On 19 October 2014, Mars experienced a very close encounter with Comet C/2013 A1 Siding Spring. Using data from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on board Mars Express (MEX), we assess the interaction of the Martian ionosphere with the comet’s atmosphere and possibly magnetic tail during the orbit of their closest approach. The topside ionospheric electron density profile is evaluated from the peak density of the ionosphere to the local plasma around Mars Express. We find unusual, complex and rapid variability in the ionospheric profile along the MEX orbit, not seen even after the impact of large coronal mass ejections. Before closest approach, large electron density reductions predominate, which could be caused either by comet water-damping, or comet magnetic field interactions. After closest approach, a substantial electron density rise predominates. Moreover, several extra topside layers are visible along the whole orbit at different altitudes, which could be related to different processes as we discuss.

The complex magnetospheric and ionospheric events during September 2017 are studied. There were 4 X-class, 27 M-class and numerous C-class flares related to ~68 coronal mass ejections (CMEs), 4 of which were halo CMEs. Of the 4 halo CMEs, only 3 reached the Earth. A fast interplanetary-CME (ICME) created an upstream sheath that caused an intense magnetic storm (SYM-H peak = -146 nT). This was followed by another intense storm (SYM-H peak = -115 nT) caused by the magnetic cloud (MC) portion of the ICME. Two moderate storms (with SYM-H peaks of -65 nT and -74 nT) were caused by a sheath associated with another halo CME and a corotating interaction region (CIR), respectively. The solar wind high-speed streams (HSSs) led to continuous substorm and convection events but no magnetic storms. Fast forward shocks (FSs) and reverse waves (RWs) associated with the fast CMEs and CIRs, and heliospheric current sheet/heliospheric plasma sheet encounters were detected. The FSs and RWs caused positive and negative sudden impulses, respectively. Half of the FSs triggered substorm onsets and the RWs caused substorm recovery phases. While the FSs led to magnetospheric relativistic electron decreases, electron accelerations were associated with the MC and the HSSs. During main phases of the intense storms, two supersubstorms (SSSs) were detected, one triggered by a FS and the other by a non-shock ram pressure pulse. The SSSs caused major geomagnetically induced currents. CME propagation codes were tested with errors in arrival times ranging from ~24 min to > 35 h.

“Eddy saturation” is the regime in which the total time-mean volume transport of an oceanic current is relatively insensitive to the wind stress forcing and is often invoked as a dynamical description of Southern Ocean circulation. We revisit the problem of eddy saturation using a primitive-equations model in an idealized channel setup with bathymetry. We apply only mechanical wind stress forcing; there is no diapycnal mixing or surface buoyancy forcing. Our main aim is to assess the relative importance of two mechanisms for producing eddy saturated states: (i) the commonly invoked baroclinic mechanism that involves the competition of sloping isopycnals and restratification by production of baroclinic eddies, and (ii) the barotropic mechanism, that involves production of eddies through lateral shear instabilities or through the interaction of the barotropic current with bathymetric features. Our results suggest that the barotropic flow-component plays a crucial role in determining the total volume transport.

Earthquakes in seismological catalogs and acoustic emission events in lab experiments can be statistically described as a linear Hawkes point process, where the spatio-temporal rate of events is a linear superposition of background intensity and the aftershock clusters triggered by preceding activity. Traditionally, statistical seismology has interpreted this model as the outcome of an epidemic branching process, where one-to-one causal links can be established between mainshocks and aftershocks. Declustering techniques have been used to infer the underlying triggering trees and relate their topological properties with epidemic branching models. Here, we review how the standard Epidemic Type Aftershock Sequence (ETAS) model extends from the Galton-Watson (GW) branching processes and bridges two extreme cases: Poisson sampling and scale-free power-law trees. We report the most essential topological properties expected in GW epidemic trees: the branching probability, the distribution of tree size, the expected family size, and the relation between average leaf-depth and tree size. We find that such topological properties depend exclusively on two sampling parameters of the standard ETAS model: the average branching ratio N_b and the exponent ratio α/b determining the branching probability distribution. From these results, one can use the memory-less GW as a null-model for empirical triggering processes and assess the validity of the ETAS model to reproduce the statistics of natural and artificial catalogs.

We present observations of the northern hemisphere auroras taken with the Far UV cameras onboard the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft during a compression of the magnetosphere by a solar wind pressure step on 30 December 2001. The compression occurs during a period of northward IMF which has given rise to the presence of a pair of transpolar arcs (TPAs) near the dawnside oval. The compression causes a brightening of the oval, from dayside to nightside over the course of 10 mins, followed by a brightening of the midnight sector oval and TPAs from nightside to dayside, again over 10 mins. We suggest that the brightening is caused by pitch angle scattering of particles trapped on closed magnetic field lines, and that the sequence of the brightening tracks the solar wind pressure step as it progresses along the length of the magnetotail. Travelling at 600 km s$^{-1}$, the step reaches up to 90 $R_E$ down-tail over the period of brightening, suggesting that the magnetic field lines which map to the TPAs are closed and stretch almost this length down-tail.

The cometary plasma environment at low gas production rates is dominated by highly compressional, large amplitude magnetic field waves in the 10-100mHz range. They are thought to be caused by an ion-Weibel instability due to a cross-field current, which is caused by the cometary ions that are accelerated along the solar wind convective electric field. We devise a new method to determine the location of the wave detection, the wave power, frequency, and bandwidth. It is found that the wave occurs everywhere in the coma, regardless of electric field direction. There is no correlation between the wave frequency and the measured plasma density. This is not in agreement with previous studies. A dependence of the frequency on the position of the spacecraft in a comet-fixed frame is in agreement with the prediction from the ion-Weibel instability. We infer a wave generation region much larger than the distances covered by Rosetta.

Observations of Jupiter’s deep atmosphere by the Juno spacecraft have revealed several puzzling facts: The concentration of ammonia is variable down to pressures of tens of bars, and is strongly dependent on latitude. While most latitudes exhibit a low abundance, the Equatorial Zone of Jupiter has an abundance of ammonia that is high and nearly uniform with depth. In parallel, the Equatorial Zone is peculiar for its absence of lightning, which is otherwise prevalent most everywhere else on the planet. We show that a model accounting for the presence of small-scale convection and water storms originating in Jupiter’s deep atmosphere accounts for the observations. Where strong thunderstorms are observed on the planet, we estimate that the formation of ammonia-rich hail (’mushballs’) and subsequent downdrafts can deplete efficiency the upper atmosphere of its ammonia and transport it efficiently to the deeper levels. In the Equatorial Zone, the absence of thunderstorms shows that this process is not occurring, implying that small-scale convection can maintain a near-homogeneity of this region. A simple model satisfying mass and energy balance accounts for the main features of Juno’s MWR observations and successfully reproduces the inverse correlation seen between ammonia abundance and the lightning rate as function of latitude. We predict that in regions where ammonia is depleted, water should also be depleted to great depths. This new vision of the mechanisms at play, which are both deep and latitude-dependent, has consequences for our understanding of Jupiter’s deep interior and of giant-planet atmospheres in general.

Russian Alpha radio navigation system (RSDN-20) emits F1=11.9kHz signals into the magnetosphere which propagate as whistler-mode waves. Observed by waveform continuous burst mode from EMFISIS on Van Allen Probes, a case is presented and featured with ducted propagation, multiple reflections and triggered emissions. Both risers and fallers appear in the triggered emissions. We use a ray-tracing method to demonstrate ducted propagation, which has a similar wave normal angle near $150^{\circ}$ as the observation. The arrival time of reflected signals is estimated using propagation analysis and compared with the observed signal arrival time. To test the non-linear cyclotron resonance theory, the order of chirping rate in triggered emission is estimated. The estimated order agrees with the observation.

A feature-based metric of the waviness of the wintertime, Northern Hemisphere polar and subtropical jets is developed and applied to three different reanalysis data sets. The analysis first identifies a “core isertel” along which the circulation per unit length is maximized in the separate polar (315:330K) and subtropical (340:355K) jet isentropic layers. Since the core isertel is, by design, an analytical proxy for the respective jet cores, the waviness of each jet is derived by calculating a hemispheric average of the meridional displacements of the core isertel from its equivalent latitude - the southern extent of a polar cap whose area is equal to the area enclosed by the core isertel. Analysis of the seasonal average waviness over the time series of the various data sets reveals that both jets have become systematically wavier while exhibiting no trends in their average speeds. The waviness of each jet evolves fairly independently of the other in most cold seasons and the slow northward creep of the polar jet is statistically significant. Finally, comparison of the composites of the waviest and least wavy seasons for each species reveals that such interannual variability is manifest in familiar large-scale circulation anomalies.

Charged particles are observed to be injected into the inner magnetosphere region from plasma sheet, and energized up to high energies over short distance and time, during both geomagnetic storms and substorms. Numerous studies suggest that it is the short-duration and high-speed plasma flows, which are closely associated with the global effects of magnetic reconnection and inductive effects, rather than the slow and steady convection that control the Earth-ward plasma transport and magnetic flux from the magnetotail, especially during geomagnetic activities. In order to include the effect of inductive electric produced by the temporal change of magnetic field on the dynamics of ring current, we implemented both theoretical and numerical modifications to an inner magnetosphere kinetic model—Hot Electron-Ion Drift Integrator (HEIDI). New drift terms associated with the inductive electric field are incorporated into the calculation of bounce-averaged coefficients for the distribution function, and their numerical implementations and the associated effects on total drift and energization rate are discussed. Numerical simulations show that the local particle drifts are significantly altered by the presence of inductive electric fields, in addition to the changing magnetic gradient-curvature drift due to the distortion of magnetic field, and at certain locations, the inductive drift dominates both the potential and the magnetic gradient-curvature drift. The presence of a self consistent inductive electric field alters the overall particle trajectories, energization, and pitch angle, resulting in significant changes in the topology and strength of the ring current.

During the COVID-19 lockdown (24 Jan to 20 March) in China low air pollution levels were reported in the media as a consequence of reduced economic and social activities. Quantification of the pollution reduction is not straightforward due to effects of transport, meteorology, and chemistry. We have analysed the NO emission reductions calculated with an inverse algorithm applied to daily NO observations from TROPOMI onboard the Copernicus Sentinel-5P satellite. This method allows the quantification of emission reductions per city, and the analysis of emissions of maritime transport and of the energy sector separately. The reductions we found are 20 to 50% for cities, about 40% for power plants and 15 to 40% for maritime transport depending on the region. The reduction in both emissions and concentrations shows a similar timeline consisting of a sharp reduction (34 to 50%) around the Spring festival and a slow recovery from mid-February to mid-March.

Measurements of solar ultraviolet radiation (UVR) performed between January and June 2020 at 10 Arctic and subarctic locations are compared with historical observations. Differences between 2020 and prior years are also assessed with total ozone column and UVR data from satellites. Erythemal (sunburning) UVR is quantified with the UV Index (UVI) derived from these measurements. UVI data show unprecedently large anomalies, occurring mostly between early March and mid-April 2020. For several days, UVIs observed in 2020 exceeded measurements of previous years by up to 140%. Historical means were surpassed by more than six standard deviations at several locations. In northern Canada, the average UVI for March was about 75% larger than usual. UVIs in April 2020 were elevated on average by about 25% at all sites. However, absolute anomalies remained below 3.0 UVI units because the enhancements occurred during times when the solar elevation was still low.

Observations of energetic charged particles associated with Io’s footprint (IFP) tail, and likely within or very near the Main Alfvén Wing, during Juno’s 12th perijove (PJ) crossing show evidence of intense proton acceleration by wave-particle heating. Measurements made by Juno/JEDI reveal proton characteristics that include pitch angle distributions concentrated along the upward loss cone, broad energy distributions that span ~50 keV to 1 MeV, highly structured temporal/spatial variations in the particle intensities, and energy fluxes as high as ~100 mW/m2. Simultaneous measurements of the plasma waves and magnetic field suggest the presence of ion cyclotron waves and transverse Alfvénic fluctuations. We interpret the proton observations as upgoing conics likely accelerated via resonant interactions with ion cyclotron waves. These observations represent the first measurements of ion conics associated with moon-magnetosphere interactions, suggesting energetic ion acceleration plays a more important role in the IFP tail region than previously considered.

Asteroids and comets have the potential to impact Earth and cause damage at the local to global scale. Deflection or disruption of a potentially hazardous object could prevent future Earth impacts, but due to our limited ability to perform experiments directly on asteroids, our understanding of the process relies upon large-scale hydrodynamic simulations. Related simulations must be vetted through code validation by benchmarking against relevant laboratory-scale hypervelocity-impact experiments. To this end, we compare simulation results from Spheral, an Adaptive Smoothed-Particle Hydrodynamics (ASPH) code, to the fragment-mass and velocity data from the 1991 two-stage light gas-gun impact experiment on a basalt sphere target, conducted at Kyoto University by Nakamura and Fujiwara. We find that the simulations are sensitive to the selected strain models, strength models and material parameters. We find that, by using appropriate choices for these models in conjunction with well-constrained material parameters, the simulations closely resemble with the experimental results. Numerical codes implementing these model and parameter selections may provide new insight into the formation of asteroid families (Michel et al., 2015) and predictions of deflection for the Double Asteroid Redirection (DART) mission (Stickle et al., 2017).

Vertical cloud overlap, the ratio of cloud fraction by area and by volume, for cumulus clouds are studied using large-eddy simulations (LES) due to the inefficient, wide-range values of cloud overlap. We can obtain information about the cloud cover of a cloud field by inspecting the individual clouds in that cloud field.We start with the maximum-random assumption and adjust this assumption for individual clouds.From this there is an under-prediction which leads to the conclusion that something can be added.We extend this by considering physical factors of cloud overlap: area variability, vertical wind shear, and turbulence. We use numerical schemes to calculate the effect of each contributor based on cloud height. We obtain great accuracy for our model of cloud overlap. Since there are multiple factors of cloud overlap, we look at the percentage of how much each contributes for a given binned cloud height. Furthermore, we get acceptable agreement for the calculated and actual total cloud cover. As such, we show that no other major contributors for cloud overlap and cloud cover exist. We end with an empirical model to describe the numerical schemes mentioned previously.

Idealized hurricanes are studied at four different sea surface temperatures (ssts) of 26°, 27°, 28°, and 29°C in six nested domains down to 62 m grid size. For the maximum and the distribution of wind speed, domain D6 (62 m) is similar to domain D5 (185 m) showing convergence near 200 m. For sst of 26°, domain 5 does not have small-scale turbulence structures like the other three cases, which is an extension of the previous work of [2009] who only obtained resolved gusts at 62 m. The distribution function of speed changes between the 27° and 28° cases, the latter one having a second peak, related to a structural change. Finally, it is suggested that the damage potential is not simply determined by the maximum wind speed because of the distribution change so a new damage indicator that represents potential damage is proposed.

Ocean acidification (OA) progression is affected by multiple factors, such as ocean warming, biological production, and runoff. Here we used an ocean-biogeochemical model to assess the impact of river runoff and climate variability on the spatiotemporal patterns of OA in the Gulf of Mexico (GoM) during 1981-2014. The model showed the expected pH and aragonite saturation state (ΩAr) decline, due to the increase in anthropogenic carbon, with trends close to values reported for the Subtropical North Atlantic. However, significant departures from the basin-averaged pattern were obtained in part of the northern GoM shelf, where pH and ΩAr increased. Model sensitivity analyses showed that OA progression was counteracted by enhanced alkalinity from the Mississippi-Atchafalaya River System. Our findings highlight that river alkalinity is a key driver of carbon system variability in river-dominated ocean margins and emphasize the need to quantify riverine chemistry to properly assess acidification in coastal waters.

In the mid-latitude Northwest Pacific Ocean, subtropical and subarctic waters meet, intermingle, and mix to form multiple ocean domains characterized by a variety of unique oceanographic structures. The majority of previous studies on the characteristics and distribution of these ocean structures were based on existing definitions of the structure and water mass of interest derived from spatiotemporally limited data. With the massive amount of data accumulated by Argo and a small spatiotemporal bias, data-driven scientific methods may be able to objectively recapture the ocean structure. In this study, we used unsupervised clustering to analyze the temperature and salinity profiles from Argo float in the mid-latitude Northwest Pacific Ocean, and the results were compared to previous knowledge of ocean structure. The results showed that classes were distributed to form specific regions and each class has different oceanographic features that generally correspond to the previously described regional divisions. A striking advantage of this new method is that it quantifies the relative abundance of each class of profiles on a 1° grid. Furthermore, we discovered that the dynamic state of Kuroshio Extension affects the distribution of some classes and the percentage of profiles that are robust to clustering. This suggests that the stability of the Kuroshio Extension flow path has an effect on the distribution of ocean structure. These findings suggest that unsupervised clustering is useful in the analysis of oceanographic structures, allowing us to investigate oceanographic structures in areas that have not previously been extensively studied.