Revisiting the structure of low-Mach number, low-beta, quasi-perpendicular shocks

Area of Science:

• Space Physics
• Plasma Astrophysics
• Interplanetary Medium

Background:

• Collisionless shock waves are fundamental in astrophysical plasmas.
• Standard models often assume laminar structures for quasi-perpendicular shocks.
• The presence and characteristics of shock precursors are key to understanding shock dynamics.

Purpose of the Study:

• To investigate the structure of low-Mach number, low-beta quasi-perpendicular interplanetary collisionless shock waves.
• To identify and characterize the presence and properties of whistler precursors in these shock waves.
• To evaluate the implications of precursor observations for existing shock wave models.

Main Methods:

• Analysis of 145 interplanetary collisionless shock waves observed by the Wind spacecraft.
• Identification and characterization of magnetosonic-whistler precursor fluctuations.
• Statistical analysis of precursor properties, including frequency, propagation direction, and amplitude.

Main Results:

• Strong evidence for large-amplitude whistler precursors in 78% of observed shocks.
• Precursor presence showed no dependence on upstream plasma beta or other shock parameters.
• Precursors exhibited specific propagation characteristics relative to the magnetic field and shock normal, with significant wave amplitudes.

Conclusions:

• The common occurrence and large amplitudes of whistler precursors challenge the laminar structure assumption for these shocks.
• Observations contradict standard models, indicating a more complex shock structure.
• These findings necessitate revisions to current theoretical frameworks for collisionless shock waves.