Motion of magnetic elements at the solar equator observed by SDO/HMI

Transcription

1 AGU 2012, #SH41D ~7 Dec Motion of magnetic elements at the solar equator observed by SDO/HMI Keiji Hayashi*, A. A. Norton, Y. Liu, X. Sun, and J. T. Hoeksema (W. W. Hansen Experimental Physics Lab., Stanford University) *)

2 Abstract / Outline Characterizing motions of the solar magnetic field near the solar equator is important for understanding the symmetry and asymmetry of large scale structures in the solar interior, solar corona, and solar wind. The SDO/HMI has been observing the full disk solar magnetic field, with a cadence of 12 minutes or 45 seconds, since April With high cadence long term observations of the solar photospheric magnetic field, we analyze the motion of the magnetic field elements, specifically latitudinal motion, near the solar equator. The regions that are divergent, convergent or cross equatorial and appear, in general, to be coherent on a spatial scale of ~15 degrees longitude and last for several days are found.

3 data We used mtrack to make data for equator regions moving with the solar differential rotation. Using HMI magnetogram data from May 1, 2010 to Oct 12, 2012, or from 180 dgr of CR2096 to 260 dgr of CR2129, we made 576 cubic data, whose size is 20 degrees by 20 degrees (lat. and lon) by 8 days. The analysis below will be done for each sub dataset (1dgr x 1dgr x 4h) This work focuses on the solar equator: For polar regions, see our neighbor (presentation #SH41D 2130 by X. Sun et al.).

4 Cubic data time Long.: West latitude:north Sol. equator Before tracing motions of magnetic elements, we here convert the values of magnetic flux density to 3 step values, 1, 0 or +1,here with a threshold of 30 Mx/cm^2, in HMI data

5 Analysis method Each large data cube (20dgr x 20dgr x 8d) is split into sub cubes whose size is 1 degree by 1 degree, and 4 hours. Then, Fourier analysis is applied. Here we note that applying Fourier analysis means we had assumed periodicity along space (latitude ad longitude) and time, which is not right: The derived coefficients, and powers, may only have qualitative meanings. For future quantitative analysis, we will apply proper treatment and analysis method.

6 Stack plot of latitudinal motions Five 1 degree latitude bins, from N2 to S2 dgr, in each CR bin. Blue: northward, Red: southward; truncated at +/ 200km/12min. Right plot: 10 deg running ave.

7 Stack plot of latitudinal motions Running average (10 degree window) along longitude was applied. We can see cross equatorial motions of magnetic elements persistent, lasting a few Carrington rotation periods, with size of 10~20 degrees. In this stack plot diagram, some patterns appear in apparent period of about 29 days: slower than the Carrington solar rotation rate. *) In stack plots, time runs from right to left, from top to bottom.

8 Comparison with H.S inferred sub surface flow Right plot: latitudinal component of sub surface flow, inferred from time distance helioseismology technique. Color: red/blue for south/northward

9 Latitudinal divergence and convergence Right plot: Divergence of latitudinal flow, longitudinally smoothed. Color : blue/red respectively shows divergence and convergence.

10 summary From HMI s long term, about 2.5 year data of the LoS magnetogram, we seek regions of divergent, convergent and equator crossing north or southward flows near the solar equator. In the latitude time plots, magnetic elements often show latitudinal motions that last a few days, comparable to (or somewhat longer than) the life time of supergranulation. In the longitude time plots, we see magnetic elements show patterns of westward motion pattern faster than the reference differential rotation rate, in this study, inferred from Doppler observation (Snodgrass, 1990). The patterns in the stack plot often last over 2 to 4 Carrington rotation periods. Such patterns appear to run from upper right to lower left; slower rotation than Carrington rotation rate. Proper filter or method to characterize the nature of the flow is needed: We anticipate motions of magnetic elements would be better defined by Correlation Tracking. Also, relating with the sub surface flows obtained from helioseismology technique will help understanding the dynamics of interior of the Sun.

11 motion v.s. activity Right plot: latitudinally squashed stack plot. Color : blue/red respectively represents positive/negative polarity.

12 motion v.s. activity Right plot: latitudinally squashed stack plot. Color : blue/red respectively represents area of B > 100G in northern/southern hemisphere.

13 Gradient of motion v.s. activity Right plot: latitudinally squashed stack plot. Color : blue/red respectively represents positive/negative polarity.

14 Gradient of motion v.s. activity Right plot: latitudinally squashed stack plot. Color : blue/red respectively represents area of B > 100G in northern/southern hemisphere.

15 Stack plot of longitudinal motions Five 1 degree latitude bins, from N2 to S2 dgr, in each CR bin. Blue: westward, Red: eastward; truncated at +/ 200km/12min