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The magnetometer instrument MPO-MAG on-board the Mercury Planetary Orbiter (MPO) of the BepiColombo mission en-route to Mercury is introduced, with its instrument design, its calibration and scientific targets. The instrument is comprised of two tri-axial fluxgate magnetometers mounted on a 2.9 m boom and are 0.8 m apart. They monitor the magnetic field with up to 128 Hz in a ± 2048  nT range. The MPO will be injected into an initial 480 × 1500  km polar orbit (2.3 h orbital period). At Mercury, we will map the planetary magnetic field and determine the dynamo generated field and constrain the secular variation. In this paper, we also discuss the effect of the instrument calibration on the ability to improve the knowledge on the internal field. Furthermore, the study of induced magnetic fields and field-aligned currents will help to constrain the interior structure in concert with other geophysical instruments. The orbit is also well-suited to study dynamical phenomena at the Hermean magnetopause and magnetospheric cusps. Together with its sister instrument Mio-MGF on-board the second satellite of the BepiColombo mission, the magnetometers at Mercury will study the reaction of the highly dynamic magnetosphere to changes in the solar wind. In the extreme case, the solar wind might even collapse the entire dayside magnetosphere. During cruise, MPO-MAG will contribute to studies of solar wind turbulence and transient phenomena.

The magnetic gradient and curvature drift of energetic ions can form a longitudinal electric current around a planet known as the ring current, that has been observed in the intrinsic magnetospheres of Earth, Jupiter, and Saturn. However, there is still a lack of observational evidence of ring current in Mercury’s magnetosphere, which has a significantly weaker dipole magnetic field. Under such conditions, charged particles are thought to be efficiently lost through magnetopause shadowing and/or directly impact the planetary surface. Here, we present the observational evidence of Mercury’s ring current by analysing particle measurements from MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft. The ring current is bifurcated because of the dayside off-equatorial magnetic minima. Test-particle simulation with Mercury’s dynamic magnetospheric magnetic field model (KT17 model) validates this morphology. The ring current energy exceeds 5 × 10 10  J during active times, indicating that magnetic storms may also occur on Mercury. Ring currents have been observed in the magnetospheres of Earth, Jupiter, and Saturn. Here, the authors show observational evidence of Mercury’s ring current that is bifurcated because of the dayside off-equatorial magnetic minima.

Cassini’s encounter with Saturn’s magnetotail — the long magnetosphere region stretching into space — has revealed that plasma exits the magnetosphere through long-duration magnetic reconnection, which ejects ten times more mass than estimated. Magnetic reconnection is a fundamental process in solar system and astrophysical plasmas, through which stored magnetic energy associated with current sheets is converted into thermal, kinetic and wave energy 1 , 2 , 3 , 4 . Magnetic reconnection is also thought to be a key process involved in shedding internally produced plasma from the giant magnetospheres at Jupiter and Saturn through topological reconfiguration of the magnetic field 5 , 6 . The region where magnetic fields reconnect is known as the diffusion region and in this letter we report on the first encounter of the Cassini spacecraft with a diffusion region in Saturn’s magnetotail. The data also show evidence of magnetic reconnection over a period of 19 h revealing that reconnection can, in fact, act for prolonged intervals in a rapidly rotating magnetosphere. We show that reconnection can be a significant pathway for internal plasma loss at Saturn 6 . This counters the view of reconnection as a transient method of internal plasma loss at Saturn 5 , 7 . These results, although directly relating to the magnetosphere of Saturn, have applications in the understanding of other rapidly rotating magnetospheres, including that of Jupiter and other astrophysical bodies.

We survey the Cassini magnetometer data during the deep tail orbits in 2006, and find 34 direct encounters with plasmoids. They occur as single, isolated events but also in groups of two or more plasmoids as is frequently observed at Earth. We show a case study example of three such plasmoids over three hours, where we estimate an upper limit of 5.68 GWb of flux closure, and derive a reconnection rate over this interval of 526 kV. We show the results of a superposed epoch analysis of all 34 plasmoids indicating that, on average, plasmoids at Saturn are ∼8 min in duration and they tend toward a loop‐like, as opposed to flux rope‐like topology, with little or no axial core magnetic field. Our analysis shows that plasmoids at Saturn are followed by an extended interval of the post‐plasmoid plasma sheet (PPPS) lasting ∼58 min. The average open magnetic flux disconnected by the continued reconnection following plasmoid formation that creates the PPPS is ∼3 GWb. We calculate expected recurrence rates for plasmoids, and compare these with a derived observational recurrence rate of one plasmoid every ∼2.4 days, explaining the reasons why the spacecraft has not observed as many plasmoids as we predict will be released. We conclude that the Cassini magnetometer measurements require a combination of Vasyliunas‐type closed‐flux plasma sheet and Dungey‐type open‐flux lobe reconnection to account for the observed properties of the plasmoids and PPPS in Saturn's magnetotail. Key Points Derive magnetotail reconnection rates at Saturn Calculate flux transport in post‐plasmoid plasma sheet Estimate Saturn plasmoid recurrence rate

A statistical study of flux ropes and traveling compression regions (TCRs) during the Time History of Events and Macroscale Interactions during Substorms second tail season has been performed. A combined total of 135 flux ropes and TCRs in the range GSM X ∼ −14 to −31 RE were identified, many of these occurring in series of two or more events separated by a few tens of seconds. Those occurring within 10 min of each other were combined into aggregated reconnection events. For the purposes of this survey, these are most likely the products of reconnection occurring simultaneously at multiple, closely spaced x‐lines as opposed to statistically independent episodes of reconnection. The 135 flux ropes and TCRs were grouped into 87 reconnection events; of these, 28 were moving tailward and 59 were moving Earthward. The average location of the near‐Earth x‐line determined from statistical analysis of these reconnection events is (XGSM, Y*GSM) = (−30RE, 5RE), where Y* includes a correction for the solar aberration angle. A strong east‐west asymmetry is present in the tailward events, with >80% being observed at GSM Y* > 0. Our results indicate that the Earthward flows are similarly asymmetric in the midtail region, becoming more symmetric inside −18 RE. Superposed epoch analyses indicate that the occurrence of reconnection closer to the Earth, i.e., X > −20 RE, is associated with elevated solar wind velocity and enhanced negative interplanetary magnetic field BZ. Reconnection events taking place closer to the Earth are also far more effective in producing geomagnetic activity, judged by the AL index, than reconnection initiated beyond X ∼ −25 RE.

The magnetopause deflects the solar wind plasma and confines Earth’s magnetic field. We combine measurements made by the four spacecraft of the Magnetospheric Multiscale mission to demonstrate how the plasma and magnetic forces at the boundary affect the interaction between the shocked solar wind and Earth’s magnetosphere. We compare these forces with the plasma pressure and examine the electron distribution function. We find that the magnetopause has sublayers with thickness comparable to the ion scale. Small pockets of low magnetic field strength, small radius of curvature, and high electric current mark the electron diffusion region. The flow of electrons, parallel and antiparallel to the magnetic field, reveals a complex topology with the creation of magnetic ropes at the boundary.

Spacecraft observations have established that all known planets with an internal magnetic field, as part of their interaction with the solar wind, possess well-developed magnetic tails, stretching vast distances on the nightside of the planets. In this review paper we focus on the magnetotails of Mercury, Earth, Jupiter and Saturn, four planets which possess well-developed tails and which have been visited by several spacecraft over the years. The fundamental physical processes of reconnection, convection, and charged particle acceleration are common to the magnetic tails of Mercury, Earth, Jupiter and Saturn. The great differences in solar wind conditions, planetary rotation rates, internal plasma sources, ionospheric properties, and physical dimensions from Mercury’s small magnetosphere to the giant magnetospheres of Jupiter and Saturn provide an outstanding opportunity to extend our understanding of the influence of such factors on basic processes. In this review article, we study the four planetary environments of Mercury, Earth, Jupiter and Saturn, comparing their common features and contrasting their unique dynamics.

We present a multi‐instrument study of a hot flow anomaly (HFA) observed by the Venus Express spacecraft in the Venusian foreshock, on 22 March 2008, incorporating both Venus Express Magnetometer and Analyzer of Space Plasmas and Energetic Atoms (ASPERA) plasma observations. Centered on an interplanetary magnetic field discontinuity with inward convective motional electric fields on both sides, with a decreased core field strength, ion observations consistent with a flow deflection, and bounded by compressive heated edges, the properties of this event are consistent with those of HFAs observed at other planets within the solar system. Key Points We have made the first unambigious identification of a hot flow anomaly at Venus We have proven that HFAs can form even at unmagnetized planets HFAs have dramatic effect on the induced magnetosphere and ionosphere of Venus

We compare estimates for the ion fluxes of twelve expected constituents of the lunar exosphere with estimates for the ion fluxes ejected from the lunar surface by solar wind ions and electrons. Our estimates demonstrate that measurements of lunar ions will help constrain the abundances of many undetected species in the lunar exosphere, particularly Al and Si, because the expected ion flux levels from the exosphere exceed those from the surface. To correctly infer the relative abundances of exospheric ions and neutrals from Kaguya Ion Mass Analyzer (IMA) measurements, we must take into account the velocity distributions of local ions. The predicted spectrum underestimates the measured levels of O+ relative to other lunar ion species, a result that may suggest contributions by molecular ions to the measured O+ rates. Key Points Fluxes of ions from the lunar exosphere can exceed fluxes of ions from surface We could infer abundances of undetected lunar neutrals from ion measurements Different exo‐ions map into Kaguya Mass Analyzer with different efficiencies

Magnetic activity in the Northern Hemisphere auroral region was examined during solar cycles 22 and 23 (1993–2008). Substorms were identified from ground‐based magnetic field measurements by an automated search engine. On average, 550 substorms were observed per year, which gives in total about 9000 substorms. The interannual, seasonal and solar cycle‐to‐cycle variations of the substorm number (Rss), substorm duration (Tss), and peak amplitude (Ass) were examined. The declining phases of both solar cycles 22 and 23 were more active than the other solar cycle phases due to the enhanced solar wind speed. The spring substorms during the declining solar cycle phase (∣Ass,decl∣ = 500 nT) were 25% larger than the spring substorms during the ascending solar cycle years (∣Ass,acs∣ = 400 nT). The following seasonal variation was found: the most intense substorms occurred during spring and fall, the largest substorm frequency in the Northern Hemisphere winter, and the longest‐duration substorms in summer. Furthermore, we found a winter‐summer asymmetry in the substorm number and duration, which is speculated to be due to the variations in the ionospheric conductivity. The solar cycle‐to‐cycle variation was found in the yearly substorm number and peak amplitude. The decline from the peak substorm activity in 1994 and 2003 to the following minima took 3 years during solar cycle 22, while it took 6 years during solar cycle 23. Key Points We found interannual variations in substorm number, duration and peak amplitude The largest substorm activity is during declining solar cycle phase A winter‐summer asymmetry was found for substorm number and substorm duration