Ion-Sound Model of Microwave Spikes with Fast Shocks in the Reconnection Region

A new model for solar spike bursts is considered based on the interaction of Langmuir waves with ion-sound waves: l+st. Such a mechanism can operate in shock fronts, propagating from a magnetic reconnection region. New observations of microwave millisecond spikes are discussed. They have been observed in two events: 4 November 1997 between 05:52–06:10 UT and 28 November 1997 between 05:00–05:10 UT using the multichannel spectrograph in the range 2.6–3.8 GHz of Beijing AO. Yohkoh/SXT images in the AR and SOHO EIT images testify to a reconstruction of bright loops after the escape of a CME. A fast shock front might be manifested as a very bright line in T _e SXT maps (up to 20 MK) above dense structures in emission measure (EM) maps. Moreover one can see at the moment of spike emission (for the 28 November 1997 event) an additional maximum at the loop top on the HXR map in the AR as principal evidence of fast shock propagation. The model gives the ordinary mode of spike emission. Sometimes we observed a different polarization of microwave spikes that might be connected with the depolarization of the emission in the transverse magnetic field and rather in the vanishing magnetic field in the middle of the QT region. Duration and frequency band of isolated spikes are connected with parameters of fast particle beams and shock front. Millisecond microwave spikes are probably a unique manifestation of flare fast shocks in the radio emission.     

Multiple wavelength observations of a solar active region

The Solar Maximum Mission Satellite, the Sacramento Peak Vacuum Tower Telescope, the Very Large Array and the Westerbork Synthesis Radio Telescope have been used to observe active region AR 2490 on two consecutive days at soft X-ray, ultraviolet, optical and radio wavelengths (2, 6, and 20 cm), with comparable angular resolution (2 to 15) and field of view (4 × 4). The radio emissions at = 6 cm and 20 cm show a double structure in which one component is associated with bright H plage, C iv and soft X-ray emission, and the other component is associated only with sunspots. No radiation at = 2 cm is detected in this latter component. Coronal temperature and emission measure derived from X-ray lines indicate that the dominant radiation mechanism of the plage-associated component is due to thermal bremsstrahlung while the gyroresonance absorption coefficient must be invoked to account for the high brightness temperature (T _b 2×10⁶K) observed in the sunspot associated component. The high magnetic field strength needed (600 G at a level where T2×10⁶K) is explained assuming a thin transition zone, in order to reach a high electron temperature close to the sunspot, where the magnetic fields are stronger. A higher temperature gradient above sunspots is also consistent with the absence of detectable C iv emission.Cooperative study of the SMY-FBS Project.On leave from the University of Napoli.On leave from the University of Torino.On sabbatical leave 1980–81 at the Arcetri Observatory.On leave from Toyokawa Observatory, Japan.     

Very Large Array-RATAN 600 observations of a solar active region

The Very Large Array (VLA) and the RATAN 600 were used to observe a solar active region on two consecutive days around the time of a partial solar eclipse in July 1990. VLA synthesis maps at 2.0, 3.5, and 6.2 cm wavelength reveal bright (T _b = 0.2 – 2.2 × 10⁶ K), compact ( = 10–40) sources above the penumbra of the leading sunspot while maps at 20 cm wavelength reveal an extended ( 4.5) looplike structure (T _b 10⁶ K) between the dominant spots. Total flux and brightness temperature spectra of both components were obtained by the RATAN at nine wavelengths between 1.7 and 21 cm. The relatively-flat spectrum of the extended emission is attributed to the optically thin thermal brems Strahlung of electrons trapped in a magnetic loop at coronal temperatures. Step-spectrum sunspot-associated emission is attributed to thermal gyroresonance radiation at different heights along the leg of a loop joining regions of opposite magnetic polarity. Comparisons with predicted distributions of gyroresonance radiation indicate that the compact sunspot-associated sources lie at heights of h = 2500–17500 km above the photosphere. Although potential fields of sufficient strength appear to exist at coronal heights, differences n the observed and predicted brightness distributions suggest some role for non-potential fields or for an inhomogeneous distribution of electron density or temperature above the sunspot.     

Noise storms and the structure of microwave emission of solar active regions

Solar radio and microwave sources were observed with the Very Large Array (VLA) and the RATAN-600, providing high spatial resolution at 91 cm (VLA) and detailed spectral and polarization data at microwave wavelengths (1.7 to 20 cm - RATAN). The radio observations have been compared with images from the Soft X-ray Telescope (SXT) aboard theYohkoh satellite and with full-disk phoptospheric magnetic field data from the Kislovodsk Station of the Pulkovo Observatory. The VLA observations at 91 cm show fluctuating nonthermal noise storm sources in the middle corona. The active regions that were responsible for the noise storms generally had weaker microwave emission, fainter thermal soft X-ray emission, as well as less intense coronal magnetic fields than those associated with other active regions on the solar disk. The noise storms did, however, originate in active regions whose magnetic fields and radiation properties were evolving on timescales of days or less. We interpret these noise storms in terms of accelerated particles trapped in radiation belts above or near active regions, forming a decimetric coronal halo. The particles trapped in the radiation belts may be the source of other forms of nonthermal radio emission, while also providing a reservoir from which energetic particles may drain down into lower-lying magnetic structures.Presented at the CESRA-Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994.     

Structure of radio sources at wavelengths 10.5 and 12.0 cm from observations of the Solar eclipse on March 29, 2006

The eclipse observations were performed at the Laboratory of Radio Astronomy of the CrAO in Katsiveli with stationary instrumentation of the Solar Patrol at wavelengths of 10.5 and 12.0 cm. The data obtained were used to determine the brightness temperature of the undisturbed Sun at solar activity minimum between 11-year cycles 23 and 24: T _d10.5 = (43.7 ± 0.5) × 10³ K at 10.5 cm and T _d12.0 = (51.8 ± 0.5) × 10³ K at 12.0 cm. The radio brightness distribution above the limb group of sunspots NOAA 0866 was calculated. It shows that at both wavelengths the source consisted of a compact bright nucleus about 50 × 10³ km in size with temperatures T _b10.5 = 0.94 × 10⁶ K and T _b12.0 = 2.15 × 10⁶ K located, respectively, at heights h _10.5 = 33.5 × 10³ km and h _12.0 = 43.3 × 10³ km above the sunspot and an extended halo with a temperature T _b = (230–300) × 10³ K stretching to a height of 157 × 10³ km above the photosphere. The revealed spatial structure of the local source is consistent with the universally accepted assumption that the radiation from the bright part of the source is generated by electrons in the sunspot magnetic fields at the second-third cyclotron frequency harmonics and that the halo is the bremsstrahlung of thermal electrons in the coronal condensation forming an active region. According to the eclipse results, the electron density near the upper boundary of the condensation was N _e ≈ 2.3 × 10⁸ cm^?3, while the optical depth was τ ≈ 0.1 at an electron temperature T _e ≈ 10⁶ K. Thus, the observations of the March 29, 2006 eclipse have allowed the height of the coronal condensation at solar activity minimum to be experimentally determined and the physical parameters of the plasma near its upper boundary to be estimated.     

Radio and soft X-ray evidence for dense non-potential magnetic flux tubes in the solar corona

The source positions of solar radio bursts of spectral types I, III(U) and III(J) and V observed by the Culgoora radioheliograph are found to lie almost radially above soft X-ray loops on pictures taken by the S-056 telescope aboard Skylab. The radio source positions and the X-ray loops occur near magnetic loops on computed potential field maps. However, the magnetic induction required to explain the radio observations is much greater than the computed potential field value at that height. Dense current-carrying magnetic flux tubes emanating from active regions on the Sun and extending to 1.5R above the photosphere provide a satisfactory model for the radio bursts.     

A relation between magnetic field strength and temperature in sunspots

We present Stokes I Zeeman splitting measurements of sunspots using the highly sensitive (g = 3) Fe i line at = 1.5649 m. The splittings are compared with simultaneous intensity measurements in the adjacent continuum. The relation between magnetic field strength and temperature has a characteristic, nonlinear shape in all the spots studied. In the umbra, there is an approximately linear relation between B ² and T _b, consistent with magnetohydrostatic equilibrium in a nearly vertical field. A distinct flattening of the B ² vs T _brelationship in the inner penumbra may be due to changes in the lateral pressure balance as the magnetic field becomes more horizontal; spatially unresolved intensity inhomogeneities may also influence the observed relation.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.     

Coronal densities and magnetic fields from K-coronameter and type IV radio burst data

From K-coronameter data we have obtained an electron density profile above the active region responsible for the Type IV burst observed on 14 September 1966. If the observed frequency cutoff in the burst's spectrum is caused by the Razin effect, then the coronal electron density may be derived from the intensity variation in the burst as it propagates outwards from the Sun. We show that the electron density profiles obtained from K-coronameter data (appropriate to 1.125 <r/R < 2.0) and from the radio data (2.2< r/R < 2.5) form a continuous distribution. We conclude that the cutoff is due to the Razin effect, and that radiation in the burst is due to relativistic electrons having a steep inverse power-law energy distribution. From the electron density profile derived from the radio data, we find that the coronal magnetic field was 0.26 G at r/R = 2.2.     

Joint radio and soft x-ray imaging of an ‘anemone’ active region

The Very Large Array and the Soft X-ray Telescope (SXT) aboard the Yohkoh satellite jointly observed the rapid growth and decay of a so-called anemone active region on 3–6 April, 1992 (AR 7124). The VLA obtained maps of the AR 7124 at 1.5, 4.7, and 8.4 GHz. In general, discrete coronal loop systems are rarely resolved at 1.5 GHz wavelengths because of limited brightness contrast due to optical depth effects and wave scattering. Due to its unusual anemone-like morphology, however, several discrete loops or loop systems are resolved by both the VLA at 1.5 GHz and the SXT in AR 7124.Using extrapolations of the photospheric field and the radio observations at 4.7 and 8.4 GHz, we find that the microwave emission is the result of gyroresonance emission from a hot, rarefied plasma, at the second and/or third harmonic. The decimetric source is complex -1.5 GHz emission from the leading part of AR 7124 is due to free-free emission, while that in the trailing part of the active region is dominated by gyroresonance emission. We also examine an interesting case of a discrete radio loop with no soft X-ray (SXR) emission adjacent to a hot SXR loop. This observation clearly shows the multithermal nature of the solar corona.     

Latent Heating of Coronal Loops

This paper is aimed at establishing the relationship between the large-scale magnetic fields (LSMF), coronal holes (CH), and active regions (AR) in the Sun. The LSMF structure was analyzed by calculating the vector photospheric magnetic field under a potential approximation. Synoptic maps were drawn to study the distribution of the B field component and to isolate regions where the open field lines of the unipolar magnetic field are most radial. These are the sites of occurrence of X-ray and Hei 10830 Å coronal holes detected from the SXT/Yohkoh images. It is shown that coronal holes are usually located in LSMF regions with a typical pattern of divergentB vectors and a so-called saddle configuration.B vectors from the conjugate (spaced by 90°) coronal holes converge towards the active regions between CH. Variations in AR distort coronal holes and change their boundaries. This implies that the energy regime in CH depends on the energy supply from the active region. The LSMF structure is more stable than coronal holes, remaining practically unchanged during tens of rotations of the Sun. Thus, a peculiar magnetically coupled system of LSMF/CH/AR has been revealed. A model has been suggested to describe the interaction of the emerging toroids in the convection zone and in the photosphere. The cellular convection, that develops at the center of the toroids, is responsible for the occurrence of active regions. The model qualitatively describes the observed particularities of the LSMF/CH/AR system.     

VLA,SOHO and TRACE Observations of Nonthermal Burst Activity,Coronal Mass Ejections,and EUV Transient Events

Very Large Array (VLA) observations of the Sun at 91 and 400 cm wavelength have been used to investigate the radio signatures of EUV heating events and coronal mass ejections (CMEs) detected by SOHO and TRACE. Our 91 cm observations show the onset of Type I noise storm emission about an hour after an EUV ejection event was detected by EIT and TRACE. The EUV event also coincided with the estimated start time of a CME detected by the LASCO C2 coronagraph, suggesting an association between the production of nonthermal particles and evolving plasma-magnetic field structures at different heights in the corona. On another day, our VLA 400 cm observations reveal weak, impulsive microbursts that occurred sporadically throughout the middle corona. These low-brightness-temperature (T _b=0.7–22×10⁶ K) events may be weak Type III bursts produced by beams of nonthermal electrons which excite plasma emission at a height where the local plasma frequency or its first harmonic equals the observing frequency of 74 MHz. For one microburst, the emission was contained in two sources separated by 0.7 R ₀, indicating that the electron beams had access to widely-divergent magnetic field lines originating at a common site of particle acceleration. Another 400 cm microburst occurred in an arc-like source lying at the edge of EUV loops that appeared to open outward into the corona, possibly signaling the start of a CME. In most instances the 400 cm microbursts were not accompanied by detectable EUV activity, suggesting that particles that produce the microbursts were independently accelerated in the middle corona, perhaps as the result of some quasi-continuous, large-scale process of energy release.     

New observations of solar noise storm radiation at decameter wavelengths

We report multifrequency observations of storm continuum and other radio bursts. Based on their positional study and their correlation with other coronal and photospheric features, we deduce that the storm source is located in the magnetic field lines lying above a single bipolar active region. Energetic electrons trapped in the magnetic structures above the spots must be responsible for the storm radiation. We show that spontaneous emission of Langmuir waves by anisotropic distributions can explain both storm continuum and bursts self-consistently. Whenever the collisional damping ( _c ) is more than the growth (- _A ), there is a steady emission responsible for the continuum, and whenever _c = - _A (which may be satisfied randomly) there is a sudden jump in T _b giving rise to bursts. The number density of energetic particles required to explain the storm continuum at 73.8, 50, and 30.9 MHz frequencies is estimated to lie in the limits n _b /n _e 10^–10–10^–9 in the context of the present observations. The brightness spectrum of the storm continuum is computed and compared with observations.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

Microwave radiation from magnetic stars

Cyclotron microwave emission from magnetic stars is considered, assuming that they have coronae with the temperatureT10⁷ K and the emission measureEM10⁵⁴ cm^–3. It has been shown that the cyclotron radiation from a star with a dipole magnetic field has a specific spectrum with a maximum in the frequency rangesv _o/2 >v >sv _o/2 (s being the number of cyclotron harmonic, andv _o the gyrofrequency corresponding to the polar magnetic field) and radiation flux decreasing towards lower frequencies asv _4/3. The frequency of the spectrum maximum depends on the angle between the line-of-sight and the magnetic axis of the star. The observed radiation from a rotating magnetic star can be modulated with a modulation depth of about 0.2 at frequencies near maximum. The radiation is partially circularly-polarized in the sense of an extraordinary mode. The degree of polarization is almost constant at frequenciesv >sv _o/2 and increases with frequency atv >sv _o/2. The estimation of cyclotron radio fluxes of the nearest magnetic stars shows that they are observable in microwaves by means of modern radio astronomy.     

Microwave and Soft X-ray Study of Solar Active Region Evolution

We have studied the properties and evolution of several active regions observed at multiple wavelengths over a period of about 10 days. We have used simultaneous microwave (1.5 and 17 GHz) and soft X-ray measurements made with the Very Large Array (VLA), the Nobeyama Radio Heliograph (NRH) and the Soft X-ray Telescope (SXT) on board the Yohkoh spacecraft, as well as photospheric magnetograms from KPNO. This is the first detailed comparison between observations at radio wavelengths differing by one order of magnitude. We have performed morphological and quantitative studies of active region properties by making inter-comparison between observations at different wavelengths and tracking the day-to-day variations. We have found good general agreement between the 1.5 and 17 GHz radio maps and the soft X-rays images. The 17 GHz emission is consistent with thermal bremsstrahlung (free-free) emission from electrons at coronal temperatures plus a small component coming from plasma at lower temperatures. We did not find any systematic limb darkening of the microwave emission from active regions. We discuss the difference between the observed microwave brightness temperature and the one expected from X-ray data and in terms of emission of a low temperature plasma at the transition region level. We found a coronal optical thickness of 10^-3 and 1 for radiation at 17 and 1.5 GHz, respectively. We have also estimated the typical coronal values of emission measure ( 5 × 10²⁸ cm^-5), electron temperature ( 4.5 × 10⁶6 K) and density ( 1.2 × 10⁹ cm₃). Assuming that the emission mechanism at 17 GHz is due to thermal free-free emission, we calculated the magnetic field in the source region using the observed degree of polarization. From the degree of polarization, we infer that the 17 GHz radiation is confined to the low-lying inner loop system of the active region. We also extrapolated the photospheric magnetic field distribution to the coronal level and found it to be in good agreement with the coronal magnetic field distribution obtained from microwave observations.     

Some features of the solar microwave emission and their connection with geomagnetic activity

A statistical investigation of the slowly varying component of the 9.1-cm solar radio emission is based upon the Stanford radioheliograms covering the years 1962–66. On the average the peak value of the brightness temperature T _b is proportional to the area covered by the corresponding spot group. However, in individual cases the observed T _b is definitely lower or higher than is to be expected from the size of the spot group. We introduce the concept microwave importance I _m of the spot group, which is the T _b to be expected from the Zürich class and spot number; and the concept relative brightness B _r, which is the ratio of the observed T _b to I _m. This leads to the distinction of faint, normal and bright sources with B _r 0.8, 0.8 < B _r < 1.2 and B _r 1.2 respectively. B _r is correlated with the maximum magnetic-field strength H observed in the spot group and with the flux-density spectrum of the source. The yearly average of B _r and the average flux-density spectrum vary with the phase of the solar cycle.An analysis of the results is based upon the electron-density distribution in the condensation, which was visible at the solar limb during the eclipse of February 5, 1962, and on an adopted temperature distribution with a central value of 4 × 10⁶ K. The computed T _b, including gyro-resonance absorption, agrees with the value derived from the microwave importance of the spot group and B _r = 0.5, which shows that in the current gyro-resonance models the electron density is underestimated. The variation of T _b with the size of the spot group can be explained by varying the dimensions of the condensation in area and in height, if the central density and temperature remain constant. The statistical relationships between B _r and H and between B _r and the flux-density spectrum yield a model for the differences between faint and bright sources: B _r increases with the contribution from the gyro-resonance absorption and with the central electron density.For paper I see Solar Phys. 7, 448. For paper III see Solar Phys. 8, 450.     

Excitation of whistler waves driven by an electron temperature anisotropy

A 3-D particle simulation of excitation of whistler waves driven by an electron temperature anisotropy (T > T ) is presented. Results show that whistler waves can have appreciable growth driven by the anisotropy. The maximum intensity of the excited whistler waves increases as a quadratic function of the anisotropy. Due to the presence of a threshold, one needs a relatively large electron temperature anisotropy above threshold to generate large-amplitude whistler waves. The average amplitude of turbulence in the context of whistler waves is up to as large as about 1% of the ambient magnetic field when T /T . The total energy density of the whistler turbulence is adequate for production of relativistic electrons in solar flares through stochastic acceleration.     

On the physical conditions in the photospheric network: An improved model of solar faculae

Semi-empirical models of solar faculae, cospatial with strong photospheric magnetic fields, have been constructed from continuum observations. The center-to-limb contrast of the various models was computed taking into account their geometrical shape. The adopted model whose horizontal size was taken to be 750 km, indicates that, in field regions, the temperature begins to rise outwards at z -125 km (above ₅₀₀₀ = 1) and that the extrapolated temperature at z -400 km is about 1500 K above that of the undisturbed atmosphere; the electron density is higher by a factor of about 30.     

The scale and strength of the galactic magnetic field according to the pulsar data

In accordance with the data on the Faraday rotation, angular coordinates, and dispersion measurements and distances of 38 pulsars, the strengthB=2.1±1.1 G and directionl=99°±24°,b0° of the large-scale galactic magnetic field and the mean electron density in the galactic discN _e=0.03±0.01 cm^–3 are determined. A comparison with the results of a study of the measures of rotation of extragalactic radio sources enabled us to estimate the characteristic half-width of the distribution of the electron density on the Z-coordinate (h400 ps). The characteristic size of galactic magnetic field flucturations is shown to be =100–150 ps.     

North-south asymmetry in the heliospheric current sheet and the IMF sector structure

It is shown that in a heliomagnetic field the presence of a magnetic quadrupole in addition to a magnetic dipole introduces a north-south asymmetry in the heliospheric current sheet (HCS) about the heliographic equator. The dominant polarity of the interplanetary magnetic field (IMF) for the above type of current sheet reverses sign at a transition latitude _T, which lies in a heliohemisphere opposite to the one in which the HCS has more heliolatitudinal extension. The position of _T in the heliosphere and the north-south asymmetry introduced in the HCS change with the relative phase of the dipole and quadrupole components present in the solar magnetic field. The effect of the above type of asymmetric HCS in the IMF mean sector width is evaluated and the results are in agreement with the observations during the minima of solar cycle 21.     

Properties of the noise storm source at wavelength 1 m from observations of the solar eclipse on March 29, 2006

Observations of the solar eclipse on March 29, 2006, at the Laboratory of Radio Astronomy of the CrAO showed that the radio radius of the Sun at a wavelength of 1 m in the direction of the first contact was R _d = 1.12 R _⊙ during solar activity minimum between cycles 23 and 24. The brightness temperature of the undisturbed Sun was T _d = (0.6 ± 0.06) × 10⁶ K. There was a noise storm source above the sunspot group NOAA 0865 whose bright nucleus had a size of 1′.3 and a brightness temperature T _b = 16 × 10⁶ K. The noise storm bursts were emitted from the region of the bright nucleus above the group NOAA 0865 and were absent during its covering by the disk of the Moon. Thermal radiation from a coronal condensation with a brightness temperature of (1?2) × 10⁶ K extending out from the visible solar disk to 2′.7 was observed during the eclipse above the eastern limb sunspot group NOAA 0866. The bright nucleus in this limb source appeared 42 min after eclipse termination and persisted in the ensuing days. This may be indicative of the time of its emergence from behind the radio horizon formed by regular refraction of radio waves in the corona. The refractive displacement was measured by comparison with the eclipse observations at a shorter wavelength of 12 cm. Its value of 0′.96 is close to the calculated value of 0′.8.