Research on energy input into the polar upper atmosphere and its response

(The following sentences are extracts from Tsuda et al., A proposal on the study of solar-terrestrial coupling processes with atmospheric radars and ground-based observation network, Radio Science, 51, doi: 10.1002/2016RS006035, 2016.)

The EISCAT_3D radar will give us great opportunities to make major breakthroughs in the science of Solar-Terrestrial Physics. High energy particles precipitate from the magnetosphere into the polar upper atmosphere along Earth's magnetic field lines and create many interesting phenomena such as "aurora borealis display" [e.g., Brekke and Egeland, 1994]. Part of the energy from the solar wind changes its form, and is transported to the lower atmosphere and to lower latitude regions. On the other hand, the polar region is also the area where part of the Earth's atmosphere outflows to the space. The phenomena occurring in the polar upper atmosphere are characterized by their rapid variability in time and space [e.g., Arnoldy 1970; Sandholt et al., 2002]. The EISCAT_3D radar will have a unique capability to investigate three-dimensional structures of the upper atmosphere and ionosphere with high temporal and spatial resolution. We aim to contribute to the development and preparation of transmitter modules of the EISCAT_3D radar system to make Japanese scientists work on such breakthrough research topics under international collaboration with the EISCAT scientific association.

The EISCAT scientific association was founded in 1975, conducting observations of the polar upper atmosphere over northern Scandinavia since 1981, and Japan joined the EISCAT scientific association in 1996 [Röttger et al., 1995; Matuura et al., 1995]. The EISCAT radars enable us to measure ionospheric parameters from the D-region ionosphere (above about 60 km) to the topside ionosphere (up to about 1500 km). In addition, the combination of the EISCAT systems in northern Scandinavia and Svalbard allows extended investigations of the polar ionosphere in the auroral zone and the polar cap. Coordinated measurements with other ground-based instrumentation, satellites, and rockets have been actively carried out, in order to understand various geophysical and plasma-physical phenomena occurring in the polar upper atmosphere. However, there are some unavoidable limitations to investigate physical phenomena characterized by their rapid variability in time and space. To overcome such limitations, the EISCAT scientific association has been proposing a project to construct a new state-of-the-art incoherent scatter radar system, named EISCAT_3D, in northern Scandinavia. The EISCAT_3D project has been selected in the Roadmap 2008 for large-scale European research infrastructures (as one of ten projects in environmental sciences) by the European Strategy Forum on Research Infrastructures (ESFRI).

EISCAT user communities have discussed science cases to be realized by the EISCAT_3D radar. Documents of the EISCAT_3D science cases were published by international science working groups [e.g., EISCAT scientific association, 2014b; McCrea et al., 2015]. New mathematical principles of phased-array radar experiment design and data analysis have been also discussed in the communities [Lehtinen et al., 2014]. Using the EISCAT_3D radar under international collaboration Japanese scientists will investigate physical processes of the following important phenomena occurring in the polar ionosphere and thermosphere/mesosphere: Auroral substorms and fine structures, the three-dimensional electric current system, ion outflow, heating and dynamics in the thermosphere, interaction between plasma and neutral particles, atmospheric variations in the vicinity of auroral arcs, and dissipation of atmospheric gravity waves. Moreover, it is expected that we will obtain a lot of knowledge about the vertical coupling of the upper and lower atmosphere and also interaction between the polar atmosphere and the mid- and low-latitude atmosphere. The new knowledge will significantly contribute to improvement of climate models. The EISCAT_3D radar can contribute to space weather forecasting under collaborations with the other IS radars as well as new satellite missions by providing operational weather centers with high quality and continual ionospheric parameters. In addition, the EISCAT_3D radar further can contribute even terrestrial weather forecasting since the EISCAT_3D radar can measure winds from the troposphere to the lower stratosphere that are important for long-term weather forecasting.

Figure 1. A schematic illustration of the EISCAT_3D radar system. It consists of 109 sub-array system (hexagonal shape). Each sub-array system has 91 antennas.

References:
Arnoldy R. L., Rapid fluctuations of energetic auroral particles, J. Geophys. Res., 75, 228-232. 1970;
Brekke, A. and A. Egeland, The northern lights, Their heritage and science, Grøndal Dreyer, 1994.
EISCAT scientific association, "EISCAT 3D Science Case" https://eiscat3d.se/project/fp7/science-case, 2014b.
Matuura et al., Japan-EISCAT collaboration on the Svalbard radar: Scientific significance, J. Geomag. Geoelectr., 47, 681-684, 1995
McCrea et al., The science case for the EISCAT_3D radar, Progress in Earth and Planetary Science, doi:10.1186/s40645-015-0051-8, 2015.
Lehtinen, M., I. I. Virtanen and M. R. Orispää (2014), EISCAT_3D Measurement Methods Handbook, Deliverable D6.7 of Work Package 6: Performance Specification of the European Commission 7th Framework Programme project. https://eiscat3d.se/sites/default/files/Handbook_D67v100_0.pdf
Röttger et al., The EISCAT Scientific Association and the EISCAT Svalbard Radar Project, J. Geomag. Geoelectr., 47, 669-679, 1995.
Sandholt et al., The cusp in rapid transition, J. Geophys. Res., 107(A12), 1427, doi:10.1029/2001JA009214, 2002

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System Description of EISCAT_3D Radar

(The following sentences are extracts from Tsuda et al., A proposal on the study of solar-terrestrial coupling processes with atmospheric radars and ground-based observation network, Radio Science, 51, doi: 10.1002/2016RS006035, 2016.)

The EISCAT_3D radar is equipped with a multi-static active phased array radar system, making it possible to observe three dimensional plasma parameters in the upper atmosphere. The EISCAT_3D radar system will have the following key capabilities [EISCAT scientific association, 2014b]: Resolution of space-time ambiguity, three-dimensional volumetric capability, sub-beam width measurements, increased sensitivity and the resulting temporal resolution, continuous monitoring of the solar variability on the terrestrial atmosphere and climate, and model validation for space weather and global climate change. The whole concept of the EISCAT_3D radar system was discussed and summarized by members of the "EISCAT_3D Design Study" project (between May 2005 and April 2009), financially supported by the Sixth Framework Programme (FP6) of the European Union [e.g., EISCAT scientific association, 2009; Wannberg et al., 2010]. The system design has been updated and specified during the FP7 "Preparatory Phase" project (between October 2010 and September 2014). The latest technical specifications of the EISCAT_3D radar are summarized in the document prepared by the EISCAT scientific association [EISCAT scientific association, 2014a].

The EISCAT_3D radar system will be installed at five locations in northern Scandinavia. It consists of one transmitting and receiving site, named core site, and four receiving only sites, named remote sites. The core site will be constructed in Skibotn, Norway. Two remote sites will be located in Bergfors, Sweden and Karesuvanto, Finland, about 100-150 km apart from the core site in Skibotn. The rest of two remote sites will be constructed in Andenes, Norway and Jokkmokk, Sweden, about 150-250 km apart from the core site. Information on observational sites are summarized in Table 3.

An overview of specifications of the EISCAT_3D radar system is shown in Table 4. Each site will have antenna arrays of about 10,000 individual antennas. The antenna element is characterized by the crossed-dipole elements tilted back towards the ground plane. The tilted elements provide excellent steering, up to a zenith angle of 60 degrees, without excessive changes in polarization ratio or antenna terminal impedance [EISCAT scientific association, 2014a]. The antenna array will consist of 109 sub-arrays, and the sub-array will consist of 91 dual-polarization antenna elements. They will be organized into a hexagonal grid. A schematic illustration of the EISCAT_3D radar site is shown in Fig. 5. The central array of each site will have a size of about 70 m across, and the outlying arrays will be distributed within a few km from the central array.

Signals with a frequency of 233 MHz will be transmitted at the core site. The maximum transmitting power will be about 10 MW. The transmitted signals are scattered in the ionosphere and return back to the ground. The returned signals will be detected by sensitive receivers at the core and remote sites. The EISCAT_3D radar system will have capabilities of digital beam forming of at least 100 beams simultaneously, and adaptive polarization control at all sites. The minimum pulse length of the EISCAT_3D radar will be 0.5 μs.

There are four stages to complete the whole EISCAT_3D radar system. At the first stage, the core site in Skibotn and two remote sites in Bergfors and Karesuvanto will be developed. Transmitting power at this stage is about 5 MW, but it will be doubled (~10 MW) at the second stage. The rest of two remote sites in Andenes and Jokkmokk will be constructed at the three and four stages, respectively.

Table 1. Specifications of the EISCAT_3D radar system, based on the technical description and part of the design study description (EISCAT scientific association, 2009 and 2014a)

References:
EISCAT scientific association, "EISCAT 3D Final Design Study Report" https://eiscat3d.se/project/fp6, 2009.
EISCAT scientific association, "EISCAT 3D: The next generation international atmosphere and geospace research radar Technical Description" https://eiscat3d.se/content/eiscat3d-technical-description, 2014a.
EISCAT scientific association, "EISCAT 3D Science Case" https://eiscat3d.se/project/fp7/science-case, 2014b.
Wannberg et al., EISCAT_3D: A Next-Generation European Radar System for Upper-Atmosphere and Geospace Research, The Radio Science Bulletin 332(75-88), 2010.

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