Features of Microwave Radiation and Magnetographic Characteristics of Solar Active Region NOAA 12242 Before the X1.8 Flare on December 20, 2014

This paper continues the cycle of authors’ works on the detection of precursors of large flares (M5 and higher classes) in active regions (ARs) of the Sun by their microwave radiation and magnetographic characteristics. Generalization of the detected precursors of strong flares can be used to develop methods for their prediction. This paper presents an analysis of the development of NOAA AR 12242, in which an X1.8 flare occurred on December 20, 2014. The analysis is based on regular multiazimuth and multiwavelength observations with the RATAN-600 radio telescope in the range 1.65–10 cm with intensity and circular polarization analysis and data from the Solar Dynamics Observatory (SDO). It was found that a new component appeared in the AR microwave radiation two days before the X-flare. It became dominant in the AR the day before the flare and significantly decreased after the flare. The use of multiazimuth observations from RATAN-600 and observations at 1.76 cm from the Nobeyama Radioheliograph made it possible to identify the radio source that appeared before the X-flare with the site of the closest convergence of opposite polarity fields near the neutral line in the AR. It was established that the X-flare occurred 20 h after the total gradient of the magnetic field of the entire region calculated from SDO/HMI data reached its maximum value. Analysis of the evolution of the microwave source that appeared before the X-flare in AR 12242 and comparison of its parameters with the parameters of other components of the AR microwave radiation showed that the new source can be classified as neutral line associated sources (NLSs), which were repeatedly detected by the RATAN-600 and other radio telescopes 1–3 days before the large flares.     

Microwave radiation of solar active regions before X flares according to the RATAN-600 observations in 2011

The evolution of the microwave radiation from four active regions, where strong X-ray flares (X-class, GOES) occurred in 2011, has been studied. Daily multiwavelength RATAN-600 radio observations of the Sun in the 1.6–8.0 cm range have been used. It has been indicated that the radiosource above the photospheric magnetic field neutral line (above the region with the maximal convergence of the fields opposite in sign) becomes predominant in the structure of the active region microwave radiation one to two days before a powerful flare as in the eruptive events previously studied with RATAN-600. The appearance of such a radiosource possibly reflects the current sheet formation in the corona above the active region. The energy necessary for a flare is stored in the magnetic field of active region, which can be considered as a factor for predicting a powerful flare.     

Anomalous polarization features of the cyclotron radiation source in the NOAA 7123 active region: The structure, dynamics, and reasons of occurrence

The research results of the unique NOAA 7123 active region (April 1992) are presented; its anomalous polarization features (long-term predominance of O-mode radiation above the central part of the main sunspot) were previously revealed by VLA observations with a resolution of (2–4)” at wavelengths of 3.6 and 6.4 cm (Vourlidas et al., 1997). Additional observations in a wider band of electromagnetic radiation, including, in particular, spectrum-polarization observations performed with the RATAN-600 and SSRT radio telescopes, are considered. The analysis of these data allowed for the conclusion that the anomaly was caused by the presence of a cold substance in the upper coronal layers, starting from the height where (6–7) cm radiation was generated. A possible transportation agent of this substance—Hα filament—one of the bases of which fell on the sunspot’s umbra-penumbra boundary, was found.     

Post-Eruptive Arcade Formation in the 25 January 2007 CME/Flare Limb Event: Microwave Observations with the RATAN-600 Radio Telescope

A CME/flare event occurred at the eastern limb on 25 January, 2007. Seven successive multi-wavelength scans in the range 1.8 cm?–?5.0 cm were obtained with the RATAN-600 radio telescope starting just at the beginning of the post-eruptive arcade formation (30 min after a C6.3 flare peak) and lasting for 3.5 hours. The conditions were favorable to study the off-limb microwave radio source associated with the post-eruptive arcade in different phases of its formation. Microwave radio emission of the arcade was rather intense initially and then considerably decreased; its maximum was co-spatial with the 195 Å Fe xii loop tops. The RATAN-600 total flux spectra of the off-limb radio source were practically flat during the first two hours indicating a predominant contribution of thermal emission. The X-ray spectrum was thermal (according to RHESSI data) at that time. Data available in the meter wavelength range during this phase were indicative of weak non-thermal processes likely due to accelerated particles. However, free?–?free emission of an isothermal source dominated in microwaves. This is indicative of the presence of a large amount of plasma in the region of arcade formation at the initial stage of the event. The weak microwave emission during the decay phase might be interpreted as the thermal cyclotron emission of the loops in the arcade.     

Transition region above sunspots inferred from microwave observations

We report the results of spectral-polarization observations of the active region NOAA 10848 made with the RATAN-600 radio telescope. High spectral resolution (1%) of observations allowed obtaining detailed temperature height profiles for the sources located above the sunspots in the transition region from the chromosphere to the corona. The resulting vertical profiles indicate that the transition region above the sunspots may extend over a considerable height interval and be characterized by a gradual increase of temperature—a pattern that is inconsistent with model atmospheres having a sharp temperature increase in the transition region.     

Computation of the polarization characteristics of RATAN-600 in the “Southern sector with a flat reflector” mode with the diffraction effects considered

We report the Mueller matrix elements for RATAN-600 computed with the allowance for the diffraction effects in the space between the main, secondary, and flat mirrors in the Souther sector with a flat reflector (“South+flat”) observing mode and in the mode of single-sector observations at the horizon throughout the entire operating wavelength range. We show that the vertical size of the M ₄₁ and M ₃₂ elements decreases, the elements shift relative to the central horizontal section, and the lobe structure of the elements changes with increasing wavelength. In the “South+flat’ mode these changes, which are due to diffraction effects, begin to show up at shorter wavelength compared to the single-sector mode. We investigated the variation of the structure of the elements M ₄₁ and M ₃₂ with the elevation angle of the flat reflector and in the case of longitudinal and transversal off-focus offsets of the primary feed.     

Polarization effects in RATAN-600 solar observations in the “Southern sector with a flat reflector” mode

We report the results of numerical simulations of the instrumental signal in the Stokes V channel of circular polarization as observed by RATAN-600 radio telescope operating in the “Southern sector with a flat reflector” mode. Our simulations are based on an improved algorithm of the beam pattern computation that takes into account diffraction in the space between the telescope mirrors. The computations cover a wide range of wavelengths in the case of a focused antenna and in the presence of aberrations. We analyze the structure and properties of the element M ₄₁ of the Mueller matrix and of the parasite signal from the solar disk in various cases of the antenna irradiation. We estimate the differences between the left- and right-polarization power-beam patterns of the telescope. We report the computed M ₁₁ and M ₄₁ elements for the case of observation of the right- and left-polarized radiation at different points of the focal line of the secondary mirror, and analyze their variations as a function of a number of parameters, including, in particular, the shift applied to correct the displacement of scans.     

Evolution of Solar Active Regions Before Large Flares: Overview of the Events of 2010–2017

Geomagnetism and Aeronomy - The paper presents a study of the evolution of the photospheric magnetic field of active regions (ARs) of the Sun in which flares (larger than the M9 class in the...     

Very Large Array-RATAN 600 observations of a solar active region

The Very Large Array (VLA) and the RATAN 600 were used to observe a solar active region on two consecutive days around the time of a partial solar eclipse in July 1990. VLA synthesis maps at 2.0, 3.5, and 6.2 cm wavelength reveal bright (T _b = 0.2 – 2.2 × 10⁶ K), compact ( = 10–40) sources above the penumbra of the leading sunspot while maps at 20 cm wavelength reveal an extended ( 4.5) looplike structure (T _b 10⁶ K) between the dominant spots. Total flux and brightness temperature spectra of both components were obtained by the RATAN at nine wavelengths between 1.7 and 21 cm. The relatively-flat spectrum of the extended emission is attributed to the optically thin thermal brems Strahlung of electrons trapped in a magnetic loop at coronal temperatures. Step-spectrum sunspot-associated emission is attributed to thermal gyroresonance radiation at different heights along the leg of a loop joining regions of opposite magnetic polarity. Comparisons with predicted distributions of gyroresonance radiation indicate that the compact sunspot-associated sources lie at heights of h = 2500–17500 km above the photosphere. Although potential fields of sufficient strength appear to exist at coronal heights, differences n the observed and predicted brightness distributions suggest some role for non-potential fields or for an inhomogeneous distribution of electron density or temperature above the sunspot.