DO SOLAR FLARES AFFECT TOTAL OZONE? DEGO MARIN. I.N.T.A., Atmospheric Sounding Station "ElArenosillo", Mazagon, Spain' JAN LASTOVICKA

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1 DO SOLAR FLARES AFFECT TOTAL OZONE? DEGO MARIN I.N.T.A., Atmospheric Sounding Station "ElArenosillo", Mazagon, Spain' JAN LASTOVICKA Institute of Atmospheric Physics, Acad. Sci. Czech Rep., Prague 4, Czech Republic* Summary: An analysis of total ozone from Hradec Kralove (50,25"N, E) and of radio wave absorption in the lower ionosphere at 1539 khz (reflection point 50.3"N, ]J.8 E) shows that there is no detectable effect of strong solar flares in total ozone, no correlation between total ozone and absorption on a day-to-day time scale, and that strong solar flares do not affect this correlation. Thus the long-term correlation of monthly average values (Alberca et al, 1996) is not reproduced on a day-to-day time scale, and the effects of strong geomagnetic storms in total ozone (LaStovicka et al., 1992; Mich and LaStovicka, 1996) have no counterpart in effects of strong solar flares. Key words: total ozone, lower ionosphere, solar flares 1. INTRODUCTION Atmospheric ozone is affected primarily by internal atmospheric processes, namely by dynamics and to some extent by chemistry. However, there are also some effects of extraterrestrial origin on ozone. The variations of stratospheric and mesospheric ozone and total ozone with the 11 -year solar cycle and the 27-day solar rotation due to variations of solar UV irradiance are a well-known phenomenon. In total ozone, the solar cycle variation attains 1.5-2%, about 85% of this variation being from heights below 25 km (e.g. Hood, 1997}. The total ozone changes by less than 0.5% in the 27-day cycle (e.g. Brasseur, 1993). Among the solar wind and high energy particle-related effects on ozone layer, the best known effects are those of solar proton events (SPEs). SPEs reduce ozone concentration (e.g. Jackman et al., 1995) mainly in the mesosphere. For the SPEs of August 1972, the decrease in total ozone attained only 2% (Reagan et al., 1981), for the SPE of October 1981 only slightly more than 2% (Jackman et al., 1995). Shumilov et al. (1992) found, for ground level SPEs, a significant decrease of total ozone in the polar cap, but no detectable effect at auroral latitudes. Highly relativistic electrons (HRE) affect ozone, as well. Goldberg et al. (1995) found an ozone depletion of more than 25% near 75 km above Poker Flat, Alaska as a consequence of a HRE event. 1 Address: I.N.T.A., Space Sci. Div., Remote Sensing Aeron. Dept., Atmospheric Sounding Station "El Arenosillo", Mazagon (Moguer, Huelva), Spain; marinsd@inta.es 2 Address: Institute of Atmospheric Physics, Acad. Sci. Czech Rep., Bocni II, Prague 4, Czech Republic; jla@ufa.cas.cz Studia geoph. et geod. 42 (1998), StudiaGeo s.r.o., Prague 533

2 D. Marin andj. Lastovicka Another factor are strong geomagnetic storms. Lastovifka et al. (1992), Mich (1994) and Mich and LaStovicka (1995) analysed the total ozone along the three latitudinal circles, 40, 50 and 60"N. They found that the effect was developed best near 50 N, was much less systematic near 60 N, and was negligible near 40 N; it occurred in winter, was much weaker in equinoctial periods, and was absent in summer. The effect was well-developed and statistically significant only for strong geomagnetic storms under high solar activity and the east phase of QBO. The effect is qualitatively consistent with storm-related changes in dynamics. It consists in ozone redistribution. The possible effect of solar flares on total ozone have not yet been studied. It is impossible to study effects of flare-enhanced solar UV-irradiance. First, at UV wavelengths in question the flare effect is weak, if detectable. Second, continuous total ozone measurements during flares at one place with a time resolution of a few minutes are not available. Thus, we concentrate on longer-term effects of flares on total ozone, e.g. through changes of dynamics. Alberca etal. (1996) found positive correlations between monthly values of total ozone and radio wave absorption in the lower ionosphere. Therefore, we investigate the possible effect of solar flares on the relationship between them. We do not study effects of solar flares on absorption. These effects (SWFs) are well known. 2. DATA The central European data (analysed also by Alberca et al., 1996), are analyzed in this paper. The data from two other regions, used by Alberca et al. (1996) - Bulgaria and Spain - cannot be used to analyze daily values because too many data are missing on the day-to-day time scale. The period was investigated. Altogether 114 events with good quality data were selected. They were all analyzed individually as case studies from both points of view, solar flare effect on total ozone and solar flare effect on the correlation between total ozone and absorption variations. The preliminary results based on a limited data sample (de la Moreno et al., 1997) and confirmed by the analysis of the 114 events with absorption at 6090, 1539 and 243 khz, indicate that some effects can be expected only for strong flares associated with strong sudden ionospheric disturbances (SIDs) and apparently correlating absorption only occurs at 1539 khz (closest to the site of ozone measurements). Some ozone data from the winter of 1989/1990 were used in Fig. 4. The homogenized Dobson total ozone data from Hradec Kralove (50.25 N, E; Vanidek, 1991) are used to represent ozone in central Europe. The radio wave absorption at 1539kHz (reflection point <p = 50.3 N, A= 11.8 E) is used as measured by the A3 method (oblique incidence on the ionosphere) and reduced to % = 15. Solar flares were selected according to their effects in the lower ionosphere, SIDs (information on flare-related enhancement of UV flux is not available). Data were taken from Solar-Geophysical Data of ERL/NOAA, Boulder. Only flares with SIDs of importance 3- and higher, i.e. mighty flares with very profound ionospheric effects, were taken into account. We found only ten such events with good quality ozone and absorption data for the period Five other events were selected in winter 1989/ Studia geoph. et geod. 42 (1998)

3 Do Solar Flares Affect Total Ozone? 3. RESULTS The data have been analyzed by the linear cross-correlation technique, and using the Fisher criterion with a reliability of 95%, to obtain the significance levels of correlations for different time lags between the absorption and ozone series. First we will present the results obtained for the three solar flares that occurred on 28/01/81, 8/02/81 and 19/08/81, which were accompanied by SIDs of importance 3, 3 and 3+, respectively. Figure 1 shows the evolution of the total ozone from Hradec Kralove for all three events. The vertical dotted lines represent the days when solar flares occurred. Some events, as that of 28/01/81, perhaps seem to indicate an apparent slight flare-related decrease of total ozone. However, when we compare all three events shown in Fig. 1, no evident and persistent flare effect on total ozone is found. Short-term changes of total ozone are substantially affected by meteorological changes. In the periods of 26/01-07/02 (event 28/01) and 03/02-16/02 (event 08/02), Hradec KrSlove was first located in the anticyclonic area both at the surface and at 500 hpa with a relatively weak circulation changing between coming from west (W) to coming from northeast (NE), i.e. with no southern component (27/01-02/02). Then it changed to a regime with everyday frontal passages and prevailingly cyclonic circulation from W to NW (04/02-11/2). This was followed by a period (12/02-16/02) of weaker circulation from W to NE in a less Fig. 1. Response of total ozone at Hradec Kralov6 to strong solar flares of 28 January, 8 February and 19 August 1981 (marked by dotted vertical lines). Studia geoph. et geod. 42 (1998) 535

4 D. Marin and J. Lastovicka developed pressure field. Thus the meteorological regime at surface, as well as at 500 hpa was relatively stable, and its effect on total ozone was less than is typical for winter. For the event of 19/08 (14/08-26/08), the tropospheric pressure field was rather undeveloped, circulation was from W-N, weak to medium; the 500 hpa level displayed almost permanently a medium wind from W, usually in the external part of a relatively weak low. Figure 2 presents the coefficients of absorption-ozone correlation with their significance bands, calculated for time-lags from -6 to +6 days, for all three events. No statistically significant relation between absorption and ozone variations can be found for any event and any time-lag,. Moreover, flares do not seem to reveal any detectable and persistent influence on cross-correlation coefficients. The above case studies indicate rather no longer-time effect of solar flares on total ozone and its relation to ionospheric radio wave absorption. Figure 3 shows the average effect of strong solar flares on total ozone and ozone-absorption cross correlations, obtained for the ten events studied by the overlapping epoch method. There is no detectable effect of strong solar flares in total ozone at Hradec Kra"love", particularly in comparison with the magnitude of the effects of strong geomagnetic storms (LaStovidka et al., 1992). There is evidently no significant cross-correlation between day-to-day values of the 1539 khz absorption and total ozone with any time-lag. Long-term correlation between absorption and total ozone, found by Alberca et al. (1996), seems to be developed best with a time-lag of 1-2 months, thus absence of day-to-day correlations does not contradict to the existence of delayed long-term correlation. Fig. 2. Cross correlations between the 1539 khz radio wave absorption and total ozone from Hradec Kralove; lag -6 to +6 days; band of 95% significance level is shown. 536 Studia geoph. et geod. 42 (1998)

5 Do Solar Flares Affect Total Ozone? Fig. 3. The ten-event average effect of strong solar flares in total ozone (upper panel) and the tenevent average cross correlation of the 1539 khz absorption with total ozone (lower panel). Effects of geomagnetic storms on total ozone in central Europe occur and are distinct only for strong storms in winter under high solar activity and the E-phase of QBO (E-QBO) (Lastovidka et al., 1992). Under these conditions all events reveal a similar effect in total ozone in central Europe. Consequently, it is necessary to check the possible influence of QBO on the effect of solar flares on total ozone. All the examined effects occurred under high solar activity conditions - the period of was the period of maximum of solar cycle 21. Two of the three events shown in Figs. 1 and 2 occurred in winter. However, they both occurred under W-QBO conditions, i.e. under conditions when according to LaStovidka et at. (1992) a significant effect of strong geomagnetic storms cannot occur. Figure 3 consists almost exclusively of events under W-QBO conditions due to much more data gaps for events under E-QBO conditions. Due to the missing data in the winter of 1979/1980, we had to select several events from another high solar activity/e-qbo winter, that of 1989/1990. Figure 4 shows the average effect of strong solar flares of 8 February 1980, 26 November 1989, 29 December 1989,20 January 1990,23 February 1990 and 28 February Again we do not see any effect, which can be attributed to flares, particularly when we consider the standard deviations of the data points shown. For instance, for key day (day 0) <r= 44 D.U., which is twice as much as the difference between the local peak values on days 0 and 2. Studia geoph. et geod. 42 (1998) 537

6 D. MarinandJ. LaStovic'ka Fig. 4. The six-event average effect of strong solar flares in total ozone under high solar activity/e-qbo conditions. Moreover, individual events provide quite a different pattern. Consequently, even under high solar activity/e-qbo conditions we do not find a detectable total ozone response to strong solar flares, contrary to the existence of such a response to strong geomagnetic storms. 4. CONCLUSION An analysis of the response of total ozone from Hradec Kra'love' (central Europe) to strong solar flares with profound ionospheric effects shows that there is no detectable effect of strong solar flares in total ozone. Such an effect has not been observed even under conditions when a remarkable response to strong geomagnetic storms has occurred (LaStovicka et al., 1992; Mich and LaStovidka, 1995). A similar analysis of correlation of total ozone with radio wave absorption in the lower ionosphere reveals no correlation on a day-to-day time scale and no effect of strong solar flares on such correlation. The existence of the long-term correlation of monthly average values (Alberca et al., 1996) does not contradict the absence of short-term correlation, because the former is expressed best with a time lag of 1-2 months, which is out of the time scale of day-to-day variations. Acknowledgement: The work of J.L. was supported by grant of the Grant Agency of the Academy of Sciences of the Czech Republic No. A /1998. Manuscript received: 23 March 1998; Revisions accepted: 19 May Studia geoph. et geod. 42 (1998)

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