Space weather: the solar perspective

The Sun, as an active star, is the driver of energetic phenomena that structure interplanetary space and affect planetary atmospheres. The effects of Space Weather on Earth and the solar system is of increasing importance as human spaceflight is preparing for lunar and Mars missions. This review is focusing on the solar perspective of the Space Weather relevant phenomena, coronal mass ejections (CMEs), flares, solar energetic particles (SEPs), and solar wind stream interaction regions (SIR). With the advent of the STEREO mission (launched in 2006), literally, new perspectives were provided that enabled for the first time to study coronal structures and the evolution of activity phenomena in three dimensions. New imaging capabilities, covering the entire Sun-Earth distance range, allowed to seamlessly connect CMEs and their interplanetary counterparts measured in-situ (so called ICMEs). This vastly increased our knowledge and understanding of the dynamics of interplanetary space due to solar activity and fostered the development of Space Weather forecasting models. Moreover, we are facing challenging times gathering new data from two extraordinary missions, NASA’s Parker Solar Probe (launched in 2018) and ESA’s Solar Orbiter (launched in 2020), that will in the near future provide more detailed insight into the solar wind evolution and image CMEs from view points never approached before. The current review builds upon the Living Reviews article by Schwenn from 2006, updating on the Space Weather relevant CME-flare-SEP phenomena from the solar perspective, as observed from multiple viewpoints and their concomitant solar surface signatures.

IPSCAT: A Catalogue of Solar Transients Identified through Interplanetary Scintillation Analysis

<p>We present a catalogue, IPSCAT, of the results of Interplanetary Scintillation (IPS) analysis applied to observations that are compiled using data from three European radio networks, EISCAT, MERLIN and LOFAR, during the early science phase of the STEREO mission, from 2007 to 2012. These analyses provide a means to study the solar wind and interplanetary transients, which we complement with observations from the Heliospheric Imagers on-board STEREO. Within the IPS data set we identify transient phenomena, specifically Coronal Mass Ejections (CMEs) and Stream Interaction Regions (SIRs), via both visual inspection and an automatic feature-finding algorithm. We study the effectiveness of the automated detection algorithm and find it to be successful at classifying CMEs, whilst the identification of SIRs is less easily established. A discussion of the statistical properties of IPSCAT is presented, together with a comparison between the IPS and HI results. Finally, we present a case study of successive CMEs within the IPSCAT data set, which were also observed by the HIs on both STEREO spacecraft and analysed using the Stereoscopic Self-Similar Expansion (SSSE) method. This work was carried out as part of the EU FP7 HELCATS (Heliospheric Cataloguing, Analysis and Techniques Service) project (http://www.helcats-fp7.eu/).</p>

Improving CME modelling with data assimilation of Heliospheric Imager observations into the HUXt solar wind numerical model.

<p>Coronal Mass Ejections that impact Earth drive the most severe space weather. To better enable effective space weather mitigation plans, there is much interest in improving the quality of CME arrival time predictions, particularly by quantifying and reducing the prediction uncertainty. A limited set of observatories, challenges in interpreting observation data, and limiting assumptions in CME parameterisations all play important roles in the uncertainty of the predicted CME evolution.</p><p>Data assimilation techniques provide a path for improving the predictive skill, by integrating observations into a modelling framework in a way that returns model states that better reflect the true state of a system. Furthermore, such techniques can self-consistently account for uncertainty in the observations, and uncertainty in the models structure and parameterisations.</p><p>We present some early results from our work to build a particle filter data assimilation scheme around the HUXt solar wind model. Assimilating the time-elongation profiles of CME flanks observed by the Heliospheric Imagers on NASAs STEREO mission, we demonstrate that such methods have good potential to improve modelled CME arrival time predictions. Using a simulation study, we present an estimate of the potential CME arrival time prediction improvements gained by using this particle-filter approach with an L5 Heliospheric Imager.</p>

A Catalogue of Coronal Mass Ejections Observed by the Heliospheric Imagers throughout the STEREO Mission

<p>Understanding the evolution of the solar wind is fundamental to advancing our knowledge of energy and mass transport in the solar system, rendering it crucial to space weather and its prediction. The advent of truly wide-angle heliospheric imaging has revolutionised the study of Coronal Mass Ejections (CMEs) by enabling their direct and continuous observation out to 1 AU and beyond. A catalogue of CMEs has been compiled using data from the Heliospheric Imagers (HIs) on board the two STEREO spacecraft, which began as part of the FP7 HELCATS project. The mission was launched in 2006 and continues to provide data, therefore spanning 13 years, over which more than two-thousand CMEs have been observed using HI. To these CMEs, we apply geometric models that make use of both single-spacecraft and stereoscopic observations in order to determine their kinematic properties. These include CME speed, acceleration, propagation direction and launch time. The resulting kinematic properties and their statistics are discussed in the context of existing CME catalogues produced from coronagraph observations. This is done with emphasis on how the different models we apply influence our results and how these differences evolve over the solar cycle and as the angular separation of the STEREO spacecraft increases throughout the mission.</p>

In situ dust measurements in the solar wind from S/WAVES TDS instrument on STEREO mission

<p>Dust particles represent an important fraction of the matter composing the interplanetary medium. At 1 A.U. dust mass density is comparable to the one of the solar wind. The large number and broad diversity of dust particles detected by the radio instrument on the STEREO satellites recommend this mission for a closer dust investigation. In situ dust measurements are based on the detection of the charges generated by dust impacts, recorded by the S/WAVES instrument near 1 A.U. since the beginning of the STEREO mission. We study the electric signals produced by these impacts, using the waveform sampler data produced by the TDS subsystem of the radio instrument, connected to three monopole antennas. For this study, we concentrate on macroscopic dust particles (~0.1 microns) whose impact generated nearly simultaneous pulses on the antennas. In particular, we present statistics of typical shapes and features of these signals based on the TDS electric potential time-series and compare the data to a theoretical model of how pulses are generated by charge collection.<br>These results will have implications on dust detection from Parker Solar Probe and Solar Orbiter missions.</p>

Effects of neighbouring planets on the formation of resonant dust rings in the inner Solar System

Context. Findings by the Helios and STEREO mission have indicated the presence of a resonant circumsolar ring of dust associated with Venus. Attempts to model this phenomenon as an analogue to the resonant ring of Earth – as a result of migrating dust trapped in external mean-motion resonances (MMRs) – have so far been unable to reproduce the observed dust feature. Other theories of origin have recently been put forward. However, the reason for the low trapping efficiency of Venus’s external MMRs remains unclear. Aims. Here we look into the nature of the dust trapping resonant phenomena that arise from the multi-planet configuration of the inner Solar System, aiming to add to the existent understanding of resonant dust rings in single planet systems. Methods. We numerically modelled resonant dust features associated with the inner planets and specifically looked into the dependency of these structures and the trapping efficiency of particular resonances on the configuration of planets. Results. Besides Mercury showing no resonant interaction with the migrating dust cloud, we find Venus, Earth, and Mars to considerably interfere with each other’s resonances, influencing their ability to form circumsolar rings. We find that the single most important reason for the weakness of Venus’s external MMR ring is the perturbing influence of its outer neighbour – Earth. In addition, we find Mercury and Mars to produce crescent-shaped density features, caused by a directed apsidal precession occurring in particles traversing their orbital region.

Oscillations of cometary tails: a vortex shedding phenomenon?

Context. During their journey to perihelion, comets may appear in the field of view of space-borne optical instruments, showing in some cases a nicely developed plasma tail extending from their coma and exhibiting an oscillatory behaviour. Aims. The oscillations of cometary tails may be explained in terms of vortex shedding because of the interaction of the comet with the solar wind streams. Therefore, it is possible to exploit these oscillations in order to infer the value of the Strouhal number S t, which quantifies the vortex shedding phenomenon, and the physical properties of the local medium. Methods. We used the Heliospheric Imager (HI) data of the Solar TErrestrial Relations Observatory (STEREO) mission to study the oscillations of the tails of comets 2P/Encke and C/2012 S1 (ISON) during their perihelion in Nov 2013. We determined the corresponding Strouhal numbers from the estimates of the halo size, the relative speed of the solar wind flow, and the period of the oscillations. Results. We found that the estimated Strouhal numbers are very small, and the typical value of S t ~ 0.2 would be extrapolated for size of the halo larger than ~106 km. Conclusions. Although the vortex shedding phenomenon has not been unambiguously revealed, the findings suggest that some kind of magnetohydrodynamic (MHD) instability process is responsible for the observed behaviour of cometary tails, which can be exploited for probing the physical conditions of the near-Sun region.

Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission

Circumterrestrial Lyman-α column brightness observations above 3 Earth radii (Re) have been used to derive separate 3-D neutral hydrogen density models of the Earth's exosphere for solar minimum (2008, 2010) and near-solar-maximum (2012) conditions. The data used were measured by Lyman-α detectors (LAD1/2) onboard each of the TWINS satellites from very different orbital positions with respect to the exosphere. Exospheric H atoms resonantly scatter the near-line-center solar Lyman-α flux at 121.6 nm. Assuming optically thin conditions above 3Re along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H-column density. We found significant differences in the density distribution of the terrestrial exosphere under different solar conditions. Under solar maximum conditions we found higher H densities and a larger spatial extension compared to solar minimum. After a continuous, 2-month decrease in (27 day averaged) solar activity, significantly lower densities were found. Differences in shape and orientation of the exosphere under different solar conditions exist. Above 3 Re, independent of solar activity, increased H densities appear on the Earth's nightside shifted towards dawn. With increasing distance (as measured at 8Re) this feature is shifted westward/duskward by between −4 and −5° with respect to midnight. Thus, at larger geocentric distance the exosphere seems to be aligned with the aberrated Earth–solar-wind line, defined by the solar wind velocity and the orbital velocity of the Earth. The results presented in this paper are valid for geocentric distances between 3 and 8Re.