STATISTICAL ANALYSIS OF SOLAR PROTON FLUX INFLUENCE ON THERMODYNAMICS OF MIDDLE ATMOSPHERE IN THE NORTH HEMISPHERE

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1 Доклади на Българската академия на науките Comptes rendus de l Académie bulgare des Sciences Tome 67, No 1, 2014 EXPLORATIONS COSMIQUES STATISTICAL ANALYSIS OF SOLAR PROTON FLUX INFLUENCE ON THERMODYNAMICS OF MIDDLE ATMOSPHERE IN THE NORTH HEMISPHERE Yordan Tassev, Natalia Kilifarska, Dimitrinka Tomova (Submitted by Corresponding Member P. Velinov on September 20, 2013) Abstract Processes of interaction between highly energetic particles emitted from the Sun and Earth s atmosphere are of great importance for understanding the mechanisms of solar-terrestrial interactions. In this paper we present analysis of the impact of solar energetic protons on the middle-lower stratosphere thermodynamics during and after solar proton event (SPE) in January In attempt to discriminate protons the energy of which has the greatest influence on the stratosphere, seven energetic bands have been examined. We found statistically significant (at 2σ-level) response in temperature and zonal wind characterized by: (i) warming of the extra-tropical lower-middle stratosphere, (ii) weakening of westerly winds. In polar stratosphere (poleward of N), however, the stratosphere responds with cooling and zonal wind strengthening. Analysis of Eliassen-Palm (EP) fluxes reveals enhanced meridional heat transfer in extra-tropics, being sensitive to the energy of precipitating protons i.e., the lower the protons energy, the stronger the z-component Eliassen-Palm fluxes EP z flux is. These results are quite unexpected in the frame of current understanding, that SPE affects mainly upper atmospheric levels mesosphere and rarely upper stratosphere. Analysis of ozone response shows furthermore an enhancement of ozone density in the extratropical middle-low stratosphere, which is a reasonable explanation for the warmer stratosphere and enhanced meridional heat flux at these levels. Key words: solar proton flux, lower-middle stratosphere, zonal wind, Eliassen-Palm flux, stratospheric ozone Introduction. The solar proton event (SPE) from January 2005 is one of the most investigated phenomenons in the last years, together with its impact on the terrestrial atmosphere. The great interest in this event is due to the facts that: (i) it is the most powerful event which occurred in a period of minimum solar activity; (ii) it was accompanied by a powerful geomagnetic storm, modifying the 95

2 rigidity thresholds for particles precipitation. The showers of energetic particles initiate ion-molecular reactions in the middle atmosphere, leading to a serious disturbance of its chemical balance. The change in chemical composition, in its turn, changes thermal and dynamical characteristics of the medium, leading to a modification of the chemical reactions constants and redistribution of chemical composition. There are many papers devoted to investigation of particles effects on the chemical composition of the middle atmosphere [ 1 15 ]. The changes in thermal and dynamical properties of the medium - more specifically in the middlelower stratosphere, however, are less investigated. This motivates us to analyze the particles effect on the stratospheric thermodynamics and their effect on the planetary wave distribution. Data and methods. For this analysis we have used daily mean values of stratospheric temperature, zonal wind and ozone (taken from ERA Interim), Eliassen-Palm (EP) fluxes calculated within the EP5 project CANDIDOS (Chemical and Dynamical Influences on Decadal Ozone Change) by courtesy of Climate Science Division of Alfred Wegener Institute for Polar and Marine Research, and proton fluxes in seven energetic bands for the period January-April The energetic intervals of proton fluxes, which are measured by the geostationary satellite GOES 11 are: E = MeV, E = 4 9 MeV, E = 9 15 MeV, E = MeV, E = MeV, E = MeV and E = MeV. The altitudes where the thermodynamic parameters are measured are the following: 200 hpa km; 150 hpa km; 100 hpa km; 70 hpa km; 50 hpa km; 30 hpa km; 20 hpa km; 10 hpa km. The statistical analysis is based on cross correlation between time series of proton flux (in a given energy interval) and the corresponding atmospheric parameter (temperature, wind, ozone or EP flux) at fixed altitude and latitude. The deviations of each examined atmospheric parameter from its monthly mean are preliminarily smoothed by 10 points running medians, as well as by proton fluxes. Lagged correlation analysis has been performed with time lag of 60 days (i.e. we have looked for an atmospheric response up to 60 days after the particles forcing). The Eliassen-Palm (EP) flux [ 16 ] and its correlation with solar protons has been analyzes in order to estimate the changes in stratospheric ability to conduct wave-like disturbances in temperature and momentum. The two components of the EP vector can be written in pressure coordinates on the sphere as [ 17 ]: EP = {EP h,ep z } = a cos φ { u v + ψ u p ; u w ψ [ (u cos φ) acos φ φ f where EP h and EP z are /( horizontal and vertical components of EP flux, and ψ is kt defined as: ψ = v T p T ). Here φ represents latitude, p pressure, p 96 Y. Tassev, N. Kilifarska, D. Tomova ]}

3 Fig. 1. Cross-correlation coefficients of the T (temperature), as measured by MLS (Aura satellite), with proton fluxes of different energies for the period January March 2005 (left column); time lag of the T response (right column); dashed lines indicate negative cross-correlation

4 Fig. 2. Cross-correlation coefficients of the zonal wind, as measured by MLS (Aura satellite), with proton fluxes of different energies for the period January March 2005 (left column); time lag of the zonal wind response (right column); dashed lines indicate negative cross-correlation

5 Fig. 3. Cross-correlation coefficients of the EP z flux, with proton fluxes of different energies for the period January March 2005 (left column); time lag of the EP z flux response (right column); dashed lines indicate negative cross-correlation

6 Fig. 4. Cross-correlation coefficients of the O 3, as measured by MLS (Aura satellite), with proton fluxes of different energies for the period January March 2005 (left column); time lag of the O 3 response (right column); dashed lines indicate negative cross-correlation

7 u, v, w the velocity vectors in (longitude, latitude, pressure) coordinate system, T the temperature, a the Earth s radius, f the Coriolis parameter and k the ratio of the gas constant to the specific heat at constant pressure. Overbars denote zonal Eulerian averages at a constant latitude and pressure, and primes denote departure therefrom. Results of statistical analysis. The goal of this investigation is to analyze the medium-term impact of solar proton flux on the thermodynamical parameters of middle-lower stratosphere and wave propagation conditions during and after proton events. The results from the cross correlation analysis are shown in four figures, presenting the cross correlation coefficients in height interval km and latitudes from 30 to 80 N, at Greenwich meridian. Figure 1 presents the latitude-altitude distribution of cross correlation coefficients (CCC) between solar proton flux (at seven different energy intervals) and stratospheric temperature profile (left column). The right column illustrates the time delay of temperature response to proton forcing. Two regions are clearly formed in the left column of Fig. 1. The first region is characterized by positive CCC and is extended from 30 N to N latitude. The maximal positive CCC reaches 0.6 at 47 N latitude and 25 km altitude. In the second region, from 65 to 80 N, CCC are negative. The maximal negative CCC reach 0.4 in all energy intervals and only in the interval E = MeV occurs a value of 0.5. This CCC distribution is tracked in all protons energy intervals. Reference to the right column of Fig. 1 shows that in the areas with positive CCC (above 20 km), the time delay has a minimum value of 5 days. In the regions with negative CCC, however, the temperature response to the proton forcing is delayed by up to 30 days. Consequently, following proton eruptions on the Sun in January 2005, the stratospheric temperature northward of 65 N latitude decreases, while extratropics become warmer. Interestingly, the positive reaction to particles forcing is much faster (about 5 days in daily mean values of atmospheric parameters), while the negative reaction is delayed by days. Figure 2 shows the CCC distribution between solar proton fluxes and deviations of zonal wind from its monthly mean values. Three regions are found in the following sequence: positive CCC, negative CCC and after that positive CCC once again. The first region with positive CCC includes the area between N latitude at altitude 15 km. The region between 35 and 65 N latitudes is characterized by negative CCC with maximum values equal to 0.5. The line with zero CCC (between the zone with negative and the second area with positive correlation coefficients) is strongly inclined toward the equator, starting at 65 N. The maximum value of positive correlation (0.6) is found with proton fluxes having the lowest energy (E = MeV), while the coherence with higher energetic protons (above 80 MeV) is much weaker. Analysis of the zonal wind response to particles forcing shows that its weakening, in the extratropics, is practically without delay (see right column in Fig. 2). In the other 7 Compt. rend. Acad. bulg. Sci., 67, No 1,

8 areas with negative CCC, the decrease of zonal wind occurs with time lag of 5 to 10 days. The region with positive CCC has a time delay of 10 to 20 days. In Figure 3 are shown CCC between the proton fluxes with different energies and EP z flux. It can be seen in the left column of the figure, that the highest positive CCC (equal to 0.6) is obtained in the four energy bands having the lowest energy (i.e., E = MeV, E = 4 9MeV, E = 9 15 MeV, E = MeV). It is clearly seen that CCC values increase with decreasing the energy of the solar protons and this enhancement propagates downward. For example CCC with value 0.6 found for protons with E = MeV moves down to km, while the same correlation value obtained for protons with E=15 40 MeV is found at much higher altitude at 28 km. The time delay in all cases is 3 days or less. The regions with negative CCC are not important as geographic areas, and also as CCC values. Analyses of connectivity between EP h flux and solar protons show that the impact of the most energetic bands has a negligible impact in the meridional transfer of momentum. Only the most energetic particles have some influence near 50 N the correlation coefficient rises to 0.4 (not shown). As a result we conclude that changes in thermo-dynamical conditions of the lower-middle stratosphere, by energetic particles, modify the wave propagation condition in the region, stimulating meridional heat transfer but having small influence on the meridional transfer of momentum. Comparison with Figs 1 and 2 makes this result understandable warming of extratropics during and after the SPE event forces the heat diffusion, leading to weakening of westerlies (equatorward of the zone of heating) and their strengthening poleward. Consequently, stronger EP z correlation with proton fluxes is a result from the warming of extratropical middlelower stratosphere. Increased EP z on the other hand should hamper the vertical propagation of planetary waves originating in the troposphere. This conclusion is in line with the works of Chshyolkova et al. [ 18 ] and Belova et al. [ 19 ] reporting very low wave activity during the examined period. Discussion. The above results reasonably raise the question about the mechanism of such stratospheric warming (within 10 days of SPE). Numerous investigations show that direct impact of solar protons is highly improbable. One possible explanation of this result could be found in the ozone response to protons forcing. Lagged correlation coefficients, presented in Fig. 4, show that equatorward of 60 N lower-middle stratospheric O 3 responds with a positive anomaly to the impinging solar protons (left column in Fig. 4). The time delay of this response is 5 10 days (similar to temperature response to particles forcing). Enhanced ozone density is a good explanation for detected stratospheric warming (due to the stronger absorption of solar UV radiation by O 3 ). However, many measurements and modelling evidences show that the effect of solar particles is found predominantly at mesospheric levels. Our additional analyses show, however, that destruction of ozone aloft stimulates its formation at lower altitudes 98 Y. Tassev, N. Kilifarska, D. Tomova

9 [ 20 ] an effect known also as ozone self-healing. There is some misunderstanding of this effect in literature some authors relate it to the transformation of active chlorine and bromine families into their reservoir species by dynamically re-distributed NOx in the polar lower stratosphere. This in turn leads to a relatively small enhancement of the lower stratospheric O 3 in polar region because it is less destructed by ClO x and BrO x families [ 21 ]. The actual ozone self-healing is a result from the reduction of its optical depth aloft. To our knowledge, none of the climatic models reproduce this effect, because the ozone optical depth is not calculated interactively. This means that reduced O 3 optical depth (resulting from the severe destruction of mesospheric ozone) is not taken into account in the calculation of the ozone profile. We have examined exactly this effect of the reduced O 3 optical depth, which allows more UV radiation to penetrate deeper into the atmosphere, producing ozone there. We also show that the deeper the negative O 3 anomaly penetrates, the stronger the ozone self-healing effect is [ 22 ]. Conclusion. Applying lagged correlation analysis between solar proton fluxes with different energies and several lower-middle stratospheric parameters (temperature, zonal wind, ozone and EP flux), we have found that: (i) extratropical stratosphere (equatorward of N) is warmer than its monthly median values, up to 10 days after the solar proton event in January 2005; (ii) stratospheric westerly winds are reduced in the same region and approximately for the same period up to 10 days after the solar event; (iii) meridional heat flux (EP z ) in the region is intensified, while momentum flux is almost insensitive to the particles forcing; (iv) lower-middle stratospheric ozone density is enhanced within the 10 days period after the SPE event. Our interpretation of this connectivity between solar protons with different energies and atmospheric parameters is that increased O 3 density (initiated by the reduction of its optical depth aloft) warms the extra-tropical stratosphere over the the Greenwich meridian, thus altering the dynamical and wave propagating properties of the media. Acknowledgements. We are very thankful to the project teams of ERA- Interim reanalyses, as well as to SPIDR data centre for making their data available. REFERENCES [ 1 ] Banks P. M. J. Geophys. Res., 84, 1979, [ 2 ] Reagan J. B., R. E. Meyerott, R. W. Nightingale, R. C. Gunton, R. G. Johnson, J. E. Evans, W. L. Imhof, D. F. Heath, A. J. Krueger. J. Geophys. Res., 86, 1981, [ 3 ] Jackman C. H., R. D. McPeters. J. Geophys. Res., 90, 1985, [ 4 ] Roble R. G., E. C. Ridley. Ann. Geophys., 5A, 1987, [ 5 ] Reid G. C., S. Solomon, R. R. Garcia. Geophys. Res. Lett., 18, 1991, Compt. rend. Acad. bulg. Sci., 67, No 1,

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