Effects of the Solar Wind Dynamic Pressure on the Martian Topside Ion Distribution: Implications on the Variability of Bulk Ion Outflow

With the aid of the ion densities measured by the Neutral Gas and Ion Mass Spectrometer and the solar wind dynamic pressures measured by the Solar Wind Ion Analyzer on board the Mars Atmosphere and Volatile EvolutioN, we investigate the modulation of a sequence of ion species in the Martian topside ionosphere by the upstream solar wind condition. Almost all ion species, except for CO 2 + and OCOH+, are very sensitive to the variation of the solar wind condition, and their densities decrease with increasing solar wind dynamic pressure. The response of the topside ion distribution to the variation of the solar wind condition is also found to be remarkably related to the magnetic field orientation, in that the solar wind modulation occurs mainly over regions with near-horizontal field lines. These observations imply substantially enhanced outflow velocities for all ion species under high solar wind dynamic pressures when the ambient magnetic fields are near-horizontal.

The Relationship Between Solar Wind Dynamic Pressure Pulses and Solar Wind Turbulence

Solar wind dynamic pressure pulses (DPPs) are small-scale plasma structures with abrupt and large-amplitude plasma dynamic pressure changes on timescales of seconds to several minutes. Overwhelming majority of DPP events (around 79.13%) reside in large-scale solar wind transients, i.e., coronal mass ejections, stream interaction regions, and complex ejecta. In this study, the intermittency, which is a typical feature of solar wind turbulence, is determined and compared during the time intervals in the undisturbed solar wind and in large-scale solar wind transients with clustered DPP events, respectively, as well as in the undisturbed solar wind without DPPs. The probability distribution functions (PDFs) of the fluctuations of proton density increments normalized to the standard deviation at different time lags in the three types of distinct regions are calculated. The PDFs in the undisturbed solar wind without DPPs are near-Gaussian distributions. However, the PDFs in the solar wind with clustered DPPs are obviously non-Gaussian distributions, and the intermittency is much stronger in the large-scale solar wind transients than that in the undisturbed solar wind. The major components of the DPPs are tangential discontinuities (TDs) and rotational discontinuities (RDs), which are suggested to be formed by compressive magnetohydrodynamic (MHD) turbulence. There are far more TD-type DPPs than RD-type DPPs both in the undisturbed solar wind and large-scale solar wind transients. The results imply that the formation of solar wind DPPs could be associated with solar wind turbulence, and much stronger intermittency may be responsible for the high occurrence rate of DPPs in the large-scale solar wind transients.

Dusk side clockwise vortex generation and aurora intensity decrease after Solar Wind Dynamic Pressure Decrease

<p>A solar wind dynamic pressure increase/decrease leads to the compression/expansion of the Earth’s magnetosphere. In response, field-aligned currents, which are carried by precipitating or escaping plasma particles, are generated in the magnetosphere and in lead to variations in the auroral intensity. In this study, we investigate magnetospheric and ionospheric responses (including magnetospheric plasma vortex, ionospheric currents and aurorae) to a sudden decrease in solar wind dynamic pressure (SW P<sub>dyn</sub>), which is critical for further understanding of the solar wind-magnetosphere-ionosphere coupling. We focused on a SW P<sub>dyn</sub> decrease event that monitored by OMNI. A counter-clockwise plasma vortex was generated in the dusk side magnetosphere uncovered by using MHD simulation method and a clockwise equivalent ionospheric currents (EIC) vortex was generated in the dusk side ionosphere within about ten minutes after the pressure pulse arrival. Simultaneously, the observation results of Spherical Elementary Currents (SECs) showed that the EIC vortex region is dominated by downward field-aligned currents and the ground-based All-Sky Imager (ASI) observations in the vicinity of this EIC vortex showed that the aurorae diminished. These observations are consistent with the scenario proposed by Shi et al. (2014) that flow vortices in the magnetosphere generated by SW P<sub>dyn</sub> sudden decrease carry downward field-aligned currents into the dusk side ionosphere, generating ionospheric current vortex and thereby modulating auroral activity on the dusk side.</p>

On the origin of Ultra-Low Frequency (ULF) waves in sudden and quasiperiodic solar wind dynamic pressure variations penetrating into Earth’s magnetosphere

<p>Several observational studies have shown that external (i.e. solar wind and magnetosheath) dynamic pressure variations can drive quasi-periodic perturbations of the geomagnetic field. In this study, we utilise multi-spacecraft (ARTEMIS, Cluster, GOES, and THEMIS) mission measurements and investigate step-like increases and quasi-periodic variations of solar wind dynamic pressure as the source mechanism of geomagnetic pulsations with frequencies between ~0.5 to 15 mHz. During intervals of slow solar wind and low geomagnetic activity — to exclude waves generated by velocity shear at the magnetopause and substorm contributions — common periodicities in electromagnetic field oscillations inside the magnetosphere and the solar wind driver are detected in Lomb-Scargle periodograms. The causal relationship is examined in frequency and polarisation signatures of waves detected at the various probes using continuous wavelet transform, cross-wavelet spectra and wavelet transform coherence. The observed dependence of wave properties on their localisation offers excellent source verification for ULF Pc4-5  waves originating in dynamic pressure variations in the upstream solar wind and propagating in the dayside magnetosphere through the field line resonance process.</p><p>This research is co-financed by Greece and the European Union (European Social Fund - ESF) through the Operational Programme “Human Resources Development, Education and Lifelong Learning 2014-2020” in the context of the project ULFpulse (MIS: 5048130).</p>