The measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft are analyzed for the basic properties of solar wind stream interaction regions (SIRs) and their associated shocks at 1.52 au, as well as the evolution of SIRs from 1 to 1.52 au. A total of 149 SIRs are identified during the period from 2014 October to 2018 November, and 126 SIRs with high-quality data are selected for this study. The average occurrence rate of SIRs at 1.52 au is 36.3 yr−1, which is comparable to but slightly higher than that (32.4 yr−1) at 1 au, meaning that most SIRs are well formed at 1 au. The average duration of SIRs at 1.52 au is about 37.0 hr, comparable to that at 1 au (36.73 hr), indicating that SIRs have not yet expanded more rapidly as they are convected to 1.52 au. The maximum magnetic field strength and pressure of SIRs decrease significantly from 1 to 1.52 au. The shock association rates of SIRs increase from 20.3% to 33.3% or higher as SIRs evolve from 1 to 1.52 au. The forward shocks tend to occur twice more frequently than the reverse shocks. About 75% of shocks at 1.52 au are quasi-perpendicular shocks. The strength of the shocks becomes weaker and the average shock speed remains almost unchanged from 1 to 1.52 au. These results will help us understand the solar wind conditions at Mars and their potential impact on the Martian space environment.

The measurements from the Mars Atmosphere and Volatile EvolutioN spacecraft, in orbit around Mars, are utilized to investigate interplanetary coronal mass ejections (ICMEs) near 1.52 au. We identify 24 ICMEs from 2014 December 6 to 2019 February 21. The ICME list is used to examine the statistical properties of ICMEs. On average, the magnetic field strength of 5.99 nT in ICMEs is higher than that of 5.38 nT for stream interaction regions (SIRs). The density of 5.27 cm−3 for ICMEs is quite comparable to that of 5.17 cm−3 for SIRs, the velocity of 394.7 km s−1 for ICMEs is slightly lower than that of 432.8 km s−1 for SIRs, and the corresponding dynamic pressure of 1.34 nPa for ICMEs is smaller than that of 1.50 nPa for SIRs. Using existing databases of ICMEs at 1 au for the same time period, we compare ICME average properties at 1.52 au with those at 1 au. The averages of the characteristic quantities decrease by a factor of 1.1–1.7 from 1 to 1.52 au. In addition, we analyze an unusual space weather event associated with the ICME on 2015 March 9–10, and propose that the extremely strong dynamic pressure with a maximum of ∼18 nPa on March 8 is caused by the combined effects of the enhanced density inside a heliospheric plasma sheet (HPS), the compression of the HPS by the forward shock, and the high velocity of the sheath ahead of the ICME.

The two‐fluid model for the solar wind of Hollweg (1970) is reconsidered with the inclusion of the spiral structure of the interplanetary magnetic field. In the present model, the protons are assumed to become collisionless beyond 0.1 AU from the sun, whereas the electrons are treated hydrodynamically and the electron temperature is supposed to obey the polytropic law. The electric field established from the charge separation, which is assumed to be derivable from a potential, tends to enhance the velocity of the solar wind at 1 AU to a value over 300 km/sec. The proton thermal anisotrophy T∥/T⊥ at the orbit of the earth is reduced from the value of 50 in the model with the radial magnetic field to the value of 11 in the present model.

Magnetic field and plasma measurements from the Mars Atmosphere and Volatile EvolutioN mission in orbit around Mars are analyzed for interplanetary fast shock properties and drivers from 2014 October to 2018 November. We identified 52 fast shocks, including 39 fast forward (FF) shocks and 13 fast reverse (FR) shocks. Most (79%) of the FF shocks are driven by stream interaction regions (SIRs) with only a few cases being driven by interplanetary coronal mass ejections, and all of the FR shocks are driven by SIRs. A total of 92% of the identified shocks are quasi-perpendicular shocks. On average, the shock strengths of SIR-driven forward and reverse shocks are comparable, and they are greater than that of ICME-driven forward shocks. The shock strengths show no systematic dependence on the shock locations relative to the Martian bow shock. We find no evidence that the shock shapes are affected by Mars and its bow shock as an obstacle in the propagating medium. The results can help us understand the nature of interplanetary shocks propagating in different environments.

Proton cyclotron waves (PCWs) upstream from Mars are usually interpreted as waves generated by ion/ion instabilities due to the interaction between the solar wind plasma and the pickup protons, originating from the extended hydrogen (H) exosphere of Mars. Their generation mainly depends on the solar wind properties and the relative density of the newborn protons with respect to the background solar wind. Under stable solar wind conditions, a higher solar irradiance leads to both increased exospheric H density and ionization rate of H atoms, and therefore a higher relative density, which tends to increase the linear wave growth rate. Here we show that the solar irradiance is likely to contribute significantly to PCW generation. Specifically, we present observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft indicating that, around the peak of the X8.2 flare on 2017 September 10, the increased solar irradiance gave rise to higher pickup H+ fluxes, which in turn excited PCWs. This result has implications for inferring the loss of hydrogen to space in early Martian history with more intense and frequent X-class flares as well as their contributions to the total loss.