Photospheric Electric Fields and Energy Fluxes in the Eruptive Active Region NOAA 11158

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Contents

Introduction

The advent of high-cadence vector magnetic field and Doppler velocity measurements from e.g. HMI/SDO, SOT/Hinode and SOLIS, have made the estimation of electric fields in the solar photosphere possible. The calculation of the electric field from magnetic and Doppler data is critically important for various quantitative studies of the solar atmosphere. First, if we know both electric and magnetic field vectors in the photosphere, we can estimate both the Poynting flux of magnetic energy and the flux of relative magnetic helicity entering the corona. Second, knowledge of electric and magnetic fields enables the driving of time-dependent simulations of the coronal magnetic field. This is the goal of the Coronal Global Evolutionary Model (CGEM, Fisher et al. 2015).

Method

We have recently improved the electric field inversion methods introduced by Fisher et al. (2010), to create a comprehensive technique for calculating photospheric electric fields from vector magnetogram sequences (Kazachenko et al. 2014). The new method, which we dubbed the PDFI (an abbreviation for PTD-Doppler-FLCT-Ideal technique, where PTD and FLCT stand for Poloidal-Toroidal Decomposition and Fourier Local Correlation Tracking), has been systematically tested for accuracy and robustness, using synthetic data from ANMHD simulations. Here we take the next step forward, and apply the PDFI technique to observations.

Dataset

The flare-productive active region (AR) NOAA 11158 was observed by HMI nearly continuously for a six-day period over 2011 February 10–16 from its emergence (see Bz-snapshots in Figure 1). We used the sequence of 770 vector magnetic field B and Doppler field measurements to derive the temporal evolution of electric fields E and Poynting S fluxes during these six days.

Figure 1: (A)–(D): HMI vertical magnetic field (Bz) maps at four different times of NOAA 11158 evolution. Panel (E): positive/negative vertical magnetic fluxes during the 6-day interval. Diamonds indicate the times of images on the left. An X2.2 flare is marked with the dashed line.

Results

To describe the photospheric electric fields E and energy fluxes S in NOAA 11158, we use two approaches: we first show the spatial distribution of B, E and S at two times, before and after the X2.2 flare; we then analyze their temporal spatially integrated evolution over six days of observations.

Magnetic Field: Bpreflare and Bpostflare

Figure 2. Horizontal (arrows) and vertical (grayscale) components of the magnetic field in NOAA 11158 at preflare (top left) and postflare times (bottom left), and the difference image between the two (right panel). The blue and red colors correspond to horizontal fields in areas of positive and negative Bz.

Pre- and post-flare snapshots of the vertical magnetic field (Figure 2) show that the horizontal magnetic field close to PIL increased by over 300 G during the X2.2 flare (see arrows), while the vertical magnetic field remained nearly constant, consistent with the field implosion scenario. On the difference image (right panel) we also notice two circular patterns, directed counter-clockwise in negative and clockwise in positive polarities, implying that the field connecting those polarities becomes less twisted.

Electric Field: Epreflare and Epostflare

Figure 3. Horizontal (arrows) and vertical (grayscale) components of the electric field at preflare (top left) and postflare times (bottom left), and the difference image between the two (right panel). The blue and red colors correspond to horizontal fields in areas of positive and negative Ez.

We find the photospheric electric field vector, ranging from -2 to 2 V/cm, to increase its magnitude by up to 0.5 V/cm at the PIL and 1 V/cm away from the PIL during the flare (Figure 3). The horizontal component is mostly concentrated along the PIL, while the vertical component is largest at the PIL and in the sunspots’ penumbrae. The presence of a nonzero Ez is related to changes in the vertical current, which is mostly concentrated close to the main PIL (Petrie 2013; Janvier et al. 2014).

Poynting Flux: Spreflare and Spostflare

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