Unusual declining phase of solar cycle 23: Weak semi-annual variations of auroral hemispheric power and geomagnetic activity

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1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L22102, doi: /2009gl040825, 2009 Unusual declining phase of solar cycle 23: Weak semi-annual variations of auroral hemispheric power and geomagnetic activity Xiaoli Luan, 1 Wenbin Wang, 1 Alan G. Burns, 1 Stanley C. Solomon, 1 Yongliang Zhang, 2 and Larry J. Paxton 2 Received 3 September 2009; revised 15 October 2009; accepted 19 October 2009; published 20 November [1] Seasonal variations of geomagnetic activity during the declining phase of solar cycle 23 (SC23-D, ) have been studied using auroral hemispheric power (HP), solar wind and interplanetary magnetic field (IMF) data and the Kp index. The well known semi-annual variations of geomagnetic activity, with peaks occurring during equinoxes, were virtually absent during SC23-D. This makes SC23-D markedly different from previous solar cycles which had clear semi-annual variations. In SC23-D, both Kp and HP had unusual peaks around the December solstice (in years 2003, 2004 and 2006) and August (in years 2004 and 2005), instead of at the equinoxes. These peaks appeared to be associated with solar wind/imf and the consequent merging electric field peaks in the same periods. Furthermore, the absolute values and relative changes of the Kp index were much smaller in SC23-D than in other solar cycles. The very weak dynamic pressure and southward IMF in SC23-D might also limit the regular modulation effects that contribute to the occurrence of peaks in equinoxes. Citation: Luan, X., W. Wang, A. G. Burns, S. C. Solomon, Y. Zhang, and L. J. Paxton (2009), Unusual declining phase of solar cycle 23: Weak semi-annual variations of auroral hemispheric power and geomagnetic activity, Geophys. Res. Lett., 36, L22102, doi: /2009gl Introduction [2] The ultimate driver of auroral precipitation and geomagnetic activity is the solar wind and interplanetary magnetic field (IMF) and its coupling with the magnetosphere-ionosphere system. The hemispheric power (HP, total auroral electron precipitation energy flux over the entire auroral oval in one hemisphere) has been reported to be closely correlated with geomagnetic activity and solar wind speed [Emery et al., 2008, 2009, and references therein]. The spectral characteristics of the hourly averaged solar wind speed and hemispheric power are also similar over the recent three solar cycles [Emery et al., 2009]. A prominent effect of the solar wind/imf driver is the wellknown semi-annual variation of geomagnetic activity, in which geomagnetic activity is higher around equinox than around solstice [Cortie, 1912; Bartels, 1925, 1932; Chapman and Bartels, 1940; McIntosh, 1959]. Semi-annual variations are also found in hemispheric power [Liu et al., 1 High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado, USA. 2 Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA. Copyright 2009 by the American Geophysical Union /09/2009GL040825$ ; Emery et al., 2008, 2009] and the occurrence frequency of geomagnetic storms [Svalgaard et al., 2002]. [3] In all previous studies using NOAA/POES observations [Liu et al., 2008; Emery et al., 2008, 2009], HP was estimated from single satellite passes using statistical patterns. Therefore, the HP may have been biased due to the large variability of auroral precipitation with magnetic local times and latitudes. During the declining phase of the solar cycle 23, the Global Ultraviolet Imager (GUVI), which is onboard the NASA TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) satellite, measured auroral emissions from a significant portion (1/3 1/2) of the auroral oval in each orbit and each hemisphere [Zhang and Paxton, 2008], and thus provided much better coverage of the auroral oval than in situ particle measurements along satellite tracks could. In this study, the correlation between hemispheric power, solar wind speed and geomagnetic activity is analyzed and the frequency/spectral variations of these parameters are obtained. The variations of the geomagnetic Kp index and hemispheric power are then compared with those in previous solar cycle phases to illustrate their unusual characteristics in the declining phase of solar cycle Data [4] Hemispheric power is calculated from the energy flux of electron auroral precipitation derived from the N 2 LBHs ( nm) and LBHl ( nm) emissions that were observed simultaneously by the GUVI instrument. The procedure to obtain energy flux from these auroral emissions was described in detail by Zhang and Paxton [2008]. Comparisons between the energy flux derived from GUVI and that observed by DMSP along the satellite track showed good agreement in case studies [Zhang and Paxton, 2008]. In this work, GUVI observations from February 2002 to October 2007 were used and over 55,000 images (28 30 images per day) were available. The TIMED orbit precesses 12 hours per 60 days; hence the GUVI orbits can cover all magnetic local times and latitudes almost evenly in the auroral regions within 60 days, when observations in two hemispheres are superposed. [5] The hemispheric power was obtained by hemispherically integrating the averaged energy fluxes observed in both hemispheres at each magnetic local time (half hour) and latitude (0.5 ) bin within a 60 day period. The averaged hemispheric power centered in each month was used for analysis (referred as monthly average hereafter). In addition, the same sliding window was also applied to the solar wind speed, IMF and Kp when they were compared with the HP, if not specified. The hourly-averaged solar wind speed, IMF L of5

2 Figure 1. (a) The monthly (top) Hemispheric Power, (middle) solar wind speed, and (bottom) Kp index from 60- day running means during , and (b) the Lomb- Scargle spectral analysis with periods labeled by month. and Kp index were obtained from the OMNI database with Kp remaining the same within each 3-hour period (ftp:// nssdcftp.gsfc.nasa.gov/spacecraft_data/omni). The UTs of the solar wind data observed from multiple spacecraft at the L1 point were shifted in the OMNI data to account for the propagation time from the satellite positions to the earth. 3. Results [6] Figure 1a shows the monthly-averaged values of hemispheric power (HP), solar wind speed, and the Kp index during Figure 1a shows that, during this period, maximum values of HP (top) occurred mostly between November and January, such as in years 2003, 2004 and Significant peaks of HP also occurred during July August of 2004 and 2005, June of However, one striking feature is that there were no significant HP peaks in equinoctial months, except for a peak that occurred in April of 2003, but this April peak had been significantly expanded toward May and June. There was also another minor one in April The variations of the solar wind speed (Figure 1a, middle) and Kp (Figure 1a, bottom) were similar to those of HP. The correlation coefficient was 0.84 between the solar wind speed and HP, and 0.87 between the solar wind speed and Kp. This result agreed with the good correlation between yearly substorm activity and yearly averaged solar wind speed [Tanskanen et al., 2005]. The highest correlation of 0.93 occurred between HP and Kp. In addition, the averaged Kp during GUVI observation periods is plotted in Figure 1a (bottom, black-circled line). During most of the intervals plotted, there was almost no difference between the actual averaged Kp values (red line) and the averages for the periods when GUVI was observing in the auroral regions. [7] The period between 2002 and 2007 was in the declining phase of the solar cycle 23 (SC23-D). During this period, the absence of the equinoctial peaks of HP in most years was obvious. This lack of equinoctial peaks is quite different from the variations of auroral activity that have been observed in previous solar cycles, during which the auroral precipitation energy was more pronounced during equinoxes than during solstices [Liu et al., 2008; Emery et al., 2009]. In particular, the seasonal variation of Kp was greatly different from that during previous solar cycles, especially near the solar minimum years of In previous studies, the maximum equinoctial peaks as well as the related semi-annual variations were reported to be most prominent in solar minimum years [Cliver et al., 2004; Emery et al., 2009]. However, in 2006 the December peak of Kp was comparable with that in April (Figure 1a, bottom, red line), while in 2007 the strongest peak was in February instead of the equinoxes. [8] Figure 1b gives a Lomb-Scargle spectral analysis of the parameters shown in Figure 1a, which provides a tool to deal with unequally spaced data. The spectral analysis using the hourly Kp index is also compared with those in the declining phases of previous four solar cycles. Note that the Lomb-Scargle spectral periods are delineated by month. According to the position of in solar cycle 23, the Kp indices in SC19-D (September,1959 August, 1963), SC20-D (September, 1970 April, 1975), SC21-D (November 1981 July 1985) and SC22-D (June 1991 February 1995) have been selected to exclude periods of about two years after solar maximum and one year before solar minimum. [9] During , prominent 7 and 9 month periods occurred with roughly the same amplitudes for HP (2GW), Kp (0.1) and solar wind speed (5 10 km/s). However, the amplitude of the 6-month periodic variation, which was usually associated with equinoctial peaks, was much weaker for HP, solar wind speed and Kp than those of the 7 and 9 month ones. This result is consistent with the absent or weak equinoctial peaks in most years of the declining phase of solar cycle 23 (SC23-D). The significant 7-month period variations of Kp and HP were caused by peaks occurring in the solstices rather than in the equinoxes. The 6-month periodicity was obvious in previous solar cycles, when significant equinoctial peaks occurred (e.g., SC21-D and SC22-D), as will be shown in Figure 2 in the discussion section. There was a large difference in the 6-month periodic variation between the SC23-D and the previous four solar cycles. From Figure 1b (bottom), the amplitude of 6-month period of Kp was about a factor of 3 5 smaller than that in SC22-D and SC21-D. Similar significant amplitudes of the 6-month period of Kp can be also found in SC17-D and SC18-D (not presented here). There were some similarities in the spectral structures of the 5 9 month period waves between SC19-D and SC23-D. However, the amplitudes of the 6-month period of Kp in 2of5

3 Figure 2. (left) Superposed epoch analysis for the relative variations and (right) absolute values of the Kp index, merging electric field (Em), solar wind speed (Vsw), southward IMF Bz (Bs), and the dynamic pressure of the solar wind (Psw) for the declining phases of the recent four solar cycles (SC 20-23). Note that Bs = 0 when Bz > 0. SC23-D was more than a factor of 2 smaller than that in SC19-D. The different selections of the starting and ending date of each declining phase might affect the relative amplitudes of the 6-month oscillations for each solar cycles, however, they can not change the fact that the amplitude of the 6-month oscillation in SC23-D was the smallest within recent 7 solar cycles. [10] A similar comparison for HP is not available, since the TIMED/GUVI had auroral observations only during However, the relative differences in 6-month amplitudes of HP between solar cycles should be similar to those for the Kp index due to the high correlation (0.93) between Kp and Hp. The much smaller amplitudes of both the 6-month period for HP and the Kp index made this declining phase of the solar cycle 23 unusual at least during the recent 7 solar cycles. The absence of the semi-annual variation of the geomagnetic activity in the declining phase was probably a rare occurrence in history. Le Mouël et al. [2004] showed that only in the period before 1920, (in SC15) during , was the semi-annual amplitude of the geomagnetic aa index much lower than that of SC19-D. Thus the semi-annual variations were very weak or absent one century ago as well. 4. Discussion [11] Semi-annual variations of geomagnetic activity with peaks occurring around the equinoxes are related to the enhancements of the magnitude of the solar wind speed and IMF, and to the larger response of the magnetosphereionosphere system to the driving solar wind/imf conditions and/or to the ionospheric conductivities during equinoxes [e.g., Cliver et al., 2000, 2004, and references therein], which serves as a modulation mechanism. Four mechanisms have been proposed for the semi-annual variations: the axial hypothesis [Cortie, 1912] that contributes to a larger solar wind speed in equinoxes, the Russell-McPherron effect [Russell and McPherron, 1973] that produces a larger Southward IMF Bz in equinoxes, the equinoctial effect [Bartels, 1925, 1932; McIntosh, 1959], and the effect involving ionospheric conductivity [Lyatsky et al., 2001; Newell et al., 2002; Nagatsuma, 2006]. The equinoctial effect involves the possible modulation effect of a y angle on the Solar wind-magnetosphere coupling efficiency. This y angle is the inclination between the solar wind flow direction and the dipole axis of the earth, whose diurnal and semi-annual variations were found to have a high correlation coefficient (0.91) with the geomagnetic activity as described by the am index [Cliver et al., 2000; 2004]. Cliver et al. [2000, 2004] suggested that the equinoctial effect contributes about 65 70% to the semi-annual variations of the geomagnetic activity over a long-term average. Similarly, there is a good correlation between the geomagnetic activity am index and the total solar zenith angle of both hemispheres in the polar regions, and thus between geomagnetic activity and the solar EUV induced total ionospheric conductivity [Lyatsky et al., 2001; Newell et al., 2002; Nagatsuma, 2006]. Hence some studies have suggested that the coupling between the magnetosphere and ionosphere through ionospheric conductivity could explain the semi-annual variations of geomagnetic activity and possible mechanisms were proposed [Lyatsky et al., 2001; Newell et al., 2002; Nagatsuma, 2006]. In this paper, we have shown that the semi-annual variations of both geomagnetic activity and auroral precipitation energy with equinoctial peaks were nearly absent during the declining phase of the present solar cycle (SC23-D). Thus, it is important to know whether or not those mechanisms were still in effect in SC23-D and why the semi-annual variations were not evident. [12] The physical explanation about how solar wind magnetosphere ionosphere coupling modulates the seasonal variations of the geomagnetic activity is still an open question. However, we have checked that the F 10.7 solar radio flux, which is commonly used as a proxy for solar EUV radiation, did not show similar semi-annual variations to those of the geomagnetic activity for all the solar cycles considered in this paper, and we know that the solar zenith angle and the y angle remain the same for all solar cycles, thus neither ionospheric conductivity effect nor the y angle modulation could be the source of the unusual lack of semiannual variations in geomagnetic activity and auroral precipitation energy in SC23-D. To further analyze geomagnetic activity and its solar wind and IMF origin among solar cycles, we show in Figure 2 a comparison of the superposed epoch analysis of the averaged relative and absolute seasonal variations of the Kp index, merging electric field (Em), solar wind speed (Vsw), southward IMF Bz (Bs) and the solar wind dynamic pressure (Psw) for the declining phases of the four recent solar cycles. The merging electric field Em was calculated from Em = Vsw*B T *sin 2 (q/2) introduced by Kan and Lee [1979], where B T is the projection of the total 3of5

4 IMF strength on the solar magnetospheric y-z plane and q is a clock angle. The merging electric field (Em) has been reported to be among a few solar wind-imf coupling functions that have the highest correlation coefficients (>0.7) with the Kp index [e.g., Newell et al., 2007], thus represents the combined solar wind-imf origin of geomagnetic activity. A 54-day running window was applied to obtain daily averaged data before the superposed epoch analysis. The relative variation ratio is the superposed epoch result from the ratio of daily average to the annual (365-day) running mean average. Except for SC23-D, each solar cycle includes data of limited years (4 6) thus there might be statistical errors. Especially in SC20-D, the statistics were largely based on data from October 1972 April, 1975, as large data gaps occurred within Thus the annual solar wind/imf ratio might be biased because of the lack of data in SC20-D. Therefore our discussion will be focused on the recent three solar cycles. [13] Consistent with the spectral analysis shown in Figure 1b (bottom), the relative Kp variations showed obvious peaks around both equinoxes from SC20-D to SC22-D. However, the relative Kp variations in SC23-D showed a strong peak within November January (centered on day 365) and a weak peak in August (around days ). This November January peak masked any possible equinoctial peaks and the consequent semi-annual variations. The August peaks also helped to reduce semi-annual variations of the Kp index. These two peaks were apparently associated with the peaks in the merging electric field (Em), which originally occurred in the relative seasonal variations of the solar wind speed and Bs in SC23-D, respectively. There were peaks of Bs that were larger than the average in the fall equinoxes in both SC21-D and SC22-D and around April in SC23-D, and the corresponding peaks in Kp index occurred, which suggested that the Russell-McPherron effect was occurring and contributed to the equinoctial peaks of the Kp index. However, in SC21-D and SC22-D strong spring peaks in Kp occurred and centered in March but there were no similar Bs peaks, suggesting that the Russell-McPherron effect was not a major factor in producing semi-annual variations of geomagnetic activity. [14] As shown in Figure 2 (left), the overall solsticeequinox difference in Kp was much smaller in comparison with that of Em in SC23-D, and there were no dominant equinox peaks in the Kp index. However, in SC22-D and SC21-D, the peaks in Kp dominated in both equinoctial seasons, although there were significant peaks in Em occurred in solstice season (e.g., in July of SC21, in February and November in SC21 and SC22). There was also a striking difference in the extent of the relative annual/ seasonal variations between SC23-D and the previous three solar cycles. The overall relative variations of each parameter (Kp, Em, Vsw, Bs and Psw) to its annual average were much smaller in SC23-D than those in the other three solar cycles. The above differences suggested less solar wind energy penetrating into the magnetosphere and ionosphere system in SC23-D. In Figure 2 (right), there were generally weaker averaged southward IMF and emerging electric fields, and much weaker solar wind dynamic pressure in SC23-D than in previous solar cycles. Also, the relative difference in the averaged absolute dynamic pressure between solar cycles was similar to that in the averaged absolute Kp index. The smaller magnitude of IMF, merging electric field and dynamic pressure in SC23-D would affect its interaction with and the associated energy input to the magnetosphere and ionosphere, resulting in a lower Kp index and smaller relative and absolute seasonal variations of geomagnetic activity. This might explain why the semiannual modulation due to the effect of y angle or ionospheric conductivity did not produce dominant peaks of the Kp index in equinoxes in SC23-D, whereas it did so in the previous two solar cycles. Slavin et al. [1986] also suggested that a long term variation in the IMF from SC20 to SC21 produced a significant increase in geomagnetic activity in SC21. Therefore, changes in solar wind-imf conditions might largely account for the unusual lack of semi-annual variations of the geomagnetic activity as well as the auroral precipitation energy seen in SC23-D. Further, variations in the Kp index was very similar to those in Em in SC23-D, indicating the solar wind-imf origin of the variations in geomagnetic activity. It is evident that the Bs peak in August and the solar wind peak during the November January period contributed to a four-peak structure in Em around days 365, 135, 225 and 330. These structures in Em, as well as the much weaker overall strength in solar wind dynamic pressure and IMF, could affect the seasonal variations of geomagnetic activity and cause the semiannual pattern to be absent in SC23-D. 5. Conclusions [15] In this study, we have found that the well-known semi-annual variations in auroral activity and geomagnetic activity, were virtually absent during in the declining phase of solar cycle 23 (SC23-D). This made the SC23-D different from previous solar cycles. In SC23-D, both the averaged Kp and its relative ratios had dominant peaks around the December solstice (in years 2003, 2004, and 2006) and in August (in years 2004 and 2005). These were different from seasonal variations of previous solar cycles, which had peaks around the equinoxes. The unusual peaks in SC23-D followed variations in the merging electric fields of solar wind and IMF. In addition, both the relative and absolute variations of the Kp index were much smaller in SC23-D. These changes of the Kp index in SC23-D were associated with dynamic pressure, southward IMF and merging electric fields that were weaker than those in previous solar cycles. This might also have limited the regular modulation effects that contribute to the occurrence of peaks in equinoxes. [16] Acknowledgments. We are grateful for the helpful comments from the two referees and discussion with Barbara Emery in HAO/NCAR. This research is supported by NASA grants NNH08AH37I and NNX08AQ91G, and in part by the Center for Integrated Space Weather Modeling (CISM) which is funded by the STC program under agreement ATM The National Center for Atmospheric Research is sponsored by the NSF. References Bartels, J. (1925), Eıne universelle Tagsperiode der erdmagnetischen Aktivität, Meteorol. Z., 42, Bartels, J. (1932), Terrestrial-magnetic activity and its relation to solar phenomena, Terr. Magn. Atmos. Electr., 37, 1 52, doi: / TE037i001p Chapman, S., and J. Bartels (1940), Geomagnetism, Oxford Univ. Press, New York. 4of5

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