Magnetic field fluctuations coinciding with substorm events

Abstract Using observations from the Van Allen Probes spacecraft, this study reports multifrequency banded magnetic field fluctuations during substorm-associated magnetic field dipolarization at L &lt; 6.6. A total of 11 events at MLT 18:00–06:00 h, which corresponds to the night side are analyzed, with a detailed examination of a single event followed by a discussion of other events. Dynamic power spectra characterize the structures within these fluctuations, while ion flux variations provide insights into particle dynamics. Plasma and magnetic pressures are also computed to assess plasma sheet dynamics. The results reveal that multifrequency magnetic field fluctuations coincide with substorm-associated dipolarization, with wave power extending up to the local proton gyrofrequency ( $$\Omega _{{\hbox {H}^{+}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>Ω</mml:mi> <mml:msup> <mml:mtext>H</mml:mtext> <mml:mo>+</mml:mo> </mml:msup> </mml:msub> </mml:math> ). In six substorm events, a significant wave power enhancement beyond the local helium ion gyrofrequency ( $$\Omega _{\text{He}^{+}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>Ω</mml:mi> <mml:msup> <mml:mtext>He</mml:mtext> <mml:mo>+</mml:mo> </mml:msup> </mml:msub> </mml:math> ) is observed, particularly in the magnetic field aligned (MFA) $$\text{B}_{\text{x}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>B</mml:mtext> <mml:mtext>x</mml:mtext> </mml:msub> </mml:math> and $$\text{B}_{\text{y}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>B</mml:mtext> <mml:mtext>y</mml:mtext> </mml:msub> </mml:math> components, suggesting quasi-parallel wave activity in the inner magnetosphere. In addition, the inverse relationship between plasma and magnetic pressures leads to a force imbalance, contributing to the acceleration of $$\text{H}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>H</mml:mtext> <mml:mo>+</mml:mo> </mml:msup> </mml:math> ions. The observed increase in plasma pressure further indicate that the dipolarization region expands during substorms. These findings indicates that high-frequency quasi-parallel waves play a role in magnetic field fluctuations in the inner magnetosphere. Graphical Abstract