JOP155 Polar Coronal Jets at the Declining Phase of the Solar Cycle

Version: 2002 March 08



Proposed Dates:

Scientific Objective:

The primary goal of this JOP is to obtain various types of observations of the dynamic jet structures originating in the polar coronal holes. The disk instruments (TRACE, EIT, CDS) can obtain observations of the jet as they are ejected from the bright UV points within the polar holes, while coronal instruments (UVCS, LASCO) will follow the jets as they propagate higher in the corona. Different types of observations - spectroscopy in the lower and extended corona, as well as imaging and white light data will allow us to study dynamics of the jets and will constrain jet plasma properties. The secondary objective is to investigate relation of the hot and cool HI, HeI ejections.

Scientific Justification:

So far, several SOHO JOPs have concentrated on the coronal jets. However, they were exclusively targeting jet phenomena originating from active regions. During the last solar minimum, when the polar coronal holes were dominating structures in the corona, EIT and LASCO C2 observations revealed different types of coronal jets - "polar jets". As the Sun is currently at the declining phase of its activity, the polar holes, and the north hole in particular, formed again and became permanent structures. This JOP is aimed at identifying and studying the jet phenomena that would be counterparts of the solar minimum polar jets.

At solar minimum the polar jets were occurring in LASCO's C2 coronagraph field of view with a frequency of about 3-4 per day. They appeared to be extensions of the structures observed earlier by EIT in the FeXII 195 A band near the limb of polar coronal holes. The polar jets originated near extreme-ultraviolet (EUV) bright points undergoing transient enhancements within the polar coronal holes. They appeared collimated (2-4 deg wide) and elongated rapidly as they traveled through the corona. Their leading edges propagated outward at speeds of 400-1100 km/s, whereas the bulk of the jet material, the "centroid", traveled at much lower velocities, with initial speed of 500 km/s and averaging around 250 km/s at 2.9-3.7 Rs. UVCS provided spectra of many of the polar coronal jets at heights ranging from 1.5 Rs to 2.5 Rs. They were recorded as a significant (by a factor of 1.4-3.0) enhancement with respect to the ambient corona in the integrated intensities of the strongest coronal emission lines, HI Lyalpha and OVI 1032,1037 A.

The jets are believed to be triggered by field line reconnection between emerging magnetic dipole and pre-existing unipolar field. Yokoyama and Shibata (1996, PASJ, 48, 353) performed megnetohydrodynamic simulations of such reconnection processes and found that the hot jet is ejected together with another jet of a cool material. That would relate to co-existence of the X-ray jets observed by Yohkoh/SXT and Halpha surges. Another goal of this JOP is to look for possible association of the polar jets with cool plasma ejections. So far there have not been any dedicated observations addressing this problem although there is evidence supporting above scenario; On 1996 July 13 EIT observed ejection of the polar jet in Fe XII 195 A band. The jet was immediately followed by a dark surge of macrospicule at what had been the location of the base of the jet.


The target should be a polar coronal hole (currently, the north polar hole seems to be well developed) and bright EUV points within the hole.

Instrument Plans: