Solar activity studies - basics

Solar eruption studies

The multi-wavelength analysis of solar eruptions was started at Tuorla by investigating shock signatures at radio wavelengths during 'halo' type coronal mass ejections (CMEs). The analysis included the use of satellite data in EUV and X-rays, ground-based H-alpha observations, dynamic radio spectra, and solar magnetograms. Radio precursors before CME lift-off were also studied, and some unusual sources and processes that lead to prolonged millimeter wave emission were discovered. This work has now been extended to studies of energetic flares. Especially the X-ray/gamma-ray solar satellite RHESSI provides spatial information on energy release sites and particle emission paths, together with radio observations.

Although the solar group at Tuorla is capable of analysing most of the satellite and ground-based data themselves (in the past we have worked with SOHO/EIT, SOHO/MDI, SOHO/LASCO, Yohkoh/SXT, Yohkoh/HXT, TRACE, CGRO/BATSE, GOES, and RHESSI satellite data), many of the studies have been and are conducted in close collaboration with researchers abroad, especially in France. The French CNRS research groups in Meudon are working in several different fields in solar physics: radio emission, magnetic field extrapolations and theory, optical and spectral lines using the solar facilities in the Canary Islands, and space weather issues. The senior researchers and PhD students from Tuorla have made regular visits to Meudon for data analysis and discussions. Other collaborating observatories include the Astrophysical Institute Potsdam (Germany), Sonnenobservatorium Kanzelhöhe (Austria), Ondrejov Observatory (Czech Republic), University of Athens (Greece), and observatories and researchers associated with the National Astronomical Observatory of Japan. Solar eruptions with particle events have been investigated together with the SOHO ERNE team in Turku. Collaboration with the High Energy Astrophysics team at the Observatory, University of Helsinki, has also been close for many years.

Our future work is based on selecting interesting flare events, with or without CMEs, with the detection of energetic particles, and that have good data coverage especially in radio, X-rays, EUV, and H-alpha. To do this we need radio imaging data from Nancay/Nobeyama/Metsähovi, flux density observations (Tuorla/Nobeyama) with simultaneous dynamic spectra, and imaging data from various satellites. By comparing the spatial and temporal evolution, especially time delays, at different wavelengths in each of the selected solar events we will be able to identify processes and locations that ignate flares and CMEs, cause shock waves, and accelerate energetic particles. Transient features and flare precursors have been observed at radio wavelengths before, and we will study their relationship to the magnetic explosions. As decimetric-metric radio emission traces the propagation paths of electrons high in the corona and centimeter radio emission is sensitive to dense plasma (also in the form of ejected plasmoids), the radio observations have vital importance in this study. RHESSI gamma-ray data together with ERNE particle observations are used for studying high-energy (> 10 MeV) proton acceleration processes at near-solar surface sources. In particular, for magnetically well-connected solar particle events, the onsets of proton intensities will be compared with the corresponding gamma-ray, x-ray, and radio emissions and the the time evolution of the proton spectrum will be investigated. Special attention is paid on distinguishing the effects of the near-solar surface and interplanetary processes and conditions on the observed particle intensities and spectra.

We are studying how well the observations go with the present models for energy release (magnetic breakout by reconnection, energy inflow before eruption), for particle acceleration (bow shocks or blast waves or both?), and what 'EIT waves' really are (fast or slow MHD waves, effects of plasma compression, or something else?). Our aim is to construct more precise and accurate schematic models that can be verified numerically. Under the COPAP collaboration with the Ruhr-Universität Bochum (Germany) and the University of Helsinki space physics group, we have done work on how to test our ideas with their numerical 3-D MHD models. On the other hand, our observations provide support for the modeling efforts.

The CME-driven shocks are also known to accelerate charged particles to high energies. The research on this area is conducted in collaboration with the ERNE group, who specialise in energetic particle observations, and the solar physics theoretical group, who have experience in analytical and numerical energetic particle studies.

Activity in the polar zones of the Sun

Although the best-known and most spectacular activity occurs close to the equator, polar regions also show a variety of large and small scale structures, depending on the observing wavelength. The most prominent large scale structures are polar coronal holes. Polar plumes, polar faculae, bright points and radio brightenings are small scale features that can be observed both inside and outside coronal hole areas. The polar radio features are also of considerable interest in their own right. The very nature of these radio brightenings is still unclear. They are typically 100-400 K above the quiet Sun level at 37 GHz. Although they resemble the sunspot-related activity in the millimetre radio maps, at high solar latitudes they are a different manifestation of solar activity. This is most clearly demonstrated by the fact that their activity peaks during the solar minimum, as was shown on the basis of Metsähovi radio monitoring data between 1982-1995, and confirmed later on the basis of Nobeyama Radioheliograph data between 1992-2001.

Polar faculae manifest themselves at the photospheric level of the Sun. It has been shown that they can have strong magnetic fields, comparable with sunspot magnetic fields. The high latitude radio enhanced temperature regions are connected to the polar faculae, and therefore it is possible that magnetic loops in the polar faculae structures are responsible for the radio enhancements. On the other hand, the origin of radio brightenings is in the low or middle chromosphere and show connections to CaII and H-alpha bright structures, which are typical for this chromospheric level. In some cases, EUV structures and X-bright points have also been associated with radio brightenings. Thus, our central goal is to continue the comparisons between radio, optical, EUV, and X-ray data to study the physical parameters behind the observed features and their association with, e.g., heliospheric models and solar wind flow. The collaboration between Tuorla Observatory, Metsähovi Radio Observatory, Pulkovo Observatory and Kislovodsk solar mountain station will continue with the aim of getting simultaneous data to be compared with satellite data at optical and EUV wavelengths.

Torsional oscillations and oscillations in the radio emission

Optical observations, both of the Doppler effect and of magnetograms have detected a periodic variation in the global rotation of the Sun. This effect has been called torsional oscillations. We have presented new results on torsional oscillations, based on Nobeyama Radioheliograph data between 1992-2002 at the wavelength of 1.76 cm. The existence of this weak and difficult to analyse phenomenon was confirmed using a completely independent set of data, radio instead of optical. The use of radio data has also enabled us to extend the study of torsional oscillations to chromospheric and low coronal levels; earlier optical results have related to the photospheric level. Future analysis of torsional oscillations, based on observations with all possible methods and thus referring to different levels of the solar atmosphere, is necessary in order to proceed in the understanding of their role in the physics of the solar cycle.

A recent addition to our solar investigations is to analyse the polarized radio emission (Nobeyama radio data set) of the whole Sun, which shows 3 minute oscillations at 1.76 cm wavelength in the 1992-2003 data. We have found that the 3 minute oscillations are present at different phases of solar activity, including the minimum of activity. These oscillations are especially conspicuous in the difference between the right and the left circular polarization. The intensity of the oscillations changes with the level of solar activity. Spectral analysis of the 3 minute oscillations in polarized solar radio emission shows that there exists modulations with periods of 27 and 157 days. Previously, 3 minute oscillations have been observed only in the sunspot regions, in which they are presumably connected with magnetic fields. In our study we excluded all sunspots from the full disk radio emission. The nature of the whole Sun 3 minute oscillations is not understood yet. It is difficult to understand how they could be produced by weak magnetic fields.

model

A schematic model for the repeated flaring shown in soft X-rays at the top of this page. The flare accelerates electrons which then follow the magnetic field lines to the ends of the magnetic loops. Radio emission is produced at the loop end where magnetic field strength is high - the emission mechanism is gyrosynchrotron emission and most of the electrons are mirrored back to the loop - while at the end of the other loop the field strength is much less and electrons interact with the ambient plasma at the loop footpoints and emit hard X-rays (bremsstrahlung mechanism). This work was published in the Solar Physics journal in 2003.