Lunar Prospector¶

Lunar Prospector (LP) was a NASA lunar orbiter from January 1998 to July 1999, carrying a magnetometer (MAG) and an electron reflectometer (ER). Its ER observations are the origin of electron reflectometry, which estimates the surface magnetic field from how many electrons the crustal field reflects. SOPRAN reads the daily ASCII tables of the PDS PPI Level 1B data (volumes LP_1201 to LP_1204, 1998-01-16 to 1999-07-29).

Variables¶

Entry point	Dimensions	Units	Contents
`lp.mag.magnetic_field`	`time, component`	nT	5-s average field (SSE). Alias `b`
`lp.mag.magnetic_field_sel`	`time, component`	nT	The same field in SEL
`lp.mag.magnetic_field_rms`	`time`	nT	RMS of the field within each 5-s average
`lp.mag.position`	`time, component`	km	Spacecraft position from the Moon centre (SSE)
`lp.mag.position_sel`	`time, component`	km	The same position in SEL
`lp.er.electron_flux`	`time, energy_bin`	1/(cm^2 s sr eV)	~80-s (16-spin) omnidirectional differential electron number flux; 15 energies
`lp.er.electron_flux_3d`	`time, energy_bin, look`	1/(cm^2 s sr eV)	3-D electron distribution; 15 energies x 88 look directions

Here lp = spn.lunar_prospector. Frames follow the PDS labels.

Frame	Definition
SSE (selenocentric solar ecliptic)	X points from the Moon to the Sun; Z is parallel to the northward normal of the ecliptic
SEL (selenographic)	Z along the lunar spin axis, X toward 0 degrees longitude; Cartesian

The ER files name the columns EFLUX, but the unit is CM**-2 * S**-1 * SR**-1 * eV**-1, a differential number flux. SOPRAN treats it as a number flux, as the unit says, and names it electron_flux.

lp = spn.lunar_prospector
time = spn.period("1998-02-01T00:00", "1998-02-01T06:00")

spn.stack(
    lp.er.electron_flux.spectrogram(time),
    lp.er.electron_flux_3d.spectrogram(time),     # averaged over look directions
    lp.mag.magnetic_field.lines(time, components="xyz"),
).plot()

Energy tables¶

ER energies are the per-record coordinate energy (time, energy_bin, eV). There are two tables (16.7 keV to 39 eV, and 15.9 keV to 7 eV), switched four times from March 1999 on. SOPRAN picks the table in effect for each record from the switch times in E_BINS.TAB. E_BINS.TAB is identical in every volume and is retrieved once.

3-D distributions¶

The time of electron_flux_3d is the spectrum time (one spectrum per 16 spins); the 15 rows (energies) of the file are gathered into one spectrum. Per-row values stay as coordinates:

Coordinate	Dimensions	Contents
`energy`	`time, energy_bin`	Energy of the row (eV)
`record_time`	`time, energy_bin`	Time of the row
`theta`	`look`	Detector elevation (deg), fixed values from `THETA.TAB`
`phi`	`time, energy_bin, look`	Azimuth (deg) as published; not wrapped to 0–360 degrees (values such as 450 occur)
`bx_despun`, `by_despun`, `bz_despun`	`time, energy_bin`	Field in despun spacecraft coordinates (nT)

Spectra with fewer than 15 rows get NaN for the missing energies. Pitch angles are not computed yet; they will be added after checking the sign conventions of the look directions (theta, phi) against the particle direction in the LP literature.

spectrogram() averages the 3-D distribution over look directions before drawing (reduction="mean" by default). Pass reduction="sum" to add them instead.

Retrieval and file layout¶

Source files are MAyymmdd.TAB, ELyymmdd.TAB, and 3Dyymmdd.TAB under https://pds-ppi.igpp.ucla.edu/volume/<volume>/DATA/<MAG|ER_LO_TS|ER_3DIST>/<YY_MMMON>/; the volume follows from the date. Retrieved files go to the Store's raw/lunar_prospector/pds3/ with the same layout, with their source and checksum recorded.

Data	Size per day
MAG	About 2.6 MB
ER (omnidirectional)	About 0.2 MB
ER (3-D)	About 24 MB

Days outside the mission and days without a file at the archive (404) are gaps. The default (missing="empty") returns an empty result, missing="warn" warns, and missing="error" raises DatasetNotFoundError. Loading only days outside the mission retrieves nothing.

Interpretation notes¶

Missing values use the labels' MISSING_CONSTANT (field 999.999, position 999999.99, flux 9.999E-99, and so on) and become NaN. Observed zero fluxes stay zero.
The field and ER values are calibrated by the archive. SOPRAN applies no spacecraft potential correction or background removal to ER.
The high-rate ER (ER_HI_TS, ~5 s, two energies) and the higher-level data (LP_2001) are not read yet.

Source: PDS PPI Lunar Prospector MAG/ER Level 1B