Chromospheric Anemone Jets and Magnetic Reconnection in Partially Ionized Solar Atmosphere View all abstracts by submitter (original) (raw)
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Chromospheric anemone jets and magnetic reconnection in partially ionized solar atmosphere
Physics of Plasmas, 2011
The Solar Optical Telescope (SOT) onboard Hinode with temporal resolution of less than 5 s and spatial resolution of 150 km has observed the lower solar atmosphere with an unprecedented detail. This has led to many important findings, one of them is the discovery of chromospheric anemone jets in the solar chromosphere. The chromospheric anemone jets are ubiquitous in solar chromosphere and statistical studies show that the typical length, life time and energy of the chromospheric anemone jets are much smaller than the coronal events (e.g. jets/flares/CMEs). Among various observational parameters, the apparent length and maximum velocity shows good correlation. The velocity of chromospheric anemone jets is comparable to the local Alfvén speed in the lower solar chromosphere. Since the discovery of chromospheric anemone jets by Hinode, several evidences of magnetic reconnection in chromospheric anemone jets have been found and these observations are summarized in this paper. These observations clearly suggest that reconnection occurs quite rapidly as well as intermittently in the solar chromosphere. In the solar corona (λ i > δ SP), anomalous resistivity arises due to various collisionless processes. Previous MHD simulations show that reconnection becomes fast as well as strongly time-dependent due to anomalous resistivity. Such processes would not arise in the solar chromosphere which is fully collisional and partially-ionized. So, it is unclear how the rapid and strongly time-dependent reconnection would occur in the solar chromosphere. It is quite likely that the Hall and ambipolar diffusion are present in the solar chromosphere and they could play an important role in driving such rapid, strongly time-dependent reconnection in the solar chromosphere.
Fast Magnetic Reconnection in the Solar Chromosphere Mediated by the Plasmoid Instability
The Astrophysical Journal, 2015
Magnetic reconnection in the partially ionized solar chromosphere is studied in 2.5 dimensional magnetohydrodynamic simulations including radiative cooling and ambipolar diffusion. A Harris current sheet with and without a guide field is considered. Characteristic values of the parameters in the middle chromosphere imply a high magnetic Reynolds number of ∼10 6 -10 7 in the present simulations. Fast magnetic reconnection then develops as a consequence of the plasmoid instability without the need to invoke anomalous resistivity enhancements. Multiple levels of the instability are followed as it cascades to smaller scales, which approach the ion inertial length. The reconnection rate, normalized to the asymptotic values of magnetic field and Alfvén velocity in the inflow region, reaches values in the range ∼0.01-0.03 throughout the cascading plasmoid formation and for zero as well as for strong guide field. The outflow velocity reaches ≈40 km s −1 . Slow-mode shocks extend from the X-points, heating the plasmoids up to ∼8 × 10 4 K. In the case of zero guide field, the inclusion of both ambipolar diffusion and radiative cooling causes a rapid thinning of the current sheet (down to ∼30 m) and early formation of secondary islands. Both of these processes have very little effect on the plasmoid instability for a strong guide field. The reconnection rates, temperature enhancements, and upward outflow velocities from the vertical current sheet correspond well to their characteristic values in chromospheric jets.
Spontaneous non-steady magnetic reconnection within the solar environment
Astronomy and Astrophysics, 2010
Context. A fundamental step to produce realistic models of the solar phenomena requiring fast (and high power) triggering events is to understand the feasibility of a spontaneous transition from a slow to a fast reconnection regime in the solar environment and its macroscopic evolution within the theoretical framework of pure resistive magnetohydrodynamics. Aims. We show the dynamical evolution of a reconnecting force-free magnetic field in a "simplified" solar atmosphere (high chromosphere-low corona) described by a pressure-balanced configuration with a variable density modeling the transition region. Magnetic reconnection plays a fundamental role in this region and we show the efficient working of a non-steady and self-feeding reconnection process whose development as determined by characteristic solar parameters (global resistivity, global viscosity, plasma beta) is followed. Methods. This work presents a 2.5-dimensional simulation study of the instability of force-free current-sheets located in a medium with a strong density variation along the current layer. In order to reach the needed high resolution and to reduce the influence of spurious numerical effects, we use a code with a fully-implicit-particle (or Flip) algorithm to solve an Eulerian-Lagrangian formulation of resistive and viscous magnetohydrodynamics equations. Results. The initial force-free configuration is observed to undergo a two-stage evolution consisting of an abrupt regime transition from a slow to a fast reconnection process, which leads the system to a final chaotic configuration. Yet the onset of the fast phase is not determined by any anomalous enhancement in the plasma's local resistivity. An asymmetric development of the whole structure is observed and the related magnetic field topology and energetic features are described. Conclusions. This mechanism can be used as a simple but effective model of several (explosive) processes taking place from the high chromosphere up to the low corona. Our simplified model of the solar atmosphere allows us to obtain a realistic oriented path for the evolution of the overall flow and reconnecting current-sheet. In the present work, the numerical experiment provides key information and observables (like the energetic fluxes) to be compared with observations.
Chromospheric Anemone Jets as Evidence of Ubiquitous Reconnection
Science, 2007
The heating of the solar chromosphere and corona is a long-standing puzzle in solar physics. Hinode observations show the ubiquitous presence of chromospheric anemone jets outside sunspots in active regions. They are typically 3 to 7 arc seconds = 2000 to 5000 kilometers long and 0.2 to 0.4 arc second = 150 to 300 kilometers wide, and their velocity is 10 to 20 kilometers per second. These small jets have an inverted Y-shape, similar to the shape of x-ray anemone jets in the corona. These features imply that magnetic reconnection similar to that in the corona is occurring at a much smaller spatial scale throughout the chromosphere and suggest that the heating of the solar chromosphere and corona may be related to small-scale ubiquitous reconnection.
Chromospheric Magnetic Reconnection and Its Possible Relationship to Coronal Heating
The Astrophysical Journal, 1999
Coronal heating is clearly related to the coronal magnetic Ðeld. This may be due to a passive role of the magnetic Ðeld in modifying wave propagation and dissipation or to an active role resulting from the liberation of magnetic energy by reconnection or in some other way. The purpose of this article is to examine the consequences of reconnection at the chromospheric level rather than in the corona. We note that the chromosphere is indeed a favorable site for reconnection to occur, since the resistivity is greatest in that regionÈspeciÐcally at the temperature-minimum location. Chromospheric reconnection can lead to coronal heating by Joule heating, by the generation and subsequent dissipation of high-frequency and magnetacoustic waves, or by the response of the coronal magnetic Ðeld to a sudden change Alfve n in connectivity. The second process could also contribute to heating of the solar wind, since highfrequency waves can be absorbed by cyclotron damping. We note also that chromospheric recon-Alfve n nection could inject sufficient chromospheric gas into the corona to balance the known steady downÑow of coronal gas through the transition region. It is also possible that chromospheric reconnection plays a role in the Ðrst ionization potential e †ect.
Magnetic reconnection signatures in the solar atmosphere: results from multi-wavelength observations
2011
In the solar atmosphere magnetic reconnection is invoked as the main mechanism causing very energetic events (1028 - 1032 erg), like flares and coronal mass ejections, as well as other less energetic phenomena, like microflares, X-ray jets and chromospheric surges. In the last decade, thanks to high spatial resolution, multi-wavelength observations carried out by both ground-based telescopes (THEMIS, SST, VTT, DST) and space-born satellites (SOHO, TRACE, RHESSI, HINODE), it has been possible to study these phenomena and several signatures of the occurrence of magnetic reconnection have been singled out. In this paper, we describe some results obtained from the analysis of multi-wavelength observations carried out in the last years, with special emphasis on those events that were characterized by plasma outflows from the reconnection site. The events here discussed are relevant to some active regions observed on the Sun, characterized by the interaction of different bundles of magnet...
On the Observations of Rapid Forced Reconnection in the Solar Corona
The Astrophysical Journal
Using multiwavelength imaging observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) on 03 May 2012, we present a novel physical scenario for the formation of a temporary X-point in the solar corona, where plasma dynamics is forced externally by a moving prominence. Natural diffusion was not predominant, however, a prominence driven inflow occurred firstly, forming a thin current sheet and thereafter enabling a forced magnetic reconnection at a considerably high rate. Observations in relation to the numerical model reveal that forced reconnection may rapidly and efficiently occur at higher rates in the solar corona. This physical process may also heat the corona locally even without establishing a significant and self-consistent diffusion region. Using a parametric numerical study, we demonstrate that the implementation of the external driver increases the rate of the reconnection even when the resistivity required for creating normal diffusion region decreases at the X-point. We conjecture that the appropriate external forcing can bring the oppositely directed field lines into the temporarily created diffusion region firstly via the plasma inflows as seen in the observations. The reconnection and related plasma outflows may occur thereafter at considerably larger rates.
Astronomy & Astrophysics, 2011
We aim at investigating the formation of jet-like features in the lower solar atmosphere, e.g. chromosphere and transition region, as a result of magnetic reconnection. Magnetic reconnection as occurring at chromospheric and transition regions densities and triggered by magnetic flux emergence is studied using a 2.5D MHD code. The initial atmosphere is static and isothermal, with a temperature of 20,000 K. The initial magnetic field is uniform and vertical. Two physical environments with different magnetic field strength (25 G and 50 G) are presented. In each case, two sub-cases are discussed, where the environments have different initial mass density. In the case where we have a weaker magnetic field (25 G) and higher plasma density ($N_e=2\times 10^{11}$ cm$^{-3}$), valid for the typical quiet Sun chromosphere, a plasma jet would be observed with a temperature of 2--3 times104\times 10^4times104 K and a velocity as high as 40 km/s. The opposite case of a medium with a lower electron density ($N_e=2\times 10^{10}$ cm$^{-3}$), i.e. more typical for the transition region, and a stronger magnetic field of 50 G, up-flows with line-of-sight velocities as high as 90 km/s and temperatures of 6 times\timestimes 10$^5$ K, i.e. upper transition region -- low coronal temperatures, are produced. Only in the latter case, the low corona Fe IX 171 \AA\ shows a response in the jet which is comparable to the O V increase. The results show that magnetic reconnection can be an efficient mechanism to drive plasma outflows in the chromosphere and transition region. The model can reproduce characteristics, such as temperature and velocity for a range of jet features like a fibril, a spicule, an hot X-ray jet or a transition region jet by changing either the magnetic field strength or the electron density, i.e. where in the atmosphere the reconnection occurs.
The interaction between emerging magnetic flux and the pre-existing ambient field has become a " hot " topic for both numerical simulations and high-resolution observations of the solar atmosphere. The appearance of brightenings and surges during episodes of flux emergence is believed to be a signature of magnetic reconnection processes. We present an analysis of a small-scale flux emergence event in NOAA 10971, observed simultaneously with the Swedish 1 m Solar Telescope on La Palma and the Hinode satellite during a joint campaign in 2007 September. Extremely high-resolution G-band, Hα, and Ca ii H filtergrams, Fe i and Na i magnetograms, EUV raster scans, and X-ray images show that the emerging region was associated with chromospheric, transition region and coronal brightenings, as well as with chromospheric surges. We suggest that these features were caused by magnetic reconnection at low altitude in the atmosphere. To support this idea, we perform potential and linear force-free field extrapolations using the FROMAGE service. The extrapolations show that the emergence site is cospatial with a three-dimensional null point, from which a spine originates. This magnetic configuration and the overall orientation of the field lines above the emerging flux region are compatible with the structures observed in the different atmospheric layers and remain stable against variations of the force-free field parameter. Our analysis supports the predictions of recent three-dimensional numerical simulations that energetic phenomena may result from the interaction between emerging flux and the pre-existing chromospheric and coronal field.