Magnetic Neutral Line Research Articles

Simultaneous dual wavelength observations of an impulsive microwave burst using the V.L.A.

Simultaneous observations of a microwave burst at 2 and 6 cm wavelengths were carried out with the Very Large Array (VLA). The 6 cm burst source is located close to a magnetic neutral line, presumably near the top of a flaring loop, while the 2 cm emission originates from the footpoints of the loop. It is concluded that the 6 cm emission is dominated by gyrosynchrotron radiation of the thermal electrons in the bulk heated plasma at a temperature of ∼ 4 × 10 7 K, while the 2 cm emission is due to nonthermal particles released and accelerated during the flare process. From the observed low degree of polarization and the lack of the 2 cm source cospatiality with the 6 cm source a magnetic field of 200–350 G and δ ≳ 4 are estimated in the flare energy release site. A DC electric field flare model is invoked to explain the long delay between the peaks at the two wavelengths. From the delay, the strength of the electric field is estimated to be 0.2–4 μ statvolt cm −1 in the flaring region.

Energetic particles in the vicinity of a possible neutral line in the plasma sheet

We have analyzed energetic protons in the energy range 30 to 500 keV and energetic electrons ≥75 keV obtained with the Max‐Planck‐Institut/University of Maryland sensor system on ISEE‐1 during a plasma sheet crossing on March 26, 1978. The behavior of protons with energies of more than ∼100 keV is very different from that of the ∼30 to ∼100 keV protons which represent the suprathermal tail of the plasma sheet distribution. The more energetic ions appear on a time scale of several minutes following a northward turning of the tail magnetic field. At about the same time the plasma measurements show a velocity of ∼200 km/s in the tailward direction. This velocity enhancement is first seen at ISEE‐1 and later on at ISEE‐2, which is earthward of ISEE‐1. The temporal sequence of the energetic particle, magnetic field, and plasma observations and the proton and electron anisotropies are discussed in terms of acceleration near a magnetic neutral line which occurs well within the plasma sheet. In this framework the magnetic neutral line would move earthward, followed by a magnetic island. The extent of the neutral line would be limited to the dusk side of the tail. No disruption of the plasma sheet is observed as compared to large‐scale substorm activity.

The reason for magnetospheric substorms and solar flares

It has been proposed that magnetospheric substorms and solar flares are a result of the same mechanism. In our view this mechanism is connected with the escape, or attempted escape, of energized plasma from a region of closed magnetic field lines bounded by a magnetic bottle. In the case of the Earth, it must be plasma that is able to maintain a discrete auroral arc, and we propose that the cross-tail current connected to the arc is filamentary in nature to provide the field-aligned current sheet above the arc. A localized meander of such an intense current filament could be caused by a tearing instability in the neutral sheet. Such a meander will cause an inductive electric field opposing the current change everywhere. In trying to reduce the component of the induction electric field parallel to the magnetic field lines, the plasma must enhance the transverse or cross-tail component; this action leads to eruptive behavior, in agreement with tearing theories. This enhanced induction electric field will cause a discharge along the magnetic neutral line at the apex of the magnetic arches, constituting an impulsive acceleration of all charged particles originally near the neutral line. The products of this phase then undergo betatron acceleration for a second phase. This discharge eventually reduces the electric field along the neutral line, and thereafter the enclosed magnetic flux through the neutral line remains nearly constant. The result is a plasmoid that has definite identity; its buoyancy leads to its escape. The auroral breakup (and solar flare) is the complex plasma response to the changing electromagnetic field.

Poleward migration of the magnetic neutral line and the reversal of the polar fields on the Sun

Poleward migration of the magnetic neutral line on the Sun has been calculated for the periods 1945–1950 and 1955–1981 using synoptic charts based on Hα observations. Epochs of sign reversal of the solar magnetic field at latitudes 50° to 90° have been determined for these periods. During the cycles 19 and 20 a threefold sign reversal took place in the northern hemisphere. During all the above cycles both the solar poles were of one polarity for a period ranging from 0.5 to 1 year. The poleward drift velocity of the magnetic neutral line varies from 6 to 29 m s−1 and seems to depend on the strength of the cycle.

Poleward migration of the magnetic neutral line and the reversal of the polar fields on the Sun

Poleward migration of the magnetic neutral line and the reversal of the polar fields on the Sun

Fluid motions in the solar chromosphere-corona transition region. III - Active region flows from wide slit Dopplergrams

Large-scale velocity patterns observed in C IV in active regions show close correspondence with photospheric magnetic field patterns. In a large majority of cases, magnetic neutral lines show blueshift on their sun center side and redshift on their limbward side. The large-scale flow is consistent with widespread loop structure having downflow in both legs of the loops. Studies of individual bright loops, confirm that the flow is downward in most cases. However, an important subset of loops show flow from one end of the loop to the other.

A numerical experiment relevant to line-tied reconnection in two-ribbon flares

The nonlinear evolution of a reconnecting magnetic field configuration similar to that occurring just before the onset of ‘post’-flare loops in two-ribbon flares is determined. The evolution, which is obtained by numerically solving the resistive MHD equations, shows two new features that have not yet been incorporated into contemporary models of ‘post’-flare loops. The first of these new features is the formation of a nearly stationary fast-mode shock above the region corresponding to the top of the loops. This fast-mode shock occurs just below the magnetic neutral line and between the slow-mode shocks associated with fast magnetic reconnection at the neutral line. The second new feature is the creation and annihilation of large-scale magnetic islands in the current sheet above the loops. The annihilation of the islands occurs very rapidly and appears to be a manifestation of the coalescence instability. The creation and annihilation of magnetic islands could be important in understanding the energetics of ‘post’-flare loops since the coalescence instability can produce an intermittent energy release more than an order of magnitude faster than that predicted by steady-state reconnection theories.

High-resolution observations of solar radio bursts at 2, 6, and 20 cm wavelength

The Very Large Array and the Westerbork Synthesis Radio Telescope have been used to observe eight solar bursts at 2, 6, or 20 cm wavelength with second-of-arc angular resolution. The regions of burst energy were all resolved with angular sizes between 5″ and 30″, brightness temperatures between 2 × 107 K and 2 x 108 K, and degrees of circular polarization between 10 and 90%. A series of 10 s snapshot maps are presented for the more intense bursts, and superimposed on photospheric magnetograms or Hα photographs. The impulsive phase of the radio bursts is located near the magnetic neutral line of the active regions, and between the flaring Hα kernels which mark the footpoints of magnetic loops. The impulsive phase of one 6 cm burst was smaller and spatially separated from both the preburst radio emission and the gradual decay phase of the burst. Another 6 cm burst exhibited preburst heating of the coronal loop in which the burst occurred. The plasma was probably heated at a lower level in the loop, while the burst energy was released several minutes later at a higher level. A multiple-spike 20 cm burst exhibited polarity inversions with degrees of circular polarization of 90%. The rapid changes in circular polarization are attributed to either a magnetically complex region or the emersion of new magnetic flux at coronal heights where magnetic field strengths H ≈ 300 to 400 G.

Current sheet acceleration of ions in the geomagnetic tail and the properties of ion bursts observed at the lunar distance

Recent analyses of measurements obtained from the Rice University Apollo 14 SIDE on the lunar surface have revealed the frequent appearance of fast tailward-streaming ion “bursts” near the centre plane of the geomagnetic tail. In this paper the properties of these “bursts” are tested for compatibility with tail current sheet tangential stress balance conditions assuming that they are produced by current sheet acceleration of tail lobe plasma downstream and tailward of a magnetic neutral line. Calculations are performed taking the ions to be either protons or singly charged oxygen, the latter possibility being directly suggested by several recent observations. When “burst” bulk parameters are calculated by assuming that the ion distribution functions are convecting isotropic Maxwellians, the results are found to be difficult to reconcile with current sheet stress balance conditions for either protons or oxygen. Use of a different ion distribution based on theoretical expectations and observations in the near-Earth tail, however, results in number density estimates being increased by factors of around an order of magnitude. When the revised densities and ion distribution functions are taken into account, reasonable agreement between observed and expected ion bulk speeds is obtained. In some cases the agreement is better assuming oxygen ions rather than protons, but not by a large factor.

Neutral line motion due to reconnection in two-ribbon solar flares and magnetospheric substorms

Two kinematic models of line-tied reconnection are considered which describe the motion of a magnetic neutral line (NL) during the main phase of a two-ribbon solar flare and during the recovery phase of a magnetospheric substorm in the geomagnetic tail. The models are kinematic in that they use only the magnetic induction equation, which suffices to determine the position and velocity of the NL as functions of time if the rate of reconnection is prescribed. The solar flare model shows that the observed large decrease in the rate at which “post”-flare loops rise upward from the photosphere during the main phase does not require a corresponding decrease in the rate of reconnection. Instead it is found that a constant rate of reconnection can account for the motion of the loops for almost the entire period during which they are observed. By contrast, application of the same procedures to the recovery phase of the magnetospheric substorm in the tail predicts a slightly increasing speed of NL motion if the rate of reconnection is constant. Furthermore, it is found that the motion of the NL relative to the ambient medium may account for much of the observed asymmetry in the magnetic field in the plasma sheet during recovery. Due to this motion, the plasma sheet thickness may be up to 4 times smaller and the normal magnetic field component up to 2 times weaker in the region tailward of the NL than in the corresponding region earthward of the NL.

Fluid motions the solar chromosphere-corona transition region. II Active region flows in C IV from narrow slit Dopplergrams

From a study of Dopplergrams made with two narrow slits in the opposite wings of the C IV line, N λ1548, we find widespread evidence for steady flow patterns associated with large sunspots and, on a larger scale, with active region magnetic field patterns. The characteristic sunspot flow is in the reverse Evershed sense with a substantial vertical component. Active region flows indicate oppositely directed velocities with mainly horizontal components on either side of magnetic neutral lines. Some neutral lines show flow toward the neutral line, whereas others show flow away from the neutral line.

Implosion of a uniform current sheet in a low-beta plasma

Using ideal, one-dimensional MHD equations, numerical and analytic solutions are presented which describe the nonlinear behaviour of an imploding current sheet in a low-fl plasma. Initially the current density is uniformly distributed in asheet of finite thickness, and the Lorentz force tending to pinch the plasma together is unopposed by any fluid pressure force. As the implosion develops the current density in the sheet is concentrated into a thin layer at the centre of the sheet, and both the current density and the current in this layer become infinite in a finite time if β = 0. At the moment this occurs, fast-mode shocks are produced which propagate outward from the centre of the current sheet, and as the shocks move away an infinitely thin current sheet is left behind. Although the solutions are related to electric discharges, they are also closely related to a problem posed by Dungey concerning the evolution of a uniformly distributed current in the vicinity of an X-type magnetic neutral line. The implications of these solutions for Dungey's problemare discussed.

Ion current in a magnetic neutral region: Generation of an incipient magnetopause

We calculate the current contributed by ions trapped in the vicinity of a magnetic X line. The X line is formed from the superposition of a dipole and a uniform, antiparallel magnetic field. The system (Dungey's open model for the magnetosphere) has an electric field with equipotential magnetic field lines and with an asymptotic behavior such that at a great distance the electric field is uniform in the dawn‐to‐dusk direction. In the magnetic neutral region E is directed along the X line toward dusk. Ions incident within about one gyrolength from the X line are captured and accelerated along the X line to large kinetic energies by the electric field. Most ions escape from the neutral region by Speiser's ejection mechanism into spiral orbits along open magnetic field lines. The neutral region ion current is azimuthal and varies very nearly as cos ϕ on the dayside as predicted by Alfven, where ф is the GSM longitude, but is negligible on the nightside in this zero‐order calculation. Ion energization rates and escaping pitch angle distributions are estimated. The effect of the neutral region current is to extend the magnetic neutral line into a neutral sheet in the north‐south direction. This leads to a positive feedback mechanism whereby the greater vertical extent of the neutral region enhances ion capture, ensuring a still larger current. Thus a magnetopauselike structure will be rapidly formed for even very small incident plasma densities.

Multiple wavelength observations of flaring active regions

We present observations of flaring active regions with the Very Large Array (V.L.A. at 6 cm and 20 cm wavelengths) and the Westerbork Synthesis Radio Telescope (W.S.R.T. at 6 cm wavelength). These are compared with photospheric magnetograms (Meudon) and with Hα and offband Hα photographs (Big Bear and Ottawa River Solar Observatories). The 6 cm radiation of these active regions marks the legs of dipolar loops which have their footpoints in lower-lying sunspots. The intense, million degree radiation at 6 cm lies above sunspot umbrae in coronal regions where the longitudinal magnetic field strength H ℓ = 600 Gauss and the height above the sunspot umbrae h = 3.5±0.5 × 10 9 cm. Circularly polarized horseshoe structures at 6 cm ring the sunspot umbrae. The high degree of circular polarization ( ϱ c = 95%) of the horseshoes is attributed to gyroresonant emission above sunspot − penumbrae. The 20 cm radiation of these active regions exhibits looplike coronal structures which extend across regions of opposite magnetic polarity in the underlying photosphere. The 20 cm loops are the radio wavelength counterparts of the X-ray coronal loops. We infer semilengths L = 5 × 10 9 cm, maximum electron temperatures T e(max) = 3 × 10 6 K, emission measures ∫N e 2d l = 10 28 cm −5, and electron densities N e = 10 9 cm −3 (or pressures p = 1 dyn cm −2) for the 20 cm bremsstrahlung. A total of eight solar bursts were observed at 6 cm or 20 cm wavelength with second-of-arc angular resolution. The regions of burst energy were all resolved with angular sizes between 5″ and 30″, brightness temperatures between 2 × 10 7 K and 2 × 10 8 K, and degrees of circular polarization between 10% and 90%. The impulsive phase of the radio bursts are located near the magnetic neutral lines of the active regions, and between the flaring Hα kernels which mark the footpoints of magnetic loops. In one case there was preburst heating in the coronal loop in which a burst occurred. Snapshot maps at 10 s intervals reveal interesting burst evolution including rapid changes of circular polarization and an impulsive burst which was physically separated from both the preburst radio emission and the gradual decay phase of the burst.

Flare build-up in preflare magnetic loops and nonlinear force-free magnetic fields

In this paper, we extend B. C. Low's study on nonlinear force-free magnetic fields. Based on Low's mathematical method, a revised boundary-value problem of the two-dimensional nonlinear force-free magnetic field is solved analytically. The solution shows that higher magnetic loops evolve towards preflare loops when the gradient of longitudinal magnetic field at the photospheric level and the angle (‘shear’) included between the magnetic field line and magnetic neutral line increase with time. The density, temperature and the current density are higher in the preflare loops than in the high-lying magnetic loops. We believe it is the loops that provide conditions for the eruption of the flare.

The relation of solar flares to the evolution and proper motions of magnetic fields

The second Action Interval of the FBS coincided with an extended period of gradual evolution in a large complex of activity which served as the target for a coordinated space-ground study. The complex produced a multitude of subflares, half of which were clustered around just a few sites, each with a distinctive magnetic character. The essential flare-producing conditions at these preferred sites were preserved for many hours, even days, despite disruptions by flares and despite the eroding effects that accompany the disintegration of sunspot groups. Three preferred sites were active for the entire Interval, 22–27 May 1980. A comparison of flaring with non-flaring sites which also contained strong concentrations of flux demonstrates the importance of magnetic complexity, flux emergence, and motions at the photospheric level. The most energetic events by far, a chain of five closely homologous flares, erupted within 13 hours at a site where all these factors were conspicuously combined. The incessant activity preceding and during these flares of the fine chromospheric fibrils that covered and surrounded this particularly energetic site indicates reconfiguration of flux tubes in the chromosphere in a matter of minutes. These rapid (2–5 minutes), small (∼10 arc-sec) changes are identified with emerging flux and with pores moving rapidly (≥200 m/s) very close to a magnetic neutral line.

Generation of coronal electric currents due to convective motions on the photosphere

Generation of electric currents in a magnetized plasma overlying a dense convective layer is studied, assuming that the magnetic field perturbation is small and satisfies the force-free equation. Currents are produced by rotational motions on the boundary in the case of a uniform equilibrium field. In a simple two-dimensional bipolar configuration, however, both irrotational and incompressible motions give rise to currents, and the current density has a peak at the magnetic neutral line. Scaling laws for the current density as well as for the stored magnetic energy are derived, and the possibility of heating the solar corona through the dissipation of coronal currents generated in this way is discussed.

VLA Observations of Solar Active Regions - Part Two - Solar Bursts

view Abstract Citations (21) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA Observations of Solar Active Regions. II. Solar Bursts Lang, K. R. ; Willson, R. F. ; Felli, M. Abstract The VLA has been used at wavelengths of 6 cm and 20 cm to detect seven solar bursts from two active regions with flux densities between 0. 1 and 6.0 sfu. The regions of burst energy release were resolved with angular sizes between 6" and 12" and brightness temperatures between 1 and 2 × 107 K. The angular sizes of the radio bursts are comparable to those of the soft X-ray kernels, and the brightness temperatures are comparable to the temperatures of the plasma which gives rise to soft X-ray emission. This suggests that we are detecting the radio wavelength counterpart of the flares seen at soft X-ray wavelengths. The two solar bursts observed at 6 cm wavelength occurred within a plage-associated "coronal condensation" in regions away from sunspots and near magnetic neutral lines. The Hα emission associated with one burst was, within the uncertainties of measurement, spatially coincident with the 6 cm burst. The five solar bursts detected at 20 cm wavelength were composed of individual events which had a duration of between 20 s and 60 s, but our time resolution of 10 s precluded the detection of components with shorter duration. Although thermal energies normally detected at soft X-ray wavelengths are inferred from their brightness temperatures, the train of events which make up the 20 cm bursts are similar to the elementary flare bursts detected at hard X-ray wavelengths in other active regions. The observed bursts at 20 cm wavelength can be explained by either the bremsstrahlung or the gyroresonant radiation of a hot, dense plasma with an electron temperature Te ≲ 2 × 107 K and an electron density of Ne ≍ 1010 cm-3. The gyroradiation easily explains the high observed circular polarization while indicating magnetic field strengths of H 90 G in the emitting region; while the bremsstrahlung requires polarization due to propagation effects at frequencies near the gyrofrequency in magnetic fields of strength H ≍ 500 G. We also discuss the implications of VLA observations for studies of the physical nature of the burst emitting sources. Publication: The Astrophysical Journal Pub Date: July 1981 DOI: 10.1086/159042 Bibcode: 1981ApJ...247..338L full text sources ADS |

Observations of solar filaments at 8, 15, 22 and 43 GHz

On April 3, 4, 6, and 8, 1978, solar observations were made using the Haystack 120 ft telescope at 8, 15, 22, and 43 GHz. Hα filtergrams obtained at the Sacramento Peak Observatory on the same days showed an average of more than 30 filaments or filament fragments (per day) on the disk. Most of these appeared as depressions in brightness temperature at 15 and 22 GHz. Because of the relatively low spatial resolution at 8 GHz, only a few appeared at that frequency, and presumably because of lower opacity in filaments at higher frequencies, few depressions were visible at 43 GHz. At 15 and 22 GHz, more depressions appeared than Hα filaments, but virtually all the radio depressions overlay magnetic neutral lines. Taking the data sets for each day as independent samples, we found that at 22 GHz, 46 of the 77 radio depressions were associated with Hα filaments; at 15 GHz the correlation was smaller; only 27 out of 48 being associated with the Hα filaments. The data imply that the microwave depression features are the result of absorption by filaments and perhaps also the result of other effects of the associated filament channel, but not necessarily coronal depletion. The effects of filament absorption are, statistically, about twice as effective as other phenomena (such as absorption by material invisible in Hα, for example) in creating the radio depression. A center-to-limb study of a single large filament clearly showed that at 15 and 22 GHz the absorption by cool hydrogen supported above the neutral line was the predominant factor in producing the observed depression at radio frequencies.

On the relationship between soft X-rays and H-alpha-emitting structures during a solar flare

Based on data obtained during a solar flare on March 31, 1979, soft X-ray (SXR) and hard X-ray (HXR) bursts are analyzed and compared with other available data in order to identify structures in H-alpha that may correspond to the SXR-emitting site. Measurements taken with the X-ray telescope and the XUV spectroheliograph flown on Skylab, have shown that the SXR emission from many flares comes from rather small structures of about 10-20 arcsec across. These structures appear to be loops that cross the magnetic neutral line. Understanding of the morphology of SXR was based on data of the solar flare of June 15, 1973, observed from Skylab, and the work of Moore et al., (1980). Dense, highly emissive coronal structures, not suggested to be the X-ray source, were forming, lost energy rapidly by emission and conduction, and finally formed the loops. It is concluded that bright H-alpha loops form rapidly as the SXR emission rises, and the overall decay (cooling rate) of SXR emission is much slower than the formation time of individual loops.