1. The Day-Night Terminator Prepared by Morris Cohen and Amanda Angell Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network

2. 2 What is the Day-Night Terminator?  The day-night terminator is the line where the day side of the earth meets the night side. It is also called the solar terminator, or the twilight zone.  It marks an important transition from the daytime to the nighttime ionosphere.  The solar terminator has significance in many areas of VLF research:  Causes variations in the Schumann Resonances  Creates backscatter from transmitted VLF and Radio signals  Has potential application in earthquake detection Source: www.nasa.gov Day-Night Terminator Source: http://time.gov/timezone.cgi?Eastern/d/-5/javahttp://time.gov/timezone.cgi?Eastern/d/-5/java

3. 3 How does the Ionosphere change at the terminator?  During the daytime, the sun’s rays are ionizing the plasma in the ionosphere, creating a denser, thicker ionosphere.  During the nighttime recombination occurs, making the ionosphere thinner and less dense. Because of this the ionosphere begins at a higher altitude during the night than the day Ionosphere Ground DayNight Day/Night Terminator

4. 4 Sunrise/sunset times  http://aa.usno.navy.mil/data/docs/RS_OneYear.php

5. 5 Sunrise/sunset times  Full year times

6. 6 Twilights  6º below – civil twilight  12º below – nautical twilight  18º below – astronomical twilight Horizon 6º below 12º below 18º below

7. 7 High altitude sunrise Varying height of sunrise h = R E (1-cos  )cos  h  Civil twilight = sunset/rise at 34.7 km altitude  Nautical twilight = sunset/rise at 136.2 km altitude  Astronomical twilight – sunset/rise at 296.5 km altitude  Sun sets at 85 km when sun is 9.43º below horizon

8. 8 Annual sun variations -- Sebha

9. 9 Annual sun variations -- HWU

10. 10 Annual sun variations -- NRK

11. 11 Diurnal variations Day Night Earth

12. 12 Diurnal variations at Sebha 0445 UT: Ionosphere showing typical nighttime variations

13. 13 Diurnal variations at Sebha 0515 UT: Sunrise terminator reaches Sebha

14. 14 Diurnal variations at Sebha 0545 UT: Sunrise terminator null effect in VLF data

15. 15 Diurnal variations at Sebha 0615 UT: Sunrise terminator second null begins

16. 16 Diurnal variations at Sebha 0645 UT: Sunrise terminator reaches transmitter

17. 17 Diurnal variations at Sebha 0715 UT: Ionosphere steady daytime signature

18. 18 Seasonal variations

19. 19 The Terminator and Earthquakes  Some studies have shown anomalies in the ionospheric day-night transition a few days before earthquakes  Studies require further confirmation, large-scale examination Source: T. Bleier, F. Freund, Earthquake Predictor. IEEE Spectrum, Dec. 2005. pp. 3-8.

20. 20 References  http://solar-center.stanford.edu/SID/educators/earthquakes.html http://solar-center.stanford.edu/SID/educators/earthquakes.html  Melnikov A., Price C., Satori G., Fullekrug M. Influence of solar terminator passages on Schumann resonance parameters(2004) Journal of Atmospheric and Solar-Terrestrial Physics, 66 (13-14 SPEC. ISS.), pp. 1187-1194.  Predicting earthquakes, Granite batteries, Dec 14th 2005, SAN FRANCISCO, From The Economist print edition  T. Bleier, F. Freund, Earthquake Predictor. IEEE Spectrum, Dec. 2005. pp. 3-8.  F. Freund, Cracking the Code of Pre-Earthquake Signals, National Information Service for Earthquake Engineering,University of California, Berkeley (Found at: http://solar-center.stanford.edu/SID/educators/earthquakes.html)Cracking the Code of Pre-Earthquake Signals http://solar-center.stanford.edu/SID/educators/earthquakes.html  Fraser-Smith, A., Introduction to the Space Environment: Electromagnetic Phenomena in the Lower Atmosphere