Day 5
On this day you will begin to relate electromagnetic waves and wavelengths to the ionosphere and discuss the ways to explore the ionosphere. This lesson wraps up with a brief discussion of the implications of understanding the ionosphere and how it can effects ground based operations, such as AM radio transmissions. Students should be introduced to the ability of the Millstone radar to explore the ionosphere. Effects of the ionosphere on communications, GPS, radio waves, and electric power grids can be presented as desired. Also, if resources are available, the Aurora Borealis (Northern Lights) can be presented as one of the phenomenon that takes place in the ionosphere.
Prep Work:
- Familiarize yourself with the MIT-Haystack web-site.
- Review effects of ionosphere on communication, GPS, radio waves, and electric power grids.
- If desired, obtain materials that show the aurora.
Class Outline:
The ionosphere is a portion of the Earth's atmosphere where ultraviolet radiation from the Sun has ionized the particles of the atmosphere into electrons and positive ions.
It acts like the screen......just as variations in the size in the mesh of the screen effects the wavelength/frequency that will get through, the electron density in the ionosphere effects what radio waves will reflect or get through. The incoherent scattering radar is able to detect this electron density, along with many other features.
INCOHERENT SCATTERING RADAR
Why does the dish of the telescope look like chicken wire? Shouldn't it be solid (like a mirror) so that the electromagnetic waves can't get through?
The radar dish is an open mesh, like our screen.
Practical reasons why the radar is a mesh:
- Saves weight
- doesn't blow in the wind
- snow (usually) gets through
- cost
- vegetation grows underneath....prevents erosion
As shown in the Radio Screens lesson, an open screen IS able to reflect a radio wave.
The radar operates by sending a pulse is sent up into the atmosphere. The height being analyzed is determined by the time it takes for the signal to return. The electron density is determined by the power of the signal returned. The Doppler shift of the frequency of the signal give the line-of-sight velocity. Other information (thermal electron velocity, thermal ion velocity, ion temperature) can be determined by interpreting the return signal.
At first, this seems like a simple enough task. However, the ionosphere has a very low density. As a matter of fact, the return from the ionosphere is the equivalent of the return you would get from illuminating a penny in the atmosphere, at a height of 300 km. This is equivalent to the distance from Boston to New York.
The leads to two simple requirements: the pulse sent out must have a lot of energy and the receiving dish must be very sensitive.
The Millstone radar can generate the following plots:
Data from the most recent experiment can be accessed on-line from this page.
On all four panels of this display, the x-axis (horizontal axis) gives the time of day and the y-axis (vertical axis) gives the altitude in the atmosphere. (Note that the time is in UT, or Universal Time. This is the time in Greenwich, England, and is also, in meteorological circles, referred to as Z, or Zulu time. Eastern Standard Time (EST) is four hours earlier than UT.)
The one of most interest in this lesson is the image in the upper-left corner labeled Ne (short for Number of Electrons, or Electron Density.) Similar to a weather radar that you may see on the local news, the colors indicate the intensity of the radar return. Red areas correspond to high electron densities and blue-green areas correspond to low densities. Note that, during the day electron densities are higher and the red area is closer to the Earth's surface. At night, this area is much higher. If the red area is considered a mirror, reflecting the AM radio waves, is can been shown that the higher this "mirror", the further over the horizon the radio waves can be bounced. In addition, the higher electron densities near the surface during the day can interfere with radio waves. The clearer (blue, or lower electron density) air during the evening permits radio waves to travel further without being attenuated or interfered with.
If you want your students to follow along with an experiment as it runs, you may check the Millstone Radar schedule. The MIT-Haystack web-site can be consulted from more information about following an experiment.
Here, as an example, is the August 1999 schedule:
The implications of knowing the electron density in the ionosphere goes well beyond simply determining how far away you can be from the WEEI radio tower and still be able to listen to Pedro Martinez chalking up another win. It can effect global communications, navigation systems, GPS, and electric power girds. More information can be found by consulting the Background Information page of this site.
Concerned groups include the military, short-wave radio operators,
There is an entirely new science growing out of these concerns call Space Weather. More can be learned about this at the Space Environmental Center.
The next step to take beyond this lesson plan, now that there is a basic understanding of the properties of the ionosphere and its effects on radio wave transmissions, is to explore the forces that effect the ionosphere. These include sun spot activity, solar flares, and coronal mass ejections. Theses events can greatly disturb to normal state of affairs in the ionosphere and not only lead to dramatic effects on communications, power grids, and pipeline, but also create one of natures most beautiful light shows: the aurora!