NOx and [Fraser-Smith, 1987]. On longer timescales, even more the subsequent O3 destruction have been measured during a pronounced changes of both the geomagnetic field strength number of large SPEs, and the findings are in accordance and its topology occur [see, e.g., McElhinny and Senanayake, with results from atmospheric chemistry models [e.g., 1982]. The terrestrial magnetic field has changed its polarity Solomon et al., 1983; Jackman et al., 2001; Verronen et many times at irregular intervals of about 250,000 years al., 2005; Rohen et al., 2005]. throughout geological history. During such a field reversal [3] There are fewer investigations concerning the SPE- the dipole component is depressed to lower than 25% of the caused HOx perturbations; for example, indirect observations present value for several thousand years, and then the field is are presented by von Clarmann et al. [2005]. In the study of no longer dominated by its dipole component [Merrill and Verronen et al. [2006], convincing agreements of model McFadden, 1999]. Long-term effects of the changing geo- predictions with Microwave Limb Sounder hydroxyl meas- magnetic fields with respect to space climatology were urements for the SPE in January 2005 are presented, and studied by Glassmeier et al. [2004]. Sinnhuber et al. [2003] Seppa¨la¨etal.[2006] have found that model results in terms have investigated the extreme case effect of a completely of ozone destruction by HOx in the mesosphere during a SPE vanishing magnetic field on chemical SPE impacts in the are confirmed by global ozone monitoring by occultation of middle atmosphere. They found that the atmospheric dis- stars (GOMOS) data. Anyway, for the purpose of longer-term turbances caused by large SPEs strongly increase if there is effects on ozone, as they are addressed here, the HOx no magnetic shielding applied at all. Then losses of total contribution is of minor importance. ozone reach up to 50% in polar regions, and erythemal [4] The quoted comparisons with measurements give weighted UVB increases in midlatitudes by about 10%. confidence in the tools for modeling the atmospheric However, the assumption of an atmosphere entirely exposed impacts of SPEs focusing on ozone destruction so that it to energetic solar protons is an absolute extreme case and appears justified to use them for the investigations presented quite unlikely. In order to regard more realistic scenarios it is here. The atmospheric chemistry model is described in necessary to consider the nature of the Earth’s magnetic field section 2.2. specifically. Details about geomagnetic field variations can [5] The regions in which extraterrestrial charged particles be derived from the worldwide analysis of paleomagnetic actually can enter the atmosphere are determined by their proxy data [e.g., Mankinen and Dalrymple, 1979].
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