Polar Plumes Research Articles

Magnetohydrodynamic waves in coronal polar plumes

Polar plumes are cool, dense, linear, magnetically open structures that arise from predominantly unipolar magnetic footpoints in the solar polar coronal holes. As the Alfvén speed is decreased in plumes in comparison with the surrounding medium, these structures are natural waveguides for fast and slow magnetoacoustic waves. The simplicity of the geometry of polar plumes makes them an ideal test ground for the study of magnetohydrodynamic (MHD) wave interaction with solar coronal structures. The review covers recent observational findings of compressible and incompressible waves in polar plumes with imaging and spectral instruments, and interpretation of the waves in terms of MHD theory.

An overview of coronal seismology

The idea of exploiting observed oscillations as a diagnostic tool for determining the physical conditions of the coronal plasma was first suggested several decades ago (Roberts et al. 1984 Astrophys. J. 279, 857). Until recently, the application of this idea has been very limited by a lack of high-quality observations of coronal oscillations. However, during the last few years, this situation has changed dramatically, especially due to space-based observations by the Solar and Heliospheric Observatory and the Transition Region and Coronal Explorer and waves and oscillations have now been observed in a wide variety of solar structures, such as coronal loops, polar plumes and prominences. This paper will briefly summarize MHD wave theory, which forms the basis for coronal seismology, as well as present an overview of the variety of recently observed waves and oscillations in the solar corona. The present state of coronal seismology will also be discussed. Currently, the uncertainty associated with the obtained parameters is still considerable and, hence, the results require a cautious interpretation. However, these examples do show that coronal seismology is rapidly being transformed from a theoretical possibility to a viable technique.

Polar Coronal Structures and the Global Magnetic Field Evolution Through the Cycle

We compare the shape and position of some plasma formations visible in the polar corona with the cyclic evolution of the global magnetic field. The first type of object is polar crown prominences. A two-fold decrease of the height of polar crown prominences was found during their poleward migration from the middle latitudes to the poles before a polar magnetic field reversal. The effect could be assigned to a decrease of the magnetic field scale. The second type of object is the polar plumes, ray like structures that follow magnetic field lines. Tangents to polar ray structures are usually crossed near some point, “a magnetic focus,” below the surface. The distance q between the focus and the center of the solar disk changes from the maximum value about 0.65 R ⊙ at solar minimum activity to the minimum value about 0.45 R ⊙ at solar maximum. At first glance this behaviour seems to be contrary to the dynamics of spherical harmonics of the global magnetic field throughout a cycle. We believe that the problem could be resolved if one takes into account not only scale changes in the global magnetic field but also the phase difference in the cyclic variations of large-scale and small-scale components of the global field.

Outflow Velocities in Polar Coronal Holes

Observations have been carried out using the oxygen VI multiplet ratio 1032/1038 Â from SUMER on SOHO. Analysis based on the Doppler dimming method shows that the outflow velocity in polar plumes is higher than that in the interplume region, contrary to many published suggestions. The addition of UVCS data for the interplume region, leads to a conclusion that the effective oxygen ion “temperature” in the radial direction has to rise to around 14 MK over the height range 1.5 to 1.8 R⊙.

The Contribution of Polar Plumes to the Fast Solar Wind

The Doppler dimming technique is used for the first time to study ultraviolet polar plumes in the height range of 1.05-1.35 R☉, using observations from the spectrometer SUMER on the Solar and Heliospheric Observatory. It is found that, contrary to a number of published suggestions, outflow velocities in the plumes exceed those in the interplume regions. Plume velocities are in excess of 60 km s-1 and are approximately constant throughout this height region. They tend to converge with the velocity of the accelerating interplume material at some height above our region of study. The analysis suggests that plume material makes a substantial contribution to the total line of sight, favoring either a curtain model for plumes or a chance alignment of a number of elementary cylindrical plumes. The intrinsic local density of plume material is some 20%-50% in excess of the interplume regions. Estimation of the total mass outflow indicates that approximately half of the fast solar wind at 1.1 R☉ arises from plumes, with the remainder from interplume material. This result validates the published electron temperature profile of David et al. for the fast wind onset, which had been questioned over the suggestion that the flow velocity might be negligible in solar plumes.

Variation of coronal line widths on and off the disk

We present observations of a Mg x 625 Å coronal line, obtained with the cds instrument on SoHO, extending from the disk part of the coronal hole to ~90 000 km above the limb in the north polar coronal hole. Observations were performed in polar plumes and inter-plume lanes. To obtain a sufficiently high signal-to-noise ratio the observations were made over long periods of time and subsequent time frames were summed up. For the off-limb observations we notice a turnover point, around 65 000 km above the limb, where the line widths seem to suddenly decrease or level-off. The initial linear increase of line width with altitude supports previous observations and is consistent with an interpretation of linear undamped Alfvèn waves propagating outwards in open field regions. The turnover point seems to indicate the location where a change of physics takes place. We find that this turnover occurs at approximately the same line width value for each of the datasets examined, suggesting that the turnover occurs whenever the non-thermal velocity reaches a certain key velocity. For the on-disk data we find larger widths in the deep coronal hole as compared to the adjacent quiet Sun regions, suggesting the presence of additional waves and/or turbulence in the coronal hole.

Spectroscopic characteristics of polar plumes

Extreme ultraviolet observations of plumes in polar coronal holes are presented and their spectroscopic signatures discussed. The study focuses on the base of plumes seen on the disk of the Sun with the Coronal DiagnosticSpectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) satellite. Spectroscopic diagnostic techniques are applied to characterise the plumes in terms of density, temperature, emission measure and element abundance. Attention is drawn to the particular limitations of some of the techniques when applied to plume structures. In particular, we revisit the Widing & Feldman (1992) findings of a plume having a large first ionization potential (FIP) effect of 10, showing that instead the Skylab data are consistent with no FIP effect. We present for the first time CDS-GIS (grazing incidence spectrometer) observations of a plume. These observations have been used to confirm the results obtained from normal incidence (NIS) observations. We find that polar plumes exhibit the same characteristics as the Elephant's Trunk equatorial plume. The most striking characteristic of the plume bases is that they are near-isothermal with a peak emission measure at transition region temperatures ≃8 x 10 5 K. At these temperatures, plumes have averaged densities N e ≃ 1.2 x 10 9 cm-3, about twice the value of the surrounding coronal hole region. Element abundances in the plumes are found to be close to photospheric, with the exception of neon which appears to be depleted by 0.2 dex relative to oxygen. The absence of a significant FIP effect in plumes is consistent with fast solar wind plasma, although it is not sufficient to prove a link between the two. Finally, we present a comparison between GIS spectra and the SOHO EIT (EUV Imaging Telescope) broad-band images, showing that temperatures derived from the EIT ratio technique are largely overestimated, for plumes and coronal holes. This is partly due to the fact that the so called Fe XII 195 A and Fe XV 284 A filters are not isothermal, and in coronal holes and plumes lower-temperature lines dominate the EIT signal.

Relation between Pressure Balance Structures and polar plumes from Ulysses high latitude observations

Ulysses observations have shown that pressure balance structures (PBSs) are a common feature in high‐latitude, fast solar wind near solar minimum. Previous studies of Ulysses/SWOOPS plasma data suggest these PBSs may be remnants of coronal polar plumes. Here we find support for this suggestion in an analysis of PBS magnetic structure. We used Ulysses magnetometer data and applied a minimum variance analysis to magnetic discontinuities in PBSs. We found that PBSs preferentially contain tangential discontinuities, as opposed to rotational discontinuities and to non‐PBS regions in the solar wind. This suggests that PBSs contain structures like current sheets or plasmoids that may be associated with network activity at the base of plumes.

Relationship between the coronal shape and the magnetic field topology during the solar cycle

We have observed ten solar eclipses during the 1973–1999 period, three of them being recorded during the rising phase of the present solar cycle 23. The observed shapes of the white-light corona are confronted with the magnetic field topology, as calculated for the corresponding eclipse days. A close relationship of the distinctive large-scale coronal structures (coronal streamers, coronal holes, polar plumes, etc.) with the calculated magnetic field structures and with the actual position of the heliospheric current sheet (as derived for the source surface at r = 2.5 R ⊙) is evident. The found relations suggest a new understanding of the coronal shape evolution during the solar cycle. It is shown that “the isogausses” and the coronal isophotes create two systems of mutually orthogonal curves. The nature of this finding is also well confirmed by estimating of the magnetic field strength inside the coronal holes observed during “our” eclipses.

Characterization of the polar plumes from high cadence LASCO-C2 observations

In order to understand the spatio-temporal characteristics of polar plumes, we analyzed a sequence of continuous observations obtained with the SOHO/LASCO-C2 coronagraph over a period of 3 days with a time resolution of 10 minutes. We generated the time intensity diagram (TID) where the radiance of the plumes is displayed as a function of their angular positions and time. The derivatives of the TID with respect to the phase angle on the one hand and to time on the other hand allow us to define the trajectories of individual plumes and their temporal variations. This confirms that polar plumes are enduring structures that are only transiently lit, as we found before, but the present analysis reveals that the blinking takes place on time scales as short as 2 hours. This is confirmed by the radial intensity diagrams (RID) which track each individual plume by displaying its radiance as a function of time and distance. These diagrams also indicate a constant speed of expansion with a mean value of 460 km/sec.

Solar Polar Plume Lifetime and Coronal Hole Expansion: Determination from Long‐Term Observations

We have generated off-limb polar synoptic charts of polar plume evolution at various solar altitudes using EUV Imaging Telescope and Large Angle and Spectrometric Coronagraph data from 1996 December. The charts allow direct measurement of the altitude expansion of the solar minimum coronal holes. We find expansion values that are consistent with the conventional picture of superradial expansion and inconsistent with radial expansion. Using visible red line data as a bridge between EUV and white-light images of the corona, we are able to confirm that the coronal structure seen at the base of the corona is preserved throughout the considered altitude range of 1.1-3.0 R☉. We show that polar plumes are episodic in nature, lasting perhaps 24 hr but recurring for up to weeks at a time; this strengthens the picture that they are caused by magnetic heating under the influence of supergranulation.

The three-dimensional structure of the solar corona

The surface where the radial component of the solar magnetic field changes sign is computed for a minimum corona. It is shown that (1) the projection of the folds of this surface onto the plane of the sky is consistent with the helmet structures observed during the eclipse of June 30, 1954; (2) there are type 1 and type 2 helmets, according to the well-known classification of coronal structures; (3) some elements of this sign-change surface of the radial field can be classified as so-called envelopes. The results obtained suggest that more complex coronal structures can be described in a similar way. An MHD model of polar plumes is considered.

Ultraviolet Coronagraph Spectrometer Observations of a High-Latitude Coronal Hole with High Oxygen Temperatures and the Next Solar Cycle Polarity

We announce the resurgence of extreme ion properties in a large, high-latitude coronal hole observed above the north heliographic pole in 2001 February at solar maximum. The observations were taken with the Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric Observatory. These observations are part of an ongoing campaign to determine the plasma properties of coronal holes during the current solar cycle. In this Letter, we compare the observations and analysis of O VI λλ1032, 1037 spectral lines of a high-latitude coronal hole in 2001 with observations of an equatorial solar maximum hole in 1999 and polar coronal holes observed near solar minimum (1996-1997). These lines provide spectroscopic diagnostics of O+5 velocity distributions and outflow velocities. The O VI line profiles show a narrow core and broad wings. The narrow core is attributed to foreground and background streamers and, possibly, dense polar plumes at the lowest observed heights. The broad wings are attributed to the coronal hole. The comparison of the coronal hole line widths shows that the O+5 perpendicular kinetic temperatures in the 2001 high-latitude hole are similar to those observed in polar coronal holes at solar minimum. These observations of extremely high ion kinetic temperatures (exceeding 108 K) at the north pole in 2001 occurred nearly simultaneously with the change of the Sun's magnetic field, as seen in recent magnetogram data. This coronal hole in 2001 may represent the first manifestation of the negative polar coronal holes that will dominate the Sun's open magnetic flux tubes at the next solar minimum. The reappearance of broad O VI profiles at a time when not all of the new polarity magnetic flux has migrated to the poles was an interesting development. The variations in coronal hole parameters with the solar cycle provide constraints on models of extended coronal heating.

Long Period Oscillations in Polar Plumes as Observed by CDS on SoHO

We examine time series of the transition region spectral line O v 629 Å, observed with the Coronal Diagnostic Spectrometer (CDS) onboard the SoHO spacecraft. Primarily we were looking for intensity and velocity oscillations in polar plumes, however by chance we detected a giant macro-spicule at the limb and were able to follow its dynamical structure. We have reported the dynamics of the giant macro-spicule in Banerjee et al. (2000a) and have also reported the existence of long period oscillations in the polar plumes as observed in the O v 629 Å line in Banerjee et al. (2000a,b). In this short contribution we revisit one of these dataset, namely s16834r01 to test the reliability of the observed periods. The statistical significance of the oscillations is estimated by using a randomisation method. Our analysis re-confirms the presence of compressional waves with periods of 20-25 minutes.

Solar Polar Rays Are Not Polar

We discuss the nature of polar rays, based on limb synoptic images constructed from Yohkoh/SXT and SORafElT data. In the literature, polar rays and polar plumes are often mentioned interchangably. We find that polar rays are projection effects caused by hot plasma from equatorial active areas and are not physically associated with the coronal polar holes. Instead, the rise in number and strength of polar rays toward solar activity maximum may be responsible for hiding the polar holes and polar plumes.

Observation of Polar Plumes at High Solar Altitudes

Using the Large-Angle Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft, we have imaged polar plumes extending 30 R☉ from disk center in the image plane and ~45 R☉ in three-dimensional space, a factor of 2-3 farther than previous imaging measurements and well into the constant-velocity regime of wind flow. We find that the plumes maintain their overall linear morphology and density enhancement to at least this altitude range. Using LASCO photometry and a modeled cylindrical plume geometry, we derive the density excess within the plumes 30 R☉ above the Sun (in three dimensions). At this altitude, the plumes are (2-4) × 103 cm-3 above the background interplume density, with an estimated plasma β of order 300. The excess electron densities are a factor of 20-30 greater than the average total electron density estimates obtained from extrapolation of in situ measurements by Ulysses at 1 AU. The contrast between the high plume excess densities that we observe and the uniformity of the wind seen by Ulysses may best be explained by wind models that include horizontal mixing in the lower heliosphere between 45 R☉ and Ulysses's altitude of ~200 R☉.

Helical Magnetic Structure of White Light Polar Plumes

We describe the fine structure of white light polar plumes observed using SOHO/LASCO C2 coronagraph. The evolving helical structures of different scales are clearly seen on processed images (the processing is made to reveal the faint contrast objects). The observed structures trace the magnetic field lines, so the electric currents flow along the axes of plumes. An MHD model of a plume which takes into account field-aligned electric currents is developed. The model permits to understand the existence of high-density plasma inside the plume due to the balance between the magnetic forces and the transverse pressure gradient.

Global Solar Corona Revealed by Time Series Observations

Time series observations at UV (Extreme Ultraviolet Imaging Telescope/Solar and Heliospheric Observatory) and X-ray (Soft X-Ray Telescope/Yohkoh) wavelengths reveal properties of the global solar corona that are not easily identified in a single image. A median-filtering technique that rejects features varying with time is used to isolate background corona. The coronal hole boundaries, polar plumes, and polar rays in the inner corona are clearly seen in Extreme Ultraviolet Imaging Telescope data over 5 months during the last solar minimum (1996 January through May). For the first time, we provide physical evidence for coronal hole boundaries in the inner corona. The observations show clearly that the polar coronal holes expand divergently with height. A simple latitudinal and radial electron density distribution for the inner corona is found.

Dissipation of Slow Magnetosonic Waves in Coronal Plumes

Recently, slow magnetosonic waves were identified in polar plumes, at heights up to about 1.2 R☉ using the Extreme Ultraviolet Imaging Telescope (EIT) observations of quasi-periodic EUV intensity fluctuations, and higher in the corona using the Ultraviolet Coronagraph Spectrometer (UVCS) white-light channel. First, we derive the linear dispersion relation for the slow waves in the viscous plasma. Next, we derive and solve an evolutionary equation of the Burgers type for the slow waves, incorporating the effects of radial stratification, quadratic nonlinearity, and viscosity. Finally, we model the propagation and dissipation of slow magnetosonic waves in polar plumes using one-dimensional and two-dimensional MHD codes in spherical geometry. The waves are launched at the base of the corona with a monochromatic source. We find that the slow waves nonlinearly steepen as they propagate away from the Sun into the solar wind. The nonlinear steepening of the waves leads to enhanced dissipation owing to compressive viscosity at the wave fronts. The efficient dissipation of the slow wave by compressive viscosity leads to damping of the waves within the first solar radii above the surface. We investigate the parametric dependence of the wave properties.

Comparison of 87 GHz solar polar structures with EUV and soft X-ray emission

Polar radio brightenings at 87 GHz (3.5 mm) are compared for the rst time with features seen in EUV and soft X-rays. The data consist of nearly simultane- ous full disk images and maps from Metsahovi Radio Observatory, SOHO/EIT, and Yohkoh/SXT on 9 selected days near the solar minimum (1996-1997). The observed radio brightenings corresponded to var- ious features seen in EUV, such as diuse or localized intensity enhancements (e.g., bright points and bases of polar plumes), and intensity depressions of varying sizes (e.g., coronal holes). Some of these features were also vis- ible in soft X-rays. The visibility of radio bright coronal holes seemed to depend on how much of the polar area was exposed, due to the variation of the B0-angle. The observed radio depressions near the solar poles were very well correlated with coronal holes and other EUV and/or soft X-ray intensity drops. More than half of the coronal holes, or coronal hole-like intensity drops in EUV and soft X-rays, had radio brightenings inside them. Therefore coronal holes do not have uniform radio bright- ness at 87 GHz. Many of the bright points seen at lower latitudes in the EIT and SXT images had no, or just faint, counterparts in the millimeter radio maps. It appears that for an EUV bright point to show up at 87 GHz it has to be bright and/or spatially large also in soft X-rays.