Spin-density induced by electromagnetic wave in two-dimensional electron gas with both Rashba and Dresselhaus spin-orbit couplings

We consider the magnetic response of a two-dimensional electron gas (2DEG) with both Rashba and Dresselhaus spin-orbit coupling to a microwave excitation. We generalize the results of [A. Shnirman and I. Martin, Europhys. Lett. 78, 27001 (2007).], where pure Rashba coupling was studied. We observe that the microwave with the in-plane electric field and the out-of-plane magnetic field creates an out-of-plane spin polarization. The effect is more prominent in clean systems with resolved spin-orbit-split subbands. Considered as response to the microwave magnetic field, the spin-orbit contribution to the magnetization far exceeds the usual Zeeman contribution in the clean limit. The effect vanishes when the Rashba and the Dresselhaus couplings have equal strength.Comment: 4 pages, 2 figure

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

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