اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدA Topology for the Penumbral Magnetic Fields
114
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We describe a scenario for the topology of the magnetic field in penumbrae that accounts for recent observations showing upflows, downflows, and reverse magnetic polarities. According to our conjecture, short narrow magnetic loops fill the penumbral photosphere. Flows along these arched field lines are responsible for both the Evershed effect and the convective transport. This scenario seems to be qualitatively consistent with most existing observations, including the dark cores in penumbral filaments reported by Scharmer et al. Each bright filament with dark core would be a system of two paired convective rolls with the dark core tracing the common lane where the plasma sinks down. The magnetic loops would have a hot footpoint in one of the bright filament and a cold footpoint in the dark core. The scenario fits in most of our theoretical prejudices (siphon flows along field lines, presence of overturning convection, drag of field lines by downdrafts, etc). If the conjecture turns out to be correct, the mild upward and downward velocities observed in penumbrae must increase upon improving the resolution. This and other observational tests to support or disprove the scenario are put forward.
قيم البحث
اقرأ أيضاً
We describe a scenario for the sunspot magnetic field topology that may account for recent observations of upflows and downflows in penumbrae. According to our conjecture, short narrow magnetic loops fill the penumbral volume. Flows along these field
lines are responsible for both the Evershed effect and the convective transport. This scenario seems to be qualitatively consistent with most existing observations, including the dark cores in penumbral filaments reported by Scharmer et al. Each bright filament with dark core would be a system of two paired convective rolls with the dark core tracing the lane where the plasma sinks down. The magnetic loops would have a hot footpoint in one of the bright filament and a cold footpoint in the dark core. The scenario also fits in most of our theoretical prejudices (siphon flows along field lines, presence of overturning convection, drag of field lines by downdrafts, etc). If the conjecture turns out to be correct, the mild upward and downward velocities observed in penumbrae must increase upon improvement of the current spatial resolution. This and other observational tests to support or disprove the proposed scenario are put forward.
A sunspot emanates from a growing pore or protospot. In order to trigger the formation of a penumbra, large inclinations at the outskirts of the protospot are necessary. The penumbra develops and establishes by colonising both umbral areas and granul
ation. Evidence for a unique stable boundary value for the vertical component of the magnetic field strength, $B^{rm stable}_{rm ver}$, was found along the umbra-penumbra boundary of developed sunspots. We use broadband G-band images and spectropolarimetric GFPI/VTT data to study the evolution of and the vertical component of the magnetic field on a forming umbra-penumbra boundary. For comparison with stable sunspots, we also analyse the two maps observed by Hinode/SP on the same spot after the penumbra formed. The vertical component of the magnetic field, $B_{rm ver}$, at the umbra-penumbra boundary increases during penumbra formation owing to the incursion of the penumbra into umbral areas. After 2.5 hours, the penumbra reaches a stable state as shown by the GFPI data. At this stable stage, the simultaneous Hinode/SP observations show a $B_{rm ver}$ value comparable to that of umbra-penumbra boundaries of fully fledged sunspots. We confirm that the umbra-penumbra boundary, traditionally defined by an intensity threshold, is also characterised by a distinct canonical magnetic property, namely by $B^{rm stable}_{rm ver}$. During the penumbra formation process, the inner penumbra extends into regions where the umbra previously prevailed. Hence, in areas where $B_{rm ver} < B^{rm stable}_{rm ver}$, the magneto-convection mode operating in the umbra turns into a penumbral mode. Eventually, the inner penumbra boundary settles at $B^{rm stable}_{rm ver}$, which hints toward the role of $B_{rm ver}^{rm stable}$ as inhibitor of the penumbral mode of magneto-convection.
The analyses of sunspot observations revealed a fundamental magnetic property of the umbral boundary, the invariance of the vertical component of the magnetic field. We aim to analyse the magnetic properties of the umbra-penumbra boundary in simulate
d sunspots and thus assess their similarity to observed sunspots. Also, we aim to investigate the role of plasma $beta$ and the ratio of kinetic to magnetic energy in simulated sunspots on the convective motions. We use a set of non-grey simulation runs of sunspots with the MURaM code. These data are used to synthesise the Stokes profiles that are then degraded to the Hinode spectropolarimeter-like observations. Then, the data are treated like real Hinode observations of a sunspot and magnetic properties at the umbral boundaries are determined. Simulations with potential field extrapolation produce a realistic magnetic field configuration on their umbral boundaries. Two simulations with potential field upper boundary, but different subsurface magnetic field structures, differ significantly in the extent of their penumbrae. Increasing the penumbra width by forcing more horizontal magnetic fields at the upper boundary results in magnetic properties that are not consistent with observations. This implies that the size of the penumbra is given by the subsurface structure of the magnetic field. None of the sunspot simulations is consistent with observed properties of the magnetic field and direction of the Evershed flow at the same time. Strong outward directed Evershed flows are only found in setups with artificially enhanced horizontal component of the magnetic field at the top boundary that are not consistent with the observed magnetic field properties at the UP boundary. We want to stress out that the `photospheric boundary of simulated sunspots is defined by a magnetic field strength of equipartition field value.
Vector magnetic fields of moving magnetic features (MMFs) are well observed with the Solar Optical Telescope (SOT) aboard the Hinode satellite. We focus on the evolution of three MMFs with the SOT in this study. We found that an MMF having relatively
vertical fields with polarity same as the sunspot is detached from the penumbra around the granules appeared in the outer penumbra. This suggests that granular motions in the outer penumbra are responsible for the disintegration of the sunspot. Two MMFs with polarity opposite to the sunspot are located around the outer edge of horizontal fields extending from the penumbra. This is an evidence that the MMFs with polarity opposite to the sunspot are prolongation of penumbral horizontal fields. Radshifts larger than sonic velocity in the photosphere are detected for some of the MMFs with polarity opposite to the sunspot.
Magnetic fields permeate the entire solar atmosphere weaving an extremely complex pattern on both local and global scales. In order to understand the nature of this tangled web of magnetic fields, its magnetic skeleton, which forms the boundaries bet
ween topologically distinct flux domains, may be determined. The magnetic skeleton consists of null points, separatrix surfaces, spines and separators. The skeleton is often used to clearly visualize key elements of the magnetic configuration, but parts of the skeleton are also locations where currents and waves may collect and dissipate. In this review, the nature of the magnetic skeleton on both global and local scales, over solar cycle time scales, is explained. The behaviour of wave pulses in the vicinity of both nulls and separators is discussed and so too is the formation of current layers and reconnection at the same features. Each of these processes leads to heating of the solar atmosphere, but collectively do they provide enough heat, spread over a wide enough area, to explain the energy losses throughout the solar atmosphere? Here, we consider this question for the three different solar regions: active regions, open-field regions and the quiet Sun. We find that the heating of active regions and open-field regions is highly unlikely to be due to reconnection or wave dissipation at topological features, but it is possible that these may play a role in the heating of the quiet Sun. In active regions, the absence of a complex topology may play an important role in allowing large energies to build up and then, subsequently, be explosively released in the form of a solar flare. Additionally, knowledge of the intricate boundaries of open-field regions (which the magnetic skeleton provides) could be very important in determining the main acceleration mechanism(s) of the solar wind.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
