A re-determination of the boundaries of the cusp

Ariel 4 data (Goodallet al., 1973) of February 4 to May 5 1972 have been used to investigate the cusp boundaries. Data with the same local time (LT) and magnetic local time (MLT) around noontime and at high invariant latitudes (INL) have been used to represent the cusp area. (1) For noontime hours (MLT=1200 and LT=1200) high electron density values are observed. (2) the boundary towards the equator of the high density region starts at 76°–77° INL and the poleward boundary is at about 82° INL. Therefore, high electron densities are observed along an INL interval of 5°–6° at the Ariel 4 heights of 500–600 km, agreeing with other investigations.     

Coincident observations of ionospheric troughs and the equatorial plasmapause

Explorer 45 traversed the plasmapause (determined approximately via the saturation of the d.c. electric field experiment) at near-equatorial latitudes on field lines which were crossed by Ariel 4 (~600km altitude) near dusk in May 1972 and on field lines which were crossed by Isis II (~1400km altitude) near midnight in December 1971 and January 1972. Many examples were found in which the field line through the near-equatorial plasmapause was traversed by Explorer 45 within one hour local time and one hour universal time of Ariel and Isis crossings of the same L coordinate. For the coincident passes near dusk, the RF electron density probe on Ariel detected electron density depletions near the plasmapause L coordinates when Ariel was in darkness. When the Ariel passes were in sunlight, however, electron depletions were not discernable near the plasmapause field line. On the selected near-midnight passes of Isis II, electron density depressions were typically detected (via the topside sounder) near the plasmapause L coordinate. The dusk Ariel electron density profiles are observed to reflect O⁺ density variations. Even at the high altitude of Isis near midnight, O⁺ is found to be the dominant ion in the trough region whereas H⁺ is dominant at lower latitudes as is evident from the measured electron density scale heights. In neither local time sector was it possible to single out a distinctive topside ionosphere feature as an indicator of the plasmapause field line as identified near the equator. At both local times the equator-determined plasmapause L coordinate showed a tendency to lay equatorward of the trough minimum.     

Local plasma processes and enhanced electron densities in the lower ionosphere in magnetic cusp regions on Mars

Both the MARSIS ionospheric sounder and the charged particle instrument package ASPERA-3 are experiments on board the Mars Express spacecraft. Joint observations have shown that events of intense ionospheric electron density enhancements occur in the lower ionosphere of magnetic cusp regions, and that these enhancements are not associated with precipitation of charged particles above a few hundred electron volts (<300 eV). To account for the enhancement by particle precipitation, electron fluxes are required with mean energy between 1 and 10 keV. No ionizing radiation, neither energetic particles nor X-rays, could be identified, which could produce the observed density enhancement only in the spatially limited cusp regions. Actually, no increase in ionizing radiation, localized or not, was observed during these events. It is argued that the process causing the increase in density is controlled mainly by convection of ionosphere plasma driven by the interaction between the solar wind and crustal magnetic field lines leading to excitation of two-stream plasma waves in the cusp ionosphere. The result is to heat the plasma, reduce the electron–ion recombination coefficient and thereby increase the equilibrium electron density.     

Nearly simultaneous observations of the conjugate polar cusp regions

The first simultaneous (within 6 min) observations of the low altitude polar cusp regions in the conjugate hemispheres are reported here based on two events detected by the DMSP-F2 and F4 satellites within the same geomagnetic local time sector. It is found that the electron spectra in the cusp are identical in the opposing hemispheres. In one case the observed latitudinal location and extent of the cusps are the same at the two hemispheres. However, in the other case the location of the equatorward boundary of the cusp regions differs by about 2° with drastically different spatial features. It is also found that in one of the events the plasma sheet electron precipitation regions overlap with the cusp regions at lower latitude in both hemispheres. The poleward boundary of these overlapping regions is located at the same latitude on either hemisphere, suggesting that this is the latitude of the last closed field line and that the cusp electrons are present on both closed and open magnetic field lines.     

Polar observations of plasma waves in and near the dayside magnetopause/magnetosheath

The plasma wave instrument (PWI) on board the Polar spacecraft made numerous passages of the dayside magnetopause and several probable encounters with the magnetosheath during the years 1996 and 1997. During periods of relatively high density, the PWI antenna-receiver system is coupled to the plasma and oscillates. The oscillations have been shown (cf. Radio Sci. 36 (2001) 203) to be indicative of periods of higher plasma density and plasma flows, possibly associated with magnetic reconnection. We have studied the plasma waves observed on three distinct magnetopause passes distinguished by the presence of these oscillations of the PWI receivers, and we report on the data obtained near, but not during, the times of the oscillations and the possible role of these waves in magnetic reconnection. Sweep-frequency receiver and high-resolution waveform data for some of these times are presented. The plasma wave measurements on each of the passes are characterized by turbulence. The most stable waves are whistler mode emissions typically of several hundred hertz that are seen intermittently in these regions. The data indicate the presence of impulsive solitary-like wave structures with strong electric fields both parallel and perpendicular to the magnetic field near, but not always within, suspected reconnection sites. The solitary waves show the highest occurrence when observed with electrostatic electron cyclotron waves. These latter waves have been observed in the past in the cusp, polar magnetosphere, and auroral regions and therefore may represent excursions into the cusp, but also indicate the presence of low-energy electron beams. Turbulence near the lower hybrid frequency, low-frequency EM waves, and impulsive monopolar electrostatic pulses are seen throughout the magnetopause and particularly near regions of large decrease in the local magnetic field and enhanced field-aligned flows, the suspected reconnection sites. The absence of significant solitary wave signatures within suspected reconnection sites may require modifications to some reconnection models.     

Observation of kink waves and their reconnection-like origin in solar spicules

We analyze the time series of Ca?ii H-line obtained from Hinode/SOT on the solar limb. We follow three cases of upwardly propagating kink waves along a spicule and inverted Y-shaped structures at the cusp of it. The time-distance analysis shows that the axis of spicule undergos quasi-periodic transverse displacement at different heights from the photosphere. The mean period of transverse displacement is ～175 s and the mean amplitude is 1 arc?sec. The oscillation periods are increasing linearly with height which may be counted as the signature that the spicule is working as a low pass filter and allows only the low frequencies to propagate towards higher heights. The oscillations amplitude is increasing with height due to decrease in density. The phase speeds are increasing until some heights and then decreasing which may be related to the small scale reconnection at the spicule basis. We conclude that transversal displacement of spicules axis can be related to the propagation of kink waves along them. Moreover, we observe signatures of small-scale magnetic reconnection at the cusp of spicules which may excite kink waves.     

Temporal variations in the dawn and dusk midlatitude trough and plasmapause position

The temporal development of the latitudinal position of a 600 km midlatitude electron density trough at dawn and dusk during the period 25–27 May 1967, which encompassed a large magnetic storm, was measured by the RF capacitive probe on the polar orbiting Ariel 3 satellite. The substorm-related changes in the L coordinate of the trough minimum and the point of most rapid change of density gradient on the low latitude side of the trough are similar. Oscillations of the trough position at dusk are in phase with substorm activity whereas movement of the trough at dawn is only apparent with the onset of the large storm. Detailed model calculations of the plasmasphere dynamics assuming a spatially invariant equatorial convection E-field which varies in step with the K_p index produces a plasmapause motion which parallels the observed trough behaviour, particularly at dusk, and shows that the outer plasmasphere and possibly the trough region are characterized by complex fine structured variations due to the past history of the magnetosphere convection.     

Chorus observations by the Polar spacecraft near the mid-altitude cusp

Observations of whistler-mode emission, including chorus, were made frequently by mid-altitude satellites such as Polar. In this study we examine an example of such emission when the high-frequency waveform receiver on board Polar was operating allowing direction-finding and polarization measurements in the low-altitude cusp. The results reveal at least two and possibly three distinct groups of narrow-banded chorus emission with a primary source region at the magnetic equator. The groups include emission propagating near the resonance cone angle, emission propagating along a density duct, and possibly emission reflected from the low-altitude cusp. The results indicate that the mid- and low-altitude cusp, perhaps unlike the high-altitude cusp, is not a significant source of narrowband chorus emission.     

Ionospheric measurements from the ESRO-4 satellite

Three ionospheric probes were carried on the ESRO-4 satellite, a spherical gridded probe with swept potential collecting positive ions, a Langmuir probe measuring electron temperature and vehicle potential, and a fixed potential gridded probe measuring fluctuations in total ion density. ESRO-4 was placed in a polar orbit of apogee 1177 km, perigee 245 km on 22 November 1972 and ionospheric data of excellent quality were obtained until the spacecraft's re-entry on 15 April 1974. The instrumentation is described and early results are presented.     

Mariner 9 ultraviolet spectrometer experiment: Scattering properties of Hellas

A least-squares analysis of 709 Mariner 9 uv spectra obtained over Hellas from Revolution 140 to 214 (January 22, 1972, to February 28, 1972) showed the extinction optical depth of the atmosphere above Hellas to be 0.5 ± 0.2. This is evidence of lingering dust in the Hellas basin. An atmospheric model, combining dust and Rayleigh scattering, was used to make an apparent pressure map of Hellas. Anomalously high pressures are interpreted as clouds developing over the southern part of Hellas near the time of autumnal equinox. The blue ratio (reflectance at 2680 Å divided by reflectance at 3050 Å) showed a 20% increase from Revolution 40 to Revolution 74 (December 4, 1971, to December 20, 1971), attributed to the decay of the dust storm. Thereafter, the blue ratio remained essentially constant through Revolution 214, which implies that the rate of clearing in Hellas was much slower than that planetwide.     

Stability analysis of magnetosheath lion roars

Whistler mode wave emissions in the magnetosheath, known as lion roars, are thought to be generated by an electron cyclotron instability. Using reported satellite data we model a magnetosheath medium where lion roars emissions occurred and we study the character, absolute or convective, of the associated electron cyclotron instability. We use a linear hot plasma dispersion equation for parallel and oblique propagation to the static magnetic field and apply Derfler's frequency cusp criterion to discriminate between absolute and convective instability. Our results show that an absolute instability is compatible with experimental data. From the linear temporal growth rate we extrapolate the saturated wave magnetic field and find a good agreement with the measurements.     

Magnetospheric Cusps under Extreme Conditions: Cluster Observations and MHD Simulations Compared

MHD simulations are here applied to aid in the interpretation of three apparent cusp encounters by the Cluster 4 spacecraft in unusual places when the magnetosphere was under extreme solar wind and interplanetary magnetic field (IMF) conditions associated with the passage of magnetic clouds imbedded within fast ICMEs. At the time of each cusp encounter the IMF was very strong, generally northward in one case, generally equatorial in a second case, and generally southward in the third case. In the southward IMF case, the MHD models locate the origin of the cusp-like plasma by showing that the position of the spacecraft at the time of encounter was engulfed in a tongue of high-pressure plasma extending from the magnetopause into the magnetosphere. This tongue points to the northern-hemisphere cusp as the source of the feature. In the equatorial IMF case an elevated-pressure feature that apparently marked a cusp encounter in the computations coincided, however, with a passage in the solar wind of a dynamic pressure pulse, thus giving an alternative interpretation of the feature. However, Cluster data unambiguously identified the event as an encounter with magnetosheath-like plasma. Given that the Cluster observations classify the event as a true encounter with a cusp-like plasma feature (and not a compression event), the model simulations can be interpreted as identifying the origin of the feature to have been the northern-hemisphere cusp even though?—?and this is the interesting point?—?the observation point was in the southern hemisphere. In the northward IMF case, neither cusp (defined as a magnetic funnel linking the magnetopause to the Earth) was directly connected to the observation point. Instead, this encounter of magnetosheath-like plasma appears to be an instance of boundary-layer formation by means of the Song?–?Russell mechanism in which two-point magnetic reconnection entrains magnetosheath plasma on closed field lines when the IMF is northward.     

Plasma properties in the dayside cusp region

A statistical study of the cusp plasma has been performed using mainly electron data from the LPS, Rome, plasma experiment flown onboard HEOS-2. We have located the cusp by means of 35–50 eV electrons, from 1.5 to 2.5R_E (south pole) and from 3R_E up to 11R_E (north pole) at 60–70° SM latitude within ±60° of SM longitude from the noon meridan plane. The average cusp thickness is 4.2° of invariant latitude. The location of the cusp in invariant latitude around the noon meridian plane depends on the IMF component B_zGSM according to the linear best fit: Λ = 78.7° + 0.48B_zGSM(γ). Away from the noon meridian plane the invariant latitude of the cusp decreases from 79–84° to 70–74° (at ±50° SM Longitude). At the equatorward edge of the north pole cusp, at all radial distances and at all SM longitudes, we have found a population of electrons with a harder energy spectrum than in the cusp itself. These electrons show a peak at 170–280 eV in our data. They are not the cusp (35–50 eV) electrons and are easily distinguishable from the 1 keV magnetospheric electrons. In the south pole auroral oval they are found at any SM longitude mainly poleward of the 1 keV electrons. The cusp electrons (35–50 eV) and protons have anisotropies that vary with radial distance and SM latitude, both flowing earthward more or less along the magnetic field.     

Electron temperature models of the thermosphere to 1100 km based on ESRO-4 measurements

Measurements of thermospheric electron temperatures at altitudes in the range 250–1100 km, made with a Langmuir probe carried on the polar-orbiting satellite ESRO-4, have been used to derive model functions of electron temperature in terms of altitude, magnetic latitude and local time for the periods November 1972 to June 1973 and March to October 1973. The technique used to compute the coefficients of the model functions is described, and the model electron temperatures are compared with those obtained from similar instruments on the Ariel-1 satellite in 1962 and the ESRO-1A satellite in 1968–1969, and from ground-based observatories. The models reproduce the major features of topside electron distributions viz. mid-day temperatures exceeding midnight temperatures by about 500 K, dawn enhancement leading to peak temperatures greater than mid-day values particularly around 50° magnetic latitude, and temperatures increasing with altitude at all latitudes and with latitude at all altitudes. The daytime mid-latitude temperature is used to complete a series of observations by various techniques over a solar cycle and thereby to confirm the sense and degree of solar cycle control on the thermospheric electron temperature predicted by theoretical considerations.     

Non-linear effects of VLF noise generation by electron beams in the polar magnetosphere

A theory of VLF noise excitation by electron beams in the polar magnetosphere is proposed. Two modes of excited oscillations are considered: waves with frequencies in the vicinity of the lower hybrid resonance (LHR) from about 50 to 1000 kHz and whistler-mode waves in the frequency range of several kHz.The spectral distribution and the level of turbulent noise, having been excited by means of two counterstreaming electron beams, are deduced in magnetized plasma at the LHR frequency. It is also shown that the growth of noise up to the quasistationary level oscillates with time. Energy density of oscillations at the LHR frequency in the region of the dayside polar cusp agrees with the experimental data.The processes of whistler excitation by electron beams are discussed. The growth rate of excitation of whistler-mode by electrostatic oscillations at the LHR frequency is calculated.     

Field-aligned currents above an auroral arc

Simultaneous observations of precipitating electrons and protons in the energy range from 15 eV to 35 keV and magnetic field variations were made onboard a sounding rocket payload launched from the Andoya Rocket Range. The electric current density deduced from the electron precipitation observed during the passage over an auroral arc was comparable to that determined from the magnetic field variations. In addition, a downward current was observed by its magnetic field signature at the northern edge of the arc which was, however, not accompanied by significant particle fluxes in the energy range under consideration. It will be assumed that this current was carried by thermal electrons of ionospheric origin.     

A magnetic boundary signature within flux transfer events

We present high resolution AMPTE-UKS data for a FTE signature in the magnetosphere. When the observations are confronted with the canonical model for such events, we find that we can substantiate clearly the existence of the two field regions (draping vs twisting) predicted from the model. In addition to these regions there is a separate, distinct region close to the expected boundary between open and closed field lines which we estimate to be of order ten ion gyroradii thick. This region is distinguished by a distinctive field and particle signature. The magnetic signature has not been reported before, but the electron signature bears a close resemblance to the electron heat flow layer reported in the ISEE data by Scudder et al. [1984 in Magnetic Reconnection in Space and Laboratory Plasmas (Edited by Hones E.W., Jr.), p. 153. AGU, Washington]. Other observations we report are the occurrence of a large flow burst and, in the central region of the FTE signature, the presence of magnetic field oscillations of period ∼ 3 s.     

Effects of the dayside field-aligned currents in location and structure of polar cusps

We have studied the dayside magnetosphere structure and its K_p, A_E and IMF-dependence using the magnetic data from IMP and HEOS satellites obtained during 1966–1972. An analysis of the field line configurations has been done on the basis of results of a least squares fitting of the model coefficients to the data subsets. The plots of the magnetopause subsolar point distance and of the polar cusp latitude vs K_p and A_E have been obtained. A detailed study of the model field distribution has revealed a substantial difference in the polar cusp field line geometry between the cases of weak and strong geomagnetic activity. We find that this results in a considerable longitudinal extension of the isointensity contours of particle precipitation at ionospheric heights during disturbed periods with K_p ? 3 or A_E ? 300 nT. The same effect has been detected for the data subsets corresponding to the IMF B_z < 0. In contrast, at quiet times the precipitation isolines are much closer to circles. We conclude therefore that the cleft-like structure of polar cusps pertains only to active periods and can be explained by a magnetic effect of enhanced Birkeland currents.     

Gamma-ray and microwave evidence for two phases of acceleration in solar flares

Relativistic electrons in large solar flares produce gamma-ray continuum by bremsstrahlung and microwave emission by gyrosynchrotron radiation. Using observations of the 1972, August 4 flare, we evaluate in detail the electron spectrum and the physical properties (density, magnetic field, size, and temperature) of the common emitting region of these radiations. We also obtain information on energetic protons in this flare by using gamma-ray lines. From the electron spectrum, the proton-to-electron ratio, and the time dependences of the microwave emission, the 2.2 MeV line and the gamma-ray continuum, we conclude that in large solar flares relativistic electrons and energetic nuclei are accelerated by a mechanism which is different from the mechanism which accelerates 100 keV electrons in flares.Research supported by NASA Grant 21-002-316 at the University of Maryland, College Park.