The Helium Decipherer: How to Read the Sun’s Invisible Past express
The solar corona (the outer atmosphere of the Sun, heated to millions of degrees) glows in ultraviolet light, but these rays are absorbed by the air and never reach Earth. Direct images of the corona from space have only been available since the 1990s. Yet since the 1970s, astronomers have observed the Sun in the infrared light of helium. The brightness of this glow depends on the ultraviolet 'illumination' from the corona: a helium image is a spectroscopy fingerprint of the corona. The neural network decipherer, trained on pairs of simultaneous images, learned to reconstruct the original ultraviolet picture, like a translator from the language of shadows to the language of flame.
The accuracy is such that the reconstructed frames match the few historical satellite data. Now we can assemble a continuous chronicle of the corona since the 1970s and trace how the Солнца activity changed in past cycles. This method unexpectedly turns old ground-based telescopes into time machines.
Original abstract
as written by the authors arXiv:2606.08652
Routine full-disk EUV imaging has been available only since the modern era, such as SOHO and SDO. To extend EUV coronal context into earlier periods, we leverage the multi-decade availability of full-disk \HeI{} observations, whose absorption is modulated by coronal irradiance and magnetic topology and is widely used as a proxy for open-field regions. We present a diffusion-based conditional image translation framework, Coronal Hole-aware Diffusion Model Translator (CH-aware DMT), to reconstruct synthetic SDO/AIA 193 \AA{} EUV images from \HeI{} inputs. The model is trained on temporally co-aligned SOLIS \HeI{} and AIA 193 \AA{} pairs spanning 2011--2015 using a month-based split, where January--October are used for training, November is used for validation, and December for testing. On the held-out test set, the reconstructions preserve dominant full-disk EUV morphology (CC=0.92) and recover CH-related low-intensity structure (CC=0.84). We further assess historical applicability by (1) comparing reconstructed AIA 193 \AA{} morphology with SOHO/EIT 195 \AA{} over 2005--2015; (2) comparing reconstructed AIA 193 \AA{} images generated from KPVT \HeI{} inputs against Yohkoh/SXT soft X-ray observations; and (3) evaluating long-term reconstructed disk-integrated emission statistics against observational EUV series and independent solar activity proxies (sunspot number and F10.7 radio flux over 1974--2015). These results indicate that CH-aware DMT conditioned on \HeI{} can provide a physically plausible synthetic AIA 193 \AA{} coronal proxy for historical studies, supporting multi-decade analyses of large-scale coronal evolution before the direct EUV imaging was available.
Archival helium images, accumulated over decades to track sunspots, have now found a second life as the key to the invisible corona.
One day, neural networks will reconstruct the appearance of distant stars from their faint spectral fingerprints.