The Number of Magnetic Null Points in the Quiet Sun Corona
- 125 Downloads
The coronal magnetic field above a particular photospheric region will vanish at a certain number of points, called null points. These points can be found directly in a potential field extrapolation or their density can be estimated from the Fourier spectrum of the magnetogram. The spectral estimate, in which the extrapolated field is assumed to be random and homogeneous with Gaussian statistics, is found here to be relatively accurate for quiet Sun magnetograms from SOHO’s MDI. The majority of null points occur at low altitudes, and their distribution is dictated by high wavenumbers in the Fourier spectrum. This portion of the spectrum is affected by Poisson noise, and as many as five-sixths of null points identified from a direct extrapolation can be attributed to noise. The null distribution above 1500 km is found to depend on wavelengths that are reliably measured by MDI in either its low-resolution or high-resolution mode. After correcting the spectrum to remove white noise and compensate for the modulation transfer function we find that a potential field extrapolation contains, on average, one magnetic null point, with altitude greater than 1.5 Mm, above every 322 Mm2 patch of quiet Sun. Analysis of 562 quiet Sun magnetograms spanning the two latest solar minima shows that the null point density is relatively constant with roughly 10% day-to-day variation. At heights above 1.5 Mm, the null point density decreases approximately as the inverse cube of height. The photospheric field in the quiet Sun is well approximated as that from discrete elements with mean flux 〈|φ|〉=1.0×1019 Mx distributed randomly with density n=0.007 Mm−2.
KeywordsMHD Sun: Corona Sun: Magnetic fields
Unable to display preview. Download preview PDF.
- Born, M., Wolf, E.: 1980, Principles of Optics: Electromagnetic Theory of Propagation and Diffraction of Light, 6th edn., Pergamon Press, New York. Google Scholar
- Ghatak, A.J., Thyagarajan, K.: 1978, Contemporary Optics, Plenum Press, New York. Google Scholar
- Hufnagel, R.E., Stanley, N.R.: 1964, J. Opt. Soc. Am. 52. Google Scholar
- Liu, Y., Norton, A.A.: 2001, MDI measurement errors: The magnetic perspective, Technical Report SOI Technical Note 01-144, Standford SOI. Google Scholar
- Press, W.H., Flannery, B.P., Teukolsky, S.A., Vetterling, W.T.: Numerical Recipes: The Art of Scientific Computing, Cambridge University Press, Cambridge, 1986. Google Scholar
- Sweet, P.A.: 1958, In: Lehnert, B. (ed.) Electromagnetic Phenomena in Cosmical Physics, Cambridge University Press, Cambridge, 123 – 134. Google Scholar