EPP

The Venusian dayside ionosphere, similar to other planetary ionospheres, is produced primarily by ionization of its neutral upper atmosphere due to solar extreme ultraviolet (EUV) radiation. It has become clear that the expansion of the ionosphere may be strongly controlled by the EUV level, as exhibited in data collected by the Pioneer Venus Orbiter (PVO) during one solar cycle (1978–1992). However, the EUV-dependence of the Venusian dayside ionopause altitude, which defines the outer boundary of the ionosphere, remains obscure because the PVO crossed the dayside ionopause only during the solar maximum; its periapsis lifted too high during the solar minimum. Recently, during the period 2006–2014, which included the longest and quietest solar minimum of the past several decades, Venus Express (VEX) provided measurements of the photoelectron boundary (PEB) over the northern high-latitude region. Since the photoelectron boundary is closely related to the ionopause, we have an opportunity to analyze the EUV effect on the dayside ionopause by combining PVO and VEX observations. We have evaluated and then reduced the orbit bias effect in data from both PVO and VEX, and then used the results to derive a relationship between solar EUV level and the dayside ionopause altitude. We find that the dayside ionopause altitude increases as the solar EUV level increases, which is consistent with theoretical expectations.

Proton cyclotron waves (PCWs) can be generated by ion pickup of Martian exospheric particles in the solar wind. The solar wind ion pickup process is highly dependent on the “IMF cone angle” — the angle between the solar wind velocity and the interplanetary magnetic field (IMF), which also plays an important role in the generation of PCWs. Using data from 2.15 Martian years of magnetic field measurements collected by the Mars Atmosphere and Volatile Evolution (MAVEN) mission, we have identified 3307 upstream PCW events. Their event number distribution decreases exponentially with their duration. A statistical investigation of the effects of IMF cone angle on the amplitudes and occurrence rates of PCWs reveals a slight tendency of PCWs’ amplitudes to decrease with increasing IMF cone angle. The relationship between the amplitude and IMF cone angle is weak, with a correlation coefficient r = –0.3. We also investigated the influence of IMF cone angle on the occurrence rate of PCWs and found that their occurrence rate is particularly high for intermediate IMF cone angles (~18°–42°) even though highly oblique IMF orientation occurs most frequently in the upstream region of the Martian bow shock. We also conclude that these variabilities are not artefacts of temporal coverage biases in MAVEN sampling. Our results demonstrate that whereas IMF cone angle strongly influences the occurrence of PCWs, IMF cone angle may also weakly modulate their amplitudes in the upstream region of Mars.

The planet Earth is an integrated system, in which its multi-spheres are coupled, from the space to the inner core. Whether the space environment in short to long terms has been controlled by the earth’s interior process is contentious. In the past several decades, space weather and space climate have been extensively studied based on either observation data measured directly by man-made instruments or ancient data inferred indirectly from some historical medium of past thousands of years. The acquired knowledge greatly helps us to understand the dynamic processes in the space environment of modern Earth, which has a strong magnetic dipole and an oxygen-rich atmosphere. However, no data is available for ancient space weather and climate (>5 ka). Here, we propose to take the advantage of " space-diversity” to build a " generalized planetary space family”, to reconcile the ancient space environment evolution of planet Earth from modern observations of other planets in our solar system. Such a method could also in turn give us a valuable insight into other planets’ evolution.

The present issue of Earth and Planetary Physics is dedicated to the near-space neutral and plasma environments of Mars. The issue includes nine papers that present new results on the properties of the Martian exosphere, ionosphere, and magnetosphere, from both observational and modeling points of view. Due to the similarity between the two objects, the issue also includes two additional papers on the near-Venus plasma environment.