A First Spectroscopic Measurement of the Magnetic-field Strength for an Active Region of the Solar Corona

Abstract

For all involved in astronomy, the importance of monitoring and determining astrophysical magnetic-field strengths is clear. It is also a well-known fact that the corona magnetic fields play an important part in the origin of solar flares and the variations of space weather. However, after many years of solar corona studies, there is still no direct and continuous way to measure and monitor the solar magnetic-field strength. We present here a scheme that allows such a measurement, based on a careful study of an exotic class of atomic transitions, known as magnetic induced transitions, in Fe9+. In this contribution we present a first application of this methodology and determine a value of the coronal field strength using the spectroscopic data from Hinode.

The source of the major solar energetic particle events from super active region 11944

“…The off-limb field strengths were measured as 1 to 4 G. With an AR positioned on the limb, this technique could be used to investigate the core magnetic field strength, but not during eruptive events, and line-of-sight superposition would make measurements in specific features difficult. Even more recently, a technique to measure the coronal magnetic field strength from the branching ratio of a specific magnetically induced transition (MIT) to a magnetic quadrupole (M2) transition in Fe X was developed ( 38 , 39 ). In what will likely be a landmark paper, the method was demonstrated to work on EIS data, which opens up access to a complete solar cycle archive of observations ( 40 ).…”

“…The theoretical basis of these characteristics is described elsewhere ( 38 ), and a schematic diagram showing the relevant transitions is also available [ Fig. 1 ( 39 )].…”

“…This can then be compared to the M2 intensity to determine the magnetic field strength because the MIT/M2 intensity ratio has a quadratic dependence on the external field strength. This is the idea behind recent work, and we follow the same method here ( 39 , 40 ).…”

Measurements of the magnetic field strengths at the bases of stellar coronae using the magnetic-field-induced transition theory

“…The suitability of this method for measurements of the solar coronal magnetic field has been recently demonstrated by Chen et al (2021) through a forward modeling approach. This technique has also been successfully applied to solar extreme ultraviolet (EUV) spectral observations (e.g., Si et al 2020;Landi et al 2020Landi et al , 2021Brooks & Yardley 2021). The measured field strengths in solar ARs are often several hundred Gauss, which is consistent with the typical field strengths in the lower corona as derived from magnetic field extrapolations (Landi et al 2020) and magnetohydrodynamic (MHD) models (Chen et al 2021).…”

“…According to the MIT theory, the magnetic field strength can be derived from the intensity ratio of Fe x 257 Å to either of the Fe x 174, 177, 184 Å lines (Si et al 2020;Landi et al 2020;Chen et al 2021). Since these line ratios are sensitive to both the magnetic field and electron density (Li et al 2015), we need to calculate the density before we can obtain a magnetic field strength from the MIT method.…”