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[en] In this Letter we apply a methodology, recently proposed by Bourouaine & Perez (BP19), to interpret solar wind turbulent power spectra beyond the Taylor approximation (TA). The turbulent power spectra were measured using Helios spacecraft data near 0.6 au. We use the model proposed in BP19 to reproduce the field-perpendicular power spectrum E(k ⊥) of antisunward Alfvénic fluctuations in the plasma frame (where k ⊥ is the field-perpendicular wavenumber) from the corresponding measured frequency power spectrum along the sampling angle θ b, which is the angle between the local magnetic field and the sampling direction. Here ω = 2πf and f is the frequency of the time signal. Interestingly enough, we found that for all corresponding measured frequency power spectrum the reproduced field-perpendicular power spectrum E(k ⊥) is the same and independent of the considered sampling angle θ b. This finding is consistent with the fact that the analyzed turbulence is strong and highly anisotropic with (where is the field-parallel wavenumber). Furthermore, for this specific time signal we found that the commonly used TA is still approximately valid with the important difference that a broadening in k ⊥ for each angular frequency ω is present. This broadening can be described in the context of the methodology proposed in BP19.
[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] Magnetosheath jets and plasmoids are very common phenomena downstream of Earth’s quasi-parallel bow shock. As the increase of the dynamic pressure is one of the principal characteristics of magnetosheath jets, the embedded paramagnetic plasmoids have been considered as an special case of the former. Although the properties of both types of structures have been widely studied during the last 20 years, their formation mechanisms have not been examined thoroughly. In this work we perform a 2D local hybrid simulation (kinetic ions – fluid electrons) of a quasi-parallel (θ Bn = 15°), supercritical (M A = 7) collisionless shock in order to study these mechanisms. Specifically, we analyze the formation of one jet and one plasmoid, showing for the first time that they can be produced by different mechanisms related to the same shock. In our simulation, the magnetosheath jet is formed according to the mechanism proposed by Hietala, where at the shock ripples the upstream solar wind suffers locally less deceleration and the flow is focused in the downstream side, producing a compressed and high-velocity region that leads to an increase of dynamic pressure downstream of the shock. The formation of the plasmoid, however, follows a completely new scenario being generated by magnetic reconnection between two plasma layers with opposite B-field orientation in the region just behind the shock.
[en] Energy supply sources for the heating process in the slow solar wind remain unknown. The Parker Solar Probe (PSP) mission provides a good opportunity to study this issue. Recently, PSP observations have found that the slow solar wind experiences stronger heating inside 0.24 au. Here for the first time we measure in the slow solar wind the radial gradient of the low-frequency breaks on the magnetic trace power spectra and evaluate the associated energy supply rate. We find that the energy supply rate is consistent with the observed perpendicular heating rate calculated based on the gradient of the magnetic moment. Based on this finding, one could explain why the slow solar wind is strongly heated inside 0.25 au but expands nearly adiabatically outside 0.25 au. This finding supports the concept that the energy added from the energy-containing range is transferred by an energy cascade process to the dissipation range, and then dissipates to heat the slow solar wind. The related issues for further study are discussed.
[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] 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] As an important source for large geomagnetic storms, an “ICME-in-sheath” is a completely shocked interplanetary coronal mass ejection (ICME) stuck in the sheath between a shock and host ejecta. Typical characteristics are identified from coordinated multi-sets of observations: (1) it is usually short in duration and lasts a few hours at 1 au; (2) its solar wind parameters, in particular the magnetic field, seem to keep enhanced for a large range of distances; and (3) common ICME signatures are often lost. The host ejecta could be a single ICME or a complex ejecta, being fast enough to drive a shock. These results clarify previous misinterpretations of this phenomenon as a normal part of a sheath region. The “ICME-in-sheath” phenomenon, together with a preconditioning effect, produced an extreme set of the magnetic field, speed, and density near 1 au in the 2012 July 23 case, all around their upper limits at the same time. This is probably the most extreme solar wind driving at 1 au and enables us to estimate the plausible upper limit for geomagnetic storm activity. With an appropriate modification in the southward field, we suggest that a geomagnetic storm with a minimum D st of about −2000 nT could occur in principle. The magnetopause would be compressed to about 3.3 Earth radii from the Earth’s center, well inside the geosynchronous orbit.
[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] Annually laminated sediments (varves) form in particular depositional settings, e.g., where seasonal climate produces fluctuations in runoff volume; variations in runoff affect the amount and type of sediment delivered to a catchment. Prior studies confirm that variations in selected varve traits correlate with inter-annual climate signals. In some locations, solar activity also appears to be expressed in varve characteristics, either through a direct effect or indirectly via influence of solar activity on climate. Evidence from proglacial Iceberg Lake, Alaska, indicates that solar activity may have directly contributed to varve deposition. A varve thickness sequence is compared to sunspot observations from 1610-1995 CE. Maunder and Dalton minima are clearly expressed in a varve power spectrogram; varve signal amplification beginning ca. 1950s CE coincides with increasing activity evident in a sunspot spectrogram, features that are only vaguely discernible in the raw time-series plots. Spectral relationships at sunspot periodicities are consistent with direct solar forcing of varve thickness, independent of any effect solar activity might otherwise have on climate. Simulations based on a meltwater model indicate that direct forcing could result from amplified ultraviolet (UV) emission during solar maxima, combined with lower UV albedo of glacial ice. The plausible forcing mechanism bolsters epistemology for concluding a cause-effect relationship: solar variability likely contributed directly to inter-decadal patterns in Iceberg Lake varve thicknesses. The putative effect could be enhanced at higher latitudes, where Earth’s atmosphere absorbs less of the UV energy emitted by the Sun; periods of lowered ozone concentration near the poles would exacerbate the natural abetting UV phenomena, potentially linking human activity to recent and accelerated polar ice cap melting. (author)