ARTEMIS

ARTEMIS is the NASA mission that has operated two THEMIS probes (P1 = THEMIS-B, P2 = THEMIS-C) in lunar orbit since 2011. SOPRAN retrieves the daily Level 2 CDF files from NASA SPDF (CDAWeb) and reads the FGM magnetic field and the ESA energy fluxes and moments. Reading needs cdflib and xarray ([artemis]).

Variables

Entry point (same for p2)	Dimensions	Units	Contents
art.p1.fgm.magnetic_field	time, component	nT	Spin-resolution (~3 s) field vector (FGS, GSE). Alias b
art.p1.esa.ion_energy_flux	time, energy_bin	eV/(cm^2 s sr eV)	Ion differential energy flux
art.p1.esa.electron_energy_flux	time, energy_bin	eV/(cm^2 s sr eV)	Electron differential energy flux
art.p1.esa.ion_density	time	cm^-3	Ion density (ground-processed moment)
art.p1.esa.ion_velocity	time, component	km/s	Ion velocity (GSE)
art.p1.esa.electron_density	time	cm^-3	Electron density

The energies of the energy fluxes are the per-record coordinate energy (time, energy_bin, eV). The L2 energy tables change from record to record, for example with the spacecraft potential correction, so SOPRAN keeps the native bins instead of forcing one shared axis. Bins missing from a table have NaN energy and are not drawn.

art = spn.artemis
time = spn.period("2011-07-01T06:00", "2011-07-01T08:00")

art.p1.fgm.magnetic_field.plan(time)          # dataset ID and source directories
b = art.p1.fgm.magnetic_field.load(time)
flux = art.p1.esa.ion_energy_flux.load(time)
flux.to_xarray().coords["energy"]             # (time, energy_bin) energy tables

spn.stack(
    art.p1.esa.ion_energy_flux.spectrogram(time),       # per-record energy axis; log color and log y
    art.p1.esa.electron_energy_flux.spectrogram(time),
    art.p1.fgm.magnetic_field.lines(time, components="xyz"),
).plot()

ESA modes

ESA stores each operating mode in separate variables. mode= selects one; the default is the continuous "reduced" mode.

mode	CDF variables (P1 ions)	Time resolution	Contents
"reduced" (default)	thb_peir_*	About 3 s	Continuous data with reduced angular resolution
"full"	thb_peif_*	About 2 min (P1 on 2011-07-01)	Data with full angular resolution
"burst"	thb_peib_*	Irregular	Short high-resolution intervals

art.p1.esa.ion_energy_flux.load(time, mode="full")
art.p1.esa.ion_energy_flux.spectrogram(time, mode="full")

Retrieval and file layout

Source files are daily files named like thb_l2_fgm_YYYYMMDD_vNN.cdf under https://spdf.gsfc.nasa.gov/pub/data/themis/<thb|thc>/l2/<fgm|esa>/YYYY/. The version vNN can increase with reprocessing, so SOPRAN picks the newest version for the day from the yearly directory listing. Retrieved files go to the Store's raw/artemis/themis/<thb|thc>/l2/<fgm|esa>/YYYY/ with their source and checksum recorded. When the Store already has a file for the day, SOPRAN does not connect unless download="always".

download	Behavior
missing (default)	Retrieve files for days that are missing
never	Never connect; missing days count as gaps
always	Recheck the listing and retrieve the newest version again

One day is about 37 MB for FGM and about 12 MB for ESA. When the archive has no file for a day, or download="never" and the file is not local, the default (missing="empty") returns an empty result. missing="warn" warns and missing="error" raises DatasetNotFoundError.

Interpretation notes

• Only the spin-resolution field (FGS) is read; the higher-rate fgl and fgh are not. The frame is GSE, and setting a View frame does not rotate it.
• ESA moments are the archive's ground-processed values; spacecraft potential correction and quality flags (*_data_quality) are as processed by the archive. The quality flags themselves are not read yet.
• Ion velocity is NaN during some intervals (for P1 on 2011-07-01, values exist only from 09:24 to 21:23 UTC). The archived velocity can be NaN even when density and other quantities are valid.
• Selenocentric position and the position relative to the lunar wake or the magnetosphere are not attached yet.

Source: NASA SPDF THEMIS