Search

Magnetic reconnection is a fundamental process known to play a crucial role in electron acceleration and heating, however, the mechanism of electron energization during reconnection is still not fully understood. This study introduces a novel electron acceleration mechanism in which electrons can be accelerated by secondary reconnection in the separatrix region. The secondary reconnection occurs in a thin current sheet resulted from the shear of the out‐of‐plane Hall magnetic fields of the primary magnetopause reconnection. It results in the intense electron energy fluxes toward the primary X‐line. This mechanism will likely be an important piece in the puzzle of particle acceleration by reconnection. Plain Language Summary: Magnetic reconnection is a fundamental process that plays a critical role in electron acceleration and heating. The separatrix region of magnetic reconnection, which distinguishes the inflow and outflow regions, is a crucial area in the particle acceleration of the reconnection process. However, previous studies mainly focused on the electron acceleration mechanism in the separatrix region under two‐dimensional reconnection, without considering the influence of the three‐dimensional structures. This paper reports, for the first time, the secondary reconnection occurring in the opposite Hall magnetic field in the three‐dimensional separatrix region of magnetopause primary reconnection. This secondary reconnection can accelerate and heat the inflow electrons of the primary reconnection, which provides important clues on how the particles are accelerated and heated by reconnection. Key Points: Secondary reconnection occurs between Hall magnetic fields in the separatrix region of the primary magnetopause reconnectionIntense electron enthalpy fluxes are injected toward the primary X‐line along the separatrixElectrons are accelerated by the secondary reconnection before being injected into the primary X‐line for further energization [ABSTRACT FROM AUTHOR]

Despite a growing number of observations of whistler waves in/around the reconnection diffusion region, the origin of the whistler waves in the diffusion region and their role in reconnection remain elusive. This paper investigates the whistler‐mode waves within an electron‐scale reconnecting current sheet observed by the Magnetospheric Multiscale mission on 23 December 2016. These whistlers were observed in both the electron diffusion region (EDR) and the immediate inflow region of the electron‐scale magnetic reconnection site. The dispersion relation of these whistlers is measured and compared to predictions from linear theory for the first time in these regions. The comparison shows that the field‐aligned drifting electron component critically modifies the dispersion relation of whistlers in the EDR due to the Doppler‐shift effects. We demonstrate that these whistlers propagated into the EDR from outside rather than locally excited, however, they did not provide sufficiently large anomalous dissipation in this EDR. Plain Language Summary: Whistler‐mode waves are important plasma waves in the space plasma system, which can effectively interact with electrons to exchange energy. Whistler waves are frequently observed in various areas of magnetic reconnection, for example, outflow region, separatrix region, and diffusion region. However, the nature of whistler waves in the electron diffusion region (EDR), where the magnetic field line is broken and reconnected, is still unclear. Here, we present an observation of whistler waves in the EDR and the inflow region of magnetic reconnection. These whistler waves originate from the outside and propagate into the vicinity of the reconnection site. We measured the dispersion relation of these whistler waves for the first time within these regions and compare it with the dispersion relations predicted by linear theory. The results indicate that the effects from the drift velocity of electrons play a key role in modifying the dispersion relation of these whistlers in the EDR. Additionally, we find that these whistlers did not provide effective anomalous dissipation in this EDR. These results advance our knowledge of the source and features of whistlers in the vicinity of the reconnection site. Key Points: Observations confirm that whistler waves can propagate into the electron diffusion region (EDR) from outsideThe dispersion relation of the whistler waves is measured for the first time in the EDRThe observed whistler waves contributed to negligible anomalous effects in the EDR [ABSTRACT FROM AUTHOR]

We investigate whistler‐mode waves and broadband electrostatic waves (EWs) within an ion diffusion region (IDR) at the magnetopause. The quasi‐parallel whistlers are observed in the separatrix regions associated with the electron anisotropy or loss cone, while the oblique whistlers, the Buneman‐type waves, and the oblique EWs are observed in the center of the current sheet associated with the accelerated electron or ion beams. The whistlers are linked with Buneman‐type waves by the electron Pacman distribution rather than the wave‐wave process. The accelerated cold electron beams excite the Buneman‐type instabilities and make the anisotropy or loss cone of hot electrons less apparent, which led to the conversion of the whistlers from quasi‐parallel to oblique. Additionally, the oblique EWs are associated with the ion beams produced by the multiple X‐line reconnection. These results provide a further understanding of the relation between plasma waves and plasma kinetics in the IDR. Plain Language Summary: The whistler waves and the broadband electrostatic waves are two common waves produced by magnetic reconnection in space plasma. Generally, the whistler waves have two modes, the quasi‐parallel and the oblique to the ambient magnetic field associated with different generation mechanisms. Both of them have been observed in the magnetic reconnection, but their relation is still unclear. The broadband electrostatic waves are sometimes simultaneously observed with the oblique whistlers. The relationship between them is also inconclusive. Here, we present a new view of the relation between the quasi‐parallel whistlers, the oblique whistlers, and the broadband electrostatic waves. They are linked by the evolution of electron Pacman distribution, which is composed of the hot magnetospheric population with the loss cone and the cold accelerated magnetosheath beam characteristic for magnetopause reconnection, in the reconnection diffusion region rather than the wave‐wave process. Besides, the oblique electrostatic waves associated with ion beams are observed in the diffusion region. These results advance our understanding of the relation between plasma waves and magnetic reconnection. Key Points: Large‐amplitude electromagnetic and electrostatic waves are observed in the separatrices and center of the diffusion region, respectivelyQuasi‐parallel whistlers, oblique whistlers, and Buneman‐type waves are linked via the evolution of electron Pacman distributionThe oblique broadband electrostatic waves associated with the ion beams are observed in the diffusion region [ABSTRACT FROM AUTHOR]

Magnetic reconnection and turbulence are the two most important energy dissipation processes in plasma. These two processes intertwine with each other and play important roles in their respective dynamic evolution. Here, we present the first evidence that secondary reconnections occur in the turbulent outflow driven by a primary reconnection in the Earth's magnetotail. We have identified 14 secondary reconnections in a large number of current filaments in the turbulent outflow, which persisted for about one and half an hour. Most of these secondary reconnections were electron‐only reconnection that has recently been discovered in the magnetosheath. These secondary reconnections entangled the magnetic field lines and dissipated the magnetic energy in the outflow region far away from the primary X line. Plain Language Summary: Magnetic reconnection usually evolves into turbulent state. The role of turbulence in reconnection is one of the most outstanding questions in reconnection. By using the unprecedented high‐cadence data from the magnetospheric multiscale mission, we present the first evidence that secondary reconnections occur in the turbulent outflow driven by a primary reconnection in the Earth's magnetotail. Most of these secondary reconnections occurred in electron‐scale current sheets and were without ion couplings. These secondary reconnections future released energy in the outflow region besides the primary X line. These results are important for understanding the turbulent reconnection, which is ubiquitous in the astrophysical system but barely investigated by in situ measurements. Key Points: First observation of secondary reconnections in the turbulent outflow driven by primary reconnectionThese secondary reconnections occurred in current sheets with broad shear angles and different orientationsMost of these secondary reconnections were without clear ion couplings [ABSTRACT FROM AUTHOR]

Two‐dimensional Hall reconnection model can explain the fast release of magnetic energy and most of its predictions have been demonstrated by in situ satellite observations. However, the three‐dimensional effect to reconnection remains poorly known. Recent numerical simulations have shown that three‐dimensional evolution is more complex than two‐dimensional in that reconnection occurs in multiple sites that are not necessarily in the primary neutral current sheet. Here, we present the first observational evidence for localized secondary reconnection at the separatrix surface of a magnetic flux rope (MFR). This secondary reconnection occurs between the axial magnetic field of the MFR, which points out‐of‐plane of the Earth's magnetopause reconnection, and the magnetospheric field. This three‐dimensional electron‐scale reconnection in the exhaust facilitates the cross‐scale energy conversion from the macro‐scale down to electron‐scale. Plain Language Summary: Magnetic reconnection releases large amounts of magnetic energy in astrophysical, space and laboratory plasmas. For the decades, the reconnection has been investigated largely based on two‐dimensional models while the three‐dimensional effect to reconnection remains poorly known. It has been shown by numerical simulation that three‐dimensional evolution, because of the freedom in the out‐of‐plane direction, is turbulent and distinct from the two‐dimension laminar reconnection in that reconnection occurs in multiple sites that are not necessarily in the primary neutral current sheet. The turbulent reconnection is characterized by the formation and interaction of multiple flux ropes, or chaotic magnetic field lines in the outflow region. Although small‐scale magnetic flux ropes and their interactions have been observed, one important missing piece is whether reconnection occurs at the region away from the neutral current sheet. Here we present the first evidence for localized secondary reconnection at the separatrix surface of the primary reconnection. This secondary reconnection occurs between the axial magnetic field of the flux rope, which points out‐of‐plane of the magnetopause reconnection, and the magnetospheric field. This three‐dimensional patchy reconnection in the exhaust facilitates the cross‐scale energy conversion from the macro‐scale down to electron‐scale and leads to a turbulent evolution of reconnection. Key Points: Three‐dimensional secondary electron‐scale reconnection was observed at the separatrix of a flux rope in the magnetosphereNew light on the destiny of flux rope from a three‐dimensional perspective: dissipation through the reconnection of the core fieldThe cross‐scale energy conversion is through the formation and dissipation of flux ropes [ABSTRACT FROM AUTHOR]

We report a Magnetospheric Multiscale (MMS) mission observation of magnetic reconnection occurring at the edge of a large‐scale magnetic flux rope (MFR), the cross‐section of which was about 2 RE. The MFR was observed at the duskside in the Earth's magnetotail and was highly oblique with its axis approximately along the XGSM direction. We find an electron‐scale current sheet near the edge of this MFR. The Hall magnetic and electric field, super‐Alfvénic electron outflow, parallel electric field, and positive energy dissipation were observed associated with the current sheet, which indicates that MMS detected a reconnecting current sheet with a large guide field. Interestingly, ions were not coupled in this reconnection, akin to the electron‐only reconnection observed in the turbulent magnetosheath. We further find that the electron‐only reconnection is commonly associated with a macroscopic MFR. This result will shed new light on understanding the multiscale coupling associated with an MFR in space plasmas. Key Points: MMS observed an electron‐only reconnection at the edge of a macroscopic magnetic flux rope in the magnetotailElectron‐only reconnection is ubiquitous in space plasma [ABSTRACT FROM AUTHOR]

Dipolarization fronts (DFs) are important for energy conversion, particle acceleration, and flux transport in the magnetotail. The partition of energy conversion between ions and electrons and the energy dissipation at DFs are not well understood. In this paper, we present a statistical study of energy conversion and dissipation of 122 DFs observed by Magnetospheric Multiscale mission in the magnetotail. Statistically, electromagnetic energy transfers to plasma at DF. The released energy is mainly transferred to ions rather than electrons. On average, ions gain energy across the whole DF, while electrons gain energy at the leading part but lose energy at the trailing part of DFs. Joule dissipation J · (E+ve × B) can be either positive or negative at DFs, and its average value is very small. The kinetic energy dissipation parameter Pi − D does not exhibit clear signatures at the DFs; hence, it is not suitable for quantifying the energy dissipation at DF. Key Points: Statistical study of the energy conversion and dissipation at DFs observed by MMSElectromagnetic energy released at DF mainly goes to heat and accelerate ionsAverage Joule dissipation across DF is very small, and the parameter Pi − D is not suitable for quantifying the energy dissipation at DF [ABSTRACT FROM AUTHOR]

Magnetic holes have been widely observed in various space plasma systems. The origin of magnetic holes and their influence to background plasma are under debate. In this paper, we show a kinetic‐scale electron vortex magnetic hole in a nonideal region of an active X line, which was observed by the Magnetospheric Multiscale mission at the dayside magnetopause. Intense current and nonideal electric field in the electron frame were observed within the magnetic hole, which led to a strong energy dissipation. Thus, the electron vortex magnetic hole probably provided an additional energy dissipation channel besides the electron diffusion region adjacent to the hole. We suggest that magnetic reconnection provided favorable conditions for the formation of this kinetic‐scale magnetic hole and is an important source of magnetic holes in space plasma. Key Points: First evidence for reconnection‐driven kinetic‐scale magnetic holeThe hole was associated with an electron vortex and was observed in the vicinity of an electron diffusion regionThe hole probably provided an additional energy dissipation channel besides the electron diffusion region [ABSTRACT FROM AUTHOR]

Recent studies have revealed a global, poleward migration trend of tropical cyclones (TCs) in terms of annual‐averaged latitude of lifetime maximum intensity under the global warming. The TCs in the western North Pacific (WNP) make the largest contribution to this trend. One existing hypothesis is that there is a linkage between poleward migration of TCs and anthropogenic forcing. Here we introduce a new measurement, the lifetime‐averaged latitude weighted by TC destructive potential, to detect the meridional migration of the WNP TCs, which is more reliable and meaningful as it considers not only past data uncertainty but also public concerns. Our results show that the trend of WNP TC migration reversed during the recent warming slowing‐down period (after ~1999). Different from the existing hypothesis, it is the change of regional pattern of sea surface temperature that contributes greatly to the meridional migration of WNP TCs. Plain Language Summary: Tropical cyclones (TCs) account for the majority of natural catastrophic losses in the developed world and are the second leading cause (next to floods) of death and injury among natural disasters affecting developing countries. The challenges for climate detection and attribution research with regard to TCs are to determine whether an observed change in TC activity exceeds the natural variability and to attribute significant changes to anthropogenic forcing (e.g., greenhouse gases or aerosols). Recent studies have indicated that the TCs over the western North Pacific (WNP) systematically migrated poleward, which is plausibly linked to the expansion of the tropics associated with anthropogenic forcing. However, utilizing our new measurement, we find that the poleward migration of WNP TCs reversed direction during the recent warming slowing‐down period (after ~1999). We thus propose a new mechanism associated with the regional pattern of sea surface temperature change, which is characterized by a relatively warm or cold sea surface temperature pool in the region (20–40°N, 140–160°E). Compared with the previous theory based on anthropogenic forcing and well‐known dominant modes of variability, the new mechanism provides a more convincing explanation for the migration of WNP TCs since 1980, especially for the recent warming slowing‐down period. Key Points: The trend of tropical cyclone migration over the western North Pacific reversed during the recent warming slowing‐down period (after ~1999)It is the change of regional sea surface temperature that contributed greatly to the meridional migration trend of tropical cyclonesThe effect of regional sea surface temperature change came from its impact on storm potential intensity and large‐scale circulation [ABSTRACT FROM AUTHOR]

Abstract: It has been proposed that, in the near‐Earth magnetotail, earthward propagating flux ropes can merge with the Earth's dipole magnetic field and dissipate its magnetic energy. However, the reconnection diffusion region related to this process has not been identified. Here we report the first in situ observation of magnetic reconnection between an earthward propagating flux rope and the closed magnetic field lines connecting to Earth. Magnetospheric Multiscale (MMS) spacecraft crossed a vertical current sheet between the leading edge of the flux rope (negative Bz) and the geomagnetic field (positive Bz). The subion‐scale current sheet, super‐Alfvénic electron outflow, Hall magnetic and electric field, conversion of magnetic energy to plasma energy (J·E > 0), and magnetic null were observed during the crossing. All the above signatures indicate that MMS detected the reconnection diffusion region. This result is also relevant to other planets with intrinsic magnetosphere. [ABSTRACT FROM AUTHOR]