Dynamic auroral structure in the vicinity of the polar cusp - multipoint observations during southward and northward IMF

The low- and high-latitude boundary layers of the earth's magnetosphere [low-latitude boundary layer (LLBL) and mantle] play important roles in transferring momentum and energy from the solar wind to the magnetosphere-ionosphere system. Particle precipitation, field-aligned current, auroral emission, ionospheric ion drift and ground magnetic perturbations are among the low-altitude parameters that show signatures of various plasma processes in the LLBL and the magnetopause current layer. Magnetic merging events, Kelvin-Helmholtz waves, and pressure pulses excited by the variable solar wind/magnetosheath plasma are examples of boundary phenomena that may be coupled to the ionosphere via field-aligned currents. Optical auroral observation, by photometry and all-sky TV cameras, is a unique technique for investigating the spatial and temporal structure of the electron precipitation associated with such phenomena. However, the distinction between the different boundary layer plasma populations cannot in general be unambiguously determined by optics alone. Additional information, such as satellite observations of particle boundaries and field-aligned currents, is needed in order to identify the plasma source(s) and the magnetosphere-ionosphere coupling mode(s). Two categories of auroral activity/structure in the vicinity of the polar cusp are discussed in this paper, based on combined ground and satellite data. In one case, the quasi-periodic sequence of auroral events at the polar cap boundary involves accelerated electrons (< 1 keV) moving poleward (< 1 km s-1) and azimuthally along the persistent cusp/cleft arc poleward boundary with velocities (< 4 km s-1), comparable to the local ionospheric ion drift during periods of southward IMF. A critical question is whether or not the optical events signify a corresponding plasma flow across the open/closed field line boundary in such cases. Near-simultaneous observations of magnetopause flux transfer events (FTEs) and such optical/ion drift events are reported. The reverse pattern of motion of discrete auroral forms is observed during positive interplanetary magnetic field (IMF) B(Z), i.e. equatorward motion into the cusp/cleft background arc from the poleward edge. Combined satellite and ground-based information for the latter cases indicate a source mechanism, poleward of the cusp at the high-latitude magnetopause or plasma mantle, giving rise to strong momentum transfer and electron precipitation structures within a approximately 200 km-wide latitudinal zone at the cusp/cleft poleward boundary. The striking similarities of auroral electrodynamics in the cleft/mantle region during northward and southward IMF indicate that a qualitatively similar solar wind-magnetosphere coupling mode is operating. It is suggested that, in both cases, the discrete auroral forms represent temporal/spatial structure of larger-scale convection over the polar magnetosphere.

Auroral bright spot sequence near 1400 MLT: Coordinated optical and ion drift observations

Optical observations of a dayside auroral brightening sequence, by means of all-sky TV cameras and meridian scanning photometers, have been combined with EISCAT ion drift observations within the same invariant latitude-MLT sector. The observations were made during a January 1989 campaign by utilizing the high F region ion densities during the maximum phase of the solar cycle. The characteristic intermittent optical events, covering ∼300 km in east-west extent, move eastward (antisunward) along the poleward boundary of the persistent background aurora at velocities of ∼1.5 km s−1 and are associated with ion flows which swing from eastward to westward, with a subsequent return to eastward, during the interval of a few minutes when there is enhanced auroral emission within the radar field of view. The breakup of discrete auroral forms occurs at the reversal (negative potential) that forms between eastward plasma flow, maximizing near the persistent arc poleward boundary, and strong transient westward flow to the south. The reported events, covering a 35 min interval around 1400 MLT, are embedded within a longer period of similar auroral activity between 0830 (1200 MLT) and 1300 UT (1600 MLT). These observations are discussed in relation to recent models of boundary layer plasma dynamics and the associated magnetosphere-ionosphere coupling. The ionospheric events may correspond to large-scale wave like motions of the low-latitude boundary layer (LLBL)/plasma sheet (PS) boundary. On the basis of this interpretation the observed spot size, speed and repetition period (∼10 min) give a wavelength (the distance between spots) of ∼900 km in the present case. The events can also be explained as ionospheric signatures of newly opened flux tubes associated with reconnection bursts at the magnetopause near 1400 MLT. We also discuss these data in relation to random, patchy reconnection (as has recently been invoked to explain the presence of the sheathlike plasma on closed field lines in the LLBL). In view of the lack of IMF data, and the existing uncertainty on the location of the open-closed field line boundary relative to the optical events, an unambiguous discrimination between the different alternatives is not easily obtained.

Ion flows and heating at a contracting polar-cap boundary: GISMOS observations indicating viscous-like interaction on the flanks of the magnetotail

This paper complements that in this issue by Clauer et al. concerning the international GISMOS campaign of 3–5 June 1987. From a detailed study of the EISCAT data, the polar-cap boundary, as defined by an almost shear east-west convection reversal, is found to contract across the EISCAT field of view between 04 and 07 MLT. An annulus of enhanced ion temperature and non-thermal plasma is observed immediately equatorward of the contracting boundary due to the lag in the response of the neutral-wind pattern to the change in ion flows. The ion flow inside the polar cap and at the boundary is shown to be relatively smooth, compared with that in the auroral oval, at 15-second resolution. The flow at the boundary is directed poleward, with velocities which exceed that of the boundary itself. The effect of velocity shears on the beamswinging technique used to derive the ion flows has been analysed in detail and it is found that spurious flows across a moving boundary can be generated. However, these are much smaller than the observed flows into the polar cap and cannot explain the 7 kV potential difference across the observed segment of the cap boundary between 04:30–06:30 UT. The ion temperature enhancements at the two observing azimuths is used to define the boundary orientation. The results are consistent with recent observations of slow anti-sunward flow of closed field lines on the flanks of the geomagnetic tail, which appears to be generated by some form of “viscous” coupling to the magnetosheath plasma.

Ion flows and heating at a contracting polar-cap boundary

Data recorded by the POLAR experiment run on the EISCAT radar during the international GISMOS campaign of 3–5 June 1987 are studied in detail. The polar-cap boundary, as denned by an almost shear East-West convection reversal, was observed to jump southward across the EISCAT field of view in two steps at 02:00 and 03:00 Magnetic Local Time and subsequently to contract back between 04:00 and 07:00 M.L.T. An annulus of enhanced ion temperature and non-thermal plasma was observed immediately equatorward of the contracting boundary due to the lag in the response of the neutral-wind pattern to the change in ion flows. The ion flow at the boundary is shown to be relatively smooth at 15 s resolution and directed northward, with velocities which exceed that of the boundary itself. The effect of velocity shears on the beamswinging technique used to derive the ion flows is analyzed in detail and it is shown that, for certain orientations of the cap boundary, spurious flows into the cap can be generated. However, these are much smaller than the observed flows into the polar cap and cannot explain the potential difference across the observed segment of the cap boundary (extending over 2 h of M.L.T.) which is roughly 7 kV. Similarly, an observed slowing of the zonal flow near the boundary cannot be explained as an error introduced by the use of the beamswinging technique. The results could be interpreted as being due to reconnection occurring on the dawn flank of the magnetopause (mapping to the polar cap at 04:30 06:30 M.L.T.). However, they are more consistent with recent observations of slow anti-sunward flow of closed field lines on the flanks of the geomagnetic tail, which appears to be generated by some form of “viscous” coupling to the magnetosheath plasma.

Ionospheric ion upwelling in the wake of flux transfer events at the dayside magnetopause

The effects of flux transfer events (FTE) on the dayside auroral ionosphere are studied, using a simple twin-vortex model of induced ionospheric plasma flow. It is shown that the predicted and observed velocities of these flows are sufficient to drive nonthermal plasma in the F region, not only within the newly opened flux tube of the FTE, but also on the closed, or "old" open, field lines around it. In fact, with the expected poleward neutral wind, the plasma is more highly nonthermal on the flanks of, but outside, the open flux tube: EISCAT observations indicate that plasma is indeed driven into nonthermal distributions in these regions. The nonthermal plasma is thereby subject to additional upforce due to the resulting ion temperature anisotropy and transient expansion due to Joule heating and also to ion accelerations associated with the FTE field aligned current system. Any upflows produced on closed field lines in the vicinity of the FTE are effectively bunched-up in the "wake" of the FTE. Observations from the AMPTE-UKS satellite at the magnetopause reveal ion upflows of energy ∼100 eV flowing out from the ionosphere on closed field lines which are only found in the wake of the FTE. Such flows are also only found shortly after two, out of all the FTEs observed by AMPTE-UKS. The outflow from the ionosphere is two orders of magnitude greater than predicted for the "classical" polar wind. It is shown that such ionospheric ion flows are only expected in association with FTEs on the magnetopause which are well removed from the sub-solar point-either towards dusk or, as in the UKS example discussed here, towards dawn. It is suggested that such ionospheric ions will only be observed if the center of the FTE open flux tube passes very close to the satellite. Consequently, we conclude the ion upflows presented here are probably driven by the second of two possible source FTEs and are observed at the satellite with a lag after the FTE which is less than their time-of-flight.

SURFACE ALFVEN WAVE DAMPING IN A THREE-DIMENSIONAL SIMULATION OF THE SOLAR WIND

Here we investigate the contribution of surface Alfven wave damping to the heating of the solar wind in minima conditions. These waves are present in the regions of strong inhomogeneities in density or magnetic field (e.g., the border between open and closed magnetic field lines). Using a three-dimensional (3D) magnetohydrodynamics (MHD) model, we calculate the surface Alfven wave damping contribution between 1 and 4 R(circle dot) (solar radii), the region of interest for both acceleration and coronal heating. We consider waves with frequencies lower than those that are damped in the chromosphere and on the order of those dominating the heliosphere: 3 x 10(-6) to 10(-1) Hz. In the region between open and closed field lines, within a few R(circle dot) of the surface, no other major source of damping has been suggested for the low frequency waves we consider here. This work is the first to study surface Alfven waves in a 3D environment without assuming a priori a geometry of field lines or magnetic and density profiles. We demonstrate that projection effects from the plane of the sky to 3D are significant in the calculation of field line expansion. We determine that waves with frequencies >2.8 x 10(-4) Hz are damped between 1 and 4 R(circle dot). In quiet-Sun regions, surface Alfven waves are damped at further distances compared to active regions, thus carrying additional wave energy into the corona. We compare the surface Alfven wave contribution to the heating by a variable polytropic index and find it as an order of magnitude larger than needed for quiet-Sun regions. For active regions, the contribution to the heating is 20%. As it has been argued that a variable gamma acts as turbulence, our results indicate that surface Alfven wave damping is comparable to turbulence in the lower corona. This damping mechanism should be included self-consistently as an energy driver for the wind in global MHD models.

THREE-DIMENSIONAL NUMERICAL SIMULATIONS OF MAGNETIZED WINDS OF SOLAR-LIKE STARS

By means of self-consistent three-dimensional magnetohydrodynamics (MHD) numerical simulations, we analyze magnetized solar-like stellar winds and their dependence on the plasma-beta parameter (the ratio between thermal and magnetic energy densities). This is the first study to perform such analysis solving the fully ideal three-dimensional MHD equations. We adopt in our simulations a heating parameter described by gamma, which is responsible for the thermal acceleration of the wind. We analyze winds with polar magnetic field intensities ranging from 1 to 20 G. We show that the wind structure presents characteristics that are similar to the solar coronal wind. The steady-state magnetic field topology for all cases is similar, presenting a configuration of helmet streamer-type, with zones of closed field lines and open field lines coexisting. Higher magnetic field intensities lead to faster and hotter winds. For the maximum magnetic intensity simulated of 20 G and solar coronal base density, the wind velocity reaches values of similar to 1000 km s(-1) at r similar to 20r(0) and a maximum temperature of similar to 6 x 10(6) K at r similar to 6r(0). The increase of the field intensity generates a larger ""dead zone"" in the wind, i.e., the closed loops that inhibit matter to escape from latitudes lower than similar to 45 degrees extend farther away from the star. The Lorentz force leads naturally to a latitude-dependent wind. We show that by increasing the density and maintaining B(0) = 20 G the system recover back to slower and cooler winds. For a fixed gamma, we show that the key parameter in determining the wind velocity profile is the beta-parameter at the coronal base. Therefore, there is a group of magnetized flows that would present the same terminal velocity despite its thermal and magnetic energy densities, as long as the plasma-beta parameter is the same. This degeneracy, however, can be removed if we compare other physical parameters of the wind, such as the mass-loss rate. We analyze the influence of gamma in our results and we show that it is also important in determining the wind structure.

On certain curves of genus three in characteristic two

We study curves of genus 3 over algebraically closed fields of characteristic 2 with the canonical theta characteristic totally supported in one point. We compute the moduli dimension of such curves and focus on some of them which have two Weierstrass points with Weierstrass directions towards the support of the theta characteristic. We answer questions related to order sequence and Weierstrass weight of Weierstrass points and the existence of other Weierstrass points with similar properties.

Partial projective representations and partial actions II

This paper is a continuation of Dokuchaev and Novikov (2010) [8]. The interaction between partial projective representations and twisted partial actions of groups considered in Dokuchaev and Novikov (2010) [8] is treated now in a categorical language. In the case of a finite group G, a structural result on the domains of factor sets of partial projective representations of G is obtained in terms of elementary partial actions. For arbitrary G we study the component pM'(G) of totally-defined factor sets in the partial Schur multiplier pM(G) using the structure of Exel's semigroup. A complete characterization of the elements of pM'(G) is obtained for algebraically closed fields. (C) 2011 Elsevier B.V. All rights reserved.

Assessment of the oculomotor response in human factor environments

The need to measure the response of the oculomotor system, such as ocular accommodation, accurately and in real-world environments is essential. New instruments have been developed over the past 50 years to measure eye focus including the extensively utilised and well validated Canon R-1, but in general these have had limitations such as a closed field-of-view, a poor temporal resolution and the need for extensive instrumentation bulk preventing naturalistic performance of environmental tasks. The use of photoretinoscopy and more specifically the PowerRefractor was examined in this regard due to its remote nature, binocular measurement of accommodation, eye movement and pupil size and its open field-of-view. The accuracy of the PowerRefractor to measure refractive error was on averaging similar, but more variable than subjective refraction and previously validated instrumentation. The PowerRefractor was found to be tolerant to eye movements away from the visual axis, but could not function with small pupil sizes in brighter illumination. The PowerRefractor underestimated the lead of accommodation and overestimated the slope of the accommodation stimulus response curve. The PowerRefractor and the SRW-5000 were used to measure the oculomotor responses in a variety of real-world environment: spectacles compared to single vision contract lenses; the use of multifocal contact lenses by pre-presbyopes (relevant to studies on myopia retardation); and ‘accommodating’ intraocular lenses. Due to the accuracy concerns with the PowerRefractor, a purpose-built photoretinoscope was designed to measure the oculomotor response to a monocular head-mounted display. In conclusion, this thesis has shown the ability of photoretinoscopy to quantify changes in the oculomotor system. However there are some major limitations to the PowerRefractor, such as the need for individual calibration for accurate measures of accommodation and vergence, and the relatively large pupil size necessary for measurement.

Connected Hopf algebras of finite Gelfand-Kirillov dimension

Following the seminal work of Zhuang, connected Hopf algebras of finite GK-dimension over algebraically closed fields of characteristic zero have been the subject of several recent papers. This thesis is concerned with continuing this line of research and promoting connected Hopf algebras as a natural, intricate and interesting class of algebras. We begin by discussing the theory of connected Hopf algebras which are either commutative or cocommutative, and then proceed to review the modern theory of arbitrary connected Hopf algebras of finite GK-dimension initiated by Zhuang. We next focus on the (left) coideal subalgebras of connected Hopf algebras of finite GK-dimension. They are shown to be deformations of commutative polynomial algebras. A number of homological properties follow immediately from this fact. Further properties are described, examples are considered and invariants are constructed. A connected Hopf algebra is said to be "primitively thick" if the difference between its GK-dimension and the vector-space dimension of its primitive space is precisely one . Building on the results of Wang, Zhang and Zhuang,, we describe a method of constructing such a Hopf algebra, and as a result obtain a host of new examples of such objects. Moreover, we prove that such a Hopf algebra can never be isomorphic to the enveloping algebra of a semisimple Lie algebra, nor can a semisimple Lie algebra appear as its primitive space. It has been asked in the literature whether connected Hopf algebras of finite GK-dimension are always isomorphic as algebras to enveloping algebras of Lie algebras. We provide a negative answer to this question by constructing a counterexample of GK-dimension 5. Substantial progress was made in determining the order of the antipode of a finite dimensional pointed Hopf algebra by Taft and Wilson in the 1970s. Our final main result is to show that the proof of their result can be generalised to give an analogous result for arbitrary pointed Hopf algebras.

Slip line field analysis of a simple plane strain closed‐die forging

A slip line field is proposed for symmetrical single‐cavity closed‐die forging by rough dies. A compatible velocity field is shown to exist. Experiments were conducted using lead workpiece and rough dies. Experimentally observed flow and load were used to validate the proposed slip line field. The slip line field was used to simulate the process in the computer with the objective of studying the influence of flash geometry on cavity filling.

Closed string thermal torus from thermo field dynamics

In this Letter a topological interpretation for the string thermal vacuum in the thermo field dynamics (TFD) approach is given. As a consequence, the relationship between the imaginary time and TFD formalisms is achieved when both are used to study closed strings at finite temperature. The TFD approach starts by duplicating the system's degrees of freedom, defining an auxiliary (tilde) string. In order to lead the system to finite temperature a Bogoliubov transformation is implemented. We show that the effect of this transformation is to glue together the string and the tilde string to obtain a torus. The thermal vacuum appears as the boundary state for this identification. Also, from the thermal state condition, a Kubo-Martin-Schwinger condition for the torus topology is derived. © 2005 Elsevier B.V. All rights reserved.

Manifest electromagnetic duality in closed superstring field theory

The free action for massless Ramond-Ramond fields is derived from closed superstring field theory using the techniques of Siegel and Zwiebach. For the uncompactified Type IIB superstring, this gives a manifestly Lorentz-covariant action for a self-dual five-form field strength. Upon compactification to four dimensions, the action depends on a U(1) field strength from 4D N = 2 supergravity. However, unlike the standard Maxwell action, this action is manifestly invariant under the electromagnetic duality transformation which rotates F-mn into epsilon(mnpq)F(pq).