Ina P. Robertson

X-rays from solar system objects

Abstract During the last few years our knowledge about the X-ray emission from bodies within the solar system has significantly improved. Several new solar system objects are now known to shine in X-rays at energies below 2xa0keV. Apart from the Sun, the known X-ray emitters now include planets (Venus, Earth, Mars, Jupiter, and Saturn), planetary satellites (Moon, Io, Europa, and Ganymede), all active comets, the Io plasma torus (IPT), the rings of Saturn, the coronae (exospheres) of Earth and Mars, and the heliosphere. The advent of higher-resolution X-ray spectroscopy with the Chandra and XMM-Newton X-ray observatories has been of great benefit in advancing the field of planetary X-ray astronomy. Progress in modeling X-ray emission, laboratory studies of X-ray production, and theoretical calculations of cross-sections, have all contributed to our understanding of processes that produce X-rays from the solar system bodies. At Jupiter and Earth, both auroral and non-auroral disk X-ray emissions have been observed. X-rays have been detected from Saturns disk, but no convincing evidence of an X-ray aurora has been observed. The first soft (0.1–2xa0keV) X-ray observation of Earths aurora by Chandra shows that it is highly variable. The non-auroral X-ray emissions from Jupiter, Saturn, and Earth, those from the disk of Mars, Venus, and Moon, and from the rings of Saturn, are mainly produced by scattering of solar X-rays. The spectral characteristics of X-ray emission from comets, the heliosphere, the geocorona, and the Martian halo are quite similar, but they appear to be quite different from those of Jovian auroral X-rays. X-rays from the Galilean satellites and the IPT are mostly driven by impact of Jovian magnetospheric particles. This paper reviews studies of the soft X-ray emission from the solar system bodies, excluding the Sun. Processes of production of solar system X-rays are discussed and an overview is provided of the main source mechanisms of X-ray production at each object. A brief account on recent development in the area of laboratory studies of X-ray production is also provided.

Spatial maps of heliospheric and geocoronal X‐ray intensities due to the charge exchange of the solar wind with neutrals

[1]xa0X-rays are generated throughout the heliosphere and the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and interstellar and geocoronal neutrals, respectively. Simple models of this X-ray emission have been presented in the past, but results from a more sophisticated model are described in this paper. In particular, to obtain the densities of interstellar neutrals our X-ray model uses to determine X-ray intensities, we use the Fahr [1971] hot model although with recently determined input parameters. The geocoronal X-ray model we use in this paper has also been improved, as described in a recent paper by Robertson and Cravens [2003]. In previous papers we demonstrated that there is significant correlation between the “long-term enhancement” part of the soft X-ray background measured by the Rontgen Satellite (ROSAT) and the solar wind proton flux. In the current paper we determine the steady-state X-ray intensities due to the interaction between the solar wind and both interstellar neutrals and the geocoronal neutrals as a function of look direction and time of year. X-ray intensity maps are shown for both a spherically symmetric solar wind and for a latitude-dependent solar wind (i.e., fast and slow solar wind regions). In all cases, the X-ray intensity is highest when the view direction is towards the Sun, but the intensity is also relatively high for view directions intersecting the gravitational focusing cone of interstellar helium. We also show a heliospheric/geocoronal X-ray intensity map for the conditions used by Snowden et al. [1995] in producing the 1/4 keV channel soft X-ray background map in galactic coordinates. Our preliminary conclusion is that very roughly 50% of the total background soft X-ray intensity in the galactic plane and 25% at high galactic latitudes can be attributed to the charge transfer process operating within the solar system, with the remaining emission coming from outside our heliosphere.

X‐ray emission from the terrestrial magnetosheath

[1] X-rays are generated throughout the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and geocoronal neutrals. The solar wind ions resulting from these collisions are left in highly excited states and emit extreme ultraviolet or soft X-ray photons. A model has been created to simulate this X-ray radiation. Published terrestrial exospheric hydrogen distributions and solar wind speed, density and temperature distributions were used in this model. Simulated images were created as seen from an observation point outside the geocorona. The locations of the bow shock and magnetopause are evident in these images. Perhaps this X-ray emission can be used to remotely sense the solar wind flow around the magnetosphere. Since similar X-rays are produced in the heliosphere, the challenge will be, however, to eliminate this background emission.

DXL: a sounding rocket mission for the study of solar wind charge exchange and local hot bubble X-ray emission

The Diffuse X-rays from the Local galaxy (DXL) mission is an approved sounding rocket project with a first launch scheduled around December 2012. Its goal is to identify and separate the X-ray emission generated by solar wind charge exchange from that of the local hot bubble to improve our understanding of both. With 1,000xa0cm2 proportional counters and grasp of about 10xa0cm2 sr both in the 1/4 and 3/4xa0keV bands, DXL will achieve in a 5-min flight what cannot be achieved by current and future X-ray satellites.

Astrophysics Noise: A Space Weather Signal

Imagine the accuracy of terrestrial weather forecasts if society relied on only a handful of isolated weather stations to supply all the input to meteorological models. Yet that is precisely the daunting situation faced by space weather forecasters, who seek to predict when and how ejections of plasma from the Sun will interact with the Earths magnetosphere. These interactions can damage spacecraft electronics, produce spurious global positioning and navigation readings, interfere with radio communications, and disrupt electrical power line grids on the ground. Though modern society increasingly relies on satellite technology and electrical conveniences, only a handful of operating heliophysics missions supply the bulk of space weather model inputs.

Invited article: First flight in space of a wide-field-of-view soft x-ray imager using lobster-eye optics: Instrument description and initial flight results

We describe the development, launch into space, and initial results from a prototype wide field-of-view soft X-ray imager that employs lobster-eye optics and targets heliophysics, planetary, and astrophysics science. The sheath transport observer for the redistribution of mass is the first instrument using this type of optics launched into space and provides proof-of-concept for future flight instruments capable of imaging structures such as the terrestrial cusp, the entire dayside magnetosheath from outside the magnetosphere, comets, the Moon, and the solar wind interaction with planetary bodies like Venus and Mars [Kuntz et al., Astrophys. J. (in press)].

The DXL and STORM sounding rocket mission

The objective of the Diffuse X-ray emission from the Local Galaxy (DXL) sounding rocket experiment is to distinguish the soft X-ray emission due to the Local Hot Bubble (LHB) from that produced via Solar Wind charge exchange (SWCX). Enhanced interplanetary helium density in the helium focusing cone provides a spatial variation to the SWCX that can be identified by scanning through the focusing cone using an X-ray instrument with a large grasp. DXL consists of two large proportional counters refurbished from the Aerobee payload used during the Wisconsin All Sky Survey. The counters utilize P-10 fill gas and are covered by a thin Formvar window (with Cyasorb UV-24 additive) supported on a nickel mesh. DXLs large grasp is 10 cm2 sr for both the 1/4 and 3/4 keV bands. DXL was successfully launched from White Sands Missile Range, New Mexico on December 12, 2012 using a Terrier Mk70 Black Brant IX sounding rocket. The Sheath Transport Observer for the Redistribution of Mass (STORM) instrument is a prototype soft X-ray camera also successfully own on the DXL sounding rocket. STORM uses newly developed slumped micropore (`lobster eye) optics to focus X-rays onto a position sensitive, chevron configuration, microchannel plate detector. The slumped micropore optics have a 75 cm curvature radius and a polyimide/aluminum filter bonded to its surface. STORMs large field-of-view makes it ideal for imaging SWCX with exospheric hydrogen for future missions. STORM represents the first flight of lobster-eye optics in space.

The Heliospheric Contribution to the Soft X‐Ray Background Emission

The soft x‐ray background observed from Earth contains contributions not only from outside the solar system such as the local bubble but contributions from within the solar system including from the interplanetary medium and from the terrestrial geocorona. Great effort was spent on removing non‐cosmic contamination from data collected during the ROSAT all‐sky survey. Some of the contamination, however, was due to x‐ray emission from solar wind charge exchange with interstellar and geocoronal neutrals. The time varying component of this contamination was removed but the steady state component was not. In this paper we will discuss our method of calculating the steady state component of solar wind charge exchange contamination and will present all‐sky maps of the soft x‐ray emission with this steady state component removed, which will allow for a re‐interpretation of the nature of the local interstellar bubble. This method also can be used to obtain information on solar wind fluxes and on solar wind composi...

The Lunar X-ray Observatory (LXO)

X-ray emission from charge exchange recombination between the highly ionized solar wind and neutral material in Earths magnetosheath has complicated x-ray observations of celestial objects with x-ray observatories including ROSAT, Chandra, XMM-Newton, and Suzaku. However, the charge-exchange emission can also be used as an important diagnostic of the solar-wind interacting with the magnetosheath. Soft x-ray observations from low-earth orbit or even the highly eccentric orbits of Chandra and XMM-Newton are likely superpositions of the celestial object of interest, the true extra-solar soft x-ray background, geospheric charge exchange, and heliospheric charge exchange. We show that with a small x-ray telescope placed either on the moon, in a similar vein as the Apollo ALSEP instruments, or in a stable orbit at a similar distance from the earth, we can begin to disentangle the complicated emission structure in the soft x-ray band. Here we present initial results of a feasibility study recently funded by NASA to place a small x-ray telescope on the lunar surface. The telescope operates during lunar night to observe charge exchange interactions between the solar wind and magnetosphic neutrals, between the solar wind and the lunar atmosphere, and an unobstructed view of the soft x-ray background without the geospheric component.

X‐Ray Emissions from Charge Exchange in the Heliosphere

X‐Rays are generated throughout the heliosphere and the terrestrial magnetosheath as a consequence of charge transfer between heavy solar wind ions and interstellar and geocoronal neutrals. These X‐ray intensities, as observed from Earth, depend on look direction and season. Due to rapid variation in geocoronal X‐ray emissions in response to time variations in the solar wind flux, it might very well be possible to observe Earth’s magnetosheath from an observation point outside the geocorona. In previous papers we have shown that there is significant correlation between “long term enhancements” in the soft X‐ray background measured by the Rontgen Satellite (ROSAT), and that roughly 25–50% of the soft X‐ray background maps are of heliospheric origin. In this paper we present a preliminary simulated soft X‐ray image of the heliosphere as would be seen by an external observer.