See image underneath (bottom) showing only some isolated coronal holes at latitudes below about 45 degrees.
The close spacing between and continuous emerging of the active regions during the years of solar cycle maximum limited the widths of the coronal holes and rapidly changed their boundaries ( Wang, 2017). By 2011, the northern polar coronal hole had gone, with the southern polar coronal hole following a year later. As the cycle progressed and solar activity started to increase, the polar coronal holes gradually disappeared and small but usually short-lived isolated coronal holes started to emerge between active regions and outside the active region belt. SC24 started with two well-formed polar coronal holes, as shown in image underneath (top). One can also observe the much slower rotation at the poles (more than 30 days), with extensions of the coronal holes "trailing" and moving eastward (to the left) compared to the equatorial features. As can be seen, coronal holes can appear everywhere on the solar surface, contrary to the active (sunspot) regions (bright areas) which are restricted to a band of roughly 30 degrees on each site of the solar equator.
#Sun corona hole 2016 movie
Hence a movie (underneath) was created showing one synoptic map per solar rotation (27 days) from the start of SC24 till present (April 2018). Nonetheless, complete synoptic maps were available from February 2011 till September 2014, and reasonably well-covered synoptic maps for pretty much the entire solar cycle 24 (SC24), starting from January 2009 onwards. The black area between longitude 90 and 150 degrees is the area not covered by the combination of the SDO and ST-A images, and would have been visible if ST-B was operational. Such a map can be seen underneath, showing the coronal hole from 6-8 April right in the middle of the earth-facing solar hemisphere near 0 degrees longitude and 0 degrees latitude. These are called "synoptic maps", and are available at the STEREO website. Because they make several images every hour, the different points-of-view allow to create an EUV image of the entire Sun every day. The current position of the three spacecraft can be seen in the sketch above, with SDO orbiting the Earth (the green dot) and A and B representing resp. its passage behind the Sun) back in 2015. STEREO-A and -B image the farside of the Sun, though ST-B is no longer operational after a failure just prior to its superior conjunction (i.e. Daily images of the solar corona are made by SDO's AIA instrument, and also with the two STEREO spacecraft as both are equipped with an EUV Imager (EUVI, 19.5nm). One such equatorial coronal hole was in a geo-effective position around 6-8 April (image underneath), and the associated wind stream was the source of a short-lived minor geomagnetic storm on 11 April.Ĭoronal holes are best observed in EUV wavelengths near 19.3nm, corresponding to temperatures in the corona of more than 1 million degrees. Linked to unipolar, open magnetic fields stretching into space, they are the source of the high-speed solar wind which can create geomagnetic disturbances. charged particles) at that temperature is very low compared to its surroundings, and thus they look like dark shapes in the corona as imaged in extreme ultraviolet (EUV) or in x-ray. Coronal holes are regions in the hot solar atmosphere ("corona") where the density of the plasma (i.e.
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