PASJ: Publ. Astron. Soc. Japan 65, 63, 2013 June 25
c 2013. Astronomical Society of Japan.
Suzaku Observation of Strong Solar-Wind Charge-Exchange Emission
from the Terrestrial Exosphere during a Geomagnetic Storm
Kumi I SHIKAWA,1 Yuichiro E ZOE,1 Yoshizumi M IYOSHI,2 Naoki T ERADA,3 Kazuhisa M ITSUDA,4 and Takaya O HASHI,1
1 Department
of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397
2 Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601
3 Department of Geophysics, Tohoku University, 6-3 Aoba, Aramakiaza, Aoba, Sendai, Miyagi 980-8578
4 The Institute of Space and Astronautical Science (ISAS), Japan Aerospace and eXpoloration Agency (JAXA),
3-1-1 Yoshinodai, Chuo, Sagamihara 252-5210
[email protected]
(Received 2012 July 13; accepted 2013 January 21)
Abstract
We present an analysis of X-ray data exhibiting strong solar-wind charge-exchange emission obtained with the
Suzaku satellite during a geomagnetic storm that occurred on 2005 August 31. A temporal variation of diffuse
soft X-ray emission, correlating with the solar-wind proton flux, was found. The diffuse emission consisted of
exospheric solar-wind charge exchange (geocoronal SWCX). We extracted the variable component of the spectrum,
which is dominated by a sum of C V, C VI, N VI, N VII, O VII, and O VIII emission lines, predicted by the theoretical SWCX model. An analysis of any time correlation between the solar wind and the O VII line flux was conducted
using the solar-wind data taken with the ACE and WIND satellites. We found that the observed SWCX intensity
was 4–10 times higher than that calculated using a model of exospheric hydrogen density, and the O7+ flux observed
with ACE at the L1 point. This suggests that the exospheric hydrogen density can be higher than that predicted by
the hydrogen model, and/or that additional O7+ ions to the measured ACE fluxes could exist in the magnetosphere.
Comparing this observation to past incidences of SWCX emission, as recorded by Suzaku, it was found that the
SWCX intensity in this observation was strong, in spite of the fact that the line-of-sight direction did not traverse the
sub-solar magnetosheath nor the magnetospheric cusps, where higher SWCX emission would be expected.
Key words: Earth — Sun: solar-terrestial relations — Sun: solar wind — X-rays: diffuse background
1.
Introduction
Terrestrial X-rays are emitted by different mechanisms,
depending on the spatial regions. At auroral regions, electron bremsstrahlung is the main X-ray production mechanism (Berger & Seltzer 1972; Petrinec et al. 2000; Stadsnes
et al. 1997). In the disk (non auroral) region or upper atmosphere, the X-ray emission lines are emitted due to scatterings of solar X-rays (Petrinec et al. 2000). Within the
terrestrial exosphere, solar wind charge exchange (SWCX)
induced emission lines are observed. The SWCX mechanism
was proposed to explain the X-ray emission observed from
the comet Hyakutake (Cravens 1997). The mechanism is as
follows. A highly charged-state ion in the solar wind interacts with a neutral atom or molecule, and an electron is transferred from the neutral to the ion in an excited state. When the
electron transits to the ground state, an extreme ultraviolet or
soft X-ray photon is emitted. To date, SWCX emissions from
comets and planets, and heliosphere have been observed (see
Bhardwaj et al. 2007; Dennerl 2010 for review).
The SWCX emission from the comet Hyakutake was found
to show temporal variations (Lisse et al. 1996; Dennerl et al.
1997). Cox (1998) predicted that the SWCX in the terrestrial
exosphere (geocoronal SWCX) can explain a part of the longterm enhancements (LTE) in the soft X-ray background that
was discovered during the ROSAT all-sky survey (Snowden
et al. 1994). The variation of the cometary SWCX emission can
be attributed to temporal variations in the solar-wind flux and
in the relative solar-wind heavy-ion composition (Neugebauer
et al. 2000). From an the analogy with this, the temporal variation of the geocoronal SWCX has been estimated based on
the SWCX model, and the measured solar-wind parameters as
a function of time. Geocoronal SWCX has also been suggested
to be a part of the LTE (Cravens et al. 2001). SWCX emission
is useful to investigate not only the LTE, but also exospheric
neutral densities and the transport of the solar-wind plasma
in the magnetosphere. Models to simulate the spatial distribution of geocoronal SWCX emission have been constructed,
and it has been suggested that it is possible to remotely
sense the magnetosphere using SWCX emission (Robertson
& Cravens 2003; Robertson et al. 2006; Collier et al. 2010,
2012; Branduardi-Raymont et al. 2012).
A simulation
in Robertson et al. (2006) indicated that the SWCX emission
became strong during the observation through the sub-solar
region of the Earth’s magnetosheath and the cusp regions.
Observational results, which support predictions of the
temporal variation, have been taken with Chandra (Wargelin
et al. 2004), XMM-Newton (Snowden et al. 2004; Carter &
Sembay 2008; Carter et al. 2010, 2011), and Suzaku (Fujimoto
et al. 2007; Ezoe et al. 2010, 2011). A systematic search
of geocoronal SWCX emission with XMM-Newton archive
data showed that most of the SWCX emission was seen when
XMM-Newton observed through the sub-solar side of the
magnetosheath (Carter & Sembay 2008; Carter et al. 2011),
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K. Ishikawa et al.
[Vol. 65,
Fig. 1. Average line of sight in the GSE XY and XZ planes during the observation. A filled black circle in each plot represents Earth. Two solid curves
indicate approximate positions of the magnetopause and bow shock as a guide.
where the highest SWCX flux is predicted (Robertson et al.
2006). A model by Robertson et al. (2006) predicts considerable variation in the SWCX emissivity throughout the magnetosheath region. In some cases, SWCX emission was seen
when the line-of-sight direction did not intersect the highest
expected flux region. Carter and Sembay (2008) concluded
that these cases probably originate from coronal mass ejections (CMEs). They studied the most spectrally rich example of
SWCX, and argued that this event was associated with a CME
recorded on 2001 October 21 (Carter et al. 2010).
Suzaku is the 5th Japanese X-ray astronomy satellite, which
was launched in 2005 (Mitsuda et al. 2007). The X-ray
CCD camera (XIS: X-ray Imaging Spectrometer) onboard
Suzaku has good energy resolution ( 65 eV at 1 keV) and
response, and a low background rate (Koyama et al. 2007).
The XIS is composed of four X-ray CCDs. Three CCDs
are front-illuminated (FI) types, and the other one is backilluminated (BI). The total effective area of the XIS instrument
is 1000 cm2 . Therefore, Suzaku is an ideal observatory with
which to study diffuse X-ray emission, like geocoronal SWCX.
Furthermore, since Suzaku is placed in a near-circular orbit
with an altitude of 550 km, we can obtain SWCX emission
data originating from the near-Earth region. Suzaku observed
geocoronal SWCX emission while pointing in the direction
of the north ecliptic pole (Fujimoto et al. 2007), and also
observed geocoronal SWCX emission toward the sub-solar
side of the magnetosheath (Ezoe et al. 2010). Ezoe et al. (2011)
found evidence of a strong enhancement of geocoronal SWCX
emission associated with a strong geomagnetic storm. They
established a method to extract the SWCX emission from the
original Suzaku data, including removing resolved astronomical sources from the field of view, using the the temporal
variation of the X-ray light curve and solar wind obtained
at the L1 point.
The authors are in the process of systematically analyzing
geocoronal SWCX emission with all available and appropriate
Suzaku data. In this paper, we present evidence of a case
involving strong SWCX emission observed in the direction
of the southern hemisphere. The line of sight for this emission did not traverse either the sub-solar region or the cusps
of the magnetosphere, even though both regions are expected
to be bright in geocoronal SWCX emission. The data for the
observation presented here were taken during a period of a Dst
index of < 100 nT, and shows stronger SWCX emission than
other Suzaku data. The Dst index is a measure of geomagnetic activity, and monitors disturbances in the magnetic field
at Earth’s surface. The index is an average of deviations
from a quiescent level of the horizontal magnetic field component measured at near equatorial geomagnetic observatories.
A Dst index of zero means that no deviation has occurred
due to quiescent conditions. Negative values of the Dst
index indicate that a geomagnetic storm is in progress, with
a more negative Dst index implying a more intense geomagnetic storm. Negative deflections in the Dst index are caused
by the growth of magnetospheric currents, such as the ring
current and the cross-tail current, which flow mainly westward
around Earth. These magnetospheric currents are composed
of the plasma of solar wind and Earth’s ionospheric origin,
and their strengths are coupled to the solar-wind conditions.
Therefore, a large geomagnetic storm is expected to be associated with strong SWCX emission. A Dst index of less than
100 nT is classified as a major geomagnetic storm. The Dst
index is provided by the World Data Center for Geomagnetism,
Kyoto, Japan.1 During this observation, the minimum Dst
index reached 111 nT.
2. Observation
Suzaku observed 1E 0102.27219 (hereafter E0102) on
August 31 in 2005 with the XIS. This object is one of the
brightest supernova remnants in the Small Magellanic Cloud.
The X-ray spectrum below 2 keV consists of many bright
1
hhttp://wdc.kugi.kyoto-u.ac.jp/dstdir/i.
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63-3
We analyzed the screened data using the HEAsoft version 6.4
package.3
We followed the same procedure as Ezoe et al. (2011)
for an analysis of terrestial diffuse X-ray emission (TDX) or
geocoronal SWCX. The TDX is extracted from two corner
regions in the XIS image, as shown in figure 2, to minimize the contamination from the X-ray source E0102 located
at the center of the field of view (FOV).
3.
Data Analysis
3.1.
Fig. 2. XIS images created from events in the energy band 0.2–1 keV
(panel a, BI) and 1–5 keV (panel b, FI) displayed using J2000.0 coordinates. For clarity, the images were smoothed by a Gaussian of = 15
pixels ( 1500 ). The two triangle regions are used for analysis
of the TDX.
emission lines originating from thermal plasma. These emission lines were previously resolved using observations taken
by the XMM-Newton RGS (den Herder et al. 2001), and
the energies of the line centers were identified (Rasmussen
et al. 2001). The energy-scale of the XIS in the low-energy
band was calibrated using the XMM-Newton RGS emissionline model (Ozawa et al. 2009). The observation started on
August 31 at 01:42 (the Day of Year, DOY, 243.07 in 2005),
and ended at 18:20 UT (DOY 243.76). During the observation,
Suzaku pointed towards (RA, Dec) = (16.ı 0100, 72.ı 0333),
(l, b) = (301.ı 557, 45.ı 061). Figure 1 shows the line of
sight in the GSE (Geocentric Solar Ecliptic) coordinate system
during the observation; the average normalized pointing vector
was (0.38735, 0.16779, 0.90614). The total exposure
time was 18.1 ks after the standard data screening procedure
(version 2.0.6.13) had been applied, as provided by the Suzaku
processing facility. The standard data screening removes
charged particle events, and selects good time intervals where
the instrument is stably pointed at the source without being
blocked by Earth and outside of high-background intervals.2
2
hhttp://heasarc.gsfc.nasa.gov/docs/suzaku/analysis/abc/abc.htmli.
Light Curve
We plot X-ray light curves extracted from the TDX region
in figure 3 along with four other quantities: the elevation angle
(ELV) of the line of sight from the Earth rim, the proton flux in
the solar wind measured by WIND,4 the SYM-H, which, like
the Dst index, also measures the geomagnetic field strength at
the surface of the Earth, but with a resolution in time of 1 min,
provided by the World Data Center for Geomagnetism, Kyoto,
Japan,5 and the Dst index. The SYM-H index shows the same
trend as the Dst, and has better time resolution. For a comparison, the X-ray light curves are generated in two energy bands,
0.5–0.7 keV and 2.5–5 keV. The lower energy band could
potentially contain oxygen emission lines from the scattering
of solar X-rays and/or SWCX, and the higher one is composed
of non-SWCX emission. The X-ray light curve in the energy
band of 0.5–0.7 keV has an enhancement from DOY 243.47
to 243.76 (storm period in figure 3). The average rates in the
first half of the observation (pre-storm) and storm period are
0.0202 ˙ 0.004 and 0.0434 ˙ 0.004 counts s1 , respectively.
Then, the rate increased by a factor of 2 from the pre-storm
period to the storm. Errors are of 2 significance. In contrast,
the light curve in the energy band of 2.5–5 keV has less variability. The average rates in the pre-storm and storm periods
are 0.0255 ˙ 0.003 and 0.0235 ˙ 0.003 counts s1 . In the light
curves, no rapid rises associated with the scattering of solar
X-rays at small ELV are found. Such rapid rises were, however,
detected by Ezoe et al. (2011).
The solar wind proton data were taken with the WIND satellite found at the L1 point between the Sun and Earth, at approximately 200 Earth radii (RE ).6 The average proton speed
during the observation was 380 km s1 . The expected time
delay for the solar wind to get to Earth from the L1 point is
0.04 d. A detailed calculation of the time delay is described
in subsection 5.2. The enhancement of the X-ray light curve
in the 0.5–0.7 keV band occurred with a decrease of the SYMH index and an increase of the solar-wind proton flux from
DOY 243.5 to 243.6. Therefore, the soft X-ray enhancement
should be closely related to the increased solar-wind flux near
Earth. This is indicative of geocoronal SWCX emission.
To check whether this time variability was due to leaked
photons from the bright X-ray source, we created intensity ratio
maps by dividing the images of the storm period by those of the
pre-storm period. Figure 4a shows a ratio map of the energy
band of 0.5–0.7 keV. In this figure, the TDX region increases
3
4
5
6
hhttp://heasarc.nasa.gov/lheasoft/i.
hhttp://web.mit.edu/space/www/wind data.htmli.
hhttp://wdc.kugi.kyoto-u.ac.jp/aeasy/index.htmli.
hhttp://cdaweb.gsfc.nasa.gov/cgi-bin/gif walki.
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K. Ishikawa et al.
[Vol. 65,
Fig. 3. XIS light curves in the energy band 0.5–0.7 keV (BI) and 2.5–5 keV (FI), elevation angle from Earth rim, solar wind proton flux, SYM-H, and
Dst index as a function of DOY in 2005. The vertical errors are 1 significance. The proton flux were calculated from WIND SWE data ( 100 s bin).
by a factor of 3, which is consistent with the X-ray rates.
Meanwhile, the bright X-ray source remains rather steady. In
the 2.5–5 keV image (figure 4b), the TDX region and the bright
X-ray source show little temporal variability.
4. Spectrum
Standard filtering of Suzaku data includes the removal of
time periods when the ELV is less than five degrees, so as
to avoid solar X-rays scattered by the Earth’s atmosphere,
although occasionally additional screening for the ELV angle
may be necessary. The soft X-ray light curve and the ELV
angle (figure 3), however, imply that no additional screening is
required. Therefore, we continued to use the standard screened
data. We extracted XIS spectra of the TDX region from both
the pre-storm and storm periods. Background events of the
instrument, sky and any other residual astronomical sources
are included in both periods. Due to Suzaku’s low-Earth
orbit, the instrumental background rate remained constant.
The sky background is mainly composed of the cosmic X-ray
background, which is observed from all directions, and does
not vary. Therefore, we assumed the background events to
be constant during the period of observation, and subtracted
the pre-storm period spectrum from the storm spectrum. The
resultant spectrum represents the enhanced component during
the storm period of the light curve. We looked at the spectrum
below 1 keV, where the majority of SWCX X-ray emission
lines are found. Therefore, we used the XIS BI spectrum only
because the XIS BI detects X-rays coming from the back side
of the CCD where there is no gate structure, and has a higher
sensitivity for soft X-rays than the XIS FI.
Figure 5 shows the resultant spectrum, which contains the
enhanced component during the storm period, with a best-fit
model. We used a theoretical SWCX emission-line model
by Bodewits et al. (2007), because Carter, Sembay, and
Read (2010) and Ezoe et al. (2011) have successfully fitted
geocoronal SWCX spectra using this model. Bodewits et al.
(2007) showed the relative emission cross-sections of highly
ionized ions (C V, C VI, N VI, N VII, O VII, and O VIII) in collision with atomic hydrogen for a variety of solar-wind velocities. We used the tabulated values for a velocity of 400 km s1 ,
because the average velocity measured by WIND during the
storm period was 380 km s1 . We allowed the major six
normalizations of the ions to be free, and the relative normalizations of the minor transitions to be fixed with respect to
the major transitions of the Bodewits et al. (2007) model;
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Strong X-Ray Emission from Terrestrial Exosphere
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Fig. 5. XIS BI resultant spectrum between the storm and pre-storm
periods. A model of SWCX emission is fitted to the data and the best-fit
parameters are listed in table 1. Emission lines are color coded: C V
(green), C VI (blue), N VI (cyan), N VII (magenta), O VII (orange), O VIII
(dark green).
Fig. 4. Ratio maps between the storm and pre-storm images in the
energy band 0.5–0.7 keV (panel a, BI) and 2.5–5 keV (panel b, FI).
The difference in the exposure time between the two images has been
corrected. For clarity, images are binned by 64 pixels. The color bars
indicate the ratio. Black triangles mark the TDX region.
we also added an extra Gaussian to reproduce the lowest
energy emission line around 0.25 keV, similar to Ezoe et al.
(2011). The SWCX model reproduced the data well with
a 2 =d.o.f of 25.23=23.
The spectrum was potentially influenced by the hard
continuum originating from the particle background.
Therefore, we fitted the resultant spectrum, in the energy
band of 1–5 keV, with a power-law model. The fitting
was acceptable with a 2 =d.o.f of 45.61=36. The bestfit photon index and normalization were 0.17+0:30
0:33 and
1
2
1
photons
s
cm
str
(line
unit,
LU)
at
1 keV,
32+13
11
respectively. We extrapolated this power-law continuum into
the low-energy band.
We fitted the resultant spectrum shown in figure 5 with a sum
of the SWCX model and the power-law of the fixed photon
index and normalization. In table 1, we list the best-fit parameters of the SWCX model, in which the normalizations are the
sum of the major and minor transitions of each ion species. The
relatively high line fluxes were measured in the line of sight
through the tail of the magnetosheath where the high SWCX
flux would not be expected.
Fig. 6. Comparison of the line energy flux ratio to O VIII. Black, red
and green data points indicate the best-fit parameters of this observation, Ezoe et al. (2011), and Carter, Sembay, and Read (2010),
respectively.
5.
Discussion
Below we discuss the properties of the geocoronal SWCX
emission, and in particular examine the SWCX line flux ratio
to the O VIII line, which is one of the most prominent in the
spectrum. We then discuss the time correlation between the
X-ray flux and the solar wind, as well as the expected SWCX
intensity.
5.1.
SWCX Line Flux Ratio
Figure 6 shows the SWCX line energy flux ratio to the
O VIII line compared with the XMM-Newton observation of
CME-induced geocoronal SWCX (Carter et al. 2010) and the
Suzaku observation of geocoronal SWCX during the strongest
geomagnetic storm after the Suzaku launch in 2005 July (Ezoe
et al. 2011). We used the line energy fluxes listed in table 1
for this purpose. While Ezoe et al. (2011) and the Suzaku
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K. Ishikawa et al.
[Vol. 65,
Table 1. Best-fit parameters of the SWCX model shown in figure 5.
Ion
C band lines
CV
C VI
N VI
N VII
O VII
O VIII
2 =d.o.f
Principal energy [eV]
256+18
34
299
367
420
500
561
653
Normalization
fX 14
35 +
12
+ 53
183 63
59 ˙ 10
5
29 +
3
5
26 +
4
52 ˙ 5
23 ˙ 5
5.0 1014
2.4 1013
1.0 1013
5.8 1014
6.0 1014
1.3 1013
6.4 1014
34.3=23
The fluxes apply to the set of major and minor transitions for one particular ion species.
The energy ranges of beginning at the top ion using to calculate the fluxes are 0.23–
0.29 keV, 0.27–0.33 keV, 0.34–0.4 keV, 0.39–0.45 keV, 0.47–0.53 keV, 0.53–0.59 keV, and
0.62–0.68 keV.
Normalization is in units of photons s1 cm2 str1 .
fX is the energy flux in erg s1 cm2 .
observation made in this study show a similar trend, there are
discrepancies between the Suzaku and XMM-Newton results.
This may be related to the observation period. Both of the
Suzaku storm observations were obtained in late 2005 August,
whereas the XMM-Newton observation (Carter et al. 2010)
was performed in 2001 October. Comparing the Suzaku
data with the XMM-Newton one, the absolute values in
the Suzaku observations are an order of magnitude higher,
although the relative flux ratios from C V to N VII are similar.
In the XMM-Newton observation by Carter, Sembay, and Read
(2010), the O VIII lines were prominent, while O VII lines were
prominent in both of the Suzaku observations discussed here.
The previously mentioned XMM-Newton observation was
carried out during the solar maximum, whereas the Ezoe et al.
(2011) and (this) Suzaku observations were performed close
to solar minimum. All of these observations were conducted
during the same solar cycle (solar cycle 23). The minimum
Dst index during the XMM-Newton observation was 187 nT.
The frequency by which the Dst index is measured below
100 nT varies according to solar activity, and high level
geomagnetic storms often occur in the period of the solar
maximum. The average solar-wind proton velocities were
647 km s1 during the XMM-Newton observation, 530 km s1
in Ezoe et al. (2011) Suzaku observation, and 380 km s1 in
this observation. The O8+ composition in the solar wind is
typically lower than the O7+ one, and its fraction is even lower
in fast solar winds. However, an increase of the O8+ fraction is observed during CMEs (Reisenfeld et al. 2003). CMEs
are enriched with highly charged oxygen, as well as with ions
of other elements (Richardson & Cane 2004).
A high fraction of O8+ was noticed during the
XMM-Newton observation, which was explained by the
CME effect. However, in the Suzaku observations, the O7+
content in the solar wind was higher than the O8+ one,
even though Ezoe et al. (2011) observation was affected
by CME. Therefore, the CME composition cannot simply
explain the observed composition difference. We note that
the XMM-Newton spectrum indicated emission lines at around
2 keV from highly charged silicon, which are absent in the
Suzaku spectra. This implies that, in the XMM-Newton time,
a higher temperature plasma reached Earth, compared with the
time of the Suzaku observations. Such a variation in the solar
wind composition possibly caused the observed difference
in the relative X-ray line fluxes between the XMM-Newton
and Suzaku spectra.
5.2. Time Correlation
It is expected that exospheric SWCX emission is proportional to the incident solar-wind flux (Robertson et al. 2006;
Carter et al. 2011; Ezoe et al. 2011). We checked the relation between SWCX and the solar-wind fluxes. We utilized
the XIS BI 0.52–0.6 keV O VII count rate as the SWCX emission, and the solar-wind proton flux measured by WIND, as
a proxy for the O7+ flux. Although the proton flux is not very
strongly correlated with the heavy ion flux (Neugebauer et al.
2000), a direct confirmation of the relation is difficult because
ACE SWICS O7+ data are not always available, and have a low
time resolution.
Before checking the relation, we needed to calculate any
time delay between these two data sets, because the WIND
satellite is located upstream in the solar wind from Earth. Then,
we conducted a cross-correlation analysis using a method
described in Ezoe et al. (2010). For this procedure, we
binned the XIS BI light curve and WIND data into the same
time bin (4096 s), and utilized the crosscorr software in
the HEAsoft package with the default mathematical algorithm
and normalization method.
In figure 7, we plot the cross correlation. The correlation
coefficient has a peak of 0.7 at a time delay of 8192–12288 s.
The null hypothesis is probability 4 103 , corresponding to
3 significance. Since the expected time delay was calculated to be 1 hr using the distance from the L1 point of WIND
to Earth (Suzaku is in low Earth orbit) and the average proton
speed in the solar wind, the existence of an extra time delay
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Strong X-Ray Emission from Terrestrial Exosphere
Fig. 7. Cross correlation between the WIND proton flux and
the XIS BI O VII count rate.
Fig. 8. Correlation between the WIND proton flux and the XIS BI
O VII count rate. The vertical error bar is 1 significance. The black
and red data are this observation (2005 August 31) and the other observations, respectively. The green and blue solid lines are the best-fit
linear function and constant offset due to the instrumental and sky
backgrounds.
was suggested. When we conducted the same procedure with
the 1-min averaged proton flux obtained from OMNIWeb,7
which has been corrected for the solar wind traveling time
between the L1 point to the bow shock nose, a time delay of
2048–4096 s remained. The sum of this and the expected time
delay of 1 hr is roughly consistent with the estimated delay of
8192 s between the XIS BI light curve and the WIND proton
flux. The extra delay may be due to the transport of solar wind
in the magnetosphere (see subsection 5.3).
The correlation between the O VII (0.52–0.6 keV) photon
flux and the solar wind proton flux is shown as black points in
figure 8. The proton flux was corrected for the estimated time
delay of 8192 s. The data is represented by a linear function,
expressed as
CXIS Œcounts s1 = a Cproton Œ105 cm2 s1 + b;
7
hhttp://omniweb.gsfc.nasa.gov/ow min.htmli.
to the instrumental and sky background. The best-fit parameters are a = (1.1 ˙ 0.4) 106 and b = (1.9 ˙ 3) 103 with
2 =d.o.f = 113.5=98. Errors are at the 90% confidence range.
When we converted b into the photon flux in units of LU based
on the O VII fitting shown in figure 5, b becomes 8 ˙ 13 LU.
The positive b value indicates that a certain level of background
emission remains.
To estimate the offset emission (the value b) more precisely,
we analyzed other Suzaku data of the same object (E0102).
Because E0102 has been used to calibrate the energy scale of
the XISs, Suzaku has observed it many times. In addition to
the observation used in this analysis, there had been 49 Suzaku
observations of E0102 up to the end of 2010. Of this set,
38 observations had the same pointing field, and were taken in
the same mode as the observation under study here. We used
these observations for further study; they are listed in table 2.
The count rate for each observation in the energy range 0.52–
0.6 keV was extracted from the same TDX region as used in
this analysis, and corrected for the extraction area. The XIS BI
count rate has been decreasing because the energy resolution
has degraded and the contamination on the optical blocking
filter for the XIS has increased.8 In order to correct it, we
checked the E0102 count rate by subtracting the background
rate. Here, we assumed the E0102 count rate to be constant,
and the background rate was extracted from the surrounding
region of E0102. Then, we corrected for the decline of each
count rate based on the rate of the earliest observation of the
set taken on 2005 August 13, using a correction factor, as listed
in table 2. The correction factor consists of the product of
the extraction area fraction and the count rate decline factor.
The red points in figure 8 show the average count rate and
proton flux corrected for the estimated time delay of 8192 s.
Although we tried to fit both this and the other observation
data (black and red points in figure 8) with a linear model, the
fitting was not statistically acceptable (2 =d.o.f of 302.9=137).
Therefore, we fitted a part of the data with the lower proton flux
(0–4000 105 cm2 s1 ) with a constant model (a blue line
in figure 8) and the higher one (> 4000 105 cm2 s1 ) with
a linear model (a green line in figure 8). The constant value
fitted for the lower proton flux and the gradient in the higher
proton flux indicate the offset emission and the SWCX component, respectively. From the best-fit parameters, we obtained
a = (1.2 ˙ 0.4) 106 and b = (7.9 ˙ 0.2) 103 with
2 =d.o.f = 108.4=95. In units of LU, the SWCX emissivity,
a and the offset emission, b correspond to 0.005 ˙ 0.002 LU
105 cm2 s1 and 36 ˙ 1 LU, respectively. The offset emission b is a sum of the background, composed of the instrument, sky and escaping photons from the central X-ray source.
To estimate the instrumental non X-ray background (NXB),
we created an NXB spectrum using xisnxbgen, (a tool
within the software package FTOOLS),9 and the NXB intensity
was calculated to be 5 ˙ 1 LU. The O VII line intensity from
the soft X-ray background consists of the local hot bubble,
extragalactic and heliospheric SWCX emission ranging from 2
to 10 LU (Yoshino et al. 2009). We estimated the intensity
(1)
where a is the SWCX emissivity and b is an offset emission due
63-7
8
9
hhttp://www.astro.isas.jaxa.jp/suzaku/doc/suzaku td/node10.html
#SECTION001033000000000000000i.
hhttp://heasarc.gsfc.nasa.gov/ftools/i.
63-8
K. Ishikawa et al.
[Vol. 65,
Table 2. Summary of E0102 observations used in this analysis in addition to the observation on 2005 August 31.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
Start time
End time
Exposure (ks)
2005-08-13 05:58:27
2006-01-17 23:22:57
2006-02-02 20:19:20
2006-04-16 09:42:04
2006-05-21 17:16:07
2006-06-26 20:47:26
2006-07-17 06:22:33
2006-08-25 04:55:35
2006-09-19 05:25:56
2006-10-21 15:36:54
2006-12-13 18:53:16
2007-01-15 03:20:53
2007-02-10 22:13:47
2007-03-18 21:11:19
2007-04-10 10:35:08
2007-06-13 10:10:12
2007-08-12 05:21:09
2007-09-28 06:13:48
2007-10-25 12:24:45
2007-12-01 19:25:40
2008-02-14 16:57:28
2008-03-15 05:43:27
2008-04-08 14:33:12
2008-06-05 03:50:53
2008-08-12 22:21:28
2008-10-22 02:31:56
2008-12-13 13:33:21
2009-03-09 03:07:54
2009-04-23 15:17:13
2009-06-26 03:42:21
2009-10-11 14:30:55
2009-10-26 18:14:08
2009-12-25 21:59:01
2010-02-04 17:50:39
2010-04-05 00:02:07
2010-06-19 03:06:41
2010-10-26 19:30:06
2010-12-09 00:36:56
2005-08-13 11:05:04
2006-01-20 09:00:19
2006-02-03 09:45:25
2006-04-16 23:30:19
2006-05-22 04:18:14
2006-06-27 10:11:24
2006-07-17 21:40:23
2006-08-26 09:26:24
2006-09-19 13:30:14
2006-10-22 05:26:15
2006-12-14 03:04:19
2007-01-15 15:58:2
2007-02-11 19:30:14
2007-03-19 07:11:14
2007-04-10 19:30:19
2007-06-14 03:31:19
2007-08-13 03:45:24
2007-09-28 15:20:14
2007-10-26 09:00:14
2007-12-02 09:50:19
2008-02-16 03:10:24
2008-03-15 20:45:24
2008-04-09 05:15:24
2008-06-05 20:32:14
2008-08-13 14:10:24
2008-10-22 15:02:12
2008-12-14 07:25:19
2009-03-09 15:29:25
2009-04-24 14:56:24
2009-06-26 13:42:22
2009-10-12 14:58:13
2009-10-27 10:00:24
2009-12-26 12:59:19
2010-02-05 10:25:24
2010-04-05 12:21:14
2010-06-19 18:53:15
2010-10-27 07:43:23
2010-12-09 11:42:14
4.2
42
21
21
18
22
22
68
11
37
28
23
36
18
18
28
39
25
26
25
48
28
22
21
21
25
30
24
45
22
50
20
22
21
22
19
20
20
Correction factor
1.2
1.6
1.4
1.8
1.2
1.3
2.0
1.2
1.4
1.9
1.6
1.7
1.6
1.6
2.0
1.4
1.7
1.7
1.8
1.6
1.4
1.5
2.0
1.5
1.6
1.9
1.4
1.5
2.0
1.7
1.9
1.8
1.7
1.7
2.0
1.5
2.2
1.7
Correction factor consists of the product of the area ratio (between the extraction region in each observation and
the TDX region on 2005 August 31) and the count rate decline ratio due to the energy resolution degradation and
increase in contamination.
of the escaping emission from E0102 based on the point spread
function (PSF). According to the PSF of the XIS telescope
(Serlemitsos et al. 2007), the brightness at the corner of the
FOV is < 1% of the center, and the intensity is < 27 LU. Thus,
we can interpret b as being the sum of these backgrounds.
We compared the SWCX emissivity in this observation
with those in the past Suzaku results. The past observations were conducted on (a) 2005 August 23–24 (Ezoe et al.
2011), (b) 2005 September 2–4 (Fujimoto et al. 2007), and
(c) 2005 October 28–30 (Ezoe et al. 2010). The SWCX emissivity values were (a) 0.002 ˙ 0.0002 LU 105 cm2 s1 toward
the south direction, (b) 0.004 ˙ 0.0006 LU 105 cm2 s1
toward the north ecliptic pole, and (c) 0.003 ˙ 0.0007 LU
105 cm2 s1 toward the sub-solar magnetosheath. When
we assume that all the proton flux measured at the L1 point
reaches the Suzaku observation region, the difference of the
SWCX emissivity could suggest a spatially non-uniform distribution of neutral hydrogen density. Although higher levels
of geocoronal SWCX intensity would be expected in the
cusp regions and the sub-solar magnetosheath (Robertson &
Cravens 2003; Robertson et al. 2006), in this result, the
higher SWCX intensity was observed in a different direction
from these regions. We discuss the high SWCX intensity
in the next section.
No. 3]
5.3.
Strong X-Ray Emission from Terrestrial Exosphere
Expected Intensity
We estimate the expected SWCX intensity based on the
solar wind flux and the column density of neutral hydrogen.
Here, we simply assume that only a single charge exchange
occurs, while the highly charged ions in the solar wind penetrate the terrestrial exosphere, from the analogy with the charge
exchange emission of moderately active comets (Cravens
1997; Cravens et al. 2001). We can express the SWCX
intensity by
1
(2)
˛ PSW NH ŒLU;
4
where ˛ is the charge-exchange cross section times the line
emission probability, PSW is the incident solar wind flux,
and NH is the column density of the target hydrogen atoms
in the terrestrial exosphere.
We calculated the O VII line intensity, using an ˛ value of
6 1015 cm2 based on the sum of the O VII contributions
listed in table 2 of Bodewits et al. (2007). Here, we assumed
the solar wind speed, which is related to ˛, to be 400 km s1 .
The average ACE O7+ ion flux of 9 104 cm2 s1 was used
as PSW . To evaluate the neutral column density of hydrogen
atoms in the terrestrial exosphere, we used the Østgaard et al.
(2003) model n(r) for the hydrogen density profile around
Earth, which falls off as exp(r). NH can be deduced from
integrating the density in the line of sight as
Z rend
n.r/ dr;
(3)
NH =
PSWCX =
rstart
where rstart is the distance to the nearest point in the line of
sight where the solar wind interacts with the exosphere, and
rend is the end point of the integration. From the line-of-sight
direction during the observation, we assumed that the SWCX
occurred in the magnetosheath. Then, we used the approximate positions of rstart of 10 RE and rend of 20 RE . NH was
estimated to be 1 1011 cm2 . Substituting these values
into the equation, PSWCX was calculated to be 4 LU. The
shape of the magnetosheath will change significantly over short
timescales. A minimum rstart was calculated to be 4 RE
using the TS05 magnetic field model (Tsyganenko & Sitnov
2005) during the observation. In this period, NH and PSWCX
were calculated to be 3 1011 cm2 and 13 LU, respectively.
The discrepancy between the observed high SWCX intensity
and the modelled value can be explained by uncertainties in
the assumed parameters, such as the solar-wind oxygen ion flux
measured with ACE, NH and/or ˛. The O7+ flux, in particular,
as measured by ACE SWICS, is likely to contain a large uncertainty, because the data is sparse and the ion flux in the line of
sight direction is assumed to be the same as the flux measured
at the L1 point.
To explain the observed SWCX intensity (52 LU), we
could increase the oxygen ion flux, NH and/or ˛. From
Bodewits et al. (2007), the uncertainty of ˛ was estimated to be
approximately 20%, and thus the influence of uncertainties in
this parameter on the magnitude of PSWCX are relatively negligible. If all other parameters are held at their values quoted
above, NH would need to be increased to 1.2 1012 cm2
to explain the observed SWCX intensity. The Østgaard et al.
(2003) model depends on the solar zenith angle. In the model,
63-9
the hydrogen density becomes higher as the solar zenith angle
increases (90ı –180ı ). However, the difference in the hydrogen
column density between the solar zenith angle of 90ı and 180ı
is 20%–30%. Therefore, the column density mostly depends
on rstart given by equation (3). To obtain the necessary NH ,
rstart has to be reduced to 2 RE , which is unlikely based
on the line-of-sight direction. Similarly, holding all other
parameters at their values above, the O7+ flux would need to
be increased to 1.1 106 cm2 s1 , which is about 12-times
higher than the ACE measured flux. Since Suzaku looked
through the night-side magnetosheath, it is expected that the
O7+ density in the magnetosheath in the line-of-sight direction would be almost the same as that in the ambient solar
wind. Ebihara et al. (2009) reported that the number density
of high-charge state O ions in the inner magnetosphere was
increased during magnetic storms, and suggested that the O
ions were transported from the high-latitude lobe to the inner
magnetosphere. The lobe is a region between the plasma sheet
and the magnetopause. Thus, not only O7+ in the magnetosheath, but also O7+ in the magnetosphere will contribute
to the X-ray emission, which may explain the discrepancy
observed between the O7+ flux measured by ACE and the flux
derived from Suzaku.
6.
Summary
We conducted a systematic study of the Suzaku archives for
cases of geocoronal SWCX emission. In the present work,
we investigated geocoronal SWCX emission associated with
the strong geomagnetic storm that exhibited a Dst index of
111 nT on 2005 August 31 using Suzaku XIS data. We
followed the same procedure as that of Ezoe et al. (2011), and
divided the data into pre-storm and storm periods based on
two X-ray light curves. The X-ray light curve created from
the recorded events in the energy band 0.5–0.7 keV showed
a factor of 2–3 enhancement during the storm period, while
those in the energy band 2.5–5 keV were almost constant.
The enhancement coincided with a geomagnetic storm and
an increase of the solar wind proton and O7+ fluxes. The
resultant spectrum between the storm and pre-storm periods
could be fitted with a theoretical SWCX model (Bodewits’
model). The SWCX line flux ratios to O VIII, taken with
the best-fit parameters, showed a similar trend to those from
Suzaku data obtained about one week before the observation
described in this analysis. XMM-Newton data taken in late
2001 October also indicated a similar trend, except for the
O VIII flux. In the XMM-Newton data, the O VIII line was
dominant, instead of the O VII line. This different trend of
the X-ray ion flux between the XMM-Newton and Suzaku
can be explained by the difference in the incident O7+ and
O8+ relative fluxes. As a result of a cross-correlation analysis between the SWCX O VII line and the solar-wind proton
data, obtained from the WIND satellite, the time delay was
estimated to be 2 hr between the solar wind recorded by
this satellite and the observed Suzaku O VII light curve, as
measured along the line of sight. The relation between the
O VII count rate and the solar-wind proton flux showed a positive correlation, and was well represented by a linear function.
A positive offset in the linear relation suggests that, even if
63-10
K. Ishikawa et al.
the solar wind flux is zero, there remains a certain amount of
X-ray flux due to the sum of instrumental and sky backgrounds.
To estimate the positive offset more precisely, we analyzed
other Suzaku data, from observations of the same target object
as that under study in this analysis. We evaluated the background flux and the SWCX emissivity, which corresponds to
a gradient in the linear function. As a result of the comparison
of the past Suzaku SWCX observations, the SWCX emissivity
was different in each observation. Assuming that the solar
wind parameters, as measured at L1, correspond to the Suzaku
observation region (after an appropriate delay), the difference
could indicate a spatially non-uniform distribution of neutral
hydrogen density. The SWCX intensity of this observation was
strong despite the fact that the observation pointing was not
towards the cusps or the sub-solar magnetosheath, where high
levels of the SWCX emissivity would be expected. The SWCX
intensity is estimated to be proportional to the charge-exchange
cross section and the line-emission probability, incident solar
wind flux, and column density of hydrogen atoms in the
terrestrial exosphere. Using the O VII contribution calculated
in Bodewits et al. (2007), the O7+ ion flux obtained from
the ACE satellite, and neutral column density of hydrogen
atoms predicted by the Østgaard et al. (2003) model, the
expected SWCX emissivity along the line of sight is calculated to be 4 LU. To obtain the observed O VII line flux, the
column density of neutral hydrogen atoms and/or the O7+ flux
need to be increased.
We thank the referees for comments and suggestions, which
have significantly enhanced this paper. This work was
supported by a Grant-in-Aid for JSPS Fellows.
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