Scientific Background

Many people are unaware that the atmosphere carries a continuous electric current and, even during fair weather, there is a strong electrostatic electric field, up to 200 volts per meter, close to the ground. This electric current is primarily due to the accumulated effect of thousands of thunderstorms, mostly in the tropical regions of the Earth. These storms feed a current from the ground up to the ionosphere, a highly conducting layer in the atmosphere lying above about 70 km altitude. The current spreads out around the globe through this layer and returns to Earth through the atmosphere as the ’fair weather current’ outside the thunderstorm areas (also known as the air-earth-current). Although the existence of the global electric circuit has been known for many decades, there are still many gaps in our knowledge concerning, for example, the contribution of other electric-field generators. These include the action of the solar wind (ions and electrons flowing out from the Sun) and tidal wind systems in the Earth’s upper atmosphere, between about 50 km and 200 km altitudes. Since changes in solar activity directly modify the solar wind and atmospheric tide generators, as well as the atmospheric conductivity (through cosmic rays) there is a potential for solar-activity driven changes in the global electric circuit. The nature and amplitude of any such effects is so far poorly researched (Roble and Tzur, 1986).

It has further been suggested that changes in the air-earth-current due to solar wind effects could modify clouds, and indeed statistical correlations between solar wind disturbances and clouds, and even with other meteorological parameters, have been reported. The correlations tend to have different signs at different locations, and sometimes are apparent only during restricted time periods. The history of the search for links between solar activity and weather has been dogged by many false trails (see for example Hoyt and Schatten, 1997). However, the theoretical possibility of a link involving the fair-weather current remains and at least some of the correlations reported in recent years are not inconsistent with this possibility (see Tinsley, 2000, for a review of the possible link between fair-weather current and clouds and Frank-Kamenetsky et al., 1999 as well as Troschichev el al, 2003 for recent results from Vostok, Antarctica). The possibility of a previously unrecognised (natural) solar wind effect on weather and climate is being used by some pressure groups to support arguments that current climate change may not be due to increasing greenhouse gases. It is thus also of some practical importance to study whether there really is an effect of the solar wind on climate and, if so, to find out how it occurs and whether it is of any significance in comparison to the current changes in atmospheric composition.

Since 1998 observations of air-earth current have been made in Kiruna, in northern Sweden above the Arctic Circle (67°89’N, 21°09’E). These have shown that the fair-weather current is enhanced by about 5% during magnetospheric substorms, one particular type of disturbance caused by the solar wind (Belova et al., 2000a,b). However, other types of disturbances than substorms seem to produce much larger effects in the fair-weather current, with the current increasing by 25-50% over the quiet values in bursts or pulsations. These kinds of disturbances are rare and each is unique, unlike substorms which follow a frequently repeated pattern. The non-substorm disturbances are hard to interpret with observations from only one site. By making simultaneous observations in both the Arctic and the Antarctic we can examine whether such events are global (i.e. affecting the whole global electric circuit) and, with the help of solar wind measurements from satellites, we hopefully can find physical interpretations. Comparison with meteorological measurements should ultimately allow a deeper study of the possible correlation between air-earth-current and the state of the lower atmosphere.

Field measurements at Wasa

Measurements were made during the summer expeditions to the Swedish Antarctic station Wasa (73°03’S 13°25’W) in 2002 and 2003. Air-earth-current was measured using a long wire antenna (standard copper antenna-wire, 100 m long) connected through a sensitive electrometer-type amplifier to ground. The antenna was erected next to the Wasa station, hung on hollow glass-fibre (bamboo in 2002) poles about 2 meters above the ground. Data was recorded with 10-second time resolution using a handheld computer. Since the current can be strongly affected by local effects, such as wind blowing aerosols across the antenna, precipitation-free and low-wind days have to be used to study the true fair-weather current. The effects of wind and precipitation can be readily recognised in the recorded data. In 2003, information on cloud cover was provided by pyranometer measurements (incoming solar radiation) made nearby by a team from Stockholm University.

Preliminary results

Measurements were successfully collected at Wasa from 16-29 January in 2002 and from 24 January-11 February 2003. In 2002 no significant disturbances were seen during the recording period, but the daily variation of the air-earth current (due to the daily variation of thunderstorms) was clearly represented in the measurements. The daily variation at Wasa was in good agreement with the measurements in Kiruna, showing that the technique worked as it should. The daily variation was again clearly recorded in 2003 (figure 1) and on two occasions significant disturbances were also seen close to simultaneously at Wasa and in Kiruna. The strongest of these disturbances is shown in figure 1, where a large burst of enhanced air-earth-current at both Wasa and Kiruna can be seen at about 13 UT. This is close to the time when a sharp jump in solar wind speed (a ’shock’) was observed at the ACE satellite, and it might be tempting to relate these two phenomena. However, the ACE satellite is located about 1.5 million kilometres ’upwind’ of the Earth so that the burst in air-earth-current occurs before the solar wind shock can reasonably have reached the Earth, and its origin is therefore unclear. The increased air-earth current observed at Wasa between 15 and 17 UT, on the other hand, might well be a result of the increased solar wind speed after the initial shock. The incoming solar radiation at Wasa showed a smooth variation over the day on this occasion, indicating more-or-less cloud free conditions, i.e. the kind of ’fair weather’ which should allow us to see solar wind influences on the air-earth current.

The results from the two short test periods in 2002 and 2003 are encouraging. It is now planned to start measurements of air-earth-current and incoming solar radiation early in the Antarctic summer of 2003/2004 and to continue collecting data into the following winter season.