This is a simplified introduction to the incoherent scatter radar technique used at the Millstone Hill Observatory. As time goes on, links to more detailed and advanced material will be added. For example, another introduction to incoherent scatter radar is here. This is a dynamic document which is illustrated by near-real-time data when the radar is running. The daemon which updates this document attempts to screen out bad data, but on occasion it may fail, and the figure may not make the desired point. Next to each figure there is a link to a prescreened illustration, which you may want to view if the figure in the document seems wrong.
RADAR (RAdio Detection And Ranging) is a technique for detecting and studying remote targets by transmitting a radio wave in the direction of the target and observing the reflection of the wave.
The most basic property of a target which can be measured by radar is its distance from the radar, known as the range. This is accomplished by transmitting short bursts, or pulses, and measuring the time between the transmission and the reception of the echo. Since radio waves travel at the speed of light (c = 300,000 km/sec = 186,000 miles per second), range = c*time/2, where the factor of 2 is because the measured time is for a round trip to and from the target. The range, together with the direction of the target, determine its location, which is what is needed for many radar applications such as air traffic control.
The strength of the received echo can also be measured. This will vary with the distance of the target, its size, its shape and its composition. For example, the echo from a Boeing 747 airliner will be much stronger that the echo from a small commuter aircraft. The echo from a stealth aircraft is much smaller than from a commercial aircraft of the same size because its shape and composition are carefully chosen to minimize radar returns.
The target of the Millstone Hill incoherent scatter radar is electrons in the earth's ionosphere rather than a discrete hard target like an airplane. The ionosphere extends from about 100 km (60 miles) to 1000 km (600 miles) above the earth's surface. High energy ultraviolet radiation from the sun removes electrons from some of the atoms and molecules in this region, and these electrons can scatter radio waves. The density of the electrons ranges from about 10,000 to 1,000,000 per cubic centimeter. Since the amount of energy scattered from each electron is well known, the strength of the echo received from the ionsphere measures the number of electrons in the scattering volume, and thus the electron density or pressure. Thus the radar functions much like a barometer. An example of the variation of electron density with altitude is shown in the following figure.
The data shown in this and the following figures were collected on 01/06/97 at 14:41:32 UT. The pulse-length was 410.0 microseconds, which corresponds to an altitude resolution of 61.5 km. The transmitter power was 2.16 megawatts and the density was measured at 20 ranges between 200 and 500 km above the earth.
Another important measurement made by some radars is the Doppler shift of the echo. The radar transmits a radio wave of known frequency - 440 MHz in the case of the Millstone Hill Radar. If the target is moving away from the radar, the waves in the reflected signal will be stretched, and the echo will have a lower frequency than the transmitted signal. Conversely, if the target is moving toward the radar, the waves will be compressed and the echo will have a higher frequency. This is how police radar guns work. They just transmit a continuous wave. Since the distance to the target is unimportant, the traffic officer only needs to know which vehicle he is pointing his radar at. The radar gun compares the frequency of the wave reflected from the vehicle to the frequency of the transmitted wave, and displays the vehicle's speed.
An incoherent scatter echo comes from a very large number of electrons. These are not stationary, but rather are in random thermal motion. Thus the echo will not be at a single frequency, but instead will contain a range. or spectrum of frequencies near the transmitter frequency. As the temperature increases, the average velocity of the electrons increases, and the range of velocities increases. Put another way, the width of the spectrum increases. The width of the spectrum is then a measure of the temperature of the ionosphere, and the incoherent scatter radar functions as a thermometer. In fact, it turns out that there are two temperatures in the ionosphere. When an electron is removed from an atom, the remaining atom, which is now missing an electron, is known as an ion. The ion gas may have a different temperature from the electon gas. As a result of electrical interactions between the ions and electrons, the width of the spectrum measures the ion temperature. However, the spectrum usually has two peaks, or wings, and the height of these wings measures the electron temperature. The electron/ion mixture is known as a plasma, and in addition to the thermal motions, the entire plasma is usually in motion. In other words, there is a plasma wind. As a result, the entire spectrum will be shifted instead of being centered on the transmitter frequency. Thus an incoherent scatter radar also functions as a wind speed meter. An example of a spectrum is shown in the following figure.
The yellow crosses are the measured spectrum from 263.1 km. The red curve is a theoretical spectrum corresponding to an ion temperature of 468.6 degrees Kelvin, an electron temperature of 2004.0 degrees Kelvin, and an drift velocity (wind) of -11.0 meters/second.
Incoherent scatter spectra are analized by finding the temperatures and velocity which yield a theoretical spectrum which most closely matches the measured spectrum.
The next figure show the ion temperatures (red) and electron temperatures (yellow) derived from the width of the spectrum and the height of the wings of the spectrum.
The final figure shows the line-of-sight drift determined from the offset of the spectrum from the transmitter frequency.