Chiral Fermions on the Lattice

A recently proposed method for regularizing chiral gauge theories non-perturbatively is discussed in detail. The result is an effective action which can be computed from the lattice gauge field, and which is suited for numerical simulations.Comment: Talk given by G. Schierholz at Yukawa International Seminar on Non-Perturbative QCD: Structure of the QCD Vacuum (YKIS97), Kyoto, December 1997; typos correcte

Remote determination of auroral energy characteristics during substorm activity

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94740/1/grl10101.pd

Technique to retrieve solar EUV flux and neutral thermospheric O, O2, N2, and temperature from airglow measurements

We describe a method for retrieving neutral thermospheric composition and solar EUV flux from optical measurements of the O(+)(P-2) 732 nm and O(D-1) 630 nm airglow emissions. The parameters retrieved are the neutral temperature, the O, L2, and N2 density profiles, and a scaling factor for the solar EUV flux spectrum. The temperature, solar EUV flux scaling factor, and atomic oxygen density are first retrieved from the 732 nm emission, which are then used with the 630 nm emission to retrieve the O2 and N2 densities. Between the altitudes of 200 and 400 km the retrieval technique is able to statistically retrieve values to within 3.1% for thermospheric temperature, 3.3% for atomic oxygen, 2.3% for molecular oxygen, and 2.4% for molecular nitrogen. The solar EUV flux scaling factor has a retrieval error of 5.1%. We also present the results of retrievals using existing data taken from both groundbased and spacebased instruments. These include airglow data taken by the Visible Airglow Experiment on the Atmospheric Explorer spacecraft and the Imaging Spectrometric Observatory flown on the ATLAS 1 shuttle mission in 1992

Assessment of ionospheric Joule heating by GUMICS-4 MHD simulation, AMIE, and satellite-based statistics: towards a synthesis

We investigate the Northern Hemisphere Joule heating from several observational and computational sources with the purpose of calibrating a previously identified functional dependence between solar wind parameters and ionospheric total energy consumption computed from a global magnetohydrodynamic (MHD) simulation (Grand Unified Magnetosphere Ionosphere Coupling Simulation, GUMICS-4). In this paper, the calibration focuses on determining the amount and temporal characteristics of Northern Hemisphere Joule heating. Joule heating during a substorm is estimated from global observations, including electric fields provided by Super Dual Auroral Network (SuperDARN) and Pedersen conductances given by the ultraviolet (UV) and X-ray imagers on board the Polar satellite. Furthermore, Joule heating is assessed from several activity index proxies, large statistical surveys, assimilative data methods (AMIE), and the global MHD simulation GUMICS-4. We show that the temporal and spatial variation of the Joule heating computed from the GUMICS-4 simulation is consistent with observational and statistical methods. However, the different observational methods do not give a consistent estimate for the magnitude of the global Joule heating. We suggest that multiplying the GUMICS-4 total Joule heating by a factor of 10 approximates the observed Joule heating reasonably well. The lesser amount of Joule heating in GUMICS-4 is essentially caused by weaker Region 2 currents and polar cap potentials. We also show by theoretical arguments that multiplying independent measurements of averaged electric fields and Pedersen conductances yields an overestimation of Joule heating.<br><br> <b>Keywords.</b> Ionosphere (Auroral ionosphere; Modeling and forecasting; Electric fields and currents

Compensation for Spherical Geometric and Absorption Effects on Lower Thermospheric Emission Intensities Derived from High Earth Orbit Images

Remote sensing of the atmosphere from high earth orbit is very attractive due to the large field of view obtained and a true global perspective. This viewpoint is complicated by earth curvature effects so that slant path enhancement and absorption effects, small from low earth orbit, become dominant even at small nadir view angles. The effect is further complicated by the large range of local times and solar zenith angles in a single image leading to a modulation of the image intensity by a significant portion of the diurnal height variation of the absorbing layer. The latter effect is significant in particular for mesospheric, stratospheric and auroral emissions due to their depth in the atmosphere. As a particular case, the emissions from atomic oxygen (130.4 and 135.6 nm) and molecular nitrogen (two LBH bands, LBHS from 140 to 160 nm and LBHL from 160 to 180 nm) as viewed from the Ultraviolet Imager (UVI) are examined. The LBH emissions are of particular interest since LBHS has significant 02 absorption while LBHL does not, In the case of auroral emissions this differential absorption, well examined in the nadir, gives information about the height of the emission and therefore the energy of the precipitating particles. Using simulations of the viewing geometry and images from the UVI we examine these effects and obtain correction factors to adjust to the nadir case with a significant improvement of the derived characteristic energy. There is a surprisingly large effect on the images from the 02 diurnal layer height changes. An empirical compensation to the nadir case is explored based on the local nadir and local zenith angles for each portion of the image. These compensations are demonstrated as applied to the above emissions in both auroral and dayglow images and compared to models. The extension of these findings to other instruments, emissions and spectral regions is examined

Ionospheric conductances derived from satellite measurements of auroral UV and X-ray emissions, and ground-based electromagnetic data: a comparison

International audienceGlobal instantaneous conductance maps can be derived from remote sensing of UV and X-ray emissions by the UVI and PIXIE cameras on board the Polar satellite. Another technique called the 1-D method of characteristics provides mesoscale instantaneous conductance profiles from the MIRACLE ground-based network in Northern Scandinavia, using electric field measurements from the STARE coherent scatter radar and ground magnetometer data from the IMAGE network. The method based on UVI and PIXIE data gives conductance maps with a resolution of ~800km in space and ~4.5min in time, while the 1-D method of characteristics establishes conductances every 20s and with a spatial resolution of ~50km. In this study, we examine three periods with substorm activity in 1998 to investigate whether the two techniques converge when the results from the 1-D method of characteristics are averaged over the spatial and temporal resolution of the UVI/PIXIE data. In general, we find that the calculated conductance sets do not correlate. However, a fairly good agreement may be reached when the ionosphere is in a state that does not exhibit strong local turbulence. By defining a certain tolerance level of turbulence, we show that 14 of the 15 calculated conductance pairs during relatively uniform ionospheric conditions differ less than ±30%. The same is true for only 4 of the 9 data points derived when the ionosphere is in a highly turbulent state. A correlation coefficient between the two conductance sets of 0.27 is derived when all the measurements are included. By removing the data points from time periods when too much ionospheric turbulence occurs, the correlation coefficient raises to 0.57. Considering the two very different techniques used in this study to derive the conductances, with different assumptions, limitations and scale sizes, our results indicate that simple averaging of mesoscale results allows a continuous transition to large-scale results. Therefore, it is possible to use a combined approach to study ionospheric events with satellite optical and ground-based electrodynamic data of different spatial and temporal resolutions. We must be careful, though, when using these two techniques during disturbed conditions. The two methods will only give results that systematically converge when relatively uniform conditions exist

Instantaneous ionospheric global conductance maps during an isolated substorm

International audienceData from the Polar Ionospheric X-ray Imager (PIXIE) and the Ultraviolet Imager (UVI) on board the Polar satellite have been used to provide instantaneous global conductance maps. In this study, we focus on an isolated substorm event occurring on 31 July 1997. From the PIXIE and the UVI measurements, the energy spectrum of the precipitating electrons can be derived. By using a model of the upper atmosphere, the resulting conductivity values are generated. We present global maps of how the 5 min time-averaged height-integrated Hall and Pedersen conductivities vary every 15 min during this isolated substorm. The method presented here enables us to study the time development of the conductivities, with a spatial resolution of ~ 700 km. During the substorm, a single region of enhanced Hall conductance is observed. The Hall conductance maximum remains situated between latitudes 64 and 70 corrected geomagnetic (CGM) degrees and moves eastward. The strongest conductances are observed in the pre-midnight sector at the start of the substorm expansion. Toward the end of the substorm expansion and into the recovery phase, we find the Hall conductance maximum in the dawn region. We also observe that the Hall to Pedersen conductance ratio for the regions of maximum Hall conductance is increasing throughout the event, indicating a hardening of the electron spectrum. By combining PIXIE and UVI measurements with an assumed energy distribution, we can cover the whole electron energy range responsible for the conductances. Electrons with energies contributing most to the Pedersen conductance are well covered by UVI while PIXIE captures the high energetic component of the precipitating electrons affecting the Hall conductance. Most statistical conductance models have derived conductivities from electron precipitation data below approximately 30 keV. Since the intensity of the shortest UVI-wavelengths (LBHS) decreases significantly at higher electron energies, the UVI electron energy range is more or less comparable with the energy ranges of the statistical models. By calculating the conductivities from combined PIXIE and UVI measurements to compare with the conductivities from using UVI data only, we observe significant differences in the Hall conductance. The greatest differences are observed in the early evening and the late morning sector. We therefore suggest that the existing statistical models underestimate the Hall conductance

Lifetime Studies at Metrology Light Source and ANKA

Abstract The Metrology Light Source (MLS), situated in Berlin (Germany) is an electron storage ring operating from 105 MeV to 630 MeV and is serving as the national primary radiation source standard from the near infrared to the extrem ultraviolet spectral region INTRODUCTION To provide users of synchrotron radiation with temporally stable experimental conditions, the lifetime τ of the stored beam with current I is a parameter of concern. This is valid for machines with a decaying beam such as ANKA and MLS, but as well for machines operated in top up mode such as BESSY II in Berlin. In 2012, the standard user operation at MLS yielded a lifetime of 3.5 hours at 150 mA beam current. Although reasonable due to the energy, this is a low value compared to 16 hours at ANKA and it would benefit the users of synchrotron radiation if it could be improved. THEORY There are two major loss mechanisms determining the lifetime of the electrons in an accelerator: The scattering of the electrons with residual gas atoms and the scattering of the electrons with other electrons within the bunch. The latter is known as the "Touschek effect", named after Bruno Touschek who first observed the effect at the small AdA electron-positron collider. The two contributions are called gas lifetime and Touschek lifetime respectively. The gas lifetime depends on the pressure P and a scattering cross section σ gas for particle losses, for which the interested reader is referred to The cross section itself is a function of the acceptance of the accelerator δ acc = Δp max /p 0 , while the pressure P depends in some respect on the beam current. The electrons in a bunch perform transverse betatron oscillations. Being an incoherent motion, this leads to * [email protected] Coulomb scattering. During the scattering process, transverse momentum gets transferred to longitudinal momentum. If the particles momentum deviation exceeds the momentum acceptance it will be lost. The resulting Touschek lifetime depends on the rate of scattering processes and therefore on the density within the bunch, i.e. on the bunch volume and the bunch current. Furthermore, it depends on the momentum acceptance with the power of three with σ x,y,s being the rms-bunch sizes and length and D(ξ) being a slowly varying function with respect to the acceptance δ acc . D also depends on the optical functions around the ring through ξ. The loss rates from Touschek effect and gas scattering add to the total loss rate 1/τ which can be measured. Multiplying the number of particles N (or the stored current I) to the Touschek lifetime τ T results in a constant: N · τ T = const. Therefore, when plotting I · τ for a Touschek dominated lifetime a constant can be expected with respect to current. Acceptance Touschek lifetime and gas lifetime depend on the acceptance of the accelerator. Two acceptances are important here, and whichever is the smallest is the limiting one: • RF-acceptance • Geometrical acceptance. The RF-acceptance δ acc,RF approximately depends on the applied cavity voltage V as [4] The geometrical acceptance depends on the minimal aperture of the vacuum chamber a(s) and the dimensions of the beam. For MLS a first order approximation considering only the horizontal plane is: with D x being the horizontal dispersion function. In the upper plot of Predictions In the lower part of EXPERIMENT In LIFETIME IMPROVEMENT The lifetime at MLS can be improved if the acceptance is only RF-limited even to larger voltages than 300 kV. To do so, the geometrical acceptance has to be improved. In order to find optics with an increased Touschek lifetime, brute force optics scans using a Fortran code were performed In Eq. 4, the horizontal dispersion function D x is in the denominator. By decreasing the dispersion function at the place with minimum aperture, the geometrical acceptance δ acc,geom can be improved. At MLS each quadrupole is powered independently. By tuning some quadrupoles of one family against the remaining ones of that family, the dispersion function was tuned to be zero at the septum. In The peak lifetime with respect to cavity voltage is now located at 500 kV, being the maximum applicable cavity voltage at the moment. In The total lifetime increase is as high as 80 %. By solely increasing the acceptance, and the change in optical functions, the lifetime would have been expected to increase by about 30 %. An explanation for the additional increase could be a strong halo around the beam due to intra-beam scattering. With this, the effect of leading the beam through the centre of the vacuum chamber at the septum could be explained as well. CONCLUSION AND OUTLOOK The theory of the Touschek effect describes the dependencies on energy and acceptance well. By understanding the different loss mechanisms and the methods to manipulate the different acceptances, it was possible to generate a new user optics with an by 80 % improved lifetime. To completely explain the total lifetime increase, further measurements are needed. To further increase the lifetime, alternate optics determined by optics scans will be tested ACKNOWLEDGMENT The authors like to thank Andreas Jankowiak (HZB) and Gerhard Ulm (PTB) for supporting this work. REFERENCES [1] R. Klein et al., Phys. Rev. ST-AB 11, 110701, 2008. [2] A.-S. Müller et al., "Energy Calibration Of The ANKA Storage Ring", Proceedings of EPAC 2004. [3] T. Goetsch, "Lifetime Studies at Metrology Light Source and Angströmquelle Karlsruhe" -Diploma Thesis, Karlsruhe Institut für Technologie, May 2013. [4] M. Sands, "The Physics Of Electron Storage Rings -An Introduction", National Technical Information Service, Springfield, Virginia, 1970. [5] J. le Duff, "Current And Current Density Limitations In Existing Electron Storage Rings", In: Nucl. Instr. and Meth. in Physics Research, 1985

Evidence for Supernova Signatures in the Spectrum of the Late-time Bump of the Optical Afterglow of GRB 021211

We present photometric and spectroscopic observations of the gamma-ray burst GRB 021211 obtained during the late stages of its afterglow. The light curve shows a rebrightening occurring ~25 days after the GRB. The analysis of a VLT spectrum obtained during the bump (27 days after the GRB) reveals a suggestive resemblance with the spectrum of the prototypical type-Ic SN 1994I, obtained about ~10 days past maximum light. Particularly we have measured a strong, broad absorption feature at 3770 A, which we have identified with Ca II blueshifted by ~14400 km/s, thus indicating that a supernova (SN) component is indeed powering the `bump' in the afterglow decay. Assuming SN 1994I as a template, the spectroscopic and photometric data together indicate that the SN and GRB explosions were at most separated by a few days. Our results suggest that GRBs might be associated also to standard type-Ic supernovae.Comment: 6 pages, 4 color figures. Accepted for publication in A&A Letters. Fig. 4 does not appair in the A&A version due to space restrictions. Includes aa.cls and txfonts.st