1) The study tracked umbral flashes and running waves in sunspots using IRIS telescope images of two active regions, AR 11836 and AR 12546, as they moved across the solar disk. 2) Wavelet analysis was used to determine the dominant wave periods in the images, finding periods increased from 145s to 305s outward from sunspot centers. 3) Comparison with magnetic field inclination data found a clear correlation between increasing wave period and inclination with distance from sunspot centers, providing evidence for the upward propagating wave theory over the trans-sunspot theory.

1. Tracking Sunspot Waves Across the Solar Disk Using IRIS
Heidi A. Sager1, Chad A. Madsen2, Edward E. DeLuca2
Brigham Young University – Idaho, 525 S. Center St., Rexburg, ID
Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA
Abstract
Waves can be observed in the transition region and upper chromosphere of
sunspots.Two particular phenomena, running waves and umbral flashes, can be
seen in the 1400 Å and 2796 Å bandpasses, respectively, on slit-jaw images
from the Interface Region Imaging Spectrograph (IRIS). How running waves
propagate through the solar atmosphere is still a topic of debate revolving
around two theories.The trans-sunspot theory explains that these waves
propagate radially across the sunspot from its center just as they appear
visually.The upward propagating theory illustrates that these waves are tied to
the magnetic field lines and are therefore slow magnetoacoustic waves,
indicating that the observed apparent motion of the waves is not real. Previous
studies focused on individual observations at a fixed viewing angle, leaving the
results susceptible to line-of-sight bias.To test for this bias, we observed the
leading sunspots of AR 11836 and AR 12546 as they traveled from the disk
center to the western limb.To assess the two theories, we applied global
wavelet analysis to our high-pass filtered slit-jaw images and were able to find
the spatial distribution of dominate wave periods for both phenomena.We
then compared the period distribution to magnetic field line inclinations from
the Helioseismic and Magnetic Imager (HMI). From this, we find a clear
correlation between magnetic field line inclination and wave period as both
increase outward from the sunspot center.The period increases from 180 s from
the sunspot center to 240 s near the edge of the penumbra for all of our
viewing angles, nullifying the existence of a line-of-sight bias and lending
credence to the upward propagating theory.The importance of discovering how
these waves are moving through the solar atmosphere could provide a
mechanism for transporting local seismic energy from the photosphere to the
corona.
Introduction
Umbral Flashes and Running Waves seen in sunspot atmospheres have been
detected in images taken by the IRIS telescope. Umbral Flashes are located in
the upper Chromosphere in the 2796 Å Mg II passband at a temperature of
16,000 Kelvin and appear as less coherent “brightenings.” Running Waves are
located in the Transition Region in the 1400 Å Si IV passband at a temperature
of 80,000 Kelvin and appear as “ripples” throughout the umbra and penumbra
of the sunspot. Both phenomena tend to die out as they hit the plage.
Goals of This Study
AR 11836 and AR 12546 were captured from numerous angles as they moved
across the solar disk.AR 11836 was originally viewed by Madsen et al. (2015),
but it was only analyzed near the disk center and needed to be observed
through different viewing angles. It is important to consider the inclination
angle and dominant period relationship while testing between an upward
propagating theory and a trans-sunspot theory. If there is a correlation between
the inclination angle and dominant period, then the waves are most likely tied
to the magnetic field lines – providing evidence of the upward propagating
theory.
Analysis
Images of AR 11836 were taken by the IRIS telescope as it traveled across the solar disk.Analysis showed it to have an increasing period outward from the umbra to the plage of the sunspot from 145 s to 305 s. The images were run through a high-pass
filter to remove the static background while keeping the rapidly changing phenomena of the waves. To determine the dominant period of the waves, the images were then run through a wavelet transform of each pixel to create a new image of the
dominant period of each pixel through time. If the program determined the period to be at or above 95% confidence intervals, their periods were kept.The contours are from HMI full-vector photospheric magnetic field data showing the increase of
inclination. If the period was below that confidence interval, they were negligible. Once the AR 11836 images were complete, we did the same processes to AR 12546 to see if these periods were consistent throughout new sunspots as well.
Observations and Results
Fig 1 – Image showing the
global spectral power of a pixel
near the center of the sunspot
throughout time of AR 11836.
The process of wavelet analysis
was used to determine the
dominant period. It was applied
to each pixel, in each image, in
both of the observed Active
Regions.
Fig 4 – Dominant Period
Maps of AR 12546 in the
1400 Å wavelength (Top Row)
and 2796 Å wavelength
(Bottom Row)
Fig 5 – Image made from a
cut of pixels from AR 11836
High-Pass Filter data with
high signal near the center of
the sunspot. This image
shows the apparent phase
velocities of the wave moving
through the plasma of the
penumbra.
Conclusion
It is evident that although there is some loss of signal as the sunspot moves away from the sunspot center in AR 11836 (Fig
2), the data shows that the viewing angle does not change the period of the waves.The third AR 11836 image may have had a
loss of signal, therefore the dominant period starts at about 195 s. Since the HMI data is located farther down in the
atmosphere of the sun, it makes sense that it would be shifted closer to the center of the disk. In AR 12546 (Fig 4) the
dominant periods that are closer to 145 s are easily detected, but longer periods farther from the sunspot center die out
quickly. These observations strongly support the upward-propagating theory.Therefore, the best wave - type candidate is
slow-magnetoacoustic waves.The slope of the c-shaped curve on the phase velocity diagram (Fig 5) tells us how fast the wave
is traveling tranverse to our line of sight.A steeper curve indicates a rapidly moving wave from the sunspot center.
This research could provide insight to heating of the solar corona.
References
Madsen, C.A.,Tian, H., & DeLuca, E. E. 2015,The Astrophysical Journal, 800, 129
Fig 2 – Dominant Period Maps of AR 11836 in
the 1400 Å (top row) and 2796 Å (bottom row)
wavelengths with overlayed HMI contours to
show dominant wave periods increase
outwardly as magnetic field inclination also
increases.
Fig 3 – Location of Dominant Period Maps
across the solar disk corresponding to Figure 2
taken in 193 Å wavelength by SDO/AIA
Fig 2
Fig 3
Acknowledgments
This research was funded by NSF-REU solar physics program at SAO, grant number AGS-1560313