[Paper Review] Network and Internetwork: a compared Multiwavelength Analysis

G. Cauzzi, A. Falchi+1 more|arXiv (Cornell University)|Apr 6, 2000

ICT Impact and Policies3 citations

TL;DR

This study uses coordinated ground-based (NSO/Sacramento Peak) and SOHO/MDI observations to analyze Network Bright Points (NBPs) across multiple atmospheric layers. It demonstrates that NaD 2 line center emission in NBPs is a reliable proxy for magnetic flux density, with a power-law relationship (β ≈ 0.58), and reveals distinct oscillation behavior: reduced power at p-mode frequencies in the chromosphere (NaD 2), a prominent 2.2 mHz peak in Hα core not coherent with photospheric oscillations, and evidence of downward-propagating waves in internetwork regions.

ABSTRACT

We analyze the temporal behavior of Network Bright Points (NBPs), present in the solar atmosphere, using a set of data acquired during coordinated observations between ground-based observatories (mainly at the NSO/Sacramento Peak) and the Michelson Doppler Interferometer onboard SOHO. We find that, at any time during the observational sequence, all the NBPs visible in the NaD2 images are co-spatial within 1 arcsec with locations of enhanced magnetic field. In analogy with the Ca II K line, the NaD2 line center emission can be used as a proxy for magnetic structures. We also compare the oscillation properties of NBPs and internetwork areas. At photospheric levels no differences between the two structures are found in power spectra, but analysis of phase and coherence spectra suggests the presence of downward propagating waves in the internetwork. At chromospheric levels some differences are evident in the power spectrum between NBPs and internetwork. The power spectrum of NBPs at the Halpha core wavelength shows an important peak at 2.2 mHz (7 minutes), not present in the internetwork areas. Its coherence spectrum with Halpha wings shows very low coherence at this frequency, implying that the oscillations at these chromospheric levels are not directly coupled with those present in lower layers.

Motivation & Objective

• To establish a quantitative relationship between NaD2 line excess intensity and magnetic flux density in solar network bright points (NBPs).
• To compare the temporal dynamics of NBPs and internetwork regions across multiple atmospheric layers using multiwavelength data.
• To investigate the coherence and phase relationships between oscillations in photospheric, chromospheric, and transition region layers.
• To determine whether the 2.2 mHz oscillation peak in Hα core is physically linked to photospheric p-mode oscillations.
• To assess the role of magnetic fields in modifying wave propagation and energy transport in the solar atmosphere.

Proposed method

• Acquired coordinated observations using the NSO/Sacramento Peak Dunn Solar Telescope (UBF, Zeiss, White Light) and SOHO/MDI for simultaneous multiwavelength coverage.
• Used narrowband filters to isolate NaD2 (5889.9 Å), Hα core (6562.8 Å), and Hα wings (±1.5 Å) for chromospheric intensity measurements.
• Measured temporal intensity fluctuations in NBPs and surrounding internetwork regions to compute power, phase difference, and coherence spectra.
• Applied power-law fitting to correlate NaD2 excess intensity with longitudinal magnetic flux density from MDI magnetograms.
• Analyzed coherence between oscillations in Hα core and photospheric layers to assess wave coupling and propagation.
• Performed spatial co-registration between NaD2, Hα, and magnetic field data to ensure accurate comparison of physical structures.

Experimental results

Research questions

• RQ1 Is there a quantitative relationship between NaD2 line center excess intensity and magnetic flux density in solar network bright points?
• RQ2 How do the oscillation properties (power, phase, coherence) of NBPs differ from those of internetwork regions across photospheric and chromospheric layers?
• RQ3 What is the origin and physical significance of the 2.2 mHz peak observed in the Hα core power spectrum of NBPs?
• RQ4 Are the chromospheric oscillations in NBPs coherent with photospheric p-mode oscillations, and what does this imply about wave propagation?
• RQ5 How does the presence of magnetic fields alter wave dynamics compared to non-magnetic internetwork regions?

Key findings

• The excess NaD2 intensity in NBPs shows a power-law dependence on magnetic flux density with exponent β = 0.58 ± 0.1, closely matching the established Ca II K relationship (β ≈ 0.6), confirming NaD2 as a reliable proxy for magnetic flux.
• All NBPs visible in NaD2 images are co-spatial (within 1″) with enhanced magnetic field regions, confirming their magnetic origin.
• At chromospheric levels contributing to NaD2 emission, NBPs exhibit strongly reduced oscillation amplitudes at p-mode frequencies compared to the photosphere.
• In Hα core emission, NBPs show a prominent power peak at 2.2 mHz (7 minutes), absent in internetwork regions, indicating a distinct chromospheric oscillatory mode.
• The 2.2 mHz oscillation in Hα core shows very low coherence with photospheric fluctuations, indicating decoupling and suggesting a non-photospheric excitation mechanism.
• Internetwork regions show evidence of downward-propagating waves, as indicated by phase and coherence spectra, while NBPs exhibit lower overall coherence, implying magnetic suppression of wave coupling.