How are the hot corona and the solar wind created?
When and how do the solar flares occur?

01. Science Aimed at by SOLAR-C

Fig. 1
Fig.1: Our Earth exists in the stream of a high temperature atmosphere (plasma) flowing out from the Sun. The Sun continuously affects the Earth via electromagnetic waves (light) of various wavelengths and the solar wind.

The Earth and the other solar system objects are orbiting in the high temperature medium (plasma) produced by our Sun (Fig.1). Then, how is such a high-temperature plasma created? And how does the Sun affect the Earth and other planets through emitting the plasma? These are the fundamental questions of space science and astronomy, deeply related to the origin of the solar system and the life.

The next-generation solar mission SOLAR-C (Fig.2) will answer these questions using its on-board telescope EUVST (EUV High-throughput Spectroscopic Telescope), which is the solar EUV spectrometer and slit-jaw imaging system with unprecedented sensitivity and spatio-temporal resolution. SOLAR-C will close in on these mysteries from the viewpoints of atmospheric heating and the flare eruptions.

Fig. 2
Fig. 2: SOLAR-C will reveal the mystery of the high-temperature plasma and the connection between the Sun and the Earth through performing spectroscopic observation of the solar atmospheres in detail. (c) NAOJ/JAXA

02. How the Hot Corona and the Solar Wind Are Generated?

Fig. 3
Fig. 3: The structure of the solar atmosphere is connected by magnetic field lines. Corona: the hot upper atmosphere. Transition region: the thin atmospheric layer connecting the chromosphere and the corona. Chromosphere: the atmosphere up to about 2,000 km. Photosphere: the solar surface as seen in visible light. (c) Solar images: NAOJ/JAXA, NASA

Whereas the temperature of the solar surface is about 6,000 Kelvin, the corona in the higher atmosphere is known to reach above one million Kelvin (Fig.3). However, it is not yet fully understood why the atmosphere is heated that much. There are two scenarios to explain the hot corona, which are the "nanoflare hypothesis" and the "wave-heating hypothesis" (Fig.4). The coronal heating problem is crucial because it is closely linked to the acceleration mechanism of the solar wind, which streams out into the interplanetary space.

With previous instruments, it has been difficult to seamlessly trace the transport of material and energy, mostly due to the temperature gaps between individual instruments and the lack of resolution. SOLAR-C will, however, observe the solar atmosphere over a wide range of temperatures, simultaneously without any gaps, with high spatial and temporal resolution. Our instrument will detect very small scale atmospheric heating events (via nanoflares or wave heating) and reveal the contribution of each mechanism and the way the solar wind is produced and leaks out.

Fig. 4
Fig. 4: To explain solar coronal heating, the "nanoflare hypothesis" (left), in which numerous tiny-scale explosions occur due to the magnetic fields in the corona, and the "wave-heating hypothesis" (right), where the energy in the solar surface is transported to the corona by the waves along the magnetic field lines, have been proposed. It has been suggested from the observations that the small-scale mechanisms that are not yet spatially and temporally resolved may contribute to the atmospheric heating. SOLAR-C will observe instances of nanoflares and wave heating and investigate the mystery of the high-temperature plasma. (c) ISAS/JAXA

03. When and How the Solar Flares Occur?

Solar flares are the largest-scale explosive events in our solar system, where the energies of 10,000 to 10,000,000 times of the largest earthquake are released only in tens of minutes to hours. If a solar flare takes place on the Sun and a fast solar wind hits the Earth's magnetic field, a variety of phenomena, including aurora, are observed on the Earth.

Solar flares are considered to be the phenomena in which the magnetic energy accumulated in the solar corona is converted to the heat and kinetic energy of the plasma through the process called "magnetic reconnection" (Fig.5). Then, how is the magnetic reconnection realized that is fast enough to explain the observed flares? How is the magnetic energy stored and suddenly released?

It has been difficult to unveil these processes with the ability of previous telescopes. For instance, the zone of magnetic reconnection is comparatively much darker than the surroundings, and the resolution of the telescopes was not enough. By taking the advantage of spectroscopy with high spatial and temporal resolution, SOLAR-C will reveal how the solar flares occur.

Fig. 5
Fig. 5: Solar flares are thought to occur because the magnetic energy stored in the corona is suddenly released via the process called "magnetic reconnection," whereby a pair of anti-parallel magnetic field lines reconnect with each other and change the topology. To explain the fast magnetic reconnection, several models including the Petschek-type reconnection, where the magnetohydrodynamic shocks support the effective heating, and the plasmoid-induced reconnection, in which a multitude of plasmoids (magnetic islands) enhance the efficiency of reconnection, have been proposed. SOLAR-C will approach the mystery of fast reconnection by observing the reconnection region with high temporal and spatial resolution. (c) NASA, AAS

04. The Three Approaches

The Hinode (SOLAR-B) spacecraft, launched in 2006, equipped with the optical telescope (SOT) and the EUV spectrometer (EIS), greatly advanced the understanding of magnetic activity by detecting magnetohydrodynamic waves and observing reconnection events. On the other hand, it suffered from weaknesses such as missing the seamless temperature coverage, i.e., the seamless coverage of different atmospheric layers (SOT for the photosphere/chromosphere and EIS for the corona), and lacking the common spatial resolutions (0.3 arcsec for SOT; 2 arcsec for EIS). Therefore, it was problematic to tackle the coronal heating mystery, where we need to follow the transport and releasing of material and energy in detail, and the solar flare mystery, where we need to observe the dark reconnection region with high sensitivity with high spatio-temporal resolution.

In order to solve the problems that were not resolved by previous instruments, SOLAR-C will take the following three unique approaches.

  • Seamless temperature coverage from 10,000 Kelvin to 1-15 million Kelvin
  • High spatial (0.4 arcsec) and temporal (1 sec) resolution
  • High dispersion spectroscopy (velocity resolution of 2 km/s)

05. Scientific Outcomes

SOLAR-C will unveil how the high-temperature corona and solar wind are generated as well as when and how the solar flares occur. Pursuing these science targets will ultimately lead to the following understandings (Fig. 6).

  • Revealing the formation mechanism of the solar atmosphere links to the understanding of the formation of stellar atmospheres under different circumstances.
  • Revealing the occurrence mechanisms of solar flares links to the realization of flare forecasting (space-weather forecasting) in the near future.
  • Observing and revealing the solar phenomena that we cannot experiment on the Earth link to the understanding and verification of fundamental processes of plasma physics and atomic physics.
  • Understanding the origin and evolution of the solar corona, the solar wind, and the solar flares links to the understanding of the past solar-terrestrial environments when the life was created on the Earth, and even the mystery why the life exists on the Earth.
  • Performing observations with SOLAR-C links to the acquisition and establishment of the technology of high-spatial resolution observation utilizing the small satellites.
Fig. 6
Fig. 6: (Left) Scientists point out the possibility that 3.5 billion years ago, when life started on Earth, the Sun was darker by about 30% than its current brightness and thus the Earth was too cold to support life (so-called "faint young Sun paradox"). By resolving the physical mechanisms acting on the modern Sun and speculating the solar terrestrial environments when life began on Earth, SOLAR-C will contribute to the solution of this paradox. (Right) Space weather is increasingly important to our highly digitized, modern society. Solar flares and coronal mass ejections can significantly impact critical infrastructure, such as artificial satellites, the power grid, and communication systems. This makes solar flare prediction and space weather forecasting increasingly relevant to our daily lives. SOLAR-C will reveal the physics that drives the flares and contribute to algorithms for predicting flares. (c) JAXA/ISAS