The SuperDARN Space Weather Radar System Joseph B. H. Baker The Bradley Department of Electrical and Computer Engineering Virginia Polytechnic Institute and State University

Outline Introduction to Geospace. Space Weather as an emerging discipline. What is SuperDARN? Mid-latitude SuperDARN. The Blackstone Radar (“HokieDARN”). SuperDARN Science at Virginia Tech. Summary.

Space Weather Solar Eruption collides with Earth’s magnetosphere Causing currents to flow Into high-latitude ionosphere

The Magnetosphere The Magnetosphere is the region of near-Earth space that fully contains the geomagnetic field. It spans a region that stretches from ~10 Earth radii (RE) on the dayside to many hundreds of RE on the nightside (we think!). The Magnetosphere can be sub-divided into semi-distinct regions in which the properties of the plasma (i.e. density, temperature, energy) are principally controlled by the local magnetic topology. Interaction Regions: The Bow Shock, Magnetosheath, Magnetopause and Cusp The Outer Magnetosphere: The Tail Lobes and Plasma Sheet The Inner Magnetosphere: The Plasmasphere and Van Allen Radiation Belts In addition, vast current systems couple the various regions with each other, as well as with the high latitude ionosphere: Magnetopause, Ring, Tail, Neutral Sheet, and Field-aligned currents.

The Magnetosphere x Solar Wind

The Thermosphere and Ionosphere The Thermosphere is the region of the Earth’s atmosphere between 80-500km altitude. The Ionosphere is the charged particle (or plasma) component of the upper atmosphere. The Ionosphere can be sub-divided into layers (or regions): D Region (60-90 km), E Region (90-140 km), F Region (140-500 km) At lower latitudes the dominant source of ions is photo-ionization. At high latitudes impact ionization from magnetospheric particles can be dominant (i.e. aurora). Ionospheric plasma is highly susceptible to instability processes that create density structure. These plasma irregularities can be detected by radars and their F-region motion is proportional to the strength and direction of the ionospheric electric field. The ionosphere forms an important lower boundary for magnetospheric processes.

Magnetosphere-Ionosphere Coupling SUN SUN DAWN Dungey, 1961 The outer magnetosphere and high latitude ionosphere are coupled via electric fields and currents that are transmitted along quasi-equipotential magnetic field lines. Plasma convection in the magnetosphere driven by the solar wind and interplanetary magnetic field (IMF) is mirrored in the ionospheric convection at high latitudes. A dense cluster of ground-based instrumentation (e.g. magnetometers, radars, imagers) in a relatively small area of the Earth’s high latitude ionosphere can be used to infer the dynamics of the magnetosphere over vast spatial scales (c.f. spacecraft coverage).

Space Weather: An Evolving Discipline Since the beginning of the “Space Age” it has been known that the near-Earth space environment is hazardous (e.g. Van Allen Radiation Belts, solar flares, magnetic storms). In recent years there has been an enhanced awareness of space weather hazards because of our increased dependence on space-based technologies and our renewed ambitions for manned space exploration. Using instrumentation on spacecraft we have been able to map the space environment reasonably thoroughly and we currently have a good empirical understanding of “average” behavior or states. We have also developed a number of numerical models to describe different parts of the solar-terrestrial system, but these models have varying degrees of sophistication, have not yet been properly coupled, and at the present time they do not include all of the necessary physics.

The Goal of Space Weather Is it a good day for a space-walk?

Space Weather: An Evolving Discipline Over the past 40 years, terrestrial weather prediction improved considerably because of increased data coverage and improvements in numerical modeling and data assimilation techniques. Compared to terrestrial weather forecasting there are a number of reasons why the prospects for a reliable space weather are less promising: Electromagnetic forces that drive plasma convection are much stronger than the pressure gradients that drive winds. Response time of plasma in the space environment to electromagnetic forcing is much shorter than the response of the lower atmosphere. The spatial coverage of available measurements is inadequate. Desirable features of space science instrumentation: Multipoint and continuous

What is SuperDARN? The Super Dual Auroral Radar Network (SuperDARN) is a network of high-frequency (HF) radars developed to study plasma motion in the Earth’s high-latitude ionosphere. The principle backscatter targets are decameter-scale plasma irregularities. At the present time there are 14 radars in the northern hemisphere and 7 radars in the southern hemisphere. (There are always plans to build new radars!) Each SuperDARN radar is broadband, low-power, electronically-phased, and has the following characteristics: Operates between 9-18 MHz Transmits ~10 kW of peak power in 16 or more look directions (beams) Uses multi-pulse sequences to determine range and Doppler information Range resolution is typically 45 km Temporal resolution is typically 1-2 minutes All SuperDARN radars produce identical data products that are routinely combined to produce hemispheric characterizations of ionospheric convection.

Why Operate at HF Frequencies? HF radiation is refracted in the ionosphere as it traverses 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 B Ionospheric plasma structure F-Region Advantages of operation at HF frequencies: 1) Refraction of signals provides access to targets in the F-region where E=-VxB. 2) Refraction of signals extends the radar range to > 3500 km. 3) Low power requirements allows for continuous operation.

The Reality at Goose Bay circa 1983

SuperDARN Operating (PI) Institutions Johns Hopkins University Applied Physics Laboratory (1983) British Antarctic Survey (1988) University of Saskatchewan, Canada (1993) National Center for Scientific Research, France (1994) National Institute for Polar Research, Japan (1995) University of Leicester, England (1995) University of KwaZulu-Natal, South Africa (1997) University of Alaska (2000) Communications Research Laboratory, Japan (2001) La Trobe University, Australia (2001) Nagoya University, Japan (2006) Virginia Tech (2008)

Northern Hemisphere Radars King Salmon, AK (Japan) Kodiak, AK (USA) Prince George, B.C. (Canada) Saskatoon, Sask. (Canada) Kapuskasing, Ont. (USA) Goose Bay, Lab. (USA) Stokkseyri, Iceland (France) Pykkvibaer, Iceland (UK) Hankasalmi, Finland (UK)

Southern Hemisphere Radars Halley, Ant. (UK) Syowa, Ant. (Japan) Bruny Is., Tasmania (Australia) SANAE, Ant. (South Africa) SANAE, Ant. (South Africa)

Combined Fields-of-View of SuperDARN Radars Currently in Operation Northern Hemisphere 14 operating radars. Southern Hemisphere 7 operating radars. McMurdo, Dome C (2) and Zhongshan in development.

||||| Simple Doppler Radar ))))) Stable Frequency Source Power Amplifier ||||| Rf Amplifier In-Phase Output (I) /2 Quadrature Output (Q) Mixers Digital Receiver

Continuous Transmission ))))) ||||| V0 Transmitter Receiver Transmitter produces continuous transmission Receiver is on continuously and produces continuous sinusoidal response in both I and Q caused by target moving at velocity V0. I Q Sampling frequency must be at least twice the highest anticipated Doppler frequency (Nyquist Frequency). No knowledge of the range to the target.

Uniform Pulsed Transmission ) ) ) ) ) | | | | | V0 Transmitter Receiver Transmitter produces pulsed transmissions at a frequency greater than the Nyquist frequency. Receiver observes response due to returns from each pulse reflected from the target.. Sampling rate must be high enough to determine the Doppler spectrum The range to the target is determined to an ambiguity of nctipp/2

The HF Dilemma Requirement 1: tipp > 24 ms To correctly determine the range to a target, the inter-pulse period (tipp) must be sufficiently long that all returns from an RF pulse are received before the next pulse is transmitted. But HF radars are sensitive to backscatter from ranges out to distances > 3600 km: Requirement 1: tipp > 24 ms To correctly determine the Doppler velocity of a target, tipp must be sufficiently short to avoid aliasing. But high-latitude ionospheric irregularities can have drift velocities in excess of 3 km/s. At 15 MHz, this requires a sampling frequency of 600 Hz: Requirement 2: tipp < 1.6 ms This dilemma can be resolved by using multi-pulse sequences and spectral analysis techniques (e.g. Fourier analysis, auto-correlation function analysis).

SuperDARN Multi-Pulse Sequences 2 1 4 6 Original Goose Bay sequence  = 2.4 ms A 9 3 8 2 4 1 SuperDARN sequence 1995-2002  = 2.4 ms B 19 9 3 8 2 4 1 SuperDARN Sequence since 2002  = 1.2 ms C Sequence A has all lags through 16; Lag 13 is repeated and not used. Sequence B has first missing lag at 16. Sequence C is Sequence B with  set to 1.2 ms and additional pulse at beginning to provide unambiguous power profile. First missing lag is 16.

Kapuskasing Range-Time Plot (Beam 4) January 11, 2001

Kapuskasing Doppler Velocity Map January 11, 2001 January 11, 2001 01:10:00 – 01:11:47 UT

SuperDARN Space Weather Map SUN

How Quickly Does Space Weather Change? April 6, 2000 Magnetic Storm Sudden Commencement

SuperDARN Science Achievements Hemispheric structure and dynamics of ionospheric convection. Mesoscale signatures of magnetosphere-ionosphere coupling: Convection vortices associated with magnetic field-aligned currents. Ionospheric flow bursts associated with magnetic reconnection events or FTEs. Inter-hemispheric (i.e. north-south) conjugacy of ionospheric convection. Convection associated with auroral activations (so-called “substorms”). Ionospheric irregularities and high latitude plasma structures (patches). Electromagnetic waves: MHD, ULF, Magnetic Field Line Resonances. Neutral atmosphere: Gravity waves, mesospheric winds, planetary waves. More generally, SuperDARN space weather convection patterns have been widely used to interpret localized features identified in other datasets.

Mid-Latitude SuperDARN The first generation SuperDARN radars were sited near 60 degrees magnetic latitude and have been very successful at monitoring ionospheric convection during weak to moderate geomagnetic activity. However, during magnetic storms the rate of SuperDARN data capture is reduced: (1) Equatorward motion of aurora (2) Increased HF absorption To overcome these shortcomings it was decided to build a second chain of radars at middle latitudes: Wallops Island, VA (2005) Hokkaido, Japan (2006) Blackstone, VA (2008) Hokkaido Wallops Blackstone

Wallops Radar (May 2005)

Wallops Measurements: SAPS/SAIDs 12 15 18 21 00 03 06 UT June 12,2005 June 13, 2005 Beam 4 Two-Dimensional Image of Sub Auroral Ion Drifts (SAID) within the Sub Auroral Polarization Stream (SAPS). Oksavik et al., [2006]

Hokkaido Measurements: TIDs Daytime TIDs (Sea Scatter) Nighttime TIDs (Ionospheric Scatter) The second mid-latitude SuperDARN radar became operational at Rikubetsu, Hokkaido, in December, 2006. The Hokkaido radar is being used to monitor the equatorward propagation of Traveling Ionospheric Disturbances (TIDs) generated at higher latitudes. By combining Hokkaido radar data with ground-based 630nm all-sky imaging and GPS TEC measurements it is possible to continuously monitor the propagation of wave fronts over a scale length of 6000km. Figures courtesy of T. Ogawa, STE Lab, Nagoya University

The “HokieDARN” Radar The third mid-latitude SuperDARN radar became operational at the Virginia Tech Agricultural Research and Extension Center (AREC) at Blackstone, VA, on February 2nd, 2008. The Blackstone radar is a collaboration between: Virginia Tech Johns Hopkins University Applied Physics Laboratory University of Leicester, U.K. The Blackstone field-of-view provides improved coverage of: (1) The sub-auroral plasmapause and trough regions (2) The onset region of auroral substorms The construction schedule for the Blackstone radar was accelerated so that it would be taking measurements during the first tail conjunctions of the NASA THEMIS spacecraft in February 2008.

The “HokieDARN” Radar

The “HokieDARN” Radar

New Antenna Design First generation SuperDARN radars use Sabre log-periodic antennas. Sabre antennas are sturdy and reliable but over time as electronic equipment has become cheaper the antennas have become an increasingly larger portion of the overall cost of building a SuperDARN radar. In 2004 a considerable modeling effort was directed toward identifying a simple and inexpensive alternative to the Sabre antenna. The result is a “twin-terminated folded dipole” antenna: Easy to construct. Significant reduction in cost.

New SuperDARN Antenna Front View Side View The antennas are strung between 55-foot traffic poles using Kevlar cable (dashed lines). The feed point is a 25:1 balun at the center of a plate mounted on the pole at 30 feet. The termination is in two 100-ohm resistors at each end of the plate. A rear reflector screen constructed from 24 wires runs the length of the array.

Antenna Modeling ECE graduate student Kevin Sterne is modeling the antenna performance and array factors using EZNEC software: Loads (Squares): 75 or 100 Ohms Source (Circle): Balun Effect of Corner reflector Full array (16 antennas) = 3934 segments Pattern Computation time: 30 minutes or more Array of 4 antennas = 1691 segments Pattern Computation time: minutes

Array Performance Modeling 3-D Model 4 antennas 2-D Elevation Slice on Bore Sight

Array Performance Measurements Models are idealized calculations. What is the array really doing? Measure the array pattern with an aircraft.

The Flight Plan

Summary SuperDARN is an international collaborative network of HF radars that is used to study the Earth’s upper atmosphere, ionosphere, and connection into Geospace. The primary data product is Doppler measurements of ionospheric convection. Much of the success of SuperDARN can be attributed to a unique capability for producing space weather maps of ionospheric convection at 1-2 minute cadence. The development of a new low-cost antenna design has jump-started construction of a second generation of SuperDARN radars that are providing new information about convection at middle latitudes during quiet and disturbed conditions. Plans to expand SuperDARN include several initiatives to build new radars at high and middle latitudes in both hemispheres. US efforts are centered on building 8 new radars across the North American sector. ECE Department at Virginia Tech is leading this effort!