Properties and evolution of a submesoscale cyclonic spiral

The evolution of a submesoscale cyclonic spiral of 1 km in diameter is simulated with ROMS (Regional Ocean Modeling System) using 33.3 m horizontal resolution in a triple-nested configuration. The generation of the spiral starts from a dense filament that is rolled into a vortex and detaches from the filament. During spin-up, extreme values are attained by various quantities, that are organized in single-arm and multi-arm spirals. The spin-down starts when the cyclone separates from the filament. At the same time, the horizontal speed develops a dipole-like pattern and isotachs form closed contours around the vortex center. The amplitudes of most quantities decrease significantly, but the instantaneous vertical velocity w exhibits high-frequency oscillations and more pronounced extremes than during spin-up. The oscillations are due to vortex Rossby waves (VRWs), that circle the eddy counterclockwise and generate multi-arm spirals with alternating signs by means of azimuthal vorticity advection. Experiments with virtual surface drifters and isopycnal floats indicate downwelling everywhere near the surface. The downwelling is most intense in the center of the spiral at all depth levels, leading to a radial outflow in the thermocline and weak upwelling at the periphery. This overturning circulation is driven by convergent near-surface flow and associated subduction of isopycnals. While the downwelling in the center may support the export of particulate organic carbon from the mixed layer into the main thermocline, the upwelling at the periphery effectuates an upward isopycnal transport of nutrients, enhancing the growth of phytoplankton in the euphotic zone.

Forming a cold electron population at a weakly outgassing comet

<p>The Rosetta Mission rendezvoused with comet 67P/Churyumov-Gerasimenko in August 2014 and escorted it for two years along its orbit. The Rosetta Plasma Consortium (RPC) was a suite of instruments, which observed the plasma environment at the spacecraft throughout the escort phase. The Mutual Impedance Probe (RPC/MIP; Wattieaux et al, 2020; Gilet et al., 2020) and Langmuir Probe (RPC/LAP; Engelhardt et al., 2018), both part of RPC, measured the presence of a cold electron population within the coma.</p> <p>Newly born electrons, generated by ionisation of the neutral gas, form a warm population within the coma at ~10eV. Ionisation is either through absorption of extreme ultraviolet photons or through collisions of energetic electrons with the neutral molecules. The cold electron population is formed by cooling the newly born, warm electrons via electron-neutral collisions. Assuming the radial outflow of electrons, the cold population was only expected at comet 67P close to perihelion, where outgassing rate from the nucleus was at its highest (Q > 10<sup>28</sup> s<sup>-1</sup>). However, cold electrons were observed until the end of the Rosetta mission at 3.8au when the outgassing was weak (Q<10<sup>26</sup> s<sup>-1</sup>). Under the radial outflow assumption, there should not have been sufficient neutral gas to efficiently degrade the electron energies.</p> <p>We have developed the first 3D collision model of electrons at a comet. Self-consistently calculated electric and magnetic fields from a collisionless and fully-kinetic Particle-in-Cell model (Deca et al., 2017; 2019) are used as a stationary input for the test particle simulations. We model the neutral coma as a spherically symmetric cloud of pure water, which follows 1/r<sup>2</sup> in cometocentric distance. Electron-neutral collisions are treated as a stochastic process using cross sections from Itikawa and Mason (2005). The model incorporates elastic scattering of electrons and a variety of inelastic collisions, including excitation and ionization of the water molecules.</p> <p>We show that the radial outflow of electrons from the coma is insufficient to generate a cold electron population under weak outgassing conditions. Using our original test particle model, we demonstrate the trapping of electrons in the inner coma by an ambipolar electric field and how this increases the efficiency of the electron cooling.  We also show that, at low outgassing rates, electron-neutral collisions significantly cool electrons within the coma and can lead to the formation of a cold population.</p> <p> </p>

Characterisation of the hydrodynamic atmospheric escape of HD 209458 b, HD 189733 b, and GJ 3470 b

<p>Hydrodynamic escape is the most efficient atmospheric mechanism of planetary mass loss and has a large impact on planetary evolution. However, the lack of observations remained this mechanism poorly understood. Therefore, new observations of the He I triplet at 10830 Å provide key information to advance hydrodynamic escape knowledge. In this work, we analyse the hydrodynamic escape of three exoplanets, HD209458 b, HD189733 b, and GJ 3470 b via an analysis of He triplet absorptions recently observed by the CARMENES high-resolution spectrograph, and their available Ly-alpha measurements, involving a 1D hydrodynamic model. We characterise the main upper atmospheric parameters, e.g.,  the temperature, the composition (H/He ratio), and the radial outflow velocity. We also study their hydrodynamic regime and show that HD209458 b is in the energy-limited regime, HD189733 b is in the recombination-limited regime, and GJ 3470 b is in the photon-limited regime. Details of this work can be found in [1], [2], [3].</p><p>References</p><p>[1] Lampón, M., López-Puertas, M., Lara, L.M., et al. 2020, A&A, 636, A13<br>[2] Lampón, M., López-Puertas, M., Sanz-Forcada, J., et al. 2021, A&A, 647, A129<br>[3] Lampón, M., López-Puertas, M., Czesla, S., et al. 2021, A&A, 648, L7</p>

High-Resolution Observations of a Destructive Macroburst

Shortly after 0600 UTC (midnight MDT) on 9 June 2020, a rapidly intensifying and elongating convective system produced a macroburst and extensive damage in the town of Akron on Colorado’s eastern Plains. Instantaneous winds were measured as high as 51.12 m s−1 at 2.3 m AGL from an eddy covariance (EC) tower, and a 50.45 m s−1 wind gust from an adjacent 10-m tower became the highest official thunderstorm wind gust ever measured in Colorado. Synoptic-scale storm motion was southerly, but surface winds were northerly in a post-frontal airmass, creating strong vertical wind shear. Extremely high-resolution temporal and spatial observations allow for a unique look at pressure and temperature tendencies accompanying the macroburst and reveal intriguing wave structures in the outflow. At 10-Hz frequency, the EC tower recorded a 5-hPa pressure surge in 19 seconds immediately following the strongest winds, and a 15-hPa pressure drop in the following three minutes. Surface temperature also rose 1.5°C in less than one minute, concurrent with the maximum wind gusts, and then fell sharply by 3.5°C in the following minute. Shifting wind direction observations and an NWS damage survey are suggestive of both radial outflow and a gust front passage, and model proximity soundings reveal a well-mixed surface layer topped by a strong inversion and large low-level vertical wind shear. Despite the greatest risk of severe winds forecast to be northeast of Colorado, convection-allowing model forecasts from 6-18 h in advance did show similar structures to what occurred, warranting further simulations to investigate the unique mesoscale and misoscale features associated with the macroburst.

Modelling the He I triplet absorption at 10 830 Å in the atmospheres of HD 189733 b and GJ 3470 b

Characterising the atmospheres of exoplanets is key to understanding their nature and provides hints about their formation and evolution. High resolution measurements of the helium triplet absorption of highly irradiated planets have been recently reported, which provide a new means of studying their atmospheric escape. In this work we study the escape of the upper atmospheres of HD 189733 b and GJ 3470 b by analysing high resolution He I triplet absorption measurements and using a 1D hydrodynamic spherically symmetric model coupled with a non-local thermodynamic model for the He I triplet state. We also use the H density derived from Lyα observations to further constrain their temperatures, mass-loss rates, and H/He ratios. We have significantly improved our knowledge of the upper atmospheres of these planets. While HD 189733 b has a rather compressed atmosphere and small gas radial velocities, GJ 3470 b, on the other hand with a gravitational potential ten times smaller, exhibits a very extended atmosphere and large radial outflow velocities. Hence, although GJ 3470 b is much less irradiated in the X-ray and extreme ultraviolet radiation, and its upper atmosphere is much cooler, it evaporates at a comparable rate. In particular, we find that the upper atmosphere of HD 189733 b is compact and hot, with a maximum temperature of 12 400−300+400 K, with a very low mean molecular mass (H/He = (99.2/0.8) ± 0.1), which is almost fully ionised above 1.1 RP, and with a mass-loss rate of (1.1 ± 0.1) × 1011 g s−1. In contrast, the upper atmosphere of GJ 3470 b is highly extended and relatively cold, with a maximum temperature of 5100 ± 900 K, also with a very low mean molecular mass (H/He = (98.5/1.5)−1.5+1.0), which is not strongly ionised, and with a mass-loss rate of (1.9 ± 1.1) × 1011 g s−1. Furthermore, our results suggest that upper atmospheres of giant planets undergoing hydrodynamic escape tend to have a very low mean molecular mass (H/He ≳ 97/3).