Magnetic reconnection—the process responsible for many explosive phenomena in both nature and laboratory—is efficient at dissipating magnetic energy into particle energy. To date, exactly how this ...dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the “diffusion region” at the sub‐ion scale. Here we report such a measurement by Cluster—four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O‐lines) and the separatrices. Inside each current filament, kinetic‐scale turbulence is significantly increased and the energy dissipation, E′ ⋅ j, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X‐lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O‐lines but not X‐lines.
Key Points
Strong current, turbulence, and energy dissipation at O‐lines
No current, turbulence, and energy dissipation at X‐lines
The current‐driven turbulence at O‐lines leads to dissipation
Two dipolarization front (DF) structures observed by Cluster in the Earth midtail region (XGSM ≈ −15 RE), showing respectively the feature of Fermi and betatron acceleration of suprathermal ...electrons, are studied in detail in this paper. Our results show that Fermi acceleration dominates inside a decaying flux pileup region (FPR), while betatron acceleration dominates inside a growing FPR. Both decaying and growing FPRs are associated with the DF and can be distinguished by examining whether the peak of the bursty bulk flow (BBF) is co‐located with the DF (decaying) or is behind the DF (growing). Fermi acceleration is routinely caused by the shrinking length of flux tubes, while betatron acceleration is caused by a local compression of the magnetic field. With a simple model, we reproduce the processes of Fermi and betatron acceleration for the higher‐energy (>40 keV) electrons. For the lower‐energy (<20 keV) electrons, Fermi and betatron acceleration are not the dominant processes. Our observations reveal that betatron acceleration can be prominent in the midtail region even though the magnetic field lines are significantly stretched there.
Key Points
Fermi acceleration dominates inside a decaying flux pileup region
Betatron acceleration dominates inside a growing flux pileup region
Betatron acceleration is caused by a local compression of magnetic field
Magnetic reconnection-the process typically lasting for a few seconds in space-is able to accelerate electrons. However, the efficiency of the acceleration during such a short period is still a ...puzzle. Previous analyses, based on spacecraft measurements in the Earth's magnetotail, indicate that magnetic reconnection can enhance electron fluxes up to 100 times. This efficiency is very low, creating an impression that magnetic reconnection is not good at particle acceleration. By analyzing Cluster data, we report here a remarkable magnetic reconnection event during which electron fluxes are enhanced by 10,000 times. Such acceleration, 100 times more efficient than those in previous studies, is caused by the betatron mechanism. Both reconnection fronts and magnetic islands contribute to the acceleration, with the former being more prominent.
We use the Magnetospheric Multiscale mission (MMS) to study electron acceleration at Earth's quasi‐perpendicular bow shock to address the long‐standing electron injection problem. The observations ...are compared to the predictions of the stochastic shock drift acceleration (SSDA) theory. Recent studies based on SSDA predict electron distribution being a power law with a cutoff energy that scales with upstream parameters. This scaling law has been successfully tested for a single Earth's bow shock crossing by MMS. Here we extend this study and test the prediction of the scaling law for seven MMS Earth's bow shock crossings with different upstream parameters. A goodness‐of‐fit test shows good agreement between observations and SSDA theoretical predictions, thus supporting SSDA as one of the most promising candidates for solving the electron injection problem.
Plain Language Summary
Collisionless shock waves are an important source of accelerating electrons up to cosmic ray energies throughout our universe. Common electron acceleration mechanisms, explaining the highly relativistic energies observed in cosmic rays, require a population of pre‐accelerated electrons up to mildly relativistic energies of around 0.1–1 MeV. This is known as the electron injection problem and a lot of research is currently spent on studying this pre‐acceleration phase of electron acceleration. We use spacecraft data from the Magnetospheric Multiscale mission to study an electron acceleration mechanism able to accelerate electrons from typical solar wind energies of 20 eV up to around 100 keV. One of the most promising theories for explaining electron acceleration is the so‐called stochastic drift acceleration theory, which involves electron interaction with plasma waves forming within a shock. We provide additional observational evidence supporting this theory.
Key Points
Using Magnetospheric Multiscale data to observe energetic electron events at Earth's collisionless bow shock
Electron acceleration at all crossings is well described by the Stochastic Shock Drift Acceleration Theory
Pitch angle diffusion rate depends on Alfvénic Mach number and shock angle (θBn)
We use the Magnetospheric Multiscale mission to observe a bi‐directional electron acceleration event in the electron foreshock upstream of Earth's quasi‐perpendicular collisionless bow shock. The ...acceleration region is associated with a decrease in wave activity, inconsistent with common electron acceleration mechanisms such as Diffusive Shock Acceleration and Stochastic Shock Drift Acceleration. We propose a two‐step acceleration process where an electron field‐aligned beam acts as a seed population further accelerated by a shrinking magnetic bottle process, with the shock acting as the magnetic mirror(s).
Plain Language Summary
Collisionless shock waves are believed to be an important source of accelerating particles up to cosmic ray energies throughout our universe. In this letter, we use spacecraft data from the Magnetospheric Multiscale mission to study an energetic electron event observed at Earth's bow shock. The event displays inconsistencies with common electron acceleration mechanisms previously studied at collisionless shocks. We propose a two‐step acceleration mechanism, combining two known mechanisms for charged particle acceleration, and provide observational evidence supporting our theory. We conclude that plasma wave‐particle interactions at the shock play a crucial role in the energization of these electrons.
Key Points
Using Magnetospheric Multiscale data we observe bi‐directional energetic electrons at Earth's collisionless bow shock
The observations are inconsistent with common electron acceleration mechanisms at shocks
We propose a two‐step acceleration process where a field‐aligned electron beam is further accelerated by a shrinking magnetic bottle
At Earth's dayside magnetopause asymmetric magnetic reconnection occurs between the cold dense magnetosheath plasma and the hot tenuous magnetospheric plasma, which differs significantly from ...symmetric reconnection. During magnetic reconnection the separatrix regions are potentially unstable to a variety of instabilities. In this paper observations of the separatrix regions of asymmetric reconnection are reported as Cluster crossed the magnetopause near the subsolar point. The small relative motion between the spacecraft and plasma allows spatial changes of electron distributions within the separatrix regions to be resolved over multiple spacecraft spins. The electron distributions are shown to be unstable to the electromagnetic whistler mode and the electrostatic beam mode. Large‐amplitude whistler waves are observed in the magnetospheric and magnetosheath separatrix regions, and outflow region. In the magnetospheric separatrix regions the observed whistler waves propagate toward the X line, which are shown to be driven by the loss in magnetospheric electrons propagating away from the X line and are enhanced by the presence of magnetosheath electrons. The beam mode waves are predicted to be produced by beams of magnetosheath electrons propagating away from the X line and potentially account for some of the electrostatic fluctuations observed in the magnetospheric separatrix regions.
Key Points
Whistler emission is observed in separatrix regions of asymmetric reconnection
Whistlers in magnetospheric separatrix regions propagate toward the X line
Whistlers are generated by loss cone distributions of magnetospheric electrons
The occurrence rate of earthward‐propagating dipolarization fronts (DFs) is investigated in this paper based on the 9 years (2001–2009) of Cluster 1 data. For the first time, we select the DF events ...by fitting the characteristic increase inBzusing a hyperbolic tangent function. 303 earthward‐propagating DFs are found; they have on average a duration of 4 s and aBz increase of 8 nT. DFs have the maximum occurrence at ZGSM ≈ 0 and r ≈ 15 RE with one event occurring every 3.9 hours, where r is the distance to the center of the Earth in the XYGSM plane. The maximum occurrence rate at ZGSM ≈ 0 can be explained by the steep and large increase of Bz near the central current sheet, which is consistent with previous simulations. Along the r direction, the occurrence rate increases gradually from r ≈ 20 to r ≈ 15 RE but decreases rapidly from r ≈ 15 to r ≈ 10 RE. This may be due to the increasing pileup of the magnetic flux from r ≈ 20 to r ≈ 15 RE and the strong background magnetic field at r <∼13 RE, where the magnetic field changes from the tail‐like to dipolar shape. The maximum occurrence rate of DFs (one event per 3.9 hours) is comparable to that of substorms, indicating a relation between the two.
Key Points
Nine years (2001‐2009) of Cluster 1 data are analyzed and 303 DFs are found
DF events are selected based on fitting Bz using a hyperbolic tangent function
Occurrence rate of DFs (1 event per 3.9 hours) and substorms are comparable
Abstract
Magnetic flux ropes or magnetic islands are important structures responsible for electron acceleration and energy conversion during turbulent reconnection. However, the evolution of flux ...ropes and the corresponding electron acceleration process still remain open questions. In this paper, we present a comparative study of flux ropes observed by the Magnetospheric Multiscale mission in the outflow region during an example of turbulent reconnection in Earth's magnetotail. Interestingly, we find the farther the flux rope is away from the X-line, the bigger the size of the flux rope and the slower it moves. We estimate the power density converted at the observed flux ropes via the three fundamental electron acceleration mechanisms: Fermi, betatron, and parallel electric field. The dominant acceleration mechanism at all three flux ropes is the betatron mechanism. The flux rope that is closest to the X-line, having the smallest size and the fastest moving velocity, is the most efficient in accelerating electrons. Significant energy also returns from particles to fields around the flux ropes, which may facilitate the turbulence in the reconnection outflow region.
ABSTRACT The Earth's magnetosheath is the region delimited by the bow shock and the magnetopause. It is characterized by highly turbulent fluctuations covering all scales from MHD down to kinetic ...scales. Turbulence is thought to play a fundamental role in key processes such as energy transport and dissipation in plasma. In addition to turbulence, different plasma instabilities are generated in the magnetosheath because of the large anisotropies in plasma temperature introduced by its boundaries. In this study we use high-quality magnetic field measurements from Cluster spacecraft to investigate the effects of such instabilities on the small-scale turbulence (from ion down to electron scales). We show that the steepening of the power spectrum of magnetic field fluctuations in the magnetosheath occurs at the largest characteristic ion scale. However, the spectrum can be modified by the presence of waves/structures at ion scales, shifting the onset of the small-scale turbulent cascade toward the smallest ion scale. This cascade is therefore highly dependent on the presence of kinetic instabilities, waves, and local plasma parameters. Here we show that in the absence of strong waves the small-scale turbulence is quasi-isotropic and has a spectral index −2.8. When transverse or compressive waves are present, we observe an anisotropy in the magnetic field components and a decrease in the absolute value of . Slab/2D turbulence also develops in the presence of transverse/compressive waves, resulting in gyrotropy/non-gyrotropy of small-scale fluctuations. The presence of both types of waves reduces the anisotropy in the amplitude of fluctuations in the small-scale range.
The role and properties of lower hybrid waves in the ion diffusion region and magnetospheric inflow region of asymmetric reconnection are investigated using the Magnetospheric Multiscale (MMS) ...mission. Two distinct groups of lower hybrid waves are observed in the ion diffusion region and magnetospheric inflow region, which have distinct properties and propagate in opposite directions along the magnetopause. One group develops near the ion edge in the magnetospheric inflow, where magnetosheath ions enter the magnetosphere through the finite gyroradius effect and are driven by the ion‐ion cross‐field instability due to the interaction between the magnetosheath ions and cold magnetospheric ions. This leads to heating of the cold magnetospheric ions. The second group develops at the sharpest density gradient, where the Hall electric field is observed and is driven by the lower hybrid drift instability. These drift waves produce cross‐field particle diffusion, enabling magnetosheath electrons to enter the magnetospheric inflow region thereby broadening the density gradient in the ion diffusion region.
Key Points
Two groups of lower hybrid waves are observed in the ion diffusion and magnetospheric inflow regions
In the magnetospheric inflow region lower hybrid waves develop when cold magnetospheric ions are present and can heat cold ions
In the diffusion region lower hybrid waves develop at the density gradient and can cause cross‐field particle diffusion
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