Flux Emergence (Theory) Cheung, Mark C. M.; Isobe, Hiroaki
Living Reviews in Solar Physics,
01/2014, Letnik:
11, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Magnetic flux emergence from the solar convection zone into the overlying atmosphere is the driver of a diverse range of phenomena associated with solar activity. In this article, we introduce ...theoretical concepts central to the study of flux emergence and discuss how the inclusion of different physical effects (e.g., magnetic buoyancy, magnetoconvection, reconnection, magnetic twist, interaction with ambient field) in models impact the evolution of the emerging field and plasma.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
We studied the acceleration mechanisms of chromospheric jets associated with emerging flux using a two-dimensional magnetohydrodynamic (MHD) simulation. We found that slow-mode shock waves generated ...by magnetic reconnection in the chromosphere and the photosphere play key roles in the acceleration mechanisms of chromospheric jets. An important parameter is the height of magnetic reconnection. When magnetic reconnection takes place near the photosphere, the reconnection outflow collides with the region where the plasma beta is much larger than unity. Then, the plasma moves along a magnetic field. This motion generates a slow-mode wave. The slow-mode wave develops to a strong slow shock as it propagates upward. When the slow shock crosses the transition region, this region is lifted up. As a result, we obtain a chromospheric jet as the lifted transition region. When magnetic reconnection takes place in the upper chromosphere, the chromospheric plasma is accelerated due to the combination of the Lorentz force and the whip-like motion of the magnetic field. We found that the chromospheric plasma is further accelerated through the interaction between the transition region (steep density gradient) and a slow shock emanating from the reconnection point. In the process, the magnetic energy released by magnetic reconnection is efficiently converted into the kinetic energy of jets. This is an MHD effect that has not been discussed before.
The launch of the Hinode satellite led to the discovery of rising plumes, dark in chromospheric lines, that propagate from large (~10 Mm) bubbles that form at the base of quiescent prominences. The ...plumes move through a height of approximately 10 Mm while developing highly turbulent profiles. The magnetic Rayleigh-Taylor instability was hypothesized to be the mechanism that drives these flows. In this study, using three-dimensional (3D) MHD simulations, we investigate the nonlinear stability of the Kippenhahn-Schluter prominence model for the interchange mode of the magnetic Rayleigh-Taylor instability. The model simulates the rise of a buoyant tube inside the quiescent prominence model, where the interchange of magnetic field lines becomes possible at the boundary between the buoyant tube and the prominence. Hillier et al. presented the initial results of this study, where upflows of constant velocity (maximum found 6 km s super(-1)) and a maximum plume width approx =1.5 Mm which propagate through a height of approximately 6 Mm were found. Nonlinear interaction between plumes was found to be important for determining the plume dynamics. In this paper, using the results of ideal MHD simulations, we determine how the initial parameters for the model and buoyant tube affect the evolution of instability. We find that the 3D mode of the magnetic Rayleigh-Taylor instability grows, creating upflows aligned with the magnetic field of constant velocity (maximum found 7.3 km s super(-1)). The width of the upflows is dependent on the initial conditions, with a range of 0.5-4 Mm which propagate through heights of 3-6 Mm. These results are in general agreement with the observations of the rising plumes.
Can Superflares Occur on Our Sun? Shibata, Kazunari; Isobe, Hiroaki; Hillier, Andrew ...
Publications of the Astronomical Society of Japan,
06/2013, Letnik:
65, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Recent observations of Sun-like stars, similar to our Sun in their surface temperature (5600–6000 K) and slow rotation (rotational period
$ >$
10 d), using the Kepler satellite by Maehara et al. ...(2012, Nature, 485, 478) have revealed the existence of superflares (with energy of 10
$ ^{33}$
–10
$ ^{35}$
erg). From statistical analyses of these superflares, it was found that superflares with energy of 10
$ ^{34}$
erg occur once in 800 yr, and superflares with 10
$ ^{35}$
erg occur once in 5000 yr. In this paper, we examine whether superflares with energy of 10
$ ^{33}$
–10
$ ^{35}$
erg could occur on the present Sun through the use of simple order-of-magnitude estimates based on current ideas related to the mechanisms of the solar dynamo. If magnetic flux is generated by differential rotation at the base of the convection zone, as assumed in typical dynamo models, it is possible that the present Sun would generate a large sunspot with a total magnetic flux of
$ \sim$
2
$ \times$
10
$ ^{23}$
Mx (
$ =$
G cm
$ ^{2}$
) within one solar cycle period, and lead to superflares with an energy of 10
$ ^{34}$
erg. To store a total magnetic flux of
$ \sim$
10
$ ^{24}$
Mx, necessary for generating 10
$ ^{35}$
erg superflares, it would take
$ \sim$
40 yr. Hot Jupiters have often been argued to be a necessary ingredient for the generation of superflares, but we found that they do not play any essential role in the generation of magnetic flux in the star itself, if we consider only the magnetic interaction between the star and the hot Jupiter. This seems to be consistent with Maehara et al.'s finding of 148 superflare-generating solar-type stars that do not have a hot Jupiter-like companion. Altogether, our simple calculations, combined with Maehara et al.'s analysis of superflares on Sun-like stars, show that there is a possibility that superflares of 10
$ ^{34}$
erg would occur once in 800 yr on our present Sun.
Abstract
People have probably been watching the sky since the beginning of human history. Observers in pre-telescopic ages recorded anomalous events, which now provide uniquely valuable information ...for modern scientists. Records that include drawings are particularly useful, since the verbal expressions recorded by pre-telescopic observers, who did not know the physical nature of the phenomena, are often ambiguous. However, drawings concerning specific datable events in the historical documents are far fewer than the verbal records. Therefore, in this paper we show the possible earliest drawings of datable auroras and a two-tail comet included in a manuscript of the Chronicle of Zūqnīn, a Syriac chronicle up to 775/776 CE, to interpret their nature. Careful perusing the original Syriac autograph manuscript, MS Vat.Sir.162, provide not only historical facts in the realm around Amida, but also information concerning low-latitude aurora observations due to extreme space weather events and the existence of sun-grazing comets.
Dim red aurora at low magnetic latitudes is a visual and recognized manifestation of magnetic storms. The great low-latitude auroral displays seen throughout East Asia on 1770 September 16-18 are ...considered to manifest one of the greatest storms. Recently found, 111 historical documents in East Asia attest that these low-latitude auroral displays appeared in succession for almost nine nights during 1770 September 10-19 in low magnetic latitude areas (<30°). This suggests that the duration of the great magnetic storm is much longer than usual. Sunspot drawings from 1770 reveal that the sunspot areas were twice as large as those observed in another great storm of 1859, which substantiates these unusual storm activities in 1770. These spots likely ejected several huge, sequential magnetic structures in short duration into interplanetary space, resulting in spectacular worldwide aurorae in mid-September of 1770. These findings provide new insight into the history, duration, and effects of extreme magnetic storms that may be valuable for those who need to mitigate against extreme events.
The Sun occasionally undergoes the so-called grand minima, in which its magnetic activity, measured by the number of sunspots, is suppressed for decades. The most prominent grand minima, since the ...beginning of telescopic observations of sunspots, is called the Maunder minimum (1645-1715), which occurred when the sunspots became rather scarce. The mechanism underlying the grand minima remains poorly understood as there is little observational information of the solar magnetic field at that time. In this study, we examine the records of one candidate aurora display in China and Japan during the Maunder minimum. The presence of auroras in such mid-magnetic latitudes indicates the occurrence of great geomagnetic storms that are usually produced by strong solar flares. However, the records of contemporary sunspot observations from Europe suggest that, at least for the likely aurora event, there was no large sunspot that could produce a strong flare. Through simple theoretical arguments, we show that this geomagnetic storm could have been generated by an eruption giant quiescent filament or a series of such events.
The launch of the Hinode satellite has allowed unprecedented high-resolution, stable images of solar quiescent prominences to be taken over extended periods of time. These new images led to the ...discovery of dark upflows that propagated from the base of prominences, developing highly turbulent profiles. As yet, how these flows are driven is not fully understood. To study the physics behind these phenomena, we use three-dimensional magnetohydrodynamic simulations to investigate the nonlinear stability of the Kippenhahn-Shluter prominence model to the magnetic Rayleigh-Taylor instability. The model simulates the rise of a buoyant tube inside a quiescent prominence, where the upper boundary between the tube and prominence model is perturbed to excite the interchange of magnetic field lines. We found upflows of constant velocity (maximum found 6 km s--1) and a maximum plume width 1500 km which propagate through a height of approximately 6 Mm in the no guide field case. The case with the strong guide field (initially By = 2Bx ) results in a large plume that rises through the prominence model at ~5 km s--1 with width ~900 km (resulting in width of 2400 km when viewed along the axis of the prominence), reaching a height of ~3.1 Mm. In both cases, nonlinear processes were important for determining plume dynamics.
Abstract
A filament, a dense cool plasma supported by the magnetic fields in the solar corona, often becomes unstable and erupts. It is empirically known that the filament often demonstrates some ...activations such as a turbulent motion prior to eruption. In our previous study, we analyzed the Doppler velocity of an H
α
filament and found that the standard deviation of the line-of-sight velocity distribution in a filament, which indicates the increasing amplitude of the small-scale motions, increased prior to the onset of the eruption. Here, we present a further analysis on this filament eruption, which initiated approximately at 03:40 UT on 2016 November 5 in the vicinity of NOAA Active Region 12605. It includes a coronal line observation and the extrapolation of the surrounding magnetic fields. We found that both the spatially averaged microturbulence inside the filament and the nearby coronal line emission increased 6 and 10 hr prior to eruption, respectively. In this event, we did not find any significant changes in the global potential field configuration preceding the eruption for the past 2 days, which indicates that there is a case in which it is difficult to predict the eruption only by tracking the extrapolated global magnetic fields. In terms of space weather prediction, our result on the turbulent motions in a filament could be used as the useful precursor of a filament eruption.
PDF
