The solar wind charge exchange (SWCX) process occurs when a high charge state solar wind ion (e.g., O7+) encounters a neutral atom or molecule (e.g., H) and picks up an electron in an excited state. The ion then emits a photon when it decays to a lower energy state and thus may lead to the satellite detection of soft X-rays. In the geospace environment, X-rays are emitted in the magnetosheath and cusp region where solar wind ions collide with geocoronal hydrogen.
Based on this mechanism, scientists from Chinese and Europe have put forward a joint proposal to image the Earth’s magnetosphere for the first time, named SMILE (Solar wind Magnetosphere Ionosphere Link Explorer).The geospace environment can be studied in two ways: by in situ measurements, or by remote sensing. Although in situ measurements provide precise information about plasma behavior locally, they cannot give the global view necessary to understand the large-scale configurations and overall evolution of the plasma. SMILE can provide that global view by wide field-of-view imaging of the X-ray emissions, and therefore, plays an essential role in magnetospheric studies and spaceweather predictions.
As one of the subtopics of this concept mission, Dr. SUN,Tianran, WANG Chi, and WEI Fei from the National Space Science Center (NSSC), in collaboration with Dr. Steve Sembay from the University of Leicester (UK), have simulated the X-ray emissions from the Kelvin-Helmholtz waves (KHW) at the magnetopause. Based on the global PPMLR MHD code (magneto-hydrodynamic simulation), a method is proposed to extract the KHW information from the X-ray intensity measured by a hypothetical X-ray telescope onboard a satellite assumed with a low Earth orbit. Specifically, the X-ray intensity at high latitude is subtracted from the intensity map as a background to highlight the role of KHW. To better prove that the KHW induced X-ray enhancement is detectable, the modeled SWCX emission is further converted into observed X-ray counts by using an X-ray telescope simulator. It is shown that KHW-induced X-ray is evident near the flank magnetopause, which provides vital support for the conclusion that X-ray imaging is a promising method to detect KHW.The validity of this method during intervals of solar winddisturbances is also verified.
Using this method, global features of KHW such as the vortex velocity, perturbation degree, spatial distribution, and temporal evolution could be evaluated from the X-ray intensity map. For a typical KH vortex, it is revealed that it moves tailward with a roughly constant angular velocity, i.e.,𝜔 = 0.10◦/s ≈ 0.0017 rad/s.The KHW rapidly develops from the dayside magnetopause toward the flanks, reaching the highest perturbation level just tailward of the flanks. Afterward, it gradually weakens toward the tail.In addition, itsspatial distribution also shows that the vortex develops to its mature stage slightly tailward of the flank magnetopause,where it has the largest latitudinal scale. According to these results, X-ray imaging of KHW is suggested as apromising observational technique to let us essentially “see” the large-scale configuration and evolution of KHWfor the first time.
This work was published on the Journal of Geophysical Research (JGR)---Space Physics.
Link: http://onlinelibrary.wiley.com/doi/10.1002/2014JA020497/abstract PDF
Citation: Sun, T. R., C. Wang, F. Wei, and S. Sembay (2015), X-ray imaging of Kelvin-Helmholtz waves at the magnetopause, J. Geophys. Res. Space Physics, 120, 266–275, doi:10.1002/2014JA020497.
Dr. SUN Tianran,
State Key Lab. of Space Weather, National Space Science Center, Chinese Academy of Sciences, 100190 Beijing, China
Figure: X-ray imaging of the Kelvin-Helmholtz wavesat different time (simulation results).
(Image by NSSC)