Autumnal asymmetry in geomagnetic activity in polar and pre-polar region

1. Autumnal asymmetry in geomagnetic activity in polar and pre-polar region Falayi, E.O. and Beloff, N. Space Science Centre University of Sussex, UK eof20@sussex.ac.uk

2. Overview • Summary • Geomagnetic activity • Data analysis for 1990 - 2007 period • Statistics of max H, max dH/dt, and AE index • Conclusion

3. Summary • Geomagnetic activities often manifest as geomagnetically induced current (GIC) in ground technology system. The maximum GIC values are proxy to the maximum value of time derivatives of the horizontal field (dH/dt). 289 substorm events were obtained from IMAGE magnetometer data at Northern Europe, using Dst and AE indices to determine the substorm events of varying strength (from -90nT up to -1800nT) from 1990- 2007, and were statistically analysed. In this study we found maximum time derivatives of the horizontal magnetic field statistically significant in October at auroral and subauroral regions. Seasonal variations are also seen, Autumn being more geomagnetically active than any other seasons, even Spring. Our result shows a high correlation of 0.886 and 0.854 at auroral and subauroral region. The test of correlation between dH/dt and measured ionospheric response (AE) shows closer relationship between dH/dt and geomagnetic disturbance and this can improve our space weather prediction system.

4. 3D view of Earth’s Magnetospheric cavity

5. Data Analysis • AE and Dst are used to define a substorms from satellite data from 1990 -2007. • 169 substorms were obtained from Soroya Observatory (auroral region) available from 1990-2007, also 120 substorms were also obtained from Nurmijarvi Observatory (sub auroral region) from 1992-2007 (Figure 1). • To examine horizontal geomagnetic field, the geoelcetric field is accompanied with varying field, indicating the time derivatives of the ground magnetic field dB/dt is proxy to GIC activity especially with horizontal component (Viljanen et al., 2001; Viljanen et al 2006). • dH/dt=sqrt((dX/dt)^2+(dY/dt)^2) to compute ground horizontal magnetic field which gives good reasonably measure of the induction or GIC activity.

6. Seasonal variation of maximum time derivatives and maximum horizontal magnetic field • The disturbance of max dH/dt peaks sharply in October, with another peak in April at auroral region, while at sub auroral region October and November shows maximum occurrences and with another occurrence in February and April. • The max H possesses the same features at auroral and subauroral regions with maxima occurrence in October, November, February and April (Figure 2). We also noticed that max H intensity is greater in subauroral than auroral region. • From Figure 3,Autumn has the highest value of max dH/dt with another peak in Spring at auroral region. Also at sub auroral region, Autumn has the maximum values of occurrence with another peak in Spring but the difference between Spring, Winter and Summer are not well pronounced

7. While at sub auroral region, the max H intensity is not well pronounced between Winter, Spring and Summer but there is high value of maximum occurrence in Autumn, while in auroral region Autumn and Spring shows high occurrences • seasonal variation of geomagnetic activity has been attributed to IMF- effect this agrees with general increase of geomagnetic activities at the equinoxes (Russell and Mc Pherron, 1973; Silverman, 1986; Hakkinen et al., 2003). As a result, more storms occur during Autumnal and Spring equinoxes than Winter and Summer. This implies that there must be a redistribution of the responsible ionospheric currents. • Bolduc et al (2000) concluded that the increasing part of the disturbance has greater time derivatives of horizontal magnetic field; maximum electric field is in opposite direction to the electrojet; and variation in ground observation of dH/dt may be due to intensity, location and orientation of the auroral electrojet.

8. GIS data analysis Figure 2. Monthly variation of max dH/dt from 1990- 2007 and 1992 -2007 (upper panel) and monthly variation of max H from 1990- 2007 and 1992 -2007 (lower panel).

9. Figure 3. Seasonal variation of max dH/dt from 1990- 2007 and 1992 -2007 (upper panel) and seasonal variation of max H from 1990- 2007 and 1992 -2007 (lower panel).

10. Figure 4. Hourly variation of number of time steps between 1990 – 2007 and 1992 -2007 when dH/dt exceeded 1nT/s at two IMAGE magnetometer stations. Soroya is located at auroral region and Nurmijarvi in subauroral region.

11. We also understudied the diurnal distribution for the hours with max dH/dt as shown in Figure 4, an interesting result was observed at auroral region (Soroya), with maximum occurrence at the early morning with another occurrence at the midnight, while in the pre midday the events disappear and also the events almost disappear before the midday at subauroral region (Nurmijarvi). This implies that night time is more significant than the day time. • Newell et al (1996), also relates the frequency occurrence of electron acceleration in dark and sunlit conditions indicated by the solar zenith. It was observed that electron acceleration is common in darkness indicating that darkness is a factor for the field aligned potential drop to be produced. • Lyatsky et al (2001) concluded that geomagnetic activity is maximum when the nightside auroral zones of both hemispheres are in darkness, which occur in Autumn and Spring.

12. Regression analysis of max dH/dt, max H and max AE Soroya Observatory Nurmijarvi Observatory

13. In Soroya observatory (auroral region), there is high correlation of 0.886 value between max dH/dt and max AE with a coefficient of determination (r2) of 0.7858 which implies that 78.58% of max dH/dt can be accounted for using AE index. While a correlation coefficient is 0.955 was obtained between maximum horizontal magnetic field intensity (H) and AE index, with coefficient of determination (r2) 0.9129 implying that 91.29% of max H can be accounted for using AE index. • In Nurmijarvi observatory (sub auroral region), the correlation coefficient of 0.854 exists between max dH/dt and max AE indices with coefficient of determination (r2) 0.7287 implies that 72.87% of max dH/dt can be accounted for using max AE index. We also estimate the geomagnetic activity using max H and max AE index, with high correlation coefficient of 0.8959 with coefficient of determination (r2) 0.8027 which implies that 80.27% of max H can be accounted for using AE index.

14. Figure 5: Linear regression graph

16. Conclusion • The greatest occurrence of geomagnetic activity was observed during the Autumnal season. • The early morning and midnight are more geomagnetically disturbed than the daytime. Lyatsky et al (2001) established the fact that geomagnetic activity is maximum when the nightside auroral zones of both hemispheres are in darkness which occur in Autumn and Spring. • There is a good correlation between the pairs of (max dH/dt and AE index) and (max H and AE index). • From our results, 72.88% and 80.09% of geomagnetically induced current can be predicted using max dH/dt and AE index at Nurmijarvi observatory. Also 78.58% and 91.29% of geomagnetic activity can be predicted using max H and AE index at Soroya observatory. • However, ionospheric dynamo is considered as one of the key parameters in the generation of ionospheric current and fields. When the ionospheric dynamo is disturbed, it produced ionospheric electric current and electric field at the high latitude ionosphere during geomagnetic storm. Ionospheric current subsequently produced variation in the geomagnetic field at the Earth surfaces as well as geomagnetically induced current in the ground. • From our finding, we have high GIC values when auroral electrojet were formed by coupling of the solar wind with magnetosphere when the Bz turned southward and solar wind velocity increased.