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:
On the high frequency ingredients of the secular variation C. Demetrescu, V. Dobrica, I. Vaduva Institute of Geodynamics, Bucharest, Romania e-mail: email@example.com 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)
Background from http://www.geomag.bgs.ac.uk/mercator.html 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
~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
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)
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)
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
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
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.
Comparison of H geomagnetic component - annual means, IGRF and CM4 models -
11-year variation22-year variation~80-year variation Comparison of ingredients of H
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
Dipole moment, IGRF The SC-related variation is even seen in the dipole part of the modelled main field!
Maps of the 22- and ~80-year variations 19401965
Time derivative of the steady radial field at Earth’s surface 1940, IGRF 1965, IGRF WMM 2005 westward displacement of main secular variation foci
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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