PARALLEL EVOLUTION OF QUASI-SEPARATRIX LAYERS AND ACTIVE REGION UPFLOWS

AR 10978, observed with the Michelson Doppler Imager (MDI, Scherrer et al. 1995) on board the Solar and Heliospheric Observatory, rotated onto the disk on 2007 December 7. By this time, it was a mature AR. From its state of evolution and the separation of its opposite-sign polarity spots, it is likely that the AR was at least three to five days old when it appeared on the east solar limb (van Driel-Gesztelyi & Green 2015).

We checked data from the Solar-Terrestrial Relations Observatory spacecraft (STEREO-A and B) in order to constrain its emergence time using data from the Extreme-ultraviolet Imager (Wuelser et al. 2004). On the date of the AR appearance, STEREO A and B were separated from the Sun–Earth line by about 21° in each direction. In the 195 Å data of STEREO-B, the AR coronal loops could already be cearly seen over the east limb on December 5. STEREO-A data showed a potential first sign of flux emergence at the future location of AR 10978 on November 21–22 at the west limb, indicated by a significant increase of coronal emission from that location (http://stereo-ssc.nascom.nasa.gov/cgi-bin/images). Therefore, the first indications of the AR appearance could have been as old as 18 days when its magnetic field was first mapped with MDI. However, this early flux emergence (on November 21–22) might not have been a major episode and, perhaps, the flux decayed quickly.

AR 10978 had a peak mean magnetic flux of 3 × 10²² Mx (Mandrini et al. 2014b), which corresponds to the large AR category (see, e.g., van Driel-Gesztelyi & Green 2015). Such ARs may survive several months; indeed, AR 10978 returned in the following rotation as AR 10980 (see, e.g., van Driel-Gesztelyi et al. 2012) and its location was magnetically active for several more solar rotations.

From December 9 to 13, magnetic flux emergence continued within the AR (Figure 1). We were able to follow five significant flux emergence episodes, which are indicated in the figure with red ellipses and numbered as EFR1–EFR5 (emerging flux region; EFR). These occurred between the pre-existing polarities marked as PP and PF in the top left panel of Figure 1. Emerging bipoles are characterized by the divergence of opposite-sign polarity flux concentrations; therefore, these EFR sites are locations of fast photospheric motions. The evolution of the border of the supergranule to the north of PP is also noteworthy. This supergranule evolved as minor polarities emerged within it (not visible in Figure 1 because of the chosen high saturation level). By December 11, the supergranule border is no longer visible.

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We have computed the transverse flows of the photospheric magnetic field features employing a local correlation tracking technique (LCT, November & Simon 1988). The proper motions of the magnetic elements over the MDI sequence of magnetograms (spatial resolution 198) was computed using a Gaussian tracking window with an FWHM of 10''.

Different values for the correlation window were used, varying from 6'' to 14''. The general pattern of the horizontal velocity vectors did not vary significantly from case to case, in the sense that we obtained similar results in terms of their distribution by direct visual inspection of the computed flow maps. Therefore, the size of the correlation window (10'' ) was selected following a criterion based on reducing the level of noise and producing a coherent tracking of the magnetic features. Such window size is within the interval of sizes used in previous works (e.g., Nindos et al. 2003; Chae et al. 2004; Vemareddy et al. 2012).

The time cadence of the LCT, 96 minutes, is fixed by the observations. It filters the evolution of faster timescales and, in particular, the amplitude of the deduced velocity decreases (e.g., see Figure 7 of Chae et al. 2004). However, since the upflows that we analyze evolve on a timescale of days, we expect the 96 minute cadence to be sufficient to derive the photospheric flows relevant to understand these upflows. The time series for the analysis spans from 22:23 UT on 2007 December 9 to 06:23 UT on 2007 December 11. Prior to applying the LCT method, the sequence of images was aligned to eliminate the possible jitter and the solar rotation of the observed target within the field of view (FOV), which results in flow maps derotated to the AR central meridian passage (CMP) position. Finally, it should be noted that whereas the LCT results give a reliable characterization of the flow patterns, the absolute values of the transverse velocities should be taken with caution because this technique, in general, underestimates them (November & Simon 1988; Vargas Domínguez et al. 2008).

The maps of the transverse flows in Figure 2 display the velocities, using arrows, for pixels with magnetic field values over a threshold of 800 G. The computed flows show a global diverging pattern of the main photospheric magnetic polarities. Emergence episodes are also detected in the flow map, see, e.g., the one corresponding to EFR1 in Figure 2(a). Another noticeable flow pattern is that on the western portion of the northern negative polarity (Figures 2(b)–(d)). It indicates a motion of the magnetic features from the northeast to the southwest.

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EIS is a raster-scanning instrument capable of constructing large FOV, up to ≈600'' in the dispersion direction and 512'' in the slit direction, using the 1'' and 2'' slits and the 40'' slot. It has a spectral resolution of 22.3 mÅ and 1'' pixels. EIS observes in two wavelength ranges 170–210 Å and 250–290 Å.

AR 10978 is selected for this study because, with the exception of a few other ARs (see, e.g., Del Zanna 2008; Del Zanna et al. 2011), it has the best EIS spatial and temporal coverage of an AR from limb to limb. This AR was tracked from 2007 December 6 to 19 using the full complement of EIS slits. In this article, we use a set of five large FOV slit rasters around the AR (CMP, see Figures 3 and 6), which occurred on December 11 at approximately 22:00 UT. The FOV cover both of the main AR polarities (see Table 1 in Démoulin et al. (2013) for more details). We selected EIS observations of the Fe xii emission line at 195.12 Å (T ≈ 1.4 MK) because it is the strongest line within EIS's wavelength ranges. We have also used EIS 40'' slot rasters of the AR that allow us to observe the bright coronal loops, which extend beyond the FOV of the slit rasters and are useful to constrain the free parameter of our magnetic field model (see Figure 4).

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EIS slit and slot raster data were processed using standard SolarSoft EIS routines to correct for dark current, cosmic rays, hot, warm, and dusty pixels and to remove instrumental effects of slit tilt and orbital variation in the line centroid position due to thermal drift. Doppler velocities for slit rasters were calculated by fitting a single-Gaussian function to the calibrated Fe xii spectra in order to obtain the line center for each spectral profile. Reference wavelengths were determined using the average wavelength value of a relatively quiescent Sun patch within each raster. Velocity maps follow the standard convention of blueshifts (redshifts) corresponding to negative (positive) Doppler velocity shifts along the line of sight (see Figures 3 and 6).

As an example, we show three Doppler velocity maps close to the AR's CMP in Figure 3. These are drawn using standard IDL routines. Following Démoulin et al. (2013), we have added guide marks in Figure 3 that help to visualize and track the main upflow structures. The pink circles surround weak velocity patterns (because of the projection effect), while the pink dashed lines separate flow streams.

When looking at the upflow pattern in Figure 3, a clear evolution is evident. This was interpreted by Démoulin et al. (2013) to be the signature of a projection effect on steady upflows that were inclined from the vertical at an angle that was larger to the east than to the west (see Figure 14 in Démoulin et al. 2013).

To compute the magnetic field topology of an AR, we first model its coronal field. We extrapolate the line-of-sight magnetic field of AR 10978 to the corona using the discrete fast Fourier transform method (as discussed by Alissandrakis 1981) under the linear force-free field (LFFF) hypothesis (${\boldsymbol{\nabla }}\times {\boldsymbol{B}}=\alpha {\boldsymbol{B}}$, with α constant). An example of extrapolation is shown in Figure 4(b) when the AR is at disk center on 2007 December 11. We use the MDI magnetogram at 11:11 UT as the boundary condition for the magnetic model. This magnetic map is the closest in time to the EIS slot image in Fe xii 195.12 Å (Figure 4(a)), which is large enough to identify the global shape of the coronal loops. The value of the free parameter of the model, α, is set to best match the observed loops following the procedure discussed by Green et al. (2002). The best-matching value is α = −3.1 × 10⁻³ Mm⁻¹ (Figure 4(b)).

A region four times larger than that encompassed by AR 10978, and centered on it, is selected from each MDI full-disk magnetogram as the boundary condition for each extrapolation. This magnetic map is embedded within a region twice larger padded with a null vertical field component for two reasons. First, to decrease the modification of the magnetic field values since the method to model the coronal field imposes flux balance on the full photospheric boundary (i.e., the flux imbalance is uniformly spread on a larger area, so the removed uniform field is weaker as the area is larger). Second, to decrease aliasing effects resulting from the periodic boundary conditions used on the lateral boundaries of the coronal volume. The photospheric boundary condition is then written in a 1024 × 1024 horizontal grid to maintain the spatial resolution of the observations. This allows us to distinguish field lines that connect to the surrounding quiet-Sun regions from those that are potentially "open" lines because they leave the extrapolation box.

To compute the topology at different times during the AR transit, we use the same value of α with the corresponding MDI map as the boundary condition. We have checked that the large-scale loops observed in EIS slot images on the dates used in our analysis can be globally fitted with this α value. Indeed, the magnetic configuration of AR 10978 has a low shear and the coronal magnetic field configuration is mainly evolving due to the evolution of the photospheric boundary condition rather than to the change in the α value. We also carry out a transformation of coordinates from the local AR frame to the observed one (see Démoulin et al. 1997) to obtain a model for which the QSL locations can be compared to EIS upflows (Section 4).

The origin of some EIS upflows has been attributed, either directly or indirectly, to magnetic reconnection occurring in the vicinity of coronal magnetic null points (Del Zanna et al. 2011; van Driel-Gesztelyi et al. 2012; Mandrini et al. 2014b). Therefore, after summarizing the main properties of null points in the next paragraph, we investigate if the observed upflows in AR 10978 are due to magnetic reconnection at these null points.

The field connectivity around a null point is characterized by the presence of so-called spines and fans (see, e.g., Longcope 2005; Pontin 2011). A fan surface separates the coronal volume into two connectivity domains, while all field lines in the close vicinity of the fan converge to the associated spine line. The local field connectivity around a null point can be found using the linear term of the Taylor expansion of the magnetic field (see Démoulin et al. 1994; Mandrini et al. 2006, and references therein). From the diagonalization of the Jacobian matrix of the field, one finds three eigenvectors and the corresponding eigenvalues, which add up to zero to locally satisfy the field divergence-free condition. The eigenvalues are real for coronal conditions (Lau & Finn 1990). A positive null point has two positive fan eigenvalues and conversely for a negative null. The fan surface is defined by all of the field lines starting at an infinitesimal distance from the null in the plane defined by the two eigenvectors that correspond to the eigenvalues that have the same sign. In a similar way, the spines are defined by the field lines tangent to the third eigenvector.

We computed the location of magnetic null points for the MDI magnetograms closest in time to the EIS velocity maps described in Section 2.2. None of the null points found were located above the AR and most of the ones above quiet-Sun regions had no field lines connecting to the AR polarities. Figure 5 illustrates the location of all magnetic null points at heights above 20 Mm, which are related to AR 10978 for an MDI magnetogram on December 12. The location of the null points is indicated by the intersection of three segments that correspond to the direction of the three eigenvectors of the Jacobian matrix. These segments are color coded to indicate the magnitude of the corresponding eigenvalue. For a negative null, dark blue (light blue) corresponds to the highest (lowest) negative eigenvalue in the fan plane and red to the spine eigenvalue. All null points in Figure 5 are negative. Their spines, drawn in black in the figure, are connected to a quiet-Sun polarity at one end and to the main negative polarity of AR 10978 at the other. As an example, we have drawn in orange field lines belonging to the fan of one of the null points. Since these nulls are above the quiet Sun, they are associated with a magnetic field intensity, B, lower than in the AR (B lies in the range [20, 50] G). Such field values are present in small polarities with sizes ≈6 pixels, so they are not within the magnetogram noise. In fact, the height of these nulls, above 20 Mm, confirms that they are associated with real local magnetic polarities.

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It is highly improbable that magnetic reconnection at these null points can drive all of the EIS upflows in AR 10978. On one hand, none of the null points has field lines linked to the upflow region on the eastern AR border. On the other hand, reconnection at these nulls could possibly result in upflows only in the neighborhood of the spines; then, this reconnection process can, at most, explain a fraction of the observed upflows (compare Figure 5 to Figure 6(d) using the QSL trace as a guide). Furthermore, reconnection at these nulls could only slightly affect the coronal loops since they are embedded in weak magnetic fields, which implies a low amount of energy release, and the structure of the pre- and post-reconnection loops is almost the same (except in the neighborhood of the nulls). Therefore, reconnection at these nulls, far away from the AR, is not expected to drive the upflows observed at the AR border.

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Based on our previous results (Baker et al. 2009; van Driel-Gesztelyi et al. 2012), we compute QSLs for a series of magnetic maps to confirm or refute the relationship between EIS upflow and QSLs during the AR evolution.

The method to compute QSLs was first described by Démoulin et al. (1996a). QSLs were defined using the norm, N, of the Jacobian matrix of the field-line mapping. This norm depends on the direction selected to compute the mapping; then, N has, in general, different values at both photospheric footpoints of a field line. To overcome this problem, Titov et al. (2002) proposed the introduction of a function called the squashing degree, Q, which is defined as N to the second power divided by the ratio of the vertical component of the photospheric field at the two opposite field-line footpoints. Q takes into account only the distortion of the field-line mapping, independently of the field strength, and is invariant along each field line.

To determine the QSL locations, we have to integrate a huge number of field lines. A key point is that a very precise integration method is used since derivatives of the mapping are needed to calculate N and Q. In order to decrease the computation time, we use an adaptive mesh, i.e., the mesh is iteratively refined only around the locations where the largest values of Q were found in the previous iteration. The fraction of points retained at each iteration controls the computation speed and how much the finally calculated Q-map will extend toward the lower values of Q. The iteration at a location is ended when the QSL is locally well resolved or, ultimately, when the limit of the integration precision is reached. Such computations can be performed at the photospheric level and also within the full coronal volume (Pariat & Démoulin 2012).

In complex magnetic configurations (e.g., quadrupolar or multipolar), the location of QSLs is strongly determined by the distribution of the magnetic field polarities at the photosphere (see Mandrini et al. 2014a, and references therein) and the maximum value of Q is typically very high (many orders of magnitude, up to infinity). Because the magnetic field configuration is more bipolar, the QSL locations become more influenced by the presence of magnetic shear and/or twist and the value of Q tends to be lower (see the discussion in Démoulin et al. 1997). AR 10978 is an isolated globally bipolar region within which several smaller bipoles emerged during its transit across the disk (see Figure 1 in Mandrini et al. 2014b and Figure 1 in this article); this increases the complexity of the QSL pattern and the Q values between the two main polarities.

QSLs are the expected locations where magnetic reconnection can occur efficiently at coronal heights, releasing the stored magnetic energy (see references in Section 1). Then, the energy released is transported along field lines toward the chromosphere. Indeed, several studies have found that flare brightenings are located along chromospheric QSL traces (Démoulin 2007; Mandrini et al. 2010; Aulanier et al. 2011; Sun et al. 2013; Dalmasse et al. 2015; Vemareddy & Wiegelmann 2014; Savcheva et al. 2015).

In the case of EIS upflows, the relationship between QSLs and upflow regions is more indirect because the upflows are observed over a broad range of coronal heights. A direct comparison between upflow regions and QSL locations, considering the trace of reconnected field lines, is also complicated because the EIS velocity maps result from the integration of optically thin emission over a large depth along the line of sight; this integration is also affected by projection effects. However, taking into account the results from previous studies (Baker et al. 2009; van Driel-Gesztelyi et al. 2012), upflows are expected in the vicinity of QSLs where short- and high-density loops can reconnect with large-scale low-density closed or "open" loops. After reconnection, the higher pressure plasma from the initial short loops will be injected into newly formed large-scale loops creating a pressure gradient that drives the upflows (Bradshaw et al. 2011). This process is clearly illustrated in Figure 5 of van Driel-Gesztelyi et al. (2012).

Figure 6 shows the location of QSLs for a series of MDI magnetograms around AR 10978 CMP and illustrates all of the minor bipole emergences (compare to Figure 1). These magnetic maps are also the closest in time to the respective EIS velocity maps in Fe xii obtained during the AR disk transit. Extreme care was taken to coalign EIS data with MDI magnetograms. This was done not only using the image-header informations but also comparing the position of all structures (e.g., loop traces) with the location of the magnetic polarities. The QSL traces, black continuous thick and thin lines, have been overlaid on the photospheric magnetograms shown as isocontours of the field and the velocity maps indicated by blue (red) shaded regions corresponding to EIS upflows (downflows). The value of Q is above 400 for all of the traces shown; of course, at QSL locations where the null-point spines are anchored (see Figure 5) the value of Q is extremely high (Q ≥ 10¹²).

Figure 6 shows that the upflow regions are consistently located in the vicinity of QSLs drawn with thicker black lines on both AR main polarities. In panels (a) through (d), two main QSL traces, called outer and inner, extend along the negative western polarity. Sections of these traces (those drawn with thicker lines) are linked to the upflows. It is striking how well the inner QSL traces match the projected shape of the inner border of western upflow regions. Furthermore, the outer QSL shapes match the locations where the upflow velocities change in magnitude (see Démoulin et al. 2013, and Figure 3), indicating upflow regions with different characteristics. As the AR and its upflows evolve, see Figure 6(e), only the westernmost upflows associated to the outer QSL trace remain. On the eastern AR border, the outer QSL traces also match the projected shape of the upflows quite well. Next, as new minor bipoles emerge and evolve the shapes of the inner QSLs evolve as well and their complexity increases.

We have also added sets of field lines to all panels in Figure 6, except for panel (a) to avoid overloading it. These field lines have been computed starting integration in the vicinity of the outer QSLs, to the west (east) of the one on the main negative (positive) AR polarity. The projected shape of these field lines matches the spatial extension of the upflows at the borders of the AR, providing evidence of the close relationship between upflows and QSLs.

In this section, we present a scenario that, being consistent with the previously described observations and models, can explain why some sections of the QSL traces in AR 10978 are related to EIS upflows, while others are not and, also, why some upflow regions have different characteristics (as shown by Démoulin et al. 2013). As an example, we show in detail the field-line connectivity at a particular date and time. Because photospheric footpoint motions can provide forcing for the build-up of currents and induce reconnection along QSLs, we refer to the photospheric flows found in the AR (see Figure 2), which facilitate the sequence of reconnections described below.

Figure 7(a) shows two sets of field lines starting on the outer QSL trace on the negative main AR polarity. The northern field lines are anchored along the west side of the QSL trace and their opposite footpoints are located in quiet-Sun regions. These are large-scale loops probably filled with low-density plasma (low EUV emission). To the south of the same QSL trace, we have a set of shorter higher-density loops within the AR. This is the situation we envision before reconnection occurs.

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A large-scale photospheric flow pattern is present in the AR for several days, as shown for the period of December 10–12 (see Figures 2(b)–(d)). The main negative polarity is persistently moving to the southwest. As a result of this motion, the blue field lines anchored to the north of the outer QSL in Figure 7(a) may be forced to reconnect with the blue ones anchored to its south. After reconnection, we show the two sets of field lines in Figure 7(b). On one side, the process results in the long red field lines with footpoints at the western border to the south of the outer QSL. The injection of high pressure plasma from the shorter pre-reconnected loops could drive the observed upflows at the western south border of the AR. A similar process would drive, on the other side, the upflows on the eastern border of the main positive AR polarity along the reconnected pink field lines (Figure 7(b)). We remark that we can identify both pre- and post-reconnection field lines in the same magnetic field extrapolation because we have computed both QSLs and the driving photospheric flows. This is comparable to analyzing a snapshot in an MHD simulation knowing the velocity field direction. However, we have no way of using these observations to identify a pair of pre-reconnection field lines evolving into a pair of post-reconnection field lines.

The process discussed above would occur provided a large enough asymmetry in plasma pressure exists between the pre-reconnected loops. In this way, and for as long as a forcing is at work, we would observe the persistent EIS upflows in AR 10978. We speculate that reconnection would occur across the QSLs in the slipping mode analyzed by Aulanier et al. (2006) and further quantified by Janvier et al. (2013).

Comparison between upflow and QSL locations is not straightforward due to the changing viewing angle and the resulting projection effect, overlapping flows viewed in the optically thin corona, etc. However, if the reconnection process along QSLs lies at the origin of the EIS upflows, we expect that the projected spatial extension of the reconnected field lines at both AR borders compares to the spatial distribution of the observed Fe xii upflows. This is indeed the case for the pink reconnected field lines anchored in the main positive AR polarity (Op), but only partially for the red ones anchored in the main negative polarity (In; Figure 7(b)). However, field lines computed on the side and close to the QSL, thus in lower Q values such as the red field lines in Figure 6(d), have a projected extension that compares well with the extension of the upflow region at the western AR border, as do the pink ones in the same panel in relation to the upflows at the eastern AR border. Both of these sets would correspond to previously reconnected lines at the outer QSLs if we take into account that the QSL trace will shift to the east (west) on the main negative (positive) polarity as reconnection proceeds.

Though the upflow region to the west of the AR in Figure 7(b) looks in projection to be a single extended broad stream, it is in fact composed of at least two different streams (Figure 3). The easternmost border of this region lies in the vicinity of the inner QSL trace. In Figure 7(c), we show a set of blue field lines that have been computed starting integration from the eastern side of the inner QSL trace on the negative polarity. Magnetic reconnection between lines in this set and those that are anchored on the western border of the inner QSL on the positive polarity, also drawn in blue, would result in the red and pink lines shown in Figure 7(d). This reconnection process could be at the origin of the upflows located to the west of the inner QSL (at In). The projected extension of the reconnected red field lines does not match the apparent extension of the upflows; in fact, for this set of lines we would expect upflows to the east of this QSL trace. As done for the outer QSL, we integrated field lines anchored toward the west of the QSL trace. Such field lines, drawn in black in Figure 7(d), have a projected shape first directed to the west and then to the east. Plasma upflows along these lines would have the observed spatial distribution, i.e., we would observe stronger upflows close to the QSL trace fading toward the west. We also noticed that if we increased the absolute value of the α parameter in our LFFF model, keeping the same footpoints, the projected shape of the field lines matches the upflow extension better. Then, we attribute the discrepancy between upflows and field-line projected shapes to the limitation of our LFFF model that does not represent well the probably higher-sheared small-scale AR magnetic field.

Our results explain the origin of all the upflow regions, shown in Figure 6, for 2007 December 12 as being due to magnetic reconnection at QSLs. We have done similar detailed connectivity analyses for all of the upflows shown in that figure. They seem to originate either by reconnection within the outer QSLs, which are associated with the large-scale magnetic field configuration of the AR, or in the inner QSLs that develop as new bipoles emerge and evolve during the AR disk transit.

A two-step reconnection process was proposed by Mandrini et al. (2014b) to explain the way AR 10978 upflowing plasma could access open field lines and be observed by in situ instruments on board satellites at L1. In that article, we found evidence for the second step reconnection in the process, which opens the path for the closed-field confined plasma into the solar wind. Our present results offer proof of the first reconnection step, which is related to reconnection within the outer QSLs and provides a pathway for plasma originally confined along AR loops to flow into large-scale loops connecting to the quiet Sun. The latter is summarized in Figure 8. Furthermore, our findings of a coherent evolution between magnetic field, QSLs, and upflows prove the concept first proposed by Baker et al. (2009) to explain the origin of EIS persistent blueshifts. In this article, we go one step forward and suggest how, due to the photospheric field evolution, only sections of the QSLs in the AR are linked to persistent plasma upflows. Further evidence of a persistent reconnection process along QSLs in the AR can be revealed by radio observations of a persistent metric noise storm above AR 10978.

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Following our study in Mandrini et al. (2014b), in this section, we search for evidence of energy release at QSLs in wavelengths typical of radio noise-storms. Noise-storms consist of a broadband continuum emission (bandwidth ∼200 MHz, around a central frequency of hundreds of MHz) lasting for hours to several days. Superimposed on this continuum, bursts of short duration with much smaller bandwidth (∼1 MHz) are observed. The continuum component exhibits a slow intensity variability. Mercier et al. (2014) analyzed radio observations in the range from 150 to 450 MHz with high spatial resolution and they concluded that noise storms show an internal structure with one or several compact cores embedded in a more extended and tenuous halo. The emission is due to the presence of a suprathermal electron population (energies ranging from one to a few tens of keV) injected and trapped in extended coronal structures, i.e., noise storms require a mechanism that quasi-continously accelerates electrons in the solar corona.

The onsets of noise-storms or their enhancements are often related to changes in the overlying corona (Kerdraon et al. 1983) and to energy release in the underlying AR (Raulin & Klein 1994; Crosby et al. 1996). These characteristics suggest that the plasma-magnetic field configuration is restructuring at the time and the place where the noise-storm is produced (see the next section).

We analyze the radio emission registered by the Nançay Radio Heliograph (Kerdraon et al. 1997) during the transit of AR 10978 across the solar disk. At higher observation frequencies (327, 408, 432 MHz), the emission indicates the presence of weak noise storms that remain almost unchanged during the whole observing period. Figure 9 illustrates the evolution of the noise storm above the AR. Radio emission contours at 80% of the maximum intensity are displayed over the nearest in time MDI magnetogram; these contours were built using 5 minute integrated data. They show no clear trend to strengthen (by increasing their sizes) or, conversely, to fade out (by diminishing their sizes).

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At lower frequencies (150.9, 228 MHz), the radio emission contours embrace almost the full solar disk (not shown in Figure 9), indicating that the radiation corresponds to background coronal emission of thermal origin. Nançay Decameter Array (DAM, 10–80 MHz, Lecacheux 2000) did not observe Type III bursts, which are known to be caused by electrons that are accelerated outward along the open coronal field lines. DAM observes the upper corona from 0.7 to 3 R_⊙. The lack of Type III emission seems to indicate that accelerated particles were not injected into open coronal structures during the observing periods as the AR crossed the disk.

The global coronal magnetic field of CR 2064 was modeled in the PFSS approximation (Figure 10), assuming a current-free coronal field using a synoptic magnetogram as the photospheric boundary condition. To close the upper boundary, PFSS models assume that the field becomes purely radial at a given height, called the source surface. This is a free parameter usually set to the value 2.5 R_⊙. The PFSS model in this article was computed with the Finite Difference Iterative Potential-Field Solver code described by Tóth et al. (2011), using the corresponding MDI synoptic magnetogram for CR 2064 as the photospheric boundary condition.

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As shown in Figure 10, the noise-storm radio emission is concentrated over the closed AR field lines with an extension to the south due to the projection effect. NRH isocontours fully enclose the AR. This spatial relation suggests that the same magnetic reconnection process that drives EIS upflows may accelerate the electrons that flow along the closed reconnected field lines originating from the radio noise-storm emission.