Semiannual Variation Of Geomagnetic Activity Research Articles

Relating 27-Day Averages of Solar, Interplanetary Medium Parameters, and Geomagnetic Activity Proxies in Solar Cycle 24

Correlations between solar, interplanetary-medium parameters, and geomagnetic-activity proxies in 27-day averages (a Bartels rotation) were analyzed for the 2009 – 2016 time interval. In this analysis, two new proxies were considered: i) \(B_{zs}\) GSM (Geocentric Solar Magnetic), calculated as the daily percentage of the IMF southward component along the GSM \(z\)-axis and then averaged every 27 days; ii) four magnetospheric indices (T-indices), calculated from the local north–south (\(x\)) contributions of the magnetosphere’s cross-tail (TAIL), the symmetric ring current (SRC), the partial ring current (PRC), and the Birkeland current (FAC), derived from the Tsyganenko and Sitnov (J. Geophys. Res. 110, A03208, 2005: TS05) semi-empirical magnetospheric model. Our results suggest that, among the parameters tested in this study, solar facular areas, interplanetary magnetic-field intensity and new proxies derived from the TS05 model could be taken into account in an empirical model, with a 27-day resolution, to explain geomagnetic activity felt on the Earth’s surface in terms of solar-surface features and the IMF condition. We further retrieve a clear annual oscillation in series of 27-day-mean values of toward/away asymmetries of geomagnetic-activity indices, which can be interpreted in the light of the Russell–McPherron hypothesis for the semiannual variation of geomagnetic activity.

Early Studies in Solar Wind Coupling and Substorms

AbstractThe space age began while I was a Physics student at Berkeley. NASA stimulated space research by the creation of eight centers of excellence each with a new building and very large grants. These centers became the nucleus of research activities that continue 55 years later. Data from new instruments and spacecraft provided unique opportunities for young people to make significant discoveries. In my case the concept of the auroral substorm was crucial, as later were data from the first instrumented synchronous satellite ATS‐1, and one of the first eccentric orbiters OGO‐5. In this account I describe my early experiences and the difficulties of completing my undergraduate degree. I then describe graduate work at Berkeley and multiple trips to the auroral zone. On completion of my PhD I began work at University of California, Los Angeles (UCLA) and soon became a professor. My graduate work and early research as a professor enabled me to introduce important concepts including the substorm growth phase, the concept of a magnetospheric substorm, the substorm current wedge, the near‐Earth neutral line, the dipolarization of the synchronous magnetic field during substorms, the semiannual variation in geomagnetic activity, and algorithms to predict the Dst index and AL index. I also did extensive work in investigations of ULF waves, magnetic storms, and the interaction of the solar wind with the magnetosphere, but these occurred later and are not discussed. I end with the question of whether it is possible today for young people with a background similar to mine to achieve similar success.

Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 1. Geomagnetic data

We study the semi-annual variation in geomagnetic activity, as detected in the geomagnetic indices am, aaH, AL, Dst and the four aσ indices derived for 6-hour MLT sectors (around noon, dawn, dusk and midnight). For each we compare the amplitude of the semi-annual variation, as a fraction of the overall mean, to that of the corresponding variation in power input to the magnetosphere, Pα, estimated from interplanetary observations. We demonstrate that the semi-annual variation is amplified in the geomagnetic data compared to that in Pα, by a factor that is different for each index. The largest amplification is for the Dst index (factor ~ 10) and the smallest is for the aσ index for the noon MLT sector (aσ-noon, factor ≈ 1.1). By sorting the data by the prevailing polarity of the Y-component (dawn-dusk) of the Interplanetary Magnetic Field (IMF) in the Geocentric Solar Equatorial (GSEQ) reference frame, we demonstrate that the Russell-McPherron (R-M) effect, in which a small southward IMF component in GSEQ is converted into geoeffective field by Earth’s dipole tilt, is a key factor for the semi-annual variations in both Pα and geomagnetic indices. However, the variability in the southward component in the IMF in the GSEQ frame causes more variability in power input to the magnetosphere Pα than does the R-M effect, by a factor of more than two. We show that for increasingly large geomagnetic disturbances, Pα delivered by events of large southward field in GSEQ (known to often be associated with coronal mass ejections) becomes the dominant driver and the R-M effect declines in importance and often acts to reduce geoeffectiveness for the most southward IMF in GSEQ: the semi-annual variation in large storms therefore suggests either preconditioning of the magnetosphere by average conditions or an additional effect at the equinoxes. We confirm that the very large R-M effect in the Dst index is because of a large effect at small and moderate activity levels and not in large storms. We discuss the implications of the observed “equinoctial” time-of-year (F) – Universal Time (UT) pattern of geomagnetic response, the waveform and phase of the semi-annual variations, the differences between the responses at the June and December solstices and the ratio of the amplitudes of the March and September equinox peaks. We also confirm that the UT variation in geomagnetic activity is a genuine global response. Later papers will analyse the origins and implications of the effects described.

Effect of Ap-Index of Geomagnetic Activity on S&P 500 Stock Market Return

Geomagnetic activity with global influence is an essential object of space weather research and is a significant link in the section of the solar wind-magnetospheric coupling process. Research so far provides strong evidence that geomagnetic activity affects stock investment decisions by influencing human health, mood, and human behaviours. Therefore, this research investigates the empirical association between geomagnetic activity and stock market return. Overall, we find that geomagnetic activity exerts a negative influence on the return of the US stock market. Further, market liquidity effectively magnifies the effect of geomagnetic activity. Inconsistent with previous literature, this effect is not mainly caused by the semiannual variation of geomagnetic activity. Our research contributes to the introduction of geomagnetic indices to financial economics studies on the impact of geomagnetic activity influence on stock market return.

МНОЖЕСТВЕННЫЕ КОРРЕЛЯЦИОННЫЕ МОДЕЛИ ЗАВИСИМОСТИ ПОТОКОВ ИЗЛУЧЕНИЯ КОСМИЧЕСКИХ РАДИОИСТОЧНИКОВ В ДЕКАМЕТРОВОМ ДИАПАЗОНЕ ОТ ЭКСТРЕМАЛЬНЫХ ПРОЯВЛЕНИЙ КОСМИЧЕСКОЙ ПОГОДЫ

PACS numbers: 95.55.Jz, 95.85.Bh, 96.50.Wx Purpose: Investigation of the effects of the influence of solar and geomagnetic activity on the condition of the Earth’s upper atmosphere by the method of “transmission through” it with the radiation of cosmic radio sources. Design/methodology/approach: Monitoring of the fluxes of powerful galactic and extragalactic radio sources is carried out at the URAN-4 radio telescope of the Odesa Observatory of the Institute of Radio Astronomy of the NAS of Ukraine since 1987 till now. The monitoring program includes radio galaxies 3C274, 3C405 and supernova remnants 3C144, 3C461. Changes of fluxes of radiation sources at decameter waves are determined by the condition of an ionosphere resulted due to space weather variation. Findings: The models of multiple correlation dependence of fluxes of radio sources on the factors forming space weather for the periods of extreme conditions of the solar and geomagnetic activity are considered. The explanatory variables are divided into three groups: indices of solar activity, flows of particles, magnetic field components. Over 60 models of the multiple correlation dependence were calculated. Values of a multiple coefficient of correlation within 0.86 to 0.99 were obtained. The greatest contribution to the quantity of a multiple coefficient of correlation give: solar radio emission at the wave of 10.7 cm (F10.7), the total area of groups of spots (Sp), flows of electrons (E) and protons (P). Conclusions: Monitoring of fluxes of space radio sources made at the “URAN-4” radio telescope makes it possible to “transmit through” the Earth ionosphere above the radio telescope and determines the integral effect of the solar and geomagnetic activity influence on it. The great amount of the data observed with using the “URAN-4” radio telescope allows more detailed study of the influence of space weather effects in the ionosphere to be made in our next works. Key words: upper atmosphere, solar activity, geomagnetic activity, space weather Manuscript submitted 26.06.2018 Radio phys. radio astron. 2018, 23(3): 189-202 REFERENCES 1. RYABOV, M. I., PANISHKO, S. K. and GUGLYA, L. I., 2011. Space weather impacts on the Earth’s upper atmosphere according to the monitoring of powerful radio sources fluxes at the URAN-4 radio telescope (Institute of Radio Astronomy, NAS of Ukraine). Odessa Astronomical Publications . vol. 24, pp. 159–161. (in Russian). 2. SOBITNIAK, L. I., RYABOV, M. I., SUKHAREV, A. L. and PANISHKO, S. K., 2017. Structure of variability indexes of cosmic weather as applied to data monitoring of fluxes of radio sources at the “URAN-4” radio telescope. Radio Phys. Radio Astron . vol. 22, no. 4, pp. 294–303 (in Russian). DOI: https://doi.org/10.15407/rpra22.04.294 3. BOROVIKOV, V. A., 2001. STATISTICA: The art of data analysis on a computer . St. Petersburg: Piter Publ. (in Russian). 4. BRUZEK, A. and DURRANT, C. J., eds., 1977. Illustrated glossary for solar and solar-terrestrial physics . Dordrecht, Boston: D. Reidel Pub. Co. DOI: https://doi.org/10.1007/978-94-010-1245-4 5. IVANOV-KHOLODNYI, G. S. and NIKOLSKY, G. M., 1969. The Sun and Ionosphere. Short wave solar radiation and its effect on the ionosphere . Moscow, Russia: Nauka Publ. (in Russian). 6. RUSSELL, C. T. and MCPHERRON, R. L., 1973. Semiannual Variation of Geomagnetic Activity. J. Geophys. Res. Space Phys . vol. 78, is. 1, pp. 92–108. DOI: https://doi.org/10.1029/JA078i001p00092 7. ZELENYI, L. M. and VESELOVSKIY, I. S., eds., 2008. Plasma heliogeophysics . Moscow, Russia: Fizmatlit Publ. vol. 1-2. (in Russian). 8. MIROSHNICHENKO, L. I., 2011. Physics of the Sun and solar-terrestrial connections . Moscow, Russia: Universitetskaya Kniga Publ. (in Russian). 9. OBRIDKO, V. N. and NAGOVITSYN, YU. A., 2017. Solar activity, cyclicity and prediction methods . St. Petersburg: VVM Publ. (in Russian).

Solar polar magnetic field dependency of geomagnetic activity semiannual variation indicated in the Aa index

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 Sun-Earth 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.

Seasonal and diurnal variation of geomagnetic activity: Russell‐McPherron effect during different IMF polarity and/or extreme solar wind conditions

The Russell‐McPherron (R‐M) effect is one of the most prevailing hypotheses accounting for semiannual variation of geomagnetic activity. To validate the R‐M effect and investigate the difference of geomagnetic activity variation under different interplanetary magnetic field (IMF) polarity and during extreme solar wind conditions (interplanetary shock), we have analyzed 42 years interplanetary magnetic field and geomagnetic indices data and 1270 SSC (storm sudden commencement) events from the year 1968 to 2010 by defining the R‐M effect with positive/negative IMF polarity (IMF away/toward the Sun). The results obtained in this study have shown that the response of geomagnetic activity to the R‐M effect with positive/negative IMF polarity are rather profound: the geomagnetic activity is much more intense around fall equinox when the direction of IMF is away the Sun, while much more intense around spring equinox when the direction of IMF is toward the Sun. The seasonal and diurnal variation of geomagnetic activity after SSCs can be attributed to both R‐M effect and the equinoctial hypothesis; the R‐M effect explains most part of variance of southward IMF, while the equinoctial hypothesis explains similar variance of ring current injection and geomagnetic indices as the R‐M effect. However, the R‐M effect with positive/negative IMF polarity explains the difference between SSCs with positive/negative IMFByaccurately, while the equinoctial hypothesis cannot explain such difference at the spring and fall equinoxes. Thus, the R‐M effect with positive/negative IMF polarity is more reasonable to explain seasonal and diurnal variation of geomagnetic activity under extreme solar wind conditions.

The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind–magnetosphere‐ionosphere coupling

The angle μ between the geomagnetic dipole axis and the geocentric solar magnetospheric (GSM) zaxis, sometimes called the “dipole tilt,” varies as a function of UT and season. Observations have shown that the cross‐polar cap potential tends to maximize near the equinoxes, when on averageμ= 0, with smaller values observed near the solstices. This is similar to the well‐known semiannual variation in geomagnetic activity. We use numerical model simulations to investigate the role of two possible mechanisms that may be responsible for the influence ofμon the magnetosphere‐ionosphere system: variations in the coupling efficiency between the solar wind and the magnetosphere and variations in the ionospheric conductance over the polar caps. Under southward interplanetary magnetic field (IMF) conditions, variations in ionospheric conductance at high magnetic latitudes are responsible for 10–30% of the variations in the cross‐polar cap potential associated withμ, but variations in solar wind–magnetosphere coupling are more important and responsible for 70–90%. Variations in viscous processes contribute slightly to this, but variations in the reconnection rate with μare the dominant cause. The variation in the reconnection rate is primarily the result of a variation in the length of the section of the separator line along which relatively strong reconnection occurs. Changes in solar wind–magnetosphere coupling also affect the field‐aligned currents, but these are influenced as well by variations in the conductance associated with variations inμ, more so than the cross‐polar cap potential. This may be the case for geomagnetic activity too.

Geomagnetic semiannual variation is not overestimated and is not an artifact of systematic solar hemispheric asymmetry

Mursula et al. [2011] (MTL11) suggest that there is a 22-year variation in solar wind activity that coupled with the variation in heliographic latitude of the Earth during the year, gives rise to an apparent semiannual variation of geomagnetic activity in averages obtained over several solar cycles. They conclude that the observed semiannual variation is seriously overestimated and is largely an artifact of this inferred 22-year variation. We show: (1) that there is no systematically alternating annual variation of geomagnetic activity or of the solar driver, changing with the polarity of the solar polar fields, (2) that the universal time variation of geomagnetic activity at all times have the characteristic imprint of the equinoctial hypothesis rather than that of the axial hypothesis required by the suggestion of MTL11, and (3) that the semiannual variation is not an artifact, is not overestimated, and does not need revision.

Ionospheric F2-Layer Semi-Annual Variation in Middle Latitude by Solar Activity

We examine the ionospheric F2-layer electron density variation by solar activity in middle latitude by using foF2 observed at the Kokubunji ionosonde station in Japan for the period from 1997 to 2008. The semi-annual variation of foF2 shows obviously in high solar activity (2000-2002) than low solar activity (2006-2008). It seems that variation of geomagnetic activity by solar activity influences on the semi-annual variation of the ionospheric F2-layer electron density. According to the Lomb-Scargle periodogram analysis of foF2 and Ap index, interplanetary magnetic field (IMF) Bs (IMF Bz <0) component, solar wind speed, solar wind number density and flow pressure which influence the geomagnetic activity, we examine how the geomagnetic activity affects the ionospheric F2-layer electron density variation. We find that the semi-annual variation of daily foF2, Ap index and IMF Bs appear clearly during the high solar activity. It suggests that the semi-annual variation of geomagnetic activity, caused by Russell-McPherron effect, contributes greatly to the ionospheric F2-layer semi-annual electron density variation, except dynamical effects in the thermosphere.

Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity

Abstract. Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity is investigated based on the three-hourly am index and solar wind parameters. When the z component of the interplanetary magnetic field (IMF) in geocentric solar magnetospheric (GSM) coordinates is southward, am indices are well correlated with BsVx2, where Bs is the southward component of the IMF and Vx is the solar wind velocity in the sun-earth direction. The am-BsVx2 relationship, however, depends on the range of Vx2: the am in higher ranges of Vx2 tends to be larger than am in lower ranges of Vx2 for the same value of BsVx2 for both equinoctial and solstitial epochs. Using the data sets of the same Vx2 range, it is shown that distribution of points in the am-BsVx2 diagram at the solstitial epochs overlaps with that at the equinoctial epochs and the average am values in each BsVx2 bin in solstitial epochs are closely consistent with those in equinoctial epochs, if Vx2 for each point at solstices are reduced to Vx2sin2 (Ψ) where Ψ is the geomagnetic colatitude of the sub-solar point. Further, it is shown that monthly averages of the am index in the long period is well correlated with the values of sin2(ψ) for the middle day of each month. These findings indicate that the factor that contributes to the generation of geomagnetic disturbance is not the velocity of the solar wind, but the component of the solar wind velocity perpendicular to the dipole axis of the geomagnetic field. The magnitude of the perpendicular velocity component varies semi-annually even if the solar wind velocity remains constant, which is considered to be the long-missed key factor causing the equinoctial effect.

Effects of dipole tilt angle on geomagnetic activity

The relationship between the orientation of the interplanetary magnetic field (IMF), represented by the clock angle which is the angle defined by IMF-B y and -B z components, and the AL and AU indices is examined at various dipole tilt angles for the period of 1978–1988. We use the IMF data obtained from the IMP 8 satellite, AL and AU indices with corrected seasonal variations, and the dipole tilt angle, which is the dipole magnetic latitude of the subsolar point calculated as a function of the day of year and universal time. For both positive (dipole tilted to the Sun) and negative dipole tilt angles, the values of | AL | and AU decrease as the IMF clock angle moves away from 180 ∘ , becoming more northward. The indices also tend to become smaller for larger dipole tilt angle, either toward or away from the Sun. This dependence on dipole tilt angle enhances the semiannual variation of geomagnetic activity.

&amp;lt;I&amp;gt;a&amp;lt;sub&amp;gt;p&amp;lt;/sub&amp;gt;&amp;lt;/I&amp;gt;-index solar wind driving function and its semiannual variations

Abstract. Semiannual variation of geomagnetic activity and ap-index in particular is supposed to consist of heliospheric factor (axial hypothesis and Russell-McPherron effect) and magnetospheric/ionospheric factor (equinoctical hypothesis). In our investigation we express ap-index as a magnetospheric response function to solar wind and IMF input. Seasonal variation in ap-index on average (1963–2003) is ~4 nT and consists of ~2.1–2.3 nT of magnetospheric/ionospheric part, 0.6–1.3 nT of heliospheric part (including 0.2–0.3 nT of R-M effect), 0.1–0.4 nT is due to the non-linear term. 90% confidence range of all estimates is ~0.1–0.25 nT. While autumn/spring magnetospheric response functions are almost identical, there is substantial difference between winter and summer functions. The increase of solar wind input in autumn and spring is also different by a factor of two.

Flux Transfer Events: 1. generation mechanism for strong southward IMF

Abstract. We use a global numerical model of the interaction of the solar wind and the interplanetary magnetic field with Earth's magnetosphere to study the formation process of Flux Transfer Events (FTEs) during strong southward IMF. We find that: (i) The model produces essentially all observational features expected for FTEs, in particular the bipolar signature of the magnetic field BN component, the correct polarity, duration, and intermittency of that bipolar signature, strong core fields and enhanced core pressure, and flow enhancements; (ii) FTEs only develop for large dipole tilt whereas in the case of no dipole tilt steady magnetic reconnection occurs at the dayside magnetopause; (iii) the basic process by which FTEs are produced is the sequential generation of new X-lines which makes dayside reconnection inherently time dependent and leads to a modified form of dual or multiple X-line reconnection; (iv) the FTE generation process in this model is not dependent on specific assumptions about microscopic processes; (v) the average period of FTEs can be explained by simple geometric arguments involving magnetosheath convection; (vi) FTEs do not develop in the model if the numerical resolution is too coarse leading to too much numerical diffusion; and (vii) FTEs for nearly southward IMF and large dipole tilt, i.e., near solstice, should only develop in the winter hemisphere, which provides a testable prediction of seasonal modulation. The semiannual modulation of intermittent FTE reconnection versus steady reconnection is also expected to modulate magnetospheric and ionospheric convection and may thus contribute to the semiannual variation of geomagnetic activity.

Spectral analysis of sunspot number and geomagnetic indices (1868–2001)

The multiple-taper spectral analysis was applied to a a, A p and R z annual and monthly average series (1868–2001). The significant periods at 95% confidence level for annual averages are 11.1 and 10.2 years for R z and 11.1, 5.3, 4.3, 2.7 and 2.1 years for a a. For monthly averages, the significant periods were 133 (11.1 years) and 17.7 months for R z and 133 (11.1 years), 63.7 (5.3 years), 17.5, 12.7 and 6 months for a a. The periods associated with the solar cycle are observed in both series. The a a spectrum showed in addition a peak near 4.3 years, which is not present in the R z spectrum. Other works have also shown this peak in geomagnetic activity. This peak is believed to be caused by the dual-peak structure in the a a. The 22-year Hale cycle was not detected either in R z or a a spectrum in the period 1868–2001, but it was seen in R z data for 1700–2000 annual averages. The explanation of this 22-year cycle has been questioned and while in some spectral analysis this periodicity has been found, others have failed in its detection. The a a spectrum showed in addition a peak of 6 months, associated to the semi-annual variation of geomagnetic activity. Three hypotheses have been considered to explain the seasonal activity variation. These are the Russell–McPherron effect, the equinoctial effect and the axial effect.

Annual and semiannual variations of the location and intensity of large‐scale field‐aligned currents

The present study examines seasonal variations of large‐scale field‐aligned current (FAC) systems in terms of the dipole tilt and clock angles. Magnetic field measurements from the DMSP F7 and F12‐F15 satellites are used. This data set consists of a total of ∼185,000 auroral oval crossings, out of which ∼121,000 crossings were selected for the present analysis. Focus is placed on the latitude at the demarcation between the region 2 (R2) and region 1 (R1) currents and the intensities of these currents. It is found that the dayside FAC moves poleward and equatorward in the summer and winter hemispheres, respectively, and the nightside FAC has the opposite seasonal dependence. In the midday sector the peak‐to‐peak variation of the FAC latitude over the entire range of the dipole tilt is ∼5°, whereas it is ∼4° around midnight. In the flank sectors the average FAC latitude is higher around the solstices than around the equinoxes irrespective of hemisphere. The corresponding dependence on the dipole clock angle can actually be found for almost all local time sectors, although the peak‐to‐peak variation of the expected semiannual variation, 2° around noon and &lt;1° in other local time sectors, is smaller than that of the annual variation except for the flank sectors. A comparison with a model magnetic field strongly suggests that the dipole tilt effect on the magnetospheric configuration is the primary cause of the annual variation, whereas the semiannual variation is inferred to reflect the fact that geomagnetic activity tends to be higher around the equinoxes. The average dayside FAC intensity is larger in the summer hemisphere than in the winter hemisphere, which can be explained in terms of the seasonal variation of the ionospheric conductivity. The dayside R1 current intensity depends more strongly on the dipole tilt than the dayside R2 current intensity, and it changes by a factor of 2–3 over the entire range of the dipole tilt angle. In contrast, the annual variation of the nightside FAC intensity is more complicated, and the nightside R2 current seems to be more intense in the winter hemisphere than in the summer hemisphere. The dependence of the FAC intensity on the dipole clock angle is less significant especially for the R1 system. Nevertheless, the result suggests that the FAC tends to be more intense around the equinoxes, which is consistent with the semiannual variation of geomagnetic activity.

Origins of the semiannual variation of geomagnetic activity in 1954 and 1996

Abstract. We investigate the cause of the unusually strong semiannual variation of geomagnetic activity observed in the solar minimum years of 1954 and 1996. For 1996 we separate the contributions of the three classical modulation mechanisms (axial, equinoctial, and Russell-McPherron) to the six-month wave in the aam index and find that all three contribute about equally. This is in contrast to the longer run of geomagnetic activity (1868-1998) over which the equinoctial effect accounts for ∼70% of the semiannual variation. For both 1954 and 1996, we show that the Russell-McPherron effect was enhanced by the Rosenberg-Coleman effect (an axial polarity effect) which increased the amount of the negative (toward Sun) [positive (away from Sun)] polarity field observed during the first [second] half of the year; such fields yield a southward component in GSM coordinates. Because this favourable condition occurs only for alternate solar cycles, the marked semiannual variation in 1954 and 1996 is a manifestation of the 22-year cycle of geomagnetic activity. The 11-year evolution of the heliospheric current sheet (HCS) also contributes to the strong six-month wave during these years. At solar minimum, the streamer belt at the base of the HCS is located near the solar equator, permitting easier access to high speed streams from polar coronal holes when the Earth is at its highest heliographic latitudes in March and September. Such an axial variation in solar wind speed was observed for 1996 and is inferred for 1954. Key words. Magnetosphere (solar wind – magnetosphere interactions; storms and substorms)

Annual TEC variation in the equatorial anomaly region during the solar minimum: September 1996–August 1997

The ionospheric total electron content (TEC) in the equatorial anomaly region is studied by analyzing dual-frequency signals from the global position system (GPS) acquired from a meridional chain of 9 observational sites clustered around Taiwan (21.9°–26.2°N, 118.4°–121.6°). This relatively dense GPS chain provides a powerful tool for studying ionospheric TEC in the northern hemispheric equatorial anomaly region with an unprecedented spatial resolution. Specifically, we studied seasonal and geomagnetic effects on the equatorial ionospheric anomaly during the solar minimum period between September 1996 and August 1997. We found that the surveyed data indicated semiannual variations in the magnitude of TEC, I c, at the anomaly, with maxima in the equinoxes, similar to the semiannual variation of geomagnetic activity. The values of I c were found to maximize in April, 1997 and minimize in July, 1997. Statistical studies indicate that the monthly values of I c do not correlate with the planetary K p magnetic index ( r=0.41) but correlate well with the D st geomagnetic activity index ( r=0.72). This suggests that variations of TEC are mainly driven by a low-latitude forcing for the surveyed period. The most likely time for I c to occur was 1400 local time (∼30%) at 20°N geographic latitude (∼37%).

Latitudinal effect in semiannual variation of geomagnetic activity

The study of the semiannual variation in both geomagnetic activity indices and raw geomagnetic field data at a large number of geomagnetic observatories showed a dramatic decrease in the relative magnitude of this variation from low to high latitudes. While the semiannual variation in the Dst index at low latitudes is about 3 times, for the midlatitude Ap and Am indices it is only about 20–30%, and the auroral electrojet AE index and polar cap PCN index show a very small or no semiannual variation. The study of auroral electrojet AU and AL indices showed that the semiannual variation is evident in AL index and not observed in AU index. Although relative magnitudes of the semiannual variation in geomagnetic activity indices are strongly reduced at high latitudes, absolute magnitudes of these variations in the Dst, Ap, Am, and AL indices are approximately the same. The semiannual variation in geomagnetic field H component consists of two equinoctial minima probably caused by the increasing westward ring current and substorm westward auroral electrojet in equinoctial months. The additional analysis of raw geomagnetic field data from geomagnetic observatories in two hemispheres confirmed the results inferred from the study of geomagnetic indices. Meanwhile in the polar caps, the semiannual variation in raw geomagnetic data showing a strong decrease in geomagnetic field H component reveals a strong increase in the Z component which also might be caused by increasing the ring current and westward auroral electrojet during enhanced geomagnetic activity occurring in equinoctial months. It was also found that the amplitude of the semiannual variation at all latitudes increased significantly with increasing geomagnetic activity.

The semiannual variation of geomagnetic activity: phases and profiles for 130 years of aa data

We determined the phases of the maxima (spring, fall) and minima (summer, winter) in the curve of smoothed daily averages of the aa geomagnetic index, available from 1868 to 1998. The dates we obtained are consistent with the equinoctial hypothesis which has aberration-adjusted theoretical maxima, for a ∼440 km s −1 (modern epoch) average solar wind speed, on 25 March (experimentally determined to be 27 March, with an uncertainty of ±2 days) and 27 September (27 September) and minima on 25 June (26 June) and 26 December (27 December). We also show that the overall shape of the 30-day smoothed modulation curve throughout the year (broad minima, narrow peaks) bears greater fidelity (| r|=0.96) to the aberration-shifted solar declination δ (the controlling angle, on average, for the seasonal variation under the equinoctial hypothesis) than to the solar B 0 angle ( r=0.76; axial hypothesis) or the solar P angle ( r=0.86; Russell–McPherron effect). Lastly, a three-parameter fit of the smoothed annual variation of the aa data with a function consisting of the sum of the smoothed yearly curves for the δ, B 0, and P angles yielded an amplitude of 0.58±0.07 for the δ component vs. 0.16±0.03 for B 0 and 0.20±0.04 for P. Thus, the phases and profiles of the 6-month wave in the long-running mid-latitude aa range index are consistent with control by a dominant equinoctial mechanism.