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[en] Flux transfer events (FTEs) are magnetic flux ropes that are produced via magnetic reconnection at the planetary magnetopause where the solar wind directly interacts with the magnetosphere. Previous observations show that FTEs with a duration of several seconds, corresponding to a spatial scale of ∼0.5–1 R M, can occur at Mercury. However, the formation of these macroscale FTEs at a small dimensional magnetopause with a radius of ∼1.5 R M remains unclear. Here, we report the observations of active magnetic reconnection events at Mercury’s magnetopause by the MESSENGER spacecraft. The reconnection process is dominated by the formation of a series of multi-scale FTEs. Ion-scale flux ropes, typically with durations of ∼1 s or less, may be produced by the tearing instability in the thin current sheet near the subsolar position. Moreover, the commonly observed macroscale FTEs consist of three to tens of successive small-scale FTEs. We propose that macroscale FTEs at Mercury are generated by the interaction and merging of multiple ion-scale flux ropes, probably through two or more steps. This is distinct from the formation of typical FTEs, mainly between a pair of X-lines, at Earth’s magnetopause. Thus, the formation and evolution of FTEs may differ among planetary magnetospheres with a vast range of scale sizes. We further conclude that Mercury’s magnetopause is a natural plasma laboratory to study flux rope dynamics and evolution for the upcoming Bepi-Colombo mission.
[en] Key messages: 1. Achieving 1.5°C requires an unprecedented transformation of the electricity sector; • On average, 3x nuclear and 30x solar/wind (or, deployment 50% and 650% above historical peaks). 2. Accelerating and scaling up nuclear power for 1.5°C appears to be feasible in terms of economic, resource and industrial capacity
[en] The SunSpot Numbers (SSNs), a proxy of the solar activity, were considered as a time series and their statistical characteristics were studied. The dynamic processes in both solar hemispheres are not strong coupled. Hence, the progress of the solar cycles was described by auto-regression (AR) models worked out for the first time separately for the Northern and the Southern hemispheres and by summation, the total SSNs were calculated. Semi-annual data were used. The model orders were determined by best approximation ex‑ante prognosis using AR models of different order to the observed solar cycle 24. A similar procedure was applied to the new solar cycle 25. The SSN maximum in the Northern hemisphere should be achieved before the maximum in the Southern hemisphere. The solar activity in the southern hemisphere would be dominant. The maximum of the total SSNs of about 117 (with a confidence interval from 77 to 165) is predicted for 2023. Key words: solar cycle 25, SunSpot Numbers (SSNs), forecast, autoregression (AR) models
[en] We present a case study of the in situ acceleration of solar wind suprathermal electrons at the two quasi-perpendicular-bow-shock crossings on 2015 November 4, combining the Wind 3D Plasma and Energetic Particle measurements of ambient solar wind suprathermal electrons and Magnetospheric Multiscale mission measurements of shocked suprathermal electrons. In both cases, the omnidirectional differential fluxes of shocked suprathermal electrons in the downstream exhibit a double-power-law energy spectrum with a spectral index of ∼3 at energies below a downward break ε brk near 40 keV and index of ∼6 at energies above, different from the unshocked suprathermal electrons observed in the ambient solar wind. At energies below (above) ε brk, the observed electron flux ratio between the downstream and ambient solar wind, J D/J A, peaks near 90° PA (becomes roughly isotropic). Electrons at ε brk have an average electron gyrodiameter (across bow shock) comparable to the shock thickness. These suggest that the bow-shock acceleration of suprathermal electrons is likely dominated by the shock drift acceleration mechanism. For electrons at energies below (above) ε brk, their estimated drift time appears to be roughly energy independent (decrease with energy), leading to the formation of a double-power-law spectrum substantially steepening at a break that’s determined by the shock thickness.
[en] We infer the depth of the internal sources giving rise to three-minute umbral oscillations. Recent observations of ripple-like velocity patterns of umbral oscillations supported the notion that there exist internal sources exciting the umbral oscillations. We adopt the hypothesis that the fast magnetohydrodynamic (MHD) waves generated at a source below the photospheric layer propagate along different paths, reach the surface at different times, and excited slow MHD waves by mode conversion. These slow MHD waves are observed as the ripples that apparently propagate horizontally. The propagation distance of the ripple given as a function of time is strongly related to the depth of the source. Using the spectral data of the Fe i 5435 Å line taken by the Fast Imaging Solar Spectrograph of the Goode Solar Telescope at Big Bear Solar Observatory, we identified five ripples and determined the propagation distance as a function of time in each ripple. From the model fitting to these data, we obtained the depth between 1000 and 2000 km. Our result will serve as an observational constraint to understanding the detailed processes of magnetoconvection and wave generation in sunspots.
[en] The cometary mission Rosetta has shown the presence of higher-than-expected suprathermal electron fluxes. In this study, using 3D fully kinetic electromagnetic simulations of the interaction of the solar wind with a comet, we constrain the kinetic mechanism that is responsible for the bulk electron energization that creates the suprathermal distribution from the warm background of solar wind electrons. We identify and characterize the magnetic field-aligned ambipolar electric field that ensures quasi-neutrality and traps warm electrons. Solar wind electrons are accelerated to energies as high as 50–70 eV close to the comet nucleus without the need for wave–particle or turbulent heating mechanisms. We find that the accelerating potential controls the parallel electron temperature, total density, and (to a lesser degree) the perpendicular electron temperature and the magnetic field magnitude. Our self-consistent approach enables us to better understand the underlying plasma processes that govern the near-comet plasma environment.
[en] The present work examines and discusses the response of the atmospheric layers to solar variations, whereas the solar outputs are responsible for the changes in the Earth’s environment. Galactic cosmic ray rates (GCRs), solar cycle lengths (SCLs), sunspots (Rz), coronal index (CI) of solar activities, the aa geomagnetic activity index, total solar irradiance (TSI), CO2 concentrations, global surface temperatures (GSTs), the near-Earth of the northern and southern hemispheres temperatures have been examined. Our results displayed that every SCL has different behaviors to the sensitivity of GST, according to different modulations of GCRs by solar wind/helio-magnetic field parameters. Lower cosmic rays and higher solar irradiance and geomagnetic activity occur when solar activity increases. Furthermore, the average sensitivities of global temperature to geomagnetics aa and total solar irradiance and in turn low-level cloud cover are significant and real. Our results could indicate that geomagnetic disturbances, which driven by the solar wind, may influence global temperature. Both correlations of GST–Rz displayed the same behavior to the end of SC 22nd, and a great discrepancy is observed during the SC 23rd. The observed correlations of Rz with NH and SH temperatures displayed different behaviors. Four different mechanisms are involved in the direct/indirect effect of TSI variations on the Earth’s atmosphere and temperatures. (author)
[en] The tilt of the bipolar magnetic region (BMR) is crucial in the Babcock–Leighton process for the generation of the poloidal magnetic field in the Sun. Based on the thin flux-tube model of the BMR formation, the tilt is believed to be caused by the Coriolis force acting on the rising flux tube of the strong toroidal magnetic field from the base of the convection zone. We analyze the magnetic field dependence of BMR tilts using the magnetograms of the Michelson Doppler Imager (1996–2011) and Helioseismic and Magnetic Imager (2010–2018). We observe that the distribution of the maximum magnetic field (B max) of BMRs is bimodal. Its first peak at the low field corresponds to BMRs that do not have sunspots as counterparts in the white-light images, whereas the second peak corresponds to sunspots as recorded in both type of images. We find that the slope of Joy’s law (γ 0) initially increases slowly with the increase of B max. However, when B max ≳ 2 kG, γ 0 decreases. Scatter of the BMR tilt around Joy’s law systematically decreases with the increase of B max. The decrease of observed γ 0 with B max provides a hint to a nonlinear tilt quenching in the Babcock–Leighton process. We finally discuss how our results may be used to make a connection with the thin flux-tube model.
[en] The occurrence rate of linear and pseudo magnetic holes has been determined during MESSENGER's cruise phase starting from Venus (2007) and arriving at Mercury (2011). It is shown that the occurrence rate of linear magnetic holes, defined as a maximum of 10 rotation of the magnetic field over the hole, slowly decreases from Mercury to Venus. The pseudo magnetic holes, defined as a rotation between 10 and 45 over the hole, have mostly a constant occurrence rate.
[en] Observations by the Parker Solar Probe mission of the solar wind at ∼35.7 solar radii reveal the existence of whistler wave packets with frequencies below 0.1 f ce (20–80 Hz in the spacecraft frame). These waves often coincide with local minima of the magnetic field magnitude or with sudden deflections of the magnetic field that are called switchbacks. Their sunward propagation leads to a significant Doppler frequency downshift from 200–300 to 20–80 Hz (from 0.2 to 0.5 f ce). The polarization of these waves varies from quasi-parallel to significantly oblique with wave normal angles that are close to the resonance cone. Their peak amplitude can be as large as 2–4 nT. Such values represent approximately 10% of the background magnetic field, which is considerably more than what is observed at 1 au. Recent numerical studies show that such waves may potentially play a key role in breaking the heat flux and scattering the Strahl population of suprathermal electrons into a halo population.