The magnetic field at the heliospheric base during solar minimum

Details The magnetic field at the heliospheric base during solar minimum The magnetic field at the heliospheric base during solar minimum

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsh13b1534z&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SH13B-1534

Astronomy and Astrophysics

Astrophysics

[2134] Interplanetary Physics / Interplanetary Magnetic Fields, [7509] Solar Physics, Astrophysics, And Astronomy / Corona, [7524] Solar Physics, And Astronomy / Magnetic Fields

Scientific paper

What is the base of the heliosphere? and where is the heliospheric base located? There appears no generally accepted answer to the questions. We define the heliospheric base as the spherical surface where all open magnetic field lines have become radial and start to gradually wind into Archimedes spirals. Both observationally and in models it seems clear that all of the coronal magnetic field above the cusp points of coronal streamers is open. However, except at the cusp points (located near 2.5 solar radii in most magnetostatic models), the open field lines are not generally radial. The non-radial coronal plasma expansion, especially during solar minimum, is supposed to be caused primarily by the transverse magnetic pressure gradient [Smith and Balogh, 1995]. The pressure gradient approaches zero at the Alfven critical point, so the height of the Alfven critical point, at about 15 solar radii, provides a reasonable estimate of the above-defined heliospheric base. If the non-radial coronal expansion is indeed caused mainly by the magnetic pressure gradient, the open magnetic flux is expected to be uniformly distributed at the heliospheric base. The fast latitude scans of the inner heliosphere by the Ulysses spacecraft have confirmed that distribution of open magnetic flux (i.e., the radial component of the magnetic field, Br, multiplied by the square of the radial distance) is independent of latitude [Smith and Balogh, 1995; 2008]. The STEREO A and B spacecraft in 2007.6-2008.6 were well separated in longitude. We compare the two sets of rotation-averages of unsigned daily open magnetic flux observed by the STEREO A and B during this period. We find that statistically the two sets of observations agree very well, suggesting the absence of a significant gradient in longitude as well. We calculate the strength of the radial magnetic field component at 15 solar radii using the horizontal-current current-sheet source-surface (HCCSSS) coronal field model applied to photospheric magnetic field observations [Zhao & Hoeksema, 1995; 2001]. The model successfully reproduces a uniform Br, the same as observed by Ulysses, STEREO A, and STEREO B. The result further confirms that the heliospheric base is located at the Alfven critical point at about 15 solar radii.

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