H-alpha Off-limb Carrington Synoptic Charts
Luca Bertello, Anna Hughes, Alexei A. Pevtsov
Introduction
H-alpha Integral Off-Limb Maps
Observations of solar prominences (and filaments) reveal complex structures, with significant differences in morphology,
lifetime, and complexity of their magnetic field environment. Studies of these solar features are important for understanding
not only their origin but also their role as precursors of energetic events such as flares and coronal mass ejections. The
Global Oscillation Network Group (GONG) operated by the National Solar Observatory provides, among other products,
high-cadence broad-band intensity measurements centered in the core of the H-alpha spectral line. Prominences and
filaments are quite visible in these 2Kx2K images, which have been obtained since 2010. We recently started a project to
produce an archive of H-alpha off-limb Carrington Synoptic Charts to be used for systematic studies of prominences. Charts
are produced to show both spatial and temporal variability of these features in four radius zones covering 1.01 to 1.11 solar
radii. Here we describe the applied methodology and show some preliminary results.
Methodology
1. Each H-alpha image is remapped into a 1-D (with 4 radius-bin frames) output image, where each pixel is the mean
intensity per azimuth bin within that (above limb) radius bin.
2. Off-limb maps are processed in site-day batches of observations to undergo three stages of cleanup. During this processing, the observations are stacked up and aligned according to camera-rotator angle, so that solar features appear as
curved lines, while instrumental artifacts occur in horizontal bands. Three stages of cleanup are applied to the site-day
set of observations. Stage 1: A 10-day mean instrumental curve as a function of azimuth angle is substracted from
the site-day image. Stage 2: A low-pass filter is applied individually to each observation. Stage 3: The image of all
of the stacked low-pass filter results is subtracted from the site-day image to remove streaking and reduce hour-angledependent artifacts.
Figure 2. Two H-alpha integral off-limb maps centered on the date 2014-12-15 and Carrington rotation 2158. The eastern
off-limb map (left image) is 180 degrees of Carrington longitude ahead of the western off-limb map (right image) for the same
time period. The colors correspond to radius bands A, B, and C, with center-radii: Red(A) = 1.025 R ; Green(B) = 1.055 R ;
and Blue(C) = 1.08 R . The spatial resolution of these two maps is 0.2 degrees, proceeding from -90◦ to +90◦ latitude in y and
for 360◦ of longitude in x (1800×900 pixels). The eastmap starts at 0◦ longitude, and the westmap starts at 180◦ longitude.
Example of a Region of Interest
3. Each 1-D cleaned off-limb map is re-sampled so that the bins proceed in solar latitude instead of azimuth angle around
the solar image.
4. Each 1-D off-limb map is added onto the final synoptic map, where each observation goes into a single column Carrington longitude bin. The synoptic map is then normalized by the number of input observations per longitude bin-column.
NOTE: If an observation has a Carrington longitude of 120 degrees, then on the east-limb synoptic map it will bin into the 30
degrees longitude column, and on the west-limb synoptic map it will bin into the 310 degrees longitude column.
GONG H-alpha Observation
Figure 3. Closeup of a region appearing on the western off-limb map shown in Figure 2. The top left panel displays the
standard map compiled from data taken at all six GONG stations. The other three panels display the map as seen when
using data from single sites only (here, Cerro Tololo, Big Bear, and Mauna Loa). The coverage from these three sites overlap,
allowing insights into features that are instrumental in origin (single site-map only) versus those that are solar in origin
(appearing concurrently at all overlapping sites). The axes in these figures are pixel indexes.
Preliminary Conclusions
Figure 1. This H-alpha image was taken at Mauna Loa on 2015-01-15 at 22:00 UT. Close-ups indicate off-limb features, with
contours marking radial distances from 1.00 R (dark red line) to 1.10 R (black line). For the H-alpha limb-mapping project
we chose the radius bins: A) 1.01-1.04 R , B) 1.04-1.07 R , C) 1.07-1.09 R , and D) 1.09-1.11 R .
• The east/west off-limb maps present data from solar locations observed about a week earlier/later than observations of
these locations at central meridian, helping us to trace the filaments’ evolution from rotation to rotation.
• A visual inspection of Figure 2. seems to suggest that most prominences have a comparable lifetime, as indicated by the
length of these features in the synoptic map.
Contact Information
Luca Bertello
Anna Hughes
Alexei A. Pevtsov
[email protected]
[email protected]
[email protected]
• The color distribution of most features shown in Figure 2., characterized by yellow-green regions surrounded by red
areas, suggests that most prominences reach higher altitudes in their central part. These prominences could be like flux
ropes, tied at their ends and rising in the middle. Figure 2. also shows the presence of very narrow prominences that
do not seem to follow that pattern, i.e., their height seems to be not well organized with respect to their ends or middle
portion. Those could be current sheet prominences.