ALBERT

Description: Mars Atmosphere and Volatile EvolutioN mission (MAVEN) observes a tenuous but ubiquitous flux of protons with the same energy as the solar wind in the Martian atmosphere. During high flux intervals, we observe a corresponding negative hydrogen population. The correlation between penetrating and solar wind fluxes, the constant energy, and the lack of a corresponding charged population at intermediate altitudes implicate products of hydrogen energetic neutral atoms from charge exchange between the upstream solar wind and the exosphere. These atoms, previously observed in neutral form, penetrate the magnetosphere unaffected by electromagnetic fields (retaining the solar wind velocity), and some fraction reconvert to charged form through collisions with the atmosphere. MAVEN characterizes the energy and angular distributions of both penetrating and backscattered particles, potentially providing information about the solar wind, the hydrogen corona, and collisional interactions in the atmosphere. The accretion of solar wind hydrogen may provide an important source term to the Martian atmosphere over the planet's history.

Description: Photoelectron peaks in the 20-30 e V energy range are commonly observed in the planetary atmospheres, produced by the intense photoionization from solar 30.4 nm photons. At Mars, these photoelectrons are known to escape the planet down its tail, making them tracers for the atmospheric escape. Furthermore, their presence or absence allow to define the so-called PhotoElectron Boundary (PEB), that separates the photoelectron dominated ionosphere from the external environment. We provide here a detailed statistical analysis of the location and properties of the PEB based on the Mars Atmosphere and Volatile Evolution (MAVEN) electron and magnetic field data obtained from September 2014 until May 2016 (including 1696 PEB crossings). The PEB appears as mostly sensitive to the solar wind dynamic and crustal fields pressures. Its variable altitude thus leads to a variable wake cross section for escape (up to ∼+50 % ), which is important for deriving escape rates. The PEB is not always sharp, and is characterized on average by : a magnetic field topology typical for the end of Magnetic Pile Up Region above it, more field aligned fluxes above than below, and a clear change of the altitude slopes of both electron fluxes and total density (that appears different from the ionopause). The PEB thus appears as a transition region between two plasma and fields configurations determined by the draping topology of the interplanetary magnetic field around Mars and much influenced by the crustal field sources below, whose dynamics also impacts the estimated escape rate of ionospheric plasma.

Description: It has long been a goal of the heliophysics community to understand solar wind variability at heliocentric distances other than 1 AU, especially at ∼1.5 AU not only due to the steepening of solar wind stream interactions outside 1 AU but also the number of missions available there to measure it. In this study, we use 35 months of solar wind and IMF data taken at Mars by the MAVEN spacecraft to conduct an autocorrelation analysis of the solar wind speed, density, and dynamic pressure, which is derived from the speed and density, as well as the IMF strength and orientation. We found that the solar wind speed is coherent, i.e. has an autocorrelation coefficient above 1/e, over roughly 56 hours, while the density and pressure are coherent over smaller intervals of roughly 25 and 20 hours, respectively, and that the IMF strength is coherent over time intervals of approximately 20 hours, while the cone and clock angles are considerably less steady but still somewhat coherent up to time lags of roughly 16 hours. We also found that when the speed, density, pressure, or IMF strength is higher than average, the solar wind or IMF becomes uncorrelated more quickly, while when they are below average, it tends to be steadier. This analysis allows us to make estimates of the values of solar wind plasma and IMF parameters when they are not directly measured, as well as provide an approximation of the error associated with that estimate.

Description: Using Venus Express, Zhang et al . [2012b] identified strong magnetic field enhancements at low altitudes over the north polar region of Venus as giant flux ropes. Strong fields at low altitudes were also observed during the Pioneer Venus Orbiter mission, but at low latitudes near the subsolar and midnight regions. We examine the possibility that the Venus Express observations are not giant flux ropes, but part of a low altitude magnetic belt that builds up in the subsolar region, passes over the terminator, and extends to the nightside. Our analysis indicates the magnetic belt is dominantly horizontal over the dayside and gains a radial component nightward. The peak magnetic field strength of the belt and the altitude at which it peaks also varies around the planet, with the lowest altitude and strongest field strength in the subsolar region, consistent with the idea of the belt forming on the dayside. Zhang et al . [2012b] also noted the fields in the polar region had a bias in the +By direction in Venus Solar Orbital coordinates. The multifluid magnetohydrodynamic simulation we present shows an asymmetry in the plasma flow from the subsolar region to the poles due to the oxygen ion and proton mass ratio. This causes the magnetic field to preferentially accumulate in the north for a +By interplanetary magnetic field direction, providing an explanation for this bias.

Description: The Venus ionospheric response to solar and solar wind variations is most evident in its magnetic field properties. Early Pioneer Venus observations during the solar maximum revealed that the Venus ionosphere exhibits two magnetic states depending on the solar wind dynamic pressure conditions: magnetized ionosphere with large-scale horizontal magnetic field; or unmagnetized ionosphere with numerous small-scale thin structures, so-called flux ropes. Here we report yet another magnetic state of Venus' ionosphere, giant flux ropes in the magnetized ionosphere, using Venus Express magnetic field measurements during solar minimum. These giant flux ropes all have strong core fields and diameters of hundreds of kilometers, which is about the vertical dimension of the ionosphere. This finding represents the first observation of these giant flux ropes at Venus. The cause of these giant flux ropes remains unknown and speculative.

Description: In this work we study the waves in regions adjacent to ten interplanetary (IP) shocks formed by the interactions between interplanetary coronal mass ejections and the solar wind. We analyze the STEREO data for the years 2007–2010. Shocks in our sample have low magnetosonic Mach numbers (Mms ≤ 2.3), their criticality ratios range between 0.8 and 2.3 and θBn are between 38° and 85°. We find ultra-low frequency (ULF, 0.01 Hz–0.05 Hz) waves and higher-frequency (HF, ≥ 1 Hz) whistler precursors upstream of these shocks. Downstream of them we observe irregular ULF fluctuations and regular HF waves with similar frequencies as in the upstream case. We find that IP shocks with relatively small Mms can excite waves in large regions in front of them (2.2 × 10−3 AU–4.6 × 10−3 AU), thereby forming large ULF wave foreshocks. We do not find any evidence for the steepening of these waves. We do observe suprathermal (E ≤ 30 keV) proton foreshocks upstream of some of the shocks in the sample. The extensions of suprathermal proton foreshocks range between 0.02 AU and 0.1 AU. However, not all foreshocks with suprathermal ions show ULF waves or vice versa. The extensions of ULF and proton foreshocks can be very different. Enhanced ULF waves and suprathermal protons can be observed upstream of local quasi-perpendicular shocks. We propose that the observed discordance between the shock geometries and the presence of the foreshock phenomena may be explained in terms of temporal and spatial variations of the local geometry of the IP shocks.

Description: We study atmospheric escape from Venus during solar minimum conditions when 147 corotating interaction regions (CIRs) and interplanetary coronal mass ejections (ICMEs) combined impact on the planet. This is the largest study to date of the effects of stormy space weather on Venus and we show for the first time statistically that the atmosphere of Venus is significantly affected by CIRs and ICMEs. When such events impact on Venus, as observed by the ACE and Venus Express satellites, the escape rate of Venus's ionosphere is measured to increase by a factor of 1.9, on average, compared to quiet solar wind times. However, the increase in escape flux during impacts can occasionally be significantly larger by orders of magnitude. Taking into account the occurrence rate of such events we find that roughly half (51%) of the outflow occurs during stormy space weather. Furthermore, we particularly discuss the importance of the increased solar wind dynamic pressure as well as the polarity change of the interplanetary magnetic field (IMF) in terms of causing the increase escape rate. The IMF polarity change across a CIR/ICME could cause dayside magnetic reconnection processes to occur in the induced magnetosphere of Venus, which would add to the erosion through associated particle acceleration.

Description: The Cassini-Huygens mission has been observing Titan since October 2004, resulting in over 70 targeted flybys. Titan's thermosphere is sampled by the Ion and Neutral Mass Spectrometer (INMS) during several of these flybys. The measured upper atmospheric density varies significantly from pass to pass. In order to quantify the processes controlling this variability, we calculate the nitrogen scale height for a variety of parameters related to the solar and plasma environments and, from these, we infer an effective upper atmospheric temperature. In particular, we investigate how these calculated scale heights and temperatures correlate with the plasma environment. Measured densities and inferred temperatures are found to be reduced when INMS samples Titan within Saturn's magnetospheric lobe regions, while they are enhanced when INMS samples Titan in Saturn's plasma sheet. Finally the data analysis is supplemented with Navier-Stokes model calculations using the Titan Global Ionosphere Thermosphere Model. Our analysis indicates that, during the solar minimum conditions prevailing during the Cassini tour, the plasma interaction plays a significant role in determining the thermal structure of the upper atmosphere and, in certain cases, may override the expected solar-driven diurnal variation in temperatures in the upper atmosphere.

Description: We investigate the characteristics of 9 interplanetary shocks associated with stream interaction regions observed by both STEREO-A and STEREO-B spacecraft during the years 2007–2008. Interplanetary shocks modify the plasma both upstream and downstream of the front shock. As they propagate, interplanetary shocks encounter solar wind with different characteristics (density, velocity) and different orientations of ambient magnetic field relative to the shock normal. Thus, it is interesting to compare dual observations of stream interaction shocks at the locations of the two spacecraft to determine the role of these parameters in controlling both the structure at the shock but also in the regions upstream and downstream from the shock. The range of shock normal angle (ΘBn) values observed by spacecraft covered the range from ∼20° to ∼81°. The largest difference in ΘBn for the same shock observed at two different longitudinal locations was ∼39°. The shock magnetosonic Mach numbers covered the range of ∼1.1 to ∼2.2, having a largest change for the same shock of ∼0.9. The jump in the field magnitude, i.e., the ratio of downstream magnetic field intensity to upstream magnetic field intensity (Bd/Bu), ranged from ∼1.1 to ∼2.25. The largest difference in the jump in field magnitude for the same shock at two different locations was ∼0.72. These variations with longitude of shock properties observed with the STEREO dual mission show the non-homogeneous character of the plasma in the heliosphere, and they need to be taken into account to understand in detail how these shocks modify the solar wind, and affect the acceleration processes of energetic particles in the solar wind.

Description: Several recent studies suggest that magnetic reconnection is able to erode substantial amounts of the outer magnetic flux of interplanetary magnetic clouds (MCs) as they propagate in the heliosphere. We quantify and provide a broader context to this process, starting from 263 tabulated interplanetary coronal mass ejections (ICMEs), including MCs, observed over a time period covering 17 years and at a distance of 1 AU from the Sun with Wind (1995-2008) and the two STEREO (2009-2012) spacecraft. Based on several quality factors, including careful determination of the MC boundaries and main magnetic flux rope axes, an analysis of the azimuthal flux imbalance expected from erosion by magnetic reconnection was performed on a subset of 50 MCs. The results suggest that MCs may be eroded at the front or at rear and in similar proportions, with a significant average erosion of about 40 % of the total azimuthal magnetic flux. We also searched for in situ signatures of magnetic reconnection causing erosion at the front and rear boundaries of these MCs. Nearly ~30 % of the selected MC boundaries show reconnection signatures. Given that observations were acquired only at 1 AU and that MCs are large-scale structures, this finding is also consistent with the idea that erosion is a common process. Finally, we studied potential correlations between the amount of eroded azimuthal magnetic flux and various parameters such as local magnetic shear, Alfvén speed, and leading and trailing ambient solar wind speeds. However, no significant correlations were found, suggesting that the locally observed parameters at 1 AU are not likely to be representative of the conditions that prevailed during the erosion which occurred during propagation from the Sun to 1 AU. Future heliospheric missions, and in particular Solar Orbiter or Solar Probe Plus, will be fully geared to answer such questions.