Pulsating aurora (PsA) is one of the major classes of aurora showing characteristic modulations in its luminosity. In general, PsA appears immediately after the substorm onset in the equatorward part of the auroral oval and is observed in the morning side continuously for a long time. PsA shows a wide variety of shapes, but, in many cases, PsA is composed of luminous patches of irregular shape having similar latitudinal and longitudinal scale sizes. PsA is known to have two outstanding periodicities: main pulsation (a few to a few tens of second) and higher frequency internal modulation (a few Hz, so-called 3 Hz modulation). Besides these modulations, recent high-speed optical imaging technologies have discovered rapid modulations whose frequency is higher than 10 Hz.
PsA has been believed to be caused by quasi-periodic precipitations of high-energy electrons from the magnetosphere. Such high-energy electrons have been considered to originate from the pitch angle scattering by the whistler mode chorus in the magnetosphere. The chorus waves are non-linear coherent waves of the whistler-mode; the chorus bursts appear typically every a few seconds, and rising tones that shows the frequency drift within a few hundred msec are embedded in the chorus bursts. Hence, the two outstanding modulation periods of PsA should be results of micro-structure of the wave particle interactions in the magnetosphere. In such a sense, morphological characteristics of PsA seen on the ground are manifestations of non-linear behavior of waves and particles in Geospace.
PsA has a long history of research since its discovery in 60’s. However, there are still several unclarified issues regarding the fundamental characteristics of PsA:
1) what process causes precipitation of the PsA electrons into the ionosphere
2) what mechanism determines the period of main pulsation
3) what process characterizes the interval of the 3 Hz modulation
4) what mechanism creates the wide variety of shapes
In particular, identification of the origin of the period of the two distinct pulsations is the most central scientific problem regarding PsA. In addition to these, the PsA electrons are known to ionize the lower thermosphere and mesosphere and may cause a depletion of ozone in the middle atmosphere.
To tackle the above-mentioned unresolved questions, we conduct a coordinated comprehensive research of PsA supported by a JSPS grant of KAKENHI Kiban-S in 5 years from 2015 to 2020. This project aims at better understanding the causal relationship between the micro-process of the wave-particle interactions in the magnetosphere and PsA in the ionosphere. We have assembled a team of experts of ground-based radio and optical observations, space-based satellite and sounding-rocket observations, and computer simulations to solve the focused questions on PsA, which is a manifestation of the non-linear wave particle interactions of whistler mode chorus waves.
The team is composed of outstanding researchers from different institutes in Japan, including Solar-Terrestrial Environment Lab. (STEL), Nagoya University, ISAS/JAXA, Tohoku University, National Institute of Polar Research (NIPR), and Univ. of Electro-Communications (UEC): all of the members share an expertise in space- and/or ground-based observations using a state of art technologies. The project is carried out by combining efforts of the following 4 teams:
1. Satellite observations team
2. Ground-based observations team
3. Rocket observations team
4. Computer simulation team
During the 5 years project period, we operate high-speed ground-based cameras in the northern Scandinavia (in Norway, Sweden and Finland) to observe rapid modulation of PsA. The incoherent scatter radars of Eurepean Incoherent Scatter (EISCAT) radar and the upcoming EISCAT_3D are also used to obtain plasma parameters in the ionosphere modified by PsA. These ground-based observations will be compared with the wave and particle data from the ERG satellite, which will be launched in 2016, in the magnetosphere to understand the connection between the non-linear processes in the magnetosphere and periodic variation of PsA on the ground. The computer simulation is then used to reveal the physical processes creating the causal link between the magnetosphere and ionosphere during intervals of PsA comprehensively.