Research Interests

I am interested in Solar Physics and are doing numerical investigations of different 2D and 3D MHD scenarios related to understanding the physics of the solar corona. Choose one of the following headings to get more information about the projects that I am involved in.
Courtesy of SOHO/EIT consortium. SOHO is a project of international cooperation between ESA and NASA.
Coronal heating Loop temperatures Reconnection Prominences Eruptive events Home

Coronal Heating

A long out standing problem in solar physics is to understand the physical reasons for the sun to have a hot corona. It has for a long time been accepted that the solar magnetic field has an important role in the puzzle. But, exactly how the mechanism for providing the energy and releasing it in the corona works has not yet been solved. E.J. Parker (197?) suggested that the shuffling of the magnetic field in the convection zone, where gas dynamics determines the dynamical evolution of the plasma, would propagate into the corona, where the magnetic field dominates the plasma dynamics. The continued braiding process of the magnetic field may only continue for a limited time before the braiding of the magnetic field leads to the collapse of the magnetic field structure and magnetic singularities in the form of current sheets are formed. After this the magnetic field will evolve through a phase where resistive effects alters the field line connectivity and free magnetic energy is released in the from of Joule heating, particle acceleration and bulk plasma acceleration that eventually are all contributing to heating the coronal plasma. Together with Aake Nordlund we investigate the Coronal Heating problem through a sequence of numerical flux braiding experiments. These proved that the braiding of a simple magnetic field structure leads to the formation of current sheets on many different length scales. The energy put into the magnetic field through the braiding of the field in the photosphere may therefore be released in the corona through dissipation of the developing current sheets.

Loop Temperatures

The location of the energy release in the corona has important implication on the temperature profile along the coronal loop. In collaboration with Duncan Mackay, we investigated the static changes in the temperature structure of loops as the heating were released differently along the loop. Heating profiles obtained at different times from the flux braiding experiments are used as heat input for this investigation. This showed that there are significant difference in both the temperature profiles and the summit temperatures depending on the distribution of the heating along the loop. Recently E.R. Priest et al. suggested to use observed temperature to put limits on the heating profiles along coronal loops. A statistical investigation of this problem have showed that present days observational accuracy is far from being adequate to use this method with any confidence. Two main points were reached in this paper, namely strict limits on the accuracy of the observations and the clear requirement for high time caderance to be able to follow the dynamical evolutions of localised energy releases. Again using time dependent flux braiding data, a time dependent analysis of the energy release is at present being conducted in collaboration with Robert Walsh.

Magnetic Reconnection

An important step towards understand how magnetic fields may release its free energy in 3D is required if we want to understand the basic mechanism that may heat the solar corona. In 2D Magnetic Reconnection have been investigated for many years. It is only with in the last 10 years that investigations of 3D magnetic reconnection have slowly started. 3D gives more freedom for possible types of reconnection than 2D. In 3D reconnection can occur in magnetic field regions where the magnetic field strength is no-zero, i.e. in situations as the flux braiding scenario. More like 2D magnetic reconnection may also take place in magnetic null points, location in space where the magnetic field strength goes to zero. Here three different types of reconnection may take place. Namely, fan, spine and separator reconnection of which the last is believed - but not proved - to be the most important. A number of numerical experiments have been preformed to investigate the effect of null reconnection. Especially the investigation of separator reconnection have had our interest.

Prominences

Prominences, the majestic cold and dense structures floating in the corona have mystified solar physicists for many years. It is becoming increasingly evident that they are the manifestation of dynamical interaction of opposite polarity magnetic flux and that they despite their quiet appearance are very dynamically active. In a project with Aaron Longbottom the formation phase of prominences were investigated. This showed that the convergence of flux may eventually lead to reconnection above the photosphere that then can lift cold dense plasma into the corona. In the simple setup that we investigated this leads to an adiabatic decrease in temperature as the plasma is lifted and expands due to the decrease in plasma pressure. It also showed that for a prominence region to survive for a long period of time, new material has to be constantly supplied during its life time.
Recently we (in collaboration with Duncan Mackay) have started to investigate how a dense plasma blob can be hanging suspended on a magnetic field line when gravity is in fact trying to pool it down towards the photosphere. We find that there are no problems in having the dense plasma blob hanging suspended on a vertical magnetic field line for MUCH longer than the free fall time. This means that other processes, as heating, radiation and heat conduction becomes important for finally determining the life time of the cold plasma blob.

Eruptive events

Observations of transition region lines have revealed sudden localised enhances in intensity, usually with doubler shifts of up to a few hundred kilometres per hour. These are today interpreted as being the result of magnetic reconnection taking place between different flux concentrations in the intergranular lain. Previous suggestions to explain these phenomena have used 1D HD simulations combined NNN? codes to solve for the resulting line profiles of transition region lines. Further experiments investigating 2D reconnection experiments and simple intensity profiles have been made. We have tried to expand the 2D investigations by making the environment in which the 2D reconnection takes place more realistic, by including a transition region in the initial conditions and to also include anisotropic heat conduction and optical thin radiation in the MHD experiment. The output from these experiments have then, as in the 1D case, been used to derive NNN?? line profiles for a few different transition region lines. These experiments, although the full parameter space in not investigates yet, indications that some parameter regimes may be rolled out as they are not producing line profiles that are consistent with observations. At present we are working making the models more realistic, using a more realistic initial condition including a more realistic background heating profile that depends on the actual physical parameters. This work is a part of Ilia Russow's PhD thesis. This work has been supervised by Robertus E, Jerry Doyle and myself. Last updated 14.06.00