Sounding Rocket CLASP2 Elucidates Solar Magnetic Field up to the Base of the Corona

Feb. 20, 2021

National Institutes of Natural Science, National Astronomical Observatory of Japan (NAOJ)
Instituto de Astrofísica de Canarias (IAC)
National Aeronautics and Space Administration / Marshall Space Flight Center (NASA/MSFC)
Institut D'Astrophysique Spatiale (IAS)

Abstract

An international research team, for the first time, revealed the magnetic structure from the solar surface to the base of the corona, combining the observations of the sounding rocket experiment CLASP2 with that of the Hinode satellite. The CLASP2 instrument was developed by the joint efforts of Japan, the US, and Europe and was launched with the NASA sounding rocket on April 11, 2019. For 6.5 min, it succeeded in observing the polarization in the ultraviolet (UV) originating from the solar chromosphere. This unprecedented observation revealed that confined flux tubes scattered on the solar surface rapidly expanded and merged with each other (Fig. 1). This study provides new insight into solar magnetic fields and opens up a new diagnostic window for chromospheric magnetic fields.

Fig. 1: Measuring the magnetic field strength at four different heights by using data from CLASP2 and Hinode allowed mapping of the spread of magnetic field lines in the plage chromosphere (Credit: NAOJ).

Main Text

The outer solar atmosphere, the chromosphere (10,000 K), and the corona (1 million K), are home to extremely high temperatures compared with the solar surface (6,000 K), and the mechanism to generate such high-temperature atmospheres is not yet understood. Moreover, the plasma density of the chromosphere is higher than that of the corona, suggesting that much more energy is required to maintain the chromosphere than the corona. These are known as "chromospheric and coronal heating problem". Various observational and theoretical investigations have been conducted to address this problem, and the chromosphere, which is the interface between the solar surface and the corona, has been known to play a critical role. However, the magnetic field in the chromosphere, which drives the fundamental processes for heating such as dynamical plasma motions and transfer of energy to the upper layers, has not been extensively measured, and the lack of information has prohibited astronomers from understanding the issue.

New instruments aimed at the measurement of chromospheric magnetic fields are being actively developed worldwide. Motivated by recent theoretical investigations on the magnetic sensitivity of the solar ultraviolet (UV) spectrum, the international CLASP and CLASP2 team focused on the polarization in the strongest UV spectral lines: hydrogen Lyman-α and Mg II. UV polarization has been unexplored for many years since the measurement requires observation from space and the technology needed to precisely measure the polarization in UV light was not yet mature. The team has overcome these difficulties and conducted the sounding rocket experiments CLASP (for the Lyman-α line) in 2015 and CLASP2 (for the Mg II lines) in 2019.

The CLASP2 international research team carried out the investigation published in Science Advances. The investigation has been coordinated by Ryohko Ishikawa (NAOJ) and Javier Trujillo Bueno (Instituto de Astrofísica de Canarias, IAC). During the observing time of 6.5 min, CLASP2 observed an active region for 2.5 min and succeeded in obtaining the polarization spectra around the Mg II lines (wavelength 280 nm) for the first time (Fig. 2). Interestingly, in addition to the Mg II lines, which are the primary spectral lines targeted by CLASP2 (indicated as Mg II k and Mg II h in Fig. 2), two Mn I lines also showed clear circular polarization signals. While the Mg II lines originate from the middle to the top of the chromosphere, the Mn I lines are emitted from the lower chromosphere. By exploiting the polarization signals in these spectral lines, magnetic field measurements at different heights in the chromosphere (i.e., bottom, middle, and top of the chromosphere) are enabled. Moreover, CLASP2 succeeded in coordinated observations with the Hinode satellite, which precisely measured the magnetic field on the solar surface (lower left panel of Fig. 2).

Fig. 2: Summary of data used in this study. CLASP2 obtained the polarization spectra (right two panels) at the location of the slit shown by the green solid line. The solar chromosphere around the CLASP2 slit was monitored by the Slit-Jaw monitor system (SJ) aboard CLASP2 (upper left panel). The detailed magnetic field information on the solar surface was recorded by the Solar Optical Telescope (SOT) aboard the Hinode satellite (lower left panel), where the white and black show the strong magnetic fields with N- and S-polarities. In the background is the full disk image of the solar chromosphere taken by the Solar Dynamics Observatory (SDO) satellite. (Credit: NAOJ, IAC, NASA/MSFC, IAS)

By combining the observations of CLASP2 with that of the Hinode satellite, the magnetic structure from the solar surface to the top of the chromosphere was revealed (Fig. 3). The spatial variation of the magnetic field on the solar surface with large amplitudes (green line in Fig. 3) shows that the confined flux tubes are scattered on the solar surface. On the other hand, the spatial variations of the chromospheric magnetic fields (blue, black, and red circles in Fig. 3) show different behaviors: [1] the field strengths in the chromospheric layers dramatically weaken compared with those on the solar surface, [2] the higher the chromosphere, the weaker the field strength, [3] the chromosphere harbors relatively strong magnetic fields in the location where the magnetic field is weak on the surface (black arrow shows one example). Based on these observational results, it has been revealed that flux tubes rapidly expand in the chromosphere and merge with adjacent flux tubes (Fig. 1). Such a picture had been imagined by astronomers for many years, but the observational evidence had not yet been obtained.

Fig. 3: Mapping of the magnetic fields from the solar surface to the base of the corona (top chromosphere), along the CLASP2 slit (green solid line in Fig. 2) at four different heights of the solar atmosphere. (Credit: NAOJ, IAC, NASA/MSFC, IAS)

Moreover, the team found a high correlation between the energy density (the product of electron density and temperature), which was inferred from the intensity (upper right panel of Fig. 2) and the magnetic fields at the top of the chromosphere (red circles in Fig. 3). This high correlation provides empirical evidence that chromospheric heating is of magnetic origin and that chromospheric magnetic fields, rather than those on the solar surface, are indispensable for understanding the heating mechanism. It is anticipated that the inferred picture of magnetic flux tubes connecting from the solar surface to the top of the chromosphere will allow deciphering how the magnetic field couples the different atmospheric layers, i.e., investigation of how energy is transferred between different layers of the Sun.

The CLASP2 instrument payload landed on the White Sands Desert by a parachute and was transported back to NASA/MSFC. The CLASP2 team plans to fly the instrument again as CLASP2.1. In CLASP2, the spectrograph slit was fixed at one location, and information about the magnetic fields along a single line on the Sun was obtained. In CLASP2.1, the team aims at obtaining the 3D (spatial 2D & height) map of the magnetic fields from the solar surface to the top of the chromosphere by moving the slit. Astronomers all over the world likewise are pursuing chromospheric magnetic field measurements with upcoming new instruments such as the SUNRISE-3 balloon project, the recently-completed DKI Solar Telescope in Hawaii and the future European Solar Telescope (EST) in Canary Islands. Finally, it is critically important to simultaneously observe the heating process in greater detail. The new solar satellite mission, Solar-C Extreme Ultraviolet High-Throughput Spectroscopic Telescope (EUVST), which is being developed by Japan in close collaboration with the US and European partners, will be able to perform spectroscopic observations of the chromosphere to the corona with high spatial and temporal resolutions (though without polarization diagnostics). The astronomers are very excited to tackle the "chromospheric and coronal heating problem" by exploiting various observational facilities and diagnostic methods.

Appendix

（1）Magnetic field measurements and chromospheric magnetic field
Astronomers measure the magnetic field by observing the polarization induced in the spectral lines. The magnetic fields on the solar surface have been widely measured using the Hinode satellite and ground-based telescopes, and their properties have been investigated in greater detail. However, the chromospheric magnetic fields are weaker than those on the solar surface, leading to weaker polarization signals, and the process in which the polarization is emerging is more complicated. Therefore, the detection of polarization due to the chromospheric magnetic field is difficult. Recently, suitable spectral lines and observational techniques to measure chromospheric magnetic fields have been investigated.

（2）CLASP (Chromospheric Lyman-Alpha Spectro-Polarimeter)
Preceding sounding rocket experiment of CLASP2. It was launched in September 2015 and performed polarimetric observations of the hydrogen Lyman-α line at a wavelength of 122 nm originating from the solar chromosphere, demonstrating the feasibility of UV spectropolarimetry. It detected the theoretically predicted scattering polarization and obtained evidence of the Hanle effect (modification of the scattering polarization due to the presence of magnetic fields in the upper chromosphere) for the first time. Moreover, theoretical modeling of the CLASP data revealed that the chromosphere-corona transition region is more corrugated than previously thought. However, CLASP did not measure the circular polarization induced by the Zeeman effect, a useful signal to help determine the full magnetic field vector.

Web Article: https://hinode.nao.ac.jp/en/news/results/170518claspsp/

（3）CLASP2 (Chromospheric LAyer Spectro-Polarimeter)
The development of the CLASP2 instrument was spearheaded by Japan with the US and France, with instrumental specifications based on theoretical investigations led by Spain. After the launch of the CLASP, the NAOJ team brought back the instrument to Japan and refurnished to measure the polarization around the Mg II lines at 280 nm. The development and testing were performed using the facility of the Advanced Technology Center at NAOJ and shipped to the USA. Finally, on April 11, 2019, it was launched from the White Sands Missile Range in New Mexico, USA. The CLASP2 team received the NASA/MSFC group achievement honor award in January 2021.

Web Article (CLASP2 launch): https://hinode.nao.ac.jp/en/news/topics/clasp2-launch-en-20190422/

Web Article (NASA Award): http://hinode.nao.ac.jp/en/news/notice/clasp2-nasaaward2020-en/index.html

（4）Active region
Region occupied by strong magnetic fields consisting of sunspots and plages. CLASP2 observed an active plage region, which is a bright region with strong magnetic fields.

（5）Zeeman effect
Quantum mechanical effect where the spectral lines split owing to the magnetic field. In the case of very strong magnetic fields, such as sunspots on the solar surface, splitting can be easily detected. When the splitting is not as prominent with weaker magnetic fields, polarization emerges depending on the field strength. In the CLASP2 data analysis, the longitudinal components of the magnetic fields are induced from the circular polarization in the spectral lines of the Mg II and Mn I lines.

Video

The launch, observational result, and landing of the CLASP2 instrument are summarized in the following movie.

Movie. The launch, solar observation, and landing of the CLASP2 instrument.

[Paper]

Title: "Mapping Solar Magnetic Fields from the Photosphere to the Base of the Corona"

Authers: Ryohko Ishikawa, Javier Trujillo Bueno, Tanausú del Pino Alemán, Takenori J. Okamoto, David E. McKenzie, Frédéric Auchère, Ryouhei Kano, Donguk Song, Masaki Yoshida, Laurel A. Rachmeler, Ken Kobayashi, Hirohisa Hara, Masahito Kubo, Noriyuki Narukage, Taro Sakao, Toshifumi Shimizu, Yoshinori Suematsu, Christian Bethge, Bart De Pontieu, Alberto Sainz Dalda, Genevieve D. Vigil, Amy Winebarger, Ernest Alsina Ballester, Luca Belluzzi, Jiri Stepan, Andrés Asensio Ramos, Mats Carlsson, Jorrit Leenaarts
Journal: Science Advances 19 Feb 2021: Vol. 7, no. 8, eabe8406 (Published on February 19, 2021)
DOI: 10.1126/sciadv.abe8406

The CLASP2 Principal Investigators are:

David McKenzie (NASA/MSFC, USA)
Ryohko Ishikawa (NAOJ, Japan)
Javier Trujillo Bueno (IAC, Spain)
Frédéric Auchère (IAS, France)

[Acknowledgments for Research Grants]

CLASP2 is an international partnership between NASA/MSFC, NAOJ, JAXA, IAC, and IAS; additional partners include ASCR, IRSOL, LMSAL, and the University of Oslo.

The Japanese contribution to CLASP2 was supported by:

FY2019-2021 JSPS KAKENHI JP19K14771 (for Young Scientists, PI: Ryohko Ishikawa)
FY2019 NAOJ Overseas Visit Program for Young Researchers (Ryohko Ishikawa)
FY2017-2019 JAXA Institute of Space and Astronautical Science (ISAS) Small Mission-of-Opportunity Program
FY2016-2018 JSPS KAKENHI JP16H03963 (for Scientific Research(B), PI: Ryohko Ishikawa)
FY2016 NAOJ Grant for Development Collaboration (PI: Shin-nosuke Ishikawa)
FY2015 ISAS Grant for Promoting International Mission Collaboration (PI: Ryohko Ishikawa)
FY2013-2017 JSPS KAKENHI JP25220703 (for Scientific Research(S), PI: Saku Tsuneta)

The US contribution to CLASP2 was supported by:

NASA Award 16-HTIDS16_2-0027 (PI: David McKenzie)

The contribution of Spain to CLASP2 was supported by:

The European Research Council (ERC) under the European Union's Horizon

2020 research and innovation programme (Advanced Grant agreement No. 742265; PI: Javier Trujillo Bueno).

The contribution of France to CLASP2 was supported by:

CNES funds CLASP2-13616A and 13617A (PI: Frédéric Auchère)

Sounding Rocket CLASP2 Elucidates Solar Magnetic Field up to the Base of the Corona (JAXA | What's New, Feb. 22, 2021)
Sounding Rocket CLASP2 Elucidates Solar Magnetic Field up to the Base of the Corona (ISAS | TOPICS, Feb. 22, 2021)

NASA Missions Make Unprecedented Map of Sun's Magnetic Field (NASA)
The CLASP2 space experiment achieves an unprecedented map of the Sun's magnetic field from the photosphere up to the base of the corona (IAC)

CLASP2 web site: https://solarwww.mtk.nao.ac.jp/Rocket_balloon_experiments/CLASP2/index_e.html

Solar observing satellite Hinode:

https://www.isas.jaxa.jp/en/missions/spacecraft/current/hinode.html (JAXA/ISAS web site)

https://hinode.nao.ac.jp/en/ (NAOJ web site)
Next-generation solar observing satellite Solar-C (EUVST): https://solar-c.nao.ac.jp/en/

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