Alfvén wave evolution within corotating interaction regions associated with the formation of magnetic holes/decreases

One-dimensional magnetohydrodynamics (MHD) simulations were performed to validate the evolution of large-amplitude Alfvénic fluctuations embedded in the high-speed solar wind as it interacts with the low-speed stream flowing ahead. It is well known that the interaction of the high- and low-speed streams results in the formation of a corotating interaction region (CIR) bounded by forward and reverse shocks. After passing through the reverse shock, the initial Alfvénic fluctuation disintegrates into two Alfvén modes traveling in opposite directions in a plasma rest frame. These modes bound a region where the magnetic field intensity is largely below the background level. The appearance of this magnetic depression, called a ``magnetic hole'' (MH) or ``magnetic decrease'' (MD), is highly consistent with previous solar wind observations in that the structure is often identified close to the reverse shock within CIRs and holds pressure balance. The process by which such evolution of Alfvénic fluctuations results in MH/MD formation is elucidated: A local maximum in the field components of the fluctuation, which have originally monotonic gradients (e.g., dB _y /dx < 0), is formed due to the amplification when the field passes through the reverse shock. This causes local current reversal (dB _y /dx > 0) within the fluctuation, where the initial force balance ($\nabla$ p ~ J × B) is violated. The resultant force sweeps the plasma backward to form a pressure increase and simultaneous magnetic decrease within the CIR, which can be associated with a MH/MD. This model naturally describes the formation process of a MH/MD.