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Context. Magnetic holes are ubiquitous structures in the solar wind and in planetary magnetosheaths. They consist of a strong depression of the magnetic field strength, most likely in pressure balance through increased plasma pressure, which is convected with the plasma flow. These structures are created through a plasma temperature anisotropy, where the perpendicular temperature (with respect to the magnetic field) is greater than the parallel temperature. The occurrence rate of these magnetic holes between Earth and Mercury can give us information about how the solar wind conditions develop on their way from the Sun to the outer Solar System. They also give information about basic plasma processes such as diffusion of magnetic structures. Aims. In this study we investigate the occurrence, size, and depth of magnetic holes during the cruise phase of BepiColombo and compare them with earlier studies. Methods. The BepiColombo magnetometer data were used to find the magnetic holes. We determined the size in seconds, the depth with respect to the background field, and the rotation angle of the background field across the structure. Minimum variance analysis delivers the polarization state of the magnetic holes. A direct comparison is made to the results obtained from the MESSENGER cruise phase. Results. We find an almost constant occurrence rate for magnetic holes between Mercury and Earth. The size of the holes is determined by the plasma conditions at the location where they are created and they grow in size, due to diffusion, as they move outwards in the Solar System. The greater the rotation of the background magnetic field across the structure, the larger the minimum size of the magnetic hole is. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Mercury is host to a rich and highly dynamic plasma environment, resulting from the interaction of the solar wind with Mercury's intrinsic magnetic field and tenuous exosphere. Ion cyclotron waves generated by the pick‐up of freshly photoionized neutrals are a common physical phenomenon in the exosphere of weakly and/or unmagnetized bodies. Here we study the properties of cyclotron waves generated by proton pick‐up, the so‐called proton cyclotron waves (PCWs), at Mercury. We surveyed 4 years of MESSENGER magnetometer data and identified 5455 PCW events observed over a wide spatial range, where ∼73% of the observed PCWs are located in the solar wind and ∼27% in the magnetosheath. By associating the wave properties with their spatial distribution, we evaluate the characteristics and differences of the PCWs in the two regions. Plain Language Summary: Mercury possesses an internally generated magnetic field which constitutes an obstacle to the supersonic solar wind. Upstream of the planet a bow shock emerges where the supersonic solar wind slowed and then flows subsonically in the so‐called magnetosheath. Mercury is also surrounded by a tenuous exosphere, which contains a variety of species that find their origin in the solar wind, micrometeorites that impact the planet and from Mercury's surface itself. These neutral particles can get photoionized through the solar UV radiation, and will then be picked‐up by the solar wind. Because the freshly pick‐up particles have a different velocity than the solar wind, the solar wind plasma becomes unstable for the generation of plasma waves, in particular for the so‐called ion cyclotron waves. Here we report for the first time on ion cyclotron waves generated by the pick‐up of photoionized hydrogen at Mercury. They are detected upstream and downstream of the bow shock, in the solar wind and magnetosheath. From the results found in this study, we speculate that the pick‐up ion cyclotron waves in these two regions might originate from different excitation mechanisms. Key Points: First report of ion cyclotron waves generated by the pick‐up of protons in the space environment of MercuryIn the solar wind the right‐hand beam resonant instability is dominatingIn the magnetosheath the left‐hand ring‐beam distribution instability becomes important [ABSTRACT FROM AUTHOR]

The magnetometer instrument on the Solar Orbiter mission is designed to measure the magnetic field local to the spacecraft continuously for the entire mission duration. The need to characterise not only the background magnetic field but also its variations on scales from far above to well below the proton gyroscale result in challenging requirements on stability, precision, and noise, as well as magnetic and operational limitations on both the spacecraft and other instruments. The challenging vibration and thermal environment has led to significant development of the mechanical sensor design. The overall instrument design, performance, data products, and operational strategy are described. [ABSTRACT FROM AUTHOR]

Magnetic reconnection in the magnetotail accelerates plasma in short‐duration bursts of fast flows, referred to as bursty bulk flows toward the Earth. These bursty bulk flows are typically accompanied by a sharp increase in the northward magnetic field component, the so‐called dipolarization front (DF). This rapid increase in northward magnetic field is often preceded by a decrease, the so called DF‐dip, where the northward magnetic field component sometimes even changes sign and turns southward. Here we present a statistical study of the DF‐dip of 43 events, using observations from the Magnetospheric Multiscale (MMS) mission during summer 2017, when MMS's apogee was located in the magnetotail around 25 RE. The 43 events are subdivided into two categories according to their DF‐dip: 20 events where the DF‐dip stays northward (positive DF‐dip category) and 23 events where the DF‐dip turns fully southward (negative DF‐dip category). We find that (1) the magnetic field depression ahead of the DF of the positive DF‐dip events correlates mainly with perpendicular currents and diamagnetic currents flowing ahead of the DF and (2) the magnetic field depression ahead of the DF of the negative DF‐dip events correlates mainly with field‐aligned currents and that this type of events might be (a) earthward moving flux rope‐like structures caused by multiple X‐line reconnection and/or (b) are a result of localized, single X‐line reconnection under a guide field. Plain Language Summary: When magnetic field lines in the Earth's magnetotail reconnect, they shoot earthward like a stretched elastic band. The plasma, connected to the magnetic field, is thereby accelerated. This usually happens repeatedly on short timescales and is called a bursty bulk flow. The reconnected field lines have a more vertical direction, instead of the original horizontal orientation. When the bursty bulk flow crosses over the spacecraft, we see a rapid increase in the vertical (northward) magnetic field component, referred to as dipolarization front, which is usually preceded by a small decrease, the so‐called dipolarization front‐dip. This decrease comes in two categories, one where vertical magnetic field component remains positive (northward) and one where the vertical magnetic field component changes its sign and becomes negative (southward). It is shown that these are two separate categories with different origins. Key Points: Size of the magnetic field depression ahead of DF correlates with field‐aligned currents if the depression is negative (negative DF‐dip)Size of the magnetic field depression ahead of DF correlates with perpendicular currents if the depression stays positive (positive DF‐dip)Positive DF‐dip events are found closer to flow braking region than negative DF‐dip events [ABSTRACT FROM AUTHOR]

Three-dimensional electron phase-space holes are shown to have positive charges on the plasma background, which produce a radial electric field and force the trapped electron component into an azimuthal drift. In this way electron holes generate magnetic fields in the hole. We solve the cylindrical hole model exactly for the hole charge, electric potential and magnetic field. In electron holes, the magnetic field is amplified on the flux tube of the hole; equivalently, in ion holes the field would be decreased. The flux tube adjacent to the electron hole is magnetically depleted by the external hole dipole field. This causes magnetic filamentation. It is also shown that holes are massive objects, each carrying a finite magnetic moment. Binary magnetic dipole interaction of these moments will cause alignment of the holes into chains along the magnetic field or, in the three-dimensional case, produce a magnetic fabric in the volume of hole formation. Since holes, in addition to being carriers of charges and magnetic moments, also have finite masses, they behave like quasi-particles, performing E × B, magnetic field, and diamagnetic drifts. In an inhomogeneous magnetic field, their magnetic moments experience torque, which causes nutation of the hole around the direction of the magnetic field, presumably giving rise to low frequency magnetic modulations like pulsations. A gas of many such holes may allow for a kinetic description, in which holes undergo binary dipole interactions. This resembles the polymeric behaviour. Both magnetic field generation and magnetic structure formation are of interest in auroral, solar coronal and shock physics, in particular in the problem of magnetic field filamentation in relativistic foreshocks and cosmic ray acceleration. [ABSTRACT FROM AUTHOR]

One of the goals of the NASA Solar TErestrial RElations Observatory (STEREO) mission is to study the feasibility of forecasting the direction, arrival time, and internal structure of solar coronal mass ejections (CMEs) from a vantage point outside the Sun-Earth line. Through a case study, we discuss the arrival time calculation of interplanetary CMEs (ICMEs) in the ecliptic plane using data from STEREO/SECCHI at large elongations from the Sun in combination with different geometric assumptions about the ICME front shape [fixed-Φ (FP): a point and harmonic mean (HM): a circle]. These forecasting techniques use single-spacecraft imaging data and are based on the assumption of constant velocity and direction. We show that for the slow (350 km s-1) ICME on 2009 February 13-18, observed at quadrature by the two STEREO spacecraft, the results for the arrival time given by the HM approximation are more accurate by 12 hr than those for FP in comparison to in situ observations of solar wind plasma and magnetic field parameters by STEREO/IMPACT/PLASTIC, and by 6 hr for the arrival time at Venus Express (MAG). We propose that the improvement is directly related to the ICME front shape being more accurately described by HM for an ICME with a low inclination of its symmetry axis to the ecliptic. In this case, the ICME has to be tracked to >30° elongation to obtain arrival time errors <± 5 hr. A newly derived formula for calculating arrival times with the HM method is also useful for a triangulation technique assuming the same geometry. [ABSTRACT FROM AUTHOR]

Foreshock ions are compared between Venus and Mars at energies of 0.6∼20 keV using the same ion instrument, the Ion Mass Analyser, on board both Venus Express and Mars Express. Venus Express often observes accelerated protons (2∼6 times the solar wind energy) that travel away from the Venus bow shock when the spacecraft location is magnetically connected to the bow shock. The observed ions have a large field-aligned velocity compared to the perpendicular velocity in the solar wind frame, and are similar to the field-aligned beams and intermediate gyrating component of the foreshock ions in the terrestrial upstream region. Mars Express does not observe similar foreshock ions as does Venus Express, indicating that the Martian foreshock does not possess the intermediate gyrating component in the upstream region on the dayside of the planet. Instead, two types of gyrating protons in the solar wind frame are observed very close to the Martian quasi-perpendicular bow shock within a proton gyroradius distance. The first type is observed only within the region which is about 400 km from the bow shock and flows tailward nearly along the bow shock with a similar velocity as the solar wind. The second type is observed up to about 700 km from the bow shock and has a bundled structure in the energy domain. A traversal on 12 July 2005, in which the energy-bunching came from bundling in the magnetic field direction, is further examined. The observed velocities of the latter population are consistent with multiple specular reflections of the solar wind at the bow shock, and the ions after the second reflection have a field-aligned velocity larger than that of the de Hoffman-Teller velocity frame, i.e., their guiding center has moved toward interplanetary space out from the bow shock. To account for the observed peculiarity of the Martian upstream region, finite gyroradius effects of the solar wind protons compared to the radius of the bow shock curvature and effects of cold ion abundance in the bow shock are discussed. [ABSTRACT FROM AUTHOR]

Abstract: The scant data from the historic Mariner-10 flybys indicate that the Hermean and terrestrial magnetospheres have many similarities. However, Mercury’s differences, e.g., the absence of an ionosphere, high sodium content, small dimension, and the stronger forcing by the solar wind, make it likely that processes of lesser importance at Earth play a more major role at Mercury. In addition, the proximity of Mercury in time and space to the onset of solar events, places the magnetosphere in a heretofore unexplored environment. [Copyright &y& Elsevier]

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere2,3. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum. [ABSTRACT FROM AUTHOR]

We examine the range of applicability of Taylor's hypothesis used in observations of magnetic turbulence in the solar wind. We do not refer to turbulence theory. We simply ask whether in a turbulent magnetohydrodynamic flow the observed magnetic frequency spectrum can be interpreted as mapping of the wavenumber turbulence into the stationary spacecraft frame. In addition to the known restrictions on the angle of propagation with respect to the fluctuation spectrum and the question on the wavenumber dependence of the frequency in turbulence which we briefly review, we show that another restriction concerns the inclusion or exclusion of turbulent fluctuations in the velocity field. Taylor's hypothesis in application to magnetic turbulence encounters its strongest barriers here. It is applicable to magnetic turbulence only when the turbulent velocity fluctuations can practically be completely neglected against the bulk flow speed. For low flow speeds the transformation becomes rather involved. This account makes even no use of the additional scale dependence of the turbulent frequency, viz. the existence of a "turbulent dispersion relation". [ABSTRACT FROM AUTHOR]

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 sub layers 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. [ABSTRACT FROM AUTHOR]

Abstract: Mercury possesses a weak, internal, global magnetic field that supports a small magnetosphere populated by charged particles originating from the solar wind, the planet''s exosphere and surface layers. Mercury''s exosphere is continuously refilled and eroded through a variety of chemical and physical processes acting in the planet''s surface and environment. Using simultaneous two-point measurements from two satellites, ESA''s future mission BepiColombo will offer an unprecedented opportunity to investigate magnetospheric and exospheric dynamics at Mercury as well as their interactions with solar radiation and interplanetary dust. The expected data will provide important insights into the evolution of a planet in close proximity of a star. Many payload instruments aboard the two spacecraft making up the mission will be completely, or partially, devoted to studying the close environment of the planet as well as the complex processes that govern it. Coordinated measurements by different onboard instruments will permit a wider range of scientific questions to be addressed than those that could be achieved by the individual instruments acting alone. Thus, an important feature of the BepiColombo mission is that simultaneous two-point measurements can be implemented at a location in space other than the Earth. These joint observations are of key importance because many phenomena in Mercury''s environment are temporarily and spatially varying. In the present paper, we focus on some of the exciting scientific goals achievable during the BepiColombo mission through making coordinated observations. [Copyright &y& Elsevier]

Abstract: We examined two consecutive plasma sheet oscillation and dipolarization events observed by Cluster in the magnetotail, which are associated with a pseudo-breakup and a small substorm monitored by the IMAGE spacecraft. Energy input from the solar wind and an associated enhancement of the cross-tail current lead to current sheet thinning and plasma sheet oscillations of 3–5min periods, while the pseudo-breakups occur during the loading phase within a spatially limited area, accompanied by a localized dipolarization observed by DSP TC1 or GOES 12. That is, the so-called “growth phase” is a preferable condition for both pseudo-breakup and plasma sheet oscillations in the near-Earth magnetotail. One of the plasma sheet oscillation events occurs before the pseudo-breakup, whereas the other takes place after pseudo-breakup. Thus there is no causal relationship between the plasma sheet oscillation events and pseudo-breakup. As for the contribution to the subsequent small substorm, the onset of the small substorm took place where the preceding plasma sheet oscillations can reach the region. [Copyright &y& Elsevier]

Abstract: The study of the neutral sheet is of fundamental importance in understanding the dynamics of the Earth’s magnetosphere. From the earliest observation of the magnetotail, it has been found that the neutral sheet frequently appears to be in motion due to changing solar wind conditions and geomagnetic activity. Multiple crossings of the neutral sheet by spacecraft have been attributed to a flapping motion of the neutral sheet in the north–south direction, a wavy profile either along the magnetotail or the dawn–dusk direction. Cluster observations have revealed that the flapping motions of the Earth’s magnetotail are of internal origin and that kink-like waves are emitted from the central part of the tail and propagate toward the tail flanks. This flapping motion is shown here to propagate at an angle of ∼45° with x GSM. A possible assumption that the flapping could be created by a wake travelling away from a fast flow in the current sheet is rejected. Other waves in the magnetotail are found in the ULF range. One conjunction event between Cluster and DoubleStar TC1 is presented where all spacecraft show ULF wave activity at a period of approximately 5min during fast Earthward flow. These waves are shown to be Kelvin–Helmholtz waves on the boundaries of the flow channel. Calculations show that the conversion of flow energy into magnetic energy through the Kelvin–Helmholtz instability can contribute to a significant part of flow breaking between Cluster and DoubleStar TC1. [Copyright &y& Elsevier]

Abstract: The ASPERA-4 instrument on board the Venus Express spacecraft offers for the first time the possibility to directly measure the emission of energetic neutral atoms (ENAs) in the vicinity of Venus. When the spacecraft is inside the Venus shadow a distinct signal of hydrogen ENAs usually is detected. It is observed as a narrow tailward stream, coming from the dayside exosphere around the Sun direction. The intensity of the signal reaches several , which is consistent with present theories of the plasma and neutral particle distributions around Venus. [Copyright &y& Elsevier]

Abstract: Plasma and magnetic field measurements made onboard the Venus Express on June 1, 2006, are analyzed and compared with predictions of a global model. It is shown that in the orbit studied, the plasma and magnetic field observations obtained near the North Pole under solar minimum conditions were qualitatively and, in many cases also, quantitatively in agreement with the general picture obtained using a global numerical quasi-neutral hybrid model of the solar wind interaction (HYB-Venus). In instances where the orbit of Venus Express crossed a boundary referred to as the magnetic pileup boundary (MPB), field line tracing supports the suggestion that the MPB separates the region that is magnetically connected to the fluctuating magnetosheath field from a region that is magnetically connected to the induced magnetotail lobes. [Copyright &y& Elsevier]

The effect of the solar wind dynamic pressure on the bow shock of Venus is examined. When the shock crossing distances are normalized to fixed solar wind Mach number, interplanetary magnetic field orientations, and EUV flux value, the scatter of the terminator plane crossings is considerably reduced. The normalized bow shock crossing data show that the size of the bow shock is insensitive to changes in the solar wind dynamic pressure. [Copyright &y& Elsevier]