---
abstract: 'H emission is a well-known indicator of magnetic activity in the Sun and other stars. It is also viewed as an important signature of chromospheric heating. However, the H line has not been used as a diagnostic of magnetic flux emergence from the solar interior. Here we report on Hinode observations of chromospheric H brightenings associated with a repeated, small-scale flux emergence event. We describe this process and investigate the evolution of the magnetic flux, G-band brightness, and H intensity in the emerging region. Our results suggest that energy is released in the chromosphere as a consequence of interactions between the emerging flux and the pre-existing magnetic field, in agreement with recent 3D numerical simulations.'
author:
- 'S. L. Guglielmino and F. Zuccarello'
- 'P. Romano'
- 'L. R. Bellot Rubio'
title: |
    HINODE Observations of Chromospheric Brightenings in the\
    H Line during small-scale Flux Emergence Events
---

Introduction
============

Numerical simulations predict that magnetic flux emerging into the solar atmosphere interact and reconnect with the pre-existing chromospheric and coronal field. This suggests that flux emergence is a relevant source of energy for the chromosphere [@Archontis:04; @Archontis:05]. The efficiency of the interaction and the consequent heating seem to depend on the geometry of the two flux systems [@Galsgaard:07]. While at large scale these results have been confirmed by high-resolution observations [@Moreno:08; @Zuccarello:08], the role of small-scale emergence events in the heating of the upper atmospheric layers, as described by @Isobe:08, is still lacking observational confirmation.

In the absence of other diagnostics, heating events in the chromosphere can be detected through the intensity profiles of the H and K lines. The correlation between flux emergence and chromospheric emission was analyzed in detail by @Balasu:01, who observed anomalous profiles in the K line in an emerging active region. However, very few examples of small-scale transient brightenings have been reported in the literature, and most of them are related to flux cancellation events [e.g. @Bellot:05].

The *Hinode* satellite, with its unprecedented spatial resolution, offers for the first time the possibility to investigate the processes that occur in the chromosphere during the emergence of magnetic flux at small spatial scales. In this Letter we analyze simultaneous chromospheric and photospheric observations of an emerging flux region taken with the Solar Optical Telescope aboard *Hinode*. During the emergence event, strong brightenings were detected in the H line core without relevant counterparts in G-band intensity, which suggests that chromospheric heating did occur at the site of flux emergence.

Observations and data reduction
===============================

On 2007 September 30, as part of the *Hinode* Operation Plan 14 (*Hinode*/Canary Islands Campaign), the active region NOAA 10971 was observed by the Solar Optical Telescope [SOT; @Tsuneta:08] onboard *Hinode* [@Kosugi:07]. The field of view (FoV) was centered at solar coordinates ($174\arcsec$, $-79\arcsec$), i.e., $11^{\circ}$ away from disk center.

The SOT spectro-polarimeter [SP; @Tsuneta:08] performed six raster scans of the active region from 08:00 to 14:00 UT, acquiring the Stokes I, Q, U, and V profiles of the photospheric lines at 630.15 nm and 630.25 nm. The FoV covered by the SP observations is $164\arcsec \times 164\arcsec$, with an effective pixel size of $0.32\arcsec$ (Fast Map mode). Simultaneously, the SOT Broadband Filter Imager (BFI) acquired filtergrams in the core of the H line ($396.85 \pm 0.3 \;\textrm{nm}$) and in the G band ($430.5 \pm 0.8 \;\textrm{nm}$), while the Narrowband Filter Imager (NFI) obtained shuttered Stokes I and V filtergrams in the wings of the D1 line at $589.6 \;\textrm{nm}$. The BFI images have a spatial sampling of $0.05\arcsec$/pixel (G-band) and $0.1\arcsec$/pixel ( H), while that of the NFI filtergrams is $0.16\arcsec$/pixel. The BFI and NFI time series have a cadence of one minute and extend from 07:00 to 17:00 UT, with a small gap between 10:05 and 10:20 UT.

We have corrected the SOT/SP and SOT/FG images for dark current, flat field, and cosmic rays with standard SolarSoft routines. Besides obtaining photospheric and chromospheric information through corrected G-band and H filtergrams, we have constructed magnetograms from the D1 Stokes I and V images acquired $\pm 156$ mÅ off the line center. From the ratio $$\frac{V}{I}=\frac{1}{2}\left( \frac{V_{\rm blue}}{I_{\rm blue}} + \frac{V_{\rm red}}{I_{\rm red}} \right)$$ we calculate the magnetic flux density $\Phi_{\rm d}$ using the weak fied approximation [@Stix:02] as $$\Phi_{\rm d}= 8 \times 10^3 \: \frac{V}{I} \;\;\; [\textrm{Mx cm}^{-2}]$$ [see @Guglielmino:08]. To first order, the magnetograms computed in this way are not affected by Doppler shifts. We remind the reader that, at disk center, the D1 line refers to the upper photospheric layers and not to the chromosphere.

For each raster scan of the SP, the profile with the minimum total polarization degree, $P=\left[\left(Q^{2}+U^{2}+V^{2}\right)/I^{2}\right]^{1/2}$, was selected as a reference profile. All the spectra in the scan were normalized to the continuum of this profile and corrected for limb darkening. Also, a stray light profile was computed by averaging the reference profiles of the six scans.

Adopting a grid paradigm, we have inverted the spectra with $P > 2\%$ using the SIR code [@RuizIniesta:92]. The inversion yields the temperature stratification in the range $-4.0 < \log \, \tau_{5} < 0$ ($\tau_{5}$ is the optical depth of the continuum at $500
\;\textrm{nm}$), together with the magnetic field strength, inclination and azimuth angles in the line-of-sight (*los*) reference frame, the *los* velocity, and the magnetic filling factor, assuming these quantities to be constant with height. Azimuth and inclination angles have been transformed to the local solar frame, whereas the *los* velocity has been calibrated using the mean quiet-Sun intensity profile computed from pixels with $P < 0.5\%$, following the procedure of @Marti:97.

Finally, all the SOT/FG and SOT/SP images have been aligned through cross-correlation algorithms.

Results
=======

NOAA 10971 has a classical bipolar $\beta$ configuration, as can be seen in the D1 magnetogram of Fig. \[fig1\]. The various SOT instruments recorded the emergence of a small bipolar region, which appeared at the internal edge of the main negative polarity. Figure \[fig2\] displays a temporal sequence of D1 magnetograms with a cadence of about 20 minutes for the $8 \times 8$ Mm$^2$ area marked in Fig. \[fig1\]. The first magnetogram of the sequence, acquired at 07:50 UT, shows the presence of a positive-polarity knot. In the following magnetograms we clearly recognize an increase in its area, as well as the appearance of a negative-polarity patch. The subsequent evolution is characterized by the separation of the opposite magnetic polarities, as indicated by the arrows. The corresponding temporal sequence of H filtergrams is also displayed in Fig. \[fig2\] and shows transient brightness enhancements at the location of the positive footpoint. This emergence event led to the appearance of bright points in the G band (Fig. \[fig3\], left panel) and to intensity enhancements in the H line core (Fig. \[fig3\], right panel).

Maps of the physical parameters derived from the SP raster scans are displayed in Fig. \[fig4\]. They demonstrate the rapid evolution of the small bipolar region: the changes in, e.g., the magnetic field distribution (second and third rows) indicate a very dynamic phase. The small bipolar region shows an emergence zone, i.e., a region between the two main polarities with horizontal fields [@Lites:98] in which upflows of $\sim 1 \;\textrm{km s}^{-1}$ can be seen at 9:18 UT and 12:23 UT. The footpoints of the emerging region exhibit vertical fields and downflows of $1.5 - 2 \;\textrm{km
s}^{-1}$. The initial photospheric total flux content of the emerging region is $1.4 \times 10^{19} \;\textrm{Mx}$, classifying as a small ephemeral region. The bipole axis was inclined about $45
\degr$ to the north-south direction in the first raster scan, but this angle varied with time. The negative-polarity footpoint soon merged with the dominant negative flux of the active region, disappearing as an individual feature.

In this area we have detected chromospheric H brightness enhancements with two main peaks during the observations, each one preceded by a minor peak. As can be seen in Fig. \[fig2\], the brightenings are associated with the positive-polarity footpoint. The duration of each peak is about half an hour, with an enhancement of $\sim 80\%$ with respect to the “quiet” level. The presence of these peaks points to interactions between the new emerging and the old pre-existing flux systems. We have computed the average intensity of the four most luminous pixels within the $8 \times 8 \;\textrm{Mm}^{2}$ FoV for the H and G-band filtergrams, respectively. Figure \[fig5\] shows the trend of brightness in H and G band in normalized units. The different behaviour indicates that they are not correlated, as the chromospheric brightness enhancements are much more intense than the increase observed in the G band at the same times. Thus, the observed H enhancements are genuinely due to photons coming from the chromosphere, and not to the significant photospheric contribution included in the passband of the SOT H filter [@Carlsson:07].

We have calculated the positive flux in the $8 \times 8$ Mm$^2$ area using the D1 magnetograms, which have a noise level of approximately $6.75 \times 10^{14} \;\textrm{Mx/pixel}$. The main contribution to the positive flux comes from the positive polarities of the emerging region. In Fig. \[fig6\] we show the flux evolution with time: the chromospheric brightness enhancements clearly correspond to an increase of positive magnetic flux in the upper photosphere. Interestingly, in both cases the maximum Ca II H intensities are reached some 30 minutes after the positive flux starts to increase.

Taking into account the temporal coincidence between the chromospheric brightenings and the positive flux increase, as well as the spatial coincidence between the location and morphology of the H brightenings and the emerging bipolar region (Fig. \[fig2\]; compare also the second, third, and fifth rows of Fig. \[fig4\]), we conclude that the localized chromospheric heating is a consequence of the emergence and subsequent interaction of the positive flux of the new bipole, which cancels with the negative ambient magnetic field.

Conclusions
===========

Using *Hinode* filtergrams and spectropolarimetric measurements, we have studied a small-scale flux emergence event. Two peaks of chromospheric origin have been detected in the H line-core intensity almost simultaneously to a magnetic flux increase in the upper photosphere.

We suggest that the chromospheric brightness enhancements may be indication that two different flux systems undergo magnetic reconnection: the old flux system belonging to the active region and the emerging magnetic field. The energy released in the process heats the chromosphere. The observed H brightenings are associated with a relatively modest amount of emerged magnetic flux (only $\sim 4 \times 10^{18} \;\textrm{Mx}$ compared with the total negative flux in the region of $\sim 2.5 \times 10^{19} \;\textrm{Mx}$), which points to a highly efficient heating mechanism. We conjecture that, while the positive flux increases, part of it cancels with the pre-existing negative flux, very likely in a process of magnetic reconnection.

Our result suggests that Joule dissipation may be a significant source of chromospheric heating during the reconnection of an emerging flux system with a pre-existing magnetic field. This would confirm the predictions of recent numerical simulations [e.g. @Galsgaard:05], also at small scales. Moreover, our work suggests that H brightness enhancements can be used as a valuable diagnostics of flux emergence. Further investigations should put this result into a firm observational and theoretical basis.

Financial support by the European Commission through the SOLAIRE Network (MTRN-CT-2006-035484) is gratefully acknowledged. This work has been partly funded by the Spanish Ministerio de Educación y Ciencia through projects ESP2006-13030-C06-02, PCI2006-A7-0624, and Programa de Acceso a Infraestructuras Científicas y Tecnológicas Singulares. *Hinode* is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in co-operation with ESA and NSC (Norway).

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