We present here some results obtained with the ASIS instrument, installed since October 2023. Scientific papers are currently under preparation, but we give here an overview of some key results of our instrument so far. We are measuring the auroral spectrum every 30 seconds, allowing us to monitor the auroral activity with a good time resolution. The Field of View (FoV) of the instrument is rather larger, of about 4°, and we integrate over the full slit, loosing then the spatial resolution.
- Proton Aurora
The ASIS instrument regularly monitors proton auroras, detecting emissions from the H-alpha and H-beta lines, as well as the primary spectral lines at 427.8 nm for N₂⁺, 557.7 nm for O(¹S), and 630 nm and 636 nm for O(¹D), as shown in the figure 1 below. The ASIS instrument is field-aligned, allowing us to monitor precipitating high-energy protons (approximately keV) that can collide with the ambient plasma, undergoing charge exchange. The resulting neutral hydrogen can radiate in the Balmer series. Secondary reactions also contribute to emissions at 557.7 nm and 630/636 nm. In this specific example, the intensities of the three main spectral lines are high enough to indicate the presence of auroral electrons as well.
Figure 1: Proton aurora - Red-dominated Aurora
The study of red-dominated auroras can serve as a valuable proxy for determining large fluxes of low-energy electron precipitation. In this context, we consider the red emissions (630 nm and 636.6 nm) to be more significant than the green line emissions as illustrated by figure 2. This situation is not uncommon and can actually persist for a considerable portion of the night as showed by Figure 3, with the example of the night 21-22 of November 2023. The first panel in figure 3 shows the ratio Red over Green lines emissions, and the blue-shaded period represents those Red-domintated aurora. Panel 2 and 3 represent the ratio Bue over Green and Red over Blue respectively. Panel 4 is showing the horizontal magnetic component as measured in Tromso. A statistical analysis of this phenomenon is currently underway.
Figure 2: Red-dominated Aurora
Figure 3: Ratios Red over Blue, Blue over Green, Red over Blue, and Horizontal magnetic component (nT) for the night 21-22 of November 2023 - Geomagnetic Storms and Sub-storms
During intense auroral activity, such as geomagnetic storms and sub-storms, the ASIS instrument is sensitive enough to detect faint spectral lines and molecular bands. One such example is shown in Figure 4, where emissions from the first negative band (1NG) of N₂⁺ and the first positive band (1PG) of N₂ are clearly visible. Additionally, faint spectral lines, such as the [N I] emission at 520 nm (with a lifetime of approximately 9 hours) and [N II] at 500 nm, are also distinctly observed.
Figure 4: Typical auroral spectra from an high activity - Energy estimations
In figure 3, the ratio Red over Blue line emissions has been plotted; this ratio can be used to estimate the energy of the precipiating electrons (see figure 5). Indeed, we can use look-up tables provided by various electron transport models, such as Transsolo (Barthelemy et al., 2025), using up to 32 streamns or models by Adachi et al. (2017) and Ono (1993), which use only 2 streams. These models are stationary, and are therefore only valid where the quasi-steady-state hypothesis holds, such as in quiet steady arcs and diffuse auroras (typically in the recovery phase). Auroral classifications are used following results from Nanjo et al. 2022, for the skibotn station, see https://tromsoe-ai.cei.uec.ac.jp/#/
Figure 5:(a) Red over blue emissions ratio to infer average. (b) Energy of the precipitating electrons based on the look-up tables obtaines using Adachi et al. (2017) in blue, and the Transsolo code in green (Barthelemy et al. 2025). (c) Horizontal component from Tromso observatory. Auroral classification from Nanjo et al. (2022) is indicated in panel (a) - Tools and Services
Spectra animations are generated on a daily basis to help visualize the auroral spectral dynamics throughout the night. An example of such an animation is shown below, for the night of January 1st to 2nd, 2025, during which red-dominated auroras were prevalent for a significant portion of the night. This type of animation can also be used to monitor continuum emissions, as the Field of View (FoV) of the ASIS instrument is large enough to capture large structures with continuum emissions. Those events are indeed of recent interest: recent observations of continuum emissions provide a unique opportunity to explore the mesoscale interactions between auroral precipitations, the ionosphere, and the thermosphere, offering insights into the formation of continuum emissions (Partamies et al. 2024).
Adachi, T., et al., Statistical characteristics of auroral substorms during a moderate geomagnetic storm observed by the Akasofu satellite system. Earth, Planets and Space, 69(1), 132, 2017. https://doi.org/10.1186/s40623-017-0677-4
Barthelemy, M., et al., synthetic spectra of the aurora: N2, N2+, N, N+, O2+ and O emissions, accepted for publication in J. Space Weather and Space Climate, 2025.
Nanjo, S., Nozawa, S., Yamamoto, M. et al. An automated auroral detection system using deep learning: real-time operation in Tromsø, Norway. Sci Rep 12, 8038, 2022. https://doi.org/10.1038/s41598-022-11686-8
Partamies, N., et al.,.: First observations of continuum emission in dayside aurora, EGUsphere, 2024, https://doi.org/10.5194/egusphere-2024-3669
