Download A multi-instrument analysis of sunspot umbrae

A multi-instrument analysis of sunspot umbrae
Fraser T. Watson, Matthew J. Penn, William C. Livingston, Alexandra Tritschler, Christian Beck, Valentin Martinez Pillet
National Solar Observatory
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Abstract
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The recent solar minimum and rise phase of solar cycle 24 have been
unlike any period since the early 1900s. We examine some properties of
sunspot umbrae over the last 17 years with three different instruments
on the ground and in space: MDI, HMI and BABO. The distribution of
magnetic fields and their evolution over time reveals that the field
distribution in cycle 24 is fundamentally different from that in cycle 23.
The annual average umbral magnetic field is examined for the 17 year
observation period and shows a small decrease of 375 Gauss, but the
mean intensity of umbrae does not vary significantly over this time. A
possible issue with sample sizes in a previous study is then explored to
explain disagreements in data from two of the source instruments. All
three instruments show that the relationship between umbral magnetic
fields and umbral intensity agrees with past studies in that the umbral
intensity decreases as the field strength increases. This apparent
contradiction can be explained by the range of magnetic field values
measured for a given umbral intensity being larger than the measured
375~G change in umbral field strength over time. Finally, the magnetic
field measured by BABO is compared to the magnetic field obtained by
FIRS at the Dunn Solar Telescope. We plan to continue the project by
introducing spectro-polarimetric inversions of the magnetic field using
the FIRS data.
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MDI
To explain this difference, we looked at the individual detections
from BABO. We discovered that from 1999-2003, only 181
sunspots were measured, compared to the 11773 observed by
MDI. MDI observes almost every day, which is not possible with
BABO and so to the right, we plot the number of umbrae
observed per observing day to remove that source of bias. We
can see that the MDI number follows the sunspot number of
cycle 23, even showing the double peak in 2000 and 2003.
However, the BABO numbers do not start to follow the cycle
until some time in 2003. As such, we believe that the BABO data
does not represent the full sunspot population before 2003,
although the individual sunspot measurements have been
compared against MDI and show good agreement.
This trend is in agreement with other authors such as De Toma
et. al. (2013) and Schad et. al. (2013)
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More recently, we have begun to compare magnetic
fields measured by BABO at the McMath-Pierce
Telescope with fields measured by FIRS at the Dunn Solar
Telescope. A major advantage of this is that both
telescopes use the same spectral line although BABO
only measures Stokes-I whereas FIRS measures all four
Stokes components. The plot to the left shows the
comparison of magnetic fields measured with BABO in
Stokes-I to those measured with FIRS in Stokes-V. The
advantage of using Stokes-V over Stokes-I is that stray
light is minimised with sunspots appearing light instead
of dark.
This first look at only a few active regions is already
promising, showing a good correlation between magnetic
field measurements from both instruments and we will
be expanding on this topic in the future.
HMI
Umbral intensities
This is explained by the spread of the data in the magnetic field
vs intensity plots. For a given intensity, there is a spread of a few
hundred Gauss in magnetic field strength, meaning that the field
of sunspots could change over time without an appreciable
change in the average intensities measured.
Comparison with DST/FIRS
BABO
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To further compare with the results of the Livingston et. al
(2012) article, we measured the intensities of all detected
sunspot umbrae to look for the upward trend reported in that
article. The plot below shows that we did not see the trend and
that the umbral intensities were almost constant while large
numbers of sunspots were on the disk. This seems to contradict
what we present in sections 2 and 3 of this poster. If the
magnetic field strength and umbral intensity are roughly linearly
related, how can one been observed to change and the other
stay constant?
Sunspots have been observed to correlate with various
indicators of solar activity and in their most recent article,
Livingston et. al. (2012) showed a continuation of the decreasing
sunspot magnetic field trend that was first reported in 2006. We
have since used an automated sunspot detection technique
(STARA, Watson et. al., 2009) to measure the field strength of
26921 sunspots in MDI and HMI data so that a comparison can
be made. The top plot to the right shows data from all three
instruments binned by year. It appears that the agreement is
fairly good from 2003 onwards but there is a significant
difference in the datasets before this time. This is crucial as the
1999-2003 years are a major part of the long term decreasing
trend previously reported.
Magnetic field strength vs intensity
There have been many studies that show a relationship
between magnetic field strength in umbrae, and the
corresponding intensity (Martinez Pillet and Vazquez (1993),
Kopp and Rabin (1994), Jaeggli, Lin and Uitenbroek (2012)).
We used data from the three instruments in this study to test
this relationship and found good agreement with these
studies – that umbral intensity decreases with increasing
magnetic field strength. The BABO data (taken in the infrared)
shows a significantly higher range of measured intensities
which can be explained by sunspot umbrae appearing far
brighter in the infrared.
Temporal trend of umbral magnetic fields
Conclusions and future work
This poster shows some of our recent results from
looking at large numbers of sunspots to derive
statistically meaningful properties. We see that the
previously studied magnetic field – intensity relationship
holds for our three datasets. We then compare magnetic
fields obtained by BABO, MDI and HMI and find that
there is evidence for BABO not being a true
representation of the sunspot population in the early
years of observations which affects the long term trend
previously reported. We also show that umbral
intensities appear to have been almost constant
throughout solar cycle 23 and are on track to be so in
cycle 24. Finally, we look at magnetic field measurements
from a few active regions using BABO and FIRS and find
good correlation between the instruments so far. We
plan to continue this study by obtaining magnetic fields
from inversion of the Stokes parameters in FIRS data.
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