Long-term variation of strong geomagnetic storms and its effect on ionospheric and telluric currents

Long-term variations in geomagnetic activity have gained importance since the last decade as a consequence of the increasing interest in global change, SunEarth relations and space weather.

During geomagnetic storms, ionospheric currents fluctuate rapidly in response to wide changes in the solar wind, especially at high latitudes. These currents induce a magnetic field at the Earth surface which combines with the Earth intrinsic magnetic field. The result is a changing magnetic field which induces currents flowing within the solid Earth and within the seas and oceans named geomag-netically induced currents (GIC) [1]. These currents are part of the more general telluric currents. When good conductors are present, such as pipelines and electric power transmission lines, the currents travel through them causing several kind and degrees of damages [2].

In the present work, the study of geomagnetic activity is confined to intense storms, since these induce rapidly changing ionospheric current systems, which ends with large induced telluric currents. The aa index and the Akasofu’s epsilon parameter ( ε ) are analyzed. ε , considered as a proxy of the energy input from the Sun via the solar wind to the magnetosphere [3], can also be a measure of geopotentials [4].We include it in the present study as an indicator of Earth’s surface effects induced by variations in ionospheric currents, and inductor of telluric currents.

Data analysis

Daily aa index available at the World Data Center-A for Solar-Terrestrial Physics and hourly solar wind data to assess ε contained in the NSSDC OMNIWeb system were used. The periods analyzed are: 1868–2005 for the aa index, and 1964–2005 for ε .

• ε has been calculated in SI units as:

ε (W) = 10⁷ V(m/s) B²(T) sin⁴( θ /2) l 0 (m)

V is the solar wind speed, B the interplanetary magnetic field (IMF) intensity, θ the angle of IMF seen from

Earth and l o a length related to the cross-section of the magnetosphere and taken as 7 R_E. The frequency distribution of days with strong activity levels per year was assessed from the aa index time series considering days with aa>80 nT. In the case of ε , hours with ε >10¹³ W per year were estimated first, and then transformed to the percentage with respect to the total amount of hourly values for the corresponding year. This was done due to the very different amount of data for years at the beginning of the record and years at the end.

In Figure, a long-term trend which consists of an overall increase since the 1900’s with a partial minimum around 1965–1970 can be noticed in aa time series, followed by a decrease since the 1990’s. The overall pattern may be part of the long-term Suess (or De Vries) cycle which has a periodicity ∼ 200 years, modulated by the Gleissberg cycle of periodicity ∼ 80–90 years. The trend in strong geomagnetic activity is determined by the amount of stormy periods, which affects ionospheric currents intensity and time variability with consequences over induced telluric currents.

Year

Fig. Frequency of days with aa > 80 nT per year (black filled circles) during the period 1868–2005, and frequency of hourly Akasofu’s epsilon parameter, ε >10¹³ W (red filled circles) during the period 1964–2005, and their 11 year running mean (enhanced black and red lines respectively).

В .M. Силберглейт , A. Г . Элиас

Discussion and Conclusions

A long-term trend in strong geomagnetic storms is observed compatible with the Gleissberg and Suess cycles, so widely manifested in direct and proxy indicators of different parameters of solar activity. These cycles are two basic modes of long-term solar variability believed to be generated from the solar dynamo. The last minimum of the Suess cycle occurred in 1905–1915 [5], and the last maximum around 1990–2000. Regarding the partial minimum around 1965, we suggest that it corresponds to the last minimum of the solar Gleissberg cycle (~80–90 year period), which would be modulating the much stronger Suess cycle. Extrapolation into the future, make us expect a decline after ∼ 2000–2010.