Nomura R.,Nagoya University |
Shiokawa K.,Nagoya University |
Pilipenko V.,Institute of the Physics of the Earth |
Shevtsov B.,Russian Academy of Sciences
Journal of Geophysical Research: Space Physics | Year: 2011
We have investigated Pc1 geomagnetic pulsations observed by induction magnetometers at three low-latitude stations (Paratunka (PTK), Moshiri (MSR), and Sata (STA), L=1.2-2.1). A detailed polarization analysis shows that polarization parameters (angle of polarization ellipse orientation, ψ, and polarization sense, ε) of individual Pc1 bands depend on frequency at all three stations. The dependence of ψ on frequency was seen in ∼70% of the 93 Pc1 events observed at MSR (L=1.5) from 14 July 2007 to 13 July 2009. The maxima of seasonal and diurnal variations of the occurrence rate were in winter and during the nighttime, as reported previously, indicating that the transmission of observed Pc1 pulsations to lower latitudes is controlled by the density of the F layer plasma. These facts suggest that spatially distributed Pc1 waves with varying frequencies depending on longitude or latitude at high latitudes propagated in the ionospheric duct to cause the frequency dependence of polarization parameters at low latitudes. We also suggest that the Pc1 pearl structure with a repetition period of ∼5-30 s observed at low latitudes is a beat of high-latitude waves with slightly different frequencies. Copyright 2011 by the American Geophysical Union.
Francia P.,University of LAquila |
Regi M.,University of LAquila |
De Lauretis M.,University of LAquila |
Villante U.,University of LAquila |
Pilipenko V.A.,Institute of the Physics of the Earth
Journal of Geophysical Research: Space Physics | Year: 2012
In this study we analyzed a long-duration ULF wave event detected on 18-19 February 2005 by Cluster satellites, upstream of the nose of the bow shock. The availability of simultaneous data from Geotail satellite, located in the foreshock region close to the dawn flank of the bow shock, allowed us to make a comparison between the observations at the two different sites. The results can be explained in terms of local wave generation, depending on the orientation of the interplanetary magnetic field with respect to the local bow shock normal. In addition, simultaneous data from Polar satellite in the inner magnetosphere and from ground stations in the southern polar cap and at low latitude allowed us to investigate the transmission of the external waves through the magnetosphere up to the ground. The observations suggest different paths of transmission. Waves generated upstream of the bow shock nose directly transmit near the subsolar point, progressively propagate into the magnetosphere and, after conversion into field-guided Alfven modes, reach the ground at high and low latitudes; waves generated on the flanks of the bow shock do not affect the subsolar magnetosphere, and consequently, there is no propagation along the closed field lines at both high and low latitudes. On the other hand, near the geomagnetic pole, the occurrence of pulsations can be related to the transmission across the magnetopause flanks of upstream waves, anywhere generated, as they are convected downstream by the solar wind; the compressional waves do not propagate deeply into the tail lobes but can couple to Alfven-guided waves along the outermost field lines.
Simms L.E.,Augsburg College |
Pilipenko V.A.,Institute of the Physics of the Earth |
Engebretson M.J.,Augsburg College
Journal of Geophysical Research: Space Physics | Year: 2010
Long-period ground ULF waves may be controlled by the mean values of solar wind and interplanetary magnetic field (IMF) parameters (velocity, density, and North-South IMF component Bz). We investigated the influence of these parameters on ground ULF power in the Pc5 range (2-7 mHz) during periods of quiet and during coronal mass ejection (CME) and corotating interaction region (CIR) storms from 1991 to 2004. With multiple regression and path analysis, we studied the influence of these hourly parameters as a set rather than individually. This allowed us to determine which factors were most influential and which were only correlated with influential factors. By using multiple regression, we have explained more variation in Pc5 power than has been achieved in previous studies. In both storm types (CME and CIR) and during all storm phases (initial, main phase, recovery, and a 48 h period after recovery) as well as during quiet periods, solar wind velocity and IMF Bz influenced ground Pc5 power directly. These two variables also acted on the ULF Pc5 indirectly through the intermediate parameters of Dst, and the variations in number density and IMF, although at a weaker level. Ground Pc5 power was greater during CME storms during the main phase and recovery but larger during CIR storms in the period after recovery. In addition, the effect of certain independent variables differed depending on storm type. A model such as this offers the possibility of nowcasting Pc5 power by inserting current levels of solar wind and IMF variables as predictors into the regression equation. Copyright 2010 by the American Geophysical Union.
Bindeman I.N.,University of Oregon |
Simakin A.G.,Institute of the Physics of the Earth
Geosphere | Year: 2014
Recent discoveries of isotopically diverse minerals, i.e., zircons, quartz, and feldspars, in large-volume ignimbrites and smaller lavas from the Snake River Plain (SRP; Idaho, USA), Iceland, Kamchatka Peninsula, and other environments suggest that this phenomenon characterizes many silicic unitsstudied by in situ methods. This observation leads to the need for new models of silicic magma petrogenesis that involve double or triple recycling of zircon-saturated rocks. Initial partial melts are produced in small quantities in which zircons and other minerals undergo solution reprecipitation and inherit isotopic signatures of the immediate environment of the host magma batch. Next, isotopically diverse polythermal magma batches with inherited crystals merge together into larger volume magma bodies, where they mix and then erupt. Concave-up and polymodal crystal size distributions of zircons and quartz observed in large-volume ignimbrites may be explained by two or three episodes of solution and reprecipitation. Hafnium isotope diversity in zircons demonstrates variable mixing of crustal melts and mantle-derived silicic differentiates. The low δ18O values of magmas with δ18O-diverse zircons indicate that magma generation happens by remelting of variably hydrothermally altered, and thus diverse in δ18O, protoliths from which the host magma batch, minute or voluminous, inherited low-δ18O values. This also indicates that the processes that generate zircon diversity happen at shallow depths of a few kilometers, where meteoric water can circulate at large water/rock ratios to imprint low δ18O values on the protolith. We further review newly emerging isotopic evidence of diverse zircons and their appearance at the end of the magmatic evolution of many longlived large-volume silicic centers in the SRP and elsewhere, evidence indicating that the genesis of rhyolites by recycling their sometimes hydrothermally altered subsolidus predecessors may be a common evolutionary trend for many rhyolites worldwide, especially in hotspot and rift environments with high magma and heat fluxes. Next, we use thermomechanical finite element modeling of rhyolite genesis and to explain (1) the formation of magma batches in stress fields by dike capture or deflection as a function of underpressurization and overpressurization, respectively; (2) the merging of neighboring magma batches together via four related mechanisms: melting through the screen rock and melt zone expansion, brittle failure of a separating screen of rocks, buoyant merging of magmas, and explosive merging by an overpressurized interstitial fluid phase (heated meteoric water); and (3) mixing time scales and their efficacies on extended horizontal scales, as expressed by marker method particle tracking. The en visioned advective thermomechanical mechanisms of magma segregation in the upper crust may characterize periods of increased basaltic output from the mantle, leading to increased silicic melt production, but may also serve as analogues for magma chambers made of dispersed magma batches. Although not the focus of this work, dispersed magma batches may be stable in the long term, but their coalescence creates ephemeral, short-lived eruptable magma bodies that erupt nearly completely. © 2014 Geological Society of America.
Pilipenko V.,Space Research Institute |
Kozyreva O.,Institute of the Physics of the Earth |
Belakhovsky V.,Polar Geophysical Institute |
Engebretson M.J.,Augsburg College |
Samsonov S.,Institute of Cosmophysics and Aeronomy
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2010
The dynamics of intense ultra-low-frequency (ULF) activity during three successive strong magnetic storms during 29-31 October 2003 are considered in detail. The spatial structure of Pc5 waves during the recovery phases of these storms is considered not only from the perspective of possible physical mechanisms, but as an important parameter of the ULF driver of relativistic electrons. The global structure of these disturbances is studied using data from a worldwide array of magnetometers and riometers augmented with data from particle detectors and magnetometers on board magnetospheric satellites (GOES, LANL). The local spatial structure is examined using the IMAGE magnetometers and Finnish riometer array. Though a general similarity between the quasi-periodic magnetic and riometer variations is observed, their local propagation patterns turn out to be different. To interpret the observations, we suggest a hypothesis of coupling between two oscillatory systems- a magnetospheric magnetohydrodynamic (MHD) waveguide/resonator and a system consisting of turbulence+electrons. We propose that the observed Pc5 oscillations are the result of MHD waveguide excitation along the dawn and dusk flanks of the magnetosphere. The magnetospheric waveguide turns out to be in a meta-stable state under high solar wind velocities, and quasi-periodic fluctuations of the solar wind plasma density stimulate the waveguide excitation. © 2010 The Royal Society.