Physics

Lower atmospheric global dust storms affect the small- and large-scale weather and variability of the whole Martian atmosphere. Analysis of the CO 2 density data from the Neutral Gas and Ion Mass Spectrometer instrument (NGIMS) on board NASA's Mars Atmosphere Volatile EvolutioN (MAVEN) spacecraft show a remarkable increase of GW-induced density fluctuations in the thermosphere during the 2018 major dust storm with distinct latitude and local time variability. The mean thermospheric GW activity increases by a factor of two during the storm event. The magnitude of relative density perturbations is around 20% on average and 40% locally. One and a half months later, the GW activity gradually decreases. Enhanced temperature disturbances in the Martian thermosphere can facilitate atmospheric escape. For the first time, we estimate that, for a 20% and 40% GW-induced disturbances, the net increase of Jeans escape flux of hydrogen is a factor of 1.3 and 2, respectively.

Using the observations of the EPT (Energetic Particle Telescope) onboard the satellite PROBA-V we study the dynamics of inner and outer belt electrons from 500 keV to 8 MeV during quiet periods and geomagnetic storms. This high time-resolution (2 sec) spectrometer operating at the altitude of 820 km on a low polar orbit is providing continuously valuable electrons fluxes for already 5 years. We emphasize especially that some MeV electrons are observed in low quantities in the inner belt, even during periods when they are not observed by Van Allen Probe (VAP). We show that they are not due to proton contamination but to clear injections of particles from the outer belt during strong geomagnetic storms of March and June 2015, and September 2017. Electrons with lower energy are injected also during less strong storms and the L-shell of the electron flux peak in the outer belt shifts inward with a high dependence on the electron energy. With the new high resolution EPT instrument, we can study the dynamics of relativistic electrons, including MeV electrons in the inner radiation belt, revealing how and when such electrons are injected into the inner belt and how long they reside there before being scattered into the Earth's atmosphere or lost by other mechanisms.

This work is a continuation of our previous articles (Yermolaev et al. in J. Geophys. Res. 120, 7094, 2015; Yermolaev et al. in Solar Phys. 292, 193, 2017; Yermolaev et al. in Solar Phys. 293, 91, 2018), which describe the average temporal profiles of interplanetary plasma and field parameters in large-scale solar-wind (SW) streams: corotating interaction regions (CIRs), interplanetary coronal mass ejections (ICMEs including both magnetic clouds (MCs) and ejecta), and sheaths as well as interplanetary shocks (ISs). In this work we analyze the average profile of helium abundance N{\alpha}/Np for interval of 1976-2016. Our results confirm main early obtained results: N{\alpha}/Np is higher in quasi-stationary fast streams than in slow ones; it slowly changes in compression regions CIRs and Sheaths from values in undisturbed solar wind to values in corresponding type of piston (HSS or ICME); in Ejecta it is closed to abundance in undisturbed streams and it is maximal in MCs. We obtained for first time that N{\alpha}/Np correlates with plasma {\beta}-parameter in compression regions CIRs and Sheaths and anti-correlates in ICMEs. The N{\alpha}/Np vs. {\beta} dependence is stronger in MCs than in Ejecta and may be used as an indicator of conditions in CME formation place on the Sun.

Characteristics of electromagnetic ion cyclotron (EMIC) waves detected by the Van Allen probes are statistically analyzed, particularly wave-band, bandwidth, and time duration. The Electric and Magnetic Field Instrument Suite and Integrated Science, an instrument on board the Van Allen Probes, provides the necessary magnetic field measurements to examine 33 months of EMIC wave occurrence (1 September 2013 to 31 May 2016). Upon visual identification, the waves are grouped into their respective wave-bands, H+, He+, or O+, defined by the frequencies they are observed at, manifested in the daily spectrograms as their location relative to gyrofrequency lines. Nearly 2,500 EMIC wave events are detected. Results suggest a prevalence of He+-band waves, and a rarity of O+-band waves (1,155 H+-band events, 1,176 He+-band events, and 125 O+-band events). The most prevalent bandwidth range for events in general is found to be 0.25 - 0.5 Hz. However, this appears to vary among the three wave-bands. Helium and oxygen wave-band events tend to have shorter bandwidths (0.25 - 0.5 Hz) than their hydrogen counterparts. Time duration is more consistent among wave-bands, and while H+-band events on average have a slightly shorter duration, the most common time duration for all wave-bands is between 20 and 40 minutes.

Mars Exospheric Neutral Composition Analyzer (MENCA) of Mars Orbiter Mission (MOM) measures the \emph{in-situ} neutral upper atmospheric constituents of Mars. Martian lower atmosphere predominated by the presence of C O 2 which photo-dissociates into atomic oxygen ( O ) in higher altitudes much near the exobase. Atomic O plays a significant role in invoking stronger presence of O + 2 in the Martian ionosphere. Primary photo-dissociative species C O 2 , crossover its neutral abundance with atomic O in the collisonless hetergenous atmosphere with varying local solar conditions. Initial measurements from Neutral Gas and Ion Mass Spectrometer (NGIMS) instrument on Mars Atmosphere and Volatile Evolution (MAVEN) estimated these crossover/transition altitude wavering between ≈ 225 km to 240 km during solar maximum conditions with peak solar illuminations. MENCA sampled the neutral atmospheric species, below the exobase upto periareion of ≈ 160 km, under low solar active conditions during June 2018. Observations of partial pressures of C O 2 and O in subsequent orbits reveals that solar inputs are crucial in quantifying these crossing points, where [O]/[C O 2 ] remain unity, alongside the influences from temperature. The multi-spacecraft measurements of the direct influences of solar wind charged particle fluxes and velocity on the daily variation of neutral thermospheric/exospheric compositions were observed on the local evening hours of Mars and presented. It marks the first-ever direct \emph{in-situ} observation of interaction between the energetic solar particle radiations on Martian exospheric compositions, potentially contibuting for the steady escape and differing population of atomic [O] in the exosphere.

The effect of rough structures on the electron emission under electron impact between 10 eV and 2 keV is investigated with a new version of the low energy electromagnetic model of GEANT4 (MicroElec). The inelastic scattering is modeled, thanks to the dielectric function theory and the Mott's model of partial waves to describe the elastic scattering. Secondary electron emission is modeled for grooved and checkerboard patterns of different dimensions for aluminum and silver. The analyses are performed according to two shape parameters h/L and d/L, h being the height, L the width of the structures, and d the spacing between two neighboring structures. The secondary electron emission is demonstrated to decrease when h/L and d/L ratios increase. When the height reaches 10 times the lateral dimensions, the electron emission yield is divided by two compared to that of a flat sample. The optimization of the two aspect ratios leads to a reduction of the electron emission yield of 80% for grooved patterns and of 98% for checkerboard patterns. This purely geometric effect is similar for aluminum and silver materials. A simple analytical model, capable of reproducing the effect on the electron emission yield of checkerboard and grooved patterns, is proposed. This model is found to be in good agreement with the Monte Carlo simulations and some experimental measurements performed in our irradiation facility.

This paper presents the response of the ionosphere during the intense geomagnetic storms of October 12-20, 2016 and May 26-31, 2017 which occurred during the declining phase of the solar cycle 24. Total Electron Content (TEC) from GPS measured at Indore, Calcutta and Siliguri having geomagnetic dips varying from 32.23°N, 32°N and 39.49°N respectively and at the International GNSS Service (IGS) stations at Lucknow (beyond anomaly crest), Hyderabad (between geomagnetic equator and northern crest of EIA) and Bangalore (near magnetic equator) in the Indian longitude zone have been used for the storms. Prominent peaks in diurnal maximum in excess of 20-45 TECU over the quiet time values were observed during the October 2016 storm at Lucknow, Indore, Hyderabad, Bangalore and 10-20 TECU for the May 2017 storm at Siliguri, Indore, Calcutta and Hyderabad. The GUVI images onboard TIMED spacecraft that measures the thermospheric O/N2 ratio, showed high values (O/N2 ratio of about 0.7) on October 16 when positive storm effects were observed compared to the other days during the storm period. The observed features have been explained in terms of the O/N2 ratio increase in the equatorial thermosphere, CIR-induced High Speed Solar Wind (HSSW) event for the October 2016 storm. The TEC enhancement has also been explained in terms of the Auroral Electrojet (AE), neutral wind values obtained from the Horizontal Wind Model (HWM14) and equatorial electrojet strength from magnetometer data for both October 2016 and May 2017 storms. These results are one of the first to be reported from the Indian longitude sector on influence of CME- and CIR-driven geomagnetic storms on TEC during the declining phase of solar cycle 24.

Artificial satellites perform downlink communication or transmission with earth stations. These transmissions are made from an array of antennas chosen to increase directivity and gain in the transmission of the electromagnetic signal. It occurs that the signal from the array of antennas can infer a disturbance by electromagnetic acceleration, which results in disturbances in the orbital components of the satellite. Taking the StarOne C1 satellite, with EIRP = 44 W, mass = 1918 kg and with a planar antenna array, we developed an acceleration model for this type of antennas. A satellite orbiter propagator was developed by the Runge-Kuta method, and from the technical characteristics of this satellite and the state vector values (position and velocity), we were able to calculate an electromagnetic acceleration of the order of 10^{-9} m/s^{2}.

Small-scale magnetic flux ropes (SFRs), in the solar wind, have been studied for decades. Statistical analysis utilizing various in situ spacecraft measurements is the main observational approach which helps investigate the generation and evolution of these small-scale structures. Based on the Grad-Shafranov (GS) reconstruction technique, we use the automated detection algorithm to build the databases of these small-scale structures via various spacecraft measurements at different heliocentric distances. We present the SFR properties including the magnetic field and plasma parameters at different radial distances from the sun near the ecliptic plane. It is found that the event occurrence rate is still in the order of a few hundreds per month, the duration and scale size distributions follow power laws, and the flux rope axis orientations are approximately centered around the local Parker spiral directions. In general, most SFR properties exhibit radial decays. In addition, with various databases established, we derive scaling laws for the changes of average field magnitude, event counts, and SFR scale sizes, with respect to the radial distances, ranging from ∼ 0.3 au for Helios to ∼ 7 au for the Voyager spacecraft. The implications of our results for comparisons with the relevant theoretical works and for the application to the Parker Solar Probe (PSP) mission are discussed.

The correlation scale and the Taylor scale are evaluated for interplanetary magnetic field fluctuations from two-point, single time correlation function using the Advanced Composition Explorer (ACE), Wind, and Cluster spacecraft data during the time period from 2001 to 2017, which covers over an entire solar cycle. The correlation scale and the Taylor scale are respectively compared with the sunspot number to investigate the effects of solar activity on the structure of the plasma turbulence. Our studies show that the Taylor scale increases with the increasing sunspot number, which indicates that the Taylor scale is positively correlated with the energy cascade rate, and the correlation coefficient between the sunspot number and the Taylor scale is 0.92. However, these results are not consistent with the traditional knowledge in hydrodynamic dissipation theories. One possible explanation is that in the solar wind, the fluid approximation fails at the spatial scales near the dissipation ranges. Therefore, the traditional hydrodynamic turbulence theory is incomplete for describing the physical nature of the solar wind turbulence, especially at the spatial scales near the kinetic dissipation scales.