Sunspot NOAA 12192, Figure 1, was both active and visible during October 20 - 29, 2014. It is the largest sunspot, with a diameter > 100,000 km, measured in solar cycle 24 regarding the size of the umbra [1] , [2] , and also the largest one observed in 24 years [3] producing tens of solar events [4] . The high activity and unstability of NOAA 12192 shows that the sunspot has a highly complex magnetic structure. The sunspot, on the right side in Figure 1, shows also various flux structures measured with SDO/HMI magnetogram. The black areas have magnetic fields with negative polarity, and white areas have positive polarity. In typical sunspots, like 2194 in Figure 1 on the right side, there is only one negative and one positive polarity area, and they are clearly separated from each other. In the case of NOAA 12192, marked as 2192 in Figure 1, there are several polarity areas. The negative magnetic field in the Figure 1 is −2.0 kG, and the positive magnetic field is more than 1.7 kG.

Large sunspots like NOAA 12192 are not usually observed, because they tend to lose their stability as they grow and change into more complex magnetic structure. They usually disintegrate into smaller sunspots accord-

ing to Shallow sunspot model [5] , which is the prevailing sunspot model. 5 Mm is the largest stable sunspot with B_o ≈ 2.6 - 2.7 kG according to the model. In this article we will show multiple results regarding the sunspot, and the radio flares and and other types of activity it produced, at various radio frequencies measured with MRO radio telescopes. We will compare the results to previous studies presented in [6] and [7] and show that NOAA 12192’s activity was exceptional as it produced several flares during the ten days of its visibility.

Solar maps at 37 GHz are produced with the 13.7-meter radio telescope, RT-14, which is a cassegain type antenna. RT-14 is located at Metsähovi Radio Observatory (MRO), Aalto University (Helsinki Region, Finland, GPS: N 60 13.04 E 24 23.35). The radio telescope provides full disk solar mapping, partial solar mapping, and, additionally, the ability to track any selected point on the solar disk. The beam size of the telescope is 2.4 arcmin at 37 GHz. The receiver is a Dicke type radiometer, thus, the radiometer’s own noise will be filtered out. For the temperature stabilization of the receiver, a Peltier element is used. The noise temperature of the 37 GHz receiver is around 280 K, and the temporal resolution during the observations is 0.1 s or less. The radio emission comes from the solar chromosphere (transition region), and is recorded as intensity. More information about the Metsähovi RT-14 can be found in [7] and [8] .

Metsähovi RT-1.8 is a radio telescope with a 1.8 m dish diameter dedicated for continuous solar observations. The telescope has a beam size of 81.6 arc min and its system noise temperature is 270 K. It observes the total radiation of the Sun at a frequency of 11.2 GHz. The emission measured at 11.2 GHz originates from lower corona. The Quiet Sun Level (QSL) is around 12,000 K at 11.2 GHz. The radiotelescope is used for observing solar radio bursts, as it acts as a detector of general solar activity. Also studies on solar oscillations have been done. High sampling rate (5 kHz) enables studying flare fine structure, including short periodic oscillation phenomena. The radio telescope has no protective radome, therefore it is vulnerable to prevailing weather conditions. Around 200 solar radio bursts have been detected since its launch in 2001. Full documentation of the Metsähovi RT-1.8 can be found in [9] . The telescope has a logarithmic output which can be utilized in the case of strong bursts.

MRO joined the worldwide solar radio burst observing network, e-Callisto, in the fall of 2010 [10] . The network is coordinated by ETH Zürich and it consists of 69 instruments in 38 locations all over the world, covering solar radio spectral observations 24 hours a day throughout the year [11] . The network focuses on the decimeter and meter wavelength range. The first radio emission coming from the solar corona observed with the Callisto system was measured on September 24, 2010 [10] . The current frequency coverage in Metsähovi is 50 - 1450 MHz (the lower band is 50 - 846 MHz and the higher band 790 - 1447.250 MHz). Due to the low instrumental sensitivity studies of the quiet Sun are not possible.

The changing radio emission indicates variation of solar magnetic activity. The birth of radio emission is affected by all plasma parameters, for example, temperature and density. This signifies the importance of radio observations and introduces interesting information on the subject. Gyromotion of thermal electrons in a presence of a magnetic field causes thermal gyroresonance emission. In active regions the magnetic field may have the strength to render the corona optically thick to enable gyroresonance absorption at the frequency range of 1 - 18 GHz. Plasma emission is the most prevailing emission mechanism in solar flares up to 3 GHz [12] . At mm- wavelengths gyro-resonance emission above the sunspot [13] and free-free radiation from plasma in active region loops are found [14] .

In this article we have shown a review of radio signatures of NOAA 12192 measured at MRO at various instruments and radio frequencies. The radio brightening is remarkable even when the long time series measured at MRO are considered. The number of flares measured with RT-1.8 was 12 during the active period of the brightening, October 20 - 29, 2014, which makes this brightening one of the most active during the long time series ever measured at MRO. In comparison in [6] , for example, in August, 2008, which was the most active month within the time series, 22 solar radio flares were measured. When the results were compared to the long time series [7] , it can be concluded that the now recorded radio brightening belongs to the strongest category including less than 5% of radio brightenings ever measured at MRO.

This exceptional sunspot will lead to follow-up studies. For example, several of the events measured at 11.2 GHz revealed interesting double-peak structures. Further studies of the double-peak structures will be conducted by the authors in the near future.

NOAA 12192 image created using the ESA and NASA funded Helioviewer Project.