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High Altitude Reflection Potential Structures (HARPS) with typical magnitudes of a few tens of volts have been frequently detected above Earth's sunlit polar cap. They are thought to be electrostatic structures forming between 1 to several Earth radii in altitude. Previous satellite studies have been unable to probe the temporal variability of their magnitude owing to their fast motion through the region of observation. In this study, we present a ∼ ${\sim} $10 min time series of observations of HARPS from NASA's Endurance sounding rocket. These observations were made from a spatially localized region, enabling the first experimental investigation of the temporal variability of HARPS. The magnitude of the potential drop was found to vary unexpectedly rapidly between 15.5 and 23.7 V on a time cadence of ≤10 ${\le} 10$ s. We additionally find an inverse correlation between polar rain precipitating flux and reflection potential magnitude, consistent with predictions from prior theoretical work. Plain Language Summary: There exists an electrical barrier over Earth's polar regions which acts to mirror escaping particles back down to Earth's atmosphere. These barriers have never been directly encountered by spacecraft, but the observation of the mirrored particles permits us to both infer their existence and measure their strength. The exact altitude of such barriers is unknown, but they are thought to form at many tens of thousands of kilometers above Earth's surface. Here we present the first study of how the strength of these electrical barriers change over short time scales. We use measurements from NASA's sub‐orbital rocketship Endurance to show that the barrier's strength varies surprisingly rapidly (∼ ${\sim} $10 s). The strength was shown to be sensitive to minor changes in particles raining down onto the polar cap from space. Key Points: We present the first experimental investigation of the temporal variability of HARPSThe potential drop varied unexpectedly rapidly (≤10 s) between 15.5 and 23.7 VAn inverse correlation was found between polar rain precipitation and reflection potential magnitude [ABSTRACT FROM AUTHOR]

Using the SuperThermal Electron Transport (STET) model coupled with the Super‐thermal Proton Electron Atomic Hydrogen—tRansport in the Ionosphere and Thermosphere (SPEAH‐RIT) codes, we demonstrate that temporal variability of ionospheric conductance is defined by several time scales: the temporal variations of the magnetospheric source, the start time of electron precipitation, and the termination of the corresponding source. In these cases, the time scales are defined by dissipation of energetic electrons and effective recombination processes. The results presented in this paper were applied in the regions of pulsating aurora and polar arcs, demonstrating the fact that ionospheric conductance requires some time to form and decay. These time delays constitute an effective "inertia" in the conductance calculation which is not accounted for in many global models which assume an instantaneous connection between precipitation and conductance. Ionospheric conductance inertia influences the temporal variation in ionospheric and magnetospheric electric fields, and as a result, impacts magnetosphere‐ionosphere (MI) dynamics on a global scale. Plain Language Summary: We demonstrate in this manuscript that temporal variability of ionospheric conductance is defined by several temporal scales and has inertias to its development and decay. Ionospheric conductance inertia influences the temporal variation in ionospheric and magnetospheric electric fields, and as a result, the further reconfiguration of electron precipitation dynamics. The results presented in this paper were applied in the regions of pulsating aurora and polar arcs clearly demonstrate an effective "inertia" of ionospheric system to electron precipitation dynamics. Key Points: Ionospheric conductance is the system response to electron precipitation in the regions of pulsating and discrete aurorasIonospheric conductance carries the inertias of its development and decayConductance formation in pulsating aurora and discrete auroral arcs are defined by multiple temporal scales [ABSTRACT FROM AUTHOR]

The most abundant neutral constituent in the exospheric region (i.e., beyond = 500 km altitude) is the atomic hydrogen (H); however, its density distributions predicted by physics-based models have been challenged by satellite-based observations of its far ultraviolet emissions. This discrepancy may impact magnetospheric ions' densities and velocities since numerous chemistry and ion-neutral coupling interactions rely sensitively on the underlying neutral hydrogen population. The Polar Wind Outflow Model a first-principled model for relevant ion species in the high-latitude ionosphere, is employed to investigate the role of neutral H on the ionospheric outflow. Specifically, variability in the outflow of ionospheric H+, He+, N+, and O+ as a response to systematic enhancement and depletion of H number densities were simulated. The altitudedependent ion density and energy partition profiles vary with neutral H density, solar activities, and ion species. These findings suggest that the exosphere plays a crucial role in controlling the production and loss of ions through ionospheric chemistry, as well as the energy contributions by altering ionneutral-electron collisions and the ambipolar electric field to the high-latitude ionospheric outflow. As a result, the escape rates of the ionospheric outflow are directly associated with exospheric distributions. This work potentially helps understand the dominant mechanisms of atmospheric escape, particularly during a hydrogen-rich early Earth's and exoplanet's atmosphere, which is known to play a significant role in understanding the evolution of Earth's atmosphere. [ABSTRACT FROM AUTHOR]

Magnetohydrodynamics (MHD)-based global space weather models have mostly been developed and maintained at academic institutions. While the “free spirit” approach of academia enables the rapid emergence and testing of new ideas and methods, the lack of long-term stability and support makes this arrangement very challenging. This paper describes a successful example of a university-based group, the Center of Space Environment Modeling (CSEM) at the University of Michigan, that developed and maintained the Space Weather Modeling Framework (SWMF) and its core element, the BATS-R-US extended MHD code. It took a quarter of a century to develop this capability and reach its present level of maturity that makes it suitable for research use by the space physics community through the Community Coordinated Modeling Center (CCMC) as well as operational use by the NOAA Space Weather Prediction Center (SWPC).

Every planet or moon with an ionosphere is thought to generate a weak electrical potential which helps ions overcome gravity and escape to space. A pilot study at Venus by Collinson et al. (2016, https://doi.org/10.1002/2016GL068327) indicated a planetary potential an order of magnitude stronger than expected. Here we present a statistical study of the electrical potential drop in the ionosphere of Venus, which was found to be an average of 7.04 ± 2.19 V. However, these strong potentials measured by Venus Express are likely atypical and extreme outliers associated with a transient phenomenon in the Venusian ionosphere. We posit they are associated with transient and sporadic density cavities in the ionosphere and may be the result of sporadic electrostatic double layer formation in the dayside ionosphere. Plain Language Summary: Previous investigations have suggested that Venus has a much stronger electrical potential in its ionosphere than expected. This electrical potential is important as it helps charged particles escape into space. At Venus, the potential can be strong enough to directly propel all water‐group ions (including oxygen) to escape velocity. Thus, understanding what is responsible for such a strong potential at a planet so similar to Earth is of utmost importance for understanding what makes terrestrial planets habitable. We present a statistical study of all measurements by the European Space Agency's Venus Express Orbiter (2006–2014). This statistical study revealed an average potential of 7.04 ± 2.19 V, far exceeding that at Mars and Earth. However, this measurement was only possible during 30 out of 3,189 orbits of Venus Express. The measurements could only be made at solar minimum and the reasons for this are yet unknown. More research is needed to investigate what generates this strong electric field. To accomplish this, future missions to Venus are necessary with a more comprehensive instrument suite that can measure the planet's electrical potential more consistently. Key Points: We present a statistical survey of all Venus Express measurements of the electrical potential drop in the Venusian ionosphereMean potential was 7.04 ± 2.2 V, but such strong potentials are likely due to transient and local phenomena possibly electrostatic double layersFuture missions to Venus are necessary to consistently measure its intrinsic electrical field and investigate its typical strengths [ABSTRACT FROM AUTHOR]

Electron heat flux is an important value for ionospheric space weather modeling networks. Utilizing the 2D array of Time History of Events and Macroscale Interactions during Substorms all‐sky‐imager (ASI) observations, Gabrielse et al. (2021, https://doi.org/10.3389/fphy.2021.744298) described a new method that estimates the auroral scale sizes of intense precipitating electron energy fluxes and their mean energies during two substorms on 16 February 2010. These parameters in combination with SuperThermal Electron Transport code were used to develop a new methodology to calculate electron thermal fluxes from data inputs in 2D during one of the substorms at 09:40:00 UT across Canada and Alaska. To test the effect of various precipitation lifetimes on electron heat flux values, boxcar averages ranging from 0 to 900 s were applied to the ASI data. These data are then combined with the newly developed kinetic simulation to determine the thermal fluxes associated with the observed diffuse and discrete precipitation. Plain Language Summary: Knowing the thermal electron heat flux at the upper ionospheric boundaries is the Achilles' heel of all ionospheric models. Such a thermal heat flux setting is especially difficult to justify in the region of the diffuse aurora that is connected to a large energy reservoir of electrons with energies of a few kiloelectron volts, the Earth's plasma sheet, where MI coupling processes are strongly interconnected. Utilizing the 2D array of Time History of Events and Macroscale Interactions during Substorms all‐sky‐imager white light observations, Gabrielse et al. (2021, https://doi.org/10.3389/fphy.2021.744298) described a new method that estimates the auroral scale sizes of intense precipitating electron energy fluxes and their mean energies during two substorms. These data are used as inputs to a numerical simulation to determine the thermal fluxes associated with the observed diffuse and discrete precipitation across Canada and Alaska. This is the first‐time data in two dimensions are used to inform a model in order to obtain the thermal electron heat flux values. The new method is an improvement over current calculations, since thermal electron heat fluxes cannot be observed directly and thus far have remained elusive and dependent on modeling assumptions. Key Points: Time History of Events and Macroscale Interactions during Substorms all‐sky imager observations are used across Canada and Alaska for first ever 2D data‐informed heat flux estimationData from substorm case study is coupled to SuperThermal Electron Transport model for the heat flux estimationTime resolution of energy flux and mean energy inputs shown to be remarkably important in heat flux calculation [ABSTRACT FROM AUTHOR]

Photoelectrons are crucial to atmospheric physics. They heat the atmosphere, strengthen planetary ambipolar electric fields, and enhance the outflow of ions to space. However, there exist only a handful of measurements of their energy spectrum near the peak of photoproduction. We present calibrated energy spectra of pristine photoelectrons at their source by a prototype Dual Electrostatic Analyzer (DESA) instrument flown on 11 July 2021 aboard the Dynamo‐2 sounding rocket (NASA № 36.357). Photopeaks arising from 30.4 nm He‐II spectral line were observed throughout the flight above 120 km. DESA also successfully resolved the rarely observed N2 absorption feature. Below 10 eV observations were in good agreement with the GLOW suprathermal electron. Above 10 eV fluxes substantially deviated from the model by as much as an order of magnitude. Plain Language Summary: We designed, built, and flew a new scientific instrument for the measurement of photoelectrons which are created when sunlight shines on the upper atmosphere. The instrument was launched on a suborbital rocket from NASA Wallops Flight Facility just before 2 p.m. on 11 July 2021. The rocket flew to an altitude of 131 km before splashing down in the Atlantic Ocean 8 min later. The instrument gathered scientific data during the flight, measuring the energy spectrum of electrons in Earth's ionosphere. Historical observations of electron spectra at these altitudes are extremely rare, and are often uncalibrated and/or not archived. We present calibrated observations of the pristine spectra of Earth's electrons near their source as a reference for future computer modeling and exploration of Earth's ionosphere. Key Points: We present in situ observations from a plasma spectrometer flown on a rocket to 131 km in the daytime mid‐latitude ionosphereThe instrument returned calibrated measurements of the energy spectra of pristine photoelectrons near the peak of productionThe N2 absorption feature and He‐II photopeaks were partially resolved. Observations are compared with the GLOW electron model [ABSTRACT FROM AUTHOR]

Traditionally, it is widely assumed that the source of electron precipitation in the region of diffuse aurora is wave‐particle interaction processes in the Earth's plasmasheet. It is difficult to imagine that ionosphere‐thermosphere system is also actively involved in the formation of electron precipitation and provides a very noticeable energy contribution to this process. In this study, we use specifically designed simulation scenarios of the SuperThermal Electron Transport code to clearly distinguish between the magnetospheric and ionospheric contributions to the precipitating electrons in the region of diffuse aurora. It is demonstrated that atmospheric collisional processes and the overall magnetosphere‐ionosphere‐atmosphere energy circulation dynamics, with the participation of two magnetically conjugate hemispheres, play a very important role in the formation of electron energy fluxes that are coming from the magnetosphere. Plain Language Summary: It is widely agreed that the electrons originating in space are primarily responsible for the initiation of the beautiful light displays of aurora in the sky. This is definitely true, because the ignition of polar lights is driven by the different acceleration processes in near‐Earth space. These processes, however, do not provide the total amount of energy coming into the polar ionosphere‐thermosphere system carried by precipitating auroral electrons. As we find and explain in this letter, the atmosphere by itself, provides an additional and very noticeable amount of energy to the electrons driving the aurora. This energy is drawn from a reservoir built up by the interaction of electrons with magnetically connected portions of the atmosphere, which traps and redistributes the incoming energy. The result is that more energy can appear to be carried by precipitating electrons than is provided instantaneously by the acceleration processes in space. This substantial additional energy is only possible because of the interaction with the atmosphere. We demonstrate this phenomena using carefully designed numerical simulations. Key Points: Electron precipitation dynamics in the region of diffuse auroraDistinguishing between electrons of magnetospheric and ionospheric origins using SuperThermal Electron Transport code simulation scenariosThe quantitative assessment of the role of atmospheric collisions in electron precipitation phenomena in diffuse aurora [ABSTRACT FROM AUTHOR]

On July 11, 2020, NASA's Parker Solar Probe made its third flyby of Venus. The upper hybrid resonance emission was observed below 1,100 km (a first at Venus), revealing electron densities an order of magnitude lower than at solar maximum. These observations are consistent with a substantial variation in the density and structure of the Venusian ionosphere over the Solar Cycle. Plain Language Summary: The planet Venus is in many ways the most Earth‐like planet known and is thus a perfect natural laboratory for understanding what makes Earth‐like planets habitable. It is often thought that Earth's magnetic field is important for life to exist, as it shields our atmosphere from being stripped away to space. If this is true then one might expect that Venus, with no protective magnetic field, would lose more atmosphere when the sun was more active. However, recent studies have shown the opposite to be true. To investigate this, we need measurements of the upper most part of the Venusian atmosphere (the ionosphere, the source of atmospheric escape. On July 11, 2020, NASA's Parker Solar Probe made a close flyby of Venus. During the 7 minutes around the closest approach, one of its scientific instruments detected low‐frequency radio emission of a type naturally generated by planetary ionospheres. By measuring the frequency of this emission, we can directly calculate the density of the ionosphere around Parker, finding it to be far less dense than previous missions have encountered. This supports the theory that the ionosphere of Venus varies substantially over the 11 year solar cycle. Key Points: We report the first detection of upper hybrid resonance emission at Venus, the first in situ measurement of the ionosphere at solar minimumElectron densities were an order of magnitude lower than at solar maximum, and asymmetrically denser toward the terminatorThe Venusian ionosphere varies significantly over the solar cycle, confirming past remote sensing experiments [ABSTRACT FROM AUTHOR]

This publication addresses the collisional superthermal electron dynamics below the auroral acceleration region (AAR). This region is the portion of an auroral field line with a field‐aligned electric field that leads to the formation of precipitating monoenergetic keV electron fluxes that produce the discrete auroral displays observable from the ground. It is assumed that these precipitating electron fluxes are monoenergetic and accelerated through a potential drop, V, such that these electrons are peaked at an energy E0 = eV, where e is the electron charge. Monoenergetic electrons precipitating into the upper atmosphere degrade to lower energies via many different collisional processes and produce the secondary electron population with energies of 10–100s eV which escapes back to magnetospheric altitudes and becomes geomagnetically trapped between the AAR and the upper ionosphere. The secondary electrons in this geomagnetic trap transfer energy via elastic Coulomb collisions to the thermal electrons. That energy is then returned to the topside ionosphere as heat flux carried by the electron thermal conduction which is essential to maintaining the topside electron temperature. Key Points: Superthermal electron (SE) dynamics in the geomagnetic trap below the AAR is consideredHow SE distribution function evolve in response to the existence of geomagnetic trap is discussedThe SE energy interplay in the geomagnetic trap and the formation downward energy fluxes are considered [ABSTRACT FROM AUTHOR]

An intriguing aspect of the famous September 2, 1859 geomagnetic disturbance (or "Carrington" event) is the horizontal magnetic (BH) data set measured in Colaba, India (magnetic latitude approximately 20°N). The field exhibits a sharp decrease of over 1,600 nT and a quick recovery of about 1,300 nT, all within a few hours during the daytime. The mechanism behind this has previously been attributed to magnetospheric processes, ionospheric processes or a combination of both. In this study, we outline our efforts to replicate this low-latitude magnetic field using the Space Weather Modeling Framework. By simulating an extremely high pressure solar wind scenario, we can emulate the low- latitude surface magnetic signal at Colaba. In our simulation, magnetospheric currents adjacent to the near-Earth magnetopause and strong Region 1 field-aligned currents are the main contributors to the large Colaba BH. The rapid recovery of BH in our simulated scenario is due to the retreat of these magnetospheric currents as the magnetosphere expands, as opposed to ring current dynamics. In addition, we find that the scenario that best emulated the surface magnetic field observations during the Carrington event had a minimum calculated Dst value between -431 and -1,191 nT, indicating that Dst may not be a suitable estimate of storm intensity for this kind of event. [ABSTRACT FROM AUTHOR]

The equatorward extent of the auroral oval, the region which separates the open-field polar cap regions with the closed field subauroral regions, is an important factor to take into account when assessing the risk posed by space weather to ground infrastructure. During storms, the auroral oval is known to move equatorward, accompanied by ionospheric current systems and significant magnetic field variations. Here we outline a simple algorithm which can be used to estimate the maximum extent of the auroral equatorward boundary (MEAEB) using magnetic field data from ground-based observatories. We apply this algorithm to three decades of INTERMAGNET data, and show how the auroral oval in the Northern hemisphere moves South with larger (more negative Dst) storms. We simulate a number of storms with different magnitudes using the Space Weather Modeling Framework (SWMF), and apply the same auroral boundary detection algorithm. For SWMF simulated storms with Dst > -600nT, the estimates of the MEAEB are broadly in line with the same estimates for historical events. For the extreme scaled storms (with Dst < -1,000 nT), there is considerable scatter in the estimated location of the auroral equatorward boundary. Our largest storm simulation was calculated using Carrington-like estimates for the solar wind conditions. This resulted in a minimum Dst = -1,142 nT, and a minimum estimated auroral boundary of 35.5° MLAT in places. [ABSTRACT FROM AUTHOR]

The escape of heavy ions from the Earth atmosphere is a consequence of energization and transport mechanisms, including photoionization, electron precipitation, ion‐electron‐neutral chemistry, and collisions. Numerous studies considered the outflow of O+ ions only, but ignored the observational record of outflowing N+. In spite of 12% mass difference, N+ and O+ ions have different ionization potentials, ionospheric chemistry, and scale heights. We expanded the Polar Wind Outflow Model (PWOM) to include N+ and key molecular ions in the polar wind. We refer to this model expansion as the Seven Ion Polar Wind Outflow Model (7iPWOM), which involves expanded schemes for suprathermal electron production and ion‐electron‐neutral chemistry and collisions. Numerical experiments, designed to probe the influence of season, as well as that of solar conditions, suggest that N+ is a significant ion species in the polar ionosphere and its presence largely improves the polar wind solution, as compared to observations. Plain Language Summary: Nitrogen is the most abundant element in the Earth's atmosphere. Around 78% N2 and 21% O2 form the air we breathe and expand into high‐altitude atmosphere, the thermosphere, and eventually the ionosphere. The neutral molecules in the ionosphere are ionized by solar radiation, and some of them break up into atoms, and others become charged particles. The ionospheric ions with sufficient energy can flow out into space, and the abundances of these outflowing ionospheric ions highly impact the near‐Earth plasma properties. Studies focused on outflowing O+ ions have been conducted for several years. However, the contribution of N+ ions to the outflow solution is still largely unknown due to the instrumental limitations. This letter addresses the transport and energization of ionospheric N+ ions based on theoretical predictions, and examines the role of N+ ions to the collision and chemistry in the topside ionosphere. This study shows that N+ ions are important components in the high‐altitude polar ionosphere and their abundances can be affected by the sunlight and seasonal variations. Key Points: We developed a seven‐ion polar wind model (7iPWOM), which solves for the outflowing N+ ionsN+ ions are the second most abundant ionospheric species, up to 1,200 kmThe presence of N+ ions improves the overall polar wind solution, when compared with observations [ABSTRACT FROM AUTHOR]

Whistler and electrostatic electron cyclotron harmonics waves are responsible for scattering and precipitating the energetic plasma sheet electrons that drive the diffuse aurora. These primary electrons with energies in the kiloelectron volt range, simultaneously precipitating in magnetically conjugate regions, produce the secondary electron population and can be reflected by the atmosphere back through the magnetosphere and precipitate into the conjugate region with additional follow‐up atmospheric backscatter. Primary, degraded, and secondary electrons can be trapped back into the magnetosphere as they travel back and forth between the two magnetically conjugate ionospheres and continuously delivering their energy to the cold plasma sheet electrons and form the electron thermal fluxes that deposit this energy at the upper ionospheric altitudes. We consider the formation of these heat fluxes focusing on the magnetosphere‐ionosphere energy interplay of the entire superthermal electron spectra from 1 eV up to 10 keV and discuss the efficiency of the different spectral energy intervals that contribute to the electron plasma heating at the magnetospheric altitudes. Our parametric studies at L = 6.8, with lower and upper band chorus whistler wave amplitudes of 10 pT and electron cyclotron harmonic wave amplitudes of 1 mVm−1, indicate the dominant role of the whistler mode in the formation of the electron heat flux coming from the magnetosphere to the ionosphere. Key Points: Magnetosphere‐ionosphere coupling processes of superthermal electrons in the region of diffuse auroraThe formation of electron heat flux in the region of diffuse auroraHow ECH and whistler waves contribute to the SE precipitation and electron heat flux formation at the ionospheric altitudes [ABSTRACT FROM AUTHOR]

The electron precipitation in the region of the diffuse aurora should be considered as a two‐step process (Khazanov et al., 2017, https://doi.org/10.1002/2016GL072063). The first one is the interaction of plasma sheet electrons with electrostatic electron cyclotron and/or whistler waves, moving those electrons into the loss cone to precipitate in both magnetically conjugate atmospheres. The second step is the interaction of these electrons with the ionosphere and atmosphere via their elastic and nonelastic collisions and reflection (backscatter) of degraded electrons back to magnetosphere and conjugate ionospheres. This paper presents the results of a newly developed scenario of nonsteady‐state electron precipitation dynamics that accounts for magnetosphere‐ionosphere‐atmosphere energy interplay over the entire energy range of the plasma sheet electron population and their affiliated secondary electrons. It also studies how both magnetically conjugate auroral regions work together with the magnetosphere in the formation of electron precipitation in the region of the diffuse aurora with the energy range coverage from 1 eV up to 10 keV. Key Points: The electron precipitation in the region of the diffuse aurora should be considered as a two‐step processNewly developed scenario of nonsteady‐state electron precipitation dynamics is discussedAsymmetry and MI coupling play a role in the formation of low energy electron precipitation [ABSTRACT FROM AUTHOR]

We investigate the space environment conditions near the Earth-size planet TOI 700 d using a set of numerical models for the stellar corona and wind, the planetary magnetosphere, and the planetary ionosphere. We drive our simulations using a scaled-down stellar input and a scaled-up solar input in order to obtain two independent solutions. We find that for the particular parameters used in our study, the stellar wind conditions near the planet are not very extreme—slightly stronger than that near the Earth in terms of the stellar wind ram pressure and the intensity of the interplanetary magnetic field. Thus, the space environment near TOI 700 d may not be extremely harmful to the planetary atmosphere, assuming the planet resembles the Earth. Nevertheless, we stress that the stellar input parameters and the actual planetary parameters are unconstrained, and different parameters may result in a much greater effect on the atmosphere of TOI 700 d. Finally, we compare our results to solar wind measurements in the solar system and stress that modest stellar wind conditions may not guarantee atmospheric retention of exoplanets. [ABSTRACT FROM AUTHOR]

We test the hypothesis that their dominant driver of a planetary ambipolar electric field is the ionospheric electron pressure gradient (∇Pe). 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. Plain Language Summary: Every planet with an atmosphere generates a weak electric field, called an "ambipolar field," which plays a critical role in the escape of the ionosphere. Until recently, these fields had never been measured due to their low strength. However, by measuring the subtle shifts in the energies of electrons generated in the ionosphere, the total potential drop associated with this field has recently been measured at both Venus and Mars. These measurements permit us to make the first investigation of the fundamental physics that underpins this field. Specifically, we test the long‐held hypothesis that ambipolar fields are primarily generated by the gradient of electron pressure along magnetic field lines that connect the ionosphere to space. We find the potential drop at Mars is consistent with theory, but Venus' field is 10 times stronger than expected. Key Points: We map the ionospheres of Venus and Mars to investigate whether ambipolar fields are generated by the thermal electron pressure gradientMars' ambipolar potential drop is consistent with what would be expected from existing theory (∼0.7 V peaking at 220 km)Venus' potential drop (10 V) far exceeds what can be explained by the mean electron pressure gradient (1 V), motivating further research [ABSTRACT FROM AUTHOR]