Northwest Limb Research Articles

Multiwavelength Observations of a B-class Flare Using XSM, AIA, and XRT

We present multiwavelength observations by Chandrayaan-2/Solar X-ray Monitor, Solar Dynamics Observatory/Atmospheric Imaging Assembly, and Hinode/X-Ray Telescope (XRT) of a B-class flare observed on 2021 February 25, originating from an active region (AR 12804) near the northwest limb. The microflare lasts for ∼30 minutes and is composed of hot loops reaching temperatures of 10 MK. We report excellent agreement (within 20%) for the average effective temperatures obtained at the flare peak from all the three instruments, which have different temperature sensitivities. The XRT filter combination of Be-thin and Be-med provides an excellent opportunity to measure the high temperatures in such microflare events. The elemental abundances during the evolution of the microflare are also studied and observed to drop toward photospheric values at the flare peak time, compared to coronal values during the rise and decay phase. This is consistent with previous XSM studies.

Numerical modeling of formation of the Maoping Pb-Zn deposit within the Sichuan-Yunnan-Guizhou Metallogenic Province, Southwestern China: Implications for the spatial distribution of concealed Pb mineralization and its controlling factors

The Maoping large Pb-Zn deposit is located in the Sichuan-Yunnan-Guizhou Metallogenic Province (SYGMP), southwestern China, hosting more than 3 mt Pb and Zn minerals, with 140 t associated Ge. This study uses numerical simulation methods to explore the key factors of the mineralizing system that formed the Maoping deposit which traditional analytical methods cannot easily identify, answering questions about the spatial distribution of galena and giving information for future resource exploration within the Maoping district. This Study reveals and highlights the practical value of numerical modeling in quantitatively researching the ore-forming processes of mineral deposits and how this knowledge can be used in future geological research and mineral exploration. Our model interlinks heat transfer, pressure (without deformation process), fluid-flow, and material migration during the mineralizing processes and indicates the presence of a temperature anomaly, location of fluid convergence and the spatial distribution of Pb2+ that match the known distribution of mineralization. We also analyze the whole process and find the key factors which control the spatial distribution of galena, answering the question why there is mineralization with economic value in the northwest limb and none in the southeast limb of the Maoping anticline. All these data can be used in future geological research to gain insights into the ore-forming system that formed the Maoping Pb-Zn deposit as well as other hydrothermal mineralizing systems in the SYGMP.

Radio evidence for a shock wave reflected by a coronal hole

We report the first unambiguous observational evidence in the radio range of the reflection of a coronal shock wave at the boundary of a coronal hole. The event occurred above an active region located at the northwest limb of the Sun and was characterized by an eruptive prominence and an extreme-ultraviolet wave steepening into a shock. The EUV observations acquired by the Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory and the Extreme Ultraviolet Imager instrument on board the Solar TErrestrial RElations Observatory were used to track the development of the EUV front in the inner corona. Metric type II radio emission, a distinguishing feature of shock waves propagating in the inner corona, was simultaneously recorded by ground-based radio spectrometers. The radio dynamic spectra displayed an unusual reversal of the type II emission lanes, together with type III-like herringbone emission, indicating shock-accelerated electron beams. Combined analysis of imaging data from the two space-based EUV instruments and the Nançay Radioheliograph evidences that the reverse-drifting type II emission was produced at the intersection of the shock front, reflected at a coronal hole boundary, with an intervening low-Alfvén-speed region characterized by an open field configuration. We also provide an outstanding data-driven reconstruction of the spatiotemporal evolution in the inner corona of the shock-accelerated electron beams produced by the reflected shock.

High‐Resolution Potassium Observations of the Lunar Exosphere

AbstractWe observed lunar exospheric potassium D1 (7,698.9646 Å) emissions using a high‐spectral resolution Fabry‐Perot spectrometer in 2014. We present the first potassium line profile measurements, which are representative of the potassium velocity distribution. Inferred temperatures are greater during the waxing gibbous phase, 1920±630 K and lower at waning gibbous phase, 980±200 K. Exosphere models suggest that the measured line widths are a combination of photon‐stimulated desorption and impact vaporization sources. The relative potassium emission intensity decreases by ∼2.5 between lunar phases 80° and 30° and is brightest off the northwest limb near the Aristarchus crater, which is a potassium‐rich surface region. Additionally, the emissions off the northern limb are brighter than the southern limb. The intensity decrease and the greater line width during the waxing gibbous versus the waning gibbous phase suggests a dawn‐dusk asymmetry.

Twisting/Swirling Motions during a Prominence Eruption as Seen from SDO/AIA

A quiescent prominence was observed at north-west limb of the Sun using different channels of Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO). We report and analyse twisting/swirling motions during and after the prominence eruption. We segregate the observed rotational motions into small and large scale. Small scale rotational motions manifest in the barbs of the prominence while the large scale rotation manifests as the roll motion during the prominence eruption. We noticed that both footpoints of the prominence rotate in the counter-clockwise direction. We propose that similar sense of rotation in both footpoints leads to prominence eruption. The prominence erupted asymmetrically near the southern footpoint which may be due to uneven mass distribution and location of the cavity near southern footpoint. Furthermore, we study the swirling motion of the plasma along different circular paths in the cavity of the prominence after the prominence eruption. The rotational velocities of the plasma moving along different circular paths are estimated to be $\sim$ 9-40 km s$^{-1}$. These swirling motions can be explained in terms of twisted magnetic field lines in the prominence cavity. Finally, we observe the twist built up in the prominence, being carried away by the coronal mass ejection (CME) as seen in the Large Angle Spectrometric Coronagraph (LASCO) onboard Solar and Heliospheric Observatory (SOHO).

MULTI-FREQUENCY OBSERVATIONS OF A SUPERBUBBLE IN THE LMC: THE CASE OF LHA 120-N 70

We present a detailed study of new Australia Telescope Compact Array and XMM-Newton observations of LHA 120–N 70 (hereafter N 70), a spherically shaped object in the Large Magellanic Cloud, classified as a superbubble. Both archival and new observations were used to produce high quality radio continuum, X-ray, and optical images. The radio spectral index of N 70 is estimated to be α = −0.12 ± 0.06, indicating that while a supernova (SN) or supernovae have occurred in the region at some time in the distant past, N 70 is not the remnant of a single specific SN. N 70 exhibits limited polarization with a maximum fractional polarization of 9% in a small area of the northwest limb. We estimate the size of N 70 to have a diameter of 104 pc (±1 pc). The morphology of N 70 in X-rays closely follows that in radio and optical, with most X-ray emission confined within the bright shell seen at longer wavelengths. Purely thermal models adequately fit the soft X-ray spectrum which lacks harder emission (above 1 keV). We also examine the pressure output of N 70 where the values for the hot (PX) and warm () phases are consistent with other studied H ii regions. However, the dust-processed radiation pressure (PIR) is significantly smaller than in any other object studied in Lopez et al. N 70 is a very complex region that is likely to have had multiple factors contributing to both the origin and evolution of the entire region.

Observations of intensity oscillations in a prominence-like cool loop system as observed by SDO/AIA: evidence of multiple harmonics of fast magnetoacoustic waves

Using SDO/AIA 304 A channel, we study the evolution of weak intensity oscillations in a prominence like cool loop system observed at North-West limb on 7 March 2011. We use the standard wavelet tool to produce statistically significant power spectra of AIA 304 A normalized fluxes derived respectively near the apex and footpoint of the fluxtube. We find periodicities of ≈667 s and ≈305 s respectively near apex and above footpoint with significance level >98 %. Observed statistically significant periodicities in the tube of projected length ≈170 Mm and width ≈10 Mm, are interpreted as most likely signature of evolution of various harmonics of tubular fast magnetoacoustic waves. Sausage modes are unlikely though they are compressive as they need bulky and highly denser loop system for their evolution for sustaining such large periods. We interpret the observed periodicities as multiple harmonics (fundamental and first) of fast magnetoacoustic kink waves that can generate some weak density perturbations (thus intensity oscillations) in the tube and can be observed pertaining to periodic variation in plasma column depth as tube is oblique in projection with respect to line-of-sight. The period ratio P1/P2=2.18 is observed in the fluxtube, which is the signature of the magnetic field divergence of the cool loop system. We estimate tube expansion factor as 1.27 which is typical of EUV bipolar loops in the solar atmosphere. We estimate the lower bound average magnetic fields ranging from ≈9 to 90 Gauss depending upon typical densities as 109–1011 cm−3 in the observed prominence-like cool loop system. We also observe the first signature of lowering fundamental mode period by a factor 0.85 due to cooling of this loop system.

Origin and Metamorphism of Corundum‐Rich Metabauxites at Mt. Ismail in the Southern Menderes Massif, SW Turkey

AbstractThe corundum‐rich metabauxites, found at the northwest limb of an NE–SW‐trending isoclinal recumbent fold at Mt. Ismail, are enclosed in thick‐bedded platform‐type marbles of Late Cretaceous age, surrounding the polymetamorphic core series in the southern part of the Menderes Massif (SW Turkey). The metabauxite horizons observed as typically boudine‐like structure, extend laterally over c. 3 km and are 1 to 5 m thick. These rocks have dominant mineral assemblages of corundum (∼50 modal %), chloritoid (∼30 modal %), white mica (margarite, muscovite), diaspore, Fe–Ti‐Oxides (ilmenite, ilmenohematite, rutile), and goethite, limonite, pyrite, tourmaline (uvite, schorl) as minor phases. Chemical analyses of whole rock samples and the mineral assemblage indicate that coexisting minerals of metabauxites are highly aluminous. A number of minerals (e.g. chloritoid and margarite) display a large compositional variation reflecting the initial chemical inhomogenetiy of the karstbauxites. The field observations, trace‐element accumulation coefficients, concentration of elements such as Cr, Zr, Ga and Ni and low amounts of immobile elements all suggest that the studied corundum‐rich metabauxites can be classified as karstbauxites, and are more likely to be a product of weathering of intermediate igneous or argillaceous parental rocks, similar to the karstic Tauric bauxites in the Central Taurides (Seydi şehir region) and probably are similar in age (Cenomanian–Turonian). In respect of tectono‐metamorphic evolution, the studied corundum‐rich metabauxites were regionally metamorphosed at ∼5–6 kbar pressure and 500–600°C as a consequence of the Barrovian metamorphism referred to as the ‘Main Menderes Metamorphism’ related to the ophiolitic obduction onto the Menderes platform from the Izmir–Ankara Suture during the Middle Eocene.

A new look at a polar crown cavity as observed by SDO/AIA

The Solar Dynamics Observatory (SDO) was launched in February 2010 and is now providing an unprecedented view of the solar activity at high spatial resolution and high cadence covering a broad range of temperature layers of the atmosphere. We aim at defining the structure of a polar crown cavity and describing its evolution during the erupting process. We use high cadence time series of SDO/AIA observations at 304\AA (50000 K) and 171\AA (0.6 MK) to determine the structure of the polar crown cavity and its associated plasma as well as the evolution of the cavity during the different phases of the eruption. We report on the observations recorded on 13 June 2010 located on the North-West limb. We observe coronal plasma shaped by magnetic field lines with a negative curvature (U-shape) sitting at the bottom of a cavity. The cavity is located just above the polar crown filament material. We thus observe the inner part of the cavity above the filament as depicted in the classical three part Coronal Mass Ejection (CME) model composed of a filament, a cavity and a CME front. The filament (in this case a polar crown filament) is part of the cavity and makes a continuous structuring from the filament to the CME front depicted by concentric ellipses (in a 2D cartoon). We propose to define a polar crown cavity as a density depletion sitting above denser polar crown filament plasma drained down the cavity due to gravity. As part of the polar crown filament, plasma at different temperatures (ranging from 50000K to 0.6 MK) is observed at the same location on the cavity dips and sustained by a competition between the gravity and the curvature of magnetic field lines. The eruption of the polar crown cavity as a solid body can be decomposed into two phases: a slow rise at a speed of 0.6 km/s, and an acceleration phase at a mean speed of 25 km/s.

Oroclines and paleomagnetism in Borneo and South-East Asia

Oroclinal bending of Borneo is interpreted to result from indentation and collision by the continental promontory of the Miri Zone-Central Luconia Province of northern Sundaland into southern Sundaland. The collision caused strong compression and uplift of the intervening Sibu Zone Upper Cretaceous–Eocene Rajang-Embaluh Group turbidite basin that was floored by oceanic crust of the Proto South China Sea. Timing of the collision is indicated by uplift of turbidite formations to be overlain by Upper Eocene–Lower Oligocene carbonates and intrusion of tin-mineralised granites into the turbidites at the south-east maximum inflexion of the orocline, a region complicated by juxtaposition of both shallow and deep water formations. The oroclinal model, requiring clockwise rotation of the north-west limb, is given no support from the paleomagnetic data that instead demonstrate about 50° of Cenozoic anti-clockwise rotation. Unfortunately not a single outcrop of the strongly oroclinally bent Sibu Zone rocks was measured for paleomagnetism in the north-west limb. Limited support was given for the required anti-clockwise rotation in the north-east limb. Previous syntheses emphasised anti-clockwise rotation, or stable non-rotation of the greater Borneo region as a coherent entity, without any internal deformation. Such models have ignored the oroclinal shape defined by the areal geology of the island, known since early Dutch publications. The northern Thailand–Myanmar north–south-trending geology fabric results from indentation by a promontory of continental India at the Assam–Yunnan oroclinal syntaxis, resulting in paleomagnetically-determined clockwise rotation. The bend of Peninsular Malaysia and Sumatra, from north–south changing to west–east towards Borneo in the south, remains difficult to model because of widespread remagnetisation.

Formation of the CME Leading Edge Observed in the 2003 February 18 Event

This work investigates a typical coronal mass ejection (CME) observed on 2003 February 18, by various space and ground instruments, in white light, H alpha, EUV and X-ray. The H alpha and EUV images indicate that the CME started with the eruption of a long filament located near the solar northwest limb. The white light coronal images show that the CME initiated with the rarefaction of a region above the solar limb and followed by the formation of a bright arcade at the boundary of the rarefying region at height 0.46 R(circle dot) above the solar surface. The rarefying process synchronized with the slow rising phase of the eruptive filament, and the CME leading edge was observed to form as the latter started to accelerate. The lower part of the filament brightened in H alpha as the filament rose to a certain height and parts of the filament was visible in the GOES X-ray images during the rise. These brightenings imply that the filament may be heated by the magnetic reconnection below the filament in the early stage of the eruption. We suggest that a possible mechanism which leads to the formation of the CME leading edge and cavity is the magnetic reconnection which takes place below the filament after the filament has reached a certain height.

Multialtitude Observations of a Coronal Jet during the Third Whole Sun Month Campaign

On 1999 August 26, a coronal jet occurred at the northwest limb near a sigmoid active region (AR 8668) that was the target for a joint observation plan (SOHO joint observing program 106) during the third Whole Sun Month Campaign. This jet was observed by several instruments at the limb (SOHO/CDS, SOHO/EIT, TRACE, and Mauna Loa Solar Observatory CHIP and PICS) and at 1.64 R☉ (SOHO/UVCS). At the limb, this jet event displayed both low- and high-temperature components. Both high- and low-temperature components were evident during the early phase (first 20 minutes) of the event. However, the low-temperature component is maintained for ~1 hr after the higher temperature component is gone. There is a second brightening (a possible second jet) seen by EIT and TRACE about 50 minutes after the onset of the first jet. The line-of-sight motion at the limb began with a 300 km s-1 redshift and evolved to a 200 km s-1 blueshift. At 1.64 R☉, the intensities of Lyα and Lyβ in the jet increased by a factor of several hundred compared with the background corona. The C III λ977 line also brightened significantly. This indicates low-temperature [~(1-2) × 105 K] emission in the jet, while the intensities of O VI λ1032 and O VI λ1037 increased by as much as a factor of 8. The UVCS data show evidence of heating at the early phase of the event. The Doppler shift in the lines indicates that the line-of-sight (LOS) velocity in the jet started from ~150 km s-1 in blueshift and ended at ~100 km s-1 in redshift. This LOS motion seen at 1.64 R☉ was apparently opposite to what was observed when the jet emerged from the limb. The Doppler dimming analysis indicates that the radial outflow speed correlates with the magnitude of the LOS speed. Interestingly, UVCS observations at 2.33 and 2.66 R☉ show no trace of the jet and SOHO/LASCO observations also yield no firm detection. We find that a simple ballistic model can explain most of the dynamical properties of this jet, while the morphology and the thermal properties agree well with reconnection-driven X-ray jet models.

Coronal Element Comparison Observed bySOHOSUMER in the Quiet Southeast and Northwest Limb Regions at 1.04R⊙above the Solar Disk

The composition in an isothermal region at 1.04 R☉ above a diffuse, quiet coronal region on the northwest limb is studied and compared with a similar region above the southeast limb analyzed by Warren in 1999. Elemental abundances relative to H in the corona are measured and normalized to the abundance ratios in the photosphere. The enrichment factors of the low first ionization potential elements are comparable above both limbs, but are significantly smaller than the factor of 4 in SUMER spectra obtained 2 years earlier at solar minimum in a diffuse, quiet equatorial region.

Detection and Diagnostics of a Coronal Shock Wave Driven by a Partial‐Halo Coronal Mass Ejection on 2000 June 28

A fast partial-halo coronal mass ejection (CME) was observed on 2000 June 28 by instruments on the SOHO spacecraft. The CME leading edge and filamentary cold core were detected over the northwest limb at 2.32 R? by the SOHO UV Coronagraph Spectrometer (UVCS). The broad profile of the O?VI ?1032 line gives evidence of a shock front at the leading edge, supporting the identification of white-light CME sharp leading edges as fast-mode shocks. Line-of-sight speeds are as high as 1500 km s-1, comparable to the projected speed obtained from LASCO. Pumping of the O?VI ?1032 by Ly? (v = 1810 km s-1) and of O?VI ?1037 by O?VI ?1032 (v = 1648 km s-1) were detected, which provide diagnostics of outflow speed and density. The angle of the ejecta with the plane of the sky is obtained, combining the projected speed from LASCO with the line-of sight-speed, and varies between 7? and 46?. In the latter case the projected height of 2.32 R? was at an actual heliocentric distance of 3.6 R?. An associated solar energetic particle (SEP) event was observed at the L1 point following this CME. The abundance and charge-state data are consistent with a gradual shock-accelerated SEP event. A type II radio burst was observed at the same time the shock front was detected by UVCS.

Eruption of a Quiescent Filament on Feb. 18, 2003

An eruptive process of a quiescent filament on Feb. 18, 2003 that initiated a CME was recorded by the full disk Hα telescope in HSOS (Huairou Solar Observing Station) and by the Solar and Heliospheric Observatory (SOHO). The eruption revealed a typical process of energy release and dense plasma ejection caused by magnetic instability. Filament eruptions, flares and CMEs are believed to be the same magnetic eruption and reconnection event in three different stages. This feature was shown more clearly during eruption of the quiescent filament because of the simple and regular magnetic line-of-sight field in photosphere compared to active regions with complex magnetic configurations. The filament was located near the solar northwest limb (N 30-40, W 40-80) on Feb. 18, 2003 with height about 11000 km. It began to erupt at 02:00 UT. Bright ribbons appeared at the magnetic feet of the filament in EIT/195 at 02:30 UT. The post-eruption loops formed at 04:00 UT and lasted about 6 hours. The CME appeared on EIT/C2 images at 02:30UT with an Ω shape, just the same as the filament erupted. The speed of mass ejected of the filament was about 250km/s. We also notice that the height of the filament increased and the magnetic fields with opposite polarity moved towards each other before the filament erupted. The filament eruption reveals that the filament may be held in the corona by the twisted magnetic fields which also keep the cool plasma from escaping the solar surface. If the twisted field was broken by some magnetic instabilities, the filament would erupt. The presence of post-eruption loops implies a transition process from non-potential (twisted) to potential field magnetic configuration in the filament.

Observations Indicating That ∼1 x 107K Solar Flare Plasmas May Be Produced in Situ from ∼1 x 106K Coronal Plasma

We discuss a set of flare observations obtained at a position of 0.10 R☉ above the solar northwest limb. The data were acquired by the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO). We derive time-dependent comparisons of physical properties, such as electron temperature and density, between flare plasma and background coronal plasma observed along the same lines of sight. In addition to temperature and density, we discuss emission measures, elemental abundances, nonthermal mass motions (from line widths), and bulk mass motions (from Doppler shifts). The observations appear to indicate that the flaring plasmas (4 × 106 K ≤ Te ≤ 1 × 107 K) along the lines of sight were formed by in situ heating and possibly by compression of the ambient coronal material (Te ≤ 2 × 106 K).

On the structural age of the Rhoscolyn antiform, Anglesey, North Wales

AbstractIn the Rhoscolyn area of Anglesey, the late Precambrian interbedded psammites and pelites of the Monian Supergroup are folded into a kilometre‐scale antiform, plunging about 25°NE and with an axial surface dipping about 40°NW. Numerous folds of up to a few tens of metres in wavelength are present on both limbs of this antiform. These smaller‐scale folds also plunge about 25°NE but clearly belong to two separate episodes of folding, and it has become a matter of longstanding controversy as to whether the larger antiform belongs to the first or second of these episodes. Close examination of the cleavage/bedding asymmetries from all the lithologies, however, shows that the large antiform is a second‐generation structure, and that on the gently dipping northwest limb, the sense of cleavage/bedding asymmetry of the earlier cleavage in the psammitic units has been almost uniformly and homogeneously reversed (so that it appears to be axial planar to the antiform), while in the pelitic units the sense of cleavage/bedding asymmetry of the earlier cleavage has been preserved. Many of the small‐scale complexities of the observed cleavage/bedding relationships may be explained by appealing to differences in the timing of the formation of buckling instabilities relative to this reorientation of the early cleavage in the psammites during the second deformation. A first‐order analysis of the finite strains from around the large‐scale antiform shows that the orientation of the first cleavage prior to the second deformation was steeply dipping to the southeast. The second deformation correlates with the southeast‐verging Caledonian deformation affecting the Monian and Ordovician units elsewhere in northwest Anglesey, while the northwest‐verging first deformation event, which is not present in the Ordovician rocks, must have occurred before they were deposited. Copyright © 2004 John Wiley & Sons, Ltd.

Evidence for the Formation of a Large-Scale Current Sheet in a Solar Flare

We present X-ray evidence for the formation of a large-scale current sheet in a flare observed by the Ramaty High-Energy Solar Spectroscopic Imager on 2002 April 15. The flare occurred on the northwest limb, showing a cusp-shaped flare loop in the rise phase. When the impulsive rise in hard X-rays (>25 keV) began, the cusp part of the coronal source separated from the underlying flare loop and remained stationary for about 2 minutes. During this time, the underlying flare loops shrank at ~9 km s-1. The temperature of the underlying loops increased toward higher altitudes, while the temperature of the coronal source increased toward lower altitudes. These results indicate that a current sheet formed between the top of the flare loops and the coronal source during the early impulsive phase. After the hard X-ray peak, the flare loops grew outward at ~8 km s-1, and the coronal source moved outward at ~300 km s-1, indicating an upward expansion of the current sheet. About 30 minutes later, postflare loops seen in the Solar and Heliospheric Observatory (SOHO) EUV Imaging Telescope 195 A passband rose at ~10 km s-1. A large coronal looplike structure, observed by the SOHO Large Angle and Spectrometric Coronagraph C2 and C3 detectors, also propagated outward at ~300 km s-1. These observations are all consistent with the continued expansion of the current sheet.

ChandraCCD Imagery of the Northeast and Northwest Limbs of SN 1006

We present deep images of the SN 1006 remnant (G327.6+14.6) with the ACIS instrument on the Chandra X-Ray Observatory. Two regions have been observed, the synchrotron-dominated northeast limb and the thermally dominated northwest limb, as well as a substantial portion of the interior of the remnant shell. The brightest features in the X-ray images correspond closely to radio features in the northeast and to Balmer-dominated filaments in the northwest. The spectra of the brighter filaments in the northeast are harder, with less prominent line emission than those in the northwest. In addition to highly elongated filaments, both images show enhanced clumps of emission well inside of the shock front that appear to be dominated by emission from oxygen. These probably arise from shock-heated ejecta, based on analysis of their X-ray spectra. We find no firm evidence for a halo of X-ray emission outside the shock to the northeast, as predicted by the Fermi shock-acceleration picture, in which relativistic electrons should be diffusing ahead of the shock. Our limits on upstream emission are less than 1.5% of the postshock levels in regions where the supernova remnant is brightest. This strongly suggests that the bright rims are flattened sheets nearly perpendicular to the plane of the sky and that the magnetic field strength jumps at the shock by a factor significantly larger than 4, as has been proposed if the shock puts significant energy into accelerating nonthermal ions. The spectra obtained of the northwest rim are all dominated by the helium-like ions of O, Ne, Mg, and Si expected from shocks with ionization (net) parameters of order 100 cm-3 yr and electron temperatures of 0.5-1 keV, far lower than the postshock ion temperature implied by estimates of the shock speed obtained from the shape of the Hα line.

Synfolding remagnetization and deformation: results from Palaeozoic sedimentary rocks in West Virginia

SUMMARY Palaeomagnetic analysis was conducted at 27 sites from two stratigraphically adjacent Palaeozoic units along two transects (Meldey and Rig) of the Patterson Creek anticline in the Appalachian orogen of West Virginia to better understand the relationship between deformation and remagnetization. While no primary directions were observed, we isolated identical, well-defined, secondary Late Palaeozoic magnetizations in carbonates from the Late Silurian Tonoloway Formation and the Early Devonian Helderberg Group. Standard application of conventional palaeomagnetic fold tests suggests that the magnetizations are synfolding in the Helderberg Group and pre-folding to early synfolding in the Tonoloway Formation. A fold test on all data combined indicates a synfolding magnetization (best grouping at 60–65 per cent unfolding), but the resulting distributions are not circular. An alternative interpretation, based on optimal differential unfolding at the site mean level yields one group with a nearly pre-folding, or early synfolding, magnetization (>70 per cent unfolding) that is both circular and concordant with North America's apparent polar wander path. The second group has a ‘variable synfolding’ magnetization that is elliptical at the optimum unfolding level and falls off the path. Differences in either the age of remagnetization or viscous partial thermoremanent magnetization contamination cannot account for this result. The sites falling into the pre-folding group are biased towards the Tonoloway Formation, the Medley Transect, the northwest limb, away from small folds and lower dip angle relative to the sites in the synfolding group. The two groups defined by the fold test results also have a relationship with both total strain and strain partitioning. The rocks containing the early synfolding remanence exhibit less total strain and, in particular, less pressure solution strain relative to grain boundary sliding and calcite twinning. It is likely that the remagnetization in the entire area was coincident and occurred prior to folding even though some sites reveal synfolding directions. The variation in the behaviour of the remagnetization relative to folding is unclear at present but it indicates that pressure solution and stylolite formation are important processes in remagnetization. In any event, the relationship between deformation and magnetization is not simple. Apparently identical magnetizations from adjacent sites may have very different histories and/or responses to deformation. While the exact relationship between remagnetization and deformation remains elusive this study demonstrates that significant variation in behaviour can exist at a very local scale.