Search

Mars Atmosphere and Volatile EvolutioN (MAVEN) has observed oscillations in the density, velocity, and temperature of ionospheric plasma on Mars. Atmospheric gravity waves can be an underlying mechanism. We propose a linearized wave–electron interaction model adopting a Wentzel–Kramers–Brillouin approximation to explore the electron density variations in the Martian day-side ionosphere for two regions, which are dominated by crustal magnetic fields and horizontal draped interplanetary magnetic fields. Our model results reveal that the electron density fluctuations associated with the crustal magnetic fields and the draped magnetic fields range from ∼ 40 per cent to ∼ 83 per cent and ∼ 29 per cent to ∼ 125 per cent, respectively. The wave-induced vertical electron flux peaks occur in a region ranging from ∼ 115 km to ∼ 179 km altitude. These results are comparable to the satellite observations. We further investigate the effect of the Martian magnetic topology on the wave-induced electron fluxes and demonstrate that the electron motions associated with the propagating gravity waves can be significantly influenced by the magnetic field orientations. The wave-induced variations in the electron temperature, ion density, and magnetic field combined with a comprehensive gravity wave model will be studied in further work.

The EXperiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) is a balloon-borne cryogenic telescope that will survey the spectrum of diffuse emission from both the Milky Way and the cosmic web to probe star formation, the interstellar medium, and galaxy evolution across cosmic time. EXCLAIM's primary extragalactic science survey maps 305 deg$^2$ along the celestial equator with an R=512 spectrometer over the frequency range \nu=420-540 GHz, targeting emission of the [CII] line over redshifts 2.5, Comment: 19 pages, 14 figures, will submit to MNRAS

When Mercury moves in its high-eccentricity orbit around the Sun, the background solar wind conditions are significantly changed. We investigated the effects of heliocentric distance and interplanetary magnetic field (IMF) orientation on the shape and location of Mercury’s bow shock. We fit empirical models to the bow shock crossings obtained from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging magnetometer observations under different conditions. Our results demonstrate that the bow shock moves antiplanetward when Mercury moves from perihelion to aphelion. However, this difference is not as significant as that of magnetopause due to the thickness variation of the magnetosheath. The IMF orientations show a weak influence on the bow shock shape and location, which implies that the southward magnetic field component is not the determinant factor that drives reconnection at the low-β magnetopause of Mercury.

Using the data from the Advanced Composition Explorer (ACE) and Wind spacecraft, we statistically studied the Parker spiral angle (PSA) of the solar wind magnetic field from 1998 to 2019 at 1 au. The PSA occurrences over both a Carrington rotation (CR) and a year can be well fitted by a Gaussian distribution. However, large-scale magnetic structures, such as interplanetary coronal mass ejections (ICMEs), can significantly deviate the PSA distribution of a CR from the Gaussian distribution. The PSA distributions of each CR and each year are affected by the solar activity: They are more concentrated at a relatively higher average PSA at solar maximum. There is also a weak anticorrelation between the yearly solar wind speed (v sw) and the average PSA. MESSENGER, Venus Express, and ACE observations at different heliocentric distances within 1 au show that the dominating polarities of the heliospheric magnetic field change greatly from year to year even when the solar activity is on the same level. Our results suggest that the PSA distribution in addition to the sunspot number can provide some new information on the magnetic field variation of the Sun.

The influences of the interplanetary magnetic field (IMF) on the induced magnetosphere of Venus are investigated using a global multispecies magnetohydrodynamics (MHD) model. The simulation results show that the induced magnetosphere is controlled by the IMF components perpendicular to the solar wind velocity (B Y and B Z in the Venus Solar Orbital coordinate), rather than the IMF magnitude (∣B∣). With the increase of ( B Y 2 + B Z 2 ) 1 2 , the induced magnetosphere becomes stronger in field strength and thicker in spatial scale, and the bow shock locates farther from the planet. The parallel IMF component (B X ) has relatively small impacts on the magnetic barrier and the magnetotail, regardless of the various IMF magnitudes and orientations caused by different B X . The responses of the Venusian induced magnetosphere to the change of upstream IMF are also studied. The time-dependent MHD calculations show that the dayside magnetosphere responds quickly with a timescale of 10 s–10 minutes, depending on the considered magnetospheric region. For comparison, the timescale required for the adjustment of magnetotail as driven by an IMF rotation is derived to be ∼10–20 minutes.

Plasma heating at thin current sheets in the solar wind is examined using magnetic field and plasma data obtained by the WIND spacecraft in the past 17 years from 2004 to 2019. In this study, a thin current sheet is defined by an abrupt rotation (larger than 45°) of the magnetic field direction in 3 s. A total of 57,814 current sheets have been identified, among which 25,018 current sheets are located in the slow wind and 19,842 current sheets are located in the fast wind. Significant plasma heating is found at current sheets in both slow and fast wind. Proton temperature increases more significantly at current sheets in the fast wind than in the slow wind, while the enhancement in electron temperature is less remarkable at current sheets in the fast wind. The results reveal that plasma heating commonly exists at thin current sheets in the solar wind regardless of the wind speed, but the underlying heating mechanisms might be different.

We perform a statistical study on the intermittency and the associated local heating in the front boundary layers (BLs) of 74 magnetic clouds (MCs). The intermittent structures are identified by the partial variance of increments (PVI) method. The probability distribution function of PVI-values reveals that the BLs are more intermittent than adjacent sheath regions, and they contain a greater concentration of strong intermittencies. These strong intermittencies are accompanied by local enhancement of the proton temperature, while the enhancement is not prominent at weaker intermittencies inside the BLs. Since the strong intermittencies are associated with magnetic reconnection (MR) processes according to previous studies, these results indicate that MR processes may account for the local heating in the MCBLs to a large extent.

Time History of Events and Macroscale Interactions during Substorms (THEMIS) data are used to investigate the magnetic field structures in the vicinity of the magnetopause. Generally, the tendency that the farther away from the Earth, the weaker the detected magnetic field is expected inside the dayside magnetopause. Here we show two cases which conflict with the expectation that the magnetic field gradient direction reverses from inward to outward in a short time interval. After a further analysis, it is found that the THEMIS probes encountered a magnetopause indentation moving along the magnetopause towards the dawn in one case, and for the other case, they crossed an evolutive indented magnetopause that was produced locally and then recovered to its normal state. These two magnetopause indentation may be related with the fast magnetosheath flow. Accordingly, we suppose that the fast magnetic gradient direction reverse is caused by the abnormal magnetic field distribution adjacent to the deformed magnetopause.

Magnetic reconnection is prevalent in the solar wind and is usually associated with interplanetary coronal mass ejections. We examined a Petschek-like reconnection exhaust (RE) in the front boundary of a magnetic cloud observed by the WIND spacecraft on 1998 June 2 and presented the first observation of a slow shock pair bounding the Petschek-like outflow jet in the interplanetary space. The whole structure contained an Alfvenic accelerated outflow and a pair of reverse slow shocks. The Alfvenic accelerated outflow was identified by Walen analysis. Rankine–Hugoniot relations were applied to confirm the slow shocks bounding the RE. Both shocks strictly satisfied the characteristics of slow shocks: (1) the intermediate Alfven Mach numbers were both below unit in the up/downstream region; (2) the slow Mach number was above unit in the upstream side but below unit in the downstream side. Plasma was compressed and heated across the trailing slow shock, especially in the shock jump layer that has a temperature 2.4 times that of the upstream.