Constant Angular Width Research Articles

Coherence of Coronal Mass Ejections in Near-Earth Space

Interplanetary coronal mass ejections (ICMEs) primarily move radially as they propagate away from the Sun, maintaining approximately constant angular width with respect to the Sun. As ICMEs have typical angular widths of around 60^{circ }, plasma elements on opposite flanks of an ICME separate in the non-radial direction at a speed, v_{mathrm{G}}, roughly equal to the ICME radial speed. This rapid expansion is a limiting factor on the propagation of information across an ICME at the local Alfvén speed, v_{mathrm{A}}. In this study, the 1-AU properties of ICMEs are used to compute two measures of ICME coherence. The first is the angular separation for which v_{mathrm{G}} exceeds the local v_{mathrm{A}}. The second measure is the angular extent over which a wavefront can propagate as an ICME travels from a given heliocentric distance to 1 AU. For both measures, ICMEs containing magnetic clouds show greater coherence than non-cloud ICMEs. However, even for magnetic clouds, information is unable to propagate across the full span of the structure. Thus interactions of ICMEs with other solar wind structures in the heliosphere are likely to lead to localised distortion, rather than solid-body like deflection. For magnetic clouds, the coherence length scale is significantly greater near the centre of the spacecraft encounter than at the leading or trailing edges. This suggests that magnetic clouds may be more coherent, and thus less prone to distortion, along the direction of the magnetic flux-rope axis than in directions perpendicular to the axis.

An operational software tool for the analysis of coronagraph images: Determining CME parameters for input into the WSA‐Enlil heliospheric model

Coronal mass ejections (CMEs)—massive explosions of dense plasma that originate in the lower solar atmosphere and propagate outward into the solar wind—are the leading cause of significant space weather effects within Earth's environment. Computational models of the heliosphere such as WSA‐Enlil offer the possibility of predicting whether a given CME will become geo‐effective and, if so, the likely time of arrival at Earth. To be meaningful, such a forecast model is dependent upon accurately characterizing key parameters for the CME, notably its speed and direction of propagation, and its angular width. Studies by Zhao et al. (2002) and Xie et al. (2004) suggest that these key CME parameters can be deduced from geometric analysis of the elliptical “halo” forms observed in coronagraph images on spacecraft such as the Solar and Heliospheric Observatory (SOHO) and which result from a CME whose propagation is roughly toward or away from the observer. Both studies assume that the CME presents a circular cross‐section and maintains a constant angular width during its radial expansion, the so called “cone model.” Development work at the NOAA Space Weather Prediction Center (SWPC) has been concerned with building and testing software tools to allow forecasters to determine these CME parameters routinely within an operational context, a key aspect of transitioning the WSA‐Enlil heliospheric model into operations at the National Weather Service. We find “single viewpoint” cone analysis, while a useful start, to be highly problematic in many real‐world situations. In particular, it is extremely difficult to establish objectively the correct ellipse that should be applied to a given halo form and that small changes in the exact ellipse chosen can lead to large differences in the deduced CME parameters. The inaccuracies in the technique are particularly evident for analysis of the “nearly circular” elliptical forms which result from CMEs that are propagating directly toward the observer and are therefore the most likely to be geo‐effective. In working to resolve this issue we have developed a new three‐dimensional (3‐D) graphics‐based analysis system which seeks to reduce inaccuracies by analyzing a CME using coronagraph images taken concurrently by SOHO and also by the two Solar TErrestrial RElations Observatory (STEREO) spacecraft, which provide additional viewing locations well away from the Sun‐Earth line. The resulting “three view” technique has led to the development of the CME Analysis Tool (CAT), an operational software system in routine use at the SWPC as the primary means to determine CME parameters for input into the WSA‐Enlil model. Results from the operational WSA‐Enlil system are presented: utilizing CAT to provide CME input parameters, we show that, during the first year of operations at SWPC, the WSA‐Enlil model has forecasted the arrival of CMEs at Earth with an average error 7.5 h.

THE FIRST OBSERVATION OF A RAPIDLY ROTATING CORONAL MASS EJECTION IN THE MIDDLE CORONA

In this Letter, we present the first direct detection of a rotating coronal mass ejection (CME) in the middle corona (5-15 R ?). The CME rotation rate is 60? day-1, which is the highest rate reported yet. The Earth-directed event was observed by the STEREO/SECCHI and SOHO/LASCO instruments. We are able to derive the three-dimensional morphology and orientation of the CME flux rope by applying a forward-fitting model to simultaneous observations from three vantage points (SECCHI-A, -B, LASCO). Surprisingly, we find that even such rapidly rotating CME does not result in significant projection effects (variable angular width) in any single coronagraph view. This finding may explain the prevalent view of constant angular width for CMEs above 5 R ? and the lack of detections of rotating CMEs in the past. Finally, the CME is a stealth CME with very weak low corona signatures as viewed from Earth. It originated from a quiet-Sun neutral line. We tentatively attribute the fast rotation to a possible disconnection of one of the CME footpoints early in the eruption. We discuss the implications of such rotations to space weather prediction.

Comparing and Contrasting View-Based and 3D Models of Navigation

What does it mean to store a 3D location and hence to navigate towards it? Participants in our experiment carried out a homing task on the scale of a room (ie, 3-4m square) in an immersive virtual reality environment. In interval one, they were shown three very long coloured vertical poles from one viewing location with some head movement permitted. The poles were easily distinguishable and designed to have constant angular width irrespective of viewing distance. Participants were then transported (virtually) to another location in the scene, and in interval two they tried to navigate to the initial viewing point relative to the poles. The distributions of end-point errors on the ground plane differed significantly in shape and extent depending on pole configuration and goal location. We compared the ability of two types of models to predict these variations in the distribution of errors: (i) view-based models, based on simple features such as angles between poles from the cyclopean point, ratios of these angles, or various disparity measures and (ii) Cartesian models based on a probabilistic 3D reconstruction of the scene geometry. For our data, we find that view-based models capture important characteristics of the end-point distributions very well whereas 3D-based models fare less well. In some ways, a Cartesian model is a very particular case of a view-based model: the two are not as different from one another as they first appear.

Stereoscopic study of angle-dependent interlayer magnetoresistance in the organic conductorκ−(BEDT-TTF)2Cu(NCS)2

We stereoscopically studied the angle-dependent interlayer magnetoresistance ${R}_{zz}(\ensuremath{\theta},\ensuremath{\phi})$ of the organic conductor $\ensuremath{\kappa}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}\text{Cu}{(\text{NCS})}_{2}$. A pressure of 7 kbar was applied to render the sample fully metallic. The stereoscopic view enables us to distinguish between quasi-one-dimensional (Q1D) and quasi-two-dimensional (Q2D) types of mixed angular magnetoresistance oscillations. A crossover between Q1D features and Q2D features is observed at $\ensuremath{\phi}\ensuremath{\sim}35\ifmmode^\circ\else\textdegree\fi{}$ when the field rotates in the conducting plane. While the coherence peak in the Q1D region has a constant angular width, $1.4\ifmmode^\circ\else\textdegree\fi{}$, the width of the humplike peak in the Q2D region grows with inverse $\text{cos}\text{ }\ensuremath{\phi}$ dependence up to $25\ifmmode^\circ\else\textdegree\fi{}$. In addition, differences of the $\ensuremath{\phi}$-dependence of ${R}_{zz}^{\ensuremath{\theta}=0}$ and of its temperature dependence support the crossover behavior between Q1D and Q2D dominance.

Climate sensitivity to changes in solar insolation in a simple coupled climate model

A simple coupled ocean, atmosphere and sea-ice model is presented. The idealised model consists of a zonally averaged land and ocean strip of constant angular width extending from pole to pole. The meridional energy transport in the ocean is modelled by contributions from the large scale thermohaline overturning cells and from horizontal diffusive fluxes. The atmospheric meridional energy transports are parametrised as diffusive fluxes in addition to advective transports in the Hadley domain. This parametrisation resolves the equatorward moisture transport as well as the poleward transport of potential energy in the upper branch of the Hadley circulation. The model reproduces the annual averaged meridional energy transports in the climate system with a small number of free model parameters. The basic feedbacks between the three climatic components are studied by investigating the model's sensitivity towards reductions in the solar insolation. It is found that the meridional energy transport in the ocean does not amplify the ice albedo feedback. This has important implications for modelling the climate sensitivity in atmosphere-only models, as these would exaggerate the sensitivity to changes in the solar insolation if their parametrisations of the meridional energy transport are constrained by surface temperatures. The role of the dependence of the atmospheric transports on the meridional temperature gradient is shown to have a significant influence on the sensitivity on the coupled model, and the inclusion of seasonal cycles greatly increase the models sensitivity. The Hadley circulation does significantly alter the strength of the ice-albedo feedback in the coupled model. The idealised configuration of the model makes it a useful tool for studying the feedbacks in the ocean-atmosphere-sea ice system in the context of the "Snowball Earth" hypothesis.

Radial rearrangement, harmonic measures and extensions of Beurling's shove theorem

LetI be a union of finitely many closed intervals in [−1, 0). LetI↞ be a single interval of the form [−1, −a] chosen to have the same logarithmic length asI. LetD be the unit disc. Then, Beurling [8] has shown that the harmonic measure of the circle ∂D at the origin in the slit discD/I is increased ifI is replaced byI↞. We prove a number of cognate results and extensions. For instance, we show that Beurling's result remains true if the intervals inI are not just one-dimensional, but if they in fact constitute polar rectangles centred on the negative real axis and having some fixed constant angular width. In doing this, we obtain a new proof of Beurling's result. We also discuss a conjecture of Matheson and Pruss [25] and some other open problems.

Seafloor magnetic lineation off the Otway/West Tasmania Basins: ridge jumps and the subsidence history of the southeast Australian margins

The seafloor off the Otway/West Tasmanian Basins has an east‐west magnetic lineation attributable to seafloor spreading and notionally identified with the set of seafloor spreading anomalies A8‐A20. Anomaly A20 (45 Ma) lies immediately south of a magnetic quiet zone that extends northward past the continent‐ocean boundary (COB). The Southeast Indian Ocean has a constant angular width between the formerly conjugate margins of Australia and Antarctica, consistent with spreading that started along the entire margin about 96 Ma.The proximity of A20 to the Australian COB in some spreading ridge segments is therefore postulated as due to jumps of the spreading ridge to Australia with concomitant transfer of the older oceanic part of the Australian Plate to the Antarctic Plate. Accordingly, the age of the oldest seafloor at the COB in seven original ridge segments is estimated to step from about 96 to 82, 79, and 75 Ma. Break‐up marks a change in the subsidence of the margin from rapid, during rifting by contine...