The beginnings of geography teaching and research in the University of Glasgow: the impact of J.W. Gregory

J.W. Gregory arrived in Glasgow from Melbourne in 1904 to take up the post of foundation Professor of Geology in the University of Glasgow. Soon after his arrival in Glasgow he began to push for the setting up of teaching in Geography in Glasgow, which came to pass in 1909 with the appointment of a Lecturer in Geography. This lecturer was based in the Department of Geology in the University's East Quad. Gregory's active promotion of Geography in the University was matched by his extensive writing in the area, in textbooks, journal articles and popular books. His prodigious output across a wide range of subject areas is variably accepted today, with much of his geomorphological work being judged as misguided to varying degrees. His 'social science' publications - in the areas of race, migration, colonisation and economic development of Africa and Australia - espouse a viewpoint that is unacceptable in the twenty-first century. Nonetheless, that viewpoint sits squarely within the social and economic traditions of Gregory's era, and he was clearly a key 'Establishment' figure in natural and social sciences research in the first half of the twentieth century. The establishment of Geography in the University of Glasgow remains enduring testimony of J.W. Gregory's energy, dedication and foresight

Estimates of random walk exit probabilities and application to loop-erased random walk

We prove an estimate for the probability that a simple random walk in a simply connected subset A of Z^2 starting on the boundary exits A at another specified boundary point. The estimates are uniform over all domains of a given inradius. We apply these estimates to prove a conjecture of S. Fomin in 2001 concerning a relationship between crossing probabilities of loop-erased random walk and Brownian motion.Comment: 26 pages, 0 figure

Jet vortex generators for turbulent flow separation control

A parametric study was performed with jet vortex generators to determine their effectiveness in controlling flow separation associated with low speed turbulent flow over a two dimensional rearward-facing ramp. Results indicate that flow separation control can be accomplished with the level of control achieved being a function of jet speed, jet orientation (with respect to the free stream direction), and orifice pattern (double row of jets vs. single row). Compared to slot blowing, jet vortex generators can provide an equivalent level of flow control over a larger spanwise region (for constant jet flow area and speed)

How Hot is the Wind from TW Hydrae?

It has recently been suggested that the winds from Classical T Tauri stars in general, and the wind from TW Hya in particular, reaches temperatures of at least 300,000 K while maintaing a mass loss rate of $\sim 10^{-11}$ \Msol yr$^{-1}$ or larger. If confirmed, this would place strong new requirements on wind launching and heating models. We therefore re-examine spectra from the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope and spectra from the Far Ultraviolet Spectroscopic Explorer satellite in an effort to better constrain the maximum temperature in the wind of TW Hya. We find clear evidence for a wind in the \ion{C}{2} doublet at 1037 \AA and in the \ion{C}{2} multiplet at 1335 \AA. We find no wind absorption in the \ion{C}{4} 1550 \AA doublet observed at the same time as the \ion{C}{2} 1335 \AA line or in observations of \ion{O}{6} observed simultaneously with the \ion{C}{2} 1037 \AA line. The presence or absence of \ion{C}{3} wind absorption is ambiguous. The clear lack of a wind in the \ion{C}{4} line argues that the wind from TW Hya does not reach the 100,000 K characteristic formation temperature of this line. We therefore argue that the available evidence suggests that the wind from TW Hya, and probably all classical T Tauri stars, reaches a maximum temperature in the range of 10,000 -- 30,000 K.Comment: 17 pages, 3 figures, Figure 1 in 2nd version fixes a small velocity scaling error and new revision adds a reference to an additional paper recently foun

Transition from collisionless to collisional MRI

Recent calculations by Quataert et al. (2002) found that the growth rates of the magnetorotational instability (MRI) in a collisionless plasma can differ significantly from those calculated using MHD. This can be important in hot accretion flows around compact objects. In this paper we study the transition from the collisionless kinetic regime to the collisional MHD regime, mapping out the dependence of the MRI growth rate on collisionality. A kinetic closure scheme for a magnetized plasma is used that includes the effect of collisions via a BGK operator. The transition to MHD occurs as the mean free path becomes short compared to the parallel wavelength 2\pi/k_{\Par}. In the weak magnetic field regime where the Alfv\'en and MRI frequencies $\omega$ are small compared to the sound wave frequency k_{\Par} c_0, the dynamics are still effectively collisionless even if $\omega \ll \nu$, so long as the collision frequency \nu \ll k_{\Par} c_{0}; for an accretion flow this requires \nu \lsim \Omega \sqrt{\beta}. The low collisionality regime not only modifies the MRI growth rate, but also introduces collisionless Landau or Barnes damping of long wavelength modes, which may be important for the nonlinear saturation of the MRI.Comment: 20 pages, 4 figures, submitted to ApJ with a clearer derivation of anisotropic pressure closure from drift kinetic equatio

Effects of diode parasitics on the performance of lattice mixers

It has been known for a number of years that the noise figure of a simple receiving system is directly proportional to the conversion efficiency of the mixer. Any improvement in this will have great bearing on the performance of a communications system, offering the possibility of an increased range of reception. The narrow-band open - circuit lattice mixer has been shown theoretically, under certain conditions, to produce a loss approaching zero. Exploratory low frequency circuits have given conversion power losses as low as 1 dB. The increasing use of communication systems at microwave frequencies brings with it the demand for low-loss mixers at these higher frequencies. As the frequency of operation increases, the parasitic diode reactance's have a more pronounced effect on the mixer performance. The original work presented here analyses the effect of the parasitic diode package capacitance on the performance o f a narrow-band open - circuit lattice mixer. The main conclusion to be drawn from the analysis is that for practical diodes used with local oscillator powers normally encountered in microwave mixers, the conversion power-loss of the mixer will, generally, be less than 1.5 dB. Another important result from the analysis is that the optimum terminating resistances are considerably reduced when even small amounts o f package capacitance (eg. 0.1 pF) are present. This may be considered as being an advantage when matching the lattice to a 50 Ω coaxial system. An experimental mixer at L-band constructed by the author using lumped circuit components, gave a 2.8 dB conversion power loss at 20mW of local oscillator drive. This was a considerable improvement on existing commercial models. Had it been a broad-band lattice mixer, it would have given as 3.6 dB minimum conversion loss. On the other hand a narrow-band mixer would have given 3.75 dB minimum. The practical mixer circuit was a modification of the one first analysed in detail by Kulesza, in that the input transformer was resonated. This configuration solved the problem of feeding the local oscillator at the input of the lattice. The 25 MHz. 3 dB band-width of the mixer would be considered sufficiently large enough for use in a communications network. Less power at the local oscillator, frequency was required when comparing this mixer with some 2-diode; mixers; in some cases producing a power saving of 50%. Experimental verification of the analytical work on the effect of diode capacitance gave within 0.9 dB of the theoretical values for low values of capacitance and for local oscillator drive levels used in practice. Finally, the 'K' mixer parameters for the case when diode package capacitance is present in a narrow-band open-circuit lattice mixer are solved. These will be of importance to any future analytical work on lattice mixers

Rotationally Modulated X-ray Emission from T Tauri Stars

We have modelled the rotational modulation of X-ray emission from T Tauri stars assuming that they have isothermal, magnetically confined coronae. By extrapolating surface magnetograms we find that T Tauri coronae are compact and clumpy, such that rotational modulation arises from X-ray emitting regions being eclipsed as the star rotates. Emitting regions are close to the stellar surface and inhomogeneously distributed about the star. However some regions of the stellar surface, which contain wind bearing open field lines, are dark in X-rays. From simulated X-ray light curves, obtained using stellar parameters from the Chandra Orion Ultradeep Project, we calculate X-ray periods and make comparisons with optically determined rotation periods. We find that X-ray periods are typically equal to, or are half of, the optical periods. Further, we find that X-ray periods are dependent upon the stellar inclination, but that the ratio of X-ray to optical period is independent of stellar mass and radius.Comment: 10 pages, 8 figures, accepted for publication in MNRA

Design of a 12 channel fm microwave receiver

The design, fabrication, and performance of elements of a low cost FM microwave satellite ground station receiver is described. It is capable of accepting 12 contiguous color television equivalent bandwidth channels in the 11.72 to 12.2 GHz band. Each channel is 40 MHz wide and incorporates a 4 MHz guard band. The modulation format is wideband FM and the channels are frequency division multiplexed. Twelve independent CATV compatible baseband outputs are provided. The overall system specifications are first discussed, then consideration is given to the receiver subsystems and the signal branching network

Compressed self-avoiding walks, bridges and polygons

We study various self-avoiding walks (SAWs) which are constrained to lie in the upper half-plane and are subjected to a compressive force. This force is applied to the vertex or vertices of the walk located at the maximum distance above the boundary of the half-space. In the case of bridges, this is the unique end-point. In the case of SAWs or self-avoiding polygons, this corresponds to all vertices of maximal height. We first use the conjectured relation with the Schramm-Loewner evolution to predict the form of the partition function including the values of the exponents, and then we use series analysis to test these predictions.Comment: 29 pages, 6 figure