Intranetwork Fields Research Articles

Intensity variations inside supergranules

Context: The convective origin of supergranulation is highly controversial. Past measurements of intensity variations inside supergranules have often been influenced by the brightness enhancement at the cell boundaries due to the magnetic network. Aims: We conduct a precise determination of intensity variations inside supergranules. Methods: We determine the supergranule cell boundary from smoothed divergence maps derived from horizontal flow maps. We derive these flow maps from intensity maps obtained by MDI/SOHO in high resolution mode. We discuss the different possible approaches to take into account the influence of the magnetic field which can be used to determine the intensity variations inside supergranules. Results: We observe a significant decrease of the intensity from the center to the boundary of supergranules. We also obtain additional clues from the behavior of the maximum intensities and minimum intensities around each pixel, which are related to granules and intergranules: the maximum intensity decreases from center to boundary, while the minimum intensity is constant or increases depending how restrictive the selection is. The difference between intensity profiles versus divergence and relative distance to cell center also provides complementary information. The corresponding temperature differences between cell center and boundary are in the range 0.8-2.8 K. The intensity enhancement (for the magnetic network) or deficit (for intranetwork fields) depends on the localisation inside the cell. Conclusions: .It is the first time that such a detailed analysis of intensity variations inside supergranulation is performed. Our results are compatible with a convective origin of supergranulation, as the intensity decreases toward the boundary of the cells. However, new simulations of supergranulation are necessary to verify whether the compared behavior of granule and intergranule intensity variations is in close agreement with convection.

A New Mechanism of Coronal Heating

The interaction between network magnetic fields and emerging intranetwork fields may lead to magnetic reconnection and microflares, which generate fast shocks with an Alfven Mach number MA<2. Protons and less abundant ions in the solar corona are then heated and accelerated by fast shocks. Our study of shock heating shows that (a) the nearly nondeflection of ion motion across the shock ramp leads to a large perpendicular thermal velocity (vth⊥), which is an increasing function of the mass/charge ratio; (b) the heating by subcritical shocks with 1.1 ≤ MA ≤ 1.5 leads to a large temperature anisotropy with T⊥/T∥ ≈ 50 for O5+ ions and a mild anisotropy with T⊥/T∥ ≈ 1.2 for protons; (c) the large perpendicular thermal velocity of He++ and O5+ ions can be converted to the radial outflow velocity (u) in the divergent coronal field lines; and (d) the heating and acceleration by shocks with 1.1 ≤ MA ≤ 1.5 can lead to u(O5+) ≈vth⊥(O5+) ≈ 460 km s−1 for O5+ ions, u(He++) ≈vth⊥(He++) ≈ 360 km s−1 for He++ ions, and u(H+) ≈vth⊥(H+) ≈ 240 km s−1 for protons at r=3–4 R⊙. Our results can explain recent SOHO observations of the heating and acceleration of protons and heavier ions in the solar corona.

Heating and Acceleration of Protons and Minor Ions by Fast Shocks in the Solar Corona

We propose a new mechanism of coronal heating, in which protons and minor ions are heated and accelerated by fast shocks. The interaction between network magnetic fields and emerging intranetwork fields may lead to the formation of thin current sheets, which triggers magnetic reconnection and microflares. The disruption of current sheet leads to fast shocks with an Alfven Mach number MA < 2. In addition, fast shocks that originate in the chromosphere by magnetic reconnection of a smaller scale may produce spicules and then enter the corona. The heating and acceleration by shocks are then examined based on a theoretical model and hybrid simulations. The results show the following: (1) the near nondeflection of ion motion across the shock ramp leads to a large perpendicular thermal velocity (vth⊥), which depends on the mass/charge ratio; (2) for subcritical shocks with 1.1 ≤ MA ≤ 1.5, the shock heating leads to a large temperature anisotropy with T⊥/T|| ≈ 50 for O5+ ions and a mild anisotropy with T⊥/T|| ≈ 1.2 for protons; (3) these subcritical shocks can directly drive an outward field-aligned velocity ~ 0.1VA ~ 240 km s-1 for protons; (4) the large perpendicular thermal velocity of O5+ ions can be converted to the radial outflow velocity (u) in the diverging coronal field lines; and (5) the heating and acceleration by shocks with 1.1 ≤ MA ≤ 1.5 can lead to u(O5+) ≈ vth⊥(O5+) ≈ 460 km s-1 for O5+ ions and u(H+) ≈ vth⊥(H+) ≈ 240 km s-1 for protons at r = 3-4 R☉. Our results can explain recent Solar and Heliospheric Observatory observations of the heating and acceleration of protons and minor ions in the solar corona.

Studies of solar intranetwork fields and current helicity in active regions

In this paper we begin with a brief review of the most recent observational studies of intranetwork elements. Next by analyzing well observed quantities of other magnetic elements, we suggest that the magnetic flux of all elements is proportional to its lifetime, mainly depending on each size. Then we present an empirical way to extrapolate the size and magnetic field of intranetwork elements, and find that their average size is about 210 km and the corresponding field strength is 350 G. Finally, our studies of current helicity in active regions are summarized as follows: (1) most of the active regions in solar cycle 22 follow the hemispheric helicity sign rule, and the evolution of surface large-scale current helicity has a good correlation with solar activity; (2) the active regions with reversed helicity sign tend to emerge at certain longitudes; (3) the rapid and substantial changes of current helicity in an area or in its vicinity are most likely to trigger flares.

The Granular Magnetic Fields of the Quiet Sun

We report new observations that combine high-precision infrared polarimetry and high-resolution imagery in the visible to demonstrate that most of the quiet solar surface contains a measurable magnetic field. We found that when observed at 1 arcsec2 resolution, 68% of the observed area contains magnetic flux higher than 5×1015 Mx (corresponding to an apparent average field of 1 G). The majority of these magnetic features have magnetic flux below 5×1016 Mx. Their magnetic field strengths range from below 200 to 1000 G, which means that their filling factors are on the order of 1%. The spatial distribution and time evolution of these magnetic features are closely associated with the solar granulation. The properties of these weak granular magnetic features we observed differ from those of the intranetwork fields described in earlier observations. We also observed the formation and disappearance of a kilogauss magnetic feature associated with the development of intergranular lanes, which may be evidence of convective collapse.

Power Spectra of Solar Network and Non-Network Fields

We report new properties of solar magnetic fields in a quiet region as found from their magnetic power spectra. The power spectra of network and intranetwork fields (non-network fields) are separately calculated from a Big Bear magnetogram obtained with moderately high spatial resolution of 1.5 arc sec and a high sensitivity reaching 2 Mx cm-2. The effect of seeing on the power spectrum has been corrected using Fried's (1966) Modulation Transfer Function with the seeing parameter determined in our previous analysis of the magnetogram. As a result, it is found that the two-dimensional power spectra of network and non-network fields appear in a form: Γ( $$k_0 $$ ≲ $$k $$ ≲ $$k $$ 1) ∼ $$k $$ -1 and Γ( $$k $$ ≳ $$k $$ 1) ∼ $$k $$ -3.5. Here $$k $$ 0 ≈ 0.47 Mm-1 for network fields and $$k $$ 0 ≈ 0.69 Mm-1 for non-network fields, the latter of which corresponds to the size of mesogranulation; $$k $$ 1 ≈ 3.0 Mm-1 for both, which is about the size of a large granule. The network field spectrum below $$k $$ 0 appears nearly flat, whereas that of non-network fields instead decreases towards lower wave numbers as Γ( $$k $$ ) ∼ $$k $$ 1.3. The turnover behavior of magnetic field spectra around $$k $$ 1 coincides with that found for the velocity power spectrum, which may justify the kinetic approach taken in previous theoretical studies of the solar magnetic power spectra.

The mystery of the chromosphere

We discuss many aspects of the solar chromosphere from an observational point of view, and show that most existing models are in direct contradiction to radio and eclipse measurements. We plead for attention to the actual observed radio temperatures and density gradients, as well as images of the chromosphere. We find that the chromosphere is not in hydrostatic equilibrium and suggest that the support is due to the tangled intranetwork fields.

Fast appearances and disappearances of weak intranetwork magnetic fields

Under good seeing conditions (<2′), the high resolution magnetic field observation at Huairou Solar Observing Station'is capable of detecting the intranetwork field. An analysis of some magnetograms of 1988 September 5 shows: 1. That the observed weak intranetwork magnetic fields (WINF) with maximum flux B between 5 and 10 G are real structures. 2. That About 70% of the WINF became undetectable at ± 5 G level after 3.4 min and about 30% can be seen again. 3. That the number of WINF seen decreased exponentially with time, from which we estimated an upper limit of about 3 min for the mean life of a WINF. 4. That a WINF is a collection of unresolved flux elements, with a flux between 2.0(+16) and 1.0(+18) mx. Our results are similar to those in the magnetograms of 1984 September 9 obtained by the Big Bear Solar Observatory.

The identification and interaction of network, intranetwork, and ephemeral-region magnetic fields

Network magnetic fields, ephemeral active regions, and intranetwork magnetic fields are illustrated and discussed in several contexts. First, they are presented in relation to the appearance and disappearance of magnetic flux. Second, their properties in common with all solar magnetic features are discussed. Third, their distinguishing characteristics are emphasized. Lastly, their interactions are illustrated. Network magnetic fields are no longer considered to be just the aged remnants of active regions. The network is the dynamic product of the merging and cancelling of intranetwork fields, ephemeral regions, and the remnants of active regions. Intranetwork fields are magnetic fields of mixed polarity that appear to originate continuously from localized source sites in between the network. The intranetwork magnetic fields are characterized by flow of successive fragments in approximately radial patterns away from their apparent source sites and by the relative weakness of their magnetic fields. Ephemeral active regions are small, new bipoles that grow as a unit or a succession of bipolar units and whose poles move in opposite directions from their apparent site of origin. Large ephemeral regions are not distinguishable from small active regions.