S-310-35 Experiment (DELTA Campaign)
Aurorae are luminous events visible in high-latitude regions. The events occur when electrons are precipitating from higher altitude to the upper atmosphere and the energy is deposited to excite the atmospheric particles (i.e., transfer the particles to high-energy state) existing in the lower thermosphere at around 100 to 120km altitude. Heating events caused by such precipitating particles or electromagnetic force bring large amounts of energy to the thermosphere. It was reported that atmospheric motion (wind) is induced to form a wind system with a distinctive structure in the upwardly vertical and horizontal directions. At the same time, many unsolved issues remain including the presence of significant velocity shear (sudden changes in wind's direction or velocity in a narrow range).
The purpose of the S-310-35 experiment was to elucidate the energetics and dynamics of the lower thermosphere in aurora. The experiment was conducted by observing the temperature and density of the atmosphere (nitrogen molecules) and the auroral emission rate with instruments onboard the rocket, and, by simultaneously observing the temperature and density of the plasma, the neutral wind, and the auroral emission distribution from the ground-based instruments. Since the main objective was to "elucidate the dynamics and energetics of the lower thermosphere associated with auroral energy input," the rocket was launched from the high-latitude "And°ya Rocket Range" in Norway (about Lat. 69 deg.).
One of the significant results, for example, was obtained by comparing the values derived by instrument to measure the temperature of molecular nitrogen with MSIS model, which is known as the most popular upper atmospheric model. The comparison revealed that the rotational temperature of the nitrogen molecules is high by 70K to 140K at an altitude of 110km (Fig. 2) while their molecular density is low by 70% at 95km altitude and 10% at 140km. If, in fact, the nitrogen temperature varies to this extent, the collision frequency between the neutral atmosphere and the ions should be very different. This finding requires us to reconsider the neutral wind, the ionospheric current, and the energy transfer rate.
We also carried out a detailed comparison of the upward wind velocity and the high temperature of nitrogen molecules derived from ground-based observations against the heating rate calculated based on EISCAT radar observation. As a result, we found that the upward flow occurs relatively soon after the change in heating rate, and that the wind velocity is related to the energy deposition rate provided by the precipitating particles.
Previous observations have shown that the atmospheric temperature in the region at the altitude between 100 and 120 km from the earth surface is approx. 200K to 400K and the plasma (ionized atmosphere) temperature is equivalent to or a little higher than that of the atmosphere. Observations by the sounding rocket launched from Uchinoura, however, indicated that, under certain conditions, the electron temperature in the ionosphere increases up to several times higher than usual. Subsequent research clarified that this high-temperature region is located near the center of the electric current (called the Sq current system) that flows in a horizontal direction in the ionosphere. Further research reported that this region is likely to be related to the field-aligned current flowing to connect the Sq current systems existing in the south and north hemisphere as shown in Fig. 3. The S-310-37 experiment's objective was to elucidate the generation mechanism of such a high temperature layer.
Based on the assumption that electron acceleration along the field-aligned direction is responsible for the heating, the newly developed superthermal electron-energy analyzer and the three pairs of electric-field detector played a key role in this experiment. This experiment also effectively employed the sounding rocket in a role to demonstrate the performance of the new instruments.
In the experiment, the electron temperature-probe and the Langmuir probe successfully confirmed, as expected, the presence of the hot electron layer with a temperature higher by about 500 to 600 K than that of the background, at an altitude of 95 to 101km. In addition, we successfully obtained an unexpected outcome. The fixed bias probe, which was installed to observe the small-scale electron density variation, identified significant perturbation in the electron density with several hundred Hz in the region from 97 to 120km including the region of high electron temperatures. This corresponds to several meters in the spatial scale and strongly suggests the presence of the plasma instability. Further, our observation revealed the following results:
1) The electron density perturbation is most significant as it approaches the center of the Sq current system.
2) The disturbance is modulated by rocket spin.
These facts suggest the presence of spatial distribution and disturbance anisotropy of the phenomenon, indicating that it is an anomalous region.