Figure 1– AR 11579 in the plane of view. (a-b) Sign inversion observed in circular polarization at 5.7 GHz (red to blue contours) in 2 days. (c-d) Magnitude of the coronal electromagnetic field computed with the PFSS design at points of transverse proliferation of microwaves. (e) Magnetic field lines of the fan-spine setup against the surface magnetogram. Red and yellow squares mark several perpendicular crossings by microwaves.
Conclusions
The determined coronal field follows the specifications $N_e$ and $L_d$ in the sense that these independently assessed criteria provide a worth of B consistent with the observed degree of the circular polarization at 5.7 GHz. The primary conclusion we draw is the verification of the possibility to enhance radio measurements with PFSS modelling for FSCs in peaceful non-flaring states. A broader variety of cases is required to create basic diagnostics for acknowledging quasi-transverse proliferation in FSCs.
Based upon the current paper: Ryabov B., Vrublevskis A.: 2023, Latvian Journal of Physics and Technical Sciences, 2, 11 doi: https://doi.org/10.2478/lpts-2023-0011
Recommendations
Alissandrakis, C.E. & & Gary, D.E.: 2021, Front. Astron. Area Sci., 7, 591075
Ryabov, B.: 2004, Chapter 7 in “Solar and Space Weather Radiophysics”, (Dordrecht: Kluwer Academic Publishers).
The theory of utilizing quasi-transverse propagation for coronal field measurements includes weak dependence on two specifications: plasma density $N_e$ and the magnetic field divergence $L_d$ (see Alissandrakis and Gary, 2021 for an evaluation). Additionally, we compute the scale of field divergence at the coronal points of radio measurements by means of the Potential Field Source Surface (PFSS) model. There are two systems of field lines: the lower-lying field lines surrounding the inner spine line and the overlying field lines surrounding the outer spinal column line. The magnetic fields of an FSC consist of a magnetic null point, a spinal column line, and a dome-shaped fan surface.
(e) Magnetic field lines of the fan-spine configuration against the surface area magnetogram.
We are pursuing more precise coronal magnetography based on changes in radio polarization as microwaves cross magnetic field lines at nearly best angles.
The theory of utilizing quasi-transverse propagation for coronal field measurements consists of weak reliance on 2 parameters: plasma density $N_e$ and the magnetic field divergence $L_d$ (see Alissandrakis and Gary, 2021 for a review). We assess coronal plasma density from local differential emission procedure computed from the Solar Dynamic Observatory (SDO/AIA) information. In addition, we compute the scale of field divergence at the coronal points of radio measurements by ways of the Potential Field Source Surface (PFSS) design. Thus, we have avoided the presumption of constant $N_e times L_d$ practiced for extended in latitude bipolar active areas, as in Ryabov (2004 ).
Our starting presumption is that the geography of fan-spine configurations (FSCs) could be recreated with the potential field extrapolation at the time of no flaring.
Example
The FSCs are considered locations of possible flaring activity and therefore are essential targets of examinations. There are two systems of field lines: the lower-lying field lines surrounding the inner spinal column line and the overlying field lines surrounding the outer spine line. The magnetic fields of an FSC include a magnetic null point, a spinal column line, and a dome-shaped fan surface area.
In contrast to the extended in latitude bipolar active areas, the FSCs represent a dominant sunspot with circumjacent magnetic flux of opposite polarity. When such active regions are close to the main solar meridian, the area of quasi-transverse proliferation is dome-shaped for the lower-lying system. This shape progresses during solar axis rotation in accordance with differing propagation angle.
We keep in mind stripes of inverted indication of circular polarization observed with the Siberian Solar Radio Telescope (angular resolution $theta = 20 ” $ at 5.7 GHz) on October 1 and October 3, 2012 (Figure 1). The coronal locations in the areas of quasi-transverse propagation where circular polarization turns to absolutely no are of specific interest since at such areas the radio measurements are independent of the at first given off polarization. The black arrow points to the coronal place at the height of 5 × 109 cm, where microwaves cross the shown field line transversally at $B = 23.4 G$, $N_e = 1.74 times 10 ^ cm ^ $, and $L_d = 3.7 times 10 ^ 8 cm$.
On October 5 (disappointed in Figure 1), the sign of polarization at 5.7 GHz inverted totally for the sunspot-associated microwave source despite the absence of any significant changes in the photospheric electromagnetic field. Such an inversion in a sunspot approaching the solar limb is a clear sign of the impact of quasi-transverse propagation of microwaves, while the stripes of the inverted polarity is an unexpected feature. We associate these stripes to a fragmentation in the 3 circumjacent sunspots of opposite magnetic polarity (in black colour).
