Mats Andre

We investigate ion dynamics in the ion diffusion region of an antiparallel symmetric reconnection event observed by the Magnetospheric MultiScale mission in Earth's magnetotail. Through detailed investigation of the ion velocity distribution functions, we show that the ion dynamics are initially (upon first entering the current sheet) dominated by the acceleration from the Hall electric and magnetic fields. The Hall electric field ballistically accelerates the ions toward the neutral sheet. The subsequent magnetic forces associated with the reconnecting and Hall magnetic fields have two distinct effects. First, the magnetic force due to the reconnecting magnetic field upstream of the X line delays the Speiser acceleration by gyroturning the ions in the direction opposite to the reconnection electric field. Second, the magnetic force due to the Hall magnetic field gyroturns the ions toward the outflow, kick-starting the outflow jet over short distances. These dynamics are consistent with fast reconnection rates.

At Earth's quasi‐perpendicular bow shock, electrons are reflected and accelerated to high velocities, forming beams. These beams excite Langmuir and beam‐mode waves in the electron foreshock region. The waves can then be converted to radio waves via linear or nonlinear processes. We investigate the properties of the Langmuir waves excited in the electron foreshock region using the Magnetospheric Multiscale (MMS) mission. MMS provides extended high‐resolution snapshots of the three‐dimensional electric field, enabling detailed analysis of wave properties. Probability distribution functions of the electric field are used to investigate the evolution of the waves and the role of density fluctuations. Distinct spectral peaks near the plasma frequency are often observed, suggestive of simultaneous observations of beam‐mode and Langmuir waves, as well as nonlinear electrostatic decay or reflection off density gradients. In addition, the electric fields often have large perpendicular components, consistent with low wave number Z‐mode waves. The statistical results show that the electric fields are largest near the electron foreshock boundary with the solar wind. The probability distribution functions exhibit close to log‐normal probability distribution functions, consistent with predictions from Stochastic Growth Theory. These results suggest that density perturbations, in addition to nonlinear three‐wave decay, are crucial to the evolution of Langmuir waves and the generation of radio waves at the plasma frequency and second harmonic. These results apply to Langmuir waves in the solar wind, such as in Type II and Type III solar radio bursts, where large‐amplitude beam‐driven Langmuir waves with similar properties are observed.

Mercury's magnetosphere is an extremely dynamic environment, where substorm‐like activity, including dipolarization fronts and particle acceleration, periodically disrupts the stability and radial extent of the closed field line region. The common narrowband ∼1 Hz waves at Mercury, near the local proton gyrofrequency are preferentially observed on closed field lines during quieter intervals, but their relationship to magnetospheric dynamics remains poorly understood. Here, we combine MESSENGER data sets of ∼1 Hz wave activity and dipolarization fronts to investigate whether newly closed field lines can sustain wave growth. We find that ∼1 Hz waves on the nightside are systematically absent during orbits with dipolarization intervals and typically confined within 1.3 R _M , compared to greater downtail extents during quiet intervals. The results indicate that ∼1 Hz waves require long‐lived, stable closed field lines to grow; therefore, when they are observed, they can serve as diagnostics of Mercury's magnetospheric state, establishing a clear connection between field‐line history, magnetospheric dynamics, and wave occurrence in Mercury's magnetotail.

Despite heat flux's role in regulating energy conversion in collisionless plasmas, its properties and evolution in the magnetosheath downstream of the Earth's bow shock are scarcely explored. We use Magnetospheric Multiscale measurements to quantify and characterize the electron heat flux in the magnetosheath. We find that the heat flux is shaped by the magnetosheath magnetic field as it drapes around the magnetosphere. While it is affected by solar wind upstream conditions and increases with magnetic field strength, it is not substantially changed by local magnetosheath processes. Also, the heat flux is limited by whistler instability thresholds.

Context . Measurements of spacecraft potential ( V _sc ) are an important tool for plasma diagnostics. High-resolution V _sc measurements can be calibrated with low-cadence observations of electron plasma frequency to derive high-resolution electron density data. Aims . We analyze V _sc measured by the Radio and Plasma Waves (RPW) instrument suite on board Solar Orbiter. We investigate the evolution of V _sc within the framework of the heliocentric distance and mission lifetime. We aim to obtain electron density estimates that are valid across a wider range of V _sc regimes, including the cases of negative V _sc . Methods . We determined V _sc by combining measurements of the probe-to-spacecraft potential, V _ps , and the probe-to-plasma potential, V _pn . Then, we used the appropriate electric current balance to find a relation between electron density and V _sc , which we calibrated to the observed electron plasma frequency. With this method, we obtained the electron density at the resolution given by the V _ps measurements. Results . We find that V _sc reaches negative values for a larger amount of time than expected for a sunlit spacecraft immersed in a tenuous plasma such as the solar wind. We are now able to identify when these V _sc changes occur, enabling us to estimate the electron density accordingly. Conclusions . Solar Orbiter charges negatively during periods of high plasma density and low photoelectron emission. However, the measured V _sc is also influenced by the electrical disconnection of the solar arrays from the spacecraft ground during fuse-blowing events. This disconnection raises the local plasma potential near the probes, causing the measured V _sc to appear negative even when Solar Orbiter remains positively charged. By distinguishing between the genuine negative spacecraft charging and the apparent negative charging induced by solar panels, we can refine previous electron density estimation methods to include cases when V _sc < 0 V. This approach provides reliable, high-resolution electron density estimates across both positive and negative V _sc regimes.

Collisionless shocks convert bulk flow energy into heat, electromagnetic fields, and non‐thermal particle populations. Recent studies suggest that downstream magnetic oscillations could play an important role in ion‐scale energy dissipation at low‐Mach‐number shocks; however, the specific shock and plasma parameters involved remain poorly understood. Interplanetary (IP) shocks, often characterized by low Mach numbers, provide an excellent opportunity for investigating these kinetic dissipation mechanisms. We demonstrate, using observations of an IP shock from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) missions, supported by test‐particle simulations, that gyrating protons generate the downstream magnetic oscillations. We found bursts of ion‐acoustic waves at the troughs and crests of the magnetic oscillations, suggesting their energy source is related to proton gyration. Comparing MMS and SolO observations, we conclude that the upstream flow speed to ion thermal speed ratio and magnetic compression ratio are key parameters controlling the ion kinetic behavior that produces downstream magnetic oscillations.

In the Earth’s magnetosheath, several processes contribute to energy dissipation and plasma heating, one of which is wave-particle interactions between whistler waves and electrons. The whistler-heat flux instability is known to scatter the strahl electrons in the solar wind. However, the heat flux properties and evolution across the Earth’s magnetosheath region have not yet been explored.We present Magnetospheric Multiscale (MMS) observations from 18 hours of burst-mode measurements using the unbiased magnetosheath campaign. We quantify the electron heat flux in the magnetosheath and examine the role of whistler instabilities in regulating it. Our results show that the heat flux follows the magnetosheath magnetic field as it drapes around the magnetosphere. We find that the heat flux is constrained by whistler instability thresholds and is aligned with the propagation of low-frequency whistler waves. We also present a case study investigating direct evidence for the whistler-heat flux instability in the magnetosheath.

The magnetosphere of Mercury is highly dynamic, a consequence of its small size, weak intrinsic magnetic field, and proximity to the Sun. One intriguing phenomenon is the presence of magnetic field fluctuations around 1 Hz. Here, we present a comprehensive statistical survey of these waves using the full span of the MESSENGER magnetometer measurements from 2011 to 2015. We find that ~1 Hz waves are observed during 10-20 % of the time that the spacecraft spent on closed field lines in Mercury’s magnetosphere, as determined from the KT17 magnetic field model. Wave occurrence is increased under magnetospheric conditions that favour an expanded closed field line region. We present the first global characterisation of the ~1 Hz waves at Mercury and demonstrate their dependence on both external drivers, such as upstream IMF conditions, and internal magnetospheric activity, such as the occurrence of identified dipolarization events. These results are discussed in the context of the BepiColombo mission, which will provide new opportunities to identify the nature of these waves and to assess their role in Mercury’s highly dynamic plasma environment.

Not-yet-known not-yet-known not-yet-known unknown We investigate an interplanetary shock at two different heliocentric longitudes using data from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) spacecraft. We focus on the kinetic ion behavior to understand the wave environment around the shock at these distinct locations. Solar Orbiter observed the flank of the shock, while MMS crossed closer to its nose. The shock exhibited similar properties at both locations and comparable whistler precursors are observed by the two spacecraft. Despite these similarities, the ion behavior varied significantly between locations. At MMS, we observed magnetic fluctuations associated with protons gyrating as they crossed the ramp, reflected protons and alphas, and more intense ion-acoustic waves than at SolO. Notably, the ion-acoustic activity displayed a periodicity that corresponded with the low-frequency magnetic fluctuations. These findings indicate that solar wind parameters significantly influence the characteristics of interplanetary shocks.