What we can do with SuperDARN
SuperDARN is primarily a ground-based radar system for observing the motion of the ionized part of the Earth’s upper atmosphere which is distributed in altitudes from 100 km to 500 km. This particular region of atmosphere is known as “Ionosphere” because it is partially ionized by the UV radiation from the Sun. The ionosphere is composed of charged particles such as ions and electrons, which is called plasma. Plasma particles in the ionosphere, which is produced by the influence of solar radiation as described above, are not stationary at all, but moving in the various directions. The radars of SuperDARN have been operated for more than 20 years, in order to observe such motion of ionospheric plasma in the global perspective. The sciences being performed using SuperDARN may include the following topics:
■ Study of convective motion of ionospheric plasma in the polar region
Plasma particles in the ionosphere, in particular those distributed in the high-latitude regions (i.e., Arctic & Antarctic areas), are constantly circulating by forces of incoming electromagnetic energy and momentum generated through the interaction between the Earth’s magnetic field and high-speed stream of plasma from the Sun (solar wind). The speed of such convective motion of ionospheric plasmas sometimes reaches a few kilometers per second. The radars of SuperDARN are able to observe their motion by analyzing Doppler shifts of obtained backscatter echoes from the ionosphere. Currently more than 30 SuperDARN radars are operative mainly in the northern and southern polar regions by the international collaborative efforts. By integrating all the data from two hemispheres, we can measure the motion of ionospheric plasma particles on a global scale with a temporal resolution of 1 or 2 minute(s), which enables us to reveal what physical process is driving the circulation of plasma particles in the Earth's ionosphere. For example, two SuperDARN radars at the Japanese Antarctic station Syowa operated by NIPR, and a radar in Alaska operated by NICT have been used to observe the highly variable plasma dynamics in the high-latitude regions. In recent years, several SuperDARN radars have been operative in the mid-latitude regions, for instance two Japanese radars in Hokkaido which were constructed by ISEE of Nagoya University. When the near Earth space is disturbed by the activities of the Sun (magnetic storms), high speed plasma motion is often observed not only in the polar region, but also in mid-latitudes. The radars in Hokkaido have captured several cases of such a high-speed flow event in mid-latitudes, which is now known as Subauroral Polarization Streams (SAPS). By compiling global and precise measurements of ionospheric plasma convection by SuperDARN in high and mid-latitude regions of both the hemispheres, we could better understand how the energy and momentum from the solar wind are transferred to the ionosphere, which will lead to an improvement of the accuracy of space weather forecasts in the ionosphere.
■ Study of generation mechanism of aurora
Aurora, which is seen in the lower part of the ionosphere at around 100 km altitude, is an illumination of the Earth’s atmosphere excited by precipitating high-energy electrons from the near Earth space (magnetosphere) along the Earth’s magnetic field lines. When and where aurora appears, the electric and magnetic fields, and electric currents are very much disturbed, and show very complex spatial and temporal variations. By operating SuperDARN radars in the auroral region, it is possible to image the electrodynamics in the vicinity of aurora arcs in a two dimensional fashion. In recent years, several special experiments have been conducted for observing aurora by combining ground-based optical instruments with SuperDARN radars in the polar region. Through such combined radio and optical experiments, we succeeded in elucidating the driving mechanism of disturbances leading to explosive behavior of aurora (auroral substorms). In addition, since aurora often shows a highly complex shape, the density of plasma in the ionosphere is very much disturbed during the breakup of aurora. Such disturbances are known to affect the global navigation satellite system (GNSS) such as GPS (Global Positioning System). Thus, continuous monitoring of the ionosphere using SuperDARN during courses of auroral substorm is highly expected to contribute to evaluating the space weather impacts of aurora to the ionosphere.
■ Study of atmospheric wavy structures in the ionosphere/thermosphere
Wavy structures are often seen in the ionosphere/thermosphere in the mid-latitude region. Such phenomena are called Traveling Ionospheric Disturbances (TIDs), and their fundamental characteristics have been studied extensively by using ground-based airglow measurements and ionospheric total electron content (TEC) estimation with GPS navigation signals. The radars of SuperDARN are also capable of detecting signatures of TIDs by using returned radio signals scattered from the ground or sea. A certain type of TIDs is believed to be generated directly by the energy input from the magnetosphere into the high-latitude part of the ionosphere/thermosphere, and well represents the redistribution process of the incoming energy from the near Earth space. SuperDARN can observe such a process in a global fashion. In addition, TIDs seen during night time over Japan are considered to be generated from the lower atmosphere; thus, the two-dimensional imaging measurements of such nighttime TIDs using SuperDARN can clarify the vertical coupling process between the different atmospheric layers.
■ Study of mesospheric phenomena in a context of global warming
Following the global warming of the Earth, cooling is on-going in the mesosphere, which is located in altitude from 70 to 90 km. There is a phenomenon called noctilucent cloud (NLC), which is believed to be a sign of the cooling of mesosphere. Noctilucent clouds are made of ice particles in the mesosphere and are visible from the ground by the scattering of sunlight. The expansion of NLCs to lower latitudes is believed to be an indicator of the progressing global warming. The radars of SuperDARN are able to observe peculiar radar echoes from the mesosphere during summer time in close association with the appearance of NLCs (Mesosphere Summer Echoes: MSE). MSE, so far, have been mainly observed in the northern and southern polar regions; thus, they had been called Polar Mesosphere Summer Echoes (PMSE). But, recent observations of a SuperDARN radar in Hokkaido demonstrated that MSE appear also in mid-latitudes. Integration of observational data from all the SuperDARN radars makes it possible to monitor the activity of MSE in a global manner, which would contribute to better understanding/predicting changes in the upper atmospheric environment related to global warming.