The magnetometer instrument MPO-MAG on-board the Mercury Planetary Orbiter (MPO) of the BepiColombo mission en-route to Mercury is introduced, with its instrument design, its calibration and ...scientific targets. The instrument is comprised of two tri-axial fluxgate magnetometers mounted on a 2.9 m boom and are 0.8 m apart. They monitor the magnetic field with up to 128 Hz in a
±
2048
nT range. The MPO will be injected into an initial
480
×
1500
km polar orbit (2.3 h orbital period). At Mercury, we will map the planetary magnetic field and determine the dynamo generated field and constrain the secular variation. In this paper, we also discuss the effect of the instrument calibration on the ability to improve the knowledge on the internal field. Furthermore, the study of induced magnetic fields and field-aligned currents will help to constrain the interior structure in concert with other geophysical instruments. The orbit is also well-suited to study dynamical phenomena at the Hermean magnetopause and magnetospheric cusps. Together with its sister instrument Mio-MGF on-board the second satellite of the BepiColombo mission, the magnetometers at Mercury will study the reaction of the highly dynamic magnetosphere to changes in the solar wind. In the extreme case, the solar wind might even collapse the entire dayside magnetosphere. During cruise, MPO-MAG will contribute to studies of solar wind turbulence and transient phenomena.
Intermolecular hydrogen bonds impede long-range (anti-)ferroelectric order of water. We confine H
O molecules in nanosized cages formed by ions of a dielectric crystal. Arranging them in channels at ...a distance of ~5 Å with an interchannel separation of ~10 Å prevents the formation of hydrogen networks while electric dipole-dipole interactions remain effective. Here, we present measurements of the temperature-dependent dielectric permittivity, pyrocurrent, electric polarization and specific heat that indicate an order-disorder ferroelectric phase transition at T
≈ 3 K in the water dipolar lattice. Ab initio molecular dynamics and classical Monte Carlo simulations reveal that at low temperatures the water molecules form ferroelectric domains in the ab-plane that order antiferroelectrically along the channel direction. This way we achieve the long-standing goal of arranging water molecules in polar order. This is not only of high relevance in various natural systems but might open an avenue towards future applications in biocompatible nanoelectronics.
Abstract
Within a galaxy the stellar mass-to-light ratio ϒ* is not constant. Recent studies of spatially resolved kinematics of nearby early-type galaxies suggest that allowing for a variable initial ...mass function (IMF) returns significantly larger ϒ* gradients than if the IMF is held fixed. We show that ignoring such IMF-driven ϒ* gradients can have dramatic effect on dynamical ($M_*^{\rm dyn}$), though stellar population ($M_*^{\rm SP}$) based estimates of early-type galaxy stellar masses are also affected. This is because $M_*^{\rm dyn}$ is usually calibrated using the velocity dispersion measured in the central regions (e.g. Re/8) where stars are expected to dominate the mass (i.e. the dark matter fraction is small). On the other hand, $M_*^{\rm SP}$ is often computed from larger apertures (e.g. using a mean ϒ* estimated from colours). If ϒ* is greater in the central regions, then ignoring the gradient can overestimate $M_*^{\rm dyn}$ by as much as a factor of two for the most massive galaxies. Large ϒ*-gradients have four main consequences: First, $M_*^{\rm dyn}$ cannot be estimated independently of stellar population synthesis models. Secondly, if there is a lower limit to ϒ* and gradients are unknown, then requiring $M_*^{\rm dyn}=M_*^{\rm SP}$ constrains them. Thirdly, if gradients are stronger in more massive galaxies, then accounting for this reduces the slope of the correlation between $M_*^{\rm dyn}/M_*^{\rm SP}$ of a galaxy with its velocity dispersion. In particular, IMF-driven gradients bring $M_*^{\rm dyn}$ and $M_*^{\rm SP}$ into agreement, not by shifting $M_*^{\rm SP}$ upwards by invoking constant bottom-heavy IMFs, as advocated by a number of recent studies, but by revising $M_*^{\rm dyn}$ estimates in the literature downwards. Fourthly, accounting for ϒ* gradients changes the high-mass slope of the stellar mass function $\phi (M_*^{\rm dyn})$, and reduces the associated stellar mass density. These conclusions potentially impact estimates of the need for feedback and adiabatic contraction, so our results highlight the importance of measuring ϒ* gradients in larger samples.
The South Korean meteorological and environmental satellite GEO-KOMPSAT-2A (GK-2A) was launched into geostationary orbit at
128.2
∘
East on 4 December 2018. The space weather observation aboard GK-2A ...is performed by the Korea Space Environment Monitor. It consists of three particle detectors, a charging monitor and a four-sensor Service Oriented Spacecraft Magnetometer (SOSMAG).
The magnetometer design aims for avoiding strict magnetic cleanliness requirements for the hosting spacecraft and an automated on-board correction of the dynamic stray fields which are generated by the spacecraft. This is achieved through the use of two science grade fluxgate sensors on an approximately one meter long boom and two additional magnetoresistance sensors mounted within the spacecraft body.
This paper describes the instrument design, discusses the ground calibration methods and results, presents the post-launch correction and calibration achievements based on the data which were acquired during the first year in orbit and demonstrates the in-flight performance of SOSMAG with two science cases.
The dynamic stray fields from the GK-2A spacecraft, which was built without specific magnetic cleanliness considerations, are reduced up to a maximum factor of 35. The magnitude of the largest remnant field from an active spacecraft disturber is 2.0 nT. Due to a daily shadowing of the SOSMAG boom, sensor intrinsic offset oscillations with a periodicity up to 60 minutes and peak-to-peak values up to 5 nT remain in the corrected data product.
The comparison of the cleaned SOSMAG data with the Tsyganenko 2004 magnetic field model and the magnetic field data from the Magnetospheric Multiscale mission demonstrates that the offset error is less than the required 5 nT for all three components and that the drift of the offsets over 10 months is less than 7 nT.
Future work will include a further reduction of the remaining artefacts in the final data product with the focus on lessening the temperature driven sensor oscillations with an epoch based identification and correction.
Abstract
Spatially resolved kinematics of nearby galaxies has shown that the ratio of dynamical to stellar population-based estimates of the mass of a galaxy ($M_{*}^{\rm JAM}/M_{*}$) correlates with ...σe, the light-weighted velocity dispersion within its half-light radius, if M* is estimated using the same initial mass function (IMF) for all galaxies and the stellar mass-to-light ratio within each galaxy is constant. This correlation may indicate that, in fact, the IMF is more bottom-heavy or dwarf-rich for galaxies with large σ. We use this correlation to estimate a dynamical or IMF-corrected stellar mass, $M_{*}^{\rm \alpha _{JAM}}$, from M* and σe for a sample of 6 × 105 Sloan Digital Sky Survey (SDSS) galaxies for which spatially resolved kinematics is not available. We also compute the ‘virial’ mass estimate $k(n,R)\,R_{\rm e}\,\sigma _{\rm R}^2/G$, where n is the Sérsic index, in the SDSS and ATLAS3D samples. We show that an n-dependent correction must be applied to the k(n, R) values provided by Prugniel & Simien. Our analysis also shows that the shape of the velocity dispersion profile in the ATLAS3D sample varies weakly with n: (σR/σe) = (R/Re)−γ(n). The resulting stellar mass functions, based on $M_*^{\rm \alpha _{JAM}}$ and the recalibrated virial mass, are in good agreement. Using a Fundamental Plane-based observational proxy for σe produces comparable results. The use of direct measurements for estimating the IMF-dependent stellar mass is prohibitively expensive for a large sample of galaxies. By demonstrating that cheaper proxies are sufficiently accurate, our analysis should enable a more reliable census of the mass in stars, especially at high redshift, at a fraction of the cost. Our results are provided in tabular form.
The Magnetospheric Multiscale Magnetometers Russell, C. T.; Anderson, B. J.; Baumjohann, W. ...
Space Science Reviews,
03/2016, Letnik:
199, Številka:
1-4
Journal Article, Book Review
Recenzirano
Odprti dostop
The success of the Magnetospheric Multiscale mission depends on the accurate measurement of the magnetic field on all four spacecraft. To ensure this success, two independently designed and built ...fluxgate magnetometers were developed, avoiding single-point failures. The magnetometers were dubbed the digital fluxgate (DFG), which uses an ASIC implementation and was supplied by the Space Research Institute of the Austrian Academy of Sciences and the analogue magnetometer (AFG) with a more traditional circuit board design supplied by the University of California, Los Angeles. A stringent magnetic cleanliness program was executed under the supervision of the Johns Hopkins University’s Applied Physics Laboratory. To achieve mission objectives, the calibration determined on the ground will be refined in space to ensure all eight magnetometers are precisely inter-calibrated. Near real-time data plays a key role in the transmission of high-resolution observations stored on board so rapid processing of the low-resolution data is required. This article describes these instruments, the magnetic cleanliness program, and the instrument pre-launch calibrations, the planned in-flight calibration program, and the information flow that provides the data on the rapid time scale needed for mission success.
The FIELDS instrumentation suite on the Magnetospheric Multiscale (MMS) mission provides comprehensive measurements of the full vector magnetic and electric fields in the reconnection regions ...investigated by MMS, including the dayside magnetopause and the night-side magnetotail acceleration regions out to 25 Re. Six sensors on each of the four MMS spacecraft provide overlapping measurements of these fields with sensitive cross-calibrations both before and after launch. The FIELDS magnetic sensors consist of redundant flux-gate magnetometers (AFG and DFG) over the frequency range from DC to 64 Hz, a search coil magnetometer (SCM) providing AC measurements over the full whistler mode spectrum expected to be seen on MMS, and an Electron Drift Instrument (EDI) that calibrates offsets for the magnetometers. The FIELDS three-axis electric field measurements are provided by two sets of biased double-probe sensors (SDP and ADP) operating in a highly symmetric spacecraft environment to reduce significantly electrostatic errors. These sensors are complemented with the EDI electric measurements that are free from all local spacecraft perturbations. Cross-calibrated vector electric field measurements are thus produced from DC to 100 kHz, well beyond the upper hybrid resonance whose frequency provides an accurate determination of the local electron density. Due to its very large geometric factor, EDI also provides very high time resolution (∼1 ms) ambient electron flux measurements at a few selected energies near 1 keV. This paper provides an overview of the FIELDS suite, its science objectives and measurement requirements, and its performance as verified in calibration and cross-calibration procedures that result in anticipated errors less than 0.1 nT in B and 0.5 mV/m in E. Summaries of data products that result from FIELDS are also described, as well as algorithms for cross-calibration. Details of the design and performance characteristics of AFG/DFG, SCM, ADP, SDP, and EDI are provided in five companion papers.
ABSTRACT For the first time, the dispersion relation for turbulence magnetic field fluctuations in the solar wind is determined directly on small scales of the order of the electron inertial length, ...using four-point magnetometer observations from the Magnetospheric Multiscale mission. The data are analyzed using the high-resolution adaptive wave telescope technique. Small-scale solar wind turbulence is primarily composed of highly obliquely propagating waves, with dispersion consistent with that of the whistler mode.
PDF
