Analysis of the flux growth rate in emerging active regions on the Sun

Abstract

We studied the emergence process of 42 active region (ARs) by analyzing the time derivative, R(t), of the total unsigned flux. Line-of-sight magnetograms acquired by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) were used. A continuous piecewise linear fitting to the R(t)-profile was applied to detect an interval, dt_2, of nearly-constant R(t) covering one or several local maxima. The averaged over dt_2 magnitude of R(t) was accepted as an estimate of the maximal value of the flux growth rate, R_MAX, which varies in a range of (0.5-5)x10^20 Mx hour^-1 for active regions with the maximal total unsigned flux of (0.5-3)x10^22 Mx. The normalized flux growth rate, R_N, was defined under an assumption that the saturated total unsigned flux, F_MAX, equals unity. Out of 42 ARs in our initial list, 36 event were successfully fitted and they form two subsets (with a small overlap of 8 events): the ARs with a short (<13 hours) interval dt_2 and a high (>0.024 hour^-1) normalized flux emergence rate, R_N, form the "rapid" emergence event subset. The second subset consists of "gradual" emergence events and it is characterized by a long (>13 hours) interval dt_2 and a low R_N (<0.024 hour^-1). In diagrams of R_MAX plotted versus F_MAX, the events from different subsets are not overlapped and each subset displays an individual power law. The power law index derived from the entire ensemble of 36 events is 0.69+-0.10. The "rapid" emergence is consistent with a "two-step" emergence process of a single twisted flux tube. The "gradual" emergence is possibly related to a consecutive rising of several flux tubes emerging at nearly the same location in the photosphere.

arXiv commentSolar Physics; accepted 16 pages, 8 figures, 1 table

References in corpus (16)

• Simulation of the Formation of a Solar Active Region
• Solar surface emerging flux regions: a comparative study of radiative MHD modeling and Hinode SOT observations
• Current status of turbulent dynamo theory: From large-scale to small-scale dynamos
• Magnetic flux emergence in granular convection: Radiative MHD simulations and observational signatures
• The physics of twisted magnetic tubes rising in a stratified medium: two dimensional results
• Numerical simulations of active region scale flux emergence: From spot formation to decay
• Three dimensional non-linear evolution of a magnetic flux tube in a spherical shell: Influence of turbulent convection and associated mean flows
• Numerical Experiments on the Two-step Emergence of Twisted Magnetic Flux Tubes in the Sun
• Statistical Study on the Nature of Solar Flux Emergence
• Two-step Emergence of the Magnetic Flux Sheet from the Solar Convection Zone
• Solar small-scale dynamo and polarity of sunspot groups
• The naked emergence of solar active regions observed with SDO/HMI
• Statistical Analysis of the Horizontal Divergent Flow in Emerging Solar Active Regions
• Using SDO/HMI magnetograms as a source of the solar mean magnetic field data
• Active Region Emergence & Remote Flares
• The Relationship Between Plasma Flow Doppler Velocities and Magnetic Field Parameters During the Emergence of Active Regions at the Solar Photospheric Level

Cited by in corpus (4)

• Sun-as-a-star Spectral Irradiance Observations of Transiting Active Regions
• Extended statistical analysis of emerging solar active regions
• Magnetic fluxes of solar active regions of different magneto-morphological classes: I. Cyclic variations
• On the possibility to probe the flare productivity of an active region on the early stage of emergence