Toby Clark

Increased magnetic storm activity from 1868 to 1995

The aa index provides the longest continuous data set which can be used in the analysis of magnetospheric and ionospheric phenomenology. All phases of the solar cycle show increases in activity since cycle 14. The activity increase does not appear to be associated with any instrumental, ionospheric or magnetospheric effects. Barely significant effects (in terms of the results presented in this paper) have been identified in the long-term change in magnetic latitude of the observatory sites, the positions of high-latitude ionospheric features such as the cusp, and ionospheric Pedersen and Hall conductivities due to changing magnetic field orientation and strength. The prime cause of the change in geomagnetic activity is an increase in solar activity. The number of storms at solar minimum has typically increased by 40% more than the other phases. This is principally due to increased recurrent storm activity to such an extent that conditions at minimum in recent cycles could be thought of as being more representative of the declining phase.

Geomagnetically induced currents in the UK: geomagnetic variations and surface electric fields

Abstract The geomagnetically induced current (GIC) risk to the power transmission grid in the United Kingdom is discussed with reference to an example of a geomagnetic storm during which GICs were suspected of causing abnormal transformer behaviour. A simple measure of the power of the magnetic field variation, the hourly standard deviation (HSD) in the north or east horizontal component, is used to determine the general risk to the UK power grid from rapid magnetic variations, according to season and local time. Monitoring and forecasting of HSD may be a useful means of gauging the likely risk to high-cost power engineering equipment. A simplified but representative three-dimensional geological model of the UK landmass and surrounding seas is used to provide an indication of the surface electric field for various amplitudes and orientations of external magnetic field variations. It is found that the resistivity contrast between seawater and the onshore geology, particularly around the Scottish metamorphic terranes, produces enhanced electric fields at coastal sites. These are as much as 4 V/km for a 1 A/m (or 1257 nT ) external field with 10 min period.

The causes of long‐term change in the aa index

The aa index provides the longest geomagnetic data set that can be used in the analysis of magnetospheric and ionospheric phenomena. All phases of the solar cycle show increases in storm activity since the end of cycle 14 in 1915. The activity increase does not appear to be strongly associated with any instrumental, ionospheric or magnetospheric effects. Small effects have been confirmed in the long-term change in ionospheric Pedersen and Hall conductivities due to the changing dipole moment of the Earth but not due to increasing greenhouse gases. Three instrumental effects have been identified where significant changes in quiet time conditions can be seen, that is, 1938, 1980, and 1997. These do not account for the majority of the increase in aa. Noise levels for the aa index are now close to those seen at the beginning of the data set. The prime cause of the increase in storm activity is an increase in solar activity. The average aa in cycle 23 should be about 1 nT less than that predicted from previous cycles due to the reduction in baseline noise levels at the start of the cycle (1997).

Observation of a decrease in mid-latitude whistler mode signal occurrence prior to geomagnetic storms

VLF whistler mode signals received in 1986–1992 at Faraday, Antarctica (65° S, 64° W) and Dunedin, New Zealand (46° S, 171° E) show night-long decreases in occurrence which may be caused by changes in F-region absorption levels. The whistler mode occurrence normally decreases for one night and can only be detected during periods when whistler mode activity lasting several hours per night is usual. Decreases in occurrence are observed more often at Dunedin than at Faraday, probably due to long sub-ionospheric paths that result in weaker signals being received at Dunedin. The decreases in occurrence appear to be associated with solar disturbances and often occur a day before the onset of geomagnetic activity. Several of the events recur with a 27-day cycle that coincides with favourably placed solar coronal holes.

Computer generated K indices adopted by the British Geological Survey

Abstract On 1 January 1991 the British Geological Survey adopted a computer method for generating K indices from its three geomagnetic observatories. This replaced the traditional handscaling method, resulting in saving of staff time. Other advantages are the ability to distribute K indices to users in real time and the fact that there will not be any change in bias of the K index caused by a change of handscaler in future. The computer algorithm is described. The results of a comparison between the computed and handscaled K indices are presented, which show the computer method to be compatible with handscaling.