Auroral Theory Research Articles

Johann Christian Heuson, a little-known auroral scholar of the early 18th century

Abstract. Heuson published two booklets of observations of the aurora of 17/18 February, 1/2 March 1721, and 16/17 November 1729 together with contemporary thoughts about the subject. His work characterizes him as a well-educated scholar familiar with contemporary auroral observations and theories. Heuson rejects views of the aurora as an omen or portent of divine wrath, but explains the aurora as a natural phenomenon and is thus in line with other well-known auroral scholars of that time.

Kristian Birkeland: The first space scientist

More than one hundred years ago Kristian Birkeland (1867–1917) first addressed the question as to why auroras appear overhead when the Earth's magnetic field is disturbed. He laid foundations for our current understanding of geomagnetism and polar auroras. For the first time cosmic phenomena were scaled and simulated in a laboratory. Birkeland's terrella experiments were ingenious. Even though the famous Lord Kelvin, in 1892, wrote that no matter passes between the Sun and the Earth, Birkeland's first auroral theory from 1896 is based on charged particle of solar origin, illustrated by the following quotation: “the auroras are formed by corpuscular rays drawn in from space, and coming from the sun”. Thus, the year 1896 marks the founding of space plasma physics. His most enduring contribution to auroral physics was his recognition that field-aligned currents are needed to couple auroral phenomena in the upper atmosphere to interplanetary space. The existence of field-aligned currents was controversial and disputed vigorously among scientists for more than 50 years. During The Birkeland Symposium in 1967 it was unanimously proposed that field-aligned currents in space should be called “Birkeland currents”, which was accepted by the International Union for Geomagnetism and Aeronomy. Today, plasma physicists strongly believe that many significant cosmic phenomena result from streams of Birkeland currents.

Multiple scales in auroral plasmas

A review of the observed space scales of the auroral features ranging from the whole auroral oval of bright discrete forms down to the nonlinear moving solitary structures with the scales of the order of Debye length is given. The characteristic physical scale which determines the generation process is indicated whenever possible. Some problems of the auroral theory and modeling are briefly discussed, and a cross-scale coupling between the auroral and magnetospheric altitudes is stressed. It becomes apparent that the first in situ studied real astrophysical plasma object—the Earth's magnetosphere/ionosphere/aurora—is a unified multi-scale system which seems to be ordered at large scales, but sometimes looks as nearly nondeterministic, or chaotic, at small scales. The most powerful processes in this system operate in a very wide range of scales, with multifarious cross-scale couplings. The statistical behavior of magnetospheric/auroral plasmas in the regions of active auroras often can be reasonably described as the near-critical state.

Multiple current sheets in a double auroral oval observed from the MAGION-2 and MAGION-3 satellites

Abstract. A case is described of multiple current sheets crossed by the MAGION-2 satellite in the near-midnight quieting auroral oval. The data were obtained by the magnetometer experiment onboard. Results show during a quieting period after a preceding substorm, or during an early growth phase of the next substorm, two double-sheet current bands, POLB and EQUB, located at respectively the polar and equatorial borders of the auroral oval separated by about 500 km in latitude. This is consistent with the double-oval structure during recovery introduced by Elphinstone et al. (1995). Within the POLB, the magnetic field data show simultaneous existence of several narrow parallel bipolar current sheets within the upward current branch (at 69.5–70.3° invariant latitude) with an adjacent downward current branch at its polar side at (70.5–71.3°). The EQUB was similarly stratified and located at 61.2–63.5° invariant latitude. The narrow current sheets were separated on average by about 35 km and 15 km, respectively, within the POLB and EQUB. A similar case of double-oval current bands with small-scale structuring of their upward current branches during a quieting period is found in the data from the MAGION-3 satellite. These observations contribute to the double-oval structure of the late recovery phase, and add a small-scale structuring of the upward currents producing the auroral arcs in the double- oval pattern, at least for the cases presented here. Other observations of multiple auroral current sheets and theories of auroral arc multiplicity are briefly discussed. It is suggested that multiple X-lines in the distant tail, and/or leakage of energetic particles and FA currents from a series of plasmoids formed during preceding magnetic activity, could be one cause of highly stratified upward FA currents at the polar edge of the quieting double auroral oval.

On the origin of auroral fingers

During the development of large‐scale magnetospheric substorm expansive phase activity, the auroral oval may become very wide. During such episodes, clear discrete arc structures appear from time to time in the equatorward portion of the evening sector oval in a region of space normally thought to feature structureless diffuse auroras. On occasion, these discrete auroral forms acquire a north‐south orientation and remain as identifiable forms for up to several minutes. Most auroral theories attempt to describe the origin of the normally east‐west aligned features. In this paper we propose that the particles responsible for north‐south aligned arc structures originate as plasma blobs injected into the central plasma sheet (CPS) from the adjacent low‐latitude boundary layer, as a result of enhanced wave activity at the interface between the two regions during active times. We show that the characteristic geometric forms of the CPS auroral structures follow from tracking the drift paths of the particles in the injected plasma blob as they convect earthward, heating adiabatically in the process. We also outline outstanding issues pertaining to the proposal as motivation for a continued study.

Magnetic field‐aligned electric field acceleration and the characteristics of the optical aurora

The long‐recognized association of brighter aurora with more deeply penetrating, and hence more energetic electrons is examined. Using the Knight (1973) relation between the magnetic field aligned current density j∥ and potential drop V∥, derived from the theory of single particle motion in the presence of a magnetic field‐aligned electric field, an approximate expression relating the energy flux Q of the precipitating electrons over discrete aurora and the mean particle energy EM is derived: Q(ergs/cm² s) = kEMγ, where 1.5 ≤ γ ≤ 2 and k depends on the temperature and density of the plasma in the source region. This expression is used in conjunction with an auroral optical excitation and emission model to specify the dependence of the red (O I[630.0])/blue (N2+(427.8)) ratio of auroral optical emissions on the brightness of the aurora. It is shown that the quantitative predictions of the discrete auroral theory are in accord with observations of the aurora. Thus the often observed anticorrelation of the red/blue ratio with auroral brightness is consistent with the single‐particle motion theory. It follows that in discrete auroral events where the theory is valid, the brightening of the aurora involves an increase in both the mean energy and the number flux of precipitating electrons.

Tangential electric fields in a drifting auroral arc

We report rocket‐borne ion convection velocity and ground‐based optical measurements in an early evening, equatorward drifting, NW‐SE oriented auroral arc. Our measurements indicate a detailed correspondence between the normal component of the ion convection velocity (the component normal to the auroral arc) and the drift motion of the auroral arc. The two velocities were identical on a time scale of minutes, and correlated strongly with each other in smaller‐scale temporal variations. The correspondence implies a near‐zero tangential electric field in an auroral arc, in the frame of reference of the arc. Our observations have serious consequences on several auroral theories.

On the magnetic mirroring as the basic cause of parallel electric fields

Among the different proposed mechanisms for generating parallel electric fields, magnetic mirroring of charged particles seems to be the most plausible. In the present paper, it is suggested that magnetic mirroring is the basic cause of parallel electric fields in the magnetosphere and that the magnetic mirroring effect may be able to form the basis of an auroral theory that can remove a major portion of the ambiguity of observations. In the model proposed, the parallel electric field is due to a magnetic confinement of a negatively charged hot collision-free plasma. A transfer of electron gyroenergy into wave energy tends to weaken this confinement; if this energy transfer becomes too strong, the parallel potential gradient will break down. Hence, from this model, in contrast to certain other models of parallel electric fields, only a small fraction of the total auroral particle energy may be expected to be transformed into electromagnetic wave energy during the acceleration process.

Auroral colour variations

Recently overshadowed by instrumental work, visual observation and colour vision theory can nevertheless provide useful information and tests of auroral theories, at least for bright displays. Colour calculations based on a normal auroral spectrum agree with its visual appearance, but Type B displays require drastic changes in red/green emission ratios around 85 km. These changes are consistent with direct electron excitation of atomic oxygen, whose relative concentration becomes negligible below 85 km. The cause of the red lower border is more likely to be N 2 1PG emissions rather than O 2 + 1NG. The N 2 1PG intensity is at least 360 kR, corresponding to a minimum energy flux of 7 × 10 13 eV cm −2sec −1 of electrons with energies above 50 keV. Particle and X-ray observations of similar very large fluxes are infrequent, consistent with the rarity of Type B displays. Visibility of Type A red aurora requires a minimum 6300 Å intensity of 65 kR, at altitudes where quenching is not important. Blue sunlit aurora implies an N 2 + 1NG intensity of probably 500 kR. The time constants of prominent auroral emissions account for such transient effects as coloured edges in moving auroras, brief flashes of green and abrupt changes from green to red. Together with steady state effects, these appear to explain most descriptions in the literature of auroral colours.

Polar Ionospheric Research

Defines the ionsphere and describes its three horizontal layers or regions, distinguished by physical characteristics and ionization processes: as quiet D, E and F regions and as disturbed D, E and F regions when there is a disturbed sun (sunspot magma). Representative research techniques are characterized: all-sky camera, auroral spectroscopy, ionosonde, riometer, VHF forward scatter, auroral radar, whistlers, balloons and rockets. Some of the research problems are: the magnetospheric model, auroral theory, and the polar substorm. Programs needed include routine recording, a polar meridian net and experiments utilizing satellites and rockets, all of which can use support of the Naval Arctic Research Laboratory at Barrow, Alaska.

Optical Upper Atmospheric Investigations at the University of Saskatchewan

The history and philosophy of optical instrumentation for auroral and airglow studies, as developed at the University of Saskatchewan, is reviewed. Some details are given of the applications of grating spectroscopic systems, Fabry-Perot monochromators, Michelson interferometers, and birefringent and filter photometers with both photographic, photomultiplier, and photoconductive detectors. Finally, examples are given of the applications of these techniques to upper atmosphere temperature determinations, auroral excitation theory, auroral hydrogen arc studies, atmospheric alkali metals, and dayglow molecular oxygen emissions.

‘In praise of famous men’—the John Dalton bicentenary celebrations, 1966

SIR HUMPHRY DAVY once tartly observed that John Dalton ‘was a very disinterested man, and had no ambition beyond that of being thought a great philosopher’. Whether or not the comment was deserved, the attention paid to Dalton in 1966, the bicentenary year of his birth, amply confirms that the ambition was fulfilled. John Dalton has a variety of claims to scientific fame. As the lectures, meetings, discussions, articles and books of the past year have shown, a number of modern scientific disciplines would like to claim him as ‘really’ their own. For a start, he was the first accurately to describe colour-blindness or ‘Daltonism’, a defect under which he himself laboured. Then again, he was a pioneer meteorologist, as his early Meteorological observations and essays testify, and as his auroral theories and his unbroken records of Manchester weather through more than half a century confirm. More than this, his was a name to be reckoned with when the physics of gases were under discussion. Our continued reference to ‘Dalton’s’ law of partial pressures betrays this aspect of his fame.

Particle trajectories in a model current sheet, based on the open model of the magnetosphere, with applications to auroral particles

This work is a study of charged particle trajectories about a current sheet which is based on Dungey's ‘Open model of the magnetosphere.’ The object of the study is to see if charged particles incident on this model current sheet, coming from the solar wind, and emerging as possible auroral particles, emerge in any kind of complex spatial distributions resembling auroral forms. Adiabatic theory, which is good throughout most of the magnetosphere, breaks down near a neutral point in the current sheet, and particle trajectories must therefore be computed. The computation is done on an IBM 7074 computer. Only proton trajectories are studied extensively, and the results yield accelerated output protons with high intensity in a thin sheet, with perhaps some thin beam structure within the sheet. Such a production of thin sheets or beams of accelerated particles is necessary for an auroral theory. The energy of the output protons is 900 times the energy of the particles comprising the bulk flow, but this energy is still about an order of magnitude smaller than that observed for auroral protons. Preliminary results with a few electron trajectories indicate that the model should yield electron energies of about the right order of magnitude for auroral electrons. Furthermore, a structure for the output electrons thinner than that for protons is suggested by these preliminary results.

Geomagnetic storms, auroras and associated effects

Our knowledge of the interplanetary medium is outlined and its frictionless interaction with the geomagnetic cavity, first discussed by Chapman and Ferraro, is described. An important feature of this interaction is the interplanetary field which is compressed and may possibly lead to the formation of a shock wave. The possibility of frictional interaction between the solar wind and the cavity is discussed; an effect which appears to cause friction is the instability of interpenetrating ion-electron streams. This effect will also cause strong heating and trapping of ions and the generation of electromagnetic waves. The theory of propagation of geomagnetic disturbances in the magnetosphere and ionosphere is reviewed, first in general terms and than for some of the various components of a geomagnetic storm. Sea-level disturbances are divided into stormtime (Dst) and other (DS) components and also into different phases and the experimental data is reviewed. Theories of Dst, including the ringcurrent theory and magnetic tail theory are discussed and compared. Attempts to explain the complex DS field comprise the “magnetospheric dynamo” theory and the asymmetrical ring-current theory; these are compared in the light of experimental evidence. Motions of plasma and field lines in the magnetosphere are discussed in general terms: there are motions which deform the field and there are interchange motions. The former are opposed by Earth currents; the latter are not. The two types of motion are coupled through ionospheric Hall conductivity. Theories of the DS field in terms of the two types of motion are described; in particular motions caused by frictional interaction with the solar wind are discussed. These motions cause a helical twist in the field lines which propagates into the polar ionosphere as a hydromagnetic wave. In the ionosphere the motions of the field lines drive currents (“moving-field dynamo”) which cause the DS field. Drifts of neutral ionization in the lower ionosphere lead to localized accumulations which play a vital part in storm and auroral theory: they cause polarization fields which change the DS current system; they react on the magnetospheric motions to cause particle acceleration and precipitation. Auroral morphology and theories are briefly reviewed; the solar wind friction theory, although far from complete may provide a start. Further development should take the form of determining ionospheric drifts, polarization electric fields and consequent magnetospheric effects. A brief discussion is given of some associated effects: growth and decay of belts of geomagnetically trapped corpuscules; increase in ionospheric absorption of radio waves and lower-level X-ray production, ionospheric “storm” and high-latitude irregularities, micropulsations, VLF and ELF radio emissions from the magnetosphere, atmospheric heating and wave generation.

Observations and experiments pertinent to auroral theories

This paper reviews the most recent observational results pertinent to theories on auroral primary particles in space. The available information on the properties of primary particles derived from optical observations are first discussed. It is pointed out that even if fast electrons are responsible for the main auroral forms, there is a possibility that proton precipation zones exist outside those of electrons (as seen from the poles), and where protons may be the dominating kind of energetic particles injected into the atmosphere from space. The general morphology of aurora is discussed, emphasizing the need for obtaining a consistent picture of auroral morphology when the tilting and rotation of the magnetic dipole axis is taken into account, and relating the subject to theoretical view-points. Quantitative studies of the dynamics of aurora have not yet received the attention they deserve. Available observations and theoretical interpretations of these are reviewed and discussed. The most recent and important satellite observations of energetic particles and magnetic fields in space are described and discussed. It is concluded that although storage of particles in the radiation belts is intimately connected with aurorae, it is unlikely that aurorae are due to particles which have been stored for any length of time in the Earth's magnetic field, except perhaps in a few special cases. Model experiments in the laboratory on auroral theories may yield important information. The most important parts of this work are discussed.

An extremely intense electron flux at 1000-kilometer altitude in the auroral zone

The radiation-research satellite Injun I passed through an extremely large electron flux, at 1000 km altitude and high latitudes, around 2238 UT on September 25, 1961. The peak flux was 400 ergs (cm/sup 2/ sec ster)/sup -1/ from electrons with energy above 1 kev and 10/sup 8/ particles (cm/sup 2/ sec ster)/ sup -1/ with energy above 40 kev. The electron flux varied greatly both in intensity and spectral character during the satellite pass. For example, the intensity changed by a factor of about a million in a time of about 30 seconds or a distance of several hundred kilometers. The electron spectrum changed so greatly and so rapidly that, if a differential power-law spectrum was applied, values of the exponent ranged between above 4 and more than 14. At this time, the electron flux was measured with similar detectors on Explorer XII in the heart of the outer zone on the same magnetic shell but at a different longitude. Since the electron intensity and spectrum measured by Explorer XII were normal, the anomalous flux seen by Injun was confined in longitude. The Injun observations indicated that the flux was essentially isotropic over the upper hemisphere. The energy flux wasmore » sufficient to cause a very bright aurora, and the measurements were tabulated for comparison with auroral theories. The measurements proved that an acceleration mechanism in the ionosphere was not always necessary to produce an aurora. (auth)« less

Theory of Auroral Bombardment.

view Abstract Citations (37) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Theory of Auroral Bombardment. Chamberlain, Joseph W. Abstract An auroral theory is developed from the point of view of particle orbits in an inhomogeneous plasma confined by a magnetic field. Specifically, a mechanism is proposed for ejection into the atmosphere of geomagnetically trapped protons and electrons. It is assumed that the energetic particles are distributed in longitude irregularly. The tendency for positive and negative particles to drift in opposite directions will then lead to momentary electrostatic fields, arising from excess charges of one sign aligned along a magnetic line of force. As particles drift into this potential, they lose transverse kinetic energy, and a portion of the particles immediately spiral out the ends of the flux tube into the atmosphere As the potential grows, the drift of particles into this "discharge tube" is inhibited, but more of those entering the potential with high velocity are ejected, regaining their lost transverse kinetic energy in accelerated motion along the magnetic field. The potential may rise sufficiently to discharge particles with energies of several kev within an interval of less than a second, which is rapid enough to render neutralization by ionospheric ions and electrons unimportant. If the density fluctuations of auroral plasma exceed a certain critical value, the electrostatic field will cause them to grow rapidly. This instability is identified with auroral-ray structure. A density fluctuation may maintain its identity, even though individual particles are constantly moving thorugh it. This characteristic may be associated with the fading and reappearance of rayed structures. The basic mechanism of electrostatic fields arising from the particle drifts will also produce local accelerations of particles, by tending to establish an equipartition of energy between protons and electrons. This is presumably the mechanism for the local acceleration of auroral electrons, although it will also modify, but less severely, the energy spectrum of trapped protons. Various other consequences of these macroscopic, but short4ived, electric fields are examined, with a view toward understanding auroral morphology. It is proposed that an B X B drift accounts for the statistical preference for auroral patterns to move toward the sunlit hemisphere and for the departures of auroral forms from alignment along circles of geomagnetic latitude, even in the polar cap. The B field, when transferred to the atmosphere by bombardment and by ordinary conduction, will produce a Hall current parallel to auroral motions; this current is identified with the auroral electrojet and associated magnetic disturbances. It is suggested that these considerations also have a bearing on the north-south thickness of auroral forms, on auroral breakup, and on daily variations. Publication: The Astrophysical Journal Pub Date: September 1961 DOI: 10.1086/147168 Bibcode: 1961ApJ...134..401C full text sources ADS |

The nightly variation of auroras at a subauroral station

The observing records of E.E. Barnard and more recent patrol photographs of the sky have been studied for an indication of a nightly variation in the occurrence of auroras at Yerkes Observatory. There appears to be a maximum probability in the neighborhood of local magnetic midnight, as at stations closer to the auroral zone. Some implications to auroral theory are discussed.

Plasma Dynamic Determination of Shock Thickness in an Ionized Gas.

view Abstract Citations (30) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Plasma Dynamic Determination of Shock Thickness in an Ionized Gas. Parker, E. N. Abstract It is shown that the principal interaction in a shock front in a tenuous ionized gas is a plasma interaction in which the ions transfer their translational energy to the plasma oscillations. The effect is, of course, of interest in auroral theories and in solar radio noise. It turns out to be the dominant effect in determining shock thicknesses nearly everywhere that the gas density is less than the solar photospheric value of 1016 atoms/cm3. We estimate that shock thicknesses in interstellar H ii regions and in interplanetary space will be 1 km or less, even though the mean free paths may be 108 km or more and the ion Larmor radius 100 km. Publication: The Astrophysical Journal Pub Date: January 1959 DOI: 10.1086/146607 Bibcode: 1959ApJ...129..217P full text sources ADS |

Long-Range Interactions in Ionized Gases in Thermal Equilibrium.

It is shown that the principal interaction in a shock front in a tenuous ionized gas is a plasma interaction in which the ions transfer their translational energy to the plasma oscillations. The effect is, of course. of interest in auroral theories and in solar radio noise. It turns out to be the dominant effect in determining shock thicknesses nearly everywhere that the gas density is less thnn the solar photospheric value of 10/sup 16/ atoms/cm/. It is estimated that shock thicknesses in interstellar H II reglons and in lnterplanetary space will be 1 km or less, even though the mean free paths may be 10/sup 8/ km or more and the ion Larmor radius ~100 km. (auth)