Negative-Positive-Negative ("NPN") Photospheric Complexes as an Origin of "Sigmoid" Coronal  Structures.

A. Grebinskij

Abstract

"Sigmoid" Xray structures (Hudson et al., 1998) are possibly closely related with CME's events  (Canfield et al., 1999a). We report here preliminary  results on the possible association  of such  structures with  underlined background magnetic fields at the photosphere (Mt. Wilson and especially  Stanford magnetograms), and  at the corona base with Nobeyama microwave imaging magnetography,  (Grebinsky, Bogod, Shibasaki et al., 1999) and RATAN 600 spectral observations.

We show, that reported AR's with a "sigmoid" pattern (see table 1 of Canfield et al., 1999b) are  strongly related with  one specific topology of background magnetic fields, which may be described  as a chain of  adjacent regions with alternative negative (N) and positive (P) polarities.  Most of reported
"sigmoid" associated AR's corresponds (in the northern hemisphere) with "PNPN background structures (with a reversed polarity in the southern hemisphere). The trailing (say, "P") part  of such structures are more diffuse versus leading  parts, so we treat "NPN" background photospheric  pattern as a basic structure, related with "sigmoids" coronal patterns.

We confirmed this association with results of a spectral-polarization microwave observations. Such  observations display the AR's magnetic field patterns at the upper chromosphere heights. We  discovered a  presence of presumed "NPN" patterns at some AR's with predominant bipolar sunspot  structures, as a weak satellite magnetic fluxes.

Such association may have a clear physical origin: the currents aligning  field lines from the central (P)  region of "NPN" structures flows  eastward and westward, with "S-shape" syndrome at projection on  the disc. If mirror image  of  "S" is observed in the opposite hemispheres (see discussion at Canfield et  al., 1999b), this scenario predicts the conservation of  magnetic "chirality" (a sign of scalar projection J*B) for sigmoid structures, with current "flow out" or "flow into" the central polarity region respective of its polarity. If confirmed, the discussed association of  background  and coronal structures may be important  for  the problem of origins of "sigmoid" structures and prediction of CME's events: "NPN" structures often exists at AR's without or weak sunspots, its spatial dimensions are greater than sunspot's areas, and are more relevant to observed CME extents.

We should mention also, that microwave observations reveal the presence of some other typical  structures, possibly related with CME's events and different from "NPN" and "sigmoid" patterns. We  present  here some examples of such RATAN -  Nobeyama observations for further discussions.
 
 

Authors : A. Grebinskij (SAO, Russia), V. Bogod (SAO, Russia), K. Shibasaki (NRAO, Japan)