Earth's ambipolar electrostatic field and its role in ion escape to space.

Cold plasma of ionospheric origin has recently been found to be a much larger contributor to the magnetosphere of Earth than expected1-3. Numerous competing mechanisms have been postulated to drive ion escape to space, including heating and acceleration by wave-particle interactions4 and a global electrostatic field between the ionosphere and space (called the ambipolar or polarization field)5,6. Observations of heated O+ ions in the magnetosphere are consistent with resonant wave-particle interactions7. By contrast, observations of cold supersonic H+ flowing out of the polar ionosphere8,9 (called the polar wind) suggest the presence of an electrostatic field. Here we report the existence of a +0.55 ± 0.09 V electric potential drop between 250 km and 768 km from a planetary electrostatic field (E∥⊕ = 1.09 ± 0.17 µV m-1) generated exclusively by the outward pressure of ionospheric electrons. We experimentally demonstrate that the ambipolar field of Earth controls the structure of the polar ionosphere, boosting the scale height by 271%. We infer that this increases the supply of cold O+ ions to the magnetosphere by more than 3,800%, in which other mechanisms such as wave-particle interactions can heat and further accelerate them to escape velocity. The electrostatic field of Earth is strong enough by itself to drive the polar wind9,10 and is probably the origin of the cold H+ ion population1 that dominates much of the magnetosphere2,3.

Closed magnetic topology in the Venusian magnetotail and ion escape at Venus.

Venus, lacking an intrinsic global dipole magnetic field, serves as a textbook example of an induced magnetosphere, formed by interplanetary magnetic fields (IMF) enveloping the planet. Yet, various aspects of its magnetospheric dynamics and planetary ion outflows are complex and not well understood. Here we analyze plasma and magnetic field data acquired during the fourth Venus flyby of the Parker Solar Probe (PSP) mission and show evidence for closed topology in the nightside and downstream portion of the Venus magnetosphere (i.e., the magnetotail). The formation of the closed topology involves magnetic reconnection-a process rarely observed at non-magnetized planets. In addition, our study provides an evidence linking the cold Venusian ion flow in the magnetotail directly to magnetic connectivity to the ionosphere, akin to observations at Mars. These findings not only help the understanding of the complex ion flow patterns at Venus but also suggest that magnetic topology is one piece of key information for resolving ion escape mechanisms and thus the atmospheric evolution across various planetary environments and exoplanets.

Detection of magnetospheric ion drift patterns at Mars.

Mars lacks a global magnetic field, and instead possesses small-scale crustal magnetic fields, making its magnetic environment fundamentally different from intrinsic magnetospheres like those of Earth or Saturn. Here we report the discovery of magnetospheric ion drift patterns, typical of intrinsic magnetospheres, at Mars using measurements from Mars Atmosphere and Volatile EvolutioN mission. Specifically, we observe wedge-like dispersion structures of hydrogen ions exhibiting butterfly-shaped distributions (pitch angle peaks at 22.5°-45° and 135°-157.5°) within the Martian crustal fields, a feature previously observed only in planetary-scale intrinsic magnetospheres. These dispersed structures are the results of drift motions that fundamentally resemble those observed in intrinsic magnetospheres. Our findings indicate that the Martian magnetosphere embodies an intermediate case where both the unmagnetized and magnetized ion behaviors could be observed because of the wide range of strengths and spatial scales of the crustal magnetic fields around Mars.

Exploring the Solar Wind-Planetary Interaction at Mars: Implication for Magnetic Reconnection.

The Martian crustal magnetic anomalies present a varied, asymmetric obstacle to the imposing draped interplanetary magnetic field (IMF) and solar wind plasma. Magnetic reconnection, a ubiquitous plasma phenomenon responsible for transferring energy and changing magnetic field topology, has been observed throughout the Martian magnetosphere. More specifically, reconnection can occur as a result of the interaction between crustal fields and the IMF, however, the global implications and changes to the overall magnetospheric structure of Mars have yet to be fully understood. Here, we present an analysis to determine these global implications by investigating external conditions that favor reconnection with the underlying crustal anomalies at Mars. To do so, we plot a map of the crustal anomalies' strength and orientation compiled from magnetic field data collected throughout the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Then, we create "shear maps" which calculate and plot the angle of shear between the crustal fields and a chosen external field orientation. From there we define a "shear index" to quantify the susceptibility of a region to undergo reconnection based on a given overlaid, external field orientation and the resulting shear map for that region. We demonstrate that the shear analysis technique augments analysis of local reconnection events and suggests southward IMF conditions should favor dayside magnetic reconnection on a more global scale at Mars.

A Statistical Investigation of Factors Influencing the Magnetotail Twist at Mars.

The Martian magnetotail exhibits a highly twisted configuration, shifting in response to changes in polarity of the interplanetary magnetic field's (IMF) dawn-dusk (B Y) component. Here, we analyze ∼6000 MAVEN orbits to quantify the degree of magnetotail twisting (θ Twist) and assess variations as a function of (a) strong planetary crustal field location, (b) Mars season, and (c) downtail distance. The results demonstrate that θ Twist is larger for a duskward (+B Y) IMF orientation a majority of the time. This preference is likely due to the local orientation of crustal magnetic fields across the surface of Mars, where a +B Y IMF orientation presents ideal conditions for magnetic reconnection to occur. Additionally, we observe an increase in θ Twist with downtail distance, similar to Earth's magnetotail. These findings suggest that coupling between the IMF and moderate-to-weak crustal field regions may play a major role in determining the magnetospheric structure at Mars.

Reflected Protons in the Lunar Wake and Their Effects on Wake Potentials.

The refilling of the lunar wake is facilitated by the wake ambipolar electric potential arising from the electron pressure gradient. Incident solar wind protons can be reflected by the lunar crustal magnetic fields and the lunar surface on the dayside and repicked up, entering the lunar wake due to their large gyroradii. This burst of positive charges can cause the lunar wake potential to be reduced by hundreds of volts. We utilize over 7 years of ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) measurements to systematically investigate how the reflected protons affect the lunar wake potential structure when the Moon is immersed in the solar wind. RPs have a peak occurrence rate of ~20% for downstream distances from the Moon at N × 2πR g and a preference of high occurrence rates and high densities in the direction of the motional electric field of the solar wind. We show that reflected protons in the lunar wake can significantly change the electrostatic ambipolar potentials in the wake, leading in turn to the formation of field-aligned, accelerated electron beams. Our case study also suggests a nonmonotonic field-aligned potential structure in the presence of reflected protons in the wake. Lastly, our results show that when the reflected proton density is larger than ~30% of the local proton density from refilling solar wind protons, the wake potential scales as the logarithmic density of reflected protons, which can be explained by the Boltzmann relation.

Mapping the Lunar Wake Potential Structure With ARTEMIS Data.

The refilling of the lunar wake is relatively well explained by the theory of 1-D plasma expansion into a vacuum; however, the field-aligned wake potential is not a directly measured quantity, and thus, a statistical analysis of wake potentials at high altitudes has not been previously performed. In this study, we obtain the wake potential by comparing the field-aligned electron distributions inside and outside of the lunar wake measured by the two probes of the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) mission. The derived potentials from ARTEMIS data vary with solar wind electron temperature and bulk flow velocity as the theory predicts. We also expand the 1-D plasma theory to 2-D in the plane of the interplanetary magnetic field and the solar wind velocity to examine how a tilted interplanetary magnetic field affects the wake potential structure. As the expansion time for the two sides of the wake differs, a wake potential asymmetry is developed in our model. This asymmetry is confirmed by the data-derived wake potentials. Moreover, ambipolar electric fields are obtained from both the modeled and data-derived wake potentials and show good agreement. Lastly, we examine the effects of the solar wind strahl-electron population on the wake potential structure, which appears to cause a net potential difference across the lunar shadow. This may imply that the disturbance of the wake plasma expansion extends farther outside the wake than previous plasma-expansion theories have predicted.

Ionospheric ambipolar electric fields of Mars and Venus: Comparisons between theoretical predictions and direct observations of the electric potential drop.

We test the hypothesis that their dominant driver of a planetary ambipolar electric field is the ionospheric electron pressure gradient (∇P e). The ionospheres of Venus and Mars are mapped using Langmuir probe measurements from NASA's Pioneer Venus Orbiter (PVO) and Mars Atmosphere and Volatile Evolution (MAVEN) missions. We then determine the component of the ionospheric potential drop that can be explained by the electron pressure gradient drop along a simple draped field line. At Mars, this calculation is consistent with the mean potential drops measured statistically by MAVEN. However, at Venus, contrary to our current understanding, the thermal electron pressure gradient alone cannot explain Venus' strong ambipolar field. These results strongly motivate a return to Venus with a comprehensive plasmas and fields package, similar to that on MAVEN, to investigate the physics of atmospheric escape at Earth's closest analog.