Solar-Terrestrial Magnetic Activity and Space Environment

H. Wang, R. Xu

• Publisher: Pergamon
• Year: 2002
• ISBN:
  9780080541433
• Topic: Technology & Engineering
• Subtopic: Aeronautic & Astronautic Engineering
• Index: Technology & Engineering

The COSPAR Colloquium on Solar-Terrestrial Magnetic Activity and Space Environment (STMASE) was held in the National Astronomy Observatories of Chinese Academy of Sciences (NAOC) in Beijing, China in September 10-12, 2001. The meeting was focused on five areas of the solar-terrestrial magnetic activity and space environment studies, including study on solar surface magnetism; solar magnetic activity, dynamical response of the heliosphere; space weather prediction; and space environment exploration and monitoring. A hot topic of space research, CMEs, which are widely believed to be the most important phenomenon of the space environment, is discussed in many papers. Other papers show results of observational and theoretical studies toward better understanding of the complicated image of the magnetic coupling between the Sun and the Earth, although little is still known little its physical background. Space weather prediction, which is very important for a modern society expanding into out-space, is another hot topic of space research. However, a long way is still to go to predict exactly when and where a disaster will happen in the space. In that sense, there is much to do for space environment exploration and monitoring. The manuscripts submitted to this Monograph are divided into the following parts: (1) solar surface magnetism, (2) solar magnetic activity, (3) dynamical response of the heliosphere, (4) space environment exploration and monitoring; and (5) space weather prediction. Papers presented in this meeting but not submitted to this Monograph are listed by title as unpublished papers at the end of this book.

Section III

Dynamical response of the heliosphere

Analysis of Lasco Observations of Streamer Blowout Events

A. Vourlidas¹; R.A. Howard²; J.S. Morrill²; S. Munz^{3 1} CEOSR/CSI, George Mason University, Fairfax, VA 22030, USA
² Naval Research Laboratory, E. O. Hulburt Center for Space Research, Washington, DC 20375, USA
³ Physical Sciences Dept., Scottsdale Community College, Scottsdale, AZ 85256, USA

ABSTRACT

We investigate the properties of a class of coronal eruptive events with a specific evolutionary pattern; namely, the overlying streamer brightens and swells for several hours or even days before a slow ejection removes it completely. These events are appropriately called streamer blowout events, comprise a small percentage of the total number of coronal mass ejections and occur throughout the solar cycle without changes in their properties. Their study is important because they might represent a stage in the evolution of streamers. Recently, we started cataloging streamer blowout events using the LASCO database. We report our preliminary results in this paper.

INTRODUCTION

Streamers are the familiar raylike features of the solar corona that are seen during eclipses. These structures are visible because of the Thompson scattered light from the coronal plasma trapped in them. Streamers mark the locations where the large scale magnetic neutral line is viewed edge-on and follow the solar cycle evolution of the large scale magnetic field. At solar minimum, when the magnetic field is dominated by its dipole moment, streamers are restricted along the solar equator. As the solar activity increases, greater numbers of streamers appear at progressively larger heliolatitudes until, at the solar maximum, they can be seen all around the solar disk.

Streamers are not static structures but evolve in two ways. The first is a gradual process in which the streamer field lines respond to continuous chances in the low coronal field (Wang et al., 1999). This process releases “blobs” of material as closed field lines open up. The second way in which the large scale coronal magnetic field patterns change involves coronal mass ejections (CMEs). Although not all CMEs are associated with streamers (Subramanian et al., 1999), a subclass of CMEs can affect streamers in a very dramatic way; they can blow the streamer away. Sheeley et al. (1982) described an event of this type for the first time and introduced the term “streamer blowout” CME (SB-CMEs, hereafter). Howard et al. (1985) presented a detailed classification of the CMEs observed by Solwind during 1979 – 1981 and considered SB-CMEs as a separate class of mass ejections with the following two-phase eruption process:

1. In the first phase, a pre-existing streamer gradually swells and brightens over a period of hours (> 6 hours) to several days. Continuous small-scale material ejection along the streamer usually accompanies this stage.

2. In the second phase, a large, slow CME develops and removes the coronal plasma causing the streamer to fade away gradually. Difference images usually show a dark, depleted region in the location of the preexisting streamer.

This sequence of events creates a very specific signature for the SB-CMEs on synoptic maps of coronal brightness (Hundhausen et al., 1981; Subramanian et al., 1999). Namely, the combination of the slow streamer swelling and the aburpt streamer disappearance take the form of a “bugle”, the flaring end of a trumpet. Hundhausen (1993) used this term to describe SB-CMEs observed by SMM and it has since beed used occasionally in the literature.

There have been very few studies of individual streamer blowout events (e.g., Sheeley et al., 1982; Illing and Hundhausen, 1986) and the only analysis of the properties of SB-CMEs, that we are aware of, is presented by Howard et al. (1985). The average properties of the SB-CMEs from their paper are reproduced in Table 1. We see that the SB-CMEs are much slower and more massive than the average CME. An interesting characteristic of this class is their occurence rate. Howard et al. (1985, 1986) found that between 1979 and 1985 streamer blowouts occured at a constant rate of about 0.1 CMEs/day even though the overall CME rate declined. This result led Sheeley et al. (1986) to suggest that blowouts might represent a normal phase in the lifecycle of streamers. In contrast, fast, explosive CMEs are usually associated with flares, radio type-II bursts or energetic particles and correlate well with the solar activity cycle.

Table 1

Average Properties of Solwind CMEs 1979-1981

CME Type	Percent of total	Speed (km/s)	Span (deg)	Mass (10^15 gr)	Kinetic Energy (10^30 ergs)
Streamer Blowout	5	200	44	5.4	0.56
All	100	472	45	4.1	3.5

One should keep in mind that the CME statistics from near-Earth orbiting coronagraphs, such as Solwind and SMM are subject to bias effects. The spacecrafts underwent frequent daily eclipses, had a relatively small field of view and the sensitivity of their detectors was lower than the sensitivity currently achieved by CCDs. Therefore, slower and massive CMEs, rather than fast CMEs, swould be more easily detected by these instruments. Such bias effects could account for the large percentage of SB-CMEs (~ 36%) seen by Solwind during the declining phase of cycle 21. It would be very interesting to find out if the Solwind results are verified by more recent observations. The Large Angle and Spectroscopic Coronagraph (LASCO) package (Brueckner et al., 1995) aboard the Solar and Heliospheric Obs...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Preface
• Section I: Solar surface magnetism
• Section II: Solar magnetic activity
• Section III: Dynamical response of the heliosphere
• Section IV: Space exploration and environment
• Section V: Space weather prediction
• Author index
• List of participants
• List of unpublished papers