HAO 2011 Profiles In Science: Dr. Liang Zhao

Contact:

303-497-1534
lzh@ucar.edu

Dr. Liang Zhao is a Post Doc I in the High Altitude Observatory of NCAR. Her research interests are in the global structure and origin of the solar wind, and the transport and evolution of the heliospheric magnetic field. This includes analyzing the in-situ observations, developing or modifying solar wind acceleration and magnetic field transport theory, and numerically simulating the plasma and magnetic field in the solar atmosphere and heliosphere. Especially, her recent research mainly focuses on the latest solar activity minimum between Cycles 23 and 24, which has been noticed very different from the previous solar minima since the space age.

Publications

(1) Zhao, L, and Fisk, L. A. 2011: Understanding the Behavior of the Heliospheric Magnetic Field and the Solar Wind during the Unusual Solar Minimum between Cycles 23 and 24, Solar Physics, doi:10.1007/s11207-011-9840-4.

Abstract: The properties of the heliospheric magnetic field and the solar wind were substantially different in the unusual solar minimum between Cycles 23 and 24: the magnetic-field strength was substantially reduced, as were the flow properties of the solar wind, such as the mass flux. Explanations for these changes are offered that do not require any substantial reconsiderations of the general understandings of the behavior of the heliospheric magnetic field and the solar wind that were developed in the Cycle 22–23 minimum. Solar-wind composition data are used to demonstrate that there are two distinct regions of solar wind: solar wind likely to originate from the stalk of the streamer belt (the highly elongated loops that underlie the heliospheric current sheet), and solar wind from outside this region. The region outside the streamer-stalk region is noticeably larger in the Cycle 23–24 minimum; however, the increased area can account for the reduction in the heliospheric magnetic-field strength in this minimum. Thus, the total magnetic flux contained in this region is the same in the two minima. Various correlations among the solar-wind mass flux and coronal electron temperature inferred from solar-wind charge states were developed for the Cycle 22–23 solar minimum. The data for the Cycle 23–24 minimum suggest that the correlations still hold, and thus the basic acceleration mechanism is unchanged in this minimum.

Figure 1 caption: (a) An illustration of the motions of the magnetic field on the Sun in the frame corotating with the equatorial rotation rate (Fisk, 1996, 2005; Fisk, Zurbuchen, and Schwadron, 1999b; Fisk and Schwadron 2001). The M-axis is the axis of symmetry for the expansion of the magnetic field from a polar coronal hole. The Ω-axis is the solar rotation axis. P marks one of the open lines (green) that connects to the Pole. The curves with arrows (red) are the trajectories of the open lines. (b)The open lines reconnect and diffuse outside the streamer-stalk region, which is marked in yellow.

(2) Zhao, L., Fisk, L. 2010: Comparison of Two Solar Minima: Narrower Streamer Stalk Region and Conserved Open Magnetic Flux in the Region Outside of Streamer Stalks, In: Cranmer, S. R., Hoeksema, J. T., Kohl, J. L. (ed.), Conference Series, Astron. Soc. Pacific, 428, , San Francisco, 229.
See online article.

Abtract: To explore the difference between the most two recent solar minima, we analyze the in-situ ACE and ULYSSES observations and examine the distributions of the three types of solar wind (streamer-stalk-associated wind, wind from outside the streamer stalk that can be associated, in part, with coronal holes, and interplanetary coronal mass ejections). We use the taxonomy provided by Zhao et al. (2009) to identify the three types of solar wind. We then map the in-situ observations to the 2.5 solar radii surface. With the aid of the potential-field-source-surface model (PFSS), we calculate the normal distance from the solar wind “foot point” to the local helisopheric current sheet on that surface. We find that the source region of the streamer stalk wind is narrower (15°~20°) compared to the previous minimum (~40°). The area outside the streamer stalk is accordingly larger, but the magnetic field strength is observed to be lower, with the result that the total amount of the magnetic open flux from the outside of streamer stalk region is conserved in the two successive solar minima. The implications of the conservation of open magnetic flux for models of the behavior of the solar magnetic field are discussed.

Figure 2 caption: Right (a): Origin of three types of solar wind. Background contours shows the magnetic polarities from PFSS model: the dashed (solid) lines represent the inward (outward) magnetic field and the purple line is the current sheet. The black line in the middle of the color band is the trajectory of ACE, the color bars above the black line indicate the two solar wind types (non-streamer-stalk wind in green and streamer-stalk wind in orange) and the color bars under the black line show observed magnetic polarities (inward in blue and outward in red). Left: (b) Probability densities of the normal distances from the source of streamer-stalk wind (solid line) and nonstreamer- stalk wind (dotted line) to the local heliospheric current sheet on the 2.5 solar radii surface in the last solar maximum; (c) the current solar minimum.

(3)
Fisk, L.A., Zhao, L. 2009: The heliospheric magnetic field and the solar wind during the solar cycle, In: N. Gopalswamy, Webb D. F. (ed.) Universal Heliospheric Processes, IAU Symposium 257, Cambridge Univ. Press, Cambridge, 109-120.
See online article.

Abtract: The heliospheric magnetic field and the solar wind are behaving differently in the current solar minimum, compared to the previous minimum. The radial component of the heliospheric magnetic field, and thus the average value of the component of the solar magnetic field that opens into the heliosphere, the so-called open magnetic flux of the Sun, is lower than it was in the previous solar minimum; in fact, lower than in any previous solar minimum for which there are good spacecraft observations. The mass flux, the ram pressure, and the coronal electron temperature as measured by solar wind charge states are also lower in the current minimum compared to the previous one. This situation provides an opportunity to test some of the concepts for the behavior of the heliospheric magnetic field and the solar wind that have been developed; to improve these theories, and to construct a theory for the solar wind that accounts for the observed behavior throughout the solar cycle, including the current unusual solar minimum.

Figure 3 caption:Normalized radial component of the heliospheric magnetic field, solar wind mass flux, solar wind ram pressure, and charge states of O, as observed by Ulysses from 1991 to 2008.

(4) Zhao, L., Zurbuchen, T.H., Fisk, L.A.: 2009: Global distribution of the solar wind during solar cycle 23: ACE observations, Geophys. Res. Lett., 36, 14104, doi:10.1029/2009GL039181.

Abstract: The composition of the solar wind can be used to determine its origin at the Sun; e.g., solar wind from coronal holes has demonstrably lower charge states than solar wind of other origins. The O7+/O6+ ratio as measured by Advanced Composition Explorer (ACE) during 1998–2008 is used to divide the solar wind into three categories: non-transient solar wind from coronal holes (hereafter referred to as CHW); non-transient solar wind that originates from outside of coronal holes (hereafter referred to as NCHW), and solar wind associated with transient interplanetary coronal mass ejections (ICMEs). The global distribution of the solar wind relative to the Heliospheric Current Sheet (HCS), as specified by a Potential-Field-Source-Surface model, is then determined. The solar wind from outside of coronal holes is found to originate from a band of about 40° in width about the HCS during solar maximum conditions, and a much smaller band of < 17° during solar minimum. These results are consistent with models for the global transport of the solar magnetic field during the solar cycle, and they are consistent with earlier global flow structure determinations based upon velocity alone.

Figure 4 caption: (top) Sunspot number and (bottom) three solar wind components during 1998–2008: ICMEs (yellow), CHW (green) and NCHW (orange).