Synoptic maps -- the big picture

Science Nugget: February 15, 2002

Introduction -- the Synoptic Maps

Following the style of the last three weeks, we'd like to present one more highlight from the Yohkoh 10th anniversary meeting. Featured in this week's nugget are synoptic studies, or more specifically, studies done by using 'synoptic maps' made of SXT images.

The principal meaning of the word, 'synoptic', according to the Merriam-Webster dictionary, is 'affording a general view of a whole'. Examples most familiar to us may be the synoptic charts of meteorology, which are the weather maps showing atmospheric isobars, wind speeds, etc.

 

In the context of solar physics, on the contrary, 'synoptic maps' usually represent the kind of images shown on the left. This particular image, made from 27 consecutive days of SXT full-disk images, is prepared by slicing a thin area located at the central meridian from each image and ranging them in order of time. Since the interval of the data (27 days) corresponds to nearly one solar rotation, the image approximately shows the corona above the whole surface of the sun. Of course the slices are not contemporaneous, but this helps to get a global view of the coronal structures; e.g. the distribution of active regions or extension of coronal holes, especially if they are long-lived.

Synoptic maps naturally have more variety. One can collect slices through much longer periods of time (e.g. for a solar cycle or more), or collect slices from other longitudes of the solar disk (e.g. at 30 degrees from the central meridian., or even at the limb or off-limb regions). It is needless to add that one gets different views from different sources of data (e.g. photospheric magnetic field or coronal emission-line images). Each type of synoptic map has its own suitable role, as introduced in the following sections.

Global activity patterns in the corona

The image on the right is a synoptic map of the photospheric magnetic field taken from the Mt. Wilson Observatory . Such synoptic maps covering more than one solar cycle show two outstanding intensity enhancements: these start around 30 degrees in latitude at the rising phase of the solar cycle and gradually migrate toward the equator. These main structures in both hemispheres clearly correspond to the emergence of the sunspots or smaller active regions. On the other hand, poleward-migrating structures (note the opposite colors, which denote magnetic polarity) are noticed at higher latitudes. Those are much weaker than the main structures but are well detected in the coronal line emissions, typically in the rising phase of the solar cycle. These two types of global streams on synoptic maps are sometimes referred as 'low/high-latitude coronal activity waves', respectively.

Identifying the counterpart of the 'high-latitude waves'

From the coronal EUV intensities obtained with SoHO/EIT, E. Benevolenskaya et al. at Stanford University showed in their ApJ paper (vol.554, L107-L110, 2001, from which the left image is taken) that the high-latitude waves are caused by giant magnetic loops connecting the polar magnetic fields with newly emerged active regions at mid-latitude. In the left image, synoptic maps of four different wavelength bands in EUV and of the magnetic fields are shown, from the late minimum to the early rising phase of the last solar cycle. White dashed lines represent magnetic neutral lines. High-latitude activity waves are clearly seen in EUV maps a little polewards of the neutral lines. At the Yohkoh meeting, Benevolenskaya presented their further study on this topic using SXT synoptic maps. Because SXT is sensitive to hotter plasma than EIT, the high-latitude waves are not steady structures but appear 'impulsively' when the activity at lower latitude is high enough. In the image on the right, SXT synoptic maps with two analysis filters (AlMg and thin Al.) are shown together with those of magnetic fields and EIT/171 images. The 171A wavelength shows million-degree plasma, whereas SXT typically shows coronal material at two million degrees or higher. The symbols I, II, and III with arrows show the low-latitude waves, high-latitude waves, and an example of the 'impulses' of high activity. She then added that the bright polar footpoints of the giant loops in EIT images are visible as whole loop structures in the SXT. Further, Benevolenskaya and her co-workers investigated the relation between the soft X-ray flux and the magnetic flux, and derived a power law with an averaged index close to 2, which varies with the phase of solar cycle.

Detecting the large-scale and long-lived coronal structures

We would like to introduce one more study, which is based on the SXT limb synoptic maps. As the name shows, limb synoptic maps are made of the X-ray intensities at the limb (or just above the limb), and provide a different view of the global corona.

J. Li at University of Hawaii presented whole-limb synoptic maps made of SXT images from 1991 to 1999 (image on the right). Its reference height is from 1.000 to 1.015 solar radius. She and her colleagues detected some global structures on this map: e.g. coronal butterfly patterns, which correspond to the 'low-latitude wave' of the previous section, coronal holes, and coronal polar sinusoids, which are related to large-scale streamers above active regions.

The polar sinusoids are hard to identify in the long-term image above, but are clearly seen in the image on the left (close-up around the 1996 portion). One can notice bright spots alternatively showing up on the eastern and western limbs. Those spots correspond to the active regions appearing on the east limb, rotating across the disk, showing up again on the west limb; then, rotating across the invisible hemisphere before showing up again on the eastern limb as they rotate onto the disk again. One can also notice the bright bridges connecting the neighboring spots on both limbs. They are thought to be formed by high-altitude plasma far above the active regions, and visible over the polar regions due to projection as they rotate. Polar sinusoids are defined as this system of bright spots and bridges, as named by Li et al..

They especially focused on the polar sinusoids which appeared from 1996 to 1997, and found that each sinusoids have lifetimes of several solar rotations, which is much longer than those of the active regions at their root. Checking carefully the location of the sunspots related to the sinusoids, they finally concluded that the sinusoids are produced by non-contemporaneous sunspot clusters, in which formation and decay of the sunspots are repeated up to a year. The details are described in their ApJ paper (vol.565, p. 1289-1297, 2002).

Closing remark

Yohkoh is a flare mission. Therefore, studies on the energetic and drastic phenomena like flares and CMEs were naturally flourishing at the 10th anniversary meeting. In addition, Yohkoh has acquired a tremendous asset in its unique accumulation of 10 years of synoptic data in the X-ray wavelength range. Although one must admit the session on the solar cycle, in which synoptic studies are included, was a minor one at the Y10 meeting, studies like those introduced above are important and worthwhile. Frankly speaking, the author of this nugget is one of those who enjoyed the solar cycle session most, and she hopes to add some significant contribution to this field.