Keywords

Seeing is an important index to evaluate the quality of an astronomical site. To estimate seeing at the Muztagh-Ata site with height and time quantitatively, the European Centre for Medium-Range Weather Forecasts reanalysis database (ERA5) is used. Seeing calculated from ERA5 is compared consistently with the Differential Image Motion Monitor seeing at the height of 12 m. Results show that seeing decays exponentially with height at the Muztagh-Ata site. Seeing decays the fastest in fall in 2021 and most slowly with height in summer. The seeing condition is better in fall than in summer. The median value of seeing at 12 m is 0.89 arcsec, the maximum value is 1.21 arcsec in August and the minimum is 0.66 arcsec in October. The median value of seeing at 12 m is 0.72 arcsec in the nighttime and 1.08 arcsec in the daytime. Seeing is a combination of annual and about biannual variations with the same phase as temperature and wind speed indicating that seeing variation with time is influenced by temperature and wind speed. The Richardson number Ri is used to analyze the atmospheric stability and the variations of seeing are consistent with Ri between layers. These quantitative results can provide an important reference for a telescopic observation strategy.

In order to compute the free core nutation of the terrestrial planets, such as Earth and Mars, the Moon and lower degree normal modes of the Jovian planets, we propose a linear operator method (LOM). Generalized surface spherical harmonics (GSSHs) are usually applied to the elliptical models with a stress tensor, which cannot be expressed in vector spherical harmonics explicitly. However, GSSHs involve complicated math. LOM is an alternative to GSSHs, whereas it only deals with the coupling fields of the same azimuthal order m, as is the case when a planet model is axially symmetric and rotates about that symmetry axis. We extend LOM to any asymmetric 3D model. The lower degree spheroidal modes of the Earth are computed to validate our method, and the results agree very well with what is observed. We also compute the normal modes of a two-layer Saturn model as a simple application.

Since 2007, the Intergovernmental Panel on Climate Change (IPCC) has heavily relied on the comparison between global climate model hindcasts and global surface temperature (ST) estimates for concluding that post-1950s global warming is mostly human-caused. In Connolly et al., we cautioned that this approach to the detection and attribution of climate change was highly dependent on the choice of Total Solar Irradiance (TSI) and ST data sets. We compiled 16 TSI and five ST data sets and found by altering the choice of TSI or ST, one could (prematurely) conclude anything from the warming being "mostly human-caused" to "mostly natural." Richardson and Benestad suggested our analysis was "erroneous" and "flawed" because we did not use a multilinear regression. They argued that applying a multilinear regression to one of the five ST series re-affirmed the IPCC's attribution statement. They also objected that many of the published TSI data sets were out-of-date. However, here we show that when applying multilinear regression analysis to an expanded and updated data set of 27 TSI series, the original conclusions of Connolly et al. are confirmed for all five ST data sets. Therefore, it is still unclear whether the observed warming is mostly human-caused, mostly natural or some combination of both.

The Free Core Nutation (FCN) is a rotational mode caused by non-alignment of the rotation axis of the core and of the mantle. Its period observed by VLBI and superconducting gravimetry is around 430 sidereal days (Sd) with precision of better than 1 Sd, while its "theoretical" period calculated by traditional approaches and a given Earth model ranges from 450 to 470 Sd. Their gap of about 30 Sd is significant compared with its observation precision. We propose a spectral element method to compute the period of FCN and obtain a period of 434 Sd which is very close to the observed value.

In a study that attempted to relate solar and human activity to Earth's recent temperature change, Connolly et al. committed a basic error in the choice of statistical methods and so overreported the effect of the Sun. A major theme of their study was that there are many data sets of past solar activity, and some of these allegedly provide statistical evidence of "most of the recent global warming being due to changes in solar activity." We avoid methods that are known to give inaccurate results and show that for 1970–2005 Northern Hemisphere land the corrected solar attribution fraction is −7% to +5%, compared with values of up to 64% reported in Connolly et al. Their higher values are entirely due to mistaken application of statistics. Unfortunately, we cannot test truly "recent" global warming since most of their solar data sets end before 2015, and two finish in the 1990s, but all tested post-1970 periods show similarly small solar contributions. The solar-climate linkage is an area of fascinating and ongoing research with rigorous technical discussion. We argue that instead of repeating errors, they should be acknowledged and corrected so that the debate can focus on areas of legitimate scientific uncertainty.

To reveal whether the dynamics of solar activity precede those of global temperature, especially in terms of global warming, the relationship between total solar irradiance (TSI), which is treated as a proxy of solar activity, and global surface temperature (GST) is investigated in the frequency domain using wavelet coherence. The results suggest that the effect of TSI on GST is mainly reflected on the characteristic scale around 22 yr, and variations in TSI lead to changes in GST with some delay effect as shown by the phase difference arrows. However, this implicated relationship has been perturbed by excessive CO₂ emissions since 1960. Through the combination of co-integration analysis and wavelet coherence, the hidden relationship between TSI and GST has been uncovered without the CO₂ effect and the results further indicate that TSI has a positive effect on GST at the characteristic scale around 22 yr with a 3 yr lead.

We examine the role that ions and electrons play in reconnection using observations from the Magnetospheric Multiscale (MMS) mission on kinetic ion and electron scales, which are much shorter than magnetohydrodynamic scales. This study reports observations with unprecedented high resolution that MMS provides for magnetic field (7.8 ms) and plasma (30 ms for electrons and 150 ms for ions). We analyze and compare approaches to the magnetopause in 2016 November, to the electron diffusion region in the magnetotail in 2017 July followed by a current sheet crossing in 2018 July. Besides magnetic field reversals, changes in the direction of the flow velocity, and ion and electron heating, MMS observed large fluctuations in the electron flow speeds in the magnetotail. As expected from numerical simulations, we have verified that when the field lines and plasma become decoupled a large reconnecting electric field related to the Hall current (1–10 mV m⁻¹) is responsible for fast reconnection in the ion diffusion region. Although inertial accelerating forces remain moderate (1–2 mV m⁻¹), the electric fields resulting from the divergence of the full electron pressure tensor provide the main contribution to the generalized Ohm's law at the neutral sheet (as large as 200 mV m⁻¹). In our view, this illustrates that when ions decouple electron physics dominates. The results obtained on kinetic scales may be useful for better understanding the physical mechanisms governing reconnection processes in various magnetized laboratory and space plasmas.

The polar outflows, as an important plasma source of the Earth's magnetosphere, usually exhibit significant north–south asymmetries, which can strongly affect the plasma distributions in the magnetotail lobe and perhaps contribute to the substorm triggering. But the mechanism of the asymmetric transport of these outflows is still unclear. In this Letter, 3D global magnetohydrodynamic (MHD) simulations are performed to investigate the development of the polar outflows after their escapes from the inner boundary under influences of the interplanetary magnetic field (IMF) B_x. It is found that the velocity of northern polar outflows is much stronger than the south. We suggest that the IMF B_x causes the north–south asymmetries in the magnetospheric configuration, and subsequently, great differences of the force and mass distributions appear between the two hemispheres, which lead to the significant north–south asymmetries in the transport of the polar outflows. We also discuss the differences in the acceleration mechanisms of the polar outflows between the northward and southward IMF cases.

Magnetosheath properties, particularly those related to magnetopause magnetic reconnection (MR), are investigated in this study. (1) Asymmetries are found to exist in the distributions of plasma and magnetic field parameters in the magnetosheath. These asymmetries are related to the interplanetary magnetic field (IMF) orientation, and they are produced either on the bow shock or inside the magnetosheath. Thus, one must be very cautious in directly using the upstream solar wind and IMF properties as the magnetopause MR initiation conditions, since the magnetosheath parameters are not the same everywhere. (2) A unique method is introduced to estimate how much IMF magnetic flux passes through the magnetosphere via MR on either the low-latitude or the high-latitude magnetopause. This flux mainly varies with three independent parameters: the IMF clock angle θ_CL, the magnetosheath plasma β, and the solar wind sound Mach number M_S. Surprisingly, the magnetic fluxes passing through the magnetosphere are comparable under the southward and northward IMF conditions. (3) The dipole tilt angle, the property from the inside of the magnetosphere, also controls the magnetosheath parameters. As the Earth's dipole tilt angle varies, the plasma pressure ridge shifts its location but remains near the magnetic equator. The stagnation point of the magnetosheath flow on the magnetopause, however, remains at the subsolar point no matter how large the dipole tilt angle is. These behaviors may be determinative of the locations of MR and the generation of flux transfer events on the magnetopause.

The China Seismo-Electromagnetic Satellite (CSES) aims to monitor electromagnetic, particle, and plasma perturbations in the iono-magnetosphere and inner Van Allen radiation belts originated by electromagnetic sources external and internal to the geomagnetic cavity, cosmic rays, and solar events. In particular, the objective of the space mission is to investigate lithosphere–atmosphere–ionosphere coupling mechanisms (including the effects of lightning, earthquakes, volcanoes, and artificial electromagnetic emissions) that induce perturbations of the top side of the ionosphere and lower boundary of the radiation belts. To this purpose, the mission has been conceived to take advantage of a multi-instrument payload comprising nine detectors for the measurement of electromagnetic field components, plasma parameters, and energetic particles, as well as X-ray flux. The Italian team participating in the CSES mission has built one of these devices, the High-Energy Particle Detector (HEPD), for high-precision observations of electrons, protons, and light nuclei. During its trip along the orbit, and thanks to the large set of detectors operated on board, CSES completely monitors the Earth, acting as an excellent instrument for space weather. The satellite was launched on 2018 February 2, with an expected life span of 5 yr. This article describes the CSES mission with a particular focus on the HEPD apparatus and its in-flight performance.