2:15 PM - 2:30 PM
[PEM10-15] Relationship between Evolution of IMF and Solar Magnetic Field in Solar Cycles 21-25
Keywords:Open flux, Solar cycle, Photospheric magnetic field, Interplanetary magnetic field
Space weather is the phenomenon that the solar activity influences the Earth and interplanetary space. To understand the background environment of space weather, it is important to focus on the magnetic field between the solar surface, corona, and interplanetary space as the basic structure of the heliosphere. The global magnetic field structure of the Sun changes over the 11-year solar cycle, and the open magnetic flux extending from the coronal holes produces the interplanetary magnetic field (IMF). During the solar minimum, solar activity is low and a unipolar magnetic field extends from the coronal holes in the polar regions. During the solar maximum, active regions complicate the magnetic field structure, and coronal structures change, affecting the IMF evolution throughout the solar cycle. However, the connection between open flux and the IMF is not fully understood. It is partially known which components of the solar magnetic field produce the IMF evolution, but few studies have linked them over the solar cycle. The open flux extrapolated from the photosphere is underestimated by a factor of 2 to 5 compared to the observed IMF near the Earth (the open flux problem; Linker et al., 2017, Wallace et al., 2019).
In this study, we investigate the IMF evolution over the solar cycle, highlighting common characteristics and unique ones in each solar cycle and quantitatively identifying the factors responsible for these properties.
We analyze IMF strength at 1 AU during Cycles 21-25 using wavelet analysis and long-term trends, and determine the periodic characteristics and evolution. We focus on the evolution related to the solar magnetic field, particularly during Cycles 24-25. We decompose the solar magnetic field into its components (l, m) using spherical harmonic expansion and the solar open flux into footpoint components per 15 degrees in heliographic latitude for each Carrington rotation (CR).
We present the results divided into five periods over the solar cycle. During Period I (CR2108-CR2130; March 2011- November 2012 in Cycle 24), a rising phase, IMF increase, delaying the evolution of sunspot numbers. The footpoint of open flux shifts toward the polar regions, and open flux extends from lower than ±15° latitudes, and IMF is variable. This variation corresponds to that of equatorial dipole and quadrupole fluxes. During Period II (-CR2152; July 2014), the sunspot number peak and the polar field reversal occur. The photospheric high latitude magnetic field ±[60-75°], the open flux from this region, and the axial dipole are almost zero. The open flux from only low latitude in the photosphere produces the IMF, but the closed flux increases in these regions due to sunspots, so IMF is stagnant. During Period III (-CR2157; November 2014), the axial dipole flux rapidly increases due to unipolar regions in |60-75°| latitudes in the photosphere. In Cycle 24, the photospheric magnetic field in±[45-60°] latitude and equatorial dipole flux also rapidly increase at the same period due to AR12192, and finally IMF rapidly increases and peaks. During Period IV (-CR2187; February 2017), a decreasing phase, the components mentioned above, corresponding to the equatorial dipole and quadrupole flux, gradually decrease with the sunspot number decrease. While the axial dipole is stable and the open flux extended from ±[60-75°] latitudes is dominant. During Period V (-CR2250; October 2021), the solar minimum and initial phase of the next solar cycle, the open flux extended mainly from ±[60-75°] latitudes at most 90%. The axial dipole flux is dominant and stable, while the equatorial dipole flux is variable. IMF decreases until the ratio of the open flux from ±[60-75°] latitudes peaks and then increases.
In conclusion, we identified the IMF evolution and the characteristics of the quantitative solar magnetic field for each phase over the solar cycle. This study will contribute to space weather research in terms of the long-term magnetic field evolutions.
In this study, we investigate the IMF evolution over the solar cycle, highlighting common characteristics and unique ones in each solar cycle and quantitatively identifying the factors responsible for these properties.
We analyze IMF strength at 1 AU during Cycles 21-25 using wavelet analysis and long-term trends, and determine the periodic characteristics and evolution. We focus on the evolution related to the solar magnetic field, particularly during Cycles 24-25. We decompose the solar magnetic field into its components (l, m) using spherical harmonic expansion and the solar open flux into footpoint components per 15 degrees in heliographic latitude for each Carrington rotation (CR).
We present the results divided into five periods over the solar cycle. During Period I (CR2108-CR2130; March 2011- November 2012 in Cycle 24), a rising phase, IMF increase, delaying the evolution of sunspot numbers. The footpoint of open flux shifts toward the polar regions, and open flux extends from lower than ±15° latitudes, and IMF is variable. This variation corresponds to that of equatorial dipole and quadrupole fluxes. During Period II (-CR2152; July 2014), the sunspot number peak and the polar field reversal occur. The photospheric high latitude magnetic field ±[60-75°], the open flux from this region, and the axial dipole are almost zero. The open flux from only low latitude in the photosphere produces the IMF, but the closed flux increases in these regions due to sunspots, so IMF is stagnant. During Period III (-CR2157; November 2014), the axial dipole flux rapidly increases due to unipolar regions in |60-75°| latitudes in the photosphere. In Cycle 24, the photospheric magnetic field in±[45-60°] latitude and equatorial dipole flux also rapidly increase at the same period due to AR12192, and finally IMF rapidly increases and peaks. During Period IV (-CR2187; February 2017), a decreasing phase, the components mentioned above, corresponding to the equatorial dipole and quadrupole flux, gradually decrease with the sunspot number decrease. While the axial dipole is stable and the open flux extended from ±[60-75°] latitudes is dominant. During Period V (-CR2250; October 2021), the solar minimum and initial phase of the next solar cycle, the open flux extended mainly from ±[60-75°] latitudes at most 90%. The axial dipole flux is dominant and stable, while the equatorial dipole flux is variable. IMF decreases until the ratio of the open flux from ±[60-75°] latitudes peaks and then increases.
In conclusion, we identified the IMF evolution and the characteristics of the quantitative solar magnetic field for each phase over the solar cycle. This study will contribute to space weather research in terms of the long-term magnetic field evolutions.