The Day-Night Terminator

Prepared by Morris Cohen and Amanda AngellStanford University, Stanford, CA

IHY Workshop on Advancing VLF through the Global AWESOME

Network

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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 signalsHas potential application in earthquake detection

Source: www.nasa.gov

Day-Night Terminator

Source: http://time.gov/timezone.cgi?Eastern/d/-5/java

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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 ionospherethinner and less dense. Because of this the ionosphere begins at a higher altitude during the night than the day

Ionosphere

Ground

Day NightDay/Night

Terminator

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Sunrise/sunset times

http://aa.usno.navy.mil/data/docs/RS_OneYear.php

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Sunrise/sunset times

Full year times

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Twilights

6º below – civil twilight12º below – nautical twilight18º below – astronomical twilight

Horizon

6º below

12º below

18º below

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High altitude sunrise

Varying height of sunrise

h = RE(1-cosθ)cos θ

h

Civil twilight = sunset/rise at 34.7 km altitudeNautical twilight = sunset/rise at 136.2 km altitudeAstronomical twilight – sunset/rise at 296.5 km altitude

Sun sets at 85 km when sun is 9.43º below horizon

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Annual sun variations -- Sebha

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Annual sun variations -- HWU

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Annual sun variations -- NRK

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Diurnal variations

Day Night

Earth

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Diurnal variations at Sebha

0445 UT: Ionosphere showing typical nighttime variations

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Diurnal variations at Sebha

0515 UT: Sunrise terminator reaches Sebha

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Diurnal variations at Sebha

0545 UT: Sunrise terminator null effect in VLF data

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Diurnal variations at Sebha

0615 UT: Sunrise terminator second null begins

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Diurnal variations at Sebha

0645 UT: Sunrise terminator reaches transmitter

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Diurnal variations at Sebha

0715 UT: Ionosphere steady daytime signature

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Seasonal variations

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The Terminator and Earthquakes

Some studies have shown anomalies in the ionospheric day-night transition a few days before earthquakesStudies require further confirmation, large-scale examination

Source: T. Bleier, F. Freund, Earthquake Predictor. IEEE Spectrum, Dec. 2005. pp. 3-8.

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References

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)

Fraser-Smith, A., Introduction to the Space Environment: Electromagnetic Phenomena in the Lower Atmosphere