Lab #2: Activity 5 Exploring the Structure of the Solar Magnetic Field Using the MAS Model

Transcription

1 Lab #2: Activity 5 Exploring the Structure of the Solar Magnetic Field Using the MAS Model In this lab activity we will use results from the MAS (Magnetohydrodynamics Around a Sphere) model of the solar corona to explore the shape and topology of the Sun s magnetic field using tools contained in the CISM-DX visualization package. The MAS model uses observations of the photospheric magnetic field as an inner boundary condition. From this the MHD fluid equations are used to obtain self-consistent solutions for the magnetic field and plasma parameters (density, temperature, and flow velocity) in a spherical volume out to a distance of 30 R s (Rs = Solar Radius = 695,000 km). At the outer boundary the solar wind flow is hypersonic. The goals of this lab are: to become familiar with the structure of the solar magnetic field in the coronal region (between 1 and 5 solar radii). to relate the structure of the solar magnetic field to the structure of the magnetic field at the surface of the sun to begin to understand the relationship between the solar magnetic field and the solar wind In its current version, MAS calculates a steady state solution to the boundary conditions. We will start by looking at two such solutions for near solar minimum case (CR 1920, February/March of 1997) and a near solar maximum case (CR 1960, February and March of 2000) from the last solar cycle. In each case the photospheric field observed during that 27- day rotation was used as a time stationary (i.e., constant in time) boundary condition and the MAS model was run until it reached an equilibrium. PART 1: Solar Magnetic Field at Solar Minimum, CR 1920 In a terminal window, change to the lab 2 directory [cd swss-labs followed by cd lab2] and start cismdx. From the File menu in the Visual Program Editor window (VPE), select Open Program... and open the network solar_corona.net from the Open... menu. Under the Windows menu, click on Open Control Panel by Name and select Lab 2 (mid way down the list). A window with the label Lab 2: The Solar Corona should appear. This window contains toggles and pull-down menus for you to control what is visualized. Activity 1: The Photospheric Field. In the control window the pull down menu under the Solar Surface selector should read Br and the CR folder should be the one appropriate for CR Execute this network. An image window should appear with a colored sphere. The sphere should represent the radial component of the magnetic field, B R, at the sun s surface (R = 1 R S ). This is the input boundary condition to the model, which is obtained from observations of the photosphere.

2 The z-axis is in the direction of rotation axis of the Sun. Rotate the sun and notice that the red and blue regions of strong field into and out of the Sun associated with active regions are mostly at lower latitudes near the equator and that the red and blue regions tend to come in pairs. Compare this surface the magnetogram synoptic map that you viewed previously for CR Can you relate active regions on the magnetogram to those on the spherical image displayed? Can you identify the polarity at the poles? The Solar Surface pull down menu has two other choices: Tanh(Br) and Polarity. Choosing Tanh(Br) compressed the scale of the magnetic field so that more details can be seen. The relative polarity at the poles should now be clear. Choosing the Polarity option will give the sign of the radial field with no indication of the relative strength. Activity 2: Structure of the Magnetic Field in the Corona. Now let s start to explore the structure of the field near the solar surface. On the solar surface, find an active region On the sheet of paper provided, sketch what you think the field lines that originate from that region will look like. Remember that the field is assumed to be radial at the surface of the sun. Be sure that your group has discussed this and has some agreement on an answer. In practice it is convenient to identify closed field lines as those field lines where B r (the radial component of the magnetic field) goes to zero somewhere along the field line. Closed field lines can be categorized by their furthest extension from the Sun. We have chosen a few examples to show the structure of closed field lines. Now in the control panel, turn on the field lines that close Inside of 2 R s under the Closed Field Lines section. These are field lines that extend no greater then 2 solar radii from the center of the sun (1 R s from the surface). Does the image shown agree with prediction? What are the differences. Do the field lines obey physical laws that you expect them to obey? What are those physical laws? Turn on the rest of the closed field line buttons (you may need to rescale the image to see it properly). Overall, does the field look like a dipole field that you would get from a regular magnet? If not, why not? What do you think is happening in the regions that have no field lines originating? Activity 3: Open Field Lines. Open field lines extend to the boundary of the simulation and so can be traced from points at that boundary. Turn on the First Open option in the Open Field Lines section. The first

3 open field lines are defined as those that extend to the simulation boundary near where the radial component of B is zero. In order to make sense of this image, you may want to turn off some of the closed field lines. Describe the structure of the open field lines. Where do the open field lines originate on the sun? What will happen to the field lines that start on regions of the sun that still don t have field lines? Turn on the Current Sheet option. This surface represents the isosurface where the radial component of the magnetic field is zero. What is the relative polarity of the field lines above and below the current sheet? Explain why there must be a current flowing on the current sheet. Draw a diagram on your paper. Finally, add in all of the open field lines to complete the picture of the solar magnetic field. On your paper, roughly sketch the structure of the magnetic field for this solar minimum case. PART 2: Solar Magnetic Field at Solar Maximum, CR 1960 In this second part we repeat everything done in Part 1 but using instead the CR 1960 calculation, is a magnetogram near solar maximum. The goal here is to contrast structure of the solar magnetic field near solar minimum (CR 1920) with near solar maximum (CR 1960). Activity 1: Comparing photospheric fields at solar minimum and solar maximum To look at CR 1960, change cr1920 to cr1960 in the CR Folder dialog on the control window. After you execute the network you may want to turn off the open field lines so you can look at the surface more easily. Again compare the photospheric field to the magnetogram synoptic map for CR Also compare it to the CR 1920 visualization you just looked at. How are the two Carrington Rotations different at lower latitudes? How are they different at the poles? You may want to use polarity along with the full magnetic field to answer these questions. Activity 2: Closed field lines. Again make a prediction about what you think you will see from the closed field lines. Sketch your prediction on your working paper. Now turn on the closed field lines. Are they consistent with your prediction?

4 What are some of the similarities and differences in the structure from solar minimum to solar maximum? Do these obey the same physical laws that you identified above in Activity 2 of part 1? Now turn on the open field lines. At solar maximum, what latitude do open field lines tend to originate from? Is it more or less structured then those at solar minimum? Sketch the structure of magnetic field in the solar maximum case as best you can. Turn on the Current Sheet. Compared to the solar minimum case, how is it oriented? Is the earth more likely to cross the current sheet at Solar Maximum or Solar Minimum? Activity 3: White Light Corona Below are white light coronagraphs taken by the SOHO satellite ( or for images) The light you are seeing is scattered off the electrons in the plasma that is trapped on the field lines. There is an image for each of Carrington Rotations you have looked at. These images are taken at the beginning of the rotation, so you should orient the simulation results so that the x-axis is pointing out of the screen.

5 Is the plasma that is imaged in the SOHO coronagraphs trapped on closed field lines or open ones? Why? What do you think has happened to the plasma that is on the other field lines?