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Recent work on the IMF control of geomagnetic activity is reviewed. The goal is to quantitatively express the temporal relation between the solar wind input and the ionospheric output from the magnetospheric system. Linear prediction filtering was used which treats the magnetosphere as a black box characterized by an impulse response. It is shown that an average impulse response can account for only about 40 percent of the variance in the AL index.

The asymptotic behavior of planetary bow shocks and the ability of gas dynamic theory to describe it are studied. Spacecraft observations at Venus, earth, and Mars are used to model the shapes and positions of their distant bow waves. The measured planetary Mach cone angles are compared with the mean sonic and MHD fast wave Mach numbers at 0.7, 1.0, and 1.5 AU to assess the downstream influence of the interplanetary magnetic field. Numerical gas dynamic flow solutions extending to 50 obstacle radii behind each planet are obtained and tested against the observed location of the downstream bow shock.

Bow shock models are employed to determine and compare the solar wind flows about Venus, Mars, and the earth. Initially, gas dynamic theory predictions are compared with the large data base available for the near-earth environment. The observed shape and location of the magnetopause proved sufficient for predicting the average dayside bow shock position to within 2% error. Use of the same gas dynamics theory produced highly disparate results for the flows past Venus and Mars. A variation of 510-1000 km altitude (high) for the solar wind-obstacle interface was found for Mars, thus requiring the presence of an effective magnetic moment of within 0.6 of 1.4 x 10 to the 22nd G/cu cm in the Martian magnetosphere, consistent with Viking data. The Venus bow wave was calculated to be closer to the planet than possible with a purely ionospheric interaction, indicating that solar wind-neutral atmosphere interactions in the lower ionosheath must necessarily be included in a gas dynamic modeling of the Venus obstacle to the solar wind.

No consensus has yet been reached on the nature of the solar wind interaction with Mars, especially with respect to the existence of a small, intrinsic field magnetosphere at Mars, as opposed to a Venus-like ionospheric interaction. An attempt is made to resolve this question through an improved determination of effective obstacle altitude and shape having as its basis the Spreiter and Stahara (1980) catalog of gasdynamic flow solutions. It is also shown that the Martian ionosphere cannot support a Venus-like ionopause at the obstacle altitudes inferred through bow wave observation modelling, even when maximal induced ionospheric magnetic fields and solar maximum EUV levels are assumed. Finally, a search of Mars and Mariner radio occultation measurements has produced no evidence for the existence of an ionopause. It is concluded that Mars probably possesses a small, intrinsic field magnetosphere.

A quantitative model of geomagnetic activity is developed and utilized to investigate the causes of the diurnal, seasonal, and IMF sector variations in the AL index records. This auroral index was chosen for study because of its high sensitivity to the strength of the westward electrojet and, hence, magnetospheric substorm activity. After the introduction of corrections for processes not related to substorms, ability of the function Bs to the 0.85th power x V squared to produce the observed variations in AL was examined. The annual variation of Bs was determined by superposing the contributions to Bs due to the inclination of the magnetic axis (Russell and McPherron, 1973) on an empirical mean distribution of Bz. V was assumed constant. The predicted values of ALc have been compared with observed averages for 9 years of solar cycle 20. The predicted annual variation of ALc for toward and away sectors are in good agreement with observation. While the predicted semi-annual component of ALc is in phase with observation, it is less than half the observed amplitude. The predicted diurnal variation of ALc for June is in satisfactory agreement with observation.

A three-parameter method for modeling the position and shape of planetary bow waves was chosen to model the near portion of the Venus, earth and Mars bow shocks, and its results were compared with those of models using one to six free variables. It was found that the relative effective shapes of the near Martian, Cytherean, and terrestrial bow shocks are ellipsoidal, paraboloidal, and hyperboloidal, respectively, in response to the increasing bluntness of the obstacles that the planets present to the solar wind. No significant deviations from axial symmetry were found when the near bow waves of the earth and Venus were mapped into the aberrated terminator plane, in agreement with gas dynamic theory predictions neglecting the effects of the IMF because of their minuteness.

Some of the natural analogs of proposed seeding experiments in the magnetosphere are: electric charged particle injections during substorms; lightning-induced whistlers providing wave injections; 'detachment' of plasma from the plasmapause, which is equivalent to light-ion seeding in the outer magnetosphere. In the present paper, the characteristics of some natural precedents to active experiments are reviewed, and their relative advantages and drawbacks are examined. One of these precedents - wave generation in detached plasma regions - is then discussed in some detail; the discussion reveals the generation of ELF electromagnetic noise when the energetic electrons exceed the stably trapped limit, and also indicates that a critical dimension for the seeded volume exists transverse to the geomagnetic field.

Simultaneous measurements of low-energy precipitating electrons and magnetic fluctuations from the low-altitude polar-orbiting satellite Ogo 4 have been compared. Analysis of the two sets of experimental data for isolated events led to the classification of high-latitude field-aligned currents as purely temporal or purely spatial variations. Magnetic field disturbances calculated by using these simple current models and the measured particle fluxes were in good agreement with measured field values. Although fluxes of electrons of greater than 1 keV were detected primarily on the night side, magnetometer disturbances indicative of field-aligned currents were seen at all local times, in both the visual auroral regions and the day side polar cusp. Thus electrons with energies of less than about 1 keV are the prime charge carriers in high-latitude day side field-aligned currents.

Simultaneous observations of ELF hiss amplitude and plasma density on Ogo 5 have been investigated. Passes through the region of variable plasma density in the outer plasmasphere have yielded a quantitative relation between the hiss amplitude, the plasma density, and the plasma density corresponding to the threshold of wave detection. It is suggested that this dependence of wave amplitude on plasma density is a source effect and is related to the wave-particle interaction in the outer plasmasphere that gives rise to hiss.

Wave normals of chorus in the outer magnetosphere have been determined for the first time from data obtained with the Ogo 5 search coil magnetometer. These measurements combined with simultaneous information concerning geomagnetic field, plasma density, and the electron energy and pitch angle distributions provide a consistent picture of the generation, propagation, and subsequent damping of chorus in agreement with theory. Specifically, the data are consistent with chorus generation within 25 deg of the equatorial plane on the dayside and within 2 deg on the nightside. Chorus is generated by a Doppler-shifted cyclotron resonance with electrons between 5 and 150 keV but only when the pitch angle distribution is peaked at 90 deg to the local magnetic field and the anisotropy exceeds a critical value.

In the first 6 months after its launch, Ogo 3 crossed the earth's bow shock over 500 times. From this group, a set of 494 shock crossings were chosen for analysis. These crossings, as they were recorded by the UCLA/JPL search coil magnetometer, were scanned and classified according to the nature of the plasma waves detected near the shock. More than 85% of the shocks detected fell into a single category showing the predominance of two independent wave trains near the shock, the higher frequency appearing upstream and the lower downstream. The other 15%, which constitute an upper limit, appear to be composed of shocks dominated by a single wave pattern and of chaotic shocks showing no orderly progression of wave frequencies as the shock was penetrated. This division of wave pattern was found to occur at all local times, that is, in all regions where the satellite penetrated the shock.

The Ogo 6 ELF chorus records from the search coil magnetometer have been compared with simultaneous electron precipitation records. The chorus signals observed in the vicinity of field lines passing through the outer magnetosphere were characteristically accompanied by electron precipitation in the same region. Both the chorus and the precipitation records consisted of a series of sharp peaks. Although in some cases chorus and precipitation peaks appeared to be associated, the observed peaks did not in general coincide. Comparison of the chorus measurements on Ogo 6 and Ogo 5 suggests a model in which the chorus is ducted along field lines to within less than 1 earth radius above the Ogo 6 orbit after which it diverges from the field lines and is deflected toward the local vertical. The result is equatorward skewing of the average wave pattern with respect to the precipitation pattern.

A simple model of magnetic flux transfer is reviewed along with empirical expressions for flux transfer rates based upon the auroral AL index and the eastward component of the interplanetary electric field. It is shown that the magnitude of the time integral of the AL index over isolated intervals of geomagnetic disturbance can be predicted from the the observed flux of southward interplanetary magnetic field lines incident on the front of the magnetosphere. Difficulties in the quantitative prediction of substorm onset time and intensity are discussed and magnetospheric stress index on both solar wind and ground based observations is proposed for this purpose. Finally, caveats concerning the use of both the AL index and interplanetary medium data in predicting the response of the magnetosphere to solar wind conditions are considered.

The relationship between the magnetosphere and the solar wind is addressed. It is noted that this interface determines how much of the solar plasma and field energy is transferred to the Earth's environment, and that this coupling not only varies in time, responding to major solar disturbances, but also to small changes in solar wind conditions and interplanetary field directions. It is recommended that the conditions of the solar wind and interplanetary medium be continuously monitored, as well as the state of the magnetosphere. Other recommendations include further study of the geomagnetic tail, tests of Pc 3,4 magnetic pulsations as diagnostics of the solar wind, and tests of kilometric radiation as a remote monitor of the auroral electrojet.

The determination of Mercury's magnetic moment from the spatially and temporally limited observations obtained by the Mariner 10 mission is dependent upon the assumed nature of both the intrinsic planetary magnetic field and that of the magnetospheric current systems. In this paper the methods that have been used for this purpose are reviewed. The results that have been obtained are then considered in comparison with the constraints placed on the planetary field by the solar wind dynamic pressures and stand-off distances inferred from the Mariner 10 magnetic field data by Slavin and Holtzer (1979) which are consistent with an effective dipole moment of 6 + or - 2 x 10 to the 22nd G cu cm. It is found that the models which fit the observations with substantial quadrupole and octupole moments are not consistent with the magnetospheric boundary conditions, presumably owing to incorrect assumptions regarding the magnetopause position, incorrect assumptions regarding solar wind dynamic pressure, and/or averaging over temporal variations in the Mariner 10 data.

The applied magnetic flux of the southward component of the IMF in GSM coordinates (Bz-) which impinges upon the sunward magnetopause is compared to the time integral of the auroral AL index during 56 intervals within a 3-month period in 1969 when interplanetary records from Heos 1 and Explorer 35 were available. The periods of magnetic activity were those for which Bz was greater than 0 and AL was less than 20gamma at the beginning and end of the interval. It was found that for these intervals, the time integral of the AL index was proportional to the applied magnetic flux with a correlation coefficient of 0.94. In addition, the empirical relationships between magnetic flux transfer, applied southward flux, and the time integral of AL arrived at by Holzer and Slavin (1978) on the basis of expansion and contractions of the forward magnetosphere observed with OGO 5 are reexamined and improved.

Measurements of the effect of dayside magnetic reconnection on the position of the earth's forward magnetopause have been scaled to Mercury in order to predict the mean solar wind stand-off distance for average conditions of solar wind dynamic pressure. It is found that for a significant portion of the time Mercury's magnetopause will be eroded and/or compressed to within 0.2 Mercury radii of the planet's surface. Solar wind stand-off distances and solar wind dynamic pressures are also calculated for the two Mariner 10 encounters with Mercury's magnetosphere. Values of the solar wind stand-off distance range from 1.3 to 2.1 Mercury radii.

A study to determine the effect of power line radiation on the stimulation of the ELF chorus detected on OGO-5 is presented. Chorus shows a maxima over the Eastern USSR, Greenland, and Central Siberia and minima over central and Eastern Canada, a distribution different than the OGO-3. This discrepancy is explained as due to data oversampling in the previous study; the OGO-5 data were reanalyzed with the oversampling removed, showing that none of the longitudinal maxima or minima are statistically significant. It is concluded that there is no significant correlation between longitude and chorus occurrence which implies that there is little or no evidence of power line harmonic radiation effects on chorus triggering.

Holzer and Slavin (1978) have found that the transfer of magnetic flux to and from the dayside magnetosphere as inferred from observed displacements of the magnetopause surface is correlated with both the magnitude of the auroral zone magnetic index AL and the incident flux of southward IMF. Empirical expressions specifying the rate at which magnetic flux is eroded in terms of interplanetary parameters and the rate of magnetic flux return as a function of AL have been developed. These relations are then used to predict magnetotail magnetic field enhancements from interplanetary and ground based data during an interval of substorm activity. The total magnetic flux in the tail is increased during intervals when the amount of flux transferred into its volume by dayside erosion exceeds the flux lost to the dayside by magnetospheric convection. Using Ogo-5 tail observations it is found for the sample events considered that these magnetic field enhancements can be described by empirical expressions for the magnetic flux transfer rates.

The paper shows that the use of an empirical ellipsoidal model of the sunward magnetopause - which uses Ogo 5 magnetometer data, solar wind data from three satellites, and auroral zone magnetic indices - yields quantitative statistical estimates of the relative values of magnetopause displacements due to solar wind variation, flux transfers, and other causes. While the displacements due to flux transfers are smaller on the average, they are clearly comparable with those due solar wind fluctuations. The fact that expansion and contraction events are observed throughout the region accessible to Ogo 5 satellite on the sunward magnetopause suggests that the entire sunward magnetosphere is affected by flux erosion and return.

A thick quasi-parallel bow shock structure was observed on February 14, 1969, with field and particle detectors of both Heos 1 and Ogo 5. The typical magnetic pulsation structure was at least 1-2 R-E thick radially and was accompanied by irregular but distinct (average) plasma distributions characteristic of neither the solar wind nor the magnetosheath. There appeared to be a separate 'interpulsation' regime occurring between bursts of large amplitude oscillations. This regime was magnetically similar to the upstream wave region but was characterized by disturbed plasma flux and enhanced noise around the ion plasma frequency. The shock structure appeared to be largely of an oblique whistler type, probably complicated by counterstreaming high-energy protons.

A thick, quasi-parallel bow shock structure was observed with field and particle detectors of both HEOS 1 and OGO 5. The typical magnetic pulsation structure was at least 1 to 2 earth radii thick radially and was accompanied by irregular but distinct plasma distributions characteristic of neither the solar wind nor the magnetosheath. Waves constituting the large pulsations were polarized principally in the plane of the nominal shock, therefore also in the plane perpendicular to the average interplanetary field. A separate interpulsation regime detected between bursts of large amplitude oscillations was similar to the upstream wave region magnetically, but was characterized by disturbed plasma flux and enhanced noise around the ion plasma frequency. The shock structure appeared to be largely of an oblique, whistler type, probably complicated by counterstreaming high energy protons. Evidence for firehose instability-based structure was weak at best and probably negative.

The low-frequency narrow hiss bands associated with plasma-density enhancements in the magnetosphere near and beyond L = 4 have been studied with the search coil magnetometer and ion mass spectrometer data from OGO 5. ELF hiss is found to accompany most of the detached plasmas in the outer magnetosphere and to be sharply limited by the steep ion-density gradient at their boundary. The observations indicate that the hiss originates in and is ducted by the plasma-density enhancements. In a particularly favorable case after an interval of low AE and Dst indices, a series of enhanced plasma-density peaks was observed between L = 4 and L = 6 in the afternoon local-time sector. In this case, it was possible to develop an empirical relation among hiss amplitude, plasma density, and L by using in situ simultaneous measurements.

Description of the first experimental determination of the wave-normal vector of proton whistlers in the ionosphere. Between the crossover frequency and the proton gyrofrequency, both right-hand and left-hand modes of propagation can occur for upgoing waves. Theoretically, the amount of energy in the respective modes depends on theta, the angle between the wave normal and the magnetic field. For proton whistlers with only left-hand mode energy between the crossover and proton gyrofrequency, theta ranged from 36 to 51 deg. For proton whistlers with strong right-hand and left-hand mode signals, theta ranged from 24 to 29 deg. The result is in good agreement with Wang's (1971) collisionless mode-coupling model. The angle between the wave normal and the vertical is found to increase with increasing altitude.

The perturbation vectors of waves up and downstream from the region of maximum compression in the bow shock were examined on OGO-5 under particularly steady solar wind conditions. The polarization of the upstream waves was RH, circular and of the downstream waves LH, elliptical in the spacecraft frame. By observing that the polarization of the waves remained unchanged as the shock motion swept the wave structure back and forth across the satellite three times in eight minutes, it was found that the waves were not stationary in the shock frame. A study of the methods of determining the shock normal indicates that the normal estimated from a shock model should be superior to one based upon magnetic coplanarity. The propagation vectors of the waves examined did not coincide with the shock model normal, the average magnetic field, or the plasma flow velocity. However, the major axis of the polarization ellipse of the downstream wave was nearly parallel to the upstream propagation vector.

Plasma wave instrument for measuriing AC electrical and magnetic field levels in outer planets missions

Very low frequency electric and magnetic wave measurements for Mariner Venus Mercury 1973 mission

Magnetic equator ELF noise examined with OGO 3 magnetometer, indicating unique signals in plasmasphere

Magnetic fluctuations observed by ground observatories, suggesting large amplitude waves as field line resonances driven by magnetopause motion

Magnetic fluctuations in various frequency ranges, associated with earth bow shock, detected with search coil magnetometer on OGO 3

Noise signals in earth magnetosheath interpreted as electromagnetic waves propagating in whistler mode

OGO triaxial search coil magnetometer for measuring earth magnetic fluctuations, discussing design rationale and observation results

Magnetospheric ELF noise, discussing OGO 3 spectrum analysis

OGO 3 search coil magnetometer data correlated with magnetopause crossing by ATS 1 satellite, discussing OGO 3 crossing of outer magnetosphere into interplanetary medium

OGO-3 spectrum analyzer measurements of magnetic noise in magnetosphere

Magnetopause location, boundary positions and magnetic noise spectral data obtained with triaxial search coil magnetometer aboard Ogo I satellite

Simultaneous measurements of low energy precipitating electrons and magnetic fluctuations from the low altitude polar orbiting satellite OGO-4 have been compared. Analysis of the two sets of experimental data for isolated events led to the classification of high latitude field-aligned currents as purely temporal or purely spatial variations. Magnetic field disturbances calculated using these simple current models and the measured particle fluxes were in good agreement with measured field values. While fluxes of greater than 1 keV electrons are detected primarily on the nightside, magnetometer disturbances indicative of field-aligned currents were seen at all local times, both in the visual auroral regions and dayside polar cusp. Thus electrons with energies less than approximately 1 keV are the prime charge carriers in high latitude dayside field-aligned currents. The satellite measurements are in good agreement with previously measured field-aligned current values and with values predicted from several models involving magnetospheric field-aligned currents.

A relatively steady band of ELF hiss has been detected by the OGO 5 search coil magnetometer on almost every passage through the plasmasphere; except for an anomalous region of the dayside at high geomagnetic latitudes, the emissions terminate abruptly at the plasmapause and are therefore referred to as 'plasmaspheric hiss.' A preliminary statistical study of the properties of the observed whistler mode turbulence has yielded the following characteristics: the waves are band limited with a sharp lower-frequency cutoff and a more diffuse upper-frequency cutoff; power spectra show a well-defined maximum near a few hundred hertz, the peak intensities generally ranging between 10 to the minus 7th power and 0.00001 gamma squared/Hz; the wave energy is spread over a bandwidth of a few hundred hertz, and corresponding wideband amplitudes are 5 to 50 milligamma; the waves are highly turbulent in nature and show little tendency toward definite polarization.

Results of an examination of the perturbation vectors of waves upstream and downstream from the region of maximum compression in the bow shock on Ogo 5 under particularly steady solar-wind conditions. The polarization of the upstream waves was right-hand circular, and that of the downstream waves left-hand elliptical in the spacecraft frame. By observing that the polarization of the waves remained unchanged as the shock motion swept the wave structure back and forth across the satellite three times in eight minutes, it was found that the waves were not stationary in the shock frame. A study of the methods of determining the shock normal indicates that the normal estimated from a shock model should be superior to the normal based on magnetic coplanarity. The propagation vectors of the waves examined did not coincide with the shock-model normal, the average magnetic field, or the plasma-flow velocity. However, the major axis of the polarization ellipse of the downstream wave was nearly parallel to the upstream propagation vector.