Evidence for Gently Sloping Plasma Density Profiles in the Deep Corona: Type III Observations

Details Evidence for Gently Sloping Plasma Density Profiles in the Deep Corona: Type III Observations Evidence for Gently Sloping Plasma Density Profiles in the Deep Corona: Type III Observations

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsh54c..03c&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SH54C-03

Astronomy and Astrophysics

Astrophysics

[7509] Solar Physics, Astrophysics, And Astronomy / Corona, [7534] Solar Physics, Astrophysics, And Astronomy / Radio Emissions, [7594] Solar Physics, Astrophysics, And Astronomy / Instruments And Techniques

Scientific paper

Type III radio bursts are produced near the local electron plasma frequency fp and near its harmonic 2fp by fast electrons ejected from the solar active regions and moving through the corona and solar wind. These bursts have dynamic spectra with frequency rapidly falling with time, the typical duration of the coronal burst being about 1--3 seconds. In the present paper, 37 well-defined coronal type III radio bursts (25--450 MHz) are analyzed. It is found that the dependence of the central frequency of the emission on time can be fitted to a power-law model, f(t) ∝ (t-t0)-α . The index α varies in the range 0.2 to ∞ , with mean and median values of 1.2 and 0.5, respectively. A surprisingly large fraction of events, 84%, has α ≤1.2. Assuming a constant speed of the electron beam, these results provide strong evidence that in the type III source regions within about 1 solar radius above the photosphere the electron number density scales as n(r) ∝ (r-r0)-β , with minimum, mean, and median β =2α of 0.4, 2.4, and 1.0, respectively. Hence, the typical density profiles are more gently sloping than could be expected from the existing empirical coronal models. In the case of negligible plasma acceleration and conical flow, from conservation of the number of electrons it follows that the electron number density will decrease as r-2 with α =1, like in the solar wind. Several events are found with such a wind-like dependence of burst frequency on time. Smaller power-law indices could result from the effects of non-conical geometry of the plasma flow tubes, deceleration of coronal plasma, and/or the curvature of the magnetic field lines. The effects of curvature of the magnetic field lines are shown to be too weak to explain such low power-law indices. A strong tendency is found for bursts from the same group to have similar power-law indices, thereby favoring the hypothesis that they are usually produced by the same source region.

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