Coronal mass ejections (CMEs) are large-scale eruptions of ionized gas (or plasma) from the Sun. The ejected material of a CME is associated with strong magnetic fields, which can cause substantial geomagnetic storms at Earth that enhance the radiation space environment and affect global communications and geolocation. Remote-sensing techniques such as Faraday rotation (FR), the rotation of the plane of polarization of linearly polarized radiation as it propagates through a magnetized plasma, can provide unique insights into the plasma density and magnetic-field structure of CMEs. FR provides information on the orientation of a CME’s magnetic field and can potentially be used to determine this orientation well before a CME reaches Earth (Liu et al., 2007). Further, Faraday-rotation observations of a source near the Sun can provide information on the plasma structure of a CME shortly after launch, potentially shedding light on the initiation process.
In this paper, we present the results of FR observations made using the radio galaxies (J2000 Right Ascension and Declination) 0842+1835, 0843+1547, and 0900+1832, which were occulted by CMEs on 2 August 2012, and use simultaneous Thomson-scattering brightness (TSB) data to independently determine the plasma-density structure through the occulting CMEs.
Data Analysis and Results
All radio observations reported here were performed using the Karl G. Jansky Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO). In order to derive independent estimates for the plasma density, we used white-light images from the LASCO-C3 instrument. LASCO-C3 is ideal because it is aligned with the Earth and, therefore, the line of sight (LOS) from a given radio source to the VLA in our radio data is similar to the LOS from that source to LASCO-C3 in optical data.
To determine the contribution from the background corona to the observed FR, we determined the best fit to a single power-law model for the plasma density using the TSB observations. This fit was then used along with an empirical model for the background coronal magnetic field to predict the FR. To model the effects of the CME on the observed TSB and FR, a constant-density force-free flux rope was embedded in the background corona.
Figure 1 shows the Thomson-scattering data scaled by the mean solar brightness and the Faraday-rotation measure (RM, the physical quantity retrieved in Faraday-rotation measurements) data for the LOS to the source 0843+1547. This radio source provided two LOS separated by 7.8 arcseconds (corresponding to 5700 km in the corona) and each was occulted by two CMEs: CME-1 and CME-2. It is important to emphasize that the model sum (solid curve) represents a fit to the observed data up to 18:30 UT. After 18:30 UT, the model sum represents a prediction based on (1) the model data for CME-1 determined from the fit before 18:30 UT, (2) the model data for CME-2 determined from independent measurements of 0842+1835 (which was occulted by CME-2 alone), and (3) the background coronal model.
Figure 1: Thomson-scattering brightness (top) and Faraday rotation measure (bottom) for 0843+1547 on 2 August 2012. The TSB is given for one LOS to the target source center; FR is given for the LOS to each component of the double radio source: Hot Spot 1 and Hot Spot 2. The dotted curve represents the background coronal model, the dashed curve represents the flux-rope model for CME-1, the dash–dotted curve represents the flux-rope model for CME-2, and the solid curve represents the sum of the contributions from all models together. The first and third vertical lines (LE-1) give the times (15:42 UT and 20:06 UT, respectively) at which occultation by CME-1 begins and ends, respectively. The second vertical line (LE-2) gives the time (18:30 UT) at which occultation by CME-2 begins. This appears as Figure 10 in Kooi et al. (2017).
The two-flux-rope model (one for CME-1 and CME-2) is able to reproduce the observational results of both TSB and FR. In particular, the model accurately predicts (1) the fast slope ≈ 6.6 rad m−2 hr−1 after occultation by CME-2 and (2) FR > 0 at the end of the observations. These two features result from the opposing helicities of CME-1 and CME-2. CME-1 has a helicity H = +1, as determined from observations of 0843+1547before 18:30 UT, and CME-2 has a helicity H = −1, as determined from the independent observations of 0842+1835. The azimuthal magnetic-field contributions to the FR from CME-1 and CME-2 (the dashed and dash–dotted lines in Figure 10, respectively) are negative and positive, respectively. From 18:30 UT to 20:06 UT, the net effect gives the fast slope in FR, and after CME-1 no longer occults 0843 near 20:06 UT, positive FR at the end of the observing session.
The Faraday-rotation transients that we measured were smaller than those observed by Levy et al. (1969) and Cannon, Stelzried, and Ohlson (1973) and larger than those observed by Howard et al. (2016); however, the plasma densities (6 − 22 × 103 cm−3 ) and axial magnetic-field strengths (2 – 12 mG) inferred from our models are consistent with the model predictions of Liu et al. (2007) and axial magnetic-field strengths inferred by Jensen and Russell (2008). Furthermore, the weighted mean LOS component of the magnetic field calculated from our data gives 1 – 6 mG, in agreement with the results of Bird et al. (1985). In order to advance beyond these observations, we have performed another set of VLA observations, this time triggered using near real-time LASCO-C2 data. These near real-time data were sufficient to detect a CME when it was still low in the corona and, therefore, we chose 10 radio sources likely to be occulted by this CME. Analysis and discussion of these data will appear in another paper in preparation.
Cannon, A. R., Stelzried, C. T., Ohlson, J. E., Deep Space Network Prog. Rep., 16, 87, 1973. ADS.
*Full list of authors: Jason E. Kooi, Patrick D. Fischer, Jacob J. Buffo, Steven R. Spangler