A MEASUREMENT OF ALBEDO CHANGES RESULTING FROM AFFORESTATION PRACTICES

ABSTRACT

The measurement of albedo using portable battery-powered logging equipment before and after the ploughing of drainage ditches by the Forestry Commission on a small catchment in Northern England is described. The change in the evaporation brought about by the reduction of the mean basin albedo from 15.2±0.7 per cent to 13.1 ±0.8 per cent would not in itself produce a significant change in the basin's hydrological behaviour.     

IDEAL CURRENT LAYERS AND KINK INSTABILITY IN LINE-TIED CORONAL LOOPS

The development of the kink instability in line-tied coronal loops is studied using a cylindrical MHD code. When the twist of magnetic field lines of the initial configuration exceeds a critical value, an ideal kink mode develops and drives this unstable equilibrium towards a secondary bifurcated equilibrium containing an electric current concentration. Contrary to a periodic untied configuration where a current sheet is ideally generated, the current layer is non-singular with a non-zero thickness and a finite amplitude. This current concentration extends along all the loop length and takes the form of an helical ribbon of intense current. The numerical results give an algebraic linear-like scaling of the characteristics of the current layer (amplitude and thickness) as a function of the aspect ratio of the loop. An interpretation in terms of axial field-line bending of the three-dimensional kinked equilibrium is proposed.