Emerging flux and coronal activity

Science nugget: sept 26, 1998

Solar magnetism offers us many suprises, and one of them must have been the fact that the coronal magnetic field - perhaps recognized initially from coronal morphology during eclipses - actually emerges in lumps from below the photosphere. We now suspect that these emergences (active regions and their allies) may have a different character than the global field the eclipse pictures show, but... where's the theory?

A given emerging flux region (EFR) makes a particular pattern of compact coronal loops, termed an "arch filament system" or AFS, most commonly seen in the midst of a developing active region. What do coronal X-rays, as observed by Yohkoh SXT show, while this flux emergence happens? When does the material entrained in the flux cease having photospheric properties, and start having coronal properties?

The figure below shows a synopsis of a study underway (based on data from 1992), in which high-resolution optical observations from La Palma show us the details of the emerging flux, and SXT soft X-ray observations show us the behavior of the corona nearby. The heliographic coordinates were approximately N25, W21.

  • Top left: X-rays (Yohkoh SXT)
  • Bottom left: white light, with an X-ray contour
  • Top right: area of emerging flux
  • Bottom right: H-alpha, showing regions of brightening and the direction of an ejection.
  • The X-ray and magnetic images match closely (~one minute) in time, but the phenomena do not coincide spatially (~5 arcsec). The accuracy of the coalignment between the image, based upon white-light images taken with SXT using nearby sunspots. Based upon our confidence in this coalignment (probably better than two arcsec maximum) we believe that the displacement is real. The separation is roughly OPPOSITE to the direction expected for radial projection, since Sun center is to the SE (lower left).

    Given only this single example, we will resist drawing conclusions, except - if the X-ray brightening and flux emergence are physically related, which we think is highly probable based upon other examples not shown here - the relationship between flux emergence and consequent coronal activity is complicated! We look forward enthusiastically to SOLAR-B, which will provide X-ray observations at resolutions rivaling La Palma's. In these data, our beloved SXT's 2.5 arcsec pixels look enormous.

    In the meanwhile, we thank our colleagues at the Swedish Solar Observatory and at Lockheed Martin for making these coordinated observations possible.

    Sept. 26, 1998: T. Shimizu (email shimizu@flare2.solar.isas.ac.jp) and H. Hudson (email hudson@isass0.solar.isas.ac.jp).