the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jan 2026
Mean Energy variation of Diffuse Aurora Observed with the ASIS Spectrograph
Abstract. We analyze more than 20,000 spectra of diffuse aurora recorded by the ASIS spectrograph over two winters, focusing on events classified as diffuse by an AI-based system, which includes pulsating auroras as a major subset. Electron characteristic energies were derived from calibrated red/blue emission ratios using a lookup-table approach based on forward modeling. To assess the systematic evolution of these energies with magnetic local time (MLT), we applied a Bayesian regression framework to the median of the mean-energy estimates, providing a robust characterization despite the large intrinsic variability of diffuse precipitation. The results reveal a clear hardening of the precipitation toward the dawn sector. Using the AE index as a proxy for geomagnetic activity, we further show that the MLT-dependent trend is influenced not only by the instantaneous AE level but also by the recent magnetospheric history, as revealed by time-lagged classifications. These findings indicate that diffuse auroral precipitation retains an imprint of preceding magnetospheric dynamics and are consistent with chorus-wave scattering modulated by evolving plasma-sheet conditions.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Annales Geophysicae.
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Status: open (until 28 Mar 2026)
- RC1: 'Comment on egusphere-2026-385', Kirsti Kauristie, 18 Feb 2026 reply
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- 1
The manuscript presents results from a statistical study analyzing energy of electron precipitation causing pulsating auroras. Precipitation energy is estimated with the ratio between red (630 nm) and blue (478 nm) emission lines’ intensities as derived from spectrograph data. The study gives a very robust confirmation of earlier findings proposing hardening of the precipitation energy in dawn sector with increasing Magnetic Local Time (MLT). In addition, new quantitative information about the MLT trend is presented. The data analysis methods used in the study are very professional and the manuscript is clearly written. I propose acceptance of the manuscript with some minor clarifications as specified below.
A general remark: I wonder how valid the MLT trend is at late dawn hours? Figure 2 shows that the number of data points in MLT bin 08 is rather small. I get the impression that Figure 5 has given guidance for the formula of Eq. 2, which inherently assumes the trend of increasing energy to cover also the late dawn sector. Do the already known properties of chorus waves or Arase observations support the trend to cover also early prenoon sector (with the recognition that the analysis by Ito et al., covers dawn sector until MLT 6). Furthermore, is there any risk that the energy estimates in the MLT bin 08 are somehow contaminated by illumination of the rising Sun?
Minor comments:
Line 28, Lorentz factor: This may not be a familiar term for all readers. Please, give a more detailed explanation for it.
Line 62, characteristic energies of electron precipitation causing optical pulsations: Extending the range to several hundreds of keV may not be appropriate here, because at those energy levels precipitation is a more relevant factor in D-layer chemistry than in auroral observations.
Line 84, introduction of the ASIS at Tromsö: It would be good to give here also the relationship between UT and MLT for Tromsö, because it helps the reader to interpret some data plots later in this manuscript (e.g. Figs 1, 3, and 4)
Line 102, classification by Nanjo et al.: Besides the auroral categories, it would be good to mention something about the AI-code capability to make distinction between cloudy conditions with background auroras and diffuse auroras.
Lines 127-128 introduction of the GLOW and Transsolo: The concept “stream” may not be familiar for all readers. Please clarify.
Figure 4, panel a: Colors in the strip showing AI categorization are confusing because they do not match with the colors given in the legend on the right side of the panel.
Equation 2: It becomes later clear in the text that “breakpoint” in the formula refers to MLT=4, but it would be good to mention it explicitly already here when introducing the formula. The “+” sign at the end of the formula is mysterious.
Lines 187-188: I guess that this is textbook material, but if possible with reasonable effort it would be nice to have a reference for the statement of 1.4826xMAD corresponding to the Gaussian-equivalent standard deviation.
Section 4.2, magnetospheric memory: The decision to discuss lags of 2 and 24 hours sounds a bit arbitrary…Is there a specific reason to select those two lag times? The conclusion of AE index exceeding 300 nT during the 24-hour period prior to pulsations to be a characteristic feature for the observed MLT trend is, in my opinion, somewhat weak, because ≥24h periods of continuous AE<300 nT are rather rare.