Solar Coronal Plumes

Polar plumes are thin long ray-like structures that project beyond the limb of the Sun polar regions, maintaining their identity over distances of several solar radii. Plumes have been first observed in white-light (WL) images of the Sun, but, with the advent of the space era, they have been identified also in X-ray and UV wavelengths (XUV) and, possibly, even in in situ data. This review traces the history of plumes, from the time they have been first imaged, to the complex means by which nowadays we attempt to reconstruct their 3-D structure. Spectroscopic techniques allowed us also to infer the physical parameters of plumes and estimate their electron and kinetic temperatures and their densities. However, perhaps the most interesting problem we need to solve is the role they cover in the solar wind origin and acceleration: Does the solar wind emanate from plumes or from the ambient coronal hole wherein they are embedded? Do plumes have a role in solar wind acceleration and mass loading? Answers to these questions are still somewhat ambiguous and theoretical modeling does not provide definite answers either. Recent data, with an unprecedented high spatial and temporal resolution, provide new information on the fine structure of plumes, their temporal evolution and relationship with other transient phenomena that may shed further light on these elusive features

Low-Frequency Lyα Power Spectra Observed by UVCS in a Polar Coronal Hole

The occurrence of f−1 noise in interplanetary magnetic fields (in the 1 × 10−5 to 1 × 10−4 Hz band) and other plasma parameters has now been known for about 20 years and has been recently identified also in the photospheric magnetic fields. However, the relationship between interplanetary and solar fluctuation spectra and the identification of their sources at the Sun are problems that still need to be addressed. Moreover, interplanetary density and magnetic field power spectra show a f−2 interval at frequencies smaller that ~6 × 10−4 Hz whose source on the Sun is at present not fully understood. In this work we report on the first study of low-frequency density fluctuations in the solar corona at 2.1 R☉. In 2006 June the Ultraviolet Coronagraph Spectrometer (SOHO UVCS) observed over a period of about 9.2 days H Lyα intensity fluctuations at 2.1 R☉ over a polar coronal hole. The Lyα intensity power spectra S(f) (related mainly to density fluctuations) showed a S(f) ∝ f−2 frequency interval between 2.6 × 10−6 and 3.0 × 10−5 Hz and a S(f) ∝ f−1 frequency interval between 3.0 × 10−5 and 1.3 × 10−4 Hz. The detection of a f−2 interval, in agreement with interplanetary density and magnetic field power spectra, has been also predicted in solar wind models as a consequence of phase-mixing mechanisms of waves propagating in coronal holes. High-latitude power spectra show a f−1 band approximately in the same frequency interval where f−1 noise has been detected in interplanetary densities, and interplanetary and photospheric magnetic fields, providing a connection between photospheric, coronal, and interplanetary f−1 noises

A Comprehensive Study of the Initiation and Early Evolution of a Coronal Mass Ejection from Ultraviolet and White-Light Data

In this work we analyze simultaneous UV and white-light (WL) observations of a slow CME that occurred on 2000 January 31. Unlike most CMEs studied in the UV so far, this event was not associated with a flare or filament eruption. Based on vector magnetograph data and magnetic field models, we find that field disruption in an active region (AR) was driven by flux emergence and shearing motions, leading to the CME and to post-CME arcades seen in the EUV. WL images, acquired by the Mark IV coronagraph at the Mauna Loa Observatory, allowed us to identify the CME front, bubble, and core shortly (about 1 hr) after the CME ejection. From polarized brightness (pB) Mauna Loa data we estimated the mass and electron densities of the CME. The CME mass increases with time, indicating that about 2/3 of the mass originates above 1.6 R☉. Analysis of the UV spectra, acquired by the Solar and Heliospheric Observatory Ultraviolet Coronagraph Spectrometer (SOHO UVCS) at 1.6 and 1.9 R☉, allowed us to derive the electron temperature distribution across the CME. The temperature maximizes at the CME core and increases between 1.6 and 1.9 R☉. This event was unusual, in that the leading edge and the CME core were hotter than the ambient corona. We discuss magnetic heating and adiabatic compression as explanations for the high temperatures in the core and leading edge, respectively

A New Variety of Coronal Mass Ejection: Streamer Puffs from Compact Ejective Flares

We report on SOHO UVCS, LASCO, EIT, and MDI observations of a series of narrow ejections that occurred at the solar limb. These ejections originated from homologous compact flares whose source was an island of included polarity located just inside the base of a coronal streamer. Some of these ejections result in narrow CMEs ("streamer puffs") that move out along the streamer. These streamer puffs differ from "streamer blowout" CMEs in that (1) while the streamer is transiently inflated by the puff, it is not disrupted, and (2) each puff comes from a compact explosion in the outskirts of the streamer arcade, not from an extensive eruption along the main neutral line of the streamer arcade. From the observations, we infer that each streamer puff is produced by means of the inflation or blowing open of an outer loop of the arcade by ejecta from the compact-flare explosion in the foot of the loop. So, in terms of their production, our streamer puffs are a new variety of CME

Cool-Plasma Jets that Escape into the Outer Corona

We report on observations acquired in 2003 May during a SOHO-Ulysses quadrature campaign. The UVCS slit was set normal to the radial of the Sun along the direction to Ulysses at 1.7 R☉, at a northern latitude of 14.5°. From May 25 to May 28, UVCS acquired spectra of several short-lived ejections that represent the extension at higher altitudes of recursive EIT jets, imaged in He II λ304. The jets were visible also in LASCO images and seem to propagate along the radial to Ulysses. UVCS spectra showed an unusually high emission in cool lines, lasting for about 10-25 minutes, with no evidence of hot plasma. Analysis of the cool line emission allowed us to infer the physical parameters (temperature, density, and outward velocity) of jet plasma and the evolution of these quantities as the jet crossed the UVCS slit. From these quantities, we estimated the energy needed to produce the jet. We also looked for any evidence of the events in the in situ data. We conclude by comparing our results with those of previous works on similar events and propose a scenario that accounts for the observed magnetic setting of the source of the jets and allows the jets to be magnetically driven

PHYSICAL PARAMETERS OF STANDARD AND BLOWOUT JETS

The X-ray Telescope on board the Hinode mission revealed the occurrence, in polar coronal holes, of much more numerous jets than previously indicated by the Yohkoh/Soft X-ray Telescope. These plasma ejections can be of two types, depending on whether they fit the standard reconnection scenario for coronal jets or if they include a blowout-like eruption. In this work, we analyze two jets, one standard and one blowout, that have been observed by the Hinode and STEREO experiments. We aim to infer differences in the physical parameters that correspond to the different morphologies of the events. To this end, we adopt spectroscopic techniques and determine the profiles of the plasma temperature, density, and outflow speed versus time and position along the jets. The blowout jet has a higher outflow speed, a marginally higher temperature, and is rooted in a stronger magnetic field region than the standard event. Our data provide evidence for recursively occurring reconnection episodes within both the standard and the blowout jet, pointing either to bursty reconnection or to reconnection occurring at different locations over the jet lifetimes. We make a crude estimate of the energy budget of the two jets and show how energy is partitioned among different forms. Also, we show that the magnetic energy that feeds the blowout jet is a factor of 10 higher than the magnetic energy that fuels the standard event

Current Sheet Evolution In The Aftermath Of A CME Event

We report on SOHO UVCS observations of the coronal restructuring following a coronal mass ejection (CME) on 2002 November 26, at the time of a SOHO-Ulysses quadrature campaign. Starting about 1.5 hr after a CME in the northwest quadrant, UVCS began taking spectra at 1.7 R, covering emission from both cool and hot plasma. Observations continued, with occasional gaps, for more than 2 days. Emission in the 974.8 A line of [Fe XVIII], indicating temperatures above 6 x 10(exp 6) K, was observed throughout the campaign in a spatially limited location. Comparison with EIT images shows the [Fe XVIII] emission to overlie a growing post-flare loop system formed in the aftermath of the CME. The emission most likely originates in a current sheet overlying the arcade. Analysis of the [Fe XVIII] emission allows us to infer the evolution of physical parameters in the current sheet over the entire span of our observations: in particular, we give the temperature versus time in the current sheet and estimate its density. At the time of the quadrature, Ulysses was directly above the location of the CME and intercepted the ejecta. High ionization state Fe was detected by the Ulysses SWICS throughout the magnetic cloud associated with the CME, although its rapid temporal variation suggests bursty, rather than smooth, reconnection in the coronal current sheet. The SOHO-Ulysses data set provided us with the unique opportunity of analyzing a current sheet structure from its lowest coronal levels out to its in situ properties. Both the remote and in situ observations are compared with predictions of theoretical CME models