Empirical Solar Proton Model for Orbiting Spacecraft Applications

An empirical model for energetic solar proton fluxes is presented. With this model the effects of such protons on geocentric space missions, to be flown during the next solar active period (1977-1983), and with orbits involving partial magnetospheric shielding, may be estimated. A synoptic background review is given, followed by a detailed discussion of the model's use, errors, uncertainties, and limitations, including sample calculations which demonstrate the application of specific or general project missions. Finally, for circular trajectories, percentage exposure maps are presented, depicting fractional mission times spent outside particular L shells as functions of orbit altitude and inclination. The distinguishing assumptions of this analysis are: 1) that the solar proton flux in the 10-100 MeV energy range, as accumulated over solar cycle 20 due to several discrete events, will be accumulated at a uniform rate for the seven active years of solar cycle 21; and 2) that all protons in the energy range of interest have a common geomagnetic latitude cutoff.     

Steady magnetospheric convection: A review of recent results

Theoretical pressure balance arguments have implied that steady convection is hardly possible in the terrestrial magnetotail and that steady energy input necessarily generates a cyclic loading-unloading sequence, i.e., repetitive substorms. However, observations have revealed that enhanced solar wind energy input to the magnetospheric system may either lead to substorm activity or enhanced but steady convection. This topic is reviewed with emphasis on several recent case studies of the Steady Magnetospheric Convection (SMC) events. In these cases extensive data sets from both satellite and ground-based instruments from various magnetospheric and ionospheric regions were available.Accurate distinction of the spatial and temporal scales of the magnetospheric processes is vital for correct interpretation of the observations during SMC periods. We show that on the large scale, the magnetospheric configuration and plasma convection are stable during SMC events, but that both reveal considerable differences from their quiet-time assemblies. On a shorter time scale, there are numerous transient activations which are similar to those found during substorms, but which presumably originate from a more distant tail reconnection process, and map to the poleward boundary of the auroral oval. The available observations and the unresolved questions are summarized here.The tail magnetic field during SMC events resembles both substorm growth and recovery phases in the neartail and midtail, respectively, but this configuration may remain stable for up to ten hours. Based on observations and model results we discuss how the magnetospheric system avoids pressure balance problems when the plasma convects earthward.Finally, the importance of further coordinated studies of SMC events is emphasized. Such studies may shed more light on the substorm dynamics and help to verify quantitatively the theoretical models of the convecting magnetosphere.     

A Review of Low Frequency Electromagnetic Wave Phenomena Related to Tropospheric-Ionospheric Coupling Mechanisms

Investigation of coupling mechanisms between the troposphere and the ionosphere requires a multidisciplinary approach involving several branches of atmospheric sciences, from meteorology, atmospheric chemistry, and fulminology to aeronomy, plasma physics, and space weather. In this work, we review low frequency electromagnetic wave observations in the Earth-ionosphere cavity from a troposphere-ionosphere coupling perspective. We discuss electromagnetic wave generation, propagation, and resonance phenomena, considering atmospheric, ionospheric and magnetospheric sources, from lightning and transient luminous events at low altitude to Alfvén waves and particle precipitation related to solar and magnetospheric processes. We review ionospheric processes as well as surface and space weather phenomena that drive the coupling between the troposphere and the ionosphere. Effects of aerosols, water vapor distribution, thermodynamic parameters, and cloud charge separation and electrification processes on atmospheric electricity and electromagnetic waves are reviewed. Regarding the role of the lower boundary of the cavity, we review transient surface phenomena, including seismic activity, earthquakes, volcanic processes and dust electrification. The role of surface perturbations and atmospheric gravity waves in ionospheric dynamics is also briefly addressed. We summarize analytical and numerical tools and techniques to model low frequency electromagnetic wave propagation and to solve inverse problems and outline in a final section a few challenging subjects that are important to advance our understanding of tropospheric-ionospheric coupling.     

Planetary radio astronomy experiment for Voyager missions

The planetary radio astronomy experiment will measure radio spectra of planetary emissions in the range 1.2 kHz to 40.5 MHz. These emissions result from wave-particle-plasma interactions in the magnetospheres and ionospheres of the planets. At Jupiter, they are strongly modulated by the Galilean satellite Io.As the spacecraft leave the Earth's vicinity, we will observe terrestrial kilometric radiation, and for the first time, determine its polarization (RH and LH power separately). At the giant planets, the source of radio emission at low frequencies is not understood, but will be defined through comparison of the radio emission data with other particles and fields experiments aboard Voyager, as well as with optical data. Since, for Jupiter, as for the Earth, the radio data quite probably relate to particle precipitation, and to magnetic field strength and orientation in the polar ionosphere, we hope to be able to elucidate some characteristics of Jupiter auroras.Together with the plasma wave experiment, and possibly several optical experiments, our data can demonstrate the existence of lightning on the giant planets and on the satellite Titan, should it exist. Finally, the Voyager missions occur near maximum of the sunspot cycle. Solar outburst types can be identified through the radio measurements; when the spacecraft are on the opposite side of the Sun from the Earth we can identify solar flare-related events otherwise invisible on the Earth.     

Space weather: its effects and predictability

Terrestrial technology is now, and increasingly, sensitive to space weather. Most space weather is caused by solar storms and the resulting changes to the Earth's radiation environment and the magnetosphere. The Sun as the driver of space weather is under intense observation but remains to be adequately modelled. Recent spacecraft measurements are greatly improving models of solar activity, the interaction of the solar wind with the magnetosphere, and models of the radiation belts. In-situ data updates the basic magnetospheric model to provide specific details of high-energy electron flux at satellite orbits. Shock wave effects at the magnetopause can also be coarsely predicted. However, the specific geomagnetic effects at ground level depend on the calculation of magnetic and electric fields and further improvements are needed. New work on physical models is showing promise of raising geomagnetic and ionospheric predictability above the synoptic climatological level. This revised version was published online in August 2006 with corrections to the Cover Date.     

Energy Spectra, Composition, and Other Properties of Ground-Level Events During Solar Cycle 23

We report spacecraft measurements of the energy spectra of solar protons and other solar energetic particle properties during the 16 Ground Level Events (GLEs) of Solar Cycle 23. The measurements were made by eight instruments on the ACE, GOES, SAMPEX, and STEREO spacecraft and extend from ～0.1 to ～500–700?MeV. All of the proton spectra exhibit spectral breaks at energies ranging from ～2 to ～46?MeV and all are well fit by a double power-law shape. A comparison of GLE events with a larger sample of other solar energetic particle (SEP) events shows that the typical spectral indices are harder in GLE events, with a mean slope of ?3.18 at >40?MeV/nuc. In the energy range 45 to 80?MeV/nucleon about ～50?% of GLE events have properties in common with impulsive ³He-rich SEP events, including enrichments in Ne/O, Fe/O, ²²Ne/²⁰Ne, and elevated mean charge states of Fe. These ³He-rich events contribute to the seed population accelerated by CME-driven shocks. An analysis is presented of whether highly-ionized Fe ions observed in five events could be due to electron stripping during shock acceleration in the low corona. Making use of stripping calculations by others and a coronal density model, we can account for events with mean Fe charge states of 〈Q _Fe〉≈+20 if the acceleration starts at ～1.24–1.6 solar radii, consistent with recent comparisons of CME trajectories and type-II radio bursts. In addition, we suggest that gradual stripping of remnant ions from earlier large SEP events may also contribute a highly-ionized suprathermal seed population. We also discuss how observed SEP spectral slopes relate to the energetics of particle acceleration in GLE and other large SEP events.     

The First two Years of Image

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) is the first satellite mission that is dedicated to imaging the Earth's magnetosphere. Using advanced multispectral imaging techniques along with omnidirectional radio sounding, IMAGE has provided the first glimpses into the global structure and behavior of plasmas in the inner magnetosphere. Scientific results from the two-year prime mission include the confirmation of the theory of plasmaspheric tails and the discovery of several new and unpredicted features of the plasmasphere. Neutral-atom imaging has shown how the ring current develops during magnetic storms and how ionospheric ions are injected into the ring current during substorms. The first global imaging of proton auroras has allowed the identification of the ionospheric footprint of the polar cusp and its response to changes in the interplanetary magnetic field. Detached subauroral proton arcs have been found to appear in the afternoon sector following south-north and east-west rotations of the IMF. Low-energy neutral atom imaging has shown global-scale ionospheric outflow to be an immediate response to solar-wind pressure pulses. Such imaging has also provided the first measurements of solar wind and interstellar neutral atoms from inside the magnetosphere. Radio sounding has revealed the internal structure of the plasmasphere and identified plasma cavities as the source of kilometric continuum radiation. These and numerous other scientific results now set the stage for the extended mission of IMAGE in which the imaging perspective will change markedly owing to orbital evolution while the magnetospheric environment undergoes a transition from solar maximum toward solar minimum.     

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.     

ATS-6 Solar Cosmic Ray and Trapped Particle Experiment

The Solar Cosmic Ray and Trapped Particle Experiment was designed to study the entry, propagation, and loss of solar cosmic rays and the acceleration and loss of trapped electrons and protons in the magnetosphere. Two orthogonal proton and alpha partical telescopes measure protons from 300 keV to 250 MeV and alphas from 2 MeV to 200 MeV. Electron spectrometers measure electrons from 50 keV to 1 MeV and are used in conjunction with the 300-keV to 1.2-MeV proton channels to study the injection of electrons and protons into the magnetosphere during substorms. Two solar cosmic ray events were observed during the first four months of operation. The first of these began on July 3, 1974, and is probably one of the more complicated events in recent years. There were numerous flares and sudden commencements as well as intense fluxes of low energy plasma with a severly perturbed magnetosphere. The second solar cosmic ray event was smaller and was associated with an isolated east limb flare. The first increase was observed on September 11, 1974.     

Early results from the ISEE electron density experiment

The ISEE-1 and 2 spacecraft contain two complementary experiments to measure the ambient electron density by radio techniques: a propagation experiment which measures the integrated electron density between ISEE-1 and 2, and a resonance sounder which measures the electron density in the vicinity of ISEE-1, and also provides AC electric field data. These experiments have been described elsewhere (Harvey et al., 1978). Results from these two experiments are presented here for the first time. The propagation experiment permits high time resolution studies of density fluctuations in the solar wind and magnetospheric frontier regions. The sounder experiment has detected for the first time plasma resonances in the solar wind and in the Earth's magnetosheath, as well as in the regions of the magnetosphere where resonances have already been observed by the spacecraft GEOS-1. We present here a preliminary review of the different types of electric field noise observed in the solar wind and magnetosheath, and discuss their relationship to the measured plasma density.CRPE/CNET, 92131 Issy-les-Moulineaux, France.     

Some aspects of the low latitude geomagnetic response under different solar wind conditions*

We review some aspects of low latitudes (L≤2) geomagnetic field variations associated with magnetospheric pulsations as well as with continuous and impulsive variations of the solar wind (SW) pressure. This revised version was published online in August 2006 with corrections to the Cover Date.     

Satellite measurements and theories of low altitude auroral particle acceleration

Several previous and new S3-3 satellite results on DC electric fields, field-aligned currents, and waves are described, interpreted theoretically, and applied to the understanding of auroral particle acceleration at altitudes below 8000 km. These results include the existence of two spatial scale sizes (less than 0.1 degree and a few degrees invariant latitude) in both the perpendicular and parallel electric fields; the predominance of S-shaped rather than V-shaped equipotential contours on both spatial saales; the correlated presence of field-aligned currents, low frequency wave turbulence, coherent ion cyclotron wave emissions and accelerated upmoving ions and downgoing electrons; intense waves inside electrostatic shocks and important wave-particle interactions therein; correlations of field-aligned currents with magnetospheric boundaries that are determined by convection electric field measurements; electron acceleration producing discrete auroral arcs in the smaller scale fields and producing inverted-V events in the larger scale fields; ion and electron acceleration due to both wave-particle interactions and the parallel electric fields. Further analyses of acceleration mechanisms and energetics are presented.Also Department of Physics.     

Interaction of the moon with the earth's magnetosphere

The general characteristics and dimensions of the earth's magnetosphere, turbulent transition region, and shock wave are reviewed and examined with regard to their possible active or passive interaction with the moon. The moon may, in turn, have a wake either detectable by space experiments outside the terrestrial shock wave or capable of interacting with the earth's shock wave, wake, or magnetospheric tail. The length and detectability of such a lunar wake depend, among other factors, on the selenomagnetic field and flow characteristics of the solar wind. Direct experimental information on a lunar wake is scanty, being, in fact, limited to one supposed case recorded by the IMP-1 satellite. Some evidence has been claimed, on the other hand, for statistical relationships between geomagnetic disturbance observed at the surface and the age of the moon. These results, however, are partly conflicting with each other as regards the phase of the supposed monthly variations, and partly are lacking statistical significance or merely grazing the significance level. The proximity of the lunar period with the length of the solar rotation, as well as a yearly and half-yearly modulation of the quasi-persistent 27-day fluctuations of geomagnetic disturbance contribute to encumber the question, rendering it very difficult, with the records presently available, to free the supposed lunar effects from solar interference. It seems, therefore, recommendable to postpone a judgement on the reality of such effects.Contribución del Instituto Antártico Argentino No. 109.     

DEMETER Observations of EM Emissions Related to Thunderstorms

The paper is related to specific emissions at frequency <3 MHz observed by the low altitude satellite DEMETER in relation with the thunderstorm activity. At its altitude (～700 km), the phenomena observed on the E-field and B-field spectrograms recorded by the satellite are mainly dominated by whistlers. Particular observations performed by DEMETER are reported. It concerns multiple hop whistlers and interaction between whistlers and lower hybrid noise. Two new phenomena discovered by the satellite are discussed. First, V-shaped emissions up to 20 kHz are observed at mid-latitude during night time. They are centered at the locations of intense thunderstorm activity. By comparison with VLF saucers previously observed by other satellites in the auroral zones it is hypothesized that the source region is located below the satellite and that the triggering mechanism is due to energetic electrons accelerated during sprite events. Second, emissions at frequency ～2 MHz are observed at the time of intense whistlers. These emissions are produced in the lower ionosphere in probable relation with Transient Luminous Events (TLEs).     

Conjugate features of magnetospheric electron dynamics observed at balloon altitudes

Analysis of recent observations (from balloons, spacecraft, and surface observatories) demonstrate regional, shell, and nearpoint conjugacy at L ～ 7 during precipitative events which were characterized by local acceleration as well as release of gradient-drifted electrons injected during substorms. A number of new features of magnetospheric dynamics relating to substorm development and sudden-commencement effects, have been brought to light which, though poorly understood at present, may prove of considerable importance and are worthy of further investigation.

1. During the initial period of instability in substorm evolution, preceding the slower magnetotail convective injection, precipitation of waves of electrons in rapid polewards motion exhibit L-shell conjugacy near midnight.
2. Transient, large scale expansions of the magnetospheric electron population accompanied by temporally imbedded substorms display large scale regional conjugacy and are simultaneously observed as similarly transient intensity dropouts at balloon altitudes.
3. Precipitation from gradient-drifting electrons in the dayside magnetosphere exhibits near point-conjugacy, at least down to the order of 50 km and quite probably less.

Similarly tight conjugacy applies to the release of electrons showing a specific local response to sudden commencements.

1. Analysis of the approach to and attainment of spectral equilibrium in the precipitation observed from drifting electrons may provide information about either, or both, the source spectrum at injection and the process of local release.
2. The specific precipitation effect sometimes observed at the time of an SC remains a rather puzzling feature, although it seems clear now that the acceleration and/or release process responsible is of a highly local nature and works selectively at small pitch angles well within the magnetospheric boundary. Coupling of the interplanetary shock with the magnetosphere must be an important aspect, but the details are not clear as yet.
3. On at least one occasion, a large part (perhaps all) of the magnetospheric electron population varied in a nearly synchronous manner in response to solar wind induced distortions during the variable compressive phase of a sudden commencement geomagnetic storm.

In the ongoing effort to identify and understand acceleration and release mechanisms involved in magnetospheric dynamics, balloon-borne experiments will continue to be useful, providing essential information presently unattainable by other means.     

Solar cosmic ray phenomena

This review attempts to present an integrated view of the several types of solar cosmic ray phenomena. The relevant large and small scale properties of the interplanetary medium are first surveyed, and their use in the development of a quantitative understanding of the cosmic ray propagation processes summarised. Solar cosmic ray events, in general, are classified into two phenomenological categories: (a) prompt events, and (b) delayed events. The properties of both classes of events are summarised. The properties considered are the frequency of occurrence, dependence on parent flare position, the time profile, energy spectra, anisotropies, particle species, velocity dispersions, etc. A single model is presented to explain the various species of delayed event. Thus the halo and core events, energetic storm particle events, EDP events and proton recurrent regions are suggested to be essentially of common origin. The association of flare particle events with electromagnetic phenomena, including optical, X-ray and microwave emissions is summarised. The conditions in a sunspot group, and solar flare that are considered to be conducive to cosmic ray acceleration processes are discussed. Considerable discussion is devoted to physical processes occurring near the Sun. Near Sun particle storage, and diffusion, and secondary injection processes that are triggered by a far distant solar flare are reviewed. In order to explain the considerable differences between aspects of the prompt and delayed events, we propose selective diffusion processes that only occur at early times in a solar flare. The type IV radio emissions at metric wave-lengths are suggested to yield direct evidence for the storage processes that are necessary to explain the properties of the delayed events, and also as yielding direct evidence of secondary injection processes. We conclude by briefly summarising the ionospheric effects of the solar cosmic radiation.     

Observations of low-energy electrons from AE-C in the south polar cusp during the geomagnetic storm of September 21, 1977

The AE-C spacecraft skimmed through the southern polar cusp at a 400 km altitude during a large geomagnetic storm on September 21, 1977. This period has been designated as a special IMS period, and the AE-C data were acquired close to the times that data were acquired by the DMSP satellite at nearly the same location over the southern polar cap, and by the GEOS satellite located near the noon-meridian in the northern hemisphere. Low energy electrons (1-500 eV) were measured with the photoelectron spectrometer experiment experiment onboard AE-C. This instrument was operated in the mode which measured precipitating electron fluxes and backscattered electron fluxes in alternating 4s intervals with two sensors. A region of intense precipitating electron fluxes was observed near 0924 UT on September 21, 1977 extending from 69 degree invariant latitude at 1100 MLT to 72 degree invariant latitude at 1152 MLT. From the spectra of the precipitating electrons, this region is identified as the southern polar cusp. Since the K _p equals 7- during this time, the displacement of the cusp down to these low latitudes is not unreasonable. Particle data obtained from the DMSP satellite on orbits close to AE-C, confirm that the position of the cusp was rapidly changing during this period, and was displaced to latitudes equatorward of the quiet time position. A second region of intense fluxes of precipitating electron was observed by AE-C at approximately 0933 UT from 69 degree invariant latitude near 1700 MLT to 66 degree invariant latitude near 1730 MLT. This region of low energy electron fluxes is characterized by slightly harder energy spectra and is interpreted as being the afternoon auroral zone. The remarkable and fortunate location of the AE-C, DMSP, and GEOS spacecraft during this special IMS period will allow future correlative studies aimed at the determination of the shape of the magnetosphere during very disturbed conditions.     

Substorm aspects of magnetic pulsations

The different types of magnetic pulsations occurring during magnetospheric substorms are analysed into the concept of polar substorms recently described in detail by Akasofu (1968). Special attention is thus paid, to the simultaneous occurrence of different types of micropulsations at different places around the earth, during the development of a substorm. Time lags between the appearance of micropulsations and other geophysical effects of the substorm are of fundamental importance in this respect. Relationships between the occurrence or spectral shape of micropulsations and the state of the magnetosphere, as determined by satellite measurements are also of interest. Recent theoretical studies about the origin of these micropulsations are reviewed: natural h.m. emissions are directly linked to the thermal plasma density, the high energetic particle fluxes and pitch angle distributions in the far magnetosphere (L 5–8). We can thus expect to be able to deduce some information about the changes of these quantities during substorms. New semi-quantitative work is reported, which tries to interpret the repetitive structure of SIP events in terms of thickness of the magnetospheric tail, and the frequency drift of IPDP's in terms of magnetospheric electric fields. The present knowledge about absorption and dispersion of hydromagnetic waves through the ionosphere or inside the submagneto-ionospheric guide is also stated, because not taking these effects into account could lead to misinterpretation of the data.     

Coronal transient phenomena

Solar coronal transients, particularly those caused by flares and eruptive prominences, play a major role in the fields of solar-terrestrial physics and astrophysics. In the former field, coronal transients and their associated interplanetary disturbances are responsible for solar and galactic cosmic ray modulations, as well as planetary magnetospheric and ionospheric disturbances. In the latter field, supernovae remnants are scaled-up manifestations of such disturbances; that is they are stellar, rather than solar, coronal transients. Study of the more accessible solar transients is proving invaluable in both fields and is, therefore, selected for attention in this paper.A series of coronal transient observations is discussed in the spirit of a representative overview following some introductory remarks on the background solar wind. One of these observations is chosen because its interplanetary signature-the shock wave-was detected by two spacecraft at different heliocentric radii. Other cases are chosen because of the extended observations of embedded eruptive prominences. Progress is also being made in the interdisciplinary areas of optical imagery complemented with radio astronomical techniques.Finally, several recent theoretical models and MHD computer simulation studies are summarized. It is suggested that further comparison of specific events with such models promises a rich harvest of physical understanding of the origin, structure and interplanetary progeny of coronal transients.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.