Tracing the Electron Density from the Corona to 1 au

Abstract

We derive the electron density distribution in the ecliptic plane, from the corona to 1 AU, using observations from 13.8 MHz to a few kHz by the radio experiment WAVES aboard the spacecraft Wind. We concentrate on type III bursts whose trajectories intersect the spacecraft, as determined by the presence of burst-associated Langmuir waves, or by energetic electrons observed by the 3-D Plasma experiment. For these bursts we are able to determine the mode of emission, fundamental or harmonic, the electron density at 1 AU, the distance of emission regions along the spiral, and the time spent by the beams as they proceed from the low corona to 1 AU. For all of the bursts considered, the emission mode at burst onset was the fundamental; by contrast, in deriving many previous models, harmonic emission was assumed.

By measuring the onset time of the burst at each frequency we are able to derive an electron density model all along the trajectory of the burst. Our density model, after normalizing the density at 1 AU to be ne(215 R0)=7.2 cm−3 (the average value at the minimum of solar activity when our measurements were made), is ne=3.3×105 r−2+4.1×106 r−4+8.0×107 r−6 cm−3, with r in units of R0. For other densities at 1 AU our result implies that the coefficients in the equation need to be multiplied by n _e (1 AU)/7.2.

We compare this with existing models and those derived from direct, in-situ measurements (normalized to the same density at 1 AU) and find that it agrees very well with in-situ measurements and poorly with ‘radio models’ based on apparent source positions or assumptions of the emission mode. One implication of our results is that isolated type III bursts do not usually propagate in dense regions of the corona and solar wind, as it is still sometimes assumed.