Circular Polarization Variability in Extragalactic Sources on Time Scales of Months to Decades

We present previously unpublished circular polarization (cp) measurements at 4.8 and 8.0 GHz made with the University of Michigan 26-meter prime focus telescope during 1978–1984 and results of a new observational program during the past two years. Based on the preliminary analysis of our recent data, eight sources were detected at 4.8 GHz with average degrees of cp ranging from ≤ 0.1% to ≥ 1%.The results are compared with observations at other frequencies and with linear polarization and total flux density variability in the sources. The behavior of the cp variability observed to date is consistent with stochastic variations produced by mode conversion in transient, opaque emitting regions in the sources. The observed sign changes observed between different epochs and different frequencies are not consistent with the hypothesis that sources maintain a fixed handedness of cp.     

Multi-Waveband Emission Maps of Blazars

We are leading a comprehensive multi-waveband monitoring program of 34 γ-ray bright blazars designed to locate the emission regions of blazars from radio to γ-ray frequencies. The ‘maps’ are anchored by sequences of images in both total and polarized intensity obtained with the VLBA at an angular resolution of ～0.1 milliarcseconds. The time-variable linear polarization at radio to optical wavelengths and radio to γ-ray light curves allow us to specify the locations of flares relative to bright stationary features seen in the images and to infer the geometry of the magnetic field in different regions of the jet. Our data reveal that some flares occur simultaneously at different wavebands and others are only seen at some of the frequencies. The flares are often triggered by a superluminal knot passing through the stationary ‘core’ on the VLBA images. Other flares occur upstream or even parsecs downstream of the core.     

Radio Band Observations of Blazar Variability

The properties of blazar variability in the radio band are studied using the unique combination of temporal resolution from single dish monitoring and spatial resolution from VLBA imaging. Such measurements now available in all four Stokes parameters, together with theoretical simulations, identify the origin of radio band variability and probe the characteristics of the radio jet where the broadband blazar emission originates. Outbursts in total flux density and linear polarization in the optical-to-radio bands are attributed to shocks propagating within the jet spine, in part, based on limited modelling invoking transverse shocks; new radiative transfer simulations allowing for shocks at arbitrary angle to the flow direction confirm this picture by reproducing the observed centimeter-band variations observed more generally, and are of current interest since these shocks may play a role in the ??-ray flaring detected by Fermi. Recent UMRAO multifrequency Stokes V studies of bright blazars identify the spectral variability properties of circular polarization for the first time and demonstrate that polarity flips are relatively common. All-Stokes data are consistent with the production of circular polarization by linear-to-circular mode conversion in a region that is at least partially self-absorbed. Detailed analysis of single-epoch, multifrequency, all-Stokes VLBA observations of 3C?279 support this physical picture and are best explained by emission from an electron-proton plasma.     

The East Pacific Rise and its flanks 8–18° N: History of segmentation,propagation and spreading direction based on SeaMARC II and Sea Beam studies

SeaMARC II and Sea Beam bathymetric data are combined to create a chart of the East Pacific Rise (EPR) from 8°N to 18°N reaching at least 1 Ma onto the rise flanks in most places. Based on these data as well as SeaMARC II side scan sonar mosaics we offer the following observations and conclusions. The EPR is segmented by ridge axis discontinuities such that the average segment lengths in the area are 360 km for first-order segments, 140 km for second-order segments, 52 km for third-order segments, and 13 km for fourth-order segments. All three first-order discontinuities are transform faults. Where the rise axis is a bathymetric high, second-order discontinuities are overlapping spreading centers (OSCs), usually with a distinctive 3:1 overlap to offset ratio. The off-axis discordant zones created by the OSCs are V-shaped in plan view indicating along axis migration at rates of 40–100 mm yr^–1. The discordant zones consist of discrete abandoned ridge tips and overlap basins within a broad wake of anomalously deep bathymetry and high crustal magnetization. The discordant zones indicate that OSCs have commenced at different times and have migrated in different directions. This rules out any linkage between OSCs and a hot spot reference frame. The spacing of abandoned ridges indicates a recurrence interval for ridge abandonment of 20,000–200,000 yrs for OSCs with an average interval of approximately 100,000 yrs. Where the rise axis is a bathymetric low, the only second-order discontinuity mapped is a right-stepping jog in the axial rift valley. The discordant zone consists of a V-shaped wake of elongated deeps and interlocking ridges, similar to the wakes of second-order discontinuities on slow-spreading ridges. At the second-order segment level, long segments tend to lengthen at the expense of neighboring shorter segments. This can be understood if segments can be approximated by cracks, because the propagation force at a crack tip is directly proportional to crack length.There has been a counter-clockwise change in the direction of spreading on the EPR between 8 and 18° N during the last 1 Ma. The cumulative change has been 3°–6°, producing opening across the Orozco and Siqueiros transform faults and closing across the Clipperton transform. The instantaneous present-day Cocos-Pacific pole is located at approximately 38.4° N, 109.5° W with an angular rotation rate of 2.10° m.y.^–1 This change in spreading direction explains the predominance of right-stepping discontinuities of orders 2–4 along the Siqueiros-Clipperton and Orozco-Rivera segments, but does not explain other aspects of segmentation which are thought to be linked to patterns of melt supply to the ridge axis.There are 23 significant seamount chains in the mapped area and most are created very near the spreading axis. Nearly all of the seamount chains have trends which fall between the absolute and relative plate motion vectors.     

Volcanic Morphology of the East Pacific Rise Crest 9°49′–52′: Implications for volcanic emplacement processes at fast-spreading mid-ocean ridges

Deep sea photographs were collected for several camera-tow transects along and across the axis at the East Pacific Rise crest between 9°49 and 9°52 N, covering terrain out to 2 km from the ridge axis. The objective of the surveys was to utilize fine-scale morphology and imagery of seafloor volcanic terrain to aid in interpreting eruptive history and lava emplacement processes along this fast-spreading mid-ocean ridge. The area surveyed corresponds to the region over which seismic layer 2A, believed to correspond to the extrusive oceanic layer, attains full thickness (Christeson et al., 1994a, b, 1996; Hooft et al., 1996; Carbotte et al., 1997). The photographic data are used to identify the different eruptive styles occurring along the ridge crest, map the distribution of the different morphologies, constrain the relative proportions of the three main morphologies and discuss the implications of these results. Morphologic distributions of lava for the area investigated are 66% lobate lava, 20% sheet lava, 10% pillow lava, and 4% transitional morphologies between the other three main types. There are variations in inferred relative lava ages among the different morphological types that do not conform to a simple increase in age versus distance relationship from the spreading axis, suggesting a model in which off-axis transport and volcanism contribute to the accumulation of the extrusive layer. Analysis of the data suggests this ridge crest has experienced three distinctly different types of volcanic emplacement processes: (1) axial summit eruptions within a 1 km wide zone centered on the axial summit collapse trough (ASCT); (2) off-axis transport of lava erupted at or near the ASCT through channelized surface flows; and (3) off-axis eruptions and local constructional volcanism at distances of 0.5-1.5 km from the axis. Major element analyses of basaltic glasses from lavas collected by Alvin, rock corer and dredging in this area indicate that the most recent magmatic event associated with the present ASCT erupted relatively homogeneous and mafic (>8.25 weight percent wt.% MgO) basalts compared to older, off-axis lavas which tend to be more chemically evolved (Perfit and Chadwick, 1998; Perfit and Fornari, unpublished data). The more primitive lavas have a more extensive distribution within and east of the ASCT. More evolved basalts (MgO <8.0wt.%) are concentrated in a broad area a few kilometers east of the axis, and in an oval-shaped area south of 9°50 N, west of the ASCT. Transitional and enriched (T- and E-) mid-ocean ridge basalts exist in relatively small areas (<1 km²) on the crestal plateau and correlate with scarps or fissures where pillow lavas were erupted. Mafic lavas in this area are primarily related to the youngest magmatic events. Geochemical analysis of samples collected at distances >500 m from the ASCT suggests that regions of off-axis volcanism may be sourced from older and cooler sections of the axial magma lens. Analysis of these data suggests that this portion of the EPR has not experienced large scale volcanic overprinting in the past 30 ka. The predominance of lobate flows (66%) throughout much of the crestal region, and subtle variations in sediment cover and apparent age between flows, suggest that eruptive volumes and effusion rates of individual eruptions have been similar over much of the last 30 ka and that most of the eruptions have been small, probably similar in volume to the 1991 EPR flow which had an estimated volume of 1×10⁶ m³ (Gregg et al., 1996).     

Seafloor evidence for ice shelf flow across the Alaska–Beaufort margin of the Arctic Ocean

New imagery of ～14 100 km² of seafloor along a 640 km stretch of the Alaska and Beaufort margins (ABM) in water depths from 250 to 2800 m depicts a repetitive association of glaciogenic bedforms (lineations and iceberg scours), broad erosional bathymetric features and adjacent downslope turbidite gullies. These bedforms have styles, depths and orientations similar to features discovered earlier on the Chukchi Borderland, up to 800 km northwest of the ABM. Lineations occur across the surface of a flattened bathymetric bench interpreted to have formed by an ice shelf sliding along the continental slope and scraping the seafloor at temporary grounding locations. The glacial geology of surrounding areas suggests that an ice shelf probably flowed from the mouths of overdeepened glacial troughs in the Canadian Arctic Archipelago westward along the ABM and across the Chukchi Borderland. This curved pathway indicates an obstruction to ice flow in the central Canada Basin, possibly caused by either a basin‐wide ice shelf or by a pile‐up of mega‐bergs originating from the Eurasian side of the Arctic Ocean. The ice shelf that affected the ABM may have formed between Oxygen Isotopic Stage 4 to 5b, possibly correlating to an inferred intra‐Stage 5 widespread Beringian glaciation. Evidence for glaciogenic features on the ABM corroborates suggestions that large ice volumes and extents existed in the Arctic during Pleistocene glacial periods. These findings have far‐reaching implications for Arctic climate studies, ocean circulation, sediment stratigraphy and the stability of circum‐Arctic continental ice masses. Copyright © 2007 John Wiley & Sons, Ltd.