Linear Correlation Between Radial and Normal Component Fluctuations of the Interplanetary Magnetic Field

Abstract The interplanetary magnetic field (IMF), particularly its north–south component, acts as a key parameter for controlling the space weather effect of solar wind disturbances on the Earth; therefore, accurate understanding of the behavior of the IMF is important for improvement of space weather prediction. This study reports the relation between radial ( $B_{r}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>B</mml:mi> <mml:mi>r</mml:mi> </mml:msub> </mml:math> ) and normal ( $B_{n}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>B</mml:mi> <mml:mi>n</mml:mi> </mml:msub> </mml:math> ) components of IMF by analyzing in situ observations collected by inner- and outer-heliosphere spacecraft over multiple solar cycles. A quadratic relation between $B_{r}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>B</mml:mi> <mml:mi>r</mml:mi> </mml:msub> </mml:math> and $B_{n}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>B</mml:mi> <mml:mi>n</mml:mi> </mml:msub> </mml:math> with a 22-year periodicity which corresponds to the magnetic polarity cycle of the Sun was observed in IMF data of the inner-heliosphere spacecraft. In contrast, IMF data of the outer-heliosphere spacecraft did not show such a quadratic relation but exhibited a linear relation between $B_{r}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>B</mml:mi> <mml:mi>r</mml:mi> </mml:msub> </mml:math> and $B_{n}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>B</mml:mi> <mml:mi>n</mml:mi> </mml:msub> </mml:math> with a slope and correlation coefficient depending on the latitude: positive and negative slopes (correlation coefficients) were revealed from the IMF data for north and south latitudes, respectively, and those magnitudes increased with the latitude. Slopes and correlation coefficients of the linear relation depended on neither the radial distance nor the solar activity. The same linear relation between $B_{r}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>B</mml:mi> <mml:mi>r</mml:mi> </mml:msub> </mml:math> and $B_{n}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>B</mml:mi> <mml:mi>n</mml:mi> </mml:msub> </mml:math> was found for the IMF data of the inner-heliosphere spacecraft by sorting them into two groups in terms of the latitude. Therefore, quadratic relation was ascribed to the combined effect of the latitude variation of the inner-heliosphere spacecraft and the latitude dependence of the linear relation. Although the physical process to generate the linear relation remains an open question, some kind of MHD waves may be responsible for it.