Synoptic Study of EUV Network in Transition Region
MayJuly, 2012
Submitted By: Guidance By:
Aditya Tyagi Dr. K.P Raju
PreFinal Year Undergraduate Indian Institute of Astrophysics
Birla Institute of Technology Koramangala, Bangalore
aditya1273.10@bitmesra.ac.in kpr@iiap.res.in
1
Certificate
I, Aditya Tyagi, hereby certify that the work being presented in this
report under the title “Synoptic Study of EUV Network in Transition
Region”, submitted as the fulfillment of my Internship Assignment at
Indian Institute of Astrophysics is an authentic record of the work carried
out by me during a period of 2 months from May 2012 to July 2012 under
the guidance of Dr. K.P.Raju.
Aditya Tyagi
Birla Institute of Technology
Mesra,Ranchi
This is to certify that the statement made above is correct to the best of my
knowledge.
Dr. K.P.Raju
Indian Institute of Astrophysics
Koramangala,Bangalore
2
Acknowledgements
This report evolved out of the efforts to characterize networks over a
period of almost 2 months. I would like to thank Dr. K.P.Raju for providing
the insight into the depth and scope of the problem as well as his valuable
guidance whenever I was in need.
I would also like to thank the Board of Graduate Studies, Indian
Institute of Astrophysics for providing me with this opportunity to come
over here and participate in such cutting edge research.
I would also like to express my gratitude to members of staff who
introduced us to the procedures and ideas of observational astronomy
during the Summer School at Kodaikanal Solar Observatory.
Data courtesy of SOHO/CDS consortium. SOHO is a project of
international cooperation between ESA and NASA.
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Contents
Abstract................................................................................................. . 5
1.Introduction........................................................................................ 6
1.1 The Sun........................................................................................................................ 6
1.2 The Solar Structure................................................................................................... 7
2.The EUV Network............................................................................... 11
3.Solar and Heliospheric Observatory............................................... 13
3.1 Coronal Diagnostic Spectrometer......................................................................... 14
4.Data and Analysis …........................................................................... 16
4.1 Corrections and Calibration.................................................................................... 16
4.2 The Analysis................................................................................................................ 18
5.Results and Conclusion...................................................................... 20
5.1 Average Network Properties.................................................................................. 21
5.2 Conclusion.................................................................................................................. 24
6.References........................................................................................... 24
4
Abstract
The chromospheric network, the bright emission network seen in the
chromospheric lines such as Ca-K and H-alpha, outline the supergranulation
cells. The solar EUV network is essentially the continuation of the
chromospheric network in the transition region. The Coronal Diagnostic
Spectrometer(CDS) on board Solar and Heliospheric Observatory (SOHO)
provides spectroscopic data from the solar meridian in the EUV range
everyday. Using Intensity distribution and statistical modelling, the
properties of the EUV network were examined over roughly one solar
cycle(1998-2011). Statistical central tendency measures and curve fitting
procedures were employed to separate the network and cell areas. The EUV
network index is obtained for the O V 630 A. The preliminary results from
the analysis are reported.
5
The Sun
The Sun is the star nearest to us. It is the center of our solar
system and sustains the life on earth. The Sun is a main sequence G2V
spectral class star. It is a huge ball of Hydrogen(75%), Helium(~23%) and
heavier elements(1.69%),mostly in the state of plasma. The Sun, like any
other star in the Universe, produces its energy from the nuclear reactions,
converting Hydrogen into Helium. It has a diameter of about 1,392,000 km
and a mass of 2X1030 kg. Its surface temperature is approximately 5800 K
with an absolute magnitude of +4.83.
The mean distance of Sun from Earth is 150 million km, making it the
brightest object in the sky. The energy from the Sun is responsible for
almost all the phenomena happening on earth essential for the survival of
life and hence, its study plays an important role in our continuous efforts to
explore the Universe.
The Sun rotates about the galactic center, taking some 225-250
million years for one revolution whereas taking almost 28 days to complete
one rotation about its axis. As it consists of plasma and not solid
substances, the Sun exhibits Differential Rotation, rotating faster at its
equator than the poles. This behavior is caused by Convection alongwith
steep temperature gradients.
The Sun doesn't have definite boundaries as it is made of plasma. The
gas density in its outer parts drops exponentially with increasing distance
from its centre. But its internal structure is well defined and consists of a
number of layers of differing densities and temperatures.
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The Solar Structure
The Solar interior is not directly visible and the Sun itself is opaque to
EM radiation. Solar physicists have used Helioseismology to study the
pressure waves emanating out of solar interior to measure and visualize the
star's internal structure.
Fig 1 shows the structure of Sun labeling the major regions and
features observed on it.
Fig. 1
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The Core
The region near the center of the Sun where the fusion process takes
place is called the Core. Almost all the energy is produced inside this region
only. The core extends about one quarter of the way from the center to the
photosphere. It has an average density of about 150 gm/cm 3 and a
temperature of about 15 million Kelvin, spewing out energy at a record rate
of 3.846X1026 watts per second! 99% of solar energy generation takes place
here which subsequently heats up the other layers.
The Radiative Zone
The radiative zone extends from 0.2 to about 0.7 solar radii. As the
radiation emerges out of core, it doesn't travel directly to the outer surface
of the Sun but experiences successive rebounding within this region. The
energy diffuses out of this region in form of photons. It may take the
photons thousands of years to escape from the radiative zone as the plasma
density is very high. The average temperature is around 2-7 million Kelvin.
Present hypothesis state that the magnetic dynamo within this zone causes
Sun's magnetic field.
The Convective Zone
The convective zone is where the plasma density and temperature is
not high enough to carry energy by means of Radiative processes. As a
result thermal columns carry energy by means of convective currents to the
surface. These columns on reaching the surface cool down and get spread
out where they are again heated to give rise to solar granulation and
supergranulation. The atoms in this zone are not ionized as the temperature
is not high enough to strip them of their electrons
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The Photosphere
It is the visible surface of the Sun, above which visible sunlight is free
to propagate through free space. It is tens to hundreds of kilometers thick
with an average temperature of 5800 K. This is one of the coolest regions
on Sun and has only a small fraction(0.1%) of ionized gases. It is seen as a
hot soup of alternating dark and bright regions. The bright regions, as
explained earlier, are formed by the plasma swelling up from the lower
radiative zone whereas the darker regions are a manifestation of plasma
sinking back into it. This is the solar granular network.
The Atmosphere
The layers above photosphere consist of ionized gases and hence
considered as the atmosphere of the star. This region is considerably hotter
than the photosphere although no conclusive theory has been adopted
regarding the reason. The atmosphere primarily is divided into three zones :
the Chromosphere,the Transition region and the Corona.
The Chromosphere
This layer starts 500 km above the photosphere and is some 2000 km
thick. The temperature in Chromosphere, contrary to expectation, increases
gradually with height peaking at some 20,000 K near the top. The most
prominent structures in the Chromosphere are the clusters of jets of plasma
called Spicules. They extend upto 10,000 km above the surface.
The Transition Region
Above the chromosphere is a thin layer about 100 km thick over which
the temperature rises drastically from 20,000 K to over 2 million Kelvin in
lower Corona. This is the transition region. Below this region, Helium is not
fully ionized, making it radiate energy very effectively but above this region
it is fully ionized. However, the Transition Region is not a layer at a
particular height but instead, it forms a kind of wrapper or nimbus around
chromospheric features such as spicules and filaments. Transition region is
dominated by ions such as C IV, O IV, O V, Si IV which emit electromagnetic
radiation in UV regions making the spectrum accessible only from space.
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The Corona
The outermost region of Sun's atmosphere is called the Corona. It
extends for millions of kilometers into the space filling a volume far larger
than that of Sun itself. Temperatures in Corona are in the range of 1-2
million Kelvins giving it unusual spectral features. These features have been
traced to the Fe XIV ions. Most of the light emanating from corona comes
from scattering. Physicists have regarded such high temperatures to
inductive magnetic heating and sonic pressure waves although there is no
clear consensus on this theory. Corona is not uniformly distributed around
the Sun and there are some regions of open magnetic fields where its
intensity falls drastically. Such regions have been named as Coronal Holes.
The Solar Cycle
The Solar Cycle has a typical time period of 11 years. This cycle is
characterized by the frequency and placement of sunspots on the surface of
the Sun. Solar cycle causes variation in the space weather as well as to some
degree weather and climate on Earth. It causes a periodic change in the
amount of irradiation received from the Sun.
Fig. 2 shows the variation of irradiance with Solar Cycle.
Fig. 2
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The EUV Network
The chromospheric network is a web-like pattern(Fig. 3) most easily
seen in the emissions of the red line of hydrogen (H-alpha) and the
ultraviolet line of calcium (Ca II K - from calcium atoms with one electron
removed). The network outlines the supergranule cells and is due to the
presence of bundles of magnetic field lines that are concentrated there by
the fluid motions in the supergranules. These patterns are co-spatial with
the magnetic network.
Early observations by rockets and satellites in the Extreme UltraViolet
regions(EUV) characteristic lines showed co-spatial bright structures with
the Ca-II emission network. The measurements from Skylab also
demonstrated the similarity between the observations in different spectra.
Fig. 3
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As with the coarse mottles of the K-line spectroheliograms, bright
clumps of EUV emission forming the network surround darker cells,
approximately 30,000 km across, identifiable with the photospheric
supergranules. The chromospheric network can be seen to extend almost
unchanged in structure from temperatures characteristic of the Lymann
continuum at 104 K through C II,CIII, and O IV to O VI at 3X10 5 K. At
temperatures above those characteristic of O VI, the appearance of the
network changes abruptly, and in coronal lines such as Mg X and Si XII it is
virtually unrecognizable as a semi continuous interlocking structure, with
occasional remnants.
The relations of the supergranules, magnetic field and EUV network is
illustrated in the models of Gabriel (1976) and Athay (1982). A magnetic
field that is concentrated in small areas between the supergranule cells at
the photospheric level is predicted in these models to fan out in a broad
canopy at coronal heights.
Early observations from Skylab S055 instrument(Reeves et. Al 1976)
indicated that the intensities of EUV features in Sun follow a distribution in
which there is a strong peak at lower intensities and a tail extending to
several times the peak intensity. The tail is more pronounced for features
outside coronal holes and for images made in chromospheric and transition
regions the mean intensity is assumed to separate the bright network from
the cells.
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Solar and Heliospheric Observatory
The Solar and Heliospheric Observatory(SOHO) is a spacecraft based
solar observatory which is a joint project of international cooperation
between the European Space Agency(ESA) and National Aeronautics and
Space Administration(NASA). SOHO was launched in December 1995 and
began normal operations in May 1996. The spacecraft is in a halo orbit
around Sun-Earth L1 point. Originally envisaged as a two year mission,
SOHO continues to operate and transmit real time solar data even after
fifteen years.
The spacecraft has three scientific objectives :
1. Investigation of outer layers of the Sun, including Chromosphere,
Transition Region and Corona within the ambit of 6 instruments
including CDS.
2. Making observations of solar wind and related phenomena with 2
instruments.
3. Probing the interior structure of the Sun with 3 additional
instruments.
On June 24,1998 contact with SOHO was lost. For 4 months, the
spacecraft was out of control, exposing the instruments to extreme
temperatures and radiation. Fortunately, none of the instruments were
found damaged when the spacecraft was brought under control in
October,1998. SOHO is the first spacecraft that has gyroless attitude
control.
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Coronal Diagnostic Spectrometer
The Coronal Diagnostic Spectrometer (CDS) provides density and
temperature diagnostics on the transition region and low corona, on spatial
and temporal scales appropriate for the fine structures and processes that
might be present. Also, the CDS spectral resolution allows the
measurement of modest flow patterns. The solar atmosphere is highly
structured and dynamic, and therefore observations of the solar
atmosphere require good spatial and temporal resolutions. In order to
probe the solar atmosphere using spectroscopic means one must identify
those ions which will be present and identify the emission lines from these
ions which will provide the best diagnostic information. For a particular ion
the ionisation equilibrium, which is a balance between ionisation and
recombination, is a function of the temperature of the plasma. Therefore,
CDS observes in the EUV where there are many emission lines of highly
stripped ions of characteristic temperatures 104 - 2 x 106 K which can
provide useful diagnostic information.
Figure 4 illustrates the EUV wavelength region, showing some of the
most useful emission lines.
Fig. 4
14
Since, the atmosphere of Sun shows a gradual increase in plasma
temperature, the different emission lines give information at different
atmospheric heights.
Fig 5 illustrates the atmospheric structure with temperature
increasing with height.
Fig.5
The Coronal Diagnostic Spectrometer consists of two spectrometers :
Normal Incidence Spectrometer(NIS) and Grazing Incidence
Spectrometer(GIS) operating in the EUV spectral region. The synoptic
studies are carried out by the NIS which takes data in two spectral windows :
308-381 Angstroms and 513-633 Angstroms including lines that are formed
over a large temperature range(104 -106 K) creating images from Sun that
extend from Upper Chromosphere and transition region to the Corona.
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Data and Analysis
Each day, the CDS takes 9 synoptic rasters of solar meridian effectively
imaging an area of 240X240 arcsec2/raster. Over a period of one month, this
leads to imaging whole Sun surface by the instrument. The details of CDS
observing sequence are provided in Table 1.
Table 1. Details of the Synop_F observing Sequence.
Instrument CDS/NIS
Slit 240 arcsec
Resolution 2.03 & 1.68 arcsecs
Exposure Time 0.12 arcsecs
Number of Rasters 9
Wavelength ~2.8 Angstroms
During preliminary analysis, it was found that the best contrast
between network and cells is obtained in O V line, peaking at log T e ~5.4 and
a wavelength of 629.73 Angstroms. Due to computational and time
constraints only this line is employed in the analysis.
Corrections and Calibrations
To save time and computational power, only the middle raster(shown
in fig 6) is used in the analysis. Raster is picked around the 15th day of every
month from 1998 to 2011 and first corrected for Cosmic Ray hits, CCD
readout bias, and flat fielded. The data was then calibrated to convert the
photon count rate into absolute units. (photons cm-2 s-1 arcsec-2)
The average line profile was then obtained by taking the mean of the
line profiles over all the pixels of the raster. The NIS views the emission
within two small windows(width ~2.83 A) which sometimes includes a small
amount of emission from nearby lines as well as portion of background.
Fortunately, the OV line is free from such anomalies and doesn't require as
such any spectral filtering. The spectral purity of the image was ensured by
finding the wavelength range that included only the line of interest. This
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wavelength range was defines by the mean (μ) of the line emission, fitted by
a Gaussian profile, plus and minus two standard deviation (2σ) of the
Gaussian. The intensities lying inside this wavelength range were then
integrated to get net intensity at the respective pixel.
Fig. 6
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Fig 7.1illustrates the raw average line profile for O V.
Fig 7.2 illustrates the wavelength range as defined above.
Fig 7.1 Fig 7.2
The Analysis
Different approaches were tried to arrive at the threshold values
segregating the Network from the Cells. Reeves(1976) observed that there
is no clear separation into two dominant intensities characteristics of the
centers of supergranulation cells and the network to yield a bi-modal
distribution. The average intensity of the entire data field was defined as
the the intensity emanating from the center of supergranulation cells and
hence, considered as the threshold. This threshold value worked fine for
low resolution images of Skylab but the contrast in case of CDS rasters is
very poor and a lot of cellular areas were wrongly classified as Network
components. Hence, the approach was discarded.
The second approach takes cue from the works of Reeves(1976) which
called for the statistical technique of Mixture Modeling to be applied to the
intensity distribution curve and estimating the intensities from different
regions by bi-modal normal distributions. This method has been employed
by Gallagher et al. (1998) and satisfactory results were obtained. But the
method was based on weightage approximations and found to be quite
time consuming to employ. The approach is illustrated in fig. 8.
Another approach was to fit a single Gaussian to the intensity
distribution curve and find the mean intensity and standard deviation of the
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curve. The threshold was then defined to be μ+σ and pixels above this
intensity were considered to be network areas. The approach was
computationally inexpensive but the results were crude and unsatisfactory.
Fig. 8
The final approach was to do median filtering over the image. This was
accomplished by creating a structure element of 25X25 pixels. Since each
pixel corresponds to 2.03 arcsecs and 1 arcsec roughly maps a distance of
700 km on Sun's surface, the structure element covered some 35,000
kilometers on Sun. Since, the average size of a supergranulation cell has
been estimated to be 30,000 kilometers, the element was big enough to
encompass the features of bounding network as well as the granulation
cell. The structuring element was then used to do median filtering over all
the pixels to get a smoothed image. The original image was then divided by
the smoothed image. By trial and error, pixels with values above 1.1 in the
resultant image were christened as network areas and below that the
granulation areas. This approach produced good results in terms of
separating the network features from the granulation areas as well as it was
found to be computationally inexpensive and hence employed to analyze
the rasters.
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Results and Conclusion
The IDL-SolarSoft routine was run on the software provided by the
CDS team for 155 rasters from years 1998 to 2011. Contours corresponding
to Ith were mapped on to the image and it can be seen from Fig 7 that there
is a neat separation of the network structures from the cell interiors,
indicating that this intensity provides a means of distinguishing the two
components which will be needed for further analysis.
Fig 9.1 shows a typical raster in the OV transition line.
Fig 9.2 shows the same raster with contours plotted at 1.1 level.
Fig. 9.1
Fig. 9.2
Average Network Properties
The relative area and intensity of the network structures was derived
for each image by selecting the pixels with a value above 1.1. The number of
pixels above this threshold divided by the total number of pixels in the
image, was then defined as the normalised network area or the Network
Index.
In addition, the normalised network emission was calculated by
summing the intensity of all the pixels lying within network , divided by the
total emission within the image. The transition region network area and
intensity show little variation with time(about 10%). Nearly 40% of the
raster area is covered by network which in turn contributes 60% of the total
intensity. The results are consistent with the findings of Reeves(1976) who
predicted that 60-70% of total intensity comes from network regions.
Gallagher et al.(1998) found the network contribution to be a little
higher(70%) which is reflected in the increased network area in their
analysis(as high as 55-60%).
We also investigated the relation between observation time and
contrast between the mean of bright network emission and mean of darker
inter-network regions. The contrast is defined by the standard expression :
C = Imean(network) – Imean(cell)
Imean(network) + Imean(cell)
where Imean(network) is the mean intensity of the normal disribution
associated with the network and Imean(cell) is the mean intensity of the
regions outside of network areas.
The values of normalised network area, normalised network emission
and contrast are plotted against the time of observations. Curve fitting
procedures were used to fit a curve to the respective data. As visibly
evident from the plots in figure 10.1-10.4, all the observations vary in
accordance with the solar cycle. There is clear evidence of increase in
network intensity with number of sunspots.
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Fig 10.1
Fig 10.2
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Fig 10.3
Fig. 10.4
Conclusion
Our analysis broadly agrees with the findings of Skylab data and other CDS
images. The EUV network intensity obviously varies by roughly 4% around
the mean value whereas the normalised network area and contrast show a
variation of about 2% around the mean value.
In addition, we propose a new method for segregating the network
areas from cells which is faster as well as computationally less expensive
than previous approaches.
Analysis was also carried with data in He-II(303.78 Angstroms) and
similar results were obtained. The method is robust and works for any
spectral window. O-V spectral window has a strong resonance line and
hence, offers best contrast in this approach.
References
1.Reeves,E.M.,'The EUV Chromospheric Network in the Quiet Sun',1976.
2.Gallagher, P.T.,Phillips,K.J.H.,Harra-Murnion,L.K.,Keenan,F.P.,'Properties of
Quiet Sun EUV network',1998.
3.Haugan,S.V.H.,'Analyzing CDS Data in IDL: An Observers Guide',1996.
4.Pike,C.D.,Mason,H.E.,'Rotating Transition Region features observed with
the SOHO Coronal Diagnostic Spectrometer',1998.
5.Foukal Peter,'Solar Astrophysics',John Wiley and Sons,Inc.,1998.
6.Gabriel,A.H.,Phil. Trans. R. Soc. London,Ser. A,1976.
7.Athay,R.G.,ApJ,1982.
8.www.soho.nascom.nasa.gov (SOHO's Home on the Web)
9.www.wikipedia.org (The free web based encyclopedia)
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