Electrodynamics of the outer solar atmosphere

The structure of the outer solar atmosphere and its magnetic coupling to the photospheric motions indicate the existence of large-scale current systems. The heating and the dynamics of coronal structures is therefore governed by electrodynamic coupling of these structures to the underlying photosphere. In a structured corona, the heating is enhanced because of several processes such as resonance absorption of Alfvénic surface waves, anomalous Joule heating, reconnection and the related topological dissipation. The global thermal and dynamic behaviour of coronal structures can be fruitfully described in terms of equivalent electrodynamic circuits, taking into account the paramount role of the photospheric boundaries. Coronal current systems may be stable, as in the case of coronal loops, but occassionally they show catastrophic behaviour if the current intensity surpasses a critical threshold.     

Low Latitude Ionospheric Electrodynamics

The low latitude ionosphere is strongly affected by several highly variable electrodynamic processes. Over the last two decades ground-based and satellite measurements and global numerical models have been extensively used to study the longitude-dependent climatology of low latitude electric fields and currents. These electrodynamic processes and their ionospheric effects exhibit large ranges of temporal and spatial variations during both geomagnetic quiet and disturbed conditions. Numerous recent studies have investigated the short term response of equatorial electric fields and currents to lower atmospheric transport processes and solar wind-magnetosphere driving mechanisms. This includes the large electric field and current perturbations associated with arctic sudden stratospheric warming events during geomagnetic quiet times and highly variable storm time prompt penetration and ionospheric disturbance dynamo effects. In this review, we initially describe recent experimental and numerical modeling results of the global climatology and short term variability of quiet time low latitude electrodynamic plasma drifts. Then, we examine the present understanding of equatorial electric field and current perturbation fields during periods of enhanced geomagnetic activity.     

The Near-Earth Plasma Environment

An overview of the plasma environment near the earth is provided. We describe how the near-earth plasma is formed, including photo-ionization from solar photons and impact ionization at high latitudes from energetic particles. We review the fundamental characteristics of the earth’s plasma environment, with emphasis on the ionosphere and its interactions with the extended neutral atmosphere. Important processes that control ionospheric physics at low, middle, and high latitudes are discussed. The general dynamics and morphology of the ionized gas at mid- and low-latitudes are described including electrodynamic contributions from wind-driven dynamos, tides, and planetary-scale waves. The unique properties of the near-earth plasma and its associated currents at high latitudes are shown to depend on precipitating auroral charged particles and strong electric fields which map earthward from the magnetosphere. The upper atmosphere is shown to have profound effects on the transfer of energy and momentum between the high-latitude plasma and the neutral constituents. The article concludes with a discussion of how the near-earth plasma responds to magnetic storms associated with solar disturbances.     

Flares of August 1972: Analysis of dynamics

The optical observations and analyses of the flares of August 1972 are reviewed with emphasis on their dynamics. In particular, various assessments are made of possible quantitative interpretation of observed data. Specific topics considered are the storage and release of magnetic energy, triggering mechanisms, particle acceleration and magnetic field reconnection, and coronal and solar wind responses. Supplemental discussions on possible future direction of research are presented, illustrating the need for examination of the storage and release of flare energy in the lower solar atmosphere.On leave from the High Altitude Observatory.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Effects of Magnetic Fields in the Solar Atmosphere on Global Oscillations

Helioseismology is practically the only efficient experimental way of probing the solar interior. Without it, the results of theoretical solar models would remain untested and, consequently, less reliable when applying them for investigating remote stars. Hence, having a firm understanding of the applicability and reliability of helioseismology and the awareness of its limits are essential in solar physics and also in astrophysics. One of the weaknesses of the currently popular helioseismic models is that they allow only limited interaction between the global acoustic oscillation modes and the magnetic lower solar atmosphere, although, observations confirm strong coupling of helioseismic oscillations to the atmospheric magnetic field. The present article overviews the attempts of taking into account atmospheric magnetic effects in the theoretical models of global solar oscillations.     

Solar magnetic fields

This paper is a review of our observational knowledge on solar magnetic fields. In Section 1 we make an attempt to summarize all observations of the general magnetic field (m.f.) of the Sun. Section 2 deals with the local m.f. at low latitudes and their connection with some features on the disk. The m.f. of sunspots and their peculiar character are considered in Section 3. The last section (4) is concerned with m.f. in sunspot groups, their changes and connections with solar activity.     

Hydrodynamic shock wave formation in the solar chromosphere and corona during flares

Basic mechanisms of the hydrodynamic shock wave formation in the solar atmosphere during flares are considered. Hydrodynamic plasma flows during flares arise due to fast energy release which is accumulated in the magnetic field of currents in the solar atmosphere. Shock waves arise as a result of rapid heating of the chromospheric upper layers from accelerated particles or heat fluxes. Powerful hydrodynamic phenomena can also arise due to explosive current sheet disruption in the region of strong magnetic field reconnection. Fundamental questions of shock wave formation and propagation in a non-homogeneous emitting solar atmosphere are discussed.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Photospheric Driving of Transient Solar Activities*

Transient solar activities are defined in this paper as explosive releases of magnetic energy in the solar atmosphere. By photospheric driving we mean, in this paper, the roles played by the processes observed in the photosphere in accumulating the magnetic energy and complexity in the solar atmosphere. We have tried to clarify the key elements of the driving processes, based on both theoretical considerations and observations.     

Giant Planet Ionospheres and Thermospheres: The Importance of Ion-Neutral Coupling

Planetary upper atmospheres-coexisting thermospheres and ionospheres-form an important boundary between the planet itself and interplanetary space. The solar wind and radiation from the Sun may react with the upper atmosphere directly, as in the case of Venus. If the planet has a magnetic field, however, such interactions are mediated by the magnetosphere, as in the case of the Earth. All of the Solar System’s giant planets have magnetic fields of various strengths, and interactions with their space environments are thus mediated by their respective magnetospheres. This article concentrates on the consequences of magnetosphere-atmosphere interactions for the physical conditions of the thermosphere and ionosphere. In particular, we wish to highlight important new considerations concerning the energy balance in the upper atmosphere of Jupiter and Saturn, and the role that coupling between the ionosphere and thermosphere may play in establishing and regulating energy flows and temperatures there. This article also compares the auroral activity of Earth, Jupiter, Saturn and Uranus. The Earth’s behaviour is controlled, externally, by the solar wind. But Jupiter’s is determined by the co-rotation or otherwise of the equatorial plasmasheet, which is internal to the planet’s magnetosphere. Despite being rapid rotators, like Jupiter, Saturn and Uranus appear to have auroral emissions that are mainly under solar (wind) control. For Jupiter and Saturn, it is shown that Joule heating and “frictional” effects, due to ion-neutral coupling can produce large amounts of energy that may account for their high exospheric temperatures.     

Planetary Magnetic Dynamo Effect on Atmospheric Protection of Early Earth and Mars

In light of assessing the habitability of Mars, we examine the impact of the magnetic field on the atmosphere. When there is a magnetic field, the atmosphere is protected from erosion by solar wind. The magnetic field ensures the maintenance of a dense atmosphere, necessary for liquid water to exist on the surface of Mars. We also examine the impact of the rotation of Mars on the magnetic field. When the magnetic field of Mars ceased to exist (about 4 Gyr ago), atmospheric escape induced by solar wind began. We consider scenarios which could ultimately lead to a decrease of atmospheric pressure to the presently observed value of 7 mbar: a much weaker early martian magnetic field, a late onset of the dynamo, and high erosion rates of a denser early atmosphere.     

Microwave spectrum analysis as solar energy release diagnostics

The possibilities of using spectrographic observations of microwave radio emission as a solar flare plasma diagnostic are discussed. The spectral fine structure of the emission is interpreted in the context of plasma emission mechanisms. The balance equations for particles and plasma turbulence together with the transfer equations for electromagnetic waves in a plasma are solved for a model containing a diverging magnetic loop. As a result of the analysis of the blip-type spectral feature, the structure of energy release region and the unperturbed plasma concentration in the preflare loop are evaluated. The number of accelerated electrons and the intensity of the plasma turbulence in the source region are estimated using the properties of the weak continuum emission following the blip. Based on the degree of circular polarization of both the narrow band and the continuum emission, estimates for the external magnetic field strength and the angular width of the radiating plasma turbulence have been obtained.     

Magnetic Flux Ropes in the Martian Atmosphere: Global Characteristics

We report observations of magnetic fields amplitude, which consist of a series of individual spikes in the Martian atmosphere. A minimum variance analysis shows that these spikes form twisted cylindrical filaments. These small diameter magnetic filaments are commonly called magnetic flux ropes. We examine the global characteristics of magnetic flux ropes, which are observed on 5% of the elliptical orbits of Mars Global Surveyor. Flux ropes are more often observed in Venus' atmosphere (70% of the orbits). In this paper we report some of the global characteristics of the flux ropes identified in the Martian atmosphere. No flux ropes are observed in the southern hemisphere of Mars. Most of them occur at high solar zenith angles, close to the terminator plane, and at high latitude with altitudes below 400 km. The orientation of the flux ropes appears random while in the case of Venus the orientation is more horizontal near the terminator for altitudes greater than 200 km. We have identified fewer flux ropes for SZA between 40 to 60 deg and for SZA lower than 20 deg, like in the case of Venus (Elphic and Russell, 1983b). Statistically, Mars' ionosphere with SZA range between 40^circ to 60^circ is less magnetized than near the subsolar point. As the Martian ionosphere is quite often magnetized by the magnetic components of the crustal field, this crustal magnetic field seems to inhibit the flux ropes formation in the southern hemisphere. However, some orbits without crustal magnetic field, called magnetic cavities, were observed without flux ropes. So the flux ropes formation process seems to be uppressed by another factor, like the solar wind dynamic pressure for Venus (Krymskii and Breus, 1988).     

Alfvén Waves in the Solar Atmosphere

Alfvén waves are considered to be viable transporters of the non-thermal energy required to heat the Sun’s quiescent atmosphere. An abundance of recent observations, from state-of-the-art facilities, have reported the existence of Alfvén waves in a range of chromospheric and coronal structures. Here, we review the progress made in disentangling the characteristics of transverse kink and torsional linear magnetohydrodynamic (MHD) waves. We outline the simple, yet powerful theory describing their basic properties in (non-)uniform magnetic structures, which closely resemble the building blocks of the real solar atmosphere.     

On the Coronal Magnetic Field Configuration and Solar Flare/CME Process

The coronal magnetic field configuration is important for understanding the energy storage and release processes that account for flares and/or CMEs. Here we present a model which is based on the work for potential magnetic field problems that only applies the condition at infinity with the boundary condition on the solar surface specified. We also discuss some recent progress on general force-free field models. For some event analyses, we have employed MDI/SOHO longitudinal magnetogram insected into the synoptic magnetogram to obtain whole boundary condition over the solar surface. Globally, the extrapolated global magnetic field structures effectively demonstrate the case for the disk signature of the radio CMEs and the evolution of the radio sources during the CME/flare processes.     

Solar Fine-Scale Structures. I. Spicules and Other Small-Scale, Jet-Like Events at the Chromospheric Level: Observations and Physical Parameters

Over the last two decades the uninterrupted, high resolution observations of the Sun, from the excellent range of telescopes aboard many spacecraft complemented with observations from sophisticated ground-based telescopes have opened up a new world producing significantly more complete information on the physical conditions of the solar atmosphere than before. The interface between the lower solar atmosphere where energy is generated by subsurface convection and the corona comprises the chromosphere, which is dominated by jet-like, dynamic structures, called mottles when found in quiet regions, fibrils when found in active regions and spicules when observed at the solar limb. Recently, space observations with Hinode have led to the suggestion that there should exist two different types of spicules called Type?I and Type?II which have different properties. Ground-based observations in the Ca?ii H and K filtergrams reveal the existence of long, thin emission features called straws in observations close to the limb, and a class of short-lived events called rapid blue-shifted excursions characterized by large Doppler shifts that appear only in the blue wing of the Ca?ii infrared line. It has been suggested that the key to understanding how the solar plasma is accelerated and heated may well be found in the studies of these jet-like, dynamic events. However, while these structures are observed and studied for more than 130 years in the visible, but also in the UV and EUV emission lines and continua, there are still many questions to be answered. Thus, despite their importance and a multitude of observations performed and theoretical models proposed, questions regarding their origin, how they are formed, their physical parameters, their association with the underlying photospheric magnetic field, how they appear in the different spectral lines, and the interrelationship between structures observed in quiet and active regions on the disk and at the limb, as well as their role in global processes has not yet received definitive answers. In addition, how they affect the coronal heating and solar wind need to be further explored. In this review we present observations and physical properties of small-scale jet-like chromospheric events observed in active and quiet regions, on the disk and at the limb and discuss their interrelationship.     

Energy flux in the Earth's magnetosphere: Storm – substorm relationship

Three ways of the energy transfer in the Earth's magnetosphere are studied. The solar wind MHD generator is an unique energy source for all magnetospheric processes. Field-aligned currents directly transport the energy and momentum of the solar wind plasma to the Earth's ionosphere. The magnetospheric lobe and plasma sheet convection generated by the solar wind is another magnetospheric energy source. Plasma sheet particles and cold ionospheric polar wind ions are accelerated by convection electric field. After energetic particle precipitation into the upper atmosphere the solar wind energy is transferred into the ionosphere and atmosphere. This way of the energy transfer can include the tail lobe magnetic field energy storage connected with the increase of the tail current during the southward IMF. After that the magnetospheric substorm occurs. The model calculations of the magnetospheric energy give possibility to determine the ground state of the magnetosphere, and to calculate relative contributions of the tail current, ring current and field-aligned currents to the magnetospheric energy. The magnetospheric substorms and storms manifest that the permanent solar wind energy transfer ways are not enough for the covering of the solar wind energy input into the magnetosphere. Nonlinear explosive processes are necessary for the energy transmission into the ionosphere and atmosphere. For understanding a relation between substorm and storm it is necessary to take into account that they are the concurrent energy transferring ways. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Observational Motivation for Computational Advances in Solar Flare Physics

A solar flare is a violent and transient release of energy in the corona of the Sun, associated with the reconfiguration of the coronal magnetic field. The major mystery of solar flare physics is the precise nature of the conversion of stored magnetic energy into the copious accelerated particles that are observed indirectly by the radiation that they produce, and also directly with in situ detectors. This presents a major challenge for theory and modeling. Recent years have brought significant observational advances in the study of solar flares, addressing the storage and release of magnetic energy, and the acceleration and propagation of fast electrons and ions. This paper concentrates on two topics relevant to the early phase of a flare, magnetic reconnection and charged particle acceleration and transport. Some recent pertinent observations are reviewed and pointers given for the directions that, this reviewer suggests, computational models should now seek to take.     

Energy Supply Processes for Solar Flares and Magnetospheric Substorms

It is shown that solar flares and magnetospheric substorms must primarily be caused by a dynamo process, rather than magnetic reconnection – a spontaneous, explosive annihilation of magnetic energy stored prior to the onset. Magnetic energy in the vicinity of solar flares and in the magnetotail shows often an increase at their onset, not a decrease. It is unfortunate that many observed features of solar flares and substorms have tacitly been ascribed to unproven (3-D) characteristics of the neutral line for a long time. In the future, it is necessary to study carefully their driving process and examine how the driven magnetic field system evolves, leading to solar flares and substorms.