A new technique for the real-time recovery of Fabry-Perot line profiles

A new analogue technique is proposed as a method of obtaining Fabry-Perot line profiles using an imaging photon detector. The technique employs the principle of replacing software with hardware, which increases speed and in this case removes problems due to quantisation errors. A further advantage of the system is that it allows the profile to be observed as the integration proceeds, something which was not possible using the digitised x and y coordinates. The ability to obtain the profile in real-time is of importance when recording from a light source whose intensity varies with time

Experimental statistics for undergraduates - revisited

The work described here is based on a series of articles (MacLeod et al 1976, MacLeod 1976, 1980) in which a laboratory radioactive source was used as a source of random events, which enabled an investigation of the Poisson distribution to be made, and was of considerable benefit in giving undergraduate students an understanding of counting statistics

Experimental statistics for undergraduates - revisited

The work described here is based on a series of articles (MacLeod et al 1976, MacLeod 1976, 1980) in which a laboratory radioactive source was used as a source of random events, which enabled an investigation of the Poisson distribution to be made, and was of considerable benefit in giving undergraduate students an understanding of counting statistics

A hybrid beam design for slow positron transport

The authors report on the design of a hybrid transport system for slow positrons based upon the high-intensity magnetically transported positron beam, which existed at the Brookhaven (BNL) high-flux beam reactor (HFBR) in 1986. The resulting modified transport, incorporating an initial electrostatic stage followed by a magnetic stage and completed by brightness enhancement moderation leading to a final electrostatic section, represented the minimum alteration of the existing beam to enable it to be used for crossed-beams differential scattering studies involving a variety of atomic and molecular systems, including atomic hydrogen. A recent proposal for constructing a very high-intensity positron beam at the Paul Scherrer Institute (PSI), based upon magnetic confinement premoderation, has rekindled interest in the conclusions of the study

Mesopause temperatures calculated from the O2(a1 Δg) twilight airglow emission recorded at Maynooth (53.2˚N, 6.4˚W)

Spectra of the O2(a1 Δg) airglow emission band at 1.27 μm have been recorded during twilight at Maynooth (53.2˚N, 6.4˚W) using a Fourier transform spectrometer. Synthetic spectra have been generated for comparison with the recorded data by assuming a particular temperature at the emitting altitude and modelling the absorption of each line in the band as it propagates downward through the atmosphere. The temperature used in generating the synthetic spectra was varied until an optimum fit was obtained between the recorded and synthetic data; this temperature was then attributed to the altitude of the emitting layer. Temperatures derived using this technique for 91 twilight periods over an 18-month period exhibit a strong seasonal behaviour with a maximum in winter and minimum in summer. Results from this study are compared with temperatures calculated from the OH(3,1) Meinel band recorded simultaneously. In winter OH temperatures exceed O2 values by about 10K, whereas the opposite situation pertains in summer; this result is interpreted in terms of a possible change in the altitude of the mesopause as a function of season. Estimates of the Twilight O2(0,0) total band intensity indicate that its intensity is lower and that its decay is more rapid in summer than in winter, in agreement with earlier observations

OH-equivalent temperatures derived from ACE-FTS and SABER temperature profiles – a comparison with OH*(3-1) temperatures from Maynooth (53.2 N, 6.4 W)

OH-equivalent temperatures were derived from all of the temperature profiles retrieved in 2004 and 2005 by the ACE-FTS instrument in a 5 degree band of latitude centred on a ground-based observing station at Maynooth. A globally averaged OH volume emission rate (VER) profile obtained from WINDII data was employed as a weighting function to compute the equivalent temperatures. The annual cycle of temperature thus produced was compared with the annual cycle of temperatures recorded at the ground-based station more than a decade earlier from the OH*(3-1) Meinel band. Both data sets showed excellent agreement in the absolute value of the temperature minimum (~162 K) and in its time of occurrence in the annual cycle at summer solstice. Away from mid-summer, however, the temperatures diverged and reach a maximum disagreement of more than 20K in mid-winter. Comparison of the Maynooth ground-based data with the corresponding results from two nearby stations in the same time-period indicated that the Maynooth data are consistent with other ground stations. The temperature difference between the satellite and ground-based datasets in winter was reduced to 14–15K by lowering the peak altitude of the weighting function to 84 km. An unrealistically low peak altitude would be required, however, to bring temperatures derived from the satellite into agreement with the ground-based data. OH equivalent temperatures derived from the SABER instrument using the same weighting function produced results that agreed well with ACE-FTS. When the OH 1.6μm VER profile measured by SABER was used as the weighting function, the OH equivalent temperatures increased in winter as expected but the summer temperatures were reduced resulting in an approximately constant offset of 8.6±0.8K between ground and satellite values with the ground values higher. Variability in both the altitude and width of the OH layer within a discernable seasonal variation were responsible for the changes introduced. The higher temperatures in winter were due to primarily to the lower altitude of the OH layer, while the colder summer temperatures were due to a thinner summer OH layer. We are not aware of previous reports of the effect of the layer width on ground-based temperatures. Comparison of OH-equivalent temperatures derived from ACE-FTS and SABER temperature profiles with OH*(3-1) temperatures from Wuppertal at 51.3 N which were measured during the same period showed a similar pattern to the Maynooth data from a decade earlier, but the warm offset of the ground values was lower at 4.5±0.5 K. This discrepancy between temperatures derived from ground-based instruments recording hydroxyl spectra and satellite borne instruments has been observed by other observers. Further work will be required by both the satellite and ground-based communities to identify the exact cause of this difference

OH-equivalent temperatures derived from ACE-FTS and SABER temperature profiles – a comparison with OH*(3-1) temperatures from Maynooth (53.2 N, 6.4 W)

OH-equivalent temperatures were derived from all of the temperature profiles retrieved in 2004 and 2005 by the ACE-FTS instrument in a 5 degree band of latitude centred on a ground-based observing station at Maynooth. A globally averaged OH volume emission rate (VER) profile obtained from WINDII data was employed as a weighting function to compute the equivalent temperatures. The annual cycle of temperature thus produced was compared with the annual cycle of temperatures recorded at the ground-based station more than a decade earlier from the OH*(3-1) Meinel band. Both data sets showed excellent agreement in the absolute value of the temperature minimum (~162 K) and in its time of occurrence in the annual cycle at summer solstice. Away from mid-summer, however, the temperatures diverged and reach a maximum disagreement of more than 20K in mid-winter. Comparison of the Maynooth ground-based data with the corresponding results from two nearby stations in the same time-period indicated that the Maynooth data are consistent with other ground stations. The temperature difference between the satellite and ground-based datasets in winter was reduced to 14–15K by lowering the peak altitude of the weighting function to 84 km. An unrealistically low peak altitude would be required, however, to bring temperatures derived from the satellite into agreement with the ground-based data. OH equivalent temperatures derived from the SABER instrument using the same weighting function produced results that agreed well with ACE-FTS. When the OH 1.6μm VER profile measured by SABER was used as the weighting function, the OH equivalent temperatures increased in winter as expected but the summer temperatures were reduced resulting in an approximately constant offset of 8.6±0.8K between ground and satellite values with the ground values higher. Variability in both the altitude and width of the OH layer within a discernable seasonal variation were responsible for the changes introduced. The higher temperatures in winter were due to primarily to the lower altitude of the OH layer, while the colder summer temperatures were due to a thinner summer OH layer. We are not aware of previous reports of the effect of the layer width on ground-based temperatures. Comparison of OH-equivalent temperatures derived from ACE-FTS and SABER temperature profiles with OH*(3-1) temperatures from Wuppertal at 51.3 N which were measured during the same period showed a similar pattern to the Maynooth data from a decade earlier, but the warm offset of the ground values was lower at 4.5±0.5 K. This discrepancy between temperatures derived from ground-based instruments recording hydroxyl spectra and satellite borne instruments has been observed by other observers. Further work will be required by both the satellite and ground-based communities to identify the exact cause of this difference

Evaluation of the Sensitivity of the Weather Research and Forecasting Model to Parameterization Schemes for Regional Climates of Europe over the Period 1990–95

The Weather Research and Forecasting model (WRF) is used to downscale interim ECMWF Re-Analysis (ERA-Interim) data for the climate over Europe for the period 1990–95 with grid spacing of 0.448 for 12 combinations of physical parameterizations. Two longwave radiation schemes, two land surface models (LSMs), two microphysics schemes, and two planetary boundary layer (PBL) schemes have been investigated while the remaining physics schemes were unchanged. WRF simulations are compared with Ensemble-Based Predictions of Climate Changes and their Impacts (ENSEMBLES) observations gridded dataset (E-OBS) for surface air temperatures (T2), precipitation, and mean sea level pressure (MSLP) in eight subregions within the model domain to assess the performance of the different parameterizations on widely varying regional climates. This work shows that T2 is modeled well byWRF with high correlation coefficients (0.8 , R , 0.95) and biases less than 48C. T2 shows greatest sensitivity to land surface models, some sensitivity to longwave radiation schemes, and less sensitivity to microphysics and PBL schemes. Precipitation is not well modeled by WRF with low correlation coefficients (0.1 , R , 0.3) and high root-mean-square differences (RMSDs; 8–9 mm day21). Precipitation shows sensitivity to LSMs in summer. No significant bias has been observed in theMSLP modeled byWRF. Correlation coefficients are typically in the range 0.7,R,0.8 whileRMSDs are in the range 6–10 hPa. MSLP output is sensitive to longwave radiation scheme in summer but is relatively insensitive to either microphysics or the choice of LSM. The optimum combination of parameterizations for all three state variables examined is strongly dependent on subregion and demonstrates the need to carefully select parameterization combinations when attempting to use WRF as a regional climate model

An illustration of method of finite differences in the solution of Laplace's equation

A technique is described which illustrates the method of finite differences in the solution of Laplace's equation for a particular problem. The problem is to locate contours of equal electrical potential for a simple two-tube electrostatic lens. A short basic program which achieves this objective is provided together with sample results. The technique can be applied to many more complex problems

Inferring hydroxyl layer peak heights from ground-based measurements of OH(6-2) band integrated emission rate at Longyearbyen (78 N, 16 E)

Measurements of hydroxyl nightglow emissions over Longyearbyen (78 N, 16 E) recorded simultaneously by the SABER instrument onboard the TIMED satellite and a ground-based Ebert-Fastie spectrometer have been used to derive an empirical formula for the height of the OH layer as a function of the integrated emission rate (IER). Altitude profiles of the OH volume emission rate (VER) derived from SABER observations over a period of more than six years provided a relation between the height of the OH layer peak and the integrated emission rate following the procedure described by Liu and Shepherd (2006). An extended period of overlap of SABER and ground-based spectrometer measurements of OH(6-2) IER during the 2003–2004 winter season allowed us to express ground-based IER values in terms of their satellite equivalents. The combination of these two formulae provided a method for inferring an altitude of the OH emission layer over Longyearbyen from ground-based measurements alone. Such a method is required when SABER is in a southward looking yaw cycle. In the SABER data for the period 2002–2008, the peak altitude of the OH layer ranged from a minimum near 76 km to a maximum near 90 km. The uncertainty in the inferred altitude of the peak emission, which includes a contribution for atmospheric extinction, was estimated to be±2.7 km and is comparable with the ±2.6 km value quoted for the nominal altitude (87 km) of the OH layer. Longer periods of overlap of satellite and ground-based measurements together with simultaneous onsite measurements of atmospheric extinction could reduce the uncertainty to approximately 2 km