Abstract The magnetic perturbations generated by Europa's plasma interaction undergo oscillatory variations since Jupiter's magnetospheric current sheet periodically sweeps across the moon. We determine the inductive response of Europa's subsurface ocean to this excitation signal and compare it to the field induced by the time‐varying yet locally uniform Jovian background field. We apply a hybrid model to calculate the three‐dimensional structure of Europa's plasma and electromagnetic environment at equidistant points in time across a Jovian synodic rotation. We isolate the time‐varying component of this interaction to calculate the resulting induced Gauss coefficients, then evaluate the feedback of this field on the plasma perturbations. We find that induction by time‐varying plasma currents generates largely dipolar contributions to Europa's magnetic environment that cause discernible modifications to the moon's plasma interaction. The effect of these induced fields is strongest near the center of Jupiter's magnetospheric current sheet. In this scenario, the inductive response to the uniform magnetospheric background field is minimized while the magnetic signatures from the deflection of the upstream flow maximize. Finally, we illustrate that the fields induced by time‐varying plasma currents can nearly equal the fields induced by the time‐varying magnetospheric fields within the current sheet, even at flyby altitudes. For all distances of the moon with respect to the sheet, these plasma‐induced fields are comparable to the induction signal from, for example, fluid convection within the ocean or deviations of the ocean's shape from a perfect sphere. Hence, this effect must be included when characterizing Europa's interior through magnetometer measurements.
Abstract Storm‐time substorms are known to impact the global magnetosphere–ionosphere (MI) system, however the effects of isolated substorms on the global MI system are not well understood. We present observational evidence that an intense isolated substorm (onset at 12:30 UT on 6 December 2016; minimum SML 913 nT) drove large‐scale global disturbances across the MI system. The stable solar wind conditions during this time provided a rare and clean opportunity to isolate the direct effects of substorm activity from storm‐time dynamics. Using data from GOES‐13, GOES‐15, and Van Allen Probes, we observed isolated substorm‐associated particle injections that contributed to the development of a robust partial ring current in the inner magnetosphere. Concurrently, strong dawn–dusk asymmetries (∼75 nT) were observed in the low– to mid–latitude ground magnetic data during the expansion phase of the isolated substorm. Interestingly, isolated substorm–associated electric field perturbations were observed in the equatorial ionosphere on both dayside and nightside, contrary to a recent statistical study showing no impact of isolated substorms on the day side equatorial ionosphere. During the expansion phase of the isolated substorm, westward electric field penetrations were observed from 07:00 to 22:00 Local Time over the equatorial ionosphere. These results demonstrate that even an isolated substorm can drive large–scale global disturbances in the MI system, highlighting the need to incorporate isolated substorm effects in future observational, modeling and space weather forecasting studies.
Abstract This study compares the capabilities of different sized micro‐pore optic (MPO) based soft X‐ray telescopes and their ability to observe features and phenomena of the magnetosheath at Earth. The basic science is the starting point and different instrument properties are applied to test which would be capable of studying features of the near‐Earth space, such as the magnetopause and bow shock location, as well as transients such as flux transfer events and mirror‐mode waves. Nine model optics are evaluated with optic radius of curvature ranging from 75 to 800 cm, and their effective area and production factor calculated. Soft X‐ray emission data from the day‐side magnetosphere is extracted from Vlasiator numerical simulation results (Grandin et al., 2024, https://doi.org/10.26464/epp2023052 ) and converted into counts detected by the nine different MPO telescopes with each unique production factor. A cosmic X‐ray background and Poisson noise are added to make realistic model instrument images. A natural tradeoff is presented between instrument size and science performance. Detection accuracy of the magnetopause location, a primary science goal for many magnetosphere observing MPO telescopes, is shown to range from 0.396 with a signal to noise (SNR) of 0.918 (75 cm at 10 s observation duration) to 0.014 and SNR of 7.491 (800 cm at 300 s observation duration). Flux transfer events can be observed in model telescope images at 10 and 30 s observation durations. The analysis shows that larger optic size improves instrument grasp, and permits shorter observation durations to image magnetospheric transients.
Abstract Thermospheric mass density at low‐Earth‐orbit (LEO) satellite altitudes is primarily controlled by the vertically integrated thermal state through hydrostatic balance and the ideal gas law. However, direct temperature and composition measurements in the 100–200 km region, where vertical temperature gradients are strongest and accurate temperature information is critical, remain sparse. Here we demonstrate that daily averages of disk temperature from the NASA's Global‐Scale Observations of the Limb and Disk (GOLD) mission exhibit a tight relationship with GRACE‐FO neutral density, providing a robust observational constraint on global‐mean density at LEO altitudes. This relationship is consistent with a hydrostatic mapping in which serves as a proxy for the vertically integrated thermal state controlling atmospheric expansion and contraction, but should not be interpreted as an exact local hydrostatic relationship. Allowing seasonal variability in the fitting technique further reduces systematic bias. These results establish height‐integrated as a physically meaningful and observationally ready constraint on global‐mean density variability, and provide a pathway toward near‐real‐time thermospheric density product.
Abstract Quasi‐thermal noise (QTN) spectroscopy is an important tool for plasma diagnostics in space missions, where the antenna geometry affects the measured voltage spectrum. Existing antenna response models mainly focus on linear dipole antennas, whereas the influence of finite opening angles in V‐shaped antennas has not been fully characterized. In this work, we develop a generalized response‐function model for symmetric V‐shaped dipole antennas with arbitrary opening angles. The formulation incorporates the wave‐vector projections along the two antenna arms and naturally recovers the classical linear‐dipole limit and short‐antenna asymptotic behavior. Comparisons with the projected‐equivalent linear‐dipole approximation demonstrate that the effective‐length description is valid only in the low‐wavenumber regime, whereas finite‐wavenumber deviations arise from the full two‐arm geometry. The dependence of the response function on the opening angle is investigated, showing systematic variations in the peak amplitude and characteristic wavenumber. The effects of finite center gaps and realistic current redistribution are further incorporated using an AWAS‐type current model. Solar Orbiter (SolO)‐like parameters are adopted to demonstrate the impact of antenna geometry on the impedance and absolute QTN spectrum. These results provide a generalized framework for interpreting QTN measurements with V‐shaped spacecraft antennas and highlight the importance of retaining the full antenna geometry beyond simple projected‐length approximations.
