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Register a new userQuantified Energy Dissipation Rates: Electromagnetic Wave Observations in the Terrestrial Bow Shock
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We present the first quantified measure of the rate of energy dissipated per unit volume by high frequency electromagnetic waves in the transition region of the Earths collisionless bow shock using data from the THEMIS spacecraft. Every THEMIS shock crossing examined with available wave burst data showed both low frequency (< 10 Hz) magnetosonic-whistler waves and high frequency (> 10 Hz) electromagnetic and electrostatic waves throughout the entire transition region and into the magnetosheath. The waves in both frequency ranges had large amplitudes, but the higher frequency waves, which are the focus of this study, showed larger contributions to both the Poynting flux and the energy dissipation rates. The higher frequency waves were identified as combinations of ion-acoustic waves, electron cyclotron drift instability driven waves, electrostatic solitary waves, and whistler mode waves. These waves were found to have: (1) amplitudes capable of exceeding dB ~ 10 nT and dE ~ 300 mV/m, though more typical values were dB ~ 0.1-1.0 nT and dE ~ 10-50 mV/m; (2) energy fluxes in excess of 2000 x 10^(-6) W m^(-2); (3) resistivities > 9000 Ohm m; and (4) energy dissipation rates > 3 x 10^(-6) W m^(-3). The dissipation rates were found to be in excess of four orders of magnitude greater than was necessary to explain the increase in entropy across the shocks. Thus, the waves need only be, at times, < 0.01% efficient to balance the nonlinear wave steepening that produces the shocks. Therefore, these results show for the first time that high frequency electromagnetic and electrostatic waves have the capacity to regulate the global structure of collisionless shocks.
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The propagation of Langmuir waves in plasmas is known to be sensitive to density fluctuations. Such fluctuations may lead to the coexistence of wave pairs that have almost opposite wave-numbers in the vicinity of their reflection points. Using high frequency electric field measurements from the WIND satellite, we determine for the first time the wavelength of intense Langmuir wave packets that are generated upstream of the Earths electron foreshock by energetic electron beams. Surprisingly, the wavelength is found to be 2 to 3 times larger than the value expected from standard theory. These values are consistent with the presence of strong inhomogeneities in the solar wind plasma rather than with the effect of weak beam instabilities.
We report evidence of magnetic reconnection in the transition region of the terrestrial bow shock when the angle between the shock normal and the immediate upstream magnetic field is 65 degrees. An ion-skin-depth-scale current sheet exhibits the Hall current and field pattern, electron outflow jet, and enhanced energy conversion rate through the nonideal electric field, all consistent with a reconnection diffusion region close to the X-line. In the diffusion region, electrons are modulated by electromagnetic waves. An ion exhaust with energized field-aligned ions and electron parallel heating are observed in the same shock transition region. The energized ions are more separated from the inflowing ions in velocity above the current sheet than below, possibly due to the shear flow between the two inflow regions. The observation suggests that magnetic reconnection may contribute to shock energy dissipation.
We present a statistical analysis of more than two thousand bipolar electrostatic solitary waves (ESW) collected from ten quasi-perpendicular Earths bow shock crossings by Magnetospheric Multiscale spacecraft. We developed and implemented a correction procedure for reconstruction of actual electric fields, velocities, and other properties of ESW from measurements, whose spatial scales are typically comparable with or smaller than spatial distance between voltage-sensitive probes. We determined the optimal ratio between frequency response factors of axial and spin plane antennas to be around 1.65/1.8. We found that more than 95% of the ESW in the Earths bow shock are of negative polarity and present an in depth analysis of properties of these ESW. They have spatial scales of about 10--100 m that is within a range of $lambda_{D}$ to $10lambda_{D}$, amplitudes typically below a few Volts that is below 0.1 of local electron temperature, and velocities below a few hundreds km/s in spacecraft and plasma rest frames that is on the order of local ion-acoustic speed. The spatial scales of ESW are distinctly correlated with local Debye length $lambda_{D}$. ESW with amplitudes of 5--30 V or 0.1--0.3 Te have the occurrence rate of a few percent. The ESW have electric fields generally oblique to local magnetic field and propagate highly oblique to shock normal ${bf N}$; more than 80% of ESW propagate within 30$^{circ}$ of the shock plane. In the shock plane, ESW typically propagate within a few tens of degrees of local magnetic field projection ${bf B}_{rm LM}$ onto the shock plane and preferentially opposite to ${bf N}times {bf B}_{rm LM}$. We argue that the ESW of negative polarity are ion phase space holes produced in a nonlinear stage of ion-ion ion-streaming instabilities. We estimated lifetimes of the ion holes to be 10--100 ms, or 1--10 km in terms of spatial distance.
We present general empirical analytical equations of bow shock structures historically used at Mars, and show how to estimate automatically the statistical position of the bow shock with respect to spacecraft data from 2D polar and 3D quadratic fits. Analytical expressions of bow shock normal in 2D and 3D are given for any point on the shocks surface. This empirical technique is applicable to any planetary environment with a defined shock structure. Applied to the Martian environment and the NASA/MAVEN mission, the predicted shock location from ephemerides data is on average within 0.15 planetary radius $R_p$ of the actual bow shock crossing as seen from magnetometer data. Using a simple predictor-corrector algorithm based on the absolute median deviation of the total magnetic field and the general form of quasi-perpendicular shock structures, this estimate is further refined to within a few minutes of the true crossing (0.05 $R_p$). With the refined algorithm, 14,929 bow shock crossings, predominantly quasi-perpendicular, are detected between 2014 and 2021. Analytical 2D conic and 3D quadratic surface fits, as well as standoff distances, are given for Martian years 32 to 35, for several (seasonal) solar longitude ranges and for two solar EUV flux levels. Although asymmetry in $Y$ and $Z$ Mars Solar Orbital coordinates is on average small, it is shown that for Mars years 32 and 35, Ls = [135-225$^circ$] and high solar flux, it can become particularly noticeable and is superimposed to the usual North-South asymmetry due to the presence of crustal magnetic fields.
Non-thermal pickup ions (PUIs) are created in the solar wind (SW) by charge-exchange between SW ions (SWIs) and slow interstellar neutral atoms. It has long been theorized, but not directly observed, that PUIs should be preferentially heated at quasi-perpendicular shocks compared to thermal SWIs. We present in situ observations of interstellar hydrogen (H+) PUIs at an interplanetary shock by the New Horizons Solar Wind Around Pluto (SWAP) instrument at ~34 au from the Sun. At this shock, H+ PUIs are only a few percent of the total proton density but contain most of the internal particle pressure. A gradual reduction in SW flow speed and simultaneous heating of H+ SWIs is observed ahead of the shock, suggesting an upstream energetic particle pressure gradient. H+ SWIs lose ~85% of their energy flux across the shock and H+ PUIs are preferentially heated. Moreover, a PUI tail is observed downstream of the shock, such that the energy flux of all H+ PUIs is approximately six times that of H+ SWIs. We find that H+ PUIs, including their suprathermal tail, contain almost half of the total downstream energy flux in the shock frame.
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