A diagnosis of the plasma waves responsible for the explosive energy release of substorm onset

During geomagnetic substorms, stored magnetic and plasma thermal energies are explosively converted into plasma kinetic energy. This rapid reconfiguration of Earth’s nightside magnetosphere is manifest in the ionosphere as an auroral display that fills the sky. Progress in understanding of how substorms are initiated is hindered by a lack of quantitative analysis of the single consistent feature of onset; the rapid brightening and structuring of the most equatorward arc in the ionosphere. Here, we exploit state-of-the-art auroral measurements to construct an observational dispersion relation of waves during substorm onset. Further, we use kinetic theory of high-beta plasma to demonstrate that the shear Alfven wave dispersion relation bears remarkable similarity to the auroral dispersion relation. In contrast to prevailing theories of substorm initiation, we demonstrate that auroral beads seen during the majority of substorm onsets are likely the signature of kinetic Alfven waves driven unstable in the high-beta magnetotail.   Paper: A diagnosis of the plasma waves responsible for the explosive energy release of...
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Nudging solar wind forecasts back towards reality

In order to forecast space weather, it is necessary to accurately model the solar wind, the continually expanding solar atmosphere which fills the solar system. At present, telescopic observations of the Sun's surface are used to provide the starting conditions for computer simulations of the solar wind, which then propagate conditions all the way from the Sun to Earth. But spacecraft also make direct measurements of the solar wind, which provide useful additional information that is not presently used. In this study we use a simple solar wind model to develop a method to routinely "assimilate" spacecraft observations into the model and thus improve space‐weather forecasts. This data assimilation (DA) approach closely follows that of terrestrial weather prediction, where DA has led to increasingly accurate forecasts. We use artificial and real spacecraft observations to test the new solar wind DA method and show that the error in predicting the near‐Earth solar wind can be reduced by around a fifth using...
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Modelling asymmetric current sheets in Earth’s magnetosphere

During a magnetic reconnection event, stored energy that had been bound up in stressed electromagnetic fields is released in the form of heat and the kinetic energy of particles. The NASA MMS mission is currently making diffusion-region measurements of these phenomena in the Earth’s Magnetosphere, with unprecedented levels of accuracy and resolution. Our work presents a theoretical model of a structure in space known as an asymmetric current sheet, such as the MMS mission may encounter in the magnetopause. The model can be implemented into particle-in-cell simulations of reconnection, with which to compare to the results from MMS satellite data. This will help us understand the fundamental physics of asymmetric magnetic reconnection. Exact Vlasov-Maxwell equilibria for asymmetric current sheets O. Allanson, F. Wilson, T. Neukirch, Y.-H. Liu and J.D.B. Hodgson, Geophysical Research Letters, 44, 17, 8685-8695 (2017) DOI: 10.1002/2017GL074168...
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Forecast uncertainty in the near-Earth solar wind conditions

Long lead-time space-weather forecasts require accurate prediction of the solar wind conditions in near-Earth space. The current state-of-the-art involves coupled numerical models initialised using photospheric magnetic field observations. This deterministic approach means there is no estimate of forecast uncertainty. Large ensembles with perturbed boundary conditions aren’t really feasible due to computational expense.  We have developed a method for producing a large ensemble of near-Earth solar wind conditions using the numerical model output with a simple 1-dimensional solar wind model (http://onlinelibrary.wiley.com/doi/10.1002/2017SW001679/full). This approach produces a probabilistic solar wind forecast which accurately captures the forecast uncertainty. ...
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How much magnetic flux does the heliosphere contain?

There is no direct means to measure the amount of magnetic flux dragged out from the corona by the solar wind to form the “open” solar flux (OSF) component which forms the heliosphere.  If we start at the photosphere and work upwards, the estimate of OSF is strongly technique dependent. If we start with in situ spacecraft data, there are ambiguities about the amount of twisting up of magnetic flux, which means that the resulting OSF estimate varies with the time resolution of the data used. In a new study published in JGR (http://onlinelibrary.wiley.com/doi/10.1002/2017JA024631/full), we show how suprathermal electron observations can be used to unambiguously determine the open flux from in situ observations.  There was an approximately factor 2 variation in the flux content of the heliosphere from the solar maximum of cycle 23 to the subsequent solar minimum. ...
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Propagation of information within coronal mass ejections

[This originally appeared as a UKSP science nugget] Coronal mass ejections (CMEs) are huge, episodic eruptions of solar plasma and magnetic field which travel through the solar corona and out into the heliosphere. At Earth, they drive the most severe geomagnetic storms and thus are the primary focus of space-weather forecasting. Using white-light imagers, individual CMEs can be tracked continuously from the low corona, through the solar wind, all the way to Earth [1] and beyond. Such observations show CMEs apparently bouncing off each other [2] and deflecting off other coronal and solar wind structures [3]. Thus it is tempting to think of a CME as a coherent structure; a single – perhaps even quasi-solid — body, playing out a game of solar billiards. Such structural coherence has two physical requirements. Firstly, coherence requires a restoring force which can (at least partially) resist deformation by external factors. E.g., A bubble is a coherent body as surface tension communicates external forces across the entirety...
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Testing the current paradigm for space weather prediction with heliospheric imagers

Predicting the arrival of coronal mass ejections (CMEs) in near-Earth space is a challenging and important problem, as CMEs are the main driver of severe space weather. The Heliospheric Imager (HI) cameras provide observations of the plasma between the Sun and Earth and can be used to track the evolution of CMEs as they flow away from the Sun, toward Earth. Techniques have been developed to use the HI observations to predict the arrival of CMEs in near-Earth space, potentially allowing the HI observations to be used in a space weather forecasting context. We assess how well these methods work for four CMEs observed by the HI cameras. We found that for these four events the techniques that use HI observations do not perform as well as the standard CME forecasting techniques used by the Space Weather Prediction Center. We try to improve the HI-based predictions by using a citizen science approach to develop a better method of tracking the...
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Solar wind variations over the last four centuries

The heliospheric magnetic field (HMF) varies over all observed timescales, from millennia (top), though decades/centuries (middle) to days/hours (bottom). In this study we used sunspot records to reconstruct the global solar wind and heliospheric magnetic field for the first time. We find that during the Maunder minimum, a period of anonymously low sunspot numbers from AD 1650 to 1715, the solar wind speed was approximately half its modern value, with a similar drop in HMF intensity. This is likely to have led to a much larger terrestrial magnetosphere and a much reduced heliosphere.Owens, M. J., M. Lockwood, P. Riley, Global solar wind variations over the last four centuries Scientific Reports, 7, Article number: 41548 (2017) doi:10.1038/srep41548...
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