The Dynamic Time Warping as a Means to Assess Solar Wind Time Series

During the last decades, international attempts have been made to develop realistic space weather prediction tools aiming to forecast the conditions on the Sun and in the interplanetary environment. These efforts have led to the development of appropriate metrics in order to assess the performance of those tools. Metrics are necessary to validate models, compare different models and monitor improvements of a certain model over time. In this work, we introduce the Dynamic Time Warping (DTW) as an alternative way to evaluate the performance of models and, in particular, to quantify differences between observed and modeled solar wind time series. We present the advantages and drawbacks of this method as well as applications to WIND observations and EUHFORIA predictions at Earth. We show that DTW can warp sequences in time, aiming to align them with the minimum cost by using dynamic programming. It can be applied in two ways for the evaluation of modeled solar wind time series. The first, calculates the sequence similarity factor (SSF), a number that provides a quantification of how good the forecast is, compared to an ideal and a non-ideal prediction scenarios. The second way quantifies the time and amplitude differences between the points that are best matched between the two sequences. As a result, DTW can serve as a hybrid metric between continuous measurements (e.g., the correlation coefficient), and point-by-point comparisons. It is a promising technique for the assessment of solar wind profiles providing at once the most complete evaluation portrait of a model.Comment: Accepted for publication in The Astrophysical Journal (ApJ) in January 2022. (Comment: Section 5 has been updated as well as a number of figures, compared to the previous version. None of them affected the final results and conclusions. Also, a number of typos have been corrected

Switchbacks, microstreams, and broadband turbulence in the solar wind

Switchbacks are a striking phenomenon in near-Sun coronal hole flows, but their origins, evolution, and relation to the broadband fluctuations seen farther from the Sun are unclear. We use the near-radial lineup of Solar Orbiter and Parker Solar Probe during September 2020 when both spacecraft were in wind from the Sun's Southern polar coronal hole to investigate if switchback variability is related to large scale properties near 1 au. Using the measured solar wind speed, we map measurements from both spacecraft to the source surface and consider variations with source Carrington longitude. The patch modulation of switchback amplitudes at Parker at 20 solar radii was associated with speed variations similar to microstreams and corresponds to solar longitudinal scales of around 5°–10°. Near 1 au, this speed variation was absent, probably due to interactions between plasma at different speeds during their propagation. The alpha particle fraction, which has recently been shown to have spatial variability correlated with patches at 20 solar radii, varied on a similar scale at 1 au. The switchback modulation scale of 5°–10°, corresponding to a temporal scale of several hours at Orbiter, was present as a variation in the average deflection of the field from the Parker spiral. While limited to only one stream, these results suggest that in coronal hole flows, switchback patches are related to microstreams, perhaps associated with supergranular boundaries or plumes. Patches of switchbacks appear to evolve into large scale fluctuations, which might be one driver of the ubiquitous turbulent fluctuations in the solar wind

Energy conversion through various channels in turbulent plasmas induced by the Kelvin-Helmholtz instability at the Earth’s magnetopause

Energy conversion in collisionless plasmas is central to the plasma heating and particle energization problems in space and astrophysical plasmas, which remain unsolved nowadays. Though it is known that the electromagnetic energy is converted to the flow and random kinetic energy (via J.E), a detailed understanding of how the electromagnetic energy is converted into particle energy and finally dissipated to heat is still lacking. Motivated by the rich physics of Kelvin-Helmholtz (KH) waves, we consider energy conversion in turbulent plasmas induced by the KH instability at the Earth’s magnetopause. With observations from the Magnetospheric Multiscale mission, we consider the energy conversion from (1) the electromagnetic fields into the flow and (2) from the flow into thermal energy for each plasma species through the pressure work (via P.∇.v). We find that the KH vortex regions, where the magnetospheric and magnetosheath plasmas mix, are the key sites of energy conversion activities. Considering the accumulation of the energy conversion through various channels with time, we find that the accumulated energy conversion rate through the electromagnetic channel constantly increases. However, the accumulated energy conversion rate through the pressure work channel only increases when the KH waves reach the nonlinear stage of development. Moreover, while the energy conversion between flow and heat via P.∇.v is very dynamic for electrons, we find that the main contribution, which finally dissipates the flow energy into heat, comes from ions. By separating the contributions of J.E and P.∇.v into multiple terms, we will discuss kinetic processes that are likely responsible for the energy conversion. We will also discuss the partitioning of energy conversion through the different channels for each species. This work paves the way towards an understanding of energy transfer across scales in turbulent plasmas as mediated by magnetopause KH waves

Switchbacks, microstreams, and broadband turbulence in the solar wind

International audienceSwitchbacks are a striking phenomenon in near-Sun coronal hole flows, but their origins, evolution, and relation to the broadband fluctuations seen farther from the Sun are unclear. We use the near-radial lineup of Solar Orbiter and Parker Solar Probe during September 2020 when both spacecraft were in wind from the Sun's Southern polar coronal hole to investigate if switchback variability is related to large scale properties near 1 au. Using the measured solar wind speed, we map measurements from both spacecraft to the source surface and consider variations with source Carrington longitude. The patch modulation of switchback amplitudes at Parker at 20 solar radii was associated with speed variations similar to microstreams and corresponds to solar longitudinal scales of around 5°-10°. Near 1 au, this speed variation was absent, probably due to interactions between plasma at different speeds during their propagation. The alpha particle fraction, which has recently been shown to have spatial variability correlated with patches at 20 solar radii, varied on a similar scale at 1 au. The switchback modulation scale of 5°-10°, corresponding to a temporal scale of several hours at Orbiter, was present as a variation in the average deflection of the field from the Parker spiral. While limited to only one stream, these results suggest that in coronal hole flows, switchback patches are related to microstreams, perhaps associated with supergranular boundaries or plumes. Patches of switchbacks appear to evolve into large scale fluctuations, which might be one driver of the ubiquitous turbulent fluctuations in the solar wind