Properties of stream interaction regions at Earth and Mars during the declining phase of SC 24

Abstract

Aims. We inspect the evolution of stream interaction regions (SIRs) from Earth to Mars, covering the distance range 1–1.5 AU, over the declining phase of solar cycle 24 (2014–2018). So far, studies only analyzed SIRs measured at Earth and Mars at different times. We compare existing catalogs for both heliospheric distances and arrive at a clean dataset for the identical time range. This allows a well-sampled statistical analysis and for the opposition phases of the planets an in-depth analysis of SIRs as they evolve with distance. Methods. We use in situ solar wind data from OMNI and the Mars Atmosphere and Volatile EvolutioN spacecraft as well as remote sensing data from Solar Dynamics Observatory. A superposed epoch analysis is performed for bulk speed, proton density, temperature, magnetic field magnitude and total perpendicular pressure. Additionally, a study of events during the two opposition phases of Earth and Mars in the years 2016 and 2018 is conducted. SIR related coronal holes with their area as well as their latitudinal and longitudinal extent are extracted and correlated to the maximum bulk speed and duration of the corresponding high speed solar wind streams following the stream interaction regions. Results. We find that while the entire solar wind high speed stream shows no expansion as it evolves from Earth to Mars, the crest of the high speed stream profile broadens by about 17%, and the magnetic field and total pressure by about 45% around the stream interface. The difference between the maximum and minimum values in the normalized superposed profiles increases slightly or stagnates from 1–1.5 AU for all parameters, except for the temperature. A sharp drop at zero epoch time is observed in the superposed profiles for the magnetic field strength at both heliospheric distances. The two opposition phases reveal similar correlations of in situ data with coronal hole parameters for both planets. Maximum solar wind speed has a stronger dependence on the latitudinal extent of the respective coronal hole than on its longitudinal extent. We arrive at an occurrence rate of fast forward shocks three times higher at Mars than at Earth.

Radiation environment for future human exploration on the surface of Mars: the current understanding based on MSL/RAD dose measurements

“…As the dose rate is varying in time, one can not quantify the reduction by simply calculating the dose rate difference before and during the parking. The modulation of the GCR radiation during this period (in which there were no major solar eruptions) is mainly driven by recurrent CIRs that follow the periodic rotation of the Sun (Geyer et al 2021), we expect that the measured dose rate would have been equal to the dose rate during the same phase of the previous cycles, if no extra butte shielding had been present. To illustrate this idea, we superpose the RAD measurements during each of the first five solar rotation cycles, which are marked from I to V in Fig.…”

“…SIR properties at Mars Recently, Huang et al (2019) have used the MAVEN measurements during the period from October 2014 to November 2018 and identified 149 SIRs and their associated shocks at Mars. Geyer et al (2021) further analyzed these events comparing in-situ solar wind data at Earth and Mars using as well the remote sensing image of the Solar coronal holes. Comparing the properties of SIRs at Mars to those at Earth, these studies found that (1) most SIRs are well formed at 1 au; (2) the average duration of SIRs at Mars is about 37.0 h, comparable to that at Earth (36.7 h) indicating that there is no significant expansion as they move radially outward up to 1.5 au; (3) the crest of the HSS profile broadens by about 17%, and the magnetic field and total pressure by about 45% around the stream interface; (4) the maximum magnetic field strength and pressure of SIRs decrease significantly from 1 to 1.5 au; (5) occurrence rate of fast forward shocks is about three times as high at Mars than at Earth.…”

“…Coincidentally, the opposition phase of Mars took place on May 30, 2016 when Earth and Mars were aligned on the same side of the Sun. Geyer et al (2021) have studied the evolution of the HSS properties in this period from the Sun to Earth and to Mars. They found that multiple coronal holes and several CIRs reappeared for at least 5 solar rotations between the end of May and the end of September.…”

“…Furthermore, the radial evolution of the CIR needs to be taken into account. For instance, the occurrence of shocks has been evaluated to be more frequent at 1.5 au while the shock strength becomes weaker at Mars (Huang et al 2019;Geyer et al 2021), and the SIR structure tends to expand at larger heliospheric distances (e.g., Gosling and Pizzo 1999). The radial evolution of the SIR and how this affects the corresponding GCR depression is still an open question.…”

Improvements to the Empirical Solar Wind Forescast (ESWF)

“…From real-time monitoring of the ESWF service and post-event analysis, it was recognized that the modeled solar wind speed profile is too gradual as compression and rarefaction effects are not considered by the smooth increase and decrease in the observed evolution of the coronal hole. Superposed epoch analysis has shown that the median time-velocity profile of a CIR consists of a sharp increase followed by longer recovery phase (Geyer et al, 2021). As of yet, this asymmetrical shape is not being considered by the ESWF model.…”