For decades after the discovery of the ionized portion of the Earth’s atmosphere, ionosondes were the primary radio instruments used for study of the ionosphere.
One limitation of ground-based ionosondes is the inability to study the ionosphere above the F-region peak altitude and, when the E-region peak is very dense, between the E and F regions. An incoherent scatter radar (ISR) [Gordon, 1958; Bowles, 1958]
uses much higher frequency waves and does not rely on total reflection to obtain a signal and, as such, does not have the limitations of an ionosonde.
Incoherent scatter radars are able to provide information about a variety of iono-spheric parameters. As will be discussed in more detail below, the line-of-sight velocity of the plasma, the electron and ion temperature, and the plasma density are all quantities that an ISR can measure at a variety of ranges along the radar beam.
An incoherent radar transmits a short VHF or UHF pulse into the ionosphere (all
current ISRs operate at frequencies of ∼50 MHz or higher [Kelley, 2009]). At these high frequencies, the radar signal is mostly unaffected by the ionosphere; however, a small amount of energy is scattered by the electrons along the radar beam. When the wave pulse encounters an electron, the oscillating electric field of the wave causes acceleration and oscillation of the electron. The oscillating electron behaves as a small dipole antenna and re-radiates an electromagnetic wave at nearly the same frequency as the radar wave. Some of this scattered energy travels back to the (typ-ically) co-located radio receiver on the ground. Each electron that the transmitted wave encounters will radiate some energy back to the receiver but, due to the random nature of the electrons, the signals are out of phase (the reason for naming the radars
‘incoherent’). The relative amount of energy that is scattered back to the radar by ionospheric electrons is very small so ISRs must transmit using very high power.
After a pulse is transmitted, the radar continuously receives signal. Range is determined by assuming speed of light propagation in a vacuum and halving the delay time for a return signal. Different ranges can be examined by analyzing the signal at various appropriate delay times. An example of the Doppler spectrum from an idealized incoherent scatter echo is presented in Figure 1.8. At a given range, each electron encountered by the wave pulse will scatter a small amount of signal back to the receiver. As the number of electrons at a given range increases, the power of the returned signal increases. Therefore, the total echo power received at a particular range (represented by the area under the curve in Figure 1.8) provides a measure of electron density at that range. Since the returned echo power naturally decreases with range, normalization of the signal must be performed to determine the overall electron density profile. An ISR will often have an ionosonde station nearby which can be used to calibrate the electron density profiles by providing an accurate measurement of the F-region peak density.
The electrons that scatter the incoherent radar wave pulse are not stationary.
The electrons have random thermal motions and may also have a bulk drift motion along the radar beam direction. The thermal motion of the electrons results in a broadening of the received spectrum because the motion of each electron will cause a
Figure 1.8: An example of an idealized incoherent radar Doppler spectrum.
The figure presents the received power as a function of frequency. fT represents the transmitted frequency and fo is the Doppler shift (from which the line-of-sight velocity is determined). The spectral width ∆f is a measure of the ion temperature. The area under the curve represents the received power and can be used to measure the electron density. Figure A.1 from Kelley [2009].
different Doppler shift to the returned wave. If there is more thermal motion, there will be more spread of frequencies that return to the receiver. Since the electrons are linked to the ions, the width of the returned echo, represented by ∆f in Figure 1.8, is actually a measure of the ion temperature [Bowles, 1958]. The electron temperature can also be measured by an incoherent radar by analyzing the shape of the Doppler spectrum. The bulk motion of the electrons and ions in the ionosphere along the line-of-sight of an ISR can be determined by measuring the overall Doppler shift of the entire spectrum (represented by the frequency offset fo in Figure 1.8).
The first dedicated incoherent scatter radar was built at Arecibo, Puerto Rico [e.g., Tepley, 1997] in 1959. This famous radio instrument is the largest in the world and is located at geographic latitude 18.3◦N. Although famous for radio astronomy studies, Arecibo was originally built to study the ionosphere of the Earth. Several incoherent scatter radars now operate at equatorial, middle, and high latitudes. An ISR operates on Canadian soil near Resolute Bay, Nunavit [Bahcivan et al., 2010].
This new ISR is located deep in the polar cap region (geographic coordinates: 75.0◦N, 95.0◦E).
EISCAT
Line-of-sight ion velocity and electron density data from one particular ISR, the Euro-pean Incoherent SCATter (EISCAT) radar [Rishbeth and van Eyken, 1993], has been used for the research in this thesis. The transmitter for the EISCAT system oper-ates at 931 MHz and is located in Tromsø, Norway (geographic coordinoper-ates: 69.6◦N, 19.2◦E). The EISCAT radar system also consists of two passive receiving stations located in Kiruna, Sweden (geographic coordinates: 67.9◦N, 20.4◦E) and Sodankyla, Finland (geographic coordinates: 67.4◦N, 26.6◦E). Use of these two passive receivers, along with the Tromsø receiver, allows three independent measurements of line-of-sight drift velocities to be made at a set location in the ionosphere. Analysis of these tristatic velocity values allows the full ionospheric velocity vector to be determined.
A comparison of Hankasalmi SuperDARN data to EISCAT tristatic velocity values from 1995-1999 is the primary focus of Chapter 5 of this thesis.