Ephemeral Regions on a Sequence of Full‐Disk Michelson Doppler Imager Magnetograms

Abstract

Ephemeral regions are small-scale, bipolar regions of magnetic field, emerging all over the solar surface. As structures ephemeral regions are short-lived; they can be recognized for 4.4 hr on average. This paper examines a 3.5 day sequence of full-disk Michelson Doppler Imager (MDI) magnetograms in order to estimate the importance of ephemeral regions to the total magnetic flux budget on the sun. The data were taken in 1997 October, which was around the minimum of solar cycle 22, and early in cycle 23. An algorithm was developed to automatically recognize ephemeral regions on this sequence of magnetograms. Assuming uniform emergence over the entire solar surface, the total amount of flux emerging in ephemeral regions is estimated to be 5 × 10²³ Mx^-1. This rate of emergence is sufficient to replace the magnetic field in quiet sun in 14 hr. In total, 38,000 ephemeral regions are found, with an absolute flux Φ in the range (2.6-407) × 10¹⁸ Mx. The distribution function of their fluxes follows an exponential with an average of 11.3 × 10¹⁸ Mx. This relatively low flux content may be due to the fact that these ephemeral regions are detected before they have reached a maximum. After their first recognition, they increase in flux with a rate of typically dΦ/dt = 1.6 × 10¹⁵ Mx s^-1. Only 60% of the ephemeral regions are found to have the orientation expected in cycle 22. After emergence, the outer borders of the ephemeral regions expand from a size of 8.9 Mm, with a velocity of 2.3 km s^-1. No particular pattern can be recognized from a map of all locations of flux emergence. From a χ² test it is found that the emergences occur randomly, on a scale below 20 Mm. On larger scales some order is found, but its origin remains unknown.