The Shape of Mercury's Magnetopause: The Picture From MESSENGER Magnetometer Observations and Future Prospects for BepiColombo

Abstract

The MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft orbited Mercury between March 2011 and April 2015, typically passing through the magnetosphere, magnetosheath, and interplanetary magnetic field, on each orbit. Using data from the Magnetometer, we identify magnetopause crossings for the complete orbital mission and model the average boundary shape. We find that Mercury's average magnetopause is well modeled by both an axisymmetric shape and a three-dimensional shape containing indentations in the cusp regions and a magnetotail that is wider in the north-south versus east-west direction. Examination of MESSENGER data and the use of simulated crossings show that coverage is very limited in the cusp region. Parameters describing the indentation are poorly constrained, and model fits may be biased due to the combination of orbital geometry and magnetospheric dynamics. The distribution of MESSENGER data inside the magnetopause, together with minimum variance analysis of a representative subset of magnetopause crossings, hints at the presence of a cusp indentation, though likely shallower than that estimated from the observed crossing positions. The BepiColombo spacecraft will arrive at Mercury in 2025. We simulate expected data coverage and magnetopause crossings using both axisymmetric and three-dimensional magnetopause models. BepiColombo will improve our understanding of the magnetopause shape by providing data over the southern hemisphere and regions of the northern cusp indentation not covered by MESSENGER. There is also the potential for simultaneous measurements of the interplanetary magnetic field and magnetopause position. Plain Language SummaryThe MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft orbited the planet Mercury from 2011 to 2015. On every orbit, MESSENGER crossed the boundary between the planet's magnetic field and solar wind, known as the magnetopause. We identify the location of the magnetopause on each orbit using MESSENGER magnetic field data. We use these locations to understand the three-dimensional shape of the boundary in space, including differences in the north-south versus east-west direction. The shape of MESSENGER's orbit means that there is limited information about some parts of this shape. For example, it is expected that the magnetopause has indentations known as "cusps," which occur near the north and south poles. MESSENGER magnetopause crossings provide no information on the southern cusp, and it is not possible to tell how large the northern cusp indentation is, or even whether it exists at all. The BepiColombo mission will arrive at Mercury in 2025, with two spacecraft that will orbit Mercury at different distances. We use our MESSENGER results to simulate where the two BepiColombo spacecraft are likely to cross the magnetopause. We find that BepiColombo will provide data on the shape of the magnetopause in places that MESSENGER did not and thereby provide information on the full three-dimensional sh...

The BepiColombo Planetary Magnetometer MPO-MAG: What Can We Learn from the Hermean Magnetic Field?

“…It is important to note, that Zhong et al (2015a) calculated their error using only 1σ . Recently, Philpott et al (2020) cast some doubt about the shape of the cusp indentions. Figure 14 shows a comparison of the subsolar magnetopause distance as a function of heliocentric distance of Mercury derived from the three magnetopause models mentioned above.…”

Magnetometer in-flight offset accuracy for the BepiColombo spacecraft

“…To achieve this goal, accurate magnetic field measurements are thus of crucial importance. Therefore, the components of a linear calibration matrix M and an offset vector O need to be obtained, in order to convert raw instrument outputs B raw to fully calibrated magnetic field measurements (see for example Kepko et al, 1996;Plaschke and Narita, 2016):…”

“…Here, the matrix M transforms B raw into a spacecraft-fixed orthogonal coordinate system. It comprises nine parameters: three scaling (gain) values of the sensor and an orthogonalization matrix, which is defined by the three angles that yield the magnetometer sensor directions with respect to the spacecraft reference frame (see for example Plaschke and Narita, 2016). The 3D offset vector O, on the other hand, reflects the magnetometer outputs in vanishing ambient fields.…”

“…b. Recently Plaschke and Narita (2016) introduced an alternative magnetometer offset calibration technique on the basis of the observation of compressible fluctuation, the so-called mirror mode method, which does not require pristine solar wind measurements. The idea is that for strongly compressible fluctuating fields (e.g., mirror modes) the maximum variance direction of the magnetic field should be nearly parallel to the mean (background) magnetic field (Tsurutani et al, 2011).…”

“…In this paper we test the applicability of the mirror mode method in different regions (plasma environments) around Mercury, based on 4 years of MESSENGER (MErcury Surface, Space ENvironment, GEophysics and Ranging, Solomon et al, 2007) fluxgate magnetometer (FGM, Anderson et al, 2007) data, which were calibrated using time intervals of (nearly) incompressible Alfvénic fluctuations in the solar wind. For reasons of simplicity (and because for Mio only the spin-axis offset needs to be evaluated) we apply the 1D mirror mode method (see Plaschke and Narita, 2016) and assume that two offset components are already accurately determined. We test to which degree the 1D mirror mode method yields vanishing offsets as expected when using calibrated data as input.…”