The X-class solar flares that occurred consecutively in May 2024

On the night of Saturday, May 11, 2024, and into the early hours of Sunday, May 12, a rare phenomenon of auroras was observed across various parts of Japan. This series of events was caused by solar flares, which are explosive phenomena occurring continuously on the surface of the sun. The sun has an 11-year activity cycle, and it is currently increasing in activity as it approaches its maximum. The National Astronomical Observatory of Japan's Solar Observation Science Project continuously monitors the sun using the Hinode solar observation satellite, the Mitaka Solar Flare Telescope, and the Nobeyama Radio Polarimeters. This time, they were able to observe four X-class solar flares*1 that occurred on May 3, 5, 8, and 11.

Figure 1 is an image taken by the solar flare telescope at the Mitaka, showing the full disk continuum images from May 2nd to May 11th, 2024, overlaid with the Hα line images focusing on the sunspot groups. This image shows how the sunspot groups rapidly developed over several days while changing positions along with the sun's rotation. NOAA13663 and NOAA13664 are the activity region numbers assigned by the National Oceanic and Atmospheric Administration (NOAA).

Figures 2 and 3 show two sunspot groups captured by Solar Optical Telescope(SOT) onboard the solar observation satellite "Hinode" on May 5th and May 11th, respectively. (a) shows visible light images, while (b) shows magnetic field images, with the N-pole shown in white and the S-pole in black. The images reveal complex magnetic structures with intertwined N and S poles. The times in the figure are indicated in Coordinated Universal Time (UT).*2

Figure 4 shows an image captured by the X-ray telescope (XRT) onboard the Solar Observation Satellite 'Hinode,' depicting the X5.8 flare that occurred on May 11th. Due to the significant difference in brightness with the surrounding area, the flare region appears as a bright white spot.

During the X5.8 flare on the 11th, a Moreton wave was captured. This phenomenon occurs when the shockwave from the flare propagates, causing vertical motion in the chromosphere. Figure 5 shows a series of Hα line images captured by the solar flare telescope, along with the derived velocity field images over time. Red indicates motion away from the observer, while blue indicates motion towards the observer. As indicated by the arrows, the wave propagates at extremely high speeds.

Figure 6 is a graph showing changes in solar microwave flux captured by the Nobeyama Radio polarimeters. Solar flares can be accompanied by strong electromagnetic emissions called microwave bursts, which are observed across a wide frequency range. The microwave flux sharply increased immediately after the flare occurred at 01:10 UT.

Figure 1: Full disk continuum images and Hα line images from May 2nd to May 11th, 2024, captured by the solar flare telescope at the Mitaka (excerpt, composite of only the two regions that produced X-class flares). (c)NAOJ

Figure 4: The appearance of an X5.8 class flare that occurred on May 11th, captured by the X-ray telescope (XRT) onboard the Solar Observation Satellite "Hinode. (c)NAOJ/JAXA/MSU

Figure 5: Moreton wave associated with the X5.8 flare on May 11th. The left shows the Hα line image, and the right shows the velocity field image (blue: towards the observer, red: away from the observer). The arrows indicate the position of the propagating wave. (c)NAOJ

Figure 6: Radio intensity graph from May 11th captured by Nobeyama Radio Polarimeters. The graph shows simultaneous observations in six different frequency bands. The values sharply increase around 01:10UT. (c)NAOJ


*1 Solar flares are classified into four classes, B, C, M, and X, based on the strength of the observed X-rays. There is a tenfold difference between M1.0 and X1.0.

*2 Universal Time (UT) is a time system based on mean solar time at the Prime Meridian. 0:00UT corresponds to 9:00 Japan Standard Time(JST).

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