HAO 2011 Profiles In Science: Dr. Mausumi Dikpati
Contact:
303-494-1512
dikpati@ucar.edu
Dr. Mausumi Dikpati is a Scientist III in the High Altitude Observatory of the National Center for Atmospheric Research. She received her PhD in 1996 from the Indian Institute of Science, Bangalore. She has been at NCAR since the fall of 1996 when she assumed a postdoctoral fellowship in the Advanced Study Program. Her main research interest is in modeling dynamics and MHD of the solar interior and the solar dynamo. Read More »
Professional Web Site(s):
http://www.hao.ucar.edu/Public/about/Staff/dikpati/index.html
Summary of Achievements
Over the past year, Mausumi Dikpati's research has focussed on modeling evolution of large-scale, cyclic solar magnetic fields. Simulating polar magnetic field patterns of cycles 22 and 23 using a flux-transport dynamo, she has demonstrated that, flux-transport dynamo models and surface transport models, despite some differences in their ingredients, produce remarkably similar responses in the polar fields' patterns to the changes in the meridional flow speed when the same latitudinal profile for the poleward surface flow is used.
As a collaborative effort, Dikpati has also continued investigations of the physics behind the differences in the speed and latitudinal profile of meridional circulation derived from Doppler measurements and Magnetic Feature Tracking (MFT) technique. Gustavo Guerrero, Matthias Rheinhardt and Axel Brandenburg at NORDITA, and Dikpati, have performed simulations of magnetic feature tracking speed in both 1D and 2D flux-transport models, using Alternating-Direction-Implicit 2D transport code and the PENCIL code, and have found that the simulated MFT speed is always the same as the plasma meridional flow speed in a 1D transport model, but is different in a 2D model.
Publications
(1) Dikpati, M., 2011: Polar field puzzle: solutions from flux-transport dynamo and surface-transport models, Astrophysical Journal, 733, 90, 1-7, doi: 10.1088/0004-637X/733/2/90.
Abstract: Polar fields in solar cycle 23 were about 50\% weaker than those in cycle 22. The only theoretical models that have addressed this puzzle are surface-transport models and flux-transport dynamo models. Comparing polar fields obtained from numerical simulations using surface-flux-transport models and flux-transport dynamo models, we show that both classes of models can explain the polar field features within the scope of the physics included in the respective models. In both models, how polar fields change as a result of changes in meridional circulation depends on the details of meridional circulation profile used. Using physical reasoning and schematics as well as numerical solutions from a flux-transport dynamo model, we demonstrate that polar fields are determined mostly by the strength of a surface poloidal source provided by the decay of tilted, bipolar active regions. The profile of a meridional flow with latitude, and its changes with time,have much less effect on polar fields in flux-transport dynamo models than in surface-transport models.
Figure caption: Surface radial field transport mechanisms, respectively, in a surface-flux-transport model with a poleward flow peaking at 6-degree latitude (frame a), at 37-degree latitude (frame b). Frame (c) shows the same effect in a flux-transport dynamo model with a peak flow at mid-latitudes. In frames (a) and (b), red and blue patches on the surface represent bipolar active regions. Red and blue continuous lines represent radial fields from a bipole, and dashed lines represent the new locations of radial fields after the meridional flow is increased. For example, by doubling the flow speed, a larger increase in the transport rate is obtained, respectively, on equatorward and poleward sides of the bipoles in frames (a) and (b); thus, fields from leader polarity drift closer to fields from followerpolarity in frame (a) and further apart in frame (b). Frame (c) describes a very similar situation as in frame (b), but in terms of poloidal fields in the r-$\theta$ plane. Solid contours represent poloidal fields produced from bipolar active regions and dashed contours represent these fields for an increased flow. The polarity division line of large-scale poloidal fields from a flux-transport dynamo model is shown by a dark line.
(2) Guerrero, G., Rheinhardt, M., Brandenburg, A., Dikpati, M., 2011: Plasma flow versus magnetic feature-tracking speeds in the Sun, Mon. Not. R. Astron. Soc., doi:10.1111/j.1745-33933.2011.01167.x
Abstract: We simulate the magnetic feature-tracking (MFT) speed using axisymmetric advective-diffusive transport models in both one and two dimensions. By depositing magnetic bipolar regions at different latitudes at the Sun's surface and following their evolution for a prescribed meridional circulation and magnetic diffusivity profiles, we derive the MFT speed as a function of latitude. We find that in a one-dimensional surface-transport model the simulated MFT speed at the surface is always the same as the meridional flow speed used as input to the model, but is different in a two-dimensional transport model in the meridional (r, theta) plane. The difference depends on the value of the magnetic diffusivity and on the radial gradient of the latitudinal velocity. We have confirmed our results with two different codes in spherical and Cartesian coordinates.
Figure caption: Tracking speed vø for magnetic diffusivity 108 ~ 3 × 1012 cm2 s-1. Solid line represents plasma flow speed, symbols the magnetic feature tracking speed for different diffusivities.