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Transcript
The Sun is a variable star and its irradiance has been measured and analyzed since the 1800’s. Early astronomers noted that the sun has a
distinct solar cycle in which the total radiative output of the sun is highest at solar maximum, lowest at solar minimum, and there is an average
variation in irradiance of about .1 to .15% over the 11 year solar cycle. However, recent observations of the irradiance at different wavelengths
using the Solar Irradiance Monitor (SIM) have revealed that this decline in irradiance when approaching solar minimum does not hold for all
wavelengths. There are offsetting trends in the variation of irradiance such that their additive effects produces the Total Solar Irradiance graph
seen at the top of this poster. This is an important discovery towards further understanding how the Sun works because it is now known that
the progression from solar maximum to minimum does not affect individual components of the sun in the same way. This poster aims to demonstrate that these offsetting trends can also be seen when searching for facular and plage regions in analysis of Precision Solar Photometric
Telescope (PSPT) images. I identify these regions as either dark or bright and show that their variation over the descending portion of Solar
Cycle 23 reinforce the assertion that the offsetting irradiance trends do, in fact, exist and that the sun is a much more dynamic star than first
thought.
4) Quantifying Whether Faculae and Plage are
Bright or Dark
5) Variationsof Faculae and Plage (Cont.)
Dark Blue Facula Percentage of Total Facular Area
Topka, K. P., Tarbell, T. D., & Title, A. M. 1997, The Astrophysical Journal, 484, 481
Average
Median
Faculae are produced by the presence of a concentrated flux tube
normal to the solar surface. The magnetic field inhibits convection at the
‘floor’ of the facula causing that area to be cooler. When viewing these flux
tubes from directly above, assiociating the internal magnetic pressure with
gas pressure equilibrium necesitates that the opacity within the tubes be
lower than outside, in turn allowing us to see deeper to the cool floors and
creating a viewable depression known as the Wilson depression.
Horizontal flow of plasma is minimally suppressed due to the slightly
evacuated status of the flux tube. This causes heating of the facular walls.
When observing most faculae on the edge of the solar disk, they appear
bright due to the viewing of their hot walls. However, with smaller flux
tubes, the horizontal flow of energy can heat up the center of the tube as
well, causing it to appear bright anywhere on the solar disk. Larger flux tubes
always have cool centers giving them the appearance of a micropore (Topka
et al. 1997).
Time (SORCE Days)
Bright Blue Facula Percentage of Total Facular Area
τ
Time (SORCE Days)
Geometry of a thin flux tube in the hot wall model. D is the tube diameter (on
average 350-650km), Zw is the Wilson depression (on average 100km deep), and θ
is the angle between the local vertical and the line of sight (equal to the heliocentric
angle).
The decreasing facular area combined with the decrease in the percentage of dark faculae results in an increase in
irradiance. On the other hand, the increase in the percentage of bright faculae does not significantly alter the irradiance because of the decrease in facular area. Therefore, the outcome from this is that there is a decrease in irradiance due to faculae with decreasing solar activity.
Average Value Of Dark Faculae per Total Area of Faculae In Blue
(Bin size = 81 days)
1.938
Red
1.936
488.4 nm
1.934
1/4/2005 1/4/2006 1/4/2007
Time
6000
Ca II K
SSI
0.2618
Teff = 5770 K
0.2613
1578.3 nm
0.2608
7 models are used
to place the pixels in
bins corresponding
to active features
1/4/2005 1/4/2006 1/4/2007
5500
Time
SSI
1.333
0.573
SSI
Brightness Temperature (K)
Brightening With
Increasing Solar
Activity
SSI
6500
Dimming With
IncreasingSolar
Activity
A Mask image is the
combination of the dark
pixels of sunspots in the
red wavelength and the
bright pixels of active
regions in Ca II K
5000
1.332
721.3 nm
0.568
0.563
295.0 nm
400
500
600
Time (SORCE Days)
700
800 900
1000
Dark Faculae
2000
1
Internetwork
2
Network
3
Active
Network
4
Plage
5
Facula
6
Sun Spot
Penumbra
Bright Faculae
7
Sun Spot
Umbra
Dark Plage
Bright Plage
Peak to Peak
6) Conclusion
Ratio
Ratio
1σ
Time (SORCE Days)
Time (SORCE Days)
Bright Blue Facula Percentage of Total Facular Area
Bright Blue Plage Percentage of Total Plage Area
Ratio
Ratio
Population
Population
Ca II K (393.45nm)
TOTAL SOLAR IRRADIANCE
Apr 3rd, 2006
Dec 10th, 2006
Figure 4: Contrasts of certain solar features are variable. As the graph above shows,
a feature may appear bright at the limb of the sun, but towards solar center, it may
not be visible or may even appear dark depending on the viewing wavelength. This
further demonstrates why viewing the sun at the Ca II K wavelength (393.45 nm) is
useful for identifying faculae because they always appear bright compared to the
surrounding sun.
Dark Red Facula Percentage of Total Facular Area
Ratio
SORCE DAYS
Brightness (Bin Number = 500)
Population
Mar 28th, 2010
Population
Jan 25th, 2009
Brightness (Bin Number = 500)
Dark Red Plage Percentage of Total Plage Area
Time (SORCE Days)
Time (SORCE Days)
Bright Red Facula Percentage of Total Facular Area
Bright Red Plage Percentage of Total Plage Area
Differing irradiance trends have recently been observed in different wavelengths suggesting that the sun is more of a variable star than first thought. These trends can be interpreted in
terms of brightness temperature as a function of wavelength. Irradiance increases with decreasing solar activity if the temperature brightness is greater than the effective temperature
(Teff) of the sun, defined to be the temperature of a black body that produces the TSI of 1361
Wm-2 at 1 AU. Likewise, decreasing irradiance results from a wavelength having a brightness
temperature less than Teff. This has contradicted previous beliefs that the sun varies in the
same way at all wavelengths for all solar features. What has been shown here is that not only is
the total solar irradiance an additive result of these offsetting trends, but we can specifically
see these trends when searching for faculae and plage in the PSPT solar images. With this data
on how facular and plage regions change over time coupled with knowledge of how faculae
work lets us assert that the decreasing percentage of dark facular and plage area at the blue
and red wavelengths indicates an increase in irradiance from those areas. An increase in irradiance is also brought about by the increase in bright faculae and plage.
Here’s what we have found:
Ratio
Brightness (Bin Number = 500)
Active solar features can be identified using
histograms of Ca II K and Red images. The red
continuum is useful for picking out sun spots as
their contrast relative the quiet sun is relatively
independent of disk position at that wavelength.
Facular and plage regions are identified in the Ca
II K wavelength because they always appear
bright relative to the surrounding sun (see
Figure 4).
In the histograms to the left, we can see sun
spots represented as the ‘bumps’ in the red
wavelength preceding the spike. At the calcium
wavelength to the right of the spike, the presence of long wings signifies bright active regions
that appear on the images. (The center-to-limb
variability work done here is consitant with
Figure 4)
Ratio
Population
Population
CLV REMOVED ALIGNED HISTOGRAM
Brightness (Bin Number = 500)
Time (SORCE Days)
Brightness (Bin Number = 500)
Ratio
Wm-2
Time (SORCE Days)
Brightness (Bin Number = 500)
Time (SORCE Days)
Dark Blue Plage Percentage of Total Plage Area
Average
Median
Aug 27th, 2005
µ = cos θ
Looking at the averages at different heliocentric angles also gives us the same sort of pattern. These
graphs, however, demonstrate that there are higher percentages of dark faculae and plage towards the
center of the solar disk and these precentages decrease when heading to solar minimum. Furthermore,
towards disc center, the average values of facular and plage areas have a more variable nature.
5) Variations of Faculae and Plage
Dark Blue Facula Percentage of Total Facular Area
= .9
= .8
= .7
= .6
= .5
= .4
= .3
= .2
Ratio
To the right is an example of a mask image from January,
15th 2005 where certain pixels corresponding to either faculae or plage have been blacked out for ease of viewing purposes. One can see from this the contribution to the total
area of faculae and plage (center image).
Time (SORCE Days)
Jan 15th, 2005
µ
µ
µ
µ
µ
µ
µ
µ
µ = cos θ
Figure 1: Change in irradiance vs time for different wavelengths
compared to a solar minimum-like condition indicated by the vertical
dashed line
3) Analyzing Ca II K and Red Images
Average Value Of Dark Plage per Total Area of Plage In Blue
(Bin size = 81 days)
= .9
= .8
= .7
= .6
= .5
= .4
= .3
= .2
Ratio
Recent SIM observations have unearthed offsetting irradiance trends at different wavelengths. Figure 1
shows how the irradiance at certain wavelengths varies when approaching solar minimum. Wavelengths with
a higher brightness temperature than the sun’s estimated black body temperature, Teff = 5770 K, show an
increase in irradiance with the approach to solar minumum whereas wavelengths with a lower brightness
temperature go through a decrease.
Time (SORCE Days)
µ
µ
µ
µ
µ
µ
µ
µ
Figure 3: The temperature of faculae and quiet sun as compared to pressure
Figure 2: Temperature brightness of different wavelengths
Population
µ = cos θ
Average Value Of Dark Plage per Total Area of Plage In Blue
(Bin size = 81 days)
All Faculae and Plage
Brightness (Bin Number = 500)
= .9
= .8
= .7
= .6
= .5
= .4
= .3
= .2
Time
Wavlength (nm)
Ca II K (393.45nm)
µ
µ
µ
µ
µ
µ
µ
µ
1/4/2005 1/4/2006 1/4/2007
Time
300
= .9
= .8
= .7
= .6
= .5
= .4
= .3
= .2
µ = cos θ
1/4/2005 1/4/2006 1/4/2007
200
Average Value Of Dark Faculae per Total Area of Faculae In Red
(Bin size = 81 days)
Ratio
2) Offsetting Trends at Different Wavelengths
µ
µ
µ
µ
µ
µ
µ
µ
Ratio
Next, to locate and designate pixels as faculae and
plage and then consequently as dark or bright, we use a
mask image coupled with the red and Ca II K images. A
mask is an image where the value associated with the
pixels ranges from one to seven. Different models are used
to locate active features and the pixels corresponding to
these active regions are given a corresponding value. This
way we can locate what pixels in the mask make up faculae
and then go look at the blue image, for example, to determine whether those pixels are bright or dark relative to the
surrounding sun.
Creating a Mask
Image
1σ
Peak to Peak
1) Abstract
PSPT Red
Ratio
Brendan Lamarre
Mentors: Jerry Harder and Mark Rast
August, 2010
1361 Wm-2
Ratio
in Solar Cycle 23
Total Solar Irradiance
The Time Evolution of Faculae and Plage
PSPT Ca II
Time (SORCE Days)
 With High Solar Activity
Time (SORCE Days)
For the figures above, the graphs (grey) show the ratio of dark or bright faculae or plage to the total faculae or plage, both dark and bright. The height of the red boxes corresponds to 2 σ
standard deviations while the range indicated by the ‘whiskers’ represents the peak to peak value in the 81 day bin. The green lines specify the median and the blue lines specify the average. The black lines simply connect the averages from all twenty-four 81 day bins.
What is presented in the graphs above is that as time progresses from solar maximum to minimum, the average percentage
of dark faculae and plage at the red and blue wavelengths decreases slightly whereas the bright faculae and plage show an
increase. Decreasing numbers of dark faculae and plage act to increase irradiance because there are less of these structures
inhibiting radiation. Similarly, the increase in bright faculae and plage increases irradiance. This simply reaffirms recent proposals
that at such wavelengths like the 400-691nm range, the sun, in fact, shows an increase in irradiance upon approaching solar
minimum.
 Larger areas of faculae and plage
 Larger fraction of dark faculae
 Repression of irradiance
 With Low Solar Activity
 Smaller areas of faculae and plage
 Larger fraction of bright faculae
 Irradiance from bright faculae is about constant due to smaller facular area conflicting with larger
fractional area of bright faculae
In conlusion, the evolution of faculae and plage in the descending half of Solar Cycle 23 demonstrates the validity of the claim that there are offsetting trends affecting the TSI