Radio Signatures of Sunspot NOAA 12192

In this paper, we present an overview of radio signatures of sunspot NOAA 12192 measured with various instruments with frequencies of 37 GHz, 11.2 GHz and 200 - 400 MHz at Aalto University Metsähovi Radio Observatory (MRO). The data were observed during October 20 - 29, 2014. In total, 12 solar radio bursts at 11.2 GHz and 8 at 200 - 400 MHz, with varying intensities and properties, were observed. Radio brightening was captured in several solar radio maps. NOAA 12192 is the largest observed sunspot during solar cycle 24. We show that this exceptional radio brightening belongs to the strongest category including less than 5% of radio brightenings ever measured at MRO.


Introduction
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.

Instrumentation
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 -1447.250MHz).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 af-fected 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 mmwavelengths gyro-resonance emission above the sunspot [13] and free-free radiation from plasma in active region loops are found [14].

Solar Radio Maps at 37 GHz
Both variations in brightness and in size of the radio brightening can be derived from the MRO 37 GHz observations.It should be noted, that the radio brightening located at the same location as the sunspot NOAA 12192.
The 37 GHz results are described in the two following subsections.

Variation in Brightness
Variation in brightness temperature of the radio brightening is listed in the Table 1.The largest values are measured on October 26, 2014, when the brightness temperature is close to 200% compared to the Quiet Sun Level (QSL), which is 7800 K. On other days within the duration of the radio brightenings activity the level is between about 120 and 140% compared to QSL.

Variation in Size
Four selected solar radio maps during October 20 -29, 2014 when the NOAA 12192 sunspot was both active and visible, measured at 37 GHz are shown in Figure 2. It can clearly be seen that the size of the radio brightenings varies with about the factor of 2.

Measurements at 11.2 GHz
We have listed all flares that exceed 100 Solar Flux Units (SFU) in Table 2. Peak flux values are estimations with errors approximately ±10%.An example of a solar radio burst measured with RT-1.8 on October 24, 2014 is in Figure 3. Observed solar radio bursts are associated with certain solar flares listed in Table 2.According to [15] about 20% of X-class flares have strength of X2.0 -X2.9.When we take into account all possible flares, only less than 1% are X-class flares.The strongest of the now observed radio bursts at 2100 SFU is also rare in the long time series measured at MRO [6].However, the previous time series did not include calibrated flux values.

Measurements with Callisto
In total, eight type III solar radio bursts listed in Table 3 were recorded at 200 -400 MHz band of the e-Callisto.Also some events were recorded at lower frequencies, which are not included in this study.An example of a Callisto recording measured on October 24, 2014 is shown in Figure 4.The radio bursts were also observed with several other e-Callisto stations all around the world [11].

Comparison with Long Time Series 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.During other months in the time series, from January, 2000 to January, 2014, the number of flares exceeded ten only six times at the integration time of a month.During the ten days in October, 2014, 12 flares were measured at 11.2 GHz.We also compared to the long time series study in [7], which includes data from 1978 to 2011 (solar cycles 21-14).In the study the strongest solar radio brightenings are 145% to QSL (11300 K).During October 26,    The now recorded radio brightening belongs to the strongest category including less than 5% of radio brightenings ever measured at MRO.

Figure 1 .
Figure 1.NOAA 12192 as seen by SDO AIA 1600 Å. Figure on the left is courtesy of NASA, ESA and JAXA.The marked area is > 100 000 km. Figure on the right with photosgraphic magnetic field map is measured by SDO/HMI magnetogram (at 6173 Å) on October 26, 2014 at 22:01:59 UT, courtesy of NASA/SDO and the AIA, EVE, and HMI science teams.

Figure 3 .
Figure 3. Two solar radio bursts on October 24, 2014 were measured at 11.2 GHz.The last wide spike is due to the calibration of the system.

Figure 4 .
Figure 4. Type III solar radio burst measured with the Callisto system at 200 -500 MHz on October 24, 2014.The large horizontal anomaly at about 400 MHz is due to RFI. 2014 145% QSL limit was exceeded on six occasions, and on its strongest point it reached about 200% QSL.The now recorded radio brightening belongs to the strongest category including less than 5% of radio brightenings ever measured at MRO.

Table 1 .
Brightness temperature, in per cents compared to QSL, of the radio brightening measured at 37 GHz from solar maps.Duration of each solar map scan is 7 min.

Table 2 .
Estimated maximum peak flux values at 11.2 GHz.Total of 12 flares were recorded at 11.2 GHz.

Table 3 .
Solar radio bursts, including the burst type, measured with the Callisto system (200 -400 MHz).