Evolution of stream interaction regions from 1 to 1.5 AU

<p>We inspect the evolution of stream interaction regions from Earth to Mars for the declining solar cycle 24. In particular, the opposition phases of the two planets are analyzed in more detail. So far, there is no study comparing the long-term properties of stream interaction regions and accompanying high-speed streams at both planets for the same time period. We build a catalogue covering a dataset of all measured stream interaction regions at Earth and Mars for the time period December 2014 – November 2018. The number of events (>120) allows for a strong statistical basis. To build the catalogue we use near-earth OMNI data as well as measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. For the opposition phase, we additionally use image data from the Solar Dynamics Observatory to complement the in-situ observations. Bulk speed, proton density, temperature, magnetic field magnitude and total perpendicular pressure are statistically evaluated using a superposed epoch analysis. For the opposition phase, coronal holes that are linked to individual streams are identified. The extracted coronal hole areas (using CATCH) and their longitudinal/latitudinal extension are correlated to the duration and maximum bulk speed of the high-speed stream following the passage of a stream interaction region. We find that an expansion of the stream interface from 1 to 1.5 AU is most visible in magnetic field and total perpendicular pressure. The duration of the high-speed stream does not increase significantly from Earth to Mars, however, the stream crest seems to increase. The amplitudes of the SW parameters are found to only slightly increase or stagnate from 1 – 1.5 AU. We arrive at similar correlation coefficients for both planets with the properties of the related coronal holes. There is a stronger linking of maximum bulk speed to latitudinal extent of the coronal hole than to the longitudinal. On average, the occurrence rate of fast forward shocks increases from Earth to Mars.</p>

Solar Wind Plasma Particles Organized by the Flow Speed

Recent reports of the first data from Parker Solar Probe (PSP) have pointed to a series of links, correlations or anti-correlations between the solar wind bulk speed ($V_{\mathrm{SW}}$ V SW ) and physical properties of plasma particles from less than 0.25 AU in the corona. In the present paper, we describe corresponding and additional links of solar wind properties, at 0.4 AU and 1.0 AU, in an attempt to complement the PSP data and understand their evolution. A detailed analysis is carried out for the main electron populations, comparing the low-energy (thermal) core and the collisionless suprathermal halo. We show that the anti-correlation observed at 0.4 AU between $V_{\mathrm{SW}}$ V SW and the number density (average value) is maintained also at 1 AU for both the core and halo electrons. On the contrary, only the core electrons manifest a clear anti-correlation of the temperature with $V_{\mathrm{SW}}$ V SW , while the halo temperature does not vary much. We also describe the ions, protons and helium, which have a more reduced mobility and their properties exhibit different variations with the solar wind speed. The results are used to shed more light on the mechanisms leading to a differential acceleration of these species and the origin of slow and fast wind modulation.

Ion acoustic waves near a comet nucleus: Rosetta observations at comet 67P/Churyumov-Gerasimenko

Ion acoustic waves were observed between 15 and 30 km from the centre of comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft during its close flyby on 28 March 2015. There are two electron populations: one cold at approximately 0.2 eV and one warm at approximately 4 eV. The ions are dominated by a cold (a few hundredths of eV) distribution with a bulk speed of (3–3.7) km/s. Near closest approach the propagation direction was within 50 degrees from the direction of the bulk velocity, leading to a Doppler shift of the waves that in the spacecraft frame cover a frequency range up to approximately 4 kHz. The wave power decreased over cometocentric distances from 24 to 30 km. The main difference between the plasma at closest approach and in the region where the waves are decaying is the absence of a significant current in the latter.

Thermophysical Phenomena Associated With Nano-Droplet Impingement on a Solid Surface

The thermophysical properties pertaining to the impingement of a nano-droplet onto a solid surface were investigated using molecular dynamics (MD) simulations. The MD simulations used data collection for an entire group of molecules to investigate the propagation of energy in the system. Simulations of a moving nano-droplet colliding with a stationary solid were performed to determine the heat transfer between the droplet and the surface. It was discovered that the droplet-substrate collision caused the droplet temperature to rise significantly upon impact. The substrate also experiences a temperature jump with a slower response time. A theoretical relation for the substrate temperature jump is also developed that shows reasonable agreement with the MD simulations for small droplet diameters. Increasing the diameter of the droplet from 2.0 nm to 4.5 nm showed a gain in the total added substrate kinetic energy. Varying the initial speed of the droplet from 10 m/s to 40 m/s showed no significant difference in the applied kinetic energy onto the substrate, suggesting that the acceleration of the droplet toward the surface due to intermolecular interactions produces an impact speed relatively independent of the initial droplet bulk speed. These trends were also reflected in a thermodynamically based simple theoretical prediction of collision energy, which was shown to be accurate for droplet diameters up to 3.5 nm. The collision energy was estimated to be on the order of 1–10 eV, and the applied heat flux is on the order of GW/m2.

Multiple magnetic dipolarizations observed by THEMIS during a substorm

The magnetic field dipolarization in the vicinity of substorm onset and during substorm expansion phase during the period of 06:00–06:40 UT on 15 February 2008 is investigated with observations from multiple probes of THEMIS. It is found that the magnetic dipolarization at the substorm onset (the onset time was about 06:14 UT) was not accompanied by obvious magnetic disturbance and ion bulk speed variation. The magnetic dipolarizations taking place during the substorm expansion phase observed by P4~(−10.97, 2.04, −3.03) RE and P3~(−11.32, 1.15, −3.10) RE were mostly accompanied by high speed earthward ion bulk flow, but the magnetic dipolarizations occurring during the substorm expansion phase observed by P5~(−9.45, 1.07, −2.85) RE were not accompanied by high speed earthward ion bulk flow. Before substorm onset THEMIS P3, P4, P5 all observed the Bx component fluctuation with a period of about 300 s. After substorm onset earthward high speed ion bulk flow and significant magnetic disturbances both occurred at P3 and P4 locations. These results indicate that there is no one-to-one relationship between the near-Earth magnetic dipolarization and the earthward ion bulk flow. In particular, the magnetic dipolarization occurring on the earthward side of the inner near-Earth plasma sheet is not accompanied by high speed earthward ion bulk flow. The dipolarization at substorm onset is a local and small scale phenomenon. There are multiple magnetic dipolarizations occurring during the substorm expansion phase. The dipolarization process is very complex and is not simply an MHD process. It is accompanied by some kinds of plasma instabilities, the plasma sheet azimuthal expansion not only by earthward ion bulk flow during substorm. A sharp increase of the AE index does not always give an accurate substorm onset time for substorm analysis.

Diamagnetic Acceleration of the Solar Wind

We have developed a new model to account for proton acceleration in the solar wind. The acceleration is a direct result of the diamagnetic effect on the charged particles, and hence on their velocity distribution. Our basic assumptions are: a collisionless plasma, energy conservation and magnetic moment conservation. Through simulation we explore the changes in bulk speed, between 0.3 AU and 1 AU. We find that the diamagnetic effect accounts for the observed velocity increase in bulk speed. Since any form of wave-particle interactions is exluded from our simulations but they fit well the observations, we suggest that the diamagnetic effect also accounts for wind expansion in early type stars, i.e., for stars without convective energy available at their surface.

Diamagnetic Temperature Control in the Solar Wind

We have developed a model to account for changes in velocity distributions of protons in the solar wind. The essential operative mechanism is the diamagnetic effect on charged particles. Through numerical simulation, we explore the (distance 0.3 AU to 1 AU) evolution of thermal anisotropy. We find that as the initial ratio T+/T∥is increased, there is a larger growth in bulk speed, as is observed. Since any form of wave-particle interactions is exluded from our simulations but they fit well the observations, we conclude that the diamagnetic effect is most relevant in controlling both thermal anisotropy and bulk speed evolution.