Helical waves in type-1 comet tails

Oscillations of type-1 comet tails with plasma compressibility taken into account are studied. A comet tail is treated as a plasma cylinder separated by a tangential discontinuity surface from the solar wind. The dispersion equation obtained in the linear approximation is solved numerically with typical plasma parameters. A sufficient condition for instability of the cylindrical tangential discontinuity in the compressible fluid is obtained. The phase velocity of helical waves is shown to be approximately coincident with Alfvén speed in the tail in the reference system moving with the bulk velocity of the plasma outflow in the tail. The instability growth rate is calculated.This theory is shown to be in good agreement with observations in the tails of Comets Kohoutek, Morehouse and Arend-Roland. Hence we conclude that helical waves observed in type-1 comet tails are produced due to the Kelvin-Helmholtz instability, and the model under consideration is justified. If so, one may estimate comet tail magnetic field from the pressure balance at the tangential discontinuity; it turns out to be of the order of the interplanetary magnetic field.     

Laboratory simulation of the induced magnetospheres of comets and Venus

The comparison of data obtained in laboratory experiments on the solar wind interaction with a body endowed with a plasma shell, the observations of comet type I tails and the direct measurements near Venus show that an induced magnetosphere is formed with an extended magnetic tail. This magnetosphere appears due to currents associated with unipolar induction. The distribution of electrodynamical forces associated with the formation of the induced magnetosphere makes it possible to explain the acceleration of matter towards the tail as in the motion across the tail observed in comets and Venus. The analysis of the condensation motion in Halley's comet yields an estimate of tail magnetic field of 30 to 50. A three-dimensional model of the induced magnetospheres of Venus and comets is developed.     

Disconnection Events in Comets

Cometary and solar wind data are compared with the purposeof identifying the solar wind conditions which are associated with comet plasma tail disconnection events (DEs),i.e., when the plasma tail appears disconnected from the cometaryhead. The cometary data are fromThe International Halley Watch Atlas ofLarge-Scale Phenomena (Brandt et al.,1992a). A systematic visual analysis of the atlas images(Voelzke and Matsuura, 1998)revealed, among other morphological structures, 47 DEs alongthe plasma tail of comet P/Halley. This work compares the current competitive theories, based on the triggeringmechanisms, in order to explain the cyclic phenomena of DEs, i.e., the ion production effects, the pressure effects and themagnetic reconnection effects are analysed. The distribution of the DEs in time or heliocentricdistance presents abimodal character possibly associated with the cometary passage through the magnetic sector boundaries in the interplanetary medium.The 47 DEs documented in 47 different images allowed the estimation of 19 onsets of DEs, i.e., the time when the cometsupposedly crossed a frontier between magnetic sectors of the solar wind. The solar wind data are taken from in situ measurements of IMP-8 (King, 1982), which is used to construct the actual variation of solarwind speed, density and dynamic pressureduring the analysed interval. These in situ measurements arereferenced to the comet by standard co-rotationmethods. The preliminary results ofthis research reveal that the DEs onsetsof comet P/Halley are correlated with pressureeffects only in 23% of the analysed cases,therefore these effects should not be the principaltriggering mechanism of DEs.     

Physical characteristics of the plasma tail of comet C/2009 R1 (McNaught)

We determined brightness distribution in the plasma tail of comet C/2009 R1 (McNaught) using observations with a small Newtonian reflector (200/1000) on June 9?C12, 2010. Images of the comet were detected using short exposures with a Canon CMOS APS-C camera. The brightness distribution is simulated and the parameters of the cometary plasma tail are obtained within the diffusion model. The magnetic field induction in the cometary tail, lifetime of light particles, and the lengthwise and transverse ion diffusion coefficients are estimated.     

Polar cap absorption on vlf emissions

It is shown that VLF emissions are greatly affected by the polar cap absorption caused by the bombardment of solar protons. Characteristics of the PCA effect on VLF emissions are examined and they are in agreement with those obtained by other studies, such as the polar blackout and the cosmic radio absorption. Therefore, the earlier conclusion that the occurrence frequency of VLF emissions decreases in high latitudes during magnetic storms is likely to be due to the PCA effect.

Taking this PCA effect into account, it is established that an enhancement of occurrence of VLF emissions occurs at geomagnetic latitudes lower than 67° during the magnetic storm. This suggests that enhanced VLF emissions during geomagnetic storms are generated in the co-rotating region of the magnetosphere or in the outer radiation belt, but not in the tail region.     

Measurement of the solar wind velocity with cometary tail rays

Cometary tail rays are traces of the magnetic fields caught in the cometary magnetosphere. Time variations of these rays give us a way to measure the local solar wind velocity at the location of a comet. We introduce a simple method for determining the radial velocity of the solar wind by observing the ray folding motion, and show an example of its application to comet P/Brorsen-Metcalf 1989o, which resulted in 340 ± 35 km s^–1.     

Search for circum‐planetary material and orbital period variations of short‐period Kepler exoplanet candidates

A unique short‐period (P = 0.65356(1) d) Mercury‐size Kepler exoplanet candidate KIC012557548b has been discovered recently by Rappaport et al. (2012). This object is a transiting disintegrating exoplanet with a circum‐planetary material–comet‐like tail. Close‐in exoplanets, like KIC012557548b, are subjected to the greatest planet‐star interactions. This interaction may have various forms. In certain cases it may cause formation of the comet‐like tail. Strong interaction with the host star, and/or presence of an additional planet may lead to variations in the orbital period of the planet. Our main aim is to search for comet‐like tails similar to KIC012557548b and for long‐term orbital period variations. We are curious about frequency of comet‐like tail formation among short‐period Kepler exoplanet candidates. We concentrate on a sample of 20 close‐in candidates with a period similar to KIC012557548b from the Kepler mission. We first improved the preliminary orbital periods and obtained the transit light curves. Subsequently we searched for the signatures of a circum‐planetary material in these light curves. For this purpose the final transit light curve of each planet was fitted with a theoretical light curve, and the residuals were examined for abnormalities. We then searched for possible long‐term changes of the orbital periods using the method of phase dispersion minimization. In 8 cases out of 20 we found some interesting peculiarities, but none of the exoplanet candidates showed signs of a comet‐like tail. It seems that the frequency of comet‐like tail formation among short‐period Kepler exoplanet candidates is very low. We searched for comet‐like tails based on the period criterion. Based on our results we can conclude that the short‐period criterion is not enough to cause comet‐like tail formation. This result is in agreement with the theory of the thermal wind and planet evaporation (Perez‐Becker & Chiang 2013). We also found 3 cases of candidates which showed some changes of the orbital period. Based on our results we can see that orbital period changes are not caused by comet‐like tail disintegration processes, but rather by possible massive outer companions. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Meteoroid Streams Associated to Comets 9P/Tempel 1 and 67P/Churyumov-Gerasimenko

The meteoroid streams associated to short-period comets 9P/Tempel 1 (the target of the Deep Impact mission)^. and 67P/Churyumov-Gerasimenko (the target of the Rosetta mission) are studied. Their structure is overwhelmingly under the control of Jupiter and repeated relatively close encounters cause a reversal of the direction of the spatial distribution of the stream relative to the comet* an initial stream trailing the comet as usually seen eventually collapses, becomes a new stream leading the comet and even splits into several components. Although these two comets do not produce meteor showers on Earth, this above feature shows that meteor storms can occur several years before the perihelion passage of a parent body.     

The estimates of the magnetic field in Halley's comet

The estimates of the magnetic field in the Halley's comet tail based on the observed acceleration of the matter and the pressure balance are discussed.     

3D triangulation of a Sun-grazing comet

The bright Kreutz Comet C/2007 L3 (SOHO) entered the fields of view of the twin Solar Terrestrial Relations Observatory (STEREO) COR1 telescopes on 7–8 June 2007. The 12° separation between the two spacecraft at the time afforded the opportunity to derive the position of the comet's tail in three-dimensional space using direct triangulation. The track of the comet's orbit is compared against more traditional orbital calculations using observations from the STEREO COR2 telescopes, and from the Large Angle and Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO). The shape of the comet's tail shows that it is composed of dust particles released when the comet was between 18 and 22 solar radii, with no significant dust production after that. The comet did not survive perihelion passage, but a rare faint remnant of the comet tail persisted for several hours after the break-up, and was seen by both the SOHO and STEREO coronagraphs to drift slowly away from the Sun. This tail remnant was found to be composed of particles far back from the head of the comet. The motion of the tail remnant shows a loss of angular momentum during the passage through the solar corona. Atmospheric drag is estimated to account for a significant fraction of this change in angular momentum, but indications are that other mechanisms may be required to completely account for the total amount of change.     

Comet plasma densities deduced from refraction of occulted radio sources

Eighty MHz observations of the occultation of the radio source Culgoora-1 0300 + 16 by the plasma tail of Comet Kohoutek (1973f) were made in February/March 1974 with the Culgoora radioheliograph. No detectable source broadening or change in flux density was observed, but the results showed a 2' arc anomaly in the observed position. This is greater than can be attributed to ionospheric refraction or experimental error. We suggest that it arose from refraction in the plasma tail of the comet. Similar observations of the occulation of the radio source Culgoora-1 2313-14 by the plasma tail of Comet West (1975n) were made at Culgoora in February 1976. These results were inconclusive but did suggest that the cometary plasma may have had some influence on the observed source position. The results are used to derive, from simple models, the distribution of electron density in comet tails. Peak electron densities of approximately 2 to 5 × 10⁴ cm^?3 and density gradients of ～0.05 cm^?3 km^?1 are indicated.     

On the shape and orientation of the tail of Comet Kohoutek (1973f)

Twenty-two photographs of Comet Kohoutek (1973f) have been projected onto the comet's orbital plane under the assumption of a flat tail confined in that plane. The comet has a mixed-type tail; therefore the results concerning Type I and Type II components of the tail are presented separately. The axis of the Type I tail appears to sweep back and forth with respect to the prolonged radius vector in a rather periodical way. Interpretations advanced by some authors for the analogous case of Comet Burnham (1959k) are mentioned briefly. For the Type II tail, a comparison with a theoretical tail model by Sekanina has allowed us to establish when the onset of appreciable dust production occurred. Finally, mention is made of the fact that some peculiarities of the dust tail might be also explained by assuming a three-dimensional model.     

哈雷彗星主尾流的研究

哈雷彗星在日彗距较大时出现长而直的主彗尾（尾流），这是很有趣的。尾流一般是指等离子体尾流；但是，当地球接近彗星轨道面时，尘埃尾流可能叠加到主彗尾上。在一般感光波段宽的彗星底片上很难区分这两种尾流。本文选取哈雷彗星在不同日彗距的5条主尾流，作了光度测量和比较分析。得出沿各尾轴及其垂直方向几个截面的亮度分布、亮度半极大全宽、尾轴的视风差角和真风差角及彗尾长度。在所分析的蓝敏底片上，过近日点前的2个尾流肯定是等离子体尾流，而5个尾流的相似性以及其他证据说明它们主要都是等离子体尾流，尘埃彗尾的污染是次要的。     

Physical characteristics of the plasma tail of comet C/2006 M4 (SWAN)

Wide-angle images with narrow-band filters isolating the radiation of CO⁺ ions and continuum of comet C/2006 M4 (SWAN) were acquired. The brightness distribution in the plasma tail shortly after the outburst of the comet on October 26, 2006, has been derived. The model brightness distribution was calculated, and the parameters of the diffusion model of the plasma tail have been obtained. For the date of observations, the magnetic-field induction in the plasma tail, as well as the lifetime of luminous particles and the lengthwise and transverse diffusion coefficients of ions have been estimated. Possible association between the change in the tail structure and that in the physical parameters along the plasma tail of the comet is discussed.     

Peculiarities in the ionic tail of Comet Ikeya-Seki (1965 f)

Direct photographs of Comet Ikeya-Seki obtained on four consecutive days from October 29 to November 1, 1965, are used for an analysis of the multiple helical structures in the ionised tail. The formation of these structures is explained on the basis of plasma instabilities excited in the tail containing twisted magnetic fields. The growth rate of the modes excited at the mode rational surface agrees well with the observed results. This model also accounts for the presence of harmonic structures seen in the tail of the comet.     

DE-like event in the plasma tail of comet C/2006 M4 (SWAN)

We investigate a DE-like event of October 27, 2006, in the plasma tail of comet C/2006 M4 (SWAN). A set of phenomena in the comet??s tail after the brightness outburst on October 24.04, 2006, is described. The typical dimensions of the tail structure details that developed as a result of plasma instabilities are found. The velocity and acceleration of the disconnected tail fragment and the beginning time of the event are determined. The likely cause of the DE-like event is the comet??s encounter with a high-speed stream of solar wind.     

Comet Tago-Sato-Kosaka 1969 IX: Tail structure 25 December 1969 to 12 January 1970

On photographs taken between 25 December 1969 and 12 January 1970 with the Michigan Curtis Schmidt on Cerro Tololo and the Hamburg Schmidt at the Boyden Observatory, Comet 1969 IX is seen projected on planes nearly perpendicular to its orbit. During this period the Type I tail was free of major structural disturbances, and the faintness of the Type II tail facilitated study of the Type I tail near the lead. Characteristics of the main Type I tail are described in Part II. A classification of the tail forms according as the tail had the form of a fan of rays or a wedge, the orientation of the tail axis, and the angular width of the tail are discussed in Section IIA. In Section IIB the structure of the tail from 26 to 31 December, a period within which a disconnection of the tail took place, is viewed as the possible result of the passage across the comet of an interplanetary magnetic sector boundary, as proposed by Niedner and Brandt (1977, Bull. Amer. Astron. Soc. 4). In Sections IIC and IID waves in rays and the onset of turbulence are considered. Section IIIA is devoted to unusually long and bright off-axis rays; data on longitudinal forms, closing rate to the axis, and photometry of a ray are presented. In Section IIIB the structure of a “plume” of seven nearly parallel rays is related to the configuration of magnetic field lines predicted by Schmidt and Wegmann (1976, IPP Report 6/147). The orientations of the plume rays are consistent with those of the calculated field lines; the inferred rate of generation of the rays is about 1/hr.     

Non-linear waves in type-1 comet tails

The non-linear stabilization of Kelvin-Helmholtz instability occurring when disturbance amplitudes become finite is studied. A comet tail is considered as a plasma cylinder with free boundary immersed in the solar wind plasma. Due to Kelvin-Helmholtz instability of kink modes the type-1 comet tail is shown to take stable shape. However the sausage-like modes with wavelength of cylinder circumference cannot be stabilized due to finite amplitudes. The Kelvin-Helmholtz instability of sausage modes leads to the comet tail (or individual ray) losing its regular structure and breaking into individual clouds with the characteristic scale of the order of the cylinder diameter.     

Comet Hyakutake (C/1996 B2): Spectacular disconnection event and the latitudinal structure of the solar wind

Images of comet Hyakutake (C/1996 B2) are analyzed in conjunction with solar wind data from spacecraft to determine the relationship between solar wind conditions and plasma tail morphology. The disconnection event (DE) on March 25, 1996 is analyzed with the aid of data from the IMP-8 and WIND Earth-orbiting spacecraft and the DE is found to be correlated with a crossing of the heliospheric current sheet. The comet was within of Earth at the time of the DE and data from IMP-8 and WIND show no high-speed streams, significant density enhancements or shocks.The latitudinal variation in the appearance and orientation of the plasma tail are interpreted based on results from the Ulysses spacecraft. In the polar solar wind region, the comet has a relatively undisturbed appearance, no DEs were observed, and the orientation of the plasma tail was consistent with a higher solar wind speed. In the equatorial solar wind region, the comet's plasma tail had a disturbed appearance, a major DE was observed, and the orientation of the plasma tail was consistent with a lower solar wind speed. The boundary between the equatorial and polar regions crossed by comet Hyakutake in April 1996 was near 30°N (ecliptic) or 24°N (solar) latitude.     

The Automatic Predictability of Super Geomagnetic Storms from halo CMEs associated with Large Solar Flares

We investigate the relationship between magnetic structures of coronal mass ejection (CME) source regions and geomagnetic storms, in particular, the super storms when the D _st index decreases below −200 nT. By examining all full halo CMEs that erupted between 1996 and 2004, we selected 73 events associated with M-class and X-class solar flares, which have a clearly identifiable source region. By analyzing daily full-disk MDI magnetograms, we found that the horizontal gradient of the line-of-sight magnetic field is a viable parameter to identify a flaring magnetic neutral line and thus can be used to predict the possible source region of CMEs. The accuracy of this prediction is about 75%, especially for those associated with X-class flares (up to 89%). The mean orientation of the magnetic structures of source regions was derived and characterized by the orientation angle θ, which is defined to be ≤ 90^∘ in the case of the southward orientation and ≥ 90^∘, when the magnetic structure is northwardly oriented. The orientation angle was calculated as the median orientation angle of extrapolated field lines relative to the flaring neutral line. We report that for about 92% of super storms (12 out of 13 events) the orientation angle was found to be southward. In the case of intense and moderate storms (D _st≥ −200 nT), the relationship is less pronounced (70%, 21 out of 30 events). Our findings demonstrate that the approach presented in this paper can be used to perform an automatic prediction of the occurrence of large X-class flares and super geomagnetic storms.