The Wallops Island radar is part of the Super Dual Auroral Radar Network (SuperDARN), and is operated by the Johns Hopkins University Applied Physics Laboratory.
This network consists of 32 radars operating on frequencies between 8 and 20 MHz and looking into the polar regions of the Earth. These radars can measure the position and velocity of plasma irregularities in the Earth's ionosphere, the highest layer of the Earth's atmosphere. The movements of these irregularities respond to energy coupled into the auroral regions from the solar wind and magnetosphere. Therefore, SuperDARN data provides scientists with information regarding the Earth's interaction with the space environment.
SuperDARN radars utilize an array of electronically phased antennas that can be steered in 16 to 24 beam directions stepping in azimuth every 3.3 degrees for a total sector of 50 to 80 degrees, repeating every one or two minutes. For each direction, the radar detects backscatter within 45 km long range gates that begin at 180 km and extend to a maximum range that is usually greater than 3500 km.
All the of radars are essentially identical in operational concept, although there are several antenna antenna configurations to accommodate the physical conditions at the site. Each of the radars has two arrays of antennas. The primary array consists of sixteen antennas. The secondary interferometer array consists of four antennas and is used to calculate the elevation angle of the incoming signal. A phasing matrix attached to the antenna array is used for beam-forming and electronically steers the radar into the selected beam directions. The Wallops Island SuperDARN employs wide VSWR bandwdith dipole (``twin-terminated folded dipole (TTFD)'') elements that sit in a corner reflector curtain.
The radar transmits a short sequence of pulses in the HF band and samples the returning echoes. There are sixteen transmitters capable of about 600 watts output each, for a maximum peak power of 9.6 kW. In practice, the peak power is run closer to about 8 kW for most radars and the average power is about 290 W. The radars are frequency-agile and listen before transmitting to avoid interference and to extract the highest-quality science data from the returns. The Wallops Island radar has pre-programmed forbidden bands to further avoid interference.
The sequence of pulses, referred to as a multi-pulse sequence, is carefully designed to allow the Doppler characteristics of different targets to be determined at multiple ranges by using the Auto-Correlation Function (ACF) of the received samples, while the secondary antenna array provides vertical angle-of-arrival information that can be used to determine their altitude. Many sequences are transmitted and the calculated ACFs integrated over a period of several seconds to minimize the effect of noise. The final average ACF is then used to calculate the backscattered power, spectral width and Doppler velocity of the plasma density irregularities in the ionosphere.
All SuperDARN radars operate continuously (except for very infrequent data outages) and typically operate at a temporal resolution of 1-2 minutes. Although, special processing and operating programs can be employed upon request.
The radar is located at the NASA Wallops Island Flight Facility on Virginia's Eastern Shore. The radar is located near the road to ``the Island'' and has a great view of rocket launches. JHU/APL also has several wide-field airglow imaging systems and dual-frequency GPS receivers that can be deployed at Wallops for rocket and radar campaigns.
|Geographic Field of View|
|Geomagnetic Field of View|
SuperDARN Science at APL
Multi-Instrument Data Fusion
The high-latitude Poynting flux represents electromagnetic coupling between the magnetosphere and ionosphere (as opposed to partical flux). While point observations of the Poynting flux have been conducted for many years by in situ spacecraft instruments, Waters, et al (2004) first demonstrated that magnetic and electric field data from distributed sensors could be combined to create high-temporal and spatial resolution maps of Poynting flux. In the present work, we are combining magnetic field perturbations detected by AMPERE (http://ampere.jhuapl.edu) with electric field estimates from SuperDARN to obtain the Poynting flux.
Traveling Ionospheric Disturbances
Atmospheric gravity (buoyancy) waves are created when an air parcel is displaced from hydrostatic equilibrium by orographic (land forms), convective (thunderstorms), or Auroral (Joule heating) forcing. Gravity waves of preferred scale and orientation are often observed in the thermosphere where they interact with the ionospheric plasma through ion drag and recombination. Gravity waves the modify the ionosphere are called ``traveling ionospheric disturbances'' or TIDs. TIDs are thought to be an important means of coupling of energy from high to low latitude and low altitudes to high altitudes. Because SuperDARN is an HF radar, a portion of the transmitted signal is refracted back toward surface of the earth and scattered back. TIDs change the ionosphere enough to slightly modify this ground-scatter range, as shown in the figure.
Another TID structure is thought to be coupled to the E region of the ionosphere. The figure below shows 630.0-nm airglow images (F region) from Millstone Hill, Massachusetts, overlaid with radar returns from the E region on the same geomagnetic field lines. Dark regions of the airglow correspond to uplift of the F region and are coupled to regions of increased backscatter from the E region and velocities away from the radar (red shift).
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