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Magnetosheath jets travel from the bow shock toward the magnetopause, and some of them eventually impact it. Jet impacts have recently been linked to triggering magnetopause reconnection in case studies by Hietala et al. (2018, https://doi.org/10.1002/2017gl076525) and Nykyri et al. (2019, https://doi.org/10.1029/2018ja026357). In this study, we focus on the enhancing or suppressing effect jets could have on reconnection by locally altering the magnetic shear via their own magnetic fields. Using observations from the years 2008–2011 made by the Time History of Events and Macroscale Interactions during Substorms spacecraft and solar wind OMNI data, we statistically study for the first time BZ within jets in the Geocentric Solar Magnetospheric coordinates. We find that BZ opposite to the prevailing interplanetary magnetic field (IMF) BZ is roughly as common in jets as in the non‐jet magnetosheath near the magnetopause, but these observations are distributed differently. 60–70% of jet intervals contain bursts of opposite polarity BZ in comparison to around 40% of similar non‐jet intervals. The median duration of such a burst in jets is 10 s and strength is ±10nT. We also investigate the prevalence of the type of strong BZ≤−24nT pulses that Nykyri et al. (2019, https://doi.org/10.1029/2018ja026357) linked to a substorm onset. In our data set, such pulses were observed in around 13% of jets. Our statistical results indicate that jets may have the potential to affect local magnetopause reconnection via their magnetic fields. Future studies are needed to determine whether such effects can be observed. Plain Language Summary: Fast earthward flows called jets are often observed within the magnetosheath. They form at the Earth's bow shock, where the solar wind is slowed down before diverting around the magnetosphere. These jets may hit the boundary of the magnetosphere, the magnetopause, with high dynamic pressure. Such impacts have been observed to trigger magnetic reconnection, in which the interplanetary magnetic field (IMF) of the solar wind connects with the Earth's magnetic field and solar wind can enter the magnetosphere. Magnetopause reconnection usually occurs when the IMF points southward, opposite to the Earth's northward field. However, jets have been proposed to trigger reconnection also during northward IMF. We study the magnetic field within jets using data from the Time History of Events and Macroscale Interactions during Substorms spacecraft and solar wind OMNI data from 2008–2011. We find that it is statistically more likely for a magnetosheath interval, associated with jet conditions, to exhibit short pulses of magnetic field directed opposite to the upstream IMF, than for a non‐jet interval of the same duration. Therefore, jets may have the potential to affect local reconnection at the magnetopause. Our results motivate future studies that investigate these possible jet‐related effects on reconnection. Key Points: A statistical study of BZ in jets is conducted using Time History of Events and Macroscale Interactions during Substorms 2008–2011 data from the subsolar magnetosheath60‐70% of jet intervals contain a pulse of BZ opposite to the interplanetary magnetic field BZ while majority of non‐jet intervals do notThe median duration of the longest opposite BZ burst in jets is ∼10s and strength is ∼±10nT [ABSTRACT FROM AUTHOR]

Magnetic reconnection at the magnetopause (MP) energizes ambient plasma via the release of magnetic energy and produces an "open" magnetosphere allowing solar wind particles to directly enter the system. At Saturn, the nature of MP reconnection remains unclear. The current study examines electron bulk heating at MP crossings, in order to probe the relationship between observed and predicted reconnection heating proposed by Phan et al. (2013, https://doi.org/10.1002/grl.50917) under open and closed MP, and how this may pertain to the position of the crossings in the Δβ‐magnetic shear parameter space. The electron heating for 70 MP crossings made by the Cassini spacecraft from April 2005 to July 2007 was found using 1d and 3d moment methods. Minimum variance analysis was used on the magnetic field data to help indicate whether the MP is open or closed. We found better agreement between observed and predicted heating for events suggestive of locally "open" MP. For events suggestive of locally "closed" MP, we observed a cluster of points consistent with no electron heating, but also numerous cases with significant heating. Examining the events in the Δβ‐magnetic shear parameter space, we find 83% of events without evidence of energization were situated in the "reconnection suppressed" regime, whilst between 43% to 68% of events with energization lie in the "reconnection possible" regime depending on the threshold used. The discrepancies could be explained by a combination of spatial and temporal variability which makes it possible to observe heated electrons with different conditions from the putative reconnection site. Plain Language Summary: Saturn's magnetic cavity in space is marked by a boundary called the magnetopause. Particles from the Sun can enter this region via a process called magnetic reconnection. However, the conditions under which this process can occur on the boundary remains unclear. We used the heating of electrons detected by the Cassini spacecraft during crossings of this boundary to study the effects of different conditions on the viability of reconnection at Saturn's magnetopause. We found that most of the crossings which showed evidence of significant electron heating close to theoretical predictions were also at locations where the magnetopause was open (i.e., locations where solar particles can enter Saturn's territory) and/or where the local conditions were suitable for reconnection to take place. Key Points: Electron bulk heating at Saturn's magnetopause is used to test hypotheses about magnetic reconnectionObservations suggestive of locally open magnetopause tend to exhibit electron heating closer to the theoretical prediction for reconnectionΔBeta magnetic shear parameter space discriminates well between events with evidence of energization and those without [ABSTRACT FROM AUTHOR]

Magnetosheath jets are plasma entities that feature a significantly enhanced dynamic pressure with respect to the ambient plasma. They occur more often downstream of the quasi-parallel bow shock. Jets can propagate through the entire magnetosheath and impact on the magnetopause. We reanalyze multi-spacecraft data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission to obtain the first unbiased distributions of scale sizes of the jets, in the directions parallel and perpendicular to their propagation direction. These distributions are log-normal; they fit well to the observations. We argue that jet scales should be log-normally distributed as they should result from multiplicative processes in the foreshock and in the magnetosheath. We find that typical jet scales are on the order of 0.1 Earth radii (RE), one order of magnitude smaller than previously reported. Median scale sizes of 0.12 RE and 0.15 RE in the perpendicular and parallel directions are obtained. The small scales may be related to the substructure of Short Large Amplitude Magnetic Structures (SLAMS) in the foreshock, or to the break up of larger jets within the magnetosheath. Use of the log-normal distributions also allows for an analysis of impact rates of small-scale jets: While previous results on large jets hitting the magnetopause several times per hour remain largely unchanged, we now find that hundreds to thousands of mostly small-scale jets could potentially impact the dayside magnetopause every hour. [ABSTRACT FROM AUTHOR]

Using a three-dimensional (3-D) global-scale hybrid code, the Magnetospheric Multiscale (MMS) reconnection event around 02:13 UT on 18 November 2015, highlighted in the Geospace Environment Modeling (GEM) Dayside Kinetic Challenge, is simulated, in which the interplanetary magnetic field (IMF) points southward and the geomagnetic field has a -27° dipole tilt angle. Strong southward plasma jets are found near the magnetopause as a result of the dayside reconnection. Our results indicate that the subsolar magnetopause reconnection X line shifts from the subsolar point toward the Northern Hemisphere due to the effect of the tilted geomagnetic dipole angle, consistent with the MMS observation. Subsequently, the reconnection X lines or sites and reconnection flux ropes above the equator propagate northward along the magnetopause. The formation and global distribution of the X lines and the structure of the magnetopause reconnection are investigated in detail with the simulation. Mirror mode waves are also found in the middle of the magnetosheath downstream of the quasi-perpendicular shock where the plasma properties are consistent with the mirror instability condition. As a special outcome of the GEM challenge event, the spatial and temporal variations in reconnection, the electromagnetic power spectra, and the associated D-shaped ion velocity distributions in the simulated reconnection event are compared with the MMS observation. [ABSTRACT FROM AUTHOR]

We identify and examine all MErcury Surface Space ENvironment, GEochemistry, and Ranging (MESSENGER) crossings of Mercury's dayside magnetopause with magnetospheric field intensities ≥300 nT. The eight such events, which occurred under highly compressed magnetosphere conditions, are analyzed in the identical manner utilized by Slavin et al. (2014, https://doi.org/10.1002/2014JA020319). The results suggest that the eight highly compressed magnetosphere events represent the highest solar wind dynamic pressures for which the MESSENGER's orbit still passed below the magnetopause and provided measurements of the dayside magnetosphere. Using the magnetohydrodynamic model by Jia et al. (2015, https://doi.org/10.1002/2015JA021143) that electromagnetically couples Mercury's interior with its magnetosphere, a series of global simulations are conducted to quantitatively characterize the response of Mercury's magnetosphere to solar wind forcing. Combining the MESSENGER observations with the simulations, we have obtained a consistent picture of how Mercury's dayside magnetospheric configuration is controlled, separately and in combination, by induction‐driven shielding and reconnection‐driven erosion. For solar wind pressures of ∼40–90 nPa, compared with the average ∼10–15 nPa at Mercury's orbit, the shielding effects of induction in Mercury's core in standing‐off the solar wind typically exceed the erosion of the dayside magnetosphere due to reconnection for these events, most of which occurred under low magnetic shear conditions. For high magnetic shear across the magnetopause our simulation predicts that reconnection would dominate. Mercury's effective magnetic moment as inferred from magnetopause standoff distance ranges from 170 to 250  nT−RM3 for these events. These findings are of crucial importance for understanding the space weathering at Mercury and its contribution to the generation of Mercury's exosphere. Key Points: Eight highly compressed dayside magnetosphere events are identified from MESSENGER data and analyzedThe 3‐D global magnetosphere model with self‐consistently coupled electrically conductive interior is used to simulate the HCM eventsBoth induction‐driven shielding and reconnection‐driven erosion are important for the HCM events [ABSTRACT FROM AUTHOR]

The electron dynamics within thin current sheets plays a key role both for the process of magnetic reconnection and other energy transfer mechanisms but, from an observational point of view, is not well understood. In this paper we report observations of a reconnecting current sheet with intermediate guide field BG=0.5Bin, where Bin is the magnetic field amplitude in the inflow regions. The current sheet width is comparable to electron spatial scales. It shows a bifurcated structure and is embedded within the magnetopause current layer with thickness of several ion scales. The electron scale current sheet has strong out‐of‐plane and in‐plane currents, Hall electric and magnetic fields, a finite magnetic field component normal to the current sheet, and nongyrotropic electron distributions formed due to finite gyroradius effects at the boundary of the current sheet. Comparison between test particle simulations and electron data shows that electrons approaching from the edge of the largest magnetic curvature are scattered to perpendicular pitch angles in the center of the current sheet while electrons entering from the opposite side remain close to field aligned. The comparison also shows that an observed depletion in phase space at antiparallel pitch angles can be explained if an out‐of‐plane electric field, which due to the guide field is close to antiparallel to the magnetic field, is present in the center of the current sheet. This electric field would be consistent with the reconnection electric field, and we therefore interpret the depletion of electron phase space density as a manifestation of ongoing reconnection. Key Points: A reconnecting electron scale current sheet is observed inside a filamented magnetopause current layerDue to a guide magnetic field and related magnetic curvature, electrons entering from opposite sides have different behaviorThe presence of an out‐of‐plane electric field can explain observed electron distributions [ABSTRACT FROM AUTHOR]

The data from terrestrial observations of cosmic rays at the global network of stations by the method of spectrographic global survey were used to analyze two Forbush decreases during the geomagnetic storms in March and June 2015. The spectra of cosmic ray variations, pitch angle anisotropy of cosmic rays at different phases of Forbush decrease development, and the changes in the planetary system of geomagnetic cutoff rigidities are presented. It is shown that, during the approximation of the spectra of variations by the power function of particle rigidity in the interval of 10-50 GV, the spectrum index is softer at the maximum modulation phase than during the phases of cosmic ray intensity decline and recovery. In the axisymmetric model of the bounded magnetosphere of the Earth, which takes into account the currents at the magnetopause and the ring current, the distance to the subsolar point and the radius of the ring current, as well as the contribution of the ring current to the changes in geomagnetic cutoff rigidity and to the Dst index during the studied events, are determined. [ABSTRACT FROM AUTHOR]

Variations in the planetary system of the geomagnetic cutoff rigidity during the moderate geomagnetic storm in March 2015 are calculated on the basis of data from cosmic-ray measurements at the world network of stations. The distance to the subsolar point and the radius of the ring current are calculated on the basis of the axisymmetric restricted Earth’smagnetosphere model that takes into account currents at the magnetopause and the ring current. The contribution of the ring current to variations in the geomagnetic cutoff rigidity and to the disturbance storm time (Dst) index is also determined within this model. [ABSTRACT FROM AUTHOR]

The present work has investigated the midlatitudinal magnetopause locations under radial interplanetary field (RIMF) conditions. Among 262 (256) earthward (sunward) RIMF events from years of 2001 to 2009, Cluster satellites have crossed the magnetopause 22(12) times, with 10 (7) events occurring at midlatitudes. The observed midlatitudinal magnetopause positions are compared with two empirical magnetopause models (Shue et al., 1998; Boardsen et al., 2000) (hereafter referred to as the Shue98 model and the Boardsen00 model). The observation-model differences exhibit local time asymmetry. For earthward RIMF cases, the Shue98 model underestimates the magnetopause positions in the postnoon sector, while it overestimates the magnetopause positions in the dawn and dusk sectors. The Boardsen00 model generally underestimates the magnetopause after 6 MLT (magnetic local time), with larger deviations in the postnoon sector as compared to those in the prenoon. For sunward RIMF cases, the selected events are mainly clustered around the dawn and dusk sectors. The comparison with the Shue98 model indicates contractions in the dawn and expansions in the dusk sector, while the comparison with Boardsen00 indicates general expansions, with larger expansions in the later local time sectors. The local time variations in the differences between observations and the Shue98 and the Boardsen00 models indicate that the real magnetopause could be asymmetrically shaped during radial IMF periods, which should be considered by magnetopause models. The observation-model differences in the magnetopause positions (ΔRMP) during RIMF periods correlate well with the solar wind dynamic pressure, with larger ΔRMP for larger Pd. The southern magnetopause expands further outward relative to the model prediction when the dipole tilt angle is more negative (local summer in the Southern Hemisphere). For earthward RIMF cases, the generally good correlations between ΔRMP and the IMF cone angle are consistent with the previous hypothesis (Dušík et al., 2010) that, with more radial IMF, the subsolar magnetopause will expand further outward, owever, this is not the case for the comparison with Boardsen00 during sunward IMF periods, as it shows less dependence on the IMF cone angle. [ABSTRACT FROM AUTHOR]

When the effect of a solar wind dynamic pressure pulse on the magnetospheric and ionospheric dynamics is studied, it is usually difficult to detect the effect of a sudden change in the density against the background of the other varying solar wind parameters, which often play a most pronounced role. Cases in which the solar wind plasma density gradient dominated in the dynamics of the different parameters of an interplanetary medium and its magnetic field are considered in this work. Variations in the Earth's dayside magnetopause current caused by a change in the solar wind ion density are presented for two such cases (February 11 and January 11, 1997) based on the method developed by us previously. Variations in the dayside magnetopause current for collisions of the magnetosphere with corotating interacting flows in January 2004, studied in detail by us previously, are also presented for Saturn. The estimates are comparable with the current values in the transitional three-dimensional current systems of Saturn that were previously calculated by us. [ABSTRACT FROM AUTHOR]

The model calculation of a magnetic disturbance, which was registered at Colaba observatory (India) during the historic giant magnetic storm on September 1-2, 1859, is illustrated. The calculation demonstrates that the observed, unusually fast, 2-h main phase of this storm, when the negative amplitude of the geomagnetic field vector H component was −1600 nT, and an extremely fast (1.5-h) initial field recovery phase from the maximum to the −110 nT amplitude can be generated. The following models of the magnetospheric current systems were used in the calculations: the ring current ( DR), the magnetospheric magnetopause current ( DCF), the magnetotail current system ( DT), and the high-latitude current system ( DP). The unusual time variation in the registered geomagnetic disturbance is related to the probable fast and considerable equatorward shift of the high-latitude currents during the main phase of the analyzed giant storm and to the same fast backward motion of these currents during the initial field recovery phase. The unusually large amplitude of the registered geomagnetic disturbance could have been caused by the total contribution of the indicated magnetospheric current systems during the time when the storm was generated as a result of the interaction between the magnetosphere and the solar plasma ejected during the gigantic solar flare before the storm. [ABSTRACT FROM AUTHOR]

The well-studied event of January 10, 1997, was used to study the mechanism responsible for the response of the auroral ionosphere to the solar-wind dynamic pressure jump given a southward interplanetary magnetic field. All solar-wind parameters of this event were constant except for the jump in plasma density (duration 22 min). Immediately after arrival and during the pressure jump, the inner boundary of the auroral oval moved poleward, whereas the position of the outer boundary was stable. The amplification of the magnetopause current caused by an increase in the solar-wind plasma density was calculated. We think that the amplified magnetopause current is closed by the polarization current at the front of the disturbed region in the magnetosphere, thus forming the 'transient ring current' that exists during the pressure jump. We determined the northward magnetic field of this 'temporary ring current,' the order of magnitude of which coincided with that of the field responsible for the observed poleward shift of the high-latitude boundary of the auroral oval. [ABSTRACT FROM AUTHOR]