Swiss Virtual Institute for Solar Science
Heating
Heating of the Solar and Stellar Atmospheres
The heating of the corona to millions of degrees and the stupendous energy release in flares are big questions in astrophysics. There is general agreement that both are related to energy stored in the coronal magnetic field and a suspicion that they may be two aspects of the same basic physics. In 1998 scientists at the Institute of Astronomy at ETH Zurich have discovered small energy releases in quiet regions of the solar corona, indicating minute events of coronal heating. The combination of soft X-ray observations by the Japanese-American Yohkoh satellite and simultaneous radio measurements by the Very Large Array in the USA suggested that the events had characteristics typical for flares, but are several orders of magnitude smaller in energy. Evidence later corroborated by EUV observations using the European SOHO satellite suggested that the energy released by these events was sufficient to heat the corona.
Still the flare process remains an enigma. Although the principle of magnetic reconnection is widely recognized to be the constituting element, the usual magnetohydrodynamic approach disregards that a large fraction of energy is released into accelerating electrons and ions. Non-thermal particles are the primary output of flares. They are observed at the Institute of Astronomy at ETH Zurich both in hard X-ray bremsstrahlung and various radio emissions, including coherent radiations of trapped and beamed electrons and synchrotron emission of relativistic electrons. Radio observations at ETH Zurich have focused on spectroscopy in a large band from meter to decimeter wavelength. Most remarkable have been a first survey of decimeter flare emissions and the timing and location of the radio emissions in relation to hard X-rays. Pioneering works have been published on the characteristics of narrowband spikes, suspected to be related to electron acceleration, and the statistics of radio bursts. The instrumentation, currently in operation at Bleien near Gränichen AG, includes 3 spectrometers: The wideband, frequency-agile Phoenix-2, the low-frequency Callisto, and the FFT multichannel Argos for high sensitivity observations.
Software and a data center for the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have been developed at ETH Zurich in collaboration with institutes in computer science to allow for fast browsing and data retrieval. RHESSI, a NASA Small Explorer mission, observes the hard X-ray and gamma-ray emissions of solar flares. It can measure individual photon energies with high precision and determine the spatial origin with high resolution. RHESSI data have been used at ETH to determine the spectral characteristics of electron acceleration of flares and to study the possible hard X-ray source of the accelerator in the corona. The results are consistent with stochastic acceleration by turbulent waves, excited by the reconnection process.
Left: The RHESSI spacecraft (designed and built partly at PSI). Right: RHESSI (color code) and TRACE (brightness) observations of a solar flare. Blue encodes hard emission, yellow soft emission.
Temporal fine structures and polarization of RHESSI data have been investigated by the PSI group. To this end, special algorithms and data reduction methods have been developed, and the RHESSI instrument response has been modeled by Monte-Carlo simulations. In order to improve our understanding of the physical processes involved in particle acceleration in solar flares, numerical simulations of particle orbits have been conducted at PSI in international collaborations, assuming an alternative dissipative acceleration scenario. Within a more theoretical approach, the mobility of charged particles in a turbulent plasma has been investigated, as it occurs in the solar corona. Many of the statistical methods developed for RHESSI data analysis have proven valuable for non-solar X-ray observations, which, conversely, have motivated new approaches to the RHESSI data.