have obviously put a great deal of effort into developing their classification system which differs considerably in detail if not in concept from those described by Abel 1, Wykeham et al 2 and B i eni a w s ki a, 4, 5. Unfortunately the system proposed by B a r t o net al. seems overcomplicated. As the authors themselves point out there are over 300,000 different geological combinations that can be represented. This number of possibilities is simply not "workable". The fact that this number is based on the analysis of 200 case records (commendable though that is) appears to be extrapolating to the extreme.These classification systems have definite limitations and it cannot possibly be expected that they will allow for all geological possibilities and stress conditions. The system described by B i eni a w ski a, 4, 5 recognizes this fact and is intended primarily for cases where tunnel wall displacements are the result only of rock mass loosening. Cases involving swelling rock/ gouge or extensive compressive failure of the rock mass (socalled "true" rock pressure) are specifically excluded. However, Barton et al. have attempted to cover all these possibilities. This appears to be asking too much of a classification system especially when the rock mass rating is related directly to tunnel support pressure. Particularly in the case of "true" rock pressure, many factors (e. g. excavation method, type and sequence of primary support installation), which cannot be accounted for in a classification system, will affect the load carried by the primary support G.Other drawbacks in the system proposed by B art o net al. appear to be:1. The only allowance for the strength of the rock material is in the SRF factor for "competent rock". For other categories of rock no account is taken of the material strenght. The term "competent rock" is itself not clearly defined.2. No allowance is made for the orientation of joint sets. This is unrealistic as it is well known v that the orientation of discontinuities relative to the tunnel axis can have an important influence on roof loosening.3. The system is based fundamentally on RQD which is both directionally dependent and rather sensitive to the skill of the driller.4. The system involves 9 rock mass classes and 38 categories for permanent support. These numbers are simply too large to be generally acceptable in practice. Similarly numbers such as 0.0015 or 0.018 for rock mass quality are likely to be viewed with some suspicion.

[1] Flickering aurora is characterized by optical emissions varying in intensity with frequencies typically between 5 and 15 Hz. In this report we use high-speed narrow field-of-view imaging in white light to determine the intensity variation and the apparent motion of the flickering spots. The characteristic patterns we find are compared with the spatial patterns derived from interference between electromagnetic ion-cyclotron waves. It is found that good agreement is reached, which we argue should make a strong rejection criterion for theories of flickering aurora.

Abstract. In this paper we present Naturally Enhanced Ion Acoustic Lines (NEIALs) observed with the EISCAT Svalbard Radar (ESR). For the first time, long sequences of NEIALs are recorded, with more than 50 events within an hour, ranging from 6.4 to 140 s in duration. The events took place from ∼08:45 to 10:00 UT, 22 January 2004. We combine ESR data with observations of optical aurora by a meridian scanning photometer at wavelengths 557.7, 630.0, 427.8, and 844.6 nm, as well as records from a magnetometer and an imaging riometer. The large numbers of observed NEIALs together with these additional observations, enable us to characterise the particle precipitation during the NEIAL events. We find that the intensities in all optical lines studied must be above a certain level for the NEIALs to appear. We also find that the soft particle precipitation is associated with the down-shifted shoulder in the incoherent scatter spectrum, and that harder precipitation may play a role in the enhancement of the up-shifted shoulder. The minimum energy flux during NEIAL events found in this study was ∼3.5 mW/m 2 and minimum characteristic energy around 50 eV.

[1] In this report we present incoherent scatter radar (ISR) observations of ionospheric response to precipitation causing flickering aurora. Flickering aurora is caused by electron precipitation with modulations at frequencies higher than 5 Hz. To resolve the variation at these short time-scales with ISR we have integrated together pulses at the same phase of the optical intensity variation observed with high-speed narrow field-of-view imaging in white light to determine the intensity variation in the field aligned direction, which is also the direction of the beam of the EISCAT Svalbard Radar (ESR). Further we show that the 3% modulation in ISR back-scattered power can be explained with electron heating by temporally modulated electron precipitation and electron cooling in collisions with ions and neutrals.