Submit manuscript...
eISSN: 2576-4543

Physics & Astronomy International Journal

Mini Review Volume 2 Issue 5

Rotation of the low temperature regions (LTR) at 8 mm

Juha Kallunki

Metsähovi Radio Observatory, Aalto University, Finland

Correspondence: Juha Kallunki, Metsähovi Radio Observatory, Aalto University, Metsähovintie 114 02540 Kylmälä, Finland, Tel 3582 9442 4852

Received: June 01, 2018 | Published: September 7, 2018

Citation: Kallunki J. Rotation of the low temperature regions (LTR) at 8 mm. Phys Astron Int J. 2018;2(5):403-406. DOI: 10.15406/paij.2018.02.00117

Download PDF


Background: In this work, we investigate rotation of the low temperature region (LTR) of the Sun at the radio wavelength of 8 mm. The temperature of these areas is less than the quiet Sun temperature (8200 ± 500 K at 8 mm). We found 100 representative LTR sources. The analyzed data is obtained from solar radio maps (Metsähovi Radio Observatory, Aalto University, Finland). The data is recorded between 1989 and 2014 both during the solar maximum and minimum. Our results show that the rotation rates of LTRs match the best with the coronal holes rotation. Our results also show that the rotation is quite rigid.

Keywords: differential rotation, coronal holes, quiet Sun, low temperature region (LTR), solar cycle


Coronal holes (CH) are the areas of low temperature, density and pressure. They can be seen darker than the quiet Sun area (QSA)1 at Extreme ultraviolet (EUV) range. CH have been studied very comprehensively,2 and also at radio wavelengths3 by using data from Metsähovi Radio Observatory (MRO). CH structures in area of the high brightness temperature regions (HTR) have also studied earlier4 with MRO data. Behaviour of the solar cycle is not yet understood comprehensively. For instance, some physical unsolved issues are related to the corona. A coronal heating question is still one major problem. The atmospheric rotation rates could help on this. Data series, as presented here, could give new information about the structure of the corona holes. Several investigations have showed that the coronal magnetic field rotates more rigidly than the photosphere.2,5 This is suggested caused e.g. by the magnetic reconnection at coronal hole boundaries.6

LTRs have earlier been used as determining the solar differential rotation at 8 mm.7 Just recently, there has been an aim to observe one solar radio map each day in MRO (–gallery, 29.05.2018). Thus, compared with the previous studies, we now have more extensively data collection, especially longer observing tracks (5–7 days).

Instrumentation and observations

The Metsähovi RT–14 telescope at the Metsähovi Radio Observatory (MRO), Aalto University (Helsinki Region, Finland, GPS: N 60 13.04 E 24 23.35) has a Cassegrain type antenna with a diameter of 13.7 meters. The working range of the telescope is 2–150 GHz (13.0 cm–2.0 mm). The antenna 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 arc min at 8 mm. The receiver is a Dicke type radiometer, thus the radiometer’s noise will be filtered out. For the temperature stabilization of the receiver, a Peltier element is used. The noise temperature of the 8 mm receiver is around 280 K, and the temporal resolution during the observations is 0.1 s or less. The obtained data is recorded as intensity. The Quiet Sun temperature at 37 GHz is around 8200 ± 500 K (Tb,qsl).8 The radio emission at 8 mm comes from the chromosphere. The temperature resolution is less than 100 K. The full documentation of Metsähovi RT–14 for solar observations can be found from.9 As an example, solar radio maps at 8 mm in

are presented. The dark areas are the regions of low temperature. In this case minimum temperature is around 97 % x T b,qsl =7950 K MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbdfwBIj xAHbqedmvETj2BSbqefm0B1jxALjhiov2DaerbuLwBLnhiov2DGi1B TfMBaebbnrfifHhDYfgasaacPqFH0xe9v8qqaqFD0xXdHaVhbbf9v8 qqaqFr0xc9pk0xbba9q8WqFfea0=yr0RYxir=Jbba9q8aq0=yq=He9 q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaqaafaaake aajugibabaaaaaaaaapeGaaGyoaiaaiEdacaqGGaGaaiyjaiaabcca caWG4bGaamivaKqba+aadaWgaaqcbasaaKqzadWdbiaadkgacaGGSa GaamyCaiaadohacaWGSbaajeaipaqabaqcLbsapeGaeyypa0JaaG4n aiaaiMdacaaI1aGaaGimaiaabccacaWGlbaaaa@4DE4@ .

We found 100 LTRs between 1989 and 2014, and 23 of them were confirmed as a coronal hole structure. Information of LTRs (latitude, longitude and brightness temperature) is collected from the consecutive solar radio maps. One representative solar map per day is taken to analysis. The analyzed data was selected with following criteria:

  1. Each region had a lifetime more than three days
  2. All the points were selected longitudes (rel. long.) between −60°and +60° to avoiding possible artifacts near to the solar limb
  3. The brightness temperature of selected areas is between 91.0 % x T b,qsl MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbdfwBIj xAHbqedmvETj2BSbqefm0B1jxALjhiov2DaerbuLwBLnhiov2DGi1B TfMBaebbnrfifHhDYfgasaacPqFH0xe9v8qqaqFD0xXdHaVhbbf9v8 qqaqFr0xc9pk0xbba9q8WqFfea0=yr0RYxir=Jbba9q8aq0=yq=He9 q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaqaafaaake aajugibabaaaaaaaaapeGaaGyoaiaaigdacaGGUaGaaGimaiaabcca caGGLaGaaeiiaiaadIhacaWGubqcfa4damaaBaaajeaibaqcLbmape GaamOyaiaacYcacaWGXbGaam4CaiaadYgaaKqaG8aabeaaaaa@4935@ and 99.5 % x T b,qsl MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbdfwBIj xAHbqedmvETj2BSbqefm0B1jxALjhiov2DaerbuLwBLnhiov2DGi1B TfMBaebbnrfifHhDYfgasaacPqFH0xe9v8qqaqFD0xXdHaVhbbf9v8 qqaqFr0xc9pk0xbba9q8WqFfea0=yr0RYxir=Jbba9q8aq0=yq=He9 q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaqaafaaake aajugibabaaaaaaaaapeGaaGyoaiaaiMdacaGGUaGaaGynaiaabcca caGGLaGaaeiiaiaadIhacaWGubqcfa4damaaBaaajeaibaqcLbmape GaamOyaiaacYcacaWGXbGaam4CaiaadYgaaKqaG8aabeaaaaa@4942@ .
  4. Totally 100 LTRs were found, which were tracked three days or more. The rotation rates were calculated on the basis of the geometrical center of the LTRs (the red cross in the middle of LTR in Figure 1).
  5. For each data set, the mean rotation rate was calculated. Also the Earth’s orbital velocity (0.9865 deg/day) was taken into account in the final results.

Figure 1 Six solar radio maps at 8 mm. Maps are observed on six consecutive days between 7 Jun. 2014 (A) and 12 Jun. 2014 (F). Red crosses indicate the object (position), which are under tracking. A black color indicates the areas of LTR


After the mean rotation rates were defined for each data set, they were fitted to the solar differential rotation function: y=A+Bxsi n 2 (x) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbdfwBIj xAHbqedmvETj2BSbqefm0B1jxALjhiov2DaerbuLwBLnhiov2DGi1B TfMBaebbnrfifHhDYfgasaacPqFH0xe9v8qqaqFD0xXdHaVhbbf9v8 qqaqFr0xc9pk0xbba9q8WqFfea0=yr0RYxir=Jbba9q8aq0=yq=He9 q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaqaafaaake aajugibabaaaaaaaaapeGaamyEaiabg2da9iaadgeacqGHRaWkcaWG cbGaamiEaiaadohacaWGPbGaamOBaKqba+aadaahaaqcbasabeaaju gWa8qacaaIYaaaaKqzGeWdaiaacIcapeGaamiEa8aacaGGPaaaaa@49F0@ , where y is a differential rotation rate (deg/day),x is a latitude (deg.)A and and Bare solar rotation parameters. Similar analysis methods have been used earlier.9 The rotation speed profile is presented in Figure 2. The mean speed (equatorial rate) is 13.27 deg/day. Our analysis shows that rotation is very rigidly. Comparison with other speed profiles is presented in Figure 2: Nobeyama (LTR) at 1.76 cm,10,11 previous 8 mm studies7,9 and CH studies5,12,13 as well. More carefully analysis was performed for three LTRs (Figure 3). Beside the geometrical centre point, six other points around the LTR were chosen and the rotation curves were created. The slopes of rotation curves are similar at all events. This confirms that the geometrical centre point of the LTR is usable for the analysis and the structure is rotating as a whole. In addition, this will confirm the interpretation of rigid rotation. In Figure 3, the slopes are similar and they do not depend on the latitude.

Comparison with previous studies

In Figure 2 rotation rate profiles of LTRs are presented. The mean equatorial rotation rate is 13.27 deg/day (LTR, center). This has some correlation to the other investigations. The study13 found a rate of 12.6–13.5 deg/day for coronal rotation at He I 10830 Å. However, there is bigger difference between the results obtained by Sheridan14, 14.8 deg/day at 1.76 cm. It is important to notice that they used coronal (bright) points in their analysis. Our results show that rotation is more rigidly especially compared with the previous radio observations. For the CH,7 found the equatorial rate 14.33 deg/day.

Artifacts and limitations

A limited resolution (beam size is 2.4 arc min) causes some inaccuracies to the results. It is difficult to say an exact effect of this and it also depends on the size of the observed structure. A good approximation is that error in the longitudinal direction cannot be larger than one third of the beam size (2.4 arc min). A low resolution will also might give larger errors close to the limb. However, we do not take into account these data points to the analysis.

The differences for previous studies can also be explained by the relative small amount of samples on high (> +40°) and on low latitudes (<–40°). The model, obtained now is not reliable on high and low latitudes. The height correction was not obtained for the results, thus the comparison would be easier with the previous studies. The height correction could reduce the speed around 0.5 deg/day as was mentioned in7. However, it has no significant effect on the final conclusions.

Figure 2 A comparisons between various rotation rate models. The current model matches well with CH models and 1,76 cm results at latitudes close to equator.

  • Figure 3 The rotation of LTRs. Upper panel: track in 2013–04–04–2013–04–06, middle panel: track in 2013–08–15–2013–08–17 and lower panel: 2014–06–07–2014–06–12. E=east and W=west. The number before E or W equals to the latitude value.

Discussion and conclusion

The main conclusion to this study is that LTRs are rotating very rigidly compared with HTRs and other atmospheric structures e.g. sunspot. In addition, the rotation of LTR at 8 mm matches quite accurately with CH rotation and rotation at 1.76 cm.10,11 Main, prevailing emission mechanism in the quiet Sun region (areas of the low magnetic field) is the thermal bremsstrahlung. The electron density of the CHs differs from the QSAs,14 which can explain the lower brightness temperature. Also several other investigations support the results that the coronal holes (or LTR) are rotating more rigidly than HTRs (or active region and plage).2,14 It is very difficult to say comprehensively why the rotation rates of the coronal holes and the low temperature regions are rigid in comparison with other structures. Some suggestions have been presented. Our impression is that areas of LTRs are not stable over the track period. Their size will change over the time, which could tell that the magnetic reconnection cannot be only reason for the rigid rotation. The magnetic diffusion can be other physical interpretative process as well.2 However, the most plausible reason for the rigid coronal hole rotation could be the physical structure of the coronal hole. Hiremath2 and Navarro–Peralta16 support the concept that the coronal holes are anchored to the deeper solar interior (even below the convective zone), thus they are not the atmospheric structures in that regard. Further, this can cause more rigid rotation for instance compared with the active regions or sunspots. Our results promote these conclusions. A fully confidence for this would need a detailed analysis of area variation between CHs and LTRs. Our results are almost consistent with2. They used data from SOHO / EIT 195 Å (lower corona) which has a relatively good height correlation to the data used here (upper chromosphere at 8 mm). Thus they are also consistent in this respect. If the coronal holes are really deep structure, it will still need more simultaneous observations on the lower atmospheric layers (photosphere and chromosphere) and more helioseismological investigations. In addition, the magnetic structure of LTR’s should be studied. One scenario is that LTRs without the connection to CH are slightly magnetic bipolar areas, and vice versa. It is also obvious, based on our results, those CHs and LTRs in 8 mm have some connection to each other. A behind of these must be some similar physical properties and processes. We can conclude on the basis of the results obtained in this study that millimeter wavelength observation could give versatile information about the solar corona. We could not, with this amount of data, study an effect of solar cycle on the rotation rates. We did not study coronal holes in the high temperature regions (HTR), which might be an interesting addition to this analysis. More observations, also at 8 mm, will be needed to fulfill the conclusions presented here.


Courtesy of NASA/SDO and the AIA, EVE, and HMI science teams. is an open–source project for the visualization of solar and heliospheric data. The project is funded by ESA and NASA.

Conflict of interest

Author declares that there is no conflict of interest.


  1. Cranmer SR. Coronal Holes. Living Reviews in Solar Physics. 2009;6(1).
  2. Hiremath KM, Hegde M. Rotation Rates of Coronal Holes and their Probable Anchoring Depths. The Astrophysical Journal. 2013;763(2):1–12.
  3. Brajsa R, Benz AO, Temmer M, et al. An Interpretation of the Coronal Holes' Visibility in the Millimeter Wavelength Range. Solar Physics. 2017;245(1):167–176.
  4. Pohjolainen S, Riehokainen A, Valtaoja E. Polar Radio Sources Observed in UV and EUV. Magnetic Fields and Solar Processes. The 9th European Meeting on Solar Physics, held in Florence, Italy. In: Wilson A, editor. France: European Space Agency; 1999. 635 p.
  5. Insley JE, Moore V, Harrison RA. The differential rotation of the corona as indicated by coronal holes. Solar Physics. 1995;160(1):1–18.
  6. Madjarska MS, Huang Z, Doyle JG, et al. Coronal hole boundaries evolution at small scales. III. EIS and SUMER views. Astronomy & Astrophysics. 2012;545(A67):1–16.
  7. Romstajn I, Brajsa R, Wöhl H, et al. Solar Differential Rotation Determined by Tracing Low and High Brightness Temperature Regions at 8 mm. Central European Astrophysical Bulletin. 2009;33:79–94.
  8. Landi E, Chiuderi Drago F. The Quiet–Sun Differential Emission Measure from Radio and UV Measurements. The Astrophysics Journal. 2008;675(2):1629–1636.
  9. Kallunki J, Lavonen N, Järvelä E, et al. A Study of Long–Term Solar Activity at 37 GHz. Baltic Astronomy. 2013;21(3):255–262.
  10. Gelfreikh GB, Makarov VI, Tlatov AG, et al. A study of the development of global solar activity in the 23rd solar cycle based on radio observations with the Nobeyama radio heliograph. I. Latitude distribution of the active and dark regions. Astronomy and Astrophysics. 2002;389(2):618–623.
  11. Gelfreikh GB, Makarov VI, Tlatov AG, et al. A study of development of global solar activity in the 23rd solar cycle based on radio observations with the Nobeyama radio heliograph. II. Dynamics of the differential rotation of the Sun. Astronomy and Astrophysics. 2002;389(2):624–628.
  12. Chandra S, Hari–Vats. Differential coronal rotation and solar activity. In: Choudhuri AR & Banerjee D, editors. First Asia–Pacific Solar Physics Meeting ASI Conference Series. 2011;2:285–290.
  13. Jones HP. Magnetic Fields and Flows in Open Magnetic Structures. Large–scale Structures and their Role in Solar Activity ASP Conference Series. In: Sankarasubramanian K, Penn M, Pevtsov A, editors. USA: Proceedings of the Conference in Sunspot; 2005. 229 p.
  14. Sheridan KV, Dulk GA. Radio observations of coronal holes. Solar and interplanetary dynamics. USA: Proceedings of the Symposium; 1979.
  15. Zhao XP, Hoeksema JT, Scherrer PH. Changes of the boot–shaped coronal hole boundary during Whole Sun Month near sunspot minimum. Journal of Geophysical Research. 1999;104(A5):9735–9752.
  16. Navarro–Peralta P, & Sanchez–Ibarra A. An observational study of coronal hole rotation over the sunspot cycle. Solar Physics. 1994;153(1–2):169–178.
Creative Commons Attribution License

©2018 Kallunki. This is an open access article distributed under the terms of the, which permits unrestricted use, distribution, and build upon your work non-commercially.