Voyager 1 occasionally detected the sudden jumps of the local interstellar magnetic ﬁeld strength since its heliopause crossing in August 2012. These events were believed to be associated with the outward propagating shocks that had the solar wind origin in the inner heliosphere. Here we investigate the correlation between the interstellar shocks and the large-scale solar wind events by means of the numerical MHD simulation. The solar wind is simpliﬁed as a symmetric ﬂow near the equatorial plane, and the interstellar neutrals are treated as a constant ﬂow with a ﬁxed density distribution along the upwind direction of the local interstellar medium. The charge-exchange between the solar wind plasma and the interstellar neutrals are taken into account. At a heliocentric distance of 1 au, the solar wind data from OMNI, STEREO A and B during the period between the year 2010 and 2017, are used as the inner boundary conditions to drive the simulation. The simulation results showed that the solar wind gradually merge into large-scale structures as the radial distance increases, being consistent with the observations by New Horizons. After propagating into the inner heliosheath, the shocks are fully developed and the corresponding pressure pulses roughly agree with the observation by Voyager 2 in the inner heliosheath. The arrival of the shocks beyond heliopause are estimated and found to be consistent with the observed signatures of interstellar shocks by Voyager 1. The possible origins of the interstellar shocks in the inner heliosheath are discussed based on the simulation.
An intrinsic noise of such a device as ionisation fission chamber for neutron detection has been measured during the lunar eclipse of 15 April 2014. The causes for the noise, in the absence of a particular source of neutrons, are electronics noises, alpha-decay of radioactive material inside the chamber and registration of these alpha particles by the registered system, and occasional background neutrons. The values of time intervals between moments of pulses registration were the subject of analysing. It revealed that in the moments when the Moon crossed an edge of an umbra or penumbra, the sets of the shortest intervals (increased numbers of pulses) are observed. The duration of these sets are about tens microseconds, and they clearly exceed the noise level. The possible causes for such effect are discussed: influence of some emission from Sun, gravitation field changing, changing of an interplanetary magnetic field, bursts of neutrons during Sun-Earth-Moon interaction, unknown “cosmophysical factors”.
The nearly analytic discrete (NAD) method is a kind of finite difference method with advantages of high accuracy and stability. Previous studies have investigated the NAD method for simulating wave propagation in the time-domain. This study applies the NAD method to solving three-dimensional (3D) acoustic wave equations in the frequency-domain. This forward modeling approach is then used as the “engine” for implementing 3D frequency-domain full waveform inversion (FWI). In the numerical modeling experiments, synthetic examples are first given to show the superiority of the NAD method in forward modeling compared with traditional finite difference methods. Synthetic 3D frequency-domain FWI experiments are then carried out to examine the effectiveness of the proposed methods. The inversion results show that the NAD method is more suitable than traditional methods, in terms of computational cost and stability, for 3D frequency-domain FWI, and represents an effective approach for inversion of subsurface model structures.
Energetic electron measurements and spacecraft charging are of great significance for theoretical research in space physics and space weather applications. In this paper, the energetic electron detection package (EEDP) deployed on three Chinese navigation satellites in medium Earth orbit (MEO) is reviewed. The instrument was developed by the space science payload team led by Peking University. The EEDP includes a pinhole medium-energy electron spectrometer (MES), a high-energy electron detector (HED) based on ΔE-E telescope technology, and a deep dielectric charging monitor (DDCM). The MES measures the energy spectra of 50−600 keV electrons from nine directions with a 180°×30° field of view (FOV). The HED measures the energy spectrum of 0.5−3.0 MeV electrons from one direction with a 30° cone-angle FOV. The ground test and calibration results indicate that these three sensors exhibit excellent performance. Preliminary observations show that the electron spectra measured by the MES and HED are in good agreement with the results from the magnetic electron-ion spectrometer (MagEIS) of the Van Allen Probes spacecraft, with an average relative deviation of 27.3% for the energy spectra. The charging currents and voltages measured by the DDCM during storms are consistent with the high-energy electron observations of the HED, demonstrating the effectiveness of the DDCM. The observations of the EEDP on board the three MEO satellites can provide important support for theoretical research on the radiation belts and the applications related to space weather.
We investigate the correlation between Disturbance Storm Time (Dst) characteristics and solar wind conditions for the main phase of geomagnetic storms, seeking possible factors that distinguish extreme storms (minimum Dst <−250 nT) and major storms (minimum Dst <−100 nT). In our analysis of 170 storms, there is a marked correlation between the average rate of change of Dst during a storm’s main phase (ΔDst/Δt) and the storm’s minimum Dst, indicating a faster ΔDst/Δt as storm intensity increases. Extreme events add a new regime to ΔDst/Δt, the hourly time derivative of Dst (dDst/dt), and sustained periods of large amplitudes for southward interplanetary magnetic field Bz and solar wind convection electric field Ey. We find that Ey is a less efficient driver of dDst/dt for extreme storms compared to major storms, even after incorporating the effects of solar wind pressure and ring current decay. When minimum Dst is correlated with minimum Bz, we observe a similar divergence, with extreme storms tending to have more negative Dst than the trend predicted on the basis of major storms. Our results enable further improvements in existing models for storm predictions, including extreme events, based on interplanetary measurements.
Polar mesosphere summer echoes (PMSE) are observed simultaneously with Digisonde and EISCAT VHF radar. The phenomenon of irregular Es layers is called PMSE-like or PMSE-Es (Polar Mesosphere Summer Echoes-Es) and has some relationship with real PMSE. In this paper, the characteristics of irregular Es layers at 80–100 km were observed by Digisonde at Tromsø during 2003–2014 are statistically analyzed with ionograms. The diurnal, day-to-day and year-to-year variations and discrepancies of occurrence rate between PMSE and PMSE-Es are compared with the statistical results observed by Esrange MST radar (ESRAD), and the reasons are discussed. The results show that the trends in the occurrence rate of PMSE-Es are similar to the trends in the occurrence rate of PMSE, but there are some notable differences. The occurrence rate of PMSE-Es is much lower than the occurrence rate of PMSE. The minimum value of PMSE-Es appears 1–2 hours earlier than the minimum value of the PMSE occurrence rate, while PMSE-Es appear earlier than PMSE in the year. In addition, there is a significant positive correlation between the annual average occurrence rates of PMSE and PMSE-Es. PMSE-Es is a relatively important occurrence in the polar mesopause. Analysis of its characteristics can provide new ideas and methods for studying the formation mechanism of PMSE.
For the first time, the effect of ions on complex conductivity and permittivity of dusty plasma at Polar Mesosphere Summer Echoes (PMSE) altitude is analyzed. Because of ions higher mass and smaller thermal velocity, generally, their effects are not considered in the study of electromagnetic properties of dusty plasmas. In this study, we modified the equations of conductivity and permittivity by adding the effect of ions. In the PMSE altitude region between 80 and 90 km, a local reduction in electron density (i.e., an electron bite-out), is produced by electron absorption onto dust particles. The bite-out condition contains high dust density and smaller electron density. From simulation results in comparatively strong bite-out conditions, we found that the ion effects on conductivity become significant with smaller dust size, lower electron temperature, and lower neutral density. For comparatively weak bite-out conditions, the ion effects on conductivity become significant with larger dust size, higher electron temperature, and higher neutral density. On the other hand, for different dust sizes, electron temperatures and neutral density, the ion effects on complex permittivity become significant only in very strong bite-out conditions. Based on these simulation results, we conclude that, in the absence of electron bite-out conditions, the effect of ions on complex conductivity and permittivity is not significant and can be ignored. However, during bite-out conditions, the effect of ions becomes significant and cannot be ignored because it significantly changes the conductivity and permittivity of dusty plasmas.
A particle-in-cell simulation of symmetric reconnection with zero guide field is carried out to understand the dynamics of ions along the separatrices. Through the investigation of ion velocity distributions at different moments and locations along the separatrices, a typical distribution is found: two counter-streaming populations in the perpendicular direction, with another two populations accelerated into distinct energy levels in the parallel direction. Backward tracing of ions reveals that the counter-streaming cores are mostly composed of ions initially located at the same side of the separatrix, while the other two accelerated populations in the parallel direction are composed of ions crossing through the neutral sheet. Through analysis of energy conversion of these populations, it is found that the ion energization along the separatrix is attributable primarily to the Hall electric field, while that in the region between the two separatrices is caused primarily by the induced reconnection electric field. For the counter-streaming population, the low-energy ions that cross the separatrix twice are affected by both Hall and reconnection electric fields, while the high-energy ions that directly enter the separatrix from the unperturbed plasma are energized mainly by the Hall electric field. For the two energized populations in the parallel direction, the ions with lower-energy are accelerated mainly by the in-plane electric field and the Hall electric field on the opposite side of the separatrix, whereas the ions with higher-energy not only experience the same energization process but also are constantly accelerated by the reconnection electric field.