SunGAN: Towards understanding of heliophysical processes with GANs 20 Apr 2021 Dr. Mehdi Cherti Joint work with: - Dr. Frederic Effenberger (Ruhr-University Bochum) - Dr. Ruggero Vasile (GFZ-Postdam) - Dr. Jenia Jitsev (FZJ) - Dr. Stefan Kesselheim (FZJ) 

Heliophysics + Auroras + Sunspots + Solar Wind + Magnetic Storms + Space Weather + Satellite Disturbances + Power Outages 

Heliophysics 

Solar Dynamics Observatory (SDO) SDO is a 3 ton satellite in geosynchronous orbit Instruments: - Helioseismic and Magnetic Imager (HMI): magnetic activity - Extreme Ultraviolet Variability Experiment (EVE): extreme ultraviolet irradiance - Atmospheric Imaging Assembly (AIA) - AIA Images on visible light, 2 UV and 7 EUV wavelength - 4096x4096 resolution taken every 12 seconds - Started on 2010, estimate mission end: 2030 

Objectives - Generative models for understanding of factors of variation in the solar data - Using (controllable) generative models to generate rare and interesting solar events, and use them for data augmentation in forecasting 

Solar data - We collected a subsample of ~40K images from SDO (each raw image is ~12MB in a FITS file) - 10 different wavelengths (channels), but we used only 193 Angstrom - Max resolution: 4096x4096 but we trained on 1024x1024 due to memory constraints - Intensity range from 0 to 16383 

Training: architectures - Different architectures trained: StyleGAN2, StyleALAE, BigGAN - StyleGAN2 provided the best results - BigGAN consistently mode collapsed - StyleALAE gave blurrier images than StyleGAN2 and was slower to train (progressive training) (StyleGAN2 architecture) 

Training: preprocessing - Contrary to natural images, histogram of pixel intensities of solar data is very skewed - To make the histogram less skewed we use the log transform - We found it much easier to learn generative models on “log(intensities)”, but it is still an open question if we can learn from raw data Cumulative distribution function of natural images from ImageNet (grayscale) Cumulative distribution function of Solar data 

Solving mode collapse By far the most frequent issue we had is mode collapse 

Solving mode collapse - Tuning the learning rate of discriminator with respect to generator helped - More importantly, differentiable augmentation from [1] successfully helped to prevent mode collapse - Translation and cutout augmentation operations were used [1] https://arxiv.org/abs/2006.10738 

Evaluation - While not perfect for the task, Fréchet Inception Distance (FID) was still helpful to detect mode collapse, to check for training evolution (learning curves) and find well performing models - Human evaluation is still needed, especially to make sure fine scale details are well modeled 

Evaluation Model Resolution FID No Log transform 512x512 Collapse from beginning +Log transform 512x512 108 +Tune LR 512x512 84 +DiffAug 512x512 35 +Double resolution 1024x1024 18 +Relativistic Loss[1] 1024x1024 12 FID ranges from ~200 to 0, lower is better [1] https://arxiv.org/abs/1807.00734 

Latent space control - Most techniques for latent space control need labels (e.g., smile or gender predictor), but we do not have labels - There are recent works that deal with unsupervised latent space control - GANSpace: Discovering Interpretable GAN Controls 

Latent space control - GANSpace idea: apply PCA to W empirical distribution - No labels needed - Each PCA component can can be used to modify a given W, then the image is generated and visualized 

Latent space control First PCA component: solar activity 

Latent space control Second PCA component: coronal holes (dark regions) in north and south poles 

Next steps - Investigate more closely semantics of latent space components - Improve fine scale detail using more multi-scale sophisticated architectures - Train on higher resolutions (2048x2048, 4096x4096): exploit recent DeepSpeed features such as Zero-OffLoad and model parallel training to deal with the GPU memory bottleneck - Train on more wavelengths (channels), they offer a richer and complementary information