90 research outputs found
X-ray Observation of Mars with Suzaku at Solar Minimun
Mars was observed in X-rays during April 3-5 2008 for 82 ksec with the
Japanese Suzaku observatory. Mars has been known to emit X-rays via the
scattering of solar X-rays and via the charge exchange between neutral atoms in
the exosphere and solar wind ions. Past theoretical studies suggest that the
exospheric neutral density may vary by a factor of up to 10 over the solar
cycle. To investigate a potential change of the exospheric charge exchange
emission, Mars was observed with Suzaku at solar minimum. Significant signals
were not detected at the position of Mars in the energy band of 0.2-5 keV. A 2
sigma upper limit of the O VII line flux in 0.5-0.65 keV was 4.3
ph cm s. Comparing this upper limit to the past Chandra and
XMM-Newton observations conducted near solar maximum, it was found that the
exospheric density at solar minimum does not exceed that near solar maximum by
more than 6-70 times.Comment: 17 pages, 7 figures, accepted for publication in PAS
Enhancement of Terrestrial Diffuse X-ray Emission Associated With Coronal Mass Ejection and Geomagnetic Storm
We present an analysis of a Suzaku observation taken during the geomagnetic
storm of 2005 August 23-24. We found time variation of diffuse soft X-ray
emission when a coronal mass ejection hit Earth and caused a geomagnetic storm.
The diffuse emission consists of fluorescent scattering of solar X-rays and
exospheric solarwind charge exchange. The former is characterized by a neutral
oxygen emission line due to strong heating of the upper atmosphere during the
storm time, while the latter is dominated by a sum of C V, C VI, N VI, N VII, O
VII, and O VIII emission lines due to the enhanced solar wind flux in the
vicinity of the exosphere. Using the solar wind data taken with the ACE and
WIND satellites,a time correlation between the solar wind and the strong O VII
line flux were investigated. We estimated necessary column densities for the
solar X-ray scattering and exospheric SWCX. From these results, we argue that a
part of the solar wind ions enter inside the magnetosphere and cause the SWCX
reaction.Comment: 33 pages, 16 figures, accepted for publication in PAS
Modeling of geocoronal solar wind charge exchange events detected with Suzaku
A model of geocoronal solar wind charge exchange (SWCX) emission was built
and compared to five Suzaku detections of bright geocoronal SWCX events. An
exospheric neutral hydrogen distribution model, charge exchange cross sections,
solar wind ion data taken with the ACE and WIND satellites, and magnetic field
models of the Earth's magnetosphere are all combined in order to predict
time-variable geocoronal SWCX emission depending on line-of-sight directions of
the Suzaku satellite. The modeled average intensities of O VII emission lines
were consistent with the observed ones within a factor of three in four out of
the five cases except for an event in which a line-of-sight direction was
toward the night side of the high-latitude magnetosheath and a major
geomagnetic storm was observed. Those of O VIII emission lines were
underestimated by a factor of three or more in all the five cases. On the other
hand, the modeled O VII and O VIII light curves reproduced the observed ones
after being scaled by ratios between the observed and modeled average
intensities. In particular, short-term variations due to line-of-sight
directions traversing cusp regions during an orbital motion of the Suzaku
satellite were reproduced. These results are discussed in the context of model
uncertainties.Comment: 47 pages, 23 figures, accepted for publication in PAS
Iron Emission Lines on the Galactic Ridge Observed with Suzaku
In order to elucidate origin of the Galactic Ridge X-ray Emission, we
analyzed Suzaku data taken at various regions along the Galactic plane and
studied their Fe-K emission line features. Suzaku resolved the Fe line complex
into three narrow lines at ~6.4 keV,~6.7 keV and ~6.97 keV, which are K-lines
from neutral (or low-ionized), He-like, and H-like iron ions, respectively. The
6.7 keV line is clearly seen in all the observed regions and its longitudinal
distribution is consistent with that determined from previous observations. The
6.4 keV emission line was also found in various Galactic plane regions (b~0).
Differences in flux ratios of the 6.4 keV/6.7 keV and 6.97 keV/6.7 keV lines
between the Galactic plane and the Galactic center regions are studied and its
implication is discussed.Comment: Accepted for publication in PASJ Suzaku 3rd special issu
Suzaku observation of Jupiter's X-rays around solar maximum
We report on results of imaging and spectral studies of X-ray emission from
Jupiter observed by Suzaku. In 2006 Suzaku had found diffuse X-ray emission in
1\unicode{x2013}5 keV associated with Jovian inner radiation belts. It has
been suggested that the emission is caused by the inverse-Compton scattering by
ultra-relativistic electrons ( MeV) in Jupiter's magnetosphere. To
confirm the existence of this emission and to understand its relation to the
solar activity, we conducted an additional Suzaku observation in 2014 around
the maximum of the 24th solar cycle. As a result, we successfully found again
the diffuse emission around Jupiter in 1\unicode{x2013}5 keV and also
point-like emission in 0.4\unicode{x2013}1 keV. The luminosity of the
point-like emission which was probably composed of solar X-ray scattering,
charge exchange, or auroral bremsstrahlung emission increased by a factor of with respect to 2006, most likely due to an increase of the solar
activity. The diffuse emission spectrum in the 1\unicode{x2013}5 keV band was
well-fitted with a flat power-law function () as in the
past observation, which supported the inverse-Compton scattering hypothesis.
However, its spatial distribution changed from Jovian
radius (Rj) to Rj. The luminosity of the diffuse emission
increased by a smaller factor of . This indicates that the diffuse
emission is not simply responding to the solar activity, which is also known to
cause little effect on the distribution of high-energy electrons around
Jupiter. Further sensitive study of the spatial and spectral distributions of
the diffuse hard X-ray emission is important to understand how high-energy
particles are accelerated in Jupiter's magnetosphere.Comment: 17 pages, 4 figures, 1 tabl
Spectral Study of the Galactic Ridge X-ray Emission with Suzaku
We have observed a typical Galactic plane field at (l,b) = (28.46d, -0.20d)
with Suzaku for 100 ksec to carry out a precise spectral study of the Galactic
Ridge X-ray Emission (GRXE). The field is known to be devoid of X-ray point
sources brighter than ~2 x 10^{-13} ergs s^{-1} cm^{-2} (2--10 keV), and
already deeply observed with Chandra. Thanks to the low and stable background
and high spectral resolution of Suzaku, we were able to resolve, for the first
time, three narrow iron K-emission lines from low-ionized (6.41 keV),
helium-like (6.67 keV), and hydrogenic ions (7.00 keV) in the GRXE spectrum.
These line features constrain the GRXE emission mechanisms: The cosmic-ray ion
charge exchange model or the non-equilibrium ionization plasma model are
unlikely, since they require either broad emission lines or lines at
intermediate ionization states. Collisional ionization equilibrium plasma is
the likely origin for the 6.67 keV and 7.00 keV lines, while origin of the 6.41
keV line, which is due to fluorescence from cold material, is not elucidated.
Low non-X-ray background and little stray-light contamination of Suzaku allowed
us to precisely measure the absolute X-ray surface brightness in the direction
of the Galactic plane. Excluding the point sources brighter than ~2 x 10^{-13}
ergs s^{-1} cm^{-2} (2--10 keV), the total surface brightness on the Galactic
plane is ~6.1 x 10^{-11} ergs s^{-1} cm^{-2} deg^{-2} (2--10 keV), including
the contribution of the cosmic X-ray background that is estimated to be ~1.3x
10^{-11} ergs s^{-1} cm^{-2} deg^{-2}.Comment: Accepted to PASJ second Suzaku Special issu
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