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Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index
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ScienceDirect Advances in Space Research xxx (2017) xxx–xxx www.elsevier.com/locate/asr
Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index Suyeon Oh a,⇑, Yu Yi b b
a Department of Earth Science Education, Chonnam National University, Gwangju 61186, Republic of Korea Department of Astronomy, Space Science and Geology, Chungnam National University, Daejeon 34134, Republic of Korea
Received 15 June 2017; received in revised form 9 August 2017; accepted 5 September 2017
Abstract Three major hypotheses have been proposed to explain the well-known semiannual variation of geomagnetic activity, maxima at equinoxes and minima at solstices. This study examined whether the seasonal variation of equinoctial geomagnetic activity is different in periods of opposite solar magnetic polarity in order to understand the contribution of the interplanetary magnetic field (IMF) in the SunEarth connection. Solar magnetic polarity is parallel to the Earth’s polarity in solar minimum years of odd/even cycles but antiparallel in solar minimum years of even/odd cycles. The daily mean of the aa, Aa indices during each solar minimum was compared for periods when the solar magnetic polarity remained in opposite dipole conditions. The Aa index values were used for each of the three years surrounding the solar minimum years of the 14 solar cycles recorded since 1856. The Aa index reflects seasonal variation in geomagnetic activity, which is greater at the equinoxes than at the solstices. The Aa index reveals solar magnetic polarity dependency in which the geomagnetic activity is stronger in the antiparallel solar magnetic polarity condition than in the parallel one. The periodicity in semiannual variation of the Aa index is stronger in the antiparallel solar polar magnetic field period than in the parallel period. Additionally, we suggest the favorable IMF condition of the semiannual variation in geomagnetic activity. The orientation of IMF toward the Sun in spring and away from the Sun in fall mainly contributes to the semiannual variation of geomagnetic activity in both antiparallel and parallel solar minimum years. Ó 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Solar polarity; Semiannual variation; Aa index
1. Introduction Geomagnetic activity shows well recognized seasonal variation: greater at the equinoxes than at the solstices. Rosenberg and Coleman (1969) and later Bohlin (1977) based their explanation for this variation on the axial hypothesis, which recognizes the Earth is located at a heliolatitude of 7.2° on March 6 and at a heliolatitude of +7.2° on September 6. Thus, at these times, the Earth is closer to the middle heliolatitudes of solar activity in the ⇑ Corresponding author.
form of high-speed streams (HSSs) in the solar wind velocity or coronal mass ejections. Russell and McPherron (1973) suggested that the equinoctial peaks are caused by the Bx and By sector structures of the interplanetary magnetic field (IMF), which translate to negative or positive geocentric solar magnetospheric (GSM) Bz values. Cliver et al. (2000) after McIntosh (1959) and Svalgaard (1977), suggested that the largest effects are due to equinoctial enhancement and to solstice blocking of the solar wind into the high latitudes of the Earth. Previous studies on semiannual geomagnetic activity have been performed mainly using geomagnetic indices such as am, aa, U (the approximate magnitude of the
E-mail address: [email protected] (S. Oh). https://doi.org/10.1016/j.asr.2017.09.008 0273-1177/Ó 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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difference between successive daily averages of the Dst index; e.g., see Fig. 1 of Russell and McPherron (1973)), AL, and Dst. Oh and Yi (2011) tested the dependency of geomagnetic activity on solar magnetic polarity by examining the occurrence of geomagnetic storms based on the low-latitude Dst index. They established that geomagnetic storm activity shows a clear increase in frequency during the equinoxes (March and September) in comparison with the solstices (June and December). However, they were unable to verify a significant difference between the geomagnetic activities in March and September when solar polarity was reversed. Thus, the previously analyzed Dst index is derived from a network of near-equatorial geomagnetic observations. In this study, therefore, we examine the dependency of geomagnetic activity in the high-latitude regions on solar magnetic polarity. The aa index is a linear scale index by nanotesla (nT) derived from the K indices measured at two antipodal observatories, and it reflects the amplitude of global geomagnetic activity during 3-h intervals normalized to ±50° geomagnetic latitude. The K indices are converted into amplitudes using mid-class amplitudes and then averaged with weighting factors that account for slight changes in geomagnetic disturbance intensities between successive Northern and Southern aa-observatories (http://isgi.unistra.fr/indices_aa.php). The Aa index, which is obtained from daily averages of eight aa values, was introduced to monitor geomagnetic activity over the longest possible time period. The Aa index is observed for higher latitudes than the Dst index and it is reasonable to surmise that the geomagnetic activity in the high-latitude regions reflected in the Aa index depends on solar magnetic polarity. The geomagnetic disturbances by solar activity are defined by the variation of horizontal component of geomagnetic field. The highlatitude regions can be easily affected by solar disturbance because of weak horizontal intensity (H). The polar geomagnetic variation sensitively responds to the change of IMF Bz, which is determined by solar magnetic polarity. Thus, the Aa index is considered suitable for investigating the semiannual variation of geomagnetic activity during periods of different solar magnetic polarity. The strength of the interaction of the solar wind with the magnetosphere is governed to a major degree by the contemporary relative directions of the component of the solar wind magnetic field relative to the direction of the Earth’s magnetic field. That is the interaction was strongest when those two magnetic fields were closest to being in opposite directions, which enabled magnetic reconnection. The strength of the interaction of the solar wind with the magnetosphere has nothing to do with the direction of the magnetic field relative to the solar poles. However, even though the magnetic reconnection is a local phenomenon, the statistical frequency of its occurrence can be determined by the solar global magnetic polarity as the background. Our present work begins at this point. Thus, our study con-
centrates on the effect of solar polarity in the solar minimum year, not on the solar events in the solar maximum year. Our goal is to determine whether the solar polarity can account for the difference of geomagnetic activity between the equinoxes. Fig. 1 summarizes the definition of solar magnetic polarity and related terminologies in the phase of solar cycle (SC) at the northern solar hemisphere. The polarity of the solar polar magnetic field experiences a reversal near each solar maximum. The increasing phase where the sunspot number increases with time has a different polarity from that in the declining phase where the sunspot number decreases with time within the same SC. At each solar minimum period, the solar polar magnetic field aligns with the Earth’s magnetic field in either the antiparallel or parallel direction. Thus, the solar polar magnetic field directed away from the Sun is antiparallel with the Earth’s magnetic field and that directed toward the Sun one is parallel. 2. Data We examined the daily aa index as the geomagnetic index. The longest-running index is the aa index, for which observations were commenced in 1868 by the International Service of Geomagnetic Indices (ISGI, http://isgi.unistra. fr/index.php). The aa index is observed from one northern hemisphere station in middle latitude and one approximately antipodal southern hemisphere station. Over the history of the aa index, there have been three different stations in each hemisphere. The Aa index is derived from the daily means of the aa index values, which represent the geomagnetic activity level at an invariant magnetic latitude of about 50°. Nevanlinna and Kataja (1993) extended the Aa values between 1844 and 1868 using a single station at Helsinki, Finland (http://www.geo.fmi.fi/MAGN/magn/), which we have used in this study. In order to verify the analogy of the semiannual variation of geomagnetic activity between geomagnetic indices, we also analyze the ap
Fig. 1. Definition of polarity in the SC phase in the northern solar hemisphere.
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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index. The 3-h ap index is the linearized form of the 3-h Kp index that starts in 1932, which is available from the website of the National Oceanic and Atmospheric Administration/National Centers for Environmental Information/ Solar-Terrestrial Physics Division (NOAA/NCEI/STP; ftp://ftp.ngdc.noaa.gov/STP/GEOMAGNETIC_DATA/ INDICES/KP_AP). The daily ap index is preferred over the daily sum of the nonlinear Kp index because averaging the Kp index could be inappropriate and generate misleading results. The ap index is observed from 11 mid-latitude stations in the Northern Hemisphere and two stations in the Southern Hemisphere. We use the monthly sunspot number in version 2.0 (Clette et al., 2014) from Sunspot Index and Long-term Solar Observations (SILSO, http:// sidc.oma.be/silso/). The extension of Aa by Nevanlinna and Kataja (1993) is marred by a problem with the calibration of the magnetometers in Helsinki, see http://www.leif.org/research/ Scale-Value-H-Helsinki.pdf and http://www.leif.org/research/Error-Scale-Values-HLS.pdf which makes the early part of the series suspect. Svalgaard (2014) pointed out that the scaled values of magnetometer at Hensinki were too low for some intervals. Therefore, we did not use the Helsinki data of 1886 adapted in low values. We also repeated the analysis for the Ap index reconstructed by Svalgaard (http://www.leif.org/research/Ap-1844-2012.xls). The Ap index exhibits the same trend as that of the Aa index, as shown in Fig. 4. Fig. 2a–c shows the monthly and yearly values of the Aa index, Ap index, and sunspot number, respectively. The Aa index and sunspot number represent the 15 SCs since 1844, and the Ap index represents the eight SCs since 1932. The Aa and Ap indices both show a trend of decreasing with declining sunspot number during the most recent three SCs. To test the dependency of the seasonal variation of the Aa index on solar magnetic polarity, only the data of 14 subset periods were used for the analysis. The data were collected for periods of clear solar magnetic polarity, such as the minimum years during the transition period from SC 9 to 23. The parallel and antiparallel periods address the times when the dipole moments of the Earth and Sun point in the same and opposite directions, respectively, as show in Fig. 1. Other studies (e.g., Russell and McPherron (1973)) have used the terminology of positive polarity when the northern hemisphere of the sun is positive and the Bx component of the IMF is outward from the sun (antiparallel IMF Bz). Similarly, the polarity that is termed negative in the northern hemisphere of the sun is equivalent to parallel in our terminology. For example, the solar minimum years of SC 9–10 and SC 21–22 are termed as parallel periods. The solar minimum years of SC 10–11 and SC 22–23 are antiparallel periods. Here, we excluded the period from SC 23 to 24 because of its very depressed solar activity; thus, we set the same number of years for the different polarities. Seven SCs since 1932 were examined in the Ap index; however, because our results for the Aa and Ap
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Fig. 2. Long-term variation of the monthly Aa index (a), Ap index (b), and sunspot number (c). Thin and thick solid lines indicate the monthly and yearly means, respectively.
indices were similar, only the results for the Aa index are discussed in the remaining sections. 3. Results Fig. 3 shows the Aa index as the daily mean of the aa index values in periods of opposite solar magnetic polarities. The mean values of the Aa index shown in Fig. 3 were obtained from the averages of 21 years at seven solar minima for antiparallel and parallel polarities. The smoothed values are obtained by negative exponential smoothing (Oh and Yi, 2017). The geomagnetic index, Aa, shows a seasonal variation pattern with more frequent geomagnetic activity during the equinoxes than during the solstices (Oh and Yi, 2011). The trend of the semiannual variation with peaks at the equinoxes in the Aa index exhibits the same pattern for both solar magnetic polarities. However, the mean values of the Aa index are higher, especially at the equinoxes in the antiparallel period than in the parallel, for the whole year term as shown in Figs. 3 and 4.
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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Fig. 3. The Aa index averages of three years at solar minimum years in opposite solar magnetic polarities.
Table 1 shows the dependency of the Aa index on the solar magnetic polarity for the 14 observed solar minimum years since 1856, with the three years at each solar minimum listed. For example, the three years at the solar minimum of solar cycles 9–10 are 1855, 1856, and 1857. The results for the Ap index are not shown; however, they were similar for the last seven solar minima.
Fig. 4 shows the monthly Aa and Ap indices in opposite solar magnetic polarities for 14 solar minima. Similar to Fig. 3, the values of the Aa and Ap indices in Fig. 4 are 21-year averages from seven SCs for antiparallel and parallel periods. To validate the Aa index from Helsinki, we examined the monthly Ap index reconstructed by Svalgaard (http://www.leif.org/research/Ap-1844-2012.xls). There is little difference between the figures for the equinoxes in the antiparallel periods. However, the semiannual variation of geomagnetic activity is stronger in the antiparallel periods. Fig. 5 shows the monthly average Aa index for the seven antiparallel solar minima and the seven parallel solar minima, as shown in Table 1. The months around the March equinox are most favored; however, half of the time, the September equinox is favored for the antiparallel periods, including the most recent period around 1996 (1995– 1997). Thus, it is unclear whether the dependency on solar magnetic polarity changes according to which equinox has the most geomagnetic activity. The magnitudes of antiparallel and parallel activity in the 22-year cycle are discussed later. Svalgaard (2011) reported on the semiannual geomagnetic variation using the long-term data of the aa index. In Fig. 5 of his paper, all rising and declining SC periods were normalized to 1.0, and the average normalized curve
Fig. 4. Monthly Aa index (a) and Ap index (b) averages of three years at solar minimum years in opposite solar magnetic polarities for the entire dataset since 1856. Error bars indicate standard errors of the Aa and Ap indices. Table 1 Dependency of the Aa index on solar magnetic polarity since 1856. The ‘‘month” columns show the months with the maximum value of the Aa index among the 12 months of the year for 3 years (±1 year from the listed solar minimum year). The ‘‘maximum” columns show the 3-month average value in the maximum months for 3 years. The ‘‘average” columns show the averages of the total 3-year solar minimum period. Parallel period
Antiparallel period
Solar minimum year (SC)
Month
Maximum (nT)
Average (nT)
Solar minimum year (SC)
Month
Maximum (nT)
Average (nT)
1856 1878 1901 1923 1944 1964 1986
Aug. May Jan. Mar. Mar. Sep. Feb.
16.8 8.9 9.0 16.4 24.2 25.3 27.2
13.1 7.8 6.8 13.1 20.0 17.5 20.9
1867 1889 1913 1933 1954 1976 1996
Oct. Nov. Oct. Mar. Mar. Mar. Oct.
26.2 15.7 11.1 22.4 25.5 28.4 20.3
18.2 12.9 9.5 16.3 19.0 22.1 16.4
(9/10) (11/12) (13/14) (15/16) (17/18) (19/20) (21/22)
(10/11) (12/13) (14/15) (16/17) (18/19) (20/21) (22/23)
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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Fig. 5. Monthly Aa index in opposite solar magnetic polarities for the entire dataset since 1856: (a)–(g) antiparallel periods and (h)–(n) parallel periods.
showed a peak in March for declining periods and two peaks in September and October for rising periods. His Fig. 3 shows the average without normalization, where the peaks are in March and October for antiparallel and parallel declining periods, respectively, and in September and March for antiparallel and parallel rising periods, respectively. Our study covers the solar minimum periods, which may be closest to the declining periods in terms of
the importance of HSS. However, we found that the Aa index shows stronger activity around March for five out of seven parallel solar minima, and similarly around October for four out of seven antiparallel solar minima. This observation is exactly opposite to the results of Svalgaard (2011) for the declining years, as shown in his Fig. 3. This reveals the differences between the declining and solar minima periods. However, the observed preference for March
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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in the declining periods in Fig. 5 of Svalgaard (2011) is similar to our finding of the preference for the March equinox in eight of 14 solar minima (Table 1). Ultimately, there may not be any statistically significant preference for either of the equinoxes to exhibit the greatest geomagnetic activity during any part of the SC. Even though the difference between the geomagnetic activities of the equinoxes cannot be established clearly by a comparison of their magnitudes in the periods of opposite solar magnetic polarities, it can be explained by power spectrum analysis using Lomb-Scargle periodgram (Lomb, 1976; Scargle, 1982). It is a well-known algorithm for detecting and characterizing periodicity in time-series including the unevenly sampled data. This method weights the data on a ‘per point’ basis rather than on a ‘per time interval’ basis as does the FFT (Wall and Jenkins, 2003). Fig. 6 shows the power spectrum analysis of the Aa index in opposite solar magnetic polarities. The power spectrum curves in Fig. 6 indicate the periodicity in the unit of day. The semiannual variation of the Aa index is statistically significant at the 95% level. Its periodicity is stronger in the antiparallel solar magnetic polarity period than in the parallel period. Cliver et al. (2004) proposed the favorable condition of geomagnetic activity by investigating the strong semiannual variation observed in the solar minimum years of 1954 and 1996. The favorable solar wind IMF direction is inward (toward the Sun) in spring and outward (away from the Sun) in fall. These favorable polarities occur at solar minima. Fig. 7 clearly shows the trends of such favorable conditions. Toward the Sun IMF in spring and the away from the Sun IMF in fall mostly contribute to the semiannual variation of geomagnetic activity in both antiparallel and parallel solar minimum years. This issue will be discussed in the next section in detail. As an additional test, we analyzed the data into two subsets based on the polarity of each day. Fig. 8 shows the analysis of the Aa index based on the IMF polarity of each day. We calculated the monthly means of the subsets of the Aa index based on the IMF polarity. We only analyzed the
data from 1975, since when the IMF has been directly observed by spacecraft and not merely inferred from the geomagnetic indices. Fig. 8 shows the monthly Aa index averages of three years at solar minimum years in antiparallel and parallel solar magnetic polarities. Each month has two subsets separated by IMF polarity. Toward the Sun IMF contributes to the peak geomagnetic activity in spring and away from the Sun IMF in fall on both antiparallel and parallel solar magnetic polarities. The geomagnetic activity in antiparallel periods shows the more considerable difference between equinoxes than in parallel ones as shown in Fig. 8. Thus the antiparallel periods are more favorable condition for the semiannual variation of geomagnetic activity. 4. Discussion and summary 4.1. Geometric considerations We corrected the model suggested by Oh and Yi (2011) using a combination of the polarity effect suggested by Rosenberg and Coleman (1969) and the enhanced GSM Bz effect of Russell and McPherron (1973). Other authors have different figures (e.g. Fig. 1 of Cliver et al. (1996)). Fig. 7a describes the Earth’s rotation around the Sun for four seasons, where the additional rotation of the magnetic axis around the rotation axis is relatively small and is not shown. Fig. 7b and c show the Earth’s magnetic field and the IMF Bx directions in each season from the Sun’s point of view for antiparallel (positive) and parallel (negative) polarities of the Sun and they indicate the conditions favorable for geomagnetic activity. The upper diagrams in panel b and panel c illustrate the solar meridian plane and effects of solar dipole tilt while the bottom diagrams show the plane of the current sheet and the effects of projection of the current sheet IMF Bx. The heliospheric current sheet (HCS) is the surface where the polarity of the Sun’s Bx magnetic field reverses from the solar northern hemisphere IMF direction to the southern one. This field extends throughout the Sun’s equatorial plane in the heliosphere.
Fig. 6. Periodic analysis of the Aa index averages of three years at solar minimum years in opposite solar magnetic polarities for the entire dataset since 1856: (a) antiparallel periods, (b) parallel periods.
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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Fig. 7. Earth’s rotation around the Sun (a), favorable conditions for geomagnetic activity for antiparallel (b) and parallel (c) polarities of the Sun and the Earth in each season from the Sun’s point of view (not to scale), (d) hypothetical occurrence rate of enhanced geomagnetic activity at the equinoxes.
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The shape of the HCS results from the influence of the Sun’s rotating magnetic field on the plasma in the solar wind. As the Sun rotates, its magnetic field twists into the form of an Archimedean spiral. Parker’s (1958) spiral magnetic field was divided into two by an HCS (Schatten, 1971). As the spiraling magnetic sheet changes polarity, it warps into a wavy spiral shape that has been likened to a ballerina’s skirt (Rosenberg and Coleman, 1969). For the period 1985–1987 in parallel polarity, the HCS was relatively flat, with a heliospheric tilt of 16°, whereas the degree of warping of the HCS was more pronounced (19°) in the antiparallel period of 1995–1997 (e.g., Fig. 1 of Emery et al. (2011)). However, the parallel period of 2007–2009 had an extraordinary tilt of 27°. Even though the HCS can be relatively flat with a low HCS tilt angle, the Earth will usually experience positive and negative IMF Bx values every solar rotation period of 27 days by being above and below the HCS as the sun rotates; this results in 13.5-day periodicities in the Bx and By values. Fig. 13 of Russell and McPherron (1973) shows the parallel period from 1962–1969 in SC 19–20, where the average C9 index for the favorable conditions shown in Fig. 7c is plotted. For this SC, the C9 index peaks in August through November, similar to the sharp peak in the Aa index found in September for the 1963–1965 period in Fig. 5m, which was one of two parallel periods where the Aa index favored September instead of March. Russell and McPherron (1973) stated that such equinoctial asymmetries do not occur in averages over longer periods, such as the AE study by Burch (1973). The favorable conditions for geomagnetic activity, shown at the bottom of Fig. 7b and c, are when Bx has a downward component along the rotation (near magnetic) axis, which provides a negative GSM Bz component to the Earth system. This occurs in equinoctial periods because of the tilt of the rotation axis, as shown in Fig. 7a, and as depicted as a hypothetical occurrence rate in Fig. 7d. In the antiparallel period of SC 22–23, increased geomagnetic activity occurs in March (position A in Fig. 7b) for inward positive Bx (toward the Sun) when the Earth is under the HCS, and in September (position B in Fig. 7b) for outward negative Bx (away from the Sun) when the Earth is above the HCS. Because the heliolatitude of the Earth is 7.2° on March 6 and +7.2° on September 6, the Earth’s relative position increases the likelihood of it being below (above) the HCS in March (September), adding to the negative southward Bz. In addition, the Bz southward component is averagely strengthened in the antiparallel polarity periods, as shown in the upper two figures of Fig. 7b. For the parallel polarity shown in Fig. 5n, the favorable positions occur in March (position C in Fig. 7c) for inward positive Bx above the HCS, and in September (position D in Fig. 7c) for outward negative Bx below the HCS. For these periods, the heliolatitude of the Earth tends to work against these favorable positions instead of complementing them.
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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Fig. 8. Monthly Aa index averages of three years at solar minimum years in antiparallel and parallel solar minimum years based on the IMF polarity: (a) and (c) for antiparallel periods, (b) and (d) for parallel periods.
4.2. The 22-year solar cycle (Hale Cycle) Chernosky (1966) found a 22-year sunspot cycle in the Ci index from 1884 to 1963, where even SCs were more active in the last half, and odd SCs were more active in the first half. Russell and McPherron (1973) added the axial hypothesis to show that this works in accordance with their hypothesis, such that the ‘quiet’ periods from odd cycle solar maximum to even cycle solar maximum (parallel periods) are when the heliolatitudes are opposite to our ‘favorable’ conditions. The ‘active’ periods from even cycle solar maximum to odd cycle solar maximum (antiparallel periods) are when the heliolatitudes add to the favorable conditions. Thus, we would expect Aa and Ap in Fig. 2 to be largest and most active in the antiparallel cycle from about 1991 to 2002. However, there is more activity in Aa and Ap in the latter halves of the two odd SCs (parallel) in 1984 and 2003, probably because of the unique nature of the general decrease in the solar polar magnetic fields, and because of the distribution of the HSS in these three SCs in our study. Oh and Yi (2011) reported that more storms occur in periods of antiparallel polarities of the Sun and the Earth than in periods of parallel polarities. They explained that the southward Bz component of the IMF is an important
factor in storm generation. Increased geomagnetic activity in antiparallel periods is to be expected because of the extra enhancement of the GSM Bz, as shown in Fig. 7b. In general, geomagnetic activity is stronger in antiparallel solar minimum periods than in parallel solar minimum periods, as has been observed through the long-term observations of the Aa index, with similar results being obtained for the Ap index since 1932. Graphically, the minima in the Schwabe sunspot cycle look quite similar, due to the very few sunspot numbers. But other direct and proxy measurements of solar activity, for example geomagnetic reconstructions and cosmogenic isotope reconstructions, demonstrate a significant longterm change in the near-Earth space environment between 1750 and the present day (Owens et al., 2016a, 2016b). The long-term change of SC may affect the level of geomagnetic activity and introduce the uncertainty in observation. Although it can be measured by the magnitude from such reconstructions, we do not discuss the distribution of the IMF direction. 4.3. Summary We examined the seasonal variation of geomagnetic activity. We used the Aa index during three years for each
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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of the 14 solar minimum periods since 1856. We divided the test periods by means of solar magnetic polarity. The solar magnetic polarity is parallel with the Earth’s polarity in the minimum years of SC 9–10, SC 11–12, and up to SC 21–22, and antiparallel with the Earth’s polarity in the minimum years of SC 10–11, SC 12–13, and up to SC 22–23. 1. As expected, the 22-year Hale cycle, where antiparallel geomagnetic activities are stronger than parallel activities, is shown to exist in the Aa data most of the time from 1856 during solar minimum periods, which is in agreement with previous studies. 2. The Aa index shows stronger activity around March for five out of seven parallel solar minima, and similarly around October for four out of seven antiparallel solar minima as shown in Table 1. It indicates that the semiannual variation of the Aa index shows the different trend in antiparallel and parallel solar minima. Additionally, the periodicity of semiannual variation in the Aa index is stronger in antiparallel periods than in parallel periods, as shown in Fig. 6. 3. It is reasonable to predict that the mean level of geomagnetic activity depends strongly on solar magnetic polarity because geomagnetic activity is related to the direction of the IMF Bz. The HSS produces most of the geomagnetic activity in solar minimum periods. However, there is no mechanism known to favor the HSS or to enhance the GSM Bz values observed on Earth asymmetrically at the March and September equinoxes. Toward the Sun IMF in spring and the away from the Sun IMF in fall contribute to the semiannual variation of geomagnetic activity in both antiparallel and parallel solar minimum years, as shown in Fig. 7. This favorable condition is clearly proven in the analysis on the subsets of the Aa index based on the IMF polarity, as shown in Fig. 8. The equinoctial effect is more important than the Russell-McPherron effect in creation of the stronger equinoctial maxima in geomagnetic activity. It depends on the tilt of the dipole toward and away from the Sun with the strongest reconnection near the equinoxes when the Earth’s dipole is exactly orthogonal to the solar wind flow direction. Recently, Oh and Yi (2017) showed the difference in solar parameters (sunspot number, sunspot area, and solar radio flux) and cosmic ray flux by solar magnetic polarity. In their work, odd solar cycles (antiparallel solar minimum years) show strong solar activity than even solar cycles (parallel solar minimum years). The results by Oh and Yi (2017) suggest that the sign of solar magnetic polarity can make the differences in geomagnetic activity and periodicity in semiannual variation in this work. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of
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Education (NRF-2015R1D1A1A01060598). The geomagnetic ap data were obtained from the Solar-Terrestrial Physics (STP) Division of the National Centers for Environmental Information (NCEI)/NOAA, the aa data from both the International Service of Geomagnetic Indices (ISGI) and the Finnish Meteorological Institute (FMI) for aa of 1844–1867, and the sunspot number data from WDC-SILSO, Royal Observatory of Belgium in Brussels. The authors thank Barbara Emery for providing helpful ideas and sincere discussions in improving this paper. References Bohlin, J.D., 1977. Extreme-ultraviolet observations of coronal holes. Sol. Phys. 51 (2), 377–398. Burch, J.L., 1973. Effects of interplanetary magnetic sector structure on auroral zone and polar cap magnetic activity. J. Geophys. Res. 78, 1047–1057. Chernosky, E.J., 1966. Double sunspot-cycle variation in terrestrial magnetic activity 1884–1963. J. Geophys. Res. 71, 965–974. Clette, F., Svalgaard, L., Vaquero, J.M., et al., 2014. Revisiting the sunspot number, a 400-year perspective on the solar cycle. Space Sci. Rev. 186, 35–103. Cliver, E.W., Boriakoff, V., Bounar, K.H., 1996. The 22-year cycle of geomagnetic and solar wind activity. J. Geophys. Res. 101, 27091– 27109. Cliver, E.W., Kamide, Y., Ling, A.G., 2000. Mountains versus valleys: semiannual variation of geomagnetic activity. J. Geophys. Res. 105, 2413–2424. Cliver, E.W., Svalgaard, L., Ling, A.G., 2004. Origins of the semiannual variation of geomagnetic activity in 1954 and 1996. Ann. Geophys. 22, 93–100. Emery, B.A., Richardson, I.G., Evans, D.S., et al., 2011. Solar rotational periodicities and the semiannual variation in the solar wind, radiation belt, and aurora. Sol. Phys. 274, 399–425. https://doi.org/10.1007/ s11207-011-9758-x. Lomb, N.R., 1976. Least-squares frequency analysis of unequally spaced data. Ap&SS 39, 447–462. McIntosh, D.H., 1959. On the annual variation of magnetic disturbance. Phil. Trans. Roy. Soc. A 251 (1001), 525–552. Nevanlinna, H., Kataja, E., 1993. An extension of the geomagnetic activity index series aa for two solar cycles (1844–1868). Geophys. Res. Lett. 20, 2703–2706. Oh, S.Y., Yi, Y., 2011. Solar magnetic polarity dependency of geomagnetic storm seasonal occurrence. J. Geophys. Res. 116, A06101. https://doi.org/10.1029/2010JA016362. Oh, S., Yi, Y., 2017. Variations in solar parameters and cosmic rays with solar magnetic polarity. ApJ 840, 14. https://doi.org/10.3847/15384357/aa6c62. Owens, M.J., Cliver, E., McCracken, K.G., et al., 2016a. Near-Earth heliospheric magnetic field intensity since 1750: 1. Sunspot and geomagnetic reconstructions. J. Geophys. Res. Space Phys. 121, 6048–6063. https://doi.org/10.1002/2016JA022529. Owens, M.J., Cliver, E., McCracken, K.G., et al., 2016b. Near-Earth heliospheric magnetic field intensity since 1750: 2. Cosmogenic radionuclide reconstructions. J. Geophys. Res. Space Phys. 121, 6064–6074. https://doi.org/10.1002/2016JA022550. Parker, E.N., 1958. Dynamics of the interplanetary gas and magnetic fields. ApJ 128, 664–676. Rosenberg, R.L., Coleman Jr., P.J., 1969. Heliographic latitude dependence of the dominant polarity of the interplanetary magnetic field. J. Geophys. Res. 74, 5611–5622. Russell, C.T., McPherron, R.L., 1973. Semiannual variation of geomagnetic activity. J. Geophys. Res. 78, 92–108.
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ScienceDirect Advances in Space Research xxx (2017) xxx–xxx www.elsevier.com/locate/asr
Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index Suyeon Oh a,⇑, Yu Yi b b
a Department of Earth Science Education, Chonnam National University, Gwangju 61186, Republic of Korea Department of Astronomy, Space Science and Geology, Chungnam National University, Daejeon 34134, Republic of Korea
Received 15 June 2017; received in revised form 9 August 2017; accepted 5 September 2017
Abstract Three major hypotheses have been proposed to explain the well-known semiannual variation of geomagnetic activity, maxima at equinoxes and minima at solstices. This study examined whether the seasonal variation of equinoctial geomagnetic activity is different in periods of opposite solar magnetic polarity in order to understand the contribution of the interplanetary magnetic field (IMF) in the SunEarth connection. Solar magnetic polarity is parallel to the Earth’s polarity in solar minimum years of odd/even cycles but antiparallel in solar minimum years of even/odd cycles. The daily mean of the aa, Aa indices during each solar minimum was compared for periods when the solar magnetic polarity remained in opposite dipole conditions. The Aa index values were used for each of the three years surrounding the solar minimum years of the 14 solar cycles recorded since 1856. The Aa index reflects seasonal variation in geomagnetic activity, which is greater at the equinoxes than at the solstices. The Aa index reveals solar magnetic polarity dependency in which the geomagnetic activity is stronger in the antiparallel solar magnetic polarity condition than in the parallel one. The periodicity in semiannual variation of the Aa index is stronger in the antiparallel solar polar magnetic field period than in the parallel period. Additionally, we suggest the favorable IMF condition of the semiannual variation in geomagnetic activity. The orientation of IMF toward the Sun in spring and away from the Sun in fall mainly contributes to the semiannual variation of geomagnetic activity in both antiparallel and parallel solar minimum years. Ó 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Solar polarity; Semiannual variation; Aa index
1. Introduction Geomagnetic activity shows well recognized seasonal variation: greater at the equinoxes than at the solstices. Rosenberg and Coleman (1969) and later Bohlin (1977) based their explanation for this variation on the axial hypothesis, which recognizes the Earth is located at a heliolatitude of 7.2° on March 6 and at a heliolatitude of +7.2° on September 6. Thus, at these times, the Earth is closer to the middle heliolatitudes of solar activity in the ⇑ Corresponding author.
form of high-speed streams (HSSs) in the solar wind velocity or coronal mass ejections. Russell and McPherron (1973) suggested that the equinoctial peaks are caused by the Bx and By sector structures of the interplanetary magnetic field (IMF), which translate to negative or positive geocentric solar magnetospheric (GSM) Bz values. Cliver et al. (2000) after McIntosh (1959) and Svalgaard (1977), suggested that the largest effects are due to equinoctial enhancement and to solstice blocking of the solar wind into the high latitudes of the Earth. Previous studies on semiannual geomagnetic activity have been performed mainly using geomagnetic indices such as am, aa, U (the approximate magnitude of the
E-mail address: [email protected] (S. Oh). https://doi.org/10.1016/j.asr.2017.09.008 0273-1177/Ó 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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difference between successive daily averages of the Dst index; e.g., see Fig. 1 of Russell and McPherron (1973)), AL, and Dst. Oh and Yi (2011) tested the dependency of geomagnetic activity on solar magnetic polarity by examining the occurrence of geomagnetic storms based on the low-latitude Dst index. They established that geomagnetic storm activity shows a clear increase in frequency during the equinoxes (March and September) in comparison with the solstices (June and December). However, they were unable to verify a significant difference between the geomagnetic activities in March and September when solar polarity was reversed. Thus, the previously analyzed Dst index is derived from a network of near-equatorial geomagnetic observations. In this study, therefore, we examine the dependency of geomagnetic activity in the high-latitude regions on solar magnetic polarity. The aa index is a linear scale index by nanotesla (nT) derived from the K indices measured at two antipodal observatories, and it reflects the amplitude of global geomagnetic activity during 3-h intervals normalized to ±50° geomagnetic latitude. The K indices are converted into amplitudes using mid-class amplitudes and then averaged with weighting factors that account for slight changes in geomagnetic disturbance intensities between successive Northern and Southern aa-observatories (http://isgi.unistra.fr/indices_aa.php). The Aa index, which is obtained from daily averages of eight aa values, was introduced to monitor geomagnetic activity over the longest possible time period. The Aa index is observed for higher latitudes than the Dst index and it is reasonable to surmise that the geomagnetic activity in the high-latitude regions reflected in the Aa index depends on solar magnetic polarity. The geomagnetic disturbances by solar activity are defined by the variation of horizontal component of geomagnetic field. The highlatitude regions can be easily affected by solar disturbance because of weak horizontal intensity (H). The polar geomagnetic variation sensitively responds to the change of IMF Bz, which is determined by solar magnetic polarity. Thus, the Aa index is considered suitable for investigating the semiannual variation of geomagnetic activity during periods of different solar magnetic polarity. The strength of the interaction of the solar wind with the magnetosphere is governed to a major degree by the contemporary relative directions of the component of the solar wind magnetic field relative to the direction of the Earth’s magnetic field. That is the interaction was strongest when those two magnetic fields were closest to being in opposite directions, which enabled magnetic reconnection. The strength of the interaction of the solar wind with the magnetosphere has nothing to do with the direction of the magnetic field relative to the solar poles. However, even though the magnetic reconnection is a local phenomenon, the statistical frequency of its occurrence can be determined by the solar global magnetic polarity as the background. Our present work begins at this point. Thus, our study con-
centrates on the effect of solar polarity in the solar minimum year, not on the solar events in the solar maximum year. Our goal is to determine whether the solar polarity can account for the difference of geomagnetic activity between the equinoxes. Fig. 1 summarizes the definition of solar magnetic polarity and related terminologies in the phase of solar cycle (SC) at the northern solar hemisphere. The polarity of the solar polar magnetic field experiences a reversal near each solar maximum. The increasing phase where the sunspot number increases with time has a different polarity from that in the declining phase where the sunspot number decreases with time within the same SC. At each solar minimum period, the solar polar magnetic field aligns with the Earth’s magnetic field in either the antiparallel or parallel direction. Thus, the solar polar magnetic field directed away from the Sun is antiparallel with the Earth’s magnetic field and that directed toward the Sun one is parallel. 2. Data We examined the daily aa index as the geomagnetic index. The longest-running index is the aa index, for which observations were commenced in 1868 by the International Service of Geomagnetic Indices (ISGI, http://isgi.unistra. fr/index.php). The aa index is observed from one northern hemisphere station in middle latitude and one approximately antipodal southern hemisphere station. Over the history of the aa index, there have been three different stations in each hemisphere. The Aa index is derived from the daily means of the aa index values, which represent the geomagnetic activity level at an invariant magnetic latitude of about 50°. Nevanlinna and Kataja (1993) extended the Aa values between 1844 and 1868 using a single station at Helsinki, Finland (http://www.geo.fmi.fi/MAGN/magn/), which we have used in this study. In order to verify the analogy of the semiannual variation of geomagnetic activity between geomagnetic indices, we also analyze the ap
Fig. 1. Definition of polarity in the SC phase in the northern solar hemisphere.
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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index. The 3-h ap index is the linearized form of the 3-h Kp index that starts in 1932, which is available from the website of the National Oceanic and Atmospheric Administration/National Centers for Environmental Information/ Solar-Terrestrial Physics Division (NOAA/NCEI/STP; ftp://ftp.ngdc.noaa.gov/STP/GEOMAGNETIC_DATA/ INDICES/KP_AP). The daily ap index is preferred over the daily sum of the nonlinear Kp index because averaging the Kp index could be inappropriate and generate misleading results. The ap index is observed from 11 mid-latitude stations in the Northern Hemisphere and two stations in the Southern Hemisphere. We use the monthly sunspot number in version 2.0 (Clette et al., 2014) from Sunspot Index and Long-term Solar Observations (SILSO, http:// sidc.oma.be/silso/). The extension of Aa by Nevanlinna and Kataja (1993) is marred by a problem with the calibration of the magnetometers in Helsinki, see http://www.leif.org/research/ Scale-Value-H-Helsinki.pdf and http://www.leif.org/research/Error-Scale-Values-HLS.pdf which makes the early part of the series suspect. Svalgaard (2014) pointed out that the scaled values of magnetometer at Hensinki were too low for some intervals. Therefore, we did not use the Helsinki data of 1886 adapted in low values. We also repeated the analysis for the Ap index reconstructed by Svalgaard (http://www.leif.org/research/Ap-1844-2012.xls). The Ap index exhibits the same trend as that of the Aa index, as shown in Fig. 4. Fig. 2a–c shows the monthly and yearly values of the Aa index, Ap index, and sunspot number, respectively. The Aa index and sunspot number represent the 15 SCs since 1844, and the Ap index represents the eight SCs since 1932. The Aa and Ap indices both show a trend of decreasing with declining sunspot number during the most recent three SCs. To test the dependency of the seasonal variation of the Aa index on solar magnetic polarity, only the data of 14 subset periods were used for the analysis. The data were collected for periods of clear solar magnetic polarity, such as the minimum years during the transition period from SC 9 to 23. The parallel and antiparallel periods address the times when the dipole moments of the Earth and Sun point in the same and opposite directions, respectively, as show in Fig. 1. Other studies (e.g., Russell and McPherron (1973)) have used the terminology of positive polarity when the northern hemisphere of the sun is positive and the Bx component of the IMF is outward from the sun (antiparallel IMF Bz). Similarly, the polarity that is termed negative in the northern hemisphere of the sun is equivalent to parallel in our terminology. For example, the solar minimum years of SC 9–10 and SC 21–22 are termed as parallel periods. The solar minimum years of SC 10–11 and SC 22–23 are antiparallel periods. Here, we excluded the period from SC 23 to 24 because of its very depressed solar activity; thus, we set the same number of years for the different polarities. Seven SCs since 1932 were examined in the Ap index; however, because our results for the Aa and Ap
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Fig. 2. Long-term variation of the monthly Aa index (a), Ap index (b), and sunspot number (c). Thin and thick solid lines indicate the monthly and yearly means, respectively.
indices were similar, only the results for the Aa index are discussed in the remaining sections. 3. Results Fig. 3 shows the Aa index as the daily mean of the aa index values in periods of opposite solar magnetic polarities. The mean values of the Aa index shown in Fig. 3 were obtained from the averages of 21 years at seven solar minima for antiparallel and parallel polarities. The smoothed values are obtained by negative exponential smoothing (Oh and Yi, 2017). The geomagnetic index, Aa, shows a seasonal variation pattern with more frequent geomagnetic activity during the equinoxes than during the solstices (Oh and Yi, 2011). The trend of the semiannual variation with peaks at the equinoxes in the Aa index exhibits the same pattern for both solar magnetic polarities. However, the mean values of the Aa index are higher, especially at the equinoxes in the antiparallel period than in the parallel, for the whole year term as shown in Figs. 3 and 4.
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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Fig. 3. The Aa index averages of three years at solar minimum years in opposite solar magnetic polarities.
Table 1 shows the dependency of the Aa index on the solar magnetic polarity for the 14 observed solar minimum years since 1856, with the three years at each solar minimum listed. For example, the three years at the solar minimum of solar cycles 9–10 are 1855, 1856, and 1857. The results for the Ap index are not shown; however, they were similar for the last seven solar minima.
Fig. 4 shows the monthly Aa and Ap indices in opposite solar magnetic polarities for 14 solar minima. Similar to Fig. 3, the values of the Aa and Ap indices in Fig. 4 are 21-year averages from seven SCs for antiparallel and parallel periods. To validate the Aa index from Helsinki, we examined the monthly Ap index reconstructed by Svalgaard (http://www.leif.org/research/Ap-1844-2012.xls). There is little difference between the figures for the equinoxes in the antiparallel periods. However, the semiannual variation of geomagnetic activity is stronger in the antiparallel periods. Fig. 5 shows the monthly average Aa index for the seven antiparallel solar minima and the seven parallel solar minima, as shown in Table 1. The months around the March equinox are most favored; however, half of the time, the September equinox is favored for the antiparallel periods, including the most recent period around 1996 (1995– 1997). Thus, it is unclear whether the dependency on solar magnetic polarity changes according to which equinox has the most geomagnetic activity. The magnitudes of antiparallel and parallel activity in the 22-year cycle are discussed later. Svalgaard (2011) reported on the semiannual geomagnetic variation using the long-term data of the aa index. In Fig. 5 of his paper, all rising and declining SC periods were normalized to 1.0, and the average normalized curve
Fig. 4. Monthly Aa index (a) and Ap index (b) averages of three years at solar minimum years in opposite solar magnetic polarities for the entire dataset since 1856. Error bars indicate standard errors of the Aa and Ap indices. Table 1 Dependency of the Aa index on solar magnetic polarity since 1856. The ‘‘month” columns show the months with the maximum value of the Aa index among the 12 months of the year for 3 years (±1 year from the listed solar minimum year). The ‘‘maximum” columns show the 3-month average value in the maximum months for 3 years. The ‘‘average” columns show the averages of the total 3-year solar minimum period. Parallel period
Antiparallel period
Solar minimum year (SC)
Month
Maximum (nT)
Average (nT)
Solar minimum year (SC)
Month
Maximum (nT)
Average (nT)
1856 1878 1901 1923 1944 1964 1986
Aug. May Jan. Mar. Mar. Sep. Feb.
16.8 8.9 9.0 16.4 24.2 25.3 27.2
13.1 7.8 6.8 13.1 20.0 17.5 20.9
1867 1889 1913 1933 1954 1976 1996
Oct. Nov. Oct. Mar. Mar. Mar. Oct.
26.2 15.7 11.1 22.4 25.5 28.4 20.3
18.2 12.9 9.5 16.3 19.0 22.1 16.4
(9/10) (11/12) (13/14) (15/16) (17/18) (19/20) (21/22)
(10/11) (12/13) (14/15) (16/17) (18/19) (20/21) (22/23)
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Fig. 5. Monthly Aa index in opposite solar magnetic polarities for the entire dataset since 1856: (a)–(g) antiparallel periods and (h)–(n) parallel periods.
showed a peak in March for declining periods and two peaks in September and October for rising periods. His Fig. 3 shows the average without normalization, where the peaks are in March and October for antiparallel and parallel declining periods, respectively, and in September and March for antiparallel and parallel rising periods, respectively. Our study covers the solar minimum periods, which may be closest to the declining periods in terms of
the importance of HSS. However, we found that the Aa index shows stronger activity around March for five out of seven parallel solar minima, and similarly around October for four out of seven antiparallel solar minima. This observation is exactly opposite to the results of Svalgaard (2011) for the declining years, as shown in his Fig. 3. This reveals the differences between the declining and solar minima periods. However, the observed preference for March
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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in the declining periods in Fig. 5 of Svalgaard (2011) is similar to our finding of the preference for the March equinox in eight of 14 solar minima (Table 1). Ultimately, there may not be any statistically significant preference for either of the equinoxes to exhibit the greatest geomagnetic activity during any part of the SC. Even though the difference between the geomagnetic activities of the equinoxes cannot be established clearly by a comparison of their magnitudes in the periods of opposite solar magnetic polarities, it can be explained by power spectrum analysis using Lomb-Scargle periodgram (Lomb, 1976; Scargle, 1982). It is a well-known algorithm for detecting and characterizing periodicity in time-series including the unevenly sampled data. This method weights the data on a ‘per point’ basis rather than on a ‘per time interval’ basis as does the FFT (Wall and Jenkins, 2003). Fig. 6 shows the power spectrum analysis of the Aa index in opposite solar magnetic polarities. The power spectrum curves in Fig. 6 indicate the periodicity in the unit of day. The semiannual variation of the Aa index is statistically significant at the 95% level. Its periodicity is stronger in the antiparallel solar magnetic polarity period than in the parallel period. Cliver et al. (2004) proposed the favorable condition of geomagnetic activity by investigating the strong semiannual variation observed in the solar minimum years of 1954 and 1996. The favorable solar wind IMF direction is inward (toward the Sun) in spring and outward (away from the Sun) in fall. These favorable polarities occur at solar minima. Fig. 7 clearly shows the trends of such favorable conditions. Toward the Sun IMF in spring and the away from the Sun IMF in fall mostly contribute to the semiannual variation of geomagnetic activity in both antiparallel and parallel solar minimum years. This issue will be discussed in the next section in detail. As an additional test, we analyzed the data into two subsets based on the polarity of each day. Fig. 8 shows the analysis of the Aa index based on the IMF polarity of each day. We calculated the monthly means of the subsets of the Aa index based on the IMF polarity. We only analyzed the
data from 1975, since when the IMF has been directly observed by spacecraft and not merely inferred from the geomagnetic indices. Fig. 8 shows the monthly Aa index averages of three years at solar minimum years in antiparallel and parallel solar magnetic polarities. Each month has two subsets separated by IMF polarity. Toward the Sun IMF contributes to the peak geomagnetic activity in spring and away from the Sun IMF in fall on both antiparallel and parallel solar magnetic polarities. The geomagnetic activity in antiparallel periods shows the more considerable difference between equinoxes than in parallel ones as shown in Fig. 8. Thus the antiparallel periods are more favorable condition for the semiannual variation of geomagnetic activity. 4. Discussion and summary 4.1. Geometric considerations We corrected the model suggested by Oh and Yi (2011) using a combination of the polarity effect suggested by Rosenberg and Coleman (1969) and the enhanced GSM Bz effect of Russell and McPherron (1973). Other authors have different figures (e.g. Fig. 1 of Cliver et al. (1996)). Fig. 7a describes the Earth’s rotation around the Sun for four seasons, where the additional rotation of the magnetic axis around the rotation axis is relatively small and is not shown. Fig. 7b and c show the Earth’s magnetic field and the IMF Bx directions in each season from the Sun’s point of view for antiparallel (positive) and parallel (negative) polarities of the Sun and they indicate the conditions favorable for geomagnetic activity. The upper diagrams in panel b and panel c illustrate the solar meridian plane and effects of solar dipole tilt while the bottom diagrams show the plane of the current sheet and the effects of projection of the current sheet IMF Bx. The heliospheric current sheet (HCS) is the surface where the polarity of the Sun’s Bx magnetic field reverses from the solar northern hemisphere IMF direction to the southern one. This field extends throughout the Sun’s equatorial plane in the heliosphere.
Fig. 6. Periodic analysis of the Aa index averages of three years at solar minimum years in opposite solar magnetic polarities for the entire dataset since 1856: (a) antiparallel periods, (b) parallel periods.
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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Fig. 7. Earth’s rotation around the Sun (a), favorable conditions for geomagnetic activity for antiparallel (b) and parallel (c) polarities of the Sun and the Earth in each season from the Sun’s point of view (not to scale), (d) hypothetical occurrence rate of enhanced geomagnetic activity at the equinoxes.
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The shape of the HCS results from the influence of the Sun’s rotating magnetic field on the plasma in the solar wind. As the Sun rotates, its magnetic field twists into the form of an Archimedean spiral. Parker’s (1958) spiral magnetic field was divided into two by an HCS (Schatten, 1971). As the spiraling magnetic sheet changes polarity, it warps into a wavy spiral shape that has been likened to a ballerina’s skirt (Rosenberg and Coleman, 1969). For the period 1985–1987 in parallel polarity, the HCS was relatively flat, with a heliospheric tilt of 16°, whereas the degree of warping of the HCS was more pronounced (19°) in the antiparallel period of 1995–1997 (e.g., Fig. 1 of Emery et al. (2011)). However, the parallel period of 2007–2009 had an extraordinary tilt of 27°. Even though the HCS can be relatively flat with a low HCS tilt angle, the Earth will usually experience positive and negative IMF Bx values every solar rotation period of 27 days by being above and below the HCS as the sun rotates; this results in 13.5-day periodicities in the Bx and By values. Fig. 13 of Russell and McPherron (1973) shows the parallel period from 1962–1969 in SC 19–20, where the average C9 index for the favorable conditions shown in Fig. 7c is plotted. For this SC, the C9 index peaks in August through November, similar to the sharp peak in the Aa index found in September for the 1963–1965 period in Fig. 5m, which was one of two parallel periods where the Aa index favored September instead of March. Russell and McPherron (1973) stated that such equinoctial asymmetries do not occur in averages over longer periods, such as the AE study by Burch (1973). The favorable conditions for geomagnetic activity, shown at the bottom of Fig. 7b and c, are when Bx has a downward component along the rotation (near magnetic) axis, which provides a negative GSM Bz component to the Earth system. This occurs in equinoctial periods because of the tilt of the rotation axis, as shown in Fig. 7a, and as depicted as a hypothetical occurrence rate in Fig. 7d. In the antiparallel period of SC 22–23, increased geomagnetic activity occurs in March (position A in Fig. 7b) for inward positive Bx (toward the Sun) when the Earth is under the HCS, and in September (position B in Fig. 7b) for outward negative Bx (away from the Sun) when the Earth is above the HCS. Because the heliolatitude of the Earth is 7.2° on March 6 and +7.2° on September 6, the Earth’s relative position increases the likelihood of it being below (above) the HCS in March (September), adding to the negative southward Bz. In addition, the Bz southward component is averagely strengthened in the antiparallel polarity periods, as shown in the upper two figures of Fig. 7b. For the parallel polarity shown in Fig. 5n, the favorable positions occur in March (position C in Fig. 7c) for inward positive Bx above the HCS, and in September (position D in Fig. 7c) for outward negative Bx below the HCS. For these periods, the heliolatitude of the Earth tends to work against these favorable positions instead of complementing them.
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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Fig. 8. Monthly Aa index averages of three years at solar minimum years in antiparallel and parallel solar minimum years based on the IMF polarity: (a) and (c) for antiparallel periods, (b) and (d) for parallel periods.
4.2. The 22-year solar cycle (Hale Cycle) Chernosky (1966) found a 22-year sunspot cycle in the Ci index from 1884 to 1963, where even SCs were more active in the last half, and odd SCs were more active in the first half. Russell and McPherron (1973) added the axial hypothesis to show that this works in accordance with their hypothesis, such that the ‘quiet’ periods from odd cycle solar maximum to even cycle solar maximum (parallel periods) are when the heliolatitudes are opposite to our ‘favorable’ conditions. The ‘active’ periods from even cycle solar maximum to odd cycle solar maximum (antiparallel periods) are when the heliolatitudes add to the favorable conditions. Thus, we would expect Aa and Ap in Fig. 2 to be largest and most active in the antiparallel cycle from about 1991 to 2002. However, there is more activity in Aa and Ap in the latter halves of the two odd SCs (parallel) in 1984 and 2003, probably because of the unique nature of the general decrease in the solar polar magnetic fields, and because of the distribution of the HSS in these three SCs in our study. Oh and Yi (2011) reported that more storms occur in periods of antiparallel polarities of the Sun and the Earth than in periods of parallel polarities. They explained that the southward Bz component of the IMF is an important
factor in storm generation. Increased geomagnetic activity in antiparallel periods is to be expected because of the extra enhancement of the GSM Bz, as shown in Fig. 7b. In general, geomagnetic activity is stronger in antiparallel solar minimum periods than in parallel solar minimum periods, as has been observed through the long-term observations of the Aa index, with similar results being obtained for the Ap index since 1932. Graphically, the minima in the Schwabe sunspot cycle look quite similar, due to the very few sunspot numbers. But other direct and proxy measurements of solar activity, for example geomagnetic reconstructions and cosmogenic isotope reconstructions, demonstrate a significant longterm change in the near-Earth space environment between 1750 and the present day (Owens et al., 2016a, 2016b). The long-term change of SC may affect the level of geomagnetic activity and introduce the uncertainty in observation. Although it can be measured by the magnitude from such reconstructions, we do not discuss the distribution of the IMF direction. 4.3. Summary We examined the seasonal variation of geomagnetic activity. We used the Aa index during three years for each
Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008
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of the 14 solar minimum periods since 1856. We divided the test periods by means of solar magnetic polarity. The solar magnetic polarity is parallel with the Earth’s polarity in the minimum years of SC 9–10, SC 11–12, and up to SC 21–22, and antiparallel with the Earth’s polarity in the minimum years of SC 10–11, SC 12–13, and up to SC 22–23. 1. As expected, the 22-year Hale cycle, where antiparallel geomagnetic activities are stronger than parallel activities, is shown to exist in the Aa data most of the time from 1856 during solar minimum periods, which is in agreement with previous studies. 2. The Aa index shows stronger activity around March for five out of seven parallel solar minima, and similarly around October for four out of seven antiparallel solar minima as shown in Table 1. It indicates that the semiannual variation of the Aa index shows the different trend in antiparallel and parallel solar minima. Additionally, the periodicity of semiannual variation in the Aa index is stronger in antiparallel periods than in parallel periods, as shown in Fig. 6. 3. It is reasonable to predict that the mean level of geomagnetic activity depends strongly on solar magnetic polarity because geomagnetic activity is related to the direction of the IMF Bz. The HSS produces most of the geomagnetic activity in solar minimum periods. However, there is no mechanism known to favor the HSS or to enhance the GSM Bz values observed on Earth asymmetrically at the March and September equinoxes. Toward the Sun IMF in spring and the away from the Sun IMF in fall contribute to the semiannual variation of geomagnetic activity in both antiparallel and parallel solar minimum years, as shown in Fig. 7. This favorable condition is clearly proven in the analysis on the subsets of the Aa index based on the IMF polarity, as shown in Fig. 8. The equinoctial effect is more important than the Russell-McPherron effect in creation of the stronger equinoctial maxima in geomagnetic activity. It depends on the tilt of the dipole toward and away from the Sun with the strongest reconnection near the equinoxes when the Earth’s dipole is exactly orthogonal to the solar wind flow direction. Recently, Oh and Yi (2017) showed the difference in solar parameters (sunspot number, sunspot area, and solar radio flux) and cosmic ray flux by solar magnetic polarity. In their work, odd solar cycles (antiparallel solar minimum years) show strong solar activity than even solar cycles (parallel solar minimum years). The results by Oh and Yi (2017) suggest that the sign of solar magnetic polarity can make the differences in geomagnetic activity and periodicity in semiannual variation in this work. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of
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Education (NRF-2015R1D1A1A01060598). The geomagnetic ap data were obtained from the Solar-Terrestrial Physics (STP) Division of the National Centers for Environmental Information (NCEI)/NOAA, the aa data from both the International Service of Geomagnetic Indices (ISGI) and the Finnish Meteorological Institute (FMI) for aa of 1844–1867, and the sunspot number data from WDC-SILSO, Royal Observatory of Belgium in Brussels. The authors thank Barbara Emery for providing helpful ideas and sincere discussions in improving this paper. References Bohlin, J.D., 1977. Extreme-ultraviolet observations of coronal holes. Sol. Phys. 51 (2), 377–398. Burch, J.L., 1973. Effects of interplanetary magnetic sector structure on auroral zone and polar cap magnetic activity. J. Geophys. Res. 78, 1047–1057. Chernosky, E.J., 1966. Double sunspot-cycle variation in terrestrial magnetic activity 1884–1963. J. Geophys. Res. 71, 965–974. Clette, F., Svalgaard, L., Vaquero, J.M., et al., 2014. Revisiting the sunspot number, a 400-year perspective on the solar cycle. Space Sci. Rev. 186, 35–103. Cliver, E.W., Boriakoff, V., Bounar, K.H., 1996. The 22-year cycle of geomagnetic and solar wind activity. J. Geophys. Res. 101, 27091– 27109. Cliver, E.W., Kamide, Y., Ling, A.G., 2000. Mountains versus valleys: semiannual variation of geomagnetic activity. J. Geophys. Res. 105, 2413–2424. Cliver, E.W., Svalgaard, L., Ling, A.G., 2004. Origins of the semiannual variation of geomagnetic activity in 1954 and 1996. Ann. Geophys. 22, 93–100. Emery, B.A., Richardson, I.G., Evans, D.S., et al., 2011. Solar rotational periodicities and the semiannual variation in the solar wind, radiation belt, and aurora. Sol. Phys. 274, 399–425. https://doi.org/10.1007/ s11207-011-9758-x. Lomb, N.R., 1976. Least-squares frequency analysis of unequally spaced data. Ap&SS 39, 447–462. McIntosh, D.H., 1959. On the annual variation of magnetic disturbance. Phil. Trans. Roy. Soc. A 251 (1001), 525–552. Nevanlinna, H., Kataja, E., 1993. An extension of the geomagnetic activity index series aa for two solar cycles (1844–1868). Geophys. Res. Lett. 20, 2703–2706. Oh, S.Y., Yi, Y., 2011. Solar magnetic polarity dependency of geomagnetic storm seasonal occurrence. J. Geophys. Res. 116, A06101. https://doi.org/10.1029/2010JA016362. Oh, S., Yi, Y., 2017. Variations in solar parameters and cosmic rays with solar magnetic polarity. ApJ 840, 14. https://doi.org/10.3847/15384357/aa6c62. Owens, M.J., Cliver, E., McCracken, K.G., et al., 2016a. Near-Earth heliospheric magnetic field intensity since 1750: 1. Sunspot and geomagnetic reconstructions. J. Geophys. Res. Space Phys. 121, 6048–6063. https://doi.org/10.1002/2016JA022529. Owens, M.J., Cliver, E., McCracken, K.G., et al., 2016b. Near-Earth heliospheric magnetic field intensity since 1750: 2. Cosmogenic radionuclide reconstructions. J. Geophys. Res. Space Phys. 121, 6064–6074. https://doi.org/10.1002/2016JA022550. Parker, E.N., 1958. Dynamics of the interplanetary gas and magnetic fields. ApJ 128, 664–676. Rosenberg, R.L., Coleman Jr., P.J., 1969. Heliographic latitude dependence of the dominant polarity of the interplanetary magnetic field. J. Geophys. Res. 74, 5611–5622. Russell, C.T., McPherron, R.L., 1973. Semiannual variation of geomagnetic activity. J. Geophys. Res. 78, 92–108.
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Please cite this article in press as: Oh, S., Yi, Y. Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index. Adv. Space Res. (2017), https://doi.org/10.1016/j.asr.2017.09.008