Comparative Cluster/Double Star observations of the high and low latitude dayside magnetopause

The launch of the Double Star mission has provided the opportunity to monitor events at distinct locations on the dayside magnetopause, in coordination with the quartet of Cluster spacecraft. We present results of two such coordinated studies. In the first, 6 April 2004, both Cluster and the Double Star TC-1 spacecraft were on outbound transits through the dawn-side magnetosphere. Cluster observed northward moving FTEs with +/- polarity, whereas TC-1 saw -/+ polarity FTEs. The strength, motion and occurrence of the FTE signatures changes somewhat according to changes in IMF clock angle. These observations are consistent with ongoing reconnection on the dayside magnetopause, resulting in a series of flux transfer events (FTEs) seen both at Cluster and TC-1. The observed polarity and motion of each FTE signature advocates the existence of an active reconnection region consistently located between the positions of Cluster and TC-1, lying north and south of the reconnection line, respectively. This scenario is supported by the application of a model, designed to track flux tube motion, to conditions appropriate for the prevailing interplanetary conditions. The results from the model confirm the observational evidence that the low-latitude FTE dynamics is sensitive to changes in convected upstream conditions. In particular, changing the interplanetary magnetic field (IMF) clock angle in the model predicts that TC-1 should miss the resulting FTEs more often than Cluster, as is observed. For the second conjunction, on the 4 Jan 2005, the Cluster and TC-1 spacecraft all exited the dusk-side magnetosphere almost simultaneously, with TC-1 lying almost equatorial and Cluster at northern latitudes at about 4 RE from TC-1. The spacecraft traverse the magnetopause during a strong reversal in the IMF from northward to southward and a number of magnetosheath FTE signatures are subsequently observed. One coordinated FTE, studied in detail by Pu et al, [this issue], carries and inflowing energetic electron population and shows a motion and orientation which is similar at all spacecraft and consistent with the predictions of the model for the flux tube dynamics, given a near sub-solar reconnection line. This event can be interpreted either as the passage of two parallel flux tubes arising from adjacent x-line positions, or as a crossing of a single flux tube at different positions.

Coordinated Cluster, ground-based instrumentation and low-altitude satellite observations of transient poleward-moving events in the ionosphere and in the tail lobe

During the interval between 8:00-9:30 on 14 January 2001, the four Cluster spacecraft were moving from the central magnetospheric lobe, through the dusk sector mantle, on their way towards intersecting the magnetopause near 15:00 MLT and 15:00 UT. Throughout this interval, the EIS-CAT Svalbard Radar (ESR) at Longyearbyen observed a series of poleward-moving transient events of enhanced F-region plasma concentration ("polar cap patches"), with a repetition period of the order of 10 min. Allowing for the estimated solar wind propagation delay of 75 ( 5) min, the interplanetary magnetic field (IMF) had a southward component during most of the interval. The magnetic footprint of the Cluster spacecraft, mapped to the ionosphere using the Tsyganenko T96 model (with input conditions prevailing during this event), was to the east of the ESR beams. Around 09:05 UT, the DMSP-F12 satellite flew over the ESR and showed a sawtooth cusp ion dispersion signature that also extended into the electrons on the equatorward edge of the cusp, revealing a pulsed magnetopause reconnection. The consequent enhanced ionospheric flow events were imaged by the SuperDARN HF backscatter radars. The average convection patterns (derived using the AMIE technique on data from the magnetometers, the EISCAT and SuperDARN radars, and the DMSP satellites) show that the associated poleward-moving events also convected over the predicted footprint of the Cluster spacecraft. Cluster observed enhancements in the fluxes of both electrons and ions. These events were found to be essentially identical at all four spacecraft, indicating that they had a much larger spatial scale than the satellite separation of the order of 600 km. Some of the events show a correspondence between the lowest energy magnetosheath electrons detected by the PEACE instrument on Cluster (10-20 eV) and the topside ionospheric enhancements seen by the ESR (at 400-700 km). We suggest that a potential barrier at the magnetopause, which prevents the lowest energy electrons from entering the magnetosphere, is reduced when and where the boundary-normal magnetic field is enhanced and that the observed polar cap patches are produced by the consequent enhanced precipitation of the lowest energy electrons, making them and the low energy electron precipitation fossil remnants of the magnetopause reconnection rate pulses.

The low-latitude boundary layer: Application of ISTP advances to past data

The destruction of the four Cluster craft was a major loss to the planned ISTP effort, of which studies of the magnetopause and low-latitude boundary layer (LLBL) were an important part. While awaiting the re-flight mission, Cluster-II, we have been applying advances in our understanding made using other ISTP craft (like Polar and Wind) and using ground-based facilities (in particular the EISCAT incoherent scatter radars and the SuperDARN HF coherent radars) to measurements of the LLBL made in 1984 and 1985 by the AMPTE-UKS and -IRM spacecraft pair. In particular, one unexplained result of the AMPTE mission was that the electron characteristics could, in nearly all cases, order independent measurements near the magnetopause, such as the magnetic field, ion temperatures and the plasma flow. Studies of the cusp have shown that the precipitation is ordered by the time-elapsed since the field line was opened by reconnection. This insight has allowed us to reanalyse the AMPTE data and show that the ordering by the transition parameter is also due to the variation of time elapsed since reconnection, with the important implication that reconnection usually coats most of the dayside magnetopause with at least some newly-opened field lines. In addition, we can use the electron characteristics to isolate features like RDs, slow-mode shocks and slow-mode expansion fans. The ion characteristics can be used to compute the reconnection rate. We here retrospectively apply these new techniques, developed in the ISTP era, to a much-studied flux transfer event observed by the AMPTE satellites. As a result, we gain new understanding of its cause and structure.

On the cause of a magnetospheric flux transfer event

We present a detailed investigation of a magnetospheric flux transfer event (FTE) seen by the Active Magnetospheric Tracer Explorer (AMPTE) UKS and IRM satellites around 1046 UT on October 28, 1984. This event has been discussed many times previously in the literature and has been cited as support for a variety of theories of FTE formation. We make use of a model developed to reproduce ion precipitations seen in the cusp ionosphere. The analysis confirms that the FTE is well explained as a brief excursion into an open low-latitude boundary layer (LLBL), as predicted by two theories of magnetospheric FTEs: namely, that they are bulges in the open LLBL due to reconnection rate enhancements or that they are indentations of the magnetopause by magnetosheath pressure increases (but in the presence of ongoing steady reconnection). The indentation of the inner edge of the open LLBL that these two models seek to explain is found to be shallow for this event. The ion model reproduces the continuous evolution of the ion distribution function between the sheath-like population at the event center and the surrounding magnetospheric populations; it also provides an explanation of the high-pressure core of the event as comprising field lines that were reconnected considerably earlier than those that are draped over it to give the event boundary layer. The magnetopause transition parameter is used to isolate a field rotation on the boundaries of the core, which is subjected to the tangential stress balance test. The test identifies this to be a convecting structure, which is neither a rotational discontinuity (RD) nor a contact discontinuity, but could possibly be a slow shock. In addition, evidence for ion reflection off a weak RD on the magnetospheric side of this structure is found. The event structure is consistent in many ways with features predicted for the open LLBL by analytic MHD theories and by MHD and hybrid simulations. The de Hoffman-Teller velocity of the structure is significantly different from that of the magnetosheath flow, indicating that it is not an indentation caused by a high-pressure pulse in the sheath but is consistent with the motion of newly opened field lines (different from the sheath flow because of the magnetic tension force) deduced from the best fit to the ion data. However, we cannot here rule out the possibility that the sheath flow pattern has changed in the long interval between the two satellites observing the FTE and subsequently emerging into the magnetosheath; thus this test is not conclusive in this particular case. Analysis of the fitted elapsed time since reconnection shows that the core of the event was reconnected in one pulse and the event boundary layer was reconnected in a subsequent pulse. Between these two pulses is a period of very low (but nonzero) reconnection rate, which lasts about 14 mins. Thus the analysis supports, but does not definitively verify, the concept that the FTE is a partial passage into an open LLBL caused by a traveling bulge in that layer produced by a pulse in reconnection rate.

The statistical cusp: a flux transfer event model

Traditionally, the cusp has been described in terms of a time-stationary feature of the magnetosphere which allows access of magnetosheath-like plasma to low altitudes. Statistical surveys of data from low-altitude spacecraft have shown the average characteristics and position of the cusp. Recently, however, it has been suggested that the ionospheric footprint of flux transfer events (FTEs) may be identified as variations of the “cusp” on timescales of a few minutes. In this model, the cusp can vary in form between a steady-state feature in one limit and a series of discrete ionospheric FTE signatures in the other limit. If this time-dependent cusp scenario is correct, then the signatures of the transient reconnection events must be able, on average, to reproduce the statistical cusp occurrence previously determined from the satellite observations. In this paper, we predict the precipitation signatures which are associated with transient magnetopause reconnection, following recent observations of the dependence of dayside ionospheric convection on the orientation of the IMF. We then employ a simple model of the longitudinal motion of FTE signatures to show how such events can easily reproduce the local time distribution of cusp occurrence probabilities, as observed by low-altitude satellites. This is true even in the limit where the cusp is a series of discrete events. Furthermore, we investigate the existence of double cusp patches predicted by the simple model and show how these events may be identified in the data.

Low-altitude signatures of the cusp and flux transfer events

The usual interpretation of a flux transfer event (FTE) at the magnetopause, in terms of time-dependent and possibly patchy reconnection, demands that it generate an ionospheric signature. Recent ground-based observations have revealed that auroral transients in the cusp/cleft region have all the characteristics required of FTE effects. However, signatures in the major available dataset, namely that from low-altitude polar-orbiting satellites, have not yet been identified. In this paper, we consider a cusp pass of the DE-2 spacecraft during strongly southward IMF. The particle detectors show magnetosheath ion injection signatures. However, the satellite motion and convection are opposed, and we discuss how the observed falling energy dispersion of the precipitating ions can have arisen from a static, moving or growing source. The spatial scale of the source is typical of an FTE. A simple model of the ionospheric signature of an FTE reproduces the observed electric and magnetic field perturbations. Precipitating electrons of peak energy ∼100eV are found to lie on the predicted boundary of the newly-opened tube, very similar to those found on the edges of FTEs at the magnetopause. The injected ions are within this boundary and their dispersion is consistent with its growth as reconnection proceeds. The reconnection potential and the potential of the induced ionospheric motion are found to be the same (≃25kV). The scanning imager on DE-1 shows a localised transient auroral feature around DE-2 at this time, similar to the recent optical/radar observations of FTEs.

Low-energy ion outflows from the ionosphere during a major polar cap expansion — evidence for equatorward motion of inverted-V structures

Data from the Dynamics Explorer 1 satellite and the EISCAT and Sondrestrom incoherent scatter radars, have allowed a study of low-energy ion outflows from the ionosphere into the magnetosphere during a rapid expansion of the polar cap. From the combined radar data, a 200kV increase in cross-cap potential is estimated. The upflowing ions show “X” signatures in the pitch angle-time spectrograms in the expanding midnight sector of the auroral oval. These signatures reveal low-energy (below about 60eV), light-ion beams sandwiched between two regions of ion conics and are associated with inverted-V electron precipitation. The lack of mass dispersion of the poleward edge of the event, despite great differences in the times of flight, reflects the equatorward expansion of the acceleration regions at velocities similar to those of the antisunward convection. In addition, a transient burst of upflow of 0+ is observed within the cap, possibly due to enhanced Joule heating during the event.

Thermal ion flows in the topside auroral ionosphere and the effects of low-altitude, transverse acceleration

Topside ionospheric profiles are used to study the upward field-aligned flow of thermal O+ at high latitudes. On the majority of the field lines outside the plasmasphere, the mean flux is approximately equal to the mean polar wind measured by spacecraft at greater altitudes. This is consistent with the theory of thermal light ion escape supported, via charge exchange, by upward O+ flow at lower heights. Events of larger O+ flow are detected at auroral latitudes and their occurrence is found to agree with that of transversely accelerated ions within the topside ionosphere and the magnetosphere. The effects of low altitude heating of O+ by oxygen cyclotron waves, driven by downward field-aligned currents, are considered as a possible common cause of these two types of event.

Rapid changes in the redox conditions of the western Tethys Ocean during the early Aptian oceanic anoxic event

The early Aptian (125 to 121 Ma) records an episode of severe environmental change including a major perturbation of the carbon cycle, an oceanic anoxic event (OAE 1a, 122.5 Ma), a platform drowning episode and a biocalcification crisis. We propose to trace changes in the oxygenation state of the ocean during the early Aptian anoxic event using the redox-sensitive trace-element (RSTE) distribution, phosphorus accumulation rates (PARs) and organic-matter characterization in three different basins of the western Tethys. The following sections have been investigated: Gorgo a Cerbara (central Italy) in the Umbria Marche basin, Glaise (SE France) in the Vocontian basin and Cassis/La Bédoule (SE France) located in the Provencal basin. In the Gorgo a Cerbara section, RSTE distributions show a low background level along the main part of the section, contrasted by different maxima in concentrations within the Selli level. In the Glaise section, the Goguel level displays a weak increase in RSTE contents coeval with moderate TOC values. At Cassis/La Bédoule, no significant RSTE enrichments have been observed in sediments equivalent to the Selli level. These differences in the records of the geochemical proxies of the Selli level or its equivalent indicate the deposition under different redox conditions, probably related to the paleogeography. Our data indicate the development of anoxic–euxinic conditions in the deeper part of the Tethys during OAE 1a, whereas in the shallower environments, conditions were less reducing. Moreover, at Gorgo a Cerbara, the Selli level is characterized by rapid changes in the intensity of reducing conditions in the water column. Ocean eutrophication seems to be a major factor in the development and the persistence of anoxia as suggested by the PAR evolution. Higher PAR values at the onset of OAE 1a suggest an increase in nutrient input, whereas the return to lower values through the first part of the OAE 1a interval may be related to the weakened capacity to retain P in the sedimentary reservoir due to bottom-water oxygen depletion. This general pattern is contrasted by the data of Gorgo a Cerbara, where the sediments deposited during the OAE 1a interval show P-enrichments (mainly authigenic P). This is associated with maxima in TOC values and Corg:Ptot ratios, suggesting that a part of the remobilized P was trapped in the sediments and as such prevented from returning to the water column.

Low-cost HMD and EEG: using visual stimulation by the Oculus Rift® to detect stronger ERDs induced by motion imagery

Virtual Reality (VR) can provide visual stimuli for EEG studies that can be altered in real time and can produce effects that are difficult or impossible to reproduce in a non-virtual experimental platform. As part of this experiment the Oculus Rift, a commercial-grade, low-cost, Head Mounted Display (HMD) was assessed as a visual stimuli platform for experiments recording EEG. Following, the device was used to investigate the effect of congruent visual stimuli on Event Related Desynchronisation (ERD) due to motion imagery.

The impact of the abrupt 8.2 ka cold event on the Mesolithic population of western Scotland: a Bayesian chronological analysis using ‘activity events’ as a population proxy

The potential impact of the abrupt 8.2 ka cold event on human demography, settlement patterns and culture in Europe and the Near East has emerged as a key theme in current discussion and debate. We test whether this event had an impact on the Mesolithic population of western Scotland, a case study located within the North Atlantic region where the environmental impact of the 8.2 ka event is likely to have been the most severe. By undertaking a Bayesian analysis of the radiocarbon record and using the number of activity events as a proxy for the size of the human population, we find evidence for a dramatic reduction in the Mesolithic population synchronous with the 8.2 ka event. We interpret this as reflecting the demographic collapse of a low density population that lacked the capability to adapt to the rapid onset of new environmental conditions. This impact of the 8.2 ka event in the North Atlantic region lends credence to the possibility of a similar impact on populations in Continental Europe and the Near East.

The Cariris Velhos tectonic event in Northeast Brazil

The Borborema Province in northeastern South America is a typical Brasiliano-Pan-African branching system of Neoproterozoic orogens that forms part of the Western Gondwana assembly. The province is positioned between the Sao Luis-West Africa craton to the north and the Sao Francisco (Congo-Kasai) craton to the south. For this province the main characteristics are (a) its subdivision into five major tectonic domains, bounded mostly by long shear zones, as follows: Medio Coreau, Ceara Central, Rio Grande do Norte, Transversal, and Southern; (b) the alternation of supracrustal belts with reworked basement inliers (Archean nuclei + Paleoproterozoic belts); and (c) the diversity of granitic plutonism, from Neoproterozoic to Early Cambrian ages, that affect supracrustal rocks as well as basement inliers. Recently, orogenic rock assemblages of early Tonian (1000-920 Ma) orogenic evolution have been recognized, which are restricted to the Transversal and Southern domains of the Province. Within the Transversal Zone, the Alto Pajeu terrane locally includes some remnants of oceanic crust along with island arc and continental arc rock assemblages, but the dominant supracrustal rocks are mature and immature pelitic metasedimentary and metavolcaniclastic rocks. Contiguous and parallel to the Alto Pajeu terrane, the Riacho Gravata subterrane consists mainly of low-grade metamorphic successions of metarhythmites, some of which are clearly turbiditic in origin, metaconglomerates, and sporadic marbles, along with interbedded metarhyolitic and metadacitic volcanic or metavolcaniclastic rocks. Both terrane and subterrane are cut by syn-contractional intrusive sheets of dominantly peraluminous high-K calc-alkaline, granititic to granodioritic metaplutonic rocks. The geochemical patterns of both supracrustal and intrusive rocks show similarities with associations of mature continental arc volcano-sedimentary sequences, but some subordinate intra-plate characteristics are also found. In both the Alto Pajeu and Riacho Gravata terranes, TIMS and SHRIMP U-Pb isotopic data from zircons from both metavolcanic and metaplutonic rocks yield ages between 1.0 and 0.92 Ga, which define the time span for an event of orogenic character, the Cariris Velhos event. Less extensive occurrences of rocks of Cariris Velhos age are recognized mainly in the southernmost domains of the Province, as for example in the Polo Redondo-Maranco terrane, where arc-affinity migmatite-granitic and meta-volcano-sedimentary rocks show U-Pb ages (SHRIMP data) around 0.98-0.97 Ga. For all these domains, Sm-Nd data exhibit Tom model ages between 1.9 and 1.1 Ga with corresponding slightly negative to slightly positive epsilon(Nd)(t) values. These domains, along with the Borborema Province as a whole, were significantly affected by tectonic and magmatic events of the Brasiliano Cycle (0.7-0.5 Ga), so that it is possible that there are some other early Tonian rock assemblages which were completely masked and hidden by these later Brasiliano events. Cariris Velhos processes are younger than the majority of orogenic systems at the end of Mesoproterozoic Era and beginning of Neoproterozoic throughout the world, e.g. Irumide belt, Kibaride belt and Namaqua-Natal belt, and considerably younger than those of the youngest orogenic process (Ottawan) in the Grenvillian System. Therefore, they were probably not associated with the proposed assembly of Rodinia. We suggest, instead, that Cariris Velhos magmatism and tectonism could have been related to a continental margin magmatic arc, with possible back-arc associations, and that this margin may have been a short-lived (<100 m.y.) leading edge of the newly assembled Rodinia supercontinent. (C) 2009 Elsevier Ltd. All rights reserved.

Post-collisional lamprophyric event in Sierra Norte, Cordoba, Argentina: Mineralogical, geochemical and isotopic characteristics

Over 20 lamprophyre dykes, varying in width between a few centimeters and several meters, have been identified in central Sierra Norte - Eastern Pampean Ranges, Cordoba, Argentina. Their mineralogy and chemistry indicate that they are part of the calc-alkaline lamprophyres clan (CAL). They contain phenocrysts of magnesiohomblende +/- augite set in a groundmass of magnesiohornblende, calcic-plagioclase, alkali feldspar, and opaque minerals, which designate them as spessartite-type lamprophyres. Alteration products include chlorite, calcite and iron oxides after malfic phenocrysts, though some are partially replaced by actinolite. Feldspars are replaced by carbonate and clay minerals. The dykes are relatively primitive, and show restricted major element variation (SiO(2) 51.1-55.3 wt.%, Al(2)O(3) 12-16.6 wt.%, total alkalies 1.5-4.7 wt.%), high Mg# (55-77), high Cr contents (27-988 ppm) and moderate to high Ni contents (60-190 ppm). Lamprophyre LILE (e.g. Rb averages 110 ppm, Sr 211-387 ppm, Ba 203-452 ppm) are high relative to HFSE (e.g., Ta 0.2-1.6 ppm, Nb 4-11 ppm, Y 17-21 ppm), and are enriched in LREE (30-70 times chondrite). They are characterized by relatively high (208)Pb/(204)Pb (38.8-39.9), (207)Pb/(204)Pb(similar to 15.7), and (206)Pb/(204)Pb (18.7-20.1), combined with low (epsilon)epsilon(Nd) (-4.69 to -1.52) and a relative moderately high ((87)Sr/(86)Sr)(i) of 0.7055-0.7074. The Rb-Sr whole rock isochron indicates an Early Ordovician age of 485 +/- 25 Ma. The calculated T(DM) (1.7 Ga) suggests that these rocks appear to have originated from a reservoir that was created during a mantle metasomatism event related to the Pampean orogeny. The Sierra Norte lamprophyres show affinities with a subduction-related magma in an active continental margin. Their geochemical and isotopic features suggest a multicomponent source, composed of enriched mantle material variably contaminated by crustal components. The lamprophyric suite emplacement occurred at the dawning stage of the Pampean orogeny, in a regional post-collisional extensional setting developed in the Sierra Norte-Ambargasta batholith (SNAB) in Early Ordovician times. (C) 2008 Published by Elsevier Ltd.

Timing and structure of the 8.2 kyr BP event inferred from delta(18)O records of stalagmites from China, Oman, and Brazil

Oxygen isotope records of stalagmites from China and Oman reveal a weak summer monsoon event, with a double-plunging structure, that started 8.21 +/- 0.02 kyr B. P. An identical but antiphased pattern is also evident in two stalagmite records from eastern Brazil, indicating that the South American Summer Monsoon was intensified during the 8.2 kyr B. P. event. These records demonstrate that the event was of global extent and synchronous within dating errors of <50 years. In comparison with recent model simulations, it is plausible that the 8.2 kyr B. P. event can be tied in changes of the Atlantic Meridional Overturning Circulation triggered by a glacial lake draining event. This, in turn, affected North Atlantic climate and latitudinal position of the Intertropical Convergence Zone, resulting in the observed low-latitude monsoonal precipitation patterns.

Highly Sensitive Fluorescent Method for the Detection of Cholesterol Aldehydes Formed by Ozone and Singlet Molecular Oxygen

Cholesterol oxidation gives rise to a mixture of oxidized products. Different types of products are generated according to the reactive species being involved. Recently, attention has been focused on two cholesterol aldehydes, 3 beta-hydroxy-5 beta-hydroxy-B-norcholestane-6 beta-carboxyaldehyde (1a) and 3 beta-hydroxy-5-oxo-5,6-secocholestan-6-al (1b). These aldehydes can be generated by ozone-, as well as by singlet molecular oxygen-mediated cholesterol oxidation. It has been suggested that 1b is preferentially formed by ozone and la is preferentially formed by singlet molecular oxygen. In this study we describe the use of 1-pyrenebutyric hydrazine (PBH) as a fluorescent probe for the detection of cholesterol aldehydes. The formation of the fluorescent adduct between la with PBH was confirmed by HPLC-MS/MS. The fluorescence spectra of PBH did not change upon binding to the aldehyde. Moreover, the derivatization was also effective in the absence of an acidified medium, which is critical to avoid the formation of cholesterol aldehydes through Hock cleavage of 5 alpha-hydroperoxycholesterol. In conclusion, PBH can be used as an efficient fluorescent probe for the detection/quantification of cholesterol aldehydes in biological samples. Its analysis by HPLC coupled to a fluorescent detector provides a sensitive and specific way to quantify cholesterol aldehydes in the low femtomol range.