Solar Orbiter and Parker Solar Probe jointly take a step forward in understanding coronal heating

(Solar Orbiter nugget #20 by Daniele Telloni¹, Marco Romoli², Marco Velli³, Gary P. Zank^4,5, Laxman Adhikari⁴, Cooper Downs⁶, Aleksandr Burtovoi⁷, Roberto Susino¹, Daniele Spadaro⁸, Lingling Zhao⁴, Alessandro Liberatore⁹, Chen Shi³, Yara De Leo^10,11, Stuart D. Bale^12,13, Justin C. Kasper^14,15, and the Metis team​)

Introduction

The Sun is the source of energy that sustains life on Earth. Energy is produced within the Sun’s core by thermonuclear fusion reactions, which cause electromagnetic radiation (light and heat) to be generated. Eventually, this electromagnetic radiation reaches the Sun’s visible surface (called the photosphere) before escaping through the outermost layers of the Sun’s atmosphere (namely the chromosphere, the transition region and finally the corona) into the solar system and beyond. If it is traveling in the correct direction, light reaches the Earth in as little as 8 minutes after it escapes from the photosphere.

The photosphere exists at a temperature of around 6000 degrees. In the 1940s, following correct interpretation of solar spectra it was discovered that the temperature of the corona is instead about 150 times hotter, reaching one million degrees (coronal temperatures can even be an order of magnitude higher during high-energy events such as flares). If the only energy transport processes occurring in the solar atmosphere were thermal in nature, this temperature imbalance would violate the second law of thermodynamics, which states that heat cannot flow from the photosphere to the corona, away from the energy source at the Sun’s core, to raise coronal temperatures above 6000 degrees. Clearly, there must be other physical mechanisms that heat the corona to such high temperatures. But what are they? In this consists of the long-standing problem of coronal heating.

In 1958, Eugene Parker [1] showed mathematically that such a hot corona could not be in hydrostatic equilibrium and was the first to theoretically predict the existence of the solar wind, namely a continuous flow of magnetized plasma expanding from the corona into interplanetary space. In 1962, Marcia Neugebauer [2], based on data from the Mariner 2 spacecraft, provided the first direct measurements of the solar wind and its properties. More than 60 years later, and despite the tremendous progress made from both observational and numerical/modeling perspectives, the mechanisms that heat the coronal plasma and accelerate the solar wind are still heavily debated. It is now well established that the source of energy that powers the solar wind is provided by the magnetic field and that, most likely (but direct evidence is not yet forthcoming!), it is the dissipation of turbulence that transfers the required energy from the magnetic field to the plasma [3,4,5]. However, until now, the rate of coronal heating by turbulence had not yet been directly measured.

Solar Orbiter - Parker Solar Probe synergies

The NASA Parker Solar Probe [6] and ESA/NASA Solar Orbiter [7] missions were launched in 2018 and 2020, respectively, with the goal of, amongst other things, unlocking the mystery related to the so-called “coronal heating problem” and the acceleration of plasma flows. Despite both satellites individually leading to discoveries that are revolutionizing our understanding of the Sun and its environment, it is when they operate in synergy that the most important breakthroughs can be achieved [8,9,10,11,12]. On June 1, 2022, Parker Solar Probe and Solar Orbiter were in a very special orbital configuration (a spacecraft quadrature) that allowed the first empirical measurement of the rate of energy deposition by turbulence in the corona. During its twelfth perihelion passage, Parker Solar Probe was in fact in a very peculiar position, skimming, but not entering, the field of view of the Italian coronagraph Metis aboard Solar Orbiter. 

Movie 1. An animation displaying the spacecraft trajectories and the Metis field of view during the quadrature.

The Metis team realized the potential importance of having Parker Solar Probe in the field of view of Metis. At that point, the Solar Orbiter operations team planned and executed an unprecedented (and not without risk) spacecraft maneuver: they actually rolled the spacecraft 45 degrees from its axis pointing toward the center of the Sun and then moved this pointing to the edge of the Sun to allow Parker Solar Probe to enter the portion of the plane of the sky remotely imaged by Metis. The different spacecraft maneuvers along with the position of Parker Solar Probe in the Metis field of view are sketched in Figure 1. This allowed the first-ever simultaneous measurement of the large-scale configuration of the corona and its microphysical/kinetic properties, ultimately enabling the first empirical estimate in the solar corona of the heating rate due to turbulent fluctuations.

Figure 1.  Schematic Solar Orbiter maneuvers required to have Parker Solar Probe within the Metis field of view.

Estimate of the coronal heating rate

Figure 2 shows the coordinated Solar Orbiter/Metis and Parker Solar Probe observations of the same plasma element. Relying on the basic equations of magnetohydrodynamics and on the joint Solar Orbiter/Metis and Parker Solar Probeobservations, the rate of energy deposition in the corona has then been estimated. It was found that the average turbulent heating rate per unit volume is about 5.5×10^-12 J m^-3 s^-1, while the coronal energy loss is 33% of the total energy flux of the solar wind. What has emerged from the study published in The Astrophysical Journal Letters [13], is that the energy deposited to the coronal flow is sufficient to explain its observed residual acceleration and to maintain the plasma in a non-adiabatic state. Indeed, a collisionless plasma with no additional heating is expected to cool with distance according to a definite scaling law [14]. Since the temperature decreases less rapidly than expected [15], this implies that plasma heating (via turbulence dissipation) occurs both in the extended corona and interplanetary space.

Figure 2.  Left: White-light corona imaged by Metis on 2022 June 1 at 22:40 UT with Parker Solar Probe marked in its plane of the sky as a blue dot; the Parker Solar Probe direction is indicated by a white dashed line. Right: Parker Solar Probe magnetic field and plasma time series (from top to bottom: wind bulk and Alfvén speed, magnetic field intensity and angle with the radial, and proton density and temperature) during the quadrature with Solar Orbiter.

What’s next?

In addition to being a very important result that will help refine turbulence-based modeling of coronal heating and subsequent solar wind acceleration, this is also a virtuous example of synergy between Solar Orbiter and Parker Solar Probe, showing how coordination of the two missions can lead to groundbreaking achievements. New and even more promising orbital configurations that will occur in the coming years are under consideration by the Solar Orbiter and Parker Solar Probe teams, which are already coordinating to exploit them and thus obtain further exciting results.

This study has been published in Daniele Telloni, et al. 2023, ApJL, 955, L4 https://doi.org/10.3847/2041-8213/ace112

Acknowledgements

Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. D.T. was partially supported by the Italian Space Agency (ASI) under contract 2018-30-HH.0. G.P.Z., L.A., and L.-L.Z. acknowledge the partial support of a NASA Parker Solar Probe contract SV4- 84017, an NSF EPSCoR RII-Track-1 Cooperative Agreement OIA-2148653, and a NASA IMAP grant through SUB000313/ 80GSFC19C0027. S.B. and J.K. acknowledge support from NASA contract NNN06AA01C. The Metis program is supported by ASI under contracts to the National Institute for Astrophysics and industrial partners. Metis was built with hardware contributions from Germany (Bundesministerium für Wirtschaft und Energie through theDeutsches Zentrum für Luft- und Raumfahrt e.V.), the Czech Republic (PRODEX), and ESA. The Metis team thanks the former PI, Ester Antonucci, for leading the development of Metis until the final delivery to ESA. D.T. also wishes to thank her for the helpful comments on the paper. The Metis data analyzed in this paper are available from the PI on request. Parker Solar Probe data were downloaded from NASA’s Space Physics Data Facility (https://spdf.gsfc.nasa.gov).

Affiliations

¹ National Institute for Astrophysics, Astrophysical Observatory of Torino, Via Osservatorio 20, I-10025 Pino Torinese, Italy; daniele.telloni@inaf.it

² University of Florence, Department of Physics and Astronomy, Via Giovanni Sansone 1, I-50019 Sesto Fiorentino, Italy

³ Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA

⁴ Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Huntsville, AL 35805, USA

⁵ Department of Space Science, University of Alabama in Huntsville, Huntsville, AL 35805, USA

⁶ Predictive Science Inc., San Diego, CA 92121, USA

⁷ National Institute for Astrophysics, Astrophysical Observatory of Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy

⁸ National Institute for Astrophysics, Astrophysical Observatory of Catania, Via Santa Sofia 78, I-95123 Catania, Italy

⁹ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

¹⁰ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany

¹¹ University of Catania, Department of Physics and Astronomy, Via Santa Sofia 64, I-95123 Catania, Italy

¹² Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA

¹³ Physics Department, University of California, Berkeley, CA 94720, USA

¹⁴ BWX Technologies, Inc., Washington, DC 20002, USA

¹⁵ Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA

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