Abstract
Variations in the propagation of globally-propagating disturbances (commonly called "EIT waves") through the low solar corona offer a unique opportunity to probe the plasma parameters of the solar atmosphere. Here, high-cadence observations of two "EIT wave" events taken using the Atmospheric Imaging Assembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO) are combined with spectroscopic measurements from the Extreme ultraviolet Imaging Spectrometer (EIS) onboard the Hinode spacecraft and used to examine the variability of the quiet coronal magnetic-field strength. The combination of pulse kinematics from SDO/AIA and plasma density from Hinode/EIS is used to show that the magnetic-field strength is in the range ~2-6 G in the quiet corona. The magnetic-field estimates are then used to determine the height of the pulse, allowing a direct comparison with theoretical values obtained from magnetic-field measurements from the Helioseismic and Magnetic Imager (HMI) onboard SDO using PFSS and local-domain extrapolations. While local-scale extrapolations predict heights inconsistent with prior measurements, the agreement between observations and the PFSS model indicates that "EIT waves" are a global phenomenon influenced by global-scale magnetic field.
Highlights
The solar corona is dominated by the Sun’s magnetic field, accurately determining its strength continues to be a difficult task
The kinematics measured by Hinode/Extreme ultraviolet Imaging Spectrometer (EIS) matched those using imagers, suggesting that “Extreme ultraviolet Imaging Telescope (EIT) waves” may be best interpreted as MHD waves propagating through the low corona. We examine those two “EIT wave” events observed by both Hinode/EIS and Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA)
The full-disk images from SDO/AIA were primarily used to examine the kinematics of the disturbances, as well as to allow some comparison with the magnetic field extrapolations obtained from the Helioseismic and Magnetic Imager (HMI: Scherrer et al, 2012) onboard SDO
Summary
The solar corona is dominated by the Sun’s magnetic field, accurately determining its strength continues to be a difficult task. Estimates may be obtained in a small number of long-wavelength (forbidden) emission lines by measuring their Zeeman splitting (cf Lin, Kuhn, and Coulter, 2004) or by using the Hanle effect (cf Raouafi, Sahal-Brechot, and Lemaire, 2002), but these. 1 are generally obtained near active regions where the magnetic-field strength is high and very strong lines in the near infrared may be used (such as, e.g., Fe xiii λ10 747; Lin, Penn, and Tomczyk, 2000). The strength of the coronal magnetic field can be derived from the gyro-resonance emission in radio wavelengths (e.g., White and Kundu, 1997). An alternative approach is to infer the coronal magnetic-field strength and other plasma parameters by examining how the properties of waves change with propagation: a technique called coronal seismology (Uchida, 1970; Roberts, Edwin, and Benz, 1984)
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