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Algorithms based on Empirical Mode Decomposition (EMD) and Iterative Filtering (IF) are largely implemented for representing a signal as superposition of simpler well-behaved components called Intrinsic Mode Functions (IMFs). Although they are more suitable than traditional methods for the analysis of nonlinear and nonstationary signals, they could be easily misused if their known limitations, together with the assumptions they rely on, are not carefully considered. In this work, we examine the main pitfalls and provide caveats for the proper use of the EMD- and IF-based algorithms. Specifically, we address the problems related to boundary errors, to the presence of spikes or jumps in the signal and to the decomposition of highly-stochastic signals. The consequences of an improper usage of these techniques are discussed and clarified also by analysing real data and performing numerical simulations. Finally, we provide the reader with the best practices to maximize the quality and meaningfulness of the decomposition produced by these techniques. In particular, a technique for the extension of signal to reduce the boundary effects is proposed; a careful handling of spikes and jumps in the signal is suggested; the concept of multi-scale statistical analysis is presented to treat highly stochastic signals.

Magnetic reconnection in current sheets is a magnetic-to-particle energy conversion process that is fundamental to many space and laboratory plasma systems. In the standard model of reconnection, this process occurs in a minuscule electron-scale diffusion region 1 , 2 . On larger scales, ions couple to the newly reconnected magnetic-field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed 3 – 5 . Much of the energy conversion occurs in spatially extended ion exhausts downstream of the diffusion region 6 . In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to have a major role in the dissipation of turbulent energy at kinetic scales 7 – 11 . However, evidence for reconnection plasma jetting in small-scale turbulent plasmas has so far been lacking. Here we report observations made in Earth’s turbulent magnetosheath region (downstream of the bow shock) of an electron-scale current sheet in which diverging bi-directional super-ion-Alfvénic electron jets, parallel electric fields and enhanced magnetic-to-particle energy conversion were detected. Contrary to the standard model of reconnection, the thin reconnecting current sheet was not embedded in a wider ion-scale current layer and no ion jets were detected. Observations of this and other similar, but unidirectional, electron jet events without signatures of ion reconnection reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling. Observations of electron-scale current sheets in Earth’s turbulent magnetosheath reveal electron reconnection without ion coupling, contrary to expectations from the standard model of magnetic reconnection.

The severe geomagnetic effects of solar storms or coronal mass ejections (CMEs) are to a large degree determined by their propagation direction with respect to Earth. There is a lack of understanding of the processes that determine their non-radial propagation. Here we present a synthesis of data from seven different space missions of a fast CME, which originated in an active region near the disk centre and, hence, a significant geomagnetic impact was forecasted. However, the CME is demonstrated to be channelled during eruption into a direction +37±10° (longitude) away from its source region, leading only to minimal geomagnetic effects. In situ observations near Earth and Mars confirm the channelled CME motion, and are consistent with an ellipse shape of the CME-driven shock provided by the new Ellipse Evolution model, presented here. The results enhance our understanding of CME propagation and shape, which can help to improve space weather forecasts. Coronal mass ejections from the Sun play an important role in space weather, yet a full understanding of their behaviour remains elusive. Towards this aim, Möstl et al . present a suite of observations showing that an ejection was channelled away from its source region, explaining incorrect forecasts.

Van Allen radiation belts consist of relativistic electrons trapped by Earth's magnetic field. Trapped electrons often drift azimuthally around Earth and display a butterfly pitch angle distribution of a minimum at 90° further out than geostationary orbit. This is usually attributed to drift shell splitting resulting from day–night asymmetry in Earth’s magnetic field. However, direct observation of a butterfly distribution well inside of geostationary orbit and the origin of this phenomenon have not been provided so far. Here we report high-resolution observation that a unusual butterfly pitch angle distribution of relativistic electrons occurred within 5 Earth radii during the 28 June 2013 geomagnetic storm. Simulation results show that combined acceleration by chorus and magnetosonic waves can successfully explain the electron flux evolution both in the energy and butterfly pitch angle distribution. The current provides a great support for the mechanism of wave-driven butterfly distribution of relativistic electrons. Relativistic electrons trapped in the Van Allen radiation belts sometimes exhibit a minimum of their pitch angle distribution at 90°. Here, the authors explain the origin of this phenomenon in terms of chorus and magnetosonic waves through simulations and observations of a geomagnetic storm data.

Space scientist who took humanity on a tour of the Solar System and beyond, in a journey of discovery like no other. Space scientist who took humanity on a tour of the Solar System and beyond, in a journey of discovery like no other.

The multi-million degree temperature increase from the middle to the upper solar atmosphere is one of the most fascinating puzzles in plasma-astrophysics. Although magnetic waves might transport enough energy from the photosphere to heat up the local chromosphere and corona, observationally validating their ubiquity has proved challenging. Here, we show observational evidence that ubiquitous Alfvén pulses are excited by prevalent intensity swirls in the solar photosphere. Correlation analysis between swirls detected at different heights in the solar atmosphere, together with realistic numerical simulations, show that these Alfvén pulses propagate upwards and reach chromospheric layers. We found that Alfvén pulses carry sufficient energy flux (1.9 to 7.7 kW m −2 ) to balance the local upper chromospheric energy losses (~0.1 kW m −2 ) in quiet regions. Whether this wave energy flux is actually dissipated in the chromosphere and can lead to heating that balances the losses is still an open question. Heating of the upper solar atmospheric layers is an open question. Here, the authors show observational evidence that ubiquitous Alfven pulses are excited by prevalent photospheric swirls, which are found to propagate upwards and carry enough energy flux needed to balance the local upper chromospheric energy loss.

Radio and optical spectroscopic observations of a brown dwarf reveal auroral emissions powered by magnetospheric currents, showing that aurorae may be a signature of magnetospheres much larger than those observed in our Solar System. Planet-like aurorae on a dwarf star Radio and optical spectroscopic observations of a dwarf star reveal auroral emissions powered by magnetospheric currents, the first confirmed detection of a planet-like aurora from a body outside our Solar System. The Sun-like coronal activity thought to dominate in main sequence stars is powered by processes that occur in their lower atmospheres, whereas on this dwarf star — at the boundary between stars and brown dwarfs — the aurorae are powered by processes originating much further out in the magnetosphere and coupled to the lower atmosphere. Aurorae are detected from all the magnetized planets in our Solar System, including Earth 1 . They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons 2 , as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere 3 . Here we report simultaneous radio and optical spectroscopic observations of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs.

Very-Low-Frequency (VLF) transmitters operate worldwide mostly at frequencies of 10–30 kilohertz for submarine communications. While it has been of intense scientific interest and practical importance to understand whether VLF transmitters can affect the natural environment of charged energetic particles, for decades there remained little direct observational evidence that revealed the effects of these VLF transmitters in geospace. Here we report a radially bifurcated electron belt formation at energies of tens of kiloelectron volts (keV) at altitudes of ~0.8–1.5 Earth radii on timescales over 10 days. Using Fokker-Planck diffusion simulations, we provide quantitative evidence that VLF transmitter emissions that leak from the Earth-ionosphere waveguide are primarily responsible for bifurcating the energetic electron belt, which typically exhibits a single-peak radial structure in near-Earth space. Since energetic electrons pose a potential danger to satellite operations, our findings demonstrate the feasibility of mitigation of natural particle radiation environment. Very-Low-Frequency (VLF) communication transmitters, operate worldwide, radiate emissions at particular frequencies 10-30 kHz. Here, the authors show VLF transmitter emissions that leak from the Earth’s ground are primarily responsible for bifurcating the energetic electron belt over 20–100 keV.

Coronal mass ejections (CMEs) are episodic eruptions of solar plasma and magnetic flux that travel out through the solar system, driving extreme space weather. Interpretation of CME observations and their interaction with the solar wind typically assumes CMEs are coherent, almost solid-like objects. We show that supersonic radial propagation of CMEs away from the Sun results in geometric expansion of CME plasma parcels at a speed faster than the local wave speed. Thus information cannot propagate across the CME. Comparing our results with observed properties of over 400 CMEs, we show that CMEs cease to be coherent magnetohydrodynamic structures within 0.3 AU of the Sun. This suggests Earth-directed CMEs are less like billiard balls and more like dust clouds, with apparent coherence only due to similar initial conditions and quasi homogeneity of the medium through which they travel. The incoherence of CMEs suggests interpretation of CME observations requires accurate reconstruction of the ambient solar wind with which they interact, and that simple assumptions about the shape of the CMEs are likely to be invalid when significant spatial/temporal gradients in ambient solar wind conditions are present.

Space storms 1 are the dominant contributor to space weather. During storms, rearrangement of the solar wind and Earth’s magnetic field lines at the dayside enhances global plasma circulation in the magnetosphere 2 , 3 . As this circulation proceeds, energy is dissipated into heat in the ionosphere and near-Earth space. As Earth’s dayside magnetic flux is eroded during this process, magnetotail reconnection must occur to replenish it. However, whether dissipation is powered by magnetotail (nightside) reconnection, as in storms’ weaker but more commonplace relatives, substorms 4 , 5 , or by enhanced global plasma circulation driven by dayside reconnection is unknown. Here we show that magnetotail reconnection near geosynchronous orbit powered an intense storm. Near-Earth reconnection at geocentric distances of ~6.6–10 Earth radii—probably driven by the enhanced solar wind dynamic pressure and southward magnetic field—is observed from multi-satellite data. In this region, magnetic reconnection was expected to be suppressed by Earth’s strong dipole field. Revealing the physical processes that power storms and the solar wind conditions responsible for them opens a new window into our understanding of space storms. It encourages future exploration of the storm-time equatorial near-Earth magnetotail to refine storm driver models and accelerate progress towards space weather prediction. Magnetic reconnection in the near-Earth magnetotail is observed to power a space storm, although suppression of magnetic reconnection caused by the Earth’s magnetic dipole was expected close to Earth.