3D magnetosheath jet properties finally resolved - High resolution simulations and observations

The supersonic solar wind blows through the solar system. Earth is partly protected by the bow shock, but fast plasma flows can still penetrate the magnetosheath.

Such magnetosheath jets (dynamic pressure enhancements) can impact the magnetopause and cause geoeffective disturbances due to their excess energy and momentum. However, it is not known how strong such space weather disturbances can be and if they may be harmful.

To fill in the gap, we must determine (a) the impact rates of jets, and (b) how much energy and momentum the jets transfer. For (a) we must learn more about jet generation and propagation and for (b) also more about the 3D jet structure. So far, jets have been studied using spacecraft data and simplified, e.g. 2D, simulations to reduce computing costs.

However, spacecraft only give a limited "1D snapshot´´ view, and the various jet selection criteria give ambiguous results. Moreover, 2D simulations are not enough since jets obviously exist in 3D space.

There is a clear need for 3D kinetic simulations since impacting jets is a fundamental 3D problem, which 2D simulations cannot address, and since the physics of the bow shock and magnetosheath is inherently kinetic.The aim of the project is to significantly improve our understanding of the geoeffectiveness of jets through their 3D properties.

We will compare data from 3D simulations with data from spacecraft developed by ESA and NASA.

We will use the state-of-the-art 3D hybrid-kinetic (kinetic ions, fluid electrons) Amitis simulation code that runs on multiple GPUs (Graphics Processing Units). It has been developed by a team-member.

Amitis resolves the 3D physics at relatively low computing costs thanks to GPU technology, and it is a strong competitor to other simulation codes, e.g. the Vlasiator from which no 3D jet studies yet have been published. We have selected a team of experts on jets and high-performance simulations.

Together with a 3-year postdoc, we will address the following intertwined key questions:Q1: What is the 3D structure of jets? – Here we primarily approach (b) above: How are jets distributed in 3D? Are jets multi- or single channel flows? Are the channels curved? How do the magnetic field, density, velocity, and dynamic pressure vary in the jets?

How is the excess energy and momentum affected?Q2: How do jets propagate in 3D? – Here we approach both (a) and (b): Do jets meander, spread out, or split up when propagating? How much do jets decelerate and why? How much do jets deviate in 3D space around the magnetosphere?

How does this affect jet impact rates and the energy and momentum transfer?Q3: How are jets generated? – Here we primarily approach (a): Can jets be efficiently generated both at the quasi-parallel and quasi-perpendicular bow shock? Do different jet properties correspond to different generation mechanisms?

How does this affect jet impact rates?Using results from Q1-3 above, we will obtain better estimates of jet impact rates, and the energy and moment transfer, as well as discuss the geoeffectiveness of jets.

We will also answer a spin-off question: (Q4) How can we develop better criteria for selecting jets from observed spacecraft data?

For example: how can we separate the jet boundary from the main channel, and how can we distinguish multiple jets from a single bursty jet?It is only now that the project is feasible through the use of the recently developed advanced GPU technology.

There are many obvious and approachable key questions about jets to resolve when we for the first time compare observed spacecraft data with high-resolution 3D kinetic simulations, i.e., there are many “low hanging fruits” in our project.

We consider the risk of project failure to be minimal, but the scientific return is large in relation to the required funding.