172 research outputs found
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The role of empirical space-weather models (in a world of physics-based numerical simulations)
Advanced forecasting of space weather requires prediction of near-Earth solar-wind conditions on the basis of remote solar observations. This is typically achieved using numerical magnetohydrodynamic models initiated by photospheric magnetic field observations. The accuracy of such forecasts is being continually improved through better numerics, better determination of the boundary conditions and better representation of the underlying physical processes. Thus it is not unreasonable to conclude that simple, empirical solar-wind forecasts have been rendered obsolete. However, empirical models arguably have more to contribute now than ever before. In addition to providing quick, cheap, independent forecasts, simple empirical models aid in numerical model validation and verification, and add value to numerical model forecasts through parameterization, uncertainty estimation and âdownscalingâ of sub-grid processes
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Time-window approaches to space-weather forecast metrics: a solar wind case study
Metrics are an objective, quantitative assessment of forecast (or model) agreement with observations. They are essential for assessing forecast accuracy and reliability, and consequently act as a diagnostic for forecast development. Partly as a result of limited spatial sampling of observations, much of spaceâweather forecasting is focused on the time domain, rather than inherent spatial variability. Thus metrics are primarily âpointâbyâpointâ approaches, in which observed conditions at time t are compared directly (and only) with the forecast conditions at time t. Such metrics are undoubtedly useful. But in lacking an explicit consideration of timing uncertainties, they have limitations as diagnostic tools and can, under certain conditions, be misleading. Using a nearâEarth solar wind speed forecast as an illustrative example, this study briefly reviews the most commonlyâused pointâbyâpoint metrics and advocates for complementary âtime windowâ approaches. In particular, a scaleâselective approach, originally developed in numerical weather prediction for validation of spatially patchy rainfall forecasts, is adapted to the time domain for spaceâweather purposes. This simple approach readily determines the time scales over which a forecast is and isnât valuable, allowing the results of pointâbyâpoint metrics to be put in greater context
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Radial evolution of sunward strahl electrons in the inner heliosphere
The heliospheric magnetic field (HMF) exhibits local inversions, in which the field apparently âbends backâ upon itself. Candidate mechanisms to produce these inversions include various configurations of upstream interchange reconnection; either in the heliosphere, or in the corona where the solar wind is formed. Explaining the source of these inversions, and how they evolve in time and space, is thus an important step towards explaining the origins of the solar wind. Inverted heliospheric magnetic field lines can be identified by the anomalous sunward (i.e. inward) streaming of the typically anti-sunward propagating, field aligned (or anti-aligned), beam of electrons known as the âstrahlâ. We test if the pitch angle distribution (PAD) properties of sunward-propagating strahl are different from those of outward strahl.We perform a statistical study of strahl observed by the Helios spacecraft, over heliocentric distances spanning â 0.3 â 1 AU. We find that sunward strahl PADs are broader and less intense than their outward directed counterparts; particularly at distances 0.3 â 0.75 AU. This is consistent with sunward strahl being subject to additional, path-length dependent, scattering in comparison to outward strahl.We conclude that the longer and more variable path from the Sun to the spacecraft, along inverted magnetic field, leads to this additional scattering. The results also suggest that the relative importance of scattering along this additional path length drops off with heliocentric distance. These results can be explained by a relatively simple, constant-rate, scattering process
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Quantifying the latitudinal representivity of in situ solar wind observations
Advanced space-weather forecasting relies on the ability to accurately predict near-Earth solar wind conditions. For this purpose, physics-based, global numerical models of the solar wind are initialized with photospheric magnetic field and coronagraph observations, but no further observation constraints are imposed between the upper corona and Earth orbit. Data assimilation (DA) of the available in situ solar wind observations into the models could potentially provide additional constraints, improving solar wind reconstructions, and forecasts. However, in order to effectively combine the model and observations, it is necessary to quantify the error introduced by assuming point measurements are representative of the model state. In particular, the range of heliographic latitudes over which in situ solar wind speed measurements are representative is of primary importance, but particularly difficult to assess from observations alone. In this study we use 40+ years of observation-driven solar wind model results to assess two related properties: the latitudinal representivity error introduced by assuming the solar wind speed measured at a given latitude is the same as that at the heliographic equator, and the range of latitudes over which a solar wind measurement should influence the model state, referred to as the observational localisation. These values are quantified for future use in solar wind DA schemes as a function of solar cycle phase, measurement latitude, and error tolerance. In general, we find that in situ solar wind speed measurements near the ecliptic plane at solar minimum are extremely localised, being similar over only 1° or 2° of latitude. In the uniform polar fast wind above approximately 40° latitude at solar minimum, the latitudinal representivity error drops. At solar maximum, the increased variability of the solar wind speed at high latitudes means that the latitudinal representivity error increases at the poles, though becomes greater in the ecliptic, as long as moderate speed errors can be tolerated. The heliospheric magnetic field and solar wind density and temperature show very similar behaviour
Tests of sunspot number sequences: 4. Discontinuities around 1946 in various sunspot number and sunspot group number reconstructions
We use five test data series to search for, and quantify, putative discontinuities around 1946 in five different annual-mean sunspot-number or sunspot-group number data sequences. The data series tested are: the original and new versions of the Wolf/Zurich/International sunspot number composite [RISNv1 and RISNv2] (respectively Clette et al., Adv. Space Res., 40, 919, 2007 and Clette et al., in âThe Solar Activity Cycleâ, 35, Springer, 2015); the corrected version of RISNv1 proposed by Lockwood, Owens, and Barnard (J. Geophys. Res. Space Physics, 119, 5193, 2014a) [RC]; the new âbackboneâ group number composite proposed by Svalgaard and Schatten (Solar Physics, 2016) [RBB]; and the new group-number composite derived by Usoskin et al. (Solar Physics, 2016) [RUEA]. The test data series used are: the group number [NG] and total sunspot area [AG] from the Royal Observatory, Greenwich / Royal Greenwich Observatory (RGO) photoheliographic data; the Ca K index from the recent re-analysis of Mount Wilson Observatory (MWO) spectroheliograms in the Calcium II K ion line; the sunspot-group number from the MWO sunspot drawings [NMWO]; and the dayside ionospheric F2-region critical frequencies measured by the Slough ionosonde [foF2]. These test data all vary in close association with sunspot numbers, in some cases non-linearly. The tests are carried out using both the âbefore-and-afterâ fit-residual comparison method and the correlation method of Lockwood, Owens, and Barnard, applied to annual mean data for intervals iterated to minimise errors and to eliminate uncertainties associated with the precise date of the putative discontinuity. It is not assumed that the correction required is by a constant factor, nor even linear in sunspot number. It is shown that a non-linear correction is required by RC, RBB, and RISNv1, but not by RISNv2 or RUEA. The five test datasets give very similar results in all cases. By multiplying the probability distribution functions together we obtain the optimum correction for each sunspot dataset that must be applied to pre-discontinuity data to make them consistent with the post-discontinuity data. It is shown that, on average, values for 1932 - 1943 are too small (relative to later values) by about 12.3 % for RISNv1 but are too large for RISNv2 and RBB by 3.8 % and 5.2 %, respectively. The correction that was applied to generate RC from RISNv1 reduces this average factor to 0.5 % but does not remove the non-linear variation with the test data, and other errors remain uncorrected. A valuable test of the procedures used is provided by RUEA, which is identical to the RGO NG values over the interval employed
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Coherence of coronal mass ejections in near-earth space
Interplanetary coronal mass ejections (ICMEs) primarily move radially as they propagate away from the Sun, maintaining approximately constant angular width with respect to the Sun. As ICMEs have typical angular widths of around 60â, plasma elements on opposite flanks of an ICME separate in the non-radial direction at a speed, vG, roughly equal to the ICME radial speed. This rapid expansion is a limiting factor on the propagation of information across an ICME at the local AlfvĂ©n speed, vA. In this study, the 1-AU properties of ICMEs are used to compute two measures of ICME coherence. The first is the angular separation for which vG exceeds the local vA. The second measure is the angular extent over which a wavefront can propagate as an ICME travels from a given heliocentric distance to 1 AU. For both measures, ICMEs containing magnetic clouds show greater coherence than non-cloud ICMEs. However, even for magnetic clouds, information is unable to propagate across the full span of the structure. Thus interactions of ICMEs with other solar wind structures in the heliosphere are likely to lead to localised distortion, rather than solid-body like deflection. For magnetic clouds, the coherence length scale is significantly greater near the centre of the spacecraft encounter than at the leading or trailing edges. This suggests that magnetic clouds may be more coherent, and thus less prone to distortion, along the direction of the magnetic flux-rope axis than in directions perpendicular to the axis
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Galactic cosmic rays in the heliosphere
Simon R Thomas, Mathew J Owens and Mike Lockwood discuss how neutron monitor counts can help map space weather. This won the 2014 Rishbeth Prize for the best student talk at the Hot Spring MIST Meeting in Bath, April 2014
The evolution of inverted magnetic fields through the inner heliosphere
Local inversions are often observed in the heliospheric magnetic field (HMF), but their origins and evolution are not yet fully understood.Parker Solar Probe has recently observed rapid, AlfvĂ©nic, HMF inversions in the inner heliosphere, known as âswitchbacksâ, which have been interpreted as the possible remnants of coronal jets. It has also been suggested that inverted HMF may be produced by near-Sun interchange reconnection; a key process in mechanisms proposed for slow solar wind release. These cases suggest that the source of inverted HMF is near the Sun, and it follows that these inversions would gradually decay and straighten as they propagate out through the heliosphere. Alternatively, HMF inversions could form during solar wind transit, through phenomena such velocity shears, draping over ejecta, or waves and turbulence. Such processes are expected to lead to a qualitatively radial evolution of inverted HMF structures. Using Helios measurements spanning 0.3â1 AU, we examine the occurrence rate of inverted HMF, as well as other magnetic field morphologies, as a function of radial distance r, and find that it continually increases. This trend may be explained by inverted HMF observed between 0.3â1 AU being primarily driven by one or more of the above in-transit processes, rather than created at the Sun. We make suggestions as to the relative importance of these different processes based on the evolution of the magnetic field properties associated with inverted HMF. We also explore alternative explanations outside of our suggested driving processes which may lead to the observed trend
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Long-term variations in the heliosphere
Reconstructions of long-term solar variability underpin our understanding of the solar dynamo, potential tropospheric climate implications and future space weather scenarios. Prior to direct spacecraft measurements of the heliospheric magnetic field (HMF) and solar wind, accurate annual reconstructions are possible using geomagnetic and sunspot records. On longer timescales, information about the HMF can be extracted from cosmogenic radionuclide records, particularly 14C in ancient trees and 10Be in ice sheets. These proxies, and what they reveal about the HMF and solar wind, are briefly reviewed here
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