Excess of Sodium Ions Density Required to Create a Wide Current at the Hermean Magnetopause

In this paper we consider an unusual structure, twice observed at the magnetopause of Mercury, and called the “Double Magnetopause”. Presumably, it is associated with a current sheet created by Na+ ions. Two alternative scenarios are considered. The first one: Sodium ions prevail outside the Hermean magnetosphere. The second: Sodium ions predominate inside the magnetosphere of Mercury. These ions have been observed inside and outside the magnetosphere. We analyze what Na+ density excess can be sufficient for creation of a wide diamagnetic magnetopause current and on which side of the magnetopause this current is located. For each scenario, two directions of the north-south (Z) component of the solar wind magnetic field are considered.

Investigating exospheric Na distributions at different spatial scales to disentangle between single and double peaked global patterns

<p>Since mid ‘80s the Na exosphere of Mercury has been investigated by means of both ground-based observations and spacecraft measurements, showing a wide range of variability from tens of minutes up to seasonal variations along the planetary orbit. It has been shown that the most common Na distribution is characterized by a high latitude double peak probably related to solar wind ion precipitation through the polar cusps. However, the existence of a single peaked equatorial Na emission has been frequently observed too. Generally, it is not straightforward to recognize the contributions due to different surface release processes that produces the observed Na exospheric global image.</p> <p>Here we apply the Multivariate Empirical Mode Decomposition (MEMD) to a dataset of images of the exospheric Na emission collected by the THEMIS ground-based telescope with the goal to disentangle the different contributions operating at different scales that are expected to be responsible of the occurrence of single vs. double peaked emissions or exospheric asymmetries. In particular, we found the existence of a wide range of scales characterizing both type of spatial patterns, ranging from small scales (less than 0.5 Mercury radii) up to large scales (about 1-2 Mercury radii). These scale-dependent patterns can be linked to different source mechanisms as the variability of solar wind magnetic field, different surface release mechanisms (thermal desorption, photon-stimulated desorption, micrometeoroid impact vaporization and ion-sputtering), as well as, to the whole Na exosphere surrounding the Hermean environment. Our conclusions are double checked by applying the MEMD both on Na exospheric measurements and on simulations of the Na exosphere as created by the different source mechanisms. The positive results show the great potential of the MEMD technique to study the complex environment of planetary exospheres and recognize the different components/processes that create it.</p>

A stochastic solar wind magnetic field model and probing its meandering nature by Energetic Electrons

<p>Energetic electrons in impulsive events can serve as an ideal probe of solar wind magnetic field. Using a recently developed Fractonal Velocity Dispersion Analysis (FVDA), the release time at the Sun and the path length of interplanetary magnetic field can be obtained with very small uncertainties in many impulsive events. Further knowing the source location, one can examine how much do the field lines deviate from the Parker spiral.  In this work, we present an analytic model for the angular dispersion of magnetic field lines that results from the turbulence in the solar wind and at the solar source surface. The heliospheric magnetic field lines in this  model is derived from a Hamiltonian $H_{\rm m}(\mu, \phi, r)$ in which the pair of canonically conjugated variables the cosine of the heliographic colatitude $\mu$ and the longitude $\phi$. This model naturally incorporates the effect of a random footpoint motion on the source surface since such a motion is due to the zero-frequency component of the solar wind turbulence. Assuming the footpoint motion is also diffusive, it is shown that the angular diffusivity of the stochastic Parker spirals is given by the angular diffusivity of the footpoints divided by the solar wind speed and is controlled by a unique parameter which is the Kubo number. We also present some model calculations of meandering field lines resulting from stochastic footpoint motion and statistical results of the field line path length from observations. Our model and statistical results can shed lights on observations made by Parker Solar Probe and Solar Orbiter.</p><p> </p><p> </p>

Self-Organization through the Inner Heliosphere: Insights from Parker Solar Probe

The interplanetary medium variability has been extensively studied by means of different approaches showing the existence of a wide variety of dynamical features, such as self-similarity, self-organization, turbulence and intermittency, and so on. Recently, by means of Parker solar probe measurements, it has been found that solar wind magnetic field fluctuations in the inertial range show a clear transition near 0.4 AU, both in terms of spectral features and multifractal properties. This breakdown of the scaling features has been interpreted as the evidence of a dynamical phase transition. Here, by using the Klimontovich S-theorem, we investigate how the process of self-organization is under way through the inner heliosphere, going deeper into the characterization of this dynamical phase transition by measuring the evolution of entropic-based measures through the inner heliosphere.

A two-spacecraft study of Mars induced magnetosphere's response to upstream conditions

<p>We investigate the effects of the upstream solar wind magnetic field on the Martian induced magnetosphere. This is a two-spacecraft study, for which we use Mars Express (MEX) magnetic field magnitude data from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument and Interplanetary Magnetic Field (IMF) measurements and solar wind density and velocity from the magnetometer (MAG) and the Solar Wind Ion Analyzer (SWIA) on board Mars Atmosphere and Volatile EvolutioN (MAVEN), from November 2014 to November 2018. Equally temporally spaced echoes appear in MARSIS' ionograms from which the electron cyclotron frequency and eventually the magnitude of the local magnetic field can be calculated. At the same time solar wind magnetic field data and solar wind parameters from MAG and SWIA respectively are utilized, providing the solar wind input to the Martian system. We make real time comparisons of the IMF and the induced magnetic field in the environment of Mars and we test the ratio B<sub>(MEX)</sub> /B<sub>(MAVEN)</sub>  against various parameters such as the solar wind dynamic pressure, velocity, density, Mach number as well as the Martian seasons, latitudes and heliocentric distances. Additionally, we search for disturbances in IMF which then can be traced in the induced field ultimately revealing the response time of the induced magnetosphere to the solar wind behaviour. <br />MEX and MAVEN measurements combined allow us to investigate the response of the Martian induced magnetosphere to the solar wind magnetic field. Real time comparisons of the IMF and the induced field could help us understand the mechanisms controlling the structure of the Martian induced magnetosphere. </p>

138175 (2000 EE104) and the Source of Interplanetary Field Enhancements

<p>We present the first optical observations taken to characterize the near-Earth object 138175 (2000 EE104).  This body is associated with Interplanetary Field Enhancements (IFEs), thought to be caused by interactions between the solar wind magnetic field and solid material trailing in the orbit of the parent body.  Based on optical photometry, the radius (in meters) and mass (in kilograms) of an equal-area sphere are found to be  250(0.1/p)^{1/2} and  1e11(0.1/p)^{3/2}, respectively, where p is the red geometric albedo and density 1500 kg/m3 is assumed.  The measured colors are intermediate between those of C-type (primitive) and S-type (metamorphosed) asteroids but, with correction for the likely effects of phase-reddening, are more consistent with a C-type classification than with S-type. No evidence for co-moving companions larger than 40(0.1/p) meter in radius is found, and no dust particle trail is detected, setting a limit to the trail optical depth < 2e-9.  Consideration of the size distribution  produced by impact pulverization  makes it difficult to generate the  mass of nanodust (minimum 1e5 kg to 1e6 kg) required to account for IFEs, unless the size distribution is unusually steep.  While the new optical data do not definitively refute the hypothesis that boulder pulverization is the source of IFEs, neither do they provide any support for it.</p> <p>Journal: Planetary Science Journal, submitted</p>

Hemispheric Asymmetry in the Mars Summer Ionosphere at Various Solar Forcing Conditions

<p>The MAVEN satellite has now made two Martian-years of ionosphere-thermosphere (I-T) observations enabling limited studies of seasonal changes in the upper atmosphere. Before examining the ionospheric dynamics associated with space weather, we wish to understand the climatological conditions of the system.  For example, previous studies have revealed the morning electron temperature overshoot as well as a close dependence between electron temperatures and neutral densities in the equatorial regions. In this presentation, we will examine differences in the northern and southern dayside ionosphere during the summer season of each hemisphere. The differences between these two cases will be contrasted with the seasonal dependence at the equator. Differences between the equatorial and polar regions are expected due to (A) differences in neutral scale heights, (B) differences in the solar zenith angle, and (C) the equilibration of I-T coupling due to differences in solar illumination.</p><p>In this work, we present a statistical analysis of MAVEN measurements comparing the north and south summer I-T. We find that when controlling for neutral pressure and latitude, the north and south plasma densities and temperatures are nearly identical below the demagnetization altitude (higher neutral pressures). Above the demagnetization altitude (lower neutral pressures), the southern hemisphere electron densities are higher than those in the northern hemisphere by ~100%. A significantly lower electron temperature is also observed in the south at these lower pressures. Given that the difference in solar EUV (and corresponding neutral heating) is ~20% between the two summer seasons, we postulate that the significantly lower plasma densities (above the demagnetization altitude) in the northern summer are due in part to an increase in ionospheric loss. This loss may be associated with the acceleration of ionospheric particles by the draped magnetic fields at an altitude where ions are not demagnetized. Furthermore, the loss may be diminished in the southern hemisphere where crustal magnetic fields increase the standoff distance to the solar wind magnetic field.</p>

On the horizontal currents over the Martian magnetic cusp

<p>Localized crustal magnetization over heavily cratered southern hemisphere at Mars gives rise to open magnetic field configurations which interact with the solar wind magnetic field to form magnetic cusps.  The downward acceleration of energetic electrons in these cusps can produce aurora and an extended topside ionospheric structure over regions of magnetic anomalies.  We report plasma collisions with the neutral atmosphere at one of the Martian cusps located at 82<sup>o</sup>S and 108<sup>o</sup>E, where the crustal field is strong with a radial component ~30<sup>o</sup> from the local zenith.  We find that the dynamo region in the upper ionosphere of Mars is located between altitudes of 102 km and 210 km. The electrons in this region are constrained to gyrate along magnetic field lines while ions are dragged by neutrals and move along the direction of applied force.  In the absence of the electric field, the horizontal current in the Martian dynamo is generated by the differential motion of ions and electrons.  We find that the bulk of the current density is equatorward and confined within the Martian dynamo near the ionospheric peak with a magnitude of ~3.5 µA/m<sup>2</sup>.  We also find that the westward current density of magnitude ~0.4 µA/m<sup>2</sup> peaking near the upper boundary of the Martian dynamo is generated by magnetized ions in the -<strong>F</strong> x <strong>B</strong> direction.  The model details and results in comparison with other studies will be presented.        </p>