Transequatorial Interconnecting Loops and Increase of Their Length in the New Cycle

Details Transequatorial Interconnecting Loops and Increase of Their Length in the New Cycle Transequatorial Interconnecting Loops and Increase of Their Length in the New Cycle

Oct 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999esasp.446..305f&link_type=abstract

8th SOHO Workshop: Plasma Dynamics and Diagnostics in the Solar Transition Region and Corona. Proceedings of the Conference hel

Astronomy and Astrophysics

Astrophysics

3

Scientific paper

Using Yohkoh SXT data, we show that active regions of the new solar cycle, appearing at high latitudes, can be connected across the solar equator by loops as long as 60 heliographic degrees (730000 km). This length greatly exceeds the limit of 37 degrees found for transequatorial interconnecting loops on Skylab and implies that the Skylab limit was simply due to the low latitudes of active regions during the period of Skylab observations. By modelling the loops in force-free approximation using Kitt Peak magnetograms, we find strong support for the interpretation that these long interconnecting loops originate through reconnection of magnetic field lines which extend from the two active regions towards and beyond the equator, and confirm the earlier finding by Canfield, Pevtsov, and McClymont that a favourable condition for the reconnection is the same chirality (i.e., same sign of the force-free parameter alpha and same direction of current flow) in the two active regions. As we were unable to find any longitudinal (i.e., east-west) loops of a comparable length, we suggest that an important component of the driving force for the reconnection of transequatorial interconnecting loops may be the differential solar rotation. It can help to drive the reconnection of loops extending in the north- south direction, whereas it does not help in the case of longitudinal loops. These conclusions are based on loop observations in December 1997 and May 1998, when only short-lived transequatorial loops connected the high-latitude active regions on the northern and southern hemispheres. However, in February 1999 a rich system of transequatorial loops as long as 550 000 km could be observed during its whole transit across the visible solar disk. We are now studying this system (not all supporting data have been available yet when writing this Abstract), comparing it with a similar system of much shorter transequatorial loops which crossed the solar disk in March/April 1992 during the preceeding cycle. We again try to model these long-lived loop systems in current-free approximation, using both Kitt Peak and Big Bear magnetograms and, for the February 1999 transit, also the more frequent, high-resolution full-disk magnetic maps from SOHO. This modelling verifies the location of the footpoints of interconnecting loops in low magnetic fields at peripheries of the interconnected active regions, indicates how some of the loops have been formed, and provides information about some of the reasons which lead to the shape and brightness variations in the loops system.

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