Keywords

The remnant magnetism in the crust of Martian southern highland is associated with the magnetic sources at an average depth of ∼32 km. In this work, we investigate the magnetization of Martian crust via 1-D parameterized model for the stagnant-lid mantle convection. According to our model, the magnetization of Martian crust is likely to take place in the top-down manner during 4.1–3.7 Ga. To reproduce the average depth of magnetic sources below the southern highland, magnetite and Mg-ferrite are anticipated to be the magnetic carriers in the Martian crust, implying the serpentinisation therein. If magnetite is the only magnetic carrier in the Martian crust, the early climate must be warm enough to maintain a surface temperature of 300 K during 4.1–3.7 Ga at least. Such a warm climate is more likely to be a regional phenomenon associated with the serpentinisation in the crust of the southern highland or the hot ejecta of Borialis impact depositing on the southern hemisphere.

Extrasolar satellites are generally too small to be detected by nominal searches. By analogy to the most active body in the solar system, Io, we describe how sodium (Na i) and potassium (K i) gas could be a signature of the geological activity venting from an otherwise hidden exo-Io. Analyzing ∼a dozen close-in gas giants hosting robust alkaline detections, we show that an Io-sized satellite can be stable against orbital decay below a planetary tidal ${{ \mathcal Q }}_{p}\lesssim {10}^{11}$. This tidal energy is also focused into the satellite driving an ∼10^5±2 higher mass-loss rate than Io's supply to Jupiter's Na exosphere based on simple atmospheric loss estimates. The remarkable consequence is that several exo-Io column densities are, on average, more than sufficient to provide the ∼10^10±1 Na cm⁻² required by the equivalent width of exoplanet transmission spectra. Furthermore, the benchmark observations of both Jupiter's extended (∼1000 R_J) Na exosphere and Jupiter's atmosphere in transmission spectroscopy yield similar Na column densities that are purely exogenic in nature. As a proof of concept, we fit the "high-altitude" Na at WASP-49b with an ionization-limited cloud similar to the observed Na profile about Io. Moving forward, we strongly encourage time-dependent ingress and egress monitoring along with spectroscopic searches for other volcanic volatiles.

By assuming that the solar wind flow is spherically symmetric and that the flow speed becomes constant beyond some critical distance r = R₀ (neglecting solar gravitation and acceleration by high coronal temperature), the large-scale solar wind magnetic field lines are distorted into a Parker spiral configuration, which is usually simplified to an Archimedes spiral. Using magnetic field observations near Mercury, Venus, and Earth during solar maximum of Solar Cycle 24, we statistically surveyed the Parker spiral angles and obtained the empirical equations of the Archimedes and Parker spirals by fitting the multiple-point results. We found that the solar wind magnetic field configurations are slightly different during different years. Archimedes and Parker spiral configurations are quite different from each other within 1 au. Our results provide empirical Archimedes and Parker spiral equations that depend on the solar wind velocity and the critical distance (R₀). It is inferred that R₀ is much larger than that previously assumed. In the near future, the statistical survey of the near-Sun solar wind velocity by Parker Solar Probe can help verify this result.

Jupiter's moon Europa is exposed to Galactic cosmic rays (GCRs), highly energetic particles from deep space that constantly bombard its surface. Here we have investigated the effect of GCRs on Europa's surface by carrying out particle physics simulations of secondary particle cascades that occur within surface ice due to incident GCR ions. We find that shielding by the Jovian magnetosphere prevents a significant fraction of the GCR spectrum from reaching Europa. Furthermore, we find that while GCRs are capable of affecting the surface down to depths of at least 10 m, the radiation dose due to these particles is small. Within the uppermost meter of Europa's surface, radiation due to magnetospheric particle bombardment dominates over the GCR dose except for surface locations at high latitudes, where the magnetospheric radiation flux is small.

Jupiter's internal flow structure is still not fully known, but can be now better constrained due to Juno's high-precision measurements. The recently published gravity and magnetic field measurements have led to new information regarding the planet and its internal flows, and future magnetic measurements will help to solve this puzzle. In this study, we propose a new method to better constrain Jupiter's internal flow field using the Juno gravity measurements combined with the expected measurements of magnetic secular variation. Based on a combination of hydrodynamical and magnetic field considerations we show that an optimized vertical profile of the zonal flows that fits both measurements can be obtained. Incorporating the magnetic field effects on the flow better constrains the flow decay profile. This will get us closer to answering the persistent question regarding the depth and nature of the flows on Jupiter.

On 2011 November 5, Venus Express observed the impact of an extremely strong interplanetary coronal mass ejection (ICME) on Venus. As a result, the Venusian induced magnetosphere dramatically fluctuated during the ICME passage: the bow shock was compressed and broadened by the sheath and the body of the ICME, respectively; an atypically strong magnetic barrier (over 250 nT) of Venus was detected; and the plasma sheet in the magnetotail flapped so rapidly that it was crossed by Venus Express 5 times within 1.5 minutes. The ionosphere was totally magnetized because of the very high magnetic pressure of the induced magnetosphere. However, the altitude of the ionopause did not decrease with respect to those in neighboring orbits, which is inconsistent with the ionopause descents reported by previous studies. We found that the ionosphere was greatly excited by the ICME as evidenced by the much higher heavy ion density. That is why the balance between the ionospheric thermal pressure and the strong magnetic pressure can be maintained at a relatively high altitude. We propose that a much stronger massloading effect resulting from the excited ionosphere is responsible for the anomalously high magnetic barrier because much more magnetic field lines were anchored. Our results also suggest that such ICMEs that can excite the ionosphere are substantially efficient in enhancing the atmospheric loss of Venus.

Kinetic-size magnetic holes (KSMHs) in the terrestrial magnetotail plasma sheet are statistically investigated using the observations from the Magnetospheric Multiscale mission. The scales of KSMHs are found to be smaller than one ion gyroradius or tens of electron gyroradii. The occurrence distributions of KSMHs have dawn–dusk asymmetry (duskside preference) in the magnetotail, which may be caused by the Hall effect. Most events of KSMHs (71.7%) are accompanied by a substorm, implying that substorms may provide favorable conditions for the excitation of KSMHs. However, there is a weak correlation between KSMHs and magnetic reconnection. The statistical results reveal that for most of the events, the electron total temperature and perpendicular temperature increase while the electron parallel temperature decreases inside the KSMHs. The electron temperature anisotropy (T_e⊥/${T}_{{\rm{e}}| | }\gt 1$) is observed in 72% of KSMHs. Whistler-mode waves are frequently observed inside the KSMHs, and most (92%) KSMHs associated with whistler waves have enhancements of electron perpendicular distributions and satisfy the unstable condition of whistler instability. This suggests that the observed electron-scale whistler waves, locally generated by the electron temperature anisotropy, could couple with the electron-scale KSMHs. The observed features of KSMHs and their coupling to electron-scale whistlers are similar to the ones in the turbulent magnetosheath, implying that they are ubiquitous in the space plasmas. The generation of KSMHs in the plasma sheet could be explained by an electron vortex magnetic hole, magnetosonic solitons, and/or ballooning/interchange instabilities.

An Earth-like planetary magnetic field has been widely invoked as a requirement for habitability as it purportedly mitigates the fluxes of ionizing radiation reaching the surface and the escape of neutrals and ions from the atmosphere. Recent paleomagnetic evidence indicates that the nucleation of Earth's inner core, followed perhaps by an increase in geomagnetic field strength, might have occurred close to the Edicarian period. Motivated by this putative discovery, we explore the ensuing ramifications from the growth or reversals of Earth's dynamo. By reviewing and synthesizing emerging quantitative models, it is proposed that neither the biological radiation dose rates nor the atmospheric escape rates would vary by more than a factor of ∼2 under these circumstances. Hence, we suggest that hypotheses seeking to explain the Cambrian radiation or mass extinctions via changes in Earth's magnetic field intensity are potentially unlikely. We also briefly discuss how variations in the planetary magnetic field may have impacted early Mars and could influence exoplanets orbiting M-dwarfs.

The flapping motion of the current sheet, with the period from several minutes to tens of minutes, is one common dynamic phenomenon in the planetary magnetotail. This Letter reports on one current sheet flapping event with the short semi-period of ∼6 s on 2017 July 17 in the Earth's magnetotail for the first time using the Magnetospheric Multiscale (MMS) mission. This short time period flapping event consists of five consecutive crossings of the current sheet. Based on a multipoint analysis of the MMS, it is found that the first four crossings propagated duskward and belong to kink-like flapping, and the fifth crossing belongs to steady flapping. The current sheet flapping was embedded in the diffusion region of magnetic reconnection, which was identified by the well-organized Hall electromagnetic field. The period of current sheet flapping was modulated by the reconnection electric field and perpendicular plasma flow, indicating that this flapping motion may be triggered by the periodical unsteady magnetic reconnection.

Following the arrival of two interplanetary coronal mass ejections on 2014 September 12, the Relativistic Electron–Proton Telescope instrument on board the twin Van Allen Probes observed a long-term dropout in the outer belt electron fluxes. The interplanetary shocks compressed the magnetopause, thereby enabling the loss of relativistic electrons in the outer radiation belt to the magnetosheath region via the magnetopause shadowing. Previous studies have invoked enhanced radial transport associated with ultra-low-frequency waves activity and/or scattering into the atmosphere by whistler mode chorus waves to explain electron losses deep within the magnetosphere (L < 5.5). We show that energetic electron pitch angle distributions (PADs) provide strong evidence for precipitation also via interaction with electromagnetic ion cyclotron (EMIC) waves. High-resolution magnetic field observations on Van Allen Probe B confirm the sporadic presence of EMIC waves during the most intense dropout phase on September 12. Observational results suggest that magnetopause shadowing and EMIC waves together were responsible for reconfiguring the relativistic electron PADs into peculiar butterfly PAD shapes a few hours after an interplanetary shock arrived at Earth.