Central Meridian Research Articles

Corneal Sublayers Thickness Estimation Obtained by High-Resolution FD-OCT

This paper presents a novel processing technique which can be applied to corneal in vivo images obtained with optical coherence tomograms across the central meridian of the cornea. The method allows to estimate the thickness of the corneal sublayers (Epithelium, Bowman's layer, Stroma, Endothelium, and whole corneal thickness) at any location, including the center and the midperiphery, on both nasal and temporal sides. The analysis is carried out on both the pixel and subpixel scales to reduce the uncertainty in thickness estimations. This technique allows quick and noninvasive assessment of patients. As an example of application and validation, we present the results obtained from the analysis of 52 healthy subjects, each with 3 scans per eye, for a total of more than 300 images. Particular attention has been paid to the statistical interpretation of the obtained results to find a representative assessment of each sublayer's thickness.

Optimal scale factor on the central meridian for a 2°TM map projection in TWD97

The purpose of this paper is to study the optimal scale factor on the central meridian in order to minimize distortions of the 2°Transverse Mercator map projection in TWD97. The optimal scale factor is first derived according to the Airy optimal theory of distortions. Next, the influences of the current and optimal scale factors on local length and area distortions are discussed and analyzed. The results show that the value of the optimal scale factor on the central meridian under consideration is 0.999942 and leads to minimal local distortions in both distances and areas on the island of Taiwan.

Helioseismic analysis of active regions using HMI and AIA data

AbstractWe apply the ring‐diagram technique to analyze three active regions located near the central meridian. Using Doppler, continuum intensity, and line depth images from the Helioseismic Magnetic Imager (HMI), we investigate the variation in the high‐degree mode asymmetry, frequencies, and horizontal flows. We find that the sub‐surface zonal and meridional flows do not change significantly with the choice of different observables representing different heights in the solar photosphere, while the mode frequencies differ. We also examine the 2‐d acoustic power distribution using data from HMI and the Atmospheric Imaging Assembly (AIA) 1600 and 1700 Å bands (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Isolated Sunspot with a Dark Patch in the Coronal Emission

Abstract On the base of the 17 GHz radio maps of the Sun taken with the Nobeyama Radio Heliograph we estimate plasma parameters in the specific region of the sunspot atmosphere in the active region AR 11312. This region of the sunspot atmosphere is characterized by the depletion in coronal emission (soft X-ray and EUV lines) and the reduced absorption in the a chromospheric line (He I 1.083 μm). In the ordinary normal mode of 17 GHz emission the corresponding dark patch has the largest visibility near the central solar meridian. We infer that the reduced coronal plasma density of about ~ 5 × 108 cm−3 is the characteristic feature of the dark patch.

Can a halo CME from the limb be geoeffective?

The probability for a halo coronal mass ejection (CME) to be geoeffective is assumed to be higher the closer the CME launch site is located to the solar central meridian. However, events far from the central meridian may produce severe geomagnetic storms, like the case in April 2000. In this work, we study the possible geoeffectiveness of full halo CMEs with the source region situated at solar limb. For this task, we select all limb full halo (LFH) CMEs that occurred during solar cycle 23, and we search for signatures of geoeffectiveness between 1 and 5 days after the first appearance of each CME in the LASCO C2 field of view. When signatures of geomagnetic activity are observed in the selected time window, interplanetary data are carefully analyzed in order to look for the cause of the geomagnetic disturbance. Finally, a possible association between geoeffective interplanetary signatures and every LFH CME in solar cycle 23 is checked in order to decide on the CME's geoeffectiveness. After a detailed analysis of solar, interplanetary, and geomagnetic data, we conclude that of the 25 investigated events, there are only four geoeffective LFH CMEs, all coming from the west limb. The geoeffectiveness of these events seems to be moderate, turning to intense in two of them as a result of cumulative effects from previous mass ejections. We conclude that ejections from solar locations close to the west limb should be considered in space weather, at least as sources of moderate disturbances.

THE NAKED EMERGENCE OF SOLAR ACTIVE REGIONS OBSERVED WITHSDO/HMI

We take advantage of the HMI/SDO instrument to study the naked emergence of active regions from the first imprints of the magnetic field on the solar surface. To this end, we followed the first 24 hours in the life of two rather isolated ARs that appeared on the surface when they were about to cross the central meridian. We analyze the correlations between Doppler velocities and the orientation of the vector magnetic field finding, consistently, that the horizontal fields connecting the main polarities are dragged to the surface by relatively-strong upflows and are associated to elongated granulation that is, on average, brighter than its surroundings. The main magnetic footpoints, on the other hand, are dominated by vertical fields and downflowing plasma. The appearance of moving dipolar features, MDFs, (of opposite polarity to that of the AR) in between the main footpoints, is a rather common occurrence once the AR reaches a certain size. The buoyancy of the fields is insufficient to lift up the magnetic arcade as a whole. Instead, weighted by the plasma that it carries, the field is pinned down to the photosphere at several places in between the main footpoints, giving life to the MDFs and enabling channels of downflowing plasma. MDF poles tend to drift towards each other, merge and disappear. This is likely to be the signature of a reconnection process in the dipped field lines, which relieves some of the weight allowing the magnetic arcade to finally rise beyond the detection layer of the HMI spectral line.

Periodic bursts of Jovian non-Io decametric radio emission

During the years 2000–2011 the radio instruments onboard Cassini, Wind and STEREO spacecraft have recorded a large amount of the Jovian decametric radio emission (DAM). In this paper we report on the analysis of the new type of Jovian periodic radio bursts recently revealed in the decametric frequency range. These bursts, which are non-Io component of DAM, are characterized by a strong periodic reoccurrence over several Jovian days with a period ≈1.5% longer than the rotation rate of the planet's magnetosphere (System III). The bursts are typically observed between 4 and 12MHz and their occurrence probability has been found to be significantly higher in the sector of Jovian Central Meridian Longitude between 300° and 60° (via 360°). The stereoscopic multispacecraft observations have shown that the radio sources of the periodic bursts radiate in a non-axisymmetric hollow cone-like pattern and sub-corotate with Jupiter remaining active during several planet's rotations. The occurrence of the periodic non-Io DAM bursts is strongly correlated with pulses of the solar wind ram pressure at Jupiter. Moreover the periodic bursts exhibit a tendency to occur in groups every ∼25 days. The polarization measurements have shown that the periodic bursts are right hand polarized radio emission associated with the Northern magnetic hemisphere of Jupiter. We suggest that periodic non-Io DAM bursts may be connected with the interchange instability in Io plasma torus triggered by the solar wind.

Solar Stereoscopy with STEREO/EUVI A and B Spacecraft from Small (6∘) to Large (170∘) Spacecraft Separation Angles

We performed for the first time stereoscopic triangulation of coronal loops in active regions over the entire range of spacecraft separation angles (α sep≈6∘,43∘,89∘,127∘,and 170∘). The accuracy of stereoscopic correlation depends mostly on the viewing angle with respect to the solar surface for each spacecraft, which affects the stereoscopic correspondence identification of loops in image pairs. From a simple theoretical model we predict an optimum range of α sep≈22∘ – 125∘, which is also experimentally confirmed. The best accuracy is generally obtained when an active region passes the central meridian (viewed from Earth), which yields a symmetric view for both STEREO spacecraft and causes minimum horizontal foreshortening. For the extended angular range of α sep≈6∘ – 127∘ we find a mean 3D misalignment angle of μ PF≈21∘ – 39∘ of stereoscopically triangulated loops with magnetic potential-field models, and μ FFF≈15∘ – 21∘ for a force-free field model, which is partly caused by stereoscopic uncertainties μ SE≈9∘. We predict optimum conditions for solar stereoscopy during the time intervals of 2012 – 2014, 2016 – 2017, and 2021 – 2023.

DEFLECTIONS OF FAST CORONAL MASS EJECTIONS AND THE PROPERTIES OF ASSOCIATED SOLAR ENERGETIC PARTICLE EVENTS

The onset times and peak intensities of solar energetic particle (SEP) events at Earth have long been thought to be influenced by the open magnetic fields of coronal holes (CHs). The original idea was that a CH lying between the solar SEP source region and the magnetic footpoint of the 1 AU observer would result in a delay in onset and/or a decrease in the peak intensity of that SEP event. Recently, Gopalswamy et al. showed that CHs near coronal mass ejection (CME) source regions can deflect fast CMEs from their expected trajectories in space, explaining the appearance of driverless shocks at 1 AU from CMEs ejected near solar central meridian (CM). This suggests that SEP events originating in CME-driven shocks may show variations attributable to CH deflections of the CME trajectories. Here, we use a CH magnetic force parameter to examine possible effects of CHs on the timing and intensities of 41 observed gradual E ∼ 20 MeV SEP events with CME source regions within 20° of CM. We find no systematic CH effects on SEP event intensity profiles. Furthermore, we find no correlation between the CME leading-edge measured position angles and SEP event properties, suggesting that the widths of CME-driven shock sources of the SEPs are much larger than the CMEs. Independently of the SEP event properties, we do find evidence for significant CME deflections by CH fields in these events.

AN ANALYSIS OF THE ORIGIN AND PROPAGATION OF THE MULTIPLE CORONAL MASS EJECTIONS OF 2010 AUGUST 1

On 2010 August 1, the northern solar hemisphere underwent significant activity that involved a complex set of active regions near central meridian with, nearby, two large prominences and other more distant active regions. This activity culminated in the eruption of four major coronal mass ejections (CMEs), effects of which were detected at Earth and other solar system bodies. Recognizing the unprecedented wealth of data from the wide range of spacecraft that were available—providing the potential for us to explore methods for CME identification and tracking, and to assess issues regarding onset and planetary impact—we present a comprehensive analysis of this sequence of CMEs. We show that, for three of the four major CMEs, onset is associated with prominence eruption, while the remaining CME appears to be closely associated with a flare. Using instrumentation on board the Solar Terrestrial Relations Observatory spacecraft, three of the CMEs could be tracked out to elongations beyond 50°; their directions and speeds have been determined by various methods, not least to assess their potential for Earth impact. The analysis techniques that can be applied to the other CME, the first to erupt, are more limited since that CME was obscured by the subsequent, much faster event before it had propagated far from the Sun; we discuss the speculation that these two CMEs interact. The consistency of the results, derived from the wide variety of methods applied to such an extraordinarily complete data set, has allowed us to converge on robust interpretations of the CME onsets and their arrivals at 1 AU.

Stop being neutral: Simon takes control!

The Simon task is one of the well known tasks that recruit cognitive control. The Simon effect, the reaction time (RT) difference between congruent and incongruent stimuli, has been commonly discussed as interference based. Nevertheless, in recent years some studies have referred to the facilitation component of the task. In the current research we measured effects of cognitive control by conducting sequential analysis and adding neutral conditions. Two neutral stimuli were employed in order to examine their effect and their validity as neutrals. It was found that presentation of Simon stimuli on the central meridian at the top or bottom of a screen (but not at the centre of the screen) created a valid neutral condition. Facilitation as well as interference effects were found. Adding a nonconflict condition modulated cognitive control independently of sequential effects. Namely, the Simon effect increased by adding both types of neutrals, but the decrease in the effect after incongruent trials was only present for the vertical neutral block, in which the Simon effect appeared after congruent but not after incongruent trials. We suggest the possibility of two different mechanisms of cognitive control.

THE ROLE OF STREAMERS IN THE DEFLECTION OF CORONAL MASS EJECTIONS: COMPARISON BETWEENSTEREOTHREE-DIMENSIONAL RECONSTRUCTIONS AND NUMERICAL SIMULATIONS

On 2009 September 21, a filament eruption and the associated coronal mass ejection (CME) were observed by the Solar Terrestrial Relations Observatory (STEREO) spacecraft. The CME originated from the southern hemisphere and showed a deflection of about 15 ◦ toward the heliospheric current sheet (HCS) during the propagation in the COR1 field of view. The CME source region was near the central meridian, but no on-disk CME signatures could be seen from the Earth. The aim of this paper is to provide a physical explanation for the strong deflection of the CME observed on 2009 September 21. The two-sided view of the STEREO spacecraft allows us to reconstruct the three-dimensional travel path of the CME and the evolution of the CME source region. The observations are combined with a magnetohydrodynamic simulation, starting from a magneticfield configuration closely resembling the extrapolated potential field for that date. By applying localized shearing motions, a CME is initiated in the simulation, showing a similar non-radial evolution, structure, and velocity as the observed event. The CME gets deflected toward the current sheet of the larger northern helmet streamer due to an imbalance in the magnetic pressure and tension forces and finally gets into the streamer. This study shows that during solar minima, even CMEs originating from high latitude can be easily deflected toward the HCS, eventually resulting in geoeffective events. How rapidly they undergo this latitudinal migration depends on the strength of both the large-scale coronal magnetic field and the magnetic flux of the erupting filament.

Further seasonal changes in Uranus’ cloud structure observed by Gemini-North and UKIRT

Near-infrared observations of Uranus were made in October/November 2010 with the Gemini-North telescope in Hawaii, using NIFS, an integral field spectrograph, and the NIRI instrument in imaging mode. Observations were acquired using adaptive optics and have a spatial resolution of approximately 0.1–0.2″.The observed spectra along Uranus’ central meridian were analysed using a multiple-scattering retrieval algorithm to infer the vertical/latitudinal variation in cloud optical depth, which we compare with previous observations made by Gemini-North/NIFS in 2009 and UKIRT/UIST observations made between 2006 and 2008. Assuming a continuous distribution of small particles (r∼1μm, and refractive index of 1.4+0i) with the single scattering albedo set to 0.75 and using a Henyey–Greenstein phase function with asymmetry parameter set to 0.7 at all wavelengths and latitudes, the retrieved cloud density profiles show that the north polar zone at 45°N has continued to steadily brighten while the south polar zone at 45°S has continued to fade. As with our previous analyses we find that, assuming that the methane vertical profile is the same at all latitudes, the clouds forming these polar zones at 45°N and 45°S lie at slightly lower pressures than the clouds at more equatorial latitudes. However, we also find that the Gemini data can be reproduced by assuming that the main cloud remains fixed at ∼2bar at all latitudes and adjusting the relative humidity of methane instead. In this case we find that the deep cloud is still more opaque at the equator and at the zones at 45°N and 45°S and shows the same seasonal trends as when the methane humidity remain fixed. However, with this approach the relative humidity of methane is seen to rise sharply from approximately 20% at polar latitudes to values closer to 80% for latitudes equatorward of 45°S and 45°N, consistent with the analysis of 2002 HST observations by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009]. Icarus 202, 287–302), with a possible indication of seasonal variability. Overall, Uranus appeared to be less convectively active in 2010 than in the previous 4years, supporting the conclusion that now the northern spring equinox (which occurred in 2007) has passed, the atmosphere is settling back into the more quiescent state seen by Voyager 2 in 1986.

Comparison between Cassini and Voyager observations of Jupiter's decametric and hectometric radio emissions

[1] In this paper, we attempt to clarify the relationship between Jovian hectometric (HOM) and non-Io-related decametric (non-Io-DAM) radio structure. For that purpose, we extend the analysis by including more data and investigating statistical properties of the Jovian DAM and HOM radio emissions based on Cassini and Voyager observations, especially below 16 MHz. We have investigated these emissions observed by the Cassini, Voyager 1, and Voyager 2 spacecraft for specific Jovigraphic latitudes in the range of −3.7°–7.3° and local times in the range of 0.76–21.4 hours. We show a statistical comparison of Cassini, Voyager 1, and Voyager 2 data for occurrence probability in Central Meridian Longitude (CML) versus Io phase and in CML versus Frequency. The main results are as follows: (1) the detailed frequency structures of non-Io-related components can be seen for different spacecraft's local time and Jovigraphic latitude, (2) the high frequency of HOM extends up to near 10 MHz, and (3) a new DAM component, named the non-Io-D, appears from 40° to 60° CML in the frequency range of 7–11 MHz. On the basis of additional information of different behaviors of non-Io-B and non-Io-A structures in longitude depending on pre- and post-encounter of Cassini data, we improved the DAM angular beaming model that shows the cone half-angle of the emitting cone decreases as a function of frequency. We conclude that the changing beaming angle is not affected by Jovigraphic latitude of the spacecraft, but rather due to different local time of the source regions.

Ultrasound Biomicroscopy of Anterior Segment Accommodative Changes with Posterior Chamber Phakic Intraocular Lens in High Myopia

To investigate changes in anterior chamber depth (ACD) and the distance between the Visian implantable Collamer lens (ICL) and the crystalline lens during pharmacologic accommodation in high myopia. Prospective, comparative case series. Thirty-three phakic eyes of 18 high myopic patients (range, -8.63 to -23.86 diopters) with a mean age of 29 years (range, 20-44 years) were examined at least 1 year after ICL implantation. Ultrasound biomicroscopy was used to measure distance changes between the corneal endothelium, the ICL, and the crystalline lens after inducing pharmacologic accommodation with topical pilocarpine in 1 eye. The contralateral eye served as the control. Mean changes of ACD measured from the posterior corneal surface to the crystalline lens (ACD-L), from the posterior corneal surface to the anterior surface of the ICL (ACD-ICL), and the distance between the ICL and the crystalline lens (ICL-L) at the central and peripheral regions of the eye. For each eye, the ICL was in contact with the iris, but it was never in contact with the crystalline lens. At baseline, the mean distance between the ICL and the crystalline lens was 0.609 ± 0.165 mm at the central horizontal meridian, 0.588 ± 0.157 mm at the central vertical meridian, 0.281 ± 0.106 mm at the peripheral temporal sulcus, and 0.290 ± 0.098 mm at the peripheral nasal sulcus. After instillation of pilocarpine, a significant decrease in ICL-L was accompanied by a significant reduction in ACD-L and an increase in ACD-ICL (P<0.01). There were no significant changes in the control eyes (P>0.05). The central ICL-L reduction in the study group was significantly larger than that in the control group (P<0.01), but the peripheral ICL-L changes in the study group were not significantly different from those in the controls (P>0.05). During pharmacologic accommodation, the ICL and the crystalline lens came closer as the ICL was pushed backward by the iris as a result of pupillary constriction. Simultaneously, the anterior surface of the crystalline lens became more convex and moved forward. Reduction of the distances at peripheral sulci was not as obvious as at the center. The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Study on Key Technology in the Control Surveying of Jinan-Qingdao Highway’s South Line

The south line of Jinan Qingdao highway (which is short of Ji-Qing south line) spans 281km, whose geomorphy is mainly composed by mountain lands and hills. Approximately, it runs from east to west and straddles several projection zones. Systemic conformance of its layout and measurement, high precision in company with multifunctional planes and vertical control nets make up this project’s key technological problems. The report tells of the selection of central meridian in different areas of Ji-Qing south line and the layout, observation, adjustment, height fitting and precision statistics of GPS control network; the layout, observation, adjustment and precision counting of vertical control nets; Meanwhile, it analyses and discusses that how to guarantee the systemic conformance of coordinate system and vertical datum, together with eliminating the division of points’ coordinates and elevation values’ steps when primary survey design are being made synchronously in two super highways or in different areas of the same expressway.

HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS OBSERVATIONS OF EUROPA'S ATMOSPHERIC ULTRAVIOLET EMISSION AT EASTERN ELONGATION

We report results of a Hubble Space Telescope (HST) campaign with the Advanced Camera for Surveys to observe Europa at eastern elongation, i.e., Europa’s leading side, on 2008 June 29. With five consecutive HST orbits, we constrain Europa’s atmospheric Oi 1304 A and Oi 1356 A emissions using the prism PR130L. The total emissions of both oxygen multiplets range between 132 ± 14 and 226 ± 14 Rayleigh. An additional systematic error with values on the same order as the statistical errors may be due to uncertainties in modeling the reflected light from Europa’s surface. The total emission also shows a clear dependence of Europa’s position with respect to Jupiter’s magnetospheric plasma sheet. We derive a lower limit for the O2 column density of 6 × 10 18 m −2 . Previous observations of Europa’s atmosphere with the Space Telescope Imaging Spectrograph in 1999 of Europa’s trailing side show an enigmatic surplus of radiation on the anti-Jovian side within the disk of Europa. With emission from a radially symmetric atmosphere as a reference, we searched for an anti-Jovian versus sub-Jovian asymmetry with respect to the central meridian on the leading side and found none. Likewise, we searched for departures from a radially symmetric atmospheric emission and found an emission surplus centered around 90 ◦ west longitude, for which plausible mechanisms exist. Previous work about the possibility of plumes on Europa due to tidally driven shear heating found longitudes with strongest local strain rates which might be consistent with the longitudes of maximum UV emissions. Alternatively, asymmetries in Europa’s UV emission can also be caused by inhomogeneous surface properties, an optically thick atmospheric contribution of atomic oxygen, and/or by Europa’s complex plasma interaction with Jupiter’s magnetosphere.

Energetic storm particle events in coronal mass ejection-driven shocks

[1] We investigate the variability in the occurrence of energetic storm particle (ESP) events associated with shocks driven by coronal mass ejections (CMEs). The interplanetary shocks were detected during the period from 1996 to 2006. First, we analyze the CME properties near the Sun. The CMEs with an ESP-producing shock are faster (〈VCME〉 = 1088 km/s) than those driving shocks without an ESP event (〈VCME〉 = 771 km/s) and have a larger fraction of halo CMEs (67% versus 38%). The Alfvenic Mach numbers of shocks with an ESP event are on average 1.6 times higher than those of shocks without. We also contrast the ESP event properties and frequency in shocks with and without a type II radio burst by dividing the shocks into radio-loud (RL) and radio-quiet (RQ) shocks, respectively. The shocks seem to be organized into a decreasing sequence by the energy content of the CMEs: RL shocks with an ESP event are driven by the most energetic CMEs, followed by RL shocks without an ESP event, then RQ shocks with and without an ESP event. The ESP events occur more often in RL shocks than in RQ shocks: 52% of RL shocks and only ∼33% of RQ shocks produced an ESP event at proton energies above 1.8 MeV; in the keV energy range the ESP frequencies are 80% and 65%, respectively. Electron ESP events were detected in 19% of RQ shocks and 39% of RL shocks. In addition, we find that (1) ESP events in RQ shocks are less intense than those in RL shocks; (2) RQ shocks with ESP events are predominately quasi-perpendicular shocks; (3) their solar sources are located slightly to the east of the central meridian; and (4) ESP event sizes show a modest positive correlation with the CME and shock speeds. The observation that RL shocks tend to produce more frequently ESP events with larger particle flux increases than RQ shocks emphasizes the importance of type II bursts in identifying solar events prone to producing high particle fluxes in the near-Earth space. However, the trend is not definitive. If there is no type II emission, an ESP event is less likely but not absent. The variability in the probability and size of ESP events most likely reflects differences in the shock formation in the low corona and changes in the properties of the shocks as they propagate through interplanetary space and the escape efficiency of accelerated particles from the shock front.

‘Active Longitudes’ in the Heliomagnetic Reference Frame

A new coordinate system – heliomagnetic reference frame – has been proposed in which the great circle passes through the solar pole and the north pole of the magnetic dipole is considered as the central meridian. It is shown that, in the new coordinate system, the active longitudes are defined much more clearly, are more stable in time, and are interlaced every 11 years.

DIFFERENTIAL EMISSION MEASURE ANALYSIS OF A POLAR CORONAL HOLE DURING THE SOLAR MINIMUM IN 2007

We have performed a differential emission measure (DEM) analysis for a polar coronal hole observed during solar minimum in 2007. Five observations are analyzed spanning the coronal hole from the central meridian to the boundary with the quiet-Sun corona. The observed heights ranged from 1.05 to 1.20 R ☉. The analysis shows that the plasma is not strictly isothermal anywhere, but rather has a high-temperature component that extends up to log T(K) = 6.2-6.3. The size and importance of this component depend on location, and its evolving magnitude with height marks the boundary between the coronal hole and the quiet corona, where it becomes dominant. The DEM of the coronal hole plasma below log T(K) = 6.0 decreases faster with height than that of the high-temperature component. We discuss the possible nature of the high-temperature component. Our results highlight the potential limitations of isothermal analyses. Such methods actually measure a DEM-weighted average temperature and as a result can infer artificial temperature gradients. Assuming the gas is isothermal along the line of sight can also yield incorrect electron densities. By revealing structures along the line of sight, a DEM analysis can also be used to more reliably interpret electron temperature and density measurements.