SOME FEATURES OF SOLAR MICROWAVE EMISSION AND THEIR CONNECTION WITH GEOMAGNETIC ACTIVITY .I. QUIET COMPONENT OF 9.1-CM EMISSION AND M-REGIONS

The quiet component of the 9.1-cm solar radio emission is studied from the Stanford radioheliograms covering the period April-October 1964. The distribution of the brightness temperature in heliographic coordinates is not entirely uniform, but positive and negative departures from the average value appear at a number of stable locations. The most important negative departure crosses the central meridian 4 days before the maximum of the recurrent geomagnetic activity. Two out of three less important brightness depressions are connected with geomagnetic disturbances in the same manner. It is suggested that the brightness depressions are identical with M-regions. The result is confirmed by the construction of polytrope models for the solar wind, for various values of the parameters α (the polytrope index) and T (the temperature in the inner corona). The velocities near the earth's orbit and in the inner corona are computed as functions of the model parameters, the density results from the observed proton flux at 1 AU. For quiet conditions the model with T = 1.26 × 106 K and α = 1.10 is appropriate. The corresponding density and temperature in the corona lead to a value of 4000 K for the contribution of the corona to the 9-cm brightness. For disturbed conditions the suitable model has the parameters T ≈ 2.0 × 106 K, a ≈ 1.04. It being given that the proton flux at 1 AU is relatively constant, the equation of continuity leads to a low coronal density because of the high solar-wind velocity. The corresponding coronal contribution to the 9-cm brightness is of the order of 10 K. This confirms that the brightness temperature is considerably reduced in the regions where the enhanced solar wind originates. We suggest the name 'coronal depression' for such regions. © 1969 D. Reidel Publishing Company.