RESUMO
The solar neighbourhood is the closest and most easily studied sample of the Galactic interstellar medium, an understanding of which is essential for models of star formation and galaxy evolution. Observations of an unexpectedly intense diffuse flux of easily absorbed 1/4-kiloelectronvolt X-rays, coupled with the discovery that interstellar space within about a hundred parsecs of the Sun is almost completely devoid of cool absorbing gas, led to a picture of a 'local cavity' filled with X-ray-emitting hot gas, dubbed the local hot bubble. This model was recently challenged by suggestions that the emission could instead be readily produced within the Solar System by heavy solar-wind ions exchanging electrons with neutral H and He in interplanetary space, potentially removing the major piece of evidence for the local existence of million-degree gas within the Galactic disk. Here we report observations showing that the total solar-wind charge-exchange contribution is approximately 40 per cent of the 1/4-keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports the notion of a million-degree hot bubble extending about a hundred parsecs from the Sun.
RESUMO
The Lunar Environment heliospheric X-ray Imager (LEXI) is a wide field-of-view soft X-ray telescope developed to study solar wind-magnetosphere coupling. LEXI is part of the Blue Ghost 1 mission comprised of 10 payloads to be deployed on the lunar surface. LEXI monitors the dayside magnetopause position and shape as a function of time by observing soft X-rays (0.1-2 keV) emitted from solar wind charge-exchange between exospheric neutrals and high charge-state solar wind plasma in the dayside magnetosheath. Measurements of the shape and position of the magnetopause are used to test temporal models of meso- and macro-scale magnetic reconnection. To image the boundary, LEXI employs lobster-eye optics to focus X-rays to a microchannel plate detector with a 9.1×∘9.1∘ field of view.
RESUMO
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.
RESUMO
A model describing changes in the frequency of drug use for an individual is presented. It is shown that the model predicts the evolution of a large number of long-time users who will exhibit behavior similar to heat flow, with the percentage of users per unit frequency analogous to temperature. Data from the NIDA 1985 Household Survey which suggest such behavior among users are presented, and they are shown to be consistent with a principle of increasing entropy.