Revealing the Magnetic Structure of the Solar Corona and Inner Heliosphere in the Era of Parker Solar Probe
The Sun’s atmosphere is a complex and dynamic magnetized plasma and extends all theway from its visible surface out into interplanetary space, carving out a bubble in the inter- stellar medium which is called the heliosphere. All interactions between the Sun and life on Earth are channelled through this medium. Of particular importance to making Sun-Earth connections are the regions called the corona and the inner heliosphere. These two regimes are strongly coupled together but their mutual boundary may be regarded as the location where the dynamic pressure of the outflowing solar wind overcomes the magnetic pressure of the Sun’s intrinsic field. By inner heliosphere, we focus on the portion of the Sun’s sphere of influence which extends out to 1 au and therefore is most relevant to the Earth and humanity.
Our most complete understanding of the corona and heliosphere comes from large scalephysical models which can fill in information about a plasma on a 3D grid. In 2018, Parker Solar Probe (PSP) was launched into an orbit taking it closer to the Sun than any human- made object in history. This has presented an opportunity to directly probe regions of the heliosphere which had hitherto could only be accessed with global modelling. In this body of work we use new data from PSP to improve our knowledge and understanding of this global structure and further derive novel constraints on plasma models of the corona and heliosphere.
Specifically, we first introduce a framework for evaluating models of the coronal magneticfield, which sets how the solar wind emerges and shapes the inner heliosphere. In addition to new PSP data which provides direct boundary conditions on the magnetic skeleton of the corona, we show how it is important to make use of pre-existing observational capabilities to constrain the sizes of coronal holes and the locations of high plasma density indicating the topology of the coronal streamer belt. We illustrate how models must be constrained at multiple boundaries to give an accurate representation and that focusing on individual specific metrics can lead to different conclusions about optimum model parameters.
Next, we use the full data set of the heliospheric magnetic field taken by Parker Solar Probein its first four years on orbit to directly measure the heliospheric magnetic field down to 0.13 au and compare directly to the large scale expectations of the Parker magnetic field. We present evidence that at 0.13 au the heliospheric magnetic field remains latitudinally isotropic, indicating the coronal field has already relaxed to this state within this radius. We measure the open magnetic flux and confirm it is conserved between 1 au and PSP’s closest approach to date. This conservation implies a deficit in open magnetic flux according to coronal models with typically accepted model parameters. We also compare the mean direction of the heliospheric magnetic field to the expectation of the Parker spiral model, finding very good agreement which is tending to improve with closing distance from the sun as the ratio of average field strength to random fluctuations increases.
Third, we present a study in which we determine Parker Solar Probe’s magnetic connectivityback to specific coronal sources for its first solar encounter. This exercise allows determi- nation of specific locations on the Sun which emit solar wind plasma later measured by PSP, and therefore contextualises its measurements. This application of combining coronal modelling and PSP data shows how making these connections is a vital building block for understanding other peculiar plasma physics observed as PSP as it has explored new re- gions of the inner heliosphere. Further, it allows disambiguation of spatial and temporal phenomena.
Finally, we present recent work using observations by Parker Solar Probe and other 1 auspacecraft to localise type III radio bursts, an impulsive solar ejection of electron beams, from emission at the solar surface out into the inner heliosphere. These events have the potential to act as passive tracers of coronal and heliospheric structure. We comment on the future prospects of using this localisation to constrain magnetic connectivity and density structure.
We close with a summary of these results and the outlook for further improvement of ourunderstanding of the coupled corona and inner heliosphere ans PSP continues to approach the Sun and as other advances in space based instrumentation are made, such as the gradual escape of the Solar Orbiter to higher latitudes.
The individual investigations, which are briefly introduced above, are united in highlightingseveral specific advances in our understanding of the Sun’s atmosphere facilitated by the ad- dition of Parker Solar Probe to humanity’s suite of heliospheric instrumentation. Specifically, we exemplify how multi-point, multi-spacecraft and multi-messenger observations at differ- ent heliographic locations are vital in making progress in constraining our physical models; using just one vantage point or one physical observable can lead to false conclusions about model optimisation. We also observe an underlying thread of the surprising utility of the very simplest model representations of the corona and heliosphere, for example a current- free corona and essentially hydrodynamic heliosphere can accurately predict the magnetic polarity structure, and even the velocity stream structure measured in situ by PSP. Lastly, we verify that as one would expect from sending an instrument to never-before explored regions of interplanetary space, new gaps in our understanding are identified. For example, confirming that coronal models do not open enough magnetic flux to the inner heliosphere, or showing at several points that while we make substantial progress exploring closer to the Sun, a lack of far-side and high latitude remote sensing (most critically of the photospheric magnetic field), remains a big limitation to accurately reproducing the physical structure of the heliosphere.