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On the high frequency ingredients of the secular variation C. Demetrescu, V. Dobrica, I. Vaduva Institute of Geodynamics, Bucharest, Romania

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Presentation on theme: "On the high frequency ingredients of the secular variation C. Demetrescu, V. Dobrica, I. Vaduva Institute of Geodynamics, Bucharest, Romania"— Presentation transcript:

1 On the high frequency ingredients of the secular variation C. Demetrescu, V. Dobrica, I. Vaduva Institute of Geodynamics, Bucharest, Romania e-mail: Acknowledgements: The study has been supported by the Institute of Geodynamics (Projects 2/2003-2007) and by the National Authority for Scientific Research (Projects 151/2007, 81- 021/2007)

2 Background from Outline - have a look at time series of annual means from 22 observatories (15 with 100-150 years of activity) - discuss the ingredients of the main field and of its secular variation - discuss the time-space evolution of the various main field ingredients - comparison with main field models (IGRF, CM4) - conclusions

3 ~80-year variation visible in the time series of annual means of geomagnetic elements - the solar cycle signature has long been recognized (Chapman & Bartels, 1940; Alldredge, JGR, 1976; Courtillot&LeMouel, JGR, 1976) - if not accounted for, the external contributions map into the modelled internal field when using observatory data - correction limited to the 11-year solar cycle effects (Sabaka et al., GJI, 2004; Olsen & Mandea, EPSL, 2007, Verbanac et al., EPS, 2007 ) by parameterization with Dst (Est+Ist) and Ap + External contributions Observatory data

4 Geomagnetic indices to quantify long-term external field variations Geomagnetic activity - aa (Mayaud, 1972; 1980) BV 2 - IHV (Svalgaard et al., 2004; Svalgaard&Cliver, 2007; Mursula et al., 2004) BV 2 - IDV (Svalgaard&Cliver, 2004) B - Dst for storm-time variations Good correlation with the solar activity (R) in terms of 11-year averages, but differences in each solar cycle Solar quiet daily variation Sq - x, y, z, r (Le Mouel et al., EPSL, 2005) TSI A common trend is present also at longer timescales: - 22-year magnetic cycle (MC) - “secular”, “80-90-year”, Gleissberg cycle (GC)

5 Successive extraction of the 11-, 22- and ~80-year variations (running averages) (Demetrescu&Dobrica, JGR, 2008) Curves are reduced to their means over the common time interval and scaled with their standard deviations about the mean as a unit (Demetrescu&Dobrica, JGR, 2008)

6 Ingredients of the observed field Example of data treatment Steady variation The 11- (red), 22- (blue) and ~80-year (green) variations extracted from data. Successively filtering out the sunspot cycle signature, a 22-year variation, and a ~80-year variation present in the time series of observatory annual means, by running averages with 11-, 22- and 78-year windows respectively, results in a so called “steady variation”. (Demetrescu&Dobrica, RRG, 2005) HAD

7 11-year variation~80-year variation22-year variation

8 Steady variation - carries the largest part of the field - in terms of variation relative to the mean for the time interval with data - large lateral differences in the time variation

9 Jerk ingredients - the long-term variation of the geomagnetic field, dominated by the presence of geomagnetic jerks separating intervals of a relative smooth variation of the field, - is produced by the ~80-year variation combined with the 22-year variation, both of internal origin, on which the SC-related variation is superimposed. - the 11-year variation is decisive though in establishing the very short time scale characterizing jerks, and to some extent also the amplitude and timing of the jerk.

10 Comparison of H geomagnetic component - annual means, IGRF and CM4 models -

11 11-year variation22-year variation~80-year variation Comparison of ingredients of H

12 IGRF: - no provision for external variations; the SC- related variation maps in the modelled main field CM4: - the external field is accounted for via the Dst index 11-year SC-related variation in main field models Unaccounted for, this variation leaks into the main field models IGRF: - sampling the field every 5 years results in a distorted SC variation CM4: - partially successful: CLF, NGK – yes; KAK, HER - no

13 Dipole moment, IGRF The SC-related variation is even seen in the dipole part of the modelled main field!

14 Maps of the 22- and ~80-year variations 19401965

15 ~80-year variation maps, IGRF 1940 1945 1955 1950 1960 1965

16 Time derivative of the steady radial field at Earth’s surface 1940, IGRF 1965, IGRF WMM 2005 westward displacement of main secular variation foci

17 Conclusions - data (annual means) from 22 observatories with long activity (15 with 100-150 years) have been processed to show the existence and some of the characteristics of a 22-year variation and of a ~80- year variation, superimposed on a so-called steady variation; - in terms of the present analysis, the jerks seem to be merely a result of the superposition of the 11- year solar-cycle-related on the 22-year and the ~80-year variations. The way the three combine makes the difference in timing, magnitude, and length of jerks as observed; - the steady variation, which carries the largest part of the field is characterized by a westward movement of its secular variation foci; - the ~80-year variation shows several maxima and minima with variable amplitudes, concentrated in two longitudinal bands; - the 22-year variation shows a high temporal and spatial variability; - the external variation still present in data (annual means uncorrected or incompletely corrected for the 11-year SC-related variation) leaks into the main field models; - in terms of secular variation, all ingredients presented contribute significantly;

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