Automatic Whistler Detector: Operational Results from New Zealand

1. Automatic Whistler Detector: Operational Results from New Zealand János Lichtenberger1, Craig J. Rodger2, and Greg McDowell2 1. Space Research Group, Department of Geophysics, Eötvös University, Budapest, Hungary. 2. Physics Department, University of Otago, Dunedin, New Zealand. VERSIM 2006 Sodankylä , Finland

2. Introduction Whistlers have been regarded as cheap and effective tools for plasmasphere diagnostic since the early years of VLF research. However, due to the very nature of whistlers, short lived tones often with fairly low occurrence rates, the manual examination of large datafiles to pickout whistler events is time consuming. With this in mind the Space Physics Group of Eötvös University developed a Automatic Whistler Detector (AWD) system, which was deployed to New Zealand in mid-2005. We will discuss the first results from this system.

3. Lightning Rate Contrast: Hungary and NZ Continuous VLF measurements are routinely made at an observatory in Tihany, Hungary (46.89 N 17.89 E, 200 m, L=1.81), where the Eötvös University Automatic Whistler Detector (AWD) has recorded whistler activity. The magnetic conjugate point of the Tihany observatory is located at in South Africa, near Grahemstown. Analysis of the first two years of operation (from 27 Feb 2002) found 368,000 whistler traces, suggesting an average whistler rate of 0.35 whistler traces per minute [Collier et al. Annales Geophys., 2006]. Tihany Grahemstown

4. Variability of Global Lightning activity As whistlers are caused by lightning activity it seems reasonable that there should be a linkage between lightning activity levels and whistler detection. On this basis one would expect much higher whistler rates in some parts of the world when contrasted with others. GEOGRAPHICAL ACTIVITY DISTRIBUTION GEOMAGNETIC ACTIVITY DISTRIBUTION Christian et al., J. Geophys. Res., vol. 108 (D1), 2003. Adapted from the LIS v1.0 Low Resolution Full Climatology dataset, shifted into CGM coordinates. red dots show geomagnetic conjugates (CGM) important for our study

5. Lightning Rate Contrast: Hungary and NZ At the low-magnetic latitudes of the Tihany Observatory AWD, single hop whistlers from the conjugate hemisphere will be the primary source of Tihany-observed whistlers. A high whistler rate is expected, due to the high lightning levels around the magnetic conjugate. Southern hemisphere lightning rates from OTD transformed into CGM coordinates and mirrored in the geomagnetic equator.

6. Lightning Rate Contrast: Hungary and NZ However, the lightning levels conjugate to New Zealand are very low, and thus one might expect very low whistler rates. Lightning rates in southern Alaska are very very low. Note high Antarctic Peninsula whistler rates likely to include some propagation. Northern hemisphere lightning rates from OTD transformed into CGM coordinates and mirrored in the geomagnetic equator.

7. Lightning Rate Contrast: Hungary and NZ Even if one includes lightning local to New Zealand (and thus a 2-hop whistler source), lightning rates which might produce whistlers observed in New Zealand are conservatively 200-300 times lower than those for Tihany. However, for 1-hop whistlers, which is what the Eötvös AWD detects, our lightning levels are ~1500 times lower. TOTAL lightning rates (combination of both hemispheres) from OTD transformed into CGM coordinates and mirrored in the geomagnetic equator.

8. New Zealand: Bad for whistlers? However, this does not tie-in with our historic impression that New Zealand is a good place to do VLF research, and a good place to observe whistlers. Data from Dunedin (NZ) 25 months between January 1958 and October 1960 produced an average whistler rate of more than 2 min−1 [Helliwell, 1965], which is a good high number. What's going on here? Maybe New Zealand used to be a good place for whistler research due to low industrialisation levels (low noise), and so the low lightning levels could produce few whistlers, but good detection. Our modern spectra don't look that good, with a lot of 50Hz interference.

9. Lightning Rate Contrast: Hungary and NZ The Eötvös University Automatic Whistler Detector (AWD) system has been operating in New Zealand continuously since 20 May 2005. János visited us to install the system, and it has operated flawlessly since then. conjugate Dunedin

10. AWD Operation in Dunedin: Detection eff. The Eötvös University Automatic Whistler Detector (AWD) system has been operating in New Zealand continuously since 20 May 2005. An examination of the first 115 days of observations found 25,974 whistler "events", where each event may include multiple whistler traces, and only 6384 "false triggers". System accuracy 80.3%! Spectrogram showing a strong multiple trace whistler detected at Dunedin in February 2006.

11. AWD Operation in Dunedin: L-shell range A case study was undertaken during 20-25 May and 1-2 July 2005, where a number of Dunedin-observed whistlers were scaled to determine the plasmaspheric electron density and L-shells observed. This showed that our whistlers probe a very large range of L-shells (including L=2.5-3.7).

12. AWD Operation in Dunedin: Overall statistics We have analysed data for the first 269 days of system operation from 20 May 2005 - 13 Febuary 2006. This produced 62,236 whistler events, which can include multiple traces. János then processed these event files to determine the number of whistler traces, and taking a conservative processing (which might underestimate by as much as 10%) and found these events included 157,183 whistler traces. SUMMARY Dunedin 0.16 whistler events per minute Dunedin 0.41 whistler traces per minute (typical 2.5 traces/whistler) Tihany 0.35 whistler traces per minute The whistler rate is more complex than just lightning rate, which is about 1500 times larger at Tihany's conjugate point.

13. AWD Operation in Dunedin: multitrace statistics About 57% of the Dunedin observed whistlers are made of a single trace, i.e. more than 40% are multi-trace. These range from 2-33 with the average being ~2.5 traces/event.

14. AWD Operation in Dunedin: Overall statistics The occurrence rate of whistlers observed in Dunedin is highly variable, and is likely to be combination of source (lightning/thunderstorm) and propagation (absorption, ducts).

15. Seasonal Variation: Tihany Quite strong seasonal variation in whistlers observed in Tihany, with most whistlers seen in the winter time, with peaks in late winter early spring. Possibly a combination of source (summer lightning in South Africa) and good nighttime propagation in Hungary.

16. Seasonal Variation: Dunedin Rather weak seasonal variation in Dunedin whistlers, except the strong peaks in early-mid winter. Ignore May and Feb as these are partial months. Hard to draw conclusions until at least a whole year is analysed. Accuracy ranges over ~7 km – 450km. Global median is 16km, global mean is ~19 km. Promising, and the extra stations in the South Atlantic should have assisted in North Africa. On average, 6.2 stations involved in the location of each lightning.

17. Diurnal Variation: Tihany The two years of AWD observations in Tihany (Hungary) showed that the vast majority of whistler occurred during local nighttime conditions. DAY

18. Diurnal Variation: Dunedin However, despite being ~11 hours shifted from the longitude of Tihany, the UT variation of whistlers observed in Dunedin is very similar to Tihany, with most whistlers observed at 3UT = 15 LT!!! Generally, we have smaller rates at local nighttime than daytime. DAY DAY This is not what we expected, and appears to be in disagreement with Helliwell [1965]!

19. Is something wrong with the southern hemisphere? A Hungarian AWD-system was operated in Grahamstown, South Africa, for a few weeks in August/September 2005. While the observation time was short, this seemed to confirm whistlers mostly came for nighttime conditions, similar to the Tihany conjugate location. So, no, it isn't a southern hemisphere thing. DAY

20. Lightning Source timing issues? The Collier et al. study, looking at Tihany whistlers, concluded that the observed whistler rate was a strong combination of ionospheric absorption and lightning activity (essentially confirming earlier conclusions of Helliwell). The Dunedin whistlers are not, it appears, strongly affected by ionospheric absorption. We need to look at the time variation of the lightning source.

21. WWLLN Lightning We can use lightning rates and LT variation from the experimental VLF World Wide Lightning Location Network to see how variation of the lightning sources effect our Dunedin-observed whistlers. WWLLN suggests that there are essentially no lightning in our conjugate region. While the WWLLN DE will only have been ~5% at this time, its probably roughly correct. Not a lot happening right there! Still, lets look at the UT variation of lightning in these regions.

22. Looking at the WWLLN data We divide up the WWLLN data into multiple regions and look at how the UT variation of WWLLN-detected lightning changes. We focus on a region more or less around our conjugate (1), and a similarly sized region WEST of our conjugate (2), and EAST of our conjugate (3).

23. Region 1: WWLLN Conjugate observations Lightning counts do not vary much with UT, only by a factor of 2. This is expected for a region which dominated by sea (where lightning activity is almost flat). Lightning peaks at roughly 4 UT, i.e., 18 LT, as expected for most lightning. 4,326 lightning locations from WWLLN in 2005

24. Region 2: West WWLLN Conjugate observations In region 2 the lightning rates vary more over the day, principally from lightning occurring over the far east of Russia, by say a factor of 4. Lightning peaks at roughly 6-7 UT, i.e., 17 LT, as expected for most lightning. 23,320 lightning locations from WWLLN in 2005

25. Region 3: East WWLLN Conjugate observations In region 3 the lightning rates vary a lot more over the day, principle from lightning occurring over North America, by a factor of >10. Lightning peaks at roughly 2-3 UT, i.e., 19.5 LT, dominated by local nighttime conditions (as reported in the WWLLN talk). 38,512 lightning locations from WWLLN in 2005

26. Tentative Conclusions as to our source Looking at the high whistler rate observed in Dunedin, and the diurnal variation of the observed whistlers relative to the diurnal variation of WWLLN-reported lightning, it seems reasonably likely that many of our whistlers come from North American lightning, rather than only due lightning occurring in our conjugate region. If conjugate lightning was the only source of our whistlers, the Antarctic Peninsula should have a whistler rate >400 times larger than ours (conservatively), or an average of 64 whistler events minute-1, >10 times higher than Burgess and Inan reported as their yearly average. Burgess and Inan, J. Geophys. Res., 98, 15643-15665, 1993.

27. Do we have evidence for this path? Actually, we know that transmissions from the VLF station near Seattle, NLK, demonstrate two preferred magnetospheric propagation paths to Dunedin, where they are regularly observed by Neil Thomson's Doppler Experiment [Clilverd and Thomson, 1992]. Clilverd and Thomson, J. Atmos. Sol. Terr. Phys., 54, 1075-1080, 1992. Two preferred propagation paths were found as shown. The entry point E1 is a sizeable distance from NLK (approximately 2Mm), showing sferics can travel a very large distance between entering a duct, traveling through the magnetosphere to X1, and then propagating subionospherically to the DU receiver. Clearly lightning source regions do not have to be centered on the conjugate point.

28. Conclusions • The Eötvös University Automatic Whistler Detector (AWD) has functioned very successfully in Dunedin, detecting a large population of whistlers over a wide range of L-shells and with a relatively small false trigger rate. • Dunedin whistler rates are high, higher than estimated from looking at Tihany’s whistler rates and conjugate lightning activity. • The average whistler event rate for Dunedin is 0.16 min-1, whistler trace rate is 0.41 min-1, with 2.5 traces/whistlers (over 3/4 year). • Dunedin-observed whistlers are most common during the day time, in start contrast with earlier findings from multiple sites. • This is most likely to be due to lightning in North America, suggesting whistler ducting is an important part in controlling whistler rates. • Whistlers may be generated many 1000's of km from the source lightning.

29. Acknowledgements • I’d like to thank tour Finnish hosts for making this Workshop happen! • The installation of the Automatic Whistler Detection Experiment Dunedin Receiver with the support of a University of Otago Research Grants which covered the costs of the PC and János' travel. • We thank the NASA LIS/OTD Science Team and Global Hydrology and Climate Center (GHCC) for collecting and providing the satellite observed lightning data.

30. Craig stands outside the Waihopai Valley Government Communications Security Bureau base, New Zealand's contribution to the ECHELON spy network. Waihopia is near the aptly named Spy Valley winery, which Craig was visiting [29 Aug 2006]. Thankyou!Are there any questions?