Role Of Solar Wind Research Articles

Solar control of F region radar backscatter: Further insights from observations in the southern polar cap

AbstractThe role of solar wind and illumination in production of small‐scale F region plasma irregularities is investigated using a 4 year data set collected by the Super Dual Auroral Radar Network (SuperDARN) facility at the McMurdo station, Antarctica (MCM). Statistical analysis of ionospheric echoes detected by MCM shows that radar backscatter from the polar F region occurs in wide and persistent bands in range that exhibit systematic changes with local time, season, and solar cycle. It is demonstrated that all variations considered together form a distinct pattern. A comparison with the F region model densities and ray tracing simulations shows that this pattern is largely controlled by the F region solar‐produced ionization during the day. During the night, however, MCM observations reveal a significant additional source of plasma density in the polar cap as compared with the model. An example of conjugate radar observations is presented that supports the idea of polar patches being an additional source of ionization on the nightside. Echo occurrence exhibits a clear peak near the solar terminator, which suggests that small‐scale irregularities form in turbulent cascade from large scales. Further, echo occurrence is enhanced for particular interplanetary magnetic field (IMF) orientations during the night. Observations indicate that solar illumination control of irregularity production is strong and not restricted to the nightside. Indirect solar wind control is also exerted by the IMF‐dependent convection pattern, since the gradient‐drift instability favors certain orientations between the plasma density gradients and convection velocity.

Hydrogen implantation in silicates: The role of solar wind in SiOH bond formation on the surfaces of airless bodies in space

AbstractHydroxyl on the lunar surface revealed by remote measurements has been thought to originate from solar wind hydrogen implantation in the regolith. The hypothesis is tested here through experimental studies of the rate and mechanisms of OH bond formation due to H+ implantation of amorphous SiO2 and olivine in ultrahigh vacuum. The samples were implanted with 2–10 keV H+, in the range of solar wind energies, and the OH absorption band at ~2.8 µm measured by transmission Fourier transform infrared spectroscopy. For 2 keV protons in SiO2, the OH band depth saturated at fluences F ~5 × 1016 H+/cm2 to a maximum 0.0032 absorption band depth, corresponding to a column density ηs = 1.1 × 1016 OH/cm2. The corresponding values for 5 keV protons in olivine are >2 × 1017/cm2, 0.0067, and 4.0 × 1016 OH/cm2. The initial conversion rate of implanted H+ into hydroxyl species was found to be ~90% and decreased exponentially with fluence. There was no evidence for molecular water formation due to proton irradiation. Translating the laboratory measurements in thin plate samples to the granular lunar regolith, it is estimated that the measurements can account for a maximum of 17% relative OH absorption in reflectance spectroscopy of mature soils, consistent with spacecraft observations in the infrared of the Moon.

Study of the influence of magnetic fluctuations and solar plasma density on the solar wind–magnetosphere coupling

We have identified and selected 212 geomagnetic disturbances of varying level of geomagnetic activity, based on the changes in Dst index. The selected events are divided into six different groups according to different geomagnetic activity levels. We study the average behavior of solar plasma flow speed (V), density (N), dynamic pressure (P), magnetic field (B), its north–south component (Bz) and variance (σB) and dusk-ward electric field (Ey), and their role in solar wind–magnetosphere coupling. For this purpose we adopted the method of superposed epoch (SPE) analysis. Hourly solar wind plasma and field parameters (V, B, Bz, σB, N, P, Ey) together with Dst index are analyzed using SPE analysis with respect to the start of the changes in geomagnetic activity, separately for all six groups of increasing level of activity. In particular, we study the influence of the enhancements in solar plasma density/dynamic pressure and field fluctuations on the solar wind–magnetosphere coupling, during southward field orientation within the responsible interplanetary structures. In addition to studying the average properties with the use of SPE analysis, we have also considered individual events with different level of geomagnetic activity (Dst) and studied its relationship to interplanetary field parameters. We observed that the enhancement in plasma density/pressure and fluctuations in field starting several hours before the storm/activity onset, might play a role in enhancing the solar wind–magnetosphere coupling efficiency.

Mercury's atmosphere: A perspective after Mariner 10

Measurements made during the Mariner 10 flybys of Mercury have shown that this planet has a tenuous atmosphere, somewhat similar to that of the Moon, which consists of at least helium and can be classified as an exosphere. The amount of helium observed can be supplied by either the accretion of only a fraction of the solar wind He 2+ diffusing across the magnetopause, or from outgassing of radiogenic helium from the planetary crust. The role of solar wind in the maintenance and depletion of Mercury's atmosphere is discussed in view of the density upper limits established from Mariner 10. The argon supply rate on Mercury is probably not more than that on the Earth, but it is difficult to say whether Mercury is deficient in potassium or not on the basis of the present data. The global outgassing of CO 2 and H 2O from the planet interior is estimated to be at least four orders of magnitude smaller than for Earth which indicates that either Mercury is deficient in volatiles or that this planet is very inactive.