Global Lightning Activity and the Atmospheric Electric Field Prepared by Marek Gołkowski and Morris Cohen Stanford University and Marek Kubicki Institute of Geophysics (IGF) Polish Academy of Sciences IHY Workshop on Advancing VLF through the Global AWESOME Network This presentation is about comparing global lightning activity to the ‘fair weather’ atmospheric electric field. The observations presented were made in cooperative effort between Stanford University and the Polish Academy of Sciences

Outline Global Atmospheric Circuit Stanford-IGF Correlation Study Atmospheric Electric Field Measurements (Polish Academy of Sciences) Global Lightning Activity Using VLF (Stanford AWESOME VLF Network) Results for March and May 2007 Summary References

Global Electric Circuit The global electric circuit describes how currents flow around the Earth on a global scale. There is a potential difference of about 200 kV between the ionosphere and the Earth that can be measured all over the Earth under ‘fair weather’ conditions, this is know as the ‘fair weather’ electric field. . Both the Earth and the ionosphere can be considered as good conductors at low frequencies. The potential difference is believed to be driven by thunderstorm activity (or also associated rain showers) . About 1 kA is believed to flow in the global electric circuit at all times. It is important to realize that the circuit is global in scale. Thunderstorms cover about 1 % of the Earth’s surface at any given time, while 99 % is free of thunderstorms and is where the return current flows. The global scale makes the circuit difficult to analyze as comprehensive global measurements are difficult to obtain. The global circuit experiences many complex variations with latitude and longitude, and activity in the ionosphere and magnetosphere, and pollution. Since the the global circuit is sensitive to the frequency of storms and also pollution in the atmosphere, studying the parameters of global circuit allows for the investigation of climate change. Global thunderstorms charge ionosphere and current returns to ground through fair weather conduction Fair weather electric field 100-300 V/m measured on the surface Circuit looks simple but shows complex variations with latitude, longitude, magnetopheric & ionospheric processes, also pollution and climate change

Global Effects: Carnegie Curve The Carnegie Curve is daily variation of fair weather electric field with universal time (UT). Global Lightning Activity and the Carnegie Curve show general correlation However, several differences remain, most notably, Carnegie Curve does not show a maxima for strong lightning activity in Africa/Europe Exact role of lightning and other variables still not well understood need for more global measurements Lightning Activity The Carnegie curve is the average of fair weather electric field (ionospheric potential) measurements on the globe obtained from ship voyages in the 1920’s. The similaity of the Carnegie curve to the global lightning activity led to the hypothesis that it is thunderstorms that drive the ionospheric potential. The lightning activity shown in the figure was obtained from averages of global weather stations. There are still differences between the average lightning activity and the Carnegie curve (both are heavily averaged). For example the Carnegie curve does not show a maximum at ~14 UT for the lightning activity maximum for the Africa/Europe region. Two hypothesis that have been made to explain this is the location of South American lightning in relation to the magnetic equator [Kartalec et al., 2006] or that the American thunderstorms contain more rainfall which can also lead to current flow between the Earth and ionosphere [Williams et al 2004]. Thus it can be seen that the exact role of lightning and other variables is still not well understood. More global measurements would help to shed light on these issues. Source: Whipple and Scrase, 1936

America vs. Africa Two hypothesis for the American dominance over Africa in the Carnegie Curve Current control by position of magnetic dip equator [Kartalev et al., 2006] Electrified shower curves in South America dominate over Africa [Rycroft et al., 2007 and others]

Lightning activity assessment Atmospheric electric field Pros – global measurement Cons – affected by fair weather conditions ELF/VLF measurements Pros – can be measured anywhere, anytime Cons – affected by ELF/VLF propagation Measures mostly lightning stroke activity It is important to understand what exactly is meant by ‘fair weather’ condition so that the results presented in later slides can be correctly interpreted. Fair weather as the term implies means no precipitation, minimal could cover, little wind, and no nearby thunderstorms. Since aerosols and pollution can also affect the measurements by changing the resistance of the atmosphere, it is desirable to have low concentrations of pollutants. Observations during fair weather conditions are considered to be measurements of the global atmospheric electric field although they often exhibit effects of specific location and only approach the Carnegie curve after long term averaging.

Stanford - IGF Joint Study Goal: Investigate role of global lightning activity on fair weather electric field Approach: Lightning releases large amount of energy into VLF band that propagates for long distances Use VLF data to estimate global lightnting activity Fair weather electric field (Ez) can be measured with standard scientific equipment Capture seasonal variation by looking at 2 months: March 2007 and May 2007 Stanford and the Polish Institute of geophysics (IGF) performed a joint study to investigate the role of global lightning activity on the fair weather electric field. The study was focused on two months March and May 2007. To measure lightning activity electromagnetic energy in the VLF band was measured using AWESOME receivers. Lightning strikes generate energy at many frequencies but in the VLF band, this energy can propagate to long distances across the globe. VLF Receiver E-Field Collector

Atmospheric Electric Field Measurements Hornsund - Polish Polar Station Świder Observatory HO SW Atmospheric electricity measurements are made with standard radioactive collectors The Polish Academy of Sciences operates a mid-latitude collector in Świder, Poland and a polar latitude collector at the Polish Polar Station in Hornsund on Spitsbergen island Measurements were made in “Fair Weather” conditions including low aerosol content confirmed from other instruments. The measurements at Hornsund also require quiet geomagnetic conditions since magnetospheric dynamics can have a strong effect on the ionosphere at polar latitudes. Atmospheric electricity measurements are made with standard radioactive collectors The Polish Academy of Sciences operates a mid-latitude collector in Świder, Poland and a polar latitude collector at the Polish Polar Station in Hornsund on Spitsbergen Measurements were made in “Fair Weather” conditions including low aerosol content confirmed from other instruments

Fair Weather Conditions Fair Weather Conditions are defined to involve: No precipitation (rain, snow, hail, fog, etc,) Minimal clouds < 4/8 (8/8 = complete cloud cover) Slow wind speed < 6 m/s No thunderstorms in ~75 km radius from measurement Additionally it is important to have conditions with small concentrations of aerosols. This concentration can be measured with additional instruments Only observations made during Fair Weather Conditions are considered to be measurements of the Global Atmospheric Electric Field It is important to understand what exactly is meant by ‘fair weather’ condition so that the results presented in later slides can be correctly interpreted. Fair weather as the term implies means no precipitation, minimal could cover, little wind, and no nearby thunderstorms. Since aerosols and pollution can also affect the measurements by changing the resistance of the atmosphere, it is desirable to have low concentrations of pollutants. Observations during fair weather conditions are considered to be measurements of the global atmospheric electric field although they often exhibit effects of specific location and only approach the Carnegie curve after long term averaging. 9 9

VLF Measurements CH SW TA MI AD PA Group 1 CH: Chistochina, Alaska PA: Palmer Station, Antarctica AD: Adelaide, Australia Group 2 TA: Taylor, Indiana SW: Swider, Poland MI: Midway Island CH SW TA MI AD PA For the study global lightning activity was estimated by looking at VLF data from three sites with Stanford Awesome receivers Sites were chosen to cover all global areas of lightning activity Total EM energy in a narrow band around 2 frequencies 325 Hz and 10 kHz was calculated every 15 minutes and hourly averages were later made. For the study global lightning activity was estimated by looking at VLF data from three sites with Stanford Awesome receivers Sites were chosen to cover all global areas of lightning activity Total EM energy in a narrow band around 2 frequencies 325 Hz and 10 kHz was calculated every 15 minutes

Day-Night Propagation Effects Palmer (PA) March 5:00 PA 10:00 15:00 VLF Waves propagate more efficiently at night Sites show greater activity when station it is nighttime at station Local time effects need to be taken corrected for in VLF data PA The analysis of the VLF data is made more complicated by the fact that VLF Waves propagate more efficiently at night than during the day. This is because the ionopshere is at a higher altitude at night where there are less collisions and less losses for propagation. This means that each site will be sensitive to whether it is day or night at its location. This can be seen in the example from Palmer station where for the daylight hours, low activity is observed and high activity is observed for the night hours. This makes it hard to assess the global lightning activity since each site is affected by different sunrise and sunset times. The propagation day/night effects must be somehow accounted for. A simple way to do this is to strengthen the signals that correspond to day time hours. This correction gives a better estimation of the global activity. More sophisticated ways to mitigate the propagation effects require some knowledge in advance about the location of major lightning. PA

Lightning vs. Ez Atmospheric electric field measurements (bottom panels) observed at SW and HO. Top panel plots show lightning activity derived from sites AD, CH, and PA using the propagation scalar method.

Lightning vs. Ez ELF/VLF estimate of global lightning activity and atmospheric electric field observed at SW for 25 March 2007 General similarity is observed on this day

Summary Fair weather electric field thought to be linked to thunderstorm activity Strokes? Rainfall? Something else? Ez field difficult to measure in “fair weather” conditions ELF/VLF measurements require propagation adjustments Combination of ELF/VLF and Ez presents complementary measurements Measurements sometimes correlated, sometimes not. Why? More analysis to come… ELF/VLF measurements combined with other instruments can yield exciting new results VLF data show three active global regions of lightning activity: Asia, Africa, Americas In May the African sector is more pronounced than in March, most likley due to seasonal weather changes Storms in Africa affect the 10 kHz band much more than the 325 Hz band, which might indicate that the charge moment of African lightning is smaller than for Asia and the Americas Atmospheric electric field measurements at Swider only exhibit increases for Asian and American sectors, not for African sector in both March and May For the single day (31 March) of the study for which multiple electric field measurements are available the electric field at Hornsund clearly shows the African center while the electric field at Swider does not. This suggests that African lightning has a different geographic effect on the atmospheric electric field than does Asian and American lightning Data from satellite Lightning Imaging Sensor (LIS) shows that for both March and May lightning activity was greatest in South America Further global measurements of both lightning and atmospheric electric field needed to resolve geographic variability of global electric circuit.

References B A. Tinsley, L. Zhou (2006). Initial results of a global circuit model with variable stratospheric and tropospheric aerosol. Journal of Geophysical Research , vol. 11, D16205, doi:10.1029/2005JD006988. Kartalev, M.D., M.J. Rycroft, M. Fuellekrug, V.O. Papitashvili, V.I. Keremidarska ., (2006). A possible explanation for the dominant effect of South American thunderstorms on the Carnegie curve. Journal of Atmospheric and Solar-Terrestrial Physics,  68, 457-468.   Rycroft, M.J., Israelsson, S. and Price, C., (2000) The global atmospheric electric circuit, solar activity and climate change. Journal of Atmospheric and Solar-Terrestrial Physics, 62, 1563–1576. Harrison, R. G. (2004), The global atmospheric electric circuit and climate, Surveys in Geophysics, 25, 441-484.  Whipple, F.J.W., Scrase, F.J., (1936). Point discharge in the electric field of the earth. Geophysical Memoirs of London VII, 68, 1–20. Williams E.R., S.J Heckman, (1993). The local diurnal variation of cloud electrification and the global diurnal variation of negative charge on the Earth. Journal of Geophysical Research , 98, D3, 5221-5234. Williams, E.R. (1993), Global Circuit response to Seasonal variations in global surface air temperature. Monthly Weather Review, 122,1917-1929. Williams, E. R. G. Satori, (2004). Lighting, thermodynamic and hydrological comparison of the two tropical continental chimneys. Journal of Atmospheric and Solar-Terrestrial Physics, 66, 1213-1231.