X-Ray and microwave observations of active regions

Abstract

We compare coordinated, high spatial resolution (2–3 arc sec) observations at 6 cm and in soft X-rays with photospheric magnetograms and optical filtergrams of two active regions. The correspondence of the brightest centimetric components in these regions with coronal loops, sunspots and pores, chromospheric structures and the photospheric magnetic field was determined. Our principal results are: The association between the microwave components and coronal X-ray and photospheric magnetic field structures is complex; in general X-ray emission was not associated with the microwave components. A majority of the components were not associated with sunspots, although the brightest (T _b ≥ 4 × 10⁶ K) components overlay regions of strong photospheric field or high field gradients. Several of the components coincided with the apparent bases of shorter coronal loops and 4 with the tops of X-ray loops.

The X-ray and magnetic field observations are used to constrain possible centimetric emission mechanisms. Thermal bremsstrahlung can not be a significant contributor to this bright microwave emission. Thermal gyro-resonance absorption is consistent with some of the observations, but untenable for those components which are bright in microwaves, lack X-ray emission, and overlie regions of weak magnetic field. As an explanation for the brightest (T _b ≥ 4 × 10⁶ K) components, the g-r theory requires coronal loops with significant currents but very low densities. Alternatively, a nonthermal mechanism implies that the emission arises from the transition region and suggests that discrete regions of continuous particle acceleration may be common in active regions.