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A Brief History of Dayside Magnetospheric Physics

A. Otto

Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA

ABSTRACT

Dayside magnetospheric physics has an early history that is closely related to our understanding of the magnetosphere as a whole. The early years of magnetospheric physics are somewhat reminiscent of the gold rush era or the exploration of the American west. Moving into the satellite era, our field had, for the first time, the opportunity to examine in-situ dayside plasma processes to confirm or reject theories, something that neither solar nor astrophysics can do. Since the late 1970s, with better and faster instrumentation, we have been able to develop a detailed understanding of magnetopause and bow shock plasma physics, where transient phenomena play a critical role. This article provides a brief history of these periods of time and how these led into a modern understanding of dayside physics and transient events.

1.1. SETTING THE STAGE: THE PRE-SATELLITE ERA

At the turn of the nineteenth century, it was known that the Earth's magnetic field could at times undergo strong perturbations that seemed to correlate with auroral activity. It was also hypothesized that these magnetic perturbations were caused by processes on the sun. The most prominent example of this relation was the great flare observed by Richard Carrington on 1 September 1859 (Carrington, 1859) and the geomagnetic response. However, such a connection between solar processes and geomagnetism was met by strong criticism at the time.

In the years around the turn of the nineteenth century, Kristian Birkeland undertook a number of expeditions to the auroral zone. He was the first to identify what he called the polar elementary storm which is now known as the auroral substorm. Birkeland provided a highly detailed description and analysis of his observations and implied the existence of vertical currents in the upper atmosphere as closure for the horizontal currents he inferred from magnetic observations. Based on the observations and his gas discharge "Terella" experiments studying the paths of electrons in a dipole representing Earth, Birkeland was convinced that the aurora and associated magnetic perturbations were caused by precipitating electrons from the sun (Birkeland, 1908). He also provided a reasonable estimate of the electric currents and the power associated with the auroral activity. Some years later, Sydney Chapman, a brilliant mathematician, published his first model for geomagnetic storms (Chapman, 1918a). Although most of this work involved horizontal currents in the upper atmosphere, the batteries for these currents were "vertical motions." These he assumed to be provided by a mostly single charged particle precipitation of solar origin although he noted that this idea was not well appreciated in the science community (Chapman, 1918b). It was only a year later that Frederick Lindemann pointed out that the supposed solar corpuscular stream cannot be single-charged and must contain ions and electrons to be charge neutral (Lindemann, 1919).

Based on a charge neutral, ideally conducting solar stream Chapman and Ferraro presented a new theory of magnetic storms where the geomagnetic field is compressed facing the stream and extended in its wake (Chapman & Ferraro, 1931) somewhat similar to our picture of the magnetosphere (Figure 1.1). They called this a magnetic hollow where solar wind particles could access the upper atmosphere only through "two horns" at the location of the cusps of the magnetic field. This model presented for the first time the concept of a magnetopause as the boundary between the solar plasma and the Earth's closed magnetosphere, and this model dominated the view in the science community for decades. The model agreed qualitatively with most magnetic storm properties particularly for the initial increase of the magnetic field (sudden commencement), however, it was not convincing for the main phase magnetic depression. Chapman and Bartels (1940, p. 810) remarked that a more efficient particle entry and energization were needed than provided in the closed magnetic field model. A different model for magnetic storms and plasma entry in the form of clouds was suggested by Hannes Alfvén (1940) that generated an ongoing controversy for two decades (e.g., Alfvén, 1958).

Figure 1.1 Illustration of the "magnetic hollow" (magnetic cavity) exposed to the ideally conducting solar plasma.

Source: From Chapman and Ferraro (1931).

It should be noted that, at the time, the stream of solar plasma was generally assumed to be transient and localized although Biermann (1951) demonstrated through cometary tail observations that the stream of solar material must, in fact, be continuous. However, Chapman shared the view with some in the community of an invisible solar corona that extended beyond Earth's orbit and expanded at a low velocity of a few 10 km s-1 (Parker, 1997). Eugene Parker realized that not both views on the stream of solar plasma could be true, and, almost coincident with the launch of the first satellites, and Parker (1958) published his famous theory of the solar wind and coined the name. Somewhat typical of this time is an episode around this publication (Parker, 1997). Parker had submitted his paper to ApJ where Chandrasekhar was editor. So, Chandrasekhar came to Parker's office one day and told him that all (highly qualified) reviewers regarded the paper as wrong and whether he really wanted to publish it. Parker said "yes," since the reviewers had no explicit objection to the physical arguments, and after a moment, Chandrasekhar responded "Alright, I will publish it." Still, 2 years later on an international conference, Chamberlain argued that the supposed supersonic solar wind was the result of a wrong integration constant and the limited heat supply allowed only for a slow expansion of about 20 km s-1 at 1 AU (Chamberlain, 1960, 1961). Fortunately, Parker's work and reputation were saved by the first satellite observations of the solar wind (Bridge et al., 1962; Gringauz et al., 1962; Snyder & Neugebauer, 1963). It should be mentioned, however, that for very rare conditions the solar wind can indeed be almost absent such that Chamberlain's view on the topic was not entirely wrong.

1.2. INTO THE SATELLITE ERA

Similar to the importance of a new understanding of electrodynamics and electricity for progress in the first half of the nineteenth century, plasma physics and particularly the formulation of the magnetohydrodynamic (MHD) equations by Alfvén, Schlüter, and others enabled the theoretical understanding of the newly discovered magnetosphere. Even though there had been and still is criticism for the MHD approach, the rapid progress in the late 1950s and early 1960s is inconceivable without the framework of a magnetofluid description, the work by Parker on the solar wind being an excellent example. This theoretical framework and the new in-situ satellite measurements that became available since 1958 advanced our knowledge of the dayside bow shock and magnetopause physics rapidly.

Gold (1955) realized that a shock likely propagated in the stream of solar plasma to cause the sudden rapid compression associated with the sudden commencement of magnetic storms. Based on the short duration (few minutes), he also implied that the solar plasma must be magnetized because otherwise the shock width, based on the very large mean free path, would be too large to explain the fast compression. Several years later, the existence of a bow shock in front of the newly discovered magnetosphere had been suggested (Axford, 1962; Gold, 1962; Kellogg, 1962; Zhigulev, 1959). For instance, Ian Axford produced the teardrop shape of the magnetosphere with a bow shock and discussed the stability of the magnetospheric boundary. He also provided the familiar estimate of the magnetopause standoff distance and argued correctly that the magnetic boundary encountered by Pioneers 1 and 5 (Sonett, 1960; Sonett et al., 1960) was the bow shock rather than the magnetopause as had been originally assumed.

Figure 1.2 The "open magnetosphere" as suggested with x lines on the day and night side.

Source: From Dungey (1961).

In the following years, properties of the bow shock such as shape, motion, and upstream particle acceleration were examined based on the newly available observations. Burlaga and Ogilvie (1968) carried out a detailed comparison of Explorer 34 observations with theoretical shock predictions and found good agreement. Some diffuse shock encounters were interpreted as the shock moving. Models using hydrodynamic flow around a model obstacle were employed to make predictions on the shape of the bow shock (Spreiter & Jones, 1963; Spreiter et al., 1966), and satellite observations of bow shock locations provided empirical models of the bow shock shape and distance that agreed well with hydrodynamic predictions (Fairfield, 1967; Fairfield & Ness, 1967; Gosling et al., 1967). Also, at the time, satellites provided the first evidence of upstream moving electrons (Fan et al., 1966) and ions (Asbridge et al.,...