地球の水素のハローEarth's hydrogen halo

About this image

Earth's outermost atmosphere, the exosphere, is a vast cloud of hydrogen atoms that glows faintly in ultraviolet Lyman-alpha light (121.6 nm). This glow can't be seen from the ground. NASA's Carruthers Geocorona Observatory watches it from about 1.5 million km away, near the Sun-Earth L1 point. This picture stacks 5,790 of its exposures, 5,458 hours in all, taken between November 2025 and October 2026. The frame is about 570,000 km across, with ecliptic north up. The glow is traced to about 270,000 km from Earth, where the data end, and it continues beyond.

The Moon is shown at 20 positions, about every 6 hours, as it passed in front of Earth on 17–22 February 2026. Its positions, size and brightness were measured in Carruthers frames; it is drawn without the motion blur of the hour-long exposures. The stars come from star catalogs (Gaia, Hipparcos, Tycho-2), placed where they stood behind Earth at the moment of the transit, 19 February 2026, 16:14 UT. Carruthers itself sees only a few far-ultraviolet stars.

What the colors mean

• The ultraviolet brightness is shown in false color on a logarithmic scale: deep violet where it is faint, then magenta and orange, and pale yellow around Earth
• The Moon is a soft gray; the stars are gently colored by their color index (B−V)
• These are not colors the eye could see: Lyman-alpha light is invisible to us

How it was made

• Data: Carruthers Level 0 files from the Wide Field Imager (18° field), doi:10.48322/2tde-1r42, from NASA's Space Physics Data Facility
• Each image calibrated: readout offsets from the masked rows, dark from the closed-filter images of the same day, conversion to photons per minute with the team's calibration. Flat field from 685 images pointed at least 20° away from Earth. Column stripes and a faint band along Earth's own detector rows removed; detector defects, stars and the Moon masked
• Every image placed on a common grid centered on Earth, in Earth radii, ecliptic north up, from the spacecraft's position and attitude (Earth's center measured in each image)
• The faint glow of interplanetary hydrogen fitted image by image, together with one halo model shared by all images, and removed. The zero level follows the halo's own fall-off with distance (about r^−2.07)
• Stacked by exposure time; after the masks, outlier rejection removed almost nothing
• Look: logarithmic brightness from 0.2 to 700 photons per minute per pixel, a "magma"-style color map, and a soft fade where the data coverage ends (about 43 Earth radii)
• Moon: 20 frames of 17–22 February 2026, about every 6 hours; center and brightness measured in each (frame minus the stacked halo); drawn as a disk of its true size at that moment, softened to the camera's resolution
• Stars: AT-HYG v3.2 catalog (David Nash / astronexus, CC BY-SA 4.0; positions and magnitudes from ESA Gaia DR3, Hipparcos and Tycho-2), 924 stars to magnitude 11, proper motion applied to February 2026, projected from Carruthers' position at the transit; size and brightness by magnitude, gentle color by B−V. Stars behind Earth's disk are left out
• Processed with my own Python tools. The processing code was written, and the processing was run, with the assistance of Claude (Anthropic); I directed the work and checked the results

Notes

• This is a stack and a composite: the halo from 5,458 hours of data, the Moon from measurements in 20 frames, the stars from catalogs.
• Apart from the catalog stars and the Moon disks described above, nothing was added. Nothing was generated with AI.
• The data references (DOIs) and acknowledgments are at the end of this page.