Loss of synchronization in the 27-day spectral component of geomagnetic indices and its relationship with solar activity

International audienceWe investigate the “27-day” spectral component of different global (aa and Dst) and local (ζ) geomagnetic indices in two period ranges relevant to the Sun's synodic rotation, as manifested by magnetic activity: 24-28 days (short) and 28-32 days (long). Cross-correlation analysis of the respective energies of the short and long periods of the 27-day rotation signal in the same geomagnetic index reveals well-defined de-synchronization events during certain solar cycles. The largest de-synchronization in the past century occurred during solar cycle 21. De-synchronization events first occur in indices of the Dst (and ζ) family, and then in indices of the aa family. We found no evidence that the strength of the de-synchronization of the solar rotation signal in the ζ-index would depend on geomagnetic latitude. Applying the same analysis to proper solar indices (sunspot number, F10.7 radio flux, interplanetary magnetic field (IMF) series, solar wind speed), we find that only the Bz component of the IMF demonstrates a de-synchronization, during solar cycle 21, between the energies of the 27-day solar rotation signal in the short and long period ranges. We discuss possible implications of these results with respect to the evolution of the toroidal and poloidal components of the Sun's magnetic field and to its large-scale structures

A correlation of mean period of MJO indices and 11-yr solar variation

International audienceThis paper focuses on the decadal to multi-decadal evolution of the spectral properties of the Madden-Julian Oscillation (MJO). Guided by former studies, we test whether the ∼11-yr (Schwabe) cycle of solar activity could be reflected in the spectral features of MJO indices: namely, we study the evolution of MJO mean period within different period ranges and compare these with the evolution of solar activity. We focus on solar proxies best linked to UV emission and cosmic rays: sunspot number WN, F10.7 flux, core-to-wing ratio MgII, and galactic cosmic rays (GCR). A clear solar signature in MJO spectral properties is indeed found and shown to be both statistically significant and robust. UV proxies are found to be better correlated with MJO mean period than GCR, thus supporting rather the ozone mechanism of solar impact on MJO. The overall correlation with solar activity is found to be stronger in the Indian Ocean. Long periods (e.g. 50-80 day) are better correlated with solar activity than shorter periods (e.g. 30-60 day). A marked change in the relationship between MJO mean period and solar activity takes place in the declining phase of solar cycle 23, adding to its unusual character

The 27-Day Signal in Sunspot Number Series and the Solar Dynamo

International audienceWe analyze the Wolf number daily series WN (1849 to present) as well as two other related series characterizing solar activity. Our analysis consists in computing the amplitude of a given Fourier component in a sliding time window and examining its long-term evolution. We start with the well-known 27.03- and 27.6-day periods and observe strong decadal variations of this amplitude as well as a sharp increase of the average value starting around 1905. We then consider a packet of 31 lines with periods from 25.743 to 28.453 days, which is shown to be a better representation of the synodic solar rotation. We first examine the temporal evolution of individual lines, then the energy of the packet. The energy of the packet increases sharply at the beginning of the 20th century, leading by more than two decades the well-known increase of the Wolf number. The nonaxisymmetry of sunspots increases before the total increase of activity and may be considered as a precursor. We discuss briefly and tentatively this observation in terms of solar dynamo theory

Solar forcing of the semi-annual variation of length-of-day

International audienceWe study the evolution of the amplitude A of the semi-annual variation of the length-of-day (lod) from 1962 to 2009. We show that A is strongly modulated (up to 30%) by the 11-yr cycle monitored by the sunspot number WN. A and WN are anticorrelated, WN leading A by 1-yr. A is therefore directly correlated with galactic cosmic ray intensity. The main part of the semi-annual variation in lod is due to the variation in mean zonal winds. We conclude that variations in mean zonal winds are modulated by the solar activity cycle through variations in irradiance, solar wind or cosmic ray intensity

Slow and Fast Rotating Coronal Holes from Geomagnetic Indices

International audienceThe evolution of the 27-day recurrence in the series of two solar indices (Wolf number WN and 10.7 cm radio flux F) and two geomagnetic indices ( Dst and ζ, variance of the geomagnetic field recorded at a magnetic observatory) have been studied over the 1957 - 2007 time span. Spectral energies contained in two period domains (25 - 27.3 and 27.3 - 31 days), designated as E 1 and E 2, have been computed. Whereas the evolution of E 1 is the same for the four indices, that of E 2 is essentially different for WN and F on the one hand, Dst and ζ on the other hand. Some general conclusions on the dynamics of the solar outer layers are inferred from these results. First the solar activity, as measured by WN, and when averaged over a few years, evolves in the same way whatever the latitude. Second, two families of coronal holes (CHs) are identified; the rapidly and the slowly rotating CHs evolve quite differently

Evidence for solar forcing in variability of temperatures and pressures in Europe

International audienceDaily temperature and pressure series from 55 European meteorological stations covering the 20th century are analyzed. The overall temperature mean displays a sharp minimum near 1940 and a step-like jump near 1987. We evaluate the evolution of disturbances of these series using mean squared inter-annual variations and "lifetimes". The decadal to secular evolutions of solar activity and temperature disturbances display similar signatures over the 20th century. Because of heterogeneity of the climate system response to solar forcing, regional and seasonal approaches are key to successful identification of these signatures. Most of the solar response is governed by the winter months, as best seen near the Atlantic Ocean. Intensities of disturbances vary by factors in excess of 2, underlining a role for the Sun as a significant forcing factor of European atmospheric variations. We speculate about the possible origin of these solar signatures. The last figure of the paper exemplifies its main results

On secular changes of correlation between geomagnetic indices and variations in solar activity

International audienceGeomagnetic indices can be divided in two families, sometimes called “mean” and “range” families, which reflect different interactions between solar and terrestrial processes on time scales ranging from hourly to secular and longer. We are interested here in trying to evaluate secular change in the correlations between these indices and variations in solar activity as indicators of secular changes in solar behavior. We use on one hand daily values of geomagnetic indices Dst and ζ (members of the “mean” family), and Ap and aa (members of the “range” family), and on the other hand solar indices WN (sunspot number), F10.7 (radio flux), interplanetary magnetic field B and solar wind speed v over the period 1955-2005. We calculate correlations between pairs of geomagnetic indices, between pairs of solar indices (including the composite Bv2), and between pairs consisting in a geomagnetic vs a solar index, all averaged over one to eleven years. The relationship between geomagnetic indices depends on the evolution of solar activity; strong losses of correlation occur during the declining phase of solar cycle 20 and in solar cycle 23. We confirm the strong correlation between aa and Bv2 and to a lesser extent between Dst and B. On the other hand, correlations between aa or Dst and v are non-stationary and display strong increases between 1975 and 2000. Some geomagnetic indices can be used as proxies for the behavior of solar wind indices for times when these were not available. We discuss possible physical origins of sub-decadal to secular evolutions of correlations and their relation with the character of solar activity (correlation of DP2 substorms and main storm occurrence, generation of toroidal field of a new cycle during descending phase of old cycle and prediction of next cycle, and also links with coupling of nonlinear oscillators and abrupt regime changes)