LMAG And Orbit

LMAG (Lunar MAGnetometer) measures the magnetic field at the spacecraft. The public files (MAG_TS*.dat) contain both the field and the spacecraft position, so SOPRAN also computes orbit geometry (altitude, sub-spacecraft point, solar zenith angle) and whether field lines reach the surface from the same files. Orbit geometry uses the LMAG sample times as they are.

Variables

Entry point	Dimensions	Units	Frame	Meaning
kg.lmag.magnetic_field	time, component	nT	MOON_ME	Magnetic field vector; alias b
kg.lmag.magnetic_field_gse	time, component	nT	GSE	GSE field contained in the files; alias bgse
kg.lmag.magnetic_field_magnitude	time	nT		Field strength \(\lvert B\rvert\); alias bmag
kg.lmag.magnetic_connection	time		MOON_ME	Connection of field lines to the surface (below)
kg.orbit.position	time, component	km	MOON_ME	Spacecraft position
kg.orbit.position_gse	time, component	km	GSE	Spacecraft position
kg.orbit.radial_distance	time	km		Distance from the lunar center
kg.orbit.altitude	time	km		Altitude above a 1737.4 km sphere
kg.orbit.subpoint	time, component	deg	MOON_ME	Longitude and latitude of the sub-spacecraft point on the sphere
kg.orbit.sza	time	deg		Solar zenith angle at the sub-spacecraft point

kg = spn.kaguya
time = spn.day("2008-01-01")

b = kg.lmag.magnetic_field.load(time)
altitude = kg.orbit.altitude.load(time)
spn.stack(kg.lmag.bmag.load(time).line(), altitude.line()).plot()

Orbit geometry and field-line connection are saved to the Store by default (cache="use"), so later loads only read them back. Results loaded with missing="empty" or "warn" while source files were missing are not saved.

Solar zenith angle

kg.orbit.sza computes the solar zenith angle at the sub-spacecraft point from the Sun direction given by SPICE. When no kernels are passed and the download policy is not never, NAIF generic kernels are fetched automatically (Coordinate Frames). To give the Sun direction yourself, use sun_vector=.

sza = kg.orbit.sza.load(time)
sza = kg.orbit.sza.load(time, sun_vector=[1.0, 0.0, 0.0], sun_frame="MOON_ME")

Orbit positions can be transformed to another frame while loading by passing frame= and context= (a FrameContext).

Field-line connection to the surface

magnetic_connection extends a straight line from the spacecraft along the local field direction and checks whether it meets a sphere of radius radius_km (default 1737.4 km). Both the field direction (plus) and the opposite direction (minus) are checked (direction="both").

conn = kg.lmag.magnetic_connection.load(time)
conn.plot(kind="footpoint")      # footpoint, altitude, incidence, distance
conn.to_xarray()

Variable	Meaning
connected_any, connected_plus, connected_minus	Whether either or each direction meets the sphere
footpoint_plus_lon, footpoint_plus_lat, …	Longitude and latitude of the intersection (deg)
distance_plus_km, distance_minus_km	Distance to the intersection
incidence_angle_plus_deg, incidence_angle_minus_deg	Acute angle between the field line and the surface normal
altitude_km	Spacecraft altitude

This is a straight-line geometry and does not include field-line bending by crustal magnetic anomalies. Tracing through a surface field model is available as an experimental API (sopran.experimental.moon.svm3d).

Matching another instrument's times

To match the times of another instrument such as PACE, use resample_like.

esa1 = kg.esa1.counts.load(time, missing="warn")
conn_on_esa1 = conn.resample_like(esa1, method="nearest", tolerance="2s")
altitude_on_esa1 = altitude.resample_like(esa1, method="linear", tolerance="10s")

The methods are listed in Time Alignment.

Limitations

• There are no tests comparing the SPICE Sun direction and frame transforms with SPEDAS geometry yet.
• Altitude and sub-spacecraft points use a spherical Moon without terrain (DEM).