Göran Marklund

Electron-Scale Measurements of Magnetic Reconnection in Space

Probing magnetic reconnection in space Magnetic reconnection occurs when the magnetic field permeating a conductive plasma rapidly rearranges itself, releasing energy and accelerating particles. Reconnection is important in a wide variety of physical systems, but the details of how it occurs are poorly understood. Burch et al. used NASAs Magnetospheric Multiscale mission to probe the plasma properties within a reconnection event in Earths magnetosphere (see the Perspective by Coates). They find that the process is driven by the electron-scale dynamics. The results will aid our understanding of magnetized plasmas, including those in fusion reactors, the solar atmosphere, solar wind, and the magnetospheres of Earth and other planets. Science, this issue p. 10.1126/science.aaf2939; see also p. 1176 Magnetic reconnection is driven by the electron-scale dynamics occurring within magnetized plasmas. INTRODUCTION Magnetic reconnection is a physical process occurring in plasmas in which magnetic energy is explosively converted into heat and kinetic energy. The effects of reconnection—such as solar flares, coronal mass ejections, magnetospheric substorms and auroras, and astrophysical plasma jets—have been studied theoretically, modeled with computer simulations, and observed in space. However, the electron-scale kinetic physics, which controls how magnetic field lines break and reconnect, has up to now eluded observation. RATIONALE To advance understanding of magnetic reconnection with a definitive experiment in space, NASA developed and launched the Magnetospheric Multiscale (MMS) mission in March 2015. Flying in a tightly controlled tetrahedral formation, the MMS spacecraft can sample the magnetopause, where the interplanetary and geomagnetic fields reconnect, and make detailed measurements of the plasma environment and the electric and magnetic fields in the reconnection region. Because the reconnection dissipation region at the magnetopause is thin (a few kilometers) and moves rapidly back and forth across the spacecraft (10 to 100 km/s), high-resolution measurements are needed to capture the microphysics of reconnection. The most critical measurements are of the three-dimensional electron distributions, which must be made every 30 ms, or 100 times the fastest rate previously available. RESULTS On 16 October 2015, the MMS tetrahedron encountered a reconnection site on the dayside magnetopause and observed (i) the conversion of magnetic energy to particle kinetic energy; (ii) the intense current and electric field that causes the dissipation of magnetic energy; (iii) crescent-shaped electron velocity distributions that carry the current; and (iv) changes in magnetic topology. The crescent-shaped features in the velocity distributions (left side of the figure) are the result of demagnetization of solar wind electrons as they flow into the reconnection site, and their acceleration and deflection by an outward-pointing electric field that is set up at the magnetopause boundary by plasma density gradients. As they are deflected in these fields, the solar wind electrons mix in with magnetospheric electrons and are accelerated along a meandering path that straddles the boundary, picking up the energy released in annihilating the magnetic field. As evidence of the predicted interconnection of terrestrial and solar wind magnetic fields, the crescent-shaped velocity distributions are diverted along the newly connected magnetic field lines in a narrow layer just at the boundary. This diversion along the field is shown in the right side of the figure. CONCLUSION MMS has yielded insights into the microphysics underlying the reconnection between interplanetary and terrestrial magnetic fields. The persistence of the characteristic crescent shape in the electron distributions suggests that the kinetic processes causing magnetic field line reconnection are dominated by electron dynamics, which produces the electric fields and currents that dissipate magnetic energy. The primary evidence for this magnetic dissipation is the appearance of an electric field and a current that are parallel to one another and out of the plane of the figure. MMS has measured this electric field and current, and has identified the important role of electron dynamics in triggering magnetic reconnection. Electron dynamics controls the reconnection between the terrestrial and solar magnetic fields. The process of magnetic reconnection has been a long-standing mystery. With fast particle measurements, NASA’s Magnetospheric Multiscale (MMS) mission has measured how electron dynamics controls magnetic reconnection. The data in the circles show electrons with velocities from 0 to 104 km/s carrying current out of the page on the left side of the X-line and then flowing upward and downward along the reconnected magnetic field on the right side. The most intense fluxes are red and the least intense are blue. The plot in the center shows magnetic field lines and out-of-plane currents derived from a numerical plasma simulation using the parameters observed by MMS. Magnetic reconnection is a fundamental physical process in plasmas whereby stored magnetic energy is converted into heat and kinetic energy of charged particles. Reconnection occurs in many astrophysical plasma environments and in laboratory plasmas. Using measurements with very high time resolution, NASA’s Magnetospheric Multiscale (MMS) mission has found direct evidence for electron demagnetization and acceleration at sites along the sunward boundary of Earth’s magnetosphere where the interplanetary magnetic field reconnects with the terrestrial magnetic field. We have (i) observed the conversion of magnetic energy to particle energy; (ii) measured the electric field and current, which together cause the dissipation of magnetic energy; and (iii) identified the electron population that carries the current as a result of demagnetization and acceleration within the reconnection diffusion/dissipation region.

On intense diverging electric fields associated with black aurora

Results are presented from the double-probe electric field instrument on the Freja satellite with particular focus on the fine-structured and dynamic plasma of the upper auroral ionosphere. The high-resolution measurements show frequently occuring intense and irregular fine-scale electric fields similar to those observed at higher altitudes by, for example, the S3-3 and Viking satellites. Whereas the high-altitude fields tend to be directly related to the auroral fine-structure this is not always the case for the low-altitude fields as illustrated by high-resolution data of a pair of very intense (≈ 1 V/m), narrow electric field structures in the post-midnight sector in the large-scale downward field-aligned current region. The structures are found to be associated with an excess of positive space charge (diverging electric fields), dropouts of precipitating electrons as well as depletions of thermal plasma, and significant wave activity. Combined with the scale-size of the structures (≈ 1 km) and the spacing between them (≈ 5 km) these observations suggest that the intense electric fields are related to east-west aligned vortex street structures of black aurora, similar to auroral curls but with opposite sense of rotation (clockwise seen antiparallel to B) and a total absence of auroral emissions. The detection of these structures was made possible by the relatively low inclination of the Freja orbit, which at times is almost tangential to the auroral oval. Thus, the Freja orbit provides a new perspective for studying many nightside auroral phenomena both at smaller scales (vortices) and at larger scales as exemplified by observations of north-south oriented auroral structures caused by rotational arc distortions.

On low‐altitude particle acceleration and intense electric fields and their relationship to black aurora

Recent findings by the Freja satellite have shown the existence of extremely intense (1–2 V/m) and small-scale (1 km) diverging electric fields which are interpreted to be associated with east–west aligned dark striations or black auroral curls. Precipitating or transversely energized ions, downward field-aligned currents carried by upward fluxes of ionospheric electrons and dropouts of energetic electron precipitation, are found to be characteristic features of such events. A comparison of these characteristics to those of the aurora point at a symmetry between the aurora and the black aurora, the aurora being associated with negative divergence of the electric field and the black aurora with positive divergence. The diverging field events typically occur during winter conditions within the midnight to early morning sector of the auroral oval. Estimates of the ambient conductivity due to solar EUV radiation for each of these events show a clear anticorrelation with the electric field magnitude. The black auroral structures are likely to be associated with localized ionospheric density depletions below that of the ambient density and caused by the upward flow of ionospheric electrons. The efficiency by which such density holes are created in regions of downward field-aligned current flow have recently been demonstrated in model studies. The electric field magnitudes are found to decrease with the scale size, not inversely as suggested in recent theoretical work but with a power law exponent of 0.6–0.8. At lower altitudes (around 800 km) the maximum intensities for a majority of the events are in the range of values that have been reported from rocket and radar measurements in the ionosphere, i.e., around 150–200 mV/m. However, close to magnetic midnight and during winter conditions small-scale diverging electric fields of 1 V/m are occasionally found to exist down to at least 800 km. We suggest that the diverging electric fields observed by Freja are associated with low-altitude and narrow (≈1–2 km) potential structures similar to the auroral potential structures at higher altitude but associated with a positive space charge and a downward parallel electric field. This is supported by Freja observations of narrow upward beams of 2 keV electrons in good agreement with a 2 kV positive peak in the electrostatic potential for a black aurora event. The existence of a downward parallel electric field at low altitudes is also supported by low-altitude observations by the S3–3 and Viking satellites. If such low-altitude potential structures do exist as our results suggest, an outstanding problem for future investigation is how they may be formed and maintained.

Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere

The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field. On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth. Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted, but how they could be maintained is still a matter for debate. The spatial and temporal behaviour of the electric fields—a knowledge of which is crucial to an understanding of their nature—cannot be resolved uniquely by single satellite measurements. Here we report on the first observations by a formation of identically instrumented satellites crossing a beam of upward-accelerated electrons. The structure of the electric potential accelerating the beam grew in magnitude and width for about 200 s, accompanied by a widening of the downward-current sheet, with the total current remaining constant. The 200-s timescale suggests that the evacuation of the electrons from the ionosphere contributes to the formation of the downward-pointing magnetic-field-aligned electric fields. This evolution implies a growing load in the downward leg of the current circuit, which may affect the visible discrete aurorae.

Auroral arc classification scheme based on the observed arc-associated electric field pattern

Abstract Radar and rocket electric field observations of auroral arcs have earlier been used to identify essentially four different arc types, namely anticorrelation and correlation arcs (with, respectively, decreased and increased arc-associated field) and asymmetric and reversal arcs. In this paper, rocket double probe and supplementary observations from the literature, obtained under various geophysical conditions, are used to organize the different arc types on a physical rather than morphological basis. This classification is based on the relative influence on the electric field pattern from the two current continuity mechanisms, polarization electric fields and Birkeland currents. In this context the tangential electric field plays an essential role and it is thus important that it can be obtained with both high accuracy and resolution. In situ observations by sounding rockets are shown to be better suited for this specific task than monostatic radar observations. Depending on the dominating mechanism, estimated quantitatively for a number of arc-crossings, the different arc types have been grouped into the following main categories: Polarization arcs, Birkeland current arcs and Combination arcs. Finally the high altitude potential distributions corresponding to some of the different arc types are presented.

Ion cyclotron waves during a great magnetic storm observed by Freja double-probe electric field instrument

Evolution of the great magnetic storm in April 1993 is studied using observations of electromagnetic ion cyclotron (EMIC) waves by the F1 double-probe electric field instrument onboard the Freja satellite. The almost continuous operation of the F1 instrument in the overview mode allowed us to follow the global EMIC wave activity at low altitudes above the ionosphere during several subsequent days covering the initial (compression), main, and recovery phases of the storm. During the initial phase of the storm the spatial occurrence of EMIC waves has a postnoon high-latitude maximum, in agreement with earlier statistical results. A sudden and dramatic change of this pattern was observed with the start of the storm main phase. During the main phase, wave amplitudes were greatly enhanced and the active wave region moved to considerably lower latitudes to the late evening MLT sector. Also, the existence of heavy ions in the later main phase changed the distribution of wave frequencies dramatically. Most interestingly, a number of oxygen band EMIC waves were observed during a limited period of about 7 hours in the later main phase. The observed asymmetric MLT distribution of these oxygen band waves implies that the oxygen loss rate is faster than the drift rate. The results suggest that the EMIC waves play a crucial role in the main and early recovery phase of a great storm.

Subauroral electric fields observed by the Freja satellite: A statistical study

Over 12 months of Freja electric field data have been scanned for subauroral electric fields (SAEF) to enable a comprehensive study of the ionospheric signatures of such electric fields. SAEF are encountered from 1800 to 0200 MLT in agreement with an earlier study. However, a large majority of the SAEF are encountered at a time slightly premidnight (2200 – 2300 MLT), with rather few occurrences before 2000 MLT and after 2400 MLT. Furthermore, the strength of the subauroral electric field is generally much larger for events close to 2200 MLT than for other events. The data confirm that SAEF occur during the substorm recovery phase but also show that SAEF occur earlier during recovery when located close to 2200 MLT than at other local times. The dependence on season and geomagnetic activity is studied, and it is found that the SAEF are more commonly observed at times of high activity when the subauroral electric fields are also generally stronger, except close to winter solstice, when strong electric fields are observed during low activity. The potentials associated with the SAEF and the relation to interplanetary magnetic field By are also studied. The observations are discussed in context with substorm-related field-aligned currents and the midlatitude trough, and we present evidence that support and refine one of the proposed production mechanisms.

Cavity resonators and Alfvén resonance cones observed on Freja

Multiresolution wavelet analysis of magnetic field, electric field, and plasma density records taken on Freja during strong auroral events shows evidence for cavity Alfven resonators in the topside ...

Event study on pre-substorm phases and their relation to the energy coupling between solar wind and magnetosphere

Abstract On 11 November 1976, after a magnetically quiet period with the interplanetary magnetic field (IMF) directed northward, a sudden southward turning of the IMF immediately led to a world-wide intensification of convection which was observed to start almost simultaneously at stations within the auroral zone and polar cap. The two-dimensional equivalent current system over the northern hemisphere had a typical two-cell convection pattern with a maximum disturbance of ΔH = −300 nT observed on the morningside in the westward electrojet region. This enhancement of activity ended after 35 min in a localized substorm onset in the midnight sector over Scandinavia. The recordings made in this area indicate large fluctuations of various ionospheric parameters starting several minutes before the substorm onset. Two subsequent stages can be resolved: (1) high-energy particle precipitation recorded by balloon X-ray detectors and maximum ionospheric current density increase, while the electrojet halfwidth shrinks and the total electrojet current becomes weaker; (2) the maximum ionospheric current density stays constant and the high-energy particle precipitation decreases, while the auroral brightness increases and the total electrojet current and its half-width show a growing trend prior to the final breakup. A suggestion is made that the time interval of these two stages should be called “trigger phase”. A short discussion explains the trigger phase observations in a magnetospheric scale. The energy coupling between solar wind and magnetosphere during the pre-substorm phases is discussed by utilizing the energy coupling function ϵ defined by Perreault and Akasofu ( Geophys. J. R. Astr. Soc. 54 , 547, 1978). The ϵ values appear to be on substorm level during the period of enhanced convection. A good correlation between ϵ and the growth of the Joule heating rate (estimated from the AE data) is found in the beginning, but during the last 20 min before substorm triggering ϵ is high while the Joule heating rate decreases. The behaviour of ϵ during the two stages of the trigger phase suggests that the start of the trigger phase is purely internally controlled while the length of the trigger phase and the final substorm onset may be influenced by the variation in ϵ.

A study of the dynamics of a discrete auroral arc

High resolution electric field and particle data, obtained by the S23L1 rocket crossing over a discrete prebreakup arc in January 1979, are studied in coordination with ground observations (Scandinavian Magnetometer Array—SMA, TV and all-sky cameras) in order to clarify the electrodynamics of the arc and its surroundings. Height-integrated conductivities have been calculated from the particle data, including the ionization effects of precipitating protons and assuming a steady state balance between ion production and recombination losses. High resolution optical information of arc location relative to the rocket permitted a check of the validity of this assumption for each flux tube passed by the rocket. Another check was provided by a comparison between calculated (equilibrium values) and observed electron densities along the rocket trajectory. A way to compensate for the finite precipitation time when calculating the electron densities is outlined. The height-integrated HalI-Pedersen conductivity ratio is typically 1.4 within the arc and about 1 at the arc edges, indicative of a relatively softer energy spectrum there. The height-integrated conductivities combined with the DC electric field measurements permitted calculation of the horizontal ionospheric current vectors (J⊥), Birkeland currents (from div J⊥) and energy dissipation through Joule heating (ΣpE2). An eastward current of typically 1 A m−1 was found to be concentrated mainly to the arc region and equatorward of it. A comparison has been made with the equivalent current system deduced from ground based magnetometer data (SMA) showing a generally good agreement with the rocket results. An intense Pedersen current peak (1.2 A m−1) was found at the southern arc edge. This edge constituted a division line between a very intense (> 10 μA m−1) and localized (~ 6 km) downward current sheet to the south, probably carried by upward flowing cold ionospheric electrons and a more extended upward current sheet (> 10 μA m−2) over the arc carried by measured precipitating electrons. Joule and particle heating across the arc were anticorrelated, consistent with the findings of Evans et al. (1977) with a total value of about 100mW m−2.