Energy conversion at the Earth's magnetopause using single and multispacecraft methods

We present a small statistical data set, where we investigate energy conversion at the magnetopause using Cluster measurements of magnetopause crossings. The Cluster observations of magnetic field, plasma velocity, current density and magnetopause orientation are needed to infer the energy conversion at the magnetopause. These parameters can be inferred either from accurate multispacecraft methods, or by using single-spacecraft methods. Our final aim is a large statistical study, for which only single-spacecraft methods can be applied. The Cluster mission provides an opportunity to examine and validate single-spacecraft methods against the multispacecraft methods. For single-spacecraft methods, we use the Generic Residue Analysis (GRA) and a standard one-dimensional current density method using magnetic field measurements. For multispacecraft methods, we use triangulation (Constant Velocity Approach - CVA) and the curlometer technique. We find that in some cases the single-spacecraft methods yield a different sign for the energy conversion than compared to the multispacecraft methods. These sign ambiguities arise from the orientation of the magnetopause, choosing the interval to be analyzed, large normal current and time offset of the current density inferred from the two methods. By using the Finnish Meteorological Institute global MHD simulation GUMICS-4, we are able to determine which sign is likely to be correct, introducing an opportunity to correct the ambiguous energy conversion values. After correcting the few ambiguous cases, we find that the energy conversion estimated from single-spacecraft methods is generally lower by 70% compared to the multispacecraft methods.Peer reviewe

Spatial dependence of magnetopause energy transfer : Cluster measurements verifying global simulations

Peer reviewe

The angular-momentum flux in the solar wind observed during Solar Orbiter's first orbit

Aims: We present the first measurements of the angular-momentum flux in the solar wind recorded by the Solar Orbiter spacecraft. Our aim is to validate these measurements to support future studies of the Sun’s angular-momentum loss. Methods: We combined 60-min averages of the proton bulk moments and the magnetic field measured by the Solar Wind Analyser (SWA) and the magnetometer (MAG) onboard Solar Orbiter. We calculated the angular-momentum flux per solid-angle element using data from the first orbit of the mission’s cruise phase in 2020. We separated the contributions from protons and from magnetic stresses to the total angular-momentum flux. Results: The angular-momentum flux varies significantly over time. The particle contribution typically dominates over the magneticfield contribution during our measurement interval. The total angular-momentum flux shows the largest variation and is typically anticorrelated with the radial solar-wind speed. We identify a compression region, potentially associated with a co-rotating interaction region or a coronal mass ejection, which leads to a significant localised increase in the angular-momentum flux, albeit without a significant increase in the angular momentum per unit mass. We repeated our analysis using the density estimate from the Radio and Plasma Waves (RPW) instrument. Using this independent method, we find a decrease in the peaks of positive angular-momentum flux, but otherwise, our results remain consistent. Conclusions: Our results largely agree with previous measurements of the solar wind’s angular-momentum flux in terms of amplitude, variability, and dependence on radial solar-wind bulk speed. Our analysis highlights the potential for more detailed future studies of the solar wind’s angular momentum and its other large-scale properties with data from Solar Orbiter. We emphasise the need for studying the radial evolution and latitudinal dependence of the angular-momentum flux in combination with data from Parker Solar Probe and other assets at heliocentric distances of 1 au and beyond

Solar Orbiter observations of the structure of reconnection outflow layers in the solar wind

We briefly review an existing model of the structure of reconnection layers which predicts that several more distinct layers, in the form of contact discontinuities, rotational Alfvèn waves, or slow shocks, should be identifiable in solar wind reconnection events than are typically reported in studies of reconnection outflows associated with bifurcated current sheets. We re-examine this notion and recast the identification of such layers in terms of the changes associated with the boundaries of both the ion and electron outflows from the reconnection current layers. We then present a case study using Solar Orbiter MAG and SWA data, which provides evidence consistent with this picture of extended multiple layers around the bifurcated current sheet. A full confirmation of this picture requires more detailed examination of the particle distributions in this and other events. However, we believe this concept is a valuable framework for considering the nature of reconnection layers in the solar wind

Coordination of the in situ payload of Solar Orbiter

Solar Orbiter’s in situ coordination working group met frequently during the development of the mission with the goal of ensuring that its in situ payload has the necessary level of coordination to maximise science return. Here we present the results of that work, namely how the design of each of the in situ instruments (EPD, MAG, RPW, SWA) was guided by the need for coordination, the importance of time synchronisation, and how science operations will be conducted in a coordinated way. We discuss the mechanisms by which instrument sampling schemes are aligned such that complementary measurements will be made simultaneously by different instruments, and how burst modes are scheduled to allow a maximum overlap of burst intervals between the four instruments (telemetry constraints mean different instruments can spend different amounts of time in burst mode). We also explain how onboard autonomy, inter-instrument communication, and selective data downlink will be used to maximise the number of transient events that will be studied using high-resolution modes of all the instruments. Finally, we briefly address coordination between Solar Orbiter’s in situ payload and other missions

Generic residue analysis and BV method comparison

International audienceDetermining the orientation of the normal direction to the magnetopause layer is a key issue for studying in detail the structure of this boundary. Both conservation laws methods and the new iterative BV method, that performs a fit of the magnetic field and ion normal flow velocity with an elliptic model, have been developed for this purpose. These methods have different model assumptions and validity ranges. Unlike the conservation laws methods, the BV method also provides spatial profiles inside the layer. However, it is compatible only with a subset of magnetopause crossings with a single layer current sheet. We compare here their results on artificial magnetopause data with noise, to understand their sensibility to small departures from their physical hypothesis. Then we present a statistical study on their comparison on a list of 149 flank and dayside magnetopause crossings

Magnetopause orientation: Comparison between generic residue analysis and BV method,

International audienceDetermining the direction normal to the magnetopause layer is a key step for any study of this boundary. Various techniques have been developed for this purpose. We focus here on generic residue analysis (GRA) methods, which are based on conservation laws, and the new iterative BV method, where B represents the magnetic field and V refers to the ion velocity. This method relies on a fit of the magnetic field hodogram against a modeled geometrical shape and on the way this hodogram is described in time. These two methods have different underlying model assumptions and validity ranges. We compare here magnetopause normals predicted by BV and GRA methods to better understand the sensitivity of each method on small departures from its own physical hypotheses. This comparison is carried out first on artificial data with magnetopause-like noise. Then a statistical study is carried out using a list of 149 flank and dayside magnetopause crossings from Cluster data where the BV method is applicable, i.e., where the magnetopause involves a single-layer current sheet, with a crudely C-shaped magnetic hodogram. These two comparisons validate the quality of the BV method for all these cases where it is applicable. The method provides quite reliable normal directions in all these cases, even when the boundary is moving with a varying velocity, which distorts noticeably the results of most of the other methods

Energy conversion at the Earth's magnetopause using single and multispacecraft methods

Anekallu, C.R.; Palmroth, M.; Pulkkinen, Tuija I.; Haaland, S.E.; Lucek, E.; Dandouras, I. [1] We present a small statistical data set, where we investigate energy conversion at the magnetopause using Cluster measurements of magnetopause crossings. The Cluster observations of magnetic field, plasma velocity, current density and magnetopause orientation are needed to infer the energy conversion at the magnetopause. These parameters can be inferred either from accurate multispacecraft methods, or by using single-spacecraft methods. Our final aim is a large statistical study, for which only single-spacecraft methods can be applied. The Cluster mission provides an opportunity to examine and validate single-spacecraft methods against the multispacecraft methods. For single-spacecraft methods, we use the Generic Residue Analysis (GRA) and a standard one-dimensional current density method using magnetic field measurements. For multispacecraft methods, we use triangulation (Constant Velocity Approach -CVA) and the curlometer technique. We find that in some cases the single-spacecraft methods yield a different sign for the energy conversion than compared to the multispacecraft methods. These sign ambiguities arise from the orientation of the magnetopause, choosing the interval to be analyzed, large normal current and time offset of the current density inferred from the two methods. By using the Finnish Meteorological Institute global MHD simulation GUMICS-4, we are able to determine which sign is likely to be correct, introducing an opportunity to correct the ambiguous energy conversion values. Energy conversion at the Earth's magnetopause using single and multispacecraft methods After correcting the few ambiguous cases, we find that the energy conversion estimated from single-spacecraft methods is generally lower by 70% compared to the multispacecraft methods