Daily solar mm-observations at Aalto University Metsähovi Radio Observatory

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Revision as of 08:39, 11 March 2015

Contents 1 Introduction 2 Observations at 37 GHz with MRO 13.7-meter radio telescope 3 Observations at 11.2 GHz with MRO 1.8-meter radio telescope 4 Observations at 50-845 MHz with Callisto 5 Conclusions 6 References

Introduction

At Aalto University Metsähovi Radio Observatory (MRO) located in Helsinki Region, Finland (GPS: N 60 13.04 E 24 23.35) we measure solar radiation at several radio frequency bands on a daily basis. During the summer time, due to the Northern location of Metsähovi, we are able to do daily 14-hour continuous solar observations. This ability is remarkable and brings a peculiar twist to our observations. The long time series measured at Metsähovi (since 1978) enable studying long term variability in the Sun, for example, by investigating solar radio brightenings. Long measurements observed during a single day can be used for studying short term changes (so-called oscillations studies) in different structures (for example, radio brightenings and polar region brightenings). In addition, the versatile radio burst data is analyzed for single bursts and for statistical reasons.

Observations at 37 GHz with MRO 13.7-meter radio telescope

Solar maps at 37 GHz are produced with the 13.7-meter radio telescope, RT-14, which is a cassegrain type antenna. 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 40 ms or less. The radio emission is recorded as intensity. More information about the Metsähovi RT-14 can be found in (Kallunki et al. (2012)) and (Kallunki (2013)).

We strive to produce at least one solar radio map every day with the 13.7-meter radio telescope at 37 GHz. During the summer time (April-September) the 13.7 meter radio telecope is used more extensively for solar observations. Then, we can, for example, follow a single radio brightening for several hours.

MRO solar maps can be found here: [1]

Observations at 11.2 GHz with MRO 1.8-meter radio telescope

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 Quiet Sun Level (QSL) is around 12,000 K at 11.2 GHz. The radio telescope 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 (Kallunki (2009)). The telescope has a logarithmic output which can be utilized in the case of strong bursts.

Latest MRO at 11.2 GHz can be accessed from here: [2]

A summary of MRO's daily solar observations can be found here: [3]

Observations at 50-845 MHz with Callisto

MRO joined the worldwide solar radio burst observing network, e-Callisto, in the fall of 2010 (Kallunki et al. (2013)). 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 (e-Callisto website (2015)). 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 (Kallunki et al. (2013)). The current frequency coverage in Metsähovi is 50 - 1450 MHz (the lower band is 50 - 845 MHz and the higher band 790 - 1447.250 MHz), but we are in the process of changing the system to measure two polarizations at a frequency range of 50 - 845 MHz to enable studying the solar magnetic field. Due to the low instrumental sensitivity studies of the quiet Sun are not possible.

The latest Callisto observations can be found here (choose MRO to access Metsähovi data): [4]

Conclusions

Aalto University Metsähovi Radio Observatory has been actively doing solar observations since 1978 at 37 GHz. Currently we do continuous solar observations at 11.2 GHz and 50-845 MHz, and produce daily solar maps at 37 GHz. Due to MRO's Northern location 14-hour long solar observations are possible during the summer time. MRO quick look data can be accessed via the links mentioned above. You can request for the measurement data by writing an email to: juha.kallunki@aalto.fi

References

Kallunki, J., Lavonen, N., Järvelä, E. and Uunila, M. (2012) A Study of Long-Term Solar Activity at 37 Ghz. Baltic Astronomy, 21, 255-262.

Kallunki, J. (2013) Studies of Solar Activity with Emphasis on Quasi-Periodic Oscillations. Doctoral Thesis, University of Turku, Ser. A I Tom. 467, Turku.

Kallunki, J. (2009) Possibilities of the Metsähovi Radiotelescopes for Solar Observations. Licenciate Thesis, Faculty of Information and Natural Sciences, Helsinki University of Technology, Helsinki.

Kallunki, J., Uunila, M. and Monstein, C. (2013) Callisto Radio Spectrometer for Observing The Sun―Metsähovi Radio Observatory Joins the Worldwide Observing Network. IEEE Aerospace and Electronic Systems Magazine, 28, 5-9. http://dx.doi.org/10.1109/MAES.2013.6575404

e-Callisto Web Site (2015) International Network of Solar Radio Spectrometers. http://www.e-callisto.org/