7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Magnetic Reconnection in the Solar Atmosphere: Future Plans for Solar
Observations
Shinsuke Imada1, International SOLAR-C Team1
Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyoe-mail (speaker):imada@eps.s.u-tokyo.ac.jp

As a fundamental step towards answering how theplasma universe is created and evolves, and how theSun influences the Earth and other planets in oursolar system, the proposed mission is designed tocomprehensively understand how mass and energy
are transferred throughout the solar atmosphere.
Understanding the solar atmosphere, whichconnects to the heliosphere via radiation, the solar
wind and coronal mass ejections, and energetic
particles, is pivotal for establishing the conditions
for life and habitability in the solar system.
SOLAR-C is a mission designed to provide aconclusive answer to the most fundamental question insolar physics: how does the interplay of magnetic fields
and plasma drive solar activity? The most significant
examples of this interplay are atmospheric heating andexplosive energy release, such as flares and coronal massejections (CMEs). Thus, the two primary scienceobjectives for SOLAR-C are:I.
II.

Understand how fundamental processes lead tothe formation of the solar atmosphere and thesolar wind.
Understand how the solar atmosphere becomes
unstable, releasing the energy that drives solar
flares and eruptions.

In order to advance our understanding of the mysterious
Sun, especially of the origin of the hot solar atmosphereand the occurrence of the solar flares, the SOLAR-C
mission concept tackles the scientific objectives bytaking the following unique approaches:
A. To seamlessly observe all the temperature regimesof the atmosphere from the chromosphere to thecorona simultaneously,
B. To resolve elemental structures of the solar
atmosphere and track their changes with sufficientcadence, and,C. To obtain spectroscopic information on dynamics ofelementary processes taking place in the solar
atmosphere.
Magnetic reconnection is one of the fundamental
processes for converting magnetic energy into thethermal and kinetic energy of the plasma. This process
occurs much faster than is predicted by classical theory.
SOLAR-C will observe the dynamics of magneticstructures to understand the mechanisms that lead to fastmagnetic reconnection in partially or fully ionizedplasmas. SOLAR-C will test reconnection models (e.g.,Sweet-Parker, Petschek, or plasmoid-unstable typereconnection) by observing the velocity, temperature,density, ionization rate in and around the shocks, andmagnetic islands of the reconnection region. The highspatial and temporal resolution by spectrometer, assisted
by slit-jaw context imaging of the chromosphere, willallow us to study chromospheric evaporation produced
by reconnection not only at coronal temperatures but also
at low temperatures deep in the transition region andchromosphere. This will determine how much energy isreleased through magnetic reconnection. The observationof magnetic reconnection in the chromosphere by
SOLAR-C will clarify the role of the fast reconnection
process in partially ionized plasma.
To date, many observations have been made on thesolar corona to confirm the presence of high-temperatureand high-speed plasma flows produced by magnetic
reconnection above flare arcades. In this talk, We will
introduce the study on plasma heating considers thetime-dependent ionization process during a large solar
flare on 2017 September 10, observed by Hinode/EUV
Imaging Spectrometer (EIS). The observed Fe XXIV/FeXXIII ratios increase downstream of the reconnection
outflow, and they are consistent with the time-dependent
ionization effect at a constant electron temperature Te =25 MK. Moreover, this study also shows that thenonthermal velocity, which can be related to theturbulent velocity, reduces significantly along thedownstream of the reconnection outflow, even when
considering the time-dependent ionization process[1].
The number of high-temperature lines observed byHinode/EIS is limited, so it is difficult to make asufficient diagnosis of the reconnection region. Recently,the next generation solar observation satellite SOLAR-C
has been discussed intensively. An ultraviolet imaging
spectrometer with dramatically improved spatial andtemporal resolution is planned for this satellite. In theSOLAR-C era, thermal nonequilibrium plasma will beextensively discussed. We expect that SOLAR-C(EUVST) will reveal the reconnection region in detail.
References[1] Imada, S., “Nonequilibrium Ionization Plasma duringa Large Solar Limb Flare Observed by Hinode/EIS”, The
Astrophysical Journal Letters, Volume 914, Issue 2,id.L28