New Finding on auroral electron acceleration: The acceleration region extends to unexpectedly high altitudes
Released: 18th., January, 2021, Academia Sinica, Institute of Astronomy & Astrophysics, Taiwan
Bright and elongated auroral arcs are typical aurora’s features seen in the night-side polar regions, and have been attracting people.
This type of auroras is generated by electrons downward accelerated by a static electric field above the auroras and hit/excited/relaxed with the atmosphere at ~100 km altitudes. Understanding the altitudes of the static electric field has been the key to understand the acceleration mechanism of auroral electrons. With 50-year observations made by sounding rockets and satellites, the acceleration takes place at mainly a few thousand km up to 20,000 km altitudes, the region where cold ionospheric plasmas and hot magnetospheric plasmas are mixed. However, it is unknown whether electrons accelerated above 20,000 km altitudes and precipitate to the auroral brightening heights, because simultaneously high angular electron observations onboard a spacecraft and high time resolution auroral cameras on the ground are required.
This study utilizes coordinated observations data by the Geospace exploration satellite ARASE flying at 30,000 km altitudes above typical acceleration regions, and ground-based THEMIS all-sky cameras to identify the auroral electron acceleration region. The ARASE satellite mainly targets wave-particle interactions in the Earth radiation belts, carrying a full set of scientific instruments including the high angular resolution electron instrument LEPe developed by the Taiwanese team, which enables us to measure characteristic electromagnetic fields and particle behaviors seen in auroral acceleration regions.
In this study, observation data by ARASE located at ~30,000 km altitudes were analyzed when a thin aurora moving toward low latitude was recorded by a THEMIS all-sky camera at the same field line of ARASE. Based on the analysis, as shown in Figure 1, the research team discovered that characteristic particles and electromagnetic signatures existed at much higher altitudes, which are very similar to those in the typical auroral acceleration regions usually at hundreds or thousands km low-altitudes.
Fig. 1. Time series data of latitude distribution of aurora (top panel) and Arase satellite observation. Particle, electric and magnetic field properties are consistent with the typical aurora acceleration region previously observed at low altitudes.
Notably, the monoenergetic downward electrons observed are strong evidence of an electrostatic acceleration above the satellite. Furthermore, the fine angular channels of the electron instrument LEP-e onboard ARASE measured downward accelerated electrons but no upward electrons, which indicates the downward electrons were lost at auroral brightening altitudes, for the first time (see Figure 2).
Fig.2. The velocity distribution of electrons at the time of (1) in Fig. 1. The blue line shows the boundary region predicted from the acceleration below the satellite. Downward-accelerated electrons were observed inside the fallen region, and upward-accelerated electrons were missing which indicates the downward electrons were lost at auroral brightening altitudes.
These observational results have been revealed that an auroral electron acceleration region can extend up to much higher altitudes than ever thought, and electrons can be accelerated at much higher altitudes down to the auroral brightening altitudes, as shown in Figure 3. Since parameters of background plasmas and magnetic fields are much different from those at the typical acceleration altitudes, the acceleration mechanism discovered at higher than 30,000 km cannot be explained by any theories previously proposed. The research team will focus on what is the maximum altitude possible to accelerate electrons above 30,000 km, and how the acceleration mechanism can exist at such high altitudes as the next step.
It is noteworthy that unique orbits and high performance of the ARASE spacecraft brought this achievement, which was not expected at the beginning.
Fig.3. Arase satellite and THEMIS cameras on the ground showed that the aurora acceleration region extended to the upper side of the satellite, and that electrons accelerated from ultra-high altitude were pouring into the aurora brightening region.
This research was presented in the paper Active auroral arc powered by accelerated electrons from very high altitudes, appears in Scientific Reports on January 18., 2021
Dr. Shiang-Yu Wang Email: email@example.com
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