Signature of the turbulent component of the solar dynamo on active region scales and its association with flaring activity

ABSTRACT It is a challenging problem to obtain observational evidence of the turbulent component of solar dynamo operating in the convective zone because the dynamo action is hidden below the photosphere. Here we present results of a statistical study of flaring active regions (ARs) that produced strong solar flares of an X-ray class X1.0 and higher during a time period that covered solar cycles 23 and 24. We introduced a magneto-morphological classification of ARs, which allowed us to estimate the possible contribution of the turbulent component of the dynamo into the structure of an AR. We found that in 72 per cent of cases, flaring ARs do not comply with the empirical laws of the global dynamo (frequently they are not bipolar ARs or, if they are, they violate the Hale polarity law, the Joy law, or the leading sunspot prevalence rule). This can be attributed to the influence of the turbulent dynamo action inside the convective zone on spatial scales of typical ARs. Thus, it appears that the flaring is governed by the turbulent component of the solar dynamo. The contribution into the flaring from these AR ‘violators’ (irregular ARs) is enhanced during the second maximum and the descending phase of a solar cycle, when the toroidal field weakens and the influence of the turbulent component becomes more pronounced. These observational findings are in consensus with a concept of the essential role of non-linearities and turbulent intermittence in the magnetic fields generation inside the convective zone, which follows from dynamo simulations.

Magnetizing the circumgalactic medium of disc galaxies

ABSTRACT The circumgalactic medium (CGM) is one of the frontiers of galaxy formation and intimately connected to the galaxy via accretion of gas on to the galaxy and gaseous outflows from the galaxy. Here, we analyse the magnetic field in the CGM of the Milky Way-like galaxies simulated as part of the auriga project that constitutes a set of high-resolution cosmological magnetohydrodynamical zoom simulations. We show that before z = 1 the CGM becomes magnetized via galactic outflows that transport magnetized gas from the disc into the halo. At this time, the magnetization of the CGM closely follows its metal enrichment. We then show that at low redshift an in situ turbulent dynamo that operates on a time-scale of Gigayears further amplifies the magnetic field in the CGM and saturates before z = 0. The magnetic field strength reaches a typical value of $0.1\, \mu \mathrm{ G}$ at the virial radius at z = 0 and becomes mostly uniform within the virial radius. Its Faraday rotation signal is in excellent agreement with recent observations. For most of its evolution, the magnetic field in the CGM is an unordered small-scale field. Only strong coherent outflows at low redshift are able to order the magnetic field in parts of the CGM that are directly displaced by these outflows.

Analysis of quiet-sun turbulence on the basis of SDO/HMI and goode solar telescope data

ABSTRACT We analysed line-of-sight magnetic fields and magnetic power spectra of an undisturbed photosphere using magnetograms acquired by the Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamic Observatory and the Near InfraRed Imaging Spectrapolarimeter (NIRIS) operating at the Goode Solar Telescope of the Big Bear Solar Observatory. In the NIRIS data, we revealed thin flux tubes of 200–400 km in diameter and of 1000–2000 G field strength. The HMI power spectra determined for a coronal hole, a quiet sun, and a plage areas exhibit the same spectral index of −1 on a broad range of spatial scales from 10–20 Mm down to 2.4 Mm. This implies that the same mechanism(s) of magnetic field generation operate everywhere in the undisturbed photosphere. The most plausible one is the local turbulent dynamo. When compared to the HMI spectra, the −1.2 slope of the NIRIS spectrum appears to be more extended into the short spatial range until the cut-off at 0.8–0.9 Mm, after which it continues with a steeper slope of −2.2. Comparison of the observed and Kolmogorov-type spectra allowed us to infer that the Kolmogorov turbulent cascade cannot account for more than 35 per cent of the total magnetic energy observed in the scale range of 3.5–0.3 Mm. The energy excess can be attributed to other mechanisms of field generation such as the local turbulent dynamo and magnetic superdiffusivity observed in an undisturbed photosphere that can slow down the rate of the Kolmogorov cascade leading to a shallower resulting spectrum.