The 22-year cycle in the geomagnetic 27-day recurrences reflecting on the F2-layer ionization

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1 Annales Geophysicae () : 7 76 SRef-ID: -576/ag/--7 European Geosciences Union Annales Geophysicae The -year cycle in the geomagnetic 7-day recurrences reflecting on the F-layer ionization E. M. Apostolov, D. Altadill, and M. Todorova Observatorio del Ebro, (URL-CSIC), Horta Alta 8, E5 Roquetes, Spain Geophysical Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev bl., Sofia, Bulgaria Received: March Revised: October Accepted: October Published: April Abstract. Solar cycle variations of the amplitudes of the 7- day solar rotation period reflected in the geomagnetic activity index A p, solar radio flux F.7 cm and critical frequency fof for mid-latitude ionosonde station Moscow from the maximum of sunspot cycle 8 to the maximum of cycle are examined. The analysis shows that there are distinct enhancements of the 7-day amplitudes for fof and A p in the late declining phase of each solar cycle while the amplitudes for F.7 cm decrease gradually, and the fof and A p amplitude peaks are much larger for even-numbered solar cycles than for the odd ones. Additionally, we found the same even-high and odd-low pattern of fof for other mid-latitude ionosonde stations in Northern and Southern Hemispheres. This property suggests that there exists a -year cycle in the F-layer variability coupled with the -year cycle in the 7-day recurrence of geomagnetic activity. Key words. Ionosphere (mid-latitude ionosphere; ionosphere-magnetosphere interactions) Magnetospheric physics (solar wind-magnetosphere interactions) Introduction The -year cycle of geomagnetic activity is the least well examined of the long-period geomagnetic variations which are longer than the solar cycle. The reality of the geomagnetic -year cycle has not gone unquestioned. There are some investigations based on spectral analysis of long-term series of geomagnetic indices or geomagnetic data of individual stations which have provided no evidence for a - year periodicity (Fraser-Smith, 97; Delouis and Mayaud, 975; Gonzalez et al., 99), but, on the other hand, there are studies showing the strong -year periodicity, for instance, those of Currie (97) and Courtillot et al. (977). Moreover, some investigations reveal an important -year cycle in some properties of the geomagnetic activity variations as the vernal-autumnal asymmetry in the seasonal variation of Correspondence to: E. M. Apostolov (ema@geophys.bas.bg) geomagnetic activity (Triskova, 989). The -year variation in the geomagnetic activity was clearly demonstrated by enhanced 7-day recurrent activity during the declining phase of even-numbered solar cycles (Ondoh and Nakamura, 98; Hapgood, 99) and also by transient activity in the rising phase of odd-numbered cycles (Cliver et al., 996). The period of activations of strong geomagnetic 7-day recurrent activity is longer in the even-numbered cycles nearly years than in the odd cycles, nearly years (Ondoh and Nakamura, 98). The significant peaks of occurrences are in the second half of the declining phase of each cycle, but the peaks are much higher for the even cycles than for the odd ones (Hapgood, 99). A large part of the F-layer variability is linked to the geomagnetic activity (Forbes et al., ; Rishbeth and Mendillo, ) and, therefore, we would expect to observe a -year cycle in the F-layer characteristics. The investigation of Feichter and Leitinger (997) of two consecutive solar cycles, and, show the existence of a -year period in the daytime total electron content (TEC) and over four solar cycles (9 ) for F-layer peak electron density (NmF) over Europe. They found that the annual variation of TEC and NmF shows the change in the maximum from autumn in odd cycles to a spring maximum in even cycles in European mid-latitudes. The authors conclude that the observed annual asymmetry of F-layer ionization in European mid-latitudes is in anti-phase with the vernal-autumnal asymmetry of geomagnetic activity (Triskova, 989) because of the ionization depression caused by negative storm effects. In this investigation we present another aspect of a -year cycle manifestation in upper ionosphere ionization based on the long-term variations of the 7-day amplitudes of the geomagnetic activity in even/odd numbered solar cycles. Data and method The following parameters have been used in our analysis. The hourly values of the F-layer critical frequency (fof) for the period from 97 to from Moscow (55.5 N, 7. E) are retrieved from the COST5 database for the

2 7 Apostolov et al.: -year cycle in the F-layer Solar cycle number 8 9 fof midday fof midnight In order to investigate the spectral composition of the solar rotation period reflected in the geomagnetic, ionospheric and solar radio flux variations, we use the Lomb-Scargle periodogram method. The periodogram algorithm given by Scargle (98) for the general case is for an unevenly sampled time series. The application of this periodogram method is very convenient in the case of ionospheric sounding time series which are characterized by data gaps. The probability that a given periodicity is a true signal can be estimated when the periodogram power is normalized by the total variance of the data (Horne and Baliunas, 986; Press et al., 99). Hocke (998) has extended the methodology for calculating the amplitude and phase spectra. 5 Results Period (days) Years Fig.. Dynamic amplitude spectra in the period range from to 5 days of solar radio flux F.7 cm, geomagnetic activity index A p and critical frequencies fof for midday and midnight (Moscow) over the years 97. Solar cycle maxima and minima are marked with dashed and thin vertical lines, respectively. Highest amplitudes are painted in a black tone. The 7-day period is indicated by a horizontal line in each panel. period and for the period 999 from the World Data Centre for Solar-Terrestrial Physics, Boulder, via the This time series has been used as a base for the purpose of our investigation because it is very long it covers the interval from the maximum of solar cycle 8 to the maximum of solar cycle and it is characterized by a scarcity of missing values. Two sequences of average hourly fof for midday ( LT) and midnight ( LT) bins for each day have been retrieved for the analysis. Additionally, the fof hourly data for some ionosonde stations in the mid-latitude belt of the Northern and Southern Hemisphere used in the present analysis are retrieved from WDC for STP, Chilton, via the Because the F-layer ionization variability is linked to the geomagnetic activity and solar EUV variations, we also use daily values of the geomagnetic activity index A p, and solar radio flux at.7 cm (F.7 cm), as proxies of geomagnetic and solar EUV variations for the time interval being investigated. Note that the time series of F.7 cm starts on February 97. Ap F.7 cm To investigate the temporal variations of the spectral power or amplitude of solar rotation period reflected in the investigated time series, dynamic spectra were calculated. The Lomb-Scargle periodogram was applied to a -year window within each time series and the window is then slid through the series in 7-day (.-year) increments. The -year window length was selected to reveal the dominant characteristics of temporal variations of the 7-day period amplitude by diminishing the seasonal variations and emphasizing the intervals with long-lived structures. The periodograms for A p and F.7 cm are computed in the frequency interval.5.85 day and for fof in the interval.. hour, both corresponding to the period range between and 5 days, with an over sampling factor of four. Figure depicts the dynamic amplitude spectra for F.7 cm, A p and fof for Moscow. The amplitudes which exceed 95% confidence level are taken for spectrograms only. Three features of the dynamic amplitude spectra of interest of the present study should be immediately noted: () The figure reveals a distinct -year cycle of the geomagnetic activity around 7-day period. The highest A p -amplitudes occur strongly in the late declining phase of the even cycles 8, and. () The highest amplitudes of F.7 cm are near solar cycle maxima and negligible in the declining phases before solar cycle minima. () There are significant activations of fof-amplitudes in both midday and midnight during the declining phases of each solar cycle. These activations coincide in time, near -years intervals before sunspot minimum, with those of the geomagnetic activity for the even cycles (8, and ). This is an indication of the expected manifestation of a -year cycle in F-layer ionization due to the -year cycle of geomagnetic 7-day period. The spectral behaviour of the investigated parameters in the late declining phase of solar cycles is more clearly seen in Fig.. This figure depicts the amplitude spectra in the period range from to 5 days for fof, A p and F.7 cm over -year intervals before sunspot minimum of the cycles 8 to. First of all, we see a clear double cycle variation in A p - and fof-spectra with higher amplitudes near the solar rotation period for solar cycles 8, and in comparison with

3 Apostolov et al.: -year cycle in the F-layer 7 Amplitude (. MHz) Amplitude (nt) Amplitude (sfu) Solar cycle number 8 9 Period (days) fof Ap F.7 cm Fig.. Amplitude spectra of fof (Moscow), A p and F.7cm in the period range from to 5 days over -year intervals in the late declining phase of solar cycles 8 before sunspot minima. The thick and dashed lines mean midday and midnight fof spectra, respectively. those for solar cycles 9 and. For F.7 cm-amplitudes there is no such variation, they are nearly equal in the cycles 9 to. The amplitude spectra of the three parameters have similar features including a significant band of sharply defined peaks around the 7-day solar rotation period, between 5 and days. The causes of the appearance of this multi-peaked structure were analyzed mainly for solar and geomagnetic parameters. According to the studies of Fenimore et al. (978), Letfus and Apostolov (98), and Bouwer (99), the combined effects of solar rotation and active region evolution, or its longitudinal distribution, may produce this structure. The power (or amplitude) spectra over long-time intervals of geomagnetic activity indices performed in many studies show dominant subsidiary peaks at 5. and 9.5 days around the peak of 7 7. days. Coleman and Smith (966) argued that these subsidiary peaks result from amplitude modulation of the 7.-day period by the period of year. However, the power of the solar rotation period reflected in the three parameters is distributed in a series of spectral lines mainly between 5 and days. This is why we have calculated the period-averaged amplitude as an integrated measure of the amplitude distributed within the periods from 5 to days for the three parameters, over -year windows sliding by 7 days (Fig. ). To some extent, Fig. represents a section of the results in Fig. for the limited periodic band of interest. Figure shows that there are enhancements of the period-averaged amplitudes in the declining phase of each solar cycle for both fof and A p. These enhancements are better expressed and they coincide in time during the even cycles and. The observed vernal-autumnal asymmetry of the annual variation of the F-layer ionization, coupled with the -year cycle in geomagnetic activity (Feichter and Leitinger, 997) has been obtained for European mid-latitudes but not for the Southern Hemisphere because of the lack of difference between vernal and autumnal maxima (Titheridge et al., 996). However, the solar rotation period in the EUV and geomagnetic variations is reflected strongly in the components of the F-layer variability in both hemispheres. We have reason to believe that the long-term variations of 7-day period characteristics on the F-layer variability should show a -year variation in the Southern Hemisphere as well. Figure shows the amplitude spectra of midday and midnight fof for southern station Canberra (5. S, 9 E) for the late declining phases of solar cycles 8 to, obtained by the same method as for fof (Moscow) in Fig.. Again, the amplitudes for the even cycles are higher than for the odd ones. Analogous results are also obtained for some mid-latitude sounding stations in Northern Hemisphere and for Southern Hemisphere in the Australian sector (Table ). The values of

4 7 Apostolov et al.: -year cycle in the F-layer Solar cycle number 8 9 (. MHz) 5 fof midday fof midnight Period-averaged amplitude (nt) Ap (s.f.u.) 8 F.7 cm Years Fig.. Behaviour of the period averaged amplitude of the period band 5 days for fof (Moscow) for midday and midnight, A p and F.7 cm for the interval 97. The solar cycle maxima and minima are marked as in Fig.. the period-averaged amplitude from the 5- to -day period of fof, using all the hourly data, over -year intervals, before sunspot minima for all stations, show that the amplitudes for even cycles are greater than for the odd ones. Conclusions The present investigation indicates the existence of -year cycle in the F-layer ionization at mid-latitudes for both Northern and Southern Hemisphere. The midday and midnight fof variations that reflect the 7-day solar rotation period, more precisely in the period range from 5 to days, show distinct enhancements in the late declining phases before sunspot minima of the investigated solar cycles 8. These fof enhancements coincide strongly in time with the significant peaks of 7-day geomagnetic activity in the second half of the declining phase of each cycle. The even-high and odd-low pattern of the 7-day geomagnetic activity in the late declining phase of solar cycles is one of the main characteristics of the -year cycle in geomagnetic activity (Hapgood, 99; Cliver et al., 996), and the same pattern is well developed for the investigated fof time series (Figs. ). Such behaviour is not typical for solar radio flux F.7 cm variations as a proxy of ionizing EUV-radiation. There are

5 Apostolov et al.: -year cycle in the F-layer 75 Table. Period-averaged amplitudes from 5- to -day periods of fof (all the hourly data,. MHz) over -year intervals before sunspot minima. Note that time intervals are in a fraction of years. The last two rows show the period-averaged amplitudes for daily A p and F.7 cm, respectively. Solar cycle 8 9 Time interval Slough (5.5 N, 59. E) Moscow (55 N, 7. E) Alma Ata (. N, 77 E) Tomsk (56.5 N, 8.9 E) Irkutsk (5.5 N, E) Akita (9.7 N, E) Boulder ( N, 56.7 E) Mundaring ( S, 6. E) Hobart (.9 S, 7. E) Canberra (5. S, 9 E) Christchurch (. S, 7. E) A p (nt) F.7 cm (s.f.u.) Amplitude (. MHz) Period (days) Fig.. The same as Fig., but for fof from Canberra for solar cycles 8.

6 76 Apostolov et al.: -year cycle in the F-layer no peaks of 7-day amplitudes in F.7 cm during the second half of the declining phases of solar cycles, particularly in the even cycles and. Instead of that, the amplitudes of F.7 cm decrease gradually, while for those of the geomagnetic index A p there are strong peaks also reflected in the fof (Fig. ). As an exception, the peak of the 7-day amplitude of F.7 cm coincides in time with that of A p for solar cycle 8, but this coincidence does not contradict the existence of a -year variation in fof caused by geomagnetic activity, clearly expressed in the behaviour of A p and fof during solar cycles 9 (Fig. ). The obtained results are not surprising. A large part of F- layer variability is linked to the geomagnetic activity (Forbes et al., ; Rishbeth and Mendillo, ). The possible causes of F-layer variability related to the geomagnetic activity by means of Joule heating and energetic particle precipitation, are linked with the substorms, magnetic storms, and interplanetary magnetic field/solar wind structures that depict strong solar rotation and a -year cycle variation. Acknowledgements. This work was supported by the international cooperation project BG between Geophysical Institute (Bulgarian Academy of Sciences) and Observatori de l Ebre (Consejo Superior de Investigaciones Cientificas, Spain), and by project BTE-85 MCYT of Spain. Topical Editor M. Lester thanks M. Förster and R. Leitinger for their help in evaluating this paper. References Bouwer, S. D.: Periodicities of solar irradiance and solar activity indices, II, Solar Phys.,, 65 89, 99. Cliver, E. W., Boriakoff, V., and Bounar, K. H.: The -year cycle of geomagnetic and solar wind activity, J. Geophys. Res.,, , 996. Coleman Jr., P. J., and Smith, E. J.: An interpretation of the subsidiary peaks at periods near 7 days in the power spectra of Ci and Kp, J. Geophys. Res., 7, , 966. Courtillot, V., Le Mouel, J. L., and Mayaud, P. N.: Maximum entropy spectral analysis of the geomagnetic activity index aa over a 7-year interval, J. Geophys. Res., 8, 6 69, 977. Currie, R. G.: Geomagnetic line spectra to 7 years, Astrophys. Space Sci.,, 5 8, 97. Delouis, H. and Mayaud, P. N.: Spectral analysis of the geomagnetic activity index aa over a -year interval, J. Geophys. Res., 8, , 975. Feichter, E. and Leitinger, R.: A -year cycle in the F-layer ionization of the ionosphere, Ann. Geophysicae, 5, 5 7, 997. Fenimore, E. E., Asbridge, J. R., Bame, S. J., Feldman, W. C., and Gosling, J. T.: The power spectrum of the solar wind speed for periods greater than days, J. Geophys. Res., 8, 5 57, 978. Forbes, J. M., Palo, S. E., and Zhang, X.: Variability of the ionosphere, J. Atmos. Sol.-Terr. Phys., 6, ,. Fraser-Smith, A. C.: Spectrum of the geomagnetic activity index A p, J. Geophys. Res., 77, 9, 97. Gonzalez, A. L. C., Gonzalez, W. D., and Dutra, S. L. G.: Periodic variation in the geomagnetic activity: A study based on the A p index, J. Geophys. Res., 98, 95 9, 99. Hapgood, M. A.: A double solar cycle in the 7-day recurrence of geomagnetic activity, Ann. Geophysicae,, 8 5, 99. Hocke, K.: Phase estimation with the Lomb-Scargle periodogram method, Ann. Geophysicae, 6, 56 58, 998. Horne, J. H. and Baliunas, S. L.: A prescription for periodic analysis of unevenly sampled time series, Astrophys. J.,, , 986. Letfus, V. and Apostolov, E. M.: Longitudinal distribution of recurrent solar activity sources and its reflection in geomagnetic variations, Bull. Astron. Inst. Czechosl.,, 9, 98. Ondoh, T. and Nakamura, Y.: Solar cycle effect of 7-day recurrent geomagnetic storms, in Solar-Terrestrial Predictions Proceedings, vol. IV, edited by Donnely, R.F., Space Environ. Lab., Boulder, Co., A-6 A-5, 98. Press, W. H., Teukolsky, S. A., Vetterling, W. T., and Flannery, B. P.: Numerical recipes in Fortran, Cambridge Univ. Press, , 99. Rishbeth, H. and Mendillo, M.: Patterns of F-layer variability, J. Atmos. Sol.-Terr. Phys., 6, 66 68,. Scargle, J. D.: Studies in astronomical time series analysis. II. Statistical aspects of spectral analysis of unevenly spaced data, Astrophys. J., 6, 85 85, 98. Titheridge, J., Leitinger, E. R., and Feichter, E.: Comparison of the long-term behaviour of the F-layer of the ionosphere, northern versus southern hemisphere, Kleinheubacher Ber., 9, , 996. Triskova, L.: The vernal-autumnal asymmetry in the seasonal variation of geomagnetic activity, J. Atmos. Terr. Phys., 5, 8, 989.