Advances in Space Research 33 (2004) 2113–2120 www.elsevier.com/locate/asr Comparative magnetospheres: lessons for Earth V.M. Vasyliunas * Max-Planck-lnstitut fu€r Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany Received 3 March 2003; received in revised form 3 April 2003; accepted 3 April 2003 Abstract The comparative study of various magnetospheres is not only interesting in itself but also useful for understanding better many aspects of the magnetosphere of Earth. Scaling relations can be tested over a much wider parameter range than that provided by Earth alone. Comparison of Earth with other magnetospheres tells us about the relative importance of the ionosphere, the sig- nificance of kinetic effects, the dependence on the geometrical configuration of rotation axis, magnetic dipole, and solar wind flow direction, and the role of the external magnetic field. Processes such as magnetic field line reconnection and auroral particle ac- celeration are common to many different magneto-spheres and can be better understood when observed in different contexts. Diffusive transport across closed magnetic field lines plays a special role in the magnetospheres both of Jupiter and of Earth. Finally, some properties and limitations of numerical simulation work are illuminated by application under distinct conditions provided by different magnetospheres, in particular Jupiter and Earth. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Comparative magnetospheres; Lessons for Earth; Scaling relations; Planetary magnetospheres 1. Introduction significance; the first step in trying to understand a magnetosphere that is being observed for the first time Lessons for Earth, or lessons from Earth? Both are has always been to invoke analogies from the terrestrial objectives of comparative magnetosphere studies as magnetosphere and see how far they can be pushed. But traditionally understood (e.g. Vasyliunas, 1983): ‘‘The also the first objective – lessons for Earth – is now as- study of comparative magnetospheres, which aims at a suming greater and greater importance as the field of unified general description of magnetospheric phenom- comparative magnetosphere studies becomes more ma- ena and physics applicable to many different objects, is ture, the result both of increased number and variety of important for a twofold reason. First, it provides a test observed magnetospheres and of increasingly detailed for the correctness and general applicability of our observations within some; indeeed, magnetospheric concepts and theories of magnetospheric physics, often physics now includes concepts and theories developed to developed in the first instance to fit specific phenomena fit specific phenomena of magnetospheres other than of the terrestrial magnetosphere. Second, it provides a Earth. The purpose of this paper is to highlight some of tool by which our detailed knowledge, based on exten- the ways by which our understanding of the terrestrial sive observations, of the EarthÕs magnetosphere may be magnetosphere may profit from results of comparative used to gain insights into the properties of other, less magnetosphere studies. accessible, magnetospheres where direct observations may be quite limited or even non-existent.’’ The second of these objectives – lessons from Earth – is of obvious 2. Scaling relations A magnetosphere is characterized by a set of pa- * Tel.: +49-5556-979-435; fax: +49-5556-979-169. rameters which can be divided into three groups (see E-mail address: [email protected] (V.M. Va- e.g. Vasyliunas, 1989a): solar wind (or, more generally, syliunas). external medium), planetary (or, more generally, central 0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.04.051 2114 V.M. Vasyliunas / Advances in Space Research 33 (2004) 2113–2120 object) and magnetospheric parameters. One of the first 1983 it is the distance traveled by light in vacuum during tasks of comparative magnetosphere studies is to search a time interval of 1/299,792,458 of a second (Mohr and for scaling laws that establish relations among the Taylor, 2000). With the second defined in terms of an various parameters. atomic frequency standard, the meter can be shown to be, in essence, proportional to the classical electron ra- 2.1. Dependence of size on pressure of external medium dius divided by the cube of the fine structure constant. It is obvious that neither under the old nor under the A basic question is how the size of a magnetosphere, current definition does the meter have any relevance to conveniently described by the distance RMP from the magnetospheres. center of the planet to the subsolar point of the mag- Actually, most descriptions of a magnetosphere as netopause, depends on the parameters of the solar wind large or small in absolute terms are really meant in re- and of the planet. It is well known that RMP is deter- lation to the size of the terrestrial magnetosphere – a mined primarily by the requirement that the total pres- geocentric view somewhat at variance with the universal sure just inside the magnetosphere balance the total outlook we like to attribute to modern science. pressure just outside (for a more detailed discussion in the context of comparative magnetospheres, see e.g. 2.2.2. Relative to kinetic length scales Russell, 2004, and references therein). Equating the A physically more meaningful description of magne- pressure of the dipole magnetic field to the dynamic tosphere size is to relate it to some length scale defined pressure of the solar wind gives the scaling relation by plasma physics. One fundamental scale is the ion 1=3 1=6 inertial length (or ion collisionless skin depth) given by R ¼ðnlÞ ð8pqV 2Þ ; ð1Þ MP (Gaussian units) where l is the dipole moment, q and V the solar wind ÀÁ 2 2 1=2 density and velocity, and n a numerical factor correcting ki ¼ Mic =47pne pffiffiffiÀÁ for the field of magnetopause currents (n ¼ 2 to first 3 À1=2 ¼ 227 km A n=1cmÀ ; ð2Þ approximation). The validity of Eq. (1) has been checked at Earth over and also equal to the gyroradius of an ion moving at the a range Alfven speed. In (2) Mi is the ion mass, A the atomic mass number, and n the concentration (number density). R 5  104 to 8  104 km ð8–12R Þ; MP E Kinetic effects, to be discussed in the later section, by using the normal variability of the solar wind, or depend on the relation of ki to other length scales. 4 4 The sizes of various magnetospheres in units of ki are RMP 3  10 to 11  10 km ð5–17REÞ; tabulated and discussed by Russell (2004). The ratio by including the rare extreme cases, for a total variation RMP=ki is large for all magnetospheres observed to date, of nearly a factor 4. With the use of data from all ob- ranging from 85 for Mercury to 5800 for Jupiter. For served magnetospheres, however, Eq. (1) can be checked objects (possibly to be found among the asteroids) over a range whose magnetic dipoles are weak enough to give RMP 4 6 RMP 3  10 to 8  10 km: comparable to or smaller than Ai; interesting structures are anticipated on theoretical grounds (Omidi et al., or a total variation of about a factor 2600. 2004), but no magnetospheres of such objects have yet been detected. 2.2. When is a magnetosphere large or small? 2.2.3. Relative to planetary radius It is common to hear a particular magnetosphere A third, in fact the most common way of expressing described as ‘‘large’’ or ‘‘small.’’ Large or small com- the size of a magnetosphere is as the ratio of RMP to the pared to what? One is reminded of the remark by radius of the planet Rplanet. The observed magneto- Chesterton (1959): ‘‘It is quite futile to argue that man is spheres fall into three groups, with typical RMP=Rplanet small compared to the cosmos; for man was always equal to: (1) 1.5 to 2 for Mercury and Ganymede, (2) small compared to the nearest tree.’’ There are at least 10 for Earth and 20 for Saturn, Uranus, Neptune, three ways of describing how big a magnetosphere is. and (3) 100 for Jupiter. 2.2.1. Relative to standard unit of length 2.3. Does size matter? When a magnetosphere is characterized as large or small simply on the basis of its size in kilometers, it is The size of a magnetosphere has, as we have seen, no being compared in effect to whatever defines the stan- particular significance if expressed in kilometers or dard meter. Until 1960 that was the distance between multiples thereof, and it is very large in relation to ki- two scratches on a particular metal bar in Paris; since netic length scales (at least for all the magnetospheres V.M. Vasyliunas / Advances in Space Research 33 (2004) 2113–2120 2115 observed to date). Does the size in units of planetary 3. Multifactor analysis radius make any difference, as far as magnetospheric physics is concerned? For most purposes, the answer is Magnetospheres are influenced by the various pa- no: as long as the dipole moment rameters not only through simple scaling relations but also in more complicated ways. To disentangle and l ¼ðR Þ3B ; planet surface understand these, a study of how they affect different is held fixed, changing the radius of the planet does not magnetospheres can be very useful. affect the magnetosphere. (This is largely true even for magnetosphere/ionosphere coupling, as noted below.) 3.1. Ionosphere There are, however, a few aspects for which size does matter: To understand the role of the ionosphere in magne- 1. The volume occupied by the planet is not part of the tospheric structure and dynamics, a comparison of magnetosphere.