What's SuperDARN?
■a brief answer
"SuperDARN" is an acronym for Super Dual Auroral Radar Network - which is an international high frequency (HF) radar network for collaborative scientific research since 1995.
If trying to add a little more detail, SuperDARN is an international consortium for geophysical studies where many scientific research groups and researchers participate to run and study with a special type of HF radars. Each component/element of SuperDARN is an obliquely sounding coherent HF Doppler radar whose field of view covers a large ionospheric area (or upper atmosphere at auroral altitude, i.e., at an altitude above ~90 km) over 3000 km distance horizontally within about or more than 50 degrees azimuthally - such a radar which can detect targets at very long-distance ranges over the horizon is a kind of OTH (Over The Horizon) radars. Each SuperDARN HF radar can mainly detect velocity/motions of plasma (ionised gas) in the ionosphere. More than 15 scientific research institutions in over 10 countries have joined this international SuperDARN project and have been running more than 30 HF radars globally in both northern and southern hemispheres. By combining all the SuperDARN HF radar data, we can obtain distribution and its temporal evolution of basic and essential ionospheric or upper atmospheric physical parameters in mid to high latitude regions, i.e., global "space weather map" in the upper atmosphere. Research with SuperDARN greatly helps us understand "space weather", i.e., physical conditions in the area surrounding the Earth ("geospace") and physical mechanisms of the interaction and relationships among solar activities, interplanetary space (or solar wind), Earth's magnetosphere, ionosphere and many related phenomena like aurora, as well as possible physical links between the upper atmosphere and lower atmosphere, or surface weather and/or global climate changes.
A simple answer to the question, "What is SuperDARN?" is given above. For those who would like to learn more about SuperDARN and the related scientific subjects, a little more detailed description on aurora, space weather and the SuperDARN history and present etc. is provided...
■Aurora
As inferred from the name and the brief explanation above, SuperDARN is directly and/or indirectly related to aurora phenomena.
Aurora (or northern lights) is one of the most beautiful spectacular in nature occurring in the polar upper atmosphere (higher than about 100 km altitude) in both hemispheres on the Earth. Aurora can be found even in the atmosphere on other planets in our solar system like Jupiter and Saturn and it is now thought to be rather a universal phenomenon on planets that have their own magnetic field and atmosphere.
■Aurora, ionosphere, upper atmosphere, and "window to the space"
Many scientific researchers have revealed that aurora and related phenomena like geomagnetic disturbances are closely related to solar activity and solar wind conditions of charged particles and fields in the interplanetary space between the sun and the earth, and that it is a visual consequence or manifestation of complicated interaction between solar wind and Earth's magnetosphere and magnetosphere - ionosphere coupling. In such a sense, the upper atmosphere is thought to be a boundary region between the Earth and the space, and particularly polar ionosphere is often called as the "window to the space" and auroral behaviour and ionospheric conditions directly and/or indirectly reflect the dynamic status of Earth's magnetosphere and the interplanetary space near the Earth (or so-called "geospace") as both are directly connected via magnetic field lines. Therefore observing aurora and ionospheric or upper atmospheric physical parameters greatly helps us understand the status of the geospace, detailed generation and decay mechanisms of aurora and many other related phenomena.
■Aurora, "Space Weather", and its relationship with our human life
Geospace environmental conditions can, sometimes directly, influence our human life and activities. As radio waves, broadly used for a variety of communication, broadcast and satellite positioning (e.g., for GPS navigation), may refract, reflect and be absorbed in the ionosphere, the ionospheric conditions affect the radio wave propagation, possibly leading to serious communication failures and positioning errors. In particular, large ionospheric or geomagnetic storms can damage the ground electricity system (e.g., electric power generators and power grid system) and possibly cause a serious disaster in our human life. Large solar events like solar flares and CMEs (coronal mass ejections) producing extremely high energy particles and X-ray could trigger unrecoverable faults in satellite functionalities and even affect astronaut activities in space due to dangerous radiation exposure. Therefore, "space weather" is thought to be essentially important issues to be understood. Moreover, we started to know that upper atmospheric phenomena and conditions could possibly affect lower atmospheric conditions considerably, i.e., surface weather and global shorter and/or longer term climate changes.
However, it has been still difficult to know or predict exactly when, where and how auroral breakup, substorms and geomagnetic storms would happen. Our current understanding of the geospace system is far from complete, due to many difficulties in observational methods.
■Weather map
By the way, "weather map", i.e., surface weather chart is available and accessible for granted in our daily life as the surface weather observation network is equipped all over the world and imager data etc from weather satellites are also available nowadays, and weather forecast is indispensable for our daily comfortable and safe life. Upper-air observation up to about 30 km altitude or the stratosphere where radiosonde balloons can reach is also performed on a routine basis mainly by meteorological agencies in many countries to produce "upper-level weather chart" which is important for more precise weather forecast.
■Space weather map
To obtain the global and dynamic upper atmospheric conditions is nothing less than obtaining "weather map" in upper atmosphere (above around 100-km) altitude or "space weather map" in the boundary region between the Earth and the space like the surface and upper-level weather chart.
Due to the lack of technical methods to observe the ionospheric conditions globally(*), for long years, we needed to rely on one-point in-site satellite observation data collected for many years, and averaged patterns of upper atmospheric weather map (ionospheric plasma convection and/or electric potential map) under a variety of solar wind conditions were obtained in the past. But it was difficult to know and investigate how global magnetospheric and ionospheric convection pattern (plasma and field behaviour) responds to changes in solar activities and solar wind conditions and so on.
* More precisely, ground-based magnetic field observation has been made since about 200 years before, and magnetic field observation network on the ground (in conjunction with recent satellite magnetic field observation) revealed existence of the important electric current system in the magnetosphere and ionosphere related to aurora and geomagnetic disturbances including magnetospheric storms and substorms. But it is still difficult to monitor three dimensional electric current distribution in the whole magnetosphere and ionosphere, and even if it can be achieved, it is still difficult to deduce basic and essential particle motions and electric field/potential distribution for true understanding of physical mechanisms in the geospace.
■early SuperDARN era
To measure ionospheric plasma motions in a large area, VHF (very high frequency of 30 to 300 MHz) Doppler radars were developed as powerful remote sensing ground-based observational instruments in 1970s. Although one radar can measure only a line-of-sight Doppler component/spectra of the plasma motions in the ionosphere, twin radars looking over common volume in the ionosphere can derive two dimensional horizontal ionospheric plasma motions. Such experiments successfully detected e.g., dynamic temporal evolution of plasma motions in the E-region ionosphere (at an altitude between about 90-130 km) in a large area (up to ~1000 x 1000 km) associated with auroral substorms. However, there were some limitations in the techniques, e.g., difficulties in deriving important electric fields from E-region plasma velocity data. To overcome such problems, by means of characteristics of HF radio wave (3 to 30 MHz) which refracts and/or reflects in the ionosphere and can propagate to a very long distance over the horizon, and also using a simple physical relationship between plasma velocity and electric field in F-region ionosphere (above ~150 km altitude), HF Doppler radars were developed to obtain F-region plasma convection as well as electric field distribution in horizontally much larger area up to over 3000 km field of view (FOV) successfully in 1980s. These VHF and HF twin radar pair observation methods obtaining important ionospheric conditions in large areas are the idea of the original "DARN" - Dual Auroral Radar Network. After the success of the DARN experiments or project, many research groups got together and established "SuperDARN" - Super Dual Auroral Radar Network in 1995, which can observe large portions of the whole polar ionosphere in both (northern and southern) hemispheres with many sets of pair of HF radars. After the kickoff of SuperDARN, we can now obtain global ionospheric plasma convection patterns and electric potential map in a high temporal resolution of every 1-2 min in nearly real time, which can greatly contribute to the space weather scientific researches and other application studies.
■SuperDARN now
More and more research institutes and university groups have joined SuperDARN and accordingly the number of the radars as well as its combined fields-of-view (FOVs) have remarkably been expanding since the beginning of SuperDARN. Now it extends also to the higher polar region (PolarDARN) and lower mid-latitudes (StormDARN). The SuperDARN community has kept a good balance between international cooperation and competition, and as many researchers have joined the project, research subjects and areas have also been expanding. Such sustained efforts enable SuperDARN to address many scientific questions and issues at present... 2 institutes and 1 university research group in Japan have joined SuperDARN and have been running 5 SuperDARN radars in total so far and have actively contributed to researches with SuperDARN through a variety of collaborative studies with domestic and international research community.