Observation of SLAMS-Like Structures Close to Martian Aphelion by MAVEN

According to various orientation of the interplanetary magnetic field (IMF), the planetary shock can be either quasi-parallel or quasi-perpendicular. Under quasi-parallel conditions a significant number of solar wind suprathermal particles are reflected from the shock and drift along IMF, forming an extended and highly turbulent region called the foreshock where various nonlinear plasma phenomena are observed. In this research, a case study of the structures in the foreshock region at Mars as observed by Mars Atmosphere and Volatile Evolution is performed close to Martian aphelion, when the foreshock wave activity is usually low. Data from plasma analyzer STATIC and magnetometer MAG is used to analyze ion beams angular spectrum and magnetic field dynamics. It is shown that the observed structures are consistent with Short Large-Amplitude Magnetic Structures (SLAMS), commonly detected in foreshock regions of magnetized and unmagnetized bodies throughout the Solar system. Finally, the Alfven Mach number is calculated to analyze characteristics of the observed foreshock structures. The analysis shows that the observed SLAMS-like structures at Mars are steepened waves formed by the ion cyclotron resonance between plasma waves propagating along the IMF and the back-streaming scattered solar wind H+ and exospheric O+ and O-2(+) ions, with the dominant impact of O+ ions. The steepening process is accompanied with observation of gradients in the diffuse ion density near the bowshock, like it has been previously reported near the Earth (e.g., Scholer, 1993, https://doi. org/10.1029/92JA01875; Tsubouchi & Lembege, 2004, https://doi.org/10.1029/2003JA010014).