Plasma Observations at Venus with Galileo

Modification of small- and middle-scale solar wind structures by the bow shock and magnetosheath: Correlation analysis

MHD turbulence and particle acceleration in a mass-loaded solar wind

Abstract. Magnetospheric ultra-low frequency (ULF) oscillations in the Pc 4–5 frequency range play an important role in the dynamics of Earth's radiation belts, both by enhancing the radial diffusion through incoherent interactions and through the coherent drift-resonant interactions with trapped radiation belt electrons. The statistical distributions of magnetospheric ULF wave power are known to be strongly dependent on solar wind parameters such as solar wind speed and interplanetary magnetic field (IMF) orientation. Statistical characterisation of ULF wave power in the magnetosphere traditionally relies on average solar wind–IMF conditions over a specific time period. In this brief report, we perform an alternative characterisation of the solar wind influence on magnetospheric ULF wave activity through the characterisation of the solar wind driver by its variability using the standard deviation of solar wind parameters rather than a simple time average. We present a statistical study of nearly one solar cycle (1996–2004) of geosynchronous observations of magnetic ULF wave power and find that there is significant variation in ULF wave powers as a function of the dynamic properties of the solar wind. In particular, we find that the variability in IMF vector, rather than variabilities in other parameters (solar wind density, bulk velocity and ion temperature), plays the strongest role in controlling geosynchronous ULF power. We conclude that, although time-averaged bulk properties of the solar wind are a key factor in driving ULF powers in the magnetosphere, the solar wind variability can be an important contributor as well. This highlights the potential importance of including solar wind variability especially in studies of ULF wave dynamics in order to assess the efficiency of solar wind–magnetosphere coupling.

The heating rate of the outer electron radiation belt at geosynchronous orbit is determined for the interval from 36 to 72 h after the onset of high‐speed stream‐driven storms. Multisatellite measurements of the radiation belt temperature are used for 93 high‐speed stream‐driven storms. During the storms, the outer electron radiation belt temperature changes from ∼120 keV to ∼190 keV. The average heating rate of 32 keV d−1is obtained. The heating rate during the storms is found to be positively correlated with the solar wind velocity and with the Kp index of geomagnetic activity and to be negatively correlated with the solar wind number density. When the solar wind velocity is held fixed, the correlation of the heating rate with Kp vanishes. Expressions for the change in the heating rate as function of the solar wind speed, the solar wind density, and the Kp index are fit to the data. The heating rate is uncorrelated with the amplitude of magnetic field fluctuations in the magnetosphere. Correlations between the heating rate and the level of velocity, density, and magnetic field fluctuations in the magnetosphere and in the solar wind are weaker than the correlations of the heating rate with the solar wind velocity and density. The heating rates correspond to a kinetic energy density change of 3.6 × 10−11erg cm−3d−1at geosynchronous orbit, to a specific entropy change of 4.1 × 106eV cm2d−1at geosynchronous orbit, and to a total heating rate of the geosynchronous orbit region of 5.3 × 106Watts.

The Earth’s magnetosphere is permanently influenced by the solar wind. When supersonic and superalfvenic plasma flow interacts with the magnetosphere, the magnetosheath region is formed, which is filled with shocked turbulent plasma. Varying SW parameters influence the mechanisms of formation of this boundary layer, including the dynamics of turbulence behind the bow shock. The effect of the solar wind on the development of turbulence in the magnetosheath was demonstrated recently based on broad statistics of spacecraft measurements. The present study considers the multipoint observations of turbulent fluctuations in the solar wind, in the dayside magnetosheath and at the flanks, to analyze the evolution of the turbulent cascade while the solar wind plasma enters the magnetosheath. Observations of the magnetosheath behind the quasi-perpendicular bow shock are analyzed to exclude the influence of the bow shock topology from consideration. Three basic types of solar wind flows are considered: slow undisturbed solar wind, compressed regions, and interplanetary manifestations of coronal mass ejections. The results show surviving Kolmogorov scaling behind the bow shock for steady solar wind flow and amplification of the compressive fluctuations at the kinetic scales at the magnetosheath flanks for the solar wind associated with compressed plasma streams. During interplanetary manifestations of the coronal mass ejection, the spectra in the dayside magnetosheath substantially deviate from those observed in the solar wind (including the absence of Kolmogorov scaling and steepening at the kinetic scales) and restore at the flanks.

Noble gas composition of the solar wind as collected by the Genesis mission

The in-ecliptic solar wind influences the boundaries of the Earth’s magnetosphere, e.g., by controlling the distances of the magnetopause and bow shock. The exospheric neutral hydrogen has been measured up to the Moon’s orbit. The charge exchange between the solar wind protons and the neutral geocoronal hydrogen creates an energetic neutral atom (ENA). The field-of-view of the IBEX-Hi instrument onboard the Interstellar Boundary Explorer (IBEX) encompasses various portions of the Earth’s magnetosphere during the year, including the subsolar point. Recently, Sokół et al. 2021 (ApJ 922 250) reported periodic, solar cycle enhancements of the solar wind dynamic pressure. We study the solar cycle evolution of the H ENA flux in the subsolar magnetosheath based on the IBEX measurements. We observed an enhancement of the ENA flux after the maximum of solar activity. We also analyze how the ENA flux varies with the solar wind parameters. We observe positive correlations with the solar wind dynamic pressure in the energy range from 0.7 to 4.3 keV and also opposite variations in correlation coefficients between the ENA flux and the solar wind speed and density.

Data analysis was performed using 17 years of DMSP SSJ/4/5 data to characterize the relations between the solar wind drivers and the electron low-energy fluxes measured on both magnetic poles (magnetic latitude above 55°). Inputs are solar wind velocity, density, dynamic pressure and Bz of the interplanetary magnetic field. Median of electron energy flux for each MLAT-MLT pair have been computed for given values of solar wind drivers. Results highlight that high velocity, density or pressure implies higher energy flux overall, higher polar rain energy fluxes, and wider nightside oval. There seems to be a positive correlation between polar rain and solar wind density as opposed to what Riehl & Hardy (1986) found. As a function of Bz, the oval width as a “U” shape and the polar cap activity a “V” shape, with their minimum at Bz around zero. Riehl, K. B., & Hardy, D. A. (1986). Average characteristics of the polar rain and their relationship to the solar wind and the interplanetary magnetic field. Journal of Geophysical Research: Space Physics, 91 (A2), 1557-1571. Retrieved from https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JA091iA02p01557 doi: https://doi.org/10.1029/JA091iA02p01557

The Martian bow shock represents the main interface between the upstream interplanetary space and the downstream planetary obstacle, where the solar-wind plasma and the frozen-in interplanetary magnetic field (IMF) begin to be perturbed. However, the physical characteristics of the Martian bow shock layer and the influence of the external solar wind drivers on them remain unclear. By employing a three-dimensional Hall magneto-hydrodynamic (MHD) model, this study aims to reveal the physical characteristics of the Martian bow-shock layer extracted from the maximum radially inward gradient of the solar-wind velocity (VS W), including the magnetic field, current density, electric fields, and the energy transfer between the fields and solar wind protons, as well as the influence of the VS W and the IMF on these features. Simulation results indicate that the IMF has initiated the processes of piling-up, draping, bending, and slipping at the Martian bow shock, inducing an associate current to flow from the +ZMS E pole to the −ZMS E pole along the bow-shock layer, with the strongest being located at the subsolar position. Furthermore, the total electric field at the Martian bow shock is constituted by the motional electric field (EM) with the +ZMS E direction around the ±ZMS E flanks and the outward ambipolar (EA) and Hall (EH) electric fields around the lower solar zenith angles; through these, the solar wind transfers its kinetic energy to the electromagnetic fields. A higher VS W gives rise to an enhanced magnetic field, current density, and electric fields at the Martian bow shock, thereby leading to an increase in the corresponding energy-transfer rates. A greater magnitude of the IMF cross-flow component tends to result in an intensified magnetic field, current densities, EM, and EH; while it causes a decreased EA and associated energy-transfer rate at the bow shock layer. If the Parker spiral angle of the IMF is not restricted to 90°, a portion of the quasi-parallel bow-shock layer will be formed, within which the magnitudes of the magnetic field, current density, EH, and the corresponding energy-transfer rate through EH are all lower than those of the quasi-perpendicular bow-shock layer. The results of this study provide valuable insights into the physical properties at the bow-shock layer that emerge during the Mars-solar-wind interactions.

Based on the DMSP F6 and F7 satellite observations, the characteristics of precipitating particles in different auroral precipitation regions of the dayside sector have been studied depending on the solar wind plasma density. Under quiet geomagnetic conditions (|AL| < 100 nT and B z > 0), a considerable increase in the fluxes of precipitating ions is observed in the zones of structured auroral oval precipitation (AOP) and soft diffuse precipitation (SDP). A decrease in the mean energy of precipitating ions is observed simultaneously with the flux growth in these regions. The global pattern of variations in the fluxes of precipitating ions, which shows the regions of effective penetration of solar wind particles into the magnetosphere at a change in the solar wind density from 2 to 20 cm−3, has been constructed. The maximal flux variation (ΔJ i = 1.8 · 107 cm−2 s−1, i.e., 3.5% of an increase in the solar wind particle flux) is observed in the SDP region on the dayside of the Earth. The dependence of precipitating ion fluxes in the low-latitude boundary layer (LLBL), dayside polar cusp, and mantle on the solar wind density at positive and negative values of the IMF B z component has been studied. In the cusp region, an increase in the precipitating ion flux is approximately 17% of an increase in the solar wind density. The IMF southward turning does not result in an appreciable increase in the ion precipitation fluxes either in the cusp or in the mantle. This fact can indicate that the reconnection of the geomagnetic field with southward IMF is not the most effective mechanism for penetration of solar wind particles into these regions.

Abstract. It is known that under certain solar wind (SW)/interplanetary magnetic field (IMF) conditions (e.g. high SW speed, low cone angle) the occurrence of ground-level Pc3–4 pulsations is more likely. In this paper we demonstrate that in the event of anomalously low SW particle density, Pc3 activity is extremely low regardless of otherwise favourable SW speed and cone angle. We re-investigate the SW control of Pc3 pulsation activity through a statistical analysis and two empirical models with emphasis on the influence of SW density on Pc3 activity. We utilise SW and IMF measurements from the OMNI project and ground-based magnetometer measurements from the MM100 array to relate SW and IMF measurements to the occurrence of Pc3 activity. Multiple linear regression and artificial neural network models are used in iterative processes in order to identify sets of SW-based input parameters, which optimally reproduce a set of Pc3 activity data. The inclusion of SW density in the parameter set significantly improves the models. Not only the density itself, but other density related parameters, such as the dynamic pressure of the SW, or the standoff distance of the magnetopause work equally well in the model. The disappearance of Pc3s during low-density events can have at least four reasons according to the existing upstream wave theory: 1. Pausing the ion-cyclotron resonance that generates the upstream ultra low frequency waves in the absence of protons, 2. Weakening of the bow shock that implies less efficient reflection, 3. The SW becomes sub-Alfvénic and hence it is not able to sweep back the waves propagating upstream with the Alfvén-speed, and 4. The increase of the standoff distance of the magnetopause (and of the bow shock). Although the models cannot account for the lack of Pc3s during intervals when the SW density is extremely low, the resulting sets of optimal model inputs support the generation of mid latitude Pc3 activity predominantly through upstream waves.

The Martian bow shock stands as the first defense against the solar wind and shapes the Martian magnetosphere. Previous studies showed the correlation between the Martian bow shock location and solar wind parameters. Here we present direct evidence of solar wind effects on the Martian bow shock by analyzing Tianwen‐1 and MAVEN data. We examined three cases where Tianwen‐1 data show rapid oscillations of the bow shock, while MAVEN data record changes in solar wind plasma and magnetic field. The results indicate that the bow shock is rapidly compressed and then expanded during the dynamic pressure pulse in the solar wind, and is also oscillated during the IMF rotation. The superposition of variations in multiple solar wind parameters leads to more intensive bow shock oscillation. This study emphasizes the importance of joint observations by Tianwen‐1 and MAVEN for studying the real‐time response of the Martian magnetosphere to the solar wind.

On the role of the magnetic field in the solar wind interaction with venus: Expectations versus observations

Interplanetary scintillation (IPS) observations made with the 327‐MHz multistation system of the Solar‐Terrestrial Environment Laboratory (STEL) are analyzed to investigate the global distribution of solar wind speed and density fluctuations (ΔNe) and their evolution during 1997–2009. This study aims at elucidating the evolution of ΔNe distribution during the cycle 23 and subsequent extended minimum, which is useful for improving understanding of the heliospheric response to the peculiar solar activity. The computer‐assisted tomography (CAT) method is used in the present study to deconvolve the line‐of‐sight integration of STEL IPS observations. This CAT method enables retrieval of the quasi‐stationary large‐scale structure of the background solar wind. The results show that the high (low)‐latitude region is dominated by reduced (enhanced) ΔNe plasma, being closely associated with the fast (slow) solar wind. The solar wind speed data show a distinct change with solar activity, and an excellent positive (negative) correlation is revealed between the fast (slow) wind area and the polar field strength of the Sun. In contrast, the ΔNe data do not show such a solar cycle variation, but instead reveal a significant increase in the fractional area of low‐ΔNe region in 2004 preceded by a constant value with a small amount of fluctuation. This change is observed for all latitudes, distinctly after 2007 for low latitudes. Our finding is consistent with the long‐term variation of the solar wind density revealed from in situ measurements at the Earth orbit, if ΔNe ∝ Ne (where Ne is the solar wind electron density), and also consistent with the coronal hole distribution during the last solar cycle. It is found that ΔNe is inversely correlated with the solar wind speed V. We obtain the best fit power law function ΔNe ∝ V−0.36±0.14 for V &gt; 350 km/s, which is basically consistent with our earlier result. This fact suggests that the fractional density fluctuations ΔNe/Ne are greater in the fast wind than in the slow wind. There is no systematic variation in the power law index or slope during 1997–2009 except for a null slope in 2000, which may be ascribed to an insufficient resolution of the CAT analysis. Thus, the inverse relation between ΔNe and V is regarded as a general rule for solar wind turbulence. The important point to note is that a marked drop in ΔNe occurs for the slow speed wind, V &lt; 350 km/s, particularly for 2004 and 2009. This fact may be attributed to different source conditions of the very‐low‐speed solar wind.

Plasma wave measurements on the Helios 1 and 2 spacecraft have revealed the occurrence of electric field turbulence in the solar wind at frequencies between the electron and ion plasma frequencies. Wavelength measurements with the Imp 6 spacecraft now provide strong evidence that these waves are short‐wavelength ion acoustic waves which are Doppler‐shifted upward in frequency by the motion of the solar wind. Comparison of the Helios results with measurements from the earth‐orbiting Imp 6 and 8 spacecraft shows that the ion acoustic wave turbulence detected in interplanetary space has characteristics essentially identical to those of bursts of electrostatic turbulence generated by protons streaming into the solar wind from the earth's bow shock. In a few cases, enhanced ion acoustic wave intensities have been observed in direct association with abrupt increases in the anisotropy of the solar wind electron distribution. This relationship strongly suggests that the ion acoustic waves detected by Helios far from the earth are produced by an electron heat flux instability, as was suggested by Forslund. Possible related mechanisms which could explain the generation of ion acoustic waves by protons streaming into the solar wind from the earth's bow shock are also considered.