Spacecraft Radio Frequency Fluctuations in the Solar Corona: A MESSENGER–HELIOS Composite Study

Abstract

Fluctuations in plasma electron density may play a role in solar coronal energy transport and dissipation of wave energy. Transcoronal spacecraft radio sounding observations reveal frequency fluctuations (FF) that encode the electron number density disturbances, allowing exploration of coronal compressive wave and advected inhomogeneity models. Primary FF observations from MESSENGER 2009 and published FF residuals from HELIOS 1975-1976 superior conjunctions were combined to produce a composite view of equatorial region FF near solar minimum over solar o↵set range 1.4-25R. Methods to estimate the electron number density fluctuation variance from the observed FF were developed. We created a simple stacked flux tube model that incorporated both propagating slow density waves and advected spatial density variations to explain the observed FF. Slow density waves accounted for most of the FF at low solar o↵set, while spatial density inhomogeneities advected at solar wind speed dominated above the sonic point at 6R. Corresponding spatial scales ranged 1-38 Mm, with scales above 10 Mm contributing most to FF variance. Flux-tube structuring of the model introduced radial elongation anistropy at lower solar o↵sets, but geometric conditions for isotropy were achieved as the the flux tube widths increased further out in the corona. The model produced agreement with the FF observations up to 12R. FF analysis provides information on electron density fluctuations in the solar corona, and should take into account the background compressive slow waves and solar wind-related advection of quasi-static spatial density variations.

Coronal Electron Density Fluctuations Inferred from Akatsuki Spacecraft Radio Observations

“…Theoretical models based on coronal electron density 3D spatial turbulence spectrum and solar wind outflow speed have been presented in detail (Woo, 1978;Tyler et al, 1981;Armand, Efimov, and Yakovlev, 1987;Efimov et al, 2008). Wexler et al (2019b) developed a two-component approach in the study of coronal FF over SO 1.4-25R . A composite data set consisting of MESSENGER 2009 radio occultation observations with archival Helios 1975-6 data was evaluated.…”

“…In this paper, we further evaluate the Akatsuki 2011 radio observations building on methods developed in Wexler et al (2019b). The general power spectra of FF were assessed, without restricting the analysis to QPCs.…”

Radio Occultation Observations of the Solar Corona Over 1.60–1.86 R⊙: Faraday Rotation and Frequency Shift Analysis

“…These generally describe the extended corona beyond SO =3 R ⊙ and are not intended to accurately extrapolate to the inner levels of the corona. Attempts to expand the number density description to incorporate the low‐to‐middle corona include the composite formulation given in Wexler et al () and the three‐term power law model of Leblanc et al () …”

“…The second half of the data show a fairly flat trend but with superimposed low-frequency fluctuations. Frequency fluctuations have been studied extensively in coronal radio studies; see Efimov et al (2017) and Yakovlev and Pisanko (2018) for summaries and Wexler et al (2019) for a recent MESSENGER-HELIOS composite analysis.…”

“…These generally describe the extended corona beyond SO = 3R ⊙ and are not intended to accurately extrapolate to the inner levels of the corona. Attempts to expand the number density description to incorporate the low-to-middle corona include the composite formulation given in Wexler et al (2019)…”