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Boundaries between space plasmas occur in numerous contexts and scales, from astrophysical jets to planetary magnetospheres. Mass and momentum transport across boundaries poses a fundamental problem in magnetospheric physics. Kelvin-Helmholtz instability (KHI) is a promising mechanism to facilitate transport. Although previous studies have suggested KHI occurrence in various space plasmas, theory predicts that compressibility prevents KHI excitation at boundaries with large density gradients because of previously considered boundary structures where density varies with velocity. Based on the observations of a large density gradient boundary by MAVEN at Mars, where we can observe an extreme case, in this study, we show that it is the entropy, instead of the previously considered density, that varies with the velocity in the real velocity-sheared boundary. The entropy-based boundary structure places the velocity shear in a lower-density region than the traditional density-based structure and weakens the compressibility effect. This new boundary structure thus enables KHI excitation even at large density gradient boundaries, such as at the ionopause of unmagnetized planets and the plasmapause of magnetized planets. The result suggests the ubiquitous occurrence of KHI in the plasma universe and emphasizes its important role in planetary cold plasma escape from unmagnetized planets. [ABSTRACT FROM AUTHOR]

Ionospheric heavy ions in the distant tail of the Earth's magnetosphere at lunar distances are observed using the ARTEMIS mission. These heavy ions are originally produced in the terrestrial ionosphere. Using the ElectroStatic Analyzers (ESA) onboard the two probes orbiting the Moon, these heavy ions are observed as cold populations with distinct energies higher than the baseline energy of protons, with the energy‐per‐charge values for the heavy populations highly correlated with the proton energies. We conducted a full solar cycle survey of these heavy ion observations, including the flux, location, and drift energy, as well as the correlations with the solar wind and geomagnetic indices. The likelihood of finding these heavy ions in the preferred regions of observation called "loaded" quadrants of the terrestrial magnetotail is ∼90%, regardless of the z orientation of the IMF. We characterize the ratio of the heavy ion energy to the proton energy, as well as the velocity ratio of these two populations, for events from 2010 to mid‐2023. This study shows that the "common velocity" assumption for the proton and heavy ion particles, as suggested in previous work through the velocity filter effect, is not necessarily valid in this case. Challenges in the identification of the mass of the heavy ions due to the ESA's lack of ion composition discrimination are addressed. It is proposed that at the lunar distances the heavy ion population mainly consists of atomic oxygen ions (O+) with velocities ∼25% more than the velocity of the co‐located proton population. Key Points: We surveyed ARTEMIS observations of ionospheric heavy ions in the magnetotail at lunar distances over a full solar cycleHeavy ions in the magnetotail are highly correlated with solar wind and geomagnetic activity, and preferentially found in loaded quadrantsHeavy ions drift tailward at ∼25% higher velocities than the concurrently observed protons, implying additional acceleration [ABSTRACT FROM AUTHOR]

Mars once had a dense atmosphere enabling liquid water existing on its surface, however, much of that atmosphere has since escaped to space. We examine how incoming solar and solar wind energy fluxes drive escape of atomic and molecular oxygen ions (O+ and O2+ ${\mathrm{O}}_{2}^{+}$) at Mars. We use MAVEN data to evaluate ion escape from 1 February 2016 through 25 May 2022. We find that Martian O+ and O2+ ${\mathrm{O}}_{2}^{+}$ both have increased escape flux with increased solar wind kinetic energy flux and this relationship is generally logarithmic. Increased solar wind electromagnetic energy flux also corresponds to increased O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape flux, however, increased solar wind electromagnetic energy flux seems to first dampen ion escape until a threshold level is reached, at which point ion escape increases with increasing electromagnetic energy flux. Increased solar irradiance (both total and ionizing) does not obviously increase escape of O+ and O2+ ${\mathrm{O}}_{2}^{+}$. Our results suggest that the solar wind electromagnetic energy flux should be considered along with the kinetic energy flux as an important driver of ion escape, and that other parameters should be considered when evaluating solar irradiance's impact on O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape. Plain Language Summary: Mars was once like Earth with a dense atmosphere enabling liquid water to exist on its surface. However, in the billions of years since then, Mars has lost much of its atmosphere to space. We study how energy inputs from the Sun and from the solar wind can drive escape of the ionized constituents of water from Mars' atmosphere. Ion escape is one of several processes of atmospheric loss, and it is a particularly effective process for removing species heavier than hydrogen and helium from terrestrial atmospheres. We find that previously unconsidered energy fluxes may play an important role in driving ion escape. Key Points: Increased solar wind electromagnetic energy flux increases escape of O+ and O2+ ${\mathrm{O}}_{2}^{+}$O+ and O2+ ${\mathrm{O}}_{2}^{+}$ have increased escape rates with increased solar wind kinetic energyUnclear dependence on increased solar irradiance for O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape [ABSTRACT FROM AUTHOR]

On 26 December 2022 the solar wind density dropped by over an order of magnitude and remained low for about a day. We have utilized in‐situ plasma measurements made by the Mars Atmosphere and Volatile EvolutioN mission to determine how this change affected the Mars‐solar wind interaction. During this time period, on inbound orbit segments, MAVEN sampled the terminator ionosphere, which switched from a magnetized to unmagnetized state immediately following the minimum in solar wind density. The magnetic field amplitude was typically 5–10 nT within the upper ionosphere prior to the event and consistently <1 nT after. During the event the magnetic pressure dominated immediately above the ionosphere while within the ionosphere the ionospheric plasma pressure dominated. The high altitude terminator ionosphere remained in this unmagnetized state throughout the event, suggesting that it was the new equilibrium state of the system. The terminator upper ionosphere returned to its original magnetized state once the solar wind density had recovered. The outbound orbit segments sampled the dayside subsolar region which remained magnetized throughout the event: the magnetization state of the ionosphere varied locally, dependent upon the solar zenith angle and corresponding incident solar wind dynamic pressure. Such conditions are different to the commonly reported unmagnetized ionospheric state at Venus during solar maximum conditions, where the interplanetary magnetic field is repelled from the entire dayside ionosphere. Drastic changes in the upstream solar wind are able to change the Mars‐solar wind interaction state on timescales less than one MAVEN orbit (∼3.5 hr). Plain Language Summary: Our Sun emits a continuous stream of charged particles radially outward into our solar system, known as the solar wind. Much like flowing water in a river is deflected around rocks and other objects in the river, the solar wind is also deflected around most obstacles in our solar system, including the planets. The physical forces that control this deflection differ depending upon the characteristics of the planet and the solar wind as it encounters the obstacle. We have studied a rare event at the planet Mars when the solar wind density dropped to very low values over the course of a day. Using orbiting spacecraft observations we have shown that the physical processes that control how the solar wind flows around the planet change directly in response to this event. These processes reverted back to their prior states once the solar wind density recovered a few days later, demonstrating that the planet‐wide system responds to large changes in the solar wind density on timescales of a few hours or less. Key Points: The Mars terminator ionosphere transitioned to an unmagnetized state in response to the 'disappearing solar wind event' in December 2022The Mars‐solar wind interaction system responds to changes in the solar wind density on timescales <3.5 hrThe event shares similarities with when the Venus ionosphere is unmagnetized, but the localized nature is an important difference [ABSTRACT FROM AUTHOR]

The occurrence of two major Amazonian droughts in 2005 and 2010 underscores the need for improved understanding of how drought affects tropical forest. During both droughts, MODIS land surface temperature data detected anomalously high daytime and nighttime canopy temperatures throughout drought-affected regions. Daytime thermal anomalies explained 38.6% of the variability in the reduction of aboveground living biomass (p < 0.01; n = 17) in drought-affected forest sites. Multivariate linear models of heat and moisture stress explained a greater proportion of the variability, at 65.1% (p < 0.01; n = 17), providing substantively greater explanatory power than precipitation-only models. Our results suggest that heat stress played an important role in the two droughts and that models should incorporate both heat and moisture stress to predict drought effects on tropical forests.

A reprint of the article "Toward the New Future," by J. P. McFadden, which appeared in the Fall 1983 issue of the "Human Life Review" is presented. Topics discussed include the issue concerning the decision of the U.S. Supreme Court about the value of human lives, the link between slavery and abortion, and the ethic of legalizing abortion.

The near lunar surface contains small‐scale magnetic field structures that provide a natural test bed for observing plasmas with a non‐zero Hall electric field, as well as potentially facilitating electron‐only reconnection. This study presents observational evidence of magnetized electrons as well as demagnetized ions when THEMIS‐ARTEMIS probe B reached an altitude of ∼15 km above the lunar surface. Additionally, observations suggest the presence of a field line topology change and traversal of a closed magnetic field structure containing solar wind electrons, suggestive of magnetic reconnection having occurred at some point between the solar wind interplanetary magnetic field and a lunar crustal magnetic field. Thus, the observations presented here are consistent with previous studies that predict prominent Hall electric fields near lunar crustal magnetic fields and further suggest that the solar wind interplanetary magnetic field may reconnect with lunar crustal magnetic fields, most likely via electron‐only reconnection. Plain Language Summary: While interactions between the solar wind and the Earth's magnetosphere have been well studied, there is still much to be learned by studying the interactions between the solar wind and the small‐scale lunar magnetic fields. Due to the small‐scale nature of the lunar magnetic fields, previous studies have suggested that the ions do not respond in the same manner as the electrons. The resulting effects lead to an electric field near regions of lunar magnetic fields. This study presents observational evidence of the aforementioned phenomena. Additionally, the spacecraft observations also suggest that magnetic reconnection, or the breaking of the lunar magnetic field lines and reconnection to the magnetic field in the solar wind, was occurring between the solar wind and the lunar magnetic fields. Key Points: Observations suggest magnetic reconnection occurs between the solar wind IMF and lunar crustal magnetic fieldsElectron pitch angle and velocity distributions suggest the spacecraft traversed a closed magnetic topology containing solar wind electronsWe report in‐situ observations of demagnetized ions and associated Hall electric fields near the lunar surface [ABSTRACT FROM AUTHOR]