Tuesday Apr. 19, 2011
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Homework #6 was returned today together with a set of solutions.  That will probably be the last homework assignment of the semester.

Today we will be looking at satellite detection and location of optical emissions produced by lightning.  In order to reduce the background signal from reflected sunlight the sensors isolate just a single bright emission in the lightning spectrum.  So this seems like a good place to mention lightning spectroscopy and show you an example of the optical spectrum of lightning.  We'll come back to this topic on Thursday and how spectra can be used to estimate lightning channel temperatures.

The figure below shows a conventional spectrometer or spectrograph.  The front of the spectrometer is pointed at a light source.  Light from the source first passes through a narrow slit (the width of the slit will partly determine the wavelength resolution of the spectrometer, i.e. whether it will be possible to separate closely spaced emission features in the spectrum).  Rays emerging from the slit are collimated by a lens and then passed through a prism (or a diffraction grating).  The light is refracted (bent) and dispersed (split into colors; the amount of bending depends on the wavelength).  A second lens focuses the parallel rays of light onto a detector or a piece of film.


You can eliminate the slit in the case of lightning because the lightning channel itself is narrow.  Lightning spectrometers also often use both a prism and a diffraction grating.  This has the effect of "straightening out" the spectrometer so that it is easier to point at lightning.  The diffraction grating might also increase the dispersion.


An example of an actual spectrum is shown below (source: Lightning Spectroscopy, E.P. Krider, Nuclear Instruments and Methods, 110, 411-419, 1973 (link to a PDF file) )

This spectrum extends from the near-ultraviolet to the near-infrared.  The first line in the Balmer Series for hydrogen (Halpha at 6563 Angstroms) can be seen clearly (the hydrogen comes from the dissociation of water vapor by the lightning).  Note also the bright lines at 7774  and 8683 Angstroms.  These are emissions from atomoic oxygen and nitrogen respectively (OI and NI denote atomic oxygen and nitrogen).  The OI line at 7774 is used by the satellite detectors that we will be discussing.

The satellite sensors that we will be discussing were designed by researchers at the NASA Marshall Space Flight Center in Huntsville Alabama.  An initial step in their program was to make measurements of lightning optical emissions from high-altitude aircraft flying well above the tops of the thunderclouds (a U-2 aircraft flying at appoximately 20 km altitude).  A publication "The Detection of Lightning From Geostationary Orbit," summarizes results from the aircraft measurements and discusses some of the factors that must be considered in the design of a satellite lightning sensor.



The area viewed by a lightning mapper aboard a satellite in geostationary orbit.  We will learn that most lightning activity falls between 35 S and 35 N latitude.


Lightning near-IR emissions measured above cloud top from a high-altitude airplane.  The 777.4 nm and 886.3 nm features (OI and NI) each contain 5 to 10% of the optical energy in a lightning flash.  The 777.4 nm line was chosen for the satellite detector.


Just like frosted glass, clouds will blur any image of the lightning channel inside the cloud.  The optical signal risetime and pulse width are each increased by about 150 microseconds.  The following figure helps to understand why this is true.
Photons emitted by a lightning channel are scattered (redirected) multiple times before leaving the cloud.  The path in pink undergoes a relatively low number of scatters.  The green path is scattered more times and takes longer to escape from the cloud.  It is difficult to distinquish between cloud-to-ground and intracloud discharges.

Somewhat surprisingly perhaps, there is very little absorption of the light signal by the cloud.

A lightning flash illuminates about 10 km diameter of the cloud top (sorry I do get carried away with the colored pencils).  Ideally this would fill a pixel on the CCD satellite detector.

We spent most of the remainder of the class discussing the Optical Transient Detector OTD).  This is the first of two satellite "lightning mappers" designed by the people at the NASA Marshall Space Flight Center.  You can read more about that research group and some of the activities on the Global Hydrology and Climate Center webpage.

The OTD was a prototype for the Lightning Imaging Sensor.  The OTD was launched on Apr. 3, 1995 and turned off in Mar. 2000, having operated 2 years beyond its expected lifetime.

It was launched into a nearly circular orbit 740 km (~450 miles) high.  The orbit was inclined 70o (relative to the Equator) so the satellite coverage extended from 75o S to 75o N latitude.

The field of view was about 1300 km x 1300 km (about 1/300 th of the earth's surface) and was imaged onto a 128 pixel x 128 pixel CCD sensor array (thus about 10 km x 10 km per pixel which is about the size of the cloud top illuminated by lightning)

Location accuracy (at nadir) was about 8 km.

The orbit precesses about 15 minutes (1/4 of a degree) per day.  It takes about 50 days for the satellite to review a specific location on the earth's surface at the same time of day.

In a year the OTD images most locations for a total time of > 14 hours (400 individual over passes).

The OTD detects lightning during the day and at night.  The typical daylight cloud background (sunlight reflected off the cloud tops) is 50 to 100 time brighter than lightning. 

Several steps must be taken in order to detect lightning signals superimposed on this bright background.
(i)   The pixel size corresponds roughly to the size of the cloud top illuminated by lightning.
(ii)  A 1 nm narrow band interference filter centered on the 777.4 nm OI emission was used.  Much of the cloud background   
      falls at other wavelengths and was blocked.
(iii) CCD signals were integrated for 2 ms - well over the duration of the lightning optical pulse.
(iv) successive frames were subtracted.  This subtracts out much of the slowly time varying background signal

This last process is illustrated below

The leftmost image shows a picture of the scene.  The middle two frames show the CCD data for pictures taken of the scene with and without a lightning.  It's hard to see the lightning until you difference these middle images.  The lightning appears in the 4th picture (the three pixels with values of 1 in the middle of the scene).

There are still sources of noise such as sun glint that create erroneous lightning counts.  The South Atlantic Anamoly is also a source of false triggers.  This is a region where the Van Allen Radiation Belts make their closest approach to the earth's surface.  The inner belt contains mainly high energy protons.  When the satellite passes through the belt, the high energy particles strike the CCD array and produce anamolous triggers.

We looked at some images of lightning activity derived from OTD data.



Here's the first image showing all of the lightning detected in 1999. 

Most of the lightning detected is found over land affected by the Intertropical Convergence Zone (ITCZ).  This refers to a feature in the 3-cell model of the earth's global scale circulation pattern.


The ITCZ is nominally located near the Equator (also labelled the equatorial low in the figure above).  It will move north of the Equator in summer (northern hemisphere summer) and south of the equator in the winter.  You can usually see a band of clouds on global satellites photographs that are associated with the ITCZ.


Here's the lightning detected during the months of December, January, and February 1999.  Note how the activity has shifted into the southern hemisphere.  The activity at higher latitudes in the northern hemisphere might be associated with storms forming along the polar front.


This is the June, July and August map for the same year.  Activity is now primarily in the northerm hemisphere.

78% of the lightning detected (and remember the OTD doesn't distinguish between cloud-to-ground and intracloud discharges) is found between 30 S and 30 N latitude.  88% of the lightning is found over continents, islands, and coastal regions.  There is much less activity over the oceans.

Here is a link to the source of these OTD GIF Images

One interesting result from 5 years of OTD lightning data is a new estimate for the global lightning flashing frequency:
44 +/- 5 flashes/sec

This is about half of the 100 flashes/sec value that was long thought to be true.  The 100 flashes/sec value dates back to 1925.

We were starting to run short of time so we finished with just a quick mention of the Lightning Imaging Sensor launched on Nov. 28, 1997 into an orbit with 35 degree inclination.  The LIS mapper is still operating.  You can read more about its design and look at examples of data here.