Star Formation in the Nuclei of Normal Galaxies

Biermann, Peter L.

doi:10.1017/s1539299600003944

Cited by 7 publications

(7 citation statements)

References 131 publications

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“…We believe that such changes are attributed to the flare associated charges in the oscillation modes. The background power of flaring and dormant ARs are found to decrease with magnetic activity index which reinforce the idea that high magnetic fields could hinder convection (Biermann 1941;Chandrasekhar 1961), which is the source of solar noise.…”

Section: Discussionsupporting

confidence: 67%

Activity-related variations of high-degree p-mode amplitude, width, and energy in solar active regions

Maurya

Ambastha

Chae

2014

A&A

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Context. Solar energetic transients such as flares and coronal mass ejections occur mostly within active regions (ARs) and release large amounts of energy, which is expected to excite acoustic waves by transferring the mechanical impulse of the thermal expansion of the flare on the photosphere. On the other hand, strong magnetic fields of AR sunspots absorb the power of the photospheric oscillation modes. Aims. We study the properties of high-degree p-mode oscillations in flaring and dormant ARs and compare them with those in corresponding quiet regions (QRs) to find the association of the mode parameters with magnetic-and flare-related activities. Methods. We computed the mode parameters using the ring-diagram technique. The magnetic-activity indices (MAIs) of ARs and QRs were determined from the line-of-sight magnetograms. The flare indices (FIs) of ARs were obtained from the GOES X-ray fluxes. Mode parameters were corrected for foreshortening, duty cycle, and MAI using multiple non-linear regression. Results. Our analysis of several flaring and dormant ARs observed during the Carrington rotations 1980-2109 showed a strong association of the mode amplitude, width, and energy with magnetic and flare activities, although their changes are combined effects of foreshortening, duty cycle, magnetic-activity, flare-activity, and measurement uncertainties. We find that the largest reduction in mode amplitude and background power of an AR are caused by the angular distance of the AR from the solar disc centre. After correcting the mode parameters for foreshortening and duty cycle, we find that the mode amplitudes of flaring and dormant ARs are lower than in corresponding QRs reducing with increasing MAI, suggesting a stronger mode power suppression in ARs with larger magnetic fields. The mode widths in ARs are larger than in corresponding QRs and increase with MAI, indicating shorter lifetimes of modes in ARs than in QRs. The variations in mode amplitude and width with MAI are not same in different frequency bands. The largest amplification (reduction) in mode amplitude (mode width) of dormant ARs is found in the five-minute frequency band. The average mode energy of both the flaring and dormant ARs is smaller than in their corresponding QRs, reducing with increasing MAI. But the average mode energy reduction rate in flaring ARs is smaller than in dormant ARs. Moreover, the increase in mode width rate in dormant (flaring) ARs is followed by a decrease (increase) in the amplitude variation rate. Furthermore, including the mode corrections for MAI shows that mode amplitude and mode energy of flaring ARs escalate with FI, while the mode width shows an opposite trend, suggesting excitations of modes and growth in their lifetimes by flares. The increase (decrease) in mode amplitude (width) is larger in the five-minute and higher-frequency bands. The enhancement in width variation rate is followed by a rapid decline in the amplitude variation rate.

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Section: Discussionsupporting

confidence: 67%

Activity-related variations of high-degree p-mode amplitude, width, and energy in solar active regions

Maurya

Ambastha

Chae

2014

A&A

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“…Umbral dots (UDs) are small brightness enhancements in sunspot umbrae or pores and were first detected by Chevalier (1916). The strong vertical magnetic field in umbrae suppresses the energy transport by convection (Biermann, 1941), but some form of remaining heat transport is needed to explain the observed umbral brightness (Adjabshirzadeh & Koutchmy, 1983). Magnetoconvection in umbral fine structure, such as UDs and light bridges, is thought to be the main contributor to the energy transport in the umbra (Weiss, 2002), see reviews by Solanki (2003), Sobotka (2006), and Borrero & Ichimoto (2011).…”

Section: Introductionmentioning

confidence: 99%

Vertical flows and mass flux balance of sunspot umbral dots

Riethmüller

Solanki

Noort

et al. 2013

A&A

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A new Stokes inversion technique that greatly reduces the effect of the spatial point spread function of the telescope is used to constrain the physical properties of umbral dots (UDs). The depth-dependent inversion of the Stokes parameters from a sunspot umbra recorded with Hinode SOT/SP revealed significant temperature enhancements and magnetic field weakenings in the core of the UDs in deep photospheric layers. Additionally, we found upflows of around 960 m/s in peripheral UDs (i.e., UDs close to the penumbra) and ≈600 m/s in central UDs. For the first time, we also detected systematic downflows for distances larger than 200 km from the UD center that balance the upflowing mass flux. In the upper photosphere, we found almost no difference between the UDs and their diffuse umbral background.

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“…At higher energies, several models have been proposed, most notably a galactic wind termination shock (Jokipii & Morfill 1987), and multiple shocks in an ensemble of OB superbubbles and young supernova remnants (Axford 1992). A comprehensive, albeit tentative, theory has been proposed by Biermann (1993 and later papers) that explains the cosmic ray spectrum, with its chemical abundances and the knee feature as resulting from a combination of supernova explosions into the interstellar medium, and supernova explosions into strong stellar winds of progenitor Wolf-Rayet stars (Biermann 1993;Biermann & Cassinelli 1993;Biermann 1995). Above the "ankle" at about 3 EeV (the ultra-high energy cosmic ray regime, called UHECR hereafter), a simultaneous change in spectrum and composition of the cosmic ray spectrum (Bird et al 1994) suggests a change of origin.…”

Section: Introductionmentioning

confidence: 99%

Contributions to the cosmic ray flux above the ankle: clusters of galaxies

Kang

Rachen

Biermann

1997

Monthly Notices of the Royal Astronomical Society

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Motivated by the suggestion of Kang, Ryu & Jones (1996) that particles can be accelerated to high energies via diffusive shock acceleration process at the accretion shocks formed by the infalling flow toward the clusters of galaxies, we have calculated the expected particle flux from a cosmological ensemble of clusters. We use the observed temperature distribution of local clusters and assume a simple power-law evolutionary model for the comoving density of the clusters. The shock parameters such as the shock radius and velocity are deduced from the ICM temperature using the self-similar solutions for secondary infall onto the clusters. The magnetic field strength is assume to be in equipartition with the postshock thermal energy behind the accretion shock. We also assume that the injected energy spectrum is a power-law with the exponential cutoff at the maximum energy which is calculated from the condition that the energy gain rate for diffusive shock acceleration is balanced by the loss rate due to the interactions with the cosmic background radiation. In contrast to the earlier paper we have adopted here the description of the cosmic ray diffusion by Jokipii (1987) which leads to considerably higher particle energies. Finally the injected particle spectrum at the clusters is integrated over the cosmological distance to earth by considering the energy loss due to the interactions with the cosmic background radiation. Our calculations show that the expected spectrum of high-energy protons from the cosmological ensemble of the cluster accretion shocks could match well the observed cosmic ray spectrum near 10 19 eV with reasonable parameters and models if about 10 −4 of the infalling kinetic energy can be injected into the intergalactic space as the high energy particles.

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Star Formation in the Nuclei of Normal Galaxies

Cited by 7 publications

References 131 publications

Activity-related variations of high-degree p-mode amplitude, width, and energy in solar active regions

Activity-related variations of high-degree p-mode amplitude, width, and energy in solar active regions

Vertical flows and mass flux balance of sunspot umbral dots

Contributions to the cosmic ray flux above the ankle: clusters of galaxies

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