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Incoherent & Coherent Scatter Radars
Transcript of Incoherent & Coherent Scatter Radars
Bowles executed the idea the same year (in 6 weeks)...
...and it worked...
...though not quite as expected...
...it turns out you do not need a 1000 ft antenna!
First permanent ISR built: Arecibo (1963)
Incoherent & Coherent Scatter Radars
Incoherent Scatter Radar (ISR)
ISR data analysis
CSR data analysis
Coherent Scatter Radar (CSR)
No clear consensus as to who fathered the technique
HF communications have been operated since the 1920s
The first over-the-horizon systems appeared in the 1950s
The legend says that during WWII, communication operators using HF (3-30MHz) and pointing their antennas due North could sometimes communicate between the US and Europe
(pulse compression, timing, phase...)
ISR spectrum (Sondrestrom)
CSR Doppler velocities
Ionospheric absorption (electron-wave interactions)
Focusing/defocusing (only HF)
Pulses used for the following information:
Frequency (HF to UHF in this case)
Phase (used for Doppler measurements)
Pulse length (range resolution)
Pulse repetition (range ambiguity)
Amplitude (T/R power)
n < 1
n = 1
n < 1
n < 1
The ionosphere is dispersive (mostly at HF frequencies):
delays (reduced phase velocity)
In reality, both ISRs and CSRs rely on coherent scatter!
Thomson scattering: an electron is accelerated by a harmonic electric field (transmitted pulse), and then re-radiates an electric field of the same frequency.
If electrons were moving around unimpeded , then the returned signal would be a result of incoherent scatter.
Electron distribution is constrained by ion motion, and in the ionosphere ion motion is controlled by plasma waves
Bragg condition: density fluctuation with scale size L result in constructive interferences (coherent bakscatter) if
L = λ/2
Radars measure raw voltages: plasma parameters are estimated after processing.
The auto-correlation function (ACF) is calculated from the raw voltages and used to derive all parameters except the delay (time between transmit and receive)
Bowles, K.L. (1958), Observation of vertical incidence scatter from the ionosphere at 41 Mc/sec, Phys. Rev. Lett., 1, 12, p.454-457
Evans, J.V. (1969), Theory and practice of ionosphere study by Thomson scatter radar, Proc. IEEE, 57, 4, p.496-530
Greenwald, R.A. et al. (1993), DARN/SuperDARN: A global view of the dynamics of high latitude convection, Space Sci. Rev., 71, p.761-796
Ruohoniemi, J.M. et al. (1998), Large scale imaging of high latitude convection with SuperDARN HF radar observation, JGR, 103, A9
Ponomarenko, P.V. et al. (2009), Refractive index effects on the scatter volume location and Doppler velocity estimates of ionospheric HF backscatter echoes, Ann. Geophys., 27, p.4207-4219
Skolnik, Introduction to Radar Systems, McGraw-Hill, 2008.
Davis, K., Ionospheric Radio, The Institution of Engineering and Technology, 1990
Ne, Te, Ti, Vi, Ion composition
Vi, spectral width, irregularity scale
The radar receives a Doppler shifted signal
UHF: operating frequency between 200 MHz to 1.3 GHz
Scatter from ion-acoustic waves
HF: operating frequency between 10-50 MHz
Scatter from a wide variety of decameter scale plasma instabilities
The ACF of a received pulse sequence is passed to a Fourier transform:
The plasma line (due to electron Langmuir waves) is rarely observed. The ions line is the routinely observed spectrum of ISRs.
ISR scatter due to density fluctuation created by ion-acoustic waves: the theory of such waves is complex but well known, and the shape of the observed spectra is related to plasma parameters.
Collective behavior: λ >> Debye length
500-1000 km range
60ºx5º beam (alt., azim.)
up to 4000 km range
detailed plasma diagnostics
wide variety of scale sizes
scatter always present
continuous operation (low power)
expansive and power hungry
complex fitting problem
limited plasma diagnostics
very sensitive to propagation effectes
Levenberg-Marquardt fitting of integrated spectrum
The ion mass is used to find ion fractions (oxygen, hydrogen, nitrogen and nitrogen oxide)
CSR practical issues
ISR practical issues
Coherent echoes from large scale irregularities can contaminate ISR data and mask the expected spectrum
Time dependent structures are difficult to interpret: vertical/horizontal motion yield similar observations, so do radar sweeping (azimuth and elevation scans)
2 types of scatter:
- Ground scatter: if ionosphere densities are high enough (daytime), signal reflected to the ground and back to the radar. Ground scatter moves slowly as the ionosphere background density changes.
- Ionospheric (coherent) scatter: field aligned plasma irregularities (decameter scale, i.e., gradient drift instabilities...). Ionospheric scatter moves with background plasma flows (from a few m/s to km/s). Density fluctuations much larger than ion-acoustic waves observed by ISRs (also scale size is larger).
Field-aligned irregularity scatter is sensitive to aspect angle with geomagnetic field
Wide range of plasma instabilities and flows: case by case analysis.
At high latitude, most plasma flows driven by ExB drift: can use Doppler velocities to calculate electric field in the ionosphere. Combining several radars enables mapping of large scale plasma flows.
Spherical harmonic statistical model
Scatter observation is highly sensitive to propagation effects: ionospheric refraction, delays, aspect condition (angle with geomagnetic field), ground geometry and physical properties (i.e., oceans are good reflector, but frozen oceans are not). Interferometric data helps remove some ambiguities.
Since plasma waves vary, no direct plasma parameters (temperature, density) is available: great care must be taken in interpreting the data.