Global-scale external magnetic fields at Mars from Mars Global Surveyor data

Abstract:

The martian magnetic field is unique among those of the terrestrial planets. It is the

net result of the interaction of the solar wind and interplanetary magnetic field (IMF)

with crustal remnant magnetization and a planetary ionosphere. Internal fields of crustal

origin have been the subject of extensive studies; the focus of our work is identification

and characterization of contributions from external magnetic fields using the Mars Global

Surveyor (MGS) vector magnetic field data. We investigate the magnitude, average

spatial structure and temporal variability of the external magnetic field at the MGS

mapping altitude of 400 km by first subtracting expected contributions from crustal

fields using existing global crustal field models. We identify contributions to the residual

dayside fields from two sources: the draped IMF and a source that we interpret to be of

ionospheric origin. As observed in previous work, nightside external fields are minimal at

mapping orbit altitudes. The IMF contribution changes polarity every 13 days due to the

geometry of the heliospheric magnetic field and Mars’ orbit. This allows us to calculate

the amplitude of the IMF at mapping orbit altitudes. The ionospheric contribution

results in a quasi-steady dayside signal in the MGS observations because of the limited

local time sampling of the MGS mapping orbit. The ionospheric contribution can be

isolated by averaging the external fields over timescales longer than several Carrington

rotations, to average out the IMF contribution. We present a global average of the

ionopsheric field for the duration of the mapping orbit (2000-2006) and analyze daytime

and nightime fields separately. We show that some structure in the time-averaged

ionospheric field is organized in the Mars body-fixed frame, due for example, to the

influence of crustal fields. We also show that the ionospheric fields vary in amplitude and

geometry with martian season. Broader local time coverage over a restricted latitude

band of 50-60 provides some insight into the evolution of ionospheric fields over a

martian sole.