The LEXI and SMILE missions will provide soft X‐ray images of the Earth's magnetosheath and cusps after their anticipated launch in 2023 and 2024, respectively. The IBEX mission showed the potential of an Energetic Neutral Atom (ENA) instrument to image dayside magnetosheath and cusps, albeit over the long hours required to raster an image with a single pixel imager. Thus, it is timely to discuss the two imaging techniques and relevant science topics. We simulate soft X‐ray and low‐ENA images that might be observed by a virtual spacecraft during two interesting solar wind scenarios: a southward turning of the interplanetary magnetic field and a sudden enhancement of the solar wind dynamic pressure. We employ the OpenGGCM global magnetohydrodynamics model and a simple exospheric neutral density model for these calculations. Both the magnetosheath and the cusps generate strong soft X‐rays and ENA signals that can be used to extract the locations and motions of the bow shock and magnetopause. Magnetopause erosion corresponds closely to the enhancement of dayside reconnection rate obtained from the OpenGGCM model, indicating that images can be used to understand global‐scale magnetopause reconnection. When dayside imagers are installed with high‐ENA inner‐magnetosphere and FUV/UV aurora imagers, we can trace the solar wind energy flow from the bow shock to the magnetosphere and then to the ionosphere in a self‐standing manner without relying upon other observatories. Soft X‐ray and/or ENA imagers can also unveil the dayside exosphere density structure and its response to space weather.
Interactions between solar wind ions and neutral hydrogen atoms in Earth's exosphere can lead to the emission of soft X‐rays. Upcoming missions such as SMILE and LEXI aim to use soft X‐ray imaging to study the global structure of the magnetosphere. Although the magnetosheath and dayside magnetopause can often be driven by kinetic physics, it has typically been omitted from fluid simulations used to predict X‐ray emissions. We study the possible results of soft X‐ray imaging using hybrid simulations under quasi‐radial interplanetary magnetic fields, where ion‐ion instabilities drive ultra‐low frequency foreshock waves, leading to turbulence in the magnetosheath, affecting the dynamics of the cusp and magnetopause. We simulate soft X‐ray emission to determine what may be seen by missions such as LEXI, and evaluate the possibility of identifying kinetic structures. While kinetic structures are visible in high‐cadence imaging, current instruments may not have the time resolution to discern kinetic signals.
more » « less- Award ID(s):
- 2010231
- NSF-PAR ID:
- 10412867
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 50
- Issue:
- 10
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The Earth's magnetosheath and cusps emit soft X‐rays due to the charge exchange between highly charged solar wind ions and exospheric hydrogen atoms. The Lunar Environment Heliospheric X‐ray Imager and Solar wind Magnetosphere Ionosphere Link Explorer missions are scheduled to image the Earth's dayside magnetosphere system in soft X‐rays to investigate global‐scale magnetopause reconnection modes under varying solar wind conditions. The exospheric neutral hydrogen density distribution, especially the value of this density at the subsolar magnetopause is of particular interest for understanding X‐ray emissions near this boundary. This paper estimates the exospheric density during solar minimum using the X‐ray Multimirror Mission (XMM) astrophysics observatory. We selected an event on 12 November 2008 from the XMM data archive, which detects soft X‐rays of magnetosheath origin while solar wind and interplanetary magnetic field conditions are relatively constant. During the event the location of the magnetopause was measured in situ by the THEMIS mission, thus the location of the solar wind ions responsible for the magnetosheath emission is well constrained by observation. We estimated the exospheric density using the Open Geospace Global Circulation Model (OpenGGCM) and a spherically symmetric exosphere model. The ratio of the magnetosheath plasma flux between the OpenGGCM model and the THEMIS, was nearly 1, which means the magnetohydrodynamic model reasonably reproduces the magnetosheath plasma conditions. The OpenGGCM magnetosheath parameters were used to deconvolve soft X‐rays of exospheric origin from the XMM signal. The lower‐limit of the exospheric density of this solar minimum event is 36.8 ± 11.7 cm−3at 10 R
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Abstract We investigate the exospheric neutral density near the subsolar magnetopause, assuming
X gse = 10R E 39.9 ± 8.0 and57.6 ± 8.0 cm−3 for the 4 May 2003 and 16 October 2001 events, respectively. Since our MHD tends to underestimate neutral densities due to its thicker magnetosheath, the true neutral density is likely to lie within the upper half of these error bars. -
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Abstract Revealing the formation, dynamics, and contribution to plasma heating of magnetic field fluctuations in the solar wind is an important task for heliospheric physics and for a general plasma turbulence theory. Spacecraft observations in the solar wind are limited to spatially localized measurements, so that the evolution of fluctuation properties with solar wind propagation is mostly studied via statistical analyses of data sets collected by different spacecraft at various radial distances from the Sun. In this study we investigate the evolution of turbulence in the Earth’s magnetosheath, a plasma system sharing many properties with the solar wind. The near-Earth space environment is being explored by multiple spacecraft missions, which may allow us to trace the evolution of magnetosheath fluctuations with simultaneous measurements at different distances from their origin, the Earth’s bow shock. We compare ARTEMIS and Magnetospheric Multiscale (MMS) Mission measurements in the Earth magnetosheath and Parker Solar Probe measurements of the solar wind at different radial distances. The comparison is supported by three numerical simulations of the magnetosheath magnetic and plasma fluctuations: global hybrid simulation resolving ion kinetic and including effects of Earth’s dipole field and realistic bow shock, hybrid and Hall-MHD simulations in expanding boxes that mimic the magnetosheath volume expansion with the radial distance from the dayside bow shock. The comparison shows that the magnetosheath can be considered as a miniaturized version of the solar wind system with much stronger plasma thermal anisotropy and an almost equal amount of forward and backward propagating Alfvén waves. Thus, many processes, such as turbulence development and kinetic instability contributions to plasma heating, occurring on slow timescales and over large distances in the solar wind, occur more rapidly in the magnetosheath and can be investigated in detail by multiple near-Earth spacecraft.
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Abstract Entire fields of science, most notably in astrophysics, rely on line‐of‐sight observations. In planetary science and heliophysics, the techniques of soft X‐ray and energetic neutral atom imaging also produce line‐of‐sight measurements. An important question is whether the geometry of the surface, for example, the magnetopause, can be reconstructed using only line‐of‐sight observations from a single spacecraft. Under a broad range of conditions, the peak emission corresponds to the tangent to the boundary surface, such as the planetary surface or magnetopause, the so‐called
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