Doctoral Thesis

Meridional distribution of the Earth's plasmaspherederived from extreme ultraviolet images

Go MurakamiSpace and Planetary Science Group
Department of Earth & Planetary Science
Graduate School of Science, The University of Tokyo

December 2010

© Copyright by Go Murakami 2010
All rights reserved

Abstract
The plasmasphere plays a central role in many of the complex processes inEarth’s inner magnetosphere. Four decades of studies by a variety of in-situ techniquesrevealed the dynamical aspect of the plasmasphere changing depending on geomagnetic
conditions. However, the in-situ measurements are local observations with which it isdifficult to separate the temporal development and the spatial structure of theplasmasphere. Recent advances in satellite-based imaging techniques have overcome
this difficulty and made it possible to routinely obtain full global images of theplasmasphere. The Extreme Ultraviolet Imager (EUV) on the IMAGE satellite produced
consecutive images of the plasmasphere from the high-latitude view with a time
resolution of 10 min and a spatial resolution of 0.1 Re.
The EUV images have revealed the details of a highly structured and dynamic
entity of the plasmasphere, and demonstrated that the remote sensing with EUV
techniques is a promising means to explore the plasmasphere. On the other hand, theirremains some unsolved problems which are difficult to be clarified by the imaging
from a limited perspective, i.e. only from the high-latitude view. For example, a radial
structure of enhanced brightness called a “finger” was found by the EUV instrument
but the cause of this structure is still not understood. Furthermore, the gap between the
results of the EUV measurements and the conventional theories for the plasmapause
formation has been pointed out. In order to solve these problems, the imaging of theplasmasphere from a different perspective has been emphasized.
We have developed the Telescope of Extreme Ultraviolet (TEX) onboard thelunar orbiter KAGUYA launched in September 2007. Data available from the TEX
instrument allow us for the first time to study the plasmasphere from a meridian
perspective. The TEX instrument images the He+ distribution in the plasmasphere by
detecting the resonantly scattered solar 30.4-nm radiation and produces images
encompassing the entire plasmasphere. The author has participated intensively in thelaboratory and in-orbit calibrations and data evaluation for the onboard instrument.
iThe performance of the TEX instrument after launch was verified using thein-orbit data obtained from February 2008 to May 2009. The result of this in-orbit
calibration is presented in Chapter 2. From March to June 2008, the TEX instrument
had produced the first sequential images of the plasmasphere viewed from the meridian
perspective. The analysis and evaluation of these first images are demonstrated in
Chapter 3. The corotation and the erosion of the plasmasphere were identified in thesequential images during the geomagnetically quiet (Kp < 3) and disturbed period (Kp =5), respectively. The angular rotation rate for the corotation and the inward velocity of
the nightside plasmapause for the erosion are estimated by analyzing these images, andthe results are consistent with those derived from IMAGE/EUV observations. Thisconfirms that the TEX instrument successfully detected the spatial distribution andtemporal development of the plasmasphere.
Furthermore, a new striking feature of enhanced brightness in the plasmasphere,
called as a plasmaspheric “filament”, was found in the TEX image during a prolonged
quiet period (Kp < 2). The shape of the filament was closely aligned to the dipole
magnetic field line, and this suggests that the filaments are caused by isolated magnetic
flux tubes filled with denser plasmas than their neighbors. The nature of the filament
agrees with that of the finger structure observed by IMAGE/EUV. Consequently, theresult indicates that the finger structure seen in the EUV images should be the equatorial
projection of isolated flux tubes filled with denser plasmas than its neighbors.
The formation mechanism of the plasmapause was studied in Chapter 4. The
sequential TEX images during the geomagnetic disturbance (Kp = 5) on 1-2 May 2008
were analyzed. The plasmapause positions at the post-midnight observed from themeridian perspective clearly agreed with those predicted by the dynamic simulations
based on the interchange mechanism. Furthermore, after the convection enhancement,
the He+ column density in the nightside plasmasphere decreased by ~30% only at the
low latitudes (< 20 deg). This suggests that the formation of the new plasmapause
occurs first near the equatorial region during a geomagnetically disturbed period. These
results agree with the formation mechanism of the plasmapause based on thequasi-interchange instability.
iiFor the next plasmaspheric imaging from the International Space Station (ISS),
new multilayer coatings having high reflectivity at 30.4 nm and low reflectivity at 58.4
nm were developed. Chapter 5 deals with this topic. For the He II imaging from ISS,the contamination from geocoronal He I (58.4 nm) emission should be eliminated more
effectively than the TEX instrument because ISS orbits at a low altitude of ~350 km,
inside the geocorona. The newly developed Y2O3/Al multilayer coating designed fornormal incidence reflection had higher reflectivity (24.9%) at 30.4 nm andsignificantly lower reflectivity (1.3%) at 58.4 nm than the conventional coatings suchas Mo/Si. The temporal stability of the Y2O3/Al multilayer coating was also evaluatedand found to be highly stable under vacuum, dry N2 purge, and normal atmosphere.
Then, based on these results, the Y2O3/Al multilayer coating was selected to apply forthe flight mirror of the Extreme Ultraviole Imager (EUVI) onboard ISS. The
calibration of the flight mirror was performed, and it was confirmed that EUVI can
produce the plasmaspheric images from ISS with a high SNR of 16.

iii

Contents
Abstract ............................................................................................................................ iContents.......................................................................................................................... iv
Acknowledgements ........................................................................................................ vi
1

2

General introduction .............................................................................................. 1
1.1

Early observations of the plasmasphere ........................................................... 1

1.2

Theoretical approach for the plasmasphere ...................................................... 8

1.3

Imaging the Plasmasphere .............................................................................. 13

1.4

Instruments for the plasmasphere imaging..................................................... 18

1.5

Construction of this Thesis ............................................................................. 20

In-orbit calibration of the Telescope of Extreme Ultraviolet onboard

KAGUYA....................................................................................................................... 21
2.1

Introduction .................................................................................................... 21

2.2

Instrument overview ....................................................................................... 22

2.2.1

Multilayer mirror ........................................................................................ 24

2.2.2

Metallic thin filter....................................................................................... 25

2.2.3

Microchannel plate (MCP) detector ........................................................... 25

2.3
2.3.1

Image scale of the field of view (FOV)...................................................... 26

2.3.2

Point-spread function (PSF) ....................................................................... 28

2.3.3

Background................................................................................................. 30

2.3.4

Pulse height distribution of the MCPs........................................................ 32

2.4
3

In-orbit performance....................................................................................... 26

Summary......................................................................................................... 34

First sequential images of the plasmasphere from the meridian perspectiveby KAGUYA.................................................................................................................. 35
3.1

Introduction .................................................................................................... 35

3.2

Instrumentation and observation .................................................................... 36

3.3

Results and discussion .................................................................................... 39

3.3.1

Inward motion of the nightside plasmapause ............................................. 39iv

3.3.2

Corotation of the plasmasphere .................................................................. 43

3.3.3

Plasmaspheric filament: An isolated magnetic flux tube filled with dense

plasmas ................................................................................................................... 48
3.4
4

Summary......................................................................................................... 58

The plasmapause formation seen from meridian perspective by KAGUYA .. 60
4.1

Introduction .................................................................................................... 60

4.2

Methodology................................................................................................... 62

4.2.1

Observation of the plasmapause from the meridian perspective................ 62

4.2.2

Calculation of the plasmapause location .................................................... 64

4.3
4.3.1

Analysis and discussion.................................................................................. 66
Comparisons of the plasmapause positions between the TEX observations

and numerical simulations ...................................................................................... 66
4.3.2
4.4
5

Meridional distribution of the plasmapause observed by TEX .................. 71
Summary......................................................................................................... 79

Development of a multilayer mirror for Extreme Ultraviolet Imager onboard

ISS-JEM ........................................................................................................................ 80
5.1

Introduction .................................................................................................... 80

5.2

Observations from ISS ................................................................................... 82

5.3

Designs and calculations ................................................................................ 84

5.4

Mesurements................................................................................................... 89

5.5

Results and discussion .................................................................................... 90

5.5.1

SiC/Mg multilayer coating ......................................................................... 90

5.5.2

Y2O3/Al multilayer coating ........................................................................ 93

5.5.3

Calibration of the flight mirror for the EUVI instrument........................... 96

5.6
6

Summary....................................................................................................... 100

Concluding remarks ........................................................................................... 102

References.................................................................................................................... 106v

Acknowledgements
I wish to express my sincere appreciation to associate Prof. Ichiro Yoshikawaof the University of Tokyo for giving me the wonderful opportunity to enter scientificresearch. His suggestion and guidance always led me to the right way and bringenthusiasm to me. I deeply keep in mind what I have learned from him during thisfive-year course.
I am grateful to Dr. Kazuo Yoshioka, Mr. Fukuhiro Ezawa, Mr. Gentaro
Ogawa, Mr. Hiromi Watanabe, Mr. Kouichi Sakai, Mr. Tatsuro Homma, and all of thecolleagues who are (and were) concerned with Ichiro Yoshikawa’s Laboratory in The
University of Tokyo for their continuing interest and encouragement. Especially Dr.
Kazuo Yoshioka gave me valuable suggestions and supports throughout all of my study.
I am proud to be a member of Yoshikawa Laboratory with them.
I would like to thank Dr. Atsushi Yamazaki, Prof. Makoto Taguchi, Dr.
Masayuki Kikuchi, Prof. Wataru Miyake, Prof. Shoichi Okano, and all of the membersof the UPI team and the SELENE project team for their effort in achieving a successful
mission. Thanks are also due to Dr. Yuki Obana and Dr. Masato Kagitani for theirhelpful advices about the analysis of the TEX data.
I am grateful to associate Prof. Naomoto Iwagami, Professor Masahiro
Hoshino, Professor Masafumi Hirahara, associate Prof. Yoshifumi Saito, and Professor
Masato Nakamura for providing sincere and constructive comments.
I would like to express my deep appreciation to Dr. Shingo Kameda, Dr.
Shoko Ohtsuki, and Dr. Yasuyuki Tanaka for providing useful comments and technicaladvices.
I wish to thank Mr. Satoshi Ichimaru and Mr. Hisataka Takenaka in NTT
Advanced Technology for manufacturing the multilayer coatings and giving me
significant suggestions.
I also wish to all the members of Naomoto Iwagami Laboratory and Spaceand Planetary Science Group at The University of Tokyo for their emotional assistanceviand friendship.
The acknowledgements would not be completed without giving thanks to myparents, Kazuhide and Keiko Murakami. They have supported me both financially andmentally during all my life. I am also grateful to my brother and sister-in-law, Naozumi
and Chie Murakami, for their encouragement.
Finally, I would like to express my heartfelt thanks to my soon-to-be wife,
Hazuki. She has always been with me and been the best partner. This work would not
be done without her sincere support and continuous encouragement for these longyears.

vii

1 General introduction
1.1 Early observations of the plasmasphere
The magnetosphere is a region around the Earth in which the Earth’s magnetic
field is the dominant influence and controls the motions of charged particles. Figure
1.1 shows the results of a synthesis of in-situ and ground-based observations over 40
years and represents today’s generally accepted global view of the Earth’s
magnetosphere. In the inner magnetosphere, the magnetic field configuration is dipolar
like with perturbation at its outer edge caused by the external pressure change of thesolar wind. At ionospheric altitude below 1000km in the mid and high latitude, thedipole field is nearly vertical. The charged particles in the ionosphere are free to move
upward along magnetic field lines and populate the higher-altitude regions of the
Earth’s magnetosphere. The charged particles, which are predominantly electrons andprotons with energies not exceeding several electron volts [eV], form a region of colddense plasma (shown in Figure 1.1) and is called the plasmasphere.
The existence of a sharp outer boundary of the plasmasphere, corresponding
to the plasmapause, was first discovered in the early 1960s [Carpenter, 1963]. Analysisof whistler waves recorded at ground stations established the plasmapause as apermanent feature within the magnetosphere. After the establishment of the definitionof the plasmasphere, satellite in-situ measurements were used to investigate the innermagnetosphere, particularly the plasmasphere in the 1970s. Figure 1.2 shows thedominance of H+, He+, and O+ measured by a mass spectrometer onboard the OGO 5
satellite, showing density profiles plotted versus the L-shell parameter (the distance ofthe equatorial crossing point of the field line from the Earth’s center, measured in
Earth’s radii). The density profile of He+ is very similar in shape to that of H+. The
density distribution of those ions sharply decreases by a factor of 100 or more. In
Figure 1.2, the position of the outer boundary, the plasmapause, at L=4.9 is evident inboth H+ and He+. The outer plasmasphere consists of about 90% H+ and about 10%
He+ with only a trace of O+.
1

Figure 1.1: A schematic diagram of a generally accepted global representation of
the Earth’s magnetosphere. This picture has been synthesized from a vast numberof localized observations made over the past four decades throughout many of theregions [Williams et al., 1992].

2Figure 1.2: A typical inbound plasmapause crossing observed by OGO 5 showing
the H+, He+, and O+ ion concentrations as a function of L and local time [Chappell
et al., 1970].

Intensive observation using spacecraft followed and revealed the dynamical
aspect of the plasmasphere changing depending on geomagnetic conditions. It became
clear that the plasmapause position is closely correlated with average magnetic activity
over a 2 to 6-hour period, e.g., the Kp index [Chappell, 1970]. The plasmasphereshrinks with the magnetic activity in a very well-behaved, predictable manner. Thisvariation in the radius of the plasmasphere depending on the magnetic activity isshown in Figure 1.3. This figure represents the superposed results over more than 40
passes of the satellite. The decrease in the radius of the plasmapause is accompanied byan increased steepness in the density gradient at the plasmapause, while the total
concentration levels inside and outside the plasmapause remain approximately ~103
ions/cm3 and ~1 ions/cm3, respectively.

3

Figure 1.3: A composite graph of H+ concentration measured by OGO 5 showing
the reaction of the plasmasphere in the nightside region to changes in the level ofmagnetic activity. The plasmapause is found to be steep and move to lower L
values with increasing activity [Chappell et al., 1970].

Another study on relation of the plasmapause position with the geomagnetic
condition was examined. Carpenter and Anderson [1992] statistically investigated thelocation of the plasmapause using the long-term in-situ observations by the Sweep
Frequency Receiver (SFR) instrument onboard the ISEE-1 satellite that orbits the Earthon the equatorial plane. Figure 1.4 shows a plot of the plasmapause locations (Lpp:
L-value of the plasmapause) versus the value of Kp in the preceding 24 hours. A leastsquares linear fit to the data is also indicated in Figure 1.4. It represent the relation Lpp
4

= 5.6 – 0.46Kpmax. It was found from ISEE-1/SFR data that this relationship did notchange significantly when the data were separated according to night, day and dawn
sector values. This indicates that the equatorial plasmapause corotates nearly along a
circular trajectory between the midnight sector where it is formed until at least 15 MLT
in the afternoon sector.

Figure 1.4: Locations of the plasmapause (Lpp) as a function of the maximum Kp
value in the preceding 24 hours deduced from the ISEE-1/SFR data during
1977-1983. The profile observed in the local time sector of 00-15 MLT isrepresented. A least squares linear fit to the data is indicated by the solid line.
[Carpenter and Anderson, 1992].

5Based on the ISEE-1/SFR data, Carpenter and Anderson [1992] also
investigated the equatorial density distribution in the plasmasphere. Figure 1.5 shows aplot of 11 dayside equatorial density profiles obtained along the near-equatorial ISEE-1
orbits. These profiles tended to be representative of very quiet magnetic conditions.
They appear to be well approximated by a linear relation between log10Neq and L. The
least squares linear fit to the profiles shown in Figure 1.5 is given by
log10 N eq

0.3145 L 3.9043(1.1)

where Neq is expressed in electrons/cm3. This result suggested that even after
prolonged periods of quiet, a saturated plasmaspheric flux tube contains fewer particles
than it would in the case of isothermal hydrostatic equilibrium [e.g., Pierrard andLemaire, 2001].
Numerous large-scale density irregularities in the plasmasphere were found by
the in-situ measurements. One of such irregularities was a deep density trough inside
the plasmasphere called as an “inner-trough” or a “donkey-ear” [e.g., Horwitz et al.,
1990; Oya, 1997; Carpenter et al., 2000; Fu et al., 2010]. Figure 1.6 shows an exampleof the equatorial density profile with an inner-trough obtained from the ISEE-1/SFR
data along an inbound orbit in the dusk sector. The density increased steeply at L ~ 4.2
to plasmasphere levels from values typical of the main plasma trough. Then at L ~ 3.5
a steep outer wall of another trough was encountered. Within this inner-trough thedensity was a factor of ~5 below nearby plasmaspheric levels. A relative steep interiorwall of the trough was detected at L ~ 2.8 as the density returned to plasmasphericlevels. Carpenter et al. [2000] conducted a statistical study of inner troughs using thedata from the CRESS satellite that encircled the Earth in near-equatorial orbits. The
properties of the inner density trough have been described by the occurrence rate(~13% of total passes), L shell range (L < 4 and ΔL = 0.5-1.5), occurrence in MLT(most in the nighttime), duration (up to 10 hours or more), and extent in longitude (atleast 20 deg).

6

Figure 1.5: Equatorial electron density in the saturated plasmasphere obtained
from ISEE-1 observations during the dayside sectors of 09-15 MLT. In each of the
11 cases, the plasmapause was beyond L = 5 (if identifiable). Only the profile
interior to the plasmapause are shown [Carpenter and Anderson, 1992].

Figure 1.6: A near-equatorial electron density profile observed by ISEE-1/SFR.
This profile shows an example of a plasmaspheric cavity in the dusk sector at
17-19 MLT on 17 September 1983 [Carpenter et al., 2000].
7

The thermal structure of the plasmasphere was also evaluated by the earlyin-situ observations. The plasma composition experiment on ISEE-1 measured the H+
temperature [Comfort, 1986]. Figure 1.7 shows the mean H+ temperature as a functionof L observed by ISEE-1. Statistical studies revealed that the mean temperature
increases with L, both on the dayside and the night side. On the dayside, however,there is a negative temperature gradient for L < 3, an effect that remains unexplained.
For L > 4, the temperatures are higher at night.

Figure 1.7: Mean H+ temperature as a function of L for (a) the dayside and (b) thenightside plasmasphere obtained by ISEE-1 [Comfort, 1986].

1.2 Theoretical approach for the plasmasphere
On the other hand, there have been theoretical approaches for thecomprehension of the plasmasphere [Nishida, 1966]. Nishida [1966] first represented aclear explanation of the formation of the plasmapause. There is a large-scale electricfield directed from dawn to dusk. This electric field originated from the solar windmagnetic field and penetrates into the magnetosphere. The plasma in themagnetosphere drifts to E B direction due to this electric field (E is the convection
electric field, and B is the Earth’s magnetic field). When the corotation with the Earth8

is superposed on a large-scale convection, a convection profile shown in Figure 1.8 isobtained. The motions of cold plasmas, such as those in the plasmasphere, areexplained by the E B drift convection, since their gradient and curvature drifts arevery small. In Figure 1.8, it can be seen that the streamlines of the plasma convections
can be categorized into two distinctly different flow regimes by a separatrix between
open and closed drift paths: those which pass through the magnetosphere, and thosewhich circulate around the Earth. The latter closed region inside the separatrix
continues to be filled up to high densities with ionospheric plasmas. Outside thisseparatrix, cold plasmas on an open drift trajectory generally drift sunward from thetail, and flux tubes are filled up to relatively lower densities of cold plasmas due totheir shorter lifetimes in the magnetosphere. As a result, the separatrix can berecognized as the outer boundary of dense plasma, i.e., the plasmapause.
When the geomagnetic activity level increases, the convection electric field
increases and the region where corotation is enforced shrinks. The last closed
equipotential, as well as the plasmapause, then shifts closer to the Earth. On thecontrary, when the convection electric field diminishes, the corotation region expands
to include some depleted flux tubes, which can then refill by diffusion from theionosphere and by other possible mechanisms.
Another mechanism based on quasi-interchange instability has been proposed
to explain the plasmapause formation [e.g., Lemaire, 1974; 1985; 2001]. Gold [1959]
first introduced the concept of interchange of the magnetic flux tubes in themagnetospheric context. Its so-called pure interchange model assumes a one to one
interchange between magnetic flux tubes enclosing the same magnetic flux and thus
leaving the shape of the magnetic field lines unchanged as well as the magnetic energyof the system unperturbed. Newcomb [1961] studied the influence of the gravitational
field and of stratification on the three modes of the ideal magnetohydrodynamic (theAlfvén mode, the fast and slow modes). He considered a plasma confined in ahorizontal magnetic field distribution by a uniform vertical gravitational field.
Newcomb [1961] identified two convective wave modes whose dispersion relation ω =ω(k) is influenced by the stratification of the plasma in the vertical direction, for nearly9perpendicular wave vector. These two modes can be distinguished by their behavior in
the limit of zero parallel wave vector: k║ → 0. In this limit the convective wave modes
fall into two types and were given the name of quasi-interchange modes. Figure 1.9
shows the schematic diagrams of two quasi-interchange modes. In the limit k║ → 0, thetype-1 mode corresponds to Gold’s pure interchange mode, while the type-2 mode
corresponds to a pure translational mode, i.e., the plasma motion is mainly along theunperturbed magnetic field lines.

Figure 1.8: Streamlines of the magnetospheric plasma motion obtained bysuperposing the motion due to the Earth’s rotation on the flow of thesolar-wind-induced convection. The boundary between two groups of streamlines,
i.e., those that pass the tail and those that do not, is suggested to be theplasmapause (dotted curve). The region of turbulence is also indicated [Nishida,1966].

10

Figure 1.9: Schematic diagrams of the quasi-interchange modes. (a) Type-1
mode: a plasma element with an excess density (dark blue part) exchanges its
position with an element of lower density (pale blue part) under the action of theeffective gravity g e including the effect of the centrifugal forces due to theplanetary rotation Ω. (b) Type-2 mode: an outward displacement (opposed to thedirection of the magnetic curvature vector c ) of a flux tube is accompanied by aparallel convergence of plasma along field lines (as indicated by yellow arrows)[from Andre and Lemaire, 2006].

Andre and Lemaire [2006] tested the local stability of quasi-interchange
modes for various diffusive and exospheric hydrostatic field-aligned density
distributions and concluded that the density distribution in the plasmasphere cannot
stay in hydrostatic equilibrium. According to this study, the type-2 quasi-interchange
mode appears to play an important role than the type-1 mode. As a matter ofconsequence, unstable field aligned flow tends to be set up in the equatorial region.
The enhanced azimuthal convection velocity leads to increased centrifugal
acceleration in the outermost layers of the plasmasphere and a reduction of the total
field-aligned potential barrier that ions have to overcome to reach the equatorial plane.

11This prompts the uplift of ions out of the underlying ionosphere. As a result of theenhanced outward flow and expansion of the plasma, the field-aligned density is
reduced along all geomagnetic field lines traversing the zero-parallel force surface(ZPFS). Along this surface the components of the gravitational and centrifugal
acceleration balance each other in the direction parallel to the geomagnetic field lines.
As a consequence of the plasma density depletion at high altitude along the field lines
traversing the ZPFS, a steep density gradient is formed in the plasmaspheric equatorial
density profile. This steep plasma density gradient corresponds to a new plasmapause[Lemaire, 2001]. Figure 1.10 shows the schematic diagram of the plasmapause
formation based on the quasi-interchange mechanism. In this scenario of theplasmapause formation, the sharp density gradient develops first in the equatorial
region, where the transverse cross-L velocity becomes most prominent due to the
maximum centrifugal acceleration and curvature of geomagnetic field lines.

Figure 1.10: A schematic diagram of the plasmapause formation based on theinterchange mechanism. Upward ionization flow and detachment of a plasma
element from the plasmasphere are occurred as a consequence of enhanced
magnetospheric convection. The dashed-dotted line represents the zero-parallel
force surface (ZPFS), where the field-aligned components of the gravitational andcentrifugal acceleration balance each other [Lemaire, 2001].

12

1.3 Imaging the Plasmasphere
In-situ measurements along the spacecraft’s path were intensively performedin the early 1970s, and revealed dynamic aspects of the plasmasphere. However, thein-situ measurements are local observations with which it is difficult to separate thetemporal development and the spatial structure of the plasmasphere. To overcome thisdifficulty, various efforts have sought to understand the global distribution of theplasmasphere by visualization.
Helium and oxygen ions are the second and third major components in theplasmasphere, as shown in Figure 1.2, and resonantly scatter the solar extreme
ultraviolet (EUV) emission at 30.4 nm (He II) and 83.4 nm (O II), respectively. For
this reason, detecting these emissions leads us to the global imaging of theplasmasphere. The fundamental technology to detect the EUV emission began with the
He II emission (30.4 nm), which was first detected on a sounding rocket experiment[Johnson et al., 1971]. After this discovery, proof-of-concept observations to giveinside-out views of the plasmasphere have been performed by both sounding rockets(e.g. Ogawa and Tohmatsu, 1971; Paresce et al., 1974; Yoshikawa et al., 1997) and thespacecraft (e.g. Weller and Meier, 1974: Chakrabarti et al., 1982; Meier et al., 1998).
However, it is difficult to extract the geophysical information on the outerplasmasphere from its inside.
The two-dimensional He II imaging of the Earth’s plasmasphere from its
outside was first done by the Planet-B (Nozomi) spacecraft [Nakamura et al., 2000].
Yoshikawa et al. [2000a, 200b, 2001] first presented static EUV images of theplasmasphere and the inner magnetosphere. Figure 1.11 represents the 2-D image
observed by the XUV scanner onboard the Planet-B spacecraft. The image shows theshape of the plasmasphere averaged over the noon-duskside sector. The dipole
magnetic field lines of L = 4 and 5 are also shown. While the inner magnetic field line
corresponds to the plasmapause, the outer magnetic field line corresponds to the outerboundary of the dusk bulge [Yoshikawa et al., 2000a].

13

Figure 1.11: Two-dimensional imagery assuming the symmetrical shape of theplasmasphere observed by NOZOMI. The dipole magnetic field lines of L = 4 and5 are also shown. While the inner magnetic field line corresponds to theplasmapause, the outer magnetic field line corresponds to the outer boundary ofthe dusk bulge [Yoshikawa et al., 2000a].

Recent advances in satellite-based imaging techniques have made it possible
to routinely obtain full global images of the plasmasphere. The Extreme Ultraviolet
Imager (EUV) on the Imager for Magnetopause-to-Aurora Global Exploration(IMAGE) satellite gave us complete sequential pictures [Burch et al., 2001a, 2001b;
Sandel et al., 2000, 2001]. The EUV instrument detects the distribution of He+ in theplasmasphere with a time resolution of 10 min and a spatial resolution of 0.1 RE at theapogee (~8 RE). The EUV instrument consists of three sensor heads and their fields ofview are joined to create a single image.
The EUV images of the plasmasphere reveal a host of distinct features and14

behaviors [Sandel et al., 2001, 2003]. The resulting global view provided a newcontext for more than 40 years of in situ and ground observations. One of the first
results led to a refinement in our descriptive language for plasmaspheric structures,
which is presented in Figure 1.12. The six EUV image panels provide examples ofplumes, notches, shoulders, fingers, channels and crenulations. The direction of the sun
is indicated by the yellow dot in each panel. The shadows and aurora are not featuresof the plasmasphere, but are routinely present in the images. The brightness in theseimages is proportional to the line integral of the He+ abundance along each pixel’s lineof sight. The sharp drop of intensity corresponds to the plasmapause.

Figure 1.12: Structures observed by the EUV instrument onboard the IMAGE
satellite. Various local structures, e.g. shoulders, plumes, fingers, channels,
crenulations and notches are identified in the EUV images. The direction to the
Sun

is

shown

as

a

yellow

dot

for

each

image.

http://image.gsfc.nasa.gov/poetry/discoveries/N47big.jpg)

15(AdoptedfromGlobal images have also revealed the morphology of the plasmaspheric
response to changes in convection, i.e. inward motion of the nightside plasmapause (orerosion) and corotation of the plasmaspheric structure. The typical pattern of erosion
and corotation of the plasmaspheric structure (plume) is illustrated using EUV
observations in Figure 1.13. The inward motion of the nightside plasmapause (a-d)
associated with the enhanced convection and the corotation of a drainage plume (e-i)
during the quiet period are identified in the sequential images.
IMAGE observations show that the overall erosion process starts with aformation of the sharp plasmapause near midnight [Spasojevic et al. 2003; Goldstein etal. 2003a; Goldstein and Sandel 2005; Gallagher and Adrian 2007]. The onset of theerosion and the plasmapause formation requires only 10-30 minutes after thesouthward turning of the IMF Bz [Goldstein et al. 2003a; Murakami et al. 2007]. As
for the corotation, the EUV images provide the first evidence that the main body of theplasmasphere does not always corotate with the Earth [Sandel et al., 2003]. The
corotation rate of the plasmasphere to the Earth’s rotation was studied by tracking the
localized density structures, such as notches, plumes, shoulders, and fingers, and wasestimated to be about 0.9 [e.g., Sandel et al., 2003; Burch et al., 2004; Gallagher et al.,2005; Galvan et al., 2010].
New global images of the plasmasphere viewed from different perspective
have been available. The KAGUYA (SELENE) spacecraft was launched by the H-IIA
rocket in September of 2007 and then put into the orbit around the Moon. The Telescopeof Extreme Ultraviolet (TEX) mounted in the Upper atmosphere and Plasma Imager(UPI) on KAGUYA detects the resonance scattering emissions of oxygen ions (O II:
83.4 nm) and helium ions (He II: 30.4 nm) to take images of near-Earth plasmas. It is
the first time to take the sequential images of the plasmasphere from the meridian
perspective, and they provide new aspects of plasmaspheric dynamics. The design,in-orbit performance, and results of the observations are described in Chapters 2, 3, and4.

16

Figure 1.13: (Top panels) Global plasmasphere images observed by
IMAGE/EUV on 18 June 2001, mapped to the magnetic equatorial plane (in SM
coordinates). The solar direction is to the right and dashed circles are drawn at L =(2, 4, 6, 6.6). Color contour indicates the column abundance of He+ (in arbitrary
units).

The

inwardmotionofthenightsideplasmapause

(a-d)

and

formation/corotation of a drainage plume (e-i) are identified. (Bottom panel) The
solar wind electric field (ESW), defined as product between the solar wind speed
and the IMF Bz observed by the ACE satellite. After 03:14 UT ESW was negative,
indicating a dawn-to-dusk global electric field. Vertical lines labeled (a) through(l) indicate times of EUV snapshots above. [Goldstein and Sandel, 2005]17

1.4 Instruments for the plasmasphere imaging
The recent imaging instruments in the EUV region mentioned above have
similar types of optics. Figure 1.14 shows schematic cross-sectional drawings of (a) the
XUV scanner onboard the Planet-B spacecraft and (b) one of the EUV sensor heads
onboard the IMAGE satellite. Each instrument is designed as a normal-incidence
telescope. Their optics consists of a multilayer coated mirror to focus EUV photons, a
metal thin filter to eliminate unwanted longer wavelength emissions, and microchannel
plates to pick up photon events.

Figure 1.14: (a) A schematic cross-sectional drawing of the XUV scanner
onboard the Planet-B (Nozomi) spacecraft [Nakamura et al., 2000]. The optics
consists of a multilayer-coated mirror, a thin metal filter, and microchannel plates.
(b) A cutaway drawing of the EUV sensor onboard the IMAGE satellite [Sandel etal., 2000].

18

The majority of effort for developing EUV instrument has been devoted tomake high reflective mirrors. It is difficult to build optics to collect EUV photons
effectively, because the reflectivities of any materials for normal incidence are quite low
in the EUV range (e.g. at most 5% at 30.4 nm). A newly developed multilayer coatedmirror provided a technical breakthrough in EUV optics [Yamashita et al., 1992;
Kunieda et al., 1996]. This technical achievement have made it possible to build
extreme ultraviolet telescopes for the imaging of the plasmasphere. The multilayer
coated mirrors having high reflectivity at 30.4 nm have been developed and installed on
the instruments mentioned above [Nakamura et al., 2000; Sandel et al., 2000]. Figure
1.15 shows the calculated and measured reflectivities of the Mo/Si multilayer mirror
installed in the TEX instrument onboard the KAGUYA spacecraft as a function of
wavelength [Yoshikawa et al., 2008]. The high reflectivity of 18.8% for the normal
incidence was achieved at 30.4 nm.

Figure 1.15: Reflectivity of a Mo/Si multilayer coated mirror used in the TEX
instrument onboard the KAGUYA spacecraft. The reflectivity of 18.8% isachieved at 30.4 nm (He II).

19

1.5 Construction of this Thesis
The main purpose of this thesis is to examine the plasmasphere distribution
and dynamics from the meridian perspective by means of a global imaging techniquein the extreme ultraviolet (EUV) spectral range. This thesis consists of 6 chapters.
Chapter 2 presents the in-orbit calibration of the Telescope of Extreme Ultraviolet(TEX) onboard the KAGUYA spacecraft. Chapter 3 details the first meridian images ofthe plasmasphere obtained by the TEX instrument. It is necessary to emphasize that the
TEX instrument observed not only known phenomena such as erosion and corotation
but also a new feature of the plasmasphere. In Chapter 4, the plasmapause formation
viewed from the meridian perspective by the TEX instrument is discussed. Chapter 5 isdevoted to the development of new multilayer mirrors for the next plasmasphericimaging from the International Space Station (ISS). Concluding remarks are given in
Chapter 6.

20

2 In-orbit calibration of the Telescope ofExtreme Ultraviolet onboard KAGUYA
2.1 Introduction
The Japanese lunar orbiter KAGUYA (SELENE) was launched by the H-IIA
rocket on 14 September 2007 and put into a polar orbit at an altitude of about 100 km.
The KAGUYA project had carried out the scientific observations of the Moon, at theMoon, and from the Moon. A major purpose of this mission was to obtaincomprehensive data that can be used to study the origin and evolution of the Moon andmoon utilization in the future [Sasaki et al., 2003; Kato et al., 2008]. KAGUYA
observed the surface composition, topography, underground structure, magnetic fieldanomaly, and gravity all over the Moon using remote sensing. Also, in situ
measurement of plasma density, electric and magnetic fields, and high-energy particles
were performed. Two satellites, OKINA and OUNA, which orbit in elliptic orbits,
provided support for communication between KAGUYA and the ground and gravity
measurements.
The Upper Atmosphere and Plasma Imager (UPI) onboard KAGUYA is
composed of two telescopes (Telescope of Visible Light: TVIS and Telescope ofExtreme Ultraviolet: TEX) and an equatorial mounting gimbal that points the telescopesin the desired direction. Unlike the other instruments onboard KAGUYA, UPI looks atthe Earth from the lunar orbit in order to visualize the Earth’s upper atmosphere andplasmasphere. TVIS takes pictures of the Earth's airglow and aurora at 4 wavelengths[Taguchi et al., 2009] and lunar sodium exosphere [Kagitani et al., 2009]. TEX images
the plasmasphere and polar wind using the resonance scattering emissions of oxygenion at 83.4 nm (O II) and helium ion at 30.4 nm (He II) [Yoshikawa et al., 2008].
After running primary function checks on the instrument, the TEX observation
was carried out in mainly two phases. The Earth-tracking phase lasted from March to21

June 2008 and provided us first sequential images of the Earth’s plasmasphere from themeridian perspective. The analysis of these images and the studies of the plasmasphereare performed in the following chapters (Chapter 3 and 4). The second phase wasdevoted to the all-sky survey and lasted 6 months from December 2008 to May 2009.
During this phase several hundreds of EUV sources were identified by the TEX
instrument. The detailed study of such stellar EUV radiations is beyond the scope of
this thesis.
In this study, the in-orbit data obtained by the TEX instrument were analyzed
to evaluate the instrument performance after launch. The image scale of the field ofview (FOV) and the point spread function (PSF) at the detector were calibrated byanalyzing the images of bright EUV sources. The long-term variation of the detector
performance was also verified.

2.2 Instrument overview
The KAGUYA spacecraft was launched in September 2007, and entered into
the lunar polar orbit at an altitude of 100 km. KAGUYA is a three-axial stabilized polar
orbiter, with one of the axes of the spacecraft always oriented towards the Moon surface.
The TEX instrument is mounted on 2-rotational-axis gimbal system together with TVIS,as show in Figure 2.1. The rotation around the azimuthal (AZ) axis cancels the apparent
drift motion of the Earth in the image due to the orbital motion of the spacecraft. The
other rotation (EL: elevation axis) cancels the orbital inclination of the Moon. During
the Earth-tracking phase, the Earth was always kept in the center of FOV thanks to bothrotations. On the other hand, during the all-sky survey the AZ angle was always fixed
and the EL angle was moved by ~3 deg everyday. Therefore, the FOV coverage of thesky was a sector strip with an angular width of 8.3 deg during a revolution of thespacecraft, and TEX surveyed an entire great circle on the sky in the period from
December 2008 to May 2009.
The TEX instrument is a type of normal-incidence telescope consisting of amultilayer-coated mirror, a split metallic thin filter, and a microchannel plate (MCP)22detector. Figure 2.2 shows a schematic diagram of the TEX instrument, and Table 1
summarizes its main features (including the results of in-orbit calibrations). The detailed
design and the ground calibration result [Yoshikawa et al., 2008] of each component aredescribed below.

Figure 2.1: Upper atmosphere and Plasma Imager (UPI) onboard the KAGUYA
spacecraft.

Twotelescopes

areinstalledontheequatorial

mounting(motor-controlled azimuthal and elevation axes) gimbal. After the insertion to thelunar orbit, the system is open (left panels) in order to make the two telescopes
ready to point to the Earth. Pointing accuracy of the equatorial mounting is 3.24”
around Azimuthal and 2.68” around Elevation axis.

23

Figure 2.2: A schematic diagram of the Telescope of Extreme Ultraviolet (TEX)
instrument onboard the KAGUYA spacecraft. Main optical components are a
multilayer mirror, a split metallic thin filter, and a microchannel plate (MCP)
detector. The field of view is separated to two regions by the filter; one region isfor He II (30.4 nm) radiation and the other is for O II (83.4 nm) radiation.

2.2.1 Multilayer mirror
The multilayer mirror is designed for normal incidence reflection with a peakreflectivity at 30.4 nm, because most materials have much higher single-layer
reflectivities at 83.4 nm. For example, the normal incidence reflectance of silicon at
83.4 nm is about 37%. The TEX mirror is coated by 20 alternating Mo/Si layers (periodof 16.49 nm, Mo thickness of 4.1 nm, and Si thickness of 12.39 nm). The mirror
substrate is made of Zerodur, and the mirror has a focal length of 168 mm, a diameter of
120 mm, and a parabolic shape to reduce aberration. The reflectivities of the flightmirror at 30.4 nm and 83.4 nm are 18.8% and 20.2%, respectively. Although the results

24

of ground calibrations showed that the Mo/Si multilayer coating was not particularly
subject to degradation by oxidation, we protected the coating using normal proceduresfor handling. We stored the mirror in boxes purged by N2, and when the mirror had
been installed in the telescope, we kept the overall instrument under dry N2 purge.

2.2.2 Metallic thin filter
The filter material was chosen to provide high transmittance at the target
wavelengths of 30.4 nm and 83.4 nm, while attenuating the unwanted Ly-α (121.6 nm)
emission from the Earth’s geocorona and the interplanetary medium. The filter is set at
12.3 mm off from the focal plane and is divided to two parts. One part has a carbon
coating of 31.2 nm on an Al film (162.8 nm), which attenuates emissions whose
wavelengths are longer than 40 nm; it allows the passage of He II (30.4 nm) with asmall transmittance of He I (58.4 nm). The other part consists of a 292.9-nm thick layerof In and transmits O II (83.4 nm). The filter is supported by a stainless steel meshhaving a wire spacing of 0.36 mm and an open area of 75%. The Al material was
permitted to oxidize in air on one side, and In was oxidized on both sides. The oxidized
coatings stabilize the material and provides the additional benefit of reducing thetransmission of Ly-α (121.6 nm).
From the ground calibration, the Al/C part has transmittances of 13.3% at 30.4
nm and 0.27% at 58.4 nm, and the transmittance above 120 nm is below 10-9. The
photon rejection at longer wavelengths is good enough to eliminate strong
contamination from Earh’s Ly-α geocorona. The In filter is used as band-pass filter toprevent He I, He II, and Ly-α emissions from contaminating O II emission. In particular,
the Ly-α is so strong that the In filter is designed to be as thick as 292.9 nm despite lowtransmittance of 2.5% at 83.4 nm. The transmittance at Ly-α is 106-fold lower than thatat 83.4 nm, and it is sufficient to reduce the contamination of Ly-α.

2.2.3 Microchannel plate (MCP) detector
The microchannel plate (MCP) detector is located at the focus of the telescope
mirror. The detector consists of “Z” stack of three MCPs and each MCP has a circular25

shape with an active area of 42 mm in diameter, a bias angle of 8 deg, a 12-μm pore size,an L/D (a ratio of length to pore diameter) of 40 and a total resistance of 125 MOhm.
Behind the MCPs, the resistive anode encoder (RAE) is placed to analyze the positionsof photon injections. The electron cloud emitted from the MCPs consists of an order of~107 electrons. The electron cloud is drifted from the MCP output to the RAE, and its
centroid position can be determined. Each photon location is transmitted to an (X, Y)
precision of 128 × 128. The dark count of the MCP detector in the ground calibrationwas ~0.4 cps/cm2.

2.3 In-orbit performance
The in-orbit data obtained from February 2008 to May 2009 were analyzed toevaluate the performance of the TEX instrument after launch. In this section, the resultsof the in-orbit calibrations are presented.

2.3.1 Image scale of the field of view (FOV)The image scale of the TEX instrument was calibrated using the images ofsome bright stars observed during the all-sky survey. This value is important to convert
the dimension of the image from pixel to angle. During the all-sky survey the equatorial
mounting gimbal was fixed and the exposure time of each image was 1 min. The FOVof the TEX instrument moved due to the orbital motion of the spacecraft. Therefore theimage scale can be estimated comparing the displacement of an EUV star in the image
and the rotation angle of the FOV for the exposure time of 1 min. Figure 2.3 shows anexample image of a star obtained by the TEX instrument during the all-sky survey. The
track of the bright star is detected at the upper left of the image (in the region for O-II
radiation). In this case the displacement of the star was 26 pixels and the rotation angleof the FOV was 1.66 deg, which corresponds to the image scale of 0.064 deg/pix. In thisway the dependence of the image scale on the position on the detector was investigated.
The images of bright EUV stars (> 10 count/min) observed from 25 April to 5 May
2009 (EL angle ranged from 30 to 60 deg) were used. Figure 2.4 represents the result.
26

The image scales are shown as a function of the distance from the center of the image. It
is clear that the image scale is constant and independent of its position. Thus, this result
indicates that there are no distortions in the TEX images. In average the image scale
was 0.065 ± 0.004 deg/pix, corresponding to 0.068 ± 0.004 Re/pix at the distance of the
Earth. The total FOV (128 pix × 128 pix) equals to 8.3 deg × 8.3 deg or 8.7 Re × 8.7 Re.

Figure 2.3: An example image of a star obtained by the TEX instrument during
the all-sky survey. During this observation phase the equatorial mounting gimbal
was fixed, so the FOV of the TEX instrument moved due to the orbital motion of
the spacecraft. In this case the movement of the FOV for 1 min was 1.66 deg,
which corresponds to 26 pixels in the TEX image.

27

Figure 2.4: Dependence of the image scale on the position at the detector. The dot
shows the average of the image scale at each distance from the center of thedetector.

2.3.2 Point-spread function (PSF)
To verify the spatial resolution of the TEX instrument from in-orbit data,
images of some bright stars were analyzed individually. The spatial resolution of thetelescope can be estimated by the width of the point-spread function (PSF). During the
Earth-tracking phase, some bright stars were detected in the FOV (mostly in the region
for O II radiation). Figure 2.5 shows an example image of a bright star obtained by the
TEX instrument. The white circle represents the Earth, and the white square encircles
the star. The star is brighter than the dayside of the Earth by a factor of ~3. A
two-dimensional Gaussian fitting was used to estimate the width of the PSF. In this caseshown in Figure 2.5 the full-width at half-maximum (FWHM) of the PSF is 1.8 pix. In
this way, the images of bright EUV stars observed from March to June 2008 wereanalyzed. As a result, the average FWHM of all PSFs was 3.7 ± 1.1 pix, corresponding28

to 0.24 ± 0.07 Re. Based on this result, the 2 × 2 pixel binning should be used in theanalysis of the plasmaspheric images, described in Section 3 and 4, to improve thesignal-to-noise ratio (SNR).

Figure 2.5: An example image of a star obtained by the TEX instrument. In thisperiod the TEX instrument was pointed to the Earth using the equatorial mounting
gimabl. The Earth is indicated by the white circle. A bright star is seen near thecenter of the image as indicated by the white square. In this image the FWHM of
the point-spread function (PSF) is 1.8 pix.

29

2.3.3 Background
The temporal variation of the background in the region for He II radiation
throughout the KAGUYA mission was investigated. Figure 2.6 shows the backgroundcounts as a function of time since the MCP detector initially turned on. The
backgrounds were estimated using the images of the interplanetary space without any
bright objects (the Earth or stars) in the FOV. The average value of the backgroundthroughout the mission was 1.8 cps/cm2. This value is larger than the MCP dark countsof ~0.4 cps/cm2 measured in the laboratory. The background is caused by dark counts
from MCPs in the detector, bombardments of cosmic rays and/or high energy particles,
and emissions from the interplanetary space. For the lunar environment, this increase
was mainly due to the contamination from He I (58.4 nm) emission from theinterplanetary space. Yamazaki et al. [2006] reported that the intensity of He I emission
from the interplanetary space observed by the Extreme Ultraviolet (XUV) scanner
onboard the Planet-B spacecraft is 1.5-7.5 Rayleigh. Using this value and the sensitivityof the TEX instrument (2.0 × 10-4 cps/Rayleigh/pix), the background caused by the He I
emission from the interplanetary space can be estimated to 0.8-4.2 cps/cm2. The result
shown in Figure 2.6 is consistent with this value. Nevertheless, the backgrounds wereconstantly low (1.8 cps/cm2) compared to typical signal levels from the plasmasphere(~30 cps/cm2) and there were no significant changes. This result suggests that the MCP
detector had worked throughout the mission without any problems, e.g., some hot spots
on the MCPs.

30

Figure 2.6: The backgrounds as a function of time since the MCPs were firstturned on. All the backgrounds were obtained while the TEX instrument was
directed to the interplanetary space, so the backgrounds due to the cosmic rays,
the high-energy particles, and the emissions from the interplanetary space. The
intrinsic detector background measured in the laboratory was below 0.5 cps/cm2[Yoshikawa et al., 2007]. The backgrounds are constantly low (~1.8 cps/cm2)
compared to typical signal levels from the plasmasphere (~30 cps/cm2) and there
are no prominent changes.

The signal-to-noise ratio (SNR) during the plasmaspheric observation can be
estimated using the background obtained above. The detector behaves as an ideal
photon counter so that signal corresponds to the number of photon events collectedduring an exposure period. During the measurement, a detector pixel sees the EUV
signal (Nsignal), measured in cps/bin, and the background (Nbackground: instrumental
background plus contaminations from the other EUV emissions), measured in the sameunits. The SNR is calculated as follows:31

N signal2
total

signal

N total2
backgroundN signal

SNR

N background2

2
background(2.1)

2N backgroundN signal
signal2
signal

N signal
N signal

2N background.

The sensitivity of the TEX instrument for He II (30.4 nm) radiation is 0.016
cps/Rayleigh/pix, and the typical intensity of the plasmasphere at 30.4 nm is ~1
Rayleigh. Assuming the 2 × 2 pixel binning and the exposure time of 20 min, SNR = 9
is achieved.

2.3.4 Pulse height distribution of the MCPs
The aging change of the MCP performance through the mission was verified
from the in-orbit data. An important indicator of the MCP stack performance is thestatistical variation of the gain; i.e. the pulse height distribution (PHD) of the single
photon events. It is well known that all MCPs experience gain decreases as a function ofuse, a phenomenon caused by electron scrubbing of useful mobile secondary emitting
materials out of the channels [e.g., Joseph et al., 1998]. The gain appears to be an
exponentially decreasing function of the total amount of charge extracted from the MCP.
To verify the aging change of the MCP gain throughout the mission, the PHDs of the
MCPs obtained at the beginning (on 14 February 2008) and the end (on 29 March 2009)of the TEX observation were compared, as shown in Figure 2.7. The observation targetson (a) 14 February 2008 and (b) 29 March 2009 were the Earth and a bright EUV star,
respectively. In both cases the same input voltages of –1943 V were applied to the MCP
stack. Comparing Figure 2.7a and b there are no clear changes of the gains at the peaksin the PHDs. This result indicates that the MCP stack installed in the TEX instrument
kept its gain performance throughout the mission.

32

Figure 2.7: Pulse height distributions (PHDs) of the microchannel plates (MCPs)
obtained (a) on 14 February and (b) on 29 March 2009. In both cases the same
input voltages of –1943 V were applied to the MCPs. Comparing (a) and (b),there are no clear changes in the PHDs.

33

Table 2.1: Principal TEX parameters
Prime focus
Angular resolution (1 pix)
Spatial resolution (1 pix)
Maximum time resoution
Mass
Size
Open Aperture
Format
FOV
Reflectivity of mirror
Transmittance of filter
Quantum efficiency of MCP
Sensitivity

F/1.4
0.065 deg
0.068 R E
1 min
2 kg
235 mm×φ140 mm
48.6 cm2
128×128
8.3 × 8.3 deg2
Typ. (at 30.4 nm)
Typ. (at 83.4 nm)
18.8%
20.2%
11.2%
2.1%
7.6%
9.8%
0.016 cps/Rayleigh/bin
0.0036 cps/Rayleigh/bin

2.4 Summary
The performance of the TEX instrument after launch was evaluated using thein-orbit data obtained from February 2008 to May 2009. The image scale of the field ofview (FOV) was calibrated by analyzing the images of bright EUV sources observed
during the all-sky survey, and the result of 0.065 deg/pix corresponding to 0.068 Re/pix
was obtained. The point-spread function (PSF) at the detector was also calibrated using
the images of bright EUV sources during the Earth-tracking phase. The average FWHM
of PSF was 3.7 ± 1.1 pix, corresponding to 0.24 ± 0.07 Re. This result suggests that the
2 × 2 pixel binning should be used in the analysis of the plasmaspheric images toimprove the signal-to-noise ratio (SNR). Furthermore, the long-term variation of thedetector performance was also verified. The background throughout the mission wasalmost constant, in average 1.8 cps/cm2. This value is enough low and thesignal-to-noise ratio (SNR) of 9 can be achieved in the plasmaspheric imaging. No
aging degradation was identified in the gain performance of the MCP detector
throughout the mission. These results are important not only for the scientific products
but also for the future instruments.
34

3 First sequential images of theplasmasphere from the meridian
perspective by KAGUYA
3.1 Introduction
Recent advances in satellite-based imaging techniques have made it possible toroutinely obtain full global images of the plasmasphere. The Extreme Ultraviolet
Imager (EUV) onboard the IMAGE satellite [Sandel et al., 2000], which detects He II
emission, produced consecutive images of the terrestrial plasmasphere from the aerialview [Sandel et al., 2001; Burch et al., 2001]. The IMAGE mission provided new
evidence of the dynamic and spatially-structured nature of the plasmasphere. For
example, IMAGE/EUV observations show that the nightside plasmapause moves
inward in quick (10-30 min) response to the southward turning of the IMF Bz[Goldstein et al., 2003b; Murakami et al., 2007]. The EUV images also provide the firstevidence that the main body of the plasmasphere does not always corotate with the
Earth [Sandel et al., 2003].
The IMAGE mission identified not only conventional pictures of theplasmasphere (plasmapause, tails, and duskside bulge), but also novel structures (e.g.,
depleted regions that are called "voids" and a sharp azimuthal gradient in He+ density
called “shoulder”). One of the most striking and puzzling features seen in the EUV
images is a radial structure of enhanced brightness called a “finger” [Sandel et al.,2001]. Sandel et al. [2001] interpreted these structures as isolated flux tubes that havebeen filled to higher densities than neighboring tubes. Such flux tubes viewed from a
position at high magnetic latitude would appear projected into the magnetic equatorplane, leading to the observed finger-like appearance. If this interpretation is correct,
investigation of fingers can play a key role to solve the longstanding issues of35

plasmasphere refilling. On the other hand, Adrian et al. [2004] explained fingers asradial He+ enhancements arising from some resonance of ultra-low frequency waves. It
is very important to know the cause of the finger structures for plasmasphere dynamics.
Although the EUV imager observed fingers many times, they have not been completely
understood due to the limited perspective of the EUV imager, i.e. only from highmagnetic latitude.
Now global images of the plasmasphere from another perspective become
available. The Telescope of Extreme Ultraviolet (TEX) onboard the Japan’s lunar
orbiter KAGUYA images the resonance scattering emissions of oxygen ion at 83.4 nm(O II) and helium ion at 30.4 nm (He II) [Yoshikawa et al., 2007]. The view afforded by
the KAGUYA orbit encompasses the plasma distribution in a single exposure, enabling
us to examine for the first time the globally-averaged properties of the plasmaspherefrom the "side" (meridian) perspective. In this study, the first meridional images of theplasmasphere obtained by the TEX instrument onboard the KAGUYA spacecraft wereanalyzed and discussed.

3.2 Instrumentation and observation
The KAGUYA spacecraft was launched in September 2007, and entered into
the lunar polar orbit at an altitude of 100 km. The TEX instrument is mounted on 2axis gimbal system. Thanks to the gimbal rotations, the Earth can always be kept in thecenter of FOV, providing images of the Earth’s plasmasphere every minute. The TEX
instrument detects the resonance scattering emissions of helium ions (He II) andoxygen ions (O II). In this study, only data of He II radiation are used.
The KAGUYA satellite orbits around the Moon with an orbital period of 2hours. This provides an observation window of the Earth every 2 hours, thoughobservation must satisfy some geometrical conditions. Figure 3.1 shows schematicgeometry among the Earth, the Moon, and the satellite. We can turn on the instrumentin the following conditions. (1) The satellite has a clear view of the Earth. (2) The
satellite is in the umbra of the Moon, thus reducing stray sunlight incident on the TEX
instrument. (3) The umbra period is longer than 15 min.
36

Figure 3.1: Schematic diagram of the geometry among the Earth, the Moon, andthe KAGUYA spacecraft. TEX starts to observe in the condition that (1)KAGUYA is in sight from the Earth, (2) KAGUYA is in umbra of the Moon, (3)the umbra period is longer than 15 min.

After the primary function checks of the complete spacecraft and instrument,
the observation of the plasmasphere in the Earth-tracking mode was started in March
2008. Unfortunately, the metal band-pass filer was found to be mechanically damaged.
This might have been due to the vibration or shock during the vehicle launch phase.
Two small pinholes on the filter were identified. The exact locations of these pinholes
were investigated, and two circular “ghosts” appeared on all EUV images due to theleak of geocoronal emissions from longer wavelength. The ghost locations are stable inall EUV images and predictable by calculation, independent of plasmasphericmorphology. This fact simplifies the image correction and image analysis, and ensures
our study without additional care. In this study, all pixels which were potentiallycontaminated by the artificial signals have been removed.
Figure 3.2 shows a schematic drawing of geomagnetic field lines from theperspective view of KAGUYA. We should note that a 2-dimensional view of theplasmasphere is not dipole-like, but the outer boundary is elliptic, because thebrightness reflects the column density along the line-of-sight (LOS) of themeasurement. For example, during the observation in June 2008 reported in thefollowing section, our telescope gazed down to the Earth’s geomagnetic equatorial

37

plane by 20 degree. As described in Chapter 2, the TEX instrument has a field-of-view(FOV) of 8.3 deg × 8.3 deg, which corresponds to 8.7 Re × 8.7 Re at the Earth’s
position. The format of the image is 128 × 128 pixels (i.e., 1 pix = 0.068 Re).

Figure 3.2: View of the geomagnetic field lines seen from KAGUYA. During theobservation in June 2008, our telescope gazed down to the Earth’s geomagnetic
equatorial plane by ~20 deg. Each line indicates a dipolar magnetic field (L = 4).
The bold field lines are drawn extending every 2- hour local time. The two whitecircles are in place to mask the circular ghosts due to an instrument problem.

38

3.3 Results and discussion
All the plasmaspheric images obtained by TEX from March to June 2008
were analyzed. Prominent phenomena of the plasmasphere, erosion and corotation,
were identified in a geomagnetically disturbed and quiet period, respectively.
Furthermore, a new striking feature of the plasmasphere was found from the meridian
perspective. The detailed studies of these events are presented below.

3.3.1 Inward motion of the nightside plasmapause
There was a succession of geomagnetically quiet days in 2008. Fortunately,
the TEX instrument has produced meridian plasmasphere images during a
geomagnetically disturbed period from 1 to 2 May. Figure 3.3 shows the geomagnetic
conditions (Kp and Dst) and IMF z-component (in GSM coordinates) on 1-2 May 2008.
The IMF data, measured by Advanced Composition Explorer (ACE) at the L1 libration
point, are time delayed by 54 min to account for propagation to the Earth’s
magnetopause, assuming the solar wind speed is 470 km/s. The southward turning of
the IMF Bz occurred at 20:20 UT on 1 May, and the maximum Kp was 5- during theobservation.
Figure 3.4a–c show three EUV snapshots obtained by TEX on 1-2 May 2008.
The apparent size and location of the Earth, the day-night terminator, the equator(dashed lines), and sub-solar point (dots) are indicated. The dipole magnetic field lines(L = 2, 3, and 4) at 03 and 15 MLT are also shown. To improve the signal-to-noise ratio,(1) all the images in the same revolution were accumulated to one image and (2) 2 × 2
pixel binning was used (1 bin = 0.14 Re). The sharp He+ edges in the nightside are
identified in Figure 3.4b and c. The nightside edges are aligned to L shells of L = 3.6
and L = 3.2 at 03 MLT, respectively (red broken lines). The detailed study about thetemporal and spatial variation of the meridional distribution during this period isdemonstrated in Chapter 4.

39

Figure 3.3: Geomagnetic conditions and IMF z-component (in GSM coordinates)on 1-2 May 2008. Vertical dashed lines indicate the observation periods
corresponding to each panel of Figure 3.4. We used the IMF data measured by the
MAG instrument onboard the ACE satellite. The IMF data is presented with a
54-min time delay to compensate for the propagation from the measurement point
to the Earth.

40

Figure 3.4: (a-c) Three EUV plasmaspheric images from the meridian view takenon 1-2 May 2008. The Earth is located near the upper edge of each image. The
dashed arc on the Earth represents the geomagnetic equator and the dot on the
Earth is the sub-solar point. The two gray circles are put in order to mask thecircular ghosts due to an instrumental trouble. The geomagnetic field line of eachL-shell (L=2, 3, and 4) are indicated in the meridional plane at 3-15 magnetic
local time (MLT). The inward motion of the plasmapause is clearly seen in thenightside. (d) Flux tubes seen from the lunar orbit. The nightside EUV brightnessreflects the plasma distribution at 03 MLT rather than adjacent local times.

Goldstein et al. [2003a] verified that the location of the EUV outer boundary
corresponds to the plasmapause by comparing the IMAGE/EUV images with in-situ
measurements. In addition, Obana et al. [2010] found the agreement between the EUV
outer boundary observed by the TEX instrument and the plasmapause location
observed by the ground-based magnetometers. According to these studies, we also
assume that the sharp He+ edge does correspond to the plasmapause. In this study, the41

EUV outer boundaries were determined along count rate of 0.4 count/min/bin whichcorresponds to the lower sensitivity threshold achieving SNR of ~2. This determinationis reasonable because the EUV intensity from the plasmasphere main-body is generally
higher and that from the plasmatrough is lower than the threshold. In our simple
estimation assuming the plasmaspheric density based on the statistical in-situ
measurement by Carpenter and Anderson [1992], the sharp plasmapause (ΔL = 0.05),
and the negligible plasmatrough density, the location of the lower sensitivity thresholdcoincides that of the plasmapause within 0.1 Re at the L-shell below 5. At the larger
L-shell the plasmapause location is underestimated in this method. The uncertainty ofthis plasmapause determination can be estimated to ±1 bin = ±0.14 Re.
Because the nightside EUV radiation at the edge comes from theplasmasphere at 03 MLT rather than adjacent local times, as shown in Figure 3.4d, thebrightness edge consists of the outermost L shell in the plasmasphere. If we assume afeatureless and azimuthally smooth plasmapause at the nightside, the edges should beinterpreted as the plasmapause with L = 3.6 at 03 MLT in Figure 3.4b and with L = 3.2
at 03 MLT in Figure 3.4c.
Based on the results of the long-term in-situ observations by the ISEE 1
satellite, Carpenter and Anderson [1992] modeled the plasmapause location as
follows:

Lpp

5.6 0.46 Kp max ,(3.1)

where Kpmax is the maximum of Kp value over an interval of several hours and up to a
day prior to the observation (details are mentioned in Chapter 1). The location
predicted by this conventional empirical model is 3.7 for the Figure 1b period and 3.3
for the Figure 1c period. These values are consistent with the results of the TEX
observations.
The IMF was clearly northward until 18 UT on 1 May. The geomagnetic
activity was quite low with average Kp of 1. A gradual decrease of brightness (density)42

with increasing L shell is seen in Figure 3.4a, as is typically seen in our EUV images ofthe plasmasphere obtained after prolonged periods of low geomagnetic activity. The
earlier successful mission IMAGE-EUV produced EUV images over several years, andGoldstein et al. [2005] reported a similar behavior as seen from a high-latitude
perspective by IMAGE-EUV during quiet periods.
From 20 UT to 23 UT on 1 May, the IMF polarity turned to southward, andthe geomagnetic activity (Kp) increased up to 5-. Dst decreased with a rate of 10[nT/h] at 21 UT. Comparing the three EUV images in Figure 3.4, it is clear that thenightside plasmapause moved inward. It is likely that the enhanced convection electricfield triggered by the southward turning of the IMF generated a new E × B drift
trajectory. The diffuse EUV outer boundary at the nightside (Figure 3.4a) then began
moving inward, forming a clear boundary. If we compare the outermost L shells
described in Figure 3.4b and c, the mean speed was approximately 0.2 Re/h and thetotal displacement was about 0.4 Re on the equatorial plane. The determination of theplasmapause location is some ambiguous, and we found the subjectivity to be ± 0.14
Re, as mentioned above. The subjective error is smaller than the total displacement,
and we estimate the mean inward speed with uncertainty as 0.2 ± 0.14 Re/h. The mean
speed of the EUV outer boundary, deduced from the IMAGE/EUV data obtained
during a similar geomagnetic trend on 22 April 2001, was 0.3 Re/h [Murakami et al.,2007]. This is not inconsistent with our result.

3.3.2 Corotation of the plasmasphereOn 1-2 June 2008, the geomagnetic activity was quite low, and no
geomagnetic storms had been identified since May 23. Figure 3.5 shows thegeomagnetic conditions (Kp and Dst) and IMF z-component (in GSM coordinates) on
1-2 June 2008. The maximum Kp was 3- during the observation, and that was 3 in theprevious 24-hour. The IMF, measured by ACE at the L1 libration point, had arelatively northward tendency except the very short period.

43

Figure 3.5: Geomagnetic conditions and IMF z-component (in GSM coordinates)on 1-2 June 2008. In this period the geomagnetic activity was quite low. Verticaldashed lines indicate our observation periods corresponding to each panel of
Figure 3.6. We used the IMF data measured by the MAG instrument onboard the
ACE satellite. The IMF data is presented by 44-min time-delay. The vertical solidline indicates the observation period of the plasmaspheric filament, described in
section 3.3.3 and Figure 3.8.

Figure 3.6 presents consecutive snapshots of the plasmasphere obtained by the
TEX instrument on 1-2 June 2008. All the images in the same revolution wereaccumulated into one image and the 2 × 2 pixel binning was used in order to increase
the signal-to-noise ratio. The Earth is located near the upper edge of each image. The
dashed arc on the Earth represents the Earth's geomagnetic equator and the dot on the44

Earth indicates the sub-solar point. During this period, the TEX instrument gazed down
the Earth from the Moon and the North Pole is presented as a cross mark in each image.
The inner and outer contour lines are drawn at count rates of 2.0 and 1.0 count/min,
respectively. The two white circles are put in order to mask the circular ghosts
explained in the previous section. For reference, the geomagnetic field lines (L=3) areindicated with magnetic local time (MLT) to align the enhanced density structure. The
magnetic field line in the consecutive images from (b) to (d) is identical, i.e., itcorotates with the Earth (24-hour duration). The magnetic field line described in (e)-(g)is also identical, but it is described with 2-hour lagged from that in (b)-(d). The images(b)-(d) were taken every 2-hour, and the images (e)-(g) were also taken with the sameroutine.
In Figure 3.6(a), there seems to be no prominent features in themidnight-to-dawn region except for the shadow. On the other hand, a medium-scaledensity structure, indicated by a black arrow, exists in the dawnside in (b)-(g). The
distribution of this structure inside the plasmasphere clearly agrees with the dipole
magnetic field line. Though it could not be distinguished from our view direction as
terms (e.g., shoulders) discovered by IMAGE satellite, this is the first result to succeed
in imaging a meridional density structure in the plasmasphere.
Furthermore, we clearly identify that the density structure rotates with the
Earth in the dawnside. In Figure 3.6(b), a part of the structure was likely to be in the
Earth's night shadow. Next, it rotated endways together with the field line in (c) and (d).
While it passed through the dayside, it was difficult to be seen due to the highbackground luminosity (i.e., the main body of the plasmasphere) and due to theintersection with the ghost area (two white circles). No image is shown here for thisperiod. The next day, the structure appeared in the dawnside and corotated with the
Earth as seen in (e)-(g). Comparisons of images (b)-(d) with (e)-(g) show thecorotation-lag of approximately 2 hours/day. (It should be remembered that themagnetic field line in the images from (e) to (g) is not identical to that in the upper
images, but it is lagged by 2 hours.) Thus, the corotation rate, that is the ratio of theobserved angular velocity to that of the Earth’s rotation, was ~0.9 in this study.
45

Figure 3.6: Seven EUV images obtained by the TEX instrument on KAGUYA.
The image was taken every 2-hour except for the period between (d) and (e). The
Earth is located near the upper edge of each image. The dashed arc on the Earthrepresents the geomagnetic equator and the dot on the Earth is the sub-solar point.
The inside and outside contour lines are drawn at count rates of 2.0 and 1.0
count/min, respectively. The two white circles are put in order to mask thecircular ghosts due to an instrumental trouble. The geomagnetic field lines (L=3)
are indicated with magnetic local time (MLT) to align the enhanced density
structure. Except for the image (a), an enhanced density structure, indicated by theblack arrow, is clearly seen in each image. It is noted that the magnetic field line46

in the consecutive image from (b) to (d) is identical, i.e., it corotates with the
Earth. The magnetic field line in the series of image from (e) to (g) is also
identical, but it is lagged by about 2 hours from that in (b) to (d).

Sandel et al. [2003] reported the first evidence that the main body of theplasmasphere does not always corotate with the Earth. They tracked notches, persistentdistinctive lower-density regions seen in EUV images obtained by the IMAGE satellite,
to infer the motion of particular volume elements of plasma. They reported that the
corotation rate ranges between 0.77 and 0.93 in the thirteen episodes they used. Burchet al. [2004] argued that the departures from corotation in the plasmasphere are driven
by corresponding motions of plasma in the ionosphere, where departures fromcorotation are often observed. They compared DMSP measurements of ionospheric ion
drifts to the motion of a notch in the plasmasphere observed by EUV at the same time,and found the consistency between both observations. They suggested that ionosphericcorotation lag arises from the ionospheric disturbance dynamo [Blanc and Richmond1980]. Heating of the auroral ionosphere by currents and precipitating particles drives
transport towards the equator. As the winds move to lower latitudes, conservation ofangular momentum leads to a departure from corotation that takes the form of awestward drift.
A recent study by Galvan et al. [Galvan et al., 2008] found that theplasmasphere on average sub-corotates with 28 hours. Gallagher et al. [2005] studied
the azimuthal motion of 18 notches seen in EUV pictures taken by the IMAGE satellite,
and reported the corotation rate ranges 0.85-0.97. The detailed review of such
sub-corotation in the plasmasphere was demonstrated by Darrouzet et al. [2009]. The
corotation rates reported in the past and estimated in this study are summarized in
Figure 3.7. The value of 0.9 in our EUV images seen from the meridian view is notinconsistent with their conclusion based on the observation from the aerial view.
These two results, presented in Section 3.3.1 and 3.3.2, confirmed that the
TEX instrument successfully obtained the spatial distribution and temporal variation of47

the plasmasphere from the meridian perspective. In the following section, the analysis
and discussion on a new striking feature in the plasmasphere first observed by the TEX
instrument are performed.

Figure 3.7: Summary plot of the corotation rate in the plasmasphere reported in
the past studies and in this study. The horizontal scale indicates the ratio of theobserved rotation velocity to that of the Earth’s rotation.

3.3.3 Plasmaspheric filament: An isolated magnetic flux tube
filled with dense plasmas
We discovered a new striking feature of the plasmasphere during a prolonged
quiet period. Figure 3.8 presents a snapshot of the plasmasphere obtained by the TEX
instrument on 2 June 2008. The image is presented in almost the same manner as each
image in Figure 3.6. In Figure 3.8, an arc-shape structure of enhanced brightness isclearly identified as indicated by the red arrow. Here we call this structure aplasmaspheric “filament.” The color contour in Figure 3.8 is adjusted in order toemphasize the structure. For reference, the dipole magnetic field line of L = 3.7 at 7.3
MLT is also drawn by the dashed curve.

48

Figure 3.8: An EUV image obtained by the TEX instrument onboard KAGUYA
on 2 June 2008. The Earth is located near the upper edge of the image. The
dashed arc and the dot on the Earth represent the Earth's geomagnetic equator andthe sub-solar point, respectively. The two gray circles are put in order to mask thecircular ghosts due to an instrumental trouble. The geomagnetic field line (L =3.7) at 7.3 MLT is indicated to align the structure of enhanced brightness, the“plasmaspheric filament,” pointed by the red arrow.

The shape of this structure seems to be completely agrees with the dipole
magnetic field line of L = 3.7 at 7.3 MLT. The magnetic field line that is the mostclosely aligned to the filament structure was selected as follows. The counts of the
pixels in Figure 3.8 were averaged along each dipole magnetic field line (at themagnetic latitude between ±20 deg), and the results are shown in Figure 3.9. A clear
peak can be identified at MLT = 7.3 and L = 3.7, indicated by the black circle, and this49

means that this magnetic field line is the best fitted to the filament structure. The
vertical cross-section profile at 7.3 MLT of Figure 3.9 is presented in Figure 3.10. The
increase of the average count at the peak (L = 3.7) comparing to the backgrounds (e.g.,
L = 3.4 and L =3.9) is larger than the statistical errors. This fact supports the reality ofthe enhanced brightness along the magnetic field line. This suggests that the filament
represents an isolated flux tube filled up to higher density than the neighboring tubes.
The TEX instrument first succeeded in identifying the isolated flux tube with dense
plasma in the plasmasphere because of imaging from the meridian perspective.

Figure 3.9: The average count along each dipole magnetic field line, derived
from the TEX image at 09:21-09:39 UT on 2 June 2008 (shown in Figure 3.8).
The counts of the pixel along each dipole magnetic field line (at magnetic
latitudes between ±20 deg) were averaged. There exists a peak at MLT = 7.3 andL = 3.7 as indicated by the black circle. The cross-sectional profile at MLT = 7.3
is presented in Figure 3.10. The dashed vertical lines also indicate the range of
MLT where the enhancement at the peak is larger than the error bars.

50

Figure 3.10: The cross-sectional profile at MLT = 7.3 in Figure 3.9. The clear
peak at L = 3.7 is indicated by the black arrow, and it corresponds to theplasmaspheric filament seen in Figure 3.8. The error bar gives the standard
deviation of accumulated counts along each magnetic field line.

Indeed, the filament structure shown in Figure 3.8 can be simulated byassuming an isolated flux tube filled with denser plasma than its neighbors. Figure 3.11
shows the calculated image of the plasmaspheric filament viewed from KAGUYA on 2
June 2008. In this calculation, we assumed the equatorial density distribution based on
the statistical in-situ measurement by Carpenter and Anderson [1992] (see Equation
1.1) and the circular shape of the plasmasphere. The He+ density (n) along themagnetic field line was also assumed to follow radial power law n = n0 (L Re / R)3,
where n0 is the density at in the equatorial plane and R is the geocentric radius [e.g.,
Obana et al., 2010]. Inside the flux tube at L = 3.7 (ΔL = 0.3) and MLT = 7.3 (ΔMLT =510.4) the He+ density was higher than its neighbors by a factor of 5, which was the
typical value of the density enhancement as discussed below. The calculated image in
Figure 3.11 is viewed from the same position as the KAGUYA spacecraft at 09:30 UT
on 2 June 2008. The dark region at the upper left of the Earth corresponds to theshadow. In the result of our calculation, the intensity of the filament is higher than its
neighbors by 13%, and this value is larger than the statistical errors. Comparing Figure
3.11 with Figure 3.8, it is clear that the filament structure is reconstructed by our
calculation. Therefore, it is concluded that a plasmaspheric filament corresponds to an
isolated flux tube filled with dense plasma.

Figure 3.11: A calculated image of a plasmaspheric filament viewed fromKAGUYA on 2 June 2008. Inside the isolated flux tube at L = 3.7 and MLT = 7.3
the He+ density is assumed to be higher than its neighbors by a factor of 5. This
image is presented in the same manner as that in Figure 3.8.

52

We approximately estimated the He+ density in the filament flux tube and its
neighboring tubes at L = 3.7 using the conventional method [Yoshikawa et al., 2000;
Gallagher et al., 2005]. The measured brightness is directly proportional to the He+column density along the line of sight because the plasmasphere is optically thin for
the He II radiation. Observed TEX instrument counts are converted to column density
using the following equation derived from the laboratory calibrations:

N = a * 1.0 * 1020 / F30.4,(3.2)

where N is the He+ column density in cm-2, a is the EUV signal in count/min/bin (for 2
× 2 pixel binning), and F30.4 is the solar flux at 30.4 nm in units of photons/cm2/s. The
solar 30.4-nm flux was 3.03 × 109 photons/cm2/s on 2 June 2008, measured by the SEE
instrument onboard the TIMED satellite. Column density can be converted to averagedensity by dividing by an estimate of the distance along the line of sight that
contributes most to the image intensity at each location in the field of view [Gallagher
et al., 2005]. This technique is applicable since densities in the plasmasphere rapidly
fall with increasing L-shell, and thus the inner most regions penetrated by a given lineof sight contribute the most to the observed 30.4-nm intensity. In addition to thetechnique described by Gallagher et al. [2005], we used the assumption concerning theazimuthal isotropy of the density in the plasmasphere. This assumption is based on thepast density observations which showed only small longitudinal (MLT) dependence[e.g., Clilverd et al., 2007].
In the event on 2 June 2008 shown in Figure 3.8, the average 30.4-nm
intensity of the plasmasphere at L = 3.7 was 0.5 count/min/bin, corresponding to thecolumn density of 1.7 × 1010 ions/cm2. The calculated column length through the
L-shell of 3.7 along the line-of-sight was 1.1 × 109 cm. Thus, the average He+ density
in the plasmasphere at L = 3.7 was estimated to be 15 ions/cm3. This value is consistent
with past in-situ measurements of the plasmasphere [e.g., Carpenter and Anderson,
1992], considering the low He+/H+ density ratio of ～0.05 for the minimum solar

53activity [Craven et al., 1997]. On the other hand, the 30.4-nm intensity along thefilament was higher than its neighboring bins by 0.25 count/min/bin, and then thecolumn density along the filament was higher by 8.3 × 109 ions/cm2. If we assume theouter shape of the filament as the cylinder whose diameter is ～2 bins (equal to 0.28
Re), the column length through the filament was estimated to be 2.0 × 108 cm.
Therefore, the density inside the filament flux tube is higher than its neighboring tubes
at L = 3.7 by 42 ions/cm3. This means that the filament flux tube is filled with denser
plasmas than its neighbors by a factor of ～4.
We searched all the TEX data obtained in the period of March-June 2008, andidentified 4 events of the plasmaspheric filament. Table 3.1 shows the date, UT, Kp,
L-value and MLT for each event. As the total of the TEX observation periods was ~26
days, the occurrence rate of the plasmaspheric filament identified by TEX was ~1
event / 8 days. It should be noted that all the events were observed during extremely
quiet periods, i.e. Kp < 2. Although the geomagnetic activity tended to be low during
the TEX observation, this result is not negligible. The dependence of the filament
occurrence on MLT was not found in the TEX observation.
Furthermore, all the events of the plasmaspheric filament occurred around the
L-value of 3.5. This result may not reflect the nature of the plasmasphere but be causedby some limitations of the observation. At the inner L-shells, it is difficult to identify
the filament (enhancement of the brightness) due to the high background luminosity(i.e., the main body of the plasmasphere). On the other hand, at the outer L-shells, theintensity of the filament decreases and it results in the low signal-to-noise ratio. The
field-of-view (FOV) of the TEX instrument also limits the opportunity to find thefilaments at the outer L-shells.
The plasmaspheric filaments can be compared with the structures called“fingers” observed by IMAGE/EUV [Sandel et al., 2001; Adrian et al., 2004]. The
fingers are regions of enhanced brightness having radial extents in the EUV images.
Sandel et al. [2001] interpreted that these structures represent isolated flux tubes thathave been filled to higher densities than neighboring tubes. Our result agrees with theirinterpretation. The comparisons between the features of the fingers observed by
54

IMAGE/EUV and the filaments observed by KAGUYA/TEX are summarized in Table
3.2 and described in detail below. Good agreement between them was found.
1. (Occurrence rate:) Adrian et al. [2004] found 20 events of the finger in
IMAGE/EUV images between February and August 2001. This indicates that theoccurrence rate of the fingers identified by IMAGE/EUV is ~1 event / 10 days. Thisvalue is consistent with that of the filament observed by TEX.
2. (Geomagnetic activity:) Almost all the fingers were observed by IMAGE/EUV in
geomagnetically quiet conditions. The average Kp value of the observation periods
listed in Adrian et al. [2004] is 1.8. This value is also consistent with the result in thisstudy.
3. (L-value:) The regions of enhanced brightness in the IMAGE/EUV images
radially extend to the L-shells of 2-4. In this study the L-shells of the filament
concentrates to 3.4-3.7 due to the limitation of the observation discussed above.
Nevertheless, these results are not inconsistent.
4. (Local time:) The EUV instrument observed the finger structure corotating over
12 hours [Sandel et al., 2001] and found it at every local time. This result agrees withthe result in this study that the filament appears independently of the local time.
However, it is difficult to identify the corotation of the filament in the consecutive TEX
images due to the instrumental limitations discussed above.
5. (Azimuthal extent:) The azimuthal extents of the fingers are up to 20 deg [Sandel
et al., 2001]. Although it is difficult to distinguish the azimuthal (local time) and radial(L-shell) extents in the TEX images, the azimuthal extent of the filament on 2 June2008 was at most 11 deg (ΔMLT = 0.7 h), as estimated in Figure 3.9. This value is notinconsistent with the results of IMAGE/EUV observations.
6. (Intensity enhancement:) In the case study of the IMAGE/EUV image on 7 June
2000, the EUV intensity of the finger was higher by ~25% than its neighbors at L = 3.
In this study the intensity of the filament was higher by ~35% than its neighbors asshown in Figure 3.10, and this value is comparable to that of the finger.

55

Table 3.1: The event list of the plasmaspheric filament showing the date, UT, Kp,
L-value, and MLT
Date

UT

Kp L-value MLT

1 April 2008 18:10-18:32
8 May 2008

1-

3.5

15.7

06:54-07:12 1+

3.4

8.0

26 May 2008 02:32-02:46

1-

3.5

21.0

2 June 2008

1

3.7

7.3

09:21-09:39

Table 3.2: The summary of the comparisons between the fingers in the
IMAGE/EUV images and the filaments in the KAGUYA/TEX images
Fingers (IMAGE/EUV) Filaments (KAGUYA/TEX)
Occurrence rate

~1 event / 10 daysa~1 event / 8 days

Geomagnetic activity

Average Kp = 1.8a

Kp < 2

Local time

No dependenceb

No dependence

L-value

L = 2-4

L = 3.4-3.7

Azimuthal extent< 20 degb< 11 deg

Intensity enhancement

~25%

~35%

a

[Adrian et al., 2004]b[Sandel et al., 2001]

On the other hand, Adrian et al. [2004] interpreted fingers as radial He+
enhancements arising from some resonance of ultra-low frequency waves. In all the
TEX data, we found only one event that can be interpreted as such a structure wasfound. As mentioned in the previous section 3.3.2 and shown in Figure 3.6, the TEX
instrument observed a localized density enhancement and its corotation on 1-2 June
2008. We simulated the plasmasphere images with radial He+ enhancements viewed
from KAGUYA by the same way as the previous calculation of the filament shown in
Figure 3.11. Each panel of Figure 3.12 represents the calculated image of the56plasmasphere with the radial structure at 01, 03, and 05 MLT. A radial density
enhancement by a factor of 5 was assumed at L < 4 with an azimuthal extent of 15 deg.
Comparing Figure 3.6b-d and Figure 3.12a-c, the structure obtained by the TEX
instrument on 1-2 June 2008 can be interpreted as a radial density enhancement as
proposed by Adrian et al. [2004], though it cannot be distinguished exclusively fromour view direction. Nevertheless, at most one event of the radial density enhancement
was identified, while four events of the plasmaspheric filament were identified by the
TEX instrument. This suggests that the fingers observed by IMAGE/EUV represent the
projection of isolated flux tubes into the magnetic equator plane rather than that ofradial density enhancements.
In recent years, many studies demonstrated the correlations between theplasmaspheric and ionospheric density structures. For example, recent comparisons ofmaps of ionospheric total electron content (TEC) with EUV images show good
agreements between plasmaspheric drainage plumes and ionospheric storm enhanced
densities (SEDs) [e.g., Foster et al., 2002]. Furthermore, localized plasma density
enhancements in the ionosphere (called “plasma blobs”) have been observed by in-situ
and ground-based observations [e.g., Yokoyama et al., 2007; Pimenta et al., 2007; Parket al., 2008]. This ionospheric structure may cause the localized density enhancementsin the plasmasphere through the magnetic field line. However, further investigation isneeded to understand the cause of the plasmaspheric filaments. In the future,
simultaneous observations of the plasmaspheric filaments and their foot points in theionosphere will be performed by the EUV Imager from the International Space Station(ISS) (see Chapter 5). Investigating the origin of the plasmaspheric filaments plays a
key role to understand the plasmasphere-ionosphere connections, especially thephysical process of the plasmaspheric refillngs.

57

Figure 3.12: Calculated images of radial He+ enhancements viewed fromKAGUYA on 1 June 2008. Localized density enhancement by a factor of 5 at L <4 was assumed to exist at (a) 01, (b) 03, and (c) 05 MLT with an azimuthal extent
of ΔMLT = 1.0.

3.4 Summary
The TEX instrument onboard the KAGUYA satellite obtained the first EUV
images of the plasmasphere from the meridian perspective in March-June 2008. The
inward motion of the plasmapause in the nightside during the disturbed period on 1-2
May 2008 was clearly observed by the TEX instrument. From the meridian view, theinward speed was estimated to be 0.2 Re/h on the assumption that the plasmapause wasfeatureless and azimuthally smooth on the nightside. This result is consistent with thatof the past study based on the IMAGE/EUV data.
The corotation of the plasmasphere during the quiet period on 1-2 June 2008
was also detected by the TEX instrument. A density enhancement inside theplasmasphere was tracked to estimate the angular rotation rate of the plasmasphere.
The observed corotation lag of 2 hours/day was consistent with the results from therecent studies derived from the IMAGE/EUV mission. These agreements confirmedthat the TEX instrument successfully obtained the spatial distribution and temporalvariation of the plasmasphere from the meridian perspective.
Furthermore, a new striking feature of enhanced brightness in the58

plasmasphere, called as a plasmaspheric filament, was found in the TEX image during
a prolonged quiet period. The shape of the filament was closely aligned to the dipole
magnetic field line, and this suggests that the filaments are caused by isolated magnetic
flux tubes filled with denser plasmas than their neighbors. We searched all the TEX
data obtained in the period of March-June 2008, and identified 4 events of theplasmaspheric filament. The nature of the filament was consistent with that of thefinger structure observed by IMAGE/EUV. Consequently, the result indicates that thefinger structure seen in the EUV images should be the equatorial projection of isolated
flux tubes filled with denser plasmas than its neighbors. The filament may be caused
by a localized structure in the ionosphere, such as plasma blobs. Investigation offilaments can play a key role to solve the longstanding issues such as plasmasphererefilling.

59

4 The plasmapause formation seen frommeridian perspective by KAGUYA
4.1 Introduction
The plasmasphere constitutes the extension of the ionosphere at high altitude:
cold plasma from the ionosphere moves along magnetic field lines and populates flux
tubes forming the plasmasphere. The outer surface in the plasmasphere is often
characterized by a sharp decrease in the plasma density, called the plasmapause.
Whistler and in-situ satellite observations have revealed that the equatorial position of
the plasmapause generally depends on the level of geomagnetic activity and varies
from 2.5 Re to 7 Re [Carpenter and Anderson, 1992; Moldwin et al., 2002].
It is well admitted that the configuration of cold plasma in the plasmaspheredepends on the electric field. This electric field is formed by the interplay of thecorotation electric field, which dominates near the Earth, and the convection electricfield, generated by the interaction of the solar wind with the magnetosphere. Nishida etal. [1966] proposed the first theory for the formation of the plasmapause based on the
postulate that the outer edge of the plasmasphere coincides with the last closed
equipotential (LCE) of the magnetospheric convection electric field superposed on the
Earth’s corotation electric field. The details of this longstanding idea are introduced in
Chapter 1.
On the other hand, some recent experimental evidences disagree with thisconventional theory. The first global comprehensive images of the Earth’s
plasmasphere were provided by the Extreme Ultraviolet (EUV) imager onboard the
IMAGE spacecraft launched in March 2000 [Burch et al., 2000; Sandel et al., 2001].
With these images, the large-scale dynamics of the plasmasphere were revealed. The
numerous EUV observations indicate the formation of a sharp plasmapause occurs in
the post-midnight sector soon (< 1 hour) after the sudden enhancement of the60

convection [e.g., Goldstein et al., 2003b, 2005; Murakami et al., 2007]. In addition,
when the convection is very weak for a prolonged period, smooth electron density
transitions from plasmasphere to the outer subauroral regions were often observed byin-situ measurements [e.g., Tu et al., 2007]. These results are inconsistent with theconventional theoretical formulation of the plasmasphere. In the conventional theory,the sharp plasmapause appears on the LCE of the electric field after a prolonged quiet
period rather than a disturbed period, and it takes over one whole day for theplasmapause to approach slowly its steady state position and settle down along the
LCE after the convection electric field changes. These disagreements have been
pointed out in several studies [e.g., Lemaire and Pierrard, 2008; Pierrard et al., 2009;
Darrouzet et al., 2009].
Another mechanism based on quasi-interchange instability has been proposed
to explain the plasmapause formation [e.g., Lemaire, 1974; 1985; 2001]. Thismechanism causes the peeling off of the plasmasphere as a result of sudden
enhancements of the convection velocity in the nightside (See Chapter 1). According tothis mechanism, the plasma becomes unstable above a Zero Parallel Force Surface(ZPFS) where the parallel component of the gravitational plus centrifugal accelerations
is equal to zero [Lemaire, 1985, 2001]. The plasma elements above this surface arepeeled off and drift away in the plasmatrough region toward the magnetopause or into
the magnetotail due to the type-2 quasi-interchange motion. In this scenario, (1) the
sharp plasmapause density gradient develops first in the equatorial region, (2) theplasmapause is formed along the equipotential surface along the ZPFS, and (3) theplasmapause formation is occurred in the post-midnight sector where the convection
velocity is significantly enhanced and it increases the centrifugal acceleration.
Pierrard and Lemaire [2004] performed the dynamic simulations ofplasmapause formation on the basis of this scenario. The plasmapause positions in thegeomagnetic equatorial plane predicted by these simulations were successfully
confirmed by IMAGE/EUV observations [Pierrard and Cabrera, 2005, 2006; Pierrard
et al., 2008]. Indeed, the model reproduces quite well the formation of plumes andshoulders observed by IMAGE/EUV. However, the EUV instrument onboard the61

IMAGE satellite provides information on only the azimuthal distribution of theplasmapause projected to the equatorial plane. The formation mechanism of theplasmapause remains an open issue, and more experimental evidences from different
perspectives are required to reveal it.
The first meridian images of the plasmasphere were provided by the TEX
instrument onboard the KAGUYA spacecraft in 2008. In this study, the sequential TEX
images during the geomagnetic disturbance on 1-2 May 2008 were analyzed. The
plasmapause positions at the post-midnight viewed from the meridian perspective werecompared with those predicted by the dynamic simulations. Furthermore, themeridional distribution of the plasmapause was investigated. This study provides animportant experimental evidence for the formation mechanism of the plasmapause.

4.2 Methodology
4.2.1 Observation of the plasmapause from the meridian
perspective
The TEX instruments onboard the KAGUYA spacecraft successfully obtained
the images of the plasmasphere from the meridian perspective between March andJune 2008, as demonstrated in Chapter 3. The TEX instrument detects the resonancescattering emissions of He+ (He II: 30.4 nm). Because the plasmasphere is opticallythin for the He II radiation, the measured brightness is directly proportional to the He+column density along the line-of-sight. The KAGUYA satellite orbits around the Moon
with an orbital period of 2 hours. This provides an observation window of the Earthevery 2 hours.
The meridian plasmasphere images during a geomagnetically disturbed period
from 1 to 2 May 2008 are used in this study. Figure 4.1, which is the same as Figure
3.3, shows the geomagnetic conditions (Kp and Dst) and IMF z-component (in GSM
coordinates) on 1-2 May 2008. The IMF data, measured by Advanced Composition
Explorer (ACE) at the L1 libration point, are time delayed by 54 min to account for
propagation to the Earth’s magnetopause, assuming the solar wind speed is 470 km/s.
62

The southward turning of the IMF Bz occurred at 20:20 UT on 1 May, and the
maximum Kp was 5- during the observation.

Figure 4.1: Geomagnetic conditions and IMF z-component (in GSM coordinates)on 1-2 May 2008. Vertical dashed lines indicate the observation periods
corresponding to each panel of Figure 4.2. We used the IMF data measured by the
MAG instrument onboard the ACE satellite. Southward turnings of the IMF Bz
occurred at 20:23 and 23:35 UT on 1 May 2008. The IMF data is presented with a
54-min time delay to compensate for the propagation from the measurement point
to the Earth. This figure is the same as Figure 3.3.

63

4.2.2 Calculation of the plasmapause location
In this study, the dynamic simulations based on the physical mechanisms andthe numerical method, which are explained by Lemaire and Gringauz [1998] andPierrard and Lemaire [2004], are used to calculate the plasmapause locations. Thismethod has been widely used in several recent studies [e.g., Pierrard and Cabrera,
2005; Pierrard and Stegen, 2008; Lemaire and Pierrard, 2008]. The dynamic code that
calculates the postion of the plasmapause can be available on the space weather portal(www.spaceweather.eu). The code calculates the locations of the plasmapause versus
MLT every hour UT during that given day using the successive values of Kp.
According to the interchange mechanism, the plasmapause is formed along a
Zero Parallel Force Surface (ZPFS) where the field-aligned components of thegravitational force and of the centrifugal force balance each other [Lemaire, 1985;
2001]. The radial distance of the ZPFS in the geomagnetic equator is given by

LZ

2GM E
3 2 RE

1/ 3

2/3

5.78

E

,

(4.1)

where G = 6.6726 × 10-11 m3 s-2 kg-1 is the gravitational constant, ME = 5.976 × 1024 kg
is the mass of the Earth, ΩE = 7.29 × 10-5 rad s-1 is the angular velocity of the Earth,and Ω is the local angular velocity of the plasma. When the rotation rate of the plasma
is equal to the angular velocity of the Earth, the ZPFS intersects the equatorial plane
along a circle whose radius is equal to 5.78 RE. The drift velocity of plasma is given by

E B
,
B2

and the angular drift velocity is given by

64

(4.2)

(r )

E tot

B

r2B2

r

,

(4.3)

where Etot is the total electric field of the convection and the corotation.
The Kp-dependent empirical electric field model E5D [McIlwain, 1986] isused in this dynamic simulation to determine the convection velocity, and ultimately
the location of the plasmapause. The E5D electric field model is derived from dynamic
proton and electron spectra measured onboard the geostationary satellites ATS-5 and 6.
This empirical electric field is fully determined only by the values of Kp. During a
geomagnetic disturbance, the angular rotational speed of the magnetospheric plasma isaccelerated due to the enhanced convection electric field in the post-midnight sector.
Consequently the minimum equatorial distance of the ZPFS is reduced and thenightside plasmapause moves inward. For instance, the ZPFS locates at 2.8 Re fromthe Earth for the angular rotation speed faster than the Earth’s rotation by a factor of 3
(Ω = 3ΩE).
The deepest penetration of the ZPF surface is located around 02:00 LT where
the convection electric field accelerates the angular velocity of the corotating plasma
and is most enhanced in the E5D model. Once the minimum radial distance for the
ZPF surface has been calculated as indicated above, the MHD drift velocity is used to
calculate the positions of the plasmapause in following MLT sectors.
Other magnetospheric convection electric field models are available in theliterature and some of them have been used to determine the positions of theplasmapause by Pierrard et al. [2008]. Among all most popular electric field models
tested by these authors (Volland-Stern, E5D, and Weimer), it happens that the E5Dmodel offers the results that were in slightly best agreement with the observations from
IMAGE/EUV for the cases examined in their comparative study. According to theirresult, the E5D electric field model is used in this study.
It has been inferred from IMAGE/EUV [e.g., Burch et al., 2004] andKAGUYA/TEX observations (see Section 3) that the azimuthal velocity of the65plasmasphere is slower than corotation in the outermost layers of the plasmasphere.
This effect is taken into account in the simulation by reducing the corotation potential.
In this study, the rotation rate of 90% corotation is used based on the result in Section
3.3.2.

4.3 Analysis and discussion
4.3.1 Comparisons of the plasmapause positions between the
TEX observations and numerical simulations
The plasmaspheric images obtained by the TEX instrument from 09:30 UT on
1 May 2008 to 01:29 UT were analyzed. In order to increase the signal to noise ratio,
all the images observed in the same spacecraft revolution were accumulated to one
image (i.e., exposure time of ~15 min) and 2 × 2 pixel binning was used. Figure 4.2
shows the EUV plasmaspheric images from the meridian view taken by TEX at (a)09:30-09:44 UT on 1 May, (b) 23:16-23:30 UT on 1 May, and (c) 01:14-01:29 UT on 2
May 2008. In these periods the line-of-sight of TEX was perpendicular to themeridional plane at 03-15 MLT. The apparent size and location of the Earth, theequator (red lines), the day-night terminator (green lines), and sub-solar point (greendots) are indicated. The dipole magnetic field lines at 03 and 15 MLT are shown. The
color contour is proportional to the logarithm of the count rate, corresponding to theintensity of resonantly scattered light (at 30.4 nm) from helium ions. The averagebackground of 1.8 cps/cm2 (corresponding to 0.16 count/min/bin), described in Section
2.3.3, was subtracted from each image. The sharp He+ edges, corresponding to theplasmapause at 03 MLT, are clearly identified as indicated by red lines in Figure 4.2.
As shown in Figure 4.1, southward turnings of the IMF Bz occurred at 20:23 and 23:35
UT on 1 May 2008. The inward motion of the nightside plasmapause due to theenhancement of the convection electric field is clearly seen in Figure 4.2.

66

Figure 4.2: Three EUV plasmaspheric images from the meridian view taken by
TEX at (a) 09:30-09:44 UT on 1 May, (b) 23:16-23:30 UT on 1 May, and (c)01:14-01:29 UT on 2 May 2008, which are almost the same as Figure 3.4. In these
periods the line-of-sight of TEX was perpendicular to the meridional plane at
03-15 MLT. The Earth is located near the upper edge of each image. The red arc
on the Earth represents the geomagnetic equator and the green dot on the Earthrepresents the sub-solar point. The two gray circles are put in order to mask thecircular ghosts due to an instrumental trouble. The color contour is proportional to
the logarithm of the count rate, corresponding to the intensity of resonantly
scattered light (at 30.4 nm) from helium ions. Contour lines at 1.0 (inner) and 0.4(outer) count/min are superposed. The geomagnetic field line of each L-shell (L=2,3, and 4) is shown. The location of each nightside plasmapause is indicated by redbold line. The inward motion of the plasmapause is clearly seen in the nightside.
The location of the ZPFS at 03 MLT is also represented by the dashed line in (c).

67

Dynamic simulations based on the interchange mechanism were also
performed. Figure 4.3 represents the examples of the simulation results at (a) 09:30 UT
on 1 May, (b) 23:30 UT on 1 May, and 01:30 UT on 2 May 2008. The time of eachpanel corresponds to the observation period of the TEX instrument presented in Figure
4.2. The plasmapause in the geomagnetic equatorial plane are shown as the black dots
in each panel. The inward motion of the nightside plasmapause at 03 MLT due to theconvection enhancement can be identified.
In order to compare the TEX observation and the dynamic simulation, theplasmapause locations at 03 MLT in the geomagnetic equatorial plane are identified
both in the TEX images and the simulation results on 1-2 May 2008. Figure 4.4 shows
the temporal variation of the plasmapause locations in the geomagnetic equatorial
plane at 03 MLT. The subjective error to determine the plasmapause in the TEX image
was estimated to be ±0.14 Re, as discussed in Chapter 3. The TEX images during
16-19 UT were not available. The arrival time of the southward IMF at the daysidemagnetopause (20:23 UT) is also indicated by the vertical dashed line in Figure 4.4.
After the southward turning of the IMF Bz, the plasmapause moves inward both in thecalculation and the TEX observation. The locations of the plasmapause observed by
TEX completely agrees with the calculation results, except at 15:30 and 21:30 UT on 1
May 2008.
The difference between the calculation and the observation at 15:30 UT
occurred due to a bulge formation. In the result of the calculation at 09:30 UT (inFigure 4.3a), a bulge (or a remaining plume) is located near 21 MLT. This structure
corotated and was located near 03 MLT at 15:30 UT. Thus the plasmapause location at
15:30 UT was quite far from the Earth (L = 4.8) in the calculation result. On the other
hand, the TEX observation demonstrated that the plasmapause at 15:30 UT was closer
to the Earth (L = 4.4). The bulge-like structure predicted in the calculation was notdetected by the TEX instrument. Nevertheless, this disagreement is not important forthis study because the bulge formation is largely dominated by the E×B drift motion at
the dayside rather than the plasmapause formation at the nightside.
The difference of the temporal response to the southward IMF between the68calculation and the observation can cause the disagreement of the plasmapause
location at 21:30 UT. Murakami et al. [2007] reported that the nightside plasmapause
starts to move inward 10-30 min after the southward turning of the IMF Bz. The resultof the TEX observation is not inconsistent with this timescale. However, the empiricalelectric field model E5D used in the dynamic simulation depends on the 3-hourly Kp
geomagnetic activity index, so it cannot reproduce the quick response of theplasmapause. It is needed to develop empirical models of higher time resolution for themagnetospheric electrostatic field distribution in order to test quantitatively and moredefinitely the theories for the plasmapause formation by comparing their theoreticalpredictions to observations. Nevertheless, the locations of the nightside plasmapause
predicted by the dynamic simulation agree with those identified in the TEX images(obtained every 2 hours). This study suggests that the plasmapause formation theory
based on the interchange mechanism is consistent with the TEX observation from themeridian perspective.

69

Figure 4.3: Results of the simulation based on the interchange mechanism at (a)09:30 UT on 1 May, (b) 23:30 UT on 1 May, and 01:30 UT on 2 May 2008. The
plasmasphere is viewed from a point of view above the North Pole. The
plasmapause in the geomagnetic equatorial plane corresponds to the black dots ineach panel. The dashed circles on each figure correspond to a radial distance of 2,
4, and 6 Re. The meridonal plane at 03-15 MLT, which is perpendicular to theline-of-sight from the TEX instrument, is also shown as a red dashed line in eachpanel. The time of each panel approximately corresponds to the observation
period of the TEX instrument presented in Figure 4.2.

70

Figure 4.4: Temporal variation of the plasmapause locations in the geomagnetic
equatorial plane at 3 MLT, obtained by the interchange simulations (solid line)
and TEX observations (dots) on 1-2 May 2008. The arrival time of the southward
IMF Bz is shown by the vertical dashed line.

4.3.2 Meridional distribution of the plasmapause observed by
TEX
The temporal variation of the meridional distribution in the plasmasphere wasinvestigated by analyzing the TEX images during the inward motion of the nightsideplasmapause. In Figure 4.2a, the distribution of the EUV radiation seems to be aligned
to the dipole magnetic field lines. On the other hand, as seen in Figure 4.2b and c, theEUV radiations in the nightside near the geomagnetic equatorial plane are relatively
low in comparison with those in higher latitudes. These depletions of the EUV

71

radiations at low latitudes can be seen more clearly in Figure 4.5. Figure 4.5 shows theaverage counts along the dipole magnetic field lines of L = 2.2-2.6 at 03 MLT as afunction of the magnetic latitude (MLAT). Each symbol represents the meridional
profile of EUV radiation shown in Figure 4.2a, b, and c. In Figure 4.5, the EUV
radiations clearly decrease below the magnetic latitude of 20 deg after the convection
enhancement. Each average count below the MLAT of 20 deg is also presented as thehorizontal line. After the convection enhancement, the EUV radiation at low latitudes
decreased by ~30% during the inward motion of the nightside plasmapause.
If the helium ions were in motion relative to the sun, the He II emission line
would be affected by Doppler shift in system moving with them. The wavelength λafter the shift can be expressed by λ = λ0 (1 - v/c), where λ0 is the original wavelength(30.38 nm), v is the velocity of the He+ relative to the Sun with a positive value
towards the Sun, and c is the speed of light. The FWHM of the solar He II emission
line is ~0.125 Å [Feldman and Behring, 1974]. For example, even if the helium ionswere moving away from the Sun at v = 10 km/s (~6 Re/h), the He II emission line
would shift to a wavelength 0.01 Å longer in the system and the intensity would
decrease by only 1%. As the typical drift velocity in the plasmasphere is below 10 km/s,
the decrease of the He II intensity due to Doppler shift is negligible.
Because the plasmasphere is optically thin for the He II radiation, themeasured brightness is directly proportional to the He+ column density along theline-of-sight. Therefore, the TEX instrument detected the depletion of the He+ column
density by ~30% near the equatorial plane during the geomagnetic disturbance.
This depletion of the equatorial column density can be caused by two
possibilities: (1) the decrease of the number density inside the plasmasphere at L = 2-3(i.e. the irregularity of the density inside the plasmasphere) and/or (2) the decrease of
the column length through the plasmasphere along the line-of-sight (i.e. the irregularityof the plasmapause shape). The former structures are known as the density troughs
inside the plasmasphere, called “inner-troughs” or “donkey-ears”, which werepreviously observed by in-situ measurements [e.g., Horwitz et al., 1990; Oya, 1997;
Carpenter et al., 2000; Fu et al., 2010].
72

The inner-trough is a structure with

significantly lower density (by a factor of 2-10) inside the plasmasphere compared tothat in the surrounding region, as shown in Figure 1.6. Carpenter et al. [2000]
conducted a statistical study of inner troughs using the data from the CRESS satellitethat encircled the Earth in near-equatorial orbits. The properties of the inner density
trough have been described by the occurrence rate (~13% of total passes), L shell range(L < 4 and ΔL = 0.5-1.5), occurrence in MLT (most in the nighttime), duration (up to
10 hours or more), and extent in longitude (at least 20 deg) [Carpenter et al., 2000].

Figure 4.5: The meridional distribution of the TEX counts inside theplasmasphere on 1-2 May 2008 shown in Figure 4.2. The instrument counts
averaged between L = 2.2 and 2.6 are plotted as a function of the geomagnetic
latitude. The average counts in the magnetic latitude below 20 deg is also
represented by each horizontal line.

73

Some

other

simultaneous

observations,

such

as

measurementsofmagnetospheric field line resonances by ground-based magnetometers or in-situ
measurements by the spacecrafts, are needed in order to determine whether an innertrough was present at the nightside during the disturbance period on 1-2 May. The
plasma mass density near the equatorial plane can be estimated by using the groundmagnetometer data of magnetospheric ultralow frequency (ULF) waves that couple tostanding field line eigenoscillations (field line resonances) [e.g., Obayashi and Jacobs,
1958; Menk et al., 1999]. However, this technique cannot be used at the nightside
because the resonances exist only in the dayside magnetosphere.
On the other hand, the in-situ measurement data of the THEMIS spacecrafts atthe nightside plasmasphere on 1-2 May were available. The spacecraft potential
measured by the Electric Field Instrument (EFI) can be used to estimate the electron
number density surrounding the spacecraft in the plasmasphere [Bonnell et al., 2008;
Courtesy of Y. Nishimura]. Figure 4.6a shows the orbit of the THEMIS-C (P2)spacecraft on 2 May 2008. The THEMIS-C orbits the Earth on the geomagnetic
equatorial plane with a period of ~2 days and a perigee of 1.4 Re at 07 MLT. The
spacecraft passed the inner plasmasphere in the noon sector around 09:00 UT. If theequatorial depletion of He+ column density in Figure 4.2b and c were caused by thesame inner-trough, it should exist in the noon sector indicated by the red shaded area in
Figure 4.6, assuming the pure corotation of the plasmasphere. Figure 4.6b presents theelectron number density estimated from the spacecraft potential of THEMIS-C. No
electron density depletions were identified by THEMIS-C in the red-shaded area.
Carpenter et al. [2000] reported inner-troughs are generally large (ΔL = 0.5-1.5 andazimuthal extent of at least 20 deg) and last up to 10 hours or more. Therefore, thisresult does not agree with the possibility that an inner-trough caused the equatorial
depletion of He+ column density during the erosion on 1-2 May.

74

Figure 4.6: (a) The THEMIS-C (P2) orbit in the solar-magnetic (SM) coordinate
on 2 May 2008, drawn in the same order as Figure 4.3. The positions of thespacecraft are presented by black dots every ~16 min from 08:31 to 10:07 UT. For
reference, the L-shell of 3.2 is also shown as the solid black circle. The plasma
elements in the red shade area were located at the post-midnight sector around
00:00 UT on 2 May. (b) The electron number density estimated from thespacecraft potential of THEMIS-C from 08:31 to 10:07 UT on 2 May.

75

The second possible cause of the depletion of the equatorial column density is
the decrease of the column length through the plasmasphere along the line-of-sight at
the low latitudes. For example, the flux tubes of L = 3.2 at 00 and 06 MLT were seen
over that of L = 2.3 at 03 MLT from KAGUYA on 1-2 May 2008. Using the same
method as the previous simulations in Chapter 3 (shown in Figure 3.11 and Figure3.12), the intensities of the plasmasphere were briefly calculated for various cases ofthe plasmapause shapes. Figure 4.7 shows the comparisons of the observation resultswith the calculated images of the plasmasphere with various plasmapause shapes
viewed from KAGUYA. The images are projected to the meridional plane at 03-15
MLT (only 03 MLT sectors are shown). The plasmapause shape is assumed to bealigned (a) to the dipole magnetic field line (L = 4.4), (b) to the ZPFS (Lz = 3.2) at low
magnetic latitudes below 10 deg and to the dipole magnetic field line (L = 4.4) at highmagnetic latitudes above 10 deg, (c) to the ZPFS (Lz = 3.2), and (d) to the dipole
magnetic field line (L = 3.2). The calculated shapes of the plasmapause at 03 MLT
does not agree with the observation results, especially calculation (b) with the TEX
image at 23:16-23:30 on 1 May. This may be caused by the simple assumption of thesame plasmapause shapes at all local times. Nevertheless, as indicated by the redcircles in Figure 4.7, the equatorial depletion of the intensity was clearly seen in our
calculation.
Figure 4.8 shows the meridional distribution of the intensity inside theplasmasphere shown in Figure 4.7. This figure is presented in the same manner as
Figure 4.5. Comparing Figure 4.8 (a) with the others (b)-(d), the decrease of theequatorial column density was estimated to be ~20% when the equatorial plasmapause
moves inward from L = 4.4 to 3.2. This value is in reasonable agreement with theobservational result. Furthermore, the profiles of (b) and (c) seems to be comparablewith the observational results of Figure 4.5 (b) and (c), respectively. Thus, these results
can be interpreted that the nightside plasmapause at 00-06 MLT moved inward only at
low latitudes during the disturbed period. In Lemaire’s [1974, 1985, 2001] interchange
scenario for the formation of the plasmapause, the plasmapause develops first in theequatorial region, and subsequently at lower altitudes along the magnetic field lines76

tangent to the ZPFS. On the other hand, the conventional theory can explain only the
profiles of Figure 4.8 (a) and (d), because the effects of the gravitational and thecentrifugal accelerations on the field-aligned plasma density distribution are not taken
into account. Therefore, the meridional distribution of the plasmasphere viewed fromthe KAGUYA spacecraft agrees with the interchange scenario rather than theconventional theory for the plasmapause formation.

Figure 4.7: Comparison of the observed and calculated images of theplasmasphere. (Upper) The observed images of the plasmasphere during erosion
on 1-2 May 2008. They are the same as those in Figure 4.2. (Bottom) Calculated
images of the plasmasphere with various plasmapause shapes viewed fromKAGUYA at 09 MLT on the geomagnetic equatorial plane. The plasmapause isassumed to be aligned to (a) the dipole magnetic field line (L = 4.4), (b) to the
ZPFS (Lz = 3.2) at low magnetic latitudes below 10 deg and to the dipole
magnetic field line (L = 4.4) at high magnetic latitudes above 10 deg, (c) to the
ZPFS (Lz = 3.2), and (d) to the dipole magnetic field line (L = 3.2).

77

Figure 4.8: The meridional distribution of the intensity inside the plasmasphereshown in Figure 4.7. These profiles were estimated in the same manner as those in
Figure 4.5.
The results presented in this study, however, do not prove that the interchange
scenario is exclusive. Our simulation discussed above was performed on quite briefassumptions, such as the azimuthal (LT) symmetry. Indeed, the plasmapause shape in
the calculated image of Figure 4.7 (b) is not comparable with the image obtained by
TEX (Figure 4.4). In order to compare the images from observations and calculations
more precisely, detailed numerical simulations for the plasmapause formation based on
the interchange mechanism are required. Further investigation is necessary tounderstand the plasmapause formation. Nevertheless, the unique observation by the
TEX instrument first detected the depletion of the equatorial column density by ~30%
during the inward motion of the nightside plasmapause. This equatorial depletioncannot be explained only in terms of the traditional paradigm involving the interplay ofthe magnetospheric convection and the Earth’s corotation. The results shown in thisstudy will play a key role to determine the plasmapause formation mechanism.

78

4.4 Summary
The formation mechanism of the plasmapause was studied using the meridian
images of the plasmasphere observed by the TEX instrument onboard the KAGUYA
spacecraft. The sequential TEX images during the geomagnetic disturbance (Kp = 5) on
1-2 May 2008 were analyzed. The plasmapause positions at the post-midnight observed
from the meridian perspective clearly agreed with those predicted by the dynamic
simulations based on the interchange mechanism. Furthermore, after the convection
enhancement, the He+ column density in the nightside plasmasphere decreased by~30% only at the low latitudes (< 20 deg). The depletion of the column density may becaused by the decrease of the column length through the plasmasphere along theline-of-sight at the low latitudes during the geomagnetic disturbance. This result is
consistent with the formation mechanism of the plasmapause based on thequasi-interchange instability. Although the interchange mechanism is not exclusive, thisis the first study to present the plasmapause formation viewed from the meridian
perspective.

79

5 Development of a multilayer mirror
for Extreme Ultraviolet Imager onboard
ISS-JEM
5.1 Introduction
Based on the heritage of the TEX instrument onboard the KAGUYA
spacecraft, the next EUV imaging mission to observe the meridional distribution of theplasmasphere from low altitude is proposed and now in progress. The ISS-IMAP(Ionosphere, Mesosphere, upper Atmosphere, and Plasmasphere mapping) mission will
make the imaging observation of the Earth’s upper atmosphere from the Exposed
Facility of Japanese Experiment Module on the International Space Station (EF of
ISS-JEM). It is scheduled to start observation in 2012. The objective of this mission is
to clarify the physical mechanism of the following three processes: (1) energy transport
process by the atmospheric structures whose horizontal scale is 50-500km in the upper
atmosphere (2) process of the plasma transport up to 20,000km altitude (3) effect of theupper atmosphere on the space-borne engineering system. To elucidate these processes,
ISS-IMAP consists of two imaging instruments, Visible and Infrared Spectral Imager(VISI) and Extreme Ultra Violet Imager (EUVI). VISI will measure the airglow of
630nm (O), 650nm (OH), and 762nm (O2) in the mesosphere and the ionosphere. Its
field-of-view (FOV) is in the nadir direction. EUVI will measure the resonant
scattering of 30.4nm (He+) and 83.4nm (O+) of the plasmasphere. Its FOV is in the
backward limb direction.
The EUVI instrument consists of two telescopes, one is for He II radiation(30.4 nm) and the other is for O II radiation (83.4 nm). The design of each telescope isalmost the same as that of the TEX instrument, which is a type of normal-incidence
telescope with a multilayer mirror, a thin metal filter, and a MCP detector. The filter for80

He II is composed of Al/C and that for O II is composed of In.
For the He II imaging from ISS, the contamination from geocoronal He I
(58.4 nm) emission should be eliminated more effectively than the TEX instrument,
because ISS orbits at a low altitude of ~350 km, inside the Earth’s geocorona. The
intensity of He II (30.4 nm) emission from the main body of the plasmasphere is ~5
Rayleigh [Nakamura et a., 2000], while the intensity of He I (58.4 nm) from thegeocorona is ~400 Rayleigh [Chackrabarti et al., 1983]. If completely the same optics
as TEX is used in the EUVI instrument, the signal from the plasmasphere iscomparable to the background from the geocorona. In order to solve this problem, it isnecessary to develop a new multilayer coating having high reflectivity at 30.4 nm andlow reflectivity at 58.4 nm. This specification is challenging because most materials
have much higher single layer reflectivities at 58.4 nm than at 30.4 nm, e.g., thenormal incidence reflectivity of Mo at 58.4 nm is 24%.
Periodic multilayer films have been developed in recent years for use asreflective coatings operating at normal incidence in the soft-x-ray and extreme
ultraviolet (EUV) spectral regions. The coatings comprise a stack of nanometer-scale
bilayers of optically dissimilar materials. The layers are arranged so that the reflections
occurring at each interface in the stack achieve constructive interference over a rangeof photon wavelengths and incident angles. The response of the coating can be tuned
by adjusting the multilayer period d (i.e., the bilayer thickness) according to Bragg’s
law n λ = 2 d cosθ, where θ is the incident angle and n is the Bragg order. The numberof periods and the relative thickness of each constituent material can also be adjusted
in order to control the spectral response of the coating.
In space and planetary science, a number of imaging or spectroscopic
observations in the EUV region have been and will be performed. High-reflectivity
multilayer coatings for the He-II radiation at 30.4 nm have been required anddeveloped for some of these applications [e.g., Sandel et al., 2000; Yoshikawa et al.,2008]. For the He II emission line, a multilayer mirror of Mo/Si has been usually andconventionally used, because it is highly stable. Yoshikawa et al. [2008] presented thereflectivity of the Mo/Si multilayer mirror installed in the TEX instrument is 18.8% at
81

30.4 nm.
In this work, a stable EUV mirror that has higher reflectivity at 30.4 nm withlower reflectivity at 58.4 nm than those of Mo/Si was developed for the plasmasphericimaging from ISS. The sample mirrors of SiC/Mg and Y2O3/Al multilayer coatings arefabricated and evaluated. The performance and the temporal stability of each
multilayer coating are reported in this chapter. Furthermore, the calibration results ofthe flight mirror for the EUVI instrument are also presented.

5.2 Observations from ISS
One of the targets of the EUVI instrument is the plasmaspheric filament. The
TEX instrument onboard the KAGUYA spacecraft found the isolated flux tubes filledwith dense plasma in quiet periods, as discussed in Chapter 3. However, due to theshort duration of the TEX observation, only four events of the filaments wereidentified and the cause of the filament is still remaining open issue. In order tounderstand the physical process of the filament formation, especially relationship withthe underlying ionosphere, we plan to observe the plasmasphere by the EUVI
instrument onboard ISS. Investigation of filaments can play a key role to solve thelongstanding issues such as plasmasphere refilling.
ISS orbits the Earth at an altitude of ~350 km with a period of 90 min. The
EUVI instrument has the FOV of 14 deg × 14 deg, and it looks toward the backward
limb direction of the ISS orbit. The inclination of the ISS orbit is 51 deg, so the EUVI
can observe the magnetic flux tubes below L-value of ~4 at low altitudes. We
simulated the plasmaspheric filaments viewed from ISS using the same assumptions as
those of the previous calculations in Section 3.3.3 and 4.3.2. Figure 5.1 shows thecalculated images of the plasmaspheric filament viewed from ISS during one
revolution. Each case of the ISS orbit with the ascending node at 00, 03, 06, and 09
MLT is shown in Figure 5.1(a), (b), (c), and (d), respectively. The horizontal axis
represents the azimuthal (longitudinal) angle of the FOV and the sunward direction
corresponds to 0 deg. The vertical axis represents the elevation (latitudinal) angle of82the FOV and the geomagnetic equatorial plane corresponds to 0 deg. At the altitudes
higher than 1000 km inside the flux tube at MLT = 21 and L = 3, the He+ density was
assumed to be higher than its neighbors by a factor of 5. As shown in Figure 5.1, thefilament can be identified, especially in (a) and (d), because in this case it appears at
the nightside and the intensity of its neighboring background is enough low thanks to
the Earth’s shadow. For example, in Figure 5.1(d) the intensity of the filament is higher
than its neighbors by ~50%.

Figure 5.1: Calculated images of the plasmaspheric filament viewed from ISS.
The horizontal axis represents the azimuthal (longitudinal) angle of the FOV (thesunward direction corresponding to 0 deg) and the vertical axis represents theelevation (latitudinal) angle of the FOV (the geomagnetic equatorial plane
corresponding to 0 deg). Inside the isolated flux tube at L = 3 and MLT = 21 theHe+ density is assumed to be higher than its neighbors by a factor of 5.

On the other hand, Figure 5.2 shows calculated images of the filament at 09
MLT. The images were calculated in the same manner as Figure 5.1 except for the
MLT of the filament. As shown in Figure 5.2, when the filament appears at the dayside,
it is difficult to identify the filament structure due to its neighboring bright83

backgrounds. Nevertheless, our calculation suggests that the EUVI instrument onboard
ISS can observe the plasmaspheric filament in the nightside region and detect its
corotating motion for several hours (during 2-3 revolutions of ISS).

Figure 5.2: Calculated images of the plasmaspheric filament viewed from ISS.
These images were calculated in the same manner as Figure 5.1 except for the
MLT of the filament (09 MLT).

5.3 Designs and calculations
In order to decide the materials for the multilayer mirror installed in the EUVI
instrument, the reflectivities of various multilayer coatings at 30.4 nm and 58.4 nm
were calculated by using a conventional method [e.g. Windt, 1998]. We selected thematerials which have been used for multilayer coatings in past studies and whose
optical constants at 30.4 nm and 58.4 nm are available. Table 5.1 shows the materials
and their optical constants (i.e., complex refractive indices n-ik) used in this study.
Generally, multilayer coatings consisting of pairs with small k and large differences ofn perform high reflectivities [e.g., Hotta et al., 2002]. We calculated the reflectivitiesof all the combinations between upper six materials in Table 5.1 (Y2O3, Ir, Au, C, Pt,

84and SiC) with small n values at 30.4, and lower six materials (Mo, MgF2, Si, Al, Mg,
and W) with large n values.

Table 5.1: Optical constants at 30.4 nm and 58.4 nm of various materials
30.4 nm58.4 nm

n

k

n

k

Y2O3 0.77

0.21

1.04

0.54

Ir

0.79

0.21

0.99

1.04

Au

0.80

0.31

1.06

0.84

C

0.81

0.085

0.65

0.75

Pt

0.83

0.21

0.79

0.92

SiC

0.85

0.045

0.35

0.48

Mo

0.91

0.43

0.61

0.75

MgF2 0.92

0.15

1.1

0.82

Si

0.93 0.0090 0.64

0.051

Al

0.94 0.0082 0.71 0.0024

Mg

0.99 0.0025 0.86

0.045

W

1.04

0.87

0.18

0.50

Here, the calculated results can be classified into two groups, i.e. with andwithout Mg as an element. All of the multilayer coatings with Mg performed quite
higher reflectivities at 30.4 nm than those without Mg. Figure 5.3 shows the calculated
reflectivities of various multilayer coatings with Mg. They all had high reflectivity
over 40% at 30.4 nm for the normal incidence of 10 deg, and the SiC/Mg multilayer
coating performed the highest reflectivity in our calculation. Yoshikawa et al. [2005]
first developed SiC/Mg multilayer coatings and their sample mirror achieved thereflectivity of 39.7% at the wavelength of 30.4 nm for the normal incidence of 10 deg.
However, Mg is known to be highly reactive (e.g. Mg is usually found in nature in theform of an oxide, a carbonate, or a silicate, often in combination with calsium).
85

Questions remain about the long-term stability of Mg-based multilayer coatings [e.g.,
Soufli et al., 2005].

Figure 5.3: Calculated reflectivities of the various multilayer coatings with Mg at
the normal incidence of 10 deg. The thickness of each element was optimized tonormal incidence light (10 deg) at 30.4 nm.

On the other hand, the multilayer coatings without Mg that achieved higher
reflectivities than 25% at 30.4 nm were listed in Table 5.2. In these multilayer
combinations, Y2O3/Al achieved the largest ratio of reflectivities at 30.4 nm and 58.4
nm (R30.4/R58.4). Figure 5.4 shows the calculated reflectivies of SiC/Mg, Y2O3/Al, andMo/Si multilayer coatings at the normal incidence of 10 deg. In the calculation both
SiC/Mg and Y2O3/Al multilayer coatings achieve higher reflectivity at 30.4 nm andlower reflectivity at 58.4 nm than conventional Mo/Si. However, the performance of86

Y2O3/Al multilayer coatings has not been experimentally evaluated yet. Therefore, inthis study, sample coatings of SiC/Mg and Y2O3/Al were fabricated and theperformance of each coating including the stability under various environments wasevaluated. This is the first study to verify the performance of the Y2O3/Al multilayer
coating.

Table 5.2: Parameters and calculated reflectivities at 30.4 nm and 58.4 nm ofvarious multilayer coatings without Mg
Materials Number ofMultilayer

periods (N) period (d) [nm]Thickness

R30.4

R58.4

R30.4/R58.4

ratio (Γ)

Y2O3/Si

40

17.0

0.25

0.276 0.0051

54

Y2O3/Al

40

16.7

0.24

0.301 0.0030

100

Ir/Si

40

16.9

0.24

0.263

0.057

5

Ir/Al

40

16.7

0.24

0.287

0.049

6

Au/Si

40

16.9

0.23

0.276

0.022

13

Au/Al

40

16.6

0.21

0.298

0.015

20

C/Si

40

17.1

0.33

0.279

0.077

4

C/Al

40

16.8

0.31

0.321

0.056

6

Pt/Al

40

16.6

0.23

0.261

0.042

6

SiC/Si

40

17

0.35

0.256

0.106

2

SiC/Al

40

16.8

0.36

0.314

0.077

4

87

Figure 5.4: Calculated reflectivities of SiC/Mg, Y2O3/Al, and Mo/Si multilayer
coatings by a conventional theoretical method. The mirrors are aimed for normal
incidence of 10 deg.

The designs of the samples are listed in Table 5.3. The SiC/Mg sample mirror
consists of periodic 40 pairs of SiC and Mg on a flat Si plane deposited by an ion-beam
sputtering system. The SiC/Mg sample was aimed for grazing angle reflection (27 deg).
We also fabricated two samples which were periodic 20 pairs of Y2O3 deposited by RFmagnetron sputtering and Al deposited by DC magnetron sputtering. Sample 1 isaimed at normal incidence reflection (10 deg). Sample 2 is for grazing angle reflection(22 deg). We evaluated the multilayer period (d) in each sample with an x-ray
diffractometer by measuring the small angle diffraction pattern (not shown). All thesample coatings were manufactured by the NTT-AT Company.

88Table 5.3: Design parameters of SiC/Mg and Y2O3/Al multilayer coatings
Multilayer

Multilayer

Thickness ratio

Number of

Incident angle ofcoating

period (d) [nm](Y2O3 / d)periods

peak reflectivity[deg]

SiC/Mg

18.1

0.30

40

27

Y2O3/Al (1)

17.1

0.33

20

10

Y2O3/Al (2)

18.2

0.24

20

22

5.4 Mesurements
Figure 5.5 shows the calibration setup of the EUV facility. The calibration
system for EUV measurements consists of a windowless RF excited flow lamp filledwith source gases and connected to a JOVIN YVON LHT30 30-cm scanningmonochromator. Using an atomic source with the monochromator provides radiation
limited only by the line widths of spectral lines from the light source. The use of gases
such as helium and argon enables us to calibrate mirrors at various wavelengths in the25-120 nm spectral range. The radiations at 30.4 nm (He II) and 58.4 nm (He I) areavailable from helium gas discharge. In the vacuum chamber, the mirror is set on amanipulator with three orthogonal axes of translation and a single axis of rotationaligned perpendicularly to the incident beam. The incident and reflected beams aredetected by a photon detector consisting 5 microchannel plates (MCPs) and a resistive
anode encoder (RAE). The reflectivity can be measured at the incident angle of morethan 5 deg due to configuration constraint in our facility. Hereafter we discuss thereflectivities at 30.4 nm and 58.4 nm versus incident angle measured from the normal
incidence.

89

Figure 5.5: The calibration setup of the EUV facility. A cold discharge lamp isused as an EUV light source and the target line is selected by a monochromator.
The selected light is collimated by pinhole at the entrance to the main vacuum
chamber. An assembly of 5 microchannel plates (MCPs) and a resistive anodeencoder (RAE) is used as an EUV detector.

5.5 Results and discussion
5.5.1 SiC/Mg multilayer coating
The sample of SiC/Mg multilayer coating was kept under atmosphere andvacuum alternately to identify the aging and to find its dependence on the90

environments. Table 5.4 shows the environment during each period. For example, fromthe 1st to the 2nd measurement the sample was kept in the vacuum chamber where thepressure was under 10-4 Pa.

Table 5.4: Environment during each period
Order Days after 1st measurement Environment
1st

N2 purge

0

Vacuum2nd

18
N2 purge

3rd

95
Vacuum4th

117
Vacuum5th529
N2 purge

6th

565

The reflectivity of the sample coating was measured for 6 times, and theresults are shown in Figure 5.6 and Figure 5.7. Figure 5.6 shows the reflectivities of
the SiC/Mg coating at 30.4 nm as a function of the incident angle. The calculated
reflectivity is also represented as a dashed curve. Figure 5.7 summarizes the temporalvariation of the reflectivity at the incident angle of 27 deg shown in Figure 5.6. The
white circle represents the calculated reflectivity, and the black dots represent themeasured reflectivities. During the shaded period the sample was kept in vacuum, andduring the other period the sample was kept in dry N2 purge.
As shown in Figure 5.6 and Figure 5.7, the reflectivity of the sample changes
dramatically in atmosphere (between the 2nd and 3rd measurement) and gradually even91under vacuum (between 3rd and 5th measurement). On the other hand, the incident
angle of the peak reflectivity did not change throughout all the measurements, as
shown in Figure 5.6. These results indicate that the elements of the multilayer became
opaque or clear without any change in thickness of the multilayer period. Indeed, Mg isknown to be highly reactive (e.g. Mg is usually found in nature in the form of an oxide,
a carbonate, or a silicate, often in combination with calsium). The nature of this aging,
however, seems complicated because the tendency of aging-effect was not constant.
This experimental result suggests that further investigation is necessary for practicaluse of the SiC/Mg multilayer coatings.

Figure 5.6: Reflectivities of the SiC/Mg multilayer mirror at 30.4 nm. The dashedline represents the calculated reflectivity. The storage environment during eachperiod is shown in Figure 5.9.

92

Figure 5.7: Peak reflectivity measured as a function of time for the SiC/Mg
multilayer coating. The white circle shows the calculated reflectivity, and black
dots represent the measured reflectivities. The wavelength was 30.4 nm andincident angle was 27 deg at the measurement.

5.5.2 Y2O3/Al multilayer coating
We measured the reflectivities of sample 1 at 30.4 nm and 58.4 nm as afunction of incident angle from the normal incidence. The result is shown in Figure 5.8.
The dots and triangles represent the measured reflectivities at 30.4 nm and 58.4 nm,
respectively. The calculated reflectivity at 30.4 nm is also shown. At normal incidenceof 10 deg, the reflectivity of sample 1 was 24.9% at 30.4 nm and 1.3% at 58.4 nm. The
result shows that our multilayer coating consisting of 20 pairs of Y2O3 and Al has
higher reflectivity at 30.4 nm and significantly lower reflectivity at 58.4 nm than theconventional coatings such as Mo/Si.
Figure 5.9 shows the reflectivity of sample 2 at 30.4 nm as a function of93

incident angle. The reflectivity was measured just after its fabrication. In order toinvestigate the temporal stability of Y2O3/Al multilayer coatings, we kept sample 2
under vacuum, dry N2 purge, and normal atmosphere. We have measured the peakreflectivity of sample 2 at 30.4 nm over a time span of about 3 months, and the result isshown in Figure 5.10. It is clear that there are no changes in the reflectivity. This result
indicates that the EUV performance of the Y2O3/Al multilayer coating is quite stable
over a period of at least 3 months independently of the environment, and there is noevidence to suggest that further changes in performance will occur over time.

Figure 5.8: Reflectivities at 30.4 nm and 58.4 nm of a Y2O3/Al multilayer coating(sample 1). The calculated reflectivity at 30.4 nm is also shown.

94

Figure 5.9: Reflectivity at 30.4 nm of a Y2O3/Al multilayer coating (sample 2),
measured just after the fabrication. The calculated reflectivity is also shown.

95

Figure 5.10: Peak reflectivity measured as a function of time for a Y2O3/Al
multilayer coating (sample 2). The wavelength was 30.4 nm and incident angle
was 22 deg at the measurement.

5.5.3 Calibration of the flight mirror for the EUVI
instrument
The experimental results in Section 5.5.1 and 5.5.2 suggest that Y2O3/Al
multilayer coatings are preferable and can be applied for the optics of the Extreme
Ultraviolet Imager (EUVI) onboard ISS. Therefore, the flight mirror for the EUVI
instrument was fabricated and calibrated.
Figure 5.11 shows a schematic diagram of the EUVI instrument, whose designis almost same as that of the TEX instrument onboard the KAGUYA spacecraft shownin Figure 2.2. The mirror substrate is made of Zerodur, and the mirror has a focal

96

length of 166 mm, a diameter of 120 mm, and a spherical shape to reduce aberration. A
photograph of the flight mirror is shown in Figure 5.12. The design of the flight mirror
substrate is almost the same as that of the TEX instrument. The mirror surface iscoated by the Y2O3/Al multilayer. The number of periods are 20 and the multilayer
period (d) is 17.0 nm (Y2O3 thickness of 5.1 nm and Al thickness of 11.9 nm). These
parameters are set to aim at the normal incidence of 10 deg, and the same as that ofSample 1 described above. The Y2O3 layers were deposited by RF magnetron
sputtering and the Al layers were deposited by DC magnetron sputtering. The NTT-AT
Company manufactured the coating of the mirror.

Figure 5.11: A schematic diagram of the Extreme Ultraviolet Imager (EUVI)
instrument onboard ISS. Main optical components are a multilayer mirror, ametallic thin filter, and a MCP detector. The design of the EUVI instrument isalmost the same as that of the TEX instrument onboard the KAGUYA spacecraft(shown in Figure 2.2).

97

Figure 5.12: A photograph of the flight mirror for the EUVI instrument. The
mirror has a focal length of 166 mm, a diameter of 120 mm, and a spherical shape
to reduce aberration. The surface is coated by a 20-period of Y2O3/Al multilayer.

The reflectivity of the flight mirror was calibrated by using the same
calibration system mentioned in Section 5.4. Figure 5.13 shows the reflectivities at
30.4 nm as a function of the incident angle to the mirror surface. The reflectivities were
measured at three positions of 30 mm, 40 mm, and 50 mm from the center of themirror. There are no dependences of the reflectivity on the position, and this result
indicates the uniformity of the multilayer coating on the mirror surface.
Figure 5.14 gives the reflectivity of the flight mirror (at 40 mm from thecenter) as a function of wavelength. The incident angle was set to 10 deg in thismeasurement. The difference between the calculation and measurement at longer
wavelengths may be due to the ambiguity of the optical parameter of Y2O3 in the EUV
region. The reflectivities of 23.0% at 30.4 nm and of 1.7% at 58.4 nm were achieved.
The ratio between the reflectivities at 30.4 nm and 58.4 nm (R30.4/R58.4) was 14, while98R30.4/R58.4 of the Mo/Si multilayer mirror installed in the TEX instrument was 2.
Using these results of calibrations, the signal-to-noise ratio (SNR) can be
estimated. It was reported that the intensity of the plasmasphere mainbody at 30.4 nm
is ~5 Rayleigh [Nakamura et al., 2000] and that of the geocoronal contamination at
58.4 nm is ~400 Rayleigh in the dayside [Chackrabarti et al., 1983]. Assuming thesame transmittance of the filter and quantum efficiency of the MCP as those of TEX,
the signal-to-noise ratio (SNR) of 16 can be achieved for 10-sec accumulation. This
indicates that the EUVI instrument with the newly developed Y2O3/Al multilayer
mirror is able to identify the plasmasphere through the geocorona from ISS.

Figure 5.13: Dependence of the mirror reflectivity on incident angle to the mirror
surface at 30.4 nm. The reflectivities were measured at positions of 30 mm, 40mm, and 50 mm from the center.

99

Figure 5.14: Reflectivity of the Y2O3/Al multilayer mirror used in the EUVI
instrument. The reflectivities at 30.4 nm and at 58.4 nm are 23.0% and 1.7%,
respectively. In this measurement the incident angle was set to 10 deg.

5.6 Summary
For the next plasmaspheric imaging from the International Space Station (ISS),
new multilayer coatings having high reflectivity at 30.4 nm and low reflectivity at 58.4
nm were developed. The SiC/Mg multilayer coating for grazing angle reflection of 27
deg achieved quite high reflectivity of 34.0% at 30.4 nm. However, the SiC/Mg
multilayer coating was unstable and the reflectivity changed under dry N2 purge andeven under vacuum. On the other hand, the temporal stability of the Y2O3/Al
multilayer coating was evaluated and found to be highly stable under vacuum, dry N2
purge, and normal atmosphere. The Y2O3/Al multilayer coating designed for normal100

incidence reflection had higher reflectivity (24.9%) at 30.4 nm and significantly lower
reflectivity (1.3%) at 58.4 nm than the conventional coatings such as Mo/Si. These are
the first result in the world to evaluate the performance of Y2O3/Al multilayer coatings.
Based on these results, the Y2O3/Al multilayer coating was selected to apply
for the flight mirror of the Extreme Ultraviole Imager (EUVI) onboard ISS. The
calibration of the flight mirror was performed, and the uniformity of the multilayer
coating on the mirror surface was confirmed. Finally, the reflectivities of 23.0% at 30.4
nm and of 1.7% at 58.4 nm for the normal incident angle of 10 deg were achieved.
This result indicates that the EUVI instrument with the newly developed Y2O3/Al
multilayer mirror is able to identify the plasmasphere with a high SNR of 16.

101

6 Concluding remarks
The novel meridian images produced by the TEX instrument onboard the lunar
orbiter KAGUYA has allowed us to explore several aspects of plasmaspheric dynamics
from an entirely new perspective. Through the detailed analysis of the TEX images, wehave obtained new results as well as confirmed and extended the results of previous
experimental studies of the plasmasphere. In addition, for the instrumental point of view,
the present study contributed to establishment of a remote sensing technique in the EUV
range and its improvement for the future missions. This thesis details the in-orbit
calibration of the Telescope of Extreme Ultraviolet onboard the KAGUYA spacecraft,the analysis on the plasmasphere images obtained from the meridian perspective, andthe development of a new multilayer coated mirror for the next plasmasphere imaging
from the International Space Station.
The performance of the TEX instrument after launch was evaluated using thein-orbit data obtained from February 2008 to May 2009. The imaging characteristics
were calibrated using the images of bright EUV sources. The image scale of the field ofview (FOV) is 0.068 Re/pix, and the total FOV equals to 8.7 Re × 8.7 Re. The average
FWHM of PSF was 3.7 ± 1.1 pix, corresponding to 0.24 ± 0.07 Re. These values wereconsidered in all the analysis of the TEX images demonstrated in this study. The
long-term aging of the detector performance was also verified. The backgroundthroughout the mission was constantly low (~1.8 cps/cm2) enough to achieve thesignal-to-noise ratio (SNR) of 9 in the plasmaspheric imaging. No aging degradation
was identified in the gain performance of the MCP detector throughout the mission.
These results are important not only for the scientific products but for the future
instruments.
The TEX instrument onboard the KAGUYA satellite obtained the first EUV
images of the plasmasphere from the meridian perspective in March-June 2008. The
inward motion of the plasmapause in the nightside during the disturbed period on 1-2
May 2008 was clearly observed by the TEX instrument. From the meridian view, the102

inward velocity was estimated to be 0.2 Re/h on the assumption that the plasmapause
was featureless and azimuthally smooth on the nightside. This result is consistent withthat of the past study based on the IMAGE/EUV data. Furthermore, the corotation of
the plasmasphere during the quiet period on 1-2 June 2008 was also identified in thesequential TEX images. By tracking a dipole-like density enhancement inside theplasmasphere, the corotation lag of the plasmasphere was estimated to be 2 hours/day,which agrees with the results from the recent studies derived from the IMAGE/EUV
data. These agreements validate the observation of the plasmasphere by the TEX
instrument from the meridian perspective.
A new striking feature of enhanced brightness (~30%) in the plasmasphere,
called as a plasmaspheric filament, was found in the TEX image during a prolonged
quiet period. The shape of the filament was extremely aligned to the dipole magnetic
field line, and this suggests that the filaments are caused by isolated magnetic flux tubes
filled with denser plasmas than their neighbors. We found 4 events of filaments in
March-June 2008, and the occurrence rate was estimated to be about 1 event / 8days. All
the filaments were observed during extremely quiet periods (Kp < 2). No dependence of
their occurrence on local time was identified. These features of the filaments are
consistent with those of the finger structure observed by IMAGE/EUV. Consequently,
the result indicates that the finger structure seen in the EUV images should be theequatorial projection of isolated flux tubes filled with denser plasmas than its neighbors.
Further investigation of filaments or fingers can play a key role to solve thelongstanding issues of plasmasphere refilling.
The formation mechanism of the plasmapause was studied by analyzing thesequential TEX images of the erosion event during the geomagnetic disturbance (Kp =5) on 1-2 May 2008. The temporal development of the plasmapause locations at the
post-midnight (03 MLT) observed from the meridian perspective agreed with thatpredicted by the dynamic simulations based on the interchange mechanism.
Furthermore, the He+ column density in the nightside plasmasphere decreased by ~30%
only at the low latitudes (< 20 deg) during the period of enhanced convection. On theother hand, no decreases of electron density (or inner troughs) in the plasmasphere were103

identified in the simultaneous in-situ measurement by the THEMIS satellite. Therefore,
this depletion of the column density suggests that the formation of the new plasmapause(i.e. the inward motion of the plasmapause) has occurred first near the equatorial region.
This result is consistent with the scenario of the plasmapause formation based on thequasi-interchange instability. Although the interchange mechanism is not exclusive, thisis the first study to present the plasmapause formation viewed from the meridian
perspective. The experimental result in this study is novel and should be an importantclue to completely understand the formation mechanism of the plasmapause.
For the next plasmaspheric imaging from the International Space Station (ISS),
new multilayer coatings were developed in order to reduce the contamination of
geocoronal He I (58.4 nm) emission. The SiC/Mg multilayer coating for grazing angle
reflection of 27 deg achieved quite high reflectivity of 34.0% at 30.4 nm, but it wasquite unstable and the reflectivity changed under dry N2 purge and even under vacuum.
On the other hand, the temporal stability of the Y2O3/Al multilayer coating wasevaluated and found to be highly stable under vacuum, dry N2 purge, and normal
atmosphere. The Y2O3/Al multilayer coating designed for normal incidence reflection
had higher reflectivity (24.9%) at 30.4 nm and significantly lower reflectivity (1.3%) at
58.4 nm than the conventional coatings such as Mo/Si. These are the first result in theworld to evaluate the performance of Y2O3/Al multilayer coatings. Then, based onthese results, the flight mirror with Y2O3/Al multilayer coating was manufactured andcalibrated for the ISS mission. It has the reflectivities of 23.0% at 30.4 nm and of 1.7%
at 58.4 nm for the normal incident angle of 10 deg. This indicates that the instrument
can produce the plasmaspheric images from ISS with a high SNR of 16.

Remaining problems and comments for future researchThis study reveals that the finger structure seen in the IMAGE/EUV images
are the equatorial projection of isolated flux tubes filled with denser plasmas than its
neighbors. However, the physical process to form them is not understood yet. Further
study of the plasmaspheric filaments (or fingers) should be performed. The future104

imaging by the EUVI instrument onboard ISS will give us key information on thecause of filaments. As ISS orbits at an altitude of ~350 km with an orbital period of 90
min and an inclination of 52 deg, the EUVI instrument will have enough opportunity toobserve the filaments. For example, the EUVI instrument can observe the extent offilaments at lower altitudes. Almost simultaneous observations of the ionosphere by
Visible and Infrared Spectral Imager (VISI) onboard ISS will be also useful to compare
the plasmaspheric and ionospheric features. The VISI instrument can observe theionospheric density enhancements (or plasma blobs) by detecting OI 630 nm emissionthat comes from an altitude region of 220-300 km [e.g., Pimenta et al., 2007]. Thesestudies enable us to deduce the dominant initial conditions for the refilling process,such as plasma density, flow velocity, and temperature, and finally to determine thephysical mechanism of the refilling.
As for the physical process of the plasmapause formation, further
investigations are also necessary. In this study, using the novel images observed by the
TEX instrument onboard the KAGUYA spacecraft, we have presented theexperimental result that the nightside plasmapause moves inward first at the low
latitudes during the disturbed period. However, the plasma detachment from themain-body of the plasmasphere has not been detected directly by imaging(IMAGE/EUV and KAGUYA/TEX) due to the instrumental limitations, such as
sensitivity, temporal resolution, and FOV. Additional future observations, not only ahigh-resolution (~1 min) imaging with the sensitivity of at least ~2 cps/Rayleigh but
also simultaneous in-situ measurements outside the plasmapause, are necessary to
clarify this problem.

105

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