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Funding Information: C. G. is supported by an ESA Research Fellowship. H. G. acknowledges support by the Swedish National Space Agency grant 108/18. B. S.-C. acknowledges support through UK-STFC grant ST/S000429/1. French co-authors acknowledge the support of CNES for the Rosetta and Comet Interceptor missions. I. M. acknowledges support through grants of Research Council of Norway (262941 and 275503). Publisher Copyright: © 2021, The Author(s), under exclusive licence to Springer Nature B.V. Comets hold the key to the understanding of our Solar System, its formation and its evolution, and to the fundamental plasma processes at work both in it and beyond it. A comet nucleus emits gas as it is heated by the sunlight. The gas forms the coma, where it is ionised, becomes a plasma, and eventually interacts with the solar wind. Besides these neutral and ionised gases, the coma also contains dust grains, released from the comet nucleus. As a cometary atmosphere develops when the comet travels through the Solar System, large-scale structures, such as the plasma boundaries, develop and disappear, while at planets such large-scale structures are only accessible in their fully grown, quasi-steady state. In situ measurements at comets enable us to learn both how such large-scale structures are formed or reformed and how small-scale processes in the plasma affect the formation and properties of these large scale structures. Furthermore, a comet goes through a wide range of parameter regimes during its life cycle, where either collisional processes, involving neutrals and charged particles, or collisionless processes are at play, and might even compete in complicated transitional regimes. Thus a comet presents a unique opportunity to study this parameter space, from an asteroid-like to a Mars- and Venus-like interaction. The Rosetta mission and previous fast flybys of comets have together made many new discoveries, but the most important breakthroughs in the understanding of cometary plasmas are yet to come. The Comet Interceptor mission will provide a sample of multi-point measurements at a comet, setting the stage for a multi-spacecraft mission to accompany a comet on its journey through the Solar System. This White Paper, submitted in response to the European Space Agency’s Voyage 2050 call, reviews the present-day knowledge of cometary plasmas, discusses the many questions that remain unanswered, and outlines a multi-spacecraft European Space Agency mission to accompany a comet that will answer these questionsby combining both multi-spacecraft observations and a rendezvous mission, and at the same time advance our understanding of fundamental plasma physics and its role in planetary systems.

The Mars Rover Magnetometers (RoMAG) will implement the first mobile magnetic field measurements on the surface of Mars. Two identical tri-axial fluxgate magnetometer sensors are mounted at the top and bottom of the mast of rover, respectively. The technology of Helmholtz compensation coil probe and digital closed-loop feedback circuit is utilized to realize high precision measurement. Each magnetometer measures the vector magnetic field with a resolution of 0.01 nT in the range of $\pm65000~\text{nT}$ . The sample rate is up to 32 Hz and the noise is $0.01~\text{nT}/\surd\text{Hz}$ @1 Hz. A rover magnetic compensation procedure was conducted to remove the most important magnetic disturbances. Mobile magnetic field measurements on the Martian surface would obtain fine-scale crust field and provide information about its remnant magnetization and any possible intrinsic magnetic field. It would help us to further understand Martian internal structure, coupling processes of solar wind-magnetosphere-ionosphere.

Due to the sparsity of space probes, it is still not clear on how the magnetic structure of the magnetotail looks like and how it evolves when the Interplanetary Magnetic Field (IMF) directs northw...

International audience; It has been long theorized, but not directly observed, that low-frequency magnetosonic plasma waves can steepen and form shocks. We show an example of small-amplitude, sinusoidal magnetosonic waves at the proton gyrofrequency upstream of the Martian bow shock. We hypothesize that these waves are produced by an ion beam instability associated with the ionization of hydrogen atoms by charge exchange with solar wind protons, solar photoionization, and/or electron impact ionization. As the waves are convected toward the planet by solar wind flow, the wave amplitude grows due to additional free energy put into the system by further ion beam particles. Finally, the steepened waves form shocks. Because of their development, the shocks are periodic with the separation at the proton gyroperiod. These observations lead to the conclusion that newborn ions may play a crucial role in the formation process of some collisionless plasma shocks in astrophysical and space plasmas.

It is found that the coupling between the solar wind and polar geomagnetic activity cause the annual and semiannual variation of auroral electrojet. The wavelet spectrum of the SuperMAG index, SML and SMU shows that both of the semiannual and annual variations are permanent features, and spectral power of the annual period of SML and SMU are stronger than that of the semiannual period. Due to coupling of the annual variation and the semiannual variation of SML, the diurnal variation of SML in summer months is not well correlated with results anticipated from the previous principle hypotheses [McIntosh, 1959; Russell and McPherron, 1973; Svalgaard, 1977; Cliver et al., 2000; Newell et al., 2013]. The annual/semiannual variation of SML and SMU is typically coupled together and is dependent on the local time and latitudinal variations of ionospheric conductivity and the electric field. A new index, the annual-corrected SMO index (SMO*), was suggested to indicate the semiannual variation of the westward auroral electrojet (SML). The seasonal-UT distribution of SMO* has good correlation with that predicted by the equinoctial hypothesis [McIntosh, 1959; Svalgaard, 2011].

Spring-Autumnal asymmetry (SA asymmetry) from years 1873–2010 in the geomagnetic activity is investigated based on the geomagnetic indices (aa, Ap and Kp). Results have shown that in general the SA asymmetry of the geomagnetic activity appears to be alternation, except for the interval during which the change of dominant solar dipole magnetic field polarity occurs. Our study implies that both the polarity of the main solar dipole field and the north-south asymmetry of the sunspot area in the two solar hemispheres control the strength of the south component (Bs) of the interplanetary magnetic field (IMF) near the Earth and the resulting SA asymmetry of the geomagnetic activity in the corresponding seasons. A simple sector structure model is devised to establish the relationship between the polarity of solar main dipole magnetic field, the IMF Bs component and the corresponding SA asymmetry in the geomagnetic activity. This is confirmed by examining the solar wind parameters from the spacecraft observations during the recent decades.

International audience; The location of ultra-low frequency (ULF) quasi-monochromatic wave onset upstream of Venus bow shock is explored using Venus Express magnetic field data. We report the existence of a spatial foreshock boundary behind which ULF waves are present. We have found that the ULF wave boundary at Venus is sensitive to the interplanetary magnetic field (IMF) direction like the terrestrial one and appears well defined for a cone angle larger than 30o. In the Venusian foreshock, the inclination angle of the wave boundary with respect to the Sun-Venus direction increases with the IMF cone angle. We also found that for the IMF nominal direction (θBX = 36°) at Venus’ orbit, the value of this inclination angle is 70o. Moreover, we have found that the inferred velocity of an ion traveling along the ULF boundary is in a qualitative agreement with a quasi-adiabatic reflection of a portion of the solar wind at the bow shock. For an IMF nominal direction at Venus, the inferred bulk speed of ions traveling along this boundary is 1.07 VSW, sufficiently enough to overcome the solar wind convection. This strongly suggests that the backstreaming ions upstream of the Venusian bow shock provide the main energy source for the ULF waves.

We investigate the occurrence rate of dipolarization fronts (DFs) in the plasma sheet by taking full advantage of all four Cluster satellites (C1–4) from years 2001 to 2009. In total, we select 466 joint-observation DF events, in which 318, 282, 254, and 236 DFs are observed by C1, C2, C3, and C4, respectively. Our findings are as follows: (1) the maximum occurrence rate is ∼ 15.3 events per day at X ∼ 15 RE in the XY plane, and the average occurrence rate is ∼ 5.4 events per day over the whole observation period; (2) the occurrence rate on the dusk side of the plasma sheet is larger and decreases with increasing BXY∕BLobe; (3) the occurrence rate within |Y| RE increases gradually from X ≈ −19 to −15 RE and then decreases from X ≈ −15 to −10 RE; (4) the occurrence rate when AE > 200 nT is much larger than that when AE X ≈ −19 to −15 RE. We suggest that both geomagnetic activity and multiple DFs contribute to the high occurrence rate of the DFs. In addition, the finite length of the DF in the dawn–dusk direction can affect the chance that a satellite observes the DF.

Energetic ion distributions in the near-Earth plasma sheet can provide important information for understanding the entry of ions into the magnetosphere and their transportation, acceleration, and losses in the near-Earth region. In this study, 11 years of energetic proton and oxygen observations (> similar to 274 keV) from Cluster/Research with Adaptive Particle Imaging Detectors were used to statistically study the energetic ion distributions in the near-Earth region. The dawn-dusk asymmetries of the distributions in three different regions (dayside magnetosphere, near-Earth nightside plasma sheet, and tail plasma sheet) are examined in Northern and Southern Hemispheres. The results show that the energetic ion distributions are influenced by the dawn-dusk interplanetary magnetic field (IMF) direction. The enhancement of ion intensity largely correlates with the location of the magnetic reconnection at the magnetopause. The results imply that substorm-related acceleration processes in the magnetotail are not the only source of energetic ions in the dayside and the near-Earth magnetosphere. Energetic ions delivered through reconnection at the magnetopause significantly affect the energetic ion population in the magnetosphere. We also believe that the influence of the dawn-dusk IMF direction should not be neglected in models of the particle population in the magnetosphere.

Bursty Bulk Flows (BBFs) have been correlated with Pi2 pulsations and damping oscillations of plasma velocity in many investigations. But the oscillation time scales in BBFs is still an open question. The purpose of this study is to statistically study the oscillated frequency distribution of magnetic field and plasma parameters inside BBFs. The data are obtained by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes during the period of 2008 to 2011. For 424 selected BBF events, we use the wavelet spectrum analysis to select the main, second and third period components of the magnetic field and plasma parameters according to their largest, second and third-largest values of the wavelet power spectral density, respectively. The power spectra show repeated information in the form of multiple peaks or oscillations. The quasi-Gaussian distribution is a good model for the occurrences of the main and secondary periods. The most probable main periods of the magnetic field and plasma parameters are between 143 s and 160 s, which located in the frequency band of Pi2 and Pi3 pulsations. The second and third period ranges from 63 s to 70 s and from 34 s to 37 s, respectively. Main periods of these parameters change little with the radial distance. We conclude that periods of these parameters are formed at the beginning of BBF history. Although the distribution model cannot give the dynamic processes, it identifies the intrinsic frequencies in oscillations of magnetic field and ion velocity inside of BBFs.

The “Nose-like” ion spectral structures are characterized by the deep inward penetration of ions from the near-Earth plasma sheet and can extend to low L shells in the inner magnetosphere. The stationary “Nose-like” ion structures are believed to be driven by the stationary electric and magnetic field during quiet geomagnetic times. However, the global distribution picture, especially for each MLT, for both single and double stationary nose structures is still not reached. In this study, we statistically investigate the stationary “Nose-like” ion spectral structure based on 2 years of Van Allen probes observations. We find a clear MLT dependence of the number of stationary noses, with a higher occurrence of single-nose structures from 13 to 03 MLT, while a higher occurrence of double-nose structures from 03 to 13MLT. This is confirmed by calculating the backward drift time using backward tracing method based on the dipole magnetic field and Weimer 96 electric field model for each MLT. Simulation results indicate that the ion spectral gap, which is formed due to long drift time or existence of the ion forbidden region, controls the nose structure at different MLT during geomagnetic quiet times. A possible physical explanation of this MLT dependence can be a combination of the ion spectral gaps created by the region of long drift time in the dayside within the nose energy range and the persistent dawn-dusk asymmetries in the convection electric field.

International audience; The statistical properties of ULF waves observed upstream of Venus foreshock are investigated. The study is restricted to waves which are observed well below the local proton cyclotron frequency. Using the magnetic field observations from Venus Express between May 2006 and February 2012, 115 quasi-monochromatic ULF wave trains have been identified. Statistical results show that the wave periods are mainly from 20 to 30 s in the spacecraft frame, which is about 2-3 times of the local proton cyclotron period. The transverse power dominates the power spectrum, and most of the waves display nearly circular or slightly elliptical polarization in the spacecraft frame. Moreover, these ULF waves mainly have small relative amplitudes with respect to the ambient field magnitude B0 for parallel component (δB||/B0 less than 0.3), while the range of relative amplitudes for perpendicular component δB⊥/B0 is from ~0.1 to ~1.0. Wave propagation angles are mainly less than 30° with respect to the mean magnetic field direction. The obtained results are very similar to the wave properties seen for ULF waves present in the terrestrial foreshock, which suggests that backstreaming ions in the Venusian foreshock form an important energy source for the generation of the waves.

The solar wind-magnetosphere interaction is still an open problem under the influences of the Bx component of the interplanetary magnetic field (IMF). We have performed a series of 3D global magnetohydrodynamic simulations and demonstrated the quantitative effects of the IMF Bx component on the locations and shapes of the bow shock and magnetopause during northward IMF. The IMF Bx can greatly change the configurations of the bow shock and the magnetosphere, and bring north-south asymmetries in the noon-midnight meridian plane. The larger magnetosonic Mach number in the northern hemisphere makes the northern part of the bow shock closer to the Earth, while the situation is opposite in the south. In the downstream of the southern bow shock, the magnetic fields get greatly compressed in the southern magnetosheath, leading to a drastic increased magnetic pressure above the magnetopause. As a result, the southern part of the magnetopause becomes closer to the earth while its northern part has a little change. Moreover, it is found that the subsolar points of the bow shock and magnetopause are not sensitive to the IMF Bx during northward IMF.

The polar outflows, as an important plasma source of the Earth's magnetosphere, usually exhibit significant north- south asymmetries, which can strongly affect the plasma distributions in the magnetotail lobe and perhaps contribute to the substorm triggering. But the mechanism of the asymmetric transport of these outflows is still unclear. In this Letter, 3D global magnetohydrodynamic (MHD) simulations are performed to investigate the development of the polar outflows after their escapes from the inner boundary under influences of the interplanetary magnetic field (IMF) B-x. It is found that the velocity of northern polar outflows is much stronger than the south. We suggest that the IMF B-x causes the north-south asymmetries in the magnetospheric configuration, and subsequently, great differences of the force and mass distributions appear between the two hemispheres, which lead to the significant north-south asymmetries in the transport of the polar outflows. We also discuss the differences in the acceleration mechanisms of the polar outflows between the northward and southward IMF cases.

We have used the AMR-CESE-MHD model to investigate the influences of the IMF Bz and the upstream solar wind dynamic pressure (Dp) on Earth’s magnetopause and bow shock. Our results present that the earthward displacement of the magnetopause increases with the intensity of the IMF Bz. The increase of the northward IMF Bz also brings the magnetopause closer to the Earth even though with a small distance. Our simulation results show that the subsolar bow shock during the southward IMF is much closer to the Earth than during the northward IMF. As the intensity of IMF Bz increases (also the total field strength), the subsolar bow shock moves sunward as the solar wind magnetosonic Mach number decreases. The sunward movement of the subsolar bow shock during southward IMF are much smaller than that during northward IMF, which indicates that the decrease of solar wind magnetosonic Mach number hardly changes the subsolar bow shock location during southward IMF. Our simulations also show that the effects of upstream solar wind dynamic pressure (Dp) changes on both the subsolar magnetopause and bow shock locations are much more significant than those due to the IMF changes, which is consistent with previous studies. However, in our simulations the earthward displacement of the subsolar magnetopause during high solar wind Dp is greater than that predicted by the empirical models.

Using an extended MHD model including the Hall effect and finite Larmor radius effect, we reproduce multiple dipolarization fronts (DFs) associated with the interchange instability in the braking region of bursty bulk flow in the plasma sheet. Our simulations reveal that the multiple DFs produced by the interchange instability are “growing” type DFs because the maximum plasma flow speeds are behind the fronts. Both the earthward and tailward moving DFs can be produced by interchange instability in the near-Earth region. The Hall electric field is the dominant electric field component in the dip region and the DF layer. The convective and the electron pressure gradient electric field components are smaller. The sharp Bz changes in both the dip region and DF layer correspond to the oppositely directed currents that are primarily associated with electrons. The ion diamagnetic current due to the strong ion pressure gradient causes an intense downward current in the dip region, which can produce the dip ahead ...

The five identical THEMIS Spacecraft, launched in February 2007, carry two magnetometers on each probe, one DC fluxgate (FGM) and one AC search coil (SCM). Due to the small size of the THEMIS probes, and the short length of the magnetometer booms, magnetic cleanliness was a particularly complex task for this medium sized mission. The requirements leveled on the spacecraft and instrument design required a detailed approach, but one that did not hamper the development of the probes during their short design, production and testing phase. In this paper we describe the magnetic cleanliness program’s requirements, design guidelines, program implementation, mission integration and test philosophy and present test results, and mission on-orbit performance.