Custom-Orientation Mollweide Maps in Python
Abstract
In heliophysics, directional data from observations or models are commonly visualized by all-sky maps in Mollweide projection. Existing plotting packages in Python are restricted in the map viewing orientation. Building on matplotlib.pyplot, we implemented a routine that enables creating Mollweide maps in arbitrary coordinate orientations while keeping figures fully customizable.
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Custom-Orientation Mollweide Maps in Python Jonathan Gasser*, Justyna M. Sokół, Maher A. Dayeh Southwest Research Institute, San Antonio TX, USA DASH Workshop 2025 October 19 – 24, 2025 San Antonio TX *Contact [email protected] INTRODUCTION Python ▪Widely used in science data analysis & visualization ▪Free, Open-Source, Non-commercial ▪Extensive Package Library ▪Interoperable, flexible, interdisciplinary Mollweide Maps ▪Common visualization tool for 2D imaging data (UV intensities, particle fluxes, column densities, CMB …) ▪Full-sky, pseudo-cylindrical, solid-angle (area) conserving projection ▪Geography, Astronomy & Astrophysics, Planetary & Space science, … Packages ▪Basemap (deprecated) ▪Matplotlib.Pyplot ▪Cartopy (geography-focused) ▪Healpy (for ‘healpix’ data format) Issues ⮽No Polar Rotations, limited Lon shifts ⮽Fixed Lon axis (left-to-right, geography) ⮽Incomplete coordinate grid ⮽Colorbar, labels etc. not customizable ⮽Plotting Artifacts PROCESSING m+1 longitude n+1 latitude m×n image pixels (Lons, Lats) coordinate meshgrid → apply Coordinate Rotation Draw patches on the plot: matplotlib.pyplot Figure Split map into patches along polar rotation’s great circle Insert coordinates to be rotated on boundaries Lon= 𝜑 and Lat= ± 𝛼 Rotation Angles: 𝝋, 𝜶, 𝜽 1 2 3 (𝐋𝐨𝐧’, 𝐋𝐚𝐭’) = 𝐑𝐳(𝛉) ∗𝐑𝐲(𝛂) ∗𝐑𝐳(𝛗) ∗ (𝐥𝐨𝐧, 𝐥𝐚𝐭) Coordinate Rotation: Coordinate Insertions: original coordinate meshgrid rotated coordinate meshgrid Inserted Coordinates…… map on the poles and the map boundary. Smearing over Missing pixels at poles or edges Gridlines missing or not rotated PRODUCTS APPLICATIONS ✓Arbitrary orientation Mollweide maps ✓Any image data, range, any celestial coordinate system ✓e.g.: ENA flux maps (IBEX, IMAP) Articles contributions: ▪A. Galli et al., ApJS 278:1 (2025) doi: 10.3847/1538-4365/abc451 ▪J.M. Sokół et al., ApJ 992:168 (2025) doi: 10.3847/1538-4357/adfa24 FEATURES ✓Full-Sky Mollweide Maps ✓Arbitrary Map Rotations ✓No Projection Artifacts ✓Right-to-left Longitude (Celestial) ✓Rotated Coordinates Grid ✓All labels, ticks, colorbar adjustable ✓ Add points, text ❑Make available on GitHub ❑Add curved lines ❑Alternative Map Projections Global ENA maps from IBEX-Hi at 1.1 keV, in three different map orientations: