Mirror mode structures and ELF plasma waves in the Giacobini-Zinner magnetosheath

Abstract

We show evidence for mirror mode structures at comet Giacobini-Zinner. These are plasma structures with alternating high ß and low ß regions driven unstable when ß /ß > 1+ 1/. These structures are detected in a region just adjacent to the magnetic tail and have scale sizes of ≈ 12 H2O group ion cyclotron radii. Calculations are presented to show that mirror mode instability can occur due to the perpendicular pressure associated with H2O+ cometary pickup ions in the region of mirror mode observation. Adjacent regions (in the magnetic tail and further in the sheath) are found to be stable to the mirror mode. Plasma waves are detected in relation with the mirror mode structures. Low frequency 56 to 100 Hz waves are present in the high beta portions, and high frequency, 311 Hz to 10 kHz, waves are present in low beta regions. These may be electromagnetic lion roar waves and electrostatic festoon-shaped waves, respectively, in analogy to plasma waves detected in the Earth's magnetosheath.