On the Fundamental and Harmonic Components of Low-Frequency Type III Solar Radio Bursts

Abstract

1 g c I Figure 3. Solution topology of equation (5) for as < /3 < a/(a-\).the sonic point in the flow approaches the coronal base so that ^o = to -a/2 and ^0, -a [ a + 1 -2 s ( a -l ) ] / 2 ( a -l ) .A selection of results is given in Table 1 for various values of the coronal base temperature T 0 and the polytropic index a. In these calculations it has been assumed that r" = R ©, the solar radius, and that m = (proton mass)/2. A prominent feature of the results is the demonstration that the terminal velocity of the coronal stream increases as the divergence parameter of the flow tube increases, for given values of T 0 and a. This result is in accordance with observations. Adams and Sturrock (1975) used Parker's (1963) theory to construct a coronal hole model but in their model the terminal fluid velocity above a coronal hole decreases as 5 increases, which contradicts observations. A second feature illustrated in Table 1 is that, as the coronal base temperature decreases, the model accommodates a much greater terminal fluid velocity relative to its value for purely spherically symmetric flow, i.e. Max{« 00 (i)/w 00 (2) } increases, as T 0 decreases. This means that the present model is more effective at low coronal base temperatures. Thus, largely in accordance with observations, for To = 1.5 x 10 6 K the model predicts that, as the flow becomes more divergent, the initial velocity changes from 0.29 km s" 1 to approach 165 km s~' as the critical radius changes from 9.9 R Q to approach the coronal base. However, the corresponding terminal velocity approaches 437 km s"' which is rather low for a high-speed stream. We suggest that this failing in the present model may be remedied by replacing the polytropic regime by a realistic energy equation which includes the effects of waves in the lower corona, and by considering a flow tube which has a more rapidly diverging cross-section than A(r) <x r. The degree to which heat addition in a coronal hole combines with the rapid divergence of the lines of flow to produce high-speed streams is still uncertain.In summary, we have applied the previously devised model of Parker (1963) to establish the new result that as the divergence of the flow tube increases the fluid expansion velocity increases throughout the flow, with certain parameter restrictions. The application of this particular model to coronal holes is new, and this study re-affirms the importance of the geometry of the constituent flow tubes in the modelling of gas emanating from a coronal hole. The qualitative result that u 0 and u x increase as 5 increases can be expected to carry over to a more realistic model in which energy is added to the flow to further drive the high speed streams. The model may be adapted by taking s > 2 near the coronal base and 5 = 2 downstream. This would more accurately simulate a real coronal hole, though if we change the value of s downstream of the sonic point then, for instance, such a change would not affect the value of u x .

Low-Frequency Observations of Transient Quasi-Periodic Radio Emission From the Solar Atmosphere

“…• (from the central meridian on the Sun) at 80 MHz (Suzuki & Sheridan 1982). A similar result was recently reported by Thejappa, MacDowall & Bergamo (2012) for the very low frequency solar type III radio bursts observed in the interplanetary medium.…”

Radio Observations of Weak Energy Releases in the Solar Corona

“…This is because the F emission is more directive compared to the H emission. According to Suzuki & Sheridan (1982), the F component has a limiting directivity of ±65 • (from the central meridian on the Sun) at 80 MHz. 2The estimated values of dcp in the present case are close to the average dcp (≈0.11) reported for the circularly polarized H component of type III bursts.…”

High Dynamic Range Observations of Solar Coronal Transients at Low Radio Frequencies With a Spectro-Correlator

“…We have assumed that the quasiperiodic radio bursts in Table 3 are due to second harmonic plasma emission since the viewing angle of the sunspot active regions associated with all the bursts is 30°. Furthermore, the fundamental plasma emission is more directive at low frequencies (Suzuki & Sheridan 1982).…”