Abstract
Solar wind charge-exchange (SWCX) emission is present in every X-ray observation of an astrophysical object. The emission is problematic when one cannot remove the foreground by the simultaneous measurement of a nearby field. SWCX emission is a serious impediment to the study of the diffuse hot ISM, including the galactic halo, as its contribution to diagnostic emission lines is temporally variable. Modeling the SWCX emission, in order to remove it from our observations, has proven to be more difficult than originally anticipated. This work reviews our current understanding of SWCX emission, with special attention to all the components required for future modeling tools. Since, in the absence of such a tool, observing programs can still be constructed to minimize the effect of SWCX, mitigation strategies are discussed. Although some aspects of SWCX will be very difficult to characterize, progress continues on many fronts.
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Notes
Disciplinary boundaries are usually poorly defined, and discipline labels are not always a good fit. “Heliophysics”, as defined by NASA, is the study of the heliosphere, excluding planets.
In this case, “magnetospheric” is a misnomer. The magnetosphere is the region containing the Earth’s magnetic field. The solar wind does not enter this region and, thus, the magnetosphere is free of SWCX emission. The magnetosheath is the region between the magnetopause (the boundary of the magnetosphere) and the bow shock, where the solar wind can interact with the neutral exosphere. However, “magnetosheathic” is a bit cumbersome, so common practice has been to use “magnetospheric” even though it is not correct.
The “helium focussing cone” is the region of the solar system with a higher density of neutral helium due to gravitational focussing by the sun of the neutral helium from the ISM (see Sect. 5.1). It would be better to call it the “focussed helium cone”, but the current usage is well established.
Incidently, there are \(\sim \)6 comets detected in the ROSAT All-Sky Survey, so it is likely that someone would have realized that comets emit X-rays had Lisse not done so spectacularly, but it probably would have taken much longer!
Since the study of SWCX falls at the intersection of multiple fields of study, it is very important to note confusing and conflicting conventions. For example, what an astrophysicist would refer to as an O VIII line (“oxygen eight”), a space physicist would refer to as \(\hbox {O}^{+7}\) (“oxygen plus seven” or sometimes just “oxygen seven”) line. In this context, of course, the line is due to the recombination of \(\hbox {O}^{+8}\) (“oxygen eight”) which can lead to a new level of confusion. This work will use the astrophysical convention for identifying lines, but will use the space physics convention when referring to the ions themselves.
These four observations were being used to test background subtraction software. The post doc who was working on the problem assumed that there was something wrong his software for several weeks.
For magnetospheric studies, the convenient length unit is the terrestrial radius, \({R}_\mathrm{E}\). Although that quantity is rather ambiguous, it is generally taken to be 6371 km. For scale, note that the Moon’s orbit has a semi-major axis of 60.4 \({R}_\mathrm{E}\), while a geosynchronous orbit has a radius of 6.6 \({R}_\mathrm{E}\).
The geocentric solar ecliptic (GSE) coordinate system is a right-handed coordinate system centered on the Earth that has as its + X axis the vector from the Earth to the Sun and as its + Z axis the direction to the north ecliptic pole. The + Y axis lies in the plane of the ecliptic in the direction opposite to the Earth’s motion. The GSE coordinate system is like the ecliptic coordinate system, except the zero point is the Sun rather than the vernal equinox. The geocentric solar magnetospheric (GSM) coordinate system is a right-handed coordinate system entered on the Earth that has as its + X axis the vector from the Earth to the Sun. The + Z axis is the projection of the Earth’s magnetic pole on a plane perpendicular to the + X axis. The \(X_\mathrm{GSE}\) and \(X_\mathrm{GSM}\) axes are the same, and the other two are rotated around that axis with respect to one another.
The solar rotation axis is inclined to the ecliptic pole by \(7.25{^\circ }\). The line of nodes is at an ecliptic longitude of \(76{^\circ }\). Thus, while ecliptic latitudes are similar to solar latitudes, they are not the same. This difference can be very important when assessing whether a particular line of sight is within the solar equatorial flow or not.
After 23 August 2011, only a much more restricted set of elements/ionization states are available and then only as abundance ratios.
ENLIL is not an acronym, it is the name of the Mesopotamian god of, among other things, the winds.
For those who would note that a current cannot be sustained without a loop, the cross-tail current is connected to the tail current which flows across the surface of the lobes of the magnetotail back to the other side of the plasma sheet. The above description is not an exhaustive description of all the plasmas and current systems, just a description of those most salient for the issue of MHD modeling.
The direction of the Sun’s motion with respect to the local standard of rest is towards \((\ell ,b)=(47{.^\circ }8,23{.^\circ }7)\) while the upwind direction is \((\ell ,b)=(5{.^\circ }6,19{.^\circ }57)\) though the neutrals are not entirely undeviated. Since the upwind direction is so close to the galactic center, X-ray studies of charge exchange emission from the nose of the heliosphere are infeasible. Frisch (1996) has drawn attention to this peculiarly infelicitous alignment which prevents the X-ray study of the nose of the heliosheath.
These papers would form an interesting study in the history and philosophy of science. None of them actually claims a heliopause crossing, just the abrupt crossing into a region that is clearly not the inner heliopause. However, later papers cite these in retrospect in terms of the heliopause crossing.
Should any reader wish to implement this model, be aware that the plots in Hodges (1994) were calculated with a higher order model than the model whose coefficients are published!
As shown in Fig. 8, the ionization temperatures calculated from the ratio of the number of ions in state \(n+1\) to the number in state n are a function of n. Thus, using a single ionization temperature for an element could be problematic if there a number of different ionization states represented within the bandpass of interest.
We use “hydrogen-like” rather than “hydrogenic” to maintain the parallel with “helium-like”, etc. which have no similar form. It should be noted that “hydrogenic”, when read by a chemist, would mean “producing water”.
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Acknowledgements
This work would not have been possible without a large number of individuals. When Steve Snowden and I were working on the XMM-Newton background, we ran into our first undeniable case of SWCX. In search of illumination, Steve made the dangerous trek across the street, from GSFC building 2 (now defunct) to building 21, and started knocking on doors until he found someone who actually thought that the problem was interesting, Michael Collier. Since then the SWCX family at Goddard has grown to include Scott Porter, David Sibeck, Yari Collado-Vega, Hyunju Connor, and Brian Walsh, all of whom have been pestered by me with naive questions about laboratory astrophysics, space physics and the solar wind for nearly a decade. Also suffering my naive questions have been Robert Allen, Tom Cravens, Renata Cumbee, Adam Foster, Maurice Leutenegger, Ina Robertson, and Randall Smith. Nick Thomas and Lynne Valencic, not being space physicists, have provided the appropriate astrophysics based feedback. The Leicester group, Steve Sembay, Andy Read, Jenny Carter, and Ian Whittaker have been working hard on the magnetospheric emission seen by XMM-Newton and have provided invaluable insights as well as great companionship. Massimiliano Galeazzi and the Miami group, whose work is rather underrepresented here, has provided a great deal of information about works in progress. Our late night discussions at Poker Flat helped structure this paper. Renzo Principe red-penned numerous drafts, correcting all the mangetospheres and disambiguating the acronyms. A very special thanks go to Dimitra Koutroumpa, an integral part of the SWCX family, who is the expert sine qua non on heliospheric SWCX, whose patience with me has been limitless. Numerous plots have been possible only with the loan of her models. The Coordinated Community Modeling Center (the CCMC) has provided a large number of simulations for this work and many of the works cited. Their help has been indefatigable. Leila Mays helped set up the special ENLIL simulation used here. Lutz Rastaetter provides great support for the BATS-R-US runs, helping us to set them up properly and to understand the results when they do not meet our expectations. My thanks also to OMNI (https://omniweb.gsfc.nasa.gov/) for providing a uniform set of solar wind data, as well as the ACE Science Center (http://www.srl.caltech.edu/ACE/ASC/) and the Ulysses Final Archive (http://ufa.esac.esa.int/ufa/), for providing the solar wind abundance and ionization data required for this work. I would also thank the several institutions that have hosted me during the work on this article, the Science History Institute (Philadelphia) and the International Space Science Institute (Bern). The first version of this review was a talk constructed for the Chandra X-ray Center and the Smithsonian Astrophysical Observatory in honor of Steve Murray. Finally, I must thank Joel Bregman for his patience, and Kim Weaver and the XMM-Newton GOF for support and forebearance. And thank you, Steve, without you I might not have found this problem.
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This work was supported by the XMM Guest Observer Facility at NASA GSFC.
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Kuntz, K.D. Solar wind charge exchange: an astrophysical nuisance. Astron Astrophys Rev 27, 1 (2019). https://doi.org/10.1007/s00159-018-0114-0
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DOI: https://doi.org/10.1007/s00159-018-0114-0