Volume 15, Number 4, December 1999
By Joseph King and Natalia Papitashvili
Solar coronal magnetic fields are drawn out into interplanetary space by the solar wind plasma flowing largely radially away from the Sun. Because the roots of most field lines stay attached to fixed regions of the Sun, each field line acquires a spiral shape by the combined action of the radial plasma outflow and the solar rotation. The drawn-out field lines, observed outside the Earth's magnetosphere as the interplanetary magnetic field (IMF), will on average lie near the ecliptic plane (defined by the Earth's motion about the Sun) and have a local direction of about 45 degrees from a line drawn from the Sun through the Earth owing to the mean solar wind speed (~ 400 km/s), the solar rotation rate (~ 360 deg/27 days), and the Sun-Earth distance (~ 150 million km).
The solar magnetic field is quite complex and time-variable in its detailed structure. However, it is dominantly of one polarity (i.e., pointing out from or in toward the Sun) over the north hemisphere of the Sun and of the opposite polarity over the other hemisphere. Once each 11-year solar activity, near the peak in activity, the solar magnetic polarity reverses such that the hemisphere that was of positive (outward) polarity during one 11-year cycle becomes negative polarity for the next 11 years.
The boundary between these (approximate) hemispheres of opposite magnetic polarity is not a flat surface coincident with the Sun's equatorial plane (perpendicular to the Sun's spin vector). This boundary is typically tilted and is frequently convoluted (non-planar). The tilt and the degree of non-planarity vary over the course of the solar activity cycle.
As drawn into interplanetary space, the solar magnetic field polarity pattern yields an IMF typically outward from the Sun on one side of the boundary and inward on the other side. This boundary, as extended into interplanetary space, is called the heliospheric current sheet, reflecting the fact that an electric current must flow in the boundary region separating the oppositely directed IMF.
The heliospheric current sheet is frequently likened to a ballerina's skirt, with standing waves rotating with the Sun's 27-day rotation period. Readers can see, for instance, http://nssdc.gsfc.nasa.gov/spds/coshel/coshel.html. The amplitude and structure of the waves depend on the solar cycle phase.
The Earth in its annual excursion about the Sun ranges between about 7 degrees north and south of the Sun's equator. A spacecraft held permanently near the Earth but just outside the Earth's magnetosphere will typically pass through the heliospheric current sheet twice or occasionally four times every 27-day solar rotation interval according to the extent of tilt and non-planarity of this current sheet and according to the Earth's location north or south of the solar equatorial plane.
Studies of the time history of the IMF polarity yield insights into the evolution of the heliospheric current sheet, and these insights in turn yield insights into processes in the solar atmosphere where the IF originates. Such studies have been pursued from the early days of the NASA space program, starting with data from the first Interplanetary Monitoring Platform (IMP) spacecraft launched in 1963.
The NSSDC multisource OMNI data set is intended mainly to simulate a long-lived spacecraft stationed just outside the Earth's magnetosphere making hourly resolution measurements of the IMF and of such solar wind plasma parameters as flow speed, ion density, and temperature. While it has a number of data gaps, it is an excellent data set for the study of long term variations of the IMF polarity and the associated heliospheric current sheet.
NSSDC has very recently created a compact figure illustrating the evolution of IMF polarity over the 1965-1999 period as captured in the OMNI data set. This figure is accessible through the OMNIWeb interface at http://nssdc.gsfc.nasa.gov/omniweb/.
For this figure daily resolution IMF polarity is computed from OMNI's hourly resolution data and is then represented simply via the color red for negative polarity and blue for positive polarity on vertically stacked, horizontal bars of 54-day (two solar rotation periods) length. Each horizontal bar with its color coded day segments begins 27 days after (or, with an alternative display option, before) the bar just above it. Since the IMF polarity typically remains fixed for many days before switching to the opposite polarity and since this pattern varies only slowly from one 27-day interval to the next, the polarity pattern (also called the IMF sector structure) is visible by scanning the plot left to right while the gradual evolution of this polarity pattern is clearly visible by scanning the plot up to down.
A companion table is also available through OMNIWeb through which the time of any structure seen in the color-coded figure may be determined. This determination enables users to retrieve the numeric data for that structure for further analysis. Further details on the development of the figure and table are available through OMNIWeb.
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