On the fluctuations of electron density and magnetic field in the solar mid-corona: spacecraft radio observations

On the fluctuations of electron density and magnetic field in the solar mid-corona: spacecraft radio observations

Context: The mechanisms for solar wind acceleration remain speculative despite decades of research. Magnetohydrodynamic (MHD) wave energy is believed to play a key role in powering the solar wind, but characterization of the magnetic energy transport through inner coronal regions is challenging. Data on MHD wave energy in the mid-corona (heliocentric altitude 1.4-2.5R⊙) are sparse; the region is beyond the lower coronal range suitable for imaging in the extended ultraviolet bands, yet far short of the distances accessible to in-situ measurements. Faraday rotation (FR) solar radio occultation observations, which reveal the line-of-sight (LOS) integrated product of
aligned magnetic field and electron density, help characterize the coronal environment and reveal wave-related fluctuations.

Aims: The main goals of this research program are to identify the radio signatures of MHD waves and obtain estimates of Alfvén wave energy in the middle corona, by analysis of transcoronal spacecraft radio transmissions.

Methods: Observations of radio transmissions were accomplished near spacecraft superior conjunction, with the radio LOS passing at solar offset (the heliocentric distance to the LOS point of closest solar approach) in the mid-coronal range. A combination of radio analysis techniques based on Faraday rotation and frequency variations was developed to constrain plasma parameters, yielding estimates of Alfvén wave speeds, magnetic fluctuations and MHD wave energy flux densities.

Results: Faraday rotation fluctuations (FRF) and radio frequency fluctuations (FF) are ubiquitous in the corona. In mid-corona, FF findings are consistent with acoustic/slow magnetosonic waves. The FRF are not explained by density fluctuations alone and suggest a predominance of Alfven waves. Estimates of mid-coronal magnetic field strengths determined by FR compared favourably with results from a 3-D MHD coronal model from the Community Coordinated Modeling Center. Mid-coronal magnetic field strengths vary greatly, depending on the coronal structure penetrated by the sensing radio signal. Energy flux density associated with wave-like magnetic fluctuations implied by FRF in mid-coronal closed field regions was on the order of 50 W/m2, potentially of significance to powering the solar wind.

Conclusion: Transcoronal radio sensing is a unique probe of the inner coronal regions that can not only shed light on questions of basic coronal physics, but also support modeling efforts by the broader solar and space weather research communities.