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

We show evidence for mirror mode structures at comet Giacobini-Zinner. Thes e are plasma structures with alternating high beta and low beta regions dri ven unstable when beta(perpendicular to)/beta(parallel to) > 1+1/beta(perpe ndicular to). These structures are detected in a region just adjacent to th e magnetic tail and have scale sizes of similar to 12 H2O group ion cyclotr on radii. Calculations are presented to show that mirror mode instability c an occur due to the perpendicular pressure associated with H2O+ cometary pi ckup ions in the region of minor mode observation. Adjacent regions tin the magnetic tail and further in the sheath) are found to be stable to the mir ror mode. Plasma waves are detected in relation with the mirror mode struct ures. Low frequency 56 to 100 Hz waves are present in the high beta portion s, and high frequency, 311 Hz to 10 kHz, waves are present in low beta regi ons. These may be electromagnetic lion roar waves and electrostatic festoon -shaped waves, respectively, in analogy to plasma waves detected in the Ear th's magnetosheath.