SuperDARN (Super Dual Auroral Radar Network)

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1 SuperDARN (Super Dual Auroral Radar Network) What is it? How does it work? Judy Stephenson Sanae HF radar data manager, UKZN

2 Ionospheric radars Incoherent Scatter radars AMISR Arecibo Observatory Sondrestrom Radar

3 Coherent scatter radars STARE Scandinavian Twin Auroral Radar Experiment (~1978) Goose Bay Radar (~1990) Sanae Radar (~2008) Christmas Valley SuperDARN radars (2011)

Several experiments by French researchers (C. Hanuise, J-P. Villain, et al.

4 Brief history of coherent scatter radars STARE = Scandinavian Twin Auroral Radar Experiment VHF, fixed frequency Only sensitive to plasma irregularities in the auroral E- region DARN SuperDARN VHF HF (8-20 MHz) DARN = Dual Auroral Radar Network Several experiments by French researchers (C. Hanuise, J-P. Villain, et al.) using HF frequencies demonstrated that an HF radar could be built. Ray Greenwald and J-P Villain set up a 4- antenna, HF system in Alaska (very near where AMISR now is located) in 1982.

5 SuperDARN is born! The concept moved from 4 antennas to 16, with separate transmitters on each antenna. With 16 antennas it was possible to generate 16 separate beams by controlling the phase of the HF transmission (and reception) at each antenna. Electronic control of the phase meant that the beam selection could be done virtually instantaneously. Radar was built and became operational in October, Goose Bay, circa 1990 Sanae, circa 2000 SuperDARN started in 1993 with 4 radars in the NH and 1 in the SH. When apartheid ended a radar was jointly funded by US and SA governments. Sanae first light 1997

6 Various improvements have been made to the radars over the years Addition of interferometer array Determine angle of arrival of backscattered signal Improves the determination of geographic location Increased transmitter power Improvements Initially we could only transmit 400 W at each antenna New radars transmit > 2000 W at each antenna Increased data storage (I and Q data) Real-time analysis of the data Real-time transmission of data from the radars to the home institutions. Fully digital radar most advanced are Sanae and Buckland Park

8 Improvements

Current state of SuperDARN 23 12 17 different research groups

9 Current state of SuperDARN different research groups from 11 countries PLANNED 2 more mid-latitude pairs; 1 equatorial

DOME C Dome C is a French-Italian scientific station located on the East Antarctic ice sheet. Concordia Research Station opened at Dome C in 2005. The geographic coordinates of Dome C are 75.1 S, 123.

This makes it the highest geomagnetic latitude radar and the highest altitude radar. Its field of view conjugates with those of the McMurdo radar and the just installed South Pole radar.

10 DOME C Dome C is a French-Italian scientific station located on the East Antarctic ice sheet. Concordia Research Station opened at Dome C in The geographic coordinates of Dome C are 75.1 S, E. It is one of several summits or "domes" of the Antarctic Ice Sheet. It is very close to the Antarctic geomagnetic pole, at about 3200 m above sea level. This makes it the highest geomagnetic latitude radar and the highest altitude radar. Its field of view conjugates with those of the McMurdo radar and the just installed South Pole radar.' Dome C is located on the Antarctic Plateau. Dome C is one of the coldest places on Earth. Temperatures hardly rise above 25 C ( 13 F) in summer and can fall below 80 C ( 112 F) in winter. Most of the cargo is moved to Dome C by traverse from Dumont d'urville Station, covering 1,200 km in 7 to 12 days depending on weather conditions. Station personnel and light cargo arrive by air, using Twin Otter aircraft from DDU or Mario Zucchelli Station at 1200 km.

11 Operations Each radar runs continuously, 24 hours/day In most cases the radars generate a complete scan across the field-of-view in one minute. This is called common mode and is the result of integrating along each of the 16 successive beams for 3s. This mode must be operated for at least 50% of the time. Special mode can be run for a maximum of 20% of the time. These are often modes tailored and matched for satellite data passes. Currently, the Van Allen Probes.

12 Operations Discretionary mode can be run for at most 30% of the time. In this mode individual experimenters, or a subset, may run a mode that suits their research interest. Fields-of-view have been extended in range to cover 3000 km (Digital radar over 5000 km). Most of the radars are able to send a selection of data to home institutions in real-time. Virginia Tech collects all the real-time feeds and generates real-time data products for space weather use. Data archive in Saskatoon, Canada.

13 SD targets are Ionospheric Irregularities Field-aligned Plasma can move freely along magnetic field. Density irregularities quickly disappear in the parallel direction (Lorentz force law) Irregularity structures are frozen in to the ambient plasma and move with the ambient plasma. (Faraday s Law) In the F-region the plasma convects due to the E cross B drift. Thus a radar measuring motion of the irregularities is measuring the local electric field.

Coherent backscatter = coherent radar When spacing of irregularities = λ/2 scatter of the signal in the backwards direction results in constructive interference.

14 Coherent backscatter = coherent radar When spacing of irregularities = λ/2 scatter of the signal in the backwards direction results in constructive interference. The result is a strong backscattered signal (similar to Bragg scattering in a crystal) If we transmit at 12MHz, λ is 25 m irregularities maxima are 12m apart Orthogonality of the transmitted signal with the background magnetic field (aspect condition) guarantees maximum returned power.

15 A problem with VHF radars Because irregularities are field-aligned, in order to get coherent backscatter, the transmitted wave must interact with the irregularities where the k vector is perpendicular to the magnetic field. VHF waves are essentially unrefracted by the ionosphere. The only place where the k vector is perpendicular to the field is in the auroral E- region.

16 Why Operate at HF? HF signals are refracted in the ionosphere as they traverse gradients in electron density. The transmitted signals can be reflected back to the radar by: 1) Plasma irregularities if the ray is quasi-perpendicular to the magnetic field OR 2) The ground F-Region B Ionospheric plasma irregularities Advantages of operation at HF frequencies: 1) Refraction of signals provides access to targets in the F-region ionosphere (~ km) 2) Refraction of signals extends the radar range to > 3500 km. 3) Low power requirements allows for continuous operation.

17 Layers of the atmosphere

18 SuperDARN radars are electronically steered Beam formation is done electronically by changing the relative phase of each transmitter at each antenna

19 Basic characteristics of a Uses pulses, not continuous wave transmission Uses phase-locked receiver This means that the phase of the received signal can be compared with the phase of the transmitted signal. SuperDARN radar The result is two outputs from the received, I and Q. The I component is the signal in-phase with the transmitted phase. The Q component is the signal at 90 degrees to the transmitted phase.

20 Measuring velocity with double pulse If the target is stationary then the phase of the received signal from the first pulse will be the same as the phase of the received signal from the second pulse. If the target is moving then the two phases will be different and the amount of difference is proportional to the velocity of the target.

21 Double-pulse ACF Let S1 be the complex signal received from pulse 1 and let S2 be the signal from the second pulse. S 1 = A 1 e x p (i φ 1 ) S 2 = A 1 exp (i (φ 1 + ω τ ) ) The 0 th lag of the autocorrelation function is R0. R * 2 0= S 1 S 1 = A 1 The first lag of the ACF is R1 R * 2 1= S 2 S 1 = A 1 exp (i (φ 1 + ω τ ))( A 1 e x p ( i φ 1 )= A 1 e x p (i ω τ ) Note that the random phase has canceled out.

With scatter coming from multiple ranges the random phases no longer cancel exactly.

22 What if we use three pulses? This is somewhat modified from our previous picture. Here, we have backscattered signals from an extended range of irregularities. With scatter coming from multiple ranges the random phases no longer cancel exactly. BUT, if we send out a sequence of triple pulses the random phases will be different for each set of triple pulses. When we add them all together the random phases will tend to cancel.

23 The range aliasing problem When we add up a large number of pulse sequences the random phases tend to cancel and we can ignore those terms. However, there are terms that have no random phase and yet have mixes of signals from two different locations in the ionosphere. This is the range aliasing problem We can fix it (at least partly) by using a multi-pulse pattern instead of a simple repeating pulse.

24 Multipulse pattern Here we can only get the first lag of the ACF from the first two pulses. We get a second lag from the second and third pulse and we get a third lag from the first and third pulses.

25 [0, 9, 12, 20, 22, 26, 27] Current SuperDARN Pulse Sequence 7 pulses 2.4 ms.3 ms pulse length (45 km range gate) no repeated lags first missing lags at 16τ and 19τ by Schiffler τ Note: Pulse length (typically 300μS) determines gate range (typically 45 km) Fundamental lag (typically 2.4 ms) determines Nyquist frequency which in turn determines highest velocity

26 What does an ACF look like? (saved in.rawacf files) For each range gate and beam Note that the real and imaginary components are in quadrature (i.e. 90 degrees out of phase). Bad lags. Not all data points are valid and are called bad lags. This happens when: Receiver blanking Cross range interference Empirical shape The frequency of the ACF gives you the velocity of the moving plasma.

27 How do we get the Doppler velocity? ACF is of the form: S = A e x p (i φ ) where φ = tan -1 (Im/Re) Phase varies linearly Between jumps of +- π 1. Φ = v Use the Doppler velocity equation: <VD> = c <ωd>/ (4π f transmit)

28 What does an ACF look like? Note also that the amplitude decreases with lag. The decrease in amplitude of the ACF is related to the width of the Doppler power spectrum. Faster decay of the amplitude implies a wider spectrum. The amplitude at lag 0 (only real) is related to the backscattered power.

29 What does a RADAR measure? (saved in.fitacf file) Range to target Amplitude of echo Velocity of target Single velocity? Turbulence Direction of target motion (interferometer, or back array needed)

30 Range-time plots

31 Field-of-view scan plots

You can only get directly measured vectors when two radars have an overlapping field of view AND both radars are seeing backscatter from the same geographical location.

32 You can only get directly measured vectors when two radars have an overlapping field of view AND both radars are seeing backscatter from the same geographical location. When these conditions are met that's when we have the highest quality data and the least ambiguity. Combining radars to get convection maps Restricting the global analysis of convection to directly overlapping observations eliminates a large amount of data.

33 Using Map Potential to get convection maps Map-Potential: makes use all the available data! Expand the polar cap potential in terms of spherical harmonics. Fit the complete set of line-of-sight velocities to determine the coefficients of the polar cap potential.

34 Example of result from Map Potential Convection maps are SuperDARN s most important product. The only experiment that can give a global snapshot of convection.

35 Coming up in Part 2: What can we do with SuperDARN radars?

Using the Radio Spectrum to Understand Space Weather

Using the Radio Spectrum to Understand Space Weather Ray Greenwald Virginia Tech Topics to be Covered What is Space Weather? Origins and impacts Analogies with terrestrial weather Monitoring Space Weather